Electrochemical Activity of Lignin Based Composite Membranes

Our society’s most pressing challenges, like high CO2 emission and the constant battle against energy poverty, require a clean and easier solution to store and utilize the renewable energy resources. However, recent electrochemical components are expensive and harmful to the environment, which restricts their widespread deployment. This study proposes an easy method to synthesize and fabricate composite membranes with abundantly found biomass lignin polymer to replace conventional costly and toxic electrode materials. Easier manipulation of lignin within the polymeric matrix could provide the improved composite to enhance its electrochemical activity. Our major focus is to activate the quinone moiety via oxidation in the polymeric mixture using a strong ionic acid. The physico-chemical and electrochemical characterizations of two different lignins within varied polymeric mixture compositions have been carried out to confirm that the redox properties of pure unmodified lignin could be achieved via intrinsic mutual sharing of the structural properties and intercross linkage leading to improved integrity and redox activity/conductivity.

[13] However, conversion of polymeric compounds into carbon is widely used, due to their 51 diversity in shapes and structures i.e. carbon fibres, activated carbon, and graphene, etc.
[14] These 52 materials confer great benefits, such as mechanical improvements or increased conductivity. [15] 53 Furthermore, they can be derived from natural sources such as wood, [16] which is mainly 54 composed of cellulose, hemicellulose and lignin. [17] 55 Lignin is a highly complex aromatic biopolymer, which is usually found in larger quantities around 56 the world, typically used as a source of fuel or an additive [18,19] in bio-mass material applications.

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Implementation of lignin within applications with higher added value has been extensively studied 58 for decades, however, the field of energy storage systems can be added as a novel application already 59 holding numerous investigations in various topics, though, lignin as bulk so far only have been used 60 for generating heat energy. [20] Due to its insulating nature, improvement in its electrochemical 61 properties/ redox activity could be challenging, nonetheless, lignin offers an enriched vital functional 62 groups like hydroxyl (-OH), methoxy (OCH3), aldehyde (CHO) and carbonyl (C=O) that supports 63 easy processing in synthetic monomers and polymers. The highly rich aromatic structure of lignin 64 allows fabrication of low-cost, activated and well-ordered carbons in distinctive shapes and 65 forms. [21][22][23][24] Lignin has already been exploited in different battery systems as binder, [25,26] 66 electrolyte [27] and as an additive, [28,29].

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It has been established that modified and treated lignin could affect positively towards discharge 68 performance of the battery. [30,31] In the meantime, the study of the impacts of different lignin 69 components and the charge storage capacity of lignin have shown the redox activity (combined 70 faradaic/non-faradaic charge storage)proving that by adding non-modified lignin, the capacity of the 71 Polymers 2020, 12, x FOR PEER REVIEW 3 of 20 mixture could increase due to electrical double layer (EDL) charge storage that is usually dependent 72 of the surface area. Hence, the final composite product can provide charge storage capacity 73 depending on the mixing ratio and surface area. However, the highest capacity could be achieved via 74 exposure of lignin functionalities towards electrolyte, homogeneity and high surface area, even 75 though with the unmodified commercial one. [32][33][34][35][36][37][38][39] In order to allow faster transfer of charge 76 storage, lignin needs suitable alterations, whether it is by chemical or physical inter-cross linking, 77 thereby, enhancing electronic conductivity and helping with the electroactive redox activity [40,41].

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The main goal of this study is to achieve low cost and easy-processed lignin based composite 79 membranes with improved redox activity. The commercial unmodified organosolv lignin has been 80 used, due to its higher relative amount of phenolic hydroxyl functional groups, although, freshly 81 extracted kraft lignin has also been used to observe the effect of sulphur groups over redox chemical 82 reaction of. Trials of different ratios and optimization has been carried out, the mixture was further 83 stabilise with the help of non-ionic plasticizer polymer such as polyethylene oxide (PEO), which also 84 help in providing extra -OH groups within the mixture. Mild reaction conditions and simple mixing 85 techniques have been employed to prepare the blends of lignin to emit multiple modification reaction 86 steps in favour of anaffordable and ecological approach. A strong acidic nature polymer, Nafion® 87 has been considered for the easy cleavage of covalent bonds to enhance the possibility of inter-cross 88 linkage that provides the adequate charge for the ionic transfer via chemical/ physical interaction of 89 lignin and assuming that within the process of constant stripping/plating process, these membranes 90 displays the promising longer stripping/plating cycle life. To our knowledge, this is the first attempt 91 to prepare lignin composite polymeric matrix with PEO adding a strong ionic acid i.e. Nafion® with 92 mild conditions, the aim is to coerce potential lignin electrochemical redox activity by activating its

