Hydrogen vehicles come with major pros and cons. One of their biggest downsides is that they require expensive fuel tanks. A new material looks to change that.
Researchers at Northwestern University have developed a new material which could revolutionise the clean energy automobile market. The material, described as being similar to a bath sponge, can soak up hydrogen and store it in a far more efficient way than previously thought possible.
Vehicles powered by hydrogen fuel cells have been viewed by some as an attractive alternative to traditional fossil fuel vehicles. Within a hydrogen-powered car, hydrogen and oxygen are passed over an anode and cathode resulting in electrical energy. The only waste products are water and more hydrogen. This essentially creates a zero emission source of fuel for vehicles, and if it has been produced with renewable energy, a much greener way to travel.
However, hydrogen also comes with some drawbacks. In particular, hydrogen gas is much lighter than traditional gas fuels, resulting in it requiring a much larger space to store it. At normal atmospheric pressure, one kilogram of hydrogen fuel - which could power a car for around 100 kilometres - would require a fuel tank capable of holding around 11,000 litres. For a comparison, the standard car has a fuel tank with a capacity of around 45 to 65 litres.
Therefore, in order to be practical, hydrogen vehicles require specialised fuel tanks which store the hydrogen at extremely high pressures - around 700 bar, which is 300 times greater than the pressure of the car's tyres. With this method, a single vehicle can carry around four to five kilograms of hydrogen gas - powering it for around 500 kilometres.
But in order to create a tank capable of safely maintaining this pressure, expensive materials are required, adding to the overall cost of hydrogen vehicles and making them less attractive alternatives to cheaper fossil fuel models. For example, the Toyota Mirai - one of the three main hydrogen-powered cars currently on sale - costs around 57,500 USD. Meanwhile, fossil fuel SUVs - which are becoming more popular in the US and are a major source of emissions - generally cost around half this figure. Additionally, there are also safety concerns given the highly flammable nature of hydrogen stored under such high pressures.
The Gas-Guzzling Super Sponge
The new material developed by Northwestern University could change this. A team, headed by Professor Omar K. Farha - the associate professor of chemistry in the Weinberg College of Arts and Sciences and member of Northwestern’s International Institute for Nanotechnology - has created an ultra-porous metal-organic framework (MOFs) with a huge surface area. This material, dubbed NU-1501, consists of organic molecules and metal ions which self-assemble to form multi-dimensional, highly crystalline, porous frameworks. These frameworks can then combine with liquid or gaseous molecules - for example from hydrogen or methane - and store them at much safer pressures in a smaller space. Farha states that one gram of NU-1501, which is the equivalent to six M&Ms, has the surface area of 1.3 American football fields. Professor Farha explained:
“We’ve developed a better onboard storage method for hydrogen and methane gas for next-generation clean energy vehicles. To do this, we used chemical principles to design porous materials with precise atomic arrangement, thereby achieving ultrahigh porosity. We can store tremendous amounts of hydrogen and methane within the pores of the MOFs and deliver them to the engine of the vehicle at lower pressures than needed for current fuel cell vehicles.”
It is hoped the new breakthrough could help hydrogen vehicle producers optimise fuel tanks and lower the overall costs of their models - making them more accessible to consumers. In doing so, they would also potentially help reach current goals for the future development of green energy automobiles.
Unfortunately, simply creating a more efficient fuel tank will not be the end of criticism of hydrogen-fuelled vehicles. Environmentalists are also concerned that hydrogen may not be as green as it originally seems. Currently, 98 percent of hydrogen is produced via methane steam reforming, which often uses natural gas as the feedstock, resulting in high carbon emissions. Techniques are being developed to use renewable sources as the feedstock, but once again this process is expensive and currently poorly optimised.
Reducing this expense and increasing the efficiency of both hydrogen production and hydrogen-fuelled vehicles will be critical for the long term commercial adoption of the technology.