CoFe2O4 nanoparticles decorated carbon nanotubes: Air-cathode bifunctional catalysts for rechargeable zinc-air batteries

CoFe2O4 nanoparticles decorated carbon nanotubes: Air-cathode bifunctional catalysts for rechargeable zinc-air batteries

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CoFe2O4 nanoparticles decorated carbon nanotubes: Air-cathode bifunctional catalysts for rechargeable zinc-air batteries ⁎

Nengneng Xua, Jinli Qiaoa, , Qi Niea, Min Wanga, He Xua, , Yudong Wangb, Xiao-Dong Zhoub,

a State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, College of Environmental Science and Engineering, Donghua Universtiy, 2999 Ren’min North Road, Shanghai 201620, China b Department of Chemical Engineering, Institute for Materials Research and Innovations, University of Louisiana at Lafayette, Lafayette, LA 70504, USA



Keywords: [email protected] composite Bi-functional electrocatalyst Amorphous and crystalline Rechargeable zinc-air battery

Exploring highly efficient bifunctional oxygen electrocatalysts are urgently for realizing rechargeable zinc-air batteries. Although CoFe2O4 as bifunctional oxygen electrocatalyst has been involved in less previous studies, [email protected] as the high performance air cathode has not been reported. In this paper, the influences of temperature, time and the weight ratio of CNT to CoFe2O4 on the morphology, crystallographic forms and electrochemical performances of the final products are thoroughly investigated. The conclusion that amorphous form is efficient for ORR and, the crystalline structure is beneficial for OER was proposed. Particularly, a novel class of [email protected] with amorphous form and crystalline structure has been successfully synthesized and exhibited excellent activity and stability in three electrodes test. More importantly, the [email protected] cathodes are made and applied to zinc-air battery, which is demonstrated to be superior performances to Pt/C and IrO2 catalysts. The high power density (333.7 mW cm−2), larger specific capacity (732 mAh g−1), and excellent rechargeability (1200 stable charge-discharge cycles) proves the feasibility of [email protected] application in large power energy-storage and conversion electronics.

1. Introduction Rechargeable zinc-air (Zn-air) batteries have received strong interest for use in high-energy density storage applications (such as electric vehicles) thanks to their high theoretical energy density, low cost and good safety and open cell configuration that uses O2 as a fuel [1]. One of the greatest kinetic challenges in rechargeable Zn-air batteries is to overcome the high activation energy during the oxygen reduction reaction (ORR) (where is the discharge process) and oxygen evolution reaction (OER) (where is the recharge process) [2,3]. Thus, more and more researches have been dedicated to the search or design for air cathode bifunctional electrocatalysts which are valid for both OER and ORR [4–9]. The precious metal has shown good ORR or OER catalyst performance in alkaline solutions. For example, platinumbased materials have long been regarded as the most efficient electrocatalysts for ORR [10], whereas Iridic oxide has been deemed as the most promising catalyst for OER in acidic and alkaline solutions [11]. However, their prohibitively high cost and scarcity are disincentives for large-scale application [12]. Therefore, there is a strong drive to decrease the precious metal ratio in the catalysts or to seek completely non-precious metal alternatives [13]. On the one hand, this is generally

done by alloying to reduce the precious metal content in the catalyst. And some alloy materials were reported to even surpass the activity of neat precious metal [14]. On the other hand, spinel type metal oxides as bifunctional metal-based catalysts have attracted much attention in the literature due to their redox stability, morphological flexibility, variable valence states and simple synthetic routes. The flexibility of spinel type oxides is highlighted by their lattices that are able to host different species of transition metals, resulting in a vast diversity of spinel oxide base catalysts reported in the literature, such as in recently reported Co3O4 [15], CuCo2O4 [16], LiCoO2 [17], NiCo2O4 [18–20], MnCo2O4 [21], and ZnCo2O4 [22] as efficient bifunctional air electrode materials. However, it's known that the oxygen catalytic activity of spinel type oxides is depressed due to their low electrical conductivity during ORR and OER [10]. Generally, it is an effective measure to accelerate the electronic conductivity of the spinel type oxides by growing or supporting the spinel oxide nanoparticles on carbon based materials (such as graphene-based materials) [23]. Especially, ACo2O4/GO (A = Ni or Mn) types hybrid catalyst was widely studied [24–27]. Compared to NiCo2O4/GO and CoMn2O4/GO type hybrid catalysts, CoFe2O4/Carbon type hybrid material was rarely researched. For instance, Yan et al. [28] reported FeCo2O4 nanoparticles coupled to hollow structure reduced

Corresponding authors. E-mail addresses: [email protected] (J. Qiao), [email protected] (H. Xu), [email protected] (X.-D. Zhou). Received 15 July 2017; Received in revised form 18 October 2017; Accepted 23 October 2017 0920-5861/ © 2017 Published by Elsevier B.V.

