Recently, MoS₂ has attracted attention as a HER (hydrogen evolution reaction) catalyst for replacing Pt, because it has similar hydrogen bonidng energy to Pt. Here, a HVER (high-voltage electrochemical reduction) method in a H-cell was attempted to prepare nanosized MoS₂ on graphene. A Pt wire was used as a cathode to reduce (NH₄)₂MoS₄ to MoS₂ on the graphene particles dispersed in water and a potential of 60 V was applied to the cell and a varible resistor was adjusted to allow a current of 50 mA to flow. During the electrochemical reduction, no Mo deposition on the Pt cathode was observed and MoS₂/graphene particles were simply obtained with a short preparation time of less than 15 min. Fig.1 shows LSV of MoS₂/rGO synthesized at the diffierent reduction time with a RDE and EIS Nyquist plots corresponding to the LSV. The MoS₂/rGO sample prepared at the reduction time of 10 min exhibited the highest current density of 37.6 mA/cm² at the overpotential of 200 mV. EIS analysis showed that the MoS2/rGO sample (10 min) has the lowest charge transfer resistance of 25 Ω. Particle sizes of the prepared MoS₂ were observed with TEM analysis. The particle size of MoS2 (10 min) was in the range of 4-10 nm. The particle size of MoS2 increased with increasing the electroreduction time. It was reported that the smaller the particle size of MoS2, the higher the HER activity.¹⁾ So, one reason for the decrease in the activity of the sample prepared at a reduction time of more than 10 minutes is because the particle size of MoS2 increases as the electrochemical reduction time increases. XRD diffraction patterns of the prepared samples showed that the prepared MoS₂ samples had three phase of 1T (tegragonal), 2H (hexagonal) and 3R (rhombohedral) and the 3R phase was mainly observed. The MoS₂ has been prepared in a hydrothermal method for HER, which shows mainly 2H phase. Recently, it was suggested that 3R phase of MoS2 outperformed the corresponding 2H phase for hydrogen evolution.²⁾ In that aspect, it is proposed that the HVER method is a promising technique for producing high activity MoS₂ catalysts.

References

[1] L. Lei, Z. Jie, B. Xiaojun, Z. Lina, D. Micheal, G.H. Scanlon, L. Baohong, Adv. Funct. Mater., 23, 5326 (2013). [2] T.J. Rou, S. Zdenek, L. Jan, S. David, P. Martin, ChemComm, 53, 3054 (2017)