New Membrane Materials for Energy Applications Based on 3D-Printable Ionogels

Abstract

Considering the well-known, global challenges the energy economy and climate policy face, batteries and fuel cells are widely discussed for potential applications in the future due to their many benefits. There are some difficulties connected with these technologies though. For PEM-fuel cells one of the main problems is linked to their membranes. These are based on Nafion®, a sulfonated fluorocopolymer and therefore have a limited application temperature of 80 °C at ambient pressure due to dehydration and corresponding loss of conductivity at higher temperatures. [1] Due to this limit in operating temperature there is a significant need for alternative membrane materials, preferably with precise and defined size and geometry that can exhibit high ion mobilities and ionic conductivities above 100 °C. In recent years research on ionic liquids (ILs) has experienced a revival. These salts with melting points below 100 °C are promising components in energy devices such as batteries, solar or fuel cells, owing to their high thermal and electrochemical stability, non-flammability and high ionic conductivities. However, to prevent leaking and realise proper function of these devices immobilizing the IL in a matrix is necessary. [2] The resulting ionogels (IGs) then combine the characteristics of the respective IL with the useful properties of the polymer, i.e. its mechanical stability. This immobilization can be realized in three different ways: doping of polymers with the IL, polymerization of vinyl monomers in the IL, and polymerization of polymerizable ILs. [3] To obtain membrane materials with precisely controlled sizes, shapes and geometries along with the necessary performance stereolithography and 3D-printing of suitable materials are a promising method. [4] The aim of this work therefore is the synthesis and characterization of ILs for ion- and especially proton-conduction. The ionic conductivities of these compounds range between 10-2 – 10-4 S/cm at elevated temperatures. Moreover, with wide electrochemical and thermal stability windows (e. g. ΔE up to 3 V, Tg around -90 °C and Td over 200 °C (for some of them)) these ILs are promising for ion transport in fuel cell membranes above 100 °C and their properties are in addition studied under aspects of ion mobility. The immobilization of these ILs is furthermore realized via different methods, as mentioned above. The corresponding transparent and flexible IGs, in part containing high wt% of IL, display promising thermal and mechanical stability and reach ionic conductivities of up to 10-3 S/cm at elevated temperatures. This study also demonstrates successful 3D-printing and structuring of IGs, which clearly enables the design of materials with different requirements by simply adapting the size and shape. [1] A. Martinelli, A. Matic, P. Jacobsson, L. Börjesson, A. Fernicola, S. Panero, B. Scrosati, H. Ohno, J. Phys. Chem. B, 2007, 111, 12462. [2] Y.-S. Ye, J. Rick, B.-J. Hwang, J. Mater. Chem. A, 2013, 1, 2719. [3] J. Lu, F. Yan, J. Texter, Progress in Polymer Science, 2009, 34, 431. [4] K. Zehbe, A. Lange, A. Taubert, Energy Fuels, 2019, 33, 12885.