Novel CdFe bimetallic complex-derived ultra-small Fe and N codoped

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Novel CdFe bimetallic complex-derived ultra-small Fe and N codoped carbon as a highly efficient oxygen reduction catalyst Heng Liu, Zhiqin Deng, Minqiang Wang, Hao Chen, Longcheng Zhang, Youquan Zhang, Renming Zhan, Maowen Xu, and Shu-Juan Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 24, 2019

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ACS Applied Materials & Interfaces

Novel CdFe bimetallic complex-derived ultra-small Fe and N codoped carbon as a highly efficient oxygen reduction catalyst

Heng Liu, Zhiqin Deng, Minqiang Wang, Hao Chen, Longcheng Zhang, Youquan Zhang, Renming Zhan, Maowen Xu*, Shu-Juan Bao*

Key Laboratory of Luminescent and Real-Time Analytical Chemistry (Southwest University), Ministry of Education, Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing 400715, PR China E-mail: [email protected]; [email protected]

ABSTRACT During the development of oxygen reduction reaction (ORR) electrocatalysts, transition metal nanoparticles embedded in N-doped graphene have attracted increasing attention owing to their low-priced, minimal environmental impact and satisfying performance. In this study, a new organic-Cd complex formed through Cd2+ coordination with p-phenylenediamine (PPD) was used to synthesis highly active Fe embedded N-doped carbon catalysts for the first time. It is significant that with the decreasing molar ratio of Cd/Fe, an obvious microstructure evolution was observed in Cd-Fe-PPD from diamond-like blocks to thick flakes, and further bloomed into flower-like shapes with ultrathin petals and then eventually exhibited large block starfish-like shapes. After carbonization treatment, Cd was removed, slack and porous N-doped carbon was formed, and Fe was assembled in N-doped carbon. Similar phenomenon was also observed in Co-PPD. The optimized Fe@NPC-2 material features uniform and welldispersed 3-5 nm Fe nanoparticles embedded in two-dimensional ultrathin carbon nanosheets 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

delivered excellent electrocatalytic performance (Eonset: 0.96 V vs RHE, E1/2: 0.84 V vs RHE), which very close to those of commercially Pt/C (Eonset: 0.95 V vs RHE, E1/2: 0.84 V vs RHE) and its methanol tolerance and durability also surpass that of Pt/C. Keywords: oxygen reduction reaction, organic-Cd complex, bimetallic-PPD, ultra-small nanoparticles

1. INTRODUCTION The shortage of fossil fuels has stimulated intensive research and rapid development of clean and sustainable energy. Fuel cells and metal-air batteries attracted more attention for appling in various energy relatived fields in the furture, but the slow cathodic oxygen reduction reaction (ORR) limited their improvement.1-4 Hence, high activity and long stability of ORR electrocatalysts are urgently needed for practical application of these devices on a large scale.5-9 Platinum-based metals have long been regarded as the best electrocatalysts. However, their high cost, limited reserve, weakness to fuel crossover, and poor stability hinder their commercial application.10-14 Recently, various nonprecious metal catalysts (NPMCs) have been explored to substitute noble metal platinum catalysts for boosting ORR.15-17 Specifically, various transition metal-nitrogen-carbon catalysts have been extensively pursued due to their low cost, minimal environmental impact, and satisfying performance.18-19 Generally, according to the different precursors, the synthetic methods of transition metal−nitrogen−carbon catalysts can be categorized into the following classes: (i) Pyrolysis the metal and carbon co-containing complexes. For example, Klaus et al,20 synthesized carbon supported Fe3O4 catalysts by pyrolysis of Fe ions-containing porous graphene oxide aerogels at 600 oC. (ii) Carbonization of metal-organic complex. Such as，Li et al.21 , prepared carbon wrapped metallic cobalt catalyst by annealing of cobalt-containing Prussian blue colloids at 900 oC. (iii) Pyrolysis of small molecular precursors. For instance, Chen et al.22 fabricated Fe2N@carbon by carbonization of glucose, melamine, and metal salt during a multi-step 2 ACS Paragon Plus Environment

