Spinel‐Type Oxides for Acidic Oxygen Evolution Reaction: Mechanism, Modulation, and Perspective

The development of proton exchange membrane water electrolyzers (PEMWEs) is primarily challenged by the slow and unstable oxygen evolution reaction (OER) at the anode. Thus, the pursuit of low‐cost, highly efficient, and durable electrocatalysts is the foundation for the widespread adoption of PEMWEs. In the past few decades, various types of electrocatalysts are proposed to serve as anode materials for acidic OER, but only a few demonstrate catalytic activity and stability comparable to iridium‐ and ruthenium‐based electrocatalysts. In recent years, some transition‐metal oxides are studied as potential candidates for acidic OER. For instance, spinel‐type oxides, e.g., Co3O4, and corresponding compounds demonstrate promise due to the rich coordination structure of metallic ions and moderate adoption energy for intermedia. In this review, recent advances in spinel‐type oxides for acidic OER are summarized. First, a fundamental understanding of reaction mechanisms for OER in acidic media is introduced. Thereafter, recent progress in rational design principles and optimization strategies for spinel oxides is systematically summarized. Finally, challenges and perspectives for the development of spinel‐based acidic OER electrocatalysts are discussed.


Introduction
[3][4][5][6] Conventional H 2 -preparation techniques, such as natural gas reforming, consume a lot of energy and emit a huge amount of greenhouse gas. [7]In the past decades, more attention has been paid to renewable energy, such as solar energy and wind, driven water electrolysis techniques, which are efficient and sustainable approaches to producing high-purity H 2 . [8]lectrochemical water splitting involves two separate reactions, i.e., hydrogen evolution reaction (HER) at the cathode side and oxygen evolution reaction (OER) at the anode side, where the overall reaction equation can be given as Water electrolysis can be divided into two different categories according to the electrolyte environment, i.e., alkaline and acidic electrolytes.[15][16][17][18][19][20] On the contrary, OER involves a four-electron-transfer process, which results in a complicated multistep reaction. [21,22]The sluggish OER reaction kinetics result in relatively large overpotential and energy consuming, which requires highly active and stable electrocatalysts to overcome the high energy barrier. [23]Thus, OER is considered to be the roadblock to the practical application of water electrolysis.Depending on the pH of electrolytes, the mechanisms for OER process demonstrate distinct differences.
In general, the oxygen-containing reactant is the initial raw material for OER, and it refers to hydroxyl groups (OH À ) in an aqueous solution. [24]In alkaline media, abundant OH À provides sufficient reactants for subsequent steps, which is gradually oxidized to water and oxygen (Figure 1a).However, in acidic solutions, the severe lack of hydroxide ions makes it difficult for the active sites on the surface of catalytic materials to bind with OH À and undergo subsequent reactions, which slows down the overall process.Therefore, in acidic OER reactions, water molecules are usually used as reactants, and additional water dissociation steps are required to provide reaction intermediates for the active sites (Figure 1b). [25]Another difference between alkaline OER and acidic OER lies in the stability of electrodes and electrocatalysts.Transition metal (TM) based (e.g., Fe, Co, and Ni) alkaline OER electrocatalysts exhibit comparable catalytic performance to DOI: 10.1002/aesr.202300075 The development of proton exchange membrane water electrolyzers (PEMWEs) is primarily challenged by the slow and unstable oxygen evolution reaction (OER) at the anode.Thus, the pursuit of low-cost, highly efficient, and durable electrocatalysts is the foundation for the widespread adoption of PEMWEs.In the past few decades, various types of electrocatalysts are proposed to serve as anode materials for acidic OER, but only a few demonstrate catalytic activity and stability comparable to iridium-and ruthenium-based electrocatalysts.In recent years, some transition-metal oxides are studied as potential candidates for acidic OER.For instance, spinel-type oxides, e.g., Co 3 O 4 , and corresponding compounds demonstrate promise due to the rich coordination structure of metallic ions and moderate adoption energy for intermedia.In this review, recent advances in spineltype oxides for acidic OER are summarized.First, a fundamental understanding of reaction mechanisms for OER in acidic media is introduced.Thereafter, recent progress in rational design principles and optimization strategies for spinel oxides is systematically summarized.Finally, challenges and perspectives for the development of spinel-based acidic OER electrocatalysts are discussed.
[28][29][30][31][32] However, when it comes to acidic OER, few electrocatalysts are suitable for large-scale applications with balanced activity and stability.For transition-metal oxides (TMOs), the reasons for catalytic performance degradation can be broadly categorized into several types.First, during the OER process, TMO surfaces undergo significant redox reactions that can alter the crystal structure and decrease the conductivity of electrocatalysts.Second, reconstructed surfaces formed during the OER process can dissolve easily in acidic media, leading to the leaching of metal sites, which are considered to be the active sites in the acidic OER process.Finally, in the OER process, the local concentration of H þ ions is much higher, increasing the acidity of the solution around the electrode.This can potentially damage the electrode's structure, especially for self-standing electrodes.[35][36][37][38][39][40][41][42] Despite the numerous advantages of alkaline OER, acidic OER is preferable due to its application potential in proton exchange membrane water electrolyzers (PEMWEs).
PEMWE is a new kind of electrolyzer, which is more suitable for future applications in hydrogen production than the traditional alkaline water electrolysis (AWE).PEMWEs demonstrate several unique advantages.For instance, a proton exchange membrane (PEM) only allows the passage of protons, which can avoid the mixture of H 2 and O 2 , as well as increase the safety of electrolyzers.Also, PEMWEs operate under an acidic solution (pH%2) and the conductivity of H 3 O þ (350 S cm 2 mol À1 ) is much higher than OH À (198 S cm 2 mol À1 ), which results in low Ohmic loss and high current density; [43] Moreover, PEMWEs can operate under wide temperature range and the application of PEM is beneficial for compact system design and stacking with lower cost and faster response. [44][47] Table 1 demonstrates some important parameters to evaluate the electrolyzers, and the comparisons between AWE and PEMWE.From the comparisons, PEMWE could be operated under a much higher current density with relatively lower cell voltage, which have much higher efficiency.While PEMWE is more efficient than AWE, its practical implementation is still limited by cost and lifespan challenges.50] As aforementioned, only a few noble-based materials can work stably under acidic environments and high anode voltages, which creates a more urgent demand for high-activity and inexpensive anodes for PEMWEs.65] However, these electrocatalysts suffer from low activity and poor stability, and more efficient modulation strategies and synthetic methods should be further investigated.Among all transitionalmetal-based electrocatalysts, spinel-type oxides demonstrate unique properties and optimal OER activity.The crystal structure of spinel contains 96 interstices, which can be filled with metallic ions.Moreover, 64 of all interstices are 4 oxygen-coordinated tetrahedral sites and 32 are 6 oxygen-coordinated octahedral sites.Only part of the interstices is filled with metallic cations, resulting in 8 tetrahedral and 16 octahedral sites occupied by divalent (þ2) and trivalent cations (þ3).The relatively open structure with flexible coordination environments and different oxidation states endows spinal oxides with a tunable OER behavior. [66]heoretical calculation results indicate that spinel oxides, such as Co 3 O 4 , possess moderate adsorption energy for the reaction intermediates and a lower reaction energy barrier for the deprotonation process, resulting in a relatively higher OER activity than RuO 2 . [67]Compared with other TMOs, spinel oxides offer some advantages, such as relatively high theoretical catalytic activity, different coordination structures of the metal center for modulation and atmospheric stability, making them promising candidates for acidic OER.Moreover, despite these advantages, spinel oxides also suffer from the disadvantages of being unstable under acidic conditions, and much effort has been put into modulating intrinsic catalytic behavior of spinel oxides.
Though spinel oxides as acidic OER electrocatalysts have garnered significant research attention, fewer studies have focused on spinel oxides compared to other transitional oxides, especially noble-metal-based electrocatalysts.Moreover, the modulation strategies and optimization mechanisms are rarely summarized.Considering the broad application prospects of spinel oxides as acidic OER electrocatalysts, it is of great significance to review the recent progress systematically and comprehensively  of spinel oxides.Herein, we start with the introduction of fundamentals and mechanisms of acidic OER, providing an in-depth understanding of different reaction pathways.Some important parameters for evaluating the catalytic performance are also described in detail.Then, we focus on several optimization strategies used for improving the acidic OER performance of spinel oxides.We also introduce some effective characterization techniques and preparation methods together with the modulation strategies.Finally, perspectives and challenges in the development of spinel oxides acidic OER electrocatalysts are presented based on the current research progress.

