C Composites as Highly Efficient Electrocatalyst towards Oxygen

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Energy, Environmental, and Catalysis Applications

A Comprehensive Investigation on Pyrolyzed Fe-N-C Composites as Highly Efficient Electrocatalyst towards Oxygen Reduction Reaction of PEMFCs Lin Li, Shuiyun Shen, Guanghua Wei, Xiaolin Li, Kun Yang, Qi Feng, and Junliang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22494 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Applied Materials & Interfaces

A Comprehensive Investigation on Pyrolyzed Fe-NC Composites as Highly Efficient Electrocatalyst towards Oxygen Reduction Reaction of PEMFCs Lin Li†, Shuiyun Shen†, Guanghua Wei‡, Xiaolin Li†, Kun Yang#, Qi Feng#, Junliang Zhang†§*

†Institute of Fuel Cells, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. ‡SJTU-Paris Tech Elite Institute of Technology, Shanghai Jiao Tong University, Shanghai, 200240, China #Advanced Technology Department, SAIC Motor Corporation Limited, Shanghai, 201804, China §MOE Key Laboratory of Power & Machinery Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China

KEYWORDS. Oxygen Reduction Reaction, Non-noble Metal Electrocatalyst, Fe-N-C, Phenanthrolene as Nitrogen Precursor, Annealing temperature.

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ABSTRACT. There remain great challenges in exploring cost-effective and highly efficient nonnoble metal electrocatalysts to catalyze the oxygen reduction reaction (ORR) of proton exchange membrane fuel cells (PEMFCs). And a further validation on their performances under a fuel cell operating condition also draws sustained attention. Herein, we report a comprehensive investigation on the ORR performances of a series of pyrolyzed Fe-N-C composites which use phenanthrolene as the nitrogen precursor, and the effects of carbon supports, transition metal precursors as well as the annealing temperature are detailedly examined. Electrochemical measurements combining with different physicochemical characterizations are employed to clarify the function of critical structures including the specific surface area, microstructure, nitrogen content, nitrogen types and corresponding proportion on their ORR and fuel cell performances. It demonstrates a half-wave potential of 0.79 V and almost a four-electron pathway. When using the as-optimized Fe-N-C composite as the cathode catalyst of a PEMFC, the maximum power density reaches as high as 540 mW cm-2.

1. Introduction Since possessing a large amount of distinctive characters, including fast start-up and quick power-match, low-temperature (~80 oC) polymer electrolyte membrane fuel cells (PEMFCs) have a great potential for the application in electrical vehicles and other electronic propulsion devices.1-2 However, there remains an immense economic disadvantage resulting from the employment of noble metal platinum (Pt) with a prohibitive cost, which functions as the most efficient electrocatalyst to accelerate the sluggish kinetic of cathodic oxygen reduction reaction (ORR), thus severely impeding the large-scale commercialization of PEMFCs. In this regard, extensive researches have been performed to explore non-precious metal catalysts,3-6 including metal oxides,7 metal sulfides,8-9 metal carbides,10-12 as well as nitrogen-doped carbon-based


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transition metal compounds (M-N-C). Especially, the series of M-N-C electrocatalysts bear more expectations because of their higher activity, four-electron selectivity, better stability as well as a higher flexibility in available precursors.13-14 In 1964, Jasinski first reported the ORR measurement on different macrocycles (Cu, Ni, Pt, Co) in alkaline media and proved that the cobalt phthalocyanine (CoPc) was most active.15 Later, Gupta et al. demonstrated the heat treatment of non-macrocycle of polyacrylonitrile combining with transition metal salts and higharea carbons, and the as-obtained Co/PAN/XC-72 electrocatalyst showed promising ORR performance in both alkaline and acidic media. More attractively, the application of such polymer as polyacrylonitrile provides a lower cost than CoTMPP macrocycle, thus helps greatly decrease the fuel cell cost.16 Therefore, more and more attentions have been paid to synthesize the M-N-C electrocatalysts through employing different nitrogen containing materials (heterocyclic compound like aniline and pyrrole, cyanamide, NH3, etc.), different transition metal salts (M can be replaced as Fe, Co, Ni and the inorganic salts like their sulfates, nitrates, acetates, chlorides, etc.) and a variety of carbon supports (commercial carbon black like Ketjenblack and Vulcan, carbon nanotube, grapheme, etc.) as the precursors.17-21 Wu et al. compared ORR activities of a group of PANI-Fe-C electrocatalysts obtaining from different carbon supports, and found that the half-wave potential for Black Pearls, Ketjenblack EC-300J and multi-walled carbon nanotubes (MWNTs) are 0.8 V, 0.79 V and 0.78 V, respectively. Especially, owing to the enhancement from graphene formation, the MWNT supported PANI-Fe electrocatalyst presents a greatly enhanced stability without performance loss for 500 hours in H2/air fuel cell.22 Peng et al. obtained a Fe-PANI/C-Mela electrocatalyst through pyrolyzing hybrid precursors containing polyaniline and melamine in Ar atmosphere and the half-wave potential was only 60 mV less than that of Pt/C.23 Wang et al. constructed a host-


