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Bio-inspired synthesis of hierarchically porous MoO2/Mo2C nanocrystals decorated N-doped carbon foam for lithium-oxygen batteries Yan Lu, Huixiang Ang, Qingyu Yan, and Eileen Fong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01966 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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Chemistry of Materials

Bio-inspired Synthesis of Hierarchically Porous MoO2/Mo2C Nanocrystals Decorated N-doped Carbon Foam for Lithiumoxygen Batteries Yan Lu,† ‡ Huixiang Ang, †‡ Qingyu Yan, †* and Eileen Fong†* †

School of Materials Science and Engineering, 50 Nanyang Avenue, Block N4.1, Singapore 639798

ABSTRACT: The lithium oxygen (Li-O2) battery is one of the most promising technologies among various electrochemical energy storage systems. The challenge to develop a high-performance Li-O2 battery lies in exploring an air electrode with optimal porous structure and high efficient bifunctional electrocatalyst. The present work demonstrates a bio-inspired synthesis route for the preparation of high performance Li-O2 air electrode materials that are made out of N-doped carbon foams decorated with hetero-nanostructured MoO2/Mo2C nanocrystals (MoO2/Mo2C@3D NCF). Here, recombinant proteins (ELK16-FLAG) facilitated the self-assembly of metal precursors and provided a carbon source for Mo2C formation. The as-prepared MoO2/Mo2C@3DNCF showed superior electrocatalytic activity in both OER and ORR mechanisms with a high round-trip efficiency of 89.1% (2.77 V/3.11 V) at 100 mA g-1 as well as exceptional rate performances and good cyclability in Li-O2 battery. The desirable electrochemical performance can be attributed to the unique hierarchical porous structure of the 3D carbon foam and the intimate contact between MoO2 and Mo2C nanocrystals. We demonstrate the novel, facile, environment-friendly bio-inspired approaches would open new avenues for the synthesis of 3D nitrogen doped carbon supported advanced functional materials with excellent electrochemical performances.

INTRODUCTION Rechargeable lithium-oxygen (Li-O2) batteries are emerging as next-generation batteries due to their high theoretical specific energy densities (up to 2–3 k Wh kgcell−1) compared to other existing rechargeable batteries.1-11 However, Li-O2 batteries suffer from sluggish kinetics during the ORR and OER reactions, leading to large over-potential and subsequently poor round-trip efficiency. Typical reaction mechanisms of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in non-aqueous Li-O2 batteries are written as 4, 12, 13 As such, for2 Li + + 2e − + O 2 ← → Li 2 O 2 ，E 0 = 2.96 V . mation of discharge products (e.g.Li2O2) on the cathode may eventually block the active sites of the electrocatalyst and reduce the capacity of the battery. Therefore, there is a critical need to design an optimal air electrode containing micrometer-sized open porosity for rapid oxygen diffusion and substantial nanoporosity (2–50 nm) to catalyze Li–O2 reactions while preventing excessive growth of the discharge products that hinder chemical pathways. There are many reports in the literature focusing on the exploration of high efficient electrocatalysts for both ORR and OER reactions in Li-O2 batteries. Numerous carbonaceous materials 14-19 and metal oxides 12, 14, 20-30 have shown promise. However, these materials often displayed poor OER activities (high up to 4.5 V) with high-charge overpotentials. Likewise, noble metal based catalysts (e.g. Au,

Pt, Ru, etc) are excellent candidates with high OER activities, but are too costly for practical applications. To achieve high round-trip efficiency (e.g. low charge/discharge over-potential), efforts have been devoted to fabricate alternative hybrid nanocomposites as bifunctional electrocatalysts in Li-O2 battery, including 33 PtAu/C,31 Pt@MCN,32 NiCo2S4@N/S-rGO and 34 Mo2C/CNTs . However, the synthesis strategies for those hybrid materials involve multiple steps and require high energy conditions and toxic/erosive reagents (such as HF or NaOH). Hence, alternative routes for the synthesis of low-cost hybrid bifunctional electrocatalysts are desired. Bio-inspired routes are environmentally friendly and have been used to produce a variety of 3D nanostructured materials. For instance, biomolecules including polysaccharides, proteins, viruses, DNA and peptides have been explored for the preparation of Co3O435 and FePO4 nanofibers36, 37 as well as mesoporous N-rich carbons38. Recombinant proteins in particular, can be genetically engineered to interact specifically with metallic precursors under benign conditions and to create inorganic nanostructures with high surface areas, with precise control over their compositions, phase, shape, and size.39, 40 In addition, the protein backbone contributes to nitrogendoping; nitrogen-doped carbons have improved conductivities, desirable for lithium-ion batteries (LIBs), supercapacitors and catalysts18, 19, 32, 41, 42. Yet, hierarchical heter-

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ogeneous structures as air cathode for Li-O2 batteries have not been prepared using bio-inspired routes. In this work, we will employ bio-inspired approaches for the preparation of 3D hierarchically porous carbon foam containing hetero-nanostructured MoO2/Mo2C nanocrystals (designated as MoO2/Mo2C@ 3D NFC). Here, recombinant elastin-like polypeptides (ELPs) containing FLAG tags [(VPGIG)2VPGKG(VPGIG)2]16-CDYKDDDDKL (named ELK16-FLAG) were used as templates to direct the self-assembly of molybdenum-based nanocrystal heteronanostructures. Molybdenum-based compounds are widely used as catalysts in many applications, including hydrogen evolution reaction (HER),43-45, 64, 65 water gas shift reaction (WGS) 46, 47 and fuel cells.48 Amongst them, molybdenum dioxide (MoO2) is expected to be a good catalyst for ORR reaction owing to the presence of free electrons in Mo4+ and good conductivity while molybdenum carbide (Mo2C) has been reported to show excellent OER property contributing to the similar surface chemistry as noble metal catalysts34,49. Both MoO2 and Mo2C have often investigated as catalysts, but their composite catalytic performances, especially in Li-O2 batteries, have yet to be evaluated. Here, we evaluate the electrochemical properties of the as-synthesized MoO2/Mo2C@ 3D NFC materials as oxygen cathodes in Li-O2 batteries. We obtained very low overpotentials with different current densities. The superior capacity stability and long cycling life of the cathodes could be attributed to the synergistic combination of MoO2 and Mo2C, presented within an inter-connected conductive 3D carbon matrix.

EXPERIMENTAL SECTION Expression and Purification of ELK16-FLAG The plasmid pET22b containing the gene encoding for ELK16-FLAG was constructed as the previously reported.40 E. coli bacteria BL21-(DE3)pLysS strain was transformed with pET22b plasmid encoding for ELK16-FLAG via heat shock. Colonies were picked and grown in 50 mL of TB (Terrific broth) media containing 50 mg L-1 ampicillin and 34 mg L-1 chloramphenicol overnight. The next day, 10 mL of bacteria culture was re-inoculated into 1 L of TB media containing the same antibiotics and grown to an optical density at 600 nm (OD 600) of 0.7~0.8 at 37 ℃. Isopropyl β-D-1-thiogalactopyranoside (IPTG) was added (to a final concentration of 1 mM) to induce protein expression. Bacterial cells were grown for another 4 h and harvested by centrifugation at 8000 rpm at 4 ℃ for 20 min before resuspending in TEN buffer (0.1MTris, 0.01 MEDTA, 1MNaCl). The cell mixture was sonicated on ice and subsequently centrifuged at 4 ℃ to collect the supernatant. The ELK16-FLAG was purified via inverse thermal cycling as previously described.50 Purified ELK16-FLAG was dialyzed against water for 3 days and lyophilized. Lyophilized ELP16 proteins were store at -20 ℃ for further use.

