Synergistic Modulation of Transition-Metal-Based Electrocatalysts for Water Oxidation: History
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Contributor:
Zhen Li
,
Ying Wang
,
Lawrence Yoon Suk Lee

Efficient electrochemical energy conversion techniques play a crucial role in mitigating energy and environmental challenges by replacing fossil fuels and decarbonizing the power and transportation sectors. Synergistic modulation has been extensively explored to develop highly efficient transition-metal-based electrocatalysts for oxygen evolution reaction (OER) because coupling effects among intrinsic activity, conductivity, mass transfer, mass diffusion, and intermediates adsorption can further promote catalytic activity.

  • electrocatalysis
  • oxygen evolution reaction
  • transition metal catalysts
  • synergistic modulation

1. Introduction

Efficient electrochemical energy conversion techniques, such as water splitting, CO2 reduction, and Zn-air battery, play a crucial role in mitigating energy and environmental challenges by replacing fossil fuels and decarbonizing the power and transportation sectors [1][2]. Oxygen evolution reaction (OER) can couple with reduction reactions (hydrogen reaction and CO2 reduction) to constitute a redox reaction circle. However, the effectiveness of these techniques is impeded by the thermodynamic uphill nature and sluggish kinetics of OER occurring on the anode [3][4]. As a result, it is often necessary to use precious metal (Ru/Ir)-based electrocatalysts to facilitate OER, yet the scarcity and high cost hamper their wide application [5]. Therefore, the pursuit of affordable and efficient electrocatalysts remains an important research focus. Typically, high-performance electrocatalysts require the following features: (1) high intrinsic activity to minimize the overpotential required for catalyzing OER; (2) abundant active sites for rapid reaction kinetics; (3) favorable conductivity for efficient electron transfer; (4) sufficient channels for effective mass transfer and gas evolution; (5) robust structural and chemical stability to ensure long-term durability; and (6) low cost for scalable fabrication.
Transition-metal-based OER catalysts have been extensively explored as alternatives to precious metals because of their abundant reserves, cost-effectiveness, and favorable theoretical electrochemical activity. Transition metals are characterized by their d-orbital valence electronic structure [6][7][8]. The interaction between oxygen-containing species and the transition-metal surface leads to electron transfer at the interfaces, driven by the difference in their electrochemical potentials. Specifically, the O 2p orbital of oxygen-containing species hybridizes with the d orbitals of transition metals to split into two energy levels [9]. The catalytic activity of OER catalysts, such as transition-metal oxides, hydroxides, sulfides, and phosphides, is closely associated with the electronic number of metal d band because eg orbit can bind surface anions, thus affecting the combination of oxygen intermediates [1][10][11][12][13][14][15][16]. Although transition-metal-based catalysts have demonstrated appealing OER performances, they still exhibit shortcomings such as poor conductivity, less accessible active sites, and high activation energy barriers [17][18][19]. Numerous strategies have been adopted to modify their electronic structure, enhance electron transfer, improve mass diffusion, and optimize intermediates’ adsorption and desorption. However, most efforts focusing on a single individual aspect were unable to fulfill all the requirements for high-performance catalysts. To overcome this limitation, synergistic modulation, which has exhibited significant effects in the fields of CO2 reduction, N2 reduction, and H2 evolution reactions [20][21][22][23][24], for simultaneously targeting multiple aspects has been suggested to realize advanced OER catalysts based on transition metals.
Despite significant efforts dedicated to the development of various transition-metal OER electrocatalysts, achieving industrial-level OER performance remains a formidable challenge. Applying modification to the electronic structure, morphology, crystalline, elementary reactions, and external fields to synergistically modulate intrinsic activity, active sites number, conductivity, mass diffusion, the free energy of intermediates adsorption, and external forces has been accepted as an effective approach to optimizing OER catalytic performance.

2. Metal Active Sites and Heterogeneous Atoms

The electrocatalytic OER performance is known to be influenced by intrinsic activity, which is determined by the energy barriers associated with the adsorption and desorption of oxygen-containing intermediates [25]. Nørskov’s theory proposes that the difference in adsorption energy between *O to *OH (∆G*O − ∆G*OH) serves as a descriptor for OER activity, which can be balanced by electronic modulation [26]. The introduction of heteroatoms, including both metal and non-metal atoms, is a highly effective strategy for tuning the electronic structure [27][28][29]. To effectively adjust the electronic structure of the reactive sites, the heteroatom must possess a relatively low electronegativity or induce an abundance of electrons.

