Potential Enhancement of Polyiodide Redox Couples via Solution Modification

The stability and high reversibility of the polyiodide equilibrium results in a positive redox couple useful in fuel, battery, and photoelec-trochemical solar cells I- +12~-13;1 ~ + 2e-->31; E(0.01 ml2,1 mNal)=298mVvs. Ag/AgCI A further positive shift of polyiodide redox couples which can make polyiodide more attractive as a storage cathode is accomplished by a two-step solution modification process. Solution-phase iodide is bound by complexation with cadmium salts. Then the complexation product, unstable at neutral pH, is stabilized by acidification. The redox potential is increased while maintaining facile charge transfer and electrolyte stability. For example, redox potentials of 425,430, or 440 mV are sustained in solutions of 0.01 m 12, 1 m Nal, and 1 m Cd(NO3)2 containing either 1 m H2SO4, 1 m HNO3, or 1 m HCIO4, respectively. Electrochemical fuel cells, batteries, 1-4 and photoelectrochemical solar cells S-14 (PECs) have employed iodide and other halide redox couples. In both electrochemical storage cells and PECs, the iodide couple can provide facile charge transfer resulting in low polariza-tion losses. Often PECs containing the iodide couple can attain efficient solar to electrical conversion efficiency not observed wit