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Nickel nanofoam/different phases of ordered mesoporous carbon composite electrodes for superior capacitive energy storage Kangsuk Lee, Haeni Song, Kwang-Hoon Lee, Soo-Hyung Choi, Jong Hyun Jang, Kookheon Char, and Jeong Gon Son ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06611 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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ACS Applied Materials & Interfaces

Nickel nanofoam/different phases of ordered mesoporous carbon composite electrodes for superior capacitive energy storage Kangsuk Lee,†,‡ Haeni Song,‡ Kwang Hoon Lee,† Soo Hyung Choi,§ Jong Hyun Jang, ¶ Kookheon Char‡* and Jeong Gon Son†*

†

Photo-Electronic Hybrid Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea ‡

School of Chemical and Biological Engineering, The National Creative Research Initiative

Center for Intelligent Hybrids, Seoul National University, Seoul 151-744, Republic of Korea §

Department of Chemical Engineering, Hongik University, Seoul, 121-791, Republic of Korea,

¶

Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

KEYWORDS: Ordered mesoporous carbon, block copolymer self-assembly, nickel nanofoam, supercapacitor, electric double-layer capacitance

ACS Paragon Plus Environment

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ABSTRACT

Electrochemical energy storage devices based on electric double layer capacitors (EDLCs) have received considerable attention due to their high power density and potential for obtaining improved energy density in comparison to the lithium ion battery. Ordered mesoporous carbon (OMC) is a promising candidate for use as an EDLC electrode because it has a high specific surface area (SSA), providing a wider charge storage space and size-controllable mesopore structure with a long-range order, suppling highly accessibility to the electrolyte ions. However, OMCs fabricated using conventional methods have several drawbacks including low electronic conductivity and long ionic diffusion paths in mesopores. We used nickel nanofoam, which has a relatively small pore (sub-100 nm to sub-µm) network structure, as a current collector. This provides a significantly shortened electronic/ionic current paths and plentiful surface area, enabling stable and close attachment of OMCs without the use of binders. Thus, we present hierarchical binder-free electrode structures based on OMC/Ni nanofoams. These structures give rise to enhanced specific capacitance and a superior rate capability. We also investigated the mesopore structural effect of OMCs on electrolyte transport by comparing the capacitive performances of collapsed lamellar, cylindrical and spherical mesopore electrodes. The highly ordered and straightly aligned cylindrical OMCs exhibited the highest specific capacitance and the best rate capability.


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INTRODUCTION Supercapacitors based on EDLCs are electrochemical devices which store and release electrical energy by nanoscopic charge separation on the interface between the electrode and the electrolyte.1–7 EDLCs are considered to be one of the most promising energy storage and power output devices as they have a high power density, lower pollutant emissions, high charging/discharging rates and long life-cycle stability.8–11 To obtain this superior performance, the electrodes should have high specific surface area (SSA) together with low ionic and electronic resistances, which requires the optimization of both porous active materials and electrode structures. Many research groups have recently presented studies regarding the fabrication of electrodes with the desired properties. These include investigations into highly ordered mesostructured electroactive materials, where the surface area and ionic accessibility are enhanced,12–14 binder-free electrodes for improved electronic conductivity.15,16 Recently, studies on EDLCs with 3D architectures are also presented for the ion and electron transport based on higher salt adsorption capacity.17–21 As a promising EDLC active material, ordered mesoporous carbons (OMCs) based on selfassembly process have been intensively researched, due to their high specific surface area over 800 m2/g and size-controllable mesopores (1.5 – 40 nm) with long-range order.22–24 Soft template-like amphiphilic block copolymers (BCPs), or organic surfactants with carbon precursors fabricated via solvent evaporation induced self-assembly (EISA) processes25–28, can form highly ordered nano-structures over the whole sample. The structure of OMCs can be controlled easily as the phase-separated morphologies vary depending on the ratio of the carbon precursors to the structure directing agents.25,29 Mesoporous structures with functional properties have been used widely as catalytic and adsorbent materials.30–32 Several research groups have


