Claims:We Claim
1. An energy storage device comprising
a) stainless steel substrate (1) integrated ZnO/a-Fe2O3 core-shell nanorods as negative electrode (2) and ZnO/C core-shell nanorods as positive electrode (3); and
b) electrolyte (4) consisting separator (5) between the nanorods electrodes.
2. The energy storage device as claimed in claim 1, wherein the device is an asymmetric supercapacitor.
3. The energy storage device as claimed in claim 1, wherein the stainless steel substrate is also current collector.
4. The energy storage device as claimed in claim 1, wherein the diameter of the core-shell nanorods is ranging from about 100 nm to about 150 nm; with a height of the core-shell nanorods is ranging from about 3 µm to about 4 µm.
5. The energy storage device as claimed in claim 1, wherein the electrolyte is selected from a group comprising sodium sulphate, potassium sulphate, lithium sulphate, sulphuric acid etc.
6. The energy storage device as claimed in claim 1 wherein the electrolyte is of concentration ranging from about 0.1M to about 1M, preferably 0.5 M.
7. The energy storage device as claimed in claim 1, wherein the separator is selected from a group comprising polypropylene mesh, polyethylene mesh, porous polycarbonate membrane, preferably polypropylene mesh.
8. A method of fabrication of an energy storage device of claim 1, said method comprising acts of
a) Preparing of stainless steel integrated ZnO nanorods further comprising acts of
i) preparing ZnO nanocrystal solution, and
ii) coating nanocrystal seed solution on stainless steel substrate followed by hydrothermal reaction anddrying to obtain stainless steel integrated ZnO nanorods.
b) Preparing of ZnO/a-Fe2O3 core-shell negative nanorods electrode further comprising acts of
i) coating ferric chloride hexahydrate solution onto ZnO nanorods obtained from step a) and drying,
ii) repeating step (i) and heating to obtain ZnO/a-Fe2O3core-shell nanorods electrode.
c) Preparing of ZnO/C core-shell positive nanorods electrode comprising act of immersing stainless steel integrated ZnO nanorods from step a) in glucose solution and heating to obtain ZnO/C core-shell nanorods electrode.
d) Assembling the substrate integrated core-shell positive ZnO/C and the core-shell negative ZnO/a-Fe2O3 electrodes along with electrolyte consisting a separator in an encapsulate to obtain the energy storage device of claim 1.
9) The energy storage device as claimed in claim 1, wherein device is of specific capacitance ranging from 110 F/g to about 120 F/g.
10) An energy storage unit comprising plurality of energy storage device comprising a)stainless steel substrate(1) integrated ZnO/a-Fe2O3 nanorods (2) as negative and ZnO/C nanorods (3) as positive electrodes; and b) electrolyte(4) consisting separator(5) between the nanorods electrodes and the energy storage devices are connected in series.
, Description:Field of invention

The present invention relates to the field of energy storage devices. More specifically, the invention relates to asymmetric supercapacitors. In particular,the invention relates to asymmetric supercapacitors comprising substrate integrated core-shell nanorods as its negative and positive electrodes and a method of fabrication of the asymmetric capacitor. The asymmetric supercapacitors with substrate integrated core-shell nanorodselectrodes find application in high power delivery capabilities with high energy density.
Background and Prior art of invention

Global concern over ever increasing greenhouse gas emission, owing to immense anthropogenic usage of fossil fuels is forcing human community to swing towards renewable energy sources. Since energy from renewable sources like sun and wind isnot consistent in nature, it becomes mandatory to store the energy from these sources that could be retrieved on demand. Therefore, with increasing demand of energy resource and ecological concerns it becomes necessary for efficient,cost effective and environment friendly energy storage systems. The most common storage system being batteries, wherein there is storage of large amount of energy in a small volume and weight of the system and having various applications fulfilling the need of energy in daily life. However, batteries lack power delivery capabilities and also they cannot operate for prolonged periods under high power levels due to which there is a reduction in the life expectancy of the battery leading to frequent battery replacements. Similarly, conventional capacitors exhibit high power density but have muchlower energy density when compared to batteries. Hence there is a need for storage systems which exhibit both high energy capacity and high power delivery capabilities.Electrochemical storage of energy through a supercapacitor has been known to have the traits of both energy and power densities, compelling research in developing supercapacitor materials with desirable energy storage properties.
