Sustainable and renewable energy sources from hydropower, solar energy and wind power are expected to relieve the heavy burdens on the current energy infrastructure (e.g., limited reserves but increasing depletion of fossil fuels) and environmental concerns (e.g., dramatic climate change and heavy CO2 emission) [1]. As the abundant and readily available energy sources, such as solar energy and wind power, are intermittent in nature, reliable electrochemical energy storage (EES) systems, which primarily include rechargeable batteries, supercapacitors, and fuel cells, are purposely explored to promote efficient utilization of these power sources [2], [3] and [4]. Among these promising EES systems, electrochemical capacitors, which are also known as supercapacitors (a better understanding of the nomenclature refers to [5]), are emerging as an important energy technology [5], [6], [7] and [8]. The Ragone plot in Fig. 1 compares the power and energy capabilities of EES systems. Supercapacitors can deliver 1–2 orders of magnitude higher power density than rechargeable batteries and store much more energy than conventional dielectric capacitors. In combination with the merits of fast charge/discharge rates, a long cycle life, a high Coulombic efficiency, low maintenance cost, and safe operation, supercapacitors offer versatile powering solutions to various applications ranging from portable consumer electronic devices and electric vehicles to large-scale smart utility grids.