Tuning the ion selectivity of porous poly(2,5-benzimidazole) membranes by phase separation for all vanadium redox flow batteries
Introduction
Utilization of sustainable energy is believed to help release the stress of energy shortage in our times and in the future. The mismatch between the intermittent energy production from sustainable sources, such as solar and wind sources, and the fluctuating energy consumption requires energy storage and conversion systems with large capacity [1]. Among possible solutions, vanadium redox flow battery (VRFB) has been drawing increasing attention in the past decade [2]. To make this technology more economically competitive and to achieve full-scale implementation, two challenges have to be tackled: to increase the electrolyte energy density [1], [3], and the seek of efficient and low-cost membranes as battery separators [4], [5].
The basic function of a membrane in VRFB is isolating electro-active V ions (V, V; VO, VO2 while allowing the passage of charge-carriers (mostly H) at sufficiently high rate, which essentially means high H/V transport selectivity and high H conductivity [6]. The harsh application environment of a membrane in VRFB involves strong acids (2–4 M H2SO4) and the oxidative VO2 ions present in the positive electrolyte [7]. Therefore, membranes should ideally also be chemically durable for years as suggested [8]. Perfluorinated sulfonic acid membranes, like Nafion, are employed in the established demonstrations of VRFB worldwide due to their high H conductivity and excellent chemical stability. However, the high cost and low ion selectivity of Nafion hinder the further development of VRFB [6], [7]. Therefore, the research focus on non-fluorinated ionomers [9], [10], [11] or porous separators [12], [13] is aimed at finding efficient and durable, yet low-cost alternatives to Nafion [6], [7].
Several membrane structures show great potential [12], [14], [15], [16], [13], [17], [18], [19], [20], [21]. Recently, our group reports that porous and cationic poly(2,5-benzimidazole) (referred as ABPBI) membranes could significantly reduce vanadium ion cross-over and could achieve 10% higher energy efficiency compared with Nafion 112 [14]. The benchmarking of the material's chemical stability by ex-situ oxidation tests also indicates great potential of the membranes [14]. Another research group describes their work on porous poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] (generally referred as PBI) membranes for VRFB [15]. The membranes proved to be efficient over a wide temperature range (−5 to 50 °C) and performed for more than 10,000 charge-discharge cycles successfully [15]. These porous poly(benzimidazole) membranes were prepared by a water vapor induced phase separation (VIPS) process [14], [15], which can be integrated in the industrial fabrication process for porous membranes. The promising properties of the porous poly(benzimidazole) membranes, together with the scalable fabrication method, actually show great potential for the commercialization of these membranes.
Thus, the understanding of the phase separation process of poly(benzimidazole) solution is of significant importance. It would also help develop nanoporous membranes for the applications in other redox flow batteries, high temperature polymer electrolyte membrane fuel cells (HT-PEMFC) [22] and water electrolysis [23], [24]. Poly(benzimidazole) is a class of heterocycle polymers with benzimidazole moiety. They are known for their excellent mechanical, chemical and solvent stability [25]. Dense membranes of poly(benzimidazole) imbibed with phosphorous acid have been studied as a promising separator for HT-PEMFC [26]. These dense membranes are routinely prepared by solvent evaporation of a polymer solution [27], and the phase separation process is quite simple. However, in the phase separation process of polymer solution with non-solvent (normally water, vapor or liquid), the thermodynamics and kinetics of the polymer-solvent-water system play a critical role in determining the structure and thus the property of the resulting porous membranes [28]. Actually, porous PBI membranes prepared by the non-solvent (water liquid) induced phase separation (NIPS) process were studied as the first generation reverse osmosis membranes [29], later also as nanofiltration membranes [30]. van de Ven et al. reported poly(benzimidazole) membranes as supports of charge carriers for HT-PEMFC, the membranes were also prepared by the NIPS process [22]. Poly(benzimidazole) membranes have macrovoids for immediate immersion precipitation of polymer solution, and the membrane morphology evolves to macrovoid-free nanoporous structure when solvent-water mixture is used as the coagulation bath [22].
The primary purpose of this work is to study systematically the effect of the water vapor induced phase separation (VIPS) process on the properties of porous poly(benzimidazole) membranes for their application in VRFB. ABPBI polymer is chosen because of its similar chemical properties to PBI, while the synthesis of ABPBI is more environmentally friendly [27]. The viscosity of polymer dope solution, the relative humidity (RH) and temperature of the water vapor, and the exposure time of polymer solution films to the water vapor are investigated in detail to study their influence on membrane morphology and ion transport properties. The VIPS process is also compared with the NIPS process in terms of phase separation mechanisms. Finally, the properties of porous poly(benzimidazole) membranes are correlated with the electrochemical performance of the VRFB.
Section snippets
Materials
Two batches of poly(2,5-benzimidazole) (ABPBI) were provided by FumaTech GmbH, Germany. The polymer properties were listed in Table 1. The solvent N-methyl-2-pyrrolidone (NMP, 99 wt%) was from Acros Organics. Details about the viscosity measurements of ABPBI solution are provided in the Supplementary material. MgSO4 7H2O, VOSO4 xH2O, NaOH, 1 N HCl solution, concentrated sulfuric acid were of general commercial sources. An electrolyte solution of 1.6 M total vanadium ions with 1: 1 (molar) V:VO
The influence of relative humidity (RH), polymer concentration and exposure time
To study the effect of phase separation conditions on the morphology and ion transport properties of these porous membranes, ABPBI polymers of different average molecular weight (1# and 2#) were used to prepare casting solutions with different viscosity. Data and detailed discussions about the polymer solution viscosity can be found in the Supplementary material (Fig. S1 and S2). The activity of water vapor was varied, as well as the time that the cast polymer solution film was exposed to the
Conclusions
The porous poly(2,5-benzimidazole) (ABPBI) membranes prepared by water vapor induced phase separation (VIPS) process have porous surface and macropores of a few microns through the whole thickness of membranes. The size and interconnectivity of macropores is greatly influenced by the molecular weight of polymers and viscosity of the casting solutions, other than the relative humidity of the water vapor. High viscosity of casting polymer solution is critical in obtaining membranes with
Acknowledgements
This work was supported by the German Federal Ministry of Education and Research (BMBF) under the project Tubulair . Tao Luo acknowledges the financial support of a CSC scholarship (201306240054). FumaTech GmbH, Germany is kindly acknowledged for providing the polymer samples. The authors thank Karin Faensen for the nice FESEM graphs, Elif Yalçinkaya for checking some conductivity measurements, and Korcan Perçin for help in the battery test.
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