Elsevier

Journal of Power Sources

Volume 307, 1 March 2016, Pages 782-787
Journal of Power Sources

Effect of flow field on the performance of an all-vanadium redox flow battery

https://doi.org/10.1016/j.jpowsour.2016.01.048Get rights and content

Highlights

  • Experimental study of effect of flow field for flow batteries.

  • Large cell sizes to resolve effect of flow field.

  • Electrochemical characterization under identical conditions.

  • Results show stable behaviour with low capacity loss.

  • Serpentine flow field exhibits superior performance.

Abstract

A comparative study of the electrochemical energy conversion performance of a single-cell all-vanadium redox flow battery (VRFB) fitted with three flow fields has been carried out experimentally. The charge-discharge, polarization curve, Coulombic, voltage and round-trip efficiencies of a 100 cm2 active area VRFB fitted with serpentine, interdigitated and conventional flow fields have been obtained under nearly identical experimental conditions. The effect of electrolyte circulation rate has also been investigated for each flow field. Stable performance has been obtained for each flow field for at least 40 charge/discharge cycles. Ex-situ measurements of pressure drop have been carried out using water over a range of Reynolds numbers. Together, the results show that the cell fitted with the serpentine flow field gives the highest energy efficiency, primarily due to high voltaic efficiency and also the lowest pressure drop. The electrolyte flow rate is seen to have considerable effect on the performance; a high round-trip energy efficiency of about 80% has been obtained at the highest flow rate with the serpentine flow field. The data offer interesting insights into the effect of electrolyte circulation on the performance of VRFB.

Introduction

There is increasing interest in redox flow batteries because of the requirement for large scale electrical energy storage in a world where increasing share of electricity is being generated from renewable energy sources such as wind and solar photovoltaic systems. Among various flow battery systems, the all-vanadium redox flow batteries (VRFB) are among the most studied owing to a number of desirable features such as quick response, tolerance to deep discharge, long cycle life, high energy efficiencies of over 80% in large installations and active thermal management [1], [2], [3], [4]. VRFBs employ VO2+/VO2+ as the positive electrolyte and V+2/V+3 redox couple in sulphuric acid as the negative and positive half-cell electrolytes, respectively [2]. VRFBs have been under development for the past three decades. Much of the recent research has focused on materials for electrodes, catalysts, new electrolytes or additives to increase energy density and range of operating temperature, cheaper ion exchange membranes and separators as alternatives to Nafion. Due to the need for larger cell and stack sizes and to improve efficiency further, a number of studies have focused on electrolyte circulation and especially on the configuration of the flow field which can be an important factor in determining the performance of a redox flow battery (RFB). A well-designed flow field will minimize the pressure drop required for circulating the electrolytes for charging and discharging the battery while maintaining proper distribution of the reactants over the cell and carrying reactants convectively into the reaction zone through cross-flow in the porous substrate [5]. Under-the-rb cross-flow occurs due to the pressure difference between adjacent parallel channels [6]. It has two consequences, both beneficial from the point of view of the flow battery. Firstly, it reduces the flow in the serpentine thereby reducing the pressure drop. Secondly, it feeds the reaction zone with reactants convectively, which enables a higher reactant flux than what is possible with diffusion only. It therefore reduces concentration overpotential and increases the electrochemical energy conversion. The importance of this is well-recognized in the fuel cell literature [7], [8], [9], [10] and new configurations of flow fields have been proposed in the literature [11], [12] to enhance it in a serpentine flow field. Its importance to flow battery applications has also been underscored in recent literature [5], [13]. Electrolyte distribution has been identified as one of the key factors that affect the performance of RFB [14], [15].

Several in-situ experimental studies have been reported on the role of flow field configuration on the electrochemical behaviour and performance of RFBs [16], [17], [18]. Chen et al. [16] made a comparative study of parallel and serpentine flow fields and concluded the latter to be preferable due to the non-uniformity that could arise in parallel flow fields. Aaron et al. [17] proposed zero gap cell architecture with serpentine flow field which showed significantly higher power density than the conventional cell structure. Tsushima et al. [18] studied the influence of cell geometry and operating parameters on performance of redox flow battery with serpentine and interdigitated flow fields and found better performance with interdigitated flow field in VRFB than with the serpentine flow field. However, most of these studies have been performed in small cells with active area of 25 cm2 or less and the effect of the flow field may not have had an impact on the overall performance. The effect of electrolyte circulation rate has also not been studied systematically. The aim of present work is to redress these gaps by conducting comparative and systematic experimental studies in which the hydrodynamic and electrochemical performance is measured in an all-vanadium redox flow battery of about 100 cm2 cell size fitted with conventional, interdigitated and serpentine flow fields.

Section snippets

Details of the experimental studies

The single-cell VRFB consisted of a proton exchange membrane (Nafion 117, 0.18 mm thickness - Alfa Aesar), electrodes, electrolyte distributors, Cu current collectors of 2 mm thickness and Perspex end plates of 15 mm thickness. Graphite plates (SGL Grade R7510, 10 mm thickness), engraved with either serpentine or interdigitated flow fields over a 10 cm × 10 cm active area, served as electrolyte distributors. The electrodes on the anode and the cathode sides were made of one-layer of carbon felt

Typical results

Charge-discharge curves are widely used to evaluate the electrochemical behaviour of VRFBs [1]. Typical results obtained from the present experiments are given in Fig. 2; these have been obtained for the interdigitated flow field. Fig. 2a shows the charge-discharge curves for over 50 cycles. The current density during charging and discharging was maintained at 50 mA cm−2 and the electrolyte flow rate maintained at 58 ml min−1. Excellent stability can be seen in the charge-discharge curves over

Discussion and conclusion

Direct comparison of the electrochemical performance of the conventional flow field with other flow fields has not yet been reported in the open literature. The consistent and stable performance data obtained in the present study provide two useful insights:

  • A comparison of the polarization characteristics of serpentine and interdigitated flow fields has been made by the present authors [6] in a smaller cell of 80 mm × 51 mm size. The present results in a larger cell are in contradiction with

References (24)

Cited by (138)

View all citing articles on Scopus
View full text