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Metal-organic Framework-Induced Synthesis of Ultra-small Encased NiFe Nanoparticles Coupling with Graphene as efficient oxygen electrode for rechargeable Zn-Air battery Jianbing Zhu, Meiling Xiao, Yelong Zhang, Zhao Jin, Zhangquan Peng, Changpeng Liu, Shengli Chen, Junjie Ge, and Wei Xing ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01503 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 10, 2016

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Metal-organic Framework-Induced Synthesis of Ultra-small Encased NiFe Nanoparticles Coupling with Graphene as Efficient Oxygen Electrode for Rechargeable Zn-Air Battery Jianbing Zhu†,‡, Meiling Xiao†,‡, Yelong Zhang†,‡, Zhao Jin§, Zhangquan Peng†, Changpeng Liu§, Shengli Chen&, Junjie Ge*,§ and Wei Xing*,†,‡ †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese

Academy of Sciences, Changchun, Jilin, 130022, China. ‡

University of Chinese Academy of Sciences, Beijing 100039, China

§

Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Changchun, 130022, China

&

Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Hubei Key Laboratory

of Electrochemical Power Sources, Department of Chemistry, Wuhan University, Wuhan, 430072, China

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ABSTRACT: Rational design of electrocatalysts to replace the noble-metal-based materials for oxygen reactions is highly desirable but challenging for rechargeable metal–air batteries. Herein, we demonstrate a unique two stage encapsulation strategy to regulate the

structure

performance of catalysts featured with thin graphene nanosheets coupling with full

and

encapsulated

ultra-fine and high loaded (~25wt%) of transition metal nanoparticles (TMs@NCX) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). By optimizing the electronic modulation effect from suitable metal cores, the best NiFe@NCX catalyst exhibits high stability and activity with an onset potential 1.03 V for ORR and an overpotential of only 0.23 V at 10 mA cm-2 for OER, which is superior to commercial Pt/C and IrO2 catalysts. Rechargeable Zn-air battery using NiFe@NCX catalyst exhibited an unprecedented small charge–discharge overpotential of 0.78 V at 50 mA cm-2, high reversibility and stability, holding great promise in the practical implementation of rechargeable metal–air batteries.

KEYWORDS: metal−organic framework; bifunctional electrocatalyst; non-noble metal; electronic structure, rechargeable Zn-Air battery


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Rechargeable metal-air batteries (Zn-air and Li-air) are drawing increasing attention in energy storage and conversion applications due to their high energy densities.[1-6] In order to make these devices commercially available, considerable attention has been paid to develop economically feasible, earth-abundant, active, and stable non-precious electrocatalysts (NPCs).[7-16] Recently, hybrid materials with transition metals (TMs) encapsulated in nitrogen-doped graphitic layers were found efficient electrocatalysts for reactions on both directions.[17-22] The catalytic activity of these catalysts is strongly dependent on two crucial factors: on one hand, the electronic structure of surface graphitic layer, which can be adjusted by electronic modulation effect from suitable metal cores, determines the turnover frequency (TOF) per active site (intrinsic activity); on the other hand, the density of active sites, i.e. the effective area of modulated surface carbon, is a function of metal core size, loading, and dispersion. Consequently, tremendous work can be done to controablly tune the activity in guidance of the two aspects. Unfortunately, the required high temperature synthesis of such hybrid catalysts (TMs encased in graphitic layers) puts forward severe structure control problems, where most catalysts suffer from the following disadvantages: i) the agglomeration of metal particles during pyrolysis process leads to the non-uniform metal particle distribution and large particle size, from tens to hundreds of nanometers;[23-25] ii) the loading of TMs are usually rather low, typically below 10 wt.%, due to the lack of efficient synthetic technique;[26-28] iii) the nanoparticles of TMs sometimes are partially encapsulated.[29,

30]

The coarse metal cores with non-uniform metal

particle size distribution and low metal loading are expected to significantly limit the catalytic activity due to the decreased TOF and number of active sites. What’s worse, the metal core is easily dissolved in an electrochemical environment due to partial encapsulation, leading to poor catalytic stability. Therefore, there still remains considerable room to address the intrinsic


