# Precursor Concentration-Dependent Sol–Gel Dynamics in Neodymium Oxide: From Gel Framework to Electrochemical Functionality in Asymmetric Supercapacitors

## TL;DR

This paper shows how changing the concentration of a starting material affects the structure and performance of neodymium oxide in supercapacitors.

## Contribution

The study demonstrates that precursor concentration critically influences the electrochemical performance of neodymium oxide electrodes.

## Key findings

- Nd-2 (3 mM) showed the highest crystallinity and electrochemical performance with 20 F cm−2 areal capacitance.
- Nd-2 retained 86.3% capacitance over 12,000 cycles and enabled a supercapacitor with 0.132 mWh cm−2 energy density.
- The 3 mM concentration maximized active surface area and pseudocapacitive behavior.

## Abstract

Rare-earth oxides possess distinctive electronic configurations, tunable oxidation states, and inherent structural robustness, making them highly attractive for advanced energy storage applications. Among these, neodymium oxide (Nd2O3) stands out due to its high surface redox activity, structural stability, and favorable band alignment, enabling efficient charge storage in electrochemical devices. In this study, Nd2O3 electrodes were synthesized via a sol–gel method with systematically varied precursor concentrations (1 mM, 3 mM, and 5 mM) to elucidate the impact of synthesis on crystallinity, morphology, and electrochemical performance. X-ray diffraction (XRD) confirmed the formation of the hexagonal Nd2O3 phase, with the 3 mM sample (Nd-2) exhibiting the sharpest reflections, indicative of enhanced crystallinity and reduced lattice defects. X-ray photoelectron spectroscopy (XPS) revealed trivalent Nd species and both lattice and surface oxygen, providing abundant redox-active sites. Field Emission Scanning Electron Microscope (FE-SEM) showed Nd-2 possessed a hierarchically interconnected fibrous network decorated with fine granules, maximizing active surface area and facilitating rapid ion diffusion. Electrochemical testing demonstrated that Nd-2 achieved an areal capacitance of 20 F cm−2, a diffusion-controlled pseudocapacitive contribution of ~84.9%, and retained 86.3% capacitance over 12,000 cycles. An asymmetric supercapacitor with Nd-2 and activated carbon delivered an energy density of 0.132 mWh cm−2, power density of 1.8 mW/cm2, and 81.1% capacitance retention over 7000 cycles. These results highlight the critical role of precursor concentration in tailoring structure and electrochemical performance, establishing Nd2O3 as a promising electrode for high-performance energy storage devices.