Carbon sheet-decorated graphite felt electrode with high catalytic activity for vanadium redox flow batteries

Carbon sheet-decorated graphite felt electrode with high catalytic activity for vanadium redox flow batteries

Carbon 148 (2019) 9e15 Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon Carbon sheet-decorated gra...

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Carbon 148 (2019) 9e15

Contents lists available at ScienceDirect

Carbon journal homepage: www.elsevier.com/locate/carbon

Carbon sheet-decorated graphite felt electrode with high catalytic activity for vanadium redox flow batteries Yu Gao a, b, 1, Hongrui Wang b, 1, Qiang Ma b, Anjun Wu b, Wei Zhang b, Chuanxiang Zhang a, Zehua Chen a, *, Xian-Xiang Zeng b, **, Xiongwei Wu b, ***, Yuping Wu b, c, **** a b c

College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan, 454000, China College of Science, Hunan Agricultural University, Changsha, Hunan, 410128, China College of Energy and Institute for Advanced Materials, Nanjing Tech University, Nanjing, Jiangsu, 211816, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2018 Received in revised form 12 March 2019 Accepted 14 March 2019 Available online 19 March 2019

A carbon sheet-decorated graphite felt electrode was synthesized by in situ polymerization and subsequent high-temperature calcination under an inert atmosphere. The resultant material brings an improved wettability, numerous defect sites, and abundant O, N and P elements as additional catalytic sites to elevate the reaction kinetics and efficiency of vanadium redox flow batteries (VRFBs). The unique CS modifier enriches electrolyte diffusion pathways, which even show a unique capillary flow. The [email protected] 3þ 2þ and V2þ/V3þ oxidation-reduction displays a high catalytic activity towards the VO2þ/VOþ 2 , V /VO couples and a reduced cathodic and anodic peak potential difference of 355 mV (vs. 564 mV for GF). The improvement to the electrode results in a [email protected] battery presenting an increased capacity of 20.8 Ah L1 compared to 13.0 Ah L1 of the GF-based battery and an increase in power density from 225 mA cm2 to 300 mA cm2. Furthermore, the battery exhibited a 74.79% energy efficiency (EE) at 150 mA cm2, with no attenuation even at 300 cycles. [email protected] greatly elevates the reaction kinetics and efficiency of VRFBs. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction Reliable high-capacity energy storage equipment has increasingly been attracting attention in terms of increasing power generation and developing renewable energy systems [1,2]. To maintain a stable clean energy source, a large capacity, low maintenance cost and highly efficient storage are required [3e5]. Vanadium redox flow batteries (VRFBs) are expected to be the most promising candidates among the various energy storage systems, especially when used with wind or photovoltaic power generation systems [6,7]. However, commercialized VRFBs remain limited due

* Corresponding authors. ** Corresponding author. *** Corresponding author. **** Corresponding author. College of Science, Hunan Agricultural University, Changsha, Hunan, 410128, China. E-mail addresses: [email protected] (Z. Chen), [email protected] (X.-X. Zeng), [email protected] (X. Wu), [email protected] (Y. Wu). 1 The authors equally contributed to this work. https://doi.org/10.1016/j.carbon.2019.03.035 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

to their low energy efficiency (EE) and poor charge-discharge performance [8]. VRFBs consist of a battery stack, which dominates the power density of the battery, and electrolyte tanks, which determine the energy rating, in external reservoirs. The electrolyte is pumped into the cell by peristaltic pumps and separated by an ion exchange membrane that allows the transfer of hydrogen ions individually [9,10]. The electrode plays the roles of a source of active sites and as a catalyst for the electrochemical reactions in VRFBs. To further optimize the electrode, VRFBs can achieve a smaller size, higher EE and better rate performance [3,6]. Because of their excellent stability under acidic conditions, rapid electron conduction and commercial availability, carbon-based electrodes have overtaken the use of metal and conductive plastics in a wide range of applications over the past few decades [11e14]. The microstructure of graphite felt (GF) based on polyaniline (PAN) is smooth and nonporous, with a low surface concentration of functional groups, but PAN-based materials show sluggish redox reactions with vanadium ions and poor wettability by the vanadium electrolyte [15,16]. Although significant improvements have been gained in

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Fig. 1. Schematic of the synthesis the [email protected] electrode.

Fig. 2. SEM images of GF (a and b), U-GF (c), [email protected] (d and e) and P-GF (f).

Fig. 3. Distributions of N, O, and P on the [email protected] surface.

