PEC stability. Crucial for the suitability of a HER photocathode is its long-term stability in operation, i.e. in a highly concentrated electrolyte under illumination. A bare WSe 2 photocathode is known to be stable in alkaline and acidic solutions. Due to d-d band transitions of excited electrons, the ATM and WSe 2 should be highly stable against photocorrosion similar to MoS 2 . Recently, Kibsgaard et al. , on the other hand, reported only a slight decrease in the photocurrent density during 2 hours for a p-type WSe 2 single crystal, coated with a Pt/Ru catalyst under acidic conditions (pH 10). Yu et al. showed a good stability for about 2 h at a constant photocurrent density at about 1.5 mA cm -2 for polycrystalline WSe 2 thin films coated with a Pt/Cu catalyst in sulfuric acid. In our case, the stability of the ATM/WSe 2 (200 nm thick) photoelectrode was tested in 0.5 M H 2 SO 4 (0 pH) under AM 1.5 illumination. Comparable to the literature, we also observed only a slight reduction of the photocurrent under acidic condition. The cathodic current density of 3.5 mA cm -2 measured at the beginning was reduced within 70 minutes to 2.7 mA cm -2 , i.e. a decrease by more than 20 % occured, comparable to the observed degradation of the ATM film in the dark, reported by Kibsgaard et al. (Supplementary Fig. ).