Understanding the Effects of Cationic Dopants on α‑MnO₂ Oxygen Reduction Reaction Electrocatalysis

Nickel-doped α-MnO₂ nanowires (Ni−α-MnO₂) were prepared with 3.4% or 4.9% Ni using a hydrothermal method. A comparison of the electrocatalytic data for the oxygen reduction reaction (ORR) in alkaline electrolyte versus that obtained with α-MnO₂ or Cu−α-MnO₂ is provided. In general, Ni-α-MnO₂ (e.g., Ni-4.9%) had higher <i>n</i> values (<i>n</i> = 3.6), faster kinetics (<i>k</i> = 0.015 cm s^–1), and lower charge transfer resistance (<i>R</i>_CT = 2264 Ω at half-wave) values than MnO₂ (<i>n</i> = 3.0, <i>k</i> = 0.006 cm s^–1, <i>R</i>_CT = 6104 Ω at half-wave) or Cu–α-MnO₂ (Cu-2.9%, <i>n</i> = 3.5, <i>k</i> = 0.015 cm s^–1, <i>R</i>_CT = 3412 Ω at half-wave), and the overall activity for Ni−α-MnO₂ trended with increasing Ni content, i.e., Ni-4.9% > Ni-3.4%. As observed for Cu−α-MnO₂, the increase in ORR activity correlates with the amount of Mn³⁺ at the surface of the Ni−α-MnO₂ nanowire. Examining the activity for both Ni−α-MnO₂ and Cu−α-MnO₂ materials indicates that the Mn³⁺ at the surface of the electrocatalysts dictates the activity trends within the overall series. Single nanowire resistance measurements conducted on 47 nanowire devices (15 of α-MnO₂, 16 of Cu−α-MnO₂-2.9%, and 16 of Ni−α-MnO₂-4.9%) demonstrated that Cu-doping leads to a slightly lower resistance value than Ni-doping, although both were considerably improved relative to the undoped α-MnO₂. The data also suggest that the ORR charge transfer resistance value, as determined by electrochemical impedance spectroscopy, is a better indicator of the cation-doping effect on ORR catalysis than the electrical resistance of the nanowire