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High-performance energy storage and conversion materials derived from a single metal-organic framework/graphene aerogel composite Wei Xia, Chong Qu, Zibin Liang, Bote Zhao, Shuge Dai, Bin Qiu, Yang Jiao, Qiaobao Zhang, Xinyu Huang, Wenhan Guo, Dai Dang, Ruqiang Zou, Dingguo Xia, Qiang Xu, and Meilin Liu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05004 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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High-performance materials

energy

derived

from

storage a

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single

conversion

metal-organic

framework/graphene aerogel composite Wei Xia‡1, Chong Qu‡1,2, Zibin Liang1, Bote Zhao2, Shuge Dai2, Bin Qiu1, Yang Jiao4, Qiaobao Zhang2, Xinyu Huang1, Wenhan Guo1, Dai Dang2, Ruqiang Zou1*, Dingguo Xia1*, Qiang Xu3*, and Meilin Liu2* 1

Beijing Key Laboratory for Theory and Technology of Advanced Battery Materials, Department

of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China 2

School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive,

Atlanta, GA 30332, United States 3

National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-

8577, Japan 4

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 331 Ferst

Drive, Atlanta, GA 30332, United States ‡These authors contributed equally to this work. *E-mail: [email protected]; [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment

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KEYWORDS: Metal-organic framework, N-doped graphene aerogel, ORR catalyst, supercapacitor

ABSTRACT: Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM)2 (mIM=2-methylimidazole). The presence of cobalt oxide (CoOx) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoOx/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co is removed, the resulting nitrogen-rich porous carbongraphene composite electrode (denoted as C/NG-A) displays exceptional capacitance and rate capability in a supercapacitor. Further, this method is readily applicable to creation of functional metal oxide hollow nanoparticles on the surface of other carbon materials such as graphene and carbon nanotubes, providing a good opportunity to tune their physical or chemical activities.

INTRODUCTION Metal oxides and porous carbons are very promising materials for electrochemical energy storage and conversion devices such as fuel cells, batteries, and supercapacitors.1,2 Designing the nanostructure of metal oxides for these applications is particularly important for further device


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performance optimization,3 considering the pronounced size/shape effects of active phase on the electrode reaction pathway and stability.4-6 Metal-organic frameworks (MOFs) are a new class of crystalline porous materials constructed from the combination of metal ions (or metal clusters) and organic linkers.7,8 MOFs-templated synthesis is emerging as a simple but versatile approach to prepare nanostructured materials.9 MOFs can usually be converted to porous carbon or metal oxides with unique nanostructures by selective pyrolysis in a controlled atmosphere.10,11 For instance, porous carbons with extremely high surface area and large pore volume have been prepared by pyrolyzing various MOFs (e.g. Zn-based MOF, Al-based MOF) under inert atmosphere.12,13 Metal oxides (e.g. Fe2O3, ZnO, CuO, TiO2) have been synthesized by simple oxidative decomposition of MOF precursors in air as well.14-17 Although there have been great achievements,18-21 it is still hard to precisely tune the morphology of the MOF-derived materials because of the lack of induction force during structure transformation at elevated temperatures.22 While some previous efforts have derived bulk metal oxide particle agglomerates or metal oxide-carbon composites from MOFs, the electrochemical properties of these metal oxides are still inadequate for many applications. Thus, it is highly desirable to either directly synthesize highly-functionalized nano-sized MOFderived material or grow embedded nanostructure with the help of novel substrates (e.g. carbon cloth,23 cellulose aerogel,24 carbon nanotube.25) to construct active materials with large surface area for electrochemical reactions. In this study, we have developed a graphene aerogel assisted method to break bulk MOF (Cobased MOF, Co(mIM)2 (mIM=2-methylimidazole).26) crystals into monodisperse metal oxide hollow nanoparticles (NPs) (Schematic shown in Scheme 1a). It is interesting to find that the parent Co-MOF nanocrystals derived monodisperse cobalt oxide (CoOx) hollow NPs with a