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shown that OMCs or metal-oxide-containing OMCs can be used as electrodes in EDLCs or pseudo-capacitive supercapacitors.33–35 And the OMC with various pore structures36–38 and nanocomposite electrodes with graphene or CNTs39–44, for superior capacitive performance were also recently reported. However, the intrinsically low electronic conductivity and long electronic current path of the OMCs and the existence of binders for enhancing adhesion between active materials and metal plate-shape current collector inevitably produce high internal resistance in the electrodes. In addition, relatively long ionic current path from the several micrometers of grinded OMC particles45,46 increase equivalent series resistance (ESR). This disturbs the charge storage/release process and eventually weakens the capacitive performance. Due to its high electronic conductivity and sub-mm sized 3D-porous framework structure, metal foam, such as conventional nickel foam, can be used as a current collector in OMC-based capacitive electrodes.47,48 The conventional nickel foam provides a highly conductive path to the middle of the sample but its pores (sub-mm) are too sparse and large relative to the scale of grinded OMC microparticles (several µm). Therefore, the conventional nickel foam can only act as a physical framework for supporting electroactive materials, rather than improving the long electronic and ionic path issues. Thus, binders and conductive additives are still required. Recently, Lou et al. developed a one-pot approach to synthesize scalable nickel nanofoam (NiNF) which has a relatively small pore (sub-100 nm to sub-µm) network structure.49,50 Because precisely positioned NiNF structures can significantly shorten the current and ionic paths of the electrodes, and have a plentiful surface area, enabling stable and close attachment of the electroactive materials without any binders, the NiNF may be suitable for use as a current collector in EDLC-based supercapacitors.


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In this study, we fabricated various ordered phases of OMC electrodes within synthesized NiNF with the aim of creating a current collector for use in EDLC supercapacitors. The sub-µm porous network structure of conductive NiNF can be perfectly fitted to the rarely conductive and highly porous OMCs. As the OMCs provide a wider charge storage area and nano-sized channels for better ionic transport, the NiNF successfully supports the OMCs without any binders and effectively lowers the internal charge transfer resistance due to the short current path in OMCs and adhesion between the OMCs and the NiNF. The cylindrical pore structures of OMC electrodes with NiNF exhibited a high specific capacitance of 259 F g-1 at current density 1 A g-1. They also retained their capacitance well (212 F g-1) at very high current densities of 200 A g-1 through an ideal charge storage/release mechanism. We also investigated the mesopore structural effect of OMCs on electrolyte transport and capacitive performance under very low ESR condition and compared macroporous collapsed lamellar structures and closed spherical mesopore structures to opened cylindrical mesopore structures. The cylindrical OMCs, which have a high surface area and open mesopores, exhibited the best capacitive performance and better ionic transport in comparison to the lower SSA lamellae and closed mesopores of the spherical structures.

EXPERIMENTAL SECTION

Materials. Nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98%), tetrahydrofuran (THF, 99.9%), 1-methyl-2-pyrolidinone (NMP, > 99.9%) and polyvinylidene fluoride (PVDF, Mw ~ 534 kg mol-1) were purchased from Sigma Aldrich. Glycerol (> 99%) was obtained from Alfa Aesar. Polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer was obtained


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from Polymer Source. The molecular weight was 25.5 kg mol-1 (18–7.5k) with a polydispersity index of 1.05. Phenol (≥ 99%, Junsei Chemical), chloroform (≥ 99%, Junsei Chemical), aqueous formaldehyde solution (36–38%, Kanto Chemical), sodium hydroxide (NaOH, 98%, Samchun Chemical) and aqueous hydrochloric acid (HCl, 1 M, Fluka) solution were used without any purification measures being taken. Synthesis of Carbon Precursor. The OMC was prepared using the soft-template method with PS-b-PEO block copolymer as a structure-directing material and phenol-formaldehyde (PF) resole as a carbon precursor. The PF resole was synthesized from phenol and formaldehyde using a base catalyst as previously reported25,51 and 1.3 g of 20 wt% NaOH (aqueous, 6.50 mmol) was added drop by drop to 6.1 g (65.0 mmol) of liquid phenol at 40°C. Then, 10.5 g of 37% aqueous formaldehyde solution (130 mmol) was also added drop by drop into the prepared solution and the temperature was raised to 70°C. After 1 h, we added 1 M HCl to make the solution neutral. Then it was dried at 40°C in a vacuum and the NaCl was extracted. To remove the NaCl precipitate, ethanol was added to the mixture, filtered by a Büchner funnel and dried in a vacuum oven. Preparation of nickel nanofoam. Nickel nanofoam was synthesized using previously reported methods.49 In a typical synthesis of nickel nanofoam, 2.50 g of nickel acetate tetrahydrate was added to 100 mL of glycerol and the solution was refluxed at 280°C for 40 minutes in a nitrogen ambient atmosphere. The nickel nanofoam immersed in the solution was then collected by magnet, rinsed by deionized water and dried. To make a coin-shaped current collector, 150 mg of nickel nanofoam powder prepared in the manner described was gently pressed into a coin shape by a carver press. The porosity of the nickel coin was calculated using the following formula:


