Hierarchical oxygen rich-carbon nanorods: Efficient and durable electrode for all-vanadium redox flow batteries

Journal of Power Sources 445 (2020) 227329

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Hierarchical oxygen rich-carbon nanorods: Efficient and durable electrode for all-vanadium redox flow batteries Md. Abdul Aziz, Syed Imdadul Hossain, Sangaraju Shanmugam * Department of Energy Science & Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 50-1, Sang-Ri, Hyeonpung-Myeon, Dalseong-Gun, Daegu, 42988, South Korea

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� A simple, and highly scalable zein protein-derived NCNR is successfully synthesized. � The NCNR exhibits abundant active ox­ ygen and nitrogen sites. � The NCNR has improved the electron transfer rate and vanadium ion transfer kinetics. � The NCNR/CF exhibits outstanding cy­ clic efficiency in VRB than pristine CF.

A R T I C L E I N F O

A B S T R A C T

Keywords: Vanadium redox flow battery NCNR Carbon felt electrode Electrochemical activity

We describe the fabrication of hierarchical oxygen and nitrogen enriched-carbon electrode materials from zein and polyacrylonitrile by a simple electrospinning technique for durable and high rate all-vanadium redox flow batteries (VRBs). The nitrogen-doped carbon nanorods (NCNR) provide abundant oxygen-rich and nitrogen 2þ 3þ ion redox re­ active sites, and thereby, enhancing the catalytic activity toward both VO2þ/VOþ 2 and V /V actions by improving ion transfer kinetics and faster electron transfer rate in VRB. With improving electro­ catalytic properties, the NCNR decorating carbon felt electrode (NCNR/CF) exhibits excellent battery performance with an impressive specific capacity of 37.3 Ah L 1 than pristine CF (22.8 Ah L 1) and CNR/CF (28.6 Ah L 1) electrodes. The NCNR/CF electrode also shows an outstanding coulombic efficiency (CE, 98.9%) and energy efficiency (EE, 84.3%) compared with the pristine CF (CE, 91.2% and EE, 73.4%) and the CNR/CF (CE, 95.6% and EE, 81.2%) electrodes in the VRB at 40 mA cm 2 current density. Furthermore, the NCNR/CF electrode exhibits 10.9 and 3.1% higher EE as compared to the pristine CF and CNR/CF electrodes, respectively. Therefore, the impressive cyclic rate capability with negligible capacity decay proves the superiority of NCNR as a potential electrode material for all-vanadium redox flow batteries.

1. Introduction Recently enormous attention has been devoted on present global

energy outlook with some emerging issues like fuel diversifications, limited resources, energy assurance and governing markets with field demand, which drive a massive consequence in both industrial as well as

* Corresponding author. E-mail address: [email protected] (S. Shanmugam). https://doi.org/10.1016/j.jpowsour.2019.227329 Received 20 July 2019; Received in revised form 26 September 2019; Accepted 16 October 2019 Available online 23 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.

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Journal of Power Sources 445 (2020) 227329

research communities to develop potential energy storage systems. It is indeed a crucial recognition that the affordable and clean energy development is underlying a great theme of today’s economic enhancement as well as affluent prospect [1,2]. The redox flow battery energy storage systems offer promising characteristics with scalability, low cost and enhanced energy density due to their originally unique architectures apart from conventionally solid-state rechargeable battery systems [3]. As taken advantages of the power-generating stack, power output, energy-carrying electrolyte, the decoupling of energy capacity, easy thermal management, and high safety are most fundamental properties that differentiate redox flow-based batteries from solid-state batteries. This unique architecture exhibits independent energy-to-power ratio tuned to charge precise applications. Considering the availability of various redox flow batteries, typically the all-vanadium redox flow battery (VRB) system introduced by Skyllas-Kazacos and co-workers is most promising due to its capability of large scale energy storage, superior electrochemical reversibility, and long life cycle [4]. Generally, the VRB utilized same redox species with 2þ 3þ different valence state in positive (VO2þ/VOþ 2 ), and negative V /V half-cell, which strongly prevents the cross-contamination problems of active redox species between two half-cells in comparison with other redox flow batteries. The electrode materials, where the redox reaction happens substantially determines the concentration and activation po­ larization of cells and has a greater impact on the capacity and energy efficiency of the VRBs [5,6]. Consequently, the electrode materials progress with the appropriate design is crucial for improving the final performance of the VRBs. So far, the carbon felt as implemented by carbon has been recognized to be one of the most favorable electrode materials for the VRBs due to its corrosion resistance, low cost, high stability, and high conductivity [7]. Nevertheless, its poor electrochemical reactivity and relatively low ki­ netic reversibility have hindered further the commercialization of VRBs, especially at a low electrical current density at charge-discharge cycle operation [8]. Extensive efforts have been devoted to improving the electrochemical reaction of electrode materials by hosting surface functionalities to improve electron conductivities as well as abundant active sites [3,9–12]. One of the promising technique to enhance the catalytic activity of VRB electrode materials is by introducing surface oxygen functional groups [13], heteroatom doping [14–16], and defect sites [17,18], metals [19] and metal oxides deposition [8,20] and these modifications significantly enhance the conductivity of resultant elec­ trodes [21,22]. Enhancement of contact area as well as to improve the percentage of active sites was mainly achieved by utilizing a variety of carbon nanostructures such as carbon nanotubes [23,24], porous carbon [25], graphene oxide [26,27], and wide-range heteroatom-doped car­ bon materials [28]. The aforementioned modification and methodolo­ gies have also generated potential consciousness in the VRB application. However, their complex synthesis procedure and ammonia toxicity, flammability of ethylenediamine especially at higher temperatures, and high price of metal precursors have hampered their large scale demon­ stration, and thereby restrict the commercialization of the VRBs. Electrospinning is considered as one of promising and straightfor­ ward technique, where thin fibers have been fabricated using a wide range of hydrocarbon polymers [29–35]. This technique would be an easily up-scaled and well-organized methodology to produce carbon nanofibers with enhanced active sites that can be used as carbonaceous electrode materials in VRB [36]. Wei and co-workers have studied the polyacrylonitrile (PAN)-based electrospun carbon nanofibers as an electrocatalyst and showed good performance in VRB [37]. The elec­ trochemical activities of carbon nanofibers enhance the electrochemical reversibility of active vanadium redox species and thereby promote the mass transport process and electron transfer rate in VRB operation. Herein, we fabricated carbon nanorods (CNRs) from PAN using different carbonization temperature and demonstrated as an efficient electrode for VRB. Even though CNRs exhibits high performance than pristine carbon felt but further improvement is still required of the CNRs