Possible Reaction Mechanisms for OER in Acidic Media
An in-depth understanding of OER mechanisms in acidic environments is the basic requirement for the rational design of highly efficient and stable electrocatalysts.In general, two possible reaction pathways are proposed to describe the OER process: adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM).Both mechanisms are schematically illustrated in Figure 2.

AEM
OER process following AEM involves four deprotonation processes of water molecules, as shown in Figure 2a.Specifically, one water molecule is adsorbed on the electrocatalyst surface and, after two consecutive deprotonation processes, an *O species is formed on the catalytic sites.Then, another water molecule undergoes the third deprotonation process, forming *OOH specie after compiling with already existing *O.Accompanied by the last deprotonation process, oxygen molecule (O 2 ) is formed from *OOH and then released from the active site.The overall reaction pathways for AEM can be given as follows (Equation ( 1)-( 4)) Each step requires a specific free energy (ΔG 1 to ΔG 4 ) to describe the bonding strength between the active sites and intermediates, and the free energy can be used to determine the ratedetermining step (RDS) and theoretical overpotential for the overall reaction.[69] According to Sabatier's principle, to achieve optimal catalytic performance, the bonding energy between active sites and reaction intermediates should be neither too strong nor too weak. [70]Nørskov and his co-authors have conducted an extensive theoretical study, which indicated that the ΔG ÃO -ΔG ÃOH value is a powerful descriptor to evaluate the OER activity, as discussed in the later section. [67]n the case of AEM-dictated OER, oxygen atoms only come from the electrolyte, which can protect the structural integrity of the catalyst to some extent. [71]However, the inevitable scaling relationship between *OH and *OOH results in a theoretical minimum overpotential of %370 mV for the water oxidation process, which is an intrinsic shortcoming of the AEM. [72,73]][76][77] Thus, another LOM is proposed to refine the understanding of acidic OER, which breaks the scaling relationship during OER by involving oxygen atoms in the lattice.

LOM
LOM is considered more efficient because it involves the concerted proton-electron transfer at surface oxygen sites, which has a lower activation energy and faster reaction kinetics compared to other OER mechanisms.After proper modification, the lattice oxygen can be activated and readily participate in the OER process, leading to higher OER activity and better catalytic performance.Generally speaking, there are two different LOM sub-mechanisms.The first type involves the lattice oxygen in metal oxides as the active center, which directly receives OH À from water molecules and generates the *OOH species through a nucleophilic attack process.Subsequently, the OOH species undergoes deprotonation and oxygen molecule release processes to form an oxygen vacancy.This oxygen vacancy is then filled with OH À in the subsequent reaction steps, completing a full reaction cycle (Figure 2b).In the other LOM pathway, metal atoms still act as the active center to receive oxygen species (OH À ) and undergo deprotonation.80][81] One obvious advantage of LOM pathway is that the scaling relationship between *OH and *OOH is broken, which endows the catalytic system with a much faster kinetics process with a lower energy barrier.Moreover, LOM is that there is no need to form a high coverage of reaction intermediates on the surface of electrocatalysts.Since O L serves as an intermediate, it efficiently reduces energy consumption.84][85] Though LOM demonstrates high catalytic activity, AEM and LOM compete in a real acidic OER process from the viewpoints of activity and stability.LOM exhibits a relatively lower theoretical voltage due to the absence of *OOH formation step, which bypasses the limitation of the scaling relationship in AEM. [80,86]his implies that LOM-dictated electrocatalysts possess a high acidic OER activity.At the same time, we see that LOM involves dynamic changes in the components at the surface of electrocatalysts during the catalytic process. [87]This leads to a less stable active center and seriously risks the electrochemical stability of materials. [79,88]For instance, when OER follows LOM, abundant oxygen vacancies (O V ) are generated on the surface of electrocatalysts, accelerating the dissolution of metallic atoms in harsh acidic electrolytes and resulting in the collapse of materials and rapid degradation of OER activities. [89]However, AEM presents a more stable reaction pathway because oxygen depletion is not involved in the process. [90]AEM and LOM usually coexist in a real OER process and some useful strategies have been proposed to effectively modulate the two competitive mechanisms.93][94]

The Transition of Catalytic Mechanisms
For spinel oxides used for acidic OER, both AEM and LOM for the acidic OER process may exist, depending on the specific conditions and properties of the material.As mentioned earlier, AEM involves a serious of sequential adsorption and deprotonation processes on the electrocatalysts surface, but this also brings the problem of higher energy barrier and lower acidic OER efficiency.
Usually, LOM is considered to be more efficient for acidic OER process because it involves the direct reaction of lattice oxygen atoms with protons to form OER-active intermediates, which can then evolve into oxygen gas with the release of electrons.This mechanism requires lower overpotentials and exhibits lower Tafel slopes and higher exchange current densities, indicating a more efficient catalytic activity for OER.Spinel oxides are particularly suitable for the LOM mechanism because they have a high concentration of lattice oxygen atoms due to their unique crystal structure.The edge-sharing metaloxygen (M-O) octahedra in the spinel oxide crystal structure can create more accessible lattice oxygen atoms, which can facilitate the LOM.Moreover, spinel oxides have a high surface area and high conductivity, which can enhance the mass transport of reactants and products and facilitate charge transfer during the OER process, based on LOM.
[97] Developing spinel oxide acidic OER electrocatalysts that can achieve a controllable transition of mechanisms while maintaining stability is still a challenging task.

Evaluation of Catalytic Activity
Different from the OER process in alkaline media, acidic OER usually faces substantial challenges in terms of stability.However, several reports have confirmed that the surface of electrocatalysts also undergoes an apparent reconstruction process during the alkaline OER process, and in situ formed metallic oxides could also serve as stable active species. [98]ao et al. aimed to predict the stability of materials in acidic media based on the Pourbaix diagram, which significantly narrowed the selection of possible active materials and doping elements (Figure 3). [99]However, these methods can only demonstrate the thermodynamical stability of the studied electrocatalysts, and more parameters are required to describe the intrinsic catalytic activity.Considering the complexity of high-performance acidic OER electrocatalysts design, densityfunctional theoretical (DFT) calculations are usually conducted to enhance the accuracy of material screening and prediction. [100]erein, some important descriptors are introduced, which provide useful insights into the selection of acidic OER materials.

Adsorption-Free Energy
According to the DFT calculations by Dau et al., an ideal electrocatalyst for AEM-driven acidic OER should demonstrate reactionfree energy with the same magnitude for the four continuous steps, resulting in a minimal energy barrier at the equilibrium potential (1.23 V) (Figure 4a). [101]However, the adsorption energies for the reaction intermediates in each step (*OH, *O, and *OOH) exhibit a strong linear relationship.For instance, Figure 3. Elements to form thermodynamically stable oxides for acidic OER.Reproduced with permission. [99]Copyright 2021, John Wiley & Sons.

