Has the time come for fuel cells to deliver the technology they have long promised, asks Elizabeth Willcocks.
Has the time come for fuel cells to deliver the technology they have long promised, asks Elizabeth Willcocks.
You need to make an urgent call, but your mobile phone has just run out of charge. Wouldn’t it be convenient if you could recharge it instantly, simply by plugging in a small lightweight cartridge? This is one of the many possibilities that hydrogen and methanol powered fuel cells promise, along with laptop computers that run for hours without cables, homes that are powered cleanly and efficiently and cars and buses that don’t pollute the atmosphere.
In the past few years there has been a rush to join the fuel cell bandwagon. Multinational companies, some of which have been involved in the field for many years, such as precious metal specialist Johnson Matthey (JM) and polymer giant DuPont, have identified the potentially huge market for fuel cell applications. Politicians recognise the benefits of the clean technology that fuel cells offer and the media are endlessly enthusiastic of them.
But you might be forgiven for being just a little bit cynical: it’s over 150 years since William Grove first discovered that reversing electrolysis (which is basically what a fuel cell does) could produce electricity and so far fuel cells have only appeared in a handful of uses. These include buses that are still very much at the trial stage and stationary power generators for hospitals, hotels and apartment blocks.
So where and when will we see commercial fuel cells emerge first? ’Ultimately, eyes are on the car market’, explains Lu Potter, business development specialist at Johnson Matthey (JM), ’...the automotive application is what justifies all the investment in fuel cells’. But both Potter and her colleague Graham Hards, manager of business technology, believe that the first commercial fuel cell-powered devices that we will be able to get our hands on are likely to be portable electronics, such as mobile phones, palmtop computers and power tools. And they both agree that it won’t be long before we do.
Last January, the German company Smart Fuel Cell (SFC), which was founded two years ago by Manfred Stefener, began producing a series of fuel cell systems. The Munich-based firm’s first product is a stand-alone power supply for use in remote situations, for example when camping, and uses direct methanol fuel cells (DMFCs). It is equipped with an exchangeable fuel tank that can provide, SFC claims, 2.5kWh of electric energy at up to 100W.
’The fuel tank can be replaced within a few seconds, even during operation’, says Jens Muller, director of research and development at SFC. ’The dream of a fuel cell concept "liquid fuel in - electricity out" has come true’, he observes.
SFC hopes to miniaturise the methanol fuel cell technology further for use in other portable applications like electronic notebooks and camcorders.
Fuel cells can increase the running times for electronic equipment by up to fivefold and can allow it to be recharged in seconds. At the same time they are also lighter than conventional batteries. Such qualities make them ideal for today’s increasingly mobile people, who demand ever smaller and lighter, but longer-lasting, mains-independent electronic devices. But why have SFC and the other companies developing miniature portable electronics, including big names such as Sony, Samsung and Motorola, decided to develop methanol fuel cells rather than the hydrogen fuel cells typically favoured by the automotive industry? The answer is that hydrogen is not easy to use as a direct fuel.
Compared with hydrogen, methanol is more readily available and is both safer and easier to store. A DMFC works in the same way as a conventional hydrogen powered-cell, but it has the added advantage that the methanol can be fed directly to the anode. Methanol also offers a greater energy density - for every mole of methanol you get six protons, compared to two protons for every mole of hydrogen - and so, says Hards: ’has a greater potential for compact applications’. Hydrogen supplies in the short to medium term will come from fossil fuels such as petroleum and natural gas, which must be reformed to extract the hydrogen (as is already done in many petrochemical complexes).
Methanol also has certain disadvantages, however. A methanol fuel cell produces CO2 as a by-product, making it less clean than its hydrogen counterpart, which only produces water as waste. Consumers might also face problems when trying to take their methanol-powered devices onto commercial aircraft, which currently ban liquid fuels in passengers’ cabin or hold luggage. But currently the biggest concern for manufacturers of DMFCs is methanol crossover - a process that occurs readily in the cell and drastically reduces its efficiency.
The problem arises, explains Keith Scott, professor of chemical engineering at the University of Newcastle upon Tyne, because the methanol can diffuse across the polymer electrolyte membrane (PEM) from the anode to the cathode. If the cell isn’t in use then this diffusion process occurs slowly along the concentration gradient. However, once the cell is turned on, the protons moving across the PEM drag the water, together with the methanol, with them - a process called electro-osmotic drag. The electrode potential at the cathode is sufficient to oxidise the methanol to CO2, reducing the cathode potential, and thus the overall power output.
