Investigation of activity of sputtered transition-metal (TM)-C-N (TM = V, Cr, Mn, Co, Ni) catalysts for oxygen reduction reaction

Variations with heat-treatment temperature of the structure and the electrocatalytic activity of sputtered transition-metal (TM)-C-N (TM = V, Cr, Mn, Co, and Ni ) films for the oxygen reduction reaction (ORR) in acid and alkaline electrolytes were studied. The films prepared were all originally amorphous. At a critical heat-treatment temperature (T-c, different for each composition) substantial nitrogen release occurred and the films transformed to a heterogeneous N-containing carbon structure with either Cr3C2, Co, Ni, V8C7, or Mn7C3, depending on the TM used. In acid electrolyte, heat- treated CrxC1-x-yNy, CoxC1-x-yNy, and NixC1-x-yNy films began to show oxygen reduction activity at T-c, while VxC1-x-yNy and MnxC1-x-yNy films showed little or no activity for ORR in acid electrolyte for all temperatures studied. However, all of the heat- treated TMxC1-x-yNy (TM = V, Cr, Mn, Co, and Ni) films showed oxygen reduction activity in alkaline electrolyte even when heated to temperatures below T-c. Because all the TMxC1-x-yNy films heated above T-c contained N-containing carbon, the activity of heat-treated sputtered C1-xNx and C films, as well as graphite powder, were studied for comparison. Their activities were much lower in acid electrolyte than those of the CrxC1-x-yNy, CoxC1-x-yNy, and NixC1-x-yNy films heat-treated at the same temperatures. C1-xNx films, C films, and graphite powder all showed activity in alkaline electrolyte but were less active than some heated CoxC1-x-yNy and NixC1-x-yNy films. The choice of TM and the heat-treatment temperature played important roles in determining the activity of the sputtered TMxC1-x-yNy films for ORR in acid and alkaline electrolytes. The corrosion stability of the heat-treated TMxC1-x-yNy libraries was studied as well. Heat-treated CrxC1-x-yNy libraries showed good passivation against corrosion. (c) 2007 The Electrochemical Society.