Anomalous Behavior of Anion Exchange Membrane during Operation

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Anomalous Behavior of Anion Exchange Membrane during Operation of a Vanadium Redox Flow Battery Arjun Bhattarai, Adam H. Whitehead, Rüdiger Schweiss, Günther G. Scherer, Maria SkyllasKazacos, Nyunt Wai, Tam Nguyen, Purna C. Ghimire, Moe Ohnmar Oo, and Huey Hoon Hng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01816 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Applied Energy Materials

Anomalous Behavior of Anion Exchange Membrane during Operation of a Vanadium Redox Flow Battery Arjun Bhattarai1, 2, *, Adam H. Whitehead3, Rüdiger Schweiss4, Günther G. Scherer5, Maria Skyllas-Kazacos6, Nyunt Wai1, Tam D. Nguyen7, Purna C. Ghimire1,8, Moe Ohnmar Oo1, Huey Hoon Hng7,* 1Energy

Research Institute @ Nanyang Technological University, Singapore

2Vflowtech

3redT

Pte Ltd., 32 Carpenter Street, Singapore 059911, Singapore

energy plc., Wokingham, UK (previously Gildemeister energy storage GmbH, Wiener

Neudorf, Austria) 4SGL

55607

Carbon GmbH, Meitingen, Germany Hägglingen, Switzerland

6School

of Chemical Engineering, The University of New South Wales, UNSW, Sydney, NSW

2052, Australia 7School

of Material Science and Engineering, Nanyang Technological University, Singapore

8 Interdisciplinary

*Corresponding

Graduate School, Nanyang Technological University, Singapore author.

Tel:

+65

67904140,

E-mail:

[email protected]

&

[email protected]

Abstract Vanadium redox flow batteries (VRFBs) are becoming an integral component of renewable energy solutions, micro-grids and backup storage systems. The reduction in cell resistance due to advancements in the electrode and overall component design allows flow cells to operate at 1|Page ACS Paragon Plus Environment

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higher current/power densities. During discharging of an experimental flow cell at higher current densities, under certain conditions, an unexpected voltage drop and subsequent recovery was observed. Similar effects of voltage drop and recovery have been observed in NiCd and primary lithium-thionyl chloride (LTC) batteries but have not yet been reported for the VRFB in peer-reviewed publications. This study aims to understand the causes behind the voltage drop effect in a VRFB under high power density operation. Keywords: vanadium redox flow battery, anion exchange membrane, voltage drop, power drop effect, segmented cell

1. Introduction The vanadium redox flow battery (VRFB) is the most widely commercialized redox flow battery (RFB) technology in comparison to other chemistries such as iron/chromium, zinc/bromide, bromide/polysulfide, vanadium/bromide and zinc/cerium batteries, etc.

1-3.

A

major advantage of the VRFB compared to other RFBs is the inherent robustness due to use of the same metal ions with different oxidation states in the anolyte and catholyte. This eliminates harmful effects of cross contamination and makes the VRFB capable of operating for more than 10,000 cycles at 100% depth of discharge 4-5. Ion exchange membranes are used in a VRFB to prevent mixing of the electrolytes, while maintaining ionic conduction, which is essential to operation. Cation exchange type (e.g. Nafion™.) membranes (CEMs) are popular for these applications, due to their excellent ionic conductivity and high selectivity. The drawback of the commonly used Nafion CEM however, is its high cost (typically >500 USD/m2) and relatively rapid vanadium crossover. As an alternative, anion exchange membranes (AEMs) have been used by several VRFB manufacturers 6. AEMs are potentially more attractive than CEMs, primarily due to lower 2|Page ACS Paragon Plus Environment

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costs, but also due to potentially lower crossover rates of vanadium ions, which reduces the frequency of electrolyte rebalancing or remixing operations that are required to restore the full capacity of the electrolyte 7. However, the net electrolyte volume imbalance and hence the resulting capacity loss rate is higher for AEMs as compared to CEMs mainly due to the higher net transport of water, sulfate, bisulfate and vanadium ions 8-9. In AEM, capacity can be simply retained by mechanical balancing of the electrolyte volume in two tanks but in CEM, they need complete remixing and occasional electrochemical rebalancing. With the use of high performance electrodes, low resistance membranes and improved cell architectures, the area specific resistance (ASR) of commercial VRFB stacks is decreasing 1015.

