(637c) Engineering the Ionomer-Substrate Interface to Address Ion Transport Limitation of Proton Exchange Membrane Fuel Cells (PEMFCs)

Renewable energy is an attractive alternate to fossil fuels because of their availability and eco-friendliness. Proton-exchange membrane fuel cell (PEMFC), a renewable energy technology, utilizes hydrogen to produce electricity. In a PEMFC, an ion-conducting polymer (ionomer) acts both as a solid polymer electrolyte-based separator and a catalyst binder to electrodes with the former being tens of micrometer thick, while the latter exists as nanometer thin-films. In sub-µm-thick films, proton conductivity suffers due to interfacial constraints which also results in poor phase segregation, smaller and ill-connected ionic domains, and even chain stiffness. Poor ion transport results in sluggish oxygen reduction reaction (ORR) kinetics and requires high platinum catalyst loading at PEMFC cathodes. Several reports indicated that in films thinner than a micron, ionomer chains often align parallel to the substrate and interact with the substrate/catalyst. Such interactions are unfavorable for ionic conduction and gas transport within catalyst layer. To alleviate this issue, in this work, we engineered the ionomer-substrate interface by covalently immobilizing aminosilanes on electrodes and model substrates. We hypothesized that by engineering the substrate with silane, a possible disruption of the lamellar orientation of the ionomer chains next to the substrate or electrode interface and a reduced pinning of the ionomer chains can be achieved. Upon doing so, we observed that the interfacial proton conductivity of the thin films improved when the ionomer layers were coated over the engineered electrode surfaces. Grazing incidence small angle X-ray scattering (GISAXS) experiments revealed a reorganization of ionic domains within the ionomer layer interfacing silane monolayer on the electrode surface such that it favored proton transport, while we also observed a significant decrease in the storage modulus of the films on the engineered electrodes suggesting reduced chain pinning. Such an interfacial engineering approach stands unique as how ionomer interfacial phenomena and constraints can be mitigated are rarely revealed in such level of details. Overall, this work creates an avenue to systematically engineer and identify the optimal interfacial physical/chemical parameters that will be beneficial to selectively position ionomer chains at interfaces to improve ionic conductivity and address ion transport limitation of PEMFCs.