Ta2O5-nanoparticle-modified graphite felt as a high-performance

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 3019−3028

pubs.acs.org/journal/ascecg

Ta2O5‑Nanoparticle-Modified Graphite Felt As a High-Performance Electrode for a Vanadium Redox Flow Battery Anteneh Wodaje Bayeh, Daniel Manaye Kabtamu, Yu-Chung Chang, Guan-Cheng Chen, Hsueh-Yu Chen, Guan-Yi Lin, Ting-Ruei Liu, Tadele Hunde Wondimu, Kai-Chin Wang, and Chen-Hao Wang* Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 10607 Taipei, Taiwan S Supporting Information *

ABSTRACT: To increase the electrocatalytic activity of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs) toward the VO2+/VO2+ redox couple, we prepared a stable, high catalytic activity and uniformly distributed hexagonal Ta2O5 nanoparticles on the surface of GF by varying the Ta2O5 content. Scanning electron microscopy (SEM) revealed the amount and distribution uniformity of the electrocatalyst on the surface of GF. It was found that the optimum amount and uniformly immobilized Ta2O5 nanoparticles on the GF surface provided the active sites, enhanced hydrophilicity, and electrolyte accessibility, thus remarkably improved electrochemical performance of GF. In particular, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results showed that the Ta2O5-GF nanocomposite electrode with a weight percentage of 0.75 wt % of Ta2O5 to GF exhibited the best electrochemical activity and reversibility toward the VO2+/VO2+ redox reaction, when compared with the other electrodes. The corresponding energy efficiency was enhanced by ∼9% at a current density of 80 mA cm−2, as compared with untreated GF. Furthermore, the charge−discharge stability test with a 0.75 wt % Ta2O5-GF electrode at 80 mA cm−2 showed that, after 100 cycles, there was no obvious attenuation of efficiencies signifying the best stability of Ta2O5 nanoparticles, which strongly adhered on the GF surface. KEYWORDS: Vanadium redox flow batteries, Ta2O5 nanoparticles, Redox couple (VO2+/VO2+)

■

INTRODUCTION The recent rapid growth of global economy and social development has significantly increased the demand for electrical energy. To meet this huge demand, fossil fuels have been the main source of electricity generation. However, these fuels produce greenhouse gases, which cause substantial environmental damage. Several countries have therefore focused on reducing greenhouse gas emissions and explored eco-friendly sources of energy, such as wind and solar energy, that are environmentally friendly, relatively cheap, and abundant as renewable alternatives to conventional fossil sources.1 However, the integration of renewable energy into the electrical grid is limited because of fluctuations in output, unavailability of light (e.g., sunlight in the night), unpredictability (wind power) with changes in the Earth’s climate, and the requirement of being used in conjunction with an energy storage device.2 Large-scale electrical energy storage (EES) systems are important to store energy during peak production and release it into the grid during peak demand.3 Among the most promising large-scale energy storage technologies, redox flow batteries (RFBs) are highly efficient electrochemical energy storage and conversion devices in which energy is stored and converted through chemical changes in species dissolved in the working electrolyte.4 Unlike conventional secondary batteries, RFBs employ electroactive materials © 2018 American Chemical Society

stored externally in separate liquid reservoirs and pumped to and from the power converting device when energy is being transferred. As a result, the energy capacity and power output scale up independently, with the former determined by the volume and concentration of the electrolyte and the latter depending on the electrode size.5−7 Moreover, RFBs generally have a long service life, rapid response to load changes, deep discharge tolerance, flexible design, and low environmental impact.1,8 Furthermore, the electrolyte acts as a built-in cooling system that allows heat to be readily extracted from the stack, which reduces the need for complex thermal management systems.9 In particular, the all vanadium redox flow battery (VRFB), proposed by Skyllas-Kazacos et al. in the 1980s, has emerged as a promising candidate for electrical energy storage since it involves the same vanadium ion as the active species in both the anolyte (V3+/V2+) and the catholyte (VO2+/VO2+).1,10 Hence, the all VRFBs have the following advantages: low metal cation cross contamination, long life, and low environmental impact. Received: August 10, 2017 Revised: January 18, 2018 Published: January 29, 2018 3019

DOI: 10.1021/acssuschemeng.7b02752 ACS Sustainable Chem. Eng. 2018, 6, 3019−3028

Research Article

ACS Sustainable Chemistry & Engineering

catalysis, photodegradation, batteries, and hydrogen production.28 Ta2O5 has been prepared by sol−gel, solvothermal, hot filament metal vapor deposition, electrochemical anodization, and microemulsion methods.29 Most of these methods yield poor crystalline Ta2O5 nanoparticles.30 Subsequently, hightemperature calcination is generally required to yield the final product, which can lower the surface area and decrease the catalytic activity. In particular, the electrocatalytic activity of the IrO2/Ta2O5 composite electrode on the surface of the titanium substrate has been clearly prepared and demonstrated by Raghu31 using electrochemical characterization and single cell tests for VRFBs. However, it has been noted that, in addition to high cost, the low conductivity of IrO2 and the substrate material leads to a low energy efficiency of the single cell performance. Hence, the durability and catalyst synthesis method for VRFBs requires further exploration. In this study, we successfully synthesized low cost Ta2O5 nanoparticles on a GF surface as an electrocatalyst to enhance electrochemical activity toward the VO2+/VO2+ redox reaction. Uniformly distributed and controlled amounts of Ta2O5 nanoparticles were introduced on the surface of GFs via simple, green, low cost, and one-step hydrothermal reactions for the first time. Decorating the Ta2O5 nanoparticles on the GF surface for the VO2+/VO2+ redox reaction notably improved the electrochemical activity and hydrophilicity and hence provided active sites and an enhanced electrolyte accessibility. We investigated the synthetic conditions, amount, and distribution of the electrocatalyst, as well as the favorable effects of the Ta2O5 nanoparticles on the surface of the GF.

For all VRFBs, a standard cell potential of 1.255 V is obtained through the following vanadium redox reactions of the VO2+/VO2+ and V3+/V2+ redox couples. Positive electrode: VO2 + + H 2O ↔ VO2+ + 2H+ + e−

E 0 = +1.00V (1)

Negative electrode: V3 + + e− ↔ V2 +

E 0 = −0.255V

VO2 + + H 2O + V3 + ↔ VO2+ + 2H+ + V2 +

(2) E 0 cell = +1.255V

(3)

