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Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries
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Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries Jiugen Qiu a, Baobing Huang a, Yuchuan Liu a, Dongyang Chen b,∗, Zailai Xie a,∗
Q1
a b
Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, College of Chemistry, Fuzhou University, Fuzhou 350116 Fujian, China College of Materials Science and Engineering, Fuzhou University, Fuzhou 350116 Fujian, China
a r t i c l e
i n f o
Article history: Received 14 July 2019 Revised 17 September 2019 Accepted 24 September 2019 Available online xxx Keywords: Vanadium redox flow batteries Carbon nanoparticles Graphite felts Hydrothermal carbons Glucose
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a b s t r a c t Fabricating of high performance electrodes by a sustainable and cost effective method is essential to the development of vanadium redox flow batteries (VRFBs). In this work, an effective strategy is proposed to deposit carbon nanoparticles on graphite felts by hydrothermal carbonization method. This in-situ method minimizes the drop off and aggregation of carbon nanoparticles during electrochemical testing. Such integration of felts and hydrothermal carbons (HTC) produces a new electrode that combines the outstanding electrical conductivity of felts with the effective redox active sites provided by the HTC coating layer. The presence of the amorphous carbon layers on the felts is found to be able to promote the mass/charge transfer, and create oxygenated/nitrogenated active sites and hence enhances wettability. Consequently, the most optimized electrode based on a rational approach delivers an impressive electrochemical performance toward VRFBs in wide range of current densities from 200 to 500 mA cm−2 . The voltage efficiency (VE) of GFs-HTC is much higher than the VEs of the pristine GFs, especially at high current densities. It exhibits a 4.18 times increase in discharge capacity over the pristine graphite felt respectively, at a high current density of 400 mA cm−2 . The enhanced performance is attributed to the abundant active sites from amorphous hydrothermal carbon, which facilitates the fast electrochemical kinetics of vanadium redox reactions. This work evidences that the glucose-derived hydrothermal carbons as energy storage booster hold great promise in practical VRFBs application. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction The large consumption of fossil fuels with accompanied environmental pollutions has stimulated extensive efforts to explore sustainable and clean energy technologies [1–5]. Redox flow batteries (RFB) as a prototype of electrochemical energy storage devices have gained research interests due to the ability to harvest the intermittency and distribution of renewable energies, such as wind energy, hydropower, solar energy and more [6–9]. Among species of RFB, vanadium-based redox flow batteries (VRFBs) presents significantly interest, because VRFBs enable energy storage on a large scale and provide the perfect solution for stationary applications [10–12]. Moreover, VRFBs is operational flexible, because the energy capacity and power generation can be adjusted independently of one another [13–15]. However, the energy efficiency of VRFBs is determined by the electrochemical activity of the porous electrode, which is preferable to have a high electroactive surfaces and conductivity for the redox reactions [16–18]. In view of these ∗
Corresponding authors. E-mail addresses: [email protected] (D. Chen), [email protected] (Z. Xie).
issues, a highly efficient and low-cost electrode is highly desirable for VRFBs. It is of great significance to develop advanced electrodes or electrocatalysts to reduce overpotentials of the cell in VRFBs [19–22]. Carbon nanostructures, including graphite felts, carbon papers, and carbon cloths, emerged as a most promising class of electrodes for VRFBs. These free-standing nanocarbons possess inherent advantages of low-cost, high-conductivity, acid-resistant ability, excellent chemical stability, and mechanical properties, to name just a few examples [23–27]. However, these perfect carbon nanostructures are chemical inert due to the fact that they are incompatible with aqueous solution and appearing large ohmic resistance [28,29]. These negative impacts can cause large overpotential in the flow cell system due to the slow charge transfer and unsatisfactory activation polarization, further limiting its wide-spread application in VRFB. To address the issues, their surface modification has been extensively investigated to improve electrochemical properties [17,30–32]. So far, several approaches have been applied to implement the carbon-based electrodes activation, such as surface oxidation by acid or base, heteroatom doping and surface decoration [17,30–32].
https://doi.org/10.1016/j.jechem.2019.09.030 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Although the poor electrochemical activity of graphite felts with the smooth surface, the free-standing and porous features of graphite felts indeed make them suitable to be conducting skeletons to load other active components enabling the construction of active electrodes. For this purpose, intense research has focused on the graphite felt loaded with external functional groups, metal and metal oxides [8–10]. It was realized that the electrocatalytic activities, such as the rate performance and capacity retention capability, of carbon modified graphite felts can be significantly improved with respect to pristine felts [8–10]. For instance, Kim et al. reported that the graphite felts doped with nitrogen species via the in situ polymerization of polypyrrole (PPy) in the presence of Co salts onto their surfaces could largely improve the catalytic activity towards VO2+ /VO2 + redox reactions due to the incorporaion of abundant active nitrogen groups [31]. Zhao and Cho and co-workers fabricated carbon nanoparticle-decorated graphite felt electrode for vanadium redox flow batteries by the post-deposition of biomass-derived carbons [9,33]. Xi et al. demonstrated that bismuth can effectively inhibit hydrogen side reactions over a wide temperature range, while promoting V2+ /V3+ redox reactions; the same group also modified graphite felt electrode with high activity cerium zirconium mixed oxide by thermal polymerization method [34,35]. The improved electrochemical activity was ascribed to that the biocarbons possess abundant oxygen functional groups and defects, which can serve as active sites and enhance wettability, leading to an improved electrochemical performance [8]. However, this post-deposition approach has been discouraged because of poor stability and low reactivity as a result of inhomogeneous mixing and weak interaction between external carbons and felts. Such inhomogeneity in the battery felt can lead to impermeable areas that reduce performance and stability. There is thereby an urgent need, but it is still a significant challenge to rationally design high-quality, homogeneous and chemically stable felts-based composites for redox reactions to lead a high VRFBs performance. Herein we have successfully prepared a novel kind of decorated graphite felts, on which carbon spheres or carbon layers are homogeneously anchored on the surface by using HTC of glucose. This in-situ method minimizes the drop off and aggregation of carbon nanoparticles. Moreover, hydrothermal carbons onto the surface of felts combines both advantages of the outstanding electrical conductivity of felts with the effective redox active sites provided by the doped nitrogen in the HTC coating layer. As a result, the optimized GFs by the HTC process deliver a significantly enhanced electrocatalytic performance for VRFBs under wide range of current densities (from 200 to 500 mA cm−2 ). The charge potentials for GFs-HTC are lower than that of pristine GFs at each current density while the discharge potentials for GFs-HTC are higher. This enhanced performance is attributed to the synergistic effects of the conductive felts and their N/O co-doped hydrothermal carbons layers.
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2. Experimental
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2.1. Materials
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Commercially available graphite felts (GFs) were bought from Gansu Haoshi Carbon Fiber Co., Ltd, which were rinsed in ethanol and deionized water binary solution (volume ratio=1:1) under ultrasonication to remove impurities before use. Nafion 212 membrane was purchased from Dupont Company and treated by standard acid boiling procedure before use. Glucose and urea were used of analytical grade.
