Patent Application
20240266574
The invention relates to electrolytes suitable for use in a battery, to batteries comprising the electrolytes, to kits comprising components of the electrolytes, to methods of stabilizing vanadium electrolytes and to the use of batteries of the invention for energy storage.
The intermit production of energy by renewable technologies requires reliable and effective energy storage systems to be suitable for wide scale grid use. Vanadium Redox Flow Batteries (VRFBs) are promising candidates for future large-scale energy storage. They are particularly suitable for autonomous energy supply systems in areas with no individual power supply, e.g. remote farms or mobile radio antennas, as well as for the storage of energy generated by photovoltaic systems or wind power plants. The unique feature of flow batteries lies in their ability to independently scale the energy storage capacity and the power output of the system, thus rendering this technology very versatile with respect to the local circumstances of the energy source.
The electrolyte used in VRFB comprises vanadium salts dissolved in sulfuric acid. However, the V(V) species has a relatively low solubility in sulfuric acid and tends to form a solid precipitate at high temperatures. This process starts by the deprotonation of the hydrate penta-coordinated [VO2(H2O)3]+ cation, which is the typical structure of V(V) in sulfuric acid. With increasing temperatures, the precipitation occurs faster, forming bigger precipitate particles and damaging the system. The precipitation process includes two main steps, deprotonation and condensation reactions:
Based on this mechanism, many inorganic and organic additives have been studied to prevent the precipitation, and thus improving the thermal stability of vanadium electrolyte. The first priority to restrict the precipitation is inhibiting the deprotonation of the hydrate penta-coordinated [VO2(H2O)3]+ structure. This could be achieved by adding the additive that can form the soluble neutral species with hydrate penta-coordinated V(V) ion.
Another approach is to focus on inhibiting the condensation of V2O5. It has been reported that some organic compounds can adsorb via polarized functional groups, such as OH, CHO, C═O, on the initial V2O5 nuclei, hindering further growth and formation of particles large enough to precipitate. These reported organic compounds include methanesulfonic acid, trifluoroacetic acid, polyacrylic acid, oxalic acid, and methacrylic acid [1]; L-glutamate [2]; coulter dispersant IIIA [3]; trishydroxymethylaminomethane (Tris) [4]; fructose, mannitol, glucose, and D-sorbitol [5]; inositol, phytic acid and sodium oxalate [6,7].
These polar additives were claimed to help maintain the operation of the system, and the small nuclei of V2O5 can be dissolved back to the electrolyte solution during discharging process.
To date, the most effective additive has been reported for positive vanadium electrolyte is hydrochloric acid, which can stabilize the positive electrolyte at high state of charge and high temperature (US 2012/0077079). However, using hydrochloric acid as a thermal stabilizing agent has a number of major disadvantages:
Phosphoric acid and phosphate compounds have been reported as alternatives for hydrochloric acid to stabilize the electrolyte (JP 2002216833, and J. Zhang, et al, J. Appl. Electrochem., 41 (2011) 1215-1221). Ammonium phosphate has also been used as an additive (CN 104300168). However, these additives are normally not so effective when used at high concentration and can lead to the formation of other solid precipitates such as VOxPO4 in the electrolyte. Further additives have also been suggested, though many were not tested and can in fact be shown to be completely ineffective in practice (AU 704534, WO 95/12219).
There have been few reports of the use of organic compounds to stabilize the electrolyte. In some cases it is clear that such organic additives can be oxidized by V(V) in the positive electrolyte, thereby reducing the state-of-charge (T. D. Nguyen, et al., Journal of Power Sources, 2016, 334, 94-103). At lower state-of-charge the positive electrolyte is more thermally stable, but the usable capacity of the battery is reduced, and therefore this approach is not advantageous. It is also likely that many organic compounds would foul the ion exchange membranes or coat on the electrodes, causing the internal resistance of the battery to rise.
The inventors have surprisingly found that the use of two separate additives inhibiting each of the precipitation steps provides a substantially more effective thermal stabilization of the positively charged vanadium electrolyte. The two separate additives are a first additive that is a deprotonation inhibitor for [VO2(H2O)3]+ and a second additive that is a V2O5 precipitation inhibitor.
