N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zn-air battery

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Incorporating inactive Nd2O3 into Co/N-doped carbon as bifunctional oxygen electrocatalyst for rechargeable Zn-air battery Jiabin Tanb,1, Xiaobo Hea,c,1, Fengxiang Yina,c,*, Biaohua Chena, Guoru Lia, Xin Liangb, Huaqiang Yind a

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China c Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, PR China d Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing 100084, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Oxygen evolution reaction Oxygen reduction reaction Zn-air battery Neodymium oxide Metallic cobalt

Co-Nd2O3/N-doped carbon (NC) as bifunctional oxygen electrocatalyst was synthesized by annealing a bimetal precursor (Co, Nd). It exhibits a nanostructure of Co-Nd2O3 hybrid nanoparticles coated with NC shells. The introduced Nd2O3 is beneficial to obtain high ratios of O vacancy/total O and graphitic-N/total N and modify the Lewis acidic property of Co-Nd2O3/NC-700, thus benefiting the OER activity. Besides, not only high Co2+/total Co and pyridinic-N/total N ratios, but also the enhanced Lewis base property is achieved after the introduction of Nd2O3, which is favorable for the ORR activity. As a result, the optimal Co-Nd2O3/NC-700 exhibits the enhanced OER/ORR activity. Especially, the OER activity is remarkably improved, thus leading to the excellent bifunctional catalytic activity with low ΔE of ∼0.86 V. Furthermore, the Zn-air battery with Co-Nd2O3/NC-700 shows a high specific capacity of ∼613.6 mA h g−1 Zn and superior charge-discharge cycling durability.

1. Introduction The rechargeable zinc-air battery has been regarded as the very promising energy conversion and storage system because of its theoretically high energy density (1084 Wh kg−1) [1,2]. The development of highly efficient non-noble metal bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) during the charge and discharge processes of rechargeable Zn-air battery is an important strategy for its commercialization [3]. The voltage gap between ORR and OER (ΔE = EOER-EORR) is generally used to evaluate the overall bifunctional electrocatalytic activity of the catalysts [4]. A smaller ΔE value means a better overall bifunctional catalytic activity or higher reversibility between OER and ORR. Recently, great efforts have been made to develop highly efficient non-noble metal bifunctional electrocatalysts, and the results reveal that the overpotential during the discharge process which is related with ORR has reached a very low level, whereas the overpotential during the charge process associated with OER is still at a high level [5]. In other words, there is much space for the improvement of OER activity during the charge process. Therefore, to obtain electrocatalysts with

outstanding bifunctional activity for rechargeable Zn-air battery, it is more efficient and significative to remarkably enhance OER activity. Among the non-noble metal bifunctional electrocatalysts, Co-based electrocatalysts such as metallic Co [6,7], cobalt oxides [8,9], cobalt phosphide [10,11], and single atom Co [12,13] have been studied as bifunctional electrocatalysts for both OER and ORR in recent years. It has been reported that the introduction of some inactive species into Co-based electrocatalysts can further enhance their electrocatalytic activity, especially the OER activity, thus leading to outstanding bifunctional catalysts. Ren et al. successfully introduced polypyrrole (PPy) into Co3O4/graphene nanosheets via one-step ball milling method [14]. Though PPy is inactive for ORR, the introduction of PPy can provide nitrogen atoms to coordinate cobalt ions to form Co-Nx active sites for ORR, thereby improving ORR activity. Chen et al. successfully synthesized Zn-Co-layered double hydroxide (LDH) electrocatalyst for OER, in which Zn is inactive for OER [15]. The inactive Zn2+ ions were found to improve the interactions between reactants and active sites, and alternatively arrange Zn and Co in the LDH layer, thus contributing to the improved OER activity. Recently, our group has introduced ZnO and CeO2 into Co-based electrocatalysts, and enhanced their

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Corresponding author at: Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, PR China. E-mail address: [email protected] (F. Yin). 1 These authors contributed equally. https://doi.org/10.1016/j.cattod.2019.12.018 Received 6 September 2019; Received in revised form 4 December 2019; Accepted 14 December 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved. This is an open access article under the#lictext# license (http://creativecommons.org/licenses/#lictextcc#/#licvalue#/).

Please cite this article as: Jiabin Tan, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.12.018

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recorded on a Raman spectrometer (Renishaw inVia, Renishaw, UK) using a laser (633 nm) excitation. The transmission electron microscopy (TEM) and high-resolution electron microscopy (HRTEM) images were collected using a microscope (Tecnai G2 F20, FEI, USA). The N2 adsorption-desorption isotherms were measured using a sorption analyzer at 77.3 K (ASAP 2460, Micromeritics, US). The specific surface areas were obtained using the Brunauer-Emmett-Teller (BET) method. The pore size distribution of mesopores was calculated by the BarrettJoyner-Halenda (BJH) method and the pore size distribution of micropores was calculated by the Horvath-Kawazoe (H-K) method. The Xray photoelectron spectra (XPS) were recorded on an ESCALAB 250 XPS photoelectron spectrometer with an Al-Kα X-ray resource (ThermoFisher Scientific, USA) at a constant pass energy of 30 eV. All binding energies were referenced to C 1s peak at 284.6 eV. The CO2temperature programmed desorption (CO2-TPD) was measured by a TPD apparatus (TP-5080, Tianjin Xianquan instrument, China). First, the sample was pretreated from room temperature to 200 ℃ at a heating rate of 10 ℃ min−1, and then kept at this temperature for 1 h in pure Ar. After cooling to 100 ℃, the atmosphere was switched to CO2 (5 % vol)/Ar at a flow rate of 30 mL min−1. After 30 min, the sample was purged with pure Ar at 100 ℃ for 1 h. When the baseline was stable, the sample was heated to 800 ℃ at a ramp rate of 10 ℃ min−1. Temperature-programmed desorption of NH3 (NH3-TPD) was also performed on the same TPD apparatus. Before each NH3-TPD measurement, the sample was pretreated at Ar for 1 h at 200 ℃ and then cooled down to 100 ℃. The NH3 adsorption was performed by passing a gas mixture of NH3 (5 % vol)/Ar at a flow rate of 30 mL min−1. After 30 min, the sample was purged with pure Ar at 100 ℃ for 1 h. When the baseline was stable, the sample was heated to 800 ℃ at a ramp rate of 10 ℃ min−1. The weight ratios of Nd in Co-Nd2O3/NC were determined by using an Inductively Coupled Plasma-Atomic Emission Spectrometer (Agilent ICP-OES 720, USA).

