Hydrophobic LDH‐Tailored Exfoliated Graphene Sheet Energy Harvester: Harnessing Pressure‐Driven Heating and Electrolysis for Advanced Real Applications

Abstract

Hydrovoltaic harvesting converts water–solid interactions into electricity, offering a sustainable power route across diverse settings. Yet most systems are hard to scale, suffer evaporation‐limited lifetimes, and lack multifunctionality, limiting real‐world application. To overcome these limitations, a highly optimized multifunctional hydrovoltaic harvester is developed by integrating exfoliated graphene oxide sheet (EGs) and hydrophobic layered double hydroxide (LDH) coatings onto a porous melamine foam scaffold. The porous foam ensures rapid, capillary‐driven water transport, the EGs coating forms continuous conductive pathways, lowering device resistance to 600 Ω and enabling efficient electron transfer, and the hydrophobic LDH layer suppresses evaporation, sustaining stable energy generation for up to 37 h in seawater. This 250 cm³ device delivers 0.016 Wh—more than tenfold higher than previously reported multifunctional systems—demonstrating exceptional scalability. The harvester directly powers a water electrolysis cell, achieving continuous hydrogen evolution without external power. It also exhibits a unique pressure‐responsive thermal sensing capability: mechanical loading (10–50 kPa) modulates internal resistance, producing localized Joule heating (50–70 °C) for self‐powered thermal actuation. Integrates long‐duration hydrovoltaic power, self‐powered electrolysis, and pressure‐induced thermal sensing in one device. Delivers durable, scalable, multifunctional performance for practical off‐grid energy and responsive sensing.

An EGs/hydrophobic‐LDH coated melamine foam harvests hydrovoltaic energy for >37 h in seawater, delivering ≈0.016 Wh from a 250 cm³ device and powering water electrolysis. Mechanical pressure (10–50 kPa) modulates internal resistance to induce rapid, localized Joule heating for thermal signaling. A biphilic architecture couples capillary flow, interfacial conductivity, and evaporation control in a scalable platform.

1. Introduction

The escalating global challenges of water and energy scarcity demand alternative energy solutions that are not only renewable but also scalable, robust, and multifunctional. Although conventional renewable sources such as solar and wind power have helped reduce dependence on fossil fuels, their inherent limitations—including intermittency, geographic dependence, and high installation costs—underscore the need for complementary technologies that can operate across a wider range of environmental conditions.^{[ 1 ]}

Hydrovoltaic energy harvesters, which generate electricity through the interaction of water with charged surfaces or within confined nanochannels, have emerged as a promising class of alternative energy devices.^{[ 2} , ³ , ^{4 ]} Unlike traditional renewables, they operate independently of sunlight or wind, enabling energy harvesting in shaded or enclosed environments.^{[ 5} , ^{6 ]} However, most reported hydrovoltaic devices exhibit limited energy output (typically <0.002 Wh) and short operating durations (<20 h), which restrict practical applicability. Moreover, many systems are single‐function and are not integrated with downstream applications such as hydrogen generation or pressure sensing.

Recent multifunctional approaches have explored cellulose membranes, CNT‐ or MXene‐based composites, and gel electrolytes to achieve partial improvements in output performance.^{[ 7} , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ^{16 ]} For instance, even the highest‐performing hydrovoltaic devices to date have yielded energy outputs around 0.00159 Wh at similar or larger device volumes, often without integration into practical systems. Moreover, pressure sensing platforms typically rely on external power sources and rarely support energy self‐sufficiency, while hydrogen production devices depend on high‐voltage external supplies and are difficult to miniaturize.

To address these limitations, we report a multifunctional hydrovoltaic harvester that integrates EGs and hydrophobic MgAl LDH coatings onto a porous melamine foam scaffold.^{[ 17} , ¹⁸ , ¹⁹ , ^{20 ]} The porous melamine substrate promotes capillary‐driven water transport, the EGs coating enhances in situ conductivity, lowering the device resistance to ≈600 Ω and enabling efficient electron transport, and the hydrophobic LDH layer retards evaporation, extending hydration and maintaining the electric double layer for >37 h. Consequently, the system delivers ≈0.016 Wh from a compact 250 cm³ device, exceeding previously reported multifunctional systems by approximately tenfold in both energy output and operating duration. Moreover, the EGs/LDH harvester serves as the sole power source for a water electrolysis cell, enabling continuous hydrogen evolution without external electricity.

In addition to its exceptional energy‐harvesting capabilities, our EGs/LDH‐coated melamine foam demonstrates a novel and distinct functionality as a pressure‐induced thermal sensor. Specifically, mechanical pressure applied to the foam (ranging from 10 to 50 kPa) directly modulates the internal electrical resistance by compressing the conductive network within the porous structure. This pressure‐induced change in resistance precisely controls the current density, enabling on‐demand localized Joule heating and thus generating an immediate and reversible thermal response. Unlike traditional thermal sensors that typically require external heating elements, our approach uniquely couples mechanical deformation with intrinsic thermal actuation, offering an innovative platform for direct pressure‐to‐temperature sensing and thermal control. This novel platform uniquely integrates hydrovoltaic energy harvesting and pressure‐induced thermal sensing into a single, scalable multifunctional device, effectively bridging advanced material design with real‐world applications.

