A photoelectrochemical system for hydrogen production from aqueous solutions has been developed, in which hydrogen is stored at the cathode with metal hydride formation. In this system, anodic oxygen evolution reaction is replaced by sulfide ion oxidation reaction, the occurrence of which together with cathodic hydrogen storage reaction requires lower overpotential. Moreover, an advantage of the system developed over other systems for hydrogen production by the photoelectrochemical method is the fact that hydrogen is stored by the action of sunlight immediately in a convenient technological form as a metal hydride. This system allowed one to use as photoelectrodes AIIIBV semiconductors, which absorb well light in the visible region: GaAs (whose energy gap width Eg = 1.43 eV) and InP (Eg = 1.28 eV); this allowed the efficiency of the system to be greatly increased through the use of a wide spectral range of sunlight.
The effect of electrolyte composition on the phototpotential of GaAs and InP electrodes in redox systems, which protect semiconductor from photocorrosion, has been studied. It has been found that in polysulfide electrolyte, electrode photopotential shifts fowards more negative values, and that recombination loss decreases when the sulfide ion concentration is increased, which increases the efficiency of solar energy conversion. It has been found that the surface modification of GaAs and InP electrodes by metal and semiconductor nanoparticles and fullerites increases the quantum yield of photoelectrochemical current in a wide spectral range. In an electrochemichal system for hydrogen storage using these semiconductors and metal hydrides, an efficiency of conversion of solar energy to chemical hydrogen energy of 7 – 8 % has been achieved.
Chemical and electrochemical procedures have been developed for the synthesis of photosensitive thin films of AIIBVI semiconductors (CdSe, CdSe/ZnS) for use in photoelectrochemical systems with hydrogen storage. The optimal temperature range of formation of photosensitive thin films in air atmosphere has been determined. It has been found that ion treatment of the surface of Zn2+ films increases their photocatalytic activity and reduces recombination loss, as a result of which photopotential takes on the optimum value for the use of these photoanodes together with metal hydride cathodes. It has been shown that the stability of photoelectrochemical cell as a whole is mainly determined by the stability and corrosion resistance of photoanodes and is the highest when aqueous polysulfide solutions, to which selenide and thiosulfate ions have been added, are used. Methods for the synthesis of polycrystalline CdSe thin films ensure their high photosensitivity, prevention of photocorrosion, long and reliable operation of solar batteries with hydrogen storage, as a result of which their economic efficiency increases. It has been found that in the case of exposing photoanodes to light, the efficiency of hydrogen storage by cathodes in this system is 85 – 98 %.
Procedures for the formation of photosensitive films based on CdSexTe1-x solid solutions (x = 0.50 – 0.65) and modification of their surface by ZnS and CdS nanoparticles have been developed; it has been found that after the modification of electrodes, the photoelectrochemical processes in them become more efficient. As a result of development of procedures for the formation of CdSe and CdSe0.65Te0.35 films, photoelectrodes with large area (~ 30 cm2) for use in photoelectrochemical cell have been obtained. It has been found that the photoanodes are fully compatible with the cathodes developed at the I.N. Frantsevich Institute of Materials Science Problems of the Ukrainian NAS for use in photoelectrochemical systems. Two designs of experimental photoelectrochemical cells for hydrogen storage have been developed; full-scale tests of these cells have been carried out, which showed them to have promise in hydrogen storage.
On the basis of this photoelectrochemical device, three-electrode photoaccumulator was proposed, which ensures uninterrupted operation of the using equipment both with and without photoelectrode illumination thanks to the reversibility of the electrochemical processes involving hydrogen.
Keywords: Hydrogen, photoelectrochemical cell, solar energy conversion, photopotential, semiconductor photoanod