Facile Synthesis of Hematite Quantum‐Dot/Functionalized Graphene‐Sheet Composites as Advanced Anode Materials for Asymmetric Supercapacitors

Abstract

For building high‐energy density asymmetric supercapacitors, developing anode materials with large specific capacitance remains a great challenge. Although Fe2O3 has been considered as a promising anode material for asymmetric supercapacitors, the specific capacitance of the Fe2O3‐based anodes is still low and cannot match that of cathodes in the full cells. In this work, a composite material with well dispersed Fe2O3 quantum dots (QDs, ≈2 nm) decorated on functionalized graphene‐sheets (FGS) is prepared by a facile and scalable method. The Fe2O3 QDs/FGS composites exhibit a large specific capacitance up to 347 F g−1 in 1 m Na2SO4 between –1 and 0 V versus Ag/AgCl. An asymmetric supercapacitor operating at 2 V is fabricated using Fe2O3/FGS as anode and MnO2/FGS as cathode in 1 m Na2SO4 aqueous electrolyte. The Fe2O3/FGS//MnO2/FGS asymmetric supercapacitor shows a high energy density of 50.7 Wh kg−1 at a power density of 100 W kg−1 as well as excellent cycling stability and power capability. The facile synthesis method and superior supercapacitive performance of the Fe2O3 QDs/FGS composites make them promising as anode materials for high‐performance asymmetric supercapacitors.

α-Fe2O3-Based Core-Shell-Nanorod–Structured Positiveand Negative Electrodes for a High-Performance α-Fe2O3/C//α-Fe2O3/MnOxAsymmetric Supercapacitor

“…7c, α 44 and also some carbon/graphene based SSCs, 45,46 and are comparable to that reported for VO x //VN-ASCs, i.e., 0.61 mWh/cm 3 , 0.5 mA/cm 2 . 39 The highest volumetric-power-density values for the present ASCs are almost similar, i.e., ∼155 mW/cm 3 at the load current of 7 mA, whereas the gravimetric power density is found to be higher for α- densities of the assembled ASCs are also compared with those reported in the literature 9,10,12,41,[47][48][49][50][51] and are shown in the Fig. 7c, which also suggest superior performance for the supercapacitors reported in this study.…”

“…5c, where symmetry between charge and discharge profiles indicates good capacitive behavior for the assembled ASC within 0-2 V. The calculated volumetric capacitance (considering total volume of the device) and energy density of the ASC from charge-discharge data are plotted against different potential windows in Fig. 5d, exhibiting significant enhancement of these performance parameters with extension of potential window from 1 V to 2 V. For instance, volumetric capacitance increases from 0.37 F/cm 3 to 0.75 F/cm 3 (∼200% enhancement), whereas the energy density shows an increment of about 820% when the potential window is extended from 1 V to 2 V. 4,6,9,11,24,37 Furthermore, both of the ASCs show excellent rate capability ranging between 78% and 80% when the scan rate is increased from 10 mV/s to 400 mV/s. The superior performance of these ASCs is further confirmed from the galvanostatic charge-discharge plots at varying load-currents as shown in the Figs.…”

“…8 The energy density of supercapacitors can be enhanced by increasing their capacitance as well as their operating-potential window, which can be achieved more easily with asymmetric supercapacitors (ASCs) than with the symmetric supercapacitors (SSCs) since, with suitable combination of positive and negative electrode materials, the operational potential window as high as 2 V has been achieved even with aqueous electrolytes. 6,[9][10][11][12][13][14] In recent years, efforts have been expended to explore several ASCs with different electrode materials based on redox-active transition-metal oxides, like RuO 2 , 15 SnO 2 , 23 etc., which are not commercially viable owing to their high cost and environmental incompatibility. Accordingly, there is need to explore certain alternative combinations of positive and negative electrode materials to design a high-performance, cost-effective and environmentally benign ASC.…”

“…4,[6][7][8][9][10][11]13,14,19,23,24 Besides MnO 2 suffers from poor electri- * Electrochemical Society Fellow. z E-mail: deb.sarkar1985@gmail.com; akshukla2006@gmail.com; sarma@iisc.ac.in cal conductivity (10 −5 -10 −6 S/cm), which affects the rate capability of the supercapacitors, but this limitation could be circumvented through nanostructured core-shell electrode design.…”

Hierarchically nanostructured transition metal oxides for supercapacitors

“…Multiphase composites based on TMOs could effectively improve the specific capacitance and cycling performance of the devices compared with single-phase TMOs because of the complex chemical compositions and the synergetic effect of both individual components [128][129][130][131][132][133][134][135]. In the following section, four types of composites were introduced, namely: 1) composites of manganese oxides and other TMOs; 2) composites of gold NP-doped TMOs; 3) composites of carbon materials and TMOs; and 4) other forms of TMOs composites.…”