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uniform size of 35 nm were successfully dispersed in N-doped graphene aerogels (NG-A) and uniformly spread on graphene sheet surface after a simple thermal activation. This material (denoted as CoOx/NG-A) served as a new electrocatalyst with excellent activity for oxygen reduction reaction (ORR) in an alkaline solution. In addition, upon removal of Co from the CoOx/NG-A, we have obtained nitrogen-rich three-dimensional carbon material (denoted as C/NG-A), which exhibited high specific capacitance (421 F g-1 at a current density of 1 A g-1), outstanding rate capability (305 F g-1 at 50 A g-1) and cycling performance (99.7% capacitance retention after 20,000 cycles in an all-solid-state symmetric device) when utilized as an electrode material in supercapacitors. Scheme 1. a, Illustration of the concept of breaking MOF into monodisperse metal oxide NPs. b, Schematic of the formation process of CoOx/NG-A and C/NG-A.


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In a typical synthesis process of CoOx/NG-A and C/NG-A (Scheme 1b), N-doped graphene hydrogel was first prepared through a hydrothermal reaction at 160 °C by mixing graphene oxide (GO) dispersion with ammonia (chemical dopant). The residual solvent in the as-prepared hydrogel was exchanged with fresh deionized water and methanol before Co-MOFs polyhedrons were in-situ grown on N-doped graphene sheets by immersing the hydrogel in MOF precursors.27 The color of the hydrogel changed from black to dark purple after reaction (Figure S1a, Supporting Information), and SEM image in Figure S1b clearly demonstrated the success growth of nanoscale Co-MOF within N-doped hydrogel (sample I in Scheme 1b). Then, the MOF loaded hydrogel was dried by a supercritical carbon dioxide drying method and thermally activated to obtain sample II. In one case, sample II was taken out from the furnace and kept at 100 °C in air for 24 h, forming CoOx/NG-A. Another option is treating sample II in concentrated hydrochloric acid to remove cobalt and obtain all-carbon C/NG-A. For ORR catalytic performance comparison, neat Co-MOF (structure confirmed by PXRD with simulated pattern in Figure S2a, Supporting Information) was prepared (without NG-A) and went through the same treatment. The product was denoted as Co/NC. RESULTS AND DISCUSSION Characterizations of CoOx/NG-A The as-prepared CoOx/NG-A is composed of interconnected networks of graphene sheets, making it an open porous structure with a very low density of ~0.012 g cm-3 (Figure S2b, Supporting Information). Nitrogen adsorption-desorption test shows a high BET surface area (1359 m2 g-1) and large pore volume in CoOx/NG-A (7.2 cm3 g-1) (Figure S3a, Supporting Information). Detailed pore size distribution generated from nitrogen sorption experiment


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suggest the existence of multiscale pores varied from micro- to macropores (Figure S3b, Supporting Information). Figure 1b shows a typical high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the CoOx/NG-A. The corrugated grey sheets, corresponding to N-doped graphene, are woven into a porous, interconnected network. Hollow bright dots with an external size of ~35 nm and internal cavity of ~10 nm are cobalt oxide (The size distribution (with histogram) of the observed cobalt oxide particles is depicted in Figure S4a, Supporting Information) NPs which are uniformly dispersed on the graphene network. From the TEM image at wide field of vision, it is confirmed that the CoOx NPs possess well-defined hollow structures (Figure 1d). High resolution TEM (HR-TEM) in Figure 1f indicates a lattice spacing of 0.287 and 0.242 nm in the shell of the hollow NP, corresponding to the Co3O4 (220) and (311) planes. (Figure S4b, Supporting Information). Moreover, Pt-based nanoscale catalysts with rich edge sites have shown enhanced ORR activity compared with regular Pt NPs recently28,29 as the rich-edged nanostructure demonstrates exceptional oxygen adsorption and activation ability.30 Interestingly, the as-obtained CoOx NPs also display highly defective surfaces with rich edges or corner sites as shown in Figure 1e, which potentially makes the rich-edged Pt-free CoOx/NG-A a novel ORR catalyst considering the hollow interior can help decreasing the nonfunctional atoms, and the unusual surface geometry can provide another opportunity to tune the activity. The oxidation state of the surface cobalt species was examined by X-ray photoelectron spectroscopy (XPS). The high resolution Co 2p spectrum reveals the co-existence of Co3+ (779.8 eV) and Co2+ (781.4 eV) (Figure S5, Supporting Information).31 Oxidized cobalt sites, particularly the Co3+ cations, have been suggested as active donor-acceptor reduction sites.5 XPS analysis also reveals a high N content in CoOx/NG-A (~5.34 wt.%), the high resolution of N 1s