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porosity = 1 −

/ =1− ℎ

where is bulk density of Ni, is the mass, is the radius and ℎ is the thickness of the nickel coin, respectively. Fabrication of OMC/Ni nanofoam electrode. To manufacture OMC film with a cylindrical morphology (denoted as OMC-C), 225 mg of PF resole and 75 mg of PS-b-PEO diblock copolymer mixture was dissolved in a solvent containing 300 mg of THF and 300 mg of chloroform for EISA process. A 40 mg of the solution was casted onto the Ni nanofoam current collector and the solvent was evaporated for 3 h at 50°C using a hot plate. It was then heated in a tube furnace at 120°C for 24 h to thermoset the PF resole. OMC-C/NiNF was finally obtained after calcination at 450°C for 3 h and carbonization at 800°C for 3 h in a nitrogen atmosphere. The temperature was raised by 1°C·min-1, 2°C·min-1 and 5°C·min-1 from room temperature to 450°C, 450°C to 600°C, and 600°C to 800°C, respectively. OMC/NiNF with lamellar and spherical morphologies (denoted as OMC-L/NiNF and OMC-S/NiNF, respectively) were obtained using the same procedure but different PF resole:PS-b-PEO ratios. 150 mg of PF resole and 150 mg of PS-b-PEO were used to make OMC-L, and 255 mg of PF resole and 45 mg of PSb-PEO were used to make OMC-S. A reference binder-containing electrode was prepared by mixing 90 wt% crushed OMC, 5 wt% PVDF and 5 wt% carbon black. Conventional Ni foam, pressed by a carver press, was used as the current collector of the reference electrode. Structural characterizations. Small angle X-ray scattering (SAXS) measurements were made along the 3C beamline of a Pohang Light Source to obtain information about the long-range morphologies of freestanding OMC films. The surface morphologies of freestanding OMC films were observed using field emission scanning electron microscopy (FE-SEM, JEOL JSM-6701F) at 10 kV and high-resolution transmission electron microscopy (HR-TEM, FEI Cryo Tecnai F20


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G2) at 200 kV. For the TEM observations, freestanding OMC films were grounded in powder using a mortar and a tip sonicator. The dispersed OMC was then dropped onto a carbon-coated copper grid. Nitrogen sorption isotherms were investigated at 77 K using a BELSORP-mini II (BEL Japan) instrument. The freestanding OMC films were smashed into small particles and dried at 150 °C in a vacuum for 5 h as a pretreatment process. From the nitrogen sorption results the Brauner-Emmett-Teller (BET) surface area (ABET) and pore size distribution were calculated using the Barrett-Joyner-Halenda (BJH) method. Electrochemical measurements. The electrochemical performance was measured using a potentiostat/galvanostat (PGSTAT-128N, Metrohm Autolab). Using a platinum coil as a counter electrode, an Hg/HgO reference electrode, and a 6 M KOH aqueous electrolyte solution, cyclic voltammetry (CV) and galvanostatic charging-discharging measurements were performed over a potential window of 0 to -0.8 V at various scan rates and current densities. The specific gravimetric capacitance of the electrodes derived from the discharging curves was calculated using following equation: C =

⁄

where and ⁄ are the current density and the gradient of the middle potential region of the discharging curve after the ohmic drop, respectively. A 0 V (vs. Hg/HgO) AC voltage of 50 mV amplitude was applied for electrochemical impedance spectroscopy (EIS) measurement and the frequency range was 1 MHz-100 mHz. The energy density and the power density are calculated from the following equations: 1 E = C(∆) 8 P=

$ ∆


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where E is the energy density, P is the power density and ∆t is discharging time.

