Hydrophilic microporous membranes for selective ion separation and flow-battery energy storage

Abstract

Membranes with fast and selective ion transport are widely used for water purification and devices for energy conversion and storage including fuel cells, redox flow batteries, and electrochemical reactors.However, it remains challenging to design cost-effective, easily processed ion-conductive membranes with well-defined pore architectures. Here, we report a new approach to designing membranes with narrow molecular-sized channels and hydrophilic functionality that enable fast transport of salt ions and high sizeexclusion selectivity towards small organic molecules. These membranes, based on polymers of intrinsic microporosity (PIMs) containing Tröger's base or amidoxime groups, demonstrate that exquisite control over subnanometer pore structure, the introduction of hydrophilic functional groups, and thickness control all play important roles in achieving fast ion transport combined with high molecular selectivity. These membranes enable aqueous organic flow batteries with high energy efficiency and high capacity retention, suggesting their utility for a variety of energy-related devices and water purification processes.In addition to conventional membrane separation processes 1, 2 , there is a rapidly growing demand for iontransport membranes in applications related to energy 1-3 . With greater reliance on renewable but intermittent energy sources such as solar and wind power, energy conversion and storage technologies are required to integrate low-carbon energy into the power grid. These include electrochemical water splitting and electrolysis for H 2 production 4 , proton-exchange membrane (PEMs) and alkaline fuel cells for energy conversion 5 , electrochemical reduction of CO 2 and N 2 to fuel and chemicals 6 , and scalable redox flow batteries (RFBs) 3,7 . In all of these established and emerging electrochemical processes, ion-selective membranes transport ions whilst isolating the electrochemical reactions in separate cells. In the new generation of RFBs 8-14 , low-cost and high-performance membranes need to have precise selectivity between ions and organic redox-active molecules [15][16][17][18] .Whilst various new electrochemical processes have been developed, the use of expensive commercial ion-exchange membranes, such as the poly(perfluorosulfonic acid) (PFSA)-based Nafion Council through grant agreement number 758370 (ERC-StG-PE5-CoMMaD). Q.S. acknowledges the financial support by Imperial College Department of Chemical Engineering Start-up Fund, seed-funding grant from Institute of Molecular Science and Engineering (IMSE, Imperial College) and seed-funding from EPSRC centres CAM-IES and Energy SuperStore (UK Energy Storage Research Hub). R.T. acknowledges a full PhD scholarship funded by China Scholarship Council. A.W. acknowledges a full PhD scholarship funded by Department of Chemical Engineering at Imperial College. B.P.D. acknowledges the Statoil scholarship. K.E.J. acknowledge the Royal Society University Research Fellowship. A.I.C. and L.C. acknowledge the Leverhulme Trust for supporting the Lev...

The immobilisation and reactivity of Fe(CN)63−/4− in an intrinsically microporous polyamine (PIM-EA-TB)

“…The presence of this base then causes a lower degree of polymer protonation and 4− is immobilised. The absence of a voltammetric response when attempting immobilisation of Fe(CN) 6 4− without protons into PIM-EA-TB is consistent with the recently reported low permeation of Fe(CN) 6 4− under conditions of redox flow battery application [52].…”

Hydrophilic Microporous Polymer Membranes: Synthesis and Applications

“…Song et al used another amidoxime-containing PIM membrane (Figure 1b, AO-PIM-1 and AO-PIM-SBF) for K 4 [Fe(CN) 6 ]/DHAQ ARFB (Figure 7a-c). [25] They found that the microporosity of polymers and the content of amidoxime functionality show a strong correlation with battery performance (Figure 7c-f). The increase in BET surface area or AO functional group content could dramatically decrease the membrane resistance, thereby improving the energy efficiency and power density of the corresponding cells.…”

“…[24] With the same method, Tan et al successfully modified the spirobifluorene-containing PIM (PIM-SBF) and obtained AO-PIM-SBF. [25] The hydrophilic amidoxime group can be either positively charged or negatively charged in aqueous solutions, depending on pH. It is positively charged at pH below 4 and becomes negatively charged when the pH is above 13.…”

Recent Development in Composite Membranes for Flow Batteries

“…[53] Even hydrophobic regular structurem aterials, such as PIMs, have been modified utilizing their distinct pore sized to endow hydrophilicity.H ydrophilic ordered channels and well-controlled thickness of the selective layer all can induce ah ighi onic conductivity. [86] Due to the above virtues, composite membranes with ordereds elective layersh old great potentiala sm embranes in FBs with high efficiencies (Coulombic efficiency,v oltage efficiency,a nd energy efficiency were all included) and low capacity decay. [86] Kim et al [20] designed aP TFE composite membranew ith a graphene oxide framework (GOF) selectivel ayer.Across-linked GOF with around0 .4 mmt hickness was formed owing to the reaction between GO nanosheets and ethylenediamine (EDA).…”