ZrO2-Nanoparticle-Modified Graphite Felt ... - ACS Publications

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ZrO2‑Nanoparticle-Modified Graphite Felt: Bifunctional Effects on Vanadium Flow Batteries Haipeng Zhou,†,‡ Yi Shen,§ Jingyu Xi,*,† Xinping Qiu,*,†,‡ and Liquan Chen†,∥ †

Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China § College of Light Industry and Food Sciences, South China University of Technology, Guangzhou 510640, China ∥ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100084, China ‡

S Supporting Information *

ABSTRACT: To improve the electrochemical performance of graphite felt (GF) electrodes in vanadium flow batteries (VFBs), we synthesize a series of ZrO2-modified GF (ZrO2/GF) electrodes with varying ZrO2 contents via a facile immersion-precipitation approach. It is found that the uniform immobilization of ZrO2 nanoparticles on the GF not only significantly promotes the accessibility of vanadium electrolyte, but also provides more active sites for the redox reactions, thereby resulting in better electrochemical activity and reversibility toward the VO2+/VO2+ and V2+/V3+ redox reactions as compared with those of GF. In particular, The ZrO2/GF composite with 0.3 wt % ZrO2 displays the best electrochemical performance with voltage and energy efficiencies of 71.9% and 67.4%, respectively, which are much higher than those of 57.3% and 53.8% as obtained from the GF electrode at 200 mA cm−2. The cycle life tests demonstrate that the ZrO2/GF electrodes exhibit outstanding stability. The ZrO2/GF-based VFB battery shows negligible activity decay after 200 cycles. KEYWORDS: vanadium flow battery, graphite felt, ZrO2, electrolyte accessibility, electrocatalyst graphene,17 graphene oxide,18 and graphite−graphite oxide19 have been studied as electrode materials in the VFB. Despite the good electrode performance, these powdered CBMs are very difficult to handle during the assembly of the VFB. On the contrary, self-supporting CBMs, such as graphite felt (GF), carbon felt, and carbon cloth, can be directly employed as electrodes without the tedious fabrication process.7,20 Among the reported self-supporting CBMs, GF is deemed to be one of the most potential electrode materials by virtue of its outstanding stability, low cost, and high electrical conductivity.4,7,13,20 Generally, GF is highly hydrophobic due to the high graphitization temperature and possesses a very low surface area, which leads to poor electrolyte accessibility and inadequate active sites.13,20 To address this issue, one common strategy is to introduce some oxygen-containing groups into the GF surface by acid treatment,21−23 thermal treatment,24 electrochemical activation,25,26 hydrothermal treatment,27 Fenton’s reagent treatment,28 and so on.29−32 Indeed, this facile strategy is capable of improving the hydrophilicity of GF electrodes. Nevertheless, the modified GF electrodes showed

1. INTRODUCTION Owing to its long cycle life, high reliability, environmental friendliness, independent tunable power and capacity, and low cost of operation and maintenance, the vanadium flow battery (VFB) has drawn comprehensive attention in energy storage and conversion.1−4 Compared with other energy storage systems, one prominent advantage of the VFB lies in the utilization of a single vanadium sulfate solution with different valence states of vanadium as the electrolyte, which significantly suppresses the cross-contamination of electrolytes.5,6 The performance of the VFB is highly dependent on the electrodes, membrane separators, and electrolytes.7−12 Recently, numerous efforts have been devoted to searching for high-performance electrode materials.8,13 In general, the electrode materials of the VFB can be categorized into noblemetal materials such as Pt−Ti and IrO2−DSA, and carbonbased materials (CBMs).7,14 Although noble-metal-based electrodes have excellent electrochemical activity and reversibility, their wide application is limited due to the extremely high cost.13,14 Thanks to their electrochemical inertness as well as abundance, CBMs are considered to be the most feasible electrode materials in the VFB. So far, in the literature, numerous CBMs such as multiwalled carbon nanotubes (MWCNTs),15 single-walled carbon nanotubes (SWCNTs),16 © XXXX American Chemical Society

Received: March 29, 2016 Accepted: May 27, 2016

A

DOI: 10.1021/acsami.6b03761 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. SEM images of (a) GF, (b) 0.1 wt % ZrO2/GF, (c) 0.2 wt % ZrO2/GF, (d, g) 0.3 wt % ZrO2/GF at different magnifications, (e) 0.4 wt % ZrO2/GF, (f) 0.5 wt % ZrO2/GF, and (i) 0.3 wt % ZrO2/GF after 200 cycles in the cycle life test. (h) TEM and corresponding FFT images of the ZrO2 nanoparticles. EDX images of the corresponding areas: (j) selective area of (a), (k) selective area of (d), (l) selective area of (i).