Nørskov et al. have indicated that the O atom in OH* and OOH*
prefers the same bonding site on the surface of electrocatalysts with a similar single bond.This phenomenon results in a linear correlation between the adsorption energy of *OOH (ΔG ÃOOH ) and OH* (ΔG ÃOH ) with a constant energy difference (ΔG ÃOOH À ΔG ÃOH = 3.2 eV) (Figure 4b). [67,102]This implies that the adsorption energy of ΔG OOHÃ can be directly obtained from the value of ΔG OHÃ according to the so-called scaling relationship regardless of the binding energy of *O (ΔG ÃO ).As described earlier in Section 2.1.1,the formation of *O or *OOH is usually regarded as the RDS.Considering the constant difference between ΔG ÃOOH and ΔG ÃOH , we can further draw the theoretical over potential (η OER ) according to Equation ( 5) [67] Interestingly, when η OER is plotted as a function of (ΔG ÃO -ΔG ÃOH ), the theoretical catalytic performance demonstrates a volcano-type relationship, which is independent of the catalytic material, as shown in Figure 4c,d.These DFT calculations clearly show that the adsorption-free energy, especially ΔG ÃO -ΔG ÃOH , is a powerful descriptor to predict the AEMdictated OER activity of materials.The advantage of ΔG ÃO -ΔG ÃOH lies in the reduced number of parameters to be considered, and the OER catalytic activity can be evaluated by only one parameter (ΔG ÃO -ΔG ÃOH ).Another important aspect of the volcano plot is that the adsorption energy of an optimal OER electrocatalyst should be neither too high nor too low, which is consistent with Sabatier's principle.According to DFT calculations, Co 3 O 4 is an excellent OER electrocatalyst, which is even comparable to the state-of-the-art Ir-, Ru-, and Rh-based oxides, implying the broad application prospects of spinel-type oxides in the field of OER.

d-Band Center of Metal and p-Band Center of Bulk O
d-band center of metal and p-band center of bulk O are two important parameters to predict the acidic OER activity of electrocatalysts from the viewpoint of the intrinsic electronic structure of electrocatalysts.The binding strength between reaction intermediates and catalytic active sites is determined by the electronic interactions, which makes these electronic structure parameters of TM oxides more intrinsic descriptors. [103]he d-band center theory was first proposed by Nørskov and Hammer.In their pioneer work, the d-band center is described as the indicator of binding strength between the adsorbed oxygen intermediates and transitional-metal center. [104]When reaction intermediates are adsorbed on the catalytic active sites, the p-band of O intermediates is hybridized with 3d orbitals of the metal center.Such orbital hybridization results in two different states: bonding states and antibonding states (Figure 5a). [105]sually, the binding strength depends on the filling of antibonding states, and the decrease of filling shifts the d-band center toward the Fermi level (E f ) together with a strong bonding strength (Figure 5b). [106]Another feature of antibonding states is that their filling mainly relies on the type of metal.Thus, the adsorbate-metal center interaction can be easily determined by calculating the d-band of specific elements. [107]In 2014, Nørskov et al. further took the d-band shape into account and proposed the concept of an upper d-band edge to refine the dband center theory, making it suitable for some complex metal alloy systems. [108]To date, the d-band center theory has been successfully applied to explain the origin of excellent OER activity, but it is still unable to well integrate the real surface information of the acidic OER electrocatalysts and cannot be used for nonmetallic electrocatalysts.
For spinel oxides, the d-band center can be effectively modulated through elemental doping or substitution.Elements such as Ru, Cr, Cu, F, N, and others have relatively small ionic radius and high electronegativity, allowing them to form strong adsorption bonds with hydroxide ions and shift the d-band center of spinel oxides. [109]Additionally, the transformation between the oxidized and reduced states of these elements can facilitate the adsorption and release of hydroxide ions, making them useful for electrocatalytic reactions.Another possible method for modulating the d-band center is creating oxygen vacancies (V O ) in spinel oxides.By creating an appropriate amount of V O , the dangling bond of metal cations on the surface can be effectively lifted and serve as the adsorption site of *OH. [110]Overall, the mechanism of d-band center modulation of spinel oxides depends on the specific method used and can involve changes in the coordination environment, M-O bond lengths and angles, surface chemistry, and strain-induced changes in the electronic structure.
Moreover, the p-band center of bulk O is an easily obtainable bulk-phase property and can serve as an important descriptor for OER activity. [111]In the case of TM-based oxides, such as spinel oxides, the p-band center of bulk is significantly influenced by the Madelung potential and affinity of oxygen electrons, which can be modulated by changing electronegativity and oxidation state of the coordinated metal atoms.The relative position of the O pband center to the metal d-band center influences the OER mechanism of spinel oxides.For example, when the O p-band center is higher than the metal d-band, lattice oxygen demonstrates a higher degree of freedom, leading to the LOM-assisted OER.In contrast, if the O p-band center is lower than the metal d-band, lattice oxygen is restricted and AEM dominates the OER process (Figure 5c). [112]The relative location of O p-band center to the E f or metal d-band center with different coordination structures also demonstrates a volcano-type relationship, i.e., OER activity can only be obtained when the O p-band possesses a relatively suitable distance to other energy-related parameters.If the O p-band center is too far from the optimal value, either the adsorption ability or stability of the electrocatalysts is compromised (Figure 5d,e). [112,113]Though the O p-band center is widely used to describe OER activity and predict highly efficient catalytic materials, it has also been pointed out that this descriptor could only be applied to conductive oxides, requiring further investigations to refine the theory. [114]he d-band center of metal and p-band center of bulk O are two important descriptors, which can be used to study the adsorption and activation energies of reaction intermediates on different sites (metal or oxygen sites).These two parameters should be considered synergistically in TMOs to make up for their respective shortcomings, providing a deeper understanding of the reaction processes and designing standards.

e g Orbital Occupancy
In the case of TMOs, the electronic structure is influenced by the hybridization of d orbitals of TM atoms and p orbitals of O atoms, resulting in orbital splitting with various bond formats and bond angles.The concept of e g theory is first applied in perovskites, which demonstrates that excessive e g filling hinders the formation of O-O bonds in *OOH and limited e g filling limits the generation of O 2 -2 due to the deprotonation of *OOH. [115,116]Then, this descriptor is found to be suitable for spinel oxides, which contain both octahedral and tetrahedral coordination structures.In spinel oxides, the 3d orbital of TM splits into two different types of orbitals.For octahedral site cations, the 3d orbital splits into a low-energy triplet t 2g orbital (containing d xy , d xz , and d yz ) and a high-energy doublet e g (containing d z 2 and d x 2 -y 2 ).For tetrahedral site cations, 3d orbital splits into high-energy triplet t 2 orbital (containing d xy , d xz , and d yz ) and low-energy doublet e orbital (containing d z 2 and d x 2 -y 2 ) (Figure 6a).The four different orbitals (t 2g , e g , t 2 , and g) show different point directions (Figure 6b). [117]It can be seen that the e g orbitals for the octahedral site directly point its shape toward six adjacent O ligands, generating a significant orbital overlap (d-p orbital hybridization), forming strong σ-σ interaction and effectively influencing the OER process (Figure 6c). [118]wever, at tetrahedral sites, the orbital direction significantly deviates from the adjacent O atom.Thus, neither t 2 nor g orbital demonstrates direct spatial orbital overlap with O 2p orbital, resulting in relatively weak π-π interactions between the TM site and adsorbed oxygen intermediates (Figure 6d). [118]Similar to perovskite oxides, the filling degree of e g orbital and OER activity also follows a volcano-type relationship.Xu and co-workers revealed that the e g orbital filling degree of octahedral cation for spinel oxides dominates the adsorption ability of reaction intermediates, while the tetrahedral structure does not account for the significant OER difference (Figure 6e,f ).The optimal OER activity is achieved when e g filling reaches a moderate degree.Excessive filling (e g > 1) or limited filling (e g < 1) hinders the reaction step and decreases the OER activity (Figure 6e).
The e g orbital occupancy provides a clear and efficient way to understand the OER process from the viewpoint of electron orbital coupling, and reveals the intrinsic relationship between catalytic activity and coordination structure.However, some works also reported that the tetrahedral cations also demonstrate high OER activity, which is contradictory to e g occupancy theory. [119,120]Copyright 2010, John Wiley & Sons.b) The relationship of adsorption energy between *OOH and *OH for some representative TM oxides, showing a linear correlation.c,d) Volcano-type plots of theoretical OER activity for perovskite oxides, spinel oxides, rutile oxides, and anatase oxides with ΔG ÃO -ΔG ÃOH as the descriptor.Reproduced with permission. [75]Copyright 2010, John Wiley & Sons.

