The Effect of Hydroxide Ion on Cd‐Chalcogenide/Aqueous Polysulfide Photoelectrochemical Cells

Abstract

Alkali hydroxide, added to the aqueous polysulfide electrolyte in n‐Cd photoelectrochemical solar cells (PEC's), is shown to be detrimental to cell performance. It is demonstrated that the added hydroxide increases visible light absorbtion in the polysulfide solution and decreases the solution lifetime. Even after compensation for the decrease in light transmission by the electrolyte, added hydroxide is shown to decrease the PEC photocurrent, photovoltage, and optical‐to‐electrical conversion efficiency. In a cell of 1 cm path length, the transmittance at 580 nm, for solutions containing , 3m sulfur, and , was, respectively, 66, 55, 44, and 37.5%. Analysis of the distribution of ionic species reveals a shift from to with increasing hydroxide. Compared to , the peak absorbance of is shifted 50 nm toward the visible, causing the variation in solution spectral response with hydroxide. K+ activity measurements were interpreted as indicative of increasing ion pairing with increased added hydroxide which may adversely effect charge‐transfer kinetics. A measured negative shift in polysulfide redox potential with increasing hydroxide is evidently not paralleled by a comparable shift in flatband potential, resulting in the observed decrease in open‐circuit voltage. Relative conversion efficiency for an electroplated thin film electrode was 36% less in polysulfide with compared to the PEC without added . The electrode immersed in 2/2/2, 2/2/3, 0/2/3, or 0/1.3/2 (molality ) exhibited conversion efficiencies of 4.72, 4.80, 5.24, and 5.44, respectively, at 100 mW/cm2 tungsten‐halogen lamp illumination.