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...

Large-scale energy storage is becoming increasingly critical to balance the intermittency between renewable energy production and consumption 1. Organic redox flow batteries (RFBs), based on inexpensive and sustainable redox-active materials, are promising storage technologies that are cheaper and have fewer environmental hazards than the more mature vanadium-based batteries (typically < 15 Wh/dm 3 , vs. 20-35 Wh/dm 3 , respectively) 2,3. Unfortunately, they have shorter calendar lifetimes and lower energy-densities and fundamental insight at the molecular level is thus required to improve performance 4,5. Here we report two in situ NMR methods to study flow batteries, which are applied on two separate anthraquinones, 2,6-dihydroxyanthraquinone, DHAQ and 4,4'-((9,10-anthraquinone-2,6diyl)dioxy) dibutyrate, DBEAQ as redox-active electrolytes. In one method we follow the changes of the liquids as they flow out of the electrochemical cell, while in the second, we observe the changes that occur in both the positive and negative electrodes in the full electrochemical cell. Making use of the bulk magnetisation changes, observed via the 1 H NMR shift of the water resonance, and the linebroadening of the 1 H shifts of the quinone resonances as a function of state of charge, we determine the potential differences of the two one-electron couples, identify and quantify the rate of electron transfer between reduced and oxidised species and the extent of electron delocalization of the unpaired spins over the radical anions. The method allows electrolyte decomposition and battery self-discharge to be explored in real time, showing that DHAQ is decomposed electrochemically via a reaction which can be minimized by limiting the voltage used on charging. Applications of the new NMR metrologies to understand a wide range of redox processes in flow and other battery systems are readily foreseen. The two in situ NMR setups Ex situ characterization of RFBs can be challenging due to the high reactivity, sensitivity to sample preparation and short lifetimes of some of the oxidised and/or reduced redox-active molecules and ions within the electrolytes. However, one of the distinct features of RFBs is the decoupling of energy storage and power generation, providing different opportunities for in situ monitoring. To date, methods such as in situ optical spectrophotometry 6 and Electron Paramagnetic Resonance (EPR) 7 have been used to study, for example, crossover of quinones and vanadyl ions, but considerable opportunities remain to improve characterization methods to address limitations inherent to each method and to probe different phenomena. Nuclear Magnetic Resonance (NMR) spectroscopy was used to study benzoquinone and polyoxometalate redox reactions in an in situ

Membranes whichallow fast and selective transport of protons and cations are required for aw ide range of electrochemical energy conversion and storage devices,such as proton-exchange membrane (PEM) fuel cells (PEMFCs) and redox flowbatteries (RFBs). Herein we report anew approach to designing solution-processable ion-selective polymer membranes with both intrinsic microporosity and ion-conductive functionality.P olymers are synthesized with rigid and contorted backbones,w hich incorporate hydrophobic fluorinated and hydrophilic sulfonic acid functional groups,t op roduce membranes with negatively charged subnanometer-sized confined ionic channels.T he ready transport of protons and cations through these membranes,a nd the high selectivity towards nanometer-sized redox-active molecules,e nable efficient and stable operation of an aqueous alkaline quinone redox flowb attery and ahydrogen PEM fuel cell.

Redox flow batteries using aqueous organic-based electrolytes are promising candidates for developing cost-effective grid-scale energy storage devices. However, a significant drawback of these batteries is the cross-mixing of active species through the membrane, which causes battery performance degradation. To overcome this issue, here we report size-selective ion-exchange membranes prepared by sulfonation of a spirobifluorene-based microporous polymer and demonstrate their efficient ion sieving functions in flow batteries. The spirobifluorene unit allows control over the degree of sulfonation to optimize the transport of cations, whilst the microporous structure inhibits the crossover of organic molecules via molecular sieving. Furthermore, the enhanced membrane selectivity mitigates the crossover-induced capacity decay whilst maintaining good ionic conductivity for aqueous electrolyte solution at pH 9, where the redox-active organic molecules show long-term stability. We also prove the boosting effect of the membranes on the energy efficiency and peak power density of the aqueous redox flow battery, which shows stable operation for about 120 h (i.e., 2100 charge-discharge cycles at 100 mA cm−2) in a laboratory-scale cell.

Redox-active organic materials have emerged as promising alternatives to conventional inorganic electrode materials in electrochemical devices for energy storage. However, the deployment of redox-active organic materials in practical lithium-ion battery devices is hindered by their undesired solubility in electrolyte solvents, sluggish charge transfer and mass transport, as well as processing complexity. Here, we report a new molecular engineering approach to prepare redox-active polymers of intrinsic microporosity (PIMs) that possess an open network of subnanometer pores and abundant accessible carbonyl-based redox sites for fast lithium-ion transport and storage. Redox-active PIMs can be solution-processed into thin films and polymer− carbon composites with a homogeneously dispersed microstructure while remaining insoluble in electrolyte solvents. Solutionprocessed redox-active PIM electrodes demonstrate improved cycling performance in lithium-ion batteries with no apparent capacity decay. Redox-active PIMs with combined properties of intrinsic microporosity, reversible redox activity, and solution processability may have broad utility in a variety of electrochemical devices for energy storage, sensors, and electronic applications.

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Redox flow batteries (RFBs) have great potential for long‐duration grid‐scale energy storage. Ion‐conducting membranes are a crucial component in RFBs, allowing charge‐carrying ions to transport while preventing the cross‐mixing of redox couples. Commercial Nafion membranes are widely used in RFBs, but their unsatisfactory ionic and molecular selectivity, as well as high costs, limit the performance and the widespread deployment of this technology. To extend the longevity and reduce the cost of RFB systems, inexpensive ion‐selective membranes that concurrently deliver low ionic resistance and high selectivity toward redox‐active species are highly desired. Here, high‐performance RFB membranes are fabricated from blends of carboxylate‐ and amidoxime‐functionalized polymers of intrinsic microporosity, which exploit the beneficial properties of both polymers. The enthalpy‐driven formation of cohesive interchain interactions, including hydrogen bonds and salt bridges, facilitates the microscopic miscibility of the blends, while ionizable functional groups within the sub‐nanometer pores allow optimization of membrane ion‐transport functions. The resulting microporous membranes demonstrate fast cation conduction with low crossover of redox‐active molecular species, enabling improved power ratings and reduced capacity fade in aqueous RFBs using anthraquinone and ferrocyanide as redox couples.

With the rapid development of renewable energy harvesting technologies, there is a significant demand for long-duration energy storage technologies that can be deployed at grid scale. In this regard, polysulfide-air redox flow batteries demonstrated great potential. However, the crossover of polysulfide is one significant challenge. Here, we report a stable and cost-effective alkaline-based hybrid polysulfide-air redox flow battery where a dual-membrane-structured flow cell design mitigates the sulfur crossover issue. Moreover, combining manganese/carbon catalysed air electrodes with sulfidised Ni foam polysulfide electrodes, the redox flow battery achieves a maximum power density of 5.8 mW cm−2 at 50% state of charge and 55 °C. An average round-trip energy efficiency of 40% is also achieved over 80 cycles at 1 mA cm−2. Based on the performance reported, techno-economic analyses suggested that energy and power costs of about 2.5 US$/kWh and 1600 US$/kW, respectively, has be achieved for this type of alkaline polysulfide-air redox flow battery, with significant scope for further reduction.