2. Results and Discussion

2.1. Design, Fabrication, and Characterization of High‐Energy Generator with Hydrophobic Layer

Figure 1a illustrates the design and operating mechanism of the hydrovoltaic energy‐harvesting generator, which integrates a porous melamine foam substrate coated with EGs and a hydrophobic MgAl LDH layer. The melamine foam enables rapid capillary‐driven water flow, induces a streaming potential via the movement of water within the charged porous network. The EGs coating enhances conductivity and facilitates efficient charge transport, while the hydrophobic LDH layer suppresses evaporation, sustaining vertical moisture gradient and extending the duration of energy generation. This synergistic combination allows the device to harvest energy stably over extended periods under ambient conditions. To clarify the fluid and charge‐transport processes in the harvester, a schematic illustration is provided in Figure S1 (Supporting Information), and a real‐time demonstration is shown in Video S1 (Supporting Information). The water is injected at one end of the harvester, and capillary action enables rapid wicking throughout the melamine foam structure. The presence of a hydrophobic LDH layer on the top side retards vertical evaporation, maintaining higher internal humidity and enabling sustained ion transport. Consequently, the moisture gradient and electrochemical potential difference are maintained, allowing continuous streaming current generation. This design allows the harvester to operate stably for extended durations without the need for additional water input or external actuation. Figure 1b illustrates the heat generation mechanism of the hydrovoltaic harvester under applied pressure. The EGs within the melamine foam create conductive pathways, which undergo resistance changes under applied pressure. This phenomenon enables localized heat generation, allowing the device function as a thermal energy sensor. The schematic highlights how mechanical compression reorganizes the conductive network, facilitating enhanced electron transport and Joule heating effects. This pressure‐responsive behavior underscores the multifunctionality of the harvester, extending its applications beyond energy generation to include thermal management and pressure sensing. Figure 1c presents the energy output of the device at a load resistance of 600 Ω as a function of harvester volume. As the volume of the harvester increases, the energy output scales proportionally, with the largest harvester (250 cm³) generating approximately 0.016 Wh of energy. This demonstrates the scalability of the device, enabling precise control of energy output by adjusting the harvester volume and the amount of seawater add. Such scalability highlights the potential for tailoring the device to meet specific energy demands in practical applications. Figure 1d assesses the long‐term stability of the harvester. When activated with a single 20 mL droplet of seawater, the device maintained a stable open‐circuit voltage (Voc) of approximately 0.4 V and a short‐circuit current (Isc) of 1.6 mA for up to 37 h. This extended operational period emphasizes the effectiveness of the hydrophobic LDH layer in controlling evaporation and ensuring consistent performance. The experiments, conducted under room temperature and 50% relative humidity (RH), confirm the robustness of the EGs/LDH‐coated harvester in generating sustained electrical power. These results validate the effectiveness of combining EGs and hydrophobic LDH coatings on melamine foam for hydrovoltaic energy harvesting. The device demonstrates the ability to generate stable power over extended periods and offers scalability for various energy demands. This highlights its practical potential for real‐world applications, such as powering small‐scale systems or supplying electricity to drive processes like water electrolysis for hydrogen production. Figure 1e presents the thermal response of the device under different applied pressures (10, 30, and 50 kPa). The temperature profiles indicate a rapid increase in heat generation immediately after pressure application, followed by a gradual decline as the heat dissipates. The maximum temperature reached scales with the applied pressure, with higher pressures leading to more pronounced temperature elevations. This behavior confirms the strong correlation between mechanical deformation and resistive heating within the EGs‐coated melamine foam structure. The results demonstrate that the harvester can serve as a self‐powered pressure sensor, where variations in resistance and heat output provide valuable information about applied loads. The efficient conversion of mechanical pressure into heat suggests the potential use of the device in self‐powered thermal sensors, smart textiles, and wearable electronics.

Figure 1.

Hydrovoltaic Generator and Thermal Energy Sensor: Structure, Mechanisms, and Performance Overview. a) Energy harvesting design and mechanism. Schematic of the hydrovoltaic generator using melamine foam with EGs and hydrophobic LDH coating. b) schematic diagram of the heat generation mechanism under pressure. c) Scalable energy output. Energy harvested as a function of melamine foam volume under seawater droplet application, demonstrating scalability and efficiency for practical use. d) Long‐term energy stability. Voltage and current profiles under ≈600 Ω resistance with small seawater droplets. e) Pressure‐driven thermal response. Temperature profiles under pressures of 10, 30, and 50 kPa.

