New Approach for High-Voltage Electrical Double-Layer Capacitors

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A New Approach for High-Voltage Electrical Double-Layer Capacitors Using Vertical Graphene Nanowalls with and without Nitrogen Doping Yu-Wen Chi, Chi-Chang Hu, Hsiao-Hsuan Shen, and Kun-Ping Huang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02401 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 26, 2016

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A New Approach for High-Voltage Electrical Double-Layer Capacitors Using Vertical Graphene Nanowalls with and without Nitrogen Doping Yu-Wen Chia,b, Chi-Chang Hua,*, Hsiao-Hsuan Shena, and Kun-Ping Huangb a.

Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, KuangFu Road, Hsin-Chu 30013, TAIWAN.

b.

Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, 195, Sec. 4, Chung Hsing Road, Chutung, Hsin-Chu 31040, TAIWAN.

KEYWORDS: vertical graphene, nitrogen doping, high voltage, supercapacitors, organic electrolyte

ABSTRACT: Integrating various devices to achieve high-performance energy storage systems to satisfy various demands in modern societies become more and more important. Electrical double-layer capacitors (EDLCs), one kind of the electrochemical capacitors, generally provide the merits of high charge-discharge rates, extremely long cycle life, and high efficiency in electricity capture/storage, leading to a desirable device of electricity management from portable electronics to hybrid vehicles or even smart grid application. However, the low cell voltage (2.52.7 V in organic liquid electrolytes) of EDLCs lacks the direct combination of Li-ion batteries (LIBs) and EDLCs for creating new functions in future applications without considering the issue of a relatively low energy density. Here we propose a guideline, “choosing a matching pair

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of electrode materials and electrolytes”, to effectively extend the cell voltage of EDLCs according to three general strategies. Based on the new strategy proposed in this work, materials with an inert surface enable to tolerate a wider potential window in commercially available organic electrolytes in comparison with activated carbons (ACs). The binder-free, vertically grown graphene nanowalls (GNW) and nitrogen-doped GNW (NGNW) electrodes respectively provide good examples for extending the upper potential limit of a positive electrode of EDLCs from 0.1 V to 1.5 V (vs. Ag/AgNO3) as well as the lower potential limit of a negative electrode of EDLCs from -2.0 V to ca. -2.5 V in 1 M TEABF4/PC (propylene carbonate) compared to ACs. This newly designed asymmetric EDLC exhibits a cell voltage of 4 V, specific energy of 52 Wh kg-1 (ca. a device energy density of 13 Wh kg-1), and specific power of 8 kW kg-1 and ca. 100% retention after 10,000 cycles charge-discharge, reducing the series number of EDLCs to enlarge the module voltage and opening the possibility for directly combining EDLCs and LIBs in advanced applications. Introduction Supercapacitors with high-rate charge-discharge and long cycle life characteristics have received much attention in recent years because of the urgent electricity storage and management demands in wide applications, such as boosting the myriad applications from portable consumer electronic devices to electric vehicles1 as well as buffering the power grids of wind/solar power plants for their intermittent availabilities.2 Electrical double-layer capacitors (EDLCs), one kind of supercapacitors, are considered a desired energy storage device because the electric energy is electrostatically stored at the interface between the electrode material and electrolyte without involvement of faradaic reactions. This purely physical process theoretically offers the possibility of no degradation, extremely high efficiency, and safety for infinite cycles of charge storage and delivery. A combination of Li-ion batteries (LIBs) and


