The performance of all vanadium redox flow batteries at below-ambient temperatures

The performance of all vanadium redox flow batteries at below-ambient temperatures

Energy 107 (2016) 784e790 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy The performance of all ...

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Energy 107 (2016) 784e790

Contents lists available at ScienceDirect

Energy journal homepage: www.elsevier.com/locate/energy

The performance of all vanadium redox flow batteries at below-ambient temperatures Jianxin Pan a, Mianyan Huang b, Xue Li a, Shubo Wang a, Weihua Li a, Tao Ma a, Xiaofeng Xie a, *, Vijay Ramani c a b c

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China Prudent Energy (Beijing) Technology Co., Beijing, 100083, China Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, 60616, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 October 2015 Received in revised form 26 March 2016 Accepted 17 April 2016

Temperature is a key parameter influencing the operation of the VFB (all vanadium redox flow battery). The electrochemical kinetics of both positive and negative vanadium redox couples were studied using CV (cyclic voltammetry). The CV results showed that the anodic peak current for the VO2 þ /VO2þ couple and the cathodic peak current for the V3þ/V2þ decreased with temperature. The peak potential difference DEp for VO2 þ /VO2þ couple varied slightly with decrease in temperature while that for V3þ/V2þ increased sharply from 276 mV at 30  C to 481 mV at 10  C, indicating that this reaction become more irreversible at the low temperature. Two VFB single cells operating at 0 and 20  C were cycled and their performance was compared. While the low temperature reduced vanadium crossover and benefitted the coulombic efficiency, a concomitant lowering in the rate of proton transport resulted in an increase in ohmic overpotential and hence a lower voltage efficiency. The efficiencies and capacity of a VFB stack were monitored in controlled environments. The static resistance of the stack varied slightly between 26.5 and 29.0 mU at 5 and 10  C, but increased to 38.1 mU on average at 2  C. © 2016 Published by Elsevier Ltd.

Keywords: VFB (all vanadium redox flow battery) Temperature effect CV (Cyclic voltammetry) VE (Voltage efficiency)

1. Introduction Large scale energy storage technologies are attracting increasing attention for the grid-connection of intermittent renewable energy sources such as solar PV and wind, as well as for applications in peak load shifting, uninterrupted power supply and so on [1,2]. With the advantages of high efficiency, fast response, deep discharge ability, flexible module design and long lifetime [3e5], the VFB (all vanadium redox flow battery) is one of most promising technologies and perhaps the technology closest to commercial application. Temperature is a key parameter that significantly influences the performance characteristics of the VFB. The processes influenced by temperature include the electrochemical redox reactions occurring at the electrodes, the vanadium ion diffusion within the electrolyte and through the membranes, and the solubility of vanadium salts in the electrolyte. Badrinarayanan and coworkers calculated the vanadium diffusion though the membrane at

* Corresponding author. E-mail addresses: [email protected], [email protected] (X. Xie). http://dx.doi.org/10.1016/j.energy.2016.04.075 0360-5442/© 2016 Published by Elsevier Ltd.

different temperatures and simulated the capacity decay due to crossover. They predicted that at the end of 200 cycles, the capacity loss at 40  C was higher (14.2%) than that of 30  C (13.3%) and 20  C (11.2%) [6,7]. Studies have shown that the V2þ solution begins to crystallize when the electrolyte is cooled down below 10  C [8], and it has been suggested that the minimum reasonable VFB operating temperature should be 10  C [9,10]. Li and coworkers reported that by using sulfate-chloride mixed electrolytes, a 2.5 mol L1 vanadium solubility was attained, and the electrolyte remained stable over a wider temperature range of 5 to 50  C compared with a traditional VFB [11]. Liu and coworkers examined the effect of temperature on the electrochemical oxidation of VO2þ using steady-state polarization, and measured the exchange current density for this reaction from 20 to 60  C. They discovered that the rate constant for the anodic oxidation of VO2þ increased from 1.2  105 cm s1to 17.7  105 cm s1 as the temperature was varied from 20 to 60  C. The diffusion coefficient of VO2þ increased from 1.60  105 cm2 s1 to 3.15  105 cm2 s1 as the temperature was varied from 15 to 70  C [12]. Wang investigated the electrochemical kinetics of VO2 þ /VO2þ in the positive electrolyte using CV (cyclic