The reactions involved in the degradation of [VO2(H2O)3]+ are shown in
Thus, the invention provides an electrolyte for a battery, the electrolyte comprising:
A deprotonation inhibitor for [VO2(H2O)3]+ may hinder the initial condensation of V(V) by complexing with the VO2+ ion, while the V2O5 precipitation inhibitor may block the surface of initial V2O5 nuclei, hindering further growth and formation of particles large enough to precipitate. This type of additive is generally not oxidized by V(V) and is therefore stable in positive electrolyte.
Based on the inventor's surprising findings, the invention provides the following numbered statements.
As used herein, the combination of the deprotonation inhibitor for [VO2(H2O)3]+ and a V2O5 precipitation inhibitor may be referred to as a combined additive.
The invention provides an electrolyte for a battery, the electrolyte comprising:
The invention is based on the surprising finding that the use of two additives in combination—a deprotonation inhibitor for [VO2(H2O)3]+ and a V2O5 precipitation inhibitor—is able to provide a thermally stable vanadium electrolyte, while avoiding health and safety risks.
The use of the two additives in combination provides a number of advantages, which are listed below.
The electrolyte of the invention comprises a deprotonation inhibitor for [VO2(H2O)3]+. The deprotonation inhibitor for [VO2(H2O)3]+ may in general be a compound/species that is able to complex with the V(V) species (i.e. [VO2(H2O)3]+) to provide a stable soluble neutral species. An example of a suitable species that is able to complex with the V(V) species is a phosphate ion (e.g. one or more of [PO4]3−, [HPO4]2− and [H2PO4]−; see M. J. Gresser, et al., Journal of the American Chemical Society, 1986, 108, 6229-6234; N. V. Roznyatovskaya, eta, Journal of Power Sources, 2017, 363, 234-243). Without being bound by theory, it is believed that organophosphate ions [RPO4]2− and [R2PO4]− may also form the same complexes with the V(V) species. Thus, any species that is able to produce one of these ions in the solvent of the electrolyte (i.e. a solvent comprising water and sulfuric acid) may act as a deprotonation inhibitor for [VO2(H2O)3]+. Without being bound by theory, it is believed that high concentrations of phosphoric acid may result in precipitation of vanadium phosphate species. Therefore, in some embodiments of the invention that may be mentioned herein, the deprotonation inhibitor for [VO2(H2O)3]+ may comprise an anion selected from [HPO4]2− and [H2PO4]−.
For the avoidance of doubt, the term “phosphate” as used herein may refer to any of the following ions: [PO4]3−, [HPO4]2−, [H2PO4]−, [RPO4]2− and [R2PO4]−, where R is an organic group, such as a C1-10 (e.g. C1-6) organic group (e.g. an aliphatic group such as alkyl). In some embodiments of the invention that may be mentioned herein, the phosphate may be selected from the group consisting of [PO4]3−, [HPO4]2−, and [H2PO4]−.
A skilled person will understand that when in aqueous solution the ions [PO4]3−, [HPO4]−, and [H2PO4]− may exist in a pH-dependent equilibrium with the fully protonated form, H3PO4. A skilled person will understand that in the sulfuric acid/water electrolyte used in the battery of the invention, the main phosphate ions present will be the partially protonated forms, e.g. [HPO4]2− and [H2PO4]−, particularly [H2PO4]−. Corresponding logic may be applied to organic phosphate ions, which may predominantly exist as [R2PO4]− and [RHPO4]−.
In some embodiments of the invention that may be mentioned herein, the deprotonation inhibitor for [VO2(H2O)3]+ may be present in an equimolar amount to the V(V) species. For example, the deprotonation inhibitor for [VO2(H2O)3]+ may result in a molar concentration of phosphate that is at least as high as the concentration of [VO2(H2O)3]+ in solution. For example, the deprotonation inhibitor for [VO2(H2O)3]+ may be able to provide phosphate ([PO4]3−) ions in an amount of at least 0.034 mM (3.26 mg/L of [PO4]3−) in 4 M H2SO4. This corresponds to 0.001 wt. % of 2 M VOSO4 in 4 M H2SO4, which is an effective vanadium electrolyte (F. Rahman, et al., J. Power Sources 1998, 72, 105).
