Water splitting material requirements

In any case, no electrode material or approach fulfills the requirements for a successful photoelectrochemical process in all respects, i.e., for routine practical use hence novel materials and approaches are constantly pursued. Note that beside the robust performance needed, the most important figure of merit for a semiconductor photoelectrode used for water splitting is the photoconversion efficiency, which is... 

Of the photocatalytic systems and structures composed of a single active material, eventually coupled with redox catalysts and/or metals, only a wide band gap oxide semiconductor, like Pt/Ti02, requiring UV irradiation, showed some photoactivity for water photosplitting. Water splitting with visible light requires the irradiation of multiple band gap photoelectrochemical cells (PEC) or Z-scheme systems (like the photosynthesis system of plants etc.). 

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... 

Both these reactions take place at temperatures between 800 and 900 C. The iron oxide acts as a solid state oxygen source for the oxidation of the combustible part of the producer gas (reaction 1) and as an oxygen receptor in the subsequent water splitting reaction. It is of central importance that the bulk of the oxide material can be cycled between the two oxidation states without breaking apart. The close relationship between the crystallographic structure of Fej04 and FeO is an inqjortant prerequisite to fulfil this requirement. In this context it is important to keep the reaction conditions such that reduction to metallic iron does not occur. 

In developing PEC hydrogen systems that will meet these requirements, research efibrts are focused in two areas (1) chemical modification of the semiconductor electrode surface material to improve system energetics, and (2) evaluation of triplejunction amorphous silicon structures as low-cost thin-film water splitting systems. 

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