| Summary: | 博士 === 國立臺灣科技大學 === 材料科學與工程系 === 106 === High production cost and the relatively low energy efficiency of the vanadium redox flow battery (VRFB) still limit their practicability. Further efforts to reduce the production cost and improve the performance of VRFB should therefore be considered. Developing highly active electrocatalysts and electrode materials with low cost are crucial in VRFB design.
First, we use a simple, green, novel, time-efficient, and potentially cost-effective water activation method to enhance the electrochemical activity of graphite felt (GF) electrodes for vanadium redox flow batteries (VRFBs). The GF electrode prepared with a water vapor injection time of 5 min at 700 °C exhibits the highest electrochemical activity for the VO2+/VO2+ couple among all the tested electrodes. This is attributed to the small, controlled amount of water vapor that was introduced producing high contents of oxygen-containing functional groups, such as –OH groups, on the surface of the GF fibers, which are known to be electrochemically active sites for vanadium redox reactions. Charge–discharge tests further confirm that only 5 min of GF water activation is required to improve the efficiency of the VRFB cell. The average coulombic efficiency, voltage efficiency, and energy efficiency are 95.06%, 87.42%, and 83.10%, respectively, at a current density of 50 mA cm−2. These voltage and energy efficiencies are determined to be considerably higher than those of VRFB cells assembled using heat-treated GF electrodes without water activation and pristine GF electrodes.
Second, we report a facile hydrothermal method to synthesize low-cost, high-catalytic-activity, and stable niobium-doped hexagonal tungsten trioxide nanowires (Nb-doped h-WO3 NWs); these NWs were employed as catalysts to improve the electrocatalytic activity of GF electrodes for use as positive electrodes in an all-vanadium redox flow battery. The effect of Nb doping and its composition on the electrochemical performance of GF electrodes for a VRFB was investigated. Cyclic voltammetry and electrochemical impedance spectroscopy results showed that Nb-doped h-WO3 NWs with a Nb/W atomic ratio of 0.03 exhibited the highest electrocatalytic activities for VO2+/VO2+ couples among all the tested electrodes. This observation was attributed to the optimal Nb-doping concentration producing moderate defect states, thereby creating structural disorders, such as oxygen vacancies, in WO3 and leading to the generation of more active sites for the VO2+/VO2+ redox reaction on the electrode. Moreover, in charge–discharge tests, a VRFB single cell using the Nb-doped h-WO3 NWs (Nb/W = 0.03) catalyst demonstrated an excellent energy efficiency of 78.10% with a current density of 80 mA cm−2. This efficiency is much higher than that demonstrated by VRFB cells with untreated GF (67.12%) and heat-treated GF obtained through conventional method (72.01%). Furthermore, in the stability test of a VRFB single cell with the Nb-doped h-WO3 NWs (Nb/W = 0.03) catalyst, almost no decay of the cell was observed even after 30 cycles. This observation indicates the outstanding stability of the cell during the redox reaction of vanadium ions under highly acidic conditions.
Third, we present a three-dimensional annealed tungsten trioxide nanowire/graphene sheet (3D annealed WO3 NWs/GS) foam as an excellent and low-cost electrocatalyst. It was prepared using VRFB electrodes through the in-situ self-assembly of graphene sheets prepared by mild chemical reduction, followed by freeze-drying and annealing. The 3D annealed WO3 NWs/GS foam exhibits the highest electrocatalytic activities toward the V2+/V3+ and VO2+/VO2+ redox couples among all the tested samples. Charge–discharge tests further confirm that a single flow cell of VRFB using the 3D annealed WO3 NWs/GS foam demonstrates excellent energy efficiencies of 79.49% and 83.73% at current densities of 80 mA cm−2 and 40 mA cm−2, respectively, which is much higher than those of cells assembled with pristine GF and 3D WO3 NWs/GS foam without annealing treatment. Moreover, it shows no obvious degradation after 50 charge–discharge cycles. These results are attributed to the formation of new W–O–C bonds, confirming that the WO3 NWs are anchored strongly to the GS, which is key to facilitating the vanadium redox couples redox reactions. Moreover, the 3D annealed WO3 NWs/GS foam exhibits a 3D hierarchical porous structure, which can provide more surface electroactive sites to improve the electrochemical performance of VRFBs.
Keywords: Vanadium redox flow battery; energy storage device; electrode; electrocatalytic activity; water activation; water vapor; Nb-doped h-WO3 NWs; graphite felt; WO3 nanowires, graphene; foam
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