State-of-the-art fuel cell catalyst layers consist of a random mixture of functional components (catalyst, ionomer, and pore) that are formed in an uncontrolled ink deposition process. The random nature of these structures makes it difficult to optimize the functional domains and hence causes severe mass transport limitations during high-power operation, resulting in a loss in performance and requiring that the fuel cell be oversized to maintain an acceptable level of performance. The ionomer binder adds an additional transport resistance and becomes significant at lower Pt loadings [1]. Decreasing this transport resistance would remove the main barrier to low-cost, ultra-high power density fuel cells. Rational design of the electrode structure in PEFCs could improve performance and reduce cost. By separating the different electrode functions into discrete electrode elements, each element can be optimized for specific functions. Arranging these optimized discrete elements in a controlled, low-tortuosity array configuration enables transport limitations to be reduced or eliminated.

We have demonstrated this approach through fabrication of freestanding arrays of vertically-oriented ionomer channels with different aspect ratios, and incorporation of these channels into fuel cell electrodes where they serve as non-tortuous proton-transport highways (Figure 1). Providing effective proton transport through these low-tortuosity percolating highways allows the catalyst domain to have a lower ionomer/catalyst ratio, reducing transport resistance. Proof of concept was demonstrated by achieving a 15% increase in performance with channels with an aspect ratio of 8. Further work is underway to increase channel aspect ratio and improve integration of channels with surrounding catalyst layer, leading to further transport improvements and increases in performance.

Fig. 1. Left: freestanding array of ionomer channels. Right: catalyst layer incorporating ionomer channels.

References

1. N. Nonoyama, S. Okazaki, A. Z. Weber, Y. Ikogi, and T. Yoshida, J. Electrochem. Soc., 158, B416 (2011).

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium.