Photo-assisted 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... 

Bockris JOM, Murphy OJ (1982-1983) The two efficiency expressions used in evaluating photo-assisted electrolysis. Appl Phys Commun 2 203-207. 

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... 

Practical utilization of photo-assisted electrolysis systems is hampered by poor overall conversion efficiencies. Essentially, this problem results from poor quantum efficiencies at low bias. 

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. 

The photo-assisted electrolysis current vs. time scans were obtained with the following experimental set-up ... 

Occurrence of the photo-assisted electrolysis reaction, from left to right, produces an increased H+ concentration at the photoanode surface. 

We have previously analyzed the photo-assisted corrosion of n-type materials during photoanode applications (8). This work suggested that the H+ ions produced during the electrolysis reaction underwent desorption from the photoanode surface much less readily than would be suspected for fully hydrated H+ ions, and is summarized in Appendix I. 

Bokris and Uosaki (1) have studied transient photo-assisted electrolysis current for systems including a p-type semiconductor photocathode and dark Pt anode. A set of current vs. time scans taken with a ZnTe photocathode system is shown in Figure 6. 

We have performed an experimental study of photo-assisted electrolysis for illuminated n-type TiC>2 photoanode/dark Pt cathode systems. Analysis of these results indicates that the electronic state of the semiconductor/electrolyte interface is influenced by the electrolysis reaction products, in a manner not previously accounted for. 

A review of photo-assisted electrolysis studies performed with p-type semiconductor photocathode/dark Pt anode systems suggests that a complementary phenomena arising from the presence of OH ions produced during the reduction half-cell reaction,... 

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. 

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