2. EXPERIMENTAL SECTION

limited enhancement on VFB performance owing to the low surface area. To increase the specific surface area, carbon materials such as MWCNTs,15 SWCNTs,16 graphene nanosheets,17 and carbon nanofibers (CNFs),33,34 metals such as Pt,35 Ir,36 and Bi,37 and metal oxides such as Mn3O4,38 WO3,39 TiO2,40 CeO2,41 PbO2,42 and Nb2O543 were deposited onto the GF supports to prepare composite electrodes. Notably, the decoration of metal oxides onto the GF could significantly enhance the performance of the VFB. Notwithstanding numerous studies in metal oxide−GF composite electrodes for the VFB, the promoting role of metal oxides in the electrode reactions still remains unclear.38−43 To make up such a deficiency, this study evaluates the performance of ZrO2-modified GF (ZrO2/GF) electrodes in the VFB. At present, the most widespread adopted utilization of ZrO2 lies in oxygen sensors, fuel cells, and ceramic membranes due to its unique physicochemical properties.44,45 The central objective of this work is to reveal the promoting role of ZrO2 in the VO2+/VO2+ and V2+/V3+ redox reactions. The selection of ZrO2 as a promoter is attributed to the presence of abundant oxygen-containing functional groups as well as unsaturated Lewis acid−base Zr4+−O2− pairs in the surface of ZrO2,46,47 which renders it as an ideal model system to elaborate the promoting mechanism of metal oxides in the electrode reaction of the VFB. It is expected that this mechanistic study could provide general information for the metal oxide−carbon composite electrodes of the VFB.

2.1. Preparation of the Electrodes. Polyacrylonitrile (PAN)based graphite felts were first thermally treated in air at 420 °C for 10 h, and are denoted as TGF. TGF was immersed into Zr(NO3)4·5H2O solutions accompanied by addition of NH3·H2O diluted solutions until the pH was 8. Subsequently, the sample was dried at 70 °C for 12 h and then heated to 500 °C for 5 h under a flow of N2. In the case in which the quantity of GF was 2 g, 7 mg of Zr(NO3)4·5H2O was dissolved in 12.5 mL of deionized water to prepare a 0.1 wt % precursor solution (14, 21, 28, and 35 mg of Zr(NO3)4·5H2O for 0.2, 0.3, 0.4, and 0.5 wt %, respectively). TGF without ZrO2 modification was also heated to 500 °C for 5 h under a flow of N2 for comparison, and is denoted as GF. 2.2. Characterization. The morphologies of GF and ZrO2/GFs were characterized by scanning electron microscopy (SEM) (S-4800, Hitachi), and the surface chemical element was analyzed by energydispersive X-ray spectroscopy (EDX). The structures of the ZrO2/GF composite were further examined by transmission electron microscopy (TEM) (Tecnai G2 F30, FEI). The crystallographic structures of the electrodes were investigated by X-ray diffraction (XRD) (D/max-2500 PC, Rigaku). The XRD profiles were recorded between 10° and 80° with a scan rate of 4 deg min−1. Raman spectra were recorded by a LabRam HR800 Raman spectrophotometer (LabRam HR800, Horiba) at an excitation radiation wavelength of 532 nm. 2.3. Electrochemical Measurements. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were conducted in a three-electrode configuration with a PARSTAT 2273 electrochemical workstation as reported previously.41 The CV test was performed from 0 to 1.4 V in 0.1 M VO2+ + 2 M H2SO4 and from B


