Degradation of oxynitride based photoanodes.

The production of green hydrogen via photoelectrochemical water splitting has the potential to play a vital part in the decarbonization of our energy economy. To commercialize this technology, the stability of the current photoelectrode materials needs to be improved. Therefore, it is essential to understand the processes causing degradation and define suitable figures of merit. This work investigates the degradation mechanisms of oxynitride electrodes focusing on TiO2 necked LaTiO2N particle-based photoanodes. Their degradation behaviour was assessed by chronoamperometries at 1.23 V vs. RHE in basic electrolyte. We identified two current decay processes based on a semi-empirical correlation between the measured chronoamperometries and a sum of two exponential decay terms. The time constant of the second exponential function is proposed as an alternative figure of merit for quantifying the stability of oxynitride based photoanodes. The applicability of this figure of merit to a wider range of oxynitride-based photoanodes and measurement conditions is demonstrated by evaluating previously reported chronoamperometries. By in depth analysis before and after chronoamperometry using STEM-EDX/EELS, HREM, ICP-MS, and XPS, we find experimental evidence that the performance decrease of the LaTiO2N photoanodes is caused by a combination of surface oxidation and cocatalyst dissolution.