Electrochemical Degradation of Lignin by ROS

: 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 e ﬃ ciency and insu ﬃ cient 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.


Introduction
At present, most of the carbon-containing bulk chemicals are transformed from non-renewable fossil resources, such as petroleum and coal, through a complicated conversion process, which brings a series of problems such as the greenhouse effect, climate change, and environmental pollution. According to the European Commission's report "World energy, technology and climate policy outlook 2030 (WETO)", the total world energy consumption would increase from 1.21 billion tons of oil equivalent (2010) to 1.45 billion tons of oil equivalent (2020), and up to 1.71 billion tons of oil equivalent by 2030. It is estimated that global carbon dioxide emissions will increase from 29.3 × 10 9 tons (2010) to 44.5 × 10 9 tons (2030), so the development and utilization of sustainable and renewable energy are imminent for contemporary people [1]. Lignin is the second most abundant component of lignocellulosic biomass after cellulose, and is the largest source of renewable aromatic compounds [2,3]. Therefore, a sustainable chemical industry based on lignin degradation to obtain aromatic platform chemicals has been proposed. Considering the complexity of the lignin molecular structure, the cleavage of the linkage bonds among the phenylpropane units during degradation is essential to improve product selectivity, which might simplify the subsequent separation process of the desired products [4][5][6]. Researchers have tried various methods, including pyrolysis, enzymatic processes, and chemical catalysis methods, to breakdown the linkage bonds in lignin [7][8][9], in which

The Generation of ROS through WOR at the Anode
There are three possible pathways for H 2 O molecules on anode: one molecule losing one electron to form *OH (1e-WOR, reaction R1); two molecules losing two electrons to form H 2 O 2 (2e-WOR, reaction R2); and two molecules losing four electrons to form O 2 (4e-WOR, reaction R3), which is referred to as the oxygen evolution reaction (OER) [28]. The redox potentials versus the normal hydrogen electrode (NHE) are marked as follows: Both *OH and H 2 O 2 could be used in degradation of lignin due to their unique oxidizing abilities. In the aqueous solution, *OH groups exists on the anode surface in the form of chemical absorption, but their lifetime is limited. Thus, trying to produce H 2 O 2 through the 2e-WOR process is considered to be a better choice [29,30]. To improve the selectivity of the 2e-WOR pathway, the practical approach is tuning the binding energies of the adsorbed oxygenic species (*OOH, *O, and *OH) on the electrode surface by choosing suitable electrochemical catalysts [31]. The moderate binding energy of hydroxyl intermediate (∆G *OH = 1.76 eV) is a benefit for the 2e-WOR pathway, because it is strong enough for the initial water adsorption but insufficient for H 2 O 2 to escape from further oxidation. BiVO 4 , CaSnO 3 , and ZnO were reported as electrochemical catalysts for 2e-WOR [32][33][34], because the very weak *OH adsorption corresponding to the optimal free energy rarely occurs on other materials. Furthermore, owing to their wide bandgaps, these electrodes also serve as photoelectrochemical catalysts for H 2 O 2 production. BiVO 4 exhibited a high Faraday efficiency for producing H 2 O 2 in a photo-electrocatalytic process, which benefits from the excellent matching between its bandgap and the redox potential of 2e-WOR [35]. Another important fact for improving the selective of 2e-WOR is the introduction of bicarbonate ions in the electrolyte, either in the electrocatalytic or photo-electrocatalytic process [36].

