Amorphous Molybdenum Sulfide on Graphene–Carbon Nanotube Hybrids as Highly Active Hydrogen Evolution Reaction Catalysts

Abstract

In this study, we report on the deposition of amorphous molybdenum sulfide (MoSx, with x ≈ 3) on a high specific surface area conductive support of Graphene-Carbon Nanotube hybrids (GCNT) as the Hydrogen Evolution Reaction (HER) catalysts. We found that the high surface area GCNT electrode could support the deposition of MoSx at much higher loadings compared with simple porous carbon paper or flat graphite paper. The morphological study showed that MoSx was successfully deposited on and was in good contact with the GCNT support. Other physical characterization techniques suggested the amorphous nature of the deposited MoSx. With a typical catalyst loading of 3 mg cm(-2), an overpotential of 141 mV was required to obtain a current density of 10 mA cm(-2). A Tafel slope of 41 mV decade(-1) was demonstrated. Both measures placed the MoSx-deposited GCNT electrode among the best performing molybdenum sulfide-based HER catalysts reported to date. The electrode showed a good stability with only a 25 mV increase in overpotential required for a current density of 10 mA cm(-2), after undergoing 500 potential sweeps with vigorous bubbling present. The current density obtained at -0.5 V vs SHE (Standard Hydrogen Electrode potential) decreased less than 10% after the stability test. The deposition of MoSx on high specific surface area conductive electrodes demonstrated to be an efficient method to maximize the catalytic performance toward HER.

The Origin of MoS2 Significantly Influences Its Performance for the Hydrogen Evolution Reaction due to Differences in Phase Purity

“…230.9 eV corresponds well with the literature value of Mo 5 + 3d 5/2 . [16] In accordance with as tudy conducted by Tang et al, [17] the binding energy differenceb etween Mo 4 + and Mo 5 + has an approximate value of 1.3 eV.T his is in good agreement with the binding energyd ifferenceo bserved for the Mo 4 + 3d 5/2 at 229.5 eV and Mo (A) 3d 5/2 at 230.9 eV in the Riedel-de Hans ample. Another possible explanation is the presence of Mo 4 + in molybdenumo xysulfides (MoO x S y ).…”

“…The difference in binding energies between S (A) and S (B) in this study is ca. 1.33 eV and is similar to the difference of 1.4 eV that was reported by Pham et al It is possible that the sample from Riedel‐de Haën contains MoO x S y impurities, or amorphous MoS 3 impurities, which could exist in the form of Mo 5+ (S 2 2− ) 1/2 (S 2− ) 2 accounting for the presence of Mo (A) . The amorphous nature of MoS 3 would not allow it to be identified by X‐ray diffraction (XRD), which was also performed (Figure S1) .…”

“…The deviation from the expected results could be accounted for by a few reasons. Firstly, a “reductive‐activation” process is noted across many of the amorphous molybdenum sulfide literature . Secondly, the Mo oxidation state identified in these studies was attributed to Mo 4+ instead of the Mo 5+ that we identified here.…”

“…This is in good agreement with the binding energy difference observed for the Mo 4+ 3d 5/2 at 229.5 eV and Mo (A) 3d 5/2 at 230.9 eV in the Riedel‐de Haën sample. Another possible explanation is the presence of Mo 4+ in molybdenum oxysulfides (MoO x S y ) . Similarly, deconvolution of the S 2p spectra of the Riedel‐de Haën sample again reveals it to be different from the rest of the samples (Figure f).…”

“…The S (A) doublet at ca. 162 and 163 eV, corresponding to 2p 3/2 and 2p 1/2 , respectively, could be assigned to either S 2− (as in MoS 2 ) or terminal S 2 2− . S (B) at higher binding energies of ca.…”