Currently, more than 80% of commercial supercapacitors utilize chemically synthesized carbon nanomaterials which are expensive and necessitates non-renewable resources. Employing renewable, environment friendly and naturally available biomass feedstock as precursor for producing carbon materials is a low-cost and sustainable way for designing the electrodes of supercapacitors. In the present study, high surface area hierarchical porous multilayered graphene-like carbon is obtained via room temperature sono-exfoliation of the activated carbon synthesized via simple and environmentally friendly hydrothermal carbonization and potassium bicarbonate activation of waste hazelnut shells as the precursor. The high surface area graphene-like carbon showed excellent electrochemical performance with specific capacitance of 320.9 F g À1 at 0.2 A g À1 current density and exceptional capacitance retention of 77.8% at 2 A g À1 current density after 10 000 cycles in 1 M Na 2 SO 4 electrolyte. Moreover, flexible supercapacitors fabricated using sonoexfoliated graphene-like activated carbon coated stainless steel mesh electrodes and biopolymer gel electrolyte exhibits an outstanding energy density of 38.7 W h kg À1 and power density of 198.4 W kg À1 . These results show that mechanically exfoliated graphene-like activated carbon derived from hazelnut shells exhibit superior electrochemical performance that can compete with other activated carbon materials used in energy storage devices for real time applications.

Lithium-ion batteries (LIBs) have gained significant market share in the field of consumer electronics, grid storage, and hybrid electric vehicles, because of its exceptional energy densities per unit volume (area or weight) compared to other electrochemical energy storage systems. However, they do approach its maximum practical capacities and efforts are underway to explore future battery systems with enhanced energy density than LIBs. There has been growing interest in incorporating multivalent ions such as Zn 2+ , Mg 2+ , and Al 3+ , since they offer greater volumetric energy densities than its monovalent counterparts like LIBs and sodium-ion batteries. It has long been acknowledged that replacing lithium with magnesium (Mg) ions in battery systems has many potential benefits such as low cost, excellent rate capability, high energy density, ease of handling, and eco-friendly. Yet, a few breakthroughs has been made in the advancement of rechargeable magnesium batteries (RMBs) since its first discovery in 2000. The success of RMBs has been hindered by the slow electrode kinetics and also the formation of passive layers on Mg metal surfaces seriously affecting its performance during Mg 2+ ion magnesiation/demagnesiation. This chapter provides comprehensive knowledge on the background of rechargeable Mg batteries, principles, and cell configuration, discussing key advancements made in the last two decades in terms of its electrode materials and electrolytes. Finally, a brief note on future research strategies is outlined. Highlights• Mg-ion batteries may replace Li-ion batteries to meet the demands of both consumer and industrial energy storage.