Flexible Solid-State Supercapacitors with Outstanding Capacitive Performance Enabled by N/B-Codoped Porous Carbon Nanosheets

With the rapid development of flexible solid-state supercapacitors, there is a pressing need for designing and fabricating thin-film electrodes with excellent capacitive performance. Herein, we report the fabrication of freestanding N/B-codoped carbon films by pyrolyzing ammonium pentaborate-templated tris(2-chloroethyl)amine hydrochloride, followed by vacuum filtration. The as-fabricated N/B-codoped carbon nanosheets (namely, BNC-850) with a thickness of around 4.0 nm have a hierarchical pore structure, a high specific surface area (883 m2 g–1), and high N/B contents (9.6 atom % N and 3.9 atom % B). Benefiting from the graphene-like structure and the high level of N/B-codoping, the freestanding BNC-850 film electrode manifested both high specific capacitance (361 F g–1 at 0.5 A g–1) and high rate capability (83.1% capacitance retention at 1.0–100 A g–1) in an aqueous alkali electrolyte. In addition, flexible solid-state supercapacitors assembled with the BNC-850 film electrode also manifested high capacitance (281 F g–1 at 1.0 A g–1) and high rate capability (76.6% capacitance retention at 1.0–50 A g–1) in an alkali poly(vinyl alcohol) (PVA)–KOH gel electrolyte together with high energy density (30.14 Wh kg–1) in a neutral sodium carboxymethyl cellulose (CMCNa)–Na2SO4 gel electrolyte. Considering that such N/B-codoped carbon nanosheets can be easily fabricated on a large scale using inexpensive and commercially available precursors and templates, they should have great potential for application in flexible solid-state supercapacitors.