2D MOF Nanoflake-Assembled Spherical Microstructures for Enhanced Supercapacitor and Electrocatalysis Performances
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Metal–organic frameworks (MOFs) are of great interest as potential electrochemically active materials. However, few studies have been conducted into understanding whether control of the shape and components of MOFs can optimize their electrochemical performances due to the rational realization of their shapes. Component control of MOFs remains a significant challenge. Herein, we demonstrate a solvothermal method to realize nanostructure engineering of 2D nanoflake MOFs. The hollow structures with Ni/Co- and Ni-MOF (denoted as Ni/Co-MOF nanoflakes and Ni-MOF nanoflakes) were assembled for their electrochemical performance optimizations in supercapacitors and in the oxygen reduction reaction (ORR). As a result, the Ni/Co-MOF nanoflakes exhibited remarkably enhanced performance with a specific capacitance of 530.4 F g−1 at 0.5 A g−1 in 1 M LiOH aqueous solution, much higher than that of Ni-MOF (306.8 F g−1) and ZIF-67 (168.3 F g−1), a good rate capability, and a robust cycling performance with no capacity fading after 2000 cycles. Ni/Co-MOF nanoflakes also showed improved electrocatalytic performance for the ORR compared to Ni-MOF and ZIF-67. The present work highlights the significant role of tuning 2D nanoflake ensembles of Ni/Co-MOF in accelerating electron and charge transportation for optimizing energy storage and conversion devices.
KeywordsMetal–organic frameworks Nanoflakes Spherical microstructure Supercapacitor Oxygen reduction reaction
A solvothermal method was used to improve the conductivity and electrochemical activity of metal–organic framework (MOF) materials by tuning their morphology and components.
Ni/Co-MOF nanoflakes exhibit remarkably enhanced performances including enhanced electrocatalytic performance for the oxygen reduction reaction.
The synthetic strategy driven by rational design gives the first example of exploring MOF-derived nanomaterials to achieve improved efficiency energy storage and conversion devices.
The design and building of metal–organic frameworks (MOFs) with controllable structures have received significant attention due to their wide range of applications such as in gas storage and separation [1, 2, 3, 4], optoelectronics and energy storage [5, 6, 7, 8, 9, 10], catalysis [11, 12, 13, 14, 15, 16, 17], and drug delivery and imaging [18, 19, 20]. With their exceptionally large surface area, abundant micropores, and variable sites for redox reactions, MOFs are considered advanced promising electrode materials for electrochemical energy storage and conversion devices such as batteries, supercapacitors, and fuel cells [8, 21, 22, 23, 24, 25, 26, 27]. The biggest problem facing individual MOFs is that they suffer from relatively low conductivity and poor electrolyte ion transport behavior, thereby restricting their efficiency for energy storage and conversion. A general approach to addressing this issue is to apply MOFs as sacrificial templates to generate porous carbon with metal or metal oxides, which can afford high conductivity and electrochemical reactivity. While high-temperature treatment inevitably results in cost increase, the intricate porous structure of MOFs cannot be employed generally [22, 28, 29, 30, 31]. The direct application of a series of MOFs as electrode materials for supercapacitors, presented in Yaghi and co-workers’ pioneering study, seems promising , but sluggish kinetics and poor long-term stability for electrochemical capacitors and electrochemical catalysis greatly limit their utilization.
