Compact Electrolytic Hydrocapacitor for Direct Harvesting and Storing Energy from Water Droplet Achieving High Voltage

As a renewable and low‐carbon emission energy source, water‐droplet‐based devices are attracting increasing interest. However, the uncertain mechanistic explanation, output shortage, and lack of storability remain huge barriers to their large‐scale applications. Herein, a compact quasi‐solid‐state electrolytic hydrocapacitor (HCelect) is successfully designed and fabricated with dual‐function of self‐charging and storage by employing novel bilayer graphene oxide/graphite flake composite films symmetrically sandwiched by poly(vinyl alcohol)–phosphoric acid electrolyte. The sandwich‐like HCelect can achieve a high‐level and repetitive self‐generation voltage of 0.94 V for over a month under a small‐scale water droplet. A possible model is proposed to explain the operating principle in detail based on the capillarity, ion diffusion, and streaming potential mechanisms. Simultaneously, the generated energy is continuously stored for ≈2.1 h due to water evaporation. In addition, unlimited output extension can be realized by integrating hydrocapacitors in an array for powering commercial devices. Interestingly, it is ascertained that prolonged finger touches on the HCelect can produce a high voltage. Therefore, this novel hydrocapacitor has notable potential applications in small portable and wearable devices owing to its compactness, low cost, environmental friendliness, safety, and flexibility, thereby providing a design idea for new innovative electronic devices.

][44][45][46][47][48] The device that simultaneously harvests and stores this type of hydropower, referred to as a "hydrocapacitor," was first proposed by Fan et al. in 2018. [39]Their centimeter-sized hydrocapacitor constructed from hydrophilic CNT/polyaniline (CNT/PANI) composite as symmetric electrodes and poly(vinyl alcohol) (PVA) gel as the electrolyte separator could convert energy from smallscale water movements and instantly store it.The device reached an output voltage of approximately 60 mV after the addition of deionized water at a 20 μL min −1 dropping rate, which further increased to 90 mV with the incorporation of wind.Another type of hydrocapacitor uses a hydrophilic porous membrane, such as an eggshell, and weighing paper as the separator to bridge the two CNT/PANI electrodes.Upon adding tap water, an improved open-circuit voltage (V oc ) of 0.26 V was recorded through the eggshell membrane.However, this output power is still insufficient for practical applications.53] Most previous devices are usually built based on a single principle without any combination of operating principles to construct more efficient devices.Among these, the electric generation of the streaming potential in porous materials, where counterions accumulate at the liquid-solid interface in a Debye layer through the flowing liquid, is known as the most widely accepted explanation.Although a new direction for harvesting energy from smallscale water movement through hydrocapacitors is provided, the output power remains low, and external boosts, such as wind or continuous water dropping, are required to achieve the maximum voltage.
In this paper, based on novel bilayer graphene oxide/graphite flake (GOGr) electrodes and PVA-phosphoric acid (PVA-H 3 PO 4 ) quasi-solid-state gel, we present a centimeter-sized hydrocapacitor with dual functions of energy conversion and storage through an efficient and low-cost method.Owing to the superior hydration of the GOGr electrode, the self-powered hydrocapacitor reached a maximum V oc of 0.94 V within 1 min when a water droplet is dropped, which is among the highest reported output.The V oc in the proposed work is considerably higher than that of other recent hydrocapacitors whose electrodes are CNT/PANI.The generated energy could be stored directly and maintained for ≈2.1 h until the water was dried out due to the water evaporation.Daily repetitive voltage was examined by repeatedly adding water droplets daily for over a month, demonstrating the reusability of the device.The factors affecting the power output, including the separator, dropping solution, and electrode, were fully investigated.Meanwhile, the working mechanism was discussed and explained based on the combination of capillarity, ion diffusion, and streaming potential mechanisms through a proposed model.In addition, the unlimited expandability of the hydrocapacitors was demonstrated through series-parallel topologies of HC elect cells.An electrical network including two cells in series was built to power a light-emitting diode (LED) and commercial digital timer.Impressively, the device could effectively convert water molecules from the human skin to electricity through a prolonged finger touch on it.These properties demonstrate the great potential applications of the novel hydrocapacitor in small portable and wearable devices, thereby providing a reference for future nanogenerators with self-generating high-voltage and storability.

