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

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 (POxN3-x)2/NPC and Co (POxN3-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 (POxN3-x)2/NPC and Co (POxN3-x)2/NPC as the cathodes show gravimetric energy densities of 894 and 836 W h kgZn-1, which are superior to that of Pt/C (793 W h kgZn-1). In addition, the rechargeable zinc-air battery assembled with Ni (POxN3-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.