Researchers led by Genki Kobayashi at the RIKEN Cluster for Pioneering Research in Japan have developed a solid electrolyte for transporting hydride ions (H−) at room temperature. This breakthrough means that the advantages of hydrogen-based solid-state batteries and fuel cells are within practical reach, including improved safety, efficiency, and energy density, which are essential for advancing toward a practical hydrogen-based energy economy. The study was published in the scientific journal Advanced Energy Materials.
For hydrogen-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. Efficient, high-speed hydrogen movement in these fuel cells requires water, meaning that the membrane must be continually hydrated so that it does not dry out. 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. To overcome this problem, scientists have been struggling to find a way to conduct negative hydride ions through solid materials, particularly at room temperature.
“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) for several reasons; 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. But, at room temperature, the number of hydrogens attached to lanthanum fluctuates between two and three, making it impossible to have efficient conduction. This problem is called hydrogen non-stoichiometry, and was the biggest obstacle overcome in the new study. When the researchers 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 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. 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. With an optimal value of at least 0.2 strontium, they observed complete 100 percent conversion of titanium-to-titanium hydride, or 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,” 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. 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.
Here is an exclusive Tech Briefs interview with Genki Kobayashi, edited for length and clarity.
Tech Briefs: What was the biggest technical challenge you faced while developing this new material?
Kobayashi: The use of ionic conductors as solid electrolytes requires not only ionic conductivity but also chemical and electrochemical stability. In hydride ionic conductors, there are no investigations from this point of view, so it was necessary to devise how the performance thresholds could be experimentally proven. In this paper, a test cell was actually assembled to investigate whether the hydrogenation reaction proceeds according to the theoretical values.
Tech Briefs: Can you explain in simple terms how it works?
Kobayashi: Although it is still in the verification stage, we want to create an electrochemical device that can store hydrogen through an electrochemical reaction and retrieve it when it is needed. In the long term, we would like to consider applications in fuel cells.
Tech Briefs: What are the pros and cons of it?
Kobayashi: Negatively charged hydrogen (hydride ions) are more reactive with O2 than the normally used protons (H+), so if they can be used in fuel cells, the resistance to reaction will be reduced, resulting in higher power density and/or reduced use of rare metal catalysts.
The materials we have developed cannot be handled in the atmosphere, so there are many challenges that need to be overcome for their application in batteries and fuel cells. Device configurations will need to be innovated.
Tech Briefs: How soon could we see these types of batteries implemented on a commercial scale?
Kobayashi: I would like to show specific application directions in 5-10 years. I think it will be more than 10 years before practical-based industry-academia collaboration will be underway.