Hydrogen fuel cells
Hydrogen fuel cells operate by reacting hydrogen and oxygen gases together to make water. This process is exothermic – it releases usable energy. Although it uses both hydrogen and oxygen, it is called a hydrogen fuel cell because hydrogen is a fuel that’s burnt in the presence of oxygen to make water, exactly the same way that a fossil fuels burn, except that there’s no carbon dioxide made from hydrogen.
Hydrogen is considered a green fuel, and because of this it is an attractive future energy source. Scientists have envisaged a future where hydrogen gas could be a major source of everyday energy, reducing greenhouse gas emissions, reducing the problem of climate change, and letting us use our televisions and computers to our hearts' content.
The hydrogen fuel cell is a simple bit of kit. It’s lightweight, with no moving parts, so doesn't malfunction easily. To work, hydrogen loses electrons that then travel round an electrical circuit to join up with oxygen atoms, producing hydrogen ions (or protons) and oxygen ions. The hydrogen ions are small enough to travel through a semi-permeable barrier to where the oxygen is. Here, they react together to produce pure, clean drinking water.
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| Fuel cell. H2 is catalysed to H+ at the anode; electrons travel through the circuit to provide a direct current and meet oxygen gas at the cathode, forming O2-. H+ ions then travel through a semi-permeable membrane to combine with O2- ions and make water. Image via © TWDK. |
Hydrogen: sourcing the resource
Hydrogen gas is not found in nature as a raw resource. Sure, there's a lot of it in the sun, but that's a very long way to import from, and not the safest nor most energy efficient journey. There is some of it in the air – but much much less than 1%. Hydrogen ions dissociate in solution when an acid is dissolved, but these ions have already given away their electrons, and cannot make water with oxygen (they can make water with alkalis, but no current flows round a circuit: this is roughly how batteries work).
In practice, hydrogen fuel is made rather than found. This isn’t necessarily a problem: sometimes it’s cheaper and less damaging to the environment to synthesise something in the lab rather than harvest it. You can make hydrogen gas in a lab by sticking an alkali metal, like potassium, in water The products are the alkali metal hydroxide and hydrogen gas, which burns with a squeaky pop. The reaction gives off extra useful heat energy (so much so the reaction is dangerous), but the alkali metal starting materials are neither easy nor cheap to procure, and this isn’t how hydrogen is made industrially.
Another way to make hydrogen is electrolysing water – running a current through it to form oxygen and hydrogen. There is, afterall, water everywhere. This is essentially the reverse process to what happens in a hydrogen fuel cell, when oxygen and hydrogen combine to make water, releasing energy. Splitting water costs the same energy that will later be released, and also produces pure oxygen (although there is plenty of that in the air around us, and theoretically a hydrogen fuel cell can work in air, 21% O2). However, water must first be purified to remove salts which could change the electrolysis reaction. This means it is a chemical energy store, or battery – useful for preserving energy we can't use now to selectively release it later – but it’s not a generator. To reduce the energy cost, the oxygen can be mixed with a fuel to provide extra energy as it burns: this is called chemically assisted electrolysis. However, some people are concerned that these processes waste clean drinking water.
Methane can also be used to make hydrogen gas, and in fact 95% of the hydrogen made industrially for uses like the Haber process comes from methane. In theory, this could be made from any hydrocarbon, but methane is the most efficient, with a higher hydrogen to carbon ratio and a lower boiling point. It contains one carbon atom and four hydrogen atoms, so you can make twice as much hydrogen per methane molecule as water, which only has two hydrogens. This still costs energy, but is more efficient than electrolysis. At temperatures above 700°C, steam is reacted with methane to generate syngas, a gas containing lots of hydrogen and carbon monoxide. As the temperature is lowered to 360°C, the carbon monoxide reacts with water again, making carbon dioxide and more hydrogen, this time taken from water rather than methane molecules. This second reaction is exothermic and provides energy. However, not only is methane a fossil fuel, producing carbon dioxide when it is burnt, but most of the energy required to create high temperature steam comes from burning other methane: hydrogen fuel is not yet a clean resource.
One carbon neutral alternative is using landfill gas, methane released from our waste that we have easy access to. Unlike methane formed from natural decay, which is found in small, distributed pockets, often deep underground or underwater, we have good access to methane from landfill sites. If unused, this waste methane will eventually escape into the atmosphere and contribute to global warming.
Hydrogen: the problem of size
Hydrogen fuel is also attractive because it’s light. 7 kg of hydrogen produces as much fuel energy as 21 kg of petrol – almost enough to fill a 60 litre petrol tank in a car. This is because hydrogen is a gas and the molecules are small. However, gases also take up a lot of space. 1 mole of any gas occupies 24 dm3, no matter how heavy it is: it only depends on how many molecules there are. Molecules like their space. 7 kg of hydrogen gas would take up 84 m3, whereas as a liquid, it would only occupy 0.1 m3. This would be equivalent to a car with a 60 litre fuel tank carrying a balloon of hydrogen gas with a radius greater than the length of the vehicle. Since 1 m3 of hydrogen gas lifts 1.1 kg of weight, 14 petrol tanks provide enough upwards thrust to lift an empty Ford Focus. Unsurprisingly, this creates a problem for storage and transport.
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To-scale model of a car with hydrogen fuel balloon. Hydrogen gas might be light, but it is enormous (impactical on a windy day). Image via © TWDK. |
Hydrogen fuel may not be that practical for cars in its current form, but it is used on spacecrafts, where volume is not a problem, but weight is. It could also be used in stationary locations like homes and industries (providing we have enough storage space on our roofs!).
Hydrogen: moving it about
Transporting hydrogen gas isn't easy. We could pipe it, much like natural gas used to cook in gas ovens, or carry it in trucks across the roads network. Pipes would require new infrastructure, with a high capital cost and inital delay, and would be much more hazardous than natural gas because hydrogen is significantly more explosive. To transport hydrogen in trucks, hydrogen could be condensed to a liquid using high pressures to crush the hydrogen molecules together until they are so close they start to act like a liquid. However, high pressures are extremely dangerous. ...Imagine a hot day, the metal truck heats up… and the hydrogen starts to boil and expand. Eventually, the pressure build up from the expanding gas will cause the truck to rupture – and explode. For this reason, transport vehicles are designed with deliberate leaks, which let excess hydrogen boil off to avoid accidents. About 4% of hydrogen is lost per day – not a very efficient method.
Whilst it seems unlikely we could have a future world powered entirely by hydrogen, it might work mixed with other power sources. Hydrogen fuels cells are great for supporting kinds of energy generation that make energy that must be used immediately, like solar panels and wind turbines. Hydrogen fuel cells can store that excess energy as chemical energy and later release it on demand, such as during the middle of the night. Any leftover energy can then be used to split water into hydrogen and oxygen fuels – electrolysis – to supply the fuel cell later.
Current research continues to investigate ways to make a hydrogen future more viable. Current fuel cells are only 60% efficient and cost 1000 times more than liquid hydrocarbon fuels per energy unit. Some researchers are investigating ways to store hydrogen in solids, which are more densely packed than liquids. The frameworks need to be light and the hydrogen needs to be bonded within the structure, but readily released to supply the fuel cell at the operating temperature of the engine. Metal hydrides and amides are two possible prospects, and researchers are enthusiastic.
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