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Synthesis of 3D nitrogen doped carbon foam supported MoO2, MoO2/Mo2C or Mo2C nanocrystals The lyophilized ELK16-FLAG protein (25 mg) was first dissolved in cold DI H2O (250 µL) at 4°C. A solution of (NH4)6Mo7O24 •4H2O (0.143 mol L-1, 300 uL) was added to the protein solution and mixed for 30 min at 4°C. The mixture was then kept stagnant for 1 h and subsequently frozen in liquid nitrogen before lyophilization. The lyophilized sample was annealed at either 400°C, 600°C or 700°C for 30 min under H2 (5%) /Argon (95%) atmosphere with a heating rate of 20°C min-1. Control samples prepared with a different Mo precursor ((NH4)2MoO4) and a repetitive lysine-free protein template (GPG-AG3)66, were obtained after annealing at 600 °C for 30 min under H2 (5%) /Argon (95%) atmosphere with a heating rate of 20°C min-1. Materials characterization Scanning electron microscopy (FESEM, JEOL 7600F) and transmission electron microscopy (TEM, JEOL 2100F) operated at an accelerating voltage of 200 kV were used to examine the morphologies of the as-annealed samples. Powder X-ray diffraction (XRD, Bruker D8 Advance, 40 kV/40 mA, Cu-Ka radiation) was employed to identify the crystalline phases. XPS measurements were carried out with a conventional monochromated X-ray source using the Al Kα line (SPECS XR50, hѵ = 1486.6 eV) and a multichannel energy analyzer (SPECS Phoibos 100 MCD-5) using CasaXPS software for data analysis. 1H NMR analysis was carried out on Bruker 300 NMR spectrometer using D2O as solvent and C6H6 as an internal standard. The nitrogen adsorption–desorption isotherms were acquired using an ASAP Tri-star II 3020 analyzer. Electrochemical Measurements The electrochemical performances of the prepared materials as oxygen cathodes in the Li-O2 battery were examined in our in-house battery case. The working electrodes were prepared by coating the slurry of the as-obtained active materials (70 wt%), carbon black (20 wt%) and polyvinylidene fluoride (PVDF) (10 wt%) dissolved in Nmethyl pyrrolidinone (NMP) onto a nickel foam square (with edge length of 0.6 cm) substrate and dried in a vacuum oven at 80°C for 2 days. The average mass loading per electrode is about 0.6 mg. The cells were assembled in the argon-filled glove box (Unilab, MBRAUM, Germany) with oxygen and moisture levels maintained at below 0.1 ppm. High-purity lithium foil was used as counter and reference electrodes and the glass fiber (GF/C, Whatman) as a separator. A solution of 1 M lithium triflate (LiCF3SO3) in tetraethylene glycol dimethyl ether (TEGDME) was used as the electrolyte. The Li–O2 cells were purged with O2 at one atmospheric pressure and were tested on the NEWARE multichannel battery test system. The cyclic voltammogram of the Li-O2 battery was performed in a potential window of 2 to 4.2 V with a scan rate of 0.2 mV s-1 under O2 atmosphere. All electrochemical tests were performed at room temperature and specific capacity was calculated based on the prepared active materials.


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Results and discussion Synthesis strategy of MoO2/Mo2C@3D NCF in protein-templating method A bioinspired approach was use for the synthesis of 3D N-doped carbon foam supported molybdenum-based composite nanocrystals. Scheme 1 illustrates the synthesis procedure. The protein (ELK16-FLAG) has been shown previously to have self-assembling capabilities.51 During the synthesis process, the lyophilized protein was dissolved in cold water followed by mixing with Mo source (e.g. (NH4)6Mo7O24•4H2O) at 4°C. After lyophilizing the mixture of protein with metal precursors, a fibrous hydrogel foam was obtained (Figure S1a). However, in the absence of metal precursor, we did not observe any 3D porous structure (Figure S1b). We conclude that there are likely molecular interactions between Mo ions and the ELK-FLAG template. The lysine (K) residues incorporated periodically within the ELK-FLAG backbone provide positively-charged amine side groups (-NH2) that interact with (Mo7O24)6- ions through hydrogen bonding and/or dative bonding (Scheme 1, step 2). These interactions serve as “cross-links” between two adjacent ELK16-FLAG molecules, resulting in the formation of protein fibers. When we repeated the synthesis with another Mo precursor with a different anion (i.e., (NH4)2MoO4), we obtained a layered porous structure instead of a fibrous structure (Figure S2a – c). Likewise, when we employed a repetitive lysine-free elastin-based recombinant protein (i.e., GPGAG3)66 as template, we were also unable to obtain a 3D fibrous structure (Figure S2d – e). Both results led us to conclude that interactions between the amine side groups (-NH2) in ELK-FLAG backbone and the negativelycharged molybdate anions in the Mo precursors are responsible for the fibrous morphology. The chelation of molybdate ions with the periodically-spaced amine groups also resulted in the homogenous distribution of the metal precursor in the protein hydrogel matrix (Scheme 1, step 3). After annealing under H2/Ar gas at 600°C, ELK16-FLAG degraded into N-doped carbon matter. In the same annealing step, carbon-coated MoO2/Mo2C NPs were also obtained, dispersed within a 3D conductive N-doped carbon aerogel network as shown in Scheme 1, step 4. Structure and morphology characterization of MoO2/Mo2C@3D NCF The morphology of the as-prepared 3D foam (inset of Figure 1a) were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A nanofibrous, microporous network covered by nanoparticles was obtained (Figure 1a). The TEM image clearly showed the presence of nanocrystallites embedded within the microfibrous carbon matrix (Figures 1b). The porous structure remained intact even when subjected to intensive ultrasonification (inset of Figure 1b). Interestingly, we found curved graphite-like lattices and onion-like carbon in the carbon matrix (Figure S3), even though the annealing temperature was only 600°C. Partial graphitization of the protein matrix was also noted at an