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ACS Applied Materials & Interfaces −1.0 to 0 V in 0.1 M V3+ + 2 M H2SO4 at ambient temperature. The EIS test was obtained under a polarization potential of 0.7 V in 1.5 M VO2+ + 3 M H2SO4 and −0.4 V in 1.5 M V3+ + 3 M H2SO4 with a potential of 5 mV in the frequency ranging from 100 kHz to 10 mHz. The setup of the VFB single cell was represented in detail in our previously published works.48−50 ZrO2/GF (5 × 5 × 0.5 cm) was employed as the positive and negative electrodes, respectively. For comparison, GF and TGF were also studied under the same conditions. Nafion 115 (7 × 7 cm) was used as the ion exchange membrane.51,52 The flow-cell test was charged and discharged ranging from 50 to 250 mA cm−2 under a potential between 0.8 and 1.65 V.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Electrode. The morphologies of GF and ZrO2/GFs are characterized by SEM and TEM. As shown in Figure 1, it can be seen that the average diameter of the carbon fibers in GF is ca. 15 μm and that the surface of the carbon fibers is quite smooth. As shown in Figure 1b−f, ZrO2 nanoparticles are uniformly distributed on the surface of the carbon fibers. With an increase of the mass ratio of ZrO2, more precipitates are found on the surface of the carbon fibers, and an agglomeration is observed when the mass ratio of ZrO2 is over 0.5 wt %. It is expected that the strong interaction of carbon fibers and ZrO2 nanoparticles could prevent the agglomeration and maintain the homogeneous particle size of ZrO2 at ratios in the range of 0.1−0.5 wt %. As shown in Figure 1g, the particle size of the ZrO2 nanoparticles ranges from 10 to 35 nm. The structures of the ZrO2/GF are further examined by TEM as shown in Figure 1h. The spacing value of the lattice fringe is determined to be 0.3 nm, which agrees well with the (101) planes of tetragonal ZrO2 crystals. EDX is further employed to examine the element content of the electrodes. It is verified that only C (97.9 wt %) and O (2.1 wt %) elements are found in GF as shown in Figure 1j. As shown in Figure 1k, it is also verified that C (91.16 wt %), O (8.6 wt %), and Zr (0.24 wt %) elements are found in the 0.3 wt % ZrO2/GF sample. By means of the EDX results, we can also verify the mass ratio of ZrO2 to guarantee accurate quality. Take 0.3 wt % ZrO2/GF for example. It is acceptable that the mass ratio of ZrO2 measured and calculated by EDX is 0.32 wt %. In contrast to the GF, the content of the O element is increased after the ZrO2 decoration. This shall be ascribed to the abundance of oxygen-containing groups (e.g., hydroxyl groups) anchored onto the ZrO2 surface. As shown in Figure 2a, characteristic peaks at 2θ values of 25.1° and 43.2° are attributed to the diffraction peaks of the GF support. Two additional peaks at 30.3° and 50.4° are found in the ZrO2/GF samples, which correspond to the typical XRD patterns of tetragonal ZrO2 (JCPDS no. 49-1642). Moreover, no fluctuations of the main peaks associated with the GF are detected, which indicates that no structural distortions have been achieved by the modification of ZrO2. Figure 2b presents the Raman spectra of the pristine GF and ZrO2/GF composite electrodes. The Raman spectrum of GF displays a D band at 1353 cm−1, which is related to the disordered structures. In addition, a G band around 1600 cm−1 can be assigned to the well-ordered sp2 carbon. The ratio of the intensities (ID/IG) is an indicator of the number of carbon defects; however, the value of ID/IG is almost the same for all the electrodes. It is confirmed that no structure defects are generated after ZrO2 modification, which is in agreement with the XRD results. 3.2. Electrolyte Accessibility Evaluation. The digital photos of electrolyte accessibility to GF, TGF, and ZrO2/GF are shown in Figure 3. It is revealed that the spherical

Figure 2. XRD patterns (a) and Raman spectra (b) of GF and ZrO2/ GFs.

Figure 3. Digital photographs of electrolyte accessibility on various electrodes: (a) GF, (b) TGF, (c) 0.3 wt % ZrO2/GF.

