1 Vanadium Redox Flow Batteries for the storage of electricity
2 produced in wind turbines
3 Esperanza Menaa, Rubén López-Vizcaínoa, María Millánb, Pablo Cañizaresb, Justo
4 Lobatob, Manuel A. Rodrigo*b.
5 aDepartment of Chemical Engineering, Institute of Chemical & Environmental
6 Technologies, University of Castilla-La Mancha, Campus Universitario s/n, 13071
7 Ciudad Real, Spain
8 bDepartment of Chemical Engineering, Faculty of Chemical Sciences & Technologies,
9 University of Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain
10
11 *Corresponding Author
12 Prof. Manuel Andrés Rodrigo. Phone +34 902 204 100 Ext 3411; Fax: +34 902 204
13 130; e-mail: [email protected]
14
15 SUMMARY
16 In this work, accelerated degradation charge-discharge tests have been applied to
17 compare the performance of a bench-scale vanadium redox flow battery (VRFB), when
18 charged under galvanostatic conditions and under the highly variable conditions of
19 current produced by wind turbines. Wind speed patterns applied for the VRFB charge
20 were obtained during three representative days in winter, in Ciudad Real (Spain). The
21 accumulated and delivered charge capacities and the different efficiencies (coulombic,
22 voltage and energy) were analyzed during three charge and discharge cycles. The
23 conversion of the different vanadium species during the charge-discharge cycles,
24 indicated that the operation mode had a strong influence on the performance of the VRFB
25 and helped to explain the charge profiles obtained. Although, similar efficiencies and
1
1 charge/discharge capacities were found, the VRFB operated in wind-charging mode
2 performs slightly worse than the VFRB operated in galvanostatic mode. Increased
3 crossover of vanadium species in the negative electrolyte compartment explains the
4 differences found. Nevertheless, it can be concluded that this type of technology seems
5 to be promising for the storage of electricity produced by wind turbines.
6
7 KEY WORDS
8 Redox Flow Battery; Vanadium; Renewable Energy Source; Wind Turbine, Accelerated
9 Degradation
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1 1. INTRODUCTION
2 The use of renewable energy sources has a great number of advantages, among
3 which two should be highlighted: the absence of pollutant emissions and the possibility
4 of using them inexhaustibly. However, there are some disadvantages related to these
5 green energies, above all, to the use of the wind as energy resource. These are its
6 instability, unpredictability and fluctuations, due to changes in the weather conditions.
7 Because of these drawbacks, the use of energy storage systems is required in order to be
8 able to maintain an uninterruptable electricity supply as it should be the required for
9 houses, hospitals or official facilities [1-3]. In other applications, such as environmental
10 powering of electrochemical devices, this necessity is not as important because of the
11 large dynamic time constants of the processes which contribute to make softer the effect
12 of fluctuations on the performance of the application [4-9].
13 There is a great variety of energy storage systems, among which it could be
14 highlighted compressed air, pumped hydro, superconducting magnetic energy, flywheels
15 or electrochemical systems [10, 11]. Well-known lead-acid batteries are included in the
16 electrochemical systems group. This technology can be used for the storage of renewable
17 sources providing-energy. However, the environmental problems related to the use of this
18 technology represent an important drawback [12, 13]. Taking into account this
19 background, in the recent years some studies have been published about a new technology
20 for the efficient storage of energy: Redox Flow Batteries [12-15]. A flow battery consists
21 of two usually aqueous electrolytes that contain two redox couples and are pumped
22 through an electrochemical cell in which chemical energy is converted to electricity [16].
