Description:Form 2

THE PATENT ACT, 1970
(39 of 1970)
&
THE PATENT RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)

“HIGH-POWER AND HIGH-ENERGY AQUEOUS AND NON-AQUEOUS HYBRID SUPERCAPACITOR DEVICES”

in the name of PONDICHERRY UNIVERSITY an Indian National having address at, PONDICHERRY UNIVERSITY, CHINNA KALAPET, KALAPET, PUDUCHERRY, PUDUCHERRY, PUDUCHERRY – 605014, INDIA.

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION:

The present invention generally relates to energy storage devices. More particularly, the present invention relates to a method for fabricating high-power and high-energy aqueous/non-aqueous hybrid supercapacitor device.

BACKGROUND OF THE INVENTION:

With the development of many electronic technologies, the use of energy storage devices increases rapidly. To satisfy the fast-growing demands, supercapacitors have paid close attention on the fields due to its fast charge-discharge ability, high energy/power density and long cycling lifetime compared with batteries.

There are reports available in the state of art revealing the existence of methods for fabricating supercapacitors.

US6187061B1 discloses a method of making a supercapacitor structure comprising providing a positive electrode member formed as a flexible membrane of polymeric matrix composition having an activated carbon powder uniformly distributed therein bonded to an electrically-conductive current collector element; providing a negative electrode member formed as a flexible membrane of polymeric matrix composition having an activated carbon powder uniformly distributed therein bonded to an electrically-conductive current collector element; providing a separator member formed as a flexible membrane of polymeric matrix composition; arranging contiguously said separator member between said positive electrode member and said negative electrode member; and bonding said separator member to said positive electrode member and to said negative electrode member to form a unitary flexible laminate structure.
WO2016053079A1 discloses a flexible supercapacitor comprising an electrolyte sandwiched between nickel foams electrodeposited with a nanocomposite. The nanocomposite comprises of a conducting polymer, graphene oxide and a metal oxide. Process of fabricating the flexible supercapacitor is also provided. The process comprises electrodepositing a nanocomposite electro-potentio-statically on a nickel foam from an aqueous solution comprising of a conducting monomer, graphene oxide and a metal salt, placed in one compartment cell followed by compressing an electrolyte between at least two layers of electrodeposited nickel foams.

US20130155579A1 discloses an electrochemical redox supercapacitor. The supercapacitor includes two thin films of electrically conducting polymer separated by an ion-permeable membrane and including an electrolyte disposed between the two thin films. Electrical contacts are disposed on outer surfaces of the two thin films. The supercapacitor is flexible and may be rolled, folded on itself, or kept substantially flat. A suitable conducting polymer is polypyrrole. In another aspect, the invention is a method for making a redox supercapacitor.

US20140087192A1 discloses a composite comprising a conducting polymer and a graphene-based material is provided. The composite includes a graphene-based material doped with nitrogen or having a nitrogen-containing species grafted thereon, and a conducting polymer arranged on the graphene-based material. Methods of preparing the composite, and electrodes formed from the composite are also provided.

US11437199B1 discloses a layered dual hydroxide (LDH) composite is provided. The LDH composite includes a nickel (Ni)-cobalt (Co)-LDH, and a nitrogen (N) and sulfur (S) co-doped reduced graphene oxide (rGO-NS), where the Ni—Co-LDH is at least partially enfolded by the rGO-NS to form the LDH composite. An electrode including the LDH composite is also provided.
One of the major problems in existing supercapacitors is the weak cycling stability upon continual charge/discharge. Electrostatic storage mechanism of an Electrical Double Layer Capacitance (EDLC)-based electrode stores only limited charges. Further, LDH materials shows evidence of interlayer hydrogen bonding, which leads to the clumping and blocking of the active sites, resulting in low conductivity and a restricted charge transfer process. Furthermore, the fabrication process of such supercapacitors is very complex and comprises of multiple steps.

Accordingly, there remains a need in the state of art to have an alternative method for fabricating supercapacitors which overcomes the aforesaid problems and shortcomings.

Hence, an attempt has been made to develop a simple and cost-effective method for fabricating aqueous/non-aqueous supercapacitor device having excellent electrochemical and simple fabrication process overcoming the limitations of prior supercapacitor structures and fabrication procedures.

OBJECT OF THE INVENTION:

The main object of the present invention is to develop a cost-effective method for fabricating energy storage devices.

Another object of the present invention is to fabricate a high-power and high-energy aqueous/non-aqueous hybrid supercapacitor device.

Yet another object of the present invention is to prepare exfoliated nickel cobalt layered double hydroxide (NiCoLDH) quantum sheets(QS) as positive electrode, reduced graphene-oxide(rGO) as negative electrode along with electrolytes and separators for fabricating aqueous/non-aqueous supercapacitor devices.
Further object of the present invention is to utilize the fabricated aqueous/non-aqueous supercapacitor device for delivering high-power density and high-energy density.