bifunctional electrocatalytic activities. In the MO-Co@N-doped carbon (M = Zn or Co, NC) [16], the introduction of ZnO was found to improve active species contents like pyridinic N for ORR, and Co-Nx as well as the Co3+/Co2+ ratios for OER, thus enhancing the bifunctional activity. The Zn-air battery with the optimized ZnO-Co@NC-700 exhibits the high specific capacity of ∼578 mA h g−1 Zn and the initial dischargecharge round-trip efficiency of such Zn-air battery is ∼63 % with the low charge voltage of ∼1.98 V due to its significantly enhanced OER activity. As for CeO2-Co3O4@NC electrocatalysts [17], the introduction of CeO2 efficiently increases the contents of active species including Co2+, Co3+ and N and modifies the acid-base properties of the electrocatalysts, providing the positive influences to the oxygen electrocatalytic activity, especially the OER activity, thus significantly enhancing the bifunctional electrocatalytic activity. It can be seen that the introduction of inactive materials into Co-based electrocatalysts is an effective method to enhance the bifunctional electrocatalytic activity for ORR and OER. Nd2O3 as rare earth metal oxides has been applied in various fields like heterogeneous catalysis, glass industry and supercapacitor in recent years [18]. For example, in PANI/Nd2O3 composites [19], Nd2O3 was found to modify the density of electron cloud and the absorption properties, which is in favor of the faradic reaction, thus improving the specific capacitance of the capacitor. In this work, Nd2O3 as inactive materials were successfully introduced into Co-based electrocatalysts by a facile pyrolysis method. It is revealed that the introduction of Nd2O3 can increase the surface contents of active species for OER and ORR and modify the Lewis acidic-base properties in the prepared electrocatalysts. As a result, the OER/ORR activity is enhanced, especially for the OER activity. Consequently, the optimal Co-Nd2O3/NC700 exhibits the superior overall bifunctional electrocatalytic activity (ΔE of ∼0.86 V) and outstanding charge-discharge performance of Znair battery, which is also comparable to the recent reported Co-based bifunctional electrocatalysts.

2.3. Electrochemical measurements 2. Experimental The electrochemical measurements of the electrocatalysts were performed in a three-electrode system with a rotating disk electrode (RDE) connecting a CHI760E electrochemical workstation (Chenhua Instrument, China) in 0.1 M KOH aqueous solution (pH = ∼13) at room temperature. A KCl-saturated Ag/AgCl electrode and a carbon rod were used as the reference and counter electrodes, respectively. All the potentials reported in this work are converted a reversible hydrogen electrode (RHE) via the following equation [16]:

2.1. Synthesis of electrocatalysts All reagents were used without any further purification. 1.76 g (6 mmol) of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1.76 g (4 mmol) of neodymium hexahydrate (Nd(NO3)3·6H2O) were dissolved in a mixed solvent containing 10 mL of deionized water and 10 mL of N, N-dimethylformamide (DMF) as solution A. 2.10 g (30 mmol) of 1, 2, 4triazole, 2.14 g (10 mmol) of 1, 3, 5-benzenetricarboxylic acid and 2.40 g of polyvinyl pyrrolidone (PVP, Mw = 58,000) were dissolved in another solvent containing 20 mL of anhydrous methanol and 110 mL of DMF as solution B. Then, solution A was added dropwise to solution B under stirring. After stirring for another 30 min, the resulting mixed solution was refluxed at 90 ℃ for 4 h. After naturally cooling to room temperature, the resulting bimetal precursor (Co, Nd) was filtered and washed with anhydrous ethanol and then dried at 60 ℃ for 12 h. Subsequently, the bimetal precursor (Co, Nd) was annealed at 600, 700 and 800 ℃ under N2 atmosphere for 2 h with a ramp rate of 2 ℃ min−1. The obtained samples were labeled as Co-Nd2O3/NC-600, Co-Nd2O3/ NC-700 and Co-Nd2O3/NC-800, respectively. Reference samples, i.e., Co/NC-700 and Nd2O3/NC-700, were prepared at 700 ℃ under similar synthetic conditions. It should be noted that the feeding amount of Co (NO3)2·6H2O or Nd(NO3)2·6H2O was set as 10 mmol during the preparation of precursor (Co) for Co/NC-700 or precursor (Nd) for Nd2O3/ NC-700, respectively.

ERHE = EAg / AgCl + 0.059 × pH + 0.197

(1)

Before the electrochemical measurements, the working electrode was prepared as follows. Briefly, 2.5 mg of the prepared samples and 50 μL of Nafion (5 wt%, DuPont) solution were ultrasonically dispersed in 1 mL ethanol to obtain a homogeneous ink. Then ∼20 μL of ink was dropped on the clean glassy carbon electrode. The working electrode was obtained after drying. The loading of the catalysts is ∼0.24 mg cm−2. For comparison, the working electrode with IrO2 (∼0.24 mg cm−2) or 20 wt% Pt/C (∼0.24 mg cm−2) was prepared with similar procedures. For the OER measurements, cyclic voltammetries (CVs) were carried out at a sweep rate of 50 mV s−1 within the range of 1.2–1.9 V in N2saturated 0.1 M KOH to obtain a stable CV profile. Then linear sweep voltammetry (LSV) curves were recorded at 5 mV s−1 within the range of 1.2–1.9 V in N2-saturated 0.1 M KOH at the rotation speed of 1600 rpm. The OER durability was tested by chronoamperometric measurement at 1.56 V for 12.5 h. For ORR measurements, CVs were performed within the range of 0–1.1 V at 50 mV s−1 in N2-saturated 0.1 M KOH to obtain a stable CV profile. The currents at 5 mV s−1 in N2-saturated 0.1 M KOH electrolyte were collected as the background currents. Then, the LSV curves for ORR were tested at 5 mV s−1 at 400, 625, 900, 1225, 1600, and

2.2. Structural characterizations The X-ray powder diffraction (XRD) patterns of the samples were recorded on a diffractometer (D/max-2500/PC X-ray, Rigaku, Japan) with Cu Kα radiation (Cu Kα, λ = 1.5406 Å). The Raman spectra were 2