23 These batteries can be divided into different groups, according to the phases of the
24 electroactive species involved in the system and the couples involved in the redox system
25 reactions [14, 17, 18]: liquid-RFB, where chemical energy is stored in the electrolyte;
3
1 solid-RFB, where chemical energy is stored in an active material on the electrode plate;
2 and hybrid-RFB, with solid and gaseous species involved in the redox couples. Among
3 the liquid-RFB, it is worth to mention the iron-chromium[19, 20], all vanadium (VRFB)
4 [21, 22] and other vanadium-based systems [23-26], bromine-polysulphide [27] and all
5 uranium [28] and neptunium [29, 30] and non-aqueous electrolytes system [31-33]. The
6 solid-RFB includes soluble lead-acid [34, 35], zinc-nickel [36, 37] and zinc-manganese
7 dioxide [38]. Finally, the hybrid-RFB includes several zinc-based systems [10, 39-42],
8 vanadium-air [43], organic redox couples [44, 45] and lithium flow batteries [46, 47].
9 In this work, a redox flow battery using the different oxidation states of the
10 vanadium as energy storage system is evaluated (Vanadium Redox Flow Battery, VRFB).
11 This type of batteries was proposed in the mid-1980s by the Professor Skyllas-Kazacos’s
12 group, belonging to the University of New South Wales (UNSW) [48-52], obtaining
13 patent in 1986 (AU Patent 575247-1986). Since then, the development of this technology,
14 even for the industrial application, has been world-wide spread [53-55]. Reactions
15 involved on the performance of the all vanadium redox flow batteries include equation 1
16 on the positive electrode and equation 2 on the negative electrode, being the overall
17 reaction shown in equation 3.
Discharge 2 18 VO2 2H e VO H2O E0=1.00 V (1) Charge
Discharge 2 3 19 V V e E0=-0.26 V (2) Charge
Discharge 2 2 3 20 VO2 V 2H VO V H2O E0=1.26 V (3) Charge
21 Taking into account these considerations, the main objective of this work is to
22 evaluate the application of vanadium redox flow battery system (VRFB) as storage of the
4
1 energy provided by a wind turbine. The most relevant property of this renewable energy
2 source is its highly random value. For this reason, a new study of the VRFB behavior
3 under a discontinuous charge produced by wind turbine has been developed. The obtained
4 results have been compared with a similar test where a galvanostatic charge process has
5 been applied using the same total electric charge. In both kind of tests, a STERION LT105
6 membrane was used to separate the positive and negative electrode compartments. This
7 membrane allows the correct performance of the VRFB, but it degrades quickly due to
8 highly corrosive electrolyte. This strategy allows us to simulate an accelerate degradation
9 in the performance of the VRFB carrying out very low number of charge/discharge cycles
10 2. MATERIALS AND METHODS
11 A scheme of experimental setup is shown in Figure 1. It is composed of a wind
12 turbine that supplies the renewable energy, the potentiostatic unit, which supplies the
13 energy according to the renewable energy profiles to the VRFB and the VRFB setup itself.
14 2.1. Wind turbine
15 Electricity production profile obtained from a Bornay 600 wind turbine (Bornay
16 Aerogeneradores, Alicante, Spain) was used. The turbine is composed of two fiberglass
17 blades (1.0 m long) controlled by an electronic regulator of 24 V and 30 A. The regulator
18 acts on the turbine as an automatic overspeed-governing system to keep the rotor from
19 spinning out of control in very high winds. In-house computer software (developed with
20 Labview, National Instruments) was used to control the process and monitoring the actual
21 wind speed. Further details can be found elsewhere [4, 5].
22 2.2. Vanadium Redox Flow Battery
23 The experimental device used in this work has been supplied by Baltic Fuel Cells
24 and it is composed of two differentiated modules: a quick connect fixture (qCf-RFB) that
25 allow the compression of the internal components of the commercial RFB cell (cF40-
5
1 RFB) using a centric clamping vise. The fore of the vise is directly applied to the active
2 area, enabling fully reproducible test conditions. In addition, qCf-RFB provides a
3 temperature regulation by pumping of a thermal fluid (Huber P20.275.50) located in
4 external thermostatic bath (Huber). cF40-RFB is a commercial electrochemical cell with
5 an active area of 40 cm2 (w=52 mm, h=77mm). Only graphite and plastic materials are
6 exposed to the electrolyte.
