Nanostructured Electrode Materials Derived from MetalâOrganic

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Nanostructured Electrode Materials Derived from Metal-Organic Framework Xerogels for High Energy Density Asymmetric Supercapacitor Asif Mahmood, Ruqiang Zou, Qingfei Wang, Wei Xia, Hassina Tabassum, Bin Qiu, and Ruo Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10725 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 1, 2016

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ACS Applied Materials & Interfaces

Nanostructured Electrode Materials Derived from Metal-Organic Framework Xerogels for High Energy Density Asymmetric Supercapacitor Asif Mahmood, Ruqiang Zou,* Qingfei Wang, Wei Xia, Hassina Tabassum, Bin Qiu, Ruo Zhao Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Department of Material Science and Engineering, College of Engineering, Peking University, Beijing 100871, China KEYWORDS: Metal-organic framework xerogel, Iron oxide, Nanoporous carbon, Aqueous asymmetric supercapacitor, High energy density.

Abstract This work successfully demonstrates metal-organic framework (MOF) derived strategy to prepare nanoporous carbon (NPC) with or without Fe3O4/Fe nanoparticles by the optimization of calcination temperature as highly active electrode materials for asymmetric supercapacitors (ASC). The nanostructured Fe3O4/Fe/C hybrid shows high specific capacitance of 600 F/g at a current density of 1 A/g and excellent capacitance retention up to 500 F/g at 8 A/g. Furthermore, hierarchically NPC with high surface area also obtained from MOF gels displays excellent electrochemical performance of 272 F/g at 2 mV/s. Considering practical applications, aqueous ASC (aASC) was also assembled, which shows high energy density of 17.496 Wh/Kg at the

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power density of 388.8 W/Kg. The high energy density and excellent capacity retention of the developed materials show great promise for the practical utilization of these energy storage devices.

1. Introduction

Supercapacitors (SCs) have attracted tremendous attention due to their high power density and long cyclic life. Therefore, a large commercial rise is expected in next decade that might take the current SCs market value to several billion dollars by 2023.1-2 Currently, carbon constitutes the heart of commercial electrode materials which stores charges by electrochemical double layer (EDL) formation with fast charge/discharge kinetics, providing high power densities (> 10000 W/Kg) but suffer from low energy densities (< 10 Wh/Kg).3 Hence it is extremely desirable to develop new electrode materials which could enhance the energy density exponentially with increased cyclic stability. In this regard, aqueous asymmetric SCs (aASCs) have gotten a lot of attention recently due to several advantages such as low cost, non-flammability, high ionic conductivity, wide voltage window (0-1.6 V), and ease of assembly under ambient conditions.4-6 The aASCs are assembled with two types of electrode materials in a single cell (the battery type electrode which undergo Faradaic redox reaction and capacitor type electrode materials).7-8 High surface area nanoporous carbon (NPC) is excellent electrode material, acting primarily as EDL capacitors, however, finding the best electrode materials which could effectively undergo Faradaic redox reaction upon cycling to maximize the energy density, has always been a great hurdle in current research society.9 Several nanomaterials have been synthesized in conquest towards designing efficient redox-active electrode materials such as RuO2, Co3O4, Fe3O4, Co(OH)2, FeOOH etc.10-14 Despite the higher performance of those metallic nanostructured


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electrodes, their cyclic performance is largely limited due to low active surface area and the increased aggregation of the nanomaterials upon cycling.9,15 A widely accepted solution to this problem is to distribute the redox-active nanomaterials onto highly conductive and elastic substrates with high surface area, like porous carbon materials.16-17 However, the preparation of the ideal composite structure is quite challenging with complex synthesis route at considerably higher costs which hinders their commercial applications.18 Hence, it is quite important to find cost effective methods to develop high performance nanostructured electrode materials in bulk quantities under industrially acceptable conditions.3,19

Recently, there is an increasing tendency to use metal-organic frameworks (MOFs) as precursors to synthesize ideal composite structures due to their ordered structure, higher surface areas, controlled porosity and inherent presence of heteroatoms.20 Traditionally MOFs could be carbonized to several products including high surface area carbon, metal oxides (MO) and metal decorated carbonaceous materials.20-21 Deriving products at higher temperatures using MOF templates is highly dependent on the coordinating metals. For instance, Zn based MOFs provide high surface area porous carbon owing to low boiling point of Zn (ca. 922 °C). However, MOFs based on other coordinated metals (Fe, Co, Cu etc.) provide electrochemically inactive metallic particles dispersed throughout the carbonaceous matrix. Presence of inactive metallic contents severely affects the application of these materials as supercapacitor electrode materials and has rarely been reported before this work.20 Moreover, under properly controlled carbonization conditions, perfect nanostructures could be obtained with ultra-small nanoparticles (NPs) uniformly dispersed in porous carbon matrix.22 Hence, the selection of carbonization temperature is critical to obtain active electrode materials which still lacks complete understanding and needs further exploration.20,23-24