Investigation of the Materials Used in the Fabrication of HC elect
The schematic and photograph of the flexible compact HC elect is shown in Figure 1a,b, respectively.The hydrocapacitor possesses thin, light, and flexible features owing to the properties of its structural materials.Its cross-section is illustrated in Figure 1b.The Al top collector was manufactured with a small hole at the center and directly pasted onto the graphite (Gr) porous surface.What is more, the Al collector works as a sheath, which could reduce the influence of outside moisture, thereby increasing the accuracy of the measurement results.The microscopic morphology of the bilayer GOGr composite film is presented in the scanning electron microscope (SEM) images (Figure 1c-e), in which two obvious layers, namely the graphene oxide layers (GOls) and microporous graphite layer (Grl), are exhibited.As shown in Figure 1c, the GO sheets are tightly stacked at the top, whereas the bottom comprises graphite microparticles and capillary microchannels.Small amounts of graphite microparticles, which are broken off from the larger particles during ultrasonication, are interspersed between the GOls.The GOl surface topography, which is minimally decorated with few graphite particles, as shown in Figure 1d, is relatively similar to that of conventional GO films (Figure S1a, Supporting Information).Meanwhile, the Grl surface consists of loosely aggregated microparticles with a length of <8 μm (Figure 1e).
The surface area and pore size of the GOGr film was determined by the Brunauer-Emmet-Teller (BET) analysis using the N 2 gas adsorption-desorption analysis.The recorded BET surface area (S BET ) and desorption average pore diameter were 6.765 m 2 g −1 and 10.54 nm, respectively.The narrow capillary channels have a width of <25 μm, as shown in the inset of Figure 1e, which facilitated the permeation and diffusion of water and oxygen.These properties achieved a superhydrophilic contact angle (CA) of 25.3°caused by the capillary action, which will be explained in Section 2.4.This value outperformed that of the top surface and conventional GO films (Figure S1b, Supporting Information).The humid absorbability was further investigated, which confirmed the results in Figure S1c (Supporting Information).Water and oxygen absorbability plays a major role in boosting the output of the hydrocapacitor.Thus, directly incorporating the Al top collector onto the graphite porous surface permits tap water and oxygen to easily infiltrate the device (Figure 1b).Besides, the Grl exhibits higher conductivity than that of the GOl and conventional GO film, which is demonstrated by sheet resistivity measurements (Figure S2a, Supporting Information), thereby improving the measurement output results.
The chemical compositions and atomic bonding of the surfaces of the GOGr and GO films were examined by X-ray photoelectron spectroscopy (XPS).As shown in the full spectrum survey in Figure 1f, the main constituent peaks of C 1s (280-300 eV) and O 1s (525-545 eV) are detected, suggesting the presence of oxygen functional groups.Meanwhile, the spectrum of conventional GO and two surfaces of the GOGr samples has three deconvolution peaks, corresponding to the aromatic rings of C═C/C─C (284.8 eV), epoxy C (C─O, 286.9 eV), and carbonyl C (C═O, 288.3 eV), respectively.Compared to the conventional GO film, the C═C/C─C bonds of GOl is slightly higher, whereas a more significant increase is observed at the Grl surface.The addition of graphite flake is believed to enhance the intensity of C═C/C─C.Microscopic analysis and crystallographic structure were characterized through Raman Spectroscopy and X-ray diffraction technique (XRD) to validate the XPS results (Figure S2b,c, Supporting Information).Obviously, the graphitization in the crystal structure is understandable in response to the addition of graphite flake.Moreover, the widespread pres-ence of oxygen functional groups is confirmed, which play an enormous role in increasing the charge carriers in response to water absorption.The above surveys demonstrate the incomplete mixing of Gr in the GO solution.In particular, most of the heavier graphite particles sank to construct a microporous Grl, whereas most GO lattices remained on top to form GOl.
In addition to the electrodes, the electrolyte channel also plays an important role in the operation.The PVA-H 3 PO 4 electrolyte was previously reported to possess good viscosity and better ionization than those of other electrolytic channels of PVA mixed with frequently used conductive substances, such as sulfuric acid (H 2 SO 4 ), potassium hydroxide (KOH), sodium hydroxide (NaOH), potassium chloride (KCl), and sodium chloride (NaCl), thereby facilitating conductive ion transport as well as electrode attachment. [54]These material features demonstrated the suitability of HC elect for watermovement-based self-powered wearable electronics owing to its excellent hydrophilicity and flexibility.These above assertions will be further verified by discussing the electrical output results below (Section 2.2).