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thermal treatment and further annealed at 800 oC. In order to enhance the degree of graphitization and conductivity of the carbon supporter, the metal-nitrogen-carbon catalysts obtained from above synthetic strategies are often performed with a pyrolysis process at high temperatures.23-26 However, the pyrolysis process always results in products that inevitably face irreversible fusion and seriously aggregation because of the drastic reactive processes that occur at high temperature. Further with the temperature continue increasing, the degree of graphitization of the carbon, and their conductivity becomes better. However, the problem of more serious agglomeration of the nanoparticles occurs. This is an urgent problem that needs to be solved, and many efforts have been made in recent years. Such as, Zhang et al.27 utilized silica-protected strategy to control the growth of Co particles after high temperature pyrolysis to remove the mSiO2 shell by chemical etching. Li et al,21 using HCl etching method to eliminate large particles formed during annealing cobalt-containing Prussian blue. Such posttreatments can remove largely agglomerated nanoparticles and expose more catalytic activity sites. Yet, the post-treatments pose other serious issues, such as acid corrosion, and the secondary high-temperature treatment will result in more energy loss and complicated process. Thereby, overcoming these current problems to prepare catalysts with high dispersibility, small size particles, and high activity using a simple method remains a great challenge. Eric P. et al.28 found that Zn could vaporize and escape through its volatile organic-Zn complex during a carbonization process at high temperatures over 800

oC.

Inspired by this

phenomenon, some effective Fe(Co)-N-C catalyst with high metal dispersibility were developed through intentionally doped Fe(Co) ions in ZIF-8 (a kind of Zn-MOF) followed by pyrolysis in inert atmosphere at high temperatures.29-32 In addition, the presence of zinc was determined to increase the distance between different atoms, preventing the leftover metal particles from agglomeration, and ultimately eliminating zinc during pyrolysis. 33 In this work, a new organic-Cd complex formed by coordination with p-phenylenediamine (PPD) was used to design highly active Fe(Co)-based catalysts for the first time. Since Cd can 3 ACS Paragon Plus Environment


be evaporated like Zn at even lower temperatures (e.g. 400 oC), and through adjusting the molar ratio of Cd/Fe, the leftover metal particles following catalyst synthesis can effectively disperse and embed in the as-formed carbon. Further, the texture of the as-synthesized catalysts can be tailored into fluffy flowers composed by ultrathin carbon petals. Similar results were observed by adjusting the molar ratio of Cd/Co. To properly address this effective and facile method, a schematic diagram is displayed in Scheme 1. When the molar ratios of Cd/Fe(Co) = n, the as-prepared bimetallic-PPD complex were denoted BMPPD-n. Surprising that a certain proportion of Cd2+ sites were replaced by Fe2+, interesting stripping phenomenon happened, from diamond-like Cd-PPD blocks transformed into very thick flakeshaped BMPPD-3, then further bloomed into BMPPD-2 flowers with ultrathin petals. When Fe sites dominate in the BMPPD-1 complex, the products restored to a large block of starfishshaped. After carbonization treatment, all of these products further evolved into a porous structure. Surprisingly, similar results were found in the synthesis experiment of Co assembled in N-doped carbon. Compared with the Zn-MOF, Cd can be carried away from CdFe-PDD at a certain temperature, and its microstructure can be tailored. While the morphology of ZIF-8 or MOF-74 does not exhibit any obvious changes by introducing different metal ions, the obtained materials still retain the original dodecahedral or large block shape.28-29,

