Oxide electrodes photocorrosion

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

The band-gap excitation of semiconductor electrodes brings two practical problems for photoelectrochemical solar energy conversion (1) Most of the useful semiconductors have relatively wide band gaps, hence they can be excited only by ultraviolet radiation, whose proportion in the solar spectrum is rather low. (2) the photogenerated minority charge carriers in these semiconductors possess a high oxidative or reductive power to cause a rapid photocorrosion. 

While freshly cleaved surfaces clearly contain some steps and other defects, the surfaces are remarkably stable to corrosion. Dramatic evidence of the stability of MoSe2 surfaces was provided by Stickney et al. who obtained LEED and Auger spectra of surfaces that had been exposed to the atmosphere and a variety of oxidizing solutions (35). Except for the presence of a ubiquitous carbon which was attributed to the epoxy resin or cleaving tape, no evidence for surface reactions was found. Long term stability tests of a photoelectrochemical cell made with a WSe2 electrode, where over 400,000 coulombs/cm2 were passed through the cell with no detectable photocorrosion, also attest to the durability of these surfaces (36). 

Photocorrosion can be prevented by adding a redox couple to the electrolyte whose potential is more favourable than the decomposition potential such that the redox reaction occurs preferentially. When n-CdS is used as photoanode in aqueous electrolytes, the electrode is photocorroded since the reaction, CdS -1- 2h - S -1- Cd, occurs readily. By adding NaOH and sodium polysuphide to the electrolyte (Ellis et al, 1976), photocorrosion is prevented. The /S redox couple preferentially scavenges the photoholes. At the anode, sulphide is oxidized to polysulphide (free sulphur) and free sulphur is reduced back at the dark cathode. Similarly n-Si anodes have been stabilized by using a nonaqueous electrolyte containing a ferricinium/ferrocene redox couple (Legg et al, 1977 Chao et al, 1983). Unfortunately, a similar stabilization technique cannot be applied to photoelectrolysis cells. Some examples of electrode... 

An interesting example of the kinetic effect in semiconductor photocorrosion is photopassivation and photoactivation of silicon (Izidinov et al., 1962). Silicon is an electronegative element, so it should be dissolved spontaneously and intensively in water with hydrogen evolution. But in most of aqueous solutions the surface of silicon is covered with a nonporous passivating oxide film, which protects it from corrosion. The anodic polarization curve of silicon (dashed line in Fig. 20) is of the form characteristic of electrodes liable to passivation as the potential increases, the anodic current first grows (the... 

Anodic oxidation of pyrrole and N-substituted pyrroles results in the formation of polypyrroles in an oxidized state, which can be useful for the preparation of conducting organic polymers.185-188 Oxidation of 2,5-di-substituted pyrroles produces soluble products and no layer of polymers.187 One of the proposed applications of such a layer of conducting polymer is the protection of semiconductor electrodes from photocorrosion.189-191... 

The main obstacle to creating liquid junction solar cells is photocorrosion of semiconductor electrodes, which reduces considerably their lifetime. In order to prevent, for example, anodic photocorrosion, a well-reversible redox couple is introduced into an electrolyte solution, so that the reaction of oxidation of the red component competes for photoholes with the reaction of photodecomposition of the electrode material (see Section IV.2). With the aid of this method, photocorrosion has been practically prevented in certain types of photocells and the duration of their continuous operation has been increased up to about one year. Yet, there are other, more subtle mechanisms of electrode degra-dation, which has hitherto prevented the lifetime of photoelectrochemical cells from becoming comparable with the 20-year lifetime of solid-state solar cells. 

Among the approaches taken to solve this problem, the most successful concern the matching of an electolyte to the semiconductor. The rate of corrosion is reduced if the semiconductor is in equilibrium with the corrosion products. The rate of corrosion can also be reduced by using a redox couple which oxidizes easily. The oxidation of the redox couple SeJ /Se2- for example, has been shown to compete successfully with photocorrosion reactions for holes in n-type GaAs electrodes.28,113-116... 

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