Lignin Functionalized with Catechol for Large‐Scale Organic Electrodes in Bio‐Based Batteries

Lignin, obtained as a waste product in huge quantities from the large‐scale cellulose processing industries, holds a great potential to be used as sustainable electrode material for large‐scale electroactive energy storage systems. The fixed number of redox‐active phenolic groups present within the lignin structure limits the electrochemical performance and the total energy storage capacity of the lignin‐based electrodes. Herein, the way to enhance the charge storage capacity of lignin by incorporating additional small catechol molecules into the lignin structure is demonstrated. The catechol derivatives are covalently attached to the lignin via aromatic electrophilic substitution reaction. The increased phenolic groups in all functionalized lignin derivatives notably increase the values of capacitance compared to pristine lignin. Further, solvent fractionation of lignin followed by functionalization using catechol boosts three times the charge capacity of lignin electrode.


Introduction
The need for environment-benign sustainable energy sources is becoming more apparent looking at the exponential growth in global energy demand. [1]The global energy consumption is projected to increase by about 30% and electricity production is expected to double by 2050. [2]In order to meet this increasing energy demand while mitigating the climate change, lots of efforts have been directed toward developing renewable supply systems like solar and wind energy. [3]nfortunately, these energy sources are subject to fluctuations depending on the weather parameters. [4,5]When it is not sunny nor windy, very small amounts of energy can be generated.In contrast, surplus energy can be produced when weather conditions are favorable.Therefore, inexpensive, eco-friendly, high-performance large-scale energy storage systems are critical to level out these fluctuations and ensure a constant and reliable power supply on the grid.[11] Lignin is the most abundant electrochemically active natural polymer on earth, isolated as a waste product from forest-based large-scale industries such as biorefinery, pulping, and papermaking.[14] In this context, we have previously reported the cost-effective, safe, scalable fabrication process of lignincarbon composite electrodes. [15]The intimate contact at nanolevel between lignin and conductive carbon particles, achieved by mechanical ball milling, allowed us to harvest the maximum energy storage capacity of the lignin molecule in the bulk electrodes.However, the commercial feasibility of lignin-based electrodes for low-cost manufacturing organic batteries (Figure 1) is still hampered by their low energy density.Among the diverse functional groups present in the lignin, phenol groups, more specifically the substituted catechol groups, are the most important groups from the energy storage perspective.During electrochemical redox reaction, these phenolic groups get reversibly oxidized to quinones via two-electron redox reaction and thereby give the charge storage capacity to lignin (Figure 1).The number of these redox-active phenolic groups is fixed in the commercially available lignin, [16,17] thus limiting the performance of lignin-based electrodes.To improve the charge density and enhance the capacitance of lignin electrodes, new chemical methodologies that can incorporate the redox-active phenolic moieties in the commercially available lignin are highly desirable.
Hereby, we present the chemical derivatization of technical grade lignin to enhance the redox-active catechol content while DOI: 10.1002/aesr.202300146Lignin, obtained as a waste product in huge quantities from the large-scale cellulose processing industries, holds a great potential to be used as sustainable electrode material for large-scale electroactive energy storage systems.The fixed number of redox-active phenolic groups present within the lignin structure limits the electrochemical performance and the total energy storage capacity of the lignin-based electrodes.Herein, the way to enhance the charge storage capacity of lignin by incorporating additional small catechol molecules into the lignin structure is demonstrated.The catechol derivatives are covalently attached to the lignin via aromatic electrophilic substitution reaction.The increased phenolic groups in all functionalized lignin derivatives notably increase the values of capacitance compared to pristine lignin.Further, solvent fractionation of lignin followed by functionalization using catechol boosts three times the charge capacity of lignin electrode.
retaining its reversible electron transfer ability with conductive matrix in the lignin-conductor composite.

