A fibrous hydroelectric generator derived from eco-friendly sodium alginate for low-grade energy harvesting

Abstract

With the development of renewable energy technologies, the recovery and utilization of low-grade energy based on hydroelectric effect have drawn much attention owing to its environmental friendliness. Herein, a novel hydroelectric generator utilizing sodium alginate-graphene oxide (SA-GO) fibers is proposed, which is eco-friendly and low-cost. These fibers with a length of 5 cm and a diameter of 0.15 mm can generate an open circuit voltage (V_oc) of approximately 0.25 V and a short circuit current (I_sc) of 4 μA. By connecting SA-GO fibers in either series or parallel, this combination can power some electronic devices. Furthermore, these fibers enable the recovery of low-grade energy from the atmosphere or around the human body. Both experimental and theoretical analysis confirm that the directional flow of protons driven by water molecules is the main mechanism for power generation of SA-GO fibers. This study not only presents a simple energy transformation method that is expected to be applied to our daily life, but also provides a novel idea for the design of humidity electricity-generation devices.

Graphical abstract

Keywords

fibrous hydroelectric generator / sodium alginate (SA) / graphene oxide (GO) / power generation

1 Introduction

Since the 21st century, energy and environmental issues have become bottlenecks restricting the progress of the global economy and society [1,2]. To solve the problems of fossil fuel depletion and environmental pollution, new and renewable energy technologies have become the guidance of technological development. For example, solar energy, wind energy, water energy, and nuclear energy have been successfully applied in our daily life [3–6]. However, there are many types of irregular and ubiquitous low-grade energies in the daily life, such as microfluidic potential energy, human motion energy, and mechanical vibration energy [7–10]. In addition, the Internet of Things (IoTs) technology has been developing rapidly, in which portable and wearable electronic products are an essential part of our lives [11–14]. However, the bulky and rigid power generators limit their development. A power source with miniaturization, integration, sustainability, and portability is expected for energy supply in IoT. In this context, the concept of self-powered system has been proposed for such electronic products [15]. With the continuous progress of electronic science and technology, various environmental energy harvesting approaches have been developed, such as vibration-driven electromagnetic energy collectors, piezoelectric energy harvesters, electrostatic vibration energy collectors, and friction nano generators [16–19]. These low-grade collection generators have great application prospects, such as wearable electronics, robotics, and environmental monitoring systems [20–22].

In addition to the abovementioned forms of micropower, the potential energy in microflows is also considered as a valuable source for driving electricity generation. In recent years, utilizing electricity from water molecules migration behavior to design moisture-triggered electric generators has become a novel option [23,24]. For example, the liquid gating energy conversion system, which exhibits discontinuous control behavior, has been demonstrated to work in wearable fall-alert devices [25]. However, there are still many problems and deficiencies in the current research on humidity power generation based on dynamic electron action, such as the low volume (or area) energy density of power generation materials, difficult degradation of humidity power generation materials and environmental pollution [26]. Besides, methods for collecting and utilizing energy while harvesting water from the environment are less developed. Compared with the high energy conversion efficiency of other kinds of low-grade energy sources, the efficiency of humidity electricity generation is relatively low. For instance, a well-aligned single-walled carbon nanotube rope is able to generate electricity when dropping water, with an energy conversion efficiency of only 11.6% [27]. Yang et al. [28] showed that the energy conversion efficiency of graphene-based hygroelectric generators (GHEGs) reaches 52%. Previous studies have revealed that certain carbon nanomaterials can generate electricity through coupling with ions in water droplets or other liquids [29–32]. Under certain humidity conditions, a raw poly (4-styrenesulfonic acid) (PSSA) film (1 cm × 1 cm) can provide an open circuit voltage (V_oc) of up to 0.8 V and a high current density of 0.1 mA/cm² [33]. An annealed multiwalled carbon nanotubes (CNTs) film (1 cm × 2.5 cm) can produce an induced voltage of 1 V and a short-circuit current of 150 nA [34]. However, most of these carbon-based humidity power generators are in the form of films or sheets [35,36]. Due to the low ductility and unformed structure, films or sheets cannot be applied well to wearable devices. In contrast, fiber materials are able to be woven, allowing them to be applied in different scenarios [37]. Li et al. [38] successfully synthesized stretchable and scalable thermoelectric fibers by a wet-spinning process, which could be used to harvest low-grade, wearable body heat energy.