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problems associated with the reported alternative catalysts, such as the inefficiency in synthesis, improper building architecture, and limited metal content/or active sites density. Metal−organic framework (MOF) constructed by organic linkers and metal ions can act as perfect encapsulators for metal ions due to their well-defined coordination environment and ordered arrangement in the framework.[31-33] However, high temperature pyrolysis often leads to structure collapse, thus making further growth of metal particles uncontrollable. In this paper, we have designed a unique two stage encapsulation technique to acquire carbon-encapsulated Ni, Fe, or NiFe2 alloy nanocrystrals (denoted as Ni@NCX, Fe@NCx, NiFe@NCx, respectively) with ultra-small core size, high metal loading, and high dispersion. As illustrated in Scheme 1, MOF based NiFe (or single Ni, Fe metals)-MILs (MIL-88b, details seen in Supporting Information, crystal structure shown in Figure S1) is employed as a metal ion encapsulator at low temperature. The melamine is adopted as both N source and soft template at high temperature (after breakdown of MOF), which forms graphitic carbon cages on TMs, thus effectively confining the metal particles from further growth. Moreover, the newly developed technique allows MOF without nitrogen source to be used in catalyst synthesis, thereby opening more choices for the selection of metal cores. By optimizing the electronic modulation effect from suitable metal cores, the synthesized catalyst exhibited the extraordinary performance for both ORR and OER. As shown in Figure 1a and S2, the TMs@NCX catalysts synthesized through the two stage encapsulation technique exhibit a typical three-dimensional, flake-like structure under SEM characterization, with the original ordered MOF structure disappeared during pyrolysis (800 oC) (Figure S3). Transmission electron microscopy (TEM) images (Figure 1b-c and S4, S5) show the flake like structure consists of thin graphene nanosheets embedded with highly dispersed and ultra-fine TMs nanoparticles. The average particle sizes (Figure 1b, Figure S5) of


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Scheme 1 Schematic illustration of the synthetic strategy of the TMs@NCxcomposite.

NiFe@NCx, Ni@NCx, and Fe@NCx samples are 7.8, 10.8, and 9.2 nm, respectively. Highresolution TEM (HR-TEM) reveals that TMs are well encapsulated by 3~5 graphitic layers (Figure 1e, S5), indicating the formation of TMs@NCX core-shell structure. The catalyst with NiFe core shows Ni and Fe are mixed at atomic level, with lattice fringe of 2.06 Å, corresponding to plane in cubic NiFe2 phase, as shown in Figure 1e inset. Scanning TEM energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping further verifies the uniform dispersion of Fe, Ni, C and N in the hybrids (Figure 1f-i). The total metal loadings in all three catalysts were at approximately 25 wt.%, as confirmed by ICP analysis (Table S1). To our best knowledge, this is the first report that TMs@NCX catalysts with such small core size and high metal loading are simultaneously acquired using high temperature pyrolysis method, evidencing the high effectiveness of the two stage encapsulation technique. The respective function of MOF and melamine in structure directing are further elucidated by carrying out fractionated pyrolysis experiments. Temperature dependent structure evolution of


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NiFe-MIL (without melamine) is shown in Figure 2a, b and S6. At low temperatures ( 0.9 indicates the presence of channels connecting macropores, which might correspond to the 3D architecture built by the graphene nanosheets.[40,

41]

The pore size distribution results

(Barrett-Joyner-Halenda (BJH) method use the N2 desorption branch) confirm the mesoporous characteristics of the synthesized catalysts (Figure S10 and Table S3), which provides plenty of catalytic active sites and facilitates electron and mass transport during electrocatalysis process. Raman spectra were measured, with D and G bands noticed at 1320 and 1590 cm−1, respectively (Figure S11). Low ID/IG (ratio of integrated intensities between the D band and G band) values were acquired on all the samples, suggesting that highly ordered graphitic structure has been formed.

Interestingly,

the

ID/IG

values

decrease

in

the

order

of

NiFe@NCx-

P>Ni@NCx>NiFe@NCx >Fe@NCx (Figure S10b), probably due to the better catalysis effect of Fe on the formation of ordered graphitic structure. The low value of ID/IG indicates the good


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Figure 3 (a) XRD pattern of the TMs@NCX samples; (b) desorption isotherms of TMs@NCX samples; (c) deconvoluted high-resolution N 1s XPS spectra of NiFe@NCx; (d) deconvoluted high-resolution C 1s XPS spectra of NiFe@NCx; (e) Fe 2p and (f) Ni 2p XPS spectra of these TMs@NCX samples.

