New paradigms of water‐enabled electrical energy generation

Nanotechnology‐inspired small‐sized water‐enabled electricity generation (WEG) has sparked widespread research interest, especially when applied as an electricity source for off‐grid low‐power electronic equipment and systems. Currently, WEG encompasses a wide range of physical phenomena, generator structures, and power generation environments. However, a systematic framework to clearly describe the connections and differences between these technologies is unavailable. In this review, a comprehensive overview of generator technologies and the typical mechanisms for harvesting water energy is provided. Considering the different roles of water in WEG processes, the related technologies are presented as two different scenarios. Moreover, a detailed analysis of the electrical potential formation in each WEG process is presented, and their similarities and differences are elucidated. Furthermore, a comprehensive compilation of advanced generator architectures and system designs based on hydrological cycle processes is presented, along with their respective energy efficiencies. These nanotechnology‐inspired small‐sized WEG devices show considerable potential for applications in the Internet of Things ecosystem (i.e., microelectronic devices, integrated circuits, and smart clothing). Finally, the prospects and future challenges of WEG devices are also summarized.


INTRODUCTION
After the second industrial revolution in the 1870s, human society entered the Electrical Age. 1 The exploitation of electrical energy has resulted in the rapid development of social economy, culture, science, and the military. 2However, the increasing demand for electricity has driven the consumption of fossil fuels, resulting in environmental pollution and climate change (e.g., the greenhouse effect, acid rain, and eutrophication of water bodies). 3,4Thus, significant efforts have been undertaken to develop renewable and low-carbon-footprint technologies that can generate electricity efficiently while simultaneously reducing environmental pollution. 5ater occupies 70% of the surface of the Earth, supports life, and is a source of abundant energy. 6,7Harnessing the energy stored in water and transforming it into electricity is a well-known and environment-friendly strategy for energy production.Hydroelectricity, alternatively referred to as hydropower or water power, has been under continuous development for centuries. 1,8However, when compared to coal-based electricity generation, hydroelectric generation is significantly limited.For example, the theoretical hydropower reserves in China are 680 million kW, ranking first worldwide.However, as of 2020, the cumulative hydropower generation in China has only reached 17.5 trillion kWh.The hydroelectric power output in China accounts for no more than 20% of its annually generated power, although the demand for hydroelectric power is gradually increasing with the development of human society. 9The inherent development speed of commercial hydroelectricity research has been unable to keep pace with the rapid development of human society.[11][12] Generally, the potential and kinetic energies of seawater are harvested using electromagnetic generators (EMGs).However, extracting low-frequency or microscopic quantities of mechanical energy, low-grade thermal energy, and decentralized chemical energy from water at various stages of the hydrological cycle is difficult using traditional large-sized EMGs.Moreover, EMGs cannot float on water without support because of the weight of the magnets and coils. 9,13,14The engineering-based mindset of hydroelectricity development often hinders the achievement of significant breakthroughs in basic research.
In recent years, varied nanotechnologies have been applied to certain centuries-old technologies, resulting in significant enhancements in their efficiency and ecofriendliness.For example, previous printing technology (i.e., 2D flat printing) has been transformed into nanogreen printing technology (such as the 3D printing technology implemented by Jiang et al. 15 ).Nano-green printing can not only expand the range of printable materials to a significant extent (i.e., from paper to ceramics and glass) but can also reduce the pollution that occurs throughout the printing process.Furthermore, traditional textile technology has evolved into nano-textile technology (such as temperature-adaptive clothing prepared by Cui et al. 16 ).These temperature-control fabrics enabled by nanotechnology exhibit the potential to improve quality of life and reduce fossil energy consumption.Moreover, nanomaterials have been used to deliver novel nanomedicines. 17ong et al. used nanomaterials as carriers for anticancer drugs to enhance their therapeutic effect. 180][21][22][23][24][25] In comparison to the large-sized generators used in hydroelectric AGE-I, micro/nanomaterials and tiny devices are used in the assembly of smallsized generators.Using these novel micro/nanomaterials, more convenient and sensitive small-sized generators can be developed, enabling the exploitation of previously ignored sources of hydroelectricity. 12Thus, emerging novel material systems and technological concepts can be used to improve centuries-old technologies and pave new research routes.
6][27][28][29][30][31][32][33][34] Thus, hydroelectric AGE-II primarily relies on various small-sized functionalized generators (e.g., triboelectric nanogenerators [TENGs], 27,35,36 piezoelectric nanogenerators [PENGs], 20,37 and hydrovoltaic generators 23,38 ) to collect and transform wasted water energy, rather than large-sized devices.Hydroelectric AGE-II has obvious advantages, as explained subsequently.(i) Hydroelectric AGE-II technology can support photovoltaic (PV) energy generation (another green-energybased production technology). 39The performance of both technologies is affected by the weather in contrasting ways, that is, PV performance is reduced under cloudy and rainy conditions, whereas hydroelectric AGE-II generator performance is predominantly enhanced under similar environmental conditions.(ii) When compared to coal-based and nuclear energy electricity generation, hydroelectric AGE-II generators exhibit no carbon emissions or pollutants, involve renewable resources, and have a minimal impact on the environment. 25,402][43][44] Recently, significant progress has been achieved in the effective exploration of novel mechanisms of hydroelectric AGE-II, establishing its foundation for future development. 21,35,40,45Some prior significantly important reviews have only discussed one aspect of small-sized power generation.Owing to differences in the disciplinary backgrounds of researchers, the different mechanisms and technologies of hydropower AGE-II have not yet been systematically integrated into a coherent logical framework.For example, Guo et al. have proposed the concept of hydrovoltaic effect and reported on its fundamental mechanism and various applications. 22ydrovoltaic effect primarily refers to the generation of electrical energy based on the direct interaction of nanomaterials with water.Moreover, Nie et al. introduced the mechanism and applications of liquid-enabled energy harvesting using triboelectric materials. 46Triboelectricity is the electrical energy generated by the synergistic effect of contact electrification and electrostatic induction when the material is in contact with water.However, the WEG process in hydropower AGE-II systems contains multiple mechanisms, which have not been directly discussed and compared from the same perspective.
In this review, the central theme is to construct a framework for presenting the latest advances in WEG in the hydrological cycle.Furthermore, electricity generation mechanisms are discussed in two categories based on the different roles of water in WEG (Section 2), and a comprehensive analysis involving various generator systems and mechanisms is provided.Importantly, this analysis focuses on analyzing current and potential formative processes in these WEG systems, delving into established theories while also exploring new and controversial ones.Section 3 highlights existing electricity generation processes that use the hydrological cycle, evaluating their respective efficiencies.The different designs and their advantages as well as limitations are discussed.Finally, an overview on WEG, specifically addressing the output power of different designs, is provided.The performance of various generator systems and key considerations for optimizing their power output are highlighted.Furthermore, future research directions for advancing the hydroelectric AGE-II are proposed, emphasizing areas for further investigation and development.

MECHANISM OF WATER-ENABLED ELECTRICITY GENERATION (WEG)
WEG involves multiple mechanisms that are directly or indirectly related to other molecular structures of water (Figure 2A).The mechanism of electricity generation (e.g., the electromagnetic induction effect) 9,47 in hydroelectric AGE-I systems is well known.The mechanical energy of water can be converted into electrical energy through an EMG. 48In this process, water is considered the working medium for transmitting mechanical energy, while ignoring the potential ionization and electrification processes that may occur when it comes into contact with materials.The generator primarily transfers the kinetic or gravitational potential energy from the water to the coil, generating motion within a magnetic field.This movement is then converted into electrical energy through electromagnetic induction.The current and potential (voltage) that are generated are called induced current and induced potential, respectively (Figure 2B).This principle is based on Faraday's law of electromagnetic induction 19 : where ε is the voltage (V), n denotes the number of turns of the coil, ΔΦ is the change in the magnetic flux (Wb), and Δt denotes the period required for the change in conductor motion (s).Thus, in a hydroelectric AGE-I system, a conductor is pushed by a turbine, which is driven by the flow of bulk water, to generate electricity.The continuous operation of such generators relies heavily on the low-viscosity properties of bulk water, which are driven by other external forces, which in turn produce mechanical movements that favor EMGs.
In contrast, several working mechanisms can be considered for nanotechnology-inspired hydroelectric AGE-II, and some remain controversial. 20This wide range of electricity generation modes has motivated us to conduct research on multi-angle generator systems to improve hydroelectric AGE-II.In this section, a new framework to delve into various small-sized electricity generation technologies is proposed, focusing on the different roles of water.These effects are categorized into the following two distinct scenarios (Figure 2).(i) The mechanical/thermal energy of water that is collected using different types of nanogenerators and converted into electrical energy is introduced.Under this category, water serves solely as an energy storage medium and facilitates energy transport to generators.Water is typically considered to be electrically neutral.Moreover, one focus of this category is on analyzing the capture and conversion of low-frequency and weak mechanical energy into electrical energy using TENGs and PENGs.Furthermore, the low-grade thermal energy of bulk water is transferred and converted into electrical energy using thermoelectric nanogenerators and pyroelectric generators.The primary reason for the formation of this subcategory is that water molecules are small and possess relatively low viscosity, providing them with good mobility as carriers of mechanical energy.Notably, the electrical double layer (EDL) formed when water and materials are in direct contact is not considered in the output power path.(ii) Water interacts with micro/nanomaterials or tiny devices, leading to the conversion of different forms of weakened water energy into electricity.Here, the role of water extends beyond being a mere medium; its functional properties and the processes of ionization and electrification must be carefully considered during this conversion.Importantly, ionization or hydrolysis of water (e.g., pure water or solution) to produce ions and charges has a significant impact on the output electrical energy path.The evolution of the EDL and changes in its formation are important topics that must be discussed in this category.In summary, the aforementioned two scenarios are based on the basic mechanisms discussed in this section.These mechanisms form the basis for the design of each generator.However, certain generators integrate multiple electrical energy production mechanisms during the hydrological cycle, as discussed in Section 3.