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Fig. 1b. All the electrocatalysts show two distinct peaks at 1335 cm−1 and 1597 cm−1, which correspond to D-bands and G-bands, respectively. The D-bands are attributed to the structural defects of carbon, while the G-bands are associated with sp2 carbon [20]. The intensity ratio of the D and G bands (ID/IG) is generally used to reflect the defect degree or graphitization degree of carbon materials [21]. The ID/IG values of Nd2O3/NC-700 and Co/NC-700 are ∼1.22 and ∼1.12, respectively. Co-Nd2O3/NC-700 displays a higher ID/IG value of ∼1.25 than Co/NC-700, suggesting that the introduction of Nd2O3 can result in low graphitization degree of carbon matrices and more defects in the carbon matrices. In addition, the Eg, F12g, F22g, and A1g bands assigned to crystalline Co3O4 are observed in Co/NC-700 [22]. However, the XRD pattern of Co/NC-700 doesn’t show any obvious diffraction peak for Co3O4, which is probably because that the amount of Co3O4 in Co/NC700 is below the detection limit of the used XRD diffractometer. For comparison, Co-Nd2O3/NC-700 doesn’t show the similar peaks assigned to Co3O4 in its Raman spectra, which is consistent with its XRD pattern. As a result, Co-Nd2O3/NC-700 is mainly composed of carbon, Nd2O3 and metallic Co nanoparticles. Fig. 2a displays the TEM image of Nd2O3/NC-700, which shows a tube-like structure. In its HRTEM (Fig. 2b), the interlayer spacing value of ∼0.29 nm can be observed, which is assigned to Nd2O3 (101). Fig. 2c and d display the TEM and HRTEM images of the Co/NC-700. It can be clearly seen that many nanoparticles with the mean size of ∼15.9 nm are distributed on the carbon matrices. As revealed in the HRTEM image, the interlayer spacing values are ∼0.19 nm and ∼0.20 nm, which correspond to the hcp Co (101) and fcc Co (111), respectively (orange ranges refer to hcp Co nanoparticles and red ranges refer to fcc Co nanoparticles). In addition, the interlayer spacing value of ∼0.24 nm is assigned to Co3O4 (311) (blue ranges refer to Co3O4 nanoparticles), which is consistent with the Raman results above. The outer interlayer spacing of ∼0.34 nm corresponds to the (002) planes of graphitic carbon shells. After the introduction of Nd2O3 (Fig. 2e and f), Co-Nd2O3/NC-700 has the larger average size of ∼21.3 nm with broader size distribution compared with Co/NC-700. In the HRTEM of Co-Nd2O3/NC-700, the interlayer spacing values of ∼0.20 nm and ∼0.29 nm can be observed, which are consistent with the fcc Co (111) and Nd2O3 (101), respectively (red ranges refer to fcc Co nanoparticles and green ranges refer to Nd2O3 nanoparticles). The interlayer spacing of the outer shells is ∼0.34 nm, corresponding to graphitic carbon (002). Thus, Co-Nd2O3/NC-700 has Co-Nd2O3 hybrid nanoparticles coated with graphitic carbon shells. The N2 adsorption-desorption isotherms of the electrocatalysts are shown in Fig. 3a. The solid and hollow marks represent the adsorption and desorption branches, respectively. Obviously, the N2 uptake of all the samples sharply increases within the low relative pressure (P/ P0 < 0.03), indicating the existence of micropores in all the samples [23]. Nd2O3/NC-700 shows an H2-type hysteresis loop at the high relative pressure (P/P0 = 0.6–1.0), which is likely related to the in-bottleshaped mesopores [24]. These results reveal that Nd2O3/NC-700 has hierarchical pores including micropores and mesopores, which is further confirmed by its pore size distribution (Fig. 3b). As for Co/NC-700, the H3-type hysteresis loop at the high relative pressure (P/ P0 = 0.6–1.0) is observed, suggesting the presence of slit-shaped mesopores [25]. It also reveals the hierarchical porous structure of Co/NC700. The BET specific surface area (SSA) of Co/NC-700 is∼165.1 m2 g−1, and its pore volume and mean pore size are ∼0.75 cm3 g−1 and 18.8 nm, respectively (Table S1). After the introduction of Nd2O3, the obtained Co-Nd2O3/NC-700 shows the similar N2 adsorption-desorption isotherm as Nd2O3/NC-700, indicating its hierarchical porous structure with micropores and mesopores. Compared with Co/NC-700, Co-Nd2O3/NC-700 has lower BET SSA (∼122.2 m2 g−1), lower pore volume (∼0.23 cm3 g−1) and smaller mean pore size of ∼7.8 nm. The XPS results confirm the presence of C, O, N, Co and Nd in CoNd2O3/NC electrocatalysts and the corresponding atomic percentages

2025 rpm in O2-saturated 0.1 M KOH. The resulting currents calibrated by the background currents were regarded as ORR currents. The ORR durability was tested by chronoamperometric measurement at 0.66 V for 12.5 h. The Koutecky-Levich (K-L) equation is widely applied to investigate ORR kinetics. The K-L equation can be described as follows [17]:

1 1 1 = + j jk jd

(2)

jk = nFkCo

(3)

jd = 0.62nFCo D2/3ω1/2υ−1/6

(4)

where j is the rotating electrode current density, jk and jd are the kinetic current density and diffusion-limiting current densities, respectively, n is the electron transfer number per O2 molecule, F is the Faraday constant, k is the electron transfer rate constant, Co is the bulk oxygen concentration in electrolyte, D is the diffusion coefficient of oxygen molecule in the electrolyte, ω is the electrode rotation speed (rad s−1) and v is the kinematic viscosity of the electrolyte. The electrochemical active surface area (ECSA) can be estimated by the electrochemical double-layer capacitance (Cdl) of the electrocatalysts within a potential window without Faradic response. Typically, a series of CV curves were carried out at various scan rates (10, 20, 30, 40 and 50 mV s−1) within a potential range of 0.91-0.96 V. The Cdl was estimated by plotting the variance between anodic and cathodic currents (Δi=ia-ic) at 0.95 V against the scan rates. The Cdl value was determined to be approximately half of the linear slope of the linear fitting line. The ECSA can be calculated as follows:

