Co 3 O 4 nanostructures for oxygen evolution reaction: a mini review

1 Introduction

The rapid growth in global energy consumption and increasing environmental concerns associated with fossil fuel utilization have accelerated the development of sustainable and clean energy technologies. In recent years, renewable energy resources such as solar, wind, and hydroelectric power have attracted considerable attention as alternatives to conventional fossil fuels. However, the intermittent nature of renewable energy sources necessitates efficient energy conversion and storage technologies for stable and long-term energy utilization. Among various approaches, electrochemical water splitting has emerged as a promising strategy for producing high-purity hydrogen fuel using renewable electricity. Green hydrogen generated through water electrolysis is considered an environmentally friendly and sustainable energy carrier for future carbon-neutral energy systems (Roger et al., 2017; Seh et al., 2017).

Electrochemical water splitting consists of two half-cell reactions, namely, the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. Compared with HER, the OER process is kinetically sluggish because it involves a complex four-electron transfer mechanism and multiple intermediate reaction steps, resulting in high overpotential and low energy conversion efficiency (Suen et al., 2017). Therefore, the development of highly efficient, stable, and low-cost electrocatalysts for OER is essential to improve the overall efficiency of water-splitting systems and industrial electrolyzers.

Currently, noble metal-based catalysts such as ruthenium oxide (RuO₂) and iridium oxide (IrO₂) are considered benchmark electrocatalysts for OER because of their excellent catalytic activity and electrochemical stability (Song and Hu, 2014). Nevertheless, their high cost, scarcity, and limited long-term availability significantly hinder their large-scale commercial applications. Consequently, extensive research efforts have focused on developing earth-abundant transition metal-based electrocatalysts including nickel, iron, manganese, and cobalt oxides (Zhang et al., 2016). Among earth-abundant transition-metal oxides, Co₃O₄ is particularly attractive for OER because its spinel framework provides chemically distinct tetrahedral Co²⁺ and octahedral Co³⁺ sites, offering multiple redox pathways that can participate in alkaline water oxidation. The coexistence of Co²⁺ and Co³⁺ ions facilitates reversible redox reactions and enhances catalytic activity during oxygen evolution reactions (Li et al., 2010).

Recent advances in nanotechnology have significantly improved the electrocatalytic performance of Co₃O₄ materials through nanostructure engineering. Various Co₃O₄ nanostructures including nanoparticles, nanosheets, nanowires, nanoflowers, hollow spheres, and porous architectures have been successfully synthesized using hydrothermal, sol–gel, chemical precipitation, microwave-assisted, and electrodeposition methods (Wang et al., 2016). These nanostructures provide large electrochemically active surface area, abundant catalytic active sites, shortened ion diffusion pathways, and enhanced electron transport characteristics, thereby improving OER performance (Smith et al., 2013). Furthermore, the integration of Co₃O₄ with conductive carbon materials such as graphene, carbon nanotubes (CNTs), and MXenes has emerged as an effective strategy for enhancing electrical conductivity, interfacial charge transfer, and structural stability (Grimaud et al., 2013). The electrical properties and charge-transfer mechanisms of electrocatalysts play a critical role in determining catalytic efficiency during oxygen evolution reactions. Improved electrical conductivity facilitates rapid electron transfer between the catalyst surface and electrolyte, while optimized charge-transfer pathways reduce interfacial resistance and accelerate electrochemical reaction kinetics (Gong et al., 2015). In addition, defect engineering, oxygen vacancy formation, heteroatom doping, and heterostructure construction have recently emerged as promising approaches for tuning the electronic structure and improving the intrinsic catalytic activity of Co₃O₄ -based electrocatalysts (Kanan and Nocera, 2008). Advanced strategies such as machine learning-assisted catalyst design and operando characterization techniques are also gaining increasing attention for the development of next-generation high-performance OER catalysts.

