Preparation And Characterization Of Lignin-based Cathode Materials For Electro-Fenton Reaction

Electro-Fenton is an advanced oxidation process for removing organic pollutants,in which the in situ electro-generated H₂O₂ can react with Fe²⁺to produce highly reactive radicals?hydroxyl radical,·OH?.The yield and efficiency of H₂O₂ electro-generation are important factors affecting the electro-Fenton process,which is in turn dependent on the type and properties of the cathode materials.The widely used cathodes for the electro-Fenton reaction are carbonaceous materials due to their characteristics such as high surface area and low cost,but these materials might suffer the problem of relatively low electrode kinetics.Some cathode materials such as boron-doped diamond and mercury can offer favorable electrode kinetics,but might have drawbacks in terms of high cost,low safety and environmental unfriendliness.Therefore,the development of renewable,low-cost,and high-efficiency cathode materials for oxygen reduction to H₂O₂ is desirable.This work was made to synthesize a renewable biopolymer with lignin?lig?incorporated into a polypyrrole?PPy?framework via a simple one-step electropolymerization method.This allows the formation of PPy/lig composites that are uniformly coated on a graphite felt?GF?substrate,having large surface area,increased mechanical stability,and abundant C=O?including quinone-type?groups.These features improved the electrocatalytic activity of the PPy/lig-GF cathode for oxygen reduction to H₂O₂ when an optimal cathodic potential?-0.5 V vs.SCE?was applied,as evidenced by the observed higher H2O2 electro-generation yield and efficiency compared to the raw GF and PPy/ClO₄^--GF cathodes.Electro-Fenton systems equipped with the GF cathodes?either modified or unmodified?for the decolorization and mineralization of AO7 followed a pseudo-first-order kinetic model.The PPy/lig-GF cathode achieved substantially higher rate constants and degradation efficiencies.In addition,the incorporation of lignin significantly improved the cycling stability of the cathode material.Focusing on the chemical structure of lignin itself,this work also explored the performance of the cathodes modified with different lignin monomers in the electro-Fenton reactors.The modification of raw GF by different lignin monomers showed good oxygen reduction ability to produce H₂O₂.Due to the difference in the content of phenolic compounds among lignin monomers,the amount of quinone groups formed during the electrochemical oxidation of electrodes was different.Both polypyrrole/syringic acid?PPy/SA?-and polypyrrole/ferulic acid?PPy/FA?-modifed GF cathodes achieved the maximum H₂O₂ yield at-0.5?vs.SCE?,which was 267.75 and 197 mg L^-1,respectively.On the contrary,the maximum value of H₂O₂ yield(147.44 mg L^-1)associated with the polypyrrole/p-coumaric acid?PPy/p-CA?-modified cathode was attained at-0.8 V?vs.SCE?.In addition,it was revealed that the PPy/SA-GF cathode exhibited higher H₂O₂ generation rate and current efficiency,and lower energy consumption in comparison to the PPy/FA-GF and PPy/p-CA-GF cathodes.The physicochemical characterization results indicate that the resulting high surface area,high conductivity,large amounts of active sites?quinone groups and C=O?,and increased defect degree are responsible for the good oxygen reduction ability.