Water solely as a medium for energy storage and transmission
In this section, we have not considered the ionization and contact electrification of water. 49In other words, only the mechanical and thermal energy changes of water are considered to drive the generator here, and the direct interaction between water and materials is not considered.Notably, the mechanical motion and temperature changes in bulk water are essentially microscopic phenomena involving the movement of water molecules or water clusters. 50,51These movements may also result in the breaking and reorganization of intermolecular hydrogen bonding, which has a direct impact on the mobility of individual molecules or clusters within water. 52Particularly, the dynamics of intermolecular hydrogen bonding and the small size of water molecules contribute to the low viscosity and superior mobility of bulk water. 53Consequently, water can serve as a carrier for mechanical energy storage and transport. 54When bulk water is in motion, water molecules undergo continuous random motion and exchange energy and momentum through collisions.
When bulk water comes into contact with a solid surface during its movement, the water molecules in motion facilitate the movement of solid molecules or atoms.However, solid molecules usually exhibit orderly arrangements and strong intermolecular forces; consequently, the motion state and morphological changes of solids are relatively small.Collecting relatively weak mechanical energy is difficult, and therefore, converting it into electrical energy requires advanced micro/nanomaterials and specialized devices. 22,31,40These existing mechanisms are based on the generation of electric charges through various processes, such as triboelectric, piezoelectric, or electrostatic induction effects.Efficient energy conversions from low-frequency mechanical to electrical energy in watersolid systems are typically achieved by using advanced micro/nanomaterials.These materials are designed to enhance the interaction between water and solids, optimize charge transfer processes, and improve energy conversion efficiency (i.e., ratio of output electrical energy to input mechanical energy involving water).6][57] By employing these advanced materials and equipment designs, the weak mechanical energy contained in water can be converted into useful electrical energy.Further research and development of micro/nanomaterials and device engineering is necessary to improve the efficiency of this energy conversion process.
During the mechanical movement of bulk water, water molecules constantly undergo random motion, exchanging energy and momentum through collisions. 58Similarly, the formation and breaking of hydrogen bonds between water molecules requires considerable energy.Therefore, water exhibits a high specific heat capacity and plays the role of a heat carrier.During the heating/cooling process of water, the motion of water molecules undergoes changes.As heat energy is transferred to water, the kinetic energy of the water molecules increases, causing faster motion.The rise in temperature intensifies the vibrations of water molecules, resulting in the instability of their relatively stable structures.Even at varying temperatures, water as a whole is constantly in motion.When it comes into contact with a solid, it can induce changes in the internal molecules of the solid. 58A smaller change in temperature results in weaker changes in molecular motion.Therefore, advanced micro/nanomaterials and intricate structural designs can aid in capturing and utilizing such weak thermal energy for electricity generation.
As a carrier of mechanical and thermal energy, water plays a guest role in the power generation process.In particular, the vibration or rotation of water molecules activated by mechanical or thermal energy induces the movement of corresponding molecules or atoms of the solid, when water molecules are in contact with a solid material.Therefore, this section primarily discusses and summarizes the charge transfer processes involved in various potential generation mechanisms.It provides some valuable insights for developing improved devices or mate-rials to capture mechanical and thermal energy from water.

Piezoelectric effect
The piezoelectric effect, which is the primary mechanism of a piezoelectric generator, involves the conversion of the mechanical force that is generated in water to electrical energy (Figure 2C). 37,59Particularly, in a hydroelectric AGE-II system, piezoelectric nanomaterials are marginally deformed by the mechanical action of water flow or impact of water droplets.Moreover, the positive and negative charges inside the material move, forming a potential difference in the process.Here, water only plays the role of prompting the deformation of the piezoelectric material, and the contact between the water and the piezoelectric material that induces electricity is not considered.
The polarization charge density generated by pressure, including applied pressure, is expressed as where F, l, and p denote the polarization charge density, piezoelectric coefficient, and applied pressure, respectively.The working principle of a PENG can be explained as follows.An insulating piezoelectric material covers two electrodes at two different areas (light-colored rectangles in Figure 2C), and vertical mechanical deformation causes electrical charging at the two ends of the material.As the external pressure increases, the polarization charge density increases.The constant processes of compression and release of water pressure on the surface of the material form a time-varying current and electric field.In a pioneering work, Wang et al. formally proposed a PENG based on ZnO nanowires in 2006. 37They determined that their ZnO nanowire PENGs generated piezoelectric polarized charges and time-varying electric fields to drive electron flow through an external circuit under hydrodynamic excitation.In this generator, raindrops or droplets of condensation fall under the effect of gravity and impact a piezoelectric material.As a result of the applied pressure, an electric current and voltage was generated, and the energy conversion efficiency ranged from 17% to 30%.Notably, this study demonstrates the capture of low-frequency mechanical energy in water, which cannot be captured using conventional EMGs.

2.1.2
Triboelectric effect (solid-solid mode) The generation of electricity as a result of friction is an everyday phenomenon and is known as triboelectricity.
The triboelectric effect can be observed when substances having different physical properties come into contact or are rubbed against each other owing to the transfer of electric charge, and it can be used to generate electricity. 35lmost all materials exhibit triboelectric effect 19 ; however, this type of electricity is difficult to collect.
To efficiently collect electrical energy that is obtained from frictional processes (e.g., low-frequency waves, flow, or water droplets in motion act to generate electricity), 19 a triboelectric collection equipment called as TENG was fabricated and the corresponding mechanism was proposed by Wang et al. in 2012. 60In these devices, the triboelectric effect occurs when the surfaces of two dielectric materials come into contact with each other and then separate (Figure 2D), resulting in the generation of electricity. 61,62By carefully designing the surface structure of micro/nanomaterials, the contact area can be maximized, resulting in a superior output performance.Once an unshielded charge accumulates within the generator, energy can seamlessly traverse through the conductive wires.
The physical mechanism behind the triboelectric effect involves the transfer of charge carriers, such as ions, to balance the electrochemical potential formed between the two different materials. 63On the application of mechanical force to separate these materials, the transferred charges remain, leaving a charged surface.That is, when two different substances (dielectric constants ε 1 and ε 2 ) are in physical contact, electrostatic charges are exchanged.Consequently, the surface becomes partially charged, and the surface charge density (б c (t)) increases with increase in contact time and area, finally reaching saturation.The formed electrostatic field drives the flow of electrons through an external load, resulting in the accumulation of free electrons in the electrodes (б 1 (k, t)), which is a function of the distance (k(t)) between the two dielectrics.
The electric fields in dielectrics 1 (V 1 ) and 2 (V 2 ) are obtained as follows: Thus, in a triboelectric generator, the movement of bulk water provides mechanical energy that moves the electrodes with respect to each other, resulting in polarization.][66][67][68] The piezoelectric and triboelectric (solid-solid) generators described in Sections 2.1.1 and 2.1.2produce electrical energy because of the contact between moving bulk water and micro/nanomaterials to achieve the transfer of mechanical energy.Water in this process only serves as an energy-transfer medium, and the direct charge transfer and transmission between water and materials are not considered.The aforementioned generators that utilize the mechanical energy of water can also be used to capture the mechanical energy generated during human and mechanical operations.Different application scenarios primarily rely on different material and device designs.The design of different generators and proposal of new mechanisms have significantly contributed to the efficiency of mechanical energy capture in water.Moreover, in recent years, some other generators such as dynamic Schottky generators 65,[67][68][69][70][71] (based on depletion layer establishment and destruction), twistron generators [72][73][74][75][76][77][78] (based on the variation of electrochemical double-layer capacitance model), and elastomeric dielectric generators [79][80][81] (based on varying capacitance models) have exhibited significant potential for capturing mechanical energy in water.However, their development is still at a more basic stage, and the relevant mechanisms and processes require further exploration and research.

Pyroelectric effect
Changes in the thermal energy contained in water are exhibited through the acceleration or deceleration of molecular vibrations.Temperature changes also affect the contact of water with materials, that is, the mechanical collision of molecules in contact with a solid surface.The pyroelectric effect is the core mechanism of pyroelectric nanogenerators, which utilize special piezoelectric materials, especially those with polar crystal structures.When affected by temperature changes, spontaneous polarization occurs at both ends of these materials. 82,83Similar to the piezoelectric effect, positive and negative charges inside the material migrate to opposing ends of the material, thereby generating a potential difference with changes in time (Figure 2E).Notably, piezoelectric materials do not necessarily exhibit the pyroelectric effect; however, pyroelectric materials definitely exhibit the piezoelectric effect.
A spontaneously polarized single crystal is permanently charged.However, the surface-bound charges created by spontaneous polarization are shielded by free charges on the external surface; thus, the electric field created by the bound charges is canceled.When the temperature changes, the total electrical moment of the crystal changes, causing the surface-bound free-charge layer to be released, and the crystal becomes charged or generates a current in a closed circuit. 20,82,84This is the basis of the pyroelectric effect, and, in WEG devices, the thermal changes that occur during the phase transition of bulk water can be used to change the internal temperature of pyroelectric materials (dT/dt > 0 or dT/dt < 0).Correspondingly, it will weaken or enhance the spontaneous polarization, which then drives the migration of electrons in the external circuit, achieving a new state of electrostatic equilibrium.

Thermoelectric effect
In the natural hydrological cycle, endothermic or exothermic processes occur during bulk-water phase changes.By effectively configuring thermoelectric materials, this waste heat can be converted into electrical energy.The thermoelectric effect is derived from the Seebeck effect 4,[85][86][87] in which a temperature difference between the two ends of a material causes the positive and negative charges to migrate to each end and accumulate to form a potential difference (Figure 2F).To evaluate the potential of thermoelectric materials, the dimensionless parameter ZT is used, which is defined as 85 where S is the Seebeck coefficient, σ is the electrical conductivity, S 2 σ is the thermoelectric factor, k is the thermal conductivity, and T is the absolute temperature.The value of ZT can be increased by increasing S 2 σ or decreasing k.Regardless of the type of materials (n-or p-type; e.g., Bi 2 Te 3 , GeTe, and SnSe), their thermoelectric properties are always interrelated, and achieving a high thermoelectric potential is difficult. 88Similarly, heat changes during phase changes in water produce different temperature distributions in different areas of the thermoelectric material, helping to generate continuous current and voltage.Notably, the thermoelectric effect is related to spatial temperature variations (dT/dx ≠ 0), whereas the pyroelectric effect is caused by time-dependent temperature variations (dT/dt ≠ 0).The aforementioned generators based on the pyroelectric and thermoelectric effects rely on the contact between the bulk water and material to achieve a spatial or temporal change in temperature, thus inducing charge transport to produce electricity.Note that pyroelectric and thermoelectric nanogenerators can not only use the thermal energy from water but also collect and store the waste heat from electronic devices, human body temperature, PV devices, and so on. 89