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guest electrocatalyst (Fe, Co)/N-C with Fe-Co dual sites in metal-organic frameworks (MOFs) structure and demonstrated its onset potential and half-wave potential were 1.06 V and 0.863 V, respectively. The dual metal sites, (Fe, Co)/N-C could reduce the cleavage barrier of O-O bond, thus leading to a much better performance than Fe SAs/N-C and Co SAs/N-C with only one metal site, while comparable to commercial Pt/C.3 Wang et al. obtained different Fe-N-C electrocatalysts by pyrolyzing the composite of carbon supported graphitic carbon nitride (gC3N4) at different temperatures and the best sample pyrolyzed at 750 °C had a half-wave potential of 0.75 V, and showed a better stability than commercial Pt/C.24 Moreover, the selection of nitrogen precursor needs to be deliberative which is critical to doping content and types, thus highly influencing their ORR activities. Lai et al.25 utilized aniline, pyrrole and ammonia, respectively, as nitrogen precursors and obtained PANi/RG-O, Ppy/RG-O, N-RG-O samples with different nitrogen types with different proportions. It was concluded that the increase in the graphitic N content improves activity while the increase in the pyridinic N content enhances four-electron selectivity. The Dodelet’s group proved that an additional heat treatment in NH3 will help increase the content of doped nitrogen, thus improving ORR activity. It was demonstrated that a Fe/Phen/Z8-derived catalyst with 15 min treatment in NH3 possessed a higher nitrogen content than that annealed in Ar only (5.3 at% vs. 3.7 at%), and presented a much higher catalytic activity of 1120 A g-1 vs. 254 A g-1.26 Among various nitrogen precursors, phenanthrolene is very popular owing to its carbon matrix similar structure as well as strong chelation effects with abundant metal ions. Michel et al. compared different electrocatalysts either with nitrogen-free precursor and being pyrolyzed in NH3 atmosphere or with phenanthrolene as the nitrogen precursor and being pyrolyzed in Ar, and found the latter one represents a higher nitrogen content.21 Besides, phenanthrolene is easy to chelate with Fe2+ and


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form stable compound [Fe(phen)3]2+, which is helpful to incorporate both iron and nitrogen into the catalyst. To achieve a high ORR activity, it is very critical to efficiently dope nitrogen into carbon together with an appropriate metal amount. In this regard, we proposed to employ phenanthrolene as the nitrogen precursor to pyrolyze a series of Fe-N-C composites via a facile and well-controlled method. The effects of carbon support, transition metal precursor as well as the annealing temperature on their ORR performances were detailedly investigated. Both physicochemical characterizations and electrochemical measurements were employed to evaluate their corresponding ORR and fuel cell performances and clarify the function of critical structures. 2. Experimental section 2.1 Materials Ethanol (C2H6O, ≥99.7 %, Sinopharm Chemical Reagent), iron(II) acetate (C4H6FeO4, 99.99 %, Sigma-Aldrich), ammonium iron(II) sulfate hexahydrate (H8FeN2O8S2·6H2O, 99.99 %, Aladdin) 1,10-Phenanthroline (C12H8N2, 99 %, Aladdin), Ketjenblack EC 300J (AkzoNobel), Ketjenblack EC 600JD (AkzoNobel), Pt/C (TEC10V50E, 46.7 wt%, TKK), Nafion solution (20 wt%, Dupont) were all used as received without further treatment. Deionized water was applied for all experiments. 2.2 Synthesis of the series of Fe-N-C composites 1, 10-Phenanthroline was adopted as nitrogen precursor and will be further complexed with Fe(II) to form chelate compound. Commercial Ketjenblack EC 300J (named C1) and Ketjenblack EC 600JD (named C2) were, respectively, employed as carbon supports without further treatment. The influence of Fe(II) salts on the ORR activity was investigated, and ammonium iron(II) sulfate hexahydrate and iron(II) acetate (labelled as Fe1 and Fe2, respectively) were


