Unveiling the Role of Heteroatom Gradient-Distributed Carbon Fibers

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Unveiling the Role of Heteroatoms Gradient-Distributed Carbon Fibers for Vanadium Redox Flow Batteries with Long Service Life Xiong-Wei Wu, Qi Deng, Chang Peng, Xian-Xiang Zeng, An-Jun Wu, Chunjiao Zhou, Qiang Ma, Ya-Xia Yin, Xiang-Yang Lu, and Yu-Guo Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22521 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Unveiling the Role of Heteroatoms Gradient-Distributed Carbon Fibers for Vanadium Redox Flow Batteries with Long Service Life Xiong-Wei Wu,† Qi Deng,*,† Chang Peng,† Xian-Xiang Zeng,† An-Jun Wu,† Chun-Jiao Zhou,† Qiang Ma,† Ya-Xia Yin,‡,# Xiang-Yang Lu,*,† and Yu-Guo Guo*,‡,#

†College

of Bioscience and Biotechnology, College of Science, Hunan Agricultural

University, Changsha, Hunan 410128, P. R. China. ‡CAS

Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS

Research/Education Center for Excellence in Molecular Sciences, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China. #University

of Chinese Academy of Sciences, Beijing 100049, P. R. China.

KEYWORDS: Vanadium redox flow batteries, Electrocatalysis, Carbon fibers, Heteroatoms gradient distribution, Long service life

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ABSTRACT The fundamental understanding of electrocatalytic reaction process is anticipated to guide electrodes upgradation and acquirement of high-performance vanadium redox flow batteries (VRFBs). Herein, a carbon-fiber prototype system with a heteroatom gradient distribution has been developed with enlarged interlayer spacing and a high graphitization that improve the electronic conductivity and accelerate the electrocatalytic reaction, and the mechanism by which gradient-distributed heteroatoms enhance vanadium redox reactions was elucidated with the assistance of density functional theory calculations. All these contributions endow the obtained electrode prominent redox reversibility and durability with only 1.7% decay in energy efficiency (EE) over 1000 cycles at 150 mA cm-2 in the VRFBs. Our work sheds light on the significance of elaborated electrode design and impels the in-depth investigation of VRFBs with long service life.

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INTRODUCTION Issues such as fossil-fuels environmental pollution, energy shortage,1 and energy security,2 have stimulated people to seek safe and clean renewable energy sources. However, many sustainable renewable energy is unstable, and large scale energy storage devices are required to maintain a stable power output.3-4 Vanadium redox flow batteries (VRFBs) are recognized as the most potential large scale energy storage system to handle this issue because of the outstanding characteristics of VRFBs such as flexible capacity design, long cycle life and environmental friendliness. To achieve satisfactory performance, the electrode, as the crucial component of VRFBs, needs to be elaborately designed because it provides abundant active sites for negative and positive redox couples and determines the energy storage and conversion efficiencies between chemical energy and electrical energy. In this regard, numerous researchers committed to enhancing the performance of VRFB’s electrodes. For example, the deposited noble metal (Pt5-6 and Ir7) catalysts exhibit high activity for positive or negative redox reactions, but these metals are expensive and prone to gas evolution. Inexpensive metal (Bi8) or metal oxides (Nb2O5,9 Mn3O4,10-11 TiO2,12 RuO2,13 and WO314) have been used as substitutes for noble metal catalysts to enhance the activity of vanadium ions reaction. However, the electrocatalytic activity of the abovementioned electrodes is inhibited by the nonhomogeneous distribution

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and low activity caused by the large size, and their preparation processes are tedious and intricate. Therefore, it is urgent to find efficient methods to improve the electrocatalytic performance of original carbon-based electrode due to the following considerations. First, carbon materials are inexpensive and widely used in flow batteries and can serve a solid foundation to enhance the electrocatalytic ability in VRFBs. Using heteroatom doping as an example, size and bond length differences can homogenize the charge distribution.15 In addition, the covalent bond between heteroatoms and carbon is thought to extend

the

oxygen-doped

electrode

operation

carbon-based

life.16-18

materials

Especially, possess

nitrogen19-20

excellent

or

catalytic

performance21-25 and large specific surface areas,26-27 resulting in good electrochemical performances. However, it should be noted that previous works have only concentrated on surface modification without taking internal structural variations in carbon electrode materials into account. For instance, phosphorus possesses the same valence electron number and similar chemical properties as nitrogen but has a larger atomic radius. The larger atomic radius of phosphorus leads to a greater electron-donating ability, which accelerates vanadium ion reactions,28-29 but makes it more difficult to embed phosphorus into a carbon electrode. Additionally, the cognition of the corresponding structure and vanadium ions reaction mechanism is a seriously inadequate.30-31 Herein, we designed a novel electrode with heteroatoms (phosphorous and oxygen) containing a functional group gradient distribution of graphite felt 4