Despite these persuasive merits, the VRFB market presence has been limited owing to its low energy efficiency. Previous studies have proven that, on the negative side (V3+/ V2+), the redox reaction rate is faster, but on the positive side (VO2+/VO2+), the reaction kinetics is slower and more complicated owing to the adverse surface of the electrode materials and the complex reaction steps.11 In addition, the range of electrocatalytic materials for this couple is limited because of the high redox potential of the couple, for which few electrode materials are stable in acidic media.11,12 Therefore, the efficiency of VRFBs should be improved before their broader market dissemination. The efficiency of a VRFB as the main component of the battery is mainly determined by the surface chemistry of the electrode, which provides an electroactive surface for conducting electrons, and ions for redox reactions.13 Since the supporting electrolyte in VRFBs is sulfuric acid, among the reported self-supporting carbon-based molecules (CBMs), polyacrylonitrile (PAN)-type graphite felts (GFs) are preferred as electrodes by virtue of their corrosion resistance in acidic solution, wide operation potential range, high electrical conductivity, and high surface area at a reasonable cost.14,15 In general, owing to its high graphitization temperature, GF is highly hydrophobic and possesses a low surface area, which leads to insufficient surface active sites and poor electrolyte accessibility, resulting in poor kinetic reversibility and electrochemical activity.13−16 Thus, attempts have been made to modify these materials to improve their electrochemical activities. Effective treatment methods have been suggested to improve the electrochemical properties of GFs in VRFBs; these methods include thermal treatment, acid treatment, and heavy metal deposition, such as Pt, Au, Ir, Pd, and Ru.17−19 However, these approaches are not practical because of the tedious treatment time, dangerous concentrated acids, high cost, limited mineral resources, and susceptibility to hydrogen evolution.16 As in another approach, an efficient, high activity, and low cost catalyst is required to promote the surface activity of GF. In this regard, low cost metal oxides, such as CeO2, Mn3O4, WO3, and PbO2, were reported as electrocatalysts for modification of the surface of GF for VRFB application.20−23 However, the electrochemical performance of the catalyst depends on the nanosized particles as well as their uniform distribution on the surface of the GF support.24 In addition, the preparation methods of such a catalyst involve complex and tedious steps.25 Tantalum oxide (Ta2O5) is among the most important metal oxides owing to its interesting chemical and physical properties. It has an excellent chemical stability, good conductivity, corrosive resistance, and strong adhesion with substrates, and it is less toxic, environmentally friendly, and low cost.26,27 Thus, its usage is essential in a variety of chemical reactions such as

■

EXPERIMENTAL METHODS

Preparation of Ta2O5 Nanoparticles on the Surface of GFs. All chemicals for this experiment were used directly without further purification. To avoid impurities and enhance wettability, untreated graphite felt (U-GF) was heat treated in air at 400 °C for 30 h and cool down to room temperature to obtain heat-treated graphite felt (GF). The detailed synthetic process for the Ta2O5 nanoparticles was adapted for our purpose from the literature.32 A typical synthesis process is briefly discussed below. First, the commercial Ta2O5 powder was dissolved in 10 mL of a 0.1 M hydrofluoric acid (40%, HF) solution; then the pH value of the solution was turned to 9, and white precipitate formed using 5 mL of a 1 M ammonia solution (30%, NH4OH). The as-prepared precipitate was again dissolved in a 30 mL of a mixed solution containing a 1 M ammonia solution and 1 M hydrogen peroxide (25%−28%, H2O2) (5:1 by volume ratio) to obtain the desired precursor material. The prepared solution mixture was heated in an oil bath at 70 °C for 1 h to increase H2O2 decomposition (Scheme 1). The GF was then immersed into this solution and transferred into a Teflon-lined autoclave to allow for hydrothermal reaction at 240 °C for 12 h. After the reaction was completed, the autoclave was cooled to room temperature naturally. Finally, the

Scheme 1. Schematic Representation for the Synthesis of Ta2O5-GF with Different Weight Percents of Nanocomposite Materials

3020


Research Article

ACS Sustainable Chemistry & Engineering modified GF was taken out, washed thoroughly with deionized water and then with ethanol, and dried overnight at 60 °C in the air before being characterized. In the case in which the quantity of GF was 2.5 g, 120 mg of Ta2O5 was dissolved in a solution to prepare a 0.5 wt % precursor solution (180 and 240 mg of Ta2O5 for 0.75 and 1 wt %, respectively), which could be controlled by adjusting the amount of Ta2O5 added into the solution (Figure S1, see the diagram in the Supporting Information). During the preparation of the working electrodes, the average measured amount of active material loading on GF was 3.2, 4.1, and 4.9 mg cm−2 for electrodes denoted previously as 0.5 wt % Ta2O5-GF, 0.75 wt % Ta2O5-GF, and 1 wt % Ta2O5-GF electrodes, respectively. Calculations were made on the basis of differences in weight of GF before and after loading. U-GF without Ta2O5 nanoparticles was also heat treated for comparison. Physicochemical Characterization. The crystallographic structure of the Ta2O5-GF powder was determined by X-ray diffraction (Bruker D2-phaser diffractometer, source of radiation (Cu Kα, λ = 1.540 Å), scanning between 10° to 70° (2θ) at a scanning rate of 2° min−1). The morphologies and microstructure of U-GF, GF, and Ta2O5-GF were investigated by field-emission scanning electron microscopy (FESEM, JSM 6500F, JEOL) and field-emission transmission electron microscopy (TEM, JEOL-2100). The TEM sample was prepared by scratching the surface of Ta2O5 modified GF, dispersed in ethanol, sonicated, and dripped onto regular carboncoated Cu grids. Energy-dispersive X-ray spectroscopy (EDX) was employed to analyze the surface elements of the samples. X-ray photoelectron spectroscopy (XPS) was performed to characterize the surface states and degrees of the functional groups. Raman spectra were recorded (Jobin-Yvon Lab-RAM HR800-confocal micro-Raman spectrometer) to identify the position and intensity of the D- and Gbands of the samples. The wetting property of the samples was analyzed by a contact angle test (FTA-125). Electrochemical Measurements. The electrochemical activity of the prepared electrodes was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), and the results were obtained under the control of EC-Lab software (Bio-Logic (SP-240) galvanostat/potentiostat). For CV measurements, a three-electrodesystem was used with a GF-modified electrode prepared by cutting circular samples with an area of 1.58 cm2, a standard Hg/Hg2SO4/ saturated K2SO4 electrode, and a platinum wire as the working, reference, and counter electrodes, respectively. The CV test was carried out in the potential range between −0.3 and 1.1 V vs Hg/ Hg2SO4 at different scan rates in a 0.05 M VOSO4 and 2 M H2SO4 electrolyte solution at room temperature. Electrochemical impedance spectra (EIS) were measured by sweeping within the frequency range 105 to 10−2 Hz under an open-circuit potential (OCP) in 0.05 M VOSO4 and 2 M H2SO4 with an excitation signal of 10 mV. The electrocatalytic performance of the as-prepared catalyst was measured using a single cell charge−discharge test as follows: The cell was assembled by sandwiching U-GF, GF, and Ta2O5-GFs electrodes on both the positive and negative sides with the total active area of 25 cm2 (5 cm × 5 cm). An untreated Nafion 117 (7 cm × 7 cm) membrane was employed as the separator. Two glass containers placed in both the negative and positive sides were filled with electrolytes containing a mixture of 1.6 M VOSO4 in a 2.5 M H2SO4 solution with a total volume of 60 mL for each as catholytes and anolytes, respectively. Electrolytes were pumped in and out of the stack using FMI pumps (QG 400) with a constant flow rate of 0.5 mL s−1. The charge−discharge performance of the flow cell was investigated using potentiostat/galvanostat (Bio-Logic, EC-Lab) with a potential window of 0.7−1.6 V at a constant current mode, under current densities of 40, 80, and 120 mA cm−2.

