Pt is a widely used catalyst for fuel cells in industry, but the irreversibility of the oxygen reduction reaction (ORR) restricts its efficiency and incurs about 40% of all irreversible energy losses¹. Furthermore, the dissolution of Pt degrades the catalyst and limits its lifetime. Pt oxide is known to play a role in promoting the dissolution² and is thought to degrade the performance of the ORR. Even though studies have been done for many years, the details of the oxide structure and the mechanism of oxide formation and reduction are still not fully resolved. In the case of the kinetics, most work has been directed at the oxide formation, and the reduction kinetics have been relatively neglected. We report here kinetic studies on the Pt oxide reduction on polycrystalline Pt by cyclic voltammetry and various potential programs that combine sweep and hold periods. Differential-equation-based models were investigated to simulate the oxide reduction and estimate kinetic parameters. 

The simplest sweep-hold method involves preparing the surface with potential sweeps and then holding the potential at selected potentials. Constant potential is typically better for quantitative analysis of kinetics since the rate constants are fixed. Compared to conventional large potential steps, the sweep preparation enables the initial conditions to be closely controlled, and the double-layer charging is less. Adding different negative-going sweeps after identical sweep-hold experiments allows for study of sweep rate dependence under identical coverage and potential initial conditions, which is not the case for regular cyclic voltammetry at different sweep rates.

Most simple mechanisms are based on rate laws in which the rates and current at time t are only dependent on the coverages and potential at time t, and not on the history of how those coverages were prepared. In contrast, rates for nucleation-and-growth type mechanisms can depend on the history. Since the oxidized surface is a restructured surface, different histories may also produced different 3-D structures at the same coverage. By varying the preparation step to produce the same coverage at the same potential but with different histories, evidence was found that the reduction peak position and shape only depend on oxide coverage and potential, and not on the history.

The results are presented and modelled with a simple adsorption mechanism, which gives reasonable agreement with experiment. Refinement will use the idea that the relationship between coverage and available Pt sites may not be linear³.

References

[1] S.G. Rinaldo, W. Lee, J. Stumper and M. Eikerling, Electrocatalysis, (2014) 5:262-272.

[2] A.A. Topalov, I. Katsounaros, M. Auinger, S. Cherevko, J.C. Meier, S.O. Klemm, K.J.J Mayrhofer, Angew. Chem. Int. Ed. Engl. (2012) 51:12613–5.

[3] P.K. Dahlstrøm, D.A. Harrington and F. Seland, Electrochim. Acta, (2012) 82:550-557.