PdNi thin films for hydrogen oxidation reaction and oxygen reduction reaction in alkaline media

Abstract

Pd-based catalysts are considered to be among the most promising electrocatalysts for both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in alkaline media. Although major progress in finding effective catalysts has been made, the reasons for the activity enhancement in alkaline conditions remain to be elucidated. Herein, we report the fabrication of alloyed PdNi thin films as a tool to study the HOR and ORR reactions. Annealing of physically evaporated PdNi thin films at different temperatures results in different surface compositions and a range of HOR and ORR activities in 0.1 M KOH. Moreover, annealed samples were acid treated to remove the surface Ni and the HOR and ORR were investigated to elucidate the effect of surface and subsurface Ni. For the HOR, it was found that the addition of Ni decreases the hydrogen binding energy (HBE) of Pd through an electronic effect, which results in an increase in activity. In addition, it was found that the HOR activity was further increased by the bifunctional effect induced by surface Ni, which provides OH− adsorption sites. In contrast to HOR, it was concluded that surface Ni was disadvantageous for the ORR. However, subsurface Ni was found to induce an electronic effect on Pd that resulted in a somewhat improvement of the ORR kinetics by improving its oxygen desorption. These results provide insights for more tailored design of electrocatalysts in alkaline media by shedding new light on the mechanisms through which the HOR and ORR kinetics are improved in alkaline media.