Supercapacitors are of two types depending on the energy storage mechanism, (i) electrochemical double layer capacitors (EDLCs) in which there is accumulation of electrical charges on the electrode and in the electrolyte solution near the electrode and (ii) pseudocapacitors (PCs), wherein the electrochemical reactions occur at the electrode surface. Combination of the above two supercapacitivestorage propertiesin a single device results in hybrid or asymmetric supercapacitors. These electrochemical processes being surface phenomena, by transforming the electrodes or the electrode surface one can manipulate the performance of the supercapacitor. It has been observed that modification of the surface of the electrodes affectsthe electronic and ionic conductivity and electrolytic properties of the supercapacitors. A large number of materials especially composites, layered materials in different combinations have been explored for their application as a supercapacitor electrodematerial to yield high energy density.However, maximum energy density that can be achieved from currently available supercapacitors is still below 10 Wh/kg and hence has limited applications. Therefore, a great deal of effort is being expended to further energy density of the supercapacitors without sacrificing their power density.
Literature provides methods wherein, electrodes have been fabricated by pasting slurry of electroactive materials prepared through mixing of active materials with binder and activated carbon, on a mesh or a similar substrate. However, such processes enhance internal resistance between the electrode material and the current collector, thus affecting overall power output, self-discharge and cycle life of the cell.
Nanostructured electrodeshave been found to increase the surface area leading to improved specific capacitance and also,due to their confined dimensions they exhibit unusual mechanical,and electrical properties.
The document titled, “Hierarchical Fe3O4@Fe2O3 Core-Shell Nanorod Arrays as High-Performance Anodes for Asymmetric Supercapacitors” (Appl. Mater. Interfaces, 2015,7(49), pp 27518–27525) depicts a hierarchical hetero-structure comprising Fe3O4@Fe2O3 core-shell nanorod arrays as the negative electrode for asymmetric supercapacitors. The Fe3O4@Fe2O3 electrode exhibits better supercapacitive performance, compared to the bare Fe2O3 and Fe3O4nanorods electrodes, and has amass loading of 1.25 mg/cm2). The hybrid electrode design is also adopted to prepare Fe3O4@MnO2 core-shell nanorods as the positive electrode for asymmetric supercapacitors. The 2 V asymmetric supercapacitor device deliver anenergy density of 0.83 mWh/cm3at a power density of 15.6 mW/cm3.
The article titled “Asymmetric supercapacitors based on ZnCo2O4@MnO2 core–shell electrode” (J. Mater. Chem. A, 2015,3, 5442-5448) explains a hierarchical ZnCo2O4@MnO2 core–shell nanotube arrays electrode developed by a facile two-step method. The electrode exhibits specific capacitance of 1981 Fg-1 (2.38 F cm-2) at a current density of 5 A g-1and cycling stability (5000cycles). Furthermore, a low-cost, high-performance asymmetric supercapacitor (ASC) with ZnCo2O4@MnO2 core–shell nanotube arrays on Ni foam (as positive electrode) and 3D porous a-Fe2O3 on Fe foil (as negative electrode) designed with an extended operating voltage window of 1.3 V.
US2016104581discloses integrated supercapacitors wherein the supercapacitor has a substrate, at least two porous electrodes integrated within the substrate,and an electrolyte extending between the at least two porous electrodes. The electrolyte is integrated with the substrate, and is positioned within the substrate. The, at least two porous electrodes and electrolyte are configured to store charge as a super-capacitor. However, the substrate integrated electrolyte makes the capacitor bulkier and thereby affecting the practical usage of the capacitor.