The Generation of ROS through ORR at the Cathode
The cathodic reaction process of an O 2 molecule might go through three competing pathways: receiving one electron to form a *O 2 − (1e-ORR, reaction R4); receiving two electrons coupled with two protons to form a H 2 O 2 (2e-ORR, reaction R5); and receiving four electrons coupled with four protons to form a H 2 O (4e-ORR, reaction R6) [37]: Owing to the high compatibility and stability of *O 2 − in aprotic solvents, 1e-ORR could occur steadily in an aprotic solvent [38]. As a fact, the generated *O 2 − would rapidly transform to *OOH (R7) if certain protons exist, followed by accepting another electron from the cathode (R8) or reacting with another *O 2 − to form OOH − (R9) [39]. It means that the 1e-ORR pathway could convert to the 2e-ORR pathway just by introducing some protons into the supporting electrolyte. The same phenomenon has been observed by introducing some metal ion (such as Mg 2+ ) into aprotic ionic liquids (ILs) [40]: In an aqueous solution or polymer electrolytes (protonic/alkaline ion exchange membrane), ORR mainly proceeds via 4e-pathway because of the thermodynamic advantage. However, the 2e-ORR pathway should be regulated for producing H 2 O 2 , from which several other ROS (including OOH − , *OH and *O 2 − ) could be formed via ionization, oxidation and rearrangement in the electrochemical system (reaction R10-R12) [26]: As the reverse reaction of WOR, the selectivity of the ORR pathways also depends on the Gibbs free energy of the three adsorption intermediates mentioned above. Along this line, the suitable free energy of adsorbed hydrogen peroxide intermediate (∆G *OOH ) is the key to control the selective two-electron oxidation of water molecules. On the other hand, to inhibit the oxygen evolution reaction, catalysts with an enormous binding energy of oxygenic species (∆G *O or ∆G *OH ) are necessary to impede 4e-ORR. Unfortunately, it is challenging to change ∆G *OOH without influencing ∆G *O or ∆G *OH because there is an individual proportional relationship between these adsorption energies [41].
Recently, significant development has been achieved in the manipulation of the 2e-ORR process. It was reported that Au nanoparticles or Au(111) and Au(110) demonstrated good 2e-ORR reactivity with small overpotentials due to strong O 2 adhesion in the surface of Au nanoparticles [42]. By controlling the size and shape of nanoparticles or the facets of Au or Au-Pd nano-alloys, the selectivity of H 2 O 2 reached 95% [43,44]. Other studies focused on graphene and other carbon-based electrocatalysts to improve the selectivity of 2e-ORR [45,46]. It is worth noting that, due to the large ion volume, steric hindrance and solvation effect of IL, the 1e-ORR process could only steadily observed in aprotic ionic liquids (ILs) because of the stabilization of *O 2 − [47]. Additionally, the excellent reversibility of the O 2 /*O 2 − redox couple in aprotic IL acts as the fundamental for the metal (Na or Mg)-O 2 battery [48].
If the specific protonic agent is present in the aprotic IL, then the formed *O 2 − might be protonated rapidly, while the protonated *O 2 − (*OOH) proceeds another proton-coupled electron transferring processes to form H 2 O 2 (electrochemical-chemical-electrochemical-chemical, ECEC mechanism) [49]. The above phenomenon implies that the type of ROS generated in situ through ORR in IL could be adjusted by manipulation of the electrolysis conditions, including the cathodic potential and protonic agent.

Degradation of Lignin and Lignin Model Compounds by ROS Generated in Situ in Aqueous Electrolytes
Electrochemical oxidation has been described as an effective alternative in wastewater treatment due to its mild treatment conditions and environmental friendliness [50]. Organics can be oxidized by the hydroxyl radicals (*OH) and other active oxygen species produced during the electrochemical reaction [51,52]. In the early studies, the direct electrooxidation method was employed to degrade lignin in aqueous electrolytes, mainly in a strong alkaline medium, in which the *OH generated near the anode was the oxidizing agent [53,54]. Typical linkage bonds in lignin, such as C-C bonds and C-O bonds, were reported to be cleaved with appropriate potential.