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supercapacitors has been demonstrated to promote the existed functions and/or generate new functions2,3 but the unmatched cell voltage creates the parallel/series issue. Although ionic liquids (ILs, e.g., EMImBF4)4 have been employed as electrolytes to achieve a high cell voltage of 4 V, the high cost, humidity control, and handling issues5 make them hard to be commercialized without considering the poor performance of IL-based EDLCs at low temperatures.6 Through combining a battery-type and capacitor-type electrodes, the so-called hybrid supercapacitors or asymmetric supercapacitors also show large cell voltages (e.g., 3.8 V for Li-ion capacitors7 and 3-4 V for nanohybrid supercapacitors8) and generally provide a relatively high specific energy. However, the improved energy density for such type of supercapacitors usually comes with certain tradeoff in the power performance without considering the possible degradation of electroactive materials for the very long-cycling and wide-rate application of supercapacitors.9 Hence, the applications of EDLCs are practically limited on account of their low energy density and cell voltage which are insufficient for future and advanced applications. Three possible strategies effectively enlarging the cell voltage of EDLCs are summarized here to enhance the specific energy because E = CV2/2 where E, C, and V respectively indicate energy, cell capacitance, and cell voltage of EDLCs. Clearly, enlarging the cell voltage is more efficient than increasing the cell capacitance and the simplest way is to change the electrolyte. For example, the cell voltage of aqueous EDLCs is significantly enlarged from 1.23 V10 to 2.52.7 V6 for organic EDLCs employing acetonitrile (ACN) or propylene carbonate (PC) as solvents. According to the degradation mechanisms reported by Naoi et al.,11 however, the oxidation and removal of surface oxygen-functional groups of activated carbon (AC) in PC constrain the upper cell voltage limit of PC-based EDLCs. The residual water molecules within


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ACs12 and the reductive decomposition of solvents to form H2 and CO2 also limit the cell voltage of organic EDLCs because of the lower working potential end.11 Based on the same strategy (i.e., changing electrolytes), the cell voltage of IL-based EDLCs can even reach 4 V.4,5 Recently, Andrea Balducci, et al.13,14 tried to develop innovative solvents and salts for further extending the cell voltage from 2.5-2.7 V to 3.2 V, in fact, using the same idea. The second strategy is surface “passivation” of carbons to depress the reaction and/or interaction among solvents, salts, and surface defects of carbons in order to enlarge the cell voltage. This idea has been successfully demonstrated by the electrochemical modification of ACs (without the long-time removal of residual water) in PC under a specified modification program in our previous work.15 The third way is to directly synthesize carbons with a very inert surface which enables to tolerate a wider potential window in commercially available organic electrolytes compared to ACs. To the best of our knowledge, this concept is never explored before although the very wide potential window of B-doped diamond electrodes in aqueous media for the electrochemical detection of heavy metal ions supports our unique idea. From all the above strategies, how to choose a matching pair of electrode materials and electrolytes is the key for effectively extending the cell voltage of EDLCs. This concept is applicable for all electrode materials of EDLCs and other energy storage systems. We test the above idea through employing the binder-free, vertically grown graphene nanowalls (GNW) and nitrogen-doped GNW (NGNW) electrodes to respectively extend the upper potential limit of a positive electrode from 0.1 V to 1.3−1.5 V and the lower potential limit of a negative electrode from -2.0 V to -2.4 ~ -2.5 V (vs. Ag/AgNO3) in 1 M TEABF4/PC for EDLCs in comparing with ACs. Consequently, an advanced design of asymmetric EDLCs in 1 M TEABF4/PC can achieve a cell voltage of 3.8-4 V with the specific energy level of 52 Wh kg-1


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(i.e. a device energy density of 13 Wh kg-1) without power loss. Hence, the practical limitations of EDLCs, due to low energy density and low cell voltage, for future and advanced applications can be solved. Synthesis of GNW and NGNW. All GNWs were synthesized by means of the microwave plasma torch (MPT) tool coupled with the plasma-enhanced chemical vapor deposition (PECVD) method. The argon gas was excited with back-ground plasma using 2.45 GHz microwave in the MPT to directly grow vertical GNW onto Ti current collectors without any binder. Methane with a flow rate of 20-40 mL min−1 mixed with argon in a volume ratio varying from 20 to 100% was fed into the reaction chamber. The microwave power was set between 500 and 1500 W for 10 min under a pressure from 20 mTorr to 760 Torr. NGNW was directly grown onto Ti substrates by the MPT-PECVD method using a mixed flow of CH4 and N2 with a total flow rate of 40 mL min-1 and CH4/N2 volume ratio = 1. The microwave power was set at 1000 W for 10 min under a pressure of 40 mTorr. Materials Characterization. High-resolution transmission electron microscopic (HRTEM) images were taken using a FEI Tecnai G2 F30 S-Twin microscope operating at 200 kV equipped with an energy dispersive X-ray (EDS) detector for element mapping/analysis. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo VG ESCALAB250 spectrometer with an Al Kα (hv = 1486.69 eV) X-ray source. Raman spectra were determined by Micro-Raman analysis (Thermo DXR HR Raman Microscope, HOROBA) using a 532-nm laser. Electrochemical Characterization. Two identical GNW/Ti electrodes were assembled in a symmetric EDLC meanwhile the asymmetric EDLC consisted of a positive GNW/Ti electrode and a negative NGNW/Ti electrode. The geometric surface area of the Ti foil substrate is


