Metal Phosphates and Pyrophosphates as Proton Conductors

Metal Phosphates and Pyrophosphates as Proton Conductors: Comparison

Please note this is a comparison between Version 1 by Montse Bazaga-García and Version 4 by Amina Yu.

It We is aboutreview the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure–property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH2PO4, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure–conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.

metal phosphate
proton conductivity
H-bond network
proton carriers
super protonic
metal pyrophosphate

1. Introduction

Metal phosphates (MPs) comprise an ample class of structurally versatile acidic solids, with outstanding performances in a wide variety of applications, such as catalysts ^[1][2][3], fuel cells ^[4][5][6], batteries ^[7], biomedical ^[8], etc. Depending on the metal/phosphate combinations and the synthetic methodologies, MP solids can be prepared in a vast diversity of crystalline forms, from 3D open-frameworks, through layered networks, to 1D polymeric structures.

The advantages of the solids are, among others, their low cost and easy preparation, hydrophilic and thermally stability and structural designability to a certain extent. Additionally, their structures are amenable to post-synthesis modifications, including the incorporation of ionic or neutral species that significantly affect their functionality and other important properties, such as the formation of hydrogen bonding networks or insertion of proton carriers.

While tetravalent and monovalent metal phosphates are usually prepared by conventional synthetic routes, the synthesis of many divalent and trivalent metal phosphates requires the presence of organic molecules/ions as structure-directing agents, some of which also act as charge-compensating ions^[9]. In any case, the metal phosphates contain acidic phosphate groups (H₂PO₄⁻ and HPO₄²⁻) or they can result from specific post-synthesis acid–base reactions, thus, allowing a mostly intrinsic proton conduction.

Additionally, an extrinsic proton conduction, associated with surface proton transport, can also result from specific chemical modification and/or induced morphological changes, e.g., with formation of nanoplatelets or nanorods particles ^[10][11]. Water molecules act both in the formation H-bonding networks inside the framework and as proton carriers. Moreover, other guest species, including organic moieties, can be inserted in their structure following different strategies, such as intercalation ^[12][13][14], template ^[15][16], and post-synthesis adsorption ^[17][18][19].

Accordingly, proton transport in MPs results from the interplay between water/guest molecules and the hydrophilic network/framework walls in specific ways. Thus, in metal phosphates, such as CsH₂PO₄ ^[20] or CaHPO₄·2H₂O ^[21] the proton-transfer mechanism involves not only the jump of H⁺ but also the free rotation of the phosphate groups. In other cases, such as for KH₂PO₄ the proton-transfer mechanism behavior is by defects ^[22].

A more complex proton-transfer process arise from the so called “quasi-liquid” state, such as described for H₃OUO₂PO₄·3H₂O ^[23][24], or it may stem from various mobile species moving on the surface, each one using different paths at different rates, e.g., in Ce(HPO₄)₂·nH₂O ^[25][26]. The proton transport in the quasi-liquid state is often characterized by a “proton jump” or the Grotthuss mechanism, with low activation energy (E_a) ^[21].

Determining proton conduction mechanisms is key to designing proton conductor materials (Figure 1). Thus, if a vehicle-type mechanism is needed, proton-containing counter ions, such as hydronium ions, NH₄⁺, protonated amine groups, and protonated organic molecules in general, should be incorporated into the structure. Additionally, the immobilization of specific functional groups and the corresponding counter ions onto the framework could be required.

Figure 1. Examples of proton transport in phosphate-containing materials: (a) structural reorientation-mediated proton transfer. Zn (magenta), P (orange), O (red) and H (pale pink). (b) hopping through H-bond networks, and (c) carrier-mediated proton conduction. Zn (magenta), Zr (green), Cu (cyan), Cl (light green), P (orange), O (red), N (blue) and H (pale pink) atoms.

On the other hand, solids exhibiting proton transport by a Grotthuss-type mechanism basically consist of continuous H-bond networks that favour H⁺ conduction with low activation energy (E_a). A main drawback for the latter materials is that, above certain temperatures, breaking of the H-bond networks can result, accompanied by the release of water molecules, which makes the construction of permanent H-bonding networks with hydrophilic channels or exploring new alternative conductive media necessary ^[27].

A research area of paramount interest is that relative to fuel-cell (FC) development regarding energy efficiency, environmental issues, and the gradual depletion of conventional fuel reserves ^[28][29]. The operating temperature regime of these electrochemical devices is determined by the electrolyte. Among the many types of solid electrolytes that have been developed over the past several decades ^[30], perfluorinated sulfonic acid (PFSA) ionomers, such as Nafion^® and Aquivion^®, are the most widely used in PEMFCs and DMFCs as polymer electrolyte membranes (PEM) for operating temperatures <120 °C.

This is due to superior conductive properties, ranging from 9 × 10⁻³ S·cm⁻¹ to 1.2 × 10⁻¹ S·cm⁻¹, and chemical and mechanical stability ^[31][32]. This PEM acts as a separator for the reactants and, at the same time, provides pathways for proton transport ^[33][34]. However, Nafion^® also shows some important drawbacks, such as methanol permeability, high costs, and complex steps of preparation, working below 100 °C to maintain the hydration of the membrane and other issues related to catalyst efficiency. All this makes necessary the development of new membrane materials ^[35][36].

Intermediate Temperature fuel cells (ITFCs), which operate between 120 and 300 °C, circumvent some of these problems, achieve less catalyst poisoning and better handling of the hydration conditions, and enable the use of non-precious or low-loading precious metal catalysts ^[37][38][39]. Nevertheless, for the case of methanol HT-PEMFCs, which must operate above 250 °C and with high pressure, possible degradation of the electrolytes should be a subject of study ^[40]. Some MP-based electrolytes gather proton conductivity properties to be used as single electrolytes or main components for ITFC applications. Examples are superprotonic CsH₂PO₄ ^[41] and M(IV)P₂O₇-based electrolytes ^[42].

This review summarizes the state of the art on proton conductive metal phosphates with special attention to the last two decades of developments. A review of publication trends, from the CAS SciFinderⁿ database, is given in Figure 2, which confirms the increasing interest in this topic during this period. In this article, our focus is placed on updating recent advances in proton conductors based on tetravalent metal phosphates/pyrophosphates and superprotonic Cs₂HPO₄. In addition, the design and development of new proton carrier systems relying on divalent and trivalent metal phosphates are also revised.

Metal

Figure 2. Publications on metal phosphates and pyrophosphate proton conductors over time.

2. Tetravalent Metal Phosphates and Pyrophosphates

2.1. Zirconium Phosphates

The prototype of layered metal(IV) phosphates is zirconium hydrogen phosphate, which presents mainly three topologies, known as the forms α: Zr(HPO₄)₂·H₂O (α-ZrP), γ: Zr(PO₄)(O₂P(OH)₂) 2H₂O (γ-ZrP), and λ: (Zr(PO₄)XY, X = halides, OH⁻, HSO₄⁻, and Y = DMTHUS, H₂O, etc.) (λ-ZrP). α-ZrP is the most thermal and hydrolytically stable phase and, thus, the most studied compound following the pioneering works by Giulio Alberti and Abraham Clearfield groups ^[43][44][45].

In the structure of α-ZrP, planar Zr atoms are coordinated by the three oxygen atoms of HO-PO₃⁻ groups, while the P-OH bond points toward the interlayer space giving rise to small cavities, where the lattice water is located. Although this water forms hydrogen bonds with P-OH groups, an extended interlayer hydrogen bond network is absent in this structure ^[44]. On the other hand, the structure of γ-ZrP consists of a biplanar Zr atomic system connected through bridge PO₄³⁻ groups, while the externally located H₂PO₄⁻ groups complete the octahedral coordination of the Zr atoms.

Due to the higher stability of the α form, proton conductivity studies have been conducted mainly with this phase. Typical values of 10⁻⁶–10⁻⁴ S·cm⁻¹ have been determined at RT and 90% RH with activation energy (E_a) ranging between 0.26 (90% RH) and 0.52 eV (5% RH). This conductivity originates from surface transport and is dominated by surface hydration since the long distance between adjacent P-OH groups hinders proton diffusion along the internal layer surface ^[46].

According to theoretical estimations ^[47], the interactions between POH and water molecules on the zirconium phosphate surface are relatively large and generate strong local hydrogen-bond networks. The high density of acid groups should produce continuous short O-O distances and generate proton-transfer paths with low activation energy, even under high-temperature and low-humidity conditions.

Studies on microcrystalline α-ZrP revealed that both the morphology and the electric field orientation are important aspects to consider. Thus, pellicular samples of α-ZrP, prepared by intercalation-deintercalation of n-propylamine ^[48], exhibited a proton conductivity of 10⁻⁴ S·cm⁻¹ at 300 °C when it was measured with the electric field parallel to the pellicles. This conductivity was found to be approximately two orders of magnitude higher than that measured on applying the electric field perpendicular to the pellicles ^[49].

Several other strategies to increase the proton conductivity of α-ZrP have been developed (Table 1). One of the first approaches was the formation of polyhydrated n-propylamine-intercalated compounds (ZrP·xPrNH₂·nH₂O). For a composition with x = 0.8 and n = 5, a proton conductivity of 1.2 × 10⁻³ S·cm⁻¹ at 20 °C and 90% RH was determined ^[50]. Another more sophisticated way of enhancing the proton conductivity of zirconium phosphate was the preparation of mesoporous zirconium phosphate/pyrophosphate, Zr(P₂O₇)_0.81(O₃POH)_0.38.

This material, obtained from the thermal decomposition of zirconium phosphite benzenediphosphonate, displayed a high specific surface area (215 m²/g) with an enhanced proton conductivity of 1.3 × 10⁻³ S·cm⁻¹ at 20 °C and 90% RH ^[51]. One of the most successful ways of obtaining zirconium phosphate derivatives with outstanding proton conductivity properties consisted in preparing mixed zirconium phosphate/phosphonates, an example being [Zr(O₃POH)_2−x(O₃PC₆H₄SO₃H)_x], whose proton conductivity was found to increase with x, up to a value of 0.07 S·cm⁻¹ (x = 1.35 at 100 °C and 90% RH) ^[52].

The great versatility of zirconium phosphate preparation under numerous experimental conditions, has been exploited to synthesize various novel derivatives, which displayed remarkable proton conductivity properties. Two examples are layered (NH₄)₂[ZrF₂(HPO₄)₂] ^[53] and the 3D open framework zirconium phosphate (NH₄)₅[Zr₃(OH)₃F₆(PO₄)₂(HPO₄)] ^[54]. In both structures, the participation of NH₄⁺ ions in the formation of extended hydrogen-bond networks (Figure 3) resulted in proton conductivities of 1.45 × 10⁻² S·cm⁻¹ (at 90 °C and 95% RH) and 4.41 × 10⁻² S·cm⁻¹ (at 60 °C and 98% RH), respectively. From the E_a values found, 0.19 and 0.33 eV, respectively, a Grotthuss-type proton-transfer mechanism was inferred for these materials (Table 1).

Figure 3. Layered structure of compound (NH₄)₂[ZrF₂(HPO₄)₂] and possible proton-transfer pathways (adapted from ^[53]).

A one-dimensional zirconium phosphate, (NH₄)₃Zr(H_2/3PO₄)₃, consisting of anionic [Zr(H_2/3PO₄)₃]_n³ⁿ⁻ chains bonded to charge-compensating NH₄⁺ ions was reported ^[55]. In this structure, the phosphates groups are disorderly protonated, while NH₄⁺ ions occupy ordered positions in between adjacent chains (Figure 4). This arrangement generates H-bonding infinite chains of acid–base pairs (N-H···O-P) that lead to a high proton conductivity in anhydrous state of 1.45 × 10⁻³ S·cm⁻¹, at 180 °C (Table 1). This material also showed a remarkable high performance in PEMFC and DMFC under operation conditions.

Figure 4. Views of chain packing and H-bond network (dashed grey lines) in compound (NH₄)₃Zr(H_2/3PO₄)₃ (adapted from ^[55]).

2.2. Zirconium Phosphate Composite Membranes

ZrP has been widely studied as a suitable filler for the preparation of Nafion^®-composite electrolytes due to its good thermal and chemical stability, proton conductivity, hydrophilic nature, very low toxicity, and stability in hydrogen/oxygen atmospheres. Nafion^®, a sulfonated tetrafluoroethylene-based fluoropolymer copolymer is the most widely used proton conductor for polymer electrolyte membrane fuel cells (PEMFCs) ^[56].

Despite its excellent properties, this polymer is limited to operations at low temperatures of <100 °C, due to dehydration, which drastically reduces the proton conductivity. As operating at higher temperatures is desirable for better efficiency, Nafion^® composite membranes have been investigated in which fillers of diverse nature were incorporated to the polymer using different approaches.

Various methodologies of preparing Nafion^®/ZrP membranes have been devised ^{[46][57][58][59][60][61][62][63][64][65]}. Among them, treatment of the Zr(IV)-exchanged ionomer with phosphoric acid ^[59], filler/ionomer co-precipitation ^[61] and casting of ionomer solution/filler gel mixtures ^[60][61][62]. In general, improvements in mechanical properties and efficiency ^[61] were highlighted, although high filler loads beyond 25% led to decreased proton conductivity in certain cases.

A derivation of the above methods consisted of using nanosized ZrP particles-containing ionomer Aquivion^® as fillers ^[63]. The resulting composite membranes showed enhanced elastic modulus and lower proton conductivity as compared to Nafion^® itself, although they were high enough for fuel cell application ^[64] with values ranging between 0.16 and 0.23 S·cm⁻¹ at 90% RH ^[62][65].

Derivatising ZrP with hydrophobic groups resulted in composite membranes mechanically stronger than the pristine Nafion^® membrane while displaying high proton conductivities ^{[66][67][68][69][70]}. Furthermore, integrating both hydrophobic and hydrophilic ZrP nanofillers showed a synergistic effect with mechanical reinforcement of the composite membrane and better fuel cell performance than that found when single ZrP filler were employed ^[60]. Other ionomer alternatives to Nafion^®, such as sulfonated poly(arylene ether) and poly(ether ether ketone) polymers (SPAEK and SPEEK, respectively), polybenzimidazole (PBI), and vinyl-type (PVA and PVP), have been employed as components of polymer electrolyte membranes ^[71].

The sulfonated poly(fluorenyl ether ketone) (SPFEK) polymer presents acceptable proton conductivity and stability; however, its performance is lesser in comparison to the benchmark Nafion^® ionomer. To overcome this handicap, several approaches have been implemented, one of them being hybridization with α-zirconium phosphate. Recently, SPFEK/ZrP-SO₃H composite membranes have been reported with improved proton conductivity, oxidative stability, methanol cross-over barrier and improved H₂/O₂ fuel cell performance ^[72][73].

ZrP has been also used as a filler in other polymeric matrices, such as poly(vinylidene fluoride) (PVDF) and (sulfonatedpoly(styrene-block-(ethylene-ran-butylene)-block-styrene) (S40). The resulting composite membranes were tested for potential applications in DMFC, with improved performance especially relative to reducing the methanol permeability ^[72][74]. PA-PBI/Zr(HPO₃) composite membranes (PA-PBI = phosphoric acid-doped polybenzimidazole) exhibited enhanced proton conductivity as compared to a pristine polymer electrolyte membrane at 200 °C and 5% RH ^[75].

A different approach is based on the substitution of the polymer membranes for others more eco-friendly, inexpensive, and biodegradable materials. Although poly(vinyl alcohol) (PVA) is a suitable candidate for this purpose, it still requires some enhancements related to its ionic conductivity due to its rigid structure. As an example, Gouda et al. ^[76] doped iotan carrageenan (IC)/sulfonate PVA (SPVA) membranes with different loads of zirconium phosphates. Enhancement of the membrane properties was attributed to abundant hydrogen bond interactions established between the zirconium phosphate particles and the polymers.

2.3. Titanium and Tin(IV) Phosphates

Structurally, titanium(IV) phosphates are more diverse than ZrP, given that structures containing an oxo/hydroxyl ligand or mixed-valence (Ti^III/Ti^IV) titanium ions have been reported. In addition to the zirconium phosphate analogues, α- ^[77] and γ-TiP ^[78], two more other layered titanium phosphates, TiO(OH)(H₂PO₄)·2H₂O ^[79] and Ti₂O₃(H₂PO₄)₂·2H₂O are known ^[80]. Furthermore, the mixed-valence titanium phosphate Ti^IIITi^IV(HPO₄)₄·C₂N₂H₉·H₂O contains microporous channels ^[81], where monoprotonated ethylenediamine and lattice water are accommodated, which favours its transformation into a porous phase, Ti₂(HPO₄)₄, at 600 °C with a 3D structure similar to that of τ-Zr(HPO₄)₂ ^[82].

Impedance spectroscopy experiments combined with dynamics simulations revealed that this compound was a good proton conductor, with a σ value of 1.2 × 10⁻³ S·cm⁻¹ (at room temperature and 95% RH) and E_a of 0.13 eV (Table 1). From these data, a water-mediated proton transport originating from the H-bond interaction of water molecules with the bridging oxygen of the porous framework was proposed ^[83].

The crystal structure of two other open-frameworks Ti₂O(PO₄)₂·2H₂O polymorphs (ρ-TiP and π-TiP) has been reported ^[84][85]. Fibrous ρ-TiP and π-TiP crystallize in triclinic and monoclinic unit cells, respectively, and are composed of TiO₆ and TiO₄(H₂O)₂ octahedra bridged through the orthophosphate groups (Figure 5). This type of connectivity creates two types of 1D channels running parallel to the direction of the fibre growth ^[84][85][86]. ³¹P MAS-NMR spectroscopy studies revealed that π-TiP has capability of adsorbing superficially protonated phosphate species (H₃PO₄/H₂PO₄⁻/HPO₄²⁻), which affects the proton conductivity properties as confirmed by AC impedance measurements.

Figure 5. Structure of Ti₂O(PO₄)₂·2H₂O, a fibrous compound with capability of enhancing surface proton transport by H₃PO₄ impregnation (adapted from ^[86]).

Thus, this solid reached a proton conductivity value of 1.3 × 10⁻³ S·cm⁻¹, at 90 °C and 95% RH (Table 1). This behaviour was explained as being due to an extrinsic vehicle-type proton transport mechanism. Additionally, H₃PO₄-impregnated π-TiP solid was used as filler for the preparation of composite membranes of Chitosan (CS) matrices exhibiting a proton conductivity of 4.5 × 10⁻³ S·cm⁻¹ at 80 °C and 95% RH, which is 1.8-fold higher than that of the pristine CS membrane ^[86].

