Water Phenomena in PEFC Catalyst Layers as the Origin of the Pt Loading Effect: A Modelling Study

摘要

The foremost objective of research on polymer electrolyte fuel cells is to design and fabricate catalyst layers with high performance at drastically reduced platinum loading. Achieving this objective demands an understanding of the impact of composition and porous structure on the water balance in the layer [2–4]. Experimental studies have shown a marked increase in the resistance to oxygen diffusion when the Pt content and, correspondingly, the thickness of the catalyst layer were lowered [5–7]. We employ a physical porous electrode model to explain these trends We present a water balance model to explain these trends. Figure 1 illustrates the modeling domain and the processes considered. The set of 1D continuity and flux equations is formulated and solved for the liquid water pressure, vapor pressure, oxygen pressure, liquid water flux and vapor flux. Model solutions reveal the impact of composition, porous structure, and operating conditions on the aforementioned properties. Reducing the Pt loading (i.e. reducing the catalyst layer thickness) results in lowering of the liquid to vapor interfacial area and hence the vaporization capability. Liquid water builds up at the interface of catalyst layer and diffusion medium resulting in poor oxygen diffusivity. Findings of these model should be considered in the efforts to design and fabricate advanced catalyst layers and diffusion media with improved performance.