Nano-thickness dependence of supercapacitor performance of the ALD-fabricated two-dimensional WO3

Effect of nano-thickness of the two-dimensional (2D) WO3 electrodes on their supercapacitor performance was investigated at the nanometers' thickness level. The 2D WO3 films with the thickness of ∼6.0, 1.2 and 0.7 nm were fabricated by the atomic layer deposition (ALD) technique. Subsequently, their electrochemical behaviors as supercapacitor electrodes were analyzed. A mechanism transition from the electric double-layer capacitance behavior to pseudo-capacitive behavior was observed as the film thickness decreased down to monolayer. Considerable improvement of the specific capacitance from 225.4 to 650.3 F g−1 was also obtained with the 2D film becoming thinner, whereas its rate capability decayed from 83.9% to 65.4%. In addition, the capacitance retention dropped from 91.7% to 65.8% and exhibited worse cycling stability over 2000 cycles with the reduction of film thickness. The Nyquist spectra further suggested that the thinnest 2D WO3 electrode had the smallest charge transfer resistance and ion diffusion resistance. These findings are attributed primarily to the crystallinity of the 2D material and to fast transport through the ultra-thin active layers used. The results obtained could pave the way for the development of atomically thin 2D energy storage devices.