Incorporation of Fe3C and Pyridinic N Active Sites ... - ACS Publications

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Incorporation of Fe3C and Pyridinic N Active Sites with a Moderate N/C Ratio in Fe-N Mesoporous Carbon Materials for Enhanced Oxygen Reduction Reaction Activity Changqing Li, Chuansheng He, Fengzhan Sun, Manchao Wang, Jiahui Wang, and Yuqing Lin ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00235 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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ACS Applied Nano Materials

Incorporation of Fe3C and Pyridinic N Active Sites with a Moderate N/C Ratio in Fe-N Mesoporous Carbon Materials for Enhanced Oxygen Reduction Reaction Activity Changqing Li§, Chuansheng He§, Fengzhan Sun, Manchao Wang, Jiahui Wang, Yuqing Lin * Department of Chemistry, Capital Normal University, Beijing 100048, China

E-mail address: [email protected] (Y. Lin) Tel.: +86 1068903047; Fax. +86 1068903047 §

These two authors contributed equally to this work 1 / 26

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Abstract A facile and effective route was reported to synthesize iron, nitrogen co-doped mesoporous carbon nanomaterials (Fe-N-C) as a high-performance oxygen reduction reaction (ORR) catalyst by pyrolyzing glycine, polyvinyl pyrrolidone (PVP), citric acid and FeCl3 through a heat-treatment (700-900 °C) process accompanied by an acid leaching technique. Among the as-prepared catalysts, the Fe-N co-doped carbon-800 after treatment with acid (Fe-N-C-800-acid) and the Fe-N-C-900 catalyst possessed excellent activity with onset potentials of 0.93 V and 0.92 V, respectively, and basically identical half-wave potentials of 0.75 V as well as a higher current density of 4.3 mA cm-2, 2.7 mA cm-2 at -0.56 V (vs RHE) through selective four-electron transfer kinetics (n= 3.5, 3.7, respectively). Moreover, they exhibit comparable Tafel slope values of 100 mV dec-1 and 108 mV dec-1, respectively, compared to 20 % Pt-C (99 mV dec-1) as well as excellent durability and resistance to methanol. The relevant abundance of Fe3C and pyridine N in the Fe-N-C-800-acid and Fe-N-C-900 has been proposed as the active sites in facilitating the desired ORR reaction. Interestingly, the physical and chemical characterization of all as-prepared catalysts indicated that the high Fe content in these catalysts does not play a key role in affecting the ORR performance but a moderate N/C ratio, i.e., 1 % in Fe-N-C-800-acid catalyst that, is pivotal in achieving the high activity towards the ORR.

Keywords: Fe-N co-doped mesoporous carbon material; acid treatment; Fe3C; N/C ratio; oxygen reduction reaction (ORR).

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1. Introduction The oxygen reduction reaction (ORR), which is a fundamental reaction in energy conversion, is significant for the development of fuel cells and metal-air batteries, and it occurs through a sluggish four-electron transfer pathway. High-efficiency catalysts, such as noble metal Pt, Pd or commercial Pt-C, have been successfully exploited to overcome complicated drawbacks, including the sluggish kinetics and higher overpotential of the ORR.1-4 However, the rare reserves, poor stability, and easy poisoning of those noble metals severely limit their large-scale application in fuel cells and hinder the development of mobile devices in fuel-cell-based technologies.5,6 Therefore, it is desirable and crucial to design and synthesize a viable alternative catalyst with outstanding activity and low-cost property to replace precious metal catalysts. Within this context, many efforts have focused on searching for low-cost precursors, such as graphene, nanotube, graphite carbon matrix materials7-13 and doping non-precious metal elements, such as Fe, Co, Ni, Cu, etc., into carbon nanostructures to design excellent ORR catalysts.14-19 Our group recently designed metal-free catalysts of amorphous carbon20 and carbon black21 that exhibited high activities towards the ORR using a facile method. Furthermore, many catalysts based on carbon materials, such as N-graphene, Fe-N-C, Fe-P-C, Co-N-C and metal oxides, have been and continue to be investigated by many researchers.22-27 Among them, transition metal and nitrogen co-doped carbon (M-N-C) materials with pronounced activity have received considerable attention as viable substitutes for noble metal-based catalysts towards the ORR on cathodes.28-33 Although the mechanism at the active sites in those non-noble metal catalysts remains unclear and the related interpretation may not be consistent when the M-N/C catalysts were prepared from different precursors, it has been shown that doping the carbon substrate with heteroatoms such as Fe, Co, N, P, etc. can greatly enhance the activity towards ORR.34-37 Most researchers have attributed the well-acknowledged active sites in the Fe-N-C catalysts to the Fe3C crystalline species and Fe-NX bonds or their synergistic effects.38,39 For example, Jiang et al. synthesized Fe@C-FeNC electrocatalysts and found that the coexistence of Fe/Fe3C nanocrystals and Fe-Nx contributed to the desirable ORR performance.29 Yang and co-workers reported a 3 / 26



