Introduction

Solid-liquid interface science is a crucial discipline within chemistry1, catalysis2,3, and energy4. In particular, the issue of charge transfer at the solid-liquid interface has received widespread attention. A recent paper by Ben-Amotz suggests that hydrogen bonds in water may hold special significance for charge transfer5. In fact, even at solid-solid interfaces, there remains controversy over whether the transferred charge involves electrons, ions6, or free radicals7,8,9. The fluidity and dispersion of liquids, along with the adsorption of ions, introduce greater uncertainty at solid-liquid interfaces.

Solid-liquid contact electrification is a common phenomenon, and researchers have recently utilized techniques such as thermal electron emission and other analytical methods to provide evidence of electron transfer occurring at solid-liquid interfaces10,11,12. Research has shown that contact electrification and electrostatic induction between water droplets and polymer surfaces can generate instantaneous voltages of several hundred volts13,14. However, the instability of water droplets and limitations in power output greatly constrain its application value15. Additionally, the underlying mechanism of liquid-phase electron transfer remains uncertain. Material selection and structural development hinge on understanding the mechanism, which is crucial for enhancing performance16.

Based on solid-liquid interfacial electron transfer, it has been proposed that this process can drive chemical reactions17,18, including the synthesis of H2O219,20. Zare’s group21. employed microfluidic injection to induce H2O2 generation via friction between the liquid and the capillary wall. They proposed that the production of H2O2 is associated with the hydroxyl groups (OH-) present on the solid surface. Wang’s group22 and Fan’s group23 utilized dielectric powders dispersed in water to enhance solid-liquid friction efficiency (hundreds-watt of external power supply). They believe that the process of H2O2 production involves the adsorption of O2. Although the mechanism behind the spontaneous generation of H2O2 at the solid-liquid interface remains controversial, it is widely accepted that interfacial electron transfer leads to the synthesis of H2O2. This suggests that when the electrode is part of the solid-liquid system, electron transfer not only generates electrical signals24,25 but may also lead to the generation of non-catalytic26,27 H2O2 in the liquid phase.

Here, we present a spontaneous H2O2 & energy generator (HEG). The main component is a sealed Fluorinated ethylene propylene (FEP) tube injected with a small amount of water. The fluid continuously slides within the chamber as the tube is shaken in the external environment. By investigating the charge transfer mechanism at the solid-liquid interface, we demonstrate that the generation of H2O2 and the output of electrical properties occur simultaneously in this process, both linked to the electronegativity of the solid phase. By designing a cyclic sliding structure between the liquid and solid phases, the electrical properties of the solid-liquid interface can be continuously collected, and the occurrence of interfacial chemical reactions can be enhanced. Moreover, by studying the electron donors and electron transfer products in the liquid phase, the electrical performance output of the solid-liquid power generation system can be further improved, achieving an output power of up to 5.8 kW/m3. It also demonstrated its applications in wave energy harvesting, wireless communications, corrosion prevention, and other fields.

Results

Solid-liquid power generation and H2O2 production during water flow

Water molecules at the interface exhibit different properties from those in bulk water28,29. In particular, during water flow, the breaking and rearranging of hydrogen bonds between water molecules cause the solid-liquid interface to continuously disappear and reappear. This dynamically active solid-liquid interface holds the potential for chemical reactions and energy transfer during water flow (Fig. 1a and Supplementary Fig. 1).

Fig. 1: Electron transfer mechanism at the solid-liquid interface and H2O2 generation.
Fig. 1: Electron transfer mechanism at the solid-liquid interface and H2O2 generation.
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a During the water flow process, chemical reactions and energy transfer occur at the solid-liquid (S/L) interface. b The circulation of water flow promotes interfacial chemical reactions, resulting in the accumulation of H2O2. c Solid-liquid interface electric double layer model. d After deionized water and Fluorinated ethylene propylene (FEP) undergo cyclic contact and separation, brightfield and fluorescence images of mixtures containing deionized water and 2’,7’-dichlorofluorescein diacetate (DCF-DA) were obtained.