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2.2. Electrode preparation
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The glucose and urea were dissolved in 30 ml deionized water with strong stirring for 30 min. Then, above mixtures and GFs
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(the mass ratio of GFs (3 cm × 3 cm, 0.45 g), glucose (0.1125 g) and urea (0.1125 g) is 4:1:1) were transferred into a Teflon-lined autoclave and stayed for 12 h. After that, the Teflon-lined autoclave was transferred to oven at 180 °C for 12 h. When cooling down naturally in air, the sample was washed with deionized water, and then dried in a vacuum oven at 80 °C for 6 h. Finally, the sample was annealed in a tube furnace at 10 0 0 °C for 2 h under N2 atmosphere. The product was denoted as GFs-HTC-1. The mass ratio of 2:1:1, 1:1:1 for the GFs, glucose and urea were also prepared, which were denoted as GFs-HTC-2 and GFs-HTC-3, respectively. For comparison, the GFG or GFU were fabricated by the same procedure but only with glucose or urea. A GFs-10 0 0 was synthesized by the same process without glucose and urea.
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2.3. Physical characterization
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The surface morphology of GFs and the GFs-HTC-1 to 3 were characterized by a scan electron microscopy (SEM, Hitachi S-4800) operating at 5 kV. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area, which was measured by Micromeritics ASAP 2020 plus/2060. Elemental composition was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250), The Raman spectra was measured by HR Evolution Raman system with a 532 nm laser excitation. The wettability was tested by contact angle tester (Theta, Attension, Sweden).
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2.4. Electrochemical measurements
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Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed by IVIUM-n-Start electrochemical workstation (instrument from IVIUM, Netherland) with three-electrode system. Platinum electrode and standard Calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. GFs/GFs-HTC-1 to 3 (1 cm × 1 cm) was employed as working electrode. The CV of positive and negative tests were carried out in 0.1 M VO2+ + 3 M H2 SO4 and 0.1 M V3+ + 3 M H2 SO4 , respectively. The voltage windows of 0.5 to 1.25 V and −0.8 to −0.2 V were used for the positive and negative tests. The EIS of VO2+ /VO2 + and V2+ /V3+ redox reactions was performed on fixed voltages of 1.0 V and −0.4 V, respectively. All CV and EIS reports were referenced to SCE in this work. The single-cell tests were the same as that reported in the previous literatures, which were carried out on integrated liquid flow battery test system (instrument from China Guangzhou Lambowang Machinery Co., Ltd. ZSHG-ATG-18). The cutoff voltage of charge and discharge is 1.7 and 0.7 V, respectively. Two pieces of GFs-HTC-1 (3 cm × 3 cm) were use as electrode, the separator was use Nafion 212 membrane (Dupont, America, 7 cm × 7 cm). The electrolytes of the positive electrode and the negative electrode are 1 M VO2+ + 3 M H2 SO4 and 1 M V3+ + 3 M H2 SO4 , which volume was 40 mL each side. The charge-discharge performance test was carried out at the current density of 100 to 500 mA cm−2 , and the cycle performance test was performed at a current density of 200 mA cm−2 for 150 cycles.
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3. Results and discussion
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3.1. Structure and surface chemistry of electrode materials
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Our fabrication methodology to synthesis HTC carbons decorated graphite felts is illustrated in Scheme 1. During the HTC assembly process, glucose first adsorbed to graphite felts and then dehydrated to 5-hydroxymethyl furfural [20]. The condensation reaction between 5-hydroxymethyl furfural and urea took place on graphite felts, resulting in the formation of hydrothermal carbon
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Scheme 1. Illustration of the synthetic route to graphite felts covered by N-doped hydrothermal carbon nanoparticles.
Fig. 2. Raman spectra of the GFs and hydrothermal treated GFs.
Fig. 1. SEM images of (a) pristine felt, (b) GFs-HTC-1, (c) GFs-HTC-2, (d) GFs-HTC-3, (e, f) high magnification images of GFs-HTC-1 and GFs-HTC-3.
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nanoparticles [20,21]. The prepared composites were further subjected to carbonization at 10 0 0 °C for 2 h in N2 atmosphere to obtain conductive carbons. It is found that HTC carbon particles strongly adhere to the surface of graphite felts, and thus exposed HTC carbons can be regarded as an active center for VRFBs redox reactions. With the introduction of nitrogen, HTC carbon materials show enhanced higher conductivity and enhanced wettability towards aqueous solution, as confirmed by electrochemical impedance spectroscopy and contact angle measurement. Scanning electron microscopy (SEM) revealed the surface morphology and microstructure of the GFs and GFs-HTC after hydrothermal carbonization and annealing process. The pristine felt presents a clean and smooth surface as shown in Fig. 1(a), while the hydrothermal treated felt GFs-HTC-1, in which the initial ratio of the felt and glucose was 4:1, appears a great number of the homogeneous and sphere-like carbon particles on the surface of felts. The formed carbon nanoparticle on the felt shows the mean size of about 300 nm which is very close to the standard HTC material of uniform spherical particles (Fig. S1). The microstructure of HTC
carbon on the felt changes dramatically when the initial ratios of the felt and glucose were 2:1 for GFs-HTC-2 or 1:1 for GFs-HTC3, resulted in a layer/aggregation of carbon on the surface of felt (Fig. 1c, d and f). This significant change in microstructure by the HTC process could be due to the strong interaction between the polar functional groups of hydrothermal carbon and the topological structure of the felt. In consistent with the SEM images, the specific surface area of GFs and GFs-HTC-1 to 3 are 4.17, 5.87, 6.70, 9.98 m2 g−1 , respectively. This may be related to the presence of hydrothermal carbon, providing abundant pathways for electrolyte transport. Raman spectra were presented in Fig. 2 to illustrate the defect concentration of the GFs and hydrothermal treated GFs. Two typical strong bands located at ca. 1347 cm−1 (D band) and 1580 cm−1 (G band) were clearly observed, arising from the disordered carbon structures and the vibration mode to the movement in opposite directions of two carbon atoms in a graphene domain, respectively [36]. Following the reported strategy, the Raman spectra were deconvoluted in order to calculate the ID /IG ratio. The derived ID1 /IG values for the HTC treated felts present an increased trend, which are 1.80∼1.85 for GFs-HTC compared to GFs (1.73). This indicates that a large quantity of defects and more structural disorder were included within felts by HTC process. X-ray photoelectron spectroscopy (XPS) in Fig. 3(a) show the coexistence of C, N and O elements in GFs after hydrothermal treatment, confirming the successful doping of heteroatoms into the graphite felt. The N contents are found to be at 0.78 at.%– 1.3 at.% for three materials, while the O content fluctuates within a certain range from 5 at.% to 10 at.%. High resolution XPS
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 4. The water contact angles of the materials (a) pristine felt, (b) GFs-HTC-1, (c) GFs-HTC-2, (d) GFs-HTC-3.
Fig. 3. (a) XPS survey scans of GFs and three GFs after HTC treatment, (b, c) high resolution of C 1 s and N 1 s spectrum of GFs-HTC-3.