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10×30×1 mm. The mass loading on every electrode is 2±0.1 mg. The assembled cells were tested with 1 M tetraethylammonium tetrafluoroborate (TEABF4) in propylene carbonate (PC) using a beaker cell without separator or a Swagelok cell with a cellulose separator. The electrochemical properties and specific capacitance were measured under both three- and two-electrode modes by cyclic voltammetric (CV) and galvanostatic charge/discharge methods. The CV and constant current charge-discharge tests were conducted by an electrochemical workstation, CHI 760e. Cyclic voltammograms were measured in a cell voltage window varying from 2.5 to 4.0 V at 50 mV s-1. For the 4-V galvanostatic charge/discharge test, an Ag/AgNO3 reference electrode was added to monitor the variation in potentials of the positive electrode. The potential difference between the positive electrode and cell voltage indicates the corresponding potential on the negative electrode (e.g., see Figure S1 in Supporting Information). The constant-current chargedischarge tests were measured under a cell voltage varying from 2.5 to 4.0 V at various current densities. Vertical growth of binder-free GNW for extending the upper limit of potential window. The graphene quality and layers can be controlled by the fine tune of the process parameters such as carbon source, pressure, flow rate, and microwave power in order to meet the design concept that carbons with a very inert surface can tolerate a wide potential window in commercially available organic electrolytes. The ways for controlling the quality and layer number of vertically grown GNWs are shown in Figure S2 in the Supporting Information. Based on the D-to-G band (ID/IG) ratio, the Raman spectra shown in Figure 1a clearly reveal that the graphene domain size and defect density of GNWs can be controlled by the fabrication parameters. Accordingly, GNWs with the ID/IG ratios equal to 0.3 and 1.7 can be denoted as high-quality (HQ) and lowquality (LQ) GNWs, respectively. Moreover, the layers of the above two GNWs are


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approximately the same (below or equal to 4 layers) since the I2D/IG (2D-to-G band) ratios are equal to 0.91 and 0.89 for HQ- and LQ-GNWs, respectively. A comparison of the two XPS spectra in Figure 1b reveals the extremely low oxygen content of both GNWs grown by the MPT-PECVD method. From spectrum 1, LQ-GNW shows a pure carbon material without obvious peaks corresponding to O although LQ-GNW with a relatively high density of defects might be easier to be oxidized by oxygen molecules in comparison with HQ-GNW when it was exposed to the ambient air. This result reveals the extremely high purity of GNWs grown by means of the MPT-PECVD method with an additional merit for the precise control of GNW quality (i.e., ID/IG independently varying from 0.3 to 1.7). The morphologies of the above two GNWs are similar while the density of LQ-GNWs seems to be higher than that of HQ-GNWs (see Figure S3 in Supporting Information). The HR-TEM image in Figure 1c reveals the excellent quality of HQ-GNWs. The graphene lattice is clearly visible in a relatively large area (i.e., ca. 200 nm2). The Fourier transform electron diffraction patterns in insets A and B of Figure 1c also reveal that the thickness of this HQ-GNW is about 2 atomic layers. Due to the extremely high purity and binder-free nature, both HQ- and LQ-GNWs can obviously extend the upper potential limit of charge-discharge in 1 M TEABF4/PC from ca. 0.1 to 1.3-1.5 V (vs. Ag/AgNO3; see Figure 2) since the upper potential limit for AC (a typical EDLC electrode material) in the same electrolyte is only 0.1 V (see Figure S4).15 From Figure 2a, only a pair of minor redox peaks between 0.1 and 0.3 V (vs. Ag/AgNO3) coincides with the irreversible oxidations,15 which reveals the inertness of GNWs to the TEABF4/PC electrolyte. Since graphite and graphite-like materials in the dual-carbon rechargeable batteries exhibit anion intercalation behavior at potentials above ca. 1.3 V (vs. Ag/AgNO3),11 there should be a space for extending the upper limit of the double-layer potential window from ca. 0.1 to 1.3 V