Many studies employed dimensionally stable anodes (DSA, e.g., IrO 2 [55], PbO 2 [56], SnO 2 [57]) to oxidize lignin through *OH generated on the anode surface. IrO 2 -based electrodes (Ti/SnO 2 -IrO 2 , Ti/RuO 2 -IrO 2 , Ti/Ta 2 O 5 -IrO 2 , and Ti/TiO 2 -IrO 2 ) were fabricated by overlaying the binary metal oxides on pretreated Ti substrates, and the Ti/RuO 2 -IrO 2 electrode showed the highest stability and the most increased activity for lignin degradation under 500 mA/cm 2 , which was a large current that can be provided in the process of electrochemical degradation of lignin [12]. Additionally, PbO 2 -based electrodes were also applied in electrochemical degradation of lignin due to their high oxygen evolution potential (+2.0 V vs. SCE). Moreover, electro-hydrogenation, as well as electrooxidation, were designed in the electrochemical reaction system, in which lignin was firstly oxidized and cleaved into intermediates through the oxidation of *OH formed by β-PbO 2 crystal on the electrode surface. Afterwards, these intermediates went through the electro-hydrogenation process at the cathodes (such as Ni, Cu, Cu/Ni-Mo-Co, alloyed Steel, Ti/Cu/Sn). In the above system, vanillin, syringaldehyde, acetosyringone, 3-hydroxy-4-methoxyphenyl-ethanone, and 4-methoxy-3-methyl-phenol were obtained as the degradation products [18,53,[58][59][60][61]. Recently, Movil-Cabrera et al. [14] prepared a Co core/Pt partial shell nanoparticle alloy anode coupled with a Pt ring cathode to electrochemically degrade lignin, suggesting that the *OH generated from water electrolysis. A suggested mechanism on the Pb/PbO 2 electrodes as follows: Co core/Pt partial shell anode: It was reported that vanillin could be obtained by using Ni as anode under mild conditions [17]. The researchers investigated different shapes of Ni electrodes and found the apparent mass transfer coefficient was different. As shown in Figure 1, they found that the use of nickel electrodes with high surface area greatly enhanced the degradation percent of lignin under the same electrolysis conditions. Waldvogel et al. prepared a layer of electrochemical active NiO(OH) on Ni anode and used it to directly electrochemical oxidize lignin [62]. To increase the products selectivity on the Ni anode, a simple undivided high-temperature electrolytic cell for the electrooxidation of lignin at 160 • C has been designed recently. Under the optimized electrolysis conditions, the vanillin can be obtained with a yield of 4.2 wt%, which in high selectivity with 67% efficiency compared to the common nitrobenzene oxidation [63].  However, the degradation of lignin through electro-generated *OH on the anode is accompanied by a strong oxygen evolution reaction (OER). As a competitive reaction, OER inhibits the charge transfer of lignin on the electrode surface, requires a higher voltage to be applied, and may have a more significant impact on electrode corrosion. Additionally, it is supposed to be attributed to over-oxidation of degradation products to organic acids and CO 2 . To prevent over-oxidation and achieve the enrichment of the target products, Wessling et al. [64] attempted to design a new type of electrochemical membrane reactor. Combined with the in situ nanopore filtration method, the electrochemical oxidation products of lignin were removed from the oxidizing environment to achieve the effect of sustainable electrolysis (see Figure 2). Separation of low-molecular-weight (LMW) products by a tubular ceramic nanoporous filtration (NF)-membrane was shown to double the number of products isolated from Kraft lignin.
Sustain. Chem. 2020, 1, FOR PEER REVIEW 2 However, the degradation of lignin through electro-generated *OH on the anode is accompanied by a strong oxygen evolution reaction (OER). As a competitive reaction, OER inhibits the charge transfer of lignin on the electrode surface, requires a higher voltage to be applied, and may have a more significant impact on electrode corrosion. Additionally, it is supposed to be attributed to overoxidation of degradation products to organic acids and CO2. To prevent over-oxidation and achieve the enrichment of the target products, Wessling et al. [64] attempted to design a new type of electrochemical membrane reactor. Combined with the in situ nanopore filtration method, the electrochemical oxidation products of lignin were removed from the oxidizing environment to achieve the effect of sustainable electrolysis (see Figure 2). Separation of low-molecular-weight (LMW) products by a tubular ceramic nanoporous filtration (NF)-membrane was shown to double the number of products isolated from Kraft lignin. The inside of the ceramic membrane is used as the reactor, the volume between the ceramic and the ion-exchange membrane is used for the collection of the permeate and the cathodic work is only used to close the electrical circuit [64]. Copyright 2015, Elsevier.
