Prior efforts to harvest energy from the salinity differential between saltwater and freshwater have focused primarily on a process known as pressure-retarded osmosis. In this approach, freshwater and saltwater are housed in separate chambers, which are divided by an artificial membrane. The higher salinity of the saltwater draws freshwater through the membrane, increasing the pressure on the saltwater side. The pressurized water is then used to drive a turbine and generate electricity.

Norwegian electric company Statkraft is currently testing pressure-retarded osmosis at a pilot plant outside Oslo and also working to develop more efficient and durable membranes. Statkraft officials say their goal is to convert 80 percent of the available chemical energy to electricity. Cui says he doubts that the approach will be able to exceed an efficiency of 40 percent. "Efficiency-wise we are certainly much better," he says.

To achieve high efficiency, Cui's group used manganese-dioxide nanorods for its battery's positive electrode. The material gives the sodium ions roughly 100 times more surface area to interact with than conventional electrode materials do. And the nanostructure allows the ions to quickly attach and detach from the electrode, making the entire battery more efficient.

Cui's team used a silver electrode to bond with the negatively charged chlorine ions. Silver, however, is prohibitively expensive for large-scale deployments, and it's also toxic, capable of causing environmental harm if it dissolves into the water being cycled through the battery. Cui says his group is looking for a substitute, but an alternative may be hard to find.