Flexible Solid‐State Asymmetric Supercapacitors with Enhanced Performance Enabled by Free‐Standing MXene−Biopolymer Nanocomposites and Hierarchical Graphene−RuOx Paper Electrodes

Abstract Two‐dimensional (2D) transition metal carbides and carbonitrides, called MXenes, with metallic conductivity and hydrophilic surfaces, show great promise as electrode materials for supercapacitors. A major drawback of 2D nanomaterials is the re‐stacking of the nanosheets, which prevents full utilization of surface area and blocks the access of the electrolyte. In this study, a free‐standing nanocomposite paper electrode is realized by combining Mo 1.33 C MXene and positively charged biopolymer lignin (the second most abundant biopolymer in nature, L‐DEA). The self‐assembled layered architecture with alternating polymer and MXene flakes increases the interlayer space to promote ion transport, and with combining charge storage capability of the lignin derivative and MXene in an interpenetrating MXene/L‐DEA nanocomposite, which offers an impressive capacitance of 503.7 F g −1 . Moreover, we demonstrate flexible solid‐state asymmetric supercapacitors (ASCs) using Mo 1.33 C@L‐DEA as the negative electrode and electrochemically exfoliated graphene with ruthenium oxide (EG@RuO x ) as the positive electrode. This asymmetric device operates at a voltage window of 1.35 V, which is about two times wider than that of a symmetric Mo 1.33 C@L‐DEA based supercapacitor. Finally, the ASCs can deliver an energy density of 51.9 Wh kg −1 at a power density of 338.5 W kg −1 , with 86 % capacitance retention after 10000 charge‐discharge cycles.