2.2. Characterization of EGs and Hydrophobic LDH Layer for Energy Generator

We optimized the energy‐harvesting performance of the EGs‐ and LDH‐coated melamine foam by carefully tuning the morphology and distribution of the LDH layers. As shown in Figures 2 and S2 (Supporting Information), different configurations and coatings of LDH layers were evaluated to understand their influence on the electrical properties and overall energy generation. Figure S2a (Supporting Information) shows the X‐ray photoelectron spectroscopy (XPS) characteristics of carbon in EGs. The O 1s XPS profiles provide useful information on the surface elemental composition and functional groups. The O 1 s spectrum indicated four types of carbon bonds, assigned to O─C, O═C─OH, O─H, and C─O─O─H bonds, in EGs.^{[ 21 ]} Figure S3b (Supporting Information) shows that the EGs were stacked in multiple layers. The Raman spectrum (Figure S3c, Supporting Information) shows the D band (≈1350 cm⁻¹), G band (≈1580 cm⁻¹), and 2D band (≈2700 cm⁻¹) of the EGs. The intensity ratio of the D to G band (I_D/I_G) is 1.02, indicating the presence of structural defects in the EGs. Figure 2a shows the structure and electrical characteristics of the melamine foam coated solely with EGs. SEM images show the morphology of the EGs coating after one and six application cycles, revealing a gradual increase in thickness and surface coverage. The resistance of the EGs‐coated foam decreased significantly with additional coating cycles, starting from 7.5 MΩ with one coating, 217 kΩ with two coatings, and further reducing to 16 kΩ after three coatings. This reduction in resistance is essential for enhancing the overall electrical conductivity and efficiency of energy harvesters. Additionally, 400 µL of water was dropped onto the foam, which spread efficiently through capillary wicking, enhancing the water distribution for energy harvesting. The FT‐IR results in Figure S4 (Supporting Information) illustrate the differences between the hydrophilic and hydrophobic LDH spectra. In Figure S4a (Supporting Information), the hydrophilic LDH spectrum shows broad absorption bands at ≈3500 cm⁻¹ owing to O─H stretching, indicative of hydrogen bonding and water molecules.^{[ 22 ]} Additional peaks are observed for NO₃ ⁻ and Mg─Al─O bonds. Figure S2b (Supporting Information) shows the effects of adding a hydrophilic LDH layer to the EGs‐coated melamine foam. The SEM images show that, as the amount of hydrophilic LDH increases (from 0.05 g to 0.20 g), the layer becomes denser and more uniform, directly influencing the electrical resistance. As the quantity of hydrophilic LDH increases, the resistance increase from 7.5 MΩ to 43 MΩ. This increase suggests improved interfacial contact between the LDH and EGs layers, optimizing water absorption and energy generation. Capillary wicking further aids in evenly distributing water across the foam, which enhances the energy‐harvesting process. To elucidate the charge generation mechanism more rigorously, we refer to the streaming potential model described by Yin et al., in which confined moisture transport through porous channels results in electric double layer (EDL) formation and net voltage generation.^{[ 23 ]} In our system, the EGs/LDH‐coated melamine foam provides a similar ionic pathway where capillary‐driven water flow induces ion displacement and EDL overlap at the solid–liquid interface. Furthermore, Das et al. demonstrated that ion‐interactive surface coatings such as modified graphene structures promote enhanced electrostatic interactions and pseudocapacitance.^{[ 24 ]} The LDH domains in our harvester act similarly by offering selective ion adsorption sites, thus increasing the surface charge density and Coulomb drag effects. Additionally, the biphilic interface structure consisting of hydrophobic EGs and hydrophilic LDH sustains directional moisture flow and prevents back diffusion, as also reported by Qiu et al.^{[ 25 ]} This continuous moisture and ion gradient helps maintain a stable electric output over time, which is essential for long‐term energy harvesting. Figure S4b (Supporting Information) shows the hydrophobic LDH, where the O─H stretching band is less intense, indicating reduced hydrogen bonding. Peaks corresponding to the C═O and R─COOH functional groups^{[ 26 ]} confirm the presence of hydrophobic modifications, suggesting that surface functionalization altered the hydrophobicity of LDH. Figure 2b shows the effect of integrating a small quantity of hydrophobic LDH (0.01 g) onto the EG‐coated melamine foam. The SEM images reveal a rough, textured surface created by the hydrophobic LDH layer, which minimally affected the electrical resistance compared with larger quantities of hydrophobic LDH. Specifically, the increase in resistance was relatively minor, suggesting that even a small amount of hydrophobic LDH was sufficient to achieve the desired water‐repellent properties without significantly increasing the resistance. This hydrophobic layer not only slows water evaporation, but also enables extended energy generation by maintaining a stable potential difference across the electrodes. When 400 µL of water was dropped onto the foam, capillary wicking and diffusion were significantly more pronounced in systems where the hydrophobic LDH was formed on the lower layer, resulting in more effective water spreading and prolonged energy generation.

Figure 2.

Characterization and Droplet evaporation dynamics on the energy of EGs/hydrophobic LDH coated energy harvester. a) Schematic, SEM image and resistance (6 coating (201 Ω) by number of EGs coating cycle. b) Schematic, SEM image and resistance by hydrophobic LDH on EGs coated melamine foam. c) Experimentally measured (solid lines) and numerically calculated (dashed lines) weight change over time for EG (blue)‐ and EGs/hydrophobic LDH (red)‐coated sponges while evaporating a 0.4 mL water droplet. The moisture (vapor phase) contents are expressed as the RH when evaporation occurs, and the liquid water contents are expressed as the liquid fraction over time for both sponges.

To investigate the evaporation dynamics on EGs and hydrophobic LDH porous structures, we conducted numerical simulations using commercial computational fluid dynamics software (COMSOL Multiphysics 6.2, COMSOL Inc., Sweden). The timescales for wicking in the porous medium (${\tau }_{wick}=\frac{\mu L^2}{\gamma r^3}$) and evaporation (${\tau }_{evap}=\frac{{\rho }_l{r}_d^2}{D({\rho }_{v,\infty }-{\rho }_s)}$) were observed to be ${\tau }_{wick}\sim {10}^{1}s$ << ${\tau }_{evap}\sim {10}^{3}s$, indicating that the evaporation of the water droplet is much slower than wicking in the porous structure. Therefore, we first simulated the wicking of a 0.4 mL water droplet into the porous medium for the first hour of the time step, and we simulated evaporation from the initial time step until the droplet completely evaporated (≈36 000 s). A moisture transport model was employed for the evaporation simulation, assuming laminar ambient flow conditions consistent with the quiescent laboratory environment in which the experiments were conducted. Figure 2c and Figure S5 (Supporting Information) show the weight measured over time during the experiments and numerical study after dispensing 0.4 mL of a droplet on an EGs‐ or EGs/hydrophobic‐LDH‐coated melamine foam. The evaporation rate of water did not change until the droplets completely evaporated from the EG‐coated hydrophilic porous medium. This is because the wetting front, where evaporation occurs, remains the same at the top interface between the air and the wetted porous media because of the combined effects of evaporation and capillary wicking. This phenomenon can be understood by analogy with transpiration on plant leaves, where evaporation at the surface drives capillary wicking in hydrophilic channels. The capillary wicking and evaporation occurring in the upper part of the structure are depicted in the time‐lapse images showing the RH in the vapor phase and the liquid water phase fraction from the numerical simulation. The changes in mass and visual representations obtained from the numerical study were consistent with the experimental results and theoretical analyses of wicking and evaporation. In contrast, for a droplet deposited on the EGs/LDH‐coated melamine foam, the water infiltrating the hydrophobic layer underneath was not fully wicked into the upper layer, unlike the behavior on the EGs‐coated melamine foam. The initial decreases in mass were similar until the liquid water in the upper layer completely evaporated from both the EGs‐ and EGs/LDH‐coated melamine foam. Once the droplets in the upper layer were depleted, the evaporation front receded into a porous layer. The receding evaporation front further delays the diffusion of the vapor phase and evaporation because the newly evaporated water vapor must diffuse through the hydrophilic upper layer. The increased RH in the porous media owing to attenuated diffusion slowed the overall evaporation rate of the sponge. Therefore, once the wicked water in the hydrophilic layer completely evaporated, the evaporation rate decreased, as observed in the experimental data and the time‐lapse images obtained from the numerical study. Based on this understanding, we adopted a biphilic porous structure to delay evaporation and extend the energy‐harvesting period of the EGs‐coated energy harvester.

