Edge-substituents and center metal optimization boosting oxygen electrocatalysis in porphyrin-based covalent organic polymers
Hongyan Zhuo, Qiming Ye, Shaoze Wang, Yu Han, Tianle Yang, Binghan Jiang, Chuangyu Wei, Linlin Feng, Tenglong Jin, Xue Liu, Zhuang Shi, Hao Song, Zhen Fu, Wenmiao Chen, Yuexing Zhang, Yanli Chen
Abstract
The promising non-noble electrocatalyst with well-defined structure is significant for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for the renewable energy devices like zinc-air batteries (ZABs). Herein, the four phenyl-linked cobaltporphyrin-based covalent organic polymers (COPs-1–4) with the different edge substituents (1 = − t Bu, 2 = −Me, 3 = −F, and 4 = −CF 3 ) are firstly designed and synthesized via a simple, efficient one-pot method. With the increase of electron donating capacity of the substituents, the highest occupied molecular orbital energy ( E HOMO ) gradually increases in the order of COP-4 < COP-3 < COP-2 < COP-1. Consequently, the optimal COP-1 with − t Bu edge groups exhibits the highest half-wave potential ( E 1/2 ) of 0.84 V (vs. RHE) among the four COPs, which is comparable with commercial Pt/C in alkaline media. The DFT calculations further reveal that with strong electron donating capacity, the Gibbs free energy decreases in the order of COP-4 > COP-3 > COP-2 > COP-1 by modulating the adsorption energy of OOH* at rate-determining step (RDS) to promote ORR activity. Furthermore, introducing Ni (II) and Co (II) into porphyrin centers afford the bimetallic CoNi-COP-1 with both Co-N 4 , Ni-N 4 active sites and edge substituted − t Bu. The synergistic effect of Co, Ni bimetallic active sites and strong electron-donating − t Bu substituents renders the CoNi-COP-1 the highest HOMO and smallest energy gap between the E LUMO and E F among the as-prepared five COPs, which leads to more filling electrons of its LUMO level, and thus exhibits the excellent ORR and OER bifunctional catalytic activities with an E 1/2 as high as 0.85 V and an overpotential (η) of 0.34 V at 10 mA cm −2 in alkaline media, superior to monometallic Co-containing COPs-1–4. In particular, the assembled ZABs with bifunctional catalyst CoNi-COP-1 possesses high power density (94.10 mW cm −2 ), high specific capacity (841.71 mAh g Zn −1 ) and long durability of over 160,000 s. This work exemplifies the rational design of pyrolysis-free non-noble metal COP-based electrocatalyst through optimizing the intrinsic metal center and its secondary coordination environment.