DESC:TECHNICAL FIELD
The present invention is in relation to batteries. In particular the invention is in relation to Redox flow batteries (RFBs) comprising Diketopyrrolopyrrole (DPP) derivative as redox active anolyte compound. The invention is also about method of fabrication of the Redox flow batteries comprising Diketopyrrolopyrrole (DPP) derivatives as anolyte.
BACKGROUND
Lately, redox flow batteries are offering propitious results in the domain of energy storage systems. The results in terms of charge/discharge cycle life, efficiency and cost endurance in its fabrication has escalated research activities to achieve better performance.
A redox flow battery is a rechargeable battery in which electroactive species is dissolved in electrolyte. Thus, as compared to conventional rechargeable batteries, the electroactive species dictate the quality of the battery along with other parameters. Among the various redox flow batteries, non-aqueous redox flow batteries are gaining impetus due to promising performance in terms of cell voltage up to 4V and change density, which is on a par or better as compared to conventional batteries.
While the aqueous organic redox flow batteries are limited to 1.23V, the focus has shifted to non-aqueous redox flow batteries utilizing organic solvents like Acetonitrile, Propylene carbonate along with suitable redox active couples. Parallelly, many organic molecules have been reported as potential candidates acting as organic redox active couples, anolyte or catholyte. Duan et al in document titled “Wine Dark Sea” in An Organic Flow Battery: Storing Negative Charge in 2,1,3 -Benzothiadiazole Radicals Leads to Improved Cyclability; ACS Energy Lett. 2017, 2,1156; reports about the usage of compound benzothiadiazole to improve the cyclability. WO2015148357 inform about symmetric redox flow battery, wherein anthraquinone has been used as redox molecule with suitable electrolyte both as anolyte and catholyte.
Oh et al, J Mater. Chem. A 2014, 2,19994 in “A metal-free and all-organic redox flow battery with polythiophene as the electroactive species” provides redox flow battery wherein polythiophene microparticles dispersed in electrolyte solutions is used as the redox couple.
However, there are challenges relating to limited solubility, lack of high-performance membranes and low electrolyte conductivity, stability of Redox molecules which is impeding the adoption of the non-aqueous Redox flow batteries. The Redox molecules need to be stable during charge-discharge cycles, while being repeatedly oxidized and reduced with negligible degradation in performance. Considerable research has been focused on improving the stability of the anolyte as they display low stability in the presence of oxygen and moisture. The demanding characteristics of the anolyte necessitate to develop organic molecules with considerable electron affinity and electrochemical stability.
STATEMENT OF INVENTION
Accordingly, the present invention aims provides redox flow battery comprising organic anolyte molecules which provide stability over considerable number of charge discharge cycles, and improve the performance in terms of voltage and charge density.
The invention adopts diketopyrrolopyrrole derivatives of formula A, which are potential anolyte candidates in a non-aqueous redox flow battery.
The invention also provides method of fabrication of the battery involving the potential diketopyrrolopyrrole derivatives of formula A.

