Restricting Growth of Ni3Fe Nanoparticles on Heteroatom-Doped

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Energy, Environmental, and Catalysis Applications

Restricting Growth of Ni3Fe Nanoparticles on Heteroatoms Doped Carbon Nanotube/Graphene Nanosheets as Air-Electrode Electrocatalyst for Zn-Air Battery Chenglong Lai, Jie Wang, Wen Lei, Cuijuan Xuan, Weiping Xiao, Tonghui Zhao, Ting Huang, Lingxuan Chen, Ye Zhu, and Deli Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13751 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

Restricting Growth of Ni3Fe Nanoparticles on Heteroatoms Doped Carbon Nanotube/Graphene Nanosheets as AirElectrode Electrocatalyst for Zn-Air Battery Chenglong Laia‡, Jie Wanga,b‡, Wen Leia, Cuijuan Xuana, Weiping Xiaoa, Tonghui Zhaoa, Ting Huanga, Lingxuan Chena, Ye Zhub, Deli Wanga* a

Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of

Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b

Department of Applied Physics, The Hong Kong Polytechnic University, Hung Horn,

Kowloon, Hong Kong 999077, China * Corresponding author * E-mail: [email protected]. ‡ These

authors contributed equally to this work

KEYWORDS: Zn-air battery, Bifunctional electrocatalyst, Heteroatoms doped carbon material, NiFe based, Synergistic effects ABSTRACT: Exploring bifunctional oxygen electrode catalysts with efficient and stable ORR/OER performance is one of the limitations for high performance Zinc-air battery. In this work, Ni3Fe alloy nanoparticles incorporated in a 3D carbon nanotube/graphene nanosheet composites with N and S co-dopping (Ni3Fe/N-S-CNTs) as bifunctional oxygen electrode electrocatalysts for Zinc-air battery. The main particle size of Ni3Fe nanoparticles 1

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could be well restricted because of the unique 3D structure of carbon nanotube/graphene nanosheets composites (N-S-CNTs). The large specific area of N-S-CNTs is conducive to the uniform dispersion of Ni3Fe nanoparticles. Based on the synergistic effect of Ni3Fe nanoparticles with N-S-CNTs, and the sufficient exposure of reactive sites, the synthesized Ni3Fe/N-S-CNTs catalyst exhibits excellent OER performance with a low overpotential of 215 mV at 10 mA cm-2, and efficient ORR activity with a half-wave potential of 0.877 V. When used as a electrocatalyst in Zinc-air battery, the device exhibits a power density of 180.0 mW cm-2 and long term durability for 500 h. Introduction Developing advanced electrochemical energy devices has drawn great concerns with the continuous development of electric vehicles and various types of electronic products. Rechargeable Zn-air battery has been widely investigated and regarded as one of promising candidates for renewable energy devices because of its safety operation and high theoretical energy density.1-12 The reaction of oxygen cathode in Zn-air battery is O2 + 2H2O + 4e- ↔ 4OH–, which involves OER during the charge process, and ORR in the discharge process.13, 14 However, the huge overpotential gap during the charge/discharge would seriously hinder the performance of zinc-air battery.15, 16 Therefore, it is of critical importance to explore bifunctional catalysts with efficient OER/ORR activity for the development of Zinc-air battery. Although precious metal based material exhibits excellent electrocatalytic activity, 17, 18 the high cost and poor stability of precious metal pose a great challenge for large-scale 2


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practical application.19,

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Recently, non-precious metal catalyst has aroused increasing

interests because of the good electrocatalytic activity and stability in alkaline electrolyte.1926

Among them, NiFe based material has been widely studied as one of the most efficient

OER catalyst since Corrigan first put forward that the OER performance of nickel would be dramatically enhanced with little Fe decoration.27-32 However, most of the reported NiFe based catalysts exhibit poor ORR catalytic activity, restricting the development of bifunctional NiFe based catalysts in practical applications.30 One of promising strategies is to support NiFe based materials on heteroatoms doped carbon by using the merits of carbon materials.28, 33 Heteroatoms (P, S, or N) doped carbon material exhibits high-efficiency ORR activity which could be demonstrated as alternative materials to replace Pt based catalysts.34-37 Furthermore, the unique 2D/3D structure of carbon materials could restrict the growth of alloy particles and hinder particles aggregation. Meanwhile, the large specific area of carbon materials allows metal particles to be more evenly dispersed, which benefit to exposure more reactive sites and thus improve the electrocatalystic performance.38,

