Method and system for improving the energy efficiency and for reconditioning of a vanadium flow battery

Abstract

The present invention comprises a method and system for improving the energy efficiency of a vanadium flow battery, VFB. This is achieved by simultaneously reconditioning the VFB through in-situ activation of the electrodes.

Claims

1. A method for improving the energy efficiency of a vanadium flow battery, VFB, comprising a cell comprising a negative half-cell and a positive half-cell, the negative half-cell comprising a negative electrode and a negative electrolyte, and the positive half-cell comprising a positive electrode and a positive electrolyte, the method comprising: reconditioning the negative electrode and/or the positive electrode of the VFB by applying an activation potential to the negative electrode resulting in the over discharge of the negative electrolyte from V.sup.II/V.sup.III to at least V.sup.IV and/or applying an activation potential to the positive electrode resulting in the over discharge of the positive electrolyte from V.sup.IV/V.sup.V to at least V.sup.III while controlling the current through the cell or the potential at the negative or the positive electrode.

2. The method of claim 1, wherein the step of applying a potential resulting in the overdischarge of the negative electrolytes and/or the positive electrolyte comprises preventing pumping of the positive electrolyte through the positive half-cell to/from a positive reservoir and/or preventing pumping of the negative electrolyte through the negative half-cell to/from a negative reservoir of the VFB during a discharge cycle.

3. The method of claim 1, further comprising the step of re-establishing the state of charge, SoC, of the positive electrolyte in the positive half-cell and/or the negative electrolyte in the negative half-cell to the operating SoCs prior to restarting the operation of the VFB.

4. The method of claim 1, wherein the step of re-establishing the state of charge, SoC, of the positive electrolyte in the positive half-cell and/or the negative electrolyte in the negative half-cell to the operating SoCs comprises the step of pumping the positive electrolyte through the positive half-cell to/from the positive reservoir and/or pumping the negative electrolyte through the negative half-cell to/from the negative reservoir of the VFB prior to commencing a charging cycle of the VFB.

5. The method of claim 1 further comprising applying the potential to the positive electrode and/or applying the potential to the negative electrode for a number of charge and discharge cycles corresponding to an initial controlled charging cycle.

6. The method of claim 1, wherein the step of reconditioning the negative electrode and/or the positive electrode of the VFB is performed prior to a first charge/discharge cycle of the battery.

7. The method of claim 1, wherein the step of reconditioning the negative electrode and/or the positive electrode of the VFB is performed after a number of cycles.

8. The method of claim 1, wherein the step of reconditioning the negative electrode and/or the positive electrode of the VFB is performed a number of cycles after a previous reconditioning.

9. The method of claim 1, wherein the step of applying the activation potential to the negative electrode comprises applying a potential more positive than −0.8V (vs Hg/Hg.sub.2SO.sub.4) to the negative electrode and the step of applying the activation potential to the positive electrode comprises applying a potential more negative than +0.4V (vs Hg/Hg.sub.2SO.sub.4) to the positive electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows the cell potential, cell open-circuit potential, each half-cell working potential and the positive and negative probe potentials for the second charging and discharging cycle of a vanadium flow battery (VFB);

(3) FIG. 2 shows a plot of the positive and negative overpotential for the charging and discharging data shown in FIG. 1;

(4) FIG. 3 shows a plot of the positive and negative overpotentials during (a) the first 5 cycles and (b) the final 5 cycles of 47 cycles for the VFB of FIG. 1;

(5) FIG. 4 shows a plot of the positive overpotentials during (a) the first 5 cycles and (b) the final 5 cycles for the VFB of FIG. 1;

(6) FIG. 5 shows a plot of the negative overpotentials versus probe potential during (a) discharging and (b) charging for the first 5 cycles and final 5 cycles of 47 cycles for the VFB of FIG. 1;

(7) FIG. 5c shows a table of the approximate positive overpotentials at probe potentials of 1.01 V and approximate negative overpotentials at probe potentials of −0.42 V during charging and discharging for the first 5 and the final 5 cycles of 47 cycles for the VFB of FIG. 1;

