Flexible robust binder-free carbon nanotube membranes for solid state and microcapacitor application

We present a liquid phase post synthesis self-assemble protocol that transforms trillions of carbon nanotubes (CNTs) in powder form into densely packed flexible, robust and binder-free macroscopic membranes with a hierarchical pore structure. We employ charge transfer engineering to spontaneously disperse the CNTs in a liquid medium. The processing protocol has limited or no impact on the intrinsic properties of the CNTs. As the thickness of the CNT membrane is increased, we observed a gradual transition from high flexibility to buckling and brittleness in the flexural properties of the membranes. The binder-free CNT membranes have bulk mass density greater than that of water (1.0 g cm^-3). We correlate the mass of the CNTs in the membrane to the thickness of the membrane and obtained a bulk mass density of ∼1.11 g cm^-3 ± 0.03 g cm^-3. We demonstrate the use of the CNT membranes as electrode in a pristine and oxidized single/stacked solid-state capacitor as well as pristine interdigitated microcapacitor that show time constant of ∼32 ms with no degradation in performance even after 10 000 cycles. The capacitors show very good temperature dependence over a wide range of temperatures with good cycling performance up to 90 °C. The specific capacitance of the pseudocapacitive CNT electrode at room temperature was 72 F g^-1 and increased to 100 F g^-1 at 70 °C. The leakage current of bipolar stacked solid state capacitor was ∼100 nA cm^-2 at 2.5 V when held for 72 h.