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Although some progress has been made towards preparing the NiFe and carbon composites, the enhancement of ORR activity on NiFe based material is limited, so the electrocatalytic performance of NiFe based materials is still needed to be further improved. Recently, N and S co-doped partially exfoliated multi-walled carbon nanotubes (N-SCNTs) has been successfully prepared in our group and exhibits excellent ORR performance.40 Considering of the unique 3D structured and large specific area of N-S3



CNTs, it can be used as a suitable carbon carrier for enhancing the ORR performance of NiFe based materials. Herein, a highly efficient bifunctional NiFe based electrocatalyst was synthesized by introducing the N-S-CNTs as carbon support (Ni3Fe/N-S-CNTs). The well-constructed Ni3Fe/N-S-CNTs catalyst exhibits more efficient bifunctional activity for OER/ORR relative to Ir/C and Pt/C. As oxygen electrode catalysts in Zinc-air battery, a power density of 180.0 mW cm-2 and long term durability of 500 h are obtained. 1. Results and discussions Scheme 1. Schematic illustration for the preparation of Ni3Fe/N-S-CNTs catalyst.

The synthesis procedure of Ni3Fe/N-S-CNTs composites is illustrated in Scheme 1. The N-S-CNTs was prepared according to our previous work.40 The carbon nanotube/graphene nanosheet composite was prepared by partially exfoliation of multi-walled carbon nanotubes (MWCNT) to form graphene nanoribbons attached to intact inner walls of nanotubes using the modified Hummers method, which was denoted as partially exfoliated carbon nanotubes (pe-CNTs). The pe-CNTs is subsequently mixed with thiourea which is used as N and S dopant, and heat-treated at 900 oC in an inert atmosphere to form N-SCNTs. Ni3Fe nanoparticles were then supported on N-S-CNTs via an impregnation4


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reduction method by using NiCl2, FeCl2 as precursors. XRD measurement (Figure S1a) was conducted to analyse the structure of as-prepared catalysts. The peak at 26 ° is corresponding to (002) plane of carbon and these peaks at 75.8°, 51.5° and 44.2°, which attributes to the (220), (200) and (111) planes of Ni3Fe, respectively, revealing that Ni3Fe alloy is successfully synthesized. Furthermore, TGA measurement (Figure S1b) was conducted to investigate the specific metal content of as-prepared catalyst. The weight percentage of Ni3Fe/N-S-CNTs catalyst is almost same when the temperature is below 360 °C, indicating a good thermal stability. There is a weight increase with gradually increasing the temperature to 450 °C, demonstrating the oxidation process of Ni3Fe alloy into metal oxide. A sharp weight loss could be observed with further increasing the temperature due to the combustion of carbon. The remained metal oxide mass fraction is 55.98%, and the specific content of Ni3Fe in Ni3Fe/N-S-CNTs is about 42.72% after calculation.

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Figure 1. (a) TEM image of Ni3Fe/N-S-CNTs. (b) HRTEM image of Ni3Fe/N-S-CNTs. (c) HAADF-STEM image and corresponding EELS maps of Ni (d), Fe (e), O (f), the composites of Fe, Ni, and O (g) in a Ni3Fe particle. The microstructure of synthesized catalysts was characterized by transmission electron microscopy (TEM). The carbon support, N-S-CNTs, is characterized via high-angle annular dark field STEM which exhibits unique 3D structure with graphene nanosheets attached to intact inner walls (Figure S2a-b). Besides, the specific surface area of N-SCNTs could be apparently increased after the structure conversion from one dimension to three dimensions.40 The BET measurement indicates that N-S-CNTs and Ni3Fe/N-S-CNTs exhibit specific surface area of 451.12 and 302.35 m2 g-1 (Figure S3), respectively, which is much larger than Ni3Fe/CNTs (89.12 m2 g-1). Besides, Ni3Fe/N-S-CNTs exhibits a main pore size distribution at 16 nm. The overview TEM image (Figure 1a) shows that Ni3Fe 6


