Photoelectrochemical water splitting process

Carbon is widely used in the catalytic processes of the chemical industry due to its unique characteristics, such as chemical inertness, high surface area and porosity, good mechanical properties and low cost. It is used for the production of chlorine and aluminum, in metal refining (gold, silver, and grain refinement of Mg-Al alloys) as well as for the electrolytic production of hydrogen peroxide and photoelectrochemical water splitting. 

Despite the many advances in computational materials science in the last decade, photoelectrochemical water splitting is still very much an experimental field. This is because many of the relevant properties of photoelectrodes are determined by factors that are either difficult to control or not yet well understood, which makes it difficult to predict a priori their influence on the actual performance. An example of this is the presence of certain defects, such as dislocations or impurities. These are often impossible to avoid, especially when low-temperature and low-cost synthesis methods are used. An important stage in the development process of a photoelectrode is therefore to systematically optimize the synthesis procedure in order to achieve a maximum photoresponse. 

Parkinson, B. and Turner, J. (2013) The Potential Contribution of Photoelectrochemistry in the Global Energy Future, in Photoelectrochemical Water Splitting Materials, Processes and Architectures (eds H-J. Lewerenz and L.M. Peter), Royal Society of Chemistry, Cambridge, UK. 

In this work, the present status of the field of semiconductor electrochemistry with regard to energy conversion processes is reviewed. Naturally, this includes the derivation of the basic concepts and a (selected) overview of systems that operate either in the photovoltaic or the photocatalytic mode. Due to their importance for storable renewable energy, the principles of solar fuel generation are treated in some detail with emphasis on photoelectrochemical water splitting. In the outlook (part 6), selected advanced concepts will be described. 

As discussed earlier, overall water-splitting process is a combination of two reactions oxidation and reduction. These two half reactions can also be termed - the two-electron stepwise process. The evolution of oxygen at the photoanode and that of hydrogen at cathode in a photoelectrochemical cell is observed as shown in reactions (2.4) and (2.5) [3, 9]. 

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... 

The early work of photoelectrochemical hydrogen production using Ti02 as catalyst, was reported by Fujishima and Honda [61]. Subsequently, the interest for the photocatalytic processes has grown significantly, although the number of the reported photocatalysts used for water splitting is still limited. 

It is generally accepted that three major processes limit the photoelectrochemical current in semiconductors after a bandgap excitation [76]. These processes are schematically illustrated in the band diagram shown in Fig. 3.2. The bold arrows show the desired processes for efficient water splitting PEC cell after a bandgap excitation the transport of electrons to the back contact, the transfer of the hole to the semiconductor surface and the oxidation of water at the semiconductor/electrolyte interface. The three major limiting processes are a) bulk recombination via bandgap states, or b) directly electron loss to holes in the... 

An alternative way is to divide the overall process of water splitting into two stages, each being conducted in a separate photoelectrochemical cell. Both cells are coupled with the aid of a certain intermediate substance, which acts as a charge transfer agent and is not consumed in the course of the overall process, but only provides the connection in series of the chemical potentials developed in both cells. Apparently, such a scheme imitates the combination of two photosystems in photosynthesis occurring in green plants. 

Already at an early stage of the research in the semiconductor dispersions, attempts have been made to carry out water splitting, CO2 reduction, etc., in other words, the same photoelectrochemical processes as in the macroscopic PEC cells. The results obtained are summarized in [51-55]. We shall confine ourselves, however, to the processes that might underly some methods of purification, eg., of waste waters etc. These processes are stimulated by electrons and holes produced in the particles by light. As only one type of the current carriers is consumed in the "useful" reaction, measures should be taken to remove the other type from the particle in order to preserve its electroneutrality and sustain the process. For this purpose a sacrificial electron donor (or acceptor) is to be introduced into the electrolyte solution. Often it is the solvent that plays sacrifice. Some examples are listed below. 

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