Based on previous studies, it is known that the fiber for hydroelectric power generation should possess the ability to form capillary channels and carry a significant number of functional groups such as −COOH and −OH, which can ionize H⁺ in solution. Graphene oxide (GO) and CNTs also possess these functional groups, as well as other oxygen-containing functional groups to generate substantial moving conductive ions [39,40]. Interestingly, the strong water absorption of GO would cause H⁺ delocalization, which can trigger spontaneous diffusion and ion gradient, allowing the humidity gradient to be maintained over time to ensure a continuous output of electrical energy [41]. However, the microchannels of porous materials are normally tortuous, which are not conducive to electrical output [42]. Hence, biomass materials have been widely considered because of biocompatibility and biodegradability [43,44]. Cellulose nanofiber (CNF) has several advantageous properties, including large specific surface area, excellent mechanical flexibility, good chemical stability and environmental friendliness, and interlaced fibers [45]. It is easy to form a porous structure for ion and electron transport. Hydrophilic functional groups (hydroxyl and carboxyl groups) are attached to the surface of the fiber, which enhance its moisture retention ability in electrolyte solution [46]. Thus, the derived functional materials have a wide application prospect in the field of energy storage. However, due to natural huge internal resistance of cellulose-based moisture-enabled electric generation, the current output is mostly below 100 nA. Therefore, it needs to composite with other materials to reduce the internal resistance, which could be a direction to develop these materials for practical applications [22]. Sodium alginate (SA) is a kind of natural water-soluble polysaccharide polymer extracted from seaweed, which has non-toxicity, biocompatibility, biodegradability, and other excellent characteristics [47,48]. The presence of hydroxyl, carboxyl, and other active functional groups in the molecular structure of SA allows for the functional properties of SA products to be further swelled by chemical modification, mixing and recombination [49,50]. SA is a biomass material known for its stability, adhesion, solubility and safety [51]. It has the ability to form a viscous liquid under mild conditions. In weakly alkaline solutions, SA is extremely hydrophilic and can be evenly dispersed. Moreover, the SA solution remains insoluble in organic solvents, allowing it to maintain the original form without dispersion and condense into various shapes. The fiber with SA as the main material has an obvious pore structure, which is the channel for the flow of water molecules and ions. Therefore, SA was chosen in this work as the main material to form the fiber, and GO as the main additive to carry the functional groups.

In this paper, a fibrous hydroelectric generator (as seen in Fig.1) is reported that can generate stable and continuous electricity based on the streaming potential in the sodium alginate-graphene oxide (SA-GO) fiber. When moisture comes into contact with the SA-GO fiber, the hydrophilicity of fiber and lithium chloride (LiCl) coating with humidity gradient keeps the water flowing constantly through the microchannels smoothly. Subsequently, the functional groups are hydrolyzed while the positive charges accumulate on one side of the fiber and diffuse, resulting in a continuous current. Through the systematic control of ingredient, dispersion, and humidity, a single fiber generator can provide a high and constant voltage of 0.25 V and current of 4 μA.

2 Experiment

2.1 Fabrication of sodium alginate-graphene oxide fiber

3 wt.% (wt.% representing mass fraction) SA solution (Macklin, Shanghai), 15 mg/mL of GO (The Sixth Element Materials Technology Co., Ltd.) solution and oxidized CNFs were mixed up in a 5:1:2 ratio. Subsequently, 5 mL of 5 mg/mL graphitized carboxyl multiwalled CNTs solution (Macklin, Shanghai) was added into the mixture. Then, the resulting solution was injected into 10 wt.% CaCl₂ (Macklin, Shanghai) ethanol coagulation bath to form fibers. After that, the fibers were washed by anhydrous ethanol and deionized water, and dried in the air. Finally, the fibers were treated with LiCl (Macklin, Shanghai) solution for several hours. Upon drying, the final fibers were obtained which could generate electricity in air humidity. The fabrication process was shown in Fig.2(a).