RESULTS AND DISCUSSION Scheme 1 describes the overall method for fabricating a hierarchical supercapacitor electrode using OMCs and NiNF. Using an amphiphilic PS-b-PEO BCPs as a structure-directing agent, OMCs were synthesized via the EISA process. Solvent evaporation is a simple and strong factor which can induce long range lateral order.52,53 As the solvent evaporates, a vertically directed concentration gradient occurs over a large area. The concentration gradient and controlled evaporation rate supplies a highly oriented external field to the film, which induces directed microphase separation with long range order. PF resole, a carbon precursor, selectively interacted with the PEO chain of the PS-b-PEO polymer through hydrogen bonding and the PEO portion mixed with PF resole behaves as a single phase. As a result, this mixture of PS-b-PEO and PF resole forms self-assembled mesostructures like simple diblock copolymers, which decide the morphology of the OMCs after calcination. The OMCs with lamellar, cylindrical and spherical mesopore structure were easily controlled by volume fraction of the two phases, in other words, the ratio between PF resole and PS-b-PEO polymer.54,55 PF resole/BCP solution was drop-casted onto the Ni nanofoam networks. Once we have obtained controlled solvent evaporation, microphase separation and self-assembly of the PF resole/BCP film can occur within the NiNF networks. PF resoles were cured to form phenolic resin with condensation polymerization at 120°C for 24 hours. After the calcination process, which took place in a nitrogen atmosphere at 450°C and removed the BCP template, the phenolic resin was carbonized at 800°C. This process enabled us to create OMC/NiNF electrodes with various pore structures within the NiNF nanowire structures.


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We analyzed structural properties such as the pore morphology and SSA before measuring the electrochemical properties, because these properties are important factors in determining the capacitive performance of the fabricated electrodes. Figure 1a shows SAXS patterns from OMCC. Clear diffraction peaks in the (10), (11), (20), (21), (31) planes were observed, which implies that the OMC-C had a well-ordered cylindrical pore structure with space group p6m. The characteristic q-value of the (10) peak was q* = 0.1669 nm-1 and the corresponding domain spacing was d = 2π/q* = 37.6 nm. The observed peaks indicated that the PF resoles and BCPs formed a highly ordered self-assembled structure even after the carbonization process. The highly ordered and straightly aligned cylindrical pore structure of the OMC-C was visually confirmed in both the in-plain and out-of-plain directions with the aid of SEM images (Figure 1b and inset) and a TEM image (Figure 1c). The average pore diameter was 25 nm and the cell parameter was 38.2 nm, which agrees with the domain spacing value calculated from the SAXS results. The highly ordered and straightly aligned cylindrical mesopores of OMC-C can act as a highway for ionic transport. This is because the pore size is large enough for the ions, and the straight cylindrical pore easily and quickly transports ions from the electrolytes into the pore without any interference.56 The nitrogen sorption isotherm of OMC-C (Figure 1d) is a type-IV curve with a hysteresis loop. Capillary condensation occurs at very high relative pressures (p/p0 = 0.95), which corresponds to a large pore diameter of 25 nm. The pore size distribution (Figure 1d, inset) was derived using the BJH calculation method. This distribution confirms the coexistence of both mesopores (24.4–32.3 nm diameter) and micropores (3.5–4.5 nm) in the hierarchical pore structure. The micropores were formed in the PEO domains of the PS-b-PEO templates after the calcination step and play the role of microchannels that connect mesopores to each other. Hence,


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they also expand the internal surface area. The OMC-C showed a sufficiently high BET surface area of 814.5 m2·g-1 to be used as an EDLC electrode. To enable the NiNF to be used as a current collector in an OMC electrode, the NiNF structures were prepared by synthesis of sub-100nm scale necklace structure of nickel nanowires from the nickel acetate precursor and slight compression into coin shape by carver press (Figure 2a, inset). After the pressing, the sub-µm 3D nano-network structure was successfully formed and sufficiently provided internal space for the OMC, as shown in the SEM image of Figure 2a. The porosity of the NiNF coin was calculated to be approximately 80%. Figure 2b and 2c are SEM images of the OMC-C/NiNF cell at two different magnifications. The sub-µm Ni nanonecklaces densely propagated through the OMCs like capillary vessels (schematically described in Figure S1 in supporting information) which hierarchical structures effectively reduced electronic current path of OMCs. In addition, the NiNF provided much larger contact surface with OMCs compared with Ni foam, thus larger contact area with nanoscopically curved texture of the NiNF/OMC can maintain stable structures without any binders, while the Ni foam/OMC system needed sufficient amount of binders for maintaining stable electrode structure. This close contact can serve as an effective electron pathway with low OMC/NiNF contact resistance. The hierarchical OMC/NiNF structures also had shortened average mesopore lengths to sub-µm range, facilitating ionic transport. We measured the properties of EDLCs with these hierarchically structured OMC/NiNF electrodes using a typical three-electrode system in a 6M KOH electrolyte. Figure 3a shows the CV curves of the OMC-C/NiNF electrode with a potential window of 0 to -0.8 V versus a Hg/HgO reference electrode with a scan rate ranging from 10 mV·s-1 to 300 mV·s-1. While only 150 mg of NiNF current collector showed negligible capacitance (Figure S2), the OMC/NiNF