electrolyte droplets of different sizes are formed with higher contact angles of more than 90°, when the electrolyte is dropped on the surface of the GF as shown in Figure 3a. Meanwhile, the electrolyte droplets on the GF surface almost remain unchanged for 24 h. Parts b and c of Figure 3 show that 200 μL of the electrolyte solution is immediately adsorbed once the electrolyte is dropped on the surface of TGF and 0.3 wt % ZrO2/GF. Although there is a smaller contact angle in contrast with that of GF when the electrolyte is dropped on TGF, the electrolyte droplet is gradually adsorbed within 5 s. Besides that, it is also identified that the electrolyte droplet is immediately adsorbed as long as the electrolyte is dropped on 0.3 wt % ZrO2/GF. On account of the electrolyte accessibility, it is suggested that the significantly improved hydrophilicity of GF is ascribed to the modification of ZrO2. 3.3. CV and EIS Studies. The configuration of the working electrode (WE) used for the CV and EIS tests is shown in Figure 4a. As shown in Figure 4b,c, it is obvious that the corrosion currents of the Ti plate in positive and negative electrolytes are so small that they can be ignored. CV curves of the GF, TGF, and 0.3 wt % ZrO2/GF at a scan rate of 1 mV s−1 in 0.1 M VO2+ + 2 M H2SO4 are compared in Figure 4b, and the data obtained from the CV results are listed in Table S1. It is evident that two main peaks correspond to the VO2+/VO2+ redox reaction and this redox reaction is a typical quasireversible process. The anodic and cathodic peak current C


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current density. The enhanced catalytic activity originates from the well-dispersed ZrO2 nanoparticles on GF. The CV curves of GF and ZrO2/GFs in 0.1 M VO2+ + 2 M H2SO4 at different scan rates are shown in Figure S1, and the relationship between ΔE of the electrodes and the scan rates is shown in Figure S2. It is obvious that the onset potential of the VO2+/VO2+ redox couples on 0.3 wt % ZrO2/GF is quite smaller and the peak current density is higher than that on the pristine GF, indicating a better electrochemical kinetics on 0.3 wt % ZrO2/GF. Moreover, as shown in Figure S2, ΔE of 0.3 wt % ZrO2/GF is the smallest among all the electrodes under different scan rates. To further investigate the reaction kinetics, the peak current density as a function of the square root of scan rate is demonstrated in Figure S3, and the slope value of Figure S3 is also presented in Figure S4. As shown in Figures S1−S4, it can be seen that 0.3 wt % ZrO2/GF exhibits the best electrochemical activity toward the VO2+/VO2+ redox reaction. According to the Randles−Sevcik equation shown in eqs S1 and S2,53 the D value ranges of GF and 0.3 wt % ZrO2/GF are exhibited in Table 1. As shown in Table 1, an evident improvement of the diffusion process is gained after ZrO2 modification due to the higher D values. Table 1. Diffusion Coefficient (D) Data of Vanadium Ion Species in the Solution of 0.1 M VO2+ + 2 M H2SO4 on GF and 0.3 wt % ZrO2/GF sample GF 0.3 wt % ZrO2/GF

D of VO2+ (cm2 s−1) −6

(4.35−7.04) × 10 (7.26−11.76) × 10−6

D of VO2+ (cm2 s−1) (5.98−9.67) × 10−6 (9.39−15.20) × 10−6

Figure 5a shows the Nyquist plots of GF, TGF, and ZrO2/ GFs under a polarization potential of 0.7 V in 1.5 M VO2+ + 3 M H2SO4 at ambient temperature. All Nyquist plots consist of a semicircle part at high frequency which is related to the charge transfer process and a linear part at low frequency representing the diffusion process. It is verified that the electrochemical process is mix-controlled by the charge transfer and diffusion process.25,54 The Nyquist plots are fitted with the equivalent circuit of the inset in Figure 5a, where Rb is the bulk solution resistance, Rct is the charge transfer resistance, CPE is the double-layer capacitance, and W is the Warburg impedance.25,54 It can be seen that the Rct values of 0.1−0.5 wt % ZrO2/GFs are 0.936, 0.457, 0.186, 0.438, and 0.385 Ω, respectively, while the Rct of TGF is 1.693 Ω (GF, 85.9 Ω). Figure 5b shows the Nyquist plots of GF, TGF, and ZrO2/GFs under a polarization potential of −0.4 V in a negative electrolyte. It can be seen that the Rct values of 0.1−0.5 wt % ZrO2/GFs are 0.894, 0.581, 0.385, 0.558, and 0.490 Ω, respectively, while the Rct of TGF is 1.789 Ω (GF, 97.8 Ω). In conclusion, ZrO2/GF could reduce the Rct value significantly and enhance the electrochemical activity, which is also in consonance with the CV results. 3.4. XPS Analysis. To understand the elemental composition and functional groups on GF and 0.3 wt % ZrO2/GF, the surface chemistries of the electrodes were investigated by X-ray photoelectron spectroscopy (XPS) measurement, and the relative contents of various functional groups obtained from curve fitting of C 1s spectra are listed in Table 2. It can be obviously found from Figure 6a,b that C 1s has several electronic states. On the basis of the reported literature,23,25 the main peak at 284.3 eV is assigned to the graphitized carbon and the rest of the peaks correspond to the defective carbon (285.0 eV), C−OH (286.3 eV), and COOH

