Fitting the porous texture of carbon electrodes to a binary ionic liquid electrolyte for the realization of low temperature EDLCs

Abstract

We report on electrical double-layer capacitors (EDLCs) operating effectively at low temperature (down to −40 °C) while implementing nanoporous carbon electrodes and an ionic liquid (IL) electrolyte. For this purpose, the binary mixture of [EMIm][FSI] and [EMIm][BF4] in 1:1 mol ratio has been selected, since it has been previously shown that it remains liquid down to its vitrification at −97 °C, unlike the parent ILs with melting points of −13 °C and 14 °C, respectively. To enhance the mass transport of the IL ions, especially at a low temperature, where the IL electrolyte exhibits a high viscosity, carbon electrode materials with a substantial share of mesopores serving as passageways for bulky ions were selected. These were a carbon black (SC2A) with a broad range of interparticle mesopores in addition to a reasonable amount of micropores, as well as a home-made templated carbon (MP98B) with hierarchical porous texture made of interconnected micropores and well-defined mesopores. From 20 to −40 °C, the capacitor with MP98B electrodes displayed greater specific capacitance, energy and power as well as better charge propagation than the cell with SC2A, whereas on volumetric basis its performance was inferior due to lower electrode density. Advanced SCMP composite electrodes of intermediate density were formulated by mixing these two carbons in the 1:1 mass ratio and harnessed to realize EDLCs with enhanced both specific and volumetric energy. Owing to the adjusted electrode properties, the performance of SCMP-based EDLCs surpassed literature data obtained with analogous constructions based on IL electrolytes and lightweight mesoporous materials, which suffered from poor volumetric metrics.