Ammonia Defective Etching and Nitrogen-Doping of Porous Carbon

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Ammonia Defective Etching and Nitrogen-Doping of Porous Carbon toward High Exposure of Heme-Derived Fe-Nx Site for Efficient Oxygen Reduction Jiaoxing Xu, Chuxin Wu, Qiangmin Yu, Yi Zhao, Xun Li, and Lunhui Guan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02841 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Sustainable Chemistry & Engineering

Ammonia Defective Etching and Nitrogen-Doping of Porous Carbon toward High Exposure of Heme-Derived Fe-Nx Site for Efficient Oxygen Reduction Jiaoxing Xu,a,b Chuxin Wu, a,b Qiangmin Yu, a,b Yi Zhao, a,b Xun Li, a,b Lunhui Guan a,b * a. CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. Fuzhou, Fujian, P. R. China.. b. Fujian Provincial key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, P. R. China. Correspondence should be addressed to: Lunhui Guan, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, YangQiao West Road 155#, Fuzhou, Fujian 350002, P. R. China. Email: [email protected]

ABSTRACT: The utilization of metal and nitrogen doped carbon as a Pt-free oxygen reduction electrocatalyst depends largely on the homogeneous composition of the metal-nitrogen sites with limited content. Herein a simple and feasible ammonia defective activation strategy is explored on ordered mesoporous carbon (APC) to confine hematin precursor and suppress the formation of inorganic Fe-based derivatives during pyrolysis. Thus, a hierarchically nanoporous Fe/N/APC catalyst with high exposed iron-nitrogen sites exhibits an impressive performance for oxygen reduction reaction in alkaline media, with large diffusion-limited current density and positive half-wave potential with respect to commercial Pt/C catalyst. The enhanced ORR properties can be majorly ascribed to synergistic contributions of high exposed catalytic sites completion from high contents of Fe-N and pyridinic N along with the fast mass-transport properties arising from the etched high permeable porous structure. When applied as cathodic catalyst in Zn-air battery, it demonstrate a power density of 200 mW cm-2 and a specific capacity of 605 mA h g-1Zn higher than those of Pt/C catalyst.

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KEYWORDS: fuel-cells, electrocatalyst, ORR, Fe/N/C catalyst, ammonia pre-synthetic activation, nanoporous

INTRODUCTION Nowadays, the growing energy and environmental concerns have stimulated intense research on the development of green energy conversion and storage technologies such as fuel cells and metal-air batteries.1-3 In these clean and renewable energy devices, the cathodic oxygen-reduction reaction (ORR) is a key step that affects the overall energy conversion efficiency, which, however, suffers from the sluggish reaction kinetics even for state-of-the-art catalysts.4 To date, catalytic materials with Pt-containing are known to be the most-efficient electrocatalyst for ORR. However, the unacceptable cost and scare reserves of precious-metal make them questionable for widespread applications. Thus, much effort has been devoted to searching earth-abundant and efficient nonprecious metal catalysts (NPMCs) that can replace the precious metal counterparts, including transition-metal (Fe, Co, etc) 5-6 or metal oxides (Fe3O4, CoOx, MnOx, etc),7-9 phosphide10 as well as metal-nitrogen heteroatomic doped carbons11-13 or even metal-free doped carbonaceous materials14. Among NPMCs, metal-nitrogen decorated carbons particularly with a family of Fe coordinated with nitrogen functional groups in the carbon matrix have been regarded as the most promising substitute due to their high ORR activity and cost effectiveness.15 Their catalytic activity is believed to be originated from the crucial parts of the nitrogen-coordinated Fe site, through the actual nature of catalytic site remains elusive.16 Based on this, various types of Fe-Nx organometallic complex located on electronic-conducting carbon materials such as graphene or carbon nanotubes have been synthesized and applied as cathodic ORR electrocatalysts for competitive energy conversion efficiency.12,17-18 Despite significant progress, however, most synthetic approaches to Fe/N/C catalysts are involved a high-temperature pyrolysis step of macrocyclic metal porphyrins precursor to generate highly active and durable Fe-Nx sites, which unpredictably produces a significant amount of less-active inorganic Fe-based species and lower the density of highly active iron-nitrogen sites. Therefore, a rational synthetic approach that can restrain the aggregation of ACS Paragon Plus Environment

