Oxygen Nucleation of MoS 2 Nanosheet Thin Film Supercapacitor Electrodes for Enhanced Electrochemical Energy Storage

A direct thin film approach to fabricate large‐surface MoS 2 nanosheet thin film supercapacitors using the solution‐based diffusion of thiourea into an anodized MoO 3 thin film was investigated. A dense MoS 2 nanosheet thin film electrode (D‐MoS 2 ) was obtained when the anodized MoO 3 thin film was processed in a low thiourea solution concentration, whereas a highly porous MoS 2 nanosheet thin film electrode (P‐MoS 2 ) was formed at a higher thiourea solution concentration. The charge storage performances of the D‐MoS 2 and P‐MoS 2 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 MoS 2 supercapacitors are attributable to the nucleation of a new MoS 2‐ x O x phase upon cycling. For the D‐MoS 2 nanosheet thin film, the formation and growth of the MoS 2‐ x O x phase during cycling was accompanied by a volumetric expansion of the MoS 2 layer. For the P‐MoS 2 thin film, the nucleation and growth of the MoS 2‐ x O x phase occurred in the pores of the MoS 2 layer. The propagation of the MoS 2‐ x O x phase also shifted the charge storage process in both films from a diffusion‐limited process to a capacitive‐dominant process.