(8) FIG. 6 shows the normalised activities obtained from both cyclic voltammetry and electrochemical impedance spectroscopy for each of 6 carbon materials for both V.sup.II/V.sup.III and V.sup.IV/V.sup.V. Also shown are the bands of operating potentials experienced by both the positive and negative half-cells during charging and discharging for the VFB of FIG. 1;

(9) FIG. 7 shows the process flow of the first embodiment of the method of the present invention;

(10) FIG. 8 shows the process flow of the second embodiment of the method of the present invention;

(11) FIG. 9 shows a plot of the positive and the negative overpotentials during (a) the first 5 cycles and (b) final 5 cycles of 40 cycles after switching the electrodes of the VFB of FIG. 1 in accordance with the first embodiment of the method of the present invention;

(12) FIG. 9c shows a table of the approximate positive overpotentials at probe potentials of 1.01 V and approximate negative overpotentials at probe potentials of −0.42 V during charging and discharging for the first 5 cycles and the final 5 cycles of 47 cycles of the VFB of FIG. 1 before switching the electrodes and the first 5 cycles and final 5 cycles of 40 cycles after switching the electrodes in accordance with the first embodiment of the method of the present invention;

(13) FIG. 10 shows a plot of the positive and the negative overpotentials during (a) the first 5 cycles of 47 cycles of the VFB of FIG. 1 before switching the electrodes and (b) the first 5 cycles of 40 cycles after switching the electrodes in accordance with the first embodiment of the method of the present invention;

(14) FIG. 11 shows a plot of the positive overpotentials during (a) the first 5 cycles and (b) the final 5 cycles of 47 cycles the VFB of FIG. 1 before switching the electrodes and (c) the first 5 cycles and (d) the final 5 cycles of 40 cycles after switching the electrodes in accordance with the first embodiment of the method of the present invention;

(15) FIG. 12 shows a plot of the negative overpotentials versus probe potential during (a) discharging and (b) charging for the final 5 cycles of 47 cycles before switching the electrodes of the VFB of FIG. 1 and the first 5 cycles of 40 cycles after switching the electrodes in accordance with the first embodiment of the method of the present invention;

(16) FIG. 13 shows a plot of both the positive and the negative overpotentials, both before (left hand side) and after (right hand side) the activation step. The last 5 cycles of 34 cycles before the activation step are shown on the left hand side and the first 5 cycles of 54 cycles after the activation step are shown on the right hand side. The activation step is in accordance with the second embodiment of the method of the present invention; and

(17) FIG. 14 shows a plot of the negative overpotential at a probe potential of −0.43 V (corresponding to a particular SoC) for both the 34 cycles before (left hand side) and the 54 cycles after (right hand side) the activation step in accordance with the second embodiment of the method of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(18) The present invention provides a method and system for increasing the energy efficiency of a vanadium flow battery by reducing the overpotential which occurs during the charging and discharging process. This is achieved by reconditioning the VFB electrodes. This reconditioning of the VFB electrodes can be performed simultaneously. The reconditioning can be performed before normal operation of the VFB has commenced. The reconditioning can also be performed after operating the VFB for a number of charge and discharge cycles. Each reconditioning can be performed for any duration. Furthermore, the reconditioning can be repeated on many occasions, such as for example whenever the voltage efficiency of the VFB has reduced.

(19) An understanding of how the overpotential can be reduced by reconditioning the VFB after operation for a number of charge and discharge cycles can be obtained from an analysis of further figures. In this regard, FIG. 5 shows plots of negative overpotentials versus probe potential during (a) discharging and (b) charging for the first 5 cycles (dashed lines) and final 5 cycles (dotted lines) cycles (of 47 cycles) of the evaluation described in the background to the invention with respect to FIGS. 1 to 4. The negative probe potential is a function of [V.sup.II]/[V.sup.III], and hence a function of the SoC. As previously explained, the overpotentials during discharging (FIG. 5(a)) increase with cycling, and by cycle 43, the negative overpotential has approximately tripled. It can also be seen that the probe potential region of operation has changed with cycle number, but there are regions of probe potential that are common to both sets of curves (i.e. regions of the same SoC) and at these common potentials, the overpotentials for the later cycles are much larger than those for the earlier cycles. Thus, the change in overpotential is not due to a change in the SoC. Furthermore, the change in overpotential is not due to changes in temperature, since the temperature was relatively constant for the duration of these 47 cycles (the temperature for the first 5 cycles was 23.7±0.7° C., and the temperature for the last 5 cycles was 23.0±0.6° C.). It will further be appreciated that the change in overpotential is also not due to changes in concentration of either vanadium or acid, since these would not account for such large changes in overpotential (i.e. a factor of three). It follows that the increase in overpotential is due to deactivation of the negative electrode.

