Ta–TiOx nanoparticles as radical scavengers to improve the durability of Fe–N–C oxygen reduction catalysts

Abstract

Highly active and durable platinum group metal-free catalysts for the oxygen reduction reaction, such as Fe–N–C materials, are needed to lower the cost of proton-exchange membrane fuel cells. However, their durability is impaired by the attack of oxidizing radicals such as ·OH and HO2· that form from incomplete reduction of O2 via H2O2. Here we demonstrate that Ta–TiOx nanoparticle additives protect Fe–N–C catalysts from such degradation via radical scavenging. The 5 nm Ta–TiOx nanoparticles were uniformly synthesized on a Ketjenblack substrate using a high-temperature pulse technique, forming the rutile TaO2 phase. We found that Ta–TiOx nanoparticles suppressed the H2O2 yield by 51% at 0.7 V in an aqueous rotating ring disk electrode test. After an accelerated durability test, a fuel cell prepared with the scavengers showed a current density decay of 3% at 0.9 ViR-free (internal resistance-compensated voltage); a fuel cell without scavengers showed 33% decay. Thus, addition of Ta–TiOx provides an active defence strategy to improve the durability of oxygen reduction reaction catalysts. Low-cost catalysts for oxygen reduction, such as Fe–N–C materials, often suffer from poor stability in fuel cells due to the generation of oxidizing radical species. Here the authors locate Ta–TiOx additives in the vicinity of Fe–N–C catalysts and show that they can successfully scavenge radicals, improving durability.