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Here we utilized the MOF structure to develop the high surface area metal-organic xerogels (MOXs) using a facile solvothermal method. These xerogels take benefit from small MOF NPs aggregated by physical interactions (π-π interaction, Van der Waal’s forces and hydrogen bonding) during the gelation process.25 To develop MOF derived aASC, MOXs based on Fe and Al were used for selective tailoring of electrode materials for positive and negative electrodes, respectively. Owing to wide abundance, cheap precursors, higher conductivities and redox-active nature of Fe, high surface area xerogels (MOX-Fe) were fabricated from MIL-100 (Fe) to derive electrode materials for positive electrode of aASC. Additionally, Fe readily transform to electrochemically active Fe3O4 in basic solution which overcome the limitation of MOF derived materials for supercapacitor due to existence of inactive species.26 MOX-Fe was carbonized at several temperatures to optimize the composite structure, with Fe3O4/Fe NPs homogenously dispersed in the high surface area three dimensional (3D) hierarchical porous carbon matrix. The resulted electrode materials show excellent specific capacitance. Similarly, for negative electrode, high surface area nanoporous carbon (NPC) was obtained from MIL-100 (Al) based xerogels (MOX-Al) by a simple post treatment with NaOH after carbonization, combining the effect of activation and metal removal in the same step.27-28 Finally, aASC was constructed by using Fe3O4/Fe/C as positive electrode and NPC as negative electrode (Fe3O4/Fe/C//NPC), as shown in Scheme 1. The aASC exhibits much improved capacitance of 202.5 F/g at a current density of 0.5 A/g which is the highest reported value so far for Fe based aASC. Although MOFs have been used to derive electrode materials for supercapacitors before, the derivation of composite electrode materials from MOFs for capacitive energy storage and full device fabrication has rarely been reported prior to this work.20 Hence, the present work could be highly


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important for realization of SCs in daily life due to ease of synthesis and high performances and will open an avenue to tailor the MOF based materials for energy harvesting.

Scheme 1. Schematic representation for the derivation of MOXC-Fe as positive and NPC as negative electrode materials for configuration of aASC 2. Experimental Section 2.1. Synthesis of MOF xerogels (MOX-Fe, MOX-Al): Two different kinds of xerogels, namely MOX-Fe, MOX-Al, were synthesized using Fe(NO3)3⋅9H2O and Al(NO3)3⋅9H2O as sources of Fe and Al, respectively.25,27 For the synthesis of Fe xerogel, Fe(NO3)3⋅9H2O (15 mmol) was treated with benzene tricarboxylatic (H3BTC) (10 mmol) in ethanol. The gel was formed as soon as the solutions were mixed which were then transferred to Teflon stainless steel autoclave and heated for 24 h at 120 ᵒC for ageing. The product was then washed repeatedly to remove non-reacted ligands from the reaction mixture with ethanol for three successive times. The products of MOX-Fe were then dried at 80 ᵒC in oven under air. Similar method was used to synthesize MOX-Al using Al(NO3)3⋅9H2O (15 mmol) and H3BTC (10 mmol) in ethanol. 2.2. Derivation of Electrode Materials:


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2.2.1. The Positive Electrode Material. MOX-Fe was used as sacrificial template to obtain the redox-active positive electrode materials for aASC. In a typical experiment, MOX-Fe was carbonized at elevated temperatures under inert conditions. The carbonization temperatures were identified from thermogravimetric analysis (TGA). MOX-Fe was carbonized at several temperatures (550, 700, 800 and 900 ᵒC) under constant flow of argon (Ar) and a constant heating rate of 5 ᵒC per minute. The samples were kept for 5 h at the target temperature. 2.2.2. The Negative Electrode Material. The MOX-Al was carbonized at 1000 ᵒC (heating rate 5 ᵒC, calcination time 5 h, Ar flow) to obtain MOXC-Al. The product was then treated with 6 M NaOH to remove the metallic species to obtain high surface area NPC. In a typical experiment, the as carbonized sample was dispersed in 6 M NaOH and forced vigorous stirring for 12 h followed by continuous washing repeatedly until normal pH was achieved. The process was repeated three times successively to ensure the removal of all metallic species. 2.3. Characterization: The structural study of the products was carried out using powder X-ray diffraction (XRD) using Bruker D8 advanced diffractometer (Cu-Kα irradiation, 2ϑ = 10-70, scan rate of 4 °/min), thermo-gravimetric analysis (TGA) using (TA Instruments SDT Q600 Analyzer) up to 1000 °C at a heating rate of 10 °C /min in N2 and X-ray photoelectron spectroscopy (XPS, Kratos Analytical Ltd). The microstructure was studied using a field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM, FEI Tecnai T20). The surface area was calculated using N2 sorption test in Quantachrome autosorb-IQ gas adsorption analyzer at 77 K. All samples were evacuated at 150 °C for 5 hours under dynamic vacuum before adsorption test. Using sorption data, non linear density functional theory (NL-DFT) was