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metal atoms and improve the desired homogeneous composition of iron-nitrogen site needs to be explored. For example, recently, Lai et al. reported a host-guest chemistry strategy to construct Fe-mIm nanoclusters@ZIF-8 and transform into Fe-Nx coordinated structure with molecular level for ORR performance improvement.19 Kim’s group developed a general “silica-protective-layer-assisted” pyrolysis approach to generate high density of catalytically active Fe-Nx sites and suppress the formation of large Fe-based nanoparticles, thus yielded catalyst of CNTs coated with thin layer porphyrinic carbon (CNT/PC) demonstrates a record high current and power density in fuel cells.17 A hierarchically nanoporous carbon with high permeable mesostructure is favored for nanocasting porphyrins and the inherent Fe-N coordinated doping.20-22 However, because of the large steric structure of precursor, the required uniform and intimate contact cannot be guaranteed by the simple mixing porphyrins precursor with conventional carbons surface. During high-temperature treatment, the organometallic complex might detach easily from carbon support and aggregate into inorganic nanoparticles. More importantly, the Fe-N sites located deeply inside the carbon matrix of the pyrolysis product are hardly accessible to the reactants and consequently contribute to the catalytic increase scarcely. To solve the problems, Cao et. al. stabilized a novel iron phthalocyanine ORR electrocatalyst covalently bonded on the single-walled carbon nanotube through surface pyridinic-functionalization.23 Ammonia heat-treatment can not only introduce desired nitrogen functionalities on carbon for active doping, but also enrich nanopores in the carbon framework to improve the exposure of active site.4 With these considerations in mind, herein an ammonia pre-synthetic activation strategy is explored on ordered mesoporous carbon to confine hematin precursor, and suppress the growth of hematin-derived inorganic Fe-based nanoparticles during the pyrolysis. Beneficial from the surface pre-synthetic activation and desired N-doping, especially enriched pyridinic-N moiety created on carbon matrix that might facilitate for the intimate adsorption of hematin precursor and the structural completion of iron-nitrogen sites during pyrolysis, thus a heteroatom-doped Fe/N/APC catalyst with high exposed iron-nitrogen site exhibits an impressive performance for oxygen reduction reaction in alkaline media, with large half-wave potential and diffusion-limited current density with respect to commercial Pt/C catalyst. ACS Paragon Plus Environment

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When used as a cathode catalyst for Zn-air battery, it also demonstrate a high power density of 200 mW cm-2 and a specific capacity of 600 mA h g-1 (only considering the mass of Zn consumed) outperformed the Pt/C (20 wt %) catalyst. EXPERIMENTAL SECTION Materials Preparation Reagents: The poly (ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic 123) and Hematin were purchased from Aladdin (Shanghai, China), furfuryl alcohol, mesitylene, dimethylformamide (DMF), tetraethyl orthosilicate (TEOS) and hydrochloric acid were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without further purification.. Synthesis of porous carbons (PC) and ammonia activation: SBA-15 silica templates were prepared similar to the previous reported method.24 Porous carbon (CMK-3) was synthesized via a template method.25 In a typical procedure, 1 g of SBA-15 silica was ultrasonicately dispersed in 70 mL of mesitylene that contains 1 mL of furfuryl alcohol, and then 5 mg of oxalate was added as catalyst for polymerization. Then, the mixture was transferred into an autoclave for solvothermal treatment at 180 oC for 24 h. The product was collected by filtration, washed by DMF for two times and directly calcined at 850 oC for carbonization for 4 h in Ar protective atmosphere. After that, the replicated ordered mesoporous carbon (CMK-3) from SBA-15 silica was obtained with HF etching. Before preparation of Fe/N/C catalyst through an adsorption-reaction method,26,27 an ammonia-induced pre-synthetic activations of porous carbon were carried out for carbon etching and N-doping in ammonia atmosphere at high-temperatures of 800, 900 and 1000 oC for 15 min, respectively. The resultant carbon supports were denoted as APC-800, APC-900 and APC-1000 correspondingly. Synthesis of Fe/N/APC catalysts and referred samples: To synthesize the Fe/N/APC catalyst, equal mass ratio of APC and hematin was dissolved in DMF and the suspension was solvothermally-treated at 180 oC for 24 h for adsorption. After that, the solid ACS Paragon Plus Environment