In our previous research, ROS generated through the ORR process in an aqueous solution was introduced to degrade lignin [65]. A non-membraned cylindrical electrolytic cell with the inner layer of graphite felt cathode and the outer layer of a mesh RuO2-IrO2/Ti anode with lignin in alkali solution as an electrolyte was designed ( Figure 3A), in which led to O2 evolution on the anode and O2 reduction to form H2O2 on the cathode. The degradation products were extracted by diethyl ether and the yield was calculated by the weight of the extractant. The yield of lignin degradation products after electrolysis for 1 h in this non-membraned cell was 10.1%, which was more than the sum of the In our previous research, ROS generated through the ORR process in an aqueous solution was introduced to degrade lignin [65]. A non-membraned cylindrical electrolytic cell with the inner layer of graphite felt cathode and the outer layer of a mesh RuO 2 -IrO 2 /Ti anode with lignin in alkali solution as an electrolyte was designed ( Figure 3A), in which led to O 2 evolution on the anode and O 2 reduction to form H 2 O 2 on the cathode. The degradation products were extracted by diethyl ether and the yield was calculated by the weight of the extractant. The yield of lignin degradation products after electrolysis for 1 h in this non-membraned cell was 10.1%, which was more than the sum of the yields in the anodic and cathodic chamber of a membrane cell (4.1% and 3.4%, respectively). The result implied that a synergic effect existed between the anodic and cathodic electrode reaction. It seems that the electrolysis conditions for producing a higher concentration of H 2 O 2 and ROS were in favor of giving higher yields of LMW products. In another study, 59.2% of lignin was degraded into LMW products after 1 h-electrolysis at 80 • C under a current density of 8 mA/cm 2 with extra O 2 . More than 20 kinds of lignin degradation products were identified, which originated from the over-oxidation during the degradation due to the strong oxidation capability of *OH, generated through the decomposition of H 2 O 2 ( Figure 3B) [66]. On the other hand, the electric quantity in this electrochemical reactor is decreased comparing with the others [53,54,57]. yields in the anodic and cathodic chamber of a membrane cell (4.1% and 3.4%, respectively). The result implied that a synergic effect existed between the anodic and cathodic electrode reaction. It seems that the electrolysis conditions for producing a higher concentration of H2O2 and ROS were in favor of giving higher yields of LMW products. In another study, 59.2% of lignin was degraded into LMW products after 1 h-electrolysis at 80°C under a current density of 8 mA/cm 2 with extra O2. More than 20 kinds of lignin degradation products were identified, which originated from the overoxidation during the degradation due to the strong oxidation capability of *OH, generated through the decomposition of H2O2 ( Figure 3B) [66]. On the other hand, the electric quantity in this electrochemical reactor is decreased comparing with the others [53,54,57].  Several simple molecules with alkyl-O-aryl bonds, such as PBP (benzyl-O-phenol) and BPE (benzyl-O-phenyl), were used as lignin model compounds to study the mechanism of the bond cleavage during the electrolysis. By comparing the degradation yield and products of PBP and BPE, we deduced that the phenolic hydroxyl group in PBP might be the active site to be selectively attacked by ROS and finally trigger the cleavage of alkyl-O-aryl bond (as shown in Figure 3C). Based on several well-designed experiments, it was confirmed that the main ROS was *OH. The degradation rate and product yield displayed a good positive linear relationship with the concentration of the electro-generated *OH on the ORR cathode [67]. This result indirectly proves that, compared with a single electrode, the synergistic effect of anode and cathode greatly increase the degradation efficiency of lignin.