SUMMARY OF THE INVENTION:

The present invention discloses a method of fabricating aqueous/non-aqueous supercapacitor devices. The method of the present invention comprising of following steps;
preparation of bulk NiCoLDH comprises of mixing 0.03M Ni(NO3)2 and 0.01M Co(NO3)2 in 30 ml methanol with 0.1M urea to form a homogenous solution and autoclaving (sovothermal treatment) in a muffle furnace at 120 °C for 12 h followed by cooling to room temperature and centrifuging to obtain precipitate followed by washing with distilled water & ethanol and drying in a vacuum oven at 60 °C overnight to form bulk NiCoLDH;
preparation of exfoliated NiCoLDH quantum sheets comprises of
mixing 50 mg of the bulk NiCoLDH and 50 ml of 5M KNO3 salt solution followed by sonication in a bath sonicator for intermittent 5 h to form colloidal solution;
centrifuging the colloidal solution and washing with distilled water to to obtain sediment;
mixing the sediment with 50 ml of formamide and sonicating for intermittent 5 h in a bath sonicator to form translucent colloidal solution;
centrifuging the translucent colloidal solution and washing with distilled water to form a precipitate and drying in vacuum oven at 60°C overnight to form exfoliated NiCoLDH quantum sheets;
characterized in that
fabrication of aqueous/non-aqueous symmetric supercapacitor employing the exfoliated NiCoLDH quantum sheets of step(b) on carbon as positive electrode, rGO as negative electrode along with electrolytes and separators.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1 depicts preparation of the exfoliated NiCoLDH quantum sheets in flowchart.

Figure 2a depicts Positive and Negative electrode preparation.

Figure 2b depicts Fabrication of aqueous and non-aqueous asymmetric supercapacitor devices.

Figure 3 depicts depicts (a) XRD patterns recorded for the bulk NiCoLDH, anion-intercalated NiCoLDH, and exfoliated NiCoLDHQS, (b) Raman spectrum recorded for the bulk NiCoLDH and the exfoliated NiCoLDHQS, (c) FT-IR spectrum recorded for the exfoliated NiCoLDHQS. Sample.

Figure 4 (a, b) depicts HR-TEM images,(c) depicts lattice fringes, (d) SAED pattern recorded for the exfoliated NiCoLDHQS sample

Figure 5 depicts (a) CV plots at different scan rates, (b) GCD plots at different current densities, (c) Current dependent on specific capacity recorded for the exfoliated NiCoLDHQS in 1M KOH.

Figure 6 depicts (a,b) CV curves for varying potential window, (c,d) CV curves at varying scan rates for the exfoliated NiCoLDH quantum sheet electrode based hybrid capacitor (Exfoliated NiCoLDHQS|1M KOH|rGO) in 1M KOH (a,c)) and (Exfoliated NiCoLDHQS|1M LiPF6 (non-aq)|rGO in 1M LiPF6 (b,d))
Figure 7 depicts CV curves at varying scan rates for the exfoliated NiCoLDHQS electrode based non-aqueous lithium-ion capacitor (Exfoliated NiCoLDHQS|1M LiPF6|rGO) in 1M LiPF6 (a) and aqueous lithium-ion capacitor (Exfoliated NiCoLDHQS|1M LiClO4|rGO) in 1M LiClO4 (b).

Figure 8 depicts (a, b) GCD profiles at different current densities, (c, d) Ragone plot for the exfoliated NiCoLDHQS electrode based hybrid capacitor (Exfoliated NiCoLDHQS|1M KOH|rGO) in 1M KOH and (Exfoliated NiCoLDHQS|1M LiPF6 (non-aq)|rGO in 1M LiPF6.

Figure 9 depicts (a, b) cycling studies of 10000 cycles at 10 Ag-1 for the exfoliated NiCoLDHQS electrode based hybrid capacitor (Exfoliated NiCoLDHQS|1M KOH|rGO) in 1M KOH (a), Insets: First and last three cycles CD curves and (Exfoliated NiCoLDHQS|LiPF6 (non-aq.)|rGO in 1M LiPF6 (b), Insets: First and last three cycles CD curves.

DETAILED DESCRIPTION OF THE INVENTION:

The present invention discloses a method of fabricating aqueous/non-aqueous supercapacitor device.
The method of the present invention comprising of following steps;
Preparation of bulk NiCoLDH
Preparation of exfoliated NiCoLDH quantum sheets
Fabrication of aqueous/non-aqueous asymmetric supercapacitor
Preparation of bulk NiCoLDH: The bulk NiCoLDH was prepared by thorough mixing of 0.03M Ni(NO3)2 and 0.01M Co(NO3)2 in 30 ml methanol with 0.1M urea to form a homogenous solution. It was then transferred to a 100 ml Teflon-lined stainless-steel autoclave. The autoclave was tightly sealed and heated in a muffle furnace at 120 °C for 12 h. After solvothermal treatment(ST), the autoclave was cooled to room temperature and the content was centrifuged. The obtained precipitate was washed with distilled water and ethanol to remove the impurity. Then, it was dried in a vacuum oven at 60 °C overnight to form bulk NiCoLDH. The following reactions occurs in the LDH formation under the solvothermal conditions:

Methanol and nitrate ions undergo a reaction at 120 °C where the nitrate is converted to ammonia and slowly releases precipitating ion, OH-. The formed ammonia on protonation releases ammonium ion ?(NH?_4^+) and also OH-. The formation of OH- ion increases the pH in the reaction vessel and results in the LDH formation as stated in Eq. (3).

Preparation of exfoliated NiCoLDH quantum sheets: The exfoliated NiCoLDH quantum sheets were synthesized from the as-prepared bulk NiCoLDH via two step sonochemical route. Figure 1 Shows the schematic for the preparation of exfoliated NiCoLDH quantum sheets.