Electricity Generation and Storability of HC elect
The measurement components and connections were built on the breadboard to create equivalent circuits without soldering to simplify the implementation of the V oc and short-circuit current (I sc ) measurements.On the circuit board, the bus strips were connected to the meters, whereas the hydrocapacitors and components were built on the terminal strips, as shown in Figure S3 (Supporting Information).As the tap water taken directly from a laboratory faucet was dropped onto the HC elect , high-efficiency electricity generation and storage are repeatedly induced, as shown in Figure 2.An obvious maximum V oc of up to 0.94 V, shown in Figure 2a and Movie S1 (Supporting Information), could be typically detected in response to dropping 28 μL tap water until the water inside the device was dried in the dry chamber due to evaporation in ≈2 h.As the chamber is an ideal dry environment, the interior evaporation rate is higher than that of the ambient environment, so the voltage maintenance time might be longer if put the HC elect outside the chamber.The addition of another tap water droplet could reactivate and reproduce the voltage signal for consecutive measurements.The device reached the maximal working stage with only a drop of tap water, which was ascertained by the stability of maximum voltage when adding other droplets, as shown in Figure S4a (Supporting Information).This is attributed to the homogeneous state reached by the GO parts inside the electrodes [7] with the application of a small amount of water, and the gradual formation of an electric field between the two electrodes.The generated electric field is sufficient to prevent the current of positive ions, thereby maintaining the stability of the voltage.Another reason might be that the membranes and electrodes would attain saturation when the liquid reached a specific level, trapping any additional droplets on the higher electrode and preventing them from penetrating the membrane deeply.When not in use, tap water should be not added.The device was kept in dry conditions to prevent unwanted reactions in the subsequent measurements, such as moisture absorption by the non-covered portions of the device after prolonged exposure to the environment.
Three distinguishable stages based on the V oc curve trend are noted (Figure 2b), namely the (I) rapid rise, (II) long stable stage, and (III) relatively slow drop.At stage I, the voltage rapidly increased and reached its maximum value within 60 s.Then gradually keeps stable at the high voltage status for over 5000 s at the II stage.After the prolonged interaction with the water molecules, the device totally absorbed the water droplets, thereby gradually dropping the voltage signals to their initial level in ≈2540 s, as represented in stage III.Totally, the voltage was maintained for ≈2.1 h by adding only a drop of tap water.The daily V oc of the same device was measured daily for 47 days with an equal dropping amount of 28 μL tap water (inset Figure 2b), resulting in the stable output of over 0.9 V. Simultaneously, the rapid decrease in the resistance of the entire device coincided with the rapid rise of voltage response with the dropping of tap water (Figure S4b, Supporting Information).From the second day onwards, the electrolytic bridge completely transformed into a solid state without appreciable effect on the V oc .The initial quasi-solid and solid states in the HC elect after 1 day could greatly reduce the negative leakage of the electrolyte and corrosion or deformation of the device.
In addition to tap water, moisture from ambient humidity is also an abundant and sustainable resource.Thus, a similar device with a manufactured 4 × 4 holes array (1.5 mm in diameter hole) on Al top collector was constructed to further test the electricity generation from moisture.As shown in Figure S5 (Supporting Information), moisture could be effectively harvested when the device was incorporated into a self-powered system under constant moisture feeding and various humidity levels.The observed maximal voltage was 0.96 V (Figure S5a, Supporting Information).By increasing the humidity, the self-charging process is accelerated (Figure S5b, Supporting Information), and the device can operate a wide range of relative humidity (RH) conditions from 25% to 80%, which ascertains its potential application (Figure S5c, Supporting Information).
The comparison of the different self-charging devices based on the water molecule movement reported in the literature is provided in Table 1, including the important parameters, such as a method to achieve V max , maximal outputs, and power generation method.Previous studies have various limitations, including the need for complex configurations with overlapping multilayer structures, large amounts of required water droplets, and wind boosts to achieve voltage values as expected.Besides, another comparison of hybrid devices with various pseudo-capacitive selfcharging devices is summarized in Table S1 (Supporting Information).Each type of device has its own advantages and weakness.Generally, the main dilemma of other pseudo-capacitive devices is instability with the changes in weather or surroundings, so the self-charging efficiency is not very high.And the achieved voltage is quite low (typically below 0.8 V), which limits their practical application.Relative to these devices, our device exhibits significant advantages in terms of its simplified structure, quick self-charging process with a small amount of tap water, and is uninfluenced by weather or surroundings while still achieving high-level output voltage.
Moreover, the discharging performance was further investigated.An equivalent circuit was built to calculate the output energy, where resistors R L , R in , and the current source A represent the external resistance, internal resistance, and induced current in the HC elect , respectively.Under S1 ON, an R L of 100 Ω was connected with the HC elect , which was charged up to a maximum voltage of 0.93 V at the beginning, as shown in the inset of Figure 2c.The voltage declined immediately when the applied load and remained stable at 0.012 V.The current output curve of the device with R in of 100 Ω applied to the circuit by turning on switch 2 (inset of Figure 2d), is shown in Figure 2d.The current flow was immediately activated upon the addition of the tap water until reaching the maximum value of 49.99 μA (499 μA cm −2 ), which corresponds to a maximum instantaneous power density of 6 μW cm −2 (calculated equation was followed by Equation S1, Supporting Information).Subsequently, the current gradually declined owing to a constant discharge through the internal circuit and remained stable.According to Equation S2 (Supporting Information), the calculated maximum power density (P max ) is 0.12 mW cm −2 .
More impressive, the HC elect not only owns a fast self-charging possibility but also can be charged by external power supplies.The galvanostatic charge-discharge GCD curves of the device are shown in Figure S6 (Supporting Information).The specific capacitance is 27.5 mF cm −2 , and the internal resistance is 72 Ω, as calculated from Equations S3 and S4 (Supporting Information), respectively.The energy harvested by a typical V oc is ≈12.15 mJ cm −2 , as calculated from Equation S5 (Supporting Information).Compared to the recently reported water moleculeinduced generators without an energy storage function, the device could effectively store the generated energy and release the stored electricity with the addition of an external load, which was demonstrated by Coulombic efficiency of 95.2% (Figure S6a, Supporting Information).In addition, self-charging by dropping tap water and external charging by an external power source were combined to maximize the function of the device as a capacitor, as shown in Figure S6b (Supporting Information).During the galvanostatic charging process, an initial voltage of ≈0.9 V is recorded owing to the maximum voltage self-charged by adding tap water.Subsequently, the voltage of the HC elect was charged up to 1.5 V (charging current of 0.5 mA cm −2 ).Carbon-based materials have been demonstrated as outstanding electrodes for supercapacitors and hydrocapacitors. [40,41,48,55,56]The voltage of the discharging process remained stable at ≈23 mV.The discharging current curve after adding tap water is charged up to 1.5 V (applied current of 0.5 mA cm −2 ).As shown in Figure S6c (Supporting Information), the maximum discharging current is ≈0.55 mA, which is considerably higher than that with self-charging only, as shown in Figure 2d.In Table S1 (Supporting Information), the specific capacitance (C s ), energy densities (E s ), power densities (P s ), and maximum self-charging voltage of the HC elect were compared with various kinds of self-charging devices based on carbon materials.Owing to its good pseudocapacitance, the HC elect can be considered a supercapacitor.Moreover, to improve capacitance and self-charging voltage, traditional selfcharging devices require a complex configuration and sophisticated material preparation.Herein, the fabricated water-based self-charging hydrocapacitor has an instantaneous response to small-scale water movement, a high-level output voltage, and a good pseudocapacitance, thereby demonstrating its potential as a water-motion-based supercapacitor and the new design idea of future innovative portable and wearable devices.

