Combining Promising Electrolyte Research and 3D Printing: Ionic Liquid Based Printable Membrane Materials

Abstract

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 e.g., high thermal and electrochemical stability, high ionic conductivities and non-flammability, making them interesting candidates for use as electrolytes.[1] One of the most pressing global issues of our time, when disregarding the current COVID19 crisis, is the climate change with its well-known challenges concerning the climate policy and the energy economy. In this context due to their many benefits, the aforementioned fuel cells and batteries are being discussed as potential appliances, although there are some disadvantages attached to these technologies. Most polymer-electrolyte-membrane (PEM) fuel cells work with Nafion®, a sulfonated fluorocopolymer, as their membrane. Nafion® however loses conductivity at higher temperatures due to dehydration, limiting its operation temperature to around 80 °C at ambient pressure.[2] ILs are considered compounds that can overcome these limitations as they offer high conductivities even at elevated temperatures.For the applications as electrolytes in the aforementioned systems the immobilization of the ILs is necessary, however, to prevent leaking and realize proper function.[1] The resulting ionogels (IGs) then combine the properties of the polymer matrix, i.e. its mechanical stability, with the characteristics of the respective IL. A suitable method to realize precise geometries and shapes for these membranes is given by 3D printing, which offers adaptable electrolyte design.[3] The synthesis and characterization of ILs for ion (specifically proton) conduction is the aim of this work. These ILs exhibit wide electrochemical and thermal stability windows. The ionic conductivities of the investigated compounds range between 10-2 - 10-4 S/cm at elevated temperatures. The ILs are furthermore investigated under aspects of ion and proton transport via different spectroscopy methods.[3,4] Moreover, the ILs are immobilized in different matrix materials to provide flexible and transparent IGs that contain up to 80 wt% of IL. Additionally, successful 3D-printing and structuring of the IGs is also demonstrated.[3] [1] M. Martinez, et al, Journal of Power Sources, 2010, 195, 5829-5839.[2] J. Le Bideau, et al., Chem. Soc. Rev., 2011, 40, 907-925.[3] K. Zehbe, et al., Energy Fuels, 2019, 33, 12885-12893.[4] Z. Wojnarowska, et al., ACS Appl. Mater. Interfaces, 2021, 13, 30614–30624.