Hydrogen – the great hope for transport?

Materials World magazine
,
1 Dec 2017

Anna Ploszajski examines the current use of hydrogen in the transport sector and its growth potential.

It’s 1970. The Apollo 13 crew has safely splashed down, Paul McCartney has announced that he is leaving The Beatles and Michael Eavis has held the first of what would become Glastonbury Festival. Meanwhile, a meeting is taking place at General Motor’s Technical Centre in Michigan, USA, to discuss the future of energy. During this, energy expert Professor John Bockris coined the term the hydrogen economy.

His idea was that our future lives could be powered by hydrogen. The promise was, and still is, very alluring – an energy-dense fuel that can be extracted from water and that burns cleanly to liberate its energy. The wider implications were more appealing still – no more over-reliance on unstable oil-producing countries, no more cities choked in smog and an ease-off of the threat of global warming. 

But, 47 years later, Bockris’ dream has yet to materialise. Why? That is a question that stirs plenty of debate, but let’s start by taking a trip through the hydrogen economy using the example of a hydrogen-powered vehicle, since this, after all, was where it all began.  

Using fuel cells

The hydrogen-powered car produces electrical power using a hydrogen fuel cell. These electrochemical devices combine hydrogen gas with oxygen to produce electricity and water. While many different types of fuel cell exist, the proton exchange membrane (PEM) is the one used for portable applications because it is lightweight and operates at low temperatures.  

A fuel cell on its own is not particularly powerful – each cell only produces a voltage of about 0.7V. However, any number of cells can be stacked in series to create more power. This scalability is one of the factors that makes fuel cells so attractive, although, until recently, their price tag had diluted some of the enthusiasm. The cost mostly comes from their catalysts, so therefore much of the current research has concentrated on either using nanotechnology to reduce the amount needed, or replacing them with cheaper metals. This has seen the cost of fuel cells half since 2007, according to a US Department of Energy document published in 2016.

Hydrogen is lightweight, making it an attractive fuel for portable applications such as cars. However, carrying the 5kg of hydrogen that it would take to drive 300 miles would require a balloon nearly 5m across. Clearly, there is a volumetric hydrogen storage problem. 

In 2015, Toyota was one of the first to commercialise a hydrogen fuel cell vehicle, the Mirai. Hyundai followed with their ix35 FCEV, and Honda with the Clarity. All have opted to store hydrogen in high-pressure (700 bar) gas cylinders. This is because of strong, lightweight materials, that offer a relatively familiar refuelling experience to consumers. 

Nevertheless, these carbon fibre-reinforced tanks are not perfect. Toyota had to incorporate two into the Mirai to give the car competitive range and still provide boot space. Gaseous hydrogen storage will always be limited by the fact that increasing the storage pressure requires thicker tank walls. This eventually negates the volume gain of increasing the storage pressure – and this limit has been reached with current materials. Research efforts in this area, therefore, involve incorporating sorbent materials inside the tanks to increase capacity, which should see next-generation cylinders reduce in size and/or increase in capacity.

The alternative to pressurised hydrogen is solid-state storage, either by physical adsorption onto the surfaces of highly porous materials like metal organic frameworks, or by chemical means in compounds that intrinsically contain hydrogen. The most commercially successful solid hydrogen stores are metal hydrides. These are light metals such as lithium, magnesium or aluminium, or alloys thereof, which can store the small hydrogen atoms interstitially in their crystal structures. The hydrogen is liberated by warming the materials. 

Metal hydrides have found niche commercial success, powering portable fuel cell phone chargers, but the systems are still considered too heavy for use in road vehicles.        

What’s the catch?

Hydrogen energy is only as clean as its source. As a highly reactive and lightweight gas, most of the hydrogen on earth exists tied up with other elements, for example with oxygen in our oceans and atmosphere. In an ideal world, it would be produced by splitting water into its elemental components by electrolysis – a fuel cell in reverse. By powering these electrolysers with electricity from renewable sources, such as wind or solar power, hydrogen production would be completely carbon neutral.