Formula A
Wherein,
R is selected from a group comprising electron withdrawing or donating groups such as cyano, dicyanomethylene, halides, nitro, sulfonyl, haloformyl, trihalomethyl, alkyl, substituted alkyl, amine, substituted amine, alkoxy, substituted alkoxy. The R1 is selected from a group alkyl, substituted alkyl, glycol, including- ethylene, triethylene, propylene or polyethylene glycol derivatives, which aid in improving the solubility of compound in organic solvents. X is selected from group of Sulphur and Selenium
The invention provides battery comprising diketopyrrolopyrrole (DPP) derivatives as anolyte.
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: Schematic diagram of a redox flow battery.
Figure 2: provides graphs of cyclic voltammetric studies.
Figure 3: Figure (3a) shows the capacity profiles while charging and discharging for 50 cycles and figures 3b-3d provides cycling performance of organic redox electrolyte flow battery at 0.5 mA.
Figure 4: Cyclic voltammograms of aromatic DPP derivatives with different donor groups, thiophene (TDPP-Hex) (4a), pyridine (PyDPP-Hex) (4b) compared with those of quinoidal DPP analogues with terminal dicyanomethylene units, SeDPP-Hex-CN4 and TDPP-Hex-CN4 (4c and 4d).
Figure 5: Cyclic voltammograms of 1 mM DBBB (200cycles) (a) in 0.1 M LiTFSI in DME as solvent at scan rate of 0.2 V/s. Cyclic voltammograms of redox pair and DBBB/TDPP-Hex-CN4 (b) in 0.1 M LiTFSIdimethoxyethane solution at 1.2 mm diameter platinum disc working electrode at a scan rate of 0.1 V/s.
Figure 6: Cycling performance of all-organic unisol Blue/TDPP-Hex-CN4 flow battery at 0.5 mA using 1 mM each of unisol Blue and TDPP-Hex-CN4 in inert atmosphere. Charge discharge profiles (a) plotted with increment of 10 cycles up to 100 cycles. Charge discharge capacities vs. cycle number (b);plot of efficiencies vs. number of cycles (c).
Figure 7:7(a) shows cycling measurements for DBBB/TDPP-Hex-CN4 flow battery,7b shows the data for charge and discharge capacity variation with number of cycles and 7c shows the coulombic, voltage and energy efficiencies as a function of number of cycles.
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, the singular “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 non-aqueous redox flow batteries comprising organic redox active electrolytes.
In another embodiment, the organic redox active electrolyte, anolyteis based on diketopyrrolopyrrole (DPP) derivative. The general molecular structure of DPP derivative is as in formula A.

Formula A
Wherein the R is selected from a group comprising electron withdrawing or donating groups such as cyano, dicyanomethylene, halides, nitro, sulfonyl, haloformyl, trihalomethyl, alkyl, substituted alkyl, amine, substituted amine, alkoxy, substituted alkoxy. The R1 is selected from a group alkyl, substituted alkyl, glycol, including- ethylene, triethylene, propylene or polyethylene glycol derivatives, which aid in improving the solubility of compound in organic solvents. X is selected from group of Sulphur and Selenium.
The redox active electrolyte, the catholyte is selected from a group comprising 1, 4-bis(isopropylamino)anthraquinone (Unisol Blue), (1,4-Di-tert-butyl-2,5-bis(2-methoxyethoxy)benzene (DBBB), (2,2,6,6-Tetramethylpiperidin-1-yl) oxyl(TEMPO) and the like.
The compound of formula A typically facilitate electron donation at an electric potential Vp and electron withdrawal is facilitated at an electric potential Vn that is greater than Vp.
The compounds of formula A may be used per se or as a composition along with a supporting electrolyte, which dissociates into a positive ion and negative ion in a solvent with dielectric constant greater than 2(from 2 to110). The composition comprises compound of formula A ranging from about1 mM to 2 mM and the supporting electrolyte is in the range of 0.1 M to 0.2 M. The supporting electrolyte concentration is in excess as compared to the electrolyte (typically 1 order of magnitude higher) depending on its solubility in specific organic solvent medium.
The supporting electrolyte may be chosen from a group comprising but not limiting to Lithium bis(trifluoromethanesulfonyl)imide, Sodium bis(trifluoromethanesulfonyl)imide, Sodium Trifluoromethanesulfonate, Lithium Trifluoromethanesulfonate, Lithium tetrafluoroborate, Tetrabutylammonium tetrafluoroborate, Tetrabutylammonium hexafluorophosphate, Tetrabutyl ammonium perchlorate or combination thereof.
The solvent is selected from a group of non-aqueous solvents comprising but not limiting to chloroform, alcohol, dimethoxyethane, triglyme, tetraglyme, polypropylene carbonate, ethylene carbonate, dimethylcarbonate, diethylcarbonate, acetonitrile, toluene.
Diketopyrrolopyrrole (DPP) derivative of formula B with different substituted alkyl groups are synthesized to adopt to analyse the feasibility of the usage of diketopyrrolopyrrole derivatives as anolyte in the redox battery.Some specific examples of formula B and related compounds with 1,4-thiophthene moiety instead of thiophene in the skeleton of formula A but not limiting to same, are depicted in Table 2.

