Electronic tuning of Ni-Fe-Co oxide/hydroxide as highly active electrocatalyst for rechargeable Zn-air batteries.

As a bifunctional oxygen electrocatalyst (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)), spinel copper cobaltite (CuCo 2 O 4 ) is attracting significant research interest owing to the tailored Co, Cu electronic structure and ease of adjusting the electrochemically active area. However, its poor OER performance (>300 mV at 10 mA cm -2 ) limits its practical application for rechargeable zinc-air batteries. Therefore, we construct a CuCo 2 O 4 /NiFe LDH oxide/hydroxide interface to tune the properties of Ni, Fe and Co for enhancing OER activity and decreasing the charging overpotential of rechargeable zinc-air batteries. The obtained electrocatalysts show a low overpotential of 251 mV (10 mA cm -2 ), which is 91 mV lower than the overpotential (342 mV) of CuCo 2 O 4 . By in situ Raman, XPS and electrochemical analyses, we ascribe the enhanced OER activity to the increasing Ni/Fe oxidation state triggered by the charge transfer of Ni/Fe and Co, which prompts CuCo 2 O 4 /NiFe LDH to rapidly form an active surface layer. Benefiting from enhanced OER performance, zinc-air batteries with a CuCo 2 O 4 /NiFe LDH electrode display a high round-trip efficiency with a low voltage gap of ∼0.78 V (10 mA cm -2 ) due to the obvious decrease in the charging overpotential. These results suggest the importance of tuning the charge transfer on interfaces for designing high-efficiency electrocatalysts.