2.3. Tuning the Resistance in EG‐Coated Energy Harvesters

In Figure 3a, as the resistance of the device increases, the generated voltage increases, reaching 0.6 V. Conversely, Figure 3b shows that, as the resistance increases, the current decreases. This is consistent with Ohm's law,^{[ 27 ]} which indicates that, for a given voltage, an increase in resistance leads to a proportional decrease in current. The ability of the device to generate current is inversely related to the resistance, at lower resistances, the device can sustain higher current levels, whereas higher resistances limit the current flow. Figure 3c confirms these observations by plotting the voltage and current against varying resistance values. The highest voltage of 0.6 V occurs at the highest resistance, whereas the highest current of approximately 250 µA is observed at the lowest resistance of 201 Ω. This inverse relationship is a direct consequence of Ohm's law, illustrating how changes in the resistance can significantly influence the electrical output characteristics of the device. The energy and power output results are shown in Figure 3d and Figure S6 (Supporting Information) The highest energy output occurs at the lowest resistance (201 Ω), which can be understood by considering the power generated by the device. Although a lower resistance results in a lower voltage, it also allows for a higher current, which in turn leads to greater power generation and, consequently, higher energy and power output. Thus, the observed energy optimization at lower resistance arises from the effective balance between voltage and current, leading to enhanced power efficiency. Figure 3e,f, and Figure S7 (Supporting Information) further demonstrate the impact of connecting multiple energy‐harvesting devices in series and parallel configurations, specifically highlighting how electrical output can be scaled by modifying the configuration of the 201 Ω harvester. Initially, five EGs‐coated samples (7.5 MΩ each) were connected in a series–parallel configuration, yielding a combined output of ≈ 2.5 V and 3.48 µA. Subsequently, increasing the coating cycles reduced the individual resistance to 201 Ω. When six of these lower‐resistance harvesters were connected, the overall resistance of the system decreased significantly from approximately 17 MΩ to 250 Ω. This reduction in resistance led to a substantial increase in power output, with the system generating approximately 100 µW, which is more than 10 times higher than the power produced by the initial 7.5 MΩ configuration. In Figure 3g–i, the characteristics of the energy harvester are examined with respect to the device size. Figure 3g schematizes the volume configurations tested. The volumes were increased progressively from 5 cm³ to 15 cm³, and finally to 50 cm³, to observe the corresponding effects on the energy output. The results indicated an evident trend in energy generation as the volume of the harvester increased. Specifically, the energy outputs measured for the different volumes were approximately 34 uWh for 5 cm³, 38 uWh for 15 cm³, and 48 uWh for 50 cm³. These values suggest that larger harvester sizes led to higher energy output. Furthermore, when the energy output was plotted against the volume, a strong linear relationship was observed, as shown in Figure 3i and Figure S8 (Supporting Information). The linear fit resulted in an R² value of approximately 0.9908, indicating a high degree of correlation and suggesting that the energy performance of the harvester can be reliably predicted based on its size. Figure S9 (Supporting Information) shows the repeatable V_OC behavior of the EG‐coated melamine foam in a 3.5 wt% NaCl solution at 60% RH. Over 10 cycles, the voltage remained constant, maintaining an average V_OC of approximately 0.6 V. Although minor fluctuations were observed in each cycle, the overall voltage stability across repeated tests demonstrated the robustness and reliability of the harvester.

Figure 3.

Tuning the resistance of EGs coated energy harvesting performance. a) Behavior of (a) Voltage and b) Current, c) Plot of voltage and current, d) Energy of EGs on melamine foam at dropping 400 µL at NaCl 3.5 wt% solution different coating cycle (resistance) at RH 40%. e) Voltage and current of EGs on melamine foam at 6 coating (each 201 Ω, 6 coat.), inset of schematic of serial parallel system. f) Power and resistance, inset of snap shot of serial parallel system under 400 µL of NaCl 3.5 wt% solution. g) Schematic of larger volume size of EGs coated melamine foam. h) behavior of voltage i) linearity of energy by EGs on melamine foam with different volume size (5cm³;7.5 MΩ, 15cm³;280 kΩ and 50cm³;500 kΩ) and NaCl 3.5wt% solution.

2.4. Enhanced Energy and Extended Harvesting time: The Effect of Hydrophobic LDH

In Figure S10a (Supporting Information), the effect of increasing the amount of hydrophilic LDH from 0.05 g to 0.20 g on the V_OC is demonstrated. The device with 0.10 g of hydrophilic LDH exhibited the highest voltage output, reaching approximately 0.7 V, due to enhanced water retention and improved capillary wicking dynamics that sustained the streaming potential over time. Conversely, the I_SC, shown in Figure S10b (Supporting Information), was highest at 0.05 g of LDH loading, where minimal internal resistance allowed for more efficient ion transport. These findings indicate that while moderate LDH loading improves voltage output by stabilizing the hydration gradient, excessive coating (e.g., 0.2 g) can increase structural resistance and impede fluid transport, ultimately reducing ionic mobility. Figure S10c (Supporting Information) further supports this interpretation by showing that the device with 0.05 g of LDH produced the highest energy output (≈2.2 nWh), suggesting an optimal balance between capillary‐driven flow and electrical conductivity. In contrast, the 0.2 g LDH sample, despite having higher hydrophilic mass, exhibited reduced energy output due to hindered absorption and ion diffusion.

