The role of Cu-ion doping in α-MnO2 electrocatalysts for the oxygen reduction reaction in alkaline electrolyte was investigated. Cu-doped α-MnO2 nanowires (Cu-α-MnO2) were prepared with varying amounts (up to ∼3%) of Cu2+ using a hydrothermal method. The electrocatalytic data indicate that Cu-α-MnO2 nanowires have up to 74% higher terminal current densities, 2.5 times enhanced kinetic rate constants, and 66% lower charge transfer resistances that trend with Cu content, exceeding values attained by α-MnO2 alone. The observed improvement in catalytic behavior correlates with an increase in Mn3+ content at the surface of the Cu-α-MnO2 nanowires. The Mn3+/Mn4+ couple is the mediator for the rate-limiting redox-driven O2/OH– exchange. O2 adsorbs via an axial site (the eg orbital on the Mn3+ d4 ion) at the surface or at edge defects of the nanowire, and the increase in covalent nature of the nanowire with Cu-ion doping leads to stabilization of O2 adsorbates and faster rates of reduction. A smaller crystallite size (roughly half) for Cu-α-MnO2 leading to a higher density of (catalytic) edge defect sites was also observed. This work is applicable to other manganese oxide electrocatalysts and shows for the first time there is a correlation for manganese oxides between electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline electrolyte and an increase in Mn3+ character at the surface of the oxide.

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.

Graphene-like carbon-Ni-α-MnO(2) and -Cu-α-MnO(2) blends can serve as effective catalysts for the oxygen reduction reaction with activities comparable to Pt/C.

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.