Sluggish oxygen reduction reaction (ORR) kinetics, even on the best available catalyst (Pt), is one of the major challenges in widespread commercialization of polymer electrolyte membrane fuel cells (PEMFC).¹ Significant advances have been made towards making highly active Pt catalysts through nanostructuring, alloying, size, and shape control to mitigate these kinetic losses.² An additional strategy for ORR activity enhancement is though the targeted engineering of the electrochemical interface. For example, Ionic liquid (IL) interlayers improve the ORR kinetics on bulk and nanostructured catalysts for both Pt and alloyed-Pt materials.³ Along with the kinetic enhancement, ionic liquid interlayers also mitigate the sulfonate adsorption on Pt and improve the durability of Pt/C thin films.^4,5

Despite remarkable improvement in performance at half-cell and MEA-level, the mechanism of this improvement is not completely understood. In this work, we combine single crystal voltammetry with microkinetic modeling to analyze the mechanism of ORR enhancement in presence of ionic liquids. With iterative feedback between single crystal voltammetry and microkinetic model output, we establish the importance of accounting for lateral interactions between adsorbed oxygenated species by using real data informed adsorption isotherms. With our experimentally validated model, we show that the mechanism of impact of ILs on ORR activity is through exclusion of water and reduction in electrochemical interface solvation, yielding lower spectator hydroxyl coverages and a reduced barrier for hydroxyl removal, resulting in higher surface site availability during the ORR. This improved understanding will help design Pt-interfaces to moderate the interaction of the interfacial water network with Pt active sites, providing another strategy for improving ORR kinetics.

References:

1. Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 486, 43–51 (2012).
2. Kulkarni, A., Siahrostami, S., Patel, A. & Nørskov, J. K. Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction. Chem. Rev. 118, 2302–2312 (2018).
3. Snyder, J., Fujita, T., Chen, M. W. & Erlebacher, J. Oxygen reduction in nanoporous metal-ionic liquid composite electrocatalysts. Nat. Mater. 9, 904–907 (2010).
4. Li, Y., Hart, J. L., Profitt, L., Intikhab, S., Chatterjee, S., Taheri, M. & Snyder, J. D. Sequential Capacitive Deposition of Ionic Liquids for Conformal Thin Film Coatings on Oxygen Reduction Reaction Electrocatalysts. ACS Catal. 9, 9311–9316 (2019).
5. Li, Y., Intikhab, S., Malkani, A., Xu, B. & Snyder, J. Ionic Liquid Additives for the Mitigation of Nafion Specific Adsorption on Platinum. ACS Catal. 10, 7691–7698 (2020).