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Highly efficient Fe-N-C oxygen reduction electrocatalyst engineered by sintering atmosphere
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Highly efficient Fe-N-C oxygen reduction electrocatalyst engineered by sintering atmosphere Qiulin Li, Heng Liu, Long-Cheng Zhang, Hao Chen, Hongjiu Zhu, Yuanke Wu, Maowen Xu, Shu-Juan Bao * Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, 400715, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Sintering atmosphere can engineer the microstructure and active sites of catalysts. � Hydrogen sintering can promote the formation of core-shell iron carbide. � Uniformed 150 nm precursor were pre pared by a simple one-pot stirring method. � Ammonia sintering can increase the content of Fe-N and pyrridinic-N of catalyst. � The half-wave potential of Fe-N-C-NH3 is more positive than that of Pt/C. A R T I C L E I N F O
A B S T R A C T
Keywords: Fe-N-C catalyst Oxygen reduction reaction Electrocatalyst Sintering atmosphere Porous carbon
Fe-N and pyridinic-N generally are considered the active sites of Fe-N-C catalyst. In this featured work, small uniform Fe/N-rich polymer spheres are selected to prepare the Fe-N-C catalyst and study how the sintering at mosphere (NH3, Ar, and H2) to engineer their products. The findings reveal that the sintering atmosphere greatly affected not only the composition, aggregate state, and morphology of Fe, but also the graphitization degree and pore structure of carbon. In addition, XPS results suggest that Fe-N-C-NH3 possesses the highest Fe-N and pyridinic-N content. Combining its abundant pores, high graphitization, and large specific surface area, the catalyst obtained in NH3 atmosphere delivers best ORR catalytic performance (E0 ¼ 0.97 V, E1/2 ¼ 0.85 V vs RHE), which is even comparable to commercial Pt/C (E0 ¼ 0.97 V, E1/2 ¼ 0.83V vs RHE). Moreover, the catalyst exhibits greater stability and methanol resistance than those of Pt/C.
1. Introduction Developing sustainable and efficient energy conversion technologies is an urgent need in response to the rapid increasing energy demand, serious environmental pollution, and energy shortage depletion [1–4]. Zinc-Air Batteries (ZABs) and Proton Exchange Membrane Fuel Cells
(PEMFCs) are considered to be very promising and highly efficient electrochemical energy storage and conversion devices because of their high power density, low cost, and safety [5–7]. However, an efficient electrocatalyst is needed to speed up the sluggish kinetics of cathodic oxygen reduction (ORR) of ZABs and PEMFCs [8,9]. Pt and Pt-based materials have long been regarded as the best active catalysts for the
* Corresponding author. E-mail address: [email protected] (S.-J. Bao). https://doi.org/10.1016/j.jpowsour.2019.227497 Received 21 September 2019; Received in revised form 9 November 2019; Accepted 21 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Qiulin Li, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227497
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ORR (E0 ¼ 0.97 V, E1/2 ¼ 0.83V vs RHE), but their high cost, low sta bility, and poor methanol resistance limits their large-scale commercial application, thus exploring highly efficient nonprecious metal-based ORR catalysts is extremely urgent [10–13]. So far, transition metals and their compounds are considered promising alternatives to noble metal ORR catalysts due to their good performance, low cost, better stability, and resistance to toxicity [14–16]. Among the various transition metal-based catalysts, Fe-N-C catalysts exhibit high catalytic efficiency and natural abundance and, thus, have attracted much attention [17]. Generally, Fe-N-C catalysts are prepared by the pyrolysis of different Fe-, C-, and N-containing pre cursors at high temperatures (700–1000 � C) in various inert atmospheric conditions [18–20]. Numerous works have confirmed that the sintering temperature significantly affects the microstructure and electrocatalytic performance of as-prepared Fe-N-C catalysts [21–23]. Although higher sintering temperatures are conducive to improve the graphitization degree and electronic conductivity of carbon, the Fe-N and pyridinic-N content is greatly decreased due to the expulsion of nitrogen-containing volatiles at high temperatures [24]. At the same time, the sintering atmosphere is an important factor affecting the structure and activity of a catalyst. To date, hardly any works have systematically studied the effects of different sintering atmospheres on catalysts. In this feature work, small uniform Fe/N-rich polymer spheres were prepared by simply stirring FeCl3 and 1,8-diaminonaphthalene in ethanol. The as-obtained ~150 nm spheres were selected as a faultless platform to construct the Fe-N-C catalyst and investigate the effects of different sintering atmospheres (NH3, Ar, and H2) on the microstructure, active sites, and catalytic performance of the as-prepared materials. Encouragingly, experimental observation revealed that the Fe in Fe-poly (1,8-diaminonaphthalene) transformed into iron nitride in NH3 and into iron carbide in both Ar and H2. Subsequently, oriented graphite layers were found covering the Fe-based particles very well in both Fe-N-C-NH3 and Fe-N-C-Ar materials, which suggests that amorphous carbon can be easily transformed into highly graphitic carbon in NH3 and Ar due to the catalysis of Fe [25]. More surprisingly, the carbon structure formed in H2 was far denser than that obtained in NH3 and Ar, and core-shelled hollow spheres were found well dispersed in the H2-sintered product. XPS results indicate that more Fe-N and pyridinic-N were formed during NH3 sintering, while most pyrrolic-N was destroyed in H2. As excepted, Fe-N-C-NH3 delivered excellent ORR catalytic activity as well as more stable and better methanol resistance in alkaline conditions compared to commercial Pt/C. Finally, a liquid zinc-air battery was assembled to investigate the practical application of our designed catalyst. Results show that the Zn-air battery assembled by Fe-N-C-NH3 as an air elec trode can maintain a stable charge and discharge efficiency for 25-h cycling, which further confirms Fe-N-C-NH3 as a promising electrocatalyst.
