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Figure 5: Catalytic stability of PdSx- heterocatalyst. (a) HER polarization curves of -Ni3S4 catalyst before and after 2,000 potential cycles between -200 and 200 mV versus RHE. Inset shows the Nyquist plots before and after stability tests. The potential for the Nyquist plot measurement was -0.204 V versus RHE. (b) Multistep chronopotentiometric curve for the -Ni3S4 catalyst, showing that this new heterocatalyst runs robustly even at a high current density of 200 mA cm−2. (c) TEM image (scale bar: 10 nm) and STEM-EDX mapping (inset; scale bar: 5 nm) of -Ni3S4 heterocatalyst after 2,000 cycles of accelerated stability test, respectively. (d) Current efficiency for H2 evolution catalyzed by the -Ni3S4 catalyst, showing a Faradic efficiency close to 100%.