Pre-implanting metal oxides to endow the N-doped carbon with boosted bifunctional catalytic activities towards oxygen reduction and oxygen evolution reactions

Nitrogen-doped carbon-based materials exhibit promising prospect as the catalysts to oxygen reduction reaction (ORR). Incorporating transition metal oxides with the catalysts can endow them with oxygen evolution reaction (OER) activity to construct bifunctional catalysts for rechargeable zinc-air batteries. In this work, a catalyst (named as 300NiFe-Mi-C) was prepared by a metal oxide-implanting strategy. The mixed metal salts of Ni and Fe were first heated to produce metal oxides, and then blended with 2-methylimidazole and carbon black, and subsequently pyrolyzed at a high temperature. In the pyrolysis, a part of metal oxides was reduced to metallic state to facilitate the doping of nitrogen atoms into carbon to form the ORR active sites while a part of metal oxides was retained to afford OER activity. Benefiting from the pre-implanting strategy of metal oxides, the resultant 300NiFe-Mi-C presents enhanced OER performance with 1.56 V of OER potential at 10 mA cm−2, outperforming the 1.68 V of the controlled sample NiFe-Mi-C (without pre-implanting) and 1.70 V of RuO2. The 0.83 V of ORR half-wave potential of 300NiFe-Mi-C is also comparable to the 0.82 V of NiFe-Mi-C and 0.86 V of Pt/C, revealing satisfactory bifunctional catalytic activities. The rechargeable zinc-air battery equipped with 300NiFe-Mi-C can stably operate at ~1.25 V with 10 mA cm−2, being higher than ~1.21 V of Pt/C+RuO2. The battery also presents outstanding durability and rechargeability, demonstrating the bifunctional activities of 300NiFe-Mi-C can be realized in practical applications.