Based on the mechanism of solid-liquid charge transfer, solid-liquid triboelectric nanogenerators can harvest energy from low-frequency and irregular water sources, such as rainwater. To enhance the electrical performance of this green energy, in addition to designing the electrode structure and constructing solid-phase roughness, it is essential to study the liquid phase properties at the solid-liquid interface. Using Kelvin probe force microscopy (KPFM)18 (Supplementary Fig. 2), we demonstrate that electron transfer is dominant in the charge transfer process between FEP and water (Supplementary Fig. 3, 4). Notably, while the solid phase gradually accumulates electrons to generate a continuous electrical signal, the liquid phase continuously loses electrons (Fig. 1b). OH- provide some of the transferred electrons at the solid-liquid interface, converting into OH19, and then combine with each other to form H2O2. This indicates that electron transfer at the interface between FEP and water simultaneously drives both energy collection by the generator and the generation of H2O2 (Fig. 1c).

To establish an ideal solid-liquid interface environment, we designed a sealed FEP tube. It contains a small amount of deionized water to minimize the influence of external factors such as air. Gentle shaking the FEP tube ensures a continuous flow of water inside, promoting sustained contact and separation between water and the inner surface of the FEP tube. The circulation of water flow promotes interfacial chemical reactions, resulting in the accumulation of H2O2.

We utilized 2’,7’-dichlorofluorescein diacetate (DCF-DA), a sensitive probe for H2O2 (Supplementary Fig. 5). After cyclic shaking of the tube, we mixed the water-soluble fluorescent probe with the deionized water inside. Under 488 nm laser excitation, laser confocal microscopy detected a strong green fluorescence reflection (Fig. 1d), absent in untreated deionized water (Supplementary Fig. 6). This confirms the production of H2O2 when deionized water periodically collides with FEP.

Mechanisms of solid-liquid electric power generation and H2O2 production

Electron transfer at the solid-liquid interface can generate electrical signals when electrodes are included in the system. Continuous contact and separation between the solid and liquid phases can lead to the accumulation of a high charge density on the surface of the fluorinated material, resulting in a non-negligible energy output. The main body of our HEG power generation module is a FEP tube (10 cm in length). Two Al electrodes are encased around the exterior of the tube, while two Ag wires are threaded through the small apertures in the center of the caps at either end of the tube (Fig. 2a). This design resembles a field-effect transistor13 (including source, drain, and gate) (Supplementary Fig. 7). When the flowing water in the tube contacts the drain Ag electrodes at both ends, it generates a huge voltage (Supplementary Figs. 8, 9). In the structural design, the Al electrodes of the source function to collect induced charges. The type of metal electrode has almost no impact on the electrical performance (Supplementary Fig. 10). However, there is a tendency for interface charge recombination to occur between the high charge density FEP and the closely fitting metal electrode (Supplementary Fig. 11). To address this issue, we employ electrospinning technology to apply a layer of Polystyrene (PS) nanofibers onto the Al sheet surface (Supplementary Fig. 12). PS is a common insulating material for triboelectricity, possessing robust charge capture and storage layer properties (Fig. 2b). This enhances electrical properties by reducing the recombination of interface charges (Fig. 2c and Supplementary Fig. 13).

Fig. 2: Mechanisms of solid-liquid electric power generation and H2O2 production.
Fig. 2: Mechanisms of solid-liquid electric power generation and H2O2 production.
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a Structure of HEG. b Polystyrene (PS) mitigates interface charge recombination. c Enhancement of electrical performance through the PS isolation layer. d Using 1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane (PFDTMS) for inner tube wall modification. e X-ray photoelectron spectroscopy (XPS) pattern of the inner wall of Fluorinated ethylene propylene (FEP) tube before (top panel) and after (bottom panel) modification. f Surface potential distributions of Kelvin probe force microscopy (KPFM) results. g The absorption spectrum of an aqueous potassium titanium oxalate (PTO) solution with added H2O2. Examples microdroplet spectra in red and blue. h Calibration curve of absorption at 400 nm acquired from the UV absorption spectrum. i The friction cycle affects the concentration of H2O2 produced. The error bars in (c) represent the standard deviations from four parallel measurements. Source data are provided as a Source Data file.