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particles are uniformly scattered on N-S-CNTs supports, with a mean particle size of 31.6 nm, which is smaller than that of Ni3Fe/CNTs (130 nm, Figure S2c and Figure S4). The smaller Ni3Fe nanoparticles on N-S-CNTs could be ascribed to the unique 3D structure and relatively large specific surface area of N-S-CNTs which could not only restrict the growth of particles, but also hinder particles aggregation, in some extent. The high-resolution TEM image of Ni3Fe/N-S-CNTs catalyst shows characteristic spacing of 0.205 nm (Figure 1b), which attributes to the (111) plane of Ni3Fe alloy. The EELS mapping for a selected particle was obtained to analyse elemental distribution. Iron is enriched on the surface of the particle, while nickel is located biased toward the interior of the particle (Figure 1c-e). This phenomenon could be interpreted by the segregation theory, where Ni serves as host in Ni3Fe particles and Fe as solute, after forming alloy nanoparticles at moderate annealing temperature, such alloy would exhibit obvious antisegregation phenomenon.41 Additionally, oxygen is appeared at the outer side of the particle (Figure 1f-g), which is attributed to surface oxidation of the particle when exposed to air. XPS characterization was conducted to further understand the surface valence states and bonding configurations of Ni3Fe/N-S-CNTs catalyst. The typical peaks of N/S in Ni3Fe/N-S-CNTs and N-S-CNTs catalysts are observed in the full-range XPS spectra (Figure S5a), which demonstrates N/S have successfully doped into pe-CNTs. Besides, the fine spectrum of C 1s in Ni3Fe/N-SCNTs (Figure S5b) is fitted into six peaks, in which peaks at 286.10 eV and 284.30 eV correspond to the -C-N and -C-S bond, further confirming the successfully doping of N/S elements into the pe-CNTs frameworks.40 Besides, the Electrochemical impedance 7



spectroscopy for synthesized catalysts (Figure S6) were obtained to study the charge transfer resistance. The N-S-CNTs and Ni3Fe/N-S-CNTs exhibit similar polarization resistance (Rp) of 0.45 Ω and 0.42 Ω, respectively, which is slightly smaller than Ni3Fe/CNTs (0.6 Ω) and pe-CNTs (2.1 Ω). These results indicate the excellent electrical conductivity of the N-S-CNTs and Ni3Fe/N-S-CNTs catalysts which could enhance electron transfer rate during electrochemical process. The OER performance of synthesized catalysts was investigated in 1.0 M KOH solution. The polarization curve of Ni3Fe/N-S-CNTs catalyst exhibits a clear negative shift and higher current density at identical potential compared with other catalysts (Figure 2a and Figure S7a). For further understanding the OER activity of Ni3Fe/N-S-CNTs catalyst, the overpotential is calculated (EOER= Ej=10mA cm-2 - 1.23 V, Figure 2b), which follows in the order: Ni3Fe/N-S-CNTs (0.215 V)  Ni3Fe/CNTs (0.252 V)  Ir/C (0.294 V)  N-S-CNTs (0.359 V). This result indicates the excellent OER catalytic activity of Ni3Fe/N-S-CNTs. The better OER catalytic activity of Ni3Fe/N-S-CNTs than Ni3Fe/CNTs could be elucidated by the enlarged electrochemical active area (ECSA). The ECSA could be qualitatively compared by electrochemical double-layer capacitance (Cdl). The Ni3Fe/NS-CNTs exhibits a much larger Cdl value (7.18 mF cm-2) relative to Ni3Fe/CNTs (1.34 mF cm-2) (Figure S8). The result suggests that the ECSA of Ni3Fe/N-S-CNTs is about 4 times higher than that of Ni3Fe/CNTs, which is ascribed to the large specific area (Figure S9) and unique 3D structure (Figure S2b) of N-S-CNTs. Besides, Ni3Fe nanoparticles with smaller particle size homogeneously dispersed in the N-S-CNTs (Figure 1a), which is 8


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beneficial to expose more reactive sites for the OER.14, 21 The Tafel plots of synthesized catalysts obtained from Figure 2a and Figure S7a are used to further study the electrode kinetics (Figure 2c and Figure S7b). Ni3Fe/N-S-CNTs exhibits a Tafel slope value of 44.1 mV dec-1, which is smaller than Ni3Fe/CNTs (50.1 mV dec-1), Ir/C (55.2 mV dec-1), and N-S-CNTs (264.5 mV dec-1), suggesting its superior electrocatalytic reaction kinetics. The OER durability, a key parameter for the application aspects, was tested via chronopotentiometry measurement (Figure 2d and Figure S7c). Specifically, Ni3Fe/N-SCNTs electrode exhibited negligible change of potential after continuous operations of OER for 10 h, while the other catalysts showed severe activity loss, indicating the superb OER catalytic stability of Ni3Fe/N-S-CNTs. All the aforementioned results (superb catalytic stability, superior reaction kinetics, and low overpotential) indicate that the excellent OER catalytic performance of Ni3Fe/N-S-CNTs catalyst.