Insights in the structure of the isomorphous series of layered α-metal(IV) phosphates [M(IV) = Zr, Ti, Sn) showed appreciable differences in the hydrogen bond interaction of the lattice water with the layers, associated with a variable layer corrugation degree along the series; α-TiP and α-SnP displaying H-bonds stronger than the prototype α-ZrP ^[87], which, in turn, might have significant implications in making internal surfaces accessible to proton conduction paths.

Nevertheless, studies on structure–property correlations are still lacking, due, in part, to appreciable differences in chemical stability under different conditions, particularly in the case of α-SnP. Recent studies carried out on a nanolayered γ-type tin phosphate, Sn(HPO₄)₂·3H₂O, have revealed that this solid could be obtained in a water-delaminated form, which showed a high proton conductivity of ~1 × 10⁻² S·cm⁻¹ at 100 °C and 95% RH (Table 1). This high value was attributed to strong H-bonds between water and the SnP layers, in combination with a high surface area of 223 m²·g⁻¹ ^[88].

2.4. Other Tetravalent Phosphates

POH groups-containing silicophosphates have received attention as proton-conducting electrolytes ^[42] but its solid-state structural characterization is elusive, at least, under conditions similar to those used in operating fuel cells ^[89]. A conductivity value of 2.5 × 10⁻³ S·cm⁻¹ at 180 °C, under more than 0.4% RH, was reported for a silicophosphate-based composite membrane. However, it was noted that the formation of Si₅O(PO₄)₆ crystals, under dry conditions, was detrimental for the proton conductivity.

NASICON-type compounds with the general formula, A_xB₂(PO₄)₃, where A = H⁺, H₃O⁺, or NH₄⁺ and B = Zr, Ti, Hf, Sn, Ge, etc., are also potential proton conductors ^[90][91]. HZr₂(PO₄)₃ was firstly prepared by Clearfield et al. ^[92] and Komorowski et al. ^[93] described the synthesis of a hydrogen form from Na_{1 + x}Zr₂Si_xP_3–xO₁₂ by replacing Na⁺ ions with hydronium ions through ion exchange. This solid showed a bulk proton conductivity of 5.0 × 10⁻⁴ S·cm⁻¹ at 100% RH and 25 °C, the proton transport mechanism being a Grotthuss-type one ^[93].

Other materials, such as HZr₂(PO₄)₃·nH₂O, NH₄Zr₂(PO₄)₃, or H_{1 ± x}Zr_{2 x}Mx(PO₄)₃ · H₂O (M = Nb, Y) ^[94][95], have been reported. Some NASICON-type compounds based on zirconium or titanium hydrogen phosphate exhibit remarkable proton conductivity while showing a high thermal stability, at least for the case of zirconium derivatives ^[96][97]. Less studied are the NASICON-type hafnium hydrogen phosphates. Among them, (NH₄)_0.4H_0.6Hf₂(PO₄)₃, obtained by thermal decomposition of the cubic NH₄Hf₂(PO₄)₃ at 550 °C, showed a proton conductivity of 1.2 × 10⁻⁶ S·cm⁻¹ at 500 °C ^[90].

2.5. Tetravalent Pyrophosphates

In addition to acid metal(IV) phosphates, tetravalent metal pyrophosphates (MP₂O₇; M = Sn, Ce, Ti, Zr), in particular tin(IV) derivatives, have received considerable attention as proton conductors for intermediate temperature electrolyte applications. These solids combine high thermal stability (T ≥ 400 °C) with conductivities of 10⁻³–10⁻² S·cm⁻¹ in the temperature range of 100–300 °C, under anhydrous or low humidity conditions ^{[98][99][100][101][102]}. The origin of this proton conductivity was initially explained because of protons being incorporated into the framework, formed by metal(IV) octahedra and corner-sharing phosphate tetrahedral, after interaction with water vapor.

Defect sites, such as electron holes ^[103] and oxygen vacancies are believed to favour proton generation ^[104]. These protons may occupy hydrogen-bonding interstitial sites on either the M−O−P or the P−O−P bonds, thus, generating a hopping proton transport between these sites ^[105][106]. In support of this, ¹H NMR studies revealed two signals corresponding to hydrogen-bonded interstitial protons at phosphate tetrahedral and metal octahedral sites in In³⁺-doped SnP₂O₇. Furthermore, indium doping seemed not to affect the proton conduction occurring by a hopping mechanism between octahedral and tetrahedral sites.

Thus, the increased proton conductivity is instead attributable to increased proton concentration ^[107]. The strategy of dopant insertion into the M(IV) pyrophosphate has been further extended to include other dopant divalent and trivalent metal ions, such as Mg²⁺, Al³⁺, Sb³⁺, Sc³⁺, and Ga³⁺, with a maximum conductivity of 0.195 S·cm⁻¹ (Table 1) being reported for In_0.1Sn_0.9P₂O₇ ^[108]. However, recent studies on metal doped and undoped SnP₂O₇ suggested that it is in co-precipitated phosphorous-rich amorphous phases where the proton conductivity mostly resides, while the crystalline phase exhibit a very low conductivity (~10⁻⁸ S·cm⁻¹ at 150−300 °C) ^[109].

Notwithstanding, the proton conductivity of the high temperature-sintered materials could be recovered by phosphoric acid treatment ^[110] or by carefully choosing the phosphate precursor. For the latter case, tetrabutylammonium phosphate-based SnP₂O₇/Nafion^® composite membranes showed an increased fuel cell performance, as compared to other analogous SnP₂O₇/Nafion^® composite membranes ^[111]. A major challenge in using metal(IV) pyrophosphates as electrolytes is forming dense pellets and the associated low open circuit voltages (OCVs) and considerable gas crossover ^[108].

Several studies to test applicability of composite membranes for ITFCs have been conducted. For this purpose, dispersion of Sb_0.2Sn_0.8P₂O₇ nanoparticles into H₃PO₄-doped PBI membranes were shown to exhibit enhanced conductivities and greater power density relative to the pristine membranes in the single cell tests ^[112]. Similar claims were made for composite membranes based on Sn_0.95Al_0.05P₂O₇ ^[113]. Other approaches, such as using graphite oxide (GO)/SnP₂O₇ or biphosphate-containing SnP₂O₇/Nafion^® composite membranes have been accomplished.

The former showed improved proton conductivity (~7.6 × 10⁻³ S·cm⁻¹) peak power density of 18 mW·cm⁻² at 220 °C and reduced fuel crossover ^[114]. The latter, aimed at increasing stability by establishing strong ammonium cation-biphosphate ion interactions, which prevents the loss of biphosphate and, hence, optimises the three-phase interface at the fuel cell electrodes. With these improvements, the H₂/O₂ fuel cell delivered a power density of 870 mW·cm⁻² at 240 °C, with minimal performance loss even under exposure to gas containing 25% carbon monoxide ^[37].

Mg-doped cerium pyrophosphate, with optimal composition of Ce_0.9Mg_0.1P₂O₇, showed a remarkable performance as electrolyte into a fuel cell, generating electricity up to 122 mA·cm⁻² at 0.33 V and displaying a peak power value of 40 mW·cm⁻² when using 50% H₂ at 240 °C. Prepared in disk form and sintered at 450 °C, the Mg-doped material exhibited a significantly high proton conductivity in the presence of moist air (P_H2O = 0.114 atm) between 160 and 280 °C. A maximum value of 4.0 × 10⁻² S·cm⁻¹ was found at 200 °C, (Table 1), which was even higher than that of CeP₂O₇ (3.0 × 10⁻² S·cm⁻¹, at 180 °C) ^[115].

Low crystallinity titanium pyrophosphates also exhibit high proton conductivity ^[116]. Values of 0.0019–0.0044 S·cm⁻¹, at 100 °C and 100% RH were reported for these compounds. The proton conduction is attributable to the presence of H₂PO₄⁻ and HPO₄²⁻ species, demonstrated by ³¹P MAS NMR and FTIR, and it occurs through a water-facilitated Grotthuss-type proton transport mechanism. Titanium phosphates are also prone to be functionalized with phosphonates groups. Thus, mixed titanium phosphate/phosphonates, Ti(HPO₄)_1.00(O₃PC₆H₄SO₃H)_0.85(OH)_0.30·nH₂O ^[117], displayed an exceptional proton conductivity of 0.1 S·cm⁻¹ at 100 °C (Table 1).

A structurally different proton-conductor metal pyrophosphate is the mixed template-containing vanadium nickel pyrophosphate, (C₆H₁₄N₂)[NiV₂O₆H₈(P₂O₇)₂]·2H₂O (Figure 6). Its crystal structure is composed of octahedrally coordinated V(IV) and Ni(II) interconnected through bridging pyrophosphate groups, thus creating a 3D framework of [NiV₂O₆H₈(P₂O₇)₂]²⁻ unit charged-compensated by protonated DABCO molecules. Hydrogen-bonding networks inside 3D channels facilitate the proton transport, and thus this material exhibits a remarkable proton conductivity of 2.0 × 10⁻² S·cm⁻¹ at 60 °C and 100% RH (Table 1) through a Grotthuss-type proton-transfer mechanism (E_a = 0.38 eV) ^[118].

Figure 6. (a) View, along b-axis, of the chain packing and (b) 3D-framework of compound (NH₄)₃Zr(H_2/3PO₄)₃, with H-bond network (dashed blue lines) highlighted (adapted from ^[118]).

3. Super Protonic Metal(I) Phosphates

Solid acid proton conductors, with stoichiometry M^IHyXO4 (MI = Cs, Rb; X = S, P, Se; y = 1, 2), have received much attention because they exhibit exceptional proton transport properties and can be used as electrolytes in fuel cells operated at intermediate temperatures (120-300 °C). The fundamental characteristics of these materials are the phase transition that occurs in response to heating, cooling or application of pressure ^[6][119], accompanied by an increase in proton conductivity of several orders of magnitude—referred to as super protonic conductivity (Figure 7).

Figure 7. The characteristic high and low temperature proton conductivity and the corresponding structures for CsH₂PO₄, adapted from ^[6][120].

This property has been associated with the delocalization of hydrogen bonds ^[120]. For CsH₂PO₄, a proton conductivity of 6 × 10^-2 S·cm^-1 (Table 1) was measured above 230 °C corresponding to the super protonic cubic (Pm-3m) phase ^[121] while it drastically drops in the low temperature phases. Recently, from an ab initio molecular dynamics simulation study of the solid acids CsHSeO₄, CsHSO₄ and CsH₂PO₄, it was concluded that efficient long-range proton transfer in the high temperature (HT) phases is enabled by the interplay of high proton-transfer rates and frequent anion reorientation.

In these compounds, proton conduction follows a Grotthuss mechanism with proton transfer being associated with structural reorientation ^[122]. The super protonic conductor CsH₂PO₄ is stable under humidified conditions (P_H2O = 0.4 atm) ^[6], but it dehydrates to CsPO₃, via the transient phase Cs₂H₂P₂O₇, at 230-260 °C, according to the relationship log(P_H2O/atm) = 6.11(±0.82) - 3.63(±0.42) × 1000/(T_dehy/K) ^[123].

Several studies ^{[6][124][125]} have shown that CsH₂PO₄ can be employed as the electrolyte in fuel cells with good long-term stability. Thus, a continuous, stable power generation for both H₂/O₂ and direct methanol fuel cells operated at ~240 °C was demonstrated for this electrolyte when stabilized with water partial pressures of ~0.3-0.4 atm. In fact, high performances, corresponding to single cell peak power densities of 415 mW·cm^-2, was achieved for a humidified H₂/O₂ system provided with a CsH₂PO₄ electrolyte membrane of only 25 μm in thickness ^[126].

Various strategies, including saltmodification by cation and anion substitution ^{[127][128][129][130]}, and mixing with oxide materials ^{[131][132][133][134][135][136][137]} or organic additives ^{[121][138][139][140]}, have been explored to improve the solid acid performance. The preparation of these derivatives and composites involved different synthesis techniques, such as sol-gel, impregnation, thin-casting and electrospinning, depending on the state of the precursor materials and the

desired product ^[6].

Studies on the effects of incorporating different substituents in the structure of CsH₂PO₄have shown mixed results. Thus, while incorporation of Rb⁺, up to 19%, led to a maximum conductivity of 0.03 S·cm^-1, at 240 °C , Ba²⁺-doping only increased the conductivity of the low temperature phase in two orders of magnitude ^[127]. On the other hand, substitution of phosphorus with Mo or W hardly modified the proton conductivity, as compared to the parent compound. For S-doping, a shift to lower temperatures of the phase transition was observed at low S-contents, while the conductivity increased only for the low temperature phase.

Mixing CsH₂PO₄ with H₃PO₄ or CsH₅(PO₄)₂ resulted in composites Cs_1-xH_2+xPO₄ (x_mole= 0.01-0.05) with excesses of protons. In comparison with CsH₂PO₄, these composites exhibited super protonic phase transition at lower temperature (up to ~200 °C) and increased lower-temperature conductivities (σ_{150 °C}~ 2 × 10-3 S·cm-1, x = 0.1) ^[141]. Addition of Cs₂HPO₄·2H₂O to CsH₂PO₄ (up to an xmole = 0.03) also resulted in an enhanced low-temperature conductivity and decreased the activation energy to Ea = 0.56 eV. For all these composites no significant changes in the high-temperature conductivity were observed, although the super protonic phase transition became more diffuse and almost disappeared in some case ^[141][142].

Imphosphates (MPs) comprise an ample class of structurally versatile acidic sol-ids, with outstrovements in the mechanical and proton conductivity properties, as well as in thermal stability of CsH₂PO₄-banseding performances in a wide variety of application electrolytes have been addressed by mixing with oxide materials, such as catalysts ^[1][2][3]zirconia, fuesil cells ^[4][5][6]ica, alumina, batteriend titania (Table 1). Thus, ^[7],the b(1-x)Cs₃(HSO₄)₂(H₂PO₄)/xSiO₂ compositedical with x = 0.7 increase the ^[8],proton acond so ouctivity up to 10^–2 S·cm⁻¹ in the range 60–200 °C. Depending on the metal/phosphate combinations and the synthetic methodologies, MP solids can be prepared in a vast diver-sity of crystallinMoreover, the introduction of fine-particle silica reduced the jump in conductivity at the phase-transition temperature and shifted it to lower temperatures. Higher silica contents led to a decrease in the conductivity due to the disruption of conduction paths ^[132].

The formus, from 3D open-frameworks, through layered networks, to 1De of acid-modified silica confers high thermal stability at low H₂O polymearic structures. The advantages oftial pressure while maintaining high proton conductivity (10⁻³–10⁻² S·cm⁻¹, athese 130–250 °C) ^[134]. A soimilids are, among others, thear trend was found for the composites (1−x)CsH₂PO₄/xTiO₂ and (1−x)CsH₂PO₄/xZrO₂ low (cost and ntents x = 0.1 and 0.2) ^[135][136]. High performasy prepara-tion, hydrophilic nce was also reported for the composite 8:1:1 CsH₂PO₄/NandH₂PO₄/ZrO₂, withermally stability and structural designability to a a stable conductivity of 2.23 × 10⁻² S·cm⁻¹ for 42 h bertain extent. Additionally, their structures arg measured for the high temperature phase ^[137]. Nanodiamenable to post-synthesis modifications, including the inonds (ND) are another type of heterogeneous additive giving rise to the (1−x)CsH₂PO₄-xND (x = 0–0.5) cormporation of ionic or neutral species that significantly affect their functionality and othersites exhibiting enhanced low temperature proton conductivities, while maintaining almost unaltered that of high temperature i^[140].

CsH₂PO₄comporsitant properties, such as the formation of hydrogen bonding networks or inser-tion ofes based on organic additives have been also intensively investigated. For example, binary mixtures containing N-heterocycles (1,2,4-triazole, benzimidazole and imidazole) displayed enhanced proton carriers.

Donductivity at temperatures below termininghe super proton conduction ic phase transition (2 − 8 × 10⁻⁴ S·cm⁻¹ at 174–190 °C) ^[138]. Another approachanisms is key to de was reported in which the solid acid CsH₂PO₄ was combigning proton ned with fluoroelastomer p(VDF/HFP), producing high conductor materialive composite membranes (Figure 11−x)CsH₂PO₄-xp(VDF/HPF) (x = 0. Thus, if a 05–0.25 wt%) with improved mechanical and hydrophobic properties, along with flexibility and reduced thickness.

However, a hicle-type mechanism is needed,gh concentration of p(VDF/HFP) rendered membranes with a reduced proton-containing counter ions, conductivity for the HT phase ^[141]. By such as hydroniusing the polymer butvar (polyvinyl butyral) ^[121], composite ions, Nmembranes (1−x)CsH4⁺,₂PO₄-xButvar (x < 0.2 wt%) pwerotonated amine groups, ande obtained showing, in a wide range of composition, a protonated organic molecules in general, should be incorporated into the structure. Additionally, the immobilizat conduction behaviour analogous to the pure salt in the high temperature region but with increased low temperature conductivity by three orders of magnitude at x = 0.2.

Table 1. Proton conductivity data for selected monovalent and tetravalent metal phosphates and pyrophosphates.