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applied to calculate the pore size distribution while pore volumes were calculated at relative pressure (P/Po) of 0.995. 2.3.1. Electrochemical testing: The electrochemical testing was carried out using Zahner Zennium universal electrochemical workstation between potential windows of -0.25 to 0.4 V using standard calomel electrode (SCE) as reference electrode in 6 M KOH. The working electrode was prepared on nickel foam using 80% active material (average weight ca. 2mg). The remaining 20% of the electrode material included binder (PTFE 10%) and active carbon (10%). NPC was also tested in similar way against SCE in the potential range of -0.7-0.1 V in 1 M KOH. The electrodes were kept in KOH solution overnight prior to analysis. The capacitance was calculated from following relationship C = I × ∆t/(∆V × m) 2.3.2. Design of aASC: The aASC were assembled in a coin type cell where the active materials were deposited on the Ni foam prior to cell formation process. The NPC was used as a negative electrode material while Fe3O4/Fe/C was used as positive electrode separated by glass fiber (GF/D) from Whatman as separator. 6 M KOH was used as the electrolyte and the aASC were tested in the voltage window of 0-1.6 V. The specific capacitance of aASC was calculated using following relationship29 C = I × ∆t/(∆V × m ) Where m is the total mass on the two electrodes. Also keeping in mind the charge balancing between anode and cathode (q+/q-), the charge on the individual electrode was found using following relationship. q = C × ∆V × m


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Where q represent charge, ∆V is the voltage window and C is the specific capacitance of the respective material. Using above relationship, the relative masses on the positive and negative electrode was calculated. The following relationship was used to calculate the relative masses on the two electrodes m /m = (C × V )/(C × V ) Where m , C and V represent the mass, capacitance and voltage window of the positive electrode when tested in three electrode system. Similarly m , C and V represent the mass, capacitance and voltage window of the negative electrode. The mass ratio of the positive to negative electrode was found to be 0.42. The capacitance of single electrode in aASC was calculated from following equation29 C = 4 × I × ∆t/(∆V × m ) The energy density of the aASC was calculated using the following relationship. E = (C × V^2)/8 (Wh/Kg) Where E is the energy density, C is capacitance of aASC and V is voltage window. Similarly, the power density of the aASC was calculated using following relationship P = E/t (W/Kg) Where P represents the power density and t is the discharge time. 3. Results and Discussion The MOXs are hierarchically porous metal organic monoliths, formed as a result of heteroaggregation of nano MOF particles (nMOFP).30 These nMOFP have the chemistry similar to MOFs at molecular level and act as subunits to make a 3D network, as shown in Scheme 1.


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These nMOFP are formed as a result of two step process, i) Strong metal ligand coordination leads to MOF clusters ii) polymerization of these clusters to form nMOFP.25,27

Figure 1. View of (a) Photographic Images of MOX-Al (i) and MOX-Fe (ii), (b) comparison of diffraction patterns of simulated (MIL-100 (Fe)31 and MIL-100 (Al)32) and developed MOXs, and (c) pore size distribution of the as prepared MOXs. Two different MOXs were developed, based on Fe and Al as shown in Figure 1a. MOXs offer several advantages including high surface areas, tunable porosities, very low densities and large


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channels. The powder XRD patterns of MOXs match with the simulated patterns of crystalline MIL-100 (Fe and Al) as shown in Figure 1b. This clearly suggests that basic building block of MOXs is MOF structure, however, considerable broadness in the powder XRD patterns arise due to the amorphous properties of nMOFP. Furthermore, the MOXs possess hierarchical porosity as shown in Scheme 1 and proved by Brunauer-Emmet-Teller (BET) measurements shown in Figure 1c. The N2 sorption isotherm of MOXs indicates that the MOX-Fe and MOX-Al have high surface area of 1643 and 1761 m2/g respectively and respective sorption isotherms are shown in Figure S1 (see Supporting Information). 3.1. Derivation of positive electrode materials from MOX-Fe The MOX-Fe was used as a sacrificial material to derive high capacity electrode materials to act as positive electrode in aASC. The pyrolysis behavior of the MOX-Fe was studied using TGA as shown in Figure 2a, which indicated two significant weight losses at 480 and 570 ᵒC, respectively. The first weight loss has been attributed to decomposition of organic ligands, while the second from 570 ᵒC arise due to the reduction of iron oxide into Fe. The final weight of the product accounts for 35% of the original weight. Hence, carbonization of xerogels at 480 ᵒC leads to products with partially decomposed frameworks while carbonization at temperatures above 570 ᵒC lead to completely decomposed frameworks into carbon. To further verify this argument and investigate the effect of carbonization temperature on nature of resulting product, MOX-Fe was carbonized at several temperatures (550, 700, 800 and 900 ᵒC, denoted as MOXC550, MOXC-700, MOXC-800 and MOXC-900 (in general, MOXC-Fe)), respectively. The nature of the resulting products was examined by using XRD analysis and the representative diffraction patterns are shown in Figure 2b. At relatively lower carbonization temperature (550 ᵒC), significant oxygen is still present in the product, bonded to Fe in the form of Fe3O4