Formula B

Table 2: Examples of the compound of formula B
Sl No Compound
1
Formula I
2
Formula II
3
Formula III
4
Formula IV
5
Formula V

The schematic diagram of a redox battery is given in figure 1. It comprises a reservoir 1 containing a redox active electrolyte, (Catholyte or positive electrolyte) R1, an inlet I1 and outlet O1 for circulating said electrolyte R1, and a conducting electrode C1 for facilitating redox of the said electrolyte R1. A reservoir 2 containing a redox active electrolyte ( Anolyte or negative electrolyte) R2, an inlet I2 and outlet O2 for circulating the said electrolyte R2, and a conducting electrode C2 for facilitating redox of the said electrolyte R2. An ion-conducting but electronically insulating layer or membrane(S) that facilitates the transport of ions from chamber 1 to chamber 2 or vice versa,but prevents the transport of any other constituents of R1 or R2 or C1 or C2 across said membrane.
The chambers 1 and 2 are of a corrosion resistant material selected from a group comprising but not limiting to stainless steel, Teflon or polypropylene. The electrodes C1 and C2 are selected from group comprising but not limiting to carbon felts, graphite felts carbon foam, nickel foam, platinum or carbon polymer composite electrodes. At least one of the electrodes C1 and C2 has a thickness ranging from about 0.1mm to 10 mm and are of BET specific surface area of 20,000 m2/m3to 70,000 m2/m3
A redox battery is fabricated in accordance with the figure 1, wherein the anolyte is compound of DPP derivative i.e.,compound 3 (TDPP-Hex-CN4) as depicted in Table 2, the catholyte is Unisol blue. Lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) is used as supporting electrolyte.The catholyte is prepared by dissolving 1 mM of Unisol blue and 0.2 M LiTFSI in dimethoxyethane. Similarly, anolyte is prepared by dissolving 1 mM of TDPP-Hex-CN4 and 0.2 M LiTFSI in dimethoxyethane. Graphite felt of 6 mm thickness is used as an electrode substrate for both the anode(A) and cathode(C). The active area of the electrode for both anode(A) and cathode (C) is 5 cm2. Stainless steel (SS) plates of 50 mm thickness are used as current collectors and end plates. The activated and lithiated Nafion 117 membrane (Ion exchange membrane, S) is used as a separator. Two micro diaphragm pumps (P1 and P2), stable in organic solvents, manufactured by KNF (model no. NF1.1TTDCB) are used for pumping the electrolyte solution. The cell is fitted with 1/8” polypropylene tubes and connected to the inlet and outlets of the pump (P1 and P2). The Flow rate is maintained at 10 ml per minute for all the studies.
The redox battery is tested for its performance in terms of current density and charge-discharge studies and the results are discussed below.
Cyclic voltammetry:
The cyclic voltammograms of organic redox active electrolyteare recorded in a three-electrode electrochemical cell under ambient conditions at room temperature (about 20-35°C). The Figures 2(a-c) providethe cyclic voltammetry scans of catholyte (fig 2a) and DPP derivative (fig 2b) in LiTFSI as supporting electrolyte and DME as solvent for 50 cycles. Figure 2c provides comparison of redox potentials for organic redox electrolytes. The experiment is performed by fixing the total volume of the electrolyte to 10 ml. The scan rate is fixed at 100 mV/s and the CVs are recorded up to 50 cycles for both electrolytes,compound 3 as in table 2 (anolyte,TDPP-Hex-CN4) and Unisol blue (catholyte). It can be noted fromthe figures that both CVs shows the two-electron reversible redox activity during oxidation andreduction processes. Also, both CVs are stable up to 50 cycles. Considering the two electronredox reactions, the overall cell voltage is calculated to be 1.26 V as shown in Figure 2c.
Galvanostatic charge-discharge studies:
Charge-discharge studies are performed in the battery fabricated as discussed herein under theflowing condition. The flow rate of the electrolytes fixed at 10 ml per minute. The rate capability tests are carried out at ambient atmospheric conditions (20-35oC) by varying current charge and discharge current densities. Figure 3a shows the capacity profiles obtained from the charge-discharge data at various current densities. The theoretical capacity of the cell is estimated to be 59.55 mAh/L. As it can be seen from the Figure 2, the charging and discharging capacities both decrease with the increasing current density. At 0.1 mA current (i.e, 0.02 mA/cm2 current density) the charge and discharge capacities obtained is about 69% and 67% of the theoretical capacity, respectively. The overpotential while charging and discharging also increases with the increasing operating current densities. The Table 1 provides the coulombic, voltage and energy efficiencies calculated for the same.