34

In here, our optimized material Fe@NPC-2 obtained from the BMPPD-2

complex, which features uniform and well-dispersed 3-5 nm Fe nanoparticles embedded in two-dimensional (2D) ultrathin carbon nanosheets and delivers excellent ORR electrocatalytic performance. Contrary to the N-doped porous carbon (NPC) prepared from the Cd-PPD, Fe@NPC-2 offers enhanced performance with an improved onset potential, half-wave positive, and diffusion-limiting current, which is comparable to the commercially available Pt/C. Moreover, the methanol tolerance and durability of Fe@NPC-2 is more stable than those of Pt/C. The excellent electroactivity of our designed Fe@NPC-2 is benefited from the uniform and ultra-small Fe nanoparticles embedded into the 2D carbon nanosheets, which not 4 ACS Paragon Plus Environment

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only could provid more chance to form Fe-N-C, but increse the density of active Fe sites. This work provides a simple and effective approach to design and fabricate ultra-small sized transition metal embedded carbon complexes to replace Pt-based catalysts for boosting ORR.

Scheme 1. (A) Diagram illustrating metal vaporizing and escaping from volatile metal organic complexes in inert atmosphere at high temperatures; (B) The molecular structure of metal organic complexes with different Cd/Fe molar ratio; (C) the morphology evolution of bimetallic-PPD complex (BMPPD-n) with different Cd/Fe molar ratio; (D) After carbonization, BMPPD-n trasformed to corresponding Fe@NPC-n. 2. Experimental Section Synthesis

of

Cd-PPD.

Cd(NO3)2·4H2O

(277.6

mg,

1.0

mmol)

and

p-

phenylenediamine (973.3 mg, 9.0 mmol) were dissolved in 50 mL of methanol, respectively.

Then

the

Cd(NO3)2·4H2O

solution

5 ACS Paragon Plus Environment

was

poured

into

the

p-


phenylenediamine solution under stirring at room temperature. After mixing together, the resulting solution was setting for 6 h. The product was collected and dried at 80 ℃ overnight in a vacuum oven. Synthesis of BMPPD-n. Cd(NO3)2·4H2O (277.6 mg, 0.9 mmol) and FeCl2·4H2O (178.9 mg, 0.9 mmol) was dissolved in 50 mL of methanol. p-phenylenediamine (973.3 mg, 9.0 mmol) was also dissolved in 50 mL of methanol. Then the Cd(NO3)2·4H2O solution was poured into the p-phenylenediamine solution, the resulting solution was stirred at room temperature for 6 h. The product was collected and dried at 80 ℃ overnight in a vacuum oven. The product is denoted as BMPPD-1. According to same process, The BMPPD-2 and BMPPD-3 were also prepared just decreasing FeCl2·4H2O content to 99.4 and 65.6 mg, respectively. Synthesis of NPC and Fe@NPC-n. The powder of Cd-PPD and BMPPD-n was calcinated under flowing Ar gas at 800, 900 and 1000 ℃ for 2 h, respectively. Cool naturally to room temperature Synthesis of Co-BMPPD-n and Co@NPC-n. The Co-BMPPD-n were prepared by a similar procedure with Fe-BMPPD-n except that replace FeCl2·4H2O with CoCl2·6H2O. The Co-BMPPD-n was calcinated at 900 ℃ for 2 h under flowing Ar to prepare the representative samples and donated as Co@NPC-n. Electrochemical measurements. 1 mg of catalyst powder was mixed with 10 uL 5wt% Nafion solution and ultrasonically dispersing in 0.2 mL of ethanol aqueous solution (ethanol and water with 1:1 vol). Then, 10 μL above ink was loaded onto a pre-polished glass carbon rotating ring disk electrode (RRDE, D = 5.0 mm, catalyst loading = 0.2 mg cm-2 ). In our work, 20 wt% commercial Pt/C was used for comparison. All electrochemical were carried on Autolab bi-potential station (CHI Instruments Inc.). RRDE loaded with electrocatalyst was


