Fuel cell vehicles

Background

A fuel cell electric vehicle (FCEV) is an electric vehicle (EV)—that is, a vehicle propelled using only electric motors—powered by the electricity generated by passing fuel and oxidant through a fuel cell.

So what is a fuel cell?

A fuel cell creates an electric current from the catalysed reaction of a fuel (say, hydrogen gas, H₂) and an oxidant, often oxygen (O₂) from air. First demonstrated in the early nineteenth century, modern fuel cells are based on the hydrogen-powered alkaline or "Bacon" fuel cells invented in the 1930s by English engineer Francis Bacon.

Most fuel cells produce low voltages, typically less than one volt, so they are usually arranged in series into fuel cell "stacks" for practical high voltage use such as powering vehicles.

Fuel cell efficiency

Fuel cells can have high thermal efficiencies of up to 85% in combined heat and power (CHP) applications, despite having lower electrical efficiencies, but this means running at very high temperatures. Conversely, the smaller fuel cell designs suitable for vehicles have a higher electrical efficiency and operate at more manageable temperatures.

The fuel cell stack powering the 2019 Hyundai Nexo has a reported 60% electrical efficiency.

Cost

Fuel cells are very expensive. This is mainly because they rely on platinum-group metals as catalysts for the REDOX reactions that extract electrons from the fuel without overtly combusting it.

Platinum group metals are very expensive.

They are very rare, and difficult to refine. The huge increase in demand that would be needed for fuel cell vehicles globally would only make them even more expensive.

In contrast, the principal limiting metal in the production of battery electric vehicles, lithium, is abundant and dirt cheap, even considering the similar huge increase in demand anticipated over the next few decades for batteries in EVs and grid scale applications.

Hydrogen as a fuel

Hydrogen is an attractive fuel for many reasons. It can be produced from water through electrolysis, and when combusted produces the same amount of water it was derived from. There are however a lot of problems to overcome for it to be a sensible option for powering private transportation.

Hydrogen interests are fossil fuel interests

First and most significantly of all, water electrolysis is expensive because it requires huge amounts of electricity. Consequently, over 95% of the world's hydrogen is produced from fossil fuels. Specifically, from the steam reformation of natural gas (methane), which is approximately:

CH₄ + H₂O → 3H₂ + CO

The carbon monoxide is used in further industrial/chemical processes, or burnt to produce energy, which produces CO₂.

This means of course that currently, powering your car with hydrogen is effectively powering it with natural gas, but in a much less efficient and far more expensive manner than if you had just powered it with methane directly.

Hydrogen is being researched and hyped by the fossil fuel industry, desperate to re-purpose their colossal global infrastructure assets, which are about to be ruinously stranded over the coming decades by the highly disruptive combination of exponential growth in solar photovoltaics, electric vehicles, battery technology advances, and ongoing global fossil fuel divestment.

Hydrogen is thus a massive waste of energy, resources, investment and time, for very little gain.

Power density

Hydrogen fuel tank in a Hyundai Nexo

Hydrogen is the lightest element, so while its energy-to-mass ratio is impressive, its very low density works against it. To store hydrogen at the sorts of high power-to-volume efficiencies that we are used to with liquid hydrocarbon fuels, we need to compress it in tanks capable of extremely high pressure, or as a cryogenic liquid kept at -252°C, which is practical only with space programme budgets.

For example, travelling 600 km in a modern FCEV requires only 4-5 kg of H₂ gas to power the fuel cell, but to store this seemingly modest amount of fuel on-board requires an 80 litre tank pressurised to 70 MPa, three times higher than a typical CNG tank.

Safety

As you can imagine, not everyone wants to drive around sitting on a tank of explosive gas compressed to 700 atmospheres. Reliable safety data for how such storage tanks perform in accidents and collisions is not available.

With a detonation velocity at standard pressure of about 3-4 km/s, hydrogen is highly explosive and thus much more dangerous in failure situations than petrol, given that at the point of any detonation it will already be a rapidly decompressing gas mixing with air across a large expanding surface area.

The safest and most power-dense way to store hydrogen remains bonding it to carbon; hydrocarbons have a very high energy-to-volume ratio, are stable liquids at room temperature, and are relatively safe when handled correctly, even in failure situations.

Loss from leaks, transportation and storage

As already mentioned hydrogen is the lightest element, its nucleus consisting of a single proton. It is very difficult for anything made of matter to be any smaller. This means that molecular hydrogen gas, H₂, being the smallest molecule that can exist, will inevitably escape through the walls of all storage devices, given sufficient time. Consequently, loss through the process of generating, pumping, transporting and storing hydrogen is both measurable and significant.