annealing temperature of 600°C previously.38, 40 The nanoparticles embedded in the carbon matrix were found to be around 10 - 15 nm in diameters (Figure 1c). The selected area electron diffraction (SAED) pattern (inset of Figure 1c) confirmed the presence of crystalline Mo2C (JCPDF no. 00-035-0787) and MoO2 (JCPDF no. 00-032-0671). The high-resolution transmission electron microscopy (HRTEM) images of the sample is shown in Figure 1d. The red arrows indicate well-dispersed nanocrystals embedded within the carbon support. Notably, the lattice fringes of two adjacent crystals (outlined by yellow circles in Figure 1d), were measured to be approximately 2.44 Å and 2.59 Å, ascribing to the (200) plane of MoO2 and the (100) plane of Mo2C, respectively (inset of Figure 1d). The HRTEM images also show that MoO2 and Mo2C nanocrystals are in fact overlapping. The X-ray diffraction (XRD) pattern (Figure 2a) of the as-prepared hierarchical porous MoO2/Mo2C@3DNCF sample matches well with the monoclinic MoO2 (JCPDF no. 00-032-0671) and hexagonal Mo2C (JCPDF no. 00-0350787) structures. The three diffraction peaks located at 34.5°, 38.0°, and 39.5° are ascribed to the (100), (002), and (101) planes of hexagonal Mo2C structure, while the diffraction peaks located at 26.0°, 37.0°, and 53.6° are assigned to the (-111), (-211), and (022) planes of the monoclinic MoO2 structure. We also noted a broad peak around 20~25° contributed by the carbon matrix (JCPDF no. 00-050-0926). Using the Scherrer's equation, the estimated grain sizes for MoO2 and Mo2C were around 16.8 nm and 16.3 nm, respectively, which was consistent with our TEM observations. Moreover, based on the XRD result and using TOPAS software calculation, the composition of the MoO2 and Mo2C in the MoO2/Mo2C@3DNCF sample was 57.5 and 42.5 wt% , respectively (Table S2). To determine the degree of graphitization, Raman spectroscopy analysis was performed on the MoO2/Mo2C@3DNCF sample. The Raman spectrum (Figure 2b) reveals two characteristic peaks of carbonaceous material at around 1348 cm-1 (D band, defects and disorders mode) and 1605 cm-1 (G band, C-C stretching mode bonds of typical graphite).38, 52 The ratio of G band intensity to D band intensity (IG/ID) is estimated to be 0.87. This low IG/ID value suggests that the carbon matrix is partially graphitized, consistent with the result observed from TEM image (Figure S3). The scanning transmission electron microscopes-energy-dispersive X-ray spectroscopy (STEM-EDX) mapping images of MoO2/Mo2C@3DNCF sample (Figure S4) shows that the Mo, C, N, O elements are evenly distribtued throughout the whole material. In order to understand the surface chemical properties of the as-prepared sample, the X-ray photo-electron spectroscopy (XPS) measurement was carried out. The XPS spectrum (Figure S5) shows five distinct signals centered around 234.4 eV, 286.4 eV, 398.7 eV, 416.5 eV and 532.4 eV, ascribed to Mo 3d, C 1s, N 1s, Mo 3p1/2 and O 1s, respectively. The composition of these species are calculated to be 35.84 (Mo 3d), 21.06 (C 1s), 6.39 (N 1s), 5.82 (Mo 3p1/2), and 30.89 wt% (O 1s), as shown in Table S1. Figure 2c presents a high-resolution (HR) Mo 3d XPS spectrum, which



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can be deconvoluted into six peaks. The peak at 228.9 eV is ascribed to Mo2C and the peak at 232.4 eV can be assigned to MoO2, confirming the presence of MoO2/Mo2C in the sample. We note that the peaks centered at 230.4 eV, 232.9 eV, 235.7 eV and 236.2 eV are associated with the intermediate oxidation states of Mo (MoOx).53 The high-resolution XPS spectrum of C 1s (Figure 2d) shows the peak at the lower binding energy of 284.5 eV is the characteristic of the Mo-C bond in Mo2C.54 In addition, the peaks at binding energies of 284.9 eV, 285.8 eV and 287.4 eV correspond to Csp2–Csp2, N–Csp2 and N–Csp3 bonds, respectively.55 This result indicates that nitrogen atoms have been introduced into the carbon matrix. The HR-XPS spectrum of N1s (Figure 2e) can be deconvoluted into four peaks. The peaks at 397.3 eV, 398.8 eV and 400.4 eV corresponded to pyridinic, pyrrolic and quaternary type N atoms, respectively,56 while the weak peak at 395.5 eV was ascribed to the Mo–N bond. It is noteworthy that the Mo-N chemical bond formed between Mo-based compound NPs and 3D NCF matrix can prevent the ultrafine NPs from eroding from the carbon matrix during the charge-discharge processes.57-59 Our XPS results confirmed that nitrogen was successfully doped in the carbon matrix.

