Thermodynamic evaluation of the n-AgIn 5Se 8 and n-CuIn 5S 8 stability in photoelectrochemical cells

Abstract

The thermodynamic stability of n-AgIn 5Se 8 photoelectrodes in several electrolytes was evaluated and the results were compared to those recently published concerning their behaviour in photoectrochemical cells. The bond cleavage photoreactions and the oxidative photodecomposition reactions possibly taking place in aqueous solutions containing either the iodide-polyiodide, the sulphide-polysulphide or the Ce 3+/Ce 4+ redox systems were examined. Reactions forming chalcogen atoms (eventually solvated to sulphide or polysulphide ions in the sulphide-polysulphide electrolyte) plus cations and/or chalcogenides, oxides and iodides were considered. Their Gibbs free energy changes and their decomposition potentials in standard conditions reffered to the normal hydrogen electrode were evaluated. Experimental data were utilised for constructing the compounds energy diagram in open-circuit conditions in order to compare the hole quasi-Fermi level at the surface with the Fermi level relevant to the above processes and to predict the materials stability in different electrolytes. In aqueous sodium sulphate solutions, AgIn 5Se 8 results to be highly unstable even in rest conditions, in agreement with experimental findings, whereas CuIn 5S 8 photodecomposes at high reverse bias only. In the I −/I 3 − containing electrolyte, the AgIn 5Se 8 situation is even worse since many photocorrosion reactions are thermodynamically possible, while the Fermi level of the redox system is located below the hole quasi-Fermi level at the electrode surface. On the contrary, owing to its favourable energy position, the redox process may positively compete with the photodecomposition reactions of CuIn 5S 8 in the sulphide-polysulphide electrolyte, though thermodynamics by itself is not able to protect the material. Hence, the observed behaviour depends on the kinetic activation barriers and therefore, on the parameters individually related to the sample quality, surface pretreatment, electrolyte composition, etc. In the Ce 3+/Ce 4+ solution, CuIn 5S 8 has a more positive flatband potential than in the other electrolyte studied, and several photodecomposition processes are more probable than the redox process.