A spontaneously sustainable bioinspired interfacial evaporation driven power generation strategy

In liquid evaporation occurs everywhere all the time. This low-grade energy absorption to drive liquid evaporation is greatly potential for sustainable spontaneously power generation. Here, a natural liquid evaporation strategy of interfacial evaporation driven nanogenerator (IENG) is developed in this work. Coupled by the phonon wind and a fluctuating Coulomb field, an induced direct current is generated. Simultaneously, inspired by the light-trapping properties of moth eye, a simple and efficient BLT-IENG including a hierarchical surface of bionic light-trapping and electrospinning perovskite conductivity with an enhanced thermally insulating and water storage capability is designed. This enhancement of the output performance is greatly attributed to the improvement of the interfacial evaporation characteristics driven by natural solar and wind energies. Hence, our BLT-IENG achieves a breakthrough in the unit area open-circuit voltage in the marine environment, which is improved by a factor of 7.6 over the currently reported average value. This work provides an unexplored strategy for multi-energies inspired natural interfacial evaporation driven power generation. At the same time, the ITW-layer is designed by using enhanced thermally insulating particles of ionic liquid (1-n-butyl-3-methylimidazolium chloride (BMIMCl))/cellulose/ titanium dioxide (TiO 2 )/silica (SiO 2 ). Our BLT-IENG shows excellent interfacial evaporation characteristics, including high light absorption efficiency (96.7%), low thermal conductivity (0.766 W m -1 h -1 ), large thermal insulating capacity (increased by 1.5 times), excellent evaporation rate (2.78 kg m -2 h -1 ) and outstanding energy conversion efficiency (97.9 %) under a light intensity of 1.0 kW·m -2 . Hence, our BLT-IENG exhibits a short-circuit current of 222.07 μA, an output power density of 34.15 μW cm -2 , and a unit area open-circuit voltage of 276 mV cm -2 in the marine environment with a light intensity of 2.0 kW·m -2 and a wind speed of 1 m s -1 . This demonstrates a new idea to develop natural interfacial evaporation driven Coulomb field. This work demonstrates an interfacial evaporation inspired sustainable spontaneously power generation technology and provides a foundation for the multiple utilization of natural energies. Meanwhile, it can also generate an opportunity to develop the offshore power generation platforms and the fresh water supply devices.


Introduction
In nature, liquid evaporation occurs everywhere all the time when water is present and exposed [1][2][3] . Compared to boiling, the required threshold for water evaporation is very low. Among them, interfacial evaporation represents the vaporization of water molecules on a solid-liquid surface or a gas-liquid surface 4,5 . The temperature, surface area of the liquid, and airflow rate on the liquid surface are essential factors for the speed of interfacial evaporation 6,7 . As a normal natural phenomenon, interfacial evaporation provides many inspirations to the sustainable spontaneously power generation. Therefore, the tentative exploration of the simple, natural interfacial evaporation driven power generation strategy is very technically promising.
Although the photothermal conversion characteristics are still the limiting bottleneck of the interfacial evaporation performance, the functional structure of natural organisms provides us with new inspiration [8][9][10][11] . In view of the interfacial light and heat absorption of solar energy, the lighttrapping structure on the surface of moth eyes gives us much inspiration due to its zero reflection of sunlight [12][13][14] . The all-inorganic-type perovskite (Cs4PbBr6) with a crystal structure similar to the moth eye structure exhibits a wide light absorption spectrum, achieving effective absorption of sunlight 15,16 . Moreover, interfacial evaporation also largely depends on the water supply/ storage and thermal energy retention 17,18 . In addition, the use of natural wind to accelerate the airflow velocity at the surface liquid will provide effective support for interfacial evaporation 19,20 .
Here, we developed a simple and efficient interfacial evaporation driven nanogenerator (BLT-IENG) including a surface bionic light-trapping and conductivity (BLC) structure and a thermal insulating and water storage (ITW) structure driven by solar and wind energies. The bionic lighttrapping structure is transplanted to the surface of the BLT-IENG by using the 3D template method.
The electrospinning photothermal conductivity layer with light-absorbing particles of Cs4PbBr6type perovskite/ MWNTs is sprayed on the bionic light-trapping structure surface by a layer-bylayer self-assembly method to construct the BLC-layer. At the same time, the ITW-layer is designed by using enhanced thermally insulating particles of ionic liquid (1-n-butyl-3-methylimidazolium chloride (BMIMCl))/cellulose/ titanium dioxide (TiO2)/silica (SiO2). Our BLT-IENG shows excellent interfacial evaporation characteristics, including high light absorption efficiency (96.7%), low thermal conductivity (0.766 W m -1 h -1 ), large thermal insulating capacity (increased by 1.5 times), excellent evaporation rate (2.78 kg m -2 h -1 ) and outstanding energy conversion efficiency (97.9 %) under a light intensity of 1.0 kW·m -2 . Hence, our BLT-IENG exhibits a short-circuit current of 222.07 μA, an output power density of 34.15 μW cm -2 , and a unit area open-circuit voltage of 276 mV cm -2 in the marine environment with a light intensity of 2.0 kW·m -2 and a wind speed of 1 m s -1 . This demonstrates a new idea to develop natural interfacial evaporation driven power generation systems and provides a new attempt to obtain multiple energies harvesting from natural environments.

