A comparative study of Nafion series membranes for vanadium redox flow batteries

Highlights

• Nafion 112, 1135, 115 and 117 membranes are evaluated for VRFB application.

• VRFBs with various Nafion membranes are tested at current density of 40–320 mA cm⁻².

• Nafion 115 presents the highest EE and electrolyte utilization at 120–240 mA cm⁻².

• An online monitoring system is developed to record the electrolyte imbalance process.

Abstract

In this study, a series of the commercial Nafion membranes (equivalent weight of 1100 g mol⁻¹) with thickness of 50 μm (Nafion 112), 88 μm (Nafion 1135), 125 μm (Nafion 115), and 175 μm (Nafion 117) are selected to investigate the thickness impact of Nafion on the performance of vanadium redox flow battery (VRFB). VRFBs are evaluated at a broad current density range from 40 to 320 mA cm⁻². The self-discharge process, cell efficiencies, electrolyte utilization, and long-term cycling stability of VRFBs based on various Nafion membranes are compared comprehensively. Owing to the good balance between vanadium ions permeability and membrane resistance, Nafion 115 membrane presents the highest energy efficiency and electrolytes utilization at current density from 120 to 240 mA cm⁻². A simple and powerful online monitoring system is developed to monitor the electrolyte imbalance process during the cycle-life test of VRFB. The capacity fading mechanism of VRFBs with different Nafion membranes is also discussed.

Introduction

With the increasing requirement of medium- and large-scale energy storage for the efficient utilization of renewable energy such as wind and solar energy [1], [2], [3], all-vanadium redox flow battery (VRFB) has received tremendous attention because of its fast response to energy demand change, low environmental impact, long life cycle and flexible operation [4], [5], [6]. Compared to other flow batteries, VRFB utilizes VO²⁺/VO₂⁺and V³⁺/V²⁺redox couples as positive and negative electrolytes respectively, which avoids the elements' cross-contamination of electrolytes [7], [8], [9].

The membrane (separator), which is used to separate the positive and negative half-cell and to provide ion conduction, is a key component of VRFB since it decides the efficiency, lifespan and cost of VRFB system [10], [11], [12], [13], [14], [15], [16]. An ideal membrane for VRFB should possess high proton conductivity, low vanadium ion permeability, excellent stability, and outstanding mechanical strength. Currently, perfluorinated sulfonic acid membranes such as Nafion have been commonly used in VRFB for its high proton conductivity and superior chemical stability [17], [18], [19], [20]. However, high cost and fast crossover of vanadium ions limit its practical application in VRFB [10], [11]. Recently, the research attention has ever been shifted to seeking the alternative non-perfluorinated membranes, such as sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(ether sulfone) (SPES), sulfonated poly(imide) (SPI) and polybenzimidazole (PBI) [10], [11], [12], [13], [14]. Interestingly, inspiration usually comes from previous studies in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC) [21]. Other than cation exchange membranes (CEM), anion exchange membranes (AEM) [15], [16] and porous nanofiltration membranes [22] have also been investigated towards VRFB application. Although lower cost and probably better cell performance than Nafion, the non-perfluorinated membranes are easy to degrade under the harsh acid and oxidation environment of vanadium electrolytes, resulting in poor cycling life [23], [24], [25], [26]. Therefore, Nafion membranes are by now still the best choice for most large scale VRFB energy storage systems due to its extremely chemical stability [17], [18], [19], [27]. Currently, much effort has been contributed to decrease the cost (by using recast ultrathin membrane) and improve the ionic selectivity (by the strategy of surface modification or organic–inorganic hybrid) of Nafion based membrane for VRFB application [28], [29], [30].

After thorough literature browsing in the field of VRFB, we found that Nafion membranes with thickness ranging from 25 μm (recast Nafion) [28] to 220 μm (wet Nafion 117) [31] have been reported. The performance of VRFBs is highly related to the properties of membrane, since it governs the coulombic efficiency (CE) of cell and is also responsible for a large percentage of the ohmic loss which directly affects the overall voltage efficiency (VE) and energy efficiency (EE) of the systems [10], [11], [12]. For instance, thick membrane has high cell resistance, which straightforwardly reduce the VE of cell and then further leads to the drop of EE, while thin membrane yields fast crossover of vanadium ions, which diminish the CE of cell and contribute to lower EE, and simultaneously increases the imbalance of electrolytes [32], [33], [34]. In addition, the thickness of membrane will also affect the life and cost of the total VRFB system [35]. In order to find a balance between price and performance (efficiency and cycling stability), an optimized thickness of membrane should be chosen.

Recently, a few preliminary studies in terms of membrane thickness-related performance of VRFB have been reported [18], [32], [33], [34]. Researchers from Pacific Northwest National Laboratory have compared the use of Nafion 115 (125 μm), Nafion 212 (50 μm) and Nafion 211 (25 μm) membranes in a 3-cell 1 kW class VRFB stack based on sulfate-chloride mixed acid electrolytes at current density of 80, 160 and 240 mA cm⁻². They found that Nafion 212 could be used as a direct replacement for Nafion 115 with improved performance and lower costs [18]. On the contrary, Jeong et al. revealed that vanadium ions crossover would play more dominant role than electrochemical reaction resistance in deciding performance of VRFB; as a result, VRFB using thick Nafion 117 (175 μm) membrane was more stable than that using thin Nafion 212 (50 μm) under the current density of 6.0–8.4 mA cm⁻² [32]. Apart from Nafion membranes, the thickness impact of other ion exchange membrane (IEM) has also been studied. Jung et al. investigated the different thickness (30, 50, 100 and 150 μm) of SPEEK membranes on performance of VRFB and found that 50 μm thick SPEEK exhibited low over potential, high energy efficiency, high maximum power density and small capacity loss rate during charge–discharge cycles at current density of 40 mA cm⁻² [33]. Hickner et al. studied three different thicknesses (28, 45 and 80 μm) of sulfonated fluorinated poly(arylene ether) (SFPAE) membranes for VRFB application and concluded that 45 μm was the optimized membrane thickness under the current density of 20–80 mA cm⁻² [34]. It can be seen that relatively narrow current density ranges and only short-term cycle life test (up to 60 cycles) have been applied to study the effect of membrane thickness on VRFB performance in previous works. The systematic evaluation of Nafion membrane thickness impact on VRFB performance such as electrolyte utilization, imbalance of electrolyte, and capacity fading mechanism have not been revealed yet.

In this study, we chose the commercial Nafion series of membranes (equivalent weight of 1100 g mol⁻¹) with thickness of 50 μm (Nafion 112), 88 μm (Nafion 1135), 125 μm (Nafion 115), and 175 μm (Nafion 117) [36] to investigate the thickness impact of Nafion on VRFB performances under current density of 40–320 mA cm⁻². The self-discharge process, cell efficiencies, electrolyte utilization, and long-term cycling stability of VRFBs with different Nafion membranes are presented comprehensively. A powerful online monitoring system (OMS) is developed for the first time to monitor the electrolyte imbalance during the cycle-life test of VRFB. The capacity fading mechanism of VRFBs with different Nafion membranes is also discussed in details.