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added as the metal precursors. As illustrated in Scheme 1, in a typical approach, 500 mg carbon material, 500 mg 1,10-Phenanthroline and a proper amount of iron source (0.7 wt.%, 1.5 wt.% and 2 wt.%) were completely mixed for 3 h to refined powder by planetary ball-milling at 350 rpm. Then, the mixture was heat treated at high temperature (800 °C, 900 °C and 1000 °C) in protection of nitrogen gas for two hours. Following the pyrolysis, the tube furnace was cooled down to room temperature naturally. Finally, the samples were obtained and denoted here as Fe(X)-N-C-Y, where X and Y represents the iron content and annealing temperature. When X=0.7%, it is omitted. Scheme 1 A schematic representation of the synthesis strategy on Fe-N-C electrocatalysts.

2.3 Characterizations Both the microstructural morphologies and element distribution were examined by transmission electron microscopy (TEM, JEM-2100F, JEOL, 200 kV). X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTRA DLD, Al Kα) was applied to analyze the chemical state and element composition of all the electrocatalysts. Raman spectra (Renishaw inVia, wavelength at 532 nm) was employed to understand the defect and graphene degree of the catalysts. Specific surface area was recorded on 3H-2000PS1 (Beishide Instrument) and the results given were based on Brunauer Emmett Teller (BET) method and the distribution of pore size was calculated using Barrett-Joyner-Halenda (BJH) methods.


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2.4 Electrochemical measurements Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were carried on electrochemical workstation (CHI 760e, CH Instruments) accompanied with rotating ring disk electrode (RDE, Pine Instruments) to evaluate the electrochemical performance of all the assynthesized electrocatalysts for ORR. In the electrochemical measurements, three electrode system (work electrode: glass carbon (Φ=5 mm) coated with catalysts, reference electrode: saturated calomel electrode (SCE) and counter electrode: platinum foil) was used throughout. All the potentials described in this paper were relative to reversible hydrogen electrode (RHE) by calibrating versus SCE in 0.1 M HClO4. Catalyst inks were prepared by dispersing 5 mg catalyst in a solution containing 0.5 mL 0.25 wt% Nafion and sonicating for 30 minutes. Afterwards, 12 µL ink was deposited onto a glossy glass carbon to fabricate the working electrode (loading ~0.6 mg cm-2). The electrochemical tests were carried out with a potential range between 1.1 and 0 V (vs. RHE) in 0.1 M HClO4 with a scan rate of 5 mV s-1. 2.5 Testing of fuel cell The fuel cell performance using the as-synthesized Fe-N-C as the cathode catalyst of membrane electrode assembly (MEA) was evaluated the under PEMFC operating conditions with the temperature of 80 °C, the humidity of 100 RH% and the backpressure of 1.5 bar. The Fe-N-C cathode was fabricated by ball-milling and ultrasonically mixing the as-synthesized FeN-C electrocatalyst with Nafion solution, and then coating onto the gas diffusion layer (GDL, GDS3250, Ballard) until the loading reached 4 mg cm-2. Commercial Pt/C catalyst (0.2 mgPt cm-2) was used as the anode catalyst. The cathode and anode together with proton exchange membrane (Nafion 211, Dupont) composed a MEA (5 cm2). Fuel cell testing was performed on multi-range fuel cell test system (850e, Scribner Associates Inc.). H2 (300 mL min-1) and O2 (400 mL min-1)