materials specifically to enhance rate capability with improved cyclic efficiency, and long-term charge-discharge cycling durability to assure the compliments of significantly high energy density and superior sta­ bility output for next-generation VRB technology. Bio-derived amino acid obtained from natural materials such as cysteine, niacin, glycine, and alanine have been demonstrated as a harmless, inexpensive, and safe heteroatom-doped carbon for electrocatalysts [38]. Therefore, we have proposed the fabrication of modified CNRs using zein protein-derived polyacrylonitrile carbonized at different temperatures, and that is using the first time as an actively modified electrode support material for VRB application. Zein has been considered as a major source of protein in the corn and contains hydrophobic amino acids, e.g. alanine, proline, and leucine with high amount α-helix, including β-sheet fractions [39]. Furthermore, the low cost ($10 per kg) and extensively produced zein protein have a unique structure with amphiphilic characteristics, which is one of the main driving forces to form ordered-film structure without external action, and their self-formation within two-dimensional periodic structures [40–42]. Thus, fundamentally well-organized zein molecules and film-forming properties promoted the heteroatom doping on CNRs without using any external nitrogen-containing gases. These bio-derived nitro­ gen-enriched CNRs facilitated the abundant oxygen and nitrogen active sites for vanadium redox reactions. We demonstrate that the NCNR could be successfully integrated into the VRB cell. The NCNR/CF shows significantly higher coulombic efficiency of 98.9% and outstanding rate capability than those of the pristine CF (91.2%), and CNR/CF (95.6%) electrodes. Furthermore, excellent retention of EE over cycling is particularly outstanding, resulting 10.9 and 3.1% higher EE of NCNR/CF than pristine CF, and CNR/CF, respectively. 2. Experimental 2.1. Fabrication of zein-derived polyacrylonitrile nanorods The Polyacrylonitrile (PAN, Mw: 150,000 g mol 1) of 1 g was perfectly dissolved in N, N-dimethylformamide (DMF) at 80 � C. Simul­ taneously, zein powder (Sigma Aldrich) of the different amount (15, 30 and 45 mg) was also dissolved in DMF at the same temperature. The prepared solutions were then mixed and stirred the whole night at 90 � C. At ambient atmosphere, the clear homogeneous solution was set up in electrospinning machine (NanoNC.ltd) to produce electrospun nano­ fiber mat. The moving distance was fixed in 10 cm from spinneret to collector with 1.0 mL h 1 feeding rate. Generally, a high voltage of 14 kV was supplied during operation, and maintained collector is rotating speed of 300 rpm. The fabricated electrospun nonwoven web was then thermally annealed in a tubular furnace (Wisd Laboratory In­ struments) at Ar atmosphere for 3 h with different temperatures (700900 � C). The heating rate of 3 � C min 1 was maintained for all samples. Finally, the calcined NCNR was collected from the furnace at room temperature and ground using the agate mortar. Based on the zein amounts, the obtained NCNR samples are assigned by NCNR-1, NCNR-2 and NCNR-3, respectively. For comparison, the pristine CNR was also fabricated form PAN only and followed the same procedure as a control sample. 2.2. Electrode preparation Generally, simple coating technique was used to prepare electrode materials for VRB test. Firstly, the CNR and NCNR catalysts ink were prepared by dispersing 27 mg (3 mg cm 2) of CNR or NCNR powders in a solution contains 135 μL of Nafion (5 wt%) and 1215 μL of ethanol. The fabricated mixture was ultrasonically dispersed for 30 min and coated on the carbon felt (CF-03, 5 mm). Pristine carbon felt was activated by thermally heating at 500 � C for 30 min before coating the catalyst ink. The catalyst coated carbon felt was finally dried in the vacuum oven at 60 � C for 2 h and used on both anode as well as cathode electrode 2