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precipitate was collected by filtration and further annealed at 850 oC for another 4 h in Ar inert gas. The product was grilled and named Fe/N/APC catalyst. With the pre-synthetic activation by ammonia at 800, 900 and 1000 oC, the final Fe-Nx/C catalysts were named as Fe/N/APC-800, Fe/N/APC-900 and Fe/N/APC-1000, respectively. For comparison, two referred samples based on the untreated porous carbon (PC) were prepared by a similar solvothermally-assisted adsorption condition and pyrolysis in respective inert Ar-gas and active ammonia atmospheres. They were denoted as Fe/N/PC and AFe/N/PC, respectively. Here it should be noted that in the preparation process of the latter referred sample (AFe/N/PC), a higher temperature (>800 oC) treatment severely deteriorate the porous structure or even complete gasification at 900 oC, possibly due to the catalyst-promoted carbon etching by ammonia in the presence of Fe.28 Therefore, the pyrolysis step was performed at relative low temperature of 700 oC for 1 h. Material Characterizations The Fe/N/APC catalysts were morphologically observed on an SU-8010 field emission scanning electron microscope (FESEM. Hitachi Ltd., Tokyo, Japan). Scanning transmission electron microscopy (STEM) image and elemental mapping were obtained by using a FEI TITAN CHEM-Stem operated with an acceleration voltage of 80 kV to minimize the radiation damage. Surface composition and electronic properties were detected by X-ray photoelectron spectroscopic (XPS) measurements on a VG Scientific ESCALAB MK II using Al Ka radiation (1486.71 eV) and the C 1s peak at 284.5 eV as internal standard. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku diffractometer (Rigaku miniflex 600 X-ray generator, Cu Kα radiation) to study the crystallographic information of samples. Electrochemical Test Electrochemical measurements were performed on an electrochemical workstation (CHI 760C, CH Instruments, Inc., Shanghai, China) coupled with a rotating disk electrode (RDE) system (Pine Instrument Company, USA). A standard three-electrode electrochemical system equipped with a gas flow pipe was used, where a glassy carbon electrode with a diameter of 5 mm was used as working ACS Paragon Plus Environment

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electrode, Ag/AgCl electrode (saturated KCl-filled) and Pt wire were used as the reference and counter electrodes, respectively. The ORR performance of the catalysts was studied by cyclic and linear-scanned voltamogram (CV and LSV) measurements in an aqueous solution of 0.1 M NaOH. CVs were recored at a scan rate of 50 mV·s-1. LSVs were measured at a rate of 10 mV·s-1 under rotation speeds of 500, 900, 1200, 1600, and 2500 rpm. The NaOH solution was bubbled with pure N2 (99.999%) or pure O2 (99.999%) both before (for at least 30 min) and during the electrochemical measurements. The ORR activity was determined based on steady-state polarization curves, which were recorded after 30 sweeps. Rotating ring-disk electrode (RRDE) measurement was used to detect the by-product of peroxide species (HO2-) during the ORR process. The durability was assessed by the current-time (i-t) chronoamperometric measurement at 0.81 V (vs. RHE) under continuous O2-bubblings.

All electrochemical experiments were conducted at ambient condition (~25 oC).

The Fe/N/APC catalyst was used as the air electrode to fabricate a Zn-air battery. Homogeneous ink including catalyst, ionomer (Nafion solution, 5 wt.%) and isopropanol was pasted onto a carbon paper with a catalyst loading of 1.0 mg cm-2. The air electrode layer with an effective area of 1 cm2 allows O2 from ambient air to reach the catalyst sites. A polished zinc plate was used as anode and 6M KOH was used as electrolyte for Zn-air batteries. For comparison, commercial 20 wt% Pt/C catalysts with the same catalyst loading were prepared using the similar method. RESULTS AND DISCUSSIONS In this work, the synthesis of highly active Fe/N/APC includes the ammonia-induced pre-synthetic activation of ordered mesoporous carbons inversely replicated from SBA-15 and then the hematin precursor adsorption and pyrolysis, as illustrated in Scheme 1. The templated-synthesis of ordered mesoporous carbon (CMK-3) was performed in solvothermal condition, in which furfuryl alcohol was used as carbon resource and oxalic acid as polymerized catalyst.25 After a conventional thermal carbonization and HF etching, ordered mesoporous carbon (PC) with a large BET specific surface area (1083 m2.g-1) and pore volume (1.1 cm3.g-1) was obtained. Upon ammonia-induced defective etching ACS Paragon Plus Environment

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Scheme 1. Schematic illustration of the synthesis of Fe/N/APC samples, including the ammonia-induced surface activation of ordered mesoporous carbons inversely replicated from SBA-15 (Step 1) and then the hematin precursor adsorption and pyrolysis (Step 2, 3).

Figure 1. (a) SEM image and (b) TEM image of Fe/N/APC-900 sample. (c) The HAADF and the corresponding elemental-mapping of C, N and Fe; (d) XRD patterns of the Fe/N/APC-900 and the referred Fe/N/PC samples, the standard diffraction data of Fe3O4(JCPDS#75-0033) and Fe(JCPDS#87-0721) were given for index.