US7576971 discloses asymmetric supercapacitors which comprises a positive electrode comprising a current collector and a first active material selected from the group consisting of metal oxides, and a combination comprising at least one of the foregoing active materials; a negative electrode comprising a carbonaceous active material; an aqueous electrolyte solution and a separator plate.
Since safety, economical value and environment friendliness also needs to be considered in designing supercapacitorsalong with its energy and power output, a substrate integrated high performance asymmetric supercapacitor with core-shell nano-rods comprising stable, cost effective and environment friendly materials for both positive and negative electrodes has been developedand tested for its performance in the present invention.
In the present invention the nano structured electrodes are grown on a stainless steel substrate to provide better reaction site, also the stainless steel substrate plays a dual role, and functions as a current collector. The super capacitor of the present invention exhibits a specific capacitance ranging from about 110 F/g to about 120 F/g at a scan rate of 10 mV/s in a potential window of 1.8V.
Summary of the invention:
Accordingly, the present invention relates to asymmetric supercapacitors comprising a stainless steel substrate integrated ZnO/a-Fe2O3 core-shell nanorods as negative electrode and ZnO/C core-shell nanorods as positive electrode and an electrolyte consisting separator between the nanorods electrodes; a method of fabrication of an energy storage device wherein said method further comprising acts of preparing of stainless steel integrated ZnO nanorods,preparing of ZnO/a-Fe2O3 core-shell negative nanorods electrode,preparing of ZnO/C core-shell positive nanorods electrode, assembling the substrate integrated core-shell positive ZnO/C and the core-shell negative ZnO/a-Fe2O3 electrodes along with electrolyte consisting a separator in an encapsulate to obtain the energy storage device; wherein the asymmetric supercapacitors comprising substrate integratedZnO/a-Fe2O3//ZnO/C core-shell nanorod electrodes showing a specific capacitance of ~115 F/g; wherein the energy storage unit comprising plurality of energy storage device comprising stainless steel substrate integrated ZnO/a-Fe2O3 nanorods as negativeand ZnO/C nanorods as positive electrodes; and electrolyte consisting separator between the nanorods electrodes and the energy storage devices are connected in series.
Brief description of figures:
The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.
Figure 1: depicts the schematic diagram of an asymmetric supercapacitor with a stainless steel substrate(1) integrated ZnO/a-Fe2O3 core-shell nano-rods electrodes (2) and ZnO/C core-shell nanorods electrodes (3) as its respective negative and positive electrodes and electrolyte (4) consisting separator (5) between the nanorods.
Figure 2: shows a)SEM image of ZnO/a-Fe2O3 core-shell nanorods grown on stainless steel substrate; (b) TEM image for ZnO/a-Fe2O3 core-shell nanorods, with high resolution TEM image; (c) depicts high density ZnO/C core-shell nanorodsgrown on stainless steel substrate and d) TEM image for one such core-shell nanorod.
In Figure 3: (a)shows Cyclic voltammograms recorded for ZnO, a-Fe2O3 and ZnO/a-Fe2O3 core-shell nanorods electrodes at the scan rate of 100 mV/s within the potential window between 0V and -0.85V, (b) shows the comparison of specific capacitances of a-Fe2O3 and ZnO/a-Fe2O3 core-shell nanorods electrodes, calculated from CV curves, as a function of scan rate, (c) depicts comparison of cyclic voltammograms collected for ZnO, C and ZnO/C core-shell nanorods electrodes at a scan rate of 100 mV/s within potential window between 0V and 0.95V, and(d) depicts the variation of specific capacitances for C and ZnO/C core-shell nanorods electrodes as a function of scan rate.