Increasing the operating current densities is a major focus for developers of flow batteries,

as it results in increased power density, reduced footprint and weight of stack, as well as reduced capital costs per kW. Charging at high states of charge (SOC) is still restricted in terms of the maximum current density, due to the potential occurrence of detrimental side reactions at high voltages (typically above 1.6 V per cell), but discharging can be performed at no risk at much higher current densities. A promising achievement was reported at the International Flow Battery Forum (IFBF) in 2017 involving the operation of a VRFB multi-cell stack up to a current density of 1200 mA cm-2

11.

The research group used a thin electrode based on a

multiwall carbon nanotube sheet and reported a cell resistivity of 0.58 Ω cm2. Using zero gap architecture and low resistance membranes, minimum cell ASRs of ~0.3 Ω cm2 has been achieved in VRFB16-17. In addition, several other studies

12-13, 18-23

employing flow field

configuration and modified electrodes, also allow the VRFB to operate at high current densities (>200 mA cm-2). By comparison, in conventional cell/stack architecture using thick porous electrodes in planar contact with bipolar plates without flow fields, an ASR > 1 Ω cm2 is expected. A further advantage of the VRFB is its safety compared with other battery systems. As a large 3|Page ACS Paragon Plus Environment


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volume of electrolyte is continuously circulated through the battery stack, heat is continuously removed from the cells, so operation at a high current density or even in the event of sudden short-circuiting between the stack terminals, do not lead to a dangerous rise in temperature or other related safety issues 24. The porous carbon electrodes, graphite bipolar plates and polymer membranes are expected to be compatible with operation at elevated current densities, although this has yet to be fully investigated however. This study reports for the first time an interesting feature observed during high rate discharging of a vanadium redox flow cell at high SOC. A sudden voltage drop during constant current discharging was observed followed by a recovery to “normal” voltages profiles after a few minutes. This phenomenon is denoted as the “power drop effect (PDE)”. Similar effects of voltage depression followed by a recovery have been reported in operation of different electrochemical devices like NiCd and primary lithium-thionyl chloride (LTC) batteries 25, and even fuel cells 26. The formation of a thick, resistive crystalline layer on the cadmium electrode reduced the active surface area resulting in voltage depression 27. In nonrechargeable LTC batteries, a very similar voltage decline and recovery was observed when the load was applied to a passivated battery. In this case, it was due to accumulation of solid compounds on the lithium metal electrode 28. The voltage recovery time was dependent on the degree of passivation. In fuel cell operation, a sudden voltage drop might occur during anode flooding

26.

However, due to the inert nature of the graphite felt electrode, a completely

different reason for the occurrence of PDE was anticipated for the VRFB. The investigation was conducted in VRFB flow cell and stack, assembled with commercial anion exchange membranes and porous carbon felt electrodes.

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2. Experimental The PDE was investigated on two sizes of single cell (20 cm2 and 100 cm2) and at stack level (625 cm2, 3-cell). The different components used in the cell and stack assembly are given in Table 1. Table 1 Material used in the cell/stack assembly Flow frame

Made of PVC with integrated flow guides

Gasket

Silicone gasket of thickness 0.5 mm

Electrodes

PAN-based carbon felt (SIGRACELL® GFD 4.6 EA, SGL Carbon, Germany), thermally activated at 600 °C for 5 hours, uncompressed thickness = 4.6 mm

Bipolar plate (BP)

0.6 mm flexible graphite plate (SIGRACELL PV 15® from SGL Technic LLC, Valencia, CA, USA)

Membrane

Anion

exchange

membrane

(non-reinforced

partially

fluorinated, counterion : none) Dry thickness: 45 µm

Vanadium

1.6 M V+3.5 in 4.5 M total sulfate, (purchased from GFE,

Electrolyte

Nuremberg, Germany)

An in-house developed single cell setup was used for the investigation of the PDE. The electrolyte volume of 100 mL per tank was continuously circulated through the cell at a