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were humidified at 80 °C and supplied at anode and cathode, respectively. Fuel cell polarization curves were obtained by staircase voltammetry with the voltage from open circuit voltage (OCV) to 0.3 V and the potentials are given after iR-correction. 3. Results and discussion 3.1 Effects of the annealing temperature on ORR The process of annealing treatment, including annealing temperature, heating time as well as annealing atmosphere greatly influences the structure, morphology and compositions of Fe-N-C electrocatalysts, thus impacting their ORR catalytic activities and stabilities.27-30 Figure 1a and 1b, respectively, illustrate CV and LSV curves on the series of as-synthesized Fe1-N-C1 electrcatalysts pyrolyzed at different temperatures ranging from 800 °C, 900 °C to 1000 °C in O2-saturated 0.1 M HClO4. It is significantly observed that the Fe1-N-C1 sample pyrolyzed at 900 °C exhibits the highest catalytic performance towards ORR in terms of both the peak potential (0.78 V vs. 0.76 V (Fe1-N-C1-800) and 0.73 V(Fe1-N-C1-1000)) and half-wave potential (0.76 V vs. 0.72 V (Fe1-N-C1-800) and 0.67 V(Fe1-N-C1-1000)).

Figure 1 (a) CV and (b) polarization curves (1600 rpm) on the series of pyrolyzed Fe1-N-C1 samples in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1.


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Furthermore, the polarization curves at various rotation rates on the series of Fe1-N-C1 electrocatalysts pyrolyzed at different temperatures are given in Figure 2. Correspondingly, the electron transfer number can be determined using the Koutecky-Levich (K-L) equation.31 1 𝑗

1

1

𝑘

𝑑𝑙

1

(1)

= 𝑗 + 𝑗 + 𝐵𝑤1/2

Where j is the measured current density, jk is the kinetic current density, jdl is the diffusionlimited current density and w is the rotation rate. The parameter B is defined by the following equation. −1/6

B = 0.62𝑛𝐹𝐶0 𝐷0 2/3 𝑣0

(2)

where n is the electron transfer number, F is the Faraday constant (96485 C mol-1), C0 is the concentration of O2 in the electrolyte (1.2×10-6 mol cm-3), D0 is the diffusion coefficient of O2 in the electrolyte (1.9×10-5 cm2 s-1), v0 is the kinetic viscosity of the electrolyte (0.01 cm2 s-1).

Figure 2 Linear sweep voltammetry (LSV) in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1 at different RDE rotation rate and their corresponding K-L plots of (a, b) Fe1-NC1-800, (c, d) Fe1-N-C1-900 and (e, f) Fe1-N-C1-1000.


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The electron transfer number can be thus calculated from the slope of the K-L plots (Figure 2) derived from the current density at 0.7, 0.65, 0.6, 0.55 V in LSV curves at various rotating rate. All the as-calculated electron transfer numbers for different electrocatalysts at different potentials are close to 4, indicating the fact that the four-electron path dominates the ORR on all the as-pyrolyzed Fe1-N-C1 electrocatalysts. It can be proved from TEM images (Figure S1) that there exist no metal or metal oxide particles for the series of Fe1-N-C1 electrocatalysts pyrolyzed at different temperatures ranging from 800 °C, 900 °C to 1000 °C, which can be due to the addition of little amount of Fe content (ca. 0.7 wt.%) in the precursors. Raman spectra was obtained to evaluate the graphitic structure in the series of pyrolyzed Fe1-N-C1 electrocatalysts, as displayed in Figure 3a. G band and D band respectively represent the degree of carbon graphitization and disordered carbons like defect position or amorphous carbon. The corresponding ratios (ID: IG) are 1.00, 0.99, 0.98 for Fe1-N-C1-800, Fe1-N-C1-900 and Fe1-N-C11000, respectively. The similar graphitization degrees indicate there exists no significant distinction in the electrical conductivities of all the catalysts.