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spectrum of C 1 s region for GFs-HTC-3 is shown in Fig. 3(b), which can be deconvoluted into three peaks, i.e. sp2 carbon, C—O— /C—N—containing function groups and O=C—O containing function groups, assigned to binding energies of ca. 285, 285.8 and 287.5 eV, respectively [37]. More importantly, the N1s spectrum in Fig. 3(c) is deconvoluted into three types of configurations located at 398.3, 400.1 and 401.3 eV and assigned to pyridinic-N (N1), pyrrolic-N (N2), and graphitic-N (N3) species, respectively [21,31]. The presence of nitrogen is expected to exert a considerable influence on electrochemical performance, because the planar pyridinic-N with lone electron pair is regarded as the most active configuration to endow adjacent carbon atoms with Lewis basicity and high electron density, thus beneficial for promoting the adsorption of positively charged vanadium ions and thus improving ion change during redox process [38,39]. The changes in surface chemistry and microstructure of GFs by HTC process were further investigated by water contact angle measurements which were performed using the water droplet method.
The surface hydrophobicity/hydrophilicity of the materials can be expressed by the water contact angle, as smaller contact angle means better wettability. As observed in Fig. 4, the contact angle of pristine GFs is 134.09°. By contrast, the values are 120.64°, 98.21°, and 89.48° for GFs-HTC-1 to 3, respectively. There is a slight increase in the contact angle of GFG, GFU and GFs-10 0 0 as shown in Fig. S2. This indicates that the wettability of hydrothermal treated GFs is substantially improved compared with GFs because of N/O functional groups and defects on the surface of GFs generated in hydrothermal treatment process, which is in agreement with XPS and Raman measurements. The good wettability of electrode materials is favorable to the mass transfer of active species.
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3.2. Performance of vanadium redox flow batteries
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Based on the unique properties of GFs after hydrothermal treatment, we suppose that the HTC modified GFs can deliver high VRFBs performance. Generally, the potential gap between redox peaks, onset redox potential, redox peak currents and their ratio (Ipc /Ipa ) are key parameters for comparing electrocatalytic activities. Fig. 5(a) presents the CV curves of the positive electrode reaction corresponding to VO2+ /VO2 + redox couples. The onset reduction potential of GFs-HTC-3 appeared a positive shift to 0.755 V (vs. SCE) at the anodic reaction associated with VO2 + →VO2+ , while the onset oxidation potential from VO2+ →VO2 + reaction showed a negative shift to 1.025 V (vs. SCE). The potential gap (࢞Ep ) between redox peaks of GFs-HTC-3 towards the VO2+ /VO2 + couple was 0.27 V after 10th cycles, much smaller than that of the pristine GFs (0.445 V). Besides, both the oxidation and reduction peak current density of GFs-HTC-3 were quite stable after 30th cycles, indicating the high electrochemical stability of GFs-HTC (Figs. 5b and S3a), which was probably due to strong interaction between N-doped hydrothermal carbons and GFs. The improved onset potentials for both anodic and cathodic processes were beneficial for electron transfer kinetics at lower applied voltage, leading to enhanced energy storage efficiency for VRFBs. As for the negative electrode (Fig. 5d), the anodic and cathodic peaks of GFs-HTC corresponding to the V2+ /V3+ couple appear at −0.3 to −0.4 V and −0.7 to −0.8 V (vs. SCE), respectively. This redox reaction was enhanced significantly for GFs-HTC-3, as its redox peaks nearly mirrored each other. Notably, the performance V2+ /V3+ appeared unstable for all electrodes after 20th cycles, implying that the poor performance of the V2+ /V3+ reduction reaction in negative half-cell was substantially affected by undesired hydrogen evolution known as a side reaction in the VRFBs system [38,40]. This phenomenon can also be observed in the pristine GFs
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Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 5. CV curves at a scan rate of 5 mV s−1 in positive electrode: (a) GFs/GFs-HTC-1 to 3, (b) 1–30 cycles CV curves of GFs-HTC-3 and (c) comparison of the 10th cycle of CV curves. The CV curves at scan rate of 5 mV s−1 in negative electrode: (d) GFs/GFs-HTC-1 to 3, (e) 1–20 cycles GFs-HTC-3 and (f) comparison of the 2nd cycle of CV curves. (g, h) CV curves at different scan rates of GFs-HTC-3 and GFs. (i) Plots of the redox peak current density versus the square root of scan rate for GFs and GFs-HTC-3.
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(Fig. S3b). However, the potential gap for the GFs-HTC-3 (࢞Ep = 0.315 V) was much smaller than that of GFs (0.48 V) at the 2nd cycle, suggesting that the electron transfer kinetic for the V2+ /V3+ redox couple were largely improved for GFs-HTC-3. These CV results clearly show that the hydrothermal carbons are efficient energy storage booster for VRFBs, by enhancing the performance of both positive (VO2+ /VO2 + ) and negative (V2+ /V3+ ) electrode reactions. The CV curves of positive and negative electrode reaction of GFG, GFU, and GFs-10 0 0 are show in Fig. S3(c, d). To understand the kinetics of VO2+ /VO2 + redox reaction, the CV curves of the GFs-HTC-3 and pristine GFs electrodes at different scan rates were recorded. The ratios of the redox peak currents are used to evaluate the reversibility of the reaction. It was found that the Ipc /Ipa value of GFs-HTC-3 was ∼ 0.9, much higher than that of GFs (∼ 0.5), indicating much better reversibility and higher catalytic activity of GFs-HTC-3. The mass transfer rate for vanadium ion couples was calculated by plotting the peak current density versus the square root of scan rate. The slope of GFs-HTC-3 was slightly steeper than that of GFs, suggesting the larger mass transfer rate of the former. These results are in line with the above results on the surface chemistry and microstructure of GFs-HTC, in which the excellent wettability of N-doped hydrothermal carbon decorated GFs leads them to having better reversibility and catalytic activity of vanadium redox reaction. The catalytic mechanism of GFs-HTC was further studied by electrical impedance spectroscopy (EIS). As shown in Figs. 6 and S4, the Nyquist plots of all electrodes showed a small
quasi-semicircle in the high frequency region, which was related to the interfacial charge transfer resistance at the electrodeelectrolyte interface [15,41]. It could be seen that the GFs-HTC had much smaller semicircle radiuss than the pristine GFs, demonstrating much lower charge transfer resistances [42]. The lowest charge transfer resistance of GFs-HTC-3 indicated the highest reaction rate and excellent catalytic activity towards vanadium redox reactions, which agreed well with the CV results. We also measured the galvanostatic charge-discharge profiles of the electrodes with current densities ranging from 100 to 500 mA cm−2 . Fig. 7(a–d) showed the charge-discharge profiles of all samples between 0.7 and 1.7 V (the charge-discharge profiles of GFU, GFG and GFs-10 0 0 are showed in Fig. S5). It could be seen that all the charge potentials for GFs-HTC were lower than that of pristine GFs at each current density while the discharge potentials for GFs-HTC were higher. This implied the glucosederived carbon decorated GFs had much lower polarization than pristine GFs during the battery operation at all current densities. The coulombic efficiency (CE) increased with the increasing current density, because of the shorter time at higher current density for the permeation of vanadium ions across the membrane (self-discharge); the voltage efficiency (VE) decreased with the increasing current density, because of the higher polarization at higher current density. It could be seen that while the CEs for all carbons were similar, the VEs of GFs-HTC were much higher than the VEs of the pristine GFs, especially at high current densities. The GFs-HTC-3 exhibited the highest VE at each current density,
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 6. Nyquist plots for different graphite felt electrodes across the in 0.1 M VO2+ + 3 M H2 SO4 and 0.1 M V3+ + 3 M H2 SO4 solution at an open-circuit potential (1.0 V for positive reaction and −0.4 V for negative reaction).
