Performance and Stability of Membrane–Electrode Assemblies Using a Carbon-free Connected Pt–Fe Catalyst and Polyphenylene-Based Electrolytes for Direct Formate Anion-Exchange Membrane Fuel Cells

A major challenge in direct formate anion-exchange membrane fuel cells (DF-AEMFCs) is the low chemical durability of membrane–electrode assemblies (MEAs). Here, we developed MEAs that combined polyphenylene-based electrolytes and a carbon-free cathode catalyst layer (CL). The polyphenylene-based electrolytes with a three-dimensionally twisted spirobifluorene (SBF) backbone possess excellent chemical stability. The carbon-free catalyst formed by a nanonetwork of connected Pt–Fe nanoparticles showed four–five times higher specific activity for oxygen reduction reaction than a conventional Pt/C catalyst in an alkaline electrolyte solution. The carbon-free structure in the connected Pt–Fe catalyst enhanced the durability against potential cycling. The MEA using SBF-based electrolytes and a connected Pt–Fe catalyst achieved a high power density of 219 mW cm^–2 for DF-AEMFCs through MEA testing under different conditions. Notably, the high performance was retained even after 150 h of operation at 0.2 A cm^–2 and 80 °C. Detailed structural analysis of the catalyst and polyelectrolyte materials used in the MEA indicated minor chemical degradation after long-term DF-AEMFC operation. The anode and cathode CLs were not delaminated and the membrane/CL interfaces were bonded properly after the MEA stability test. The cathode catalyst retained the connected Pt–Fe nanonetwork and hollow capsule structures. A small amount of Fe leached out from the catalyst; however, a chemically ordered fct phase was maintained in the catalyst. Cryo-transmission electron microscopy observations showed a swollen SBF-based ionomer layer with a coating thickness of ∼50 nm on the catalyst surface, which remained unchanged after the stability test. This study successfully demonstrated that carbon-free connected nanoparticle catalysts are more advantageous than Pt/C for AEMFCs and that MEAs for DF-AEMFCs with both high performance and stability can be developed, providing design guidelines for the development of advanced MEAs.