This mini review summarizes recent advances in Co₃O₄ nanomaterials for oxygen evolution reactions, with particular emphasis on synthesis strategies, nanostructure engineering, electrical properties, charge transport mechanisms, and electrochemical performance. In addition, current challenges and future research directions for developing highly efficient, stable, and scalable Co₃O₄ -based electrocatalysts for renewable energy and green hydrogen applications are discussed.

2 Fundamentals of oxygen evolution reaction

The oxygen evolution reaction (OER) is a crucial anodic electrochemical process involved in water splitting, metal–air batteries, regenerative fuel cells, and various renewable energy conversion systems. The overall efficiency of electrochemical water splitting is largely limited by the sluggish kinetics of OER due to its complex multistep reaction mechanism involving four-electron transfer processes (Burke et al., 2015a). Therefore, the development of highly active and durable electrocatalysts is essential for reducing overpotential and improving electrochemical reaction kinetics.

Electrochemical water splitting consists of two half-cell reactions, namely, the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. In alkaline electrolyte, the OER process can be represented as follows:

4 O H − → O 2 + 2 H 2 O + 4 e −

The OER mechanism generally proceeds through several intermediate adsorption and desorption steps occurring on the catalyst surface. Initially, hydroxide ions (OH⁻) are adsorbed onto active catalytic sites, followed by the formation of intermediate species such as OH ^*, O ^*, and OOH ^*. Finally, molecular oxygen is released from the catalyst surface (Friebel et al., 2015). The reaction pathway can be represented as:

O H * → O * → O O H * → O 2 O H

The adsorption energy and stability of these intermediate species strongly influence the catalytic activity and reaction kinetics of electrocatalysts. An ideal OER catalyst should possess optimized binding energy toward reaction intermediates to facilitate rapid oxygen evolution while minimizing energy losses (Trotochaud et al., 2014). Among various transition metal oxides, Cobalt Oxide (Co₃O₄) has attracted significant attention due to its unique spinel crystal structure and mixed valence states of Co²⁺ and Co³⁺ ions occupying tetrahedral and octahedral lattice positions, respectively. These mixed oxidation states promote reversible redox reactions and facilitate efficient electron transfer during oxygen evolution reactions (Lu and Zhao, 2015). In addition, Co₃O₄ exhibits excellent chemical stability, environmental compatibility, and relatively low cost compared with noble metal-based catalysts. Nanostructure engineering has emerged as an effective strategy for improving the OER performance of Co₃O₄ electrocatalysts. Various nanostructures including nanoparticles, nanosheets, nanowires, hollow spheres, and porous frameworks provide larger electrochemically active surface area, abundant exposed active sites, shortened ion diffusion pathways, and enhanced charge-transfer efficiency (Li et al., 2017). Furthermore, introducing oxygen vacancies, heteroatom doping, and conductive carbon supports such as graphene, carbon nanotubes, and MXenes can significantly improve electrical conductivity and catalytic activity by modifying the electronic structure and facilitating rapid interfacial charge transport (Yu et al., 2013).

Several electrochemical parameters are commonly used to evaluate the OER performance of electrocatalysts, including overpotential, Tafel slope, exchange current density, electrochemical active surface area (ECSA), and long-term stability. Lower overpotential and smaller Tafel slope values indicate superior catalytic activity and faster reaction kinetics (Fabbri et al., 2014; Suntivich et al., 2011). Electrochemical impedance spectroscopy (EIS) is also widely employed to analyze charge-transfer resistance and conductivity behavior of electrocatalysts during oxygen evolution reactions.

Electrochemical water splitting and oxygen evolution reaction (OER) mechanisms on Co₃O₄ electrocatalysts in alkaline electrolyte (Figure 1). The figure presents the hydrogen evolution reaction (HER) and OER half-cell reactions, along with OH⁻ adsorption, intermediate formation (OH^*, O^*, and OOH^*), and oxygen evolution pathways on the catalyst surface. The spinel Co₃O₄ structure containing mixed Co²⁺/Co³⁺ redox centers and oxygen vacancies facilitates rapid electron transport, enhanced charge-transfer kinetics, and improved electrocatalytic OER performance. Various synthesis techniques have been developed to fabricate Co₃O₄ nanostructures with controlled morphology and physicochemical properties. As summarized in Table 1, each synthesis route offers distinct advantages and limitations in terms of morphology control, scalability, crystallinity, and processing temperature. The choice of synthesis method plays a crucial role in determining the structural characteristics and subsequent electrocatalytic performance of Co₃O₄ -based OER catalysts.

