GRAPHENE-BASED ELECTROACTIVE NANOFLUIDS AS LIQUID ELECTRODES IN FLOW CELLS

Abstract

The invention relates to VERY stable electroactive nanofluids comprising graphene-based compounds, and the production methods thereof. The invention also relates to the use of said electroactive nanofluids as liquid electrodes for the storage of energy in flow cells.

Claims

1. A electroactive nanofluid characterized in that it comprises a) a liquid medium selected from an organic solvent or a water solution of acidic, neutral or basic compounds and said liquid medium optionally comprising a surfactant, and b) a graphene-based compound or composite, homogenously dispersed in the liquid medium, and said graphene-based compound or composite optionally comprising an electroactive substance associated to the graphene based compound or composite.

2. The electroactive nanofluid according to the preceded claim, wherein the liquid medium is an organic solvent selected from the list consisting of acetonitrile, dimethylformamide and dimethylacetamide.

3. The electroactive nanofluid according to the preceded claim, wherein the surfactant is in a weight percent between 0.01% and 5% based on the total weight of the liquid medium.

4. The electroactive nanofluid according to any of claims 1 to 3, wherein the graphene based compound is in a weight percent between 0.01% and 10% based on the total weight of the electroactive nanofluid.

5. The electroactive nanofluid according to the preceded claim, wherein the graphene based compound is in a weight percent between 0.025% and 0.4% based in the total weight of the electroactive nanofluid.

6. The electroactive nanofluid according to any of claims 1 to 5, wherein the graphene-based compound further comprising polyoxometalates clusters as electroactive material.

7. The electroactive nanofluid according to the preceded claim wherein the poluoxometalates clusters are in a weight percent between 0.01% and 10% based on the total weight of the electroactive nanofluid.

8. The electroactive nanofluid according to any of claim 6 or 7, wherein the polyoxometalates clusters are selected from the list consisting of phosphotungstate and phosphomolybdate.

9. The electroactive nanofluid according to any of claims1 to 8, comprising activated carbons or carbon nanotubes.

10. Use of the electroactive nanofluid according to any of claims 1 to 9 as electrode of a flow electrochemical cell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1. Raman spectra of GO and rGO, respectively

[0029] FIG. 2(a) XRD patterns of graphite, graphene oxide (GO) and reduced graphene oxide (rGO), (b) XPS spectrum of rGO, inset shows core-level C1s spectrum

[0030] FIG. 3. (a, b) Scanning electron micrographs and (c, d) transmission electron micrographs of rGO at two different magnifications.

[0031] FIG. 4. (a, b) Nitrogen adsorption/desorption isotherm of rGO sample with corresponding BJH pore size distribution plot.

[0032] FIG. 5. Schematic diagram of the flow cell setup used in Example 3 in which charged and discharged ENFs are stored in separate containers. Two peristaltic pumps with automatic control of flow direction and flow rate were used. 1 Tanks 2 Pump 3 Separator 4 Cell.

[0033] FIG. 6 Cyclic voltammetry (CV) curves of rGO ENFs of different concentration at 20 mV/s scan rate in static condition.

[0034] FIG. 7(a, b) CV curves of rGO electroactive nanofluid of 0.025 wt % concentration at different scanning rates starting from lowest scan rate 1 mV/s to highest scan rate 10,000 mV/s, respectively.

[0035] FIG. 8 Cyclic voltammetry (CV) curves of (a, b) 0.1 wt % rGO electroactive nanofluid and (c, d) 0.4 wt % rGO electroactive nanofluid at different scanning rates starting from lowest scan rate 1 mV/s (0.001 V/s) to 10,000 mV/s (10 V/s). Impressively, rGO electroactive nanofluid-based flowing electrode showed a rectangular shaped CV (typical of capacitive behavior) at the very high scan rate of 10 V/s confirming excellent power density of rGO electroactive nanofluid-based electrochemical flow capacitor (EFC). This is the highest scan rate used to measure CV curves for flow cells.

[0036] FIG. 9 Variation of specific capacitance with scan rate for rGO ENFs of different concentrations.

[0037] FIG. 10 (a) Nyquist plots for the rGO ENFs of different concentrations across the frequency range from 10 mHz to 10 kHz.

