Electric investigation of a photo-electrochemical water splitting device based on a proton exchange membrane within drilled FTO-covered quartz electrodes: under dark and light conditions

Abstract

I-V characteristics of a photo-electrochemical (PEC) water splitting device based on a proton exchange membrane with fluorine doped tin oxide (FTO) covered quartz drilled electrodes were investigated. A nanostructured TiO2 film has been used as the anodic photo-catalyst and a supported Pt thin film has been employed as the cathodic electro-catalyst. To interpret the experimental electric data in the cell, in both dark and sunlight conditions (air mass, AM 1.5G, 100mW/cm2), appropriate descriptions have been used for each electrode. The cathode was modelled in terms of the Butler-Volmer approach and for the anode a semiconductor diode equation was used. Taking into account the experimental evidence that the oxygen evolving electrode overpotential in the working cell is rather large respect to the cathode (at least one order of magnitude), an approximated expression for the current is proposed. The influence on I-V of the covering of the electrode by atomic and molecular oxygen is estimated, in the linear limit, and the important role of the bubble covering factor in the behavior of the device is discussed. A model for the AC response of the cell to an external AC stimulus of small amplitude, based on a generalization of the Poisson-Nernst-Planck model with linear boundary conditions is also developed and used to interpret the electrochemical impedance spectroscopy (EIS) measurements on the PEC device allowing the determination of the two transfer coefficients at the electrodes and their dependence on the applied bias for both dark and sunlight conditions. The developed models provide useful tools to determine and compare important parameters affecting the photo-catalytic and electro-catalytic performance of the electrodes and the final efficiency of the PEM photo-electrolyzer.