TABLE 1. Common synthesis methods for Co₃O₄ nanostructures, highlighting the resulting morphology, key advantages, limitations, and typical processing temperatures.

3 Electrical properties and charge transport mechanisms of Co₃O₄ nanomaterials

The electrical properties and charge-transfer characteristics of electrocatalysts play a crucial role in determining oxygen evolution reaction (OER) performance. Efficient electron transport between the catalyst surface and electrolyte facilitates rapid electrochemical kinetics, reduces overpotential, and enhances catalytic activity (Grzelczak et al., 2013). The charge-transport behavior of Co₃O₄ is closely associated with its normal spinel structure, in which Co²⁺ ions occupy tetrahedral sites while Co³⁺ ions preferentially occupy octahedral sites. The coexistence of these oxidation states enables redox-mediated charge redistribution and electron hopping within the oxide lattice. Consequently, the electronic structure of Co₃O₄ provides an important basis for understanding its interfacial charge-transfer behavior during OER.

Co₃O₄ possesses a normal spinel crystal structure in which Co²⁺ ions occupy tetrahedral lattice sites while Co³⁺ ions occupy octahedral sites. The coexistence of Co²⁺ and Co³⁺ ions enables reversible redox reactions and electron hopping mechanisms within the crystal lattice, thereby facilitating charge transport and improving catalytic efficiency (Sivachidambaram et al., 2019). During oxygen evolution reactions, electron transfer primarily occurs through hopping between Co²⁺ and Co³⁺ redox centers, which accelerates electrochemical reaction kinetics and enhances OER activity. Although pristine Co₃O₄ exhibits semiconducting behavior with moderate electrical conductivity, nanostructure engineering significantly improves its charge-transfer characteristics. Various nanostructures including nanoparticles, nanosheets, nanowires, hollow spheres, and porous frameworks provide shortened electron transport pathways, larger electrochemically active surface area, and increased exposure of catalytic active sites (Li et al., 2016). These structural advantages contribute to faster ion diffusion and enhanced interfacial electron transfer during electrochemical reactions.

Defect engineering and oxygen vacancy formation have emerged as effective strategies for improving the electrical conductivity and catalytic performance of Co₃O₄ electrocatalysts. Oxygen vacancies can modify the electronic structure, increase carrier concentration, and create additional active sites for hydroxide ion adsorption (Hunter et al., 2016). Furthermore, heteroatom doping with elements such as Fe, Ni, Mn, and Cu has been demonstrated to optimize the electronic configuration and improve charge-transfer efficiency. In recent years, integrating Co₃O₄ with conductive carbon materials such as graphene, carbon nanotubes (CNTs), and MXenes has attracted significant interest for enhancing electrical conductivity and interfacial charge transport (Morales-Guio et al., 2014). These conductive supports provide rapid electron transport pathways and reduce charge-transfer resistance between the catalyst surface and electrolyte. In addition, hybrid heterostructures can improve catalyst stability and facilitate efficient adsorption/desorption of reaction intermediates during OER.

Several electrochemical characterization techniques are commonly employed to investigate the electrical properties and charge-transfer behavior of Co₃O₄ electrocatalysts. Linear sweep voltammetry (LSV) is widely used to evaluate catalytic activity and overpotential behavior, while Tafel slope analysis provides information regarding electrochemical reaction kinetics (Bajdich et al., 2013). Electrochemical impedance spectroscopy (EIS) is an important technique for analyzing charge-transfer resistance and conductivity characteristics. Lower charge-transfer resistance obtained from EIS measurements generally indicates improved electron mobility and enhanced catalytic performance.