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Figure 3 (a) Raman spectra of the series of pyrolyzed Fe1-N-C1 electrocatalysts; N 1s XPS spectra of (b) Fe1-N-C1-800, (c) Fe1-N-C1-900 and (d) Fe1-N-C1-1000. It is noted that the Fe content in the series of pyrolyzed Fe1-N-C1 electrocatalysts raises with the increase in the annealing temperature due to the further decomposition at higher temperature, while the N content first increases and then decreases (As shown in Figure S2), thus implying that 900 °C is the most appropriate temperature to incorporate nitrogen into carbon matrix. XPS analysis was further used to examine the effects of annealing temperature on the specific states of nitrogen in the pyrolyzed Fe1-N-C1 electrocatalysts. The peaks of high resolution N1s (Figure 3b-d) spectra can be divided into four species, including pyridinic N (N1 and N1*, 398.0-399.5 eV), pyrrolic N (N2, 400.1-400.9 eV), quaternary N (N3, 401-402 eV) and oxidized N (N4, 402410 eV), in which pyridinic N can coordinate with Fe in the form of Fe-Nx (labeled as N1*)24, and all the corresponding calculated contents are listed in Table 1.


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Table 1 The content of Fe, N as well as different nitrogen species in pyrolyzed Fe1-N-C1 electrocatalysts. Sample

Fe1-N-C1-800

Fe1-N-C1-900

Fe1-N-C1-1000

Fe content (wt.%) (ICP)

1.294

1.471

1.489

Nitrogen content (at.%)

0.62

0.91

0.49

pyridinic N

（%）

35.2

4.6

7.8

Fe-Nx

（%）

21.9

22.7

20.3

pyrrolic N

（%）

/

13.8

14.5

quaternary N

（%）

39.1

54.0

50.5

oxidized N

（%）

3.8

4.9

6.9

After annealing, the proportions of quaternary N and pyridinic N are higher than pyrrolic N, suggesting that N atoms were successfully incorporated into the carbon matrix located at the edge or inside of graphitic carbon. When comparing the content of N species, it can be seen the proportion of N1* actually first increases and then decreases, and reaches the maximum at 900 °C. It agrees well with the trend of ORR activities on the series of pyrolyzed Fe1-N-C1 electrocatalysts, implying the critical role of Fe-Nx structure. Currently, the controversy of active site mainly focuses on pyridinic N32-33, quaternary N34-35 and M-N sites36-37. Based on our results, it seems that the Fe-Nx structure is the main active sites. 3.2 Effects of carbon support on ORR The choice in carbon supports influences the structure, morphology and specific surface area of Fe-N-C catalysts, which impacts on the formation as well as utilization of active sites, thus resulting in different ORR activities. In this work, EC 300J and EC 600JD were employed


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because these two carbon supports have similar morphology while different specific surface areas, helping to examine the effect of specific surface area on ORR performance.

Figure 4 (a) CV and (b) polarization curves (1600 rpm) on Fe1-N-C1-900 and Fe1-N-C2-900 electrocatalysts in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1, respectively, using EC 300J and EC 600JD as carbon support. Figure 4 displays both CV and polarization curves (1600 rpm) on two samples, i.e., Fe1-N-C1900 and Fe1-N-C2-900 electrocatalysts, which, respectively, used EC 300J (labelled as C1) and EC 600JD (labelled as C2) as carbon support. It is noted that there exist similar onset potential and half-wave potential for the two kinds of electrocatalysts but different limited diffusion current densities, implying the fact that carbon support affects the mass transfer, which may be related to the different specific surface areas of EC 300J and EC 600JD. The carbon support of EC 600JD possesses a much higher specific surface area than EC 300J (1462 m2 g-1 vs. 808 m2 g1

, detailed in Figure S3), which is believed to benefit the transfer of both oxygen and water in

PEMFCs. Besides, polarization curves on Fe1-N-C1-900 and Fe1-N-C2-900 at various rotation rate ranging from 100 rpm to 2500 rpm and their corresponding K-L plots are shown in Figure 5 and the calculated electron transfer number are 3.6 and 3.9 for Fe1-N-C1-900 and Fe1-N-C2-900, respectively. It is thus suggested that the selectivity of four-electron-transfer reaction on the Fe1-


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N-C2-900 electrocatalyst is higher than that on Fe1-N-C1-900, indicating the carbon support also have an impact on ORR activity.