Figure 4: shows (a) Cyclic voltammograms (CV) of the ZnO/a-Fe2O3//ZnO/C core-shell nanorods asymmetric supercapacitor cell recorded within different operational potential windows, the figure in the inset shows digital image of the prototype cell having dimension of 1×1cm2; (b) variation of specific capacitance and energy density calculated from CV curves as a function of operating potential window; (c) variation of specific capacitance as calculated from CV curves as function of potential scan rates; (d) Galvanostatic charge/discharge curves recorded at different current densities within a potential window of 1.8V; (e) Nyquist plot (-Z'' vs Z' plot) for asymmetric capacitor cell within frequency range from 10 mHz to 1 MHz with an ac field amplitude of 5 mV, the sketch in the inset shows high frequency region of Nyquist plot and (f) cycling performance of asymmetric supercapacitor cell recorded at a scan rate of 100 mV/s during 5000 cycles.
Figure 5: shows Ragone plot for ZnO/a-Fe2O3//ZnO/C core-shell nanorods asymmetric supercapacitor cell; the energy and power density values are compared with the same for other iron oxide based asymmetric supercapacitors devices; the inset images provided are the digital photographs of different LEDs powered by the assembled asymmetric cell.
Figure 6: depicts the schematic diagram of two asymmetric supercapacitors connected in series.
Figure 7: shows the digital photographs of a single supercapacitor cell and the stacked supercapacitor cell with cell potentials. Inset images show LEDs lighted with two differing supercapacitor configurations.
Detailed description of invention:
The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the figures, description and claims.It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.
The present invention is in relation to an energy storage device comprising stainless steel substrate integrated ZnO/a-Fe2O3 core-shell nanorods as negative electrode and ZnO/C core-shell nanorods as positive electrode, and electrolyte consisting separator between the nanorods electrodes.
In another embodiment of the invention, the energy storage device is an asymmetric supercapacitor.
In yet another embodiment of the invention, the stainless steel substrate is also current collector.
In another embodiment of the invention, the diameter of the core-shell nanorods is ranging from about 100 nm to about 150 nm; with a height of the core-shell nanorods is ranging from about 3 µm to about 4 µm.
In another embodiment of the invention, the electrolyte is selected from a group comprising sodium sulphate, potassium sulphate, lithium sulphate, sulphuric acid, preferably sodium sulphate.
In yet another embodiment of the invention the electrolyte is of concentration ranging from about 0.1M to about 1M, preferably 0.5 M.
In yet another embodiment of the invention the separator is selected from a group comprising polypropylene mesh, polyethylene mesh, porous polycarbonate membrane, preferably polypropylene mesh.
The present invention is also in relation to a method of fabrication of an energy storage device comprising acts of preparing of stainless steel integrated ZnO nanorods further comprising acts of preparing ZnO nanocrystal solution, andcoating nanocrystal seed solution on stainless steel substrate followed by hydrothermal reaction and drying to obtain stainless steel integrated ZnO nanorods; preparing ZnO/a-Fe2O3 core-shell negative nanorods electrode, further comprising acts of coating ferric chloride hexahydrate solution onto ZnO nanorods followed by drying and heating the coated nanorodsto obtain ZnO/a-Fe2O3 core-shell nanorodselectrode, preparing of ZnO/C core-shell positive nanorods electrodes comprising acts of immersing stainless steel integrated ZnO nanorods in glucose solution and heating to obtain ZnO/C core-shell nanorods electrode, assembling the substrate integrated core-shell positive ZnO/C and the core-shell negative ZnO/a-Fe2O3 electrodes along with electrolyte consisting a separator in an encapsulate to obtain the energy storage device.
In another embodiment of the invention the device is of specific capacitance ranging from about 110 F/g to about 120 F/g.
The present invention is also in relation to the energy storage unit (figure 6) comprising plurality of energy storage devices (figure 1) comprising stainless steel substrate integrated ZnO/a-Fe2O3 nanorods as negative and ZnO/C nanorods as positive electrodes; and electrolyte consisting separator between the nanorods electrodes, wherein energy storage devices are connected in series.
