Abstract
Slow reaction rates for oxygen reduction reaction (ORR) is one of the major barriers for improving PEMFC performance. Optimizing electrode structures for enhanced transport properties can improve high current density performance of PEMFCs. Conventional porous electrodes are fabricated by coating or spraying techniques with minimal control of its micro-structure, rendering random and tortuous transport pathways (Fig. 1a left). As a result, proton transport through the ionomer and oxygen diffusion in primary and secondary pores are sub-optimal and often become a performance limiting factor depending on the cell's operation conditions. Therefore, a carefully designed electrode micro-structure should significantly improve proton and oxygen transport. In this work we developed structured electrodes (Fig. 1a right) for Platinum based PEMFCs where different components of the electrode serve a dedicated function (i.e., transport of proton or oxygen). These electrodes are highly ordered to reduce transport losses associated with tortuosity and non-percolation. In addition, the structural attributes are controlled on multiple length scales to minimize transport resistance and improve the PEMFC performance. For design optimization and to improve the fundamental understanding of the transport mechanism in the electrode, we developed a state-of-the-art multi-physics electrode model to perform parametric study. The model accurately accounts for ORR kinetics, agglomerate structure, proton transport in bulk membrane and thin film ionomer, oxygen diffusion in the pores, heat transfer, as well as vapor and liquid phase water transport in the electrode. By varying geometric and cell operating conditions, we conducted more than 12,000 simulations using COMSOL Multiphysics to identify critical electrode design parameters that enable fast reaction and improved transport phenomena. Results from our continuum scale simulation show significant performance improvement for structured electrodes compared to conventional electrodes (Fig. 1b). The performance improvement is due to the reduced proton and oxygen transport resistances. Altogether, our findings unveil structured electrodes as a new resource for efficient charge and mass transport, and as a promising material design platform towards obtaining reliable PEMFCs with enhanced energy conversion performance.
Figure 1