Hydrogen Production by Water Electrolysis

Electrolysis of water to generate hydrogen and oxygen has a history of more than 200 years [5]. It is the simplest of all the water-splitting techniques and comparatively efficient. The net reaction is 

Oxygen, a highly useful gas, is the only by-product. Water electrolyzers today satisfy approximately 3.9% of the world s 

This is the maximum amount of useful work that can be derived from the system on driving the reaction in the opposite direction. Thus, Vrev corresponds to the reversible work and is consequently called the thermodynamic reversible potential. At 25°C and 1 bar, the AG for the water-splitting reaction is 237.178 kj/mol [10]. Therefore, 

The reaction at this voltage is endothermic and hence at isothermal conditions heat energy (=TAS, where is S is entropy and T is the absolute temperature) must be absorbed from the surrounding environment for the increase in entropy associated with water 

When energy exactly equal to the enthalpy AH = AG + TAS (=285.83 kj/mol at 1 bar and 25°C) for water splitting is supplied, no heat is absorbed or evolved by the system [10]. The voltage corresponding to this condition, the thermoneutral voltage Vm is given by 

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Srinivasan S., Salzano F.J., Prospects for hydrogen production by water electrolysis to be competitive with conventional methods, Int. J. Hydrogen Energ., 2,53-59,1977. 

In a process of methane regeneration form C02, a two-step reaction of hydrogen production by water electrolysis and methanation of C02 with the hydrogen [Eqs. (6) and (7)] is available ... 

Mergel, J., Carmo, M., Fritz, D.L. Status on teehnologies for hydrogen production by water electrolysis. In Stolten, D., Scherer, V. (eds.) Transition to Renewable Energy Systems, pp. 425 50. Wiley-VCH, Weinheim (2013)... 

Let us now revisit the case of hydrogen production by water electrolysis. 

For a given cell voltage U, the (minimum) energy consumption qH2 of hydrogen production by water electrolysis in W per kg hydrogen is given by Eq. (6.19.7) ... 

Laoun, B. (2007) Thermodynamics aspect of high pressure hydrogen production by water electrolysis. Rev. Energ. Renouvelables, 10, 435-444. 

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