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(magnetite syn, JCDPS No. 88-0315) while the peak broadness suggests the presence of large amorphous contents. Increasing carbonization temperature leads to complete decomposition of framework and partial reduction of Fe3O4 into Fe (Fe JCDPS No. 89-7194). Since the carbonization temperature is not very high (700 ᵒC), a considerable quantity of Fe3O4 is still present in the product. A peak around 26° depicts the reflections from graphitic planes of carbon. Hence, the product obtained at 700 ᵒC consists of Fe3O4/Fe distributed throughout the porous carbon. Furthermore, an increase in carbonization temperature (800 and 900 ᵒC) leads to further reduction of Fe3O4 to FeO along with conversion to Fe3C as identified by powder XRD analysis. XRD analyses were further extended to calculate the average crystallite sizes of Fe and Fe3O4. An average crystallite size of ca.34 nm and 18 nm was found for Fe and Fe3O4 for MOXC-700, respectively. Crystallite sizes of Fe and Fe3O4 from MOXC-500, MOXC-800 and MOXC-900 were also calculated and found in the range of ca. 30 nm. In addition, large crystals (30-35 nm) of Fe3C were also observed in MOXC-800 and MOXC-900 which show increased reduction of Fe to Fe3C upon calcination at further higher temperatures. The morphological aspects of carbonized products were studied using FESEM (Figure 2c and Figure S3, see Supporting Information). Large particles (ca. 200 nm) covered with thick carbon shell were observed in the carbon matrix at lower carbonization temperature (550 ᵒC) which confirm the incomplete decomposition of framework at this temperature (Figure S3a, see Supporting Information). Since carbon plays very active role in reducing the metal oxides at higher temperatures, carbon reduces Fe3O4 to Fe which causes the oxidation of outer carbon shell.21 Thus, at higher carbonization temperature of 700 ᵒC, much smaller NPs were observed that are homogenously dispersed over hierarchically porous carbon. A further increase in temperature to 800 and 900 ᵒC leads to increase in particle size due to conversion of surface Fe


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Figure 2. (a) TGA curve of MOX-Fe under N2 atmosphere. (b) Powder XRD patterns of carbonized MOX-Fe under different calcining temperatures. (c) FESEM analysis of MOXC-700. (d) TEM analysis of MOXC-700 (inset represent (i) the HRTEM analysis (scale 5nm) and (ii) SAED diffraction patterns). (e) N2 sorption isotherms of various carbonized MOX-Fe samples. (f) Deconvolution of Fe from MOXC-700.


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into iron carbide as proved by powder XRD (Figure 1b) and FESEM (Figure S3, see Supporting Information ). TEM analyses were carried to obtain more insight into the product. From Figure 2d (i), it can be clearly observed that the particles of Fe3O4/Fe in MOXC-700 have an average size of 50 nm and are covered with thin layer of carbon (Figure 2d (ii)) which is highly favorable to protect surface and structural changes of NPs in the electrode. The selected area electron diffraction (SAED) (Figure 2d (ii)) further shows that the product is polycrystalline and match with the standard cards of Fe and Fe3O4. It is quite clear that decomposition of MOX-Fe leads to drastic decrease in surface area (from 1700 to 200 m2/g) due to the formation of heavy metallic particles as represented by N2 adsorption isotherms in Figure 2e. However, once the structure is decomposed, no significant changes in surface area are observed upon increase in carbonization temperatures as shown in Table S1 (see Supporting Information). Furthermore, XPS was used to investigate the composition and oxidation states of the carbonized products, and the results show the existence of core levels of Fe, C, O and N confirming the presence of all species in the product (Figure S4, see Supporting Information). The deconvolution of Fe from MOXC-700 clearly indicate the presence of mixed oxidation states (Fe2+ and Fe3+) along with metallic Fe in the product as shown in Figure 2f. It is also believed that the presence of heteroatoms in the carbonaceous matrix could largely improve the conductivity of the products along with overall electrochemical properties by disturbing the density of state (DOS). The energy dispersive x-ray spectroscopy (EDS) analyses were used to determine the concentration of heteroatoms in the product as shown in Figure S5 (Supporting Information). The product was found to contain nitrogen species derived from the metal nitrates precursor accounting for 3.97 atm% of the product. The nitrogen deconvolution reveals two different forms (pyrrolic and pyridinic) in the carbon as shown in the


