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Nanostructured Cementite/Ferrous Sulfide Encapsulated Carbon with Heteroatoms for Oxygen Reduction in Alkaline Environment Hsin-Chih Huang, Cheng-Yi Su, Kai-Chin Wang, Hsueh-Yu Chen, YuChung Chang, Yen-Lin Chen, Kevin C.-W. Wu, and Chen-Hao Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05033 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Nanostructured

Cementite/Ferrous

Sulfide

Encapsulated Carbon with Heteroatoms for Oxygen Reduction in Alkaline Environment Hsin-Chih Huang,† Cheng-Yi Su, † Kai-Chin Wang,† Hsueh-Yu Chen, † Yu-Chung Chang, † Yen-Lin Chen, ‡ Kevin C.-W. Wu,*, ‡ and Chen-Hao Wang*,† †Department

of Materials Science and Engineering, National Taiwan University of

Science and Technology, No.43, Keelung Rd., Sec.4, Da'an Dist., Taipei 10607, Taiwan *Email: [email protected] (C. H. Wang) ‡ Department

of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt

Rd., Taipei 10617, Taiwan *E-mail: [email protected] (Prof. Kevin C.-W. Wu) KEYWORDS: Oxygen reduction reaction, Non-precious metal catalysis, Fuel cells, Prussian blue, PEDOT

ACS Paragon Plus Environment

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ABSTRACT: Non-precious metal catalysts of the oxygen reduction reaction (ORR) are highly preferred in anion exchange membrane fuel cells (AEMFCs) because of the high stability and low cost. Here, a novel pyrolyzed poly(3,4-ethylene dioxythiophene) hydrate (PEDOT)-Prussian blue (PB) catalyst (FeCN-S) for the ORR in an AEMFC cathode demonstrated high catalytic performance. After pyrolysis at 800 °C, the templated PBPEDOT formed the nanostructured Fe3C/FeS encapsulated carbon with heteroatom contribution, which exhibited optimized ORR activity with a direct four-electron transfer pathway for AEMFC applications. This improvement in activity is attributed to the specific structure, the heteroatom contribution, and the coordination structure.


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INTRODUCTION Hydrogen energy is a major green alternative for decreasing the reliance on petroleum. A fuel cell is one of the environmental technologies which uses hydrogen energy and produces pure water. Anion exchange membrane fuel cell (AEMFC) efficiently converts chemical energy into electrical energy and is a preferred system because it does not require the use of a platinum catalyst in the electrode. The application contains not only the green energy but also the environment-friendly which is correlated to sustainability. However, AEMFC operation is limited by the slow oxygen reduction reaction (ORR) at the cathode,1 a problem that is conventionally overcome by using a large number of platinum catalysts. However, platinum utilization is limited by its low natural abundance and high cost.2 Moreover, in an alkaline environment, OH species adsorb on the platinum surface and block the active sites, leading to low electrochemical activity. Hence, replacing platinum with an efficient, stable, and abundant material is one of the current major research goals in the field of fuel cell development. Non-precious metal catalysts, such as transition metal nitrogen-containing complexes, conductive polymer-based catalysts, transition-metal chalcogenides, metal oxides, metal carbides, metal nitrides, and enzymatic compounds, are potential materials for cathode applications.3 Among these compounds, iron- and cobalt-based compounds with heteroatoms possess excellent ORR activity.4-6 In a fundamental study, Mukerjee et al. examined the mechanisms of non-precious metal catalysts in acid and alkaline media and


