3D Graphene–Ni Foam as an Advanced Electrode for High

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3D Graphene−Ni Foam as an Advanced Electrode for HighPerformance Nonaqueous Redox Flow Batteries Kyubin Lee,† Jungkuk Lee,† Kyoung Woo Kwon,§ Min-Sik Park,*,‡ Jin-Ha Hwang,*,§ and Ki Jae Kim*,† †

Graduate School of Energy and Environment, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin Gyeonggi 17104, Republic of Korea § Department of Materials Science and Engineering, Hongik University, Seoul 04066, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Electrodes composed of multilayered graphene grown on a metal foam (GMF) were prepared by directly growing multilayer graphene sheets on a three-dimensional (3D) Ni-foam substrate via a self-catalyzing chemical vapor deposition process. The multilayer graphene sheets are successfully grown on the Ni-foam substrate surface, maintaining the unique 3D macroporous structure of the Ni foam. The potential use of GMF electrodes in nonaqueous redox flow batteries (RFBs) is carefully examined using [Co(bpy)3]+/2+ and [Fe(bpy)3]2+/3+ redox couples. The GMF electrodes display a much improved electrochemical activity and enhanced kinetics toward the [Co(bpy)3]+/2+ (anolyte) and [Fe(bpy)3]2+/3+ (catholyte) redox couples, compared with the bare Ni metal foam electrodes, suggesting that the 2D graphene sheets having lots of interdomain defects provide sufficient reaction sites and secure electric-conduction pathways. Consequently, a nonaqueous RFB cell assembled with GMF electrodes exhibits high Coulombic and voltage efficiencies of 87.2 and 90.9%, respectively, at the first cycle. This performance can be maintained up to the 50th cycle without significant efficiency loss. Moreover, the importance of a rational electrode design for improving electrochemical performance is addressed. KEYWORDS: nonaqueous redox flow battery, electrode, Ni metal foam, graphene, energy storage system

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INTRODUCTION In recent years, the growing demand for efficient and reliable energy storage systems has become more crucial than ever. Redox flow batteries (RFBs) are currently regarded as an important class of energy storage system and need to integrate with energy harvesting systems that are suitable for large-scale energy storage systems (ESSs) for grid supporting and utilization of renewable energy sources.1−4 Even though the power and energy densities of RFBs are relatively lower than those of other competing systems (e.g., lithium-ion batteries), they offer unlimited power and capacity through increasing the number of stacks and expanding the size of the electrolyte storage tanks.5,6 Moreover, RFBs also have many advantages such as a fast response time, flexible layout, and extremely long cycle life.7,8 The principle of RFBs is based on the electrochemical redox reactions of electro-active species (i.e., redox couples) dissolved in separated electrolytes. Given the unique operating mechanism of RFBs, both the electrolytes that govern the electrochemical redox reactions and the electrodes must be chosen carefully to achieve high-energy-efficiency RFBs;4,8 the electrodes must be robust and offer sufficient reaction sites. To © 2017 American Chemical Society

this end, intensive efforts have been devoted to developing nonaqueous electrolytes that exhibit higher operating voltages (>2 V) than aqueous electrolytes, which are limited to an operating voltage of less than 1.7 V due to water hydrolysis.8−17 However, only a few studies have investigated metal electrodes as replacements for commercial carbon felt electrodes.8 Recently, the feasibility of using metal-foam electrodes in nonaqueous RFBs has been examined.8,9,18 Although using metal-foam electrodes can improve the electrochemical reactivity of RFBs, additional surface modifications such as carbon coating and chemical etching are still necessary to generate sufficient reaction sites due to their low specific surface area. Nevertheless, using metal-foam electrodes still holds great promise because of their excellent mechanical stability, low specific resistance, porous structure, and cheaper price than carbon-based electrode in this research field.8−10 As a further stage in our efforts toward developing practical metal-foam electrodes, we herein designed and synthesized a Received: April 5, 2017 Accepted: June 20, 2017 Published: June 20, 2017 22502

DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of preparation of multilayered graphene grown metal foam (GMF) electrode by a self-catalyzing chemical vapor deposition (CVD) process.

multilayered graphene grown Ni foam (GMF) electrode by chemical vapor deposition (CVD) and examined its potential use in nonaqueous RFBs, in particular, using [Co(bpy)3]+/2+ and [Fe(bpy)3]2+/3+ redox couples. Multilayered graphene can be easily grown on a variety of metal substrates (e.g., Cu, Ni, Ru, Ir, and Co). In particular, CVD-grown graphene exhibits superior electrochemical performance to chemically prepared graphene, due to the low number of defects and good uniformity; additionally, the production of CVD-grown graphene is relatively easy to scale-up.19,20 Taking into account these features, we propose an ideal electrode structure composed of a graphene surface and 3D metal-foam framework for facilitating efficient charge transport during operation. We expect that depositing 2D graphene sheets on the Ni-foam surface will offer sufficient reaction sites without a significant loss in electric conduction, thus leading to improved electrochemical performance. By employing GMF electrodes, voltage efficiencies of nonaqueous RFBs can be effectively increased by reducing the charge-transfer resistance during cycling.