Please cite this article as: Xu, N., Catalysis Today (2017),

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graphene oxide spheres (FeCo2O4/HrGOs), manifesting a comparable ORR and OER activity to that of the Pt/C and RuO2/C catalysts. Bian et al. [29] synthesized CoFe2O4 supported on graphene as an efficient ORR and OER catalyst. It was believed that the improvement in the catalytic activity and stability in the CoFe2O4/rGO for the ORR and the OER mainly arises from the strong coupling between CoFe2O4 and rGO. Compared to GO, carbon nanotubes (CNTs) is a conductive material with high surface area and good stability under a wide range of environmental conditions. However, to the best of our knowledge, CoFe2O4/Carbon type hybrid material is still largely unexplored, particular to zinc-air battery applications. It is, therefore, the need for carrying on in-depth study for obtaining experimental evidence to guide the research efforts for improving these approaches. In this work, we report the CoFe2O4 nanoparticles in situ coupling with CNTs hybrid catalyst ([email protected]), which is synthesized via a simple one-step hydrothermal approach. The effect of CNT and CoFe2O4 ratio, synthesis temperature and time on the catalytic activity and battery performances were systematically investigated combine with catalyst morphology and crystalline structure (TEM image, HRTEM image and SAED image). It was found that the different crystal forms are existence in [email protected] composite catalyst, which is beneficial for ORR and OER. Additionally, in order to meet real battery operation conditions, all battery tests must use air in ambient conditions instead of pure O2. Therefore, the [email protected] cathode exhibit good zincair battery performance as evidenced by the high discharge power density when it was used to primary battery, and long-term cyclic stability when it was used to researchable battery.

work electrode. For 20 wt% Pt/C (Johnson Matthey), IrO2 (Johnson Matthey) were also prepared for a loading of 100 μg cm−2. A Hg/HgO (SCE, in saturation KCl, aq) reference electrode and a Pt rod counter electrode were used together with the GC working electrode. The measurements were carried out under a continuous flow of O2 through the 0.1 M KOH electrolyte to maintain an O2-saturated environment. All potentials in this paper was referenced to the reversible hydrogen electrode (RHE) at pH 13 (=E [vs. SCE] + 0.9934 V). ORR curves were recorded from 0.2 to 1.1 V vs RHE at a scan rate of 5 mV s−1 with an O2-saturated electrolyte under various electrode rotation speeds (300, 600, 900, 1200 and 1500 rpm). OER curves were evaluated from 1 to 2 V vs RHE at a rotation speed of 1500 rpm.

2. Experimental

3.1. Microstructure and morphology of [email protected] nanocatalyst

2.1. Synthesis of sample

In general, a catalyst with crystalline phase or amorphous would be considered a great performance because of abundant accessible active sites [30–32]. Fig. 1(b) shows the XRD profile of the [email protected] nanocomposites. XRD patterns of CNTs and CoFe2O4 are also included for comparison. The characteristic peaks in [email protected] and pure CoFe2O4 can be well indexed as CoFe2O4 (cubic spinel phase, JCPDS no: 22-1086), and amorphous and crystalline structure were existence of composite catalyst. The broad peak among [email protected] at around 26° corresponding to (002) peak of CNTs. So it can be concluded that the [email protected] were synthesized successfully. And the average crystallite size of the CoFe2O4 nanoparticles in [email protected] hybrid was calculated to be ∼6.5 nm by Scherrer formula [33]. The morphologies and the size of these catalysts ([email protected] z%) were investigated using TEM images (Figs. 1 (c) and S1–S4). As observed from Figs. 1 (c) and S1(a), CoFe2O4 nanoparticles were uniformly and selectively decorated on the CNTs without aggregation and detachment. The CoFe2O4 nanoparticles size distribution in composite catalyst is in the range of 3.5–10.5 nm with an average particle size of 7 nm (Fig. S1(c)). It is in great agreement with the value obtained from the XRD patterns (Fig. 1(b)). The crystal structure of the [email protected] was further revealed by high-resolution TEM (HRTEM). As shown as Fig. 1(c) inset, these measured d spacing were determined to be 0.29 nm and 0.25 nm that corresponds to the lattice spacing of the (220) and (311) fringes of [email protected] It is worth noting that these are two crystal forms of composite catalyst: crystalline and amorphous (Fig. S1(b)). Selective area diffraction pattern (SAED) also described the good crystalline and the indistinctive amorphous nature (Fig. 1(d)). The seven distinct concentric diffraction rings can be indexed to the (220), (311), (222), (400), (331), (422), (511) and (440) fringes of cubic spinel CoFe2O4, which quite agrees with the results of the XRD pattern (Fig. 1(b)). The above TEM characterization results suggest that different crystal forms were existence in composite catalyst (e.g. amorphous form is efficient for ORR [34]). The CNTs network which improves the mass transport and conductivity and, the cubic spinel CoFe2O4 which enhances the catalyst activity for both ORR and OER in [email protected] may result in extraordinary electro-catalysis activity.