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used as the working electrode, a Ag/AgCl (saturation KCl) as reference electrode, a platinum foil as counter electrode, the electrolyte was 0.1 M KOH. JK, kinetic-limiting current was calculated by Koutecky-Levich equation that reported in literature.22 n = 4IDisk / (IDisk + IRing/N) was used to calculate the electron transfer number based on the RRDE measurement. where IDisk and IRing are current of disk electrode and Pt ring electrode; N is the collection efficiency of (0.27). Zn-air battery test. Air cathode was prepared by dispersing the catalyst (1.0 mg cm-2) onto carbon fiber paper, and then dried at 80 oC for 4 h in an oven. And then the as-prepared cathode was assembled with a Zn foil anode. 6 M KOH containing 0.2 M Zn(CH3OO)2 was employed as electrolyte. All the battery performance were analyzed by CHI 760e workstation (CH Instrument Co.). 3. RESULTS AND DISCUSSION 3.1 Catalyst Optimization and Structural Characterizations. In this work, the advantages of employing Cd-PPD complex as a skeleton and template to design Fe nanoparticles embedded into carbon materials are based on several major aspects: (i) PPD with abundant amine groups has a strong tendency to coordinate with inorganic cations and can bind with metal ions to form metal-PPD coordination complexes via an -NH2 group, increasing the feasibility to form metal-Nx and nitrogen-doped carbon structures under high temperatures. (ii) PPD can not only coordinate with Cd2+, but also with Fe2+, Co2+, Ni2+, Cu2+, etc. This makes it possible to introduce other reactive species into the reaction process. (iii) During the calcination process, Cd with low melting point can be easily removed at high temperature, even at 400 oC (Figure S1), and the evaporation of Cd further favor the formation of porous structure that ensures efficient ion diffusion and proton transport during catalytic reactions. (iv) The texture of the bimetal-PPD complex can be tailored. Before calcination, the thermal characterization of Cd-PPD and BMPPD-n were investigated by thermal gravimetric analysis (Figure S2). With the increasing Fe content in 7 ACS Paragon Plus Environment


the complexes, the first weight loss stage of the samples gradually shifted from 280-300 ºC to 360-400 ºC. This may be due to the Fe-N coordination bond energy is stronger than that of Cd-N, which improves the thermostability of the relative materials. On the other hand, compared with Cd, Fe more easily interacts with C and is further reduced and fixed on carbon to encapsulate Fe nanoparticles in carbon. At the same time, in the first weight loss, the weight loss ratio of Cd-PPD and BMPPD-3 was larger than others, where the large amount of gas produced in the decomposition process caused serious destruction of the morphology of the materials. Subsequently, the original massive structure formed a porous carbon structure, while the smallest weight loss ratio was observed in BMPPD-2, and its morphology was well maintained after the decomposition process.

Figure 1. (A-C) FESEM images of the Fe@NPC-2; (D, E) TEM images of the Fe@NPC-2; (F) high-resolution TEM image of the Fe@NPC-2. The morphology and microstructure of the as-prepared samples were first observed by scanning and transmission electron microscopy (SEM and TEM). Cd-PPD exhibits a dense diamond-like block structure. As the Fe content in the samples increased, their morphology also gradually changed from adiamond-like structure to a thick flake-shaped structure, then 8 ACS Paragon Plus Environment

