Oxygen Nucleation of MoS₂ Nanosheet Thin Film Supercapacitor Electrodes for Enhanced Electrochemical Energy Storage

A direct thin film approach to fabricate large-surface MoS₂ nanosheet thin film supercapacitors using the solution-based diffusion of thiourea into an anodized MoO₃ thin film was investigated. A dense MoS₂ nanosheet thin film electrode (D-MoS₂) was obtained when the anodized MoO₃ thin film was processed in a low thiourea solution concentration, whereas a highly porous MoS₂ nanosheet thin film electrode (P-MoS₂) was formed at a higher thiourea solution concentration. The charge storage performances of the D-MoS₂ and P-MoS₂ thin films displayed an unusual increase in capacitance on extended cycling, leading to a capacitance as high as around 5–8 mF cm⁻². X-ray diffraction and cross-sectional microscopy revealed the capacitance enhancements of the MoS₂ supercapacitors are attributable to the nucleation of a new MoS_2-xO_x phase upon cycling. For the D-MoS₂ nanosheet thin film, the formation and growth of the MoS_2-xO_x phase during cycling was accompanied by a volumetric expansion of the MoS₂ layer. For the P-MoS₂ thin film, the nucleation and growth of the MoS_2-xO_x phase occurred in the pores of the MoS₂ layer. The propagation of the MoS_2-xO_x phase also shifted the charge storage process in both films from a diffusion-limited process to a capacitive-dominant process.