2.4. Battery fabrication and measurement A home-made zinc–air battery was used for the rechargeable batteries tests at room temperature. The air electrode was prepared by spraying the catalyst ink onto a gas diffusion layer (Toray TGP-H-090, 1 cm × 1 cm). And the loading of air electrode was 2 mg cm−2. 6 M KOH solution was used as the electrolyte and a clean zinc plate was the anode. Power density plots, discharge and charge polarization were measured by a galvanodynamic method. Discharge-charge cycling curves were carried out by a pulse method using recurrent galvanic pulses at a current density of 5 mA cm−2 (5 min of discharge followed by 5 min of charge with no rest period in-between). 3. Results and discussion

0.20 g Iron(III) nitrate (Fe(NO3)3 9H2O, > 99.5%, Sinopharm chemical reagent Co., Ltd, China), 0.20 g cobalt nitrate (Co(NO3)2 4H2O, > 99.5%, Sinopharm chemical reagent Co., Ltd, China) and CNTs (the weight ratio of CNT to Fe and Co varied from 0.04, Alpha Nano Technology Co. Ltd., China) were dissolved in 30 mL ammonium hydroxide solution (NH3 H2O, 28.0-30.0 vol%, Sinopharm chemical reagent Co., Ltd, China) and stirred for 1 h. Then, it was transferred to a 100 mL autoclave, sealed and heated at 160 °C for 6 h in a drying oven. After the heated procedure, the autoclave was cool to room temperature naturally. These solid products were collected, washed with water several times and then dried at 60 °C for 6 h in a drying oven. Finally, these dried precursors were calcined in air at 350 °C for 1 h to obtain [email protected] The schematic illustration for the preparation of [email protected] was shown in Fig. 1(a). 2.2. Characterization Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-2100F microscope operating at 200 kV accelerating voltage. Field-emission scanning electron microscope (FESEM) images were obtained using a using an FEI Sirion 200 scanning electron microscope (SEM) operating at 5 kV. The x-ray diffraction XRD patterns were obtained from a Philips PW3830 x-ray diffractometer using Cu Kα radiation (λ = 0.15406 nm) with a current of 40 mA and voltage of 40 kV. 2.3. Electrochemical measurements The electrochemical activity of samples was carried out in a traditional three-electrode cell reactor at room temperature. A 2.5 mg mL−1 catalyst ink was prepared by adding 5.0 mg catalyst to 2 mL of a Nafion mixture (0.08: 1 v/v 5 wt% Nafion solution-ethanol) followed by at 30 min of sonication. 8 μL of the catalyst ink was then drop-cast onto a clean GC electrode surface to a catalyst loading of 100 μg cm−2 and as 2

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Fig. 1. (a) Schematic illustration of the synthesis process of [email protected] composite and its battery behavior. (b) XRD patterns of CoFe2O4, CNTs and [email protected] composite. (c) TEM image of [email protected] composite, (Inset: HRTEM image of the interface). (d) Corresponding SAED pattern of [email protected] composite.

to obtain [email protected], [email protected], [email protected] CNTs160-14 and [email protected] (Fig. S4). At 2 h, a small number of cubic spinel CoFe2O4 nanoparticles were formed. The characteristic lattice fringes and the concentric diffraction rings are distinct. Compared with 2 h, a large numbers of cubic spinel CoFe2O4 nanoparticles begins to form and reunite at the long reaction time (i. e. 10 h, 14 h and 18 h). In particular, the catalyst nanoparticles are reunited to form nanosheets at 18 h, suggesting that crystallinity of the catalyst was obviously improved with the prolonging of reaction time. The above TEM characterization results suggest that the specific morphologies and the size of these catalysts can be successfully tailored by changing the synthesis conditions. And the specific morphologies and the size of these catalysts may promote the catalytic activity.