Other Descriptors
The aforementioned descriptors are normally obtained from the perspective of molecular orbitals, which are effective for predicting or explaining the origin of OER activity in TMO-based electrocatalysts.Some other useful descriptors are also applied to explain the unique OER activity of spinel oxides, which is consistent with experimental results. [121,122]or instance, M-O covalency is a useful descriptor to explain the effect of electron transfer process on OER performance.Yang et al. have indicated that, even though some oxides demonstrate the same e g occupancy, the OER activities may differ due to electron-sharing along the M-O bond. [116]Xu et al. have further proved that the Co-O covalency could be enhanced by doping appropriate amounts of Fe into spinel ZnCo 2 O 4 , which facilitates the electron transfer between TM sites and reaction intermediates.The enhanced injection/extraction of electrons from oxygen further promotes the OER activity (Figure 7a). [123]he coordination environment, such as the density of coordinatively unsaturated metal centers (M CUS ) and coordination  [105] Copyright 2005, Springer Nature.b) The density of states (DOS) plots of 110-cus and 110-Cu-cus, and the corresponding schematic illustration of bond formation between the reaction surface and adsorbate.Reproduced with permission. [106]Copyright 2018, John Wiley & Sons.c) Schematic illustration of the change in OER mechanism caused by the relative position of O p-band center and metal d-band center in spinel oxides.d) Experimentally obtained OER activity as a function of the calculated O p-band center and metal d-band center distance.Reproduced with permission. [112]Copyright 2020, Springer Nature.e) Experimentally obtained OER activity against O p-band center relative to E f of some representative TM oxides.Reproduced with permission. [113]opyright 2013, Springer Nature.numbers (CNs) of active sites, is also an important aspect to understand the catalytic mechanism of spinel oxides.M CUS is a descriptor developed from the molecular orbital and energy band theory, which describes the adsorption strength for reaction intermediates with the highest occupied d-state (E d ) energy relative to the E f .Different from other indicators, the description of catalytic performance with M CUS depends on the semiconductor type of TM oxides.For n-type semiconductors, E d is far below E f , which results in fulfilled antibonding states with relatively weak adsorption ability on the surface.In this case, a surface with a higher density of M CUS can effectively increase E d and upshift the antibonding states, leading to excellent OER activity.In contrast, for p-type semiconducting oxides, E d is located relatively close to E f , resulting in a partially filled antibonding state and strong adsorption ability for intermediates.To further enhance the OER activity, a lower density of M CUS is needed to promote the desorption process (Figure 7b). [124]In the case of spinel oxides, porous structure and oxygen vacancies can create many unsaturated metal sites with a large number of dangling bonds.127][128] Although these CN-related descriptors can be used to predict the OER activity of TM oxides, such geometric configuration parameters are usually not quantitatively correlated with the OER performance.Also, most of the parameters are developed for bulk materials and demonstrate several limitations to deal with composites or supported oxides.
Oxidation enthalpy (ΔH 0 t ) is another descriptor to estimate the adsorption ability for reaction intermediates, which is the standard enthalpy needed to oxidize metal cations from lower valence to higher valence.One should note that a clear volcano curve can be obtained by plotting acidic OER potential against ΔH 0 t (Figure 7c). [129,130]The plot shows that oxides with optimal acidic OER activity should possess moderate redox properties.As a result, RuO 2 and IrO 2 demonstrate optimal acidic OER performance, and TM oxides exhibit the following order: Moreover, ΔH 0 t can also be used to predict the catalytic performance of binary spinel oxides, and NiCo 2 O 4 is believed to possess the best activity among Ni-Co spinel oxides. [129,131]n addition to these main descriptors, some other descriptors, such as oxidation state, [132] optimality of tolerance factor, [133] and M-O bond length, [134] have also been proposed to predict or evaluate the OER activity of TM oxides.However, considering the complexity of actual OER process, these descriptors, including e g , d-band center and p-band center, may not be able to provide a complete picture of the real situation, and it is of great significance to develop a multi-parameter joint analysis theory to provide comprehensive and precise explanations of the OER mechanism.In the case of spinel oxides, more situations should be considered in detail for designing effective modulation strategies due to the complex coordination structure of metal centers.
on spinel oxides for acidic OER and corresponding evaluating parameters are summarized in Table 2.For the spinel oxides used in acidic OER, most of the studies are focused on the modulation of Co-based spinel oxides because Co is one of the few first-row transition metals that are relatively stable in acid media (Figure 3). [99]Also, Co possesses a moderate adsorption strength of reaction intermediates. [143]For an intuitive comparison of the current results, the acidic OER performance of various electrocatalysts is further plotted according to Table 2 (Figure 8).It can be seen clearly that most of the spinel oxides demonstrate stabile performance for 100 h.However, since most of the studies only demonstrate partial stability data, e.g., chronopotentiometry, it is hard to determine whether the electrocatalysts can still maintain such a stable catalytic activity or not.146][147] For most of the noble metals containing spinel oxides, the overpotential ranges from 200 to 300 mV, which is much lower than most of the noble-metal-free spinel oxides (>300 mV).150][151] Also, Ir-incorporated spinel oxides usually demonstrate better stability than Ru-incorporated oxides, originating from the intrinsic stability of Ir under high voltage under acidic media. [152]owever, by utilizing some proper optimization strategies, the noble-metal-free spinel oxides can also achieve relatively low overpotentials with satisfying stability. [96,145]he commonly used optimization strategies for spinel oxides can be divided into the following categories: coating, atomic doping, constructing heterostructure and coordination structure optimization.

Coating Strategy
Though spinel oxides, especially Co 3 O 4 -based electrocatalysts, have shown promising acidic OER performance, MO x species generated during the OER process at high applied potentials rapidly dissolve, resulting in the collapse of crystal structure and degradation of catalytic performance. [153]cidic OER stability of spinel oxides, some protective layers are proposed to be coated on their surface, which can effectively enhance the stability and catalytic performance.For instance, Yu et al. have proposed carbon-covered Co 3 O 4 nanoparticles serving as stable acidic OER electrocatalysts using the pyrolysis of zeolitic imidazolate framework-9 (ZIF-9), followed by ball milling with a mixture of graphite and paraffin oil (GPO) (Figure 9a). [154]The as-prepared Co 3 O 4 @C/GPO demonstrated a relatively low overpotential and robust acidic OER performance for over 40 h.The authors claimed that the enhanced acidic OER activity for Co 3 O 4 @C/GPO could be attributed to the large surface area and hydrophobic support, which suppressed the leaching of Co species and preserved active sites (Figure 9b,c).A similar result has also been reported by Yang et al., [55] where a thin carbon layer (%3.6 nm) was successfully coated on Co 3 O 4 nanosheets (NSs) prepared by electrodeposition (Figure 9d).After a two-step calcination process, the interfacial strength between Co 3 O 4 NSs and carbon cloth substrate was effectively enhanced, which reduced the contact resistance and improved adhesive ability.Also, the coated carbon layer at the edges of NSs could Reproduced with permission. [123]Copyright 2018, John Wiley & Sons.b) Volcano plot of the OER overpotential as a function of M CUS .Reproduced with permission. [124]Copyright 2019, American Chemical Society.c) Acidic OER performances of different TMOs as a function of ΔH 0 t , extracted from ref. [129,130]   at a current density of %4.5 mA cm À2 ; b) at a current density of 7.91 mA cm À2 ; c) pH value of the electrolyte is 1; d) at the current density of mA cm À2 ; e) at the current density of 0.1 mA cm À2 ; f ) pH value of the electrolyte is 2.5.
avoid the exfoliation of electrocatalysts, which further promoted stability (Figure 9e,f ).In addition to carbon, TiO 2 is also found to be a suitable material to protect Co 3 O 4 during acidic OER process.Chen et al. have introduced an adjustable TiO 2 layer on the surface of Co 3 O 4 by changing the deposition cycles of atomic layer deposition.The modified thickness (4.4 nm) provided an enhanced acidic OER stability of 80 h, which is 3 times larger than the uncoated Co 3 O 4 (Figure 9g,h). [155]However, different from the previous report, [156,157] these results demonstrate that TiO 2 coating drastically reduces the OER activity of Co 3 O 4 , which can be ascribed to the increased charge-transfer resistance between nonconducting TiO 2 layer and isolated active sites in Co 3 O 4 (Figure 9i).This work reveals that the coating layer should be carefully regulated to enhance stability, as well as maintain OER activity.Yeh et al. have further investigated the influence of coating order on the acidic OER performance of the proposed core-shell structure prepared with fluorine-doped tin oxide (FTO) and Co 3 O 4 . [158]Co 3 O 4 -coated FTO (FTO@Co 3 O 4 ) and FTO-coated Co 3 O 4 (Co 3 O 4 @FTO) were prepared with different synthetic paths (Figure 9j,k).The OER activity followed the following sequence: FTO@Co 3 O 4 /CP > Co 3 O 4 > Co 3 O 4 @FTO/CP (Figure 9l).Also, FTO@Co 3 O 4 /CP electrode demonstrated a minimum decay rate of 2% after 21.5 h during chronopotentiometry tests, which is better than Co 3 O 4 /CP (6.1%) and Co 3 O 4 @FTO/CP (3.2%) (Figure 9m).The excellent stability and catalytic performance of FTO@Co 3 O 4 /CP can be attributed to the dispersion of Co 3 O 4 on FTO, which maximizes the exposure of active sites, and oxygen-vacancy-free characteristic can reduce the leaching of Co species.In contrast, Co 3 O 4 @FTO/CP exhibited the worst OER activity due to the coverage of active sites by the coating layer.
The protective coating is a useful method to protect spinel oxides from metallic leaching, but may arise the problem of isolation for catalytic active sites.Thus, the coating thickness and coating material should be carefully modified and selected to maintain the exposure of active sites, while protecting the intrinsic catalytic characteristics of spinel oxides.