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Figure 2. (a) The OER polarization curves comparison of Ir/C, Ni3Fe/N-S-CNTs, Ni3Fe/CNTs, and N-S-CNTs catalysts in 1.0 M KOH (sweep rate: 5.0 mV s-1; rotation rate: 1600 rpm). (b) Histogram of overpotentials at 10.0 mA cm-2 (EOER = Ej=10mA cm-2 - 1.23 V). (c) Tafel plots for synthesized catalysts were obtained from Figure 2a. (d) OER durability evaluation by chronopotentiometry response at 10 mA cm-2 of the N-S-CNTs, Ni3Fe/N-S-CNTs and Ni3Fe/CNTs (rotation rate: 1600 rpm) The CV curves of synthesized catalysts (Figure. S10) were obtained to investigate ORR activity. According to the observed oxygen reduction peaks in O2-saturated solution, Ni3Fe/N-S-CNTs exhibits a more positive peak potential (0.855 V) relative to N-S-CNTs (0.830 V), Fe/N-S-CNTs (0.825 V), Ni3Fe/CNTs (0.740 V) and Ni/N-S-CNTs (0.820 V) 10


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catalysts. Besides, the polarization curves of synthesized catalysts were investigated to further illustrate ORR activity (Figure 3a and Figure S11a). Ni3Fe/N-S-CNTs shows a more positive half-wave potential (0.877 V) relative to Pt/C (0.848 V), N-S-CNTs (0.855 V), Ni/N-S-CNTs (0.857 V), Fe/N-S-CNTs (0.836 V) and Ni3Fe/CNTs (0.648 V) catalysts. The overpotential (EORR = 1.23 V - E1/2) of synthesized catalysts are calculated and shown in Figure 3b, which follows in the order: Ni3Fe/N-S-CNTs (0.353 V)  N-S-CNTs (0.375 V)  Pt/C (0.382 V)  Ni3Fe/CNTs (0.582 V). Obviously, Ni3Fe/N-S-CNTs catalyst exhibits a much lower overpotential than others, demonstrating the excellent ORR activity. The improved ORR electrocatalytic activity of Ni3Fe/N-S-CNTs relative to N-S-CNTs is probably due to the synergistic effect of Ni3Fe nanoparticles with N-S-CNTs.44-46 Besides, the Tafel plots of synthesized catalysts were obtained from the ORR polarization curves (Figure 3c and Figure S11b).16, 47 Ni3Fe/N-S-CNTs exhibits the smallest Tafel slope value (43.2 mV dec-1) relative to N-S-CNTs (52.3 mV dec-1), Pt/C (56.7 mV dec-1) and Ni3Fe/CNTs (57.1 mV dec-1), indicating fast catalytic reaction kinetics on Ni3Fe/N-SCNTs. Moreover, the ORR curves at different rotation speeds are conducted to calculate electron transfer number (Figure 3d), and it is calculated to be close to 4 at potentials between 0.8 and 0.9 V, suggesting the 4-electron reduction pathway. The stability of Ni3Fe/N-S-CNTs catalysts was investigated by using chronoamperometric method (Figure 3e and Figure S11c). Ni3Fe/N-S-CNTs shows only 13% loss of initial current, which is better than N-S-CNTs (19.4%), Pt/C (29.1%), Fe/N-S-CNTs (21%) Ni3Fe/CNTs (18.2%) and Ni/N-S-CNTs (22.6%) after 10 h measurement for ORR, indicating excellent long11



term durability of Ni3Fe/N-S-CNTs. Generally, the overall overpotential (Eall = EOER +

EORR) is used to evaluate the bifunctional performance of catalysts for oxygen electrodes, so the Eall of synthesized catalysts are calculated, as illustrated in Figure 3f, which follows in the order: Ni3Fe/N-S-CNTs (0.568 V)  Pt/C + Ir/C (0.676 V)  N-S-CNTs (0.734 V)  Ni3Fe/CNTs (0.834 V). Ni3Fe/N-S-CNTs catalyst exhibits the lowest overpotential among the electrocatalysts, indicating the best bifunctional catalytic performance. Moreover, compared with other NiFe based catalysts in the recent literatures (Table S1), the asprepared Ni3Fe/N-S-CNTs catalyst also exhibits superior bifunctional catalytic activity.