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demonstrated that non-precious metal catalysts facilitated ORR in alkaline media,7 improved the activity of the anion intermediate, and enabled the complete transfer of the four-electron reduction pathway. This evidence demonstrates the feasibility of using nonprecious metal catalysts for ORR in alkaline media. Prussian blue (PB) is a nitrogen-containing transition metal compound, having a large surface area, high-density Fe-N4 coordination structure, and low cost of procurement and can be used for ORR. In 1984, Itaya et al. demonstrated that the PB crystal is containing zeolitic in nature and that its metal centers exhibit the catalytic ORR activity.8 Sawai et al. used cobalt sulfate and ferricyanides to form transition metal hexacyanometallate precursors and then heated with carbon black to facilitate ORR.9-10 The heat-treated catalysts exhibited high activity in acid and alkaline solutions on the addition of methanol, making it suitable for air cathode applications. Barman et al. used PB as a precursor to synthesize Fe/Fe3C encapsulated nitrogen-doped graphitic nanostructures and enabled bi-functionality of ORR and oxygen evolution reaction (OER). After post-treatment through acid leaching, the stability of the catalysts was comparable with that of traditional catalysts.11 Liu et al. anchored PB nanoparticles on graphene oxide (GO) through the hydrothermal method.12 The obtained three-dimensional (3D) Fe/Fe3C nitrogen-doped graphene aerogel structure exhibited improved ORR activity and stability in acid and alkaline media, enabling the formation of various metal-organic frameworks-based structures.


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In addition to nitrogen, electronegative atoms such as sulfur and phosphor are other options for the development of compounds. Poly(3,4-ethylene dioxythiophene) hydrate (PEDOT) is a conductive polymer, and its sulfur-containing structure, specific morphology, and conductive matrix are beneficial as a potential electrocatalyst. Chowdhury et al. synthesized CoMn2O4-PEDOT nanocomposites as bi-functional catalysts for ORR and OER.13 Because of the synergistic chemical coupling effect, the catalysts in CoMn2O4PEDOT exhibited enhanced catalytic activity and stability. Zhang et al. mixed PEDOT:PSS with reduced GO and prepared ORR catalysts through the solution-processed method. After acid-leaching post-treatment, it exhibited improved activity and higher stability compared with platinum catalysts in alkaline media.14 Guo et al. fabricated hemin-doped PEDOT with 3D nanostructures that exhibited ideal ORR activity and stability in neutral media.15 PEDOT nanostructures possess a large surface area and can be used as an ORR catalyst over a wide pH range. These experimental and theoretical results evidence that PEDOT is a potential candidate for catalyzing ORR. Singh et al. used density functional theory to understand the ORR mechanism and the catalytic property of PEDOT.16 The nitrogen-containing transition metal compound and conductive polymer are used to fabricate non-precious metal catalysts for investigating ORR. Wang et al. used Prussian blue analogue (PBA) and polyaniline (PANI) to prepare platinum-free ORR catalysts.17 During pyrolysis and acid leaching, the structure exhibited uniform dispersion and nanoporous formation, leading to superior catalytic activity in alkaline and acid media


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compared with commercial platinum catalysts. During the process, cobalt nitrate and potassium ferricyanate were used to prepare PBA. After mixing with PANI, the PANI/PBA composite was pyrolyzed at 1000 °C; X-ray diffraction (XRD) data revealed that the product contains FeCo alloy. The composite was then treated through acid leaching to remove most of the FeCo alloy and thus create a nanoporous structure. The PBA functioned as a template for the pore formation, and PANI contributed to nitrogen doping and conductivity. This work elucidates the synthesis of pyrolyzed PB-PEDOT (FeCN-S) for ORR. At a specific temperature, the pyrolyzed FeCN-S catalyst formed the nanostructured Fe3C/FeS encapsulated carbon with heteroatom contribution, accelerating the ORR with efficient catalytic activity through the direct four-electron reduction pathway. Thus, the pyrolyzed FeCN-S catalyst is a good candidate to replace platinum catalysts.


