For the assessment of the catalytic activity of the ATM/WSe 2 photocathode, a carbon (C) counter electrode is used instead of Pt counter electrode, since in acidic electrolyte Pt particles could be oxidatively etched form the Pt counter electrode and reductively deposited on the working electrode. From the beginning, we checked 8 IV cycles using the C counter electrode and further 2 IV cycles using Pt counter electrode. However, the performance of the ATM/WSe 2 photocathode remains almost similar for the Pt and C counter electrodes, which proves that there is no influence of Pt particles on the working electrode that could increase the photocurrent density (Supplementary Figure ).