Silver-modified porous 3D nitrogen-doped graphene aerogel: Highly efficient oxygen reduction electrocatalyst for Zn−Air battery

Accepted Manuscript Silver-modified porous 3D nitrogen-doped graphene aerogel: Highly efficient oxygen reduction electrocatalyst for Zn−Air battery Jie Hu, Ziwei Shi, Xueqian Wang, Huici Qiao, Hao Huang PII:

S0013-4686(19)30294-4

DOI:

https://doi.org/10.1016/j.electacta.2019.02.051

Reference:

EA 33642

To appear in:

Electrochimica Acta

Received Date: 20 October 2018 Revised Date:

20 December 2018

Accepted Date: 11 February 2019

Please cite this article as: J. Hu, Z. Shi, X. Wang, H. Qiao, H. Huang, Silver-modified porous 3D nitrogen-doped graphene aerogel: Highly efficient oxygen reduction electrocatalyst for Zn−Air battery, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.02.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Silver-modified porous 3D nitrogen-doped graphene aerogel: highly efficient oxygen reduction electrocatalyst for Zn−Air battery Jie Hua, b, Ziwei Shib, Xueqian Wangb, Huici Qiaob, Hao Huanga ∗ State Key Laboratory of Metastable Materials Science & Technology, Yanshan University, Qinhuangdao, 066004, P.R. China b

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a

Hebei Key Laboratory of Applied Chemistry, Department of Environment and

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Chemistry, Yanshan University, Qinhuangdao, 066004, P.R. China

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Abstract: Developing highly efficient electrocatalyst is crucial to improve the efficiency of oxygen reduction reaction for Zn-air battery. Herein, Ag-modified porous 3D nitrogen-doped graphene aerogel is synthesized by one-step hydrothermal method for promoting the electrocatalytic performance and stability toward oxygen

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reduction reaction. Interestingly, in this study, N doping process, reduction of AgNO3 and graphene oxide, and the three-dimensional self-assembly can be finished during

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the hydrothermal synthesis at the same time. The obtained Ag-modified 3D nitrogen-doped graphene hybrid presents an interconnected 3D porous framework and

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Ag nanoparticles homogeneously distribute on the inner and surface of 3D N-doped graphene networks. The unique structure results in excellent oxygen reduction reaction catalytic activity with a superior stability to commercial 20wt.% Pt/C. The high electrochemical activity and durability of the hybrid is also confirmed in Zn-air batteries that outperform Pt/C in discharge voltage plateau and long-term durability, showing a promising oxygen reduction catalyst for Zn-air batteries.

∗

Corresponding author. Tel.: +86 15369700375. E-mail address: [email protected]. 1

ACCEPTED MANUSCRIPT Keywords: Silver-modified, Nitrogen-doped, 3D graphene aerogel, Oxygen reduction reaction, Zn-air batteries.

1. Introduction

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Zn-air batteries are receiving intense interest owing to their high energy density, cost effective, and environmentally benign in energy storage and conversion devices

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[1-3]. However, the intrinsic sluggish kinetics of oxygen reduction reaction (ORR), as the main cathode reaction in batteries, greatly restricts the large-scale application of

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zinc-air batteries [4-6]. Generally, noble metal Pt and its alloys are commonly used to improve the reaction rate and overcome the large over potential in ORR. Although Pt-based catalysts have exhibited excellent performance in ORR process, the high costs and poor stability of Pt severely hamper their commercialization in clean energy

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technology [7-11]. Faced with these challenges, it is very important to develop efficient, low-cost and comparable electrocatalyst for ORR.

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Recently, researchers have found that N-doped carbon materials, such as N-doped carbon nanotubes (CNTs)[2,12-13], nitrogen-doped hollow graphene

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spheres(N-HGSs)[8], and N-doped graphitic layers[14] can be used as the excellent ORR catalysts because of their high electrical conductivities and large specific surface areas[15-16]. More importantly, doping nitrogen can provide abundant structural defects, active sites and adjust the charge distribution of carbon, which can greatly improve the electrocatalytic activity of catalysts [15, 17-18]. In particular, graphene, a two-dimensional carbon matrix, has drawn increasing attention and commonly used as substrate for supporting metal due to its excellent conductivity, mechanical, 2

ACCEPTED MANUSCRIPT electronic, and stable chemical property[19-21]. But in practical applications, random aggregation and the strong planar stacking of 2D graphene (2DG) sheets can lead to the loss of active sites and a lower specific surface area [22-23]. Consequently, some

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efforts have been done to assembly 2DG sheets into three-dimensional forms such as hydrogels [21], aerogels [24], and other carbon frameworks [25-26]. Such 3D graphene (3DG) structures are not only possess the merits and properties of graphene

rapid transfer channels of ions, which greatly enhance its

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surface utilization and

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sheets but also for preventing graphene sheets from agglomerating, providing higher

applicability in real applications[27-31].

