A highly ordered multi-layered hydrogenated TiO2-II phase nanowire array negative electrode for 2.4 V aqueous asymmetric supercapacitors with high energy density and long cycle life

Abstract

To increase the active material loading coupled with the improvement of electrical conductivity and ionic transport in the TiO₂ based negative electrode for 2.4 V aqueous asymmetric supercapacitors (ASCs), a highly ordered binder-free multi-layered hydrogenated TiO₂-II phase nanowire array (ML-HTO) electrode is synthesized via a multi-step method. Due to the highly ordered scaffold-like multi-layered structure, a hydrogenated TiO₂-II phase, small TiO₂ grains (4.4 nm), high oxygen vacancy (Ti³⁺) concentration and functionalized hydrophilic surfaces, endowing the electrode with high electrical conductivity (low band gap of 2.39 eV), more positive flat-band potential (−0.35 V) and a high Na⁺ diffusion coefficient (3.08 × 10⁻⁸ cm² s⁻¹), the as-prepared ML-HTO electrode with high active material loading (3.0 mg cm⁻²) exhibits a high areal specific capacitance (710.7 mF cm⁻²), which is much higher than that of a pristine TiO₂ nanowire array (TO) electrode (28.3 mF cm⁻²) and the single layered hydrogenated TiO₂ nanowire array (HTO) electrode (258.7 mF cm⁻²). Consequently, a 2.4 V aqueous ASC was firstly assembled with the ML-HTO electrode as the negative electrode, presenting a high energy density of 90.3 W h kg⁻¹ at 349.0 W kg⁻¹, as well as a high volumetric energy density of 9.6 mW h cm⁻³ at 61.0 mW cm⁻³, which is much higher than most of previously reported aqueous ASCs. The capacitance retention of the as-prepared MnO₂//ML-HTO ASC is 65.7% after 10 000 cycles, demonstrating its long cycle life.