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and N-doping, it imparts the porous carbon with high permeable nanoporosity and nitrogen functionalities (Step 1). The unique textural structure and surface chemical properties of APC make it an ideal carrier for hematin precursor adsorption and subsequent pyrolysis. The adsoroption of hematin precursors was performed in a solvothermal method (Step 2). The surface activation for defective etching and nitrogen doping of porous carbon might improve the intimate contact of carbon with hematin precursor17 and suppress the inorganic growth of heme-derived Fe-based nanoparticles during pyrolysis, thus a Fe/N/APC catalyst with plenty amounts of homogeneous iron-nitrogen sites was obtained (Step 3), as identified firstly by physical characterization of SEM and TEM observations. From SEM image of the Fe/N/APC-900 sample in Figure 1a, the typical replicated mesoporous carbon kept rod-like shape of the SBA-15 template, with average length and thickness of about 1 µm and 0.5 µm, respectively. The rods link one after another to form lotus-root joints similar to previous observation.24 With TEM observation on Fe/N/APC-900 sample, textural mesopores within the body of micrometer-sized carbon along the [100] direction can be seen (Figure 1b and the enlarged region). No iron oxides with large grain size attached on the porous carbon were clearly observed in low-resolution TEM image (Figure S1a), greatly differed from those observations in referred Fe/N/PC sample based on unactivated porous carbon (Figure S1b). From high-resolution TEM image in Figure S1c and d, it is hard to clearly observe the iron-based nanoparticles, although the nano or atomic clusters of iron cannot be excluded. The polycrystalline diffraction ring of the SAED pattern in the inset of Figure 1b indicates the poor crystallinity of Fe/N-doped carbon structure. The STEM-HAADF image (Figure 1c) confirmed the ordered nanoporous structure of Fe/N/APC-900 sample, and the corresponding elemental mappings indicate the uniform distribution of C, N, and Fe within the carbon matrix (Figure 1c). In addition, the XRD pattern of the Fe/N/PC-900 sample was recorded for the analysis of the hematin-derived inorganic nanoparticles with comparison to those of referred Fe/N/PC sample. As shown in Figure 1d, the XRD pattern of Fe/N/PC sample displays three sets of obvious diffraction peaks which can be indexed to well-known graphitic carbon, magnetite (Fe3O4) and metallic Fe according to the standard JCPDS card (JCPDS#75-0033, JCPDS#87-0721). The strong diffraction peaks ACS Paragon Plus Environment

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associated with large magnetite (Fe3O4) and metallic Fe nanoparticles might result from non-uniform stacking of porphyrins precursor and strong sintering behavior during pyrolysis. While in case of Fe/N/APC-900 sample based on surface chemical activated carbon support, its XRD pattern shows much weaker diffraction peaks, suggesting the aggregation of hematin-derived inorganic iron-based nanoparticles was significantly inhibited during pyrolysis and more homogeneous composition of Fe-Nx sites were formed. More interestingly, with a careful examination on the diffraction peaks at 43.5, 50.6o, the hematin-derived inorganic nanoparticles on APC were selectively grown into metallic iron and iron nitride species (JCPDS#75-2128), quite different from the large magnetite (Fe3O4) nanocrystal present in case of Fe/N/PC, which have been proved to be positively affect the turn over frequency (TOF) of catalytically active centers. 29

Figure 2. (a,c) High-resolution XP C 1s and N 1s spectra of Fe/N/APC-900. (b) High-resolution XP Fe 2p spectra of Fe/N/PC and Fe/N/APC-900. (d) The calculated contents of different nitrogen moieties versus total atoms. With surface chemical properties analysis by XPS, compared to PC, the ammonia activated porous carbon (APC) possess new doped nitrogen functional groups with a concentration of 3.5 at.%, including 65 % pyrrolic- and 27 % pyridinic-N components. While the oxygen component was contrarily reduced ACS Paragon Plus Environment

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from 8.05 at.% to 3.7 at.%, indicating a surface purification and substitutional N-doping (Table S1). The tailored carbon surface properties contributed significantly to the selective formation of iron-based nanoparticles during pyrolysis. With Fe/N doping on porous carbon, like Fe/N/PC sample, the XPS survey spectra of Fe/N/APC sample in Figure S2 reveals the presence of carbon, nitrogen, oxygen and Fe. Among the chemically-bonded C, N, Fe and O atoms, Fe and N are “active” elements for the completion of active catalytic sites on carbon.16 A Gaussian fit of C 1s spectrum in Figure 2a shows three different peaks at 284.6, 285.3 and 288.6 eV, corresponding to C-C/C=C, C-O/C=N, and HO-C=O, respectively. In the spectra of Fe 2p (Figure 2b), the peaks centered at 705-717 and 722-730 eV with two main peak and a satellite peak are attributed to the Fe 2p3/2 and Fe 2p1/2 spin-orbit levels of Fe-based compounds.30 Compared to Fe/N/PC, the only Fe2+ 2p peaks of Fe/N/APC shifted to a lower binding energy, indicating the higher electrons density of the centered Fe2+ ions contributed from the more neighbouring N atoms when hematin-derived Fe-Nx species anchored on APC.31 The whole nitrogen content from the relative intensity reach a level of ~1.87 at.% for Fe/N/APC-900, near two times as that of Fe/N/PC catalyst (0.96 at.%). The higher level of Fe and N contents in Fe/N/APC-900 is also confirmed from ICP and elemental analysis (Table S2). The high-resolution N 1s spectra in Figure 2c can be fitted into five peaks: pyridinic-N (~398.0 eV), Fe-N (~399.0 eV), pyrrolic-N (~400.1 eV), quaternary-N (~401.0 eV), and oxidized N species (~402.2eV), respectively.19 Among them, the peak associated with metal-N coordinated structure including Fe-Nx site at ~399.0 eV was widely observed in previously reported Fe-N/C catalyst,

