Water photo-electrolysis

O.K. Varghese, C.A. Grimes, Appropriate strategies for determining the photoconversion efficiency of water photo electrolysis cells a review with examples using titania nanotube array photoanodes. Sol. Energy Mater. Sol. Cells 92, 374—384 (2008)... 

Electrolysis of a water solution of KI gives a saturated solution of iodine at the anode (see photo at top of page 499). 

Photoelectrochemical water-splitting is a combination of solar cell with electrolysis in a electrolyte, and has been actively studied. However, the selection of the photo semiconductors is so tightly limited that photoelectrochemical methods can hardly compete with the combined system of solar cell with electrolysis. 

Figure 2). Hydrogen was evolved at the cathode, the over-all process being the decomposition of water. This process, involving an applied potential difference between the anode and cathode, we shall refer to as photo-assisted electrolysis.

Photo-assisted Electrolysis of Water Using n-type Anode and Metal... 

Sastri MVC, Nagasubramanian G (1982) Studies of ferric oxide electrodes for the photo-assisted electrolysis of water. Int J Hydrogen Energy 11 873-876... 

Noda M (1982) Photo-assisted electrolysis of water hy Si photoelectrodes. Int J Hydrogen Energy 7 311-320... 

Below stepped-illumination experiments are presented for the photo-assisted electrolysis of water using n-type TiC or SnC photoanode/dark Pt cathode systems. An analysis of these results will be performed, focusing on the influence of the anodic halfcell reaction products upon the electronic state of the semiconductor /electrolyte interface. 

Consider a photo-assisted water electrolysis cell, incorporating a photoanode and dark metal cathode. Illumination of the n-type semiconductor photoanode with a depletion space charge region results in a net flow of positive vacancies, or holes, to the semiconductor/electrolyte interface. Here the hole (h+) may be accepted by the reduced form of the oxygen redox couple. 

Pourbaix (16) has prepared theoretical stability diagrams of potential vs. pH for many common metals and nonmetalloids. A review of these results indicates that semiconductor compounds of Au, Ir, Pt, Rd, Ru, Zr, Si, Pd, Fe, Sn, W, Ta, Nb, or Ti should serve as relatively acid-stable photoanodes for the electrolysis of water. Indeed, all of the stable photo-assisted anode materials reported in the literature, as of March, 1980 (see Table III) contain at least one element from this stability list, with the exception of CdO. Rung and co-workers (18) observed that the CdO photoanode was stable at a bulk pH of 13.3. The Pourbaix diagram for Cd (16) shows that an oxide film passivates Cd over the concentration range 10.0 < pH < 13.5. Hence the desorption of the product H+ ion for the particular case of CdO must be exceptionally facile without producing an effective surface pH lower than 10.0. This anamolous behavior for CdO is not well understood. 

Hydrogen can be produced from splitting of water through various processes ranging from water electrolysis, photo(solar)-electrolysis, photo-biological production to high-temperature water decomposition. 

Fig. 1. Alternate configurations varying the number of photo harvesting units and electrolysis units for solar water splitting.3 The photoconverter in the first system generates the requisite water electrolysis voltage and in the second system generates twice that voltage, while the photoconverter in the third and fourth units generate respectively only half or a third this...

Ottova et al. looked at two-compartment semiconductor-septum electrochemical photovoltaic cells with cadmium selenide and cadmium selenide telluride for water photolysis [126], They used cells consisting of two chambers separated by a CdSe or CdSe/CdTe bipolar electrode. The bipolar electrodes were prepared by painting a CdSe slurry on a metal substrate or by ultrasound-aided electrodeposition from CdSe solution in ZnCl2. The photoresponse (voltage and current output) and hydrogen yield from photo-induced electrolysis of H20 in the dark chamber of the cell were evaluated as a function of CdSe preparation method. The ultrasound-aided deposition technique gave excellent coatings of CdSe. 

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