In Figure 4a–c, the introduction of a hydrophobic LDH layer (0.01 g) forms an evaporation‐suppressing layer beneath the EGs, which significantly affects the performance of the device. Specifically, in Figure 4a, the device generates a voltage of approximately 0.45 V and a current of approximately 100 µA, and these outputs are sustained for approximately 10 h. This extended duration of energy generation is directly attributed to the ability of the hydrophobic LDH layer to control the evaporation rate, ensuring a more consistent moisture level within the device over time. Moreover, when comparing the performance of the device before and after forming the hydrophobic layer at similar resistance levels, the power output increased by approximately 1.5 times, reaching approximately 40 µW (Figure 4c). This comparison highlights that the inclusion of the hydrophobic LDH not only extends the operational time but also enhances the power output, demonstrating the dual benefits of this layer in both improving the efficiency and prolonging the duration of energy harvesting. Figure S11 (Supporting Information) shows the thermal imaging camera analysis comparing the water wicking behavior in EGs and EG/hydrophobic LDH systems over a period of 5 h with 400 µL of seawater. The images reveal a distinct difference in the water transport between the two configurations. In the case of EGs, the wicking process was less pronounced, with slower and more limited water migration toward the right side. This suggests that water was not efficiently transported across the EG surface, potentially limiting the amount of water available for evaporation‐driven energy generation. In contrast, the EG/phobic LDH system showed a more substantial wicking effect, with water migrating further to the right and accumulating more efficiently over time. This enhanced wicking behavior is likely due to the influence of the hydrophobic LDH layer underneath it, which controls the movement and accumulation of water. Figure S12 (Supporting Information) show the relationship between the salt concentration in the electrolyte solution and the electrical performance of the EG‐coated energy harvester. As the NaCl concentration increased, both the V_OC and the I_SC increased.^{[ 28 ]} Specifically, at higher concentrations, such as 5.0 M NaCl, the highest voltage and current levels were observed. This increase in the electrical output at higher ionic concentrations can be attributed to the enhanced availability of charge carriers in the solution, which facilitates more efficient ion transport and charge separation at the electrode interfaces. The presence of more ions in a higher‐concentration solution leads to increased ionic conductivity, reducing the internal resistance of the electrolyte, and thus enabling a greater flow of current and a higher potential difference across the harvester. In Figure 4d, when using CaCl₂, the device exhibits sustained voltage output, maintaining its energy‐harvesting characteristics even after 140 h of operation. This long‐term stability is notable compared with that of other salts, such as KCl and LiCl, which show a more rapid decline in performance over time. CaCl₂ owing to its strong hygroscopic nature, can absorb moisture from the surrounding air even under relatively low humidity conditions.^{[ 29 ]} This property facilitates continuous water uptake into the porous structure through capillary wicking, enabling sustained ionic movement and stable charge transfer within the hydrovoltaic system. CaCl₂, a divalent salt, provides a high charge density per molecule, which enhances the conductivity of the solution and supports continuous energy generation without significant degradation. Figure 4e shows a schematic of the different configurations of EG‐coated melamine foam harvesters, with volumes ranging from 5 cm³ to 250 cm³ and the corresponding resistances ranging from 201 Ω to 800 Ω. Figure 4e and Figure S13 (Supporting Information) show the V_OC behavior over time across various volume configurations. As the volume increased, the voltage output also increased. For instance, the 5 cm³ device generated approximately 0.1 V and 200 µA, whereas the 250 cm³ device produced a significantly higher voltage of approximately 0.4 V and a current of 1200 µA. This substantial increase in both the voltage and current with larger volumes demonstrates the enhanced energy‐harvesting potential of larger devices, highlighting the benefits of increased volume on the electrical performance. It shows an evident trend of increasing energy output with larger harvester volumes, with the 5 cm³ device generating approximately 0.00017 Wh, whereas the 250 cm³ device produced approximately 0.016 Wh—over 95 times more energy. This finding emphasizes the scalability of the system, demonstrating that larger harvesters can directly contribute to a significantly higher energy production. When harvesters were connected in series (as shown in Figure 4f–h), the voltage output significantly increased, reaching approximately 1.5 V with four devices. However, when the harvesters were connected in parallel (as demonstrated in Figure 4f), the current output was notably enhanced, reaching approximately 4 mA with the same four‐device setup. A comparison between the single‐device and four‐device configurations showed that the power output increased substantially, with the four‐device setup generating approximately 12 times more power. The inset of Figure 4h visually reinforces the scalability potential of the system, demonstrating how more devices can be integrated to achieve higher performance levels. This figure shows the repeatability of the energy‐harvesting process across four consecutive cycles (Figure S13d, Supporting Information). The V_OC was measured during each cycle to assess the stability of the harvester over four repeated uses. The results showed that the device consistently generated a voltage of approximately 0.4–6 V during each cycle. This indicates that the harvester reliably maintained its performance across multiple cycles, confirming its durability and potential for long‐term operation. Figure 4i and Table 1 compare the energy output and operational time of the current harvester with those reported in recent literature.^{[ 7} , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ^{16 ]} The 3D plot shows that the EG‐coated harvester with hydrophobic LDH integration and multidevice scaling generates a significantly higher energy output and operates for a longer duration than the other devices. Specifically, this study achieved an energy output of up to 0.016 Wh and maintained operation for an extended period, significantly outperforming other systems that typically generate lower energy outputs and have shorter operational times. This comparison underscores the superior performance of the proposed design, highlighting its potential for establishing a new standard in the field of hydrovoltaic energy harvesting by combining high efficiency with long‐term stability.