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both wettability and electrochemical activity by a variety of surface treatment procedures and composite catalysts, including treatment by plasma, microwaves, Hummers' reagent, Fenton's reagent, and alkalis [9,10,17e19] and the deposition of metal oxides, metallic elements, one-dimensional nanowires, and three-dimensional conductive networks on GF [20e28], the stability and catalytic activity of the electrode is limited by the low specific area for catalysis, and reactions with the active species are still not satisfactory [29,30]. In this work, a well-controlled surface treatment was adopted to fabricate a carbon sheet-decorated graphite felt ([email protected]) electrode that exhibits remarkably improved reaction kinetics and EE as a result of abundant O, N and P elements, which act as extra catalytic sites, and electrolyte diffusion pathways. Compared with the action of the original electrode, V2þ/V3þ, V3þ/VO2þ, and VO2þ/VOþ 2 redox couples and a pseudo-capacitance are clearly observed by cyclic voltammetry (CV), and they supply additional electrons for the electrochemical reaction. In addition, the [email protected] electrode exhibits a higher charge/discharge power density (300 mA cm2) and EE (74.79%, 150 mA cm2) than the original electrode.

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2. Experimental 2.1. Fabrication of the [email protected] composite electrode The pristine graphite felt was immersed into a 20 ml phytic acid solution (2 ml phytic acid was dispersed in 18 ml deionized water), submerged in an ice water bath. Urea (0.5 g) was added to the mixed solution, followed by ultrasonication for 10 min. The graphite felt was removed from the solution and left drying in an oven at 60 C overnight. Finally, the composite electrode precursor was placed in a tube furnace under an argon atmosphere at a temperature of 800 C. In contrast, urea-treated GF (U-GF) and phytic acid-treated GF (P-GF), with solely urea or phytic acid, were prepared under the same conditions. 2.2. Material characterization The morphology of GF and the [email protected] electrode were characterized by a scanning electron microscope (SEM) (Hitachi S4800) operating at 8 kV and Transmission Electron Microscope (TEM, JEM-2100F). The Raman (Thermal fisher Lab RAM HR800) spectra

Fig. 4. (a) Raman spectra of GF and [email protected] (b) XPS spectra of GF and [email protected] (cef) High-resolution C 1s, O 1s, N 1s, and P 2p XPS spectra.

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were obtained with a 532 nm laser excitation. The X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific ESCA Lab 250Xi with 200 W Al Ka radiation. 2.3. Electrochemical testing The CV measurements were accomplished by a typical threeelectrode system with the [email protected] electrode (0.5  0.5 cm2) as the working electrode, Ag/AgCl as the reference electrode and Pt as the counter electrode in a 0.1 M VOSO4 þ 3 M H2SO4 electrolyte on an electrochemical work station. The electrochemical impedance spectroscopy (EIS) test was conducted with the same device for frequencies ranging from 0.01 Hz to 1 MHz at an amplitude of 15 mV. The galvanostatic charging and discharging tests were performed with a single cell on the battery testing system (Land CT2001A, China). The GF or [email protected] (2  2 cm2) electrode was used as the working electrode, and Nafion 115 (DuPont, USA) was employed as the separator. The electrolyte for positive and negative tests was 1.5 M VOSO4 þ 1.5 M V2(SO4)3 þ 3 M H2SO4 (15 ml) (Hunan Yinfeng Co. Ltd.). The operation voltage was from 0.8 V to 1.65 V. All the measurements were carried out at room temperature. 3. Results and discussion The [email protected] electrode was prepared by phytic acid-urea polymerization followed by high-temperature calcination under an inert gas. The detailed preparation process is shown in Fig. 1. A comparison of the SEM images in Fig. 2a and b and Fig. 2d and e shows that the carbon sheets were successfully loaded onto the GF. Furthermore, under the same experimental conditions, neither phytic acid (P-GF) nor urea (U-GF) alone could form uniform carbon sheets, as shown in Fig. 2c and 2f. Further TEM results (Fig. S1)

