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How does a fuel cell work, and should I buy a hydrogen car?

Andrew English
The hydrogen fuel cell has been around since the 19th Century but, as Andrew English explains, it has an important place in the 21st

It was Welsh scientist and justice of the peace Sir William Grove who, in 1839, discovered the principle of producing electricity from an electro-chemical reaction between hydrogen and air.

He called it the 'gas battery', though what we now know as the fuel cell wasn't really a feasible electricity producer until the mid 20th century. In 1955 General Electric’s Willard Grubb and Leonard Niedrach produced the first hydrogen-and-oxygen fuelled, proton-exchange membrane (PEM) fuel cell, the type used in almost all automotive applications. It was one of these PEM fuel cells that was used by Nasa to generate power for the Gemini project in the 1960s.

A fuel cell is effectively an electricity-generating device using chemical reactions. There are lots of different types of fuel cell but the chemical reaction we are looking at here is that of combining hydrogen and oxygen to produce water and energy in the form of electricity.

It might not look like much, but this little machine could hold the key to a sustainable future Credit: Andrew Crowley 

The cell consists of a couple of electrodes; a positively-charged anode, and a negatively-charged cathode. These are in contact with an electrolyte, which has a synthetic polymer membrane to keep the oxygen and hydrogen apart and only permit the passage of certain types of ions.

So hydrogen atoms enter the cell at the anode side, where they are stripped of their negatively-charged electrons by the platinum catalyst, and then pass through the anode, travelling round an external electrical circuit to give electricity.

The positively charged hydrogen ions travel through the centre of the electrolyte membrane to the cathode side, where they are be joined by the oxygen atoms and rejoined by the electrons, which have travelled 'round the circuit. The output of the cell is therefore electricity, heat and water.

Each cell produces only a tiny amount of current so many cells are stuck together in what are known as 'stacks'. In these an ink consisting of the carbon, catalyst, and electrode are sprayed on to the solid electrolyte with carbon end papers which act as electrodes.

Filling up a Toyota Mirai, a hydrogen fuel cell car, at Chobham services on the M25 Credit: Andrew Crowley 

The beauty of the fuel cell is its theoretical simplicity and elegance, water-to-water (2H2 + O2 = 2H2O) but getting there isn't so simple. Hydrogen is the most plentiful element in the universe, but it doesn't exist as a free gas, instead forming strong bonds with other elements. As a result it's difficult and expensive to produce.

"Hydrogen is like money," said one expert, "it is all around us, but it doesn't necessarily belong to you."

The advantages of hydrogen fuel cells over internal combustion engines and battery-electric motors as vehicular propulsion include emissions, range and charging time. The hydrogen fuel cell car emits only pure water at the tailpipe, which is clean enough to drink and therefore does not contribute to local pollution. It only takes a minute or two to fill the tank of a hydrogen car, putting it broadly in line with a petrol or diesel car, and the range from this charge will be several hundred kilometres – something that could take hours for a battery-electric car to achieve.

hydrogen car puff

One kilogram of hydrogen has the energy equivalent of one US gallon of petrol, so by weight, hydrogen has a pretty efficient bang, but depending on how you store it, its volume can be up to four times bigger than petrol. It's expensive and energy-intensive to store as a gas where it can cost up to 50 per cent of calorific value of the gas to compress, still more so when stored as a liquid at cryogenic temperatures, when it also boils off and vents to the air. It's also highly inflammable, loves to escape from containment and causes embrittlement of metals.

Extraction of sustainable hydrogen from water via a process known as electrolysis, which is basically a fuel cell operating in reverse, requires electricity. The source of this can be something as clean as a hydroelectric dam or as dirty as a diesel generator; a decentralised network of electrolysis plants powered by renewables is possible, but H2 will inevitably involve consumption of fossil fuels for some time.

The fuel cell only produces direct current so further losses are experienced in inverting the current into alternating current suitable for efficient vehicle drive motors. Many of these problems are being overcome, particularly by car makers such as Toyota and Hyundai.

Hydrogen | is H2 the fuel of the future?

Another issue is that a fuel cell will only operate with water, not steam or ice, so temperature management is essential. Heat has to be carefully dispersed, which is why fuel cell cars often have so many cooling ducts. And water needs to be carefully drained from the cell after use and restarting in very low temperatures has been a major hurdle, although Toyota claims its Mirai fuel-cell car will restart at temperatures as low as minus 30 degrees celsius.

There are plenty of naysayers about fuel-cell technology and you can see them scoffing on the internet, but most are either ignorant or are heavily invested in alternative technologies such as battery vehicles. It might have been some time in coming, but fuel cells are here and their number will grow as part of a wide variety of automobile power solutions of the future. Japan is wedded to the technology and its car companies have invested heavily, not just on developing fuel cells but also in the infrastructure of hydrogen filling stations.

Should I buy a hydrogen fuel cell car?

Probably not yet, writes Ed Wiseman. Unless you live close to a reliable hydrogen filling station, you're likely to find the UK's embryonic recharging infrastructure a tad frustrating. There are only around four public stations in the London area, with a further handful of limited-access pumps. The wider UK's network is growing but still sparse.

The majority of hydrogen adoption thus far has been in 'captive' fleets; groups of vehicles based at a single depot or around a specific geographic area. Police forces and local authorities were among the first to build fuel cell fleets, from scooters to road sweepers, with efforts underway to introduce hydrogen to London's minicab operators.

These early adopters are part of the process that makes hydrogen infrastructure economical – the so-called 'chicken-and-egg' situation means that it's unviable to build H2 refuelling stations without any customers driving fuel cell cars, but foolish to buy a fuel cell car with nowhere to charge it up. You do see the Toyota Mirai out in the wild from time to time, but we estimate the UK's entire hydrogen fuel cell fleet to be in the low hundreds. 

It's hard to tell when this will change. The new generation of fuel cell cars will go a long way to making hydrogen mainstream; we drove the Hyundai Nexo in London the other day and absolutely loved it. Indeed, the Nexo's claimed 497-mile range (easily twice that of most plug-in battery-electric cars) will impress consumers and alleviate some of the pressure on infrastructure – if you only have to refuel every 500 miles, and it only takes a few minutes, suddenly a fuel cell car is a practical choice rather than an environmental one. 

Advantages of hydrogen over battery-electric systems are numerous. Ultimately, though, widespread household ownership of fuel cell cars is likely to come after the adoption of hydrogen in other applications. Uptake by (for example) haulage and fleet operators will fuel infrastructure development, which will make fuel cell cars easier to recommend to the average family. And as prices for both the fuel and the cars start to fall, those machines will make more compelling economic sense. 

At the moment, the Hyundai Nexo is by far the most accomplished hydrogen fuel cell car. It's one of the best models in its category full stop, and probably our favourite zero-emission car of all time. Sales will commence in Britain at the start of 2019 – we'd recommend waiting 'til then if you're in the market for a FCEV.