bamboo-like carbon nanotube/Fe3C hybrid nano-electrocatalyst, on which the active sites were attributed to the wrapping of Fe3C NPs into the b-CNTs and a higher percentage of pyridinic N (33.3 %).40 On the other hand, the synthesized temperature and precursors are considered as another factor in determining activity of the electrocatalysts towards the ORR. Among that, the pyrolysis temperature can be adjusted to obtain the optimized N/C ratio accompanied by various types of active sites. In addition, many inexpensive precursors (thiourea, tannic acid, dopamine, iron porphyrin, urea, etc.) have been exploited to prepare N and Fe co-doped carbon materials with superior ORR activity.41-43 Notably, the acid leaching process was adopted as an effective method to post-treat the M-N-C material for the exposure and activation of metal active sites.44,45 Herein, Fe-N-C-X (X= 700, 800, 900) catalysts were synthesized through a heat-treatment method at different temperatures (700, 800, 900 °C) using inexpensive precursors composed of glycine, polyvinyl pyrrolidone (PVP), citric acid and FeCl3. Moreover, the Fe-N-C-X catalysts after acid leaching in 0.5 M H2SO4 were also synthesized to investigate the ORR performance. Among six prepared catalysts, Fe-N-C-800-acid and Fe-N-C-900 exhibited the best ORR performance considering the onset and half-wave potentials, electron transfer number, current density and dominant resistance to methanol. Specifically, the Fe-N-C-800-acid catalyst possesses a higher current density of 4.3 mA cm-2 compared with that of the Fe-N-C-900 catalyst (2.7 mA cm-2) at a potential of 0.56 V (vs RHE). However, the average electron transfer number (3.7) and the H2O2 production yield (4.1 %-10 %) of Fe-N-C-900 is slightly better than that of Fe-N-C-800-acid with an average electron transfer number (n) of 3.5 and a H2O2 production yield of 8.5 %-17.2 %. In addition to the catalytic evaluation, we focused on exploring the relationship between the ORR property and different active sites and found that the high content of Fe in these catalysts did not play a key role in the ORR activity outcome, but a moderate N/C ratio, i.e., 1 % in Fe-N-C-800-acid catalyst in this study, is pivotal to achieving the high activity towards the ORR.

2. Experimental 2.1 Synthesis of Fe-N-C-X and N-C-X Electrocatalyst 4 / 26


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In a typical preparation process, 3 g of glycine, 500 mg of citric acid and 500 mg of FeCl3 are dispersed in ultra-pure water (80 mL) and stirred for two hours at room temperature to form a homogenous solution. Then, 500 mg of PVP was added into the above homogenous solution and stirred continuously for an additional 20 min. Subsequently, the final homogeneous solution was dried in a baking box at 80 °C for 3 h to acquire a condensed solution. Next, the condensed solution was transferred into a corundum boat and baked at 150 °C for 3 h to obtain the preliminary carbonized yellow-black sticky jelly. Finally, above carbonized material was subjected to complete carbonization under 700 °C, 800 °C and 900 °C at a heating rate of 3 °C min-1 for 2 h in a N2 flow atmosphere to prepare the Fe-N-C-700, Fe-N-C-800, and Fe-N-C-900 samples, respectively. Additionally, the Fe-N-C-X-acid materials were prepared after the leaching of Fe-N-C-X in 0.5 M H2SO4 with stirring for 12 h, followed by a rinsing procedure with DI water for at least three times and kept it under drying. N-C-800-acid and N-C-900 as comparison materials were synthesized using the same schematic process as that of the Fe-N-C-X-acid and Fe-N-C-X catalysts just without the involvement of FeCl3. 2.2 Materials Characterization SEM images and energy dispersive X-ray spectroscopy (EDX) mapping of the as-prepared catalyst were obtained using a Hitachi S-2600N scanning electron microscope. HRTEM images of the samples were obtained on a JEM-2100F transmission electron microscope (Japan) operated at 200 kV. Thermogravimetric (TG) measurements were evaluated on a TGA-DSC analyzer (STA 449F3+ASC). XPS (X-ray photoelectron spectroscopy) measurements were conducted on a VG Micro-tech ESCA 2000. X-ray powder diffraction (XRD) of the samples was conducted on a Bruker D8 advance powder diffractometer with Cu Kα radiation. Raman spectra of the catalysts was obtained by Raman spectroscopy (EQUINOX 55, German). 2.3 Electrocatalytic Measurements In this present work, all the electrochemical studies (CV, LSV) were carried out on a conventional three-electrode setup (glassy carbon as the working electrode, Ag/AgCl and platinum wire as the reference and counter electrodes) cell from CHI 701E (Chenhua, 5 / 26