32,33

indicating that part of Fe-Nx sites in the hematin precursor in

our present work survive after high temperature pyrolysis. More importantly, the pyridinic N component in Fe/N/APC-900 retains a level of 0.63 at.%, as nearly triple times as that of Fe/N/PC (0.22 at.%). Accordingly, the Fe-N species on the Fe/N/APC-900 is about 60 % higher than that of Fe/N/PC (Figure 2d). These suggested that as a carrier, the porous carbon with ammonia activation can reduce the less active hematin-derived Fe-based nanoparticles and improve the homogeneous component of pyridinc N and Fe-Nx, which are believed to participate in the formation of active sites for the ORR. 15


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To gain insight the superb textural properties of porous carbon by ammonia defective etching for highly disperse iron-nitrogen sites, nitrogen adsorption-desorption test were firstly performed. From Figure 3a, the isotherm of porous carbon (PC) replicated from SBA-15 is of mixed type I and type IV with disctinct H1 hysteresis loop, indicating a mixed meso/microporous characteristic with a high specific surface of 1083 m2.g-1 similar to previous report.24 The pore size distribution (PSD) was derived to be micro/mesoporous

regions peaked at 1.3 and 3.0 nm (Figure 3b). Upon ammonia pre-synthetic

activation, the BET specific surface area of APC increase from 1083 to 1770 m2.g-1, with a total pore volume increase from 1.1 to 1.66 cm3.g-1. From PSD analysis (Figure 3b), the increased surface area and pore volume (~60%) mainly resulted from the enriched micro/mesopores by ammonia defective etching,2 which was evidenced by the obvious enhancement of D band associated with defect concentration in Raman profile (Figure S6a). The enrichment of nanopores in carbon framework is favorable for implanting hematin precursor and hosting the generated Fe-Nx active site with pyrolysis.20,21 With Fe/N doping by metallicporphyrin, the Fe/N/PC catalyst demonstrates an identical isotherm curve with similar N2 adsorption amount with respect to PC (no shown), suggesting a negligible pore-fillings. While in case of Fe/N doping on APC with ammonia etching, the resultant Fe/N/APC-900 catalyst demonstrates a considerable reduce in N2 adsorption amounts compared to carbon support (Figure 3a and 3c), indicating a significant micropores filling by hematin-derived Fe-Nx species. Thus, the Fe/N/APC catalyst demonstrate a calculated decreases of BET specific surface area from 1770 to 1118 m2.g-1 and total pore volume from 1.66 to 1.18 cm3.g-1. Both are similar to those of referred Fe/N/PC (SBET=1127 m2.g-1, Vtotal=0.99 cm3.g-1). The slight difference of total pore volume comes from the enriched mesopores of 3 nm based from PSD analysis (Figure 3d). With this respect, the pre-synthetic ammonia etching endows porous carbon with more developed and permeable textural structure as well as tailored chemical properties, which contributes to the feasible hematin filling and the high exposed Fe-Nx catalytic active sites.


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Figure 3. (a, c) N2 adsorption/desorption isotherm of PC, APC, Fe/N/PC Fe/N/APC-900; (b, d) The corresponding pore size distribution (PSD) derived from DFT method.

Figure 4. (a) LSV curves of Fe/N/APC-900 and Pt/C (20 wt%) in O2-saturated 0.1

M KOH. b) LSV

curves of Fe/N/APC in O2-saturated 0.1 M KOH at various rotation speeds. c) RRDE voltammograms ACS Paragon Plus Environment

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and (d) peroxide yields with regarding to the total oxygen reduction products and the calculated electron transfer number of Fe/N/APC and Pt/C in O2-saturated 0.1 M KOH. e) RDE polarization plots of PC, APC, Fe/N/APC-900, the referred Fe/N/PC, AFe/N/PC and commercial Pt/C (20 wt%) catalysts at a rotation speed of 1600 rpm and (f) Tafel plots of Fe/N/APC and commercial Pt/C catalyst, inset shows the kinetic limiting current density (JK) at 0.9 V.

