Hydrovoltaic Energy on the Way

Jun Yin obtained his PhD from Nanjing University of Aeronautics and Astronautics (NUAA) in 2016, after which he worked in the University of Manchester as a research associate for 3 years. Then, he joined NUAA in 2019 as a professor. His research focuses on surface/interface interaction of nanomaterials and their applications.Jianxin Zhou is an associate professor of mechanics at NUAA. He obtained his PhD (2007) in condensed matter physics from Nanjing University. His research focuses on the utilization of energy conversion and energy transfer properties of low-dimensional materials. His research interests also include fabrication, assembly, and microscopic characterization of nanosized structures for sensing and energy applications.Sunmiao Fang received his BS in College of Science from NUAA in 2013. He is currently a PhD candidate in Nanomechanics at Institute of Nanoscience, NUAA. His main research interests focus on functional materials associated with charge transport and energy conversion.Wanlin Guo obtained his PhD from Northwestern Polytechnical University in 1991, and in 2017 he was elected as an academician of the Chinese Academy of Science. He is a Chair Professor at NUAA and the founding director of the Key Laboratory of Intelligent Nano Materials and Devices of Ministry of Education. His research focuses on intelligent nano materials and devices, novel conception and technology for efficient energy conversion, molecular physical mechanics for neuronal signaling and molecular biomimics, as well as strength and safety of aircraft and engine. Jun Yin obtained his PhD from Nanjing University of Aeronautics and Astronautics (NUAA) in 2016, after which he worked in the University of Manchester as a research associate for 3 years. Then, he joined NUAA in 2019 as a professor. His research focuses on surface/interface interaction of nanomaterials and their applications. Jianxin Zhou is an associate professor of mechanics at NUAA. He obtained his PhD (2007) in condensed matter physics from Nanjing University. His research focuses on the utilization of energy conversion and energy transfer properties of low-dimensional materials. His research interests also include fabrication, assembly, and microscopic characterization of nanosized structures for sensing and energy applications. Sunmiao Fang received his BS in College of Science from NUAA in 2013. He is currently a PhD candidate in Nanomechanics at Institute of Nanoscience, NUAA. His main research interests focus on functional materials associated with charge transport and energy conversion. Wanlin Guo obtained his PhD from Northwestern Polytechnical University in 1991, and in 2017 he was elected as an academician of the Chinese Academy of Science. He is a Chair Professor at NUAA and the founding director of the Key Laboratory of Intelligent Nano Materials and Devices of Ministry of Education. His research focuses on intelligent nano materials and devices, novel conception and technology for efficient energy conversion, molecular physical mechanics for neuronal signaling and molecular biomimics, as well as strength and safety of aircraft and engine. Water is the largest energy reservoir, regulator, and balancer on Earth. On the 510 million km2 surface of the planet, solar radiation brings a power density of 340 W·m−2, from which water absorbs more than 115 W·m−2 (Figure 1).1Stephens G. Li J. Wild M. Clayson C.A. Loeb N. Kato S. L’Ecuyer T. Stackhouse P.W. Lebsock M. Andrews T. An update on Earth’s energy balance in light of the latest global observations.Nat. Geosci. 2012; 5: 691-696Crossref Scopus (432) Google Scholar If the global primary energy consumption of human beings is also averaged over the global area, it is 0.036 W·m−2, only approximately 1/4,500 of the energy absorbed by water. Additionally, energy adsorbed by atmosphere, around 75 W·m−2, can also be utilized by water. The energy drives water evaporation, which takes around 60 W·m−2 and gives rise to moisture, rainfall, snowfall, rivers, and so on, establishing a global water cycle. From the water wheels that have been used since ancient China and Greece, the hydroelectric power plants that are being built throughout the whole industrial era till today, to the recently emerged hydrovoltaic technology,2Zhang Z. Li X. Yin J. Xu Y. Fei W. Xue M. Wang Q. Zhou J. Guo W. Emerging hydrovoltaic technology.Nat. Nanotechnol. 2018; 13: 1109-1119Crossref PubMed Scopus (114) Google Scholar various ways have been devised to harness the tremendous energy contained in water. In contrast to conventional technologies that harvest kinetic energy of water, hydrovoltaic technology generates electricity from the direct interaction of materials with water.1Stephens G. Li J. Wild M. Clayson C.A. Loeb N. Kato S. L’Ecuyer T. Stackhouse P.W. Lebsock M. Andrews T. An update on Earth’s energy balance in light of the latest global observations.Nat. Geosci. 