New material allows for better hydrogen-based batteries and fuel cells

Researchers led by Genki Kobayashi at the RIKEN Cluster for Pioneering Research in Japan have developed a solid electrolyte to transport hydride ions (H−) at room temperature.

This advancement means that the benefits of solid-state batteries and hydrogen-based fuel cells are within practical reach, including increased safety, efficiency and energy density, which are essential to moving towards a practical hydrogen-based energy economy. The study was published In the diary Advanced energetic materials.

For hydrogen fuel and energy storage to become widespread, they need to be safe, highly efficient and as simple as possible. Current 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 across a polymer membrane when generating power.

The efficient, high-speed movement of hydrogen in these fuel cells requires water, meaning the membrane must be continually hydrated to not dry out. This limitation adds a layer of complexity and cost to battery and fuel cell design, limiting the viability of a next-generation hydrogen-based energy economy. To overcome this problem, scientists have been struggling to find a way to conduct negative hydride ions through solid materials, particularly at room temperature.

The wait is over. “We have achieved a real milestone,” says Kobayashi. “Our result is the first demonstration of a solid electrolyte conducting hydride ions at room temperature.”

The team had been experimenting with lanthanum hydrides (LaH3-δ) for several reasons: hydrogen can be released and captured relatively easily, the conduction of the hydride ion is very high, they can operate below 100°C and they have a crystalline structure.

But, at room temperature, the number of hydrogens attached to lanthanum fluctuates between 2 and 3, making efficient conduction impossible. This problem is called hydrogen nonstoichiometry and was the biggest hurdle overcome in the new study. When the researchers replaced some of the lanthanum with strontium (Sr) and added just a hint of oxygen, to obtain a basic formula of La1xMr.xh3-x-2yohandThey got the results they expected.

The team prepared crystalline samples of the material using a process called ball milling, followed by annealing. They studied the samples at room temperature and found that they could conduct hydride ions at high speed. They then tested its performance in a solid-state fuel cell made of the new material and titanium, varying the amounts of strontium and oxygen in the formula. With an optimal value of at least 0.2 strontium, they observed a complete 100% conversion of titanium to titanium hydride, or TiH.2. This means that almost no hydride ions were wasted.

“In the short term, our results provide guidelines for the design of materials for hydride ion-conducting solid electrolytes,” Kobayashi says. “In the long term, we believe this is a turning point in the development of batteries, fuel cells and electrolytic cells that run on hydrogen.”

The next step will be to improve performance and create electrode materials that can reversibly absorb and release hydrogen. This would allow the batteries to be recharged, as well as allowing hydrogen to be stored and easily released when needed, which is a requirement for the use of hydrogen-based energy.