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constant flow rate of 100 mL min-1 using a peristaltic pump. A battery tester from NEWARE, China was used for cycling. For testing in a 3- cell stack, a larger setup was used with electrolyte volume of 1.5 L per tank. Two magnetically-coupled, centrifugal pumps were used to maintain electrolyte circulation at a constant flow rate of 300 mL min-1 per cell. The theoretical flow rate for discharge at I = 25 A to a final SOC 2% is 15 mL min-1 per cell. This was equivalent to a stoichiometric flow factor of 20. Cell cycling was performed at different current densities, and the voltage profiles over time were monitored. For investigating the PDE, various other operating conditions were studied, which will be explained in the respective sections. In addition to the cell cycling, half-cell potential measurements were performed in a 20 cm2 cell to determine the specific half-cell responsible for the PDE. For these measurements, a setup as shown in Figure 1 was used.

Figure 1. Set-up developed for half-cell voltage measurement, with reference cell compartments on the outlet fluid lines. Reference electrodes (saturated mercurous sulfate electrode, |Hg/Hg2SO4| sat. K2SO4) were inserted near the outlet of each half cell, using purpose-built holders. The tip of the inserted 6|Page ACS Paragon Plus Environment

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reference electrode was only in contact with outflowing electrolyte. The cell was charged at 80 mA cm-2 and discharged at 120 mA cm-2. Simultaneously, using two channels from a multichannel potentiostat (VMP3-BioLogic, France), the half-cell potentials (measured against the reference electrodes) were recorded. From the measurements, cell voltages of both half-cells and the full cell during charging/discharging could be determined. This procedure allowed the separation of the potential difference terms in the cell. A segmented cell was designed to scrutinize whether the PDE is a localized effect or if it occurs uniformly over the entire active area. For this, the bipolar plate and copper collector of the positive side of a 100 cm2 active area cell were split into sixteen segments, whereas the thick porous electrode was used undivided. The segmented cell setup is shown in Figure 2.

Figure 2. Schematic of the experimental set up for segmented cell study (a), sixteen cables on the positive side along with electrolyte inlet and outlet tubes (b) and position of sixteen segments and showing direction of electrolyte flow (c). 7|Page ACS Paragon Plus Environment


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Further details on the design of the segmented cell and test setup was reported previously29. All the segments were connected to different terminals in a battery tester. Similar to normal cells, all sixteen segments were charged/discharged together in galvanostatic mode and the local voltages were recorded. The resistance of the AEM in the presence of H2SO4, V2+, V3+, VO2+ and VO2+ was determined concomitantly using electrochemical impedance spectroscopy (EIS). The concentration of blank sulfuric acid solution was 4 M. For preparing V2+, V3+, VO2+ and VO2+ solutions, a 1.6 M V+3.5 in 4.5 M total sulfate solution was electrolyzed in a flow cell and samples were taken at fully charged (V2+ from the negative tank, and VO2+ from the positive tank, at OCV = 1.59 V) and fully discharged conditions (V3+ from the negative tank and VO2+ from the positive tank, at OCV = 1.2 V). For EIS measurement, a set-up with glassy carbon rods (⌀ = 0.5 cm) encased in a PTFE tube were pressed vertically onto the base plate, as shown in Figure 3.

Figure 3. Schematic of EIS set-up for measuring the impedance of membrane. A special holder and tripod stand were used to align the glassy carbon rod. For measurement of EIS, the glassy carbon electrode was used as working electrode and the base plate (graphite plate, SIGRACELL TF6) served as the counter electrode. A blank measurement of the initial


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resistance without membrane was performed. This was then followed by pressing the membrane between the glassy carbon electrode and the base plate. The high frequency intercept of the EIS with the real axis was used to obtain the Ohmic resistance value. The membrane ASR was calculated as follows: Membrane ASR = (Ohmic resistance with membrane – Ohmic resistance without membrane) x geometric area of glassy carbon electrode

Eq. (1)

The membranes were immersed into the respective electrolyte ion solution for 24 h prior to conducting the EIS measurement. During the measurement, a few drops of corresponding vanadium solution were also spread around the glassy carbon electrode to prevent dehydration.

