Ta2O5-Nanoparticle-Modified Graphite Felt As a High-Performance

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TaO-nanoparticle-modified graphite felt as a highperformance electrode for vanadium redox flow battery Anteneh Wodaje Bayeh, Daniel Manaye Kabtamu, Yu-Chung Chang, Guan-Cheng Chen, HsuehYu Chen, Guan-Yi Lin, Ting-Ruei Liu, Tadele Hunde Wondimu, Kai-Chin Wang, and Chen-Hao Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02752 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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Synopsis – Decoration optimum and uniformly distributed Ta2O5 nanoparticles on GF surface provided the active sites, enhanced hydrophilicity and electrolyte accessibility, thus remarkably improved electrochemical performance of GF. 415x188mm (96 x 96 DPI)

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Ta2O5-nanoparticle-modified graphite felt as a high-performance electrode for 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. *Corresponding author, E-mail: [email protected] Tel: +886-2-2730-3715; Fax: +886-2-2737-6544

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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 stable, high catalytic activity, and uniformly distributed hexagonal Ta2O5 nanoparticles on the surface of GF by varying the Ta2O5 contents. 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 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 weight percentage of Ta2O5 to GF of 0.75 wt% 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 0.75 wt% Ta2O5-GF electrode at 80 mA cm−2 showed that after 50 cycles, there was no obvious attenuation of efficiencies signifying, the best stability of Ta2O5 nanoparticles which strongly adhered on the GF surface.

Key words: 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 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,

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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 active species in both the anolyte (V3+/V2+) and the catholyte (VO2+/VO2+).10-11 Hence, the all VRFBs have the following advantages: low metal cation cross contamination, long life, and low environmental impact.


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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: + ↔ + 2 + , = +1.00 (1) Negative electrode: + ↔ , = −0.255 (2) + + ↔ + 2 + , cell = +1.255 (3) Despite these persuasive merits, VRFBs 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.12 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.12-13 Therefore, the efficiency of VRFBs should be improved before their broader market dissemination. The efficiency of 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.14 Since the supporting electrolyte in VRFBs is sulfuric acid, among the reported selfsupporting 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.15-16 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.14-17 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.18-20 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.17 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 4 ACS Paragon Plus Environment


for modification of the surface of GF for VRFB application.21-24 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.25 In addition, the preparation methods of such a catalyst involves complex and tedious steps.26 Tantalum oxide (Ta2O5) is among the most important metal oxides owing to its interesting chemical and physical properties. It has excellent chemical stability, good conductivity, corrosive resistance, less toxicity, environmentally friendly, strong adhesion with substrates, and low cost.27-28 Thus, its usage is essential in a variety of chemical reactions such as catalyst, photodegradation, batteries, and hydrogen production.29 Ta2O5 has been prepared by sol‐gel, solvothermal, hot filament metal vapor deposition, electrochemical anodization, and microemulsion methods.30 Most of these methods yield poorly crystalline Ta2O5 nanoparticles.31 Subsequently, high‐temperature 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 IrO2:Ta2O5 composite electrode on the surface of titanium substrate has been clearly prepared and demonstrated by Raghu32 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 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 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 a 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 enhanced electrolyte accessibility. We investigated the synthetic conditions, amount, and distribution of the electrocatalyst, as well as the favorable effects of Ta2O5 nanoparticles on the surface of the GF.


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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.33 A typical synthesis process is briefly discussed below. First, the commercial Ta2O5 powder was dissolved in 10 mL of 0.1 M hydrofluoric acid (40%, HF) solution and then the pH value of the solution was turned to 9 and white precipitate formed using 5 mL of 1 M ammonia solution (30%, NH4OH). The as-prepared precipitate was again dissolved in a 30 mL mixed solution containing 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).

Scheme 1 Schematic representation for the synthesis of Ta2O5-GF with different wt% nanocomposite materials. 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 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 6 ACS Paragon Plus Environment


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 were 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 = 1.540Å), 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 carbon-coated 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 G-bands 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 electrode-system was used with a GF-modified electrode prepared by cutting circular samples with an area of 1.58 cm2, a standard Hg/Hg2SO4/sat.K2SO4electrode, 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 0.05 M VOSO4 + 2 M H2SO4 electrolyte solution at room temperature. Electrochemical impedance spectra (EIS) were measured by sweeping within the frequency range of 105 to 10−2


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Hz under open circuit potential (OCP) in 0.05 M VOSO4 + 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 x5 cm). An untreated Nafion 117 (7 cm x 7 cm) membrane was employed as the separator. Two glass containers placed in both the negative and positive sides were filled with electrolyte containing a mixture of 1.6 M VOSO4 in a 2.5 M H2SO4 solution with 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, ECLab) with a potential window of 0.7 to 1.6 V at a constant current mode, under current densities of 40, 80, and 120 mA cm−2.