Water as a functional material
Water is not only considered a carrier of mechanical and thermal energies but also exhibits certain unique properties similar to charged functional materials.For instance, water acts as a weak electrolyte and undergoes the ionization process: 2H 2 O ⇌ H 3 O + +OH − . 50Moreover, water exhibits superior hydration properties and can dissolve various substances such as NaCl and MgCl 2 .Therefore, when water comes into contact with different micro/nanomaterials, the electrification process may occur.However, the process of liquid-solid contact electrification is always a controversial topic, representing a divergence in various electricity generation mechanisms. 50ere, considering the different contributions of ions and electrons in water-material contact electrification, electrical energy production is further divided into the following two important mechanisms.(i) Hydrovoltaic effect, 40 which primarily relies on the electrokinetic effect, includes the formation and boundary movement of the EDL.The relative motion of water and materials is driven by forces such as pressure, gravity, tension, latent heat, and chemical potential energy differences.Thus, the ions in water are adsorbed on the surface of the material through electrostatic adsorption or chemical reaction, and the relative positions of the stern and diffuse layers are shifted to constitute the cause of the potential difference.(ii) Triboelectric effect (liquid-based effect), 35 where the generator mechanism primarily depends on the formation of the Wang hybrid layer, involves the coupling of EDL theory with the theory of electron-cloud potential wall.In this context, the direct interaction between water and a material is considered not only for the adsorption or migration of ions in water but also for the exchange of electrons between water and the material.The overlap of electron clouds outside the atomic nuclei of water and the material can also induce electron migration.Particularly, electron and ion domination are determined by the difference in properties between the aqueous solution and the contact material.For instance, on the surfaces of hydrophilic materials, ion migration is often the primary cause of electrical energy formation.Conversely, on the surfaces of hydrophobic materials, electron movement plays a key role in the electrical energy generation process.

Hydrovoltaic effect
The electrokinetic effect as the core physical mechanism of the hydrovoltaic effect (or hydrovoltaic phenomenon) refers to the fact that a moving liquid can generate an electric potential difference in a narrow capillary channel or gap driven by a pressure gradient or capillary force. 12rál and Shapiro predicted that the flow of water in metal nanotubes would result in electrokinetic effects. 90oreover, in recent years, researchers have primarily focused on carbon-based materials (graphene oxide [GO], carbon black, and graphene) using flowing liquids or gases to generate streaming, evaporating, drawing, and waving potentials. 12,22,40The hydrovoltaic generator is based on the interaction of nanomaterials and water molecules for direct electrical energy generation and relies on the abovementioned potentials.This has been described in detail by Guo et al. 22 and, thus, is not discussed here.
To clarify the EDL mechanism, streaming potential is briefly introduced here.As shown in Figure 2G,H, the electrokinetic effect is closely related to the dynamic formation of the EDL. 5 Particularly, when an aqueous solution comes into contact with the surfaces of charged materials, their surface charges attract counterions in the solution to form an EDL, which screens the surface charge.Thus, the EDL consists of two parallel layers (i.e., stern and diffuse layers) having opposing polarity at the solid-liquid interface (Figure 2H).In the stern layer, opposite charges are tightly adsorbed onto the charged surface as a result of chemical interactions and/or an applied electrical potential, whereas the diffuse layer is composed of attractive counterions that screen the first layer via coulombic forces.Therefore, if an electrolyte enters pore channels via capillary force or a pressure gradient (p), the ions neutralize the charges on the pore wall surfaces, and an EDL is formed.As a result of the formation of this charged layer, the pressure-driven ion flow leads to a net charge transfer and the formation of a flow potential.Moreover, the confinement of the charger water in the nanoscale channels or pores can result in the formation of a diffusion layer.In particular, when the pore size is comparable to the Debye length (i.e., the length scale at which the charge carriers shield the electric field) of the solution, the EDLs overlap significantly, resulting in a counterion-dominated enclosed solution. 22herefore, to enhance electricity generation efficiency, the size of the nanochannels should be the same as the Debye length of the solution to ensure the overlap of the EDL.The streaming current (I) and voltage (U) are determined as follows: where ε 0 represents the dielectric constant of the vacuum, ε r , η, and L 1 refer to the relative dielectric constant, dynamic viscosity, and specific conductivity of the liquid, respectively, a and y are the capillary radius and capillary length of the nanochannel, respectively, ΔP is the pressure difference, and C is the zeta potential of the nanochannel surface.Therefore, when nanomaterials are used to extract energy from hydrological processes, different forces driv-ing the movement of water with respect to the nanomaterial surface result in fluctuations in the EDL, thus allowing the continuous generation of current and voltage.Based on the discussion of streaming potential in the hydrovoltaic phenomenon, certain other special and significant processes must be discussed here.

Humidity-gradient-induced potential
Humidity is expressed as the proportion of water molecules in the air, which is widely distributed both spatially and geographically.The shape and movement rules of water molecule clusters are relatively more complex.Humidity-related small-sized power generation focuses on the changes in water, gas, and liquid states and interaction of substances, thus deriving more generator systems and mechanisms.Electrical energy can be generated owing to the differences in humidity gradients formed on the surface of the nanomaterial.In this type of generator, the energy released by the conversion of water molecules from the gaseous to adsorbed state (i.e., the interaction of water with the functional groups of the hygroscopic material) results in charge separation and the continuous generation of electrical energy.However, even when a large humidity-gradient difference exists between the two ends of the hygroscopic material, the obtained energy output is often less than predicted.Thus, research on the exploitation of humidity gradients for WEG is significantly challenging. 21,25Fortunately, recently, self-maintaining protein nanowire devices have been proposed. 91In this system, one end of the hygroscopic material is rich in hydrophilic functional groups that can trap water molecules in the gaseous state via the formation of hydrogen bonds.Under the stimulation of the humidity gradient, the oxygen-containing groups are ionized by the humidity to release free protons and form a built-in humidity gradient.In this process, chemical energy is converted to electrical energy.Water molecules in humid air are spontaneously absorbed by functional groups (e.g., with -OH, -COOH, and -SO 3 H), which results in the release and migration of positively/negatively charged ions and current flow in an external circuit.This is an exciting discovery; however, the underlying generator mechanism and charge transfer process must be further explored by future researchers.
Moreover, streaming currents and potentials as typical hydrovoltaic effects can also explain some of the potentials induced by humidity gradients.Water molecules are adsorbed by hygroscopic materials to form an EDL.Gaseous water is adsorbed and converted into liquid water by hygroscopic materials.Then, relying on the narrow nanochannels of the hygroscopic material, capillary forces are induced to promote the movement of ions in water, forming an EDL.Based on the theory of electrokinetic effect, this process enables the production of electrical energy.Therefore, this type of electricity generation from moisture can also be referred to as a special hydrovoltaic effect.
To sum up, certain mechanisms in humidity-gradientbased power generation cannot be easily analyzed using the electrokinetic effect theory (evolved from the hydrovoltaic effect) and require further exploration and research.The systematic development and overview of moisture-induced electricity generation has been recently studied by Qu et al. and therefore will not be discussed in detail here. 92In their research, the mechanisms involved in moisture generation were clarified into the following three types.(i) Hydroelectricity is a method of electrical energy generation between water molecules and material surfaces based on physicochemical reactions.Note that some of these reactions are not discussed here because of their low reproducibility and persistence.(ii) Asymmetric ion movement occurs under moisture in various classical material designs, leading to the formation of diverse ion gradients.In Section 3.4, a detailed introduction to two types of classical cases is provided.(iii) The primary difference among humidity-related, waving, dragging, and evaporation potentials is the difference in driving force, which can be attributed to the different types of hydrovoltaic effects, and these result in the streaming potential/current.

Salinity-gradient-induced potential
Salinity gradient effect occurs when two salt solutions having different concentrations are mixed, for example, at the mouth of a river as the freshwater meets the seawater and at the interface between the bulk solution and water-air interfacial region during solar-thermal interfacial seawater desalination.When solutions of different concentrations are mixed, the increase in ionic disorder causes an increase in entropy. 93Thus, by controlling the mixing of these solutions, electrical energy can be obtained, 94 and some common methods of achieving this include reverse electrodialysis (RED), 95 interfacial evaporation-induced salinity difference, 42 and capacitive mixing. 23,96Here, the mechanism of the salinity gradient effect is described in detail using the well-known RED systems as an example.
][97][98] The IEM membrane preferentially transports counterions (i.e., species with opposite charges to the ions) and promotes the transport of anions and cations in different directions, thereby forming an ionic current.By placing electrodes on both ends of the IEM stack, the ionic current can be easily converted into electrical current, whereas a reversible redox reaction occurs at both ends of the IEM stack.The potential energy of this generator is given by where R is the gas constant, B is the absolute temperature, z is the ion valence, F is Faraday constant (96 485 C mol −1 ), Q is the selectivity (where , J + and J -are the cation and anion fluxes, respectively), and c H and c L are the molar concentrations of the high-and low-concentration salt solutions of the two layers of the membrane, respectively.The salinity-gradient-induced potential generally relies on the difference in chemical potential energy caused by the gradient of the ion concentrations in aqueous solutions, as well as electron transport in an external circuit to return the system to energy balance.Importantly, the constant presence of a difference in salinity between the two sides of the membrane structure induces the generation of continuous voltage and current in the external circuit.
Moreover, the uses of nanopore-based membranes as IEMs for capturing salinity gradient energy have exhibited significant potential.The above-mentioned IEM can also be replaced by well-designed nanoporous membranes that are driven by the osmotic pressure difference; then, the salt solution diffuses from the high-to low-concentration region.The electrolyte fluid is forcibly pushed through the nanopore channel by applying pressure, thus taking advantage of the nature of the net residual charge on the inner surface of the nanoflow channel (generally, a negative charge).The electrolyte fluid is forced through the nanopore channel by the applied pressure, and charge separation occurs when the electrolyte fluid passes through the EDL near the inner surface of the pore channel, resulting in a streaming current and potential.Therefore, the potential induced based on the difference in salinity can also be classified as a special hydrovoltaic phenomenon.The efficient structural adjustment, size optimization, and ion-selective construction of the nanomembrane structure can effectively optimize the performance of various architectural designs of permeable energy generators.