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inset of the Figure 2f. This pyrrolic and pyridinic nitrogen constitute 82% and 18%, respectively. The existence of heteroatoms is also confirmed from the deconvolution of carbon component as considerable quantity of carbon is bonded to nitrogen and oxygen (Figure S4b-c, see Supporting Information). The electrochemical properties of the MOXC-Fe were investigated in 6M KOH using a standard three-electrode system (vs. SCE). Figure 3a represents the comparison of electrochemical performance (calculated after initial activation during cycling) of several carbonized samples, in which MOXC-700 exhibits the highest capacitance. Despite the fact that MOXC-550 consists of Fe3O4/C as proved by powder XRD analysis (Figure S2a, Supporting Information), it exhibits poor electrochemical performance due to limited charge conductivity of the carbon matrix and poor diffusion of electrolyte ions through thick carbon shell which is mainly due to partial decomposition of the organic ligand (Figure S3a, see Supporting Information). Similarly, at higher carbonization temperatures (MOXC-800 and MOXC-900), the Fe NPs are converted into Fe3C. The presence of Fe3C reduces the charge storage process of the SCs due to its poor redox activity. Conversely, MOXC-700 consists of both Fe3O4/Fe NPs covered with a thin carbon shell, dispersed throughout the hierarchically porous carbon making MOXC-700 ideal for high capacitive performance. Furthermore, Fe is readily oxidized into Fe3O4 upon exposure to strong alkaline solution.26 To investigate, the MOXC-700 was dispersed in 6 M KOH, and the powder XRD experiment was performed to characterize the product (denoted as MOXC-700T) (Figure 3b). It is quite clear that Fe is converted into Fe3O4 upon exposure to electrolyte solution resulting in higher performance. Deconvolution of O1s from MOXC-700T (Figure S4b) clearly shows large concentration of oxygen moieties bonded to metal in the derived electrode material.


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It further elaborates the use of MOXC-700 directly for electrochemical application by exposing the electrode into KOH solution prior to analysis. The porosity of the electrode material is another very important factor for efficient electrochemical performance. As shown in Figure 3c, all the products consist of hierarchical porosity with the PSD lying in the range of 1-15 nm. Developing hierarchical porous structure is very important for mass transport, since smaller pores help in complete wettability of the product, while large pores provide basis for fast mass transfer during higher discharge rates. Compared with other reported Fe based electrode materials (performance falling in the range of 200-300 F/g), MOXC-700 shows high capacitive performance of 600 F/g at discharge rate of 1 A/g.11,33-35 Such a high performance can be attributed to high activity of homogeneously dispersed redox-active particles inside the hierarchical carbon matrix, which participates actively in reversible redox process to deliver excellent performance. Furthermore, 3D hierarchical porous nature of the carbon with suitable PSD (1.2 and 12 nm) provides ideal conductive support through pore walls with efficient transport of electrolyte ions by pores.36 Moreover, carbon also provides support for double layer charge storage which further enhances the capacitance of the electrode materials. Owing to these characteristics, MOXC-700 also shows excellent capacitance retention of 500 F/g at higher scan rate of 8 A/g. Figure 3d represents the capacitance of MOXC-700 at different current densities. It can be further noted that increasing current density does not affect the performance as observed in most of the reported electrode materials.37-38 The inset of Figure 3d represents the cyclic voltamogram (CV) of the MOXC-700 at the scan rate of 2 mV/s which shows well defined oxidation (+0.242 V) and reduction (-0.078 V) peaks. Well defined redox peaks confirm the Faradaic behavior of the carbonized product due to quasielectron transfer that involves the Fe2+/Fe3+ and Fe/Fe2+ redox reactions.39


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Figure 3. (a) Comparative analysis of electrochemical capacitances of MOXC-Fe. (b) Powder XRD patterns of MOXC-700 and MOXC-700T. (c) PSD of MOXC-Fe. (d) Capacitance at different current densities. The inset shows CV curve of MOXC-700. (e) Cyclic performance of the MOXC-700 for 5000 cycles at current density of 8 A/g.


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The electrode shows excellent capacity retention over longer charge discharge cycles (5000 cycles) at high current density of 8 A/g (Figure 3e). A sharp increase in capacitance (from 280 F/g for the 1st cycle to 440 F/g for 30th cycle) is observed which could be attributed to the activation of the electrode materials such as (i) increased electrode/electrolyte interface and (ii) electrochemical conversion of Fe to Fe3O4.40-41 The excellent stability of the electrode materials could be attributed to shorter electrolyte diffusion pathways, higher mechanical stability of the electrode materials derived from MOFs, presence of protective carbon coating and reduced stresses in the electrode materials.9,15 3.2. Derivation of NPC from MOX-Al as negative electrode materials Keeping in mind the basic properties needed for the negative electrode materials in aASC, high surface area carbon was derived from MOX-Al as shown in scheme 1. Since higher carbonization temperature enhances the conductivity of the derived carbon, MOX-Al used here was carbonized at 1000 ᵒC to obtain MOXC-Al. To get high surface area NPC, the carbonized product further activated by soaking in NaOH solution. This activation process not only improves the surface area remarkably (from 495 m2/g for MOXC-Al to 1757 m2/g), but also removes the electrochemically inactive Al species inside the carbon matrix. It should be noted that comparing with the former reported activation method (HF-soaking or KOH-heating), this new activation method is very convenient and environmental friendly, without using any toxic chemicals or high temperature processing. The NPCs were characterized for their microstructure using FESEM and TEM. It is quite clear from Figure 4a that NPC is 3D hierarchical carbon retaining the structure even after removal of metallic species. The inset of Figure 4a further represents that NPC consist of interconnected particles generating porous networks in the carbon.


