Biomass-Derived Cu, S, and N-Doped Biochar as a Green Electrocatalyst for CO2 Reduction to Formate and Acetate

Biomass-derived chars offer a sustainable platform for electrocatalytic CO2 reduction (ECO2R) when functionalized with nontoxic metals and heteroatoms. In this work, Cu, S, and N-doped biochar were synthesized via pyrolysis of waste pine needles with addition of copper acetate and thiourea, providing a green, low-cost, and scalable route. Three catalysts with varying Cu loadings were prepared, and electrodes were fabricated by drop-casting the doped biochar with poly(2,6-dimethyl-1,4-phenylene oxide) pentyl trimethylammonium ionomer (PPO-LC) on carbon paper. Structural characterization by Raman spectroscopy revealed a balance between defect density and sp2 domain size, while X-ray photoelectron spectroscopy confirmed the presence of Cu+, nitrogen in aminic/pyridinic positions, and thioether-type sulfur, key functionalities for CO2 adsorption and activation. Scanning electron microscopy imaging showed a homogeneous, porous catalyst layer, facilitating gas transport and electron transfer. Electrochemical testing demonstrated that the catalyst with an intermediate Cu loading (3.1 at%) and copper(I) as active site achieves the highest selectivity toward acetate, surpassing the performance of catalysts with lower or higher Cu loading. The results highlight that careful tuning of Cu content and oxidation state, combined with heteroatom doping and controlled structural defects, is essential for designing efficient, selective, and durable biomass-derived ECO2R electrocatalysts.