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(or even above). This idea works well using our vertically grown GNW/Ti electrodes with high purity (i.e., without any binder and conductive agent in the electrode). When the upper potential limit of CV is gradually extended to 1.0 and 1.5 V, the i–E curves on the positive sweeps completely follow the same trace, even though a minor anion intercalation phenomenon into GNWs is visible at potentials positive than 1.3 V. These results reveal that the upper potential limit of charge-discharge on the vertically grown GNWs can be substantially extended from 0.1 to 1.3-1.5 V. In Figure 2b, the double-layer current density of LQ-GNW is much higher than that of HQ-GNW, reasonably attributed to the fact that the specific double-layer capacitance of the basal plan of graphite and graphene materials is significantly lower than that of the edge plan.16 Hence, the vertically grown GNWs are promising positive electrode materials for highvoltage supercapacitors. Vertical growth of binder-free GNW and NGNW for extending the lower limit of potential window. The open-circuit potential of GNWs in 1 M TEABF4/PC is about -0.6 V and the positive limit of potential window has been effectively extended to 1.3-1.5 V (i.e., a potential window of 1.9-2.1 V) using binder-free, vertically grown GNW/Ti. Consequently, the potential window of the negative electrode has to be enlarged in order to meet the charge- and ratebalanced requirements.17 An irreversible reduction due to hydrogen evolution in TEABF4/PC electrolytes generally starts at ca. -1.9 V

-2.1 V (vs. Ag/AgNO3) on AC because of electrolyte

and residual water decomposition.15, 18 This irreversible reduction commences at ca. -2.2 V (vs. Ag/AgNO3) for both LQ- and HQ-GNW/Ti electrodes (see Figure 3). Therefore, replacing AC with the vertically grown GNWs could extend the lower limit of double-layer potential window from ca. -2.0 V to -2.2 V in TEABF4/PC. More importantly, from Figure 4a, the lower limit of potential window can be further extended to -2.4 V when nitrogen is directly doped into


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graphene during the vertical growth of GNWs (i.e., NGNWs). In addition, ideal capacitive responses of NGNW between 0 and -2.4 V are clearly visible. This result reveals that the binderfree, vertically grown, nitrogen-doped GNWs significantly depress the irreversible reduction of organic electrolyte and residual water at the negative potential end, further enlarging the working potential window to 1.8 V. Since a higher capacitive current density is obtained (presumably attributed to that nitrogen doping could improve the ion bonding energy in electrolyte, leading to an increase in the specific capacitance19), it is possible to balance the charges stored in the positive potential window of a GNW/Ti electrode and the negative one of a NGNW/Ti electrode. Thus, vertically grown NGNWs are a suitable negative electrode material for the PC-based EDLCs. Consequently, the cell voltage of an asymmetric EDLC with a positive electrode of GNW/Ti and a negative electrode of NGNW/Ti in 1 M TEABF4/PC should be able to reach 3.8 V or even higher. On the other hand, the CV curves of this NGNW electrode in 1 M TEABF4/PC reveal worse performance in the positive potential window in comparison with LQ-GNW from Figure 4b. The pair of redox peaks between 0.1 and 0.3 V is obvious meanwhile the double-layer responses are visible at potentials negative to 1.0 V. The irreversible oxidation at potentials positive than 1.0 V on this NGNW is reasonably due to the relatively high density of defects, leading to a significant content of oxygen on NGNWs (see Figure 5). These results support our proposal that the oxygen-free, vertically grown GNWs are inert to TEABF4/PC at highly positive potentials, circumventing the electrolyte decomposition issue resulting from the presence of oxygencontaining functional groups. Based on all the above new findings and discussion, the key for effectively extending the cell voltage of EDLCs is to find matching pairs of electrode materials and electrolytes for positive and negative electrodes because of different degradation