(20) FIG. 5(b) shows a plot of negative overpotentials versus probe potential during charging. The overpotential can be seen to have tripled at selected probe potentials (i.e. SoC). The substantial increase in negative overpotential is shown more clearly in FIG. 5c, where the negative overpotentials at negative probe potentials of −0.42 V during charging and discharging for the first 5 and the final 5 cycles (of 47 cycles) are summarised. Also shown are the positive overpotentials at positive probe potentials of 1.01 V during charging and discharging for these cycles. This shows that there is very little change (˜1 mV) in the positive overpotential.

(21) In this regard, FIG. 6 shows the bands of operating potentials for both the positive (right hand vertical line) and negative (left hand vertical line) half-cells from the results discussed with reference to FIG. 1 to FIG. 5. Also shown in FIG. 6 are the normalised activities obtained from both cyclic voltammetry and electrochemical impedance spectroscopy for each of 6 carbon materials for both V.sup.II/V.sup.III and V.sup.IVV.sup.V. It is clear from this figure that the negative half-cell operating potential is negative enough to cause deactivation of the electrode for V.sup.II/V.sup.III. Thus, it is unsurprising that the overpotential of the negative half-cell during cycling became progressively worse over the 47 cycles. It is also clear that the positive half-cell operating potential is not positive enough to cause significant deactivation of the electrode for V.sup.IV/V.sup.V. Thus, it is unsurprising that the overpotential of the positive half-cell remained at a very low value for the duration of the 47 cycles.

(22) It is also clear in FIG. 6 that the positive half-cell working potential is positive enough to cause activation of the electrode for V.sup.II/V.sup.III oxidation-reduction. Similarly, the negative half-cell working potential is negative enough to cause activation of the electrode for V.sup.IV/V.sup.V oxidation-reduction.

(23) Thus, it will be appreciated that during operation of a flow battery, the overpotential increases at the negative electrode and sometimes at the positive electrode. This results in a decrease in the energy efficiency of the battery. The present invention reduces this overpotential by applying activation potentials to the negative and positive electrode, in order to minimise the overpotentials of both electrodes. Accordingly, by changing the activity (i.e. the kinetics of the V.sup.IV/V.sup.V or V.sup.II/V.sup.III redox couples) at the electrodes by anodisation or cathodisation of the electrode, the energy efficiency of the system can be increased.

(24) It should be noted that these treatments are reversible, and the states of activity can be toggled by switching from one treatment to another. Anodic treatment of carbon electrodes leads to enhancement of the rates of the V.sup.II/V.sup.III reactions, but inhibition of the rates of the V.sup.IV/V.sup.V reactions. Conversely, cathodic treatment leads to inhibition of the V.sup.II/V.sup.III reactions but enhancement of the V.sup.IV/V.sup.V reactions. In this regard, there are three distinct regions of potential, corresponding to three different surface states consisting of an oxidised, an intermediate and a reduced state. The intermediate state is responsible for activation of the electrodes, and the oxidised and reduced states are responsible for deactivation of the electrodes.

(25) The present invention describes a number of different embodiments for the in-situ activation treatment of the electrodes in a VFB. However, it will be appreciated that any other suitable method for treatment of the electrodes could equally well be used in order to reduce the overpotential during battery operation.

