1 Introduction
The growing imbalance between resource demand and natural supply has made access to clean water and sustainable energy one of the most pressing global challenges of the 21st century [
1–
3]. Population growth, climate change, and rapid industrialization have intensified freshwater shortages in many arid and coastal regions, while continued reliance on fossil fuels for power generation exacerbates greenhouse gas emissions and environmental degradation [
4,
5]. Conventional centralized water-treatment and power-generation systems are often costly, infrastructure-dependent, and unsustainable in resource-limited environments [
6,
7]. To address these challenges, it is crucial to develop alternative strategies that make use of readily available environmental resources. Such strategies should minimize dependence on external energy and water supplies while maintaining autonomous operation under varying natural conditions [
8,
9]. Recent advances have demonstrated the potential to extract usable resources directly from the surrounding environment, including humidity, light, and temperature gradients [
10–
12]. These advances mark a shift toward decentralized, renewable, and low-carbon technologies capable of harvesting water and energy from ambient sources.
Among these emerging approaches, “moisture utilization”, which harnesses humidity in the air as both a water and energy source, has gained increasing attention, particularly given the widespread availability of atmospheric moisture across diverse geographic regions. Atmospheric water harvesting (AWH) captures water vapor directly from the air [
13], while moisture-enabled electricity generation (MEG) converts humidity gradients into electrical power [
14]. Both approaches operate under mild conditions, require no liquid feedstock, and can function effectively in natural environments [
9,
15]. The key challenge lies in creating materials that can efficiently capture, store, and release water [
16], or convert interactions with moisture into electrical energy [
17]. Ultimately, the performance of AWH and MEG technologies depends on the design of interfacial materials that can precisely regulate water transport and charge movement [
18]. Conventional inorganic and polymeric materials, such as metal-organic frameworks (MOFs), hygroscopic salts, and ion-conducting polymers, have shown promising performance in AWH and MEG applications [
15,
19,
20]. However, they often face trade-offs among efficiency, scalability, and environmental compatibility. Many also depend on expensive precursors, non-degradable polymers, or energy-intensive regeneration processes. These limitations underscore the need for materials that combine strong moisture responsiveness with renewability and environmental safety.
In recent years, the emergence of bio-based materials offers such possibilities. Among them, cellulose stands out as a renewable and biodegradable polymer that is abundantly available from diverse biomass sources, including wood, agricultural residues, and bacterial fermentation (Fig. 1A) [
22,
23]. More importantly, cellulose exhibits intrinsic hydrophilicity, abundant reactive sites [
24], and hierarchical architecture that can be tailored for moisture capture and energy conversion [
13,
25]. These features make it an attractive and sustainable material platform. Beyond its abundance, what truly distinguishes cellulose is its remarkable physicochemical versatility. Its multiscale structure combines hierarchical porosity, tunable crystallinity, rich surface chemistry, and strong hydrogen-bonding capability [
23]. At the nanoscale, cellulose microfibrils contain both crystalline and amorphous regions that form interconnected networks capable of adsorbing and transporting water through capillary and diffusion processes [
26,
27]. At the macroscale, cellulose fibers can be assembled into filaments, films, foams, aerogels, and hydrogels with adjustable pore size and surface wettability [
28–
31]. Such multiscale tunability allows for precise control over moisture absorption, release, and charge transport, all of which are key factors governing the performance of AWH and MEG systems. Moreover, the hydroxyl groups on cellulose chains provide abundant sites for chemical modification or hybridization with hygroscopic salts, conductive fillers, and responsive polymers [
32]. Such modifications can tailor interactions with water molecules, ion mobility, and electron transport, imparting the essential functionalities for efficient humidity-driven devices [
33–
35]. However, realizing the full potential of cellulose requires deliberate structural design that bridges molecular chemistry with macroscopic function. This review therefore focuses on how structural and compositional tuning of cellulose can link its natural properties to advanced water-energy applications.
This review aims not only to clarify the structure-property-function relationships governing cellulose performance in AWH and MEG (Fig. 1B and C), but also to treat cellulose as a unified material platform rather than discussing it within broad material classes. We first discuss the fundamental structure of cellulose and its interactions with water molecules, which form the physical and chemical basis of its moisture responsiveness. We then summarize key design strategies, such as chemical modification, morphological control, and bioinspired engineering, that translate this understanding into functional material systems. Two representative applications are highlighted: cellulose-based AWH systems, including fog collection, air condensation, and sorbent-driven processes; and cellulose-based MEG systems, where humidity-induced ion and electron transport enables power generation. By integrating insights from both areas, this review establishes a coherent framework linking cellulose-moisture interactions to device-level performance across AWH and MEG. In addition, we provide a systematic analysis of the key design trade-offs involved in integrating these two functionalities, particularly the balance between maximizing water uptake and preserving moisture and ion gradients required for continuous operation. Finally, we discuss current challenges and future directions, including the use of machine learning (ML)-assisted material discovery, and the development of sustainable manufacturing strategies guided by life-cycle assessment. The goal is to provide a roadmap for advancing cellulose from a traditional biopolymer to a high-performance platform for humidity-driven water-energy technologies.
2 Fundamentals of Cellulose and Moisture Interaction
To understand how cellulose interacts with moisture, it is necessary to first examine its intrinsic structure. Cellulose is organized hierarchically, from molecular chains and crystalline-amorphous arrangements to fibrils, fibers, and porous networks. These structural features dictate how water is bound, transported, and redistributed across different length scales. Establishing this structural foundation provides the necessary context for examining both the interaction mechanisms and the resulting structure-property-function relationships discussed in the following sections.
2.1 Hierarchical structure and composition of cellulose
Cellulose possesses a complex hierarchical structure that extends across multiple length scales, from molecular chains to macroscopic assemblies (Fig. 2A). At the molecular level, cellulose is a linear homopolysaccharide composed of β-D-glucose units linked by β-1,4-glycosidic bonds [
36]. Each anhydroglucose unit (AGU) carries three hydroxyl groups capable of forming extensive intra- and intermolecular hydrogen bonds, leading to a highly cohesive and semi-crystalline polymer network [
37]. The linear chains align and bundle into elementary fibrils and microfibrils that contain interspersed crystalline and amorphous regions [
23,
38]. The crystalline domains, classified mainly as cellulose Ⅰ (native) or cellulose Ⅱ (regenerated), feature ordered chain packing stabilized by strong hydrogen bonding and van der Waals interactions, resulting in high stiffness and structural stability [
39–
41]. In contrast, the amorphous regions exhibit disordered chain arrangements with abundant accessible hydroxyl sites, leading to greater chain mobility, local swelling, and enhanced affinity for water [
42,
43].
At larger scales, these microfibrils further assemble into bundles, fibers, and porous networks through hydrogen bonding, physical entanglement, and topological confinement [
44,
45]. This multilevel architecture dictates the mechanical robustness, permeability, and mass-transport behavior of cellulose-based materials [
23]. Variations in biological origin, whether plant, bacterial, algal, or tunicate, produce distinct chain organizations, degrees of polymerization, and microfibril dimensions [
46]. Processing routes such as mechanical fibrillation, chemical modification, and dissolution and regeneration provide additional control over morphology and surface chemistry [
23,
47,
48]. Furthermore, cellulose can be engineered into a wide range of material forms, including filaments, films, foams, aerogels, and hydrogels (Fig. 2B), each with distinct morphology and porosity [
28–
30]. Consequently, cellulose’s hierarchical structure not only defines its mechanical and physicochemical properties but also endows it with a unique capacity to mediate moisture interactions across molecular, nano-, and microscale environments. Establishing this structural foundation is essential for understanding the mechanisms underpinning cellulose-moisture interactions and their relevance to subsequent water-energy applications.
2.2 Cellulose-moisture interaction mechanisms
Cellulose-moisture interactions are primarily governed by hydrogen bonding, with additional contributions from dipole-dipole attraction and capillary condensation[
24,
49,
50]. When exposed to humid environments, water molecules interact with cellulose through three main sorption states (Fig. 2C): (1) bound water, which forms direct hydrogen bonds with hydroxyl or ether oxygen atoms; (2) intermediate water, which accumulates as loosely bound water layers on cellulose surfaces; and (3) free water, including capillary-condensed water in meso- and macropores, which behaves similarly to bulk liquid water. At low relative humidity (RH), bound water dominates, leading to limited swelling but strong molecular association [
51,
52]. As humidity increases, the formation of multilayer water and subsequent capillary condensation lead to substantial water uptake, softening of amorphous domains, and structural expansion [
24,
49]. These processes are generally reversible, enabling cellulose to undergo cyclic sorption and desorption without chemical degradation. Therefore, the relative proportions of bound, intermediate, and free water determine not only the total moisture uptake of cellulose, but also the strength, mobility, and reversibility of cellulose-moisture interactions.
