Comparative Internal Pressure Evolution at Interfaces of Activated Carbon for Supercapacitors Containing Electrolytes Based on Linear and Cyclic Ammonium Tetrafluoroborate Salts in Acetonitrile

Abstract In this study, the real‐time increase in pressure of the accumulated gases at the electrode/electrolyte interface serves as a safety criterion for four conductive electrolytes comprising acetonitrile (ACN) and organic salts. They include tetrafluoroborate as an anion and cyclic 1,1‐dimethylpyrrolidinium (Pyr11+), spiro‐(1,1′)‐bipyrrolidinium (SBP+), acyclic methyl triethyl ammonium (Et3MeN+) or standard tetraethylammonium (Et4N+) as cations. The main focus lies on the SPBF4/ACN system. While the concentrated Pyr11BF4/ACN exhibits a minimal pressure evolution (≈25 Pa) under ambient conditions at 3.0 V, its electrochemical stability is inferior to SPBF4 at high operating voltage. The electrolytes with acyclic tetrafluoroborate salts (1.0 mol L−1) reveal a 20‐fold increase in pressure due to the weak salt‐ACN interactions and the subsequent high solvent evaporation. The pressure evolution at the interface of activated carbon/electrolyte in electrochemical double layer capacitor (EDLCs) is merely related to the operating voltage and cation nature, viz. Pyr11+ < SBP+ < Et4N+ < Et3MeN+. The fixed specific capacity of 109 F g−1, volumetric capacity of 76 F cm−3, and moderate gas generation (≈190 Pa at 3.0 V, that shifts to ≈400 Pa at 3.4 V) confirm the safe character of the SPBF4/ACN electrolyte for such energy storage devices.