Osmotic energy, also known as blue energy, is a new way to harness the natural mix of seawater and freshwater to generate renewable electricity. When these two types of water meet, ions from the salt water migrate toward the less salty water through a special ion-selective membrane. This movement generates voltage, which can be interpreted as electricity.
Despite its potential, this technology faces significant obstacles. Membranes designed to allow ions to pass quickly through them often lose their ability to effectively separate charges. Furthermore, maintaining the durability of the structure has proven difficult. Because of these limitations, most osmotic energy systems have been primarily limited to laboratory experiments.
Lipid-coated nanopores improve ion flow
Scientists from the Laboratory of Nanoscale Biology (LBEN), led by Aleksandra Radenovic of the EPFL Faculty of Engineering, collaborated with researchers from the Center for Interdisciplinary Electron Microscopy (CIME) to demonstrate solutions to these problems. Their discovery is natural energy.
The researchers improved ion movement by coating the nanopores with tiny lipid bubbles known as liposomes. Under normal conditions, these nanopores allow ions to pass through with high precision but at very low speeds. However, when coated with a lipid layer, the nanopores allow selected ions to pass through much more easily. Reduced friction significantly increases ion transport and improves overall system performance.
“Our work combines the strengths of two major approaches to osmotic energy harvesting: the polymer membranes that are the inspiration for our highly porous structures, and the nanofluidic devices we use to define highly charged nanopores,” said Radenovic. “By combining a scalable membrane layout with precisely designed nanofluidic channels, we achieve highly efficient osmotic energy conversion, paving the way for nanofluidic-based blue energy systems.”
Hydration lubrication inside nanopores
The lubricious coating used in this study is based on lipid bilayers, a structure commonly found in the membranes of living cells. These bilayers naturally assemble when the two layers of fat molecules align with the water-repelling (hydrophobic) tails facing inward and the water-attracting (hydrophilic) heads facing outward.
When applied to stalactite-like nanopores embedded in silicon nitride membranes, the outwardly facing hydrophilic heads attract a very thin layer of water. This layer of water, only a few molecules thick, clings to the surface of the nanopore and prevents ions from directly interacting with it. As a result, friction is reduced and ions can pass through the pores more smoothly.
Higher output from Blue Energy
To test this design, the researchers created a membrane containing 1,000 lipid-coated nanopores arranged in a hexagonal pattern. They then evaluated the device under conditions that mimic the natural salinity found where seawater and river water meet.
The system achieved a power density of approximately 15 watts per square meter. This production rate is approximately two to three times higher than current polymer membrane technology can produce.
A step towards a practical blue energy system
Previous computer simulations suggested that improving both ion flow and selectivity in nanofluidic channels could dramatically enhance osmotic energy production. However, experiments demonstrating both improvements simultaneously are rare.
“By showing that ion transport can be fundamentally restructured through precise control of nanopore shape and surface properties, our work advances blue energy research beyond performance testing and into the era of true design,” said LBEN researcher Tzu-Heng Chen.
Lead author Yunfei Teng notes that the team’s “hydration lubrication” strategy has potential applications beyond osmotic energy systems. “The enhancement of transport behavior promoted by hydration lubrication that we observed is universal, and the same principle can be extended beyond blue energy devices,” he says.
Advanced imaging and research facilities
This project also relied on detailed analysis of the structure and chemical composition of the nanopores. The research was carried out by Dr. Victor Boureau from EPFL’s Interdisciplinary Center for Electron Microscopy (CIME). Additional support was provided by EPFL’s shared research facilities for nanofabrication, materials characterization, and high-performance computing, including CMi, MHMC, and SCITAS.
