Electronic Structure Modulation in Isomeric Carborane-Thiolated Cu4 Nanoclusters Boosts CO2 Electroreduction to Methane

Electrochemical CO2 reduction (eCO2R) to energy-dense hydrocarbons is central to sustainable carbon recycling, although achieving selective conversion at high current density and lower overpotential remains challenging. Here, we explore three molecularly precise nanomaterials, comprising nearly square planar Cu4 kernel protected by isomeric carborane-thiolate (CBT) cages, viz., Cu4-oCBT (ortho-carborane), Cu4-mCBT (meta-carborane), and Cu4-ICBT (iodinated-ortho-carborane), for eCO2R. Among them, Cu4-mCBT shows the most favorable performance, achieving ∼74% overall eCO2R selectivity and a CH4 Faradaic efficiency (FE) of ∼45% at high current densities of 500 mA cm-2 (-0.87 V vs RHE) while maintaining promising production of C2+ products. Furthermore, Cu4-mCBT and Cu4-ICBT catalysts deliver industrially relevant CH4 partial current densities (JCH4 ∼ 323 mA cm-2 at -0.99 V) at low overpotential, representing competitive performance compared to Cu nanoclusters (NCs) stabilized by conventional ligands. First-principles calculations reveal that Cu4-mCBT lowers the free energy barrier for *CHO to *CHOH hydrogenation, the rate-limiting step for methane formation, due to better orbital interactions with the intermediates. In situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) confirms the transient intermediate species formed in the predicted eCO2R-to-CH4 pathway. Charge density difference, projected density of states, and in situ X-ray absorption studies demonstrate the primary catalytic role of the Cu-core, while three-dimensional cage-like ligand shells modulate the electronic structure of exposed metal atoms, thereby optimizing the intermediate binding. This study investigates the catalytic performance of carborane-appended tetranuclear copper clusters toward CO2 reduction pathways and elucidates their structure-activity relationships.