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Shanghai). A Rotating Ring-Disk Electrodes (RRDE-3A, BAS, Japan) with a Pt ring (4 mm inner-diameter and 5 mm outer-diameter) and glass carbon electrode (3 mm diameter) were employed for screening the ORR performance of various catalysts. Before the experiments, the working electrode was first polished with 1.0 and 0.05 µm alumina slurries and subsequently washed with ultra-pure water. All the prepared sample (6 mg mL-1) inks were made with N-N-dimethlformamide (DMF). Then, 10 µL of a catalyst suspension was pipetted onto the GC electrode with a catalyst loading of 0.4 mg cm-2, and 5 µL Pt-C (20 %) (5 mg mL-1) was also modified on the GC electrode surface for a catalyst loading of 0.35 mg cm-2, followed by coating 5 µL Nafion (0.5 %) as an adhesive layer. The cyclic voltammetry (CV) experiments were conducted out at a scan speed of 100 mV s−1 on 0.1 M KOH and 0.5 M H2SO4 solution with O2 or N2 saturated, respectively. Linear sweep voltammograms (LSV) were measured with a sweep rate of 10 mV s-1 with O2 saturated in 0.1 M KOH, respectively. I-t curves for Fe-N-C-800 and 20 % Pt-C were obtained in the presence and absence of 2 M methanol in an O2-satured 0.1 M KOH solution at a constant potential of -0.3 V (vs Ag/AgCl). The electron transfer number (n) can be calculated from the following equation:46 n=4Id/ (Ir/N+Id)

(1)

Here, Id and Ir refer to the disk and ring currents, respectively, and N = 0.42 corresponds the ring collection efficiency. H2O2 production can be calculated from the following equation: % H2O2= 200(Ir/N)/(Id+Ir/N)

(2)

In evaluating the ORR performance, the working potentials versus Ag/AgCl in all experiments were referenced to the reversible hydrogen electrode (RHE) scale based on: ERHE = EAg/AgCl + 0.059 pH +0.197 V

3. Results and discussion The synthesized process and formation mechanism of the mesoporous Fe-N-C-X and Fe-N-C-X-acid catalysts is illustrated in Scheme 1A and Scheme 1B. Initially, as shown in Scheme 1A, the glycine, citric acid and FeCl3 were mixed with PVP under stirring for 20 min to form a homogeneous solution. After baking the above homogeneous solution in a furnace for 3 h at 80 °C to acquire a condensed solution, transferring them into a corundum boat and 6 / 26


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baking again in a furnace for 3 h at 150 °C to obtain a preliminary carbonized yellow-black sticky jelly. Finally, the yellow-black jelly was pyrolyzed at 700 °C, 800 °C and 900 °C in an N2 atmosphere to form Fe-N co-doped mesoporous carbon materials (Scheme 1A), and the samples were denoted as Fe-N-C-700, Fe-N-C-800, Fe-N-C-900, respectively. Furthermore, the Fe-N-C-700-acid, Fe-N-C-800-acid and Fe-N-C-900-acid catalysts were formed through an acid leaching process of the as-prepared Fe-N-C-700, Fe-N-C-800 and Fe-N-C-900 in 0.5 M H2SO4, and their proposed formation mechanism was depicted in Scheme 1B. For comparison, N-C-800-acid and N-C-900 were synthesized using the same procedure as the Fe-N-C-X material without the addition of Fe source.

Scheme 1. Schematic of (A) the synthetic route and (B) formation mechanism of Fe-N-C-X (X= 700, 800, 900) catalysts.

To test the ORR performance of the as-prepared catalysts, CV measurements of Fe-N-C-700,

Fe-N-C-800,

Fe-N-C-900,

N-C-900,

N-C-800-acid,

Fe-N-C-700-acid,

Fe-N-C-800-acid, and Fe-N-C-900-acid were carried out in O2 and N2 saturated 0.1 M KOH (Figure 1). The onset potential as a crucial factor in determining the electrocatalytic performance is referred to the potential at which the current density exceeds a threshold value of 0.1 mA cm-2.47 As displayed in Figure 1a, the ORR peak potentials of N-C-900, Fe-N-C-700, Fe-N-C-800 and Fe-N-C-900 were 0.61 V, 0.55 V, 0.58 V and 0.71 V, respectively, and the onset potentials corresponded to 0.76 V, 0.79 V, 0.8 V and 0.92 V, respectively. In Figure 1b, N-C-800-acid, Fe-N-C-700-acid, Fe-N-C-800-acid and Fe-N-C-900-acid corresponded to the ORR peak potentials of 0.59 V, 0.5 V, 0.72 V and 0.65 7 / 26



V and the onset potentials of 0.72 V, 0.70 V, 0.93 V and 0.88 V, respectively. The electrochemical data revealed that the Fe-N-C-800-acid and Fe-N-C-900 had the best electrocatalytic activities towards the ORR among two series Fe-N-C materials before and after acid treatment with respect to the onset potential and peak potential. Moreover, Figure 1c and Figure 1d further displayed the typical oxygen reduction curves for the Fe-N-C-800-acid and Fe-N-C-900 catalysts in 0.1 M O2 and N2-statured KOH electrolyte, which indicated the better performance of Fe-N-C-800-acid with larger current density, better positive peak potential and enhanced onset potential compared to the Fe-N-C-900 catalyst.

(a)

(b) -2

-2

2 mA cm

2 mA cm

Fe-N-C-900

Fe-N-C-900-acid

Fe-N-C-800 Fe-N-C-800-acid Fe-N-C-700

Fe-N-C-700-acid

N-C-900

0.0

0.3 0.6 0.9 E (V vs. RHE)

(c) 2

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Current Density (mA cm )

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Current Density (mA cm )

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0 -2 -4

N2-Sat. O2-Sat.

