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Design and NanoEngineering of Microporous Membranes for Energy Storage

Periodic Reporting for period 2 - NanoMMES (Design and NanoEngineering of Microporous Membranes for Energy Storage)

Reporting period: 2021-05-01 to 2022-10-31

In order to achieve the goal of Paris Agreement to limit global warming to 1.5℃, there is an urgent need to mitigate CO2 emissions. The European Union 2030 Framework for Climate and Energy Policy sets an EU-wide target to cut 40% greenhouse gas emissions and over 27% share of renewable energy consumption. Renewable energy supply infrastructure such as solar and wind power are being rapidly developed by many countries, particularly in the Europe. However, the fluctuating renewable energy generation leads to the mismatch between the supply and demand, and instability of power grid. In order to integrate the increasing low-carbon energy generation to energy system, there is an urgent need for large scale cost-effective energy storage systems capable of storing the renewable energy and supply at demand.
The aim of this project is to develop grid-scale long-duration energy storage. Among the various types of energy storage systems, redox flow batteries (RFBs) are one of the most promising technologies for grid scale energy storage owing to the ability to decouple power and energy. The total energy output depends on the scalable volume of the electrolyte in the tank, while the total power output is determined by the size of active area inside the stack. The membrane separator in flow battery system is a crucial component that enables the fast transport of charge carrying ions while simultaneously keeps the redox materials separated from each other. Particularly, the expensive Nafion membrane ($500/m2) can take up to 30% in the total costs of flow battery stack, which limits the scale up and widespread commercialization of flow battery technology.
The aim of my project is to perform interdisciplinary research combining the molecular engineering of novel microporous polymers, membrane science and engineering, rechargeable batteries and electrochemistry, and process engineering to create new transformative membranes that enable development of cost-effective renewable energy storage and generation technologies. The ultimate goal of the project is to generate design principles for next-generation ion-selective membranes that will have broad implications on advanced batteries for energy storage, helping the EU develop renewable energy and reduce greenhouse gas emissions.
During the first work period of the project, significant progress has been made to develop the synthetic approaches of new ion-selective membranes, characterization techniques, manufacturing, and integration with battery devices. We have developed microporous polymers with narrow pore size distribution, especially solution-processable polymers of intrinsic microporosity (PIMs). The synthetic chemistry of PIMs allows versatile combination of monomers with different geometries, such as tuning the angles of contorted sites, molecular length between contorted sites, and pendant groups on the backbone. We have synthesised the first-generation PIMs by double aromatic nucleophilic substitution polycondensation of monomers containing rigid and contorted structures. We have developed simple acid-base reactions to modify the polymers into ion-conductive membranes, including amidoxime and Troger’s Base functional groups. We developed sulfonated PIM polymers that provide high conductivity in near-neutral pH electrolytes and high ionic selectivity towards redox active species. These membranes show much higher ionic conductivity than Nafion membranes. We have performed systematic characterization of these polymers and established structure-properties-function relationships that will guide the future development of polymer membranes for electrochemical devices. We have further used computational modelling approaches to generate molecular models of the polymers and investigated the structure of polymers and formation of water and ionic transport channels.
Apart from the development of membrane materials, the project also targets at design and assembly of flow batteries combining the newly developed membranes with redox active materials and investigation of the energy storage performance as well as electrochemistry in the context of new membranes. We have demonstrated the development of metal-ion based flow batteries, and aqueous organic redox flow batteries using organic molecules as redox materials, such as anthraquinone-derivatives and ferro/ferricyanide. By pairing the newly developed ion-selective membranes with these new redox active electrolytes, we achieved high battery performance, including high energy efficiency and high cycling stability. We have also developed other aqueous RFBs, including hybrid H2-manganese, zinc-iron, and polysulfide electrolytes operated in acidic and alkaline solutions.
We have made developments beyond the state of the art:
Our new hydrocarbon polymer membranes represent a new generation of ion-selective membranes with both high ionic conductivity and high selectivity, which overcomes the trade-off between conductivity and selectivity of conventional ion-exchange membranes. Our new design strategy generates highly selective ion-transport membranes with precise control over pore architecture, pore size distribution and ionic conductive functionality.
Another major concern is the cost and sustainability of membrane manufacturing. Commercial ion-exchange membranes, such as the poly(perfluorosulfonic acid)-based Nafion is very expensive (~US$500 per m2). There are growing concerns about environmental impacts of Nafion manufacturing and the by-products perfluoroalkyl and polyfluoroalkyl substances (PFASs), known as “forever chemicals,” which can cause contamination of drinking water and pose long-term threats to human health. Therefore, development of sustainable hydrocarbon membranes with substantial cost reduction to replace the Nafion membranes have both significant economic and society impact.
We have demonstrated the potential of manufacturing and upscaling of polymer membranes to meter square by roll-to-roll casting and manufacturing. These membranes can be integrated into kilowatt scale flow battery stack and demonstrated long duration energy storage performance.
We have extended the design strategy to develop new membranes for other electrochemical technologies such as fuel cells and water electrolysers which potentially could contribute to the development of green hydrogen production and utilization.

As the project continues, we will expand the synthetic chemistries of the membrane development to develop new membrane materials, through accelerated materials discovery in collaboration with our computational collaborators. We will also continue to develop battery chemistries to develop high energy density redox flow batteries. We will also explore the applications of our membranes in other electrochemical technologies such as green hydrogen production and electrochemical CO2 conversion towards synthesis of chemicals and fuels.

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