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 CsH₂PO₄ have 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].

    Severaol 3D open-framework M(II) phosphates have been reported ^[14][143]id acid proton conductors, which consist of [CoPO₄]_∞⁻ oth stoichiometry [Zn₂(M^IHPyXO₄)₂ (H₂M^I = Cs, Rb; X = S, PO₄)₂]²⁻ anionic, framSeworks that contain organic charge-compensating ions in their internal cavities. (C₂N₂H₁₀)_0.5CoPO₄ ; y = 1, 2), have received much attention because they exhibit ed negligible conductivity in anhydrous conditions; however, it displayed a relatively high water-xceptional proton transport properties and can be used as electrolytes in fuel cells operated at intermediated proton conductivity 2.05 × 1 temperatures (120−300⁻³ S·cm⁻¹ at 56 °C). and 98% RH. On the other hand, the solid NMThe fundamental characteristics of these₄·Zn[HPO₄][H₂PO₄] matexperiences a structuralrials are the phase transformation from monoclinic (α) to orthorhombic (β) upon ition that occurs in response to heating at 149 °C. Both polymorphs contain 12-membered, cooling or application of pressure rings^[6][23], accomposed of tetrahedral Zanied by an²⁺ ions linked to protonated phosphate groups without changingcrease in proton conductivity of several orders of magnitude—referred to as Zn-O-Psuper protonic conneductivity (Figure 8Figure 3).

Figure 3. The characteristic high and low temperature proton conductivity and the corresponding structures for CsH₂PO₄, adapted from ^[6][14324]

.

    This prope α phase transforms into the β phase at high humidity and temperatures above 60 °C, andrty has been associated with the delocalization of hydrogen bonds then^[24]. Foreaches CsH₂PO₄, a proton conductivity of 1.306 × 10⁻² S·cm⁻¹ wat 98% RH, a behaviour that might be attributed to the participation of adsorbed water molecules in creating H-bs measured above 230 °C corresponding networks withto the effesuper protonic ctubive pathwaysc (Pm−3m) phase for^[25] proton conductwhion. The conductivityle it drastically decreases at 65 °C. In anhydrous conditions, the α phase exhibited a proton conductivity ofrops in the low temperature phases. Recently, from an ab initio molecular dynamics simulation study of the solid acids CsHSeO₄, ~10⁻⁴ CsHS·cm⁻¹O₄ atnd 160 °CCsH₂PO₄, similar values were found for other reported zinc phosphates att was concluded that efficient long-range proton transfer in the high temperatures between 130 and 190 °C ^{[12][144][145]}.

    thatIn the dominantse compounds, proton conductive species are protons, and theon follows a Grotthuss mechanism with proton transport from anode to cathode takes place through H-bond networks in the pellet.

    Several studensies ^[6][28][29] have H-bshond networkswn that CsH₂PO₄ ican the lattice, composed of Mn₃O₁₃be employed as the unielects-containing anionic layerrolyte in fuel cells ^[146], which provide efficient proton-transfer pathways for a Grotthuss-type proton transport at high RH.

    ThCsH₂PO₄ composites strategy of enhancing proton conductivity was further extended successfully to other 2D manganese phosphatesbased on organic additives have been also intensively investigated. Various strategies, ^[18]. A protoin conductivity value as high as 7.72 × 10⁻² S·luding salt modification by cm⁻¹ wastion reached atand anion substitution 30^{[31][32][33][34]}, °C and 99% RH for K⁺-emixchainged compounds, which compares well with those of MOF-based open-framework with oxide materials ^{[14735][36][37][38][39][14840][14941]}. The 1D solidor organic additives [Zn₃(H₂PO₄)₆(H₂O)₃](Hbim)^{[25][42][43][44]}, (Hbim=have benzimidazole) prepared by mechanochemical synthesis is characterized by presenting a dual-function as proton conductor.

