Powering Electric Vehicles: Fuel-Cells and Big Batteries
Picture credit: Alstom
The UK government has a manifesto commitment for all cars and vans on the roads to have zero emissions by 2050. So, unless ‘wireless power’ technology can be developed and installed quickly, all cars powered by electric motors will need batteries or fuel cells for energy storage. The political commitment is impressive but seems to be based more on hope than any understanding of the magnitude of the difficulties to be overcome. There are no quick fixes and each of the current ‘green’ alternatives are not necessarily as environmentally friendly as they seem at first sight. Until recently, many people thought that Hydrogen fuel cell technology would ultimately replace old-fashioned rechargeable batteries for mobile applications.
A hydrogen fuel cell is a device that converts chemical energy from Hydrogen (the fuel) into electricity through an electrochemical reaction between it and Oxygen (the oxidiser). It’s constructed in a similar way to a conventional battery: cells consisting of anode and cathode material are separated by a chemical electrolyte (Fig.1). The main difference lies in the way the chemical reactants that produce the electric current are stored. In a primary (non-rechargeable) battery, the chemicals are built-in at manufacture and get ‘used up’ as the battery is discharged. A rechargeable battery has a reversible chemical reaction: it can be recharged by applying a reverse voltage to the terminals for a time. A fuel cell has its active chemicals (Hydrogen and Oxygen gases) supplied from external storage when required. In fact, only Hydrogen has to be stored as air contains enough Oxygen for the reaction to proceed.
How it works
An Internal Combustion (IC) engine releases energy from fuel by oxidising it through the process of burning. A fuel cell also oxidises fuel, but electrochemically, without burning. Burning fuel releases Carbon Monoxide and other harmful products – the fuel cell has only one by-product: water. Hydrogen H2 is fed under pressure into one side of the cell and permeates through the anode until it reaches the electrolyte. A reaction takes place splitting the H2 atoms into electrons e- and positively-charged nuclei H+ (ions). The electrolyte won’t allow electrons to pass through so they stay in the anode and form an electric current through the load. The nuclei, actually just protons, move through the electrolyte until they hit the cathode. Another reaction now takes place as Oxygen, the Hydrogen nuclei and electrons from the load current are recombined to form water (H2O) and release heat. To speed up these reactions a layer of Platinum, a catalyst, is placed on each electrode/electrolyte boundary. The Platinum facilitates a reaction but takes no part in it – just like it does in a petrol car’s exhaust gas catalytic convertor. This explains how the most popular type of fuel cell works – a Polymer Electrolyte Membrane Fuel Cell (PEMFC) – but there are others.
Types of Fuel Cell
All fuel cells essentially work the same way, only differing in the electrolyte used. Table 1 lists the major types in use today together with their essential characteristics:
They all have their particular advantages which I won’t go into now, but if you want to know more, a good tutorial paper on fuel cells can be found here. The question we really want answering is: ‘Are fuel cells better than batteries as a source of power for electric vehicles of the future?’
Fuel Cells versus Batteries: Pros and Cons
- It is, of course, possible to burn Hydrogen as a fuel in a suitably modified car engine, but this chemical method is very inefficient with much energy wasted as heat. The fuel cell is much more efficient, but then so is a rechargeable battery.
- With the current state of technology, mobile fuel cells can’t provide the levels of instantaneous power required by electric motors for rapid acceleration. Just as the power of an IC engine is dependent on the speed with which the correct fuel-air mixture can fill the cylinder and how fast the waste gases can be removed, so the fuel cell output current depends on the rate at which Hydrogen can be processed and ‘converted’ to electricity. The problem of output is being tackled by clever shaping of the catalytic interface, greatly increasing its area. Current fuel cell vehicle designs also feature a small conventional battery to provide fast power delivery when sudden acceleration is demanded.