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spectrum in Figure S5c further suggests most of the N (~39.6 %) is the pyridinic N which exhibits highest ORR activity.32

Figure 1. a, Photograph and structure model of CoOx/NG-A. b, HAADF-STEM image of CoOx/NG-A. c, Corresponding EDS profile of Co3O4 NP. d, TEM image of CoOx/NG-A. e-f, HR-TEM images of Co3O4 phase in CoOx. There are three main factors that may influence the formation of the defective CoOx hollow NPs. First, the graphene may act like a diffusion barrier to prevent agglomeration of NPs during the synthesis. Without graphene, we obtained only large and dense particles derived from the original parent MOF (Figure S7, 8, Supporting Information) particles. Second, a suitable heat treatment temperature is important. Temperature like 450 and 600 °C instead of 750 °C were also applied, the resulting morphology was just like MOF precursor, large polyhedrons were


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observable (Figure S9, Supporting Information), whereas a higher temperature (750 °C) led to homogenous CoOx hollow NPs. Third, in this study, MOF-derived sample II in Scheme 1b displayed a clear Co core-C shell structure which was proved by TEM images in Figure S11a-d. The Co core and N-doped C shell came from the Co center and N-rich ligands of MOF, respectively (Figure S11e, f). After exposure to air, the highly active Co cores were oxidized and nanoscale hollow cavities were formed inside the NPs due to the different diffusion rates of the O atoms in air and core Co atoms, which is the well-known Kirkendall effect. Besides, due to the density difference between metallic Co and cobalt oxides, volume expansion was exacerbated, leaving behind the irregular CoOx NPs. However, oxidation of metal NPs without carbon shells is more likely to produce metal oxide hollow NPs with regular morphology from literatures.33,34 Thus, the presence of MOF ligand-derived carbon shells is important to the formation of richedged CoOx hollow NPs. Characterizations of C/NG-A C/NG-A was synthesized by refluxing of sample II in concentrated HCl (Scheme 1b). The microstructure was examined using SEM and TEM (Figure 2a, b). Similarly, C/NG-A is composed of interconnected networks of graphene sheets. However, the images demonstrate only corrugated sheets without any bright dot, suggesting the removal of Co. The co-existence of porous carbon and NG-A was indicated by comparison of the pore size distribution of NG-A, C/NG-A, and pristine Co-MOF samples after the thermal & chloride acid treatment (Co/NCAcid treated) (Figure. S13b, Supporting Information). FTIR is used to characterize the functional groups in C/NG-A (Figure S12a, Supporting Information). In the spectrum, the sharp peak at 1572 cm-1 is attributed to C=C/C=N groups (aromatic) and the broad peak at about 1150 cm-1 stems from the overlap of C-N groups (1194 cm-1) as well as alkoxy C-O groups (1110 cm-1),