Influence of Separators, Dropping Solutions, and Electrodes on the Voltage Output Results
Fiber membranes, such as weighing paper, filter paper, and paper wipers, are rich in plant fibers.Meanwhile, Nafion membrane is widely known for its ion-conductive properties, which could produce zeta potentials in devices. [41,57,60,61]Thus, these membranes were used as separators between two GOGr electrodes to compare the output voltage results.All samples were cut into the same specific shape to prevent short circuits.Subsequently, the same sandwich-like-structured membrane hydrocapacitor (HC mem ) using GOGr electrodes and various membranes was assembled and compared with the HC elect , as shown in Figure 3a.The maximum V oc of the hydrocapacitors after being exposed to the same amount of tap water is shown in Figure 3b.The HC mem s using the filter paper and weighing paper could induce the maximal V oc of 0.45 and 0.5 V, respectively, whereas using the paper wiper had the worst effect (≈0.17 V).As naturally porous plant materials have strong hygroscopicity, water immediately permeates through the plant fiber membrane, activating the ionic selection process, as explained in Section 2.4. Figure S7 (Supporting Information) shows the SEM images revealing the structure of the filter paper, weighing paper, and paper wiper.The interwoven and microporous structure of the coalescing fibers with several pores is observed, which promotes the diffusion of gas and water molecules.However, compared to filter paper and weighing paper, the pores of paper wipers are relatively large, which makes water penetrate through them immediately without selective filtration completely.In contrast, the Nafion membrane and PVA-H 3 PO 4 quasi-solid-state electrolyte had a more significant effect on the output voltage with values of 0.93 and 0.94 V, respectively.The Nafion membrane was used as a solid-electrolyte binder, resulting in a comparable output voltage to that of the PVA-H 3 PO 4 electrolyte membrane, which outperformed the plant fiber membrane.The Nafion and PVA-H 3 PO 4 lattice are decorated with oxygen-related functional groups, [62,63] thereby releasing more charge carriers.However, the Nafion film is relatively expensive and has a larger thickness, which would result in a longer H + ion transporting pathway and lower conductance compared to the PVA-H 3 PO 4 quasi-solid-state electrolyte membrane (Figure S8, Supporting Information).
Moreover, different types of polar solvents with the same dropping volume (28 μL) were tested to determine the influence of the dropping solutions on the HC elect .In this experiment, the dropping solutions were considered to be the only variable.Protic solvents (hydrochloric acid (HCl, purity 36.5%),H 3 PO 4 , tap water, and ethyl alcohol (EtOH) 85%), aprotic solvents (dimethylformamide (DMF), and deionized water), and an ionic compound (sodium chloride (NaCl, purity 98%)) were used as dropping solutions.The output voltage after exposure to different types of dropping solution on the HC elect for 60 s is shown in Figure 3c, where protic solvents and ionic compound NaCl show better results than aprotic solvents.Among the several types of protic solvents, tap water, and EtOH exhibit the best results in terms of the output voltage (0.94 and 0.95 V, respectively) and charged velocity with the same droplet volume.These results are ascribed to the influence of the H + -containing functional groups in solutions.To elucidate the originality of these H + ions from the oxygenrelated functional groups of the water molecules and EtOH, some aprotic solvents, instead of water, were used to stimulate the HC elect .Aprotic solvents render the device similar phenomena as tap water does with the reasonably low V oc , suggesting that the charge carriers (i.e., H + ions) are actually ionized when deeply penetrating into the device.Tap water and EtOH outperformed the output voltage than that of DMF and deionized water because they contain much more conductive ions (e.g., H + , H 3 O + , OH − ,…), which could raise the potential across the device based on the principles that are explained in Section 2.4.Although the ionic compound NaCl also contains conductive ions, including Na + and Cl − , it is difficult for Na + to move through the separator owing to its lower mobility than that of H + .Importantly, it cannot pass through GOl because of the inability to bond with oxygen-rich functional groups like H + ions.Therefore, the Na + accumulates on the top surface of the separator, resulting in an inefficient potential across the device.Nevertheless, even protic solvents HCl and H 3 PO 4 did not achieve significantly better results than those of the aprotic solvents.A possible reason is the reaction of these solvents with the top electrode due to the faradaic processes, [64] resulting in significantly reduced output.Previous studies indicated that the Faradaic reaction would not occur with neutral protic solvents, [64] consistent with the best obtained voltage results from dropping EtOH (pH 7.2) and tap water (pH 6.9).Another explanation is the excessive ions that could cause rapid self-discharging owing to their high conductivity.Nonetheless, the precise reason should be further investigated.In summary, there are four main factors of dropping solutions that were supposed to affect the output performance device, including the type of polar solvent, electrical mobility of charged particles, bond possibility with oxygen-rich functional groups of ions, and pH.Owing to its charging possibility without adding saline electrolytes and simply adding tap water to activate the de-vice, which could greatly prevent the corrosion of the Al electrode and leakage of harmful solutions, thereby enhancing the stability and cycle life.Furthermore, to study the effect of the electrodes on the performance of the HC elect , different construction designs using various active materials were discussed.The maximal voltage and desorption time after dropping tap water were gathered and compared, as shown in Figure S9 (Supporting Information).This result shows that HC elect outperforms harvested and stored conductive ions.Moreover, the best-obtained result and ubiquitous presence of tap water demonstrated its enormous application potential.

