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This salt battery harvests osmotic energy

This salt battery harvests osmotic energy where the river meets the sea

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An improved membrane (yellow line) dramatically increased the amount of osmotic energy harvested from salinity gradients, such as those found in estuaries where salt water (left tank) meets fresh water (right tank).

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Credit: Adapted from ACS Energy Letters 2024, DOI: 10.1021/acsenergylett.4c00320

Estuaries – where freshwater rivers meet the salty sea – are great locations for birdwatching and kayaking. In these areas, waters with different salt concentrations mix and can be sources of sustainable, ‘blue’ osmotic energy. Researchers inside ACS Energy Letters report the creation of a semipermeable membrane that collects osmotic energy from salt gradients and converts it into electricity. The new design had an output power density more than twice that of commercial membranes in laboratory demonstrations.

Osmotic energy can be generated wherever salinity gradients are found, but the available technologies to capture this renewable energy have room for improvement. One method uses a series of reverse electrodialysis (RED) membranes that act as a kind of ‘salt battery’, generating electricity from pressure differences caused by the salt gradient. To even out that gradient, positively charged ions from seawater, such as sodium, flow through the system into the fresh water, increasing pressure on the membrane. To further increase harvesting power, the membrane must also maintain a low internal electrical resistance by allowing electrons to flow easily in the opposite direction to the ions. Previous research suggests that improving both ion flow through the RED membrane and electron transport efficiency would likely increase the amount of electricity captured from osmotic energy. So Dongdong Ye, Xingzhen Qin and colleagues designed a semi-permeable membrane from environmentally friendly materials that would theoretically minimize internal resistance and maximize output power.

The researchers’ RED membrane prototype contained separate (i.e., decoupled) channels for ion transport and electron transport. They created this by sandwiching a negatively charged cellulose hydrogel (for ion transport) between layers of an organic, electrically conductive polymer called polyaniline (for electron transport). Initial tests confirmed their theory that decoupled transport channels resulted in higher ionic conductivity and lower resistance compared to homogeneous membranes made of the same materials. In a water tank simulating an estuary environment, their prototype achieved an output power density 2.34 times higher than that of a commercial RED membrane and maintained performance for 16 days of non-stop operation, demonstrating long-lasting, stable underwater performance. In a final test, the team created an array of salt batteries from 20 of their RED membranes and generated enough electricity to separately power a calculator, LED light and stopwatch.

Ye, Qin and their team members say their findings expand the range of ecological materials that can be used to create RED membranes and improve osmotic energy harvesting performance, making these systems more feasible for real-world use.

The authors acknowledge funding from the National Natural Science Foundation of China.

The abstract of the article will be available on April 24 at 8:00 am Eastern Time here: https://doi.org/10.1021/acsenergylett.4c00320

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