Phase formation control The formation of MoO2 and Mo2C and their heteronanostructures were influenced by the annealing temperatures. When annealed at 400°C, we were able to obtain a similar 3D porous structure (Figure S6a). However, only MoO2 could be obtained (sample is designated MoO2@3D NCF; Figure S8a). Conversely, when the annealing temperature was increased to 700°C, pure Mo2C was obtained instead (sample is designated as Mo2C@3D NCF; Figure S8a). For 700°C, we also noted that although the fibrous architecture of the sample was maintained, the fibers were thicker, and were covered with larger particles (Figures S6d-f). Nitrogen adsorption-desorption isotherms of the three products (Figure S7) confirmed the presence of mesopores within the fibrous carbon matrix, with a narrow pore size distribution centered at 3 nm. With increasing annealing temperatures, the specific surface areas decreased from 117 m2 g-1 for MoO2@3D NCF to 92 m2 g-1 (MoO2/Mo2C@3D NCF) and to 73 m2 g-1 for Mo2C@3D NCF. Raman spectra of MoO2@3D NCF and Mo2C@3D NCF were shown in Figure S8b, where the intensities of the D and G peaks were significantly reduced for Mo2C@3D NCF. This could be attributed to the reduction in the overall carbon content due to consumption of carbon for carbide formation. To further check the phase evolution in MoO2/Mo2C@3D NCF sample, we prepared the control samples with dwelling time of 0 h (MoO2/Mo2C@3D NCF0 h) and 1h (MoO2/Mo2C@3D NCF-1 h) at the setting annealing temperature of 600 ℃. The XRD results showed that only MoO2 phase (JCPDF no. 00-032-0671) with poor crystallization was observed without dwelling time at 600 ℃ (Figure S9a). When the dwelling time extending to 1 h,

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well crystallized MoO2/Mo2C composite (JCPDF no. 00032-0671 for MoO2 and JCPDF no. 00-035-0787 for Mo2C) was readily obtained, with a MoO2 to Mo2C ratio of 49.41: 50.59 (Figure S9b and Table S2). Given the above results, a possible formation mechanism of the MoO2/Mo2C@3D NCF was proposed. At low annealing temperatures (~400°C), the molybdate precursors (Mo7O246-) were first reduced to MoO2 nanocrystals under H2/Ar atmosphere. Simultaneously, the protein carbonizes into an amorphous carbon matrix. As temperature increases to be 600 ℃, MoO2 is further reacted to form Mo2C via: 2 MoO 2 + CH 4 + 2 H 2 = Mo 2 C + 4 H 2 O

where CH4 was derived from simple reaction of adsorbed hydrogen with the carbon support.60 A dwelling time at 600 °C was necessary to obtain MoO2/Mo2C heteronanocomposites. Finally, to obtain pure Mo2C, the annealing temperature could be increased to 700 ℃.Hence, 3D carbon foams containing pure MoO2, Mo2C or their heteronanocomposites can be readily obtained by tuning the annealing temperature and time.