Current (mA) Coulombic
Efficiency (%) Voltage
Efficiency (%) Energy
Efficiency (%)
0.1 97.2 64.9 63.1
0.5 94.6 64.1 60.6
1 96.0 53.4 51.2
2 96.7 21.5 20.8
Table 1: shows the coulombic, voltage and energy efficiencies of the redox battery

It is evident from the data that voltage efficiencies and thus energy efficiencies are decreasing with increasing the operating current densities.
The cell is also subjected for charge-discharge cycling at a current density of 0.1mA/cm2 with 1.2V as charging and 0 V as discharging cutoff voltages. Figure 3b shows the charge discharge profiles of Unisol Blue/TDPP-CN4 flow battery at 5 mA charge and discharge current for 50 cycles at ambient conditions (20-25oC, atmospheric pressure). Typically, the figure shows, charge-discharge profiles Unisol Blue/TDPP-CN4 flow cell cycling at different currents (fig 3a), cycling test of Unisol Blue/TDPP-CN4 flow cell was carried out employing 1 mM Unisol Blue and 1 mM TDPP-CN4 as the cathodic and anodic redox active species separated by lithiatednafion 117 membrane.
It is evident from the capacity profiles (Figure 3b) that there is a clear two-electron redox reaction occurring while charging and discharging processes.
The theoretical capacity is estimated to be 59.55 mAh/L. The capacity vs. cycle number plot isshown in Figure 3c. It is noted that the charging and discharging capacities are stable up to 18 cycles with acapacity range of 14.5 to 15.5 mAh/L but it slowly starts to decline after 18 cycles (Figure3c).
At the 50th cycle, the charge and discharge capacities are dropped to 6.66 mAh/L and 6.11 mAh/L, retaining 46% and 42% of initial charge and discharge capacities, respectively. Figure 3d shows the combined plot of coulombic, voltage and energy efficiencies. The flow cell delivers good coulombic efficiencies throughout its 50 charge-discharge cycles, averaging to about 96 %. The minor loss in the Coulombic efficiency could be due to overcharge beyond 1.1 V. The voltage efficiencies are calculated by taking the ratio between average discharge voltage and average charge voltage. The flow cell shows a steady voltage efficiency indicating the absence of any side reactions or other failure mechanisms. The average voltage efficiency is estimated to be 51 % over 50 cycles. Energy efficiencies can be obtained from the product of coulombic and voltage efficiencies. As observed in Figure 3d, the energy efficiency also shows its steady nature over 50cycles, with an average energy efficiency of about 49 %.
Experimentation is also carried out with other derivatives of Diketopyrrolopyrrole for comparative analysis and galvanostatic charge-discharge studies are conducted in glove box using Unisol blue / TDPP-Hex-CN4and employing different catholyte, DBBB along with TDPP-Hex-CN4.

Specifically, electrochemical properties of DPP derivatives having hexyl (DPP-Hex) side chains with thiophene (TDPP-Hex) and pyridine (PyDPP-Hex) as donor units along with quinoidal DPP analogues having terminal dicyanomethylene groups for thiophene
(TDPP-Hex-CN4) and selenophene (SeDPP-Hex-CN4) derivatives are studied.

