Polysaccharides: The Sustainable Foreground in Energy Storage Systems

Thomas, Sharin Maria; Gómez-Romero, Pedro; González-Gil, Rosa M.

doi:10.3390/polysaccharides6010005

Open AccessReview

Polysaccharides: The Sustainable Foreground in Energy Storage Systems

by

Sharin Maria Thomas

¹

,

Pedro Gómez-Romero

^1,2

and

Rosa M. González-Gil

^1,*

¹

Novel Energy-Oriented Materials Group at Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain

²

Consejo Superior de Investigaciones Científicas (CSIC), 28006 Madrid, Spain

^*

Author to whom correspondence should be addressed.

Polysaccharides 2025, 6(1), 5; https://doi.org/10.3390/polysaccharides6010005

Submission received: 31 October 2024 / Revised: 21 December 2024 / Accepted: 6 January 2025 / Published: 13 January 2025

(This article belongs to the Collection Current Opinion in Polysaccharides)

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Abstract

:

Polysaccharides offer a perfect option as raw materials for the development of a new generation of sustainable batteries and supercapacitors. This is due to their abundance and inherent structural characteristics. Polysaccharides can be chemically functionalized and engineered, offering a wide range of possibilities as electrode materials (as precursors of porous nanocarbons), binders and separators. Their hierarchical morphology also enables their exploitation as aerogel and hydrogel structures for quasi-solid and solid polymer electrolytes with high conductivity and wide voltage stability windows. In this review, we discuss how different polysaccharides, such as lignocellulosic biomass, starch, chitosan, natural gums, sugars and marine polysaccharides, can be applied in different components of energy storage systems (ESSs). An overview of the recent research work adhering to each functionality of different polysaccharides in various storage systems is provided.

Keywords:

polysaccharides; lignocellulosic biomass; chitosan; sugars; supercapacitors; batteries

1. Introduction

Polysaccharides are unique biopolymers that merge in among a fascinating multifunctionality in the natura. They form captivating structural architectures and at the same time constitute a natural energy reserve in living organisms like plants, animals and some micro-organisms [1,2]. Analogously, these abundant biopolymers have been scientifically proven to be efficient precursors in energy storage systems (ESSs) such as batteries (BAT) and supercapacitors (SC). They can be adeptly engineered to modify and reinforce the inherent properties and render versatile functionality for use as active carbonaceous materials in electrodes, binders, separators, additives in liquid electrolytes, in gel polymer electrolytes (GPEs), and solid polymer electrolytes (SPEs) well [3,4,5,6,7].

Two centuries of complete energy dependency on fossil fuels have stretched the Earth’s natural equilibrium, the alarming consequences being carbon emissions, global warming, and climate change leading to eventual uninhabitability. With the prolific expansion of miniature electronics and smart wearable devices, the need for raw materials and device components that are safe to the environment, with a regulated life cycle is of vital importance to reduce the overloading of electronic wastes. Compared to other commonly used electrode materials such as metal oxides and conducting polymers, carbon-based precursors are the most sought-after electrode materials for energy storage devices because of their natural abundance, functionality, pore structure and well-suited electrochemical performance [8,9]. Metal oxides often require expensive processing and are not compatible with aqueous electrolytes and depict narrow stability windows [9]. And some limitations of conducting polymers include poor rate, mechanical and cyclic stability [10,11]. Over the past decade, different forms of carbon such as activated carbon, hard carbons and carbon nanomaterials have been developed from plant and animal biomass via carbonization and pyrolysis paired with activation [12,13,14]. The inherent elements in the biomass structure such as nitrogen, phosphorous and sulfur will be retained in the final carbon source as a heteroatom dopant contributing to pseudocapacitive charge storage of the electrode [15]. Traditional organic and inorganic materials can also release toxic byproducts upon processing that are hazardous to the ecosystem. In addition to low-cost production and being a sustainable choice, polysaccharides offer excellent choice due to their structural diversity, complex porous network, high surface area and abundant functional groups. The various functional groups make them more eclectic, as the biopolymers can be functionalized chemically and physically for improved mechanical and electrical properties [16,17]. Bioinspired multifunctional modifications utilizing polysaccharides are beneficial for achieving advanced binder systems and electrolytes [18,19,20]. GPEs with hierarchical structures, strong mechanical characteristics and physiochemical modifications are appealing as both stable electrolytes and as separators [21]. The numerous hydrophilic functional groups helps in effective water retention and also improves the ion transport in electrolytes [22,23].

As of 2017, approximately 1 Gt of dry biomass was harvested across the EU (European Union). Of the harvested biomass, 18.8% accounts for bioenergy and 19.1% for biomaterials (biomass-based products) [24]. These statistics significantly contribute towards the electricity sector of renewable energy which was 30% in 2023 and forecasted to increase to 46% in 2030 globally. With the objective of reducing the dependency on fossil fuels and increasing reliance of renewable systems, this is expected to increase steadily [25]. Polysaccharides are considered central to this shift with their increasing use in supercapacitors and batteries as different parts such as carbon precursors (active materials, biochar, functional materials, etc.) for electrodes, as binders, separators, and semi-solid/quasi-solid polymer gel electrolytes, as well as other applications including biomedical, material science, and food technology (packaging, food structuring agents) [24]. Repurposing the polysaccharides to synthesize value-added products for different applications such as energy harvesting and storage could trap carbon for prolonged periods extending the life cycle, thus contributing to negative carbon emissions [26].

In this review, energy storage systems (batteries and supercapacitors) based on different polysaccharides such as lignocellulosic biomass (cellulose, lignin and hemicelluloses), starch, pectin, chitosan, chitin, sodium alginate, agar and different gums (Gum Arabic, xanthan gum, etc.) and some sugars such as dextran and glucose are discussed. The latest advances concerning the different components of ESSs using polysaccharides will also be discussed, with emphasis on how well they can replace different traditional polymers, with improved structural design, strong hydrogen bonding, mechanical integrity and options for engineering the functionality depending on various ESSs. Figure 1 depicts the various polysaccharides discussed in this review.

2. Plant-Derived Polysaccharides

2.1. Lignocellulosic Biomass (Cellulose, Lignin, and Hemicelluloses)

Lignocellulosic biomasses are a ubiquitous natural source derived from the plant biomass, the three main constituents of which are: cellulose, hemicelluloses and lignin. Cellulose is the most abundant polysaccharide with a linear chain of glucose molecules connected by β-1,4 glycosidic linkage. Cellulose has been extensively used as a sustainable precursor in energy storage systems due to its rich carbon content, high functional moieties, porosity, abundance and low cost [27,28,29,30]. In hemicelluloses, it is a chain of various monosaccharides such as xylose, arabinose, and galactose with abundant functional groups for modifications [31]. Akin to cellulose, lignin is being steered as a rich carbon source in energy storage applications. Unlike cellulose, lignin is an aromatic hydrocarbon polymer and is amorphous in nature. It is not considered a polysaccharide itself but considering its increasing impact on energy storage solutions, we decided to also include it in this review. Lignin constitutes a 3D network structure of three phenylpropane units connected by C-O and C-C bonds. It is rich in aliphatic and aromatic hydroxyl groups along with reactive groups like quinones [32]. The redox couple (quinone/hydroxyquinone) in the phenolic group containing lignin contributes to its pseudocapacitance in ESSs [33,34]. The carbon content of lignin is the highest among the other two constituents; cellulose and hemicellulose [35]. Hence, the interest in the development of lignin-derived carbons for electrodes has increased tremendously [36,37,38], among other related applications. The pyrolysis of lignin at higher temperatures will pave the way for oxygen functional groups from the carbon framework to remove defects and yield porous structures [39]. The introduction of heteroatoms such as O/N to the lignin precursor improves the pseudocapacitance and enhance the surface wettability of the carbonaceous materials [40]. The pyrolysis process is in addition affected by several factors, including the lignin origin, its pretreatment and its activation process [40]. Unlike cellulose, the pyrolysis of lignin is more complex due to the heterogeneity in lignin composition and tends to form a part of biochar. However, several examples of carbon-derived lignin for energy storage have been reported [41,42,43,44,45].

2.1.1. ESSs Based on Lignocellulosic Biomass

Active Material (Carbon Precursors)

KOH is the most commonly used activator for the development of nanopores and achieving the required surface area in the activated carbon materials (AC) followed by acid treatments. An eco-friendly method without the use of any activators or any strong acids was proposed by Lin et al. for the fabrication of a hierarchical porous carbonaceous active material using lignin and nanocellulose (LN). A unidirectional freeze-drying of LN aerogel was carried out followed by carbonization. Nanocellulose acted as a template for lignin and facilitated the extent of graphitization and specific surface area (1770 m² g⁻¹). The electrode showed high electrochemical performance with a power density of 349.9 Wkg⁻¹ and an energy density of 11.7 W.hkg⁻¹ in a two-electrode system [46]. A lignin–carbon composite has been used as a cathode in Zn–lignin battery coupling reversible redox activity of lignin with Zn anode plating and stripping with fast charge transport, maintaining high cyclic stability up to 8000 cycles at 1 A g⁻¹ [47]. There is another example of the use of lignin for obtaining lignin nanofibers by electrospinning technique [48], and normally in combination with other polymers and biopolymers, for free-standing electrodes in supercapacitors [49], and batteries [50], Figure 2.

Figure 2. (1) Schematic draw about the use of lignin-derived nanofibers for symmetric supercapacitor assembly [49]. (2) Synthesis scheme for the pristine and densified materials from lignin mats (above) and their electrochemical performance (bottom). Reproduced from Ref [51]. (3) Lower left; (a) Schematic illustration of the transformation of lignin into 3D hierarchical porous carbons; Scanning Electron Microscopy Image (SEM) image of (b) the lignin precursor; (c) hydrochar pretreated at 180 °C; (d,e) with KOH activator; (f,g) the magnified images. Reproduced with permission from [52]. © 2017 Royal Society of Chemistry.

Nanocellulose such as CNF provide mechanically strong nanoporous structures but are not conductive. Integrating them with electroactive polymers such as PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) resulted in highly conductive electrodes. CNF also facilitated adequate rheology required for the ink to be spray coated, resulting in thick scalable high-performance electrodes of 30 µm thickness with a volumetric power density of 1.78 Wcm⁻³ and an areal power density of 26.5 mW cm⁻² [53]. An all nanocellulose-based Li-metal battery was designed by Wang et al. using nanocellulose fibers (NCF) as the main precursor to electrodes and separators. A combination of carbon nanofibers with NCF was used to develop the anode, which displayed steady cyclability up to 120 h at high current density. A paper-like cathode with scalable potential was fabricated with a mix of NCF, carbon nanotubes and LiFePO₄ particles (lithium iron phosphate). Even after 1000 cycles, the assembled battery retained 85% of the capacity at a rate of 2 C (1.27 mA cm⁻²) [54]. Liu et al. developed a novel composite containing lignin–nanocellulose and Mxene (M_n+1X_n; M—early transition metal, and X will be carbon or nitrogen) that functions as a zinc gating layer in zinc aqueous batteries, meaning the layer selectively guides Zn ions towards the anode and prevents any excess water molecule interference. Hence, it acts a protective layer at the electrode–electrolyte interface, enhancing zinc-ion mobility and preventing corrosion. The integrated laminate composite structure renders good structural stability and the lignin–nanocellulose in addition ensures good wettability between the gating layer and electrolyte [55].

Carbonization and pyrolysis of biomass yield carbon biochar as a major product, which is an economical and sustainable alternative for obtaining heteroporous carbon. There is extensive literature on biomass-based carbon as an integral part of EES [56]. The quality of the byproduct is defined by the type of precursors, processing temperature and post-treatment process that the biomass undergoes [57]. Biochars produced at temperatures between 400 and 700 degrees are used as porous electrode materials in supercapacitors and Li-ion batteries. The high surface area, porosity, and integrated heteratoms such as oxygen and nitrogen of biochar contribute to maximizing the electrochemical performance [58]. With the commercialization of Li-ion batteries, the challenges due to limited resources, environment and safety concerns are hard to ignore. Na-ion (NaI) batteries are slowly gaining momentum as a green alternative to Li-ion batteries in terms of resource abundance, cost-effectiveness and safe assembly. The low graphitization of biochar prevents its use as an anode in Na-ion batteries due to weak intercalation of sodium ions, thus severely degrading the electrochemical activity [59]. Yu et al. realized an in situ modification of microcrystalline cellulose with ionic liquids (ICC-X, where X is the temperature in degrees) at temperatures varying from 750 to 1400 °C, inducing graphitic structures with nitrogen doping in the biochar. The doping opened more active sites for Na⁺ ion storage, resulting in high capacity along with dilated interlayer graphitic spaces. As anodes, the biochar showed superior reversible specific capacities of 391 and 136 mAhg⁻¹ at stable 100 and 1000 discharge/charge cycles at current densities of 100 and 500 mA g⁻¹, respectively [60], Figure 3. By tuning the microstructures of the active material, the pores, heteroatoms and defects of carbon can be conformed to reduce the solid electrolyte interface (SEI), thereby reducing the irreversible capacity. Xie et al. showed that controlling porosity in nanocrystalline cellulose by hydrothermal treatment and pyrolysis resulted in an optimized structure with a high initial coulombic efficiency (ICE) of 90.4% and a reversible capacity of 314 mAh g⁻¹ [61]. Thus, temperature plays a crucial role in different attributes of carbon that directly influence the electrochemical performance of the energy storage systems. Graphitization increases with temperature between 850 and 1000 degrees while the defects, surface area, heteroatoms and other functional groups maybe reduced at higher temperatures. A balance should be struck between these parameters as heteroatoms and defects are complimentary towards pseudocapacitive charge storage. A high surface area and structured porosity are essential for the active sites for ion transport while keeping the capacitive drop in check by careful optimization.