At the molecular scale, cellulose-moisture interactions are highly dynamic. Hydroxyl groups on the polymer chains continuously form and break hydrogen bonds with water molecules, enabling rapid reorganization of the hydrogen-bonding network in response to environmental changes [
53]. In this process, bound water mainly reflects direct cellulose-moisture association, whereas intermediate water provides a more mobile hydration environment for hydrogen bonds exchange. This dynamic exchange stabilizes the hydrated structure and contributes to dielectric relaxation and local ionic conduction [
54,
55], phenomena that are central to MEG. Experimental and simulation studies have shown that such reconfiguration facilitates proton hopping and transient charge redistribution [
56,
57], linking water adsorption directly to functional electrical responses.
At the mesoscale, the interaction of water with cellulose is governed by the interplay between crystalline rigidity and amorphous flexibility (Fig. 2D). Crystalline regions, with their tightly packed and ordered hydrogen-bonding network, hinder the penetration of water molecules and limit swelling [
58]. In contrast, the amorphous and interfacial regions provide accessible sorption sites and diffusion pathways [
59]. These regions are therefore more likely to accommodate intermediate water and, at higher RH, free water generated through multilayer adsorption or capillary condensation. These dynamic boundaries accommodate volume changes and facilitate moisture redistribution, directly influencing water uptake kinetics, swelling behavior, and, in electrically active systems, ion and charge transport. Together, the cellulose-moisture interactions at both molecular scale and mesoscale define the basis for translating humidity into functional responses.
2.3 Structure-property-function relationships
The moisture-responsive behavior of cellulose is intrinsically linked to its hierarchical structure and composition. Understanding these mechanisms is essential for establishing the structure-property-function relationships that govern the performance of cellulose-based systems in AWH and MEG. At the nanoscale, the degree of fibrillation and fiber diameter directly influences water diffusion pathways. Highly fibrillated nanocellulose offers a large surface area, short diffusion distances, and abundant hydroxyl groups, enabling rapid water uptake and efficient adsorption [
49,
60,
61]. In contrast, larger fibrils or less processed fibers provide longer diffusion paths but contribute to structural robustness, highlighting the inherent trade-off between transport efficiency and mechanical stability.
At the mesoscale, porosity and network architecture are critical determinants of both water storage and transport kinetics. Open and interconnected pores facilitate capillary condensation and rapid moisture absorption, whereas closed or tortuous pore networks can trap liquid water, slowing desorption and prolonging retention [
62,
63]. Hierarchical or multiscale porosity, combining micro-, meso-, and macropores, allows cellulose-based materials to simultaneously capture water effectively while maintaining controllable release rates. Furthermore, gradient or asymmetric pore distributions can create directional water transport [
64], a feature particularly advantageous for fog collection of AWH or for generating sustained voltage in MEG systems.
Surface chemistry provides an additional means of tuning the structure-property-function relationships. Chemical modifications such as oxidation, esterification, or ionic functionalization can introduce carboxylate, sulfate, or quaternary ammonium groups [
65,
66]. These functional groups modulate the binding affinity of water molecules, modify local surface energy, and influence the mobility of hydrated ions. In MEG, such modifications are particularly important as they regulate ion diffusion and charge separation, directly affecting voltage and current output. Similarly, in AWH, surface chemistry determines wettability, droplet nucleation, and the efficiency of water collection and release.
Quantitative indicators, including equilibrium water uptake, diffusion coefficients, contact angle, and ionic conductivity, are commonly used to correlate structural features with functional performance. For example, higher amorphous content or increased surface area generally corresponds to greater water adsorption [
42,
43], while optimized pore architecture balances uptake and desorption rates. Likewise, specific ionic functionalization can enhance charge transport without compromising mechanical stability. Together, these parameters outline how specific structural modifications translate into moisture-responsive behaviors. In practical material development for AWH or MEG, performance ultimately depends on finding an appropriate balance among hydrophilicity, porosity, and mechanical robustness. The material must absorb moisture efficiently, facilitate rapid transport, and tolerate repeated cycling. Achieving this combination of properties generally requires a multiscale design strategy that integrates targeted modification with control over structural design.
3 Design Strategies for Cellulose-based Functional Materials
Building upon the fundamental understanding of cellulose-moisture interactions, the next milestone is to manipulate these interactions through rational cellulose design. The design principles for cellulose-based functional materials focus on enhancing their responsiveness and controllability toward moisture. This specifically includes the processes of moisture sorption, transportation, and storage, as well as the utilization of ions and charges involved in these processes. Given the structural diversity of cellulose-based materials, this section summarizes rational design strategies across molecular scale, mesoscale, and macroscopic level for achieving effective management of moisture and charge in cellulose-based systems.
3.1 Molecular and chemical design
At the molecular level, chemical functionalization and hybridization are employed to deliberately tailor the interactions between cellulose and water molecules or charges (Fig. 3A and B). This strategy bridges molecular structure and macroscopic function by converting the intrinsic hydroxyl-rich cellulose backbone into a tunable chemical interface, where specific functional groups regulate water binding strength, ion generation, and charge mobility.
3.1.1 Molecular surface functionalization
Surface functionalization bridges chemical structure and moisture-responsive performance by introducing polar, ionic, or thermo-responsive groups that directly tune hydrogen-bonding density, hydration strength, and mobile ion concentration. The hydroxyl groups located at the C2, C3, and C6 positions of cellulose molecules form a hydrophilic network for moisture sorption and storage. However, the dense hydrogen-bonding network affects their moisture sorption efficiency. Through molecular modification, functional groups can be introduced to enhance this performance.
Carboxylation, one of the most common chemical modification strategies for cellulose, effectively increases its hygroscopicity and local charge density [
70–
73]. For example, Zhu et al. introduced carboxylate groups at the C
6 position via a 2,2,6,6-Tetramethylpiperidinyloxy (TEMPO)-mediated oxidation reaction and used the resulting TEMPO-oxidized cellulose nanofibers (TOCNF) to fabricate films. Owing to the hygroscopicity of carboxylate groups, a TOCNF film with a carboxyl content of 0.52 mmol·g
–1 exhibited a moisture content of 8.0% at 70% RH, which increased to 13.4% when the carboxyl content was raised to 1.45 mmol·g
–1. Furthermore, the carboxylate groups introduced on the TOCNF deprotonate in the presence of adsorbed water, releasing sodium ions that can migrate under an applied electric field. A higher charge density further enhances the mobility of these ions [
70]. In another study, TOCNF-based fibers showed an increase in wetting rate from 1.2 to 1.6 g·g
–1·s
–1 as the surface carboxyl content increased from 0.35 to 0.53 mmol·g
–1 [
71].
Grafting zwitterionic groups onto cellulose molecules not only enhances the hydrophilicity of the cellulose network but also increases its water sorption capacity through network swelling. For example, cellulose grafted with 3-[(3-chloropropyl) dimethylammonio] propane-1-sulfonate demonstrates a broad range of water uptake, with 0.86 g·g
–1 at 15% RH and 2.18 g·g
–1 at 60% RH [
74]. This high water uptake is attributed to the dissociation of zwitterions associated on the cellulose surface after binding with hygroscopic salt ions. The resulting cellulose network swells to accommodate more water, leading to a peak swelling ratio of 11.4 g·g
–1. Building on this, alkylation can be further employed to tune the water solubility and lower critical solution temperature of cellulose (LCST), thereby imparting thermoresponsive behavior [
74–
76]. For example, hydroxypropyl groups can be grafted onto cellulose chains via an epoxide ring-opening reaction [
74]. This modification restructures the hydrogen-bonding network and enhances water solubility. At 60 °C, strengthened hydrophobic interactions promote water desorption, releasing up to 95% of the bound water.