The electrochemical polarization behavior of Co₃O₄(111), Co₁₊δFe₂−δO₄ (111), and Fe₃O₄(111) electrocatalysts before and after the OER (Figure 2). The current density versus potential (V vs. RHE) curves demonstrate the catalytic activity and charge-transfer characteristics of the investigated oxide materials under electrochemical conditions. The comparison between the “before OER” and “after OER” measurements reveals significant changes in electrocatalytic performance due to surface reconstruction and activation during oxygen evolution reactions. The Co₃O₄-based electrocatalyst exhibits enhanced OER activity with lower onset potential and higher current density compared with Fe₃O₄, indicating superior catalytic kinetics and improved electron-transfer behavior. After OER operation, the increased current response suggests the formation of more active surface sites and improved electrochemical conductivity. Furthermore, the reduced overpotential and enhanced polarization behavior confirm the important role of mixed valence Co²⁺/Co³⁺ redox centers in facilitating rapid charge transport and oxygen evolution kinetics. Overall, the figure demonstrates the excellent electrochemical performance of Co₃O₄ -based materials for efficient oxygen evolution reaction applications.

Figure 3 presents a comprehensive electrochemical evaluation of Co₃O₄-based electrocatalysts toward the oxygen evolution reaction (OER). The linear sweep voltammetry (LSV) curves shown in Figure 3a reveal that the Co₃O₄ nanoflower catalyst exhibits the highest catalytic activity, delivering a current density of approximately 115 mA cm^-2 at 1.8 V versus RHE, significantly outperforming Co₃O₄ nanosheets, nanoparticles, commercial IrO₂, RuO₂, and bare Ni foam. The superior OER performance is attributed to the unique hierarchical nanoflower morphology, which provides abundant active sites and enhanced mass transport. The corresponding Tafel plots (Figure 3b) further confirm the accelerated reaction kinetics of the Co₃O₄ nanoflowers, exhibiting the lowest Tafel slope of 37 mV dec⁻¹, compared with 48, 62, 72, and 89 mV dec⁻¹ for Co₃O₄ nanosheets, nanoparticles, IrO₂, and RuO₂, respectively. The lower Tafel slope indicates faster charge-transfer kinetics and more favorable OER reaction pathways.

The electrochemical impedance spectroscopy (EIS) Nyquist plots in Figure 3c reveal reduced charge-transfer resistance for Co₃O₄ nanoflower structures, confirming improved electrical conductivity and rapid interfacial electron transport. Figure 3d compares the overpotential values at a current density of 10 mA cm^-2, demonstrating superior OER activity for optimized Co₃O₄ nanostructures. In addition, the double-layer capacitance (Cdl) measurements shown in Figure 3e indicate a larger electrochemically active surface area and abundant catalytic active sites for nanostructured catalysts. Furthermore, the long-term stability measurements presented in Figure 3f confirm excellent electrochemical durability during continuous OER operation. The improved catalytic performance is mainly attributed to enhanced charge-transfer pathways, larger active surface area, efficient ion diffusion, and favorable Co²⁺/Co³⁺ redox behavior in Co₃O₄ nanostructures. The electrocatalytic performance of Co₃O₄ strongly depends on its morphology. Table 2 compares representative Co₃O₄ nanostructures, highlighting their structural advantages, limitations, and OER benefits. Among these, hierarchical and carbon-supported architectures generally exhibit enhanced conductivity, faster charge transport, and superior catalytic stability, making them promising candidates for efficient water oxidation.

TABLE 2. Comparison of representative Co₃O₄ nanostructures for oxygen evolution reaction (OER), including their typical size, advantages, limitations, and electrocatalytic benefits.