For the anion intercalation treatment, the bulk NiCoLDH (50 mg) was mixed with 50 ml of 5M KNO3 salt solution and sonicated in a bath sonicator for intermittent 5 h for the intercalation of nitrate anions into the LDH interlayers. After the anion exchange treatment, the interlayer gallery of the NiCoLDH filled with excess nitrate ions and resulted in the swollen phase of the LDH. The colloidal solution was centrifuged, washed with distilled water to obtain sediment. The obtained sediment was poured with 50 ml of formamide and sonicated for further intermittent 5 h in a bath sonicator. The introduction of formamide leads to the formation of new hydrogen bonding in the interlayer gallery and the penetration of large amount of formamide molecules leading to the exfoliation of the bulk NiCoLDH into quantum sheets. Then the obtained translucent colloidal solution was subjected to centrifuge and washed with distilled water to form precipitate. The precipitate was dried in vacuum oven at 60 °C overnight to obtain the exfoliated NiCoLDH quantum sheets.

The obtained exfoliated NiCoLDH quantum sheets were then subjected to characterization studies.

The crystal structure of the synthesized exfoliated NiCoLDH quantum sheet was examined using X-ray diffraction (XRD) with Cu Ka radiation (? = 1.5418 Å) (PANalytical, Aeris Research 2D), Raman (Varian 5000) and Fourier transformed infra-red (FT-IR) spectroscopies (Nicolet 6700). A high-resolution transmission electron microscope (HR-TEM JOEL JEM 2100 Plus) was utilized to study surface morphology.

Fabrication of aqueous/non-aqueous asymmetric supercapacitor employing the exfoliated NiCoLDH quantum sheets on carbon cloth as positive electrode, rGO on the carbon cloth as negative electrode along with electrolytes and separators.

Initially, the three-electrode setup was constructed to assess the electrochemical activity of the exfoliated NiCoLDH quantum sheet. The exfoliated NiCoLDH quantum sheet, super P carbon, polyvinylidene difluoride (PVDF) as a binder and N-methyl-2-pyrrolidone (NMP) solvent were mixed in a weight ratio of 70:20:10 to prepare the working electrode slurry. The homogeneous slurry prepared was coated on a carbon cloth (1x1 cm) by means of the doctor-blade method and kept in a vacuum oven for drying at 60°C for overnight (Figure 2a). The three-electrode cell was constructed using the exfoliated NiCoLDH quantum sheet as the working electrode, Ag/AgCl as a reference electrode and platinum (Pt) (1x1 cm) as the counter electrode in 1M KOH solution. The three-electrode cells constructed were tested for cyclic voltammetry (CV) studies in the potential window of 0-0.55 V at several scan rates in 1M KOH. Further, galvanostatic charge-discharge (GCD) profiles were performed at differing current densities in the aforementioned potential range of 0-0.5 V using an electrochemical workstation (Biologic, SP-150). Each of the hybrid aqueous and non-aqueous capacitors the (Exfoliated NiCoLDH|1M KOH|rGO) and (Exfoliated NiCoLDH |1M LiPF6|rGO) was made in the coin type(CR2032) and their performance was evaluated. The hybrid device comprised the rGO as negative electrode(cathode) and the exfoliated NiCoLDH as positive electrode(anode). The carbon cloth-coated exfoliated NiCoLDH was cut into a 16 mm diameter disc to make the positive electrode and the 16 mm diameter rGO was also punched to use as a negative electrode for the hybrid capacitor fabrication. Whatman-filter paper was used as a separator for aqueous and Celgard was used as a separator for the non-aqueous supercapacitor.

The two CR-2032-coin cells containing the positive electrode, negative electrode and separator were assembled using 1M KOH aqueous electrolyte (aqueous supercapacitor device) and 1M LiPF6 non-aqueous electrolyte(non-aqueous supercapacitor device). Figure 2b shows the fabrication steps for the aqueous and non-aqueous supercapattery devices. By fixing the suitable potential window, the CVs were recorded at several scan rates for the coin cells. The GCD profiles at several current rates were recorded in a fixed voltage limit of 0-1.8 V for the (Exfoliated NiCoLDH|1M KOH|rGO) aqueous hybrid capacitor and 0-2.8 V for the (Exfoliated NiCoLDH|1M LiPF6 |rGO) non-aqueous hybrid capacitor using the above-mentioned workstation.
Crystal Structure and Morphology: Exfoliation of the bulk NiCoLDH in a solvent can lead to pealing of the MO6 layers and form nanocolloid. The Tyndall effect can serve as one of the means for confirming the formation nanocolloidal suspensions, in this case, the exfoliated NiCoLDHQS. Hence, using a side-incident laser pointer light beam, the nanocolloidal suspensions of the exfoliated NiCoLDHQS exhibited a clear evidence of a light scattering effect, commonly referred to as Tyndall scattering (or Tyndal effect). The appearance of the clear light path implies that the colloids contain suspended LDH quantum sheets (QS) that scatter the incident light. A normal solution without suspended solid particles do not scatter the light. As evidently seen, the bulk NiCoLDH and the ion exchanged NiCoLDH did not show the Tyndall effect, whereas, a clear Tyndall effect has been exhibited by the exfoliated NiCoLDHQS colloids. The heterogeneous mixture containing the suspended exfoliated NiCoLDHQS is denser than the normal solution. The suspended particles do not allow the light to pass through, instead cause scattering in all directions, leading to the trajectory of the light beam clear visibility. The confirmation of the Tyndall effect reveals that the colloidal solution contains the exfoliated NiCoLDHQS solid particles whose size is comparable or less than the wavelength of the incident light. It is assumed that the QS are in the range of 1-100 nm which usually scatter the incident light.

