Simultaneous Passive Water and Electricity Generation in Arid Regions via Fe³⁺/Li⁺ Cross‐Linked Hygroscopic Hydrogels

ABSTRACT

Humans are confronting global water scarcity and energy shortages in data centers, and utilizing moisture from the air to address both water and electricity issues is a promising direction. Here, a porous hygroscopic hydrogel is integrated, which is composed of sodium alginate that is co‐loaded with Fe³⁺ and Li⁺. Hydrophilic adsorption sites are constructed via ionic cross‐linking and salt loading, enabling low desorption enthalpy (<40 kJ mol^{− 1}) for efficient water desorption. The water uptake of the dual‐ionic hydrogel is over 5.1 g g⁻¹ at 90% RH, and this value is 1.1 g g⁻¹ at 30% RH. By designing a water harvesting device with rotating adsorption/desorption zones, with a daily production of 11.8 L kg_hydrogel ⁻¹ at 70% RH. The hydrogel not only provides abundant sites for moisture adsorption and storage but also offers channels for ion transport to enhance electricity generation. By constructing an asymmetric humidity structure, the dissociated cations in the hydrogel will diffusion to the substrate by concentration gradient and the electricity is evocated, producing a highly short‐circuit current (250 µA) and open‐circuit voltage (7.6 V), illuminating the multi‐LED lights.

Porous hygroscopic hydrogels are prepared by Fe³⁺/Li⁺ cross‐linking reaction, and the water uptake is enhanced to 1.1 g g⁻¹ at 30% RH. Combined with this hydrogel to integrate bifunctional devices, to produce drinking standard fresh water (11.8 L kg⁻¹) and electricity (7.6 V) are concurrently produced in arid regions.

1. Introduction

Freshwater resources are a critical cornerstone for human society [1, 2, 3]. Globally, the total amount of water in the air reaches up to 12,900 cubic kilometers, nearly 1/7 of the total volume of lakes and rivers [4, 5]. If we can directly utilize atmospheric water, the challenges posed by uneven freshwater distribution in arid regions can be alleviated. Atmospheric circulation enables sufficient moisture, and adsorption‐based atmospheric water harvesting (AWH) technology can produce freshwater, such as in deserts and plateaus [6, 7]. Although progresses have been made in adsorbents, efforts to develop hydrogels with high adsorption capacity and low desorption energy consumption are still ongoing. Meanwhile, a large amount of moisture is accompanied by energy conversion during the adsorption process [8, 9]. The energy of the moisture‐absorbing generator device is provided by the Gibbs free energy of adsorbed water, which supplies energy for charge separation. The charges redistribution by humidity gradient forms a potential difference, generating usable electricity to electronic devices [10, 11]. Therefore, developing a hydrogel to both water and electricity generation has a huge competitive advantage.

The efficiency of AWH is determined by the material‐level characteristics, including sorption capacity, kinetics, enthalpy, stability, and system‐level characteristics [12, 13, 14]. For example, nickel‐based hydrogel was formed via coordination reactions, generating water uptake of 5.0 g g⁻¹ at 95% relative humidity (RH) [15]. Functionalized cellulose hydrogels achieve high water uptake by integrating zwitterions, capturing 0.86–1.32 g g⁻¹ of moisture at 15%–30% RH, achieving a water production of 14.19 kg kg_hydrogel ⁻¹ day⁻¹ [16]. The thermoresponsive hydrogel nanofiber can adsorb moisture to saturation within 2 h at 15%–30% RH. Under sunlight, it transforms to hydrophobic, and exudes liquid water within 5 min. The transmissive radiative cooling film can reduce the temperature of the condensation end by 5°C, accelerating the water condensation rate by 50% [17]. Rational design of surface hydroxyl, amino, and carboxyl groups, the water uptake can be controlled. The interaction between oxygen‑containing groups and metal ions can regulate the water adsorption active sites [18], and the water adsorption efficiency can be improved through thermoelectric devices [19]. Further, the water yield can be improved by increasing the condensation efficiency of the water harvesting system [20, 21, 22].

In moisture adsorption, the water content differences will be established, and the ions with different dissociation degrees will diffuse in the hydrogel under the action of chemical potential [23, 24, 25]. When the soft hydrogel adsorbs moisture at the surface, the H⁺ in the water will be released and diffuse into the bulk, generating electricity [26, 27]. Fe nanoparticle serial hydrogels loaded onto leaves to form a dry–wet asymmetric structure generate a voltage of 0.5 V [28]. The multilayer stacked photocatalysis and moisture‐powered generator can produce a current of 1.8 µA, light illumination can reduce the ion aggregation [29]. A 3D porous generator is formed through the layered structure of cationic and anionic polymers, accelerating the water adsorption rate [30]. Integrating a moisture‐powered device with a capacitor, generating a power density of 49.4 µW cm⁻². Thus, the bulk skeletal structure of the hydrogel is one of the key components of electricity generation [31, 32, 33].