Compounds/Dimensionality

Temperature

(

C)/RH(%)

	Conductivity (S·cm^-1)	Ea (eV)	Ref.
*Tetravalent Metal Phosphates*
ZrP·0.8PrNH₂·5H₂O/2D	20/90	1.2 × 10^-3	1.04	^[50]
Zr(P₂O₇)_0.81(O₃POH)_0.38/2D	20/90	1.3 × 10^-3	0.19	^[51]
Zr(O₃POH)_0.65(O₃PC₆H₄SO₃H)_1.35/2D	100/90	7.0 × 10^-2	---	^[52]
(NH₄)₂[ZrF₂(HPO₄)₂]/3D	90/95	1.45 × 10^-2	0.19	^[53]
(NH₄)₅[Zr₃(OH)₃F₆(PO₄)₂(HPO₄)]/3D	60/98	4.41 × 10^-2	0.33	^[54]
(NH₄)₃Zr(H_2/3PO₄)₃/1D	90/95	1.21 × 10^-2	0.30	^[55]
Ti₂(HPO₄)₄/1D	20/95	1.2 × 10^-3	0.13	^[83]
Ti₂O(PO₄)₂·2H₂O (π-TiP)/3D	90/95	1.3 × 10^-3	0.23	^[86]
Ti(HPO₄)₁(O₃PC₆H₄SO₃H)_0.85(OH)_0.30·nH₂O/2D	100/--	0.1	0.18	^[117]
Sn(HPO₄)₂·3H₂O/2D	100/95	1.0 × 10^-2	---	^[88]
α-ZrP₂O₇/3D	300	1.0 × 10⁻⁴	---	^[49]
(NH₄)₃Zr(H_2/3PO₄)₃/1D	180	1.45 × 10⁻³	0.26	^[55]
*Tetravalent Pyrophosphates*
TiP₂O₇/3D	100/100	4.4 × 10^-3	0.14	^[116]
(C₆H₁₄N₂)[NiV₂O₆H₈(P₂O₇)₂]·2H₂O/3D	60/100	2.0 × 10^-2	0.38	^[118]
In_0.1Sn_0.9P₂O₇/3D	300	0.195	---	^[108]
Ce_0.9Mg_0.1P₂O₇/3D	200	4.0 × 10⁻²	---	^[115]
CeP₂O₇/3D	180	3.0 × 10⁻²	---	^[115]
*Super protonic Cesium Phosphates*
CsH₂PO₄/3D	>230	6.0 × 10⁻²	---	^[120]
Cs_1-xRb_xH₂PO₄/3D	240	3.0 × 10⁻²	0.92	^[127]
Cs_1−xH_2+xPO₄/3D	150	2.0 × 10⁻²	0.70	^[141]
(1−x)Cs₃(HSO₄)₂(HPO₄)/xSiO₂	200	1.0 × 10⁻²	---	^[132]
(1-x)CsH₂PO₄/xTiO₂	230	2.0 × 10⁻²	---	^[135]
(1-x)CsH₂PO₄/xZrO₂	250	2.6 × 10⁻²	---	^[136]
CsH₂PO₄/NaH₂PO₄/ZrO₂	230	2.23 × 10⁻²	---	^[137]

4. Divalent and Trivalent Metal Phosphates

4.1. Divalent Metal Phosphates

Diovalen of specific functional groups and the corresponding counter ions onto thet transition metal phosphates show a great structural versatility, from 1D polymeric topologies through layered framework could be required. On the other hand, to 3D open-framework structures. Most of these solids exhibiting proton transport by a Grotthuss-type mecha-nism basically consist of continuous H-bond networks that favour H⁺are synthetized in the presence of organic molecules, which are retained as protonated guest species (amines, iminazole derivatives, etc.), thus, compensating the anionic charge of the inorganic framework. This is cfonduction with low activation energy (Ea). A main drawback for the latter materials is that, above certain temperatures, breaking of the H-bond networks can result, accompanied byrmed by the metal ion, mainly in octahedral or tetrahedral coordination environments, linked to the phosphate groups with different protonated degrees (H_xPO₄). tThe release of water molecules, which makes the construcpresence of the latter makes possible the formation of permanent H-bondingeffective and extensive hydrogen bond networks with hydrophilic channels or exploring new alternative conductive media necessaryparticipation of water molecules. In addition, protonated guest species and water itself can act as proton carriers, thus, boosting proton conduction ^[9].

Figur

(Table 1.

Examples of proton transport in phosphate-containing materials: (a) structural reorientation-mediated proton transfer. Zn (magenta), P (orange), O (red) and H (pale pink). (b) hopping through H-bond networks, and (c) carrier-mediated proton conduction. Zn (magenta), Zr (green), Cu (cyan), Cl (light green), P (orange), O (red), N (blue) and H (pale pink) atoms.

^[10][11]. Water molecules act both in the formation H-bonding networks inside the framework and as proton carriers. Moreover, other guest species, including organic moieties, can be inserted in their structure following different strategies, such as intercalation ^[12][13][14], template ^[15][16], and post-synthesis adsorption ^[17][18][19]. Accordingly, proton transport in MPs results from the interplay between water/guest molecules and the hydrophilic network/framework walls in specific ways. Thus, in metal phosphates, such as CsH₂PO₄ ^[20] or CaHPO₄·2H₂O ^[21] the proton-transfer mechanism involves not only the jump of H⁺ but also the free rotation of the phosphate groups. In other cases, such as for KH₂PO₄ the proton-transfer mechanism behavior is by defects ^[22].

It is about the state of the art on proton conductive metal phosphates with special attention to the last two decades of developments. A review of publication trends, from the CAS SciFinderⁿ database, is given in Figure 2, which confirms the increasing interest in this topic during this period. The focus is placed on updating recent advances in proton conductors based on tetravalent metal phosphates/pyrophosphates and superprotonic Cs₂HPO₄. In addition, the design and development of new proton carrier systems relying on divalent and trivalent metal phosphates are also revised.

Figure 2. Publications on metal phosphates and pyrophosphate proton conductors over time.

2. Super Protonic Metal(I) Phosphates

Soeveralid acid proton conductors 3D open-framework M(II) phosphates have been reported ^[14][143], whith stoichich consist of [CoPO₄]_∞⁻ ometry M^I [Zn₂(HyXPO₄ )₂(M^IH₂PO₄)₂]²⁻=anionic Cs, Rb; X = S, P, Se; y = 1, 2), have received much attention because theyframeworks that contain organic charge-compensating ions in their internal cavities. (C₂N₂H₁₀)_0.5CoPO₄ exhibited exceptional proton transport properties and can be used as electrolytes in fuel cells operated at internegligible conductivity in anhydrous conditions; however, it displayed a relatively high water-mediate temperatures (120−3d proton conductivity 2.05 × 100⁻³ S·cm⁻¹ at 56 °C) and 98% RH. The fundamental characteristics of thesOn the other hand, the solid NMe₄·Zn[HPO₄][H₂PO₄] matexperials are the phaseences a structural transition that occurs in response toformation from monoclinic (α) to orthorhombic (β) upon heating, cooling or application of pressure at 149 °C. Both polymorphs contain 12-membered ^[6][23],rings accompanied by aosed of tetrahedral Zn²⁺ ioncrease in s linked to proton conductivity of several orders of magnitude—referred to asated phosphate groups without changing super protonicZn-O-P condunectivity (Figure 38).

Figure 3. The characteristic high and low temperature proton conductivity and the corresponding structures for CsH₂PO₄, adapted from

Figure 8. Irreversible structural transformation in NMe4Zn[HPO4][H2PO4]4 adapted from

^[6143][24].

. N (sky-blue), O (red), Zn (magenta), P (orange), C (grey) and H (pale pink) atoms.

Thise α property has been associated with the delocalization of hydrogen bondshase transforms into the β phase at high humidity and temperatures above 60 °C, and ^[24].then For CsH₂PO₄,eaches a proton conductivity of 61.30 × 10⁻² S·cm⁻¹ wat 98% RH, as measured above 230 °C corresp behaviour that might be attributed to the participation of adsorbed water molecules in creating H-bonding to the super protonic networks with effecubtic (Pm−3m) phaseve pathways for proton ^[25]conduction. wThile ite conductivity drastically drops in the low temperature phases. Recently, from an ab initio molecular dynamics simulation study of the solid acids CsHSeO₄,ecreases at 65 °C. In anhydrous conditions, the α phase exhibited a proton conductivity of ~10⁻⁴ CsHSO₄·cm⁻¹ andt 160 CsH₂PO₄°C, sit was concluded that efficient long-range proton transfer in the highmilar values were found for other reported zinc phosphates at temperature (HT) phasess between 130 and 190 °C is^{[12][144][145]}.

Menabled by the interplay of high proton-transfer rates and fmbrane-electrode assembly prepared with the pelletized solid, gave an observed open circuit voltage (OCV) of 0.92 V at 190 °C measured in a H₂/air cequent anion reorientation.

Inll, suggesting that these compounds, protondominant conduction follows a Grotthuss mechanism withve species are protons, and the proton transfer being associated with structural reorieport from anode to cathode takes place through H-bond networks in the pellet.

As ant exationmple of 2D ^[26].metal Tphosphate super protonicwater-assisted proton conductors, (Cs₂H₁₀N₂) [Mn₂(HPO₄)₃](H₂O), dis stable under humidified conditions (P_H2Oplayed a proton conductivity of 1.64 =× 10.4⁻³ atS·cm)⁻¹ ^[6], but it ndehydrates to CsPO₃,viar 99% RH at 20 °C. This prothe transient phase Cs₂H₂P₂O₇, on conductivity was at 230−260 °C, accotrdingibuted to the relationship log(P_H2O/atm)formation of = 6.11(±0.82) − 3.63(±0.42) × 1000/(T_dehy/K) ^[27].

Sdevnseral studie H-bond networks ^[6][28][29]in thave shown that CsH₂PO₄ e lattican be employed as, composed of Mn₃O₁₃ unithe electrolyte in fuel cells-containing anionic layers ^[146], whith good long-term stability. Thus, a continuous, stable power gech provide efficient proton-transfer pathways for a Grotthuss-type proton transport at high RH.

Anotheration for both H₂/O₂ way of improving the proton conductivity in land direct methanol fuel cells operated at ~240 °C was demonstyered divalent metal phosphates is favouring the formation of hydrogen bond networks by ion exchange. For instance ^[17], partiated fl exchange of Na⁺for ethis electrolyte when stabilizedylendiammonium yielded two new crystalline phases, with water particomposition (C₂H₁₀N₂)_xNal_1−x[Mn₂(PO₄)₂] pressures(x of ~0.3−0.4 atm. In fact, high performances, corresponding to single cell peak power densities of 415= 0.37 y 0.54), which influenced formation of extended hydrogen bond networks and concomitant increase in proton conductivity, from 2.22 × 10⁻⁵ mWS·cm⁻²,¹ for the was achieved for a-synthesized material (in ethylendiammonium form) to 1.3 × 10⁻² huS·cmi⁻¹ (x = 0.37) andified 2.1 × H₂/O₂10⁻² systeS·cm⁻¹ provided(x = 0.54) with a Cat 99% RH and 30 °C.

ThisH₂PO₄ elecstrolyte membrane of only 25 µm in thicknessategy of enhancing proton conductivity was further extended successfully ^[30].

CsH₂PO₄ctomp osites based on organic additivther 2D manganese phosphates have^[18]. A beeproton also intensively investigated. Vconductivity value as high as 7.72 × 10⁻² S·cm⁻¹ warious strategies, including salt modifireached at 30 °C and 99% RH for K⁺-exchation by cation and anion substitutionnged compounds, which compares well with those ^{[31][32][33][34]},of MOF-band mixing with oxidesed open-framework materials ^{[35147][36148][37149][38][39][40][41]}. The 1D sor orgalid [Zn₃(H₂PO₄)₆(H₂O)₃](Hbicm) additives(Hbim= ^{[25][42][43][44]}, have benzimidazolen explored to improve the solid acid performance. The preparation of these derivatives and comp) prepared by mechanochemical synthesis is characterized by presenting a dual-function as proton conductor.

It losies tes involved different synthesis techhe coordinated water by heating transforming into [Zn₃(HPO₄)₆](Hbiquesm), such as sol-gel, impregnation, thin-casting and elewhich exhibits an intrinsic proton conductivity higher than the hydrated form, reaching a value of 1.3 × 10⁻³ S·cm⁻¹ atrospinning 120 °C, depending on the statebelieved to be due to a rearrangement of the precursor materialsconduction path and the desired product ^[6].

3. Divalent Metal Phosphates

Divaliquid-like behaviour of benzimidazole molecules. Int transition metal phosphates show a great structural versatility, from 1D polymeric topologies through layered framework to 3D open-framework structures. Most addition, this solid also showed porosity, thus, enabling the adsorption of gaseous methanol that further improved the proton conductivity of these solids are synthetized in the presence of organic molecules, which are ret anhydrous phase. This enhancement of proton conductivity in methanol-adsorbed samples was explained as protonatedby the effective participation of the guest species (amines, iminazole derivatives and so on.), thus, compensatingmolecule in formation of extended hydrogen-bond interactions the^[19].

aAnionic charge of the inorganic framework. This is formed by the metal ion, mainly in octahedral or tetrahedral coordination environments, linked to the p ordered-to-disordered structural transformation and its implication in proton conduction were investigated for the 1D copper phosphate gro[ImH₂][Cups(H₂PO₄)₂Cl]·H₂O (Im = wimith differentdazole). In this structure, the protonated degrees (imidazole (ImH_xPO₄₂). and The presence of the latter makes possible the formation of effectivethe water molecule are located in interspaces of the anionic chains [Cu(H₂PO₄)₂Cl]^-. aUpond extensive hydrogen bond networks with particip heating a structural transformation of water molecules. In addition, protonated guest species and water itself can act as protonfrom an ordered crystalline state to a disordered state occurred. Highly mobile and structurally disordered H⁺ carriers, were thus, boostingsupposed to be responsible of the high proton conductivity 2 × 10⁻² S·cm⁻¹ at 130 °C, under anhydrous conditions ^{[12150][13][14]}.

A 1D zinc phosphate-based proton conductor, [Zn₃(H₂PO₄)₆(H₂O)₃](BTA) (BTA = 1,2,3-benzotriazole) has been reported ^[45151] that exhibits high proton conductivity, 8 × 10⁻³ S·cm⁻¹ in anhydrous glassy-state (120 °C). The glassy-state, developed via melt-quenching, was suggested to induce isotropic disordered domains that enhanced H⁺ dynamics and conductive interfaces. In fact, the capability of the glassy-state material as an electrolyte was found suitable for the rechargeable all-solid-state H⁺ battery operated in a wide range of temperatures from 25 to 110 °C.

Focus han example of 2Ds been also put on metal phosphate-based solid solutions ^[152][153]. Vacancies wcan be generater-assistedd that introduce extra protons into the structure and increase the proton conductors, (C₂ion. This effect was investigated for the 1D rubidium and magnesium polyphosphate compound, RbMg_1-xH₁₀N₂) [Mn_2x(HPO₄₃)₃](·yH₂O), displayed a for which system a maximum proton conductivity of 1.645.5 × 10⁻³ S·cm⁻¹ was measunder 99% RH at 20 °Cred at 170 °C with a vehicle-type mechanism of H₃O⁺conduction.

Thise proton conductivity was attributed to results from H-bond interactions between water molecules and corner-sharing PO₄ chains that provide formation of dense Hsandwiched edge-sharing RbO₆-boMgO₆ chaind ^[152]. Anetworks in the lattice,other example is the solid solution with composed ition Cof M_1-xZn_3x(H₂PO₄)₂·2H₂O₁₃ (0 < x < 1.0), unwhits-contach showed the highest conductivity value, 2.01 × 10⁻² S·cm⁻¹ at 140 °C, for a composinting aon Co_0.5Zn_0.5(H₂PO₄)₂·2H₂O ^[153].

4.2. Trivalent Metal Phosphates

Zeolite-like onic layerspen framework ^[46],metal(III) wphosphich provide efficieates consist of metal(III) ions-phosphate species (PO₄^3-, HPO₄^2-, H₂PO₄^-) linkages feat proton-transfer pathways for a Grotthuss-type proton transport at high RH.

Seveuring internal cavities, where charge-compensating cations and/or neutral species are located and, thus, dense hydrogen bond networks frequently result. In addition, the ralobust 3D open-inorganic framework M(II) phosphates have been reportendues this porous material with better thermal and chemical stability compared ^[14][47], which consist of [CoPO₄]_∞⁻th porous coordination polymers/metal or [Zn₂(HPO₄)₂(H₂PO₄)₂]²⁻ganionic frameworks (PCPs/MOFs) ^[13].

Regarding thao prot conton conductivity (Table 2), alumin organic charge-compium phosphate-based solids are by far the most studied compounds ^{[154][155][156][157][158][159][160][161][162][163][164][165][166][167]}. The species insating ions in their internal cavities. (C₂ide channels affect in different ways to proton conduction. Thus, while water adsorption is key to assist proton transfer in (NH₄)₂Al₄(PO₄)₄(H₁₀)_0.5CoPO₄)·H₂O by a hopping mexhibitchanism along H-bond chains ^[166], densely packed NH₄⁺ ions show negligible conductivity in anhydrous conditiontribution because of hampered migration.

By us;ing an however, it displayed a relativeorganic template-free synthetic methodology, a 3D open-framework aluminophosphate Na₆[(AlyPO₄)₈(OH)₆]·8H₂O high(JU103) water-meds prepared ^[158], whiatch showed a proton conductivity 2.05of 3.59 × 10⁻³ S·cm⁻¹, at 5620 °C and 98% RH. On the other hand, the solid NMe₄·Zn[ It was argued that the enhanced conductivity of the as-synthesized material as compared to its NHPO₄][H₂PO₄]⁺- or Ag⁺-expchangeriences a structural trd forms is indicative of a beneficial effect of hydrated Na⁺ ionsfo in genermation from monocating proton-transfer pathways. The 3D cesium silicoaluminophosphate, Cs₂(Al_0.875Sin_0.125)₄(P_0.875Sic _0.125O₄)₄(αHPO₄), to orthorhombibelonging to the structural family of SAPOs, was shown to exhibit a remarkable proton conductivity of 1.70 × 10⁻⁴ S·cm⁻¹ (β)at upon heating at 149 °C. Both polylow temperature and RH (20 °C and 30%, respectively) ^[168][169].

Several 2D aluminorphs contain 12-membered ringphosphates have been reported as proton conductors ^[154][156]. These composed of tetrahedral Zunds (denoted as AlPO-CJ70/2) are structurally characterized by displaying an²⁺ anions ic layer: [Alink₂P₃O₁₂]^3-, formed by alto protonernating Al³⁺ atend phosphate groups without changing Zn-O-P connectivity (Figure 4).

Figure 4. Irreversible structural transformation in NMe₄Zn[HPO₄][H₂PO₄]₄ adapted from ^[47]. N (sky-blue), O (red), Zn (magenta), P (orange), C (grey) and H (pale pink) atoms.

The α phase transforms into the β phase at high humidity and temperatures above 60 °C, and then reaches a proton conductivity of 1.30 × 10⁻² S·cm⁻¹ at 98% RH, a behaviour that might be attributed to the participation of adsorbed water molecules in creating H-bonding networks with effective pathways for proton conduction. The conductivity drastically decreases at 65 °C. In anhydrous conditions, the α phase exhibited a proton conductivity of ~10⁻⁴ S·cm⁻¹ at 160 °C, similar values were found for other reported zinc phosphates at temperatures between 130 and 190 °C ^[12][48][49].

