Breakthrough in solid electrolyte for hydrogen-based batteries, fuel cells

Researchers in Japan have recently developed a solid electrolyte for transporting hydride ions (H−) at room temperature, which opens up new avenues for improving the efficiency, energy density and practicality of hydrogen-based batteries and fuel cells.

Led by Genki Kobayashi at the RIKEN Cluster for Pioneering Research, the scientists claim that the latest breakthrough signifies that the advantages of hydrogen-based solid-state batteries and fuel cells are within practical reach, a step towards advancing a hydrogen-based energy economy. The study was published in the scientific journal Advanced Energy Materials last month.

"The wait is over. We have achieved a true milestone," said Kobayashi. "Our result is the first demonstration of a hydride ion-conducting solid electrolyte at room temperature." 

The team had been experimenting with lanthanum hydrides (LaH3-δ), as hydrogen can be released and captured relatively easily and hydride ion conduction is very high. But, at room temperature, the number of hydrogens attached to lanthanum fluctuates between 2 and 3, making it impossible to have efficient conduction. 

This limitation, called as 'hydrogen non-stoichiometry', was the biggest obstacle overcome in the new study, according to the researchers. When they replaced some of the lanthanum with strontium (Sr) and added just a pinch of oxygen, for a basic formula of La1-xSrxH3-x-2yOy, they have got the results they were hoping for.

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 a high rate. 

Further, 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. With an optimal value of at least 0.2 strontium, they have observed a 100 percent conversion of titanium to titanium hydride (TiH2). This means that almost zero hydride ions were wasted.

"In the short-term, our results provide material design guidelines for hydride ion-conducting solid electrolytes," observed 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 researchers claim that the next step will be to improve performance and create electrode materials that can reversibly absorb and release hydrogen. This would allow batteries to be recharged, as well as make it possible to place hydrogen in storage and easily release it when needed, which is a requirement for hydrogen-based energy use.

For H2-based energy storage and fuel to become more widespread, it needs to be safe, very 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 through a polymer membrane when generating energy.

The limitation at present is that high-speed hydrogen movement in these fuel cells requires water, which warrants membrane to be continually hydrated, thus adding an additional layer of complexity and higher cost to design. The latest research finds a way to conduct negative hydride ions through solid materials at room temperature.