However, the experimental performance of 3DG networks still lie far behind so far because of the as-prepared graphene by a modified Hummer preparation contains

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lots of oxygenic functional groups, such as carboxyl, carbonyl and so on, therefore lead to low conductivity. An effective method is to modify the 3DG with a small amount of noble metal [32-34]. For instance, Qin et al. [35] synthesized Pt/Fe,

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N-codoped 3D graphene as an effective electrocatalyst for ORR. Among noble metals,

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Ag has been regarded as the most contender: (1) relatively low cost; (2) high conductivity and stability; (3) can catalyze the direct four electron (4 e-) reaction for ORR and the HO2 anion disproportionation. For example, D.J. Davis et al. [36] synthesized silver-graphene nanoribbon composite as the catalyst for ORR and it showed excellent catalytic activity and outstanding tolerance to CH3OH. F T Goh et al.[37] prepared a Ag nanoparticle-modified MnO2 nanorods catalyst and applied it in the Zn-air battery. Such silver-modified catalysts exhibit excellent electrochemical 3

ACCEPTED MANUSCRIPT activity and can be used as capable catalysts for the air electrode. In this paper, we report a simple method to synthesize a high conductive silver-modified porous 3D nitrogen-doped graphene (3DNG) aerogel through

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one-step hydrothermal method, wherein, ethylenediamine is not only the nitrogen source but also acted as the reducing agent used to restore AgNO3 to Ag. Interestingly, it has been found that N doping, the three-dimensional self-assembly of graphene

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oxide, reduction of graphene oxide (GO) and AgNO3 are going on simultaneously.

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The as-prepared hybrid presents an interconnected 3D porous framework, large specific surface area and Ag nanoparticles homogeneously distribute on the inner and surface of 3DNG networks. The resultant Ag-modified 3D N-doped graphene (Ag-3DNG) exhibits superior catalytic activity for ORR compare with the commercial

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Pt/C (20 wt %) catalyst, such as a better onset potential of +0.97 V (vs. RHE), half-wave potential of +0.81 V (vs. RHE) and excellent stability. Furthermore, the Zn−air batteries by using Ag-3DNG as catalyst for air electrode present higher

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discharge voltage plateau and better durability than the Pt/C.

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2. Results and discussion Insert Fig.1

In this work, we synthesized a porous Ag-3DNG aerogel via one-step

hydrothermal method, the schematic procedure is shown in Fig. 1. It is well known that GO has a double conjugate structure with hydrophobic plane and hydrophilic functional group. The hydrophilic-hydrophobic equilibrium of the Van der Waals force and electrostatic repulsion between graphene sheets regulates their solution 4

ACCEPTED MANUSCRIPT properties, when the equilibrium is broken, the hydrogel is produced. During the process of gelation, the part of graphene sheets overlaps to form hydrogels with 3D porous structures. In this study, the AgNO3 precursors are adsorbed on the surface of

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GO by the electrostatic attraction, and then the ethylenediamine plays three roles during the hydrothermal process: (1) N is incorporated into graphene through the ethylenediamine as the nitrogen source; (2) ethylenediamine can be used as a

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complexant to react with AgNO3; The complexation of Ag+ and ethylenediamine (en)

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is similar with Ag+ and NH3·H2O, Ag+ will be in the form of [Ag(OH)2en]- and its reaction is as follows: Ag+ + 2H2O + en → [Ag(OH)2en]- + 2H+, then

[Ag(OH)2en]-

will be reduced to Ag. (3) Ethylenediamine can also restore GO to graphene as a reducing agent. Thus, N doping process, reduction of AgNO3 and graphene oxide, and

hydrothermal process. Insert Fig.2

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the three-dimensional self-assembly can be completed simultaneously during the

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Fig. 2a, 2b and 2c show the SEM images of 2DG, 3DG and 3DNG, it is observed

