Hydrogen cars use hydrogen fuel as their power source. Although this technology is being expored for rockets and other transport vehicles, it looks set to have the greatest impact on automobiles in the future.
The chemical energy of hydrogen is converted to mechanical energy through a REDOX (reduction/oxidation) reaction between hydrogen and oxygen within a specially developed fuel cell.
As hydrogen is not found in reservoirs or natural deposits, as with fossil fuels, it needs to be produced from natural gas or biomass, or electrolysed from water. One benefit of hydrogen power is the decrease in greenhouse gas emissions, particularly when the gas is produced using renewable electricity to convert water into hydrogen.
While hydrogen can be produced from fossil fuels like natural gas, this results in greenhouse gas emissions, thereby removing or minimising the environmental benefits. Therefore, renewable energy sources seem to be the answer, such as in Iceland where geothermal power is being used for hydrogen production or in Denmark, where wind power is being used.
How Do Hydrogen Fuel Cells Work?
A hydrogen fuel cell converts potential chemical energy into electrical energy using a proton exchange membrane (PEM) that uses hydrogen gas (H2) and oxygen (O2). However, since oxygen is readily available in the atmosphere, the fuel cell only needs to be supplied with the hydrogen required to power the vehicle.
Hydrogen fuel cells are made up of a negatively charged cathode and a positively charged anode which are put in contact with an electrolyte. The electrolyte is the proton exchange membrane, a specially treated material. Hydrogen gas enters the fuel cell on the anode side and is forced through the catalyst by pressure. The PEM only conducts positively charged ions, while blocking the electrons. The anode conducts the electrons, which have been freed from the hydrogen molecules, through an external circuit. These electrons provide the power to drive the electric motor, light bulbs, and so forth.
Meanwhile, oxygen is forced through the catalyst from the cathode side, where the negative charge of the atoms attracts the hydrogen atoms that have been pushed through the external circuit, before the hydrogen ions and the oxygen recombines to form water.
The following hydrogen fuel cell equation shows the process:
O2 + 4H+ + 4e– → 2H2O
2H2 → 4H+ + 4e–
2H2 + O2 → 2H2O (net reaction)
Hydrogen fuel cells vary and use different materials for the catalyst, mainly platinum nanoparticles. These nanoparticles face the PEM and the catalyst is rough and porous so as to expose the maximum surface area to the hydrogen or oxygen.
The fuel cells are placed together in stacks. The stacks are embedded in a module including fuel, water and air management, and coolant control hardware and software.
Hydrogen Fuel Cells Advantages and Disadvantages
Hydrogen fuel cells offer both advantages and disadvantages compared to traditional engines. Fuel cells are not only more reliable due to a lack of moving parts, but they are more efficient too. This greater efficiency is because the chemical potential energy is converted directly into electrical energy rather than having to first be converted into heat and then again for the mechanical work – which is known as the ‘thermal bottleneck.’ Emissions from hydrogen fuel cell cars are also cleaner than from traditional engines, as they emit just water and some heat, rather than the plethora of greenhouse gases associated with traditional combustion engines.
However, there are a number of challenges with hydrogen fuel cells, including being expensive to produce. This is primarily due to the expense of the rare substances, such as platinum, required for the catalyst. The earliest fuel cell designs also struggled to perform at low temperatures, but later modifications to the technology have ensured that this has now been addressed. The service life of fuel cells is also now comparable to that of other vehicles, with a PEM expected to last for 7,300 hours under cycling conditions.
Read our complete guide on the pros and cons of hydrogen fuel cells here
There have been concerns raised over the storage of hydrogen in the cars themselves. Once pumped into the car, the gas is held in a high-pressure cylinder, leading some to worry about the safety of storing a highly flammable gas in the vehicle. However, all of the cars on the market need to pass stringent safety tests.
With regard to transportation, there has been research related to using ammonia borane, a hydrogen storage compound, from which the hydrogen can be separated using a membrane. This offers transportation advantages as ammonia is easier to safely store in tankers than pure hydrogen.
In addition to the fuel tanks of vehicles and transportation issues, the hydrogen needs to be stored at hydrogen filling stations. The low ignition energy, coupled with hydrogen’s high combustion energy, and the fact that the gas tends to leak from tanks, has led to explosions at hydrogen filling stations. Again, this is an obvious factor that needs to be addressed ahead of the widespread use of hydrogen vehicles.