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electrodes exhibited rectangular shape of CV curves and maintained at all scan rate, which indicates that the electrode had stable EDLC performance even at high scan rates. The galvanostatic charging-discharging curve (Figure 3b) has a symmetric triangular shape, which implies stable electrochemical behaviors during the charging and discharging processes. No ohmic drop occurred at any current density, indicating that the electrode had a very low equivalent series resistance (ESR). To compare the capacitive performance of our electrodes to those prepared by conventional methods, we also prepared binder-containing OMC electrodes with conventional Ni foam (bOMC-C/Ni) and on stainless steel (bOMC-C/SS). The bOMC-C/Ni and bOMC-C/SS were made by mixing crushed freestanding bulk OMC-C film, PVDF binder and carbon black, and then smearing it onto the commercial Ni foam or stainless steel plate. The morphology of bOMC/Ni with binder can be seen in Figure S3a. The bOMC-C/Ni electrode also had rectangular CV curves (Figure S3b) but its shape gradually deformed as the scan rate increased. The charging-discharging curves of the OMC-C/NiNF and the bOMC-C/Ni at a current density of 2 A g-1, as shown in Figure S3d, indicates that the bOMC-C/Ni electrode had a lower capacitive performance and a small ohmic drop, caused by electrical energy loss. Figure 3c shows the gravimetric and volumetric capacitances of OMC-C/NiNF and bOMC-C/Ni and bOMC-C/SS, derived from the discharging curve (Figure 3b and S3b). OMC-C/NiNF exhibited high capacitance, with values of 259 F·g-1 at 1 A·g-1 and high rate capability of 89 % at 10 A·g-1 and 82 % even at 200 A·g-1, whereas bOMC-C/Ni had a capacitance of 139 F·g-1 at 1 A·g-1 and relatively low rate capability of 65 % at 10 A·g-1. The bOMC-C/SS showed 146 F·g-1 at 1 A·g-1 and the rate capability of 40 % at 10 A·g-1. Even compared to other studies based on OMC-based EDLC electrodes,26,57 the OMC-C/NiNF electrode capacitor exhibited superior capacitance and


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rate capability. The result of ref. (26) showed specific capacitance of 136 F·g-1 at 0.5 A·g-1 and low rate capability of 63 % at 5 A·g-1. Similarly, ref. (57) exhibited relatively higher capacitance of 205 F·g-1 at 1 mV·s-1, but its rate capability was 64 % at 20 mV·s-1. OMC-C/NiNF also showed superior capacitance compared to other recent studies which show high capacitive performance using various carbon electroactive materials, such as activated carbons (196 F·g-1 at 0.2 A·g-1, 88% rate capability),58 reduced graphene oxides (237 F·g-1 at 1 A·g-1, 72% rate capability)59 and reduced graphene oxide/super-short carbon nanotubes (244 F·g-1 at 50 mV·s-1, 85% rate capability).60 This capacitive performance difference from the same specific surface area of OMCs implies that the charge transfer mechanism was more efficient when the NiNF current collector and binder-free fabrication process were used. While the bOMC-C/Ni could not fully use porous structures of OMC for the capacitance even at 1 A·g-1, the OMC-C/NiNF electrode fully elicit the maximized capacitance from SSA of OMCs. It mainly originated from the kinetic issues such as short electronic current paths of capillary vessel structures, low contact resistance between OMC and NiNF and short ion transfer distance from the sub-µm OMC/NiNF hierarchical structures. To investigate the cause of superior capacitive performance, EIS analysis was also executed. Figure 3d shows that the gradient of the OMC-C/NiNF is much steeper than that of bOMC-C/Ni in the low frequency region. Because an ideal capacitor has a fully vertical EIS curve, the steeper slope measured for the OMC/NiNF based capacitor indicates that the charge transfer mechanism is close to that of ideal capacitors. The intercept value of the real impedance value corresponds to the summation of bulk electrolyte resistance (Rbulk) and electrical resistance of the electrode (Relectrode). Because we controlled the gap between the working and reference and resistivity of bulk electrolyte, the difference of intercept values of the real impedance between OMC-C/NiNF