4. Trivalent Metal Phosphates

A few examples of phosphate-based proton conductors of other trivalent metals do exist. Among them, two Fe(III) phosphates, 1D (C₄H₁₂N₂)_1.5[Fe₂(OH)(H₂PO₄)(HPO₄)₂(PO₄)]·0.5H₂O ^[13] and 3D open-framework iron(III) phosphate (NH₃(CH₂)₃NH₃)₂[Fe₄(OH)₃(HPO₄)₂(PO₄)₃]·4H₂O ^[50], have been reported. Both compounds contain Fe₄O₂₀ tetramers as a common structural feature. The 1D solid is composed of chains of tetramers bridged by PO₄^3- groups and having terminal H₂PO₄^- and HPO₄^2- groups ^[51], while piperazinium cations and water molecules are disorderly situated in between chains. This arrangement gives rise to extended hydrogen bonding interactions and hence proton conducting pathways. The proton conductivity measured at 40 °C and 99% RH was 5.14 × 10⁻⁴ S·cm⁻¹, and it was maintained upon dispersion of this solid in PVDF ^[13]. In the case of the 3D solid, infinite chains of interconnected tetramers are interlinked, in turn by phosphate groups that generate large tunnels (Figure 5). The diprotonated 1,3-diaminopropane and water molecules, localized inside tunnels, form an extended hydrogen bond network with the P–OH groups pointing toward cavities. These interactions favour proton hopping, the measured proton conductivity being of 8.0 × 10⁻⁴ S·cm⁻¹, at 44 °C and 99% RH, and with an E_a of 0.32 eV ^[52]. Furthermore, the proton conductivity of this compound increased up to 5 × 10⁻² S·cm⁻¹ at 40 °C upon exposure to aqua-ammonia vapors from 1 M NH₃·H₂O solution. This result confirms this treatment as an effective way of enhancing proton conductivity, which has been elsewhere demonstrated for the case of coordination polymers ^[53][54].

Figure 5. Open-framework structure of (NH₃(CH₂)₃NH₃)₂[Fe₄(OH)₃(HPO₄)₂(PO₄)₃]·4H₂O showing guest species inside channels and H-bond interactions. Fe (green), O (red), P (orange), and C (grey) atoms.

Other trivalent metal phosphates have been reported, for example, BPO_x ^[55] and CePO₄ ^[56]. The former exhibited a proton conductivity of 7.9 × 10⁻²S·cm⁻¹ as self-supported electrolyte and 4.5 × 10⁻² S·cm⁻¹ as (PBI)−4BPO_x composite membrane, measured at 150 °C and 5% RH, but structure/conductivity correlations were not established because of its amorphous nature.

There are only a few examples of metal(III) pyrophosphates displaying proton conductivity. Among them is the open framework magnesium aluminophosphate MgAlP₂O₇(OH)(H₂O)₂(JU102) ^[57]. Its structure is composed of tetrahedral Al³⁺ and octahedral Mg²⁺ ions coordinated by pyrophosphate ions. This connectivity results in an open framework with unidirectional 8-ring channels. The proton conduction properties originate from the existence of an H-bond network in which coordinated water molecules participate. Thus, the proton conductivity measured at 55 °C on water-immersed samples was 3.86 × 10⁻⁴ S·cm⁻¹, which raised to 1.19 × 10⁻³ S·cm⁻¹ when calcined at 250 °C and measured at the same conditions, while the E_a value hardly changed from 0.16 to 0.2 eV. This behaviour was explained as being due to a dehydration–rehydration process that enhances proton conductivity by altering the H-bonding network and the pathway of proton transfer.

Another example of 3D open framework metal(III) pyrophosphate is the compound NH₄TiP₂O₇ ^[58]. The structure of this solid is composed of negatively charged [TiP₄O₁₂]⁻ layers, forming one-dimensional six-membered ring channels, where the NH₄⁺ ions are located. Its proton conductivity increased from 10⁻⁶ S·cm⁻¹ under anhydrous conditions to 10⁻³ S·cm⁻¹ at full-hydration conditions and 84 °C. The low E_a value, 0.17 eV, characteristic of a Grotthuss-type proton-transfer mechanism, was associated with the role played by the NH₄⁺ ions in the channels as proton donors and promoters of proton migration. A drop in proton conductivity was observed when the triclinic TiP₂O₇ phase formed by thermal decomposition of NH₄TiP₂O₇ ^[58].

5. Tetravalent Metal Phosphates and Pyrophosphates

5.1. Zirconium Phosphates

Torus tetrahedra, which is charge-compensated by N,N-dimethylbenzylamine or α-methylbenzylamine ions, respectively. In these prototype of layered metal(IV) phosphates is zirlayered structures, extended H-bond networks are formed through interactions of the amine N atoms, H₂O moleconium hydrogenles and protruding phosphate, which presents mainly three topologies, known as the forms groups of the anionic layer. Consequently, water-mediated proton conduction processes occurred upon immersion in water, with σ values around α:10⁻³ Zr(HPO₄)₂S·H₂O (α-ZrP)cm⁻¹, γ:at Zr(PO₄)(O₂P(OH)₂)80 2H₂O (γ-ZrP)°C, and λ:E_a (Zr(PO₄)XY, X= hvalidues, OH^-, of 0.16−0.2 HSO₄^-eV, typicand Y=DMTHUS, H₂O, l of and so on) (λ-ZrP)Grotthuss-type proton-transfer mechanism.

α-ZrPBy is the most thermal and hydrolytically stable phase and, thusfollowing a synthetic route in which methylimidazolium dihydrogenphosphate was used as a solvent, structure-directing agent, and a phosphorus source, the mosolid (C₄H₇N₂)(C₃H₄N₂)₂·Al₃(PO₄)₄·0.5H₂O (SCU−2) was obtained ^[164]. Its layered studied compound following ructure, built up from corner-sharing Al³⁺ and P^V tetrahe pioneering works by Giulio Alberti and Abraham Clearfield groupsdra, features 8-member rings where guest imidazolium ions and water molecules are hosted. At 85 °C and 98% RH, this solid showed a proton conductivity of ^[59][60][61]5.

In the structure of α-ZrP, planar Zr atoms are coordinated by the three oxygen atoms of HO-PO₃^- groups, while the P-OH bond points toward the interlayer space giving rise to small cavities, where the lattice water is located. Although this water forms hydrogen bonds with P-OH groups, an extended interlayer hydrogen bond network is absent in this structure ^[60]. On the other hand, the structure of γ-ZrP consists of a biplanar Zr atomic system connected through bridge PO₄³

9 × 10

⁻ groups, while the externally located H₂PO₄^- groups complete the octahedral coordination of the Zr atoms.

The great versatility of zirconium phosphate preparation under numerous experimental conditions, has been exploited to synthesize various novel derivatives, which displayed remarkable proton conductivity properties. Two examples are layered (NH₄)₂[ZrF₂(HPO₄)₂] ^[63] and the 3D open framework zirconium phosphate (NH₄)₅[Zr₃(OH)₃F₆(PO₄)₂(HPO₄)] ^[64]. In both structures, the participation of NH₄⁺ ions in the formation of extended hydrogen-bond networks (Figure 6) resulted in proton conductivities of 1.45 × 10⁻² S·cm⁻¹ (at 90 °C and 95% RH) and 4.41 × 10⁻² S·cm⁻¹ (at 60 °C and 98% RH), respectively. From the E_a values found, 0.19 and 0.33 eV, respectively, a Grotthuss-type proton-transfer mechanism was inferred for these materials.

Figure 6. Layered structure of compound (NH₄)₂[ZrF₂(HPO₄)₂] and possible proton-transfer pathways (adapted from ^[63]).

A one-dimensional zirconium phosphate, (NH₄)₃Zr(H_2/3PO₄)₃, consisting of anionic [Zr(H_2/3PO₄)₃]_n³ⁿ

S·cm

⁻ chains bonded to charge-compensating NH₄⁺ ions was reported ^[65]. In this structure, the phosphates groups are disorderly protonated, while NH₄⁺ ions occupy ordered positions in between adjacent chains (Figure 7). This arrangement generates H-bonding infinite chains of acid–base pairs (N-H···O-P) that lead to a high proton conductivity in anhydrous state of 1.45 × 10⁻³ S·cm⁻¹, at 180 °C. This material also showed a remarkable high performance in PEMFC and DMFC under operation conditions.

Figure 7. Views of chain packing and H-bond network (dashed grey lines) in compound (NH₄)₃Zr(H_2/3PO₄)₃ (adapted from ^[65]).

5.2. Titanium and Tin(IV) Phosphates.

and Structurally, titanium(IV) phosphates are more diverse than ZrP, given that structures containing an oxo/hydroxyl ligand or mixed-valence (T low activation energy (0.20 eV), characteristic of a Grotthuss-type proton-transfer mechanism. An efficient pathway for the proton transfer was attributed to the hydrogen bond network established between the i^III/Tmi^IV) titdanzolium ions have been reported. In addition to the zirconiuand water molecules interacting with the host framework.

A few examples of phosphate analogues, α--based proton ^[66] acond γ-TiP ^[67], uctwo morers of other layered titanium trivalent metals do exist. Among them, two Fe(III) phosphates, Ti1D (C₄H₁₂N₂)_1.5[Fe₂(OH)(H₂POH₄)(HPO₄)₂(PO₄)]·20.5H₂O ^[6813] and T3D open-framework iron(III) phosphate (NH₃(CH₂)₃NH₃)₂[Fe₄(OH)₃(HPO₄)₂(PO₄)₂₃]·24H₂O ^[170], harve knowbeen ^[69]. Furthermore, the mixed-valencported. Both compounds contain Fe₄O₂₀ tietanium phosphate Tramers as a common structural feature. The 1D soli^IIITd i^IV(HPO₄)₄·C₂N₂H₉·H₂Os compontains micsed of chains of tetramers bridged by PO₄^3- grouporous channels and having terminal H₂PO₄^- and HPO₄^2- groups ^[70171], whilere monoprotonated ethylenediamine and lattice water are accommodated, which favours its transformation into a porous phase, T piperazinium cations and water molecules are disorderly situated in between chains. This arrangement gives rise to extended hydrogen bonding interactions and hence proton conducting pathways. The proton conductivi₂(HPO₄)₄,ty measured at 6040 °C with a 3Dand 99% RH was 5.14 × 10⁻⁴ struS·cm⁻¹, and iture similar to that of τ-Zr(HPO₄)₂was maintained upon dispersion of this solid in PVDF ^[7113].

In Tthe crystal structure of two other open-frameworks Ti₂Oase of the 3D solid, infinite chains of interconnected tetramers are interlinked, in turn by phosphate groups that generate large tunnels (PO₄Figure 9)₂·2H₂O. The dipolymorphs (ρ-TiP and π-TiP) has been reportedrotonated 1,3-diaminopropane and water molecules, localized inside tunnels, form an extended hydrogen bond network with the ^[72][73].P–OH Fibgrous ρ-TiP and π-TiP crystallize in triclinic and monoclinic unit cells, respectively, and areps pointing toward cavities. These interactions favour proton hopping, the measured proton conductivity being of 8.0 × 10⁻⁴ S·compose⁻¹, at 44 °C and of99% TiO₆RH, and TwiO₄(H₂O)₂th an E_a ocf 0.32 eV ^[172]. Furtahedra bridged through the orthophosphrmore, the proton conductivity of this compound increased up to 5 × 10⁻² S·cm⁻¹ at 40 °C upon e groups (Figure 8)xposure to aqua-ammonia vapors from 1 M NH₃·H₂O solution. This resultype of connectivity creates two types of 1D channels running parallel to the direction of confirms this treatment as an effective way of enhancing proton conductivity, which has been elsewhere demonstrated for the case of coordination polymers ^[173][174]. tThe fibre groobserved variation of E_a with ^[72][73][74].the ³¹Pammonia MAS-NMR spconcectroscopy studies revealentration also suggested that π-TiPNH₃, has capabilitywell as H₂O, mof adsorbing superficiallylecules contribute to create protonated phosphate species (-transfer pathways, by a Grotthuss mechanism. H₃PO₄/H₂PO₄^-/HPO₄^2-)owever, when ammonich affectsa concentrations were lower than 0.5 M, the proton conductivity properties as confirmed by AC impedancon mechanism tended to be vehicle-type one measurements.

Figure 8. Structure of Ti₂O(PO₄)₂·2H₂O, a fibrous compound with capability of enhancing surface proton transport by H₃PO₄ impregnation (adapted from ^[74156]).

Figure 9. ThOpe isomorphous series of layn-framework structure of (NH₃(CH₂)₃NH₃)₂[Fered α-metal₄(IVOH) phosphates [M₃(IVHPO₄) = Zr, Ti, Sn₂(PO₄)₃]·4H₂O showing gued appreciable differences in the hydrogen st species inside channels and H-bond interaction of the lattice water with the layers, associated with a variable layer corrugation degree along the series; α-TiP and α-SnP displaying H-bonds stronger than the prototype α-ZrP ^[75]s. Fe (green), O (red), P (orange), which, in turn, might have significant implications in making internal surfaces accessible to proton conduction pathnd C (grey) atoms.

Recent studies carried out on a nanolayered γ-type tin phosphate, Sn(HPO₄)₂·3H₂O, have revealed that this solid could be obtained in a water-delaminated form, which showed a high proton conductivity of ~1 × 10⁻² S·cm⁻¹ at 100 °C and 95% RH. This high value was attributed to strong H-bonds between water and the SnP layers, in combination with a high surface area of 223 m²·g^{−1 ^[76]}.

Other tetravalent phosphates such as silicophosphates with POH groups have received attention as proton-conducting electrolytes ^[77] but its solid-state structural characterization is elusive, at least, under conditions similar to those used in operating fuel cells ^[78].

5.3. Tetravalent Pyrophosphatesble 2.

Proton conductivity data for selected Tdivaletrant and trivalent metal pyrophosphates.

Compounds/Dimensionality

Temperature

(

C)/RH(%)

Conductivity

(S·cm

^-1

)

(eV)

Ref.

Divalent Metal Phosphates



(C₂N₂H₁₀)_0.5CoPO₄/3D	56/98	2.05 × 10^-3		1.01	^[143]
NMe₄·Zn[HPO₄][H₂PO₄] (β phase)/3D	60/98	1.30 × 10^-2		0.92	^[14]
(C₂H₁₀N₂) [Mn₂(HPO₄)₃](H₂O)/2D	20/99	1.64 × 10^-3		0.22	^[146]
(C₂H₁₀N₂)_xNa_1−x[Mn₂(PO₄)₂]/2D	30/99	2.1 × 10^-2		0.14	^[17]
(C₂H₁₀N₂)_1-xK_x[Mn₂(PO₄)₂]·2H₂O/2D	30/99	7.72 × 10^-2		0.18	^[18]
(C₂H₁₀N₂)_1-xK_x[Mn₂(HPO₄)₃] (H₂O)/2D	30/99	0.85 × 10^-2		0.081	^[18]
[Zn₃(H₂PO₄)₆(H₂O)₃](Hbim)/1D	120		1.3 × 10⁻³	0.50	^[19]
[ImH₂][Cu(H₂PO₄)₂Cl]·H₂O/1D	130		2.0 × 10⁻²	0.1	^[150]
[Zn₃(H₂PO₄)₆(H₂O)₃](BTA)/1D	120		8.0 × 10⁻³	0.39	^[151]
RbMg_0.9H_0.2(PO₃)₃·yH₂O/1D	170		5.5 × 10⁻³	---	^[152]
Co_0.5Zn_0.5(H₂PO₄)₂·2H₂O/1D	140		2.01×10⁻²	---	^[153]
*Trivalent Metal Phosphates*
Na₆[(AlPO₄)₈(OH)₆]·8H₂O/3D	20/98		3.59×10^-3	0.21	^[158]
[C₉H₁₄N]₈[H₂O]₄·[Al₈P₁₂O₄₈H₄]/2D	80, in water		9.25×10^-4	0.16	^[154]
[R-,S-C₈H₁₂N]₈[H₂O]₂·[Al₈P₁₂O₄₈H₄]/2D	90/98		3.01×10^-3	0.20	^[156]
(C₄H₇N₂)(C₃H₄N₂)₂·Al₃(PO₄)₄·0.5H₂O/2D	85/98		5.94×10^-3	0.20	^[164]
In(HPO₄)(H₂PO₄)(D,L-C₃H₇NO₂)/3D	85/98		2.9 × 10^-3	0.19	^[16]
(NH₃(CH₂)₃NH₃)₂[Fe₄(OH)₃(HPO₄)₂(PO₄)₃]· 4H₂O/1D	40/99		5.0 × 10^-2	---	^[170]

Hbim = benzimidazole; Im =imidazole; BTA = 1,2,3-benzotriazole.

(MP₂O₇;For Mthe = Sn, Ce, Ti, Zr), in particular tin(IVseries of isostructural imidazole cation (ImH₂)-templated derivatives, combilayered metal phosphates, [ImH₂][X-(HPO₄)₂(H₂O)₂] (FJU−25-X, X = Al, Ga, and Fe high thermal sta), it was found that the proton conductivity was dependent on mobility (T ³ 400 °C) with of imidazole guests, FJU−25-Fe exhibiting the highest proton conductivities ofy (5.21 × 10⁻³–10⁻²⁴ S·cm⁻¹in athe temperature range of 10 90 °C). The determined activation energies (~0.20–300 eV) °C,were under anhydrous or low humidity conditions ^{[79][80][81][82][83]}indicative of a Grotthuss-type mechanism of proton conduction.

The amino acid-template indium phosphate, In(HPO₄)(H₂PO₄)(D,L-C₃H₇NO₂) (SCU−12), represents a sigin of thisngular example of 3D metal phosphate-based proton conductiviors ^[16].

Its cry was initially explained because of protons being incorporated into the framework, formed by metal(IV) octahedra and corner-sharingstal structure is formed by edge-sharing four-ring ladders, with the amino acid molecules attached to the ladders through In–O bonds. Further bridging the indium phosphate tetrahedral, afteladders by the H₂PO₄^- groups interaction with water vapor.

Dgives rise to a threfect sites, such as electron holes-dimensional structure. The presence of two kinds of proton carriers, ^[84]H₂PO₄^- ions and zwitterioxygen vacancies are believed to favournic alanine molecules, favours the development of high proton generconductivities (2.9 × 10⁻³ S·cm⁻¹ ati 85 °C and 98% RH) thronugh ^[85].a TGrotthesuss-type protons may-transfer mechanism (E_a = 0.19 occupyeV).

Othydrogen-bonding interstitial sites on either the M−er trivalent metal phosphates have been reported, e.g., BPO−P_x ^[175] orand the P−CePO−P₄ bonds,^[176]. tThus, generating a hoppinge former exhibited a proton trconductivity of 7.9 × 10⁻²S·cm⁻¹ ans self-sport between theupported electrolyte and 4.5 × 10⁻² S·cm⁻¹ ase (PBI)−4BPO_x composites ^[86][87]. Imembrane, support of this,measured at 150 °C and 5% ¹RH, NMR studies revealed two signals correspondingbut structure/conductivity correlations were not established because of its amorphous nature. The latter showed a low-temperature (RT) proton conduction < 10⁻⁵ S·cm⁻¹ ato hydrogen-bonded inte 100% RH through a structure-independent proton-transport mechanism ^[176].