Figure 5 Linear sweep voltammetry (LSV) in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1 at different rotation rate and their corresponding K-L plots of (a, b) Fe1-N-C1900 and (c, d) Fe1-N-C2-900. The comparison of Raman spectra (Figure S4) between Fe1-N-C1-900 and Fe1-N-C2-900 electrocatalysts states that the sample using EC 600JD as the carbon support leads to an increase in the D-band/G-band ratio compared to the former one (ID:IG=0.99 and 1.10 for Fe1-N-C1-900 and Fe1-N-C2-900, respectively). As seen in Figure 6, TEM images display only carbon structure without metal or metal oxide particles in both Fe1-N-C1-900 and Fe1-N-C2-900 electrocatalysts, and the N 1s XPS spectra are divided into five peaks fitting well with the original data. The total nitrogen content as well as relative concentrations of each N species are listed in Table 2. As is seen, the total N content in Fe1-N-C2-900 is a little higher than that in Fe1-N-C1-900.


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Furthermore, more Fe-Nx nitrogen are formed in the Fe1-N-C2-900 electrocatalyst than in Fe1-NC1-900, indicating the importance of Fe-Nx for ORR.

Figure 6 TEM images of (a) Fe1-N-C1-900 and (b) Fe1-N-C2-900. Scale bar: 50 nm. N 1s XPS spectra of (c) Fe1-N-C1-900 and (d) Fe1-N-C2-900 Table 2 The content N as well as different nitrogen species in Fe1-N-C1-900 and Fe1-N-C2-900 electrocatalyst 1

1

1

2

Sample

Fe -N-C -900

Fe -N-C -900


0.91

1

pyridinic N

（%）

4.6

18.5

Fe-Nx

（%）

22.7

22.8

pyrrolic N

（%）

13.8

25.9

quaternary N

（%）

54.0

30.9

oxidized N

（%）

4.9

1.9


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3.3 Effects of iron precursor on ORR Metal precursor is also a key factor influencing the ORR performance either from anion aspect or cation aspect. Considering the chelation of phenanthroline and Fe2+, the cation was determined and two different anion salts were applied to probe into the effect of metal precursor. In addition, the optimal iron content seems to be 1.5 wt.% with reference of ORR activity comparison (Figure S5). When further increasing the iron content, metallic particles emerge in the catalysts obviously seen in TEM image (Figure S6). Hence, the effects of iron precursors including ammonium iron(II) sulfate and iron(II) acetate on the ORR performance were evaluated while EC 600JD was used as the carbon support, the annealing temperature was kept at 900 °C and the iron mass proportion was assured to be 1.5 wt. % in the precursors during the synthesis.

Figure 7 (a) CV and (b) Polarization curves (1600 rpm) of Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)N-C2-900 in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1. Figure 7 compares CV and polarization curves (1600 rpm) on Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 electrocatalysts, in which Fe1 and Fe2 mean Ammonium iron(II) sulfate hexahydrate and Iron(II) acetate, respectively. It is revealed that the Fe2(1.5 %)-N-C2-900 sample displays a more positive peak potential of oxygen reduction reaction in CV curves and more positive onset potential and 10 mV higher half-wave potential in LSV curves than Fe1(1.5 %)-NC2-900, demonstrating the crucial role of metal precursors in the Fe-N-C electrocatalysts. The


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half-wave potential of the optimal catalyst Fe2(1.5 %)-N-C2-900 is 0.79 V, only 60 mV less than that of commercial Pt/C (Figure S7).

Figure 8 Linear sweep voltammetry (LSV) in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1 at different RDE rotation rate and their corresponding K-L plots of (a, b) Fe1(1.5 %)-N-C2-900 and (c, d) Fe2(1.5 %)-N-C2-900 Figure 8 presents the LSV curves tested at various rotation speeds from 100 to 2500 rpm and corresponding K-L plots as-derived at different voltages from 0.55 to 0.7V. The electron transfer number can be estimated as 3.7 and 4.0 for Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900, respectively, revealing the fact that both electrocatalysts possess an excellent selectivity to fourelectron-transfer reaction. Finally, the as-synthesized Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 electrocatalysts were examined in an H2-O2 PEMFC. The membrane electrode assemblies (MEAs) are fabricated with Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 as the cathode while commercial Pt/C (0.2 mgPt cm-2) as the anode, and Figure 9a demonstrates both the polarization and power density curves. It is found that the maximum power density of the fuel cell with the Fe2(1.5 %)-N-C2-900