Further, the phase and structure of the exfoliated NiCoLDHQS crystals were studied using X-ray diffraction. The recorded XRD patterns are given in Figure 3(a). The bulk NiCoLDH XRD pattern matches with the standard pattern (JCPDS PDF# 38-0715). The (006) and (012) planes are in the multiples of (003) plane, ensuring the layered nature of the bulk NiCoLDH sample. It is seen that the XRD pattern of the ion-exchanged NiCoLDH still has the Bragg peaks of (006) and (012), though the identity of the (006) Bragg peaks has drastically reduced compared to the intensity obtained for the same peaks of the bulk NiCoLDH. This implies that the treatment with the solvent lead to intercalation of the solvent molecules into the layers of the bulk NiCoLDH and initiated de-lamination. Due to the limited ability of the solvent treatment, exfoliation did not happen clearly. The XRD pattern of the exfoliated NiCoLDHQS has Bragg peak only at 2?, 11?, all other peaks are almost diminished. The absence of other Bragg peaks except the (003) plane confirms the exfoliation of the bulk NiCoLDH into the NiCoLDH quantum sheets. The FWHM of (003) is large for the ion-exchanged NiCoLDH (21°) compared to the bulk NiCoLDH (19°), which is likely an indication of a wider dispersion of water and nitrate in tilt angles in the interlayer space, and also the spatial random orientations of the water, whereas the FWHM of the (006) Bragg peak of the exfoliated NiCoLDHQS is significantly large, implying the presence of few layers of QS in the grains. The crystallite size obtained for the bulk NiCoLDH was 7.34 nm and that for the exfoliated NiCoLDHQS was 5.79 nm. The exfoliated NiCoLDHQS were suspended in a translucent colloidal solution due to the smaller crystallite size. The exfoliated NiCoLDHQS mostly consists of the (003) planes, although a minor intense (012) plane. The solvent treatment with KNO3 seems initiation of the bulk NiCoLDH into exfoliation. The ultrasonication had a clear effect that lead to complete de-lamination of the MO6 layers of the bulk LDH leading to the exfoliated NiCoLDH quantum sheets. It is believed that the bath sonication having the ultrasonic power of 60 W is sufficient to break the bonds present between the MO6 layers by dismantling the lamellar structure. Such dismantling of the layers has been also obtained in other layered materials, such as MAX into Mxene and MoS2. Figure 3(b) shows the obtained Raman spectrum for the bulk NiCoLDH and the exfoliated NiCoLDHQS. The Raman shifts at 526 and 458 cm-1 for the bulk NiCoLDH correspond to the Ni-O and Co-O symmetric stretching vibrations. A weak peak at 310 cm-1 is due to the F2g vibrational mode of the spinel phase of NiCo2O4. Interestingly, the Raman profile of the exfoliated NiCoLDHQS shows broad and weak Raman shifts, with a shift towards the positive side. As fewer Mo6 layers are present, the Raman shifts are resulted in low intensity in the case of the exfoliated NiCoLDHQS. Also, as there no much neighbours, the wavenumber of the Raman peaks has shifted to higher.

The FT-IR spectrum studied for the exfoliated NiCoLDHQS shown in Figure 3(c) reveals wide peak in the range of 3732-2988 cm-1, confirming the presence of hydrogen-bonded OH. The MO-H (M denotes Ni or Co) bonds deformation mode is confirmed by the presence of the peak at 1115 cm-1. The peaks at 1634 and 1367 cm-1 arise due to the CO32- and NO3- ions, respectively. The presence of the Ni-O and Co-O bonds significantly marked by the observation of the peaks at 405 and 641 cm-1. Furthermore, the exfoliated NiCoLDHQS sample’s surface morphology was analysed using HR-TEM as shown in Figure 4. The presence of individual LDH sheets with an irregular shape is observed. The lattice fringes are clearly visible that confirm the crystallinity of the prepared exfoliated NiCoLDHQS. The aforementioned XRD data are substantiated from the HR-TEM analysis as the interplanar spacings of 0.34 and 0.23 nm of the (006) and (015) Braggs planes match very well in both cases. Further, the SAED pattern shown in Figure 4d confirms the presence of clear rings which are indexed to the (003) and (006) planes, substantiating the crystallinity and the XRD results.

Electrochemical performance of exfoliated NiCoLDH quantum sheets: The electrochemical performance of the exfoliated NiCoLDHQS was initially assessed using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) studies in three-electrode cell configuration in 1M KOH electrolyte. Fig. 5a shows the CV profiles recorded at various scan rates, 1, 3, 5, 7, 10 and 20 mV s-1 in the 0 – 0.55 V potential window. It is observed that there is a pair of redox peaks in the CV profiles, implying that the exfoliated NiCoLDHQS electrode is electrochemically active and exhibits a Faradaic (battery-type) behaviour. The reversibility and the high charge storing capacity are well demonstrated by the trend of linearly increasing peak current with an increase of the scan rate. The current peaks at 0.31 and 0.13 V are attributed to the oxidation and reduction of Ni/Co to the +3/+2 states in the exfoliated NiCoLDHQS, respectively. The following reactions are considered to occur on the exfoliated NiCoLDHQS:

LDH-Ni(OH)2 + OH- ? LDH-NiOOH + e- + H2O Eo = 0.34 V (4)

LDH-Co(OH)2 + OH- ? LDH-CoOOH + e- + H2O Eo = 0.28 V (5)

