Current state-of-the-art fuel cell cathodes rely on high loading of platinum (Pt) or Pt-alloy on high surface area carbon supports mixed with Nafion. The cost of the high Pt loading is still a hindrance to the commercialization of polymer electrolyte fuel cells (PEFCs) ¹. The electrode structure consists of a random aggregation of the functional components (catalyst, ionomer, and pore space) 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 a higher loading and active area of Pt to maintain an acceptable level of performance. The ionomer binder adds an additional transport resistance and becomes significant at lower Pt loadings². Decreasing this transport resistance would remove a significant barrier to low-cost, ultra-high power density fuel cells. Rational design of the electrode structure in PEFCs could improve performance and reduce cost.

However, in current state-of-the-art fuel cells, each unit volume of electrode contains a random mix of components that must simultaneously provide catalytic activity, water transport, O₂transport, electronic conductivity, and protonic conductivity. Since each unit must provide all functions, units cannot be optimized for individual functions. By separating the different electrode functions into discrete electrode elements, each element can be optimized for specific functions as shown in Figure 1a. Arranging these optimized discrete elements in a controlled, low-tortuosity configuration enables transport limitations to be reduced or eliminated. In previous work, Middlemen et al. showed under optimal conditions oriented catalyst layer structure can be fabricated but didn’t present any electrochemical evaluation ³. Komini Babu et al. fabricated vertically oriented Nafion nanofibers and deposited Pt via vapor deposition ⁴. The Nafion nanofiber electrode showed increase in performance compared to Pt sputtered on Nafion 115 electrode but lower in performance to Pt/C based electrode.

In this work, we propose an alternate electrode structure to enable good transport behavior and high cell performance through controlled deposition of electrode components in an optimized, organized structural configuration. Figure 1b shows the SEM image of Nafion nanofibers for proton transport, fabricated by solution casting on a porous template. Providing effective proton transport through these low-tortuosity percolating highways allows the catalyst domain to have a lower ionomer/catalyst ratio, reducing transport resistance.

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.

References

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