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EXPERIMENTAL SECTION Preparations of pyrolyzed FeCN-S Potassium ferricyanide is used as the precursor in PB synthesis, and polyvinylpyrrolidone (PVP) is used as the capping and a reducing agent. The formula of PB is Fe4III[FeII(CN)6]3, one of the mixed-valence transition metal hexacyanometalates which are used in the manuscript.18 PEDOT is used not only as the sulfur source of heteroatom contribution but also as the nanostructured template. A 3 mg of PVP, 135 mg of K3[Fe(CN)6], and 25 mg of PEDOT were dissolved in 200 mL of 0.1 M HCl accompanied by stirring for 30 min at room temperature. The mixture was heated to 80 °C for 20 h in the oven. The particles were collected through centrifugation and washed several times with deionized water and ethanol. Pyrolyzed FeCN-S was prepared at temperatures of 600 °C, 800 °C, and 1000 °C. The resultant catalysts are labeled FeCN-S-600, FeCN-S-800, and FeCN-S-1000, respectively. The pyrolyzed pristine PB at 800 °C is labeled FeCN-800 for the comparison of ORR activity. In the pyrolysis process, the sample was loaded into the aluminum oxide boat leading into a furnace; the temperature was ramped at 4 °C per min in a nitrogen environment for 2 h, following which the furnace was allowed to cool to room temperature naturally. Material Analysis


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In thermogravimetric analysis (TGA, TA Instrument Q500), the temperature was increased from room temperature at a rate of 5 °C per min to 800 °C under constant N2 flow. High-resolution scanning electron microscopy (HRSEM, JEOL-6500F) was adopted to determine the microstructural surface morphologies. A field-emission transmission electron microscope (FEGTEM, FEI Tecnai™ G2 F-20 S-TWIN) was used to capture the images for the elemental analysis of the catalysts. XRD (Bruker D2) data were obtained using Cu Kα1 radiations (λ =1.54056 Å). Xray photoelectron spectroscopy (XPS) data were obtained at beamline 24A1 from the National Synchrotron Radiation Research Center (NSRRC). The photoemission spectra were collected using an analyzer (SPECS PHOIBOS 150). The X-ray absorption spectrum (XAS) at the Fe Kedge was obtained at beamline 17C1 (NSRRC). All spectra were received at room temperature and a transmission mode was used to record it. The XAS data were processed in the IFEFFIT software package. Electrochemical Measurements Electrochemical measurements used the potentiostat/galvanostat instrument’s threecompartment approach (Biologic VSP Bi-stat). A rotating-ring disk electrode (RRDE, PINE AFE6R2GCAU) with a disk made of glassy carbon (GC) and a ring made of gold was used as the working electrode. The counter electrode was Pt foil, and the reference electrode was saturated calomel (0.242 V vs. NHE). All potentials were referenced to the reversible hydrogen electrode (RHE). The ORR was tested in oxygen-saturated 0.1 M KOH electrolyte. Catalyst ink was prepared by mixing 12 mg of catalyst, 3 mL deionized water, and 2 mL isopropanol. A solution of 20 μL of the ink and 5 μL of 0.1 wt.% Nafion was dropped onto


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the disk electrode, which was then left to dry in air at ambient temperature. The ORR curves were scanned at a rate of 10 mV per second and at 1600 rpm rotation. The low scan rate reduced the non-Faradic current produced by the catalysts. The ring used the applied potential of 1.1 V vs. RHE to oxidize the hydrogen peroxide ion, which was generated by the catalysts on the disk during ORR. The yield of hydrogen peroxide ions was thus determined. Fuel Cell Test A membrane-electrode-assembly with an area of 5 cm2 was created using two electrodes on both sides of a Fumapem® FAA-3 (OH-, FuMA-Tech GmbH). In the preparation of the specific electrodes, the catalysts were dispersed in 10 wt.% Fumion FAA-3 solution as the catalyst ink. This ink was hand-painted onto the carbon cloth, with a FeCN-S-800 loading of 3.0 mg cm-2. The cathode was then kept at room temperature in 1 M KOH for 12 hours. Moreover, the commercial Pt/C (John Matthey) with a loading of 0.8 mg cm-2 was used as the catalyst for anode and comparison electrodes. A polarization experiment was conducted on the AEMFC at 50 °C, using hydrogen and oxygen through the anode and the cathode, respectively, without the back pressure. The AEMFC performance was measured using a fuel-cell test station (Tension Energy, Inc.) by recording the cell voltage and current when the cell had reached steady values. RESULTS AND DISCUSSION


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The ORR pathway in an alkaline environment mostly involves the following reactions: Reaction 1: 𝑂2 +2𝐻2𝑂 + 4𝑒 ― →4𝑂𝐻 ― …𝐸0 = 0.401 𝑉