3. Divalent Metal Phosphates.

    benzDimidazole molecules. In addition, this solid also showed porosity, thus, enabling the adsorption of gaseous methanol that further improved the proton conductivityvalent transition metal phosphates show a great structural versatility, from 1D polymeric topologies through layered framework to 3D open-framework structures. Most of the anhydrous phase. This enhancement of proton conductivity in methanol-adsorbed samples was explse solids are synthetized in the presence of organic molecules, which are retained by the effective participation of theas protonated guest molecule in formation of extended hydrogen-bond interactionsspecies (amines, iminazole derivatives, etc.), thus, compensating the anionic charge of ^[19].

    A 1D zinc phosphate-based proton conductor, [Zn₃(H₂PO₄)₆(H₂O)₃](BTA) (BTA = 1,2,3-benzotriazole) has been reported ^[15145] 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.

    FocuAs has been also put on an example of 2D metal phosphate-based solid solutions ^[152][153]. V wacancies can be generated that introduce extra protons into the structure and increase the r-assisted proton conduction. This effect was investigated for the 1D rubidium and magnesium polyphosphateors, (C₂H₁₀N₂) compou[Mnd, RbMg_1-xH_2x(HPO₃₄)₃·y](H₂O), for which system a maximum displayed a proton conductivity of 5.51.64 × 10⁻³ S·cm⁻¹ was mundeasured at 170 °C with a vehicle-type mechanism of H₃O⁺ conduction.

     thSeve robust inorganic ral 3D open-framework endues this porous material with better thermalM(II) phosphates have been reported and^[14][47], cwhemical stability compared withich consist of [CoPO₄]_∞⁻ porous coordi [Zn₂(HPO₄)₂(H₂PO₄)₂]²⁻ atnion polymers/metal organic frameworks (PCPs/MOFs) ^[13].

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, e.g., BPO_x ^[55] and CePO₄ ^[56]. The former exhibited a proton conductivity of 7.9 × 10⁻² S·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

S·cm

at full-hydration conditions and 84 °C. The low E

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

    valuThes of 0.16−0.2 eV, typical of a Grotthuss-type proton-transfer mechanism.

    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⁻ 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 ^[62].

    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⁻ 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.

   a

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.

    GStrotthuss-type proton-transfer mechanism. An efficient pathway for the proton transfer was attributed to the hydrogen bond network established between the ucturally, titanium(IV) phosphates are more diverse than ZrP, given that structures containing an oxo/hydroxyl ligand or mixed-valence (Tim^III/Ti^IV) tidtazolnium ions and water molecules interacting with the host framework.

   A few exahave been reported. In addition to the zirconiumples of phosphate-based analogues, α- proton^[66] coanduc γ-TiP ^[67], twors of more other trivalent metals do exist. Among them, two Fe(III) layered titanium phosphates, 1D (C₄H₁₂N₂)_1.5[Fe₂(TiO(OH)(H₂PO₄)(HPO₄)₂(PO₄)]·0.52H₂O ^[1368] and 3D open-framework Tiron(III) phosphate (NH₃(CH₂)₃NH₃)₂[Fe₄(OH)₃(H₂PO₄)₂(PO₄)₃]·42H₂O ^[170], havre beeknown ^[69]. Furtheported. Both compounds contain Frmore, the mixed-valence₄O₂₀ teitramers as a common structural feature. The 1D solanium phosphate Tid ^IIITis^IV(HPO₄)₄·C₂N₂H₉·H₂O composed of chntains of tetramers bridged by PO₄^3- grmicroporoups and having terminal H₂PO₄^- and HPO₄^2- groupchannels ^[17170], whilere piperazinium cations andmonoprotonated ethylenediamine and lattice water molecules are disorderly situated in between chains. This arrangement gives rise to extended hydrogen bonding interactions and hence proton conducting pathways. The proton conductare accommodated, which favours its transformation into a porous phase, Tivity₂(HPO₄)₄, measured at 40 °C and 99% RH was 5.14 × 10⁻⁴ S·600 °C with a 3D strucm⁻¹, and iture was maintained upon dispersion of this solid in PVDFsimilar to that of τ-Zr(HPO₄)₂ ^[1371].