Figure 3. (a) The ORR polarization curve comparison on Ni3Fe/N-S-CNTs, N-S-CNTs, Ni3Fe/CNTs, and Pt/C catalysts. (b) Histograms of overpotentials at 1.88 mA cm-2 (EORR = 1.23V- E1/2). (c) Tafel plots for the synthesized catalysts. (d) Koutecky-Levich (K-L) plots from liner sweep voltammetry (LSV), and the corresponding LSV curves (inset) for ORR of Ni3Fe/N-S-CNTs catalyst at various rotation speeds. (e) ORR durability evaluation 12


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by i–t chronoamperometric response of the synthesized catalysts (Rotation rate:1600 rpm; Constant potential: 0.70 V). (f) Histograms of the overall overoptential (Eall = EOER + EORR) for the synthesized catalysts. XPS measurement was performed to provide further insights into the excellent bifunctional performance of Ni3Fe/N-S-CNTs composite catalysts. Three fitted peaks could be observed in the high-resolution spectra of Ni 2p and two fitted peaks in the fine spectra of Fe 2p for Ni3Fe/N-S-CNTs and Ni3Fe/CNTs catalysts (Figure 4a-b).25, 48 It is clearly observed that the binding energy of Ni 2p peak and Fe 2p peak in Ni3Fe/N-S-CNTs show a lower energy shift compared with that of Ni3Fe/CNTs. For the pristine N-S-CNTs, the fine N 1s spectra could be fitted into three components (Figure 4c), which correspond to pyridinic-N (398.41 eV), pyrrolic-N (399.21 eV) and graphitic-N (400.80 eV),49 and the high-resolution S 2p spectrum (Figure 4d) could be fitted into three peaks, which correspond to sulfur in -SOn- bonds (168.43 eV), conjugated -C=S- (165.10 eV) and C-SC bonds (163.90 eV), respectively.50 However, after coordinated with Ni3Fe nanoparticles, both of the N 1s peak and S 2p peak in the Ni3Fe/N-S-CNTs move to a higher binding energy relative to N-S-CNTs. These phenomena reveal the electronic interaction between Ni3Fe and N-S-CNTs, which is in charge of the synergistic enhancement of OER and ORR catalytic activitiy.45, 51-54 The CV of the Ni3Fe/N-S-CNTs and Ni3Fe+N-S-CNTs hybrid material were conducted (Figure S12a-b). The reduction peak of Oads and the redox peak of Ni(OH)2/NiOOH in Ni3Fe/N-S-CNTs shift positively when compared with Ni3Fe+N-SCNTs hybrid material. These phenomenon further demonstrate the interaction between 13



Ni3Fe and N-S-CNTs. Besides, it can be seen from Figure S12c-d that the Ni3Fe+N-SCNTs hybrid material exhibits poorer ORR and OER catalytic activity than the Ni3Fe/NS-CNTs catalyst. These results indicate that there are the synergistic catalytic effects between Ni3Fe nanoparticles and N-S-CNTs in Ni3Fe/N-S-CNTs catalyst during the electrochemical process. Further analysis data of the bonding energy change were shown in Table S2, the binding energy shift of the N 1s and S 2p in Ni3Fe/N-S-CNTs relative to N-S-CNTs is due to the electronic interaction of Ni3Fe with N-S-CNTs, and thus tuning the adsorption energy of oxygen (Oads), which improving the ORR activity. 45, 51, 52 It can be concluded from the mechanism of ORR at heteroatom doped carbon materials (Figure S13) that the Oads ((O·2― )ads and (HO·2)ads ) adsorption strength change on catalyst will affect the reaction rate of ORR. Besides, the Ni3Fe/N-S-CNTs exhibits more positive reduction peak potential of Oads than N-S-CNTs (Figure S14), demonstrating the adsorption strength of Oads on Ni3Fe/N-S-CNTs weakens relative to N-S-CNTs, thus improve the ORR activity of Ni3Fe/N-S-CNTs. However, electronic interaction between Ni3Fe and N-S-CNTs could cause the lower energy shift of the Ni 2p and Fe 2p relative to Ni3Fe/CNTs, which would lead to an upward shift in the Ni, Fe d-band center.53 Based on Density Functional Theory, the upward shift of d-band center could result in the stronger adsorbate bonding of H2O/OH- with Ni3Fe active sites, thus enhance the H2O/OHadsorption on Ni3Fe active sites.52 Besides, it can be concluded from the mechanism of OER (Figure S15) that the enhancement of OH- adsorption on active sites could improve the OER reactive rate.54 Therefore, the excellent bifunctional electrocatalytic performance 14


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of Ni3Fe/N-S-CNTs catalysts is ascribed to the synergistic effect of Ni3Fe nanoparticles with N-S-CNTs.