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RESULTS AND DISCUSSION

Multilayered graphene can be grown on the surface of a 3D Ni foam substrate through chemical vapor deposition (CVD) using a methane (CH4) precursor, as illustrated in Figure 1. Commercial Ni foam was employed as the substrate and also acts as a catalyst for the direct growth of multilayered graphene by reducing the energy barrier to form graphene and promoting its lateral growth. The graphene growth behavior is highly dependent on the type of metal substrate and we expected that multilayered graphene possessing many interdomain defects would grow on a polycrystalline Ni foam substrate.21−25 Graphene islands nucleated randomly on the Ni-foam surface during the initial stage of the synthesis. The initial graphene domains had different lattice orientations and, as the process proceeded, they grew and finally merged to form many interdomain defects, as illustrated in Figure 1. We expect that the imperfect bonding structure of the interdomain defects will provide favorable reaction sites for the redox reactions of electro-active species.21−25 Figure 2a,b shows field-emission scanning microscopy (FESEM) images of the bare Ni foam at different magnifications. The Ni foam has straight grain boundaries, a smooth surface, and a 3D configuration. After 10 h of

Figure 2. (a, b) FESEM images of pure Ni foam; (c, d) FESEM images of GMF electrode after 10 h of deposition; (e, f) FESEM images of GMF electrode after 20 h of deposition at different magnifications.

deposition, a thin multilayered graphene had formed evenly on the Ni-foam surface and a small number of interdomain defects were seen (Figure 2c,d); this graphene was not thick enough to blur fully the grain boundaries. Interestingly, the number of interdomain defects and thickness of the deposited multilayered graphene increased with increasing deposition time. As shown in Figure 2e,f, the Ni-foam surface was fully wrapped with a thick multilayered graphene and a large number of interdomain defects were distinctly observed after 20 h of deposition. The structure of the deposited multilayered graphene on the Ni-foam surface was analyzed using X-ray diffraction (XRD) 22503

DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508

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Figure 3. (a) X-ray diffraction patterns of pure Ni foam and GMF electrodes; (b) Raman spectra of pure Ni foam and GMF electrodes; (c) highresolution XPS spectrum of C 1s peak; (d) functional groups obtained from curve fitting of C 1s XPS spectrum.

deposition time. GMF-20 was composed of 17.7% sp3 C−C species, which indicates that the thickness of the deposited graphene sheets is highly dependent on the deposition time, as shown in Figure 3d. The surface morphology and roughness of GMF-20 was investigated using atomic force microscopy (AFM) as shown in Figure 4a. There were a large number of microscale interdomain defects, in accordance with the aforementioned FESEM observations. Such interdomain defects are generally formed by the continuous lateral growth of individual graphene domains, with different orientations, during the CVD process. Transmission electron microscopy (TEM) investigation confirmed that the multilayered GMF-20 has a thickness of approximately 10 nm (Figure 4b) and displays the general characteristics of 2D materials: confined thickness, high specific surface area, and chemically reactive facets. More interestingly, the existence of interdomain defects between graphene sheets of different orientations was confirmed (Figure 4c); this indicates that the disordered carbon structure will be advantageous for facilitating electrochemical redox reactions by providing additional reaction sites. The electrochemical performance of the GMF electrodes (GMF-10 and GMF-20) toward [Fe(bpy)3]2+/3+ (catholyte) and [Co(bpy)3]+/2+ (anolyte) redox couples was assessed using cyclic voltammetry (CV) measurements in a three-electrode cell. Figure 5a shows the CV profiles of bare Ni foam, GMF-10, and GMF-20 for the [Fe(bpy)3]2+/[Fe(bpy)3]3+ redox couple at a scan rate of 50 mV s−1. As expected, both GMF-10 and GMF-20 displayed superior electrochemical performance to the bare Ni-foam. The GMF-20 exhibited the largest anodic and cathodic peak currents and the smallest peak potential separation (ΔV = 0.23) for the [Fe(bpy)3]2+/[Fe(bpy)3]3+ redox couple (ΔV for bare Ni foam = 0.34, ΔV for GMF-10