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mechanisms. Although the growth mechanism of vertical graphene nanosheet structures (GNSSs) model has been proposed through the in-situ scanning electron microscopy (SEM) and the morphology of vertical graphene was affected by the SiO2 surface condition,20 knowledge about the direct growth of nitrogen-doped vertical graphene is very limited. The present study also demonstrates a unique process for vertical growth of nitrogen-doped graphene nanowalls in one step (see Scheme S1 in the Supporting Information). In growing NGNWs, the MPT background plasma was changed from argon to nitrogen gas, meanwhile the nitrogen content of NGNW could be controlled by the fine tune of preparation parameters. Material characteristics of NGNW examined by means of Raman and XPS analyses as well as SEM cross-section and HR-TEM images are shown in Figure 5. The cross section image of NGNWs in Figure 5a displays a continuous and vertical architecture with total thickness of 50 µm. The network-like structure provides uniform porous structure (characteristics length of pores ≈ 50 nm, see Figure S3) and large surface area for the double-layer charge storage (see Scheme 1). The HR-TEM image in Figure 5b reveals good quality of NGNW with the film thickness of ca. 1 nm (i.e., 2-3 layers). Therefore, the MPT-PECVD technique under the N2 atmosphere is an effective tool in manufacturing high quality, vertical NGNWs. From curve 1 in Figure 5c, the XPS spectrum of NGNW indicates an implanted nitrogen content of 7.6 at% and the presence of a few oxygen-containing functional groups. The depth-profile XPS results (every 30-sec argon plasma etching with one XPS measurement, see Figure S5 in the Supporting Information) reveal the uniform distribution of N atoms doped onto the 3D NGNWs. Figure 5d displays a good lattice arrangement of NGNWs from the sharp 2D band and G band peaks. The high D band intensity of NGNWs indicates a relatively higher defect density in comparison with LQ- and HQ-GNWs because nitrogen doping significantly reduces the graphene domain size. In addition,


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the carbon defects formed by nitrogen doping can react with oxygen molecules in the air, leading to the presence of oxygen functional groups. Due to the very little knowledge about the heteroatom doping during the vertical growth of graphene, this work demonstrates an efficient, one-step method for growing the vertical nitrogen-doped graphene with the nitrogen content reaching 7.6 at% and a few oxygen (< 2 at%). The amount of sp2-bonding of NGNWs can be ascertained by the ratio between π* bonding and π* + σ* bonding through the electron energy loss spectroscopy (EELS, see Figure 5e).21 Table S1 shows a comparison of EELS results for the carbon K near-edge structure between NGNW and graphite of equivalent thickness. On the assumption that the sp2 bonding in the graphite reference spectrum is 100%, the sp2 bonding of our NGNW sample is about 93%. From Scheme 1, both GNWs and NGNWs display a vertical, binder-free structure which favors the penetration of electrolytes and electron transport in the whole graphene matrix. The direct growth of GNWs and NGNWs onto current collectors avoids the complicated coating process and reduces the contact resistance of coatings without considering adhesion and cost. Therefore, the binder-free, oxygen-free, vertical GNWs effectively circumvent the issue of oxygenfunctional group removal at highly positive potentials meanwhile carbon atoms within such GNWs are inert to the irreversible oxidation of organic electrolytes in order to extend the positive potential window. Moreover, the uniform doping of nitrogen on the binder-free, vertical NGNWs significantly depresses the irreversible reduction of residual water and organic electrolyte at the negative potential end, further enlarging the working potential window. The superior performances of an asymmetric EDLC consisting of a negative NGNW/Ti electrode and a positive LQ-GNW/Ti electrode in Figure 6 reveal the successful development of a high-performance, 4-V EDLC in 1 M TEABF4/PC. In Figure 6a, the CV curves of this full cell