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(HGGF) for vanadium redox reactions in the VRFBs. The embedded heteroatoms enlarge the interlayer spacing of the crystallization zone in amorphous carbon, which improves the electronic conductivity. This is the first report of embedding phosphorous in the carbon fibers of graphite felt and its radial-distribution in the amorphous carbon layer. Importantly, such a carbon structure accelerates the electrocatalytic processes of vanadium redox couples and this was verified for the first time with the assistance of density functional theory calculations. This new electrode not only exhibited excellent electrocatalytic activity for vanadium ion redox couples but also showed ultrastability with only 1.7% decay of the energy efficiency (EE) over 1000 cycles at 150 mA cm-2, and an electrochemical performance comparison analysis is summarized in Figure S1 and Table S1. The HGGF may has great promising in VRFBs application due to the low cost of material (potassium dihydrogen phosphate for 800 $/t) and processing.

EXPERIMENTAL SECTION General Experimental Procedures All chemicals were of analytic purity. The graphite felt (GF) was immersed into potassium dihydrogen phosphate solution with 0.84 mol/L, then ultrasonic for 30 minutes, and dried in oven at 80oC to constant weight. Subsequently, the obtained materials were placed in the tube furnace to anneal at 800oC under the atmosphere of Argon flow for 30 minutes. After cooled to room temperature, 5



the modified electrode was immersed into diluted hydrochloric acid to remove residual impurities, washed to neutrality with deionized water, and dried in air at 80oC, and the prepared sample (C-PO-O) was marked as HGGF. The pure graphite felt was marked as a pure-GF electrode, which was composed of pure carbon without any heteroatom elements and was considered an ideal electrode. To create a control, the GF was put in a quartz tube and annealed at 800oC under an Argon atmosphere for 30 minutes without potassium dihydrogen phosphate, and the control was marked as HGF. Materials and Reagents Potassium dihydrogen phosphate (Analytic purity, Sinopharm Chemical Reagent Co., Ltd); Nafion membrane (Dopont, America); GF (thickness of 6 mm) and electrolyte (0.05 M VOSO4 + 3 M H2SO4, 0.375 M V2(SO4)3 + 0.75 M VOSO4 + 3.5 M H2SO4) (Hunan Province YinFeng New Energy Co. Ltd, Changsha, China). All of the water used was deionized water. Characterization of Samples The scanning electron microscope (SEM) (S4800, Hitachi, Japan) was operated under the condition at 10 kV. The structures of electrodes were determined by X-ray diffraction (XRD) (scan rate of 5°/min with a Cu Kα radiation source (λ = 1.5418 Å)). Raman spectra of the electrodes were collected at 514 nm. The distribution of phosphorus in the carbon fiber was scanned by an electron microprobe (EPMA-1720). The XPS operation conditions were 200 W Al K and atmospheric pressure of 3×10-10 mbar. An 6


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electrochemical workstation (CHI760D) was used with an Ag/AgCl electrode (reference electrode) and platinum sheet (counter electrode). The working frequency of the Autolab instrument (PGSTAT 302N) was from 0.01 Hz to 1000 kHz, and the EIS results were fit using Zview software. The batteries with the different electrodes were tested by charge-discharge cycling with a cycling instrument (CT2001A) at 25oC and 35oC, respectively.

RESULTS AND DISCUSSIONS Morphology and Structure Property HGGF with a gradient distribution of heteroatoms was prepared by calcinating graphite felt (GF) after loading with potassium dihydrogen phosphate (KH2PO4), and the fabrication process is schematically illustrated in Figure 1a. Obviously, there was no change in the surface morphology after treatment from that of the pristine GF (Figure S2). The GF was heated under the same conditions as HGGF without phosphorus as a control, which was marked HGF. The elements distribution was tracked by Energy dispersive X-ray spectroscopy (EDS) (Figure 1b and 1c). Carbon (C), oxygen (O) and phosphorus (P) were detected, and C shows a relatively equal radial distribution. The O and P elements present a parabolic distribution and are mainly dispersed in the outer layer of carbon fibers. The phosphorus containing groups were thought to embed into carbon layer of graphite felt and interact with the carbon atoms during the calcination process. To further investigate the O and P distribution on a larger scale, the planar features of the carbon fibers were further 7