Therefore, the morphologies of pure CoFe2O4, [email protected] 5% and [email protected] 20% were investigated using TEM images, HRTEM image and SAED (Fig. S2). When the weight ratio of CNT to Fe and Co was 0, pure CoFe2O4 presents a lamellar morphology and the size is estimated to be > 200 nm. In contrast, the smaller size of CoFe2O4 nanoparticles of [email protected] nanohybrid is due to the dispersing effect of CNTs support in preventing the CoFe2O4 nanoparticles from aggregation. The HRTEM and SAED characterization shows obvious polycrystalline crystal forms. When the weight ratio of CNT to CoFe2O4 was 5%, large amounts of CoFe2O4 nanoparticles were dispersed in a small amount of CNTs. So CoFe2O4 nanoparticles was easy to reunited. When the ratio was increased to 20%, the quantity of CoFe2O4 nanoparticles of [email protected] 20% decreased sharply at the same area. Therefore, whether to reduce or increase the proportion of CNTs are not conducive to improve the effective specific surface area of the catalyst. It is interesting to note that the crystallinity of catalyst was negatively related to the ratio of CNTs to CoFe2O4. To explore the effects of the hydrothermal synthesis temperature and time on the morphology, the crystal forms, and the size of [email protected], the [email protected] samples were studied by using TEM images, HRTEM image and SAED (Figs. S3 and S4). First, when the hydrothermal reaction time was maintained on 6 h, we change the hydrothermal reaction temperature at 100 °C, 120 °C, 140 °C and 180 °C to obtain [email protected], [email protected], [email protected] CNTs140-6 and [email protected] From Figs. S1 and S3, it can be seen that the crystal nucleus of cubic spinel CoFe2O4 (the size of 3–5 nm) was formed and are surrounded by a large number of amorphous phases at low temperature (100 °C). At 120 °C, the crystal nucleus of cubic spinel CoFe2O4 nanoparticles (the size of 4–8 nm) was grown, and the amorphous phases of [email protected] was decreased. At 140 °C, the amorphous phases of [email protected] rapidly disappeared. A large number of cubic spinel CoFe2O4 nanoparticles (the size of 4–9 nm) appeared from CoFe2O4 nanosheet. However, the cubic spinel CoFe2O4 does not evenly dispersed until the temperature at 160 °C, where the crystallinity was further improved. At a high temperature (180 °C), the cubic spinel CoFe2O4 was aggregated and the size distribution is in the range of 7–15 nm. Then, we change the hydrothermal reaction time for 2 h, 10 h, 14 h and 18 h at same temperature (160 °C)

3.2. Electrochemical activity of [email protected] nanocatalyst In O2-saturated 0.1 M KOH solution, LSV curves of [email protected] nanocatalyst for ORR and OER are shown in Fig. 2(a) and (b). For a comparison, Pt/C, IrO2 and [email protected] z% were also measured (Fig. S5). The [email protected] shows the highest ORR performance among Pt/C, IrO2 and the [email protected] z% materials. It even has a slightly lower half-wave potential (0.78 V) than commercial Pt/C (0.82 V), and a largely higher half-wave potential than IrO2 (0.58 V). The [email protected] shows the largest limiting current density than Pt/ C and IrO2. The average numbers (n) of electrons transferred for ORR process of [email protected] was 3.95 calculated by the Koutecky−Levich (K-L) equations [35] throughout the tested potential range of 0.3 V–0.5 V (Fig. 2c). The average electron transfer numbers of Pt/C and IrO2 are 3.97 and 3.07 (Fig. S6). [email protected] also shows the much higher OER activity than Pt/C. Especially, in high potential, the OER activity of [email protected] is even superior to IrO2 which is the state-of-the art commercial OER catalyst. The Tafel slope values for OER of the Pt/C, IrO2 and [email protected] are 89, 67 and 63 mV dec−1 (Fig. 2c). These results reconfirm the ORR and OER performance of the [email protected] catalyst. For bifunctional electrocatalyst, the difference in potential (ΔE) between ORR current density at 3 mA cm−2 and OER current density at 10 mA cm−2 is often used to evaluate the bifunctionality [36]. The smaller the ΔE, the closer the catalyst is to an 3