The RDE experiments were performed to examine the ORR electrocatalytic activity of the Fe/N/APC catalyst in O2- and Ar-saturated 0.1 M KOH. A commercial Pt/C catalyst (20 wt%) was used for comparison. The linear sweep voltammetric (LSV) curves in Figure 4a suggests that Fe/N/APC-900 exhibit a exceptional electrocatalytic performance with an onset potential (Eonset) of 0.96 V and an half-wave potential (E1/2) of 0.88 V (vs. RHE). These values are respectively close to and more positive than those of Pt/C tested under similar conditions (Eonset = 0.965 V and E1/2 = 0.85 V), and outperform most previously reported Fe-Nx/C ORR catalysts especially in term of half-wave potential (Table S3).30, 32, 34

More importantly, the diffusion-limiting current density also demonstrates a better performance of

Fe/N/APC (e.g., 6.05 mA·cm-2 at 0.3 V vs RHE) than that of Pt/C (5.0 mA·cm-2). With increasing rotation rates from 500 to 2500 rpm, the analogous ORR polarization curves of the Fe/N/APC-900 (Figure 4b) were obtained and the current density was enhanced. Moreover, the linear Koutecky-Levich plots and the identical fitting lines suggested the first-order reaction kinetics toward the concentration of the dissolved oxygen and similar electron-transfer numbers at different potentials, which correlated with ORR selectivity (Figure 4b, inset). 24 To further evaluate the ORR selectivity of the Fe/N/APC-900 catalyst, we conducted rotating ring-disk electrode (RRDE) measurements. As shown in Figure 4c, similar Eonset and superior E1/2 of Fe/N/APC-900 was observed with respect to Pt/C. The large diffusion-limiting current density was also observed on Fe/N/APC-900 (e.g., 6.3 mA·cm-2 at 0.3 V vs RHE) than that of Pt/C (5.5 mA·cm-2). The average electron transfer number calculated from the RRDE measurements at all potentials is ~3.97,


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which is similar or even slight higher than the value calculated from Pt/C electrode (3.95), indicating the desired four-electron oxygen reduction process.35 RRDE results also revealed that the H2O2 yield measured with Fe/N/APC-900 remained below 3% at all potentials and dropped to 0.5% at 0.8 V vs RHE, implying Fe/N/APC-900 catalyst has extremely high ORR catalytic efficiency and selectivity. The Fe/N/APC-900 catalyst also shows high durability for ORR, as confirmed by the current-time (i-t) chronoamperometric record for the ORR (Figure S3). Continuous O2 reduction (30000 s) at 0.81 V (vs. RHE) s. on the Fe/N/APC-900 electrode resulted in only 18% loss of current density before becoming constant. The corresponding current loss over Pt/C electrode under the same conditions was as high as approximately 30%. These results suggested that Fe/N/APC-900 have much better selectivity and stability than the commercial Pt/C catalyst. To identify the role of ammonia defective activation on porous carbon and its contribution as a carrier to the high performance of the final Fe/N/APC catalysts, RDE measurement on PC, APC, the referred Fe/N/PC, AFe/N/PC electrodes were also carried out and the polarization curves were plotted in Figure S4 and Figure 4e with those of Fe/N/APC-900 and commercial Pt/C catalysts for comparison. From the RDE curves in Figure 4e, it can be seen that the porous carbon (PC) exhibited poor electrocatalytic activity with a large ORR overpotential. The surface N-doping did not promote the ORR activity considerably, as a slight reduced activation overpotential was observed, resultant from the major formation of the inactive pyrrolic N (65 %) species on carbon support.36 The Fe/N/PC and Fe/N/APC-900 prepared by hematin-derived Fe/N doping on respective PC and APC supports demonstrate significantly improved ORR activity, indicating that hematin-derived Fe-N doping are responsible for the main catalytic increase. With a comparison of ORR activities between Fe/N/PC, Fe/N/APC-900 and commercial Pt/C catalyst, It is seen that Fe/N/PC demonstrates a competitive ORR performance with respect to Pt/C catalyst, with a similar half-wave potential at 0.85-0.86 V and analogous diffusion-limiting current density, through the onset potential is about -20 mV negative. In case of Fe/N/APC-900, it demonstrated substantial enhanced ORR activity when compared with Fe/N/PC catalyst, as reflected by the respective 20~30 mV positive shift of the onset potential (Eonset) ACS Paragon Plus Environment

