Designing High-Performance Composite Electrodes for Vanadium

Jun 15, 2018 - Designing High-Performance Composite Electrodes for Vanadium Redox Flow Batteries: Experimental and Computational Investigation...
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Functional Inorganic Materials and Devices

Designing High-Performance Composite Electrodes for Vanadium Redox Flow Batteries: Experimental and Computational Investigation Qiang Ma, Xian-Xiang Zeng, Chunjiao Zhou, Qi Deng, Peng-Fei Wang, Tong-Tong Zuo, Xu-Dong Zhang, Ya-Xia Yin, Xiong-Wei Wu, Li-Yuan Chai, and Yu-Guo Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04846 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

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Designing High-Performance Composite Electrodes for Vanadium Redox Flow Batteries: Experimental and Computational Investigation Qiang Ma,†,‡, ⊥ Xian-Xiang Zeng,†, ⊥ Chunjiao Zhou,† Qi Deng,†,ǁ Peng-Fei Wang,‡ Tong-Tong Zuo,‡ Xu-Dong Zhang,‡ Ya-Xia Yin,‡ Xiongwei Wu,*,†,ǁ Li-Yuan Chai,*,§ and Yu-Guo Guo*,‡ †

College of Science, Hunan Agricultural University, Changsha, Hunan 410128, China



CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, CAS Research /

Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China §

School of Metallurgy and Environment, Central South University, Changsha, Hunan 410012,

China ǁ

Hunan Province Yin Feng New Energy Co. LTD, Changsha, Hunan 410000, China

KEYWORDS: Vanadium redox flow batteries, Composite electrodes, Density functional theory, Catalytic activity, High performance

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ABSTRACT

Highly catalytic electrodes play a vital role in exploiting the capability of vanadium redox flow batteries (VRFBs) but suffer from a tedious synthesis process and ambiguous interaction mechanisms for catalytic sites. Herein, a facile pyrolysis of urea was applied to prepare graphitic carbon nitride-modified graphite felt (GF@CN), and by the virtue of a density functional theory-assisted calculation, the electron-rich pyridinic nitrogen atom of CN granules is demonstrated as the adsorption center for redox species and plays the key role in improving the performance of VRFBs, with 800 cycles and an energy efficiency of 75% at 150 mA cm−2. Such experimental and computational collaborative investigations guide a realizable and cost-effective solution for other high-power flow batteries.

INTRUDUCTION A high-efficiency and scalable electrochemical energy storage system has been recognized as a promising choice in fulfilling the incremental demand for renewable energy.1-6 Among them, redox flow batteries, especially vanadium redox flow batteries (VRFBs),7 have gained increasing attention owing to their quick response time, long life-span, deep discharge capability, etc.8-10 The conversion of a vanadium ion with several valence states allows the transformation between chemical and electrical energy during the VRFB operation. The electrodes, as the redox reaction zone, are critical for catalyzing the redox reactions of the vanadium ion species.11-13 The catalytic capability of the electrodes determines the electrochemical performance of entire battery.14-16 To date, carbon-based materials are a prominent candidate for VRFBs due to their superior stability, high conductivity and corrosion resistance.6, 17 However, the inferior electrochemical reaction kinetics and reversibility of carbon-based materials are major problems that 2

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negatively impact the cycle lifespan and energy efficiency of VRFBs. Hence, improving the reactivity of the electrodes is of profound importance in augmenting the energy and power densities for the VRFBs. Recently, intensive research efforts have been made to create redox reaction sites on the electrode materials, including the modification of noble metals18-21 or transition metal oxides,22-24 heteroatom doping25-30 and the fabrication of composite electrodes.31-34 Nitrogen doping and compositing with nitride are effective ways to increase the catalytic sites on the surface of carbon-based materials, including carbon nanotubes, graphene, and mesoporous carbon.31,