Fig. 7. Charge-discharge curves of batteries with the presented GFs electrodes at various current densities (a–d); and coulombic efficiency, voltage efficiency and energy efficiency (e,f).
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Fig. 8. (a) The specific discharge capacity of VRFBs as a function of cycle number at different current densities, (b) cycling performance of the optimized GFs-HTC-3 and GFs at a current density of 200 mA cm−2, (c) capacity fading curve of GFs-HTC3 and GFs at a current density of 200 mA cm−2 .
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which was in good agreement with the EIS result that it had the lowest charge-transfer resistance. Consequently, the GFs-HTC-3 exhibited the highest energy efficiency (EE) than all the other samples (Figs. 7e and S6). Remarkably, the pristine GFs was unable to be charged and discharged when current density was higher than 400 mA cm−2 . In contrast, the GFs-HTC-3 was able to deliver an EE of 65% at the very high current density of 500 mA cm−2 , which was outstanding even when compared with many other advanced electrode materials in literatures. The rate performance of GFs and GFs-HTC were presented in Fig. 8(a). It can be seen that the discharge capacity of GFs-HTC were much higher than that of the pristine GFs. For example, the discharge capacity of GFs-HTC-3 was boosted to 209.2 mAh, which was 4.18 times higher than that of GFs (50.1 mAh) at 400 mA cm−2 . What’s more, capacity retention at current density of 500 mA cm−2 was 58.53%. This proved that GFs-HTC-3 had excellent rate performance. In view of above, the GFs-HTC-3 was the most optimized electrode based on a rational approach delivering an impressive catalytic performance in VRFBs. This electrode was chosen to perform a long-term cycling test to evaluate the durability under a high current density of 200 mA cm−2 . As shown in Fig. 8(b), the CE for GFs-HTC-3 was nearly the same during the whole cycling test (96.5%), while the EE for GFs-HTC-3 remained at 82.9% with a small decay rate of 1.43% per cycle, which is much better than GFs. The capacity fading curve was shown in Fig. 8(c), it was obvious that capacity decay rate of GFs-HTC-3 is less than that of GFs, GFsHTC-3 can still reach 64.02 mAh with 150 cycles, while GFs has a capacity of only 13.4 mAh with 76 cycles. The good stability can be further supported by the SEM images of the GFs-HTC-3 after electrochemical testing, in which HTC carbon particles are still strongly adhere to the surface of GFs (Fig. S7). The excellent electrochemical activity and stability are presumably due to the N-doped amorphous carbon coating, which promotes the mass/charge transfer and thus increasing the electrochemical activity, and creates oxygenated/nitrogenated active sites and hence enhances wettability. In order to further prove that GFs-HTC-3 has excellent battery performance, we compared the VRFBs performance of the Nafion 212 (N212, 50 μm) with that of the benchmark commercial Nafion 115 (N115, 125 μm) membrane at a current density of 200 mA cm−2 . The results show that GFs-HTC-3 still performs well after N212 was replaced by N115. CE, EE and charge-discharge curves were shown in Fig. 9. It can be seen that greater CE was achieved by replacing N212 with N115, this is due to CE is mainly determined by vanadium ions permeability and side reactions. Before the test, N2 was injected into the negative electrode, thus inhibiting the side reaction of V2+ in the negative electrode. So CE is determined by the permeability of vanadium. The thick membrane can inhibit the vanadium ions crossover to improve the CE of the
Fig. 9. The coulombic efficiency, energy efficiency and charge-discharge curves of GFs-HTC-3 electrodes with different Nafion membranes.
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 10. The charge-discharge curves at the current densities of (a) 20 0 and (b) 40 0 mA cm−2 ; (c) the coulombic and voltage efficiencies and (d) the energy efficiency for VRFBs for GFs-HTC-3 to 5.
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battery, so use N115 can get greater CE than N212. As for the decrease of EE, the main reason is that the thick membrane has large Ohmic resistance, which directly reduces the VE of the battery and further leads to the decrease of EE [43]. Because of the larger thickness and area resistance, higher charge voltage and lower discharge voltage of N115 were observed in Fig. 9(b), resulting in a slight drop in battery capacity, that is consistent with previous reports [43,44]. This work further evidences that GFs-HTC-3 electrode hold great promise in practice VRFBs application. To optimize the electrode materials, the mass ratio of GFs, glucose and urea increased to 2:3:3 and 1:2:2, which were denoted as GFs-HTC-4 and GFs-HTC-5, respectively. Fig. 10(a, b) show the charge-discharge curves of GFs-HTC-4 and GFs-HTC-5 at current densities of 300 and 400 mA cm−2 . The corresponding coulombic and voltage efficiencies, charge-discharge curves are presented in Fig. 10(c, d). It is found that the battery with GFs-HTC-3 still presents the highest capacities, voltage efficiency and energy efficiency. With the increase of glucose and urea mass, the performance for VRFBs show a decreasing trend. It may be due to the aggregated HTC particles, which tend to detach from the surface of graphite felt under flowing condition.
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4. Conclusions
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In summary, we have successfully synthesized a new and highly efficient GFs-based electrode for VRFBs by using hydrothermal carbonization of glucose and urea. This facile synthetic strategy allows the homogeneous deposition of HTC amorphous carbons on the graphite felts and endows them with the features of abundant oxygen and nitrogen-containing functional groups. Further advantages of our approach are the use of glucose and urea which is sustainable and has a much lower cost compared with other C/N precursors. As a result, the optimized GFs-HTC electrode delivers a
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significantly boosted electrocatalytic performance for VRFB under wide range of current densities. The VEs and EEs of GFs-HTC are much higher than the VEs of the pristine GFs, especially at high current densities. Long term cycling test indicates an exceptional durability of the GFs-HTC electrode at a high current density of 200 mA cm−2 . The boosted performance is most likely due to the coating of hydrothermal carbons that can promote mass/charge transfer and thus reduce charge transfer resistance. We thus believe that these unique HTC carbon decorated felts hold great promise for practice VRFBs application.
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Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
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This work was supported by the Award Program for Fujian Minjiang Scholar Professorship and the National Natural Science Foundation of China (21571035).
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Supplementary material
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.030.
