ACS Appl Energy Mater. 2023 Jan 9;6(1):267-277. doi: 10.1021/acsaem.2c02873. Epub 2022 Dec 22.
The mixture of CO and H2, known as syngas, is a building block for many substantial chemicals and fuels. Electrochemical reduction of CO2 and H2O to syngas would be a promising alternative approach for its synthesis due to negative carbon emission footprint when using renewable energy to power the reaction. Herein, we present temperature-controlled syngas production by electrochemical CO2 and H2O reduction on a cobalt tetraphenylporphyrin/multiwalled carbon nanotube (CoTPP/MWCNT) composite in a flow cell in the temperature range of 20-50 °C. The experimental results show that for all the applied potentials the ratio of H2/CO increases with increasing temperature. Interestingly, at -0.6 V RHE and 40 °C, the H2/CO ratio reaches a value of 1.2 which is essential for the synthesis of oxo-alcohols. In addition, at -1.0 V RHE and 20 °C, the composite shows very high selectivity toward CO formation, reaching a Faradaic efficiency of ca. 98%. This high selectivity of CO formation is investigated by density functional theory modeling which underlines that the potential-induced oxidation states of the CoTPP catalyst play a vital role in the high selectivity of CO production. Furthermore, the stability of the formed intermediate species is evaluated in terms of the pKa value for further reactions. These experimental and theoretical findings would provide an alternative way for syngas production and help us to understand the mechanism of molecular catalysts in dynamic conditions.