Efficiency of light-energy conversion in photogalvanic cells and water cleavage systems

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Abstract
In this paper we assess the thermodynamic efficiency of models for a well-known photogalvanic cell and a recently developed water cleavage system in order to determine, for comparable light-flux conditions, their relative effectiveness as light-energy conversion devices. The photogalvanic cell considered is the Ru(bipy)3+2/Ru(bipy)3+3–Fe+2/Fe+3 system and we base our calculations on the experimental work of Creutz and Sutin, and Lin and Sutin, and the theoretical analysis of Albery and Archer. The cyclic water cleavage system considered is the Ru(bipy)3+2/Ru(bipy)3+3–MV+1/MV+2 system (using a dispersed metal/metal oxide catalyst) discovered by Kalyanasundaram and Grätzel, and our calculations are based on their experimental work and the recent stability analysis of this system reported by Dung and Kozak. Although superficially different, the analysis draws attention to the common (photochemical, electrochemical, and mathematical) features of these two systems and, where possible, this correspondence is stressed and implemented in our simulations. Although other examples of each type of device are known, for this pair of systems (as described by the models presented herein) we find that the efficiency of conversion of light energy into electrical energy via a photogalvanic cell is similar to the efficiency of conversion of light energy into chemical energy via the (cyclic) water cleavage system, if the concentration of the dispersed metal/metal oxide catalyst in the system is <10−4 M. However, one finds a pronounced increase in the efficiency of the water cleavage system (under the same light-flux conditions) when the concentration of catalyst is increased, and this enhancement is quantified in our simulations. Alternatively, simulations also show that the efficiency of the photogalvanic system can improve markedly upon suppressing the back reaction between Ru(III) and Fe(II).