[0038] FIG. 11 Galvanostatic charge-discharge curves for rGO electroactive nanofluid of 0.025 wt % at different current densities in static condition.

[0039] FIG. 12 The power density versus energy density of rGO electroactive nanofluid in a Ragone plot.

[0040] FIG. 13 Variation of coulombic efficiency of rGO ENFs over 1500 charge and discharge cycles.

[0041] FIG. 14 Chronoamperometry for rGO electroactive nanofluid (0.025 wt %) at different applied voltages such as 0.2, 0.4, 0.6 and 0.8 V which shows high coulombic efficiency of 98.2% when charged to a cell potential of 0.8 V and subsequently discharged to 0 V.

[0042] FIG. 15 Self-discharge i.e. shows the time-dependent loss of the open circuit cell potential for rGO ENFs (0.025 wt %).

[0043] FIG. 16 Cyclic voltammograms (20 mV/s) of 0.025 wt % rGO ENFs for different flow rates.

[0044] FIG. 17 Variation of specific capacitance of rGO ENFs with flow rates.

[0045] FIG. 18 Nyquist plots for the 0.025 wt % rGO electroactive nanofluid for different flow rates (frequency range 10 mHz to 10 kHz).

[0046] FIG. 19 Chronoamperometry for rGO electroactive nanofluid (0.025 wt %) during flow condition of 10 ml/min which shows high coulombic efficiency of 96.8% when charged to a cell potential of 0.9 V and subsequently discharged to 0 V.

[0047] FIG. 20(a, b, c) Scanning electron micrograph (SEM) and (d, e, f) scanning transmission electron micrographs of rGO, rGO-PMo12, rGO-PW12, respectively

[0048] FIG. 21 CV curves of a) rGO-PW12 and b) rGO-PMo12 electroactive nanofluids (0.025 wt %) electrode at different scanning rates (from 5 mV/s to the highest scan rate of 200 mV/s).

[0049] FIG. 22 Variation of specific capacitance rGO-PW12 and rGO-PMo12 electroactive nanofluids (0.025 wt %) electrode with scan rate

[0050] FIG. 23 Galvanostatic charge-discharge curves for 0.025 wt % of (a) rGO-PW12 and (b) rGO-PMo12 electroactive nanofluids at different current densities in static condition.

[0051] FIG. 24 The power density versus energy density of rGO-PW12 and rGO-PMo12 electroactive nanofluids (0.025 wt %) in a Ragone plot

[0052] FIG. 25 Chronoamperometry for (a) rGO-PW12 and (b) rGO-PMo12 electroactive nanofluids at different applied voltages such as 0.4, 0.6, 0.8 and 1.0 V which shows high coulombic efficiency of 95.2% when charged to a cell potential of 1.0 V and subsequently discharged to 0 V.

[0053] FIG. 26 Galvanostatic charge/discharge cycling test for rGO-POM (rGO-PW12 and rGO-PMo12) electroactive nanofluids at different current densities from 4 A/g to 16 A/g for 200 cycles

[0054] FIG. 27 CV curves (at 100 mV/s scan rate) of 0.025 wt % rGO-POM electroactive nanofluid (a) rGO-PW12 and b) rGO-PMo12) for different flow rates.

EXAMPLES

Example 1

Synthesis of Reduced Graphene Oxide (rGO).

[0055] Graphene oxide (GO) was synthesized from natural graphite using the modified Hummers method. Briefly, 5 g NaNO.sub.3 and 225 ml H.sub.2SO.sub.4 were added to 5 g graphite and stirred for 30 min in an ice bath. 25 g KMnO.sub.4 was added to the resulting solution, and then the solution was stirred at 50 C. for 2 h. 500 ml deionized water and 30 ml H.sub.2O.sub.2 (35%) were then slowly added to the solution, and the solution was washed with dilute HCl. Further, the GO product was washed again with 500 ml concentrated HCl (37%). The reduced graphene oxide (rGO) was prepared by high temperature treatment of the GO sample at 800 C. under nitrogen.

