Development of Direct Solar Thermal Hydrogen

The direct thermochemical process, to generate hydrogen by splitting water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Although conceptually simple, the single-step thermal dissociation of water has been impeded by the need for a high-temperature heat source to achieve a reasonable degree of dissociation, and—to avoid ending up with an explosive mixture—by the need for an effective technique to separate H2 and O2. Unfortunately, the decomposition of water does not proceed substantially until the temperature is very high. Generally temperatures of 2500 K have been considered necessary for direct thermal water splitting. The Gibbs function (AG, or free energy) of the gas reaction H2O — H2 + / O2, does not become zero until the temperature is increased

Thermochemical and Thermal/Photo Hybrid Solar Water Splitting 91 Table 1. The pressure equilibrium constants of the water dissociation reaction.24

Temperature (K)

2500

3000

3500

Ki

I.34xi0-4

8.56xi0-3

I.68xi0-i

K2

4.22xi0-4

I.57xi0-2

2.i0xi0-i

K3

I.52xi03

3.79xi0l

2.67xi00

K4

4.72xi03

7.68xi0l

4.0ixl00

to 4310 K at 1 bar pressure of H2O, H2 and O2.3 At a water pressure of 0.1 bar or greater, significant mole fractions of hydrogen are not spontaneously formed at temperatures below 2200 K. The entropy (AS), driving the negative of the temperature derivative of the Gibbs function change, is simply too small to make direct decomposition feasible at this time.4

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