The lower FE can also be attributed to the lower stability and faster degradation of the [Mo3S4] 4+ species. On the other hand, some of the [Mo3S13] 2-samples were stable for over 1.5 hours under harsh working conditions with high FE. This can be observed in Table , where FE measurements at different times are presented. CV measurements of bare Sb2Se3 photocathode without catalysts (Figure ) were measured under illumination with light chopping which showed negligible dark currents and no photocurrent. [Mo3S4] 4+ catalyst, which is insoluble in pure H2O (in contrast to [Mo3S13] 2-), was dissolved in 1 M HCl. To test the effects of the HCl solution, a bare Sb2Se3 was soaked in HCl which showed similar performance to the bare Sb2Se3 device (Figure ). The only observable difference is that the oxidation and reduction peaks seen in bare Sb2Se3 are considerably reduced in magnitude, implying that the HCl may have an etching property, removing oxides from the surface of the Sb2Se3 as previously studied. As Pt is the benchmark catalyst for HER, a bare Sb2Se3 film was deposited with 2 nm of Pt and measured under illumination with light chopping. As shown in Figure , the dark current observed with the Pt catalyst is considerably increased compared to that of the bare Sb2Se3 alone. However, no photocurrent is observed with the Pt catalyst (the Pt catalyst is known to be most effective on the Sb2Se3 when combined with a TiO2 overlayer, which was not investigated in this work). The stability of each molybdenum sulfide cluster as a catalyst was investigated by measuring them through 60 cycles of CV at 10 mV s -1 between 0.1 V to -0.3 V versus RHE, corresponding to 80 minutes of measurement time (Figure ). The [Mo3S13] 2-shows an improved onset and current in the first 5 cycles, then stabilizes for 20 cycles, and then starts to degrade slowly. Furthermore, chronoamperometric stability tests were conducted to evaluate the long-term stability of the catalysts (Figure ). The [Mo3S13] 2-catalyst exhibits a characteristic stability curve similar to the CV sweeps. The initial improvement in the current can be attributed to an increase in activity resulting from the partial exchange of disulfide ligands with aqua ligands. This substitution affects the kinetics of the hydrogen evolution reaction, making it more favorable. However, a complete exchange of disulfides for aqua ligands can lead to decreased activity due to the loss of these hydrogen evolution active sites leading to very high free energies of the Volmer step. In contrast, the [Mo3S4] 4+ catalyst shows only degradation with time, with the film (Au, Sb2Se3 and catalyst) peeling off after approximately two hours, rendering longer stability tests impossible. A partial peeling also occurs with the [Mo3S13] 2-catalyst, contributing to the decrease in current, which could imply that the catalyst is not the limiting factor in such stability tests. This catastrophic failure is likely due to aggressive bubble formation penetrating beneath the film, which must be addressed and optimized for further long-term stability studies.