Functionalized Bimetallic Hydroxides Derived from Metal–Organic

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Functionalized Bimetallic Hydroxides Derived from Metal-Organic Frameworks for High Performance Hybrid Supercapacitor with Ex-ceptional Cycling Stability Chong Qu, Bote Zhao, Yang Jiao, Dongchang Chen, Shuge Dai, Ben M deGlee, Yu Chen, Krista S. Walton, Ruqiang Zou, and Meilin Liu ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 2, 2017

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ACS Energy Letters

Functionalized Bimetallic Hydroxides Derived from Metal-Organic Frameworks for High Performance Hybrid Supercapacitor with Exceptional Cycling Stability Chong Qu†,‡, Bote Zhao‡, Yang Jiao§, Dongchang Chen‡, Shuge Dai‡, Ben M. deglee‡, Yu Chen‡, Krista S. Walton§, Ruqiang Zou*,†, and Meilin Liu*,‡ †

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

‡

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States

§

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, United States

ABSTRACT: A hybrid supercapacitor consisting of a battery-type electrode and a capacitive electrode could exhibit dramatically enhanced energy density compared with a conventional electrical double-layer capacitor (EDLCs). However, advantages for EDLCs such as stable cycling performance will also be impaired with the introduction of transition metal-based species. Here, we introduce a facile hydrothermal procedure to prepare highly porous MOF-74-derived double hydroxide (denoted as MDH). The obtained 65%Ni-35%Co MDH (denoted as 65Ni-MDH) exhibited a high specific surface area of up to 299 m2 g-1. When tested in a three-electrode configuration, the 65Ni-MDH (875 C g-1 at 1 A g-1) exhibited excellent cycling stability (90.1% capacity retention after 5,000 cycles at 20 A g-1). After being fabricated as a hybrid supercapacitor with N-doped carbon as the negative electrode, the device could exhibit not only 81 W h kg-1 at a power density of 1.9 kW kg-1 and 42 W h kg-1 even at elevated working power of 11.5 kW kg-1, but also encouraging cycling stability with 95.5% capacitance retention after 5,000 cycles and 91.3% after 10,000 cycles at 13.5 A g-1. This enhanced cycling stability for MDH should be associate with the synergistic effect of hierarchical porous nature as well as existence of inter-layer functional groups in MDH (proved by Fourier transform infrared spectroscopy (FTIR) and in situ Raman spectroscopy). This work also provides a new MOF-as-sacrificial template strategy to synthesize transition metalbased hydroxides for practical energy storage applications.

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In recent years, the increasing concern about environmental issues and the rapidly expanded demand for next-generation power sources for electric vehicles, smart grids evoke the optimizations of energy storage systems and technologies. Supercapacitors (SCs) have been considered as a promising choice because of their high power density, long life time, and fast charging capability. However, the relatively low energy density hinders their application in areas that require both energy and power. 1-4 Conventional electrical double-layer capacitors (EDLCs) store charge by fast ion adsorption based upon an electrostatic mechanism, which limits their capacitance. 5-7 Compared to EDLCs, hybrid supercapacitors, which is composed of a carbon-based capacitive electrode and a transition metal-based battery-type electrode, have recently attracted significant attention because of the comparatively higher capacitance and broader operation voltage. 8,9 However, advantages such as good cycling stability of EDLCs will also be impaired with the introduction of a transition metal-based electrode. Accordingly, numerous efforts have been devoted to optimize the electrochemical performance of the battery-type electrode materials, including nanostructure engineering, 10,11 formation of composite with carbon-based materials (such as graphene and carbon nanotubes), 12-14 and transition metal doping, 10,14 etc.

than 20 wt% of the MOFs structure, will not make contributions in MOF-based EDLCs. 25 Accordingly, it is highly desirable to develop novel MOF-derived materials, which will not only take advantage of the metal ions in the framework for faradaic contribution (high capacity), but also inherits intrinsic features of MOFs (e.g. exceptional surface areas) for high performance energy storage applications.

The as-synthesized Ni/Co-based MOF-74 samples were dispersed in a 2 M KOH aqueous solution and the stirred gently for 2 h at room temperature before being transferred to Teflon-lined stainless autoclave (Scheme 1). The autoclave was placed in an oven at 120 ˚C for 2 h. After being washed

and filtered, MDHs were obtained and named 25Ni-MDH (25% nickel nitrate-75% cobalt nitrate), 50Ni-MDH, 65NiMDH, 75Ni-MDH, and 85Ni-MDH, respectively, based on the initial ratios of Ni(II): Co(II).