and Raman spectroscopy measurements. XRD patterns of the GMF samples prepared with different deposition times (Figure 3a) displayed a crystalline peak with a low Bragg angle (2θ = 26.6°) that corresponds to the (002) peak of graphite, which is typical of multilayered graphene. This indicates that multilayered graphene was successfully formed on the Ni-foam surface. Raman spectra of the GMF samples (Figure 3b) displayed the characteristic peaks of multilayered graphene at 1588 and 2701 cm−1, corresponding to the G and 2D bands, respectively, of graphitic carbon.26,27 The intensity of the Gband, arising from the in-plane vibration (E2g symmetry) of sp2 carbon, increased notably with increasing deposition time, indicating that more graphene layers are stacked and form thicker multilayered graphene with extended deposition times. The disorder-induced D-band (around 1350 cm−1) was not observed in either GMF sample, indicating that the multilayered graphene possessed a perfect crystal structure with no significant defects. The number of graphene layers in the multilayered graphene samples was estimated by comparison of the 2D band intensity. The intensity ratio (I2D/IG) of GMF-20 was estimated to be 0.58. Considering that single-layer graphene has a typical I2D/IG value of 4, the small value for GMF-20 indicates that multilayer graphene sheets were formed during growth. The chemical bonding on the GMF surface was investigated by X-ray photoelectron spectroscopy (XPS) in order to understand clearly the microstructure of the deposited multilayered graphene. Figure 3c shows a deconvoluted C 1s spectrum for GMF-20, with a strong peak at 284.5 eV that corresponds to C−C (sp2) bonds. A small peak attributed to C−C (sp3) bonds and a minor C−O-bond peak were also visible at 285.3 and 286.3 eV, respectively. The percentage of sp3 C−C species present increased, and the percentage of sp2 C−C species correspondingly decreased, with increasing 22504

DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508

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Figure 4. (a) AFM image of GMF-20 electrode; (b) TEM image of multilayer graphene on top of the Ni foam, (c) TEM image of the interdomain defect between graphene sheets of different orientations.

current density versus the square root of the scan rate.9,28 Figure 5c,d shows that the oxidation and reduction peakcurrent densities for the [Fe(bpy)3]2+/[Fe(bpy)3]3+ and [Co(bpy)3]+/[Co(bpy)3]2+ redox couples are nearly proportional to the square root of the scan rate, which suggests that these redox reactions are diffusion controlled on all tested electrodes. The ECSA was calculated from the Randle−Sevcik slope for all electrodes in both the catholyte and anolyte. The ECSA values for the GMF-20 in the catholyte (1.074) and anolyte (1.470) were about 2.1 and 1.66 times, respectively, larger than those of the bare Ni foam, indicating that the GMF20 possesses many more electrochemically active sites. The increased number of electrochemically active sites on GMF-20 might originate from the abundant interdomain defects formed during graphene growth. Standard rate constants, k, (Figures S1 and S2) were calculated from the CV measurements using the following eq 1:28

= 0.30), which is attributed to the large number of interdomain defects in GMF-20. The ratio of the anodic peak current to the cathodic peak current (Ipa/Ipc) is another important factor to consider when assessing redox reversibility. It is well-known that the reversibility of a redox reaction improves as Ipa/Ipc approaches 1.0.9,28 The GMF-20 Ipa/Ipc ratio was 1.48 for the [Fe(bpy)3]2+/[Fe(bpy)3]3+ redox couple, whereas the bare Nifoam and GMF-10 Ipa/Ipc ratios were 3.31 and 2.13, respectively. This indicates that the [Fe(bpy)3]2+/[Fe(bpy)3]3+ redox-couple reversibility is relatively high when using the GMF-20. Additionally, the anodic and cathodic peak currents were 9.45 and 11.42 mA cm−2, respectively, when using GMF20, which are about 2.0 and 1.8 times, respectively, larger than those for the bare Ni-foam (Figure 5b). Moreover, the GMF-20 demonstrates significantly reduced polarization for [Co(bpy)3]+/[Co(bpy)3]2+ redox couple in comparison with bare Ni foam, accompanied by an Ipa/Ipc ratio (0.82) that is close to 1.0. This indicates that the [Co(bpy)3]+/[Co(bpy)3]2+ redoxcouple reversibility is greatly improved by the growth of multilayered graphene with a large number of interdomain defects. The effective electrochemical surface area (ESCA) of an electrode is considered one of the most important factors when developing electrodes for RFBs, as it directly affects the electrochemical performance of an electrode.4,9,28 The effective ESCA of an electrode can be evaluated by plotting the peak-

i p = 0.227nFAC0k exp[−αnF(Ep − E 0)/RT ]