ethanol under stirring for 1 h. Then, 54 mg ferric trichloride hexahy drate (0.2 mmol) was dissolved in 10 mL anhydrous ethanol, followed by the drop-wise addition of the ferric trichloride solution to the above solution under stirring condition, and continuous stirring for another 24 h. The reaction mixture was centrifuged and washed with ethanol several times. The as-obtained sample dried at 30 � C under vacuum overnight and named as Fe-poly(1,8-diaminonaphthalene). 2.3. Synthesis of Fe-N-C catalysts The obtained Fe-poly(1, 8-diaminonaphthalene) was pyrolyzed in NH3 (Ar/NH3 ¼ 4/1), Ar, and H2 (Ar/H2 ¼ 9/1) atmosphere, respec tively. The specific calcination process was performed as follows: the product was heated to 400 � C at a rate of 5 � C min 1 and maintained for 1 h, then the temperature was increased to 900 � C at a rate of 1 � C min 1 and maintained for 2 h. After naturally cooled to room temperature, the obtained materials were denoted as Fe-N-C-NH3, Fe-N-C-Ar, and Fe-N-CH2, respectively. 2.4. Electrochemical measurements of ORR The electrochemical measurements referenced to our previous work [26], and performed on CHI 760 electrochemical workstation (CHI In struments Inc.) and Autolab potentiostat (PGSTAT302 N) system coupled with a Pinerotator (AFMS-LXF). 5 mg catalysts dispersed in a solution containing 50 μl Nafion, 475 μl deionized water and 475 μl anhydrous ethanol by sonicating for 30 min to form a homogeneous ink, which was used to modify polished glassy carbon electrode (GCE, Φ ¼ 5 mm) as working electrode with a loading of 0.38 mg cm 1. Ag/AgCl and Pt foil as reference and counter electrode, respectively. Cyclic voltam metry (CV) was performed on a rotating disk electrode (RDE) in O2- or N2- saturated 0.1 M KOH solution at 5.0 mV s 1. Linear sweep voltam metry (LSV) was performed in O2 saturated 0.1 M KOH solution with rotating rate 1600 rpm. Electrochemical stability of catalysts were investigated by chronoamperometric responses (i-t) at 0.75 V vs RHE. Methanol resistance test of Fe-N-C-NH3 and 20% Pt/C were evaluated by LSV in 0.1 M KOH containing 2 M methanol solution. All of the poten tials were calibrated to the reversible hydrogen electrode (RHE) ac cording to Nernst equation ERHE ¼ EAg=AgCl þ 0:197 þ 0:0592 pH. 3. Results and discussion 3.1. Physical characterization of as-prepared catalysts As shown in Fig. 1A, ferric ions are able to oxidize 1,8-diaminonaph thalene and initiate the polymerization process. During the polymeri zation process, the free amine groups can capture ferric ions to form strong complexes with ferric ions [27]. Because the large steric hin drance of the amino groups of 1,8-diaminonaphthalene limits its poly merization rate and polymerization degree [28], the size of Fe-poly(1, 8-diaminonaphthalene) can be well controlled on the nanoscale, around 150 nm (Fig. S1). Hence, Fe-poly(1,8-diaminonaphthalene) was selected an ideal precursor to construct the Fe-N-C catalyst for investi gating the effects of different sintering atmospheres (NH3, Ar, and H2) on the microstructure, active sites, and catalytic performance of the as-prepared materials. The crystal structure of the samples was first recorded by X-ray diffraction (XRD). As can be seen from Fig. 1B, the XRD pattern of the sample obtained by sintering in NH3 matched well with that of PDF # 75–2127, suggesting that Fe in Fe-poly(1,8-diaminonaphthalene) transformed into FeN0.0324 in NH3 atmosphere. Comparatively, Fe transformed into C0.12Fe1.88 (PDF # 44–1293) and Fe3C (PDF # 35–0772) in Ar and H2, respectively. A relatively wide diffraction peak around 25.5� can be clearly observed from the three XRD patterns, which correspond to the (002) facets of graphitic carbon. This confirms that poly(1,8-diaminonaphthalene) was carbonized to graphitic carbon.
2. Experimental section 2.1. Materials and chemicals 1,8-diaminonaphthalene, ethanol was purchased form Admas-beta Co. Ltd. Ferric trichloride hexahydrate (FeCl3⋅6H2O) was purchased prom Aladdin Co. Ltd, China. Pt/C (20-wt % on Vulcan XC-72R) were purchased from Hesen Co. Ltd, Shanghai, China. Nafion® solution (5 wt %) was purchased from Alfa Aesar Co. Ltd. The deionized water (DI, 18.25 MΩ cm 1) was obtained in a purification instrument (Millipore Elix® Advantage 10, MERCK MILLIPORE). All chemicals were analytical grade and use as received without any further purification. 2.2. Synthesis of Fe-poly(1, 8-diaminonaphthalene) To prepare Fe-poly(1,8-diaminonaphthalene), 316 mg 1,8-diamino naphthalene (2 mmol) was first dissolved in 140 mL anhydrous 2
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Fig. 1. (A) Schematic illustration of the preparation process and features of as-designed catalysts; (B) XRD patterns of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2; (C) Raman spectra of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2.
Fig. 2. Low-magnification TEM images of (A) Fe-N-C-NH3, (E) Fe-N-C-Ar and (I) Fe-N-C-H2. High-magnification TEM images of (B, D) Fe-N-C-NH3, (F, H) Fe-N-C-Ar and (J, L) Fe-N-C-H2. HRTEM images of (C) Fe-N-C-NH3, (G) Fe-N-C-Ar and (K) Fe-N-C-H2. 3
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The sharpest (002) peak in the XRD pattern of Fe-N-C-NH3 demonstrates the highest graphitic degree of Fe-N-C-NH3 among the three samples [29], which were further studied by the Raman spectra. As depicted in Fig. 1C, the peaks located at 1370 cm 1 and 1544 cm 1 are ascribed to the characteristic D and G bands of the carbon material. The intensities of the G-band (IG) and D-band (ID) are related to the vibration mode of graphitic and disorder carbon, respectively. The ID/IG ratio of Fe-N-C-NH3, Fe-N-C-Ar, and Fe-N-C-H2 is 0.94, 1.07, and 1.20, respec tively, and the low ID/IG ratio correlates to the high graphitic degree of carbon materials. This result suggests the Fe-N-C-NH3 has the highest graphitic degree among of these materials [30]. The microstructure and morphology of the samples were first observed by Field Emission Scanning Electron Microscope (Fe-SEM). As shown in Fig. S2, after sintering in NH3, Ar, and H2, respectively, the size and spherical morphology of the precursor were well maintained. Although all three materials retained similar spheres, Transmission Electron Microscope (TEM) (Fig. 2 A, E, I) results show that the NH3- and Ar- sintered samples display a slack and porous structure, while the H2sintered sample is highly compact and solid. At the same time, aggre gation of Fe-based particles occurred in Fe-N-C-NH3 and Fe-N-C-Ar, while uniform core-shelled hollow heres formed in Fe-N-C-H2. Multi layers carbon shell with an obvious fingerprint-like structure were observed around Fe-based particles in Fe-N-C-NH3 (Fig. 2B) and Fe-N-CAr (Fig. 2F), whereas disordered amorphous carbon was formed in Fe-NC-H2 (Fig. 2J). Clear 2.07 Å lattice fringes of Fe-N-C-NH3 match well with the (111) facet of FeN0.0324 (Fig. 2C), and the 2.01 Å lattice fringes of Fe-N-C-Ar correspond to the (110) crystal plane of Fe0.12C1.88 (Fig. 2G). A blurred interplanar spacing of 2.39 Å was observed in Fe-NC-H2, ascribing to the (121) planes of Fe3C (Fig. 2K). It is worth noting that during TEM observation, clear lattice fringes of Fe-based particles can be easily