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

Nickel-doped alpha-MnO2 nanowires (Ni-alpha-MnO2) were prepared with 3.4% or 4.9% Ni using a,hydrothermal method. A comparison of tile electrocatalytic data for the oxygen reduction reaction (ORR) in alkaline electrolyte, versus that obtained with alpha-MnO2 or Cu-alpha-MnO2 is provided: In general, Ni-a-MnO2 (e.g, Ni-4.9%) had higher n values (n = 3:6), faster kinetics (k = 0.015 cm s-'), and lower charge, transfer resistance (R-CT = 2264 Omega at half-wave) values than MnO2 (n = 3.0, k = 0.006 cm s(-1), R-CT = 6104 Omega at half-wave) or Cu-alpha-MnO2 (Cu-2.9%, n = 3.5, ku 0.015 cm s(-1), R-CT = 3412 Omega at half-wave), and the overall activity for Ni-a-MnO2 trended with, increasing Ni content, Ni-4.9% > Ni-3.4%. As observed for Cu-alpha-MnO2, the increase in ORR activity.correlates with the ainount of Mn at the-surface of the Ni-alpha-MnO2 nanowire. Examining, the activity for both Ni-alpha-MnO2 and Cu-alpha-MnO2-rxiaterials indicates that the Mn'+ at the surface of the electrocatalysts dictate' s the activity trends within the overall series. Single riannwir, C resistance measurements conducted on 47 nanowire devices (is of a-MnO2, 16 of Cu-alpha-MnO2-2,9%, and 16 of Ni-alpha-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 alpha-MnO2. The data also suggest that the ORR charge transfer resistance value, as deteriniridd by electrochemical impedance spectroscopy, is a better indicator--of the cation-doping effect on QRR. catalysis than the, electrical resistance of the nanowire.