Bimetallic MetalâOrganic Frameworks as Efficient Cathode Catalysts

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Bimetallic Metal-Organic-Frameworks as Efficient Cathode Catalysts for Li-O2 Batteries Su Hyun Kim, Young Joo Lee, Do Hyung Kim, and Yun Jung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15499 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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ACS Applied Materials & Interfaces

Bimetallic Metal-Organic-Frameworks as Efficient Cathode Catalysts for Li-O2 Batteries Su Hyun Kim, Young Joo Lee, Do Hyung Kim and Yun Jung Lee* Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea

KEYWORDS. Li-O2 battery, MOF-74, bimetallic MOF, bifunctional catalyst, Mn, Co.

ABSTRACT. Metal organic frameworks (MOFs) have the potential to improve the electrochemical performance of Li-O2 batteries with high O2 accessibility and catalytic activity of the open metal sites. Here, we explored bimetallic MnCo-MOF-74 as cathode catalysts in LiO2 batteries. MnCo-MOF-74 was synthesized with Mn to Co ratio of 1:4 by a simple hydrothermal reaction. Compared to monometallic Mn-MOF-74 and Co-MOF-74 with only single catalytic activity for LiOH formation or oxygen evolution reaction, bimetallic MnCoMOF-74 demonstrated a capability to facilitate improved reversibility and efficiency during both discharge and charge cycles. Benefitting from the porous structure of MOF as well as the complementary contribution from both Mn- and Co-metal clusters, MnCo-MOF-74

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outperformed Mn-MOF-74 and Co-MOF-74. A high full discharge capacity of 11150 mAh g-1 at 200 mA g-1 was achieved in MnCo-MOF-74. During the cycling test, MnCo-MOF-74 stably delivered a limited discharge capacity of 1000 mAh g-1 for 44 cycles at 200 mA g-1, which is remarkably longer than those of KB, Mn-MOF-74, and Co-MOF-74 with cycle lives of 8, 22, and 18 cycles, respectively.


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1. INTRODUCTION Rechargeable Li-O2 batteries have attracted attention as one of the promising next generation energy systems, owing to their high theoretical energy density beyond the limit of commercial Li-ion batteries (LIBs).1-3 However, there are still many challenges in the development of practically applicable Li-O2 batteries.4 Various side reactions due to the reactive oxygen radical5 and carbon instabilities6 still limit the energy efficiency and poor cyclability.7-8 Employing catalysts that promote oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER) is a representative research direction for addressing these challenges. Noble metal catalysts (Ru9, Ir10, and Pt11) are known to improve the cycle life and roundtrip efficiency of batteries, however, their stability in a Li-O2 system is still under debate. Moreover, the low economic feasibility of these precious metals has prompted researchers to develop cost-effective catalysts with acceptable performances. Transition metal oxides (MnO212, Co3O413, and NiO14) and bimetal composites (FeCo15, Ni@Co3O416, Pt3CO,17 and MnCo18) have been evaluated for this purpose. In most literature reporting the catalytic performances of these catalysts in Li-O2 battery cathodes, the working mechanism of the catalysts was speculated to facilitate formation and/or decomposition Li2O2 discharge products. Interestingly, some reports ascribed the improved performances, such as reduced charge overpotentials, increased capacity, and particularly, enhanced reversibility, to the decomposition of LiOH in dimethylethane (DME)19-21, tetraglyme (G4),22-23 and dimethyl sulfoxide (DMSO).24-25 For example, in a Ru/MnO2/SP system with a trace amount of water,24 the trace water reacts with Li2O2 generating H2O2 and LiOH, and MnO2 (electrolytic manganese dioxide (EMD), γ-MnO2) facilitates the regeneration of water (H2O2 → H2O + O2), aiding continuous formation of LiOH even with the initial trace amount of water.