Producing drinking water and harvesting environmental energy from atmospheric are the feasible approaches to achieving a sustainable society [34, 35]. Due to the abundance of hydrophilic groups in the hydrogel, it exhibits excellent water absorption capacity; meanwhile, the dissociation of ions occurs upon water absorption, enabling the hydrogel to achieve simultaneous water and electricity production (Figure 1a). The hydrogel is designed by a cross‐linking process, which is thermodynamic regulated by introducing ─OH, ─COOH, graphene oxide (GO), Fe³⁺, and Li⁺ as hygroscopic functional groups, the binding energy between water molecules and the hydrogel can be reduced. In ion transport kinetics regulation, the continuous water channels in the porous network can reduce the ion migration resistance. Through a passive adsorption/desorption process, the moisture in air can be transferred to liquid water and generate a voltage due to the anisotropic accumulation of anions and cations. The harvested water can be utilized in scenarios such as arid areas, saline alkali land, mountains, plateaus, and space stations. The generated electricity from a sandwich structure (containing dry and wet separated regions) can charge electronic devices (e.g., mobile phones and indicator lights).

FIGURE 1.

Fabrication and morphologies of hydrogels. (a) Scheme of the hygroscopic hydrogel for water collection and electricity generation by use of humidity. (b) Fabrication process of the hydrogel, including the freeze–drying and cross‐linking process (the Fe³⁺ and Li⁺ are co‐linked with SA). (c) Optical microscope images corresponding to different moisture absorption times at 20°C and 70% RH. (d) SEM image of the prepared porous hydrogel.

Uninhabited areas (such as desert no‐man's‐lands) are confronted with the dilemma of both water and energy scarcity. To achieve concurrently water and energy production, a bimetal composite hydrogel was developed (Table S1). Through intramolecular/intermolecular design, the synergistic improvement of moisture absorption and power generation is achieved from two levels: thermodynamics (reducing water binding energy, optimizing moisture adsorption enthalpy change) and kinetics (reducing ion migration resistance). The sodium alginate (SA), FeCl₃, and LiCl cross‐linked hydrogel exhibits a water uptake of 5.1 g g⁻¹ and desorbs 80% of the combined moisture at 65°C. At 70% RH, the water production reaches 11.8 L kg_hydrogel ⁻¹ per day. By constructing an asymmetric humidity generator onto carbon paper, an open‐circuit voltage of up to 1.2 V (2 cm²) is achieved through ions’ diffusion, enabling continuous energy harvesting.

2. Results and Discussion

2.1. Fabrication and Characterization of Hydrogels

By cross‐linking reactions with metal ions and hydrogel frameworks, a metal‐ion cross‐linked porous hydrogel with a robust skeleton is prepared. Ice crystal sublimation during freeze–drying removes water constituents and thereby generates hierarchical porous structures across the entire hydrogel matrix (Figure 1b) [36]. The freeze–dried hydrogel is sequentially dropped by Fe³⁺ and Li⁺ solutions, electropositive ions replace Na ions, and SA chains cross‐link with Fe ions to form a stable structure, ensuring the porous structure does not collapse (Figure S1). The higher of the LiCl loading, the more LiCl crystals are visible, to decrease the size of the pores.

The synthesized porous hydrogel captures moisture from the air, enabling the phase transition from vapor to liquid states. Figure 1c presents optical microscope images of the hydrogels. The surface of the dehydrated hydrogel is rough and uneven, featuring abundant air‐hydrogel interfaces. Upon exposure to ambient air (20°C, 70% RH), the hydrogel immediately adsorbs moisture, and its surface is gradually covered by tiny water droplets. Within 10 min, water spreads over the surface and diffuses into the bulk, confirming the completion of gas–liquid phase transition and water storage via the surface porous channels. Scanning electron microscopy (SEM) images reveal the microstructure of the hydrogels with porous structures, and the skeleton structure is visible (Figure 1d). The size of the pores and skeleton is around 10 µm and 5 µm. This structure has a highly mechanical stability and water storage capacity. Atomic Force Microscopy (AFM) also reveals nanoscale surface structures beyond pores that are tens of micrometers in size (Figure S2).