The deprotonation inhibitor for [VO2(H2O)3]+ may comprise any suitable counterion. A particular example of a counterion that may be present is ammonium, NH4+. However, other counterions, such as substituted ammonium ions (whether primary, secondary, tertiary or quaternary), may also be used, provided they are soluble in the electrolyte.
It is desirable that the deprotonation inhibitor for [VO2(H2O)3]+ does not include halide ions, because halide ions cause a number of disadvantages in vanadium batteries.
The electrolyte comprises a V2O5 precipitation inhibitor. As explained above, deprotonation of [VO2(H2O)3]+ results in VO(OH)3, which condenses in solution to provide V2O5, which accumulates and precipitates to form particles that grow over time and eventually damage the system. Species that are able to interact with dissolved V2O5 to prevent accumulation and precipitate formation are therefore useful in the electrolyte. In general, soluble polymers that are able to coat/surround nuclei of V2O5 in solution will prevent the accumulation of sufficient amounts of V2O5 for precipitation. The polymers must be soluble in the aqueous sulfuric acid solvent used in the battery, and also should not comprise repeating units having functional groups that are susceptible to oxidation by the V(V), such as OH and COOH (T. D. Nguyen, et al., Journal of Power Sources, 2016, 334, 94-103). A skilled person would easily be able to identify such groups that are susceptible to oxidation from basic knowledge of organic chemistry.
In some embodiments of the invention that may be mentioned herein, the V2O5 precipitation inhibitor is at least as soluble as V2O5. For example, the V2O5 precipitation inhibitor may have a solubility at least 0.326 mg/L in 4 M H2SO4 (corresponding to 0.0001 wt. % of 2 M VOSO4 in 4 M H2SO4).
Particular aspects and embodiments of the invention are discussed below.
In some embodiments of the invention that may be mentioned herein, the deprotonation inhibitor for [VO2(H2O)3]+ may comprise an inorganic compound.
In some embodiments of the invention that may be mentioned herein, the deprotonation inhibitor for [VO2(H2O)3]+ may be present at an amount of from 0.001 to 3 wt % of the combined weight of the vanadium salt and the solvent, for example from 0.25 to 1 wt %.
In some embodiments of the invention that may be mentioned herein, the inorganic compound may be selected from one or more of the group consisting of a phosphate salt and a non-halide ammonium containing compound. For example, the inorganic compound may be selected from one or more of the group consisting of a sodium phosphate, a potassium phosphate and an ammonium phosphate. Specific inorganic compounds that may be mentioned herein include one or both of NH4H2PO4 and (NH4)2HPO4.
In some embodiments of the invention that may be mentioned herein, the V2O5 precipitation inhibitor may comprise an organic compound.
In some embodiments of the invention that may be mentioned herein, the V2O5 precipitation inhibitor may be present in an amount of from 0.0001 to 0.5 wt % of the combined weight of the vanadium salt and the solvent, for example from 0.025 to 0.1 wt %.
In some embodiments of the invention that may be mentioned herein, the organic compound may be selected from one or more of the group consisting of a water-soluble polymer and a water-soluble gelatin compound.
In some embodiments of the invention that may be mentioned herein, the organic compound may be selected from one or more of the group consisting of polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA) and polyacrylate. In alternative embodiments of the invention, the organic compound may be selected from one or more of the group consisting of polyvinylpyrrolidone (PVP) and a water-soluble polyalkylene glycol. For example, the organic compound may be selected from one or both of polyvinylpyrrolidone and polyethylene glycol. In some embodiments of the invention that may be mentioned herein, the organic compound may be polyvinylpyrrolidone.
In some embodiments of the invention that may be mentioned herein, the V2O5 precipitation inhibitor may be a water-soluble polymer (e.g. PVP or polyethylene glycol) having an average molecular weight (e.g. a weight average molecular weight) of at least 10,000, such as 10,000 to 100,000, or 10,000 to 50,000.
Specific combinations of organic compound and inorganic compound that may be mentioned herein include the combinations where the inorganic compound includes NH4H2PO4 and the organic compound includes PVP, and where the inorganic compound includes (NH4)2HPO4 and the organic compound includes PVP.
In some embodiments of the invention that may be mentioned herein, the vanadium salt may comprise a vanadium sulfate.
In some embodiments of the invention that may be mentioned herein, the electrolyte may comprise vanadium ions in a concentration of from 1.0 to 3.0 M, for example from 1.6M to 2M.