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Owing to the excellent catalytic activity of Ru for LiOH decomposition, the cell showed significantly lowered charge potentials. These previous studies opened a new strategy for improving the cathode performance. Metal organic frameworks (MOFs) are promising platforms for improving the electrochemical performance of Li-O2 batteries with high O2 accessibility and possible catalytic activity of the open metal sites. MOFs composed of organic linkers and metal clusters as secondary building units (SBUs). By controlling the complex linkage of the metal and organic linkers, different MOFs could be designed to achieve different characteristics such as electrical conductivity, catalytic activity,26 adsorption,27-28 surface areas,29 and interaction at the open metal sites.30 MOF-74, as a highly porous (hexagonal) structure with one-dimensional (1D) channel, has many open sites on the channel surface that affect O2 adsorption. In addition, open metal sites of MOF74 in air electrodes could be potential catalysts for easy formation and decomposition of discharge products during ORR and OER. M-MOF-74 (M: transition metals) was previously applied to Li-O2 batteries as air cathodes and Mn-MOF-74 demonstrated the highest discharge capacity compared to M-MOF-74 (M = Co, Mg), MOF-5, HKUST-1, and Super P.31 In addition, there are literatures on the performance of Mn, Co, Ni, or their bimetallic combination in MOF.32-35 In this study, we synthesized bimetallic MnCo-MOF-74 materials to be used as cathode catalysts in Li-O2 batteries. A simple hydrothermal method was applied for the synthesis of monometallic Mn- and co-MOF-74 and bimetallic MnCo-MOF-74. The bimetallic MnCo-MOF74 cathodes provided a high full discharge capacity (11150 mAh g-1) and excellent cyclability (44 cycles) with low overpotential at a limited capacity of 1000 mA h g-1. MnCo-MOF-74 outperformed the single metal counterparts, Mn- and Co-MOF-74. Through the discharge


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product analysis and performance analysis in a galvanostatic electrochemical test, we identified the origin of the superior performance of MnCo-MOF-74. Mn and Co metal clusters act complementarily MnCo-MOF-74 leading to improved reversibility and efficiency of the Li-O2 battery cathodes. Mn-metal clusters transformed Li2O2 to LiOH, and Co-metal clusters efficiently decomposed LiOH. In contrast, monometallic Mn-MOF-74 and Co-MOF-74 with only single catalytic activity for LiOH formation or OER showed relatively poor reversibility. Owing to the synergistic integration of Mn- and Co-metal clusters, MnCo-MOF-74 demonstrated significantly improved property.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis All reagents were purchased from Sigma-Aldrich. The bimetallic organic framework composites (MnCo-MOF-74) were synthesized following a modified method reported by Li et al 31

and Wang et al.34 Manganese(II) chloride tetrahydrate (0.6485 g, 3.28 mmol) and cobalt(II)

nitrate hexahydrate (0.75 g, 2.58 mmol) were dissolved in 60 mL solution mixture of deionized water (D.I.) and ethanol (1:1, by volume). A solution of 2, 5-dihydroxyterephthalic acid (H4DOBDC, 333 mg, 1.68 mmol) in 30 mL of N, N-dimethylformamide (DMF) was added dropwise to the above solution and mixed under stirring for 30 min. The solvent composition in the resulting solution was DMF: DI: EtOH = 1:1:1 by volume. The solution was heated for 24 h at 135 °C in 180 mL Teflon-lined autoclave. The precipitated products were washed thrice with DMF via centrifugation and immersed in methanol for three days, periodically changing methanol several times. Finally, solid products were obtained after drying under vacuum at 80 °C for 24 h. Mn-MOF-74 and Co-MOF-74 were synthesized following a similar procedure with


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only one metal precursor. For obtaining Co-MOF-74, cobalt(II) nitrate hexahydrate (0.75 g, 2.5 mmol) was used and the solvent composition of the final solution was the same as that for MnCo-MOF-74. For Mn-MOF-74, manganese(II) chloride tetrahydrate (1.098 g, 5.54 mmol) was used, and the solvent composition of the final solution was DMF: DI : EtOH = 15:1:1 by volume.34

2.2. Characterization The chemical bonding of the M-MOF-74 materials was analyzed by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50 spectrometer, Thermo Fisher Scientific Co.). The crystal phase of powders and electrodes was examined by high resolution X-ray diffraction (HR-XRD, 9k, SmartLab, Rigaku) using Cu-Kα radiation, in the 2θ range of 5.0 - 60.0° at scan rate of 1° min-1 in steps of 0.02°. The morphologies of the synthesized materials and surfaces of the air cathodes were observed by a scanning electron microscope (SEM, Nova Nano SEM 450) and the elemental mapping of MnCo-MOF-74 was conducted by energy dispersive X-ray spectroscopic analysis (EDS) coupled with SEM. The synthesized materials and cathodes after discharge and recharge were characterized by X-ray photoelectron spectroscopy (XPS, Theta probe base system, Thermo Fisher Scientific Co.). The water source of M-MOF-74 was confirmed by Thermogravimetric analysis (TGA/DSC 1, METTLER TOLEDO Co). The electrochemical stability of M-MOF-74 was evaluated by cyclic voltammetry (CV) under Ar atmosphere (Ivium N stat, Ivium Technollogies Co.).

2.3. Li-O2 cell assembly and Electrochemical tests


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The cathodes were prepared by casting a slurry of the material on a stainless steel mesh. The slurry consisted of 40 wt% synthesized-materials as catalyst, 40 wt% KB (or pristine KB used at 80 wt%) and 20 wt% polyvinylidenefluoride (PVDF) dispersed in N-methyl-2-pyrrolidone (NMP). The casted air cathodes were dried in 80 °C vacuum oven for 24 h. The coin-type Li-O2 batteries (R2032) were assembled in an argon-filled glove box with water and oxygen contents of less than 0.1 ppm. The Li-O2 batteries were assembled with a glass fiber separator (GF/D, Whatman), a lithium foil anode (thickness, 500 µm), and 300 µm of 1.0 M bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) in tetraethylene glycol dimethylether (TEGDME) electrolyte. After cell assembly, the cells were stabilized for 3 h at room temperature in an oxygen-filled chamber. All the cell performance evaluations were based on the weight of carbon black (KB) with a total loading of 0.5–0.8 mg cm-2. The full charge-discharge profiles of Li-O2 cells were tested in the voltage window from 2.3 V to 4.5 V and at a rate of 200 mA g-1. The cycling test was conducted at a limited capacity of 1000 mA h g-1 at a rate of 200 mA g-1. The LiOH-preloaded electrodes were prepared by making free-standing PTFE-bound sheets of active materials. To maximize catalytic decomposition of LiOH by M-MOF-74, the electrodes were prepared with composition of M-MOF-74: Carbon: PTFE: LiOH = 6: 3: 1.5: 1.5. The total mass of electrode was 13.4 mg. All performance evaluations were based on the weight of LiOH (1.675 mg). The LiOH-preloaded electrodes were charged at 0.1 mA for 18 h. The decomposition efficiency of LiOH for OER is the ratio of the charge capacity to theoretical capacity of LiOH (1119.05 mAh g-1) loaded. The Li2O2-preloaded electrodes were prepared similarly. The decomposition efficiency for Li2O2 was calculated by the theoretical capacity of Li2O2 (1168.3 mAh g-1) based on the weight of Li2O2 (1.238 mg) with a total loading of 9.9 mg.


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3. RESULTS AND DISCUSSION 3.1 Characterization of synthesized M-MOF-74 During the synthesis of M-MOF-74, Mn2+ and Co2+ ions in DMF/H2O/EtOH solvent combined with H4DOBDC to form a 1D metallic-organic polymer as an intermediate phase. The hydrothermal treatment transformed this intermediate phase into the three-dimensional (3D) structure of MOF-74. In the resulting M-MOF-74, metal ions are coordinated with carboxylates. The COO− coordinated to the metal cluster in MOF-74 gives rise to characteristic asymmetric and symmetric vibration modes in FT-IR.36 To confirm the successful synthesis of M-MOF-74, the synthesized products were investigated by FT-IR spectroscopy. The absorption bands at 1550 and ~1380–1404 cm-1 in Figure 1a coincide with the reported characteristic vibration modes of COO− in MOF-74. The crystal structure of the synthesized materials was identified by XRD analysis (Figure 1b). All XRD patterns of M-MOF-74 synthesized in this study were almost identical to the previously reported XRD pattern of Zn-MOF-74.37 All diffraction patterns have two main peaks around 2θ = 6.8 ° and 11.8 ° corresponding to (210) and (300) planes of the MOF-74 crystal, respectively.33 The oxidation state and bonding characteristic of the metal ions in the synthesized M-MOF-74 was analyzed by XPS. The XPS survey scan spectra (Figure S1) reveal the constituent elements for Mn-MOF-74, Co-MOF-74, and MnCo-MOF-74. The designed metal ions are successfully incorporated into the corresponding M-MOF-74. Figure 1c shows the high-resolution Mn 2p XPS spectra of the synthesized M-MOF-74. Mn-MOF-74 provides two well-defined peaks at 653.6 eV and 641.4 eV, corresponding to Mn 2p1/2 and Mn 2p3/2 of Mn2+ in Mn-O bonds.34 In addition, the satellite peak of Mn 2p3/2 at 645.6 eV also indicates the presence of Mn2+ in Mn-O


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bonds.38-39 There is minor amount of Mn3+ state in Mn-MOF-74. We therefore concluded that the main oxidation state of Mn in Mn-MOF-74 is Mn2+ with minor quantities of Mn3+. Co 2p XPS spectrum of Co-MOF-74 in Figure 1d shows two characteristic peaks at 797.7 eV and 781.7 eV, corresponding to Co 2p1/2 and Co 2p3/2 of Co2+ in Co-O bonds. Satellite peaks of Co 2p1/2 and Co 2p3/2, corresponding to Co2+ are also observed. As in Mn-MOF-74, Co-MOF-74 also contains a minor peak of Co3+ at 779.2 eV.40 Thus, the predominant oxidation state of Co ion in Co-MOF74 is mainly Co2+ with minor Co3+. The oxidation states of Mn and Co in MnCo-MOF-74 are similar to those of Mn-MOF-74 and Co-MOF-74, respectively. This suggests that each metal cluster building block (Mn, and Co based) is maintained in MnCo-MOF-74 with the binding of metal ions and organic linkers. The morphology of M-MOF-74 was observed using SEM (Figure 2). All M-MOF-74 samples showed hexagonal crystallization with the crystal growing along one direction. The mixed bimetallic MnCo-MOF-74 was also successfully obtained with the morphology of the homogenous nanorod framework as those of Mn- and Co-MOF-74. The uniform distribution of Mn and Co metal ions in MnCo-MOF-74 was obvious in the EDS elemental mapping images. The ratio of Mn: Co in MnCo-MOF-74 was 1:4.24 (Figure S2). Although the molar amount of Mn was higher than that of Co in the precursor, the amount of Co incorporated into the final MnCo-MOF-74 is ~4 times higher than that of Mn. This implies that the efficiency of forming secondary building blocks with organic linkers is different for the two metal ions, and probably Co, ions more effectively form metal cluster-organic links.41 Synthetic conditions such as the relative amount of the precursors and solvent composition was varied to determine if the ratio of Mn to Co in the final MnCo-MOF-74 could be controlled. As shown in Figure S3 and S4,


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however, synthesis conditions other than the one reported in the experimental section failed to generate stable hexagonal rod-shaped morphology of MOF-74.

3.2 Analysis of the Discharge Products In order to characterize the discharge products in air cathodes employing M-MOF-74 catalysts, the cathodes were examined after the first discharge and recharge process (Figure 3). Figure 3a shows the XRD patterns of the discharged and recharged electrodes of KB only, Mn-MOF-74, Co-MOF-74, and MnCo-MOF-74 electrodes. On one hand, the crystalline discharge product of catalyst-free KB and Co-MOF-74 electrodes was Li2O2 with peaks at 2θ = 33°, 35°, and 58.9° observed in the XRD pattern. On the other hand, Mn-MOF-74 only showed peaks corresponding to LiOH with peaks at 2θ = 32.4°, 35.6°, 51.5° and 55.8°. The discharge products of MnCoMOF-74 cathodes were a mixture of Li2O2 and LiOH. The results of this XRD analysis indicate that the incorporation of the Mn-metal cluster in M-MOF-74 is related to the formation of LiOH. In previous literature, manganese oxide was reported to catalyze the conversion of Li2O2 to LiOH in the presence of water.24 The water content of the electrolyte in this study was carefully controlled below ~20 ppm, however, formation of LiOH in Mn-MOF-74 and MnCo-MOF-74 electrodes was consistently observed. To further characterize the discharged and recharged electrodes, XPS analysis was conducted (Figure 3b). The discharge products of KB electrodes in XPS appeared as a mixture of Li2O2 and Li2CO3, though the latter was not detectable in XRD, probably due to its amorphous nature. After the first recharge, Li2CO3 remained in the KB cathode. The remaining Li2CO3 would accumulate over cycles and lead to the deposition of an electrically passivating layer. On the contrary, the discharge product of Co-MOF-74 electrode is Li2O2, consistent with the XRD result, and it completely decomposed on recharge. This implies


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that Co-MOF-74 has a catalytic activity for OER, facilitating the decomposition of Li2O2. The discharge products of Mn-MOF-74 electrode were a mixture of Li2CO3 and LiOH, and LiOH remained after re-charge. While Mn in M-MOF-74 promotes the formation of LiOH, the catalytic activity of Mn for decomposition of LiOH seems inadequate for complete decomposition. For bimetallic MnCo-MOF-74, Li2O2 and LiOH were observed in the discharged electrode, as in the XRD result. Unlike Mn-MOF-74, however, the discharge products were almost completely decomposed during the charge cycle. The morphology of the discharged and recharged electrodes was observed using SEM (Figure 4). After the first discharge, the cathodes under study showed different morphologies of the discharge products. Catalyst-free KB cathode showed discharge products of film-like morphology. After recharge, the KB electrode almost recovered the original clean surface, however, a little residue was noticed, which is probably the remaining Li2CO3 detected in XPS. For Mn-MOF-74, discharge products formed crumpled sheet-like film, a typical morphology of LiOH.42 Some residues remained in the re-charged electrode of Mn-MOF-74. The discharge products of Co-MOF-74 were disc-shaped toroids. After recharge, the surface became relatively clean, consistent with the XPS result. In case of MnCo-MOF-74 electrode, large toroidal particles were observed in the discharged state. The recharged electrodes of MnCo-MOF-74 recovered a clean pristine state. According to previous reports from Kwabi et al.21 and Wu et al.23, the presence of water could lead to the formation of large toroidal Li2O2. The XRD and SEM analysis of Mn-containing MOF-74 (Mn-MOF-74 and MnCo-MOF-74) indicated the possibility of the presence of water in the system. Since the origin of water is not the electrolyte, we have considered other possible water sources. First, there could be water in M-MOF-74. Although M-MOF-74 was thoroughly dried, water could be persistently adsorbed in


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MOF owing to the high affinity of water to MOF. In the TGA of M-MOF-74 (Figure S5), the weight loss at temperatures below 100 ℃ is from the water removal. Water was detected, although in small quantities, in thoroughly dried M-MOF-74 (dried at 80 ℃ under vacuum for more than 2 days). This might be one source of water.30 Second and more important, the ability of MnO2 to decompose H2O2 should be considered. In the presence of trace H2O, the discharged product Li2O2 reacts according to the reaction, Li2O2 + H2O → H2O2 + 2 LiOH. As MnO2 catalyzes the decomposition of H2O2 to water, MnO2 can regenerate H2O for further formation of LiOH or formation of large toroidal Li2O2.21, 23 Similarly, Mn in the M-MOF-74 could boost the catalyzed conversion of H2O2, resulting in the continuous formation of LiOH (in Mn-MOF-74 and MnCo-MOF-74) and large toroidal Li2O2 (in MnCo-MOF-74).

3.3 Electrochemical Properties The electrochemical stability of M-MOF-74 catalysts was confirmed by measuring cyclic voltammetry (CV) in Ar atmosphere (Figure S6), and M-MOF-74 catalysts synthesized in this work were stable in voltage ranges from 2.3 V to 4.5 V. The catalytic air cathode performances of KB, Mn-MOF-74, Co-MOF-74, mixture-MOFs and MnCo-MOF-74 were galvanostatically tested. As a comparison, the physical mixture of Mn-MOF-74 and Co-MOF-74 with the same Mn to Co atomic ratio with MnCo-MOF-74 (Mn:Co=1: 4.24) was also evaluated (mixture-MOF74). Figure S7 shows the first full discharge and charge profiles of electrodes at 200 mA g-1. Among the tested samples, MnCo-MOF-74 presented the highest discharge capacity (11150 mAh g-1), which is significantly higher than those of mixture-MOF-74 (7767 mAh g-1), MnMOF-74 (6040 mAh g-1), Co-MOF-74 (5630 mAh g-1), and KB (3020 mAh g-1).Compared to the previous reports with capacities of Mn-MOF-74 (9420 mAh g-1), Co-MOF-74 (3630 mAh g-1),


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and super P carbon (2170 mAh g-1) at 50 mA g-1,31 MnCo-MOF-74 cathode shows a higher capacity even at a high current, confirming the porous nature of the M-MOF-74 synthesized in this study. The cyclic performances were tested with a limited capacity of 1000 mAh g-1 at 200 mA g-1 in the voltage range of 2.3 and 4.5 V. “Figure 5a–5e show the discharge-charge profiles of KB, Mn-MOF-74, Co-MOF-74, mixture-MOF-74, and MnCo-MOF-74, respectively. The overpotential in the graphs was measured at mid capacity (500 m h g-1). Figure 5f and Figure S8 summarize the results shown in Fig. 5a–5e. Figure 5f is the plot of the end voltage versus cycle number and Figure S8 is the discharge and charge capacity versus cycle number. In the cycling test, KB showed very limited cycle life (8 cycles at a limited discharge capacity of 1000 mAh g-1 and 200 mA g-1) with a large overpotential of 1.74 V. Among the profiles, noteworthy is the charge profile and overpotential of Mn-MOF-74. The charge profile of Mn-MOF-74 is different from those of the other electrodes, and the overpotential is lower (1.07 V) than other cathodes. This result supports the observation of LiOH as the discharge product of Mn-MOF-74 cathode by XRD and XPS analyses. LiOH is known to decompose at a lower potential than Li2O2,23 thus Mn-MOF-74 demonstrates lower charge potential than others. The air cathodes adopting M-MOF-74 catalysts delivered the discharge capacity of 1000 mAh g-1 much longer than KB: Mn-MOF-74 (for 22 cycles), Co-MOF-74(for 18 cycles), mixtureMOF-74 (for 28 cycles), and MnCo-MOF-74 (for 44 cycles). Although Mn-MOF-74 shows high catalyst activity for the formation of LiOH, Mn-MOF-74 fails to show sufficient activity for completely decomposing LiOH as observed by XPS, resulting in lower cyclability (Figure 5). Co-MOF-74 shows slightly better cyclability than that of KB owing to the OER effect, however, it still presented limited cycle life. MnCo-MOF-74 demonstrated much improved reversibility as reflected in an extended cycle life as well as a low overpotential of 1.26 V. The mixture-MOF-74


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showed inferior cycling stability of 28 cycles and overpotential of 1.32 V, verifying the beneficial effect of bimetallic-MOF formation for bi-functionality. As shown in Figure 5f, MnCo-MOF-74 maintains the lowest charge end voltage and the highest discharge end voltage, revealing improved ORR and OER kinetics compared to the other electrodes. The bimetallic MnCo-MOF-74 thus presented the highest catalytic activity for the formation and decomposition of discharge products among the tested materials. This performance might originate from the synergistic effect of the combination of Mn and Co metals, as in bifunctional catalysts,15, 43-44 though the roles seem to be different. Owing to the presence of Mn metal cluster in MnCo-MOF74, it has a higher tendency for the formation of LiOH, which however is lower than that of MnMOF-74. Thus, the discharge product of MnCo-MOF-74 consists of a mixture of LiOH and Li2O2. LiOH did not completely decompose in Mn-MOF-74 owing to the low activity of Mn metal cluster for the decomposition of LiOH, resulting in poor cyclability. In MnCo-MOF-74, however, the reversibility and thus the cycling property remarkably improved despite the presence of LiOH. This implies that the decomposition of LiOH is facilitated in MnCo-MOF-74 than in Mn-MOF-74. Co metal cluster might have superior catalytic activity for the oxidation of LiOH. To assess the ability of Co-metal cluster to decompose LiOH, LiOH-preloaded electrodes were prepared with KB, Mn-MOF-74, Co-MOF-74, and MnCo-MOF-74 (Figure S9). As expected, the Co-metal cluster has superior catalytic activity for LiOH decomposition, and thus, Co-MOF-74 and MnCo-MOF-74 show 100 % decomposition of LiOH. Co-MOF-74 also possesses catalytic OER activity for Li2O2, however, the activity might be relatively poor compared to the activity for LiOH decomposition. In the Li2O2-preloaded electrodes, Co-MOF74 actually showed 87% decomposition efficiency (Figure S10). While the discharge products Li2O2 completely decomposed in in-situ discharged electrodes in Figure 3a, the pre-loaded