ECSA=

Cdl Cs

(5)

where Cs is the specific capacitance for an atomically smooth planar surface under homogeneous electrolyte conditions. Electrochemical impedance spectra (EIS) were obtained under an open-circuit voltage within the frequency range from 106 to 10−1 Hz. The performance of Zn-air battery containing the prepared catalysts was tested in a home-made electrochemical cell. A polished Zn-plate (∼0.2 mm in thickness) was used as the anode, and the air cathode was prepared by coating the catalysts with the loading of ∼2 mg cm−2 on a hydrophobic carbon paper. 6 M KOH with 0.1 M ZnCl2 was used as the electrolyte. The performance of primary and rechargeable Zn-air batteries was evaluated on a LAND testing system (CT2001A, Wuhan LAND Electronic Co.Ltd, China) under open air condition at room temperature. For comparison, the performance of the Zn-air battery containing the mixed 20 wt% Pt/C + IrO2 was also tested under similar conditions. 3. Results and discussion 3.1. Structure and characterizations The Scheme 1 illustrates the synthesis process of Co-Nd2O3/NC electrocatalysts. The bimetal (Co, Nd) was synthesized via reflux condensation and followed by facile pyrolysis to obtain the Co-Nd2O3/NC. The detailed procedures are described in the experimental section. Fig. 1a shows the XRD patterns of the electrocatalysts prepared at 700 °C. Nd2O3/NC-700 displays the distinct diffraction peaks at (2θ=) ∼26.9°, ∼30.8°, and ∼47.4°, assigned to Nd2O3 (PDF No. 41-1089). The diffraction peaks for the face-centered cubic (fcc) Co (at ∼44.2°, ∼51.5°, and ∼75.9°) (PDF No. 15-0806) and the hexagonal closepacked (hcp) Co (at ∼41.7° and ∼47.6°) (PDF No. 05-0727) are observed in Co/NC-700. After the introduction of Nd2O3, Co-Nd2O3/NC700 exhibits the distinct diffraction peaks for Nd2O3 (PDF No. 41-1089) and fcc Co (PDF No. 15-0806). The Raman spectra of the prepared electrocatalysts are shown in 3

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Scheme 1. The scheme diagram for the synthesis of Co-Nd2O3/NC.

The high-resolution XPS spectra of Co 2p in Co/NC-700 and CoNd2O3/NC-700 (Fig. 3e) are composed of Co 2p3/2 and Co 2p1/2. After the deconvolution of Co 2p3/2 peaks, the peaks at ∼778.4 eV, ∼781.5 eV and ∼780.0 eV are assigned to Co0, Co2+ and Co3+, respectively, and the ones at∼783.5 eV and ∼787.0 eV correspond to the satellite peaks of Co2+ and Co3+, respectively [30,31]. As indicated in Table S2, Co-Nd2O3/NC-700 shows lower total Co surface content (∼1.93 at%) than Co/NC-700 (∼3.72 at%). The surface contents of Co2+ and Co3+ in Co-Nd2O3/NC-700 (∼0.74 at% and ∼0.71 at%, respectively) are also lower than those in Co/NC-700 (∼1.67 at% and ∼1.24 at%, respectively). Fig. 3f shows the Nd 3d XPS spectra of Nd2O3/NC-700 and Co-Nd2O3/NC electrocatalysts. The Nd 3d5/2 peaks can be deconvoluted into Nd3+ peak (∼982.6 eV) and Nd3+ satellite peak (∼979.6 eV) [32]. The Nd2O3/NC-700 shows the higher surface content of Nd than Co-Nd2O3/NC electrocatalysts (Table S2).

of the above elements are shown in Table S2. As shown in Fig. 3c, the O 1s XPS spectra of the prepared samples can be deconvoluted into three peaks, denoted as O1, O2 and O3. The O1 peaks at ∼530.1 eV, O2 peaks at ∼531.3 eV and O3 peaks at ∼532.5 eV are assigned to lattice oxygen, oxygen vacancy and adsorbed O, respectively [26,27]. The binding energy of these O species of the prepared electrocatalysts keeps unchanged (Table S3). As shown in Table S2, Co-Nd2O3/NC-700 shows higher surface content of total O species (∼17.23 at%) than Co/NC-700 (∼9.48 at%). Moreover, the O vacancy surface content of Co-Nd2O3/ NC-700 (∼6.78 at%) is much higher than that of Co/NC-700 (∼3.02 at %) (Table S4). As shown in Fig. 3d, the N 1s XPS spectra of Nd2O3/NC-700 can be deconvoluted into three peaks, which are assigned to pyridinic-N (∼398.7 eV), pyrrolic-N (∼400.3 eV) and graphitic-N (∼401.4 eV), respectively [28,29]. However, the additional peak at ∼406.1 eV is observed in the N 1s XPS spectra of Co/NC-700 and Co-Nd2O3/NC-700, which corresponds to oxidized-N. As shown in Table S2, Co-Nd2O3/NC700 exhibits the lower surface content of total N species (∼3.82 at%) than Co/NC-700 (∼6.30 at%). In addition, Co-Nd2O3/NC-700 also has lower pyridinic-N (∼1.30 at%) and graphitic-N (∼1.16 at%) surface content than Co/NC-700 (∼1.83 at% and ∼1.54 at%, respectively) as shown in Table S4.

3.2. OER/ORR electrocatalytic activity and durability Figs. 4a and S1a show the OER LSV curves of the prepared samples and IrO2. Table S5 summarizes the corresponding OER activity. Obviously, the Nd2O3/NC-700 doesn’t reach the 10 mA cm−2 target even at ∼1.9 V, indicating it is inactive for OER. Co/NC-700 delivers a

Fig. 1. (a) XRD patterns; (b) Raman spectra of Nd2O3/NC-700, Co/NC-700 and Co-Nd2O3/NC-700. 4