shown in Figure 1a. The XRD patterns of all samples showed a unique and broad characteristic peak at 26.1°, which can be

Figure 1. (a) X-ray diffraction patterns of GF and Ta2O5-GFs at various wt % ratios and (b) Raman spectra of U-GF, GF, and Ta2O5GFs at various wt % ratios.

attributed to the typical diffraction peak of the (002) graphitic plane in GFs.33 By comparing the XRD patterns of GF and Ta2O5-GF nanoparticles in all compositions, it can be concluded that decorating Ta2O5 nanoparticles did not change the position of graphitic diffraction peaks. Hence, the structures of GFs were not affected by the decoration of Ta 2 O5 nanoparticles. Additional peaks were also observed at 14°, 28°, 29°, 34°, and 37° in the Ta2O5-GF nanocomposite samples, which agrees well with the typical XRD patterns of Ta2O5 (JCPDS-no. 65-5464). Thus, Ta2O5-GFs exhibited a hexagonal crystal structure that belongs to the space group of P6/mmm. The intense peak pointed at 14° corresponds to the (100) diffraction, which is dominant over the (200) diffraction peak providing the evidence that Ta2O5 nanoparticles are highly oriented in the direction on the GF support, which is in agreement with the growth axis of hexagonal Ta2O5 nanoparticles. The XRD pattern clearly reveals the crystalline nature of the Ta2O5 nanoparticles. (This is in agreement with the selected area electron diffraction (SAED) pattern shown in the inset of Figure 3b.) In addition, no other impure phases were observed. Furthermore, no additional Ta2O5-GF nanocomposite peaks were observed at around 23.1° and 25°, which could be because the broad and dominant peak of GF appeared and overlapped with the Ta2O5-GF nanocomposite peaks. Raman spectroscopy can provide sufficient information for the characterization of graphite-based materials. From the Raman spectra shown in Figure 1b, the two typical peaks are the D-band at 1350 cm−1, which is an indicator of the vibrational stretching at sp3 defect sites, and the G-band at 1580 cm−1, which is generally associated with the vibrational stretching at sp2 defect sites.34 The intensity ratio of the Dand G-bands indicates the density of structural defects on the surface of the catalyst support. As can be seen from the figure, the ID/IG ratios with their corresponding values for each electrode are as follows: U-GF (ID/IG = 1.01), GF (ID/IG = 1.01), 0.5 wt % Ta2O5-GF (ID/IG = 1.00), 0.75 wt % Ta2O5-GF (ID/IG = 1.03), and 1 wt % Ta2O5-GF (ID/IG = 1.03). The ID/ IG ratios of all samples are approximately one, and the heat treatment and the additional decoration of Ta2O5 nanoparticles do not cause any obvious defects on the GF surface. In traditional GF surface modification methods, such as acid treatment, thermal treatment, and heavy metal deposition, an increment in catalytic activity toward the VO2+/VO2+ redox couple is attributable to the structural disordering formed on the surface graphite layer.35 However, the origin of enhance-

■

RESULTS AND DISCUSSION Structural and Morphology Characterization of GF and Ta2O5-GF Electrodes. The crystal structures and possible phase changes of the samples were investigated by wide-angle X-ray diffraction (XRD) spectra. The data for GF, 0.5 wt % Ta2O5-GF, 0.75 wt % Ta2O5-GF, and 1 wt % Ta2O5-GF are 3021


Research Article


Figure 2. SEM images of (a) U-GF, (b) GF, (c) 0.5 wt % Ta2O5-GF, (d and e) 0.75 wt % Ta2O5-GF, and (f) 1 wt % Ta2O5-GF.

Figure 3. (a) TEM image, (b) high-resolution TEM image and the corresponding selected area electron diffraction (SAED) pattern (the inset), (c) the high annular dark-field (HAADF) STEM image and the corresponding elemental mapping of (d) carbon, (e) oxygen, and (f) tantalum for the 0.75 wt % Ta2O5-GF nanocomposite electrode.

contains relatively lower amounts of Ta2O5 nanoparticles, its dispersion on the surface of GF is not uniform, and consequently, the carbon fiber is not completely covered by the Ta2O5 nanoparticles. Well-dispersed Ta2O5 nanoparticles are formed on the surface of GF, when the weight ratio of the powder to GF is found to be 0.75 wt % (Figure 2e). The EDX analysis verified the presence of Ta, C, and O as elemental constituents in the 0.75 wt % Ta2O5-GF nanocomposite sample (Figure S2). After careful scratching of the Ta2O5 nanoparticles from the GF surface, the morphology of the sample was further examined. Figure 3a shows the TEM image of Ta 2 O 5 nanoparticles with particle sizes of around 10−13 nm, which are strongly adhered onto the GF surface. Figure 3b indicates the crystal nature of Ta2O5 nanoparticles, and the lattice fringe is seen to be perpendicular to the growth direction . The measured spacing value of the lattice fringe is determined to be 0.23 nm, which agrees well with the (100) planes of the hexagonal Ta2O5 crystal. The selected area electron diffraction (SAED) pattern of the nanoparticles in the inset of Figure 3b

ment in activity for Ta2O5-GF nanocomposite electrodes is only attributed to the high catalytic activity and uniformly immobilized Ta2O5 nanoparticles on the surface of GF. Surface morphologies of U-GF, GF, and Ta2O5-GFs with different weight ratios were characterized by SEM and TEM. As shown in Figure 2a, the surface of U-GF has some impurities attached to it, which can impede electron transfer and ion adsorption.36 After the heat treatment (Figure 2b), the GF shows a smooth and clean surface without observable defects, which can provide suitable substrates for the growth of Ta2O5 nanoparticles.37 Figure 2c−f show that Ta2O5 nanoparticles are grown successfully on the surface of GF using the hydrothermal method; this might result in an increase in specific surface area of the Ta2O5-GF composite electrode.38 As observed in Figure 2f, when the weight ratio of Ta2O5 nanoparticles on the surface of GF increases to 1 wt % (1 wt % Ta2O5-GF electrode), the nanoparticles tend to form more precipitates and this is followed by agglomeration. Hence, it is believed to be unfavorable for electrocatalytic applications.38 In addition, for the 0.5 wt % Ta2O5-GF electrode (Figure 2c), since the sample 3022


Research Article


Figure 4. (a) The wide-scan XPS of GF and 0.75 wt % Ta2O5-GF and curve-fitting narrow-scan XPS spectra for (b) C 1s of GF, (c) C 1s of 0.75 wt % Ta2O5-GF, (d) O 1s of GF, (e) O 1s of 0.75 wt % Ta2O5-GF, and (f) Ta 4f of 0.75 wt % Ta2O5-GF electrodes.