seen in Fe-N-C-NH3 and Fe-N-C-Ar but are much more difficult to capture in Fe-N-C-H2. This difference in the Fe-N-C-H2 may be due to the Fe-based particles that were tightly covered by solid car bon. Further observation of the carbon structure of the three samples based on TEM reveals that Fe-N-C-NH3 exhibits a slacker and more porous structure, thus a higher graphitization degree (Fig. 2B and D), while the carbon of Fe-N-C-H2 is more disordered and compact (Fig. 2J and L). In general, the presence of more slack and porous carbon could create abundant graphene edges, which could increase the active sites of ORR and lower the oxygen adsorption barrier on the surface of the graphene edges [31]. Hence, the structure of the Fe-N-C-NH3 catalyst can promote a highly efficient electrochemical reaction of O2. The N2 adsorption/desorption measurement was used to charac terize the specific surface area and pore size distribution of the asprepared materials. As shown in Fig. S3A, the isothermal adsorption curves of the three samples quickly increased as the relative pressure was increased at 0–0.1, suggesting the adsorption of microspores in the samples [32]. Based on IUPAC classification, no hysteresis loop was observed in the isothermal adsorption desorption curve of Fe-N-C-H2, revealing its typical microporous structure, while the pronounced hys teresis loops of Fe-N-C-NH3 and Fe-N-C-Ar indicate their mesoporous structure [33]. Similar results were observed from the pore size distri bution curves of these samples and agree well with TEM observation (Fig. S3B). The BET specific surface areas of Fe-N-C-H2, Fe-N-C-Ar, and Fe-N-C-NH3 were determined to be 287.4 m2 g 1, 386.5 m2 g 1 and 628.95 m2 g 1, respectively. Subsequently, we investigated the elec trochemical surface area (ECSA) of as-prepared catalysts using 0.2 M [Fe (CN)6]3-/4- as the redox probe and 0.1 mol KCl as the supporting elec trolyte. From the CV curves (Fig. S4), the specific capacitances of the Fe-N-C-H2, Fe-N-C-Ar, and Fe-N-C-NH3 were calculated as 226.0 F g 1, 277.2 F g 1, and 496.2 F g 1, respectively, which are consistent with the BET results and TEM observations. This ECSA order of the as-prepared catalysts was also confirmed by the double layer capacitance (Cdl) of three materials displayed in Fig. S5 [34]. Based on the above results, it is suggested that different sintering atmospheres have a great influence on the microstructure of as-prepared
Fe-N-C materials, which can be explained by the following reasons. (1) In the Ar and NH3 sintering process, iron will catalyze the graphitization of carbon to form highly graphitized carbon layers. In comparison, a more solid carbon structure forms in H2, since hydrogen can effectively reduce the decomposition rate of hydrocarbons [35]. In addition, hydrogen can effectively fill the dangling bonds of surface carbon atoms, which maintains their sp3 configuration instead of graphitic sp2 or sp structure [36]; thus, no obvious fingerprint-like lattice fringes can be observed in Fe-N-C-H2. (2) Solid carbon formed by H2-sintering plays a key role in assembling core-shell iron carbide. In high temperatures, iron reacts with carbon to produce iron carbide, subsequently gathering together via ripening agglomeration to form a nucleus. As the reaction continues, the agglomeration of iron carbide is confined by dense carbon and has difficulty passing through solid carbon, eventually forming a core shell structure. (3) Argon only provides an inert atmosphere, while ammonia is a corrosive gas. Therefore, carbon will further be etched and produce more pores during NH3-sintering than in argon atmosphere. X-ray photoelectron spectroscopy (XPS) is a great technique for sensitively detecting the elemental composition and chemical state of the as-prepared materials’ surfaces. The XPS survey spectra indicate the presence of C, N, O, and Fe elements in the as-prepared catalysts (Fig. S6A). Fig. S6B shows the high-resolution spectra of C1s of the three samples. The peak at 383.6 eV is usually considered correspond to Fe-C bond, and the very weak Fe-C peak may due to the low content of Fe in the as-prepared materials [37,38]. Fig. 3A displays the high-resolution spectra of Fe2p of the three samples. The peaks at 721.0 eV, 723.2 eV, and 725.4 eV correspond to the 2p1/2 peaks of Fe0, Fe2þ, and Fe3þ, and those at 706.7 eV, 711.1 eV, and 714.4 eV correspond to the 2p3/2 peaks of Fe0, Fe2þ, and Fe3þ, respectively [39]. A satellite peak at 718.8 eV further indicates the co-existence of Fe2þ and Fe3þ in the three catalysts. The peak at 711.1 eV can be correspond to the coordination of Fe-N species, which are considered to be the ORR active sites [40]. It is worth noting that the Fe0 content close to zero in Fe-N-C-Ar is far lower than that of the catalysts obtained in H2 and NH3. This difference may be due to the reduction power of H2 and NH3, whereas Ar only provides an inert atmosphere. In Fig. 3B, the N1s spectra for three samples can be deconvoluted into several distinct characteristic peaks at 398.3 eV, 399.3 eV, 400.1 eV, 400.8 eV, and 403.5 eV, corresponding to pyridinic-N, Fe-Nx, pyrrolic-N, graphitic- N, and oxidized-N, respec tively [41]. The peak at 399.3 eV indicates the Fe-Nx moiety in our sample, which is similar to that in FePc [42]. The contents of different N1s in the three samples were calculated and are displayed in Fig. 3C. It is clear that the highest pyridinic-N and Fe-NX content were formed in the NH3-sintered sample, while few pyrrolic-N was observed in Fe-N-C-H2. This confirms that ammonia-sintering can help form pyridinic-N by nitrogen doping and more Fe-NX through reaction with NH3. However, in the H2-sintering process, pyrrolic-N was more easily destroyed. As calculated from thermogravimetric analysis (TGA) (Fig. S7), the iron content of Fe-N-C-H2, Fe-N-C-Ar, and Fe-N-C-NH3 is 5.6%, 8.4%, and 17.3%, respectively, which results from the different pyrolysis degrees of their precursors in different atmospheres. 3.2. Electrochemical characterization of as-prepared catalysts Cyclic voltammetry (CV) measurements in N2- and O2-saturated 0.1 M KOH solutions were first performed to evaluate the ORR catalytic activity of the samples. As displayed in Fig. 4A, no ORR characteristic peaks were observed under N2-saturated solution for the three catalysts, while obvious ORR characteristic peaks appeared in O2-satureated so lutions, which suggests that the as-prepared catalysts have good ORR catalytic activity. The linear sweep voltammetry (LSV) curves at 1600 rpm for all catalysts are depicted in Fig. 4B. Fe-N-C-NH3 exhibits a higher one-set potential (E0 ¼ 0.97 V vs RHE) and half-wave potential (E1/2 ¼ 0.85 V vs RHE) than both Fe-N-C-Ar (E0 ¼ 0.92 V, E1/2 ¼ 0.8 V vs RHE) and Fe-N-C-H2 (E0 ¼ 0.89 V, E1/2 ¼ 0.75 V vs RHE), indicating that Fe-N-C-NH3 possesses the best ORR catalytic performance. Moreover, 4
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Fig. 3. (A) Fe2p XPS spectra and (B) N1s XPS spectra of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2; (C) the percentage of pyridinic-N, graphitic-N, pyrrolic-N, oxidized-N and Fe-N of as-prepared samples; (D) structural features of pyridinic-N, graphitic-N, pyrrolic-N and Fe-N.
