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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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