The present invention relates to a substrate-integrated high-performance asymmetric supercapacitor comprising ZnO/a-Fe2O3 core-shell nanorods and ZnO/C core-shell nanorods as its respective negative and positive electrodes (Figure 1). The substrate being a stainless steel substrate plays a dual role. Apart from being a substrate for the growth of the nanorods, it also acts like the current collector in the capacitor. The stainless steel substrate /current collector not only improves mechanical integrity of the system but also eases electron transfer from the electrolyte to the current collector enhancing the power density limit and reducing the self-discharge, thereby improving the cell voltage. Moreover, higher mechanical stability and structural integrity of these stainless steel substrate integrated nanostructures help to sustain volume changes by repetitive ion intercalation/deintercalation processes during long term cycling, thus improving cycle life/calendar life of the asymmetric supercapacitor cells. The stainless steel substrate integrated growth of nanostructures in this asymmetric supercapacitor cell improves contact resistance of electrodes.
The nanorods electrodes grown on the stainless steel substrate have a core-shell arrangement. The positive nanorods electrode comprises carbon and ZnO and the negative nanorods electrodes comprises a-Fe2O3 and ZnO. In the core-shell architecture, thin shell layers of highly electroactive a-Fe2O3 and C effectively provide large reaction sites and shorten ion-diffusion path significantly, whereas the highly conductive ZnO, as the core material, serves as highway for fast electrontransfer to current collector. Each of these nanorods have a diameter ranging from about 100 nm to about 150 nm and with height of the nanorods ranging from about 3 µm to about 4 µm. The SEM image in Figure 2 shows the ZnO/a-Fe2O3and ZnO/C core-shell nanorods grown on stainless steel substrate. Each of the ZnO/a-Fe2O3and ZnO/C core-shell nanorods are also depicted under high resolution TEM images.
The electrochemical behaviour of these core-shell nanostructures is profound as compared to the individual component materials. Hematite (a-Fe2O3) has been considered as negative electroactive material due to its high electrochemical activity thereby providing enhanced specific capacitance, a suitable negative potential window because of its high hydrogen over-potential in aqueous electrolyte, environment friendliness and abundance in nature. However, poor electrical conductivity (~10-14 S/cm) and short ion-diffusion length, limit its electrochemical performance significantly, although this drawback can be overcome by creating oxygen vacancies within thin a-Fe2O3 films. Hence, ZnO/a-Fe2O3 core-shell nanorods as negative electrode instead of individual component-materials will potentially improve its capacitive performance and hence of the cell. This is substantiated by the cyclic voltammetry studies. Cyclic voltammograms (Figure 3(a)) recorded for ZnO, a-Fe2O3 and ZnO/a-Fe2O3 core-shell nanorods electrodes at the scan rate of 100 mV/s within the potential window between 0V and -0.85V is observed. Current density for a-Fe2O3 thin film is seen to be significantly higher than ZnO because of higher electrochemical activity of the former. The electrochemical performance of ZnO/a-Fe2O3 core-shell nanorods is found to be substantially higher as compared to both of its individual components, namely ZnO and a-Fe2O3. Also, the comparison of specific capacitances of a-Fe2O3 and ZnO/a-Fe2O3 core-shell nanorods electrodes, calculated from CV curves, is done as a function of scan rate (Figure 3(b)). ZnO/a-Fe2O3 core-shell nanorods electrode exhibits maximum specific capacitance of 491.7 F/g at a scan rate of 10 mV/s, which is significantly higher than a-Fe2O3 thin film electrode that exhibits only 295 F/g at the same scan rate. Moreover, ZnO/a-Fe2O3 core-shell nanorods electrode shows a capacitance retention of 65.7 % when the scan rate is increased from 10 mV/s to 400 mV/s while a-Fe2O3 thin film electrode could retain only 50.3% of initial capacitance during the same course of scan rate.
