Lignin–Chitosan Gel Polymer Electrolytes for Stable Zn Electrodeposition

Lignin−Chitosan Gel Polymer Electrolytes for Stable Zn Electrodeposition Naroa Almenara, Robin Gueret, Alberto J. Huertas-Alonso, Unnimaya Thalakkale Veettil, Mika H. Sipponen,*and Erlantz Lizundia* Cite This: ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Electrochemical energy storage technologies offer means to transition toward a decarbonized society and carbon neutrality by 2050. Compared to conventional lithium-ion batteries, aqueous zinc-ion chemistries do not require scarce materials or toxic and flammable organic-based electrolytes to function, making them favorable contenders in the scenario of intensifying climate change and supply chain crisis. However, environmentally benign and bio-based materials are needed to substitute fossil-based battery materials. Accordingly, this work taps into the possibilities of lignin together with chitosan to form gel polymer electrolytes (GPEs) for zinc-ion chemistries. A simple fabrication process enabling free-standing sodium lignosulfonate− chitosan and micellar lignosulfonate−kraft lignin−chitosan GPEs with diameters exceeding 80 mm is developed. The GPEs combine tensile strength with ductility, reaching Young’s moduli of 55 ±4 to 940 ±63 MPa and elongations at break of 14.1 ±0.2 to 43.9 ± 21.1%. Competitive ionic conductivities ranging from 3.8 to 18.6 mS cm−1and electrochemical stability windows of up to +2.2 V vs Zn2+/Zn were observed. Given the improved interfacial adhesion of the GPEs with metallic Zn promoted by the anionic groups of the lignosulfonate, a stable cycling of the Zn anode is obtained. As a result, GPEs can operate at 5000 μA cm−2with no short-circuit and Coulombic efficiencies above 99.7%, outperforming conventional separator−liquid electrolyte configurations such as the glass microfiber separator soaked into 2 M ZnSO4aqueous electrolyte, which short-circuits after 100 μA cm−2. This work demonstrates the potential of underutilized biorefinery side-streams and marine waste as electrolytes in the battery field, opening new alternatives in the sustainable energy storage landscape beyond LIBs. KEYWORDS: lignin, chitosan, bioeconomy, circular economy, gel polymer electrolyte, zinc-ion battery (ZIB), zinc plating/stripping ■INTRODUCTION Our society faces serious global challenges associated with the depletion of finite non-renewable resources, environmental pollution, and climate crisis. The current linear economy, following a “take−make−dispose” approach, requires extensive amounts of raw materials that are then processed into goods and once used are finally discarded as a non-biodegradable waste. The extraction and processing of non-renewable materials such as metals or petroleum-based polymers is associated with notable environmental costs, including large carbon dioxide footprints, air and drinking water pollution, biodiversity loss, or eutrophication, among others. 1 Therefore, ambitious initiatives have been set to transition toward a completely circular economy by 2050. 2 In a circular economy, the materials and products are used for as long as possible, which, coupled with lower rates of extraction and exploitation of renewable natural resources, reduces resource depletion and avoids uncontrolled waste accumulation during the end-oflife. 3 The development of efficient renewable energy conversion and storage technologies is a pressing need to reach sustainable production and consumption patterns that regenerate natural systems. 4,5 The energy transition is pivotal toward climate change mitigation as it may shift from fossil-energy production to renewable-energy sources. 6 Electrochemical energy storage systems are particularly relevant for renewable-energy exploitation given their ability to store and deliver on-demand power. 7 So far, rechargeable lithium ion batteries (LIBs) have been the predominant solution given their relatively high energy-densities, low self-discharge rates, and long operation lifespans. 8 Unfortunately, current LIBs rely on scarce, expensive, and often harmful materials such as lithium, cobalt, manganese, or nickel to function. In addition, flammable organic-based electrolytes are often required, increasing the risk of undesired fires or explosions. As a result, their Received: September 29, 2022 Revised: January 16, 2023 Published: January 30, 2023 Research Article pubs.acs.org/journal/ascecg © 2023 The Authors. Published by American Chemical Society 2283 https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 Downloaded via UNIV DEL PAIS VASCO on March 14, 2023 at 16:14:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. fabrication, use, and end-of-life management poses a number of challenges from an environmental point of view, especially in the long run. 9,10 Seeking for alternative battery chemistries that offer acceptable electrochemical performance, good safety, and use of earth-abundant and environmentally friendly materials is becoming increasingly urgent. Rechargeable zinc-ion chemistries fulfill these stringent requirements as they rely on zinc ions (Zn2+) as charge carriers. 