Mechanism Discussion
The experimental results mentioned above indicated that the HC elect can easily convert electricity and effectively store generated energy from the movement of water molecules.All components play a role in generating the high-voltage of the HC elect , which is discussed as follows.The role of the novel bilayer GOGr composite film will be discussed, in terms of the morphology and chemical properties of the layers.From the SEM image and CA measurements of the GOGr electrode, as shown in Figure 1c,d, and Figure S2b (Supporting Information), the bilayer GOGr film is constructed from the microporous Grl and GOls.Grl exhibits good water absorbency and conductivity, as demonstrated in Figures S1a and S2b (Supporting Information), which is ideal for transporting conductive particles and osmosis of the water flow.Its narrow capillary inlet could simultaneously facilitate the flow of water and oxygen due to the pressure difference across the liquid surface, according to Equation S6-S8 (Supporting Information). [65]In addition, tightly stacked GOls are widely known as lattice-decorated rich-oxygen functional groups, such as carboxylic groups, [63,66] which make it can work as a proton (i.e., H + ) conductive electrolyte when absorbing water.The abundance of free H + is closely released to the electrolyte channel owing to its positive ion-sieving possibility in response to water, [67] which further accelerated the output results with small-scale water movement and short charging time.These features suggest that these two layers could effectively support each other for providing fast water infiltration and partial conversion of water to conductive ions at the cathodic part.The specific mechanism of the water-movement-based electricity generation in the hydrocapacitor will be fully explored below.
In this work, we proposed a potential model to explain the working mechanism of the hydrocapacitors based on the combination of the capillarity, ion diffusion, and streaming potential model (Equations S6-S11, Supporting Information), as shown in Figure 4. Tap water infiltrated the hydrocapacitor through a small hole in the Al top current collector, thereby activating a sequential reaction.After dropping the tap water onto the device, tap water and oxygen easily infiltrate through the capillary channels of the microporous Grl, while carrying hydronium (H 3 O + ) ions in the water to the GOl.The movement of the water and H 3 O + ions could stimulate GOl.On the one hand, the layer-bylayer structure lattice with hydrophilic groups, such as hydroxyl (─OH), and carboxylic (─COOH) groups, and their narrow pore size [68,69] further allowed the deep water penetration into the electrolyte channel; [62,63] On the other hand, the H + ions are simultaneously guided according to the ion jumping principle [70][71][72] (also called the Grothuss mechanism) owing to the top-down movement of water and H 3 O + .Thus, abundant free H + was released close to the upper surface of the electrolyte channel.Additionally, the cathode reactions follow (1) of Equation S12 (Supporting Information).Thus, the middle of GOGr electrode was formed by OH − and immobile negatively charged functional groups bonded on GO (e.g., ─COO − ), [7] which raised the zeta potential of the electrolyte channel and facilitate more H + flow down.Hence, a larger potential difference was boosted across the two electrodes.Though the movement of water through the individual top part or the top part coated with PVA-H 3 PO 4 quasisolid state electrolyte could also produce potential when PVA-H 3 PO 4 electrolyte was used to bridge two parts of HC elect , a noticeable difference in V oc -t curves trend was observed, as demonstrated in Figure S10 (Supporting Information).Thus, it was necessary to further explore the role of PVA-H 3 PO 4 electrolytes in hydrocapacitor.
The separation of electrolyte channels plays a major role in generating current and potential across the device, similar to other hydrocapacitors or MEGs, where traditional streaming potential was supposed to be mainly responsible for the electrolyte operation. [39,41,48]Thus, the PVA-H 3 PO 4 electrolyte has three main possible roles based on the streaming potential theory as follows.First, its better ionization and viscosity than those of other common PVA-conductive substance solid-state electrolytes, [54] improving the H + ion diffusion through the electrolyte channel as well as electrode attachment.Second, separating the two electrodes caused a net separation of the conductive ions or induce a current from the water movement in the hydrocapacitors, which occurred in the electrolyte channel or porous structure underwater diffusion. [48,57,73]Third, the intrinsic hydrophilicity of the PVA-H 3 PO 4 electrolyte allows its PVA part to produce additional H + flow in response to water and transport water up to the bottom electrode.Based on the proton jumping process, the PVA chains could transport H + through the electrolyte channel due to its hydrogen bond network of the alcoholic hydroxyl groups, driven by the top-down movement of water, while repelling negative ions and forming an electrical double layer on the surface of the PVA chain, as shown in Figure 4.According to the streaming potential theory, [52,74,75] when water infiltrated the electrolyte channel, the H + ions are simultaneously carried by jumping over the PVA chain.Consequently, a net positive charge flow is released downstream, while negative ions are repulsed upstream.This theory is consistent with the higher potential obtained at the downstream electrode.In addition, as the water approached the device, the aqueous H 3 PO 4 would dissociate positive ions, such as H + and H 3 O + , and negative ions, such as dihydrogen phosphate (H 2 PO 4 − ), hydrogenphosphate (HPO 4 2− ), and phosphate (PO 4 3− ), according to the dissociation equation (Equation S13, Supporting Information), which can contribute to the asymmetric reorientationdislocation process and potential across the electrolyte.As the H + ions flow downstream to the bottom electrode, water then activates the reaction of the anodic electrode.Owing to the relatively similar structure to the Al-air batteries, Al was oxidized to Al 3+ following Equation S12 (Supporting Information), whereas O 2 was reduced at the top electrode, and the energy was stored in the hydrocapacitor as the same Al-air battery reaction. [55,58,76]arious separators were also studied, including Nafion and plant fiber membranes, for output and structure optimization of the device, as shown in Figure 3.According to previous studies, porous plant fiber membranes, such as weighing paper, filter paper, and paper wiper, have a negative zeta potential from ≈ −20 to −30 mV, [60,[77][78][79][80] resulting in the selective downstream passage of positive ions [61] and residual negative ions at the top electrode.These theories were consistent with the downstream conductive ion flow with positive potential.As ion exchange membranes, the working principle of the Nafion membrane is similar to that of the electrolytic membrane. [57]