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Journal of Power Sources 445 (2020) 227329

materials. 2.3. Electrochemical measurements The cyclic voltammograms (CVs) was measured using biologic bipotentiostat (VSP-modular), were executed in the three-electrode setup cell containing 0.1 M VOSO4 in 2 M H2SO4 aqueous solution. The glassy carbon (GC), Ag/AgCl, platinum wire was used as the working, reference, and counter electrode, respectively. The 5 mg of CNR or NCNR was dispersed in 160 μL of isopropyl alcohol, 30 μL of DI water, and 10 μL of Nafion solution (5 wt%) using an ultrasonic bath for 1 h and a loading of 2 μL, on GC (0.092 cm2), was kept constant for all the samples. The CV for all samples measured at a scan rate of 10 mV s 1, and CV results are presented without IR correction. A single cell of VRB was constructed by inserting the polymer membrane (Nafion-212) be­ tween two modified carbon felt electrodes (9 cm2). Vanadium electro­ lyte with a concentration of 1.6 M VOSO4 in 2 M H2SO4 solution was employed as electrolyte solution in both positive and negative electro­ lyte tank. A peristaltic pump (Reglo ICC 2ch Pump) was used to recir­ culate the vanadium electrolytes solution with a flow rate of 12 mL min 1. A continuous N2 flow was passed through the electrolyte vessels and de-aerated to escape the chemical oxidation of the vanadium electrolyte solutions. An eight channel battery analyzer (BST8-3, MTI Corp.) was used to test the VRB electrochemical performance with a constant voltage range (1.6–1.0 V). The VRB cyclic efficiencies combining of coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) were investigated based on our previous litera­ ture [43].

Fig. 1. Schematic illustration of the NCNR and NCNR/CF fabrication process.

3. Results and discussion 3.1. Characterization and electrochemical activity of CNR and NCNR

Fig. 2. Schematic presentation of the proposed mechanism of NCNR for the VO2þ/VOþ 2 redox reaction.

The highlights of excellent electron conductivity of the electrode materials and efficient active sites for the vanadium ions redox reactions are crucial to enhance electron transfer kinetics and mass transfer rates

Fig. 3. FE-SEM images of (a) CNR, and (b) NCNR, FE-TEM images of (c) CNR and (d) NCNR. 3

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Journal of Power Sources 445 (2020) 227329

amorphous carbon nature for both CNR and NCNR (Fig. S1). Energy dispersed X-ray spectroscopy (EDXS) from FE-TEM presented the amount and distribution of carbon, nitrogen and oxygen elements of CNR and NCNR catalyst (Figs. S2 and S3). Furthermore, NCNR exhibited higher oxygen content (3.82 wt%), which could provide more active sites for the vanadium redox reaction as compared to the pristine CNR (2.74 wt%). Fig. S4 represents the powder X-ray diffraction (XRD) patterns of CNR, NCNR-1, NCNR-2 and NCNR-3 samples prepared of different carbonization temperatures (700–900 � C). A broad reflectance of 24 (2θ) and a small peak of 44 (2θ) was observed for all samples prove the maintenance of original carbon structure in CNR and NCNR [36]. The surface chemistry of CNR and NCNR was determined by Fourier-transform infrared (FTIR) spectra in attenuated total reflection (ATR) mode, and the results are illustrated in Fig. S5. The FTIR spectra of the NCNR-1, NCNR-2 and NCNR-3 samples with different carbon­ ization temperatures in the range of 800–2600 cm 1 displays charac­ –O teristics bands conforming to amide I, II, and III at 1641 (C– stretching), 1530 (N-H bending) and 1240 cm 1 (C-N stretching), respectively, except a slight change depending on temperatures [45]. Moreover, the characteristic peaks at 2295 and 1045 cm 1 matching – N) and C–O vibrational stretching, respectively. with nitrile groups (C– – The illustrated all peaks were perfectly matched with a representative band of pyrolyzed PAN samples [46], which further proves that the NCNR maintains of original CNR chemistry even after zein incorporated in PAN derived CNR. Even though the surface chemistry of all samples obtained at different temperatures appear almost similar based on FTIR characteristics, but to understand the actual chemical composition of samples, quantitative elemental analysis (C, H, N and S) was carried for all samples (Table 1 and Fig. S6). The results show that increasing temperature (700–900 � C), the weight per cent of nitrogen and hydrogen is decreased whereas the carbon content is increased. The highest amount of carbon (90.3 wt%) was obtained for CNR at 900 � C sample. The fabricated NCNR samples show higher nitrogen amount (NCNR-1, NCNR-2, and NCNR-3: 9.79, 9.96 and 9.67 wt%, respectively) than a pristine CNR (8.81 wt%) at 900 � C. Therefore, it proved that NCNR contains more nitrogen species as compared to pristine CNR prepared at the same temperature (900 � C). Raman spectroscopy was further performed to characterize the carbon nature of the fabricated CNR and NCNR. As shown in Fig. S7, all samples exhibited two char­ acteristics peaks at 1350 cm 1 for D-band and at 1580 cm 1 for G-band are accompanying with the vibrational stretching of sp3 and sp2 defect sites, respectively. The peak intensity ratio between the D- and G-band indicates the quality and nature of the carbon. Moreover, it is worth noting that the ID/IG value at a near zero suggests high crystallinity and a value near one indicates the presence of more defect sites in the carbon framework. Consequently, the peak intensity ratio between the D- and G-band was calculated and their corresponding values are listed in