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Fig. 2. TEM and HRTEM images of Nd2O3/NC-700 (a) and (b); Co/NC-700 (c) and (d) (inset 1: the corresponding particle size distribution histogram); and CoNd2O3/NC-700 (e) and (f) (inset 2: the corresponding particle size distribution histogram) (orange ranges refer to hcp Co nanoparticles, red ranges refer to fcc Co nanoparticles, blue ranges refer to Co3O4 nanoparticles, and green ranges refer to Nd2O3 nanoparticles) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

current density of 10 mA cm-2 (Ej=10) at ∼1.69 V (the corresponding overpotential η10 = ∼460 mV). After the introduction of Nd2O3, the Ej=10 for Co-Nd2O3/NC-700 significantly decreases to ∼1.65 V (η10 = ∼420 mV), which is even lower than that of IrO2 (Ej=10 = ∼1.68 V, η10 = ∼450 mV). The low Ej=10 of Co-Nd2O3/NC-700 is also comparable to the recent reported Co-based electrocatalysts for OER, such as CoNPC (∼1.63 V) [33], Co-CoO@3DHPG (∼1.68 V) [34], and Co3O4/CoFe (∼1.67 V) [4]. In addition, the trend of the OER activity for Co-Nd2O3/NC electrocatalysts prepared under different temperature

follows: Co-Nd2O3/NC-700 > Co-Nd2O3/NC-800 > Co-Nd2O3/NC-600, suggesting the high OER activity of Co-Nd2O3/NC-700. Furthermore, as shown in Figs. 4b and S1b, Co-Nd2O3/NC-700 exhibits a much lower Tafel slope of ∼66.9 mV dec-1 than Co/NC-700 (∼122.8 mV dec-1) and IrO2 (∼114.1 mV dec-1). The Tafel slopes for Co-Nd2O3/NC electrocatalysts prepared under different temperature decrease first and then increase with temperature increasing, further indicating the more superior reaction kinetics of Co-Nd2O3/NC-700 during the OER process. Figs. 4c and S1c show the ORR LSV curves collected at 1600 rpm 5

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Fig. 3. (a) N2 adsorption-desorption isotherms; (b) pore size distributions; (c)-(f) XPS spectra of O 1s, N 1s, Co 2p and Nd 3d, respectively.

under different temperature follows the trend: Co-Nd2O3/NC700 > Co-Nd2O3/NC-800 > Co-Nd2O3/NC-600. However, the limiting current density of Co-Nd2O3/NC-700 (∼4.34 mA cm−2) is lower than those of Co/NC-700 (∼4.69 mA cm−2) and 20 wt% Pt/C (∼4.82 mA cm−2), which is likely due to its low porosity (lower BET SSA of ∼122.2 m2 g-1 and pore volume of ∼0.23 cm3 g-1 as discussed above). The limiting current density of Co-Nd2O3/NC electrocatalysts prepared under different temperature decrease first and then increase with the increase of temperature (Table S5). As for the Tafel plots shown in Figs. 4d and S1d, Co-Nd2O3/NC-700 shows a low Tafel slope of ∼92.7 mV dec−1, which is lower than that of Co/NC-700 (∼107.8 mV dec−1) and slightly higher than that of 20 wt%

and Table S5 lists the corresponding values for ORR activity. The onset potential (Eonset) and half-wave potential (E1/2) of Nd2O3/NC-700 are as low as ∼0.80 V and ∼0.56 V, respectively, indicating it is inactive for ORR. Co/NC-700 exhibits an Eonset of ∼0.88 V and Co-Nd2O3/NC-700 shows a close Eonset of ∼0.86 V. Compared with the E1/2 of Co/NC-700 (∼0.77 V), the slightly enhanced ORR activity is achieved on CoNd2O3/NC-700 with higher E1/2 of ∼0.79 V, which is close to that of 20 wt% Pt/C (∼0.82 V). The ORR activity of Co-Nd2O3/NC-700 is not so outstanding, but still comparable to the recent reported Co-based electrocatalysts for ORR, such as Co@CoO@Co3O4-N/C (∼0.74 V) [35], CoNC-CNF (∼0.80 V) [36], and Co@Co3O4@NC (∼0.80 V) [37]. Besides, the ORR activity for Co-Nd2O3/NC electrocatalysts prepared 6

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Fig. 4. (a) OER LSV curves of Nd2O3/NC-700, Co/NC-700, Co-Nd2O3/NC-700, and IrO2 at 1600 rpm at 5 mV s−1; (b) the corresponding Tafel plots; (c) ORR LSV curves of Nd2O3/NC-700, Co/NC-700, Co-Nd2O3/NC-700 and 20 wt% Pt/C in O2-saturated 0.1 M KOH at 1600 rpm at 5 mV s−1; (d) the corresponding Tafel plots; (e) The electron transfer numbers for Nd2O3/NC-700, Co/NC-700, Co-Nd2O3/NC-700 and 20 wt% Pt/C.

Pt/C (∼79.1 mV dec−1). Among the Co-Nd2O3/NC electrocatalysts prepared under different temperature, their Tafel slopes decrease first and then increase as the temperature increasing, indicating the favorable ORR kinetics of Co-Nd2O3/NC-700. The kinetics of ORR process was further investigated by the K-L equation (Eq. (2)–(4)) and the corresponding K-L plots derived from their RDE curves (Fig. S2) are

shown in Fig. S3. Obviously, except Nd2O3/NC-700, all the prepared electrocatalysts display good linearity and high parallelism, suggesting the first-order reaction kinetics versus the concentration of dissolved oxygen [38]. The kinetic current densities (jk) at 0.8 V calculated from K-L plots are shown in Table S5. The jk for Co/NC-700 is only ∼1.44 mA cm-2. Co-Nd2O3/NC-700 exhibits a higher jk of ∼2.26 mA 7

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Fig. 5. (a) OER mass activity through normalization of the current density at an overpotential of 470 mV to the mass loading on the glassy carbon disk electrode; (b) ORR mass activity through normalization of the kinetic current density at 0.8 V to the mass loading on the glassy carbon disk electrode.

