Agradecimentos: The authors are very grateful for the financial support from the Brazilian funding agencies CNPq (310544/2019-0 - PQ-2 grant), CAPES PROEX 88887.374731/2019-00, Print-CAPES (8887.572651/2020-00) and FAPESP (2017/11958-1 & 2014/02163-7). The authors gratefully acknowledge the support Shell and the strategic importance of the support given by ANP (Brazil's National Oil, Natural Gas and Biofuels Agency) through the R&D levy regulation. R. G. Freitas wishes to thank CNPq (PQ-2 grant: Process 304442/2019-4). This study was financed in part by the CAPES (Brazil) - Finance Code 001 Ver menos

Abstract: This study investigated flexible, freestanding niobium pentoxide (Nb2O5) decorated multiwalled carbon nanotube (MWCNT) electrode material in a sodium-ion pseudocapacitor and its respective energy storage mechanism. Sodium is an abundant element in the Earth's crust, with attractive sustainability and low-cost appeal for the scientific community and energy markets such as electric vehicles and renewable energies. Combined with niobium pentoxide and carbon nanotubes, sodium ions can perform ultra-fast intercalation into niobium pentoxide and electrostatic adsorption onto carbon high surface areas. Niobium pentoxide particles were investigated using X-ray diffraction via Rietveld refinement, a powerful technique to study crystalline materials' electronic and structural properties that directly influence Na+-ion diffusion. The niobium pentoxide-decorated MWCNT electrode material was investigated within a symmetric supercapacitor (SIC) and as an anode for sodium-ion batteries (SIBs). A finite-length transmission line was used to model the impedance behavior of the solid and liquid phases that comprised the material/solution interface. This numerical modelling allowed investigators to infer the presence of MWCNT as a nanostructured matrix since Nb2O5 dispersive nanoparticles increased the overall pseudopacacitance by 63.2% for Nb2O5/MWCNT compared to MWCNT nanostructured electrodes. Electrochemical findings revealed a maximum capacitance of 192 F g-1 for the SIC full cell, with capacitance retention of 96% after 10,000 cycles. In addition, the SIC delivered outstanding power density from 98.7 to 24,671 W kg-1 and high energy density from 5.4 to 2.7 W h kg-1. In an SIB half-cell configuration, the electrode delivered a reversible sodium-ion storage capacity of ~163 mA h g-1, with a retention capacity of ~68 mA h g-1 after 100 cycles at 25 mA g-1. These systems bridge the gap between supercapacitors and batteries by employing the best properties of them both Ver menos