Nitrogen-doped carbon sheets coated on CoNiO2@textile carbon as bifunctional electrodes for asymmetric supercapacitors

This work reports a facilemethod to prepare N-doped carbon sheets that are uniformly coated on the surface of CoNiO2@activated textile carbon (CS-CoNi@aTC). We used easily polymerized dopamine as the carbon precursor and hierarchical NiCo-LDH nanosheets grown on activated textile carbon as the flexible substrate. The subsequent thermal annealing treatment at 400 degrees C in a nitrogen atmosphere converts polydopamine into amorphous N-doped carbon and simultaneously decomposes the NiCo-LDH into CoNiO2. Both the thickness and mass loading of carbon sheets can be facilely controlled by changing the concentration of dopamine. Besides enhancing the areal capacitance of aTC by adding pseudocapacitance, these carbon sheets also significantly enhance the cycling stability of CoNiO2 through reinforcing the interfacial coupling of CoNiO2 nanosheets and carbon fibers. Acid etching of CoNiO2 leaves behind vertical carbon sheets connected on the aTC substrate (CS@aTC). An aqueous asymmetric supercapacitor (ASC) built with CS-CoNi@aTC and CS@aTC can exhibit remarkable cycling stability with 93% capacitance retention and 100% coulombic efficiency after continuous charging-discharging for 45 000 cycles. An assembled solid-state ASC delivers an areal capacitance of 284 mF cm(-2) and a maximum volumetric energy density of 1.4 mW h cm(-3) while exhibiting good flexibility and mechanical robustness.