3. Results and discussion Charge discharge curves during cycling under normal conditions and with appearance of the PDE are shown in Figures 4-a and 4-b, respectively. As indicated in Figure 4-b, an unusual drop in discharge voltage was observed. This drop of discharge voltage at a set current corresponds to a drop in power output. As the charging and discharging of commercial flow batteries are usually regulated in terms of power, this effect was named as “power drop effect (PDE)”.

a

b



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Figure 4. A normal charge-discharge voltage curve (a) and a charge-discharge voltage curve with PDE (b). In energy storage applications requiring a constant supply of power from a battery, if a PDE occurs, this creates a voltage sag, which can be detrimental. However, if the effect is brief, it has minimal consequences in terms of round-trip efficiency.

3.1 Appearance of PDE under various operating conditions The PDE was usually observed during a high current discharge from a high SOC and in some cases following electrolyte imbalance and exposure of the cell at high VO2+ concentrations for long time. A single cell of 100 cm2 size was charged/discharged continuously for 50 cycles. The cell was then cycled further at increasing discharge currents, while keeping the charging current density constant at 40 mA cm-2. The discharge current density was increased from 80 mA cm-2 up to 130 mA cm-2, as shown in Figure 5. With the increase in current density during discharging, the PDE appeared first when the current density reached 100 mA cm-2 and became more pronounced with further increase of discharge current. The drop appeared sooner after a discharge from a high SOC. After a sharp drop in voltage, the time taken for the discharge voltage to recover to normal was about 4 minutes and 5 minutes at a discharge current density of 120 mA cm-2 and 130 mA cm-2, respectively.

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Figure 5. Emergence of the power drop effect with increase in discharge current density. The flow cell was exposed to a high SOC of 85% for six days by continuously pumping the charged electrolyte through the cell. Thereafter, the cell was discharged at 80 mA cm-2. A similar feature, but more pronounced, was observed as shown in Figure 6. It took more than 10 min to recover to the normal voltage profile. However, on the second and subsequent cycles at the same current density the PDE was no longer apparent.

Figure 6. PDE observed after long exposure to a high SOC.



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During the long-term cycling test of a 100 cm2 single cell at a current density of 40 mA cm-2, PDE appeared after 70 cycles and become pronounced in subsequent cycles, as shown in Figure 7.

Figure 7. PDE observed between 70th and 80th cycle in a 100 cm2 single cell measured at 40 mA cm-2. As the cell was charged/discharged continuously without electrolyte rebalancing, a significant imbalance of the electrolyte occurred, with a higher volume on the negative tank and lower volume on the positive tank observed. The AEM membrane has low permeability to vanadium ions (Coulombic efficiencies typically > 98%). As a consequence of the imbalance of the electrolyte, a higher SOC is reached in the positive tank resulting in higher concentration of V (V) in the positive half-cell compared to a fully balanced cell charged to the same voltage limit. Under this condition, the PDE could be observed even at the lower current density of 40 mA cm-2. The PDE was also briefly observed in a 625 cm2 size 3-cell stack for about 80 s (Figure 8), indicating that PDE can occur irrespective of the number and size of cell employed. Due to the 12 | P a g e ACS Paragon Plus Environment

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limitation of the battery tester (max. 60 A), the current density could not be further increased to observe the feature more prominently.

Figure 8. Appearance of PDE in a 3-cell stack. To monitor the effect of temperature on the PDE, the electrolyte temperature was increased from 25 °C to 40 °C at intervals of 5 °C, and the cell cycling was performed. The PDE was less pronounced with increasing electrolyte temperature, as shown in Figure 9.



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Figure 9. Effect of temperature on the PDE. The charging and discharging current density were 70 and 130 mA cm-2, respectively. The test was performed on a 20 cm2 size flow cell.

In addition, the PDE was more pronounced, and appeared for longer time when the upper charging cut-off voltage limit was increased (results not shown).

3.2 Localization of the cause of the PDE An attempt was made to identify the location of the PDE in the cell by isolating the different components of the cell voltage. This was accomplished by half-cell potential measurements as shown in Figure 10. A 20 cm2 size flow cell was cycled (charged at 70 mA cm-2 and discharged at 100 mA cm-2) using the battery tester. At the same time, the half-cell potential (vs SCE) across each half cell terminal and reference electrode was recorded. The black line shows the voltage measured across the battery terminals, which recorded a significant PDE. However, the voltage across the negative half-cell (negative terminal vs reference electrode) and across the positive half-cell (positive terminal vs reference electrode) did not show evidence of the PDE. As the voltage drop across the membrane was not included in the half-cell potential measurements, this suggests that the membrane is responsible for the PDE.