2.1. Metal Active Sites and Heterogeneous Metal Atoms

Sargent’s group demonstrated that introducing metallic dopants (W, Mo, Nb, Ta, Re, and MoW) with high-valence charges can lower the energy barriers for valence charge transition in 3d metals, such as Fe, Co, and Ni, thereby improving catalytic OER performance [30]. Furthermore, the adsorption energy of NiFe-LDH towards oxygen intermediates can also be optimized after the introduction of those high-valence metals. Zhao’s group investigated the in situ structural reconstruction from V-doped Ni2P to NiV oxyhydroxides, where the synergistic interaction between Ni hosts and V dopants can modulate the electronic structure of NiV oxyhydroxides, facilitating the adsorption of *OH and deprotonation of *OOH intermediates [31]. Wang et al. incorporated high-valence state tantalum (Ta) into the pristine NiFe-LDH through the hydrothermal method [32]. Structural characterizations and DFT results revealed that Ta doping induced electronic structure modulation around Ni, Fe, and Ta, and the eg orbital of Ta, resulting from charge transfer, promoted the adsorption of OH species on Ta sites and improve the conductivity of NiFe-LDH. It is worth noting that surface reconstruction can occur easily before OER. Rare earth metals containing unique 4f sub-shell electrons have also attracted significant attention [33][34]. Sun et al. synthesized Ce-doped LaNiO3 and found that low-concentration Ce doping at the A-site can promote surface reconstruction into a highly active NiOOH phase by optimizing the O 2p level [35].
In addition to doping, loading single atoms on catalyst surfaces has emerged as a promising strategy to achieve outstanding catalytic properties by utilizing low-coordination and unsaturated active sites [36].

2.2. Metal Active Sites and Heterogeneous Non-Metal Atoms

Opposite to metal elements, highly electronegative non-metal atoms can attract electrons from metals to form adsorption sites for oxygen-containing intermediates during the OER process. Li et al. demonstrated the OER performance of NiFeP catalysts can be improved by partially replacing P with S [37]. The formation of metal-sulfur bonds modulated the electronic structure of the catalysts, leading to a decrease in the energy barrier during the adsorption process and reaction pathway of OER. Moreover, S doping facilitated the generation of *OOH and the release of O2 during the OER process. N-doped NiS2 exhibited enhanced OER activity because of its well-defined morphology, fast charge transfer, and enriched N doping [38]. Specifically, the presence of N atoms adjacent to the active sites of Ni shifted the position of Ni d-states closer to the Fermi level, and the strong electron-withdrawing property of N atoms endowed adjacent Ni atoms with a higher oxidation state. Moreover, the introduction of N atoms also promoted the value of (∆G*OH − ∆G*OOH) close to the volcano center, which indicated optimized adsorption energy towards oxygen-containing intermediates during the OER process. Additionally, halogen atoms (F, Cl, and Br) have been proven to effectively modulate the electronic structure of the matrix to improve OER performance [39][40][41][42][43].

3. Heterogeneous Atoms and Crystallographic Structure

In addition to the conventional method of incorporating heterogeneous atoms, manipulating the crystalline nature of catalysts provides an alternative approach to modulating their electronic properties. By synergistically combining heteroatoms with the creation of vacancies, lattice distortion, and grain boundaries, researchers have demonstrated the effectiveness of this strategy in precisely adjusting the electronic properties of catalysts, leading to significant improvements in catalytic performance or even the mechanism of OER.