Elemental Doping
Though coating with a protective layer can enhance the stability of spinel oxides during the acidic OER process, it does not change the intrinsic catalytic characteristics because the electronic structure is not effectively regulated, which does not reduce the energy barrier of the RDS step. [97,159,160]Doping with heteroatoms into the lattice of target materials is a useful strategy to further enhance the intrinsic catalytic performance. [122,161,162][165][166][167] 10f ).Other similar works also proved that elements doped into the lattice of spinel oxides host reduces the energy barrier of RDSs for acidic OER, which indicates that atomic substitution is a more effective doping form than conventional surfacesupported single-atom catalysts or bulk metals. [169,170]In general, noble-metal-doping-induced catalytic activity enhancement relies heavily on the intrinsic catalytic activity of the doped noble metal, and the support materials  10g). [96]Moreover, the doping-induced shortened Co-Co distance could also enrich OH adsorption and change the OER mechanism from traditional AEM to a more efficient oxide path mechanism with moderate reaction free energy (Figure 10h,i).In addition to the cation-doping methods, some anion-doping methods are also used to regulate the coordination structure of active sites.For instance, Shang et al. have prepared a P-doped Co 3 O 4 (P-Co 3 O 4 ) with PO 6 geometric configuration and investigated its acidic OER performance. [171]Owing to the high electronegativity of P atom, the PO 6 unit could effectively increase the valence state of Co 2þ and introduce more oxygen vacancies around active sites, inhibiting LOM for acidic OER and stabilizing the crystal structure of P-Co 3 O 4 under acidic conditions.
Moreover, heteroatom doping could significantly regulate the electronic structure of active sites and reduce energy barriers  [154] Copyright 2022, Springer Nature.d) HRTEM image and corresponding electronic diffusion spectra maps of Co 3 O 4 @C nanosheets.e) Acidic OER activities of various Co 3 O 4 electrocatalysts and f ) corresponding stability test results.Reproduced with permission. [55]Copyright 2016, Elsevier, Inc. g) Transmission electron microscopy (TEM) and HRTEM images of TiO 2 /coated Co 3 O 4 .h) Stability test and i) linear sweep voltammetry (LSV) curves of TiO 2 /Co 3 O 4 /FTO electrode with different coating thicknesses.Reproduced with permission. [155]Copyright 2022, American Chemical Society.HRTEM images of the core-shell structure for j) FTO@Co 3 O 4 /CP and k) Co 3 O 4 @FTO/CP.l) Acidic OER activity of different Co 3 O 4 -based electrocatalysts and m) corresponding stability results.Reproduced with permission. [158]Copyright 2023, Royal Society of Chemistry.
during the acidic OER.However, two important issues need to be further studied.First, the coexistence of two coordination structures (octahedral and tetrahedral sites) brings different sites in spinel oxides.However, doping heteroatoms at a specific target site is still a difficult task, which makes it hard to investigate the structure-property relationship between various doping sites and catalytic activities.Another issue is the selection of doping elements.It is impractical for people to try every possible doping element and doping amount, especially for the scarce noble metals.With the development of artificial intelligence (AI) and machine learning (ML) techniques, the possible efficient spinel oxides-based acidic OER performance with different dopants can Reproduced with permission. [168]Copyright 2021, American Chemical Society.g) EXAFS spectra of Ba-doped Co 3 O 4 , demonstrating a shortened Co-Co bond length.h) Modified free energy and theoretical overpotential of Ba-doped Co 3 O 4 and i) corresponding reaction mechanism diagram.Reproduced with permission. [96]Copyright 2023, American Chemical Society.[174] Unfortunately, there is still a lack of application of AI or ML in screening doped spinel oxides for acidic OER.

Heterostructure Construction
[177][178] In the case of spinel oxides, constructing a heterostructure renders two main influences for enhancing acidic OER activity.On the one hand, the heterostructure interfaces provide active sites due to abundant vacancies or defects.On the other hand, the coupling of spinel oxides and other heterogeneous materials can modify the redox properties and local electronic structure, providing moderate reaction free energy for the acidic OER process.Niu et al. have prepared a RuO 2 -loaded Co-Mn spinel oxide (RuO 2 /(Co, Mn) 3 O 4 ), which demonstrated an overpotential of 270 mV with a relatively low Ru loading (2.51 wt%).They have used both atomic doping to modify the Co 3 O 4 substrate and RuO 2 loading to enrich the density of active sites (Figure 11a,b).DFT calculation results further confirmed that RuO 2 was the adsorption center for oxygen intermediates, and its loading could enhance the intrinsic acidic OER activity of pristine Co 3 O 4 .However, its over-strong bonding strength limited the formation of *OOH.After Mn doping, the d-band center of Ru shifted far away from the Fermi level, resulting in a weak adsorption strength and facilitating the formation of *OOH (Figure 11c).In contrast, the modified substrate could further enhance the distribution of electrons around RuO 2 / Co 3 O 4 interfaces, reducing the energy barrier of RDS.Usually, noble metal oxides loaded with spinel oxides demonstrated better acidic OER performance than their pristine counterparts.The heterostructure interfaces are considered to be active sites due to abundant vacancies, defects, and unsaturated metallic sites. [179]The precisely designed heterostructure interfaces in spinel-oxide-based acidic OER electrocatalysts can also modify redox properties around interfaces, effectively reducing Reproduced with permission. [36]Copyright 2021, Springer Nature.
the required applied potential to drive the catalytic process.Our recent work demonstrated that introducing nanocrystalline CeO 2 into Co 3 O 4 /CeO 2 composites could break the activity/ stability trade-off, which enhances the intrinsic acidic OER activity. [36]In our work, Co 3 O 4 /CeO 2 composites were prepared on the conductive substrates (FTO or carbon paper) by electrodeposition and subsequent annealing process (Figure 11d,e).The detailed electrochemical characterization results exhibited that the introduction of CeO 2 could improve the number of active intermediates that participated in the OER process without changing the RDS of Co 3 O 4 /CeO 2 composites (Figure 11f,g).Moreover, the introduction of CeO 2 resulted in the oxidation of Co 3þ , facilitating the oxidation of Co 3þ into active Co 4þ species (Figure 11h).Ex situ extended X-ray absorption fine structure (EXAFS) spectra further confirmed that incorporated with CeO 2 reduced the Co-O bond length due to the charge redistribution from Co 3 O 4 to CeO 2 , leading to a higher valence of Co species around Co 3 O 4 /CeO 2 interfaces (Figure 11i).Furthermore, in situ Raman spectra also demonstrated a flexible bonding environment of Co 3 O 4 /CeO 2 with a more readily oxidized feature and the formation of dimeric Co 4þ -Co 4þ , allowing rapid OER kinetics.
It is worth noting that rational designing of heterostructure and active interfaces in spinel oxides is an effective strategy to modulate the catalytic performance.Owing to the lattice mismatch and massive defects at the interfaces, the stability of these spinel oxide heterojunctions is usually unsatisfying (<50 h, Table 2).[181][182] Thus, designing heterostructure interfaces requires more delicate regulations to balance competition between catalytic stability and activity.