(1)

Reaction 2: 𝑂2 + 𝐻2𝑂 + 2𝑒 ― →𝑂𝐻 ― +𝐻𝑂2― …𝐸0 = ―0.065 𝑉

(2)

Reaction 3: 𝐻𝑂2― + 𝐻2𝑂 + 2𝑒 ― →3𝑂𝐻 ― …𝐸0 = 0.867 𝑉

(3)

Reaction 4: 2𝐻𝑂2― →2𝑂𝐻 ― + 𝑂2

(4)

The direct pathway of a four-electron reduction belongs to reaction 1, and the twoelectron reduction pathway belongs to reaction 2. Because the direct ORR pathway generates a higher thermodynamically reversible potential than does the indirect ORR pathway, Reaction 1 is preferred over Reaction 2 as the ORR for the AEMFC application. When the PB was pyrolyzed at a specific temperature, new active sites were created because the structure was destroyed partially or completely. To realize the destruction of the structure after pyrolysis, Figure 1 depicts the TGA of the pristine PB. The decomposition of pristine PB was divided into three regions: region I below 50 °C is the loss of H2O, and region II between 50 and 200 °C as well as region III between 275 and 450 °C are the release of (CN)2; then the curve is relatively stable after 350 °C.19 The remaining weight was 46.5% of the initial weight at 800°C. Studies of transition metal complexes have confirmed that the metal-containing nitrogen chelates are bound to


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other atoms, and the M-Nx-Cy structure is formed like a poly-aromatic hydrocarbons structure, enhancing the electrical conductivity.20-23

Figure 1. TGA curve of pristine PB. To clarify the effect of the pyrolysis temperature, PB-PEDOT was pyrolyzed at temperatures of 600 °C, 800 °C, and 1000 °C. The resultant catalysts were FeCN-S-600, FeCN-S-800, and FeCN-S-1000, respectively. Figure 2a presents the ORR activities of FeCN-S-600, FeCN-S-800, and FeCN-S-1000. In Figure 2a, the lower part shows disk current (Id), and the upper part shows the ring current (Ir) as a function of applied potential. After pyrolysis at 800 °C, the catalyst presented the absolute values of highest Id and lowest Ir. The electron-transfer number (n) and the yield of hydrogen peroxide ion (%HO2-) for ORR were calculated using the following equations, where N is the collection efficiency of the RRDE, and the value was determined to 0.368.


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𝑛=

4𝐼𝑑

Page 12 of 42

(5)

𝐼𝑟

𝐼𝑑 + 𝑁

2𝐼𝑟

%𝐻𝑂2― =

𝑁 𝐼𝑟

(6)

× 100%

𝐼𝑑 + 𝑁

Figures 2b and 2c display the n values and %HO2-, respectively. The highest n and the lowest %HO2- are observed from the pyrolysis at 800 °C. Figure S1 displays the Tafel plots of FeCN-S-600, FeCN-S-800, and FeCN-S-1000 and assist in determining the performances at high and low potentials, especially in the kinetic region. FeCN-S-800 exhibits the highest reaction rate as the Tafel slopes of FeCN-S-800 and FeCN-S-1000 at the low overpotential region are 84 and 125 mV dec-1 in an alkaline environment.


