This paper performs a thermodynamic analysis of a modified dual photo-electrochemical cell for water electrolysis, previously developed by Grätzel. In a typical Grätzel cell, a semi-transparent photo-electrode is used in conjunction with a dye photo-sensitized cell to generate a bias voltage necessary to overcome the inherent over-potentials caused by irreversibilities within the cell. The modified method implements hybrid photo-catalysis instead of heterogeneous catalysis. In the hybrid photo-catalysis approach, both homogeneous and heterogeneous photo-catalysts are used to enhance the speed of the water splitting reaction and to widen the portion of the solar radiation spectrum for the process. The dual cell consists of two tandem units. The first unit is a photo-electrolysis cell exposed to solar radiation (at both cathodic and anodic sides) via a transparent window, whereas the second unit is a dye-sensitized solar cell, which assists the photo-electrolysis cell. In the cathodic solution, there are dissolved Brewer-type supra-molecular complexes for photo-catalytic water reduction to generate hydrogen. They absorb solar radiation in the upper visible spectrum and dislocate multiple electrons at the active center. The complexes accept electrons donated from a GaP-based semi-transparent photo-cathode. The remaining un-absorbed radiation (mainly in the infrared range) crosses the semitransparent counter-electrode and the back glass. It is absorbed by a solar thermal collector for enhancing the solar radiation utilization by co-generating low-grade heat. The tandem cell with hybrid photo-catalysis has promising potential of improved solar radiation utilization. This paper analyzes the system efficiency and shows that 4% energy efficiency can be obtained for hydrogen generation. About 20% of the incident solar spectrum can be captured by the cell and used for hydrogen generation. Around 60% of solar radiation is recovered in the form of heat on a flat plate solar thermal collector placed behind the cell. The influence of catalyst concentration and pH also is studied. The device forms a four-gap solar absorber system, which is coupled to a cogeneration sub-system for heating, so the solar energy utilization is maximized. The four-gap system absorbs photons at 1.6, 2.1, 2.3, and 2.7 eV and generates five reversible potentials of 0.42, 0.9, 1.6, 2.1, and 2.3 V. Based on the predicted results, the reaction rate appears to be enhanced with respect to other solar electrolysis cells (such as photo-electrolyzers and a dual photo-electrochemical cell) because homogeneous catalysis enhances the electrode kinetics. Copyright © 2012 John Wiley & Sons, Ltd.
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