Construction of Binary Metal Sites
There are two different kinds of structures in spinel oxides, i.e., 4 oxygen-coordinated tetrahedral sites and 6 oxygen-coordinated octahedral sites.Hence, acidic OER activity could be effectively enhanced by selectively substituting metals at different coordination structures.Kundu et al. have comparatively investigated the catalytic performance of tetrahedral-sites-substituted and octahedral-sites-substituted Co 3 O 4 (Co 2 TiO 4 and CoCr 2 O 4 ). [183,184][187][188] These results demonstrate that the tetrahedral sites are more active than the octahedral sites, which seems contrary to our previous discussion in Section 2.2.3 and indicates that octahedral sites serve as active sites for the OER process due to strong σ-σ interactions (Figure 12a,b).Electrochemical impedance spectroscopy demonstrates similar charge-transfer resistance (R ct ) for tetrahedral-sites-substituted and octahedralsites-substituted Co 3 O 4 , ruling out the influence of resistance (Figure 12c,d).The theoretical and experimental discrepancies may arise from the following two aspects: 1) the morphologies of catalysts are different from each other, which may expose different number of active sites (Figure 12e-h), and 2) though Cr and Ti do not demonstrate high acidic OER performance, their substitution can still modify the electronic structure of Co sites, which may further enhance the intrinsic activity and stability in acidic electrolytes even for theoretically inert tetrahedral sites. [189,190]Another representative work by Xiao et al. also demonstrated that the partial substitution of Co oct with Mn 3þ/4þ could significantly enhance the acidic OER stability, while maintaining the acidic OER activity of traditional Co 3 O 4 . [145]They have prepared Co 2 MnO 4 on different substrates, such as FTO, carbon plates, and Pt/Ti plates, using a simple drop coating method, followed by low-temperature annealing.EXAFS spectra were used to confirm that Mn occupied the octahedral sites during the synthesis process (Figure 12i), which was further proved by Rietveld refinement and Raman analysis.This self-supported electrode demonstrated exhilarating stability after 1500 h stability test at a high current density of 200 mA cm À2 , which, to the best of our best knowledge, is the best spinel oxide for acidic OER (Figure 12l).DFT calculations were conducted to further investigate the origin of excellent catalytic activity and stability.According to DFT results, the formation of *O and *OOH were considered to be the RDS for Co 3 O 4 (311) and Co 2 MnO 4 (311) models, respectively (Figure 12j).The theoretical activity of different electrocatalysts is consistent with experimental results, following the given order: Co 3 O 4 % Co 2 MnO 4 > CoMn 2 O 4 > MnO 2 at 1.7 V (vs.reversible hydrogen electrode (RHE)), exhibiting that the catalytic activity of Co 2 MnO 4 is as high as Co 3 O 4 (Figure 12k).It should be noted that the catalytic activity is reduced by about an order of magnitude when Co oct atoms are completely substituted by Mn (CoMn 2 O 4 ), further confirming that the octahedral sites in spinel oxides act as active centers for the OER process.Moreover, DFT calculations are employed to presume the origin of the high stability of Co 2 MnO 4 .[193][194] The authors claimed that the dissolution of lattice O in Co 2 MnO 4 and Co 3 O 4 was the main reason for the degradation of acidic OER activity according to the Pourbaix diagrams.It is found that more electrons transfer from Mn to O atom after Mn substitution, forming a strong Mn-O bond (Figure 12m).PDOS results indicate that the interactions of lattice O 2p orbitals and Mn 3d orbitals result in a more stable Mn-O bond than the Co-O bond in Co 3 O 4 , effectively enhancing the stability and preventing the dissolution of lattice O atoms (Figure 12n).Moreover, the d-band center of Co 3d orbitals in Co 2 MnO 4 is lower than Co 3 O 4 , resulting in a stronger *O adsorption energy and better OER activity of Co 2 MnO 4 (Figure 12o).
Selectively substituting tetrahedral or octahedral sites with acidic stable elements in spinel oxides is an effective way to enhance the stability and modify the electronic structure of active sites. [189]When designing spinel oxides from the perspective of substituting metallic cations at different coordination structures, two important issues should be further considered.One is the selection of substitution elements, which mainly determines the intrinsic acidic OER activity of the final products.For example, Co, Ni, and Fe are considered to be OER-active substances, while Mn, Ti, and Cr facilitate the formation of the M-O bond and enhance the stability of TMOs.The effective combination of different substitutes effectively balances the competing relationship between activity and stability.[197] Another urgent issue is the evidence of the exact substituting location, which hinders the precise explanation of the structure-property relationship at the catalytic active sites.00]

Comprehensive Comparisons for Different Strategies
Although spinel oxides have shown promising potential as electrocatalysts for acidic OER, significant research is still needed to improve their performance, which involves selecting the right optimization strategy for different systems.To show more clearly the application scenarios of the various optimization strategies, comprehensive comparisons for different modulation strategies  c,e) Reproduced with permission. [183]Copyright 2019, American Chemical Society.b,d,f-h) Reproduced with permission. [184]Copyright 2022, Springer Nature.would be given, which will give an overview of the common optimization strategies.
For now, strategies used to modulate the acidic OER performance of spinel oxides could be divided into three categories: The first one is adjusting the electronic structure of intrinsic spinel oxides, and in essence this kind of strategy can usually result in a reduction of reaction free energy of the RDS in acidic OER process.To achieve this target, elemental doping and constructing heterostructure are the most commonly used methods.For example, by doping Mn into Co 3 O 4 , the spinel oxides obtained a stronger adsorption energy of *O, which is benefit for the formation of O-O bond in the following step. [145]y adjusting the electronic structure, the intrinsic acidic OER performance could be effectively enhanced.Moreover, by doping proper element into the spinel oxides, more elementals bonding would be generated, and the resistance of acidic environment may be further improved.
The second one is changing the acidic OER mechanism.Different OER mechanism would demonstrate different catalytic performance.Usually, LOM mechanism demonstrated better performance than traditional AEM mechanism, although it may result in easier degradation than the latter one.For now, there still lack proper strategies for controlled regulation of catalytic mechanisms.However, based on the recent report, by doping different element into the spinel or perovskite oxides, the catalytic mechanism could be effectively modulated. [96,201,202]tability is a crucial factor that must be considered when designing materials for electrochemical applications.In acidic electrolytes, the material can erode due to the dissolution and detachment of surface atoms, which can lead to a loss of active sites and decreased performance.While coating the material with carbon-based materials as a sacrificial layer can reduce erosion, it can also limit the contact between active sites and reactants.Therefore, a more effective approach is to increase the stability of the metal-metal or M-O bonds, which can resist erosion while maintaining active sites.One way to achieve this is by constructing heterostructures or designing highentropy spinel oxides.These methods introduce more stable bonds and functional interfaces into the spinel oxides framework, which further enhances stability and helps to maintain active sites.
Overall, the strategies used to enhance the acidic OER activities of spinel oxides still require exploration when compared to the situation in alkaline media.Other valuable strategies, such as crystal phase modulation, crystal face engineering, and stress engineering, have not yet been reported in acidic OER.Exploring these strategies could be a potential direction to broaden the application of spinel oxides in this area.