Solid-liquid power generation involves characteristic processes represented by t0, t1, t2, t3, t4, t5, and t6 (Supplementary Fig. 14). Upon water contacting the Ag electrodes at the left and right ends, a closed loop forms (t3, t6), resulting in an instantaneous and significant electrical signal. As water disconnects from the Ag electrodes (t2, t5), charge transfer decreases. When water diffuses to or from the top of the Al electrodes (t1, t4), charge transfer either begins or stops. It is noteworthy that the electrical signals of the left and right HEGs are generally similar, the difference is that one of the signals slightly lags behind (Supplementary Fig. 15).

To further confirm the correlation between the electron transfer capability of the solid-liquid interface and the electrical signal output, as well as the production of H2O2, we employed the electronegative material 1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane (PFDTMS) to modify the inner wall of FEP tube (Fig. 2d and Supplementary Fig. 16). In the X-ray photoelectron spectroscopy (XPS) test chart in Fig. 2e, the surface of FEP modified by PFDTMS exhibits the appearance of the -CF3 peak and the increase of -CF2 peak. The heightened presence of the electron-attracting F element enhances the electronegativity of the solid phase surface, thereby improving electron transfer performance at the solid-liquid interface. (Supplementary Fig. 17). The generation of HEG electrical signals is inseparable from electron migration. We employed KPFM to scan the inner wall of the FEP tube before and after solid-liquid contact. The resulting KPFM image (Fig. 2f and Supplementary Figs. 3, 4) clearly shows the potential change before and after such contact, consistent with the electron transfer theory proposed by the potential well model11 (Supplementary Fig. 1).

In addition to the potassium titanium oxalate (PTO) method (Supplementary Fig. 18), the KI method30 and the potassium permanganate method23 are also commonly used for quantitative testing of H2O2. Figure 2g shows different concentrations of H2O2 alongside the UV absorption spectrum of the sample. The concentration of H2O2 in the modified tube increased from about 8 μM to 18 μM (Fig. 2h and Supplementary Figs. 19, 20), indicating that the increase in solid phase electronegativity promotes the production of H2O2. Additionally, hydroxyl radicals play a crucial role in the production of H2O2 through electron transfer at the solid-liquid interface (Supplementary Fig. 21). Nevertheless, the H2O2 generated through electron transfer at the solid-liquid interface will not continue to increase as the friction cycle rises (Supplementary Fig. 22), tending to reach saturation after 900 cycles (Fig. 2i and Supplementary Fig. 23).

Optimization of solid-liquid contact power generation

Solid-liquid power generation consists of the charging stage and the saturation stage13 (Supplementary Fig. 24). In the charging stage, periodic contact between the solid and liquid phases causes electrons to transfer from the liquid to the solid phase. During this stage, the electrical signal arises from contact electrification and electrostatic induction. As the solid phase charge density accumulates, the electrical performance gradually increases. When the solid-phase charge density approaches saturation and the solid-liquid electron transfer reaches equilibrium, the system enters the saturation stage. At this point, the electrical signal, driven by electrostatic induction, reaches its maximum and stabilizes.