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The lignins have been thoroughly characterized within our research group "BioRP", the ratios and 119 values of H, G, S, and ratio of S/G (

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Electrochemical measurements were carried out at room temperature with mild flow of N2 gas using

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The water uptake capacity of lignin/ polymer composite membranes showed lower rehydration 168 ratios, probably due to the pore generation/swelling owing to their lower thickness. The thickness 169 can be a crucial factor for the pore sizes and porosity of material. PONF, OLPO and KLPONF

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The shoulder of the broad peak at 3226 cm -1 could be due to the intermolecular -H bonds of aliphatic 218 hydroxyl groups. The peak at 3404 cm −1 corresponds to intermolecular dimer OH peak. 223 FTIR spectra ( Figure 5) of OLPONF and KLPONF has been compared with the NF, OL and KL 224 spectra, mostly the peak resembles the mixture of lignin and Nafion®, however, the peaks of PEO 225 seems to be overlapped by the OLNF mixture. In the spectra, the peaks which seems to have a 226 Polymers 2020, 12, x FOR PEER REVIEW 8 of 20 shoulder in the region of ~980.64 cm -1 in OL/KLNF and OL/KLPONF are probably corresponding to 227 the C-O-C group of Nafion® that usually appears slightly shifted (981.21 cm -1 ) in pure NF. The peak 228 at ~960 cm -1 corresponds to Si-OH, Si-O-Si at 804cm -1 instead of 809 cm -1 , and CF2 within the region of 229 1100-1200 cm -1 . C=O peak has shifted from 1656cm -1 to 1712 cm -1 , which in OL/KLNF also appears 230 around same shift. The changing behaviour and impact of NF on the functionality of lignin is still yet 231 to be discovered in depth, however, the possible interconnection could be proven on the bases of 232 slight shifting of C-O-C and C=O stretching vibration bands (Table S2)

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Resistivity and c) Conductivity with reference to applied pressure.

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In order to test the theory of increased conductivity of lignin via composite mixture within ionically 254 rich polymer matrix, the tests have been repeated with the membrane only with lignin and nafion ® 255 (OLNF) (Figure. 7b). The attempts to record the resistance and conductivity of the KLNF mixture 256 membrane was unsuccessful due to brittleness of membrane, which broke upon applying pressure, 257 however, sometimes we achieved a gel type membrane that usually stick to the surface of Cu 258 electrodes and made it quite difficult to follow the calculation procedure.

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An attempt to measure the conductivity in a liquid phase (organic solvent) was done, in order to have 263 an assurance of the direct influence of Nafion® on the conductivity. It seems to decrease up to ~10 -4 , 264 owing to the fact that Nafion®'s hydrophilic sulfonate groups could improve the solubility of 265 quinone in water resulting ionization and production of aromatic anion. However, in the case of 266 organic solvents the ionization -of quinone was restricted, which affected the conductivity. Usually, 267 in the case of liquids, the conductivity shown within a mixture solution is proportional to its ion 268 concentration.

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To explain the loss of hydrophilic groups during the solution submersion, the conductivity has been 273 noted after the wetting test, the OLPONF membranes were chosen on the basis of their higher 274 mechanical intactness, the resistance and resistivity surprisingly increased after submersion, and

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The sweep of different potential rate has been performed from 5-100 mVs -1 , however there wasn't 296 any significant changes in the current rate, that could be due to the possibility of constant ion-297 adsorption dependent redox processes, this constant behaviour could be explained by the fact that 298 the acidic electrolyte has been changed after each experiment, which could provide the constant ionic      Table S2 and S5.

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Mikel Alberro Astarbe has taken part in formal analysis, data curation, writing-review and editing, 397 and visualization.

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Jalel Labidi has taken part in conceptualization, resources, writing-review and editing, 399 visualization, supervision, project administration, and funding acquisition.