Figure 4. Photograph of the configuration of the working electrode (a). CV curves of the Ti plate blank, GF, TGF, and 0.3 wt % ZrO2/ GF: (b) in 0.1 M VO2+ + 2 M H2SO4, (c) in 0.1 M V3+ + 2 M H2SO4.

densities of GF are 45.4 and 40.8 mA cm−2 (TGF, 52.8 and 50.3 mA cm−2), while those of 0.3 wt % ZrO2/GF are increased to 57.6 and 53.3 mA cm−2, respectively. The peak separations (ΔE) of the VO2+/VO2+ redox couples on GF are 0.308 and 0.296 V on TGF. However, the ΔE of VO2+/VO2+ redox couples on 0.3 wt % ZrO2/GF is dramatically decreased to 0.224 V. Therefore, the electrochemical activity of the VO2+/ VO2+ redox couple is in the order 0.3 wt % ZrO2/GF > TGF > GF. As presented in Figure 4c, the V2+/V3+ redox reaction is also a typical quasi-reversible process. The anodic and cathodic peak current densities of GF are 41.6 and 45.8 mA cm−2 (TGF, 47.2 and 50.6 mA cm−2), and those of 0.3 wt % ZrO2/GF are 51.6 and 54.0 mA cm−2, respectively. ΔE of V2+/V3+ redox couples on GF (0.291 V) is close to that on TGF (0.285 V), while ΔE on 0.3 wt % ZrO2/GF is decreased to 0.223 V. These results indicate that the electrochemical activity toward the V2+/V3+ redox couple is in the order 0.3 wt % ZrO2/GF > TGF > GF. In conclusion, it can be seen that 0.3 wt % ZrO2/GF displays the best electrochemical activity for both negative and positive redox reactions due to the smaller ΔE and higher peak D


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Figure 5. Nyquist plots of GF, TGF, and ZrO2/GFs: (a) under a polarization potential of 0.7 V in 1.5 M VO2+ + 3 M H2SO4, (b) under a polarization potential of −0.4 V in 1.5 M V3+ + 3 M H2SO4.

Table 2. Contents (%) of Various Functional Groups Obtained from Curve Fitting of C 1s Spectra sample

graphitized carbon (284.3 eV)

defective carbon (285.0 eV)

C−OH (286.3 eV)

COOH (289.0 eV)

total (C−OH, COOH)

GF 0.3 wt % ZrO2/GF

59.4 43.2

34.3 41.2

2.3 6.3

4.0 9.3

6.3 15.6

Figure 6. Curve-fitting spectra of XPS C 1s of GF (a) and 0.3 wt % ZrO2/GF (b). (c) Curve-fitting spectra of XPS Zr 3d of 0.3 wt % ZrO2/GF.

at 184.8 eV is attributed to the Zr 3d3/2 spintronics signal, which reveals that we have successfully synthesized the uniform immobilization of ZrO2 nanoparticles on GF via this facile immersion-precipitation approach. It is indicated that considerable quantities of −OH functional groups are absorbed on the surface of ZrO2 nanoparticles and the ZrO2/GF composite electrode tends to generate various forms of oxygen-containing functional groups (C−OH, COOH), thereby promoting the catalytic activity of the redox reaction of VO2+/VO2+. In addition, modification with ZrO2 nanoparticles is beneficial for the activation of the O−H bond and the scission of the C−H bond; therefore, the ZrO2/GF composite electrode can facilitate the oxygen transfer process and reduce the reaction activation energy, which results in the obvious enhancement of the redox reaction. 3.5. VFB Single-Cell Performance. The performances of VFBs with GF, TGF, and ZrO2/GFs at current densities of 50, 100, 150, 200, and 250 mA cm−2 (five cycles for each current