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that 2DG presents a planar structure with gauze-shaped wrinkles and folds, whereas 3DG and 3DNG display interconnected 3D porous network with hierarchical pore sizes, suggesting graphene sheets are efficiently assembled during the hydrothermal process. It can be seen that the 3D porous structure after doping N atoms has no obvious change, which is according with the XRD results. As shown in Fig. 2d, after the formation of Ag-3DNG, the hybrid still maintain the interconnected 3D porous network and Ag nanoparticles are uniformly distributed and tightly attached onto the 5

ACCEPTED MANUSCRIPT outer and inner walls of graphene sheets in the network. Fig. 2e shows that the 3DNG are transparent with some wrinkles. For Ag-3DNG, Fig. 2f clearly indicates that Ag particles are distributed uniformly on the surface of 3DNG with no aggregation. The

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HRTEM image of Ag-3DNG in Fig. 2g shows a typical nanocomposite structure with Ag nanoparticles embedded in the N-doped graphene nanoshell. Furthermore, it further confirms the well-defined lattice fringe of Ag, where the lattice distance of

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0.236 nm and 0.204 nm are corresponded to the (111) and (200) plane, respectively.

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The selected-area electron diffraction pattern (Fig. 2h) gives diffraction pattern agreeing with fcc Ag crystals as well. Insert Fig.3

The crystal structural of the Ag-3DNG were further characterized by X-ray

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diffraction, as shown in Fig. 3. It can be seen that the key indicator of GO at 11.5o is assigned to the (001) plane, demonstrating there are some oxygen-containing functional groups between the graphite layers [38]. In the pattern of 3DG, 3DNG and

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Ag-3DNG, the diffraction peak of GO disappeared and a new diffraction peak at 24.5o

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is observed, which can be indexed to the (002) plane of graphene with an interlayer distance of 0.36 nm. This indicates that the elimination of oxygen-containing functional groups from GO and also proved the existence of strong π-π stacking interactions between the graphene sheets as the major cause of gelation [23, 39]. Moreover, the XRD pattern of Ag-3DNG also shows four more diffraction peaks located at 38.1o, 44.3o, 64.4o, and 77.4o, corresponding to the (111), (200), (220), and (311) planes , respectively, which are commendably accord with the standard card of 6

ACCEPTED MANUSCRIPT the fcc Ag crystal (PDF#04-0783). This result confirms that AgNO3 has been successfully restored to Ag and attached onto the surface of 3DNG. Insert Fig.4

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The structure of samples was further confirmed by Raman spectra. Fig. 4 shows that all of spectra appear two characteristic peaks of graphite: the D peak is at ~1350 cm-1 and the G peak is at ~1590 cm-1, which are corresponded to disordered sp3

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carbon and graphitic sp2 carbon, respectively [40-41]. The intensity ratio of ID/IG, is

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an indication to evaluate the extent of defects in graphitic structures [42-43]. The ID/IG values are 1.47 for 3DNG, 1.24 for 3DG and 0.87 for GO, respectively, indicating that the hydrothermal process results in a large number of sp3 defects on 3DG. Moreover the high ID/IG value of 3DNG demonstrates that doping N atoms into the carbon

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matrix can lead to highly disordered [44]. For Ag-3DNG, because metal Ag has no Raman peak, its shape is similar to 3DNG spectrum, except there is a little peak at ~490 cm-1, corresponding to Ag2O. This shows the existence of silver in the form of

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Insert Fig.5

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trace Ag2O and a large amount of mental silver in composite materials.

Insert Table 1

The BET specific surface areas and respective pore size distribution of 3DG,

3DNG and Ag-3DNG are measured according to N2 adsorption/desorption (Fig. 5). It is apparent that all samples present the typical type (IV) isotherm which is indicative of the mesoporous nature of samples with a H3 hysteresis loop. As plotted in Fig. 5a, the 3DNG presents a greater adsorption capacity, therefore it has a larger surface area, 7

ACCEPTED MANUSCRIPT as listed in Table 1, which may be attributed to the N atoms doped into the carbon matrix resulted in more defects as illustrated from the Raman analysis. Moreover, the larger surface area can expose more active sites and enhance the catalytic activity. For

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Ag-3DNG, the specific surface area and pore diameter decreased comparing with 3DNG, which is due to the introduction of Ag occupying the position of some holes. The corresponding pore-size distributions curves via the Barrett-Joyner-Halenda (BJH)

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method are shown in Fig. 5b. It can be seen that the pore size distribution after doping

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N atoms has no obvious change, except the pores centered at 3.3 nm has greatly increased, and the loading of silver particles slight reduced the pore size. Insert Fig.6 Insert Table 2