0.0

0.3

0.6 0.9 E (V vs. RHE)

1.2

0.3 0.6 0.9 E (V vs. RHE)

2

1.2

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0

-2 N2-Sat.

-4

O2-Sat.

0.0

0.3

0.6

0.9

1.2

E (V vs. RHE)

Figure 1. (a) CV curves for the ORR with the N-C-900, Fe-N-C-700, 800, and 900 catalysts, respectively, and (b) with N-C-800-acid, Fe-N-C-700, 800, 900-acid catalysts, respectively, in O2-saturated 0.1 M KOH at a speed of 100 mV s-1; (c) CV curves for the ORR of Fe-N-C-800-acid; and (d) Fe-N-C-900 in 0.1 M O2 and N2 saturated KOH electrolyte.

To investigate the kinetics of Fe-N-C-800-acid and Fe-N-C-900 catalyst, the benchmark 20 % Pt-C catalyst was compared using the CV and LSV measurements. As shown in Figure 2a, the ORR peak current densities of the Fe-N-C-800-acid, Fe-N-C-900 and 20 % Pt-C 8 / 26


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catalysts were 5.8 mA cm-2, 3.0 mA cm-2 and 6.8 mA cm-2, which indicated the close activity of Fe-N-C-800-acid to the 20 % Pt-C catalyst. In addition, LSV curves of the Fe-N-C-800-acid, Fe-N-C-900 and 20 % Pt-C catalysts were conducted in an O2-saturated 0.1 M KOH solution with a rotating speed ranging from 400 rpm to 2025 rpm at a scan speed of 10 mV s-1. As described in Figure 2b, the Fe-N-C-800-acid and Fe-N-C-900 also possessed excellent activities with onset potentials of 0.93 V and 0.92 V, and basically the same half-wave potentials of 0.75 V, respectively, whereas the onset potential and half-wave potential were observed of 1.02 V and 0.87 V for 20 % Pt-C. During a mixed kinetic- and diffusion-controlled zone, the current density at E=0.56 V (RHE) of these catalysts was determined as 4.3 mA cm-2, 2.7 mA cm-2 and 5.9 mA cm-2, respectively. Notably, the current density and the onset potential of Fe-N-C-800-acid catalyst were close to those of the 20 % Pt-C catalyst, indicating it may be a suitable substitute for noble-metal-based catalysts with respect of its high ORR performance. The ORR performance of Fe-N-C-800-acid and Fe-N-C-900 catalysts was also evaluated in 0.5 M H2SO4 (Figure S1), and they possessed undesirable activities with a poor selectivity to O2 owing to the different state of the active sites under acidic conditions48 and the easy accessibility to the reactant (O2, H+) as well as other intermediate products (i.e., H2O2).49 However, the optimal Fe-N-C-800-acid catalyst exhibited an excellent activity compared with the reported studies in alkaline media with respect to the catalyst loading, onset potential and current density (Table 1). Meanwhile, the electron transfer numbers (n) of the Fe-N-C-X/acid and Pt/C catalysts were calculated from the RRDE tests over a potential window between 0.46 V and 0.76 V. The current density of the as-prepared material for the ORR increased with ascending rotating speed owing to the decreased diffusion distance (Figure 2c-d, Figure S2). In Figure 2e, the average electron transfer number (n) of the Fe-N-C-900 and 20 % Pt-C catalysts was calculated as 3.70 and 3.93 at four potentials of 0.46, 0.56, 0.66, and 0.76 V, while that was 2.28 and 3.5 for Fe-N-C-800 and Fe-N-C-700. Accordingly, the Fe-N-C-700-acid, Fe-N-800-acid and Fe-N-C-900-acid catalysts had n values of 3.38, 3.50 and 3.45, respectively (Figure 2f). Based on the comparisons of the electron transfer number (n), it was demonstrated that the two best catalysts of Fe-N-C-800-acid and Fe-N-C-900 in each series underwent the desirable four-electron pathway. In addition, the H2O2 production yield of the 9 / 26



Fe-N-C-800-acid and Fe-N-C-900 samples together with 20 % Pt-C catalyst was also calculated in Figure S3a over the same potential window ranging from 0.46 V to 0.76 V. The H2O2 production of the Fe-N-C-800-acid and Fe-N-C-900 was 8.5 %-17.2 % and 4.1 %-10.0 %, respectively, which both indicate higher H2O2 yield than that of 20 % Pt-C catalyst (0.8 %-1.6 %). The H2O2 yield results are consistent with the calculated (n) of the as-prepared catalysts discussed above, and Fe-N-C-800-acid has a pronounced ORR activity compared to that of other Fe-N-C types catalysts with and without acid leaching. Table 1. Comparison of the ORR catalytic performances in terms of the onset potential, current density and catalyst loading for reported well-developed Fe-based electrocatalysts and our Fe-N-C-800-acid catalyst at 1600 rpm with a potential of 0.56 V vs RHE in 0.1 M KOH.

