Oxygen Evolution Reaction Pathway Research Articles

Minimal doping approach to activate lattice oxygen participation in K2WO4 electrocatalysts for oxygen evolution reaction

Understanding oxygen redox processes is pivotal for advancing oxygen evolution catalysts, critical in electrochemical devices for renewable fuel synthesis. Thermodynamic instability of high entropy metal oxides can interfere with oxygen redox reactions via electrochemical reconstruction. Transforming a W5N4 pre-catalyst into thermodynamically stable K2WO4 through electrochemical treatment in KOH, and incorporating minimal Fe doping, modifies the oxygen evolution reaction (OER) pathway towards oxygen redox activity, significantly influenced by the pH of the electrolyte. The Fe-doped K2WO4 catalyst demonstrates superior OER performance, achieving 10 mA/cm^2 at an overpotential of merely 181 mV (without iR compensation) and sustaining a lifetime of 1200 h at 1000 mA/cm2. Electrochemical methods and DFT calculations confirmed Fe doping creates oxygen vacancies and increases electron density near the Fermi level, enhancing the OER pathway. This methodology offers a versatile approach to adapt high entropy catalysts for broad electrochemical energy conversion applications, highlighting its potential for innovation in renewable energy technologies.

How to Break the Activity‐Stability Conundrum in Oxygen Evolution Electrocatalysis: Mechanistic Insights

AbstractTechnically viable electrocatalysts for the oxygen evolution reaction (OER) must be both active and stable under the harsh conditions at an electrolyser anode. While numerous highly active metal‐oxide catalysts have been identified, only very few are sufficiently stable, with iridium oxides being the most prominent. In this perspective, we draw insights from OER mechanisms to circumvent the activity‐stability conundrum generally plaguing the development of OER catalysts. In the commonly considered OER mechanisms, one or several metal‐oxygen (M−O) bonds are required to be broken along the OER pathway, providing a mechanistic link between the OER and oxide decomposition. However, a recently discovered mechanism on crystalline iridium dioxide provides a new OER pathway without M−O bond breakages, thus enabling the combination of sufficient activity and stability.

Unraveling the competition between the oxygen and chlorine evolution reactions in seawater electrolysis: Enhancing selectivity for green hydrogen production

Selective oxidation of water without production of chlorine during the electrolysis of seawater is a critical impediment towards obtaining green hydrogen. Indeed, understanding the complex competitive mechanisms of oxygen and chlorine formation at the anode is an analytic challenge. An argument for direct seawater electrolysis is presented with a dissection of the complications that arise at the anode in the presence of seawater ionic constituents such as the chloride ion. Electrolyser system durability and the impact on the current and voltage efficiencies are discussed. Critical challenges at the anode under the acidic conditions of proton exchange membrane water electrolysis interrelate the thermodynamic and kinetic constraints of the oxygen evolution reaction (OER) and the chlorine evolution reaction (CER) to elucidate the heterogenous mechanisms of the OER and the CER and the crucial stability predicament under selective OER electrocatalysis. The selectivity circumstances resolved by Density Functional Theory computations shed insight onto the reaction conditions that select for the preferred OER adsorbate chemisorption on the surface and substantiates the observed overpotentials required for the OER and CER; experimental rotating ring disk electrode analysis further indicate competitive adsorption of OER and CER reactants under an assumed Langmuir isotherm model. Identifying the rate determining step and breaking the scaling relationship of the AEM OER pathway may both improve the stability of the catalyst and achieve lower OER overpotentials. Critical insight is given into designing the heterogeneous electrocatalyst structure with selective facets, additional point defects, and augmented active site density through single atom catalysts. An argument for the utilization of ruthenium for its high natural ubiquity and modulable valence states that can facilitate atomic configurations with optimal active site d-band center energies to promote selective adsorbate binding is presented. Studies of in-situ filtration of the chloride ion under acidic conditions highlight the utility of manganese oxide and silica; the augmentation of the conductivity through manipulation of polaronic interactions; and the design of heterogeneous electrocatalysts with self-healing characteristics demonstrated in select molecular catalysts that may decrease the overpotential of the OER and achieve selectivity. It is with the hope that these design strategies provide insights into future research efforts to uncover an electrocatalytic surface selective for OER evolution under the perilous acidic conditions and reveal an effective solution as serendipitous as the abundancy of a natural resource such as seawater.

P-Incorporation Induced Enhancement of Lattice Oxygen Participation in Double Perovskite Oxides to Boost Water Oxidation.