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voltammetry) and chronopotentiometry, and pointed out that an elevated temperature facilitates the redox reaction kinetics and ion diffusion, but reported no significant effect on reversibility [13]. Some numerical models of VFB single cells that considering the effect of temperature reported that the temperature rises by 2e3  C during the charging and discharging process [14,15]. While the effect of temperature on VFB performance has been evaluated, very little in the way of systematic work ranging from fundamental electrochemistry to the stack level has been conducted to investigate the effect of low temperature conditions. Such a study is important as VFBs find many applications in regions where the atmospheric temperature is well below freezing. In this work, the electrochemical kinetics of both the positive and negative vanadium redox couples were studied in the temperature range 10 to 30  C using CV. Subsequently, two VFB single cells operating at 0  C and 20  C were cycled at different applied current densities and their characteristics were compared. Finally, a demonstration VFB system operating in a realistic environment was assessed. 2. Experimental details 2.1. Electrochemical measurements A typical three-electrode cell was employed for the CV measurements. A pure graphite rod was used as the working electrode, a platinum plate was used as the counter electrode and a SCE (saturated calomel electrode) was used as the reference electrode. Concentrated solutions of 1.5 mol L1 VOSO4 in 3.0 mol L1 H2SO4 were prepared and stored in a glass jacketed cell. The solutions were placed under an argon shield and maintained at different temperatures from 10 ± 1 to 30 ± 1  C using the glycol cold bath or a water bath. The CV experiments were performed at a scan rate of 50 mV s1 in the potential range 1.0~1.6 V. The CV experiments were obtained with a Zahner electrochemical workstation (Zahner IM6ex, German). 2.2. VFB single cell and stack tests The schematic diagram of the VFB and test system is illustrated in Fig. 1. A laboratory VFB single cell was assembled with two pieces of 90 mm  84 mm graphite felt (SGL Group, Germany) as the positive and negative electrodes, a Nafion117® membrane (DuPont, USA) as the separator, and two sheet of copper plates as the current collectors. The 1.5 mol L1 V3þ negative electrolyte (0.75 mol L1 V2(SO4)3 in 3.0 mol L1 H2SO4) was obtained by electrolyzing the VO2þ positive electrolyte. 50 mL of these two electrolytes were stored in two separate glass reservoirs with jeckets and circulated into each half cell using peristaltic pumps at the rate of 50 mL min1. The glycol/water fluid is cycled from circulating cooler (LSJ-5D, Zhixin Experimental Instrument Tech., Shanghai, China) to the jeckets of reservoirs, to maintain the electrolytes in the reserviors is at the setting study temperature points. All the pipes and utilidors were thermal insulation. The whole VFB battery and test system was operating in a thermoalstat environment. VFB cell was charged and discharged at constant current with the voltage bounds maintained between 0.75 and 1.65 V, by the battery test system that integrated by controller (PFX2512, Kikusui Electronics, Japan), power source (PWX1500L, Kikusui Electronics, Japan) and load (PLZ664WA, Kikusui Electronics, Japan), with the test accuracy were 0.1 mV, 0.1 mA and 0.1 ms. Two sets of experiments were conducted, with the electrolyte solutions maintained at 0 and 20  C. The battery was not cooled and was run at room temperature. Only the electrolytes were cooled and hence the stated temperatures refer to the electrolyte inlet temperature that

Fig. 1. A schematic diagram of VFB with test system.

monitored by the temperature probe in the controller with the accuracy was 0.1  C. The charging/discharging output performance and cycle performance of the VFB were evaluated in each case. Both cells were operated in the same mode, namely galvanostatic charging/discharging at the current density of 50 mA cm2 for 10 cycles, followed by 75 mA cm2 for 5 cycles, 25 mA cm2 for 5 cycles and 50 mA cm2 for 5 cycles. Subsequent to the test of single cell, a VFB stack, made up of five cells with an electrode area of 250 cm2, was constructed and integrated with the HMU (heat management unit) and other auxiliaries. Besides the larger capacity and power, the VFB system differed from the single cell mainly in the integration of heat management unit, which had a better ability to adapt to temperature variation. The VFB stack was charged and discharged at 60 mA cm2 and the whole VFB system was exposed in a simulated temperature-vary environments that according to the realistic meteorological records in winter including some extreme conditions.