In this study, we report on the synthesis of highly porous MOF-derived double hydroxide (denoted as MDH) with classic nickel-cobalt MOF-74 (M2(DOBDC); M=metal, DOBDC=2,5-dihydroxy-1,4-benzenedicarboxylic) as precursor through a facile process of alkaline treatment. 26 The obtained MDH exhibited a high specific surface area of up to 299 m2 g-1 with a hierarchical porous nature compared to previously reported hydroxides (typically < 100 m2 g-1). 9,27 As the crystalline structures of Ni-MOF-74 and Co-MOF-74 are exact the same, 28,29 by altering atomic ratio between initial Ni(II) and Co(II), the 65%Ni-35%Co MOF-74 (denoted as 65Ni-MOF-74) derived MDH (denoted as 65Ni-MDH) exhibited the best energy storage performance which delivered a high capacity of up to 875 C g-1 in a three-electrode configuration with 2 M KOH as electrolyte at a current density of 1 A g1 , as well as excellent cycling stability (90.1% capacity retention after 5,000 cycles at 20 A g-1) compared to 25Ni-MDH, Emerging as a new class of materials with tunable structure 50Ni-MDH, 75Ni-MDH, and 85Ni-MDH. A hybrid supercapacitor was assembled using a 65Ni-MDH as positive elecand exceptional porosities that far surpass active carbon, metal-organic frameworks (MOFs) are drawing great attention as trode and N-doped porous carbon (denoted as N-C) as negaelectrode materials in energy storage and conversion. 15-18 tive electrode. 30 The as-fabricated supercapacitor demonstratGenerally, MOF-based SCs can be classified into three types ed an energy density of 81 W h kg-1 at a power density of 1.9 according to different surface chemistry and device configurakW kg-1, which retained 42 W h kg-1 even at elevated working tion, (i) utilizing pristine conductive MOFs to store charge power of 11.5 kW kg-1. The hybrid device could exhibit even based upon an electrostatic mechanism in organic electrolyte; better cycling stability with 95.5% capacitance retention after 19 5,000 cycles and 91.3% retention after 10,000 cycles at 13.5 A (ii) pyrolyzing MOFs to highly porous carbons with good g-1. The outstanding cycling performance may be a result of conductivity for conventional EDLCs electrode materials; 20-24 (iii) combining MOF-derived materials with conductive addico-existence of inter-layer oxygen-containing functional tives such as conductive carbons (e.g. graphene), organic polgroups which were further studied and confirmed by Fourier ymers (e.g. polyaniline).16,17 Although several MOFs or MOFtransform infrared spectroscopy (FTIR) and in situ Raman spectroscopy. This work also provides a new strategy to synderived materials based SCs listed above can achieve high operating voltage, while the relatively low capacitance will thesize MOF-derived battery-type materials for energy storage, which could be extended to other transition metal based still significantly limit their practical application. Meanwhile, MOFs for further performance promotion. it is also notable that metal ions which usually occupy more Scheme 1. Illustration of synthesis process from MOF-74 to Ni-Co MDH

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ACS Energy Letters

The microstructures of MOF-74 and derived MDHs were examined using electron microscopy. Most of the previous MOF-related efforts receive bulk agglomerates or composites as the products, 26,31 in which a large part of pores was not effectively utilized for energy storage in case of poor conductivity. Figure 1a, 1b show typical scanning electron microscope (SEM) images of micron-sized 65Ni-MOF-74 with a spear-shaped morphology ranging from ~10 to 15 µm in length and ~2 to 4 µm in diameter. By altering the mole ratio between Ni and Co, the morphologies were similar to each other with particle size of ~10 µm (Figure S1a-S1d). In order to reduce the size of MOFs and make them more energy storage favorable, KOH treatment of as-synthesized MOF-74 under controlled hydrothermal condition was processed which resulted in the destruction of micron-sized MOF-74 and formation of nanoscale 65Ni-MDH (~ 500 nm wide as shown in Figure 1c, 1d), other Ni-Co ratios MOF-74 derived MDHs have similar shrunken particle sizes after hydrothermal process (depicted in Figure S1e-S1h).

