Hydroxy-Group-Functionalized Single Crystal of Copper(II)-Porphyrin Complex for Electroreduction CO₂ to CH₄

Purposefully developing crystalline materials at molecular level to improve the selectivity of electroreduction CO₂ to CH₄ is still rarely studied. Herein, a single crystal of copper(II) complex with hydroxy groups was designed and synthesized, namely 5,10,15,20-tetrakis(3,4-dihydroxyphenyl)porphyrin copper(II) (Cu-PorOH), which could serve as a highly efficient heterogeneous electrocatalyst for electroreduction of CO₂ toward CH₄ . In 0.5 m KHCO₃ , Cu-PorOH gave a high faradaic efficiency of 51.3 % for CH₄ and drove a partial current density of 23.2 mA cm^-2 at -1.5 V versus the reversible hydrogen electrode in H-cell. The high performance was greatly promoted by the hydroxy groups in Cu-PorOH, which could not only form stable three-dimensional frameworks through hydrogen-bonding interactions but also stabilize the intermediate species by hydrogen bonds, as supported by density functional theory calculations. This work provides an effective avenue in exploring crystalline catalysts for CO₂ reduction at molecular level.