Hydrogen Fuel Cells: Are They the Future of Transport?
As governments across the world seek to achieve a ‘Net Zero’ future, policymakers are looking at ways in which transport can be decarbonised. Amongst the options being discussed are hydrogen fuel cells. But, what exactly are they? And, do they represent the future of transport? The Lab investigates…
What is a hydrogen fuel cell?
Before delving deeper into the case for and against hydrogen fuel cells as a means of powering transport, let’s first define what is meant by the term ‘hydrogen fuel cell’.
Mention the words hydrogen and transport to many people, and they’ll likely imagine that the hydrogen is burnt to provide the combustion required to propel a vehicle in much the same way that petrol or diesel does.
The truth is rather different.
The majority of hydrogen-based vehicles on the road today are in fact electric vehicles.
They use an electric motor and powertrain for propulsion, with the electricity for those components being delivered by a hydrogen fuel cell.
A hydrogen fuel cell, then, is rather like a battery.
Using hydrogen as a fuel, a hydrogen fuel cell turns this hydrogen into electricity, which is then used for the vehicle’s powertrain.
So, whilst you may fill up a vehicle’s tank with hydrogen, it’s the fuel cell that does the hard work and converts it into usable energy for the vehicle.
How do hydrogen fuel cells work?
So, how exactly do hydrogen fuel cells work? To provide an ‘at a glance’ explanation, the typical hydrogen fuel cell works as follows:
- The fuel cell contains two electrodes - a negative electrode (also known as an anode), and a positive electrode (also known as a cathode).
- These two electrodes sit either side of an electrolyte (also known as a proton exchange membrane).
- Upon operation, hydrogen (H₂) is fed to the anode, whilst oxygen is fed simultaneously to the cathode.
- A catalyst (such as platinum black or platinum supported on carbon), is used at the anode to separate hydrogen molecules into protons and electrons.
- The newly created protons and electrons then move towards the cathode.
- However, the protons and electrons take different routes:
- The electrons travel along an external circuit, creating a flow of electricity which can be used to power the vehicle’s motor.
- The protons move through the electrolyte before arriving at the cathode, where they meet up with the electrons (which have just passed through the external circuit) and oxygen. The result of this meeting is heat and water. These are the by-products generated by a hydrogen fuel cell.
- The electrons travel along an external circuit, creating a flow of electricity which can be used to power the vehicle’s motor.
What are the different types of hydrogen fuel cells?
The above section simplistically describes how a hydrogen fuel cell works. However, as fuel cell technology has developed, variations on elements of the technology - and the way it works - have emerged.
These variations have primarily concerned the electrolyte (or proton exchange membrane) that is used.
Take the following fuel cell types for example…
Polymer electrolyte membrane fuel cells
Polymer electrolyte membrane (PEM) fuel cells - as their name suggests - use a solid polymer as an electrolyte, and porous carbon electrodes containing a platinum catalyst.
PEM fuel cells offer a great combination of both high power density and low weight - making them ideal for transport applications.
Having said that, however, PEM fuel cells do require a very pure supply of hydrogen. This is because the platinum used as the cell’s catalyst is very susceptible to carbon monoxide poisoning.
Alkaline fuel cells
Alkaline fuel cells are amongst the earliest fuel cells to receive widespread adoption and application.
This type of fuel cell uses a water solution that contains potassium hydroxide as the electrolyte.
Phosphoric acid fuel cells
Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte medium, with the acid contained within a Teflon-bonded silicon carbide matrix.
One of the benefits of this type of fuel cell, is it is typically more resistant to carbon monoxide poisoning than PEM cells.
To date, phosphoric acid fuel cells have been mainly used for stationary power generation, however in recent years, they have been used in large vehicles such as HGVs and buses.
The above are the most common types of hydrogen fuel cell in use at present, however a range of other fuel cells are increasingly finding applications in a variety of fields.
- Direct methanol fuel cells - which use methanol as the anode gas rather than hydrogen.
- Molten carbonate fuel cells - which use an electrolyte composed of a molten carbonate salt mixture, which is suspended in a porous, chemically-inert lithium aluminium oxide matrix.
- Solid oxide fuel cell - this type of fuel cell uses a hard, non-porous ceramic compound as the electrolyte.
How is hydrogen for hydrogen fuel cells created?
One of the main arguments behind switching to hydrogen fuel cells for transport applications is the low-carbon nature of hydrogen.
As we have seen from the above points, hydrogen fuel cells generate only heat and water during operation. This sits in stark contrast to traditional fossil fuel-based internal combustion engines, which spew a whole host of climate changing elements into the atmosphere.
But, dig a little deeper and the contrast between the two fuel sources is less stark.
Is hydrogen as green as its advocates are making out?
The answer is - it depends on how the hydrogen is created in the first place.