Fig. 6. (a) Chronoamperometric curves of Co-Nd2O3/NC-700 and IrO2 at 1.56 V; (b) Chronoamperometric curves for Co-Nd2O3/NC-700 and 20 wt % Pt/C at 0.66 V.

is lower than 20 wt% Pt/C (∼22.17 A g−1 cat.), but slightly higher than Co/NC-700 (∼6.00 A g−1 cat.) and the Co-Nd2O3/NC electrocatalysts obtained under different temperatures. Its ORR mass activity is still comparable to other ORR electrocatalysts, such as ZIF derived CoeNeC −1 (∼18.6 A g−1 cat. at 0.82 V) [42], MoS2/P-ICPC (∼23.0 A gcat. at 0.61 V) [43] and BaxSm1-xMn2O5-δ/C (∼0.69 A g−1 at 0.90 V) [44]. These recat. sults above further demonstrate the introduction of Nd2O3 can remarkably improve the OER mass activity but slightly enhance ORR mass activity. The overall bifunctional electrocatalytic activity ΔE here is calculated by the equation ΔE=Ej=10-E1/2. As listed in Table S5, the ΔE of Co/NC-700 is ∼0.92 V. Though Co-Nd2O3/NC-700 exhibits close ORR activity to Co/NC-700, it still shows the smallest ΔE of ∼0.86 V among the prepared electrocatalysts mainly due to its significantly enhanced OER activity. Moreover, compared with the recently reported Co-based bifunctional electrocatalysts, such as Co@CoO@Co3O4-N/C (∼0.92 V) [35], Co3O4/CoFe (∼0.87 V) [4] and N-CoOx/CNTs (∼0.87 V) [45] (Table S7), Co-Nd2O3/NC-700 still exhibits a comparable bifunctional electrocatalytic activity.\ As shown in Fig. 6a, Co-Nd2O3/NC-700 shows outstanding OER durability investigated by chronoamperometry at 1.56 V. Co-Nd2O3/ NC-700 exhibits a ∼96.5 % retention of its initial OER current of after 12.5 h, which is much higher than that of IrO2 (∼37.2 %). The ORR durability of Co-Nd2O3/NC-700 and 20 wt% Pt/C was measured at 0.66 V (Fig. 6b). Co-Nd2O3/NC-700 can keep the ∼90.2 % retention of its initial current, while 20 wt% Pt/C only keeps ∼64.7 % of its initial current after 12.5 h, demonstrating the remarkable ORR durability of

cm-2, but lower than that of 20 wt% Pt/C (∼5.32 mA cm-2). As for the Co-Nd2O3/NC electrocatalysts prepared under different temperature, the jk follow the same order as their ORR activity order, further implying the favorable ORR kinetics of Co-Nd2O3/NC-700. The electron transfer number (n) calculated from the K-L equation (Eqs. (2)–(4)) is shown in Figs. 4e and S1e. Clearly, the average n value of Co/NC-700 is ∼3.83, indicating a mixed pathway with two- and four- electron during the ORR process. Co-Nd2O3/NC-700 exhibits a higher n value of ∼3.89, which is close to that of 20 wt% Pt/C (∼3.95), indicating its high selectivity of mixed two- and four- electron pathway. The similar mixed pathway also occurs on Co-Nd2O3/NC-600 and CoNd2O3/NC-800, whose n values are ∼3.81 and ∼3.86, respectively. The electrocatalytic activity of the prepared electrocatalysts is further evaluated by the OER mass activity (at 1.7 V, or η = ∼430 mV) and the ORR mass activity (at 0.8 V) normalized to the mass loading on the glassy carbon disk electrode. The OER and ORR mass activity of the Co-Nd2O3/NC electrocatalysts are shown in Fig. 5 and Table S6. As for OER mass activity, Nd2O3/NC-700 exhibits the low mass activity of ∼1.46 A g−1 cat.. The OER mass activity of Co-Nd2O3/NC-700 (∼113.96 A −1 g−1 cat.) is much higher than those of Co/NC-700 (∼50.58 A gcat.), other Co-Nd2O3/NC electrocatalysts prepared under different temperature, and even IrO2 (∼46.04 A g−1 cat.). The high OER mass activity of CoNd2O3/NC-700 is also comparable to other OER electrocatalysts, such as selenized Ni3(BO3)2 nanoflowers (∼35 A g−1 cat. at η = 370 mV) [39], α-Co(OH)2 nanomeshes (∼31.3 A g−1 cat. at η = 303 mV) [40] and NiMOF (BTC) (∼79.0 A g−1 cat. at η = 400 mV) [41]. Furthermore, CoNd2O3/NC-700 exhibits the ORR mass activity of ∼9.42 A g−1 cat., which 8

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Fig. 7. (a) EIS Nyquist plots under open-circuit voltage for Nd2O3/NC-700, Co/NC-700 and Co-Nd2O3/NC-700; (b) NH3-TPD curves; (c) CO2-TPD curves of Nd2O3/ NC-700, Co/NC-700 and Co-Nd2O3/NC-700.

introduction of Nd2O3 is favorable for the formation of O vacancy, thus leading to the higher surface O vacancy contents of Co-Nd2O3/NC-700 (∼6.78 at%) than that of Co/NC-700 (∼3.02 at%). Also, Co-Nd2O3/ NC-700 exhibits a higher O vacancy/total O ratio of ∼0.39 than that Co/NC-700 (∼0.32) (Table 1). Numerous reports have demonstrated that O vacancy can provide a moderate bonding strength with intermediate hydroxyl and facilitate the adsorption of H2O, thus enhancing the OER [50–52]. Therefore, the high O vacancy/total O ratio of CoNd2O3/NC-700 is favorable for its remarkable OER activity. Additionally, graphitic-N species in N-doped carbon are also found to modify the adsorbed interaction between intermediates and carbon substrates, thus benefiting the OER process [53,54]. As shown in Table 1, Co-Nd2O3/NC-700 has a higher graphitic-N/total N ratio of

Co-Nd2O3/NC-700. The Nyquist plots of the prepared electrocatalysts are shown in Fig. 7a. As shown in the Nyquist plots, the charge-transfer resistance (Rct) is determined by the diameter of the semicircular arc in the high frequency region [46]. The Rct of Co/NC-700 is ∼6.71 Ω, however, CoNd2O3/NC-700 exhibits a higher Rct value of ∼12.97 Ω likely due to the low graphitization of N-doped carbon [47]. Besides, the decrease of electron-rich N contents (∼6.30 at% for Co/NC-700 and ∼3.82 at% for Co-Nd2O3/NC-700) (especially the graphitic-N species (∼1.54 at% for Co/NC-700 and ∼1.16 at% for Co-Nd2O3/NC-700)) and the introduction of Nd2O3 with poor electrical conductivity may also account for the low charge-transfer efficiency of Co-Nd2O3/NC-700 [48,49]. Although Co-Nd2O3/NC-700 shows the inferior charge transfer efficiency to Co/ NC-700, its OER and ORR activity are better than Co/NC-700. Therefore, there are other factors affecting OER and ORR activity of the prepared electrocatalysts. As shown in Fig. 3a, the introduction of Nd2O3 decrease the BET SSAs of Co-Nd2O3/NC-700 compared with Co/NC-700. The ECSA was evaluated by calculating Cdl value, which was estimated by the linear slope of the fitting line plotted by capacitive currents versus scan rates (Fig. S4f). Their corresponding CV curves without Faradaic responses are shown in Fig. S4. Co-Nd2O3/NC-700 shows a Cdl value of ∼5.26 m F, close to that of Co/NC-700 (∼5.60 m F), suggesting their close ECSA. These results reveal that BET SSAs and ECSA are not main factors affecting the OER and ORR activity. As discussed in the XPS results of O 1s (Fig. 3c and Table S4), the