Results and Discussion Structural and morphology characterization of GF and Ta2O5-GFs 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% Ta2O5GF, and 1 wt% Ta2O5-GF are shown in Figure 1a. The XRD patterns of all samples showed a unique and broad characteristic peak at 26.1°, which can be attributed to the typical diffraction peak of (002) graphitic plane in GFs.34 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 Ta2O5 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 hexagonal crystal structure 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 selected area 8 ACS Paragon Plus Environment


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.

Figure 1 (a) X-ray diffraction patterns of GF and Ta2O5-GFs at various wt% ratio and (b) Raman spectra of U-GF, GF, and Ta2O5-GFs at various wt% ratios. Raman spectroscopy can provide sufficient information for characterization of graphitebased materials. From the Raman spectra shown in Figure 1b, the two typical peaks are the Dband 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.35 The intensity ratio of the D and G band 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 VO2+/VO2+ redox couple is attributable to the structural disordering formed on the surface graphite layer.36


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However, the origin of enhancement in activity for Ta2O5-GF nanocomposite electrodes are 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.37 After the heat treatment (Figure 2b), the GF shows a smooth and clean surface without observable defects, which can provide suitable substrate for the growth of Ta2O5 nanoparticles.38

Figure 2 SEM images of (a) U-GF, (b) GF, (c) 0.5 wt% Ta2O5-GF, (d, e) 0.75 wt% Ta2O5-GF and (f) 1 wt% Ta2O5-GF. Figures 2(c-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 Ta2O5GF composite electrode.39 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.39 In addition, for the 0.5 wt% Ta2O510 ACS Paragon Plus Environment


GF electrode (Figure 2c), since the sample 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 constituent 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 Ta2O5 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 .

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 0.75 wt% Ta2O5-GF nanocomposite electrode. 11 ACS Paragon Plus Environment

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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 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 Figures 3(c-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 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) O1s of GF, (e) O1s of 0.75 wt% Ta2O5-GF, and (f) Ta 4f of 0.75 wt% Ta2O5-GF electrodes. Figure 4a shows the XPS wide-scan spectra of the GF and 0.75 wt% Ta2O5-GF electrodes in the binding energy range of 0–1300 eV, which consists of the photoemission spectra from C, O, and 12 ACS Paragon Plus Environment


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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 elemental concentration of C, O and Ta in the 0.75 wt% Ta2O5-GF are 94.61%, 4.52% and 0.87%, respectively. Compared with the pristine GF, the atomic concentration of oxygen increased after decoration of Ta2O5 nanoparticles (Table 1). Table 1 elemental concentration of C, O and Ta from Figure 4a. Sample

Atomic contents (at. %)

GF 0.75 wt% Ta O -GF 2

5

C

O

Ta

89.14

8.81

---

81.4

13.8

5.4

From Figure 4a and Figure S3b, in contrary to the 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.40 Figures 4b and c present the peak fitting of C1s for the GF and 0.75 wt% Ta2O5-GF electrodes, respectively, and the corresponding results are summarized in Figure S4a and Table S1. The C1s 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).22, 41-42

From the C1s 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 0.75wt% Ta2O5-GF (16.9%). Owing to the involvement of oxygen-containing functional groups to activate the VO2+/VO2+ redox reaction, the O1s 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,40, 43-45 respectively (Figures 4d and 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 group increases tremendously from 12.66% for GF to 28.19% for 0.75 wt% Ta2O5-GF electrode. This increment is attributed to C=O bond breakage and formation of –OH and/Ta=O bonds, which mainly acts as a vital 13 ACS Paragon Plus Environment

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precursor for electron transfer with the VO2+/VO2+ redox couple.41 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 modified electrode highly hydrophilic and enhances the adsorption of vanadium ions.46 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 towards the VO2+/VO2+ redox couple.47 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 Ta+5 cation.48 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 droplet remains the same for more than 18 h since the GF surface is highly hydrophobic in nature.46 The contact angle measurement of 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).