spectra can be deconvoluted into four peaks at the binding energies as follows: graphitized carbon (284.7 eV), hybridized carbon (285.6 eV), hydroxyl species (286.6 eV), and carbonyl species (289.9 eV).21,40,41 From the C 1s spectra, the amount of oxygen-containing functional groups (COH, OC−O) on the 0.75 wt % Ta2O5-GF electrode increased remarkably from that on GF (8.1%) and 0.75 wt % Ta2O5-GF (16.9%). Owing to the involvement of oxygen-containing functional groups to activate the VO2+/VO2+ redox reaction, the O 1s spectra can also be deconvoluted into four peaks at the binding energies of 531.8, 532.5, 533.1, and 534.2 eV, corresponding to the C O, OH and TaO, CCO, and HOH bonds,39,42−44 respectively (Figure 4d,e), and the fitting results are also summarized in Figure S4b and Table S2. After modification with Ta2O5 nanoparticles, the peak intensity associated with OH and TaO groups increases tremendously from 12.66% for GF to 28.19% for the 0.75 wt % Ta2O5GF electrode. This increment is attributed to CO bond breakage and formation of OH and TaO bonds, which mainly acts as a vital precursor for electron transfer with the VO2+/VO2+ redox couple.40 In addition, the extensive increment in the contents of adsorbed water (0% for GF) and (22.82% for 0.75 wt % Ta2O5-GF) makes the surface of the modified electrode highly hydrophilic and enhances the adsorption of vanadium ions.45 Skyllas-Kazacos et al. previously reported that increasing the amount of oxygen-containing functional groups on the surface of the electrode can provide more active sites and hence improve the electrochemical activity toward the VO2+/VO2+ redox couple.46 Moreover, the two peaks of Ta 4f in Figure 4f includes a peak at 26.5 eV attributed to the Ta 4f7/2 spin orbital and the other peak at 28.4 eV attributed to the Ta 4f5/2 spin orbital, which corresponds to the Ta5+ cation.47 Electrolyte Accessibility Evaluation. The interaction forces between the liquid and solid surfaces play a key role in the phenomenon of “wettability” for the solid phase and “wetting” for the liquid. As shown in Figure 5a, the contact angle measurement of GF is 91.0°, and the size of the water

shows two rings with some dots on the rings, which are attributed to the (100) and (200) faces of the polycrystalline characteristics. Furthermore, the elemental mapping analysis results shown in Figure 3c−f clearly elucidate the coexistence and homogeneous distribution of Ta, O, and C within the electrode material. These results thus clearly validate that Ta2O5 nanoparticles are successfully and uniformly anchored on the surface of GF to form the electrode material. To obtain a greater understanding of the elemental composition and functional groups on the samples, X-ray photoelectron spectroscopy (XPS) was carried out. Figure 4a shows the XPS wide-scan spectra of the GF and 0.75 wt % Ta2O5-GF electrodes in the binding energy range 0−1300 eV, which consists of the photoemission spectra from C, O, and Ta. In contrast to GF, new peaks are shown in the spectrum of 0.75 wt % Ta2O5-GF, which can be attributed to the Ta peaks. Besides, the atomic percentage (at %) of C, O, and Ta in the 0.75 wt % Ta2O5-GF is 81.1%, 13.5%, and 5.4%, respectively. Compared with the pristine GF, the atomic concentration of oxygen increased after the decoration of Ta2O5 nanoparticles (Table 1). From Figure 4a and Figure S3b, in contrast to the Table 1. Atomic Percentage (at %) of C, O, and Ta from Figure 4a GF 0.75 wt % Ta2O5-GF

C

O

Ta

91.19 81.1

8.81 13.5

5.4

case of GF, the ratio between O/C increased drastically after Ta2O5 nanoparticles were decorated on the GF surface. Therefore, the surface compositions such as oxygen-containing functional groups (CO, OH, OCO, and HO H) increased on the surface of the 0.75 wt % Ta2O5-GF electrode, which plays a major role in enhancing the electrochemical performance of the redox reaction.39 Figure 4b,c presents the peak fitting of C 1s for the GF and 0.75 wt % Ta2O5-GF electrodes, respectively, and the corresponding results are summarized in Figure S4a and Table S1. The C 1s 3023


Research Article


Figure 5. Digital photographs of electrolyte accessibility and contact angle measurements of (a) GF, (b) 0.5 wt % Ta2O5-GF, (c) 0.75 wt % Ta2O5GF, and (d) 1 wt % Ta2O5-GF.

droplet remains the same for more than 18 h since the GF surface is highly hydrophobic in nature.45 The contact angle measurement of the 0.5 wt % Ta2O5-GF is far lower than that of GF (32.0°), and the water droplet remains on the surface of the electrode for only 3 s (Figure 5b). In addition, it is also clearly observed from Figures 5c,d that, along with an increment in the weight percentage ratio of Ta2O5 nanoparticles on the surface of GF to those on the surface of the 0.75 and 1 wt % Ta2O5-GF electrodes, it is difficult to measure the contact angle for both samples. For both electrodes, the water droplet sags immediately on the surface, which is due to the super hydrophilicity of the oxygen-containing functional groups such as hydroxyl and carboxyl.40 The wettability and hence electrolyte accessibility of the electrodes are therefore arranged in the following order: 0.75 wt % Ta2O5-GF and 1 wt % Ta2O5-GF > 0.5 wt % Ta2O5-GF > GF, which would be favorable for the performance of VRFBs.13 Electrochemical Characterization of GF and Ta2O5GFs. The peak potential separation (ΔEp), peak current densities (Jpa and Jpc), and peak current density ratio (Jpa/Jpc) of the redox reaction are employed to analyze the electrochemical performance of the VRFB electrodes. To examine the performance toward the VO2+/VO2+ redox couple, the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were utilized on GF and Ta2O5-GFs with different contents. Figure 6a,b shows the CV curves of all

Table 2. Electrochemical Property Results Obtained from CV Curves (Figure 6) electrode GF 0.5 wt % Ta2O5-GF 1 wt % Ta2O5-GF 0.75 wt % Ta2O5-GF

Jpa/Jpc

ΔEp (V)

0.745 0.8

1.1 1.06

0.67 0.58

1.37

0.81

1.04

0.56

1.30

0.85

0.95

0.45

Jpa (mA cm−2)

Jpc (mA cm−2)