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and half-wave (E1/2) potentials, as well as the significantly increased diffusion-limiting current density (Figure 4e). However, the as-prepared Fe/N/APC-900 demonstrates a moderate electrocatalytic ORR activity in acidic media (Figure S5), with a much more negative onset potential with respect to Pt/C catalyst (~-100 mV), like previously report nonprecious metal catalyst.2, 19 Here it should be noted that if ammonia was participate in the pyrolysis step at a selected temperature of 700 oC, the obtained AFe/N/PC displayed an inferior ORR activity with negative half-wave (E1/2) potential than the Fe/N/APC-900, possibly due to the failure of inorganic growth inhibition (Figure S6). Thus, through optimizing porous carbon support by ammonia defective activation and nitrogen doping, the resultant Fe/N/APC-900 exhibited the best ORR activity that substantially outperformed Pt/C catalyst. The enhanced ORR activity of the Fe/N/APC-900 was also gleaned from the much smaller Tafel slope of 30 mV/decade at low overpotentials (Figure 4f) than that measured with Pt/C (43 mV/decade) in 0.1 M KOH. The calculated kinetic current density (Jk) at 0.9 V is 2.0 mA·cm-2, higher than the values of Fe/N/PC (1.5 mA·cm-2) and Pt/C catalyst (1.6 mA·cm-2) (Figure 4f, inset). All these suggested that the ammonia pre-synthetic activation strategy upon porous carbon resulted in the tailored textural and surface properties by defective etching and N-doping, which are beneficial for the formation of homogeneous Fe-Nx sites and then ORR performance enhancement. On basis of the observations of the considerably enhancement of ORR performance of Fe/N/APC by using APC as a carrier, and in combination with the direct evidence that enriched homogeneous component of iron-nitrogen site were observed from XRD and XPS ananlysis, it can be concluded that the enhanced ORR activity of the Fe/N/APC-900 mainly benefit from the surface activation and N-doping of porous carbon support, which lead to inorganic derivatives inhibition and preferable generation of Fe-Nx homogeneous composition during pyrolysis. According to the literatures,15,23 it is known that the interfacial interaction between ORR catalyst layer and underlying support is the key for developing efficient ORR NPMCs. The surface pyridinic-functionalization is an effective route to enhance the interfacial intimate contact of metalloporphyrins precursor with carbons. Thus, the temperature of ammonia activation for ACS Paragon Plus Environment

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Figure 5. (a) XRD patterns and (b) High-resolution N 1s of the Fe/N/APC-800 (1), Fe/N/APC-900 (2) and Fe/N/APC-1000 (3) samples; (c) Nitrogen content, the (pyridinic+Fe)-N component in total atoms and the atomic ratio of N/Fe of three Fe/N/APC samples. (d) LSV curves of Fe/N/APC samples. Inset in (d) shows the corresponding calculated kinetic limiting current densities at 0.8 V. optimal porous carbon is a key to the high activity of the Fe/N/APC for ORR. To this aim, porous carbon was thermally activation by ammonia at another two temperatures of 800 and 1000

o

C. With further hematin-derived Fe/N-doping, the obtained Fe/N/APC-800 and

Fe/N/APC-1000 were systematically characterized and tested on RDE to evaluate the ORR activities. With XRD analysis in Figure 5a, it is found that the XRD patterns of Fe/N/APC-800 and Fe/N/APC-1000 demonstrate weak diffraction peaks, which was indexed to be Fe and iron nitride similar to those observed in Fe/N/APC-900, through the FeOx was also detected in Fe/N/APC-1000 sample. While from the Raman profiles (Figure S7), it can be seen that two prominent D band and G band are present at around 1330 and 1580 cm-1, respectively. The ratio of the intensity of D mode normalized to G mode (ID/IG ratio) was used for estimating the defect ACS Paragon Plus Environment

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concentration. The identical ID/IG ratio between Fe/N/APC-900 and APC revealed that the carbon defects by ammonia etching were kept unchanged with the subsequent hemein-derived Fe/N doping. With further XPS analysis, the XP survey spectra revealed the presence of carbon, nitrogen, oxygen and Fe (no shown). The XP N 1s spectra of the three Fe/N/APCs were listed in Figure 5b for comparison. The calculated nitrogen content increased from 1.6 at.% to 1.9 at.%, and then decreased significantly to 1.1 at.% with activation temperature. Moreover, with a careful deconvolution of high resolution N 1s, it is seen that N1s can be fitted into five peaks: pyridinic-N (~398.0 eV), Fe-N (~399.0 eV), pyrrolic-N (~400.1 eV), quaternary-N (~401.0 eV), and oxidized N species (~402.2eV), respectively.19, 32, 33 The nitrogen configurations were tuned considerably with varying activation temperature. With ammonia pre-activation at 800 and 900 o