Structure design and characterization analysis of the BLT-IENG
As conceptually shown in Fig. 1a, the bionic light-trapping structure was prepared on the surface of the BLT-IENG by 3D printing based on the microscopic size and array structure of the moth eye.
Then, its surface was covered with a high-efficiency Cs4PbBr6 photothermal conductivity substrate by electrospinning to prepare the BLC-layer. Meanwhile, an ionic liquid (BMIMCl), nanoparticles (TiO2/SiO2) and α-cellulose were uniformly dispersed in the thermal insulation substrates to construct the ITW-layer ( Supplementary Fig. 1). Our BLT-IENG exhibits efficient solid-liquid interfacial evaporation characteristics for the power generation. With the action of interfacial evaporation driven by solar and wind energies, a flowing potential is formed by the accumulated pressure difference in the multiwalled carbon nanotube (MWNT)-microchannels. After connection, electrons move along the wire from the wet end at the bottom to the dry end exposed to air by the coupling action of the phonon wind and a fluctuating Coulomb field, and an induced direct current is finally generated.
The bean hawk moth eyes are selected as the experimental object in this work ( Supplementary   Fig. 2a). The inside of the moth eye is composed of a hexagonal structure ( Supplementary Fig. 2b).
The partial enlargement shows that the inside of the hexagon is formed by an array of cone-like structures (Fig. 1b). After partial extraction, mesopores with a diameter of approximately 20 nm are dispersed in the BLT-IENG ( Supplementary Fig. 2c). The bionic moth eye structure was fabricated and screened by the 3D template method (Fig. 1c, Supplementary Fig. 3 and Supplementary Fig.   4). The experimental samples with the BLC-layer (Fig. 1d) were prepared by surface spraying nanocomposite materials (Cs4PbBr6, MWNTs, etc.) ( Supplementary Fig. 5). The BLC-layer with the Cs4PbBr6-type perovskite enhances the light absorption efficiency and stability by changing the reflection or refraction light path. The SEM image shows that the surface of the perovskite is composed of a massive structure ( Supplementary Fig. 2d), and a cone-like structure is presented in the TEM image (Fig. 1a), whose shape is close to that of the moth eye.

Power generation performance of the BLT-IENG induced by the interfacial evaporation
Studies show that under the light intensity of 2 kW·m -2 and the wind speed of 1 m·s -1 , the open circuit voltage of the BLT-IENG is as high as 1.106 V (Fig. 2a), and the short-circuit current is as high as 222.067 μA (Fig. 2b). By comparison with Control 2, it is found that the design of the BLT-IENG with bionic light-trapping structure can effectively increase its output voltage and output current by nearly 4 times. The power generation performance of the BLT-IENG is closely related to the interface conductivity and the evaporation capacity. The strength of the electrical signal generated by Control 1 is related to the conductive properties of the Ti derivatives presented in the evaporation interface. The open circuit voltage and the short-circuit current of INEG (Control 2) after MWNTs composite enhancement with high conductivity has been significantly improved by nearly 3 times and 8 times than Control 1. In order to eliminate the interference of external factors, the power generation performance parameters of the interfacial evaporation of pure α-cellulose was experimented in a nearly vacuum environment. The transient current and voltage signals are very weak, which is almost close to zero ( Supplementary Fig. 6c, d). Fig. 2c and Fig. 2d show in detail the voltage and current changes produced by the BLT-IENG during the interfacial evaporation inspired power generation process. When the evaporation rate of the BLT-IENG gradually rises to a stable maximum value, the water absorption and evaporation loss reach a dynamic equilibrium state, which makes the voltage and current generated by the BLT-IENG during this process show corresponding dynamic changes. This process effectively reflects the relationship between the BLT-IENG's power generation and water evaporation. The excellent power generation of the BLT-IENG is mainly attributed to its evaporation performance under the BLC-layer and the ITW-layer