Results and Discussion
Commercially available LignoBoost kraft lignin (KL) was modified by covalently linking the ortho-catechol (KLOC), mono-methoxy protected ortho-catechol (KLOMC), and paracatechol (KLPMC) under solvent free conditions, using sulfuric acid as a catalyst.Scheme 1 represents the general strategy of lignin modification.Under acidic conditions, the aliphatic hydroxyl (Al-OH) groups (green line) present within the lignin molecule get protonated and become good leaving groups.These protonated hydroxyl groups are then substituted by catechol moieties (blue line) via aromatic electrophilic substitution reaction.
The degree of substitution and increased phenolic content was analyzed by comparing the 13 P NMR of catechol-modified lignin derivatives with pristine lignin.After the modification of lignin, the peak for Al-OH at 145-150 ppm disappeared and the peaks for phenolic hydroxyl (Ph-OH) at 137-144 ppm increased significantly (Figure 2a).Further, integration of signals from obtained 31 P NMR spectrum indicates that Al-OH groups are substituted almost quantitatively by catechol moieties (Table 1).
The molecular weights of the modified lignin derivatives were monitored by gel permeation chromatography (GPC) (Figure 2b).The high-temperature and acidic conditions during the modification lead to a cleavage of the ether linkages, which slightly reduced the molecular weight of the modified lignin compared to the pristine KL (Table 1).However, the small fragments were removed during the purification process which resulted in lignin derivatives with high phenolic content and small polydispersity index (PDI).
The cyclic voltammetry (CV) and the galvanostatic chargedischarge (GCD) studies in three-electrode systems were performed on lignin/carbon electrodes to evaluate the electrochemical performance of modified lignin derivatives compared to pristine lignin.The results of the electrochemical measurements, taken in 0.1 M HClO 4 electrolyte, are presented in Figure 3.
The cyclic voltammograms of lignin/carbon are shown in Figure 3a.The two distinct peaks at around 0.35 and 0.55 V correspond to the quinone/hydroquinone redox activity of lignin. [18]here is a clear increase in the intensity, indicating increased charge storage in the modified lignin samples.Furthermore, the peak-to-peak separation is found to be increased in all the modified lignin samples compared to the pristine sample.Peak-to-peak separation for KL, KLOC, KLPMC, and KLOMC is 265, 327, 433, and 423 mV, respectively.Compared to all the samples, KLPMC shows a broad peak in CV (blue curve in Figure 3a).This may be due to the presence of two different catechol moieties, ortho-and para-catechol, in KLPMC-modified lignin structure.][20] In addition to pH, the kinetics of the redox reactions might also be dependent on the local environment around the active sites. [21]In the case of KLPMC, due to the possibility of forming both ortho-and para-substituted catechol moieties during the chemical modification of lignin, the overall redox process occurs at rather broad potential window as compared to the KLOC and KLOMC.
Figure 3b,c shows the galvanostatic discharge plots of the lignin/carbon electrodes at charge/discharge rates of 1 and Scheme 1.General strategy for functionalization of lignin using different catechol derivatives via electrophilic substitution reaction.The aliphatic hydroxyl groups (green) are substituted by catechol derivatives (blue).0.1 A g À1 , respectively.The plateau in the discharge plot is observed at slightly higher potential for KLOMC, whereas KLPMC shows the plateau at lower potential than the pristine KL composite.Compared to all the samples, KLOC shows better discharge profile both at 1 and 0.1 A g À1 .Maximum capacity does not vary more than 14% among the modified samples.Maximum capacity of 63 AE 3 mAh g À1 is obtained for the KL composite at 0.1 A g À1 rate followed by 81 AE 4 mAh g À1 for KLOC, 77 AE 3 mAh g À1 for KLPMC, and 88 AE 4 mAh g À1 for KLOMC composite samples.The chemical modification with catechol attached covalently to pristine lignin led to about 40% increase in the charge capacity for the lignin samples.