Degradation of Lignin and Lignin Model Compounds by ROS Generated in Situ in Ionic Liquids
As alternative electrolytes, ionic liquids (ILs) are used in many electrochemical systems because of the unique properties such as wide electrochemical window, thermal stability, high conductivity, and low volatility [68,69]. Moreover, the electrochemical catalytic depolymerization of lignin in ILs has the following advantages: (1) ILs can improve the dissolution of lignin and its model compounds, thereby promoting the efficiency of electrocatalytic degradation; (2) under mild reaction conditions, different products selectivity and higher electrolysis efficiency can be obtained by applying different cell voltage on the electrode; (3) effectively inhibit OER or over oxidation of lignin, the current intensity generated in the electrochemical reaction process is directly proportional to the yield of intermediate or final products; and (4) avoid using harmful solvents and reagents to pollute the environment, while also reducing the energy consumption of separation and recovery processes. In 2010, Compton's group [70] firstly The oxidation peak A1 was attributed to the formation of *OH, which can be further oxidized to H 2 O 2 (peak A2) at the higher potential. This indicates that an attempt can be made to generate ROS in situ through anodizing in these electrolytes. At the applied voltage of 2.5 V in 24 h, TMS-H 2 O and [emim][OTf]-H 2 O exhibited electrochemical lignin degradation rates of 22% and 23% (w/w), respectively. It was found that in the electrolysis and proton transfer reactions, the presence of water (compared with the absence) displayed a significant impact on the degradation efficiency. Meanwhile, it was also found that the protic ionic liquid TMS is more beneficial to the electrocatalytic generation of H 2 O 2 .
PBP as the lignin model compound was electrolyzed using an ORR cathode in [BMIM]BF 4 (an aprotic IL), and the highest conversion rate of 83.6% for 4 h at a current density of 0.4 mA/cm 2 was observed (as shown in Figure 5A) [72]. The ORR process in [BMIM]BF 4 was studied and the number of the transferring electrons was estimated to be 1. 36   We also studied the degradation of PBP in a protic ionic liquid [73]. The comparison of ORR process in [BMIM]BF 4 and [NEt 3 ]HSO 4 was shown in Figure 6, the anodic peak at 0.51 V (peak 1a) and the cathodic peak at −0.45 V (peak 1c) in both cases attributed to the oxidative reaction of PBP on the anode and the reductive reaction of the oxidation products of PBP, respectively. However, peak 2a and 3a which were revolved with *O 2 − disappeared in [NEt 3 ]HSO 4 , indicating that hydrogen peroxide (H 2 O 2 ) was generated through the 2e-ORR process in the protic IL (see Figure 6).
Sustain. Chem. 2020, 1, FOR PEER REVIEW 6 We also studied the degradation of PBP in a protic ionic liquid [73]. The comparison of ORR process in [BMIM]BF4 and [NEt3]HSO4 was shown in Figure 6, the anodic peak at 0.51 V (peak 1a) and the cathodic peak at −0.45 V (peak 1c) in both cases attributed to the oxidative reaction of PBP on the anode and the reductive reaction of the oxidation products of PBP, respectively. However, peak 2a and 3a which were revolved with *O2 − disappeared in [NEt3]HSO4, indicating that hydrogen peroxide (H2O2) was generated through the 2e-ORR process in the protic IL (see Figure 6). After continuous electrolysis in protic ionic liquid [NEt3]HSO4 for 1 h under a current density of 1 mA/cm 2 at 20 °C, 48.2% of PBP conversed and the faradaic efficiency was calculated as 29.5%. The deduced mechanism is shown in Figure 7. Recently, in order to verify the mechanism of bond cleavage of Cα-Cβ bond or β-O-4 bond in lignin by different kinds of ROS, GGE (guaiacylglycerol-β-guaiacyl ether) was chosen as a lignin model compound to be electrolysis using an ORR cathode in an aprotic ionic liquid ([BMIM]BF4). The results indicated that when the main ROS was *OOH, coniferyl alcohol, guaiacol, and vanillin were detected as the degradation products which is supposed to follow β-O-4 bond cleavage mechanism; Otherwise, when OOH − was the main ROS, coniferyl alcohol could not be detected and the degradation reaction path is assumed to follow Cα-Cβ bond cleavage mechanism [74]. These results might provide more possibility to improve the product selectivity in the electrochemical lignin degradation by carefully controlling the electrolysis conditions. Recently, in order to verify the mechanism of bond cleavage of C α -C β bond or β-O-4 bond in lignin by different kinds of ROS, GGE (guaiacylglycerol-β-guaiacyl ether) was chosen as a lignin model compound to be electrolysis using an ORR cathode in an aprotic ionic liquid ([BMIM]BF 4 ). The results indicated that when the main ROS was *OOH, coniferyl alcohol, guaiacol, and vanillin were detected as the degradation products which is supposed to follow β-O-4 bond cleavage mechanism; Otherwise, when OOH − was the main ROS, coniferyl alcohol could not be detected and the degradation reaction path is assumed to follow C α -C β bond cleavage mechanism [74]. These results might provide more possibility to improve the product selectivity in the electrochemical lignin degradation by carefully controlling the electrolysis conditions.