Fig. 2. Cyclic voltammograms of 1.5 mol L1 VOSO4 þ 3.0 H2SO4 mol$L1 solutions at various temperatures.

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Table 1 The CV results of 1.5 mol L1 VOSO4 þ 3.0 H2SO4 mol$L1 solutions, calculated from voltammograms in Fig. 1. Couples

Temp./ C

ipa/mA

VO2 þ /VO2þ

10 0 10 20 30 10 0 10 20 30

7.35 10.68 11.43 14.13 13.71 1.27 1.55 2.44 5.21 6.37

V3þ/V2þ

± ± ± ± ± ± ± ± ± ±

ipc/mA 0.08 0.12 0.10 0.04 0.08 0.11 0.09 0.07 0.11 0.12

7.32 7.84 8.76 11.32 12.39 4.22 5.61 9.24 14.14 17.73

± ± ± ± ± ± ± ± ± ±

0.07 0.12 0.08 0.02 0.04 0.13 0.07 0.09 0.08 0.11

Epa/V

Epc/V

DE/mV

jipa/ipcj

1.020 1.017 1.020 1.014 1.013 0.438 0.428 0.433 0.436 0.427

0.819 0.828 0.833 0.841 0.843 0.919 0.894 0.835 0.712 0.665

201 189 187 173 170 481 466 402 276 238

1.004 1.362 1.304 1.248 1.107 0.301 0.276 0.264 0.368 0.359

Fig. 3. CE, VE and EE of VFB single cells operated at 0  C and 20  C and at various current densities.

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3. Results and discussion 3.1. Electrochemical kinetics Fig. 2 shows the CV curves at different temperatures ranging from 10 to 30  C. Peaks corresponding to two redox couples were observed. The peaks in the high potential zone (0.9~1.1 V) belong to the VO2 þ /VO2þ couple and those in the lower potential zone (0.9 ~ 0.4 V) belonged to the V3þ/V2þ couple; which correspond to the reactions at the positive and negative electrode, respectively. The peaks corresponding to the VO2þ/V3þ transition that should be located theoretically between 0.2 and 0.4 V, were not obviously detected in practice because of the concentrated solution used. The ratio between the cathodic peak current (ipa) to anodic peak current (ipc) and the separation between the peak potentials (DEp) for both VO2 þ /VO2þ and V3þ/V2þ couples were calculated, and are listed in Table 1. The data shows that all the peak currents, especially the anodic peak current of the VO2 þ /VO2þ couple and the cathodic peak current of the V3þ/V2þ couple, decreased as the temperature was lowered. This suggested that the facility of the redox reaction decreased with temperature in this range. In practice, this finding should translate to a poorer voltage efficiency for the battery. In addition, the jipa/ipcj value for the V3þ/V2þ couple is far removed from 1 as compared with the corresponding ratio for VO2 þ /VO2þ. One reason for this could be the chemical reaction of V2þ with VO2þ near the electrode surface, which lowers its concentration. While the DEp for the VO2 þ /VO2þ couple varies slightly with lowering temperature; that for the V3þ/V2þ couple increased sharply from 276 mV at 30  C to 481 mV at 10  C. The biggest contributor to this increase was the shift of the cathodic peak potential of this couple toward more negative values. The broadening of the DEp for both redox couples with the decreases of temperature is an indication of the increasing irreversibility of these reactions as the temperature is lowered, which would lead to a further lowering of battery performance. 3.2. VFB single cell performance Fig. 3 presents the charging and discharging efficiencies of VFB single cell operated with the electrolyte inlet temperature at 0  C and 20  C. Typically, the coulombic efficiency (CE, the ratio of discharge capacity to the charge capacity for each cycle), energy efficiency (EE, the ratio of discharged electrical energy to the

