Review:The Rise of Hydrovoltaics

Water contains tremendous energy,which fuels the Earth’s water cycle.Technology for generating electricity directly from interaction between water and nanomaterials is referred to as hydrovoltaic technology,providing versatile ways to harvest energy from most steps of the water cycle.Due to its attractive potential,intensive efforts have been devoted into this area,and lots of notable developments have been made during the last few years,supporting the progress of hydrovoltaics.In this article,a brief review of recent progress made in hydrovoltaic energy harvester for mechanical and environmental energy of water is presented.Following that,the future directions for hydrovoltaic energy and its potential on hydrovoltaic ecology and intelligence are envisioned.

Hybrid hydrovoltaic electricity generation driven by water evaporation

Water evaporation is a ubiquitous natural process exploiting thermal energy from ambient environment.Hydrovoltaic technologies emerged in recent years offer one prospective route to generate electricity from water evaporation,which has long been overlooked.Herein,we developed a hybrid hydrovoltaic generator driven by natural water evaporation,integrating an“evaporation motor”with an evaporation-electricity device and a droplet-electricity device.A rotary motion of the“evaporation motor”relies on phase change of ethanol driven by water-evaporation induced temperature gradient.This motion enables the evaporation-electricity device to work under a beneficial water-film operation mode to produce output of~4 V and~0.2μA,as well as propels the droplet-electricity device to convert mechanical energy into pulsed output of~100 V and~0.2 mA.As different types of hydrovoltaic devices require distinctive stimuli,it was challenging to make them work simultaneously,especially under one single driving force.We here for the first time empower two types of hydrovoltaic devices solely by omnipresent water evaporation.Therefore,this work presents a new pathway to exploiting water evaporation-associated ambient thermal energy and provides insights on developing hybrid hydrovoltaic generators.

Role of electrodes in study of hydrovoltaic effects

The last decade has witnessed the emergence of hydrovoltaic technology,which can harvest electricity from different forms of water movement,such as raindrops,waves,flows,moisture,and natural evaporation.In particular,the evaporation-induced hydrovoltaic effect received great attention since its discovery in 2017 due to its negative heat emission property.Nevertheless,the influence of electrode reactions in evaporation-induced power generation is not negligible due to the chemical reaction between active metal electrodes and water,which leads to“exceptional”power generation.Herein,we designed a series of experiments based on air-laid paper devices with electrodes of different activities as the top and bottom electrodes.To verify the contribution of electrodes,we compared the output performance of different electrode combinations when the device was partially-wetted and fully-wetted.The device hydrophilicity,salt concentration,and acidity or basicity of solutions were also comprehensively investigated.It is demonstrated that the chemical reaction of active metals(Zn,Cu,Ag,etc.)with different aqueous solutions can generate considerable electrical energy and significantly distort the device performance,especially for Zn electrodes with an output voltage from~1.26 to~1.52 V and current from~1.24 to~75.69μA.To promote the long-term development of hydrovoltaic technology,we recommend use of inert electrodes in hydrovoltaic studies,such as Au and Pt,especially in water and moisture environment.

Materials for evaporation-driven hydrovoltaic technology

Water constitutes the largest energy carrier on earth,absorbing more than 70%of the solar energy received by the earth's surface,yet its low exploitation has been a constant concern.The hydrovoltaic effect is an emerging technology that generates electricity through the direct interaction between nanomaterials and water of various forms(raindrops,waves,flows,moisture,and natural evaporation).Especially,the evaporation-driven hydrovoltaic effect is a spontaneous and ubiquitous process that can directly convert thermal energy from the surrounding environment into electricity without the demand for additional mechanical work,which shows unique advantages compared with other hydrovoltaic effects.A variety of nanostructured materials have been steadily developed for evaporation-driven hydrovoltaic devices(EHDs)in recent years.However,there has been a lack of a clear specification on the selection and design of materials for improving device performance.Herein,we first analyze the mechanisms of EHDs followed by a summarization of the recent advances in materials,including carbon materials,biomass-based materials,metal oxides,composite materials,and others.We then discuss the strategies for improving the energy conversion efficiency and the output power in terms of structural design,surface modification,and interface treatment.Finally,we provide an outlook on the potential applications of electricity generation,sensors,and desalination technology,as well as the challenges and prospects for the development of this emerging technology in the future.

