N,P-Doped Carbon-Based Freestanding Electrodes Enabled by Cellulose Nanofibers for Superior Asymmetric Supercapacitors

The preparation of freestanding electrodes with excellent conductivity and mechanical strength plays a vital role in simplifying the electrode fabrication process and reducing the electrode cost. Herein, a N,P-doped self-supporting carbon electrode is synthesized via a combined templating/activating coassisted carbonization procedure and vacuum filtration process. The glucose precursor was first transformed into N,P-doped carbon nanosheets (NPCNs) with a large specific surface area (2073 m 2 g –1 ) and rich heteroatom-doping (phosphorus: 2.1 atom %; nitrogen: 4.1 atom %) with the assistance of P 2 O 5 and dicyandiamide, and then the compact freestanding electrode (NPCN-f) with outstanding mechanical strength was constructed via vacuum filtration using a mixture of NPCN and conductive cellulose nanofibers. The resultant freestanding electrode exhibits a high capacitance of 318 F g –1 at 1 A g –1 and retains 188 F g –1 at 100 A g –1 in an alkaline electrolyte. Furthermore, excellent electrochemical performances are also exhibited in the asymmetric supercapacitor assembled using NPCN-f as the negative electrode and NPCN/MnO 2 -f synthesized by a self-controlled redox process as the positive electrode. The asymmetric device can deliver a high energy density of 41.5 Wh kg –1 at a power density of 182.0 W kg –1 and excellent cyclability with 93% capacitance retention after 10 000 cycles in a neutral electrolyte. This work sheds light on the design of self-supporting electrodes for advanced energy storage devices.