Scott and his team, together with researchers at Cranfield University, are investigating various solutions to the methanol crossover problem. One option, he suggests, is to make the membrane less permeable to methanol. This can be achieved by changing the polymer used in the PEM or by modifying the existing polymer in some way, for example by adding palladium metal. Another solution is to use a cathodic catalyst that is selective to oxygen reduction and so will not oxidise the methanol - possible catalysts are based on Ru/Se and Ru/S systems, says Scott.
DuPont manufactures the Nafion PEM, which it claims is the ’world standard for the fuel cell industry’ and, recognising the potential market for fuel cells, has recently started manufacturing other fuel cell components, including its own membrane-electrode assemblies (MEAs). The company has also been investigating technology that could reduce the rate of methanol crossover. Dave Reichert, the technology marketing manager for direct methanol fuel cells at DuPont, revealed that the company was engaged in scaling up a potential new PEM product that may significantly reduce the problem. Meanwhile, other companies are moving away from DMFCs altogether for their portable electronics. For example, a US firm Lilliputian Systems, in conjunction with Lawrence Livermore National Laboratory, US, plans to develop a prototype miniaturised high-temperature solid oxide fuel cell (SOFC) for hand-held electronic devices, using funding from a federal Advanced Technology Programme (ATP) award.
A driving force
Although the consumer’s first encounters with fuel cells may be portable devices like mobile phones, there is no doubt that the major driving force for their development is their application in the vehicle industry. Cutting harmful exhaust emissions and reducing our reliance on ultimately finite fuels are both issues of paramount importance in today’s world. A fuel cell-powered car could run on hydrogen, which can potentially be derived from renewable resources such as solar powered electrolysis, and the only by-product is water.
The attraction is obvious, yet there are still a number of technical hurdles. For instance, the UK does not currently have a fuel infrastructure for producing hydrogen. The problem may be overcome by using so-called ’bridging technologies’, which involve reforming fuels, such as gasoline, that are readily available in the UK, to produce a hydrogen-rich stream either at the pump or on board the vehicle. Both options present difficulties: as Hards explains, it’s quite complicated to get all the complex reforming chemistry into a system that is compact enough to fit an ordinary car. At the same time, reforming fuel at the pump leads to the problem of how to store the hydrogen in the car. It’s not impossible, but large and heavy tanks would be required to withstand the high pressures that are reached. Potter points out that this option won’t really get car drivers very far in terms of mileage.
Solving the problem of hydrogen storage is thus an important factor to consider in the fuel cell story. The attraction of fuel cells for car manufacturers is not just about taking cars out of the carbon emission debate but also about providing, as JM’s Lu Potter puts it, ’a better product’. Car manufacturers could, for example, use the excess heat produced by the fuel cells. If the fuel is reformed on board it can be used to power, say, air conditioning in the car, while reforming at the pump offers the possibility of powering heaters or chiller cabinets at the fuelling station.
Firoz Rasul, CEO of the Canadian company Ballard Power Systems, appears to agree with Potter. Speaking at last year’s Greenpeace Business conference in London, he predicted that customers would not necessarily buy a fuel cell car because of the benefit it might bring to the environment - they are really interested in what he calls the ’added extras’. Debby Harris, corporate communications manager at Ballard, pointed out some of benefits that a fuel cell-powered car could bring to the customer, including a smoother and quieter ride, and a whole host of electronic devices, including satellite navigation, in-car entertainment hardware and internet access. Such applications require a lot of electricity and in a conventional car would mean a pretty hefty battery as well as the combustion engine. A fuel cell, on the other hand, could power the car and all the electronics directly, which means a lot less weight in the car. Despite the technical problems, fuel cell cars have already been introduced onto the market, albeit in small numbers and as expensive luxuries. BASF, for example, has been keen to advertise its involvement in the development of DaimlerChrysler’s prototype electric car, Necar 5 and Toyota and Ford hope to bring a few tens of vehicles out at the start of this year.
Potter anticipates that by 2015 fuel cell cars will begin to take off and we may see up to half a million of them on the road. However, this is still only a very small percentage of the total number of cars. How soon the fuel cell cars concept begins to make a real mark, says Potter, ’depends on how well the consumer accepts it’.