Besides, elemental mapping was acquired using energy dispersive spectroscopy (EDS) to elucidate the elemental composition of 65Ni-MDH (Figure 1e). It can be seen that 65NiMDH is composed of Ni, Co, O, and C exclusively. The images also unambiguously reveal that these elements are homogenously distributed in the 65Ni-MDH sample. The crystallinity and purity of as-prepared MOF-74 samples were confirmed from single crystal X-ray diffraction (XRD)(Figure S2b). Similarities among all the patterns are evident, all the assynthesized MOF-74 had peaks that match with simulated pattern perfectly, suggesting that the as-obtained Ni-Co MOF74 structure has 1-D hexagonal pores forming a honeycomblike lattice lined with square-pyramidal open metal sites, 32 and altering the initial Ni(II): Co(II) ratios didn’t observably affect the phase structures of MOF-74. Figure 2a presents the comparison XRD patterns between 65Ni-MOF-74 and 65NiMDH. It is easy to find that all MOF-74 characteristic peaks disappear, the main peaks of 65Ni-MDH could be indexed to a hexagonal nickel-cobalt hydroxide phase (ICDD: 04-0134410) very well, suggesting the decomposition of 65Ni-MOF74 and formation of 65Ni-MDH during the material synthesis. It is also notable that the existence of broaden peaks may be an impact of inhomogeneously strained crystallite in the MDH structure, 33,34 which is most likely induced by nonuniform distribution of MOF-74 functional groups derived from organic ligands in MDH. Figure 2b and Figure S3a, 3b demonstrate the N2 adsorption-desorption isotherms and pore size distribution plots of 65Ni-MOF-74 and 65Ni-MDH. The BET surface areas of 65Ni-MOF-74 and 65Ni-MDH are 1095 and 299 m2 g-1, respectively, which indicate the inheritance of MOFs porous nature by 65Ni-MDH hydroxide (much larger than other reported hydroxides 9,27) even after hydrothermal process. The pore size distribution plots reveal the presence of rich micropores peaked at around 1.2 nm in the 65Ni-MOF-74, while the 65Ni-MDH exhibited hierarchical pore distribution peaks between 1 and 5 nm, suggesting the formation of both micropores and small meso-pores. The surface electronic state and the composition of 65Ni-MDH were analyzed by X-ray photoelectron spectroscopy (XPS). The presence of Ni, Co, O, and C elements in the 65Ni-MDH sample has been supported by the survey spectrum (Figure S3c), in agreement with the EDS results. Figure 2c shows the high resolution C 1s spectrum, which can be fitted and divided into four peaks: the carbon to carbon bond (284.6 eV), the carbon single-bonded to oxygen (286.2 eV), the carbonyl carbon (287.8 eV), and the carboxylate carbon (289.0 eV), 35 which along with the high resolution O 1s spectrum (Figure S3d) indicate: there are oxygencontaining functional groups engaged in the structure of 65NiMDH. Figure S4a, 4b show the high-resolution Ni 2p and Co 2p spectra. The Ni 2p1/2 and Ni 2p3/2 peaks centered at 873.3 and 855.7 eV, respectively, were observed with two corresponding satellites, in accord with that of Ni(OH)2 36, 37 and Ni-Co hydroxide. 38 The Co 2p3/2 was centered at 781.1 eV, corresponding to cobalt hydroxide, 39 confirming the formation of double hydroxides. The spin-orbit splitting of 15.5 eV between Co 2p1/2 and Co 2p3/2, and the low intensity of the satellite line, suggest that both Co2+ and Co3+ exist in the MDH. 27,

Figure 1. (a), (b) SEM images of as-synthesized 65Ni-MOF74, (c), (d) SEM images of 65Ni-MDH at various scale bar, (e) EDS spectrum of 65Ni-MDH and elemental mapping and corresponding SEM images of 65Ni-MDH

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within the same potential window. The 65Ni-MDH electrode delivers the longest discharge time, which is in accord with CV results.