In order to make hydrogen fuel cell cars the transport of the future, there is a real need to improve the infrastructure around the vehicles. This will involve increasing the number of global and UK hydrogen fuel stations, which will either need to be supplied by compressed hydrogen tube trailers, liquid hydrogen tank trucks, hydrogen pipelines or, alternatively, use some form of dedicated on-site production. Creating this infrastructure to match that of the needs of the consumer could prove costly, even as some propose the creation of home hydrogen fuel stations.
Codes and Standards
Another factor that could delay the widespread use of hydrogen are the necessary codes and standards for safety and storage of the gas. These will need to be developed for a variety of hydrogen electric vehicles and across different nations.
Vehicle Cost and Production
Hydrogen powered cars are currently expensive to buy, with the first to hit the market including the Toyota Mirai, the Hyundai Nexo, and the Honda Clarity. Not only can current costs exceed an entry price of around £50,000, but the production rates for these vehicles are still relatively low, meaning a potentially long wait after an order is placed for a car. However, with more companies looking to enter the hydrogen fuel cell car market, the prices should start to drop over time as is the case with most new technologies.
What are the Pros and Cons?
- Cheaper Tax
Like electrically powered vehicles and plug-in hybrids, hydrogen fuel cell cars are ‘ultra low emission vehicles,’ meaning that they are classed in a low tax bracket. In addition, the lack of CO2 emissions mean that owners won’t have to pay vehicle excise duty (road tax) – aside from a low annual supplement for cars over £40,000 for the first five years.
- Congestion charges
Those who travel into Central London or other areas with congestion charges will not have to pay if they are driving a fuel cell vehicle.
As already mentioned, the lack of harmful emissions mean that hydrogen fuel cell cars are kinder on the environment. This benefit needs to be qualified by how the hydrogen is produced, with renewable energy sources being the most favourable. However, even where fossil fuels are used for hydrogen production it still produces fewer harmful emissions than burning petrol or diesel. Plus, hydrogen cars avoid the environmental issues associated with the extraction of petrol or diesel for traditional combustion engines.
Range anxiety has become a factor for electric vehicles, even with new technologies to dramatically improve charging times. However, this is unlikely to be such an issue for hydrogen cars, which can cover 300 miles from one tank.
- Filling up
With very few hydrogen refuelling stations currently in existence, filling up your vehicle is a real issue right now. However, the UK government has created a multi-million pound fund to improve the refueling infrastructure to support the new technology, meaning that refuelling shouldn’t be such a problem in the future.
- Running costs
Aside from finding somewhere to fill up, there are also concerns over fuel costs. With hydrogen currently being more expensive than petrol or diesel, running a hydrogen car could hit consumers in the pocket for the time being, at least.
- Fuel tanks
As mentioned in the hydrogen storage section (above) there are some concerns over the safety of driving around with a tank full of highly flammable hydrogen gas. While it is highly combustible, so is petrol, and the tanks used for hydrogen have been designed to be especially strong. While this adds weight and cost to the vehicles, they need to pass safety standards before being made commercially available, meaning that this shouldn’t really be an issue.
Improvements to Hydrogen Vehicle Technology and Infrastructure
Many of the perceived negatives of hydrogen cars can be addressed with investment in infrastructure and technology. Dedicated fuelling stations for hydrogen are more expensive than implementing charging stations for electric vehicles and, unless the take-up of hydrogen vehicles increases, this investment is unlikely to be promoted. This creates something of a Catch-22 situation, whereby the infrastructure is needed to support the take-up of hydrogen vehicles but, without the take up of hydrogen vehicles, the need for the infrastructure could be brushed aside as unnecessary. That said, the UK government and the EU are already backing a drive to increase the number of available hydrogen filling stations.
The technology driving the vehicles themselves is also set to improve over time and this technology is also set to become cheaper as the range for hydrogen cars increases. Cheaper costs, improved efficiency and greater supporting infrastructure will all serve to drive consumer confidence and take-up of hydrogen cars in the future.
Differences Between Fuel Cell Cars and Electric Cars
While the traditional combustion engine looks set to become a thing of the past, hydrogen fuel cell vehicles face stiff competition from other electric vehicles.
While both battery electric vehicles and hydrogen fuel cell vehicles offer emission-free driving, battery-powered vehicles can use the existing infrastructure to recharge, although they need to be plugged in for longer periods of time and still have questions over range.
It is a question of which technology can address their particular challenges to become the favoured form of transport for the future.