Notably, the asymmetry of the CV profiles implies participation of more than one species as an involvement of both Ni2+ and Co3+ ions with the OH- ions in the redox reaction. Further, to quantify the charge storage behaviour of the exfoliated NiCoLDHQS electrode, GCD studies were performed in the aforementioned potential range and the obtained profiles are displayed in Fig. 5b. It is observed that the GCD profiles also exhibit the asymmetric characteristic, substantiating the CV profiles. The battery-type behaviour is demonstrated by the clear plateau potentials seen in the ranges of 0.3–0.5 V and 0.3–0.2 V during charging and discharging, respectively. The higher charge/discharge times at 1 A g-1 signify the enhanced charge storage behaviour. The specific charge was quantified by means of the following Eq.:

where C represents the specific charge of the electrode (C g-1), i represents discharge current (A), t represents discharge time (s), and m represents electrode material active mass (g). Fig. 5c shows the current density-dependent specific capacity of the exfoliated NiCoLDHQS electrode. Interestingly, the exfoliated NiCoLDHQS electrode shows a specific charge as high as 655 C g-1 at 1 A g-1. It is perceived that the specific capacity tends to decrease upon increase of the current density due to the limited utilization of the active material at high current density. Even at a high current density of 3 Ag-1, the QS electrode exhibited significant charge capacity of 100 Cg-1.

Performances of aqueous (Exfoliated NiCoLDHQS|1M KOH|rGO) and non-aqueous (Exfoliated NiCoLDHQS|1M LiPF6|rGO) hybrid supercapacitor: The enhanced charge storage of the exfoliated NiCoLDHQS in the three-electrode configuration reveals that the LDH electrode could be a candidate for an attractive positive electrode in the hybrid supercapacitor device. Thus, the hybrid supercapacitor device was fabricated using the exfoliated NiCoLDHQS as the positive electrode and the rGO as a negative electrode in the form of a laboratory prototype CR-2032 coin-cell using 1M KOH aqueous electrolyte (Exfoliated NiCoLDHQS|1M KOH|rGO) and non-aqueous electrolyte containing lithium salt (Exfoliated NiCoLDHQS|1M LiPF6|rGO). The non-aqueous electrolyte 1M LiPF6 was made by dissolving LiPF6 in EC: DEC (1:1 ratio) solvent. The positive and the negative electrode mass balance were achieved by the charge balance using the following formula.:

m_+/m_- =Q_-/Q_+ =(C_- V_-)/(C_+ V_+ ) (7)

where m+ and m- are the respective mass loadings, C+ and C- are respective specific capacity (in C g-1), V+ and V- signify the negative and the positive electrode potential window, respectively. The detailed charge storage features of the rGO electrode in three-electrode cell was examined and reported elsewhere. The performance of the fabricated hybrid capacitors was studied using the CV and GCD analyses. Initially, to optimise the working potential, each of the hybrid aqueous and the non-aqueous lithium-ion capacitor devices was cycled at various potential windows from 0 to 2.0 V and 0 to 3.0 V, respectively. Figure 6(a, b) shows the obtained CV profiles of the hybrid aqueous capacitor and the non-aqueous lithium-ion supercapacitor devices. It is inferred that the potential window 0–1.8 V is optimum for the aqueous hybrid supercapattery device, beyond this potential HER seems to occur which is non-desirable. On the other hand, the potential range of 0-2.8 V is optimum for the non-aqueous lithium-ion capacitor. Figure 6(c), (d) shows the scan rate-dependent CV profiles recorded for the aqueous hybrid capacitor and non-aqueous lithium-ion capacitor in the fixed potential ranges. As clearly seen, the aqueous hybrid capacitor shows clear redox peaks as well as hysteresis current loop in the CV profiles recorded at each scan rate as shown in Figure6(c). The appearance of the current peaks and their dependent on the scan rate imply the perfect hybrid device nature contributed by the exfoliated NiCoLDHQS and the rGO electrodes. The appearance of the peaks in the hybrid device is due to the reaction stated in Eqs. 4 and 5. It is clear that there is involvement of OH- in the redox reactions, although the peak potentials are different due to the two-electrode nature. The exfoliated NiCoLDHQS contributes Faradaic charge storage, whereas the rGO has notable non-Faradaic contribution for the charge storage. On the other hand, a close observation of the CV profiles recorded for the non-aqueous lithium-ion capacitor shown in Figure 6(d) exhibiting an EDLC behaviour resulted from the surface adsorption of lithium ions on the exfoliated NiCoLDHQS and also have peaks, rather multiple peaks with low current response. As is seen there are peaks at 1.5 and <0.5 V in the cathodic region and the corresponding peaks in the anodic region. The peak current response is pronounced at high scan rates. As the electrode is exfoliated NiCoLDH and the electrolyte contains Li+ ions only in the non-aqueous, there is no way that the said redox reactions (Eqs. 4 and 5) be possible. An interesting observation is the potentials seen in the CV profiles. The obtained peak potentials are very well related to lithiation/delithiation due to intercalation/de-intercalation of Li+ into the exfoliated NiCoLDHQS or conversion/re-conversion reaction occurring on the exfoliated NiCoLDHQS as has been observed elsewhere. Thus, the obtained peak potentials are attributed to the following conversion/reconversion or intercalation/de-intercalation reactions:

NiCoO + Li+ + e- ? LiNiCoO (8)

NiCoO + 2Li+ + 2e- ? NiCo + Li2O (9)