Figure 3. (1) Graphical illustration of N-doped graphitic nanosheets derived from lignocellulosic biomass in the presence of BMIMAcO. (2) Electrochemical performance of ICC samples from [60]: (a) short-term cycling at a current density of 100 mA g⁻¹ after 10 cycles at 50 mA g⁻¹; (b) long-term cycling stability at 500 mA g⁻¹; (c) rate performance; (d) comparison of ICC-1000 with state-of-the-art carbons; (e) first three CV curves of ICC-1000 at 0.1 mV s⁻¹; (f) initial three discharge–charge cycles of ICC-1000 at 50 mA g⁻¹; (g) different storage sites of Na+ ion on the ICC-1000 electrode. Reproduced from [60]. © 2021 Elsevier B.V.

Compared to other lignocellulosic counterparts, hemicelluloses have an upper hand in terms of easy alkali extraction and availability as a byproduct in bioethanol production. Lin et al. reported a porous AC derived from hemicellulose (HPAC x-y, where x is the mass ratio of ZnCl₂ to hemicellulose and y is the activation temperature) by simple chemical activation using ZnCl₂ at a nitrogen-inert low temperature of 500 °C. The obtained carbon showed a very high active surface area (approximately 1361 m²g⁻¹) and combined with the high porosity, this contributes to superior capacitance, Figure 4 [62]. Another sustainable source that is not commonly used is soybeans. Soybean dreg is a promising precursor due to its natural porosity and rich electroactive functional groups (O and N). Li et al. developed a 3D self-doped honeycomb structured with high packing density, engineered pore volume and specific surface area (SSA). When assembled with a GPE of (sodium carboxymethyl cellulose) CMC-Na/Na₂SO₄, the supercapacitor delivers a superior energy density of 23.4 Wh kg⁻¹. Tailoring the pore volume and SSA resulted in high volumetric and areal capacitances of 204.7 F cm⁻³ and 4.7 F cm⁻², respectively, with a 6.0 M KOH electrolyte [63].

Figure 4. (1) Schematic process of hemicellulose-derived materials (HPAC x-y). (2) Cyclic voltammetry (CV) curves and galvanostatic charge–discharge (GDC) curves of HPAC 1–500 materials in all-solid-state supercapacitor from Ref. [62]: (a) CV curves from 1 to 1.8 V at 50 mV s⁻¹; (b) CV curves of HPAC-1-500 material from 5 to 100 mV s⁻¹; (c) GCD curves of HPAC 1–500 material at different current densities; (d) cycling stability at 1 A g⁻¹ (Ragone Plot in inset). Reproduced with permission from [62]. © 2019 Published by Elsevier Inc.

Hard carbons (HCs), a part of biochar and activated carbon, are non-graphitizable carbon prepared from biomass at a high temperature of 1000 degrees and above. Cellulose-derived HCs are mostly used as anode electrodes in Na, K and Li-ion batteries [64]. Hard carbons can be prepared by direct pyrolysis of cellulose in an inert atmosphere [28]. HC anodes are preferred over biochar in NaI batteries due to easy regulation of microstructures, higher reversible specific capacity and low sodium storage voltage [65]. Tuning biomimetic microtubular structures in hard carbon from renewable cotton with octet truss (combination of micropores and mesopores) microstructures resulted in a high reversible capacity of 314 mAh g⁻¹ in Na-ion BATs [66]. Simone et al. have prepared cellulose HCs at different temperatures (from 1000 °C to 1600 °C) obtaining excellent rate capability where the sodium extraction rate varies from C/10 to 5 C [67]. Hard carbons derived from cellulose acetate microspheres have been used as active materials in Li-ion batteries by Fischer et al. The modified porous electrode enabled Li-ion intercalation with rapid charging rates and was operational in high temperatures such as 60 degrees. In addition, the carbon materials were well dispersed with CMC compared to PVDF (polyvinylidene difluoride) due to homogenous adhesion rendering stable electrochemical activity after 50 cycles with a reversible capacity of approximately 300 mA hg⁻¹ at 0.1 C [68].

Binders

Even though CMC is considered the state of the art in aqueous electrolytes, compared to PVDF in most cases, CMC has weak mechanical strength and may not sustain under high stress like long cycling and mass loading [69]. Nanocellulose has been reported to be used as a binder and/or support material in developing novel nanocomposite materials to improve the overall structural integrity. Bacterial nanocellulose (BC) was employed as a crosslinking agent with two-dimensional conductive MXene nanosheets that mechanically reinforced the free-standing electrode and improved the electrochemical performance of the supercapacitor due to improved interlayer spacing rendering more active sites for the ions [70]. Cellulose nanofibers (CNF) were shown to be more befitting binders in combination with biochar-based active material compared to the PVDF binders. The hydrophilicity of nanofibers results in good wettability and reduced bulk electrolyte resistance. CNF retains the micropores on biochar, thus facilitating continuous ion mobility and higher capacitance [71]. Cellulose nanocrystals (CNC) have been used as a template to create graphitic carbon and doped with redox CoFe₂O₄, imparting pseudocapacitive behavior. The specific capacitance of the electrode material is greatly augmented by the crosslinking between the nanocellulose and CoFe₂O₄ due to the sizable electrode–electrolyte interfacial area. The nanocomposite exhibited a specific capacitance of 629 F/g at a current density of 0.5 A/g and maintained 95.8% of its capacitance after 5000 cycles [72], Figure 5.

Figure 5. CNC is used as a graphitic carbon template/support for pseudocapacitive CoFe₂O₄ in the preparation of CoFe₂O₄@CNC composite for hybrid supercapacitors application. (1) Electrochemical performance of CoFe₂O_{4 and} CoFe₂O₄@CNC: (a,b) cyclic voltammetry (c,d) charge–discharge cycles, (e) capacitance vs. current density, (f,g) electrochemical impedance spectra, and (h,i) cyclic retention, respectively. (2) FE-TEM images of (a,b) CoFe₂O₄ and (e,f) CoFe₂O4@CNC nanocomposites; HRTEM images (c,g), and selected area electron diffraction (SAED) pattern (d,h) of CoFe₂O₄ and CoFe₂O4@CNC, respectively. (3) Scheme of the synthesis procedure of CoFe₂O₄ and CoFe₂O₄@CNCs composite structures. Adapted from Ref. [72]. © 2022 Elsevier Ltd. All rights reserved.

Electrolytes

Electrolytes are vital for the transport of ions between the electrodes while maintaining chemical stability. Traditional liquid electrolytes are being replaced by solid and gel polymer electrolytes (SPEs and GPEs) due to flammability and corrosion. As they provide a physical separation, separators can be eliminated. In a GPE, the liquid salt is substituted by a polymeric-salt system integrated with plasticizers [73]. One crucial aspect to be considered with GPEs is the optimal balance between mechanical stability and ionic conductivity. Cellulose hydrogels are three-dimensional porous crosslinking structures with solid mechanical flexibility. The large number of hydroxyl groups bonds well with aqueous electrolytes locking up water for the conductive medium and retaining continual ionic conductivity. Electrochemically active components have been proposed to integrate with the flexible cellulose hydrogel for developing high-performance devices. One such approach was to introduce a hollow polypyrrole (PPy) network in situ to the cellulose hydrogel. The hydrogel acted as a ‘protective jacket’ reinforcing the electrochemical system even under mechanical load and stretching, resulting in a high ionic conductivity of 0.606 S/cm and stable cycling with 80% capacity retention after 10,000 cycles at 10 A/g [74].

A double-network hydrogel strategy has been engineered to reinforce the mechanical strength of the hydrogels. Li et al. combined cellulose with a biocompatible polymer, polyacrylamide (PAM), resulting in an interpenetrating dual network with numerous ion channels and excellent mechanical integrity such that multi-angle bending or heavy loads did not alter the electrochemical performance of the flexible supercapacitors. A high specific capacitance of 989 mF cm⁻² and an energy density of 0.132 mWh cm⁻² were noted [75]. A polyelectrolyte dual-network CNF hydrogel with charged monomers in PAM was recently developed by Zhang et al. The conductivity of the hydrogel was significantly improved by optimizing the amount of CNF to 103 mS cm⁻¹ and immersing the hydrogel network in a redox electrolyte of K₃ [Fe(CN)₆] widened the potential window of the flexible supercapacitor up to 1.8 V, with an ultrahigh energy density (139.65 mWh cm⁻² at the power density of 0.88 mW cm⁻²), Figure 6 [76]. Due to the presence of hydroxyl groups, the wettability is enhanced and the hydrogel is easily adhered without any extra binding.

Figure 6. (a) Schematic of the dual-energy storage mechanism. (b) Schematic diagram of the preparation process, crosslinked polymer networks, conductive mechanism and energy storage mechanism of hydrogel electrolyte. (c) GCD plots of (acrylamide-[2-(methacryloyloxy)-ethyl]trimethyl ammonium chloride-sodium acrylate anhydrous) AM4DMC1AA-Na/CNF-ClFeIII with various maximum voltages (current density of 1 mA cm⁻²) (1.7, 1.8 and 1.9 V). (d) CV curves for flexible supercapacitors AM4DMC1AA-Na/CNF-ClFe^III at different sweep speeds (20, 50, 100, 200, 500 mV/s). (e) Conductivity of hydrogels with different CNF contents. (f) GCD plots of FSC AM4DMC1AA-Na/CNF-ClFe^III under various current densities (0.5, 1, 2, 5 and 10 mA cm⁻²). (g) GCD plots of two devices connected in series and in parallel; (h) CV curves at sweep rate of 100 mV/s for FSC with K₃ [Fe(CN)₆] and FSC without K₄ [Fe(CN)₆]/K₃ [Fe(CN)₆]. Reproduced and adapted from [76]. © 2023 Elsevier Ltd. All rights reserved.

While ionic liquids (ILs) are widely employed to dissolve cellulose, they are also used as plasticizers to make GPEs. A printable GPE of nanocellulose (CNF) with ionic liquid was reported by González-Gil et al. Compared to CMC, CNF was found to be a better precursor to the hydrogel due to more available hydroxyl groups. At an optimized ratio, the hydrogel showed a printable viscosity and mechanical stability with a better capacity retention of 80% after 10,000 cycles at 1 mA cm⁻² up to a wide voltage window of 2 V [77]. Nanocomposite GPEs with polyvinyl alcohol (PVA)/cellulose acetate (CA) blend along with dopant salts such as lithium acetate have been studied to produce excellent ionic conductivities. With the addition of nanofillers such as TiO_2, the potential window of the system expands up to 2.2 V [78]. Self-healable polymer electrolytes are enormously beneficial when the system is subjected to immediate stress or damage. With crosslinking ability and large carboxyl anions, CMC can easily form ionic crosslinks. This is a dual-crosslinking approach, where the hydrogel was first assembled by freeze drying followed by immersion in a Ca²⁺ solution, which adds to enhanced structural integrity of the hydrogel by linking to excess carboxyl groups and free ions for ionic conductivity. In a symmetric supercapacitor, hydrogel exhibits good flexibility and improved electrochemical performance due to ionic mobility due to Ca²⁺ ions. When cut, an intrinsic healing ability is exhibited due to hydrogen bonding and ionic interactions. A total of 93.9% of capacitance is retained after 5 continuous cutting and healing cycles and a striking energy density of 63.3 Wh kg⁻¹ at 0.5 A g⁻¹ within a potential window of 1.2 V [79].

Solid polymer electrolytes (SPEs) are safe alternatives to conventional liquid and gel electrolytes. However, it is important to regulate the crystalline nature of the electrolyte as the amorphous domains favor better ion conduction [80]. To engineer an optimum SPE, glass transition temperature (T_g), mechanical integrity and favorable ion mobility should be carefully modulated. Higher glass transition would mean high mechanical stability at room temperature, but the system will be too rigid, impeding ion mobility. The high T_g and abundant polar functional groups of cellulose derivatives such as cellulose acetate (CA) and oxidized CMC (OCMC) help in developing physically crosslinked SPEs with high mechanical strength and thermal stability. Hamad et al. studied the CA and OCMC with two different crosslinkers such as PVA and citric acid for use as GPEs and SPEs in Li-ion batteries. Crosslinking is intended to reduce the resistance of electrolytes. Free volume and long chains of PVA favor Li⁺ ion movement. However, CA with a lower T_g than OCMC also immensely favors ionic conductivity in both GPEs and SPEs. Due to high water retention and free volume with long chains, the CA-PVA-GPE performed better than the OCMC-PVA-SPE due to less solvent uptake. An ionic conductivity of 10⁻² S cm⁻¹ was noted in both states and a superior electrochemical stability greater than 4.5 V in the Li-ion batteries, Figure 7 [81].

Figure 7. (1) Cell assembly of GPEs and SPEs for LiCoO₂/cell/Li. (2) Synthesis of CMC and OCMC from cotton, and synthesis of CA from sugarcane. (3) (a) Cyclic voltammetry curve of (a) LiCoO₂/CA-citric acid/Li cell and LiCoO₂/OCMC-citric acid/Li and (b) LiCoO₂/CA-PVA/Li cell and LiCoO₂/OCMC-PVA/Li. (4) Galvanostatic charge−discharge profiles (a–h) of LiCoO₂/GPEs or SPEs/Li at different current densities (0.1, 0.2, 0.5, 1, and 2 C). Reproduced and adapted from [81]. © 2022 Elsevier B.V. All rights reserved.

