The Fuel Cell
It seems strange to think these could be the last days of the internal combustion engine, which has dominated personal transportation for the second half of the 20th century. Much as it has contributed to extraordinary mobility for many people, countries to rely heavily on oil from politically unstable regions, and intensified the long-term threat to global warming.
It seems possible that some time in the next decade, auto-manufacturers will be ready to market new, super energy-efficient cars powered by fuel cells that drive powerful, high-torque electric motors. Such vehicles will produce virtually no polluting gases such as carbon dioxide, which contributes to the greenhouse effect, or oxides of nitrogen or sulphur that are the main causes of acid rain. They can be fuelled by hydrogen from a variety of secure energy sources.
The development of fuel cell depends on a detailed understanding of one of the simplest chemical reactions: the burning of hydrogen to make water.
2H2 (g) + O2 (g) = 2H2O (l)
The reaction is highly exothermic and the equilibrium constant, of the order of 1080, is most definitely on the side of reactants going to products. However, a glass jar containing two volumes of hydrogen and one volume of oxygen at room temperature, though potentially highly explosive, could exist in that state forever.
This is because at room temperature, the collisions between hydrogen and oxygen molecules are just not of the right type, or energetic enough, or of the right frequency to ensure that they react. The activation energy is too high for the reaction to go at a measurable rate at room temperature. But place a glowing splint in the jar and the explosion can shatter the glass.
What happens is that the hot glowing splint excites some molecules of hydrogen and oxygen to such an extent that their collisions are now energetic enough to overcome the activation energy and cause the reaction. Then, the energy given out is so great that it excites more hydrogen and oxygen molecules into reacting. More energy is given out and in a fraction of a second the reaction spirals out of control and becomes an explosion.
There is another way to induce this reaction. Throwing a pinch of palladium on charcoal into the gas jar as the same effect as the glowing split. The palladium on charcoal acts as a catalyst. The surface of the palladium ha sites where reactant molecules can attach themselves. Reactant molecules absorbed onto these sites can find themselves close enough together to react and in the right orientation. Bond breaking and formation occurs much more easily when the reactant molecules are aligned in the right manor, so the catalysts lowers the activation energy of the reaction.
A heterogeneous catalyst, such as palladium, is effectively a ‘dating agency’ bringing together with ease molecules that by themselves have difficultly meeting productivity. Catalysts like this are now set to produce a revolution in the way we use fuels.
Producing usable energy by fuels in highly inefficient process. Petrol and diesel engines are never more than 25-30% efficient. The rest of the energy produced during the burning of these fuels merely go to heat up the surroundings.
In a fuel cell, with the help of a catalyst, it is feasible to combine hydrogen and oxygen is such a way that, instead of the energy in the reaction heating up the surroundings, it produces electricity.
A hydrogen-oxygen fuel cell has two porous electrodes and hot concentrated potassium hydroxide solution as the electrolyte. Hydrogen gas circulates under pressure around one electrode where it is oxidised to water, while oxygen gas is pumped round the other electrode where it is reduced to hydrogen ions:
O2 (g) + 2H2O (l) +4e- = 4OH- (aq) (at the
2H2 (g) + 4HO- (aq) = 4H2O (l) + 4e- (at
the negative electrode)
The efficiencies of these cells can be as high as 75% and they were used in early American Gemini spacecrafts where they also provided the astronauts with drinking water.
Until quite recently, fuel cells were expensive and cumbersome pieces of equipment. But fuel-cell technology has moved on since the space-race days of the 1960s. The new fuels cells, produced by Ballard Power Systems in Vancouver, Canada, are very highly compact pieces of equipment. They are based on a design pioneered for the space programme, but Ballard have made significant improvements to the design, so that the electrodes consists of thin sheets of a porous carbon-based conducting material coated with a small amount of platinum catalyst. These sandwich the electrolyte, which is an ultra-thin conducting membrane.
The first vehicles powered by fuel cells include a bus cruising the Vancouver streets, and commercial vehicles on test at Daimler-Benz’s research headquarters near Ulm in Germany. They use stacks of 24 of the fuel cell units, weighing a total of about 50 Kg. In the case of the Vancouver bus, this amounts to the delivery of 125 horsepower to the bus’s powerful electric motor, and gives a range of about 100 miles.
Fuel cells score better than vehicles running off conventional rechargeable batteries. No matter how efficient these batteries can be made be to run, ultimately they depend on non-renewable, polluting forms of electricity generators. Instead of pollution coming from the vehicle, the use of conventional batteries merely shifts those emissions back to the power stations, which generates the electricity to recharge the batteries.