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in 1 M TEABF4/PC with the cell voltage varying from 2 to 4 V at 50 mV s-1 are generally symmetric and rectangular in shape, revealing an ideal EDLC with excellent reversibility. The above achievement obviously results from the highly reversible capacitive responses of GNW and NGNW in 1 M TEABF4/PC within the positive and negative potential windows, respectively. Figures 6b and 6c show the charge-discharge curves of this asymmetric cell at different cell voltages and various current densities. The highly symmetric curves of this cell at various cell voltages and current densities strongly support the above statements. Moreover, the cell capacitance retention is good in the above current density region (see Figure 6d), indicating the high power characteristics of this asymmetric cell. This statement is strongly supported by the shiny light of a 9-LED lamp boosted by a single pouched cell (see Figure 6e; dimensions: 30×15 mm). From Figure 6c, the coulombic efficiency for the first cycle at 0.5 A g-1 in a new cell is about 92%, attributable to the irreversible but minor oxidation and reduction reactions on the positive and negative electrodes during the first charge process. In the following chargedischarge tests with gradual increase in current density, the coulombic efficiency is above/equal to 99%. Accordingly, our novel asymmetric design of EDLCs with a negative NGNW/Ti electrode and a positive GNW/Ti electrode is a promising 4V supercapacitor employing commercial organic liquid electrolytes. Figure 7a shows the typical charge-discharge test for evaluating the stability of our newly designed asymmetric EDLCs in a Swagelok cell where the cell voltage is between 3 and 4 V for 1000 cycles at 2 A g-1. Figure 7b shows the chargedischarge curves after every 1000-cycle charge-discharge test for 10000 cycles, revealing good stability of this asymmetric EDLC with a cell voltage of 4 V. Figure 7c demonstrates a 5% increase in the charge capacity of this 4V EDLC after 1000 cycles of the above charge-discharge measurement. In addition, after the 10000-cycle test, its capacitance is still higher than the


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original value, showing a good capacitance retention. Note that both positive and negative electrodes after the above 10000-cycle stability test show no significant changes in microstructures confirmed by TEM, SEM, and Raman spectra analyses (see Figures S6-S8). In Figure S9, the XPS results show the presence of B and F elements in few amounts on LQGNW/Ti after the stability test, suggesting minor adsorption of anions. The relatively obvious O signal may be attributed to the residual of minor oxidation of solvent molecules. Similar phenomena are found for NGNW/Ti after the stability test, indicating the inert reactivity of NGNW to the electrolyte at very negative potentials. All these results support the excellent stability of GNW/Ti and NGNW/Ti in this 4V EDLC test. Because of the high cell voltage, the specific energy and power of this device (based on the total mass of active materials on both positive and negative electrodes) respectively achieve 52 Wh kg-1 and 8 kW kg-1, which are superior to the other articles under the same comparison basis (see Table S1). Naoi et al.18 recommended the floating test at specified cell voltages to be a simple method for evaluating the stability of high-voltage supercarpacitors. The floating test results of the above asymmetric cell (freshly prepared) at 4 V for 6 h are shown in Figure S10 to demonstrate the excellent stability. Due to that a minor curvature between 4 and 3.8 V at low current densities (e.g., 0.5 A g-1) is found, the 6-h floating test of this cell is expected to be over 10000 chargedischarge cycles on the basis of the charge-discharge time spent in this potential region. This 6-h floating test clearly demonstrates the stability of the cell at 4 V since the current density of the floating test is generally lower than 30 µA cm−2 (Figure S10a). In addition, the charge-discharge curves measured after every hour floating test are very stable and retain ca. 100% of the initial capacity after the above 6-h test (Figure S10b). The electrochemical impedance spectra (EIS in the negative Nyquist plots) of this asymmetric EDLC during the 6-h floating test in the


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frequency range from 100 kHz to 0.01 Hz are overlapped in the whole frequency range (Figure S10c), revealing the excellent stability of our cell in the floating test.