30.1 30.96

−27.53 −29.05

1.412 1.38

32.14

−32.31

36.18

−38.54

Epa (V) Epc (V)

wt % Ta2O5-GF, and 0.5 wt % Ta2O5-GF electrodes are increased to 36.18/38.54, 32.14/32.31, and 30.96/29.05 mA cm−2, respectively. The Jpa/Jpc ratio of GF is 1.1. Conversely, all Ta2O5-GF electrodes show a Jpa/Jpc ratio of nearly one, indicating that the Ta2O5 nanoparticles may improve the reversibility of the VO2+/VO2+ redox couple. Considering the values of ΔEp, the peak current density, and the Jpa/Jpc ratio, the electrochemical activity of the electrodes is in the order of 0.75 wt % Ta2O5-GF > 1 wt % Ta2O5-GF > 0.5 wt % Ta2O5-GF > GF. This suggests that the catalytic activity and the reversibility of the electrodes are enhanced by the decoration of Ta2O5 nanoparticles on the GF surface. The CV curves of GF and Ta2O5-GFs at different scan rates are shown in Figure S5, and the relationship between ΔEp of the electrodes with different scan rates is shown in Figure S6. From the figures, it is noticeable that the onset potential of the VO2+/VO2+ redox couple on the 0.75 wt % Ta2O5-GF electrode is smaller and the peak current density is higher than that for all other electrodes, indicating that the best electrochemical kinetics occurs on the surface of the 0.75 wt % Ta2O5-GF electrode. In addition, as shown in Figure S6, among all electrodes, the ΔEp of the 0.75 wt % Ta2O5-GF is the smallest under different scan rates. Similarly, the anodic and cathodic peak current densities of the VO2+/VO2+ redox couple are linearly proportional to the square root of the scan rates (Figure 6b), suggesting that the faster mass transfer processes on the surface of the 0.75 wt % Ta2O5-GF electrode.48 The slopes of the electrode are also slightly steeper than all other electrodes, which indicates that the faster diffusion process of VO2+/VO2+ occurs on the surface of the 0.75 wt % Ta2O5-GF.35 (The detail estimation for the apparent diffusion coefficient (D) and electrochemical surface area (ECSA) of the electrodes are shown in the Supporting Information.) Such results agree well with the previous analyses that the excellent wettability of the electrode improves the reversibility of the vanadium redox reaction.49 The electrocatalytic activity of each electrode material toward the VO2+/VO2+ redox couple was further investigated using EIS. As shown in Figure 7, all samples demonstrate the Nyquist plots composed of a typical Randles model with the Warburg impedance. Thus, the higher catalytic activity of the VO2+/

Figure 6. (a) CV curves of various sample electrodes in 0.05 M VOSO4 and 2 M H2SO4 solutions at a scan rate of 3 mVs−1 and (b) peak current densities vs square root of scan rates with VO2+/VO2+ redox couple for various samples.

electrodes at a scan rate of 3 mVs−1 and the corresponding peak current density of various samples vs the square root of scan rates, respectively. All parameters are summarized and listed in Table 2. The ΔEp for the VO2+/VO2+ redox couple on GF is 0.67 V. However, the ΔEp values are significantly decreased to 0.45, 0.56, and 0.58 V for the 0.75 wt % Ta2O5-GF, 1 wt % Ta2O5-GF, and 0.5 wt % Ta2O5-GF electrodes, respectively. The anodic/cathodic peak current densities of GF are 30.1/ 27.53 mA cm−2, while for those of the 0.75 wt % Ta2O5-GF, 1 3024


Research Article


transfer of an oxygen atom, which is likely to be the ratedetermining step in the overall mechanism.54 First VO2+ ions diffused from the bulk electrolyte and ion-exchange with H+ of the oxygen-containing functional groups on the GF surface, and then they bond onto the electrode surface. Second, electron transfer occurs from the VO2+ to the electrode along the Ta OV bond as well as the transfer of one of the oxygen atoms on the functional group to the VO2+ forming VO2+. Finally, the VO2+ exchanges with H+ from solution and diffuses back into the bulk solution. For the discharge process, the reverse reactions would occur. Single Cell Performance. To understand the influence of decorating Ta2O5 nanoparticles on the surface of GF, a VRFB single cell was assembled for electrodes consisting of U-GF, GF, and various Ta2O5-GF electrodes. Figure 8 presents the

Figure 7. EIS curves of GF and Ta2O5-GFs with various weight % ratios in 0.05 M VOSO4 and 2 M H2SO4 solutions with an excitation signal of 10 mV under an open-circuit potential (OCP).

VO2+ redox couple reaction on the surface of GF can be attributed to the combined action of faster charge and mass transfer rates, consistent with the CV results.35,50 From the fitting results, as listed in Table 3, the charge transfer resistance Table 3. Parameters Resulting from the Nyquist Plot Obtained from EIS Results of Figure 7 electrode

Rs (Ω)

Rct (Ω)

GF 0.5 wt % Ta2O5-GF 1 wt % Ta2O5-GF 0.75 wt % Ta2O5-GF

3.8 3.5 3.2 3.75

47.3 38.9 19.8 13.1

Figure 8. (a) Charge−discharge curves of U-GF, GF, and Ta2O5-GFs with varying weight % ratios at a current density of 80 mA cm −2 and (b) efficiencies of the cell with 0.75 wt % Ta2O5-GF at different current densities of 40, 80, and 120 cm−2.

(Rct) values of the electrodes in decreasing order are listed as GF > 0.5 wt % Ta2O5-GF > 1 wt % Ta2O5-GF > 0.75 wt % Ta2O5-GF and are in good agreement with the CV results, as shown in Figure 6. The low Rct values for all modified Ta2O5GF electrodes indicate an enhanced electron transfer rate for the VO2+/VO2+ redox couple. The high improvement in kinetic polarization could be due to the presence of abundant oxygencontaining functional groups on the electrode surface that facilitate the VO2+/VO2+ redox reaction.51,52 The detailed electrocatalytic process and the proposed mechanism on Ta2O5-GF toward the VO2+/VO2+ redox reaction are shown in Scheme 2. Unlike most transition-metal oxides, the presence of four TaO and two TaO bonds in tantalum oxide (Ta2O5) might be the critical factor for the notable improvement in electrochemical activities of the Ta2O5GF nanocomposite electrode.53 As previously reported, the oxygen-containing functional groups (OH, COOH,  CO, and CO) on the electrode surface probably behave as active sites, catalyzing the VO2+/VO2+ redox reactions.49,13 It can be seen from this reaction mechanism that the charging and discharging processes at the positive electrode involve the

charge−discharge curves of those electrodes at a current density of 80 mA cm−2. To avoid unnecessary side reactions, such as O2 (g) and H2 (g) evolution, the potential window was limited between 1.6 and 0.7 V.55 During the constant current charging and discharging test, Coulombic efficiency (CE), energy efficiency (EE), and voltage efficiency (VE) are defined and calculated as follows: CE for the battery is determined by the ratio of charging and discharging capacity of the flow cell; VE can be calculated by taking the difference between the charging and discharging voltage, and EE is the derivative of the multiple products of CE and VE, (EE = VE × CE). The values of CE, VE, and EE are listed in Table 4. As shown in Figure 8a and Table 4, owing to the notable decrease in overpotential, the 0.75 wt % Ta2O5-GF electrode reveals the highest discharge voltage and the lowest charge voltage among all electrodes, with a VE of 78% and EE of 73.73% at a relatively higher current density of 80 mA cm−2. The high performance of the cell could

Scheme 2. Schematic Illustration of the Mechanism for the VO2+/VO2+ Redox Reaction Occurring in the Presence of Ta2O5 Nanoparticles on the Surface of the GF Electrode

3025


Research Article

ACS Sustainable Chemistry & Engineering Table 4. Efficiencies of All Electrodes at a Current Density of 80 mA cm−2 electrode

CE (%)

VE (%)

EE (%)

