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We report an electrode material for supercapacitors composed of nickel oxide (NiO) nanoparticles supported onto radially oriented multi-walled carbon nanotubes (CNTs) using a stainless-steel fine-mesh as the support (AISI:CNT-NiO). CNT scaffolds showed a turbostratic multi-walled structure with an interplanar spacing of 0.32 ± 0.02 nm and a diameter of ∼20–100 nm. NiO nanoparticles exhibited a diameter of ∼2–7 nm. X-ray data confirmed the presence of NiO in the scaffold. A large pseudocapacitive voltage range of 2.0 V was obtained in a 1.0 M Li2SO4 aqueous solution. The main contribution to the overall pseudocapacitance is due to the presence of reversible solid-state surface Faradaic reactions involving the Ni(II)/Ni(III) redox couple. High specific capacitance values of ∼1200 F g−1 at 5 A g−1 for the AISI:CNT-NiO electrode were extracted from galvanostatic discharge curves. Considering the contribution of negative voltages, the specific power and energy determined using cyclic voltammetry exhibited values of ∼140 Wh kg−1 and ∼9 W kg−1, respectively, at 0.02 V s−1. A specific capacitance of ∼1028 F g−1 was obtained at this scan rate. Even after 40,000 cycles carried out under galvanostatic conditions, the symmetric coin cell remained stable with a very high coulombic efficiency of ∼99%, which is a remarkable result. Also, we attributed to carbon nanotubes an extraordinary stability as electron drain on the current collector. The morphology factor analysis revealed that 19% of the electrochemically active surface area is confined to the inner surface regions of the porous nanostructured active layer. A low value of 0.15 mΩ g was extracted for the equivalent series resistance. New insights are presented concerning the true meaning of negative voltages for coin cells. Interesting findings regarding the porous nature of electrodes were elucidated using the impedance technique.

Different nanostructured mixed metal oxide (MMOs) electrodes containing Sn, Ru, and Ir supported on titanium with the nominal composition Ti/[Sn0.5Ru(0.5−x)Ir(x)]O2 were prepared using the drop-coating method. SEM analysis revealed the presence of oxide islands in the microsized domains of ca. 50 μm separated from each other by cracks. On the contrary, analysis of the nanosized domains revealed the presence of nanostructures containing rectangular grains with an average diameter of ca. 200 nm. The discrepancy between the nominal and real oxide compositions was attributed to volatilization of Sn during the heat treatment. XRD analysis revealed a poor degree of crystallinity and an average crystallite size of 15 ± 4 nm. Different electrochemical studies revealed higher specific pseudocapacitances in the range of 175–222 F g−1 for the MMO containing 50 mol% Ir. The low values obtained for the morphology factor (0.11 ≤ φ ≤ 0.33) indicate a relatively narrow interval of 11–33% for the active surface regions confined to the inner surface regions of the oxide layers. The chronoamperometric findings obtained for short times (t ≤ 50 ms) permitted evaluation of the electrical double-layer capacitance (Cedl), with verified maximum values in the range of 68–75 F g−1 for the MMO containing 50 mol% Ir. On the contrary, the experimental findings obtained for long times (0.1 s ≤ t ≤ 3 s) revealed that the reversible solid-state Faradaic reactions did not exhibit a Fickian behavior. The impedance analysis revealed that MMOs exhibit a porous electrode behavior (De Levie’s model) permitting the determination of the pseudocapacitance and the resistance of the electrolyte inside the pores.

A novel and scalable method is reported to produce flexible and freestanding fabrics composed of high surface area activated carbon nanofibers (aCNF) for applications in supercapacitors. The electrochemical behavior of a symmetric supercapacitor using the aCNF electrodes and the operando Raman spectra study accomplished under polarization conditions are reported. The evolution of Raman spectra during polarization supported the aCNF stability and suggested the insertion of the HSO4− ions in carbon micropores. These findings evidenced a new symmetry at the aCNF/electrolyte interface, where electronic and ionic charges accumulate. The distributed capacitance in the time domain was studied by numeric differentiation of galvanostatic charge-discharge findings. The impedance behavior of the solid and liquid phases composing the aCNF/solution interface was modeled using a macro homogeneous description of two closely mixed phases represented by a single-channel transmission line incorporating the anomalous transport of the ionic charges in the disordered structure of aCNF. Electrochemical findings revealed outstanding charge-storage properties in neutral aqueous electrolyte resulting in long lifespan, low equivalent series resistance (12 mΩ g), high coulombic efficiency (∼99.8 %), a maximum distributed capacitance of 82 F g−1 (0.25 A g−1-1.1 V), maximum specific energy and power of 2.98 W h kg−1 and 72,672 W kg−1, respectively.

Vertically aligned carbon nanotubes/graphene oxide nanocomposite (VACNT-GO) has been prepared and applied as electrode material. First, dense packets of VACNT were prepared in microwave chemical vapor deposition reactor, and then functionalized by oxygen plasma etching. We observed that oxygen plasma could exfoliate carbon nanotubes tips and provide oxygen group attachment on its surface, changing its wettability as well. This change in wettability of the VACNT is crucial for its electrochemical application, since as-grown VACNT is super-hydrophobic. After exfoliation and functionalization, the electrochemical tests were performed using potassium ferrocyanide. The cyclic voltammetry (CV) and impedance spectroscopy revealed fast electron transfer kinetics on this new material. The CV peak potential separation was 59 mV, suggesting ideal reversibility at the electrode. The Nyquist and Bode plots were well-fitted as modified Randles equivalent electrical circuits with non-charge transfer impedance. This new highly porous nanostructures have been intensively characterized by scanning electron microscopy, Brunauer–Emmett–Teller surface area, surface wettability, Raman and X-ray photoemission spectroscopy. Our results suggest this new material has a relevant potential for future applications in electrocatalysis and (bio)sensors.