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The XPS of Ag-3DNG was measured in order to investigate the chemical composition and valence state. As shown in Fig. 6a, the XPS survey spectrum reveals

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that the Ag-3DNG hybrid mainly consists of C, N, O and Ag. The atomic ratio and mass ratio of C, N, O and Ag are shown in Table 2. The high-resolution C1s spectrum

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shown in Fig. 6b exhibits four peaks, those corresponding to C-C (284.4eV), C-O (285.4eV), C-N (286.6eV) and O=C-OH (288.1eV), respectively [31,45], which reveals the successful doping of N in the grephene. Fig. 6c can be deconvolved into three peaks, which centered at 398.6, 399.5 and 400.5 eV, corresponding to pyridinic-N, pyrrolic-N and graphitic-N, respectively [8,15]. According to previous reports, the N atom doped into the carbon matrix can change the electronic structure, improve chemical activity and reduce Fermi level of the adjacent carbon atoms, 8

ACCEPTED MANUSCRIPT beneficially adsorbing and activating O2 molecules [46]. In particular, the pyridinic-N and graphitic-N play the key role in ORR process. It has been established that Pyridinic-N occurs in the edge of graphite planes and links with two carbon atoms, it

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can donate a electron to the aromatic π system to impart Lewis basicity to the carbon, which is able to facilitate the reductive adsorption of O2 and improve the onset potential of electrocatalysts. Graphitic-N bonds with three carbon atoms in the

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graphene basal plane, which is as electrocatalytic active sites for ORR and greatly

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increasing the limiting current density [47-48]. In the Ag3d spectrum (Fig. 6d), two peaks at 368.4 and 374.4 eV can be assigned to Ag3d5/2 and Ag3d3/2, corresponding to the 3d bonding energy of metallic silver, which further illustrates the presence of Ag in a metallic state in the Ag-3DNG hybrid.

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Insert Fig.7

As shown in Fig. 7a, the obvious cathode peaks corresponding to ORR are observed in O2-saturated solution, whereas no reaction in N2-saturated solution.

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Furthermore, the potential of peak for Ag-3DNG is 0.75 V (vs RHE), which is more

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positive than that for 2DG (0.64 V), 3DG (0.66 V) and 3DNG (0.68 V), indicating the better ORR catalytic activity of Ag-3DNG. Fig. 7b shows the LSV curves of 2DG, 3DG, 3DNG, Ag-3DNG and commercial 20 wt% Pt/C. Apparently, Ag-3DNG exhibits relatively positive onset potential (0.97 V vs RHE), half-wave potential (0.81 V vs RHE) and larger diffusion-limited current density (5.25 mA⋅cm-2), which comparable to those of commercial Pt/C (0.98 V, 0.85 V, 5.25 mA⋅cm-2) and much better than 2DG (0.83 V, 0.75 V, 3.61 mA⋅cm-2), 3DG (0.88 V, 0.78 V, 4.16 mA⋅cm-2) and 3DNG 9

ACCEPTED MANUSCRIPT (0.89 V, 0.76 V, 5.06 mA⋅cm-2), respectively, indicating the excellent ORR activity of Ag-3DNG. In order to highlight the excellent oxygen reduction catalytic performance of self-made catalyst Ag-3DNG, the catalytic performance of some other 3D graphene

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or carbon hybride was listed in Table S1. As shown in table, the Ag-3DNG shows comparable and better ORR performance than them in an alkaline medium.

To further unravel the reaction kinetics for ORR, the LSV measurements at

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400-1600 rpm were performed and the kinetic parameters were calculated by

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Koutecky-Levich (K-L) equation, as shown in Fig. S1. As can be seen, the diffusion-limited current densities of different samples exhibit an increasing trend due to the more high speeds diffusion. The fitting K-L plots are linear relationships between j-1 and ω-1/2 under each potential. The number of transferred electrons n for

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Ag-3DNG is figured to be ~4 at different potentials from 0.2 V to 0.6 V, which approached the theoretical value 4 of Pt/C measured in the same condition, indicating a dominated four-electron transfer pathway for ORR.