(1)

where ip is the peak current, n is the number of electrons involved in the reaction, F is Faraday’s constant, A is the active surface area of the electrode, C0 is the bulk concentration of the oxidant, α is the charge-transfer coefficient, Ep is the peak potential, and E0 is the equilibrium potential. Standard rate constants were calculated as described in reference 26 and the 22505

DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508

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Figure 5. Cyclic voltammograms of the three prepared electrodes: (a) 0.01 M [Fe (bpy)3]2+/3+ and (b) 0.01 M [Co(bpy)3]+/2+ in PC with 1 M TEABF4 at a scan rate of 50 mV s−1. Plot of the peak current density versus the square root of the scan rate for (c) 0.01 M [Fe (bpy)3]2+/3+ and (d) 0.01 M [Co(bpy)3]+/2+.

values for the oxidation of [Fe(bpy)3]2+ and the reduction of [Co(bpy)3]2+ on the different electrodes are presented in Table 1. As expected, the GMF-20 exhibited higher rate constants for

given experimental conditions, allowing for reaching a higher SOC at the cutoff voltage of cell. Figure 6b,c shows the coulomb and voltage efficiencies of the bare Ni foam and GMF20 cells, respectively, during cycling. As expected, the GMF-20 cell exhibited higher voltage efficiencies (>90%) and more stable coulomb efficiencies (>90%) than those of the bare Ni foam cell. As a result, the GMF-20 cell exhibited high energy efficiency of 81.8%, which is 8% higher than that of the pure Nifoam cell (73.8%). Such improvements are likely to originate from the increased number of active sites, rapid ion transport, and superior charge-transfer rates arising from the introduction of graphene sheets, with a large number of interdomain defects, on the pure Ni foam.

Table 1. Standard Rate Constants (k) for Pure-Ni Foam, GMF-10, and GMF-20 Electrodes in Both the Anolyte and Catholyte of a Nonaqueous Redox Flow Battery reaction rate constant, k (10−2 m s−1) electrode

oxidation of [Fe(bpy)3]2+

reduction of [Co(bpy)3]2+

pure Ni foam GMF-10 GMF-20

3.579 4.304 4.868

3.343 3.609 5.496

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the oxidation of [Fe(bpy)3]2+ and the reduction of [Co(bpy)3]2+ than those of the bare Ni foam, suggesting that charge transfer at the electrode/electrolyte interface is greatly enhanced by introducing graphene sheets with abundant interdomain defects on the Ni foam surface. The practical use of GMF-20 was assessed using an RFB cell that was galvanostatically charged and discharged at a constant current density of 20 mA for 50 cycles. As shown in Figure 6a, the operation time during the first cycle was 1.7 times longer when using the GMF-20 (14.6 h) compared with bare Ni foam (8.6 h) under identical conditions. More importantly, it was evident that the electrochemical polarization of the GMF-20 cell significantly reduced during repeated charging and discharging. The longer operation time and lower electrochemical polarization of the GMF-20 cell is attributed to the greater participation of active species (i.e., redox couples) dissolved in the electrolytes during charging and discharging. In other words, the utilization of the active materials is improved by employing the more-active GMF-20 in the cell under the

CONCLUSION

We developed a multilayered GMF electrode via a thermal CVD process and the feasibility of the proposed electrode was carefully examined in a nonaqueous RFB composed of [Fe(bpy)3]2+/[Fe(bpy)3]3+ and [Co(bpy)3]+/[Co(bpy)3]2+ redox couples. The GMF electrodes exhibited much improved electrochemical activity and lower polarization compared with a bare Ni foam electrode toward [Fe(bpy)3]2+/[Fe(bpy)3]3+ and [Co(bpy)3]+/[Co(bpy)3]2+ redox couples. As a result, a high energy efficiency of more than 80% was attained with a stable cycle performance when employing GMF electrodes in the nonaqueous RFB. This was attributed to the large number of interdomain defects in the multilayered graphene providing additional reaction sites for electrochemical redox reactions. Consequently, we postulate that multilayered graphene grown on a Ni-foam electrode can be seriously considered as an advanced electrode for high performance nonaqueous RFBs. 22506

DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508

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Figure 6. (a) Charge−discharge profiles of the first cycle and (b) Coulombic, (c) voltage, and (d) energy efficiencies of the nonaqueous RFB cells with pure Ni foam (black) and GMF-20 electrode (red).