Separators

Separators are crucial to upkeep the electrochemical performance and long-term dis(charge) in EES while electrically separating the two electrodes thereby preventing short circuits. Rather than its original role, it can be functionalized to allocate porosity, and mechanical strength and attract the charges permitting electrolyte permeation [82]. In Li-S batteries, porous cellulose-based membranes as separators have proven to reduce dendrite growth. The corrosion of Li anodes is also prevented by inhibiting the transfer of polysulfides [83]. Li et al. showed grafting phosphate groups to cellulose nanofibers rendered enhanced blocking of polysulfides by electrostatic repulsion and facilitating Li⁺ ion transfer facilitating faster kinetics and reversible reactions at both electrodes. The modified CNF separators showed high mechanical strength [84]. Lignocellulose-based materials in different ESSs are summarized in Table 1.

Table 1. Lignocellulose as different functional parts of energy storage systems.

Ref	Polysaccharide (Modified Form)	Functionality	Energy Density	Power Density	Specific Capacity/ Specific Capacitance	ESS
[46]	Lignin/Nanocellulose	Active material	11.7 W h kg⁻¹	349.9 W kg⁻¹	216.2 F g⁻¹ at 0.5 A g⁻¹	SC
[47]	Lignin (lignin carbon nanocomposite	Cathode	23 W h kg⁻¹	610 W kg⁻¹	75 mAh g⁻¹	Zn–lignin BAT
[39]	Lignin (heteroatom-doped)	Porous carbon	-	-	348 F g⁻¹ at 1 A g⁻¹	SC
[49]	Lignin (lignin carbon nanofiber)	Active material	1.77 W h kg⁻¹	156 kW kg⁻¹	11.95 F g⁻¹	SC
[85]	Lignin (lignin carbon nanofiber)	Active material	2245 μW h cm⁻³	60 kW kg⁻¹	-	SC
[51]	Lignin (lignin carbon nanofiber)	Active material	6 W h L⁻¹	-	130 F cm⁻³	SC
[86]	Lignin (lignin carbon nanofiber)	Active material	37.1 W h kg⁻¹	400 W kg⁻¹	442.2 F g⁻¹	SC
[50]	Lignin (lignin carbon nanofiber)	Anode	-	-	1783.8 mA h g⁻¹ at 0.2 C	Li-ion BAT
[50]	Lignin (lignin carbon nanofiber)	Anode	-	-	453.2 mA h g¹ at 0.2 C	Na-ion BAT
[87]	Lignin (lignin carbon nanofiber)	Anode	-	-	330 mA h g¹ at 100 mA g⁻¹	Na-ion BAT
[56]	Commercial biochar pellet (bio-graphite)	Anode	-	-	293 mA h g⁻¹ at 20 mA g⁻¹	Li-ion BAT
[88]	Biochar from olive tree and spent malt rootlets	Electrocatalysts	-	-	-	Zn-air BAT
[89]	Biochar(tetra-heteroatom-doped N, O, S, Cl)	Active carbon	30.2 W h kg⁻¹	164.0 W kg⁻¹	638 F g⁻¹ at 0.5 A g⁻¹	SC
[60]	IL-induced microcrystalline cellulose biochar	Anode	-	-	391 mA h g⁻¹ at 100 mA g⁻¹	Na-ion BAT
[61]	Chiral nematic nanocrystalline cellulose-based biochar	Anode	-	-	314 mA h g⁻¹ at 0.1 C	Na-ion BAT
[62]	Hemicellulose (porous AC)	Porous active material	11.7 W h kg⁻¹	349.9 W kg⁻¹	302.4 F g⁻¹ at 0.5 A g⁻¹	SC
[63]	Soybean dreg	Honeycomb-like carbonaceous material	23.4 W h kg⁻¹	225 W kg⁻¹	281.4 F g^–1 at 0.5 A g^–1	Quasi-solid-state SC
[63]	Soybean dreg	Honeycomb-like carbonaceous material	23.4 W h kg⁻¹	225 W kg⁻¹	247.6 F cm^–3 at 0.5 A g^–1	Quasi-solid-state SC
[14]	Biomass (forestry waste)	Anode	-	-	370 mA h g^–1 at 100 mA g^–1	Li-ion BAT
[14]	Biomass (forestry waste)	Anode	-	-	147.7 mA h g^–1 at 50 mA g^–1	Na-ion BAT
[28]	Cellulose (HCs)	Anode	-	-	334.9 mA h g⁻¹ at 30 mA g⁻¹	Na-ion BAT
[90]	Glucose, sucrose, maltose, cellulose, glycogen, and amylopectin (HCs)	Anode	-	-	353 and 290 mA h g⁻¹ at 25 mA g⁻¹	Na and K-ion BATs
[66]	Renewable cotton (HCs)	Anode	-	-	315 mA h g⁻¹ at 0.1 C	Na-ion BAT
[67]	Cellulose (HCs)	Anode	-	-	310 mA h g⁻¹ at C/10 rate	Na-ion BAT
[68]	Regenerated spherical cellulose (HCs)	Anode	-	-	300 mA h g⁻¹ at 0.1 C	Li-ion BAT
[71]	Cellulose nanofibers	Binder	2.9 Wh kg⁻¹	21.3 kW kg⁻¹	268.4 F g⁻¹ at 5 A g⁻¹	SC
[72]	Cellulose nanocrystals (nanocomposite)	Template for electrode	17.9 Wh kg⁻¹	530.2 W kg⁻¹	629 F g⁻¹ at 0.5 A g⁻¹	SC
[74]	Cellulose	Hydrogel electrolyte	20.4 Wh kg⁻¹	194 W kg⁻¹	255 F g⁻¹	Flexible SC
[75]	Cellulose (cellulose-polyacrylamide double network)	Hydrogel electrolyte	0.09 mWh m⁻²	4.5 mW cm⁻²	989 mF cm⁻² at 2 mA cm⁻²	Flexible solid-state SC
[27]	PAM–hydroxy propyl cellulose hydrogel	Porous carbon electrode	17.2 W h kg^–1	550 W kg^–1	102.5 F g^–1 at 0.5 A g^–1	SC
[76]	Cellulose nanofibers	Hydrogel electrolyte	139.65 mWh cm⁻²	0.88 mW cm⁻²	396.30 mF cm⁻²	Flexible SC
[77]	Carboxymethylated cellulose nanofibers (NC-ionogel)	GPE	-	-	160 F g⁻¹	SC
[70]	MXene-bacterial cellulose	Flexible electrodes	15μW h cm⁻²	41 μW cm⁻²	346 mF cm⁻²	SC
[30]	Bacterial cellulose (N/O/S-doped)	Carbon electrode	5.14 Wh/kg	5000 W/kg	268.2 F/g at 0.5 A/g	SC
[29]	Bacterial Cellulose/MXene	Mesoporous Electrodes	9.63 Wh kg^–1	250 W kg^–1.	594 F g^–1 at 1 A g^–1	SC
[78]	Cellulose acetate (nanocomposite polyvinyl alcohol/cellulose acetate)	GPE	0.39 Wh kg⁻¹	2.84 W kg⁻¹	26.23 F g⁻¹ (Electrode)	SC
[78]		GPE	0.39 Wh kg⁻¹	2.84 W kg⁻¹	8.01 F g⁻¹ (EDLC)	SC
[79]	CMC	Hydrogel electrolyte	63.3 Wh kg⁻¹	-	309 F g⁻¹	Quasi-solid-state SC
[81]	Cellulose acetate and oxidized carboxymethyl cellulose	Solid and GPE	-	-	230 mA h g⁻¹	Li-ion BAT
[83]	Cellulose	Porous membrane separator	-	-	947 mAh g⁻¹	LiS BAT
[84]	Phosphorylated CNF	Separator	-	-	5.37 mA h cm⁻²	LiS BAT
[7]	Cellulose	Separator	33.4 Wh kg⁻¹	46 W kg⁻¹	25 mAh g⁻¹	Li structural BAT
[3]	Rice husk	Porous carbon	-	-	1032 mA h g⁻¹ at 0.1 C	Li–S BAT

2.2. Seaweed Polysacharides: Alginate, Agar and Gums

Marine algae such as red, and brown seaweed are important polysaccharide reservoirs such as alginate, agar and carrageenans beneficial for energy storage systems [91]. Alginate is a polysaccharide found in brown algae, composed of d-mannuronic acid and l-guluronic acid, which contain multiple carboxyl and hydroxyl groups. The guluronate block (G-block) from the alginate chains can form strong crosslinks with divalent cation ions, creating a structure known as the “egg box structure” (Figure 8) which allows a self-healing behavior, allowing the material to be recovered after damage.

2.2.1. ESSs Based on Alginate

In ESSs, these properties can be extended and used to provide better electrodes. The high and uniform content of carboxy and hydroxy groups in its surface provides strong adhesion of the active materials when it is used as a binder, being a biodegradable, water-soluble and cost-effective alternative (7–8 USD/kg) to the commonly used PVDF (8−10 USD/kg) in the current electrode formulations, allowing the transition to aqueous electrode processing [92]. It could also contribute to better cycling stability due to the carboxylic groups’ presence that restricts particle movement during volume expansion during cycling, improving the mechanical strength. In electrode composition, sodium alginate (SA) has been used in zinc-ion batteries by Xie et al., who developed Mn-, Zn-, and Ca-SA systems as binders for a commercial β-MnO₂ electrode development, where the resulting cathode exhibited improved electrochemical performance compared to the cathode that employs PVDF as binder [93]. Moreover, Kovalenko et al. have compared different binders applied for Li-ion batteries, such as PVDF, CMC, and SA. They found that alginate shows better transport of Li+ ions than CMC, which could mainly be attributed to the fact that -COOH moieties are more homogeneously distributed in alginate but have a random distribution in the chemically modified CMC [94]. Additionally, SA binders were also used in other systems such as Li-S batteries [95] and Na-ion batteries [96].

Electrolytes

The same properties are beneficial for the application of sodium alginate in GPE preparation due to its ability to form hydrogels with mechanical robustness, self-healing and anti-freezing properties, but also its high operational safety and electrochemical performance. The rich hydrophilic groups in alginate provide a high water retention capacity but can facilitate the rapid diffusion of ions through the GPE structure. For example, Wang et al. have used alginate-based anti-freezing hydrogel electrolytes for Zn-ion batteries (ZIBs,) combining it with guar gum (GG) and ethylene glycol (EG). The resulting GPEs were synthesized after GG/SA hydrogels containing an electrolyte solution were immersed in an EG solution and showed an improved ionic conductivity and anti-freezing performance exhibiting the highest discharge capacity of 354.9 mA h g⁻¹ at 0.15 A g⁻¹ and a capacity retention of 91.52% after 1000 cycles at 6 A g⁻¹ [97]. Another example can be found in Badawi et al., where an alginate/PEDOT: PSS composite hydrogel has been used as a self-healing GPE for supercapacitors, showing excellent ability to self-heal and repair its original composition within 10 min of cutting, proving a high specific capacitance of 356 F g⁻¹ at 100 mV/s g⁻¹ [98]. The all-hydrogel supercapacitors constituted the hydrogel electrolyte that was sandwiched between two electrodes, which tightly adhered together because of the existence of hydrophilic interaction/hydrogen bonds. Zeng et al. prepared all-hydrogel soft supercapacitors, which were capable of resisting stretchable and compressible deformation from the combination of a dual-network sodium alginate and polyacrylamide (PAM) with the introduction of redox-active pair K₃Fe(CN)₆/K₄Fe(CN)₆ and Na₂SO₄ into the hydrogel matrix to increase the conductivity and electrochemical activity [99]. Feng et al. reported another example of a multifunctional hydrogel electrolyte based on sodium alginate crosslinked by dynamic catechol–borate ester bonding, which can withstand −10 °C with a self-healing ability [100]. Chen et al. developed a SPE by integrating sodium alginate nanofibers in polyethylene oxide (PEO) matrix. The SPE showed great flame-retardant properties due to the complex heat decomposition properties of alginate and low heat combustion byproducts that slow down heat transfer. This safe SPE offers a great solution to the flammability of organic electrolytes conventionally used in Li-ion batteries. The efficient crosslinking in the composite SPE leads to a wide electrochemical window extending up to 4.6V. SA also facilitates high ionic conductivities at lower temperatures due to the introduction of non-crystalline regions by SA hindering the PEOs original orderly structure [101].

Separators

Sodium alginate can also be applied in preparing functional separators, due to the alginate properties to improve thermal and chemical stability compared with pristine commercial polyethylene (PP)-derived Celgard. It would also improve the wettability of the electrolyte as Song et al. reported for Li-ion batteries. They fabricated a composite separator of SA and attapulgite (ATP) nanofibers by simple phase inversion and evaporation processes (Figure 8). ATP nanofibers provide enhanced thermal stability and mechanical performance, while SA provides good porosity and chemical stability. After being heated at a temperature of 250 °C, the commercial Celgard 2325 separator was burned away, while the SA/ATP separator remained intact [102]. Another practical application of alginate in separators for Li-ion is its use in runaway systems based on bifunctional separators. A thermal-response sodium-alginate-modified poly(m-phenylene isophthalamide) (PMIA) separator has been reported by Hu et al. as a successful tool to improve Li-ion safety. The PMIA polymer has flame-retardant and self-extinguishing properties as well as a good electrolyte wettability. The introduction of a sodium alginate coating layer increases the mobility and transport of Li⁺ ions compared with non-modified PMIA separators by 2.5 fold, improving the performance [103]. Similar results have been reported by different polymers such as polyetheramine [104]. Apart of non-solvent phase inversion (NIPS) preparation or papermaking processes [105], sodium alginate-based nonwoven separators in batteries also attract wide attention. Similar to the previous example, a heat-resistant alginate-based nonwoven separator has been prepared by electrospinning by Wen et al. and applied to Li-ion batteries [106]. Interestingly, the SA separator provides larger cycling stability (79.6% of its initial discharge capacity, exceeding the 69.3% for PP separator) after 200 cycles at an elevated temperature of 55 °C.