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Figure 2. (a) ORR curves for FeCN-S catalysts at different temperatures, (b) the n values, and (c) %HO2- of the catalysts against disk potentials. The ORR behaviors of pristine PB, pristine PB-PEDOT, FeCN-800, and FeCN-S-800 were demonstrated using the RRDE method. Figure S2a presents the RRDE curves of pristine PB, pristine PB-PEDOT, FeCN-800, and FeCN-S-800. The typical ORR curve has three dominant regions of potential: the kinetic range above 0.8 V, the mixed range between 0.8 and 0.6 V, and the mass-transfer range below 0.6 V. The Id curve of FeCN-S-800 is clearly higher than the pristine PB, pristine PB-PEDOT, and FeCN-800 in all dominant regions, representing the highest ORR activity of FeCN-S-800. Figures S2b and S2c plot the n values and %HO2- in the ORR against the applied the potential of the disk, respectively. The n values for the pristine PB, pristine PB-PEDOT and FeCN-800 are observed between 3.60 and 3.80 at a large over-potential of 0.3 V. For FeCN-S-800, the n value and %HO2- are almost constant, which are estimated to be 3.99 and below 0.5%, respectively, exhibiting that the ORR of FeCN-S-800 progresses mostly through the direct reduction pathway. The direct reduction pathway confirms that PEDOT improves not only the conductivity but also the ORR activity. To compare the ORR activities of non-precious metal catalysts in alkaline medium, the ORR performances of PB-based and non-precious metal catalysts reported in the papers are shown in Table 1. Compared with other stateof-the-art non-precious metal catalysts, our catalyst demonstrated competitive ORR activity. The ORR activity can be influenced by various effects, including transition metal


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nitrogen-containing compounds, the heteroatom content, the types of the transition metal, the structure of the catalyst, the surface properties of the support material, etc. Among these effects, the ligand-metal interaction has an important effect on ORR activity. This improvement in activity is attributed to not only the type and content of heteroatom atoms but also the specific structure and the coordination structure according to the following analyses.


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Table 1. The ORR performances of Prussian blue-based and non-precious catalysts reported in the papers. Electrolyte: 0.1 M KOH. Catalyst

Onset potential

Electron transfer number

Tafel slope

Half-wave Reference potential

~4.00

78.0 mV dec-1 0.82 V

12

Prussian blue-based catalysts Fe/Fe3C@N-rGO

0.95 V vs. RHE

NIC

-0.20 V vs. Ag/AgCl 3.70

--

--

24

C-2PANI/PBA

--

--

0.85 V

17

N-HG

-0.05 V vs. Ag/AgCl 3.82

--

--

11

3.75-3.98

N-doped 1.00 V vs. RHE Fe/Fe3C@C/RGO

3.08-3.52

72.0 mV dec-1 0.93 V

25

Non-precious metal catalysts Co1-xS/SNG/CF

0.99 V vs. RHE

3.93-3.98

85.3 mV dec-1 --

26

Ionogel-Fe-C-N

1.00 V vs. RHE

3.99

74.0 mV dec-1 0.87 V

27

(Co@NPC-900)12

1.05 V vs. RHE

3.96

62.8 mV dec-1 0.82 V

28

Fe-Cu-N/C

--

3.98

--

0.89 V

29

CoO@NS-CSs

0.95 V vs. RHE

>3.90

60.0 mV dec-1 0.82 V

30

Fe(1)-Cu(3)-N-C1000

--

3.85-4.00

--

31

Fe-N/C-900

0.04 V vs. Ag/AgCl

3.94

63.0 mV dec-1 -0.07 V vs. Ag/AgCl

32

FeNC-20-1000

1.04 V vs. RHE

3.99

49.0 mV dec-1 0.77 V

33

S-Fe/N/C

0.91 V vs. RHE

3.95-4.00

--

0.80 V

34

FeCN-S-800

0.91 V vs. RHE

3.99

84 mV dec-1

0.76 V

This work

0.85 V


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The ORR stability of FeCN-S-800 in an alkaline environment could be determined through the potential cycling method, which is typically applied to measure the stability of catalysts. The ORR curves of FeCN-S-800 in the alkaline environment after 10000, 20000, and 30000 cycles with potential cycling between 0.6 and 1.0 V are shown in Figure 3. FeCN-S-800 demonstrates good stability of 44 mV decay at half-wave potential after 30000 cycles.

Figure 3. ORR stability test of FeCN-S-800 by the potential-cycling method. To compare the performance of pyrolyzed FeCN-S catalyst with commercial Pt/C catalyst in the single cell, Figure 4a shows the polarization plots of the AEMFCs using Pt/C and FeCN-S-800 in the cathodes, which produces a maximum power density of 142 mW


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and 125 mW cm-2, respectively. Additionally, the durability test of FeCN-S-800 in the cathodes of the AEMFCs through hydrogen and oxygen flowed into the anode and the cathode, respectively, is demonstrated in Figure 4b. At 0.5 V, the performance of FeCNS-800 remains constant 87% of initial value, which is much better compared with Pt/C. Because the anion-exchange membrane strongly influences cell performance,35-38 the effect of the electrolytic membrane in the fuel cell warrants further examination.