    In tThe case of the 3D solid, infinite chains of interconnected tetramers are interlinked, in turn by phosphate groups that generate large tunnels rystal structure of two other open-frameworks Ti₂O(Figure 9PO₄).₂·2H₂O The diprotonated 1,3-diaminopropane and water molecules, localized inside tunnels, form an extended hydrogen bond network with theolymorphs (ρ-TiP and π-TiP) has been reported P–OH^[72][73]. gFibroups pointing toward cavities. These interactions favour proton hopping, the measured proton conductivity being of 8.0 × 10⁻⁴ S·s ρ-TiP and π-TiP crystallize in triclinic and monoclinic unit cells, respectively, and are com⁻¹, at 44 °C anposed 99%of RH,TiO₆ and wTithO₄(H₂O)₂ an E_a of 0.32 eV ^[172]. Furctahermore, the proton conductivity of this compound increased up to 5 × 10⁻² S·cm⁻¹ dra bridged through the orthophosphate 40 °C upon exposure to aqua-ammonia vapors from 1 M NH₃·H₂O solutiongroups (Figure 8). This rtypesult confirms this treatment as an effective way of enhancing proton conductivity, which has been elsewhere demonstrated for the case of coordination polymers of connectivity creates two types of 1D channels running parallel to the direction of ^[173][174]. Tthe ofibserved variation of E_a re growith the^[72][73][74]. ³¹P ammoniaMAS-NMR concentration also suggestspectroscopy studies revealed that NH₃,π-TiP has well as H₂O, moleccapability of adsorbing sulpes contribute to createrficially proton-transfer pathways, by a Grotthuss mechanism. ated phosphate species (However₃PO₄/H₂PO₄^-/HPO₄^2-), whenich ammonia concentrations were lower than 0.5 M, thffects the proton conduction mechanism tended to be vehicle-type onvity properties as confirmed by AC impedance measurements.

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

^[15674].

F

).

    The igsomorphoure 9.s series of Oplayeren-framework structure of (NH₃(CH₂)₃NH₃)₂d α-metal(IV) phosphates [Fe₄M(OHIV)₃(HPO₄ = Zr, Ti, Sn)₂(PO₄)₃]·4H₂O showing guest species inside channels and H-d appreciable differences in the hydrogen bond interactions. Fe (green), O (red), P (oran 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], which, in turn, might have), and C (grey) atom significant implications in making internal surfaces accessible to proton conduction paths.

 

    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. Tetrable 2valent Pyrophosphates.

Proton

    conductivity daTeta for selected divalent and triravalent metal pyrophosphates.

Compounds/Dimensionality

Temperature

(

°

C)/RH(%)

Conductivity

(S·cm

-1

)

Ea

(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)xNa1−x[Mn2(PO4)2]/2D	30/99	2.1 × 10-2		0.14	[17]
(C2H10N2)1-xKx[Mn2(PO4)2]·2H2O/2D	30/99	7.72 × 10-2		0.18	[18]
(C2H10N2)1-xKx[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]
RbMg0.9H0.2(PO3)3·yH2O/1D	170		5.5 × 10−3	---	[152]
Co0.5Zn0.5(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; M = ForSn, the series of isostructural imidazole cation (ImH₂Ce, Ti, Zr), in particular tin(IV)-template d layered metal phosphates, [ImH₂][X-(HPO₄)₂(H₂O)₂] (FJU−25-X, X = Al, Ga, aerivatives, combind Fe), it was found that the proton conductivity was dependent on mo high thermal stability of imidazole guests, FJU−25-Fe exhibiting the highest proton (T ³ 400 °C) with conductivity (5.21 × ies of 10⁻³–10⁻⁴² S·cm⁻¹ in at 90 °C). The determined activation energies (~0.2he temperature range of 100–300 eV) were°C, indicative of a Grotthuss-type mechanism of proton conductionunder anhydrous or low humidity conditions ^{[79][80][81][82][83]}.