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As concluded from FESEM, the TEM (Figure 4b) also provides evidence that the carbon consists of interconnected particles generating hierarchical porosity. The powder XRD pattern of NPC does not show any prominent peaks, clearly showing successful removal of Al from MOXC-Al (Figure 4c). Furthermore, it is strongly believed that disorderness in the product improves the efficient diffusion of electrolyte ions through the electrode materials enhancing the electrochemical properties of the resulting system.3,18 The Raman spectroscopy was used to analyze inner structure of the carbon matrix. As shown in Figure 4d, two major bands were observed at 1336 and 1594 cm-1, which could be assigned to D and G bands respectively. The intensity ratio between the D and G bands (ID/IG) was found to be 1.02, which confirms the existence of graphitic carbon with highly disordered contents due to existence of heteroatoms e.g. N or/and O. The electrochemical properties of the NPC is strongly dependent on type of carbon and heteroatoms present in the inherent structure.42 The core levels of C, N and O in NPC were studied using XPS and C1s has been deconvoluted into various peaks at 284.5, 285.6, 286.1, 289.1 and 291.7 eV corresponding to C-C, C=N, C-O-C, O-C=O and π-π* transitions as shown in Figure 4e. Similarly, the deconvolution of N1s leads to two prominent peaks at the binding energies of 399.6 and 401.0 eV clearly showing the presence of N in form of amine and graphitic centers as shown in Figure 4f. Moreover, two oxygen functionalities (C-O and H-O-C) were identified in the NPC which further improve the electrochemical response of the NPC and increase the wettability of the electrode.


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Figure 4. (a) FESEM and (b) TEM images (c) XRD pattern and (d) Raman analysis of NPC High resolution XPS spectra of (e) C1s and (f) N1s and O1s of the NPC The activation of MOXC-Al leads to significant improvement in porosity and pore volume in the resulting product. The NaOH removes residual Al species from MOXC-Al by converting Al to Na3Al(OH)6 as shown in Figure 5a.43 Most of the micropores present on the outer domain of the


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carbon matrix tend to widen when exposed to harsh conditions resulting in mesopores.44-45 With the widening of micropores to mesopores, the micropores are also regenerated which are coupled by the removal of Al species. These micro- and mesopres are hierarchically interconnected in the resulting NPC as shown in Figure 5b. The as-carbonized samples show low pore volumes (0.59 cc/g) due to higher metallic contents which increased substantially to 1.95 cc/g after activation with NaOH. The resultant NPC exhibits high BET surface area up to 1757 m2/g as compared to 495 m2/g of the MOXC-Al. Thus, the improved pore volume and surface area confirmed that NaOH activation is much effective method to improve these properties of carbon as well as environmentally benign. Besides the surface area, the PSD of the nanoporous materials also play very important role in their electrochemical properties. Usually, mismatch between electrolyte ions and pore size of carbon materials inhibit the ion transportation inside the materials, leading a poor performance at higher scan rates.46 Considering the PSD of the NPC developed here, the NPC serve as ideal electrode materials for high rate analysis, as the NPC consists of two different sizes of interconnected pores (i.e. 1.2 and 5.8 nm) as shown in Figure 5c. The size of hydrated K+ ion is much similar to H3O+ within the range of 0.36 to 0.42 nm.47 It is quite clear that even the smaller pores (1.2 nm) are three times larger than the size of fully hydrated K+, H3O+ and un-hydrated OH- ions which enable the performance retention even at very high discharge rates without much compromise over the performance. The electrochemical tests were performed in 1 M KOH solution (vs. SCE) in a wide potential window of -0.7-0.1 V. The NPC shows typical rectangular CV curve at various scan rates from 2-100 mV/s which confirms its EDLC behavior (Figure 5d).


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Figure 5. Schematic presentation of Al removal from (a) MOXC-Al and (b) the pore structure of NPC after NaOH activation. (c) PSD of MOXC-Al and NPC (the inset shows their respective N2 sorption isotherms). (d) CV curves and (e) capacitance at various scan rates of NPC. (f) Capacitance retention under cyclic testing. The inset shows the CV profiles for 1st and 1000th cycles of the NPC electrodes.


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Owing to the high surface area and well interconnected pores, the NPC show high capacitance of 272 F/g and 258 F/g at the scan rates of 2 and 10 mV, respectively. As shown in Figure 5e, NPC retains capacitance up to 207 F/g at higher scan rates of 250 mV which is much higher than most of the reported carbon materials (including MOF derived carbon).28,48-50 Hence, NPC show excellent capacitance retention due to high surface area, favorable porosity, 3D interconnected networks and presence of heteroatoms in form of nitrogen (1%). The NPC further exhibits excellent capacity retention up to 1000 cycles without any changes in CV characteristics as shown in Figure 5f. Thus, these excellent properties of NPC make it a promising candidate in commercial aASC to boost up their power density as well as energy density without compromising on the life of these devices. 3.3. Development and performance of aASC The aASCs were assembled in a coin type cell using the MOXC-700 as the positive electrode (redox-active Faradaic electrode) due to its higher energy density, and the NPC as negative electrode material because of its excellent power density as shown in Figure 6a. To get an overall voltage window of 1.6 V for aASC, the operating voltage window of NPC was adjusted in the range of -1.2 V to 0.1 V and MOXC-700 was tested in the voltage window of -0.25 V and 0.4 V as shown in Figure 6b. The voltage range of the aASCs was also tested in the various voltage windows as shown in Figure S9. It was clearly observed that voltage window of the aASC device could be extended up to 1.6 V. Moreover, unlike the common organic/ionic electrolytes used for hybrid cells, the aqueous electrolytes (e.g. KOH, NaOH and LiOH) could provide higher conductivities, opportunity to assemble the cell under ambient conditions, safer to use and offer wider potential windows. Therefore, due to the aforementioned advantages, 6 M KOH was used as electrolyte in the assembled cell and used in the voltage range of 0-1.6 V.


