Metal species-dependent structural formation in metal oxide-based oxygen reduction reaction catalysts synthesized via ammonia nitridation of metal polyacrylates

Abstract In this study, we investigated alternatives to platinum catalysts in proton-exchange membrane fuel cells. Our goal was to clarify the relationship between the metal species and the synthesis conditions that enable (1) active site formation through nitrogen doping of metal oxides and (2) the development of electron-conducting pathways via carbon deposition during the ammonia nitridation of metal oxide-based oxygen reduction reaction catalysts. Polyacrylic acid–metal complexes of titanium, niobium, and zirconium were used as precursors and thermally treated under ammonia or nitrogen atmospheres. The resulting catalysts were characterized to evaluate their microstructural features. Under the ammonia nitridation conditions employed in this study, the Ti-based catalyst exhibited an appropriate balance between nitrogen incorporation into titanium oxides and the formation of conductive deposited carbon pathways, whereas the Nb-based catalyst showed possible nitrogen incorporation into niobium oxides but insufficient development of conductive networks. By contrast, the Zr-based catalyst exhibited neither effective nitridation of the zirconium oxides nor adequate conductive path formation under the same baseline conditions. These results indicate that under the common baseline heat-treatment conditions, the metal species determines the extent to which active sites form and electron-conducting pathways develop. Graphical abstract