the number of electron transfer (n) and yield of H2O2 (H2O2%) are sig nificant indicators for evaluating the ORR kinetics of catalysts. RRDE voltammograms of the three catalysts at 1600 rpm are shown in Fig. 4C, where solid and dashed lines indicate disk current (ID) and ring current (IR), respectively. The lower IR means the less intermediate product in the ORR process, which demonstrates a higher n and lower H2O2%. The n and H2O2% calculated from ID and IR are displayed in Fig. 4D. The n values of Fe-N-C-NH3, Fe-N-C-Ar, and Fe-N-C-H2 involved in the ORR process were determined to be 3.99, 3.97, and 3.9, and the H2O2% values were 6.3%, 7.5% and 12.5%, respectively. It is clear that the n of all catalysts is close to the theoretical value of 4.00 for Pt/C, indicating a complete 4e ORR pathway. Further, the NH3-sintered catalyst deliv ered better ORR kinetics than the catalysts obtained in Ar and H2. Similar results were also obtained from their LSV curves at different rotating speeds (Fig. S8 A to C) and the Koutecky Levich (K L) plots (Fig. S8 D to F) indicate first-order reaction kinetics toward the con centration of dissolved oxygen [43]. Diffusive resistance is a key indi cator for evaluating high efficiency ORR catalysts. The electrochemical impedance spectroscopy (EIS) of different catalysts is displayed in Fig. S9. The straight line at lower frequencies is ascribed to the diffusive resistance of electrolyte/reaction raw in electrode materials. The straight line of Fe-N-C-NH3 is straighter than that of Fe-N-C-Ar and Fe-N-C-H2, which indicates that the Fe-N-C-NH3 has much lower diffu sion resistance. Subsequently, we found that ORR catalytic performance of the Fe-NC-NH3 catalyst is comparable to a commercial 20% Pt/C catalyst (Fig. 4E) and is better than most previously reported Fe-N-C materials judged from E0 and E1/2 (Fig. S10). Besides the ORR activity, the antimethanol toxicity and durability of Fe-N-C-NH3 were investigated. Generally, when using Pt/C as an electrocatalyst to test ORR electro catalytic performance, a representative methanol oxidation peak always appears after adding methanol to the electrolyte [44] (Fig. S11A). In contrast, no response was obtained toward methanol when Fe-N-C-NH3
was used as the ORR electrocatalyst (Fig. S11B), which is contributable to the better anti-methanol toxicity of Fe-N-C-NH3 than 20% Pt/C. After 10 h i-t test at 0.75 V vs RHE, the current density of the Fe-N-C-NH3- based electrode retained up to 94% of its initial current density, while the current density of commercial Pt/C only retained up to 65% under same experimental conditions (Fig. 4F). This further demonstrates that Fe-N-C-NH3 has better durability than 20% Pt/C. The above advantages of Fe-N-C-NH3 can be ascribed to the following reasons: (1) Fe-N-C-NH3 contains high contents of Fe-N and pyridinic-N; (2) the slack and porous structure creates abundant nitrogen-doped graphene edges, which could lower the oxygen adsorption barrier on the catalyst’s surface; (3) the highly graphitized carbon shell can reinforce the corrosion resistance of the catalyst. 3.3. The practical application possibility of Fe-N-C-NH3 based ZAB A homemade liquid zinc-air battery (Fig. S12A) was further assem bled to explore the feasibility of practical application of our designed catalyst. An ultrahigh open circuit potential of 1.51 V was observed for Fe-N-C-NH3, which is higher than 20% Pt/C (1.44 V) (Fig. S12B). As displayed in Fig. S12C, a higher peak power density (79.2 mW cm 2) of Fe-N-C-NH3, close to that of Pt/C (84.6 mW cm 2), was obtained with an increase in current density and decrease in voltage. Further, no obvious decrease was observed from the charge-discharge curves of our assem bled zinc-air battery after 25-h of cycling (Fig. S12D), suggesting its good stability. 4. Conclusion In summary, small uniform Fe/N-rich polymer spheres were fabri cated using a simple stirring method at room temperature. The Fe-N material was subsequently employed as a faultless precursor to construct Fe-N-C catalysts and study the effects of the sintering 5
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Michel Lef� evre, Fr�ed� eric jaouen, jean-pol dodelet, iron-based catalysts with improved oxygen reduction activity in polymer electrolyte Fuel Cells, Science 234 (2009) 71–74. [18] Z.-Y. Chen, Y.-N. Li, L.-L. Lei, S.-J. Bao, M.-Q. Wang, H.-L. Heng-Liu, Z.-L. Zhao, M.w. Xu, Investigation of Fe2N@carbon encapsulated in N-doped graphene-like carbon as a catalyst in sustainable zinc-air Batteries, Catal. Sci. Technol. 7 (2017) 5670–5676. [19] Y. Zhang, L. Qian, W. Zhao, X. Li, X. Huang, X. Mai, Z. Wang, Q. Shao, X. Yan, Z. Guo, Highly efficient Fe-N-C nanoparticles modified porous graphene composites for oxygen reduction reaction, J. Electrochem. Soc. 165 (2018) H510–H516. [20] S. Stariha, K. Artyushkova, M.J. Workman, A. Serov, S. McKinney, B. Halevi, P. Atanassov, PGM-free Fe-N-C catalysts for oxygen reduction reaction: catalyst layer design, J. Power Sources 326 (2016) 43–49. [21] H. Peng, Z. Mo, S. Liao, H. Liang, L. Yang, F. Luo, H. Song, Y. Zhong, B. 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Fig. 4. Electrochemical properties of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2 in 0.1 M KOH electrolyte: (A) CV curves; (B) Linear sweep voltammetry (LSV) curves; (C) RRDE voltammograms; (D) Electron transfer number and H2O2 yield; (E) LSV curves of Fe-N-C-NH3 and 20% Pt/C; (F) Chronoamperometric responses of Fe-N-C-NH3 and 20% Pt/C catalyst.
atmosphere (NH3, Ar, and H2) on the catalysts’ microstructure and ac tivity sites. It was found that hydrogen-sintering leads to a dense carbon structure with an evenly distributed core-shell structure of iron carbide, while ammonia- and argon-sintering result in a porous carbon structure and aggregated Fe-based nanoparticles. It is important to note that NH3sintering will produce a loose and porous carbon structure, creating more Fe-N and pyridinic-N active sites. As an ORR catalyst, Fe-N-C-NH3 delivered a more positive onset potential (0.97 V vs RHE), half-wave potential (0.85 V vs RHE), as well as better stability and methanol resistance compared to 20% Pt/C. These results indicate the effect of a calcination atmosphere on engineering the microstructure and ORR performance of Fe-N-C catalysts. In short, our work introduces an effi cient ORR Fe-N-C-NH3 electrocatalyst, for which the preparation method can be extended to other Metal-N-C materials with special function and application. Declaration of competing interest The authors declare no known competing financial interests or per sonal relationships that could have appeared to influence the work re ported in this paper. Acknowledgements We appreciate support from the National Natural Science Foundation of China (21972111, 21773188), Fundamental Research Funds for the Central Universities (XDJK2019AA002, XDJK2019B052), Natural Sci ence Foundation of Chongqing (cstc2018jcyjAX0714), Chongqing En gineering Research Center for Micro-Nano Biomedical Materials and Devices, Chongqing Key Laboratory for Advanced Materials and Tech nologies. We also thank Dr. Ding-Yu Liu for help with XPS test and 6