Ionovoltaic device
In 2014, Kim et al. proposed the concept of an "ionovoltaic device."In this special device, electrical energy is generated by moving water droplets on top of a solid interface. 56,99The theory behind these devices has been extensively debated, and researchers have proposed that electrostatic adsorption, EDL modulation, and electrodynamic effects play important roles in the functioning of this device.However, by controlling the type and concentration of ions in water droplets, 100 Park et al. observed that electrical energy generation is primarily achieved through EDL modulation, that is, the adsorption and desorption of ions.Notably, the movement of ionized water droplets (i.e., those containing Li + , Na + , K + , F − , Cl − , Br − ) is a key factor in the EDL modulation process. 101o date, several hydrovoltaic generators based on the ionovoltaic device have been reported as part of the attempts to increase the power output.In particular, a promising "electrolyte-insulator-semiconductor" structure has been reported. 99This device comprises a hydrophobic layer and an insulating substrate.The hydrophobic layer facilitates the slippage of ionized water droplets at the liquid-solid interface and prevents the wetting of the generator; it also acts as a dielectric layer.Thus, the insulating layer prevents the direct transfer of electrons or molecules in the water droplet toward the electrode.As the ionized droplet rolls over the hydrophobic layer, the contact area between the droplet and solid surface changes constantly.Particularly, a new contact area is formed in front of the droplet, and the cations in the droplet are adsorbed by electrostatic attraction to the solid surface, which is predominantly negatively charged.As the cations are adsorbed onto the electrode, the electron density increases instantaneously as a result of coulombic interactions.In contrast, at the back of the droplet, the adsorbed ions are released via droplet movement, and the electron density decreases.Thus, as a result of changes in the electron density, a current can be obtained.Note that the major difference between the ionovoltaic device and traditional hydrovoltaic generator is the difference in the contact angle on the hydrophilic solid surface, thus achieving differences in electron induction and ion migration mechanism, which provides a new breakthrough direction for its development.
Overall, the hydrovoltaic effect is a creative discipline that can systematically and efficiently integrate phenomena related to the direct action of water and solids for electrical energy production.The core physical mechanism is the conventional electrodynamic effect, that is, the formation of EDL and the moving movement of the EDL boundary.In addition to considering the direct contact of varied states of water with nanomaterials and the resultant production of electrical energy, the driving forces of pressure, gravity, latent heat of evaporation, and so on may also be considered.The developments of special hydrovoltaic effects such as humidity-gradientinduced potential, salinity-gradient-induced potential, and ionovoltaic effect are also discussed.The aforementioned important hydrovoltaic effects have significantly enriched the study of WEGs; however, certain profound issues must also be considered, such as the corrosiveness of salt solutions and periodicity of power generation.

2.2.2
Triboelectric effect (liquid-based) The hydrovoltaic effect is predominant in the traditional EDL formation process, giving priority to the role played by ion migration in charge migration during the contact between water and material.In the exploration of hydrovoltaic science, few researchers have considered electron migration during liquid-matter (e.g., liquid-solid, liquid-gas, and liquid-liquid) 102 contact because of the triboelectric effect.Generally, this friction is assumed to not contribute to the formation of electric potential or may cause material depletion, and others; therefore, this perspective must be explored.
Conversely, the liquid-solid triboelectric effect, which integrates ion migration with electron transfer, is used to analyze the charge transfer process.The underlying mechanisms of liquid-solid triboelectric effect not only consider the electron-cloud barrier model (detailed discussion of the solid-solid triboelectric effect is presented) but also consider the EDL formation at the liquid-solid interface.
Here, water is considered to work as a triboelectric material for the construction of liquid-solid TENG with insulating polymer films. 35,46Furthermore, various types for liquid-solid TENGs based on contact separation, sliding, single-electrode models exist. 46The key processes involved in the contact electrification of these devices are electron transfer (primary) and ion transfer (secondary) during the liquid-insulator contact process.As for the electrokinetic effect, EDL theory is also important for liquid-solid triboelectricity generation, 35,103,104 the details of which are presented in Section 2.1.2.Recently, the formation of an EDL at a solid-liquid interface was clarified not only from the perspective of ionization or dissociation reactions but also from that of electron transfer.This new concept involving electron transfer in EDL formation is called the "Wang transition" model (Figure 2I). 103This model is based on electron transfer as a result of the strong electron-cloud overlap at the liquid-solid interface, which results in contact electrification via electron transfer as well as partial ion migration and a continuous increase in the voltage; moreover, the generation of electrostatic energy primarily relies on electrostatic induction and the triboelectric effects induced by the interactions between water and a frictional material.Researchers are considering the collection of liquid-solid triboelectricity using specific devices; moreover, both the triboelectric and streaming potentials can evidently be collected simultaneously. 105 schematic of a liquid-solid TENG is shown in Figure 2J. 106As shown in the figure, a single electrode is placed at the top surface of a solid material.This is advantageous because the triboelectric charge is not required to pass between the top and bottom electrodes, as in other generator configurations, and the electromagnetic shielding effects are avoided.The current-generation process can be summarized as follows 107 : (i) A water droplet carrying a marginally positive charge falls onto the negatively charged generator surface.(ii) Then, the water droplet spreads out, coming into contact with the top electrode, which is in contact with the negatively charged surface of the TENG.(iii) Consequently, the electrode becomes polarized, and positive and negative charges migrate toward the electrode and surface, respectively.Overall, the electrical potential between the electrode and ground is disrupted, and the negative charge in the ground flows toward the electrode, thus generating a current.(iv) Then, the droplet contracts as it rebounds from the surface, and the negative charge on the electrode flows back toward the ground.Moreover, the charge on the surface is lost or neutralized.Subsequently, the process is repeated.Importantly, the single-electrode architecture differs from that of conventional TENGs, and this is critical for efficient energy generation in liquid-solid systems.
Section 2.2 describes that hydroelectric generators (based on the hydrovoltaic effect) and TENGs (based on the liquid-based triboelectric effect) rely on the direct interaction of protons and electrons generated by the ionization and electrification of water molecules with micro/nanomaterials to generate electrical energy.Certain other generator systems, such as semiconductor generators based on water polarization, also treat water as a dielectric material. 108However, this approach cannot be effectively explained by the aforementioned theory.In this case, water, as a functional material, directly interacts with other materials to generate electricity.
In summary, seven important electrical generation mechanisms have been discussed related to WEG.However, WEG is not limited to these mechanisms.For example, by exploiting the reverse electrowetting phenomenon and thermogalvanic effects of liquid droplets, 109 Zhou et al. prepared a sensor that can capture temperature and pressure information from electrical signals resulting from interactions at the water-nanomaterial interface.Therefore, the discussed theories are key to the development of WEG devices, but we should not be limited to these effects alone.

DESIGN PRINCIPLES FOR HYDROELECTRIC AGE-II GENERATORS
A series of proof-of-concept electricity generation mode designs based on the various generator mechanisms and hydrological cycle processes (evaporation, wave, infiltration, flow, condensation, etc.) have been presented for hydroelectric AGE-II.A typical hydrological cycle process, such as seawater evaporation, involves variations in salinity, temperature, and humidity, along with the flow of water and steam.The weak energy in the evaporation process can be collected and converted into electrical energy using a rational design of the generator architecture and the mechanisms mentioned in Section 2. Thus, the natural water cycle is a dynamic process in which water and vapor undergo continuous movement and transformation.During this process, the mass and energy differences at various stages can be harnessed and converted into electrical energy.Among these methods, each electricity generation technology relies on devices made of special materials, such as dielectric, thermoelectric, piezoelectric, and photothermal materials. 20In this section, we systematically explain the designs of typical electricity generation models for different water cycle processes, focusing on their design advantages and internal connections.

Flow-related WEG designs
Utilizing micro/nanomaterials to generate electricity from water flow, including fresh water, seawater, and other solutions, is a promising and versatile research direction with numerous mechanisms that must be explored. 110articularly, the direction and speed of water flow, ion concentration, and length of the flow channel, as well as the number, position, and length of the electrode materials affect the power from the incoming electrical energy, which is generated from the flowing water.In this section, several recent important studies are discussed in detail, focusing on the experimental setup, nanomaterials, and construction and factors influencing electric power generation.

Electrodes
The types, number, and position (and other factors) of electrodes in a hydroelectric generator can significantly affect the efficiency and stability of the power output.For example, Sun et al. investigated the influence of the electrode position on the voltage generated by the flow of an NaCl solution across graphene sheets 111 and determined that tilting the generator at 45 • provided the maximum voltage output.Therefore, to efficiently produce electrical energy and improve the accuracy of the tests in WEG, multiple electrodes must be explored to minimize the possibility of errors in the results.

Device assembly
The use of nanomaterials can also enhance hydroelectric power output.For example, Xu et al. prepared flow-based hydroelectric nanogenerators based on highly linear structured multi-wall carbon nanotubes (MWCNTs) wrapped around a polymer substrate. 112When this generator came into contact with an aqueous solution, an EDL was formed between the generator surface and solution (Figure 3A).Consequently, the anions in the diffusion layer were slowed down by the opposing net charge in the stern layer.Therefore, electrons in the MWCNTs moved to balance the excess charge, leading to the formation of a surface potential difference and generation of electrical energy when an external circuit was connected.As the flow distance increased, the surface potential difference also increased, returning to its original value when flowing back at the same speed (Figure 3B).At a given velocity, the output current and voltage of this hydroelectric nanogenerator can be continuously increased and stabilized.Interestingly, ordered mesoporous carbon spheres (pore size 3-5 nm) could be compounded onto the generator, thus increasing the output voltage efficiency as the content of porous carbon spheres was increased, and reached a maximum voltage of 341 mV (Figure 3C,D).Interestingly, the maximum voltage output of the flow generator doubled after the inclusion of carbon spheres.This change is attributed to their abundance of pores, and a higher specific surface area increases the number of ion adsorption sites, which facilitates the creation of a potential difference, enhancing the electrical power output.

