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

Abstract

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.