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respectively. The peaks for C=O (aromatic) at 1740 cm-1 and O-H vibration at 1390 cm-1 could also be observed.35 XRD pattern is shown in supplementary Figure S12b, two broad peaks can be seen at 23.6 ° and 44.3 °, corresponding to (002) and (101) reflections of graphitic carbon (sp2boned carbon), respectively. Nitrogen adsorption-desorption isotherms at 77 K were performed to reveal the porosity property. The spectra exhibited a high BET surface area of 814 m2 g-1, which is not as high as CoOx/NG-A due to the shrinkage of carbon shells upon Co removal. The sorption curves are identified as IUPAC type IV with a hysteresis loop above P/P0 = 0.5, suggesting the presence of meso-pores in the material (Figure S13a, Supporting Information). Therefore, non-local density functional theory (NL-DFT) was taken to access the pore size distributions (Figure S13b, Supporting Information). The sharp distribution of pores with width of 5 nm, indicating the uniformed mesoporous characteristics in the material. The XPS survey spectrum of C/NG-A reveals the presence of C, N, and O without any other impurity in the sample. The atomic percentage of N, C, and O is 8.26% (higher than that of CoOx/NG-A), 85.46%, and 6.27%, respectively (Figure S14a, Supporting Information). The high resolution N 1s spectrum can be deconvoluted into three peaks centered at 398.2, 400.0, and 401.0 eV, corresponding to the pyridinic, pyrrolic, and graphitic nitrogen, respectively (Figure 2d).36 It is need to be noticed that the atomic percentage of both graphitic N and pyridinic N which exhibit better energy storage performance in C/NG-A enhance greatly.37 Likewise, Figure S13b displays the high resolution C 1s spectrum, which could be assigned to five components: SP2 C=C (284.2 eV), SP3 C-C (284.8 eV), C-OH (285.8 eV), C=O (287.5 eV), and π-π* electronic transitions (290.8 eV). Elemental mapping was acquired using EDS to elucidate the elemental composition of C/NG-A (Figure 2c). It can be seen that the material is composed of C, N, O exclusively. The atomic percentage for N, C, and O is 8.11%, 85.91%, and 5.98%,


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respectively, which is in good agreement with XPS survey scan results. Accordingly to a more accurate elemental analysis, combustion CHNS, the atomic percentage of C and N is 84.65% and 8.14%, respectively. It is also notable that nitrogen is homogenously distributed throughout the C/NG-A. Raman spectroscopy has been used to evaluate the quality of carbon material (Figure 2e). Both G and D bands of carbon appear in the Raman spectra of three materials. However, the C/NG-A has a higher ID/IG ratio, indicating the reduction of GO and the generation of more defects in the MOF-derived porous carbon/NG-A system.


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Figure 2. a, SEM image of C/NG-A. b, HRTEM image of C/NG-A. c, Elemental mapping of C/NG-A. d, The XPS high resolution N 1s spectrum of C/NG-A. e, Raman spectra of C/NG-A and related materials. Electrochemical evaluation of CoOx/NG-A The catalytic activity of CoOx/NG-A was evaluated using electrochemical measurements such as cyclic voltammetry (CV) and rotating disk electrode (RDE). Figure 3a shows the CV signals in O2 versus N2-saturated 0.1 M KOH electrolytes. The neat Co/NC sample, consisting of particles directly derived from Co-MOF, exhibited relatively low ORR activity. In contrast, the CoOx/NG-A sample exhibited a more positive reduction peak potential (0.786 V vs. RHE) and higher cathodic current density, suggesting a prominent improvement in ORR catalytic activity. Figure 3b displays the typical polarization curves of Co/NC, CoOx/NG-A, and commercial Pt/C (20 wt% Pt on Vulcan XC-72) catalyst. The onset potential (determined for J = -0.1 mA cm-2) of CoOx/NG-A reaches 1.019 V, which is much more positive than that of Co/NC (0.872 V), Co3O4 (poor activity), and NG-A (0.887 V, Figure S15, Supporting Information). The half-wave potential is 54 mV more positive than that of Co/NC and almost identical to that of Pt/C. Moreover, CoOx/NG-A shows a much higher limit current density than that of Co/NC (Figure 3b). In the mixed kinetic and diffusion controlled zone of the ORR polarization curve (around 0.6-0.9 V), CoOx/NG-A exhibits a steeper increase in the current density than Pt/C, indicating higher catalytic activity of CoOx/NG-A. The kinetic current densities of all samples were calculated from the polarization curves after mass-transport correction and normalization with respect to the geometric area of the electrode (Figure 3c). The CoOx/NG-A catalyst demonstrated the highest value of 32.5 mA cm-2 at 0.750