Figure 8. (1) (a) Structure of sodium alginate where crosslinkers are divalent, trivalent, and quadrivalent ions, respectively and (b) schematic of the “egg-box” structure; Reprinted from [107]. (2) (a) synthesis process of SAM-700 melanine-alginate material, (b) schematic draw of all-solid-state symmetric supercapacitor in PVA/SA/KOH gel electrolyte, showing the interaction between SA and PVA using borax as crosslinker, (c) specific capacitance and capacitance retention of SAM-700-4//SAM-700-4 all-solid-state symmetric supercapacitor, (d) Ragone Plot and picture of the device in the inset; Reproduced from Ref. [108]. © 2022 Published by Elsevier B.V. (3) Schematic illustration on the preparation process of N-PCNFs from cobalt crosslinking of sodium alginate. Reproduced with permission from [109]. © 2015 American Chemical Society. (4) (a) The synthesis routine of SA/ATP separators via a phase inversion method. (b) Digital photos of Celgard 2325 and SA/ATP separators before and after being heated to 250 °C. (c) The SEM images of the porosity of SA/ATP separators before and after being heated at 250 °C. (d) The fire retardancy of the SA/ATP separator (left) and Celgard 2325 (right). Reproduced with permission from Ref. [102]. Copyright 2019, Elsevier.

Active Material (Carbon Precursors)

Porous carbon and nanocarbons can also be synthesized to form sodium alginate-based precursors, being used as electrode materials for energy storage devices, i.e., in supercapacitors. The high content of functional groups (-OH and -COONa) enables the introduction of other functional groups and moieties such as heteroatoms like nitrogen while providing extra pseudocapacitance. The introduction of this kind of group can improve the wettability, conductivity and capacitance. In this sense, SA is used as a promising carbonaceous precursor for electrode preparation [110], (Figure 8). Sodium alginate aerogels have been employed as electrodes for supercapacitors [111] and batteries [110]. For example, Ye et al. fabricated nitrogen and oxygen co-doping porous carbon nanospheres using SA and PANI (Polyaniline) as precursors which showed a specific capacitance of 627 F g⁻¹ at 1 A g⁻¹ in a 3 M H₂SO₄ electrolyte with an enhanced wettability and performance [112]. In addition to PANI, melamine [108], urea [113] and ammonium alginate [114] have also been used as a co-dopant (Figure 8). Another option for adding pseudocapacity is to incorporate metal oxides or metal sulfides/phosphides. Sodium alginate-derived carbons can strongly bind these nanomaterials, providing good performance with larger capacitance retention [115], and as mentioned, SA-based aerogels are also often used as anode materials in batteries [116]. Wang et al. synthesized hybrid Co₃O₄/SA-C materials from crosslinking of alginate chains with Co²⁺ ions, followed by a pyrolysis between 600 °C and an oxidation process at 200 °C. The supercapacitor performance displayed a specific capacitance of 645 F/g at 5 mV/s in 2 M KOH, and a 99% capacitance retention after 10,000 charge−discharge cycles at 20 A g⁻¹. The same hybrid Co₃O₄/SA-C material has been used as anode for Li-ion which retains a Coulombic efficiency of 99.5% and a specific discharge capacity of 779 mAh g^–1 after 50 cycles [117]. Zhang et al. have also reported nitrogen-doped SA-carbon aerogel as an anode material [108], and Lv et al. have used FeSe nanoparticles embedded into carbon nanofiber aerogels as anode materials for Na-ion batteries [118].

2.2.2. ESSs Based on Agar and Gums

Agar in turn is made up of two polysaccharides; agarose (70%) and agaropectin (30%) which are primarily differentiated by the presence of sulfate groups. Agarose is a linear polysaccharide with repeating D-galactose and 3,6-anhydro-L-galactopyranose connected by glycosidic bonds and has numerous hydroxyl groups, while agaropectin is a complex branched polysaccharide with sulfate groups. The numerous hydroxyl groups present in agar are beneficial in crosslinking and retaining water molecules in hydrogels and GPEs [119]. Agarose is soluble in hot water and gelates upon cooling. Hydrogel structures are formed by physical crosslinking of hydrogen bonds which are intricate structures with tunable porosity, reinforcable mechanical strength and flexibility and to improve functionality depending on use as different parts of ESSs.

On the other hand, natural gums such as Xanthum gum (XG), Gellan gum (GG), Gum Arabic (GA) and κ-carrageenan (CG) have inherent viscous characteristics that make them relatively better precursors, notably for gel electrolytes and binders compared to the other polysaccharide counterparts which require advanced chemical modifications. The versatile and rich functional groups make it possible to transform the natural gums into multifunctional gums to sustain in an EES [120]. However, there are reports of gums used as carbon sources and other parts like separators as mentioned below. Gellan gum is an anionic linear polysaccharide with four different repeating polymeric units namely, two 1,3-β-D-glucose, one 1,4-β-D-glucuronic acid, and one 1,4-α-L-rhamnose unit. Guar gum is a complex structure with a mix of polysaccharides and glycoproteins. The main chain is a β-1,3-linked D-galactose core with multiple branching to 3- and 6-linked galactose and 3-linked arabinose.

Active Material (Carbon Precursors)

Freeze-dried agarose hydrogels (aerogel) upon carbonization and activation are fine-tuned to have microporous and mesoporous structures with high surface area and pore volume. Hwang et al. showed that using potassium oxalate (K₂C₂O₄) as an activator could lead to a fine interconnected pore structure which will mediate efficient charge transfer compared to KOH and KCl that forms closed pores. The porosity contributes to a superior specific capacitance of 166.0 F g⁻¹ at 0.125 A g⁻¹ (Figure 9). In addition K₂C₂O₄ releases gaseous byproducts (CO, CO₂) that act as a surfactant to stabilize the pore structure [121]. Moreover, introducing heteroatoms has been investigated to boost the electrochemical performance due to redox activity and pseudocapacitance [122]. In one such instance, co-doping multiple heteroatoms have been shown to synergistically to improve the K⁺ ion storage in supercapacitors and batteries. O/N/B (oxygen, nitrogen, boron) atoms were doped into carbon aerogel (CA) from agarose precursor, forming a porous 3D honeycomb structure with multiple active sites for charge transfer. ON-CA showed symmetric GCPL profiles and high voltage plateaus. ON doping resulted in reversible redox reactions with a mass-specific capacitance of 385.3 F g⁻¹ at a current density of 0.5 A g⁻¹ [123].

The rapid commercialization of Li-ion batteries can be sustained effectively by anodes that facilitate uninterrupted lithiation/delithation with controlled volume expansion. Silicon (Si) is an abundant alternative to graphite anodes with a very high theoretical capacity of 4200 mA h g⁻¹, but it comes with the downside of serious volume expansion and breakage of thick SEI layers disassociating anode from current collectors. To overcome these challenges, Si can be cladded with carbon and other conductive nanoparticle composites to devise porous structures with high conductivity [124]. Sang et al. developed an algal seaweed (Gelidium amansii) derived from naturally nitrogen-doped biochar decorated with Si nanoparticles (NPs) to productively regulate the volume expansion during dis(charge) and a stable SEI. The interconnected biochar network structure encapsulates Si NPs enhancing the electrode performance with high conductivity and cyclic stability, (Figure 9). A reversible capacity at 1111.61 mA h g⁻¹ was maintained after 500 cycles at a current of 1 A g⁻¹ and reduced to 714.08 mA h g⁻¹ after 1000 cycles under the same current density [125].

Figure 9. (1) Preparation of Si@N-GAm-5 composites (SEM in inset) (a) capacity-voltage curves of Si@N-GAm-5 at the first, fifth, 10th, 50th, and 100th cycles at 1 A g^–1 current density and the CV curves of the Si@N-GAm-5 composites. Reproduced from [125]. Copyright © 2024, American Chemical Society; (2) (a) SEM images of the carbon prepared from agarose (PC-6-700, a′–c′); (b) Galvanostatic charge–discharge profiles; (c) Rate capability tests. Reproduced from [121]. Copyright © 2021, the authors. Published by American Chemical Society; (3) Left: In situ optical microscope photographs of the symmetric transparent cells under various deposition times in (c) 1 M Zn(CF₃SO₃)₂ aqueous electrolyte and (d) PAAm/agar/Zn(CF₃SO₃)₂ hydrogel electrolyte. Right above: (a) Schematic model of the assembled quasi-solid-state zinc-ion hybrid supercapacitor. (b) Rate performance of the Zn//PAAm/agar/Zn(CF₃SO₃)₂//NC ZHSC. Right bottom: (a) Voltage–time curves of the Zn//Zn symmetric cells in hydrogel and aqueous electrolytes. (b) Tafel curves. Reprinted from [126]. Copyright © 2022, American Chemical Society; (4) Preparation process of the Ag/GO-GPE. (a) CV and (b) GCD profiles. Bottom part: (a) Ag/GO₀-GPE, Ag/GO₁-GPE, Ag/GO₂-GPE, Ag/GO₃-GPE membranes in contact with a hot electric iron tip (b) Photos of the Ag/GO₀-GPE membrane after different burning times. (c) Photos of the Ag/GO2-GPE membrane after different burning times. Reproduced with permission from [127]. © 2022 Elsevier B.V. All rights reserved.

Electrolytes

A highly ionic conductive and flexible hydrogel using agarose with graphene oxide (GO) and lithium acetate (LiOAc) was fabricated by Lv et al. GO nanosheet facilitated the impedance reduction between layers allowing easy movement of Li⁺ ions. An optimized ordered lamellar GPE showed a high ionic conductivity (73.8 mS cm⁻¹) with a high operating window of 1.8V and electrochemical performance did not recoil with bending, Figure 9 [127]. A guar gum (GG) composite with vinyl acetate was used as a corrosion inhibitor in the electrolyte in a lead acid battery by forming a protonated thin film on the Pb surface by electrostatic interactions preventing corrosion of electrode by sulfuric acid (5 M) at a fully charged state and due to hydrogen evolution reaction [128]. Over time agar has been notably used as a corrosion inhibition additive in metal batteries. One such case is where agar as an additive is used in aluminum-air batteries to deter the self-corrosion reaction of aluminum anodes by shifting the corrosion potential values to negative and depending on the concentration of the additive, significantly inhibiting the hydrogen evolution rate. However, unlike GG the inhibition is due to viable physisorption of sulfate ester function groups (R–O–SO₃⁻) of agar molecules rather than film forming [129].

On the other hand, aqueous ZIBs are firmly gaining momentum due to their low cost and eco-friendly nature. Still, the parasitic reactions due to water electrolytes and limiting operating potential further lead to corrosion and hydrogen evolution reaction (HER) prevails. So there is a gradual shift towards GPEs. Nevertheless, all biopolymers in this context are not suited for Zn-ion batteries due to incompatibility with the sulfate ions in the electrolyte (ZnSO₄). Wu et al. proposed a Gum Arabic (GA)-based gel polymer electrolyte for ZIBs. The extensive hydroxyl groups facilitated a network with strong hydrogen bonding preventing any detrimental effects of aqueous solvent, thus extending the voltage window up to 2.26 V vs. Zn^2+/Zn while facilitating Zn ion diffusion. Thus, inhibiting corrosion and HER compared to electrolytes without GA. The GPE showed an ionic conductivity of 1.10 × 10⁻² S cm⁻¹ and was stable for 1300 h at 0.2 mA cm⁻² with 0.2 mAh cm⁻² [130]. The physical crosslinking of hydroxyl groups alone may not be able to render the mechanical stability; tensile and compressive strength in the long term. Complementing the hydrogen bonding of agar, a covalently bonded dual-hydrogel network of PAAm (polyacrylamide) with Zn(CF₃SO₃)₂ as the aqueous electrolyte was reported by Ji et al. They assembled a Zinc-ion hybrid supercapacitor (ZHSC) with a Zn anode and a nitrogen-doped biopolymer-derived carbonaceous electrode as cathode with the hydrogel electrolyte (Figure 9). The hydrogel was highly flexible and could withstand extreme mechanical deformations with a compressive stress of 118.0 kPa under 60% strain condition and depicts a high ionic conductivity of 1.55 S m⁻¹ indicating smooth ion migrations between the electrodes. A capacity retention of 56.8% was noted as the current densities increased from 0.25 to 20.0 A g⁻¹. Even after 10,000 cycles, the EES system showed an ultrahigh capacity retention of 112.4% and almost 100% Coulombic efficiency at 5.0 A g⁻¹. The hydrogel limits the anode platting due to the absence of any side reactions because of free water molecules [126].

Binders

Maneuvering binder functionality can significantly reduce the limitations regarding battery performance because a good binder with adequate adhesion between the current collector, conductive and active materials that ions can easily shuttle through, ensuring long-term cycling and stability. Compared to traditional binders (PVDF) and other biopolymer-based binders (CMC and agarose), a carboxymethylated gellan gum was proposed as a green binder with great adhesion and water solubility. The inherent viscous nature also comes in handy here. The large number of carboxylic and acetyl groups in the structure favor robust hydrogen bonding and secure interfacial interaction among binder and other electrode materials, respectively. Thus, restraining volume expansion and with long cyclability. At a higher mass loading (>1.0 mg Si/cm²), the battery was stable up to 100 cycles with a charge capacity retained till 900 mA h g⁻¹ at a current density of 1000 mA g⁻¹ and an excellent ICE of 82.5% [131] Marine polysaccharides and natural gum-based materials in different ESSs are summarized in Table 2.