Electrochemical performances The electrochemical performances of Li-O2 batteries were evaluated using a two-electrode system, in which Li foil is the reference electrode, with lithium triflate (LiCF3SO3) in tetraethylene glycol dimethyl ether (TEGDME) as the electrolyte and oxygen cathodes containing MoO2@3D NCF, Mo2C@3D NCF and MoO2/Mo2C@3D NCF as electrocatalysts. The first discharge/charge profiles of MoO2/Mo2C@3D NCF batteries were compared with those with MoO2@3D NCF and Mo2C@3D NCF batteries at a current density of 100 mA g-1 (Figure 3a). The MoO2/Mo2C@3DNCF, MoO2@3DNCF and Mo2C@3D NCF batteries delivered discharge voltages of 2.77 V, 2.65 V and 2.54 V and the charge voltages of 3.11 V, 3.17 V, 3.13 V, respectively. The round-trip efficiency of the battery is defined as the ratio of discharging voltage value to the charging voltage value. Hence the round-trip efficiencies of MoO2/Mo2C@3D NCF, MoO2@3D NCF and Mo2C@3D NCF batteries are estimated to be 89.1%, 83.6% and 81.1%, respectively. It is notable that the discharging voltage of MoO2@3D NCF battery is higher than the discharging voltage of Mo2C@3D NCF battery, indicating that MoO2@3D NCF sample is a better ORR electrocatalyst as compared to Mo2C@3D NCF sample. The enhancement of ORR activity in MoO2@3D NCF battery is likely to be associated with the strong affinity of MoO2 towards oxygen molecules, resulting in facile formation of Li2O2 product. On the other hand, Mo2C@3D NCF battery exhibits a lower charging voltage as compared to MoO2@3D NCF battery, implying the former is an efficient OER electrocatalyst. The presence of hetero-nanostructured MoO2/Mo2C nanocrystals significantly improved the ORR and OER kinetics. The enhanced polarization performance of MoO2/Mo2C@3D NCF battery with higher discharging voltage compared to MoO2@3D NCF sample and lower charging voltage to Mo2C@3D NCF is attributed to


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the synergistic effect between MoO2 and Mo2C. In particular, the intimate contact of MoO2 and Mo2C nanocrystals in the nano domain (Figure 1d) not only facilitated the formation/decomposition of discharge products, their favored interfacial structures also further enhanced the transport of electrons and ions during catalytic processes.61,62 Both factors contributed to an improved OER/ORR performance of the Li-O2 battery. In addition, previous studies have reported that chemically doped nitrogen in carbon-based materials enhanced the overall conductivity of the electrode as well as improved the ORR activity due to the alteration of electronic structure. 18, 19, 32 Taken together, MoO2/Mo2C@3DNCF sample showed superior bifunctional electrocatalytic property for both ORR and OER reactions in Li-O2 battery with high round-trip efficiency compared to the previous reported results (Table S3). Figure 3b shows that the specific capacity of MoO2/Mo2C@3D NCF battery was limited at 500 mAh g-1 and the cyclability can be maintained beyond 120th cycle, indicating excellent cycling performance. However, we observed a sharp drop in the capacity at around 135th cycle. This might be due to the accumulation of irreversible lithium alkyl carbonate (LAC) on the electrode surface after prolonged cycling, and blocking the active sites for efficient ORR and OER reactions. We also noted that the MoO2/Mo2C@3D NCF battery has a stable charging voltage, but it has an unstable discharging voltage. The unstable discharging voltage might be caused by the strong interaction between the MoO2 surface and the peroxide radical, which promoted the formation of LAC that gradually increases the charge transfer resistance of the MoO2 (responsible for ORR), leading to a slight raise in discharging voltage. Meanwhile, we believe that this aforementioned side reaction has a lesser occurrence on the Mo2C surface and therefore, the charging voltage remained stable after 120 consecutive cycles. We further examined the effect of various current densities on the discharge/charge profiles of MoO2/Mo2C@3D NCF batteries (Figure 3c). The MoO2/Mo2C@3D NCF batteries are discharging/charging at various current densities of 100 mA g-1, 200 mA g-1, 500 mA g-1, 1000 mA g-1 and 2000 mA g1 with discharge/charge ratios of 2.77/3.11 V, 2.68/3.15 V, 2.61/3.19 V, 2.49/3.42 V and 2.19/3.68V, respectively. With increasing current densities, the over-potentials of discharging/charging voltages increased considerably. However, even at high current density of 1000 mA g-1, the polarization of the Li-O2 battery with MoO2/Mo2C@3D NCF was very low as compared to the reported results, as summarized in Table S3. It is noteworthy that the MoO2/Mo2C@3D NCF air electrode exhibited superior OER catalyst activity with a low charge plateau at 3.42 V even under a high current density of 1000 mA g-1. Furthermore, we have conducted a cyclic voltammetry on the MoO2/Mo2C@3D NCF battery at a scan rate of 0.2 mV s-1 in LiCF3SO3/TEGDME and under O2 atmosphere (Figure S10). The MoO2/Mo2C@3D NCF electrode shows a cathodic peak at 2.74 V (representing ORR), and displays an anodic peak at 3.22 V (representing OER). These results

