Researchers at the Laboratory of Nanoscience for Energy Technology (LNET) at the Swiss Federal Institute of Technology Lausanne (EPFL) have engineered a nanodevice that uses light and heat to generate a stable current by evaporating saltwater.
The research is unique because the heat and light are used not only to facilitate evaporation but also to control the flow of ions in saltwater and the flow of electrons in the nanodevice.
Hydrovoltaic (HV) technology is an emerging technology that generates electricity by leveraging the interaction of water with functional materials. Unlike conventional methods of using water as a fluid to drive turbines, HV effect uses processes such as evaporation or droplet flow to generate continuous electricity.
In 2024, researchers at LNET reported building a nanoscale HV device using silicon nanopillars, in which fluid samples evaporated in the channels created by the spaces between them. Two years on, the device can now produce a power output exceeding that of similar technologies, and it can also control the flow of ions and electrons within it.
Roles of light and heat
The effect of heat on evaporation is well known. We understand that supplying heat to water speeds up evaporation. Scientists working on the HV effect have also used this information to speed up electricity generation. However, a research team led by Giulia Tagliabue, an associate professor at the LNET, recognized the role of light in the process as well.
The nanodevice used by the researchers is made from silicon. A semiconductor by nature, the device’s electrons are excited by photons from a light source, while heat enhances its surface charge to a negative value.
The device also has three separate layers – one each for evaporation, ion transport, and electrical charge collection. Heat-driven evaporation in the saltwater layer also causes ions to shift, creating a separation of positive and negative charges. This charge separation at the solid-liquid interface creates an electric field, producing electricity in the circuit.
Schematic of the hydrovoltaic device with a top evaporating electrode surface and a bottom array of silicon nanopillars immersed in water. Image credit: 2026 LNET EPFL CC BY SA
Impact on electricity generation
“Our work shows that due to this surface charge effect, the addition of solar light and heat can enhance energy production by a factor of 5,” said Tagliabue in a press release. “This natural effect has always existed, but we are the first to harness it.”
The improved output obtained by harnessing this effect is evident in the device’s excellent 1 V voltage and 0.25 W/m2 power density. The trilayer device design also helped the researchers develop a model for their observations and further enhance power output by tweaking salt concentrations and nanopillar structures.
The team also addressed a major disadvantage that HV devices often face with saltwater. Attempts to improve device performance through heat and light have often led to material degradation.
To overcome this, the researchers used an oxide coating on their nanopillars to protect against unwanted chemical reactions with saltwater while maintaining stable performance.
The researchers are hopeful that their work will help advance hydrovoltaic devices that can power battery-free devices using water, heat, and sunlight. This could help build devices ranging from wearables to environmental sensors and beyond.
The research findings were published in the journal Nature Communications.