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further bloomed into a fluffy flower shape with ultrathin petals. When Fe sites dominate in the BMPPD-1 complex, the products restored to a large block of starfish shape (Figure S3). After further carbonization at high temperature, Cd-PPD decomposed and transfered to porous carbon materials (Figure S4), but its morphology is very different from its precursor due to a large amount of Cd evaporation, while with the increasing of iron content, the morphology of products remained well with their precursors gradually. This mainly due to the decreasing of Cd content, the destruction degree from Cd evaporation also decreased greatly under high temperature, and which is consistent with the TGA results. Figure S5 shows the FESEM images of BMPPD-2 and its uniformly distributed EDS mapping of all elements. Combining with the reports in literature,35-38 abundant nitrogen content is conducive to form Fe-Nx moieties and nitrogen-doped carbon, and the existence of Fe provides the possibility of forming Fe-based catalysts, which could enrich effective active sites. The morphology of Fe@NPC-2, shown in Figure 1A-C, was maintained perfectly even after carbonization, and each lamellar structure possesses abundant pores. Figure 1D shows the TEM images of Fe@NPC-2, clearly, the Fe nanoparticles are uniformly loaded on two-dimensional graphenelike carbon. Figure 1E display more detailed characteristics of Fe@NPC-2 and that about 3-5 nm Fe nanoparticles distributed in carbon layers. Figure 1F reveals that the Fe nanoparticles were wrapped by 4 layers of the graphene shell and the 0.202 nm lattice fringe corresponding to the (1 1 0) plane of Fe. Inspired by introducing iron into Cd-PPD leads to interesting morphology and structue changes of Cd-PPD, we also using cobalt to replace parts of Cd in Cd-PPD and the similar results were displayed in Figure S6. The 2D graphene-like structure can increase the specific surface area of the material and the small-sized nanoparticles can improve the catalytic activity. The above advantages will be favorable to O2 diffusion and electron conduction in ORR process.



Figure 2. (A) XRD patterns of NPC and Fe@NPC-2; (B) N2 sorption isotherms at 77 K of NPC and Fe@NPC-2; (C) Pore size distributions; XPS spectra of (D) C 1s, (E) N 1s, and (F) Fe 2p for Fe@NPC-2. The XRD pattern of BMPPD-n is well consistent with Cd-PPD (Figure S7), indicating that only some cadmium ions were replaced during the introduction of iron ions and without destroying the original coordination binding method. NPC and Fe@NPC-2 were prepared via pyrolysis of Cd-PPD and BMPPD-2, and the XRD pattern shows broad shoulder peak at ~25º, corresponding to (0 0 2) diffraction plane of graphite.39 Nothing else phase peak indexed to Cd-based was found in the XRD patterns of both samples. At the same time, there is no any Cd characteristic peak was observed in the XPS spectrum of the two materials (Figure S8). Above result suggesting that Cd was removed during the high-temperature annealing process. However, the Fe@NPC-2 following peaks match well with metal Fe (JCPDS 87-7194), as shown in Figure 2A and and Figure S9. In The Raman spectra, the D and G peaks (1332 cm-1, 1588 cm-1) of samples corresponding to defect and graphitic carbon, respectively.40 In Figure S10, the value of the peak intensity ratio (ID/IG) is 0.98 for NPC and 1.08 for Fe@NPC-2,


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suggesting that more defect carbon structure formed in Fe@NPC-2, which is important for the adsorotion of O2 for ORR. 41 The porous properties of the prepared materials were characterized by Brunauer-EmmettTeller (BET) measurement. Type-IV isotherm hysteresis was observed for our prepared two samples (Figure 2B), revealing the mesoporous characters of NPC and Fe@NPC-2. According to measurements, Fe@NPC-2 has a larger specific surface area of 1347.98 m2/g compared to 866.98 m2/g of NPC. The pore size distribution of the two samples shown in Figure 2C, which revealed that pore sizes for Fe@NPC-2 and NPC mainly centered at 0.6 and 4.0 nm, respectively. The difference of their pore structure mainly from the following reason. (1) the Cd-PPD is huge solid blocks, while the precursor of Fe@NPC-2 is slack flowers with ultrathin petals. (2) during carbonization process, more Cd evaporation from CdPPD destroyed the microstructure of its product greatly, while the introduction of Fe in BMPPD-2, not only decreased the Cd evaporation, but also enlarged the distance between the Cd atom, which prevents the agglomeration of Cd and causes more smaller pores during carbonization process. As we all know that a large specific surface area and abundant pores could facilitate the rapid diffusion of O2 and electrolyte to the active sites of electrocatalyst, further improve its ORR activity. 42 The C 1s XPS spectra of the Fe@NPC-2 and NPC are shown in Figure 2D and Figure S11A. Three subpeaks of sp2-hybridized at 284.8, 286.0, and 288.8 eV corresponding to C-C, C-N(C-O), and C=N.

