Electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors: A case study with MgO-decorated few-layer graphene as an electrode material

Herein, electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors are elucidated by using MgO-decorated few-layer graphene (FLG) as an electrode material. MgO-decorated FLG (MgO/FLG) is chosen in this study because on the one hand it has good physical characteristics making it lucrative as an electrode material in graphene based aqueous and non-aqueous electrochemical supercapacitors and on the other hand in MgO/FLG, MgO is chemically bonded to FLG (through either oxygen-mediation (Mg-O-C) or direct bonding with carbon (Mg-C)) and thereby reduces the possibility of exothermic reactions upon charge-discharge cycling leading to an unsafe failure of the supercapacitor. As anticipated, coin cell supercapacitors fabricated with MgO/FLG symmetric electrodes and a non-aqueous BMIM BF₄ electrolyte popped up exothermically (the temperature rise was in the range of 110-165 °C) during the initial charge cycles. Post-cycling X-ray photoelectron spectroscopic analysis of the electrode material revealed that the chemical bonding between MgO and FLG triggered highly exothermic reactions leading to the formation of products such as MgF₂ and Mg(OH)₂. This study elucidates that the heat production and the gaseous species release during charging-discharging cycles are serious safety concerns if the choice of the electrode material and non-aqueous electrolyte (BMIM BF₄ and plausibly PF₆ based electrolytes) combination is not properly made. On the other hand when MgO/FLG was used in combination with a 6 M KOH aqueous electrolyte in an aqueous supercapacitor, it delivered a discharge capacitance as high as 168 F g^-1 at a current density of 0.5 A g^-1. Even at the end of the 5000th cycle with a current rate of 3 A g^-1, a discharge capacitance of 101 F g^-1 corresponding to 95% retention was recorded.