A Salty Energy Source

The difference in salinity between ocean and river water can be tapped as a renewable energy source by a battery that stores the energy electrochemically, according to work led by Stanford University materials scientists (Nano Lett., DOI: 10.1021/nl200500s). Exploiting the enormous quantity of entropic energy dissipated as freshwater and saltwater mix in estuaries is a decades-old idea and technical challenge. Most of the strategies for capturing that energy of mixing, such as a recently reported sandwich device based on porous carbon electrodes and ion-exchange membranes, rely on membranes to prevent freshwater and saltwater from mixing directly. As such, developing those technologies depends heavily on the cost, lifetime, and ion-transport efficiency of the membranes. In contrast, the battery demonstrated by Stanford’s Fabio La Mantia, Mauro Pasta, Yi Cui, and coworkers features an electrode made from sodium manganese oxide nanorods that selectively incorporates and releases sodium ions and a silver electrode that interacts selectively with chloride ions. By alternately submerging the electrodes in freshwater and saltwater, the team charged and discharged the battery, measuring an energy extraction efficiency of up to 74%.