High energy efficiency and stability of vanadium redox flow battery using pore-filled anion exchange membranes with ultra-low V4+ permeation

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Abstract

Anion exchange membranes for vanadium redox flow battery are prepared by filling pore of polytetrafluoroethylene (PTFE) substrates with poly(arylene ether ketone) with imidazole (PAPI). For facile impregnation of PAPI molecules into pore of hydrophobic PTFE, catechol and polyethyleneimine (PEI) are co-deposited to hydrophilize the surface of PTFE. Compared with the pristine PAPI membranes, dimensional and mechanical properties of the pore-filled membranes improved due to the reinforcing effect of PTFE substrate. While this process reduces water uptake and swelling ratio by 10 % and 8 %, respectively, it enhances the tensile strength by 5 folds. Strikingly, the ion conductivity even slightly increases along with the huge reduction of vanadium ion permeability. Pore filed membranes reveal vanadium ion permeability of 1.4 × 10−7 cm2 min−1–2.08 × 10−7 cm2 min−1 at room temperature, which is 10 times and 3 times lower than those of the commercial Nafion117 and FAP450 membranes, respectively. For the cycling test of VRFB cell with 200 cycles, it shows the coulombic and energy efficiency over 96.5 % and 85 %, respectively, because of the low vanadium permeability. In association with this, the capacity retention behavior is more stable than the commercial membranes.

Introduction

In recent years, approximately 79 % of electricity was produced from fossil fuels [1]. Although this energy source has an advantage related with high efficiency and low production cost, it also has a critical drawback associated with limited amount of resource and environmental pollution [2]. Although the renewable energy sources, such as solar, wind, and hydro power become more important for their eco-friendliness and un-limitedness [3], [4], [5], they often confront the efficiency issues related with intermittence of power generation caused by the environmental change. Recently, great efforts for continuous and wide application of renewable energy sources promoted the development of cost effective energy storage systems balancing its supply and demands [6].

The redox flow battery (RFB) is considered as one of the most promising large-scale energy storage systems because of its flexible design, low maintenance cost, fast response time, and long lifetime [7], [8]. As a representative type of redox flow battery systems, vanadium redox flow battery (VRFB) is operated by redox reactions between two different couples of vanadium ions at each side. The redox couple of positive electrolyte is VO2+(V4+)/VO2+(V5+), while that of negative electrolyte is V3+/V2+ [9]. Using the same metal ions with different oxidation states in electrolyte solutions, the electrodes and membrane would not be cross-contaminated, which is a noticeable feature compared with other types of RFB including Fe-Cr and Zn-Br [10].

As the one of the major parts for VRFB, the ion exchange membrane acts as a separator dividing two electrolyte solutions to prevent physical mixing of vanadium ions [11], which eventually prohibits self-discharge reaction. When the charged vanadium ions in each solution are cross-permeated, the oxidation and reduction reactions concurrently occur in both solutions, leading to a serious damage on cell performance. As VO2+ is often generated at positive side and V3+ is generated at negative side by undesired vanadium ion cross-flow [12], it prolongs the charging but accelerates the discharging reaction, and thus the efficiency of VRFB is degraded. Therefore, the ion exchange membranes for VRFB application require several important properties including low vanadium ion permeability, high ion conductivity, and good chemical and mechanical stability. One of the well-known ion exchange membranes for VRFB is Nafion for its excellent chemical stability and high proton conductivity. Its high vanadium ion permeability is, however, a critical obstacle for its wide application, because it often accelerates the self-discharge reaction leading to significant loss of capacity and coulombic efficiency in long term operation. In order for replacement of Nafion, a number of hydrocarbon based proton exchange membranes such as poly(ether ether ketone) (PEEK), poly(fluorenyl ether ketone sulfone), and poly(arylene ether ketone) (PAEK) have been studied [13], [14], [15]. Even though the smaller ion cluster size of hydrocarbon based membranes reduced the vanadium ion permeation more effectively than Nafion, huge improvement is still needed in its permeation characteristics because those cationic exchange membranes basically have limitations in repelling the vanadium ions with positive charge. The anion exchange membrane (AEM), which was widely applied to alkaline fuel cell or water electrolysis, can be an alternative for the proton exchange membranes. Because it has positively charged functional group such as quaternary ammonium and pyridinium [16], [17], it repulses vanadium ion by Donnan exclusion effect, and thus it may establish low vanadium ion permeability and high coulombic efficiency [18], [19], [20], [21]. Despite such an advantageous feature, the ion transport phenomena through the AEM for VRFB application are complicated [22], as the protons are also migrating along with the sulfate anions via the free volume or the acid molecules absorbed in membranes [23], [24], [25]. Thus, its clarification would be beneficial to the application of AEMs for VRFB. In addition, it still has a weakness compared with proton exchange membranes in association with the lower redox stability especially at positive half-cell side [26].

