EPFL researchers have discovered that nanoscale devices using the hydroelectric effect can harvest electricity from the evaporation of liquids with higher ion concentrations than purified water, unlocking a large untapped energy potential.

Evaporation is such a common natural process that most of us take it for granted. In fact, about half of the solar energy reaching Earth results in evaporation processes. Since 2017, researchers have been working to harness the energy potential of evaporation through the hydrovoltaic (HV) effect; This allows electricity to be collected when passing a liquid across the charged surface of a nanoscale device. Evaporation creates a continuous flow inside the nanochannels inside these devices, which act as passive pumping mechanisms. This effect is also observed in plant microcapillaries, where water transport occurs due to the combination of capillary pressure and natural evaporation.

Although hydroelectric devices currently exist, there is little operational understanding of the conditions and physical phenomena that govern the production of high voltage energy at the nanoscale. Julia Tagliabue, director of the Energy Technologies Nanoscience Laboratory (LNET) in the Faculty of Engineering, and PhD student Tarık Anwar wanted to fill this knowledge gap. They used a combination of experiments and multiphysics simulations to characterize liquid flows, ion flows, and electrostatic effects resulting from solid-liquid interactions to optimize high-voltage devices.

“Thanks to our new, highly controlled platform, this is the first study to quantify these hydroelectric phenomena, highlighting the importance of various interfacial interactions. But we also made an important finding in the process: hydroelectric devices can operate across a wide range of salinity levels, allowing for optimal performance.” It contradicts the previous understanding that highly purified water is needed for water,” Tagliabue says.

The LNET study was recently published in Cell Press. Device.

An explanatory multiphysics model

The researchers’ device represents the first hydroelectric application of a technique called nanosphere colloidal lithography, which allows them to create a hexagonal network of precisely spaced silicon nanopillars. The gaps between the nanocolumns formed ideal channels for the evaporation of liquid samples and could be fine-tuned to better understand the effects of liquid retention and the solid-liquid interface.

“Most liquid systems containing salt solutions have equal numbers of positive and negative ions. But when you confine the liquid to a nanochannel, only ions with opposite polarities of surface charge will remain,” Anwar explains. “This means that if you allow a liquid to flow through the nanochannel, you will produce a current and voltage.”

“This leads to our main conclusion that the chemical balance of the nanodevice surface charge can be used to enhance the operation of hydroelectric devices at the salinity scale,” Tagliabue adds. “In fact, as the ion concentration of the liquid increases, the surface charge of the nanodevice increases. As a result, we can use larger fluid channels when working with higher concentration liquids. This makes it easier to build devices that will be used not only with purified water, but also with tap or sea water.”

Water, water everywhere

Since evaporation can occur continuously over a wide range of temperature and humidity ranges, and even at night, there are many exciting potential applications for more efficient high voltage devices. The researchers hope to explore this potential with the support of a seed grant from the Swiss National Science Foundation, which aims to develop “a completely new paradigm for waste heat utilization and renewable energy production at large and small scales.” real world conditions. -Earth conditions in Lake Geneva.

Since HV devices can theoretically be used anywhere there is liquid or even moisture, such as sweat, they can also be used to power sensors in connected devices, from smart TVs to health and fitness wearables. Thanks to LNET’s expertise in light energy harvesting and storage systems, Tagliabue also wants to see how light and photothermal effects can be used to control surface charges and evaporation rates in HV systems.

Finally, researchers also see significant synergies between HV systems and clean water production.

“Natural evaporation is used to control desalination processes because fresh water can be collected from salt water by condensing the steam produced by the evaporation surface. You can now imagine using a high voltage system to produce clean water and use electricity at the same time,” explains Anwar.