Assuming that the exfoliated NiCoLDHQS have predominantly NiCoO, based on the obtained peak potentials and literature reports, the stated intercalation, conversion/re-conversion are the most probable in the Li+ ion containing electrolyte. This means that an electrolyte containing any other lithium salt in aqueous should also exhibit similar intercalation/de-intercalation or conversion/reconversion reactions as has observed in the case of LiPF6 in EC: DEC. To confirm this, a hybrid lithium-ion capacitor in aqueous electrolyte containing lithium salt (Ex. LiClO4) was newly done. Thus, a new lithium capacitor device (Exfoliated NiCoLDHQS|1M LiClO4|rGO) was constructed in 1M LiClO4 (Aq.). The aqueous device using LiPF6 could not be constructed as the dissolution of the LiPF6 in water lead to formation of hydrofluoric acid (HF), which is not suitable as electrolyte. Hence, LiClO4 salt was dissolved in water and used as an electrolyte. Figure 7 shows the comparison of CV profiles recorded at different scan rates on the non-aqueous (Exfoliated NiCoLDHQS|1M LiPF6|rGO) and a new device having Li+ ion in aqueous electrolyte (Exfoliated NiCoLDHQS|1M LiClO4|rGO). It is clearly visible that the CV profiles of both the devices are almost similar, having peaks, rather multiple peaks both in cathodic and anodic regions, confirming the said intercalation/de-intercalation or conversion/reconversion reaction also occurring in the containing Li+ ions aqueous electrolyte. Thus, the multiple peaks obtained in the case of the non-aqueous lithium-ion capacitor device is due to lithiation/de-lithiation of Li+ ions. To quantify the specific energy and specific power of the hybrid supercapacitors, the GCD curves were studied in the optimised potential windows at various current rates. Figure 8(a-b) shows the charge-discharge profiles of the aqueous hybrid capacitor and the non-aqueous lithium-ion capacitor. Both the device profiles are asymmetric in nature, substantiating the aforementioned CV results. The following Eqs. were used to quantify the specific energy and the specific power:

E=(CV^2)/7.2 W h/kg (10)

P=(3600 X E)/?t W/kg (11)

where E denotes the specific energy (Wh kg-1), C denotes the specific capacity (C g-1), V denotes the voltage of the device (V), P is the specific power (W kg-1) and ?t denotes the discharging time (s). Figure 6(c-d) shows the Ragone plots constructed for the aqueous hybrid device and the non-aqueous lithium-ion capacitor at various current rates. The aqueous hybrid supercapacitor exhibits an enhanced energy density of 250 W h kg-1 at a specific power of 4 kW kg-1. Interestingly, the aqueous hybrid capacitor device showed an extremely high specific power of 48 kW kg-1 at a high specific energy of 95 W h kg-1. The non-aqueous lithium-ion capacitor exhibits an enhanced specific energy of 60 W h kg-1 at a specific power of 2.3 kW kg-1. Interestingly, the non-aqueous device also exhibited an enhanced specific power of 10 kW kg-1 even at a specific energy of 20 W h kg-1. The specific power observed by the aqueous hybrid device is almost matching with the power density of the commercial supercapacitors. Such a high specific power and specific energy could be attributed to the quantum tunnelling effect associated with the exfoliated quantum sheets which lead to fast electron/ion transports in its 2D layers. Table 1 shows the performance evaluation of the exfoliated NiCoLDHQS in both the half (three electrode) and full cell (device) configuration. Undoubtedly, the specific energy and the specific power obtained for the (Exfoliated NiCoLDHQS|1M KOH|rGO) or the (Exfoliated NiCoLDHQS|1M LiPF6|rGO) in the present work are exceedingly high and have never been observed before for the similar materials.
Table 1: Performance Evaluation

Experiment/Parameter 3 Electrode Cell
(Exfoliated NiCoLDH|1M KOH|Pt) 2 Electrode (Device)
Exfoliated NiCoLDH|1M
KOH|rGO Exfoliated NiCoLDH|1M LiPF6|rGO
Cyclic voltammetry Potential window: 0-0.55 V
Scan rate: 1,3,5,7,10,20 mV sec-1 Potential window: 0-1.8 V
Scan rate: 5, 10, 20, 30, 50, 100 mV sec-1 Potential window: 0-2.8 V
Scan rate: 5, 10, 20, 30, 50, 100 mV sec-1
Galvanostatic charge
discharge Potential window: 0-0.5 V
Current density: 1, 1.5, 1.75, 2, 2.5, 3, 3.5, 5 Ag-1 Potential window: 0-1.8 V
Current density: 2.5, 5, 7.5, 10, 12.5, 15, 17.5, 22.5, 30 Ag-1 Potential window: 0-2.8 V
Current density: 0.6, 1, 1.5, 2, 2.5,5 Ag-1
Specific capacity (C g-1) 655 - -
Specific energy (Whkg-1) NA 240
95 60
20
Specific power (Wkg-1) NA 4
48 2.3
10
Cycling stability
(10000 cycles) NA 100% coulombic efficiency throughout the cycles 100% coulombic efficiency throughout the cycles
The practicality of the hybrid device was evaluated by long charge-discharge cycling. Figure 9(a, b) shows the cycle-life data with the coulombic efficiency recorded for the aqueous hybrid and the non-aqueous lithium-ion capacitor. The coulombic efficiency is 100% for the initial cycles and attains almost 100% in the subsequent charge-discharge cycles. The inset shows the first three and last three cycle GCD profiles, which are almost similar indicating the robustness stability of the electrode.