The expensive of ILs causes concern about the recycling of ILs. Fortunately, the difference between ILs and the degradation products makes it possible to separate the products and recycle the ILs. In our previous study, [NEt 3 ]HSO 4 was re-used for electrolysis of lignin after the degradation products of lignin being extracted by diethyl ether and water being removed by rotary evaporation. Figure 8 shows the color of [NEt 3 ]HSO 4 was gradually darkened after re-used for several times due to a specific residue of lignin. The yield of degradation products in each cycle of electrolysis was 64.1%, 60.7%, and 53.4%, respectively.
The recently reported studies on electrochemical degradation of lignin or lignin model compounds are listed in Table 1. It is found that more extended time and higher voltage is required in the electrolysis by anodic oxidation. However, the competitive reaction of OER with the traditional alkaline solution system results in a lower degradation efficiency. Due to the non-selective mechanism of electrochemical degradation of lignin, in addition to electrochemical conversion rate, researchers are paying more and more attention to the yield of target products, such as vanillin, syringaldehyde, vanillic acid, and guaiacol. On the other hand, the current method for expressing the selectivity of lignin products cannot be unified. It suggests that electrochemical degradation of lignin by anodic oxidation to generated *OH needs longer time and higher current density, most of the conversion rate or current efficiency of these studies were not mentioned (No. 1, 2, 5, 8, 9 in Table 1). The lack of data indicates that the topic is not yet ripe for review. Different ORR cathodes such as graphite felt or C-PTFE gas diffusion electrode (GDE), and a variety of ROS synergistic degradation of lignin in ILs have been investigated, the efficiencies have been promoted to a different extent (No. 10, 11, 13, 14 in Table 1). The 1e-ORR or 2e-ORR processes in ILs could continuously produce ROS, which avoids consuming excess reagents or expensive metal catalysts. Too many protons might quench the ROS, and too less protons would not generate H 2 O 2 . It is worth noting that the number of protons in protic IL is appropriate to in situ electro-generated H 2 O 2 , which is the reason for the improved current efficiency.

Concluding Remarks and Prospects
In summary, lignin degradation by the generated ROS in situ through WOR and ORR processes provides the possibility to improve the degradation efficiency. Firstly, the different kinds of ROS could be generated through manipulating the electrolysis conditions of the WOR anode or ORR cathode, such as choosing the protic or aprotic supporting electrolyte. Secondly, the different kinds of ROS presents different reactive properties and might selectively attack the different linkages in lignin, resulting in good products selectivity. Thirdly, the current efficiency of lignin degradation in ILs is much better than that in aqueous electrolyte, since the lifetime of ROS is prolonged in ILs. Finally, the wide difference between the degradation products and ILs makes it possible to recycle ILs easily.
Nowadays, with the great efforts of scientists and the development of in situ analytical instruments, it is a great significance to reveal the complex reaction mechanism of electrochemical degradation of lignin for improving the selectivity and current efficiency of value-added chemicals. However, sustainable electrochemical degradation of commercial lignin still has a long way to go, due to the complex reactions on the electrode surface. We hope these results could lighten the application of electrochemical methods to degrade lignin to aromatic platform molecules in the near future.
Author Contributions: Investigation and data curation: H.J., A.X., Z.W., R.X., L.W.; resources: Y.C. and P.W.; writing-original draft preparation: Y.C. and H.J.; writing-review and editing: Y.C., H.J., and Y.T. All authors have read and agreed to the published version of the manuscript.
Funding: China National Science Foundation (no. 22075012) and Beijing National Science Foundation (no. 2182046).