Lignin/polypyrrole interpenetrating networks decorated Lignin-containing cellulose nanofibril composite membrane for High-performance supercapacitors.

• Lignin-polypyrrole interpenetrating networks were developed for supercapacitor (SCs). • LCNFs was used as both flexible substrates and active materials for SCs. • The capacitive strengthening mechanism of lignin was systematically investigated. • The mechanical strengthening mechanism of composite membrane was studied. Lignin-containing cellulose nanofibrils (LCNFs) are emerging and promising multifunctional nanomaterials for economical and eco-friendly flexible supercapacitors (SCs). However, the strengthening mechanism of lignin on the capacitive and mechanical performance of LCNF-based nanocomposite electrodes has not been deeply studied. Herein, a mechanically strong nanocomposite electrode with excellent capacitive performance was fabricated by decorating a sodium lignosulphonate/polypyrrole (LS/PPy) interpenetrating network onto LCNFs, the relationship between lignin content, LCNFs morphology and electrodes performance was systematically investigated. Results showed that lignin could effectively strengthen the capacitive and mechanical performance of the electrodes via providing pseudocapacitance generated by the reversible conversion of quinone/hydroquinone and controlling the fibrillation degree of LCNFs. With decreasing the lignin content, LCNFs gradually transformed from nanofibril bundles to uniform nanofibrils, forming an electrode with hierarchical porous structure, highly interconnected conductive network, high active site exposure rate and excellent capacitive and mechanical performance. However, insufficient lignin could weaken the capacitive and mechanical performance of the electrodes via reducing quinone/hydroquinone content, electrodes porosity and LCNFs strength. Therefore, electrodes with lignin content of 11.4% displayed excellent tensile strength (20.46 MPa) and areal specific capacitance (2567 mF cm−2, 1 mA cm−2). The assembled all-solid-state supercapacitor (ASSC) delivered high coulombic efficiency (∼98%) and energy density, 88.6 μWh cm−2 (499.9 μW cm−2) and 1.74 mWh cm−3 (9.8 mW cm−3) for areal and volumetric energy (power) density, respectively. This work systematically reveals the strengthening mechanism of lignin on the capacitive and mechanical performance of the electrodes and provides theoretical guidance for constructing high-performance LCNF-based nanocomposite electrodes. [ABSTRACT FROM AUTHOR]