Table 2. Seaweed polysaccharides: alginate, agar, and gums as different functional parts of energy storage systems.

Ref	Polysaccharide (Modified Form)	Functionality	Energy Density	Power Density	Specific Capacity/ Specific Capacitance	ESS
[4]	Kelp (sodium alginate)	Hydrogel electrolyte, membrane separator and binder	8 W h kg⁻¹	25 W kg⁻¹	277 F g⁻¹ and 88.2 F cm⁻³	solid-state SC
[4]	Kelp (sodium alginate)	Hydrogel electrolyte, membrane separator and binder	2.5 W h L⁻¹	8 W L⁻¹	277 F g⁻¹ and 88.2 F cm⁻³	solid-state SC
[19]	Alginate (mussel-inspired catechol conjugation)	Binder	-	-	600 mAh g ⁻¹	Li-ion BAT
[98]	Alginate (alginate/PEDOT:PSS)	Hydrogel electrolyte	40.08 Wh kg⁻¹	400.35 W kg⁻¹	356 F g⁻¹ at 100 mV/s g⁻¹	Flexible solid-state SC
[102]	Sodium alginate (sodium alginate/attapulgite)	Separator	-	-	115 mAh g⁻¹ at 5 C	Li-ion BAT
[103]	Sodium alginate/PMIA	Separator	-	-	101.1 mAh g⁻¹ at 2 C	Li-ion BAT
[104]	Alginate fiber-grafted polyetheramine	Separator	-	-	100 mAh g^–1 at 2 C	Li-Metal BAT
[105]	Cellulose and calcium Alginate fiber	Separator	-	-	160 mAh/g at 0.5 C	Li-ion BAT
[106]	Alginate	Separator	-	-	151 mAh g⁻¹ at 1 C	Li BAT
[111]	Sodium alginate	porous carbon aerogel	28 W h kg⁻¹	400 W kg⁻¹	204 F g⁻¹ at 0.2 A g⁻¹	SC
[110]	Alginate nanofibers from seaweed (N-doped graphitic nanofibers)	Electrocatalyst and anode material	-	-	625 mAh g⁻¹ at 1 A g⁻¹ and 197 F g⁻¹ at 1 A g⁻¹	Li-ion BAT and SC
[108]	Sodium alginate /melamine fibers	Electrode/gel electrolyte /separator	20.87 Wh kg⁻¹	4000 W kg⁻¹	441.80 F g⁻¹ at 0.5 A/g	Solid-state SC
[113]	Alginate (polyurea-crosslinked nitrogen/metal co-doped alginate)	Carbon aerogel	6.5 Wh kg⁻¹	37.5 W kg⁻¹	95.3 F g⁻¹ at 1 A/g	SC
[114]	Phenolic resin/ammonium alginate	Coral-like carbon structure	9.82 Wh kg⁻¹	115 W kg⁻¹	282.8 F g⁻¹ at 0.5 A g⁻¹	SC
[117]	Co₃O₄ nanoparticles anchored carbon aerogel	Carbon aerogel	-	-	779 mAh g^–1	Li-ion BAT
[118]	FeSe nanoparticles/carbon nanofiber aerogels	Carbon nanofiber aerogels	-	-	313 mA h g⁻¹ at 2000 mA g⁻¹	Na-ion BAT
[132]	Agar-PVA/GO	Gel electrolyte	-	123.7 mW/cm²	595.8 mA h g⁻¹	Zn-Air BAT
[133]	Agar/PAM	Hydrogel electrolyte	-	-	300 mAh g⁻¹ at 0.1 A g⁻¹	Zinc-ion BAT
[121]	Agarose/K₂C₂O₄	Porous carbon	18.9 Wh kg⁻¹ at 0.125 A g⁻¹	20 kW kg⁻¹ at 10 A g⁻¹	166.0 F g⁻¹ at 0.125 A g⁻¹	SC
[123]	Agarose (O/N)	Carbon aerogel	51.8 Wh kg⁻¹	443 W kg⁻¹	174 mA h g⁻¹	K-ion BAT and SC
[125]	Seaweed/Si N-doped	Biochar-anode	-	-	1111.61 mA h g^–1 at 1 A g^–1	Li-ion BAT
[127]	Lamellar agarose/graphene oxide	Gel polymer electrolyte	-	-	791.67 mF cm⁻² at 5 mA cm⁻²	Solid-state SC
[128]	Guar gum/vinyl acetate	Anode	-	-	-	Lead-acid BAT
[129]	Agar	Electrolyte additive	2912 Wh kg⁻¹	-	2366.86 mA h g⁻¹	Aluminum-air BAT
[130]	Gum arabic	Gel polymer electrolyte	-	-	0.2 mA cm⁻² with 0.2 mA h cm⁻²	Zinc-ion BAT
[20]	Xanthum gum (Millipede inspired)	Structured binder	-	-	2150 mA h g⁻¹	BAT
[126]	PAM/agar/Zn(CF₃SO₃)₂)	Cathode/hydrogel electrolyte	61.3 Wh kg⁻¹	-	92.8 mA h g^–1 73.4 mA h g^–1	Zinc-ion hybrid solid-state SC
[134]	Sodium alginate and pectin	Solid blend biopolymer electrolyte (1.26 × 10^–7 S cm^–1 at 303 K)	-	-	-	ESS
[135]	Potato starch and guar gum (mass loading—7.0 mg cm⁻²)	Binder	-	-	20 F g⁻¹ at 10 A g⁻¹	SC

2.3. Starch

One of the best-known polysaccharides is starch, the main form of energy storage in plants and is made from two polysaccharides: amylose and amylopectin. It has a large amount of hydroxyl groups that make starch in a well-suited biopolymer for aqueous ESSs [136]. The amylose chains coil into a spiral shape by hydrogen bonding, acting as a compact structure that provides remarkably high retention of water and insolubility, making starch a very suitable choice for hydrogel formation [137]. Indeed, the high concentration of hydroxyl groups present in the starch surface can be increased after the dextrinization process when heated in water: it goes through an order to disorder transition where the coils break down small chains in solution [138]. Also, the high presence of amylopectin (normally > 80%) in the starch structure, which is branched, often providing superior mechanical properties when compared with other similar polysaccharides, such as CMC. Starch can be easily prepared, and even artificially synthesized and functionalized [139,140] making it itself a versatile material for many applications such as green binder, gel-polymer matrix and/or carbon precursor.

2.3.1. ESSs Based on Starch

Binders

Starch is a natural commercial available and cost-effective binder which is used in the preparation of conductive glues and electrodes. It has been used in non-aqueous ESSs, providing a reduction in the electrode resistance, compared with only CMC, due to the noticeably lower pore clogging by starch [141]. In this work from Jeżowski et al., starch is used in supercapacitors with a double strategy: as binder in the formulation of the activated carbon slurry, but also forming a conductive intermediate layer in combination with carbon SuperP (carbon black) on the top of stainless steel current collector, reducing the internal resistance and allowing a better adhesion. Other authors, such as Varzi et al., has also reported the use of starch as binder for supercapacitors, where its use provides higher electrode loadings (>5 mg cm⁻²) without cracking thanks to the presence of high content of amylopectin, when compared with linear polymer such as CMC [142]. Modified starch has also been used as binder for stabilizing silicon nanoparticles in anodes for Li-ion batteries due to the interaction of Si with the starch hydroxyl groups [143]. For example, Rohan et al. has used an in situ thermally crosslinking of starch with maleic anhydride during the preparation of the silicon anodes in water. This strategy allows the generation of a 3D-structure suppressing swelling issues, thus providing better mechanical properties for the composite electrodes. In comparison with PVDF, crosslinked starch anodes shown a higher performance in terms of stability and efficiency [144].

Electrolytes

As a GPE component, starch can be found in combination with ionic liquids, such as [Amim]Cl [145], but also with other biopolymers like chitosan [146] and lignocellulosic biomass [147]. Those sustainable GPEs were being used in flexible supercapacitors, but also in batteries such as Li-S, but also Zn-ion and Zn-air. El Sharkawy et al. prepared a LiCl@starch-based hydrogel which has an ultrahigh ionic conductivity of 0.079 S·cm^–1 with excellent thermal stability and nonflammability, and the assembled symmetric supercapacitor (AC//2-LiCl@starch//AC) withstands a wide operating voltage window of 2.4 V with a specific capacitance of 62.3 F g⁻¹ [148]. Another example, but in quasi-solid Li-S batteries, is found in Lin et al. work [149]. Lin developed a starch-based GPE which has an average discharge capacity of 562 mA h⁻¹ at 0.5 C for 1000 cycles at room temperature, that could reach the 2000 cycles at 2 C and 45 °C with a capacity of 388 mA h g⁻¹ (Figure 10).

Figure 10. (1) Preparation of starch-hosted electrolyte films and molecular structural characterization of the starch host. Electrochemical stability of the starch-hosted electrolyte. (a) Cyclic voltammetry of SS/Starch + LiTFSI/Li cells at the scan route of 2.8 V → −0.8 V → 6.0 V and at a rate of 10 mV s⁻¹. (b) Polarization tests of Li/Starch + LiTFSI/Li cells at 25 and 45 °C at 0.01 mA. Reproduced from [150]. Copyright © 2016 The Royal Society of Chemistry. (2) (a) Zn anode protection from SS gel layers on Zn metal. (b) Galvanostatic cycling of bare Zn and Zn@SS symmetrical cells; (c) SEM images of the Zn electrode after 10 cycles (d) Linear polarization curves of bare Zn and Zn@SS. Reprinted from [151]. © 2022 American Chemical Society. (3) Electrochemical performances of the FAZAB based on nano-SFQ, SFQ, and PVA–KOH GPEs. (a) GCD curves at 1 mA cm⁻². (b) Rate profiles at different current densities of 0.25, 0.5, 1, 2, 4, and 8 mA c⁻². (c) The galvanostatic discharging curves. (d) Polarization and corresponding power density curves. (e) Comparison of the cycling life of the nano-SFQ-based FAZAB with previously reported FAZABs. Inset: The photos of PVA–KOH and nano-SFQ in the original state and after exposure to air for different period of time. Reprinted from [152]. © 2022 Wiley-VCH GmbH.

Starch hydrogels are also useful as protective anode layers. In the case of zinc-based batteries, such as Zn-ion, starch has been deposited on top of the Zn anode, creating a protective layer that enables Zn electroplating homogeneity reducing adverse reactions. Starch has the chelating ability to coordinate Zn²⁺ ions, forming ionic conductive gels, which can increase ion transportation, regulating the plating to metallic Zn. Using this system, Wang et al. reported symmetrical Zn||Zn cells with this starch layer on top, showing a prolonged long lifetime of 1000 h at 5 mAh cm^–2 [151], Figure 10. A similar strategy was followed by Fan et al. but using a sulfonated functional composite (SFQ) starch in the composition of a gel polymer layer with KOH-based electrolytes to develop flexible aqueous zinc air batteries (FAZAB), Figure 10. The presence of sulfonate groups contributes to exposing the preferent 002 plane from Zn, which is less prone to dendrite formation [152]. A part of Zn, starch can also be used in Al-ion batteries as a gel electrolyte promotor. Using AlCl₃ as salt a stable GPE can be obtained due to Al³⁺ cation interaction with the hydroxyl group of starch and increases polymer solubility, making gel electrolyte viable for battery applications [153].

Separators

In addition to the GPE, starch can be used in the development of membranes, but there are not many examples in the literature [154]. Jeżowski et al. have prepared a membrane for Li-ion capacitors by ultrasonic-assisted polymerization from a solution 1:1 of potato starch and water, followed by a soaking step with 1 M Li₂SO_4.-The prepared separators are flexible and can be stored for up to 12 months without any degradation, withstanding extended cyclability (up to 50,000 cycles) [155]. In addition, starch has been used in the preparation of solid gel polymer electrolytes as a substitute for Na-ion membranes in redox flow batteries. Aday et al. demonstrated its potential use in passive quinone-based redox flow batteries but further investigation needs to be carried out to improve the mechanical strength of the membranes [156].

Active Material (Carbon Precursors)

Another important application of starch in energy storage is as a carbon precursor for electrode preparation. Mesoporous carbons can be obtained with a controlled porosity by pretreatment with IL, followed by the Starbon process^® [157], where the starch is treated in water with temperature to expand its structure, creating a mesoporous network. Here, the starch type and its content in amylose have an impact on the porosity of the carbonized material [158]. Normally, the higher this content, the higher the resultant porosity. Uriburu-Gray et al. has demonstrated the suitability of these mesoporous carbons in Li-O₂ materials, resulting in good cyclability up to 75 cycles at 300 mAh/g. Controlled porosity can also be achieved using the exothermic pyrolysis method [159]. Cao et al. used Mg salts to generate an in situ MgO template as a pore creator during the pyrolysis of the Mg–starch composites, obtaining a specific surface area up to 2300 m² g^–1 with the hierarchical porous architecture of interconnected micro-, meso- and macropores. The prepared material was used in symmetrical supercapacitors using 6 M KOH, obtaining a good specific capacitance (229 F g^–1 at 1 A g^–1) with a large cycling stability (94% capacitance retention after 10,000 cycles at 2 A g^–1) [160]. Other suitable methods are the pyrolysis of starch structures with a typical chemical activation with KOH [161]. On the other hand, Titirici et al. have produced hard carbons for Na-ion batteries from a novel technique applying a pretreatment by bio-fermentation. This step can effectively facilitate the pyrolysis for producing starch-based HCs, generating a larger interlayer spacing and disordered structure with abundant closed micropores, which is favored for sodium storage. They reported a high reversible capacity of 335 mA h g⁻¹ at 30 mA h g⁻¹ with a high rate performance with a reversible capacity (140.6 mA h g⁻¹) and long cycling stability [162]. A hybrid ternary composite electrode was prepared by integrating two 2D materials such as MoS₂ (molybdenum disulfide) and reduced graphene oxide (rGO) with plasticized starch. The two former 2D materials act as nanofillers and the biopolymer acts as the matrix. The composite electrode exhibits superior electrochemical performance due to reduced crystallinity because of rGO and the defects created by MoS₂. The electrode retained 85% of its capacitance up to 10,000 cycles with an improved specific capacitance of 124.91 F g⁻¹ 0.10 mA g⁻¹ [163]. Starch biopolymers in different ESSs are summarized in Table 3.