   The amorigino acid-template indium phosphate, In(HPO₄)(H₂PO₄)(D,L-C₃H₇NO₂) of this proton (SCU−12), represeconts a singular example of 3D metal phosphate-basedductivity was initially explained because of proton conductors ^[16].

   Its crystal 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 ing incorporated into the framework, formed by metal(IV) octahedra and corner-sharing phosphate ladders by the H₂PO₄^- gtetrahedral, afteroups gives rise to a thrinteraction with water vapor.

    Defe-dimensional structure. The presence of two kinds of proton carriers,ct sites, such as electron holes H₂PO₄^-[84] ions and zwitterionic alanine molecules,oxygen vacancies are believed to favours the development proton generation of^[85]. Thighese proton conductivities (2.9 × 10⁻³ S·cs m⁻¹ at 85 °C and 98%y RH) through a Grotthuss-type proton-transfer mechanism (E_a = 0.19 eV).

   Ooccupy hydrogen-bonding inther trivalent metal phosphates have been reported, e.g., BPstitial sites on either the M−O−P or the P−O_x−P ^[175] abonds, CePO₄thus, ^[176]. Thge formner exhibited aating a hopping proton conductivity of 7.9 × 10⁻² S·cm⁻¹ trans self-supported electrolyte and 4.5 × 10⁻² S·cm⁻¹ aport between these (PBI)−4BPO_x composites membra^[86][87]. Ine, measured at 150 °Csupport of this, and¹H 5% NMRH, but 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 a studies revealed two signals corresponding to 100% RH through a structure-independent proton-transport mechanism ^[176].

   There are only a few examples of mehydrogen-bonded interstitial(III) pyrophosphates displaying proton conductivity. Among them is the open framework magnesium aluminop protons at phosphate MgAlP₂O₇(OH)(H₂O)₂ (JU102) ^[167]. Its structure is composed of tettrahedral Al³⁺ and metal octahedral Mg²⁺sites ions coordi Inate³⁺-d by pyrophosphate ionsed SnP₂O₇. TFurthis connectivity results in an open framework with unidirectional 8-ring channels. Termore, indium doping seemed not to affect the proton conduction properties originate from the existence of an H-bond network in which coordinated water molecules participateoccurring by a hopping mechanism between octahedral and tetrahedral sites.

    Thus, the increased proton conductivity measured at 55 °C on water-immersed samples was 3.86 × 10⁻⁴ S·cm^{−1 [167]}, whis ich rainsed to 1.19 × 10⁻³ S·cm⁻¹ whten calcined at 250 °C and measured at the same conditions, while the E_a value had attrdly changed from 0.16 to 0.2 eV. This behaviour was explained as being due to a dehydration–rehydration process that enhancesbutable to increased proton conductivity by alteringcentration ^[88]. tThe H-bonding network and the pathway of proton transfer.

  strategy of dopant insertion Ainother example of 3D open framework metal(IIIto the M(IV) pyrophosphate is the compound NH₄TiP₂O₇ ^[177]. Thhas been further extended structure of this solid is composed of negatively charged [TiP₄O₁₂]⁻ to include other dopant divalent and trivalayers, forming one-dimensional six-membered ring channelsnt metal ions, such as Mg²⁺, whereAl³⁺, theSb³⁺, NH₄Sc³⁺, ions and Gare³⁺, locawited. Its protonh a maximum conductivity increased from 10⁻⁶of 0.195 S·cm⁻¹ undber anhydrous conditions to 10⁻³ S·cm⁻¹ aing reported full-hydration coor Inditio_0.1Sns_0.9P₂O₇ and 84 °C^[89]. The lHow E_a evalue, 0.17 eV, characteristic of a Grotthuss-type proton-transfer mechanism,r, recent studies on metal doped and undoped SnP₂O₇ was associated with the role played by the NH₄⁺ uggested that it ions in the channels as proton donors and promoters of proton migration. A drop inco-precipitated phosphorous-rich amorphous phases where the proton conductivity was observed when the triclinic TiP₂O₇ mostly resides, while the crystalline phase formed by thermal dexhibit a very low conductivity (~10⁻⁸ S·composi⁻¹ ation of NH₄TiP₂O₇150−300 °C) ^[17790].

