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.
Nanostructured Ni(x)Co(3-x)O4 films serve as effective electrocatalysts for both the oxygen reduction and oxygen evolution reactions in alkaline electrolyte.
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