Table 3. Starch as different functional parts of energy storage systems.

Ref	Polysaccharide (Modified Form)	Functionality	Energy Density	Power Density	Specific Capacity/ Specific Capacitance	ESS
[141]	Starch	Binder	12 Wh kg ⁻¹	100 kW kg⁻¹	-	SC
[146]	Chitosan/potato starch	Electrolyte	3.17 Wh kg⁻¹	855 W kg⁻¹	16.1 F g⁻¹	SC
[148]	LiCl/starch	Hydrogel electrolyte	28 Wh kg⁻¹	-	62.3 F g⁻¹	SC
[149]	Starch–lithium acetate and titania	Solid polymer electrolyte	-	-	119.77 F g−1	solid-state SC
[151]	Starch	Anode (hydroxyl layer)	-	-	115 mAh g⁻¹ at 1.0 A g⁻¹	zinc-ion BAT
[153]	Starch/Al₂(SO₄)₃·18H₂O	Gel electrolyte	-	-	193 mA h g⁻¹ (Al\|MoO₃)	Al-ion BAT
[153]	Starch/Al₂(SO₄)₃·18H₂O	Gel electrolyte	-	-	140 mA h g⁻¹ (Al\|MnO₂)	Al-ion BAT
[155]	Potato Starch	Separator	8 Wh kg⁻¹	1 kW kg⁻¹	30 F g⁻¹	EDLC
[159]	Starch	Porous cathode	-	-	3978 mAh/g at 1 C	Li-O₂ BAT
[160]	Starch–magnesium nitrate	Porous carbon	-	-	229 F g^–1 at 1 A g^–1	SC
[161]	Starch	Biocarbon	60.16 Wh kg⁻¹ (@140 W kg⁻¹)	24,590 W kg⁻¹ (@51.24 Wh kg⁻¹)	192 F g^–1	SC
[164]	Corn starch	Porous carbon	24.5 Wh kg^–1	695 W kg⁻¹	372 F g^–1 at 0.5 A g^–1	SC

2.4. Pectin

One further biopolymer frequently used in ESS studies is pectin. Natural pectin is a polysaccharide which can be found in abundance in the peel of many fruits, especially apples and oranges, and can be extracted under mildly acidic conditions. Chemically, pectin is a linear chain of poly(α-1,4-galacturonic acids) linked by α-(1,4)-glucosidic bonds, residues with additional sugar residues such as arabinose or rhamnose depending on its source, which can vary the degree of methylation of carboxylic acid presence (Figure 11).

2.4.1. ESSs Based on Pectin

Electrolytes

Its water solubility and ability to form gels in the presence of cations make it suitable for hydrogel formation, among other applications such as binder, for energy storage, enabling free-standing electrolyte gelation. Thus, GPE formation is the wider use of pectin due to its gelling properties, induced in high-esterification pectin due to the self-crosslinking by hydrogen bonding and hydrophobic interactions between the methyl-ester terminations. Chelfouh et al. have used apple pectin to generate a GPE for a Zn-Li-ion hybrid supercapacitor using different ratios of affordable Zn(SO₄) and Li₂(SO₄), observing good conductivities and performance, similar to the same liquid electrolytes, with the addition of withstanding a wide range of operating temperatures (50 to −20 °C) [165], Figure 11. In the case of primary Mg-ion batteries, a pectin GPE can be obtained by a simple solvent casting method using only Mg(NO₃)₂, showing an ionic conductivity in the range of 10⁻⁴ S cm⁻¹, and good stability until 3.8V [166]. A similar study can be found for Zn-ion batteries in which ZnCl₂ is reported as the supporting salt [167]. Other examples of the use of pectin in forming GPEs can be found in the studies of Wilson and Botte, where they use small amounts of CaCl₂ as a crosslinking agent in low-esterified pectin, and NaCl and KCl salts as supporting electrolytes, obtaining a proton ion transport GPE. This is due to KCl dissociates and K⁺ preferentially taking the proton site at the end of a carboxylic acid group from pectin. This allows the transfer of protons through the channels in the pectin matrix, thanks by helical strands arrangement as an “egg box” junction effect that we can also find in other polysaccharides such as cellulose. The prepared GPE was evaluated in a coin cell symmetrical supercapacitor showing good capacitance retention over 5000 cycles [168]. (Figure 11). Other suitable salts to obtain a proton-conducting GPE based on pectin are ammonium chloride or bromine [169], as well as ammonium thiocyanate [170].

Figure 11. (1) Pectin structure and (a) electrochemical performance of LVO electrode with different binder (a) PVDF and (b) pectin at a current density of 0.02 A g⁻¹ in the potential range of 0.01–3 V; (c,d) show the lithiation/delithiation profile of LVO-PVDF electrode and LVO-pectin electrode; (e,f) are the lithiation/delithiation rate performance of LVO-PVDF and LVO-pectin electrode; (g) The XRD patterns of the LVO-pectin electrode after 100 cycles; (h) XRD patterns of the LVO-PVDF electrode after 100 cycles. Reproduced from [171]. © 2023 Elsevier Ltd. All rights reserved. (2) Electrochemical properties of Zn plating/stripping for (a) mono-salt electrolytes and (b) bi-salt electrolytes. Current applied: 0.250 mA/cm² for 2 h [165]. (3) (a) Proposed ion transport mechanism for pectin matrix doped with aqueous CaCl₂ and KCl. The KCl dissociates and K+ preferentially takes the place of an H+ on the end of a carboxylic acid functional group. This allows the Cl−, H₂O, and dissociated OH− to transfer protons through the channels in the pectin matrix. The pectin matrix is characterized by helical strands that find arrangement through the Ca⁺ “egg box” junction effect. (b) CVs measured from 0 to 0.8 V, then back to 0 V over a range of measurement rates are displayed on the 5th cycle of each speed to allow for relaxation to a sustained periodic state. (c) Specific capacitance retention after 5000 GCD cycles between 0 and 0.8 V is displayed with the measurement rate of CVs used in the calculation. Reproduced from [168].

Binders

As a binder, pectin is less used, in comparison with other polysaccharides, but we can find a few examples. Su et al. have reported a recyclable Li₃VO₄-pectin anode (LVO) for Li-ion battery (Figure 11), where the authors claim that the utilization of pectin as an eco-friendly binder demonstrated nearly 90% material recovery and 100% capability for subsequent re-lithiation [171]. The same authors recently used iron-doped pectin as the binder in graphite electrodes for Li-ion, showing an improvement in performance compared with PVDF due to an increase in surface-controlled processes instead of diffusion-dominated processes, and the presence of Fe⁺²/Fe³⁺ ions [172]. In the case of supercapacitors, Harikumar and Batabyal reported in a recent article the use of pectin as a binder for free-standing electrodes. Using the combination of graphite, PEG and pectin, they obtained biodegradable electrodes with a capacitance retention of 98% and a specific capacitance of 11.48 mF/cm² over 2500 cycles [173]. Pectin-based biopolymers in different ESSs are summarized in Table 4.

Table 4. Pectin as different functional parts of energy storage systems.

Ref	Polysaccharide (Modified Form)	Functionality	Energy Density	Power Density	Specific Capacity/ Specific Capacitance	ESS
[165]	Apple pectin	Hydrogel Electrolyte (>10^–3 S cm^–1)	-	-	-	ESS
[166]	Pectin–Mg(NO₃)₂	Electrolyte (10⁻⁴ S cm⁻¹)	-	-	-	ESS
[167]	Pectin–ZnCl₂	Electrolyte (4.49 × 10−3 S/cm)	-	-	-	Zn-ion BAT
[169]	Pectin doped with NH4X (X=Cl, Br)	Electrolyte (NH4Cl—4.52 × 10⁻⁴ S cm⁻¹ NH4Br—1.07 × 10−3 S cm⁻¹)	-	-	-	Solid-state devices
[170]	Pectin–ammonium thiocyanate (NH₄SCN)	Electrolyte (1.5 × 10⁻³ S cm⁻¹)	-	-	-	ESS
[171]	Pectin–Li₃VO₄	Anode			230 mAh g⁻¹ at 0.02 A g⁻¹	Li-ion BAT
[171]	Pectin–Li₃VO₄	Anode			400 mAh g⁻¹ 1 A g⁻¹	Li-ion BAT
[173]	Pectin/PEG/graphite	Electrode and electrolyte	-	0.86 mW cm⁻²	11.48 mF cm⁻²	SC

2.5. Sugars (Dextran and Glucose)

2.5.1. ESSs Based on Dextran

Electrolytes

Complex sugar molecules such as dextran produced by lactic acid bacteria could be an effective multifunctional electrolyte for Zn-air batteries (ZAB). Among different electrolytes tested for ZAB, there might be a problem of high overpotential for Zn deposition with hydrogel electrolytes, water in salt and high-concentration electrolytes due to their high viscosity [18]. Li et al. showed that dextran as an electrolyte additive could improve the electrochemical performance of ZAB. Dextran surprisingly exhibits traits similar to common ZAB electrolytes such as Zn(CF₃SO₃)₂, ZnSO₄, Zn(ClO₄)₂, and ZnCl₂. The multifunctional polysaccharide forms an SEI passive layer and facilitates gradual desolvation of [Zn(H₂O)₆]²⁺ ensuring adsorption on Zn (002) planes selectively while supplying Zn²⁺ ions which are uniformly deposited due to homogenous electric field at to electrode–electrolyte interface. With 50 mg mL⁻¹ additive with Zn(CF₃SO₃)₂ electrolyte, the Zn anode depicts the highest cumulative plating capacity (CPC) of 3400 mAh cm⁻² at 5 mA cm⁻² [18].

2.5.2. ESSs Based on Glucose

Active Material (Carbon Precursors)

Glucose is derived from the byproducts of biorefinery, as a major product of the hydrolysis of starch [174]. Direct carbonized glucose and hydrothermally carbonized (HTC) glucose can be used to produce biochar and activated carbons which can be used in supercapacitors and batteries. The HTC treatment of glucose, introduced by Falco, Sevilla and Titirici [175], before the activation process allows the production of carbons with superior rate capability and lower resistance, obtaining better robustness and performance (Figure 12). These carbons can be used in many applications of energy storage [176]. Cai et al. showed that electrochemical performance is influenced by the atomic percentage of heteroatoms in a glucose-derived biochar and eventually independent from heteroatoms to pore size at higher current. Egg white was used as a green source of N. As the proportion of N/O heteroatoms increased, the specific capacitance and cyclability improved but at higher current densities, the capacitance decreased where pore size was a more relevant factor considering the ionic mobility [177].

Figure 12. (1) Synthetic pathway for glucose- and trehalose-derived nanocarbons through HTC and pyrolysis steps. Reproduced from [178]. (2) (a) Spherical glucose biochar development from chestnuts and its use as an electrocatalyst for ZAB combined with MnO₂. (b) Charge–discharge cycling test at 5 mA/cm^3; (c′) Inset from (b). Reproduced from [179]. © 2023 Elsevier Ltd. All rights reserved. (3) Proposed model for cellulose HTC under mild processing conditions (180 °C < T < 280 °C). Dotted lines represent minor reaction routes. Reproduced from [180]. (4) Glucose-derived carbons comparison performance obtained by combined HTC and pyrolysis, or direct pyrolysis. (a) Ragone Plot; (b) SEM image of the porous nanocarbon obtained by combined method. Reproduced with permission from [175]. © 2014 WILEY-VCH Verlag GmbH & Co.

Glucose-derived biochar composite material is found to be a good bifunctional catalytic (OER/ORR) cathode material in ZAB (Figure 12). A highly crystalline spherical glucose biochar is used in combination with MnO₂ and PEDOT: PSS. The cathode material is polymerized in situ with Fe(ClO₄)_3. The orderly structure with high surface area facilitates high catalytic activity and the Fe loading complementarily enhances the electrical conductivity that is similar to the traditionally used Pt/C catalyst. A stable cycling up to 100 with an efficiency of 61.7% was noted and a notable power density of 101.2 mW/cm at 139.4 mA/cm³. This material is also promising towards large-scale production as 9540 ZABs (electrode area: 4 cm²) were produced from 1000 g of raw material [179]. In addition to biochar and activated carbon, hard carbons can also be synthesized from glucose through HTC pretreatment and pyrolysis [181]. Wortmann et al. have prepared hard carbon microspheres with hierarchical porosity using trehalose (glucose dimer) as a carbon source [178], Figure 12. In the same way, other researchers such as Tang and co-workers have prepared hollow carbon nanospheres as sodium ion battery anode materials [182]. Using latex in the hydrothermal decomposition of glucose, and after nonoxidative thermal treatment at 1000°, hollow carbon spheres with thin amorphous carbon walls are produced. This strategy can boost mass transport by obtaining better rate capabilities and capacity. Different sugars as utilized in different ESSs are summarized in Table 5.

Table 5. Sugars (dextran and glucose) as different functional parts of energy storage systems.