Onset

Half-wave

Current Density

Potential

Potential

(mA cm-2)

(V)

(E1/2)

E=0.56 V

Fe/N4/GN-2.7

0.95

0.85

3.5

0.33

Fe-N-C-HNS-750

0.89

0.72

4.8

0.255

0.91

0.76

4.2

0.3

Fe/Co-NPGr

0.93

0.81

3.8

2.5

Fe-N-CNF-S

0.94

0.80

5.0

0.6

Fe-Nx/C

0.9

0.79

4.8

0.24

Fe/N/C

0.94

0.74

4.0

0.6

Fe-N-C-800-acid

0.93

0.75

4.3

0.4

Catalyst

Fe/N/C/CNT@PC F

(a) -2

3 mA cm

Fe-N-C-800-acid

Loading References

(mg/cm2)

Nano Energy. 2017, 32, 353-358 Nanoscale. 2015,7, 1501-1509 J. Mater. Chem. A. 2015, 3, 9658-9667. Adv. Funct. Mater. 2016, 26, 2150-2162 Angew. Chem. Int. Ed. 2015, 54, 8179-8183 Chem. Comm. 2014, 50,14760-14762 J. Power Source. 2014, 271, 87-96. This work

(b) 0

-2

Fe-N-C-900

urrent Density (mA cm )

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-4

ERHE= 0.56 V 2.7 mA cm-2 4.3mA cm-2

20% ACSPt-C Paragon Plus Environment 5.9 mA cm-2

-6

Fe-N-C-900 Fe-N-C-800-acid 20% Pt-C

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Figure 2. (a) CV curves of Fe-N-C-800-acid and Fe-N-C-900 materials in O2-saturated 0.1 M KOH with a scan rate of 100 mV s-1; (b) current density of Fe-N-C-800-acid, Fe-N-C-900 and 20% Pt-C obtained from LSV curve (RRDE) at a rotating speed of 1600 rpm; (c) LSV curves with the RRDE for Fe-N-C-800-acid; (d) Fe-N-C-900 in O2-saturated KOH at 10 mV s−1 with rotation rates increasing from 400 to 2025 rpm; (e) the calculated electron transfer number (n) of Fe-N-C-700, 800, 900 together with 20% Pt-C; and (f) Fe-N-C-700, 800, 900-acid, together with 20 % Pt-C. The performance of these materials was also evaluated from their mechanistic and kinetic properties based on diffusion-corrected Tafel plots (η=a+b*logj, where b is the Tafel slope and j is current density). The Tafel curve of Fe-N-C-700, 800, and 900; Fe-N-C-700, 800, 900-acid and 20 % Pt-C are obtained from the LSV curves (RRDE) at a rotating speed 1600 11 / 26



rpm (Figure 3). As displayed in Figure 3, Fe-N-C-800-acid and Fe-N-C-900 had a lower Tafel slope of 110 mV dec-1 and 108 mV dec-1, separately, which were close to that of 20 % Pt-C catalyst (99 mV dec-1). However, other prepared materials, including Fe-N-C-700, Fe-N-C-800, and Fe-N-C-700-acid possessed higher Tafel slope over a range of 164-274 mV dec-1, which were inferior to that of the 20 % Pt-C catalyst. Generally, a lower Tafel slope is associated with a better ORR activity of catalyst. The lower slope of Fe-N-C-800-acid (110 mV dec-1) and Fe-N-C-900 (108 mV dec-1) catalysts may indicate a faster kinetic response when catalyzing the oxygen reduction reaction.

E (V vs. RHE)

0.4

Fe-N-C-800-acid Fe-N-C-900 20 % Pt-C Fe-N-C-700 Fe-N-C-700-acid Fe-N-C-800 Fe-N-C-900-acid

0.2 99

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dec -1 110 mv

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-1

ec

ec vd m

16 3

4 28

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108 m

vd ec

v dec -1

-1

-0.4 -2

-1 0 1 -2 Log (J/mA cm )

2

Figure 3. Tafel curve of Fe-N-C-700, 800, and 900; Fe-N-C-700, 800, 900-acid and 20 % Pt-C obtained from the LSV curves (RRDE) at a rotating speed 1600 rpm.

The stability and methanol resistance ability of the relative better catalysts of Fe-N-C-800-acid, Fe-N-C-900 materials and commercial 20 % Pt-C catalysts were evaluated and compared using chronoamperometric measurements at a potential of 0.66 V (vs RHE) in an O2-saturated 0.1 M KOH electrolyte. As shown in Figure S4a, after 10000 s of operation, the current of the 20 % Pt-C catalyst decreased sharply by 40 % of its initial current, suggesting its poor stability, and Fe-N-C-900 had a similar decrease of 38 %. Among these three catalysts, the Fe-N-C-800-acid retained pronounced stability with only a 23 % decrease of current. The results of methanol resistance test of the above catalysts are displayed in Figure S4b. After the addition of 2 M methanol, the 20 % Pt-C catalyst exhibited poor resistance and an instability to methanol with an initial current decrease by 50 %, while a 12 / 26