7 A commercial vanadium electrolyte, supplied by Golden Energy Fuel Cell Co. Ltd.
8 (China), was used in this work. It had a 2 M vanadium concentration (VO2+ in the positive
3+ 9 electrolyte and V in the negative electrolyte) in 3 M H2SO4. At the beginning of every
10 single test, positive and negative electrolyte tanks were filled up with 200 mL of the
11 corresponding solution. A peristaltic pump (Heidolph Pumpdrive 5001) was used to
12 deliver electrolyte to the RFB cell at a constant flow rate (20 mL min-1). The negative
13 electrode tank reservoir (containing V2+, which is an instable specie, and V3+) was purged
14 with a nitrogen stream to avoid the oxidation of the V2+ due to the presence of oxygen
15 during the charge and discharge experiments.
16 Carbon soft felt, provided by SGL Group was used as electrodic material both in
17 the cathode and in the anode. Carbon felt pieces were submitted to a previous treatment
18 with boiling 1 M NaOH before being used as electrodes, in order to increase the
19 hydrophilicity of the material. A cationic exchange membrane (STERION L-105) was
20 used to separate the anodic and cathodic compartments inside the cell. As it is going to
21 be explained in the text, this membrane undergoes a rapid degradation during the tests,
22 reflected on an increase in the vanadium species permeability. This allows us to evaluate
23 with a short test, the effects of a rapid ageing of the system, because the influence of the
24 harsh conditions reached in the VRFB on the cell materials is known to be the main
25 weakness of the technology.
6
1 2.3. Chemical and Electrochemical Analytical Techniques
2 Total vanadium concentration was measured off-line using an inductively coupled
3 plasma spectrometer, Liberty Sequential from Varian. Vanadium ions concentration used
4 as reactive in charge VRFB process was measured employing a UV-visible spectrometer,
5 Cary 300 from Agilent, using the Beer-Lambert-law. [56, 57]
6 Electrochemical parameters were controlled and monitored using an Autolab
7 PGSTAT302N (Metrohm) potentiostat controlled by the NOVA 1.11 program. Charge
8 and discharge procedures were programmed using the chrono potentiometry method. In
9 this way, the temporal variation in the OCV and voltage values was monitored during the
10 application of the corresponding value of current density.
11 3. RESULTS AND DISCUSSION
12 As stated before, the main objective of this research is focused on the study of the
13 behaviour of a VRFB charged under different conditions: galvanostatic and under an electrical
14 current pattern provided by wind turbines. To make this possible, accelerated ageing tests have
15 been carried out using highly degradable membranes in the VRFB device.
16 Figure 2 shows the time-course of the wind speed, measured during three
17 consecutive days (25th to 27th January, winter) in Ciudad Real, a town in the centre of
18 Spain, (38.59 N 3.55 O). This variation is going to be used as patterns of wind velocity
19 in the tests carried out in this work to compare the performance of VRFB charged
20 galvanostatically and with wind turbines intensity patterns. As it can be observed, the
21 wind speed evolution was discontinuous, just as it may be expected from this energy
22 source. The average wind speed was 1.4 m s-1. For additional information, Part b of Figure
23 2 shows the distribution of measured wind speed according to the International Beaufort
24 scale. According to this classification, the conditions selected as wind pattern for this test
7
1 could be classified as light air, and they are typical conditions of wind in this region during
2 winter.
3 Taking into consideration the patterns obtained in the variation of wind speed, a
4 directly proportional current profile was calculated for being applied for the charge
5 process of the VRFB. To do this, it was considered the total energy required to achieve
6 the charge of the battery by applying a galvanostatic procedure at 250 mA (corresponding
7 to 6.25 mA cm-2). This value was 11.64 Ah in the first charge stage. From this target
8 value, it was obtained the current profile that is represented in Figure 3 and it is the
9 powering pattern that is going to be used in this work. In order to evaluate the performance
10 of the battery after successive charge and discharge cycles, three consecutive cycles (130
11 h in total) were applied both for the galvanostatic and for the wind charging procedures.