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cathode reaches 540 mW cm-2 at 0.47 V, which is higher than Fe1(1.5 %)-N-C2-900 and competitive with those with other attractively reported non-precious cathodes.38-41 Figure 9b displays the iR-free cell voltage to logarithm of current density, and the Tafel slops are 98.8 and 91.6 mV dec-1 for Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900, respectively. In high potential region, the polarization curves are dominated by kinetic overpotentials, which is closely related to the activity of catalyst. Thus, Fe2(1.5 %)-N-C2-900 possesses higher ORR kinetic activity than Fe1(1.5 %)-N-C2-900. Overall, considering the polarization curves as well as Tafel plots, Fe2(1.5 %)-N-C2-900 exhibits better ORR performance than Fe1(1.5 %)-N-C2-900, which is consistent with the RDE results.

Figure 9 (a) The polarization and power density curves of H2-O2 fuel cell. Cathodes: ~ 4 mg cm-2 of Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900; O2 400 mL min-1; 100 RH% and 1.5 bar back pressure. Anode: 0.2 mgPt cm-2 Pt/C; H2 300 mL min-1; 100 RH% and 1.5 bar back pressure. Cell tests at 80 °C. (b) Tafel plots of PEMFC performance for Fe1 (1.5%)-N-C2-900 and Fe2 (1.5%)N-C2-900 catalysts. To gain a deep insight to the relationship between transition metal precursors and ORR performance, both the structure and element compositions were compared between Fe1(1.5 %)-


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N-C2-900 and Fe2(1.5 %)-N-C2-900 electrocatalysts. According to TEM images (Figure 10a-b), both Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 electrocatalysts display only carbon structure without metal or metal oxide particles with the increase of iron content, while Raman spectra (Figure S8) indicates the Fe1(1.5 %)-N-C2-900 sample presents a little higher ratio of ID:IG than that of Fe2(1.5 %)-N-C2-900 (1.10 and 1.05, respectively). N 1s XPS spectra (Figure 10 c-d) of Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 were analyzed, and the total nitrogen content as well as relative concentrations of each N species are listed in Table 3.

Figure 10 TEM images of (a) Fe1 (1.5%)-N-C2-900 and (b) Fe2 (1.5%)-N-C2-900. Scale bar: 50 nm. N 1s XPS spectra of (c) Fe1(1.5 %)-N-C2-900 and (d) Fe2(1.5 %)-N-C2-900 It is noted that the total nitrogen content in Fe1(1.5 %)-N-C2-900 is higher than that in Fe2(1.5 %)-N-C2-900, which may due to a double N doping from the application of ammonium iron(II) sulfate hexahydrate as iron source. However, by comparing the proportion of Fe-Nx nitrogen, the Fe2(1.5 %)-N-C2-900 electrocatalyst owns more Fe-Nx structure than Fe1(1.5 %)-N-C2-900,


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further confirming the enhancement of Fe-Nx for ORR, which is consistent with our discussion above. It can be observed that the proportion of active site Fe-Nx in Fe2(1.5 %)-N-C2-900 is higher than that of Fe1(1.5 %)-N-C2-900, thus leading to both better RDE and fuel cell performances.

Table 3 The content N as well as different nitrogen species in Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 Sample

Fe1(1.5 %)-N-C2-900

Fe2(1.5 %)-N-C2-900


1.95

1.55

pyridinic N

（%）

21.3

18.3

Fe-Nx

（%）

25.6

47.9

pyrrolic N

（%）

5.8

/

quaternary N

（%）

22.9

27.7

oxidized N

（%）

24.4

6.1

4. Conclusion In summary, a series of highly active Fe-N-C electrocatalysts were obtained and the effects of carbon support, transition metal precursor as well as the annealing temperature were comprehensively investigated. The optimal catalyst Fe2(1.5 %)-N-C2-900 synthesized with EC 600JD, phenanthroline and iron (II) acetate as precursors and pyrolyzed at 900 °C behaves the highest ORR performance, including a half-wave potential of 0.79 V and almost a four-electron pathway. When using it as the cathode electrocatalyst under the H2-O2 condition, a maximum power density up to 540 mW cm-2 was obtained, implying the as-optimized Fe2(1.5 %)-N-C2-900


