Sacrificial ZnO nanorods drive N and O dual-doped carbon towards trifunctional electrocatalysts for Zn-air batteries and self-powered water splitting devices

Integrated energy systems (IES) have attracted increasing attention in recent years. Zn-air battery powered water splitting devices require the development of highly active and durable trifunctional electrocatalysts for the oxygen evolution, oxygen reduction, and hydrogen evolution reactions (OER/ORR/HER). However, engineering rational nano-scaled designs and achieving the required synergy are major challenges due to the lack/weak control of synthesis processes. Herein, ZIF-67 regular polyhedra were fabricated for the first time to incorporate single ZnO nanorods. Thereafter, pyrolysis sacrificed the nanorods and stimulated intriguing modifications on the ZnONR@ZIF-67-derived CoO_x@N, O-doped hierarchical carbon (CoO_x@NOC), not only from the outside-in, but also from the inside out. Consequently, an outstanding enhancement in OER/ORR/HER trifunctional activity was achieved. The CoO_x@NOC based Zn-air battery showed a small initial charge-discharge voltage gap of 92 mV at 10 mA cm⁻²and a high specific capacity and maximum power density of 757.39 mA h g_Zn⁻¹and 141.65 mW cm⁻², respectively. A CoO_x@NOC-based all-solid-state Zn-air battery (SS ZAB) was fabricated, which showed a high open circuit potential of 1.49 V. Two SS ZABs in series drove an overall water splitting system, which showed an intriguingly low potential of 1.51 V at 10 mA cm⁻², surpassing most reported electrocatalysts. Thus, the excellent performance of CoO_x@NOC implies its great potential to compete with noble metal electrocatalysts.