Photoanodes of n-type semiconductors with solar matched bandgaps are generally difficult to stabilize in conventional aqueous redox electrolytes. The use of highly concentrated electrolytes which reduce water activity in the design of photoelectrochemical systems has been an attractive strategy. Modification of the kinetics and energetics of selected electrode reactions in these media can lead to enlarging the redox potential range of stability and enhancement of power conversion efficiency. The Cu(I)-Cu(II) couple in concentrated chloride media is of interest because of its reversible behavior, favorable shift of potentials with increasing ligand (chloride) concentration, and effective competition to InP oxidation. Rotating ring-disk electrode voltammetry has been used to demonstrate efficient hole transfer from n-InP to Cu(I) in saturated CaCl2electrolyte and singular increase in operating lifetime over that of conventional aqueous (0.5M CaCl2) media. The spectral and transport properties of the electrolyte system have been measured. Open-circuit voltages above 600 mV for the n-InP/Cu(I)-Cu(II)-saturated CaCl2/C cell are readily attainable for solar intensities.