Figure 4.

Effect of hydrophobic LDH with EGs coated energy harvesting performance. Behavior of a) Voltage and b) Current, c) Calculated power of EGs with hydrophobic LDH coated on melamine foam under 3.5 wt% of seawater 400 uL. Behavior of d) 3.3 M of KCl, LiCl and CaCl₂ 400 µL solution at RH 40% of EGs/0.1 g LDH on melamine foam of 20MΩ. e) energy by EGs on melamine foam with different volume under seawater solution. Behavior of f) voltage and g) current, h) power, inset of snap shot by 1 and 4 devices of EGs on melamine foam at volume size 250 cm³;400 Ω under seawater solution at RH 40%. i) Recently reported literature survey compared to our work.

Table 1.

Literature survey of hydrovoltaic generators.

Energy (Wh)	Energy generation time[min]	Voltage [V]	Current [mA]	Dropping solution	Relative humidity [%]	Materials or structural design	Reference
0.0168	2 100	0.40	1.20	Seawater	50	EGs/hydrophobic LDH	This study
0.00063	167	0.47	0.48	CaCl2	25	cellulose sponge‐based hydrovoltaic power generator	[7]
0.00144	150	0.66	0.87	FeCl3	20	CNTs/CB/PVDF@CC//Al device	[8]
1.14X10−5	13	0.33	0.16	NaCl	–	MXene/Sepiolite‐based generator	[9]
3.59 × 10−⁵	167	0.43	0.03	NaCl	55	o‐MWCNTS/o‐CB@MS devices	[10]
0.000108	270	0.60	0.04	Deionized water	25	3D hierarchical TiO2 nanoflower (3D HTNF)‐based WDHPGs	[11]
1.67 × −10−5	167	0.60	0.01	NaCl	34	fluorine‐incorporated Mg‐Al hydrotalcite nanosized clay	[12]
5.36X10−6	67	0.12	0.04	NaCl	78	Titanium dioxide nanotubes coated with polypyrrole (PPy@TiO2 NTs)	[13]
0.00159	27	0.78	4.54	Seawater	70	bilayer hydrogel electricity generator	[14]
3.85X10−5	20	0.77	0.15	CaCl2	40	Delignified balsa wood cellulosic generators	[15]
0.00026	400	0.55	0.07	Deionized water	32	LM/C@CC	[16]

2.5. Mechanical Durability and Real Application in Electrolysis

As shown Figure 5a, the EGs/LDH‐coated melamine foam harvester exhibits excellent mechanical robustness and diverse energy output capabilities under various deformation modes, including twisting, bending, folding, and rolling. As shown in Figure 5b, the voltage and current generated during these mechanical tests were consistent, with rolling producing the highest values of approximately 0.25 V and 300 µA, resulting in an energy output of ≈0.0003 Wh. Folding and twisting followed with slightly lower outputs, while bending generated the least energy. These results confirm the harvester's mechanical flexibility and its ability to reliably generate energy across a range of deformations, making it well‐suited for dynamic and variable operational environments. Additionally, the practical applicability of the harvester was validated by integrating it into a water electrolysis system. Over a 1 h electrolysis experiment, the hydrogen concentration steadily increased from approximately 14 ppm to 92 ppm, as shown in Figure 5c,d and Figures S14 and S15 (Supporting Information). This demonstrates that the harvester provided stable electrical energy to sustain the electrolysis process continuously. For this application, the energy harvester was used as the sole power source for the electrolysis cell without mechanical deformation, relying purely on energy generated through capillary wicking. The stability of the hydrogen production during the experiment confirms the effectiveness of the harvester in powering electrolysis, showcasing its capability as a reliable and consistent energy source for sustainable industrial applications. To further validate this, we compared the theoretical hydrogen output calculated using Faraday's law (2.96 µL over 60 minutes at 7.1 µA) with the experimentally measured hydrogen concentration change in a sealed 50 mL chamber (14 ppm to 92 ppm, equivalent to 3.9 µL). The close agreement between these values corresponds to an estimated Faradaic efficiency of approximately 80%, demonstrating effective charge utilization and confirming the system's electrochemical compatibility for direct water splitting. These findings highlight the dual advantages of the harvester's mechanical resilience and energy generation efficiency, as well as its potential for enabling clean hydrogen production.

Figure 5.

Energy harvesting and hydrogen generation using EG‐coated melamine foam for real application. a) Photographs of the harvester under various mechanical deformations: twisting, bending, folding, and rolling, mechanical robustness. b) Electrical output performance (voltage, current, and energy) under different deformation modes, stable energy generation. c) Application of the harvester in water electrolysis for hydrogen production. The experimental setup includes a multimeter, hydrogen gas detector, and electrolysis cell. The graph shows hydrogen generation (ppm) over time. d) Real‐time monitoring of hydrogen production during electrolysis over 60 minutes, demonstrating stable and sustained performance.