In some embodiments of the invention that may be mentioned herein, the electrolyte may comprise sulphate ions in a concentration of from 2M to 6M, for example from 4M to 5M.
In some embodiments of the invention that may be mentioned herein, the invention provides an electrolyte for a battery, the electrolyte comprising:
In some aspects of this embodiment, the at least two additives include:
The invention provides a redox flow battery comprising the electrolyte of the invention. The redox flow battery according to the invention may be useful in energy storage, and the invention therefore also provides the use of a redox flow battery according to the invention for energy storage.
The invention also provides a kit of parts for stabilizing a vanadium electrolyte for use in a vanadium redox flow battery, the kit of parts comprising:
In the kit of parts according to the invention, the deprotonation inhibitor for [VO2(H2O)3]+, and the V2O5 precipitation inhibitor may be as defined hereinabove in relation to the electrolyte of the invention.
In the kit of parts according to the invention, the V2O5 precipitation inhibitor may comprise an organic composition. For example, the V2O5 precipitation inhibitor may comprise an aqueous solution of an organic compound, such as an aqueous solution of an organic compound selected from one or more of the group consisting of a water-soluble polymer and a water-soluble gelatin. In these cases, the organic compound may be present in a concentration of greater than or equal to 50 mg/mL in the aqueous solution.
The invention also provides a method of stabilizing a vanadium electrolyte for use in a vanadium redox flow battery, the vanadium electrolyte comprising:
In the method of stabilizing a vanadium electrolyte for use in a vanadium redox flow battery according to the invention, the deprotonation inhibitor for [VO2(H2O)3]+, V2O5 precipitation inhibitor and vanadium salt may be as defined hereinabove in relation to the electrolyte of the invention or the kit of parts according to the invention.
In some embodiments of the method of the invention, adding the deprotonation inhibitor for [VO2(H2O)3]+ comprises dissolving the deprotonation inhibitor for [VO2(H2O)3]+ (e.g. an inorganic compound) in the solvent.
In some embodiments of the method of the invention, the deprotonation inhibitor for [VO2(H2O)3]+ (e.g. an inorganic compound) is present in an amount of 0.001 to 3 wt % of the stabilized electrolyte, such as 0.25 to 1 wt %.
In some embodiments of the method of the invention, adding the V2O5 precipitation inhibitor may comprise dissolving the V2O5 precipitation inhibitor (e.g. an organic compound) in water to obtain an aqueous solution of the V2O5 precipitation inhibitor (e.g. organic compound) and adding the aqueous solution of the V2O5 precipitation inhibitor (e.g. organic compound) to the vanadium electrolyte.
The invention is illustrated by the below Examples, which are not to be construed as limitative.
For the avoidance of doubt, the deprotonation inhibitor for [VO2(H2O)3]+ and the V2O5 precipitation inhibitor may be referred to below, whether alone or in combination, as “additives”, e.g. as mono additives or dual additives.
For brevity and ease of reference herein, specific combinations of additive are assigned indicators as shown below.
Vanadium electrolyte (1.6 M VIII/IV) in 4 M total SO42−) were purchased from AMG Titanium Alloys & Coatings, Germany and used as-received.
The additives were purchased and used as-received: ammonium dihydrogen phosphate (Sigma-Aldrich, ≥99.99% trace metals basis), ammonium hydrogen phosphate (Sigma-Aldrich, ≥99.99% trace metals basis), ammonium phosphate (Reagent, Astral Scientific).
Graphite felt (GFD 4.6 EA, SGL Carbon Group) was exploited as porous electrode. To improve the cell efficiency, the felt was thermo-activated by heating at 600° C. for 5 h). The bipolar plate is expended graphite (TF 6/PV 15, 0.6 mm, SGL Carbon Group) for 20 cm2 active cell, and monolithic carbon plate (F100, 2 mm, SGL USA) for 3-stack cell.
The separator was anion exchange membrane (AEM, Fumatech FAP 450, 50 μm thickness) and cation exchange membrane (CEM, Nafion 117, 177.8 μm thickness).
Moreover, the cell also used PVC flow frame and copper plate as current collector.
