Anodized Nb_2O_5: application for gas sensors, lithium-ion batteries and dye-sensitized solar cells

In this research, the PhD candidate pursued the development of hydrogen (H₂) gas sensors, lithium-ion batteries (LIBs) and dye-sensitized solar cells (DSSCs) based on anodized nanoporous niobium pentoxide (Nb₂O₅). The PhD candidate successfully synthesized two types of nanoporous Nb₂O₅ morphologies via electrochemical anodization techniques: highly ordered nanoporous vein-like structures and nanochannels. The physical and chemical properties of Nb₂O₅, at its various stoichiometries, can be favorability tuned to obtain enhanced sensitivity to selected target gas species. Highly ordered nanoporous structures offer high surface to volume ratios, suitable surface energies, and an optimum spacing for the interactions with many target gas molecules. As such, the anodized Nb₂O₅ was investigated for gas sensing in this PhD research. The sensor found to exhibit better performance with faster response and recovery time when compared to that Nb₂O₅ H₂ gas sensors previously reported. The study showed that the enhanced performance was due to increased surface area and the reduced embedded ionic defects. Considering the continuous and highly packed vein-like network with many lateral interconnections, which provide excellent channels for the fast transfer of both Li⁺nanoporous vein-like structures films for the development of superior electrodes for LIBs. From the experiment, the LIBs had delivered durable capacity within the operating voltage window of 1.0–3.0 V vs. Li/Li⁺, with a reversible capacity of 201 mAh g^–1
Furthermore, this device also demonstrates safe LIBs operation due to a higher, V ≥ 1.0, discharge cut-off voltage which reduces dangerous high-temperature reactions, some of the features that evidenced the augmented performance of the created electrodes. In this research, the PhD candidate chose Nb₂O₅ as a suitable photoanode material for DSSCs due to its wide band gap, the favourable location of the conduction band edge and long electron lifetime. Photoanode with nanoporous vein-like structures and nanochannels morphologies were also expected to absorb a high amount of loaded dye. After a comprehensive investigation, the PhD candidate believed that the high efficiencies of DSSCs based on anodized Nb₂O₅ films were prompted by the combination of their reduced electron scattering, wider bandgap and higher conduction band edge, as well as longer effective electron lifetime. However, the anodized Nb₂O₅ films impeded by a higher number of recombination centres, promoting more obstructed free charge carrier transport has hindered the performance of DSSCs. As a result, the PhD candidate discovered that the selection of electrolyte composition during anodization is essential in order to produce reduced impurity-driven defect states in anodized nanoporous Nb₂O₅. In summary, the author believes that the outcomes of this PhD research provide readers with an in-depth knowledge regarding the capabilities that anodized Nb₂O₅ films provide in enhancing effects for specific applications. The author also believes that this study has contributed significantly towards in the advancement of functional transition metal oxides field and creating exciting new knowledge.