Charge carrier transport in materials with disordered structures

Ana Šantić

Glass Laboratory, Ruđer Bošković Institute, Zagreb, Croatia; E-mail: asantic@irb.hr

Materials with disordered structures such as glasses, polymers, molten salts, gels have a crucial role in the rapid development of new technologies. Their specific electrical properties make them attractive for application in various electrochemical devices from Li-batteries, supercapacitors, fuel cells, electrochromic windows to photovoltaic cells. Improvement of these materials for particular application, as well as preparation of novel composites, is based on a thorough understanding of the basic mechanisms of charge carrier transport. This contribution will present the recent studies of electronic and ionic transport in three types of structurally disordered materials: i) oxide glasses e.g. iron phosphates, ii) ionic liquids and iii) ionic liquid gels.

Iron phosphate glasses are electronically conducting glasses with polaronic conduction mechanism where conduction takes place by electrons hopping from Fe²⁺ to Fe³⁺. Consequently, polaron transport directly depends on Fe²⁺/Fe_tot ratio and overall Fe₂O₃ content. Depending on composition (addition of Cr₂O₃, MoO₃, PbO, Na₂O) and preparation conditions, the electrical conductivity of iron phosphate based glasses may vary over seven orders of magnitude.¹

            Ionic liquids are low temperature molten salts that show exceptional properties: high ionic conductivity, non-volatility, non-flammability, chemical and thermal stability.

The ionic conductivity of these liquids is closely related to their fluidity.² The same elementary step of an ion is involved in both flow of charge and fluid flow. However, in order to contribute to charge transport or fluid flow, this elementary step of an ion has to be followed by suitable movements of neighbouring ions. For fluid flow, these latter movements need to fulfil more specific requirements than for flow of charge, which results in slower ion dynamics.

From the standpoint of application, a relatively high fluidity of ionic liquids could be a considerable disadvantage due to possible leakage which can result in malfunction of the electrochemical device. Therefore, the most recent investigation is focused on the preparation of ionic liquid gels, so called ionogels, which exhibit specific combination of high ionic conductivity and mechanical flexibility. In such ionogel the liquid component is trapped in a sponge-like matrix of the gelator. The leakage of the electrolyte is therefore prevented and at the same time the high ionic conductivity is preserved.

References:

¹ A. Šantić, C.W. Kim, D.E. Day, A. Moguš-Milanković, J. Non-Cryst. Solids, 2010, 356, 2699.

² A. Šantić, W. Wrobel, M. Mutke, R.D. Banhatti, K. Funke, Phys. Chem. Chem. Phys., 2009, 11, 5930.