X‐Ray Diffraction (XRD) plots confirm that the dehydrated hydrogel has a distinct crystalline structure (Figure 2a). The diffraction peaks at 34.9°, 50.1°, and 62.5° correspond to LiCl crystals, while other peaks are attributed to LiCl·(H₂O)_n. When the hydrogel adsorbs moisture, its crystalline structure is disrupted, accompanied by the disappeared peaks. The located large peak with a non‐sharp at 30° implies the hydrated hydrogel belongs to an amorphous structure, identifying that the moisture harvesting destroys the crystal structure of the hydrogel and dissociates the constituted groups/ions. Transmission Electron Microscopy (TEM) images demonstrate that the hydrogel is mostly amorphous but contains partial crystalline regions, with the most prominent lattice spacings being 0.148, 0.182, and 0.257 nm (Figure 2b). It can be observed that graphene oxide is distributed in a sheet‐like manner in the hydrogel, which exerts a certain supporting effect on the hydrogel skeleton. The three d‐spacings correspond to the XRD peaks at 34.9°, 50.1°, and 62.5°, the characteristic crystal planes of LiCl [37]. TEM elemental mapping shows the uniform distribution of elements in the hydrogel (Figure S3). For the hydrogel with excessive LiCl content (30% wt), strong and sharp LiCl crystalline peaks are still visible moisture adsorption. Such persistent crystallinity results in pore blockage, reducing both specific surface area and adsorption capacity, also decreasing the number of adsorption sites and mass transport channels. Selected Area Electron Diffraction (SAED) data indicate these dominant lattices appear as sharp diffraction rings (Figure 2c). These inner rings reveal the partial crystal structure, and marched well with the distinct interplanar spacings.

FIGURE 2.

Structural characteristics of the hydrogels. (a) XRD of the hydrogel before and after moisture absorption at 20°C and 70% RH. (b) HRTEM image of hydrogel. (c) SAED pattern corresponding to the HRTEM image. (d) The molecular structure of the hydrogel by DFT stimulation. (e) The calculated water embedding energy of the hydrogel. (f) Raman spectra of the hydrogel before and after water absorption. (g) XPS data of the hydrogel corresponding to O 1s. (h) XPS data of the hydrogel with C1s orbit. (i) In situ FTIR spectra of the hydrogel at different moisture absorption times in air.

To shed light on the moisture adsorption, a molecular hydrogel structure was constructed via density functional theory (Figure 2d). A corresponding stable structural model with minimum energy, Fe ions were embedded in SA for cross‐linking. Fe³⁺ undergoes an ion exchange reaction with Na ions, which cross‐links with three SA chains, enhancing the stability of the hydrogel [38]. For this structure, compared with the Fe/Li hydrogel, the Fe‐Li hydrogel (SFL) exhibits lower water injection energy, indicating the excellent hygroscopicity (Figure 2e). This binding embedding energy (−1.15 eV) proves the physisorption, the weak van der Waals interaction can guarantee the large adsorption capacity and fast adsorption/desorption rate. Raman spectroscopy implies the hydrogel has characteristic peaks at 1609 and 2949 cm⁻¹, corresponding to C─C and C─H bonds, and the peaks at 1343 and 1609 cm⁻¹ are attributed to GO (Figure 2f) [39]. Compared to the dehydrated hydrogels, the peak intensity of the hydrated hydrogel is different, the broad peak in the range of 3000–3600 cm⁻¹ can be assigned to O─H bonds (proving the super hydrophilic structure). Notably, this peak within 3000–3600 cm⁻¹ of O─H can be deconvoluted into two sub‐peaks: the spectral peak at ∼3200 cm⁻¹ to strongly hydrogen‐bonded water (corresponding to a highly ordered tetrahedral hydrogen‐bonding network, ice‐like structure); the spectral peak at ∼3400 cm⁻¹ to weakly hydrogen‐bonded water (corresponding to a disordered structure with perturbed hydrogen‐bonding network).

X‐Ray photoelectron spectroscopy (XPS) was used to characterize the surface chemical information of the hydrogels, with the C 1s peak was calibrated to 284.8 eV as the reference. The O 1s spectrum shows two peaks at 531.2 and 532.5 eV (Figure 2g), assigning to O─C bonds and ─OH groups. After Fe coupling, the peak shifts to 531.0 eV, suggesting Fe coordinates with oxygen‐containing functional groups, causing the O peaks from different chemical environments to a single peak. Usually, Fe³⁺ forms complexes with oxygen‐containing ligands (such as Fe‐COOH complexes), causing a decrease in the electron density of oxygen atoms and an increase in the O 1s binding energy. The hydrogel exhibits more abundant oxygen species. The binding energy of 533.0 eV is a typical value for neutral oxygen atoms in polymers, which is higher than the binding energy of oxygen in free water molecules. For the C 1s spectrum (Figure 2h), the C─C, C─O, and C═O bonds in the hydrogel precursors correspond to 284.5, 286, and 288 eV [40, 41]. After cross‐linking with Fe ions, the binding energy of C─O and C═O is increased by 0.2 and 0.1 eV, indicating a coordination reaction between Fe ions and SA. After LiCl loading, the binding energy of C─O and C═O increased by 0.1 eV, proving the coordination reaction between Li ions and SA. In situ Fourier Transform Infrared (FTIR) spectroscopy confirms the hydrogel has a chemically coordinated structure and strong hydrophilicity (Figure 2i). The characteristic peak at 3000–3600 cm⁻¹ is broad with time, displaying the gradual accumulation of moisture on the surface. The characteristic peaks at 1621 and 1023 cm⁻¹ are attributed to H₂O and C─O─C bonds [42].

