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

Lithographically Defined Three-Dimensional Graphene Structures

A simple and facile method to fabricate 3D graphene architectures is presented. Pyrolyzed photoresist films (PPF) can easily be patterned into a variety of 2D and 3D structures. We demonstrate how prestructured PPF can be chemically converted into hollow, interconnected 3D multilayered graphene structures having pore sizes around 500 nm. Electrodes formed from these structures exhibit excellent electrochemical properties including high surface area and steady-state mass transport profiles due to a unique combination of 3D pore structure and the intrinsic advantages of electron transport in graphene, which makes this material a promising candidate for microbattery and sensing applications