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[34] F. Meng, H. Zhong, D. Bao, J. Yan, X. Zhang, In situ coupling of strung Co4N and intertwined N-C fibers toward free-standing bifunctional cathode for robust, efficient, and flexible Zn-air Batteries, J. Am. Chem. Soc. 138 (2016) 10226–10231. [35] M. Frenklach, The role of hydrogen in vapor deposition of diamond, J. Appl. Phys. 65 (1989) 5142–5149. [36] M. Frenklach, K.E. Spear, Growth mechanism of vapor-deposited diamond, J. Mater. Res. 3 (2011) 133–140. [37] S. Meguro, T. Sasaki, H. Katagiri, H. Habazaki, A. Kawashima, T. Sakaki, K. Asami, K. Hashimoto, Electrodeposited Ni-Fe-C cathodes for hydrogen evolution, J. Electrochem. Soc. 147 (2000) 3003–3009. [38] H.-x. Zhong, Q. Zhang, J. Wang, X.-b. Zhang, X.-l. Wei, Z.-j. Wu, K. Li, F.-l. Meng, D. Bao, J.-m. Yan, Engineering ultrathin C3N4 quantum dots on graphene as a metal-free water reduction electrocatalyst, ACS Catal. 8 (2018) 3965–3970. [39] T.-N. Tran, C.-H. Shin, B.-J. Lee, J.S. Samdani, J.-D. Park, T.-H. Kang, J.-S. Yu, FeN-Functionalized carbon electrocatalyst derived from a zeolitic imidazolate framework for oxygen reduction: Fe and NH3 treatment effects, Catal. Sci. Technol. 8 (2018) 5368–5381. [40] X. Liu, H. Liu, C. Chen, L. Zou, Y. Li, Q. Zhang, B. Yang, Z. Zou, H. Yang, Fe2N nanoparticles boosting fenx moieties for highly efficient oxygen reduction reaction in Fe-N-C porous catalyst, Nano Res 12 (2019) 1651–1657. [41] J. Li, S. Ghoshal, W. Liang, M.-T. Sougrati, F. Jaouen, B. Halevi, S. McKinney, G. McCool, C. Ma, X. Yuan, Z.-F. Ma, S. Mukerjee, Q. Jia, Structural and mechanistic basis for the high activity of Fe-N-C catalysts toward oxygen reduction, Energy Environ. Sci. 9 (2016) 2418–2432. [42] T. Li, H. Deng, J. Liu, C. Jin, Y. Song, F. Wang, First-row transition metals and nitrogen Co-doped carbon nanotubes: the exact origin of the enhanced activity for oxygen reduction reaction, Carbon 143 (2019) 859–868. [43] Z.-y. Chen, H. Liu, L.-c. Zhang, Q.-l. Li, M.-w. Xu, S.-J. Bao, Facile and scale synthesis of Co/N/S-doped porous graphene-like carbon architectures as electrocatalysts for sustainable zinc-air battery Cells, ACS Sustain. Chem. Eng. 7 (2019) 7743–7749. [44] H. Xu, A.-L. Wang, Y.-X. Tong, G.-R. Li, Enhanced catalytic activity and stability of Pt/CeO2/PANI hybrid hollow nanorod arrays for methanol electro-oxidation, ACS Catal. 6 (2016) 5198–5206.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Highly efficient Fe-N-C oxygen reduction electrocatalyst engineered by sintering atmosphere Qiulin Li, Heng Liu, Long-Cheng Zhang, Hao Chen, Hongjiu Zhu, Yuanke Wu, Maowen Xu, Shu-Juan Bao * Key Laboratory of Luminescent and Real-Time Analytical Chemistry, Ministry of Education, Institute for Clean Energy and Advanced Materials, Faculty of Materials and Energy, Southwest University, Chongqing, 400715, PR China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Sintering atmosphere can engineer the microstructure and active sites of catalysts. � Hydrogen sintering can promote the formation of core-shell iron carbide. � Uniformed 150 nm precursor were pre pared by a simple one-pot stirring method. � Ammonia sintering can increase the content of Fe-N and pyrridinic-N of catalyst. � The half-wave potential of Fe-N-C-NH3 is more positive than that of Pt/C. A R T I C L E I N F O
A B S T R A C T
Keywords: Fe-N-C catalyst Oxygen reduction reaction Electrocatalyst Sintering atmosphere Porous carbon
Fe-N and pyridinic-N generally are considered the active sites of Fe-N-C catalyst. In this featured work, small uniform Fe/N-rich polymer spheres are selected to prepare the Fe-N-C catalyst and study how the sintering at mosphere (NH3, Ar, and H2) to engineer their products. The findings reveal that the sintering atmosphere greatly affected not only the composition, aggregate state, and morphology of Fe, but also the graphitization degree and pore structure of carbon. In addition, XPS results suggest that Fe-N-C-NH3 possesses the highest Fe-N and pyridinic-N content. Combining its abundant pores, high graphitization, and large specific surface area, the catalyst obtained in NH3 atmosphere delivers best ORR catalytic performance (E0 ¼ 0.97 V, E1/2 ¼ 0.85 V vs RHE), which is even comparable to commercial Pt/C (E0 ¼ 0.97 V, E1/2 ¼ 0.83V vs RHE). Moreover, the catalyst exhibits greater stability and methanol resistance than those of Pt/C.
1. Introduction Developing sustainable and efficient energy conversion technologies is an urgent need in response to the rapid increasing energy demand, serious environmental pollution, and energy shortage depletion [1–4]. Zinc-Air Batteries (ZABs) and Proton Exchange Membrane Fuel Cells
(PEMFCs) are considered to be very promising and highly efficient electrochemical energy storage and conversion devices because of their high power density, low cost, and safety [5–7]. However, an efficient electrocatalyst is needed to speed up the sluggish kinetics of cathodic oxygen reduction (ORR) of ZABs and PEMFCs [8,9]. Pt and Pt-based materials have long been regarded as the best active catalysts for the
* Corresponding author. E-mail address: [email protected] (S.-J. Bao). https://doi.org/10.1016/j.jpowsour.2019.227497 Received 21 September 2019; Received in revised form 9 November 2019; Accepted 21 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Qiulin Li, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227497
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ORR (E0 ¼ 0.97 V, E1/2 ¼ 0.83V vs RHE), but their high cost, low sta bility, and poor methanol resistance limits their large-scale commercial application, thus exploring highly efficient nonprecious metal-based ORR catalysts is extremely urgent [10–13]. So far, transition metals and their compounds are considered promising alternatives to noble metal ORR catalysts due to their good performance, low cost, better stability, and resistance to toxicity [14–16]. Among the various transition metal-based catalysts, Fe-N-C catalysts exhibit high catalytic efficiency and natural abundance and, thus, have attracted much attention [17]. Generally, Fe-N-C catalysts are prepared by the pyrolysis of different Fe-, C-, and N-containing pre cursors at high temperatures (700–1000 � C) in various inert atmospheric conditions [18–20]. Numerous works have confirmed that the sintering temperature significantly affects the microstructure and electrocatalytic performance of as-prepared Fe-N-C catalysts [21–23]. Although higher sintering temperatures are conducive to improve the graphitization degree and electronic conductivity of carbon, the Fe-N and pyridinic-N content is greatly decreased due to the expulsion of nitrogen-containing volatiles at high temperatures [24]. At the same time, the sintering atmosphere is an important factor affecting the structure and activity of a catalyst. To date, hardly any works have systematically studied the effects of different sintering atmospheres on catalysts. In this feature work, small uniform Fe/N-rich polymer spheres were prepared by simply stirring FeCl3 and 1,8-diaminonaphthalene in ethanol. The as-obtained ~150 nm spheres were selected as a faultless platform to construct the Fe-N-C catalyst and investigate the effects of different sintering atmospheres (NH3, Ar, and H2) on the microstructure, active sites, and catalytic performance of the as-prepared materials. Encouragingly, experimental observation revealed that the Fe in Fe-poly (1,8-diaminonaphthalene) transformed into iron nitride in NH3 and into iron carbide in both Ar and H2. Subsequently, oriented graphite layers were found covering the Fe-based particles very well in both Fe-N-C-NH3 and Fe-N-C-Ar materials, which suggests that amorphous carbon can be easily transformed into highly graphitic carbon in NH3 and Ar due to the catalysis of Fe [25]. More surprisingly, the carbon structure formed in H2 was far denser than that obtained in NH3 and Ar, and core-shelled hollow spheres were found well dispersed in the H2-sintered product. XPS results indicate that more Fe-N and pyridinic-N were formed during NH3 sintering, while most pyrrolic-N was destroyed in H2. As excepted, Fe-N-C-NH3 delivered excellent ORR catalytic activity as well as more stable and better methanol resistance in alkaline conditions compared to commercial Pt/C. Finally, a liquid zinc-air battery was assembled to investigate the practical application of our designed catalyst. Results show that the Zn-air battery assembled by Fe-N-C-NH3 as an air elec trode can maintain a stable charge and discharge efficiency for 25-h cycling, which further confirms Fe-N-C-NH3 as a promising electrocatalyst.
