An old diesel freight train in British Columbia, Canada is about to get a new lease of life. Local firm Hydrogen in Motion (H2M) is currently converting the Green Goat locomotive to run on a mix of hydrogen and battery power. The so-called switcher locomotive performs tasks such as transporting small loads of lumber or animal feed at rail yards. If all goes to plan, H2M will have the engine running by the end of this year, or early next. "With the successful demonstration of this we would be looking at much larger trains as well," says H2M president and chief executive Grace Quan. "We'd be looking at converting entire fleets." Hydrogen, which emits water but no carbon dioxide when burned, is often touted as a fuel of the future.There are already a few hydrogen-powered trains out there, such as the ones currently being rolled out in the German state of Lower Saxony. The technology made its debut there in 2018.In the UK, a hydrogen locomotive, HydroFlex2, is undergoing testing in Long Marston, Warwickshire. But current methods of storing hydrogen in tanks, as a highly pressurised gas or extremely cold (cryogenic) liquid, are expensive and potentially unsafe. Scientists have long aimed to find ways of storing hydrogen in more inert solid forms. In H2M's case, the company has developed a nanomaterial within which hydrogen is stored at relatively low pressure compared to other techniques. By simply opening a valve and lowering the pressure, the hydrogen comes off, just like when the CO2 in a carbonated drink gets released when you open the bottle, says Ms Quan. She adds that hydrogen could be especially useful for powering heavy vehicles that go on long journeys, such as trains and lorries, though the company has also developed a hydrogen-powered three-wheeler tuk tuk as a small-scale demonstration. "Depending on the size of the fuel cell, we could give up to a week's worth of power," says Ms Quan of the tuk tuk. But she adds that sourcing investment funds to develop her firm's technology has not been easy. And many other solid state hydrogen storage solutions are at an early phase of development.Though some of them are exciting, the huge challenge they face is to prove that they are commercially viable and worth choosing over, say, batteries for certain applications. Duncan Gregory at the University of Glasgow says that those working on hydrogen storage technology at the moment are aiming to achieve a density of 5 wt% (weight per cent) or greater within the storage medium. "It's really difficult to get to that," he explains. Srikanth Mateti (left) and Ian Chen are working on a technique to make hydrogen powderThere's plenty of effort, though. Take the method that Ian Chen at Deakin University in Australia and colleagues are currently beavering away on. It involves a technique called ball milling, a sort of grinding process using tiny balls inside a canister. This grinding activity raises pressure levels, encouraging gases to become absorbed by a powder inside the canister. Prof Chen and his colleagues have found that they can use this to store gases including hydrogen in boron nitride powder. "We like it because it's stable, it's not toxic, it's lightweight," says Prof Chen, who adds that 5 wt% should be possible with this method. In July, the team described how this process could be used to store hydrocarbons in an academic paper. The detail of experiments in which they managed to store hydrogen and other gases such as CO2 and ammonia are yet to be made public. Prof Chen is enthused by the possibility of storing hydrogen in this form, which simply requires the application of heat to release the gas again. But he admits that a long road lies between this research and commercial success. The team would need to design large-scale equipment and show that the method would be truly cost-effective at scale. "We don't claim that we have solved all the major problems," says Prof Chen. The cost challenges were underlined in October when the German state of Baden-Württemberg ruled out replacing diesel locomotives with hydrogen-powered trains. A study commissioned by the state concluded that the installation of overhead electricity lines or battery hybrid trains provided much better value for money over a 30-year period. Prof Gregory, who is currently working with a separate, undisclosed firm on hydrogen technology, adds that one of the potential issues with the ball milling process is how long it takes. The small-scale experiments reported by Prof Chen and his colleagues so far took 20 hours of milling. Besides transportation, hydrogen stored in a solid state could have other uses. A trial currently under way in Scotland will test hydrogen generation and storage on one of the Orkney IslandsA machine-learning system will monitor weather patterns and decide when to use electricity from nearby wind turbines to power the electrolysis process, which is how hydrogen is extracted from water.