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and bOMC-C/Ni can be originated from the difference of Relectrode of OMC-C/NiNF and bOMCC/Ni. The measured intercept value of OMC-C/NiNF was 343 mΩ, which was much smaller than 791 mΩ of bOMC-C/Ni. This difference indicates that the large contact area between OMC and NiNF that stems from 3D porous nanowire network structure in OMC-C/NiNF considerably reduced Relectrode. The ion accessibility in the pore structures can be quantified as pore resistance (Rp), which can be obtained as the value of the three times of the real impedance value as the gradient varies.61 The value of Rp in the case of OMC-C/NiNF-based electrodes was measured to be 159 mΩ, which was much smaller than that of bOMC-C/Ni, which was 1.16 Ω. This implies that the shortened mesopore lengths to sub-µm range in the leaf-vein-like hierarchical structure of the OMC-C/NiNF helps electrolyte ions to access the carbon surface of the pores more easily. Additionally, the structural stability arising from the increased contact-area between mesoporous carbon and NiNF gave rise to long-term stability of the capacitive performance. The specific capacitance retained almost 100% of its initial value after 4,800 continuous cycles, as shown in Figure 3e. Ragone plot, which shows the relation between the energy density and the power density, is an effective indicator of the capacitive performance of the supercapacitors. Figure 3f exhibits the Ragone plots of OMC-C/NiNF and a few examples of the carbon supercapacitor systems with aqueous electrolytes reported in other literatures. OMC-C/NiNF showed a maximal energy density of 5.72 Wh·kg-1 at a power density of 106 W·kg-1 for a 0.8 V potential window and maximal power density of 11.85 kW·kg-1 with 1.32 Wh·kg-1 of energy density. Importantly, high power density of 4.99 kW·kg-1 was obtained at a reasonable energy density of 3.60 Wh·kg-1. These values are higher than the other values reported by other literatures that show capacitive performances of the ordered mesoporous carbon-based aqueous supercapacitor systems.42,44,62


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To verify how the mesopore structure affects the capacitive performance, OMCs that have other structures, such as lamellar and spherical pore structures, were tested by adjusting the component ratio between PF resole and PS-b-PEO BCP. Under the same EISA condition, the lamellar pore structure appeared at a much lower ratio of PF resole (OMC-L, 150:150, w/w ratio of PF resole:BCP) than the cylinder (225:75) and the spherical structure was obtained at a much higher component ratio (OMC-S, 255:45). Figure 4a shows the collapsed lamellar structure of OMC-L, which is like stacked paper. The domains in the lamellar structure were not selfsupported and it was difficult to maintain their periodic structures during the calcination process. Therefore, these collapsed lamellar structures were expected to have a lower SSA. The spherical mesopore structure of OMC-S was investigated using SEM, as shown in Figure 4b and confirmed by the SAXS experiment shown in Figure S4. The SEM image shows the spherical mesopores, which had a diameter of approximately 20 nm. The SAXS pattern exhibited many peaks corresponding to the (110), (200), (211), (220), (310), (222), (321) planes. These high order peaks indicate that the structure of OMC-S is body-centered cubic with space group Im3'm, which has a characteristic q-value of q* = 0.1855 nm-1 in the (110) plane and a domain spacing of d = 33.9 nm. Unlike the cylindrical structure of OMC-C, the spherical mesopores of OMC-S are not interconnected, so the structure prevents the access of electrolyte ions. Figures 4c and 4d show the nitrogen sorption isotherms of OMC-L and OMC-S. Even though both of them are type-IV adsorption curves, the hysteresis loop barely appears in the isotherm curve of OMC-L. The nitrogen sorption isotherm of OMC-L is similar to that of a nonporous material. This is because the pores in OMC-L are too large, about a few micrometers in diameter, and each surface in the lamellar pore is regarded as a plane. The pore size distribution in the inset of Figure 4c, shows a broad pore distribution ranging from sub-nm to sub-100 nm, indicating


