Physical modeling of local reaction conditions inside of cathode catalyst layer of polymer electrolyte fuel cells

The foremost practical objective in research on polymer electrolyte fuel cells is to design catalyst layers with high performance at markedly reduced platinum loading. The overarching goal is thus to enhance the effectiveness factor of platinum utilization inside the cathode catalyst layer. This requires design modifications in fuel cell components, understanding of local reaction conditions inside the cathode catalyst layer, accounting for the impact of surface charging phenomena at pore walls on catalyst activity, as well as understanding water distribution and fluxes in porous electrode media and how the water balance affects all the aforementioned effects. As a contribution towards this objective, this thesis presents models to understand the local reaction conditions inside the cathode catalyst layer. This refers to rationalizing the oxygen and proton density in the cathode catalyst layer from the macroscopic level to the nanopore level. This work has been divided into three parts. The first part focuses on understanding of surface charging phenomena and catalytic activity in water-filled pores that are bounded on one side by an ionomer-skin layer. The model-based analysis reveals that the density of charged side chains at the ionomer shell exerts a pronounced impact on the surface charge density at the Pt surface and thereby on the activity of the pore for the oxygen reduction reaction. In the second part, we employed physical models of catalyst layer operation to analyze large sets of experimental performance data of fuel cells with gradually decreased Pt loading. The analysis reveals systematic variations in physical properties of cathode catalyst layers with Pt loading that can be consistently explained with a variation in the fuel cell water balance. A correlation exponent was introduced, which can be used to assess the design of a catalyst layer in terms of the propensity to flooding. The last part serves the need for a comprehensive water balance model as revealed by research described in the previous paragraph. We present a basic 1D +1D model to rationalize variations in water distribution and water fluxes in catalyst layers, diffusion media, and flow fields in response to changes in structure, composition and operating conditions. The model-based analysis consistently reproduces major trends in performance upon a systematic reduction in Pt loading. The tools and analyses provided in this thesis could thus inform strategies for minimizing the Pt loading without running into the water trap.