Coupling enhancement effect on interfacial evaporation of the BLT-IENG
The light absorbance of the BLT-IENG with the BLC-layer was measured at wavelengths of 190-2500 nm (Fig. 3a). The average light absorption efficiency is approximately 94.7 %, which is due to the angle of incidence being reduced by the bionic light-trapping structure. The 3D printing manufacturing process method for the BLT-IENG with the bionic light-trapping structure leads to the best photothermal characteristics ( Supplementary Fig. 8a). According to the equations given below, the theoretical reflectivity is 3 % (the theoretical model, see Supplementary Fig. 8b) 23 . x y d f x y n n x y d where n2 is the refractive index of the substrate, n1 is the refractive index of air, d is the diameter of the cylinder, f(x,y) is a single periodic function, and * is the convolution symbol. The refractive index of the moth eye structure is n2=30 in the long-wave infrared band. The microstructure period Λ is (50±0.1) µm, depth h is (30±0.5) µm, and bottom diameter d is ( Simultaneously, experiments show that the BLC-layer surface has a high thermal conductivity, while the thermal conductivity of the ITW-layer is very low (0.743 Wm -1 k -1 ). The TiO2 and SiO2 particles inside can reflect not only light but also heat radiation. Thus, the reflected heat radiation can diffuse back to the inside, thereby storing the absorbed heat converted from light in the ITWlayer. This improves the overall temperature of the BLT-IENG system ( Supplementary Fig. 10a).
Comparing the surface temperature of different ITW-layer, the thermal insulation performance of the ITW-layers with TiO2 or SiO2 is greatly improved (Supplementary Fig. 10b). Among them, the surface temperature of the ITW-layer with TiO2 and SiO2 is basically stable, which is approximately 30 °C after 1 h of evaporation (Fig. 3d). Meanwhile, the water absorption capacity of the ITW-layer can be expressed by its water content (Q) in the saturated state, which is represented by Q=(m1-m2)/m1 24 , where m1 is the weight of the ITW-layer in the saturated state (g) and m2 is the weight of the ITW layer after drying treatment (g). Result shows that the highest water content ratio is approximately 82 % ( Supplementary Fig. 10c).
In general, the temperature gradient of the BLC-layer in the natural environment after illuminated by the sunlight is relatively large, and relatively speaking, the temperature of the ITW-layer can be maintained stably (Fig. 3e). The BLT-IENG with the light-trapping structure, whose upper surface is covered with the photothermal BLC-layer and lower surface existing thermal insulating ITWlayer can achieve efficient interface photothermal conversion (Fig. 3f). A large temperature shock is generated at the evaporation interface to achieve efficient interface photothermal conversion, and the moisture storage maintains good thermal insulating properties to ensure a stable moisture supply.
The Meanwhile, the electrons and holes in TiO2, TiNx and SiO2 particles constitute carriers in the semiconductor. In actual situations, these electrons are easily affected by various scattering factors, causing them to transit between different electronic states. When the system is in dynamic equilibrium, there will be many electron-hole pairs in the particles. When the BLT-IENG is stimulated by the sunlight, the concentration of electrons and holes will change due to changes in external conditions, and the system will no longer maintain a dynamic balance. In the case of nonequilibrium, the excess carriers will recombine. When the external environment becomes stable and the external excitation is stopped, the system will return to a dynamic equilibrium state. In the Auger process, the recombination of conduction band electrons and valence band holes transfers energy to the third carrier through the collision process. When electrons are excited by light or heat, the electrons absorb energy, so the entire particle is excited to a higher energy level 21,22 . When the electrons return to the ground state, the particles return to the ground state, thereby converting the excess energy into thermal energy that spreads inside the BLT-IENG (Supplementary Fig. 11b).

