image: A conductive two-dimensional metal-organic framework (Cu3(THT)2) well-defined Cu-S4 sites were prepared, where S atoms can finely tune the electron state of Cu and offer additional stabilizing sites for the *CO. Therefore, the Cu3(THT)2 can switch the main products from simple CO to high-valued CH4 with high performance.
Credit: ©Science China Press
The Cu3(THT)2 catalyst achieves the optimal Faradaic efficiency (FE) towards CH4, reaching 63.5% at –1.4 V versus the reversible hydrogen electrode (RHE; all subsequent potentials are referred to RHE and were not iR-corrected), along with an industrial level CH4 partial current density of –189.6 mA cm–2. Additionally, the Cu-S4 sites in Cu3(THT)2 display good stability and can sustain its initial activity even after continuous electrolysis for 21,000 seconds. In stark contrast, Cu3(HITP)2, featuring Cu-N4 sites, exhibits markedly inferior CO2RR performance, with a CO Faradaic efficiency (FECO) of merely 40%, and a tendency to decompose and reconstruct into Cu2O particles after a mere 300 seconds of electrolysis.
Theoretical calculations reveal that S coordination atoms, featuring low electronegativity, can endow Cu-S4 sites with higher electron density compared to Cu-N4 site. This leads to a stronger binding with the 5σ/1π-orbital of *CO and a better π-backbonding with 2π*-orbital of *CO. In addition, the divalent S atom in Cu-S4 motifs serves as an electron acceptor, capable of binding with the O atom in *CO through S∙∙∙O weak interactions, which further optimizes the binding strength of *CO. As a consequence, the combined effect of the elevated electronic density in Cu-S4 and high oxophilic S atoms is envisioned to break the scaling relationship in CO2-to-CH4.
To the best of our knowledge, this work is the first attempt to reveal the role of non-metallic S center in CO2 electrocatalyst for switching the main products from simple CO to high-valued CH4 with high selectivity and large current density.