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electrical energy required for charging in each cycle), and voltage efficiency (VE (Voltage efficiency), can be calculated from the ratio of EE to CE in galvanostatic mode) are the key parameters to evaluate the cyclic performance of a VFB. As shown in Fig. 3, the CE of the single cell operating in 0  C was larger than that operating in 20  C at all applied current densities. CE losses are caused by the hydrogen and oxygen evolution side reactions as well as by selfdischarge due to vanadium ions crossover though the membrane. These processes are all accelerated at higher temperatures, which explain the trend seen in Fig. 4. In general, the CE varied only slightly during the cycles conducted at the same current density, and increased with the increasing current density for both operating conditions. At a higher current density, the charge and discharge time become short hence the influence of vanadium crossover is minimized. At the same current density, the VE did not change for a given inlet electrolyte temperature. The VE gradually decreased with the increase of the current density, due to the influence of cell resistance as the reported in the literature [16,17]. The temperature also had a significant influence on VE, and the VE of the cell operating at 20  C was larger than that operating at 0  C at all current densities. As depicted in Fig. 4, while the low temperature reduces vanadium crossover (a positive effect), it is a double-edged sword, leading to a concomitant lowering in the transport of protons, which results in a decrease in the proton conductivity of the membrane. The end result is an increase in the ohmic over-potential and hence a lowering in the voltage efficiency as seen in the data. The difference value in VE between 0  C and 20  C increased from 1.5% at 25 mA cm2 to 4.5% at 75 mA cm2. The higher drop in VE at higher current densities is both due to the higher ohmic polarization, and due to the lower facility of the redox reactions (especially the V3þ/ V2þ couple) at lower temperatures as shown in the CV results. The latter leads to enhanced overpotentials for a given current density. The EE describes the overall efficiency of the VFB cell. Lower temperatures benefit the CE but not the VE as discussed above. As the product of CE and VE, the EE value was higher at 20  C compared to 0  C except at very low current densities. Since the focus of operating VFB systems is to run at high current densities to minimize the system weight and volume, we believe that operating at extremely low temperatures (0  C) would be detrimental. Fig. 5 shows the representative profiles of charging/discharging curves of the VFB single cell at 0  C and 20  C. The voltage at 0  C was larger during the charging process and lower during the

Fig. 4. Schematic representation for the differences of VFB operating and higher and lower temperature.

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Fig. 5. Charge/discharge curves of a VFB single cell operating at 0  C and 20  C: (a) 8th cycle @50 mA cm2, (b) 13th cycle @ 75 mA cm2, (c) 18th cycle @ 25 mA cm2 and (d) 23rd cycle @ 50 mA cm2

discharging process than the corresponding values at 20  C. This trend was seen at all current densities, confirming the added polarization of the battery at lower temperature. Additionally, the increased charge potential plateau and lower discharge potential plateau lead to the battery voltage reach their bounds faster, resulting in the charging time and discharging time of 0  C being shorter than those at 20  C at all current densities. All of these observations reinforce the finding that the VFB operating at 0  C had a poorer capacity. In the Al-Fetlawi and coworkers’ research [18] on the mathematic modeling of a 100 mm  100 mm VFB single that charging and discharging at different environment temperatures of 30, 45, 60  C. The simulation results showed that with the increase of environment temperatures, the charging voltage decreased and the discharging voltage increased, nearly 6e8 mV per 15  C. Compared with the simulated results of VFB at high operation temperature zone and the experimental results of VFB at low operation temperature zone, the trends of temperature effect on the charge and discharge behavior are the same, the extends of voltage variation with temperature are different, mainly caused by the difference of geometry construction of VFB single cell and operation parameters such as current density and environment temperature. And the results indicate that VFB should operation at a reasonable temperature windows, excessively high and low temperature will lead to a performance deterioration. The discharge capacity decay and the OCV (open circuit voltages) before each charging and discharging process are shown in Fig. 6. Temperature had only a marginal impact on the capacity decay in this set of experiments. The OCVs before charging at 0  C were 25e30 mV larger than those at 20  C. The reason for this observation was that during the pervious discharge process, the battery operating at lower temperature terminated earlier as discussed above, and hence more capacity remained in the electrolytes instead of being discharged. The weaker discharging ability and the persistent accumulation of capacity in the electrolytes at low temperature leads to battery capacity decay. In this instance,

the difference in the decay rate was not evident given the flow rates used. But we believe the difference would be more pronounced at higher flow rates. 3.3. VFB stack performance A demonstration VFB stack integrated with a heat management unit was tested at different temperatures in a controlled environmental. The stack operating temperature was set at 20  C, while the environmental temperatures were varied between four levels, namely 20 ± 1  C, 10 ± 1  C, 5 ± 1  C and 2 ± 1  C, as shown by the rectangles in Fig. 7d (The points represent the actual measured temperature values). The stack coulombic efficiency (Fig. 7a) was maintained at nearly 98%; this uniformity was ascribed to the HMU,

Fig. 6. Comparison of capacity decay and the open circuit voltage of VFB cells operating at 0 and 20  C.