2.2. Water Adsorption/Desorption Performances

To investigate the dynamics of moisture adsorption, LiCl with different concentrations was prepared, and the cross‐linking time of hydrogel frameworks was adjusted accordingly. The results demonstrate the hydrogel loaded with 18% LiCl has the water uptake of 1.86 g g⁻¹ at 50% RH. The water absorption rate of the hydrogel increases with decreased thickness, while the total water absorption capacity increases with thickness. (Figure S4). Lower LiCl provides insufficient moisture adsorption sites, failing to form effective hydration centers to capture moisture. Higher LiCl will lead to crystallization or agglomeration during the cross‐linking process, blocks the porous channels within the hydrogel. The blocked pores hinder the diffusion and transfer of water vapor into the hydrogel, reducing the water uptake, and the 18% wt LiCl in the hydrogel shows the highest water uptake. The adjustment of hydrogel cross‐linking time is intended to match the salt loading, and the 30 min of cross‐linking suggests the highest water uptake of 1.91 g g⁻¹ at 50% RH (Figure S5). The water uptake Fe³⁺ cross‐linked hydrogel is higher than that of the Ca²⁺, Ni²⁺, and Co²⁺ cross‐linked hydrogel. This higher hydrophilicity can be ascribed to: Divalent metal and sodium alginate usually form a classic “egg box model” 2D planar cross‐linking structure. Trivalent Fe³⁺ has a higher charge density (ion charge/radius ratio) and can form a 3D cross‐linked network with functional groups. The cross‐linked hydrogel with an Fe³⁺ mass fraction of 2% exhibits the highest water uptake, The lower FeCl₃ concentration cannot maintain its porous structure, while the higher concentration weakens the transport dynamic of water in bulk hydrogel. Appropriate cross‐linking time ensures the formation of a stable porous structure to accommodate LiCl, excessive cross‐linking may increase the matrix density and further exacerbate pore blockage with high salt concentrations [43].

To evaluate the moisture adsorption capacity of hydrogels, water uptakes of various adsorbents at different RHs were measured. At 30% RH, the water uptake of SFL hydrogel is 1.1 g g⁻¹ only in 60 min (Figure 3a), implying the rapid adsorption kinetics. The higher of the RH, the time to saturation is longer, and the hydrogel can achieve its 90% water uptake (2.75 g g⁻¹) within 120 min at 70% RH. Figure 3b compares the water uptakes of the SFL, SA, LiCl, SA and LiCl mixture (SL), and SA and FeCl₃ mixture (SF). At 20°C and 50% RH, the SFL hydrogel (co‐loaded with Fe and Li) exhibited the highest water uptake of 1.91 g g⁻¹, almost 2 times of SF. When the water first contacts the hydrogel surface (Figure S6), the contact angle is 54°. Within <0.5 s, the contact angle decreases to 13°. This phenomenon illustrates that when water is on the hydrogel surface, the hydrogel can store the water in its pores within 0.5 s.

FIGURE 3.

Moisture adsorption properties of hydrogels. (a) Water uptake of the SFL hydrogel at different RH. (b) Water uptake of various adsorbents at 20°C and 50% RH. (c) DSC and (d) TGA curves of the hydrous hydrogel. (e) In situ FTIR data of evaporated water. (f) Adsorption isotherms of the hydrogel from 15°C to 35°C. (g) ln P versus 1/T at different water uptakes. (h) Enthalpy of adsorption of the SL, SF, SA, and SFL at different water uptakes. (i) Comparison of the adsorption isotherms of SFL hydrogel and other adsorbents.