2012; 5: 691-696Crossref Scopus (432) Google Scholar The underlying mechanisms include streaming potential, waving potential, drawing potential, evaporation-induced electricity, and gradient-induced ion diffusion. With these versatile hydrovoltaic effects, energy from flowing, waving, dropping, condensing, and evaporating water can now be harvested, significantly extending our capability in harvesting water energy. If 1% of energy adsorbed by water could be utilized with an efficiency of 1% through hydrovoltaic technology, it could provide nearly 1/3 of the global energy consumption, comparable to that of crude oil (Figure 1). To achieve such a goal, intensive efforts have been devoted to hydrovoltaic technology with notable developments made since the introduction of this field 2 years ago. The power generation has been improved by several orders with incorporation of new materials and devices. Here, we will review hydrovoltaic technology for harvesting kinetic energy, evaporation energy, and gradient energy, mainly focusing on the recent developments, and envision the future directions for hydrovoltaics. When solid materials come in contact with water, ions taking charges of opposite polarity (counter ions) to the surface charge will be attracted, forming an electrical double layer (EDL) at the water-solid interface. The EDL-based classical electrokinetic effects, especially the streaming potential, can convert kinetic energy of flowing water through narrow channels to electricity. However, the production of notable streaming potential usually requires quite large pressure gradient, hindering its practical applications. Instead of the steady EDL, moving boundary of EDL leads to novel hydrovoltaic effects. It was initially demonstrated that drawing or dropping a water droplet on graphene supported on insulating substrates can give rise to a voltage of tens of millivolt, referred to as drawing potential, with an output power density up to 40 μW·m−2.3Yin J. Li X. Yu J. Zhang Z. Zhou J. Guo W. Generating electricity by moving a droplet of ionic liquid along graphene.Nat. Nanotechnol. 2014; 9: 378-383Crossref PubMed Scopus (267) Google Scholar Then, by adapting polarized or charged materials as substrates, or applying bias potential, volt-level drawing potential can be produced in graphene layer with the output power density enhanced by two orders, up to 3 mW·m−2 (Figure 2A).4Yang S. Su Y. Xu Y. Wu Q. Zhang Y. Raschke M.B. Ren M. Chen Y. Wang J. Guo W. et al.Mechanism of Electric Power Generation from Ionic Droplet Motion on Polymer Supported Graphene.J. Am. Chem. Soc. 2018; 140: 13746-13752Crossref PubMed Scopus (32) Google Scholar It has been widely demonstrated that drawing potential enables harvesting the kinetic energy of raindrops and solar cells working on rainy days as well.5Tang Q. Duan Y. He B. Chen H. Platinum Alloy Tailored All-Weather Solar Cells for Energy Harvesting from Sun and Rain.Angew. Chem. Int. Ed. 2016; 55: 14412-14416Crossref PubMed Scopus (44) Google Scholar Similarly, waving energy can be harvested by placing a single layer of graphene or carbon film across the surface of waving water into electricity referred to as waving potential.6Yin J. Zhang Z. Li X. Yu J. Zhou J. Chen Y. Guo W. Waving potential in graphene.Nat. Commun. 2014; 5: 3582Crossref PubMed Scopus (140) Google Scholar More excitingly, by introducing electret materials and electrostatic induction effect, it has been demonstrated recently that the dynamic formation and vanishing of EDL between a water bridge and a metallic electrode can give rise to a power density up to 50 W·m−2, managing to power one hundred commercial LEDs with one dropping droplet (Figure 2A).7Xu W. Zheng H. Liu Y. Zhou X. Zhang C. Song Y. Deng X. Leung M. Yang Z. Xu R.X. et al.A droplet-based electricity generator with high instantaneous power density.Nature. 2020; 578: 392-396Crossref PubMed Scopus (182) Google Scholar But the recorded high output power density is instantaneous with a typical duration of only several microseconds. Water evaporation always occurs around us, during which a huge amount of thermal energy in the environment turns into latent energy of water vapor. But human beings had never managed to harvest electricity from the process directly, up until 3 years ago. It was first reported in 2017 that natural water evaporation through a small piece of porous carbon black film in ambient environment can produce sustainable voltage over 1 V and current up to 100 nA.8Xue G. Xu Y. Ding T. Li J. Yin J. Fei W. Cao Y. Yu J. Yuan L. Gong L. et al.Water-evaporation-induced electricity with nanostructured carbon materials.Nat. Nanotechnol. 2017; 12: 317-321Crossref PubMed Scopus (324) Google Scholar Compared to the aforementioned hydrovoltaic effects for harvesting kinetic energy, this evaporating potential and current persist as long as the evaporation continues. Since this pioneering work, a great upsurge has been spreading worldwide for energy harvesting through water evaporation. A large variety of functional materials have been demonstrated to construct water-evaporation generators, including carbon nanomaterials, hydroxides, oxides,9Sun J. Li P. Qu J. Lu X. Xie Y. Gao F. Li Y. Gang M. Feng Q. Liang H. et al.Electricity generation from a Ni-Al layered double hydroxide-based flexible generator driven by natural water evaporation.Nano Energy. 