Demonstration of the Expandability and Applicability of the Proposed Device
Figure 5 shows the experimental results of the HC elect to demonstrate its application and extensibility.The HC elect is suitable as a small portable and wearable power source and can be infinitely expanded by integrating the HC elect cells in series or parallel to linearly enhance output results for practical applications.Figure 5a,b shows the electrical properties of one, two, three, and four cells of the hydrocapacitor connected in series and parallel, respectively, after adding the same amount of tap water.A high voltage of 3.4 V was achieved by a series connection of four hydrocapacitors, respectively (Figure 5a).Similarly, four hydrocapacitors connected in parallel increased the I sc from 49.65 to 161.7 μA (Figure 5b).The compact structure of the HC elect allows it easy to achieve continuous expansion and broadens applications by providing a sufficient output voltage.By loading a potentiometer with various resistances, the electric output of the external devices provided by the HC elect was investigated in an equivalent circuit (Figure 5c).Correspondingly, the resistance level increased from 1 kΩ to 4 mΩ, the voltage of the potentiometer increased from 0.01 to 0.94 V, and the current decreased from 49.55 to 1.24 μA.The resistor had a maximum instantaneous power of 52.47 μW corresponding to a resistance of 0.1 mΩ.With only two HC elect cells arranged in series, the entire device can generate sufficient electrical output to directly activate a digital timer and LED, corresponding to the onset voltages of 1.5 and 1.8 V, as shown in Figure 5d and Movies S2 and S3 (Supporting Information).Moreover, dividing the device array into small pieces could enhance output power because the electrical output was independent of the film area.In this work, we focused on the applicability of the hydrocapacitor; thus, tap water was used in most of the experiments because of its prevalence in the environment, resulting in a wider application range.
In addition to the inexhaustible water source in the environment, the human skin provides a steady and constant water supply.In Figure 5e, the HC eclect was attached to the wrist, and contact was maintained until the water on the fingers dried up.A maximum voltage of 0.91 V was recorded after 48 s of contact.After ≈860 s, the skin surface became dry, leading to voltage instability and rapidly decreasing.The rapid rise of the V oc trend was supposed to be affected by both moisture and mechanical force caused by the finger.This test demonstrated the potential for the energy conversion of the device from the water present in the human skin.Furthermore, the device could also be transformed into complex shapes to incorporate wearable devices owing to the bendability of its structural layers.
Although previous research has enhanced the performance of hydrocapacitors by adding more working layers, this has consequently increased the cost and complexity of the fabrication and construction.In this study, we introduced a novel hydrocapacitor and its working mechanism.This novel hydrocapacitor integrated a water-induced electric generator and energy storage device, which addresses the problems of stable and continuous environment sources and device complexity.In addition, the HC elect could be easily integrated with other rechargeable supercapacitors [81][82][83] in a series or parallel to simultaneously enhance their specific energy and power.These combinations demonstrate great application potential for portable and wearable devices.

