New technique produces electricity with nothing but saltwater and freshwater

Technology designed to extract energy from the mixing of fresh and saltwater — based on the difference in salt concentrations between the two water sources — has been around for a while. This is an attempt to derive power from the “osmotic pressure” that builds up when fresh and saltwater mix and nature tries to balance out the salinity gradient.

However, the three main methods for carrying out this energy extraction — including pressure retarded osmosis (PRO), reverse electrodialysis (RED) and capacitive mixing (CapMix) — all come with their own drawbacks. These range from clogged membranes that no longer allow water to move through them (in the case of PRO) to an inability to produce sufficiently large amounts of power (in the case of RED and CapMIX).

The Penn State researchers have developed a new method combining the RED and CapMix methods to create an electrochemical flow cell in which two channels are separated by a membrane. A copper hexacyanoferrate electrode is then placed in each channel, with graphite foil used as a current collector. In their study, one channel was then fed with synthetic seawater, and the other with synthetic freshwater. Switching the water’s flow paths then charges the cell to produce an unprecedented amount of electricity, compared with previous RED and CapMIX solutions.

“There are two things going on here that make it work,” Christopher Gorski, assistant professor in environmental engineering at Penn State, said in a statement. “The first is you have the salt going to the electrodes. The second is you have the chloride transferring across the membrane. Since both of these processes generate a voltage, you end up developing a combined voltage at the electrodes and across the membrane.”

Going forward, scalability will need to be tested, and one imagines that there may be a few hurdles in the form of potential interference with river traffic. But it’s definitely exciting news — and we’ll be eager to see how this develops.