12 Discharge stages in both studies were carried out galvanostically, by applying a constant
13 value of current density in all cases of 6.25 mA cm-2.
14 The effect of the type of energy supply used for the charging step, was assessed on
15 a bench-scale VRFB, in which 200 mL of 2 M V3+/VO2+ in 3 M sulfuric acid was used at
16 constant volumetric flow rate of 20 mL/min and constant temperature 30 ºC. In Figure 4
17 the variation in the voltage of the cell during the three consecutive charge / discharge
18 cycles in galvanostatic and wind charging are compared. In the same Figure, the variation
19 in the open circuit voltage (OCV) of the system is also shown.
20 Charge stages were maintained until an abrupt increase in the voltage of the cell
21 was observed, which corresponds to an OCV of 1.5 V. In the same way, at the end of the
22 discharge stages, an abrupt decrease in the cell voltage was observed, corresponding to
23 an OCV of 1.3 V, approximately. These OCV values are usually indicated in the literature
24 to establish the final charge and discharge states of the vanadium redox flow batteries [56,
25 58].
8
1 OCV measurements were carried out during the charge and discharge stages to
2 know an estimation of the SOC with the time and to assess the cell performance when it
3 is charged by different methods. The OCV was measured between the changes of applied
4 current in the wind charge procedure and every 4 hours in the galvanostatic charge
5 procedure. As it can be observed in the Figure 4a, a progressive and smooth variation of
6 the cell voltage and the OCV was obtained when the cell was charged by galvanostatic
7 mode. However, when a non-constant current density was applied the increasing of the
8 cell voltage and the OCV become bumpy. When the wind rate is null, the cell does not
9 charge and therefore the cell voltage and the OCV values remain constant or drop. A
10 slight self-discharge can happen when the current density remains null for a long
11 time. This could explain the higher times of charge when the cell was supplied by wind
12 power against the galvanostatic mode to achieve the complete charge of the battery, as it
13 can be observed in Table 1.
14 On the other hand, a decrease in the charge and discharge times was observed with
15 the cycles. This drop is a direct consequence of the accelerated degradation undergone by
16 the used membrane, which in turn produces an accelerated degradation in the performance
17 of the VRFB. The STERION membrane used in the VRFB is known to be easily attacked
18 by the electrolytes used in both compartments of the VRFB, undergoing severe changes
19 in its structure, which can be reflected on the increase in the crossover of vanadium
20 species from the electrolyte compartment in which they are contained to the other
21 electrolyte compartment. To understand this important degradation of the performance of
22 the VRFB, permeability of the membrane was measured before use and after their use in
23 the three cycles. Thus, permeability to V3+ in mixtures 50% V3+ - 50% VO2+ changed
24 from 3.11 10-5 up to 3.28 10-5 cm2 h-1, while permeability to VO2+ increased from 1.26
25 10-4 up to 2.93 10-4 cm2 h-1. These values are high enough to explain the accelerated
9
1 ageing of the membranes, in particular in the case of VO2+ and supports the important
2 effects of the electrolyte on the degradation of the membrane of the VRFB and hence in
3 the degradation of its performance, because it favor the crossover of vanadium species to
4 the other VRFB compartment.