[0056] Results of Raman analysis are depicted in FIG. 1). The intensity ratio of the Raman D band (at 1348 cm.sup.1) and G band (at 1591 cm.sup.1) was D/G=1.02, thereby confirming the formation of reduced graphene oxide.

[0057] Crystallographic study was carried out using Panalytical X'pert Pro-MRD instrument (CuKalpha radiation and PIXel detector). X-ray photoelectron analyses were carried out by X-ray photoelectron spectroscopy (XPS, SPECS Germany, PHOIBOS 150). FIGS. 2a) and b) refers to (a) XRD patterns of graphite, graphene oxide (GO) and reduced graphene oxide (rGO), (b) XPS spectrum of rGO, inset shows core-level C1s spectrum. The oxygen content in this rGO, as determined by XPS was 5.8%.

[0058] Surface morphological analysis of an rGO sample was carried out by scanning electron microscopy (FEI Quanta 650F Environmental SEM). TEM images were obtained with a field emission gun transmission electron microscope (Tecnai G2 F20 S-TWIN HR(S) TEM, FEI). See FIG. 3a)-d). HRTEM revealed that the rGO sheets are almost single layer with highly transparent aspect whereas FESEM and TEM images of a bulk sample showed layers completely spread out to form a highly porous layered structure.

[0059] N.sub.2 adsorption/desorption was determined by Brunauer-Emmett-Teller (BET) measurements using Micromeritics instrument (Data Master V4.00Q, Serial#:2000/2400). Results are shown in FIG. 4. A distinct hysteresis loop observed is ascribed to the presence of a mesoporous structure in the interleaving nanosheets. In addition, rGO nanosheets exhibits pores in mesopores as well as macroporous region.

Example 2

Synthesis of rGO Electroactive Nanofluid

[0060] rGO Electroactive nanofluids were prepared by direct mixing of rGO in the liquid medium also called base fluid. In this analysis the base fluid was 1 M H.sub.2SO.sub.4 in distilled water. Electroactive nanofluids with different concentrations were prepared by mixing 0.025, 0.1 and 0.4 wt % of rGO in 1 M H.sub.2SO.sub.4 aqueous solution. In order to get stable suspension, 0.5 wt % of surfactant (triton X-100) was added and the mixture kept in an ultrasonic bath up to 2 h. The resulting sols were directly used as flowing electrodes in a home-made flow cell described in the text.

[0061] rGO electroactive nanofluids were prepared with different concentrations (0.025, 0.05, 0.1, 0.2 and 0.4 wt %) after different time intervals. The as-prepared rGO electroactive nanofluid looks dark black indicating stable and uniform dispersion of rGO in 1 M H.sub.2SO.sub.4 aqueous solution. rGO dispersions begun to precipitate after standing almost 10 hours and completely settled down after 40 hrs. Moreover, it is interesting to note that rGO electroactive nanofluids with low concentrations (0.025 and 0.05 wt %) remain stable for even longer time. Finally all rGO electroactive nanofluids could be easily re-dispersed by just mild shaking, looking again like the as-prepared products, and remaining stable for more than 5 hours, suggesting high stability of rGO electroactive nanofluids.

Example 3

Flow Cell Design

[0062] The electrochemical characterization of these rGO electroactive nanofluids of example 2 was carried out both under static and continuous flow conditions using a specially designed flow cell. See FIG. 5. The cell body (7 cm6 cm1 cm) was made of two stainless-steel plates, acting as current collectors with a carved serpentine flow channel 5 mm wide and 5 mm deep. The cell compartments were separated by a polyvinylidene fluoride (PVDF) membrane (Durapore; Merck Millipore, Germany) and oil paper was used as a gasket providing airtight sealing. The contact area between the ion-permeable membrane and the flow electrode was 12.7 cm.sup.2. Finally, the cell was designed with the high level of flow control required for prototype testing in order to meet the expectations for a large-scale Effective Function Code (EFC) device. Thus, dual (back and forth) automated flow control of peristaltic pumps for each, positive and negative compartments was implemented.

Example 4

Characterization of rGO Electroactive Nanofluid Electrodes

[0063] The electrochemical performance of rGO electroactive nanofluids electrodes of different concentrations under static conditions was investigated by cyclic voltammetry (CV) using the flow cell design of example 3.