11 Conventional zinc-ion batteries benefit from the large capacity of Zn (820 mAh g−1). 12 These batteries are composed by a Zn metal negative electrode (anode), a positive electrode (cathode) that enables the reversible Zn2+ (de)insertion, and a glass microfiber separator soaked into a liquid electrolyte. 13 Importantly, they do not require dry atmospheric assembly conditions as zinc-ion chemistries could operate with aqueous-based electrolytes, which further enhances their safety and environmental sustainability over LIBs and other multivalent systems (Mg, Ca, and Al). 11 Thereby, aqueous zinc-ion chemistries present significant advantages for stationary energy storage. 14 The separator−electrolyte pair is a relevant battery component because it determines to a large extent its electrochemical performance (energy density or cycle stability) and safety (thermal stability or resistance against dendrite puncture). 15 This component must ensure adequate ion transference between electrodes at the same time that electronically and physically insulates the anode and the cathode so that internal short circuits and eventual ignition or explosion risks are avoided. 16 The substitution of the porous non-renewable separator soaked into an aqueous electrolyte by a gel polymer electrolyte (GPE) is now pursued by both academia and industry to enhance battery safety by avoiding electrolyte leakage. 17 GPEs typically show an increased compatibility with electrodes given their mechanical flexibility, surpassing liquid electrolyte designs in terms of interfacial compatibility and operating lifetime. 18,19 In addition, GPEs offer increased room-temperature ionic conductivities in comparison with solid-state Zn electrolytes, which usually present conductivities below 10−4mS cm−1due to the high charge density of Zn2+. 20 Usually, GPEs are obtained from petrochemical sources, with polyethylene oxide, polyvinylidene fluoride and its copolymers, or polyacrylonitrile being the most widely found systems. 19 In contrast, if the development of GPEs is accompanied by the use of renewable sources, additional environmental benefits may be obtained. Interestingly, recent works have proven the suitability of biopolymerderived GPEs to achieve highly reversible and stable metal deposition in batteries. 18,21 Another advantage of many natural polymers is their hydrophilic nature that facilitates ion exchange across the GPE. As the batteries are becoming common in increasing number of everyday applications, there is a need for large scale biomass resources for GPEs. Underutilized biorefinery wastes offer plenty of materials with abundant functional groups and tailored properties to be exploited as battery electrolytes. So far, most of the efforts to develop Zn2+ conducting materials have been directed toward the use of celluloses, 22 agarose, 23 or chitosan. 24 Among the bio-based materials not yet fully exploited, lignin is particularly attractive given its aromatic structure and abundant production as a by-product from industrial biomass processing. 25 Currently, ∼98% of lignin production (from the estimated 80 million ton year−1) is combusted for energy recovery purposes, which clearly collides with circular economy principles. 26 This resource waste represents a missed opportunity, particularly when considering recent life cycle assessment (LCA) studies highlighting that when processed into materials and (nano)fillers, 27 lignin can lower the environmental impacts (including CO2footprint) over their petro-based counterparts. Therefore, the valorization of lignin into high-value-added products will positively contribute to economics and enhance carbon efficiency of biorefineries. 28 Among lignin-derived materials, sodium lignosulfonate (LS) and kraft lignin (KL), products arising from the sulfite and sulfate pulping process show interesting properties for GPEs. In addition to their phenolic hydroxyl and sulfonic acid groups, the aromatic groups and ether bonds of lignosulfonates can interact with different metal ions through cation−πinteractions to enhance ion conduction, 29 yielding GPEs with large ionic conductivities. In addition, the adhesive properties of lignin 30 can be exploited to obtain materials that adhere onto metallic surfaces, enhancing the stability of the electrolyte−electrode interface and enlarging the lifespan of a battery. Sulfonation and subsequent chlorination of pine acid hydrolysis lignin and its combination with poly(vinyl alcohol) has been reported to produce GPEs with an ionic conductivity of 0.25 mS cm−1. 