C, pyridinic-N is the major nitrogen component on Fe/N/APC. While the temperature was

elevated to 1000 oC, graphitic-N becomes predominant. The nitrogen content, (pyridinic+Fe)-N component as well as N/Fe atomic ratio were reploted in Figure 5c. It is clearly that the N/Fe atomic ratio decreased with increasing temperature of ammonia pre-synthetic activation. The Fe/N/APC-900 possesses the highest total nitrogen content and (Pyridinic+Fe)-N component, which are the crucial parts of the catalytic active sites and therefore demonstrated the best ORR activity among the Fe/N/APC catalysts. Figure 5d showed the LSV curves of the Fe/N/APC catalysts. Compared to Fe/N/APC-900, it is seen that the other two Fe/N/APCs displayed either negative onset or half-wave potential, demonstrate relative lower ORR catalytic efficiency. When the ORR apparent current was corrected by mass transports, the Fe/N/APC-900 exhibits the optimized catalytic activity in term of kinetically limited current density (57.5 mA·cm-2 at 0.8 V), much higher than the values of Fe/N/APC-800 (30.8 mA·cm-2 @0.8 V) and Fe/N/APC-1000 (37.5 mA·cm-2 @0.8 V), respectively. Considering the optimized ORR activity is present in Fe/N/APC-900 with highest (Pyridinic+Fe)-N component and medium BET specific surface area (Table S1), therefore, it is likely to conclude that the formation of homogeneous Fe-Nx structure is the key to the high ORR activity of Fe/N/C catalyst. ACS Paragon Plus Environment

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Figure 6. a,b) Galvanostatic discharge curves of Zn–air batteries with Fe/N/APC-900 and Pt/C as cathodic catalysts at two different current densities: (a) 10 mA cm-2 and (b) 25 mA cm-2. c) Long-term galvanostatic discharge curves of Zn–air batteries with Fe/N/APC-900 and Pt/C as cathode catalysts for 24 h. The specific capacity was normalized to the mass of consumed Zn. d) Power density against current density of zinc-air batteries with Pt/C and Fe/N/APC-900 cathodes.

To assess the utility of Fe/N/APC-900 as air cathodic catalyst in actual energy conversion devices, a Zn-air battery was fabricated and the energy outputs were measured. For comparison, commercial Pt/C catalysts were also tested under the similar conditions. From the galvanostatic discharge curves (Figure 6a, b), it is seen that the discharge potentials plateaus decreased with increasing current densities. At the discharge current densities of 10 and 25 mA cm-2, the battery with the Fe/N/APC-900 cathodic catalyst showed voltage plateaus of about 1.33 and 1.27 V, respectively. These values are slightly higher than those of Pt/C catalyst and other reported results. In the galvanostatic discharge curves (Figure 6c), no significant decay of voltage was observed for Fe/N/APC-900 in long-term discharge after 24 h, highlighting the good stability of the Fe/N/APC-900 catalyst toward ORR. When normalized to the mass of Zn consumed, the ACS Paragon Plus Environment

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specific capacity of the Zn-air battery integrated with Fe/N/APC-900 as cathodic catalyst was calculated to be 605 mA h g-1 at the current density of 10 mA cm-2, higher than that of Pt/C catalyst (590 mA h g-1). In Figure 6d, the rate energy output exhibit a peak power density of 200 mW cm-2 for Fe/N/APC-900, which is superior to that of Pt/C catalysts (180 mW cm-2), showing a potential of replacing noble metal-based catalyst.

CONCLUSIONS A simple and feasible ammonia pre-synthetic activation strategy is explored to achieve highly efficient heme-derived Fe/N/C electrocatalysts with high exposed Fe-N sites. This Fe/N/APC catalyst exhibits an exceptional perormance for ORR in alkaline media with a higher current density and a higher power density output of 200 mW cm-2 in Zn-air battery with respect to commercial Pt/C catalyst. The enhanced ORR properties was ascribed to synergestic contributions of high exposed catalytic sites completion from high contents of Fe-N and pyridinic N along with the fast mass-transport properties arising from the etched high permeable porous structure. This study demonstrates a simple and feasible synthetic strategies focusing on the inorganic derivative inhibition and preferably formation of accessible homogeneous Fe-Nx site on high permeable porous structure for ORR.

ACKNOWLEDGEMENTS. We acknowledge the financial support by the Natural Science Foundations of China (No.21101154, No.21171163, No. 91127020), NSF for Distinguished Young Scholars of Fujian Province (Grant No. 2013J06006), the Science and Technology Planning Project of Fujian Province (Grant 2014H2008), and the strategic Priority Research Program of the Chinese Academy of Sciences (XDA09010402).


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Supporting Information Avaiable. Some additional characterization data such as TEM, XRD, Raman, XPS and the ORR electrochemical properties are present here. This material is available free of charge via the Internet at http://pubs.acs.org.

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TABLE OF CONTENTS (TOC) GRAPHIC

Ammonia defective pre-activation is explored on ordered mesoporous carbon toward homogeneous Heme-derived Fe-N/C catalyst with an impressive oxygen reduction performance.


Ammonia Defective Etching and Nitrogen-Doping of Porous Carbon

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