2.2 Measurement of electrical output performance

The fiber was attached to the slide as a sample by conductive silver paint (Macklin, Shanghai) and copper tape (Jingdong Online). The V_oc and short circuit current of SA-GO fiber were measured and recorded by an electrometer (2400, Keithley, USA). A humidity detector (HC-05B, Sieval, Guangzhou, China) and an electric pump (Sobo, Guangzhou, China) were designed as a constant humidity device to control the humidity. When requiring a humidity higher than the ambient one, moist air was pumped into the test chamber. On the contrary, when requiring a lower humidity, dry air was pumped into the test chamber to replace the moist air. The detailed measurement can be referred to in Refs. [52,53].

3 Results and discussion

Fiber was stretched and wrapped around a cylindrical solvent tube in Fig.2(b), showing the flexibility and swelling property of fibers. Moreover, the SA-GO fibers demonstrated an excellent strength: a fiber can withstand 50 g of weight. When three fibers are combined into a group, it can easily withstand 100 g of weight (Fig.2(c)).

The experiment verifies that the water absorption and swelling characteristics of the fiber are not related to the humidity of the air. Hence, if there is enough time to absorb water, the fiber can always reach a certain water absorption rate. In this way, the volume swelling rate and water content are used in this work to indicate the water absorption rate (Fig.2(d) and Fig.2(e)). The mass of a 5 cm dry fiber is 10.8 mg measured on the electronic scale, and after swelling its mass is 12.3 mg, which indicates that the water content is 1.5 mg accounting for 12.2% of the total weight of wet fiber. Compared with the fiber length before and after swelling, the volume swelling rates are around 12% (Fig.2(d)).

The scanning electron microscope (SEM) images of the SA-GO fiber are shown in Fig.2(f) and Fig.2(g), which show the surface structure of the fiber. The macromolecular clumps of SA or GO on the fiber surface can be observed after zooming in 10000 times, and the micro-channels can be shown after zooming in 5000 times. The channels range in diameter from 0.1 to 1 μm, allowing plenty of water to flow through and interact with the functional groups to produce hydrogen ions [54]. These results suggest that through chemical or physical treatment, the material undergoes the generation of numerous capillary channels, while the pore surface carries functional groups such as −COOH through oxidation. Thus, two conditions for generating streaming potential are satisfied, i.e., the capillary channels and charged surface.

In terms of power output, the streaming potential generated by the SA-GO fiber is closely related to the air humidity. The output voltage of the fiber was accurately measured at various air humidities from 50% relative humidity (RH) to 90% RH using the humidity voltage test device (Fig.3(a)). When the humidity is low (50% RH), it takes a longer time for the fiber to absorb water and swell. During this process, the voltage of the fiber also rises slowly, and finally reaches saturation and stabilizes at about 0.08 V (Fig.3(b)). When the air humidity increases, the fiber swelling rate increases, and the time to reach stable voltage decreases. The final stable voltage is higher than the voltage at a low humidity. At 90% RH, the voltage reaches a stable value in about 400 s, which is more than 0.2 V (Fig.3(c)). With the enhancement of RH, the increased voltage proves that water is crucial in the electricity-generation process, similar to electricity generation of carbon nanomaterials when exposing to moisture diffusion or water evaporation [55]. The fibers can also generate electricity for a long time in a changing environment, and the voltage of the fibers fluctuates slightly with fluctuations in air humidity. For example, in a laboratory atmosphere with 50% of RH, the voltage at both ends of the fiber fluctuates around 0.08 V, and the power generation time is longer than 50 h (Fig.3(d)). By varying the atmosphere humidity around the fiber, it will respond quickly and exhibit the change in voltage output. Fig.3(e) manifests that if the humidity changes from 80% RH to 60% RH, the voltage changes from 0.15 to 0.1 V within 300 s.

In practical applications, the output power of the SA-GO fiber can be increased by in series or parallel connection. Several SA-GO fibers were connected in series to power a light emitting diode (LED) bulb as a self-powered device (Fig.4(a)). As demonstrated in Fig.4(b), it can be easily achieved by more than 0.2 V for output power by connecting two fibers of the same size in series. Besides, the output voltage of the fiber group in series is also related to humidity. However, the influence of humidity is not as significant as that of a single fiber, and the output voltage of the fiber group can reach about 0.25 V at different humidities. A 5 cm long SA-GO fiber has a short-circuit current of about 4 μA at 70% RH (Fig.4(c)), which is superior to other moist-electric power generations in the previous work (Tab.1).

