Copper Bipyridine Polymers for Selective Electrocatalytic Oxygen Reduction to Water

Developing efficient, nonprecious metal catalysts for the oxygen reduction reaction (ORR) is critical for fuel cell technologies. In this study, we develop Cu2+ bipyridine polymers as selective ORR electrocatalysts for the reduction of O2 to water. Two polymer variants, based on 4,4′- and 5,5′-substituted bipyridine ligands, were synthesized and evaluated at varying Cu2+:ligand ratios. Electrochemical studies using linear sweep voltammetry and rotating ring-disk electrode analyses reveal that Cu2+ bipyridine polymers exhibit enhanced ORR activity and selectivity compared to their monomeric counterparts when heterogenized. The 4,4′-substituted Cu2+ bipyridine polymer at a 2:1 Cu2+:ligand ratio demonstrates the highest selectivity for the four-electron reduction of O2 to H2O, with an average electron transfer number of 3.8–3.9 across a wide pH range. While monomeric Cu2+ bipyridine complexes show faster kinetics in homogeneous catalysis, they exhibit poor selectivity in heterogeneous systems. In contrast, Cu2+ bipyridine polymers significantly improve selectivity in heterogeneous catalysis, making them superior candidates for practical ORR applications. These results establish Cu2+ bipyridine polymers as promising nonprecious metal ORR catalysts, contributing to the design of alternatives to Pt-based systems for fuel cells.