Band structure optimization and interface engineering of porphyrin-based covalent organic polymer heterojunction for boosted oxygen electrocatalysis in zinc-air batteries.

Developing efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) is crucial for advancing zinc-air batteries (ZAB). Herein, a novel interface engineering strategy was developed to fabricate covalent organic polymer (COP) COP/ZnCo2O4 heterojunction catalysts, denoted as COP-R/ZnCo2O4 (R = Me, H, Br) by in-situ integrating COP with spinel ZnCo2O4 as the auxiliary active site. The presence of porphyrin-based CoN4 sites after integration is supported by comprehensive study of FT-IR, UV-vis and solid-state NMR spectroscopy. Moreover, through systematic modulation of the electron-negativity of substituents on COP-R (R = Me, H, Br), the electron configuration of the heterojunction was modulated, leading to optimized catalytic performance. Among the three heterojunctions, COP-Me/ZnCo2O4 exhibits outstanding catalytic activity for ORR with a half-wave potential (E1/2) of 0.83 V, comparable to commercial Pt/C. Through a combination of spectroscopic analysis, band structure analysis and DFT calculation, we clearly demonstrate that electrons spontaneously migrate from ZnCo2O4 to the COP-Me at the heterojunction interface. The optimized band structure between ZnCo2O4 and COP-Me facilitates the electron transfer, which promotes formation of oxygen intermediate and thus contribute to its superior ORR performance. When assembled into the ZAB, COP-Me/ZnCo2O4 exhibits an open-circuit voltage of 1.48 V, a maximum power density of approximately 121.00 mW cm-2, and excellent cyclic charge-discharge stability over 300 cycles.Thereby this work paves a new avenue for heterojunction catalysts for electrochemical energy conversion and storage.