A cascade of in situ conversion of bicarbonate to CO2 and CO2 electroreduction in a flow cell with a Ni-N-S catalyst

Combination of CO2 capture using inorganic alkali with subsequently electrochemical conversion of the resultant HCO3- to high-value chemicals is a promising route of low cost and high efficiency. The electrochemical reduction of HCO3- is challenging due to the inaccessible of negatively charged molecular groups to the electrode surface. Herein, we adopt a comprehensive strategy to tackle this challenge, i.e., cascade of in situ chemical conversion of HCO3- to CO2 and CO2 electrochemical reduction in a flow cell. With a tailored Ni-N-S single atom catalyst (SACs), where sulfur (S) atoms located in the second shell of Ni center, the CO2 electroreduction (CO2ER) to CO is boosted. The experimental results and density functional theory (DFT) calculations reveal that the introduction of S increases the p electron density of N atoms near Ni atom, thereby stabilizing *H over N and boosting the first proton coupled electron transfer process of CO2ER, i.e., *+e–+*H+*CO2→*COOH. As a result, the obtained catalyst exhibits a high faradaic efficiency (FECO ∼ 98%) and a low overpotential of 425 mV for CO production as well as a superior turnover frequency (TOF) of 47397 h–1, outcompeting most of the reported Ni SACs. More importantly, an extremely high FECO of 90% is achieved at 50 mA cm–2 in the designed membrane electrode assembly (MEA) cascade electrolyzer fed with liquid bicarbonate. This work not only highlights the significant role of the second coordination on the first coordination shell of the central metal for CO2ER, but also provides an alternative and feasible strategy to realize the electrochemical conversion of HCO3- to high-value chemicals.