(26) In accordance with a first embodiment of the method of the invention, the overpotential is reduced through a technique which involves switching the positive and negative half-cells. This switching results in an increase in electrode kinetics and, therefore, an increase in energy efficiency. It will be appreciated that the number of cycles is arbitrary. The ideal number can be system dependent since it can be dependent on the ratio of electrolyte in the cells and electrolyte in the reservoirs. Furthermore the number can be dependent on how the battery was cycled, for example whether the SoC is between 20 and 80% or some other range and whether the current density was 100 mA m.sup.−2 or some other value.

(27) The switching of the positive and negative half-cells involves a number of steps, which are shown in FIG. 7. In step 700, the electrolyte in the negative and positive half-cells are drained into their respective reservoirs. The electrolyte tubes are then clamped (step 705). All electrolyte inputs and outputs from each side of the cell can then be disconnected (step 710). In step 715, the negative reservoir is then connected to the positive half-cell and the positive reservoir connected to the negative half-cell and the electrolyte allowed to flow again. In this manner, the electrode that formerly contained V.sup.II/V.sup.III and was the negative electrode now contains V.sup.IV/V.sup.V and is now the positive electrode while the electrode that formerly contained V.sup.IV/V.sup.V and was the positive electrode now contains V.sup.II/V.sup.III and is now the negative electrode. As a result, the new positive electrode had been treated at reducing potentials while the new negative electrode had been treated at oxidising potentials. This results in minimising the overpotentials on both electrodes and re-establishing energy efficiency through the reconditioning of the electrodes.

(28) A second embodiment of the method of the present invention will now be described with reference to FIG. 8. In accordance with this embodiment, the electrolyte flow is turned off during discharging of the system when the current is still on by turning off the pumps (step 800). This results in the electrolyte in the positive and negative half-cells being overdischarged while the electrolyte outside of the cell, such as in the reservoirs, stays at the original operating state of charge (SoC). Therefore, as the electrolyte is overdischarged—from V.sup.II/V.sup.III to V.sup.IV (or further) at the negative electrode and from V.sup.IV/V.sup.V to V.sup.III (or further) at the positive electrode—the electrode and electrolyte potentials will correspond to activation potentials for the respective electrodes causing both the negative and positive electrodes to be reactivated (see FIG. 6).

(29) The method of the second embodiment thus simultaneously reconditions the positive and negative electrodes using suitable potentials while the electrolyte pumps are turned off. As this method is fast, efficient and well understood, it allows VFB electrodes to be routinely reconditioned in-situ.

(30) It is important that the electrolyte flow is turned off in this second embodiment of the method of the present invention. In this manner, the treatment is performed quickly (since only a fraction of the electrolyte is used), with a region of treatment potentials that causes activation can be easily accessed. In addition, the electrolyte that is left in the reservoirs and tubes can be used to facilitate the re-establishment of SoC to normal operating levels without electrochemical charging of the electrolyte. Furthermore, during periods of non-use, there would be advantages associated with leaving the electrolyte in these discharged states within each half-cell, since not only will the activity of the electrodes be maintained, but these discharged electrolytes (V.sup.III and V.sup.IV) are more stable than the charged electrolytes (V.sup.II and V.sup.V) over a wide range of temperatures.

(31) Once cycling is to be restarted, the pumps should be turned on again before the application of any significant current, so that the electrolyte in the reservoirs will re-establish the SoC of the electrolyte in the half-cells back to operating SoCs (step 805). It is important to re-establish SoC of the electrolyte in this manner, or by using a very small charging current, since the large overpotentials that are required for conversion of V.sup.III to V.sup.IV and vice-versa can diminish or cancel the beneficial effects of the in-situ electrochemical treatment.

(32) The second embodiment of the invention can also be implemented after the initial charge from the starting 50:50 V.sup.III/V.sup.IV electrolyte (which is often used when commissioning batteries and after ‘mixing’ of electrolytes for re-establishing of battery capacity). Charging at these SoCs requires relatively high overpotentials for conversion of V.sup.III to V.sup.IV and vice-versa. These overpotentials and resulting high cathodic and anodic potentials at the negative and positive electrodes, respectively can cause the activity of the electrodes to decrease. Therefore, since the kinetics of the V.sup.III/V.sup.IV redox couple are very slow, very small currents should be used during initial charging until all V.sup.III is converted to V.sup.IV at the positive electrode and all V.sup.IV is converted to V.sup.III at the negative electrode. For the same reason, the new V.sup.III/V.sup.IV electrolyte should be added at a slow pump rate, so that most of the charging occurs under normal operating conditions, that is under conditions of fast kinetics such as those of the V.sup.II/V.sup.III and V.sup.IV/V.sup.V couples. In this manner, the V.sup.III/V.sup.IV is converted primarily by chemical reactions in solution to the redox couples of operating electrolytes.

