Efficient electroreduction of CO2 to syngas over ZIF-8 derived oxygen vacancy-rich ZnO nanomaterials

Abstract

Electroreduction of CO₂ is a sustainable approach to produce syngas with tunable CO/H₂ ratios, which are required as specific reactants for the optimization of desired products. Herein, ZnO-d and ZnO-n nanomaterials were derived from zeolitic imidazolate framework-8 (ZIF-8) precursors with different morphologies, which exhibit good performance for CO₂ electroreduction to syngas when using 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF₆, 30 wt%)/acetonitrile/water (H₂O, 5 wt%) as the supporting electrolyte. Regulation of the number of oxygen vacancies in different ZnO samples and the applied potentials during electrolysis can obviously affect the CO/H₂ ratios in syngas, which would be changed from 1/3 to 8.5/1. Meanwhile, ZnO-n with abundant oxygen vacancies displays the highest CO faradaic efficiency (FE) of 73.2% with the total current density of 9.8 mA cm⁻² at −1.9 V vs. Ag/Ag⁺. More oxygen vacancies and higher electrochemical specific surface area of ZnO-n can provide more active sites and facilitate the adsorption of CO₂ and its intermediates. The smaller charge transfer impedance of ZnO-n can accelerate the electron and proton transfer, and thus improve the catalytic activity and the selectivity of products. In addition, the good synergistic effect between the [Bmim]PF₆-containing electrolyte and ZnO-n can enhance the formation of syngas.