4 Nanostructure engineering of Co₃O₄ electrocatalysts

4.1 Morphology-dependent OER activity

Nanostructure engineering provides an effective route to regulate the surface area, pore architecture, exposed facets, and accessibility of catalytic sites in Co₃O₄. By controlling these structural parameters, the diffusion of hydroxide ions, release of oxygen bubbles, and utilization of surface-active sites can be optimized. Designing nanostructured Co₃O₄ materials can significantly enhance electrochemically active surface area, expose abundant catalytic sites, facilitate rapid ion diffusion, and improve electron-transfer kinetics during electrochemical reactions (Zhao et al., 2014). Various Co₃O₄ nanostructures including nanoparticles, nanosheets, nanowires, nanorods, nanoflowers, hollow spheres, and porous frameworks have been extensively investigated for OER applications. Among these structures, Co₃O₄ nanoparticles provide short electron transport pathways and high surface-to-volume ratio, resulting in improved catalytic activity and reduced overpotential (Guan et al., 2017). However, severe agglomeration of nanoparticles may decrease the accessibility of active sites and limit long-term electrochemical stability.

Two-dimensional Co₃O₄ nanosheets have attracted considerable attention because of their ultrathin layered morphology and large exposed surface area. The nanosheet architecture provides abundant active edge sites and facilitates efficient electrolyte penetration, thereby improving charge-transfer behavior and oxygen evolution kinetics (Zhou et al., 2023). Similarly, one-dimensional nanowires and nanorods exhibit continuous electron transport pathways and enhanced structural stability during long-term electrochemical operation. Recently, hierarchical and porous Co₃O₄ nanostructures have demonstrated superior OER performance due to their interconnected pore networks and large electrochemically active surface area. Hollow spheres and porous nanoflowers can effectively accelerate electrolyte diffusion and increase the accessibility of catalytic active sites (Huang et al., 2019). In addition, hierarchical architectures improve structural robustness and accommodate volume changes during electrochemical cycling, thereby enhancing catalytic durability.

Figure 4a shows uniformly distributed Co₃O₄ nanoparticles with diameters typically ranging from 20 to 60 nm. The nanoparticles exhibit rough surfaces and moderate agglomeration while maintaining a high density of exposed active sites. Such nanoscale particles provide a large electrochemically active surface area (ECSA), short electron-transport distances, and abundant catalytic centers, leading to enhanced OER activity (Zhao et al., 2014). Figure 4b displays Co₃O₄ nanocubes with well-defined cubic morphology and particle sizes of approximately 180–220 nm. The highly crystalline facets exposed on the cubic structures can modify the adsorption energies of oxygen-containing intermediates (*OH, *O, and *OOH), thereby improving the intrinsic catalytic activity and structural stability during prolonged electrolysis (Guan et al., 2017). The interconnected Co₃O₄ nanosheets shown in Figure 4c form an ultrathin porous network with sheet thicknesses of approximately 30–40 nm. This two-dimensional architecture offers a large accessible surface area and interconnected pore channels that facilitate electrolyte penetration, rapid OH⁻ diffusion, and efficient release of evolved oxygen bubbles. The open nanosheet framework also minimizes diffusion resistance and promotes faster reaction kinetics (Zhou et al., 2023). Figure 4d presents one-dimensional Co₃O₄ nanowires with diameters of approximately 70–80 nm, forming interconnected conductive pathways. The anisotropic nanowire architecture enables continuous electron transport along the longitudinal direction while simultaneously providing abundant surface-active sites and shortened ion-diffusion pathways. Such features contribute to reduced charge-transfer resistance and improved electrocatalytic efficiency (Huang et al., 2019). Figure 4e illustrates hollow Co₃O₄ nanospheres with diameters of approximately 300–320 nm. The hollow interior and porous shell structure considerably increase the specific surface area while reducing the diffusion distance for electrolyte ions. The hierarchical hollow architecture exposes both the inner and outer surfaces to the electrolyte, resulting in enhanced utilization of catalytic active sites and improved oxygen evolution kinetics (Feng et al., 2019). Figure 4f shows Co₃O₄ nanoballs uniformly anchored on a three-dimensional carbon aerogel framework. The spherical Co₃O₄ nanoballs (∼220 nm) are homogeneously dispersed throughout the conductive porous carbon network, which prevents nanoparticle aggregation and establishes efficient electron-conduction pathways. The carbon aerogel significantly enhances electrical conductivity, accelerates charge-transfer kinetics, improves structural integrity, and provides excellent mechanical stability during long-term OER operation (Wang et al., 2019). In additionally, the SEM images clearly demonstrate that nanostructure engineering plays a crucial role in optimizing the electrocatalytic performance of Co₃O₄. Zero-dimensional nanoparticles maximize active-site density, one-dimensional nanowires facilitate directional electron transport, two-dimensional nanosheets improve electrolyte accessibility, hollow nanospheres enhance mass transport through internal cavities, and carbon-supported nanoballs combine high conductivity with structural robustness. These complementary structural characteristics collectively improve oxygen evolution kinetics, reduce charge-transfer resistance, lower the overpotential, and enhance the long-term durability of Co₃O₄ -based electrocatalysts, highlighting morphology-controlled design as an effective strategy for developing efficient and stable OER catalysts (Zhao et al., 2014; Guan et al., 2017; Zhou et al., 2023; Huang et al., 2019; Feng et al., 2019; Wang et al., 2019). The correlation between morphology and electrochemical properties is summarized in Table 3. Variations in surface area, electron transport, electrolyte diffusion, and structural stability significantly influence OER activity. In particular, Co₃O₄/carbon composites demonstrate the most balanced electrochemical characteristics, emphasizing the importance of morphology engineering for designing high-performance OER electrocatalysts.