Another notable feature of an electrode material is the rate at which it can be charged and discharged.If the charge capacity of an electrode does not vary much with the current density, it means that all charge transport and transfer phenomena are fast (electronic conductivity, proton conductivity, electron transfer reaction).Figure 3d shows the compiled data of the discharge capacity at various charge/discharge rates.There is a decrease in capacity when the rate of charge/discharge is increased from 0.1 A g À1 up to 8 A g À1 with 73%, 60%, 75%, and 88% for KL, KLOC, KLPMC, and KLOMC, respectively, indicating the best possible charge storage characteristics by KLOC among the modified samples.This result indicates the possible difference in the substitution of lignin during catechol modification, which,  in turn, can affect the lignin/carbon composite formation while ball milling leading to differences in the discharge capacity.One of the representative electrodes of the catechol modification, namely, KLOC/carbon, was subjected to the galvanostatic cycle at a charge/discharge rate of 1 A g À1 in 0.1 M HClO 4 electrolyte (Figure S1b, Supporting Information).A capacity retention of 60 % was obtained after 2200 cycles for KLOC/C composite in 0.1 M HClO 4 electrolyte due to the possible degradation of the ether bonds in modified lignin (KLOC).Among the several side-chain moieties in native lignin, the presence of significant amounts of α-O-alkyl and γ-O-alkyl ethers has been suggested. [22]Under acidic conditions, these ethers bonds get cleaved leading to slow degradation of lignin which can lead to the loss of capacity.
In a previous study by Crestini et al., the acetone-soluble kraft lignin (ASKL) and acetone-insoluble kraft lignin (AIKL) fractions were analyzed in terms of chemical composition and molecular weights. [17]The AIKL had a low content of phenol (3.5 mmol g À1 ) compared to the ASKL fractions (5.2-5.7 mmol g À1 ), but a more than three times higher content of reactive arylglycerol-β-aryl ether (β-O-4 0 ) substructure.This triggered our interest of this as a starting material for catechol substitution reactions.The reactive β-O-4 0 ether bond linkages can also undergo substitution reaction with catechol and further improve the charge storage capacity.In order to test our hypothesis, lignin fractions were extracted from the crude kraft lignin using acetone as solvent in a single-step fractionation process.The ASKL and AIKL fractions were mechanically milled with carbon black to form the composites with a 1:1 weight ratio of lignin/carbon.These are the reference samples providing a low current level in the CV (Figure 4a) and low storage capacity of 34 AE 2 mAh g À1 (AIKL) and 51 AE 3 mAh g À1 (ASKL) (Figure 4b).The ASKL has better charge storage ability in the composite electrode compared to AIKL.This is believed to be due to a higher phenolic content in ASKL compared to AIKL (Table S1, Supporting Information).Both ASKL and AIKL fractions were then functionalized with catechol (C-ASKL, C-AIKL) followed by the mechanical milling with carbon black to form the composite electrode material of similar lignin/carbon ratio (1:1).During the functionalization synthesis with catechol, the precipitation step purifies the modified lignin such that only low molecular weight fraction with narrow polydispersity is obtained (Table S1, Supporting Information).The resulting charge storage capacities are as high as 98 AE 5 mAh g À1 (C-ASKL) and 106 AE 5 mAh g À1 (C-AIKL) at 0.1 A g À1 (Figure 4b).The highest charge capacity is obtained from modified AIKL and as we hypothesized it is due to its higher content of the reactive β-O-4 0 substructure compared to the ASKL fraction, which makes AIKL prone to incorporate more catechol units into the lignin structure.It is also important to note that the total phenolic content after catechol modification for ASKL fraction increased from 4.75 to 5.97 mmol