31 However, the potential of sodium lignosulfonate, which is the dominant technical lignin based on today’s production quantities, in combination of natural polysaccharides has remained unexplored in GPEs. Accordingly, here, we report a simple yet effective scalable fabrication of free-standing GPEs consisting of chitosan and lignosulfonate or micellar dispersion of lignosulfonate with softwood kraft lignin, each of the formulations combining renewability, low cost, thermomechanical resistance, ionic conductivity, and electrochemical stability. The gel character of the biopolymer-electrolyte enhances battery safety and environmental sustainability over conventional designs based on microporous fossil-based separators soaked in liquid electrolytes. Importantly, the presence of free water inherent to glass microfibers in the 2 M ZnSO4aqueous electrolyte system is avoided, suppressing side reactions onto Zn surfaces and thus obtaining longer operation lifespans in symmetric Zn/Zn cells. 32 Obtained results represent a step forward in the development of batteries that use upcycled biorefinery waste streams. ■EXPERIMENTAL SECTION Materials. Softwood kraft lignin (KL, BioPiva 100 pine kraft lignin (UPM, Finland), lignosulfonate (LS, DS10, Domsjo, Sweden). Chitosan (50,000−190,000 Da), glutaraldehyde (50 wt % in H2O), and zinc sulfate monohydrate (ZnSO4·H2O) have been purchased from Sigma-Aldrich. The carbon paper (TP-060-T5) was purchased from QuinTech. Electrolytes were prepared using ultrapure water (18.2 MΩcm). All chemicals were used as received without any further purification. For electrochemical studies, glass microfiber separators (GF/A, Whatman) were employed as received. Zn foil (0.25 mm thickness) was obtained from Thermo Fisher Scientific. Fabrication of Gel Polymer Electrolytes. As summarized in Figure 1, lignosulfonate−chitosan gel polymer electrolytes were synthesized using LS or a micellar dispersion of lignosulfonate− kraft lignin (LSKL). 33 Four samples of two different compositions of LSKL−chitosan and LS−chitosan were prepared, obtaining a lignin content of 10 and 30 wt % in the final membrane. Chitosan was added to a minimum of 70 wt % to improve the film-forming ability while ensuring adequate electrochemical properties (lower chitosan concentrations fail to form free-standing membranes). Table S1 in the Supporting Informationgives additional details on each sample ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2284 composition along with the nomenclature. Glutaraldehyde was used as a chemical cross-linker to enhance the physical integrity of the gels. Preparation of Lignosulfonate−Kraft Lignin−Chitosan Membranes. The chitosan solution (1 wt % in 1 wt % of acetic acid) was added to a vial containing a predetermined amount of KL-LS gel dispersion (43 wt %). The mixture was stirred for 3 days until homogenization was attained. Then, for the cross-linked samples, 50 wt % glutaraldehyde was added to the homogenized solution and the mixture was stirred at 500 rpm for 30 min. After that, the samples were casted on polypropylene Petri dishes (⌀= 80 mm) and allowed to cross-link for 3 days at room temperature in the fume hood. GPEs of 250 ±70 μm thickness were obtained after immersing the films into a 2 M ZnSO4aqueous solution overnight. Preparation of the Lignosulfonate−Chitosan Samples. Lignosulfonate solution (1 wt %) was dropwise added to a chitosan 1 wt % solution in 1 wt % of acetic acid under magnetic stirring using a syringe pump at a flow rate of 15 mL h−1. The mixture was stirred for 3 days until homogenization was attained. Then, for the cross-linked samples, 50 wt % glutaraldehyde was added to the homogenized solution and the mixture was stirred at 500 rpm for 30 min. After that, the samples were casted on polypropylene Petri dishes (⌀= 80 mm) and allowed to cross-link for 3 days at room temperature. GPEs of 250 ±70 μm thickness were obtained after immersing the films into a 2 M ZnSO4aqueous solution overnight. Membrane Characterization. Field-emission scanning electron microscopy (FE-SEM) analyses were carried out using a Hitachi S4800 at an acceleration voltage of 5 kV. Before the morphological observations of the cryo-fractured surfaces, the samples were sputtered with a 10 nm-thin gold−palladium layer. Powder X-ray diffraction (XRD) patterns were obtained with a D8 Discover diffractometer in reflection mode using Cu Kαradiation (45 kV, 40 mA). Attenuated total reflectance Fourier transform infrared (ATRFTIR) spectroscopy results were obtained using a Varian 610-IR FTIR spectrometer equipped with diamond ATR optics. The thermal degradation behavior of gel electrolytes was studied by means of thermal gravimetric analysis (The Discovery TGA) in platinum oxide pans under air atmosphere at a heating rate of 10 °C min−1and 50 mL min−1for each sample (2 to 7 mg). The mechanical behavior of samples was studied by uniaxial tensile tests using a universal testing machine (Trapezium Shimadzu AGS-X) equipped with a 100 N load cell at a deformation rate of 1 mmmin−1. Specimens 20 mm long and 6 mm wide with thicknesses of 250 ±70 μm were used. The mean average value and standard deviation Young’s modulus (E) (from 0.5−1% strain region), stress and strain at yield (σyand εy, respectively), and stress and strain at break (σband εb, respectively) were determined from four measurements. The electrolyte uptake (EU) of lignin gel electrolytes was measured after immersion of the LSKL−chitosan and LS−chitosan membranes in 0.5, 