Ref	Polysaccharide (Modified Form)	Functionality	Energy Density	Power Density	Specific Capacity/ Specific Capacitance	ESS
[18]	Dextran	Electrolyte additive	-	-	3400 mAh cm⁻² at 5 mA cm⁻²	Zn-ion BAT
[18]	Dextran	Electrolyte additive	-	-	400 mAh cm⁻² at 5 mA cm⁻²	Zn-ion BAT
[179]	Glucose	Biochar electrode	-	101.2 mW cm at 139.4 mA cm⁻³	-	Zn-air BAT
[177]	Glucose-derived N/O co-doped AC	Electrode	-	-	417 F g⁻¹ at 0.5 A/g	SC
[175]	HTC glucose-derived AC	Electrode	2.6 Wh L⁻¹	0.64 kW L⁻¹	240 F g⁻¹ at 0.1 A/g	SC
[175]	Pyrolyzed gucose-derived AC	Electrode	2.7 Wh L⁻¹	0.14 kW L⁻¹	220 F g⁻¹ at 0.1 A/g	SC
[181]	D-gucose-derived HCs	Anode	-	-	250 mA h g⁻¹ at 50 mA g⁻¹	Na-ion BAT
[182]	HTC gucose-derived hollow carbon nanospheres	Anode	-	-	200 mA h g ⁻¹ at 50 mA g ⁻¹	Na-ion BAT

3. Animal-Derived Polysaccharides

3.1. Chitin

Chitin is the second most abundant biopolymer in the world after cellulose. Chitin is composed of a long chain of N-acetyl-n-glucosamine residues by a 1, 4-.S-bond structure, Figure 13. Chitin can be considered as an analogue of cellulose, but with an aminoacetyl group instead of a C-2 hydroxyl group, and can be found in many marine creatures (such as crabs and shrimps), mushrooms and in the cell walls of bacteria. Chitin extraction can be performed at a relatively low price from these organic resources especially shells of selfish, and waste of seafood processing industries [183]. As a material, chitin is non-toxic, hard, and presents a low chemical reactivity, being almost insoluble in most organic solvents. In comparison with cellulose, due to the strong micelle structure of the aminoacetyl group in the chitin molecule, it protects the structure from chemical attacks. These difference between chitin and cellulose in their chemical properties and solubility limit its usage in many applications in ESSs [184]. However, many strategies to control its solution properties have been applied by many researchers. The most significant is functionalization with many groups such as acetyl [185] or carboxymethyl groups to make it more soluble [186]. For example, if chitin is reacetylated up to 51% or more in the presence of acetic anhydride, then a water-soluble derivative can be obtained. The other is to fragment it into nanofibers or nanowhiskers to be able to include it in functional components [187].

Figure 13. (1) Chitin and chitosan structure. (2) Graphical illustration of the formation process for the nanofibrous carbon microspheres derived from chitin. Reproduced with permission from [184]. © 2021 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. (3) Schematic illustration of the preparation process of carbonization or activation (CRG) and its performance in supercapacitors; (g) Ragone Plots and (h) cycling stability at 10 A g⁻¹ for 10,000 cycles. Reproduced from [188]. © 2020 Elsevier Ltd. All rights reserved. (4) (a)The fabrication process of self-doped chitosan-derived carbon; (b) rate capability in a Li-ion battery; (c) SEM image of N-doped carbon. Reprinted with permission from [189]. Copyright © 2023, the author(s), under exclusive license to the Korean Carbon Society. (5) Fabrication of the chitosan-derived (CCS/PAM) hydrogel electrolyte membrane. Electrochemical performances of the solid-state supercapacitor based on the CCS/PAM hydrogel electrolyte membrane (CV and GCD). Reproduced with permission from [190]. © 2023 Elsevier B.V. All rights reserved.

3.1.1. ESSs Based on Chitin

Separators

Once chitin was treated, many applications in energy storage have been tested. Thanks to its low chemical reactivity, it is a good candidate for the development of separators. For example, Wang et al. have synthesized a low-cost chitin-based nanofiber separator which can stabilize zinc anodes in front of dendrites in aqueous ZIB. Thanks to its zincophilic character (due to nitrogenated groups) and its homogeneous nanopores, it can facilitate the desolvation of hydrated Zn²⁺ ions, homogenizing the ion flux of Zn, while maintaining excellent mechanical properties [191]. A similar strategy was followed with a ciano-derived chitin separator by Zhang et al. in recent studies on Li-ion batteries [192]. The grafting of cyanoethyl groups onto the chitin nanofibers increased ionic conductivity, enabling higher Li⁺ ion transport, weakening the interaction between Li⁺ and [PF₆]⁻ within the electrolyte.

Active Material (Carbon Precursors)

In addition to separators, chitin has also been used as a source for activated carbon preparation. In a very recent study, Itoi et al. have been able to prepare a chitin-derived porous N-doped carbon (Figure 13) using an activation-free synthesis, by simply carbonizing chitin at 850 °C for 1 h under a N₂ atmosphere. The EDLC (electrochemical double-layer capacitors) developed from this material show an area-normalized capacitance of 54–57 μF cm^–2, which is larger than previously reported for chitin-derived carbons, and a retention of 53%. This improvement is attributed to the presence of a high content of micro- and mesopores in combination with the N-doping which increases the conductivity. Symmetrical two-electrode cell measurements achieved energy and power densities of 4.9 W h kg⁻¹ at 11.3 W kg⁻¹ and a long cycling of 10,000 cycles in 1 M H₂SO₄ electrolyte [193].

3.2. Chitosan

However, the most important chitin-derived material is chitosan. Chitosan is obtained by removing enough acetyl groups from chitin. This de-acetylation provides higher solubility for the molecule to be soluble in most diluted acids. Chitosan is widely used to produce hydrogels as it is one of the most valuable functional materials in the field of polymer electrolytes due to its exceptional properties, including biocompatibility, biodegradability and good absorption behavior. Chitosan can retain its chemical and thermal stability up to 200 °C while providing excellent mechanical strength. In addition, the high content of hydroxyl and amino groups present enhances its hydrophilicity, which is advantageous for the development of many applications in energy storage and also facilitates its functionalization [194].

3.2.1. ESSs Based on Chitosan

Separators

Chitosan’s good mechanical strength and absorption retention have generated great interest in the development of chitosan-derived separators and GPEs. In the case of functional separators, Song et al. demonstrated its use for Li-ion batteries, where their chitosan nanoporous separator can be obtained by a simple filtration and ethanol-exchange method, showing improved electrolyte wettability (324% of uptake) and half the thickness in comparison with commercial Celgard 2335 [195]. Filter papers modified by chitosan were used as active separators in aqueous Zn batteries by Yang et al., alllowing biased adsorption of ions and protons in the electrolyte. Side reactions and corrosion activity are inhibited by the reduced proton activity. This low-cost modification facilitated stable cyclic performance in both zinc symmetric cells (over 2900 h) and Zn||MnO₂ batteries [196].

Electrolytes

On the other hand, its use in GPE development is wider due to the presence of electron pairs at the oxygen atom from hydroxyl and a nitrogen atom from amino groups, making it an appropriate polymer host for ionic conduction. Cruz-Balaz et al. generate a green chitosan–avocado starch GPE using epichlorohydrin as a crosslinking agent for primary Zn-air batteries, which can withstand 12 KOH solution without dissolving and can reach a maximum power density of 8.82 mW·cm² using Pt/C as a cathode catalyst [197]. It can also be combined with CMC using citric acid as a crosslinker as Bósquez-Cáceres et al. reported for primary alkaline Zn-air batteries, obtaining good performance with a specific capacitance of 1026 mA∙h g⁻¹_. Finally_, functionalized chitosan can also be a good candidate for GPE formation [194]. The inherent sol–gel transition of carboxylated chitosan (CCS), meaning the ability to exist in colloidal and gel form regulated by external parameters such as temperature, pH, or crosslinking, makes it a promising candidate for GPEs. The hydrogel is fully renewable and electrochemical performance does not alter significantly. With the introduction of glycerol as a humectant, the hydrogel (CCS/PAM) displays good water retention and is adaptable at extreme conditions ranging from −11 to 70 °C (Figure 13). An excellent ionic conductivity of 2.59 × 10⁻² S cm⁻¹ is displayed [190].

Active Material (Carbon Precursors)

The amino functional groups in chitosan are rich with innate heteroatoms such as nitrogen and oxygen. The heteroatoms are introduced within the carboxylic rings by direct pyrolysis, thus initiating pseudocapacitance and wettability of carbon, improving the overall electrochemical activity of the EESs. It was shown that chitosan with a hydrothermal pretreatment, activation by ZnCl₂ followed by calcination resulted in a rich porous structure with abundant micropores and mesopores showing a high specific surface area of 1679 m² g⁻¹. With a 3-electrode system, a high specific capacitance of 287.94 F g⁻¹ at 1.0 A g⁻¹ was noted as a result of double-layer capacitance and pseudocapacitance [198]. Microwave hydrothermal heating has been reported by Gao et al. as a faster method than hydrothermal heating with carbonization to produce chitosan/rGO in a sandwiched structure with dominant N and O functional groups (Figure 13). The good wettability of the composite structure enabled its use as an additive-free electrode with superior conductivity. When assembled with a lignin hydrogel electrolyte, the solid-state supercapacitors showed a high capacitance retention rate of 92.57% and good stability after 10,000 cycles with a retained coulombic efficiency of 96.96%. High energy storage of 8.4 Wh kg⁻¹ was achieved at a power density of 50 W kg^{− 1} [188]. In addition to the specific functional groups imparting pseudocapacitance, researchers have tried integrating redox materials with carbon. Quan et al. introduced NiO and Ni(OH)₂ nanoparticles to carbon aerogel structures derived from chitosan. The aerogel structures will complement the redox properties depicting combined electrochemical activity. The carbonized aerogel composite with carbon black and chitosan binder was pressed onto a Ni foam and assembled in a 3-el system with 6 M KOH electrolyte. Characteristic redox peaks were noted as well as a capacity of 316 mAh g⁻¹ at 1 Ag⁻¹ at an optimized carbonization temperature of 300 degrees with a high specific surface area facilitating effective ion transport. The agglomeration of Ni-based nanoparticles is prevented by CS allowing high conductivity [199].

Depending on the precursor source, combination ratio, type of activator used, heteroatom content, carbonization temperature, etc., the final functionality varies greatly and can be optimized. This is important to balance the mass-specific capacitance and volumetric-specific capacitance as shown by Xue et al. to employ the electrode in practical compact and portable electronics. An aerogel with 90% porosity shows 5-fold lower volumetric-specific capacitance than a carbon with 50% porosity [200]. Chitosan and calcium gluconate were used as the carbon and heteroatom precursors with an optimized ratio of KOH as the activator. The micropore to mesopore proportion and the biochar bulk density were hence controlled enabling advantageous double-layer capacitance and the pseudocapacitance improved linearly with the percentage of heteroatoms. With a 1 M H₂SO₄ aqueous electrolyte, similar specific capacitances of 386.3 F g⁻¹ and 320.6 F cm⁻³ and high energy densities of 16.23 Wh kg⁻¹/13.47 Wh L⁻¹ were observed [201]. Gradual strengthening of porosity at various processing stages to favor stable electrochemical dynamics was reported by Jiang et al. an initial mesoporous xerogel structure with liquified lignin (O) and chitosan (N) was chemically crosslinked by β-glycerophosphate (P), forming a hierarchical porous structure with an impressive SSA of 2107.3 m² g⁻¹. Pseudocapacitive properties were realized by the well-preserved heteroatoms depicting a high capacitance of 534.9 F g⁻¹ at 1 A g⁻¹ and excellent energy densities of 11.5 Wh kg⁻¹ at 4500 W kg⁻¹ [202]. Controlling the N-doping in chitosan-graphitized carbon can enhance the specific doping of pyrrolic-N at the periphery of graphitic layer, which improves the diffusion prospects of Li+ ions in LIB as electron donating tendency and charge mobility favor the carbon catalytic activity (Figure 13). Xia et al. introduced 2ZnCO₃· 3Zn(OH)₂ to the carbon precursor, selectively improving the doping percentage with high pyrrolic-N (52.5%) while suppressing graphitic-N in the bulk that is detrimental to anodic performance. The composite electrode showed improved rate capability and cycling. A reversible capacity of 518 mAh g⁻¹ after 100 cycles at 200 mA g⁻¹ was observed as well. The charge storage behavior was found due to both capacitive and diffusion-controlled process [189].

Binders

In addition to their original role as adhesives between the active material and current collectors, binders are being investigated to favor the surface reaction, mechanical stability, etc., in various EES. Chitosan is widely reported as a binder material for various rechargeable batteries due to its amino and hydroxyl functional groups. However, chitosan with high viscosity cannot always bind well with other components to form slurries and should be modified. CNC was used as a rich hydroxyl source with chitosan as a free-standing substrate and binder with hard carbon for Na-ion batteries by Idamayanti et al. The optimal synergy between -NH and -OH groups can influence the surface chemistry of anode materials by allowing effective sodiation/de-sodiation, and self-healability by strong hydrogen bonding controlling effects due to volume expansion and formation of a stable SEI when exposed to electrolytes. A high discharge initial capacity of 285 mAh g⁻¹, a reversible capacity of 244 mAh g⁻¹ at 25 mA g⁻¹ for 5 cycles and steady cyclability for 100 cycles [203]. Chitosan can be reinforced to maintain the mechanical integrity while cycling due to volume expansion. In Li-S batteries, a methanesulfonic acid-modified chitosan was used as a binder to contain the polysulfide shuttling by anchoring the sulfides via polar functional groups of chitosan. The plasticizer acid ensures the protonation of amino groups, preventing crystallization and thus improving mechanical characteristics. The modified binder composite retains its capacity (212 mA h g⁻¹) at a higher C rate (2 C) due to controlled volume changes upon dis(charge) [204]. Hamzelui et al. showed that in addition to crosslinking, optimizing the degree of acetylation (DA) and degree of polymerization of chitosan as a binder is vital to sustaining the structural stability and accommodating Si particles during de-lithiation in the case of Li-ion batteries. The modified binder used in anodes showed high capacities and stable coulombic efficiencies up to 200 cycles of different full cells of (Si/Gr||Li metal and (lithium nickel manganese cobalt oxide) NMC622||Si/Gr and in free-standing electrodes too [109]. A list of various chitin and chitosan-based functional materials in different ESSs are summarized in Table 6.