electrical conductivity of the as-prepared catalysts (even better than the commercial conductive carbon black, Vulcan XC-72), which is confirmed by the results from electrochemical impedance spectroscopy (Figure S12). X-ray photoelectron spectroscopy (XPS) results reveal the presence of C, O, N, Ni and/or Fe (Figure S13) in the catalysts. Particularly, nitrogen doping level was found decrease in the order of Ni@NCx>NiFe@NCx>NiFe@NCx-P>Fe@NCx (Table S1). The higher nitrogen content in NiFe@NCx compared to NiFe@NCx-P may be attributed to the more effective capture of carbon nitride gases (e.g. C2N2 +, C3N2 +, C3N3 +) during pyrolysis, benefitted from the ultrafine nanoparticles and the porous frame of MOF. High-resolution N 1s XPS spectra confirm the presence of pyridinic N (398.9 eV) and quaternary N (400.9 eV) (Figure 3c and S14),[42, 43] with quantified results summarized in Table S1. Of particular interesting is the highresolution C 1s XPS spectrum (Figure 3d and S14), where a new peak at 282.2–282.7 eV is observed. The newly formed peak, also observed in our previous work[44], is attributed to the


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graphene surface signals modulated by the electron penetrated from the encased TMs. Ni 2p and Fe 2p spectra are shown in Figure 3e, f, which suggest the metallic state of Ni and Fe in all TMs@NCx samples. After forming NiFe alloy, signals of both Ni and Fe were shifted, thus resulting in fine-tuning on the electronic structure of the outer carbon layers, as confirmed by the more suitable bonding energy and distinct peak intensity of the modulated C 1s signal. The electrocatalytic performances of the prepared materials for ORR were evaluated systematically in 0.1 M KOH (Figure 4a-d, S15, 16, Table S4). The structure advantage of NiFe@NCx over NiFe@NCx-P is confirmed in Figure 4a, with a 66 mV positive shift in half wave potential been observed. The much higher E1/2 of NiFe@NCX (0.86V) than Ni@NCX (0.78V) and Fe@NCx (0.84V) suggests the effectiveness of electronic structure modulation from core, which was further supported by kinetic currents evaluation (Figure 4b) and onset potential (1.03V for NiFe@NCx). Considering that NiFe@NCX possesses a much lower N content (3.63wt.%) than the Ni@NCX (6.11 wt.%), the much enhanced activity of the former indicates the successful boosting of catalytic activity by the rational design of metal cores. The Tafel slop of the synthesized catalysts were also measured and compared with the Pt/C catalysts (Figure 4c and Figure S17), interestingly, the NiFe@NCx catalyst outperformed the Pt/C catalyst (40 µg/cm2Pt) by 26 mV in E1/2, with the same rate determining step observed in Tafel slope. A 4-electron pathway is found to dominate the ORR using Kouteckey-Levich plots (n = 4.1) and RRDE experiments (n = 3.96) (Figure 4d, S15). In order to be used in metal-air rechargeable batteries, reactions on both directions needs to be fast, thus, the catalytic activities of different electrodes for OER were evaluated and shown in Figure 4e and S18. Excitingly, NiFe@NCx substantially outperformed all other catalysts in the aspect of onset potential and current densities at fixed potentials. Specifically, the NiFe@NCx


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catalyst exhibited onset overpotential of only 0.23 V and required an overpotential of only 0.32 V to reach current density of 10 mA cm−2. This overpotential is not only much smaller than

Figure 4 (a) ORR polarization curves for TMs@NCX samples in O2-saturated 0.1 M KOH (5 mV/s, 1600 rpm); (b) kinetic current density (jk) of ORR at different potentials on different catalysts; (c) Tafel plots for NiFe@NCX and Pt/C catalysts (a); (d) liner sweep voltammetry (LSV) for oxygen reduction on NiFe@NCX catalyst in O2-saturated 0.1 M KOH at various rotation speeds; insert: Koutecky-Levich (K-L) plots derived from the LSV results； (e) OER polarization curves of TMs@NCX samples and commercial IrO2, Pt/C catalysts; (f) the Tafel slops of OER for TMs@NCX samples; (g) ORR polarization plots of NiFe@NCX before and after 20,000 potential cycles in O2-saturated electrolyte; insert: chronoamperometric measurement (CA) of NiFe@NCX catalyst within 36,000 s at 0.8 V. (h) OER polarization curves of NiFe@NCX catalyst before and after 1,000 potential cycles; insert: CA of NiFe@NCX catalyst within 36,000 s at 1.55 V.