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Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries Jiugen Qiu a, Baobing Huang a, Yuchuan Liu a, Dongyang Chen b,∗, Zailai Xie a,∗
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a b
Fujian Provincial Key Laboratory of Electrochemical Energy Storage Materials, College of Chemistry, Fuzhou University, Fuzhou 350116 Fujian, China College of Materials Science and Engineering, Fuzhou University, Fuzhou 350116 Fujian, China
a r t i c l e
i n f o
Article history: Received 14 July 2019 Revised 17 September 2019 Accepted 24 September 2019 Available online xxx Keywords: Vanadium redox flow batteries Carbon nanoparticles Graphite felts Hydrothermal carbons Glucose
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a b s t r a c t Fabricating of high performance electrodes by a sustainable and cost effective method is essential to the development of vanadium redox flow batteries (VRFBs). In this work, an effective strategy is proposed to deposit carbon nanoparticles on graphite felts by hydrothermal carbonization method. This in-situ method minimizes the drop off and aggregation of carbon nanoparticles during electrochemical testing. Such integration of felts and hydrothermal carbons (HTC) produces a new electrode that combines the outstanding electrical conductivity of felts with the effective redox active sites provided by the HTC coating layer. The presence of the amorphous carbon layers on the felts is found to be able to promote the mass/charge transfer, and create oxygenated/nitrogenated active sites and hence enhances wettability. Consequently, the most optimized electrode based on a rational approach delivers an impressive electrochemical performance toward VRFBs in wide range of current densities from 200 to 500 mA cm−2 . The voltage efficiency (VE) of GFs-HTC is much higher than the VEs of the pristine GFs, especially at high current densities. It exhibits a 4.18 times increase in discharge capacity over the pristine graphite felt respectively, at a high current density of 400 mA cm−2 . The enhanced performance is attributed to the abundant active sites from amorphous hydrothermal carbon, which facilitates the fast electrochemical kinetics of vanadium redox reactions. This work evidences that the glucose-derived hydrothermal carbons as energy storage booster hold great promise in practical VRFBs application. © 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
1. Introduction The large consumption of fossil fuels with accompanied environmental pollutions has stimulated extensive efforts to explore sustainable and clean energy technologies [1–5]. Redox flow batteries (RFB) as a prototype of electrochemical energy storage devices have gained research interests due to the ability to harvest the intermittency and distribution of renewable energies, such as wind energy, hydropower, solar energy and more [6–9]. Among species of RFB, vanadium-based redox flow batteries (VRFBs) presents significantly interest, because VRFBs enable energy storage on a large scale and provide the perfect solution for stationary applications [10–12]. Moreover, VRFBs is operational flexible, because the energy capacity and power generation can be adjusted independently of one another [13–15]. However, the energy efficiency of VRFBs is determined by the electrochemical activity of the porous electrode, which is preferable to have a high electroactive surfaces and conductivity for the redox reactions [16–18]. In view of these ∗
Corresponding authors. E-mail addresses: [email protected] (D. Chen), [email protected] (Z. Xie).
issues, a highly efficient and low-cost electrode is highly desirable for VRFBs. It is of great significance to develop advanced electrodes or electrocatalysts to reduce overpotentials of the cell in VRFBs [19–22]. Carbon nanostructures, including graphite felts, carbon papers, and carbon cloths, emerged as a most promising class of electrodes for VRFBs. These free-standing nanocarbons possess inherent advantages of low-cost, high-conductivity, acid-resistant ability, excellent chemical stability, and mechanical properties, to name just a few examples [23–27]. However, these perfect carbon nanostructures are chemical inert due to the fact that they are incompatible with aqueous solution and appearing large ohmic resistance [28,29]. These negative impacts can cause large overpotential in the flow cell system due to the slow charge transfer and unsatisfactory activation polarization, further limiting its wide-spread application in VRFB. To address the issues, their surface modification has been extensively investigated to improve electrochemical properties [17,30–32]. So far, several approaches have been applied to implement the carbon-based electrodes activation, such as surface oxidation by acid or base, heteroatom doping and surface decoration [17,30–32].
https://doi.org/10.1016/j.jechem.2019.09.030 2095-4956/© 2019 Published by Elsevier B.V. and Science Press on behalf of Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Although the poor electrochemical activity of graphite felts with the smooth surface, the free-standing and porous features of graphite felts indeed make them suitable to be conducting skeletons to load other active components enabling the construction of active electrodes. For this purpose, intense research has focused on the graphite felt loaded with external functional groups, metal and metal oxides [8–10]. It was realized that the electrocatalytic activities, such as the rate performance and capacity retention capability, of carbon modified graphite felts can be significantly improved with respect to pristine felts [8–10]. For instance, Kim et al. reported that the graphite felts doped with nitrogen species via the in situ polymerization of polypyrrole (PPy) in the presence of Co salts onto their surfaces could largely improve the catalytic activity towards VO2+ /VO2 + redox reactions due to the incorporaion of abundant active nitrogen groups [31]. Zhao and Cho and co-workers fabricated carbon nanoparticle-decorated graphite felt electrode for vanadium redox flow batteries by the post-deposition of biomass-derived carbons [9,33]. Xi et al. demonstrated that bismuth can effectively inhibit hydrogen side reactions over a wide temperature range, while promoting V2+ /V3+ redox reactions; the same group also modified graphite felt electrode with high activity cerium zirconium mixed oxide by thermal polymerization method [34,35]. The improved electrochemical activity was ascribed to that the biocarbons possess abundant oxygen functional groups and defects, which can serve as active sites and enhance wettability, leading to an improved electrochemical performance [8]. However, this post-deposition approach has been discouraged because of poor stability and low reactivity as a result of inhomogeneous mixing and weak interaction between external carbons and felts. Such inhomogeneity in the battery felt can lead to impermeable areas that reduce performance and stability. There is thereby an urgent need, but it is still a significant challenge to rationally design high-quality, homogeneous and chemically stable felts-based composites for redox reactions to lead a high VRFBs performance. Herein we have successfully prepared a novel kind of decorated graphite felts, on which carbon spheres or carbon layers are homogeneously anchored on the surface by using HTC of glucose. This in-situ method minimizes the drop off and aggregation of carbon nanoparticles. Moreover, hydrothermal carbons onto the surface of felts combines both advantages of the outstanding electrical conductivity of felts with the effective redox active sites provided by the doped nitrogen in the HTC coating layer. As a result, the optimized GFs by the HTC process deliver a significantly enhanced electrocatalytic performance for VRFBs under wide range of current densities (from 200 to 500 mA cm−2 ). The charge potentials for GFs-HTC are lower than that of pristine GFs at each current density while the discharge potentials for GFs-HTC are higher. This enhanced performance is attributed to the synergistic effects of the conductive felts and their N/O co-doped hydrothermal carbons layers.
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2. Experimental
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2.1. Materials
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Commercially available graphite felts (GFs) were bought from Gansu Haoshi Carbon Fiber Co., Ltd, which were rinsed in ethanol and deionized water binary solution (volume ratio=1:1) under ultrasonication to remove impurities before use. Nafion 212 membrane was purchased from Dupont Company and treated by standard acid boiling procedure before use. Glucose and urea were used of analytical grade.