Table 6. Chitosan and chitin as different functional parts of energy storage systems.

Ref	Polysaccharide (Modified Form)	Functionality	Energy Density	Power Density	Specific Capacity/ Specific Capacitance	ESS
[186]	Carboxymethylated chitin-IL	SPE (1.16 × 10⁻³ S·cm⁻¹)	-	-	-	SC
[191]	Chitin nanofibers	Separators	-	-	251.9 mAh g⁻¹ at 0.15 A g⁻¹	Zinc-ion BAT
[192]	Cyanoethyl-chitin nanofiber	Separators (0.33 mS/cm)	-	-	~230 mA h g⁻¹ at 0.2 C	Li-ion BAT
[205]	Cyanoethyl-chitin nanofiber	Separators	-	-	155.1 mA h g⁻¹ at 0.2 C	Li-ion BAT
[193]	Chitin	Porous Carbon	4.9 W h kg^–1	11.3 W kg^–1	173–177 F g^–1 at 0.05 A g^–1	ESS
[194]	Chitosan–carboxymethylcellulose	Hydrogels Electrolyte (0.19 S∙cm⁻¹)	-	85 mW∙cm⁻²	1026 mA∙h g⁻¹	Zinc–air BAT
[195]	Chitosan nanofiber	Separators (0.68 mS cm⁻¹)	-	-	117 mAh g⁻¹ at 1 C	Li-ion BAT
[197]	Chitosan–avocado Starch	Hydrogels Electrolyte (0.61 S·cm⁻¹)	-	90 mW·cm⁻²	618 mA h·g⁻¹	Zinc–air BAT
[190]	Carboxylated chitosan and PAM	Hydrogel electrolyte (2.59 × 10⁻² S cm⁻¹)	14.28 Wh kg⁻¹	449.98 W kg⁻¹	37.28 F g⁻¹ at 0.2 A g⁻¹	Solid-state SC
[198]	Chitosan–N, O self-co-doped	Porous carbon	28.74 Wh kg^− 1	399.92 W kg^− 1	287.94 F g^− 1 at 1.0 A g^− 1	ESS
[188]	Chitosan/reduced graphene oxide	Electrode material	8.4 Wh kg⁻¹	50 W kg⁻¹	274 F g⁻¹ at 0.5 A g⁻¹	SC
[199]	Chitosan-derived carbon aerogel	Active electrode material	65 Wh kg−1	1500 W kg⁻¹	209 F g⁻¹ at 1.0 A g⁻¹	SC
[201]	Calcium gluconate and chitosan	N, O co-doped biochar	16.23 Wh∙kg⁻¹	152.16 W·kg⁻¹	386.3 F∙g⁻¹, 320.6 F∙cm⁻³ at 0.5 A·g⁻¹	SC
[202]	Lignin-modified chitosan xerogel	N,P,O-doped porous carbon electrode material	58 Wh/kg	375 W/kg	534.9 F/g at 1 A/g	SC
[202]	Lignin-modified chitosan xerogel	N,P,O-doped porous carbon electrode material	11.5 Wh/kg	4500 W/kg	534.9 F/g at 1 A/g	SC
[189]	Chitosan	Nitrogen self-doped carbon anode	-	-	518 mAh g⁻¹ at 200 mA g⁻¹	Li-ion BAT
[203]	CNC-reinforced chitosan	Hard carbon anode	-	-	285 mAh g⁻¹ at 25 mA g⁻¹	Na-ion BAT
[5]	Chitosan	Binder	-	-	342.3 mAh g⁻¹ at 2 C	Li-ion BAT
[204]	Chitosan–methanesulfonic acid	Binder	-	-	212 mA h g⁻¹ at 2 C	Li-S BAT
[109]	Chitosan	Binder	-	-	625.76 mAh g⁻¹ at 0.05 C	Li-ion BAT

4. Hybrids and Composites

Hybrid storage systems and polysaccharides have been implemented to synergetically obtain the best multi-components and systems involved and also create new suitable characteristics to improve the overall performance of the systems. Some examples can be found in the application of binders, where increasing the mass loading is a beneficial way to increase the active material and hence the specific energy of an EDLC. CMC is considered to be the state-of-the-art aqueous binder in EDLC; however, the coating cracks with increasing mass loading. Different natural biopolymers such as potato starch, wheat starch and GG were studied in comparison with CMC by Ruschhaupt et al. They saw that PS/GG in a 3:1 ratio showed a 45% increase compared to CMC with improved flexibility and adhesion to the current collector for a mass loading of 7.0 mg cm⁻² [135].

In GPE and SPE applications, owing to the need for an amorphous domain for a SPE, different polymers are considered. Both chitosan and potato starch are semi-crystalline in nature; however, when combined, their amorphous regions dominate. The polysaccharides were mixed with a compatible ammonium salt (NH₄SCN) and glycerol as a plasticizer. The ionic conductivity of the solid electrolyte is boosted by optimizing the glycerol content and the solvating and viscous characteristics (1.62 × 10⁻³ S cm⁻¹ at 24% of Gly). The SPE assembled with AC/PVDF/carbon black electrodes in NMP showed a power density of 855 W/kg and an energy density of 3.17 Wh/kg over a wide operating window of 2.1V [146]. Another example is the combination of sodium alginate and pectin. Different compositions of sodium alginate were blended with pectin via the casting technique to show their potential as conducting electrolytes (providing an ionic conductivity of approximately 10^–7 at 303 K) [134].

5. Conclusions and Future Perspectives

This review has offered a recent overview promoting the use of polysaccharides in ESSs. These natural materials are highlighted for their role in promoting a cleaner, recyclable, and circular economy, arriving just on time to boost the energy transition. This review also emphasizes the potential of biomass and biowastes in developing carbon materials, which can be customized for specific applications in ESSs. In addition, multifunctional modifications can enhance polysaccharides to improve their mechanical strength and flexibility, allowing them to be seamlessly integrated with other components, and enabling the development of tailor-made systems for targeted smart applications.

In perspective, polysaccharides have over the years proved to be a significant green alternative in energy storage systems. Some important observations are:

−: Polysaccharides, being biodegradable and biocompatible, can fend against the challenges of electronic wastes. The innate structure of polysaccharides with strong hydrogen bonding and numerous functional groups is of great advantage when it comes to their use in ESSs. Tuning the functional groups to derive desired conductive and mechanical properties can render the final material or device flexible and transparent and meet the commercial requirements.
−: Strong hydrogen bonding promotes ionic conductivity and water retention in hydrogel structures and establishes a wider electrochemical stability window. The presence of functional groups also allows the formation of hybrid structures with metal ions and other conducting polymers. Nanostructured polysaccharides further improve mechanical strength and maintain electrolyte stability. The presence of amorphous regions in polysaccharides facilitates better ionic mobility.
−: The flammability of organic solvents in batteries is a major concern. Alginate-based electrolytes have exhibited superior flame retardancy and thermal stability. SPEs based on PEO and alginate salts have been reported to exhibit self-extinguishing properties.
−: The complex network structures of polysaccharides make them good raw materials for carbonaceous materials. Polysaccharides with self-doped heteroatoms, high porosity and hierarchical structures are greatly useful as electrode materials of supercapacitors and batteries.

Some existing bottlenecks include to improving the long-term durability of natural biopolymers. Long processing and extraction techniques can also be challenging. The extensive hydroxyl groups are both beneficial in tuning functionality as well making the polysaccharides hydrophilic in nature. This can increase the tendency of the material or device to absorb the atmospheric moisture which proves detrimental to the ESS. It is important to optimize and regulate the characteristics and to develop materials with long-term cyclic stability.

Despite commercialization challenges, polysaccharides show promise in competing with traditional petroleum-derived materials (for example substituting PVDF binder) often outperforming them. Binders based on polysaccharides such as CMC and alginate have exhibited superior roles as binders, with crosslinking structures maintaining the structural integrity of electrodes. Separators made from nanocellulose and or with functionalization can render properties such as ion-selective transfer and durability for long-term cycling. With the promising electrochemical performances in accord with the abundant reserve for biomass, the prospect of translation to commercial value-added products can be expected in the future. There is still room for improvement. Polysaccharides, being naturally less conductive materials, can be chemically functionalized to improve conductivity. Depending on the requirement, they can be synergistically combined with other functional metal ions or inorganic counterparts. Their mechanical strength can be reinforced for structural stability of the device. Binders can be functionalized to promote an active role rather than structural integrity. ESSs can be potentially upscaled for commercial applications due to the natural abundance of polysaccharides. Incorporating plasticizing agents at an optimized level can improve the ionic conductivity of electrolytes, development of GPEs and structural pliability when used in binders. Looking ahead, we envision that this rapidly growing field will become increasingly essential in our transition to a greener energy model, thanks to the numerous advantages highlighted in this work.

Author Contributions

Conceptualization, S.M.T. and R.M.G.-G.; methodology, S.M.T. and R.M.G.-G.; validation R.M.G.-G. and P.G.-R.; investigation, S.M.T. and R.M.G.-G.; writing—original draft preparation, S.M.T. and R.M.G.-G.; writing—review and editing, S.M.T., R.M.G.-G. and P.G.-R.; visualization, S.M.T., R.M.G.-G. and P.G.-R.; supervision, R.M.G.-G. and P.G.-R.; project administration, P.G.-R.; funding acquisition, P.G.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministerio de Ciencia Innovacion y Universidades (MCIU), Agencia Estatal de Investigación (AEI) and the European Regional Development Fund (FEDER) (grants PID2021-128390OB-I00, TED2021-130205B-C21, and PLEC2022-009328). The ICN2 is supported by the Severo Ochoa Centres of Excellence programme, grant CEX2021-001214-S, funded by MCIU/AEI/10.13039.501100011033. The ICN2 is also funded by the CERCA programme/Generalitat de Catalunya. This work has been carried out within the framework of the doctoral program (PhD) of Material Science (Department of Physics) of Universitat Autònoma de Barcelona (UAB) and supported by the grant PRE2021-097210 from the call Ayudas para contratos predoctorales para la formación de doctores/as 2021 from the Agencia Estatal de Investigación, with the support of the Severo Ochoa Centres of Excellente programme, grant CEX2021-001214-S funded by MCIU/AEI/10.13039/501100011033 and FSE+.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Polysaccharides treated in this review with their chemical structure and origin.

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Thomas, S.M.; Gómez-Romero, P.; González-Gil, R.M. Polysaccharides: The Sustainable Foreground in Energy Storage Systems. Polysaccharides 2025, 6, 5. https://doi.org/10.3390/polysaccharides6010005

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Thomas SM, Gómez-Romero P, González-Gil RM. Polysaccharides: The Sustainable Foreground in Energy Storage Systems. Polysaccharides. 2025; 6(1):5. https://doi.org/10.3390/polysaccharides6010005

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Thomas, Sharin Maria, Pedro Gómez-Romero, and Rosa M. González-Gil. 2025. "Polysaccharides: The Sustainable Foreground in Energy Storage Systems" Polysaccharides 6, no. 1: 5. https://doi.org/10.3390/polysaccharides6010005

APA Style

Thomas, S. M., Gómez-Romero, P., & González-Gil, R. M. (2025). Polysaccharides: The Sustainable Foreground in Energy Storage Systems. Polysaccharides, 6(1), 5. https://doi.org/10.3390/polysaccharides6010005

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Polysaccharides: The Sustainable Foreground in Energy Storage Systems

Abstract

1. Introduction

2. Plant-Derived Polysaccharides

2.1. Lignocellulosic Biomass (Cellulose, Lignin, and Hemicelluloses)

2.1.1. ESSs Based on Lignocellulosic Biomass

Active Material (Carbon Precursors)

Binders

Electrolytes

Separators

2.2. Seaweed Polysacharides: Alginate, Agar and Gums

2.2.1. ESSs Based on Alginate

Electrolytes

Separators

Active Material (Carbon Precursors)

2.2.2. ESSs Based on Agar and Gums

Active Material (Carbon Precursors)

Electrolytes

Binders

2.3. Starch

2.3.1. ESSs Based on Starch

Binders

Electrolytes

Separators

Active Material (Carbon Precursors)

2.4. Pectin

2.4.1. ESSs Based on Pectin

Electrolytes

Binders

2.5. Sugars (Dextran and Glucose)

2.5.1. ESSs Based on Dextran

Electrolytes

2.5.2. ESSs Based on Glucose

Active Material (Carbon Precursors)

3. Animal-Derived Polysaccharides

3.1. Chitin

3.1.1. ESSs Based on Chitin

Separators

Active Material (Carbon Precursors)

3.2. Chitosan

3.2.1. ESSs Based on Chitosan

Separators

Electrolytes

Active Material (Carbon Precursors)

Binders

4. Hybrids and Composites

5. Conclusions and Future Perspectives

Author Contributions

Funding

Data Availability Statement

Conflicts of Interest

References

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