that of Ni@NCX (Eonset: 0.33V), Fe@NCX (0.38V, 0.45 V), and NiFe@NCx-P (0.25V, 0.37 V), but also significantly smaller than the state-of-the-art IrO2 catalysts (0.25 V, 0.37V, consistent with reported values). At 1.65 V, the NiFe@NCX catalyst generated current density of 22.42 mA cm-2, outperforming IrO2, Ni@NCx, and Fe@NCx by 4.1, 47.4, and 15.5 times, respectively. The catalytic kinetics for oxygen evolution was evaluated by Tafel plots (Figure 4f, S19), where lower Tafel slope was obtained on the NiFe@NCX (60.6 mV dec−1) catalyst in comparison to the


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Ni@NCx (226 mV dec-1), Fe@NCX (83.9 mV dec−1), NiFe@NCX-P (101 mV dec-1), and IrO2 (97 mV dec−1), suggesting a more favorable OER kinetics on the NiFe@NCX electrode.[45-49] Thus, our NiFe@NCX is found as one of the best non-precious bifunctional catalysts reported (Table S5, S6). Strong durability toward ORR/OER is of great significance for practical applications. The stability of the NiFe@NCx catalyst was evaluated by accelerated durability tests (ADT) and chronoamperometric measurements (CA). For ORR, the NiFe@NCx catalyst shows only 5 mV negative shift in half-wave potential and almost unchanged Tafel slop after 20,000 cycles (Figure 4g and Figure S20), while the counterpart Pt/C catalyst shows a value of 17 mV (Figure S21). CA results (0.8 V, insert Figure 4g) shows a current density attenuation of 2.7% for the NiFe@NCx catalyst, in comparison to much higher decrease for the Pt/C catalyst (31.9%) in the CA test (Figure S22). For OER, after 1,000 continuous potential cycles, the required overpotential for a current density of 10 mAcm−2 remained almost the same (Figure 4h) during cycling with the similar Tafel slop (Figure S23). Decrease in current density at higher overpotential was observed, which may be ascribed to the catalysts detachment from the glassy carbon electrode caused by large volume of O2 generated. The chronoamperometric result further demonstrates the outstanding stability of NiFe@NCX, showing an anodic current attenuation of 13.8% within 36,000s (insert Figure 4h), whereas IrO2 displays a 2.2 times larger current attenuation of 30.4% (Figure S24, S25). The structure properties of the NiFe@NCX catalyst were further examined after ADT test: i) XRD pattern of the catalyst after ADT test shows no phase alternation in comparison to the fresh NiFe@NCX (Figure S26); ii) no obvious change in the catalyst micro-structure occurred and the crystalline size of NiFe alloy maintained almost the same, as confirmed by TEM (Figure S27); iii) no Ni and Fe signals were detected from the


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electrolyte after ADT test by ICP-OES (Table S7). All these results indicate the highly stable nature of NiFe@NCx material even in the harsh testing condition for OER, due to the steady encapsulation structure and anticorrosive graphitic layers. Thus, the hybrid NiFe@NCX successfully meets our satisfactory in terms of both activity and stability when acting as bifunctional catalysts. To reveal the real effect of encased TMs on their intrinsic catalytic activities, we calculated the turnover frequency (TOF) of the synthesized TMs@NCx (calculation detail is shown in experimental section and Figure S28). As shown in Figure 5a and Table S8, the NiFe@NCx catalyst gives an extraordinary TOF value of 3.58αCs×10-4 for ORR at 0.85V and 1.02αCs×10-2 for OER at 1.70V (α donate as the ECSA generated by 1mol active carbon sites, Cs is the specific capacitance of the samples in 0.1 M KOH). While the TOF for ORR is 34.6 times and 1.32 times that of Ni@NCx (1.03αCs×10-5) and Fe@NCx (2.72αCs×10-4) catalysts, the intrinsic activity for

Figure 5 (a) TOF of the TMs@MCx catalysts for ORR at 0.85 V and OER at 1.70 V; (b) the N/C ratio versus onset potential for ORR and OER of TMs@NCX catalysts, (c) the pyridinic N content versus onset potential for ORR and OER of TMs@NCX catalysts.