3.1. Synergistic Modulation on Heterogeneous Atoms and Cation/Anion Vacancies

The construction of oxygen vacancies is a prevalent strategy in the design of transition-metal-based catalysts, owing to their low formation energy. An oxygen vacancy is a type of point defect that arises from the removal of oxygen atoms in the metal oxide lattice without causing a phase transition. The resultant reduction in oxygen concentration induces electron deficiency in neighboring metal species, leading to a redistribution of electron density towards the metal atoms and a subsequent reduction in electron density around oxygen atoms. This electronic modulation promotes the interaction between hydroxyl ions and OER reaction intermediates. Yang et al. incorporated N doping and oxygen vacancy into the Co3O4 catalyst and demonstrated that N atoms redistributed electronic configuration of Co atoms to facilitate OER kinetics, while generating rich oxygen vacancies could activate lattice oxygen oxidation mechanism during the OER process [44]. The synergistic effect of N doping and oxygen vacancies optimized the adsorption behavior of oxygen-containing intermediates. Additionally, electronic states can be regulated by integrating heterogeneous metal atoms with oxygen vacancies [45]. Li et al. constructed W-doped NiFeW-LDHs with oxygen vacancies on nickel foam and demonstrated that the weakening of metal-oxygen bonds and the shift of the O 2p band center towards the Fermi level induced the formation of oxygen vacancies, thereby enhancing the adsorption capacity of OER intermediates [46]. The positive shift of the d-band center and generation of oxygen vacancies enhanced the adsorption capacity of intermediates in the OER process. While anion vacancies (e.g., P, S, and Se) have received considerable attention [47][48][49], cationic vacancies have been relatively less explored due to their higher hopping barriers. Recent studies indicated that cationic vacancies can play a similar role to their anionic counterparts in improving OER activity [50][51][52]. For example, Zhao et al. designed and synthesized highly efficient Fe-doped La0.5Sr0.5−δCoO3 with Sr vacancies for OER and proposed that the synergistic effect of Fe active sites and Sr vacancies activated the lattice oxygen mechanism [53]. Theoretical calculations revealed that surface Fe sites acted as the catalytic centers to trigger lattice OER, while Sr vacancies could promote oxidation of surface lattice oxygen through uplifting O 2p levels to facilitate OER.

3.2. Heterogeneous Atoms and Lattice Distortion/Grain Boundaries

Lattice distortion and grain boundary engineering have emerged as effective strategies for boosting the kinetics of OER by creating additional active sites. Liao et al. explored the introduction of cerium (Ce) atoms into NiFe-LDH to induce lattice distortion [54]. Experimental and theoretical results demonstrated that the incorporation of Ce and lattice distortion regulated the electronic structure of Ni atoms in active sites and lowered the Gibbs free energy of the potential-determining step: *OH → *O. Additionally, the creation of a high density of grain boundaries has been proposed as a promising strategy for augmenting the number of active sites for OER due to the loose distribution of atoms along these boundaries [55][56]. Qiao et al. synthesized (FexCo1-x)B OER electrocatalyst with controllable grain boundary density [57]. Physical characterizations and DFT calculations confirmed that the presence of Fe atoms and manipulation of grain boundaries could effectively modulate the electronic states and provide more efficient active sites, respectively, thus synergistically enhancing the OER process.
Inducing heteroatoms and creating lattice distortion also can cause lattice strain due to the change in atom–atom bond length or by the induced lattice mismatch. The electronic structure of the catalysts’ surface is sensitive to lattice strain, which makes strain a useful strategy for regulating electrocatalysis [58][59][60]. Ma et al. induced tunable lattice strain into NiFeMo alloys through dual doping of Mo and Fe, which in turn changed d-band center and electronic interaction on catalytic active sites, thus improving OER performance [61]. In addition, combining lattice strain with other modifications can synergistically modulate the OER property of catalysts. Liu et al. investigated the coupling effect of lattice strain and oxygen defects on electrocatalytic OER activity of La0.7Sr0.3CoO3−δ thin films [62]. Experimental results and computational calculations indicated that excessive oxygen defects induced by strain increased the eg state occupancy and expanded the energy gap between Co 3d and O 2p bands, leading to lower OER activity.

4. Electronic Structure and Morphology

The OER enhancement strategies mentioned above mainly involve regulating the electronic structure, conductivity, and adsorption-free energy of the active intermediate species. On the other hand, regulating morphology is another effective strategy to improve the OER efficiency of transition-metal catalysts by increasing specific surface area, exposing more active sites, and accelerating the release of bubbles. By integrating morphology engineering with electronic modulation to enhance intrinsic activity, catalytic performance can be further boosted.

