Nanoscale Carbon Modified α-MnO2 Nanowires: Highly Active and Stable Oxygen Reduction Electrocatalysts with Low Carbon Content.

ABSTRACT

Carbon-coated α-MnO2 nanowires (C-MnO2 NWs) were prepared from α-MnO2 NWs by a two-step sucrose coating and pyrolysis method. This method resulted in the formation of a thin, porous, low mass-percentage amorphous carbon coating (<5 nm, ≤1.2 wt % C) on the nanowire with an increase in single-nanowire electronic conductivity of roughly 5 orders of magnitude (α-MnO2, 3.2 × 10-6 S cm-1; C-MnO2, 0.52 S cm-1) and an increase in surface Mn3+ (average oxidation state α-MnO2, 3.88; C-MnO2, 3.66) while suppressing a phase change to Mn3O4 at high temperature. The enhanced physical and electronic properties of the C-MnO2 NWs-enriched surface Mn3+ and high conductivity-are manifested in the electrocatalytic activity toward the oxygen reduction reaction (ORR), where a 13-fold increase in specific activity (α-MnO2, 0.13 A m-2; C-MnO2, 1.70 A m-2) and 6-fold decrease in charge transfer resistance (α-MnO2, 6.2 kΩ; C-MnO2, 0.9 kΩ) were observed relative to the precursor α-MnO2 NWs. The C-MnO2 NWs, composed of ∼99 wt % MnO2 and ∼1 wt % carbon coating, also demonstrated an ORR onset potential within 20 mV of commercial 20% Pt/C and a chronoamperometric current/stability equal to or greater than 20% Pt/C at high overpotential (0.4 V vs RHE) and high temperature (60 °C) with no additional conductive carbon.