ethanol under stirring for 1 h. Then, 54 mg ferric trichloride hexahy drate (0.2 mmol) was dissolved in 10 mL anhydrous ethanol, followed by the drop-wise addition of the ferric trichloride solution to the above solution under stirring condition, and continuous stirring for another 24 h. The reaction mixture was centrifuged and washed with ethanol several times. The as-obtained sample dried at 30 � C under vacuum overnight and named as Fe-poly(1,8-diaminonaphthalene). 2.3. Synthesis of Fe-N-C catalysts The obtained Fe-poly(1, 8-diaminonaphthalene) was pyrolyzed in NH3 (Ar/NH3 ¼ 4/1), Ar, and H2 (Ar/H2 ¼ 9/1) atmosphere, respec tively. The specific calcination process was performed as follows: the product was heated to 400 � C at a rate of 5 � C min 1 and maintained for 1 h, then the temperature was increased to 900 � C at a rate of 1 � C min 1 and maintained for 2 h. After naturally cooled to room temperature, the obtained materials were denoted as Fe-N-C-NH3, Fe-N-C-Ar, and Fe-N-CH2, respectively. 2.4. Electrochemical measurements of ORR The electrochemical measurements referenced to our previous work [26], and performed on CHI 760 electrochemical workstation (CHI In struments Inc.) and Autolab potentiostat (PGSTAT302 N) system coupled with a Pinerotator (AFMS-LXF). 5 mg catalysts dispersed in a solution containing 50 μl Nafion, 475 μl deionized water and 475 μl anhydrous ethanol by sonicating for 30 min to form a homogeneous ink, which was used to modify polished glassy carbon electrode (GCE, Φ ¼ 5 mm) as working electrode with a loading of 0.38 mg cm 1. Ag/AgCl and Pt foil as reference and counter electrode, respectively. Cyclic voltam metry (CV) was performed on a rotating disk electrode (RDE) in O2- or N2- saturated 0.1 M KOH solution at 5.0 mV s 1. Linear sweep voltam metry (LSV) was performed in O2 saturated 0.1 M KOH solution with rotating rate 1600 rpm. Electrochemical stability of catalysts were investigated by chronoamperometric responses (i-t) at 0.75 V vs RHE. Methanol resistance test of Fe-N-C-NH3 and 20% Pt/C were evaluated by LSV in 0.1 M KOH containing 2 M methanol solution. All of the poten tials were calibrated to the reversible hydrogen electrode (RHE) ac cording to Nernst equation ERHE ¼ EAg=AgCl þ 0:197 þ 0:0592 pH. 3. Results and discussion 3.1. Physical characterization of as-prepared catalysts As shown in Fig. 1A, ferric ions are able to oxidize 1,8-diaminonaph thalene and initiate the polymerization process. During the polymeri zation process, the free amine groups can capture ferric ions to form strong complexes with ferric ions [27]. Because the large steric hin drance of the amino groups of 1,8-diaminonaphthalene limits its poly merization rate and polymerization degree [28], the size of Fe-poly(1, 8-diaminonaphthalene) can be well controlled on the nanoscale, around 150 nm (Fig. S1). Hence, Fe-poly(1,8-diaminonaphthalene) was selected an ideal precursor to construct the Fe-N-C catalyst for investi gating the effects of different sintering atmospheres (NH3, Ar, and H2) on the microstructure, active sites, and catalytic performance of the as-prepared materials. The crystal structure of the samples was first recorded by X-ray diffraction (XRD). As can be seen from Fig. 1B, the XRD pattern of the sample obtained by sintering in NH3 matched well with that of PDF # 75–2127, suggesting that Fe in Fe-poly(1,8-diaminonaphthalene) transformed into FeN0.0324 in NH3 atmosphere. Comparatively, Fe transformed into C0.12Fe1.88 (PDF # 44–1293) and Fe3C (PDF # 35–0772) in Ar and H2, respectively. A relatively wide diffraction peak around 25.5� can be clearly observed from the three XRD patterns, which correspond to the (002) facets of graphitic carbon. This confirms that poly(1,8-diaminonaphthalene) was carbonized to graphitic carbon.
2. Experimental section 2.1. Materials and chemicals 1,8-diaminonaphthalene, ethanol was purchased form Admas-beta Co. Ltd. Ferric trichloride hexahydrate (FeCl3⋅6H2O) was purchased prom Aladdin Co. Ltd, China. Pt/C (20-wt % on Vulcan XC-72R) were purchased from Hesen Co. Ltd, Shanghai, China. Nafion® solution (5 wt %) was purchased from Alfa Aesar Co. Ltd. The deionized water (DI, 18.25 MΩ cm 1) was obtained in a purification instrument (Millipore Elix® Advantage 10, MERCK MILLIPORE). All chemicals were analytical grade and use as received without any further purification. 2.2. Synthesis of Fe-poly(1, 8-diaminonaphthalene) To prepare Fe-poly(1,8-diaminonaphthalene), 316 mg 1,8-diamino naphthalene (2 mmol) was first dissolved in 140 mL anhydrous 2
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Fig. 1. (A) Schematic illustration of the preparation process and features of as-designed catalysts; (B) XRD patterns of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2; (C) Raman spectra of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2.
