Influence of membrane structure on the operating current densities of non-aqueous redox flow batteries: Organiceinorganic composite membranes based on a semi-interpenetrating polymer network Sung-Hee Shin, Yekyung Kim, Sung-Hyun Yun, Sandip Maurya, Seung-Hyeon Moon * School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-Gwagiro, Buk-gu, Gwangju 500-712, Republic of Korea highlights graphical abstract  Organiceinorganic composite AEMs based on semi-interpenetrating polymer network.  Adjustment of physical and chemical membrane structure by monomer type.  Determination of operating current densities by physical membrane structure.  Effect of chemical membrane struc- ture on ion transport phenomena.  High performance of porous charged membrane at high current density. article info Article history: Received 14 April 2015 Received in revised form 2 July 2015 Accepted 15 July 2015 Available online xxx Keywords: Organiceinorganic composite membrane Non-aqueous redox flow battery Operating current density Poly(vinylidene fluoride) Semi-interpenetrating polymer network abstract We develop three types of organiceinorganic composite membranes based on a semi-interpenetrating polymer network (SIPN) to explore the effects of membrane structure on the possible operating cur- rent densities of a non-aqueous redox flow battery (RFB) system. Poly(vinylidene fluoride) (PVdF) is selected as a supporting polymer matrix for improving the chemical and thermal stability of the organic einorganic composite membranes. We also introduce silica nanoparticles (5 wt% of PVdF) into the membranes to ensure the low crossover of active species. The fabrication of SIPN through the addition of glycidyl methacrylate, 4-vinylpyridine, or N-vinylcarbazole enables control of the membrane structure. Depending on monomer type, the membrane structure is determined to be either aliphatic or aromatic in terms of chemical properties and either dense or porous in terms of physical properties. These chemical and physical structures affect the electrochemical properties that correspond to charge/discharge per- formance and to the range of possible operating current densities. An important requirement is to examine charge/discharge performance at the possible range of operating current densities by using various membrane structures. This requirement is discussed in relation to a proposed design strategy for non-aqueous RFB membranes. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Non-aqueous redox flow batteries (RFBs) are being developed as a promising grid-level energy storage system with high power density because such batteries provide a stable supply of electricity * Corresponding author. E-mail address: shmoon@gist.ac.kr (S.-H. Moon). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2015.07.045 0378-7753/© 2015 Elsevier B.V. All rights reserved. Journal of Power Sources 296 (2015) 245e254