show that carbon nano sheets are thin layer with random spatial structure. No obvious graphite-carbon lattice fringes have been observed that may be attributed to a lower degree of graphitization. Apparently, the wettability of the electrode for the distinctive [email protected] structure has been improved greatly as shown in Fig. S2. Meanwhile, the presence of the carbon sheets led to the unique conductivity of the electrolyte flow, diverse diffusion pathways, and a higher local flow velocity of the electrolyte [31]. Besides, the existence of carbon sheets can improve the space utilization ratio of carbon fibers, enlarge the reaction area and accelerate electron transport, enhance the catalytic efficiency with maximized space utilization towards redox species dramatically and further improve the catalytic activity of [email protected] [32,33]. We examined the presence of O, N, and P on the surface of [email protected] by energy dispersive spectroscopy (EDS). As shown in Fig. 3, abundant O, N, and P atoms were uniformly distributed across the surface of the composite electrode. Among these heterocarbons, the atomic contents of O, N and P are 34.73%, 3.06% and 13.81%, respectively. D and G bands were clearly resolved close to 1350 and 1600 cm2 by Raman spectroscopy (Fig. 4a). The intensity ratios of the D to G bands for GF and [email protected] were 0.899 and 0.961, respectively. Thus, the two-dimensional (2D) [email protected] electrode formed from phytic acid and urea had a lower degree of graphitization than did GF, which corresponds to the presence of a larger number of defect sites and a higher catalytic activity [34,35]. These also correspond to the test results of TEM images that no obvious graphite-carbon lattice fringes is observed. To further determine the valence states of the elements on the surface of [email protected], XPS measurements were obtained in the range of 100e800 eV. As shown in Fig. 4b, the introduction of CS increased the O/C atom ratio of GF from 0.12 to 0.80, which indicates that the surface of the [email protected] contained abundant oxygen-containing functional groups. Moreover, certain amounts of N and P derived

Fig. 5. (a) CV curves of GF, U-GF, P-GF and [email protected] at a scan rate of 10 mV s1from 0.8e1.4 V obtained from an electrolyte of 0.1 M VOSO4 þ 3.0 M H2SO4. (b) CV curves of GF and [email protected] at a scan rate of 10 mV s1 from 0.2e1.0 V in the solution of 0.1 M VOSO4 þ 3.0 M H2SO4. (c) The trend of the peak current versus the square root of various scan rates towards the VO2þ/VOþ 2 couple for GF and [email protected] (d) A comparison of EIS plots for GF, U-GF, P-GF and [email protected]

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from CS were detected, making the atom ratios of the electrode surface increase from 1.7% to 0.1%e3.6% and 10.3%. The highresolution C 1s spectra (Fig. 4c) exhibited five bands at 289.9 eV (C5), 287.4 eV (C4), 286.1 eV (C3), 285.4 eV (C2), and 284.7 eV (C1), which are ascribed to -COOH, C]O or C-P, C-O, C-N and C]C groups [36e38]. The O 1s peak (Fig. 4d) was divided into four peaks, which may be assigned to carbonate (O4, 534.0 eV), C-C]O (O3, 532.9 eV), C-O (O2, 531.8 eV), and C]O or O-P (O1, 530.6 eV) groups [25,30,39,40]. Introducing more functional groups that contain oxygen can enrich vanadium ion catalytic active sites [40e42]. Fig. 4e shows that the N 1s spectra contained three characteristic peaks, N3 (401.1 eV), N2 (399.8 eV) and N1 (398.3 eV), which can be attributed to graphitic-N, pyrrolic-N, and pyridinic-N, respectively [43]. At the same time, the high-resolution P 2p spectra (Fig. 4f) exhibited three main peaks at 134.1 eV (P3), 133.5 (P2), and 132.7 (P1), which can be assigned to P-O, P-N, and P-C bonds, respectively [30,37]. Overall, the connected N, P, O and carbon atoms in various functional groups co-promote the reaction of vanadium ions. We concluded that the composite electrode with abundant N and P atoms and several oxygen functional groups displayed a high 3þ 2þ catalytic activity towards VO2þ/VOþ and V2þ/V3þ 2 , V /VO oxidation-reduction couples based on Fig. 5a and Table S2. The

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cathodic and anodic peak potential differences (DE) of GF and [email protected] towards the VO2þ/VOþ 2 couple were 564 mV and 355 mV, respectively. The ratios of the peak redox currents for the VO2þ/VOþ 2 couple of the GF electrode (positive Ipa/Ipc ¼ 2.60 and negative Ipa/ Ipc ¼ 0.03) indicated its poor electrochemical performance relative to that of [email protected] (positive Ipa/Ipc ¼ 1.36 and negative Ipa/Ipc ¼ 0.31). In contrast, the CV properties of GF in the presence of pure phytic acid and urea are also shown in Fig. 5a. Both phytic acid (P-GF) and urea (U-GF) individually played a role on improving the catalytic activity, but the improvement was far less than [email protected] electrode. Furthermore, the [email protected] electrode showed a unique catalytic activity that dramatically promoted the reaction of V3þ/VO2þ (Fig. 5b); the unique oxidation-reduction peak indicated that the electrode presented a large effect on the redox reaction of vanadium ions [44]. The test results show that a better electrocatalytic activity, higher electrochemical reversibility and lower overpotential of V2þ/V3þ, V3þ/VO2þ and VO2þ/VOþ 2 were achieved after the in situ growth of CS [38,44,45]. The mass transfer of vanadium ions was further estimated based on the Randles-Sevcik equation by changing the potential scan rate (Fig. 5c and Fig. S3). Compared with GF, the [email protected] electrode exhibited a steeper slope, suggestive of faster mass transfer kinetics. The electrochemically active surface