3.1.2 Molecular hybridization
Molecular hybridization bridges cellulose’s structural scaffold with external functional components, allowing hygroscopic salts, MOFs, or conductive fillers to provide additional sorption sites, ion reservoirs, or electron pathways while cellulose maintains mechanical integrity and vapor/liquid transport channels. Hygroscopic salts exhibit outstanding moisture sorption capacity across a wide range of RH (11%–100%) [
77–
79]. This behavior arises from their multistep water uptake mechanism, including chemisorption, deliquescence, and salt solution absorption. Typical hygroscopic salts include LiCl, LiBr, and CaCl
2. When integrated with cellulose, the hybrid system shows improved water uptake. This enhancement arises from the increased reactive surface area of the salt and the confinement of the salt solution. For example, hybridizing LiCl with nanofibrillated cellulose combines the advantages of both components [
80,
81]. LiCl converts water vapor from air into liquid water, while the nanofibrillated cellulose network transports and stores the liquid water within its structure. The resulting hybrid gel achieves a water uptake of 0.6–2.36 g·g
–1 at 18%–95% RH. Compared with LiCl, CaCl
2 has a lower moisture sorption capacity but is more cost-effective. It can also form crosslinked networks with cellulose to reinforce the network structure. For example, Chen et al. embedded CaCl
2 into a TOCNF network through electrostatic interactions, which not only imparted mechanical elasticity but also increased the water uptake to 41%–56% at 43% RH [
82]. Other hygroscopic salts, such as CoCl
2, also show high water uptake when combined with microfibrillated cellulose (MFC), reaching 0.49–2.38 g·g
–1 at 25%–85% RH [
83]. In addition, CoCl
2 exhibits humidity-responsive color changes that can be used to indicate the end point of the sorption process. This color transition arises from the interaction between unsaturated Co coordination complexes and water molecules.
As a new class of sorbents, MOFs provide exceptionally high specific surface area and tunable pore channels through their metal nodes and organic linkers [
84–
86]. These features enable efficient capture of water molecules via physisorption and capillary condensation. Numerous MOFs have been employed for water sorption, such as HKUST-1 [
85], MIL-101 [
87] MOF-801 [
68,
88], MOF-303 [
89,
90]. Similarly, the powdery form of most MOFs limits their practical capture moisture. Hybridizing them with cellulose network offers a viable strategy to overcome this limitation. For instance, embedding MOF-303 into a TOCNF network enables a water uptake of 0.5–0.7 g·g
–1 at 90% RH [
89]. Owing to the tunable pore channels of MOFs, hygroscopic salts can be incorporated into the framework to further enhance moisture sorption. LiCl-loaded MOF-303@TOCNF then exhibits a high water uptake of 4.69 g·g
–1 at 90% RH. A similar effect can be observed at low RH. At 30% RH, MIL-101@BC shows a water uptake of nearly 0.2 g·g
–1, which increases to about 0.8 g·g
–1 after embedding CaCl
2 [
87]. Meanwhile, hybridization with conductive components introduces additional pathways for charge transport. These hybrid interfaces generate localized electric fields that promote directional charge migration under humidity gradients, thereby enabling synergistic coupling between moisture and charge transport. For example, TOCNF and carbon nanotubes (CNTs) were assembled into hybrid composites [
26,
74]. The hydrophilic groups on the TOCNF surface adsorb moisture and facilitate electron transfer from water molecules to the CNTs. As a result, the hybrid paper exhibits a humidity-dependent charge response.
3.2 Mesostructural and morphological design
Molecular engineering defines the chemical environment for cellulose-moisture interactions, while mesoscale design significantly influences how these molecular interactions are translated into collective water transport behaviors within cellulose systems. Mesostructural design bridges nanoscale cellulose-moisture interactions and bulk material performance by organizing pores, channels, and gradients into continuous pathways for vapor diffusion, liquid transport, and ion migration (Fig. 3C).
3.2.1 Pore engineering
Pore engineering bridges pore geometry and moisture-management function by matching micro-, meso-, and macropores with different stages of water uptake, storage, and release. A well-designed porous structure provides not only abundant sorption sites but also continuous pathways for vapor diffusion and liquid flow. Cellulose nanofiber (CNF) are ideal building materials owing to their high aspect ratio and abundance of hydrophilic groups [
91]. Through controlled assembly methods such as ice templating, ionic crosslinking, and pH triggering, followed by dehydration techniques including freeze-drying, supercritical drying, or air drying, CNF can be transformed into aerogels or foams with high porosity [
92–
99]. Water vapor first condenses into liquid water on the pore walls. Then, the liquid water is transported along the hydrophilic interconnected pore network into the interior of the aerogel [
80,
81]. However, the highly tortuous structure limits the kinetics of moisture sorption. To reconcile fast sorption kinetics, hierarchical porous structures are highly advantageous.
The coexistence of micro-, meso-, and macropores achieves multistage moisture management. Macropores accelerate water transport through the porous structure. Mesopores and micropores increase the interfacial area between cellulose and air. They also provide a confined environment for hygroscopic salts and other functional components. This hierarchical configuration ensures moisture sorption speed, mitigating the diffusion and transport limitations that often occur in tortuous porous structure. For example, Duan et al. selected melamine foam with a pore size of 150 µm as the structural framework [
100]. Chitosan and sodium alginate were then introduced to coat the melamine foam scaffold and generate submicron pores, followed by carboxymethyl cellulose to construct nanoscale pores. As a result, the hierarchical porous structure significantly enhanced water diffusion and transport, achieving a peak water uptake of 3.97 g·g
–1 at 90% RH [
101–
103]. Yu and co-workers used hydroxypropyl cellulose and konjac glucomannan as a hybrid matrix to construct hierarchical pores [
21]. The 20–50 µm microscale pores increase the interfacial area between the hydrogel and water vapor, while additional submillimeter pores accelerate the water transport. Moreover, 3D printing offers new opportunities for directly controlling hierarchical porous structures. Zhu et al. used direct ink writing combined with freeze-drying to assemble TOCNF and LiCl into aerogels with micron-nanometer- and millimeter-scale pores [
93]. Compared with TOCNF/LiCl aerogels prepared by direct freeze-drying, the hierarchically structured aerogel increased the water uptake rate from 0.08 to 0.13 g· g
–1·h
–1.
3.2.2 Directional and gradient architectures
Directional and gradient architectures bridge anisotropic structure with directional transport by reducing tortuosity and creating chemical-potential, moisture, or ion-concentration differences that drive water and charge migration. Directional freezing, as a representative technology, induces the formation of directional structures by guiding the growth of ice crystals [
104,
105]. This oriented architecture minimizes tortuosity and promotes anisotropic transport [
106,
107]. Meanwhile, gradient architecture based on directional structure, water content, and salt solution concentration reinforces local chemical potential differences, thereby self-driving ion redistribution, and generating electrochemical potential within cellulose systems [
108–
110]. In a recent example, Cai et al. fabricated a MXene/cellulose nanocrystal (MXene/CNC) aerogel via unidirectional freezing casting technology [
110]. The directional freezing guides ice growth to form vertically aligned, low-tortuosity channels. This structure accelerated water transport within the channels, where it is further absorbed by LiCl distributed in the channels. Compared with aerogels with random pore structures, COMSOL Multiphysics simulations showed that the water transport rate in the aligned channels increased from 0.5 µm·s
–1 to 1.5 µm·s
–1. In addition, gradient differences in moisture content along the vertically aligned channels further induce ion concentration gradients. Specifically, the dry side generated a positive pole, while the wet side formed a negative one. Therefore, the MXene/CNC aerogel can generate continuous electrochemical potentials [
111,
112].
Natural wood cells possess an inherent aligned structure and hydrophilicity, which are well suited for water and ion transport [
113]. However, the lack of micro- and nanoscale porosity and the relatively weak interactions with water limit the performance of wood-based devices. Controlled dissolution-regeneration methods can introduce micro- and nanoscale pores within aligned channels [
114–
116]. For example, Gu et al. constructed micro- and nanoscale pores within wood cell walls via LiCl/N,N-Dimethylacetamide (LiCl/DMAc) dissolution and acetone regeneration [
115]. This structure increased the water transport rate by 39%. In addition, the directed water flow and its interaction with carboxylate and hydroxyl groups amplify the streaming and electrokinetic potentials. Water evaporation can further enhance the ionic concentration gradient between the top and bottom of modified wood. Consequently, the open-circuit voltage was increased to 254.5 mV, which is 70 times higher than that of delignified wood. Gao et al. employed delignification combined with gradient oxygen plasma etching to optimize the pore structure and induce a gradient oxygen-to-carbon ratio within wood [
109]. Oxygen plasma treatment promoted the formation of pores larger than 50 nm, which accelerated water transport. As a result, the water contact angle decreased from 117.69° to 80.32°. In addition, enriched hydroxyl, carbonyl, and carboxylate groups on the cell wall enhanced interfacial water dissociation and ion migration, thereby boosting the streaming potential. The open-circuit voltage was improved from 26 mV for delignified wood to 250 mV.