Conclusion
In summary, novel bilayer GOGr composite films symmetrically sandwiched by PVA-H 3 PO 4 electrolyte were developed through a facile method.Herein, we proposed a novel dual-function 2D centimeter-sized device, referred to as the hydrocapacitor, which can easily transform electricity from a small amount of water droplet and simultaneously store the generated energy.The device reached the maximal voltage of 0.94 V with the addition of only a 28 μL droplet of tap water, which could be explained by the combination of water-movement-induced electricity generation principles, including capillarity, ion diffusion, and streaming potential.The performance of the hydrocapacitor is influenced by the types of separators, dropping solutions, and electrodes.Moreover, we demonstrated the performance of the hydrocapacitor cells arranged in series and parallel to further enhance the output without a limit.Two single cells in a series could power an LED and digital timer.More impressive, this flexible hydrocapacitor effectively converted electricity from the water on the human skin by prolonged finger touching.The hydrocapacitor has considerable properties, such as being small in thickness, flexible, low cost, environmentally friendly, and wide range of applications.It can provide a reference for the new generation of small portable, and wearable devices.

Experimental Section
Preparation of the Bilayer GOGr Composite Film: GO powders were synthesized through the chemical exfoliation of graphite flake (Alfa Aesar, 7-10 μm) according to the modified Hummers method [62] via freezedrying.An aqueous GO homogenous solution (8 mg mL −1 ) was obtained by sonicating the GO powders in 30 mL distilled water for 30 min.Subsequently, the GO homogenous solution and 0.08 g Gr were well mixed using an ultrasonic stirrer in a glass beaker (capacity of 50 mL and diameter of 42 mm) for 45 min at 5 °C.The stirring process included three 15 min stages corresponding to different positions of the ultrasonic probe at 25, 15, and 5 mL levels, respectively, to obtain a homogeneous composite solution.The resultant solution was spread on a standard 120 mm petri dish and then dried in a vacuum oven for 24 h at 60 °C to obtain a 15 ± 3 μm thick bilayer GOGr composite film, as shown in Figure S11a-c (Supporting Information).
In this study, conventional GO films with an average thickness of 13 μm were also fabricated for comparative measurements and examinations.In detail, 30 mL of the same aqueous GO homogenous solution was ultrasonicated for 15 min at 5 °C, poured into a petri dish of the same dimensions, and vacuum dried in a vacuum oven for 18 h at 60 °C.
Preparation of the PVA-H 3 PO 4 Electrolyte and Separators: The weighing paper, paper wiper, and filter paper with thicknesses of 30, 80, and 200 μm, respectively, are common laboratory materials, which were cut-tosize sheets.In addition, 127 μm thick proton exchange membrane DuPont Nafion N117 was purchased directly from the internet, which was also cut into the same shape.Each piece of the Nafion membrane was boiled in 3 wt.%H 2 O 2 , 1.0 M H 2 SO 4 , and deionized water.
Preparation of the Dropping Solutions: Under magnetic stirring, 100 mg mL −1 hydrolyzed PVA uniform gel was obtained by stirring PVA powders (Sigma Aldrich, M w 13 000-23 000) in 10 mL boiling distilled water at 90 °C for 2 h.Subsequently, 1 g H 3 PO 4 (Sigma Aldrich, purity of 99%) was slowly added to obtain the PVA-H 3 PO 4 gel electrolyte.To prevent the effects of temperatures, the electrolyte solution was cooled to room temperature before fabrication.
Deionized water, tap water, and EtOH 85% are common laboratory solutions.NaCl, HCl, DMF, and H 3 PO 4 (purity 99%) were directly purchased from the internet.
Fabrication of Hydrocapacitors: The GOGr film was cut into a certain shape pieces with a working area of 1 × 1 cm 2 using a laser (Figure S11a, Supporting Information).
The weighing paper, filter paper, paper wiper, and Nafion separator membrane were cut into a 1.1 × 1.1 cm 2 working size, which is larger than the working area to prevent short circuits.
As shown in Figure S11d (Supporting Information), an Al foil adhesive tape (Teraoka, Japan) was cut into two suited sizes to prevent short circuits as the substrate/positive and negative current collectors at the top and bottom surfaces, respectively.A small hole with a diameter of ≈2 mm on the top collector allows the dropped solutions to penetrate the hydrocapacitor.First, the Al current collectors were pasted onto the porous surface visible to the naked eye of two separate GOGr electrodes as the bottom and top parts.These two parts were sandwiched by a 45 mg PVA-H 3 PO 4 gel electrolyte to obtain an HC elect and membrane separators to obtain HC mem s.Subsequently, the HC elect was placed in a fume hood for 3 h at 60 °C until the electrolyte changed to a quasi-solid state.After drying, the compact HC elect was ≈85 ± 5 μm and 0.3 g in thickness and weight, respectively.
Material Characterization and Electrical Measurement: The surface morphology was observed by a scanning electron microscope (SEM, Hitachi High Technologies, Japan).The GOGr film was characterized by the X-ray diffraction technique (XRD, scan rate of 5°min −1 , Rigaku Co. Ltd, JP).The elemental composition was identified using X-ray photoelectron spectroscopy (XPS, Axis-Ultra HSA SV, Kratos Analytical, Japan).The surface hydrophilicity was determined by contact angle measurements (Digidrop, GBX, Whitestone Way, France, 65%, 25 °C).Surface resistivity measurement was examined using a resistivity meter (Loresta-AX MCP-T370, Nittoseiko Analytech, Japan).The S BET and pore properties of the GOGr electrode were determined by N 2 adsorption-desorption experiments at −196 °C using a surface area and porosity analyzer (TriStar II Series, Micromeritics instrument Ltd., USA).
Electric output measurements versus time (V/I/R-t) curves were recorded by a dual measurement multimeter GDM-8342 (GW Instek, Taiwan).GCD experiments were performed on a digital ADCMT 6240 source/monitor meter (ADCMT Advantest, Japan) using the battery charge/discharge test function controlled by the Labview software.A digital thermometer/hygrometer recorder (SwitchBot, China) was used to record the ambient temperature and RH via the smartphone app.A dry chamber filled with N 2 gas and desiccant beads was used to obtain a dehydration environment at an RH of 0 +5 humidity and reset the device to its initial state after each test.All output measurements were conducted in natural light and ambient conditions at 24 °C and RH of 34%.