Figure 4. The fine XPS spectrum of Ni 2p (a) and Fe 2p (b) for the Ni3Fe/N-S-CNTs (upper) and Ni3Fe/CNTs (bottom). The fine XPS spectrum of N 1s (c) and (d) for the Ni3Fe/N-SCNTs (upper) and N-S-CNTs (bottom). To evaluate the practical application of Ni3Fe/N-S-CNTs as electrocatalyst for bifunctional oxygen electrode, a self-made rechargeable Zinc-air battery (Figure 5a) was built. Figure 5b and Figure S16a show the polarization curves of Zinc-air battery. Ni3Fe/N15



S-CNTs catalyst exhibits a high peak power density of 180.0 mW cm-2, which is much larger than Pt/C-Ir/C (125.4 mW cm-2), N-S-CNTs (113.1 mW cm-2), Ni/N-S-CNTs (125.3 mW cm-2), Fe/N-S-CNTs (118.7 mW cm-2) and Ni3Fe/CNTs (77.9 mW cm-2). Besides, the discharge/charge curves of synthesized catalysts at different current density are shown in Figure 5c and Figure S16b. Clearly, the Ni3Fe/N-S-CNTs-based Zinc-air battery (black cubes in Figure 5c) shows much lower voltage gaps than Pt-based, Ir-based, N-S-CNTs, Fe/N-S-CNTs, Ni3Fe/CNTs and Ni/N-S-CNTs batteries, which is consistent with its superior bifunctional performance (Figure 2 and Figure 3). Moreover, the Zinc-air battery with Ni3Fe/N-S-CNTs exhibits a large specific capacity of 718 mAh gZn-1 with a discharge platform of 1.28 V (Figure 5d) at 5mA cm-2, which is better than that using Ni3Fe/CNTs (657.7 mAh gZn-1, 1.18 V) and Pt/C-Ir/C (694.5 mAh gZn-1, 1.22 V). This result demonstrates that the Ni3Fe/N-S-CNTs catalyst can promote specific capacity of Zinc-air battery.

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Figure 5. (a) Schematic diagram of a self-made Zn-air battery. (b) The polarization curves of Zn-air battery by using Ni3Fe/N-S-CNTs, N-S-CNTs, Pt/C-Ir/C, and Ni3Fe/CNTs as oxygen cathode catalysts. (c) The anodic/cathodic polarization curves of Zinc-air battery with the Ni3Fe/N-S-CNTs, Ni3Fe/CNTs, Ir/C and Pt/C catalysts. (d) Specific capacities of Zn-air battery with Pt/C-Ir/C, Ni3Fe/N-S-CNTs and Ni3Fe/CNTs catalysts (current density: 5.0 mA cm-2) Apart from the activity of air electrode, the operation durability of Zn-air battery is another key parameter for its potential commercial application. Therefore, galvanostatic charge/discharge cycle (30 min charge and 30 min discharge) was conducted (Figure 6a). The Zn-air battery with Ni3Fe/N-S-CNTs exhibits superb cycling stability, which is confirmed by 500 discharge/charge cycles over a long period of 500 h with a slight 17