To conclude, hierarchical porosity, directional channels, and gradient architectures collectively regulate water sorption, liquid transport, ion migration, and electrochemical potential generation in cellulose-based materials. These mesostructural strategies provide the structural foundation for the macroscopic and bioinspired designs discussed below, where internal transport pathways are further translated into larger-scale geometries and surface architectures for application-oriented water management.
3.3 Macroscopic and bioinspired design
The assembly and biomimetic design of cellulose networks integrate tailored surface chemistry, geometric hierarchy, and controlled structure to sustain continuous and reversible water- and ion-management systems (Fig. 3D). Nature provides abundant inspiration for water capture and transport [
117,
118]. Cactaceae species inhabit arid environments, where one of their key survival strategies is efficient fog collection mediated by their spines [
119–
121]. Fog condenses and coalesces on cactus spines, followed by spontaneous droplet movement from the tip to the base owing to geometry-induced Laplace pressure and surface-energy differences. Some beetles in desert collect drinking water by condensing fog on their elytra. The elytra feature a patterned arrangement of hydrophobic, wax-coated regions and hydrophilic, non-waxy regions [
122]. This design promotes fog condensation and directional water transport. Beyond water capture, rapid water transport is also widely observed in nature.
Nepenthes alata peristome has an asymmetric micro- and nanoscale grooved surface that generates unidirectional capillary forces [
123]. This structure enables continuous, directional water transport from the outer to the inner rim at high transport speed.
Translating these principles into cellulose systems requires the coordination of surface chemistry and structural hierarchy [
124]. This approach enhances water capture, transport, and energy coupling, enabling the reproduction of such natural mechanisms in engineered cellulose-based materials. For example, Zhang et al. combined the structural features of cactus spines and desert beetle elytra to create an asymmetric amphiphilic surface with enhanced fog capture and collection efficiency [
125]. The amphiphilic cellulose ester promotes efficient droplet nucleation, while the laser-engraved spine-like geometry guides directional liquid flow. This bioinspired design achieves a high water-harvesting rate of 85.47 kg·m
–2·h
–1. Similarly, Duan et al. used cellulose, polyvinylpyrrolidone, and poly(vinylidene fluoride) as raw materials and fabricated fibers via wet spinning followed by multistep dip-coating [
126]. This process mimicked the axial microgroove structure of cactus spines and the wettability gradient of
Nepenthes alata peristome. The resulting bioinspired fibers achieved a fog collection rate of 180.62 g·cm
–2·h
–1. These macroscopic and bioinspired designs illustrate how natural architecture can be translated into cellulose-based systems to integrate water capture, collection, and regeneration within a continuous and reversible water-management cycle.
At this macroscopic scale, the essential design goal is not merely to mimic biological forms but to preserve stability, scalability, and function under practical environmental conditions. Through multiscale structural assembly and multimodal coupling, cellulose-based materials can sustain prolonged humidity interaction. This enables efficient water capture, directional transport, and adaptive response to environmental changes. For example, a cellulose/sodium alginate/lignin hydrogel was prepared as a hydrophilic network [
76]. LiCl was added to enhance its hygroscopicity, resulting in a water uptake rate of 1.74 kg·kg
–1·h
–1 at 30% RH. This sorption hydrogel was then integrated into a rotary collector, where accelerated air circulation and continuous adsorption-desorption further increased the overall water uptake. In another example, LiCl was embedded into a TOCNF/poly(N-isopropylacrylamide) hydrogel, which achieved a water uptake of 0.99 g·g
–1 at 20% RH [
127]. When integrated into a circulation device, this system significantly improved the overall water collection efficiency. By integrating chemical modification, pore engineering, bioinspired design, and structural assembly across multiple scales, cellulose-based systems can be rationally tailored to enhance the controllability of moisture. This controllability is fundamental to the operation of humidity-driven systems such as AWH and MEG.
4 Application I: Atmospheric Water Harvesting (AWH)
Based on the regulation of cellulose-moisture interactions, many functional applications have been explored [
25]. One emerging direction is AWH, a sustainable approach for decentralized freshwater production. Earth’s atmosphere holds nearly 13 trillion tons of water vapor, far exceeding all accessible surface freshwater, making AWH a promising route to alleviate growing water scarcity [
128].
4.1 Fundamental mechanisms
4.1.1 Water sorption
The key to turning this diffuse water reservoir into usable liquid is efficiently capturing the vapor [
129]. Current strategies are generally classified into two major categories: condensation-based and sorption-based mechanisms [
79]. The sorption pathway can be further divided into adsorption on porous solids and hygroscopic absorption, including deliquescence-driven water uptake. Condensation-driven harvesting captures moisture by cooling humid air below its dew point, enabling vapor to nucleate and grow on solid surfaces. Heterogeneous nucleation, droplet growth, and subsequent transport are controlled by surface energy, temperature gradients, and curvature-dependent Laplace pressure [
130]. Adsorption offers a versatile route to capture water through physical adsorption via hydrogen bonding and van der Waals forces or by chemical coordination at ionic sites. The process is governed by pore architecture. Micropores (< 2 nm) serve as primary adsorption sites; mesopores (2–50 nm) facilitate capillary condensation; macropores (> 50 nm) act as mass-transport channels that accelerate vapor diffusion [
131,
132]. Hygroscopic salts, including deliquescent materials, such as LiCl and CaCl
2, can uptake substantial amounts of water and form a liquid brine under high RH; notable examples can capture water even at RH below 20% [
77]. After water sorption, its release is equally important. Unlike air condensation and fog collection, which can directly yield liquid water, sorption-based systems require the release of water captured by the sorbents, and then the water release mechanism can be introduced.
From the perspective of water states, moisture uptake reflects their complementary roles across humidity ranges and structural conditions. At low RH, water is mainly adsorbed as bound water on accessible hydrogen-bonding or ionic sites, particularly in amorphous or interfacial regions, enabling moisture capture even at low vapor pressure [
21,
133]. As RH increases, intermediate water contributes to multilayer adsorption and promotes swelling of amorphous domains, facilitating diffusion along fibril interfaces and mesopore walls [
134]. At high RH, free water becomes dominant via capillary condensation or salt deliquescence in meso- and macropores, governing overall uptake through capillary flow and continuous liquid pathways [
135]. Temperature and surface chemistry further regulate these processes by modulating hydrogen-bonding strength, diffusion kinetics, and sorption affinity [
136]. The coexistence of these states therefore underpins efficient water capture across varying environmental and structural conditions.
4.1.2 Water release
While water uptake is dictated by the density and strength of hydrophilic interactions, water release in cellulose-based AWH systems reflects the relaxation of these interactions across molecular, mesoscale, and architectural levels [
137]. Unlike inorganic sorbents that rely on strong coordination bonds and therefore require high temperatures for regeneration, cellulose binds water primarily through hydrogen bonding and weak polar interactions. These bonds reorganize readily under mild thermal or mechanical perturbation, enabling a multi-stage desorption process beginning with the removal of loosely held surface water, followed by the breakup of hydration shells, and finally the release of deeply confined water from reorganizing fibrillar networks. This intrinsic responsiveness allows cellulose systems to operate with low regeneration energy, particularly when desorption triggers are embedded directly into the molecular or structural design of the material.
Correspondingly, release behavior is governed by water states, binding strength, and environmental conditions. Free water confined in larger pores or interconnected channels is removed rapidly through evaporation or capillary-driven transport [
138]. Intermediate water, associated with weaker interactions, can be released under mild thermal or solar input, enabling reversible cycling [
75]. Bound water, strongly coordinated with functional groups, requires higher energy input and often limits regeneration efficiency [
21]. Temperature accelerates desorption by enhancing molecular mobility but may reduce equilibrium uptake. Structural factors, including pore architecture, crystallinity, and surface chemistry, jointly determine desorption pathways and kinetics [
139]. The interplay among these factors governs the trade-off between energy consumption and water recovery rate in cellulose-based AWH systems.