Recent studies revealed that two-dimensional (2D) nanomaterials with short pathways for mass transport and multiple metallic ions create an opportunity for energy storage and conversion devices relative to their counterparts with other dimensionalities. The electron confinement in two dimensions of the ultrathin 2D nanomaterials renders compelling electronic properties [33, 34, 35, 36]. Prominent examples include the MOF@graphene oxide designed for the lithium-sulfur battery that functioned as a battery separator to selectively sieve Li+ ions while blocking polysulfides , the 2D porphyrin paddlewheel framework-3 (PPF-3) nanoflake successfully used as an electrode for a supercapacitor , the 2D metal oxide/hydroxide graphene nanohybrids that exhibited outstanding catalytic behavior for the oxygen reduction reaction (ORR) , and metal-nitrogen-containing mesoporous carbon/graphene nanoflakes exhibiting enhanced ORR performance . Despite the fact that these state-of-the-art 2D nanoarchitectures show great potential in optimizing energy storage and catalysis, they usually show limited performance because of the following two issues: (I) most MOFs possess micropores with diameters less than 2 nm, thus blocking the transport of atoms, ions, and large molecules. Therefore, the coexistence of micropore–mesopore–macropores in MOFs is highly desired; and (II) the reactivity of MOFs on the pseudocapacitance and catalytic performance should be enhanced by optimizing the valence variability of the metallic ions as redox centers. In this regard, it will be of great significance to develop MOFs with multiple structures that consist of hierarchical pores and 2D nanoflakes, and also a 3D large interconnected network that can afford efficient charge, mass exchange, and low internal resistance.
Herein, we report a facile strategy to synthesize 2D MOF nanoflake-assembled spherical microstructures composed of ultrathin Ni/Co- and Ni-imidazolate framework nanoflakes as subunits (simplified as Ni/Co-MOFs nanoflakes). This unique superstructure with 3D accessible sites, maximized surface area, and synergistic effect of dual metallic (Ni3+/Ni2+ and Co3+/Co2+) ions is highly beneficial in enhancing electrochemical storage and conversion. As a result, the Ni/Co-MOFs nanoflakes exhibit remarkable performances with a specific capacitance of 530.4 F g−1 at 0.5 A g−1 in 1 M LiOH aqueous solution, 1.72 and 3.15 times higher than that of Ni-MOF nanoflakes (306.8 F g−1) and ZIF-67 (168.3 F g−1), respectively; good rate capability and robust cycling performance with no capacity fading after 2000 cycles. Additionally, Ni/Co-MOF nanoflakes show higher electrocatalytic activity for the ORR than Ni-MOF nanoflakes and ZIF-67. The smart synthetic strategy employing rational design gives the first example of exploring MOF-derived nanomaterials in achieving more efficient energy storage and conversion devices.
3 Materials and Methods
Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was purchased from Zhengzhou Chemical Reagents Co., Ltd. Anhydrous methanol was purchased from Tianjin Chemical Reagents Co., Ltd. 2-Methylimidazole (MW = 82.10, C4H6N2) was purchased from Sigma-Aldrich. Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O) was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals used were of analytical grade and used without further purification.
3.2 Synthesis of ZIF-67
ZiF-67 was synthesized according to the previous literature . A solution of 2-methylimidazole (7.5 mM, 15 mL) in methanol was slowly added to 15 mL of a Co(NO3)2·6H2O (1.9 mM) methanol solution using a syringe at room temperature. After ultrasonic irradiation for 15 min, the ZIF-67 nanocrystals were separated via centrifugation.
3.3 Synthesis of Ni-MOF Nanoflakes
Ni-MOF nanoflakes were synthesized by a solvothermal method. The as-prepared ZiF-67 nanocrystals were dispersed in 15 mL methanol followed by the addition of 15 mL Ni(NO3)2·6H2O methanol solution (1.9 mM). The above mixture was transferred to a Teflon-lined stainless-steel autoclave and kept at 120 °C for 1 h. Finally, the product was obtained by centrifugation, washed three times with methanol, and dried at 60 °C for 12 h.
3.4 Synthesis of Ni/Co-MOF Nanoflakes
As-prepared ZiF-67 nanocrystals were dispersed in 15 mL methanol followed by the addition of 15 mL methanol containing Ni (NO3)2·6H2O (0.95 mM) and Co(NO3)2·6H2O (0.95 mM). The product was obtained by centrifugation, washed three times with methanol, and dried at 60 °C for 12 h.
