Chinese scientists develop new technology to produce electrical energy from water

Water is indispensable to life. About 70% of the human body and 80% of a cell is water. The water covers about 71% of the Earth’s surface area, absorbing nearly 70% of the solar radiation energy on the earth surface. The annual average power of water state transiting could reach 60 trillion kilowatts, which is three orders higher than the annual average energy consumption of all the human beings in the world (about 18 billion kilowatts in 2016). The annual average power of water evaporation can reach 40 trillion kilowatts, indicating that there is a huge space for exploitation. Unlike solar energy and wind energy, water energy can change into various forms of energy such as evaporation energy, raindrop energy, flow energy, waterfall energy, and wave energy in the process of the water cycle.

In human history, people have invented numerous tools based on the water-energy transforming principle such as the water-driven wheels, the steam-driven locomotives, and the water-driven generators. In the 19th century, it has already been discovered that the flow of water through a narrow channel or gap will lead to an electrokinetic effect like streaming potential. The electrokinetic effect is that electrical signal is produced through shearing the electric double layer at the solid-liquid interface by fluid-solid coupling, which is based on principles of both thermodynamics and electrostatics.

In the early researches on the interaction between water and low-dimensional materials, it was discovered that water molecules flowing through carbon nanotubes could generate electrical signals in carbon nanotubes as a result of phonon and Coulomb drag effect. As it is operated on the principle of Secondary effect of quantum mechanical phenomena, the signal is very weak, lacking potential for improvement.

In recent years, the multidisciplinary research team led by Guo Wanlin of the Institute of Nanoscience of NUAA has implemented systematic researches on Fluid-solid-electrical coupling of two-dimensional coating systems, for example, graphene. Systematic experimental results verified that carbon material flow could lead to electricity production which was reported worldwide was actually caused by the interaction of metal electrodes and liquids on graphene. The complete immersion in the flowing liquid of graphene will not lead to electricity production.

What’s more, the team also found that the movement of droplets on the surface of a solid coated with a single layer of graphene generates “drawing potential” proportional to the velocity of the droplets (Nature Nanotech. 9, 378, 2014). And the liquor fluctuates up and down along the surface of the solid coated with graphene generates “waving potential” proportional to the wave velocity. (Nature Commun. 5, 3582, 2014). Based on previous two newly discovered electrokinetic effect, the team proposed a new electrokinetic effect theory of electric generation of double-layer electrical boundary motion. New technologies are created such as painting sensing and cloud and raindrop energy collection based on the theory. In addition, the team collaborated with Huazhong University of Science and Technology and discovered that inexpensive nanostructured materials such as carbon black can constantly generate volt-level electrical energy through the process of water evaporation in the atmosphere (Nature Nanotech. 12, 317, 2017). The biggest advantage of that is that it does not require any mechanical input. Under the desired environment, a centimeter-sized carbon black sheet can stably output volt-level voltage. The electrical energy generated by a few square centimeters of the film is able to directly drive a liquid crystal display.

Similar to photovoltaics and piezoelectrics effect, the team referred to this phenomenon of direct transformation of water energy into electricity through the interaction of water and material as “Hydrovolcanic effect”. This finding makes obtaining electrical energy from the water cycle possible. Researches in this area are receiving more and more international attention.

The currently implemented Hydrovoltaic device is expected to be compatible with intelligent clean energy supply system. Theoretically, Hydrovoltaic technology enjoys bigger potential than photovoltaic technology water system because of the unaffected water evaporation. Wind energy, solar energy, and waste heat can be co-exploited to increase electricity generation. The Hydrovoltaic system is innately compatible with the fluid and biological environment and can capture energy from every step of the earth water circulation, converting low-quality energy such as latent heat into high-quality electric energy. Besides generating electricity, this system has the merits of producing clean water and regulating air temperature. Fluid information such as flow orientation, flow rate, and ion concentration can be measured through drawing potential. The Hydrovolcanic device combined with a micro-nano sensor device can form a self-powered sensing system. Most of the existing electronic devices work at the surface of solid structures or solid and solid interfaces. However, the Hydrovoltaic system works on the solid-liquid or solid-liquid-gas interface, which is different from traditional electronic devices, thus enjoying the unique prospect.

Although some progress has been made on the Hydrovoltaic effect, the reported electricity generation so far is not enough for the daily electricity consumption. There’s a long way on making this industrialized. We expect that this review article will attract more researchers to join the field to promote the rapid development of Hydrovoltaic technology.

The study of Hydrovoltaic effect can further inspire people to better understand brains, helping to develop brain-like artificial intelligence technology. The nerve signals are primary electrical signals. Traditional neuroscience theory believes that the transmission of these electrical signals with the ion acting as the main carrier between neurons is the basis of human intelligence. However, more and more researches suggest that there might be more mechanisms with respect to the generation and transmission of electrical signals in biological pathways. Unlike computers, which rely on transistors, brain operations rely mainly on liquids and soft materials. However, up to now, we understand less of the microscopic Hydrovoltaic effect of liquids than solids. The in-depth researches depend greatly on breakthroughs in basic theory.

The news was first surfaced at NUAA website. The complete research can be read at Nature Nanotechnology‘s December issue.