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2.2. Electrode preparation
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The glucose and urea were dissolved in 30 ml deionized water with strong stirring for 30 min. Then, above mixtures and GFs
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(the mass ratio of GFs (3 cm × 3 cm, 0.45 g), glucose (0.1125 g) and urea (0.1125 g) is 4:1:1) were transferred into a Teflon-lined autoclave and stayed for 12 h. After that, the Teflon-lined autoclave was transferred to oven at 180 °C for 12 h. When cooling down naturally in air, the sample was washed with deionized water, and then dried in a vacuum oven at 80 °C for 6 h. Finally, the sample was annealed in a tube furnace at 10 0 0 °C for 2 h under N2 atmosphere. The product was denoted as GFs-HTC-1. The mass ratio of 2:1:1, 1:1:1 for the GFs, glucose and urea were also prepared, which were denoted as GFs-HTC-2 and GFs-HTC-3, respectively. For comparison, the GFG or GFU were fabricated by the same procedure but only with glucose or urea. A GFs-10 0 0 was synthesized by the same process without glucose and urea.
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2.3. Physical characterization
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The surface morphology of GFs and the GFs-HTC-1 to 3 were characterized by a scan electron microscopy (SEM, Hitachi S-4800) operating at 5 kV. The Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area, which was measured by Micromeritics ASAP 2020 plus/2060. Elemental composition was analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250), The Raman spectra was measured by HR Evolution Raman system with a 532 nm laser excitation. The wettability was tested by contact angle tester (Theta, Attension, Sweden).
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2.4. Electrochemical measurements
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Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed by IVIUM-n-Start electrochemical workstation (instrument from IVIUM, Netherland) with three-electrode system. Platinum electrode and standard Calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. GFs/GFs-HTC-1 to 3 (1 cm × 1 cm) was employed as working electrode. The CV of positive and negative tests were carried out in 0.1 M VO2+ + 3 M H2 SO4 and 0.1 M V3+ + 3 M H2 SO4 , respectively. The voltage windows of 0.5 to 1.25 V and −0.8 to −0.2 V were used for the positive and negative tests. The EIS of VO2+ /VO2 + and V2+ /V3+ redox reactions was performed on fixed voltages of 1.0 V and −0.4 V, respectively. All CV and EIS reports were referenced to SCE in this work. The single-cell tests were the same as that reported in the previous literatures, which were carried out on integrated liquid flow battery test system (instrument from China Guangzhou Lambowang Machinery Co., Ltd. ZSHG-ATG-18). The cutoff voltage of charge and discharge is 1.7 and 0.7 V, respectively. Two pieces of GFs-HTC-1 (3 cm × 3 cm) were use as electrode, the separator was use Nafion 212 membrane (Dupont, America, 7 cm × 7 cm). The electrolytes of the positive electrode and the negative electrode are 1 M VO2+ + 3 M H2 SO4 and 1 M V3+ + 3 M H2 SO4 , which volume was 40 mL each side. The charge-discharge performance test was carried out at the current density of 100 to 500 mA cm−2 , and the cycle performance test was performed at a current density of 200 mA cm−2 for 150 cycles.
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3. Results and discussion
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3.1. Structure and surface chemistry of electrode materials
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Our fabrication methodology to synthesis HTC carbons decorated graphite felts is illustrated in Scheme 1. During the HTC assembly process, glucose first adsorbed to graphite felts and then dehydrated to 5-hydroxymethyl furfural [20]. The condensation reaction between 5-hydroxymethyl furfural and urea took place on graphite felts, resulting in the formation of hydrothermal carbon
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Scheme 1. Illustration of the synthetic route to graphite felts covered by N-doped hydrothermal carbon nanoparticles.
Fig. 2. Raman spectra of the GFs and hydrothermal treated GFs.
Fig. 1. SEM images of (a) pristine felt, (b) GFs-HTC-1, (c) GFs-HTC-2, (d) GFs-HTC-3, (e, f) high magnification images of GFs-HTC-1 and GFs-HTC-3.
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nanoparticles [20,21]. The prepared composites were further subjected to carbonization at 10 0 0 °C for 2 h in N2 atmosphere to obtain conductive carbons. It is found that HTC carbon particles strongly adhere to the surface of graphite felts, and thus exposed HTC carbons can be regarded as an active center for VRFBs redox reactions. With the introduction of nitrogen, HTC carbon materials show enhanced higher conductivity and enhanced wettability towards aqueous solution, as confirmed by electrochemical impedance spectroscopy and contact angle measurement. Scanning electron microscopy (SEM) revealed the surface morphology and microstructure of the GFs and GFs-HTC after hydrothermal carbonization and annealing process. The pristine felt presents a clean and smooth surface as shown in Fig. 1(a), while the hydrothermal treated felt GFs-HTC-1, in which the initial ratio of the felt and glucose was 4:1, appears a great number of the homogeneous and sphere-like carbon particles on the surface of felts. The formed carbon nanoparticle on the felt shows the mean size of about 300 nm which is very close to the standard HTC material of uniform spherical particles (Fig. S1). The microstructure of HTC
carbon on the felt changes dramatically when the initial ratios of the felt and glucose were 2:1 for GFs-HTC-2 or 1:1 for GFs-HTC3, resulted in a layer/aggregation of carbon on the surface of felt (Fig. 1c, d and f). This significant change in microstructure by the HTC process could be due to the strong interaction between the polar functional groups of hydrothermal carbon and the topological structure of the felt. In consistent with the SEM images, the specific surface area of GFs and GFs-HTC-1 to 3 are 4.17, 5.87, 6.70, 9.98 m2 g−1 , respectively. This may be related to the presence of hydrothermal carbon, providing abundant pathways for electrolyte transport. Raman spectra were presented in Fig. 2 to illustrate the defect concentration of the GFs and hydrothermal treated GFs. Two typical strong bands located at ca. 1347 cm−1 (D band) and 1580 cm−1 (G band) were clearly observed, arising from the disordered carbon structures and the vibration mode to the movement in opposite directions of two carbon atoms in a graphene domain, respectively [36]. Following the reported strategy, the Raman spectra were deconvoluted in order to calculate the ID /IG ratio. The derived ID1 /IG values for the HTC treated felts present an increased trend, which are 1.80∼1.85 for GFs-HTC compared to GFs (1.73). This indicates that a large quantity of defects and more structural disorder were included within felts by HTC process. X-ray photoelectron spectroscopy (XPS) in Fig. 3(a) show the coexistence of C, N and O elements in GFs after hydrothermal treatment, confirming the successful doping of heteroatoms into the graphite felt. The N contents are found to be at 0.78 at.%– 1.3 at.% for three materials, while the O content fluctuates within a certain range from 5 at.% to 10 at.%. High resolution XPS
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 4. The water contact angles of the materials (a) pristine felt, (b) GFs-HTC-1, (c) GFs-HTC-2, (d) GFs-HTC-3.
Fig. 3. (a) XPS survey scans of GFs and three GFs after HTC treatment, (b, c) high resolution of C 1 s and N 1 s spectrum of GFs-HTC-3.