- Although the fuel cell has ‘zero emissions’, there is the question of how the Hydrogen is created in the first place. Unlike Oxygen, it’s not present as a free element in the air. It can be obtained using a process called Electrolysis to split water molecules (H2O) using electricity. Obtaining Hydrogen by electrolysis is very expensive. Instead, Hydrogen is usually produced in bulk by the ‘Steam Reforming’ of natural gas or Methane – a very complicated process requiring a large amount of heat. Unfortunately, CO2 is also produced as a by-product. The lack of a process to produce Hydrogen without unwanted by-products is problematic and makes the rechargeable battery alternative seem more attractive. Of course, this assumes that all the required electricity for recharging such a battery will be derived from ‘renewable sources’. If all vehicles are to be electric by 2050 that concept is probably borderline fantasy. Local power grid infrastructures are also likely to need uprating to support whole streets of consumers charging their cars at the same time!
- Fuel cells are easy to scale in both voltage and capacity. Like batteries, cells can be ‘stacked’ in series to create the desired output voltage. Unlike batteries, but like IC engines, fuel cell capacity is easily increased by using a larger fuel tank. Scaling a battery for increased capacity is a much more difficult design exercise. However, a litre of petrol (gasoline) contains far more energy than a litre of Hydrogen – even when liquified. Given current technology, a car may not be able to carry enough Hydrogen to give it the same range as an equivalent IC engine or even a battery-only vehicle.
- Relative to a battery, a fuel cell system is more complex and thus very expensive to make. For the time being, this factor alone is enough to limit their use in automobiles. Costs are falling, however, as the technology is refined. For example, efforts at finding a replacement for Platinum as the catalyser are proving fruitful.
- The lack of a refuelling infrastructure for fuel cell cars is often cited as a major obstacle for consumer acceptability. That, and the attractive concept of charging a battery car at home using ‘cheap’ electricity. However, uprating local cabling, not to mention the installation of a special electricity meter in every home will be very costly. I don’t believe the UK government is prepared to lose all that duty (tax) revenue if petrol/diesel fuels are banned, so a way will have to be found for that revenue stream to be maintained. Hence the concept of a separate meter. All this could be avoided if your car could be refuelled with Hydrogen at a conventional petrol station. But that too would require considerable investment from somebody.
- The issue of safety is frequently brought up in discussions about mobile fuel cells – all that explosive Hydrogen! Well, petrol is just as dangerous and we’ve got used to that. In fact, Hydrogen, a ‘light’ gas which dissipates quickly in the event of a leak, is safer than petrol vapour which sinks into hollows like the boot floor just waiting for a spark to ignite it. Batteries are not ‘safe’ either. Both Lead-Acid and Lithium-Ion are prone to catching fire or exploding if short-circuited or seriously overcharged. There have been a number of aircraft and electric vehicle fires recently caused by overcharging or accidental damage to Lithium-Ion batteries.
- Recent research has highlighted a problem with fuel cells that use air as the oxidiser. Air is mostly Nitrogen, a small proportion of Oxygen and a little Carbon Dioxide. What about the effect of trace pollutants such as nitric oxide, nitrogen dioxide, ammonia and sulphur dioxide? The research report’s conclusion is that some caused temporary power loss, others permanent damage. Some of these gasses come from IC engines, so it might be a problem that solves itself.
- A concern with both the common rechargeable battery technologies is the limited lifetime, which can be made worse by the particular operating conditions in use. Lead-Acid batteries do not like being deeply discharged or being left for long periods in a discharged state. In a well-maintained IC engine car, the battery never has to endure either of these conditions. When used as a traction battery which frequently undergoes ‘deep-cycling’, it may have a short life. Lithium-Ion chemistry replaced Lead-Acid for high-power applications years ago because of its better energy density and lighter weight. A price has to be paid for this better performance: Lithium-Ion batteries require tightly-controlled charging/discharging and can age (lose capacity) with repeated deep-cycling or strangely if stored in a fully charged state.
And the winner is…
At the time of writing, the general consensus is that the best power source for electric cars has to be the Lithium-Ion battery. Fuel cells are deemed to be too complicated and too expensive to be practical. Fuel cell technology is improving and there are other applications: a static installation providing power to an individual home is definitely possible, especially if waste heat can be put to good use in cold climates.
Meanwhile, the electric railway train in the heading picture has just started running a passenger service in Germany. And guess what? It uses Hydrogen fuel cells as a power source.
