Abstract
In this paper, thiourea was successfully grafted onto the surface of acid preprocessed graphite felts [sulfuric acid-treated graphite felt (SA-GFs)] by thiol-carboxylic acid esterification. The thiourea-grafted graphite felts (TG-GFs) were investigated as the positive electrode for vanadium redox flow battery (VRFB). X-ray photoelectron spectroscopy results suggested that thiourea was grafted into the surface of graphite felts. The cyclic voltammetry showed that the peak potential separation decreased by 0.2 V, and peak currents were greatly enhanced on TG-GF electrode compared with SA-GF electrode, implying improved electro-catalytic activity and reversibility of TG-GF electrode toward VO2+/ redox reaction. The initial capacity of TG-GF-based cell reached 55.6 mA h at 100 mA cm−2, 22.6 mA h larger than that of SA-GF-based cell. The voltage and energy efficiency for TG-GF-based cell increased by 4.9% and 4.4% compared with those of SA-GF-based cell at 100 mA cm−2, respectively.
Highlights
Vanadium redox flow battery (VRFB) as energy storage system has caused more and more attention because vanadium redox flow battery (VRFB) displays some advanced characteristics, such as long cycle life, high energy efficiency (EE), and excellent electrochemical reversibility (Bhushan et al, 2019; Li et al, 2019; Xiang and Daoud, 2019; He et al, 2020; Lv et al, 2020)
We report a novel, simple, and mild method for in situ functionalizing graphite felt electrode by grafting thiourea for VRFB
X-ray photoelectron spectroscopy (XPS) spectra for S2p shown in Figures 1D,E indicate that thiourea-grafted graphite felts (TG-graphite felts (GFs)) sample exhibits S2p3/2 and S2p1/2 signals at 163.9 and 165.1 eV, respectively, as well as a trace peak at 168.2 eV (Baker et al, 2004; Huang et al, 2014)
Summary
Vanadium redox flow battery (VRFB) as energy storage system has caused more and more attention because VRFB displays some advanced characteristics, such as long cycle life, high energy efficiency (EE), and excellent electrochemical reversibility (Bhushan et al, 2019; Li et al, 2019; Xiang and Daoud, 2019; He et al, 2020; Lv et al, 2020). The nitrogen-containing groups have been reported to be active toward vanadium redox reactions. He et al (2013) added two organic additives in positive electrolyte, which provided -NH2 group on the surface of the graphite felt and could be employed as active sites for vanadium ion reactions. Thiourea Grafted Graphite Felt Electrode graphene could be greatly improved due to introducing the amine and sulfur functional groups into graphene. The electrocatalytic properties of multi-walled carbon nanotubes toward the VO2+/VO+2 redox couple were improved by surface functional treatments using thiourea as nitrogen and sulfur sources (Li et al, 2017). We report a novel, simple, and mild method for in situ functionalizing graphite felt electrode by grafting thiourea for VRFB. The cells were assembled using TG-GF and SAGF (3 × 3 cm2) as positive electrode, SA-GF as corresponding negative electrode, and perfluorinated ion-exchange membrane as separator in 1.2 M V(III)/V(IV) + 3 M H2SO4 electrolyte
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