Self-powered/self-cleaned atmosphere monitoring system from combining hydrovoltaic,gas sensing and photocatalytic effects of TiO_(2)nanoparticles

A new self-powered/self-cleaned atmosphere monitoring system has been fabricated from TiO_(2)nanoparticles through combining hydrovoltaic,gas sensing and photocatalytic effects.The TiO_(2)nanoparticle film can convert natural thermal energy into electricity(hydrovoltaic effect)by the spontaneous water evaporation.The hydrovoltaic/gas-sensing coupling effect of TiO_(2)nanoparticle offers the waterevaporation-powered gas detection performance,and the outputting voltage/current has a good response to the surrounding gas atmosphere,directly acting as the gas sensing signal.The zeta potential of TiO_(2)is changed by the surface adsorption of gas molecules,and thus affects the electricity output of the system.The outputting electricity can directly power a wireless transmitter for transmitting the sensing information to external platform,and the whole system can work independently without electricity power supply.The rainwater can be used as the fuel of the system,and thus the system can be used outdoors without scheduled maintenance.Moreover,the photocatalytic activity of TiO_(2)can effectively degrade the organic pollutants on the film under photo illumination,leading to a self-clean behavior of the system.The system can probably promote the development of green sensing techniques with evaporation-induced ability.

Wearable Battery-Free Perspiration Analyzing Sites Based on Sweat Flowing on ZnO Nanoarrays

We fabricated wearable perspiration analyzing sites for actively monitoring physiological status during exercises without any batteries or other power supply.The device mainly consists of ZnO nanowire(NW)arrays and flexible polydimethylsiloxane substrate.Sweat on the skin can flow into the flow channels of the device through capillary action and flow along the channels to ZnO NWs.The sweat flowing on the NWs(with lactate oxidase modification)can output a DC electrical signal,and the outputting voltage is dependent on the lactate concentration in the sweat as the biosensing signal.ZnO NWs generate electric double layer(EDL)in sweat,which causes a potential difference between the upper and lower ends(hydrovoltaic effect).The product of the enzymatic reaction can adjust the EDL and influence the output.This device can be integrated with wireless transmitter and may have potential application in constructing sports big data.This work promotes the development of next generation of biosensors and expands the scope of self-powered physiological monitoring system.

Mechanism of water-evaporation-induced electricity beyond streaming potential

Since its first discovery in 2017,evaporation-induced electricity has attracted extensive attention and shown significant advantages in green energy conversion.While the streaming potential-related electrokinetic effect has been intensively explored and widely recognized as the underlying mechanism,the role of coupling between water molecules and charge carriers in the material remains elusive.Here we show through carefully designed experiments that the streaming potential effect indeed plays a role but can only contribute about half to the total water-evaporation-induced voltage occurring within the partially-wetted region of the carbon black film where the solid-liquid-gas three-phase interface exists.It is also shown that water evaporation from carboxyl and amino-functionalized carbon black films produces opposite voltage signals.Detailed first-principles calculations unveil that the adsorption of water molecules can lead to reversed charge transfer in the carboxyl and amino-functionalized carbon substrates.Finally,an evaporation-driven charge transport mechanism is proposed for the induced electricity mediated by the coupling between water molecules and charge carriers in the material.The results reveal the important role of direct interaction between water molecules and materials,deepening our understanding of the mechanism for evaporation-induced hydrovoltaic effect beyond streaming potential.

Integrating reduced graphene oxides and PPy nanoparticles for enhanced electricity from water evaporation

Developing high-performance nanostructured materials is key to deliver the potential of hydrovoltaic technology into practical applications.As single-component materials have approached its limit in generating hydrovoltaic electricity,the development of multi-component hydrovoltaic materials has been necessary in continuously boosting the electricity output.Here,we report a hydrovoltaic material by integrating reduced graphene oxides and polypyrrole nanoparticles(rGO/PPy),where the rGO contributes improved conductivity and large specific surface area while PPy nanoparticles enable enhanced interaction with water.The device fabricated with this material generates a short-circuit current of 6μA as well as a maximum power density of over 1μW/cm3 from natural evaporation of water.And the substantial ion-PPy interaction enables robust voltage generation from evaporation of various salt solutions.Moreover,an outstanding scaling ability is demonstrated by connecting 10 devices in series that generate a sustainable voltage of up to~2.5 V,sufficing to power many commercial devices,e.g.LED bulb and LCD screen.

Geometry effect on water-evaporation-induced voltage in porous carbon black film

Water-evaporation-induced electricity generation shows great application prospect due to the volt-level voltage induced by natural water evaporation in ambient environment.Increasing investigations have been conducted on water-evaporation-induced electricity in different materials,surface treatments,working mechanisms and wide ranges of practical applications.However,to further enhance the electricity output remains a great challenge.In this study,we fabricated porous carbon black films with isosceles trapezoid shapes to generate water-evaporation-induced electricity,in contrast with the widely used rectangle samples.We found that an isosceles trapezoid film produces two different voltages by inversing the film,because of different flow rates and capillary heights after inversion.The ratio of these voltages varies with the length of longer base when lengths of shorter base and heights are fixed,and the maximum ratio can reach 1.77:1 in our study.This work shows the importance of geometry design in improving the output voltage.