Osmotic energy, often referred to as blue energy, is an emerging method for producing renewable electricity by harnessing the natural mixing of saltwater and freshwater. When these two types of water meet, ions from the saltwater move through a specialized ion-selective membrane toward the lower-salinity water. This movement generates a voltage that can be captured as electricity.
Despite its potential, the technology has faced significant obstacles. Membranes designed to allow ions to pass through quickly often lose the ability to separate charges effectively. In addition, maintaining structural durability has proven difficult. Because of these limitations, most osmotic energy systems have remained largely confined to laboratory experiments.
Lipid-Coated Nanopores Improve Ion Flow
Scientists from the Laboratory for Nanoscale Biology (LBEN), led by Aleksandra Radenovic in EPFL’s School of Engineering, together with researchers at the Interdisciplinary Centre for Electron Microscopy (CIME), have now demonstrated a solution to these problems. Their findings were published in Nature Energy.
The team improved ion movement by coating nanopores with tiny lipid bubbles known as liposomes (liposomes). Under normal conditions, these nanopores allow ions to pass through with high precision but at a very slow rate. When coated with the lipid layer, however, the nanopores allow selected ions to move through far more easily. The reduced friction significantly increases ion transport and boosts the system’s overall performance.
“Our work brings together the strengths of two main approaches to osmotic energy harvesting: polymer membranes, which inspire our high-porosity architecture; and nanofluidic devices, which we use to define highly charged nanopores,” says Radenovic. “By combining a scalable membrane layout with precisely engineered nanofluidic channels, we achieve highly efficient osmotic energy conversion and open a route toward nanofluidic-based blue-energy systems.”
Hydration Lubrication Inside Nanopores
The lubricating coating used in the study is based on lipid bilayers, structures commonly found in the membranes of living cells. These bilayers naturally assemble when two layers of fat molecules align with their water-repelling (hydrophobic) tails facing inward and their water-attracting (hydrophilic) heads facing outward.
When applied to the stalactite-shaped nanopores embedded in a silicon-nitride membrane, the outward-facing hydrophilic heads attract an extremely thin layer of water. This water layer is only a few molecules thick, yet it clings to the nanopore surface and prevents ions from directly interacting with it. As a result, friction is reduced and ions can pass through the pore more smoothly.
Higher Power Output From Blue Energy
To test the design, the researchers produced a membrane containing 1,000 lipid-coated nanopores arranged in hexagonal pattern. They then evaluated the device under conditions that mimic the natural salt concentrations found where seawater and river water meet.
The system achieved a power density of about 15 watts per square meter. This output is roughly 2-3 times higher than what current polymer membrane technologies can produce.
A Step Toward Practical Blue Energy Systems
Previous computer simulations had suggested that improving both ion flow and selectivity in nanofluidic channels could dramatically enhance osmotic energy generation. However, experiments demonstrating both improvements at the same time have been rare.
“By showing how precise control over nanopore geometry and surface properties can fundamentally reshape ion transport, our study moves blue-energy research beyond performance testing and into a true design era,” says LBEN researcher Tzu-Heng Chen.
First author Yunfei Teng notes that the team’s “hydration lubrication” strategy may have applications beyond osmotic energy systems. “The enhanced transport behavior we observe, driven by hydration lubrication, is universal, and the same principle can be extended beyond blue-energy devices,” he says.
Advanced Imaging and Research Facilities
The project also relied on detailed analysis of nanopore structure and chemical composition. This work was carried out by Dr. Victor Boureau at EPFL’s Interdisciplinary Centre for Electron Microscopy (CIME). Additional support came from EPFL’s shared research facilities for nanofabrication, materials characterization, and high-performance computing, including CMi, MHMC, and SCITAS.