4.2 Cellulose-based AWH systems
Within this landscape, cellulose and its derivatives provide a versatile material platform for constructing AWH architectures. Their hydrophilic chemistry promotes rapid vapor capture, whereas their hierarchical porous structures facilitate mass transport, water storage, and energy-efficient regeneration [
137,
140]. To understand the contribution of cellulose within these architectures, it is necessary to distinguish between the chemical role of active sorbents and the structural function of their supporting matrices. MOFs and hygroscopic salts are responsible for water capture through their porosity or intrinsic affinity toward water vapor [
141]. In contrast, cellulose-based materials generally do not serve as the primary active sorptive phase. Instead, they function as structural scaffolds that support, organize, and stabilize the active components within macroscopic architectures [
93]. The importance of cellulose arises from its ability to form hierarchically organized and mechanically robust biopolymeric networks that convert otherwise unprocessable particulate sorbents into coherent monolithic systems with defined geometries and interconnected porosity [
142]. Such structural organization facilitates vapor diffusion and water transport, provides mechanical stability, and improves processability at the device scale. Consequently, cellulose-based scaffolds bridge molecular-scale sorption phenomena and the engineering requirements necessary for practical AWH devices, enabling the integration of highly efficient sorbents into scalable and functional water-harvesting platforms.
4.2.1 Water capture in cellulose-based materials
In cellulose-based AWH systems, performance is not determined solely by intrinsic water affinity, but also by how captured moisture is transported and released within the material [
75,
89]. This behavior is governed by the hierarchical structure of cellulose, which defines capillary pathways, vapor diffusion, and liquid transport dynamics. This interplay between moisture sorption and structural organization provides the basis for the following structural design strategies.
The hierarchical pore structure of cellulose provides natural capillary networks that can be further optimized through directional alignment or gradient porosity. Vertically aligned cellulose microchannels, produced via freeze-casting or 3D printing, enable anisotropic liquid transport driven by Laplace pressure gradients. Surface patterning into alternating hydrophilic and hydrophobic regions can also improve droplet mobility and prevent flooding; for example, beetle-inspired Janus meshes with one hydrophobic side and one hydrophilic side guide water to drip off efficiently [
143]. Building on these advantages, Duan et al. designed multi-biomimetic regenerated cellulose fibers using a scalable wet-spinning and dip-coating approach (Fig. 4A) [
126]. The fibers integrate cactus-like microgrooves for enhanced droplet capture, spider-silk-inspired spindle knots for directional transport, and Nepenthes-type wettability gradients that promote rapid shedding. These synergistic structural motifs generate strong Laplace-pressure differences and capillary-driven flow, enabling exceptionally fast droplet migration and achieving a record fog-collection rate of 180.62 g·cm
–2·h
–1 for exceeding traditional single-biomimetic designs.
These composite systems combine the high hygroscopicity of salts with the structural robustness and recyclability of cellulose fibrillar network, resulting in highly efficient sorbents. For example, Zhu et al. demonstrated that incorporating LiCl into a 3D printed CNF scaffold produces a hierarchically porous structure that immobilizes LiCl within micrometer-scale pores while maintaining vertically aligned channels of cellulose robust framework for rapid vapor transport (Fig. 4B) [
93]. As a result, the CNF/LiCl scaffold delivers a 1.6-fold higher water sorption rate compared with conventional freeze-dried aerogels (Fig. 4C). Inspired by the plant leaves, another strategy to further address leakage and improve long‐term stability is developing a core-shell CNF architecture made up of a salt-loaded core in a hydrophobic shell to retain water within the core during deliquescence while permitting controlled vapor exchange through the outer shell [
144]. This biomimetic configuration significantly improves low-RH sorption capacity and eliminates brine seepage during cycling. All of the aforementioned strategies set the stage for real-world application of AWH systems.
Moreover, incorporating high-surface-area additives such as MOFs or porous oxides into a cellulose scaffold can significantly enhance water uptake capacity while simultaneously improving structural stability [
87,
145–
147]. Figure 4D–F illustrates the morphology of ROS-039 (one type of MOFs) paper composites and its AWH cycling behavior mapping, highlighting how its S-shaped isotherm supports rapid uptake at low RH and efficient sorption-desorption oscillations (Fig. 4E). In the composite form, cellulose functions as a structural stabilizer and vapor-transport scaffold, distributing ROS-039 uniformly and preserving open diffusion pathways (Fig. 4D). This hierarchical support reduces kinetic limitations seen in the pure powder, enabling smoother uptake curves and faster turnover. As shown in the heatmaps, optimized partial-loading cycles yield water productivities approaching 7–8 L·kg
–1·day
–1 (Fig. 4F), demonstrating how cellulose enhances both working capacity and cycling frequency by preventing particle agglomeration, maintaining pore accessibility, and supporting rapid regeneration essential for practical AWH operation. Therefore, in MOF-cellulose hybrid aerogels, the MOF particles are uniformly distributed throughout the cellulose network, while the ambient-dried scaffold retains a highly interconnected pore structure that facilitates rapid vapor diffusion and efficient water transport [
142]. The composite retains the mechanical flexibility and low-density architecture of cellulose, yet gains the strong, high-affinity adsorption sites characteristic of MOFs [
148]. As a result, these hybrids exhibit enhanced uptake at low humidity.
A visual comparison of four major AWH material classes, including synthetic polymeric scaffolds, hygroscopic salts, MOFs, and biomass-derived scaffolds, is shown in the radar plots in Figure 4G, which map their relative strengths across five criteria: water uptake, cycling stability, energetic ease of regeneration, processability, and sustainability [
131]. Each radar plot highlights the characteristic trade-offs associated with these sorbents. Synthetic polymer scaffolds perform well in processability and mechanical tunability, but their moderate sorption capacity and low sustainability limit their large-scale deployment. Hygroscopic salts despite extremely high water uptake, show poor cycling stability and high regeneration energy [
149]. MOFs, with their crystalline microporosity and tailored adsorption energetics, achieve high uptake and relatively low regeneration temperatures, but they suffer from the lowest sustainability and processability and limited structural robustness [
84]. In contrast, biomass scaffolds reflect a far more balanced performance profile. Their absolute moderate water uptake capacity compared with salts or MOFs is compensated by sustainability, processability, low-energy regeneration and cycling stability. All combined, the comparative maps prove the central argument of this section: while advanced crystalline sorbents excel in single metrics such as uptake, biomass-derived scaffolds offer the most practical combination of performance, durability, cost efficiency, and environmental compatibility, positioning cellulose as a leading platform for AWH devices. Recent cellulose-based systems for AWH are listed in Table 1.
4.2.2 Water release in cellulose-based materials
One of the key advantages of cellulose is its ability to incorporate thermoresponsive chemical motifs whose phase transitions improve water-release efficiency. As shown in Figure 5A, a representative molecular-engineering strategy converts natural cellulose into functional AWH sorbents through two sequential steps: hydroxypropylation and zwitterionization [
75]. In lower critical solution temperature (LCST)-type systems such as alkylated zwitterionic biomass hydrogels, the cellulose-rich network swells at low temperature but undergoes hydrophobic collapse above the LCST, which reorganizes hydrogen bonds and drives a deswelling front that expels stored water. The cycling data in Figure 5B demonstrates how these materials behave under real outdoor conditions. At night, when RH increases, the hydrogel network expands and absorbs moisture; during daytime heating, the network contracts and releases water, enabling repeated sorption and desorption without any external energy supply. This combination of humidity-driven swelling and temperature-induced contraction produces steadily increasing cumulative water output across multiple day-night cycles. These results show that cellulose-based hydrogels link molecular phase responsiveness with natural environmental fluctuations, allowing continuous AWH through passive and reversible structural transitions.
