| Summary: | 博士 === 國立臺灣科技大學 === 材料科學與工程系 === 107 === As one of the most promising electrochemical energy storage systems, vanadium redox flow battery (VRFB) has received increasing attention due to its attractive features for large-scale storage applications. However, high production cost and the relatively low energy efficiency still limit their feasibility. Therefore, developments of powerful electrocatalyst and electrode materials with low cost are critical for the design of VRFB. To improve the energy density and overall performance for large scale applications, extensive research has been carried out on the electrode modification methods for VRFB.
First, to increase the electrocatalytic activity of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs) toward the VO2+/VO2+ redox couple, we prepared stable, high catalytic activity, and uniformly distributed hexagonal Ta2O5 nanoparticles on the surface of GF by varying the Ta2O5 contents. Scanning electron microscopy (SEM) revealed the amount and distribution uniformity of the electrocatalyst on the surface of GF. It was found that the optimum amount and uniformly immobilized Ta2O5 nanoparticles on GF surface provided the active sites, enhanced hydrophilicity and electrolyte accessibility, thus remarkably improved electrochemical performance of GF. In particular, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results showed that the Ta2O5-GF nanocomposite electrode with weight percentage of Ta2O5 to GF of 0.75 wt% exhibited the best electrochemical activity and reversibility toward the VO2+/VO2+ redox reaction, when compared with the other electrodes. The corresponding energy efficiency was enhanced by ~ 9% at a current density of 80 mA cm−2, as compared with untreated GF. Furthermore, the charge–discharge stability test with 0.75 wt% Ta2O5-GF electrode at 80 mA cm−2 showed that after 50 cycles, there was no obvious attenuation of efficiencies signifying, the best stability of Ta2O5 nanoparticles which strongly adhered on the GF surface.
Second, we synthesized simple, inexpensive, and conductive W18O49 nanowires (W18O49NWs) as electrocatalysts on the surface of GF through the one-step solvothermal process. Cyclic voltammetry and electrochemical impedance spectroscopy studies revealed that W18O49NWs exhibit electrocatalytic effects on a VO2+/VO2+ redox couple on the positive side, which enhance the electrochemical kinetics of the redox reactions. To further improve the electrochemical performance of the W18O49NWs, the sample was thermally annealed with a controlled amount of H2/Ar atmosphere to form oxygen-vacancy–rich hydrogen-treated W18O49NWs (H-W18O49NWs). When used as an electrode in a VRFB single cell, this material demonstrated outstanding performance with 9.1% and 12.5% higher energy efficiency than cells assembled with W18O49NWs and treated GF, respectively, at a high current density of 80 mA cm−2. The superior performance of the H-W18O49NW electrocatalyst-based electrode can be attributed to the presence of numerous oxygen vacancies, which were proven to act as active sites for the VO2+/VO2+ redox reaction. Moreover, the uniformly immobilized and 1D nature of the W18O49NWs facilitated the charge-transport process, enhanced hydrophilicity and electrolyte accessibility, and thus remarkably reduced electrochemical polarization during the mass transfer of active species. The long-term cycling performance confirmed the outstanding durability of the as-prepared H-W18O49NWs–based electrode with negligible activity decay after 100 cycles.
Third, we use a simple, low-cost, and powerful titanium niobium oxide–reduced graphene oxide (TiNb2O7–rGO) nanocomposite electrocatalyst which was synthesized through dispersion and blending in aqueous solution followed by freeze-drying and annealing for all-vanadium redox flow battery (VRFB). The TiNb2O7 nanoparticles are uniformly anchored between the rGO sheets; simultaneously, the rGO sheets are separated using TiNb2O7 nanoparticles. The synergistic effects between them prevent the agglomeration of the nanoparticles and restacking of the rGO sheets. Cyclic voltammetry and electrochemical impedance spectroscopy results reveal that among all prepared samples, the TiNb2O7–rGO nanocomposite electrocatalyst exhibits the most favorable electrocatalytic activity toward VO2+/VO2+ and V3+/V2+ at the positive electrode and the negative electrode, respectively, to facilitate the electrochemical kinetics of the vanadium redox reactions. The corresponding energy efficiency is improved by ~11.1% and 12.34% at current densities of 80 and 120 mA cm−2, respectively, compared with pristine graphite felt. The superior performance of the TiNb2O7–rGO nanocomposite electrode may have been due to the synergistic effects related to the high electronic conductivity of rGO nanosheets and the interfacial properties created within TiNb2O7 and rGO. Furthermore, the charge-discharge stability test demonstrates the outstanding stability of the TiNb2O7–rGO electrodes. The TiNb2O7–rGO-based VRFB exhibits negligible activity decay after 200 cycles. The remarkable electrocatalytic activity and mechanical stability are achieved due to the TiNb2O7–rGO nanocomposite being strongly anchored on the graphite felt surface for a substantial time during repetitive cycling.
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