(289.0 eV). As shown in Table 2, 0.3 wt % ZrO2/GF demonstrates a total oxygen-containing functional group (C− OH, COOH) content of 15.6%, which is significantly higher than that of GF with a value of 6.3%. In addition, 0.3 wt % ZrO2/GF also possesses more defective carbon content (41.2%) than that of GF (34.3%). The trend of XPS C 1s spectra reveals a decrease of the graphitized carbon and an increase of defective carbon after ZrO2 modification; therefore, more defect sites can serve as the active sites and increase the specific surface area as well, resulting in a better electrochemical activity. According to the redox reactions of VO2+/VO2+, it is indicated that the oxygen transfer may be the rate-controlling step. As a consequence, 0.3 wt % ZrO2/GF displays a substantially higher electrocatalytic performance than GF, which is in agreement with the CV and EIS results. On the other hand, Zr 3d has two electronic states. On the basis of the published literature,44 the peak at 182.6 eV is attributed to the Zr 3d5/2 spintronics signal and the other peak E


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Figure 7. Digital photograph and testing parameters of VFB single cells during the charge−discharge test and the cell performance of VFBs assembled with GF, TGF, and ZrO2/GFs at certain current densities: (a) Coulombic efficiency (CE), (b) voltage efficiency (VE), (c) energy efficiency (EE). The cycle life test of VFB was performed under a current density of 100 mA cm−2.

rarely been reported in previous works.3,20 The outstanding performance of the VFB with the 0.3 wt % ZrO2/GF electrode could be ascribed to the bifunctional effects introduced by the ZrO2 nanoparticles, which can improve the electrolyte accessibility and enhance the electrochemical activity by facilitating the diffusion kinetics, providing more oxygen functional groups, and reducing the reaction activation energy. To further investigate the stability of TGF and 0.3 wt % ZrO2/GF electrodes in the VFB, the cycle life test continued to be performed under a current density of 100 mA cm−2 after the charge−discharge test at a current density of 250 mA cm−2. As shown in the right part of Figure 7, the CEs of the TGF and ZrO2/GF are around 92% at a current density of 100 mA cm−2 and remain almost unchanged during the cycle life test. The EE value of the 200th cycle of 0.3 wt % ZrO2/GF is 76.6%, while that of the TGF is only 68.4%. Moreover, no obvious EE decay of the ZrO2/GF electrode is observed during the cycle life test, indicating that the outstanding stability of the ZrO2 nanoparticles remained over repetitive cycles in flowing electrolyte, which can be confirmed by the SEM images and EDX results shown in Figure 1i,l. The charge−discharge curves of VFBs with GF and ZrO2/ GFs are exhibited in Figure 8. It is notable that the ZrO2/GF electrodes all possess a lower charge voltage and a higher discharge voltage compared with GF at specific current

density) are shown in Figure 7. All VFB single cells were tested at the same time as shown in the top photograph of Figure 7. For the GF, TGF, and ZrO2/GFs, the Coulombic efficiency (CE) values are almost the same at the same current density and the CE values increase with increasing current density due to the decreased time of vanadium ion crossover through Nafion 115 membranes.51 It is notable that the VFBs with GF and TGF electrodes at 250 mA cm−2 even seem to be incapable of performing in the charge and discharge process because of their poor electrochemical activity, while ZrO2/GFs demonstrate their superior performance due to the enhanced electrochemical activity after ZrO2 modification. The detailed data from Figure 7 are shown in Table S2. As shown in Figure 7b,c, it is obvious that the voltage efficiency (VE) and energy efficiency (EE) of ZrO2/GFs are extremely superior to those of the GF and TGF under the same current density, especially at large current density. The VFB with the 0.3 wt % ZrO2/GF electrode demonstrates the best performance, and the corresponding VE and EE at a current density of 200 mA cm−2 are 71.9% and 67.4%, respectively, which are obviously higher than those of the VFBs assembled with TGF (VE = 61.8% and EE = 58.4%) and GF (VE = 57.3% and EE = 53.8%). Moreover, even at a current density of 250 mA cm−2, the CE, VE, and EE of the VFB with the 0.3 wt % ZrO2/GF electrode are 94.6%, 65.7%, and 62.1%, respectively, which have F