Power generation performance of the BLT-IENG in the marine environment
After determining that the BLT-IENG has the best water evaporation properties, we improved its power generation performance by varying marine environmental factors. Interestingly, unlike the common triboelectric nanogenerators (TENGs) generating an alternating current, our BLT-IENG outputs a direct current. By increasing the external light to 2 kW·m -2 , the total input power of the external environment is increased, which greatly improves the water evaporation rate of the BLT-IENG. Therefore, it has a higher open-circuit voltage (0.894 V), as shown in Fig. 4a, and a higher short-circuit current (230.2 μA), as shown in Fig. 4b. After connecting a variable load (0-10 6 Ω), its maximum power density is 12.86 μW·cm -2 (Fig. 4c). Fig. 4d shows that the open-circuit voltage of the BLT-IENG in sea water is increased to 1.01 V, and it has a larger short-circuit current (215.6 μA) in Fig. 4e, due to the ion concentration increase. However, when the ion concentration is too high, thus blocking the pores in the mesoporous BLT-IENG during the interfacial evaporation process, the water evaporation rate decreases. At the same time, the increase in the ion concentration increases the number of ions crystallized inside the BLT-IENG, thereby increasing its resistance and reducing its short-circuit current. Comparison studies show that the power generation performance of the BLT-IENG in sea water is better than that in the other solutions overall. After connection to various loads (0-10 6 Ω), the maximum power can reach 13.61 μW·cm -2 (Fig. 4f). To further improve the power generation of the BLT-IENG, we change the wind speed on the surface of the BLT-IENG. Fig. 4g shows that the open-circuit voltage of the BLT-IENG increases to 1.106 V and its short-circuit current increases to 222.067 μA under a wind speed of 1 m·s -1 (Fig. 4h).
After connection to various loads (0-10 6 Ω), the maximum power density can reach 15.46 μW·cm -2 (Fig. 4i). The increase in wind speed can effectively increase the water evaporation rate of the BLT-IENG. However, when the wind speed is too high, part of the water molecules is subjected to excessive force from the wind, so that they cannot interact with the BLC-layer and directly diffuse into the air. Although the increase in wind speed can increase the water evaporation rate of the BLT-IENG, the ultra-high wind speed can also reduce the effective evaporation rate of the BLT-IENG for the power generation, thereby significantly weakening the power generation performance.
Studies have shown that our BLT-IENG in sea water has the best power generation performance under a light intensity of 2 kW·m -2 and a wind speed of 1 m·s -1 . It has a maximum open-circuit voltage of 1.106 V, a maximum short-circuit current of 222.067 μA and an output power of 245.61 μW without an external load.

Working principle induced by interfacial evaporation and application of the BLT-IENG
The above advantages of the BLT-IENG are mainly attributed to the following reasons (Fig. 5a). Therefore, there will be no difference in the ion concentrations on both sides of the MWNTmicrochannels, and the BLT-IENG appears to be electrically neutral. However, when water molecules pass through the MWNT-microchannels, which are narrow enough to overlap the electric double layer, the MWNT-microchannels will have ion-selective permeability. As a result, the flux of cations through the MWNT-microchannels will be larger than that of anions, and the solution in the MWNT-microchannels will no longer be electrically neutral. Moreover, cations and anions will accumulate at the exit and entrance of the MWNT-microchannels, respectively, thereby forming a flowing potential of the electrokinetic phenomenon.
At last, some anions and cations inside and outside the MWNT-microchannels can be removed by vapor during the interfacial evaporation, and the number of ions passing will decrease, thereby reducing the potential of the electric double layer. However, more water molecules participate in the interfacial evaporation are changed into vapor in the initial stage. Thus, there are more interfacial ions than those removed by vapor, and the voltage has a rising tendency at this moment.
As time passes, the evaporation rate of the BLT-IENG gradually increases, and the pressure difference between the entrance and exit of the MWNT-microchannels increases. This results in the numbers of anions and cations produced by water molecules and MWNTs gradually increasing at the entrance and exit of the MWNT-microchannels, thereby gradually increasing the streaming potential. With the water evaporation rate reaching a stable value, anions and cations continuously accumulate at the entrance and exit of the MWNT-microchannels, and the number of anions and cations removed by vapor tends to stabilize, thereby forming a stable and maximum streaming potential. However, because no external circuit is connected, there is only a potential, and no current is formed. After the circuit of the two ends of the BLT-IENG is connected, electrons move along the wire from the wet end at the bottom to the dry end exposed to air via the action of the phonon wind and the fluctuating Coulomb field 21 , and a phonon-Coulomb field coupling direct current is generated (Fig. 5a).
Through the comparison of the evaporation performance under simulated marine environment, the water evaporation rate of the proposed BLT-IENG with sea water increases to 4.385 kg m -2 h -1 (increased nearly twice) at a 2.0 kW·m -2 light intensity and a 1 m s -1 wind speed (Fig. 5b). The produced fresh water can fully meet the WHO's requirements for human drinking standards ( Supplementary Fig. 12a) 25 . The produced fresh water in the BLT-IENG is proven to be derived from sea water instead of the water inside the ITW-layer, further certifying the sustainability of our BLT-IENG ( Supplementary Fig. 13). In addition, we designed a self-powered electronic integrated system (Fig. 5d). After simple signal processing, this integrated system can drive some low-voltage devices by absorbing solar and wind energy (Fig. 5e for working system circuit). Therefore, our BLT-IENG can provide time reminders for the marine equipment, and we can also collect electricity conveniently by charging batteries. In this way, this device can also be widely deployed on the sea for the freshwater production and the electric energy harvesting ( Supplementary Fig. 14), which creates an opportunity to develop the offshore power generation platforms and the fresh water supply devices.

Conclusion
In summary, we demonstrated a solar and wind natural energies driven interfacial evaporation

Data availability
The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Figures and Tables
Title page of bioinspired interfacial evaporation driven nanogenerator.