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

Although many electrochemical properties of 2D materials depend sensitively on the nanosheet dimensions, there are no systematic, quantitative studies which analyze the effect of nanosheet size and thickness on electrochemical parameters. Here we use size-selected WS₂ nanosheets as a model system to determine the effect of nanosheet dimensions in two representative areas: hydrogen evolution electrocatalytic electrodes and electrochemical double layer capacitor electrodes. We chose these applications, as they depend on the interaction of ions with the nanosheet edge and basal plane, respectively, and so would be expected to be nanosheet-size-dependent. The data show the catalytic current density to scale inversely with mean nanosheet length while the volumetric double layer capacitance scales inversely with mean nanosheet thickness. Both of these results are consistent with simple models allowing use to extract intrinsic quantities, namely the turnover frequency and the double layer thickness, respectively

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

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

Effect of Percolation on the Capacitance of Supercapacitor Electrodes Prepared from Composites of Manganese Dioxide Nanoplatelets and Carbon Nanotubes

Here we demonstrate significant improvements in the performance of supercapacitor electrodes based on 2D MnO₂ nanoplatelets by the addition of carbon nanotubes. Electrodes based on MnO₂ nanoplatelets do not display high areal capacitance because the electrical properties of such films are poor, limiting the transport of charge between redox sites and the external circuit. In addition, the mechanical strength is low, limiting the achievable electrode thickness, even in the presence of binders. By adding carbon nanotubes to the MnO₂-based electrodes, we have increased the conductivity by up to 8 orders of magnitude, in line with percolation theory. The nanotube network facilitates charge transport, resulting in large increases in capacitance, especially at high rates, around 1 V/s. The increase in MnO₂ specific capacitance scaled with nanotube content in a manner fully consistent with percolation theory. Importantly, the mechanical robustness was significantly enhanced, allowing the fabrication of electrodes that were 10 times thicker than could be achieved in MnO₂-only films. This resulted in composite films with areal capacitances up to 40 times higher than could be achieved with MnO₂-only electrodes

Preparation of Gallium Sulfide Nanosheets by Liquid Exfoliation and Their Application As Hydrogen Evolution Catalysts

Here, we demonstrate the production of large quantities of gallium sulfide (GaS) nanosheets by liquid exfoliation of layered GaS powder. The exfoliation was achieved by sonication of the powder in suitable solvents. The variation of dispersed concentration with solvent was consistent with classical solution thermodynamics and showed successful solvents to be those with Hildebrand solubility parameters close to 21.5 MPa^1/2. In this way, nanosheets could be produced at concentrations of up to ∼0.2 mg/mL with lateral sizes and thicknesses of 50–1000 nm and 3–80 layers, respectively. The nanosheets appeared to be relatively defect-free although oxygen was observed in the vicinity of the edges. Using controlled centrifugation techniques, it was possible to prepare dispersions containing size-selected nanosheets. Spectroscopic measurements showed the optical properties of the dispersions to vary strongly with nanosheet size, allowing the elucidation of spectroscopic metrics for in situ estimation of nanosheet size and thickness. These techniques allow the production of nanosheets with controlled sizes, which will be important for certain applications. To demonstrate this, we prepared films of GaS nanosheets of three different sizes for use as hydrogen evolution electrocatalysts. We found a clear correlation between performance and size, showing small nanosheets to be more effective. This is consistent with the catalytically active sites residing on the nanosheet edges