Conclusions and Perspectives
The production of green hydrogen from water electrolysis using renewable electricity is an important route to simultaneously solve the energy crisis and environmental pollution.Theoretical and experimental results have proven that PEMWEs are efficient electrolyzers for hydrogen production; however, their large-scale applications are constrained by the instability and relatively low catalytic activity of the anode catalytic materials, especially the noble-metal-free materials.Thus, the exploration of low-cost and high-efficiency electrocatalysts for acidic OER is the core issue for the development of PEMWEs.Spinel oxides are promising noble-metal-free electrocatalysts for acidic OER. [66,203,204]However, the recent progress on the application of spinel oxides for acidic OER has not been summarized.In this review, starting from the basic catalytic mechanism and catalytic activity descriptors, we comprehensively reviewed the recent progress of spinel oxides-based acidic OER electrocatalysts.It can be concluded that the intrinsic acidic OER activity for most spinel oxides is still relatively low when compared with the stateof-the-art noble-metal-based oxides, i.e., the overpotential is usually larger than 300 mV.However, this deficiency could be effectively rectified with some strategies, such as elemental doping, including noble metal doping, non-noble metal doping and non-metal elements doping, morphological modulation, and compositional regulation.Compared with other transitionmetal-based acidic OER electrocatalysts, the investigation of spinel oxides in alkaline media reveals that there are still some critical challenges before practical applications of spinel oxides for acidic OER. 1) First of all, more spinel oxides should be further investigated to extend the application potential in practical use.To date, the commonly known spinel oxides, i.e., Co 3 O 4based materials, have been mainly investigated for acidic OER (Table 2); however, the spinel oxide family could be divided into three different types: normal spinel phases with a generic formulation of (A 2þ th )(B 3þ oct ) 2 O 4 , inverse spinel phases with a generic formulation of (A 2þ oc )(B 3þ th )(B 3þ oct )O 4 , and complex spinel phases, where cations (A 2þ and B 3þ ) partially occupy both tetrahedral and octahedral sites.Due to abundant occupation sites in spinel oxides, both the component and number of active centers could be regulated flexibly.In contrast, the diversity of crystal structures in spinel oxides also brings more challenges for the screening of possible high-activity materials.Thus, the combination of theoretical guidance (DFT calculations or ML methods) and experimental verification should be organically combined to maximize the efficiency of material screening; 2) In situ characterization techniques should be applied to investigate the evolution of electrocatalysts and unveil the catalytic mechanism.The surface of electrocatalysts would inevitably undergo some evolutionary processes during both AEM and more efficient LOM.However, due to the lack of solid experimental evidence, it is difficult to determine the exact reaction route in most cases, which is unfavorable for the investigation of the reaction mechanism, especially for the OER process in harsh acidic media.[207][208] Some in situ spectroscopic studies can also probe the existence of surface-adsorbed species, which can be used to determine the reaction path. [40,84,209]Most recently developed in situ liquid-phase TEM technique is a powerful tool for the direct observation of structure evolution during the electrochemical process.Moreover, the detailed evolution process could be obtained with the help of multiple integrated spectroscopic characterization techniques, such as electronic diffusion spectra, electron energy loss spectra, and selected area electron diffraction. [210,211]owever, due to the obstruction of electron beams in liquid, it is usually hard to obtain a clear atomic-level crystal structure of the electrocatalysts and more effort should be put into the optimization of liquid cells used in TEM equipment; 3) Furthermore, the studies on the external field modulation of acidic OER activities in spinel oxides should be strengthened.[214][215] However, there is still no report about the influence of external fields on the acidic OER performance of spinel oxides.For example, as a family of magnetic materials, the physical properties of spinel oxides can be modulated by magnetic fields, leading to enhanced coupling with adsorbed O species.Light excitation produces more photogenerated carriers, which can improve redox properties at the surfaces of spinel oxides and enhance the overall acidic OER performance; 4) Accurate theoretical explanations for the catalytic mechanism should be focused on.Theoretical calculations are widely used to explain or predict the catalytic mechanisms in transition-metalbased electrocatalysts.However, the models built for DFT calculations are relatively arbitrary.For spinel oxides, it has been proved that the crystal surfaces render a significant influence on catalytic activity because different exposed surfaces contain different amounts of octahedral sites. [145,216]Unfortunately, most of the DFT calculations select facets in terms of proving the superiorities of materials without concern for the actual situation, which may lead to unreliable conclusions about the origin of electrocatalysts.Moreover, most of the DFT calculations are based on AEM only, ignoring the existence of LOM path.To reveal the true catalytic reaction mechanism more accurately, the combination of theoretical calculations and analytical techniques should be further developed, which would deepen the understanding of the structure-property relationship in spinel oxides; and 5) The role of metallic cations with different coordination structures should be further investigated.For example, according to theoretical predictions, the octahedral sites should demonstrate higher catalytic activities than the tetrahedral site.However, a few studies have shown that the metallic cations at the tetrahedral sites are more favorable for the formation of reaction intermediates, indicating that tetrahedral sites may have some indirect influence on the overall acidic OER activity. [119,217]Moreover, tetrahedral sites can also influence the geometrical structure and coordination of octahedral sites, and the interactions between both sites are still unclear.Thus, a deeper understanding of the function of metallic cations with different coordination structures is essential for the rational design of spinel oxides with high catalytic performance.
In summary, with a deeper understanding of acidic OER mechanism and continuous exploration of novel materials, the application of spinel oxides in PEMWEs would hopefully be realized, and more attention should be paid to the development of modulation strategies in future studies.

Figure 1 .
Figure 1.The oxygen evolution reaction (OER) mechanisms for a) alkaline media and b) acidic media.

Figure 2 .
Figure 2. Schematic illustration of acidic OER mechanisms: a) adsorbate evolution mechanism (AEM) pathway, b) lattice oxygen mechanism (LOM) pathway for OER on metal sites, and c) LOM pathway for OER on oxygen sites.

Figure 4 .
Figure 4. a) Real and ideal OER Gibbs free energy diagram for the four-step process.Reproduced with permission.[101]Copyright 2010, John Wiley & Sons.b) The relationship of adsorption energy between *OOH and *OH for some representative TM oxides, showing a linear correlation.c,d) Volcano-type plots of theoretical OER activity for perovskite oxides, spinel oxides, rutile oxides, and anatase oxides with ΔG ÃO -ΔG ÃOH as the descriptor.Reproduced with permission.[75]Copyright 2010, John Wiley & Sons.

Figure 5 .
Figure 5. a) Schematic diagram of the formation of bonding states and antibonding states.Reproduced with permission.[105]Copyright 2005, Springer Nature.b) The density of states (DOS) plots of 110-cus and 110-Cu-cus, and the corresponding schematic illustration of bond formation between the reaction surface and adsorbate.Reproduced with permission.[106]Copyright 2018, John Wiley & Sons.c) Schematic illustration of the change in OER mechanism caused by the relative position of O p-band center and metal d-band center in spinel oxides.d) Experimentally obtained OER activity as a function of the calculated O p-band center and metal d-band center distance.Reproduced with permission.[112]Copyright 2020, Springer Nature.e) Experimentally obtained OER activity against O p-band center relative to E f of some representative TM oxides.Reproduced with permission.[113]Copyright 2013, Springer Nature.

Figure 6 .
Figure 6.a) The energy levels of the degenerate d-orbitals in octahedral and tetrahedral coordination.b) Schematic diagram of the spatial arrangement of oxygen ligands.Reproduced with permission.[117]Copyright 2017, John Wiley & Sons.Interactions of O 2p orbital with TM 3d orbital at c) octahedral and d) tetrahedral sites.Reproduced with permission.[118]Copyright 2019, John Wiley & Sons.OER activity of representative spinel oxides as a function of e) e g occupancy of octahedral cation and f ) e occupancy of the tetrahedral cation.Reproduced with permission.[117]Copyright 2017, John Wiley & Sons.