characterized with the X-ray electron probe microanalysis (EPMA), which is an accurate tool to evaluate elemental composition and distribution. The O and P elements in the cross-sectional image shows an intensive distribution of O on the surface and a sparse distribution of P in the inner layer of electrode, indicating a gradient distribution in the radial direction that is consistent with EDS and EPMA results (Figure 1b-1f). X-ray photoelectron spectroscopy (XPS) was used to detect the abovementioned heteroatom composition and valence states in the carbon fibers (Figure S3), which indicate a phosphorus content in total atoms of 1.84%. From the high-resolution spectra, the O 1s and P 2p spectra were extracted and studied in detail (Table 1). Compared with the GF, the O/C ratio of HGGF was increased by 65% and additional oxygen and phosphorus containing bonds such as C-C=O (532.9 eV),

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Figure 1. (a) Schematic diagram of the fabrication process of the HGGF electrode. (b) SEM image of HGGF and (c) the corresponding EDS signals of C (black), O (red), and P (green) elements. (d) EPMA of the cross section of the HGGF electrode and corresponding element mapping of (e) O and (f) P. C-PO-O (532.0 eV), C-P=O (530.4 eV), C-P-O (132.3 eV) and C-P=O (133.4 eV) appeared (Figure 2a and 2b).21,32-33 The introduction of heteroatoms will cause a structural alteration. In the X-ray diffraction (XRD) patterns (Figure 2c), three kinds of electrode exhibit two peaks at 25.3o and 43.8o corresponding to the crystallographic planes of (002) and (100), respectively,34 and these peaks shifted toward lower angles at 25.1o and 43.6o. Compared to the interlayer space (d) of GF (3.60 Å) and HGF

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(3.57 Å), a slightly larger d of 3.63 Å was calculated with the Bragg equation for (002) plane of HGGF after the P atoms insertion (Figure 2e and Table 1). Meanwhile, the ratios of the D band and G band of all the treated samples in the Raman spectra decreased (Figure 2d), and indicated that the sp2 hybrid orbitals of carbon were restored and the degree of graphitization improved after calcination due to the metal accelerating the graphitic structure formation in heat treatment.35-38 Table 1. The XPS survey results, XRD peaks and Raman spectra peaks of the GF, HGF and HGGF electrodes. Electrode

GF

HGF

HGGF

RO/C

0.066

-

0.109

RP/C

-

-

0.021

XRD

d002

3.60 Å

3.57 Å

3.63 Å

Raman

ID/IG

1.002

0.998

0.962

XPS

R = Atomic intensity ratio; d = Interlayer space

10


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Figure 2.

(a) O 1s and (b) P 2p XPS spectra and fitting curves for the HGGF

electrodes. (c) XRD patterns and (d) Raman spectra of the GF, HGF and HGGF electrodes. (e) The change in the carbon layer of HGGF. DFT Calculations The energy of adsorption between vanadium ions and the electrode surface sites is a key point in the electrocatalytic process,39 and the electrocatalytic activity was explored using Gaussian software-based density functional theory (DFT) calculations with a model coronene molecule (C24H12) and its optimized derivatives, e.g. HO-C24H12 and O-OP-C24H12 (see details in supporting information).40 The adsorption energy of the C24H12, HO-C24H12 and O-OP-C24H12 for VO2+ are -0.360 A.U, -0.368 11



A.U, and -0.379 A.U, respectively. A lower absorption energy means a stronger interaction between the electrode and vanadium ions.41 Obviously, O-OP-C24H12 that represents the HGGF electrode, possesses a higher electrocatalysis than the other two counterparts. Additionally, the energy separation between the lowest-unoccupied molecular orbital (LUMO) and the highest-occupied molecular orbital (HOMO) is a significant index of kinetic stability and chemical reactivity, and we calculated the LUMO and HOMO energy separation for the C24H12, HO-C24H12 and O-OP-C24H12 (Figure 3). The HOMO-LUMO energy gaps of C24H12, HO-C24H12 and O-OP-C24H12 are 4.038 eV, 3.834 eV, and 3.482 eV, respectively. Compared to the C24H12 and HO-C24H12, the O-OP-C24H12 has a smaller energy gap. The smaller energy gap results in a low kinetic stability and an outstanding chemical activity because O-OP-C24H12 tends to contribute or receive corresponding electrons from adjacent LUMO or HOMO levels.42