Hydrogen is made using a number of techniques, including natural gas reforming/gasification, electrolysis, and pyrolysis. The key point is what energy source is used to power those techniques.
The reality is that these hydrogen-production techniques are powered in a number of ways - some of which are greener than others. Below, we’ve set out the main ways in which hydrogen production is powered.
When hydrogen advocates wax lyrical about the eco-friendly nature of hydrogen fuel cells, it’s generally green hydrogen that they’re referring to.
Green hydrogen refers to hydrogen which uses electricity from renewable sources (such as wind power, solar power etc) for the electrolysis process that creates the hydrogen.
White hydrogen refers to naturally-occurring hydrogen - that is, hydrogen which is found in deposits in its gaseous form.
At present, there isn’t a commercially-viable way to extract and use this naturally-occurring hydrogen, however it is understood that there are a number of companies currently engaged in test drilling.
After green hydrogen, blue hydrogen is one of the most environmentally-friendly ways of producing hydrogen.
Blue hydrogen uses the methane in natural gas as a feedstock in what’s called the ‘steam reforming’ process. This involves bringing together natural gas and steam to create hydrogen. However, the steam reforming process also generates carbon dioxide as a by-product.
The blue hydrogen process maintains its eco credentials by capturing this carbon dioxide via carbon capture and storage (CCS).
This results in hydrogen which has very low carbon emissions associated with its production. According to studies conducted by Volvo, the blue hydrogen production process results in 0.5 kg of CO2 per kg/H₂.
According to the International Energy Agency (IEA), blue hydrogen will become the biggest source of hydrogen by 2050.
A particular novel form of hydrogen is turquoise hydrogen. This is a type of hydrogen which is produced via methane pyrolysis.
Methane pyrolysis involves subjecting methane (within natural gas) to pyrolysis. This pyrolysis is powered by electricity and results in the cracking of the methane. As the molecules of the methane stretch and eventually crack, the result is both hydrogen and carbon.
Unlike blue hydrogen - where the carbon by-product takes the form of a gas - the turquoise hydrogen process results in the carbon by-product taking the form of solid carbon.
The bonus of solid carbon is that it’s a valuable raw material in its own right, which can be used in the production of goods such as tyres.
A rather unusual - and heretofore - little known form of hydrogen is pink hydrogen. This refers to a hydrogen production process which uses nuclear energy to power the electrolysis process.
The production of pink hydrogen is at present largely limited to those countries that have large fleets of nuclear reactors, such as France.
Grey, black and brown hydrogen
Grey, black and brown hydrogen refers to hydrogen which is produced using either natural gas, black coal, or brown coal via either steam reforming or coal gasification.
As you would expect, grey, black and brown hydrogen are not being touted as the future of hydrogen production due to the carbon emissions that are associated with them.
So, as you can see, there are hydrogen fuel cells suitable for light and heavy vehicles alike, and a range of low-carbon hydrogen production methods. So, why aren’t we seeing vast fleets of hydrogen-powered vehicles on our roads?
The answer can be summed up with a single word - infrastructure.
The hydrogen infrastructure problem
For over a century, an extensive infrastructure has developed to cater for the production, storage, and transport of petrol and diesel.
The challenge facing advocates of using hydrogen as a transport fuel, is how this pre-existing infrastructure can be converted to hydrogen use.
Doing this is going to be easier said than done. Below, we’ve set out some of the most pressing challenges facing the build-out of hydrogen infrastructure.
Arguably the most difficult challenge posed by hydrogen is its storage.
Hydrogen can be stored in either a gaseous or liquid form. In its gaseous state, hydrogen must be stored in high-pressure tanks of 350-700 bar. In its liquid form, hydrogen is stored cryogenically.
Temperatures and pressures are only one part of the challenge.
Hydrogen is an ultralight gas, (being around 11 times lighter than the air we breathe). This means it is very diffuse and occupies a much larger volume than other gases.
Consider the following fact - to store a single kilogram of hydrogen requires a storage volume of 11,000 litres (at atmospheric pressure). Even when compressed to 700 bar, hydrogen requires a storage volume of 24 litres.
This means that unless very high pressure, compact storage tanks are developed, hydrogen vehicles will have a limited range (around 1 kg of hydrogen allows a vehicle to travel 100 km).
An additional challenge associated with the storage of hydrogen is leakage. As hydrogen molecules are around eight times smaller than methane molecules, they are far harder to store, requiring much more robust storage solutions than petrol or diesel.
Whilst many of the properties of hydrogen make it safer than other fuels such as petrol or diesel (for example, it is non-toxic), it does have some dangers, too.
Hydrogen has a lower ignition energy than petrol or natural gas, meaning it can ignite more easily. It also has a wide range of flammable concentrations, meaning that leak detection is crucially important. According to research from the American National Standards Institute, hydrogen requires only a tenth as much energy to ignite as petrol does - in other words, a spark of static electricity from an individual’s finger would be enough to ignite hydrogen.