Since the fibers have good toughness and strength in the dry state, it is possible to weave several fibers into different shapes to generate electricity. The experiment indicates that the fiber can not only withstand the tensile strength of 0.5 N but also be bended or twisted more than 30 times without breaking. Three fibers which are twisted into a bundle can provide a voltage of 0.25 V at 50% RH (Fig.4(d)), three times that of a single fiber. Hence, attributed to its physical and chemical properties, the SA-GO fiber can be easily integrated into wearable devices to absorb moisture and generate electrical output from gaseous environments such as human breathing.

In addition, the voltage and current of fibers were detected to confirm that the power generation performance was stable by conducting cycle experiments. As indicated in Fig.4(e), the current output of a 5 cm SA-GO fiber rises with water droplets on the one side of the fiber and slowly falls as the water evaporates. Similarly, due to evaporation equilibrium, the voltage reaches its peak and gradually decays to a lower but stable value (Fig.4(f)) [63].

The electricity generated by the fiber can be explained by the electrokinetic phenomena between the solid–liquid interface [64,65]. The study on electrodynamic–electric double layer phenomenon has been conducted for many years, and many researchers have proposed different theoretical models of double layer. The Gouy–Chapman–Stern–Grahame (CGSG) model divides the solid–liquid interface electrical layer into two layers, i.e., the Stern layer and the diffusion layer [66]. The Stern layer includes the inner Helmholtz layer and the outer Helmholtz layer, in which the ions are considered to be immobile [67]. The potential of the inner Helmholtz layer decreases linearly, while that of the outer Helmholtz layer and the diffusion layer decreases exponentially. The boundary where the flow occurs in the diffusion layer is called the shear plane, which is one or two radii away from the solid surface. The potential of the shear plane is corresponding to the zeta potential, which is the main factor influencing the dynamics of the water–solid contact [68].

When the fluid is driven by an external force, the double electric layer between the solution and the pore channel slides, causing the ions in the diffusion layer to flow with the fluid. This phenomenon is called streaming current (I_str). The accumulation of free charge at one end results in the formation of streaming potential (U_str). When the capillary radius is much larger than the double layer thickness, the I_str and U_str can be calculated by the Helmholtz–Smoluchovski (HS) formula [69]

(1)I_{\text{str}}\text{=}\frac{{\epsilon }_{\text{r}}{\epsilon }_0\zeta A_{\text{c}}}{\eta L}\mathrm{\Delta }P,

(2)U_{\text{str}}\text{=}\frac{{\epsilon }_{\text{r}}{\epsilon }_0\zeta }{\sigma \eta }\mathrm{\Delta }P,

where ε_r is the relative dielectric constant of the liquid, ε₀ is the vacuum dielectric constant, ζ is zeta potential, ΔP is the pressure difference, η is the viscosity of the liquid, A_c is the cross-sectional area of capillary pipes, L is the tube length, and σ is the conductivity of the solution.

Hence, the streaming potential, generated by the electrokinetic effect, is the mechanism of the electricity generation. At first, water comes into contact with one end of the SA-GO fiber, and then water is absorbed inside by the hydrophilic materials. The capillary force and evaporation balance allow water to flow continuously along the fiber, thus a potential difference occurs at both ends which is known as the streaming potential [70]. In this process, electrical double layers are formed [71].

To investigate the different substances to increase the electric output of the fiber, fibers with various combinations of substances were developed and tested at different humidity levels (Fig.5(a)). Among these fibers, the one composed of SA, GO, CNT, and CNF can generate a higher voltage than those of others.