Figure 3. (a) CV curves in a potential range from 0 to 0.5 V (vs. Ag/AgCl) at a scan rate of 10 mV s-1, (b) GCD curves at 1 A g-1, (c) specific capacity and (d) cycling performance of MDH electrodes with different initial Ni(II) to Co(II) ratios (i.e., 25Ni-MDH, 50Ni-MDH, 65Ni-MDH, 75Ni-MDH, and 85Ni-MDH). For the CV curves of 65Ni-MDH electrode (Figure S6a), the current response increases accordingly with increasing scan rates from 10 to 40 mV s-1, indicating good rate capability. The specific capacities of the MDH electrodes were calculated based on the GCD curves (Figure S6b, Figure S7) according to Equation S (1) and the results were summarized in Figure 3c. All the Ni-Co hydroxides show promising but similar rate capability while the 65Ni-MDH-based electrode exhibits the highest specific capacity of 875, 857, 821, 765, 744, and 666 C g-1 at the current densities of 1, 2, 4, 8, 10, and 20 A g-1, respectively. These numbers are among the bests for MOF-related supercapacitor materials. 15-17,19 Figure 3d demonstrates the cycling performance of the MDH electrodes at a high current density of 20 A g-1 for 5,000 cycles, the corresponding capacity retentions after 5,000 cycles for 25NiMDH, 50Ni-MDH, 65Ni-MDH, 75Ni-MDH, and 85Ni-MDH are 78.4%, 83.0%, 90.1%, 85.4%, and 87.3%, respectively. After compared with literatures (27,41-43) (Table S1), it is very encouraging to find these MOF-74 derived MDHs are much more stable than most of the previously reported Ni-Co oxides/hydroxides synthesized by traditional methods like chemical bath or electrodeposition. 9,10

Figure 2. (a) XRD patterns of as-synthesized 65Ni-MOF-74 and 65Ni-MDH, (b) N2 adsorption-desorption isotherms and corresponding pore size distribution plot (inset) of 65NiMDH, (c) High resolution XPS spectra of C 1sin the 65NiMDH sample. The electrochemical performance of MOF-74 derived NiCo hydroxides were firstly evaluated in a three-electrode configuration in a 2 M KOH aqueous electrolyte and the results are shown in Figure 3 and Figure S5. For comparison, the electrochemical performance of 65Ni-MOF-74 (see in Figure S5, 65Ni-MDH exhibited better electrochemical performance than 65Ni-MOF-74), 25Ni-MDH, 50Ni-MDH, 75Ni-MDH, and 85Ni-MDH-based electrodes were also measured under the same condition. Cyclic voltammetry (CV) curves of the asprepared electrodes at a scan rate of 10 mV s-1 are demonstrated in Figure 3a. The sloping profile of 25Ni-MDH indicates a dominant CoO2/CoOOH 1-phase reaction. However, when the Ni(II) ratio increases in the MDH, 2-phase plateau corresponding to the Ni(OH)2/NiOOH faradaic reaction gradually takes place, and a pair of well-defined redox peaks are observed. 40 Among the electrodes with different initial Ni(II) to Co(II) ratios, the 65Ni-MDH electrode exhibits the highest current density and largest integrated area, indicating the highest charge storage capability for the 65Ni-MDH sample. Figure 3b shows the corresponding GCD curves of all MDH electrodes with different initial Ni(II) to Co(II) ratios at 1 A g-1

To further evaluate the practical application potential of 65Ni-MDH electrode, a hybrid supercapacitor was assembled using 65Ni-MDH as the positive electrode material and N-C (three-electrode performance is shown in Figure S8) as negative electrode material (denoted as 65Ni-MDH//N-C) in a 2 M KOH aqueous electrolyte, with photograph and schematic shown in the inset of Figure 4b. The corresponding electro-

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ACS Energy Letters even at elevated working power of 11.5 kW kg-1 (Equation S (6), S (7)). These numbers are superior to most of reported MOF-derived active material-based energy storage systems so far, such as MOF-derived NixPyOz// MOF-derived NixPyOz, 45 C-GMOF (MOF-5/GO hybrid materials derived porous carbon)//C-GMOF, 46 and ZIF-67-derived C3O4//ZIF-67 derived carbon. 47 It is also comparable with other recent published energy storage systems composed of nickel hydroxide-based electrodes, such as CBC-N2 (nitrogen-doped carbonized bacterial cellulose) @LDH (layered double hydroxide)-0.4//CBCN2, 48 Ni-Co-Fe hydroxide//AC (active carbon), 49 and MWCNT (multiwalled carbon nanotubes)/amorphousNi(OH)2//rGO/CNT. 50

