Robust proton exchange membrane for vanadium redox flow batteries reinforced by silica-encapsulated nanocellulose
Introduction
With the increasing demand for energy and the depletion of fossil energy, the exploitation of renewable energy resources, such as solar and wind energy, is a top priority [1]. Due to the intermittency and volatility of these natural resources, there is an urgent need to develop large-scale energy storage technology for stable power output [2]. Among a host of candidates, all-vanadium redox flow batteries (VRFBs) stand out as a promising electrochemical energy storage device relying on its merits of high efficiency, long lifetime, good safety and flexible design [3,4]. Proton exchange membrane (PEM), one of key components in VRFBs, acts as the separator for anolyte and catholyte to avoid vanadium ions crossover. The PEMs also serve as proton conductors so that it can contribute to high voltage efficiency in VRFBs [5].
The PEMs in VRFBs are expected to have high proton conductivity, low vanadium permeability, strong mechanical strength and good chemical stability [6]. Currently, perfluorosulfonic acid (e.g. Nafion® membrane) is widely used in VRFBs, but its high cost and serious vanadium permeation have hindered its further commercialization in VRFBs [7]. The migration of various vanadium ions in VRFBs would cause self-discharge reaction and capacity loss, and therefore decrease cell efficiency and electrolyte utilization [8]. In order to solve this problem, various approaches have been studied [9], including: (a) replacing perfluorosulfonic acid with low-cost polymer materials [[10], [11], [12], [13], [14]], such as sulfonated poly(ether ether ketone) (SPEEK), sulfonated poly(ether sulfone) (SPES), sulfonated polyimide (SPI); (b) introducing functional fillers like SiO2, TiO2, GO as barriers for vanadium species crossover [[15], [16], [17], [18], [19]]; (c) preparing highly selective porous membranes by non-solvent induced phase inversion to improve H+/VO2+ selectivity [[20], [21], [22], [23]].
Nanocellulose (NC) is receiving much attention as membrane material in batteries due to its availability, sustainability and excellent mechanical property [24,25]. It has been reported that NC membrane showed good hydrophilicity, high mechanical strength and stability in lithium-ion battery [26]. However, few studies focus on its application in vanadium redox flow battery, where inhibiting vanadium migration across the membrane is additionally required.
In this work, NC is chosen as substrate material, which is filled with SPES to prepare NC-reinforced proton exchange composite membranes. The good hydrophilicity of nanocellulose is expected to create favorable condition for proton conduction. To improve the chemical stability of the NC fibers and suppress vanadium permeation, compact silica layer is covalently synthesized on the surface of the nanocellulose fibers via an in-situ sol-gel method. Properties of the membrane were characterized to evaluate the membrane’ applicability in VRFBs, including morphology, mechanical strength, proton conductivity, vanadium permeability, chemical and thermal stability, as well as the single cell performance.
Section snippets
Materials
Nanocellulose membranes were supplied by Hainan YeGuo Foods CO., LTD. Nafion®212 (N212) was purchased from DuPant Co. SPES with a sulfonation degree (SD) of 50% was bought from Changzhou KTCHEM. Co. Ltd. Graphite felt with a thickness of 6 mm was bought from Liaoning JinGu Carbon Materials Co., Ltd., China. All chemicals were analytical reagent and used as received.
Synthesis of silica-encapsulated NC composite membrane (CNC)
Silica dense layer was formed on the surface of the nanocellulose fibers via an in-situ sol-gel method reported by our group [26].
Membrane morphology
A dense proton exchange membrane is particularly essential in VRFBs as any minor defect or crack would accelerate vanadium crossover and cause self-discharge reactions. The nanofibrous structure of the NC membrane can be observed in Fig. 3a. The NC fibers were naturally cross-linked and possessed an average diameter of 100–200 nm. During the sol-gel process, SiO2 sol was firstly anchored on the surface of NC fibers through the Si–O–C bonding and then grew via self-condensation of Si–OH groups
Conclusions
In this work, sulfonated poly(ether sulfone) membranes reinforced by core-shell structured nanocellulose was successfully prepared. Due to the hydrophilic surface of NC and CNC, the membrane showed good compatibility between the porous matrix and SPES, leading to promising membrane performance. Ultra-high tensile strengths of 92.57 MPa for NC-SPES and 54.46 MPa for CNC-SPES were obtained, while that of the pristine SPES membrane was only 16.30 MPa. Both the NC-SPES and CNC-SPES membranes showed
Acknowledgement
The work described in this paper was fully supported by the Key Research and Develop Project of Hainan Province (Grant No. ZDYF2019004).
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Authors contribute equally to the paper.