Thickness Dependence and Percolation Scaling of Hydrogen Production Rate in MoS₂ Nanosheet and Nanosheet–Carbon Nanotube Composite Catalytic Electrodes

Abstract

Here we demonstrate that the performance of catalytic electrodes, fabricated from liquid exfoliated MoS2 nanosheets, can be optimized by maximizing the electrode thickness coupled with the addition of carbon nanotubes. We find the current, and so the H2 generation rate, at a given potential to increase linearly with electrode thickness to up ∼5 μm after which saturation occurs. This linear increase is consistent with a simple model which allows a figure of merit to be extracted. The magnitude of this figure of merit implies that approximately two-thirds of the possible catalytically active edge sites in this MoS2 are inactive. We propose the saturation in current to be partly due to limitations associated with transporting charge through the resistive electrode to active sites. We resolve this by fabricating composite electrodes of MoS2 nanosheets mixed with carbon nanotubes. We find both the electrode conductivity and the catalytic current at a given potential to increase with nanotube content as described by percolation theory.

Electrical, Mechanical, and Capacity Percolation Leads to High-Performance MoS2/Nanotube Composite Lithium Ion Battery Electrodes

“…51 Recently, a number of papers have described the electrical properties of films consisting of mixtures of exfoliated nanosheets and both nanotubes and graphene. 35,48,52 Here, we measured the electrical conductivity of the SWNT/MoS2 composite films using the four probe technique, plotting the results versus Mf in Figure 2C. For the MoS2-only film, we find an-plane conductivity of ~10 -6 S/m, close to that reported for similar systems.…”

“…Similar findings were reported for capacitance percolation in MnO2/SWNT composite supercapacitors, 35 and recently, percolation of catalytic activity in MoS2/SWNT composite electrocatalysts. 52 While this is not fully understood, we suggest that the percolative nature of the capacity is due to the scaling of the extent of the interconnected nanotube network with . When >c, nanotubes can either belong to the network spanning the entire film or be isolated from it.…”

Electrochemical Applications of Two-Dimensional Nanosheets: The Effect of Nanosheet Length and Thickness

“…This is important for electrochemical applications as it allows the electrolyte access to the internal structure of the electrode. Indeed recent measurements in our group 44 on MoS2 nanosheet networks applied as hydrogen evolution electrocatalysts showed complete catalyst penetration for electrodes of thickness up to at least 5 µm. Here, the film thicknesses were measured by profilometry (see SI methods) and ranged from 680 nm to 210 nm so we expect to whole internal surface to be exposed to electrolyte.…”

“…This value compares with 0 RB112.5 H2 molecules s -1 m -1 , estimated recently for LPE MoS2 electrodes. 44 We note that if all edge disulphides were active, 44 then the maximum possible value of B would be 1.56 nm -1 , leading to a minimum value of R0=2.4×10 -3 s -1 . This can be compared with Jaramillo's value of R0=20×10 -3 s -1 , measured for MoS2.…”

“…We have found that in most cases, nanosheet networks have porosity close to 50% independent of nanosheet type. 44,74,75 Characterization equipment…”