Platinum and power

One of the classes I took in university was on electrochemistry and fuel cells. It was very interesting, but was also a reality check on the hype of hydrogen fuel cells vs. the reality. One item in particular that stood out for me was that the catalyst required for efficient, low temperature hydrogen fuel cell operation was platinum. While platinum isn't the most expensive metal out there, gold having passed it in price not too long ago, it's way up there. As I recall, the raw platinum required to make a fuel cell cost a significant fraction of the cost of a normal car, and that was before they processed it into a useful catalyst. Since then, they've improved the structure here and there and reduced the amount of platinum required bit by bit, but it's still a lot.

Not long ago, however, some researchers in Finland figured out a way to reduce the amount of platinum by more than half.

A hydrogen fuel cell converts hydrogen and oxygen into water, but hydrogen and oxygen will do that on their own when mixed. To make it a fuel cell, the hydrogen and oxygen steps have to be separated.

On the anode side, the hydrogen is split from H2 and the electrons captured for use in the motor:

\[\mathrm{H}_2 \rightarrow 2\mathrm{H}^+ + 2\mathrm{e}^-\]

The hydrogen cations then migrate over to the cathode through a membrane that allows them to pass but not oxygen, to where the oxygen is added to the fuel cell.

At the cathode, the oxygen is split from O2 by taking in electrons—the ones that travelled through the wire, through whatever the fuel cell is powering, and into the cathode:

\[\mathrm{O}_2 + 4\mathrm{e}^- \rightarrow 2\mathrm{O}^{2-}\]

It turns out that on both anode and cathode, platinum acts as a high efficiency catalyst for both of the two half cell reactions. For the obvious reason of cost mentioned above, the entire anode and cathode aren't made of platinum, but are only coated with it. The particular method of coating the electrodes has been under development and improving about since fuel cells were first made. This method, atomic layer deposition, uses less platinum (or palladium, a catalyst for alcohol fuel cells and the one studied in the linked paper) and produces a thinner, more even layer on the electrode surface. Atomic layer deposition works on platinum as well, as it does on many elements.

Earlier this year, researchers in Ontario discovered how platinum is so efficient: it actually repels water once it's been exposed to hydrogen, which pushes the water away and lets the hydrogen gas reach the catalyst surface. This isn't just a coating of hydrogen, either, but hydrogen embedded within the platinum catalyst. Hydrogen is known to diffuse into solid metal, which makes it rather hard to keep in a metal tank for long periods of time. Fortunately there are many other ways of storing hydrogen than as a compressed pure gas, and more still under development.

As hinted at by the Finnish study, however, it may be alcohol fuel cells, rather than hydrogen, which prove to be commercially viable. Alcohol is much easier to store and transport than hydrogen is, for one thing—although corn ethanol for fuel in internal combustion engines is already competing for arable land with food crops, so even that is not completely certain.

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