2.6. Pressure‐Driven Heat Generation and Sensing Stability

The EGs‐coated melamine foam exhibits reliable heat generation and pressure sensing capabilities, as demonstrated in Figure 6 . Figure 6a presents infrared (IR) thermal images under time‐resolved loading, highlighting the evolving heat distribution across the foam. The thermal response is highly reproducible and increases monotonically with applied pressure, indicating the foam's ability to efficiently convert mechanical stress into thermal energy. This behavior underscores the suitability of the material for thermal management under variable mechanical loads. Figure 6b illustrates the temperature profiles corresponding to pressures of 10 kPa, 30 kPa, and 50 kPa. The generated heat ranged from approximately 50 °C under 10 kPa to over 70 °C under 50 kPa, emphasizing the tunable nature of the foam's thermal output based on the applied pressure. Inset thermal images confirm near‐uniform heat distribution during sensing, supporting consistent device performance. Furthermore, the behavior of current and voltage change under applied pressures was monitored, showing a strong‐correlation with the temperature rise (Figure S16 and Video S2, Supporting Information). In this system, the primary heating element is the graphene network embedded in the foam, which generates heat via the Joule heating effect. According to Joule's first law;

$$\mathrm{P}={\mathrm{I}}^2·\mathrm{R}$$ (1)

where P is the Joule heating power, I is the current, and R is the resistance of proposed foam material. The decrease of volume of the foam leads to the reducing inter‐sheet distance between graphene sheets and increasing conductive pathway under applied pressure, thereby reducing the resistance and promoting current flow. This behavior was supported by experimental data showing a monotonic increase in current with pressure, highlighting the pressure‐induced reinforcement of the conductive network and its role in enhancing the electrothermal performance. Moreover, the enhanced current flow under reduced resistance leads to a quadratic increase in power further supporting the observed pressure‐enhanced thermal generation. To evaluate the thermal response stability of the foam material under repeated mechanical loading, we monitored the temperature changes using an infrared thermal camera during cyclic pressure application and release. The resulting temperature profiles over multiple cycles (Figure S17, Supporting Information) demonstrated consistent heating and cooling behavior, confirming both the reproducibility and reliability of the pressure‐induced thermal switching effect. These results highlight the potential of the foam for use in responsive and durable electrothermal applications. Capacitance measurements under repeated pressurization cycles (Figure 6c) reveal the foam's stability and reproducibility as a pressure sensor. Different EG concentrations (0.1 wt%, 1 wt%, and 2 wt%) were tested, and all samples showed minimal variation in capacitance across cycles, indicating reliable sensor performance. The foam's consistent response is further demonstrated in Figure 6d, where capacitance changes are tracked over a wide pressure range (8 kPa to 1942.69 kPa). The foam maintains steady performance, even under high pressures, highlighting its mechanical durability. Finally, Figure 6e shows the normalized capacitance response (Ca/C₀) across different pressures, which remained stable and linear over time. This reliable performance under variable pressures confirms the foam's precision and repeatability as a pressure sensor. Overall, these results underscore the foam's ability to generate heat energy with a tunable output (50 °C to 70 °C) during pressure sensing and its robustness in maintaining stable capacitance changes across multiple cycles and a wide range of pressures. These findings position the EG‐coated melamine foam as a highly efficient and reliable material for multifunctional energy harvesting, sensing, and thermal management applications.

Figure 6.

Heating energy‐driven pressure sensors based on melamine foam coated with EGs. a) Infrared images showing the heat distribution over time for EG‐coated melamine foam under different pressures. b) Temperature profiles of the EG‐coated melamine foam at different applied pressures (10 kPa, 30 kPa, 50 kPa) over time. Insets show infrared images corresponding to each pressure. c) Capacitance and force signals of the sensor under repeated pressurization cycles for different EG concentrations (0.1 wt%, 1 wt%, 2 wt%). d) Capacitance changes at various pressures (8 kPa to 1942.69 kPa) under cyclic loading, demonstrating sensor performance across a wide pressure range. e) Capacitance response (Ca/C₀) plotted over time for different pressures.

3. Conclusion

This study introduces a highly optimized, multifunctional EGs/LDH‐coated melamine‐foam hydrovoltaic harvester designed for efficient energy harvesting, thermal management, and pressure sensing. By integrating EGs and hydrophobic LDH, the harvester lowers internal resistance and suppresses evaporation, enabling prolonged energy generation (≈0.016 Wh) with stable performance for up to 37 h. The optimized architecture also exhibits exceptional mechanical robustness, maintaining energy output under twisting, bending, folding, and rolling deformations. The practical applicability of this harvester was demonstrated by directly powering a water electrolysis system, where it consistently supported hydrogen production, increasing hydrogen concentration from ≈ 10 ppm to ≈ 92 ppm over 1 h. Additionally, the harvester provides pressure‐tunable heating (50–70 °C) under varying loads while maintaining stable capacitance across a wide pressure range (8–1942.69 kPa). Collectively, these results validate the device's versatility and reliability, positioning it as a promising platform for clean‐energy generation and advanced sensing applications.

4. Experimental Section

Methods—Materials

All the reagents were used without further purification or treatment. The reagents used in this study were graphite foil (0.5 mm, 99.8% and Alfa Aesar), platinum foil (0.5 mm, 99.99%, Sigma‐Aldrich), ammonium sulfate ((NH₄)₂SO₄, 99.0%, Sigma‐Aldrich). magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, ≥99.0%, Sigma‐Aldrich), aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O, ≥98.0%, ACS reagent, Sigma‐Aldrich), sodium hydroxide (NaOH, ≥98.0%, Daejung), and stearic acid (≥97%, Sigma‐Aldrich).

Synthesis of Exfoliated Graphene Oxide Sheet

EGs sheets were synthesized via electrochemical exfoliation. The graphite and platinum foil were placed in a 0.1 M ammonium sulfate solution with a positive DC potential of 10 V applied to the graphite electrode. The EG was collected and rinsed several times with deionized water. The as‐obtained sample was dispersed in deionized water by bath sonication, centrifuged at 10,000 rpm for 5 min, and finally placed in a 50 mL ethanol solution. The supernatant dispersion was carefully separated, and the graphene content in the solution was fixed at 1.6 wt% for further processing.

