Hydrogen fuel cell technology has been around for as long as battery technology suitable for cars, yet its success has not been nearly as great. In 2019, 4.8 million electric cars were in use, while only 7,500 hydrogen fuel cell cars had been sold. So why have hydrogen fuel cells not taken off?
One of the difficulties in hydrogen fuel cell cars is the lack of available infrastructure. Across the whole of the UK, only 20 hydrogen refuelling points exist, in contrast with 30,000 electric charging points. This is partly due to the availability of electricity in contrast to hydrogen. Electricity can be taken from the grid, to which most buildings are connected, while hydrogen is usually manufactured by steam methane reforming (95% of the hydrogen produced in the US and 48% of the hydrogen produced globally is made using this method).
In steam methane reforming, methane is reacted with steam in the presence of nickel at a temperature of approximately 800ºC, producing hydrogen gas and carbon monoxide. The carbon monoxide is then reacted with steam to produce carbon dioxide and hydrogen gas. This method releases a significant amount of emissions: roughly 9 tonnes of carbon dioxide are released for every tonne of hydrogen gas produced. As such, hydrogen fuel cell cars fuelled by this method are not as low carbon as battery-powered electric cars, if the electricity is produced by renewable sources.
However, hydrogen production can be made renewable by using electrolysis. Traditional electrolysis is very inefficient, losing about 30% of the energy stored in the hydrogen through the intensive electricity requirements of electrolysis. Proton Exchange Membrane (PEM) is slightly more efficient, losing under 20% of the energy stored. However, the lower efficiency still makes hydrogen fuel cells less competitive than battery technology.
The greatest advantage of hydrogen fuel cells over batteries is their high energy density. Hydrogen can either be compressed at very high pressure or cooled to a very low temperature, of below -250ºC, where it condenses and can be stored. Either way, hydrogen has a fuel density far greater than that of lithium-ion batteries. This helps it to solve a problem facing electric car manufacturers. Unfortunately, both methods use a significant amount of energy, again increasing the cost of hydrogen.
Cars achieve greater electrical efficiency if they are lighter, so less energy is needed to move them. In order to achieve long ranges, comparable to that of petrol or diesel cars, battery-powered cars need to have larger batteries. This significantly increases the weight of the car, both decreasing efficiency and the gain in range. In contrast, pressurised or liquefied hydrogen has such a high energy density that the range of the car can be increased without increasing the weight of the car by much at all.
The hydrogen is then used to generate electricity using another proton exchange membrane. First, the hydrogen electrons are split from the atoms at the anode in the presence of a catalyst. Then, the protons are attracted to the cathode and move through the proton exchange membrane, while the electrons are unable to pass through it. The electrons are then used to power the axis of the car, before returning to the other side of the fuel cell, where the hydrogen reacts with oxygen in the presence of a platinum catalyst, producing water.
Though they may not become a viable replacement for battery-powered electric cars, they may find another niche, in the form of planes. Large commercial passenger planes exclusively run on jet fuel, produced from crude oil. At current rates, oil reserves will run out by the middle of the century, and even before that, dwindling oil stocks will lead to a huge increase in prices, making flight not commercially viable. For this reason, a renewable alternative is needed. Due to the low energy density of batteries, they are not suitable for planes, where the weight should ideally be as low as possible. However, hydrogen fuel cells' incredibly high energy density, three times higher than jet fuel itself, makes it a feasible, but costly, alternative.
Much has been made of the Japanese government's plan to make the country a "hydrogen society". This plan involves shipping hydrogen from Australia to power affordable hydrogen fuel cell vehicles. While on the surface this plan appears to be a revolutionary idea designed to decarbonise Japan's economy, this impression doesn't hold up to scrutiny. Firstly, the hydrogen from Australia is to be produced from brown coal, releasing a huge amount of carbon dioxide emissions. These emissions are then meant to be injected into geological rock formations, to prevent them from reaching the atmosphere. This might be a good short-term solution, but it is uncertain whether it would work in the long term, and even if it did work, the cost might be prohibitively expensive. However, this does appear to be one of Japan's only options in decarbonising the economy, as the island lacks enough wind potential, or space for solar, for either to constitute the majority of Japan's electricity generation. The Japanese government's investment in hydrogen may, however, significantly reduce the cost of hydrogen around the world, making hydrogen-fuelled planes and cars more viable.
Overall, hydrogen fuel cells are unlikely to replace battery-powered cars in the electric vehicle market, due to the much higher cost of the fuel, and the lack of availability of hydrogen. However, there is a real opportunity for hydrogen fuel cell planes, as potentially the only alternative to the current jet fuel-powered planes.
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