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which kept the VFB stack working at the set-point temperature as opposed to drifting with the external environment temperature. However, the CE dropped sharply when the environment temperature deceased below 0  C abruptly, but retained its normal value after a brief interval once the HMU had time to respond to the abrupt drop. The voltage efficiency (Fig. 7a) and energy efficiency (Fig. 7a) varied similarly, because the ratio of VE to EE, namely the CE, was almost unchanged. Under normal operating conditions, the VE deceases from 87% to 83.5% as the environment temperature dropped from 20  C to 5  C; the reasons have been discussed in Section 3.2. The reason for the VE varying periodically at 10 and 5  C is that the HMU works only intermittently (the details of HMU operation were deemed proprietary by our industry partner). However, when the environment temperature dropped below 0  C, the VE dropped to about 80%. That VE recovered quite slowly and recovered to normal values only several cycles after the environment temperature transitioned back to normal

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temperatures. The VE will be worse if the stack persistently operated below 0  C, implying that it is beyond the ability of the stack to tolerate prolonged low temperature operation despite the presence of the HMU. The ohmic resistance of the stack was measured after the end of some sampled charging processes (Fig. 7c). The static resistance of the stack was 24.0 mU (the hollow block seen at 0 cycles). During operation, the R value varied slightly between 26.5 and 29.0 mU at 5 and 10  C, but increased to 38.1 mU on average at 2  C. This confirmed that the rapid ohmic polarization due to decreased conductivity at this condition led to a lowering in the voltage efficiency. The discharge capacity is illustrated in Fig. 7b capacities at 5  C and 10  C were only slightly smaller than that at 20  C, which is primarily attributable to the HMU. The HMU guarantees the VFB stack will operate in a reasonable temperature window even the surrounding environment temperature changes. The capacity varied periodically, in a trend similar to the VE, because when the

Fig. 7. VFB system operation profile: Efficiencies (a), the ohmic resistance (b), the discharge capacity (c) and sampled environment temperature T (d).

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VFB stack temperature reached a bound in the design temperature window, the HMU was activated under the feedback control, preventing significant capacity decay. However, when the VFB was operated at 2  C, the capacity decreased drastically, from 85% to nearly 70% just in 2 or 3 cycles. This was because at these low temperatures, the heat exchange balance between the VFB stack and HMU broke down; exposure to the extremely low environmental temperature was beyond the ability of the HMU to regulate. This further confirmed the practical problems that arise when operating a VFB (especially a stack) at temperatures below 0  C. 4. Conclusions In the present paper, the effect of low temperature conditions on fundamental electrochemistry of VFB electrode couples and the performance of VFB single cell and the stack were investigated. The CV results showed that the electrochemical activities and reversibility of both the positive and negative vanadium redox couples decreased obviously as the temperature decreased from 30 to 10  C indicating that the facility of these redox reactions decreased with temperature. Compared with VFB single cell operated at 0 and 20  C, operating at the lower temperature reduced vanadium crossover and benefitted the coulombic efficiency, a concomitant lowering in the transport of protons resulted in an increase in the ohmic overpotential and hence a lowering in the voltage efficiency. A demonstration VFB stack integrated with a heat management unit was evaluated at various surrounding environment temperature according to the realistic meteorological records in winter. The heat management unit allowed the VFB stack to maintain its performance at despite of the environment temperature being varied from 20  C to 5  C. But the heat management unit could no longer cope with maintaining the thermal balance of the stack when the environment temperature reached below 2  C. The stack was unable to recover rapidly from low temperature transients, though a rapid recovery from transients at higher temperatures. Altogether, our results suggest that operating a VFB at very low temperatures (<0  C) will result in significant losses in efficiency. Acknowledgments The work was funded by National High Technology R&D Program of China (2012AA051201) and National Natural Science

Foundation of China (51573083). Vijay Ramani would like to acknowledge the Hyosung S. R. Cho Endowed Chair Professorship at Illinois Institute of Technology for partially funding the collaboration with Tsinghua University.

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