43

The deconvoluted N 1s peak were into four sub-peaks, namely,

pyridinic-N (398.4 eV), pyrrolic-N (399.9 eV), graphitic-N (401.3 eV), and oxidized-N (402.9 eV), and for both NPC and Fe@NPC-2 (Figure S11B and Figure 2E).44 The pyridinic-N and graphitic-N are the governing species in the two materials and are from the decomposition of organic ligand under carbonization process. Meanwhile, pyridinic-N has potential to obtain Fe-Nx structure and graphitic-N produce the lowest barrier for the rate-limiting first electron 11 ACS Paragon Plus Environment


transfer as well as the high selectivity toward the four-electron reduction pathway, which acts an improtant role for ORR.45 The Fe 2p survey spectrum of Fe@NPC-2 shows Figure 2F, The peaks at 711.5, 713.3, 718.6, 723.4 and 726.9 eV corresponding to the Fe-N structure, 2p3/2 of Fe2+ and Fe3+, satellite peak, and 2p1/2 of Fe2+, respectively.41 As reported,46 Fe group possess a high catalytic activity, which can promote the adsorption of O2 for further reduction reaction. 3.2 ORR Activity and Stability. In our work, cyclic voltammetry (CV) tests were first used to assess the ORR performance of our synthesized catalysts. As shown in Figure 3A, in the N2-saturated electrolyte, no any Faradaic currents appeared for all samples. By contrast, obviously ORR peaks appear in O2-saturated KOH. It can be clearly observed that the Fe@NPC-2 electrode exhibited an ORR peak very close to 20% Pt/C, which is more positive than that of NPC. To further evaluate the ORR activity of each catalysts in more detail, linear sweep voltammetry (LSV) curves were performed by rotating disk electrode (RDE) (Figure 3B). Among the samples, Fe@NPC-2 shows an onset potential (Eonset) of 0.96 V and a halfwave potential (E1/2) of 0.84 V, which is superior to the ORR catalytic activity of the commercial Pt/C catalyst (Eonset: 0.95 V, E1/2: 0.84 V) and of NPC (Eonset: 0.85 V, E1/2: 0.76 V). The limited diffusion current density increased from 4.08 mA cm2 of the NPC to 5.43 mA cm2 of Fe@NPC-2, suggesting that the presence of Fe species play an essential role for ORR activity of catalysts.


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Figure 3. (A) CV curves of NPC, Fe@NPC-2, and Pt/C in N2 and O2 saturated 0.1 M KOH; (B) linear sweep voltammograms (LSV) curves of various catalysts and Pt/C at a rotation speed of 1600 rpm in O2 saturated 0.1 M KOH with a scan rate of 10 mV s-1; (C) LSV curves of Fe@NPC-2 at different rotating speeds; (D) Percentage of peroxide yield and electron transfer number of various catalysts and Pt/C; (E) Current-time (i-t) curves of Fe@NPC-2 and Pt/C at 0.7 V with an addition of 2 M methanol; (F) relative retention i-t curves at 0.7 V in 0.1 M KOH solution for Pt/C and Fe@NPC-2. As we all know, the calcining temperature and the content of active sites are crucial to the ORR activity of the catalytic. The degree of graphitization of the material at a too-low temperature will influence the electronic conduction, and a too-high temperature may cause serious agglomeration of the metal particles.47 In this work we also explored the effect of temperature on material properties. In Figure S12, the Fe@NPC-2 obtained at 900 oC exhibited better performance compared to those obtained at other temperatures. At the same time, the performance of the materials from the precursors with different molar ratio of Cd/Fe was also studied and displayed in Figure S13. It is clearly that when the molar ratio of Cd/Fe equel to 2, the as-prepared catalyst delivered the best ORR activity, which may due to that 13 ACS Paragon Plus Environment