U-GF GF 0.5 wt % Ta2O5-GF 1 wt % Ta2O5-GF 0.75 wt % Ta2O5-GF

94.70 94.48 94.95 94.41 94.82

64.10 68.06 73.04 75.62 78.10

60.70 64.30 69.35 71.73 73.73

be attributed to the uniformly immobilized Ta2O5 nanoparticles on the surface of the GF fibers, creating more oxygencontaining functional groups on it. Hence, this provides the active sites for the adsorption of energy containing redox couples (VO2+/VO2+) and improves the electrolyte accessibility. In general, decorating the optimum amount of Ta2O5 nanoparticles on the surface of GF reduces electrochemical polarization by increasing the active sites toward the VO2+/ VO2+ redox couple for fast electron and mass transfer reactions.51 To explore the rate capability of the 0.75 wt % Ta2O5-GF electrode, the charge−discharge performance was measured at 40, 80, and 120 mA cm−2 current densities, which are shown in Figure 8b. When the current densities for the given electrode are increased, owing to the reduced time for vanadium ion to pass through the Nafion 117 membrane, the CE increases. However, the VE and consequently EE decreases since the fast charging and discharging rate process is always accompanied by an increase in the ohmic resistance and overpotential.1 The single cell performance of the electrode on the negative side (V3+/V2+) was also assembled using GF as the anode and 0.75 wt % Ta2O5-GF as the cathode. The electrochemical performance revealed that the electrodes exhibit almost similar EE, CE, and VE compared with that of the cell consisting of 0.75 wt % Ta2O5-GF electrodes on both anode and cathode sides (Figure S7 and Table S4). To further assess the stability and suitability of the 0.5 wt % Ta2O5-GF, 0.75 wt % Ta2O5-GF, and 1 wt % Ta2O5-GF electrodes in a strongly acidic solution during a battery test, 100 cycles of the charge−discharge operation were completed at a relatively higher current density of 80 mA cm−2. From Figure 9a−c, the results clearly show a consistent performance for all electrodes over the consecutive 100 cycles of the charging and discharging operation. The results indicate that all electrodes possess an excellent stability under the strongly acidic flowing electrolyte even after 100 cycles. In addition, there was no obvious attenuation of efficiencies, signifying that the best stability and electrocatalytic effect was created by Ta2O5 nanoparticles, which strongly adhere for a long time to the GF surface during repetitive cycling. Furthermore, the discharge capacity decay test of the 0.75 wt % Ta2O5-GF and GF (Figure 9d) shows that, compared to GF, the 0.75 wt % Ta2O5-GF electrode has a low capacity decay and stable EE, which demonstrate the outstanding stability of the electrocatalyst for VRFBs. The excellent performance of the Ta2O5-GF nanocomposite electrode could be attributed to the following reasons: (1) Decorating the optimum amount of Ta2O5 nanoparticles on the GF made it a highly hydrophilic surface, which enhances the wettability and hence increases accessibility for the VO2+/VO2+ redox couple. (2) The presence of the TaO bond within the Ta2O5 nanoparticles could facilitate efficient electron transport and play a critical role in the interfacial charge transfer process, which can facilitate and increase the overall cell performance.

Figure 9. (a) CE, (b) VE, (c) EE, and (d) discharge capacity of 100 charge−discharge cycles for the cell with the 0.5 wt % Ta2O5-GF, 0.75 wt % Ta2O5-GF, 1 wt % Ta2O5-GF, and GF at a current density of 80 mA cm−2.

(3) Owing to the small particle size of Ta2O5 in the Ta2O5-GF nanocomposite electrode, the distance for electrons to travel to surface becomes shortened.56 Meanwhile, small particle sizes increase the contact area between Ta2O5 and GF, leading to more charge transfer to GF and electrolyte. Moreover, as illustrated in many other studies,29,54 the increased surface area of the Ta2O5-GF nanocomposite due to the formation of small Ta2O5 particles on the GF surface can supply more surface active sites for the VO2+/VO2+ redox couple.

■

CONCLUSION In summary, to improve the electrocatalytic activity of GF electrodes toward the VO2+/VO2+ redox couple, hexagonal Ta2O5 nanoparticles were decorated on the surface of the electrode using a hydrothermal method, and these modified electrodes were applied to a VRFB application for the first time. The amount and distribution of Ta2O5 nanoparticles on the surface of the GF electrode play an important role in determining the electrochemical activity and the performance of the electrocatalyst. The electrochemical activity of the GF modified with Ta2O5 nanoparticles for the VO2+/VO2+ redox couple was notably improved, which could be attributed to the uniformly immobilized Ta2O5 nanoparticles on the surface of the GF fibers, creating more surface active oxygen-containing functional groups. Hence, this can provide the active sites and electrolyte accessibility for the adsorption of energy-containing redox species (VO2+/VO2+). In particular, the 0.75 wt % of the Ta2O5-GF electrode shows a high stability and provides a substantial enhancement to the battery performance in a highly acidic flowing solution. At a current density of 80 mA cm−2, the CE, VE, and EE of the cell are 94.82, 78, and 73.73%, respectively, which is much higher than all other electrodes. The charge−discharge stability test at 80 mA cm−2 shows that, after 100 cycles, there was no obvious attenuation of efficiencies, signifying that the best stability and electrocatalytic effect is created by Ta2O5 nanoparticles, which strongly adhere for a long time to the surface of GF during repetitive cycling. 3026


