Towards enhanced energy density of graphene-based supercapacitors: Current status, approaches, and future directions

Despite high power density, fast charging/discharging rate, and long operational lifetime, large-scale application of supercapacitor (SC) is limited by its intrinsically low energy densities (of 5-8 Wh kg−1 (gravimetric) and 5-8 Wh L−1 (volumetric)), which are at least 10-fold lower than battery. Since the invention of graphene in 2004, graphene-based SCs have set the upper performance limit of the symmetric carbon-based SCs due to superior electrical conductivity and very high accessible surface area of graphene. Still, only two companies have commercialised graphene-based supercapacitors thus far. Their maximum achievable energy density (i.e., 11.65 Wh kg−1) is too low to make them competitive against batteries in high-energy applications. To this end, this comprehensive review focuses on the material- and device-level approaches to high energy density graphene-based conventional macroscale SCs (≥11.65 Wh kg−1) and flexible SCs and microsupercapacitors (≈0.3-10 mWh cm−3; ≈300-16000 μWh cm−2). It includes a description on how each approach is implemented and an explanation of how each can provide effective research results. This review also meticulously discusses the underlying challenges and possible solutions to achieve high energy density graphene-based SCs in practice.