5 Figure 5 shows the total charge (Q) applied in three consecutive charge-discharge
6 tests. Due to the decreasing duration of the consecutive charge/discharge cycles, the total
7 electric charge required to achieve the fixed state of charge was also lower as the number
8 of cycles increases. In the same way, the amount of energy obtained in the discharge
9 stages was lower. In turn, the obtained efficiency, calculated as the ratio between the
10 energy applied for the charge and the energy obtained in the discharge was 40.7%, 48.8%
11 and 38.9% in the three galvanostatic-charging cycles, and 36.1%, 36.5% and 28.5% in
12 the three wind-charging cycles. It is suggested that, during the wind-charging processes,
13 in the temporal periods for which the value of current applied is too low, the charge
14 process is less efficient, and consequently, to achieve the same value of state of charge,
15 the supply of a higher total amount of energy is necessary. Nevertheless, although starting
16 from the same value of OCV, the efficiency obtained in the windy discharge processes is
17 lower than the obtained in the galvanostatic. Other efficiency parameters obtained from
18 the results of these tests, and calculated according to literature[56], are shown in Table 2
19 and their values support this worse performance of the VRFB that undergoes the wind-
20 charging procedure.
21 Considering this significant reduction in the efficiency during the operation of the
22 battery, it may be suggested that the values of OCV, that have been used as indicative of
23 the state of charge of the battery, do not correspond with the total conversion of VO2+ to
+ 3+ 2+ 24 VO2 and of the V into V , as it was expected when the vanadium redox flow battery
25 is in its total charge state. Taking into account this consideration, during the three
10
1 consecutive charge/discharge cycles, it was monitored the variation of the vanadium
2 species. Figure 6 shows the variation in the UV-Vis spectra for the positive and negative
3 electrolytes during the first charging cycle.
4 As it can be observed, the maximum peak of absorbance for the VO2+ decreases
+ 5 during the charge procedure until it disappears when it was totally oxidized to VO2 . In
6 the same way, the peak of absorbance obtained at 400 nm for the V3+ decreases until a
7 minimum value, obtained when it was totally reduced to V2+. Taking into account this
8 consideration, in the consecutive charge/discharge cycles, achieving the same value of
9 OCV, the same maximum absorbance peaks must be obtained in the UV-Vis
10 spectrophotometric analysis.
11 Figure 7 shows the total amount of vanadium present in the positive and negative
12 electrolytes as VO2+ and V3+, respectively. In this Figure, it can be observed that in the
13 consecutive charge/discharge cycles, the total amount of VO2+ and V3+ contained in the
14 electrolyte decrease, both under the galvanostatic mode and also under the wind charging
15 procedures. Although, these results could indicate an irreversible character in the redox
16 reactions involved in the performance of the vanadium redox flow battery, they can also
17 be associated to the crossover of the electrolyte, which in turn is caused by the degradation
18 of the membrane. In this way, despite the same value of OCV was obtained in the different
19 charge/discharge cycles, this parameter does not indicate the real state of charge of the
20 battery with the ageing. Thus, the same value of OCV is obtained with lower amounts of
+ 2+ 21 VO2 and V in the total state of charge of the battery. The same observation was pointed
22 out by other authors, which have related different degradation procedures occurring in
23 the elements of the VRFB (membrane, electrolyte and electrodes) with the decrease in
24 the SOC during successive charge / discharge cycles [59].
25
11
1 From the data show in Figure 7, the SOC values according to the equations 4
2 (negative electrolyte) and 5 (positive electrolyte) were calculated [60].
C 2 3 SOC V (4) CV2 CV3
C 4 SOC VO 2 (5) C C 2 VO 2 VO
5 As it can be seen, the SOC values depend on the concentration of the ions involved
6 in the reaction of each compartment. The obtained values are shown in Table 3.
7 These values indicate that the charge was not complete in the second and third
8 cycle, mainly in the negative compartment. This fact could explain the lower values of
9 charge observed in the Figure 4 during those cycles. On the other hand, if both operation
10 modes are compared, the SOC values of the wind-charging mode are smaller than the
11 ones for the galvanostatic-charging mode. Thus, it seems that the wind profiles, (i.e. when
12 a variable current is supplied with random periods during which the VRFB is not charged
13 and some discharge process could occurs due to the instability of the V2+ specie) lead to
14 lower values of SOC in the negative compartment.
