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˙OH selectively attacks the active sites opposite to phenolic hydroxyl groups and leads to bond-cleavage of ether bonds.

p-benzyloxyl phenol (PBP), as a lignin model compound, was electrochemically degraded in [BMIM]BF 4 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 ( • O 2 − ), the one-electron reductive product of dioxygen, is the main resultant of ORR in [BMIM]BF 4 . However, the generated • O 2 − radical transformed into hydroperoxyl radical ( • OOH) in the presence of protons donated by PBP, and then received one more electron to form H 2 O 2 , which then attacked and broke down the alkyl-O-aryl bonds in PBP molecules. The continuous electrolysis of PBP in [BMIM]BF 4 provided indirect evidence to reveal that the in situ generated H 2 O 2 on ORR cathode was responsible for the cleavage of ether bonds in PBP.

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The electrochemical degradation of p-benzyloxyl phenol (PBP) in [HNEt3][HSO4] is investigated using an oxygen reductive reaction (ORR) cathode in a non-membrane cell. It is disclosed that the two-electron reductive products of O2 (that is H2O2) is the main reactive oxygen species (ROS) in [HNEt3][HSO4], a protic ionic liquid (PIL). The degradation of PBP in PIL with the degradation rate of 48.2% and the current efficiency of 29.5% are obtained, which is higher than that in [BMIM][BF4], an aprotic ionic liquid (AIL). Based on the products identified by GC-MS, the effect of electro-generated H2O2 on the cleavage of ether bond in PBP is proposed. The result confirms that the appropriate number of protons in supporting electrolyte plays an important role in electrochemical degradation of lignin model compounds.

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.

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