Operando X-ray absorption spectroscopy studies of Fe and Co based catalyst on WO3/BiVo4 photoanodes for water splitting

The goal of this thesis is to experimentally study the local atomic and electronic structure of two compounds used as co-catalysts of photoelectrochemical reactions using X-ray absorption spectroscopy (XAS) with synchrotron radiation. Specifically, CoFe-prussian blue and amorphous CoFe-oxide layers deposited on WO3/BiVO4 were studied before, during and after photoelectrocatalysis. Firstly, ex-situ XAS at the Fe and Co K-edges were performed in order to understand the initial local structure around these cations. Then, the samples spectroscopical features were investigated by employing a specifically designed photoelectrochemical cell, and operando measurements were performed under the application of a bias potential and illumination in order to reproduce the operating conditions. In absence of light excitation, we investigated the bias-dependence of the XAS spectrum on Co-K edge for both catalysts, observing that the cocatalyst oxidizes while in the PEC cell under bias. When the electrolyte is removed, it tends to revert to the initial state. Successively, to see the effect of light, a second operando measurement were performed illuminating the samples (while in PEC with electrolyte) in two different methods: by illuminating constantly and intermittently. We found that illuminating the sample produces a slight modification in the absorption spectra, which was further investigated by FEXRAV (fixed energy X-ray absorption voltammetry). This last technique was performed to map better the oxidation states of the samples in the applied potential window, finding out that CoFe−PB and CoFeOx have different behavior. In fact, for CoFe−PB it takes an higher voltage in the dark than in the light to start the oxidation process whereas for CoFeOx the opposite behavior is found. A preliminary interpretation is that this is due to the different recombination processes of electrons between the co-catalyst and BiVO4.