Role of Cu-Ion Doping in Cu-α-MnO2 Nanowire Electrocatalysts for the Oxygen Reduction Reaction

The role of Cu-ion doping in α-MnO 2 electrocatalysts for the oxygen reduction reaction in alkaline electrolyte was investigated. Cu-doped α-MnO 2 nanowires (Cu-α-MnO 2 ) were prepared with varying amounts (up to ∼3%) of Cu 2+ using a hydrothermal method. The electrocatalytic data indicate that Cu-α-MnO 2 nanowires have up to 74% higher terminal current densities, 2.5 times enhanced kinetic rate constants, and 66% lower charge transfer resistances that trend with Cu content, exceeding values attained by α-MnO 2 alone. The observed improvement in catalytic behavior correlates with an increase in Mn 3+ content at the surface of the Cu-α-MnO 2 nanowires. The Mn 3+ /Mn 4+ couple is the mediator for the rate-limiting redox-driven O 2 /OH – exchange. O 2 adsorbs via an axial site (the e g orbital on the Mn 3+ d 4 ion) at the surface or at edge defects of the nanowire, and the increase in covalent nature of the nanowire with Cu-ion doping leads to stabilization of O 2 adsorbates and faster rates of reduction. A smaller crystallite size (roughly half) for Cu-α-MnO 2 leading to a higher density of (catalytic) edge defect sites was also observed. This work is applicable to other manganese oxide electrocatalysts and shows for the first time there is a correlation for manganese oxides between electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline electrolyte and an increase in Mn 3+ character at the surface of the oxide.