Experimental setup
The experimental setup is important for effectively performing the testing process.A good test setup ensures accuracy and comparability.Importantly, the interference of additional irrelevant electrical signals must be avoided during the test.Kuriya et al. designed a setup for generating electricity by coupling graphene with flowing water, 113 as shown in Figure 3E.The setup consisted of (i) a glass top plate, (ii) supporting layer, (iii) nanomaterialbased WEG generator, (iv) electrodes, (v) spacer, (vi) electrical connection ports, (vii) a channel with (viii) size counter, (ix) R L , and (x) source meters.Driven by a syringe pump, water is pushed into the device, and the mechanical energy of the flowing water can be converted to electrical energy as it passes through the electrode material.During testing, the experiments were performed at room temperature and the equipment was electrically shielded, thus ensuring the accurate assessment of this flow design; the authors focused on the effects of different flow rates and channel lengths, as shown in Figure 3F-H.Figure 3F,G shows the linear relationship among the voltage, output power, and flow rate.The electric potential was observed to increase linearly with flow rate for devices having different channel lengths (Figure 3H).The data obtained for the standard electric potential and force were also fitted.
Here, we analyzed the efficiency improvement of electricity generation through water flow, emphasizing the importance of electrodes, device assembly, and experimental apparatus.These points are not repeated in the subsequent chapters, but instead, the classical electricity generation designs for various water circulation processes are described.These designs include mechanism application or material structure considerations, showcasing diverse electricity generation systems utilizing varied mechanisms in different water circulation scenarios.

Hydrovoltaic system design
Water flows on the surface of nanomaterials to form an EDL, enabling the production of electrical energy efficiency.In Section 2.2.1, a detailed introduction was given, and in this section, the focus is on introducing the concept of a heterogeneous water flow for the generation of electrical energy.For example, Boamah et al. designed a special water flow generator where a fluid with a certain salt concentration gradient passes through the surface of metal nanolayers (such as iron, vanadium, and nickel with thicknesses of approximately 10-30 nm) to produce a voltage of tens of millivolts at flow velocities as low as a few centimeters per second. 114Owing to its nanoscale construction, this device can sensitively collect weakened energy from water flow and convert it into electrical energy.The specific process occurs as follows.Ultrathin nanometal layers are deposited through physical vapor deposition onto various substrates.The metal oxidizes in the air, forming metal oxides.If the oxide nanocoating is sufficiently thin, the electrostatic potential extends beyond it, polarizing the underlying metal, similar to the charging of metal atoms on the ultrathin oxide layer.Once the iron nanolayer is exposed to the water flow, an EDL forms at the interface between water and the iron layer.This design achieves a novel material system for converting weak mechanical energy in water into electrical energy.Moreover, considering the thinness of the metal layer, it can be deposited on different surfaces such as umbrellas, cars, and clothing, allowing for a wider range of applications.This work also demonstrates the applied phenomenon of hydrovoltaic effect.

Triboelectric system design
Water is also used as a carrier of mechanical energy, and it can be used for the production of electricity based on the mechanism of triboelectric effect.Wang et al. designed a composite TENG that could collect both sources of energy in an integrated system. 115The generation of electrostatic energy is attributed to the frictional charges in water, that is, friction generated between water and a surrounding material, such as air or a pipe.In this generator design, water was circulated over nanostructured superhydrophobic (SHS) surfaces polytetrafluoroethylene (PTFE) thin films to capture the electrostatic energy and over a wheel to capture mechanical energy (for mechanistic details, refer to the sections related to solid-solid and liquid-solid triboelectric effects).At a water velocity of 54 mL s −1 , the open-circuit voltages for capturing electrostatic and mechanical energies were 72 and 102 V, respectively, and the short-circuit currents were 12.9 and 3.8 µA, corresponding to maximum instantaneous power densities of 0.59 and 0.03 W m −2 , respectively.This is a significantly outstanding design that cleverly fuses the capture of two types of water energies based on device mechanism design and nanomaterial coupling.

Wave-related WEG designs
Waves contain abundant energy and have been studied as another source of electrical energy.The driving force of ocean waves is primarily solar energy.7][118] The mechanical energy generated by ocean waves is proportional to the wind speed at sea level, and the motion of the bulk seawater interface leads to the characteristic undulation of waves, which results in differences in the potential energy, reciprocating forces, and changes in buoyancy.Using micro/nanomaterials or devices, the mechanical energy of waves can be converted into electricity based on the aforementioned fundamental theories (mechanism is described in Section 2).However, most studies have focused on converting wave energy to electrical energy using conventional EMGs, and the exploitation of wave energy remains relatively low. 14In particular, low-frequency wave energy is difficult to harvest using conventional macroscopic hydroelectric power generation equipment.

Triboelectric system design
Here, two generator models for wave energy utilization proposed by Wang et al. in 2015 and 2017 based on triboelectric effect (solid-solid mode) are highlighted. 119,120In one model, a nanogenerator with an arch-shaped structure was prepared, 119 and the primary mechanism is discussed in Section 2.1.2(Figure 2C).As shown in Figure 4A, this nanogenerator is composed of acrylic, copper-aluminum, PTFE, and polyethylene terephthalate layers.Notably, the use of conventional polymers reduces the preparation costs and facilitates large-scale applications of this generator.When the mechanical energy of a simulated wave is applied to the top of the arch, the two PTFE layers and the central aluminum layer are forced into contact.These two materials have different frictionally charged properties, and charge is transferred at their interface.Subsequently, as the wave motion is repeated, these layers separate, forming a directional flow of current as a result of the formed dipole moment, and achieving continuous generation of electricity.This WEG generator generated a voltage and current of 569.9 V and 0.93 mA, respectively, at an impact acceleration of 10 m s −2 and a displacement of 9 cm (Figure 4B,C).
In the second model, silicone rubber was used to manufacture a spherical WEG device to harvest wave energy. 120wing to its spherical structure, the device is not affected by its angle of tilt, enabling the capture of a large amount of mechanical energy from the waves.A schematic of the TENG structure is shown in Figure 4D,E.The device consists of a hollow sphere coated with Ag-Cu containing a ball.During testing, a DC motor was used to move the generator back and forth, thus simulating wave motion.Figure 4F-I shows the effects of different wave amplitudes and frequencies on the open-circuit voltage and shortcircuit current of the device.Figure 4F shows that, at 3 Hz, the transferred charge quickly increases with increase in displacement amplitude, and subsequently plateaus.The voltage and current show similar trends, reaching 1780 V and 1.8 µA, respectively, in the stable region (Figure 4G,H).Figure 4I shows the frequency dependence of the shortcircuit current.The frequency increases initially, and the maximum current occurs at 5 Hz.Furthermore, a significant increase in power (greater than 10 times) can be achieved by coupling multiple WEG devices.Moreover, because of the variability in wave frequency, the variation in the output power with frequency was also tested.The findings of this study indicate the potential of WEG devices in effectively harnessing wave energy.
These results show that Wang et al. have made a series of outstanding contributions to research into WEG devices that exploit the triboelectric effect.They also proposed the concept of "blue energy dream" 19,33 by which millions of spherical WEG cells can be connected by cables to achieve large-scale power generation.In our view, the achievement of the "blue energy dream" in the near future is within reach when synergistically integrated with other technological strategies.

Hydrovoltaic system design
Unlike the above use of water waves as a mechanical energy transmission medium, water can also interact directly with the material during fluctuations, resulting in the formation of EDL via this behavior, which enables the production of electrical energy.Yin et al. inserted graphene into water and identified that moving the gasliquid boundary on the surface of severe graphene can induce a voltage of 0.1 V. 121 Moreover, the rate of voltage generation and fluctuation as well as the size of the graphene surface showed a linear variation relationship.The generated potential is named waving potential and this case serves as a typical case of hydrovoltaic effect.The electric energy production of the boundary moving process of different concentrations of water and graphene formation has been elaborately studied.

Other important designs
In addition to the above situations, an interesting case was presented where wave energy was harvested through the use of a twistron generator.Baughman et al. prepared a socalled twistron generator based on carbon nanotube (CNT) yarn. 77The change in capacitance of the electrolyte-coated yarn during stretching results in the production of electrical energy.Notably, when the yarn was stretched by 30%, the open-circuit voltage increased by 140 mV, and, when the yarn was stretched by 50% and the stretching frequency was 12 Hz, a peak power output of 179 W kg −1 was achieved.
The twistron generator was tested in 0.6 M NaCl, which is a concentration similar to that found in seawater; however, the authors observed that the voltage output exhibited minimal variations when using NaCl concentrations ranging from 0.6 to 5 M (peak power dropped by less than 20%, even at concentrations as low as 0.1 M).Thus, this generator can be used in marine environments of varying salinity.In contrast to bulky EMGs, twistron generators can capture energy from significantly small wave movements because of their small scale (thinner than a human hair).Furthermore, the yarn can be attached to a balloon, and, when the balloon is disturbed by waves, the yarn is stretched, allowing the twistron generator to produce electricity.

Droplet-related WEG designs
Water on the surface of the Earth evaporates when exposed to the heat of the sun. 39,122When this water vapor meets cold air at high altitudes, it condenses into small droplets and falls to the ground as rain.Rainfall is influenced by climate, geography, and the time of day, making it an unpredictable source of energy.Therefore, droplet-based designs for energy generation also include processes such as evaporation and condensation.

Triboelectric system design
A typical design of a droplet-based WEG is shown in Figure 5A.In this device, water droplets fall on top of a PTFE layer on an indium-tin-oxide-coated glass substrate. 106Droplets of tap water (ion concentration of 3.1 × 10 −3 mol L −1 ) were used to simulate rainwater, and the device produced a stable charge (approximately 1.6 × 10 4 droplets resulted in a surface charge of 19.8 nC).Further, the output power was increased by connecting four devices together (Figure 5B).The principle of energy generation of this droplet-based WEG is based on the liquid-solid triboelectric effect.Notably, this design could also capture energy from water drops separated from seawater.
A key problem facing WEG devices is performance degradation under certain operating conditions, such as low temperatures, as a result of changes in the physicochemical properties of the interacting materials.Xu et al. reported a novel device that they called a slippery lubricant-impregnated porous surface (SLIPS)-based TENG (SLIPS-TENG). 123The SLIPS-TENG could be operated under a wide range of conditions, and the maximum power output was 2.5 nW (with a 100 MΩ load).To show the superiority of the SLIPS-TENG, a SHS-TENG was prepared for comparison.The output power of the SLIPS-TENG (200 nW) was significantly larger than that of the SHS-TENG under the same conditions, and the SLIPS-TENG device could power an array of lightbulbs at both 25 and −3 • C (Figure 5C), whereas the SHS-TENG could not provide sufficient power at −3 • C, thereby demonstrating the superior low-temperature performance of the SLIPS-TENG.
Overall, WEG designs involving water droplets are based on the droplets falling onto or dripping down a functional micro/nanomaterial, and the water-material interface exhibits a triboelectric effect on the performance of these devices.Thus, different types of WEGs can be developed by tailoring the material properties.Importantly, the operating conditions, such as temperature and humidity, must be carefully considered to increase the practical scope of WEG devices.
Using another design, Shin et al. prepared a condensation-drop-based WEG generator (Figure 5D). 124n this device, instead of converting mechanical energy into electrical energy, the energy involved in condensation was exploited.Specifically, water droplets condense, nucleate, and grow on a PTFE surface and are then shed; subsequently, they sweep across the surface and pass over several electrodes and, finally, separate from the PTFE surface (Figure 5D,E).This process occurs because, as the condensing droplet grows, the downward force increases until it exceeds the frictional force between the droplet and substrate.At this point, the droplet flows downward, passing over an upper electrode, which is not affected by the electric potential of the droplet (Figure 5D).The main reason for this occurrence is that the residual cations in the droplet only affect the electrode when the droplet is in contact with the electrode (Figure 5E).In contrast to the conventional triboelectric effect, where the generation of free charges occurs ubiquitously, in this scenario, the induction of free charges is confined solely to the upper electrodes owing to the grounding of each electrode.While the remaining cations and negative free charges are momentarily balanced through electrostatic induction, the resulting output voltage consistently maintains a positive value.The authors have studied the exploration of the value of electrical energy generation in each process of condensation, which is a valuable inspiration for the study of the WEG process, and researchers should learn to study in more detail the various stages of electrical energy production involved in each water cycle process.

