Free‐Standing Tunnel‐Structured MnO2 Nanorods‐Doped with Nickel and Cobalt Cations as Bifunctional Electrocatalysts for Zn–Air Batteries

Abstract

AbstractThe rational design of the electrocatalysts is paramount to alleviating the global energy and environment crisis. Despite a bright future of MnO2 shown in energy storage and conversion, the intrinsic low electrical conductivity glooms its application in electrocatalytic reactions like oxygen reduction/evolution reactions (ORR/OER). The doping strategy is applied to equip the self‐supported MnO2 with enhanced ORR/OER and zinc‐air battery performance. In this work, a class of free‐standing MnO2 nanorods arrays on carbon paper‐doped with either cobalt or nickel cations are engineered through a simple hydrothermal method. The substitutional doping by Co or Ni that partly replaces the Mn ions in the [MnO6] octahedra brings about the Jahn–Teller distortion that exhibits excellent catalytic performance for ORR and OER. Indeed, the ORR performance reveals that the doping resulted in more positive half‐wave potential (by >20 mV), higher limiting current densities, and an electron transfer number close to four. As for the OER, the doping not only decreases the overpotential at 10 mA cm−2 but also brings about an enhancement in the current density at 1.76 V six times greater than with the undoped MnO2 catalyst. An optimal concentration of 0.25 in the molar ratio Co/Mn or Ni/Mn is discovered based on the ORR/OER bifunctionality. Homemade rechargeable Zn–air aqueous batteries assembled with doped MnO2 deliver higher peak power density, higher specific capacity, lower charge voltage, lower charge/discharge voltage, and robust stability.