Involvement of coordination chemistry during electron transfer in the stabilization of the pyrite (FeS 2)photoanode

Abstract

The exchange of electrons between reducing agents and illuminated pyrite (FeS 2 has been studied using rotating ring-disc techniques with special emphasis on the stabilization of the semiconductor liquid junction. A systematic investigation of the redox systems I −/I 2 ( E 0 = 0.534 V/SHE), Br −/Br 2 ( E 0 = 1.087 V/SHE), Fe(phen) 2+/3+ 3( E 0 = 1.056 V/SHE) and Fe(bipy) 2+/3+ 3( E 0= 1.096 V/SHE) has been made to find out whether the ability of the reducing species to interact chemically with the pyrite surface has an effect on stability against corrosion. It turned out that under identical experimental conditions the negatively charged halogenide ions I − and Br − stabilize pyrite efficiently while the positively charged complexes Fe(phen) 2+ 3 and Fe(pipy) 2+ 3 cannot stabilize it. These results support the conclusion that the pyrite photoanode requires ligand mediated inner sphere electron transfer to iron states in the electrode surface for stabilization. Complementary evidence comes from the observation that addition of Cl − ions to a Fe(phen) 2+ 3 containing electrolyte improves stabilization by this complex. The consequences of this study are that (1) electron transfer theory based on weak interaction cannot be applied to this system and (2) development of efficient stabilization of photoelectrodes for regenerative solar cells appears to be feasible on the basis of mechanisms involving interfacial coordination chemistry.