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Comparing to 2DG, the brilliant catalytic performances of Ag-3DNG are mainly

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due to its unique structure, as illustrated in SEM images. On one hand, their interconnected 3D porous network formed during hydrothermal process can minimize charge transfer and diffusion resistance. On the other hand, the hierarchical porous structures with a large pore volume and specific surface area provide more active sites as well as smooth transport channels of reactants. Comparing to 3DG, the incorporation of N in the Ag-3DNG is a critical factor to improve ORR performance. According to the literature, the amount of active sites in carbon materials can be 10

ACCEPTED MANUSCRIPT increased by doped with N [35, 48, 49-50]. As illustrated in XPS, pyridinic-N can affect the electron cloud distribution of adjacent carbon atoms, which can lead to produce active sites that can directly participate in ORR process. Therefore,

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pyridinic-N is also able to facilitate the reductive adsorption of O2 and improve the onset potential of electrocatalysts. Moreover, graphitic-N bonds with three carbon atoms in the graphene basal plane can increase the limiting current density. Finally,

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Ag-3DNG has superior activities that comparable to Pt/C and better than Ag-free

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catalysts (3DNG), indicating Ag nanoparticles plays essential role in the catalyst due to their high conductivity and electrochemical activity.

The high ORR activity of Ag-3DNG is also confirmed by the results determined from rotating ring-disk electrode (RRDE) measurements (Fig. 7c, d). The low yield of

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the H2O2 (<7%) reflects the high electron transfer number (>3.8) between 0.2-0.8 V for Ag-3DNG, which is lower than that of Pt/C but is significantly higher than that for 2DG, 3DG and 3DNG, suggesting the ORR on Ag-3DNG mainly proceeds through

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four-electron pathway, coinciding well with the results of RDE. Furthermore, the

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corresponding Tafel slop (Fig. 7e) of Ag-3DNG at low over potential region is calculated to be 70.2 mV·dec-1, which is smaller than 3DNG (111.8 mV·dec-1), 3DG (114.4 mV·dec-1), 2DG (136.6 mV·dec-1) and Pt/C (73.5 mV·dec-1), indicating the fast kinetics process and high transfer coefficient toward ORR. Durability of the catalyst is another key parameter for a practical application. As shown in Fig. 7f, Ag-3DNG exhibits 14.5% decrease after 28800 s continuous operation. In contrast, Pt/C displays a faster current loss with about 35% under the 11

ACCEPTED MANUSCRIPT same conditions, indicating the better stability of Ag-3DNG catalyst. The durability of the Pt/C is lower than Ag-3DNG, which is owing to the carbon black constituent in commercial Pt/C catalyst are poorly stable, leading to Pt particles are easily dissolved

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and aggregation under working conditions, thus reducing the durability. Insert Fig.8

In order to futher explored the activity and stability of samples, we incorporated

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the catalysts into a real air cathode for the primary zinc-air batteries. The discharge

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curves of different catalysts shown in Fig. 8a indicate that 3DG, 3DNG and Ag-3DNG all exhibit a steady state and almost no decay, while Pt/C and 2DG presents obvious and a relatively slight decay, respectively, which are attributed to the 3DG possesses an interconnected 3D porous structure and numerous active sites, increasing

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three-phase interface and providing more transmission paths for O2. In addition, the modification of Ag in the Ag-3DNG results in the highest discharge voltage plateau 1.21 V because of the high electrical conductivity of Ag.

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Fig. 8b shows the constant-current discharge cycles of Ag-3DNG in the primary

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zinc-air battery. In this experiment, the battery required to be filled with the fresh electrolyte after every cycle, due to the decline of KOH concentration in the electrolyte. As can be seen, the discharge plateau is higher after one cycle because the active material is adequately in contact with electrolyte. The discharge plateau begins to decrease after the second cycle, and the discharge plateau can be maintain about 1.192 V until the ninth cycle, indicating the Ag-3DNG catalyst shows superior stability. 12

ACCEPTED MANUSCRIPT Insert Fig.9 The EIS is shown in the Nyquist plots in Fig. 9, which is referred to investigate the electrode kinetics and resistance in electrochemical systems. The impedance plots

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of all samples are almost composed of a semicircle and a straight line, which are assigned to charge transfer and diffusion processes of oxygen, respectively. The impedance spectra is evaluated by fitting the spectra to an equivalent circuit as shown

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in the inset of Fig. 9, where R1 represents the ohm resistance between the reference

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electrode and current collector, C1 represents the limit capacitance, and all these impedance spectra almost has the same values of R1 and C1. A main difference is the semicircles in the high frequency regions of the Nyquist plot are attributed to the charge transfer resistance R2 comes from the Faradaic reactions and double-layer