After the soaking treatment using LiCl, the fiber surface retains a significant amount of LiCl molecules, which readily hydrolyze and aid in the absorption of water from the air [72]. Compared with other common hygroscopic salts (e.g., CaCl₂ and FeCl₃), fibers with LiCl as the hygroscopic agent could show a better performance of electricity generation, owing to the effective ion diffusion with a small ionic radius and a low electrostatic repulsive force with a low ionic valence [73]. As a result, a layer of alkaline solution forms on the surface of the SA-GO fiber. Additionally, the SA molecules extend in weak alkaline solution. As a result, the fiber can swell in a moist atmosphere. Consequently, both the length and diameter of the fiber increase (Fig.5(b)).

The power generation mechanism of the SA-GO fiber is illustrated as follows. The dry fiber is unable to ionize H⁺ due to the absence of water, and no electricity signal could be detected. However, when the fiber is placed in a moist atmosphere, it swells rapidly or slowly according to the humidity. In the process of humidity power generation, the SA-GO fibers first absorb water from the air because of the hygroscopic agent (LiCl molecules) on one side of the fiber. Part of the absorbed water is ionized to produce H⁺ owing to the presence of GO. Then, numerous water molecules gradually move through the microchannels within the SA-GO fiber driven by a moisture gradient. The transportation of water will carry H⁺ to diffuse to the other side of the SA-GO fiber. It is worth noting that the aggregation of hydrogen ions was detected by pH paper which turned red when in contact with one side of the SA-GO fiber, confirming the aggregation of hydrogen ions (Fig.5(c)). As water molecules flow past functional groups (e.g., −OH and −COOH), a large number of H⁺ would be further ionized and move with the current (Fig.5(d)). These hydrogen ions are driven by water to concentrate, forming an ion concentration gradient and resulting in the formation of a streaming potential. Furthermore, it is indicated that the output voltage reaches a consistent peak value of 0.2 V when the fiber is flipped in Fig.5(e), proving that water flowing in one direction is the key to generate electricity. Therefore, the performance of the moist-electric generator comes from the streaming potential.

4 Conclusions

In summary, a humidity power generation was designed and developed based on the eco-friendly SA. The output voltage of a single SA-GO fiber increases from 0.08 to 0.2 V with the air humidity rising from 50% RH to 90% RH. The voltage remains stable without attenuation during a long period of power generation and can be further increased by series or braided bunches. Moreover, the fiber exhibits a high toughness and strength, making it resistant to damage. Hence, these characteristics allow the SA-GO fiber to be integrated into wearable devices, which can recycle water from human breath or environment to generate electricity for sustainable wearable and self-powered electronics. This work opens a new avenue for further developing high-performance humidity generators using SA from moisture in the environment.

References

[1] Kılkış Ş, Krajačić G, Duić N. . Research frontiers in sustainable development of energy, water and environment systems in a time of climate crisis. Energy Conversion and Management, 2019, 199: 111938

[2] Zhou Q, Gong F, Xie Y L. . A general strategy for designing metal-free catalysts for highly-efficient nitric oxide reduction to ammonia. Fuel, 2022, 310: 122442

[3] Gong F, Wang W B, Li H. . Solid waste and graphite derived solar steam generator for highly-efficient and cost-effective water purification. Applied Energy, 2020, 261: 114410

[4] Kempton W, Pimenta F M, Veron D E. . Electric power from offshore wind via synoptic-scale interconnection. Proceedings of the National Academy of Sciences of the United States of America, 2010, 107(16): 7240–7245

[5] Gong F, Li H, Wang W B. . Scalable, eco-friendly and ultrafast solar steam generators based on one-step melamine-derived carbon sponges toward water purification. Nano Energy, 2019, 58: 322–330

[6] Moriarty P. Global nuclear energy: An uncertain future. AIMS Energy, 2021, 9(5): 1027–1042

[7] Wang Z, Tan L, Pan X M. . Self-powered viscosity and pressure sensing in microfluidic systems based on the piezoelectric energy harvesting of flowing droplets. ACS Applied Materials & Interfaces, 2017, 9(34): 28586–28595

[8] Cai M J, Yang Z S, Cao J Y. . Recent advances in human motion excited energy harvesting systems for wearables. Energy Technology, 2020, 8(10): 2000533

[9] Fang L, Zheng Q W, Hou W C. . A self-powered vibration sensor based on the coupling of triboelectric nanogenerator and electromagnetic generator. Nano Energy, 2022, 97: 107164

