Electrochemical capacitors and supercapacitors represent the typical energy storage devices possessing rapid charge/discharge kinetics and a high power density (Zhu et al. in Science 332:1537–1541, 2011; Lang et al. in Nat Nanotechnol 6:232–236, 2011; Simon et al. in Science 343:1210–1211, 2014; Sheberla et al. in Nat Mater 16:220–224, 2017). In general, supercapacitors can be classified as the electrochemical double-layer capacitors (EDLCs) or pseudocapacitors according to the energy storage and conversion mechanisms.