g À1 and for AIKL fraction it increased from 4.42 to 6.08 mmol g À1 (Table S1, Supporting Information).These results are in good agreement with our above hypothesis that β-O-4 0 ether groups in AIKL can undergo catechol substitution reaction leading to higher phenolic content.Figure 4d shows that the capacity decreases faster versus current density for the C-ASKL than C-AIKL (21% capacity retention for C-AIKL vs 0.14% for C-ASKL at 8 A g À1 ).Finally, we compare the effective redox activity of these samples by measuring the ratio (R) of the total current due to the redox peaks (oxidation and reduction) and the corresponding capacitive current at 0 V (vs Ag/AgCl), represented as R = I redox activity /I capacitive (see inset in Figure 4d).I capacitive is mostly due to the creation of electric double layer between the conducting carbon phase and the electrolyte, while I redox activity reflects the amount of phenol redox groups involved in the charge storage on the lignin phase. [23]Because the electric double-layer contribution is small and likely similar between the various four samples, the large current ratios R found for the two catechol modifications (C-AIKL with R = 7.87 and C-ASKL with R = 7.61) compared to the nonfunctionalized fraction (ASKL with R = 5.35 and AIKL with R = 5.27) clearly indicate the success of the approach to increase charge storage capacity by the introduction of aromatic phenolic groups in lignins.
We shortly review the values found in the literature for ligninbased electrodes to be able to localize our finding.lignosulfonate (LS) composited with graphite was obtained with 35 mAh g À1 . [24]esulfonated lignosulfonate composited with carbon black via ball milling reached 38 mAh g À1 . [15]The conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) can self-assemble with lignosulfonate by electrostatic interactions to form insoluble PEDOT-LS composite.However, the heavy mass of the monomer limits the value of specific capacity (45 mAh g À1 ). [14]Kraft lignin composited with carbon black via ball milling led to a much cheaper alternative with slightly better performance (49 mAh g À1 ). [23]A significant improvement is obtained by fine tuning the composition of a synthetic polymer made of syringol (S), guaiacol (G), and hydroquinone (HQ).S and G are monomers in natural lignins.On top of that, single-walled carbon nanotube (SWCNT) is used to have an intimate mixing with the redox polymer.The resulting PolySGHQ/SWNT electrode has a capacity of 72 mAh g À1 . [13]When the same synthetic polymer PolySGHQ is coupled to poly-pyrrole, very high values were obtained (94 mAh g À1 ). [13]This can be regarded as an ultimate value.However, it is important to note that this result is not based on natural lignin and it is obtained with an expensive carbon conductor (SWCNT).The use of a conducting polymer with low monomer weight like pyrrole is a way to decrease the active mass of the electrode and combine directly with lignosulfonate to obtain the decent value of 75 mAh g À1 . [25]More complex polymer electrodes including redox activity at various potentials could broaden the voltage storage range and, consequently, enhance the charge capacity.It is the case with the combination of PEDOT/lignin composited with polyanthraquinone (PAAQ) (75 mAh g À1 ). [26]The largest capacity found with a cheap material combination is based on kraft lignin and activated carbon with a value of 80 mAh g À1 . [18]For the later result, it is important to mention that most of the energy stored in those electrodes is via the electric double layer with the activated carbon/electrolyte interface rather than via the redox chemistry of lignin.So that type of electrode is rather for supercapacitors than batteries.From this overview of the literature, it appears clearly that we are breaking a record for the energy storage of low-cost lignin-carbon electrode with 106 mAh g À1 .Moreover, in those novel lignin electrodes, the energy is dominantly stored by the redox activity of the lignin functionalized with catechol.