1, and 2 M ZnSO4aqueous solution for 24 h as m m mEU 100 ( ) dry wet dry = × (1) where mwet and mdry are the weight of the wet and dry lignin gels, respectively. Electrochemical studies were carried out using a VMP3 Biologic electrochemical workstation. Gel electrolytes (diameter = 13 mm; area = 1.327 cm2) were assembled into Swagelok-type cells at room temperature. For ionic conductivity measurements, gel electrolytes were sandwiched between two stainless steel rods. Electrolyte resistance was measured using a two probe AC impedance spectroscopy analyzer with a 5 mV voltage amplitude in the frequency range from 1 Hz to 5 MHz. The resistance was measured from the high-frequency intercept on the real axis in the Nyquist plots, and the ionic conductivity (σi) was obtained according to d R A i b = × (2) where dis the gel thickness, Rbrepresents the bulk resistance extracted from the intercept of the curve with the real impedance axis in the Nyquist plot, and Aaccounts for the contact area of the gel electrolyte and the stainless steel rod. In addition, the electrochemical stability window was studied by voltammetric measurements, where gel electrolytes were sandwiched between a carbon paper as the working electrode and a Zn metal disk as the reference and counter electrode. The voltammograms were obtained in the potential range of −0.25 V to +2.4 V vs Zn2+/Zn with a scan rate of 1 mV s−1using a VMP3 Biologic instrument. Zn stripping and plating performance was studied under different current densities from ±50 to ±500 μA·m−2. To that end, Figure 1. Schematic representation of the fabrication process of LS−chitosan and LSKL−chitosan gel polymer electrolytes. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2285 lignosulfonate−chitosan and LSKL−chitosan GPEs were mounted between two Zn-metal discs. For the sake of comparison, Zn stripping and plating was also performed using a glass fiber separator soaked in a 2 M ZnSO4aqueous solution. For post-mortem SEM studies, the cycled Zn surfaces were washed with water before drying under vacuum. ■RESULTS AND DISCUSSION Morphological and Structural Characterization. A scalable fabrication process for GPEs is developed using underutilized biorefinery wastes as a source material while enabling a rapid and stable Zn2+ transport. Our efforts have been focused on the use of lignosulfonate and chitosan as the polymeric constituents for free-standing membrane formation, which after immersion in 2 M ZnSO4H2O yield ionically conducting GPEs. Preliminary experiments showed the inability to achieve lignin-only GPEs due to their poor filmforming properties. In this sense, chitosan was incorporated at 70 and 90 wt % as a model polysaccharide to improve the filmforming ability. 34 The morphology of the resulting GPEs after freeze-drying was investigated by scanning electron microscopy (SEM), and the cryo-fractured cross sections are shown in Figure 2a. The obtained dense and smooth structure with no voids or crystalline aggregates observed for all the compositions indicates the uniform dissociation of the zinc salt within the polymeric gel. This feature is particularly interesting as it is considered a prime requisite to achieve a homogeneous ion transport across the GPE and offer a stable ion electrodeposition onto Zn. An increased amount of lignin yields brownish GPEs (Figure S1). As shown in Figures S2 and S3, the developed approach is simple and scalable enough to result in homogeneous, free-standing, and mechanically flexible GPEs with diameters above 80 mm. Amorphous electrolytes are preferred as the crystalline regions generally present an increased resistance for the transport of ions. 35 Lignins are known to be amorphous, but chitin, the precursor of chitosan, can form crystalline domains. 36 Accordingly, the occurrence of crystalline phases has been assessed by X-ray diffraction experiments. As shown in Figure 2b, all the GPEs present an amorphous halo with a broad peak centered at 2θ= 20.5°originating from the overlap of the (110) crystal plane of chitosan (2θ=∼19.2°), 37 together with the broad band at 2θ= 22°of stacked lignin aromatic layers into the (002) plane (see the diffraction patterns of the biopolymer GPE components in Figure S4). 38 As opposed to bare ZnSO4salt, which forms a crystalline structure with well-differentiated and narrow diffraction peaks (Figure S4), 39 the GPEs here synthesized present low intensity sharp peaks, indicating the coexistence of large amorphous regions with few crystalline phases. Overall, the predominant amorphous halo for all the GPEs suggests the ability of lignin− chitosan blends for complexation and dissolution of ZnSO4, enabling a low barrier for ion diffusion for the migration of Zn2+ through these amorphous phases (note, on the contrary, the highly crystalline character of the ZnSO4salt in Figure S4). 