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Correspondingly, the electrochemical reaction

kinetics and reversibility of the electrodes are improved. Nevertheless, these composite electrodes suffer from a tedious synthesis process and cannot fulfill the desire for a cost-effective and large-scale application of VRFBs. Furthermore, the interaction mechanism between nitrogen-containing groups and redox species is ambiguous, which is critical to design high-performance electrodes for the VRFBs. Herein, to disclose the inherent connection between active species and nitrogen-containing groups, we explored the electrostatic potentials (ESPs) of the van der Waals (vdW) surface of a prototype CN molecule through experimental and computational investigation for the first time. ; we determined that pyridinic nitrogen was the best choice to promote catalysis in the CN granule cladding layer,37,

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which contains plentiful pyridinic nitrogen and supplies

abundant redox sites for absorbing active species during the charging-discharging process. The electrocatalytic activity and mass transport of VRFBs are vastly boosted, leading to an outstanding lifespan of 800 cycles and an energy efficiency of 75% at 150 mA cm−2.

EXPERMENTAL SECTION 2.1 Preparation of the GF@CN electrodes First, the GF was immersed into an aqueous solution (50 mL) containing different amounts of urea (0, 6 and 8 g) with ultrasonic treatment. Afterwards, the sample was dried at 80 ℃, and 3

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the GF precursor was transferred to a muffle furnace under air atmosphere for heat treatment at 550 ℃ for one hour with a heating rate of 5 ℃ min−1. The GF@CN treated with 0 g, 6 g and 8 g of urea were denoted as GF@CN-0, GF@CN-1 and GF@CN-2, respectively. 2.2 Characterization of the GF@CN Electrodes The sample morphologies and microstructure were obtained from scanning electron microscopy (SEM, S-4800, Hitachi, operating at 8 kV) and transmission electron microscope (TEM, JEM-2100F). The Fourier transform infrared (FTIR) spectra of GF and GF@CN-2 were collected with the help of KBr pellets in transmission mode at room temperature on a Bruker Tensor 27 spectrometer. The Raman tests were conducted using a micro-Raman spectrometer (LabRAM HR Evolution) with a 532 nm laser. The X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALab 250Xi using 200 W Al Kα radiation. The base pressure in the analysis chamber was approximately 3×10−10 mbar. The electrolyte wettability of electrodes was tested using 40 µL electrolytes (0.05 mol L−1 (M) VOSO4 in 3 M H2SO4) drop on the surface of electrodes. 2.3 Electrochemical tests The cyclic voltammetry (CV) tests were performed on an electrochemical workstation (CHI760D, Chenhua Co. Ltd.) with 0.05 mol L−1 (M) VOSO4 in 3 M H2SO4, and the testing area was 0.5×0.5 cm2 at a scan rate of 10 mV s−1. A three-electrode system was adopted in the electrochemical tests: the GF and GF@CN were used as the working electrodes, and the Ag/AgCl electrode and platinum sheet served as the reference and counter electrodes, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (PGSTAT 302N) with a bias potential of 5 mV in the frequency range of 0.1 Hz-100 kHz at room temperature using a scan rate of 5 mV s−1. The EIS tests use

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0.05 mol L−1 (M) VOSO4 within 3 M H2SO4 as electrolytes. The impedance spectra were fitted with the Z-view software. The charge-discharge performances of the batteries were evaluated by a battery testing system (CT2001A, China) under the constant current mode. Both the positive and negative electrolytes consisted of 15 mL of 0.75 M VOSO4 + 0.75 M V2(SO4)3 in 3 M H2SO4 (Hunan Yinfeng) with a measurement area of 2×2 cm2. GF@CN and GF were used as the reaction electrodes, and the Nafion 115 membrane (DuPont, USA) was employed as a separator. 1.6 V (vs. Ag/AgCl) and -0.8 V were selected as the cutoff voltages for cathode and anode, respectively. Furthermore, the diameter of inlet for electrolyte flowing was 2 mm and the flow rate was 20 mL min−1 during charge-discharge process. The charge-discharge rate capability and capacity recovery were examined from 50 to 200 mA cm-2 and then returned to 50 mA cm−2 with each rate for 5 cycles. 2.4 Computational details All configurations reported herein were calculated with the M062X functional in a standard Pople 6-31+g(d, p) basis set,39,

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as implemented in the Gaussian 09 program.41

