Abstract
Nano-sized Mo-doped titania (Mo0.1Ti0.9O2) and Nb-doped titania (Nb0.25Ti0.75O2) were directly synthesized via a continuous hydrothermal flow synthesis process. Materials characterization was conducted using physical techniques such as transmission electron microscopy, powder x-ray diffraction, x-ray photoelectron spectroscopy, Brunauer–Emmett–Teller specific surface area measurements and energy dispersive x-ray spectroscopy. Hybrid Li-ion supercapacitors were made with either a Mo-doped or Nb-doped TiO2 negative electrode material and an activated carbon (AC) positive electrode. Cells were evaluated using electrochemical testing (cyclic voltammetry, constant charge discharge cycling). The hybrid Li-ion capacitors showed good energy densities at moderate power densities. When cycled in the potential window 0.5–3.0 V, the Mo0.1Ti0.9O2/AC hybrid supercapacitor showed the highest energy densities of 51 Wh kg−1 at a power of 180 W kg−1 with energy densities rapidly declining with increasing applied specific current. In comparison, the Nb0.25Ti0.75O2/AC hybrid supercapacitor maintained its energy density of 45 Wh kg−1 at 180 W kg−1 better, showing 36 Wh g−1 at 3200 W kg−1, which is a very promising mix of high energy and power densities. Reducing the voltage window to the range 1.0–3.0 V led to an increase in power density, with the Mo0.1Ti0.9O2/AC hybrid supercapacitor giving energy densities of 12 Wh kg−1 and 2.5 Wh kg−1 at power densities of 6700 W kg−1 and 14 000 W kg−1, respectively.
Highlights
Introductionelectrochemical double layer capacitance (EDLC) suffer from significantly reduced energy densities (Hu et al 2015), unless the specific surface area of the active carbon is sufficiently high to allow high energy and power densities simultaneously (Lust et al 2002, Stoller et al 2011, Härmas et al 2016)
Whilst Li-ion batteries benefit from low self-discharge and high energy density, supercapacitors utilizing electrochemical double layer capacitance (EDLC) show more promise for applications with a higher power demand such as regenerative breaking in hybrid electric vehicles (Ibrahim et al 2008, Li et al 2016)
constant charge/discharge (CCD) were performed at current densities in the range 0.5–20 mA cm−2 in the potential windows of 0.5–3.0 V and 1.0–3.0 V, respectively
Summary
EDLCs suffer from significantly reduced energy densities (Hu et al 2015), unless the specific surface area of the active carbon is sufficiently high to allow high energy and power densities simultaneously (Lust et al 2002, Stoller et al 2011, Härmas et al 2016) This problem can be addressed by developing battery-supercapacitor hybrid devices (Li et al 2015, Zuo et al 2017). Some of its polymorphs (including anatase (Wang et al 2007), Nb-doped anatase (Lübke et al 2015), TiO2(B) (Dylla et al 2013), and TiO2 bronze (Liu et al 2011)) have been shown to exhibit pseudocapacitive charge storage behaviour It is important, to consider the imbalances in energy and power densities between the Faradaic charge storage in the batterylike electrode and the non-Faradaic charge storage on the surface of the AC electrode. The authors demonstrate, for the first time, that doped anatase nanomaterials such as Mo0.1Ti0.9O2 or Nb0.25Ti0.75O2 synthesised via a continuous hydrothermal flow method can be incorporated into hybrid supercapacitors with an AC cathode to show high energy densities at high power densities
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