In addition to the electronegativity of the solid phase, the components of the liquid phase also affect the solid-liquid electrical properties. This is manifested as changes in the electrical properties of the HEG due to change in the liquid phase composition during experiments (Fig. 3a and Supplementary Fig. 25). After friction, the hydroxide ions in the liquid phase lose electrons and convert into hydroxyl radicals, which subsequently produce H2O2 (Supplementary Figs. 26, 27). On the other hand, Fig. 2i shows that the H2O2 concentration reaches a critical value. Does H2O2 affect the charge transfer? In Fig. 3b, we tested the electrical properties of aqueous solutions containing varying concentrations of H2O2, revealing that even trace amounts of H2O2 can adversely affect the electrical properties. This is primarily because the added H2O2 obstructs the solid-liquid electron transfer during the charging stage, thereby affecting the electrical energy output in the final saturation stage. For instance, when 0.1 mM of H2O2 is added to the solid-liquid system, the solid surface potential decreases from −840 V to −512 V (Supplementary Fig. 28). It is worth noting that unlike ions that directly affect the solid-liquid interface performance by intervening in the double electrical layer, after the charge on the solid surface is balanced (saturation stage), the addition of H2O2 will not affect the electrical properties of the solid-liquid system (Supplementary Fig. 29). This is because, at this point, solid-liquid power generation mainly relies on electrostatic induction.

Fig. 3: Optimization of solid-liquid contact power generation.
Fig. 3: Optimization of solid-liquid contact power generation.
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a The equivalent circuit diagram when a liquid contacts a Ag electrode. b Effect of H2O2 concentration in liquid phase on electrical properties. c Effect of NaOH concentration in liquid phase on electrical properties. d The output voltage when the external resistance changes from 1 Ω to 5 × 108 Ω, and the calculation of power output under different resistive loads. e Comparative analysis of power output performance among different wave energy harvesting devices33,34,35,36,37,38,39,40,41. The blue area represents the triboelectric nanogenerator (TENG), and the green area represents the electromagnetic generator (EMG). f Stability of voltage and current performance of power generation unit. The error bars in (b, c, f) represent the standard deviations from four parallel measurements. Source data are provided as a Source Data file.

Based on the solid-liquid electron transfer mechanism, a moderate increase in OH- may be beneficial for the output of electrical performance. Upon reaching an OH- concentration of 0.05 mM in water, the output voltage increases to about 312.5 V, and the current rises to 103.8 μA (Fig. 3c). However, with further increases in OH- concentration, the electrical performance decreases. This decline stems from the increase in OH- concentration weakening the interfacial capacitances C1 and C2. In contrast, a decrease in pH has a great negative impact on the electrical output (Supplementary Fig. 30).

In terms of energy output and storage, half of the HEG (either the left or right end) charges capacitors of 22 μF, 47 μF, and 100 μF to 5.4 V, 2.5 V, and 1.27 V (Supplementary Fig. 31), respectively, within 100 s. Upon integration, half, two, and four devices can charge the 100 μF capacitor to 1.27 V, 4.7 V, and 9.1 V in the same time, demonstrating a linear relationship between output voltage and the number of units. This enables practical applications such as integration and collaborative energy harvesting of HEG. When the external resistance is 8 × 105 Ω, Fig. 3d demonstrates a maximum power output of 9.35 W/m2 (2.9 kW/m3) (Supplementary Fig. 32). In previous works on Fig. 3e (Supplementary Table S2), wave energy harvesters (triboelectric nanogenerator (TENG) and electromagnetic generator (EMG)) mostly fall below 400 kW/m3 in power density, while a single HEG with two power generation units (Supplementary Fig. 7) increases the power output of water to 5.8 kW/m3. Another major advantage of solid-liquid power generation is its stability. After 7 days of cyclic power generation, the HEG maintains stable electrical performance output (Fig. 3f).

