Nanocage for efficient hydrogen storage

Materials World magazine
,
1 Jun 2018

A project aiming to improve the safety and efficiency of hydrogen storage is developing composite materials to allow controllable release of the gas. Kathryn Allen reports.

The NanoComposites for Active Gas Encapsulation (nanoCAGE) project, funded by the Engineering and Physical Sciences Research Council, aims to improve hydrogen storage in porous materials by creating new composites using polymeric and nanoporous materials, which can control hydrogen capture and release using external stimulus. 

While hydrogen, which can be sourced from water using excess renewable energy, is a potential replacement for fossil fuels, its gaseous state makes storing expensive and difficult as it must be kept at high pressures or very low temperatures. It is also flammable, therefore requires safety features on storage tanks. 

Current methods of storing hydrogen for industry include compressing hydrogen in tanks at pressures above 350 bar, or using liquid helium to cool it down, transforming it to liquid hydrogen. 

The materials developed in the nanoCAGE project could be used for hydrogen storage on board vehicles, without the need for this ancillary equipment. This would increase the fuel economy of the car and allow for smaller tanks, as hydrogen is condensed. It would also improve safety as the hydrogen is encapsulated in the material and shouldn’t be released if the vehicle crashes. 

It is a five-year project, currently at the proof of principle and fundamental science stage. The Principal Investigator of the project, Dr Valeska Ting, Associate Professor in Smart Nanomaterials, University of Bristol, UK, expects that, by 2023, the team will have a demonstration system and should be confident enough of the parameters that they can propose an optimisation.

Functional composites 

The developed composites will encapsulate nanoporous materials – those that have a pore size on the nanometre or angstrom level – such as metal organic frameworks, activated carbons, nanostructured carbons, zeolites, and porous polymers, in an active polymeric-based material. 

External stimulus, such as light, heat, or the application of a magnetic field, will be used to cause a reversible structural change in the material, which will allow or prevent hydrogen gas from moving in or out of the pores. This method could allow the amount of hydrogen stored at room temperature to be increased by up to 10 times. 

Ting said, ‘We’re looking at combining different types of materials for different properties, to optimise them, so we can maximise the amount of hydrogen stored, and at reasonable temperatures and pressures. We’re aiming for something close to room temperature, possibly down to –10/20°C, as this can be achieved with standard heating and cooling on board a vehicle. In terms of pressure, we’re hoping to be dealing with no more than 10 bar.’

The amount of hydrogen, which these composites will be capable of storing, is limited. However, according to Ting, they will be easy to make and, therefore, scalable. The team is also working on taking waste carbon materials from the pulp and paper industry – for example, waste lignin, which is usually binned – and converting it into activated carbon – a nanoporous material that could be used to store hydrogen. 

Industry applications

Once these parameters are reached, there are two main applications in mind for the composites. One is the large-scale transportation of hydrogen from a wind or solar farm – where hydrogen could be generated via electrolysis using surplus electricity – to a filling station at a different location by putting the encapsulation material on board a tanker. Equipment would be needed at the destination site to alter the conditions of the material to release the hydrogen. 

The second application includes the aforementioned use in cars. Ting explained, ‘In your car you’ve automatically got sources of heat and cooling in the engine, so we’re aiming to get materials that are optimised for those sorts of small temperature fluctuations. The magnetic trapping or light-induced changes would be applied at the destination site and the material tailored to different applications.’

Despite current high investment in battery electric vehicles, Ting maintains that it is crucial to invest in a range of technologies, pointing out that both have pros and cons. ‘Materials, lifetime, charging times, and range limit battery electric vehicles,’ said Ting. ‘The range of hydrogen vehicles currently depends on the size of the tank. The real benefit of hydrogen is that it’s just like filling up your tank at the petrol station, you can fill it up in a few minutes and go, you wouldn’t have to stay plugged in for half an hour or longer depending on your battery size.’