Fabrication and evaluation of intercalation electrodes for the storage of electrical energy generated in photoelectrochemical storage cells are the project objectives.
Layered chalcogenides of general formula MX2 were investigated for the storage of electrical energy generated by photoelectrochemical solar cells. These storage materials can be highly economical and have excellent volumetric energy densities, allowing direct incorporation into flat plate photovoltaic modules. Two approaches were evaluated: (1) intercalation electrodes, such as CuxTiS2, which can be charged by a separated regenerative photoelectrochemical cell, and (2) 'active' photointercalation/photodeintercalation cells in which the storage step is effected directly by irradiating a layered semiconducting photoelectrode.
A cell of the first kind was demonstrated in a cell consisting of an n-GaAs photoelectrode and TiS2 intercalation electrode. The intercalant was Cu in an acetonitrile-tetrabutylammonium chloride electrolyte. During the charging reaction, CuC12 -1 was oxidized to CuC14 -2 at the n-GaAs photoanode and reduced to CuxTiS2 at the cathode. The best results were obtained with TiS2, which had been pressed onto Ta exmet screen. The cell was charged under tungsten lamp irradiation of 40 mW/cm 2 until the TiS2 was 36 percent utilized. The spontaneous discharge cycle was continued until 64 percent of the stored charge was recovered. The major problem encountered in the cycling experiments was deterioration of the physical structure of the TiS2 electrode, although the system itself appears to be chemically reversible between 0 < x < 0.6.
A new electrode bonding procedure based on presintered MX2/Teflon/ graphite mixtures was developed and produced much more robust electrodes. Use of VSe2 as the intercalation material resulted in a spontaneous intercalation rate of Cu in aqueous solutions, which was almost six times that obtained for TiS2.
Studies of 'active' photointercalation devices were made on four singlecrystal" semiconducting layered dichalcogenides: HfS2, HfSe2, ZrSe2, and ZrS2. These were prepared as single crystals by iodine vapor transport. The photoelectrochemical properties of the four compounds were measured in electrolytes with different redox potentials. HfS2 and ZrSe2 showed both pand n-type photoresponse indicative of nearly intrinsic, highly compensatedtype material. No photoresponse was observed with the ZrS2 crystals. Cu was spontaneously intercalated into HfS2 in aqueous solution. As the intercalation progressed toward 0.5 equivalent/mole passed, the photovoltage and photocurrent increased, and the crystal showed pronounced n-type charac-