2019; 57: 269-278Crossref Scopus (38) Google Scholar polymers, semiconductors,10Qin Y. Wang Y. Sun X. Li Y. Xu H. Tan Y. Li Y. Song T. Sun B. Constant Electricity Generation in Nanostructured Silicon by Evaporation-Driven Water Flow.Angew. Chem. Int. Ed. 2020; 59: 10619-10625Crossref PubMed Scopus (15) Google Scholar natural wood, and so on. The power density has been enhanced by greater than two orders of magnitude from an initial 100 μW·m−2 to 60,000 μW·m−2 (Figure 2A). The output electricity of a single device is already sufficient to power low-consumption devices like calculators and LCD displays directly (Figure 2B). Through optimizing both the materials and device configurations, it has potential to further improve the power output. Porous materials with proper pore size, hydrophilic channels for water transport, and notably charged channel surfaces are highly desired. Currently, all the devices for harvesting water evaporation energy are simply based on films partially immersed in water. Development of other configurations, such as bulk materials floating on water surface, could significantly facilitate its large-scale application. Also, integrating technology for solar-powered water evaporation, such as adapting photothermal materials, has potential to gain additional energy from sun radiation. In spite of the significant enhancement in output power density, fundamental understanding of the underlying mechanism of evaporating potential remains a challenge. One clear point is that it is tightly related to the EDL and interaction of water molecules with the porous film, as both the voltage value and polarity can be significantly modified by introducing a charged material surface.11Li J. Liu K. Ding T. Yang P. Duan J. Zhou J. Surface functional modification boosts the output of an evaporation-driven water flow nanogenerator.Nano Energy. 2019; 58: 797-802Crossref Scopus (33) Google Scholar However, simple models based on streaming potential and other known electrokinetic effects cannot explain everything, and quantum mechanics faces difficulty even in understanding the structure of water. The most important missing part is how ion/proton motion/accumulation at the water-material interface could give rise to a constant electronic current within the material. In contrast to the measurement of streaming current, where non-polarizable electrodes, such as most commonly used Ag/AgCl electrode, are adapted to convert ion current in liquid to electron current in lead wire at the electrode-electrolyte boundary, the evaporating potential and current are formed in the porous films and measured directly using metal or carbon nanotube films as the electrodes. Thus, a comprehensive understanding of evaporating potential requires further efforts. Generally, any kind of energy gradient is equivalent to a driving force that can do work in the system, such as moving charge carriers. Thus, a gradient always contains energy. The most common gradient energy is available from directional diffusion of ions along a concentration gradient in water, which is specifically termed as blue energy, or osmotic energy. While conventional technologies to harvest osmotic energy are rather inefficient, 2D materials with supra-nanometer pores are promising for enhancing the energy conversion efficiency, but their scaling up is a challenge. Exposing some porous films with asymmetric structures or functional groups to moisture can artificially introduce a concentration gradient of dissociated charge carriers, leading to a membrane potential due to the diffusive motion of the charge across the films. It was first reported in a graphene oxide film with a functional group gradient12Zhao F. Cheng H. Zhang Z. Jiang L. Qu L. Direct Power Generation from a Graphene Oxide Film under Moisture.Adv. Mater. 2015; 27: 4351-4357Crossref PubMed Scopus (184) Google Scholar and soon extended to other systems, including polymers, TiO2 nanowires, and graphene quantum dots.13Huang Y. Cheng H. Shi G. Qu L. Highly Efficient Moisture-Triggered Nanogenerator Based on Graphene Quantum Dots.ACS Appl. Mater. Interfaces. 2017; 9: 38170-38175Crossref PubMed Scopus (36) Google Scholar The moisture induced potential ranges from tens of millivolt to nearly 1 V, and a maximum output power density up to 18.6 W·m−2 has been claimed.13Huang Y. Cheng H. Shi G. Qu L. Highly Efficient Moisture-Triggered Nanogenerator Based on Graphene Quantum Dots.ACS Appl. Mater. Interfaces. 