p-benzyloxyl phenol (PBP), as a lignin model compound, was electrochemically degraded in [BMIM]BF₄ ionic liquid (IL) using an oxygen reduction reaction (ORR) cathode. The utilization of this kind of aprotic IL made it possible to identify which kind of reactive oxygen species (ROS) played an important role in the electrochemical depolymerization of lignin. Electrochemical study confirmed that superoxide radical (·O₂⁻), the one-electron reductive product of dioxygen, is the main resultant of ORR in [BMIM]BF₄. However, the generated ·O₂⁻ radical transformed into hydroperoxyl radical (·OOH) in the presence of protons donated by PBP, and then received one more electron to form H₂O₂, which then attacked and broke down the alkyl-O-aryl bonds in PBP molecules. The continuous electrolysis of PBP in [BMIM]BF₄ provided indirect evidence to reveal that the in situ generated H₂O₂ on ORR cathode was responsible for the cleavage of ether bonds in PBP.

Hydroxyl radicals (˙OH) generated by oxygen reduction reaction (ORR) were believed to be responsible for the electrochemical depolymerization of lignin in our previous study. However, the mechanism research was hard to carry out due to the recalcitrant nature of lignin. In this paper, 4-benzyloxyl phenol (PBP) and benzyl phenyl ether (BPE) were employed as lignin model compounds (LMCs). Based on the qualitative and quantitative analysis of the degradation products, a mechanism was put forward, which is that the in situ generated ˙OH selectively attacked the active site opposite to the phenolic hydroxyl group and induced the cleavage of the alkyl-O-aryl ether bond. The proposed mechanism was further verified by the electrochemical degradation of PBP under controlled conditions, which showed a positive correlation between the degradation efficiency of PBP and the concentration of in situ generated ˙OH radicals.

Lignin is a unique renewable aromatic resource in nature. In the past decades, researchers have attempted to breakdown the linkage bonds in lignin to provide aromatic platform chemicals that used to come from the petrochemical industry. In recent years, electrochemical lignin degradation under mild conditions has drawn much attention from the scientific community owing to its potential to scale up and its environmental friendliness. Sustainable electrochemical degradation of lignin consumes less energy and usually requires mild conditions, but low degradation efficiency and insufficient product selectivity are still significant challenges. The method for lignin degradation by reactive oxygen species (ROS) generated through the water oxidation reaction (WOR) at the anode and oxygen reduction reaction (ORR) at the cathode are more attractive for sustainable electrochemical degradation. The present contribution aims to review advancements in electrochemical degradation of lignin in aqueous or non-aqueous supporting electrolytes, focusing on the regulation of ROS in situ generated on the electrode.