Electromagnetic induction system design
Unlike the above design, Ma et al. prepared an SHSdroplet-based WEG generator consisting of a conductive coil, liquid droplets, and an SHS magnetic powder/Ecoflex base. 47When simulated liquid droplets come into contact with this assembled generator, the transferred mechanical energy causes the magnetic fluid to pass through the coil, thereby generating electrical energy (Figure 5F).In contrast to that of the above designs, the principle of this generator is the same as that of EMGs in hydroelectric AGE-I.Importantly, when 50 µL of water was dropped from a height of 10 cm onto this generator, a voltage and current of 19.8 mV and 1.2 mA, respectively, were obtained.

Hydrovoltaic system design
Wang et al. reported a droplet-based WEG device constructed of PTFE on a glass substrate with copper and gold electrodes above and below the PTFE layer, respectively (Figure 5G,H). 125In this device, a stable negative charge can be stored on the PTFE surface, thus forming an electrostatic field.The two electrodes accumulate positive charge via electrostatic induction.To achieve electrostatic equilibrium, the charge density of the PTFE should be twice that of the top electrode.When a water droplet falls on the top electrode and then spreads out, the area of water surrounding the electrode increases, and this extends the electrode area.The disruption of electrostatic equilibrium between the two electrodes induces electron transfer, thus restoring equilibrium and forming a potential difference.A single drop of water falling on the surface of such a WEG device having a 2.5 cm × 7.5 cm × 0.03 mm PTFE substrate produced positive and negative voltages of up to 143.8 and −17.1 V, respectively.

Humidity-related WEG designs
The gaseous state of water in the atmosphere is key to the water cycle.Although the atmosphere contains abundant water, this water as an energy resource is often overlooked because most of the water is present as vapor.Moreover, the use of humidity-gradient differences is a new mode of electrical energy production. 7[128][129]

Bulk design
A significant challenge in the development of humiditybased WEGs is the discontinuous nature of energy gen-eration resulting from the varying and transient nature of atmospheric water vapor.To solve this problem, Liu et al. prepared a bulk thin-film device composed of nanoscale protein wires that could produce a continuous electrical output (Figure 6A). 91A gradient energy group design on the surface of the device is not required, and it can function continuously for extended durations.This device with an area of 50 mm 2 and film thickness of 7 µm could produce a stable DC voltage output of approximately 0.4-0.6V for more than 2 months (Figure 6B).A single device could even power a semiconductor nanowire transistor.The device surface does not require a gradient-based functional group design, utilizing its functional groups such as carboxyl groups.Driven by the moisture gradient, an ionization or a mobile proton concentration gradient may be generated in the carboxyl group, and the proton gradient can in turn promote proton diffusion displacement.The device can achieve continuous production of electrical energy by using electrode sheets of different sizes covered with nanofilms, without the need for constructing a hygroscopic material with different hygroscopic performance layers.This is a considerably creative and inspiring design that has significantly advanced the development of moisture-absorbing materials and their device design.Importantly, the dynamic adsorption-desorption of water molecules at the interface provides a continuous input of energy as a result of the high affinity of the surface functional groups of the protein nanowires for water molecules, as well as a self-sustaining electric field that promotes ionization and charge transfer.

Asymmetric design
Qu et al. achieved superior results in humidity-based electricity generation 5,[130][131][132][133][134] through the preparation of varied humidity generators using polymer electrolytes and GO.For example, a bilayer polymer film has been reported (Figure 6C). 126The polymer film contains polycation polydiallyl dimethyl ammonium chloride and a polyanionic hybrid comprising polystyrene sulfonic acid and polyvinyl alcohol.The polymer layers initially absorb atmospheric water; subsequently, H + and Cl − are generated in the different layers, thus forming a concentration gradient.
Driven by this ionic concentration gradient, H + and Cl − diffuse in opposite directions, yielding a current and voltage (0.95 V at 25% relative humidity [RH] and 25 • C).Moreover, they also reported a hygroscopic reduced GO (rGO) device with a diameter of 2 cm that achieved a voltage output of 1.5 V. 135 The working principle relies on the poor ability of the device to capture water from the atmosphere because of the hydration of the gradient-type rGO and the adsorbed water molecules, and the positively charged protons can spontaneously diffuse from the GO side to the rGO side.
In addition to GO and rGO, Liu et al. reported a device based on the adsorption of water vapor by protonated functional groups on the surface of porous carbon fiber (PCF) membranes. 127A significant electric potential was achieved when the numbers of proton donors varied on the different sides of the PCF (Figure 6D).Notably, a 5 cm × 1 cm PCF membrane in a high-humidity environment (RH > 95%) achieved an open-circuit voltage of approximately 65 mV for 6 h.Moreover, the functional groups in the PCF were changed to modify the adsorption of water vapor and generate different voltages at the same RH.A comparison of four devices is shown in Figure 6E.These devices were subjected to different functionalization treatments: (i) Half the PCF was functionalized with plasma, (ii) half the PCF was functionalized with acid, (iii) the entire PCF was functionalized with plasma, and (iv) the PCF was untreated.
Based on the material design of the bulk structure or asymmetric structure design, the researcher aims to convert gaseous water into liquid water, thus creating a certain ion concentration gradient inside the material, which results in the production of electricity.

3.4.3
Coupling design for WEG and solar-based system The performance of humidity-gradient-based WEG system can be enhanced by coupling them to solar-based systems, such as PV-based systems, to increase electrical production or evaporation to increase the humidity, which can enhance the power generation efficiency (Figure 6F,G).Duan et al. prepared an all-inorganic solar cell with a custom carbon electrode to simultaneously harvest solar and water vapor energies (Figure 6F). 128The device produced a maximum power conversion efficiency of 9.43% under one sun irradiation, and a voltage of 0.35 V and current of 0.45 mA at 80% RH were obtained as a result of the streaming potential at the carbon black/atmospheric water interface.Moreover, Bai et al. demonstrated an increase in the electrical power of humidity generator under sunlight coupling (Figure 6G,H). 129They achieved a light-coordinated humidity-gradient difference related to WEG devices, which is composed of a hydrophilic polyelectrolyte with a photosensitive phytochrome, and the mediation of the electrode/material interfaces was also achieved.This WEG device produces a stable open-circuit voltage of up to 0.92 V and short-circuit current density of 1.55 mA cm −2 .
In this subsection, key studies related to the harvesting of humidity-gradient-based energy have been presented in detail.As shown by the findings of these studies, the effects of factors such as solar and wind input on the energy output of humidly-gradient-based WEG devices should not be neglected.

Salinity-related WEG designs
Salinity gradient energy (also known as osmotic energy) is an abundant, recyclable, and nonpolluting green energy source that can produce a stable energy output. 41,43In particular, at the mouths of rivers, freshwater enters the sea, and a significant salinity gradient is formed.The potential power of such an interface is estimated to be 0.8 kWh m −3 , and its global total is estimated to reach 30 TW. 23,32,136 With the rapid development of nanotechnology and membrane science, salinity gradient energy, as a type of blue energy, has inspired scientists in the development of high-performance WEG systems.The streaming currents and streaming potentials (also known as the hydrovoltaic effect) that are generated by solvents (e.g., saline solutions) through nanochannels have also been explored.The nanotechnology has facilitated more efficient modulation of nanochannels (e.g., pore size and type), leading to greater differential energy harvesting of salt.The characteristic dimensions of the nanochannels possessed by the membranes correspond to Debye length (the Debye length is inversely proportional to the square of the ion concentration), enabling efficient differential energy harvesting of salt as compared to that of the bulk solution in RED.This subsection focuses on the use of materials with subnano-and nanochannel designs in the salinity-gradient-based capture process and the setup of the membrane system device structure.

Sub-nanochannel design
Hong et al. prepared a 2D metal carbide and nitride (MXene) film to harvest energy from salinity gradients (Figure 7A). 137The MXene film possesses subnanochannels that act as cation-selective channels and achieves an output power of 21 W m −2 at room temperature.The current measurement test device was configured with Ag/AgCl on both sides of the MXene film for experiments with different KCl concentrations and pH conditions, as shown in Figure 7B.Moreover, Gao et al. reported a membrane-scale nanofluidic device with a symmetric architecture (Figure 7C). 138The composite film has a heterogeneous structure comprising negatively charged mesoporous carbon and positively charged macroporous alumina.When the mixed membrane was placed in solutions of different concentrations, power density of 3.46 W m −2 was achieved as a result of the modulation of the nanofluid properties to achieve an asymmetric electrostatic structure.