Activating the lattice oxygen in the catalysts to participate in the oxygen evolution reaction (OER), which can break the scaling relation-induced overpotential limitation (> 0.37V) of the adsorbate evolution mechanism, has emerged as a new and highly effective guide to accelerate the OER. However, how to increase the lattice oxygen participation of catalysts during OER remains a major challenge. Herein, P-incorporation induced enhancement of lattice oxygen participation in double perovskite LaNi0.58Fe0.38P0.07O3-σ (PLNFO) is studied. P-incorporation is found to be crucial for enhancing the OER activity. The current density reaches 1.35mA cmECSA -2 at 1.63V (vs RHE), achieving a sixfold increase in intrinsic activity. Experimental evidences confirm the dominant lattice oxygen participation mechanism (LOM) for OER pathway on PLNFO. Further electronic structures reveal that P-incorporation shifts the O p-band center by 0.7eV toward the Fermi level, making the states near the Fermi level more O p character, thus facilitating LOM and fast OER kinetics. This work offers a possible method to develop high-performance double perovskite OER catalysts for electrochemical water splitting.

Unlocking the Transition of Electrochemical Water Oxidation Mechanism Induced by Heteroatom Doping

AbstractHeteroatom doping has emerged as a highly effective strategy to enhance the activity of metal‐based electrocatalysts toward the oxygen evolution reaction (OER). It is widely accepted that the doping does not switch the OER mechanism from the adsorbate evolution mechanism (AEM) to the lattice‐oxygen‐mediated mechanism (LOM), and the enhanced activity is attributed to the optimized binding energies toward oxygen intermediates. However, this seems inconsistent with the fact that the overpotential of doped OER electrocatalysts (<300 mV) is considerably smaller than the limit of AEM (>370 mV). To determine the origin of this inconsistency, we select phosphorus (P)‐doped nickel‐iron mixed oxides as the model electrocatalysts and observe that the doping enhances the covalency of the metal‐oxygen bonds to drive the OER pathway transition from the AEM to the LOM, thereby breaking the adsorption linear relation between *OH and *OOH in the AEM. Consequently, the obtained P‐doped oxides display a small overpotential of 237 mV at 10 mA cm−2. Beyond P, the similar pathway transition is also observed on the sulfur doping. These findings offer new insights into the substantially enhanced OER activity originating from heteroatom doping.

Unlocking the Transition of Electrochemical Water Oxidation Mechanism Induced by Heteroatom Doping.

Heteroatom doping has emerged as a highly effective strategy to enhance the activity of metal-based electrocatalysts toward the oxygen evolution reaction (OER). It is widely accepted that the doping does not switch the OER mechanism from the adsorbate evolution mechanism (AEM) to the lattice-oxygen-mediated mechanism (LOM), and the enhanced activity is attributed to the optimized binding energies toward oxygen intermediates. However, this seems inconsistent with the fact that the overpotential of doped OER electrocatalysts (< 300 mV) is considerably smaller than the limit of AEM (> 370 mV). To determine the origin of this inconsistency, we select phosphorus (P)-doped nickel-iron mixed oxides as the model electrocatalysts and observe that the doping enhances the covalency of the metal-oxygen bonds to drive the OER pathway transition from the AEM to the LOM, thereby breaking the adsorption linear relation between *OH and *OOH in the AEM. Consequently, the obtained P-doped oxides display a small overpotential of 237 mV at 10 mA cm-2. Beyond P, the similar pathway transition is also observed on the sulfur doping. These findings offer new insights into the substantially enhanced OER activity originating from heteroatom doping.

Metal-Oxygen Hybridization of Bi/Bife(oxy)Hydroxide for Sustainable Lattice Oxygen Mechanism at High Current Density