Fig. 6. (a and b) EE and VE of GF and [email protected] at current densities of 50e300 mA cm2. (c) Charge-discharge profiles of GF and [email protected] in single cells at 150 mA cm2. (d) The response line of the overpotential vs current density. (e) The cycling EE of the [email protected] electrode at 150 mA cm2.

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area (ECSA) of carbon based materials could be estimated by double-layer capacitance (Cdl), which could be calculated from CV curves within non-Faradaic potential range. As shown in Eq. S(1) and Eq. S(2), the slop of fitting lines of current density (ip) at 0.3 V versus scanning rate (V) is equal to Cdl, and the specific capacitance (Cs) is a constant with 5 mF cm2 [46,47]. Thus, the Cdl can be quantified, and the values are 0.592 mF cm2 and 2.18  103 mF cm2 for [email protected] and GF, respectively. Therefore, the ECSA are 118.4 and 0.436 for [email protected] and GF, which indicates that the N and P functional groups based on carbon sheet can enhance the electrochemical activity of [email protected] electrode and perform strong faradaic pseudo capacitance behaviors. The details are shown in Fig. S4. The EIS plots (Fig. 5d) show the ultra-low interface transfer resistance of [email protected] (6.5 U) based on the equivalent circuit corresponding to the Nyquist plot (Fig. S5), which is much lower than those for GF (499), U-GF (345) and P-GF (209) [48]. The result indicates that [email protected] exhibited faster electron transmission between the electrode interface and electrolyte. The better electrochemical performance is inextricably linked to the unique morphology, which produced a larger number of catalytically active sites and abundant mass diffusion pathways. VRFB single cells were assembled to further assess the charge/ discharge performance. As shown in Fig. 6a and b, the [email protected] electrode showed a higher EE and voltage efficiency (VE) for charge/discharge current densities of 50e300 mA cm2; even at 300 mA cm2, the [email protected] battery continued to produce a considerable EE, whereas the GF-based battery was unable to carry out an effective charge or discharge beyond 225 mA cm2. In particular, at 200 mA cm2, the assembled battery exhibited a 6.6% increase in EE from 61.6% for GF to 68.3% for [email protected] Fig. 6c shows the charge/discharge profiles of the GF (black) and [email protected] (red) electrodes at 150 mA cm2. The clear increase in the discharge capacity (GF, 13.0 Ah L1; [email protected], 20.8 Ah L1) and decrease in the overpotential during the charging and discharging processes (Fig. 6d) demonstrate the better charge and discharge performance. A cycling charge and discharge test was performed at 150 mA cm2, and the cell based on [email protected] showed a stable EE of 74.6% in the 300th cycle (Fig. 6e). The enhanced capacity and charge-discharge performance could be attributed to the high wettability and abundant catalytically active sites of the [email protected] electrode. Furthermore, the existence of carbon sheets enriched the diffusion paths of the electrolyte on the electrode. 4. Conclusion To summarize, the 2D [email protected] electrode exhibited a greatly improved wettability, electrocatalytic performance and redox reversibility. The unique 2D structure enriched the mass diffusion pathways of the electrolyte, enlarged the reaction area, improved the space utilization and accelerated the conduction of vanadium ions. The increased catalytic activity of the electrode contributed a [email protected] battery presenting an increased capacity of 20.8 Ah L1 compared to 13.0 Ah L1 of the GF-based battery and a 74.79% energy efficiency (EE) at 150 mA cm2, with no attenuation even at 300 cycles. This study introduces a new method for the development of high-performance VRFB electrodes. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51772093, 51803054, 51425301 and U1601214), the Key Scientific Research Project for Higher Education of Henan Province (No. 16A150009), the Natural Science Foundation of Henan Province (General Program) (No. 162300410119, 162300410115), and the Innovative Research Team (in Science and

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