TABLE 3. Morphology–property relationship of Co₃O₄ electrocatalysts, summarizing the influence of different nanostructures on surface area, electron transport, electrolyte diffusion, stability, and overall OER activity.

4.2 Heteroatom doping and carbon-assisted interface engineering

Heteroatom doping provides an effective strategy for regulating the electronic structure and intrinsic catalytic activity of Co₃O₄. Incorporation of transition-metal dopants such as Fe, Ni, Mn, and Cu into the Co₃O₄ lattice can modify the local coordination environment, oxidation-state distribution, defect concentration, and electronic density around the Co. sites (Hu et al., 2022; Ede and Luo, 2021; Xu et al., 2016). Such modifications can alter the Co²⁺/Co³⁺ redox equilibrium and facilitate more favorable charge redistribution during OER (Hu et al., 2022; Ede and Luo, 2021). In particular, dopant-induced lattice distortion and oxygen-vacancy formation can generate additional catalytically active sites and modify the adsorption energetics of key OER intermediates, including OH, O, and OOH* (Xu et al., 2016; Tong et al., 2019; Chen et al., 2024). Consequently, optimized doping can decrease the kinetic barrier for the OER pathway and improve the intrinsic catalytic activity of Co₃O₄. However, excessive dopant incorporation may disrupt the spinel lattice, reduce structural stability, or generate unfavorable electronic states; therefore, an appropriate dopant concentration is essential for achieving an optimum structure–activity relationship (Hu et al., 2022; Ede and Luo, 2021).

The integration of conductive carbon materials provides a complementary approach for overcoming the relatively limited electrical conductivity of Co₃O₄. Graphene, carbon nanotubes (CNTs), carbon aerogels, and MXenes can form continuous conductive networks around or within Co₃O₄ nanostructures (de Lima et al., 2024; Zhao et al., 2015; Ahmed et al., 2018). The resulting Co₃O₄/carbon interfaces reduce electron-transport distances and facilitate rapid electron transfer between the catalyst and conductive support. At the same time, carbon frameworks can suppress Co₃O₄ nanoparticle agglomeration, increase the accessible electrochemical surface area, and improve electrolyte penetration (de Lima et al., 2024; Zhao et al., 2015). Strong interfacial coupling may additionally modify the electronic structure of Co₃O₄ and promote charge redistribution across the heterointerface, thereby improving the adsorption and conversion of OER intermediates (de Lima et al., 2024; Zhao et al., 2015; Ahmed et al., 2018).

