Enhanced photoelectrochemical hydrogenation of green-house gas CO2 to high-order solar fuel on coordinatively unsaturated metal-N sites containing carbonized Zn/Co ZIFs

Abstract

Porous carbon-based catalysts facilitate CO2 photoelectrochemical reduction reaction (CO2PRR) through the high charge transfer ability and CO2 adsorption ability. However, the design of catalysts with high selectivity towards high-order solar fuels (such as ethanol and propanol) remains challenging. Herein, catalysts of carbonized Zn/Co Zeolitic Imidazolat Frameworks with various pyrolysis hours (xh-C-Zn/Co ZIFs) were synthesized and accessed for high selective CO2PRR for the first time. XRD and TEM analyses show that the C containing functional groups in pristine Zn/Co ZIF are carbonized and turned into amorphous porous carbon, while many Co and ZnO nanoparticles are formed during the pyrolysis process. Electrochemical characterizations of the catalysts prove that increasing pyrolysis time of Zn/Co ZIF from 1 h to 3 h, resulting in a decreased light current density from 3.3 mA/cm2 to 2.3 mA/cm2, which is much higher than that of Zn/Co ZIF (1.9 mA/cm2). Meanwhile, increasing pyrolysis time of Zn/Co ZIF from 1 h to 3 h results in decreased BET surface area and CO2 adsorption ability. XPS spectra suggest a decreased content of metal-nitrogen bonds in C–Zn/Co ZIF, which leads to the formation of coordinatively unsaturated metal-nitrogen sites. Density functional theory (DFT) calculations reveal that the coordinatively unsaturated CoN3V sites in C–Zn/Co ZIF had the highest catalytic activity towards high-order organics. The total carbon atom conversion rate reaches 5459 nmol/h·cm2 when 1h-C-Zn/Co ZIF is employed as catalyst in the CO2PRR system and the selectivity towards high-order solar fuel reaches 84%.