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decrease in the current of 25 % and 20 % was seen for Fe-N-C-800-acid and Fe-N-C-900 materials, suggesting a better methanol resistance ability of the two catalysts than that of the 20 % Pt-C catalyst. In addition, the effect of SCN- ions on the ORR activity of the Fe-N-C-800-acid and Fe-N-C-900 catalysts was evaluated in 0.5 M H2SO4. As shown in Figure S4c-d, the kinetic current density at 0.75 V decreased by 114 mV and 80 mV for the Fe-N-C-800-acid and Fe-N-C-900 after the introduction of 5 mM NaSCN. This result clearly indicates that the active sites of Fe-N-C-800-acid and Fe-N-C-900 should contain Fe because SCN- ions have a high affinity to the Fe ions.50 The microstructural morphology of the as-prepared catalysts was investigated in SEM images. As shown in Figure 4a-f, all the catalysts revealed a porous structure. Figure 4g-j and Figure S5 described the uniform distribution of the C, N and Fe elements in the synthesized Fe-N-C-800-acid and Fe-N-C-900 materials, respectively, which also validated the successful doping of Fe in those materials. The HRTEM images of Fe-N-C-800-acid validated the formation of small and well-dispersed Fe3C particles, which were nearly spherical with average diameters of approximately 50 nm (Figure 4k). The nanoparticles with similar darkness were inlaid in the nano-capsule structure of the carbon layers with inner diameters of 40-100 nm and a thickness of 5-20 nm (Figure 4k). The Fe3C particles were tightly encircled by the walls of the carbon nano-capsules composed of some amorphous graphite and well-ordered graphitic sheets with a spacing of approximately 0.34 nm (Figure 4k,l) in accordance with the (002) plane of the graphitic carbon lattice.51 In addition, a lattice spacing of 0.247 nm matches well with the (111) plane of Fe3C,40 demonstrating that Fe-N-C-800-acid nanocomposite could be synthesized by an inexpensive precursor through a three-step in situ carbon thermal-reduction method. The content percentage of the C, N and Fe elements in these Fe-N-C-X (X= 700, 800, 900) materials and acid-treated Fe-N-C-X (X= 700, 800, 900) materials are listed in Table S1. The carbon content increased with ascending temperatures, whereas the nitrogen content revealed an irregular trend with percentage in 900 °C (1.4 %) being higher than that at 800 °C (0.9 %), which may arise from the double carbonization process at 150 °C and 700 °C under different environments. Additionally, the iron content in these catalysts indicated a reversible trend like that of carbon owing to the decomposition of the inorganic iron salt at higher temperatures. The TGA measurement was 13 / 26



further performed to detect the basic content of iron in Fe-N-C-800-acid and Fe-N-C-900 at 800 °C in an air flow with a heating speed of 10 °C min-1. The TGA curve (Figure S6) showed that the residual Fe2O3 amount was 1.15 %, and 13.2 % for Fe-N-C-800-acid and Fe-N-C-900, respectively. Hence, the Fe content was approximately calculated to be 0.9 % and 10.3 % in the Fe-N-C-800-acid and Fe-N-C-900, which shows some variation with the EDX results. In contrast, the carbon and nitrogen percentage of prepared catalysts increased after acid treatment, while the N/C ratio and iron percentage decreased, with an especially sharp decrease in the iron content.

Figure 4. (a-c) SEM images of Fe-N-C-700, 800, and 900, and (d-f) Fe-N-C-700, 800, and 900-acid materials; (h-j) mapping distribution of the C, N, and Fe elements for Fe-N-C-800-acid; and (k-l) HRTEM image of Fe-N-C-800-acid. To investigate structural influence on the ORR performance of as-prepared catalysts, XRD patterns were obtained. Apparently, in Figure 5a, a peak located at a 2θ of 25.5 ° corresponds well to the graphite 2h (002) plane in the Fe-N-C-X (X=700, 800, 900) materials (PDF: 89-7213). Another peak centered at 45.1 ° corresponds to the Fe3C (031) (PDF:89-7213) species, which also indicated the successful Fe doping in Fe-N-C catalysts. However, the intensity of the Fe3C in Fe-N-C-700 material is weak owing to a larger 14 / 26


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inorganic iron species content compared with that in the other materials, i.e., Fe-N-C-800 or 900. The order of the Fe3C peak intensity is as follows: Fe-N-C-900> Fe-N-C-800> Fe-N-C-700, and the difference between the Fe3C intensities is inconsistent with the results of ORR activity in Figure 1 because the higher content of Fe3C active site is favorable for improving the catalytic activity.52 However, in Figure 5b, the post-acid-treated materials of Fe-N-C-X (X=700, 800, 900) also revealed two peaks at 25.5° and 43.2° assigned to the (002) graphite plane and the Fe3C (111) plane, respectively with different Fe3C peak intensities. In contrast to the materials before acid treatment, the acid-treated materials (Fe-N-C-X-acid) indicated smoother and more ordered curves, showing effective removal of crystal impurities in those materials.