The resulting structure–property relationship can therefore be described as follows: heteroatom doping regulates the local electronic structure and catalytic active sites, whereas carbon integration primarily strengthens electrical connectivity, interfacial charge transport, and structural stability (Hu et al., 2022; de Lima et al., 2024; Zhao et al., 2015; Ahmed et al., 2018). When these effects are appropriately balanced, Co₃O₄ -based heterostructures can exhibit lower charge-transfer resistance, faster OER kinetics, reduced overpotential, and improved operational stability (de Lima et al., 2024; Zhao et al., 2015; Ahmed et al., 2018). Thus, combining controlled doping with conductive carbon integration represents a promising strategy for simultaneously optimizing the intrinsic activity and transport properties of Co₃O₄ electrocatalysts (Hu et al., 2022; de Lima et al., 2024; Ahmed et al., 2018).

5 Perspectives and challenges

Despite the progress achieved through morphology control, defect engineering, heteroatom doping, and interface engineering, several challenges remain before Co₃O₄ -based OER catalysts can be translated into practical electrolyzers. These challenges include limited intrinsic electrical conductivity, structural reconstruction under operating conditions, incomplete understanding of the catalytically active surface phase, and difficulties in simultaneously achieving high activity and long-term stability. From a research perspective, future development of Co₃O₄ is strongly directed toward rational nanostructure engineering, where controlled synthesis of porous, hollow, and hierarchical morphologies can increase the density of accessible active sites, shorten mass-transport pathways, and improve electrolyte accessibility (Zhao et al., 2014; Guan et al., 2017). Hybridization with conductive carbon materials such as graphene, carbon nanotubes, activated carbon, and graphitic carbon nitride can further improve electrical connectivity and interfacial charge transfer while suppressing nanoparticle aggregation (Zhou et al., 2023; Huang et al., 2019; de Lima et al., 2024; Zhao et al., 2015; Ahmed et al., 2018).

Further improvements can be achieved through heteroatom doping and interface engineering, which modify the local electronic structure, oxidation-state distribution, defect chemistry, and adsorption characteristics of OER intermediates (Feng et al., 2019; Hu et al., 2022; Ede and Luo, 2021; Xu et al., 2016; Tong et al., 2019; Chen et al., 2024). Such strategies can regulate the Co²⁺/Co³⁺ redox behavior and optimize the balance between catalytic-site activity and charge-transfer efficiency. Nevertheless, excessive defect generation or dopant incorporation may adversely affect structural integrity and long-term stability, indicating that precise control of dopant concentration and defect density is essential for obtaining an optimum structure–property relationship (Hu et al., 2022; Ede and Luo, 2021; Xu et al., 2016; Tong et al., 2019; Chen et al., 2024). Recent reviews also emphasize morphology, doping, defect engineering, and heterostructure construction as major approaches for improving Co₃O₄ -based OER performance.

Despite these promising developments, several critical challenges still limit the large-scale practical application of Co₃O₄ -based systems. One major issue is the relatively limited intrinsic electrical conductivity of Co₃O₄, which can restrict electron transport and increase charge-transfer resistance during electrochemical operation. Although carbon integration and heterostructure construction can alleviate this limitation, maintaining strong and stable interfacial coupling during prolonged electrolysis remains challenging (de Lima et al., 2024; Zhao et al., 2015; Ahmed et al., 2018). Experimental studies of Co₃O₄/carbon systems have demonstrated that improved charge-transfer characteristics and conductive networks can contribute substantially to enhanced OER performance.

Another important challenge is structural instability during long-term OER operation. Surface reconstruction, changes in cobalt oxidation states, dissolution, and formation of catalytically active oxyhydroxide-like surface species can alter the structure of Co₃O₄ under operating conditions (Luo et al., 2024; Geng et al., 2025). Therefore, the structure of the pristine catalyst before electrolysis cannot always be directly correlated with the actual active phase during OER. Recent studies and reviews emphasize the importance of understanding surface restructuring and the evolution of active sites under realistic electrochemical conditions.