Synthesis of Hydrophilic and Hydrophobic Layered Double Hydroxides

First, hydrophilic MgAl LDH was synthesized via a coprecipitation method. Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O) and aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) were used as the metal precursors. A solution containing 0.1 mol of magnesium nitrate and 0.05 mol of aluminum nitrate was prepared in 100 mL of deionized water, resulting in a Mg²⁺ to Al³⁺ molar ratio of 2:1. The metal nitrate solution was slowly added to deionized water (200 mL) under continuous stirring. Concurrently, a 2 M NaOH solution was prepared and added dropwise to the metal nitrate solution until the pH reached 10, ensuring slow addition to prevent localized high‐pH conditions that could lead to premature precipitation. Hydrophobic MgAl LDH was synthesized by modifying the surface of hydrophilic MgAl LDH using stearic acid. In this case, the reaction temperature was controlled at 80 °C, and 100 g of stearic acid was added. In each case, the mixtures were allowed to cool slowly to the ambient temperature. The solids were recovered via centrifugation and washed once with distilled water, four times with ethanol, and once with acetone.

Fabrication of the EG/LDH‐Coated Melamine Foam Harvester

The melamine foam was first immersed in an EGs dispersion and subsequently dried in an oven at 60 °C for 3 hours to form a conductive network. This dipping and drying process was repeated under identical conditions to ensure consistency across coating cycles. As the number of EGs coating cycles increased, the electrical resistance of the foam decreased significantly, indicating improved surface coverage and conductivity. Specifically, the resistance values measured after each coating cycle were as follows: 7.5 MΩ after one cycle, 217 kΩ after two cycles, 16 kΩ after three cycles, 2 kΩ after four cycles, 432 Ω after five cycles, and 201 Ω after six cycles. The substantial drop in resistance saturated beyond the fifth coating cycle, suggesting that the conductive pathways were sufficiently formed. Following the EGs coating, the entire foam was immersed in a hydrophilic MgAl LDH solution and dried at 60 °C for 3 hours. Last, for the hydrophobic LDH layer, only the bottom portion (≈2 cm) of the foam was selectively dipped into the hydrophobic LDH solution and similarly dried at 60 °C for 3 hours.

Hydrogen Evolution via Water Electrolysis

The practical applicability of the EGs coated melamine foam harvester was demonstrated by integrating it into a water electrolysis system to generate hydrogen. The electrolysis setup consisted of a sealed cell containing two platinum electrodes (10 × 10 × 0.1mm, PTFE, Wizmac) submerged in a 1 M KOH electrolyte solution. To activate the harvester, 200 µL of 3.5 wt% NaCl solution (seawater) 3.3M CaCl₂ 200 µL was droplet onto the foam surface. The electrical output of the harvester, including voltage and current, was continuously monitored using a multimeter. The hydrogen gas generated during electrolysis was measured over a 60‐minute period using a hydrogen gas detector.

Pressure Sensing and Heat Generation

The multifunctional capabilities of the EGs/LDH‐coated melamine foam were evaluated through its application as a pressure sensor with heat generation capabilities. The foam was placed on a thermally insulated platform to minimize heat loss, and controlled pressures ranging from 10 kPa to 50 kPa were applied using a force gauge. Heat distribution and intensity were captured using an infrared thermal imaging camera, while capacitance changes were measured with an LCR meter to assess pressure sensitivity. For each applied pressure, the foam's thermal response was recorded at 5‐second intervals, and infrared images were used to track the spatial distribution of heat over time. Capacitance was measured during cyclic loading and unloading to evaluate the sensor's stability and reproducibility. To study the effect of material composition, foam samples with varying EG concentrations (0.1 wt%, 1 wt%, and 2 wt%) were tested under identical conditions.

Characterization

High‐resolution XPS was performed at the Korea Basic Science Institute in Daejeon, Korea. The sample morphologies were characterized using SEM (S‐4800, Hitachi Ltd., Japan) equipped with an energy‐dispersive X‐ray spectroscopy (EDS) detector. The solid sample was characterized via FT‐IR (Nicolet 380, Thermo Fisher Scientific) in the range of 4000 cm⁻¹ to 500 cm⁻¹ with a resolution of 4 cm⁻¹. All the experiments were conducted under controlled conditions, with the room temperature and RH maintained at approximately 25 °C and 50%, respectively. Following the addition of seawater solution to an electrode to initiate power generation, the V_oc and I_sc were measured using a source measurement unit (Keithley 2400, Cleveland, OH, USA). The measurement apparatus was controlled using a PC software (I. V. Solution, Seoul, Republic of Korea). All the energy‐harvesting experiments were conducted at room temperature and an RH of 50%. The solution was dropped onto the device 30 s after the start of each test, and the energy generation was measured. The energy values were calculated by the power over time, using the measured open‐circuit voltage (V) and short‐circuit current (I) data. Specifically, the energy (E) was determined by E = V*I*t.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

J.Y.P. performed conceptualization, wrote, review and edited the original draft, methodology, formal analysis, investigation, data curation. S.‐H.L. performed formal analysis, investigation, and data curation. H.‐R.L. performed formal analysis, investigation, and data curation. S.N. performed resources and software. J.O. performed software and data curation. H.K. performed formal analysis, data curation. D.‐W.J. performed resources and validation. Y.‐H.C. performed supervision, project administration, and funding acquisition. All the authors discussed the results and commented on the manuscript.

Supporting information

Supporting Information

Supplemental Video 1

Supplemental Video 2

Park J. Y., Lee S.‐H., Lim H.‐R., et al. “Hydrophobic LDH‐Tailored Exfoliated Graphene Sheet Energy Harvester: Harnessing Pressure‐Driven Heating and Electrolysis for Advanced Real Applications.” Small 21, no. 41 (2025): e07492. 10.1002/smll.202507492

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.