There are only a few examplestitial of metal(III) pyrophosphates displaying protons at conductivity. Among them is the open framework magnesium aluminophosphate MgAlP₂O₇(OH)(H₂O)₂(JU102) ^[167]. Its structuret is composed of tetrahedral Al³⁺ and metal octahedral Mg²⁺ ions coordites in In³⁺-dnated by pyrophosped SnP₂O₇hate ions. FurtThermore, indium doping seemed not to affect tis connectivity results in an open framework with unidirectional 8-ring channels. The proton conduction occurring by a hopping mechanism between octahedral and tetrahedral sitesproperties originate from the existence of an H-bond network in which coordinated water molecules participate.

Thus, the increased p proton conductivity measured at 55 °C on water-immersed samples was 3.86 × 10⁻⁴ S·cm⁻¹ ^[167], which roaised to 1.19 × 10⁻³ S·cm⁻¹ when calconductivity is insteined at 250 °C and measured at the same conditions, while the E_a value hard attributable to increasedly changed from 0.16 to 0.2 eV. This behaviour was explained as being due to a dehydration–rehydration process that enhances proton concentrationductivity by altering ^[88]. Tthe sH-bonding netrategy of dopant inserwork and the pathway of proton transfer.

Anotiher example of 3D on into the M(IVpen framework metal(III) pyrophosphate has been further extendis the compound NH₄TiP₂O₇ ^[177]. Thed to include other dopant divalent and trivstructure of this solid is composed of negatively charged [TiP₄O₁₂]⁻ lalyent metal ions, such as Mg²⁺rs, forming one-dimensional six-membered ring channels, Al³⁺,where Sb³⁺,the Sc³NH₄⁺, and Gions a³⁺,re wilocath a maximumed. Its proton conductivity of increased from 10.195⁻⁶ S·cm⁻¹ bundeing reporr anhydrous conditions to 10⁻³ S·cm⁻¹ ated for Iull-hydration condition_0.1Ss an_0.9P₂O₇d 84 ^[89]°C. The Howlow E_a valuev, 0.17 er, recent studies on metal doped and undoped SnP₂O₇V, characteristic of a Grotthuss-type proton-transfer mechanism, was asuggested that itsociated with the role played by the NH₄⁺ ions in co-precipitated phosphorous-rich amorphous phases where thethe channels as proton donors and promoters of proton migration. A drop in proton conductivity mostly resides, while the crystallinewas observed when the triclinic TiP₂O₇ phase formexhibit a very low conductivity (~10⁻⁸d by thermal decomposition of S·cm⁻¹ atNH₄TiP₂O₇ 150−300 °C) ^[90177].

5. Outlook

LGreat effow crystallinity titanium pyroprts have been devoted to the synthesis and characterization of metal phosphates also exhibit high -based proton conductivityors over more than ^[91].three Vdecalues of 0.0019–0.0044 S·cdes. Among them, zirconium⁻¹, phosphat 100 °C and 100% RH were reported for these compounds. The proton conduction is attributables are prominent not only because of the feasibility of Zr(IV) and phosphate ions to form a rich variety of crystallographic architectures (from 1D to 3D open frameworks) but also due to the presence of H₂PO₄^- ir outstanding HPO₄²⁻ spropecrties, demonstrated b and workability while being environmentally ³¹Pbenign MAS NMR and FTIR, and it occurslow cost materials.

Althrough a water-facilitated Grotthuss-type proton transport mechanism. Titanium pough the prototype layered α-zirconium phosphate has been commonly proven as a filler for PEMFCs devices, new synthetic designs of M(IV) phosphates, including pyrophosphates are also prone to be functionalized with compounds, are promising candidates to broaden their applicability in different electrochemical devices. Other M(IV) phosphonates groups. Thus, mixed titanium phosphate/phosphonates, Ti(HPO₄)_1.00(O₃PC₆H₄SO₃H)_0.85(OH)_0.30·nH₂O ^[92]ates and pyrophosphates (M = Ti, Sn) are less known due, in part, to their amorphous nature or strong tendency to amorphise at working temperatures, though, displayed an exceptionaln some cases, these compounds presented remarkable proton conductivity ofproperties.

CsH₂PO₄, a 0.1super S·cprotonic m⁻¹aterial, wat 100 °C.

s proven as Aa structurally uitable electrolyte for both H₂/O₂ and different proton-conductor metal pyrophosphate is the mixrect methanol fuel cells operated at ~240 °C, and provides excellent performance when controlling its thermal stability. In addition, combinations with other materials are possible to adjust the specific characteristics of the composite CsH₂PO₄electrolyte, thus, offering a wid template-containie range of compositions with tuning properties.

Recengtly, research vanadium nickel pyrophosphate, (C₆H₁₄N₂)[Nand developments in metal phosphate proton conductors have been addressed to divalent or triV₂O₆H₈(P₂O₇)₂]·2H₂Ovalent (Figure 9).metal Iphosphats crystales, which present a remarkable structure is composed of octahedrally coordinated V(IV) and Ni(II) interconnected throughal versatility and tunable conductivities; however, more in-depth studies are required to assess their potential use and applicability for low and intermediate temperature fuel cells.

brApplidging pyrocations of metal phosphate groups, thus creating a 3D framework of [NiV₂O₆H₈(P₂O₇)₂]²⁻s as electrolytes or as electrolyte components are in continuous progress, although their use for energy storage and conversion remains a challenge. For practical applications in funit charged-compensated by el cells at low/intermediate temperatures, phosphate-based protonated DABCO molecules. Hydrogen-bonding networks inside 3D channels facilitate the proton transport conducting electrolytes have demonstrated acceptable proton conductivity values; however, other features, such as their mechanical strength, chemical/thermal stabilities, film-forming ability (in the case of composite membranes), durability, and thus this materialfuel cross-over, are key factors to be improved.

Funding: exThibits a remarkable proton conductivity ofs research was funded by PID2019−110249RB-I00 (MICIU/AEI, Spain) and PY20−00416 (Junta de Andalucia, Spain/FEDER) research projects.

Acknowledgments: 2M.B.G.0 × 1 thanks PAIDI2020⁻² S·researcm⁻¹h grat 60 °C and 100% RH through a Grotthuss-type proton-transfer mechanism (E_a = 0nt (DOC_00272 Junta de Andalucia, Spain) and R.M.P.C. thanks University of Malaga under Plan Propio de Investigación for financial support.38 eV) ^[93].

Figure 9. (a) View, along b-axis, of the chain packing and (b) 3D-framework of compound (NH₄)₃Zr(H_2/3PO₄)₃, with H-bond network (dashed blue lines) highlighted (adapted from ^[93]).