OER is increased by 25.5 (4.01αCs×10-4 for Ni@NCx) and 4.2 (2.2αCs×10-3 for Fe@NCx) times, respectively. The above calculations suggest the intrinsic activity of TMs@NCx is highly dependent on the metal cores, where the electronic structure of surface active sites was efficiently turned (see C 1s XPS). The more effectiveness of NiFe core on catalytic performance


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enhancement was further evidenced by the N-doping-actvity relationship analysis. It has been long accepted that nitrogen doping in carbon materials (or metal/carbon composites) is correlated with the increase in catalytic activity of the ORR/OER. Especially, the recent work demonstrated that for N-doped carbon ORR catalysts, the active sites are carbon atoms adjacent to pyridinic N.[50] Here, total N content as well as pyridinic N content in varied catalysts was plotted against the ORR and OER performances, as shown in Figure 5b, c. Clearly, the NiFe@NCX sample exhibited the highest ORR and OER activity but possesses relative lower total and pyridinic nitrogen content. Combining above analysis and the structure features of TMs@NCX catalysts, we may conclude: i) the electronic structure of the graphene surface can better be modulated by the NiFe core, thus offset the contribution of the nitrogen dopants to the catalytic activity; ii) decreasing the size of NiFe nanoalloy further facilitates the electron penetration from the metal core to surface graphitic layer; iii) the abundant pore structure in MOF ensures high nitrogen content in the final material. A homemade single-cell Zn-air battery was fabricated and tested to reveal the in-situ cell performance of our bifunctional catalyst under real battery operating conditions (Figure 6a). Figure 6b presents polarization and power density curves for Zn–air batteries based on the NiFe@NCx and Pt/C+IrO2 cathodes, respectively. The NiFe@NCx catalyst showed larger current density and peak power density than those of the Pt/C+IrO2 catalyst. The specific capacity of the NiFe@NCX based battery was estimated to be 583.7 mAhg-1 (Figure S29), corresponding to a gravimetric energy density of 732.3 Wh kgZn-1, which is higher than the Pt/C+IrO2 based battery (specific capacity 499.1 mAh g-1, energy density 543.2 Wh kgZn-1). Figure 6c shows the discharge and charge polarization curves for Zn-air batteries with NiFe@NCx and commercial catalysts (Pt/C and IrO2) cathode. Obvious lower charge-discharge


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voltage gap was observed with NiFe@NCx cathode compared to the commercial one, indicating a better rechargeability. Especially, the sum of the charge-discharge overpotential for the NiFe@NCx cathode-based Zn-air battery is 0.78 V at a current density of 50 mA cm-2, which is

Figure 6 Performances of rechargeable Zn-air batteries employed NiFe@NCX and IrO2+20% Pt/C catalysts: (a) schematic depiction of the Zn-air battery structure; (b) discharge curves and power density of zinc-air batteries; (c) charge and discharge polarization curves for the batteries; (d) the cycle stability of rechargeable Zn-air battery, the galvanostatic discharge-charge cycling curves were performed at 10 mA cm−2 and a duration of 600 s per cycle.

significantly lower than that of the Pt/C+IrO2 cathode (1.1 V). The overpotential is the most crucial benchmark in estimating a bifunctional electrocatalyst. The NiFe@NCx cathode-based battery showed a initial voltage gap of 0.39 V and a high round-trip efficiency of 76.7% at 10 mA cm-2. After 205 cycles, slight performance loss was found on the NiFe@NCx cathode (0.29


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V increasing in the voltage gap), while commercial cathode demonstrated a higher increase in the voltage gap (0.48 V, Figure 6d). The relatively lower overpotential exhibited by NiFe@NCx cathode indicate good rechargability of the battery with the exception of the prominent ORR and OER activities. The single-cell results clearly signify that the more applicable of NiFe@NCXbased electrocatalysts than Pt/C+IrO2 in practical conditions. In summary, we have constructed a hierarchical architecture that consists of ultra-small transition metal nanoparticles encased in graphitic layers coupling with graphene sheet with the aid of MOF (TMs@NCx). Electrochemical measurements showed that NiFe@NCx displays the best ORR and OER activity among the carbon-based electrocatalysts tested in alkaline medium to date. The superb electrocatalytic performance originates from the modulation of the electronic structure of outer carbon layers by electron penetration from the NiFe core. Reducing the size of encapsulated nanoalloy can significantly increase the active sites density and the electron density in the graphene shells, which further enhances the ORR and OER activity. We also demonstrate that the NiFe@NCX is a talented non-noble metal electrocatalyst for both ORR and OER in rechargeable Zn–air batteries comparison to noble metal (Ir and Pt) based ones. These findings pave new ways towards the development of high-performance, inexpensive ORR and OER electrocatalysts. ASSOCIATED CONTENT Supporting Information Detailed experiments, TEM images, elements mapping, EDX, XRD, XPS spectra and additional electrochemical results. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Corresponding Author Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21373199), the National Basic Research Program of China (973 Program, 2012CB215500) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA09030104).

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