High-Temperature, Indirect-Solar Thermal Hydrogen Processes

3 High-Temperature, Indirect-Solar Thermal Hydrogen Processes 

More recently, higher temperature processes have been considered (at T 2000 K), such as two-step thermal chemical cycles using metal oxide reactions.2 The first step is solar the endothermic dissociation of the metal oxide to the metal or the lower-valence metal oxide. The second step is non-solar, and is the exothermic hydrolysis of the metal to form FI2 and the corresponding metal oxide. The net reaction is H2O = H2 + 0.5 O2, but since FI2 and O2 are formed in different steps, the need for high-temperature gas separation is thereby eliminated  

Elements in cycle Maximum temperature, K Total reaction steps in cycle 

Other redox pairs, such as Mn304/Mn0 and C03O4/C0O have also been consi dered, but the yield of H2 in the reaction has been too low to be of any practical in terest.53 H2 may be produced instead by reacting MnO with NaOH at above 900 K in a 3 step cycle.56 Steinfeld further suggests2 that partial substitution of iron in Fe304 by other metals (e.g., Mn and Ni) forms mixed metal oxides of the type (Fei xMx)304 that may be reducible at lower temperatures than those required for the reduction of Fe304, while the reduced phase (Fei-xMx)i-yO remains capable of splitting water.57 59 

4 Hybrid Solar Thermal/Electrochemical/Photo (STEP) Water Splitting 

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