Nanochannel design
Wu et al. designed membranes having opposing charges that were derived from bacterial cellulose and applied these to extract energy from salinity gradients (Figure 7D). 139By controlling the permeation of high-saltconcentration seawater and low-salt-concentration river water through the nanocellulose-based material mixture, a maximum output power of 0.23 W m −2 was obtained.This high output power could be achieved because of the high surface charge density and narrow nanochannels, which enhanced ion selectivity and the ion permeation rate.Similarly, Ding et al. proposed a high-power WEG device comprising oppositely charged Ti 3 C 2 T x MXene membranes combined with closed 2D nanofluidic channels (Figure 7E). 97The MXene nanochannels exhibited different polarities, a surface charge dominated by ion transport, and superior cation or anion selectivity.On the mixing of 0.5 mol L −1 seawater and 0.01 mol L −1 river water, a maximum output power of 4.6 W m −2 was obtained.
Research into salinity-gradient-based WEG devices is currently focused on increasing the efficiency and power output based on nanochannels and pores, although efforts to mitigate fouling and reduce the cost of membranes and electrodes are equally important to minimize the levelized cost of salinity gradient energy production.The use of nanomaterials provides new ideas for traditional osmotic energy capture such as RED and reverses capacitive deionization for osmotic energy capture.An EDL is formed by the net charge on the surface of the nanopore and the ions in the solution that are squeezed into the pore based on osmotic pressure, which continuously drives the movement of the EDL boundary as the concentration changes at both ends of the membrane, thus enabling the formation of flow currents and flow potentials.Follow-up studies must also focus on the differences in mass transfer and ion transport between sub-nanochannel 140 and nanotubular channels.

Evaporation-related WEG designs
Evaporation facilitates the transformation of surface seawater or groundwater into atmospheric water, enabling the harnessing of this mass transfer and the concurrent alterations in energy for electricity generation. 20roughout the evaporation process, variations occur in the salinity, temperature, humidity, and water flow.Solardriven interfacial water evaporation concentrates thermal energy at the air-liquid interface and can be exploited for desalination. 7In this context, we explored the generation of electrical energy through the utilization of a solar-driven interfacial evaporation process.

Pyroelectric and piezoelectric designs
Zhu et al. prepared nitrogen-coupled carbon sponges to transport water to an interface based on capillary forces for in situ photothermal evaporation (Figure 8A). 141The thermal energy produced during the condensation of the resulting vapor was then converted to electrical energy using ferroelectric fluoropolymers (e.g., polyvinylidene fluoride [PVDF]).This device capitalizes on the pyroelectric and piezoelectric effects triggered by temperature variations induced by evaporation and subtle oscillations of the PVDF film.The temperature fluctuation in a 5 cm × 1.5 cm PVDF device yielded a peak open-circuit voltage of 20 V and short-circuit current of 80 nA (Figure 8B-D).The close correlation between the fluctuations in current-voltage magnitude and temperature variations suggests that the principal energy-generating mechanism is the pyroelectric effect.When water vapor contacts the PVDF membrane surface, it condenses into several small droplets, and the gas-to-liquid transformation releases a large amount of heat to the PVDF membrane.As the humidity around the PVDF film decreases, the small water droplets evaporate rapidly, thus cooling the PVDF film.

Thermoelectric design
Zhu et al. 86 studied the temperature differences between the evaporating material and seawater that enable thermoelectric materials to generate electrical energy (Figure 8E).They prepared organic button sponges by coating solutions of CNT/cellulose nanocrystal nanocomposites on polydimethylsiloxane sponges.In the course of the water evaporation process propelled by solar heating, a temperature disparity emerges between the organic sponge and bulk water mass because of thermal localization.Subsequently, by harnessing the Seebeck effect, a thermoelectric module can transform this temperature contrast into electrical energy.Ion thermoelectricity, which is emerging as an innovative approach to thermoelectric technology, is employed for the generation of electrical energy in evaporation processes.This is primarily attributed to variations in the carrier types, resulting in distinct regulatory parameters for thermoelectric conversion.Zhou et al. designed a solar-evaporation-based generator that was not based on the PV or solid-state thermoelectric effects. 142Owing to the pronounced light absorption of the thin film and limited thermal conductivity within this apparatus, a substantial temperature gradient emerges at the surface when exposed to light, leading to the rapid evaporation of water from the aqueous electrolyte.The temperature gradient resulting from water evaporation propels selective ion transport within the charged nanochannels, leading to the creation of ionic thermoelectric and mobile potentials.Under one sun irradiation, the device achieved an output voltage of 1.1 V and a power density of 15 W m −2 .

Salinity-gradient-induced design
In addition to temperature gradients, salinity gradients also occur during the evaporation process and can be exploited for electrical energy production.Yang et al. designed a two-layer device to achieve both freshwater production and electricity generation. 42The structure of the device is shown in Figure 8F.The top layer is a CNTmodified filter paper and bottom layer is a Nafion membrane.The CNT-modified filter paper absorbs sunlight to achieve desalination with high efficiency.In contrast, the ion-selective Nafion membrane separates surface brine and highly concentrated seawater, and the large tion difference can be used to generate electricity.Under one sun, the device produced pure water and simultaneously generated a power output of 1 W m −2 .In this work, driven by solar-thermal energy, various regions with distinct ion concentrations were formed during the evaporation process, resulting in a chemical potential difference.Subsequently, an external circuit connected these diverse regions, resulting in a current driven by the chemical potential difference.

Hydrovoltaic system design
In another study, Qin et al. reported silicon-nanowirebased WEG devices. 143In this device, which is schematically illustrated in Figure 8G, an electron density gradient forms because of the internal water flow through the Debye screening effect and coulombic interactions.The device achieved a short-circuit current density of more than 55 µA cm −2 and an open-circuit voltage of up to 400 mV.This work, which achieved electrical energy production through the latent heat of evaporation and capillary forces driving the flow of water in nanotubes, is typical of the process of evaporation-induced potential formation (also known as the hydrovoltaic effect).

Triboelectric system design
Similar to the previous process, a flowing nanogenerator was prepared by Wang et al. using porous alumina. 105The sample is immersed in water (Figure 8H), and capillary forces drive the water from the bottom electrode to the top electrode, establishing a pressure equilibrium at a specific height within the nanotube channel.The water in the top region continuously evaporates, creating a pressure difference between the two ends of the capillary flow.This pressure difference continuously drives the capillary water in the upward direction.Here, the contact between water and alumina was considered, and the transfer of electrons from water to alumina was achieved based on the recombination of the outermost electron clouds of atoms.Note that the water ion is unstable, forming two radicals OH and H 3 O + .Simultaneously, ionization reactions occur on the solid surface, providing anions for alumina and thus forming an EDL, which is negatively charged on the surface of the alumina nanopore (Figure 8I).Moreover, the direct transfer of electrons from water to the solid surface is easier to remove than the ionization to electrons.During the flow of water through the nanotubes, the transportation of cations in the EDL along with the flow results in a net positive charge transport, known as the streaming current.Meanwhile, the induced electric field created by the resulting polarization of charge distribution along the flowing axis will result in a streaming potential.
The evaporation-based WEG model is one of the most complex processes discussed in this review, involving the movement of bulk water, the functionalization properties of water, and its phase transition (i.e., interconversion of gas and solid states).Consequently, the mechanism behind evaporation-based electricity generation process is complex and remains controversial.Therefore, a significant challenge for future research lies in identifying the most effective generator design.
The aforementioned six WEG designs are typical representatives of hydroelectric AGE-II.A summary of the corresponding design key points is shown in Figure 9.The basic properties of water (volume, size, ion concentration, ion type) and motion (velocity, width, frequency, amplitude, angle) play a crucial role in power generation performance.After determining the basic parameters, the researcher should identify the driving force (e.g., temperature difference, humidity difference, and salinity difference) for electricity generation.Subsequently, they can select the appropriate material system and mechanism for preparing the generator.In particular, the type, size, and position of the electrodes are important factors when testing the power generation performance.In addition to the six designs discussed in this review, certain special electricity generation designs have been proposed; one such example includes adjusting the water bridge between two conducting plates. 101Water can also be used as an electrolyte for the development of water-based batteries. 144,145ater as a charge transport medium is also of significant research value.

COMMENTS AND OUTLOOK
The utilization of micro/nanomaterials with unique architectures and physicochemical properties enables the construction of WEG devices that can generate electricity from a range of hydrological processes.To compare varied technologies, an overview of the electricity generation process for different WEG designs is shown in Figure 10.The power generation performances reported and summarized here are based on a systematic analysis of recent reviews and research articles.The gray squares in Figure 10 represent values that have not yet been reported.Compared to traditional EMG, the new WEG devices or groups of devices produce a significantly small amount of power (ranging from a few milliwatts to a few hundred watts).The primary reasons for this limitation are as follows.First, the size of conventional devices is considerably larger, 117 making them more efficient, especially as compared to the efficiency of small-sized WEG devices.Moreover, certain WEG devices based on triboelectric and piezoelectric effects tend to use dielectric materials with high resistance. 19,28,35Consequently, this type of generator has a higher internal resistance and therefore a lower current value, resulting in a lower output power.Finally, the WEG is dependent on the small-sized water molecular motion in hydrocycle processes. 19,46,116In this condition, the water molecular motions tend to be irregular and with a low frequency; therefore, they produce less electrical energy.A commonly employed strategy to significantly increase the power generated by the same WEG device is to connect the device in series or parallel in a circuit or to increase the size of the device.However, these strategies undoubtedly increase the energy consumption and cost, limiting the range of applications.Therefore, the development of high-power WEG equipment is the current research focus of hydroelectric AGE-II.Nanotechnology-inspired water-energy networks include a variety of technologies that are not limited to electricity generation.For instance, the utilization of solardriven interfacial evaporation presents a viable means to generate potable water, [146][147][148][149] which can seamlessly integrate with the electricity generation process. 6,7In this review, an analysis of hydroelectric AGE-II is presented, focusing on the incorporation of various new technologies in the hydrological process.Moreover, the various theories behind these energy conversion techniques, such as the electrokinetic, triboelectric, and piezoelectric effects, were elaborately explained.Utilizing these different mechanisms as well as creative nanostructured materials, novel WEG devices can be prepared to capture the energy embedded in the different roles assumed by water.Finally, a systematic overview of six WEG device designs (Figure 11, left) that can inspire researchers in the fields of materials and water sciences to develop new materials for capturing energy and converting it into electrical energy is presented.
In summary, the key limitations of small-sized WEG systems arise from the inherent challenges associated with the unpredictable, low-frequency, and low-energy-density hydrologic cycle.Nevertheless, the benefits are unmistakable: minimal carbon emissions, renewable energy source, ample reserves, low energy consumption, and a diverse array of applications (Figure 11, right).Therefore, small-sized WEG technology exhibits significant potential for deployment in diverse environments featuring water.Moreover, WEG devices of hydroelectric AGE-II can serve various purposes, such as powering electronics, environmental sensors, health monitoring, information storage, and human-computer interactions, particularly in the context of the Internet of Things (IOT). 12,19,22,150n the future, WEG holds the potential to extend its applicability to various scenarios.One notable example is small-sized WEG device utilization in facilitating the transmission of subtle signals related to forest humidity, temperature, and vibrations.These devices will enable the prediction of natural disasters, such as fires and earthquakes, leveraging the superior perceptual properties inherent in micro/nanomaterials.Moreover, compact WEG devices are designed for coping with challenging environments, including deep sea environments, glaciers, and volcanoes.These small-sized WEG devices can be used an IOT energy source to reduce the risk of humans engaging in explorations in such dangerous environments.Furthermore, employing WEG devices to energize sensors has the potential to yield data on various parameters such as sound, light, heat, electricity, mechanics, chemical processes, biology, and location in off-grid settings.2][153][154][155][156][157][158] Therefore, small-sized WEG technologies are truly interdisciplinary, having applications in materials, life, marine, and climate sciences.Most importantly, small-sized WEG devices will contribute to increasing the sustainability of society.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Schematics of hydroelectric AGE-I (left) and hydroelectric AGE-II (right).

