Tailored Local Electronic Environment of Co-N 4 Sites in Cobalt Phthalocyanines for Enhanced CO 2 Reduction Reaction.

Atomically dispersed Co-N 4 -based catalysts have been recently emerging as one of the most promising candidates for facilitating CO 2 reduction reaction (CO 2 RR). The local electronic environment of Co-N 4 sites in these catalysts is considered to play a critical role in adjusting the catalytic performance, the effort of which however is not yet clearly verified. Herein, a series of cobalt phthalocyanines with different peripheral substituents including unsubstituted phthalocyanine Co(II) (CoPc), 2,9,16,23-tetramethoxyphthalocyaninato Co(II) (CoPc-4OCH 3 ), and 2,9,16,23-tetranitrophthalocyaninato Co(II) (CoPc-4NO 2 ) are supported onto the surface of the multi-walled carbon nanotubes (CNTs), affording CoPc@CNTs, CoPc-4OCH 3 @CNTs, and CoPc-4NO 2 @CNTs. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure measurements disclose the influence of the peripheral substituents on the local electronic structure of Co atoms in these three catalysts. Electrochemical tests indicate the higher CO 2 RR performance of CoPc-4OCH 3 @CNTs compared to CoPc@CNTs and CoPc-4NO 2 @CNTs as exemplified by the higher Faraday efficiency of CO, larger part current densities, and better stability displayed by CoPc-4OCH 3 @CNTs at the applied voltage range from -0.6 to -1.0 V versus RHE in both H-cell and flow cell. These results highlight the effect of the electron-donating -OCH 3 substituent on the enhanced catalytic activity of CoPc-4OCH 3 @CNTs, which will help develop Co-N 4 -based catalysts with promising catalytic performance toward CO 2 RR.