Solar cells merged with a liquid battery

US researchers have been seeking a solution to a fundamental limitation of solar cells: the dark of the night.

While lithium-ion batteries are these days being used in everything from hybrid vehicles to laptop computers, they are too expensive a solution to use on something as massive as the electric grid. Professor Song Jin, from the University of Wisconsin–Madison, had a better idea: integrating the solar cell with a large-capacity battery.

Along with colleagues from UW–Madison and the King Abdullah University of Science and Technology in Saudi Arabia, Jin made a single device that eliminates the usual intermediate step of making electricity and instead transfers the solar energy directly to the battery’s electrolyte. He chose a ‘redox flow battery’ (RFB), which stores chemical energy in a tank of liquid electrolyte.

“The RFB is relatively cheap and you can build a device with as much storage as you need, which is why it is the most promising approach for grid-level electricity storage,” said Jin.

In the new device, standard silicon solar cells are mounted on the reaction chamber and energy converted by the cell immediately charges the water-based electrolyte, which is pumped out to a storage tank. According to Jin, discharging the battery to power the electric grid at night could hardly be simpler: “We just connect a load to a different set of electrodes, pass the charged electrolyte through the device and the electricity flows out.”

This solar-charged device directly transfers energy from sunlight into a liquid battery and stores it in the container at lower right. During the discharge cycle, electricity leaves the device through electrodes at the top. Image credit: David Tenenbaum, University Communications/UW–Madison.

Wenjie Li, a UW–Madison graduate student and first author on the study, explained that the team’s device “harvests sunlight to liberate electrical charges and directly changes the oxidation-reduction state of the electrolyte on the surface of the cells”.

“It’s essentially a solar battery, and we can size the RFB storage tank to store all the energy generated by the solar cells,” he said.

According to Jin, having the solar cells directly charge the electrolyte “makes for simplicity, cost reduction and potentially higher efficiency”. Solar charging and electrical discharging can be repeated for many cycles, he said, with little efficiency loss.

With their concept now proven and published in the journal Angewandte Chemie International Edition, the researchers are now working on improvements. One improvement would be to match the solar cell’s voltage to the chemistry of the electrolyte, minimising losses as energy is converted and stored. Another would be searching for electrolytes with larger voltage differential, which currently limits energy storage capacity.

“It’s not just about the efficiency of converting sunlight into electricity, but also about how much energy you can efficiently store in the device,” said Jin.

Top image: This solar-charged battery directly stores energy from sunlight in a tank. Image credit: David Tenenbaum, University Communications/UW–Madison.