References

Shivhare, A.; Kumara, A.; Srivastava, R. Metal phosphate catalysts to upgrade ligno-cellulose biomass into value-added chemicals and biofuels. Green Chem. 2021, 23, 3818–3841.
Zhao, H.; Yuan, Z.-Y. Insights into Transition Metal Phosphate Materials for Efficient Electrocatalysis. ChemCatChem 2020, 12, 3797–3810.
Chen, L.; Zhao, Y.; Yang, J.; Liu, D.; Wei, X.; Wang, X.; Zheng, Y. New Versatile Synthetic Route for the Preparation of Metal Phosphate Decorated Hydrogen Evolution Photocatalysts. Inorg. Chem. 2020, 59, 1566–1575.
Goñi-Urtiaga, A.; Presvytes, D.; Scott, K. Solid acids as electrolyte materials for proton exchange membrane (PEM) electrolysis: Review. Int. J. Hydrog. Energy 2012, 37, 3358–3372.Goñi-Urtiaga, A.; Presvytes, D.; Scott, K. Solid acids as electrolyte materials for proton exchange membrane (PEM) electrol-ysis: Review. Int. J. Hydrog. Energy 2012, 37, 3358–3372.
Paschos, O.; Kunze, J.; Stimming, U.; Maglia, F. A review on phosphate based, solid state, protonic conductors for intermediate temperature fuel cells. J. Phys. Condens. Matter 2011, 23, 234110.Paschos, O.; Kunze, J.; Stimming, U.; Maglia, F. A review on phosphate based, solid state, protonic conductors for interme-diate temperature fuel cells. J. Phys. Condens. Matter 2011, 23, 234110.
Haile, S.M.; Chisholm, C.R.I.; Sasaki, K.; Boysen, D.A.; Uda, T. Solid acid proton conductors: From laboratory curiosities to fuel cell electrolytes. Faraday Discuss. 2007, 134, 17–39.
Cheng, Q.; Zhao, X.; Yang, G.; Mao, L.; Liao, F.; Chen, F.; He, P.; Pan, D.; Chen, S. Recent advances of metal phosphates-based electrodes for high-performance metal ion batteries. Energy Storage Mater. 2021, 41, 842–882.
Habraken, W.; Habibovic, P.; Epple, M.; Bohner, M. Calcium phosphates in biomedical applications: Materials for the future? Mater. Today 2015, 19, 69–87.Habraken, W.; Habibovic, P.; Epple, M.; Bohner, M. Calcium phosphates in biomedical applications: Materials for the fu-ture? Mater. Today 2015, 19, 69–87.
Li, J.; Yi, M.; Zhang, L.; You, Z.; Liu, X.; Li, B. Energy related ion transports in coordination polymers. Nano Select. 2021, 1–19.Mohammad, N.; Mohamad, A.B.; Kadhum, A.A.H.; Loh, K.S. A review on synthesis and characterization of solid acid mate-rials for fuel cell applications. J. Power Sources 2016, 322, 77–92.
Pica, M.; Donnadio, A.; Casciola, M. From microcrystalline to nanosized α-zirconium phosphate: Synthetic approaches and applications of an old material with a bright future. Coord. Chem. Rev. 2018, 374, 218–235.
Wong, N.E.; Ramaswamy, P.; Lee, A.S.; Gelfand, B.S.; Bladek, K.J.; Taylor, J.M.; Spasyuk, D.M.; Shimizu, G.K.H. Tuning Intrinsic and Extrinsic Proton Conduction in Metal−Organic Frameworks by the Lanthanide Contraction. J. Am. Chem. Soc. 2017, 139, 14676–14683.Wong, N.E.; Ramaswamy, P.; Lee, A.S.; Gelfand, B.S.; Bladek, K.J.; Taylor, J.M.; Spasyuk, D.M.; Shimizu, G.K.H. Tuning In-trinsic and Extrinsic Proton Conduction in Metal−Organic Frameworks by the Lanthanide Contraction. J. Am. Chem. Soc. 2017, 139, 14676–14683.
Horike, S.; Umeyama, D.; Inukai, M.; Itakura, T.; Kitagawa, S. Coordination-network-based ionic plastic crystal for anhydrous proton conductivity. J. Am. Chem. Soc. 2012, 134, 7612–7615.Horike, S.; Umeyama, D.; Inukai, M.; Itakura, T.; Kitagawa, S. Coordination-network-based ionic plastic crystal for anhy-drous proton conductivity. J. Am. Chem. Soc. 2012, 134, 7612–7615.
Zhang, K.-M.; Lou, Y.-L.; He, F.-Y.; Duan, H.-B.; Huang, X.-Q.; Fan, Y.; Zhao, H.-R. The water-mediated proton conductivity of a 1D open framework inorganic-organic hybrid iron phosphate and its composite membranes. Inorg. Chem. Commun. 2021, 134, 109032.
Yu, J.-W.; Yu, H.-J.; Ren, Q.; Zhang, J.; Zou, Y., Luo, H.-B.; Wang, L.; Ren, X.-M. Humidity-sensitive irreversible phase transformation of open-framework zinc phosphate and its water-assisted high proton conduction properties. Dalton Trans. 2021, 50, 8070–8075.Yu, J.-W.; Yu, H.-J.; Ren, Q.; Zhang, J.; Zou, Y., Luo, H.-B.; Wang, L.; Ren, X.-M. Humidity-sensitive irreversible phase trans-formation of open-framework zinc phosphate and its water-assisted high proton conduction properties. Dalton Trans. 2021, 50, 8070–8075.
Su, X.; Yao, Z.; Ye, Y.; Zeng, H.; Xu, G.; Wu, L.; Ma, X.; Chen, Q.-H.; Wang, L.; Zhang, Z.; et al. 40-Fold Enhanced Intrinsic Proton Conductivity in Coordination Polymers with the Same Proton-Conducting Pathway by Tuning Metal Cation Nodes. Inorg. Chem. 2016, 55, 983–986.
Shi, J.; Wang, K.; Li, J.; Zeng, H.; Zhang, Q.; Lin, Z. Exploration of new water stable proton-conducting materials in an amino acid-templated metal phosphate system. Dalton Trans. 2018, 47, 654–658.Shi, J.; Wang, K.; Li, J.; Zeng, H.; Zhang, Q.; Lin, Z. Exploration of new water stable proton-conducting materials in an ami-no acid-templated metal phosphate system. Dalton Trans. 2018, 47, 654–658.
Zhang, K.-M.; He, F.-Y.; Duan, H.-B.; Zhao, H.-R. An alkali metal ion-exchanged metal-phosphate (C2H10N2)xNa1−x[Mn2(PO4)2] with high proton conductivity of 10−2 S·cm−1. Inorg. Chem. 2019, 58, 6639–6646.
Zhang, K.-M.; Jia, Y.; Gu, Y., He, F.-Y.; Zhao, H.-R. A facile and efficient method to improve the proton conductivity of open framework metal phosphates under aqueous condition. Inorg. Chem. Commun. 2020, 120, 108128.
Umeyama, D.; Horike, S.; Inukai, M.; Kitagawa, S. Integration of Intrinsic Proton Conduction and Guest-Accessible Nanospace into a Coordination Polymer. J. Am. Chem. Soc. 2013, 135, 11345–11350.Umeyama, D.; Horike, S.; Inukai, M.; Kitagawa, S. Integration of Intrinsic Proton Conduction and Guest-Accessible Nano-space into a Coordination Polymer. J. Am. Chem. Soc. 2013, 135, 11345–11350.
Baranov, A.I.; Khiznichenko, V.P.; Sandler, V.A.; Shuvalov, L.A. Frequency dielectric dispersion in the ferroelectric and superionic phases of CsH2PO4. Ferroelectrics 1988, 81, 183–186.
Colomban, P. Chemistry of Solid-State Materials. In Proton Conductors: Solid, Membranes and Gels-Materials and Devices; Cambridge University Press: Cambridge, UK, 1992.Colomban, P. Chemistry of Solid-State Materials. In Proton Conductors: Solid, Membranes and Gels-Materials and Devices; Cam-bridge University Press: Cambridge, UK, 1992.
Fragua, D.M.; Castillo, J.; Castillo, R.; Vargas, R.A. New amorphous phase KnH2PnO3n+1(n>>1) in KH2PO4. Rev. Latin Am. Metal. Mat. 2009, 2, 491–497.
Botez, C.E.; Tackett, R.J.; Hermosillo, J.D.; Zhang, J.; Zhao, Y.; Wang, L. High pressure synchrotron x-ray diffraction studies of superprotonic transitions in phosphate solid acids. Solid State Ion. 2012, 213, 58–62.Skou, E.; Andersen, I.G.K.; Simonsen, K.E.; Andersen, E.K. Is UO2HPO4·4H2O a proton conductor? Solid State Ion. 1983, 9, 1041–1047.
Baranov, A.I. Crystals with disordered hydrogen-bond networks and superprotonic conductivity. Review. Crystallogr. Rep. 2003, 48, 1012–1037.Cabeza, A.; Martinez, M.; Benavente, J.; Bruque, S. Current rectification by H3OUO2PO4 3H2O (HUP) thin films in electrolyte media. Solid State Ion. 1992, 51, 127–131.
Bagryantseva, I.N.; Gaydamaka, A.A.; Ponomareva, V.G. Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and Butyral polymer. Ionics 2020, 26, 1813–1818.Barboux, P.; Morineau, R.; Livage, J. Protonic conductivity in hydrates. Solid State Ion. 1988, 27, 221–225.
Dreßler, C.; Sebastiani, D. Effect of anion reorientation on proton mobility in the solid acids family CsHyXO4 (X = S, P, Se, y = 1, 2) from ab initio molecular dynamics simulations. Phys. Chem. Chem. Phys. 2020, 22, 10738–10752.Barboux, P.; Livage, J. Ionic conductivity in fibrous Ce(HPO4)2·(3+x)H2O. Solid State Ion. 1989, 34, 47–52.
Taninouchi, Y.K.; Uda, T.; Awakura, Y.; Ikeda, A.; Haile, S.M. Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C. J. Mater. Chem. 2007, 17, 3182–3189.Li, J.; Yi, M.; Zhang, L.; You, Z.; Liu, X.; Li, B. Energy related ion transports in coordination polymers. Nano Select. 2021, 1–19.
Mathur, L.; Kim, I.-H.; Bhardwaj, A.; Singh, B.; Park, J.-Y.; Song, S.-J. Structural and electrical properties of novel phosphate based composite electrolyte for low-temperature fuel cells. Composites Part B 2020, 202, 108405.Steele, B.C.H.; Heinzel, A. Materials for fuel-cell technologies. Nature 2001, 414, 345–352.
Otomo, J.; Tamaki, T.; Nishida, S.; Wang, S.; Ogura, M.; Kobayashi, T.; Wen, C.-J.; Nagamoto, H.; Takahashi, H. Effect of water vapor on proton conduction of cesium dihydrogen phosphate and application to intermediate temperature fuel cells. J. Appl. Electrochem. 2005, 35, 865–870.Wei, J. Proton-Conducting Materials Used as Polymer Electrolyte Membranes in Fuel Cells (ch 9). In Polymer-Based Multi-functional Nanocomposites and Their Applications; Song, K., Liu, C., Guo, J.Z., Eds.; Elsevier: Amsterdam, The Netherlands, 2019; Chapter 9, pp. 245–260.
Uda, T.; Haile, S.M. Thin-membrane solid-acid fuel cell. Electrochem. Solid State Lett. 2005, 8, A245–A246.Sazali, N.; Salleh, W.N.W.; Jamaludin, A.S.; Razali, M.N.M. New Perspectives on Fuel Cell Technology: A Brief Review. Membranes 2020, 10, 99.
Navarrete, L.; Andrio, A.; Escolástico, S.; Moya, S.; Compañ, V.; Serra, J.M. Protonic Conduction of Partially-Substituted CsH2PO4 and the Applicability in Electrochemical Devices. Membranes 2019, 9, 49.Dupuis, A.-C. Proton exchange membranes for fuel cells operated at medium temperatures: Materials and experimental techniques. Prog. Mater. Sci. 2011, 56, 289–327.
Ponomareva, V.G.; Bagryantseva, I.N.; Gaydamaka, A.A. Study of the Phase Composition and Electrotransport Properties of the Systems Based on Mono- and Disubstituted Phosphates of Cesium and Rubidium. Chem. Sustain. Dev. 2019, 27, 238–245.Peighambardoust, S.J.; Rowshanzamir, S.; Amjadi, M. Review of the proton exchange membranes for fuel cell applications. Int. J. Hydrog. Energy 2010, 35, 9349–9384.
Martsinkevich, V.V.; Ponomareva, V.G. Double salts Cs1-xMxH2PO4 (M = Na, K, Rb) as proton conductors. Solid State Ion. 2012, 225, 236–240.Mauritz, K.A.; Moore, R.B. State of understanding of Nafion. Chem. Rev. 2004, 104, 4535–4585.
Ponomareva, V.G.; Bagryantseva, I.N. Superprotonic CsH2PO4-CsHSO4 solid solutions. Inorg. Mater. 2012, 48, 187–194.Hickner, M.A.; Ghassemi, H.; Kim, Y.S.; Einsla, B.R.; McGrath, J.E. Alternative polymer systems for proton exchange mem-branes (PEMs). Chem. Rev. 2004, 104, 4587–4612.
Ponomareva, V.G.; Uvarov, N.F.; Lavrova, G.V.; Hairetdinov, E.F. Composite protonic solid electrolytes in the CsHSO4-SiO2 system. Solid State Ion. 1996, 90, 161–166.Han, X.; Xie, Y.; Liu, D.; Chen, Z.; Zhang, H.; Pang, J.; Jiang, Z. Synthesis and properties of novel poly(arylene ether)s with densely sulfonated units based on carbazole derivative. J. Membr. Sci. 2019, 589, 117230.
Ponomareva, V.G.; Shutova, E.S.; Matvienko, A.A. Conductivity of Proton Electrolytes Based on Cesium Hydrogen Sulfate Phosphate. Inorg. Mater. 2004, 40, 721–728.Kuzmenko, M.; Poryadchenko, N. Perspective materials for application in fuel-cell technologies. In Fuel Cell Technologies: State and Perspectives; Sammes. N., Ed.; Springer: Dordrecht, The Netherlands, 2005; pp. 253–258.
Otomo, J.; Minagawa, N.; Wen, C.-J.; Eguchi, K.; Takahashi, H. Protonic conduction of CsH2PO4 and its composite with silica in dry and humid atmospheres. Solid State Ion. 2003, 156, 357–369.Lee, K.-S.; Maurya, S.; Kim, Y.S.; Kreller, C.R.; Wilson, M.S.; Larsen, D.; Elangovan, S.E.; Mukundan, R. Intermediate temper-ature fuel cells via an ion-pair coordinated polymer electrolyte. Energy Env. Sci. 2018, 11, 979–987.
Ponomareva, V.G.; Shutova, E.S. High-temperature behavior of CsH2PO4 and CsH2PO4-SiO2 composites. Solid State Ion. 2007, 178, 729–734.Norby, T. Solid-state protonic conductors: Principles, properties, progress and prospects. Solid State Ion. 1999, 125, 1–11.
Singh, D.; Singh, J.; Kumar, P.; Veer, D.; Kumar, D.; Katiyar, R.S.; Kumar, A.; Kumar, A. The Influence of TiO2 on the Proton Conduction and Thermal Stability of CsH2PO4 Composite Electrolytes. S. Afr. J. Chem. Eng. 2021, 37, 227–236.Li, Q.; He, R.; Jensen, J.O.; Bjerrum, N.J. Approaches and Recent Development of Polymer Electrolyte Membranes for Fuel Cells Operating above 100 °C. Chem. Mater. 2003, 15, 4896–4915.
Singh, D.; Kumar, P.; Singh, J.; Veer, D.; Kumar, A.; Katiyar, R.S. Structural, thermal and electrical properties of composites electrolytes (1−x) CsH2PO4/x ZrO2 (0 ≤ x ≤ 0.4) for fuel cell with advanced electrode. SN Appl. Sci. 2021, 3, 46.Loreti, G.; Facci, A.L.; Ubertini, S. High-Efficiency Combined Heat and Power through a High-Temperature Polymer Elec-trolyte Membrane Fuel Cell and Gas Turbine Hybrid System. Sustainability 2021, 13, 12515.
Veer, D.; Kumar, P.; Singh, D.; Kumar, D.; Katiyar, R.S. A synergistic approach to achieving high conduction and stability of CsH2PO4/NaH2PO4/ZrO2 composites for fuel cells. Mater. Adv. 2021, 3, 409–417.Boysen, D.A.; Uda, T.; Chisholm, C.R.I.; Haile, S.M. High-Performance Solid Acid Fuel Cells Through Humidity Stabiliza-tion. Science 2004, 303, 68–70.
Aili, D.; Gao, Y.; Han, J.; Li, Q. Acid-base chemistry and proton conductivity of CsHSO4, CsH2PO4 and their mixtures with N-heterocycles. Solid State Ion. 2017, 306, 13–19.Zhang, J.; Aili, D.; Lu, S.; Li, Q.; Jiang, S.P. Advancement toward Polymer Electrolyte Membrane Fuel Cells at Elevated Temperatures. AAAS Res. 2020, 2020, 9089405.
Bagryantseva, I.N.; Ponomareva, V.G.; Khusnutdinov, V.R. Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and poly(vinylidene fluoride-co-hexafluoropropylene). J. Mater. Sci. 2021, 56, 14196–14206.Clearfield, A.; Smith, S.D. The crystal structure of zirconium phosphate and the mechanism of its ion exchange behavior. J. Colloid Interface Sci. 1968, 28, 325–330.
Ponomareva, V.G.; Bagryantseva, I.N.; Shutova, E.S. Hybrid systems based on nanodiamond and cesium dihydrogen phosphate. Mater. Today 2020, 25, 521–524.Clearfield, A.; Smith, G.D. Crystallography and structure of alpha-zirconium bis(monohydrogen orthophosphate) mono-hydrate. Inorg. Chem. 1969, 8, 431–436.
Ma, N.; Kosasang, S.; Yoshida, A.; Horike, S. Proton-conductive coordination polymer glass for solid-state anhydrous proton batteries. Chem. Sci. 2021, 12, 5818–5824.Troup, J.M.; Clearfield, A. Mechanism of ion exchange in zirconium phosphates. 20. Refinement of the crystal structure of .alpha.-zirconium phosphate. Inorg. Chem. 1977, 16, 3311–3314.
Zhao, H.-R.; Xue, C.; Li, C.-P.; Zhang, K.-M.; Luo, H.-B.; Liu, S.-X.; Ren, X.-M. A Two-Dimensional inorganic−organic hybrid solid of manganese(II) hydrogenophosphate showing high proton conductivity at room temperature. Inorg. Chem. 2016, 55, 8971–8975.Casciola, M. From layered zirconium phosphates and phosphonates to nanofillers for ionomeric membranes. Solid State Ion. 2019, 336, 1–10.
Wang, M.; Luo, H.-B.; Liu, S.-X.; Zou, Y.; Tian, Z.-F.; Li, L.; Liu, J.-L.; Ren, X.-M. Water assisted high proton conductance in a highly thermally stable and superior water-stable open-framework cobalt phosphate. Dalton Trans. 2016, 45, 19466–19472.Ogawa, T.; Ushiyam, H.; Lee, J.-M.; Yamaguchi, T.; Yamashita, K. Theoretical Studies on Proton Transfer among a High Density of Acid Groups: Surface of Zirconium Phosphate with Adsorbed Water Molecules. J. Phys. Chem. C 2011, 115, 5599–5606.
Umeyama, D.; Horike, S.; Inukai, M.; Itakura, T.; Kitagawa, S. Inherent Proton Conduction in a 2D Coordination Framework. J. Am. Chem. Soc. 2012, 134, 12780–12785.Alberti, G.; Casciola, M.; Costantino, U. Inorganic ion-exchange pellicles obtained by delamination of α-zirconium phos-phate crystals. J. Colloid Interface Sci. 1985, 107, 256–263.
Inukai, M.; Horike, S.; Chen, W.; Umeyama, D.; Itakurad, T.; Kitagawa, S. Template-directed proton conduction pathways in a coordination framework. J. Mater. Chem. A. 2014, 2, 10404–10409.Alberti, G.; Casciola, M.; Costantino, U.; Leonardi, M. AC conductivity of anhydrous pellicular zirconium phosphate in hydrogen form. Solid State Ion. 1984, 14, 289–295.
Zhao, H.R.; Jia, Y.; Gu, V.; He, F.Y.; Zhang, K.M.; Tian, Z.F.; Liu, J.L. A 3D open-framework iron hydrogenophosphate showing high proton conductance under water and aqua-ammonia vapor. RSC Adv. 2020, 10, 9046–9051.Casciola, M.; Costantino, U.; D’Amico, S. Protonic conduction of intercolation compounds of α-zirconium phosphate with propylamine. Solid State Ion. 1986, 22, 127–133.
Zima, V.; Lii, K.-H. Synthesis and characterization of a novel one-dimensional iron phosphate: [C4H12N2]1.5[Fe2(OH)(H2PO4)(HPO4)2(PO4)]·0.5H2O. J. Chem. Soc. Dalton Trans. 1998, 24, 4109–4112.Alberti, G.; Casciola, M.; Cavalaglio, S.; Vivani, R. Proton conductivity of mesoporous zirconium phosphate pyrophos-phate. Solid State Ion. 1999, 125, 91–97.
Lii, K.-H.; Huang, Y.-F. Large tunnels in the hydrothermally synthesized open-framework iron phosphate (NH3(CH2)3NH3)2[Fe4(OH)3(HPO4)2(PO4)3]·xH2O. Chem. Commun. 1997, 9, 839–840.Alberti, G.; Casciola, M.; Donnadio, A.; Piaggio, P.; Pica, M.; Sisani, M. Preparation and characterisation of α-layered zirco-nium phosphate sulfophenylenphosphonates with variable concentration of sulfonic groups. Solid State Ion. 2005, 176, 2893–2898.