[0064] FIG. 6 shows the CV curves of symmetric rGO electroactive nanofluid cells with different concentrations (from 0.025 wt % to 0.4 wt % rGO) at a scan rate of 20 mV/s. The currents under the curves increase and specific capacitances decrease as the concentration of electroactive nanofluids is increased, thus showing similar behavior to conventional supercapacitors with solid electrodes. The shape of CV curves is quasi-rectangular indicating a dominant capacitive mechanism of energy storage.

[0065] FIG. 7(a, b) shows the CV curves of rGO electroactive nanofluid (0.025 wt %) electrode at different scanning rates (from 1 mV/s to the highest scan rate of 10 V/s). The rectangular CV shape remains even at the very high scan rate of 10,000 mV/s; indicating that rGO electroactive nanofluids possess excellent rate capabilities, as needed for high-power supercapacitors.

[0066] Similar results are observed even for high concentration electroactive nanofluids. FIG. 8 shows cyclic voltammetry (CV) curves of (a, b) 0.1 wt % rGO electroactive nanofluid and (c, d) 0.4 wt % rGO electroactive nanofluid at different scanning rates starting from lowest scan rate 1 mV/s (0.001 V/s) to 10,000 mV/s (10 V/s). Impressively, rGO electroactive nanofluid-based flowing electrode showed a rectangular shaped CV, typical of capacitive behavior, at the very high scan rate of 10 V/s confirming excellent power density of rGO electroactive nanofluid-based electrochemical flow capacitor (EFC). It should be noted that the shape of CV curves becomes more and more rectangular with increase in scan rate. This indicates an increasingly important relative contribution of capacitive (Double Layer) energy storage vs. pseudocapacitive (faradaic) energy storage at high rates.

[0067] The behavior observed for electroactive nanofluids implies that the bulk of the liquid can be polarized, which in turn implies percolative electronic conduction through the electroactive nanofluid which could therefore be rightly considered as a true liquid electrode.

[0068] The values of specific capacitance of the cell for rGO electroactive nanofluids were calculated from the CV curves and are shown in FIG. 9 for the different concentrations and scan rates tested.

[0069] As it could be expected, specific capacitances decrease gradually with increasing scan rate. A specific capacitance value of 169 F/g(rGO) was obtained for 0.025 wt % rGO electroactive nanofluid at a scan rate of 1 mV/s. Please note that in order to store that amount of charge on rGO within the flow cell, the rGO flakes must be electrically connected to the external load via a conductive pathway. Unlike conventional supercapacitors in which solid film electrodes benefit from well-defined fixed conduction paths, our cell utilizes a liquid electrode in which charge must percolate through a dynamic network of conductive particles.

[0070] The values of specific capacitances represented in FIG. 9 are very much comparable or even higher than the values reported for thicker carbon slurries.

[0071] For instance, Presser et al. [The electrochemical flow capacitor: A new concept for rapid energy storage and recovery. Adv. Energy Mater. 2, 895-902 (2012)] prepared a thick carbon slurry of carbide-derived carbon powder obtained from titanium carbide (TiC: CDC) and 1 M Na.sub.2SO.sub.4 with electroactive nanofluids of 3:1 and 4:1 (electrolyte: carbon by mass). The highest specific capacitance reported for toothpaste-like TiC:CDC slurry (3:1, electrolyte: carbon) was 109 F/g at the scan rate of 2 mV/s.

[0072] Furthermore, Zhang et al. [Zhang, C. et al. Highly porous carbon spheres for electrochemical capacitors and capacitive flowable suspension electrodes. Carbon, 77, 155-164 (2014)] reported a specific capacitance of 154 F/g at 2 mV/s in 1 M H.sub.2SO.sub.4 for a thick slurry of porous carbon spheres with concentration ranging from 16 wt % to 23 wt %.

