Highly active bifunctional oxygen electrocatalysts derived from nickel– or cobalt–phytic acid xerogel for zinc–air batteries

Abstract

Developing highly efficient non-noble metal electrocatalysts for oxygen electrode reactions is highly desirable for industrial scale application in energy related devices. Herein, two new kinds of Ni (PO_xN_3−x)₂/NPC and Co (PO_xN_3−x)₂/NPC (NPC: N, P-co-doped carbon) are synthesized through a facile post-treatment of nickel– or cobalt–phytic acid xerogel, followed by an annealing procedure under an argon and ammonia atmosphere at 800 °C. The as-prepared catalysts exhibit outstanding catalytic activities for both the oxygen reduction and evolution reactions, which are comparable to those of Pt/C and IrO₂. Furthermore, the primary zinc–air batteries assembled with Ni (PO_xN_3−x)₂/NPC and Co (PO_xN_3−x)₂/NPC as the cathodes show gravimetric energy densities of 894 and 836 W h kg_Zn⁻¹, which are superior to that of Pt/C (793 W h kg_Zn⁻¹). In addition, the rechargeable zinc–air battery assembled with Ni (PO_xN_3−x)₂/NPC exhibits an excellent round-trip efficiency, which is shown by a slight increase in the sum of the overpotentials for discharge–charge cycling at a current density of 20 mA cm⁻², even after experiencing 33 h of testing. To the best of our knowledge, there are few reports on metaphosphate salts where oxygen is partially replaced by nitrogen as bifunctional oxygen electrode catalysts for zinc–air batteries. This work provides an easy, low-cost and scalable avenue to develop new kinds of catalyst for application in energy devices.