Hydrogen energy production through the electricity-driven water electrolysis has been broadly studied to deal with growing energy demands and environment pollutions. Oxygen evolution reaction (OER) which is the half anodic reaction of water electrolysis determines overall water electrolysis due to OOH* coordination with high energy barrier. Recently, alternative reaction kinetics detouring sluggish OOH intermediate in OER pathway has been proposed as breakthrough for efficient water electrolysis. That is the strategy which directly conjugates activated lattice oxygen species to form O-O coupling instead of OOH intermediate. However, absence of facile method to realize lattice oxygen activation and structural instability during OER cycles remain as challenge, hindering practical applications of water electrolysis. In this work, metal-oxygen hybridization method has been demonstrated as not only a simple and facile strategy to activate lattice oxygen species but also sustain lattice oxygen mechanism (LOM) during OER cycles at a practical current density (> 1000 mA cm-2). Using redox potential difference between bismuth (Bi) and iron (Fe) as driving force, galvanic replacement and Kirkendall effect take place in binary metal system, resulting in heterostructure composed of amorphous BiFe(oxy)hydroxides and molecular bismuth (Bi) metal nanoparticles (BM/BiFeOxHy) with abundant oxygen non-bonding states. In 1 M KOH solution, the BM/BiFeOxHy electrocatalyst requires low overpotential of 232 and 359 mV at the current densities of 10 and 1,000 mA cm-2, respectively. Moreover, long-term catalytic stability is demonstrated up to 1,000 hours at a practically high current density of 1,000 mA cm-2 without significant degradation by virtue of the balanced hybridization of Bi/Fe-O. Electrochemical/physicochemical analysis and density functional theory (DFT) calculation reveal that the excellent OER performance and stability of BM/BiFeOxHy electrocatalyst are attributed to the optimized Fe/Bi-O hybridization and resulting heterostructure with increased oxygen non-bonding states.

Direct Dioxygen Radical Coupling Driven by Octahedral Ruthenium-Oxygen-Cobalt Collaborative Coordination for Acidic Oxygen Evolution Reaction.

The acidic oxygen evolution reaction (OER) has long been the bottleneck of proton exchange membrane water electrolyzers given its harsh oxidative and corrosive environments. Herein, we suggest an effective strategy to greatly enhance both the acidic OER activity and stability of Co3O4 spinel by atomic Ru selective substitution on the octahedral Co sites. The resulting highly symmetrical octahedral Ru-O-Co collaborative coordination with strong electron coupling effect enables the direct dioxygen radical coupling OER pathway. Indeed, both experiments and theoretical calculations reveal a thermodynamically breakthrough heterogeneous diatomic oxygen mechanism. Additionally, the active Ru-O-Co units are well-maintained upon the acidic OER thanks to the electron transfer from surrounding electron-enriched tetrahedral Co atoms via bridging oxygen bonds that suppresses the overoxidation and thus dissolution of active Ru and Co species. Consequently, the prepared catalyst, even with a low Ru mass loading of ca. 42.8 μg cm-2, exhibits an attractive acidic OER performance with a low overpotential of 200 mV and a low potential decay rate of 0.45 mV h-1 at 10 mA cm-2. Our work suggests an effective strategy to significantly enhance both the acidic OER activity and stability of low-cost electrocatalysts.

Electrochemical Etching Switches Electrocatalytic Oxygen Evolution Pathway of IrOx /Y2 O3 from Adsorbate Evolution Mechanism to Lattice-Oxygen-Mediated Mechanism.

Oxygen evolution reaction (OER) plays key roles in electrochemical energy conversion devices. Recent advances have demonstrated that OER catalysts through lattice oxygen-mediated mechanism (LOM) can bypass the scaling relation-induced limitations on those catalysts through adsorbate evolution mechanism (AEM). Among various catalysts, IrOx , the most promising OER catalyst, suffers from low activities for its AEM pathway. Here, it is demonstrated that a pre-electrochemical acidic etching treatments on the hybrids of IrOx and Y2 O3 (IrOx /Y2 O3 ) switch the AEM-dominated OER pathway to LOM-dominated one in alkali electrolyte, delivering a high performance with a low overpotential of 223mV at 10mA cm-2 and a long-term stability. Mechanism investigations suggest that the pre-electrochemical etching treatments create more oxygen vacancies in catalysts due to the dissolution of yttrium and then provide highly active surface lattice oxygen for participating OER, thereby enabling the LOM-dominated pathway and resulting in a significantly increased OER activity in basic electrolyte.