Use of graphitic carbon as shell layer over ZnO in ZnO/C core-shell nanorods as positive electro-active material not only increases active sites for double layer formation on carbon but also its higher electronic conductivity ensures good electrical contact of ZnO/C nanorods with the substrate (current collector), thus facilitating charge/ion transfer even at higher scan rates. Additionally, as carbon has enough mechanical strength, it can sustain a high level of strain caused by the volume variation in ZnO core and thus preventing its disintegration during continuous ion intercalation/deintercalation processes which would reflect on the cell performance also.Comparison are done for cyclic voltammograms (Figure 3(c)) collected for ZnO, C and ZnO/C core-shell nanorods electrodes at a scan rate of 100 mV/s within potential window between 0V and 0.95V. The current density increases significantly after carbon coating on ZnO nanorods, which is also higher than the carbon thin film electrode, reflecting improved electrochemical performance for ZnO/C core-shell nanorod electrode.The variation of specific capacitances for C and ZnO/C core-shell nanorods electrodes as a function of scan rate is observed (Figure 3(d)).
As calculated from the cyclic voltammograms collected at different potential scan rates between 10 mV/s and 200 mV/s within the potential window of 0 to 0.95V, ZnO/C core-shell nanorods electrode exhibits a specific capacitance of 229 F/g at a scan rate of 10 mV/s while carbon thin film electrode exhibits 198 F/g at the same scan rate. As the scan rate increases from 10 mV/s to 200 mV/s, ZnO/C core-shell nanorods electrode shows a capacitance retention of 56% which is more than twice of that observed for carbon thin film based electrode, which shows only 25% retention at 200 mV/s.
The electrolyte incorporated between the nanorods electrodes of the asymmetric supercapacitor is selected from a group comprising sodium sulphate, potassium sulphate, lithium sulphate, sulphuric acid, preferably sodium sulphate. Electrolyte with concentration ranging from about 0.1M to about 1M, preferably 0.5 M is integrated in the capacitor. The supercapacitor consists of the separator selected from a group of separators comprising polypropylene mesh, polyethylene mesh, porous polycarbonate membrane, preferably polypropylene mesh.
The assembled ZnO/a-Fe2O3//ZnO/C core-shell nanorods asymmetric supercapacitor cell exhibits aspecific capacitance ranging from 110 F/g to about 120 F/g more specifically 115 F/g at a scan rate of 10 mV/s in a potential window as wide as 1.8V and with response time as low as 39 ms. The asymmetric supercapacitor can deliver an energy density of ~ 41 Wh/kg and power density of ~ 7 kW/kg. Figure 4(a) depicts cyclic voltammetry data obtained for the ZnO/a-Fe2O3//ZnO/C core-shell nanorod asymmetric supercapacitor (ASC) prototype cell in varying potential windows, in 0.5M Na2SO4 aqueous electrolyte. It is observed that there is an improvement in the electrochemical performance of ZnO/a-Fe2O3//ZnO/C core-shell nanorods asymmetric supercapacitor when the potential window is extended from 0.8V to 1.8V.The evolution of performance parameters with increasing cell potential where the specific capacitance is found to increase from 9.3 F/g to 37.1 F/g (~ 400 % increment) with increase in cell potential from 0.8V to 1.8V, is depicted in Figure 4(b). It is also observed that, the energy density (E) of the asymmetric supercapacitor, calculated using equation, E = 1/2 CspV 2, where, Csp being the specific capacitance and V is the potential window, increases from 0.83 Wh/kg to 16.7 Wh/kg, which is ~ 2000% improvement, with increase in cell potential. Therefore, a stable potential window of 0 to 1.8V has been chosen for further electrochemical measurements on ZnO/a-Fe2O3 // ZnO/C core-shell nanorods asymmetric supercapacitor cell.
