Jean LE BIDEAU1, Nicolas DEMARTHE1, Thibaud GUILLEMIN1, Yann CLAVEAU1
1Nantes Université - Institut des Matériaux de Nantes, Nantes, France
After almost 20 years since their first developments, ionogels continue to be the subject of intensive research. Their performances, the increased safety they bring, their industrializable formulations motivate this attraction. The current work sometimes shows an increasing complexity of the confining network, as well as confirming the versatility of ionogels for most ionic liquids.
Within an ionogel, a solid network confines a major part of the ionic liquid, with the formation of two continuous interconnected networks. These are biphasic electrolytes, with a real liquid state demonstrated at the micron scale. The solid state of ionogels does not affect the performance, which remains competitive with conventional liquid electrolytes, and sometimes even with enhanced performances.
The effect of the interface is one of the determining elements: it locally modifies the physics of ionic liquids, through short-range anion-cation interactions, but also limits the size of the aggregates. In other words, the interface moderates some of the detrimental characteristics of ionic liquids. Examples of a wide range of studies, both with rigid inorganic confining arrays and with polymeric and dynamic confining arrays at the nanometer scale will be presented. Preferential segregation effects of certain species at the interfaces can also be exploited in order to modify the cation-anion interaction, and thus favor the diffusion at the interface of certain ions of interest: it is here the balance of the chemistry of the confining network and that of the confined species that must be found. Evidences of such preferential segregation will be presented. Modelling will be of interest for a wide screening of the various parameters.
The degree of confinement itself, i.e. the quantity of ionic liquid confined and the dimensions of the confinement space, influences the macroscopic performances: on the one hand the ionic conductivity is proportional by definition to the quantity of ionic species, but on the other hand the ratio between the interface surface area and the quantity confined must be optimized. Some physical effects such as changes in phase transition temperatures are often reported, while others such as the equivalence of the pressure increase in situ are less so.
The diversity of applications is increasing: supercapacitors, batteries, photovoltaics, low temperature fuel cells, sensors, actuators, electronics, vacuum or high temperature environment ... Their processing is undergoing spectacular developments, motivated by the diversity of applications, but also by the versatility of the chemistry, from liquid to solid phase, used for the synthesis of ionogels. As an example, a focus on microdevices with interdigitated 3D electrodes with 40-fold area enhancement surface factor will be presented.