Unraveling Pathways of Photocatalytic Oxygen Evolution on Fluorinated Anatase TiO2

Recent experiments reported that the efficiency of the oxygen evolution reaction (OER) was greatly improved in fluorinated TiO2 by forming the surface hydrogen bonds, but the mechanism remains ambiguous. Herein, we systematically investigated dual OER pathways on the fluorinated anatase TiO2(101) surface by using the first-principles calculations. The dual H-bonding water is significantly activated by trapping a first hole in the system and facilitates its proton transfer in pathway I. The inactive Ti-coordinated water also transfers a proton to a bridge O2– ion with a similar barrier and simultaneously traps the first hole in pathway II. Both ·OH radicals separately transfer a proton to the F– ion and bridge O2– ion very quickly and trap the second hole to produce the ·O radical. Consequently, the ·O radical directly couples with the bridge O2– ions with a high barrier of about 0.72 eV to produce the flat peroxo dimer (O22–) in pathway I. The Ti-coordinated ·O radical undergoes two steps of the proton transfer and O–O coupling with both high barriers of about 0.75 eV to produce the oblique O22– species in pathway II. Finally, the O22– spontaneously and successively traps the third and fourth holes to quickly form the oxygen molecule. Additionally, the fluorine atoms are further demonstrated to accelerate the proton transfer and the O–O coupling steps by comparing the pathways on the fluorinated and pure TiO2 surfaces. These results may provide new insights into the OER mechanism in fluorinated TiO2 and roles of the dopant in improving OER efficiency of the TiO2 photocatalysts.

Shifting Oxygen Evolution Reaction Pathway via Activating Lattice Oxygen in Layered Perovskite Oxide

AbstractDeveloping high‐performance oxygen evolution reaction (OER) catalysts are critical for the practical application of many electrochemical energy devices. In this study, taking layered perovskite oxide thin films as the model system, it is demonstrated that the OER pathway can be effectively shifted by activating lattice oxygen, leading to strongly enhanced intrinsic activity. The OER performance of Ruddlesden‐Popper (RP)‐phase cobaltite is significantly enhanced as Sr doping at the A site increases, which is attributed to the shift of the reaction pathway from adsorbate evolution mechanism (AEM) to lattice oxygen‐mediated mechanism (LOM). Advanced spectroscopic techniques and density functional theory calculations reveal that the Sr dopant effectively facilitates oxygen ligand hole formation, charge transfer from the oxygen sites, and the formation and migration of oxygen vacancy, hence promoting lattice oxygen to participate in surface reactions. The results provide critical insight into the role of oxygen activity and offer a potential way for constructing highly active electrocatalysts.

Activating Lattice Oxygen in Spinel ZnCo2O4 through Filling the Oxygen Vacancies with Fluorine for Electrocatalytic Oxygen Evolution.

The development of productive catalysts for oxygen evolution reaction (OER) remains a major challenge that requires significant progress in both mechanism and material design. Conventionally, the thermodynamic barrier of the lattice oxidation mechanism (LOM) is lower than that of absorbate evolution mechanism (AEM) because the former could bypass certain limitations. However, it remains a challenging task to control the OER pathway from the AEM to the LOM by taking advantage of the intrinsic properties of catalyst. Herein, we incorporate F anion into oxygen vacancies of spinel ZnCo2O4 and establish the link between electronic structure and the OER catalytic mechanism. Density theoretical calculation reveals that F upshifts O 2p center and activate the redox capability of the lattice O, successfully triggering the LOM pathway, achieving a low overpotential of 350 mV at 10 mA cm-2. Moreover, the large electronegativity of F anion is favorable for the balance of residual protonation, which can stabilize the structure and locally trigger LOM without surface reconstruction during OER reaction. This finding provides a feasible strategy to concurrently enhance the activity and stability of OER, and demonstrates the significance of considering lattice oxygen participation for understanding the OER trend of highly active spinel.

Activating Lattice Oxygen in Spinel ZnCo2O4 through Filling Oxygen Vacancies with Fluorine for Electrocatalytic Oxygen Evolution

AbstractThe development of productive catalysts for the oxygen evolution reaction (OER) remains a major challenge requiring significant progress in both mechanism and material design. Conventionally, the thermodynamic barrier of lattice oxidation mechanism (LOM) is lower than that of absorbate evolution mechanism (AEM) because the former can overcome certain limitations. However, controlling the OER pathway from the AEM to the LOM by exploiting the intrinsic properties of the catalyst remains challenging. Herein, we incorporated F anions into the oxygen vacancies of spinel ZnCo2O4 and established a link between the electronic structure and the OER catalytic mechanism. Theoretical density calculations revealed that F upshifts the O 2p center and activates the redox capability of lattice O, successfully triggering the LOM pathway. Moreover, the high electronegativity of F anions is favourable for balancing the residual protonation, which can stabilize the structure of the catalyst.

Strong-Proton-Adsorption Co-Based Electrocatalysts Achieve Active and Stable Neutral Seawater Splitting.