Formula I Formula II Formula III Formula IV
Cyclic voltammetry studies
Figure 4 provides the Cyclic voltammograms of aromatic DPP derivatives with different donor groups, thiophene (TDPP-Hex) (fig 4a), pyridine (PyDPP-Hex) (fig 4b) and compared with those of quinoidal DPP analogues with terminal dicyanomethylene units, SeDPP-Hex-CN4 and TDPP-Hex-CN4 (fig 4c and fig 4d). A reversible two-step reduction process is observed for both molecules.
While the CVs of TDPP-Hex exhibit quasi-reversible redox peaks, however, the CV for PyDPP-Hex found to be irreversible. This evidently shows the influence of donor moiety on the resonance stabilization of radical cation/anion formed during electrochemical redox reactions. The LUMO energies are calculated from the onset of reduction potentials in CVs of TDPP-Hex and SeDPP-Hex,which indicate that the incorporation of selenium in DPP backbone reduces the LUMO energy level from -3.1 eV for TDPP-Hex to -3.2 eV for SeDPP-Hex. The 0.1 eV stabilization of LUMO energy level of TDPP-Hex is possibly due to the lower ionization energy of selenium atom in comparison to sulphur atom.
The LUMO energies calculated for the DPP systems lie around -3.1 eV, which is not sufficiently low to be within the electrochemical stability range of n-type organic material. Therefore, the energy of the LUMO level is further reduced to below -4 eV by substituting terminal position with an electron-accepting functional group. The p-conjugated quinoidal core of DPP with terminal dicyanomethylene moiety is modified and evaluated for the electrochemical properties of DPP derivatives (TDPP-Hex-CN4and SeDPP-Hex-CN4) as redox-active materials for non-aqueous redox flow battery.

The CVs of TDPP-Hex and PyDPP-Hex are recorded in 0.1 M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DME as the supporting electrolyte. CVs of SeDPP-Hex-CN4 and TDPP-Hex-CN4 are carried out in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in chloroform as the supporting electrolyte.
Theelectrochemical properties of all-organic non-aqueous flow cells employing TDPP-Hex-CN4as anolyte andUnisol blue (UB) dye (1,4-Bis (isopropylamino)anthraquinone) as the catholyte in one cell ( flow cell 1) and DBBB (1,4-Di-tert-butyl-2,5-bis (2-methoxyethoxy)benzene) as the catholyte ( flow cell 2) in another.The redox chemistries of both active materials involve the formation of positively and negatively charged ions at cathode and anode, respectively are shown in scheme 1 below.

¬
Scheme 1: Redox reactions inTDPP-Hex-CN4/(Unisol blue or DBBB) flow batteries.
TDPP-Hex-CN4 at the anode side undergoes two-stage reduction leading to the formation of a dianion during charging, that gets oxidized to neutral species during discharge. The oxidation of Unisol blue/DBBB occurs at the cathode forming a dication/radical cation, during charge and reverse reaction during discharge.
For DBBB (Figure 5a), the1e- redox event is observed to be reversible, for more than 200 cycles based on the peak-current ratio of ~0.93. Cyclic voltammogram shown in Figure 5a is typical of a one-electron transfer redox reaction between neutral DBBB to DBBB.+ radical cation at E1/2 value of 1.2 V vs. Ag/AgCl.
For the system employing TDPP-Hex-CN4 and DBBB, ?E0 value between two discrete reduction peaks of TDPP-Hex-CN4 and single oxidation peak of DBBB is calculated to be 1.2 V and 1.4 V considering 1 e- and 2 e- redox reaction process of TDPP-Hex-CN4 respectively (Figure 5b).

Typically, figure 5 provides Cyclic voltammograms of 1 mM DBBB (200cycles) ( fig 5a) in 0.1 M LiTFSI in DME as solvent at scan rate of 0.2 V/s. Cyclic voltammograms of redox pair and DBBB/TDPP-Hex-CN4 ( fig 5b) in 0.1 M LiTFSIdimethoxyethane solution at 1.2 mm diameter platinum disc working electrode at a scan rate of 0.1 V/s. All the CVs are performed at temperature around 20oC-25oC and ambient atmospheric conditions in a three-electrode electrochemical cell with Ag/AgCl reference, Pt disc working and Pt wire counter electrodes.

