Vlovepeugeot ชมรมคนรักเปอโยต์ (เปอร์โยต์) ประเทศไทย

A pure stoichiometric mixture is most easily obtained by water electrolysis, which uses an electric current to dissociate the water molecules:

    electrolysis: 2 H2O ? 2 H2 + O2
    combustion: 2 H2 + O2 ? 2 H2O

William Nicholson was the first to decompose water in this manner in 1800. The energy required to generate the oxyhydrogen always exceeds the energy released by combusting it.

More energy is therefore required to drive the electrolysis cell than can be extracted from burning the resulting hydrogen-oxygen mixture.

Efficiency

Water electrolysis does not convert 100% of the electrical energy into the chemical energy of hydrogen. The process requires more extreme potentials than what would be expected based on the cell's total reversible reduction potentials. This excess potential accounts for various forms of overpotential by which the extra energy is eventually lost as heat. For a well designed cell the largest overpotential is the reaction overpotential for the four electron oxidation of water to oxygen at the anode. An effective electrocatalyst to facilitate this reaction has not been developed. Platinum alloys are the default state of the art for this oxidation. The reverse reaction, the reduction of oxygen to water, is responsible for the greatest loss of efficiency in fuel cells. Developing a cheap effective electrocatalyst for this reaction would be a great advance (see also[6]).

The simpler two-electron reaction to produce hydrogen at the cathode can be electrocatalyzed with almost no reaction overpotential by platinum or in theory a hydrogenase enzyme. If other, less effective, materials are used for the cathode then another large overpotential must be paid.

The energy efficiency of water electrolysis varies widely with the numbers cited below on the optimistic side. Some report 50?80%[7].[8] These values refer only to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency may be closer to 30?45%.[9]