Conclusion
In summary, commercially available technical grade lignin was functionalized by covalently attaching the small redox-active aromatic molecules to lignin backbone.The cleavage of the ether linkages during the modification resulted in lignin derivatives with high phenolic content and narrow mass distribution compared to pristine lignin.The increased redox-active phenolic groups led to 40% higher charge storage capacity.The charge storage capacity of lignin was further enhanced by solvent fractionation of lignin followed by functionalization using catechol to reach 100 mAh g À1 , which is a new record for low-cost lignin-carbon electrodes.
The modification of lignin via electrophilic substitutions on aromatic catechol moieties is a scalable, cost-effective method that makes "ligno-catechol" a promising electrode material for large-scale batteries and eventually opens new routes to add other functionalities to tailor the properties of lignin for various applications.

Experimental Section
Technical grade LignoBoost lignin was purchased from Valmet.All other chemicals were purchased from Sigma, unless otherwise mentioned.
General Procedure for Lignin Modification: LignoBoost kraft lignin (10.0 g) was dissolved in 20 g of molten catechol (dihydroxybenzene, 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol).Conc.H 2 SO 4 (5 mL) was then added dropwise to the reaction mixture and heated at 180 °C, while stirring, for 60 min.The reaction mixture was cooled down and dissolved in 200 mL acetone and then precipitated by adding slowly into 1.5 L of dilute H 2 SO 4 (2 N).The precipitate was separated using sintered funnel.The crude product was washed with water (4 Â 250 mL) followed by diethyl ether (4 Â 200 mL).The dark brown material obtained was dried under vacuum at 120 °C for 5 h.
P NMR Analysis: The hydroxyl groups of lignin samples were phosphorylated and quantified using 31 P NMR spectroscopy by adjusted method according to Granata and Argyropoulos. [27]Briefly, lignin sample was dissolved using a mixture of dimethylformamide (DMF) and pyridine in the presence of an internal standard (IS) and a relaxation reagent (RR) and then phosphorylated using a mixture of derivatization reagent (DR) and deuterated chloroform.The phosphorylated sample was then scanned using liquid state 31 P NMR spectroscopy and the hydroxyl groups were quantified by integration of the corresponding signals from obtained 31 P NMR spectra.GPC Analysis: GPC analyses were performed using Agilent 1260 Infinity II instrument with triple detectors (refractive index (RI), light scattering (LS), viscometry).Two GPC columns (Agilent PolarGel M, PL1117-6800, 8 μm, 300 Â 7.5 mm) were connected in series for analyses.Highperformance liquid chromatography (HPLC)-grade DMF with 0.1% LiBr was used as the eluent (0.5 mL min À1 , at 30 °C).Standard calibration was performed with polymethyl methacrylate standards (EasiVials, MW range 535-2 210 000 g mol À1 ).The dissolved samples (DMF) were shaken for 10 min at room temperature and filtered over a 0.45 μm syringe filter.Sample detection was performed with triple detectors.All data were recorded using Agilent GPC/SEC V2.2 Software.
Lignin-Carbon Composite Electrode: The lignin/carbon electrode material was fabricated via solvent-free mechanical milling (ball milling) of conductive carbon (ENSACO 360 G from IMERYS Graphite and Carbon, with the supplier mentioned Brunauer-Emmett-Teller (BET) surface area of 780 m 2 g À1 ) and the modified lignin, in a planetary ball mill, Retsch PM 100 using ZrO 2 milling media.The dry ratio of the lignin and carbon was 1:1.The milled powder was then combined with 6 wt% of CMC/SBR (carboxymethyl cellulose/styrene butadiene rubber; 2:4) binder system, and the resulting slurry was applied onto the current collector.For electrochemical measurements, stainless steel current collector, precoated with conductive carbon, was used as the current collector.

Figure 1 .
Figure 1.Sketch of a vision for organic batteries based on forest materials.Chemical structure of lignin and its redox activity via proton-electron coupled transfer.

Figure 2 .
Figure 2. a) Quantitative 31 P NMR spectra and signal assignment of pristine and modified lignin derivatives; b) molecular weight distributions of pristine lignin and modified lignin derivatives obtained by GPC.

Figure 3 .
Figure 3. a) CV of the modified lignin/carbon composites at a scan rate of 5 mV s À1 ; b,c) are the galvanostatic discharge plots at a charge/discharge rate of 1 and 0.1 A g À1 , respectively; and d) discharge capacity at different charge/discharge rate for the composites.

Figure 4 .
Figure 4. a) CV at a scan rate of 5 mV s À1 ; b) the galvanostatic discharge plots at a charge/discharge rate of 0.1 A g À1 ; c) discharge capacity at different charge/discharge rate; and d) the I redox activity /I capacitive comparison plot for the composite electrode containing fractionated kraft lignin in acetone before and after the catechol modification.The measurements are done in 0.1 M HClO 4 .

Table 1 .
Degree of modification quantified by 31 P NMR; molecular weight distribution and PDI measured by GPC.