To determine the configuration of moisture adsorption, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) tests were conducted. The exothermic crystallization process of LiCl occurs at under 126°C (Figure 3c). After the temperature exceeded 150°C, the hydrogel further desorbed bonded water; no endothermic or exothermic reactions occurred at >180°C [44]. The hydrogel mainly released free water below 100°C, corresponding to the evaporation of physically adsorbed water (Figure 3d). The result indicates that the hydrogel produced a large amount of liquid water during the adsorption process. The evaporation of bound water is occurred >100°C, assigning to the high energy of chemically adsorbed water. The mass of the hydrogel is unchanged at this stage, declaring the moisture in the hydrogel was completely desorbed. The evaporated substances exhibit the broad peaks in the wavenumber ranges of 1500–1600 cm⁻¹ and 3500–3700 cm⁻¹ with in situ TG‐FTIR plots (Figure 3e), confirming the water molecules are evaporated [45]. Peaks gradually appear from 40°C, suggesting the free water inside the hydrogel begins to release. Most of the water is released at 40°C–126°C, with the decreased intensity of mass loss (observed in TGA). The FTIR peak intensity weakens at 126°C, indicating the complete release of free water. At this temperature, LiCl undergoes predominant crystallization, while the bound water of LiCl·H₂O starts to be released above 150°C.

According to the Langmuir adsorption isotherm, the adsorption capacity is expressed as αp/(1+αp), where α represents the interaction force between the adsorbent and water, and p is the water vapor pressure RH [13]. Consequently, the water uptake is proportional to RH, which is exceeding 5.1 g g⁻¹ at 90% RH (Figure 3f). Even in 30% RH, the water uptake is 1.1 g g⁻¹, attesting that the SFL hydrogel can be applied in low‐humidity areas such as arid deserts and high salt plateaus. The water uptake is comparable from 15°C to 35°C, manifesting that the hydrogel is usable within a broad temperature range. To check the adsorption enthalpy of the hydrogel, the relation of temperature and water uptake is concluded (Figure 3g). The adsorption capacity is correlated with vapor pressure, and a dynamic adsorption equilibrium was established for each humidity level at a fixed temperature. Based on the Clausius–Clapeyron equation, the heat released during water adsorption could be determined from its adsorption isotherms. Thus, the adsorption enthalpy (ΔH _{s o r p}) was calculated as the slope of the lnP vs. 1/T curve, which is related to the gas constant R,

$$\ln \frac{P_1}{P_2}=-\frac{\mathrm{\Delta }{H}_{sorp}}{R}·\left(\frac{1}{T_1}-\frac{1}{T_2}\right)$$ (1)

when comparing SA, SL, SF, and SFL, the ΔH _{s o r p} of all samples exceeded 50 kJ mol⁻¹ at low adsorption capacities (<1 g g⁻¹), this value is dropped below 40 kJ mol⁻¹ under >1.5 g g⁻¹ (Figure 3h) [46]. These results certify the chemical adsorption dominated at low adsorption capacities (monolayer coupling)—characterized by the formed high‐bond‐energy bonds. For instance, water molecules coordinate with Li⁺ ions to form stable hydrated ions. In contrast, physisorption dominated at high adsorption capacities. After the initial monolayer adsorption, water molecules continued to be adsorbed via interactions between hydroxyl groups, with van der Waals forces serves as the primary interaction [47]. This type of adsorption featured low binding energy and a multi‐layer structure. When compared with previously reports for AWH (Figure 3i), the water uptake of SFL hydrogel is higher than most of the others in 30%–90% RH [48, 49, 50, 51, 52]. Stability tests demonstrate the hydrogel retained over 90% of its water uptake even after 30 adsorption–desorption cycles (Figure S7).

2.3. AWH Device and Application

Low desorption energy consumption is a key factor in achieving sustainable and cost‐effective water production [53, 54, 55]. To evaluate the desorption energy consumption of the SFL hydrogel, moisture‐saturated hydrogels were conducted at different temperatures (Figure 4a). At 55°C, the hydrogel can only desorb 60% of the adsorbed moisture, the hydrogel will desorb all the adsorbed water under 85°C in 60 min. The hydrogel can be completely dried in 1 h, while silica gel or LiCl can only desorb 75% or 50% of the adsorbed water (Figure S8). The intermolecular interactions within the hydrogel network weaken the strong binding between LiCl and water, reducing the desorption energy consumption. The lower of LiCl content in hydrogel, the water can be released in a shorter time, but the lower of LiCl, the lower of water uptake. Taking into the overall consideration, we choose 18% wt of LiCl for further investigation, which has the highest water uptake and only extends a small amount of time for water releasing. Based on this desorption temperature, solar energy is an appropriate choice. To evaluate the hydrogel's ability to absorb solar light, ultraviolet‐visible (UV–vis) absorption was performed in the wavelength of 200–2500 nm (Figure 4b) [56]. The absorption intensity of the SFL hydrogel without GO is lower than the hydrogel with GO, exhibiting a higher solar light absorption (>90%). When the hydrogel was exposed to one sun illumination, its surface temperature was recorded using a real‐time infrared camera. After 15 min, the surface temperature of the hydrogel reached to 93°C (Figure 4c), enabling complete desorption of water. Under one sun illumination, >75% of the adsorbed moisture could be desorbed within 30 min (Figure S9).

FIGURE 4.

