Redox-Active Electrolyte based MnWO4//AC Asymmetric Supercapacitors

Finding supercapacitive materials with high energy and power densities has attracted signicant interest in recent years. Herein, we are reporting layered MnWO 4 nanostructure for supercapacitor applications. MnWO 4 //AC asymmetric cell was fabricated by using hydrothermally synthesized MnWO 4 nanostructure as a cathode and activated carbon as an anode. Prior to device fabrication, the structural and electrochemical properties of MnWO 4 were thoroughly studied. MnWO 4 //AC asymmetric cell with KOH electrolyte showed specic capacitance and energy density of 90 F/g (at 1 mA/cm 2 ) and 51 Wh/kg, respectively. Upon addition of redox-active KI into KOH, both the specic capacitance and energy density were signicantly enhanced (144 F/g and 90 Wh/Kg, respectively). The enhanced electrochemical properties of MnWO 4 //AC asymmetric cell can be attributed to the high-speed solution-phase Faradic reactions contributed by KI redox species in the KOH electrolyte.


Introduction
Increasing environmental pollution and depletion of traditional energy resources has aroused extensive attention on energy storage and energy conversion devices [1]. Especially, electrochemical energy storage devices are projected to be a vital part of electronic portable devices, electric vehicles, and airplanes [2,3].
Among all the energy storage systems, supercapacitors (SCs) are the most promising candidates for the above-mentioned applications because they deliver relatively high power, fast charging, good cyclic stability, and low maintenance cost [4][5][6]. However, SCs possess very low energy density in comparison to fuel cells and batteries [2,3]. The energy density of SCs can be further enhanced by developing asymmetric supercapacitors (AS) where one electrode stores the charges by faradaic redox reactions and the other one stores the charges based on the electrical double layer capacitance (EDLC) mechanism [3]. In AS con guration, different voltage windows of two different electrode materials are added to maximize the resultant operating potential window, which intern increases the resultant energy density according to the equation, E = 0.5 CV 2 . Typically, carbonaceous materials and their derivatives such as graphene, activated carbon, or CNTs [7][8][9] are used as an anode. On the contrary, the pseudo-capacitive materials (e.g. NiO, MnO 2 , Fe 3 O 4 , and RuO 2 ) and conducting polymers, in which high pseudocapacitance comes from reversible surface faradaic redox reactions are used as positive electrodes [10][11][12][13][14][15][16].
In recent years, redox-active nanomaterials have gained a lot of interest as positive electrodes. The nanodimensions and redox behavior of these materials simultaneously offer multiple oxidation states and higher surface area for charge storage. Among different metal oxides, metal tungstate is interesting candidate because of its reversible electrochemical redox reactions, good electrical conductivity, good theoretical speci c capacitance, and low cost [17][18][19]. Recently, manganese tungstate (MnWO 4 ) has gained a lot of attention due to its outstanding physicochemical properties [20][21][22][23]. MnWO 4 has indeed been reported as a good supercapacitor electrode since Mn and W species are electrochemically active [24][25]. Moreover, the additional W metal helps to improve the conductivity of the MnO 2 electrode [26].
However, these reports are focused on the preliminary study of MnWO 4 as the SC electrode and not an SC device.
Another approach to enhancing the speci c capacity of the SC cell in terms of enhancing the potential window is to modify the electrolyte by the addition of redox-rich species like halides (KI, KBr), quinones, sodium vanadium complexes, and phenylenediamine, etc. in conventional electrolyte systems [2,3]. The redox-active species enhance the pseudocapacitive contribution of the electrolyte which helps to enhance the potential window of SC cell [3]. KI is found to be one of the best candidates due to its low cost, ecofriendly and Iodide can produce redox pairs (like I -/I 3 and I 2 /IO 3 -) during the electrochemical process [3].
Hereby, we have designed MnWO 4 nanostructures by the facile hydrothermal method. We have designed  Figure 3c shows the XPS spectra of W 4f in which two peaks appearing at the binding energies of 34.58 eV and 36.71 eV were corresponded to the W 4f 7/2 and W 4f 5/2 ; respectively which con rms the +6 valence state of W in MnWO 4 [27]. The O1 s peak was associated with the binding energy of 529.57 eV (Fig. 4d), which could be assigned to Mn-O-W bond in MnWO 4 [27]. The presence of all the characteristic peaks further proves the single phase formation of MnWO 4 .
The morphology of the SC electrode strongly in uences the performance of supercapacitors. In particular, hierarchical nanostructures such as nanorodes, nanosheets, and nanoplates undoubtly contribute to enhance the performance of the supercapacitors because of their high surface areas, and short electronand ion-transport pathways. The FESEM images of the MnWO 4 nanostructures (Fig 2a,b) show edge-   Figure 3b shows the GCD curves of the MnWO 4 electrode recorded at different current densities. The charging and discharging curves are nonlinear at all current densities, which might be due to the absorption/desorption process at electrode and electrolyte interface, and faradic redox reaction of MnWo 4 (Mn 2+ / Mn 3+ ) with the electrolyte [27]. The speci c capacitance as well as areal capacitance were calculated and presented in Fig. 3c. MnWO 4 electrode showed maximum speci c (areal) capacitance of 430 F/g (320 mF/cm 2 ) at a current density of 6 mA/cm 2 and it is retained to 70 F/g (50 mF/cm 2 ) even at 20 mA/cm 2 . Comparably high values of capacitances at high current densities con rm the good rate capability of the MnWO 4 electrode. Further, the MnWO 4 electrode showed about 90 % of capacitance even for 5000 cycles, suggesting good cyclic stability (Fig. 3d).
For practical relevance, an asymmetric cell MnWO 4 //AC was fabricated using 2 M KOH electrolyte and KI added KOH electrolyte. The CV curves of the MnWO 4 //AC asymmetric cell in KOH electrolyte were studied at different scan rates of 10 to 100 mV/s as shown in Fig 4a. The CV curves show quasi-rectangular shapes even at high scan rates of 100 mV/s, revealing good rate capability and excellent reversibility of the device. Similarly, GCD curves of the MnWO 4 //AC asymmetric cell show nearly linear variation within the potential window of 0 to 1.6 V (Fig. 4b). The speci c capacitance as a function of current density was estimated using the following equation.
where I is the discharge current, V is the potential window, dt is the discharge time and m is the mass of the material on electrodes (both electrodes, 5.9 mg). Figure 4c represents the speci c capacitance of MnWO 4 //AC asymmetric cell at various current densities. MnWO 4 //AC asymmetric cell showed a maximum speci c capacitance value of 90 F/g at a lower current density of 1 mA/cm 2 . Figure Figure 5b shows the CV curves for MnWO 4 //AC asymmetric cell with KI added electrolyte at different scan rates. The weak redox peaks are well sustained even at high scan rates, indicating better rate capability behavior of MnWO 4 //AC asymmetric cell. However, MnWO 4 //AC asymmetric cell with KI added KOH electrolyte did not show much change in the shape of GCD curves (Fig. 5c). The maximum speci c capacitance calculated from GCD curves for MnWO 4 //AC asymmetric cell for KI added electrolyte was obtained to be 140 F/g (Fig. 5d), which is almost 1.5 times higher than the one obtained for MnWO 4 //AC asymmetric cell without redox additive (90 F/g). The enhancement in speci c capacitance is due to the addition of redox actives species from KI, which not only improved the ionic conductivity of the electrolyte but also enhanced the electron transfer redox reactions.
We have further calculated the speci c energy and speci c power for both the asymmetric cells and presented in Fig. 6a  Declarations