Specific capacitance values for the asymmetric supercapacitor calculated from cyclic voltammetry data are plotted against scan rates in Figure 4(c). Maximum specific capacitance for the asymmetric capacitor is found to be ~ 115 F/g at a scan rate of 10 mV/s, however, retaining a capacitance value of ~ 47 F/g as the scan rate increases from 10 mV/s to 100 mV/s. Galvanostatic charge/discharge data for this asymmetric capacitor collected at varying current densities ranging from 1 to 5 A/g are shown in Figure 4(d). As can be seen from the data, wherein the discharge profile is almost symmetrical to charging profile at lower discharging currents with very small IR-voltage drop (0.08V at 0.5 A/g) implying smaller equivalent series resistance for the asymmetric supercapacitor (Figure 4(d)). However, at higher current densities charge/discharge profiles become more symmetrical and linear, representing ideal capacitive behaviour and fast charge/discharge characteristic for the asymmetric supercapacitor.
To estimate the internal resistance and response time of the ASC, electrochemical impedance spectroscopy (EIS) are carried out and corresponding Nyquist plot (-Z'' vs Z' plot) is noted (Figure 4(e)). At higher frequencies, intercept of Nyquist curve with real axis yields equivalent series resistance (ESR) which is found to be ~ 2.8 ? in this case, implying good electrical conductivity for the electrolyte and low internal resistance for the electrode. Low frequency region of Nyquist curve is more inclined towards imaginary axis which signifies ideal capacitive behaviour of the cell.
Long-term cycling performance of assembled ZnO/a-Fe2O3//ZnO/C core-shell nanorods ASC is observed during cyclic voltammetry analyses for 5000 cycles at a scan rate of 100 mV/s within the potential window of 0 to 1.8V (Figure 4(f)). With the exception of the initial increase which might be due to activation process, capacitance remains almost constant up to 2000 cycles with a capacitance retention of almost 99%. Moreover, the asymmetric capacitor cell can retain more than 81% of its initial capacitance after 4000 cycles which however decreases to ~70% after 5000 cycles.
The Ragone plot (Figure 5) that represents energy and power performance of ZnO/a-Fe2O3//ZnO/C core-shell nanorods ASC cell is plotted. The energy and power values of the asymmetric supercapacitor is compared with other reported iron oxide based aqueous and solid state asymmetric supercapacitor devices. Maximum energy density that has been achieved for this asymmetric supercapacitor is 41.2 Wh/kg at a load-current density of 0.5 A/g with a corresponding power density of 471 W/kg. However, energy density retains a value of 13 Wh/kg with a maximum power density of 7 kW/kg when current density is increased by 10 times, representing excellent rate performance for the ASC.
The present invention also relates to the method of fabrication of the supercapacitor which involves the steps of
i) Preparation of ZnO nanorods on Stainless Steel (SS) substrate as the core material.
ii) Preparation of ZnO/a-Fe2O3 core-shell nanorods as the negative electrode material
iii) Preparation of ZnO/C core-shell nanorods as the positive electrode material.
iv) Assembling the asymmetric supercapacitor cell along with electrolyte and separator in an encapsulate.
Experimental:
Preparation of ZnO nanorods on Stainless Steel (SS) substrate as the core material
Well aligned ZnO nanorods are grown on stainless steel substrate by a modified seed-assisted hydrothermal synthesis procedure reported elsewhere. In first step, ZnO nanocrystals seed are prepared by reducing zinc acetate dihydrate (0.01M) with sodium hydroxide (0.03M) in ethanol solution at 60°C with continuous stirring for 2 h. The resultant nanocrystals are uniformly dispersed in the solution. Subsequently, the nanocrystals were drop-casted on properly cleaned and polished SS substrates for several times followed by drying at 150°C for 15 min to increase particle adhesion on substrate surface. After uniformly coating with seed layer, the substrates are placed horizontally inside a teflon-lined stainless steel autoclave containing zinc nitrate dihydrate (0.025M) and hexamethylenetetramine (0.025M) followed by heating at 90°C for 6 h for the growth ZnO nanorods arrays. The substrate with white ZnO layer is taken out from autoclave, copiously cleaned with water and ethanol and dried in an oven at 60°C overnight. Finally, ZnO nanorods are annealed in a tube furnace in Argon atmosphere at about 350°C for about 3 h.
