Cu,S,N Heteroatom-Tailored Carbon Quantum Dots Enabling Efficient Electrochemical CO₂ Reduction to Acetate and Formate

Abstract

Electrochemical CO₂ reduction (ECO2R) offers a promising route to mitigate CO₂ emissions while generating value-added chemicals. Here, we report the synthesis of heteroatom-modified carbon quantum dots (CQDs) via a green hydrothermal method using citric acid, glucosamine, urea, and cysteine as precursors, with copper incorporation. Structural characterization by XPS and Raman spectroscopy confirmed successful Cu, S, and N doping, while water contact angle measurements revealed precursor-dependent surface hydrophilicity. Electrodes prepared with 80 wt% CQDs and 20 wt% anion-exchange ionomer exhibited high electrochemically active surface areas. Among the series, Cu,S,N-CQD-GAH derived from glucosamine, showing lower hydrophilicity, delivered the most effective ECO2R performance. This catalyst selectively produced formate and acetate, as verified by HPLC and ¹H NMR, achieving acetate formation rates up to 1.5 mmol•g cat ⁻¹•h⁻¹ at -0.4 V vs RHE. The half-cell energy efficiency peaked at 52.3% with a Faradaic efficiency of 66.8% at -0.2 V vs RHE. The enhanced activity is attributed to nitrogen functionalities in aminic/pyridinium oxide forms and the presence of Cu(I), which promotes C-C coupling. In contrast, sulfur-free or Cu(II)-rich CQDs exhibited diminished activity. Long-term chronoamperometry confirmed stable current densities after an initial activation period, highlighting the durability of the Cu,S,N-CQD catalysts.