chemical results are shown in Figure 4, S9. According to Equation S (2) and S (3), the mass ratio between positive and negative electrodes is R=0.42 to achieve charge balance. The Faradaic efficiencies (FE) of the hybrid device at various current densities are depicted in Figure S10a, by using overcapacitive positive electrodes (R=0.5 and 0.6), 44 the FE at 2 A g-1 can be improved to 84.6% and 92.8% (Figure S10b), respectively. Figure 4a shows the CV curves of the 65NiMDH//N-C device, the profiles exhibit overlapped characteristics of two different energy storage mechanisms. The corresponding GCD curves at current densities from 1 to 13.5 A g-1 are shown in Figure S8a, where a sloping discharge plateau from 1.3 to 0.7 V are observed, consistent with the CV results. The capacitances of 65Ni-MDH//N-C are summarized in Figure S9b (calculated by Equation S (4)), that is, 215, 186, 147, 135, and 112 F g-1 at current densities of 2, 3.5, 7.5, 9, and 13.5 A g-1, respectively.

In order to further probe in possible reason for the outstanding cycling stability, additional characterizations were carried out. Raman spectroscopy does not require vacuum compared to other electron-based surface analysis techniques such as XPS, 51 which makes it eligible to in situ identify the phase evolution MOF-74 during the MDH synthesis process. The process of 65Ni-MDH synthesis was characterized in situ, and Raman spectroscopic acquisition time for each spectrum was 90 s (CCD camera response time included), each spectrum was collected from 90 cm-1 to 2000 cm-1. From the first cycle in Figure 5a, MOF-74 characteristic peaks which can be indexed very well to previous reported results, 52 were observed when the as-synthesized Ni-Co MOF-74 was immersed in KOH solution initially. The signal intensities of the MOF-74 characteristic peaks (highlighted in magenta) decreased over time, while new peaks centered at ~510, 1350, and 1610 cm-1 (highlighted in turquoise) appeared after immersion in KOH solution for ~1 h, indicating the formation of new phase. The emerging 65Ni-MDH characteristic peaks demonstrate relatively steady intensities pre/post hydrothermal process as well as after 5,000 cycles (Figure S12), proving the stability of the as-synthesized 65Ni-MDH in KOH electrolyte. The magnified spectrum of the peak marked by four-point star shown in Figure 5a was acquired through integrating of 10 cycles from 200 to 800 cm-1 after in situ Raman measurement. Two Raman bands can be clearly observed and match very well with Raman spectrum of previously reported γ-Ni hydroxide, 53 which is actually the oxidized form of α-Ni hydroxide with interlayer species existed. These two bands ν1 (~550 cm-1) and ν2 (~485 cm-1) could be assigned as the A1g mode and Eg mode of the Ni-Co hydroxide framework, respectively. To further analyze the emerging bands at 1350 and 1610 cm-1 (marked by five-point stars), which are generally regarded as carbon-related species in Raman spectrum, Fourier transform infrared spectroscopy (FTIR) analysis was performed with the typical results shown in Figure 5b. The results further indicate the highly functionalized feature of 65Ni-MDH, where the absorption band at 519 cm-1 which is corresponding to the Raman peak at 510 cm-1 could be assigned to the O-H bending vibration; the broad band at 3450 cm-1 is the typical O-H stretching vibration; both of which prove the existence of hydroxyl in 65Ni-MDH. 54 Other peaks centered at ~1609, 1413, and 1244 cm-1 in the FTIR spectrum are in agreement with the five-point star peaks in Raman, which indicate the presence of C=C (overlapped with C=O) stretching vibration, C-OH bending vibration, and C-O stretching vibration, respectively. To

Figure 4. (a) CV curves of the 65Ni-MDH//N-C device in a voltage window of 0-1.7 V at various scan rates from 5 to 50 mV s-1, (b) Cycling stability and Faradaic efficiency of the 65Ni-MDH//N-C device at 13.5 A g-1 for 10,000 cycles (insets are photograph and schematic of as-fabricated HS full cell). The cycling performance and Faradaic efficiency of the 65Ni-MDH//N-C device at a high current density of 13.5 A g-1 is shown in Figure 4b. More than 95% of its initial capacitance was retained after 5,000 cycles, and the capacitance retention of 91.3% was achieved even after 10,000 cycles, and the FE of the electrochemical storage process during the cycling process is close to 100%. The long-term cycling stability, which has always been a problem for previously reported Ni-Co hydroxide-based energy storage material, is significantly optimized in case of the 65Ni-MDH//N-C device. 8,31 Figure S11 presents the Ragone plot (energy density vs. power density) for the 65Ni-MDH//N-C device which shows a high energy density of 81 W h kg-1 at a power density of 1.9 kW kg-1 and 42 W h kg-1