Table 1 Quantitative elemental analysis and charge transfer resistance of the samples pyrolyzed at different temperature. Sample

Temperature (� C)

C (wt %)

H (wt %)

N (wt %)

Charge transfer resistance (Ω)

CNR CNR CNR NCNR1 NCNR1 NCNR1 NCNR2 NCNR2 NCNR2 NCNR3 NCNR3 NCNR3

700 800 900 700

81.94 85.47 90.32 81.7

1.27 0.95 0.87 1.14

16.79 13.58 8.81 17.16

466.50 182.19 152.32 376.85

800

85.15

0.88

13.97

167.33

900

89.45

0.76

9.79

146.83

700

81.89

1.13

16.98

302.92

800

85.14

0.88

13.98

160.22

900

89.29

0.75

9.96

115.14

700

81.69

1.15

17.16

385.56

800

85.16

0.91

13.93

170.06

900

89.58

0.75

9.67

131.79

towards vanadium redox couples, respectively [44]. A simple, highly scalable and environmentally friendly zein-derived polyacrylonitrile NCNRs with different carbonization temperature are synthesized as shown in Fig. 1. The prepared NCNR electrocatalyst by electrospinning technique followed by thermal heating facilitated the abundant active oxygen and nitrogen sites, which are giving faster electron transfer rate and improving the vanadium ion transfer kinetics (Fig. 2). Conse­ quently, high battery performance is expected using the fabricated NCNR/CF electrode in VRB. The field-emission scanning electron microscope (FE-SEM) images as depicted in Fig. 3a and b shows the nanorods morphology of pristine CNR and NCNR catalyst, respectively. It was revealed that the original morphology of CNR has retained in NCNR even zein mixed with PAN. The diameter of nanorods is about 150 nm and expected to improve the adsorption of vanadium ions by facilitating abundant active sites in CF electrode of the VRB. The morphological advancement in CNR and NCNR was further emphasized by field-emission transmission electron microscope (FE-TEM) technique as shown in Fig. 3 (c and d). Addi­ tionally, the CNR and NCNR samples show a typical cylindrical rodshaped morphology with an average fiber diameter of 136 and 150 nm, respectively, which are obtained by checking 50 arbitrary nanorods for each sample. The TEM images showed in Fig. 3c and d, depict the amorphous carbon for both CNR and NCNR samples. Furthermore, high magnification FE-TEM images confirmed the

Fig. 4. (a) Cyclic voltammograms (CVs) of all samples measured at a scan rate of 10 mV s 1, and (b) EIS at amplitude of 10 mV and a frequency range of 100 kHz to 100 mHz in 0.1 M VOSO4 in 2 M H2SO4 solution for the GC, CNR, NCNR-1, NCNR-2, and NCNR-3 pyrolyzed at 900 � C. 4

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Journal of Power Sources 445 (2020) 227329

Fig. 5. (a) XPS survey and (b) Ratio of oxygen and carbon atoms of CNR, NCNR-1, NCNR-2, and NCNR-3 pyrolyzed at 900 � C.