Damage to the ozone layer

Since H₂ is also far too light to be held by Earth's gravity, it escapes upwards through the atmosphere and into space. On its way to space it passes through and damages the ozone layer, where it permanently reacts with ozone to form water:

3H₂ + O₃ → 3H₂O

This is an irreversible (non-equilibrium) reaction. It also never gets mentioned anywhere, and seems to only be known to meteorologists.

Overall efficiency

To measure this we have to take into account where the hydrogen comes from. Currently, hydrogen fuel is just refined natural gas, with the CO₂ emissions occurring at the point of production.

For a hydrogen-powered fuel cell vehicle to play the sustainability game, its hydrogen must be sourced from the electrolysis of water, which requires a large amount of energy, which we'll call e. Specifically, hydrolysis requires electricity, and involves loss due to DC conversion and heat, so only about 70% of the input energy is available as hydrogen fuel.

Note that already, before the hydrogen has even been siphoned, pumped, compressed and transported to a nearby hydrogen filling station, the battery-powered electric vehicle (BEV) can use 100% of the generated electricity.

After generation of electricity and hydrogen fuel:

• FCEV: e = 70%
• BEV: e = 100%

We now have hydrogen gas which needs to be pressurised and transported to your local filling station, each at about a 10% loss (either through actual loss of gas or the consumption of extra energy). For the sake of argument let's assume this is all done using more borrowed electricity, which leaves us with just over half of the energy spent available as hydrogen fuel.

Meanwhile the full unit of electricity is transmitted to your garage at about 5% transmission loss and stored in your battery EV with about 85% efficiency; there is some loss from charger equipment AC/DC conversion and as heat.

After fuel transportation and fueling/charging the vehicle:

• FCEV: e = 56%
• BEV: e = 80%

Now we consume the on-board energy to propel the vehicle. The FCEV converts the H₂ fuel into electricity with 60% efficiency at best, which powers the electric motors which are about 90-95% efficient. Meanwhile the BEV simply dumps the battery charge directly to the motors, at about 90% efficiency overall.

At the point of propelling the vehicle forward:

• FCEV: e = 31%
• BEV: e = 72%

Compared to a fuel cell EV, a battery EV is more than twice as efficient, and this is the best imaginable scenario. In reality, compression and transportation of fuel gases is done with existing ship rail and road infrastructure, which currently involves high usage of fossil fuels and thus CO₂ emissions.

It is a sad fact that in 2020 most of the world's electricity is still generated by burning fossil fuels and emitting CO₂. Using this electricity to refine water into hydrogen, just to power a fuel cell powered electric vehicle at a dismal 31% efficiency is not really helping, when you could just replace the hydrogen tank and fuel cell with a big battery and go more than twice the distance.

Comparison with petrol-powered cars

A battery EV also wins against petrol-powered cars even when we burn all of the petrol to generate its electricity, because while the combustion emits CO₂, it is still far more efficient to dump the electricity into an EV battery directly than to use trucks to transport petrol to filling stations and use it to power an internal combustion engine. Even in 2020 most car engines operate at a pretty terrible 20-25% efficiency.

Many pundits think we are truly witnessing a revolution in transport not seen since the disappearance of steam traction engines and horses a hundred years ago; it's just that nobody in 2020 has noticed yet, the same way nobody noticed the internet in 1995.

Is there still a case for Hydrogen?

Yes.

Hydrogen fuel cells make more sense at scale, where batteries would take up too much space or weight, or when connection to the grid is not practical. On large vessels, they could replace the use of fossil fuels and positively transform entire sectors, such as shipping and aviation. They also have potential for use in large scale CHP applications, remote installations, and certain grid-scale operations.

Glossary

BEV

Battery Electric Vehicle. An electric vehicle powered with electricity stored in an on-board battery.

CNG

Compressed Natural Gas.

EV

Electric Vehicle. A vehicle propelled by electric motors.

FCEV

Fuel Cell Electric Vehicle. An electric vehicle powered with electricity generated from a fuel cell.

References

• Naam, Ramez, May 2020. "Solar’s Future is Insanely Cheap"
• Voelcker, John, October 2018. "2019 Hyundai Nexo: first drive of 380-mile fuel-cell crossover utility", Green Car Reports.
• Smith, Colin, 2 September 2020. "Hydrogen cars and their future in New Zealand", Stuff.
• "Current Hydrogen projects in New Zealand", Hydrogen New Zealand Association
• Moyle, M. P., et al., March 1960. "Detonation Characteristics of Hydrogen-Oxygen Mixtures", AIChE Journal, American Institute of Chemical Engineers.
• Ball, P., June 2003. "Hydrogen fuel could widen ozone hole", Nature News. DOI: 10.1038/news030609-14
• Bossel, Ulf, October 2006. "Does a Hydrogen Economy Make Sense?" Proceedings of the IEEE 94:10.