Galvanostatic Charge-Discharge Studies:
In order to improve the capacity retention for unisol blue/TDPP-Hex-CN4flow cells, cell measurements are performed inside an argon glove box providing inert atmospheric conditions, with H2O and O2 content maintained below 0.1 ppm and 0.2 ppm, respectively. All other experimental conditions such as concentrations of redox active molecules, flow rate and charge discharge current are not changed and kept similar to the cycling experiment carried out under atmospheric conditions. As shown in Figure 6a, the transition among neutral and charged species (TDPP-Hex-CN4 or UB/TDPP-Hex-CN4.- or UB.+/TDPP-Hex-CN42- or UB2+) is visible as two discrete plateaus during charging and discharging. The flow cell performance improved with an enhanced capacity of 24mAh/L compared to 13-14mAh/L in ambient conditions (Figure 6b). The capacity retention over the course of 100 cycles is improved to 70% in comparison to 42% after 50 cycles for the cell measured outside the glove box. The average coulombic efficiency as well as voltaic efficiency in this case is observed to be higher, i.e. 99% and 68% respectively, thereby increasing the overall average energy efficiency for 100 cycles to 67 % (Figure 6c).

Typically, figure 6 shows the Cycling performance of all-organic Unisol Blue/TDPP-Hex-CN4 flow battery at 0.5 mA using 1 mM each of Unisol Blue(UB) and TDPP-Hex-CN4 in inert atmosphere. Charge discharge profiles (a) plotted with increment of 10 cycles up to 100 cycles. Charge discharge capacities vs. cycle number (b); plot of efficiencies vs. number of cycles (c).

The results suggest UB/TDPP-Hex-CN4 flow cell performs much better when moisture and/or O2are minimized. Presence of said impurities even in minor quantities could be detrimental to the stability of radical anions or cations and thus likely influence flow cell performance characteristics such as, efficiency, cycle life and shelf life.

Similarly, another flow cell is assembled employing 2 mM DBBB and 1 mM TDPP-Hex-CN4(Flow cell 2) as catholyte and anolyte respectively. All the other components used in flow cell 2 such as electrochemical cell, electrode substrate, membrane and supporting electrolyte are identical to flow cell 1. Figure 7 provides cycling performance of all-organic DBBB/ TDPP-Hex-CN4 flow battery at 0.5 mA employing 2 mM DBBB and 1 mM TDPP-Hex-CN4 as the catholyte and anolyte. Charge-discharge profiles (a) plotted with increment of 10 cycles from 1-100 cycles; charge and discharge capacities vs. cycle number (b); coulombic, voltage and energy efficiencies with number of cycles (c). The theoretical capacity is estimated to be 48mAh/L.

Cycling measurements for DBBB/TDPP-Hex-CN4 cells are carried out at a current density of 0.1 mA/cm2 (0.5 mA current) in inert conditions maintained inside the glove box. Charge and discharge cut-off voltages are fixed at 1.45 V and 0 V, respectively.Figure 7a shows the charge-discharge profiles of DBBB/ TDPP-Hex-CN4 flow cell ( flow cell 2). Two distinct plateaus close to 1.1 V and 1.3 V are observed during charging. The first plateau at 1.1 V likely corresponds to reduction of TDPP-Hex-CN4 to TDPP-Hex-CN4.-radical anion at the anode half-cell and concomitant oxidation of DBBB to DBBB.+ radical cation on the catholyte side of the cell. Since twice the redox concentration of DBBB (2 mM) compared to TDPP-Hex-CN4 (1mM) is taken, only 50% of DBBB is expected to be converted to DBBB.+ radical cationic form and 50% of DBBB to be remained in its neutral form. A second plateau around 1.3 V corresponds to the complete oxidation of neutral DBBB to DBBB.+ radical cation at the cathode and reduction of TDPP-Hex-CN4 radical anion to its dianion (TDPP-Hex-CN42-) at the anode. Similarly, two plateaus are observed close to 1.1 V and 0.6 V during discharge, that correspond to the reverse reactions at both electrodes recovering the active materials to their neutral form.

