(528c) Ionic Liquid Composite Electrocatalysts for the Oxygen Reduction Reaction in PEM Fuel Cells

The cathode catalyst layer (CCL) of the proton exchange membrane fuel cell (PEMFC) cathode requires a high amount of Pt to compensate for the sluggish oxygen reduction reaction (ORR) kinetics. Reducing the Pt loading in the CCL remains the major challenge in commercializing fuel cell devices. Recent developments enable up to 20-fold improvement in the activity of ORR electrocatalysts and facilitate a lower amount of Pt in the catalyst layer.¹ However, these low-Pt-loaded cathodes show significant voltage losses at high current densities (HCD).² These performance losses are associated with local oxygen transport resistances due to higher particle-particle distance and the interfacial resistance between Pt and PFSA-based ionomers.³ The interfacial resistance observed at the catalyst-ionomer interface is exacerbated with increase in the thickness of the ionomer film on catalyst particles⁴ and specific adsorp­­tion of sulfonate groups.⁵

These challenges can be potentially addressed by modifying the catalyst-ionomer interface at fuel cell cathode. Protic and hydrophobic Ionic liquids (ILs) are known to enhance the ORR performance of state-of-the-art catalysts in half-cell experiments.⁶ Our research group has previously shown that an ionic liquid interlayer on the Pt surface mitigates the specific adsorption of sulfonate groups. Despite remarkable half-cell performance of ionic liquid modified catalysts (100-150%), ionic liquid incorporated CCLs show only a modest improvement (10-15%).⁷ Inhomogeneous distribution of ionic liquids in the catalyst layer and potential loss of ionic liquids during fuel cell operation are possible reasons behind this discrepancy. The use of sulfonated polymerized ionic liquid block copolymer (SPILBCP) as an ionomer presents a novel method to incorporate ionic liquids in the catalyst layer. SPILBCP has sulfonated domains to enable proton conduction and ionic liquid domains to facilitate oxygen transport and mitigate sulfonate adsorption. In this work, we implement rotating disk electrode (RDE) techniques to analyze a sulfonated polymeric ionic liquid block copolymer (SPILBCP) as an ionomer for the PEMFC cathode. We found that Pt/C catalyst films prepared using PILBCP show two-fold improvement over those prepared using Nafion, a state-of-the-art ionomer. We use the CO displacement technique to show that PILBCP thin films exhibit a lower degree of sulfonate adsorption than Nafion on a highly ordered Pt surface. Furthermore, thin films of SPILBCP and Nafion show comparable proton conductivity and oxygen permeability. These results support the recent finding showing up to two-fold improvement in the kinetic activity of PEMFCs using SPILBCP as a catalyst binder. ⁸

References

1. Shao M, Chang Q, Dodelet JP, Chenitz R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem Rev. 2016;116(6):3594-3657. doi:10.1021/acs.chemrev.5b00462

2. Kongkanand A, Mathias MF. The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells. J Phys Chem Lett. 2016;7(7):1127-1137. doi:10.1021/acs.jpclett.6b00216

3. Weber AZ, Kusoglu A. Unexplained transport resistances for low-loaded fuel-cell catalyst layers. J Mater Chem A. 2014;2(41):17207-17211. doi:10.1039/c4ta02952f

4. Sasikumar G, Ihm JW, Ryu H. Dependence of optimum Nafion content in catalyst layer on platinum loading. J Power Sources. 2004;132(1-2):11-17. doi:10.1016/j.jpowsour.2003.12.060

5. Garrick TR, Moylan TE, Yarlagadda V, Kongkanand A. Characterizing electrolyte and platinum interface in PEM fuel cells using CO displacement. J Electrochem Soc. 2017;164(2):F60-F64. doi:10.1149/2.0551702jes

6. Snyder J, Fujita T, Chen MW, Erlebacher J. Oxygen reduction in nanoporous metal-ionic liquid composite electrocatalysts. Nat Mater. 2010;9(11):904-907. doi:10.1038/nmat2878

7. Snyder J, Livi K, Erlebacher J. Oxygen reduction reaction performance of [MTBD][beti]-encapsulated nanoporous NiPt alloy nanoparticles. Adv Funct Mater. 2013;23(44):5494-5501. doi:10.1002/adfm.201301144

8. Li Y, Cleve T Van, Sun R, et al. Modifying the Electrocatalyst-Ionomer Interface via Sulfonated Poly ( Ionic Liquid ) Block Copolymers to Enable High-Performance Polymer Electrolyte Fuel Cells. 2020.