Cellulose-based materials also support desorption strategies using optical and thermal fluxes. Radiative-cooling fabrics shown in Figure 5C exploit high solar reflectivity (0.3–3 µm) and strong mid-IR emissivity (8–13 µm) to drive day-night adsorption-extraction cycles [
154]. During regeneration, the fabric cools below the dew point, weakening cellulose-moisture interactions so loosely bound water transitions from “damp fiber” states to droplets that can be extracted under mechanical compression or heating (Fig. 5D). As shown in Figure 5E and F, solar heating enables complete recovery of the stored water over a wide RH range, while mechanical squeezing can effectively release water once RH exceeds 60%. The release kinetics under solar heating generally scale with RH, as higher humidity increases both the amount of adsorbed water and the effective light-absorbing area of the sorbent. By contrast, water release through mechanical compression occurs almost instantaneously, exhibiting negligible extraction time.
In addition to thermal and photothermal pathways, electrothermal and electromechanical effects offer complementary desorption mechanisms in cellulose systems. Incorporating conductive networks such as MXenes, CNTs, or polypyrrole enables Joule heating under low voltage, producing localized thermal gradients that accelerate the dissociation of hydration shells without heating the entire system. In covalent organic framework/cellulose (COF/cellulose) hybrid aerogels, proton conduction through the COF framework provides distributed electrothermal dissipation, enabling rapid water release that occurs simultaneously with moisture-driven electrical output [
145]. Moreover, electromechanical vibrations, first demonstrated in cellulose-based triboelectric systems [
159], can be adapted to hygroscopic cellulose matrices to shake loose brine pockets, reduce water adhesion, and disrupt stagnant domains, thereby enhancing evaporation during regeneration.
Building on electrically driven mechanisms, Figure 5J illustrates a complementary approach where hygroscopic cellulose aerogels are integrated with solar modules to provide simultaneous water release and passive cooling [
158]. In this configuration, the aerogel sits beneath a photovoltaic panel, capturing moisture during cooler nighttime conditions and releasing it under daytime solar irradiation. The evaporative cooling during desorption can reduce the temperature of the solar panel, a trend confirmed by thermal images and temperature-time curves that display several-degree reductions compared with panels without the aerogel (Fig. 5K). This cooling effect results in measurable performance improvements, including increases in open-circuit voltage and maximum power output (Fig. 5L). Measurements of evaporative mass loss and evaporation rate demonstrate that the aerogel maintains continuous water release under 1 kW·m
–2 illumination, and outdoor tests show stable cycling across day-night variations in temperature, humidity, and sunlight. These results indicate that photothermal desorption in cellulose aerogels can support both water regeneration and passive thermal management, enabling improved photovoltaic efficiency within a fully passive AWH cooling system. Effective AWH operation requires coordinated vapor capture, internal transport, and controlled release [
160], and cellulose provides an adaptable platform capable of integrating these functions within a single hierarchical material. By combining directional liquid-transport features inspired by natural surfaces with hybrid material design, cellulose-based architectures can merge condensation, adsorption, and deliquescence processes into one recyclable system [
161].
5 Application II: Moisture-Enabled Electricity Generation (MEG)
Apart from atmospheric water harvesting, another important application of cellulose-moisture interactions is MEG. In MEG, cellulose is prepared into porous structures that take advantage of its hydrophilic nature and abundant ionizable groups to support moisture-driven ion diffusion and electricity generation. MEG describes the phenomenon in which hygroscopic materials convert moisture-induced chemical potential differences into electrical output through moisture adsorption, ion generation, and directional ion transport [
162–
164]. The first evidence of moisture-induced charge separation was reported in 2009, when Gouveia and Galembeck observed spontaneous charge accumulation on cellulose paper and metal surfaces under high RH [
165]. A functional MEG device based on this discovery was developed in 2015, when Qu et al. prepared a graphene oxide (GO) MEG generator in which proton migration along an oxygen-containing functional group gradient generated voltage of ~30 mV and current density of 10 μA·cm
–2 [
166]. Subsequent studies confirmed that even uniform GO films can generate electricity when a moisture gradient exists [
167]. Since then, performance of MEG devices has improved significantly through material selection and optimized structural design [
168]. For example, Wang et al. achieved over 1000 V output using a MEG device built from sequentially stacked bilayer polyelectrolyte films, highlighting the evolution of MEG and its potential in self-powered electronics [
169].
5.1 Fundamental mechanisms
When moisture contacts the surface of hygroscopic materials, water molecules adsorb and interact with functional groups such as hydroxyl or carboxylate groups, leaving fixed charges on the solid surface and releasing counter ions into the thin interfacial water layer [
170]. This interfacial charge separation forms an electric double layer (EDL) with a compact layer of bound ions and a diffuse layer of mobile counter ions [
171]. For MEG devices, an asymmetric moisture gradient along the material creates differences in hydration and ion concentration between wet and dry regions, so that mobile ions diffuse from the wetter side toward the drier side to generate diffusion potential and current to maintain charge balance [
164,
172]. Based on how moisture participates in the energy conversion process, moisture-enabled generators can be divided into indirect routes, where adsorbed water triggers other conversion processes, and direct routes, in which a static moisture gradient drives directional ion diffusion and contributes to electricity output [
162,
163,
173]. Here we focus on the direct, flow-free, moisture gradient-induced ion diffusion mechanism that is most relevant to cellulose-based MEG [
174,
175].
In MEG devices, cellulose serves as both moisture sorption sites and the medium for ion diffusion. Its hydroxyl and carboxylate groups release mobile ions upon hydration [
176]. The hierarchical porous network of cellulose-based foams, aerogels, and porous films confines water and EDL within narrow nanochannels, amplifying diffusion potential even under small RH difference [
177]. In addition, extended hydrogen-bonding network along cellulose fibrils facilitates fast proton conduction, further enhancing directional ion diffusion [
178]. Moreover, the surface chemistry of cellulose can be modified through interactions such as oxidation, esterification, and etherification to adjust surface charge density and hydrophilicity. This affects the amount of generated mobile ions upon hydration, the intensity of EDL, and eventually the performance of the MEG device [
179]. All of these properties enable cellulose-based MEG to maintain stable internal moisture and ionic gradient to deliver continuous electricity output.
The three water states also play distinct roles in cellulose-based MEGs under different environmental and structural conditions. Bound water, associated with hydroxyl and carboxylate groups, facilitates surface ionization and the formation of fixed charges, influencing the EDL and surface charge density [
25]. Intermediate water supports proton and ion transport through partially connected hydrogen-bonding networks, sustaining conductivity under moderate RH [
180]. At higher RH, free water enhances ionic conductivity by forming continuous transport pathways; however, excessive accumulation may homogenize moisture and ion distributions, weakening the chemical potential gradient required for directional transport [
65]. Structural features such as pore connectivity, crystallinity, and surface functionalization, together with temperature-dependent diffusion and ion mobility, jointly regulate these processes. This trade-off underlies the need for asymmetric and gradient designs that balance transport efficiency with gradient preservation for continuous MEG operation.
5.2 Design strategies of cellulose-based MEG systems
Recent studies on cellulose-based MEG have mainly focused on two directions: ion- and electron-transport-oriented material modifications to enhance ion and electron transport, and asymmetrical structure designs that help maintain moisture and ion gradients.
5.2.1 Material composition modifications
5.2.1.1 Ion-transport-oriented material modifications
Chemical interactions between water molecules and functional groups in cellulose are critical in EDL formation. Higher concentration of these groups provides additional active sites for water adsorption and ionization, increasing the concentration of dissociated protons, therefore enhancing ionic conductivity and charge generation. One effective route is the chemical modification of cellulose, which introduces additional acidic or polar groups such as carboxylate or sulfate groups. Yang et al. demonstrated that a citric acid-induced gradient of carboxylate groups generated an internal proton gradient (Fig. 6A and B), leading to 275 mV output [
181]. Even without a gradient, simply increasing the concentration of these groups can boost charge carrier availability. Mo et al
. prepared an MEG with sulfated cellulose nanofibrils (SCNF) grafted with sulfamic acid, which achieved 0.9 V output under 80% RH (Fig. 6C), much higher than an unmodified PVA hydrogel [
180]. Similarly, TOCNF and carboxymethyl CNF introduce carboxylate groups that release protons upon hydration and facilitate ion diffusion. Xie et al. optimized this strategy by embedding a sulfonated covalent organic framework into a carboxylated CNF network, taking advantage of both sulfonated and carboxylated groups and obtaining a continuous output voltage of ~0.55 V for over 5 h under ambient humidity[
145]. Overall, increasing the concentration of acidic or polar functional groups through chemical modification improves moisture adsorption, proton release, and EDL density, leading to higher and more stable electricity output than unmodified cellulose.
