Thermal evolution of the structure and activity of magnetron-sputtered TM-C-N, TM = Fe, Co) oxygen reduction catalysts

Thin-film libraries of TMxC1-x-yNy (TM = Fe, Co; 0 < x < 0.09; 0 < y < 0.5) have been prepared by combinatorial sputter deposition. The libraries were subsequently annealed at 700-1000 degrees C to induce structural and compositional changes. Using grazing-incidence X-ray diffraction and scanning electron microscopy, structural changes were followed as a function of annealing temperature. At temperatures above 700 degrees C, the previously homogeneous and amorphous thin films became a heterogeneous mixture of (partially) graphitized nitrogen-containing carbon and either Fe3C or beta-Co. The onset of this transformation is accompanied by a rapid decrease in N content and occurs as a function of both transition metal content and temperature. Catalytic activity for oxygen reduction is at its maximum partway through this transformation. (c) 2006 The Electrochemical Society.