**5.2 Recent exploration in Co3O4 to enhance bifunctional catalysis**

Another simple and most investigated metal oxide for bifunctional catalysis is Co3O4 due to its superior activity and durability. As mentioned earlier, Co3O4 is a spinel-type metal oxide, where Co2+ and Co3+ occupy the Td and Oh sites, respectively. The Co2+ tetrahedral sites are the active sites for ORR, and the Co3+ octahedral sites are the active sites for OER. The optimal amount of both ions would lead to overall bifunctional activity and will minimize the potential difference between the two reactions.

### **5.3 Effects of particle size and surface area**

Esswein et al. elucidated the size dependence activity of Co3O4 crystallites on electrocatalytic OER in an alkaline medium [63]. They prepared cubic Co3O4 nanoparticle materials with the average size of 5.9, 21.1 and 46.9 nm. Then, the prepared materials were loaded onto a Ni foam support to evaluate the OER performance with a constant loading amount of 1 mg cm<sup>2</sup> . They attained 10 mA cm<sup>2</sup> current density at 328, 363 and 382 mV for small (5.9 nm), medium (21.1 nm) and large (46.9 nm) sized Co3O4 particles, respectively. The activities were correlated with the surface area of the isolated Co3O4 particles.

Menezes et al. established a method to produce nanochains of cobalt oxide (Co3O4) via low-temperature degradation of cobalt oxalate dehydrate [64]. In fact, they were able to display exceptional OER performance at low overpotentials in both basic and neutral media with the as-prepared Co3O4 nanochains. They also additionally prepared nanostructured Co3O4 materials by the solvothermal method and compared the activity of commercial Co3O4 of various morphologies. Remarkably, the ORR performance of carbon-supported Co3O4 nanochains displays remarkable activity compared to that of Pt. Surprisingly, they found that even though the nanochain Co3O4 prepared by the reverse micelle route possesses a lower BET surface area (12 m<sup>2</sup> g<sup>1</sup> ) than solvothermal Co3O4 (18 m<sup>2</sup> g<sup>1</sup> ) and commercial Co3O4 (49.4 m<sup>2</sup> g<sup>1</sup> ), it displays a low overpotential toward OER in both alkaline and neutral media.

#### **5.4 Effect of mesoporosity and morphology**

Sa et al. reported ordered mesoporous Co3O4 spinels with a gyroid mesostructure, obtained by the assistance of a KIT-6 mesoporous silica template through the nanocasting method, which emerged as one of the best methods to obtain ordered mesoporous structured materials. Particularly, silica supports such as KIT-6, which possess a double gyroid mesostructure, can deliver large active sites than that of analogous materials with a 2D hexagonal structure [65]. Furthermore, the ordered mesoporous Co3O4 spinels template from KIT-6, with a highly interconnected

network structure, is expected to show enhanced stability under harsh catalytic or electrocatalytic reaction conditions.

#### **5.5 Effect of the nature of hydrous oxide**

Zhan et al. demonstrated the bifunctional activity of hydrothermally prepared Co (OH)2 hexagonal nanoplates and cobalt oxides (CoO and Co3O4) in basic medium, where the hydroxide of cobalt (Co(OH)2) displayed a superior activity than the oxides of cobalt (CoO and Co3O4) [66]. The bifunctional catalysis ability mostly shown by the OER/ORR potential difference (ΔE) could be achieved as lower as 0.87 V (RHE), comparable to that of metal-based catalysts, when the Co(OH)2 nanoplates were anchored on N-doped reduced graphene oxide. In addition, both Co3O4 and CoO possess a lower *n* value of 2.6–2.7 for ORR, which testified that pristine cobalt oxide catalysts predominantly follow a two-electron pathway of ORR. Co(OH)2 was therefore highly ORR active than other cobalt catalysts. Similarly, in the case of ORR, Co (OH)2 was most OER active than Co3O4 and CoO even though it has a lower number of cobalt site and surface area. Therefore, it is understood that the hydroxide of cobalt is a better bifunctional catalyst than the oxides of cobalt.

#### **5.6 Existing challenges and future directions**

After the detailed review of each stairway of non-precious metal oxide (i.e., MnO2, Co3O4), it is absolutely necessary to address the existing challenges and future working directions that could be highly supportive for the research community for finding more insightful information on the catalysts and their further development to enhance the bifunctional activity.

The exact mechanisms and underlying fundamental process of the oxygen electrode reactions are still unclear; they vary from material to material. There was a drastic difference at the electrode electrolyte interface of each electrocatalyst, and it possessed different binding energies with the reactant species. So it is highly recommended to adopt sophisticated in situ spectroscopic techniques to study the interface during the electrochemical reaction so that its exact mechanism could be identified. Along with this, the theoretic prediction (DFT calculation) of interface especially for the possible way of reactant species adsorption and desorption has to be developed. There is another study called post-experimental analysis that is the analysis of the catalyst after the employment of electrochemical reaction for a certain period. The deeper analysis and its comparison with its fresh nature provides more insightful information about the moiety, which is the exact reason for the catalytic activity. And finally, the collective analysis and interpretation of all these, that is, in situ spectroscopic study and theoretical prediction of the interface and post-experimental analysis, will lead to a loophole for the identification of the exact mechanism of the oxygen electrode reactions for the particular catalyst. The understanding of underlying mechanisms is the heart of knowledge, which is the key to trigger the researcher for the establishment of marvelous candidates for the oxygen electrode reactions.

Now the question is why the pinpoint mechanism of oxygen electrode reactions is not yet strongly declared even after knowing the route to identify it. The reason is that the existing issues in the aforementioned sequence of the route are (1) insufficient knowledge in theoretical prediction and (2) lack of the appropriate in situ spectroscopic technique and its integration into the electrochemical setup. Hence, it has been suggested that more efforts have to be devoted in the field of experimental

characterization, and theoretical study leads to better fundamental understanding of electrochemical reactions. In addition, the integration of various characterization techniques into the electrochemical setup is also recommended to track the reaction.

If we look at the Co3O4 catalyst, it has a multi-oxidation state (Co2+ and Co3+) compound, where there is still no clear-cut idea about the optimum level of Co2+/Co3+ ratio for better performance of bifunctional activity. Moreover, there are some reports of Co3O4 highlighting that Co2+ and Co3+ are responsible for ORR and ORR, respectively. In recent years, some researchers have reported the exact opposite trend of responsibility in bifunctional reaction (ORR/ORR) with strong evidence, which creates more puzzles. In addition, Co3O4 is a good catalyst for OER but not for ORR; the intentional tuning on it for the improvement of ORR activity affects the OER performance. Therefore, it is recommended that a more in-depth study of this material has to be explored well rather than its advanced study.

In the case of MnO2, it is a good candidate for ORR (can follow the four-electron transfer mechanism), but it is not so for OER. As stated earlier, the intentional tuning of one side of the reaction drastically affects the other side of the reaction. Hence, such a way of fine tuning the material for the catalytic enhancement of OER activity without affecting its ORR performance is highly desirable. Therefore, the existing challenges strongly direct the researcher to explore things in the area of understanding the mechanism of ORR/OER, tuning the composition of the existing atom of the catalyst to obtain optimum composition and development of novel synthetic approaches for selectively attaining certain properties of the catalyst.