Application of HEG in the maritime internet of things

By using interface charge transfer, HEG can simultaneously achieve H2O2 generation and energy harvesting in a wave environment (Fig. 4a), and has practical application capabilities in the fields of interface chemistry and energy harvesting. For the field of interface chemistry, in this green and pollution-free production of H2O2, the presence of free radicals such as ·OH has the ability to promote the chemical degradation of certain pollutants (such as methyl orange)18. In the experiment, we injected 0.5 ppm methyl orange aqueous solution into HEG and used a shaker instrument to simulate wave. After continuous solid-liquid friction, the methyl orange aqueous solution degraded by about 93.5% (Fig. 4b and Supplementary Fig. 33), which showed the potential for wastewater treatment. This chemical degradation is influenced not only by free radical scavengers. Notably, adding H2O2 also decreases the degradation efficiency of methyl orange (Supplementary Fig. 34), indicating that the presence of H2O2 impacts free radical generation. The H2O2 and free radicals generated at this interface, along with the local electric field31 between the electrode and water, all contribute to the antibacterial properties of HEG.

Fig. 4: Application demonstration of maritime Internet of Things.
Fig. 4: Application demonstration of maritime Internet of Things.
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a Utilizing wave energy, the power generation units can serve various purposes such as coastal warning lights, wireless water level detection, and ship anti-corrosion. b Comparison of methyl orange aqueous solution content before and after friction cycle. c The left and right ends of a power generation unit can each drive a warning sign for illumination. The distance between the warning signs is about 3 meters. d Warning signs that alternate glow during the night. e Block diagram of wireless alarm signal transmission and reception. f The signal receiving system triggers a powerful light and sound alarm upon receiving the signal (~15 m). g Schematic diagram of cathodic protection of mild steel in 3.5 wt% NaCl aqueous solution. h Surface morphology images of low carbon steel with and without electrochemical cathodic protection after 4 h, 8 h, and 12 h respectively. Source data are provided as a Source Data file.

For the field of energy collection, HEG also has broad application prospects as solid-liquid power generation devices. The following demonstrates the applications of offshore energy harvesting, wireless monitoring, and corrosion prevention (Supplementary Fig. 37).

As depicted in Fig. 4c, in the application of night warning lights along riversides or seashores, we utilize red and blue LED lights to form warning symbols (Supplementary Fig. 38). A single HEG harvests wave energy to make two warning signs glow alternately (Fig. 4d and Supplementary Movie 1), achieving self-driven lighting (Supplementary Fig. 39). Water level monitoring plays a crucial role in disaster prevention. We have integrated 8 rectified HEGs with wireless systems. When the water level exceeds the warning threshold, the waves drive the HEGs to oscillate and generate electricity. This energy is then collected in the capacitor module to power the wireless module (Fig. 4e). The signal can be transmitted over a long distance (~15 m) to the signal receiving module coil (Supplementary Fig. 40), where it is decoded into flashing bright lights and audibly alerts (Fig. 4f). This ensures the transmission of water level warning signals both visually and audibly (Supplementary Movie 2). For the switch, an electronic switch32 was constructed using a Zener diode, silicon-controlled rectifier (SCR), and resistor, enabling fully wireless, autonomous monitoring (Supplementary Fig. 41).

Furthermore, for ocean-going ships and offshore sensors, corrosion of metal materials by seawater significantly impacts their service life. In Fig. 4g, we collect and utilize wave energy to effectively mitigate metal corrosion through the electrochemical cathodic protection method. In the experiment, we directly connected the positive electrode of the rectified wave energy harvesting unit to the carbon sheet and the negative electrode to the protected mild steel. The wave composition consisted of a 3.5 wt% NaCl aqueous solution, simulating seawater. By observing the surface morphology through a microscope (Fig. 4h), we found that the unprotected low carbon steel exhibited yellow rust after 4 h, whereas the protected low carbon steel only showed partial rust marks after 12 h, indicating a greatly reduction in metal corrosion.

Discussion

In summary, through our investigation of the charge transfer mechanism at the solid-liquid interface, we have confirmed that chemical reactions and energy transfer occur simultaneously during water flow. Leveraging the flow of the liquid phase, we achieved the production of H2O2 at the solid-liquid interface and continuous solid-liquid energy harvesting (5.8 kW/m3). Applications in coastal warning lights, wireless water level detection, and corrosion protection for ships illustrate the wide-ranging applicability of this technology in the maritime Internet of Things.

