One-Step Preparation of Trinary Co(III)/Co(II)/Co(0) Bifunctional Catalysts with Controllable Co Valence Distribution via Solution Combustion Synthesis and Its Application to Oxygen Reduction and Evolution in Zinc–Air Batteries

Regulating the electronic structure and chemical valence of the active site for electrocatalysis is highly crucial and challenging. In this work, a cost-effective and facile strategy for regulating the cobalt valence distribution through one-step solution combustion synthesis via cobalt nitrate (oxidizer) and glycine (fuel) was developed to prepare a bifunctional Co(III)/Co(II)/Co(0) catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in a zinc-air battery. Experimental findings show that when increasing the fuel/oxidizer ratio φ, the composition of the synthesized catalyst gradually changes from binary Co(III)/Co(II) to trinary Co(III)/Co(II)/Co(0), and the average Co valence keeps decreasing. As the content of Co(II)/Co(0) increases, the ORR performance of the sample gradually improves. The sample synthesized at φ = 1.2 shows the best bifunctional catalytic activity and was employed to assemble a rechargeable zinc-air battery. Overall, compared with the published data, the proposed catalyst has a comparable ORR/OER activity and catalyst stability, with an excellent battery life, efficiency, and stability (up to 160 h). This work provides a promising pathway for designing the valence state distribution of non-noble cobalt-based catalysts, which can be easily prepared on a large scale with low cost and used in various technologies involved in ORR and OER reactions.