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.

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.