Direct electrolysis of pH-neutral seawater to generate hydrogen is an attractive approach for storing renewable energy. However, due to the anodic competition between the chlorine evolution and the oxygen evolution reaction (OER), direct seawater splitting suffers from a low current density and limited operating stability. Exploration of catalysts enabling an OER overpotential below the hypochlorite formation overpotential (≈490mV) is critical to suppress the chloride evolution and facilitate seawater splitting. Here, a proton-adsorption-promoting strategy to increase the OER rate is reported, resulting in a promoted and more stable neutral seawater splitting. The best catalysts herein are strong-proton-adsorption (SPA) materials such as palladium-doped cobalt oxide (Co3- x Pdx O4 ) catalysts. These achieve an OER overpotential of 370mV at 10mAcm-2 in pH-neutral simulated seawater, outperforming Co3 O4 by a margin of 70mV. Co3- x Pdx O4 catalysts provide stable catalytic performance for 450h at 200mAcm-2 and 20h at 1Acm-2 in neutral seawater. Experimental studies and theoretical calculations suggest that the incorporation of SPA cations accelerates the rate-determining water dissociation step in neutral OER pathway, and control studies rule out the provision of additional OER sites as a main factor herein.

Implications of the M-OO∙∙OO-M recombination mechanism on materials screening and the oxygen evolution reaction

Identification of active electrocatalysts for the oxygen evolution reaction (OER), corresponding to the bottleneck in electrolyzers to produce gaseous hydrogen as energy vector, by electronic structure calculations relies on the assumption of the mononuclear mechanism, comprising the *OH, *O, and *OOH intermediates. This mechanistic description is thermodynamically hampered by a scaling relation between the *OH and *OOH adsorbates, which may serve as an explanation why OER catalysts commonly require large overpotentials to reach sufficient current densities. Recently, an alternate OER pathway was proposed that, in contrast to the mononuclear description, consists of the formation of two adjacent *OO adsorbates, and gaseous oxygen is produced by chemical recombination of the neighboring *OO intermediates. In the present manuscript, a data-driven model based on a dedicated assessment of the elementary reaction steps is deduced, which enables evaluating the mononuclear and *OO pathways by the same set of parameters. Potential-dependent volcano plots are constructed to comprehend the energetics of the competing mechanisms. It is demonstrated that the alternate OER pathway consisting of the *OO∙∙*OO recombination step may excel the mononuclear description at overpotentials corresponding to typical OER conditions. Consequently, it is suggested that future studies, aiming at the identification of OER materials, may not omit the *OO∙∙*OO recombination mechanism when using concepts of materials screening in a heuristic fashion or multiscale modeling.

Oxygen Evolution Electrocatalysts for the Proton Exchange Membrane Electrolyzer: Challenges on Stability.

Hydrogen generated by proton exchange membrane (PEM) electrolyzer holds a promising potential to complement the traditional energy structure and achieve the global target of carbon neutrality for its efficient, clean, and sustainable nature. The acidic oxygen evolution reaction (OER), owing to its sluggish kinetic process, remains a bottleneck that dominates the efficiency of overall water splitting. Over the past few decades, tremendous efforts have been devoted to exploring OER activity, whereas most show unsatisfying stability to meet the demand for industrial application of PEM electrolyzer. In this review, systematic considerations of the origin and strategies based on OER stability challenges are focused on. Intrinsic deactivation of the material and the extrinsic balance of plant-induced destabilization are summarized. Accordingly, rational strategies for catalyst design including doping and leaching, support effect, coordination effect, strain engineering, phase and facet engineering are discussed for their contribution to the promoted OER stability. Moreover, advanced in situ/operando characterization techniques are put forward to shed light on the OER pathways as well as the structural evolution of the OER catalyst, giving insight into the deactivation mechanisms. Finally, outlooks toward future efforts on the development of long-term and practical electrocatalysts for the PEM electrolyzer are provided.

Lightest Metal Leads to Big Change: Lithium‐Mediated Metal Oxides for Oxygen Evolution Reaction

AbstractAs the lightest metal, the reversible insertion/extraction properties of lithium have been key findings in lithium metal oxide chemistry. Lithium has been widely used in the oxygen evolution reaction (OER), and the reaction mechanism of lithium‐mediated metal oxides has both similarities and uniqueness compared to typical dual metal oxides. Notably, the insertion/extraction of lithium during the OER is also crucial for the construction of novel surface reconstruction models. This review aims to provide the concepts of general OER pathways and key features of dual metal oxides for the OER. As a comparison then the development of lithium metal oxides for the OER is introduced and the chemistry underlying lithium metal oxide catalysts is unveiled. This review also examines the challenges remaining for the relevant catalysts, with prospects for further improving their OER activities.