Conclusions. From literature review and the results in this work, we clearly demonstrate a guideline that choosing matching pairs of electrode materials and electrolytes can effectively extend the cell voltage of EDLCs. The binder-free, vertically grown GNW/Ti and NGNW/Ti electrodes respectively provide good examples for extending the upper potential limit of a positive electrode of EDLCs from 0.1 V to 1.3-1.5 V (vs. Ag/AgNO3) and the lower potential limit of a negative electrode of EDLCs from -2.0 V to -2.4 -2.5 V in 1 M TEABF4/PC in comparison with activated carbons. We clearly reveal that GNWs with a very inert surface to the commercially available organic electrolytes enable to tolerate a very positive potential limit meanwhile NGNWs, inert to hydrogen evolution and PC reduction, show ideal capacitive behavior at a very negative potential limit. Moreover, the MPT-PECVD method is an interesting and unique technique for one-step growth of vertical GNWs and NGNWs (with 7.6 at% N content uniformly distributed in the vertical structure) which provide facile penetration of electrolytes and electron transport in the whole graphene matrix, circumvent the complicated coating process, and reduce the contact resistance between graphene and current collector. Hence, a novel asymmetric EDLC consisting of a negative NGNW/Ti electrode and a positive GNW/Ti electrode reaches 52 Wh kg-1 and 8 kW kg-1, respectively (i.e., a device energy density of 13 Wh kg-1). Therefore, it opens the possibility for direct combining EDLCs and Li-ion batteries to extend the practical applications in the future.


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Supporting Information Available: The supporting information includes 10 figures, 1 scheme, and 1 table with a TEM image. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION. *: Corresponding author; email: [email protected] ACKNOWLEDGMENT. The financial support of this work, by the National Science Council of Taiwan, under contract no. NSC 102-2221-E-007-120-MY3, 101-2221-E-007-112-MY3, Ministry of Science & Technology of Taiwan, under contract no. MOST103-3113-E-006-009, 104-2923-E-007-003-MY3, and the boost program in the Low Carbon Energy Research Centre in NTHU, is gratefully acknowledged.

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(12) M. Morita, Y. Noguchi, M. Tokita, N. Yoshimoto, K. Fujii, T. Utsunomiya, Electrochim. Acta 2016, 206, 427-431. (13) A. Krause, A. Balducci, Electrochem. Commun. 2011, 13, 814. (14) S. Pohlmann, A. Balducci, Electrochim. Acta 2013, 110, 221. (15) H.-H. Shen, C.-C. Hu, J. Electrochem. Soc. 2014, 161, A1828. (16) W. Yuan, Y. Zhou, Y. Li, C. Li, H. Peng, J. Zhang, Z. Liu, L. Dai, G. Shi, Sci. Rep. 2013, 3, 2248. (17) T.-H. Wu, C.-T. Hsu, C.-C. Hu, L. J. Hardwick, J. Power Sources 2013, 242, 289. (18) S. Ishimoto, Y. Asakawa, M. Shinya, K. Naoi, J. Electrochem. Soc. 2009, 156, A563. (19) H. M. Jeong, J. W. Lee, W. H. Shin, Y. J. Choi, H. J. Shin, J. K. Kang, J. W. Choi, Nano Lett. 2011, 11, 2472. (20) D. H. Seo, S. Kumar, K. Ostrikov, Carbon 2011, 49, 4331. (21) S. D. Berger, D. R. McKenzie, P. J. Martin, Phil. Mag. Lett. 1988, 57, 285.


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Figures and Captions

(a)

Scheme 1 (a) in comparing with AC powder-coated electrodes, GNW/Ti and NGNW/Ti display a binder-free, vertical structure, favoring the penetration of electrolytes and electron transport in the whole graphene matrix. (b, top) The oxygen-free, binder-free GNWs circumvent the issue of oxygen-functional group removal, which are inert to the irreversible oxidation of organic electrolytes, enlarging the upper limit of working potential window. (b, bottom) The uniform N doping on the binder-free, vertical NGNWs significantly depresses the irreversible reduction of residual water and organic electrolyte at the negative potential end, further enlarging the working potential window.


