Researchers develop water-based battery

Stanford University has reported that its scientists have developed a water-based manganese-hydrogen battery that could meet future grid energy storage requirements.

The prototype manganese-hydrogen battery, reported on 30 April in Nature Energy, stands just 7.6 cm tall and generates a mere 20 mWh of electricity, which is on par with the energy levels of LED flashlights that hang on a key ring. Despite the prototype’s small output, the researchers are confident they can scale up this tabletop technology to an industrial-grade system that could charge and recharge up to 10,000 times, creating a grid-scale battery with a useful lifespan well in excess of a decade.

Yi Cui, a professor of materials science at Stanford and senior author on the paper, said manganese-hydrogen battery technology could be a way to store unpredictable wind or solar energy so as to lessen the need to burn carbon-emitting fossil fuels when the renewable sources aren’t available.

“What we’ve done is thrown a special salt into water, dropped in an electrode and created a reversible chemical reaction that stores electrons in the form of hydrogen gas,” said Cui.

The team that dreamed up the concept and built the prototype was led by Wei Chen, a postdoctoral scholar in Cui’s lab. In essence, the researchers coaxed a reversible electron-exchange between water and manganese sulfate, a cheap, abundant industrial salt used to make dry cell batteries, fertilisers, paper and other products.

When the researchers attach a power source to the prototype, the electrons flowing in react with the manganese sulfate dissolved in the water to leave particles of manganese dioxide clinging to the electrodes. Excess electrons bubbled off as hydrogen gas, thus storing that energy for future use. Engineers know how to re-create electricity from the energy stored in hydrogen gas so the important next step was to prove that the water-based battery can be recharged.

The researchers did this by reattaching their power source to the depleted prototype, this time with the goal of inducing the manganese dioxide particles clinging to the electrode to combine with water, replenishing the manganese sulfate salt. Once this salt was restored, incoming electrons became surplus, and excess power could bubble off as hydrogen gas, in a process that can be repeated.

Cui estimated that, given the water-based battery’s expected lifespan, it would cost about 1 cent to store enough electricity to power a 100 W lightbulb for 12 hours.

“We believe this prototype technology will be able to meet Department of Energy goals for utility-scale electrical storage practicality,” Cui said.

The US Department of Energy (DOE) has recommended batteries for grid-scale storage should store and then discharge at least 20 kW of power over a period of an hour, be capable of at least 5000 recharges and have a useful lifespan of 10 years or more. To make it practical, such a battery system should cost $2000 or less, or $100 per kWh.

Former DOE secretary and Nobel laureate Steven Chu, now a professor at Stanford, has a longstanding interest in encouraging technologies to help the nation transition to renewable energy.

“While the precise materials and design still need development, this prototype demonstrates the type of science and engineering that suggest new ways to achieve low-cost, long-lasting, utility-scale batteries,” said Chu, who was not a member of the research team.

Cui said there are several types of rechargeable battery technologies on the market, but it isn’t clear which approaches will meet DOE requirements and prove their practicality to the utilities, regulators and other stakeholders who maintain the nation’s electrical grid.

For instance, Cui said rechargeable lithium-ion batteries, which store the small amounts of energy needed to run phones and laptops, are based on rare materials and are thus too pricey to store power for a neighbourhood or city. Cui said grid-scale storage requires a low-cost, high-capacity, rechargeable battery. The manganese-hydrogen process seems promising.

“Other rechargeable battery technologies are easily more than five times of that cost over the lifetime,” Cui added.

Chen said novel chemistry, low-cost materials and relative simplicity made the manganese-hydrogen battery ideal for low-cost grid-scale deployment.

“The breakthrough we report in Nature Energy has the potential to meet DOE’s grid-scale criteria,” Chen said.

The prototype needs development work to prove itself. For one thing it uses platinum as a catalyst to spur the crucial chemical reactions at the electrode that make the recharge process efficient, and the cost of that component would be prohibitive for large-scale deployment. But Chen said the team is already working on cheaper ways to coax the manganese sulfate and water to perform the reversible electron exchange. “We have identified catalysts that could bring us below the $100-per-kilowatt-hour DOE target,” he said.

The researchers reported doing 10,000 recharges of the prototypes, which is twice the DOE requirements, but said it will be necessary to test the manganese-hydrogen battery under actual electric grid storage conditions in order to truly assess its lifetime performance and cost. Cui said he has sought to patent the process through the Stanford Office of Technology Licensing and plans to form a company to commercialise the system.

Image: Postdoctoral scholar Wei Chen holds a prototype of a water-based battery developed in the lab of Stanford materials scientist Yi Cui. (Image credit: Jinwei Xu)