Wavefunctions were obtained with the M062X functional in the 6-311+G(d, p) basis set.42 The analysis of electrostatic potentials (ESPs) on the molecular van der Waals (vdW) surface was done using Multiwfn program.43, 44

RESULTS AND DISCUSSIONS 3.1 Characterizations of composite electrode The GF@CNs composite electrodes were prepared through a facile urea pyrolysis, and the synthetic route is illustrated in Figure 1a. The SEM images of GF and modified GF are shown (Figure 1b-f and Figure S1), and the CN granules with a 100 nm diameter (Figure 1g) are uniformly coated on the GF when the amount of urea reached 8 g. It can be reasonably

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speculated that the abundant CN granules could provide a large amount of catalytic active sites for the adsorption of vanadium redox couples during operation.

Figure 1. (a) Schematic diagram of the preparation process for GF@CN electrodes and the redox reactions of vanadium active species on the GF@CN surface, (b, c) SEM images of the pristine GF and (d-f) as-prepared GF@CN-2 electrodes, (g) TEM image of CN. In addition, the GF@CN-2 and untreated GF electrodes were further measured by FTIR spectroscopy (Figure 2a). A typical peak at 800 cm−1 is observed, corresponding to the deformation of tri-s-triazine ring modes, and the region between 1100 cm−1 and 1700 cm−1 is the characterized stretching mode for the C-N heterocyclic of the triazine ring. The sharp absorptions at approximately 3200 and 3400 cm−1 that involve the vibrations and deformation modes of -NH2 groups are examined.45 The abovementioned characteristic peaks of CN appear and confirm that the CN granules are successfully coated onto the GF surface. Furthermore, the carbon vibrational modes for PAN-based GF also arise, such as C-C, C=C, C-H, C-N, C=N, locating respectively at 1035 cm−1, 1585 cm−1, 2924 cm−1, 1021 cm−1 and 1650 cm−1. The microstructure of the as-prepared electrodes was changed and was characterized by Raman spectroscopy (Figure 2b). The typical D and G bands are obtained at 6

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approximately 1350 and 1600 cm−1,46 and the ID/IG values are 1.04, 1.02, 1.01 and 0.98 for GF@CN-2, GF@CN-1, GF@CN-0 and untreated GF electrodes, respectively. Apparently, after being modified by the CN, a mass of defects is introduced and facilitates the reaction of the vanadium redox couples.

Figure 2. (a) FTIR spectra of the pristine GF and GF@CN-2, (b) Raman spectra of GF, GF@CN-0, GF@CN-1 and GF@CN-2. From the structural formula of the CN, the GF@CN electrodes contain four types of nitrogen groups. The three primary forms are pyridinic nitrogen (N1), pyrrolic nitrogen (N2) and quaternary nitrogen (N3) (Figure 3a). The oxidic nitrogen (N4) is mainly the oxidation state of the pyridinic nitrogen existing on the GF (Figure S2a). Therefore, the electron-rich N1 atom possesses a lone pair of electrons and will show a strong interaction with positively charged species.47 The chemical characteristics of nitrogen and related elements were further characterized through the X-ray photoelectron spectra (XPS) (Figure 3b-e). The high-resolution C1s spectrum shows five fitting peaks located at 289.8, 287.2, 285.9, 285.1, and 284.6 eV (Figure 3c), which can be assigned to COO– (C5), C–O or C=O (C4), C–N or C=N (C3), C–C (C2), and C=C (C1), respectively,33 while the O1s spectrum involves four bands at 534 eV, 532.9 eV, 531.8 eV and 530.6 eV, which can be ascribed to carbonate (O4), C−C=O (O3), C−OH (O2) and C=O (O1), respectively (Figure 3d).48 Meanwhile, the chemical composition of nitrogen is changed after modification, and as shown in Figure 3e, the N1s spectrum can be divided into four characteristic peaks, which are N1 (398.5 eV), N2 7

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(399.3 eV), N3 (401 eV) and N4 (402.9 eV).49, 50 The nitrogen content increases to 4.7%, and the N1 and N2 content outweigh that of the pristine GF (Figure 3f), which might be the main contribution to the absorption of active species,25, 29 and is verified by the following density functional theory (DFT) calculations.