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that the periodic mesopore structures are massively collapsed during the carbonization process forming stacked paper-like structure. On the other hand, the OMC-S curve exhibits an apparent hysteresis and open loop where the desorption process was not terminated even at low relative pressures. This indicates the existence of ink-bottle pores which are described at pore I in Figure S5 and have wider dimension in the interior than that at the exit such that the adsorbates which fill the interior space cannot easily come out through the exit channel. The pore size distribution of OMC-S (Figure 4d, inset) comprised mesopores with diameter of 19.9–32.3 nm and micropores with diameter 3.7–4.5 nm. These micropores may form channels between the spherical mesopores. The BET surface areas were measured as 479.62 m2·g-1 for OMC-L and 856.76 m2·g-1 for OMC-S. In terms of the only SSA, the OMC-S can be the best capacitive mesostructures for supercapacitor electrodes among the different structured OMCs and the OMC-L would be the worst structure for electroactive materials. The EDLC characteristics of the three different types of electrodes, including CV relationships and galvanostatic charging-discharging properties, were measured. The CV curves of OMCL/NiNF and OMC-S/NiNF at various scan rates are shown in Figure S6 and the CV curves of the three different electrodes at a scan rate of 50 mV·s-1 are shown in Figure 5a. The lamellar and spherically structured electrodes exhibited the rectangular curves typical of EDLCs but the OMC-C/NiNF had the biggest area underneath the curve. This result disagrees with our results concerning the BET surface areas of the OMCs. The capacitances of OMCs were calculated from the discharging curves shown in Figure 5b. The capacitance of OMC-S/NiNF was the highest value of 262 F·g-1, followed by that of OMC-C/NiNF (259 F·g-1) and OMC-L/NiNF (199 F·g-1) for a low current density of 1 A·g-1. However, the capacitance of OMC-S/NiNF decreased significantly as the current density increased, being 198 F·g-1 when the current density


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was 20 A·g-1, whereas that of OMC-C/NiNF and OMC-L/NiNF decreased less, going to 224 F·g1

and 163 F·g-1, respectively. The low rate capability of OMC-S/NiNF is related to its ionic transport behavior and surface

accessibility, which are determined by the pore structure. The OMC-C/NiNF has highly ordered and straightly aligned cylindrical open mesopores which play the role of highways for ionic transport even in very rapid charging/discharging conditions. As a result, the OMC-C/NiNF electrode exhibits much better ionic accessibility and maintains high capacitance at high current densities. The plane-shaped pores in OMC-L/NiNF based electrodes also exhibited good rate capability behavior at high current densities. In contrast, the OMC-S/NiNF has individually isolated spherical mesopores connected by 2–4 nm diameter microchannels. Since most of the mesopores can be regarded as mesoscopically closed pores, ions in the electrolyte must pass through the microchannels to access the internal spherical mesopores. Therefore, the capacitance drastically decreases at high current densities, even though these structures have a larger specific surface area. Our sphere and cylindrical OMC structure also can be simplified as schematics in Figure S5. The sphere OMC can be described as a series of larger cubic pores and narrower/shorter channels, while the cylindrical OMC can be described as a larger/long cylinder pore connected narrower channels. Compared two porous structures, the degree of constriction factor of sphere OMCs are much higher than that of cylindrical OMCs. Therefore, much large pore resistance and low rate capability can be exhibited at the spherical OMC electrodes. We also investigated the electrochemical behaviors of the three different mesoscopic structures using EIS measurements. Because the gap between the OMC/NiNF and the Hg/HgO reference cell is controlled constantly, the ESR values of three different OMC/NiNF cells showed similar values around 0.32 Ω with only minor differences. We focused on the pore resistance (Rp) values of the


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electrolytes in the pore structures (Figure 5c). While the low-frequency gradients of the three different electrodes in the Nyquist plot were similar, the Rp value of OMC-S/NiNF (~257 mΩ) was much higher than the Rp values of OMC-L/NiNF (~130 mΩ) and OMC-C/NiNF (~159 mΩ). OMC-L/NiNF and OMC-C/NiNF had a relatively small Rp, which indicates that the pore structure hinders the ionic transport less. In the case of OMC-S/NiNF, which had a large Rp, ionic access was hindered by the closed pore structure. These pore resistance tendencies are intimately related to the rate capabilities of each OMC/NiNF electrode. The Fe(CN)63-/4- redox experiment with various OMC structures in Figure S7 also well matched with our rate capability and ESI experiment. Based on these mesoscopic structural analyses, we conclude that highly ordered and straightly aligned continuous mesopores are required to facilitate rapid ionic transport in pore structures during fast charging/discharging processes.

