Ionomer distribution control via thiophene S-modification of carbon support for high-power proton exchange membrane fuel cells

Achieving a significant reduction in the cost of proton exchange membrane fuel cells (PEMFCs) is an imminent goal, and decreasing the overall platinum loading in the membrane electrode assembly (MEA) is regarded as a critical means to achieve this objective. However, as the Pt loading decreases, the MEA performance drops sharply, which is mainly impeded by the mass transport resistance at the triple phase boundaries (TPBs) for O₂ and H⁺ to reach the catalyst active site through the ionomer. In this research, we propose an effective strategy to achieve precise control of the three-phase microenvironment within catalyst layer by introducing thiophene sulfur (C-S-C) species onto the surface of the carbon support. FIB/slice-STEM EDX mapping (STEM-EDX mapping of focused ion beam-processed and microtome-sliced samples) and molecular dynamics (MD) simulation clearly demonstrate that the interaction between thiophene sulfur and the sulfonic group on the ionomer's side chains enhances the uniformity of the ionomer distribution, thereby promoting the formation of ideal TPBs. Consequently, the local oxygen transport resistance in the catalytic layer is reduced from 0.236 s cm^-1 to 0.047 s cm^-1. This improvement achieves a high rated power density of 1.06 W cm^-2@0.67 V and a Pt utilization efficiency of 8.47 W mg_PGM^-1, with Pt loading as low as 0.125 mg_Pt cm^-2. These findings exceed the technology targets set by the United States Department of Energy (DOE).