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capacitance C2 on the grain surface. The corresponding fitting parameters are listed in Table S2, indicating the Ag-3DNG achieves a rapid charge transport due to the superior conductivity of Ag. At the same time, the 3D porous graphene acts the role of

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electron acceptor and transmission. The straight line of the curve in the low frequency

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range represents Warburg resistance W and R3 caused by ion diffusion or the transport between the electrolyte to electrode surface. The 3D porous structure, more defects and N doping in the Ag-3DNG provide more diffusion and migration pathways of absorbed oxygen and electrolyte ions. In addition, Ag also possesses good catalytic performance for adsorbing and activating O2 molecules, therefore, resulting in the Ag-3DNG catalyst shows the larger slope in the low frequency range. 3. Conclusion 13

ACCEPTED MANUSCRIPT Herein, we successfully synthesized a silver-modified 3D nitrogen-doped graphene aerogel hybrid by one-step hydrothermal method. The as-prepared Ag-3DNG composite displays excellent ORR performance and superior stability

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comparable to the commercial Pt/C. Besides, the Zn-air battery with Ag-3DNG as the catalyst shows higher discharge voltage plateau and better durability than Pt/C. The prominent electrocatalytic activities are mainly due to its 3D porous structure, the

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introduction of highly active N-containing species (pyridinic-N and graphitic-N) and

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the high conductivity and stability of Ag. The results demonstrate that the Ag-3DNG hybrid is a promising electrocatalyst in metal-air batteries. Supplemental Information

Supplementary data and Experimental associated with this article can be found,

Acknowledgements

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in the online version.

The authors gratefully acknowledge the support of the Natural Science

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Foundation of Hebei Province (Nos. B2016203172 and E2017203160), China

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Scholarship Council (No.201708130018) and the Research Program of the College Science &Technology of Hebei Province (No. ZD2017073). References

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Figure Captions

Fig. 1. Schematic procedure for the preparation of Ag-3DNG aerogel. Fig. 2. SEM images of 2DG (a), 3DG (b), 3DNG (c) and Ag-3DNG (d). TEM images of 3DNG (e) and Ag-3DNG (f). HRTEM image (g) and SAED pattern (h) of Ag-3DNG. 22

ACCEPTED MANUSCRIPT Fig. 3. XRD patterns of GO, 3DG, 3DNG and Ag-3DNG. Fig. 4. Raman spectra of GO, 3DG, 3DNG and Ag-3DNG. Fig. 5. (a) N2 adsorption and desorption isotherms of 3DG, 3DNG and Ag-3DNG; (b)

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Corresponding pore-size distributions curves of samples. Fig. 6. XPS spectra of (a) survey scan, High-resolution curves of (b) C1s, (c) N1s and (d) Ag 3d of Ag-3DNG.

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Fig. 7. (a) CV profiles of 2DG, 3DG, 3DNG and Ag-3DNG in 0.1 M KOH solutions saturated with N2 or O2 at a scan rate of 50 mV⋅s-1. (b) LSV curves, (c) RRDE

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voltammograms, (d) electro-transfer number and peroxide yield, and (e) Tafel plots of 2DG, 3DG, 3DNG, Ag-3DNG and commercial 20 wt% Pt/C in an O2-saturated 0.1 M KOH solution at a scan rate of 5 mV⋅s-1 and a rotating rate of

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1600 rpm. (f) Current-time (i-t) chronoamperometric responses of Ag-3DNG and Pt/C in an O2-saturated 0.1 M KOH solution at 0.6 V and a rotation speed of 400 rpm.

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Fig. 8. (a) Discharge curves of different catalysts at 25 mA⋅cm-2 and (b) The

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constant-current discharge cycles of Ag-3DNG in the primary zinc-air batteries. Fig. 9. Nyquist plots of the assembled Zn–air battery with different catalysts. Inset: the corresponding equivalent circuit diagram.

Table 1. Physical data of 3D, 3DNG and Ag-3DNG. Table 2. The atomic ratio and mass ratio of C, N, O and Ag in the XPS analysis of Ag-3DNG.

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Specific surface

Average pore

Pore volume

area (m2⋅g-1)

diameter (nm)

(cm3⋅g-1)

3DG

219.5521

15.1897

3DNG

240.2596

24.5509

Ag-3DNG

197.4952

14.0945

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Sample

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Table. 2 C

N

O

Ag

Atomic ratio (%)

82.43

8.37

8.37

0.83

Mass ratio (%)

74.38

8.81

10.07

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6.74

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