Phosphate-Tolerant Oxygen Reduction Catalysts

Increased oxygen reduction reaction (ORR) kinetics, improved CO tolerance, and more efficient water and heat management represent significant advantages that high-temperature polymer electrolyte fuel cells (HT-PEFCs) operating with a phosphoric acid-doped polybenzimidazole (PBI) membrane offer over traditional Nafion-based, low-temperature PEFCs. However, before such HT-PEFCs become viable, the detrimental effect of phosphate chemisorption on the performance of state-of-the-art Pt-based cathode catalysts needs to be addressed. In this study, we propose a solution to the severe poisoning of Pt-based PEFC cathode catalysts with phosphates (H2PO4 – and HPO4 2–) by replacing standard Pt/C catalysts with phosphate-tolerant, nonprecious metal catalyst (NPMC) formulations. Catalysts with a very high surface area (845 m2 g–1) were synthesized in this work from polyaniline (PANI), iron, and carbon using a high-temperature approach. The effects of metal precursors and metal loading on the morphology, structure, and ORR activity of the NPMCs were systematically studied. Electrochemical measurements indicated that as-prepared Fe-based catalysts (PANI-Fe-C) can tolerate phosphate ions at high concentrations and deliver ORR performance in 5.0 M H3PO4 that is superior to that of Pt/C catalysts. A 30 wt % Fe-derived catalyst was found to have the most porous morphology and the highest surface area among studied Fe-based catalysts, which correlates with the highest ORR activity of that catalyst. These cost-effective and well-performing ORR catalysts can potentially replace Pt/C catalysts in phosphoric acid-based HT-PEFCs.