Electrochemical CO2 Reduction by Cobalt(II)2,9,16,23-tetra(amino)phthalocyanine: Enhancement Effect of Active Sites toward Methanol Formation

The electrochemical reduction of CO2 to high-value-added fuels such as methanol is an effective avenue to alleviate the greenhouse effect and global climate change. However, reasonable design and screening of highly efficient electrocatalysts remain challenging. So, a promising cobalt(II) 2,9,16,23-tetra(amino)phthalocyanine (CoTAPc) hybrid electrocatalyst was prepared with carbon nanotubes (CNT) as the support based on the ball milling method. And its surface morphology and structure were characterized by the methods such as scanning electron microscopy, Brunauer–Emmett–Teller, energy-dispersive spectroscopy, Fourier transform infrared spectrometer, UV–vis absorption spectra, Raman spectra, and X-ray photoelectron spectroscopy, as well as the performance of CO2 reduction reactions (CO2RR) was evaluated based on those methods, such as cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, gas chromatograph, 1H nuclear magnetic resonance, and headspace gas chromatography. Then, the structure–activity relationship on the hybrid catalyst and the influence mechanisms of CNT, ball milling, and amino group on CO2RR were elucidated by combining with the density functional theory (DFT) calculations. The addition of CNT improved the dispersion and induced the structural bending deformation of CoTAPc to form more active sites for CO2 reduction toward methanol, accompanied by the increase of the Faradaic efficiency and current density by nearly 76 and 200%, respectively. The shear action of ball milling shortened the length of the CNT and enhanced the interaction between CoTAPc and the CNT so that the electron-transfer resistance during the CO2RR decreased, achieving nearly 150% increase of the partial current density of methanol. The amino group on CoTAPc affected the pathway of the CO2RR to methanol by modifying the charge distribution of the Co–N4 structure to alter the binding energy of the intermediate. According to the results of DFT calculations, the pathway of CO2RR to methanol on the hybrid catalyst was proposed to be CO2(g) → *CO2 → *CO2– → *COOH →*CO → *CHO → *CH2O → *CH2OH → *CH3OH → CH3OH(l). This work will provide a significant reference for the preparation of high-performance electrocatalysts to achieve efficient CO2 reduction to those high-value-added products such as methanol.