5. Synergistic Modulation on Elementary Reactions

The adsorbate evolution mechanism (AEM) and the lattice-oxygen-mediated mechanism (LOM) are two well-established mechanisms that play crucial roles in OER. In alkaline media, the AEM involves a series of four concurrent proton-electron transfer reactions, where metal atoms act as reaction centers. These reactions can be described by the following equations [63]:
OH− + * → *OH + e−   
*OH → *O + e− + H+   
*O + OH− → *OOH + e−   
*OOH → * + O2(g) + e− + H+   
The scaling relation among the reaction intermediates in the AEM pathway imposes a theoretical lower limit of 0.37 eV on the overpotential [64][65]. Three strategies have been proposed to break this scaling relation to obtain better activity: (1) stabilizing OER intermediate *OOH while maintaining the adsorption of *OH; (2) inducing a proton acceptor to regulate the reaction pathway; and (3) activating lattice oxygen for direct coupling of O−O radical, which is also known as LOM. This means that the LOM can bypass the formation of *OOH, and thus the limitation in scaling relation between *OH and *OOH can be avoided. For example, amorphous NiFeP nanostructures were fabricated for highly active and stable OER electrocatalysts [66]. The electronic structure of metal sites could be modulated by the ligand effect of P, consequently breaking the scaling relationships among these OER intermediates. Specifically, the adsorption energy gap between *OH and *OOH can be reduced from 3.08 to 2.62 eV by the incorporation of P atoms in NiFeOOH, which resulted in the shift of rate-determining step for OER from the formation of *O to *OH. Similarly, Liu et al. synthesized S-doped NiFe2O4 nanocone arrays which showed a current density of 100 mA cm−2 with an overpotential of 270 mV, which was superior to reported spinel-type oxides [67]. The calculation results demonstrated that the PDOS of Ni-d of Ni atoms adjacent to S atoms was localized near the Fermi level, suggesting that the coupling of Ni-d orbitals and 2p orbitals of oxygen-containing intermediates was promoted. In NiFe2O4, *O to *OOH is the rated-determining step (RDS). DFT calculations revealed that the energy barrier of RDS on the Ni site decreased to 0.25 eV, significantly lower than that on the Fe site, after the introduction of S. These findings suggest that S doping imparts appropriate electronic states and enhanced adsorption capabilities to Ni sites, breaking the scaling relation during the OER process.
Introducing a second component on host materials to form a heterostructure is considered a simple and effective route to design efficient OER electrocatalysts [68][69]. In contrast to heteroatom doping, which necessitates limiting the number of dopants to a low level (typically, <10% of the total elements) to avoid the emergence of new crystal phases that could impact the original structures and block active sites, heterostructures offer several advantages. These include synergistic effects, strain effects, and electronic interactions, all of which contribute to enhanced catalytic performance [70][71][72]. The strong interaction in the heterostructure has been proven to effectively modify the local electronic configuration around active sites and optimize the adsorption/desorption energy of intermediates on different components [71][73].

6. Synergistic Modulation on External Fields

Field-assisted electrocatalysis has emerged as a promising technique for enhancing electrochemical reactions, particularly in the context of OER. This technique utilizes external factors such as magnetic fields, strain, and light to provide additional means of engineering and optimizing the OER process.
Theoretical explanations for magnetic field-assisted OER primarily involve three key effects: magnetothermal, spin-polarized, and electron energy state enhancement effects. The overall OER performance can be improved by increasing the surface temperature of catalysts, optimizing the adsorption thermodynamic features of reactants and intermediates, and accelerating electron transfer. For example, Garcés-Pineda et al. conducted a comprehensive investigation on the influence of an external static magnetic field on a series of transition metal oxides during the electrocatalytic OER process in an alkaline electrolyte [74]. 
Light-assisted electrocatalytic OER involves two primary mechanisms: photocarrier and photothermy. When photosensitive materials are subjected to light irradiation, the carriers become excited, facilitating the overcoming of potential barriers in charge transfer and redox reactions [75][76]. Thus, coupling photo-excited carriers with electrochemical reactions can significantly accelerate catalytic rates. Bai et al. successfully hybridized CoFe-LDH with WO3/SnSe2 n–p heterojunction and demonstrated that the overpotential for OER could be decreased by 80 mV under simulated sunlight irradiation [77]. During the OER process, photo-generated holes on the valence band of SnSe2 would be transferred to CoFe-LDH and oxidize Co/Fe into higher valence states. Consequently, OH− could rapidly adsorb on metal sites and undergo deprotonation to form *O species. In other words, the photoelectric synergy system in the heterojunction led to a reduction in the energy barrier for OER and a remarkable acceleration of the OER kinetics.
In solar light-assisted electrocatalysis, the photothermal effect represents another critical aspect that can provide an additional driving force, namely thermal energy, to reduce activation energy, thus promoting the electrochemical reaction kinetics [78][79][80]. Photo-sensitive materials, including plasmonic metals, semiconductors, and carbon materials, can respond to solar light and generate in situ thermal energy to promote electron transfer. For example, Liang et al. synthesized a self-supported reduced graphene oxide (rGO) film with abundant carbon defects and broad light absorption [81]. 

This entry is adapted from the peer-reviewed paper 10.3390/catal13091230

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