Research Article


■

based redox flow battery electrolytes. J. Power Sources 2013, 241, 173− 177. (12) Prifti, H.; Parasuraman, A.; Winardi, S.; Lim, T. M.; SkyllasKazacos, M. Membranes for redox flow battery applications. Membranes (Basel, Switz.) 2012, 2 (2), 275−306. (13) Kim, K. J.; Park, M.-S.; Kim, Y.-J.; Kim, J. H.; Dou, S. X.; SkyllasKazacos, M. A technology review of electrodes and reaction mechanisms in vanadium redox flow batteries. J. Mater. Chem. A 2015, 3 (33), 16913−16933. (14) Xi, J.; Xiao, S.; Yu, L.; Wu, L.; Liu, L.; Qiu, X. Broad temperature adaptability of vanadium redox flow batteryPart 2: Cell research. Electrochim. Acta 2016, 191, 695−704. (15) Chakrabarti, M. H.; Brandon, N. P.; Hajimolana, S. A.; Tariq, F.; Yufit, V.; Hashim, M. A.; Hussain, M. A.; Low, C. T. J.; Aravind, P. V. Application of carbon materials in redox flow batteries. J. Power Sources 2014, 253, 150−166. (16) Parasuraman, A.; Lim, T. M.; Menictas, C.; Skyllas-Kazacos, M. Review of material research and development for vanadium redox flow battery applications. Electrochim. Acta 2013, 101, 27−40. (17) Sun, B.; Skyllas-Kazacos, M. Modification of Graphite Electrode Materials for Vanadium flow battery ApplicationI. Thermal Treatment. Electrochim. Acta 1992, 37, 1253. (18) Sun, B.; Skyllas-Kazacos, M. Modification of Graphite Electrode Materials for Vanadium flow battery ApplicationPart II. Acid Treatments. Electrochim. Acta 1992, 37, 2459. (19) Noack, J.; Roznyatovskaya, N.; Herr, T.; Fischer, P. The Chemistry of Redox-Flow Batteries. Angew. Chem., Int. Ed. 2015, 54 (34), 9776−9809. (20) Jing, M.; Zhang, X.; Fan, X.; Zhao, L.; Liu, J.; Yan, C. CeO2 embedded electrospun carbon nanofibers as the advanced electrode with high effective surface area for vanadium flow battery. Electrochim. Acta 2016, 215, 57−65. (21) Kim, K. J.; Park, M.-S.; Kim, J.-H.; Hwang, U.; Lee, N. J.; Jeong, G.; Kim, Y.-J. Novel catalytic effects of Mn3O4 for all vanadium redox flow batteries. Chem. Commun. 2012, 48 (44), 5455−5457. (22) Wu, X.; Xu, H.; Lu, L.; Zhao, H.; Fu, J.; Shen, Y.; Xu, P.; Dong, Y. PbO2-modified graphite felt as the positive electrode for an allvanadium redox flow battery. J. Power Sources 2014, 250, 274−278. (23) Yao, C.; Zhang, H.; Liu, T.; Li, X.; Liu, Z. Carbon paper coated with supported tungsten trioxide as novel electrode for all-vanadium flow battery. J. Power Sources 2012, 218, 455−461. (24) Wang, W.; Luo, Q.; Li, B.; Wei, X.; Li, L.; Yang, Z. Recent progress in redox flow battery research and development. Adv. Funct. Mater. 2013, 23 (8), 970−986. (25) Li, B.; Gu, M.; Nie, Z.; Shao, Y.; Luo, Q.; Wei, X.; Li, X.; Xiao, J.; Wang, C.; Sprenkle, V.; Wang, W. Bismuth nanoparticle decorating graphite felt as a high-performance electrode for an all-vanadium redox flow battery. Nano Lett. 2013, 13 (3), 1330−5. (26) Yu, X.; Wei, Y.; Li, Z.; Liu, J. One-step synthesis of the single crystal Ta2O5 nanowires with superior hydrogen production activity. Mater. Lett. 2017, 191, 150−153. (27) Momeni, M. M.; Mirhosseini, M.; Chavoshi, M. Growth and characterization of Ta2O5 nanorod and WTa2O5 nanowire films on the tantalum substrates by a facile one-step hydrothermal method. Ceram. Int. 2016, 42 (7), 9133−9138. (28) Zhou, C.; Shang, L.; Yu, H.; Bian, T.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Mesoporous plasmonic Au-loaded Ta2O5 nanocomposites for efficient visible light photocatalysis. Catal. Today 2014, 225, 158− 163. (29) Ismail, A. A.; Faisal, M.; Harraz, F. A.; Al-Hajry, A.; Al-Sehemi, A. G. Synthesis of mesoporous sulfur-doped Ta2O5 nanocomposites and their photocatalytic activities. J. Colloid Interface Sci. 2016, 471, 145−154. (30) Tao, C.; Xu, L.; Guan, J. Well-dispersed mesoporous Ta2O5 submicrospheres: Enhanced photocatalytic activity by tuning heating rate at calcination. Chem. Eng. J. 2013, 229, 371−377. (31) Chandrabose Raghu, S.; Ulaganathan, M.; Lim, T. M.; Skyllas Kazacos, M. Electrochemical behaviour of titanium/iridium(IV) oxide:

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02752. Weight ratios, EDX images, wide-scan XPS and O/C ratios, chemical composition ratios, contents of various functional groups, CV curves, plots of peak separation vs scan rates, charge−discharge curves, and efficiencies of the electrodes (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +886-2-2730-3715. Fax: +886-2-2737-6544. ORCID

Chen-Hao Wang: 0000-0003-2350-3287 Author Contributions

The article was written through contributions of all authors. All authors have given approval to the final version of the article. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research is supported by the Institute of Nuclear Energy Research, Atomic Energy Council. We would like to acknowledge Professor Kuei-Hsien Chen at Academia Sinica and Professor Li-Chyong Chen at National Taiwan University for their helpful assistance.

■

REFERENCES

(1) Ponce de León, C.; Frías-Ferrer, A.; González-García, J.; Szánto, D. A.; Walsh, F. C. Redox flow cells for energy conversion. J. Power Sources 2006, 160 (1), 716−732. (2) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical energy storage for green grid. Chem. Rev. 2011, 111 (5), 3577−613. (3) Larcher, D.; Tarascon, J. M. Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 2015, 7 (1), 19−29. (4) Lin, K.; Gómez-Bombarelli, R.; Beh, E. S.; Tong, L.; Chen, Q.; Valle, A.; Aspuru-Guzik, A.; Aziz, M. J.; Gordon, R. G. A redox-flow battery with an alloxazine-based organic electrolyte. Nature Energy 2016, 1, 16102. (5) Brushett, F. R.; Vaughey, J. T.; Jansen, A. N. An All-Organic Nonaqueous Lithium-Ion Redox Flow Battery. Adv. Energy Mater. 2012, 2 (11), 1390−1396. (6) Pan, F.; Wang, Q. Redox Species of Redox Flow Batteries: A Review. Molecules 2015, 20 (11), 20499−517. (7) Gong, K.; Fang, Q.; Gu, S.; Li, S. F. Y.; Yan, Y. Nonaqueous redox-flow batteries: organic solvents, supporting electrolytes, and redox pairs. Energy Environ. Sci. 2015, 8 (12), 3515−3530. (8) Houser, J.; Clement, J.; Pezeshki, A.; Mench, M. M. Influence of architecture and material properties on vanadium redox flow battery performance. J. Power Sources 2016, 302, 369−377. (9) Xie, Z.; Xiong, F.; Zhou, D. Study of the Ce3+/Ce4+ Redox Couple in Mixed-Acid Media (CH3SO3H and H2SO4) for Redox Flow Battery Application. Energy Fuels 2011, 25 (5), 2399−2404. (10) Skyllas-Kazacos, M.; Robins, R. G.; Fane, A. G. New AllVanadium Redox Flow Cell. J. Electrochem. Soc. 1986, 133 (5), 1057− 1058. (11) Vijayakumar, M.; Wang, W.; Nie, Z.; Sprenkle, V.; Hu, J. Elucidating the higher stability of vanadium(V) cations in mixed acid 3027