The morphologies of the samples were studied by field-emission scanning electron microscopy (FE-SEM, JEORJSM-6700F) and transmission electron microscopy (TEM, FEI Tecnai G2 20) with an accelerating voltage of 200 kV. Powder XRD patterns were collected using a Y-2000 X-ray diffractometer with copper K α radiation (λ = 1.5406 Å) at 40 kV and 40 mA. Fourier transform infrared (FTIR) spectra of the products were recorded on a TENSOR 27 FTIR spectrometer (Bruker) in the absorption mode with a resolution of 2 cm−1. The X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCA LAB 250 spectrometer using a focused monochromatic Al-K α line (1486.6 eV) X-ray beam with a diameter of 200 μm. Thermal gravimetric analysis (TGA) was conducted on an SMP/PF7548/MET/600 W instrument from 50 to 800 °C with a heating rate of 10 °C min−1 in a nitrogen atmosphere.
3.6 Electrochemical Measurements
3.6.1 Supercapacitor Measurements (Three-Electrode System)
The capacitance performances of the samples were evaluated with a three-electrode system on an electrochemical workstation (CHI 760E, CH Instrument, China) at room temperature. The suspension with active materials at a concentration of 1.0 mg mL−1 was prepared by sonicating 1 mg of active materials in 1 mL ethanol containing Nafion (Sigma-Aldrich, 5 wt%) at a volume ratio of 995:5. Then, 20 μL of ink was dropped onto a glassy carbon disk (diameter 5 mm) and dried thoroughly in air, resulting in a catalyst loading of 0.1 mg cm−2. The auxiliary and reference electrodes were Pt wire and Ag/AgCl, respectively. The electrochemical measurements were carried out in 1 M LiOH solution.
3.6.2 Oxygen Reduction Reaction (ORR)
To prepare the working electrode, 5 mg of catalyst and 5 mg carbon black (Alfa Aesar, 99.9+ wt%) were dispersed in a mixture of 950 μL ethanol and 50 μL Nafion (Sigma-Aldrich, 5 wt%) under sonication for 30 min to obtain a homogeneous slurry. Then, 8 μL of this catalyst ink was loaded onto a glassy carbon rotating disk electrode of diameter 5 mm, resulting in the catalyst loading of 0.2 mg cm−2. The electrode was dried under dissolvent conditions for 5 h.
Electrochemical impedance spectral measurements were carried out in the frequency range from 100 kHz to 10 mHz on a CHI 760E electrochemical workstation. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements (Pine Research Instruments, USA) were conducted using a standard three-electrode system. The catalyst-coated glassy carbon electrode, an Ag/AgCl electrode in saturated KCl solution, and Pt wire were used as the working, reference, and counter electrodes, respectively. The electrolyte was 0.1 M potassium hydroxide (KOH) aqueous solution. The potential measured against the Ag/AgCl electrode was converted to the potential versus the reversible hydrogen electrode (RHE) according to E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 + 0.059 pH. All measurements were carried out at room temperature.
4 Results and Discussion
The TGA data of Ni/Co-MOF and Ni-MOF nanoflakes were collected under air at the heating rate of 10 °C min−1 from room temperature to 800 °C. The slight weight loss (<8 wt%) before 220 °C is attributed to water that is bonded to the imidazole group of the MOF framework via H-bonds. The typical weight loss of Ni/Co-MOF and Ni-MOF nanoflakes calculated from the TGA data (Fig. 2c) was determined to be 31.26 and 32.07 wt% at 250 and 220 °C, respectively, attributed to the removal of the organic ligands. FTIR spectra of Ni/Co-MOF and Ni-MOF nanoflakes were measured to identify the surface functional groups and observe the formation of coordinated polymers. As shown in Fig. 2d, the FTIR spectrum has been studied extensively and every absorption peak was assigned to its corresponding vibration. The characterization peaks at 2923 and 584 cm−1 were attributed to the aliphatic C–H stretch and the C=N stretching vibrations of 2-methylimidazole. Therefore, the stretching of aliphatic C–H and vibration of C=N in 2-methylimidazole at 624 and 3100 cm−1 shifted to 584 and 2923 cm−1 after assembling Ni/Co-MOF or Ni-MOF, indicating the strong interaction of C=N and C–H groups in 2-methylimidazole with the Ni2+ or Co2+ ions . Based on the above analysis, the linker between the Ni2+ and Co2+ and 2-methylimidazole remained unchanged during phase transformation.