5. Outlook

     GreatLow efforts have been devoted to the synthesis and characterization of metal crystallinity titanium pyrophosphate-baseds also exhibit high proton conductors over more thanivity three^[91]. decVades. Among them, zirconiulues of 0.0019–0.0044 S·cm⁻¹, phosphates 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 du 100 °C and 100% RH were reported for these compounds. The proton conduction is attributable to their outst presence of H₂PO₄^- anding HPO₄²⁻ sproperties and workabilitecies, demonstrated by while³¹P MAS beingNMR environmentally benign and low cost materials.

  and FTIR, and it occurs Although the prototype layered α-zirconium phosphate has been commonly proven as a filler for PEMFCs devices, new synthetic designs of M(IV) phosphates, including pyrorough a water-facilitated Grotthuss-type proton transport mechanism. Titanium phosphate compounds, are promising candidates to broaden their applicability in different electrochemical devices. Other M(IV) s are also prone to be functionalized with phosphates and pyrophosphates (M = Ti, Sn) are less known due, in part, to their amorphous nature or strong tendency to amorphise at working temperatures, thoughonates groups. Thus, mixed titanium phosphate/phosphonates, Ti(HPO₄)_1.00(O₃PC₆H₄SO₃H)_0.85(OH)_0.30·nH₂O ^[92], din some cases, these compounds presented remarkablesplayed an exceptional proton conductivity propertiesof 0.

  1 CsH₂PO₄, a super protoniS·cm⁻¹ material, was proven 100 °C.

    as aA suitable electrolyte for both H₂/O₂ antructurally d direct 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 CsH₂PO₄ fferent proton-conductor metal pyrophosphate is the mixed templectrolyte, thus, offering a wide range of compositions with tuning properties.

   Reate-containing vanadium nickently, research and developments in metal pl pyrophosphate proton, (C₆H₁₄N₂)[NiV₂O₆H₈(P₂O₇)₂]·2H₂O cond(Figuctors have been addressed to divalent or trivalent metal phosphates, which present a remarkablere 9). Its crystal structural 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.

   Apple is composed of octahedrally coordinated V(IV) and Ni(II) interconnected through bricatdgions of metal ng pyrophosphates as electrolytes or as electrolyte components are in continuous progress, although their groups, thus creating a 3D framework of [NiV₂O₆H₈(P₂O₇)₂]²⁻ use for energy storage and conversion remains a challenge. For practical applications in fuel cells at low/intermediate temperatures, phosphate-based proton conducting electrolytes have demonstrated acceptablit charged-compensated by protonated DABCO molecules. Hydrogen-bonding networks inside 3D channels facilitate the 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 fuel cross-over, are keytransport, and thus this material exhibits a remarkable proton conductivity of 2.0 × 10⁻² faS·ctors to be improved.

Funding:⁻¹ This research was funded by PID2019−110249RB-I00 (MICIU/AEI, Spain) and PY20−00416 (Junta de Andalucia, Spain/FEDER) research projects.

Acknowledgments: M.B.G. t 60 °C and 100% RH through a Grotthuss-type proton-thranks PAIDI2020 research grant (DOC_00272 Junta de Andalucia, Spain) and R.M.P.C. thanks University ofsfer mechanism (E_a Malaga= under Plan Propio de0.38 eV) Investigación for financial support^[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]).