[10] Zhou L M, Liu Y Y, Liu S L. . For more and purer hydrogen-the progress and challenges in water as shift reaction. Journal of Energy Chemistry, 2023, 83: 363–396

[11] Wang S J, Gong F, Zhou Q. . Transition metal enhanced chromium nitride as composite nitrogen carrier for sustainable chemical looping ammonia synthesis. Applied Catalysis B: Environmental, 2023, 339: 123134

[12] Yang C R, Ko C T, Chang S F. . Study on fabric-based triboelectric nanogenerator using graphene oxide/porous PDMS as a compound friction layer. Nano Energy, 2022, 92: 106791

[13] Benadda B, Beldjilali B, Mankouri A. . Secure IoT solution for wearable health care applications, case study Electric Imp development platform. International Journal of Communication Systems, 2018, 31(5): e3499

[14] Nelson E C, Verhagen T, Vollenbroek-Hutten M. . Is wearable technology becoming part of us? Developing and validating a measurement scale for wearable technology embodiment.. HJMIR mealth and uHealth, 2019, 7(8): e12771

[15] Dolez P I. Energy harvesting materials and structures for smart textile applications: Recent progress and path forward. Sensors, 2021, 21(18): 6297

[16] Jiang G Q, Dong T, Guo Z K. Nonlinear dynamics of an unsymmetric cross-ply square composite laminated plate for vibration energy harvesting. Symmetry, 2021, 13(7): 1261

[17] Song J W, Sun G H, Zeng X. . Piezoelectric energy harvester with double cantilever beam undergoing coupled bending-torsion vibrations by width-splitting method. Scientific Reports, 2022, 12(1): 583

[18] Li Z, Yuan S M, Ma J. . Cutting force and specific energy for rotary ultrasonic drilling based on kinematics analysis of vibration effectiveness. Chinese Journal of Aeronautics, 2022, 35(1): 376–387

[19] Zhou Q, Gong F, Xie Y L. . 1+1 > 2: Learning from the interfacial modulation on single-atom electrocatalysts to design dual-atom electrocatalysts for dinitrogen reduction. ScienceDirect, 2023, 8(6): 1753–1763

[20] Yadav P, Sahay K, Srivastava M. . Emerging trends in self-healable nanomaterials for triboelectric nanogenerators: A comprehensive review and roadmap. Frontiers in Energy, 2023, 17(6): 727–750

[21] Gao H Q, Hu M G, Ding J F. . Investigation of contact electrification between 2D MXenes and MoS₂ through density functional theory and triboelectric probes. Advanced Functional Materials, 2023, 33(15): 2213410

[22] Sun Z Y, Wen X, Wang L M. . Emerging design principles, materials, and applications for moisture-enabled electric generation. eScience, 2022, 2: 32–46

[23] Luo Z L, Liu C H, Fan S S. A moisture induced self-charging device for energy harvesting and storage. Nano Energy, 2019, 60: 371–376

[24] Liang Y, Zhao F, Cheng Z H. . Electric power generation via asymmetric moisturizing of graphene oxide for flexible, printable and portable electronics. Energy & Environmental Science, 2018, 11(7): 1730–1735

[25] Zhang J, Zhan K, Zhang S S. . Discontinuous streaming potential via liquid gate. eScience, 2022, 2: 615–622

[26] Bai J X, Huang Y X, Wang H Y. . Sunlight-coordinated high-performance moisture power in natural conditions. Advanced Materials, 2022, 34(10): 2103897

[27] Ji L, Zheng K, Zheng L. . Direct transition of potential of water droplets to electric energy using aligned single-walled carbon nanotubes. Chinese Physics B, 2010, 19(6): 066101

[28] Yang C, Huang Y X, Cheng H H. . Rollable, stretchable, and reconfigurable graphene hygroelectric generators. Advanced Materials, 2019, 31(2): 1805705

[29] Zhang Z H, Li X M, Yin J. . Emerging hydrovoltaic technology. Nature Nanotechnology, 2018, 13(12): 1109–1119

[30] Guan W X, Guo Y H, Yu G H. Carbon materials for solar water evaporation and desalination. Small, 2021, 17(48): 2007176