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V, which is about 9.2 times higher than that of the Co/NC catalyst and 3.3 times larger than that of the Pt/C catalyst, revealing the higher intrinsic catalytic activity of the CoOx/NG-A. The number of electron transferred (n) in ORR is another important parameter that characterizes the ORR activity. The preferred value for n is 4, suggesting a direct reduction of O2 to OH-. The n of CoOx/NG-A was calculated to be ~3.8 at 0.750 V from the Koutechy-Levich plots, which is in agreement with that calculated from the RRDE measurement (Figure S16, Supporting Information), implying the 4-electron reduction pathway by CoOx/NG-A, similar to the ORR process catalyzed by Pt/C. In comparison, Co/NC catalyst exhibits an electron transfer number of ~2.7 in 0.1 M KOH (Figure S17, Supporting In-formation), suggesting a dominant 2-electron reduction pathway.

Figure 3. a, CVs of CoOx/NG-A at 50 mV s-1 (solid line-O2, dash line-N2). b, ORR polarization curves at 10 mV s-1 and 1600 rpm. c, Kinetic-limiting current density and electron-transfer


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number at 0.750 V. d, Chronoamperometric responses at 0.764 V. e,f, ORR polarization curves of CoOx/NG-A (e) and Pt/C (f) in ADT measurement. In addition to the high catalytic activity, CoOx/NG-A also exhibited remarkable stability for ORR catalysis. Chrono-amperometric measurement at a high voltage of 0.764 V recorded a more than 82% current retention over 30,000 s of continuous operation (Figure 3d). In comparison, Pt/C showed obvious activity decay with less than 50% retention under the same testing conditions. The catalysts were further evaluated via accelerated durability test (ADT). CoOx/NGA retained the original high activity without observable shift in the polarization curve after 3,000 cycles (Figure 3e). However, the baseline Pt/C catalyst showed decreased activity under that same condition, reflecting a 26 mV negative shift in the half-wave potential (Figure 3f). Moreover, tolerance to contaminant poisoning during electrochemical operation is also an important parameter for ORR catalyst. Pt is known vulnerable to methanol poisoning, which degrades fuel cell performance. Pt/C catalyst showed a sharp decrease in current density when methanol was added, while our CoOx/NG-A catalyst delivered a steady current upon addition of methanol (Figure S18, Supporting Information), suggesting better tolerance to methanol poisoning of CoOx/NG-A. Electrochemical evaluation of C/NG-A To evaluate the energy storage performance of the C/NG-A as a supercapacitor electrode, we also performed CV, galvanostatic charge-discharge (GCD) cycling, and electrochemical impedance spectroscopy (EIS) of the active material with areal mass loading of ~1.9 mg cm-2 using a three-electrode configuration in an aqueous H2SO4 electrolyte (1.0 M) from -1.0 to 0 V (vs Ag/AgCl). All CV curves acquired at scan rates varied from 10 to 100 mV·s-1 (Figure 4a)


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exhibit approximately semi-rectangular shapes, indicating electrostatic charge storage mechanism. When increasing scan rate, the current response increased accordingly, indicating good rate capability of the C/NG-A electrode. GCD curves collected at different current densities display typical triangular profiles (Figure 4b). The gravimetric capacitance of the C/NG-A electrode was calculated from the GCD curves at different current densities based on the mass of the active material. As shown in Figure 4c, the C/NG-A was able to yield a high capacitance of 421 F g-1 at 1 A g-1, representing one of the best achieved gravimetric capacities for nitrogen doped graphene (Table S1, Supporting Information). Furthermore, when the current density increased to 50 A g-1, a capacitance of 305 F g-1 was still retained, corresponding to a capacitance retention of 72.5% of its initial value, indicating fast ion and electron transport within C/NG-A at high current density. When the mass loading of active electrode materials was increased to ~9.5 mg cm-2, the electrochemical performances remained the same except the GCD curves at high current densities (Figure S19a-d), implying that the rate capability is largely unaffected by the mass loading and C/NG-A has great potential for practical applications.