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spectrum of C 1 s region for GFs-HTC-3 is shown in Fig. 3(b), which can be deconvoluted into three peaks, i.e. sp2 carbon, C—O— /C—N—containing function groups and O=C—O containing function groups, assigned to binding energies of ca. 285, 285.8 and 287.5 eV, respectively [37]. More importantly, the N1s spectrum in Fig. 3(c) is deconvoluted into three types of configurations located at 398.3, 400.1 and 401.3 eV and assigned to pyridinic-N (N1), pyrrolic-N (N2), and graphitic-N (N3) species, respectively [21,31]. The presence of nitrogen is expected to exert a considerable influence on electrochemical performance, because the planar pyridinic-N with lone electron pair is regarded as the most active configuration to endow adjacent carbon atoms with Lewis basicity and high electron density, thus beneficial for promoting the adsorption of positively charged vanadium ions and thus improving ion change during redox process [38,39]. The changes in surface chemistry and microstructure of GFs by HTC process were further investigated by water contact angle measurements which were performed using the water droplet method.
The surface hydrophobicity/hydrophilicity of the materials can be expressed by the water contact angle, as smaller contact angle means better wettability. As observed in Fig. 4, the contact angle of pristine GFs is 134.09°. By contrast, the values are 120.64°, 98.21°, and 89.48° for GFs-HTC-1 to 3, respectively. There is a slight increase in the contact angle of GFG, GFU and GFs-10 0 0 as shown in Fig. S2. This indicates that the wettability of hydrothermal treated GFs is substantially improved compared with GFs because of N/O functional groups and defects on the surface of GFs generated in hydrothermal treatment process, which is in agreement with XPS and Raman measurements. The good wettability of electrode materials is favorable to the mass transfer of active species.
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3.2. Performance of vanadium redox flow batteries
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Based on the unique properties of GFs after hydrothermal treatment, we suppose that the HTC modified GFs can deliver high VRFBs performance. Generally, the potential gap between redox peaks, onset redox potential, redox peak currents and their ratio (Ipc /Ipa ) are key parameters for comparing electrocatalytic activities. Fig. 5(a) presents the CV curves of the positive electrode reaction corresponding to VO2+ /VO2 + redox couples. The onset reduction potential of GFs-HTC-3 appeared a positive shift to 0.755 V (vs. SCE) at the anodic reaction associated with VO2 + →VO2+ , while the onset oxidation potential from VO2+ →VO2 + reaction showed a negative shift to 1.025 V (vs. SCE). The potential gap (࢞Ep ) between redox peaks of GFs-HTC-3 towards the VO2+ /VO2 + couple was 0.27 V after 10th cycles, much smaller than that of the pristine GFs (0.445 V). Besides, both the oxidation and reduction peak current density of GFs-HTC-3 were quite stable after 30th cycles, indicating the high electrochemical stability of GFs-HTC (Figs. 5b and S3a), which was probably due to strong interaction between N-doped hydrothermal carbons and GFs. The improved onset potentials for both anodic and cathodic processes were beneficial for electron transfer kinetics at lower applied voltage, leading to enhanced energy storage efficiency for VRFBs. As for the negative electrode (Fig. 5d), the anodic and cathodic peaks of GFs-HTC corresponding to the V2+ /V3+ couple appear at −0.3 to −0.4 V and −0.7 to −0.8 V (vs. SCE), respectively. This redox reaction was enhanced significantly for GFs-HTC-3, as its redox peaks nearly mirrored each other. Notably, the performance V2+ /V3+ appeared unstable for all electrodes after 20th cycles, implying that the poor performance of the V2+ /V3+ reduction reaction in negative half-cell was substantially affected by undesired hydrogen evolution known as a side reaction in the VRFBs system [38,40]. This phenomenon can also be observed in the pristine GFs
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Fig. 5. CV curves at a scan rate of 5 mV s−1 in positive electrode: (a) GFs/GFs-HTC-1 to 3, (b) 1–30 cycles CV curves of GFs-HTC-3 and (c) comparison of the 10th cycle of CV curves. The CV curves at scan rate of 5 mV s−1 in negative electrode: (d) GFs/GFs-HTC-1 to 3, (e) 1–20 cycles GFs-HTC-3 and (f) comparison of the 2nd cycle of CV curves. (g, h) CV curves at different scan rates of GFs-HTC-3 and GFs. (i) Plots of the redox peak current density versus the square root of scan rate for GFs and GFs-HTC-3.
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(Fig. S3b). However, the potential gap for the GFs-HTC-3 (࢞Ep = 0.315 V) was much smaller than that of GFs (0.48 V) at the 2nd cycle, suggesting that the electron transfer kinetic for the V2+ /V3+ redox couple were largely improved for GFs-HTC-3. These CV results clearly show that the hydrothermal carbons are efficient energy storage booster for VRFBs, by enhancing the performance of both positive (VO2+ /VO2 + ) and negative (V2+ /V3+ ) electrode reactions. The CV curves of positive and negative electrode reaction of GFG, GFU, and GFs-10 0 0 are show in Fig. S3(c, d). To understand the kinetics of VO2+ /VO2 + redox reaction, the CV curves of the GFs-HTC-3 and pristine GFs electrodes at different scan rates were recorded. The ratios of the redox peak currents are used to evaluate the reversibility of the reaction. It was found that the Ipc /Ipa value of GFs-HTC-3 was ∼ 0.9, much higher than that of GFs (∼ 0.5), indicating much better reversibility and higher catalytic activity of GFs-HTC-3. The mass transfer rate for vanadium ion couples was calculated by plotting the peak current density versus the square root of scan rate. The slope of GFs-HTC-3 was slightly steeper than that of GFs, suggesting the larger mass transfer rate of the former. These results are in line with the above results on the surface chemistry and microstructure of GFs-HTC, in which the excellent wettability of N-doped hydrothermal carbon decorated GFs leads them to having better reversibility and catalytic activity of vanadium redox reaction. The catalytic mechanism of GFs-HTC was further studied by electrical impedance spectroscopy (EIS). As shown in Figs. 6 and S4, the Nyquist plots of all electrodes showed a small
quasi-semicircle in the high frequency region, which was related to the interfacial charge transfer resistance at the electrodeelectrolyte interface [15,41]. It could be seen that the GFs-HTC had much smaller semicircle radiuss than the pristine GFs, demonstrating much lower charge transfer resistances [42]. The lowest charge transfer resistance of GFs-HTC-3 indicated the highest reaction rate and excellent catalytic activity towards vanadium redox reactions, which agreed well with the CV results. We also measured the galvanostatic charge-discharge profiles of the electrodes with current densities ranging from 100 to 500 mA cm−2 . Fig. 7(a–d) showed the charge-discharge profiles of all samples between 0.7 and 1.7 V (the charge-discharge profiles of GFU, GFG and GFs-10 0 0 are showed in Fig. S5). It could be seen that all the charge potentials for GFs-HTC were lower than that of pristine GFs at each current density while the discharge potentials for GFs-HTC were higher. This implied the glucosederived carbon decorated GFs had much lower polarization than pristine GFs during the battery operation at all current densities. The coulombic efficiency (CE) increased with the increasing current density, because of the shorter time at higher current density for the permeation of vanadium ions across the membrane (self-discharge); the voltage efficiency (VE) decreased with the increasing current density, because of the higher polarization at higher current density. It could be seen that while the CEs for all carbons were similar, the VEs of GFs-HTC were much higher than the VEs of the pristine GFs, especially at high current densities. The GFs-HTC-3 exhibited the highest VE at each current density,
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 6. Nyquist plots for different graphite felt electrodes across the in 0.1 M VO2+ + 3 M H2 SO4 and 0.1 M V3+ + 3 M H2 SO4 solution at an open-circuit potential (1.0 V for positive reaction and −0.4 V for negative reaction).
