《Table 1.Properties of different samples.》

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《Enhanced hydrogen evolution reaction over molybdenum carbide nanoparticles confined inside single-walled carbon nanotubes》

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aDetermined by inductively coupled plasma–optical emission spectrometry（ICP–OES，PerkinElmer ICP-OES 7300DV）.bMeasured from TEM images.cDetermined by N2adsorption/desorption，using Brunner-Emmett-Teller（BET）method.

The HRTEM shows that the MoCx@SWNTs exhibits much smaller MoCxparticles of 2.9 nm than that of MoCx/SWNTs（6.5 nm，Fig.1 and Table 1）.It was reported previously that the particle size of Mo Cx-based electrocatalysts could play a decisive role in their catalytic activity for HER[36–38].For example，breaking down the micro-sized MoCxto various nanostructures（e.g.nanooctahedrons，nanowires，nanoparticles）had been demonstrated to exhibit a significant effect on the HER activity[19，38–40].Furthermore，the activity can be further enhanced by decreasing the size of MoCxat the nanoscale.For example，Nakanishi and co-workers reduced the size of molybdenum carbonitride（MoCN）materials from hundreds of nanometers to 20–30 nm by utilizing a CO2-emission assisted synthesis protocol，which consequently decreased the onset potential of HER from 90 to 50 mV[40].Liu and coworkers reported that the Mo Cxcatalyst with particle sizes of 10–15 nm exhibited 3 times higher current density at an overpotential of 200 mV than that of20–25 nm[38].Zou and co-workers demonstrated that reducing the size of MoCxparticles from 7.3 to 3.6 nm could significantly enhance the HER activity[20].A further reduction of MoCxsize from 1.5–4.5 nm to 1–3 nm by decreasing the annealing temperature could also improve HER activity，as demonstrated by Wang’s group[41].Therefore，the significantly smaller size of the encapsulated MoCxNPs could be one of the reasons for the higher activity of Mo Cx@SWNTs than MoCx/SWNTs since smaller particles not only provide a larger amount of catalytic sites but also facilitate the access of electrolytes thereby enable rapid mass and charge transfer.