Promoting ZIF-8-Derived Fe-N-C Oxygen Reduction Catalysts via Zr Doping in Proton Exchange Membrane Fuel Cells: Durability and Activity Enhancements

The atomically dispersed iron site and nitrogen co-doped carbon catalysts (Fe-N-C) have demonstrated promising performance in replacing Pt toward the oxygen reduction reaction (ORR) in acids for proton exchange membrane fuel cells. However, the insufficient durability of Fe-N-C catalysts prohibitively hinders their practical applications. Herein, we report that the co doping of Zr and Fe dual metal sites into a ZIF-8-derived mesoporous carbon exhibited significantly improved durability for the ORR. Especially, a membrane electrode assembly from the ORR cathode catalyst only lost 25% voltage after 20 h of continuous operation at a constant current density. After an extended test of up to 100 h, the Zr-doped Fe-N-C catalyst retained 40% of its initial performance, superior to the catalyst without Zr doping with more than 70% activity loss after only 20 h. The cathode also showed significantly improved ORR activity, achieving a maximum power density of 0.72 W cm-2 under H2/air conditions. Extensive experimental characterization and density functional theory calculations suggested that the promoted catalytic activity and stability are due to the formation of Zr-based active sites with enhanced acidic tolerance than the individual Fe sites. Also, the doping of Zr could suppress the formation of H2O2 and other free radicals, thus mitigating active site degradation. The possible Fe/Zr dual-metal active sites, i.e., N2(N)-Fe-N2-Zr-N2(O2), likely have enhanced intrinsic ORR activity relative to conventional FeNx sites.