Wednesday, June 02, 2010

A Fuel Cell that Runs on Air and Water.

It seems too good to be true that water can be used as a fuel, but in a recent paper, a fuel-cell has been described which runs on water and air, in which you don't actually "burn" water but a concentration gradient of water is established between the two electrodes allowing entropy rather than enthalpy to drive the energy output from the cell. The power output is small, orders of magnitude lower than from hydrogen or methanol fuel cells, but the supply and handling of these flammable fuels is avoided. It is proposed that the cell might be used in applications which require relatively low power consumption, for example sensors of various kinds or emergency signalling units, and that the devices might be used best in desert or warm coastal regions where the water is readily evaporated from the cell, thus maintaining its concentration gradient.

On one side of the cell (anode), the reaction 2H2O ---> O2 + 4H+ + 4e- occurs;

while on the other (cathode), the reverse process occurs: O2 + 4H+ + 4e- ---> 2H2O.

The two electrodes, cathode and anode, are separated by a polymer electrolyte membrane which permits protons to cross to reach the cathode while the electrons are made to flow as part of a circuit to carry an electrical current.

The authors note that such a concentration cell avoids the logistic difficulties of using hydrogen gas; nonetheless for an application such as transportation the far greater power output of a hydrogen cell is necessary, and the provision of "green" hydrogen in quantity. All types of fuel cell also require platinum in quantity, the demand for which already exceeds world production of "new" platinum.

Thus the "prediction" by Jules Verne in his novel "Mysterious Island", published in 1874, as espoused by the fictional engineer, Cyrus Smith, "I believe that water will one day be used as a fuel, that the hydrogen and oxygen of which it is constituted will be used, simultaneously or in isolation, to furnish an inexhaustible source of heat and light, more powerful than coal can ever be. Water is the coal of the future.", remains some way off.

A very interesting piece of science, however.

Related Reading.
"A fuel cell that runs on air and water," A. M. Dreizler and E. Roduner, Energy and Environmental Science, 2010, 3, 761
DOI: 10.1039/c001381a

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