Tuning the Hydroxyl Density of MXene to Regulate the Electrochemical Performance of Anchored Cobalt Phthalocyanine for CO₂ Reduction

Precise electronic state regulation through coordination environment optimization by metal–support interaction is a promising strategy to facilitate catalysis reaction, while the limited density of functional groups in the bulk substrate restricts the regulation degree. Herein, different sizes of Ti 3 C 2 T x MXene with hydroxyl (−OH) terminal including the MXene layer (ML-OH, 3 μm), the MXene nanosheet (MNS-OH, 600 nm), and the MXene quantum dot (MQD-OH, 8 nm) were prepared to anchor CoPc, and the effect of −OH density on the performance of electrochemical CO 2 reduction was systematically investigated. Notably, a linear relationship was established by plotting reactivity vs hydroxyl density. With the highest −OH density, CoPc/MQD-OH exhibits a superior Faradaic efficiency for CO formation (FE CO ) of ∼100% at −0.9 to −1.0 V vs RHE and a high FE CO of >90% over a wide potential window from −0.8 to −1.4 V. The mechanism exploration shows that the axial coordination interaction of the −OH terminal with Co increases the electron density of the Co site, thus promoting the adsorption and activation of CO 2 . This work provides a new insight into designing of molecular catalysts with high efficiency and tunable structure for other electrochemical conversions.