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Fuel cell yet to breakthrough despite holding great promise

Fuel cell yet to breakthrough despite holding great promise

The fuel cell is one of those technologies that never quite makes it to the mainstream. Yet it is almost as old as the battery that many want it to replace.

Sir William Grove, who devised the first one in 1839, called it a 'gas voltaic battery'. Knowing that it was possible to split water molecules into oxygen and hydrogen by passing an electric current through the liquid, he found it was possible to reverse the process: combine oxygen and hydrogen to produce water and electricity. It took 50 years for the term 'fuel cell' to appear, when Ludwig Mond and Charles Langer worked on developing a commercially viable version.

NASA has used fuel cells for decades. But the alkaline fuel cells employed in the space programme had one clear design objective: to provide high energy density. The water they produced was also handy side benefit: it could be drunk by the astronauts. Operating cost, however, was not an primary concern.

Devised by British engineer Francis Thomas Bacon and subsequently named after him, the alkaline fuel cell (see fig 1) uses a solution of potassium hydroxide in water and stored in a porous solid matrix. Oxygen and hydrogen are pumped in as a charge is applied.



Hydrogen introduced at the anode decomposes into protons and electrons. The electrons pass through an electrical connection to the load; the positive ions combine with negatively charged hydroxyl ions that pass through the electrolyte solution to produce water that is then pumped out of the system.

Unfortunately, CO2 and even water itself poison the process, so the cell has to be sealed from the atmosphere. Because the unit has to be sealed, NASA could get away with using asbestos as the matrix for the electrolyte chemical. The catalysts needed for the cell can also be pretty cheap, using metals such as nickel. The Apollo programme employed platinum for its higher efficiency.

Overall, the cell is very efficient: only around 30% of the energy it generates is lost. But the cost of sealing and maintaining the purity of the chemicals needed makes the alkaline design an unlikely candidate for the fuel cell of the future.

One way around the problem of alkaline fuel cells is to change the electrolyte. The approach that the US Department of Energy reckons will work best for future vehicles is the polymer-based proton-exchange membrane. Instead of using a porous ceramic soaked in electrolyte, this uses a thin plastic membrane not unlike clingfilm in overall look and feel, although it is normally a fluoropolymer like Teflon rather than polyethylene. The idea is to allow protons to pass through the membrane, but to block electrons so electron transfer occurs only by completing an electrical circuit from anode to cathode.



Fluoropolymers are good electrical insulators, but can be turned into ionic conductors. For example, DuPont's Nafion puts sulphonic acid groups onto a Teflon polymer backbone. Protons on the acidic nodes can hop from one to the other and, in an electric field, will tend to move one way through the membrane towards the cathode.

The fuel cell can work at relatively low temperatures compared with other designs: 100°C is normal for a proton-exchange membrane design. Because the electrolyte can be reduced to a thin membrane, the cell is relatively compact and easy to scale up for large power densities. It also starts up quite quickly and is not poisoned by gases such as carbon monoxide. These factors make the proton-exchange membrane cell (see fig 2) the most likely option for hydrogen powered vehicles, even though the 50% operating efficiency for today's designs is much lower than the Bacon cell's.

The big problem with the proton-exchange cell is its reliance on hydrogen. As a component of water, hydrogen is hardly scarce, but it takes a lot of energy to obtain molecular hydrogen and then transport and store it. Research is under way to find more economic ways to liberate hydrogen from water. One possibility is biotechnology.

Schemes such as the EU-funded BioModularH2 project have been formed to develop organisms that feed on sunlight to power the production of hydrogen from water or feedstock chemicals such as acetates. Bacteria that crack water in hydrogen and oxygen exist – many are nitrogen-fixing species already useful in agriculture that produce hydrogen as a part of the process. However, they produce relatively little gas compared with the amount of energy and food they consume, so scientists are trying to redesign the enzymes they use to make them more efficient: the best naturally occurring hydrogen producing enzymes are, unfortunately, poisoned by oxygen.

Some researchers are trying to tweak the enzymes to make them less sensitive to oxygen; so far with limited success. Another approach is to stick with the natural enzyme and increase production by engineering proteins that are efficient at removing oxygen before it can do any harm. These hydrogen producers are longer-term options: no-one is looking at a time-scale shorter than ten years before even pilot production.