Figure 3. DFT calculations of the model coronene molecule with optimized structures: (a) C24H12, (b) HO-C24H12 and (c) O-OP-C24H12, corresponding molecular 12


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orbitals of (d, e, f) HOMO and (g, h, i) LUMO levels for (d, g) HO-C24H12, (e, h) C24H12 and (f, i) O-OP-C24H12 configurations. Kinetic Investigation In the cyclic voltammetry (CV) tests, the potential separation (△E) of the redox peaks and the peak current density ratio (Ipa/Ipc) were calculated to estimate the electrocatalysis of all electrodes toward negative and positive redox reactions. Figure 4a shows that both the positive and negative HGGF electrodes exhibit obvious oxidation and reduction peaks. The GF electrode also appears obvious redox peaks in the positive direction, but does not exhibit negative redox peaks because the negative GF electrode possessed severe hydrogen evolution behavior. Thus, the HGGF electrode not only exhibits prominent electrocatalytic activity but also effectively restricts hydrogen evolution during the negative electrolyte electrochemical reaction. The positive peak potential separation of the HGGF electrode (△E = 210 mV) is smaller than the GF electrode (△E = 551 mV). Furthermore, compared with the GF electrode, the HGGF electrode also shows a better electrochemical performance toward V2+/V3+ in the negative electrolyte. According to the oxidation and reduction peak current ratios of the GF and HGGF electrodes listed in Table 2, the HGGF presents a higher electrochemical activity and reversibility than the GF electrode for both V2+/V3+ and VO2+/VO2+ redox reactions. To deeply investigate the electrocatalytic activity and charge transfer behavior of the electrode, electrochemical impedance spectroscopy (EIS)43-44 was used to test the Randles circuit of all 13



electrodes (Figure 4b and Figure S4). As shown (Figure 4b), a sloped line presents in the low frequency region and a semicircle shows in the high frequency region, implying that the vanadium ions electrochemical reaction on the surface of electrode is a mixed process controlled by diffusion and charge transfer.45 In the equivalent circuit (Figure S4), R1 indicates the ohmic resistance of the electrode and solution and does not show obvious variations. However, the values of R2, which is the ohmic resistance for the electron transfer from electrode surface to solution,46 are 630 ± 41 Ω and 0.14 ± 0.01 Ω for the GF and HGGF electrodes, respectively, indicating that the HGGF electrode possesses a lower charge transfer resistance owing to its smaller HOMO-LUMO energy gap for the phosphorus groups with adsorbed vanadium ions.47 Using CV tests at different scan rates (Figure 4c and 4d), the absolute values of the Ipa/Ipc were obtained. The Ipa/Ipc value of the HGGF electrode was nearly constant over the whole scan rate range because the phosphorus functional group accelerates the electron delivery to the surface of the electrode (Figure 4e). Moreover, compared with the vast peak potential difference (△E) of the GF electrode (Figure 4f), the HGGF electrode presented an almost constant △E for the V2+/V3+ and VO2+/VO2+ reactions. The mass transfer performances of the GF and HGGF electrodes were evaluated by the corresponding plots of the peak current density vs the square root of the scan rates extracted from the CV tests at different scan rates (Figure S5).48 The correlation coefficients of the GF and HGGF electrodes are larger than 0.99, which indicated that the positive redox couple reaction was determined by a diffusion process. Moreover, the slopes of the HGGF electrode for the oxidation and reduction 14


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reactions were 11.88% and 182.30% higher, respectively, than those of the GF electrode, suggesting that the HGGF electrode possesses a faster mass transfer process. A possible reason for the faster process may be the small adsorption energy and high electrocatalytic activity induced by the presence of phosphorus functional groups. Table 2. Electrochemical performances obtained from CV tests for the VO2+/VO2+ and V2+/V3+ reactions on GF and HGGF electrodes. mA cm-2 Positive half-cell Negative half-cell

