Modelling cathode catalyst degradation in polymer electrolyte fuel cells

Nano-sized Pt particles in the cathode catalyst layer of a polymer electrolyte fuel cell afford a high initial electrochemically active surface-area. However, the gain in active surface area for desired surface reactions is offset in part by enhanced rates of degradation processes that cause losses in catalyst mass, catalyst surface-area, and electrocatalytic activity. The loss of electrochemically active surface-area of the catalyst causes severe performance degradation over relevant lifetimes of polymer electrolyte fuel cells yet a consistent theoretical approach, linking experimental observations of surface-area loss related phenomena to purported mechanisms of degradation was missing. Accordingly, a dynamic model of surface-area loss and Pt mass balance phenomena based on the theories of Lifshitz, Slyozov and Wagner, and Smoluchowski is developed. It relates kinetic rates of degradation processes to the evolution of the particle-size distribution and its moments. We pursue model validation and evaluation by analyzing an extensive set of electrochemical surface-area loss experiments probing the impact of accelerated stress test control levers. Our Pt mass balance model unifies degradation characterization approaches and accordingly discriminates the predominant degradation mechanisms. The evaluation and validation approaches established a firm link between surface-area loss, Pt dissolution and Pt oxidation. As a consequence of our evaluation results, a kinetic model for Pt(111) oxide formation and reduction is developed and validated against a wide range of electrochemical, spectroscopic and theoretical work found in the relevant literature. The model provides a comprehensive picture of surface electrochemical processes that occur at Pt(111). In closing we discuss future routes of research. Foremost is the extension of cyclic voltammetry work to polycrystalline Pt and Pt nanoparticle electrodes, we suggest that these are the logical steps towards linking dynamic Pt oxidation with surface tension, Pt dissolution, surface-area loss and the oxygen reduction reaction.