Sandwich-like high-load MXene/polyaniline film electrodes with ultrahigh volumetric capacitance for flexible supercapacitors.

Two-dimensional transition metal carbides/nitrides (MXene) have excellent physicochemical properties, but the restacking of MXene films restricts their development towards flexible supercapacitors with high energy density. Introducing Polyaniline (PANI) into the MXene layer and expanding the interlayer distance of the MXene can reduce the effect of restacking on the MXene after compounding. However, despite the excellent electronic conductivity of the composited MXene/PANI (MP), its internal slow ionic kinetics becomes a fundamental limitation of the electrochemical performance after the MP loading increases. To compensate for this weakness, MP films are often scaled down to a few micrometers in size (<2 mg cm), which limits their development. Here, we introduce α-FeO/MnO (FM) into MP for the first time by designing a sandwich structure, which significantly improves the bulk capacitance. Due to a large number of active sites and good hydrophilic properties on the MXene surface, FM can interact with the MP. By complexing with MP, the accumulation and loss of FM can be reduced. At the same time, the effect of increasing loading on the electrochemical performance of MP can be compensated. The MXene-PANI/α-FeO-MnO/MXene-PANI (MP/FM/MP) electrode still exhibits high capacitive performance (661 F g, 3138 mF cm) when the MP loading reaches 5 mg cm, with excellent mechanical properties and increased flexibility. In addition, the corresponding symmetric supercapacitor also shows a remarkable energy density of 53.32 Wh·L (17.45 Wh kg). This study provides a way to fabricate MXene-based electrodes with high loadings by designing sandwich-structured electrodes.