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Hydrogen ions enter the electrolyte and are transported to the cathode. Simultaneously, the electrons liberated in the anodic reaction flow through the external work-producing device. At the cathode, the protons and electrons produced in the anode combine with oxygen gas fed to the cathode chamber according to the half-cell reaction:

'/202(g) + 2H+(from electrolyte) + 2e ^ H2O(L) (10.17)

The sole reaction product, pure liquid water, drains from the cell as the only waste stream.

In terms of chemical potentials, the free energy change of the overall cell reaction is:

AG = H H2OCL) - >2 H 02(g) -^2(g) = g^O(L) - X(g02 + RTlnp02) - (gH + RTlnpH2)

where

is the standard free energy of formation of liquid water. At 25oC, AGH 0(l) = -239 kJ / mole . Using the above equation for AG in Eq (10.5a) with z = 2 gives the cell voltage:

where the standard EMF of the cell is:

If the gases in the electrodes are at atmospheric pressure, the cell voltage is also 1.23 V. 0peration at higher pressures would increase the available voltage but would entail construction of a more rugged unit. The actual EMF provided by the fuel cell is less than the open-circuit value given above because current is drawn to produce power. The i - V characteristics of the cell determine its performance. To obtain a usable voltage for power production, many cells must be operated in series.

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