Controlling electrochemical lignin depolymerization via halide chemistry at boron-doped diamond electrodes

Lignin, a highly abundant yet underutilized biopolymer, holds significant potential as a renewable source of aromatic chemicals. Herein, we present a sustainable electrochemical approach for lignin valorization, in which oxidative pathways are tunable via halide redox chemistry. Using boron-doped diamond electrodes, chosen for their wide potential window, high current efficiency, and fouling resistance, we elucidate the roles of halide (Cl^-, Br^-, I^-) and membrane configuration in governing electrochemical reactivity, selectivity, and surface stability. Cyclic voltammetry and extended electrolysis reveal distinct regimes: chloride and bromide mediate direct electrode-driven oxidation which suffers from surface passivation, whereas iodide enables solution-phase oxidation via electrogenerated iodine species, minimizing fouling and delivering the highest yield of aromatic products. Product analysis confirms β-O-4 cleavage as the dominant depolymerization route, with vanillin as the primary product. Incorporation of a cation-exchange membrane substantially enhances yields and promotes deeper oxidative transformations, such as the formation of isovanillic and furan carboxylic acids. These findings define key parameters for achieving sustainable and tunable electrochemical lignin conversion without the need for sacrificial oxidants.