New Material May Build Better Hydrogen Based Fuel Cells

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.

Its a breakthrough that means 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 towards a practical hydrogen-based energy economy.

The study paper was published in the scientific journal Advanced Energy Materials.

Schematic of a solid-state fuel cell made from the new material and titanium. The result of the galvanostatic discharge reaction showed that the Ti electrode was completely hydrogenated to TiH2 for x ≥ 0.2. Image Credit: RIKEN Cluster for Pioneering Research. Click the link to the press release for the largest view.

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 is a constraint that 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.

Kobayashi has said, “We have achieved a true milestone. 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) 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 hydrogen atoms attached to lanthanum fluctuates between 2 and 3, 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 La_1-xSr_xH_3-x-2yO_y, 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. Then 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% conversion of titanium to titanium hydride, or TiH₂. This means that almost zero hydride ions were wasted.

Kobayashi noted, “In the short-term, our results provide material design guidelines for hydride ion-conducting solid electrolytes. 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 “storage 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.

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This is a remarkable breakthrough in making fuel cells more practical. Not quite noted above is that today’s designs need an above freezing environment. All the time, not just when running. For much of the time and much of the world that is killer. Moreover a freeze would destroy a fuel cell, thus even a rare freeze zone would entail considerable risk. The team hasn’t said anything specific yet. But this is promising.

With that in mind this team’s work looks like quite a breakthrough. Now if the storage issue could just get a big cost and safety breakthrough . . .

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