15 Figure 8 shows the Ragone plots obtained for the battery for both operation modes.
16 Slightly lower values are obtained when the VRFB underwent wind-charging, although
17 for both charging modes the obtained values were close to the typical and expected values
18 for the flow battery technology. It is to say, power density values around 1-1.5 W kg-1
19 and energy density values around 6-25 W h kg-1 which are in agreement with other values
20 found in literature for this kind of technology [61].
21 CONCLUSIONS
22 The application of discontinuous values of electric current (characteristic of wind energy)
23 in charge cycles of VRFB results in a slightly poorer performance of the system, as
24 compared to a VRFB charged galvanostatically. Although, the differences between 12
1 operation modes are small, VRFB reached lower efficiencies and charge/discharge
2 capacities when it was used as windy energy storage system. Main problems are produced
3 in the negative compartment in which the effects of the crossover produced by the
4 degradation of the membrane on the species are raised by the fluctuations in the value of
5 current applied.
6 On the other hand, the ratio power density/energy density was very similar for the two
7 operation modes tested in this work. Thus, although further work is required to translate
8 the results achieved in the laboratory to a full-size battery, the obtained results are
9 promising enough to support the claim that the vanadium redox flow batteries are suitable
10 as storage systems of electricity produced by wind turbines.
11 ACKNOWLEDGMENTS
12 Financial support from the Spanish Ministry of Economy, Industry and Competitiveness
13 and European Union through projects CTM2013-45612-R and CTM2016-76197-R
14 (AEI/FEDER, UE) and CYTEMA E2TP Program of the University of Castilla- La
15 Mancha is gratefully acknowledged.
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1 TABLES
2
3 Table 1. Duration of the charge and discharge cycles.
Galvanostatic Wind-charging
charging mode mode
1st Charge 46.9 h 46.9 h
2nd Charge 26.5 h 29.0 h
3rd Charge 15.0 h 21.8 h
1st Discharge 19.0 h 16.8 h
2nd Discharge 13.0 h 11.0 h
3rd Discharge 6.0 h 5.6 h 4
5
6 Table 2. Efficiencies obtained during the different essays, Coulombic efficiency (C);
7 Voltage efficiency (V) and Energy efficiency (E).
Operation Mode
Cycle Galvanostatic-charging mode Wind-charging mode
C V E C V E
1st 40.68 94.48 38.44 36.10 93.21 33.65
2nd 48.82 85.81 41.89 36.45 91.61 33.39
3rd 38.88 81.01 31.50 28.53 90.83 25.92 8
9
10
11
12
13 20
1
2 Table 3. SOC values obtained from data of Figure 7 and equations 4 and 5.
Positive compartment Negative compartment
Galvanostatic Wind- Galvanostatic Wind- -charging charging -charging charging mode mode mode mode
1st Charge 0.977 0.852 1.000 1.000
1st Discharge 0.007 0.178 0.093 0.171
2nd Charge 0.991 0.979 0.749 0.713
2nd Discharge 0.044 0.371 0.094 0.179
3rd Charge 0.770 0.983 0.640 0.507
3rd Discharge 0.441 0.711 0.308 0.185 3
4
5
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9
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11
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13
14
15
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18
21
1
2 ER-17-7985
3 FIGURES
4
Potentiostat Wind VRFB turbine setup
Wind Electric current profile profile
Positive Negative RFB electrolyte electrolyte
VO2+ / V3+ / + 2+ VO2 V
Thermostatic bath 5 6 Figure 1. Scheme of experimental setup.
