Importance of hydrophilic pretreatment in the hydrothermal growth of amorphous molybdenum sulfide for hydrogen evolution catalysis

Amorphous molybdenum sulfide (MoS_x) has been identified as an excellent catalyst for the hydrogen evolution reaction (HER). It is still a challenge to prepare amorphous MoS_x as a more active and stable catalyst for the HER. Here the amorphous MoS_x catalysts are prepared on carbon fiber paper (CFP) substrates at 200 °C by a simple hydrothermal method using molybdic acid and thioacetamide. Because the CFP is intrinsically hydrophobic due to its graphene-like carbon structure, two kinds of hydrophilic pretreatment methods [plasma pretreatment (PP) and electrochemical pretreatment (EP)] are investigated to convert the hydrophobic surface of the CFP to be hydrophilic prior to the hydrothermal growth of MoS_x. In the HER catalysis, the MoS_x catalysts grown on the pretreated CFPs reach a cathodic current density of 10 mA/cm² at a much lower overpotential of 231 mV on the MoS_x/EP-CFP and 205 mV on the MoS_x/PP-CFP, compared to a high overpotential of 290 mV on the MoS_x of the nonpretreated CFP. Turnover frequency per site is also significantly improved when the MoS_x are grown on the pretreated CFPs. However, the Tafel slopes of all amorphous MoS_x catalysts are in the range of 46-50 mV/dec, suggesting the Volmer-Heyrovsky mechanism as a major pathway for the HER. In addition, regardless of the presence or absence of the pretreatment, the hydrothermally grown MoS_x catalyst on CFP exhibits such excellent stability that the degradation of the cathodic current density is negligible after 1000 cycles in a stability test, possibly due to the relatively high growth temperature.