Titanium Dioxide-Grafted Copper Complexes: High-Performance Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media

Abstract

The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathodes of fuel cells significantly hampers fuel cell performance. Therefore, the development of high-performance, non-precious-metal catalysts as alternatives to noble metal Pt-based ORR electrocatalysts is highly desirable for the large-scale commercialization of fuel cells. TiO2 -grafted copper complexes deposited on multiwalled carbon nanotubes (CNTs) form stable and efficient electrocatalysts for the ORR. The optimized catalyst composite CNTs@TiO2 -ZA-[Cu(phen${{^{{\rm NO}{_{2}}}}}$)(BTC)] shows surprisingly high selectivity for the 4 e(-) reduction of O2 to water (approximately 97 %) in alkaline solution with an onset potential of 0.988 V vs. RHE, and demonstrates superior stability and excellent tolerance for the methanol crossover effect in comparison to a commercial Pt/C catalyst. The copper complexes were grafted onto the surface of TiO2 through coordination of an imidazole-containing ligand, zoledronic acid (ZA), which binds to TiO2 through its bis-phosphoric acid anchoring group. Rational optimization of the copper catalyst's ORR performance was achieved by using an electron-deficient ligand, 5-nitro-1,10-phenanthroline (phen${{^{{\rm NO}{_{2}}}}}$), and bridging benzene-1,3,5-tricarboxylate (BTC). This facile approach to the assembly of copper catalysts on TiO2 with rationally tuned ORR activity will have significant implications for the development of high-performance, non-precious-metal ORR catalysts.

Keywords: copper complexes; non-precious metal catalysts; oxygen reduction reaction; titanium dioxide.