Stanford Team Creates Water-Based Battery to Store Wind and Solar Energy

Researchers at Stanford have built a water-based battery that is capable of providing an inexpensive way to store solar or wind energy produced when the wind is blowing and the sun is shining so it can be fed back into the electric grid and be redistributed when there is a high demand.

The prototype manganese-hydrogen battery measures just three inches in height and produces a mere 20 milliwatt-hours of electricity, which is equal to the energy levels of LED flashlights that come in a key ring. In spite of the prototype’s miniscule output, the researchers are self-assured they can expand this table-top technology into an industrial-grade system that could charge and recharge up to 10,000 times, forming a grid-scale battery with a worthwhile lifespan extending over a decade. Details of their efforts can be found in the April 30 issue of the Nature Energy.cui

Yi Cui, a professor of materials science at Stanford and the paper’s senior author, said manganese-hydrogen battery technology could be one of the missing pieces in the country’s energy puzzle—a way to store irregular solar or wind energy so as to cut the need to burn reliable but carbon-emitting fossil fuels when the renewable sources are not 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.

Yi Cui, Professor of Materials Science, Stanford

Clever chemistry

The Stanford team that came up with the concept and assembled the prototype was led by Wei Chen, a postdoctoral scholar in Cui’s lab. Essentially, the scientists coaxed a reversible electron-exchange between water and manganese sulfate, an inexpensive, plentiful industrial salt used to create fertilizers, dry cell batteries, paper, and other products.

To imitate how a solar or wind source might feed power into the battery; the team linked a power source to the prototype. The electrons flowing in reacted with the manganese sulfate mixed in the water to leave particles of manganese dioxide stuck to the electrodes. Surplus electrons bubbled off as hydrogen gas, thereby storing that energy for later use. Engineers are aware of how to re-create electricity from the energy stored in hydrogen gas so the vital follow-up step was to demonstrate that the water-based battery can be recharged.

The researchers achieved this by re-attaching their power source to the depleted prototype, this time with the aim of stimulating the manganese dioxide particles stuck to the electrode to mix with water, restocking the manganese sulfate salt. Once this salt was restocked, incoming electrons became excess, and additional power could bubble off as hydrogen gas, in a process that can be repeated over and over again.

Cui projected that, given the water-based battery’s anticipated lifespan, it would cost a penny to store sufficient electricity to power a 100-watt light bulb for twelve hours.

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

Yi Cui, Professor of Materials Science, Stanford

The Department of Energy (DOE) has suggested batteries for grid-scale storage should store and then discharge no less than 20 kW of power over a one hour period, be capable of a minimum of 5,000 recharges, and have a worthwhile lifespan of a decade or more. To make it useful, such a battery system should not cost more than $2,000, or $100/kW hour.

Former DOE secretary and Nobel laureate Steven Chu, currently a professor at Stanford, has a long-enduring interest in boosting technologies to help the country shift 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.

Shifting away from carbon

According to DOE estimates, around 70% of U.S. electricity is produced by natural gas or coal plants, which make up 40% of carbon dioxide emissions. Changing to solar and wind generation is one approach to curtail those emissions. But that brings on new challenges involving the inconsistency of the power supply. Most obviously, the sun only shines by day and, occasionally, the wind does not blow.

But another not properly understood but significant form of inconsistency comes from surges of demand on the grid—that system of high-tension wires that distribute electricity across regions and finally to homes. On a hot day, when people return home from the office, they tend to crank up the air conditioning, utilities must have load-balancing plans to match the peak demand: some way to increase power generation within minutes so as to prevent blackouts or brownouts that might otherwise collapse the grid.

Today, utilities frequently achieve this by turning on on-demand or “dispatchable” power plants that may lie unused most of the day but can come online in a matter of minutes—generating rapid energy but increasing carbon emissions. Certain utilities have established short-term load balancing that does not depend on fossil-fuel burning plants. The most widely used and cost-effective strategy is pumped hydroelectric storage: using surplus power to convey water uphill, then allowing it flow back down to produce energy during peak demand. However, hydroelectric storage only works in regions with sufficient space and water. So to make solar and wind more beneficial, DOE has encouraged high-capacity batteries as a substitute.

High capacity, low cost

Cui said there are a number of types of commercial rechargeable battery technologies, but it isn’t clear which methods will match the DOE requirements and demonstrate their practicality to the utilities, regulators, and other stakeholders who maintain the country’s electrical grid.

For example, Cui said rechargeable lithium-ion batteries, which store the small volumes of energy needed to operate laptops and phones, are based on uncommon materials and are therefore too costly to store power for a city or a neighborhood. Cui said grid-scale storage requires an inexpensive, high-capacity, rechargeable battery. The manganese-hydrogen process showed promise.

Other rechargeable battery technologies are easily more than five times of that cost over the life time.

Yi Cui, Professor of Materials Science, Stanford

Chen said innovative chemistry, inexpensive materials, and relative simplicity made the manganese-hydrogen battery perfect for cost-effective grid-scale deployment.

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

The prototype requires development work to substantiate itself. For one thing, it uses platinum as a catalyst to trigger the vital chemical reactions at the electrode that render the recharge process efficient, and the cost of that component would be exorbitant for large-scale deployment. But Chen said the team is already looking at cheaper ways to encourage 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 Stanford researchers reported performing 10,000 recharges of the prototypes, which is twice the DOE requirements, but said it will be required to test the manganese-hydrogen battery under real electric grid storage conditions so as to actually evaluate its lifetime cost and performance.

Cui said he has applied for a patent for the process via the Stanford Office of Technology Licensing and plans to start a company to market the system.

Yi Cui is also a professor in the Photon Science Directorate at SLAC National Accelerator Laboratory, a senior fellow of the Precourt Institute for Energy, and a member of Stanford Bio-X and the Stanford Neurosciences Institute. Additional co-authors include Guodong Li, a visiting scholar in materials science and engineering who is currently with the Chinese Academy of Sciences; postdoctoral scholars Hongxia Wang, Jiayu Wan, Lei Liao, Guangxu Chen and Jiangyan Wang; visiting scholar Hao Zhang; and graduate students Zheng Liang, Yuzhang Li and Allen Pei.

This research received funding from the Department of Energy.