2017; 9: 38170-38175Crossref PubMed Scopus (36) Google Scholar However, these devices cannot work sustainably and required cyclical moisture feeding with quite a high gradient in humidity. It is desirable to find materials that can spontaneously provide electricity in ambient air with a large range of relative humidity and work for a long duration. It was first achieved in directionally reduced graphene oxide membranes.14Cheng H. Huang Y. Zhao F. Yang C. Zhang P. Jiang L. Shi G. Qu L. Spontaneous power source in ambient air of a well-directionally reduced graphene oxide bulk.Energy Environ. Sci. 2018; 11: 2839-2845Crossref Google Scholar Ambient exposures of it at a relative humidity from 25% (comparable to a desert environment) to 85% could provide a potential around 0.2–0.45 V, which can be sustained for more than 100 h. However, the short-circuit current density dropped quickly from 9 mA·m−2 to less than 1 mA·m−2 in tens of seconds, setting the upper limit of the sustainable output power density at less than 0.1 mW·m−2. It was newly reported that thin protein nanowire films sheared from microbes showed even more attractive performance.15Liu X. Gao H. Ward J.E. Liu X. Yin B. Fu T. Chen J. Lovley D.R. Yao J. Power generation from ambient humidity using protein nanowires.Nature. 2020; 578: 550-554Crossref PubMed Scopus (76) Google Scholar It can work sustainably for more than 2 months in ambient humidity with a claimed power density around 50 mW·m−2, but further work is needed to fully understand the operating principles that enabled this high performance and stability. Although impressive progress has been made on harvesting electricity by hydrovoltaic technology, there is still a long way to go to make it useful for our daily energy consumption. To understand the dynamic coupling of molecules, ions, protons in liquids, and electrons in solids is one essential issue. The behavior of water, ions, and electrons in a hydrovoltaic system may differ in many ways from the framework of classical theory. Although quantum mechanics can effectively describe the physical properties of solids on the nanoscale, the behavior of liquid-solid interactions is beyond its present ability. Reliable and effective theories and methods are needed. Another essential issue is surface charge. As mentioned above, drawing potential and evaporating potential can be enhanced by using electrified surfaces. Electrostatic gating can increase the streaming current in molecular-sized slit-like channels by 20 times. The outstanding performance of protein membrane in ambient humidity may be a manifestation of charged amino acid residues in proteins. This is not surprising from a fundamental point of view because the structure of EDL, the solid-liquid interaction, and even the structure of confined water can be adjusted by charge. The question now is where and how to place the charge, and by what guidelines. With respect to energy conversion efficiency, traditional mechanics has performed exemplary work. In a classic Newtonian cradle, energy can be effectively transferred from one ball to the other with the same mass by elastic collision with low energy dissipation. To achieve this wonderful situation, we must develop and utilize the full power of the hydrovoltaic principle to guide the system design so that the energy carried by water molecules and ions can be fully transferred to electrons in solid materials. In comparison with electricity generation by fossil fuels, hydrovoltaic effects, such as evaporating potential, emit neither carbon dioxide nor other harmful contaminants, such as NOx and PM. Moreover, it can convert low-quality latent heat in ambient environment into high-quality electric energy, providing an avenue by which to slow down global warming without scarifying our demand on energy. Another merit of water evaporation is that it can produce purified water, conditioning room temperature at the same time. From this perspective, hydrovoltaic technology is environmentally friendly and has great potential for ecological sustainable development (Figure 3). Overall, hydrovoltaic technology has definitely been boosted during the last few years, with notable efforts contributed by scientists from many fields. Not only kinetic energy of water, but also water evaporation and ambient moisture, can now be utilized to generate electricity. Considering the tremendous energy carried by water, hydrovoltaic energy is on the way to becoming a renewable energy widely and abundantly distributed on Earth. Its zero-contaminant emission, ambient heat adsorption, and water purification endow it potential to bring disruption for the key societal question in balance between energy consumption and ecology destruction. The bright prospect of hydrovoltaic technology deserves collaborative and persistent efforts devoted to this field. This work was supported by National Key Research and Development Program of China ( 2019YFA0705400 ), National Natural Science Foundation of China ( 51535005 , 51702159 ), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures ( MCMS-I-0418K01 , MCMS-I-0419K01 ), NSF of Jiangsu province ( BK20170791 ), the Fundamental Research Funds for the Central Universities ( NC2018001 , NP2019301 , NJ2019002 , NE2020001 ), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions .