Ultrathin Two-Dimensional MetalâOrganic Framework Nanosheets

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Ultrathin Two Dimensional Metal Organic Frameworks Nanosheets with the Inherent Open Active Sites as Electrocatalysts in Aprotic Li-O2 Batteries Mengwei Yuan, Rui Wang, Wenbo Fu, Liu Lin, Zemin Sun, Xinggui Long, Shuting Zhang, Caiyun Nan, Genban Sun, Huifeng Li, and Shulan Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21808 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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ACS Applied Materials & Interfaces

Ultrathin Two Dimensional Metal Organic Frameworks Nanosheets with the Inherent Open Active Sites as Electrocatalysts in Aprotic Li-O2 Batteries Mengwei Yuan,a Rui Wang,a Wenbo Fu,b Liu Lin,a Zemin Sun,a Xinggui Long,b Shuting Zhang,a Caiyun Nan,a Genban Sun,a,* Huifeng Li,a,* and Shulan Ma a,* aBeijing

Key Laboratory of Energy Conversion and Storage Materials, College of

Chemistry, Beijing Normal University, Beijing 100875, China bInstitute

of Nuclear Physics and Chemistry, China Academy of Engineering Physics,

Mianshan Road 64, Mianyang, Sichuan, 621900, China KEYWORDS：Metal–organic framework, nanosheets, electrocatalyst, Li-O2 batteries Abstract Ultrathin two dimensional metal organic frameworks (2D MOFs) have the potential to improve the performance of Li−O2 batteries with high O2 accessibility, open catalytic active sites and large surface area. To obtain highly efficient cathode catalysts for aprotic Li−O2 batteries, a facile ultrasonicated method has been developed to synthesize three kinds of 2D MOFs (2D Co-MOF, Ni-MOF and Mn-MOF). Contributing from the inherent open active sites of Mn-O framework, the discharge specific capacity of 9464 mAh g-1 is achieved with the 2D Mn-MOF cathode, higher than those of the 2D Co-MOF and Ni-MOF cathodes. During the cycling test, 2D Mn-MOF cathode stably operates more than 200 cycles at 100 mA g−1 with a curtailed discharge capacity of 1000 mAh g−1, quite longer than those of other. According to the further electrochemical analysis, we observe that the 2D

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Mn-MOF outperforms 2D Ni-MOF and Co-MOF due to a superior ORR and OER activity, in particular the efficient oxidation of both LiOH and Li2O2. The present study provides new insights that the 2D MOF nanosheets can be well applications as the Li−O2 cells with high energy density and long cycling life.

Introduction Nowadays, rechargeable Li−O2 batteries are attracting more attentions as one of the promising next energy conversion and storage devices,1-2 owing to the super high theoretical energy density (11400 Wh kg-1) of Li-O2 battery beyond the limitation of conventional Li-ion battery. However, there are still many challenges for their practical application.3 Many side reactions in Li-O2 battery, resulting from the reactive oxygen-containing radical and carbon instabilities, seriously limit the cyclability of the battery.4-6 Employing electrocatalysts to promote the battery reactions, including the oxygen reduction reactions (ORR) and oxygen evolution reactions (OER), is an efficient strategy for addressing these challenges.3 In recent years, precious metals (such as Ru,7-9 Ir,10 Au,2, 11Pd,12-13 and Pt14), transition metal oxides (such as MnO2,15-16 Co3O4,17-18 NiO,19 NiFe2O4,20 and MnCo2O421-22), transition metal alloys (FeCo23, FexNiy24), metal nitrides (Co4N25), metal carbides (Mo2C26, NiZnC27) and sulfides (MoS228, Co3S429) have been widely investigated. MOFs consist of organic linkers and metal clusters as secondary building units. MOFs could be endowed with different characteristics such as catalytic, separation and adsorption, via controlling the metal ions and the ligands.30-31 Due to the special structural characteristics, MOFs could provide the potentials of both homogeneous


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and heterogeneous catalysts. Also, the inherent transportation pathways and uniform porous structure endow the MOFs with high gas accessibility, diffusibility and possible catalytic activity in the backbone. Based on these advantages, currently, MOFs and their derivatives have been considered as promising platforms for enhancing performance of the electrochemical energy storage and conversion.32-35 Ultrathin 2D MOFs nanosheets not only inherit the characteristics of the bulk MOFs, but also possess high percentage of exposed metal atoms on the surface, which are the coordinatively unsaturated metal sites with more dangling bonds, being significant for electrocatalysis.31,

36

Analogously to that of inorganic metal catalysts, nano-sized

porous MOFs possess readily open active sites and the improved conductivity. The ultrathin 2D MOFs with the nanometer thickness promote the fast electrons transfer,31, 37-39

allow the rapid mass transport and ensure the close contact between the catalyst

and electrolyte.40-41 Additionally, open metal sites in air electrodes could be potential active sites for ORR and OER during the battery cycling. Considering these advantages, 2D MOFs may offer a great opportunity as promising electrocatalyts towards the ORR and OER reactions. There are some reports for the bulk M-MOF-74 (M = Mn, Co)42 and Ni-MOFs33 applied to aprotic Li−O2 batteries as oxygen cathodes. But the application of ultrathin 2D MOFs in Li-O2 batteries is still a blank. The brilliant results of ultrathin 2D MOFs for ORR and OER36, 43 portend a quite amazing accomplishment in Li-O2 batteries. In this work, we have synthesized series of 2D MOFs to be used as cathode electrocatalysts for aprotic Li−O2 batteries for the first time. The facile ultrasonic



method endowed the MOFs with an expected ultrathin structure. The battery performance of 2D Mn-MOF outperformed the 2D Ni-MOF and Co-MOF. The 2D Mn-MOF cathode provided a higher initial discharge specific capacity (9464 mAh g−1) and superior cyclability (more than 200 cycles) with low a limited capacity of 1000 mA h g−1. Through the concrete analysis for discharge and charge product, we observed the origin of the excellent performance of 2D Mn-MOF. The Mn-O clusters illustrated the highly electrocatalytic activity for the decomposition of Li2O2 and LiOH in 2D Mn-MOF, enhancing reversibility and efficiency for the Li−O2 batteries. By contrast, 2D Ni-MOF and 2D Co-MOF with lower catalytic activity for ORR and oxidation of LiOH and Li2O2 in OER showed relatively poor cyclability. Due to the bifunctional reaction of Mn-O clusters, the 2D Mn-MOF demonstrated significantly improved property.

Experimental section Preparation of the 2D MOFs. The 2D MOFs were prepared via a modified method as previous literature reported by Qiao et al.36 32 mL N, N-dimethylformamide (DMF), 2 mL ethanol (EtOH) and 2 mL deionized water (DIW) were firstly mixed to form a homogeneous solution. Then, 0.75 mmol BDC and 0.75 mmol MnCl2·4H2O were added into the above solution, respectively, stirring for 30 min. Then 0.8 mL triethylaine (TEA) was injected into the solution. Afterwards, the mixture was continuously ultrasonicated for 8 h under ambient conditions. The 3D Mn-MOF was synthesized by replacing the ultrasonicated process with stewing. The resulting product was collected via centrifugation, washed


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with ethanol for several times, and dried under vacuum at 60 oC for 24 h. The 2D Co-MOF and Ni-MOF were prepared through a similar process. 32 mL DMF, 2 mL EtOH and 2 mL DIW were mixed to form a homogeneous solution. Then, 0.75 mmol BDC and 0.75 mmol CoCl2·6H2O or NiCl2·6H2O were added into the above solution, and stirred for 30 min. And then 0.8 mL TEA was injected into the solution. The mixture was continuously ultrasonicated for 8 h. Finally, the as-prepared samples were collected and washed with ethanol and water several times, and dried under vacuum at 60 oC for 24 h. Materials Characterization Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50 spectrometer, Thermo Fisher Scientific Co.) was used to investigate the chemical bonding of the as-prepared 2D MOFs. The structure data was collected on X-ray powder diffraction (XRD) (Phillips X’pertProMPD) using Cu Kα radiation with the generator 40 mA and 40 mV. The morphology of the as-prepared materials and oxygen cathodes states were observed via field emission scanning electron microscopy (FESEM, an acceleration voltage of 5 kV, SU-8010, Hitachi). The microstructure was observed with a transmission electronmicroscope (TEM, an acceleration voltage of 200 kV, JEM-2010, JEOL). The surface analysis was conducted by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250Xi spectrometer (Thermo Fisher). The powder electronic conductivity of 2D MOFs was measured via a four contact method. The samples were pressed to disk at 4 MPa to measure the resistance with a Keithley 2400 digital multimeter in four-wire mode. And the corresponding electronic



conductivity was calculated based on the resistance and the size of the disk. The water source of MOFs was investigated by thermogravimetric analysis (TGA/DSC 1, MettlerToledo Co.). Raman spectra were collected on a microscopic confocal Raman spectrometer (LabRAMAramis, Horiba JobinYivon) with a 532 nm laser. The electrolytes during cycles were collected by washing the glass fiber filter separators with DMSO, and subjected to 1H NMR (Bruker, 500 MHz). AFM (Nanoscope VIII MultiMode, Bruker) measurements were taken in the tapping mode. Li-O2 Batteries Assembly and Electrochemical Performance Tests The oxygen cathodes were fabricated by pasting the slurry on a current collector (carbon paper from Torray). The slurry consisted of 45 wt % 2D/3D MOFs, 45 wt % Ketjen Black (KB), 10 wt % polyvinylidenefluoride (PVDF), which were dispersed in N-mehtyl-2-pyrrolodone. Then the oxygen cathodes were dried thoroughly in a 100 °C vacuum oven for 12 h to remove the residual solvent. The cathodes preloaded with Li2O2 and LiOH were prepared via a similar approach. The composition of these electrodes were 2D MOFs/KB/PVDF/Li2O2(LiOH)=3:3:2:2. Firstly, the 2D MOFs, KB and Li2O2(LiOH) were added into the agate mortar and they were mixed as uniform as possible via long time to grind (~ 20 min). The PVDF (concentration, 1%) with certain mass was injected into the above powder and grinded swiftly. After 3 minutes, the slurry was pasted on the carbon paper (φ14 mm). Finally, the carbon paper was treated with 100 oC at vaccum to remove the residual NMP. All performance of the electrodes were evaluated in CR2032 coin-type battery. The batteries were assembled in an argon-filled glovebox (water and oxygen contents

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