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Nitrogen-doped porous carbon nanosheets derived from coal tar pitch as an efficient oxygen-reduction catalyst Dai Yu, Lihui Zhou, Jing Tang, Jinxia Li, Jun Hu, Changjun Peng, and Honglai Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01941 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 23, 2017

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Industrial & Engineering Chemistry Research

Nitrogen-doped porous carbon nanosheets derived from coal tar pitch as an efficient oxygen-reduction catalyst

Dai Yu, Lihui Zhou,* Jing Tang, Jinxia Li, Jun Hu, Changjun Peng, Honglai Liu

School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China

*Corresponding author: Dr. Lihui Zhou, Tel: 0086-21-64253847; Fax: 0086-21-64252947; E-mail: [email protected]

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ABSTRACT High value-added processing and manufacturing of coal tar pitch (CTP), an abundant and cheap by-product of the coal industry, is a crucial issue worldwide. Herein, novel nitrogen-doped porous carbon nanosheets (N-PCN) were fabricated by selecting CTP and ferric chloride (FeCl3) as the carbonaceous source and mild activating agent respectively. Due to the synergistic effects of the unique flaky graphitic structure, hierarchical porosity, high nitrogen doping contents and the trace of Fe-containing sites, N-PCN exhibited excellent oxygen-reduction (ORR) electrocatalytic activity in alkaline medium. Linear sweep voltammetry (LSV) testing results for N-PCN showed that the diffusion-limited current density was as high as 5.8 mA cm-2 and the half-wave potential was 0.85 V (vs. RHE), both better than commercial 20 wt% Pt/C. Moreover, N-PCN showed an ideal methanol tolerance and catalytic stability, indicating its potential application as cathode catalysts in direct methanol fuel cells (DMFCs).

Keywords: Coal tar pitch, Ferric chloride, Nitrogen-doped porous carbon nanosheets, Oxygen reduction reaction


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1. Introduction Growing demand for clean and sustainable energy has motivated researchers to exploit alternative energy conversion and storage systems with attractive performance, low cost, and environmental benefits.1-3 The oxygen reduction reaction (ORR) is an indispensable cathodic reaction in these renewable energy devices, including fuel cells1 and metal-air batteries,4,5 but its sluggish kinetics makes it essential to design and construct highly active and durable catalysts for the ORR. Platinum (Pt) has long been regarded as the most efficient ORR catalyst to date.6 However, the prohibitive price,7 low natural abundance, suspect stability and poor tolerance against fuel crossover/CO

poisoning

of

Pt

have

severely

hindered

the

large

scale

commercialization of Pt-based catalysts.6 Therefore, non-precious metal catalysts (NPMCs) with comparable catalytic activities to replace the expensive Pt-based catalysts for the ORR are desperately needed.8,9 Heteroatom-doped (e.g., nitrogen, phosphorus, sulfur, etc.) carbons have emerged as promising NPMCs candidates, due to their high ORR activity, stability and low cost.10,11 Among these, nitrogen-doped carbons have gained particular interest owing to their unique advantages, such as high conductivity, chemical inertness, and unlimited availability of various carbonaceous source materials.11 To achieve excellent electrocatalytic ORR performance, extensive efforts have been devoted to exploring nitrogen-doped carbon nanomaterials with a large surface area/stable structure, certain nitrogen-doping contents and good electrical conductivity.8,12 You et al.10 fabricated highly porous N self-doped carbon catalyst derived from polyacrylonitrile with ZnO as the templates/pore former, 3 ACS Paragon Plus Environment


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leading to a positive half-wave potential of 0.859 V (vs. RHE) towards ORR in an alkaline medium. Yu et al.13 synthesized nitrogen-doped carbon nanosheets using g-C3N4 as both the template and nitrogen source, achieving comparable ORR activity to a commercial Pt/C electrocatalyst. In recent years, various carbonaceous sources, such as honeysuckles,14 natural spider silk,15 saccharin,16 sargassum tenerrimum,17 soy milk,18 coprinus comatus19 and filter paper20 have been exploited for fabricating highly efficient carbon-based ORR catalysts. As an abundant by-product of the coking process of the coal industry, coal tar pitch (CTP) is a well-known fantastic carbonaceous precursor for carbon materials due to its high carbon content and low price.21 Unfortunately, most of CTP have been remained wasted. Containing many polycyclic aromatic hydrocarbon molecules with sp2-hybridized carbon atoms, CTP can be tapped to produce graphitic porous carbon.21 Moreover, it has been observed that incorporation of nitrogen into pitch enhances the electrochemical performance in supercapacitors and lithium-ion batteries.22-24 However, few studies have focused on the synthesis of nitrogen-doped CTP-based carbon materials for the efficient ORR catalysts. Herein, we report a facile synthesis of novel nitrogen-doped porous carbon nanosheets (N-PCN) with graphite-like flaky structure and rich micropores/mesopores by selecting CTP as a raw material and FeCl3 as a mild activating agent. The preparation process was displayed in Figure 1. The as-fabricated material was used as ORR catalyst and exhibited superior electrochemical activity, methanol tolerance, and comparable stability to a commercial Pt/C catalyst. 4 ACS Paragon Plus Environment

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Figure 1. Schematic illustration of the synthesis process of N-PCN.