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Figure 6c shows the CV curves of the assembled hybrid MOXC-700//NPC aASC at the scan rate of 50 mV. One broad oxidation peak was observed at 1.168 V, attributing to the oxidation of Fe to Fe3+ (involving the dual oxidation steps of Fe to Fe2+ and Fe2+ to Fe3+) as shown in equation (i) confirming the active Faradaic redox reaction in aASC. In addition, two major reduction peaks were observed at -0.627 V and -1.455 V showing the reduction of Fe3+ as shown in equation (ii) and (iii). Similar redox peaks were observed at lower scan rates of 2 and 10 mV as shown in the inset of Figure 6c, confirmed that developed cell has no effect of increasing or decreasing scan rates on its conductivity and diffusion of ions. The peak current increased linearly with increasing scan rate which verified that electronic and mass transfer is fast enough in accordance with the scan rate. Furthermore, no shift in the peak position was observed with varying the scan rate, which assures the high conductivity and faster ion transfer of electrode materials at higher scan rate, while most of the materials shows larger shift due to limited diffusion of ions at higher scan rate. Symmetric cells comprising both of the electrodes of NPC was also assembled to further investigate the origin of the redox peaks. The NPC//NPC asymmetric cell shows rectangular CV curve without any redox peaks as shown in Figure S10, which confirms that redox behavior of aASC based on MOXC-700//NPC is because of the redox active materials composed of MOXC-700. Fe→Fe2+ + 2e- and Fe2+ →Fe3+ + 1e- (+1.168 V -Oxidation) 3+

-

2+

-

2+

Fe + 1e →Fe Fe + 2e →Fe

(i)

(-0.627 V -Reduction)

(ii)

(-1.455 V -Reduction)

(iii)

To determine the rate capability of the developed aASC, the cells were tested under various current densities from 0.5 to 6 A/g. A high specific capacitance of 202.5 F/g was obtained for one electrode in aASC at lower discharge rate of 0.5 A/g due to complete activation of MOXC700 and excellent compatibility between the porosity of NPC and size of electrolyte ions.


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Furthermore, the aASC provides the capacitance of 170, 130, 120 and 105 F/g at scan rates of 1, 2, 4 and 6 A/g, respectively for single electrode in aASC, displaying high capacity retention with increasing current densities as shown in Figure 6d. Hence tailoring electrode structures at molecular level can exponentially enhance the electrochemical properties, consequently improving the energy densities. Individually, both electrode materials consisted of favorable porosities providing high capacity retention even at higher scan rates when tested in 3-electrode system. Hence, when combined together in practical two electrode coin type cell, the electrode materials retain their individual characteristics to provide higher capacities at lower scan rates and excellent capacity retention upon increasing current densities. Moreover, the electrochemical conversion of Fe further cause slightly longer charging time during initial cycling as shown in Figure 6e. However, after initial activation, the charge discharge cycles show symmetric behavior which demonstrate that the working voltage window is suitable for the aASC as shown in inset of Figure 6f. The asymmetric cell further delivers excellent cyclic stability with extraordinary capacity retention up to several thousand cycles as shown in Figure 6f. An increase in capacitance during initial cycling was observed due to enhanced wettability and electrochemical conversion of Fe to Fe3O4. The increased wettability leads to higher electrode/electrolyte interface resulting in large electrochemical surface area which in turn enhanced the capacitive properties of the cell. Moreover, the electrochemical conversion led to higher number of active sites for charge storage which further enhance the charge storage capability of the electrode materials. However, after complete activation was achieved, no further increase in capacitance was observed.


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Figure 6. View of MOXC-700//NPC asymmetric cell: (a) aASC assembly, (b) Comparative CV curves of NPC and MOXC-700 (c) CV curve of MOXC-700//NPC aASC (d) capacitance of


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single electrode in aASC at various current densities, (e) first five charge-discharge cycles, and (f) capacitance retention for assembled MOXC-700//NPC aASC The developed aASC cell shows excellent capacitance retention (>80%) after 10,000 cycles which is consistent with the other metal/C systems reported as shown in Figure 6f.4-6 The decrease in capacitance upon successive cycling could be due to partial agglomeration of the electrode materials. Figure S11 represent the TEM analysis of electrode materials after testing which clearly suggest that most of the particles retain their original size and shape while few particle undergo agglomeration. We speculate that these aggregated particles are responsible for partial decrease in capacitance. The energy and power densities are very important parameters in optimizing materials for practical applications.3,18 Traditionally, the practical applications of aASCs are enhanced by high power densities but largely limited due to lower energy densities. On the other hand, most of the recently developed SC systems present high energy densities but result in great compromise over the power densities. The asymmetric cells developed here showed high energy density of 17.496 Wh/Kg at the power density of 388.8 W/Kg (much higher than most of the reported Fe-C systems). The Ragone plot in Figure 7 shows the relationship between energy and power density and also presents comparison with other reported systems.4-5,51-57 The higher energy density values suggest the uniqueness of the MOX-derived materials for their practical use in real energy storage devices.


