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Seawater as a Renewable Energy Source

Renewable energy may now be harnessed from saltwater by means of a nanopore membrane, courtesy of researchers from Osaka University.

As natural resources are depleted and become increasingly scarce, developing new renewable sources of energy remains an important concern for scientists. In their search for new renewable sources of energy, researchers at the Institute of Scientific and Industrial Research (SANKEN) at Osaka University have developed a method to generate electricity from highly-concentrated salt solutions, like seawater.

Most people have heard of osmosis in science lessons, and may even know that it refers to the process in which aqueous ions or molecules pass through a selectively permeable membrane from a solution of higher water potential to a solution of lower water potential. The researchers successfully made use of this concept to generate electricity, by harnessing the osmotic potential of saltwater with specialised selectively permeable membranes that only allow ions to cross.

“Whenever there is a non-equilibrium situation, such as two water tanks with different salt concentrations, there is often an opportunity to covert this thermodynamic energy into electricity,” says first author Makusu Tsutsui.

The team, led by Osaka University professors, utilised semiconductor processing technology to develop a membrane precisely fabricated to have nanopores of a certain size and structure, arranged in a particular manner. As the researchers used pre-existing, decades-old silicon wafer fabrication technologies to produce the membranes from extremely thin pieces of silicon, the costs associated with the production of the entire membrane were kept low.

The key feature of the silicon wafer fabrication technology that allowed for the production of such a membrane was its precision in controlling the size and the location of pores for optimum electricity generation from saltwater.

The researchers found that using a fabricated membrane with only one 20-nm-sized nanopore allowed the device to reach its highest power efficiency of 400 kW/m2. Surprisingly, the researchers found that increasing the number of nanopores on the membrane did not result in increased power efficiency, but instead decreased it. They also discovered that the ideal configuration of the pores was that of 100-nm-sized pores arranged in a grid, 1 micrometre apart from each other.

The researchers believe that they are in the right direction to designing effective nanopore devices for power generation from ionic solutions. “Many other research groups are promising environmentally friendly ‘green’ energy, but we go one step further and propose ‘blue’ energy based on oceanwater that can be applied on an industrial scale,” senior author Tomoji Kawai says. In the future, his team also hopes to be able to increase the scale of their devices to test their real-life applications. [APBN]

Source: Tsutsui et al. (2022). Sparse multi-nanopore osmotic power generators. Cell Reports Physical Science, 101065.