[31] Zhang P X, Liu Y Y, Liu S L, et al. Precise design and modification engineering of single-atom catalytic materials for oxygen reduction. Small, 2024, 20(4): 2305782

[32] Zhang H H, Liu Y Y, Wei H J. . Atomic-bridge structure in B−Co−P dual-active sites on boron nitride nanosheets for catalytic hydrogen generation. Applied Catalysis B: Environmental, 2022, 314: 121495

[33] Xu T, Ding X T, Huang Y X. . An efficient polymer moist-electric generator. Energy & Environmental Science, 2019, 12(3): 972–978

[34] Xue G B, Xu Y, Ding T P. . Water-evaporation-induced electricity with nanostructured carbon materials. Nature Nanotechnology, 2017, 12(4): 317–321

[35] Li J, Liu K, Xue G B. . Electricity generation from water droplets via capillary infiltrating. Nano Energy, 2018, 48: 211–216

[36] Gong F, Li H, Zhou Q. . Agricultural waste-derived moisture-absorber for all-weather atmospheric water collection and electricity generation. Nano Energy, 2020, 74: 104922

[37] Liang Y, Zhao F, Cheng Z H. . Self-powered wearable graphene fiber for information expression. Nano Energy, 2017, 32: 329–335

[38] Li J H, Xia B L, Xiao X. . Stretchable thermoelectric fibers with three-dimensional interconnected porous network for low-grade body heat energy harvesting. ACS Nano, 2023, 17(19): 19232–19241

[39] Cho H, Kim S, Liang H. . Electric-potential-induced uniformity in graphene oxide deposition on porous alumina substrates. Ceramics International, 2020, 46(10): 14828–14839

[40] Han Z, Fina A. Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review. Progress in Polymer Science, 2011, 36(7): 914–944

[41] Yan H P, Liu Z, Qi R H. A review of humidity gradient-based power generator: Devices, materials and mechanisms. Nano Energy, 2022, 101: 107591

[42] Wang K Q, Xu W H, Zhang X. . Bio-inspired water-driven electricity generators: From fundamental mechanisms to practical applications. Nano Research Energy, 2023, 2: e9120042

[43] Zhang P X, Sun K, Liu Y Y. . Improving bifunctional catalytic activity of biochar via in-situ growth of nickel–iron hydroxide as cathodic catalyst for zinc-air batteries. Biochar, 2023, 5(1): 60

[44] Zhang P X, Liu Y Y, Wang S L. . Wood-derived monolithic catalysts with the ability of activating water molecules for oxygen electrocatalysis. Small, 2022, 18(34): 2202725

[45] Hassan S H, Velayutham T S, Chen Y W. . TEMPO-oxidized nanocellulose films derived from coconut residues: Physicochemical, mechanical and electrical properties. International Journal of Biological Macromolecules, 2021, 180: 392–402

[46] Li M J, Zong L, Yang W Q. . Biological nanofibrous generator for electricity harvest from moist air flow. Advanced Functional Materials, 2019, 29(32): 1901798

[47] Shakeri F, Ariaeenejad S, Ghollasi M. . Synthesis of two novel bio-based hydrogels using sodium alginate and chitosan and their proficiency in physical immobilization of enzymes. Scientific Reports, 2022, 12(1): 2072

[48] Li Y Q, Zhang H, Fan M Z. . A robust salt-tolerant superoleophobic alginate/graphene oxide aerogel for efficient oil/water separation in marine environments. Scientific Reports, 2017, 7(1): 46379

[49] Bae J, Yun T G, Suh B L. . Self-operating transpiration-driven electrokinetic power generator with an artificial hydrological cycle. Energy & Environmental Science, 2020, 13(2): 527–534

[50] Zhang R, Qu M J, Wang H. . Moist-electric films based on asymmetric distribution of sodium alginate oxygen-containing functional groups. Reactive & Functional Polymers, 2022, 181: 105421

[51] Chen Z D, Song J, Xia Y M. . High strength and strain alginate fibers by a novel wheel spinning technique for knitting stretchable and biocompatible wound-care materials. Materials Science and Engineering C, 2021, 127: 112204