Water releasing properties and water production analysis. (a) Desorption rates of water‐saturated hydrogel at different temperatures, the inset is the adsorption—desorption model. (b) UV–vis–NIR absorption spectrum of the hydrogel with/without GO. (c) A temperature rise curve one sun illumination, and the illustration is an IR image temperature (d) Scheme of the water collection device, showing the process of transforming gaseous water into liquid water. (e) COMSOL simulation of temperature (left) and humidity (right) changes inside the asymmetric heating space. (f) The physical setup of the water collection device, including PV panel, motor, control panel, and hydrogel. (g) The rate of the collected water under different RH conditions at 20°C, inset is the collected water.

Based on the moisture desorption capability of the hydrogel under solar light, an integrated AWH device was designed (Figure 4d). The hydrogel was divided into two zones: one zone for desorption under solar light and the other zone for moisture adsorption in shade. After one adsorption cycle, the positions of the hydrogel zones were switched—saturated hydrogels were rotated to the position under solar light for desorption. Through this zone‐switching, continuous desorption‐adsorption cycles were achieved. The generated water vapor was condensed into droplets upon encountering a cold surface and then collected in a water storage chamber. COMSOL simulations of air flow demonstrate the asymmetric distribution of heat sources, which induced spontaneous air flow from the high‐temperature zone to the low‐temperature zone (Figure 4e) [57]. The internal cavity has a size of 90 mm × 90 mm, where the 45 mm × 5 mm rectangular block in the lower right corner represents the hydrogel, which generates heat under sunlight. Owing to the hydrogel's excellent photothermal properties, its temperature is higher than that of other regions. Heat diffuses from the temperature gradient, and water vapor transferring from the right side of the cavity to the left. For the RH distribution, the humidity in the high‐temperature area is low, while the humidity near the condensation wall (a low‐temperature region) is high. By this integration, most water vapor can be condensed inside the desorption chamber.

The physical prototype of the water harvesting device is shown in Figure 4f, a photovoltaic (PV) panel supplied power to the microprocessor and steering gear. The hydrogels were mounted on a circular tray, which was rotated by the steering gear to switch the working states of the hydrogel (Figure S10). The diameter of the circular adsorption bed is 9 cm, and 4 grilles are installed in the middle to partition the circular adsorption bed. Under sunlight, the shadow of the PV panel creates temperature differences in different zones. The condensation wall, shaded by the PV panel, has a lower temperature than the desorption zone, which is conducive to the condensation of water vapor. At the initial stage of desorption, as the humidity inside the chamber increases, small water droplets gradually form on the condensation wall. This designation can ensure the hydrogels with adsorption and desorption are physically disconnected, which are carried out in separate zones.

Under one sun intensity, the internal humidity of the water collection device rapidly rises to 100% within 5 min to create conditions for condensing water (Figure S11). After the incorporation of hydrogel, the device was performed under different humidity. Due to the larger droplets had not yet formed at the initial time, no water was collected in the first 30 min. As more and more water accumulated on the condensation wall, large droplets gradually formed and flowed down into the water groove, eventually entering the collection chamber along the water collection groove (Figure S12). Higher of the RH, larger of the produced water. When the condition is 70% RH, 8.6 mL of liquid water was collected by use of 2.5 g hydrogel within 7 h (Figure 4g), and even at 40% RH, 3 mL of liquid water can be collected. By calculation, the water production rate is around 4 or 12 L kg_hydrogel ⁻¹ day⁻¹ at 40 or 70% RH in a self‐sustained manner. This device can be used in the temperature of 10°C–30°C, demonstrating the wide application scenarios of this technology (Figure S13). The quality of the collected water complies with the World Health Organization (WHO) drinking water standards [58], including F⁻, Cl⁻, and Fe³⁺. Considering the price of raw materials and devices, the hydrogel‐integrated AWH device enables low‐cost and distributed water production in remote arid areas, and the water production rate is higher than most of others (Table S2).

2.4. Moisture Electricity Generator Performances

When the hydrogel converts gaseous moisture into liquid, the generated water will undergo the ionization reactions as contained oxygenated groups at the surface. After moisture adsorption, hydrogen atoms in carboxyl (─COOH) and hydroxyl (─OH) will dissociate into free positive charges, leading to the formation of an electric double layer (EDL) (Figure 5a) [59, 60]. By constructing an asymmetric moisture‐absorbing structure on the surface of conductive carbon paper, an electric field could be established. Attributed to the large amount of dissociated H⁺ on the surface, part of the H⁺ will diffusion to the dry area. The negative groups are left on the surface, exhibiting a negative potential, while the dry region displays a positive potential due to the H⁺ accumulation (Figure 5b). The asymmetric distribution of the hydrogel on the carbon substrate generates an open‐circuit voltage of 0.55 V (Figure S14). To verify that the electricity generator was driven by humidity distribution differences, voltage measurements were conducted at different positions on the hydrogel and substrate by multimeter probes. We fixed the positive electrode on the carbon paper, while the negative electrode could be moved. A voltage was only generated when the positive and negative probes were placed on the wet and dry surfaces (Figure S15), and the voltage changes are negligible when the position is changed. The current output of the hydrogel without LiCl is only 20 µA. As the LiCl concentration increases, the current rises to 287 µA when the hydrogel thickness is 0.5 mm. As the thickness continues to increase, the current gradually decreases. Further, the hydrogel prepared by 18% wt LiCl has the highest current due to the highest water uptake.