Figure 5 Digital photographs of electrolyte accessibility and contact angle measurements of (a) GF, (b) 0.5 wt% Ta2O5-GF, (c) 0.75 wt% Ta2O5-GF, and (d) 1 wt% Ta2O5-GF. In addition, it is also clearly observed from Figures 5c and 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 wt% 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.41 The wettability and hence electrolyte accessibility of the electrodes are therefore arranged in the following order: (0.75 wt% Ta2O5-GF, 1 wt% Ta2O5-GF) > 0.5 wt% Ta2O5-GF > GF, which would be favorable for the performance of VRFBs.14 Electrochemical characterization of GF and Ta2O5-GFs The peak potential separation (∆Ep), 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. Figures 6a and b show the CV curves of all 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 ∆Ep for the VO2+/VO2+ redox couple on GF is 0.67 V.

Figure 6 (a) CV curves of various sample electrodes in 0.05 M VOSO4 + 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. However, the ∆Ep 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 0.75 wt% 15 ACS Paragon Plus Environment

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Ta2O5-GF, 1 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 ∆Ep, 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 ∆Ep of the electrodes with different scan rates are 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 ∆Ep 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 process on the surface of 0.75 wt% Ta2O5-GF electrode.49 The slopes of the electrode also slightly steeper than all other electrodes, which indicates that the faster diffusion process of VO2+/VO2+ on the surface of 0.75 wt% Ta2O5-GF.36 (The detail estimation for 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 vanadium redox reaction.50 Table 2 Electrochemical properties result obtained from CV curves (Figure 6). Electrode

Jpa(mA cm−2)

Jpc (mA cm−2)

Epa (V)

Epc (V)

Jpa/Jpc

∆Ep (V)

GF

30.1

-27.53

1.412

0.745

1.1

0.67

0.5 wt% Ta2O5-GF

30.96

-29.05

1.38

0.8

1.06

0.58

1 wt% Ta2O5-GF

32.14

-32.31

1.37

0.81

1.04

0.56

0.75 wt% Ta2O5-GF

36.18

-38.54

1.30

0.85

0.95

0.45



The electrocatalytic activity of each electrode material towards 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+/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.36, 51 From the fitting results, as listed in Table 3, the charge transfer resistance (Rct) values of the electrodes in decreasing order are listed as: GF > 0.5 wt% Ta2O5-GF > 1wt% 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 Ta2O5-GF 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 oxygen-containing functional groups on the electrode surface that facilitate VO2+/VO2+ redox reaction.52-53

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


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Table 3 Parameters resulting from the Nyquist plot obtained from EIS results of Figure 7 Electrode

Rs(Ω)

Rct (Ω)

GF

3.8

47.3

0.5 wt% Ta2O5-GF

3.5

38.9

1 wt% Ta2O5-GF

3.2

19.8

0.75wt % Ta2O5-GF

3.75

13.1

The detailed electrocatalytic process and the proposed mechanism on Ta2O5-GF towards VO2+/VO2+ redox reaction are shown in Scheme 2. Unlike most transition metal oxides, the presence of four Ta=O and two Ta—O bonding in tantalum oxide (Ta2O5) might be the critical factor for the notable improvement in electrochemical activities of the Ta2O5-GF nanocomposite electrode.54 As the previous report, the oxygen containing functional groups such as (—OH, — COOH, —C=O, and —C—O) on the electrode surface probably behave as active sites, catalyzing the VO2+/VO2+ redox reactions.50, 55 It can be seen from this reaction mechanism that, the charging and discharging processes at the positive electrode involve the transfer of an oxygen atom, which is likely to be the rate determining step in the overall mechanism.56

Scheme 2 The schematic illustration of the mechanism for the VO2+/VO2+ redox reaction occurring in the presence of Ta2O5 nanoparticles on the surface of GF electrode. 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 18 ACS Paragon Plus Environment


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 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.57 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 notable decrease in overpotential, the 0.75 wt% Ta2O5-GF electrode

Figure 8 (a) Charge–discharge curves of U-GF, GF and Ta2O5-GFs with varies weight % ratios at 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.