Methods

Materials

FEP hollow tube (13 mm ID × 15 mm OD) and FEP membrane (0.1 mm thick) were purchased from Fluoroplastic Materials Co., Ltd, China. Al sheet (0.03 mm thick) and Ag wire (0.2 mm) were purchased from Weng Hou Metal Materials Co., Ltd, China, PS powder (Mw = 2.2 × 105) was purchased from Shunjie Plastic Co., Ltd, China. Dimethylformamide (DMF), acetone and formic acid were purchased from Aladdin Chemistry Co., Ltd, China. 1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane was purchased from adamas, China. Potassium titanium oxalate (PTO) was purchased from Shanghai Haohong Biomedical Technology Co., Ltd, China. 2’,7’-Dichlorofluorescein diacetate (DCFH-DA) was purchased from Shanghai Yuanye Biotechnology Co., Ltd, China. Ethylenediaminetetraacetic acid (EDTA), Tert-Butanol, p-Benzoquinone, AgNO3, O-phenylenediamine (OPD) were purchased from adamas, China. KI, ammonium molybdate was purchased from Sigma-Aldrich. Ammonium molybdate and potassium permanganate were purchased from Macklin.

Fabrication process of the HEG

Preparation of PS isolation layer: 15 wt% PS (mass ratio) was dissolved in DMF at 60 °C, and PS nanofibers were uniformly deposited on the Al sheet by means of electrospinning (TEADFS-700, China). The spinning parameters were as follows: voltage of 18 kV, receiving distance of 15 cm, advancing speed of 0.5 ml/h, and needle inner diameter of 0.5 mm.

Modified tube inner wall: Prepare a 1 wt% solution of 1H, 1H, 2H, 2H-Perfluorodecyltrimethoxysilane (PFDTMS)/ethanol. Inject the solution into the tube, seal the tube for 3 h, then remove the solution and dry the tube. Ensure that the inner tube walls are modified with fluorosilane coupling agent.

Fabrication of the HEG: The main body of the HEG power generation module consists of a FEP tube (length 10 cm). Two pieces of PS/Al sheets, each 3.75 cm wide, are used to cover both ends of the tube as external electrodes. Inside the tube, 4 mL of liquid is filled. The openings at both ends of the tube are sealed with FEP membranes and secured with PET covers made by 3D printing. Two Ag wires are introduced through the center holes of the covers at both ends, and the interfaces are sealed with waterproof adhesive.

Antibacterial test

Escherichia coli BNCC269342 (Henan Engineering Research Center of lndustrial Microbiology) was cultured on LB agar (Cat#L1015, Solarbio) medium at 37 °C for 24 h. The bacterial culture was then divided into three groups: Group I was injected into a modified sealed FEP tube, kept stationary, and incubated at 37 °C for 3 h. In contrast, Groups II and III were injected into sealed FEP tubes (without electrodes) and HEG devices (with electrodes), respectively, and shaken in a shaker for 3 h. 50 µL of bacterial suspension from each group were then transferred onto LB agar plates, spread evenly using a bacterial spreader, and incubated at 37 °C for 24 h. After incubation, the plates were gently rinsed with 1X PBS and centrifuged to collect the bacterial pellet. Antibacterial activity was characterized using the SpectraMax Paradigm Multi-Mode Detection Platform. The incident light wavelength was set to 600 nm, and the optical density (OD600) was measured after the bacterial solution was diluted tenfold.

Characterization

For the investigation of sample morphologies, a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) was utilized. The near-infrared spectrum was tested using a UV-Vis-NIR spectrometer (UV3600, Japan). X-ray photoelectron spectroscopy (XPS) was performed using an Escalab250Xi photoelectron spectrometer. Kelvin probe force microscope (KPFM) measurements were performed using atomic force microscope (Park NX20). Fluorescence imaging research conducted via laser scanning confocal microscope (TCS SP5II). The Keithley 6514 was used to test the electrical output performance.