7
8
9
10
11
12
13
14
22
1
2
5 A)
4
1 -
/ m s m / 3
speed 2 Wind 1
0
1/26/16 0:34 1/26/16 5:11 1/27/16 1/26/16 5:17 1/26/16 0:28 1/27/16
1/25/16 10:25 1/25/16 15:08 1/25/16 19:51 1/25/16 10:00 1/26/16 19:45 1/26/16 14:55 1/27/16 1/26/16 15:02 1/26/16 10:12 1/27/16 19:38 1/27/16 Date
6 Strong breeze
B) 1
- 5 Fresh breeze
4 Moderate breeze / m s m /
3 Gentle breeze speed 2 Light breeze
Wind 1 Light air
0 Calm 0 20 40 60 80 100 Percentage / % 3 4 Figure 2. (A) Wind speed variation in Ciudad Real, Spain, during three consecutive days
5 (25th to 27th January, winter). (B) Distribution of wind speed monitored classified
6 according to the International Beaufort wind force scale.
7
23
1
2
1st 2nd 3rd charge/discharge charge/discharge charge/discharge 1
0.8
0.6
0.4
0.2 Current / A / Current 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 -0.2
-0.4 Time / h 3
4 Figure 3. Electric current profiles during the three charge/discharge cycles. Solid line:
5 Direct wind charging procedure; Dashed line: Galvanostatic charging procedure.
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9
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1
2
3
2 A) 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 3rd
Voltage cell and OCV / V / OCV and cell Voltage 1st 2nd charge/ 0.2 charge/discharge charge/discharge discharge 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time / h
2 B) 1.8 1.6 1.4 1.2 1 0.8 0.6
0.4 3rd Voltage cell and OCV / V / OCV and cell Voltage 0.2 1st 2nd charge/ charge/discharge charge/discharge discharge 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time / h 4
5 Figure 4. Charge/discharge response of VRFB. (A) Galvanostatic-charging procedure
6 and (B) Wind-charging procedure. Solid line: Cell voltage; Dashed line: OCV.
7 25
1
2
12 10 8 6 4 2 0
Q /Ah Q -2 -4 -6 -8 -10 -12 First First Second Second Third Third Charge Discharge Charge Discharge Charge Discharge 3
4 Figure 5. Electric charge required and obtained in the three charge and discharge cycles.
5 White bars: Galvanostatic-charging procedure; Black bars: Wind-charging procedure.
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9
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1
2
1 A) 0.9 0.8 0.7 0.6 0.5 0.4 Abosrbance 0.3 0.2 0.1 0 300 350 400 450 500 550 600 650 700 750 800 Wavelength / nm
1 B) 0.9 0.8 0.7 0.6 0.5 0.4 Absorbance 0.3 0.2 0.1 0 300 350 400 450 500 550 600 650 700 750 800 Wavelength / nm 3 4 Figure 6. UV-visible spectra variation during the first charge process for the positive (A)
5 and negative (B) electrolyte. Solid line: OCV= 1.20 V; dotted line: OCV= 1.30 V; dashed
6 dotted line: OCV= 1.36 V; medium dashed line: OCV= 1.43 V; short dashed line: OCV=
7 1.47 V; long dashed line: OCV= 1.5 V.
27
1
0.5 A) 1st 2nd 3rd 0.45 charge/discharge charge/discharge charge/discharge 0.4 0.35 0.3
mol 0.25 2+
0.2 VO 0.15 0.1 0.05 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time / h
0.5 B) 1st 2nd 3rd 0.45 charge/discharge charge/discharge charge/discharge 0.4 0.35 0.3
mol 0.25 3+
V 0.2 0.15 0.1 0.05 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 Time / h 2
3 Figure 7. Evolution of total amount of (A) VO2+ into positive half-cell and (B) V3+ into
4 negative half-cell during the charge/discharge cycles of VRFB. Solid line: Galvanostatic-
5 charging procedure; Dashed line: Wind-charging procedure.
6
28
1
2
80
1 -
40
/ W kg W / Density
20 Power
10 50 100 200 400 800 Energy Density / W h kg-1 3 4 Figure 8. Ragone plots. () Wind-charging charge procedure; () Galvanostatic-
5 charging procedure.
6
7
29