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electrodes could show lower decomposition than in-situ discharge electrode since discharge products and catalysts do not make such close contact as in in-situ discharged electrodes. When the decomposition efficiency was compared in pre-loaded configuration, Co-MOF-74 definitely showed higher decomposition efficiency for LiOH than Li2O2. This could explain the superior reversibility of MnCo compared to Mn or Co-MOF-74. Mn-MOF-74 promotes the synthesis of LiOH, but cannot decompose it. Co-MOF-74 has the ability to completely decompose LiOH, but the discharge product of Co-MOF-74 is Li2O2 with inherent low reversibility. MnCo-MOF-74 contains both Mn and Co metal clusters, and they complementarily function for the formation and decomposition of the discharge products.

4. CONCLUSION We prepared a bimetallic MnCo-MOF-74 material as a cathode catalyst for Li-O2 batteries. Simple hydrothermal treatment generated both monometallic- and bimetallic-MOF-74. The successful formation of the MOF-74 structure was confirmed by various analyses such as FT-IR, XRD, XPS, and SEM. We found that the bimetal coexists in M-MOF-74 at a ratio of Mn to Co of 1: 4.24 with a stable hexagonal-rod-type porous structure. The bimetallic MnCo-MOF-74 cathodes exhibited high full discharge capacity (11150 mAh g-1) and excellent cyclability (44 cycles) with low overpotential at limited capacity of 1000 mAh g-1. The bimetallic MnCo-MOF74 outperformed the monometallic Mn-MOF-74 and Co-MOF-74. Owing to the synergistic integration of Mn and Co metal clusters, MnCo-MOF-74 promotes the formation of discharged products during ORR, which can be easily decomposed during OER. Mn and Co metal clusters operate complementarily in MnCo-MOF-74 resulting in improved reversibility and efficiency of


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Li-O2 battery cathodes. We thus demonstrated the feasibility of tuning the catalytic activity through the versatility of the MOF platform.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XPS spectra of M-MOF-74 (M = Mn, Co, MnCo), SEM-EDS elemental mapping of MnCoMOF-74, SEM images of M-MOF-74 synthesized under different metal precursor or solvent ratios, TGA curve of M-MOF-74 after drying, CV curves of M-MOF-74 under Ar atmosphere, galvanostatic full discharge and recharge profiles, discharge and charge capacity vs. cycle number in Figure 5, electrochemical charge performance of LiOH-preloaded electrodes in a Li-O2 cell along with XRD pattern and SEM images, electrochemical charge performance of Li2O2-preloaded electrodes in a Li-O2 cell. (PDF)

FIGURES


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(a) MnCo-MOF-74

%T

Co-MOF-74 Mn-MOF-74 MOF-74 (Zn) 1400 1550

C-O stretching of C-O

500

1000

1500

=O

2000

2500

3000

3500

-1

4000

Wavenumbers(cm )

Intensity

Intensity

(b)

6.5

2theta

11.5 12.0 2theta

7.0

MnCo-MOF-74 Co-MOF-74 Mn-MOF-74 MOF-74 (Zn)

5

10

15

20

25

30

35

40

45

50

2theta

(c) Mn 2p

Intensity

Co-MOF-74

MnCo-MOF-74 Mn 2p3/2 Mn 2p1/2 Satellite 645 eV

Mn2+ 641.4 eV Mn3+ 642.9 eV

Mn-MOF-74 660

655

650

645

640

635

B.E(eV)

(d) Co 2p Co 2p3/2 Co 2p1/2 Satellite 803.5 eV Co2+ 781 eV Satellite 785.6 eV

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co3+ 779 eV

Co-MOF-74

MnCo-MOF-74

Mn-MOF-74 810

800

790

780

770

B.E(eV)


17


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Figure 1. Characterization of the synthesized M-MOF-74 (M = Mn, Co, MnCo) materials. (a) FT-IR spectra and (b) XRD patterns compared with those of MOF-74 (Zn). High resolution XPS of M-MOF-74: (c) Mn 2p and (d) Co 2p.

