Enhancing Electrochemical CO2 Reduction Performance of CoII-phthalocyanine Polymer through Axial Ligand Coordination

This work presents an approach to enhance the catalytic performance of a CoII-phthalocyanine (CoPc)-based polymer for electrochemical CO2 reduction (ECO2R) by introducing a 4-dimethylaminopyridine (DMAP) axial ligand on a Co metal center. Following a previous study on 4-aminophenoxy-substituted CoPc monomer (CoPc-1) and polymer (p-CoPc-1) catalysts, a target DMAP-coordinated CoPc polymer (p-CoPc-2) was prepared by two practical routes. One included DMAP axial coordination of CoPc-1 followed by electropolymerization of the resulting CoPc-2 monomer, while the other involved DMAP treatment of a p-CoPc-1 film. Ultraviolet−visible (UV−vis) spectrophotometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) confirmed the successful preparation of CoPc-2 and its polymer films. Results from catalytic performance evaluation revealed that the polymer from the first route (p-CoPc-2A) exhibited superior CO2-to-CO conversion performance compared to that from the second one (p-CoPc-2B). The p-CoPc-2A-catalyzed ECO2R in an H-cell at an applied potential of −1.20 V vs NHE (−0.77 V vs RHE) showed current density (jCO) and Faradaic efficiency of CO formation (FECO) of 8.13 mA·cm−2 and 99%, respectively, corresponding to a turnover frequency (TOFCO) of 2.84 s−1. At this optimal potential, p-CoPc-2A exhibited an average FECO of 96% with a turnover number (TONCO) of 4.0 × 106 over 40 h of electrolysis. In a flow cell, p-CoPc-2A showed superior catalytic performance compared to p-CoPc-1 by maintaining the average FECO of 95% and j of 167 mA·cm−2 for 25 h. Electrochemical impedance spectroscopy (EIS) and in situ XAS studies reflected a critical role of the DMAP ligand in promoting interfacial charge transfer, charge transport, and stabilization of the local coordination environment around the Co metal center during the ECO2R. Furthermore, the theoretical calculation showed the impact of the DMAP coordination in elevating the orbital energy level, lowering the free-energy barrier for *COOH intermediate formation, and stabilizing the low-spin reaction pathway.