indicate that the reactions are reversible and the voltages are quite similar to that of the galvanostatic discharge/charge curves in Figure 3b. To our best knowledge, even without the addition of redox mediator (e.g. I-/I3-), this is the lowest charging voltage value ever reported for such high current density, indicating that our material has exceptional rate performance. We applied very high current densities to Li-O2 cells to test the limits of the MoO2/Mo2C@3D NCF air electrodes. At 2000 mA g-1, the cells can cycle in a voltage range of 2~4V with a discharge voltage of 2.19 V and a charge voltage of 3.68 V. However, at a current density of 5000 mA g1 , the cells were no longer capable of cycling due to the large voltage polarization. Moreover, due to the excellent rate performance and good stability of MoO2/Mo2C@3D NCF air electrode, the Li-O2 cells were still able to cycle up to 40 cycles with a discharge depth of 5000 mAh g-1 under a current density of 1000 mA g-1, maintaining an energy efficiency of 63% (2.21/3.05 V) with a low voltage difference of 0.84 V (Figure 3d). The improved rate performance of the Li-O2 battery is attributed to the unique hybrid architecture of MoO2/Mo2C@3D NCF, comprising intimate contacted MoO2/Mo2C nanocrystals embedded in hierarchical porous carbon matrix with both micrometer-sized open porosity and mesopores. In particular, the advantages of hierarchical porous 3D nitrogen-doped carbon matrix are: (1) micrometer open pores facilitate rapid oxygen diffusion and mesopores provide active sites for catalyzing Li-O2 reactions; (2) high electronic conductivity throughout the whole electrode by offering effective electron transportation networks and (3) uniform distribution of hetero-nanostructured MoO2/Mo2C nanocrystals on the support, ensuring effective synergistic ORR/OER electrocatalysis. Overall, MoO2/Mo2C@3D NCF air electrode showed enhancement performances in Li-O2 battery with excellent bifunctional catalytic activity for both OER and ORR, superior rate performance and good cyclability. XPS, proton nuclear magnetic resonance (1H NMR), TEM and selected area electron diffraction (SAED) analyses were conducted to investigate the formation of Li2O2 product and parasitic products (e.g. HCO2Li, CH3CO2Li and Li2CO3) on the discharged/charged MoO2/Mo2C@3DNCF cathode after ~140 cycles at a current density of 100 mA g-1. The XPS spectrum of Li 1s for the discharged and charged MoO2/Mo2C@3DNCF cathode is shown in Figure 4a. The signal at a binding energy of 54.8 eV corresponds to Li2O2, while the binding energy of 55.2 eV can be attributed to Li2CO3.63 The spectrum confirmed that high amounts of Li2O2 were indeed present. Further morphological characterization (Figure S11 and S12) on the discharged and charged electrodes were conducted. We found that the 3D architecture of MoO2/Mo2C@3DNCF cathode is well maintained after 1st discharge and charge process as shown in SEM images (Figures S11a and b), providing efficient pores for oxygen diffusion and plenty sites for Li2O2 deposition. Spherical Li2O2 nanoparticles (NPs) with estimated 100 nm in diameters were observed after 1st discharge process through



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both SEM (Figure S11a) and TEM (Figure S12) images. It is noteworthy that the formed Li2O2 during discharge process was absent after recharge process, leaving clean and porous surface of air electrode (Figure S11b), showing excellent catalyst performance of MoO2/Mo2C@3DNCF cathode on the reversibility of formation/decomposition of Li2O2. During recharging, trace amount of Li2CO3 can also be detected, and the formation of Li2CO3 on the charged cathode might be attributed to the residual electrolyte adsorbed on the electrode surface and the decomposition of carbon based materials. The parasitic products of HCO2Li and CH3CO2Li were analyzed using 1H NMR. The chemical shift at 8.37 ppm and 1.82 ppm corresponded to HCO2Li and CH3COLi respectively.2 During discharging, 0.78 µmol of HCO2Li and 0.35 µmol of CH3COLi were also detected on the MoO2/Mo2C@3DNCF cathode (Figure 4b). Typically, formation of the Li alkyl carbonates results in huge over-potential and poor cycling stability of Li-O2 battery. It is notable that trace amount of HCO2Li (0.32 µmol and CH3COLi (0.24 µmol) could be detected upon charging (Figure 4c). These results suggest that Li2O2 is the main discharge product and it can be oxidized when charging the MoO2/Mo2C@3DNCF cathode.

excellent bifunctional catalytic activity. Taken together, we showed that bio-inspired approaches open new avenues for the synthesis of 3D nitrogen doped carbon supported advanced functional materials with excellent electrochemical performances.