Figure 2. Morphology and structure of the synthesized M-MOF-74; SEM images of (a) MnMOF-74, (b) Co-MOF-74, and (c) MnCo-MOF-74; (d) EDS mapping images of MnCo-MOF-74 for (e) manganese (yellow) and (f) cobalt (blue).


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Li2 O2 LiOH SUS

KB D Mn-MOF-74 D Co-MOF-74 D MnCo-MOF-74 D

KB C Mn -MOF-74 C Co-MOF-74 C MnCo-MOF-74 C

Intensity

(a)

30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60

2theta KB D Mn-MOF-74 D Co-MOF-74 D MnCo-MOF-74 D

(b)

KB C Mn -MOF-74 C Co-MOF-74 C MnCo-MOF-74 C

Li 2CO3 Li 2O2

LiOH

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


59

58

57

56

55

54

53

52

B.E(eV)

Figure 3. Characterization of discharged and recharged electrodes. (a) XRD patterns and (b) Li 1s XPS spectra of the electrodes. Cells were discharged and recharged at 0.02 mA for 100 h.


19


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Figure 4. Morphology of the discharged and recharged electrodes. SEM images of the electrodes after discharging and recharging at 0.02 mA for 100 h: (a, b) KB, (c, d) Mn-MOF-74, (e, f) CoMOF-74, and (g. h) MnCo-MOF-74.


20

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5.0

5.0

(a)

KB 1st 2nd 3rd 5th 10th 15th 20th

1.74 V

3.0 2.5

4.0 1st 2nd 3th 5th 10th 15th 18th 20th 22th 25th

3.5

1.07 V 3.0 2.5

0

200

400

600

800

1000

2.0

1200

0

200

400

600

800

1000

0

200

400

600

800

1000

0

200

1200

-1

Capacity(mAh g )KB

600

800

1000

1200

-1

MnCo-MOF-74

(f) 4.5

4.0 3.5

1st 2nd 3th 5th 10th 20th 30th 35th 37th 39th

1.26 V

3.0 2.5 2.0

400

Capacity(mAh g )KB

0

200

400

600

800

1000

1200

-1

Capacity(mAh g )KB

End voltage (V)

2.5

Voltage (V)


3.0

1.39 V

3.0

2.0

1200

4.5

4.0


5.0

(e)

4.5

1.32 V

3.5

2.5

5.0

Mixture-MOF-74

3.5

4.0

Capacity(mAh g )KB

5.0

(d)

Co-MOF-74

-1

-1

Capacity(mAh g )KB

2.0

(c) 4.5

Voltage (V)

3.5

Voltage (V)

Voltage (V)

Mn-MOF-74

4.5

4.0

2.0

5.0

(b)

4.5

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.0

KB Mn-MOF-74 Co-MOF-74 Mixture-MOF-74 MnCo-MOF-74

3.5 3.0 2.5 2.0

0

10

20

30

40

50

Cycle number

Figure 5. Galvanostatic cycling performance of Li-O2 batteries using (a) KB, (b) Mn-MOF-74, (c) Co-MOF-74, (d) miture-MOF-74, and (e) MnCo-MOF-74 electrodes at a limited capacity of 1000 mA h g-1 with a current density of 200 mA g-1. (f) End voltage versus cycle number of the electrodes.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


21


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This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), Korean Ministry of Science & ICT (grant no. NRF2014R1A2A1A11049801). This work was also supported by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), which was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (Grant No. 20174010201240).

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Table of Contents/Abstract Graphic


29

Bimetallic MetalâOrganic Frameworks as Efficient Cathode Catalysts

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