ELECTRODIC PHENOMENA AT ANODE OF TOTALLY ILLUMINATED, THIN-LAYER IRON-THIONINE PHOTOGALVANIC CELL

Properties of Corning thin-film SnO2 electrodes in contact with 0.005-0.5M solutions of sulfuric acid in aqueous solvents containing 0-50 v/o CH3CN were characterized. Schottky-Mott plots gave flatband potentials (EFB) in the range 0-0.2V vs. SCE and charge carrier densities in the range 4-7 × 1020cm-3 in 0.01M sulfuric acid, independent of solvent composition. Standard reduction potentials vs. SCE in 0.01M acid at room temperature were calculated from equilibrium compositions to be -0.04V for TH+/-TH2+, 0.33V for TH2+/TH42+ and 0.14V for TH+/TH42+ in water and -0.06V for TH+/-TH2+, 0.28V for -TH2+/TH42 + and 0.11V for TH+/TH42+ in 50 v/v/o aq. CHSCN. Voltammetric data show that establishment of protonic equilibrium is rapid compared with cathodic reduction of TH+ at SnO2 in water and in 50 v/o aq. CH3CN. In contrast, protonic equilibration between the two electron-transfer steps is slow for thionine-l-sulfonic acid. Voltammetric data show that reduction of TH+ and oxidation of TH42+ are kinetically controlled at both SnO2 and Pt electrodes with reversibility greater at platinum than at SnO2. Reversibility is slightly reduced by addition of CH3CN to the solvent. Rectification (inhibition of oxidation of TH42+) is not severe enough to prevent the use of SnO2 as a selective anode but may reduce efficiency of photogalvanic conversion. Weak but persistent adsorption of TH+ on SnO2 activates photogalvanic conversion. Adsorption of TH42+ varied considerably from sample to sample of SnO2 and was strong in some cases. Both oxidation and reduction of the Fe+3/Fe+2 couple are much less reversible at SnO2 than at Pt. Rectification (inhibition of oxidation of Fe2+) is pronounced. Both oxidation and reduction on SnO2 become more reversible with increasing fraction of CHaCN; the major effect is enhancement of oxidation of Fe2+ but even with 50 v/o CH3CN the couple is much less reversible than at Pt. Implications of the data with respect to efficiency of totally illuminated thin-layer iron-thionine photogalvanic cells are discussed. © 1978, The Electrochemical Society, Inc. All rights reserved.