F
I G U R E 2 Hydroelectric AGE-II system: (A) Different roles of water in the generation of electrical energy; (B) electromagnetic, (C) piezoelectric, (D) triboelectric (solid-solid), (E) pyroelectric, and (F) thermoelectric effects; (G and H) evaporation-induced potential design of hydrovoltaic effect and electrical double layer (EDL); (I and J) drop-based liquid-solid triboelectric design and the Wang hybrid EDL model.

F I G U R E 3
(A) Schematic of water-flow-based energy generation.(B) Voltage output of the device in saturated NaCl solution that is moving backward and forward.(C) Dependence of output voltage and current on the concentration of the aqueous solution that is flowing at the same speed.(D) Effect of doping with ordered mesoporous carbon on output voltage.(E) Schematic of flow-based water-enable electricity generation (WEG).(F) Dependence of the generated electric potential on the length of the flow pipe.(G) Electromotive force at different flow speeds (solid line indicates a linear fit).(H) Output power at different flow speeds (solid line indicates a linear fit).Source: (A-D) Reproduced from Ref. [112] with permission from Wiley-VCH, copyright [2017].(E-H) Reproduced from Ref. [113] with permission from AIP, copyright [2020].

F I G U R E 4
(A) Schematic illustration of wave-related WEG generator with arch-shaped structure.Inset picture: scanning electron microscopic image of polytetrafluoroethylene and nanopores on an aluminum electrode.(B) Variations in open-circuit voltage and (C) short-circuit current with time under the impact of the simulated wave.(D) Schematic illustration of wave-related WEG generator with spherical structure.(E) WEG has two modes of motion (stable and unstable) with a ball structure.(F−H) In the WEG using a spherical structure, changes in transfer charge, short-circuit current, and open-circuit voltage with time under different frequencies.(I) Changes in short-circuit current with time under different motion states in the WEG using a spherical structure.Source: (A-C) Reproduced from Ref. [119] with permission from the American Chemical Society, copyright [2015].(D-I) Reproduced from Ref. [120] with permission from the American Chemical Society, copyright [2018].

F G U R E 5
(A) Schematic and (B) photographs of a droplet-based water-enabled electricity generation (WEG) for harvesting energy from rainfall.(C) Photographs showing light emitting diode bulb arrays powered by the continuous flow of water droplets on the slippery lubricant-impregnated porous surface-based TENG and superhydrophobic TENG at 25 and −3 • C. (D) Condensation-based generator for extracting energy from raindrop nucleation, growth, and shedding.(E) Photos of droplet flow at different stages.Blue and gray rectangles represent the electrode area.(F) A superhydrophobic magnetoelectric system containing glycerol droplets between the top of the superhydrophobic coil and the magnetic supporting layer.(G) Device schematic and (H) voltage of the droplet-based electricity generator.T touch , T max , and T separate are the contact times between the water droplets and electrodes.Blue and green indicate the diffusion and contraction areas of water droplets, respectively.Source: (A and B) Reproduced from Ref. [106] with permission from Springer Nature, copyright [2020].(C) Reproduced from Ref. [123] with permission from Oxford University Press on behalf of China Science Publishing & Media Ltd, copyright [2019].(D and E) Reproduced from Ref. [124] with permission from Elsevier, copyright [2021].(F) Reproduced from Ref. [47] with permission from John Wiley & Sons, copyright [2020].(G and H) Reproduced from Ref. [125] with permission from Elsevier, copyright [2020].

F
I G U R E (A) Humidity-based water-enabled electricity generation (WEG) using Geobacter sulfurreducens.Inset: Transmission electron microscopy image of Geobacter sulfurreducens.Scale bar = 100 nm.(B) Output voltage and relative humidity versus time.(C) Humidity-based WEG prepared using bilayer polyelectrolyte films.(D) Schematic of porous carbon film (PCF)-based device.High-and low-functional-group regions (denoted as HFGR and LFGR in the figure, respectively) indicate high-and low-functional-group contents, respectively.(E) Output voltage of PCF-based WEGs driven by water vapor adsorption.Device types I, II, III, and IV are PCF membranes subjected to different treatments condition.(F) Schematic of tailored carbon-electrode all-inorganic perovskite solar cells for harvesting solar and water vapor energy.(G) A novel solar-coupled humidity-gradient-based WEG for collecting energy from moisture and sunlight simultaneously, thereby increasing power output.(H) A polystyrene sulfonic acid (PSSA)/R film composed of a PSSA matrix and R clusters.A poly(3,4-ethylenedioxythiophene) polystyrene sulfonate layer was spin-coated on the bottom electrode in close contact with the PSSA/R film.Source: (A and B) Reproduced from Ref. [91] with permission from Springer Nature, copyright [2020].(C) Reproduced from Ref. [126] with permission from Springer Nature, copyright [2021].(D and E) Reproduced from Ref. [127] with permission from Wiley, copyright [2016].(F) Reproduced from Ref. [128] with permission from Wiley, copyright [2018].(G and H) Reproduced from Ref. [129] with permission from Wiley, copyright [2022].

F I G R E 7
(A) Schematic of a salinity-gradient-based generator.(B) Voltage and current variation of a Ti 3 C 2 T x membrane at different salt concentrations at a pH of 5.7.(C) Schematic of salinity-gradient-based membrane-type generator.(D) Salinity gradient based on bacterial cellulose (BC) nanofluid.Positively and negatively charged aligned BC membranes represent anion-and cation-selective diaphragms, respectively.(E) Salinity-gradient-based electricity generation based on oppositely charged MXene membrane pairs for constructing nanofluidic channels.n-MXene and p-MXene indicate negatively and positively charged MXene nanosheets, respectively.RW, river water; SW, seawater.Source: (A and B) Reproduced from Ref. [137] with permission from the American Chemical Society, copyright [2019].(C) Reproduced from Ref. [138] with permission from the American Chemical Society, copyright [2014].(D) Reproduced from Ref. [139] with permission from Elsevier, copyright [2020].(E) Reproduced from Ref. [97] with permission from John Wiley & Sons, copyright [2020].

F I G U E 8
(A) Schematic diagram of steam-generation-induced electric potential using carbon sponges.(B) Temperature fluctuations on the surface of a polyvinylidene fluoride (PVDF) film.(C) Piezo-pyroelectric output currents and (D) voltages of the PVDF films during the evaporation process.(E) Schematic of the synergistic interfacial photothermal water evaporation and thermoelectricity generation process based on carbon-based polydimethylsiloxane sponge.(F) Schematic of a hybrid system for solar desalination and salinity power extraction.By equipping the system with an ion-selective membrane, electricity can be extracted from the salinity gradient.(G) Schematic diagram of a silicon-nanowire-based WEG device.Graphite and silver-painted electrodes act as a cathode and anode for the device, respectively.(H) Experimental setup for electricity generation using porous alumina.(I) Typical mechanism of evaporation-induced potential based on the hybrid layer proposed by Wang.Source: (A-D) Reproduced from Ref. [141] with permission from John Wiley & Sons, copyright [2019].(E) Reproduced from Ref. [86] with permission from John Wiley & Sons, copyright [2019].(F) Reproduced from Ref. [42] with permission from Royal Society of Chemistry, copyright [2017].(G) Reproduced from Ref. [143] with permission from John Wiley & Sons, copyright [2020].(H and I) Reproduced from Ref. [105] with permission from John Wiley & Sons, copyright [2022].

F I G R E 9 F I G U R E 1 0
Summary of the key points of the design principles for various types of water-enabled electricity generation (WEG) systems in the hydrocycle process.Current (A), voltage (V), and power (W) ranges for different mode designs.The vertical axis indicates the order of magnitude of the corresponding values.

F I G U R E 1
Schematic of water-enabled electricity generation (WEG) models and potential applications.IOT, Internet of Things.Insets in the diagram on the left side related to the flow-based design mode.Insets of the right diagram from human-computer interaction.Source: (Left inset) Reproduced from Ref.[153] with permission from Springer Nature, copyright[2014]; reproduced from Ref.[121] with permission from Springer Nature, copyright[2014]; reproduced from Ref.[106] with permission from Springer Nature, copyright [2019]; reproduced from Ref.[91] with permission from Springer Nature, copyright [2020]; reproduced from Ref.[44] with permission from Springer Nature, copyright [2019]; reproduced from Ref.[42] with permission from Royal Society of Chemistry, copyright [2017]; reproduced from Ref.[101] with permission from Springer Nature, copyright[2013].(Right inset) Reproduced from Ref.[155] with permission from Elsevier, copyright [2018]; reproduced from Ref.[154] with permission from American Chemical Society, copyright[2016]; reproduced from Ref.[156] with permission from John Wiley & Sons, copyright[2019]; reproduced from Ref.[131] with permission from John Wiley & Sons, copyright [2016]; reproduced from Ref.[157] with permission from Elsevier, copyright [2022]; Reproduced from Ref.[158] with permission from Elsevier, copyright[2020].
This work is supported by the Fundamental Research Funds for Central Universities of Hohai University (B220203014), Postgraduate Research & Innovation Program of Jiangsu Province (4200261601), National Natural Science Foundation of China (51909066), the Zhejiang Ocean University Talent Introduction Research Fund (No. JX6311103723), the ES Program (via Nagoya University), and the JST-ERATO Yamauchi Materials Space Tectonics Project (JPMJER2003).