Even if hydrogen becomes easy to produce, it remains a poor choice for portable applications which are saddled with slow-to-evolve battery technology. However, mildly acidic organic liquids, such as ethanol and methanol, make good substitutes for molecular hydrogen as they will donate protons for these reactions. The direct ethanol fuel cell can even use the same type of proton-exchange membrane as the hydrogen version. One big advantage of using ethanol is that a biomass industry has been established. The ethanol group remains on the anode side to be oxidised into carbon dioxide, donating protons that can cross over to the cathode as part of the process.

The most promising candidate for portable devices remains the direct methanol fuel cell. Here, methanol is dissolved in water, with the recombination of ions taking place at the cathode. However, this is the cell's main drawback – because methanol has to cross from anode to cathode, the reaction is much slower and even less efficient than the proton-exchange cells. Today's designs also tend to produce less than 0.5V.

Fuel cells suffer from polarisation which means that as current density increases the voltage tends to drop. Close to peak current, the voltage can drop off dramatically because of losses from the effects of transporting a high concentration of fuel across the electrolyte – it starts to take a long while to refresh the chemical at the interface and remove the waste products. As a result, cells need to be stacked to develop a usable voltage. In portable systems, this is likely to be achieved through a large number of cells constructed using microfluidic techniques to prevent the size of the cell from spiralling out of control.

Although methanol is both toxic and flammable, the International Civil Aviation Organisation's dangerous goods panel voted in late 2005 to let passengers carry micro fuel cells and fuel cartridges onto airplanes. The US Department of Transportation decided in 2008 to allow up methanol cartridges, of an approved design, to carry up to 200ml in liquid – twice that of the current volume allowed for cosmetics in carry-on baggage. However, the problem for methanol fuel cells remains less one of fear of an airline ban, more their real-world performance.

Fuel-cell construction (see fig 3) is trickier than with proton-exchange designs because it needs to use water to dilute the methanol and encourage the protons and the methanoate groups to disassociate. As water adds to the weight, it makes more sense to recycle the liquid, rather than to supply it fresh. That adds complexity to the design. Also carbon dioxide needs to be removed efficiently from the solution that passes out of the fuel cell.



Although the aim is ultimately to power laptops and phones, the main target for methanol fuel cells currently lies in military applications: replacing bulky batteries for backpack radios and other battlefield electronics.

If the fuel cell does not need to go anywhere, the options expand considerably. Stationary, grid-based cells might provide short-term generating capacity to cope with peaks in demand or for reversible systems as alternatives for batteries in power networks that have a large proportion of generation based on renewables. As they can be fixed in place and shielded, they can use more extreme fuels and processes.

For example, the protonic ceramic fuel cell needs to operate at more than 700°C in order to oxidise gas-phase molecules electrochemically. These ions can pass through a porous, solid electrolyte without demanding the use of a liquid that might leak out over time and contaminate the ground. However, this design has a low current density, although this can be increased with a thicker electrolyte layer. As it can support an efficiency of up to 65% using pipeline-borne natural gas, this is one looks to be a good candidate for grid-based cells.

Another technology suited to use in stationary cells is the phosphoric-acid fuel cell. This operates at 200°C and uses the corrosive acid as an electrode. However, pilot plants have been built and are being tested at a number of sites. Other candidates include the molten carbonate fuel cell and the solid oxide fuel cell, both of which need to work at more than 600°C. But they have demonstrated large practical efficiencies. Siemens Westinghouse, for example, has demonstrated a 250kW system that works at up to 52% efficiency.

Although large cells such as these work at high temperatures, some researchers have pursued the option of using the solid-oxide design in portable products. The Fraunhofer Institute has developed one that uses methane as its fuel source, although it is aimed primarily at military systems that need 100W. However, the Massachusetts Institute of Technology has developed one using that uses infrared light, generated by incandescent gas as an intermediate form of energy, although efficiency was just 5%.

Even though parts of the industry seem to have settled on mainstream architectures, such as proton-exchange or direct methanol, more than 170 years since its invention, the options for the fuel cell are still pretty open.

Author
Chris Edwards

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