Elsevier

Electrochimica Acta

Volume 319, 1 October 2019, Pages 210-226
Electrochimica Acta

Performance improvement of a vanadium redox flow battery with asymmetric electrode designs

https://doi.org/10.1016/j.electacta.2019.06.158Get rights and content

Highlights

  • A VRFB model with considering vanadium ions crossover and electrode compressions was presented.

  • Capacity decay behavior and vanadium ions crossover was investigated.

  • Charge-discharge voltages and efficiency of the VRFB were analyzed.

  • Asymmetric electrode design strategies were proposed and examined.

  • Overall performance was improved with asymmetric electrode designs.

Abstract

A two-dimensional transient model with considering vanadium ions crossover and incorporating the impact of electrode compression was presented for a vanadium redox flow battery (VRFB). Emphasis is located on examining the effects of the proposed asymmetric electrode structure designs on capacity degradation, vanadium ions crossover, the charge-discharge voltages and efficiency of the VRFB during long-time cycling. It was indicated that for asymmetric electrode compression with same original thickness, the capacity decay can be effectively alleviated with increasing the positive compression ratio, due to the fact that the electrolyte crossover is balanced by adjusting the convection flux during a charge-discharge cycle. The charge-discharge performance and energy efficiency of the VRFB can also be simultaneously boosted resulting from the reduced contact resistance and increased electrolyte flow velocity in the compressed positive electrode. Also, it was found that with appropriate design of asymmetric original thickness of uncompressed electrode, the capacity decay can be avoided and battery charge-discharge performance can be improved. This study will provide fundamental information for optimizing the asymmetric electrode structures such that the overall battery performance during long-time cycling can be enhanced.

Introduction

The vanadium redox flow battery (VRFB) exhibits the virtues of long cycle life, high energy efficiency and independence of power and energy ratings, making it suitable for serving as a potential energy storage technology for steady electric power supplies, emergency power backup, and stabilizing intermittent renewable power (such as solar and wind power) systems [[1], [2], [3], [4], [5], [6]]. Although the VRFBs are currently approaching commercialization, the problems associated with battery performance in terms of system capacity, power and efficiency, and the difficulties in optimizing the operating conditions and battery structures are still remained and need to be resolved.

To increase the performance of the VRFB, great efforts have been paid to develop component materials with excellent properties for electrode, electrolyte and membrane of VRFB [[7], [8], [9], [10], [11]]. Besides, with current state-of-art component materials, the improvement of battery performance also relies on optimizing the cell structure and operating strategy [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]] and addressing the critical transport issues in the VRFB [[23], [24], [25], [26], [27]].

In the VRFB, one critical transport issue is the unexpected vanadium ions permeation through the membrane, known as vanadium ions crossover, which leads to the imbalance of electrolyte in both half-cells and thus causes the capacity decay during battery cycling. Some fundamental studies [[28], [29], [30], [31]] have found that the imbalance of vanadium ions amount between two half-cells mainly results from the different diffusion coefficients of various vanadium ions across the membrane. Also, there are some studies focusing on revealing the mechanism of the convective transport of vanadium ions in the membrane [[32], [33], [34]] and the migration of vanadium ions through the membrane [[35], [36], [37]].