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ess:23,25 (1) diffusion of the VO2+ (VO2+ in the reduction process) from the solution to the surface of the graphite fibers; (2) VO2+ (VO2+ in the reduction process) adsorption and ion interchanges with the oxygen-containing groups, which take place on the surface of the graphite fibers; (3) charge transfer reaction from VO2+ to VO2+ (VO2+ to VO2+ in the reduction process); (4) VO2+ (VO2+ in the reduction process) desorption and ion exchange with hydrogen ions, forming a surface VO2+ (VO2+ in the reduction process); (5) transfer of VO2+ (VO2+ in the reduction process) back to the solution. As illustrated in Scheme 1, the abundant hydroxyl groups on the surface of the ZrO2 nanoparticles as well as the hydroxyl groups on the surface of the graphite fibers can serve as the active sites and catalyze the redox reaction of VO2+/VO2+. Combined with the CV and EIS analysis results, owing to the dramatic decrease of the R ct value and increased diffusion coefficient, the modification of the ZrO2 nanoparticles can promote the redox reaction of VO2+/VO2+ by facilitating the electron transfer reaction and reducing the reaction activation energy.13 Besides, on account of the modification of ZrO2 nanoparticles, the O−H bond can also be activated, which results in reducing the reaction activation energy, and these hydroxyl groups are able to promote the accessibility of the vanadium electrolyte. In conclusion, the enhanced electrochemical performance of redox reactions could be ascribed to the bifunctional effects introduced by ZrO2 modification. 3.7. Evaluation of Feasibility. By analyzing the technological parameters of some common modifications or treatments for the electrode in the VFB, we are now able to provide practical guidelines for further research. As summarized in Table 3, it is clearly found that the previous modification or treatments of GF can be categorized into three types. Research on surface modification of the GF electrode through some physicochemical treatments was mostly reported in the early stage. Although some of these modification methods are not particularly tedious, the modified GF electrodes show limited enhancement of VFB performance due to the low surface specific area. Shortly afterward, research was gradually turned to nanocarbon material−GF composite electrodes due to the increased surface specific area and enhanced electrocatalytic activity, but the syntheses of the composite electrodes are much complex than the methods described in this study and the performance of the VFB is also inferior. Recently, there have

Figure 8. Charge−discharge curves of VFBs with GF and ZrO2/GF electrodes at various current densities: (a) 100 mA cm−2, (b) 150 mA cm−2, (c) 200 mA cm−2.

densities of 100, 150, and 200 mA cm−2. Therefore, a better cell performance with higher charge−discharge capacities is achieved by ZrO2 modification due to its ability to reduce the electrochemical polarization of the VO2+/VO2+ redox reaction. The detailed data from Figure 8 are shown in Table S3. It is evident that 0.3 wt % ZrO2/GF achieves the highest capacity, which is consistent with the CV and EIS results. 3.6. Proposed Catalytic Mechanism. The proposed catalytic mechanism on the GF and ZrO2/GF electrodes toward the VO2+/VO2+ redox reaction is shown in Scheme 1. The catalytic mechanism may involve the following proc-

Scheme 1. Proposed Mechanism on the GF and ZrO2/GF Electrodes toward the VO2+/VO2+ Redox Reaction

G


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Table 3. Comparison of Various Modification (Activation) Methods of Graphite Felt (GF) or Carbon Felt (CF) Electrode Material for VFB Application parameters of the VFB single cell type process the GF (CF) itself

carbon nanomaterial modification

metal or metal oxide modification

modification or activation method

reagent and treatment conditions

electrode size (cm)

max j (mA cm−2)

CE (%)

VE (%)

EE (%)

60

97.3

83.0

80.8

24

60 60 40 80 60 60 20 80 40

97.4

81.1

87.6 91.6 98.0 98.8 98.8 97.6 80.2

71.4 82.5 69.7 75.1 85.6 69.6 92.4

79.0 66.0 62.5 75.6 68.3 74.2 84.6 67.9 74.1

21 22 23 25 27 28 29 30 31

ref

thermal

400 °C in air, 30 h

acid acid mixed acid electrochemical hydrothermal Fenton’s reagent NH3/O2 microwave oxygen plasma

98% H2SO4, boiling for 5 h 68% HNO3, 115 °C, 2 h H2SO4/HNO3 (3:1), 80 °C, 8 h 1 M H2SO4, 50 mA cm−2, 560 mA h g−1 25% NH3·H2O, 180 °C, 15 h (Fe2+ + H2O2 + H+), 0.5 h NH3/O2 (1:1), 500 °C, 24 h 400 °C, 15 min VO2 = 5 sccm, 100 W, 2 min

