A method for reducing pressure build-up in an electrochemical cell is provided comprising: providing an anhydrous nonaqueous electrolyte solution comprising at least one quaternary ammonium salt and at least one anhydrous nonaqueous solvent; contacting the electrolyte solution with at least one acid scavenger; and loading the nonaqueous electrolyte solution into the electrochemical cell. Also, provided is an electric double-layer capacitor (supercapacitor) comprising a free acid scavenger.