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Responding to a comment on Twitter concerning fire extinguishers on electric vehicles. Great deal of confusion around about the best way to tackle 'Lithium' fires. Non-rechargeable Lithium metal batteries - those tiny 3V coin cells for instance - will react badly if broken open and exposed to water. Dry powder (Class-D) extingiushers only. Lithium-Ion batteries contain very little Lithium metal so can be dowsed with just about anything: water, foam, powder even sand. Preferably large quantities of water to cool down adjacent cells and stop them from exploding. However, if smoke suddenly starts pouring into the car the only advice is to stop, get out and call the fire brigade. If they arrive quickly and can gain access to the batteries, a flood of water may just confine the fire to a single cell and stop others exploding. Fires like this are very rare and are caused by an internal short-circuit causing massive over-heating. The other scenario is when an accidental collision physically damages one or more cells causing short-circuits. The fire in either case may prove stubborn to put out completely while short-circuits remain.
As for hydrogen fuel cells, again the best advice is to move well away from the car and call the fire-brigade!
Power Grid would need to have support from Wind and Solar energy. Hoping in the future I rather put up solar panels on my roof and building roofs instead of tar and shingles. That would provide a large boost of energy during sunny days. The power would be localized for those electric vehicles that need to recharge in those areas.
Don't hear too much about how the power grid is going to tolerate all the new load demands. How green is the extra power generation going to be against low emission combustion engines?
@David Woodbridge Recent research in the US suggests that fast-charging an EV battery in 10 minutes will require 300 to 400 kW of power, so several cars at a 'fueling station' will need about 1MW. Forget about 240 volts - such a facility will have to get it's electricity from the 11 or 33 kV grid! I very much doubt that the UK has, or will have, anything like enough generating capacity to handle those sorts of numbers nationwide.
Great article, hydrogen is my thing so I'm biased what 'the answer' should be but...
There is light at the end of the tunnel - Dr. John Bannister Goodenough, who is attributed with the invention of the lithium-ion battery, has invented its replacement based on sodium using a glass electrolyte (with the help of his team). It has much superior charging and cycle characteristics and being glass, won't short and burn if mechanically damaged. The sodium is in plentiful supply (sea water) so we don't need to waste vast swathes of China. Best of all, Dr. Goodenough released his research findings for the good of everyone at a recent TED talk. https://www.allaboutcircuits.com/news/john-bannister-goodenough-inventor-lithium-cobalt-oxide-cathode/
Interesting article. Another significant problem with personal BEV's is the fact that some 40% of homes would require a charging lead of significant length to trail over the public highway/pavement. Such properties do not include a meaningful driveway to park a car.
Have you heard of lithium titanate batteries these can be charged and discharge very fast and are safe they csn ne punchured with nails, crushed even shorted and do not explode, these cells are created nano technology and I see them as the answer to a lot a problems including battery life.
@de-bug Yes, a number of companies are selling a Li-Ion battery where the anode has been coated with Li-titanate nanocrystals instead of carbon to massively improve electron flow. This enables much larger discharge currents and very fast charging. The downside is the cell voltage of 2.4 volts as opposed to the usual 3.7. That means the battery is much larger and heavier (more cells) for a given output than the 'normal' Li-Ion equivalent! Still, there is much on-going research in the area of battery technology and there will be other 'solutions'.
Would Carbon Capture programmes also be an alternative? They can produce carbon Neutral fuels. That way we could stick with the combustion engine for the time being and additionally reduce the carbon emission's in the atmosphere through capture. If we had enough Capture facilities. With regards to global warming, surely this is a good alternative - as mining for precious metals / mass production of a next generation of cars is just as detrimental to the Environment. Carbon Engineering (Canadian Company) seem to be pioneering this technology.
@cgray123 Carbon Engineering's Air to Fuel concept makes great sense for a country without significant fossil fuel reserves of its own, but I doubt whether it will have much impact on atmospheric CO2 levels. After all, the process captures carbon only to have it quickly released again when the fuel is used. There is in fact a cost - the energy used and stored by the conversion process itself which then reappears as the fuel burns.