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was subjected to focused ion beam thinning (FIB, FEI-Helios) and then analyzed using HRTEM (FEI, Tecnai) operated at 200 keV. Electrochemical Measurements. Cyclic voltammetry (CV) measurements were carried out using a potentiostat/galvanostat (EC-Lab, BioLogic) in a three-electrode glass cell consisting of a GMF working electrode, a Pt mesh counter electrode, and a Ag/Ag+ reference electrode at various scan rates from 5 to 100 mV s−1. The anolyte was prepared by dissolving 0.1 M tris(2,2′-bipyridine) cobalt tetrafluoroborate [Co(bpy)3 (BF4 ) 2, HANCHEM] and 1.0 M tetraethylammonium-tetrafluoroborate (TEABF4, Aldrich) as a supporting electrolyte in propylene carbonate (PC, PANAX ETEC). The catholyte was prepared using 0.1 M tris(2,2′-bipyridine) iron tetrafluoroborate [Fe(bpy)3(BF4)2] and the same supporting electrolyte. A nonaqueous RFB cell composed of GMF electrodes (3 cm × 3 cm), a cation-exchange membrane (Fumatech, F-14100), and bipolar plates (Morgan Korea Co. Ltd.) was assembled. The main components were assembled in order of the flow path and both electrolytes were stored in separate reservoirs connected to the cell. The cell was galvanostatically charged and discharged over a voltage range of 1.7 to 2.1 V at a current density of 2.2 mA cm−2. Each electrolyte was passed through an inlet with an inner diameter of 4.8 mm and circulated inside the cell at a flow-rate of 0.685 mL s−1 (Watson−Marlow 520S Peristaltic Pump).

EXPERIMENTAL DETAILS

GMF Preparation. Multilayered graphene was grown via thermal CVD on Ni-foam substrates (3 cm × 3 cm, Alantum Corp. The specification of Ni foam is provided in Table S1)). The gas atmosphere was controlled to deplete oxygen and use hydrogen during growth of the graphitic layers on the Ni-foam substrates. The fabrication process was divided into four steps: (i) the processing chamber temperature was raised to 950 °C at a heating rate of 10 °C/ min under a continuous flow of hydrogen (90 sccm); (ii) and then kept at 950 °C for 1 h under a reducing atmosphere controlled using hydrogen; and (iii) the processing chamber was subjected to the injection of methane into the synthesis chamber, where the hydrogen: methane was kept at 3:1 ratio with flow rates of 90 and 30 sccm, respectively; the working pressure was controlled at 10 Torr during growth of the multilayered graphene for 10 or 20 h; and (iv) the chamber was then cooled to ambient conditions through furnace cooling; it took approximately 2 h to cool the chamber from 950 °C to room temperature and hydrogen was continuously supplied throughout cooling. Materials Characterization. The morphology and microstructure of the GMF electrodes were examined using field-emission scanning electron microscopy (FESEM, JEOL JSM-7000F) and high-resolution transmission electron microscopy (HRTEM, JEOL ARM-200F) combined with energy dispersive X-ray spectroscopy (EDS). Powder X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (PANanalytical, Empyrean) equipped with a 3D pixel semiconductor detector and a Cu Kα radiation source (λ = 1.540 56 Å). Further structural characterization was performed using Raman spectroscopy (Bruker Senterra Grating 400) with a He−Ne laser at a wavelength of 532 nm, and X-ray photoelectron spectroscopy (XPS, Thermo Scientific Sigma Probe). To estimate the thickness of the graphitic layers, the graphitically covered Ni foam was coated with Au using thermal evaporation. The surface morphology of the graphene sheets grown on Ni foam was analyzed by noncontact atomic force microscopy (AFM) with a Pt-coated Au tip. The gold-coated Ni foam

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04777. Cyclic voltammograms of positive electrodes as a function of scan rate, cyclic voltammograms of negative electrodes as a function of scan rate, and specification of Ni foam (PDF) 22507

DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508

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AUTHOR INFORMATION

Corresponding Authors

*K. J. Kim. E-mail: [email protected]. *J.-H. Hwang. E-mail: [email protected]. *M.-S. Park. E-mail: [email protected]. ORCID

Ki Jae Kim: 0000-0002-2166-7467 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Energy Efficiency & Resources Core Technology Program (No. 20162020107060 and 20152000000350) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) of the Ministry of Trade, Industry, and Energy, Republic of Korea. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B01015557).

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DOI: 10.1021/acsami.7b04777 ACS Appl. Mater. Interfaces 2017, 9, 22502−22508