High-performance all-solid-state flexible micro-supercapacitor arrays with layer-by-layer assembled MWNT/MnO_x nanocomposite electrodes

In this study, we report on the fabrication of high performance planar-type flexible micro-supercapacitor (MSC) arrays using Au electrodes coated with a functionalized multi-walled carbon nanotube (MWNT) film and a layer of MWNT-COOH/MnO_x nanoparticle (NP) composite on top. The MWNT thin film was formed via layer-by-layer (LbL) assembly of MWNTs functionalized with amine groups and MWNTs with carboxylic acid groups in water. The hydrothermally synthesized composite of MWNT-COOH/MnO_x NPs was coated on top of the MWNT film (LbL-MWNT). The addition of MWNT-COOH/MnO_x NP composite as a top layer enhanced the performance of the MSCs dramatically, resulting in a volumetric capacitance of 50 F cm^-3 at a scan rate of 10 mV s ^-1 and a coulombic efficiency of ∼100%. By contrast, a volumetric capacitance of 3.6 F cm^-3 was obtained when using only the LbL-MWNT film. After repetitive operation up to ∼10⁴ times, the capacitance remained at ∼88.3% of the original value. With a deliberate circuit design consisting of serially connected MSC arrays, various light-emitting diodes operating at different bias voltages could be lit. The MSC circuit fabricated on a polyethylene terephthalate (PET) film showed stable electrochemical properties upon 1000 cycles of bending deformation.