Bazaga-García, M.; Colodrero, R.M.P.; Papadaki, M.; Garczarek, P.; Zon, J.; Olivera-Pastor, P.; Losilla, E.R.; Reina, L.L.; Aranda, M.A.; Choquesillo-Lazarte, D.; et al. Guest Molecule-Responsive Functional Calcium Phosphonate Frameworks for Tuned Proton Conductivity. J. Am. Chem. Soc. 2014, 136, 5731–5739.Gui, D.; Zheng, T.; Xie, J.; Cai, Y.; Wang, Y.; Chen, L.; Diwu, J.; Chai, Z.; Wang, S. Significantly dense two-dimensional hy-drogen-bond network in a layered zirconium phosphate leading to high proton conductivities in both water-assisted low-temperature and anhydrous intermediate-temperature regions. Inorg. Chem. 2016, 55, 12508–12511.
Lim, D.-W.; Kitagawa, H. Proton Transport in Metal−Organic Frameworks. Chem. Rev. 2020, 120, 8416–8467.Yu, J.-W.; Yu, H.-J.; Yao, Z.-Y.; Li, Z.-H.; Ren, Q.; Luo, H.-B.; Zou, Y.; Wang, L.; Ren, X.-M. A water-stable open-framework zirconium(IV) phosphate and its water-assisted high proton conductivity. CrystEngComm 2021, 23, 6093–6097.
Mamlouk, M.; Scott, K. A boron phosphate-phosphoric acid composite membrane for medium temperature proton exchange membrane fuel cells. J. Power Sources 2015, 286, 290–298.Gui, D.; Dai, X.; Tao, Z.; Zheng, T.; Wang, X.; Silver, M.A.; Shu, J.; Chen, L.; Wang, Y.; Zhang, T.; et al. Unique proton trans-portation pathway in a robust inorganic coordination polymer leading to intrinsically high and sustainable anhydrous proton conductivity. J. Am. Chem. Soc. 2018, 140, 6146–6155.
Pusztai, P.; Haspel, H.; Tóth, I.Y.; Tombácz, E.; László, K.; Kukovecz, Á.; Kónya, Z. Structure-Independent Proton Transport in Cerium(III) Phosphate Nanowires. ACS Appl. Mater. Interfaces 2015, 7, 9947–9956.Kusoglu, A.; Weber, A.Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 2017, 117, 987–1104.
Mu, Y.; Wang, Y.Y.; Li, Y.; Li, J.Y.; Yu, J.H. Organotemplate-free synthesis of an open-framework magnesium aluminophosphate with proton conduction properties. Chem. Commun. 2015, 51, 2149–2151.Alberti, G.; Casciola, M.; Capitani, D.; Donnadio, A.; Narducci, R.; Pica, M.; Sganappa, M. Novel Nafion–zirconium phos-phate nanocomposite membranes with enhanced stability of proton conductivity at medium temperature and high relative humidity. Electrochim. Acta 2007, 52, 8125–8132.
Petersen, H.; Stefmann, N.; Fischer, M.; Zibrowius, I.R.; Philippi, W.; Schmidt, W.; Weidenthaler, C. Crystal structures of two titanium phosphate-based proton conductors: Ab initio structure solution and materials properties. Inorg. Chem. 2022, 61, 2379–2390.Grot, W.G.; Rajendran, G.; Hendrickson, J.S. International Patent Application No. PCT/US96/03804, International Publication No. WO 96/2975, 26, September, 1996.
Clearfield, A.; Smith, S.D. The crystal structure of zirconium phosphate and the mechanism of its ion exchange behavior. J. Colloid Interface Sci. 1968, 28, 325–330.Alberti, G.; Casciola, M.; Pica, M.; Tarpanelli, T.; Sganappa, M. New Preparation Methods for Composite Membranes for Medium Temperature Fuel Cells Based on Precursor Solutions of Insoluble Inorganic Compounds. Fuel Cells 2005, 5, 366–374.
Clearfield, A.; Smith, G.D. Crystallography and structure of alpha-zirconium bis(monohydrogen orthophosphate) monohydrate. Inorg. Chem. 1969, 8, 431–436.Casciola, M.; Bagnasco, G.; Donnadio, A.; Micoli, L.; Pica, M.; Sganappa, M.; Turco, M. Conductivity and Methanol Permea-bility of Nafion–Zirconium Phosphate Composite Membranes Containing High Aspect Ratio Filler Particles. Fuel Cells 2009, 9, 394–400.
Troup, J.M.; Clearfield, A. Mechanism of ion exchange in zirconium phosphates. 20. Refinement of the crystal structure of .alpha.-zirconium phosphate. Inorg. Chem. 1977, 16, 3311–3314.Arbizzani, C.; Donnadio, A.; Pica, M.; Sganappa, M.; Varzi, A.; Casciola, M.; Mastragostino, M. Methanol permeability and performance of Nafion–zirconium phosphate composite membranes in active and passive direct methanol fuel cells. J. Power Sources 2010, 195, 7751–7756.
Casciola, M. From layered zirconium phosphates and phosphonates to nanofillers for ionomeric membranes. Solid State Ion. 2019, 336, 1–10.Pica, M.; Donnadio, A.; Casciola, M.; Cojocaru, P.; Merlo, L. Short side chain perfluorosulfonic acid membranes and their composites with nanosized zirconium phosphate: Hydration, mechanical properties and proton conductivity. J. Mater. Chem. 2012, 22, 24902–24908.
Gui, D.; Zheng, T.; Xie, J.; Cai, Y.; Wang, Y.; Chen, L.; Diwu, J.; Chai, Z.; Wang, S. Significantly dense two-dimensional hydrogen-bond network in a layered zirconium phosphate leading to high proton conductivities in both water-assisted low-temperature and anhydrous intermediate-temperature regions. Inorg. Chem. 2016, 55, 12508–12511.Pica, M.; Donnadio, A.; Capitani, D.; Vivani, R.; Troni, E.; Casciola, M. Advances in the Chemistry of Nanosized Zirconium Phosphates: A New Mild and Quick Route to the Synthesis of Nanocrystals. Inorg. Chem. 2011, 50, 11623–11630.
Yu, J.-W.; Yu, H.-J.; Yao, Z.-Y.; Li, Z.-H.; Ren, Q.; Luo, H.-B.; Zou, Y.; Wang, L.; Ren, X.-M. A water-stable open-framework zirconium(IV) phosphate and its water-assisted high proton conductivity. CrystEngComm 2021, 23, 6093–6097.Bauer, F.; Willert-Porada, M. Comparison between Nafion and a Nafion zirconium phosphate nanocomposite in fuel cell applications. Fuel Cells 2006, 6, 261–269.
Gui, D.; Dai, X.; Tao, Z.; Zheng, T.; Wang, X.; Silver, M.A.; Shu, J.; Chen, L.; Wang, Y.; Zhang, T.; et al. Unique proton transportation pathway in a robust inorganic coordination polymer leading to intrinsically high and sustainable anhydrous proton conductivity. J. Am. Chem. Soc. 2018, 140, 6146–6155.Casciola, M.; Cojocaru, P.; Donnadio, A., Giancola, S.; Merlo, L.; Nedellec, Y.; Pica, M.; Subianto, S. Zirconium phosphate reinforced short side chain perflurosulfonic acid membranes for medium temperature proton exchange membrane fuel cell application. J. Power Sources 2014, 262, 407–413.
Alberti, G.; Cardini-Galli, P.; Costantino, U.; Torracca, E. Crystalline insoluble salts of polybasic metals—I Ion-exchange properties of crystalline titanium phosphate. J. Inorg. Nucl. Chem. 1967, 29, 571–578.Yang, C.; Srinivasan, S.; Aricò, A.S.; Creti, P.; Baglio, V.; Antonucci, V. Composition Nafion/zirconium phosphate mem-branes for direct methanol fuel cell operation at high temperature. Electrochem. Solid-State Lett. 2001, 4, A31–A34.
Christensen, A.N.; Anderson, E.K.; Andersen, I.G.; Alberti, G.; Nielsen, M.; Lehmann, E.K. X-Ray Powder diffraction study of layer compounds. The crystal structure of α-Ti (HPO4)2·H2O and a proposed structure for γ-Ti (H2PO4)(PO4)·2H2O. Acta Chem. Scand. 1990, 44, 865–872.Yang, C.; Costamagna, P.; Srinivasan, S.; Benziger, J.; Bocarsly, A.B. Approaches and technical challenges to high tempera-ture operation of proton exchange membrane fuel cells. J. Power Sources 2001, 103, 1–9.
Li, Y.J.; Whittingham, M.S. Hydrothermal synthesis of new metastable phases: Preparation and intercalation of a new layered titanium phosphate. Solid State Ion. 1993, 63, 391–395.Costamagna, P.; Yang, C.; Bocarsly, A.B.; Srinivasan, S. Nafion (R) 115/zirconium phosphate composite membranes for op-eration of PEMFCs above 100 °C. Electrochim. Acta 2002, 47, 1023–1033.
Kőrösi, L.; Papp, S.; Dékány, I. A layered titanium phosphate Ti2O3(H2PO4)2·2H2O with rectangular morphology: Synthesis, structure, and cysteamine intercalation. Chem. Mater. 2010, 22, 4356–4363.Yang, C.; Srinivasan, S.; Bocarsly, A.B.; Tulyani, S.; Benziger, J.B. A comparison of physical properties and fuel cell perfor-mance of Nafion and zirconium phosphate/Nafion composite membranes. J. Membr. Sci. 2004, 237, 145–161.
Ekambaram, S.; Serre, C.; Férey, G.; Sevov, S.C. Hydrothermal synthesis and characterization of an ethylenediamine-templatedmixed-valence titanium phosphate. Chem. Mater. 2000, 12, 444–449.Lee, H.-K.; Kim, J.-I.; Park, J.-H.; Lee, T.-H. A study on self-humidifying PEMFC using Pt-ZrP-Nafion composite membrane. Electrochim. Acta 2004, 50, 761–768.
Krogh Andersen, A.M.; Norby, P.; Hanson, J.C.; Vogt, T. Preparation and characterization of a new 3-dimensional zirconium hydrogen phosphate, τ-Zr(HPO4)2. Determination of the complete crystal structure combining synchrotron X-ray single-crystal diffraction and neutron powder diffraction. Inorg. Chem. 1998, 37, 876–881.Escorihuela, J.; Narducci, R.; Compañ, V.; Costantino, F. Proton Conductivity of Composite Polyelectrolyte Membranes with Metal-Organic Frameworks for Fuel Cell Applications. Adv. Mater. Interfaces 2019, 6, 1801146.
Bortun, A.I.; Khainakov, S.A.; Bortun, L.N.; Poojary, D.M.; Rodriguez, J.; Jose, R. Garcia, J.R.; Clearfield, A. Synthesis and characterization of two novel fibrous titanium phosphates Ti2O(PO4)2·2H2O. Chem. Mater. 1997, 9, 1805–1811.Liu, K.L.; Lee, H.C.; Wang, B.Y.; Lue, S.J.; Lu, C.Y.; Tsai, L.D.; Fang, J.; Chao, C.Y. Sulfonated poly(styrene-block-(ethylene-ran-butylene)-block-styrene (SSEBS)-zirconium phosphate) (ZrP) composite membranes for direct methanol fuel cells. J. Membr. Sci. 2015, 495, 110–120.
Salvadó, M.A.; Pertierra, P.; García-Granda, S.; García, J.R.; Fernández-Diaz, M.T.; Dooryhee, E. Crystal structure, including H-atom positions, of Ti2O(PO4)2·(H2O)2 determined from synchrotron X-ray and neutron powder data. Eur. J. Solid State Inorg. Chem. 1997, 34, 1237–1247.Hu, H.; Ding, F.; Ding, H.; Liu, J.; Xiao, M.; Meng, Y.; Sun, L. Sulfonated poly(fluorenyl ether ketone)/Sulfonated α-zirconium phosphate Nanocomposite membranes for proton exchange membrane fuel cells. Adv. Compos. Hybrid Mater. 2020, 3, 498–507.
Babaryk, A.A.; Adawy, A.; García, I. Trobajo, C.; Amghouz, Z.; Colodrero, R.M.P. Cabeza, A.; Olivera-Pastor, P.; Bazaga-García, M.; dos Santos-Gómez, L. Structural and proton conductivity studies of fibrous π-Ti2O(PO4)2·2H2O: Application in chitosan-based composite membranes. Dalton Trans. 2021, 50, 7667–7677.Pandey, J.; Seepana, M.M.; Shukla, A. Zirconium phosphate based proton conducting membrane for DMFC application. Int. J. Hydrog. 2015, 40, 9410–9421.
Bruque, S.; Aranda, M.A.G.; R. Losilla, E.; Olivera-Pastor, P.; Maireles-Torres, P. Synthesis optimization and crystal structures of layered metal(IV) hydrogen phosphates, .alpha.-M(HPO4)2.cntdot.H2O (M = Ti, Sn, Pb). Inorg. Chem. 1995, 34, 893–899.He, R.; Li, Q.; Xiao, G.; Bjerrum, N.J. Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors. J. Membr. Sci. 2003, 226, 169–184.
Huang, W.; Komarneni, S.; Noh,Y.D.; Ma, J.; Chen, K.; Xue, D.; Xuea, X.; Jiang, B. Novel inorganic tin phosphate gel: Multifunctional material. Chem. Commun. 2018, 54, 2682–2685.Gouda, M.H.; Tamer, T.M.; Konsowa, A.H.; Farag, H.A.; Mohy Eldin, M.S. Organic-Inorganic Novel Green Cation Exchange Membranes for Direct Methanol Fuel Cells. Energies 2021, 14, 4686.
Zhang, J.; Aili, D.; Lu, S.; Li, Q.; Jiang, S.P. Advancement toward Polymer Electrolyte Membrane Fuel Cells at Elevated Temperatures. AAAS Res. 2020, 2020, 9089405.Alberti, G.; Cardini-Galli, P.; Costantino, U.; Torracca, E. Crystalline insoluble salts of polybasic metals—I Ion-exchange properties of crystalline titanium phosphate. J. Inorg. Nucl. Chem. 1967, 29, 571–578.
Ansari, Y.; Telpriore, G.; Tucker, C.; Angell, A. A novel, easily synthesized, anhydrous derivative of phosphoric acid for use in electrolyte with phosphoric acid-based fuel cells. J. Power Sources 2013, 237, 47–51.Christensen, A.N.; Anderson, E.K.; Andersen, I.G.; Alberti, G.; Nielsen, M.; Lehmann, E.K. X-Ray Powder diffraction study of layer compounds. The crystal structure of α-Ti (HPO4)2·H2O and a proposed structure for γ-Ti (H2PO4)(PO4)·2H2O. Acta Chem. Scand. 1990, 44, 865–872.
Sato, Y.; Shen, Y.; Nishida, M.; Kanematsu, W.; Hibino, T. Proton Conduction in Non-Doped and Acceptor-Doped Metal Pyrophosphate (MP2O7) Composite Ceramics at Intermediate Temperatures. J. Mater. Chem. 2012, 22, 3973–3981.Li, Y.J.; Whittingham, M.S. Hydrothermal synthesis of new metastable phases: Preparation and intercalation of a new lay-ered titanium phosphate. Solid State Ion. 1993, 63, 391–395.
Singh, B.; Kima, J.-H.; Park, J.-Y.; Song, S.-J. Dense composite electrolytes of Gd3+-doped cerium phosphates for low-temperature proton-conducting ceramic-electrolyte fuel cells. Ceram. Inter. 2015, 41, 4814–4821.Kőrösi, L.; Papp, S.; Dékány, I. A layered titanium phosphate Ti2O3(H2PO4)2·2H2O with rectangular morphology: Synthesis, structure, and cysteamine intercalation. Chem. Mater. 2010, 22, 4356–4363.
Nagao, M.; Kamiya, T.; Heo, P.; Tomita, A.; Hibino, T.; Sano, M. Proton conduction in In3+-doped SnP2O7 at intermediate temperatures. J. Electrochem. Soc. 2006, 153, 1604–1609.Ekambaram, S.; Serre, C.; Férey, G.; Sevov, S.C. Hydrothermal synthesis and characterization of an ethylenedia-mine-templatedmixed-valence titanium phosphate. Chem. Mater. 2000, 12, 444–449.
Nagao, M.; Takeuchi, A.; Heo, P.; Hibino, T.; Sano, M.; Tomita, A. A proton-conducting In3 + -doped SnP2O7 electrolyte for intermediate-temperature fuel cells. Electrochem. Solid-State Lett. 2006, 9, A105.Krogh Andersen, A.M.; Norby, P.; Hanson, J.C.; Vogt, T. Preparation and characterization of a new 3-dimensional zirconi-um hydrogen phosphate, τ-Zr(HPO4)2. Determination of the complete crystal structure combining synchrotron X-ray sin-gle-crystal diffraction and neutron powder diffraction. Inorg. Chem. 1998, 37, 876–881.
Jin, Y.; Fujiwara, K.; Hibino, T. High temperature, low humidity proton exchange membrane based on an inorganic–organic hybrid structure. Electrochem. Solid-State Lett. 2010, 13, B8.Mileo, P.G.M.; Kundu, T.; Semino, R.; Benoit, V.; Steunou, N.; Llewellyn, P.L.; Serre, C.; Maurin, G.; Devautour-Vinot, S. Highly Efficient Proton Conduction in a Three-Dimensional Titanium Hydrogen Phosphate. Chem. Mater. 2017, 29, 7263–7271.
Jin, Y.; Shen, Y.; Hibino, T. Proton conduction in metal pyrophosphates (MP2O7) at intermediate temperatures. J. Mater. Chem. 2010, 20, 6214–6217.Bortun, A.I.; Khainakov, S.A.; Bortun, L.N.; Poojary, D.M.; Rodriguez, J.; Jose, R. Garcia, J.R.; Clearfield, A. Synthesis and characterization of two novel fibrous titanium phosphates Ti2O(PO4)2·2H2O. Chem. Mater. 1997, 9, 1805–1811.
Chen, X.; Wang, C.; Payzant, E.; Xia, C.; Chu, D. An oxide ion and proton co-ion conducting Sn0.9In0.1P2O7 electrolyte for intermediate-temperature fuel cells. J. Electrochem. Soc. 2008, 155, B1264.Salvadó, M.A.; Pertierra, P.; García-Granda, S.; García, J.R.; Fernández-Diaz, M.T.; Dooryhee, E. Crystal structure, including H-atom positions, of Ti2O(PO4)2·(H2O)2 determined from synchrotron X-ray and neutron powder data. Eur. J. Solid State In-org. Chem. 1997, 34, 1237–1247.
Kreller, C.R.; Pham, H.H.; Wilson, M.S.; Mukundan, R.; Henson, N.; Sykora, M.; Hartl, M.; Daemen, L.; Garzon, F.H. Intragranular phase proton conduction in crystalline Sn1−xInxP2O7 (x = 0 and 0.1). J. Phys. Chem. C 2017, 121, 23896–23905.Babaryk, A.A.; Adawy, A.; García, I. Trobajo, C.; Amghouz, Z.; Colodrero, R.M.P. Cabeza, A.; Olivera-Pastor, P.; Ba-zaga-García, M.; dos Santos-Gómez, L. Structural and proton conductivity studies of fibrous π-Ti2O(PO4)2·2H2O: Application in chitosan-based composite membranes. Dalton Trans. 2021, 50, 7667–7677.
Kreller, C.R.; Wilson, M.S.; Mukundan, R.; Brosha, E.L.; Garzon, F.H. Stability and conductivity of In3+-doped SnP2O7 with varying phosphorous to metal ratios. ECS Electrochem. Lett. 2013, 2, F61−F63.Bruque, S.; Aranda, M.A.G.; R. Losilla, E.; Olivera-Pastor, P.; Maireles-Torres, P. Synthesis optimization and crystal struc-tures of layered metal(IV) hydrogen phosphates, .alpha.-M(HPO4)2.cntdot.H2O (M = Ti, Sn, Pb). Inorg. Chem. 1995, 34, 893–899.
Foran, G.Y.; Goward, G.R. Site-Specific Proton Dynamics in Indium-Doped Tin Pyrophosphate. J. Phys. Chem. C 2020, 124, 28407–28416.Huang, W.; Komarneni, S.; Noh,Y.D.; Ma, J.; Chen, K.; Xue, D.; Xuea, X.; Jiang, B. Novel inorganic tin phosphate gel: Multi-functional material. Chem. Commun. 2018, 54, 2682–2685.
Scott, K.; Xu, C.; Wu, X. Intermediate temperature proton-conducting membrane electrolytes for fuel cells. Wiley Interdiscip. Rev. Energy Environ. 2014, 3, 24–41.Ansari, Y.; Telpriore, G.; Tucker, C.; Angell, A. A novel, easily synthesized, anhydrous derivative of phosphoric acid for use in electrolyte with phosphoric acid-based fuel cells. J. Power Sources 2013, 237, 47–51.
Anfimova, T.; Lie-Andersena, T.; Pristed Jensen, E.; C. Brorson Prag, C.; Nielsen, U.G.; Sørensen, D.R.; Skou, E.M.; Christensen, E.; Bjerrum, N.J.;.Li, Q. The effect of preparation method on the proton conductivity of indium doped tin pyrophosphates. Solid State Ion. 2015, 278, 209–216.Moshareva, M.A.; Novikova, S.A.; Yaroslavtsev, A.B. Synthesis and ionic conductivity of (NH4)1–x Hx Hf2(PO4)3 (x = 0–1) NASICON-type materials. Inorg. Mater. 2016, 52, 1283–1290.
Hogarth, W.H.J.; Muir, S.S.; Whittaker, A.K.; Diniz da Costa, J.C.; Drennan, J.; Lu, G.Q. Proton conduction mechanism and the stability of sol–gel titanium phosphates. Solid State Ion. 2007, 177, 3389–3394.Clearfield, A. Structural concepts in inorganic proton conductors. Solid State Ion. 1991, 46, 34–43.
Alberti, G.; Costantino, U.; Casciola, M.; Ferroni, S.; Massinelli, L.; Staiti, P. Preparation, characterization and proton conductivity of titanium phosphate sulfophenylphosphonate. Solid State Ion. 2001, 145, 249–255.Clearfield, A.; Roberts, B.D.; Subramanian, M.A. Preparation of (NH4)Zr2(PO4)3 and HZr2(PO4)3. Mater. Res. Bull. 1984, 19, 219–226.