when the ratio is too small, the Fe-relatived active sites are insufficient, and when the ratio is too large, the Fe particles are seriously agglomerated (Figure S14). To further study the active sites of the as-prepared materials, Fe@NPC-2 was immersed in 0.5 M H2SO4 to remove or diminish Fe content and the obtained material is denoted as Fe@NPC-2-acid. Generally believed that after acid leaching, Fe doped carbon catalysts will perform better performance due to larger size nanoparticles are removed by acid and the exposed active sites are more easily to contact with the reactive species.21 However, in this work the onset potential, limited diffusion current density and half-wave potential of Fe@NPC-2-acid were shifted negatively, and its limiting current also decreases obviously (Figure.S15). This result indicates that Fe nanoparticle is important active sites in Fe@NPC-2. Hence, the coexistence of Fe-N-C and Fe active sites in Fe@NPC-2 is contribute for its superior ORR performance. ORR kinetics were analysized by RDE tests at different rotating speeds (Figure 3C), and the corresponding Koutecky-Levich plots display excellent linearity under different potentials (Figure S16), which suggests a first-order reaction kinetics process occured in this condition. According to calculation, the electron transfer number (n) eaquel to ~4.0, indicating that Fe@NPC-2 composites favor a 4e- ORR process. However, the average n value for NPC is ~3.75, (Figure S17), which suggests that existence of a less efficient 2e- pathway. The ORR reaction process of different electrocatalysts was further studied by rotating ring-disk electrode (RRDE) test (Figure S18). The electron transfer number (n) and percentage of HO2yield calculated from RRDE curves are shown in Figure 3D, which were determined to be 3.7, 4.0, and 4.0 for NPC, Fe@NPC-2, and Pt/C, respectively.The yield of HO2- at the Fe@NPC-2 electrode with a low percentage is highly similar to 20% Pt/C, revealing a desired apparent 4eORR pathway of Fe@NPC-2. The methanol tolerance and long term stability of the Fe@NPC-2 were tested by chronoamperometry response. In Figure 3E, Fe@NPC-2 exhibited excellent resistance against methanol crossover. After the addition of 2 M methanol, it recovered rapidly after a slight change, while the Pt/C dropped sharply. In Figure 3F, after 14 ACS Paragon Plus Environment

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continuous 10 h operation, the durability of the Fe@NPC-2 catalyst still keep at 89%, however, the Pt/C decrease to 73%, which suggested that Fe@NPC-2 catalyst has a better stability performance. Such superior performance is comparable to similar electrocatalysts reported in the literature (Table S1). The above results indicate that our designed catalyst has a good application prospect under alkaline conditions.

Figure 4. (A) Schematic illustration of Fe@NPC-2 catalyst was loaded on the current collector as an air electrode applied to the zinc-air battery; (B) Open circuit voltage curves; (C) Polarization and power density plots; (D) Specific capacity of the primary zinc-air batteries; (E) Charge/discharge profiles of rechargeable zinc-air batteries using Fe@NPC-2 and Pt/C as the air electrodes (5 mA/cm2, 20 min/cycle). 3.3 Zinc-Air Battery Performance. A home-made Zn-air battery was assembled to further investigative the performance of Fe@NPC-2 under practical application. It was constructed by Fe@NPC-2 dispersed on carbon paper as the air cathode, zinc foil as the anode and 6 M KOH as the electrolyte. Figure 4A contains the scheme illustration of Fe@NPC-2 catalyst was loaded on the current collector as an air electrode applied to the zinc air battery. Figure 4B shows that a stable open circuit voltage of 1.46 V, high than the Pt/C-based battery (1.45 15 ACS Paragon Plus Environment