Figure 3. (a) Structural formula of the CN. (b) XPS spectra of pristine GF and GF@CN. (c) C 1s, (d) O 1s and (e) N 1s XPS signals and corresponding fitting curves. (f) The content of N1, N2, N3 and N4 for GF, GF@CN-0 and GF@CN-2, respectively. 3.2 DFT calculations To comprehend the interaction of a vanadium ion with diverse nitrogen groups, the electrostatic potentials (ESPs) on the CN molecule van der Waals (vdW) surface were analyzed based on DFT and used to compare the adsorption effect between vanadium ions and CN.51 As shown in Figure 4a and Figure S2b, the local surface values of the ESP display the following rule: N1 (ca. -45.20 kcal mol−1) < N4 (ca. -20.37 kcal mol−1) < N2 ≈ N3 (ca. 5.16 kcal mol−1). The negative minimum value appears near N1 and suggests that the electron density is a locally concentrated N1 type, indicating that N1 types are also highly vulnerable sites for vanadium ion attack. In contrast, the N2 and N3 show positive ESP values, which means that N2 and N3 types are not vulnerable to attack from vanadium ions. The adsorption 8

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energies were calculated to further reinforce the above analysis. As shown in Figure 4b-e, the CN can exhibit flexible folding capability with adsorption energies about -8.17 eV for VO2+ (Figure 4b-c) and -16.52 eV for VO2+ (Figure 4d-e). Both values are lower than the counter values (-5.94 eV for VO2+ and -10.84 eV for VO2+) for N4 in GF (Figure S2c-d), indicating that N1 provides a feasible adsorption site for vanadium ions. Consequently, the vanadium ions prefer to attack N1 rather than to approach N4, let alone N2 or N3 sites with positive values, which demonstrates that the pyridinic nitrogen lead to a stronger catalytic activity than other nitrogen functional groups.52 The DFT calculation results are in accordance with the experimental results from the XPS analysis data.

Figure 4. (a) Optimized structure with color-mapped ESPs on the vdW surface of CN. Cyan, white, red, and blue represent C, H, O, and N atoms, respectively. Optimized configurations of the CN interacting with (b, c) VO2+ and (d, e) VO2+. Cyan, white, gray, red and blue represent V, H, C, O, and N atoms, respectively. 3.3 Electrochemical performance As previously mentioned, the electron density is locally concentrated on the N1 types, and the charge and mass transfer are effectively enhanced. The CV profiles of GF@CN-1 and 9

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GF@CN-2 exhibit four typical reversible peaks of vanadium ion redox couples (Figure 5a),53 while at the untreated GF negative electrode, there are no pronounced redox peaks, but severe voltage polarization is observed. Additionally, the onset potential and potential difference of the redox peaks (∆E = Vpa – Vpc) are important parameters to reflect the reversibility and redox reaction kinetics of the VRFBs. The onset potential of the oxidation process of GF@CN-2 electrode is reduced to 0.79 V. Lowering the polarization voltage of the redox reactions provides a great benefit for the electron transfer kinetics and energy efficiency. The ∆E of GF@CN-2 (534 mV), together with other modified electrodes (Table S1), is much lower than that of the untreated GF (858 mV). In addition, the Ipa/Ipc value of 1.68 for the GF@CN-2 electrode is much close to 1 than that of the untreated GF (Ipa/Ipc = 2.96), showing that the vanadium redox reaction is more reversible than the GF electrode. Obviously, the GF@CN-2 composite electrode exhibits large double layer capacitance behavior indicating the surface area of GF@CN-2 is larger than that of GF electrode, which is demonstrated by potential step curves (Figure S3 and Figure S4). Furthermore, the electrolyte wettability of GF@CN-2 is superior to that of GF (Figure S5). It also could illuminate the reason why GF@CN-2 electrode holds excellent electro-catalytic performance.