Zhang, L.; Liu, X.; Sun, X.; Jian, J.; Li, G.; Yuan, H. Proton conduction in organically templated 3D open-framework vanadium−nickel pyrophosphate. Inorg. Chem. 2019, 58, 4394–4398.Komorowski, P.G.; Agryropoulos, S.A.A.; Canaday, J.D.; Kuriakose, A.K.; Wheat, T.A.; Ahmad, A.; Gulens, J. The study of hydronium NASICON conductivity with deuterium. Solid State Ion. 1992, 50, 253–258.
Stenina, I.A.; Kislitsyn, M.N.; Ghuravlev, N.A.; Yaroslavtsev, A.B. Phase transitions and ionic mobility in hydrogen zirco-nium phosphates with the NASICON structure, H1 ± xZr2–xMx(PO4)3⋅H2O, M = Nb, Y. Mater. Res. Bull. 2008, 43, 377–383.
Stenina, A.; Pinus, I.Y.; Rebrov, A.I.; Yaroslavtsev, A.B. Lithium and hydrogen ions transport in materials with NASICON structure. Solid State Ion. 2004, 175, 445–449.
Stenina, I.A.; Zhizhin, M.G.; Lazoryak, B.I.; Yaroslavtsev, A.B. Phase transitions, structure and ion conductivity of zirconium hydrogen phosphates, H1 ± xZr2–xMx(PO4)3 ⋅ H2O (M = Nb, Y). Mater. Res. Bull. 2009, 44, 1608–1612.
Mieritz, D.; Davidowski, S.K.; Seo, D. Accessing alkali-free NASICON-type compounds through mixed oxoanion sol–gel chemistry: Hydrogen titanium phosphate sulfate, H1−xTi2(PO4)3−x(SO4)x (x=0.5–1). J. Solid State Chem. 2016, 242, 116–125.
Sato, Y.; Shen, Y.; Nishida, M.; Kanematsu, W.; Hibino, T. Proton Conduction in Non-Doped and Acceptor-Doped Metal Pyrophosphate (MP2O7) Composite Ceramics at Intermediate Temperatures. J. Mater. Chem. 2012, 22, 3973–3981.
Singh, B.; Kima, J.-H.; Park, J.-Y.; Song, S.-J. Dense composite electrolytes of Gd3+-doped cerium phosphates for low-temperature proton-conducting ceramic-electrolyte fuel cells. Ceram. Inter. 2015, 41, 4814–4821.
Nagao, M.; Kamiya, T.; Heo, P.; Tomita, A.; Hibino, T.; Sano, M. Proton conduction in In3+-doped SnP2O7 at intermediate temperatures. J. Electrochem. Soc. 2006, 153, 1604–1609.
Nagao, M.; Takeuchi, A.; Heo, P.; Hibino, T.; Sano, M.; Tomita, A. A proton-conducting In3 + -doped SnP2O7 electrolyte for intermediate-temperature fuel cells. Electrochem. Solid-State Lett. 2006, 9, A105.
Jin, Y.; Fujiwara, K.; Hibino, T. High temperature, low humidity proton exchange membrane based on an inorganic–organic hybrid structure. Electrochem. Solid-State Lett. 2010, 13, B8.
Jin, Y.; Shen, Y.; Hibino, T. Proton conduction in metal pyrophosphates (MP2O7) at intermediate temperatures. J. Mater. Chem. 2010, 20, 6214–6217.
Chen, X.; Wang, C.; Payzant, E.; Xia, C.; Chu, D. An oxide ion and proton co-ion conducting Sn0.9In0.1P2O7 electrolyte for intermediate-temperature fuel cells. J. Electrochem. Soc. 2008, 155, B1264.
Kreller, C.R.; Pham, H.H.; Wilson, M.S.; Mukundan, R.; Henson, N.; Sykora, M.; Hartl, M.; Daemen, L.; Garzon, F.H. Intra-granular phase proton conduction in crystalline Sn1−xInxP2O7 (x = 0 and 0.1). J. Phys. Chem. C 2017, 121, 23896–23905.
Kreller, C.R.; Wilson, M.S.; Mukundan, R.; Brosha, E.L.; Garzon, F.H. Stability and conductivity of In3+-doped SnP2O7 with varying phosphorous to metal ratios. ECS Electrochem. Lett. 2013, 2, F61−F63.
Foran, G.Y.; Goward, G.R. Site-Specific Proton Dynamics in Indium-Doped Tin Pyrophosphate. J. Phys. Chem. C 2020, 124, 28407–28416.
Scott, K.; Xu, C.; Wu, X. Intermediate temperature proton-conducting membrane electrolytes for fuel cells. Wiley Interdiscip. Rev. Energy Environ. 2014, 3, 24–41.
Anfimova, T.; Lie-Andersena, T.; Pristed Jensen, E.; C. Brorson Prag, C.; Nielsen, U.G.; Sørensen, D.R.; Skou, E.M.; Christen-sen, E.; Bjerrum, N.J.;.Li, Q. The effect of preparation method on the proton conductivity of indium doped tin pyrophos-phates. Solid State Ion. 2015, 278, 209–216.
Li, W.; Bose, A.B.; Rusakova, I.A. An approach for restoring the proton conductivity of sintered tin pyrophosphate mem-branes for intermediate temperature fuel cells. J. Power Sources 2016, 307, 146–151.
Ramaiyan, K.P.; Herrera, S.; Workman, M.J.; Semelsberger, T.A.; Atanasov, V.; Kerres, J.; Sandip Maurya, S.; Kim, Y.S.; Krel-ler, C.R.; Mukundan, R. Role of phosphate source in improving the proton conductivity of tin pyrophosphate and its com-posite electrolytes. J. Mater. Chem. A 2020, 8, 16345–16354.
Wu, X.; Mamlouk, M.; Scott, K. A PBI-Sb0.2Sn0.8P2O7-H3PO4 Composite Membrane. Fuel Cells 2011, 11, 620–625.
Jin, Y.C.; Nishida, M.; Kanematsu, W.; Hibino, T. An H3PO4-doped polybenzimidaz-ole/Sn0.95Al0.05P2O7 composite mem-brane for high-temperature proton exchange mem-brane fuel cells. J. Power Sources 2011, 196, 6042–6047.
Huang, M.; Huang, X.; Deng, Y.; Fei, M.; Xu, C.; Cheng, J. Graphite oxide-incorporated SnP2O7 solid composite electrolyte for high-temperature proton exchange membrane fuel cells. Int. J. Hydrog. Energy 2017, 42, 1113–1119.
Le, M.-V.; Tsai, D.-S.; Yang, C.-Y.; Chung, W.-H.; Lee, H.-Y. Proton conductors of cerium pyrophosphate for intermediate temperature fuel cell. Electrochim. Acta 2011, 56, 6654–6660.
Hogarth, W.H.J.; Muir, S.S.; Whittaker, A.K.; Diniz da Costa, J.C.; Drennan, J.; Lu, G.Q. Proton conduction mechanism and the stability of sol–gel titanium phosphates. Solid State Ion. 2007, 177, 3389–3394.
Alberti, G.; Costantino, U.; Casciola, M.; Ferroni, S.; Massinelli, L.; Staiti, P. Preparation, characterization and proton con-ductivity of titanium phosphate sulfophenylphosphonate. Solid State Ion. 2001, 145, 249–255.
Zhang, L.; Liu, X.; Sun, X.; Jian, J.; Li, G.; Yuan, H. Proton conduction in organically templated 3D open-framework vana-dium−nickel pyrophosphate. Inorg. Chem. 2019, 58, 4394–4398.
Botez, C.E.; Tackett, R.J.; Hermosillo, J.D.; Zhang, J.; Zhao, Y.; Wang, L. High pressure synchrotron x-ray diffraction studies of superprotonic transitions in phosphate solid acids. Solid State Ion. 2012, 213, 58–62.
Baranov, A.I. Crystals with disordered hydrogen-bond networks and superprotonic conductivity. Review. Crystallogr. Rep. 2003, 48, 1012–1037.
Bagryantseva, I.N.; Gaydamaka, A.A.; Ponomareva, V.G. Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and Butyral polymer. Ionics 2020, 26, 1813–1818.
Dreßler, C.; Sebastiani, D. Effect of anion reorientation on proton mobility in the solid acids family CsHyXO4 (X = S, P, Se, y = 1, 2) from ab initio molecular dynamics simulations. Phys. Chem. Chem. Phys. 2020, 22, 10738–10752.
Taninouchi, Y.K.; Uda, T.; Awakura, Y.; Ikeda, A.; Haile, S.M. Dehydration behavior of the superprotonic conductor CsH2PO4 at moderate temperatures: 230 to 260 °C. J. Mater. Chem. 2007, 17, 3182–3189.
Mathur, L.; Kim, I.-H.; Bhardwaj, A.; Singh, B.; Park, J.-Y.; Song, S.-J. Structural and electrical properties of novel phosphate based composite electrolyte for low-temperature fuel cells. Composites Part B 2020, 202, 108405.
Otomo, J.; Tamaki, T.; Nishida, S.; Wang, S.; Ogura, M.; Kobayashi, T.; Wen, C.-J.; Nagamoto, H.; Takahashi, H. Effect of water vapor on proton conduction of cesium dihydrogen phosphate and application to intermediate temperature fuel cells. J. Appl. Electrochem. 2005, 35, 865–870.
Uda, T.; Haile, S.M. Thin-membrane solid-acid fuel cell. Electrochem. Solid State Lett. 2005, 8, A245–A246.
Navarrete, L.; Andrio, A.; Escolástico, S.; Moya, S.; Compañ, V.; Serra, J.M. Protonic Conduction of Partially-Substituted CsH2PO4 and the Applicability in Electrochemical Devices. Membranes 2019, 9, 49.
Ponomareva, V.G.; Bagryantseva, I.N.; Gaydamaka, A.A. Study of the Phase Composition and Electrotransport Properties of the Systems Based on Mono- and Disubstituted Phosphates of Cesium and Rubidium. Chem. Sustain. Dev. 2019, 27, 238–245.
Martsinkevich, V.V.; Ponomareva, V.G. Double salts Cs1-xMxH2PO4 (M = Na, K, Rb) as proton conductors. Solid State Ion. 2012, 225, 236–240.
Ponomareva, V.G.; Bagryantseva, I.N. Superprotonic CsH2PO4-CsHSO4 solid solutions. Inorg. Mater. 2012, 48, 187–194.
Ponomareva, V.G.; Uvarov, N.F.; Lavrova, G.V.; Hairetdinov, E.F. Composite protonic solid electrolytes in the CsHSO4-SiO2 system. Solid State Ion. 1996, 90, 161–166.
Ponomareva, V.G.; Shutova, E.S.; Matvienko, A.A. Conductivity of Proton Electrolytes Based on Cesium Hydrogen Sulfate Phosphate. Inorg. Mater. 2004, 40, 721–728.
Otomo, J.; Minagawa, N.; Wen, C.-J.; Eguchi, K.; Takahashi, H. Protonic conduction of CsH2PO4 and its composite with silica in dry and humid atmospheres. Solid State Ion. 2003, 156, 357–369.
Ponomareva, V.G.; Shutova, E.S. High-temperature behavior of CsH2PO4 and CsH2PO4-SiO2 composites. Solid State Ion. 2007, 178, 729–734.
Singh, D.; Singh, J.; Kumar, P.; Veer, D.; Kumar, D.; Katiyar, R.S.; Kumar, A.; Kumar, A. The Influence of TiO2 on the Proton Conduction and Thermal Stability of CsH2PO4 Composite Electrolytes. S. Afr. J. Chem. Eng. 2021, 37, 227–236.
Singh, D.; Kumar, P.; Singh, J.; Veer, D.; Kumar, A.; Katiyar, R.S. Structural, thermal and electrical properties of composites electrolytes (1−x) CsH2PO4/x ZrO2 (0 ≤ x ≤ 0.4) for fuel cell with advanced electrode. SN Appl. Sci. 2021, 3, 46.
Veer, D.; Kumar, P.; Singh, D.; Kumar, D.; Katiyar, R.S. A synergistic approach to achieving high conduction and stability of CsH2PO4/NaH2PO4/ZrO2 composites for fuel cells. Mater. Adv. 2021, 3, 409–417.
Aili, D.; Gao, Y.; Han, J.; Li, Q. Acid-base chemistry and proton conductivity of CsHSO4, CsH2PO4 and their mixtures with N-heterocycles. Solid State Ion. 2017, 306, 13–19.
Bagryantseva, I.N.; Ponomareva, V.G.; Khusnutdinov, V.R. Intermediate temperature proton electrolytes based on cesium dihydrogen phosphate and poly(vinylidene fluoride-co-hexafluoropropylene). J. Mater. Sci. 2021, 56, 14196–14206.
Ponomareva, V.G.; Bagryantseva, I.N.; Shutova, E.S. Hybrid systems based on nanodiamond and cesium dihydrogen phos-phate. Mater. Today 2020, 25, 521–524.
Ponomareva, V.G.; Bagryantseva, I.N. Effect of the excess protons on the electrotansport, structural and thermodynamic properties of CsH2PO4. Solid State Ion. 2017, 304, 90–95.
Ponomareva, V.G.; Lavrova, G.V. The influence of Cs2HPO4·H2O impurity on the proton conductivity and thermal proper-ties of CsH2PO4. Solid State Ion. 2019, 329, 90–94.
Wang, M.; Luo, H.-B.; Liu, S.-X.; Zou, Y.; Tian, Z.-F.; Li, L.; Liu, J.-L.; Ren, X.-M. Water assisted high proton conductance in a highly thermally stable and superior water-stable open-framework cobalt phosphate. Dalton Trans. 2016, 45, 19466–19472.
Umeyama, D.; Horike, S.; Inukai, M.; Itakura, T.; Kitagawa, S. Inherent Proton Conduction in a 2D Coordination Frame-work. J. Am. Chem. Soc. 2012, 134, 12780–12785.
Inukai, M.; Horike, S.; Chen, W.; Umeyama, D.; Itakurad, T.; Kitagawa, S. Template-directed proton conduction pathways in a coordination framework. J. Mater. Chem. A. 2014, 2, 10404–10409.
Zhao, H.-R.; Xue, C.; Li, C.-P.; Zhang, K.-M.; Luo, H.-B.; Liu, S.-X.; Ren, X.-M. A Two-Dimensional inorganic−organic hybrid solid of manganese(II) hydrogenophosphate showing high proton conductivity at room temperature. Inorg. Chem. 2016, 55, 8971–8975.
Phang, W.J.; Lee, W.R.; Yoo, K.; Ryu, D.W.; Kim, B.S.; Hong, C.S. PH-dependent proton conducting behavior in a met-al-organic framework material. Angew. Chem. Int. Ed. 2014, 53, 8383–8387.
Nagarkar, S.S.; Unni, S.M.; Sharma, A.; Kurungot, S.; Ghosh, S.K. Two-in-one: Inherent anhydrous and water-assisted high proton conduction in a 3D metal-organic framework. Angew. Chem. Int. Ed. 2014, 53, 2638–2642.
Nguyen, N.T.T.; Furukawa, H.; Gándara, F.; Trickett, C.A.; Jeong, H.M.; Cordova, K.E.; Yaghi, O.M. Three-dimensional met-al-catecholate frameworks and their ultrahigh proton conductivity. J. Am. Chem. Soc. 2015, 137, 15394–15397.
Horike, S.; Chen, W.; Itakura, T.; Inukai, M.; Umeyama, D.; Asakura, H.; Kitagawa, S. Order-to-disorder structural transfor-mation of a coordination polymer and its influence on proton conduction. Chem. Commun. 2014, 50, 10241–10243.
Ma, N.; Kosasang, S.; Yoshida, A.; Horike, S. Proton-conductive coordination polymer glass for solid-state anhydrous pro-ton batteries. Chem. Sci. 2021, 12, 5818–5824.
Matsuda, Y.; Yonemura, M.; Koga, H.; Pitteloud, C.; Nagao, M.; Hirayama, M.; Kanno, R. Synthesis, crystal structure, and ionic conductivity of tunnel structure phosphates, RbMg1-xH2x(PO3)3y(H2O). J. Mater. Chem. A 2013, 1, 15544–15551.
Antraptseva, N.M.; Solod, N.V.; Povshuk, V.A. Electrical conductivity of solid solution of Co(II)-Zn dihydrophosphate and its thermolysis products. Funct. Mater. 2015, 22, 322–326.
Fan, D.; Tian, P.; Xu, S.T.; Wang, D.H.; Yang, Y.; Li, J.Z.; Wang, Q.Y.; Yang, M.; Liu, Z.M. SAPO-34 templated by dipropyla-mine and diisopropylamine: Synthesis and catalytic performance in the methanol to olefin (MTO) reaction. New J. Chem. 2016, 40, 4236–4244.
Yu, Y.; Zhu, J.; Liu, J.; Yan, Y.; Song, X. Synthesis and characterization of two layered aluminophosphates [R-C8H12N]8[H2O]2·[Al8P12O48H4] and [S-C8H12N]8[H2O]2·[Al8P12O48H4] with a mirror symmetric feature and their proton conductivity. Dalton Trans. 2017, 46, 9157–9162.
Park, S.H.; Choi, W.; Choi, H.J.; Hong, S.B. Organic-Free Synthesis of Silicoaluminophosphate Molecular Sieves. Angew. Chem. Int. Ed. 2018, 57, 9413–9418.
Wang, Y.Y.; Sun, Y.J., Mu, Y.; Zhang, C.Q.; Li, J.Y.; Yu, J.H. Organotemplate-free hydrothermal synthesis of an alumino-phosphate molecular sieve with AEN zeotype topology and properties of its derivatives. Chem. Commun. 2014, 50, 15400–15403.
Sun, Y.J.; Yan, Y.; Wang, Y.Y.; Li, Y.; Li, J.Y.; Yu, J.H. High proton conduction in a new alkali metal-templated open-framework aluminophosphate. Chem. Commun. 2015, 51, 9317–9319.
Parise, J.B. Crystal Structures of Related Novel Aluminophosphate Frameworks: Aipo4-21(py), Aipo4-EN3 (en) and A Struc-tural Model for Alpo4-25. In Studies in Surface Science and Catalysis; Drzâj, S.H.B., Pejovnik, S., Eds.; Elsevier: Amsterdam, The Netherlands, 1985; Volume 24, pp. 271–278.
Parnham, E.R.; Morris, R.E. The ionothermal synthesis of cobalt aluminophosphate zeolite frameworks. J. Am. Chem. Soc. 2006, 128, 2204–2205.
Wei, Y.; Tian, Z.; Gies, H.; Xu, R.; Ma, H.; Pei, R.; Zhang, W.; Xu, Y.; Wang, L.; Li, K.; et al. Ionothermal synthesis of an alu-minophosphate molecular sieve with 20-ring pore openings. Angew. Chem. Int. Ed. 2010, 49, 5367–5370.
Liu, L.; Yang, J.; Li, J.; Dong, J.; Šišak, D.; Luzzatto, M.; McCusker, L.B. Ionothermal sythesis and structure analysis of an open-framework zirconium phosphate with a high CO2/CH4 adsorption ratio. Angew. Chem. Int. Ed. 2011, 50, 8139–8142.
Nakayama, M.; Sugiura, Y.; Hayakawa, T.; Nogami, M. A novel proton conductor of imidazole–aluminium phosphate hy-brids in the solid state. Phys. Chem. Chem. Phys. 2011, 13, 9439–9444.
Wang, K.; Li, T.; Zeng, H.; Zou, G.; Zhang, Q.; Zhien Lin, Z. Ionothermal synthesis of open-framework metal phosphates using a multifunctional ionic liquid. Inorg. Chem. 2018, 57, 8726–8729.
Zhang, C.; Yan, Y.; Huang, Z.; Shi, H.; Zhang, C.; Cao, X.; Jiang, J. Triclinic AlPO-34 zeolite synthesized with nicotine and its proton conduction properties. Inorg. Chem. Commun. 2018, 96, 165–169.
Xue, C.; Zou, Y.; Liu, S.-X.; Ren, X.-M.; Tian, Z.-F. Two different types of channels exhibiting distinct proton transport be-haviour in an open framework aluminophosphate. J. Solid State Chem. 2018, 258, 695–701.
Mu, Y.; Wang, Y.Y.; Li, Y.; Li, J.Y.; Yu, J.H. Organotemplate-free synthesis of an open-framework magnesium alumino-phosphate with proton conduction properties. Chem. Commun. 2015, 51, 2149–2151.
Fan, D.; Barrier, N.; Vicente, A.; Gilson, J.-P.; Clevers, S.; Dupray, V.; Coquereld, G.; Valtchev, V. Organic template-free syn-thesis of an open framework silicoaluminophosphate (SAPO) with high thermal stability and high ionic conductivity. Inorg. Chem. Front. 2020, 7, 542–553.
Zhou, D.; Chen, L.; Yu, J.H.; Li, Y.; Yan, W.F.; Deng, F.; Xu, R.R. Synthesis, crystal structure, and solid-state NMR Spectros-copy of a new open-framework aluminophosphate (NH4)2Al4(PO4)4(HPO4)·H2O. Inorg. Chem. 2005, 44, 4391–4397.
Zhao, H.R.; Jia, Y.; Gu, V.; He, F.Y.; Zhang, K.M.; Tian, Z.F.; Liu, J.L. A 3D open-framework iron hydrogenophosphate showing high proton conductance under water and aqua-ammonia vapor. RSC Adv. 2020, 10, 9046–9051.
Zima, V.; Lii, K.-H. Synthesis and characterization of a novel one-dimensional iron phosphate: [C4H12N2]1.5[Fe2(OH)(H2PO4)(HPO4)2(PO4)]·0.5H2O. J. Chem. Soc. Dalton Trans. 1998, 24, 4109–4112.
Lii, K.-H.; Huang, Y.-F. Large tunnels in the hydrothermally synthesized open-framework iron phosphate (NH3(CH2)3NH3)2[Fe4(OH)3(HPO4)2(PO4)3]·xH2O. Chem. Commun. 1997, 9, 839–840.
Bazaga-García, M.; Colodrero, R.M.P.; Papadaki, M.; Garczarek, P.; Zon, J.; Olivera-Pastor, P.; Losilla, E.R.; Reina, L.L.; Aranda, M.A.; Choquesillo-Lazarte, D.; et al. Guest Molecule-Responsive Functional Calcium Phosphonate Frameworks for Tuned Proton Conductivity. J. Am. Chem. Soc. 2014, 136, 5731–5739.
Lim, D.-W.; Kitagawa, H. Proton Transport in Metal−Organic Frameworks. Chem. Rev. 2020, 120, 8416–8467.
Mamlouk, M.; Scott, K. A boron phosphate-phosphoric acid composite membrane for medium temperature proton ex-change membrane fuel cells. J. Power Sources 2015, 286, 290–298.
Pusztai, P.; Haspel, H.; Tóth, I.Y.; Tombácz, E.; László, K.; Kukovecz, Á.; Kónya, Z. Structure-Independent Proton Transport in Cerium(III) Phosphate Nanowires. ACS Appl. Mater. Interfaces 2015, 7, 9947–9956.
Petersen, H.; Stefmann, N.; Fischer, M.; Zibrowius, I.R.; Philippi, W.; Schmidt, W.; Weidenthaler, C. Crystal structures of two titanium phosphate-based proton conductors: Ab initio structure solution and materials properties. Inorg. Chem. 2022, 61, 2379–2390.