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Fig. 2. [email protected], Pt/C and IrO2 catalysis (a) ORR polarization curves, (b) OER polarization curves, (c) the number of electrons transferred and Tafel slope, and (d) the difference in potential between ORR current density at 3 mA cm−2 and OER current density at 10 mA cm−2.

high temperature (180 °C), the agglomeration of catalyst results in attenuation of catalyst performance of ORR and OER, which is in good agreement with HRTEM image and SAED image (Fig. S3). Fig. S5(e) and (f) is the ORR and OER curves of [email protected] compounds. For the hydrothermal reaction time at 2 h, 6 h, 10 h, 14 h and 18 h, the ORR performances of [email protected], [email protected], [email protected] CNTs160-10, [email protected] and [email protected] increased first and then decreased. The reason is that the active site of the catalyst may not be formed at short time (2 h). As time goes on (> 6 h), the crystallization of the catalyst was increased gradually (Fig. S4), which results in a decrease in the activity of ORR [34]. Surprisingly, the OER is more favoured as the crystallinity of the catalyst is increased, and no obvious agglomeration of catalyst nanoparticles was observed (Fig. S5(f)). This reasonably explains the activity trend of OER in Fig. S5(f). In addition, Figs. S7-9 and S10 (a)–(c) show the corresponding K-L plots, the average n and the Tafel slope values of [email protected] z%. And Fig. S10 (d)–(f) shows the ΔE values of [email protected] z%.

ideal bifunctional oxygen catalyst. As shown in Fig. 2(d), The values of ΔE of [email protected], Pt/C and IrO2 are 0.96 V, 1.18 V and 1.56 V, respectively. We also compared the bifunctionality of [email protected] with those reported in literatures recently, and the ΔE values are listed in Table S1. Obviously, the [email protected] exhibits good bifunctional catalytic activity among the reported oxygen electrocatalysts. Fig. S5(a) and (b) is the ORR and OER curves of [email protected] x% with different mass ratio between CoFe2O4 and CNTs. The [email protected] with the mass ratio between CoFe2O4 and CNTs of 1:9 shows the highest ORR and OER performance among CoFe2O4, [email protected] 5% and [email protected] 20%. The reason is that the catalyst is easy to reunite with none or bits of CNTs. But the excess CNTs (20%) would reduce the ratio of active sites of the catalyst and suppress the catalytic activity. Combining with Fig. S2, the results show that the CNTs network improves the mass transport and conductivity and enhances the catalyst activity for ORR and OER by coupling with cubic spinel CoFe2O4. In other words, the CNTs play an important role in ORR and OER by influencing the morphology and the electronconductive properties of the obtained hybrid materials. Fig. S5(c) and (d) is the ORR and OER curves of [email protected] compounds. When the hydrothermal reaction temperature being at 100 °C, 120 °C, 140 °C and 160 °C, the ORR performances of [email protected], [email protected] CNTs120-6, and [email protected] and CoFe2O[email protected] were improved successively, while for the hydrothermal reaction temperature at 180 °C, the ORR performance of [email protected] was decreased rapidly. For OER performance, the activity: [email protected] < [email protected]–6 ≈ [email protected] > [email protected] > [email protected], which is consistent with the ORR performance. The reason may be that the active site of the catalyst is gradually formed with increasing temperature. However, at

3.3. Zinc-air battery performance In order to further determine the [email protected] catalyst performance of ORR and OER under real battery operation conditions (all battery tests must use air in ambient conditions instead of pure O2), a home-made zinc–air battery was fabricated. The current density increased as the cell voltage decreased of the zinc-air battery in Fig. 3(a). As shown in Fig. 3(a), the open-circuit voltage (OCV) of [email protected], Pt/C and IrO2 cathode were as high as 1.52 V, 1.23 V and 1.21 V, respectively. The maximum power density of the [email protected] cathode was 333.7 mW cm−2, much higher than that of Pt/C cathode (101.4 mW cm−2) and IrO2 cathode (89 mW cm−2), as well as those 4

