By adjusting the proportions of the material's components, the scientists lowered the temperature at which hydrogen began to be released to about 77°C. That falls within the 60-80°C range of waste heat typically produced by fuel cells, raising the possibility that heat generated while producing electricity could help release the next batch of hydrogen.

A fuel cell converts hydrogen into electricity while also generating heat. Hydrogen flows from a storage tank into the fuel cell, where it reacts with oxygen rather than burning as it would in an engine. The reaction produces electricity, water and heat. The electricity powers a vehicle or device, while the heat is typically dissipated.

If that heat could instead be redirected to the storage tank, it could trigger the release of more hydrogen. In this way, the fuel being consumed could help prepare the next portion, making use of a byproduct that would otherwise escape into the environment.

This could help address one of the major challenges in hydrogen energy: storage. Although hydrogen has a high energy density by weight, it has a low density as a gas. It is therefore usually compressed to 350 or 700 bar in heavy, robust tanks. Hydrogen can also be liquefied, but this requires temperatures close to -253°C. Both approaches consume energy and add complexity to the system.

An alternative is to store hydrogen within a solid material. Hydrides, compounds containing hydrogen and other elements, can absorb the gas and release it when heated, potentially allowing storage at lower pressures. However, many lightweight hydrides require temperatures significantly higher than the 60-80°C range available from fuel-cell waste heat. A separate heating system is therefore needed, consuming some of the energy produced.

The researchers sought to address this problem by changing the proportions of the components in the storage material. They prepared composites consisting of magnesium amide, lithium hydride and lithium borohydride. The material containing a high proportion of the latter compound proved the most promising.

The selected composite began releasing hydrogen at 350 Kelvin, or about 77°C. This was about 100 degrees lower than in a similar system without lithium borohydride, putting it within the range of waste heat produced by a fuel cell.

Below 127°C, the material released hydrogen equivalent to about 1.2% of its mass. Its total hydrogen capacity, measured after heating to 227°C, reached 3.12%.

The result points to the possibility of a cycle in which heat generated by a fuel cell triggers the release of the next batch of hydrogen. However, the material cannot yet release its entire supply using waste heat alone. The fastest gas release occurred at around 149°C.

The researchers therefore identified two key challenges for further development: shifting most of the reaction into the 60-80°C range and increasing the material's hydrogen capacity.

Importantly, no ammonia emissions were detected during the reaction. In similar materials, ammonia can contaminate the gas and reduce storage capacity over time.

X-ray measurements at a synchrotron revealed at least two previously undescribed intermediate phases. The authors hypothesise that these phases, together with partial melting of the material, facilitate ion mobility and consequently the release of hydrogen. The proposed mechanism still requires confirmation.

The individual hydride powders were prepared through reactive milling in an argon atmosphere. Collisions between steel balls pulverised the constituents and supplied the energy required for the reaction. Col. Marek Polański, PhD, a professor at the Military University of Technology's Department of Functional Materials and Hydrogen Technologies, was responsible for this stage of the research.

The project was supported by the Polish National Science Centre, and the findings were published in the scientific journal Nature Communications.

There is still a long way to go before the material could be used in an industrial-scale hydrogen storage system. Its ability to absorb and release hydrogen was tested over only three cycles.

After the third cycle, the material could still store nearly 3% of its mass in hydrogen, although the release rate had slowed. Hydrogen release also began at about 115°C rather than 77°C. Repeated use therefore did not significantly reduce storage capacity, but it made fuel recovery more difficult.

Measurements taken at around 130°C showed an equilibrium pressure of approximately 100 bar. This is lower than the pressure used in current compressed hydrogen tanks, but remains substantial. The material will need to be tested over at least 50 cycles, as well as in combination with an operational fuel cell.

If sufficient hydrogen can eventually be released using only heat generated by a fuel cell, the tank and fuel cell could form a single integrated system. Each portion of hydrogen consumed would help prepare the next until the supply was exhausted.

For now, however, the concept remains a promising avenue for development rather than a ready-to-use technology.

Krzysztof Petelczyc (PAP)

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