40 Attenuated total reflectance−Fourier transform infrared (ATR-FTIR) spectra in Figure 2c present the characteristic absorption bands of lignin, with a band at 1000 cm−1assigned to C−O deformation and aromatic C−H in plane deformation, 1550 and 1650 cm−1(aromatic skeletal vibrations and C�O stretch), 2800−3000 cm−1C−H stretch in methyl groups in LS and methylene groups of LS and chitosan, and a broad band at 3100−3500 cm−1assigned to hydroxyl groups in chitosan and lignin. 41 Thermal and Mechanical Properties. The development of GPEs with acceptable thermal stabilities is an essential prerequisite toward battery safety so that the likelihood of thermal runaway is reduced. Thermogravimetric analysis (TGA) is commonly used to assess the thermal stability of battery separators. The TGA curves under air atmosphere Figure 2. (a) Representative SEM micrographs showing the cross sections of the GPEs with varied compositions. (b) XRD patterns and (c) ATRFTIR spectra of synthesized GPEs. The scale bar size for SEM images varies due to the slight differences in film thickness. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2286 shown in Figure 3a for LSKL−chitosan and LS−chitosan membranes (before ZnSO4soaking and no Zn salts) show an initial weight loss of 10−15 wt % at 75 °C, resulting from the adsorbed moisture evaporation. A marked thermodegradation event occurs in the temperature range of 245−400 °C. This weight loss arises from pyrolysis of the biopolymer involving fragmentation of the inter-unit linkages between phenolic and carbonyl groups and the release of monomeric phenols in lignin 30 as well as the deacetylation and cleavage of glycosidic linkages in chitosan. 42 Finally, a decomposition of the residual carbon occurs. In addition, Figure 3b shows the thermodegradation traces for the GPEs, which are obtained after soaking the membranes into 2 M ZnSO4H2O. The mass loss event at nearly 200 °C arises from the water loss resulting from ZnSO4 salt crystallization. 43 Interestingly, ZnSO4exerts a retarding effect on mass loss for all the studied formulations, particularly during the second and third degradation stages at temperatures above ∼300 °C. In particular, the GPEs reach their 50% weight loss at 390−438 °C (depending on the composition) in comparison with the 311−408 °C observed for the membranes without the electrolyte. For the sake of comparison, as highlighted by the dashed line, the microporous polyolefin Celgard 2325 separator (composed by petro-based polypropylene and polyethylene) reaches the 50% weight loss at 284 °C. 44 This flame retardancy effect of lignin is consistent with previous studies, which ascribed such behavior due to the formation of intumescent char layers efficiently limiting the heat and flammable volatile transfer. 43,45 In addition, the marked increase in the residual mass at 700 °C from 2.5 wt % for KL-LS/Chi 10/90 to 36 wt % for its GPE counterpart is explained by the effective flame retardant role of zinc sulfate by promoting the formation of charring layers. A similar effect was also found in polypropylene-based composites. 45 The amount of char residue slightly increases with lignin amount (either KL or LS) given the tendency of aromatic rings to yield carbonaceous structures. The improved thermal stability after impregnation with ZnSO4together with the larger char residue at high temperatures indicates that LSKL−chitosan and LS− chitosan GPEs are appealing to physically isolate battery electrodes (anode and cathode) at high temperatures, avoiding the risk for short-circuit upon thermal runwaway. 46 Mechanical properties of GPEs should be also considered when designing safe batteries with long-operation life spans. In particular, mechanically adaptable and ductile electrolytes typically show an enhanced interfacial contact with metallic electrodes, while stiff GPEs offer a physical barrier against undesired dendrite growth. 47 Uniaxial tensile stress−strain experiments were conducted to evaluate the Young’s modulus (E), tensile stress at yield and at break (σyand σb), and elongation at yield and at break (εyand εb). Representative stress−strain curves can be seen in Figure 4a, while Table S2 summarizes the main characteristic parameters. The glass microfiber separator presents a stiff and brittle character, with a modulus of 1470 ±120 MPa and εbof ∼5.8 ±0.1%. 48 On the contrary, as schematized in Figure 4b, LSKL−chitosan and LS−chitosan GPEs show a semi-ductile behavior characterized by Evalues ranging from 55 ±4 to 940 ±63 MPa and εb values of 14.1 ±0.2 to 43.9 ±21.1% (Figure 4c,d). For the sake of comparison, the most widely exploited GPE, the polyethylene oxide/LiTFSI blend, displays a Young’s modulus of ∼100 MPa. 15 Independently of the composition, we found that glutaraldehyde cross-linking increases stiffness and lowers ductility due to the formation of extended imine linkages in the electrolyte. An increased fraction of lignin lowers both Eand εb values as reported for chitosan-lignin films, 49,50 while the micellar LSKL dispersion results in stiffer but more brittle GPEs in comparison with LS−chitosan GEPs. 51 Overall, the GPEs here developed show an enlarged ductility while keeping acceptable tensile modulus values, offering an advantageous resistance against dendrite penetration. 