FIGURE 5.

The performance of the electricity generation device. (a) Scheme of the electricity generation device, including hydrogel and carbon paper substrate. (b) Mechanism of the moisture‐based electricity generator. (c) Open circuit voltage and current curves during charging and discharging, inset is the physical setup. (d) CV curves of the electricity generation device at different potential scan rates. (e) Open circuit voltage and short‐circuit current curves under different load resistances. (f) Voltage of multiple electricity generation devices in a series structure. (g) Six devices in series for lighting seven LED bulbs.

To investigate the output voltage, a layer of aluminum (Al) was coated on the hydrogel surface—changing the electrode configuration from “hydrogel‐carbon (C)” to “carbon‐metal”. With C and metal as electrodes, the open‐circuit voltage of the generator could reach over 1.2 V, which is higher than the hydrogel‐C combination (Figure S16). Due to the high hygroscopicity of the SFL hydrogel, it can still generate a voltage of 1 V even under low humidity. The higher of the RH, the voltage will be higher due to the accelerated dissociation of charged carriers in the hydrogel. The current of the inert electrode is smaller than Al may due to: (i) The work function (4.08 eV) of Al is the closest to the electrochemical potential of the main carriers, forming the smallest interface charge transfer barrier; (ii) The thin layer of Al₂O₃ naturally formed on the surface can be partially dissolved by ions under the water environment, the actual contact is the composite interface of Al/Al₂O₃/hydrogel, which reduces the contact resistance instead. This result suggests the hydrophilicity of the hydrogel is crucial for voltage generation. Meanwhile, the output voltages of SA, pure salts, and salt‐loaded hydrogels were compared, among which the salt‐loaded hydrogels exhibited the highest voltage. The 3D network framework of the hydrogel provides stable ion transport channels, ensuring steady voltage. The incorporation of LiCl increased the internal ion concentration in the hydrogel, thereby further enhancing their power generation. Compared with inert electrodes, the Al electrode shows a higher voltage due to the slight electrochemical reaction and a well‐contacted interface. Long‐term testing of the generator illustrates that the short‐circuit current is >200 µA (Figure 5c). When the hydrogel is freshly taken out of the oven, its water content is low, and the output voltage is only 0.3 V. As the hydrogel adsorbs moisture, the open‐circuit voltage increases, reaching 1.2 V after 30 min and maintaining for an extended period. This generator is discharging by conducting the short‐circuit current. A relatively large current was observed at the initial stage of discharge, attributed to the accumulation of internal ions. As the discharge duration increases, the movement of internal ions gradually reaches equilibrium. After stopping the discharge, the open‐circuit voltage recovers with continuous moisture absorption, demonstrating its self‐sustaining capability for continuous electrical output.

The hydrogel‐based electricity generator exhibits a capacitor‐like discharge behavior. After short‐circuit discharge, the open‐circuit voltage is decreased, the generator could spontaneously recharge through the continuous moisture capture, restoring the voltage. Since the hydrogel desorbs water under natural sunlight, the required external energy for recovery is minimal. Cyclic voltammetry curves show no obvious redox peaks from 0 to 1.2 V, indicating no obvious redox reactions occurred at the Al electrode (Figure 5d). The Nyquist plot of the generator displays a semicircle approximating a capacitive arc, which reflects the EDL capacitance effect (Figure S17). This phenomenon is attributed to the asymmetric humidity structure. For the stability tests, the water adsorption is 80 min, and desorption under 85°C with 15 min, shows there is no obvious degradation in the generated open‐circuit voltage, indicating the well stability even after multiples of water adsorption/desorption. When the hydrogel absorbs moisture, the ionization of oxygenated functional groups (e.g., carboxyl and hydroxyl groups) is located at the surface, and the dissociated H⁺ diffusion to carbon paper electrode. The capacitive arc directly corresponds to the charge storage and transfer behavior of EDL, confirming that the electricity generation mechanism is related to the redistribution of charged ions.

