Cathodic electrodeposition of MoS₂ (WS₂) atop glassy carbon (ITO) was recently reported utilizing a single-source precursor MoS₄^2- (WS₄^2-) from an aqueous (acetonitrile) electrolyte. Since MoS₂ and WS₂ are hydrodesulfurization (HDS) catalysts for sulfur removal from hydrocarbons, they immediately react with and desulfurize the electrolytes from which they are deposited, yielding amorphous deposits that contain excess sulfur. However, the desired stoichiometry MX₂ can be recovered by high temperature annealing, which removes excess sulfur and forms moderately crystallized and partially oriented thin films, as shown by x-ray diffraction (XRD) studies. This two-step process for deposition of MoS₂ and WS₂ thin films of controlled thickness facilitates scientific studies of the nature of pseudocapacitance in these materials. Most reports of MoS₂ and WS₂ pseudocapacitance study nanocomposites with carbon in order to maximize the specific capacitance, but fundamental electrochemical studies are challenging due to the complex topography and surface heterogeneity.

The specific capacitance of MoS₂ and WS₂ thin films with a thickness ranging from 10 nm to 5μm were studied by cyclic voltammetry and galvanostatic charge discharge in several electrolytes, including 1.0 M aqueous Na₂SO₄, 1.0 M aqueous LiClO₄, and 1.0 M LiClO₄ in acetonitrile. For both MoS₂ and WS₂ thin films, the specific capacitance of stoichiometric films obtained by high temperature annealing is 1-2 orders of magnitude higher than the as-deposited amorphous films. The maximum specific capacitance of 1980 F/g is obtained for 10 nm thick WS₂ films in 1.0 M aqueous LiClO₄, although this is of course arises from the small mass in the denominator. The capacitance per unit area for WS₂ films in 1.0 M aqueous Na₂SO₄ increases with increasing film thickness, and then reaches a plateau value at a thickness of 30-50 nm. This suggests that the active region thickness for charge storage in WS₂ is 30-50 nm, which is close to values previously reported for charge storage in RuO₂. Since the cation radius of Li⁺ (0.90 nm) is smaller than that of Na⁺ (1.16 nm), higher capacitance per unit area is obtained in LiClO₄- than in Na₂SO₄-containing electrolytes. In addition, the capacitance per unit area in LiClO₄-containing electrolytes does not reach a maximum plateau value until a much higher WS₂ film thickness.

Cathodic electrodeposition of Cu-doped MoS₂ atop glassy carbon can also be accomplished from an aqueous electrolyte containing 10.0 mM (NH₄)₂MoS₄, 5.0 mM CuSO₄, 0.1 M KCl, and 0.50 M KSCN at pH 6.95. Without the presence of thiocyanate (SCN^-) as a complexing agent, the cathodic potentials for electrodeposition of Cu and MoS₂ are separated by ~700 mV. However, the addition of SCN^- shifts the Cu deposition potential in the cathodic direction towards the deposition potential of MoS₂. In addition, a cathodic shoulder is observed during cyclic voltammetry experiments that may indicate an induced co-deposition effect. Electrodeposition at the potential of this cathodic shoulder yields MoS₂ thin films that contain 1-4 atom% Cu, as determined by energy dispersive x-ray spectroscopy (EDX) and Rutherford backscattering (RBS). As observed for un-doped MoS₂ thin films, high temperature annealing is required to obtain moderately recrystallized and partially oriented films. Four-point probe measurements demonstrate that Cu doping reduces the MoS₂ resistivity by 10.3x. The capacitance obtained by cyclic voltammetry and galvanostatic charge discharge for Cu-doped MoS₂ thin films is 2.5-3.5x higher than that obtained for MoS₂ films of the same thickness.