The fuel cell (H 2 / O 2) are one of the solutions under consideration for a clean automobile propulsion.
However the slowness of the reaction ( H 2 = > 2 H + + 2 e – ) (1) obliges to work at high temperature or to use a metal catalyst on the anode where this reaction is made.
The use of a fuel cell for the automobile propulsion implies an intermittent operation, which excludes a high temperature. The only fuel cells studied for the car are with catalyst. But this catalyst, generally platinum, is very expensive.
A study published by two Czech scientists in 2000 (*) seems to indicate a possible way to reduce the cost of a fuel cell. This study shows that the sonication of a mercury electrode activates the reactions which occur on its surface.
The sonication is the fact of subjecting a liquid to an ultrasonic field. This ultrasonic field generates phenomena of cavitation. Micro bubbles are formed during the depressive phase of the sound field. The compressive phase of the sound wave contracts these bubbles. This contraction is fast: very high temperature and pressure are reached during a short time when the liquid walls of the bubble come into contact at high speed.
When these bubbles implode near the anode of a fuel cell, a small surface of the electrode and small liquid volume in contact are subjected to high pressure and temperature. The multiplication of these hot spots on the surface of the electrode could prevent the accumulation of contaminants and delay its polarization. Perhaps this limitation of polarization would allow the use of a metal less noble than platinum to form the anode of the fuel cell.
On the other hand, the sonication could activate the reaction (1). In a fuel cell, the hydrogen gas is in contact at the same time with the anode and also with the electrolyte wetting this electrode. This contact gas/liquid implies that the sonication could generate bubbles containing the hydrogen gas. The implosion of the bubbles during the compressive phase of the ultrasonic field subjects the hydrogen enclosed in the bubbles to high temperature and pressure. This treatment could accelerate the reaction (1).
The sonication of the anode of a fuel cell could avoid the use of platinum or a high temperature if the energy necessary to the generation of the ultrasonic field is not too high compared to the energy produced by the fuel cell.
(*) : Organic sonoelectrochemistry on mercury pool electrode, Jiri Klima and Jiri Ludvik, Collection of Czechoslovak Chemical Communications, 2000, 65, 941-953.