Electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors: A case study with MgO-decorated few-layer graphene as the electrode
Herein, electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors is elucidated by using MgO-decorated few-layered graphene (FLG) as the electrode material. MgO-decorated FLG (MgO/FLG) is chosen for this study because on 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 educes the possibility of exothermic reactions under charge-discharge cycling leading to an unsafe failure of the supercapacitor. As anticipated, the coin cell supercapacitors fabricated with MgO/FLG symmetric electrodes and non-aqueous BMIM BF4 electrolyte popped-up exothermically (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 the highly exothermic reactions leading to the formation of products such as MgF2 and Mg(OH)2. This study elucidates that the heat production and gaseous species release during charging-discharging cycles is a serious safety concern if the choice of 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 6M KOH aqueous electrolyte in an aqueous supercapacitor, it delivered a discharge capacitance as high as 168 F/g at 0.5 A/g current density. Even at the end of the 5000th cycle with 3 A/g current rate, a discharge capacitance of 101 F/g corresponding to 95% retention was recorded.