Table 1 The relative surface compositions of Co/NC-700 and Co-Nd2O3/NC-700 determined by XPS. Samples

Co/NC-700 Co-Nd2O3/ NC-700

9

O 1s

N 1s

Co 2p

O vacancy/ total O

graphiticN/total N

pyridinicN/total N

Co3+/total Co

Co2+/total Co

0.32 0.39

0.24 0.30

0.29 0.34

0.45 0.38

0.33 0.37

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∼0.30 than Co/NC-700 (∼0.24), which contributes to the high OER activity of Co-Nd2O3/NC-700. Furthermore, Co3+ species in Co-based electrocatalysts are demonstrated to function as the electron acceptor and promoting the electrophilic adsorption, which is beneficial for the OER activity [55,56]. Compared with Co/NC-700 (∼0.45), Co-Nd2O3/ NC-700 shows a lower Co3+/total Co ratio of ∼0.38, but much higher OER activity. Thus, it can be supposed that the contribution of Co3+ to OER activity is not primary here. In addition, the Lewis acidic sites could facilitate the activation of H2O molecules, thus improving the OER activity [57]. The NH3-TPD was used to characterize the surface acidity of electrocatalysts and the results are shown in Fig. 7b. In general, a desorption peak at higher temperature means more strong acidity and the peak area can be used to measure the amount of the acidic sites. Co/NC-700 displays two distinct desorption peaks at ∼505 °C and ∼549 °C, respectively, representing two different types of acidity strength. Co-Nd2O3/NC-700 also exhibits two distinct peaks at ∼534 °C and ∼626 °C, respectively. The desorption peaks of CoNd2O3/NC-700 are higher than Co/NC-700, indicating more strong acidity of Co-Nd2O3/NC-700. The calculated acidity amount of CoNd2O3/NC-700 is ∼2.78 mmol g−1, which is much higher than that of Co/NC-700 (∼0.92 mmol g−1). Therefore, the introduction of Nd2O3 is found to enhance the acidity strength and increase acidic sites. The enhanced Lewis acidity of Co-Nd2O3/NC-700 is also beneficial for its high OER activity. As a result, although Co-Nd2O3/NC-700 has lower Co3+/total Co ratio, its higher O vacancy/total O, graphitic-N/g-1cat. N and enhanced Lewis acidic property jointly promote the OER activity. As for ORR activity, pyridinic-N species in N-doped carbon are demonstrated to modify the electronic structure of carbon, thus improving the ORR activity [58,59]. Co-Nd2O3/NC-700 shows a higher pyridinicN/total N ratio of ∼0.34 than Co/NC-700 (∼0.29), which positively contributes to its slightly enhanced ORR activity. Moreover, Co2+ species was reported to serve as electron donor and preferentially adsorb oxygen molecules, which can promote the ORR process in Cobased electrocatalysts [17,60]. After the introduction of Nd2O3, CoNd2O3/NC-700 shows a higher Co2+/total Co ratio (∼0.37) than those of Co/NC-700 (∼0.33), which is favorable for its enhanced ORR activity. Fig. 7c shows the CO2-TPD results to characterize the surface basicity of the electrocatalysts. Similarly to NH3-TPD, a desorption peak at higher temperature means more strong basicity and the peak area can be used to measure the amount of the base sites. Nd2O3/NC-700 displays a CO2 desorption peak at ∼762 °C and Co/NC-700 shows a peak at the lower temperature of ∼526 °C. The CO2 desorption peak of Co-Nd2O3/NC-700 is at ∼598 °C, which is higher than that of Co/NC700, suggesting the more strong surface basicity of Co-Nd2O3/NC-700. The calculated amount of base sites in Co-Nd2O3/NC-700 (∼1.34 mmol g−1) is much higher than that in Co/NC-700 (∼0.55 mmol g−1). These results indicate that the introduction of Nd2O3 can strengthen the basicity and increase the surface base sites of Co-Nd2O3/NC-700. The Lewis base sites are suggested to be the active site for ORR, on which O2 can be adsorbed [61,62]. The modified Lewis basicity of Co-Nd2O3/NC700 can also account for its slightly enhanced ORR activity. Therefore, the relatively higher pyridinic-N/total N ratio, Co2+/total Co ratio and the modified base property of Co-Nd2O3/NC-700 function together to achieve the enhanced ORR activity. The Nd weight ratios in Co-Nd2O3/NC electrocatalysts determined by ICP-MS measurements are shown in Table S8. As shown in Fig. 8, both OER and ORR activity change trend of Co-Nd2O3/NC electrocatalysts show the obvious positive correlation with their Nd weight ratios. Although the Nd weight ratio in Co-Nd2O3/NC-700 is close to that in Co-Nd2O3/NC-800 (Table S8), Co-Nd2O3/NC-700 exhibits much better OER and ORR activity than Co-Nd2O3/NC-800. Thus, the existence of Nd2O3 mainly affects the surface contents of active species and the Lewis acid-base property as discussed above, thereby affect the OER and ORR activity. All in all, although the introduction of inactive Nd2O3 results in the inferior electrical conductivity and unremarkable ECSA, it definitely

