Ionic liquid mixtures for supercapacitor applications: Synergy of electrochemistry, NMR and simulations

We are currently witnessing the rapid development of a large number of energy-storage technologies with a great impact in our daily life. This is visible in a wide range of industrial sectors ranging from portable applications, such as consumer electronics, through autonomous wireless sensor networks and structural health monitoring, to large-scale applications, such as environmentally friendly electric vehicles. To make these applications possible, critical technical specifications regarding fast delivery of energy on minute or even second timescale and long operational lifespan are required. Electrical double layer capacitors (EDLCs), or supercapacitors, have emerged as promising electrochemical energy storage devices. They satisfy the mentioned requirements due to their excellent capacitive properties in terms of high power density, excellent cycling stability and reversibility. Nowadays, the market of EDLCs involves already numerous commercial applications, as for example in electric mobility for stop/start systems in cars, electric drive in buses, etc. Despite the tremendous research effort, the low energy density and the limited electrochemical performance at low temperatures are currently one of the main bottlenecks, hindering the wide-spread application of these ecologically-friendly energy storage systems. Therefore, new perspectives from the material science point of view are crucial to accomplish tomorrow’s technological challenges in this field. In recent years, the synergistic effect produced between novel electrolytes based on ionic liquids (ILs) and the design of porous carbonaceous nanostructure electrodes have paved the way to conceive high performance EDLCs. This was due to the outstanding features of electrodes: a large operating cell voltage, wide working temperature range as well as excellent capacitive properties. However, the energy storage mechanisms involved at the electrode/electrolyte interface during the charging and discharging processes have not been yet fully understood. In order to reach this challenging goal, a selection of different IL mixtures by tuning their anion/cation size, shape, valence and molecular flexibility will be investigated in nanoporous carbons at different working temperatures and operating cell voltages. The correlation of electrochemical performance of the device as a function of the employed IL mixture and the pore size distribution of carbon electrodes will be scrutinized by means of a multi-disciplinary approach based on electrochemistry, modelling, and in-situ nuclear magnetic resonance measurements. As a result, the knowledge acquired in this project will enlighten new horizons to design reliable high performance EDLCs in the near future.