Hydride ion-conducting solid electrolyte works at room temperature

Led by Genki Kobayashi at the RIKEN Cluster for Pioneering Research in Japan, the breakthrough is expected to advance efforts towards a hydrogen economy with the added benefits of improved safety, efficiency, and energy density. The study has been published in Advanced Energy Materials.

Hydrogen-based fuel cells used in electric cars work by allowing hydrogen protons to pass from one end of the fuel cell to the other through a polymer membrane. Efficient, high-speed hydrogen movement in these fuel cells requires water, so the membrane must be continually hydrated. According to Riken, this constraint adds an additional layer of complexity and cost to battery and fuel cell design that limits the practicality of a next-generation hydrogen-based energy economy.

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“We have achieved a true milestone,” Kobayashi said in a statement. “Our result is the first demonstration of a hydride ion-conducting solid electrolyte at room temperature.”

The team had been experimenting with lanthanum hydrides (LaH_3-δ) as the hydrogen can be released and captured relatively easily, hydride ion conduction is very high, they can work below 100°C, and have a crystal structure.

At room temperature, the number of hydrogen atoms attached to lanthanum fluctuates between two and three, making it impossible to have efficient conduction. This problem - hydrogen non-stoichiometry - was the biggest obstacle overcome in the new study. The researchers achieved their objective by replacing some of the lanthanum with strontium (Sr) and added a small quantity of oxygen for a basic formula of La_1-xSr_xH_3-x-2yO_y.

The team prepared crystalline samples of the material using ball-milling, followed by annealing. They studied the samples at room temperature and found that they could conduct hydride ions at a high rate. Then, they tested its performance in a solid-state fuel cell made from the new material and titanium, varying the amounts of strontium and oxygen in the formula. They observed complete 100  per cent conversion of titanium to titanium hydride with almost no hydride ions wasted.

“In the short-term, our results provide material design guidelines for hydride ion-conducting solid electrolytes,” said Kobayashi. “In the long-term, we believe this is an inflection point in the development of batteries, fuel cells, and electrolytic cells that operate by using hydrogen.” The next step will be to improve performance and create electrode materials that can reversibly absorb and release hydrogen.