However, only about 4% of the hydrogen produced today comes from electrolysis, with the other 95% by steam methane reforming (SMR), which derives hydrogen from steam and methane, with carbon dioxide as a by-product. SMR is currently the cheapest production route and although it somewhat dirties hydrogen’s sustainable credentials, a fuel cell vehicle powered by hydrogen produced by SMR still achieves a 30% CO2 saving compared to diesel or petrol cars. 

In the future, hydrogen could come from photo-electrochemical processes (using light to split water), or biological processes (exploiting the metabolic processes of microorganisms). Hydrogen from biomass could even be a net-remover of CO2 from the atmosphere, though these approaches are still at the early stages of development.

Modifying infrastructure

Whatever the method of production, SMR plants and renewable energy sources are rarely located in close proximity to the end user. This is the main incentive for the hydrogen economy – to transfer renewable energy to the end user.  

For this to be rolled out on a large scale, hydrogen will need to be distributed down gas pipelines. Of course, networks exist, but much of the existing high-pressure transmission pipelines are made from high-strength steel. Hydrogen causes embrittlement of steel and ultimately failure of these old pipelines. Low-pressure pipelines are currently being upgraded to polyethylene in the Iron Mains Replacement Programme, which seeks to address failures with at risk iron gas mains. Such polymer pipes would also be suitable for hydrogen. 

The single biggest hold-up to the widespread adoption of hydrogen fuel cell vehicles is the availability of refuelling stations. The website H2stations.org maintains a map of hydrogen refuelling stations around the world. In the UK, hydrogen-fuelled journeys would be limited to just London and the M4, or local journeys in the Midlands or Aberdeen. 

Hydrogen cars have also suffered from a chicken and egg problem – manufactures are reluctant to produce cars that can’t be conveniently refuelled and governments are unwilling to invest in expensive new infrastructure that won’t be used – estimates of the cost of a refuelling station start at US$2.5m.

While Germany and the coastal regions of the USA have more comprehensive coverage, the only country to truly break this cycle so far has been Japan – Toyota took the risk to commercialise the Mirai and the infrastructure followed. 

Indeed, infrastructure is the principal reason why electric vehicles have had their strong head-start, as electricity is, on the whole, readily available. To date, the number of hydrogen fuel cell vehicles sold around the world is less than 2,000, compared to 2 million plug-in electric vehicles. 

There has, however, been some success in the UK with hydrogen buses. They only need one refuelling station and are in use in London on the RV1 route. Schemes are being rolled out across the UK and Europe, and hydrogen-powered forklift trucks are also gaining popularity.

Safety concerns

But isn’t hydrogen dangerous? The short answer for cars is, not as dangerous as petrol and diesel. Hydrogen fuel cell vehicles are equipped with many safety features to prevent ignition of the fuel.  

Since hydrogen is a lightweight gas, when it burns, it flares upwards, away from the body of the car. With a similar rupture to the fuel tank of a traditional car, the liquid fuel pours out onto the road and burns the car from beneath. A study by researchers at the University of Miami, USA, demonstrated the rupture and burning of a hydrogen-powered car and a gasoline-powered car. At the end of the experiment, the hydrogen car was left warm but otherwise undamaged, whereas the gasoline car was a smouldering wreckage. 

The negative perception of hydrogen is another reason why the hydrogen economy has struggled to gain traction. Hydrogen bombs and the burning zeppelin of the Hindenburg disaster are ever-present in our collective consciousness when the topic arises. More could – and should – be done by industry and government to improve hydrogen’s reputation. 

Electric or hydrogen fuel cell?

The question isn’t black and white. Hydrogen fuel cell cars also contain lithium-ion batteries, as the fuel cells cannot react fast enough to a step on the accelerator pedal. Their electricity output subsequently goes into charging batteries, which are connected to the power train. 

The two technologies are complementary. The hydrogen economy will not be realised without battery technology and some of the shortcomings with batteries can be rectified by hydrogen fuel cells. 

Advances in fuel cells, electrolysers and hydrogen storage technologies will see the economic argument for the hydrogen economy get stronger. Watch this space. After all, mirai is the Japanese word for future.