Hydrogen storage goal in sight

With more and more organisations looking for ways to combat climate change, a reasonably priced hydrogen sponge to soak up and release emissions looks increasingly possible. More than a decade ago, on a visit to Ford’s US research laboratory in Dearborn, Eureka was shown an internal combustion engine that ran on hydrogen, allowing cars to be effectively fuelled with water – either in the car or at hydrogen-producing stations – which could then be electrolysed to produce hydrogen to power the vehicle. The only drawback was having to pump it into cylinders for high pressure storage until it was ready to be supplied to the engine. Nobody was seriously suggesting filling up with liquid hydrogen at the petrol station. The more sensible solution was to find a material that would soak up hydrogen and then release it, either by reducing pressure or warming it. Professor Bill David and his colleagues at the Rutherford Appleton Laboratory at Didcot, Oxfordshire, are studying several of the more promising candidates. At the same time, its Isis facility is being enhanced and a Rapid Throughput Facility set up to greatly accelerate the process of finding the most precise composition for the ‘sponge’. “Realistically, our goal is to find a material that could reversibly retain 5% hydrogen by weight at room temperature at a few atmospheres pressure, which would be a very significant material,” says David. Two main routes are being followed: one, in which the hydrogen atoms sit between other larger atoms in a metal alloy that does not change its structure, are called interstitial alloys; the other is inorganic chemical compounds that can be made to take up, and then give out, hydrogen reversibly. Other possibilities include carbon nanotubes, and organic and metal organic chemical compounds, but interstitial metal alloys and inorganic compounds seem to be the frontrunners. The Rutherford Appleton team is looking at lithium boron hydrides, magnesium hydrides, and amine boranes. One of the most promising of these looks to be lithium borohydride amide, which can store 10-11% hydrogen, and would only need to be heated to 100ºC to release the hydrogen. The only problem is that at present it is ‘one shot’ – meaning you would have to buy a loaded cartridge at a filling station and exchange it for another one when it was empty. However, according to David’s post doctoral co-worker Dr Marco Somariva, they are also looking at magnesium hydrides, doped with transition metals, such as nickel, magnesium or titanium, that can be loaded reversibly, except that right now the reaction kinetics are considered to be too slow and the release temperature (300ºC) too high. Research into another promising family of compounds is also underway. One involves a new target station attached to Isis – a proton synchrotron that bleeds off protons to run them into tungsten targets where they yield large numbers of neutrons. Neutron diffraction is the only tool that allows researchers to see hydrogen entering and leaving hydrogen storage materials in real time. The other relates to the Rapid Throughput Facility, which should be up and running by December. It is based on semiconductor fabrication technology and will process 50 samples at a time, allowing a throughput of 50 to 100 samples a day. Coupled with the Isis facility, which will hopefully show up the best recipe combinations to investigate further, it should speed up the screening process to find the optimum composition for a reversible hydrogen storage compound by possibly a thousand times. And even if it does not lead to the solution to be found in the next generation of cars, buses, trains, aircraft and ships, David concludes, it “could lead to a better rechargeable hydride battery, to replace lithium and nickel hydrides, or even a new high temperature superconductor”. Pointers * Viable, reversible hydrogen storage materials should start to become available in the next few years * 5% hydrogen by weight is the immediate goal, but 10% looks feasible * At present, there are a large number of competing materials that are being investigated and optimised; which is likely to turn out to be best is not yet clear