52 The mechanical flexibility of the GPEs also facilitates battery cell assembly while ensuring a good interfacial contact with metallic Zn due to the ability to accommodate electrode volume changes upon Zn2+ insertion/extraction. 53 Electrochemical Characterization. The predominantly amorphous character of the GPEs together with the abundant hydroxyl functional groups (4.12 mmol g−1) 33 available to interact with Zn2+ of the GPEs encourage their use as ionic conductors for batteries. The electrolyte uptake was quantified based on eq 1 after soaking the membranes into 0.5, 1, and 2 M ZnSO4H2O for 24 h. EU values ranging from 78 to 319 wt % were obtained for various membrane compositions (Figure 5a). No marked changes depending on KL or LS are achieved. Glutaraldehyde cross-linking lowers electrolyte uptake values as the amount of groups to interact with water are reduced. 54 An increase in the lignin content (at expenses of a decrease of Figure 3. TGA traces under air atmosphere for (a) LS−chitosan and LSKL−chitosan membranes and (b) corresponding GPEs. Dashed line: Celgard 2325 membrane. The insets in (a) show the LS10Chi90 sample before and after TGA measurement. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2287 chitosan) also results in a lowered electrolyte uptake despite the phenolic hydroxyl groups and carbonyl groups being active water sorption sites in lignin. 55 Though KL and LS are not hydrophobic materials as such, the presence of hydrophobic rings as opposed to the abundant polar groups of chitosan lends explanation to the lower electrolyte uptake in the presence of lignin. 56 It is also observed that larger electrolyte uptake values are achieved when increasing ZnSO4concentration. It is important to note that measured water uptake values are larger than the ∼14−16 wt % shown for organosolv/ Nafion/polyethylene oxide or organosolv/Nafion membranes 57 or the 28.5−34.5 wt % obtained for sulfonated poly(ether ether ketone)/lignin membranes 58 and comparable to the 230 wt % shown by lignin membranes impregnated in solutions of organic carbonates. 59 These results indicate the suitability of Kraft lignin−chitosan and lignosulfonate− chitosan to provide efficient medium for Zn2+ transport. Zn2+ conductivity values have been obtained from the Nyquist impedance plots in Figure S5 using the eq 2 (further details are provided in Tables S3−S5). All the GPEs display straight lines with no semicircles as a result of their ionically conducting character. As summarized in Figure 5b, ionic conductivities from 3.8 to 18.6 mS cm−1are achieved for the GPEs. Larger values of 15.0 and 59.3 mS cm−1are observed for the glass microfiber separator soaked in 0.5 and 2 M ZnSO4 H2O, respectively. However, lignin−GPE conductivities are superior to the 0.12 mS cm−1reported for a carboxymethyl cellulose/ZnSO4GPE, 60 *the 14.6 mS cm−1shown for xanthan gum/ZnSO4/MnSO4GPE, 61 or the 8.9 mS cm−1of chitosan/choline nitrate GPEs. 62 We also achieved larger values compared to the reported lignin-derived GPEs intended for LIBs, including 3.73 mS cm−1showed by a GPE obtained upon soaking lignin fibers into 1 M LiPF6ethylene carbonate/ dimethyl carbonate/ethyl methyl carbonate 59 or 2.52 mS cm−1 obtained for a polyvinylpyrrolidone/lignin soaked into the same system as above. 63 Similarly, obtained conductivities are above the numbers reported for porous membranes soaked in zinc salt aqueous solution, such as the 9.1 mS cm−1obtained for a Nafion/lignin membrane soaked into 2 M ZnSO4H2O. 64 A plausible explanation may be the coordination of Zn2+ mobile charge species with the hydroxyl groups in chitosan and lignin as well as carboxylic and sulfonate groups of lignin, together with the lone pair on the N atom and O atom in chitosan coordinating with Zn ions and the phenol hydroxyl groups of lignin that dissociate the anion from the salt, 59 facilitating Zn2+ movement. In addition, the 3D structure of LS, having vast sulfonate, polar ether, and hydroxyl groups forms ionic domains with highly mobile water molecules, 54 further boosting Zn2+ conductivity. These characteristics enable obtaining GPEs with a good compromise between mechanical properties (stiff but ductile) and ionic conductivFigure 4. Mechanical characterization with (a) representative stress−strain curves together with (b) a schematic illustration showing the mechanical characteristics of glass microfiber separator and lignin GPEs. Mean values of (c) Young’s modulus and (d) elongation at break of LS−chitosan and LSKL−chitosan GPEs. Data corresponding to glass microfiber is shown for comparison. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2288 ities in the mS cm−1order, often conflicting requirements in the polymer electrolyte field. 65,66 Aqueous Zn cell failure is usually triggered by hydrogen evolution reactions and corrosion reactions of the metallic Zn anode, limiting the practical ability of zinc-ion batteries. 