[0073] Finally, electrochemical impedance spectroscopy data show a low ohmic resistance in the range of about 0.23-0.28 that suggests fast ion transport and a highly conductive network facilitating charge and ion percolation. See FIG. 10. These values are even lower than those reported for spherical carbon particles suspension electrodes in [0074] Hatzell, K. B. et al. A high performance pseudocapacitive suspension electrode for the electrochemical flow capacitor. Electrochim. Acta, 111, 888-897 (2013). [0075] Hatzell, K. B. et al. Composite manganese oxide percolating networks as a suspension electrode for an asymmetric flow capacitor. ACS Appl. Mater. Interfaces, 6, 8886-8893 (2014). [0076] Hatzell, K. B. et al. Capacitive deionization concept based on suspension electrodes without ion exchange membranes. Electrochem. Commun. 43, 18-21 (2014).

[0077] In addition, the impedance curves show a distorted semi-circle in the high-frequency region due to the porosity of rGO and a vertically linear spike in the low-frequency region. The high-frequency intercept of the semi-circle on the real axis yields the solution (electrolyte) resistance (Rsol), and the diameter provides the charge-transfer resistance (Rct) over the interface between rGO electrode and electrolyte.

[0078] The electrochemical performance of rGO electroactive nanofluids of examples 2 was further studied by galvanostatic charge/discharge cycling in static conditions as shown in FIG. 11. The shapes of charge-discharge curves are symmetric, triangular and linear for the rGO electroactive nanofluids at all different current densities used. For the 0.025 wt % rGO electroactive nanofluid, specific capacitance were 117 and 50 F/g(rGO) at current densities of 1 A/g and 2.5 A/g, respectively. This corresponds to specific energy values of 5.7-13.1 Wh/kg(rGO) and specific power of 0.45-1.13 kW/kg(rGO) as shown in FIG. 12.

[0079] The values of specific energy are significantly higher than for previous reports on slurries, for example, 5.6-8.2 Wh/kg for carbon beads slurry [Campos, J. W. et al. Investigation of carbon materials for use as a flowable electrode in electrochemical flow capacitors. Electrochim. Acta 98, 123-130 (2013)].

[0080] Moreover, the galvanostatic cycling performance was found to be stable, with cycle efficiencies greater than 97.6% after 1500 charge/discharge cycles (FIG. 13).

[0081] FIG. 14 shows a series of chronoamperometry experiments carried out for rGO electroactive nanofluids under static conditions. Initially, the cell was completely discharged for a period of 15 min and then charged to different potentials such as 0.2, 0.4, 0.6 and 0.8 V. The specific capacitances were calculated for rGO electroactive nanofluids at different potentials and are in the range 36-156 F/g(rGO), which are comparable to the values derived from CVs. The coulombic efficiency of the rGO electroactive nanofluid cell was found to be 98.2% (FIG. 14), a large value since we did include the leakage current, which is in very good agreement with the coulombic efficiency derived from galvanostatic charge/discharge experiments (98.9%).

[0082] FIG. 15 shows the time-dependent loss of the open circuit cell potential (self-discharge) for rGO electroactive nanofluids. After 30 min of charging to maximum cell potential of 0.9 V, the open circuit voltage dropped to 34% of the initial voltage (0.9 V) after 24 h.

[0083] This is still comparable or better than the optimized commercial solid-electrode packed supercapacitor cells [Kaus, M. Kowal, J. Sauer, D. U. Modelling the effects of charge redistribution during self-discharge of supercapacitors. Electrochim. Acta 55, 7516-7523 (2010)].

[0084] In order to get full insights on the potential of the electroactive nanofluids described in example 2 to be applied in flow cells, the electrochemical properties of rGO electroactive nanofluids under continuous flow conditions were also investigated. FIG. 16 shows CV curves (at 20 mV/s scan rate) of 0.025 wt % rGO electroactive nanofluid for different flow rates. It is interesting to note that the shape of the CV curves remains identical for the different flow rates used, which confirms the uniform and stable nature of the electroactive nanofluid. However, the current under the curve increases with increase in flow rate from 0 to 10 ml/min but then begins to decrease for flow rates >10 ml/min.