Figure 5. (a) XRD survey of Fe-N-C-X and (b) Fe-N-C-X-acid catalysts; (c) Raman spectra of Fe-N-C-X and Fe-N-C-X-acid; and (d) nitrogen adsorption/desorption isotherms of Fe-N-C-800, Fe-N-C-900, Fe-N-C-800-acid and Fe-N-C-900-acid, respectively.

Raman was carried out to analyze the carbon graphitization and defect degree of sp2 carbon materials. As shown in Figure 5c, all the synthesized materials showed two peaks at 1328 cm-1 and 1578 cm-1 associated with the D band and G band, respectively. The content of 15 / 26



defects in the materials is evaluated by comparing the relative strength of these two bands (ID/IG ratio).36 The ID/IG of Fe-N-C-700 and Fe-N-C-700-acid are not calculated because of their disordered peaks. Interestingly, the Fe-N-C-800-acid (ID/IG =1.16) and Fe-N-C-900-acid (ID/IG =1.11) possessed more defects than Fe-N-C-800 (ID/IG =1.04) and Fe-N-C-900 (ID/IG =1.07), which may be attributed to the removal of inactive iron from the carbon atom surface in the Fe-N-C-X catalysts. However, the Fe-N-C-900-acid sample revealed a slight increase in ID/IG after acid treatment, and the ORR activity decreased, as displayed in Figure 1a and Figure 1b, which could be explained by the loss of some active Fe (Fe3C) sites during acid treatment. The surface area difference of these Fe-N-C-X (X= 700, 800, 900) materials with and without acid treatment were studied in Figure 5d. All the prepared catalysts showed a type-IV pattern with an H4 hysteresis loop in the nitrogen adsorption/desorption isotherm curves, which validates the simultaneous formation of mesopores.37 The surface areas of Fe-N-C-800 and Fe-N-C-900 were determined to be 187.0 m² g-1 and 197.0 m² g-1, respectively. However, after acid treatment on the as-prepared catalyst, the surface areas of Fe-N-C-800-acid and Fe-N-C-900-acid increased to 196.8 m² g-1 and 263.8 m² g-1, demonstrating the great contribution of acid leaching to increasing the surface area. As seen, the pore-size distribution of Fe-N-C-800 before and after acid treatment shifted from 4.05 nm to 3.97 nm owing to the removal of inactive Fe species (Figure S7). Meanwhile, Fe-N-C-900 almost maintained its pore size at a narrower distribution of 3.96 nm, resulting from the higher graphitic carbon encapsulated with some Fe coordinates (Figure 4k-l), and thus, the pore size distribution was nearly unchanged. From these results and the electrochemical activity of the as-prepared material for oxygen reduction, it may be inferred that the larger surface area of the as-prepared optimal catalysts may accelerate the adsorption of O2 molecules on the electrode surface. To probe the chemical composition of the prepared material, XPS was performed, as shown in Figure 6a-d, and Figure S8-S10. Apparently, C 1s, N 1s, O 1s, and Fe 2p characteristic peaks can be monitored in Fe-N-C-900 (Figure 6a), and the Fe 2p peaks of Fe-N-C-800-acid and Fe-N-C-900 were both weak in the full XPS spectra, which can be explained from the HRTEM image of Fe-N-C-800-acid, as seen in Figure 4k-l, the Fe3C 16 / 26


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particles tightly encircled with well-ordered graphitic carbon and amorphous carbon. The existing carbon around the Fe3C may suppress the contribution to the Fe signal from Fe-N-C-X thus resulting in a negligible Fe 2p signal.45

Figure 6. (a) The full XPS survey; (b) N 1s spectra of Fe-N-C-800-acid and (c) Fe-N-C-900; (d) the percentage of N 1s types in the Fe-N-C-800-acid and Fe-N-C-900; (e) Fe 2p spectra of Fe-N-C-800-acid and Fe-N-C-900 material; and (f) the N/C ratio of the as-prepared materials with respect to the ORR activity. ORR activity was represented by the current density obtained from peak potential in Figure 1a-b, of which the ORR activity of Fe-N-C-800-acid was normalized to 1.

Furthermore, the deconvoluted N 1s peaks in Fe-N-C-800-acid and Fe-N-C-900 (Figure 6b-c), show four well-resolved peaks assigned to graphitic N (400.98 ± 0.3 eV), oxidized N 17 / 26



