High-performance CoII-phthalocyanine-based polymer for practical heterogeneous electrochemical reduction of carbon dioxide

Highlights: • Bithiophene-functionalized Co^II-phthalocyanine can be readily electropolymerized. • The polymer exhibits high catalytic activity for selective conversion of CO₂ to CO. • CO₂ reduction can be achieved at low overpotential under ambient aqueous condition. • Spectroelectrochemical study suggests Co^I-centered catalytic mechanism. -- Abstract: A new bithiophenyl-substituted Co^II-phthalocyanine is synthesized and electropolymerized to obtain corresponding polymer films on an indium tin oxide-coated glass and a carbon paper. Formation of the target polymer on substrates is confirmed by Ultraviolet–visible spectrophotometry, Raman spectroscopy and attenuated total reflection-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy analysis, scanning electron microscopy and energy dispersive X-ray spectroscopy. Heterogeneous electrochemical reduction of carbon dioxide (CO₂) in a 0.5 M KHCO₃ aqueous solution under catalysis of the target polymer coated on the carbon paper gives carbon monoxide (CO) as a major product. Controlled potential electrolysis at a constant potential of –0.66 V vs. reversible hydrogen electrode, corresponding an overpotential of –0.54 V from thermodynamic potential for CO production, for 2 h leads to optimum yield of CO with Faradaic efficiency (FE), turnover number (TON) and turnover frequency (TOF) of 94%, 2.10 × 10³ and 0.29 s^–1, respectively. Upon 20-h electrolysis with quantitative product monitoring, the target polymer appears to be stable throughout the process and continuously produces CO with the FE, TON and TOF of 72%, 1.24 × 10⁴ and 0.17 s^–1, respectively. Spectroelectrochemical study suggests metal-centered mechanism via formation of active reduced species having significant interaction of Co^I with CO₂. This research demonstrates the use of the synthetically practical Co^II-phthalocyanine-based polymeric material as an efficient and stable electrocatalyst for the CO₂ reduction.