[0085] The variation of specific capacitance values with flow rate is presented in FIG. 17 . We can see how those values increase from 31 to 48 F/g(rGO) as flow rate is increased from 0 to 10 ml/min, but then decrease for higher flow rates. This initial increase in specific capacitance may be attributed to the flow of fresh rGO nanoparticles taking part in charge storage. However, at high flow rates, and under the experimental conditions used, the residence time for rGO nanoparticles in the flow channels will eventually be insufficient to allow for full charge of all the dispersed material, consequently leading to a decrease of the specific capacitance. It should be noted though that this bypassing effect could take place at much faster rates through an optimized design of the electrochemical cell, for instance with higher surface area current collectors.

[0086] In addition to this, fast electroactive nanofluid flow rates can also lead to a detrimental increase in resistance (contact resistance, friction and particle-particle resistances). This is confirmed from potentiodynamic electrochemical impedance measurements (PEIS) shown in FIG. 18. The interfacial resistance associated with the current collector and rGO electroactive nanofluid interface was found to constitute a large portion of the total cell resistance.

[0087] Furthermore, when compared to static charge-discharge experiments, there is a slight decrease in coulombic efficiency in cycling under flowing conditions (96.8%) (FIG. 19). This may also be attributed to the increase in interfacial resistance during flow condition. Nevertheless, these preliminary flow experiments confirmed that EFC cells based on rGO electroactive nanofluids work very promisingly during flow conditions.

Example 5

Synthesis of Polyoxometalate (POM)-rGO Electroactive Nanofluids

[0088] Two different hybrid materials based on rGO and polyoxomelatates (POM) i) rGO-phosphotungstate (H3PW12O40) (in short, rGO-PW12) and ii) rGO-phosphomolybdate (H3PMo12O.sub.40) (in short, rGO-PMo12) have been prepared. Briefly, two samples, 0.25 g each of rGO was dispersed in 100 ml of deionized water in two separate beakers with probe sonicator (of power 1500 watt) for 2 hours. Later, 10 mM of each phosphotungstic acid (H3PW12O403H2O, (PW12)) and phosphomolybdic acid (H3PMo12O40.3H2O, (PMo12)) were added in the corresponding beakers of pre-sonicated rGO solutions. These suspensions were further sonicated for 5 hours and kept at room temperature for the next 24 hrs. Afterwards, these products were filtered-off separately and dried in vacuum oven at 80 C. overnight. The resulting products were labelled as rGO-PW12 and rGO-PMo12, for the synthesis from phosphotungstic and phosphomolybdic acids, respectively.

[0089] Hybrid Electroactive nanofluids of rGO-POMs were prepared by direct dispersion of rGO-PW12 and rGO-PMo12 solids in water. In particular, for application as flowing electrode the solids were dispersed in aqueous H.sub.2SO.sub.4 electrolyte. Thus, hybrid electroactive nanofluids were prepared by mixing 0.025 wt % of rGO-PW12 and rGO-PMo12 in 1 M H.sub.2SO.sub.4 aqueous solution, separately. In order to get stable suspension, 0.5 wt % of surfactant (triton X-100) was added and the mixture was kept in an ultrasonic bath up to 2 h.

[0090] FIG. 20(a, b and c) shows SEM images of rGO, rGO-PMo12 and rGO-PW12 samples, respectively whereas FIG. 20d), e) and f) show high resolution scanning TEM (STEM) images , showing the complete and homogeneous coverage of POMs clusters on rGO (FIG. 20e) and f)) which are seen as minuscule (1 nm size) bright specs on the graphene flakes.

Example 6

Characterization of POM-rGO Electroactive Nanofluids Electrodes

[0091] The electrochemical characterization of the POM-rGO electroactive nanofluids of example 5 was carried out both under static and continuous flow conditions using the flow cell described in Example 4.

[0092] FIG. 21 shows the CV curves of a) rGO-PW12 and b) rGO-PMo12 electroactive nanofluid (0.025 wt %) electrode at different scanning rates (from 5 mV/s to the highest scan rate of 200 mV/s). Impressively, the CV shapes are not ideal rectangular confirming the contribution from redox activities of POMs clusters. In addition, the shape of CV curves remains same even at the high scan rate of 200 mV/s; indicating that rGO-POM electroactive nanofluids possess excellent rate capabilities, as needed for high-power supercapacitors.

