Sustainable room-temperature, water-driven conversion of CO₂ to graphitic carbon quantum dots on electride electrodes

Abstract

Converting CO₂ into valuable solid carbon offers the dual benefits of permanent sequestration and economic utility. Although CO₂ reduction via reactive hydrides to produce solid carbon is thermodynamically feasible under ambient conditions, its implementation has been limited by sluggish kinetics, the energy cost of hydride regeneration, and handling challenges. Here, we report a highly efficient, water-driven process for converting CO₂ into graphitic carbon quantum dots (g-CQDs) using water-durable LaCu₀.₆₇Si₁.₃₃ intermetallic electride electrodes. The method operates via a sustainable H₂O/H⁻ cycle and a concerted hydride transfer mechanism enabled by localized electric fields within the electride’s interstitial voids. Under a negative potential (–1.5 to –2.25 V vs. Ag/AgCl), spontaneous water dissociation occurs on LaCu₀.₆₇Si₁.₃₃ (H₂O + e⁻ → H* + OH⁻), where surface-bound H* and rapid electron injection increase electron occupancy in the interstitial voids, generating strong localized electric fields. These fields promote tunneling-induced hydride transfer to localized CO₂ at the electrode–electrolyte interface, driving its reduction to hydrogenated carbon intermediates that dehydrate into g-CQDs. Meanwhile, OH⁻ ions undergo oxygen evolution at the anode, regenerating H₂O and releasing O₂. This closed redox cycle enables continuous CO₂-to-carbon turnover, producing 0.156 g of g-CQDs from CO₂ in a 50 mL electrolyte within 1.5 h, with record-low energy consumption (1.90 kWh/kg C) and stable long-term operation. A fast electron-injection rate is essential for field generation, as replacing the conductive Cu substrate with ITO completely suppresses both current and g-CQD formation. The suggested mechanism is supported by DFT-calculated electronic density of states (EDOS), Fowler–Nordheim analysis, and experimental observations, including the absence of g-CQD products without CO₂ or H₂O, exponential increases in current density and yield with potential, isotope substitution with D₂O, sustained current growth over 24 hours, and reproducibility across different electride catalysts. These findings establish a sustainable, high-yield CO₂-to-carbon conversion route and identify intermetallic electrides as practical platforms for mediating quantum