Mitigating capacity decay and improving charge-discharge performance of a vanadium redox flow battery with asymmetric operating conditions
Introduction
Redox flow battery (RFB) has been regarded as a promising energy storage technology for the stabilization of grid electricity supplies, emergency power backup, and intermittent renewable power systems such as solar and wind power, due to its virtues of long cycle-life, high efficiency and flexibilities of energy and power ratings [[1], [2], [3], [4]]. Although great progress has been made in advancing RFB technology, especially the all-vanadium redox flow battery (VRFB) technology, the problems associated with battery performance in term of system capacity, power and efficiency, and the difficulties in optimizing battery structures and operating strategies are still remained and need to be resolved.
Technically, the performance of VRFB depends not only on physical properties of battery components and materials for charge transport and electro-catalysis, but also on the management of complex flow and species transport processes inside the cell. To boost the VRFB performance, researchers have paid their attentions on improving electrolyte properties for high vanadium ions solubility and stabililty, and advancing the electrode properties for high electrochemical reversibility and electrochemical activities and designing new membranes for lower crossover of vanadium ions and water transfer across the membrane and lower resistance simultaneously in the VRFB [[5], [6], [7], [8], [9], [10], [11], [12]]. Besides, for given battery component materials, the increase of battery performance also relies on optimzing the battery structure and operating strategies [4,[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]], and addressing the critical transport issues in theVRFBs [25,26].
One major mass transport isssue in the VRFB is the vanadium ions crossover [[27], [28], [29], [30], [31]]. The unbalanced transport properties of different vanadium ions in the membrane result in the imbalance of electrolyte during battery cycling. Consequently, the half-cell with less amount of vanadium ions becomes the dominant factor limiting the battery discharge capacity. To resolve this, practically, the electrolytes in both tanks need to be periodically rebalanced by remixing to recover the battery capacity [[32], [33], [34], [35]].
Currently, there have been many fundamental studies on revealing the transport mechanisms of vanadium ions through the membrane. One major transport mode of vanadium ions crossover is molecular diffusion driven by concentration gradient through the membrane. Some researchers [36,37] measured the diffusion coefficients of four different vanadium ions () in the membrane. It was indicated that the diffusivities of various vanadium ions are different, which was believed to be the main reason that causes the electrolyte imbalance [38,39]. Kumbur and coworkers [[40], [41], [42]] also presented a two-dimensional model to investigate the mechanism of convective transport of vanadium ions and its impact on the total flux of vanadium ions crossover. Their study showed that the convective transport of vanadium ions is composed of the osmotic convection and the electro-osmotic convection and that the convective transport occurs in the same direction as applied current [40]. Since electro-osmotic convection yields zero net effect on vanadium ions crossover for an entire cycle, the difference in net convective transport is only governed by the osmotic convection [41,42]. Later, Yang and Ye [43] found that concentration gradient across the membrane gives rise to the migration and electro-osmotic convection of vanadium ions. In addition to above studies, some researchers also found that migration is in the same direction as the applied current and mitigates the crossover flux of negative electrode species and intensifies that of positive electrode species during charging conditions and the opposite happens during discharging conditions [44,45].
Based on the understanding of the mechanisms of vanadium ions crossover, many researchers have put forward various operating strategies for alleviating the vanadium crossover and the associated battery capacity decay [41,[46], [47], [48], [49], [50]]. For example, the studies by Kumbur and his coworkers [46] indicated that applying asymmetric current operating strategy (i.e., increasing charge current while keeping the discharge current constant) can suppress the capacity decay due to the reduction of net convective transport of vanadium ions across the membrane. The reduced net convective transport of vanadium ions was mainly attributed to the fact that increasing charging current increases the magnitude of the electro-osmotic convection during charging process, which in turn, compensates for the convective crossover due to the osmosis process. Besides, it was indicated that the asymmetric flow rates and asymmetric electrolyte viscosities can also effectively reduce the capacity decay resulting from the minimization of osmotic convection [46]. Also, Yang and his coworkers [47] proposed to apply asymmetric concentration operating conditions (i.e., decreasing the initial concentration of negative electrolyte while increasing the initial concentration of positive electrolyte) to mitigate capacity decay. They found that high concentration gradient of vanadium ions between two half-cells is beneficial to mitigating the imbalance of vanadium crossover. However, in their study, the electrolyte volumes in both half-cells are kept the same. As a result, the initial discharge capacity is actually decreased due to the decreased total amount of vanadium ions in negative half-cell with fixed electrolyte volume. Besides, Xi et al. [48] showed that the appropriate strategies of adding negative electrolyte solution can suppress the battery capacity decay. It was indicated that the multiple addition strategy is superior to the strategy of adding the same amount of negative electrolyte once at the beginning. Moreover, Dong and his coworkers [49] found that the battery capacity decay can be alleviated by adding a soluble draw solute into the catholyte, which can counterbalance the osmotic pressure between the negative and positive half-cell. Furthermore, Wang et al. [50] found that by regulating the gas pressures in the negative and positive electrolyte tanks, the capacity can be also stabilized. However, the influence of outlet pressure of positive electrode on transport mechanisms of vanadium ion across the membrane, characteristics of electrolyte imbalance and the variation of capacity during cycling is still not fully understood.
In this work, a two-dimensional, isothermal, transient model with considering vanadium ion transport in the membrane is presented for a VRFB. Emphasis is located on examining the influence of asymmetric electrolyte concentrations and asymmetric operation pressures on the capacity decay behavior, mechanism of vanadium ions crossover and the charge-discharge voltages of a VRFB during cycling. Notice that the asymmetric electrolyte concentration strategy (i.e., keeping higher vanadium concentration in the positive half-cell than that in negative half-cell) herein can maintain the same initial discharge capacity of the battery by keeping the same initial amount of vanadium ions in both half-cells. This study will provide fundamental information for optimizing the asymmetric operating strategies such that the battery capacity decay can be mitigated and the charge-discharge performance can be improved.
Section snippets
Mathematical model
Consider a 2-D simulation domain as indicated in Fig. 1a which represents the core of VRFB with flow-through electrode. Typically, it consists of a negative carbon plate, a negative porous electrode, a membrane, a positive porous electrode and a positive carbon plate. The electrolytes consisting of vanadium ion couples dissolved in sulfuric acid solution are stored in the respective electrolyte tanks and circulated through the battery. In Fig. 1a, x-axis represents the direction across the
Results and discussion
It is well known that the imbalance of electrolyte crossover causes the buildup of vanadium ions in one half-cell and a corresponding decrease in the other, leading to the problem of battery capacity decay [28,30,31,[38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51]]. As a matter of fact, the crossover of vanadium ions through the membrane by diffusion, convection and migration cannot be entirely avoided. However, the imbalance of electrolyte crossover during
Conclusion
A two-dimensional, isothermal, transient model with considering vanadium ions crossover through the membrane is presented for a vanadium redox flow battery. Emphasis is located on examining the influence of asymmetric electrolyte concentrations and asymmetric operation pressures on the characteristics of discharge capacity decay and vanadium ions crossover as well as the charge-discharge performance of a VRFB during cycling. The main findings are as follows:
- (1)
When the battery is operated at same
Acknowledgements
This work was financially sponsored by the National Natural Science Foundation of China (No.51876159) and Shaanxi Province Youth Science & Technology New Star Plan (2016KJXX-56).
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