Figure 4. (a) Polarization curve and (b) durability test of the H2-O2 AEMFC using Pt/C and FeCN-S-800 as the cathodes. Operation temperature: 50 °C; electrolyte: Fumapem® FAA-3 (OH-, FuMA-Tech GmbH). PB is one of the coordination polymers, which possess not only the uniform pore shape and size but also the functionalities.24 Figure 5a-e show HRSEM images of pristine PB, pristine PBPEDOT, FeCN-S-600, FeCN-S-800, and FeCN-S-1000. Pristine PB particle demonstrated cubic morphology in the range 50-150 nm, and it showed uniform coverage of the PEDOT nanotube. The morphology of PB-PEDOT did not undergo a considerable change at 600


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°C. At 800 °C, PB particle formed pore due to the removal of organics,39 exhibiting a perceptible cubic porous structure. However, the morphology was destroyed and aggregated at 1000 °C, thus lowering the catalytic behavior, besides the HRSEM images in large scale are shown in Figure S3. Table S1 presents the (Brunauer-Emmett-Teller) BET surface areas for FeCN-S-600, FeCN-S-800, and FeCN-S-1000. The BET surface areas of catalysts increased with the temperature, which altered the surface characteristics and catalytic activities.

Figure 5. HRSEM images of (a) pristine PB, (b) pristine PB-PEDOT, (c) FeCN-S-600, (d) FeCN-S-800, and (e) FeCN-S-1000. Figure 6a presents the TEM image of pristine PB-PEDOT, and the image presents a uniform coverage of PB on PEDOT. Figure 6b shows the HRTEM image of pristine PBPEDOT, and the d-spacing is determined by the lattice fringes, and the image is enlarged


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in Figure S4. The d-spacing of the cubic particle was 5.06 Å, which was associated with the (200) plane of crystalline PB, respectively. TEM-energy dispersive X-ray spectroscopy (EDS) mapping is used to realize the elemental distributions, as shown in Figure 6c-f. The mapping displays the corresponding images of iron, carbon, nitrogen, and sulfur, which demonstrate that the atoms are uniformly distributed in the catalyst.

Figure 6. TEM images of pristine PB-PEDOT catalyst showing (a) the bright-view image, (b) lattice fringes marked by d-spacing distance, (c) the corresponding EDS mapping of iron, (d) the corresponding EDS mapping of carbon, (e) the corresponding EDS mapping of nitrogen, and (f) the corresponding EDS mapping of sulfur.


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Figure 7a shows the TEM image of FeCN-S-800. The d-spacing of the catalyst is judged from HETEM images of the lattice fringes to identify the compounds, as shown in Figure 7b and 7c. The d-spacing of the particles was 2.01, and 2.08 Å, which are associated with the (031) plane of crystalline Fe3C and (114) plane of crystalline FeS, respectively. Figure S5 shows the enlarged lattice fringes of Figure 7b and 7c. Therefore, the FeCN-S-800 contains the structures of Fe3C and FeS. The iron compounds were encapsulated in the carbon shell, enhancing corrosion resistance.22, 40-42 Tylus et al. indicated that the metal and metal oxide nanoparticles could be protected by surrounding graphene-like layers, which enhanced the ORR ability.43 To understand the distributions of every element throughout the catalyst, Figure 7c-f present a series of TEM-EDS mapping images. The mapping presents the corresponding images of iron, carbon, nitrogen, and sulfur. Although the sulfur-mapping image of FeCN-S-800 is intense at the edge, the images still tend to uniform distribution in general, displaying effective utilization of the active sites.


