Solar water splitting cells

While for a solar water splitting cell, light is directly absorbed by the semiconductor electrode (anode or cathode). The separation of electron-hole pairs is achieved in the built-in electric field near the semiconductor surface. The electric field is formed due to the charge transfer between the semiconductor electrode and the electrolyte as schematically shown in Fig. 17.5(b) [28]. Take an n-type semiconductor electrode for example... 

Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, Lewis NS. Solar Water splitting cells. Chemical Reviews. 2010 110(11) 6446—6473. 

Solar Water Splitting Cells. Chem. Rev., Vol. 110, pp. 6446-6473 Wang, B. (2005). Recent Development of Non-Platinum Catalysts for Oxygen Reduction Reaction. /. Power Sources, Vol. 152, p>p. 1-15... 

Fig. 8.9 Schematic diagram of PV-electrolysis systems proposed for solar water splitting (a) Electricity generated from photovoltaic cell driving water electrolysis (b) PV assisted cell with immersed semiconductor p/n junction as one electrode.

As explained earlier, photoelectrochemical splitting of water was done for the first time in 1972 (Fujishima and Honda). However, the efficiency of this cell was very low (about 1%) and hence not practical. A number of advances have brought an economical standalone, one-step solar water-splitting technology much nearer. There have been four steps in these advances. 

Weizmann Institute Scientists Zero In on Direct High-Temperature Solar Water Splitting, Hydrogen Fuel Cell Letter, November 1996. 

Brillet, J., Comuz, M., Le Formal, F., Yum, J.H., Gratzel, M., Sivula, K. Examining architectures of photoanode-photovoltaic tandem cells for solar water splitting. J. Mater. Res. 25, 17-24 (2010)... 

Establishing an interesting reference case for multijunction solar water splitting, multijunction PV cells with adequate useable photopotential to directly drive electrolysis have been coupled with electrolyzer systems, which can be separate or fully integrated. This approach is compatible with the PV-electrolysis reactor types described in Sect. 7.2.4. However, since the PV output of the multijunction cell is designed to have direct compatibility with electrolysis, no power conditioning units are required. A device level schematic is shown in Fig. 7.26 for the case of a triple-junction PV cell driving the electrolyzer reactions. The useable potential... 

An alternative tandem device being developed for solar water splitting is the PV/PEC hybrid photoelectrode. In this approach, a PEC top cell is monolithically stacked with a solid-state single-junction PV back cell. Such a photoelectrode device is compatible with both the one- and two-electrode reactor configurations described in Sect. 7.2.4, and the reactor-level considerations described in that section apply. Figure 7.28 shows a device level schematic for a specific two-electrode implementation incorporating a hybrid photocathode and a separate counter electrode. Possible alternative variations include the use of a hybrid photoanode instead of the photocathode, and the integration of the counter and... 

Another important example of interfacial PCET can occur between semiconductor surfaces and adsorbate molecules, and is particularly relevant for some of the current energy conversion strategies, such as dye-sensitised solar cells (DSSC) or photoelectrochemical (PEC) water splitting cells. A simple proof of the involvement of PCET in interfacial redox processes is the dependence of the conduction and valence band potentials of semiconducting metal oxides, such as Ti02, with pH. The nature of the surface terminal groups (typically O or OH in metal oxides) will have a strong influence in the thermodynamics and kinetics of the system. 

Optical and electrical properties of plasma deposited films, sometimes unique indeed, as well as the easy of their deposition, at low temperature and low cost, on inexpensive substrates of almost any size and shape, render these materials very attractive for optoelectronic applications. The possibility to tailor optical parameters, such as refractive index and extinction coefficient, and what is particularly important - the ability to adjust parameters of the electronic structure, such as transport p, optical gap, density of localized states, etc., recommend these plasma films as active photoelectric elements, e.g. for solar cells and water splitting cells. 

Finally, one more type of water splitting cells should be mentioned, namely integrated photovoltaic-electrolysis (PV-PEC) cells. In this type of devices, the photovoltaic cell and the electrolyser are combined into a single system, in which the light-harvesting solar cell is one of the electrodes. Very often, thin-film solar cells fabricated by the cold plasma deposition method are employed in the PV-PEC devices (Kelly Gibson, 2006). A diagram of such a system with a simply a-Si H solar cell is shown in Fig. 8. There is no doubt that the role played by the cold plasma deposition technique in the creation of such systems is unquestionable (see Sec. 4.1.). 

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