The positive electrolyte was prepared at room temperature (22-25° C.) by a single cell with 20 cm2 active area. The main component of the cell includes PVC flow frame, expended graphite bipolar plate (TF 6, SGL Carbon Group), graphite felt porous electrode (GFD 4.6 EA, SGL Carbon Group, heat treated at 600° C. for 5 h), and anion type exchange membrane (FAP 450, Fumatech). The constant applied current is of 40 mA·cm−2 controlled by a NEWARE battery tester. The pristine vanadium electrolyte was pumped through the cell by a peristaltic pump with flow-rate of 10 mL/min. The stage of charge (SOC) of positive electrolyte was estimated based on the relation of the open circuit voltage (OCV) and SOC was calculated by Nernst equation. In this work, we employed the vanadium positive electrolyte with the 90% SOC for the thermal stability test. For ultraviolet-visible (UV-Vis) spectroscopic measurement, the 0% and 100% SOC electrolyte solution (V(V)) was prepared.
The components used to form a VRFB single cell are shown in
The electrolyte temperature was controlled by two magnetic hotplates (Heidolph, 505-30080 00 MR Silver Package-Magnetic Stirrer MR Hei-Tec, Temp. Sensor PT 1000 (V4A), Clamp). The additives were dissolved into the pristine vanadium electrolyte at room temperature prior to the cycling test.
The thermal stability test was conducted using a straightforward apparatus including a water-bath and a thermometer to control the temperature. 5 mL aliquots of positive vanadium electrolyte (90% SOC) were used for testing, with the addition of the given additives. All electrolyte samples were heated and kept at different constant temperatures (45, 50° C.). The samples were checked hourly to detect the precipitate by a physical method (see below the tested tube) when sufficiently large amounts had formed, and the time to onset of precipitation was noted. In order to gain accurate information regarding the onset time, the thermal stability test was repeated at least 3 times for each additive compound.
Results of the static thermal stability test are shown in
The results show the outstanding performance obtained by using both a deprotonation inhibitor for [VO2(H2O)3]+; and a V2O5 precipitation inhibitor in the stabilization of positive vanadium electrolytes at high temperatures. The use of the combined additives demonstrate much better thermal stabilizing ability in comparison with corresponding mono additives at both 45 and 50° C.
Specifically, at 45° C., the time to precipitate of the blank vanadium electrolyte is about 142.5±0.5 h. This increases to about 166.5±23.5, 203±11, 274.5±59.5 h with the addition of single components of 0.25 wt. % (NH4)2HPO4, 0.25 wt. % NH4H2PO4, or 0.025 wt. % PVP, respectively.
However, the dual addition of 0.025 wt. % PVP+0.25 wt. % (NH4)2HPO4 (A1) or 0.025 wt. % PVP+0.25 wt. % NH4H2PO4 (B1) increased the thermal stability to 371.5±10.5 and 395.5±10.5 h, respectively. In other words, addition of 0.025 wt. % PVP in the combined additives, increased the time to onset of precipitation by 123.1% as compared to just 0.25 wt. % (NH4)2HPO4, or by 94.8% when compared to just 0.25 wt. % NH4H2PO4.
At 50° C., we also observed the excellent performance of dual additives against the mono additives. The time to precipitate of blank electrolyte can be prolonged from 47.5±0.5 h up to about 148±6 and 172±18 h by adding A1 and B1 additives, respectively. These values are much higher than for single additives alone. As for the test at 45° C., increasing the amount of each component in such combined additives also does not significantly enhance their thermal stabilizing effectivity for the vanadium electrolyte at 50° C.
It is clear from the above results at both 45 and 50° C. that the stability increases for two additive systems are far beyond what could be achieved by doubling the concentration of the individual additives alone. It is also clear that the percentage stability increase provided by using both a deprotonation inhibitor for [VO2(H2O)3]+ and a V2O5 precipitation inhibitor is far greater than the sum of the effects provided by using either a deprotonation inhibitor for [VO2(H2O)3]+ or a V2O5 precipitation inhibitor alone. In other words, it is clear that the use of both a deprotonation inhibitor for [VO2(H2O)3]+ and a V2O5 precipitation inhibitor results in an unexpectedly beneficial synergistic effect.