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Figure 7. TEM images of FeCN-S-800 catalyst showing (a) the bright-view image, (b) lattice fringes marked by d-spacing distance of Fe3C, (c) lattice fringes marked by dspacing distance of FeS, (d) the corresponding EDS mapping of iron, (e) the corresponding EDS mapping of carbon, (f) the corresponding EDS mapping of nitrogen, and (g) the corresponding EDS mapping of sulfur.


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Figure 8 presents the XRD patterns of pristine PB, pristine PB-PEDOT, FeCN-S-600, FeCN-S-800, FeCN-S-1000, and the reference of PB (JCPDS, PDF#521907), Fe3C (JCPDS, PDF#892867), and FeS (JCPDS, PDF#801032). The characteristic peaks of pristine PB and pristine PB-PEDOT correspond to PB. The FeCN-S-600 has FeS peaks, which are the derivatives from the reaction of the PB and the PEDOT. At 800 °C and 1000 °C of pyrolysis temperatures, not only FeS peak but also Fe3C peak was formed in the XRD patterns, which is consistent with the TEM results of FeCN-S-800. However, the FeCN-S-800 has higher relative content of FeS than the FeCN-S-1000. Carbon could reduce some iron ions to a metallic state, and with part of the metal transferred to iron compounds, the ORR activity was obviously affected by the composition and type of iron compound.44-46 Studies of FeS-related structures have demonstrated improved ORR activity, electrical conductivity, and consumption of O2.47-49 From the XRD patterns, there was no Fe metal appeared, PB-PEDOT forms FeS and Fe3C crystalline structures after pyrolysis which was consistent with the TEM results. However, FeCN-S catalyst exhibited the phenomenon of aggregation at a much higher pyrolysis temperature of 1000 °C from HRSEM image which blocked the active sites and reduces the ORR ability.50-52


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Figure 8. XRD patterns of pristine PB, pristine PB-PEDOT, pyrolyzed FeCN-S at various temperatures. The reference pattern of Prussian blue (JCPDS, PDF# 521907), Fe3C Cohenite (JCPDS, PDF# 892867), and FeS (JCPDS, PDF# 801032) are shown below. According to the TEM-EDS mapping, the heteroatoms of nitrogen and sulfur uniformly dispersed on the catalyst. Moreover, nitrogen-doped carbon catalysts discovered that the nitrogen structures or incorporations contributed ORR catalytic activity. To understand the effect of nitrogen structure, the N1s XPS of pristine PB-PEDOT, FeCN-S-600, FeCN-S800, and FeCN-S-1000 are shown in Figures 9a-d, respectively. Table 2 shows the


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corresponding fitting results. Pristine PB-PEDOT demonstrates peaks at 398.5 and 400.3 eV, which are pyridinic-type nitrogen and pyrrolic-type nitrogen, respectively. After the pyrolysis, part of the structure was transferred to cyanide and quaternary-type nitrogen at 399.4 and 401.4 eV, respectively.53-55 Quaternary-type nitrogen increased the limiting current density; it was proposed that this type of nitrogen structure reduces the carbon band gap energy, which actually influenced the ORR ability.56-59 While pyridinic-type nitrogen converted the ORR to the ideal process of four-electron, improving the onset potential, it claimed that the nitrogen structure modified the band structure of carbon, and raised the density of π states near the Fermi level and reduced the work function.6062

FeCN-S-800 has much higher pyridinic-type and quaternary-type nitrogen content.

Additionally, only FeCN-S-1000 displays the cyan groups content compared to others. The CN- ions coordinate with transition metals, blocking the pathway of ORR. Li et al. discussed the CN- poisoning effect on the ORR activity, and the degree of ORR activity loss to CN- blocking was dependent on the metal level in the materials.63 Table S2 presents the elemental content for the FeCN-S-800 according to the XPS fitting results for checking the elemental compositions. The amount of sulfur and iron are too less in the XPS results, causing the hard of the fitting. Due to the XPS is a surface-sensitive analysis, once the sulfur and iron are covered by other materials, the detection will become difficult. Therefore, X-ray absorption spectra are used to analyze the existence of Fe-N and Fe-S. With the highintensity X-ray from the National Synchrotron Radiation Research Center, it can get a much clear understanding of the whole material.