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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 be attributed to the uniformly immobilized Ta2O5 nanoparticles on the surface of the GF fibers, creating more oxygen-containing functional groups on it. Table 4 Efficiencies of all electrodes at current density of 80 mA cm−2 Electrode

CE (%)

VE (%)

EE (%)

U-GF

94.70

64.10

60.70

GF

94.48

68.06

64.30

0.5 wt% Ta2O5-GF

94.95

73.04

69.35

1 wt% Ta2O5-GF

94.41

75.62

71.73

0.75 wt% Ta2O5-GF

94.82

78.10

73.73

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 towards the VO2+/VO2+ redox couple for fast electron and mass transfer reaction.52 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 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+) were also assembled using GF as the anode and 0.75 wt% Ta2O5-GF at the cathode. The electrochemical performance revealed that the electrodes exhibit almost similar EE, CE and VE compared with that of the cell consists of 0.75 wt% Ta2O5-GF electrodes on both anode and cathode side (Figure S9 and Table S4).



Figure 9 (a) CE, (b) VE, (c) EE, and (d) discharge capacity of 100 charge–discharge cycles for the cell with 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. 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 strongly acidic solution during battery test, 100 cycles of charge– discharge operation were completed at a relatively higher current density of 80 mA cm−2. From Figure 9, the results clearly show consistent performance for all electrodes over the consecutive 100 cycles of charging and discharging operation. The results indicate that all electrodes possess 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) show that, compared to GF, 0.75 wt% Ta2O5-GF 21 ACS Paragon Plus Environment

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electrode has low capacity decay and stable EE, which demonstrate that 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 optimum amount of Ta2O5 nanoparticles on the GF made it highly hydrophilic surface, which enhances the wettability and hence increase accessibility for VO2+/VO2+ redox couple. (2) The presence of 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. (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.58 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,30, 56 the increased surface area of Ta2O5-GF nanocomposite due to the formation of small Ta2O5 particles on GF surface can supply more surface active sites for VO2+/VO2+ redox couple.



Conclusion In summary, to improve the electrocatalytic activity of GF electrodes towards 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 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 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 Ta2O5-GF electrode shows high stability and provides 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 50 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.


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Associated content Supporting Information •

Figure S1 The weight% ratio of Ta2O5-GF as a function of the concentration of Ta2O5 added in the solutions.

•

Figure S2 EDX image analysis of the GF and 0.75 wt% Ta2O5-GF.

•

Figure S3 (a) The wide-scan XPS and (b) O/C ratios for GF, 0.5 wt% Ta2O5-GF, 0.75 wt % Ta2O5-GF, and 1wt% Ta2O5-GFs.

•

Figure S4 Chemical composition ratio of functional groups from curve fitting of (a) C1s and (b) O1s XPS spectra for GF and 0.75 wt% Ta2O5-GF samples.

•

Figure S5 CV curves of GF and Ta2O5-GFs at different scan rates in a solution of 0.05 M VOSO4 + 2 M H2SO4.

•

Figure S6 Plots of peak separation (∆Ep) vs. scan rates in a solution of 0.05 M VOSO4 + 2 M H2SO4.

•

Figure S7 Charge-discharge curves of (a) GF-GF (black) and 0.75 wt% Ta2O5-GF-0.75 wt% Ta2O5-GF (red), (b) GF-GF and GF-0.75 wt% Ta2O5-GF cells at the anode and cathode respectively at a current density of 80 mA cm −2.

•

Table S1 Contents (%) of various functional groups obtained from curve fitting of C1s spectra.

•

Table S2 Contents (%) of various functional groups obtained from curve fitting of O 1s spectra.

•

Table S3 Apparent Diffusion coefficient (D) data of vanadium ion species in the solution of 0.05 M VOSO4 + 2.5 M H2SO4 on GF and 0.75 wt% Ta2O5-GF electrodes.

•

Table S4 Efficiencies of the electrodes at current density of 80 mA cm−2



Author information Corresponding Author E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 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 Prof. Kuei-Hsien Chen at Academia Sinica and Prof. Li-Chyong Chen at National Taiwan University for their helpful assistance.


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56. 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. Electrochimica Acta 2014, 132 (Supplement C), 37-41. 57. 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 Catalysis 2015, 5 (12), 7288-7298. 58. 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. 2015, 5, 17836.



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Synopsis – Optimum amount of Ta2O5 nanoparticles on GF provided the active sites, enhanced hydrophilicity and remarkably improved electrochemical performance of GF.


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Ta2O5-Nanoparticle-Modified Graphite Felt As a High-Performance

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