Illuminated scanning flow cell – operando study on
light-induced power generation & degradation
Photovoltaic materials have seen rapid development in the past decades, propelling the global transition towards a sustainable and CO2-free economy. Storing the day-time energy for night-time usage has become a major challenge to integrate sizeable solar farms into the electrical grid. Developing technologies to convert solar energy directly into hydrogen fuels would not only ease grid operation, but also power other energy demands, e.g. fuel cell vehicles.
Current photovoltaic materials have excellent conversion efficiency, but they need to be shielded from the environment, precluding their use in water electrolysis. A new class of materials, photoelectrodes, is being developed which combines both functions of harvesting solar energy and converting them into hydrogen by electrolysis. As photoelectrodes are witnessing ever increasing power conversion efficiency, their lifetime has become the limiting factor for their economic return, making it pivotal to understand how these materials degrade under operation conditions.
To address this specifically, we have built up an illuminated scanning flow cell (iSFC) system to enable operando measurements, as plotted in Fig. 1. The electrolyte is confined in the flow cell to reach defined surface areas of the photoelectrode. With monochromated or broad-band illumination coupled to potentio- or galvano-static control, the photo-activity of the materials can be evaluated. At the same time, the outflow of the electrolyte is analysed by a mass spectrometer to quantify the photo-corrosion in operando.
So far, we have applied iSFC measurements to quantify the dissolution of WO3  and BiVO4 [2,3] photoanodes during operation, which have been considered stable since decades. We revealed that while stable in the dark, they can rapidly degrade once the illumination introduces enough minority carriers on their surface. The surface chemistry also plays a major role, as vastly different photo-corrosion rates are measured in various electrolytes .
Before our operando measurements, many considered a photoelectrode stable by showing a stable photocurrent with time. We have refuted this assumption in our recent work  using a life cycle study of BiVO4 (Fig. 2). The photocurrent did not decrease until the BiVO4 film degraded to less than ~60 nm, the diffusion length of the minority carrier to yield the photocurrent. We used this example to alert the community that photocurrent stability on thicker photoelectrodes cannot prove photostability, whereas independent photo-corrosion measurements are essential.
Having demonstrated the limited stability for some of the best photoanodes, they still have a bright future for application. Within the community, we are developing catalytic layers on photoanodes that simultaneously improve photo-activity and stability. To unveil the critical junctions and give them lifetime assessment, we are fostering local synergy at the MPIE to correlate the operando iSFC measurements with surface characterisation at the GO department, atom probe tomography at the MA department, and electron microscopy at the NG group.