Another strategy is to increase the concentration of mobile ions by incorporating salt or strong acids into the cellulose matrix. Hygroscopic salts such as LiCl and NaCl absorb moisture and dissociate into ions, increasing ionic conductivity in humid environments and slowing down the moisture equilibrium process, which extends the duration of electricity output. Strong acids like HCl and H
2SO
4 play a similar role by providing additional protons for ion diffusion. Song et al. developed a bilayer MEG by stacking a BG/GO/CNF composite on a NaCl/CNF layer. The salt-loaded bottom layer served as an ion reservoir, from which ions migrated toward the upper layer (Fig. 6D), nearly doubling the voltage to 1.17 V and increasing current density tenfold to 2770 µA·cm
–2 compared with a device without NaCl [
182]. In another related study, Eun and Jeon incorporated different amounts of NaCl into CNF films before laser carbonization [
185]. The optimal NaCl concentration produced 550 µA·cm
–2 and 0.65 V at 90% RH, benefiting from the Na
+ concentration gradient across the thickness. These studies confirm that introducing mobile ions such as Li
+, Na
+, Cl
–, and H
+ significantly enhances both the intensity and stability of the generated electricity.
Apart from salt and strong acids, ionic liquids (ILs) have been used to improve moisture retention and prevent device drying issues. Li et al. prepared a TOCNF ionogel that delivered a continuous output of ~0.3 V and 10 µA for several hours (Fig. 6E) [
183]. The ionogel preserves an ionic conductivity of ~0.1 S·m
-1 even when fully dried or kept below freezing, which is adequate for driving a LED (Fig. 6F). The nonvolatility and intrinsic conductivity of ILs contributed to this stable performance. Overall, the incorporation of salts, strong acids, or ILs increases charge carrier concentration and often enhances water retention, resulting in higher and more stable output in cellulose-based MEG. Recent cellulose-based materials for MEG are listed in Table 2.
5.2.1.2 Electron-transport-oriented material modifications
Apart from increasing mobile ion concentration or introducing additional functional groups, performance of cellulose-based MEG can also be improved by adding conductive or redox-active components. Conductive fillers such as CNTs, GO, and MXenes improve electron transport, serve as built-in microelectrodes that reduce internal resistance, and provide additional surface functional groups for moisture adsorption and ion dissociation. Lee et al. developed a CNF film with upper surface converted into a graphitic carbon layer through infrared laser-induced graphitization [
187]. The treatment introduced a vertical gradient of oxygen-to-carbon functional group, with more oxygenated groups at the lower cellulose-rich region and more conductive graphitic carbon near the top. This variation maintained a potential difference across the film and supported continuous directional ion diffusion even under full moisture saturation. As a comparison, films with a uniform distribution of functional groups quickly became symmetrically hydrated under high humidity and lost the driving force for ion diffusion.
In many reported cellulose-based MEG, electricity generation relies almost entirely on ion diffusion, resulting in limited output. Incorporating redox-active materials that enable faradaic reactions alongside ion diffusion has proven to be an effective way to enhance performance. Song et al. verified this approach by adding BG into the CNF/GO layer of a bilayer film [
182]. During operation, BG underwent a reversible Na
+-mediated redox reaction to Prussian Blue (PB), generating additional faradaic current (Fig. 6G–I). As a result, this device achieved a current density of 2770 µA·cm
–2, more than ten times compared to a device without BG.
5.2.2 Asymmetrical structure design
Creating asymmetric structures in MEG is a key design strategy for sustaining moisture or ion gradients, which are essential for continuous electricity output. In symmetric MEG devices, moisture quickly reaches equilibrium, which leads to the disappearance of potential difference. This explains why many early MEG devices produced only transient electricity output. In contrast, asymmetric designs where one side remains wetter or more ion-rich than the other, are able to maintain a persistent chemical potential gradient that drives directional ion diffusion and generates stable output [
65].
Several strategies have been developed to introduce asymmetry in cellulose-based MEG. A common method is to prepare Janus or bilayer membranes that enforce unidirectional moisture transport by combining a hydrophilic layer with a hydrophobic layer, or by building an internal ionic gradient with redox-active components [
186]. The second approach is to maintain a chemical or ion gradient within the material through the incorporation of salts and strong acids [
181,
182,
187]. Physical asymmetry can also be achieved by structuring pores to form a porosity gradient or oriented channels. Chen et al. fabricated a highly ordered honeycomb-like CA film with a pore-size gradient across its thickness[
65]. The smaller pores on one side and larger on the other caused asymmetric condensation and flow of moisture, forming a unidirectional ion diffusion path (Fig. 6J).
Some MEG devices combine several asymmetric designs. Chen et al. constructed a dual-gradient bilayer MEG in which one layer contained a pore-size gradient for directional moisture transport, and the other had a gradient of oxygen-rich functional groups to enhance ion diffusion [
184]. This membrane achieved ~0.67 V and 11.2 µA·cm
–2 with good stability under different RH levels; when multiple units were connected in series, they could power an LED for more than 6 h (Fig. 6K). Likewise, Yang et al. combined 1D sulfated CNF and 2D graphene sheets to introduce self-sustained moisture gradients and structural anisotropy [
189]. Attributed to the internal gradient and microcapacitive effects, the system produced a steady output of ~0.54 V for over 2160 h. Overall, asymmetric structures based on gradients of ions, functional groups, or pore size are critical for continuous and stable electricity generation in cellulose-based MEG, as they slow down the moisture equilibrium process, support directional ion diffusion, and extend device lifetime.
5.3 Toward multifunctional integration
Although cellulose-based AWH and MEG share a common foundation in moisture sorption, their design principles are intrinsically different: AWH is typically optimized toward thermodynamic equilibrium to maximize water uptake and retention [
21], whereas MEG relies on maintaining a nonequilibrium moisture and ion gradient for continuous electricity output [
110]. As a result, materials designed for high water uptake tend to homogenize moisture distribution and weaken the chemical potential gradient required for MEG, while structures that favor rapid transport often compromise water retention. Therefore, integrating AWH and MEG requires a decoupled design strategy, in which water sorption and transport are spatially or structurally regulated.
Cellulose provides a platform for constructing such decoupled designs because its hierarchical porous architecture and tunable surface chemistry allow engineered spatial organization of moisture-storage domains and ion-transport pathways within a single material, providing the structural basis for coupling water harvesting with continuous electricity generation [
190]. In cellulose-based MEG-AWH systems, both water harvesting and electricity generation are driven only by humidity and sunlight, and each AWH adsorption/desorption cycle spontaneously restores the moisture gradient required by MEG, generating a self-replenishing system [
191].
Upon moisture adsorption, functional groups in cellulose capture water via hydrogen bonding, while doped salts or dissociate into mobile ions, forming EDL along micro and nanochannels [
163]. Spatial asymmetry leads to directional ion diffusion and electricity output [
192]. Desorption takes place upon solar or ambient heating, or when the system is integrated with photothermal agents, releasing the stored moisture as vapor. This not only resets the EDL for continuous electricity output but also enables water collection via condensation [
191]. Once dried, the cellulose network spontaneously reabsorbs moisture, restoring the ionic gradient and closing this self-sustaining cycle.
This concept was first demonstrated in 2020 by Gong et al., who prepared a carbon-coated corn stalk/LiCl moisture absorber that harvested ~1.8 kg·kg
–1 water under 80% RH and generated an stable voltage output of 0.6 V for over 40 h [
193]. DFT calculations revealed that surface oxygen groups on the carbon layer created a charge asymmetry that, combined with the natural vascular channels of the stalk, enabled directional proton migration (Fig. 7A and F). Following this approach, inspired by Ficus aerial roots, Wang et al. developed a sodium alginate (SA)-CNF aerogel embedded with ethanolamine-modified CaCl
2 and photothermal gallic acid-Fe
3+ complexes (Fig. 7B) [
146]. The device achieved a water absorption capacity of 2.18 g·g
–1 and an output voltage of ~0.102 V at 90% RH and could be recovered by 91.42% within 240 min under 1 kW·m
–2 sunlight (Fig. 7C–E). Overall, the integration of cellulose-based AWH and MEG into a single system reduces material redundancy. It also offers a sustainable approach to harvest clean water and electricity at the same time, which is particularly attractive in remote or off-grid areas.
