Electrochemical pseudo-capacitor materials that can deliver high power and store high capacity energy for applications that required faster rate performance than traditional energy storage battery are needed. Amorphous Nb₂O₅ has been evaluated as a pseudo-capacitor. Amorphous thin film (550 nm) was synthesized by RF magnetron sputtering. The morphology of the Nb₂O₅ nanoparticle thin films was smooth as evaluating in field emission scanning electron microscope (FE-SEM) with nanoparticle size ranged between 5 and 15 nm. A cross-sectional FE-SEM image shows thickness was approximately 550 nm with excellent adhesion to the SS substrates. XPS revealed that the as-deposited materials are Nb₂O₅.

To understand the electrochemical redox reactions, cyclic voltammetry (CV) and galvanostatic discharge-charge (GDC) experiments were performed at different rates. The CVs were performed at scan rates of 0.1, 1, 2, and 5 mV/s. For example, for the scan rate at 1 mV/s a sharp peak current is observed at ~0.85 V during the first cycle which has been assigned to the formation of the solid electrolyte interface (SEI) and reduction of Nb⁺⁵ to Nb⁺⁴. As the cathodic sweep continues to zero volt no conversion reaction took place. The anodic sweep shows two broad and weak peak currents at ~0.7 and ~1.5 V. During the second cycle, the peak current at ~0.85 V disappeared which is typical of irreversibility and loss of capacity in the first cycle. All subsequent cycles were smooth with quasi rectangular CV curves which indicated the pseudo-capacitive characteristics of amorphous Nb₂O₅. This is opposite to sharp peak currents typically observed in CV that are characteristics of battery electrodes. Ex-situ XPS were performed on the sample stopped at 1.0 V during the reverse potential sweep and 1.84 V during the forward potential sweep. A very thick SEI layer was observed when the sample was stopped and analyzed at 1.0 V. NbOx was observed after fifteen minutes of sputtering at 1 KeV Ar+ and Nb^+4/+5 after 90 min of sputtering. The SEI is composed of carbonates (Li₂CO₃), aliphatic carbon species, and fluorides (LiF). The capacity observed during the first cycle was 436 mAh/g at C/3 rate. After 100 cycles, the coulumbic efficiency was maintained at 100% and the capacity retention was 160 mAh/g. The materials responded very well to different cycling rates.