A sample of 2 mL of the electrolyte solution having 90% state of charge (SOC) was heated at 50° C., and the precipitate nuclei in the tested solution were analyzed a laser particle size analyzer (Fritsch, Analysette 22 Compact). To analyze the precipitation rate, the tested solution (2 mL, 90% SOC) was heated at 50° C. for 3, 5 and 7 days. The resulting precipitate after each heating period was filtered and dried in air at room temperature for 24 h. Assuming that V2O5 is the sole product, the degree of precipitation was estimated with respect to the initial molar number of V(V).
To investigate the precipitate composition, the electrolyte samples (5 mL, 90% SOC) were also heated at 50° C. for 10 days and the precipitates collected by filtration and air-dried at 60° C. for over 24 h. X-ray diffraction (XRD) was conducted by a Shimadzu X-ray Diffractometer (Shimadzu XRD-6000, with the λCu-Kα=0.15418 nm).
The combined additives also reduce the amount of vanadium precipitates over various heating times as compared to the blank electrolyte.
In addition to reducing the rate of precipitation, combined additives also significantly minimize the particle size of V2O5 precipitate (
The combined additives were also examined the possibility to form any solid bi-product with the positive vanadium electrolyte.
The effect of additives on the oxidation stage of the positive vanadium electrolyte was investigated by the Ultraviolet-visible (UV-Vis) spectra, which were recorded by a Carry Series UV-Vis-NIR Spectrophotometer with a 10 mm path-length quartz cell.
In the valence stage change experiment, 1 mL solution of 1.6 M V(V) in 4 M total SO42− solution (100% SOC) was prepared with the addition of tested additives. To achieve a complete dissolution, all samples were sonicated for 1 h and kept for 5 days at room temperature before performing UV-vis measurements. A solution of 4 M H2SO4 was used as the reference solution. In practice, an aliquot of 100 μL of sample was diluted in 3 mL with reference solution to practically eliminate interference from complexes of V(IV) and V(V).
The UV-vis measurement was conducted for the combined inorganic-organic additives to affirm its stability in strong oxidative medium of V(V) electrolyte. The result showed in
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy in this work was performed using a BioLogic SP-150 potentiostat A three-electrode electrochemical cell with a reference electrode (Hg/Hg2SO4), a working electrode (glassy carbon electrode (GCE)) and a counter electrode (Pt) was used for both CV and EIS test. All measurements were done under Argon saturated condition of the electrolyte.
Results are shown in
The Nyquist plot obtained from electrochemical impedance spectroscopy (EIS) measurement of the positive vanadium electrolyte containing different thermal stable additives is also showed in
A single cell with 20 cm2 active area was used to perform the cycling test with the variation of electrolyte temperature. The main component of the cell includes PVC flow frame, expended graphite bipolar plate (TF 6, SGL Carbon Group), graphite felt porous electrode (GFD 4.6 EA, SGL Carbon Group, heat treated at 600° C. for 5 h), and ion exchange membrane (Fumatech FAP 450 AEM and Nafion 117 CEM). The cell was charged and discharged with a current density of 100 mA·cm−2 and within the potential window of 0.9-1.65 V. An amount of 100 mL vanadium electrolyte was pumped through the cell by a peristaltic pump with a flow-rate 50 mL/min. The charged/discharged cycle was controlled by a NEWARE battery testing machine.
The morphology of the graphite felt electrode after cell cycling was observed by a field emission scanning electron microscopy (FESEM, JEOL 7600F). The elemental composition of the electrode was characterized by microanalysis using an INCA EDS detector integrated with said FESEM equipment.
The cell cycling with 20 cm2 single cell indicates that the voltage efficiency (VE) of the electrolyte compositions of the invention are almost unchanged as compared to original electrolyte at both 25 and 50° C.
A further benefit of the combined additive is shown in
Energy Dispersive X-ray (EDX) analysis confirmed the presence of V, S and O in the precipitate, which can be assigned to the V2O5 with some associated sulfuric acid/sulfates. The movement of electrolyte through FAP 450 AEM may facilitate this precipitation, as the electrolyte was transferred from the positive to the negative side, therefore the positive electrolyte would have reached a higher SOC faster, leading to the faster precipitation. However, by using combined additives during cycling test, there was no obvious change on the morphology of the graphite felt (
Similar tests were also performed using a cell assembled with Nafion 117 CEM. Other cell components and testing parameters were kept the same as the cell using Fumatech FAP 450 AEM.