Figure 5. (a) Cyclic voltammograms of the GF, GF@CN-0, GF@CN-1, GF@CN-2 electrodes in a solution of 0.05 M VOSO4 + 3 M H2SO4 at a scan rate of 10 mv s-1, (b) plot of peak current density vs. square root of the scan rate for the VO2+/VO2+ redox couples, (c) the electrochemical impedance plots of GF, GF@CN-0, GF@CN-1 and GF@CN-2, the insert is the enlarged plot ranging from 0 ohm to 12 ohm. 10

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Furthermore, the peak current density of the CV curves versus the square root of the potential scan rate is plotted based on the Randles-Sevcik equation to estimate the mass transfer performance of the vanadium ion (Figure 5b). Compared to those of the GF electrode, the higher slope values of the GF@CN-2 electrodes suggest that faster mass transfer kinetics occur. The equivalent circuit is fitted from the Nyquist plots (Figure 5c and Figure S6),54 and the charge transfer resistance is lower than that of the GF electrode, indicating a faster electron transfer process at the electrode/solution interface. All aspects demonstrate that the introduction of a CN cladding layer on the GF can obviously ameliorate the electrochemical reaction kinetic and reversibility of the GF@CN-2 electrodes.

Figure 6. (a) Rate performance, (b) charge-discharge profiles of pristine GF and GF@CN-2 electrodes at 150 mA·cm-2. (c) CE, VE and (d) EE for GF and GF@CN-2 at various current densities, (e) the EE of VRFBs based on GF@CN and GF electrode with 800 charging-discharging cycles at 150 mA cm-2. 11

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The GF@CN-2 was utilized as the electrode to assemble VRFBs (Figure S7), which were applied to evaluate its effectiveness in promoting the charge-discharge and rate capability. The reversible capacities are 32.54 Ah L−1, 27.41 Ah L−1, 24.97 Ah L−1, 20.71 Ah L−1, 14.12 Ah L−1, and 8.73 Ah L−1 at constant current density from 50 to 200 mA·cm−2 (Figure 6a). The obtained VRFBs retain a prominent capacity even through the current density increases to 200 mA·cm−2, implying that the CN granules on the GF@CN-2 electrode accelerate the electron transfer and reduce the vanadium ion transfer resistance. The GF@CN-2 electrode delivers a discharging capacity of 20.67 Ah L−1 at a current density of 150 mA·cm−2, with reduced overpotential (Figure 6b). In addition, the VRFBs with GF@CN-2 electrode shows a slower loss in energy efficiency (EE) and voltage efficiency (VE) than that of GF electrode-based VRFBs ranging from 50 to 150 mA·cm−2 (Figure 6c) and sustains an even Coulomb efficiency (CE). The EE of VRFBs with GF@CN-2 electrode also reaches to 88.7% at 50 mA cm−2 and 64.1% at a current density of 200 mA cm−2 and can nearly recover to the initial value after rate measurements (Figure 6d). The performances of other modified electrodes are also superior to those of pristine GF electrode-based VRFBs (Figure S8). The VRFBs based on GF@CN-2 display excellent stability at 150 mA cm−2 for 800 cycles with an EE of 75% and only 0.008% decay ratio (Figure 6e).

CONCLUSIONS In conclusion, the GF@CN composite electrode is successfully designed using both experiments and DFT calculations, demonstrating that pyridinic nitrogen serves as the local center with high electron density and stable binding sites for active species. The corresponding VRFBs display an outstanding operation stability of 800 cycles with an energy efficiency of 75%. This work demonstrates the inherent mechanism of nitrogen-doped carbon

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materials in improving the kinetic characteristics of VRFBs and provides a new perspective for the future design of a high-power redox flow battery. ASSOCIATED CONTENT Supporting Information SEM images of GF@CN-0 and GF@CN-1; ESP calculation for the vdW surface of N4; Potential step curves of GF and GF@CN-2; The electrolyte wettability of GF and GF@CN-2; Fitting equivalent circuit based on the Nyquist curve; Table of the CV data for pristine and modified GF; The cell configuration of VRFBs; VRFB performances based on the GF@CN-0 and GF@CN-1 electrodes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMARION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] Author Contributions ⊥

Qiang Ma and Xian-Xiang Zeng contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Technology of the People’s Republic of China (Grant No. 2016YFA0202500), the National Natural Science Foundation of China (Grant No.51772093). REFERENCES 13

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