To evaluate the output power, different resistance values of loads are connected (Figure 5e). When the load resistance is smaller, the short‐circuit current is higher (250 µA), and the voltage is lower. When the load resistance is larger, the open‐circuit voltage can be enhanced to 1.2 V, but the current is small, resulting in lower power output. The maximum power output is assigning to the load resistance of 10⁴ Ω, reaching to 68.2 µW cm⁻² (Figure S18). When the generator was used to charge capacitors, it could fully charge a 470 µF capacitor in 2 s, proving the rapid power supply capability. Although the open‐circuit voltage of a single generator is relatively low, the output voltage can be increased by series connection. The negative electrode is connected to the carbon paper, and the positive electrode is connected to the other generator unit to form a parallel structure. The series connection of 6 generators yielded a voltage of 7.6 V (Figure 5f). Two power generators are connected in parallel, and three sets of these parallel‐connected power generators are connected in series, enabling the output voltage to exceed 3.5 V (> the turn‐on voltage of the light‐emitting diode) (Figure S19). This combined configuration could directly power seven LED bulbs (Figure 5g), demonstrating the promising prospect of using hydrogels to drive external electricity devices. Compared with most adsorbent‐based power generation systems, the as‐prepared SFL hydrogel exhibits higher water absorption, open‐circuit voltage, and short‐circuit current at 70% RH (Table S3). Relying on the redundant moisture in the air, the porous hydrogel provides a pathway for the supply of distributed green energy (water and electricity) [61, 62, 63].

3. Conclusion

We have developed a hygroscopic hydrogel that can be used both for water production and electricity generation from the air. The porous hydrogel was formed by freeze–drying SA, which not only provides abundant adsorption sites but also offers channels for moisture transfer and storage. The SFL hydrogel exhibits an excellent moisture adsorption performance over a wide humidity range, from 1.1 to 5.1 g g⁻¹ at 30%–90% RH. Its daily water collection capacity is 11.8 L kg_hydrogel ⁻¹ at 70% RH, which meets the daily drinking water demand of a 6‐person household. By leveraging the asymmetric distribution of the hydrogel, a dry–wet interface is formed, creating a stable potential difference of 1.2 V with a metal electrode (7.6 V for 6 units by series connection). By use of this electricity, a series‐parallel combination of LED bulbs can be powered, becoming a multifunctional device at atmospheric presence.

4. Experimental Section

4.1. Preparation of Hydrogels

For the preparation of porous hygroscopic hydrogels, 2 g of sodium alginate was dissolved in 48 mL of deionized water, and the mixture was stirred with a magnetic stirrer for 4 h to ensure complete dissolution of sodium alginate. Next, 25 mL of a graphene oxide aqueous solution with a mass concentration of 10 mg mL⁻¹ was prepared. To achieve uniform dispersion of graphene oxide, an ultrasonic instrument (Elma, Germany) was used for 30 min. After 4 h of stirring, 25 mL of the graphene oxide aqueous solution and 25 mL of deionized water were added to the sodium alginate solution, and stirring was continued for 1 h to ensure uniform dispersion. The prepared sodium alginate mixed solution was freeze–dried to obtain a porous framework. The prepared sodium alginate mixed solution is pre‐frozen at −40°C and then freeze–dried under a pressure of 3 Pa to obtain a porous framework. The freeze–dried hydrogel was dropwise added in a 2% (mass fraction) FeCl₃ solution for cross‐linking. The solution was evenly dropped onto the surface of the freeze–dried hydrogel using a dropper, ensuring that every part of the hydrogel was saturated with the solution. Followed by immersion in an 18% (mass fraction) LiCl solution. Finally, the hydrogel was heated at >80°C for 2 h to obtain an anhydrous hydrogel.

4.2. Water Production

The water production assembly consists of a solar cell, an acrylic frame (10 × 20 × 15 cm³), the microprocessor, a circular adsorption bed, and a steering gear. The hydrogel in the circular adsorption bed is around 2.5 g in 64 cm². The hydrogels were mounted on a circular tray, which was rotated by the steering gear to switch the adsorption and desorption states of the hydrogel. The generated water vapor condenses into liquid water at the condensation end and for collection. At last, the liquid water can be collected for further tests.

4.3. Electricity Generation

The power generation assembly consists of paper coated with conductive carbon paste and the hydrogel. The carbon paper is dried in an oven at 90°C for 20 min. The dried paper is cut into 1 × 2 cm², and the prepared hydrogel was cut into pieces on one side of the paper (1 × 1 cm²). After drying, the power generation test can be carried out for different electrodes. To test the voltage, the positive probe of a multimeter should be placed on the carbon paper area, and the negative probe is located on the hydrogel.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma73439‐sup‐0001‐SuppMat.docx.

Data Availability Statement

All data generated or analyzed during this study are included in this published article and its supplementary information files.