V

mV

electrode

Ipa

Ipc

Vpa

Vpc

GF HGGF GF HGGF

134.0 199.0 30.8 67.2

-39.7 -147.0 -163.0 -289.0

1.211 0.990 -0.250 -0.400

0.660 0.780 -0.799 -0.674

Abs.=Absolute value

15


Abs. (Ipa/ Ipc) 3.38 1.35 0.19 0.23

△E 551 210 549 274


Figure 4. (a) CV tests and (b) EIS spectra for the HGGF and GF electrodes. CV curves for (c) HGGF and (d) GF electrodes and corresponding absolute value of (e) Ipa/Ipc and (f) peak potential difference at different scan rates. The solutions used for these tests are a mixture of 0.05 M VOSO4 and 3 M H2SO4. Electrochemical Performance To investigate the electrochemical performances of GF and HGGF, the VRFBs were assembled (Figure S6), and the charge-discharge tests were conducted (Figure 5a). Compared with the GF electrode, the charge voltage and discharge voltages of the HGGF electrode decreased by 242.4 mV and increased by 273.0 mV, respectively, and the discharge capacity of the HGGF electrode was improved by 105.1% at 100 mA cm-2. The power capability is an important index, and VRFBs with GF and HGGF electrodes were tested at various current densities. As shown in Figure 5b, the voltage efficiency (VE) of the GF electrode gradually decayed as the VRFB operated at 50 mA cm-2, which showed that the electrochemical activity gradually decreased because of hydrogen evolution on the negative electrode (Figure 4a). In addition, the GF electrode cannot operate at 120 mA cm-2 because the charge transfer between the electrode and solution is suppressed at a larger current density, as shown in the EIS tests. Although the VE, EE and discharge capacity of the HGGF-based VRFB gradually decreased as the current density increased from 50 to 200 mA cm-2, the decrease in the VRFB with the HGGF electrode was smaller than that of the GF-based VRFB (Figure 5c and 5d). Meanwhile, the HGGF-based VRFB showed outstanding 16


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stability at a large current density, and the VE, EE, and discharge capacity of HGGF-based VRFB nearly recovered to the initial values at 50 mA cm-2. In contrast, the GF-based VRFB failed when the current density was over than 100 mA cm-2. The one-fold increment in the rate ability originates from the advanced electronic conductivity and enhanced electrocatalysis due to the embedded O and P heteroatoms. Moreover, as shown in Figure 5e, the EE of the HGGF-based VRFB can maintain at a stable level of 73.34% after 1000 charge-discharge cycling at current density of 150 mA cm-2. It should be noted that the HGGF electrode can further boost the rate capability to 300 mA cm-2 at slightly elevated temperatures (35 oC) (Figure S7).

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Figure 5. (a) The charge/discharge profiles of VRFB based on GF and HGGF electrodes at 100 mA cm-2. The (b) VE, (c) EE, and (d) discharge capacity of the VRFBs at different current densities. The long-term operation performance of the VRFB based on the HGGF electrode at 150 mA cm-2 and (e) EE.

CONCLUSIONS To sum up, we designed an innovative electrode with a gradient distribution of phosphorous-containing functional groups that ensures an ultralong cycling life and outstanding electrochemical activity. With the assistance of DFT evaluations, the narrow HOMO-LUMO energy gap and adsorption energy between vanadium ions and the obtained electrode materials have been demonstrated, and these characteristics endow the electrode with fast redox reaction kinetics and a high power capability. This work provides a prototype for VRFBs with integrated in-depth theoretical and experimental investigations and will advance the revelation of interfacial reaction mechanism in other flow batteries. ASSOCIATED CONTENT Supporting Information Density functional theory calculation information; Table of the electrochemical performance comparison of the HGGF with other works; SEM images of GF and HGGF; XPS survey of the GF and HGGF electrodes; Equivalent circuit for the EIS spectra; Plot of the anodic and cathodic peak currents for HGGF and GF versus the 18


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square root of each scan rates; Photo of the VRFB; Energy efficiencies for VRFB based on HGGF and GF electrodes under different current densities (35oC). AUTHOR INFORMARION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the Basic Science Center Project of Natural Science Foundation of China (Grant no. 51788104), the National Key R&D Program of China (Grant no. 2016YFA0202500), the “Transformational Technologies for Clean Energy and Demonstration,” Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDA21070300), the Double First-Class Construction Project of Hunan Agricultural University (Grant no. SYL201802002, SYL201802008), and the Scientific Research Fund of Hunan Provincial Education Department (Grant no. 15K058). REFERENCES [1] 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, 3577-3613. [2] Fukumoto, M.; Imanaka, T. Preface: The First Critical Workshop on the Effect of 19



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