Measurements were taken at three different potentials, which varied for each catalyst. P1(before onset potential), P2 (at onset potential) and P3 (after onset potential) were set to +0.22, +0.02 and -0.18 mV vs RHE for [Mo3S4] +4 , respectively. Moreover, P1, P2 and P3 were set to +0.05, -0.15 and -0.3 mV vs RHE for [Mo3S13] 2-, respectively. Peak fitting was performed in CasaXPS, and spectra were plotted in Origin Pro. The combined Mo 3d and S 2s region was fitted with two pairs of peaks for both Mo 3d and S2s. Two different binding configurations are further supported by the S 2p region that shows two distinct S 2p peaks. All peaks were fitted with a GL(30) function, and peak area ratio and spin-orbit splitting were constrained. The same full width at half maxima (FWHM) is used for peak doublets for both peaks.