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Figure 4. a, CV curves of C/NG-A at various scan rates in 1 M H2SO4. b, GCD curves of C/NGA at different current densities. c, Specific capacitance of C/NG-A calculated based on Equation. S3. d, Schemtic and image of flexible all-solid state symmetric device. e, CV curves of C/NG-A SSCs at various scan rates. f, Bending test of C/NG-A SSCs. g, Cycling stability of C/NG-A SSCs at 20 A g-1. h, Ragone plot of C/NG-A SSCs compared with similar N-doped materials.

Flexible and lightweight all-solid-state supercapacitors (SSCs) have recently attracted much attention for wearable or portable electronic devices. To evaluate the suitability of the C/NG-A for this application, we fabricated a two-electrode symmetric supercapacitor using 1.0 M PVA/


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H2SO4 as the gel electrolyte (Figure 4d). The obtained SSCs exhibited rectangular CV curves at various scan rates (Figure 4e), and the steep slopes at charge and discharge thresholds at both high and low scan rates (Figure S20, Supporting Information). As shown in Figure. S21, the tails of the EIS at low frequencies for the SSCs have a very large slope, indicating a faster ion diffusion rate at the electrode/electrolyte interface and thus excellent rate capability. The asprepared supercapacitor can be bent without sacrificing the electrochemical performance. Figure 4f proves that, the CV curves remained almost the same at a scan rate of 100 mV s-1 under bending angles from 0° to 180°, indicating that the integrity of the flexible SSCs when folded. The C/NG-A based SSCs also exhibited outstanding cycling stability in a potential window of 0 to 1 V at both high and low current density of 20 A g-1 (and 2 A g-1), retaining extraordinary 99.7% (95.4%) of the initial capacitance after 20,000 (15,000) cycles (Figure 4g, Figure S22). Finally, compared with other symmetric all-solid state cells based on nitrogen-rich carbon-based material reported in the literatures,38-43 the C/NG-A demonstrated outstanding power density (500 W kg-1) at the energy density of 33.89 Wh kg-1 and retained 25,000 W kg-1 at 24.86 Wh kg-1 (Figure 4h), indicating exceptional energy and power density for this kind of carbon based material. Based on the characterizations and electrochemical measurements, it can be found that the graphene has been efficiently decorated with functional metal oxide hollow NPs or MOF-derived carbon through the synthesis process and endowed with active electrochemical properties. This method is also applicable to other advanced carbon materials like carbon nanotube (CNT). As shown in Figure S23, CNT aerogel (CNT-A),44 assembled from interconnected CNTs, was successfully modified with the similar CoOx hollow NPs via the same method, implying the


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universality of this strategy. The resultant CoOx/CNT-A composite kept the original mechanical robustness of CNT-A,44 which could be compressed without impairing the 3D structure. CONCLUSION In summary, we developed a N-doped graphene aerogel assisted method and successfully synthesized both CoOx/NG-A and C/NG-A from Co-MOF/NG-A. The obtained CoOx/NG-A coupled catalyst displays excellent ORR activity and stability as well as good ORR selectivity in alkaline solution while the C/NG-A carbon-based material obtained after cobalt removal exhibits extraordinary energy storage capability in both three-electrode configuration and two-electrode all-solid-state device. This work not only develops a new strategy of building advanced nanostructure from MOFs, but also provides a novel idea of efficiently fabricating high performance electrochemical materials which satisfy both energy conversion and energy storage fields from a single MOF/NG-A-derived nanostructure. Methods Synthesis of GO. Graphite oxide (GO) was fabricated from graphite powder by a modified Hummers method45,46. Typically, 2.5 g of K2S2O8 and 2.5 g of P2O5 was dissolved in 12 mL of concentrated H2SO4. This solution was heated to 80 °C, followed by addition of 3 g of graphite powder. The mixture was kept at 80 °C for 4.5 h and then cooled down to room temperature. Next, 500 mL of de-ionized (DI) water was added into the mixture. The diluted solution was kept overnight and then filtered and washed with DI water. The resulting product was dried in air overnight to make the pre-oxidized graphite powder. The powder was added into 120 mL of cold (0 °C) concentrated H2SO4. Afterwards, 15 g of KMnO4 was added slowly under stirring during which the solution temperature was kept below 20 °C. Then, the mixture was stirred at 35 °C for


