Triggering superior sodium ion adsorption on (2 0 0) facet of mesoporous WO₃ nanosheet arrays for enhanced supercapacitance

Tungsten trioxide (WO₃) is believed to be a promising electrode candidate for supercapacitors, due to its reversible surface redox reaction resulted from the almost same W-O bond lengths with different oxidation states. However, the specific capacitance of WO₃ electrode is very low (<250 mF cm⁻²) because of its low conductivity and slow surface redox reaction kinetics. Herein, to address these issues, mesoporous WO₃ nanosheets with exposed (2 0 0) facets were anchored on activated carbon cloth (ACC) via a simple two-step process including heat treatment and alcohol-thermal process. The ACC was coated on WO₃ seeds and proceed a heat treatment at 500 °C and the time of alcohol-thermal process were 7 h, 8 h, and 9 h labeled as WO₃@ACC-n (n = 7, 8, 9). SEM and TEM images confirmed the WO₃ nanosheets were successful anchored on the ACC. The WO₃@ACC-8 electrode showed the best specific areal capacitance which could reach as high as 659 mF cm⁻² at a current density of 5 mA cm⁻² and superior stability with almost no decrease in 10,000 cycles in the presence of Na₂SO₄ as the neutral electrolyte. The capacitance is encouragingly much higher than the previously reported values in a three-electrode system in the neutral aqueous condition. DFT calculations reveal that, in comparison to the (0 0 2) and (0 2 0) facets, the exposed (2 0 0) facets of the mesoporous WO₃ nanosheets triggers the superior Na⁺ adsorption, which accelerates the Na⁺-involved surface redox reaction for the boosted supercapacitor performance. A flexible asymmetric supercapacitor (FASC) was fabricated from the WO₃@ACC electrode, which exhibits high energy density (0.5 mWh cm⁻³) and power density (62 mW cm⁻³) without any attenuation during ±180° wide-angle bending.