(33) A third embodiment of the method of the present invention will now be described. In accordance with this embodiment, the polarity of the system is changed by overdischarging a battery while pumping of the electrolyte continues. This is in contrast to the second embodiment of the invention, where the pumping of the electrolyte was turned off. As a result, the negative electrolyte changes from V.sup.II/V.sup.III to V.sup.IV/V.sup.V while the positive electrolyte changes from V.sup.IV/V.sup.V to V.sup.II/V.sup.III. Thus, the negative electrode becomes the positive electrode, and the positive electrode becomes the negative electrode. It will be appreciated that in accordance with this third embodiment of the invention, the polarity of the battery is switched without the need for the redirection of the electrolyte. This is in contrast to the method of the first embodiment, where the electrolyte is redirected. It should be noted however, that if the current used during this process is too great, the benefits of the new positive electrode having a history of being held at reducing potentials and the benefits of the new negative electrode having a history of being held at oxidising potentials will be diminished or cancelled. However, if the benefits are diminished or cancelled they can be re-established by carrying out the method of the second embodiment of the invention. As explained in respect of this second embodiment, low currents with no pumping when V.sup.III/V.sup.IV is present in the cell, as well as the gradual addition of V.sup.III/V.sup.IV electrolyte to the cell through slow pumping can be used so as to avoid large overpotentials.

(34) The reduction in the overpotential of both electrodes which results from the implementation of the first embodiment of the invention will now be described with the aid of further figures. As an example, the method of the first embodiment of the invention can be carried out directly after a cell has undergone 47 charge and discharge cycles. A series of charging and discharging cycles can then be performed between the ‘control’ potential previously mentioned and the overpotentials approximated as described for FIG. 2 after the positive and negative half cells have been switched.

(35) The results for the first 5 cycles after the switching of the positive and negative half cells in accordance with the first embodiment of the present invention are shown in FIG. 9 (a). It is clear by comparing FIG. 3(a), which illustrates the first five cycles before the switch, with FIG. 9a that the negative overpotentials have decreased after switching. The results for cycles 36-40 after switching are shown in FIG. 9(b). If this figure is compared against FIG. 3(b), which illustrates the final five cycles before switching (namely cycles 43 to 47), it can be seen that although there has been some increase in the negative overpotential over the 40 cycles (˜20 mV), the overpotential is still much less than that before switching. Thus, not only does switching appear to improve the performance of the negative half-cell initially, the improvement remains even at 40 cycles after switching. The positive and negative overpotentials during these cycles are summarised in FIG. 9c.

(36) The overpotentials during (a) the first 5 (of 47 cycles) before switching the electrodes and (b) the first 5 cycles after switching the electrodes are compared more closely in FIG. 10 by plotting them on a more sensitive y-axis scale. It can be seen that the negative overpotentials after switching are in some cases significantly less than those with no treatment.

(37) It should also be noted that although it can be seen that the switching of the electrodes significantly improves the performance of the negative half-cell, the overpotential of the negative half-cell is still significantly larger than that of the positive half-cell. This is in agreement with the results discussed in the background of the invention section, where it was noted that, in general, the kinetic rates of V.sup.IV/V.sup.V are greater than those of V.sup.II/V.sup.III. The same result can be observed for all five carbon materials tested using both cyclic voltammetry and electrochemical impedance spectroscopy in a three-electrode cell apparatus.

(38) FIG. 11 illustrates a comparison between the positive overpotentials during (a) the first 5 cycles and (b) the final 5 cycles (of 47 cycles) before switching with (c) the first 5 cycles and (d) the final 5 cycles (of 40 cycles) after switching. For all four plots, the positive overpotential remains low (<13 mV). Thus, it can be seen that switching does not cause significant changes to the positive overpotential.