Conclusion

AUTHOR INFORMATION

In summary, a facile, bioinspired synthesis approach was adopted for the preparation of 3D N-doped carbon foam with MoO2/Mo2C nanocrystals. Here, recombinant proteins (ELK16-FLAG) provides the carbon matrix and served as the carbon source for Mo2C formation. We showed that by simply tuning the annealing temperature, we were able to obtain pure MoO2 or Mo2C, as well as their heteronanostructures. We found that MoO2/Mo2C@3DNCF showed exceptional overall rate performances and good cyclability in Li-O2 battery by providing bifunctional catalytic activity in both OER and ORR mechanisms. The desirable electrochemical performance can be attributed to the unique hierarchical porous structure of the 3D carbon foam. Particularly, we show here that hierarchical porous electrodes comprising micrometer-sized open porosity and mesopores (2-50 nm) can serve as an important model for controlled porosity, and demonstrate that they can significantly enhance the cycle stability and rate capability of the Li−O2 batteries. The micrometer-sized open porous framework of the prepared electrodes can provide facile accessibility of oxygen to the inner side of the air electrode and prevent the clogging of pores by discharge product, guarantee the effective formation/decomposition of lithium peroxide. While substantial mesopores supplied enlarged electrolyte/electrode contact areas as well as more active sites for catalyzing Li-O2 reactions, contributing to the exceptional polarization performance. Additionally, nitrogen doping in the carbon matrix further enhance not only the overall conductivity of the electrode, but also improve the catalyst activity due to the alteration of electronic structure .The synergistic effect between the MoO2 and Mo2C nanocrystals in a single domain also contributed to their

Corresponding Author

ASSOCIATED CONTENT Supporting Information. SEM images of lyophilized protein treated with Mo metal precursor and pure protein; TEM images of the carbon matrix in the MoO2/Mo2C@3D NCF sample; EDX mapping, XPS survey spectrum and nitrogen adsorption/desorption isotherms of the prepared MoO2/Mo2C@3D NCF sample; chemical physical characterizations (SEM images, TEM images, XRD patterns, raman spectrum and nitrogen adsorption/desorption isotherms) for as-synthesized control samples, including MoO2 @3D NCF , Mo2C@3D NCF, MoO2/Mo2C@3D NCF-0h and MoO2/Mo2C@3D NCF-1h samples, as well as MoO2 samples obtained with a different Mo precursor ((NH4)2MoO4) and a repetitive lysine-free protein template (GPG-AG3); SEM and TEM image of discharged MoO2/Mo2C@3DNCF cathode; the compositions of species in the prepared samples; an electrochemical performance comparison of our products to previously reported air electrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

* Email: [email protected] * Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge funding from Nanyang Technological University and Singapore MOE AcRF Tier 1 grants RG2/13 and RG113/15. The characterization work was performed at the Facility for Analysis, Characterization, Testing and Simulation in Nanyang Technological University.

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Scheme 1. Schematic illustrating the synthesis of MoO2/Mo2C@3D NCF sample with the assistance of ELK16-FLAG proteins



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Figure 1. (a) SEM and (b-d) TEM images of the as-prepared MoO2/Mo2C@3DNCF. Inset in (a) shows the as-annealed material. Inset in (b) shows the well-preserved porous structure of the carbon matrix after intensive sonication. Inset in (c) is the selected area electron diffraction patterns of (c) (Scale bar: 2 1/nm). Inset in (d) is an image of the area enclosed by the red box at higher magnifications. Yellow circles and red arrows in (d) indicate positions of the nanocrystals.


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Figure 2. XRD pattern (a), Raman spectrum (b), XPS spectrum of Mo3d (c), C1s (d) and N1s (e) of MoO2/Mo2C@3DNCF sample.



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Figure 3. Electrochemical performances of the as-prepared cathodes in Li-O2 batteries. (a) The first discharge/charge profiles for MoO2@3D NCF, MoO2/Mo2C@3D NCF and Mo2C@3D NCF cathodes at cathode with a discharge capacity of -1 -1 500 mA h g at a current density of 100 mA g ; (b) Cycling performance of MoO2/Mo2C@3D NCF battery at a discharge -1 -1 capacity of 500 mA h g at a current density of 100 mA g ; (c) Rate performances of MoO2/Mo2C@3D NCF battery with -1 a discharge capacity of 1000 mA h g under different current densities at voltage range of 2~4 V ; (d) High rate (1000 mA -1 -1 g ) cycling performance of MoO2/Mo2C@3D NCF battery with a discharge capacity of 5000 mA h g .


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Figure 4. (a) XPS spectra of Li1s and (b-c) H NMR spectra for discharged and charged cathode of MoO2/Mo2C@3DNCF, using benzene as internal standard for quantification purposes.


Mo2C

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