The flow of and electric current between two electrodes in a strong alkaline water electrolyte produces the decomposition of water into its elements, hydrogen and oxygen. The electrode reactions can be expressed by the following equations:
At the cathode
2H2O + 2e ---> H2 + 2OH-
At the anode
2OH- ---> ½O2 + H2O + 2e
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Process reaction H2O ---> H2 + ½O2
A certain electrical potential difference has to be applied to the electrodes in order to make the current flow. The thermodynamics of the reaction process shows that the minimum voltage (reversible) necessary to start the current flowing is equal to 1.23 V, corresponding to the free energy (deltaG) of the processing reaction. At this voltage the system produces hydrogen, but requires and additional amount of energy corresponding to the process reaction entropy to operate isothermally. Therefore, the minimum voltage at which hydrogen can be produced without additional heat is 1,49V.
At this voltage the irreversible energy losses compensate the entropy of the process reaction. In practice, the cell voltage has to be kept well over the reversible value to account for the other losses such as the over voltage of electrodes, the bubble effect, and the electrical resistance of metals, electrolyte, an diaphragms.
SESPI system runs at a voltage as low as 1,8 – 1,9 V over long periods of time, due to:
Special activation of the electrodes for longer operational life
Separators with low gas permeability and high ion conductivity
Filter press assembly with well designed bipolar metal conductors.
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