still plays important effect on ORR and OER activity. The introduced Nd2O3 is beneficial to achieve high O vacancy/total O ratio, graphiticN/total N ratio, and the enhanced Lewis acidity, which is favorable for the significantly improved OER activity. As for ORR, the introduced Nd2O3 results in high Co2+/total Co ratio, high pyridinic-N/total N ratio, and the enhanced Lewis base property, thus benefiting its improved ORR activity. As a result, Co-Nd2O3/NC-700 displays excellent bifunctional catalytic activity. 3.3. Zn-air battery performance In consideration of the bifunctional catalytic activity of Co-Nd2O3/ NC-700, it was further employed as a Zn-air battery cathode catalyst. The discharge polarization curves and the corresponding power density plot of the Zn-air battery with Co-Nd2O3/NC-700 and mixed 20 wt% Pt/ C + IrO2 are shown in Fig. 9a. The battery with Co-Nd2O3/NC-700 exhibits a high open-circuit voltage of ∼1.41 V and a peak power density of ∼104 mW cm−2, slightly lower than that with mixed 20 wt% Pt/C + IrO2 (∼1.49 V and ∼126.9 mW cm−2, respectively). The galvanostatic deep discharge curves at a current density of 10 mA cm-2 (Fig. 9b) show that the discharge voltage plateau of the battery with CoNd2O3/NC-700 is ∼1.17 V, slightly lower than that of the battery with mixed 20 wt% Pt/C + IrO2. After normalized to the mass of the consumed Zn, the specific capacity of the battery with Co-Nd2O3/NC-700 is ∼613.6 mA h g-1 Zn, slightly lower than that of the one with mixed 20 wt % Pt/C + IrO2 (∼678.7 mA h g-1 Zn). The specific capacity of the battery with Co-Nd2O3/NC-700 is still comparable to recently reported Zn-air batteries with different electrocatalysts such as single-atom Fe-Nx-C -2 (∼641.0 mA h g-1 Zn at 10 mA cm ) [63] and N-doped NiCo2O4 -1 -2 (∼780.0 mA h gZn at 10 mA cm ) [64] (Table S9). The rechargeable performance of the battery with Co-Nd2O3/NC-700 was investigated and the corresponding cycling performance is shown in Fig. 9c. As is known, the charge process is associated with OER and the discharge process is related to ORR. At the initial stage, the discharge voltage of the battery with Co-Nd2O3/NC-700 is as same as the battery with mixed 20 wt% Pt/C + IrO2 (∼1.14 V). Notably, the charge voltage of the battery with Co-Nd2O3/NC-700 is ∼2.02 V, lower than that of the battery with mixed 20 wt% Pt/C + IrO2 (∼2.19 V), which thanks to the superior OER activity of Co-Nd2O3/NC-700. Also, the battery with CoNd2O3/NC-700 exhibits the higher initial round-trip efficiency of ∼59.3 % than the battery with mixed 20 wt% Pt/C + IrO2 (∼53.2 %). After 340 cycles, the charge voltage of the battery with mixed 20 wt% Pt/C + IrO2 exhibits a large increment value of ∼0.45 V, but the charge voltage of the battery with Co-Nd2O3/NC-700 only increased ∼0.07 V due to its excellent OER durability. After cycling, Co-Nd2O3/NC-700 exhibits a voltage gap of ∼1.72 V, which is lower than that of the battery with mixed 20 wt% Pt/C + IrO2 (∼1.96 V), indicating the better cycling durability. Such good cycling durability of the battery with Co-Nd2O3/NC-700 is comparable to recently reported Zn-air batteries with different electrocatalysts such as LaNi0.85Mg0.15O3 nanofibers (110 h cycling time at 10 mA cm−2) [65] and LaMn0.75Co0.25O3−δ nanofibers (70 h cycling time at 2 mA cm−2) [66] (Table S9). These results reveal that the Co-Nd2O3/NC-700 electrocatalyst has outstanding bifunctional catalytic activity and durability. 4. Conclusions The inactive Nd2O3 was successfully introduced to Co/NC electrocatalysts to regulate the active species for OER and ORR. In the resulting Co-Nd2O3/NC, the Co and Nd2O3 nanoparticles are well distributed on N-doped carbon matrices and coated with NC shells. After the introduction of Nd2O3, the ratios of O vacancy/total O ratio and graphitic-N/total N in Co-Nd2O3/NC-700 increase, which is in favor of the superior OER activity. Besides, the Lewis acidic property is also enhanced in Co-Nd2O3/NC-700, which also benefits the OER activity. On the other hand, the introduced Nd2O3 is beneficial to obtain high 10

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Fig. 8. (a) Relationship between Ej=10 and Nd weight ratios in Co-Nd2O3/NC electrocatalysts; (b) relationship between E1/2 and Nd weight ratios in Co-Nd2O3/NC electrocatalysts.

Fig. 9. (a) Discharge polarization curves and the corresponding power density plot of the Zn-air battery; (b) Long-time galvanostatic discharge curves of the Zn-air battery at 10 mA cm−2; (c) Galvanostatic charge-discharge cycling at 10 mA cm−2 (5 min charge and 5 min discharge).

Co2+/total Co ratio and pyridinic-N/total N ratio, and improve the Lewis base property thus contributing to the improved ORR activity. As a result, Co-Nd2O3/NC-700 exhibits a low Ej=10 value of ∼1.65 V and a high E1/2 value of ∼0.79 V. Its ΔE is as low as ∼0.86 V mainly due to its significantly enhanced OER activity. When applied in a Zn-air battery, the battery with Co-Nd2O3/NC-700 shows the open-circuit voltage of ∼1.41 with a high specific capacity of ∼613.6 mA h g−1 Zn , which are comparable to those of the battery with mixed 20 wt% Pt/C + IrO2. Moreover, thanks to the enhanced OER activity and durability, the battery with Co-Nd2O3/NC-700 exhibits better charge-discharge cycling

durability than he battery with mixed 20 wt% Pt/C + IrO2. This work could open up an efficient way to develop the oxygen electrocatalysts with high bifunctional activity for Zn-air battery via the introduction of inactive materials. CRediT authorship contribution statement Jiabin Tan: Investigation, Writing - original draft. Xiaobo He: Conceptualization, Methodology, Writing - review & editing. Fengxiang Yin: Supervision, Writing - review & editing. Biaohua 11

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Chen: Project administration. Guoru Li: Investigation, Formal analysis. Xin Liang: Software, Data curation. Huaqiang Yin: Resources. [19]

Declaration of Competing Interest [20]

The authors declare no competing financial interest. Acknowledgements

[21]

This work is supported by the National Natural Science Foundation of China (21706010), the Natural Science Foundation of Jiangsu Province of China (BK20161200). Special thanks to the support from Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University (ACGM2016-06-02 and ACGM2016-0603), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Key Laboratory of Advanced Reactor Engineering and Safety, Ministry of Education (ARES-2018-09).

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Appendix A. Supplementary data

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Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cattod.2019.12.018.

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