Protic Ionic Liquid Based 3D Printable Ionogels for Applications As Electrolytes

Abstract

When disregarding the current COVID19 crisis the climate change is one of the most pressing global issues with its well-known challenges concerning the energy economy and climate policy. In this context due to their many benefits, fuel cells and batteries are being discussed as potential applications. These technologies are not without some disadvantages themselves, though. Most polymer electrolyte membrane (PEM) fuel cells work with Nafion®, a sulfonated fluorocopolymer, as their membrane, which loses conductivity at higher temperatures due to dehydration, which limits the operation temperature to around 80 °C at ambient pressure. [1] As a result alternative membrane materials are needed which can preferably be designed with defined geometries and sizes and show high ion mobilities and corresponding ionic conductivities at elevated temperatures.As promising components in energy devices such as fuel or solar cells and batteries ionic liquids (ILs) are discussed due to their promising properties. These salts with melting points below 100 °C show high thermal and electrochemical stability, non-flammability and high ionic conductivities. For the applications mentioned above the immobilization of the ILs is necessary to realize proper function and prevent leaking. [2] The immobilization of the ILs can be realized in three different ways: 1. swelling of polymers in the IL, 2. polymerization of polymerizable ILs and 3. polymerization of vinyl-monomers in an IL. [3] The resulting ionogels (IGs) can then combine for example the characteristics of the respective IL with the useful properties of the polymer, i.e. its mechanical stability. To realize precise shapes and geometries for the membrane materials stereolithography and 3D-printing of the respective IGs provide a suitable method. [4]The aim of this study is the synthesis and characterization of ILs for proton-conduction. The ionic conductivities of these compounds range between 10-2 - 10-4 S/cm at elevated temperatures. Moreover, these ILs exhibit wide electrochemical (e. g. ΔE up to 3 V) and thermal stability windows (Td ≥ 230 °C) and are therefore promising for ion transport at elevated temperatures. NMR-studies also provide information about their ion mobility. Furthermore the ILs are immobilized via two of the aforementioned methods to provide flexible and transparent IGs that contain up to 80 wt% of IL. These IGs display promising thermal and mechanical stability and reach ionic conductivities of up to 10-3 S/cm at elevated temperatures. Successful 3D-printing and therefore structuring of IGs can also be demonstrated.[1] Martinez, et al., Journal of Power Sources, 2010, 195, 5829–5839.[2] Le Bideau, L. Viau, A. Vioux, Chem. Soc. Rev., 2011, 40, 907–925.[3] Lu, F. Yan, J. Texter, Progress in Polymer Science, 2009, 34, 431-448.[4] Zehbe, A. Lange, A. Taubert, Energy Fuels, 2019, 33, 12885-12893.