To alleviate the problem of vanadium crossover and its consequent impacts, some researchers have attempted to develop new membranes with higher ions selectivity [[38], [39], [40], [41], [42], [43], [44]] in order to hinder the permeation of vanadium ions through the membrane. In practice, however, the practical method is proposed to mitigate the problem of discharge capacity decay by periodically remixing the electrolytes in both reservoir tanks [[45], [46], [47], [48], [49]]. Besides, some researchers also proposed the strategies of asymmetric operating conditions for the VRFB to mitigate the imbalance of electrolyte and suppress the discharge capacity decay [33,[50], [51], [52], [53], [54], [55]]. For example, Kumbur and his coworkers [33] presented a two-dimensional transient VRFB model to investigate the influence of asymmetric operating conditions(i.e., electrolyte flow rate and electrolyte viscosity in negative half-cell are set to be double of those in positive half-cell) on the discharge capacity degradation of the battery. Their studies indicated that using asymmetric operating conditions can be capable of eliminiating the decay of battery discharge capacity as the result of modified osmotic convection of vanadium ions across the membrane. Their another study [50] also discovered that applying asymmetric current operating condition (i.e., the charging current is increased to be 1.67 times as much as discharging current)can also mitigate the degradation of discharge capacity of the battery. Another study by Yan and his coworkers [51] indicated that a decrease of 25% in the volume of positive electrolyte can realize the mitigation of imbalanced vanadium ions crossover and the consequent discharge capacity decay of the VRFB. Besides, the study by Xi et al. [52] also found that adding excess electrolyte in the negative half-cell can retard the discharge capacity decay of the battery and thus lengthen the service life in actual practice. They also found that the service life of the battery with the multiple adding stratey is much longer than that with adding the same volume of the negative electrolyte at a time at the beginning of the battery cycling. In addition, some researchers also found that regulating the gas pressures in positive and negative electrolyte tanks can stabilize the discharge capacity of the VRFB. For example, Wang et al. [54] mitigated the discharge capacity decay of a micporous separator-based all-vanadium battery by vacuum or filling N2 into the electrolyte tanks owing to adjustment of the convection flux across the separator (convection-dominated). Recently, Yang and his coworkers [55] proposed asymmetric outlet pressure strategy with a two-dimensional mathematical model incorporating vanadium crossover and mitigated the discharge capacity decay of a Nafion-based all-vanadium battery during cycling for the reason that enhanced convection flux from positive to negative half-cell alleviates the electrolyte imbalance.Their foundation indicates that although the net crossover of vanadium ions across Nafion membrane is dominated by diffuison, the convection flux also plays an important role under asymmetric outlet pressure strategy. Besides, in the study, they also studied the asymmetric initial concentration strategy and mitigated the discharge capacity decay during cycling, which is attributed to the fact that enhanced diffusion flux to negative side alleviates the electrolyte imbalance between two half-cells without decreasing the initial discharge capacity. Essentially, the reason that using approporate asymmetric operating conditions for the VRFB proposed in above studies can eliminate the discharge capacity decay is due to the fact that the transport mechanisms (i.e., diffsuion, convection, migration) of vanadium ions through the membane can be regulated in a charge-discharge cycle and the balance of vandium ions crossover in each cycle can be realized by the asymmetric operating conditions.

As a matter of fact, the change of electrode structure may also indirectly influence the transport of vanadium ions through the membrane. Currently, many researchers have paid their attentions to examine the impact of elctrode compression on flow behavior of electrolyte and the overall battery charge-discharge performance [[56], [57], [58], [59], [60]]. For example, Jeon et al. [58] experimentally investigated the influence of compressed carbon felt electrodes on the performance of a VRFB and found that although the vanadium redox flow battery with 30% symmetric electrode compression exhibits a 49% higher discharge capacity than that without electrode compression, it still suffers a discharge capacity decay during cycling. Another study by Ju and Oh [59] also found that the symmetric electrode compression has little impact on vanadium crossover and resultant discharge capacity decay by analyzing the total amount of vanadium species in the negative and positive half-cells. Recently, Menictas et al. [60] proposed variable porous electrode compression strategy which can successfully increase the limiting current density of the battery due to the reason that species transport in the outlet region of the electrodes was increased. Notice that in this study, it is assumed that the electrode structure is symmetric or centrosysmmetric with same compression ratio. Actually, the different electrode structure and properties caused by asymmetric electrode structure designs may also lead to different flow and transport conditions in both half-cells, thus influencing the transport of vanadium ions through the membrane. Currently, the related studies are quite scarce.

Motiviated by this, in this work, a two-dimensional, transient model with considering vanadium ion crossover and the electrode compression deformation impacts was proposed for a VRFB. The influences of asymmetric electrode structure and compression conditions on the characteristics of the capacity decay, the behavior of species crossover, the charge-discharge voltage and efficency of the VRFB during cyclic process were investigated in detail. The study will provide potential methods to mitigate the capacity decay and improve the VRFB performance by asymmetric electrode structure designs.

Section snippets

Mathematical model

Consider a two-dimensional (2D) simulation domain as sketched in Fig. 1, the typical structure of VRFB consists of a negative carbon plate, a negative porous electrode, a membrane, a positive porous electrode and a positive carbon plate. In the figure, x-axis is the direction across the thickness of the membrane while y-axis is the electrolyte flow direction. Fig. 2 also illustrates the two different cases of aysmmetric electrode structure designs. In Case A, the original thicknesses of carbon

Model validation

To validate the model, the simulated charge-discharge voltages of the VRFB are firstly compared against experimental data found in Ref. [32]. Fig. 3a shows the comparison of the simulated and measured charge-discharge voltages of the battery in a charge-discharge cycle. In the experiment, a volume of 60 mL electrolyte solution with initial vanadium ions concentration of 1.04 mol L−1 was supplied to each half-cell at the flow rate of 30 mL min−1. The battery was charged from initial SOC of 0.15

Conclusion

In this study, a two-dimensional, transient model with considering vanadium ion transport across the membrane and the compression deformation impacts on electrode properties and contact resistance was proposed for a VRFB. The influences of asymmetric electrode structure designs on capacity decay, species crossover, charge-discharge voltage and system efficency of the VRFB during long-time cycling were investigated in detail. The salient findings are as follows:

  • (1)

    For Case A (i.e., asymmetric

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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