From, the above it is evident that the aqueous and non-aqueous hybrid devices fabricated by the method of the present invention exhibited excellent cycling stability with excellent coulombic efficiency as high as 99% even at the 10,000th charge-discharge cycle. Thus, it is concluded that the developed method be a better alternative for the existing methods.

In one of the preferred embodiments, the present invention shall disclose a method of fabricating aqueous/non-aqueous supercapacitor device. The method of the present invention comprising of following steps;

preparation of bulk NiCoLDH comprises of mixing 0.03M Ni(NO3)2 and 0.01M Co(NO3)2 in 30 ml methanol with 0.1M urea to form a homogenous solution and autoclaving(solvothermal condition) in a muffle furnace at 120 °C for 12 h followed by cooling to room temperature and centrifuging to obtain precipitate followed by washing with distilled water & ethanol and drying in a vacuum oven at 60 °C overnight to form bulk NiCoLDH;
preparation of exfoliated NiCoLDH quantum sheets comprises of
mixing 50 mg of the said bulk NiCoLDH and 50 ml of 5M KNO3 salt solution followed by sonication in a bath sonicator for intermittent 5 h to form colloidal solution;
centrifuging the colloidal solution and washing with distilled water to obtain sediment;
mixing the sediment with 50 ml of formamide and sonicating for intermittent 5 h in a bath sonicator to form translucent colloidal solution;
centrifuging the translucent colloidal solution and washing with distilled water to form a precipitate and drying in vacuum oven at 60 °C overnight to form exfoliated NiCoLDH quantum sheets
characterized in that
fabrication of aqueous/non-aqueous asymmetric supercapacitor employing the exfoliated NiCoLDH quantum sheets of step(b) on carbon as positive electrode, rGO as negative electrode along with electrolytes and separators.

As per the invention, in the method of fabricating aqueous/non-aqueous supercapacitor device of the present invention, the fabrication of aqueous supercapacitor device comprising:

mixing 70 weight percentage of the exfoliated NiCoLDH quantum sheets, 20 weight percentage of Super P carbon and 10 weight percentage of polyvinylidene difluoride (PVDF) and NMP solvent to form a NiCoLDH quantum sheets slurry followed by coating the NiCoLDH quantum sheets slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form anode;
mixing 70 weight percentage of reduced graphene oxide (rGO) 20 weight percentage of super P carbon and 10 weight percentage of NMP solvent to form a rGO slurry followed by coating the rGO slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form cathode;
employing Whatman filterpaper as separator between the anode and the cathode;
employing 1M KOH as aqueous electrolyte.

In accordance with the invention, in the method of fabricating aqueous/non-aqueous supercapacitor device of the present invention, the fabrication of non-aqueous supercapacitor device comprising:

mixing 70 weight percentage of the exfoliated NiCoLDH quantum sheets, 20 weight percentage of Super P carbon and 10 weight percentage of polyvinylidene difluoride (PVDF) and NMP solvent to form a NiCoLDH quantum sheets slurry followed by coating the NiCoLDH quantum sheets slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form anode;
mixing 70 weight percentage of reduced graphene oxide (rGO) 20 weight percentage of super P carbon and 10 weight percentage of NMP solvent to form a rGO slurry followed by coating the rGO slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form cathode;
employing Celgard as separator between the anode and the cathode;
employing 1M LiPF6 as non-aqueous electrolyte.

Working example 1:

Method of fabrication of aqueous supercapacitor device:

Preparation of bulk NiCoLDH: 0.03M Ni(NO3)2 and 0.01M Co(NO3)2 was mixed in 30 ml methanol with 0.1M urea to form a homogenous solution. It was then autoclaved(solvothermal condition) in a muffle furnace at 120 °C for 12 h and cooled to room temperature and then centrifuged to obtain precipitate. The precipitate was then washed with distilled water & ethanol and dried in a vacuum oven at 60 °C overnight to form bulk NiCoLDH.

Preparation of exfoliated NiCoLDH quantum sheets: 50 mg of the bulk NiCoLDH and 50 ml of 5M KNO3 salt solution were mixed and sonicated in a bath sonicator for intermittent 5 h to form colloidal solution. The colloidal solution was then centrifuged and washing with distilled water to form sediment. The sediment was then mixed with 50 ml of formamide and sonicated for intermittent 5 h in a bath sonicator to form translucent colloidal solution. It was then centrifuged and washed with distilled water to form a precipitate and drying in vacuum oven at 60 °C overnight to form exfoliated NiCoLDH quantum sheets.

Fabrication of aqueous supercapacitor device: 70 weight percentage of the exfoliated NiCoLDH quantum sheets, 20 weight percentage of super P carbon and 10 weight percentage of polyvinylidene difluoride (PVDF) and NMP solvent were mixed to form a NiCoLDH quantum sheets slurry. The slurry was then coated on carbon cloth by doctor blade method and dried in a vacuum oven at 60oC for overnight and punched into 16mm diameter disc to form anode. 70 weight percentage of reduced graphene oxide (rGO) 20 weight percentage of super P carbon and 10 weight percentage of NMP solvent were mixed to form a rGO slurry. The slurry was then coated on carbon cloth by doctor blade method and dried in a vacuum oven at 60oC for overnight and punched into 16mm diameter disc to form cathode. Whatman filter paper was used as separator between the anode and the cathode and 1M KOH as aqueous electrolyte.

