Efficiency
The electrolysis of water requires a minimum of 237.13 kJ of electrical energy input to dissociate each mole. Since each mole of water requires two moles of electrons, the specific electrical energy required is 118.57 kJ/mole (7.40×10<sup>23</sup> eV/mole). It follows then that a minimum electrical power input per ampere is implied, namely 1.23 W/ampere. In turn, the minimum electrolytic potential for electrolysis of water or 1.23 V. Thus, any current (I) at applied voltage (V) greater than 1.23 V is an overvoltage and results in waste heat which can be estimated as I×(V-1.23).<sup id="cite_ref-8" class="reference">
[9]</sup>
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. Developing a cheap effective electrocatalyst for this reaction would be a great advance (see also<sup id="cite_ref-9" class="reference">
[10]</sup>). In 2008, a group led by
Daniel Nocera announced the development of an electrocatalyst composed of the abundant metal
cobalt and
phosphate.<sup id="cite_ref-10" class="reference">
[11]</sup> Other researchers are pursuing carbon-based catalysts.<sup id="cite_ref-11" class="reference">
[12]</sup>
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%.<sup id="cite_ref-12" class="reference"></sup>