[52] Gong F, Li H, Huang J G. . Low-grade energy harvesting from dispersed exhaust steam for power generation using a soft biomimetic actuator. Nano Energy, 2022, 91: 106677

[53] Lv Y L, Gong F, Li H. . A flexible electrokinetic power generator derived from paper and ink for wearable electronics. Applied Energy, 2020, 279: 115764

[54] Abbasi H R, Karimian S M H. Water mass flow rate in a finite SWCNT under electric charge: A molecular dynamic simulation. Journal of Molecular Liquids, 2016, 224: 165–170

[55] Xu Y F, Chen P N, Peng H S. Generating electricity from water through carbon nanomaterials. Chemistry, 2018, 24(24): 6287–6294

[56] Zhao F, Cheng H H, Zhang Z P. . Direct power generation from a graphene oxide film under moisture. Advanced Materials, 2015, 27(29): 4351–4357

[57] Daripa S, Khawas K, Behere R P. . Efficient moisture-induced energy harvesting from water-soluble conjugated block copolymer-functionalized reduced graphene oxide. ACS Omega, 2021, 6(11): 7257–7265

[58] Li Q J, Zhou M, Yang Q F. . Flexible carbon dots composite paper for electricity generation from water vapor absorption. Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2018, 6(23): 10639–10643

[59] Chen N, Liu Q, Liu C. . MEG actualized by high-valent metal carrier transport. Nano Energy, 2019, 65: 104047

[60] Gao X, Xu T, Shao C X. . Electric power generation using paper materials. Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2019, 7(36): 20574–20578

[61] Lyu Q Q, Peng B L, Xie Z J. . Moist-induced electricity generation by electrospun cellulose acetate membranes with optimized porous structures. ACS Applied Materials & Interfaces, 2020, 12(51): 57373–57381

[62] Yang W Q, Lv L L, Li X K. . Qatarized silk nanofibrils for electricity generation from moisture and ion rectification. ACS Nano, 2020, 14(8): 10600–10607

[63] Bae J, Yun T G, Suh B L. . Self-operating transpiration-driven electrokinetic power generator with an artificial hydrological cycle. Energy & Environmental Science, 2020, 13(2): 527–534

[64] Sharfarets B P, Kurochkin V E, Sergeev V A. On the operation of an electroacoustic transducer based on electrokinetic phenomena under turbulent fluid motion. Acoustical Physics, 2020, 66(5): 559–563

[65] Yun T G, Bae J, Rothschild A. . Transpiration driven electrokinetic power generator. ACS Nano, 2019, 13(11): 12703–12709

[66] Grahame D C. The electrical double layer and the theory of electrocapillarity. Chemical Reviews, 1947, 41(3): 441–501

[67] Hunter R J. Zeta Potential in Colloid Science: Principles and Applications. Cambridge: Academic Press, 1988

[68] Kirby B J, Hasselbrink E F Jr. Zeta potential of microfluidic substrates: 1. Theory, experimental techniques, and effects on separations. Electrophoresis, 2004, 25(2): 187–202

[69] Olthuis W, Schippers B, Eijkel J, et al. Energy from streaming current and potential. Sensors and Actuators. B, Chemical, 2005, 111–112: 385–389 10.1016/j.snb.2005.03.039

[70] Xue G B, Xu Y, Ding T P. . Water-evaporation-induced electricity with nanostructured carbon materials. Nature Nanotechnology, 2017, 12(4): 317–321

[71] Xu T, Ding X T, Cheng H H, et al. Moisture-enabled electricity from hygroscopic materials: A new type of clean energy. Advanced Materials, 2023, early access, https://doi.org/10.1002/adma.202209661

[72] Albayrak C, Barim G, Dag O. Effect of hygroscopicity of the metal salt on the formation and air stability of lyotropic liquid crystalline mesophases in hydrated salt-surfactant systems. Journal of Colloid and Interface Science, 2014, 433: 26–33

[73] Tan J, Fang S M, Zhang Z H. . Self-sustained electricity generator driven by the compatible integration of ambient moisture adsorption and evaporation. Nature Communications, 2022, 13(1): 3643

RIGHTS & PERMISSIONS

Higher Education Press