Water splitting energy requirements

A photochemical process could use the energy of sunlight to split water into H2 and 02, the former then being used as a fuel which can be stored indefinitely. The splitting of water requires in principle a photoactivated catalyst dissolved or dispersed in water. The energy requirement for the overall reaction H20— H2 + 02 is 1.23 eV (per electron). In conditions of electrolysis it would however proceed extremely slowly at this potential difference which pertains to a thermodynamic equilibrium, and it is well known that a substantial overvoltage of the order of 0.5 V is required in practice to drive this reaction. 

The main reaction product is water vapor. The water splitting reaction is endothermic and the energy required for a significant hydrogen production rate is high. 

From thermodynamic considerations, all semiconductors that satisfy the above-mentioned energy-gap requirements should be able to be used for photosensitised water splitting. These include Ti02 (anatase form), CaTiOa, Ta205 and Zr02. 

A portion of the supplied thermal energy is expelled with the products. The overall water splitting process, see Fig. 2.6, in general requires both heat and work input. In Fig. 2.6 qr represents the heat energy supplied at high temperature Tr, qo represents the heat rejected at lower temperature To, and Wi is the useful work input, if any, for the process. The enclosed region contains only water, or water and materials involved in different... 

In a process called direct thermolysis, at a high enough temperature thermal energy is sufficient to split water into hydrogen and oxygen. Only one reaction is involved in this process, that of equation (2.3.1). In Fig. 2.6, if the input water and output gas mixture are at the same temperature To, the minimum work input required to effect water splitting at temperature Tr can be written [60]... 

The energy required for splitting water is lessened, opening the way to the use of small band gap semiconductors. 

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