# Time-Engineered Hydrothermal Nb2O5 Nanostructures for High-Performance Asymmetric Supercapacitors

## TL;DR

This study shows how controlling reaction time during hydrothermal synthesis can create better Nb2O5 nanostructures for high-performance supercapacitors.

## Contribution

The novel approach of time-engineered hydrothermal synthesis to optimize Nb2O5 nanostructures for supercapacitors is introduced.

## Key findings

- A 12 h reaction time produces optimal hierarchically porous Nb2O5 nanostructures with enhanced electrochemical performance.
- The NbO-12 electrode shows high capacitance, fast kinetics, and excellent cycling stability over 12,000 cycles.
- An asymmetric supercapacitor using NbO-12 delivers 0.101 mWh cm−2 energy density with stable operation at 1.5 V.

## Abstract

Precise control over nanostructure evolution is critical for optimizing the electrochemical performance of pseudocapacitive materials. In this work, Nb2O5 nanostructures were synthesized via a time-engineered hydrothermal route by systematically varying the reaction duration (6, 12, and 18 h) to elucidate its influence on structural development, charge storage kinetics, and supercapacitor performance. Structural and surface analyses confirm the formation of phase-pure monoclinic Nb2O5 with a stable Nb5+ oxidation state. Morphological investigations reveal that a 12 h reaction time produces hierarchically organized Nb2O5 architectures composed of nanograin-assembled spherical aggregates with interconnected porosity, providing optimized ion diffusion pathways and enhanced electroactive surface exposure. Electrochemical evaluation demonstrates that the NbO-12 electrode delivers superior pseudocapacitive behavior dominated by diffusion-controlled Nb5+/Nb4+ redox reactions, exhibiting high areal capacitance (5.504 F cm−2 at 8 mA cm−2), fast ion diffusion kinetics, low internal resistance, and excellent cycling stability with 85.73% capacitance retention over 12,000 cycles. Furthermore, an asymmetric pouch-type supercapacitor assembled using NbO-12 as the positive electrode and activated carbon as the negative electrode operates stably over a wide voltage window of 1.5 V, delivering an energy density of 0.101 mWh cm−2 with outstanding durability. This study establishes hydrothermal reaction-time engineering as an effective strategy for tailoring Nb2O5 nanostructures and provides valuable insights for the rational design of high-performance pseudocapacitive electrodes for advanced energy storage systems.