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Figure 1 (a) Raman spectra and (b) the XPS element survey spectra of HQ- and LQ-GNW. (c) The HR-TEM image of HQ-GNW with Fourier transform electron diffraction patterns (insets) at points A and B.


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Figure 2 CV curves of (a) LQ-GNW with the upper potential limit of CV equal to 0.5, 1.0 and 1.5 V, and (b,1) HQ-GNW and (b,2) LQ-GNW with the upper potential limit of CV = 1.5 V.

Figure 3 (a) CV curves of LQ-GNWs measured at 50 mV s-1 in 1 M TEABF4/PC with the lower potential limit of CV equal to -2.0, -2.2, and -2.4 V and (b) CV curves of (1) HQ-GNW, and (2) LQ-GNW.


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Figure 4 The CV curves of NGNW/Ti measured at 50 mV s-1 in 1 M TEABF4/PC with (a) the lower potential limit of CV equal to -2.0, -2.2, -2.3 and -2.4 V; and (b) the upper potential limit of CV equal to 0.5, 1.0, and 1.5 V.


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Figure 5 (a) The SEM cross-section images of NGNW/Ti with a vertical structure equal to ca. 50 µm; (b) a typical TEM image of NGNW with thickness < 5 atomic layers; (c) the XPS element survey spectra of NGNW (surface: as-prepared sample and the sample with 5-min plasma etching); (d) Raman spectra of LQ-GNW and NGNW; (e) EELS spectra of NGNW and graphite. Quantification of the near-edge structure indicates that the NGNW provides 93% sp2 bonding (further details of the calculated data can be found in Table S1).


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Figure 6 (a) The CV curves of an asymmetric EDLC consisting of a positive GNW/Ti electrode and a negative NGNW/Ti electrode in 1 M TEABF4/PC with cell voltage of 2.5, 3.0, 3.5, and 4.0 V at 50 mV s-1. (b) The constant-current charge-discharge curves of the above asymmetric EDLC in 1 M TEABF4/PC with a cell voltage of 2.5, 3, 3.5 and 4 V at 2.0 A g-1. (c) The constant-current charge-discharge curves of the above asymmetric EDLC in 1 M TEABF4/PC with a cell voltage of 4 V at 0.5, 1, 2, 3, 4 and 5 A g-1. (d) The cell capacitance retention and coloumbic efficiency against the charge-discharge current density for the asymmetric EDLCs prepared in this work, and (e) a 9-LED lamp boosted by a single pouched cell.


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Figure 7 (a) A typical 1000-cycle charge-discharge test between 3 and 4 V for an asymmetric EDLC consisting of a positive GNW/Ti electrode and a negative NGNW/Ti electrode in 1 M TEABF4/PC with cell voltage of 4 V at 2 A g-1. (b) The 1st, 1000th, 2000th, 3000th, 4000th, 5000th, 6000th, 7000th, 8000th, 9000th and 10000th charge discharge curves of the asymmetric EDLC in (a). (c) The cell capacitance retention and coloumbic efficiency against the 1st to 10000th cycles for the asymmetric EDLC in (a); inset in (c) shows a Swagelok cell for this stability test.


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Table of Contents Yu-Wen Chi, Chi-Chang Hu*, Hsiao-Hsuan Shen, and Kun-Ping Huang A New Approach for High-Voltage Electrical Double-Layer Capacitors Using Vertical Graphene Nanowalls with and without Nitrogen Doping

GNW/Ti and NGNW/Ti display an open microstructure, favoring the penetration of electrolytes and electron transport in the whole graphene matrix. The oxygen-free, binder-free GNWs, inert to the irreversible oxidation of organic electrolytes, enlarge the upper limit of working potential window to 1.5 V (vs. Ag/AgNO3). The uniform N doping on the binder-free, vertical NGNWs inhibits the irreversible reduction of residual water and organic electrolyte at the negative potential end of ca. -2.5 V in 1 M TEABF4/propylene carbonate, leading to a newly designed 4V asymmetric EDLC.


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New Approach for High-Voltage Electrical Double-Layer Capacitors

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