Preparation of ZnO/a-Fe2O3 core-shell nanorods as the negative electrode material
ZnO/a-Fe2O3 core-shell nanorods electrodes are prepared by depositing a-Fe2O3 on ZnO nanorods by spin coating method. For this purpose, ethanol solution of ferric chloride hexahydrate (0.05M) is drop casted onto ZnO nanorods array and spin dried at 3000 rpm for 30s. The dried samples are subsequently heated at 250°C for 5 min to improve particle adhesion on nanorod surface. White colored substrate turned yellow indicating the formation of FeOOH after repeating the procedure 7 times. The as prepared core-shell nanorods are annealed in N2 atmosphere at 400°C for 1 h to completely convert FeOOH to a-Fe2O3 with change in substrate color from yellow to brick red. a-Fe2O3 nanoparticles are also grown directly on stainless steel substrate under identical conditions for comparison.
Preparation of ZnO/C core-shell nanorods as the positive electrode material
To prepare ZnO/C core-shell nanorods electrodes, ZnO deposited stainless steel substrate is immersed into 50 ml of 0.5M glucose aqueous solution for 24 h for substantial adsorption of glucose molecules onto nanorod surface. Subsequently, the stainless steel foil is taken out and dried at 60°C followed by annealing in Argon atmosphere at about 500°C for about 5 h to allow complete carbonization of glucose shell onto ZnO nanorods. For comparison, C shells are also grown directly on stainless steel substrate under similar conditions.
Assembling the asymmetric supercapacitor cell
The prototyped asymmetric supercapacitor cell is fabricated by separating ZnO/a-Fe2O3 and ZnO/C core-shell nanorods electrodes each of 1×1 cm2 in size with a separator followed by encapsulating in poly-ethylene-terepthalate (PET) films to prevent electrolyte leakage.
The performance of a typical asymmetric supercapacitor is provided in Table 1.
Table 1:Performance chart for asymmetric supercapacitor cell with stainless steel substrate integrated ZnO/a-Fe2O3 // ZnO/C core-shell nanorods electrodes
Parameters Positive electrode
(ZnO/C) Negative electrode
(ZnO/a-Fe2O3) ZnO/a-Fe2O3//ZnO/C ASC cell
Operating voltage (V) 0V to 0.95V 0V to -0.85V 0V to 1.8V
Specific capacitance
@ 10 mV/s 229F/g 492 F/g 115 F/g
Specific capacitance
@ 100 mV/s 141 F/g 382.4 F/g 47 F/g
Cycling performance - - ~ 99% up to 2000 cycles
> 81% after 4000 cycles
Response time - - ~ 39 ms
Maximum energy and power density - - 41.2 Wh/kg and 7 kW/kg

A plurality of supercapacitors can beconnected in a series to enhance the performance of the individual capacitor as shown in Figure 6.
In order to exhibit the practical implication of the aforesaid asymmetric supercapacitor, a prototype asymmetric supercapacitor (1×1 cm2) is realized to light a Light-Emitting-Diode (LED). A single asymmetric supercapacitor can light a red LED after charging at 3 A/g for about 10s while a tandem device fabricated by connecting two asymmetric supercapacitor units (each 1×1 cm2) in series, which has an extended potential window, can light blue and green LEDs after charging for about 10s(Figure 5 and Figure 7).
Use of substrate-integrated ZnO/a-Fe2O3 and ZnO/C core-shell nanostructures in fabricating asymmetric supercapacitor, help increasing its capacitance, energy and power density by improving charge transportation process, lowering its internal resistance and self-discharge. This asymmetric supercapacitor device provides higher energy and power density as compared to any other iron-oxide-based supercapacitors
The aforesaid description is enabled to capture the nature of the invention. It is to be noted however that the aforesaid description and the appended figures illustrate only a typical embodiment of the invention and therefore not to be considered limiting of its scope for the invention may admit other equally effective embodiments.
It is an object of the appended claims to cover all such variations and modifications as can come within the true spirit and scope of the invention.