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Figure 7. Ragone plot showing the relationship between energy density and power density 4. Conclusions In summary, considering advantage of MOF-based structure, we have developed gels which are easily scalable and elaborate their electrochemical applications. Furthermore, problem associated with high temperature carbonization of MOF to derive electrochemically active species has been addressed successfully. We demonstrate that careful selection of the central metal and understanding of the carbonization temperature could effectively result in electrochemically active nanostructured electrode materials. The MOX derived Fe3O4/Fe decorated in carbon network shows high capacitance of 600 F/g which is much higher than most of the reported Fe systems with high capacity retention for 5000 cycles and also shows good rate capabilities. The


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MOX system was further utilized to derive NPC by a simple and scalable activation step with NaOH. Activation with NaOH not only improved the surface area but also exponentially improved the surface area of the resulting product, showing efficacy of our proposed method. The resultant NPC possesses high surface area of 1757 m2/g, favorable porosity and excellent capacitive performance of 272 F/g at 2 mV/s. Furthermore the aASC provides high values of energy density at excellent power density which is much higher than Fe-based systems also in comparison to the state of the art new nanostructured materials which are synthesized after laborious work. It is believed that present work will open new avenues to tailor the MOF-derived nanostructures for energy storage applications. ASSOCIATED CONTENT The supporting information provides insights over BET, XRD, SEM and XPS analysis of several products presented in this manuscript. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Ruqiang Zou E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China 51322205 and 21371014, the New Star Program of Beijing Committee of Science and Technology (2012004).


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Synthesis of Hierarchical Nitrogen-Doped Porous Carbon for High-Performance Supercapacitors ACS Appl. Mater. Inter. 2014, 6, 7214-7222. (50) Liu, B.; Shioyama, H.; Jiang, H.; Zhang, X.; Xu, Q. Metal–Organic Framework (MOF) as a Template for Syntheses of Nanoporous Carbons as Electrode Materials for Supercapacitor Carbon 2010, 48, 456-463. (51) Puthusseri, D.; Aravindan, V.; Madhavi, S.; Ogale, S. 3D Micro-Porous Conducting Carbon Beehive by Single Step Polymer Carbonization for High Performance Supercapacitors: The Magic of In Situ Porogen Formation Energy. Environ. Sci. 2014, 7, 728-735. (52) Brousse, T.; Bélanger, D. A Hybrid Fe3O4-MnO2 Capacitor in Mild Aqueous Electrolyte Electrochem. Solid-State Lett. 2003, 6, A244-A248. (53) He, X.; Zhao, N.; Qiu, J.; Xiao, N.; Yu, M.; Yu, C.; Zhang, X.; Zheng, M. Synthesis of Hierarchical Porous Carbons for Supercapacitors from Coal Tar Pitch with Nano-Fe2O3 as Template and Activation Agent Coupled with KOH Activation J. Mater. Chem. A. 2013, 1, 9440-9448. (54) Long, C.; Jiang, L.; Wei, T.; Yan, J.; Fan, Z. High-Performance Asymmetric Supercapacitors with Lithium Intercalation Reaction Using Metal Oxide-Based Composites as Electrode Materials J. Mater. Chem. A. 2014, 2, 16678-16686. (55) Chen, H.; Zhou, S.; Wu, L. Porous Nickel Hydroxide-Manganese Dioxide-Reduced Graphene Oxide Ternary Hybrid Spheres as Excellent Supercapacitor Electrode Materials ACS Appl. Mater. Inter. 2014, 6, 8621-8630. (56) Salunkhe, R. R.; Tang, J.; Kamachi, Y.; Nakato, T.; Kim, J. H.; Yamauchi, Y. Asymmetric Supercapacitors Using 3D Nanoporous Carbon and Cobalt Oxide Electrodes Synthesized from a Single Metal–Organic Framework ACS Nano. 2015, 9, 6288-6296. (57) Xu, Y.; Lin, Z.; Huang, X.; Liu, Y.; Huang, Y.; Duan, X. Flexible Solid-State Supercapacitors Based on Three-Dimensional Graphene Hydrogel Films ACS Nano. 2013, 7, 4042-4049.


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Table of Contents Tailoring metal-organic framework gels provides easy method to synthesize bulk quantities of high surface area product. These products provide opportunity to tailor high surface area electrode materials which could be assembled to obtain high energy devices with excellent electrochemical performances.


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Nanostructured Electrode Materials Derived from MetalâOrganic

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