Block Copolymer Sdapp Membranes for Vanadium Redox Flow Batteries-Strategy to Address Transport and Durability

Abstract

The electrolyte separator (membrane) is a key component in vanadium redox flow batteries (VRFB) for large scale electrical energy storage. In the VRFB cell, the separator serves to physically separate positive and negative electrolyte solutions while establishing ionic conduction to pass current between two half cells. High ionic conductivity, low vanadium permeability, chemical stability and low cost are desired properties of RFB separators. To serve as a separator, the membrane should deter unwanted redox species transport while maintaining excellent ionic transport properties to minimize ohmic resistance. In the harsh electrolyte environment, the membrane must be stable enough to withstand chemical attacks in the electrolyte environment for a long lifespan. Moreover, the cost of the separator must be kept low enough to be integrated into large-scale battery system. Generally, ion exchange membranes are used as electrolyte separator in VRFBs. Sulfonated Diels Alder poly(phenylene) (SDAPP) cation exchange membranes have emerged as a good membrane candidate for both proton exchange membrane fuel cell and VRFB. The first generation SDAPP is a partially sulfonated poly(phenylene) random polymer. The random SDAPP membrane has shown comparable performance to Nafion in transport study and battery testing.1 From the perspective transport properties, SDAPP is satisfactory as anelectrolyte separator in VRFBs. However, random SDAPP is not chemically stable enough in the electrolyte environment to serve for a targeted lifespan because the unfunctionalized benzene ring in SDAPP is vulnerable to attack from V5+ (VO2 +) ion, a strong oxidant. Sulfonated polyphenylene/polysulfone block copolymer SDAPP membranes show significantly improved resistance to V5+ attack and excellent cell performance.2 The SDAPP block copolymer consists of a hydrophilic segment of fully sulfonated polyphenylene and hydrophobic segment of polysulfone. The block-SDAPP membrane provides an extended lifespan within VRFB environment in comparison to random SDAPP. Chemical stability of block-SDAPP is enhanced by full sulfonation of poly(phenylene) in the hydrophilic segment. Ionic conductivity is not sacrificed by block copolymerization. In the battery, the performance of block-SDAPP can be better than state of art Nafion 212. As is shown in the figure, at 500 mA/cm2cycling current density, cells equipped with three different block-SDAPP membranes can reach 76% energy efficiency, compared to 70% with Nafion 212, with the same electrode and electrolyte conditions. About 80% voltage efficiency was achieved with block-SDAPP membranes, because of the lowered internal resistance. Moreover, the coulombic efficiency reached by the three block SDAPPs is respectively 97%, 95% and 93%, compared to 95% of Nafion. Vanadium crossover across block SDAPP membranes is similar to that in Nafion 212. Considering that block-SDAPPs have lower ASR in the battery, the ionic selectivity of block SDAPP is essentially better than Nafion. To further optimize the structure of block-SDAPP for VRFB application, we are investigating the correlation between structure and performance of block-SDAPP. Under battery electrolyte conditions, ionic equilibrium, ionic transport and membrane morphology will be studied via the protocol established in previous studies.3Sulfuric acid, water uptake and vanadium ion partitioning in membrane will be determined after being equilibrated in electrolyte of various acid and vanadium composition to simulate battery environment. Conductivity and vanadium permeability of block SDAPP will also be measured after the same equilibration. Since water is a critical mediator for proton and cation transport in aqueous media, pulsed field gradient nuclear magnetic resonance (PFG-NMR) will be performed to investigate proton and vanadium transport mechanisms in equilibrated membranes. Membrane morphology will be studied by transmission electron microscopy and small angle X-ray scattering will be used to establish the geometry of ionic cluster channel. Acknowledgement This work is supported by Dr. Imre Gyuk, Office of Electricity Delivery and Energy Reliability, US Department of Energy.