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2 h, followed by slow addition of 250 mL of DI water in an ice bath to keep the temperature below 50 °C. The resulting mixture was stirred for another 2 h, and then 700 mL of DI water was added, thereafter 20 mL of 30 % H2O2 was immediately injected into the mixture. The solution was kept for 12 h and the supernatant was removed. The left solution was washed with 10 % diluted HCl and 1 L of DI water. In the final step, GO dispersion was purified by dialysis for 7 days. The final concentration of the GO dispersion was around 15 mg mL-1. Synthesis of nitrogen-doped graphene hydrogel. The nitrogen-doped graphene hydrogel was synthesized through a hydrothermal process using NH4OH as the chemical dopant. Typically, 2.5 mL of GO dispersion was diluted with 8 mL of DI water, followed by sonication for 2 h. Next, 10 mL of NH4OH was added into this diluted GO dispersion. The mixture was stirred for 1 h and then sealed in a Teflon-lined autoclave. The reaction was carried out at 160 °C for 24 h to form hydrogel with a shape of cylinder. The resulting hydrogel was immersed in DI water, which was exchanged with fresh DI water several times to completely remove the residual NH4OH. Finally, the DI water was exchanged with methanol. Synthesis of carbon nanotube aerogel. Carbon nanotube aerogel (CNTA) was synthesized through a CVD process28 using 1,2-dichlorobenzene and ferrocene as carbon source and catalyst precursor, respectively. Typically, ferrocene powders were dissolved in dichlorobenzene to make a solution of 0.06 g mL-1. Afterwards, the solution was injected into a 2-inch quartz tube in a furnace at a rate of 0.13 mL min-1. A mixture of Ar (2000 mL min-1) and H2 (300 mL min-1) was used as carrier gas. The temperature was maintained at 860 °C during the CVD process. A quartz substrate was put in the reaction zone to collect the CNTA.


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Synthesis of Co-MOF/NG-A. 0.144 g of Co(NO3)2·6H2O and 0.324 g of 2-methylimidazole was each dissolved in 10 mL of methanol. And then the former salt solution was poured into the latter ligand solution under vigorous stirring. After 60 s, stir was stopped and nitrogen doped graphene hydrogel was added into the mixture, kept at room temperature for 24 h. Latter, the cylinder-like hydrogel, whose color became slightly purple, was took out and immersed into ethanol. The ethanol was exchanged with fresh ethanol several times in order to remove the residual reagent. Afterwards, the resulting cylinder-like gel was dried using supercritical carbon dioxide, leading to the nitrogen doped graphene aerogel supported Co-MOF (named as CoMOF/NG-A). Synthesis of C/NG-A. Co-MOF/NG-A was pyrolyzed at 750 °C in a temperature-programmed furnace under an argon gas flow for 2 h. The furnace was cooled down to room temperature naturally in the argon atmosphere. Then the pyrolyzed sample was treated in concentrated hydrochloric acid at 80 °C for 6 h. The resulting sample was collected by centrifugation, washed with DI water and then dried at 80 °C. Synthesis of CoOx/NG-A and CoOx/CNT-A. Co-MOF/NG-A or Co-MOF/CNT-A was pyrolyzed at 750 °C under an argon gas flow for 2 h. The sample was taken out from the furnace and heated at 100 °C in air for 24 h. After cooling down to room temperature, the sample was collected and stored in dried condition. ASSOCIATED CONTENT Additional methods and experimental characterizations in the supporting Information document is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *Ruqiang Zou: [email protected] *Dingguo Xia: [email protected] *Qiang Xu: [email protected] *Meilin Liu: [email protected] Author Contributions ‡These authors contributed equally to this work. Notes Correspondence and requests for materials should be addressed to Ruqiang Zou and Qiang Xu. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (51322205 and 21371014) and Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200). REFERENCES (1)

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