increased discharge/charge overpotential (Figure 6b). For comparison, the Pt/C-Ir/C driven Zn-air battery shows severe enlargement of overpotential gap, indicating the gradually loss of battery performance during cycling. Besides, the N-S-CNTs, Fe/N-S-CNTs, Ni/N-SCNTs and Ni3Fe/CNTs air electrode have complete lost their activity, after 200, 420, 400 and 330 discharge/charge cycles, respectively (Figure S17). Furthermore, after the continuous 60 discharge/charge cycles (Figure 6c), Ni3Fe/N-S-CNTs based air-cathode exhibits a slight loss with a low charge/discharge overpotential (0.75 V) and a small decreased round-trip efficiency of 5.5%, which is smaller than the Pt/C-Ir/C based aircathode (1.01V and 14.7%). These results demonstrate the excellent stability and robustness of Ni3Fe/N-S-CNTs air-electrode. Besides, the micromorphology of the Ni3Fe/N-S-CNTs catalyst after 500 h cycling measurement was characterized by TEM (Figure S18). Most of the Ni3Fe nanoparticles uniformly distributed on carbon support with slightly agglomerated to form relatively larger particles compared with the micromorphology before 500 h charge/discharge cycling (Figure 1a). This phenomenon indicates that the excellent cyclability of Ni3Fe/N-S-CNTs air-electrode is probably attributed to the stable microstructure of Ni3Fe/N-S-CNTs catalyst. Finally, compared with various catalysts in the recent literature (Table S3), all the aforementioned results (high power density, low charge/discharge voltage gaps, large specific capacity and superb stability), indicate that the Ni3Fe/N-S-CNTs catalyst could improve Zn-air battery performance and is a promising candidate for oxygen electrode catalyst in Zinc-air battery.

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Figure 6. (a) Galvanostatic discharge/charge cycling by using Pt/C-Ir/C and Ni3Fe/N-SCNTs as air-electrode catalyst at 5.0 mA cm-2 (30 min charge and 30 min discharge). (b) The 450th to 500th galvanostatic charge/discharge cycling by using Ni3Fe/N-S-CNTs as airelectrode catalyst. (c) Galvanostatic charge/discharge cycling by using Pt/C-Ir/C and Ni3Fe/N-S-CNTs as air-electrode catalyst at 5.0 mA cm-2. 2. Conclusion In summary, we demonstrated an efficient synthetic strategy in constructing a bifunctional NiFe-based catalyst consisting of Ni3Fe alloy nanoparticles incorporated in 3D carbon nanotube/graphene nanosheet composite with heteroatoms (N/S) dopping. 19



Ni3Fe nanoparticles with small particles size are uniformly scattered on N-S-CNTs supports, which is ascribed to the large specific area and unique 3D structure of N-S-CNTs. Furthermore, based on the synergistic catalytic effect of Ni3Fe nanoparticles with N-SCNTs, and the sufficient exposure of reactive sites, the as-prepared Ni3Fe/N-S-CNTs catalyst exhibits excellent bifunctional (ORR/OER) catalytic performance. As the catalyst material for air-electrode, the Zn-air battery exhibits long durability, high power density and excellent cell efficiency, which is better than that with Pt/C-Ir/C air-electrode. The present work put forward a new idea to design and synthesize NiFe-based bifunctional (ORR/OER) catalysts. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the Internet on the ACS Publications website. Detailed additional SEM images and electrochemical performance (PDF) AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. ORCID ID Deli Wang: 0000-0003-2023-6478 Author Contributions The manuscript was written through contributions of all authors. All authors have given 20


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approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation (21573083) (to D.W.). S/TEM work was carried out at the Department of Applied Physics, The Hong Kong Polytechnic University. Dr. Ye Zhu are supported by the Hong Kong Research Grants Council through the Early Career Scheme (Project No. 25301617) and the Hong Kong Polytechnic University Grant (Project No. 1-ZE6G). J.W. and Y.Z. thanks Dr. Lu Wei for optimizing the JEOL JEM-2100F microscope. The authors thank the Analytical and Testing Center of HUST for allowing us to use its facilities. REFERENCES (1) Fu, J.; Cano, Z. P.; Park, M. G.; Yu, A.; Fowler, M.; Chen, Z., Electrically Rechargeable Zinc-Air Batteries: Progress, Challenges, and Perspectives. Adv. Mater. 2017, 29, 1604685. (2) Lee, D. U.; Xu, P.; Cano, Z. P.; Kashkooli, A. G.; Park, M. G.; Chen, Z., Recent Progress and Perspectives on Bi-Functional Oxygen Electrocatalysts for Advanced Rechargeable Metal-Air Batteries. J. Mater. Chem. A 2016, 4, 7107-7134. (3) Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J. E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H., Advanced Zinc-Air Batteries Based on High-Performance Hybrid Electrocatalysts. Nat. Commun. 2013, 4, 1805. (4) Liu, X.; Park, M.; Kim, M. G.; Gupta, S.; Wu, G.; Cho, J., Integrating NiCo Alloys 21



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Restricting Growth of Ni3Fe Nanoparticles on Heteroatom-Doped

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