Counter-electrode function in nanocrystalline photoelectrochemical cell configurations

The search for rigid or flexible photoelectrochemical solar cell counter-electrode (CE) alternatives has been a continuous effort and long on-going process in our lab, as studies in CE kinetic performance and stability on the one hand seek to improve the overall efficiency of the solar cell, while attempting to adapt to novel design concepts or new materials, on the other. The dye-sensitized TiO2 nanocrystalline solar cell utilizing the iodide/tri-iodide redox mediator served as the system of reference for a theoretical characterization and computational simulation used to scrutinize CE performance, which was coupled by experimental exploration of catalyst materials and different design options. Two basic approaches can be identified with respect to CE design. Firstly, there is the case where the catalyst has sufficient kinetic performance and can be utilized when deposited on any stable support material current collector even at monolayer quantities, and secondly, the case when the kinetics of the catalyst are insufficiently high to sustain the required currents and therefore the effective exchange current density must be enhanced by internal surface area increase, thus the need to impart porosity to either the catalyst material or the current collector or both. The kinetic/electrocatalytic performance of candidate catalyst materials, as well as the mass-transfer limitations of commonly applied cell configurations have been determined in most cases by means of experiment. However, predictions have also been made by electrochemical simulation of a variety of given systems under steady-state operation, where the CE is examined as an integral part of the energy conversion system, making clear the implications of the varying physical and geometric parameters of the comprising elements of the device, i.e. the porous photoelectrode, the spacer configuration and the CE, including also the nature and the properties of the electrolyte constituting their junction. (C) 2004 Elsevier B.V. All rights reserved.