(39) FIG. 12 illustrates the negative overpotentials versus negative probe potential (i.e. a function of the SoC) during (a) discharging and (b) charging for the final 5 cycles before switching the electrodes (dotted lines) and for the first 5 cycles after switching the electrodes (solid lines) are shown. It can be seen that although the operating SoC range of the system before and after switching are different, as seen by the change in operating probe potential range, there are regions of the two operating ranges that are common to both sets of curves. The overpotentials after switching are smaller than those before switching in these ranges: i.e. for the same SoC. This shows that the decrease in overpotential is indeed due to the treatment of the electrode. In fact, the treatment has resulted in the overpotential being divided by more than three; i.e. a reduction of the overpotential to less than that of fresh felt (as shown in FIG. 9c). The improvement is ascribed to the fact that the positive half-cell operating potential is positive enough to cause activation of the electrode for V.sup.II/V.sup.III.

(40) The reduction in the overpotential of both electrodes which results from the implementation of the second embodiment of the invention (see FIG. 8) will now be described with the aid of further figures. As an example, the method of the second embodiment of the invention can be carried out directly after a cell has undergone 34 charge and discharge cycles. After overdischarging the electrolyte in each half-cell, the overdischarged electrolyte can be allowed to sit in its respective half-cell for 1 hour or more. A series of charging and discharging cycles can then be performed between the ‘control’ potentials previously mentioned and the overpotentials approximated as described for FIG. 2 after the positive and negative half-cell electrodes have been reactivated.

(41) FIG. 13 compares the results for the first 5 cycles after the electrode activation with the last 5 cycles before electrode activation in accordance with the second embodiment of the present invention. It is clear by comparing the left hand side of FIG. 13, which illustrates the first five cycles before the switch, with the right hand side of FIG. 13 that the negative overpotentials have decreased significantly after activation. As before, the change in positive overpotential is almost insignificant, due to the fact that its magnitude is so small to begin with. To more clearly illustrate the changes in negative overpotential with time, the overpotential at a given SoC was recorded for each cycle, both before and after activation. The SoC was determined from the probe potential value, and so this amounted to recording the overpotential of the negative half-cell when the negative probe was at −0.43 V. FIG. 14 shows a plot of negative overpotential (for a probe potential of −0.43 V) vs. time for both the 34 cycles before and the 54 cycles after electrode activation. Both before and after activation, the negative overpotential increases slowly with time. However, even 54 cycles after activation, it is still much lower than the overpotential before activation. Thus, not only does activation appear to improve the performance of the negative half-cell initially, the improvement remains even at 54 cycles after activation.

(42) The present invention provides numerous advantages. By minimising overpotentials at the carbon electrodes of flow batteries through reconditioning the battery by activating the electrodes, the efficiency of the battery is improved. The method can also be easily applied to large-scale or small scale VFBs, leading to significant improvements in energy efficiencies, and thus reduction in the operating costs of VFBs.

(43) In addition, the decrease in overpotentials means that the potentials experienced by the electrodes will be less extreme. This results in a decrease in hydrogen and oxygen evolution, as well as other undesired side reactions. As a result, the colombic efficiency as well as the voltage efficiency is increased. Furthermore, less extreme potentials at the electrodes results in less degradation of the electrodes, thereby increasing battery life.

(44) The method of the present invention also has the advantage that it can be applied in-situ, and in most embodiments can be applied without the need for disassembly of the system.

(45) The embodiments in the invention described with reference to the drawings may comprise a computer apparatus and/or processes performed in a computer apparatus. The invention may comprise computer programs, particularly computer programs stored on or in a carrier adapted to bring the invention into practice. The program may be in the form of source code, object code, or a code intermediate source and object code, such as in partially compiled form or in any other form suitable for use in the implementation of the method according to the invention. The carrier may comprise a storage medium such as ROM, e.g. CD ROM, or magnetic recording medium, e.g. a floppy disk or hard disk. The carrier may be an electrical or optical signal which may be transmitted via an electrical or an optical cable or by radio or other means.

(46) In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.