67 As the extent of the initial overpotential obtained for Zn plating on a smooth Zn foil is related to the hydrogen produced, the electrochemical stability window of GPEs in the potential range of −0.25 to +2.4 V vs Zn2+/Zn has been assessed. As indicated by the low current peaks in Figure 5c, the overall stability window expands up to +2.2 V vs Zn2+/Zn with hydrogen and oxygen evolution reaction potentials enlarged in comparison to the glass microfiber separator soaked in the liquid electrolyte. Particularly, the onset overpotential for hydrogen evolution reaction is notably shifted cathodically by 20−50 mV for all GPEs. As a result, the system here explored can be useful with high-voltage cathodes such as cobalt hexacyanoferrate. 11 This observation matching previous reports that highlight the wide electrochemical stability of lignin in LIBs 59,68 is ascribed to the adhesion of the GPE to the electrode surface to provide a passivation layer. 11 Importantly, lignosulfonate−Kraft lignin−chitosan and lignosulfonate− chitosan GPEs present a 4.5to 18-fold reduction on the anodic and cathodic intensity peaks at ∼0 V in comparison with the glass microfiber separator soaked into 2 M ZnSO4 H2O as a result of the hydrogen evolution reaction depletion by lignin GPEs. 67 The reduced resistance to Zn anodic dissolution and Zn cathodic deposition process provided by the biopolymer GPEs is further confirmed by Zn/Zn symmetrical cell results. The electrochemical performance of the GPEs has been further investigated using a symmetric Zn/Zn cell configuration at room temperature. Importantly, this technique provides information regarding the reversibility of the Zn2+ transport through the GPEs. 69 Obtained results are depicted in Figure 6, where negative and positive potentials represent Znmetal stripping and plating, respectively. At a low current density of 50 μA cm−2, all the samples display a reversible Znmetal plating and stripping behavior, including the glass microfiber separator soaked in 2 M ZnSO4H2O. Voltage fluctuations are observed as current density increases, suggesting the growth of a resistive solid electrolyte interphase (SEI) onto the surface of the Zn metal. 70 The glass microfiber separator shows a sudden voltage drop identified as an internal short-circuit with dendrite penetration just after setting the current at 200 μA cm−2(42 h test). This premature shortcircuit originates from the inhomogeneous ion transport between Zn surfaces facilitated by the micrometer-sized pores of the separator. 69 On the contrary (magnified view in Figure S6a), the non-cross-linked LS-Chi 30−70 sample provides a nearly square wave shape even at a high current densities of 500 μA cm−2(>100 h test), indicating the occurrence of homogeneous metal ion electrodeposition with reversible Zn/Zn2+ redox reactions also at elevated current densities. 20,69 We conducted a symmetric cycling test under more severe conditions for the LS30Chi70 GPE and observed no shortcircuit for areal current densities as high as 5000 μA cm−2 (Figure 7a), solving the anode reversibility and stability issues observed in conventional ZIBs. This long-term stability and process reversibility is also confirmed by the high Coulombic efficiency values of above 99.8% for the majority of the compositions studied (Figure S7). More precisely, Coulombic efficiencies approaching 99.5% are observed when the areal current density is below 300 μA cm−2, which is above the values of 99.6% obtained by cyclohexanedodecol-modified ZnSO4electrolytes 71 or the 99.3% showed by the Zn(ClO4)2 electrolyte. 72 We ascribe this performance to the reversible Zn2+ insertion/extraction originating from the good electrolyte uptake and the ionic conductivity of lignin-GPEs, together with their mechanically adaptative performance (ductility), adhesive properties given by the phenolic hydroxyls, 30 and resistance against dendrite growth (relatively high Young’s modulus values). In addition, we also observed that the GPEs having lower lignin fractions present a region of large polarization during the initial three to six stripping/plating cycles for each Figure 5. (a) Electrolyte uptake; (b) ionic conductivity and (c) electrochemical stability window LS−chitosan and LSKL−chitosan GPEs. The inset shows an expanded view of the oxygen evolution reaction region. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2289 current density before reaching a steady-state polarization shape, suggesting a poorer initial interfacial contact (Figure S8). 73 The extent of overpotential is also a relevant parameter because it indicates the charge transport characteristic, an essential requisite to avoid dendrite formation. 74 Despite the lower ionic conductivity of lignin GPEs over the glass microfiber separator soaked in 2 M ZnSO4H2O, comparable overpotential values of ∼160 mV are observed at of 50 μA cm−2for the majority of the samples. It may be concluded thus that the faster Zn2+ transport in the glass separator is offset by a smoother and more homogeneous Zn plating/stripping provided by the GPEs, which enables a stable SEI layer. 