Home sweet home
As far as domestic (residential) applications go, Potter expects that we might see fuel cell-powered homes as early as 2005. ’Residential applications present more challenging cost requirements’. The consumer will expect the fuel cell to be ’reliable and safe’, she adds, and also at a cost that is competitive with current power prices. Rasul believes that Japan, where the cost of electricity is high, will be the best location initially to introduce fuel cell houses. In places like the UK, where electricity is relatively cheap, the more expensive fuel cell power is unlikely to take off as well as it might in Japan, he explained.
The optimism for fuel cell commercialisation shows no signs of diminishing and investment in their development continues to rise. With a myriad of fuel cell applications coming to the market, we are closer than ever to getting our hands on the real products. It appears that the long-promised fuel cell dream may have finally arrived.
Source: Chemistry in Britain
1. How a fuel cell works
A fuel cell is best regarded as an electrochemical engine and typically consists of an anode and a cathode separated by an electrolyte membrane. Hydrogen is delivered to the anode, where platinum, or a combination of Pt and Ru, catalyses the hydrogen’s ionisation into protons and electrons. Protons migrate through the electrolyte, but the electrons cannot and so they pass through an external circuit to produce the electricity. The protons and electrons recombine at the cathode, where they are oxidised to water. A single cell only produces a few volts of electric potential and so cells are stacked to create the required voltage - a car, for example, might contain up to 500 individual fuel cells.
Various feedstocks could potentially be used to supply the hydrogen for a fuel cell in the short to medium term, for example gasoline, liquid petroleum gas (LPG) or natural gas, but most of these fuels need to undergo reformation to create the hydrogen-rich stream that a fuel cell needs. The raw fuel can be reformed via one of three methods: via an endothermic reaction with steam; via an exothermic partial oxidation; or via a combination of the two methods to balance the heat gain and loss, known as the autothermal mode.
Each method has its pros and cons and the best route depends on the eventual application. For example, the reaction with steam produces a gas stream with a much higher hydrogen content, but the more rapid start-up of the partial oxidation means that the response to demand is faster - an important consideration in the automotive industry when a rapid injection of fuel is required as soon as your foot hits the accelerator.
The resulting hydrogen-rich gas stream - the reformate - normally contains a considerable amount of CO2. This is poisonous to the anode, and so the reformate usually goes through a further two reactions to bring the CO2 content down to a fraction of a per cent.
Ultimately, scientists hope to be able to derive hydrogen from purely renewable resources, for example solar-powered electrolysis of water or from biomass-derived molecules. Until this is achieved, however, hopes for a hydrogen economy powering clean cars remains but a dream.
3. A bio-bright future
Looking ahead, the future for fuel cells looks increasingly rosy with the development of biofuel cells. A biofuel cell works in much the same way as a conventional fuel cell but, instead of inorganic catalysts like platinum, it uses biocatalysts such as enzymes, or even whole organisms, to catalyse the conversion of chemical energy into electricity. Eugennii Katz, associate professor at the Hebrew University of Jerusalem, Israel, believes that biofuel cells could potentially be used in various biomedical applications, for example to power a pacemaker. As medicine comes to rely increasingly on implantable electronic devices for treating a number of conditions there will be a growing demand for the reliable power that a fuel cell can supply. As well as avoiding any maintenance that might require surgery, a biofuel cell could use fuel that is readily available in the body, for example glucose in the bloodstream, and it would ideally draw on this power for as long as the patient lives, explains Katz.
A basic biofuel cell can operate in two ways. It can use biological catalysts - enzymes extracted from biological systems - to oxidise fuel molecules at the anode and to enhance oxygen reduction at the cathode of the biofuel cell. Alternatively, whole microbial cells can be used to supply the biofuel cells with the fuel. In both cases the main scientific problem is the electrical coupling of the biological components of the system with the fuel cell’s electrodes. Molecules known as electron-transfer mediators can provide efficient transport of electrons between the biological components, enzymes or microbial cells, and the electrodes of the biofuel cell. ’Integrated biocatalytic systems that include biocatalysts, electron-transfer mediators and electrodes have been recently developed and applied in biofuel cells’, says Katz.
Katz is optimistic for the future of biofuel cells. ’I think that in the next few years we will see some real applications of biofuel cells with great commercial success’, he said. Although biofuel cells have not yet been considered for portable electronic devices, Katz anticipates that they soon will be, and he goes on to predict even more bizarre applications. For example, Stuart Wilkinson at the University of South Florida, has recently developed gastrobots - robots that power themselves by digesting real food.