Fabrication, morphology, and performance of fiber mat cathode MEAs with a MOF-based PGM-free catalyst

Fe–N–C platinum group metal (PGM)-free catalysts were manufactured via a metal-organic-framework (MOF) precursor by Pajarito Powder, LLC. The MOF catalyst was then incorporated into electrospun fiber mat or powder cathode membrane-electrode-assemblies (MEAs) with a Nafion 211 membrane and a Pt/C powder anode. Fiber mat and powder cathodes were examined with binders at different Nafion to polyvinylidene fluoride (PVDF) weight ratios, where the results suggest that the presence of hydrophobic PVDF helped to drive product water and hydrogen peroxide away from the surface of catalyst particles and out of the cathode, thus improving long-term catalyst stability. MEA durability was evaluated by constant voltage hold experiments at 0.3 V, 0.5 V, or 0.7 V. An unusual up-and-down swing in power density was observed during the initial stages of a 0.5 V hold experiment. The power variation was attributed to two oxygen reduction reaction active sites coexisting on MOF catalyst particles, where high-activity/unstable sites produced high power initially, but deactivated quickly with a precipitous drop in power density during the initial ca. 15 h of a constant voltage hold experiments. At the same time, there was slow activation of lower-activity but more stable sites, resulting in long-term constant power output. For a fiber cathode MEA with MOF catalyst, a stable power density of 112 mW cm⁻² was achieved at 0.5 V when the Nafion/PVDF weight ratio was either 80/20 or 83/17.