Effects of Carbon Pore Size on the Contribution of Ionic Liquid Electrolyte Phase Transitions to Energy Storage in Supercapacitors

Konstantin Schutjajew,Markus Antonietti,Martin Oschatz,Runyu Yan,Christina Roth

doi:10.3389/fmats.2019.00065

Abstract

Recent research on ionic liquid electrolyte-based supercapacitors indicated the contribution of phase transitions of the electrolytes at high cell voltages to the energy stored. This mechanism can be exploited to significantly increase the energy density of supercapacitors, which up to now remains their major drawback. It was found that these ordering transitions require the presence of mesopores within the carbon electrode materials and that porosity in general is a key factor to trigger them, but details of the mechanism remain unexplained. To get a more profound understanding of this phenomenon, carbon materials with different pore diameters and volumes were synthesized and the effect of those properties on the phase transitions in the ionic liquids was studied by means of cyclic voltammetry. A clear correlation between the peak current and the mesopore volume is revealed and an optimal pore diameter was determined, exceeding which does not improve the phase transition behavior. These findings are useful as guidelines for the rational design of carbon mesopores in order to utilize the new energy storage modes which are neither fully capacitive, nor redox-based.

Highlights

The progressing integration of modern technology into daily life has led to an ever increasing energy demand, causing the exploitation of already scarce fossil fuel resources close to their exhaustion and causing extensive damage to the environment
Carbon materials with two prevailing mesopore diameters were synthesized with different mesopore volumes by using silica templates with different primary particle sizes and different ratios between carbon precursor and template
They were used as supercapacitor electrodes to investigate the influence of mesopore structure on the potential-dependent liquid phase transition of room-temperature ionic liquids (RTILs) under practically relevant conditions

Summary

Introduction

The progressing integration of modern technology into daily life has led to an ever increasing energy demand, causing the exploitation of already scarce fossil fuel resources close to their exhaustion and causing extensive damage to the environment. The electrical double layer capacitor (EDLC) is a possible supplement to the lithium-ion battery (Simon and Gogotsi, 2008, 2013; González et al, 2016; Salanne et al, 2016). The energy in such devices is traditionally stored via electrostatic adsorption of electrolyte ions to the electrode surface, which results in very high power density and exceptionally long cycle life.

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