Importance of holes in understanding mesoporous electrodes for solar water splitting
Daniel Staff a, Alison Walker a, Thomas Risbridger b, Laurence Peter b, Petra Cameron b
a Physics, University of Bath, Department of Physics, University of Bath, Bath, 0, United Kingdom
Oral, Alison Walker, presentation 016
Publication date: 31st March 2013

The direct splitting of water into hydrogen and oxygen using light in semiconducting photo-electrochemical cells (PEC) has received much attention since the concept was first demonstrated because of hydrogen's potential as a fuel. There have been extensive efforts to discover and design suitable photo-electrode materials for this process. Although TiO2 only absorbs light in the UV part of the solar spectrum, it remains of great interest as a model system due to its ease of preparation and operational stability, as well as the possibility of reducing its band gap. Additionally, since the successful application of nanocrystalline TiO2 in dye-sensitized solar cells, creating a bulk heterojunction, the use of mesoporous photo-electrodes is an interesting alternative to bulk electrodes for PEC water splitting.

We present an electrical transport model for the TiO2 electrode that predicts the current of the water splitting cell for substrate side and electrode illumination at varying illumination wavelengths and intensities. This model solves the coupled continuity equations for electrons and holes in 1-dimension, assuming that as is the case for dye-sensitized cells, there is no space charge since the electrode is mesoporous and permeated by an electrolyte solution. Our model is quite general and includes transport, recombination, water oxidation and back reaction of electrons with oxygen. Our results are compared with experimental studies of the variation of incident photon to current efficiency with wavelength for and of current density with illumination intensity.

We show for the first time the need to consider the role that holes play and discuss how the competition between transport, recombination, water oxidation and back reaction in determining cell output.

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