75 Moreover, notable overpotential differences are observed after 100 h (500 μA cm−2; see Figure S6b), with values ranging from 130 to 318 mV. Overall, glutaraldehyde cross-linking and the substitution of LS by KL seem to increase the overpotential. We ascribe this behavior to the formation of a heterogeneous solid electrolyte interphase on the Zn anode surface. 74 In any case, it is important to note that the low overpotential obtained by the LS,Chi 30/70 GPE enables a high voltage efficiency of the battery, delaying undesired decomposition reactions that may yield to a premature cell failure or inefficient charge/ discharge. 74 It is worthy to note that the observed overpotential value for LS30Chi70 GPE is similar to that shown by a cellulose−agarose GPE recently developed by our group, which offers a lifespan extending over 8500 h (one year cycling) in a symmetric Zn|Zn configuration. 23 In fact, we postulate that developed lignin-based GPEs suppress the presence of free water as opposed to conventional membrane/ aqueous electrolyte systems, shielding Zn surfaces from the free water-induced Zn corrosion and thus enabling long-term cycling. 32 Noting this composition presents the lower Young’s modulus (55 ±4 MPa), it can be concluded that achieving a good interfacial compatibility and intimate contact of the GPEs with the metallic Zn (also promoted by the hydroxyl and ether Figure 6. Room-temperature voltage (vertical axis) vs time (horizontal axis) curves for symmetric Zn/Zn cells for Zn plating/stripping at different current densities for lignosulfonate and lignosulfonate/kraft lignin GPEs. Details on the composition of each GPE together with the sample code are provided for each composition. Colors from blue to green represent current densities from ±50 to 500 μA cm−2, while the lightning indicates the occurrence of short-circuit. The curves corresponding to the glass microfiber soaked into 2 M ZnSO4H2O are shown for comparison. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2290 groups in lignin) definitively contribute to reduce the overpotential. In fact, post-mortem SEM and XRD analyses of cycled Zn surfaces in Figure 7b and Figure S9, respectively, prove that the smooth and homogeneous electrodeposition feature of the LS30Chi70 and LS30Chi70GA GPEs is translated into a flat and dendrite-free zinc structure (JCPDS: 36-1451 of metallic zinc). This morphology contrasts with the microscale high-surface area needle-like Zn deposits observed when the symmetric cells separated by a glass microfiber soaked into 2 M ZnSO4H2O (short-circuit at 100 μA cm−2). The more abundant deposits observed for the LS30Chi70 GPE over the LS30Chi70GA one may originate from the notably larger current densities, triggering shortcircuit (5000 vs 200 μA cm−2). Considering the functional properties and their renewable/ biodegradable nature, we envisage the following potential applications for fabricated lignin-containing GPEs. First, the flexible character of developed GPEs and, in particular, of the samples containing lignosulfonate, is attractive to implement solid-state ZIBs for flexible and wearable electronic devices, where batteries could operate under repeated stretching and bending conditions. 76 Given the versatility of the gelation (physical or chemical), the large ionic conductivities, and mechanical resistance achieved, we foresee a bright future of biopolymer-electrolytes into higher-energy density zinc-ion chemistry batteries. For example, further efforts are needed to incorporate lignin GPEs in secondary zinc-air batteries 77 or in ZIBs comprising metal−organic framework (MOF) cathodes. 78 When lignin−chitosan GPEs are paired with organic cathode materials, fully renewable ZIBs could be obtained, avoiding the use of scarce and toxic critical raw materials that are causing serious resource depletion and supply chain bottleneck issues. 79 Additionally, the biodegradable character of electrolyte constituents makes these GPEs susceptible to degrade under composting conditions, opening new opportunities to fabricate transient rechargeable batteries that are rapidly degraded into harmless by-products (once the adequate trigger is activated) after a period of stable operation. 23 Thereby, lignin−chitosan GPEs have the potential to lessen the inherent environmental impact of conventional LIBs requiring non-biodegradable and harmful materials. ■CONCLUSIONS The production of battery electrolytes adhering to circular economy principles requires a cautious balance between the Figure 7. (a) Room-temperature voltage (vertical axis) vs time (horizontal axis) curves for symmetric Zn/Zn cells for the LS30Chi70 GPE at high areal current densities. (b) Post-mortem SEM images of the Zn surfaces after cycling in symmetric Zn/Zn cells using a glass microfiber separator soaked into 2 M ZnSO4H2O together with those corresponding to the LS30Chi70 and LS30Chi70 GPEs. The SEM image corresponding to the pristine Zn foil is shown for comparison. ACS Sustainable Chemistry & Engineering pubs.acs.org/journal/ascecg Research Article https://doi.org/10.1021/acssuschemeng.2c05835 ACS Sustainable Chem. Eng. 2023, 11, 2283−2294 2291