(403.46 ± 1.7 eV), pyrrolic N (400.47 ± 0.2 eV), and pyridinic N (398.12 ± 0.4 eV) species.53 The variation in the location on N 1s of the as-prepared material may be attributed to the difference in chemical environment. Additionally, the percentage of the different nitrogen types in the Fe-N-C-800-acid and Fe-N-C-900 were evaluated in Figure 6d, clearly indicating the relatively higher percentage of pyridinic N (40.5 %) in Fe-N-C-800-acid compared to that of Fe-N-C-900 material. Although the specific role of each nitrogen species is still under investigation, pyridinic N and graphitic N are broadly considered to play key roles in catalytic activities. Remarkably, pyridinic N improves the onset potential for ORR, while the graphitic N is correlated to the diffusion-limited current density.28 Both oxidized N and pyrrolic N are basically regarded as having little influence on the ORR kinetic activity. This may be explained by the enhanced ORR performance of Fe-N-C-800-acid over that of Fe-N-C-900 material. Interestingly, the characteristic peaks of Fe 2p (Fe 2p 3/2 at 710.0 eV and Fe 2p1/2 at 724.3 eV) appeared clearly in Fe-N-C-900 material compared with Fe-N-C-800-acid material owing to the relative higher Fe content (Figure 6e). As far as we know, the N/C ratio is significant in determining the ORR activity for the Fe and N co-doped non-precious metal catalysts, owing to the total electronegativity (δ) difference generated by nitrogen with respect to carbon (δN= 3.04 and δC = 2.55) in the carbon matrix.53 In our study, we found that the as-prepared Fe-N-C-700 and Fe-N-C-700-acid catalysts with higher N/C ratios demonstrated relatively worse performances compared with other catalysts, such as Fe-N-C-800-acid and Fe-N-C-900 with relative lower N/C ratios. The N/C ratios of the as-prepared materials are displayed in Figure 6f, showing that Fe-N-C-700-acid had the highest N/C ratio (8.0 %) and Fe-N-C-900-acid had the lowest (0.4 %); and the Fe-N-C-700 corresponds the ratio of 7.4 %, 1.3 % of Fe-N-C-800, 1.0 % of Fe-N-C-800-acid and 0.6 % of Fe-N-C-900, respectively. Furthermore, the N/C ratio associated with the ORR activity was also investigated in Figure 6f, and the Fe-N-C-900-acid with the lowest N/C ratio showed a better activity than that of the Fe-N-C-700 (7.4 %) and Fe-N-C-700-acid (8.0 %) catalysts. However, the Fe-N-C-800-acid catalyst with a 1.0 % N/C ratio had the best ORR activity, while the Fe-N-C-900 with an N/C ratio of 0.6 % had a large current density of approximately 70 % than that of the Fe-N-C-800-acid catalyst. Taken together, the analysis of the relationship between the N/C ratio and the ORR activity revealed 18 / 26


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that the optimal N/C ratio in Fe-N-C materials is 1 %. Fe3C and pyridine N in the Fe-N-C-800-acid and Fe-N-C-900 catalysts were considered to be the active sites for enhancing the ORR activity. Importantly, by comparing the ORR properties of the six as-prepared catalysts, it is proposed that a moderate N/C ratio, such as 1 % in the Fe-N-C-800-acid catalyst in this study, is pivotal for acquiring a desirable ORR activity.

4. Conclusions Fe-N co-doped mesoporous carbon materials were prepared by a heat-treatment approach using inexpensive precursors (glycine, PVP, citric acid and FeCl3) and followed by an acid leaching technique. Through testing of the three Fe-N-C-X (X=700, 800, 900) materials and three acid-treated materials, i.e., Fe-N-C-X-acid, it was found that Fe-N-C-800-acid and Fe-N-C-900 both exhibited the best ORR performance due to their similar onset and half-wave potentials (0.92-0.93 V, 0.75 V vs RHE), electron transfer numbers, current density and resistance to methanol. Specifically, the Fe-N-C-800-acid catalyst had a higher catalytic ORR current density (4.3 mA cm-2) compared to that of the Fe-N-C-900 catalyst (2.7 mA cm-2) at a potential of 0.56 V (vs RHE). However, the average electron transfer number (3.7) and the H2O2 production yield (4.1 %-10 %) of Fe-N-C-900 is slightly better than those of Fe-N-C-800-acid (3.5, 8.5 %-17.2 %). Furthermore, acid leaching is shown to be favorable for the removal of inactive iron complexes and helps to expose the active sites, including Fe3C and pyridinic N. This study indicates that the higher content of Fe in these catalysts does not play a key role in the ORR activity, but a moderate N/C ratio, i.e., 1 % in Fe-N-C-800-acid catalyst is pivotal for achieving the desired ORR activity.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.xxxx. Other data for characterizations and catalytic activity measurements, SEM image, elemental mapping distribution image, TGA curves, nitrogen adsorption-desorption isotherms, the corresponding pore-size distribution, full XPS spectra, N 1s spectra and Fe 2p spectra of as-prepared catalysts (PDF). 19 / 26



Author Contributions §

These authors equally contributed to this work. All authors have given approval to the final

version of the manuscript.

ORCID Yuqing Lin: 0000-0003-1501-5005

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation (21575090), Beijing Municipal Natural Science Foundation (2162009), Scientific Research Project of Beijing Educational Committee (KM201810028008), and Youth Innovative Research Team of Capital Normal University.

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For TOC Only Incorporation of Fe3C and Pyridinic N Active Sites with a Moderate N/C Ratio in Fe-N Mesoporous Carbon Materials for Enhanced Oxygen Reduction Reaction Activity Changqing Li§, Chuansheng He§, Fengzhan Sun, Manchao Wang, Jiahui Wang, Yuqing Lin * Department of Chemistry, Capital Normal University, Beijing 100048, China

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Incorporation of Fe3C and Pyridinic N Active Sites ... - ACS Publications

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