<P> The electrolysis of water in standard conditions requires a theoretical minimum of 237 kJ of electrical energy input to dissociate each mole of water, which is the standard Gibbs free energy of formation of water . It also requires energy to overcome the change in entropy of the reaction . Therefore, the process cannot proceed below 286 kJ per mol if no external heat / energy is added . </P> <P> Since each mole of water requires two moles of electrons, and given that the Faraday constant F represents the charge of a mole of electrons (96485 C / mol), it follows that the minimum voltage necessary for electrolysis is about 1.23 V. If electrolysis is carried out at high temperature, this voltage reduces . This effectively allows the electrolyser to operate at more than 100% electrical efficiency . In electrochemical systems this means that heat must be supplied to the reactor to sustain the reaction . In this way thermal energy can be used for part of the electrolysis energy requirement . In a similar way the required voltage can be reduced (below 1 V) if fuels (such as carbon, alcohol, biomass) are reacted with water (PEM based electrolyzer in low temperature) or oxygen ions (solid oxide electrolyte based electrolyzer in high temperature). This results in some of the fuel's energy being used to "assist" the electrolysis process and can reduce the overall cost of hydrogen produced . </P> <P> However, observing the entropy component (and other losses), voltages over 1.48 V are required for the reaction to proceed at practical current densities (the thermoneutral voltage). </P> <P> In the case of water electrolysis, Gibbs free energy represents the minimum work necessary for the reaction to proceed, and the reaction enthalpy is the amount of energy (both work and heat) that has to be provided so the reaction products are at the same temperature as the reactant (i.e. standard temperature for the values given above). Potentially, an electrolyser operating at 1.48 V would be 100% efficient . </P>

Electrolysis of water to form hydrogen and oxygen gases