Table S1. The obtained results concluded the existence of disordered carbon with low degree of graphitization. Cyclic voltammetry (CV) test with three electrode setup was carried out to analyze the electrocatalytic properties of the various NCNR samples at a scan rate of 10 mV s 1 (Fig. S8). The peak potential dif­ ference (ΔE ¼ Epa Epc) of the redox peaks is an important parameter to reflect the redox reaction kinetics of the VRBs [28]. The CNR and NCNR catalysts calcined at 700 � C showed very poor kinetics. However, a good vanadium ion redox behavior was obtained with increasing temperature from 800 to 900 � C for all samples. The anodic peak potential at 1.1 V is attributed to the VO2þ to VOþ 2 oxidation, and while during the backward sweep, a broad peak centered at 0.87 V was observed, which is as a result 2þ of the VOþ reduction [8]. The glassy carbon (GC), NCNR-1, 2 to VO NCNR-2, and NCNR-3 electrodes showed the peak current density for 2 the oxidation of VO2þ to VOþ 2 was 49, 55, 97 and 78 mA cm , respec­ 2þ þ tively and peak potential difference for the VO /VO2 redox couple was 0.39, 0.26, 0.28 and 0.34 V, respectively, (Fig. 4a). On the other hand, the cathodic peak potential observed at 0.65 V is attributed to the reduction of V3þ to V2þ but while return sweep for no oxidation peak (V2þ to V3þ) was observed for the pristine CNR pyrolyzed at 900 � C. Whereas, the NCNR electrodes showed an increasing trend of current improvement for V2þ/V3þ redox couple with increasing zein content, which was entirely absent for the pristine CNR and GC electrodes. Therefore, the fabricated NCNR is considered to be an effective elec­ 2þ 3þ trocatalyst for both VO2þ/VOþ redox reactions, because of 2 and V /V its abundant oxygen-rich and nitrogen functional groups. It is worth noting that with increasing the amount of zein in modified CNR, the redox behavior of vanadium ion also increased, and that nature is completely missing in the pristine CNR. To understand the improved catalytic activity towards the vanadium redox reaction, the electro­ chemical impedance spectroscopy (EIS) was further analyzed. The EIS measurements were carried out with a frequency range of 100 kHz to 100 mHz in an open circuit voltage using a perturbation potential of 10 mV AC amplitude. Fig. S9 represents, the Nyquist plots of each catalyst showed an in­ clined line within the low-frequency area, specifically coming from mass diffusion resistance through the electrolyte solution (R1), and one semicircle in a high-frequency area is recognized as the charge transfer resistance (R2). All catalysts showed low charge transfer resistance when increasing the temperature from 700 to 900 � C (Tables 1 and S1). Fig. 4b presents the equivalent circuit (inset) and the R2 value of GC, CNR, NCNR-1, NCNR-2, and NCNR-3 (pyrolyzed at 900 � C) is 498.2, 152.3, 146.8, 115.2 and 131.8Ω, respectively. Furthermore, NCNR-2 exhibited 1.3-folds lower charge transfer resistance than that with the pristine

CNR electrode. The noticeable charge transfer proficiency of the NCNR based catalyst is mainly attributed to the presence of abundant active sites, which significantly enhanced the electrode-electrolyte and het­ eroatom interfacial area [35,47]. The surface area of CNR and NCNR samples is determined by the BET N2 adsorption–desorption isothermal analysis, and results revealed the high surface area of NCNR samples than a pristine CNR. The CNR, NCNR-1, NCNR-2, and NCNR-3 samples exhibited BET surface area values of 14, 18, 19 and 20 m2 g 1, respec­ tively. The PAN based carbon fibers showed relatively a low BET surface area due to their non-porous nature [47,48]. However, the BET surface area of NCNR samples is increased with increasing zein incorporation into CNR. To understand the chemical surface of the fabricated CNR and NCNR at 900 � C, X-ray photoelectron spectroscopy (XPS) investigation was carried out for CNR, NCNR-1, NCNR-2, and NCNR-3 samples. The XPS survey of the samples showed peaks for C, N and O elements (Fig. 5a). The high-resolution C1s spectrum (Fig. S10a) were deconvoluted into – C (284.4 eV), C-C (285.1 eV), C-N/C– –N five peaks attributed to C– – O (287.5 eV). The N 1s spectrum (285.9 eV), C-O (286.3 eV) and C– (Fig. S10b) are allocated to pyridinic N (398.2 eV), pyrrolic/pyridonic N (400.3 eV), graphitic N (401.0 eV) and oxygenated N (402.5 eV) func­ tional species. On the other hand, the O 1s spectrum (Fig. S10c) con­ – O (530.6 eV), C-OH (531.8 eV), tained four peaks corresponding to C– – O (532.9 eV) and CO3 (534.0 eV) [49]. Additionally, the high C-C– resolution C 1s spectrum of CNR, NCNR-1, NCNR-2, and NCNR-3 exhibited a high-pitched graphitic carbon peak at 284.4 eV. However, NCNR samples demonstrated total oxygen-containing functional groups are higher as compared to pristine CNR sample. More specifically, the – O functional groups in the NCNR-2 catalyst is presence of C–C– straightly accompanying with active vanadium redox species, was significantly higher when compared to that of the CNR sample. There­ fore, the NCNR samples spectrum showed a reduction of the graphitized carbon, and an enhancement of the defective carbon afterwards the zein incorporation in CNR. The NCNR contains remarkably higher nitrogen species than that of the CNR, mainly as pyrrolic N and pyridinic N, which are found to improve the catalytic activity towards the vanadium ion redox reactions in comparison with the others nitrogen containing catalytic species because of the presence of a lone pair of electrons. Therefore, the high amount of nitrogen species in the NCNR facilitates a low energy barrier for the vanadium ion redox reactions [49]. Conse­ quently, a high amount of active sites of NCNR, ensuing in an enhanced electrocatalytic activity towards vanadium redox reaction than pristine CNR. As a consequence, the NCNR-2 shows a considerably higher 5