5×6 5×5 3×4 5.4 × 6

corona + H2O2 MWCNT

4 A, 15 s × 2 + 30% H2O2, 1 h coating on CF, 100 °C, 24 h

5×5 2.5 × 2.5

145.8 70

96.7 98.6

70.3 76.1

68.0 75.0

32 15

SWCNT graphene oxide mesoporous carbon N-doped carbon graphene nanosheet N-doped CNT CNF/CNT CNF metal impregnation

coating on CF, 80 °C, 5 h electroreduction deposited onto GF coconut-shell-derived and coating corn-protein-derived and coating graphite, ball-milling, 24 h, and coating CVD growth, 800 °C CVD growth, 700 °C in C2H4 CVD growth, 660−690 °C in C2H4 Pt4+, Pd2+, Au4+, Mn2+, Te4+, In3+, Ir3+

2.5 × 2.5 2.5 × 2.5 5×5 5 cm2 5 cm2 5 cm2 5 cm2 2×2

20 20 10 150 150 10 100 30

92.2

96.8

91.8 98.0 97.0 81.3 97.7 94.9

92.6 70.0 67.8 94.7 67.5 87.8

89.3 81.8 85.0 68.6 65.8 77.0 66.0 83.3

16 18 55 56 17 57 33 34 35

Ir Bi Mn3O4 WO3 TiO2/C PbO2 Nb2O5(W)

10% H2IrCl6, 450 °C, 5 min BiCl3, electrodeposited onto GF 1 M (C2H3O2)2Mn·4H2O, 200 °C, 12 h (NH4)6W7O24·6H2O, 550 °C in N2, 2 h TnBT, EG, 120 °C, 10 h pulse electrodeposited NH4[NbO(C2O4)(H2O)]·xH2O, (NH4)10W12O41·5H2O, 170 °C, 48 h Zr(NO3)4·5H2O, precipitation

10 cm2 10 cm2 ×3 ×5 ×4 cm2

60 150 40 60 200 80 150

67.1 97.2 85.4 95.1 90.0 99.7 98.3

87.2 80.3 90.2 81.8 73.0 78.3 74.3

58.5 78.1 77.0 78.1 65.4 78.1 72.8

36 37 38 39 40 42 43

5×5

150

92.5

78.3

72.4

this work

200 250

93.7 94.6

71.9 65.7

67.4 62.1

ZrO2

5 cm2 2×2 5×5

3 5 3 5

4. CONCLUSIONS

been many attempts to modify the GF electrode through precious metal or metal oxides. However, the high cost of the precious metal and less recoverable reserves hinder the application of the previously reported literature. For the VFB, it is well-known that CE is determined by the membrane while VE is the essential parameter to evaluate the activity of the electrode. As shown in Table 3, the difference in CE values of the reported literature may be caused by different types and areas of the membranes used. Therefore, to demonstrate the promoting effect of ZrO2 nanoparticles, we have to compare the VE of all electrodes in Table 3. In fact, VFB performance with ZrO2/GFs electrodes in this study is superior to that of the previous works, especially at high current density. What is more, the VFB with ZrO2/GF electrodes can operate at a current density up to 250 mA cm−2, which has rarely been reported in previous works. Consequently, ZrO2/ GFs would be an excellent candidate for commercial application which would promote the development of VFB applications with high power density and excellent energy storage efficiency.

In summary, an environmental, economic, facile, and one-step precipitation method based on ZrO2 as a bifunctional catalyst approach has been applied to improving the electrochemical activity of electrodes for the VFB. The improved performance of VFB application is ascribed to the bifunctional effects (the enhanced electrochemical activity and improved electrolyte accessibility) after ZrO2 modification. In addition, 0.3 wt % ZrO2/GF demonstrates excellent performance and stability as the electrode in flowing electrolyte after 200 cycles in the charge−discharge test. At 200 mA cm−2, the CE, VE, and EE of the VFB with the 0.3 wt % ZrO2/GF electrode are 93.7%, 71.9%, and 67.4%. The EE value is 13.6% higher than that of GF and 9.0% higher than that of TGF. Moreover, even at an especially high current density of 250 mA cm−2, the CE, VE, and EE of the VFB with the 0.3 wt % ZrO2/GF electrode are 94.6%, 65.7%, and 62.1%, respectively, which have rarely been reported in previous works so far. H


Research Article


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03761. Randles−Sevcik equation, discharge capacity of VFBs at various current densities, CV curves at different scan rates, and relationship among the peak separation, peak current density, and scan rate (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21576154) and Basic Research Project of Shenzhen City (Grant JCYJ20140417115840235).

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J


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