Fig. 7. Charge-discharge curves of batteries with the presented GFs electrodes at various current densities (a–d); and coulombic efficiency, voltage efficiency and energy efficiency (e,f).
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Fig. 8. (a) The specific discharge capacity of VRFBs as a function of cycle number at different current densities, (b) cycling performance of the optimized GFs-HTC-3 and GFs at a current density of 200 mA cm−2, (c) capacity fading curve of GFs-HTC3 and GFs at a current density of 200 mA cm−2 .
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which was in good agreement with the EIS result that it had the lowest charge-transfer resistance. Consequently, the GFs-HTC-3 exhibited the highest energy efficiency (EE) than all the other samples (Figs. 7e and S6). Remarkably, the pristine GFs was unable to be charged and discharged when current density was higher than 400 mA cm−2 . In contrast, the GFs-HTC-3 was able to deliver an EE of 65% at the very high current density of 500 mA cm−2 , which was outstanding even when compared with many other advanced electrode materials in literatures. The rate performance of GFs and GFs-HTC were presented in Fig. 8(a). It can be seen that the discharge capacity of GFs-HTC were much higher than that of the pristine GFs. For example, the discharge capacity of GFs-HTC-3 was boosted to 209.2 mAh, which was 4.18 times higher than that of GFs (50.1 mAh) at 400 mA cm−2 . What’s more, capacity retention at current density of 500 mA cm−2 was 58.53%. This proved that GFs-HTC-3 had excellent rate performance. In view of above, the GFs-HTC-3 was the most optimized electrode based on a rational approach delivering an impressive catalytic performance in VRFBs. This electrode was chosen to perform a long-term cycling test to evaluate the durability under a high current density of 200 mA cm−2 . As shown in Fig. 8(b), the CE for GFs-HTC-3 was nearly the same during the whole cycling test (96.5%), while the EE for GFs-HTC-3 remained at 82.9% with a small decay rate of 1.43% per cycle, which is much better than GFs. The capacity fading curve was shown in Fig. 8(c), it was obvious that capacity decay rate of GFs-HTC-3 is less than that of GFs, GFsHTC-3 can still reach 64.02 mAh with 150 cycles, while GFs has a capacity of only 13.4 mAh with 76 cycles. The good stability can be further supported by the SEM images of the GFs-HTC-3 after electrochemical testing, in which HTC carbon particles are still strongly adhere to the surface of GFs (Fig. S7). The excellent electrochemical activity and stability are presumably due to the N-doped amorphous carbon coating, which promotes the mass/charge transfer and thus increasing the electrochemical activity, and creates oxygenated/nitrogenated active sites and hence enhances wettability. In order to further prove that GFs-HTC-3 has excellent battery performance, we compared the VRFBs performance of the Nafion 212 (N212, 50 μm) with that of the benchmark commercial Nafion 115 (N115, 125 μm) membrane at a current density of 200 mA cm−2 . The results show that GFs-HTC-3 still performs well after N212 was replaced by N115. CE, EE and charge-discharge curves were shown in Fig. 9. It can be seen that greater CE was achieved by replacing N212 with N115, this is due to CE is mainly determined by vanadium ions permeability and side reactions. Before the test, N2 was injected into the negative electrode, thus inhibiting the side reaction of V2+ in the negative electrode. So CE is determined by the permeability of vanadium. The thick membrane can inhibit the vanadium ions crossover to improve the CE of the
Fig. 9. The coulombic efficiency, energy efficiency and charge-discharge curves of GFs-HTC-3 electrodes with different Nafion membranes.
Please cite this article as: J. Qiu, B. Huang and Y. Liu et al., Glucose-derived hydrothermal carbons as energy storage booster for vanadium redox flow batteries, Journal of Energy Chemistry, https://doi.org/10.1016/j.jechem.2019.09.030
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Fig. 10. The charge-discharge curves at the current densities of (a) 20 0 and (b) 40 0 mA cm−2 ; (c) the coulombic and voltage efficiencies and (d) the energy efficiency for VRFBs for GFs-HTC-3 to 5.
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battery, so use N115 can get greater CE than N212. As for the decrease of EE, the main reason is that the thick membrane has large Ohmic resistance, which directly reduces the VE of the battery and further leads to the decrease of EE [43]. Because of the larger thickness and area resistance, higher charge voltage and lower discharge voltage of N115 were observed in Fig. 9(b), resulting in a slight drop in battery capacity, that is consistent with previous reports [43,44]. This work further evidences that GFs-HTC-3 electrode hold great promise in practice VRFBs application. To optimize the electrode materials, the mass ratio of GFs, glucose and urea increased to 2:3:3 and 1:2:2, which were denoted as GFs-HTC-4 and GFs-HTC-5, respectively. Fig. 10(a, b) show the charge-discharge curves of GFs-HTC-4 and GFs-HTC-5 at current densities of 300 and 400 mA cm−2 . The corresponding coulombic and voltage efficiencies, charge-discharge curves are presented in Fig. 10(c, d). It is found that the battery with GFs-HTC-3 still presents the highest capacities, voltage efficiency and energy efficiency. With the increase of glucose and urea mass, the performance for VRFBs show a decreasing trend. It may be due to the aggregated HTC particles, which tend to detach from the surface of graphite felt under flowing condition.
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4. Conclusions
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In summary, we have successfully synthesized a new and highly efficient GFs-based electrode for VRFBs by using hydrothermal carbonization of glucose and urea. This facile synthetic strategy allows the homogeneous deposition of HTC amorphous carbons on the graphite felts and endows them with the features of abundant oxygen and nitrogen-containing functional groups. Further advantages of our approach are the use of glucose and urea which is sustainable and has a much lower cost compared with other C/N precursors. As a result, the optimized GFs-HTC electrode delivers a
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significantly boosted electrocatalytic performance for VRFB under wide range of current densities. The VEs and EEs of GFs-HTC are much higher than the VEs of the pristine GFs, especially at high current densities. Long term cycling test indicates an exceptional durability of the GFs-HTC electrode at a high current density of 200 mA cm−2 . The boosted performance is most likely due to the coating of hydrothermal carbons that can promote mass/charge transfer and thus reduce charge transfer resistance. We thus believe that these unique HTC carbon decorated felts hold great promise for practice VRFBs application.
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Declaration of Competing Interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgments
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This work was supported by the Award Program for Fujian Minjiang Scholar Professorship and the National Natural Science Foundation of China (21571035).
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Supplementary material
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jechem.2019.09.030.
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