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Journal of Power Sources 445 (2020) 227329

survey spectra further proved the high amount of oxygen and nitrogen functional groups of NCNR could be expected much more electro­ catalytically active sites towards vanadium redox reaction in compari­ son with the pristine CNR catalyst at same conditions. The electrical conductivity test was conducted to clarify the superior catalytic activity of the prepared NCNR catalyst. As shown in Fig. S12, the NCNR samples exhibited higher electrical conductivity as compared to the pristine CNR, which further proves the outstanding electro­ chemical properties of the zein incorporated NCNR. The N content, peak potential difference for the VO2þ/VOþ 2 redox couple, charge transfer resistance, and electrical conductivity of CNR, NCNR-1, NCNR-2, and NCNR-3 are plotted in Fig. 6 to understand the catalytic activity of NCNR samples. Considerably, there is a trade-off between N content, peak potential difference, charge transfer resistance, and electrical conductivity, which should be optimized for the highest catalytic per­ formance. With increasing the N content, the electrical conductivity increases and resulting the trend of NCNR-2 > NCNR-1 > NCNR3 > CNR. Therefore, combining with excellent electrocatalytic proper­ ties of the fabricated NCNR samples show low charge transfer resistance, and resulting low peak potential difference for the VO2þ/VOþ 2 redox couple than pristine CNR sample. Furthermore, the fabricated NCNR samples, specifically NCNR-2 shows these advanced catalytic properties trend than that of the pristine CNR at same condition and thereby ex­ pected much improved electrochemical performance in battery operation. The CNR and NCNR materials are coated on CF electrodes before testing the VRB electrochemical performance. The morphology of pris­ tine CF, CNR/CF and NCNR/CF electrodes is investigated and charac­ terized by FE-SEM. The images in Fig. S13 demonstrated that the average diameter of the carbon fiber in CF electrode is 15 μm and surface is very smooth, and the NCNR (average diameter of 150 nm) are ho­ mogeneously dispersed on the carbon fiber surface. Additionally, the distribution of CNR, NCNR-1, NCNR-2, and NCNR-3 on the carbon fiber

Fig. 6. Comparative relation of N content, peak potential difference for the VO2þ/VOþ 2 redox couple, charge transfer resistance, and electrical conductivity for the CNR and NCNR samples pyrolyzed at 900 � C.

electrochemical performance as compared to the pristine CNR, which is in good agreement with the results of CV and EIS as well. The N 1s contents of the CNR, NCNR-1, NCNR-2, and NCNR-3 are 5.4, 6.0, 6.3 and 5.7%, respectively, and follow the trend of NCNR-2 > NCNR-1 > NCNR-3 > CNR (Fig. S11). On the other hand, the O 1s contents of the CNR, NCNR-1, NCNR-2, and NCNR-3 are 5.8, 7.2, 8.7 and 7.0%, respectively, and follow the trend of NCNR-2 > NCNR-1 > NCNR-3 > CNR. Moreover, Fig. 5b shows the ox­ ygen to carbon atomic content ratio in the CNR, NCNR-1, NCNR-2, and NCNR-3 are 0.06, 0.08, 0.10 and 0.08, respectively. Therefore, from XPS

Fig. 7. VRB (a) Charge-discharge curves, (b) Efficiencies, (c) Discharge capacity retention of using pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes at 40 mA cm 2 current density, and (d) Cycling efficiencies of NCNR-2/CF electrode at a current density of 40 mA cm 2. 6

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Journal of Power Sources 445 (2020) 227329

