Pseudohalide-Based Electrolytes for High-Energy Electrochemical Capacitors

Abstract

Redox-active electrolytes and additives offer a large selection of available species serving for supercapacitor energy enhancement. The first group refers to typical organic compounds like hydroquinone with isomeric forms. There are also several reports on methylene blue and indigo carmine compounds tested on multi-walled carbon nanotubes. Nevertheless, these organic admixtures have limited solubility in water, and/or their bulkiness restricts adsorption at the interface. Apart from organic molecules, another group of electroactive species includes halide ions. Initially, the iodide/iodine and bromine/bromide redox couples were introduced to supercapacitors. To the best of our knowledge, this is the first report on the employment of alkali metal and ammonium thiocyanates (SCN-) aqueous solutions serving as an electrolyte for supercapacitors. The investigation presents a comprehensive study including the effect of salt concentration, the impact of counter anion type (K+, Na+, Li+ and NH4 +), the influence of applied current collectors as well as the effectiveness of asymmetric configuration. Our research proves that symmetric AC/AC system can easily operate up to 1.6 V with high capacitance and great cyclability. The thiocyanate redox shuttle with formal redox potential at +0.77 V vs. SHE preserves the positive electrode against oxidation and contrarily to iodides or bromides, oxidized SCN- specimen (e.g. polythiocyanogen) in a polymeric form does not evaporate from the system (unlike I2 or Br2). A comparative study on the effect of salt concentration and dependence on the type of current collectors (gold, titanium, and stainless steel) was performed using the KSCN aqueous solutions at various concentrations (1-7 mol∙L-1). It has been demonstrated that the highest concentration of SCN- specimen (7 mol∙L-1) preserved the best performance whatever the current collector used. However, on Ti and stainless steel current collectors, the activity of SCN- seems to be slightly reduced and more ‘capacitive’ profile has been observed. More in-depth studies demonstrated that the system based on 7 mol∙L-1 KSCN aqueous solution preserved high capacitance values even at 10 A∙g-1 current load and retained high energy density (~30 Wh∙kg-1) up to 10 kW∙kg-1 of power rate. Furthermore, we observed that kind of current collector strongly influences the cycle life of the system. Although the gold current collectors provided the highest capacitance during first 1000 cycles, their cyclability is limited to 2 000 cycles. For stainless steel-based systems, the capacitance retention reached 10 000 cycles at 1.6V voltage range.