What’s more, hydrogen burns invisibly. This means that special hydrogen flame detectors are required in and around hydrogen storage facilities/fuel stations.
To add to the safety challenge associated with hydrogen, some metals become brittle when exposed to the element.
Despite all of these challenges, efforts are being made to develop safety systems that can handle the challenges laid out above. A good example is Rhino HySafe’s Ultra-Fast Explosion Relief (UFER) system, which is designed to protect fuel stations from hydrogen-related explosions.
Transitioning to a hydrogen-economy, will also require significant training in the safe handling of hydrogen.
This not only applies to the distributors of hydrogen fuels, but the public at large if they are going to be fuelling hydrogen vehicles on a regular basis.
To support a widespread transition to hydrogen-powered vehicles, a fundamental review of our current fuel distribution network will need to be undertaken.
At present the UK Government, as outlined in its UK Hydrogen Strategy, sees hydrogen primarily being used by heavy goods vehicles. This is because HGVs typically operate on a back-to-depot refuelling method, which is compatible with the distributed hydrogen production which is expected during the 2020s.
In terms of a broader roll-out of hydrogen distribution infrastructure, this remains a significant challenge - and one that doesn’t appear to have any immediate answers.
Note - whilst an effective hydrogen distribution infrastructure for transport applications appears to be some way off, great strides are being made in incorporating hydrogen into the national gas grid for home heating etc. The HyNet project is a leading example of this.
The benefits of hydrogen fuel cells
So far, we’ve admittedly painted a fairly bleak picture of the prospects of hydrogen as a transport fuel. And, from an infrastructure perspective, it’s a true and accurate picture.
But, we’d be remiss not to outline many of the benefits associated with hydrogen fuel cells and how these benefits make hydrogen a viable option for the future of transport.
In contrast to fossil fuel reserves, which are depleting across the globe, hydrogen is the most abundant element in the Universe.
Yes, it does require extraction via electrolysis, steam reforming, or pyrolysis, but these techniques have come a long way. Plus, with these techniques increasingly being powered in a low-carbon way, hydrogen is an ideal fuel for a Net Zero future.
Hydrogen fuel cells provide a high-density source of energy. In fact, hydrogen has the highest energy content of any fuel by weight.
This makes it ideal for applications such as heavy transport where energy density is a particular requirement.
Hydrogen fuel cells are also highly-efficient. Especially when compared to traditional fuels like petrol or diesel.
Where internal combustion engines will only convert 40 percent of petrol or diesel energy into locomotion (the rest is lost as heat and friction), a hydrogen fuel cell vehicle will typically use 50 to 60 percent of the hydrogen fuel’s energy - whilst also reducing fuel consumption by as much as 50%.
This makes hydrogen fuel cells ideal for hard-working, long-distance vehicles such as HGVs and buses.
One of the major obstacles preventing the mass adoption of battery-based electric vehicles is their charging times.
Even with the very best battery technology, and ultra-fast chargers, electric vehicles can still take half an hour to charge. On the more basic EVs, charge times can be several hours.
Compare this to hydrogen fuel cells, which can be refuelled in just a few minutes.
This places hydrogen fuel cell vehicles in the same territory as traditional internal combustion vehicles in terms of convenience.
Reduced noise pollution
Due to the way they work, hydrogen fuel cells do not produce any noise. As such, hydrogen-powered cars are far quieter than internal combustion-powered cars, with the majority of noise likely to be road noise - just like battery-based electric vehicles.
Another of the common criticisms of battery-based electric vehicles is their range - or rather lack thereof.
Hydrogen fuel cell vehicles avoid this criticism as they have a similar range to petrol and diesel powered cars of around 300 miles.
Could hydrogen fuel cells be the future of transport?
Assuming that the rather significant infrastructure-related issues can be resolved, could hydrogen fuel cells be the future of transport?
For many heavier types of vehicles, the answer is yes.
Given the back-to-depot refuelling method of larger vehicles such as HGVs, buses and coaches, the transition to hydrogen should be fairly straightforward (albeit, with appropriate hydrogen storage solutions in place at depots).
The UK Government is certainly backing this scenario. In fact, their strategy goes further, adding that in addition to HGVs and buses, hydrogen could even power commercial shipping and aviation:
“By 2030 we envisage hydrogen to be in use across a range of transport modes, including HGVs, buses and rail, along with early stage uses in commercial shipping and aviation. Our analysis shows there could be up to 6TWh demand for low carbon hydrogen from transport in 2030.
Beyond this we expect to see an increased role for hydrogen in aviation and shipping decarbonisation which could become a large component of the overall hydrogen demand in the long term”.
So, are we all going to end up driving hydrogen fuel cell-powered cars?
But it does seem like the buses and heavy goods vehicles of the future will be running on hydrogen…
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