The XPS spectrum for the Mo region shows an oxidation state of Mo 6+ corresponding to the MoO 3 phase, which is obvious from the 3d 3/2 and 3d 5/2 peaks at 232.6 eV and 235.8 eV (Supplementary Fig. ). The sulphur 2p peaks are significantly changed after testing, confirming the formation of sulfates (oxidation state S 4+ ) at 166.5 eV and amorphous MoS 3.75 with the attributed S 2p 1/2 and S 2p 3/2 peaks at 162.8 eV and 160.9 eV, i.e. to the break of the terminal, apical, and the bridging ligands of [Mo 3 S 13 ] 2-clusters.