Electrolyte-mediated Cu⁺/Cu⁰ ratio control in Cu/graphene catalysts for divergent CO₂ electroreduction pathways

The electrochemical CO₂ reduction reaction (CO₂RR) to multi‑carbon products offers a promising route for sustainable fuel production, yet achieving high selectivity remains challenging due to the complex interplay between catalyst structure and reaction pathways. Here, we report a rational design of Cu-Cu₂O/graphene (Cu/G) composites with tunable Cu⁺/Cu⁰ ratios via electrolyte-mediated electrochemical synthesis. By varying tetraalkylammonium/phosphonium electrolytes (TBABF₄, TBAPF₆, TBACF₃SO₃, TBPBF₄), we precisely control Cu oxidation states, as validated by the results of X-ray absorption fine structure (XAFS), X-ray diffraction (XRD), and high-resolution transmission electron microscope (HRTEM). In-situ Fourier transform infrared (In-situ FTIR) spectra and in-situ Raman spectra indicate Cu⁺-rich surfaces (Cu/G-TBABF₄) favor the production of formate via O-bound intermediates, while Cu⁰-dominated interfaces (Cu/G-TBAPF₆) promote the formation of CO through C-bound pathways. Mixed Cu⁺/Cu⁰ systems (Cu/G-TBACF₃SO₃, Cu/G-TBPBF₄) enable CC coupling, with ethylene (FE = 28 %) and ethanol (FE = 30.2 %) selectivity governed by the Cu⁺/Cu⁰ balance. Ultraviolet photoelectron spectroscopy (UPS) results reveal that lower work functions enhance *CO coverage and stabilize C₂₊ intermediates, elucidating the electronic structure-activity relationship. This work demonstrates electrolyte-mediated oxidation state engineering as a versatile strategy to steer CO₂RR pathways, offering a generalizable approach for designing multi-valent electrocatalysts with tailored selectivity.