surface in CF electrode was shown in Fig. S13. It is worth noting that with increasing the mass ratio of zein in NCNR, the more nanorods are observed on the carbon fiber surface, and a little agglomeration is wit­ nessed for NCNR-3/CF sample. Therefore, it is expected that the ho­ mogeneous distributed CNR and NCNR catalyst on carbon fiber would facilitate an excellent medium to improve electrocatalytic reaction for vanadium redox couples due to their enhanced oxygen and nitrogen active sites. 3.2. Vanadium flow battery performance The excellent features of the prepared NCNR catalyst decorated on carbon felt electrode used as the positive and negative electrodes in the all-vanadium redox flow battery for demonstrating practical applica­ tion. Based on the electrochemical properties obtained in the threeelectrode setup, the NCNR-1, NCNR-2, and NCNR-3 samples pyrolized at 900 � C were coated on CF and assembled the VRB to classify their effectiveness on the VRB performance at ambient atmosphere. The pristine CF and CNR/CF also assembled as comparison samples, and the corresponding electrode materials with a single cell VRB performances are summarized in Fig. 7. As shown in Fig. 7a, the voltage profile be­ tween 1.0 and 1.6 V of the pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/ CF, and NCNR-3/CF at 40 mA cm 2 current density. The pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes exhibi­ ted discharge specific capacity of 22.8, 28.6, 34.7, 37.3 and 34.5 Ah L 1, respectively. Additionally, NCNR/CF electrodes exhibited an excellent performance on VRB, specifically the discharge capacity of NCNR-2/CF electrode is 1.6 and 1.3-times higher than those of the pristine CF and CNR/CF electrodes, respectively, at the same condition. An extreme increase of IR drop might be consigned to the pristine CF, because of its poor electrochemical reactivity and relatively low kinetic reversibility hydrophobicity as well. Additionally, the overpotential of 220, 103, 113, 117 and 173 mV was found at a discharge capacity of 500 mA h for pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF elec­ trodes, respectively. The small polarization influence and excellent ca­ pacity rate of NCNR/CF electrode was mainly due to its abundant oxygen and nitrogen active sites, which improves the utilization of vanadium electrolyte and assists the fast transfer of ions as well as electrons in the electrode. The VRB cyclic efficiency including of coulombic efficiency (CE), voltage effi­ ciency (VE) and energy efficiency (EE) were measured for all electrodes at 40 mA cm 2 current density (Fig. 7b). The pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes exhibited CE of 91.2, 95.6, 98.4, 98.9 and 98.5%, respectively. Besides, the NCNR-2/CF electrode showed 7.7 and 3.3% higher CE as compared to the pristine CF and CNR/CF electrodes, respectively. An outstanding CE is obtained for all NCNR/CF electrodes due to their super electrochemical activity to­ wards vanadium redox reaction. While for VE, the pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF showed VE of 80.5, 83.9, 85.1, 85.2 and 83.5%, respectively. The pristine CF electrode revealed the lowest VE because of its lower electrochemical reactivity, poor ki­ netics and high IR drop. On the other hand, incorporating the zein into the CNR, the VE gradually increased from 83.9 to 85.2% for NCNR-2/CF electrode. Additionally, NCNR/CF electrode exhibited higher VE in comparison with the pristine CNR/CF electrode, because reasonably more oxygen functional groups of NCNR can enhance electrocatalytic activity towards vanadium redox reaction. Therefore, the IR resistance is decreased in VRB process, resulting improved VE of using NCNR/CF electrode. The similar fashion has also obtained regarding EE, which establishes the most significant energy conversion ratio in operating a VRB calculated by multiplying CE and VE. The pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes exhibited EE of 73.4, 81.2, 83.8, 84.3 and 82.3%, respectively. Furthermore, the NCNR2/CF electrode exhibited 10.9 and 3.1% higher EE as compared to the pristine CF and CNR/CF electrodes, respectively. This comprehensively and outstanding result is attributed to the fact that the zein structure

Fig. 8. VRB cyclic efficiencies of CF, CNR/CF, and NCNR-2/CF electrodes at different current densities.

affords in NCNR of necessary dynamic contact area for electron and ion transfer through the electrode and electrolyte as well as the plenty of electrocatalytic active domains in the NCNR, significantly enhancing the electrochemical reversibility of the vanadium ion redox reactions, spe­ cifically for durable VRB. To explore the long-term stability of the NCNR/CF electrode in VRB, the cyclic charge-discharge test was carried out at 40 mA cm 2 current density of using pristine CF, CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes. The NCNR/CF electrode exhibited superior ca­ pacity retention, as illustrated in Fig. 7c. Furthermore, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes showed capacity retention of 89, 91 and 90%, respectively, after 100 charge-discharge cycles in VRB. However, at the same condition, the pristine CF and CNR/CF exhibited capacity retention in only 38 and 79%, respectively. Furthermore, NCNR-2/CF showed 83% capacity retention at 300 charge-discharge cycles in VRB, which is significantly higher than that with the pristine CF decorated VRB at only 100 charge-discharge cycles. The cycling test results are shown in Fig. S14 and Fig. 7d, reveal that increasing trend of CE along with the negligible falling-off VE and EE of the VRB assembled with NCNR/CF electrodes even after operating 100 charge-discharge cycles. Specifically, NCNR-2/CF electrode exhibited the stable cyclic efficiencies even at 300 charge-discharge cycles in VRB. In contrary, a much higher efficiency decay was observed for pristine CF, and CNR/CF at only 100 charge-discharge cycles in VRB. The outstanding perfor­ mance was ascribed mainly because of the superior electrocatalytic properties of NCNR/CF electrode during VRB operation. Again trans­ lating views to obtain the superiority of the NCNR/CF electrode in VRB, cyclic charge-discharge was carried out of using pristine CF, CNR/CF, and NCNR-2/CF electrodes at different current density, and results are summarized in Fig. 8 and Table S2. The result shows CE of using NCNR2/CF electrode increases linearly from 98.9 to 99.2% together with increasing the current density of 40–160 mA cm 2. Nevertheless, the VE gradually declines from 85.2 to 80.8%, although the current density was increased from 40 to 160 mA cm 2, attributed to a high ohmic polari­ zation in the battery at high current density and thus, resulting lower VE [50,51]. Similarly, the EE also dropped from 84.3% at 40 mA cm 2 to 80.1% at 140 mA cm 2 current density. It is worth noting that when current density is changing from 140 to 40 mA cm 2, the cell itself remarkably restored its original efficiency, which was obtained at the initial stage for NCNR-2/CF electrode. In contrast, pristine CF, CNR/CF are showed lower efficiency than that with the NCNR-2/CF at the different current density, and also completely failed to acquire initial 7

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Journal of Power Sources 445 (2020) 227329

4. Conclusions

Table 2 The performance of VRBs with CNR/CF and NCNR-2/CF electrodes in compar­ ison with previous work. Catalyst

Electrolyte

Current density (mA cm 2)

CE (%)

EE (%)

Cycle number

Ref.

