(472a) Improved Fuel Cell Performance Incorporating a Novel Ionic Liquid-Functionalized Electrolyte

Sluggish oxygen reduction reaction (ORR) kinetics is a major source of overpotential loss in polymer electrolyte fuel cells (PEFC) requiring higher catalyst loadings (often Pt) at the cathode ultimately increasing the overall stack cost.^1,2 This challenge has motivated extensive research into developing better electrode materials (catalyst and ionomers) that exhibit high performance with low Pt loadings.^3–5 One promising approach has been the incorporation of ionic liquids (IL) into the electrode to promote ORR activity in RDE studies.⁶ Despite promising early work, the practical implementation of IL in device-level experiments has been less auspicious due to challenges of controlling IL film thickness to limit additional O₂ transport resistance and maintaining IL content over the course of fabrication and testing.⁷ To overcome such challenges, we have incorporated a novel block copolymer electrolyte⁸ with ionic liquid (methylimidazolium bis(trifluoromethylsulfonyl)imide, [MIm][TFSI]) and sulfonic acid moieties (SPILBCP) into cathode catalyst layers and evaluated its cell-level performance using an array of electrochemical techniques.⁹

In this study, we illustrate how SPILBCP significantly improves kinetic performance (nearly two-fold) by suppressing coverage of Pt surface oxidation under saturated conditions. Limited ionic (H⁺) and gas (O₂)transport of SPILBCP electrodes becomes problematic when operating at lower relative humidity. To overcome these shortcomings, membrane electrode assemblies (MEAs) with mixed SPILBCP/Nafion ionomers were developed to maintain high Pt utilization by improving the connectivity throughout the electrode. The composite formulation was able to leverage properties of both polymer chemistries to achieve superior H₂/Air performance across all potentials under both wet and dry operating conditions. Overall, our work reveals a milestone for the PEMFC community as it is the first time that specific activity (i_s^0.9V) values determined from an MEA were observed to be on par with prior RDE results for Nafion-free Pt/Vu systems.⁹ Finally, these findings not only highlight the potential of IL-functionalized ionomers for fuel cell applications, but the promise for other electrochemical energy conversion devices.

References

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