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sample can be a promising non-noble metal ORR electrocatalyst. Through comprehensively combining different physicochemical characterizations and electrochemical performance analyses, we propose that the fact that Fe-Nx may be well incorporated into the carbon matrix contribute greatly to the high ORR activity. Thus, the choice of Fe-containing precursors as well as carbon supports impacts on the ORR electrocatalytic activity. ASSOCIATED CONTENT Supporting Information. Additional figures including TEM images, Raman spectra, specific surface areas, Fe and N contents, ORR polarization curves and comparison of performance in Table (PDF). AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected]. ORCID Junliang Zhang: 0000-0003-2370-9699 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21533005) and the National Key Research and Development Program of China (2016YFB0101201).


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BRIEFS. Comprehensively investigation of the effect of each component in pyrolyzed Fe-N-C catalysts on ORR activity. Table of Contents.


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Scheme 1 A schematic representation of the synthesis strategy on Fe-N-C electrocatalysts. 175x43mm (300 x 300 DPI)



Figure 1 (a) CV and (b) polarization curves (1600 rpm) on the series of pyrolyzed Fe1-N-C1 samples in O2satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1. 82x30mm (300 x 300 DPI)


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Figure 2 Linear sweep voltammetry (LSV) in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s1 at different RDE rotation rate and their corresponding K-L plots of (a, b) Fe1-N-C1-800, (c, d) Fe1-N-C1900 and (e, f) Fe1-N-C1-1000. 82x41mm (300 x 300 DPI)



Figure 3 (a) Raman spectra of the series of pyrolyzed Fe1-N-C1 electrocatalysts; N 1s XPS spectra of (b) Fe1-N-C1-800, (c) Fe1-N-C1-900 and (d) Fe1-N-C1-1000. 82x68mm (300 x 300 DPI)


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Figure 4 (a) CV and (b) polarization curves (1600 rpm) on Fe1-N-C1-900 and Fe1-N-C2-900 electrocatalysts in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1, respectively, using EC 300J and EC 600JD as carbon support. 82x31mm (300 x 300 DPI)



Figure 5 Linear sweep voltammetry (LSV) in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s1 at different rotation rate and their corresponding K-L plots of (a, b) Fe1-N-C1-900 and (c, d) Fe1-N-C2900. 82x62mm (300 x 300 DPI)


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Figure 6 TEM images of (a) Fe1-N-C1-900 and (b) Fe1-N-C2-900. Scale bar: 50 nm. N 1s XPS spectra of (c) Fe1-N-C1-900 and (d) Fe1-N-C2-900 82x65mm (300 x 300 DPI)



Figure 7 (a) CV and (b) Polarization curves (1600 rpm) of Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900 in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s-1. 82x32mm (300 x 300 DPI)


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Figure 8 Linear sweep voltammetry (LSV) in O2-satuarated 0.1 M HClO4 solution with a scan rate of 5 mV s1 at different RDE rotation rate and their corresponding K-L plots of (a, b) Fe1(1.5 %)-N-C2-900 and (c, d) Fe2(1.5 %)-N-C2-900 82x64mm (300 x 300 DPI)



Figure 9 (a) The polarization and power density curves of H2-O2 fuel cell. Cathodes: ~ 4 mg cm-2 of Fe1(1.5 %)-N-C2-900 and Fe2(1.5 %)-N-C2-900; O2 400 mL min-1; 100 RH% and 1.5 bar back pressure. Anode: 0.2 mgPt cm-2 Pt/C; H2 300 mL min-1; 100 RH% and 1.5 bar back pressure. Cell tests at 80 °C. (b) Tafel plots of PEMFC performance for Fe1 (1.5%)-N-C2-900 and Fe2 (1.5%)-N-C2-900 catalysts. 82x31mm (300 x 300 DPI)


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Figure 10 TEM images of (a) Fe1 (1.5%)-N-C2-900 and (b) Fe2 (1.5%)-N-C2-900. Scale bar: 50 nm. N 1s XPS spectra of (c) Fe1(1.5 %)-N-C2-900 and (d) Fe2(1.5 %)-N-C2-900 82x64mm (300 x 300 DPI)



Table of Contents 82x31mm (300 x 300 DPI)


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C Composites as Highly Efficient Electrocatalyst towards Oxygen

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