Carbon-Supported Manganese Oxide Nanoparticles as Electrocatalysts for the Oxygen Reduction Reaction (ORR) in Alkaline Medium:  Physical Characterizations and ORR Mechanism

MnOx/C and Me-MnOx/C (Me = Ni, Mg) electrocatalysts prepared by chemical deposition of manganese oxide nanoparticles on carbon have been characterized by Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and chemical analysis. Their Oxygen Reduction Reaction (ORR) kinetics and mechanism have been investigated in alkaline KOH solutions by using the Rotating Disk Electrode (RDE) and the Rotating Ring-Disk Electrode (RRDE) setups. Doping the MnOx/C nanoparticles with nickel or magnesium divalent cations can considerably improve their oxygen reduction activity. As a result, the Me-MnOx/C electrocatalysts exhibit ORR specific or mass activities close to the benchmark 10 wt % Pt/C from E-TEK. At low ORR current densities, the undoped MnOx/C electrocatalyst displays a reaction order with respect to PO2 and OH- of 1 and −0.5, respectively, while ∂E/∂log i is ca. −59 mV dec-1. The ORR reaction order toward OH- is unchanged with the magnesium doping, while it becomes −2 with the nickel doping. RRDE data show that doping the MnOx/C electrocatalysts directs the ORR toward the four-electron pathway. The first electrochemical step of the 4-electron ORR mechanism is probably the quasiequilibrium proton insertion process into MnO2 leading to MnOOH, while the second electron transfer, consisting of the O2,ads species electrosplitting, yielding Oads and hydroxide anion, is rate determining. The presence of the doping metal cations may stabilize the intermediate MnIII/MnIV species, which assist this second charge transfer to oxygen adatoms. As a result, the ORR rate is enhanced for the Me-MnOx/C electrocatalysts: they exhibit remarkable ORR catalytic activity and yield quantitative formation of OH- (selectivity toward the 4-electron pathway).