Ultrafast spectroscopy is highly useful to investigate the light-induced processes in photoactive materials. Notably, most ultrafast spectroscopy studies on dye-sensitized photocathodes have been carried out in air or in an organic solvent instead of under operating conditions. Though, some femtosecond transient absorption studies on dye-sensitized NiO photocathodes showed that a negative potential could increase the hole injection efficiency and slow down charge recombination. However, these studies were carried out in an acetonitrile-based electrolyte, while for proton or CO2 reduction an aqueous electrolyte is needed. Moreover, we recently observed that the working environment of a NiO-based photocathode plays an important role in both the light-induced hole injection and charge recombination dynamics. So far, it is unknown whether, and if so how, the external bias potential affects the charge separation and recombination dynamics in an aqueous electrolyte.