[0093] The values of specific capacitance of the cell for rGO-POMs (Example 5) electroactive nanofluids were calculated from the CV curves and are shown in FIG. 22 for rGO-PW12 and rGO-PMo12 nanofluid of constant concentration (0.025 wt %) at different scanning rates.

[0094] The specific capacitances decrease gradually with increasing scan rate. Interestingly, the specific capacitance values of 273 F/g(rGO-PW12) and 305 F/g(rGO-PMo12) were obtained for 0.025 wt % rGO-POMs electroactive nanofluid at a scan rate of 5 mV/s. The values of specific capacitances represented in FIG. 22 are very much comparable or even considerably higher than the values reported for carbon-POM composite solid electrodes in conventional supercapacitors.

[0095] For example, in our recent investigation by P. Gomez-Romero et al. [Hybrid energy storage: high voltage aqueous supercapacitors based on activated carbon-phosphotungstate hybrid materials, Mater. Chem. A, 2014, 2, 1014] reported a specific capacitance of 254 F/g for Activated carbon-PW12 solid composite electrodes whereas in our other report [Hybrid electrodes based on polyoxometalates-carbon for electrochemical supercapacitors] the value of specific capacitance obtained for Activated carbon-PMo12 solid electrode was 183 F/g.

[0096] The electrochemical performance of rGO-POMs electroactive nanofluids described in example 5 was further studied by galvanostatic charge/discharge cycling in static conditions as shown in FIG. 23 a) rGO-PW12 and b) rGO-PMo12. The shapes of charge-discharge curves are not ideal triangular and linear for both the rGO-POM electroactive nanofluids at all different current densities used. For the 0.025 wt % rGO-POM electroactive nanofluid, specific energy values obtained are in the range of 7-28.8 Wh/kg(rGO-PW12) and 9.3-30.9 Wh/kg(rGO-PMo12) whereas the specific power for both electroactive nanofluids is 2-8 kW/kg(rGO-POM) as shown in FIG. 24.

[0097] FIG. 25 shows a series of chronoamperometry experiments carried out for a) rGO-PW12 and b) rGO-PMo12 electroactive nanofluids under static conditions. Initially, the cell was completely discharged for a period of 15 min and then charged to different potentials such as 0.4, 0.6, 0.8 and 1.0 V. The specific capacitances were calculated for both rGO-POM electroactive nanofluids at different potentials and are in the range 124-242 F/g(rGO-PW12 and 143-293 F/g(rGO-PMo12), which are comparable to the values derived from CVs. The coulombic efficiency of the rGO electroactive nanofluid cell was found to be between 98.3-98.7% (FIG. 25), a large value since we did include the leakage current.

[0098] The cycle life is very important parameter in the supercapacitors. FIG. 26 shows the cycle stability of rGO-POM electroactive which was investigated by galvanostatic charge/discharge test at different current densities from 4 A/g to 16 A/g for 200 cycles. It is interesting to note that both the rGO-POM based liquid electrodes exhibits stability in the range of 92-94% after 2000 cycles.

[0099] Further, the electroactive nanofluids described in example 5 to be applied in flow cells, in order to investigate the electrochemical properties of rGO-POM electroactive nanofluids under continuous flow conditions. FIG. 27 a) rGO-PW12, b) rGO-PMo12 shows CV curves (at 100 mV/s scan rate) of 0.025 wt % rGO-POM electroactive nanofluid for different flow rates. It is interesting to note that the shape of the CV curves remains identical for the different flow rates used, which confirms the uniform and stable nature of the rGO-POM electroactive nanofluid. However, the current under the curve increases slightly with increase in flow rate from 0 to 10 ml/min but then begins to slight decrease for flow rates >10 ml/min.

[0100] This slight variation in current density may be attributed to the flow of fresh rGO-POM nanoparticles taking part in charge storage. However, at high flow rates, and under the experimental conditions used, the residence time for rGO-POM nanoparticles in the flow channels will eventually be insufficient to allow for full charge of all the dispersed material, consequently leading to a decrease of the specific capacitance. In addition to this, during the high flow rate the redox species in rGO-POM (example 5), could not get time for complete oxidation and reduction which consequently reduces the current under curves.