Research Article

ACS Sustainable Chemistry & Engineering Tantalum pentoxide and graphite for application in vanadium redox flow battery. J. Power Sources 2013, 238, 103−108. (32) Zhu, G.; Lin, T.; Cui, H.; Zhao, W.; Zhang, H.; Huang, F. Gray Ta2O5 Nanowires with Greatly Enhanced Photocatalytic Performance. ACS Appl. Mater. Interfaces 2016, 8 (1), 122−7. (33) Di Blasi, A.; Di Blasi, O.; Briguglio, N.; Aricò, A. S.; Sebastián, D.; Lázaro, M. J.; Monforte, G.; Antonucci, V. Investigation of several graphite-based electrodes for vanadium redox flow cell. J. Power Sources 2013, 227, 15−23. (34) Estevez, L.; Reed, D.; Nie, Z.; Schwarz, A. M.; Nandasiri, M. I.; Kizewski, J. P.; Wang, W.; Thomsen, E.; Liu, J.; Zhang, J. G.; Sprenkle, V.; Li, B. Tunable Oxygen Functional Groups as Electrocatalysts on Graphite Felt Surfaces for All-Vanadium Flow Batteries. ChemSusChem 2016, 9 (12), 1455−61. (35) Jin, J.; Fu, X.; Liu, Q.; Liu, Y.; Wei, Z.; Niu, K.; Zhang, J. Identifying the Active Site in Nitrogen-Doped Graphene for the VO2+/ VO2+ Redox Reaction. ACS Nano 2013, 7 (6), 4764−4773. (36) Gao, C.; Wang, N.; Peng, S.; Liu, S.; Lei, Y.; Liang, X.; Zeng, S.; Zi, H. Influence of Fenton’s reagent treatment on electrochemical properties of graphite felt for all vanadium redox flow battery. Electrochim. Acta 2013, 88, 193−202. (37) Kabtamu, D. M.; Chen, J.-Y.; Chang, Y.-C.; Wang, C.-H. Electrocatalytic activity of Nb-doped hexagonal WO3 nanowiremodified graphite felt as a positive electrode for vanadium redox flow batteries. J. Mater. Chem. A 2016, 4 (29), 11472−11480. (38) Fu, S.; Zhu, C.; Song, J.; Engelhard, M. H.; Du, D.; Lin, Y. Three-dimensional Nitrogen-Doped Reduced Graphene Oxide/ Carbon Nanotube Composite Catalysts for Vanadium Flow Batteries. Electroanalysis 2017, 29, 1469. (39) Flox, C.; Skoumal, M.; Rubio-Garcia, J.; Andreu, T.; Morante, J. R. Strategies for enhancing electrochemical activity of carbon-based electrodes for all-vanadium redox flow batteries. Appl. Energy 2013, 109, 344−351. (40) Kabtamu, D. M.; Chen, J.-Y.; Chang, Y.-C.; Wang, C.-H. Wateractivated graphite felt as a high-performance electrode for vanadium redox flow batteries. J. Power Sources 2017, 341, 270−279. (41) Thu Pham, H. T.; Jo, C.; Lee, J.; Kwon, Y. MoO2 nanocrystals interconnected on mesocellular carbon foam as a powerful catalyst for vanadium redox flow battery. RSC Adv. 2016, 6 (21), 17574−17582. (42) Das, T.; Mahata, C.; Mallik, S.; Varma, S.; Sutradhar, G.; Bose, P. K.; Maiti, C. K. Interface Properties of Mixed (TiO2)1−x(Y2O3)x and (Ta2O5)1−x(Y2O3)x (0 ≤ x ≤ 1) Gate Dielectrics on Sulfur-Passivated GaAs. J. Electrochem. Soc. 2012, 159 (3), H323−H328. (43) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Hähner, G.; Spencer, N. D. Structural Chemistry of Self-Assembled Monolayers of Octadecylphosphoric Acid on Tantalum Oxide Surfaces. Langmuir 2000, 16 (7), 3257−3271. (44) Kabtamu, D. M.; Chang, Y.-C.; Lin, G.-Y.; Bayeh, A. W.; Chen, J.-Y.; Wondimu, T. H.; Wang, C.-H. Three-dimensional annealed WO3 nanowire/graphene foam as an electrocatalytic material for all vanadium redox flow batteries. Sustainable Energy & Fuels 2017, 1, 2091. (45) Goulet, M.-A.; Skyllas-Kazacos, M.; Kjeang, E. The importance of wetting in carbon paper electrodes for vanadium redox reactions. Carbon 2016, 101, 390−398. (46) Sun, B.; Skyllas-Kazacos, M. Modification of graphite electrode materials for vanadium redox flow battery application. Electrochim. Acta 1992, 37 (7), 1253−1260. (47) Devan, R. S.; Lin, C.-L.; Gao, S.-Y.; Cheng, C.-L.; Liou, Y.; Ma, Y.-R. Enhancement of green-light photoluminescence of Ta2O5 nanoblock stacks. Phys. Chem. Chem. Phys. 2011, 13 (29), 13441− 13446. (48) Gao, C.; Wang, N.; Peng, S.; Liu, S.; Lei, Y.; Liang, X.; Zeng, S.; Zi, H. Influence of Fenton’s reagent treatment on electrochemical properties of graphite felt for all vanadium redox flow battery. Electrochim. Acta 2013, 88, 193−202. (49) Wu, L.; Shen, Y.; Yu, L.; Xi, J.; Qiu, X. Boosting vanadium flow battery performance by Nitrogen-doped carbon nanospheres electrocatalyst. Nano Energy 2016, 28, 19−28.

(50) Wang, S.; Zhao, X.; Cochell, T.; Manthiram, A. Nitrogen-Doped Carbon Nanotube/Graphite Felts as Advanced Electrode Materials for Vanadium flow battery. J. Phys. Chem. Lett. 2012, 3, 2164. (51) Liu, T.; Li, X.; Xu, C.; Zhang, H. Activated Carbon Fiber Paper Based Electrodes with High Electrocatalytic Activity for Vanadium Flow Batteries with Improved Power Density. ACS Appl. Mater. Interfaces 2017, 9 (5), 4626−4633. (52) Zhou, Y.; Liu, L.; Shen, Y.; Wu, L.; Yu, L.; Liang, F.; Xi, J. Carbon dots promoted vanadium flow batteries for all-climate energy storage. Chem. Commun. 2017, 53 (54), 7565−7568. (53) Yao, C.; Zhang, H.; Liu, T.; Li, X.; Liu, Z. Carbon paper coated with supported tungsten trioxide as novel electrode for all-vanadium flow battery. J. Power Sources 2012, 218, 455−461. (54) Shen, Y.; Xu, H.; Xu, P.; Wu, X.; Dong, Y.; Lu, L. Electrochemical catalytic activity of tungsten trioxide- modified graphite felt toward VO2+/VO2+ redox reaction. Electrochim. Acta 2014, 132, 37−41. (55) Shao, Y.; Cheng, Y.; Duan, W.; Wang, W.; Lin, Y.; Wang, Y.; Liu, J. Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow Batteries: A Selected Review. ACS Catal. 2015, 5 (12), 7288− 7298. (56) Lin, C.; Yu, S.; Zhao, H.; Wu, S.; Wang, G.; Yu, L.; Li, Y.; Zhu, Z.-Z.; Li, J.; Lin, S. Defective Ti2Nb10O27.1: an advanced anode material for lithium-ion batteries. Sci. Rep. 2016, 5, 17836.

3028


Ta2O5-nanoparticle-modified graphite felt as a high-performance

Recommend Documents