Figure 7 .
Figure 7. a) Computed partial electronic DOS (PDOS) of ZnCo 2 O 4 and Zn v -Fe-ZnCo 2 O 4 .Reproduced with permission.[123]Copyright 2018, John Wiley & Sons.b) Volcano plot of the OER overpotential as a function of M CUS .Reproduced with permission.[124]Copyright 2019, American Chemical Society.c) Acidic OER performances of different TMOs as a function of ΔH 0 t , extracted from ref.[129,130] Zhu et al. have prepared Irincorporated Co 3 O 4 (Ir-Co 3 O 4 ) NSs with a simple mechanochemical method and NSs demonstrated an overpotential of 236 mV with the durability of 30 h, which is significantly higher than the pristine Co 3 O 4 NSs.Operando X-Ray absorption spectroscopy (XAS) is used to probe catalytic mechanisms after Ir doping.It is found that the valence states of Ir and Co species in Ir-Co 3 O 4 are irreversibly increased during the OER process, forming strong covalent M-O sites (Figure 10a,b).The enhanced M-O sites further facilitate the formation of O-O bonds by nucleophilic attack of electrophilic O ligands, resulting in an enhanced OER performance.DFT calculation results also demonstrate that Ir sites can effectively regulate charge redistribution and electrons transfer from Ir to the adjacent Ir-O bond.The strong electronic interactions between Ir and Co 3 O 4 support promote the conductivity of the material and positively shift the d-band center, stabilizing the adsorbates.Thus, the energy barrier of RDS steps on both Ir and Co sites can be reduced, leading to an excellent acidic OER performance of Ir-Co 3 O 4 (Figure 10c).Qiao et al. have further investigated the influence of Ir configuration on catalytic performance. [168]They applied different synthesis routes to obtain Ir-substituted Co 3 O 4 (Ir 0.06 Co 0.29 O 4 ) NSs and isolated Ir-loaded Co 3 O 4 nanoparticles surface (Ir SA@Co 3 O 4 ), where Ir 0.06 Co 0.29 O 4 exhibited a four times higher mass activity than Ir SA@Co 3 O 4 (Figure 10d).It was found that Ir atoms substituted in Co octahedral sites (Co oct ) ensured an identical spatial correlation between dopants and cation sites, leading to strong interactions with oxygen atoms in the Co 3 O 4 lattice.Compared with isolated Ir atoms in Ir SA@Co 3 O 4 , the d-band center of Ir-Co oct nanodomain demonstrated an obvious downshift, which regulated the adsorption energy of oxygen intermediates and effectively enhanced the OER activity (Figure 10e).Moreover, Ir substitution can also result in a deeper valence band, which enhances the oxidation resistance and stability of Ir 0.06 Co 0.29 O 4 NSs under acidic OER conditions (Figure (spinel oxides) usually demonstrated limited enhancement.A recent study by Wang et al. has demonstrated that the local geometry of Co ions could be changed by doping cations with large ionic radius (Ba atom) into Co 3 O 4 , resulting in a shortened Co-Co and Co-O distance (Figure

Figure 9 .
Figure 9. a) High-resolution transmission electron microscopy (HRTEM) image of Co 3 O 4 @C/GPO.b) Comparison of Co 3 O 4 @C/GPO and uncoated Co 3 O 4 .c) Stability test of Co 3 O 4 @C/GPO.Reproduced with permission.[154]Copyright 2022, Springer Nature.d) HRTEM image and corresponding electronic diffusion spectra maps of Co 3 O 4 @C nanosheets.e) Acidic OER activities of various Co 3 O 4 electrocatalysts and f ) corresponding stability test results.Reproduced with permission.[55]Copyright 2016, Elsevier, Inc. g) Transmission electron microscopy (TEM) and HRTEM images of TiO 2 /coated Co 3 O 4 .h) Stability test and i) linear sweep voltammetry (LSV) curves of TiO 2 /Co 3 O 4 /FTO electrode with different coating thicknesses.Reproduced with permission.[155]Copyright 2022, American Chemical Society.HRTEM images of the core-shell structure for j) FTO@Co 3 O 4 /CP and k) Co 3 O 4 @FTO/CP.l) Acidic OER activity of different Co 3 O 4 -based electrocatalysts and m) corresponding stability results.Reproduced with permission.[158]Copyright 2023, Royal Society of Chemistry.

Figure 10 .
Figure 10.a,b) Operando X-Ray absorption spectroscopy (XAS) spectra of Ir-L 3 and Co K edge.c) Gibbs free energy diagram at Ir and Co sites under different applied potentials (left panel) and d-band centers for pristine Co 3 O 4 and Ir-Co 3 O 4 .Reproduced with permission. [218]Copyright 2022, Springer Nature.d) Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy image of Ir 0.06 Co 0.94 O 4 NSs.e) Comparison of acidic OER activity and d-band center for Ir 0.06 Co 0.94 O 4 and Ir SA@Co 3 O 4 .f ) DOS of Ir 0.06 Co 0.94 O 4 and Ir SA@Co 3 O 4 .Reproduced with permission.[168]Copyright 2021, American Chemical Society.g) EXAFS spectra of Ba-doped Co 3 O 4 , demonstrating a shortened Co-Co bond length.h) Modified free energy and theoretical overpotential of Ba-doped Co 3 O 4 and i) corresponding reaction mechanism diagram.Reproduced with permission.[96]Copyright 2023, American Chemical Society.

Figure 11 .
Figure 11.a) HRTEM image of RuO 2 /(Co,Mn) 3 O 4 heterostructure.b) OER activity of different heterostructures.c) PDOS of Ru d-band orbitals of RuO 2 / Co 3 O 4 and RuO 2 /(Co, Mn) 3 O 4 , which demonstrates that the modified substrate can downshift the d-band center of Ru d orbital.Reproduced with permission. [219]Copyright 2021, Elsevier, Inc. d) HRTEM image and corresponding selected area electron diffraction pattern of CeO 2 /Co 3 O 4 .e) Comparison of OER activity of Co 3 O 4 and CeO 2 /Co 3 O 4 .f ) Cyclic voltammetry (CV) curves obtained in 0.5 M H 2 SO 4 in H 2 O and D 2 O. f ) Apparent activation energy (E app ) of Co 3 O 4 and CeO 2 /Co 3 O 4 at different temperatures.g) Ex situ XAS and i) in situ Raman spectra of Co 3 O 4 and CeO 2 /Co 3 O 4 .Reproduced with permission.[36]Copyright 2021, Springer Nature.

Figure 12 .
Figure 12.OER activities of a) tetrahedral-and b) octahedral-sites-substituted Co 3 O 4 .Corresponding electrochemical impedance spectroscopy spectra for c) Co 2 TiO 4 and d) CoCr 2 O 4 .Different morphologies of e) Co 2 TiO 4 and f-h) CoCr 2 O 4 .a,c,e) Reproduced with permission.[183]Copyright 2019, American Chemical Society.b,d,f-h) Reproduced with permission.[184]Copyright 2023, American Chemical Society.i) Fourier-transformed EXAFS spectra of Mn in Co 2 MnO 4 .j) Activity map with the adsorption energy of *OH and *O as descriptors for different electrocatalysts at 1.7 V (vs.RHE).k) Correlation between experimental and theoretical activities of various spinel oxides.l) Schematic diagram of the catalyst dissolution mechanism in acidic media.m) Oxidation state of Co 3 O 4 and Co 2 MnO 4 and schematic diagram of the charge-transfer resistance.PDOS calculation results of n) lattice O 2p orbitals and connected metal 3d orbitals, as well as adsorbed *O 2p orbitals and o) surface Co 3d orbitals.Reproduced with permission.[145]Copyright 2022, Springer Nature.
Figure 12.OER activities of a) tetrahedral-and b) octahedral-sites-substituted Co 3 O 4 .Corresponding electrochemical impedance spectroscopy spectra for c) Co 2 TiO 4 and d) CoCr 2 O 4 .Different morphologies of e) Co 2 TiO 4 and f-h) CoCr 2 O 4 .a,c,e) Reproduced with permission.[183]Copyright 2019, American Chemical Society.b,d,f-h) Reproduced with permission.[184]Copyright 2023, American Chemical Society.i) Fourier-transformed EXAFS spectra of Mn in Co 2 MnO 4 .j) Activity map with the adsorption energy of *OH and *O as descriptors for different electrocatalysts at 1.7 V (vs.RHE).k) Correlation between experimental and theoretical activities of various spinel oxides.l) Schematic diagram of the catalyst dissolution mechanism in acidic media.m) Oxidation state of Co 3 O 4 and Co 2 MnO 4 and schematic diagram of the charge-transfer resistance.PDOS calculation results of n) lattice O 2p orbitals and connected metal 3d orbitals, as well as adsorbed *O 2p orbitals and o) surface Co 3d orbitals.Reproduced with permission.[145]Copyright 2022, Springer Nature.

Table 1 .
Main characteristics of AWE and PEMWE.

Table 2 .
The OER performance of representative spinel oxides in acidic electrolytes. a)