Abstract
A comprehensive study on the hydroxyl multiwalled carbon nanotubes (hydroxyl MWCNTs) as catalysts in a positive reaction was performed to improve the efficiency of the vanadium redox flow battery (VRFB). The physicochemical properties of the hydroxyl MWCNT-modified electrode were characterized by using a scanning electron microscope (SEM), conductivity measurement, Brunner–Emmet–Teller (BET) measurement, X-ray photoelectron spectroscopy (XPS) analysis, cyclic voltammetry (CV) tests, electrochemical impedance spectroscopy (EIS) analysis, and charge-discharge tests. The prepared composite electrode possesses a huge amount of oxygen-containing groups, high-specific surface area, high electrical conductivity, and high catalytic activity towards the VO2+/VO2+ reaction based on physicochemical characterization. The hydroxyl MWCNT-modified graphite felt (hydroxyl MWCNTs/GF) shows the best cell performance with the energy efficiency of 79.74% and remains in high stability after 50 cycles. The improved cell performance is probably ascribed to the increase in active sites, fast charge transfer, and mass transfer rate of the introduced hydroxyl MWCNTs.
Highlights
With the gradual depletion of fossil energy, people around the world pay more and more attention to the development and utilization of renewable energy resources
Hydroxyl MWCNTs were used as electrode catalysts, while pristine MWCNTs were used as a comparison. e characteristics of the prepared hydroxyl MWCNTs/GF were tested by using scanning electron microscope (SEM), four-point probes, BET method, and X-ray photoelectron spectroscopy (XPS). e electrochemical performance of the hydroxyl MWCNT-modified electrodes for VO2+/VO2+ redox couple was analyzed by carrying out the tests of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge
Charge-discharge tests were conducted by using an assembly home-made static battery test system. e prepared hydroxyl MWCNT-modified electrode was used as a positive electrode in charge-discharge tests. e anolyte for vanadium redox flow battery (VRFB) was 1 M V3+ + 4.2 M H2SO4, and the catholyte was 1 M VO2+ + 4.2 M H2SO4. e upper and lower voltage limits for the charge-discharge test were 1.65 V and 0.8 V, respectively. e durability of the prepared hydroxyl MWCNTs/GF electrode was evaluated by 50 cycles of charge-discharge tests
Summary
With the gradual depletion of fossil energy, people around the world pay more and more attention to the development and utilization of renewable energy resources. Most investigations are focused on the modification of GF to improve its electrochemical performance. Some literatures have focused on the investigation of CNTs as catalysts for VRFB electrodes. E kinetic properties of V2+/V3+ and VO2+/VO2+ redox couples are improved by employing the graphite/CNT composite as electrodes. Among various kinds of catalysts, hydroxyl MWCNTs exhibit remarkable electrochemical performance. Li et al investigated the MWCNTs functionalized with hydroxyl groups for the VO2+/VO2+ redox reaction [28]. Compared with the pristine MWCNTs, the hydroxyl MWCNTs used as positive-electrode reaction catalysts exhibit better storage efficiency. The catalytic mechanism of the hydroxyl modified MWCNTs for the VO2+/VO2+ redox reaction still remains unclear. E electrochemical performance of the hydroxyl MWCNT-modified electrodes for VO2+/VO2+ redox couple was analyzed by carrying out the tests of CV, EIS, and charge-discharge Hydroxyl MWCNTs were used as electrode catalysts, while pristine MWCNTs were used as a comparison. e characteristics of the prepared hydroxyl MWCNTs/GF were tested by using SEM, four-point probes, BET method, and XPS. e electrochemical performance of the hydroxyl MWCNT-modified electrodes for VO2+/VO2+ redox couple was analyzed by carrying out the tests of CV, EIS, and charge-discharge
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