Figure 1 .
Figure 1.a) Schematic illustration and a digital photograph of the quasi-solid-state HC elect .b) Schematic of the cross-section construction of HC elect .c) Cross-section microscopic morphologies, and surface topography at the d) top surface and e) bottom surface of the bilayer GOGr film.f) XPS survey spectra and g) high-resolution XPS C 1s spectra of the top and bottom surfaces of the GOGr film and conventional GO film.

Figure 2 .
Figure 2. a) V oc output of the HC elect in response to dropping tap water.b) One cycle of the typical open-circuit voltage variation of the HC elect after dropping tap water until got dried.The inset graph shows the maximal open-circuit voltage of the same HC elect every day for 47 days.c) V oc and d) I sc versus time during self-charging and discharging processes with a load of 100 Ω.The arrows indicated that 28 μL of tap water was start dropped onto the HC elect .Insert map shows equivalent electrical circuits corresponding to charge and discharge processes.

Figure 3 .
Figure 3. a) Schematic diagram of the HC mem on the plant fiber of the membrane working under dropping 28 μL of tap water.b) Comparison of the reached output voltage of every HC mem , which used GOGr electrodes, tap water, and different separators, including plant fiber, Nafion, and electrolyte membranes.c) Comparison of the reached output voltage of every HC mem , which used GOGr electrodes, PVA-H 3 PO 4 electrolytic separator, and different types of dropping solution.

Figure 4 .
Figure 4. a) Schematic illustration for the possible mechanism of self-charging power system integrating a cell of HC elect and practical device.b) Schematic diagram for ion diffusion in the nanofluidic electrolyte channels driven by the induced streaming potential.

Figure 5 .
Figure 5. a-e) Properties and applications of HC elect .Voltage and current outputs for HC elect connected a) in series and b) in parallel.c) Electric output of potentiometer with different resistances level connected to HC elect after dropping tap water.d) Photograph of digital timer and commercial light-emitting diode (LED) powered by two HC elect cells connected in series.e) V oc of HC elect that worked as a self-powered device from human body moisture.The inset shows the experiment method, in which the device was attached to the wrist as a flexible device and was charged by long-lasting finger touch.In this experiment, Al top collector here was manufactured with a 4 × 4 holes array (1.5 mm in hole diameter) to increase the contact area with the finger.

Table 1 .
Summary and comparison of summary and comparison of water molecules movement-based self-charging devices..