Figure 7b shows the data for charge and discharge capacity variation with number of cycles. The charge and discharge capacity for 1st cycle is observed to be 8mAh/L and 7.8mAh/L, respectively. The values of charge and discharge specific capacities and the percentage of capacity retention after 50 and 100 cycles are given in Table 2. Capacity retention at the end of 100 cycles is found to be more than 70% of the initial capacities, indicating good cycling performance of DBBB/TDPP-Hex-CN4 flow cell. Figure 7c shows the coulombic, voltage and energy efficiencies as a function of number of cycles. Remarkably, DBBB/TDPP-Hex-CN4 flow cell has excellent average coulombic efficiency of 99% and average voltage and energy efficiencies of about 37%.

Table 2. Charging and discharging capacity retentions after 50 and 100 cycles.
Charge capacity (mAh/L) Capacity retention (%) Discharge capacity
(mAh/L) Capacity retention (%)
Cycle 1 8.00 100 7.60 100
Cycle 50 6.25 78.12 6.20 81.54
Cycle 100 5.60 70.00 5.55 73.02

Thus, the present invention provides a potential redox flow battery with steady performance in terms of current density and charge discharge cycles. The adoption of diketopyrropyrrole derivatives as anolyte offer the advantage in terms of stability, cost and efficiency as observed through the performance of the redox flow battery.
,CLAIMS:WE CLAIM:

1. A redox flow battery comprising solutions of organic anolyte and catholyte; wherein the anolyte is a compound of formula A;

Formula A
wherein,
R is selected from a group comprising electron withdrawing or donating groups such as cyano, dicyanomethylene, halides, nitro, sulfonyl, haloformyl, trihalomethyl, alkyl, substituted alkyl, amine, substituted amine, alkoxy, substituted alkoxy;
R1 is selected from a group alkyl, substituted alkyl or glycol-ethylene, triethylene, propylene or polyethylene glycol derivatives, which aid in improving the solubility of compound in organic solvents;and
X is selected from group of Sulphur and Selenium
2. The redox flow battery as claimed in claim 1, wherein the battery is a non- aqueous battery.
3. The redox flow battery as claimed in claim 1, wherein the solutions are in solvents with dielectric constant between 2 and 110
4. The redox flow battery as claimed in claim1, wherein the solution of anolyte is selected from group of compound comprising
Formula I Formula II FormulaIII Formula IV
5. The redox flow battery as claimed in claim 1, wherein the catholyte is selected from a group comprising but not limiting to 1, 4-bis(isopropylamino)anthraquinone (Unisol
Blue), (1,4-Di-tert-butyl-2,5-bis(2-methoxyethoxy)benzene (DBBB), (2,2,6,6- Tetramethylpiperidin-1-yl) oxyl (TEMPO) and the like.
6. The redox flow battery as claimed in claim 1, wherein the anolyte of formula A is combined with a supporting electrolyte, selected from a group of compounds comprising but not limiting to Lithium bis(trifluoromethanesulfonyl)imide, Sodium bis(trifluoromethanesulfonyl)imide, Sodium Trifluoromethanesulfonate, Lithium Trifluoromethanesulfonate, Lithium tetrafluoroborate, Tetrabutylammonium tetrafluoroborate, Tetrabutylammonium hexafluorophosphate, Tetrabutyl ammonium perchlorate or combination thereof.
7. The redox flow battery as claimed in claim 1, wherein the battery is stable for 75-100 charge-discharge cycles.

8. A redox flow battery, wherein anolyte is a solution of compound of formula I

Formula I
9. A redox flow battery, wherein anolyte is a solution of compound of formula IV

Formula IV