Working example 2:

Method of fabrication of non-aqueous supercapacitor device:

Preparation of bulk NiCoLDH: 0.03M Ni(NO3)2 and 0.01M Co(NO3)2 was mixed in 30 ml methanol with 0.1M urea to form a homogenous solution. It was then autoclaved(solvothermal condition) in a muffle furnace at 120 °C for 12 h and cooled to room temperature and then centrifuged to obtain precipitate. The precipitate was then washed with distilled water & ethanol and dried in a vacuum oven at 60 °C overnight to form bulk NiCoLDH.

Preparation of exfoliated NiCoLDH quantum sheets: 50 mg of the bulk NiCoLDH and 50 ml of 5M KNO3 salt solution were mixed and sonicated in a bath sonicator for intermittent 5 h to form colloidal solution. The colloidal solution was then centrifuged and washed with distilled water to obtain sediment. The sediment was then mixed with 50 ml of formamide and sonicated for intermittent 5 h in a bath sonicator to form translucent colloidal solution. It was then centrifuged and washed with distilled water to form a precipitate and drying in vacuum oven at 60 °C overnight to form exfoliated NiCoLDH quantum sheets.

Fabrication of non-aqueous supercapacitor device: 70 weight percentage of the exfoliated NiCoLDH quantum sheets(QS), 20 weight percentage of super P carbon and 10 weight percentage of polyvinylidene difluoride (PVDF) and NMP solvent were mixed to form a NiCoLDH quantum sheets slurry. The slurry was then coated on carbon cloth by doctor blade method and dried in a vacuum oven at 60oC for overnight and punched into 16mm diameter disc to form anode. 70 weight percentage of reduced graphene oxide (rGO) 20 weight percentage of super P carbon and 10 weight percentage of NMP solvent were mixed to form a rGO slurry. The slurry was then coated on carbon cloth by doctor blade method and dried in a vacuum oven at 60oC for overnight and punched into 16mm diameter disc to form cathode. Celgard was used as separator between the anode and the cathode and 1M LiPF6 as non-aqueous electrolyte.

Although the invention has now been described in terms of certain preferred embodiments and exemplified with respect thereto, one skilled in the art can readily appreciate that various modifications, changes, omissions, and substitutions may be made without departing from the scope of the following claims.
, Claims:WE CLAIM:

1. A method of fabricating aqueous/non-aqueous supercapacitor device comprising of following steps:

a. preparation of bulk NiCoLDH comprises of mixing 0.03M Ni(NO3)2 and 0.01M Co(NO3)2 in 30 ml methanol with 0.1M urea to form a homogenous solution and autoclaving(solvothermal condition) in a muffle furnace at 120 °C for 12 h followed by cooling to room temperature and centrifuging to obtain precipitate followed by washing with distilled water & ethanol and drying in a vacuum oven at 60 °C overnight to form bulk NiCoLDH;
b. preparation of exfoliated NiCoLDH quantum sheets comprises of
i. mixing 50 mg of the said bulk NiCoLDH and 50 ml of 5M KNO3 salt solution followed by sonication in a bath sonicator for intermittent 5 h to form colloidal solution;
ii. centrifuging the said colloidal solution and washing with distilled water to obtain sediment;
iii. mixing the said sediment with 50 ml of formamide and sonicating for intermittent 5 h in a bath sonicator to form translucent colloidal solution;
iv. centrifuging the said translucent colloidal solution and washing with distilled water to form a precipitate and drying in vacuum oven at 60 °C overnight to form exfoliated NiCoLDH quantum sheets;
characterized in that
c. fabrication of aqueous/non-aqueous asymmetric supercapacitor employing the said exfoliated NiCoLDH quantum sheets of step(b) on carbon as positive electrode, rGO as negative electrode along with electrolytes and separators.

2. The method of fabricating aqueous/non-aqueous supercapacitor device as claimed in claim 1, wherein the said fabrication of aqueous supercapacitor device comprising:

a) mixing 70 weight percentage of the said exfoliated NiCoLDH quantum sheets step(a) of claim 1, 20 weight percentage of super P carbon and 10 weight percentage of polyvinylidene difluoride (PVDF) and NMP solvent to form a NiCoLDH quantum sheets slurry followed by coating the said NiCoLDH quantum sheets slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form anode;
b) mixing 70 weight percentage of reduced graphene oxide (rGO) 20 weight percentage of super P carbon and 10 weight percentage of NMP solvent to form a rGO slurry followed by coating the said rGO slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form cathode;

c) employing Whatman filterpaper as separator between the said anode and the cathode
d) employing 1M KOH as aqueous electrolyte.

3. The method of fabricating aqueous/non-aqueous supercapacitor device as claimed in claim 1, wherein the said fabrication of non-aqueous supercapacitor device comprising:

a. mixing 70 weight percentage of the said exfoliated NiCoLDH quantum sheets step(a) of claim 1, 20 weight percentage of super P carbon and 10 weight percentage of polyvinylidene difluoride (PVDF) and NMP solvent to form a NiCoLDH quantum sheets slurry followed by coating the said NiCoLDH quantum sheets slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form anode;
b. mixing 70 weight percentage of reduced graphene oxide (rGO) 20 weight percentage of super P carbon and 10 weight percentage of NMP solvent to form a rGO slurry followed by coating the said rGO slurry on carbon cloth by doctor blade method and drying in a vacuum oven at 60oC for overnight and punching into 16mm diameter disc to form cathode;
a) employing Celgard as separator between the said anode and the cathode
b) employing 1M LiPF6 as non-aqueous electrolyte.

Dated this 12th day of JUN 2024

For PONDICHERRY UNIVERSITY
By its Patent Agent

Dr.B.Deepa
IN/PA 1477