Fig. 2. Low-magnification TEM images of (A) Fe-N-C-NH3, (E) Fe-N-C-Ar and (I) Fe-N-C-H2. High-magnification TEM images of (B, D) Fe-N-C-NH3, (F, H) Fe-N-C-Ar and (J, L) Fe-N-C-H2. HRTEM images of (C) Fe-N-C-NH3, (G) Fe-N-C-Ar and (K) Fe-N-C-H2. 3
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The sharpest (002) peak in the XRD pattern of Fe-N-C-NH3 demonstrates the highest graphitic degree of Fe-N-C-NH3 among the three samples [29], which were further studied by the Raman spectra. As depicted in Fig. 1C, the peaks located at 1370 cm 1 and 1544 cm 1 are ascribed to the characteristic D and G bands of the carbon material. The intensities of the G-band (IG) and D-band (ID) are related to the vibration mode of graphitic and disorder carbon, respectively. The ID/IG ratio of Fe-N-C-NH3, Fe-N-C-Ar, and Fe-N-C-H2 is 0.94, 1.07, and 1.20, respec tively, and the low ID/IG ratio correlates to the high graphitic degree of carbon materials. This result suggests the Fe-N-C-NH3 has the highest graphitic degree among of these materials [30]. The microstructure and morphology of the samples were first observed by Field Emission Scanning Electron Microscope (Fe-SEM). As shown in Fig. S2, after sintering in NH3, Ar, and H2, respectively, the size and spherical morphology of the precursor were well maintained. Although all three materials retained similar spheres, Transmission Electron Microscope (TEM) (Fig. 2 A, E, I) results show that the NH3- and Ar- sintered samples display a slack and porous structure, while the H2sintered sample is highly compact and solid. At the same time, aggre gation of Fe-based particles occurred in Fe-N-C-NH3 and Fe-N-C-Ar, while uniform core-shelled hollow heres formed in Fe-N-C-H2. Multi layers carbon shell with an obvious fingerprint-like structure were observed around Fe-based particles in Fe-N-C-NH3 (Fig. 2B) and Fe-N-CAr (Fig. 2F), whereas disordered amorphous carbon was formed in Fe-NC-H2 (Fig. 2J). Clear 2.07 Å lattice fringes of Fe-N-C-NH3 match well with the (111) facet of FeN0.0324 (Fig. 2C), and the 2.01 Å lattice fringes of Fe-N-C-Ar correspond to the (110) crystal plane of Fe0.12C1.88 (Fig. 2G). A blurred interplanar spacing of 2.39 Å was observed in Fe-NC-H2, ascribing to the (121) planes of Fe3C (Fig. 2K). It is worth noting that during TEM observation, clear lattice fringes of Fe-based particles can be easily seen in Fe-N-C-NH3 and Fe-N-C-Ar but are much more difficult to capture in Fe-N-C-H2. This difference in the Fe-N-C-H2 may be due to the Fe-based particles that were tightly covered by solid car bon. Further observation of the carbon structure of the three samples based on TEM reveals that Fe-N-C-NH3 exhibits a slacker and more porous structure, thus a higher graphitization degree (Fig. 2B and D), while the carbon of Fe-N-C-H2 is more disordered and compact (Fig. 2J and L). In general, the presence of more slack and porous carbon could create abundant graphene edges, which could increase the active sites of ORR and lower the oxygen adsorption barrier on the surface of the graphene edges [31]. Hence, the structure of the Fe-N-C-NH3 catalyst can promote a highly efficient electrochemical reaction of O2. The N2 adsorption/desorption measurement was used to charac terize the specific surface area and pore size distribution of the asprepared materials. As shown in Fig. S3A, the isothermal adsorption curves of the three samples quickly increased as the relative pressure was increased at 0–0.1, suggesting the adsorption of microspores in the samples [32]. Based on IUPAC classification, no hysteresis loop was observed in the isothermal adsorption desorption curve of Fe-N-C-H2, revealing its typical microporous structure, while the pronounced hys teresis loops of Fe-N-C-NH3 and Fe-N-C-Ar indicate their mesoporous structure [33]. Similar results were observed from the pore size distri bution curves of these samples and agree well with TEM observation (Fig. S3B). The BET specific surface areas of Fe-N-C-H2, Fe-N-C-Ar, and Fe-N-C-NH3 were determined to be 287.4 m2 g 1, 386.5 m2 g 1 and 628.95 m2 g 1, respectively. Subsequently, we investigated the elec trochemical surface area (ECSA) of as-prepared catalysts using 0.2 M [Fe (CN)6]3-/4- as the redox probe and 0.1 mol KCl as the supporting elec trolyte. From the CV curves (Fig. S4), the specific capacitances of the Fe-N-C-H2, Fe-N-C-Ar, and Fe-N-C-NH3 were calculated as 226.0 F g 1, 277.2 F g 1, and 496.2 F g 1, respectively, which are consistent with the BET results and TEM observations. This ECSA order of the as-prepared catalysts was also confirmed by the double layer capacitance (Cdl) of three materials displayed in Fig. S5 [34]. Based on the above results, it is suggested that different sintering atmospheres have a great influence on the microstructure of as-prepared
Fe-N-C materials, which can be explained by the following reasons. (1) In the Ar and NH3 sintering process, iron will catalyze the graphitization of carbon to form highly graphitized carbon layers. In comparison, a more solid carbon structure forms in H2, since hydrogen can effectively reduce the decomposition rate of hydrocarbons [35]. In addition, hydrogen can effectively fill the dangling bonds of surface carbon atoms, which maintains their sp3 configuration instead of graphitic sp2 or sp structure [36]; thus, no obvious fingerprint-like lattice fringes can be observed in Fe-N-C-H2. (2) Solid carbon formed by H2-sintering plays a key role in assembling core-shell iron carbide. In high temperatures, iron reacts with carbon to produce iron carbide, subsequently gathering together via ripening agglomeration to form a nucleus. As the reaction continues, the agglomeration of iron carbide is confined by dense carbon and has difficulty passing through solid carbon, eventually forming a core shell structure. (3) Argon only provides an inert atmosphere, while ammonia is a corrosive gas. Therefore, carbon will further be etched and produce more pores during NH3-sintering than in argon atmosphere. X-ray photoelectron spectroscopy (XPS) is a great technique for sensitively detecting the elemental composition and chemical state of the as-prepared materials’ surfaces. The XPS survey spectra indicate the presence of C, N, O, and Fe elements in the as-prepared catalysts (Fig. S6A). Fig. S6B shows the high-resolution spectra of C1s of the three samples. The peak at 383.6 eV is usually considered correspond to Fe-C bond, and the very weak Fe-C peak may due to the low content of Fe in the as-prepared materials [37,38]. Fig. 3A displays the high-resolution spectra of Fe2p of the three samples. The peaks at 721.0 eV, 723.2 eV, and 725.4 eV correspond to the 2p1/2 peaks of Fe0, Fe2þ, and Fe3þ, and those at 706.7 eV, 711.1 eV, and 714.4 eV correspond to the 2p3/2 peaks of Fe0, Fe2þ, and Fe3þ, respectively [39]. A satellite peak at 718.8 eV further indicates the co-existence of Fe2þ and Fe3þ in the three catalysts. The peak at 711.1 eV can be correspond to the coordination of Fe-N species, which are considered to be the ORR active sites [40]. It is worth noting that the Fe0 content close to zero in Fe-N-C-Ar is far lower than that of the catalysts obtained in H2 and NH3. This difference may be due to the reduction power of H2 and NH3, whereas Ar only provides an inert atmosphere. In Fig. 3B, the N1s spectra for three samples can be deconvoluted into several distinct characteristic peaks at 398.3 eV, 399.3 eV, 400.1 eV, 400.8 eV, and 403.5 eV, corresponding to pyridinic-N, Fe-Nx, pyrrolic-N, graphitic- N, and oxidized-N, respec tively [41]. The peak at 399.3 eV indicates the Fe-Nx moiety in our sample, which is similar to that in FePc [42]. The contents of different N1s in the three samples were calculated and are displayed in Fig. 3C. It is clear that the highest pyridinic-N and Fe-NX content were formed in the NH3-sintered sample, while few pyrrolic-N was observed in Fe-N-C-H2. This confirms that ammonia-sintering can help form pyridinic-N by nitrogen doping and more Fe-NX through reaction with NH3. However, in the H2-sintering process, pyrrolic-N was more easily destroyed. As calculated from thermogravimetric analysis (TGA) (Fig. S7), the iron content of Fe-N-C-H2, Fe-N-C-Ar, and Fe-N-C-NH3 is 5.6%, 8.4%, and 17.3%, respectively, which results from the different pyrolysis degrees of their precursors in different atmospheres. 3.2. Electrochemical characterization of as-prepared catalysts Cyclic voltammetry (CV) measurements in N2- and O2-saturated 0.1 M KOH solutions were first performed to evaluate the ORR catalytic activity of the samples. As displayed in Fig. 4A, no ORR characteristic peaks were observed under N2-saturated solution for the three catalysts, while obvious ORR characteristic peaks appeared in O2-satureated so lutions, which suggests that the as-prepared catalysts have good ORR catalytic activity. The linear sweep voltammetry (LSV) curves at 1600 rpm for all catalysts are depicted in Fig. 4B. Fe-N-C-NH3 exhibits a higher one-set potential (E0 ¼ 0.97 V vs RHE) and half-wave potential (E1/2 ¼ 0.85 V vs RHE) than both Fe-N-C-Ar (E0 ¼ 0.92 V, E1/2 ¼ 0.8 V vs RHE) and Fe-N-C-H2 (E0 ¼ 0.89 V, E1/2 ¼ 0.75 V vs RHE), indicating that Fe-N-C-NH3 possesses the best ORR catalytic performance. Moreover, 4
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Fig. 3. (A) Fe2p XPS spectra and (B) N1s XPS spectra of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2; (C) the percentage of pyridinic-N, graphitic-N, pyrrolic-N, oxidized-N and Fe-N of as-prepared samples; (D) structural features of pyridinic-N, graphitic-N, pyrrolic-N and Fe-N.