NCNR-2/CF

1.6 M V in 2 M H2SO4 1.6 M V in 2 M H2SO4 1.6 M V in 2.5 M H2SO4 1.6 M V in 2.5 M H2SO4 1.6 M V in 3 M H2SO4 1 M V in 2.5 M H2SO4 1 M V in 2.5 M H2SO4 1 M V in 4 M H2SO4 0.8 M V in 3 M H2SO4

40

98.9

84.3

300

40

95.6

81.2

100

40

93.2

85.4

200

This work This work [51]

80

94.8

73.7

100

[52]

20

92.2

83.3

180

[53]

75

82.3

73.2

50

[54]

100

84.9

72.1

NA

[55]

80

92.0

78.0

50

[56]

50

95.8

79.3

50

[57]

CNR/CF TiNb2O7–rGO/ GF 0.75 wt% Ta2O5/GF PWA-CNF/CF NiCoO2/GF CoO/GF Ni/CNF/CF CNF–N9/GF

In summary, we have synthesized and characterized a highly active metal-free carbon-based catalyst with an inexpensive, simple technique and environmentally friendly for high-performance VRB. The NCNR-2/ CF electrode showed an outstanding coulombic efficiency of 98.9% and superior rate capability as compared to pristine CF (91.2%), and CNR/ CF (95.6%) electrodes. Moreover, NCNR-2/CF electrode exhibited 10.9 and 3.1% higher EE as compared to the pristine CF, and CNR/CF elec­ trodes, respectively, indicating a preferred electrode material in VRB application. The amphiphilic nature of the zein protein facilitates abundant oxygen and nitrogen functional species in NCNR, effectively enhance the contact area for vanadium ion and making fast electron transfer kinetics between electrolyte and electrode. Therefore, a super improved electrochemical performance was achieved of using NCNR catalyst. Acknowledgements The authors acknowledge the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2017R1D1A1A09000838) for financially supported.

efficiency at 40 mA cm 2 (Fig. 8). Moreover, NCNR-2/CF electrode exhibited an excellent efficiency at different current densities as compared to the pristine CF, and CNR/CF, indicating the super elec­ trochemical reactivity and relatively high kinetic reversibility of the NCNR-2/CF. In order to demonstrating the potential of the NCNR/CF electrode, FE-SEM analysis was conducted to investigate the surface morphology of using CNR/CF, NCNR-1/CF, NCNR-2/CF, and NCNR-3/CF electrodes after charge-discharge cycles in VRB. As depicted in Fig. S15, the FESEM micrograph of two different magnification shows that CNR, NCNR-1, and NCNR-3 uniformly exist on the surface of CF electrode after 100 charge-discharge cycles in VRB. Fig. S15, also showed a similar trend of the NCNR-2/CF even after 300 charge-discharge cycles in VRB. Furthermore, FTIR spectrum analysis was also revealed that the NCNR-2 catalyst intactly present at the carbon felt electrode at 300 chargedischarge cycles operation in VRB (Fig. S16). UV–vis. absorption spec­ trum of the vanadium electrolyte solution was analyzed, and results showed that the vanadium electrolyte solution demonstrated neither a wavelength shift nor a new absorption peak in comparison with pristine vanadium electrolyte solution even after 300 charge-discharge cycling operation in VRB (Fig. S17). The observation clarifies that the vanadium electrolyte solution was strongly maintained its original properties without any changes over VRB charge-discharge cycling. The perfor­ mance of CNR/CF and NCNR-2/CF electrodes are compared with the other catalyst support for VRB electrode and results are summarized in Table 2. The NCNR-2/CF electrode exhibited higher CE of 98.9% and EE of 84.3% than those with the titanium niobium oxide–reduced graphene oxide nano­ composite electrocatalyst (TiNb2O7–rGO, CE, 93.2% and EE, 85.4%) [51], Ta2O5 nanoparticles (0.75 wt% Ta2O5, CE, 94.8% and EE, 73.7%) [52], heteropolyacid on nitrogen-enriched carbon nanofiber (PWA-CNF, CE, 92.2% and EE, 83.3%) [53], binary nickel cobalt oxide (NiCoO2, CE, 82.3% and EE, 73.2%) [54], cobalt oxide (CoO, CE, 84.9% and EE, 72.1%) [55], electrospun nickel-carbon nanofibers (Ni/CNF, CE, 92.0% and EE, 78.0%) [56], and electrospun nitrogen-doped carbon nanofiber (CNF–N9, CE, 95.8% and EE, 79.3%) [57]. Moreover, NCNR-2/CF showed impressive durability in VRB operation as compared to the other catalyst summarized in Table 2. Therefore, the proposed NCNR catalyst proved great potential as a new electrode material for VRB application. Further research, including the optimization and working mechanism of NCNR for application in energy devices is under progress.

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