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 n values (n = 3.6), faster kinetics (k = 0.015 cm s–1), and lower charge transfer resistance (R CT = 2264 Ω at half-wave) values than MnO2 (n = 3.0, k = 0.006 cm s–1, R CT = 6104 Ω at half-wave) or Cu–α-MnO2 (Cu-2.9%, n = 3.5, k = 0.015 cm s–1, R 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.