To provide a clearer comparison of the strategies discussed above, Table 3 summarizes representative cellulose-based AWH and MEG systems in terms of modification methods, structural features, performance improvements, and underlying mechanisms. By organizing these elements within a single framework, the table shows how specific structural designs translate into functional performance and clarifies the structure-property-function relationships in both AWH and MEG.
6 Conclusion and Perspectives
6.1 Conclusion
Cellulose-based materials have demonstrated great potential for AWH and MEG, offering a sustainable alternative to conventional inorganic and polymeric systems. Throughout this review, we have summarized the fundamental understanding of cellulose’s hierarchical structure, its interactions with moisture, and the resulting structure-property-function relationships that enable water and energy conversion. We have also discussed how molecular design, structural engineering, and bioinspired strategies collectively contribute to optimizing the performance in terms of AWH and MEG of cellulose-based systems under diverse humidity conditions.
At the molecular level, the arrangement of crystalline and amorphous regions governs hydrogen bonding, sorption kinetics, and dimensional stability. At the mesoscale, porosity and morphology define diffusion pathways and capillary effects, while surface chemistry determines the balance between hydrophilicity and ionic conductivity. Through controlled modification, such as introducing functional groups, tailoring pore geometry, or constructing asymmetric and gradient structures, cellulose can achieve both high water uptake and efficient charge transport. Together, these factors shape the macroscopic performance of AWH and MEG devices.
In AWH, their hygroscopic nature, large surface area, and tunable morphology enable water capture through adsorption, condensation, and fog interception. Optimized porosity and surface chemistry improve uptake and release kinetics, while photothermal coatings and bioinspired microstructures facilitate regeneration and droplet shedding. Gradient or channel-guided designs promote directional liquid transport, enabling integrated systems that combine capture, release, and collection. In MEG, functionalized cellulose enhances ion mobility under humidity gradients, while cellulose-carbon hybrids convert ionic motion into electronic currents. Gradient and asymmetric architectures sustain continuous voltage generation, and multilayer composites integrate ion and electron pathways to improve output and durability. Overall, the collective body of research reviewed here establishes a coherent scientific framework that links molecular interactions to device-level performance, providing a roadmap for advancing cellulose as a key material in sustainable water-energy technologies.
6.2 Current challenges
Despite notable advances, cellulose-based systems for AWH and MEG still face fundamental scientific and performance-related challenges that limit their efficiency and practical deployment.
(1) Incomplete understanding of moisture-structure-charge coupling: The dynamic interplay between water adsorption, transport, and charge redistribution in cellulose networks remains only partially understood. Existing models often treat moisture sorption and ionic motion as independent processes, overlooking the feedback between capillary flow, ion hydration, and local potential gradients. Direct experimental evidence correlating molecular-scale hydrogen bonding dynamics with macroscopic electrical outputs is scarce. A deeper mechanistic understanding supported by in situ spectroscopy, advanced imaging, and multiscale simulation is essential to guide rational material design.
(2) Trade-offs between hydrophilicity, sorption kinetics, and regeneration: Enhancing hydrophilicity improves water uptake but often leads to slower desorption and hysteresis in both AWH and MEG systems. Highly polar surfaces bind water too strongly, impeding regeneration and cyclic stability. Conversely, excessive hydrophobic modification suppresses sorption kinetics and disrupts continuous moisture-charge coupling. Achieving an optimal balance between adsorption strength, mobility, and reversibility remains a key scientific bottleneck.
(3) Limited control over hierarchical structure and interfacial pathways: While cellulose’s multiscale architecture enables diverse functionalities, quantitative control over pore connectivity, surface polarity gradients, and nanochannel geometry is still lacking. Irregular pore networks can cause nonuniform water distribution, localized stress, and inefficient charge transport. Precise engineering of anisotropic or asymmetric structures, which can guide directional water and ion transport, requires deeper insight into how molecular organization translates to macroscopic function.
(4) Insufficient correlation between structure, dynamics, and performance metrics: Most studies report macroscopic results, such as water uptake and voltage output, without linking them to underlying physicochemical parameters such as diffusion coefficients, interfacial charge density, or relaxation time of bound water. The absence of such correlations limits predictive design and cross-material comparison. Establishing universal parameters that connect nanoscale interactions with device-level efficiency is critical for developing performance models and benchmarking standards.
(5) Coupled degradation mechanisms under dynamic environments: Under cyclic humidity, ultraviolet exposure, and thermal fluctuations, cellulose undergoes reversible swelling, chain rearrangement, and partial hydrolysis, which alters its dielectric properties and interfacial charge behavior. These processes are not yet systematically characterized, and their impact on long-term electrochemical stability is poorly understood. Designing materials that maintain both functionality and biodegradability under real-world conditions remains a major scientific and engineering challenge.
(6) Instability of salt distribution and structural integrity: Many cellulose-based AWH and MEG systems rely on hygroscopic salts to enhance water uptake or maintain ion reservoirs. Under cyclic humidity, these salts undergo dissolution, redistribution and even leakage, leading to reduced sorption capacity and loss of the moisture/ion asymmetry required for sustained MEG operation. Repeated swelling-deswelling further induces mechanical fatigue and pore collapse, degrading transport pathways. A quantitative understanding of these coupled effects remains lacking.
(7) Limitations in scalable processing and controlled biodegradability: Most cellulose-based AWH and MEG systems remain at laboratory scale, and their scalability is yet to be demonstrated. Fabrication methods such as freeze-drying and 3D printing are energy-intensive and difficult to scale, limiting practical deployment. Meanwhile, although biodegradability is advantageous, its behavior under operating conditions is not well controlled, which may compromise stability. Chemical modification can improve durability but often reduces biodegradability, creating a trade-off between performance and sustainability. Achieving scalable fabrication with controlled lifetime remains a key challenge.
6.3 Future perspectives
Looking forward, several promising directions are expected to guide the next phase of development for cellulose-based water-energy technologies:
(1) Adaptive and intelligent material design: Future systems may employ cellulose matrices integrated with stimuli-responsive polymers, nanoparticles, or conductive fillers to dynamically regulate pore structure, surface wettability, or charge distribution. These adaptive materials could respond autonomously to changes in humidity, temperature, or light, improving both efficiency and durability under real-world conditions.
(2) Data-driven design and ML-assisted optimization: The combination of machine learning (ML), high-throughput experimentation, and multiscale simulations can accelerate the discovery of optimal cellulose compositions and structures. By correlating molecular indicators with performance metrics, predictive models could guide the rational design of materials with targeted water uptake, ion mobility, and mechanical resilience. For example, ML has been used to screen over 6000 MOFs for their water adsorption performance in AWH, predicting the top candidates with high accuracy[
194]. Neural networks have been trained to predict moisture penetration dynamics in paper [
195]. A transfer learning approach has been applied to GO-based water flow enabled electric generators, effectively optimizing device performance with limited experimental data [
196]. Extending these approaches to cellulose systems could guide the rational design of composition and architecture under specific operating conditions, such as tuning charge density or pore gradients for a given RH range.
(3) Sustainable manufacturing and life-cycle assessment: Advancing green fabrication routes, such as solvent-free processing and utilization of waste biomass, will be crucial for minimizing the environmental footprint. Comprehensive life-cycle assessments are needed to evaluate trade-offs between performance, cost, and sustainability, ensuring that cellulose-based systems align with circular-economy goals.
(4) Cross-scale integration and field validation: Bridging molecular-scale understanding with device engineering requires in situ characterization under realistic humidity, temperature, and solar exposure conditions. Pilot-scale demonstrations and long-term outdoor testing will be essential to assess reliability, durability, and energy-water efficiency under variable climates.
In summary, cellulose-based materials offer a uniquely versatile and sustainable platform for converting atmospheric moisture into usable water and electrical energy. By integrating insights from hierarchical structure, cellulose-moisture interactions, and structure-property-function relationships, researchers have developed AWH and MEG systems that combine efficient capture, transport, and energy conversion. Despite remaining challenges in mechanistic understanding, structural control, and long-term stability, continued interdisciplinary research, adaptive material design, and data-driven optimization promise to advance cellulose-based technologies from laboratory demonstrations to practical, self-sustained water-energy solutions capable of addressing global resource challenges.
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