The pore filled membrane system is an option to resolve the weakness of the pristine proton exchange or anion exchange membrane for VRFB application [27], [28], because it can control the ion selectivity and conductivity by pore size and porosity, maintaining the mechanical and chemical stability [29], [30], [31], [32]. For the pore filled membrane system, the porous polytetrafluoroethylene (PTFE) substrate is a good choice as a reinforcing material for its excellent mechanical and chemical stability [33]. By reinforcement with this PTFE substrate, the resulting pore filled membranes are expected to show basically better mechanical and chemical properties than the pristine ones against the highly acidic electrolyte solution. In addition, the permeation of vanadium ion is expected to be further reduced by the synergy of size exclusion effect of PTFE pores along with Donnan exclusion effect of positively charged PAPI. On the other hand, the loss of ion conductivity caused by the presence of insulative PTFE frame can be compensated by reduction of the thickness of membrane. In this study, imidazolium grafted PAEK (PAPI) was filled in the pore of PTFE substrate. Imidazolium group was specifically chosen as the N-heterocyclic structure exhibits excellent chemical stability against the highly acidic environment of VRFB [34]. As the hydrophobic PTFE has quite low surface energy associated with C-F bonds, it disturbs facile impregnation of the hydrophilic PAPI molecules into PTFE substrate [35], [36]. Thus, PTFE surface hydrophilization is quite prerequisite for the feasible impregnation of PAPI into PTFE [37], [38]. After the preparation of the pore-filled membranes, properties of prepared membranes such as ionic conductivity, vanadium ion permeability, mechanical properties and chemical stability were investigated, Also, the prepared membranes were assembled to VRFB single cell and its performances were evaluated.

Section snippets

Materials

For synthesizing PAEK backbone, dimethyl sulfoxide (DMSO), 4,4′-difluorobenzophenone, dimethyl formamide (DMF), and tetrahydrofuran (THF) were purchased from Tokyo Chemical Industry (TCI, Japan). N,N′-dicyclohexylcarbodiimide (DCC), N-hydroxysuccinimide (NHS), potassium carbonate, and 4,4′-bis(4-hydroxyphenyl)-valeric acid were obtained from Sigma-Aldrich (St. Louis, MO, USA), and toluene was obtained from Samchun Chemicals (South Korea). Iodomethane, N,N’-dimethylacetamide (DMAc), and

Surface modification of membranes

The contact angle of water droplet on the pristine PTFE, moPTFE, and PTFE/PAPI membranes was measured to confirm surface modification. The water droplets on the pristine PTFE and moPTFE are shown in Fig. 3(a) and (b), and the contact angle was 132° and nearly zero, respectively, implying that hydrophilization of the surface of PTFE was successful. The water contact angle on PTFE/PAPI 2.5 membrane slightly increased to 38° after pore filling process as shown in Fig. 3(c). For the reference,

Conclusion

The reinforced membranes were fabricated by filling pores of surface modified porous PTFE substrate with PAPI. Despite the decrease of ion exchange capacity due to the use of insulative PTFE substrate, the ionic conductivity was compensatively increased by thickness reduction. In addition, the dimensional stability and mechanical properties of pore-filled membrane were greatly improved by the reinforcing effect of PTFE substrate. The PTFE/PAPI membranes exhibited quite low vanadium ion

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was sponsored by the National Research Foundation of Korea (NRF 2018M3D1A1058642) funded by the Ministry of Science and ICT.

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