V). Figure 4C presents the polarization and power density curves. Which displays the peak power density of 99.3 mW cm2 assembled from Fe@NPC-2 catalyst, superior to the battery with Pt/C (86.1 mW cm-2). For primary zinc–air battery, the Fe@NPC-2 cathode at the current density of 10 mA cm-2 exhibit a specific capacity of 760 mAh g-1, demonstrating a high energy utilization of the theoretical capacity (~820 mAh g-1) and a comparable high specific capacity of 780 mAh g-1 for the Pt/C catalyst (Figure 4D). Furthermore, the electrocatalytic oxygen evolution performance of Fe@NPC-2 was tested and compared with that of commercial Pt/C (Figure S19). Galvanostatic charge/discharge cycling was conducted under ambient air conditions and the electrolyte replaced to 6 M KOH add to 0.2 M Zn(CH3OO)2, and a fixed current density of 5 mA cm-2 with each galvanostatic pulse 20 min for charge and discharge. Figure 4E shows 100 charge/discharge cycles, and the initial overpotential for Fe@NPC-2 was determined to be 1.1 V, and the roundtrip efficiency of 44.8%. After 100 cycles, the discharge-charge voltage gap increased to 1.15 V, corresponding to the roundtrip efficiency was decrease only 0.2% (Figure S20). However, the battery with Pt/C behave a relatively high overpotential and discharge-charge voltage losses after 83 cycles, which probably due to the poor stability performance of Pt/C and easier aggregation in long-term cycle period.

4. CONCLUSIONS In summary, this work reports a new synthesis method for preparation of highly active catalysts. In the synthetic process, a new metal organic complex (Cd-PPD) is initially designed as a template and carbon precursor. By adjusting the molar ratio of Cd/Fe(Co) and calcination temperature, both Fe and Co can be efficiently assembled very well on the surface of carbon during the Cd vapored away, and the particle size, dispersion, and microstructure of carbon can be tailored subtly. The optimized Fe@NPC-2 has a giant specific surface area (1347.98 m2/g), uniformly dispersed 3-5 nm Fe nanoparticles can be embedded in two16 ACS Paragon Plus Environment

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dimensional (2D) ultrathin carbon nanosheets, which make the catalyst exhibit superior electroactivity compared to commercial Pt/C due to its higher half-wave potential and onset potential, resistance against methanol crossover, and durability. Furthermore, the Fe@NPC-2 based Zn-air battery delivered a power density of 99.3 mW cm-2 and better rechargeability. This study provides valuable guidance for the direct preparation transition metal embedded carbon based ORR catalysts for future energy storage and conversion application. Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: Figures S1-S20 and Table S1 ACKONWLEDGEMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21773188), Natural Science Foundation of Chongqing (cstc2018jcyjAX0714), Chongqing Key Laboratory for Advanced Materials and Technologies and Chongqing Engineering Research Center for Micro-Nano Biomedical Materials and Devices. REFERENCES (1) Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R., Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116 (6), 3594-657. (2) Fu, J.; Hassan, F. M.; Zhong, C.; Lu, J.; Liu, H.; Yu, A.; Chen, Z., Defect Engineering of Chalcogen-Tailored Oxygen Electrocatalysts for Rechargeable Quasi-Solid-State Zinc-Air Batteries. Adv. Mater. 2017, 29 (35). (3) Xu, Y.; Chen, B.; Nie, J.; Ma, G., Reactive Template-Induced Core-Shell FeCo@C Microspheres as Multifunctional Electrocatalysts for Rechargeable Zinc-Air Batteries. Nanoscale 2018, 10 (36), 17021-17029. (4) Zhang, Z.; Luo, Z.; Chen, B.; Wei, C.; Zhao, J.; Chen, J.; Zhang, X.; Lai, Z.; Fan, Z.; Tan, C.; Zhao, M.; Lu, Q.; Li, B.; Zong, Y.; Yan, C.; Wang, G.; Xu, Z. J.; Zhang, H., One-Pot 17 ACS Paragon Plus Environment


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Mesoporous-Silica-Protected

Calcination

as

Efficient

Oxygen

Reduction

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ToC figure


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Novel CdFe bimetallic complex-derived ultra-small Fe and N codoped

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