The key challenge for the development of highperformance molybdenum sulfide HER catalysts lies in the limited fundamental understanding for the correlation between the catalytic activities and physical features of the materials.Here we have demonstrated an unambiguous correlation between the catalytic performance and the composition/ crystallinity of molybdenum sulfide. The results indicate that the crystallinity plays an overwhelming role in determining the catalytic performance, while the composition does not matter much. The crystallinity can affect the three figures of merit of the catalytic performance (Tafel slope, turnover frequency (TOF), and stability) in opposite directions. Generally, the materials with low crystalline quality may provide low Tafel slopes (∼40 mV/dec), while highly crystalline molybdenum sulfide shows higher TOFs (by 2 orders of magnitude) and better stability. DFT calculations suggest that the terminal disulfur complex S 2 2− , which may exist in MoS 3 and also likely MoS 2 of low crystalline quality due to its structural disorder, could be the true catalytically active site responsible for the low Tafel slope. Our results indicate that one key issue for the rational design of high-performance molybdenum sulfide HER catalysts is to engineer the crystallinity such that balancing its contradictory effects on the different aspects of the catalytic performance. We show that nanocrystalline MoS 2 with few-layer nanoclusters in a lateral size of 5−30 nm provides a more promising platform than either amorphous or highly crystalline molybdenum sulfide due to its combination of low Tafel slopes and good stability. As a way to illustrate this notion, we have developed a MoS 2 catalyst by engineering the crystallinity that shows Tafel slopes of 40 mV/dec, exchange current densities of 3.5 μA/cm 2 , and extraordinary stability with constant performance over >10000 cycles, which are among the best values ever reported. The performance of this catalyst could be further improved by using rougher substrates or doping to improve the relatively low exchange current density.

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No abstract

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