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Fig. 3. The zinc-air battery using the [email protected], Pt/C and IrO2 cathode, with zinc plate with a thickness of 0.5 mm as the anode. (a) Polarization curve and corresponding power density plots, (b) rate discharge curves of the as-prepared zinc-air battery array at different current densities (5 mA cm−2–50 mA cm−2), (c) the long time discharge and corresponding to specitific capacity, (d) C–D polarization curves, (e) C–D cycling data at 5 mA cm−2 with 10 min per cycle (Inset: part of the amplification of cycling data).

maintain within 1 V), evidencing its excellent rechargeability. The value of voltage gap and the stabilization time of the [email protected] z% cathode at C-D cycles process are shown in Fig. S11(d)–(f). When cycled at 50 mA cm−2 with each C-D cycle being 120 min, the [email protected] cathode exhibited charge and discharge potentials of nearly 2.30 V and 0.99 V (Fig. S12), respectively. When compared to [email protected] z%, Pt/C, IrO2, and even other studied spinel catalysts (Table S3), obviously, the [email protected] catalyst is one of the best bifunctional catalysts with substantially improved stability and minimized overpotentials during the C-D processes.

reported for the batteries in the literatures (Table S2). In addition, Fig. S11(a)–(c) indicates that the OCV values and the maximum power density of [email protected] z% all lower than that of [email protected] These results conform to the ORR performance. The galvanostatic discharge performance was obtained in order to investigate the battery’s stability by a multi-current step method at different current densities. Fig. 3(b) shows the rate discharge curves from 5 mA cm−2 to 50 mA cm−2, keeping 25 min at each current density. The voltage plateaus varies from 1.24 to 0.91 V, batter than Pt/C (1.32–0.70 V), IrO2 (1.21–0.63 V) and other reported catalyst [37]. The discharge curves of each step are smooth and stable in the whole discharge duration. It was interesting that the battery of [email protected] showed higher voltage platform at large current density (> 30 mA cm−2). This suggests that the battery of [email protected] can stably work over a wide range of discharge currents, especially at large current density. Fig. 3(c) reflects that the stable discharge time (voltage > 1 V) was 210 h at 5 mA cm−2 more than Pt/C (102 h) and IrO2 (67 h). The specific capacity of [email protected], Pt/C and IrO2 were 732 mAh g−1, 643 mAh g−1 and 532 mAh g−1, corresponding to a high energy density ∼935 Wh kg−1 (energy efficiency: 69%), ∼790 Wh kg−1 (energy efficiency: 58%) and ∼621 Wh kg−1 (energy efficiency: 46%), implying a faster and more stable ORR kinetics of the [email protected] cathode than that of Pt/C and IrO2 catalyst. As shown in Fig. 2d, the charge and discharge (C-D) voltages of [email protected], Pt/C and IrO2 were obtained by the galvanodynamic testing at all current densities measured. The C-D voltages of [email protected] clearly show significantly reduced overpotentials compared to those of state-of-art Pt/C and IrO2 catalyst, particularly at small current density or charge process. Therefore, it strongly demonstrates the effectiveness of the [email protected] hybrid as a bifunctional catalyst. Actually, as shown in Fig. 3(e), the [email protected] cathode shows the highest round-trip efficiency (negligible voltage change at the back-end) and the smallest value of voltage gap of only 0.8 V when compared with Pt/C and IrO2 cathodes (where the galvanostatic C–D cycling curves were obtained at 5 mA cm−2, with each cycle being 10 min). The [email protected] cathode exhibited 1200 stable C-D cycles with pimping degradation (in other words, the stabilization time is 200 h for the voltage drop

4. Conclusions In conclusion, a variety of catalysts ([email protected] z%) with different morphologies and properties were synthesized via a simple approach by tailoring the reaction conditions. At the same time, the results reveal that the amorphous form is efficient for ORR, but the crystalline structure is beneficial for OER of [email protected] The [email protected] cathode exhibited excellent ORR and OER performance in a rechargeable zinc–air battery, evidenced by high power density, high discharge voltage plateau at different current density, high energy density, high round-trip efficiency, and remarkable stability up to 200 h (1200 cycles) under ambient air. Especially, the [email protected] catalyst demonstrated comparable or even superior performance relative to Pt/C, IrO2 catalysts. This work will open a new feasibility of the simplistic synthesis of the bifunctional catalyst as a replacement of precious metals for high performance electrochemical energy storage and conversion technologies.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (U1510120) and the International Academic Cooperation and Exchange Program of Shanghai Science and Technology Committee (14520721900). 5

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