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Figure 9. XPS showing the N1s spectra of (a) pristine PB-PEDOT, (b) FeCN-S-600, (c) FeCN-S-800, and (d) FeCN-S-1000. Table 2. The XPS fitting results of N1s spectra in Figure 8. N1s

Quaternary-type nitrogen

Pyrrolic-type nitrogen

(401.4 eV)

(400.3 eV)

--

33.6 %

--

66.4 %

FeCN-S-600

29.6 %

53.0 %

--

17.4 %

FeCN-S-800

36.1 %

26.1 %

--

37.8 %

FeCN-S-1000

60.2 %

--

39.8 %

--

(atomic %) Pristine

Cyanide (399.4 eV)

Pyridinic-type nitrogen (398.5 eV)

PB-PEDOT


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Extended X-ray absorption fine structure (EXAFS) spectrum of FeCN-S catalyst under different pyrolyzed temperatures is shown in Figure 10. Here, the atomic distance of FeN and that of Fe-C were very close and hard to separate. Therefore, Fe-N/Fe-C was marked for the peak. It indicated that Fe-N/Fe-C, Fe-S, and Fe-Fe are at 1.8 Å, 2.3 Å and 2.7 Å, respectively.64-68 Pristine PB-PEDOT shows the obvious atomic distance of Fe-N/Fe-C, which comes from the PB. During pyrolysis, the atomic distance of Fe-S is considerably noticeable in the structure. At the pyrolysis temperature of 800 °C and 1000 °C, both the Fe-N and Fe-S are dominant and FeCN-S-800 has a higher ratio of Fe-N than that of FeCN-S-1000. Studies of metal-nitrogen moieties for the ORR have concluded that the Fe-N-C structure might be the most effective improvement on the ORR activity.69-71 The covalent incorporation of Fe-Nx sites into π-conjugated carbon basal plane modified the electron donating/withdrawing capability of the carbonaceous ligand and improved the ORR activity.43 This improvement represents that the enhanced ORR activity is attributed to the co-incorporation nitrogen and sulfur types, and nitrogen effect effectively plays a vital role in catalytic ability.


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Figure 10. The EXAFS spectra of Fe K-edge in pristine PB-PEDOT and FeCN-S catalysts with various pyrolyzed temperature from 600 oC to 1000 oC. Based on the data with the hypothetical interpretations, the pyrolyzed FeCN-S catalyst formed the nanostructured Fe3C/FeS encapsulated carbon with not only nitrogen but also sulfur. This formation contributed to the ORR abilities including onset potential and limiting output current. Furthermore, with the effect of specific structures and heteroatoms, the catalyst potentially contributes remarkable catalytic activity in fuel cell applications.


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CONCLUSION Nanostructured Fe3C/FeS encapsulated carbon with heteroatom contribution was formed by the pyrolysis of PB-PEDOT at 800 °C, illustrating high potential ORR activity for AEMFC application. The RRDE technique demonstrated that the catalyst revealed an almost ideal pathway of direct four-electron reduction. The properties of the specific structure, heteroatom contribution and coordination structure from the catalyst resulted in the increase of ORR activity. The ideal to modify the FeCN-S structure is believed to be the potential candidate of the non-precious metal catalyst for AEMFC application.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Electrochemical Tafel plots, ORR results, HESEM images in large-scale, TEM images in enlarged lattice fringe, BET surface area and the elemental content from XPS fitting results. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. H. Wang) *E-mail: [email protected] (Prof. Kevin C.-W. Wu) Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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The author would like to thank financial support from the Ministry of Science and Technology of Taiwan (MOST 104-2628-E-011-003-MY3). The authors acknowledge the National Synchrotron Radiation Research Center (Beamline 17C1 and 24A1), Hsinchu, Taiwan for XAS and XPS analysis facility.


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A pyrolyzed poly(3,4-ethylene dioxythiophene) hydrate (PEDOT)-Prussian blue (PB) catalyst (FeCN-S) is applied in the clean and eco-friendly fuel cell.


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Ferrous Sulfide ... - ACS Publications

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