Nonconformal Particles of Hyperbranched Sulfonated Phenylated Poly(phenylene) Ionomers as Proton-Conducting Pathways in Proton Exchange Membrane Fuel Cell Catalyst Layers

Characteristic poor electrochemical kinetics, high ionic resistance, and high mass transport resistance within the catalyst layer (CL) are chief among parameters that cause poor performance of proton exchange membrane fuel cells (PEMFCs) utilizing hydrocarbon-based proton-conducting ionomers. Herein, the design and addition of nondimensionally swellable, nonconformal, hyperbranched sulfo-phenylated poly(phenylene) ionomer particles (HB-sPPT-H+) are reported to introduce a direct pathway for proton conduction in hydrocarbon ionomer-based CLs, resulting in an eight times reduction in ionic resistance of the CL, a 71% increase in catalyst mass activity, and a >90% increase in power at 0.6 V (H2/air) compared to state-of-the-art hydrocarbon ionomer-based CLs. The benefits of incorporating HB-sPPT-H+ ionomer particles are also shown when employed in perfluorosulfonic acid (PFSA) ionomer-based PEMFCs. These results dispel a commonly held conception that hydrocarbon ionomers possess limitations of gas permeability and electrochemical activity and open up previously unexplored avenues of ionomer development for nonfluorous, wholly hydrocarbon PEMFCs.