The results from the electrochemical performances for the supercapacitor and ORR studies indicate that the unique structure of the Ni/Co-MOF nanoflakes plays a role in optimizing the electrochemical performance of MOF materials. Significantly, the spherical microstructure assembled by interconnected MOF nanoflakes networks offers a continuous pathway for mass transportation, and the hollow nanocage formed in the textures may act as an ion-buffering reservoir for promoting transport of the electrolyte ion and oxygen, which bring more effective transmission (compared to the microporous only ZIF-67) leading to a high electrochemical performance for supercapacitors and the ORR. The synergistic effect of dual metallic (Ni3+/Ni2+ and Co3+/Co2+) ions in Ni/Co-MOF nanoflakes raised the reaction activity during the energy storage and conversion, thereby optimizing electron and charge transportation and accelerating the reaction kinetics (enhancing the activity). The enhanced conductivity of Ni/Co-MOF can be further demonstrated by the Nyquist plots. As shown in Fig. S5, Ni/Co-MOF exhibited a lower resistance (4.0 Ω) than ZIF-67 (8.5 Ω) and Ni-MOF (6.2 Ω), suggesting a higher conductivity of the Ni/Co-MOF that is probably attributed to electron hopping between cations with different valences [46, 51]. Moreover, the spherical microstructure of Ni/Co-MOF nanoflakes provides favorable layer-by-layer assembled nanoflakes, which effectively prevent the collapse of the nanoflakes (enhancing the stability). Additionally, to investigate the cation substitution effect, we replaced Ni2+ with Fe2+ ions in the structure; solid spheres were obtained instead of the nanosheet morphology, and they showed poor electrochemical performance (not shown here). Therefore, the choice of the Ni3+/Co2+ couple may be suitable for constructing a high-efficiency electrode. Accordingly, the Ni/Co-MOF nanoflakes exhibit better electrochemical performances for supercapacitors and the ORR than Ni-MOF nanoflakes and ZIF-67.
We demonstrate an effective wet-chemical approach to achieve spherical hollow microstructures of assembled 2D Ni/Co-MOF nanoflakes and Ni-MOF nanoflakes. This approach leads to favorable Ni/Co-MOF nanoflake spherical microstructures with many exposed active sites. When applied in supercapacitors and the ORR, the Ni/Co-MOF exhibits remarkable performances with a specific capacitance of 530.4 F g−1 at 0.5 A g−1 (higher than that of Ni-MOF (306.8 F g−1) and ZIF-67 (168.3 F g−1)), good rate capability, and robust cycling performance with no capacity fading after 2000 cycles. Besides, Ni/Co-MOF nanoflakes can be used as an advanced non-noble metal catalyst for the ORR with its excellent oxygen reduction catalytic activity, good durability, and good methanol tolerance. Taking their robust electrochemical performance in supercapacitors and ORR into account, our work provides a new concept to design and synthesize rationally tunable structures of 2D MOF nanoflakes to improve the electrochemical performance for energy storage and conversion.
This work was financially supported by the National Natural Science Foundation of China (Nos. 21571157, U1604123, and 51473149), Outstanding Young Talent Research Fund of Zhengzhou University (1521320001), and the Open Project Foundation of Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) (2017–29), Nankai University, and Open Project Foundation of Key Laboratory of Inorganic Synthesis and Preparation of Jilin University.
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