Mechanisms of the Oxygen Evolution Reaction on NiFe2O4 and CoFe2O4 Inverse-Spinel Oxides.

Spinel ferrites, especially Nickel ferrite, NiFe2O4, and Cobalt ferrite, CoFe2O4, are efficient and promising anode catalyst materials in the field of electrochemical water splitting. Using density functional theory, we extensively investigate and quantitatively model the mechanism and energetics of the oxygen evolution reaction (OER) on the (001) facets of their inverse-spinel structure, thought as the most abundant orientations under reaction conditions. We catalogue a wide set of intermediates and mechanistic pathways, including the lattice oxygen mechanism (LOM) and adsorbate evolution mechanism (AEM), along with critical (rate-determining) O–O bond formation barriers and transition-state structures. In the case of NiFe2O4, we predict a Fe-site-assisted LOM pathway as the preferred OER mechanism, with a barrier (ΔG⧧) of 0.84 eV at U = 1.63 V versus SHE and a turnover frequency (TOF) of 0.26 s–1 at 0.40 V overpotential. In the case of CoFe2O4, we find that a Fe-site-assisted LOM pathway (ΔG⧧ = 0.79 eV at U = 1.63 V vs SHE, TOF = 1.81 s–1 at 0.40 V overpotential) and a Co-site-assisted AEM pathway (ΔG⧧ = 0.79 eV at bias > U = 1.34 V vs SHE, TOF = 1.81 s–1 at bias >1.34 V) could both play a role, suggesting a coexistence of active sites, in keeping with experimental observations. The computationally predicted turnover frequencies exhibit a fair agreement with experimentally reported data and suggest CoFe2O4 as a more promising OER catalyst than NiFe2O4 in the pristine case, especially for the Co-site-assisted OER pathway, and may offer a basis for further progress and optimization.

Surface self-reconstruction of telluride induced by in-situ cathodic electrochemical activation for enhanced water oxidation performance

Metal tellurides attract recent attention because of their promising applications as effective catalysts for the oxygen evolution reaction (OER). However, inappropriate adsorption energy between OER intermediates and telluride leads to an unsatisfactory electrocatalytic intrinsic activity. Herein, we adopt a unique in-situ cathodic electrochemical activation process to facilitate the surface self-reconstruction to form oxygen vacancy (OV)-rich TeO2 layer onto Fe-doped NiTe (OV@Fe-NiTe). Characterizations and theoretical calculation demonstrate that the presence of the OV-rich TeO2 layer realizes the adjustment of d-band center of the active site that translates into an enhancement of the adsorption of *OOH intermediate and thus the optimization of the OER pathway. Consequently, the OV@Fe-NiTe only requires an ultralow overpotential of 245 mV to drive 100 mA cm-2 in 1 M KOH, 95 mV lower than that of Fe-NiTe, and hence becoming the best water oxidation electrocatalysts amongst recently reported telluride electrocatalysts. This study presents a unique strategy to exploit telluride-based catalysts through electrochemical surface engineering.

Structural Design Strategy and Active Site Regulation of High-Efficient Bifunctional Oxygen Reaction Electrocatalysts for Zn-Air Battery.

Zinc-air batteries (ZABs) exhibit high energy density as well as flexibility, safety, and portability, thereby fulfilling the requirements of power batteries and consumer batteries. However, the limited efficiency and stability are still the significant challenge. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial cathode reactions in ZABs. Development of bifunctional ORR/OER catalysts with high efficiency and well stability is critical to improve the performance of ZABs. In this review, the ORR and OER mechanisms are first explained. Further, the design principles of ORR/OER electrocatalysts are discussed in terms of atomic adjustment mechanism and structural design in conjunction with the latest reported in situ characterization techniques, which provide useful insights on the ORR/OER mechanisms of the catalyst. The improvement in the energy efficiency, stability, and environmental adaptability of the new hybrid ZAB by the inclusion of additional reaction, including the introduction of transition-metal redox couples in the cathode and the addition of modifiers in the electrolyte to change the OER pathway, is also summarized. Finally, current challenges and future research directions are presented.