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summarize, 65Ni-MDH is supposed to be γ-Ni-Co hydroxide with interlayer functional groups (hydroxyl, carboxyl, etc.) from the in situ Raman and FTIR results.

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promising electrochemical performance with excellent cycling stability (90.1% capacity retention after 5,000 cycles at 20 A g-1). A device composed of a 65Ni-MDH-based positive electrode and a nitrogen-doped porous carbon negative electrode demonstrated an energy density of 81 W h kg-1 at a power density of 1.9 kW kg-1 while exhibiting even better cycling stability with a capacitance retention of 91.3% after 10,000 cycles at 13.5 A g-1. From further analysis like in situ Raman, the MDH was studied thoroughly and revealed to be γ-Ni-Cohydroxides with interlayer species, which might contribute to the high surface area and ultimately helped enhancing the long-time cycling stability. This work not only provides a new method of synthesizing advanced energy storage material with highly porous structure, which could be extended to other transition metal-based classic MOFs, but also demonstrates a new MOF-as- sacrificial template strategy to synthesize battery-type material for highly stable energy storage applications.

ASSOCIATED CONTENT Supporting Information. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Experimental section, SEM images, XRD, XPS, N2 adsorptiondesorption isotherms, ex situ Raman spectra, and extra electrochemical experiment results (CV, GCD curves, and Ragone plot).

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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Figure 5. (a) In situ Raman spectra of 65Ni-MDH during the synthesis process and the high resolution Raman spectrum of the peak marked by four-point star, (b) FTIR spectrum of 65Ni-MDH.

This work was supported by the National Natural Science Foundation of China (No. 21371014), National Program for Support of Top-notch Young Professionals, Changjiang Scholar Program, and Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200). C. Q. acknowledges the financial support of a scholarship from the China Scholarship Council (CSC).

After all, there are several factors that may contribute to the high performance of the MDH. First, the pore size distribution of MDH is optimized by developing considerable meso-pores (as transport channels) and micro-pores (as active sites). 4,19 Then, the transition metal ion in the hierarchically porous MDH structure will be utilized efficiently for faradaic contribution. Second, the presence of rich functional groups engaged between the layers contribute significantly to the high surface area of the derived MDH structure. Third, the size of MDHs are reduced compared to the initial micron-sized MOF-74, which is more favorable for fast energy storage.

REFERENCES (1) Wang, G.; Zhang, L.; Zhang, J. A review of electrode materials for electrochemical supercapacitors. Chem. Soc. Rev. 2012, 41, 797828. (2) Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors. Nat. Mater. 2008, 7, 845-854. (3) Zhao, B.; Ran, R.; Liu, M.; Shao, Z. A comprehensive review of Li4Ti5O12-based electrodes for lithium-ion batteries: The latest advancements and future perspectives. Mater. Sci. Eng., R 2015, 98, 1-71. (4) Li, B.; Dai, F.; Xiao, Q.; Yang, L.; Shen, J.; Zhang, C.; Cai, M. Nitrogen-doped activated carbon for a high energy hybrid supercapacitor. Energy Environ. Sci. 2016, 9, 102-106. (5) Chen, Z.; Wen, J.; Yan, C.; Rice, L.; Sohn, H.; Shen, M.; Cai, M.; Dunn, B. Lu, Y. High-performance supercapacitors based on hierarchically porous graphite particles. Adv. Energy Mater. 2011, 1, 551-556.

In summary, novel Ni-Co MOF-74 derived metal functionalized hydroxides were successfully synthesized via a facile method. These hydroxides inherited the high porosity of parent MOFs and exhibited high surface area of 299 m2 g-1. By using these MOF derived battery-type MDHs as supercapacitor electrodes and altering the initial Ni(II): Co(II) mole ratio, it is found that the 65Ni-MDH sample exhibited the most

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