Irreversible structural dynamics on the surface of bimetallic PtNi alloy catalyst under alternating oxidizing and reducing environments

In this work changes in surface chemistry and morphology of PtNi alloy induced by redox environments were investigated by photoelectron spectroscopy, atomic force microscopy and substantiated by theoretical calculations. PtNi was exposed to alternating oxidation (O2) and reduction (H2) gaseous atmospheres at different temperatures (298, 373, 523 K) to simulate its behavior as a cathode catalyst in proton exchange membrane fuel cells. Results showed that the PtNi alloy undergoes surface nickel enrichment under switched reactive environments. The most significant effects occurred at 523 K, when the Ni/Pt surface atomic ratio increased from 0.8 to 6.7 after five redox cycles, while the bulk Ni/Pt value reached 2.3. Along with compositional changes catalyst coarsening was observed. The revealed behavior of the PtNi alloy allows a better understanding of the structural dynamics of a bimetallic catalyst during its interaction with reactive environments that is a prerequisite for development of efficient fuel cell catalyst.