Facet-dependent CO₂ reduction reactions on kesterite Cu₂ZnSnS₄ photo-electro-integrated electrodes

Photoelectrochemical CO₂ reduction by Cu₂ZnSnS₄ (CZTS) photocathodes is a potentially low-cost and high-efficiency CO₂ conversion approach. However, the current CZTS-based photocathodes for the CO₂ reduction reaction (CO₂RR) are challenged by the active side reaction of the hydrogen evolution reaction (HER) and the incompatibility with efficient electrocatalysts. In this work, by means of density functional theory (DFT), we predict that a (220)-facet-suppressed kesterite CZTS could be an efficient photo-electro-integrated photocathode for formic acid production in the CO₂RR. The results show that the competitive HER is mostly favored on the (220) facet. And the CO₂RR for formic acid production on the (112) and (312) facets exhibits a thermodynamic energy barrier lower than 0.26 eV. Different from the d-band theory in metal electrocatalysts, it is found that the density of low energy unoccupied states in the S 3p orbital plays a key role in determining the CO₂RR reaction path of the kesterite CZTS. Furthermore, two different trends of adsorption energy depending on the chemical characteristic of adsorbates are analyzed. Our study unveils the potential for selectively reducing CO₂ into formic acid with kesterite CZTS and provides a possible route for manipulating the electrocatalytic properties of metal sulfide catalysts.