Ta-TiOx nanoparticles as radical scavengers to improve the durability of Fe-N-C oxygen reduction 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. 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 center dot OH and HO2 center dot that form from incomplete reduction of O-2 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.