UC Irvine

The current generation of sulfo-phenylated polyphenylene (sPPP) ionomers represent a significant improvement compared to prior generations of hydrocarbon ionomers, renewing interest for their use in proton exchange membrane fuel cells (PEMFCs). However, gaps in understanding remain regarding integration as both the membrane and catalyst layer ionomer. While sPPP membranes have been shown to be more chemically robust than prior generations of hydrocarbon ionomers, the effect of operating conditions, as well as membrane failure mechanisms have not been adequately explored. To close this gap, catalyst-coated sPPP membranes were subjected to the US Department of Energy chemical durability accelerated stress test. Relative humidity (90 °C, 30% RH & 80% RH) as well as the sampling rate (every 24 hr or beginning and end of test only) were varied to deconvolute chemical and mechanical stressors. At the standard condition (30% RH) and without intermittent sampling, sPPP membranes exhibit excellent durability, surpassing the target of 500 hr by a factor of two; while at 80% RH membranes fail at 300 hours. The addition of sampling further reduces cell lifetimes by a factor of three in both cases. Ex-situ characterization, conducted by cross-section SEM and x-ray CT, shows that the membranes do not thin during testing, instead failing through cracking. Within the catalyst layer, understanding the triple phase boundary created by the reactant gases, the ion conducting phase, and catalyst surface is critical to creating PEMFCs with high activity. This work has been well documented for perfluorinated sulfonic acid ionomers; however, sPPP ionomers are relatively novel, and their ionomer-catalyst interface is not well understood. This work quantifies the adsorption of sulfonic acid moieties on the platinum surface via CO displacement and stripping. These measurements are performed across the humidity window through repeated “voltage recovery” operations to evaluate the evolution of the ionomer domain through cell life. While sulfonic acid coverage at 100% RH is consistent throughout testing, at 25% RH, coverage drops from 15% after mounting to less than 4% following two recoveries. This drop in ionomer coverage correlates with a two-thirds reduction in the current obtained at 0.7 V in H2/Air polarization curves as well as increases in the proton transport resistance by a factor of four and ten at 100% and 25% RH, respectively. These results suggest that the ionomer phase is mobile during high humidity steps, with stabilization of the ionomer phase needed to achieve both high performing and durable PEMFCs with sPPP ionomers.