Another limitation is the incomplete utilization of catalytically accessible sites. Although nanostructuring can substantially increase the exposed surface area, not all surface cobalt sites are necessarily accessible to electrolyte ions or participate equally in OER. Consequently, future catalyst design should focus not only on increasing the geometric surface area but also on maximizing the electrochemically active surface area and optimizing the accessibility and intrinsic activity of catalytic sites (Liu et al., 2025). In addition, complex and multistep synthesis procedures for advanced nanostructured Co₃O₄ may hinder scalability, reproducibility, and cost-effective manufacturing (Reikowski et al., 2019). The development of simple, scalable, and environmentally sustainable synthesis routes is therefore essential for translating laboratory-scale Co₃O₄ catalysts toward practical electrolyzer applications.

Finally, the precise identification of the catalytically active phase and the dominant OER reaction pathway remains an important scientific challenge. The Co₃O₄ surface may undergo potential-dependent restructuring, making it difficult to distinguish the role of the original spinel structure from that of the reconstructed surface during OER (Luo et al., 2024; Geng et al., 2025). Advanced *in situ* and operando spectroscopic, microscopic, and electrochemical techniques, combined with density functional theory calculations, are therefore required to establish clear correlations among catalyst structure, electronic properties, surface reconstruction, intermediate adsorption, and OER activity (Poshan Kumar Reddy et al., 2026; Scharf et al., 2026). Such mechanistic understanding will be critical for rationally designing next-generation Co₃O₄ catalysts rather than relying primarily on empirical optimization.

Overall, future research should focus on scalable and reproducible synthesis, controlled defect and interface engineering, conductivity enhancement through rational composite design, improved structural stability, and deeper mechanistic understanding under realistic operating conditions. In particular, integrating controlled heteroatom doping with conductive carbon frameworks offers an attractive route for simultaneously optimizing the intrinsic catalytic activity and charge-transport properties of Co₃O₄. These approaches, together with operando characterization and theory-guided catalyst design, can help translate the promising laboratory-scale performance of Co₃O₄ -based electrocatalysts into durable and practical systems for sustainable hydrogen and oxygen production.

6 Conclusion

Co₃O₄ -based electrocatalysts have emerged as promising non-noble-metal catalysts for the oxygen evolution reaction (OER), owing to their multivalent Co²⁺/Co³⁺ redox chemistry, spinel structure, compositional flexibility, and tunable nanostructure. The major advantages of Co₃O₄ include its relatively low cost, availability of cobalt-based oxide materials, diverse morphologies, and ability to provide catalytically active sites through defect and surface engineering. Nanostructuring, heteroatom doping, conductive carbon integration, and interface engineering can further increase active-site accessibility, improve electrical conductivity, facilitate charge transfer, and enhance OER kinetics. Despite these advantages, Co₃O₄ also exhibits several important limitations. Its relatively poor intrinsic electrical conductivity can restrict charge transport, while surface reconstruction, oxidation-state changes, and structural degradation during prolonged electrochemical operation can affect catalytic stability. In addition, not all cobalt sites are electrochemically accessible, and the actual active surface phase formed under OER conditions remains an important subject of investigation. Complex synthesis procedures for highly engineered nanostructures can further limit scalability and reproducibility.

Overall, the advantages and limitations of Co₃O₄ can be summarized as follows: (i) advantages—low-cost transition-metal oxide chemistry, multivalent redox states, structural tunability, abundant opportunities for defect engineering, and compatibility with conductive heterostructures; and (ii) limitations—limited electrical conductivity, possible structural reconstruction, incomplete active-site utilization, stability concerns, and challenges associated with scalable synthesis. Future research should therefore focus on rational control of electronic structure and defects, stabilization of active interfaces under operating conditions, development of conductive heterostructures, and scalable synthesis strategies. In particular, combining heteroatom doping, carbon-assisted charge transport, and operando characterization will be important for establishing clear structure–property–activity relationships and advancing Co₃O₄ toward durable and practically relevant OER applications.

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Footnotes

Edited by: Manisha Das, Japan Society for the Promotion of Science (JSPS), Japan

Reviewed by: Soumitra Payra, West Bengal Chemical Industries Limited, India

Sayan Halder, Uniwersytet im Adama Mickiewicza w Poznaniu Szkola Jezykowa, Poland