How Nanotechnology Promises to Greatly Reduce the Manufacturing Cost of Fuel Cells

There has been a lot of talk in the mainstream media regarding fuel cell powered vehicles as a greener and cleaner alternative to petrol engines over the past few years. A major obstacles to this revolution is the cost of hydrogen fuel cell power plants. A fuel cell is a device that converts fuel, normally in the form of a gas such as hydrogen or oxygen, into electricity. For road vehicles, proton exchange membrane (PEM) cells are widely seen as the most promising option.

Proton exchange membrane (PEM) fuel cells provide high power density and the advantages of low weight and volume compared to other types of fuel cells. In a PEM cell, hydrogen and oxygen gas are fed to catalytic electrodes at opposite sides of a special membrane. This special membrane is porous to protons but not electrons. The protons and electrons are separated by the action of a platinum catalyst in the electrodes. The protons can diffuse directly through the membrane but the electrons have to make their way through an external circuit to reach the other side, providing power for an electric motor in the process.

Proton Exchange Membrane Fuel Cell. (Source: Dept. of Energy)

Professor Rod Boswell and the Space Plasma, Power & Propulsion Group at ANU have been working with plasmas for many years and recently became interested in the possibility using plasma deposition technology to dramatically reduce the cost of making fuel cells.

Professor Boswell also commented "Production of current cells frequently relies on wet chemical stages which are messy, inefficient and consume large amounts of expensive materials. Our aim is to develop plasma based techniques to create both the membranes and the catalytic electrodes needed in fuel cells."

The manufacture of electrodes starts with a substrate of carbon paper. Carbon paper is chosen because it is both porous to the gaseous fuels used in the final cells and is also an excellent conductor of electricity. This is loaded into the plasma reactor chamber and a very fine layer of nickel is deposited on the surface. Under the right conditions the nickel forms nanoscale droplets all over the carbon surface.

The next stage is to introduce methane and hydrogen into the plasma chamber. Many complex reactions ensue leading to a very surprising situation where carbon complexes diffuse through the nickel seeds to form multi-carbon complexes below. The highly reactive hydrogen protons in the chamber etch away any carbon atoms that aren't strongly bonded to each other. The practical upshot of this is that carbon nano fibres grow below the nickel droplets lifting them from the substrate as they extend. The result is a carpet-like covering of carbon nanofibres on the paper.

Once the forest of nanofibres has been created the next step is to sputter coat the surface with platinum. Professor Boswell explains: "During the sputtering process the nanofibre tips get thickly coated with platinum with the droplets becoming progressively sparser further down the fibre. It's very much like snow falling in a forest, a lot gets deposited on the tree tops which greatly reduces the amount on the ground."

The tremendous advantage of this nanotechnology electrode is that its vast surface area and microscopically thin platinum coat reduce the amount of platinum required to about 15% of that in a conventional electrode of the same power specification.

To make the finished fuel cell, the membrane is sandwiched between the hairy sides of two of the carbon catalytic electrode sheets and the whole assembly is hot pressed into a single sheet.

The new fuel cell technology is exciting stuff and may well be a key part of the transition to clean transport. "It has to be a holistic approach to clean transport. If you buy a cylinder of hydrogen today, chances are it was made from fossil fuels - it would be better to just burn the fossil fuel directly. What we need are fuel cell vehicles running on hydrogen that is in turn generated by clean electricity from solar or hydro. Then we'd be getting ahead" Professor Boswell warns. At the moment it costs about six times as much to run on hydrogen as petrol. However, with petrol costs continuing to climb and the possibility of economies of scale in hydrogen production and distribution, it may not be all that long before that economic balance shifts.