Development of α-MnO2 Nanowire with Ni- and (Ni, Co)-Cation Doping as an Efficient Bifunctional Oxygen Evolution and Oxygen Reduction Reaction Catalyst

Abstract

Manganese oxides (MnO2) with nanowire morphology materials are promising candidates for improving oxygen evolution and oxygen reduction reaction (OER/ORR) performance. In this work, we developed transition metal cation doping strategy into the α-MnO2 tunnel structure to tune the Mn oxidation states and control the uniform nanowire morphology, crystalline structure to investigate the effect of doping over bifunctional activity. The single Ni2+ cation doping in α-MnO2 with various loading concentrations resulted in 8Ni−MnO2 exhibiting remarkable OER and ORR activity owing to their excessive concentration of Mn3+ and Mn4+ octahedral sites respectively. Further, Co2+ cation doping in 8Ni−MnO2 leads to an enhanced synergistic effect that significantly improves the fraction of Mn3+ quantity which is confirmed by average oxidation state. For electrochemical OER performance, 8Co−8Ni−MnO2 exhibits a potential of 1.77 V, Tafel slope value of 68 mV dec−1 and lower charge transfer resistance and it is active in ORR with more positive onset potential of 0.90 V, half-wave potential of 0.80 V, better current density (4.7 mA cm−2) and a four-electron pathway. Moreover, bifunctional activity (ΔE=EOER@10 mA cm−2 – ORR@E1/2) of 8Co−8Ni−MnO2 demonstrated 0.97 V, indicates an excellent activity in alkaline electrolyte solution.