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Figure 10. Half-cell and full cell voltage measurements using the set-up shown in Fig. 1, during multiple galvanostatic cycles, in which PDE events are evident. A segmented cell study was used to unravel whether the power drop effect was a localized phenomenon or observed throughout the membrane surface (active area). A segmented cell assembled using the AEM, was charged to an SOC of 90% at a current density of 40 mA cm-2 and discharged to an SOC of 5% at a current density of 80 mA cm-2. The flow rate of the electrolyte was maintained at 100 mL min-1. Figure 11-a shows the initial OCV, discharge voltage and final OCV of all sixteen segments. The applied current in all sixteen segments are shown in Figure 11-b.



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Figure 11. PDE observed in a segmented cell study. Despite the fact that the stoichiometry of the electrolyte was very high (~14), the PDE was observed equally throughout all sixteen segments. This demonstrates that the PDE is neither related to mass transport polarization nor confined to certain areas of the electrode, but appears across the whole surface area of the membrane. To further investigate this phenomenon, the resistance of the membrane was measured ex-situ. In addition to the AEM investigated for PDE, the membrane resistance of another AEM and two other CEMs was measured. These membranes are proprietary, although considered suitable and representative of VRFB membranes. Figure 12 shows the change in ASR of AEMs and CEMs in the presence of electrolyte solution containing vanadium ions at different oxidation states. The corresponding data are given in Table 2.


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Figure 12. Membrane ASR in different electrolyte solutions. Table 2. Membrane resistance measured by EIS. AEM-1* Area specific

AEM-2

CEM-1**

(non-reinforced partially (non-reinforced fluorinated, (non-reinforced per-

CEM-2 (Reinforced, per-

fluorinated, counter ion :

counter ion: in

None)

chloride/methylsulfate form)

ion in H+ from)

ion in H+ form)

(45 µm)

(30 µm)

(183 µm)

(30 µm)

H2SO4 (4 Molar)

0.13

0.07

0.15

0.10

V2+

0.22

0.12

0.27

0.18

V3+

0.26

0.19

0.25

0.16

VO2+

0.28

0.22

0.28

0.20

VO2+

2.61

0.97

0.17

0.22

resistance @25 °C (Ohm cm2) in

fluorinated, , counter fluorinated, counter

* PDE described in this manuscript was only investigated for AEM-1. **This membrane was tested over various current densities, but no PDE was observed. 17 | P a g e ACS Paragon Plus Environment


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The resistance of all four types of membrane in the presence of H2SO4, V2+, V3+ and VO2+ ions was less than 0.3 Ohm cm2. However, the resistance of both AEMs increased by more than an order of magnitude in the presence of VO2+ ions compared to the values observed in H2SO4. By comparison, the resistance of the CEMs showed a minimal change in the different solutions, indicating that AEMs are undergoing fouling in the VO2+ solution. Hence, it may be anticipated that VRFB cells employing AEMs would exhibit increased resistivity when the positive electrolyte is at a high SOC. However, in this case, any increase in resistivity due to exposure of the membranes to V(V) would be limited to one face of the membrane (the opposing one being exposed to the negative electrolyte). The relationship between PDE and membrane ASR is critical. For AEM 1, the membrane ASR increased from 0.28 Ohm cm2 (in the presence of V(IV) solution) to 2.61 Ohm cm2 (in fully charged V(V) solution). In a 100 cm2 cell operated at 100 mA cm-2, this could lead to an additional voltage drop of 233 mV (i.e. (2.61-0.28) Ohm cm2 x 100 mA cm-2). This is significant voltage drop for a VRB cell, which is usually charged and discharged between OCV limits of 1.5 V and 1.25 V, respectively. In flow battery operation at system level, a sudden high voltage drop due to the PDE can cause several problems, such as, prematurely reaching the discharge voltage limit, failure of the power conversion system to supply the power demand, misinterpretation by inverter and battery management system (BMS) of the battery state-of-health, etc. In many cases this could lead to early termination of discharge and possibly trigger a fault alarm. In a further test, the current was interrupted several times during the complete discharging of a cell through the PDE, as shown in Figure 13. The objective of this test was to evaluate how long any extra membrane resistivity persisted after interruption of the discharge current at different SOCs. 18 | P a g e ACS Paragon Plus Environment

