Ultrathin supercapacitor electrodes with high volumetric capacitance and stability using direct covalent-bonding between pseudocapacitive nanoparticles and conducting materials

We introduce high-performance ultrathin supercapacitor electrodes obtained through the direct covalent-bonding layer-by-layer (LbL) assembly (or ligand-exchange LbL assembly) of amine-functionalized multiwalled carbon nanotubes (CNTs) and transition metal oxide nanoparticles (TMO NPs). The main characteristic of our approach is that the internal interfacial resistance of the electrodes can be minimized through the direct covalent-bonding adsorption of densely packed, high-quality TMO NPs onto CNTs without the aid of nonactive binders or insulating NP ligands, and the resulting volumetric capacitance and cycling stability of the electrodes can be significantly enhanced. For this study, well-defined oleic acid-stabilized pseudocapacitive metal oxide nanoparticles (i.e., OA-Fe₃O₄ and OA-MnO NPs) prepared in toluene were densely adsorbed onto the CNT layer due to the high affinity between the surface of the TMO NPs and the NH₂ moieties of the CNTs. The (CNT/OA-Fe₃O₄ NP)₂₀ multilayer electrode exhibited a high volumetric capacitance of 248±15Fcm^-3 (128±7Fg^-1) at 5mVs^-1 despite the intrinsically low specific capacitance of the Fe₃O₄ NPs. Additionally, these film electrodes exhibited high performance stability, maintaining 99.2% of their initial capacitance after 1000 cycles. Furthermore, upon the insertion of OA-MnO NPs with high crystallinity and a high theoretical pseudocapacitance value within multilayers instead of OA-Fe₃O₄ NPs, the formed electrodes (i.e., (CNT/OA-MnO NP)₂₀ multilayers) exhibited a higher volumetric capacitance of 305±10Fcm^-3 (183±5Fg^-1) (at a scan rate of 5mVs^-1) than other conventional ultrathin supercapacitor electrodes, including manganese oxide or iron oxide NPs.