Rational Design of the Composite Perfluoro-Sulfonic Ionomer for High-Performance Proton Exchange Membrane Fuel Cells

Scaling proton exchange membrane fuel cells (PEMFCs) is constrained by high cathodic overpotential and platinum usage for the oxygen reduction reaction, challenges exacerbated by catalyst poisoning from perfluorosulfonic acid (PFSA) ionomers. Though compositing PFSA with additives can mitigate this poisoning, progress has remained largely empirical due to the lack of quantitative assessment tools. Here, we introduce an electrochemical probe leveraging ionomer-coated single-crystal Pt(111) to quantify the coverage and strength of PFSA adsorption, elucidating how cationic additives suppress Pt poisoning. We identify an inverse correlation between cation hydration energy and PFSA adsorption, guiding the rational design of a poorly hydrated tetramethylammonium-anchored covalent organic framework (TMA+-COFs) as an ionomer additive. Electrochemical and spectroscopic analyses reveal that the composite TMA+-COFs/PFSA layer significantly inhibits sulfonate adsorption and poisoning onto Pt(111) through robust electrostatic interactions, which translates to a 1.7- and 4-fold activity enhancement for industrial Pt/C in rotating-disk and gas-diffusion electrodes, respectively. We also demonstrated a high mass activity of 1.08 A mgPt-1 in a PEMFC cathode with TMA+-COFs. Our work provides an alternative avenue to conventional catalyst engineering through the rational design of advanced composite ionomers for high-performance, low-Pt PEMFCs.