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Figure 13. Termination of discharging at different SOCs to investigate the recovery of PDE. The voltage drop decreased when the current was interrupted for few seconds but reappeared when current was applied again. This means that the PDE seems to be correlated to charge passed through the membrane. The dependence of resistivity on the direction and magnitude of current indicates that migration, with species accumulation/depletion, must be important in the PDE mechanism. On discharge, cations migrate from negative to positive sides of the membrane and anions from positive to negative. This strongly supports the idea that it is an anionic species of V(V) that is responsible for the PDE, and would also explain why the AEM tends to adsorb V(V) species much more strongly than other vanadium oxidation states. Hence, combining all the findings mentioned above, we hypothesize that the PDE is caused by the movement of negatively charged V(V) complexes, e.g. vanadium (V)-sulfate complexes (VO2SO-4 and VO2(SO4)23-), vanadium(V)-bisulfate (VO2(HSO4)2-) towards and possibly into the membrane pores from the positive half-cell. These sulfate and bisulfate complexes of V(V) were captured in highly acidic conditions using Raman spectroscopy and their concentration depends upon the V(V) and total sulfate concentrations as well as on S to V and H+ to V ratios 19 | P a g e ACS Paragon Plus Environment


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in the positive half-cell30. Within the pores, V(V) could precipitate temporarily at high SOC and block the pores, however, the larger size of these complexes within the pores of the membrane could restrict the ion transport through the membrane. These negatively charged complexes would move through the AEM during the discharge process due to the direction of the electron flow in the external circuit that drives anions from the positive to the negative side. Therefore, the observed resistivity rise is only observed during discharge and not during charging. Furthermore, PDE is not observed in CEMs, which have a low uptake of anions. The blocking of the pores acts to increase the membrane resistivity and limit the maximum current density that can be supported (reduced number of active channels). The PDE was most apparent when; (i) the positive electrolyte is highly charged (Figure 7, Figure 9), (ii) the current density is high so that the membrane resistivity becomes the dominant resistive term (Figure 4, Figure 5) and (iii) the temperature is lower so that membrane conductivity is lower as compared to membrane conductivity at high temperature (Figure 9). When there was no rebalancing of the electrolyte during long-term cycling, the PDE appeared even at low current density (Figure 7) as the positive electrolyte SOC continued to increase leading to higher VO2+ concentrations with continued cycling. As per our investigation and hypothesis proposed, the PDE is likely to appear in all types of AEMs under certain conditions. The depth of the PDE may depend upon various conditions such as, electrolyte temperature, current density, SOC, charge history, active/passive/no rebalancing, membrane thickness, etc. This study only covered a few parameters and opens the door for further investigation with a wider range of membranes.

4

Conclusion


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Due to the advancement in the electrode materials and improvements in cell architecture, VRFBs can now be operated at much higher current densities (>100 mA cm-2) than was possible originally. Considering this trend, the behavior of a cell with commercial AEM was investigated at a high current density. In this study, a sudden drop in discharge voltage was recorded for several minutes, which recovered as discharging continued. This feature, reported for the first time, was named as “power drop effect” and was more pronounced at higher current density, lower temperature and higher SOC of the positive electrolyte. Although quite reversible, it was also sensitive to the recent history of the cell. This abnormal voltage drop was attributed to an increase in membrane resistance, caused by the accumulation of V(V) complexes or temporary precipitation of V2O5 within the pores of the membrane facing the positive half-cell. This effect is only expected to occur with anion exchange membranes that also displayed an increased membrane resistance in V(V) solutions, not observed in the case of CEMs. Any sudden drop in voltage or power during the operation of a flow battery should be avoided in real operation as delivery of constant power is required. Hence, this effect should be further investigated in other types of membranes used in the VRFB in order to determine their suitability for high power density applications.

Acknowledgements The authors thank SGL Carbon, Germany for financial and technical support in this project. We would also like to acknowledge Dr. Bernd Bauer and Dr. Tomas Klicpera from FUMATECH BTW GmbH, Bietigheim-Bissingen, Germany for their valuable support in exploring this effect.



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