the number of electron transfer (n) and yield of H2O2 (H2O2%) are sig nificant indicators for evaluating the ORR kinetics of catalysts. RRDE voltammograms of the three catalysts at 1600 rpm are shown in Fig. 4C, where solid and dashed lines indicate disk current (ID) and ring current (IR), respectively. The lower IR means the less intermediate product in the ORR process, which demonstrates a higher n and lower H2O2%. The n and H2O2% calculated from ID and IR are displayed in Fig. 4D. The n values of Fe-N-C-NH3, Fe-N-C-Ar, and Fe-N-C-H2 involved in the ORR process were determined to be 3.99, 3.97, and 3.9, and the H2O2% values were 6.3%, 7.5% and 12.5%, respectively. It is clear that the n of all catalysts is close to the theoretical value of 4.00 for Pt/C, indicating a complete 4e ORR pathway. Further, the NH3-sintered catalyst deliv ered better ORR kinetics than the catalysts obtained in Ar and H2. Similar results were also obtained from their LSV curves at different rotating speeds (Fig. S8 A to C) and the Koutecky Levich (K L) plots (Fig. S8 D to F) indicate first-order reaction kinetics toward the con centration of dissolved oxygen [43]. Diffusive resistance is a key indi cator for evaluating high efficiency ORR catalysts. The electrochemical impedance spectroscopy (EIS) of different catalysts is displayed in Fig. S9. The straight line at lower frequencies is ascribed to the diffusive resistance of electrolyte/reaction raw in electrode materials. The straight line of Fe-N-C-NH3 is straighter than that of Fe-N-C-Ar and Fe-N-C-H2, which indicates that the Fe-N-C-NH3 has much lower diffu sion resistance. Subsequently, we found that ORR catalytic performance of the Fe-NC-NH3 catalyst is comparable to a commercial 20% Pt/C catalyst (Fig. 4E) and is better than most previously reported Fe-N-C materials judged from E0 and E1/2 (Fig. S10). Besides the ORR activity, the antimethanol toxicity and durability of Fe-N-C-NH3 were investigated. Generally, when using Pt/C as an electrocatalyst to test ORR electro catalytic performance, a representative methanol oxidation peak always appears after adding methanol to the electrolyte [44] (Fig. S11A). In contrast, no response was obtained toward methanol when Fe-N-C-NH3
was used as the ORR electrocatalyst (Fig. S11B), which is contributable to the better anti-methanol toxicity of Fe-N-C-NH3 than 20% Pt/C. After 10 h i-t test at 0.75 V vs RHE, the current density of the Fe-N-C-NH3- based electrode retained up to 94% of its initial current density, while the current density of commercial Pt/C only retained up to 65% under same experimental conditions (Fig. 4F). This further demonstrates that Fe-N-C-NH3 has better durability than 20% Pt/C. The above advantages of Fe-N-C-NH3 can be ascribed to the following reasons: (1) Fe-N-C-NH3 contains high contents of Fe-N and pyridinic-N; (2) the slack and porous structure creates abundant nitrogen-doped graphene edges, which could lower the oxygen adsorption barrier on the catalyst’s surface; (3) the highly graphitized carbon shell can reinforce the corrosion resistance of the catalyst. 3.3. The practical application possibility of Fe-N-C-NH3 based ZAB A homemade liquid zinc-air battery (Fig. S12A) was further assem bled to explore the feasibility of practical application of our designed catalyst. An ultrahigh open circuit potential of 1.51 V was observed for Fe-N-C-NH3, which is higher than 20% Pt/C (1.44 V) (Fig. S12B). As displayed in Fig. S12C, a higher peak power density (79.2 mW cm 2) of Fe-N-C-NH3, close to that of Pt/C (84.6 mW cm 2), was obtained with an increase in current density and decrease in voltage. Further, no obvious decrease was observed from the charge-discharge curves of our assem bled zinc-air battery after 25-h of cycling (Fig. S12D), suggesting its good stability. 4. Conclusion In summary, small uniform Fe/N-rich polymer spheres were fabri cated using a simple stirring method at room temperature. The Fe-N material was subsequently employed as a faultless precursor to construct Fe-N-C catalysts and study the effects of the sintering 5
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Fig. 4. Electrochemical properties of Fe-N-C-NH3, Fe-N-C-Ar and Fe-N-C-H2 in 0.1 M KOH electrolyte: (A) CV curves; (B) Linear sweep voltammetry (LSV) curves; (C) RRDE voltammograms; (D) Electron transfer number and H2O2 yield; (E) LSV curves of Fe-N-C-NH3 and 20% Pt/C; (F) Chronoamperometric responses of Fe-N-C-NH3 and 20% Pt/C catalyst.
atmosphere (NH3, Ar, and H2) on the catalysts’ microstructure and ac tivity sites. It was found that hydrogen-sintering leads to a dense carbon structure with an evenly distributed core-shell structure of iron carbide, while ammonia- and argon-sintering result in a porous carbon structure and aggregated Fe-based nanoparticles. It is important to note that NH3sintering will produce a loose and porous carbon structure, creating more Fe-N and pyridinic-N active sites. As an ORR catalyst, Fe-N-C-NH3 delivered a more positive onset potential (0.97 V vs RHE), half-wave potential (0.85 V vs RHE), as well as better stability and methanol resistance compared to 20% Pt/C. These results indicate the effect of a calcination atmosphere on engineering the microstructure and ORR performance of Fe-N-C catalysts. In short, our work introduces an effi cient ORR Fe-N-C-NH3 electrocatalyst, for which the preparation method can be extended to other Metal-N-C materials with special function and application. Declaration of competing interest The authors declare no known competing financial interests or per sonal relationships that could have appeared to influence the work re ported in this paper. Acknowledgements We appreciate support from the National Natural Science Foundation of China (21972111, 21773188), Fundamental Research Funds for the Central Universities (XDJK2019AA002, XDJK2019B052), Natural Sci ence Foundation of Chongqing (cstc2018jcyjAX0714), Chongqing En gineering Research Center for Micro-Nano Biomedical Materials and Devices, Chongqing Key Laboratory for Advanced Materials and Tech nologies. We also thank Dr. Ding-Yu Liu for help with XPS test and 6
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