Potential dependence of composition and structure of electrodeposited Fe–Pt films

Composition and structure of electrodeposited Fe–Pt–O films are investigated in dependence on deposition potential. At potentials positive to the hydrogen evolution, a solid solution of Pt with some Fe is present that can be explained by underpotential deposition of Fe. In the potential region where hydrogen evolution by proton reduction is a side reaction, oxygen is detected in the films. The O content scales with the Fe content and at least part of the Fe is oxidized. When applying more cathodic potentials, the fraction of Fe and O increases, grain size is reduced and an amorphous/nanocrystalline phase forms. The Fe content is pH dependent in this potential region. At even more cathodic potentials, overpotential deposition of Fe in the bcc structure sets in, the O content is reduced. The results are discussed with respect to thermodynamics in the Fe–Pt–(O) system. The concepts of underpotential deposition of Fe during Fe-Pt alloy deposition and of hydroxide formation due to a pH rise at the cathode are evaluated. These simple models can only partially describe the deposition process in agreement with the compositional and structural information obtained and a coupled mechanism is proposed.