At 50° C., the cell cycling performance is also significantly improved with the presence of combined additives in the electrolyte (
After cycling test, the possible formation of V2O5 precipitate in the flow-frame and electrode was also examined. There is no obvious presence of V2O5 precipitate as in the case of the cell assembled with Fumatech FAP 450 AEM, probably due to the crossover of negative electrolyte species that can dissolve the precipitate. Despite the lack of macroscopic deposits, it was possible to detect the formation of several V2O5 precipitates on the surface of the felt by SEM after cycling at 50° C. (
The different behaviors of combined additive in the ceiling cycling using different ion exchange membranes is surprising, particularly for Nafion 117 CEM. In contrast to the results in static condition, the cell performance with the electrolyte containing dual-state additives is significantly improved. The most suitable explanation is that PVP in dual-state additives may form a very thin in situ anion-exchange layer on the Nafion membrane, which would then dramatically reduce cross-over. It should be noted that PVP-based membrane was found to highly improve the VRFB performance. In the case of FAP 450 AEM, as PVP molecules will adopt a positive charge in acidic solution, it cannot be absorbed on the membrane surface to improve the cell performance. Eventually, FAP 450 AEM may interact strongly with phosphate ions, resulting to a decrease in cell activity, since it was found to vigorously interact with positive electrolyte species in our previous study.
Overall, the formation of solid V2O5 precipitate during cell cycling can obviously block the channel of the flow-frame as well as the surface of porous electrode. This will become a big problem in a large scale VRFB system, as the precipitate may stuck not only in the flow-frame of the cell but also in the complicated pump line system, and resulting in dangerous failure. Therefore, the advantageous excellent thermal stabilizing performance of novel combined additives for vanadium electrolyte will be highly essential in safely maintaining the system operation. The electrolyte crossover and capacity fading due to the addition of combined additives can be minimized with various electrolyte rebalancing approaches in larger scale VRFB.
The A1 and B1 additive formulae were chosen to be further evaluated in a 3-stack VRFB system.
To demonstrate the long-term performance of new thermal stable vanadium electrolyte, a large scale VRFB with 3-stacked cell was cycled using electrolyte with and without combined additives.
The 3-stack VRFB was cycled for over 200 cycles with 1.5 L of vanadium electrolyte in each tank. The electrolyte was pumped through the VRFB system using magnetic pump. The pressure sensor and thermometer were also integrated into the system to continuously measure the electrolyte flow-rate, pressure and temperature. The charged/discharged current was controlled by a NEWARE battery testing machine. The thermal stable additives were dissolved into the pristine electrolyte prior to the cycling test.
For the B1 electrolyte, it was observed that the electrolyte temperature is varied from 32.5 to 35.5° C. in the negative tank, and from 33.2 to 36.2° C. in the positive tank. By the injection of combined additive, no increment of electrolyte temperature was found after more than 16 h operating time (
The influence of B1 recipe on the electrochemical property of 3-stack cell performance is shown in
The self-discharge current density is steady even with the addition of combined additive, which is measured to be about 0.72 and 0.78 mA·cm2 for pristine electrolyte and novel electrolyte. However, the cell resistivity rises from about 1.73 Ω·cm2 for the blank electrolyte to around 1.96 Ω·cm2 for the novel electrolyte, due to the resistance of PVP as discussed before.
Cell: 625 cm2 active area; electrolyte volume: 10 L; current density: 80 mA/cm2; membrane: Fumatech FAP 450; bipolar plate: PV15; electrode: GFD 4.6 graphite felt; number of cycle: 150. Ammonium phosphate concentration: 0.25% wt; PVP concentration: 0.025% wt (both expressed as a percentage of the weight of the blank (i.e. without the additives) vanadium electrolyte).
The novel combined additives (B1) also demonstrate excellent performance as compared to the pristine electrolyte when operating in 1 kW VRFB system as shown in
The excellent improvement of thermal stability indicated that as-reported additives can be used in the commercial vanadium redox flow battery. In particular, the combined additives give much higher thermal stabilizing ability for vanadium electrolyte. This is a significant factor to reduce the cost of cooling system, and therefore the overall cost of VRFB.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10202106540Q | Jun 2021 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2022/050416 | 6/16/2022 | WO |
