Insights into Electrochemical Behavior of Manganese Oxides in Catalyzing the Oxygen Reduction and Evolution Reactions and the Effect of Operation Conditions

To address the needs for clean energy, the regenerative fuel cell (RFC) and rechargeable metal-air batteries are considered as very promising approaches. Notwithstanding the abundant investigation in the past decades, their viability is still limited by the sluggish oxygen reactions on the oxygen electrode and the scarcity of conventional noble metal electrocatalysts (e.g. Pt, Ir, Ru, and their oxides) for the oxygen reduction (ORR) and oxygen evolution reactions (OER). For enhancing the bifunctional catalytic activity on the oxygen electrode, manganese oxides-based materials are under the spotlight due to their bifunctional electrocatalysis performance for both fuel cell (FC) and water electrolysis (WE) working modes in alkaline media at a low cost. However, the multivalent and diverse polymorphism of MnOx can lead to a series of complex electrochemical reactions that present different oxygen catalytic activities towards ORR and OER. To date, only a limited amount of study has been done on ultrafine manganese-based core-shell bifunctional oxygen catalysts and there remains the insufficient understanding of the ORR/OER catalysis mechanism as well as the redox pathway of the Mn sites. To address the knowledge gap in the literature, we have investigated the oxygen catalysis bifunctionality and the Mn site electrochemical behavior of Mn/Mn3O4 core-shell structural nanomaterial, and compared this to the commonly used β-MnO2, γ-MnO2 commercial catalysts in 5 M potassium hydroxide electrolyte. The crystalline structures of these commercial samples were characterized by X-ray diffraction (XRD). It should be noted that in the case of the nano core-shell structural sample, besides the diffraction from the Mn (core) and the Mn3O4 (shell) planes, the diffraction pattern also possesses intensive ramsdellite diffraction peaks. Its surface defects (oxygen vacancies), amorphous shell structure and hybrid Mn oxidation states lead to a facilitated potassium uptake in the MnOx polyhedrons1, suppression of Mn dissolution by the K-MnOx bonding structure reinforcement2, and higher oxygen adsorption capacity3 with an enlarged surface adsorption energy4. With the help of cyclic voltammetry of different types of manganese oxide, we have uncovered a series of electrochemical and chemical reactions involved with the change of Mn oxidation states. The difference in the crystallographic structure between different samples is revealed in their electrochemical response (Fig. 1 and Fig. 2). In terms of potassium uptake, it is favoured in ramsdellite crystalline through their wide 1 X 2 tunnels and in the Mn3O4 amorphous area by absorption instead of the narrow 1 X 1 tunnels in pyrolusite crystalline. This crystallographic dissimilarity results in different Mn site oxidation and reduction pathways. After initial cycling, some parallel behaviors among these oxides can be observed. In addition, we also investigated the effect of the operating voltage range. In comparison to the ORR or OER operating mode, when the voltage range was extended to cycle between the ORR and OER potential range, the Mn/Mn3O4 electrodes delivered enhanced O2 and HO2 - reduction activities along with boosted OER performance. With respect to O2 in the electrolyte, the dissolved O2 not only acts as a reactant for the ORR reaction but also influences the OER activity. Notwithstanding the oxygen molecule may block the active sites on the bulk catalysts (e.g., β-MnO2 and γ-MnO2) and thereby diminish their OER capability by half, the Mn/Mn3O4 shows double the OER current density in O2 saturated 5 M KOH electrolyte. These interesting results require a better understanding of the Mn site oxidation and reduction pathways along with oxygen reaction catalysis. The influence of operational conditions should also be considered in the protocol for bifunctional catalytic activity assessment. Among the investigated samples, the structural crystalline ramsdellite shows better electrochemical behavior and performance in comparison to the pyrolusite material. In particular, the core-shell structural Mn/Mn3O4 offers a potential approach to meet the needs of the practical reversible oxygen reactions in the RFC without losing OER catalytic activity or the need to purge the electrolyte between charge and discharge. References P. H. Benhangi, A. Alfantazi, and E. Gyenge, Electrochim. Acta, 123, 42–50 (2014). G. Fang et al., Adv. Funct. Mater., 29, 1808375 (2019). S. Yan, Y. Xue, S. Li, G. Shao, and Z. Liu, ACS Appl. Mater. Interfaces, 11, 25870–25881 (2019). B. C. Han, C. R. Miranda, and G. Ceder, Phys. Rev. B - Condens. Matter Mater. Phys., 77, 075410 (2008). Figure 1

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