CONCLUSIONS

We presented a high-performance EDLC electrode based on OMC, using nickel nanofoam as a current collector. The electrode was fabricated using a solvent evaporation-induced selfassembly method. We used a direct carbonization method to obtain OMC structures inside the nanoscale 3D network structures of NiNF without resorting to the use of less conductive binders. Our strategy effectively shortened the electron/electrolyte transport pathways through the hierarchical structure and reduced the contact resistance between the OMCs and current collectors. Therefore, the OMC/NiNF electrode exhibited superior capacitive performance, owing to highly improved internal electrical conductivity and enhanced electrolyte ion


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accessibility. Moreover, we also fabricated mesoscopically different OMC/NiNF electrodes, including collapsed lamellar, cylindrical and spherical mesopore structures, and compared their electrochemical capacitances. Due to its high SSA and easily accessible open mesopore structure, the cylindrical mesopore structure exhibited the best performance among the three electrodes. The shortened current path and direct contact from the leaf-vein-like hierarchical structures and the ionic highways from EISA based OMCs may be ideal for use as electrodes in EDLC supercapacitors

with

high

gravimetric

and

volumetric

energy

density

properties.


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Scheme 1. Overall scheme for the fabrication process of a binder-free ordered mesoporous carbon/Ni nanofoam coin cell using block copolymer as a structure-directing agent.


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Figure 1. (a) Small angle X-ray scattering (SAXS) curve of the ordered mesoporous carbon film with a cylindrical morphology (OMC-C). (b) Scanning electron microscopy (SEM) images of the OMC-C; a side-view image of the cylindrical pattern (inset). (c) Transmission electron microscopy (TEM) image of the OMC-C. (d) Nitrogen sorption isotherm of the OMC-C and the corresponding pore size based on the Barrett-Joyner-Halenda (BJH) method (inset).


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Figure 2. (a) SEM image of a Ni nanowire network in a Ni coin (inset shows an optical photo of a Ni coin). (b, c) SEM images of the OMC-C/NiNF at two different magnifications.


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Figure 3. (a) Cyclic voltammograms at various scan and (b) galvanostatic charging-discharging curves at different current densities of the OMC-C/ nickel nanofoam (NiNF). (c) Gravimetric and volumetric specific capacitances of the OMC-C/NiNF and the bOMC-C/Ni and bOMC-C/SS. The calculations are based on the galvanostatic charging-discharging results. (d) Nyquist plots of the OMC-C/NiNF and the bOMC-C/Ni at 0 V with an Hg/HgO reference electrode. (e) Longterm performance of OMC-C/NiNF at 5 A·g-1. The inset shows the galvanostatic charging/discharging curves for the 5th and 4,800th cycles. (f) Ragone Plots of OMC-C/NiNF that compared with performances of other ordered mesoporous carbon based supercapacitor systems.


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Figure 4. SEM images of (a) OMC film with lamellar morphology (OMC-L) and (b) OMC film with spherical morphology (OMC-S). Nitrogen sorption isotherm and pore size distribution (inset) of (c) OMC-L and (d) OMC-S.


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Figure 5. (a) Cyclic voltammograms of OMC-L/Ni, OMC-C/NiNF and OMC-S/NiNF at scan rate of 50 mV·s-1. (b) Calculated specific capacitance of various morphologies of mesoporous carbon. (c) Nyquist plots of OMC-L/NiNF, OMC-C/NiNF and OMC-S/NiNF. The inset is the Nyquist plots comparing the pore resistances from 3 different samples.


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ASSOCIATED CONTENT Supporting Information. The detailed cyclic voltammograms and galvanostatic chargingdischarging curves of bulk OMC-C with conventional Ni foam, collapsed lamellar and spherical OMC with Ni nanofoam and SAXS profiles of spherical OMCs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT This research was financially supported by the WCU(World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-2008-000-20012-0) and the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (The National Creative Research Initiative Program for “Intelligent Hybrids Research Center” 2010-0018290), the Global Frontier Research Program (2011-0032156) and Korea Institute of Science and Technology (KIST) internal project. The experimental support by the staffs at the 3C1 beamline of the Pohang Light Source is also gratefully acknowledged.


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Different Phases of Ordered ... - ACS Publications

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