2. Experimental Section 2.1. Materials All chemicals were used as received. CTP (softening point: 105 oC) was obtained from Jining Lu coal-chemical Co., Ltd. HCl (38 wt%), HNO3 (68 wt%), H2SO4 (98 wt%) and n-hexane were purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. FeCl3 was provided by Sinopharm Chemical Reagent Co., Ltd. Commercial 20 wt% Pt/C (Hispec 3000) was provided by Johnson Matthey Corp.

2.2. Preparation of catalysts 5 ACS Paragon Plus Environment


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Pretreatment pitch (PP) was prepared by extracting CTP with n-hexane and oxidizing the extracts with mixed acid. The CTP with a softening point of approximately 378 K was provided by Jining Lu coal-chemical Co., Ltd. Specifically, CTP was pulverized and sifted through a 200-mesh sieve, and then n-hexane was selected to extract CTP at 105 °C in a Soxhlet extractor for 24 h.25 Subsequently, 1 g extracts were oxidized in a 20 mL mixture of concentrated nitric acid and sulfuric acid (v/v=1:4) for 40 min at room temperature. After the reaction, the suspension was poured into 500 mL deionized water and then filtrated. The filtration residue was washed with deionized water and dried at 80 °C to obtain PP. N-doped porous carbon nanosheets (N-PCN) were prepared as follows: FeCl3 (1.2 g) as activating agent was uniformly dispersed in 10 mL ethanol by vigorous stirring. PP (0.3 g) was dissolved in the above suspension and the mixture solution was evaporated into a black viscous liquid with stirring at 80 °C. Then, the sample was further dried in a vacuum oven at 80 °C overnight. The obtained product was pyrolyzed at 900 °C in a tube furnace for 2 h at a heating rate of 5 °C min-1 in a flowing nitrogen atmosphere. After cooling down to room temperature, the pyrolytic product was immersed in a 2 M HCl solution at 80 °C for 6 h with vigorous stirring to etch away any Fe species, followed by filtering and washing with deionized water to remove the impurities thoroughly until the filtrate pH reached neutral, and then dried at 100 °C for 12 h. Finally, the acid-leached product was pyrolytically treated again at 900 °C in a N2 atmosphere for 2 h to obtain N-doped porous carbon nanosheets (N-PCN). 6 ACS Paragon Plus Environment

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For comparison, a controlled sample N-doped carbon (denoted as N-C) was prepared without FeCl3 via the similar route.

2.3. Preparation of working electrodes A glassy carbon electrode (GCE, 5.0 mm in diameter, Pine Instruments, USA) was used as the working electrode substrate. The catalyst inks were fabricated by dispersing 5.0 mg of corresponding carbon catalysts into a mixture of 666 µL ultrapure water (18.2 MΩ cm), 333 µL iso-propyl alcohol, and 30 µL of 5 wt% Nafion aqueous solution (Alfa Aesar) as a binder, followed by an ultra-sonication treatment for 40 min. Then, 13 µL of the above suspension was dropped on the polished GCE surface and dried at room temperature, leading to a catalyst loading of 0.3 mg cm-2. For comparison, the commercial 20 wt% Pt/C (Johnson Matthey Corp.) catalyst ink was also fabricated according to the same procedure.

2.4. Characterization The crystalline structures of the samples were identified by X-ray diffraction (XRD) patterns performed on a D/max 2550 diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 100 mA. The morphology and structure of the samples were characterized by scanning (SEM, HITACHI S-3400N, Japan) and transmission electron microscopy (TEM, JEOL JEM-2100, Japan). Raman spectra were obtained on a laser Raman microscope (Iuvia reflerx, Renishaw). Fe content was detected by inductively coupled plasma atomic emission spectrometry (ICP-AES, 7 ACS Paragon Plus Environment


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Agilent 725ES). X-ray photoelectron spectroscopy (XPS) measurements were conducted on the ESCALAB 250Xi spectrometer (Thermo Scientific) with Al Kα X-ray source for excitation. The surface area and porosity of the samples were measured by nitrogen adsorption/desorption at 77 K using ASAP-2020 instrument (Micromeritics, USA). Elemental analysis (EA) was carried out by an elemental analyzer (Vario EL III, Germany).

2.5. Electrochemical measurements All of the electrochemical analyses were performed at room temperature using rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques on a Parstat 4000 (Princeton Applied Research) electrochemical workstation combined with a conventional three-electrode cell, that is, the catalyst modified GCE as the working electrode, an Ag/AgCl, KCl (3 M) electrode as the reference electrode and a Pt wire as the counter electrode. All potentials in this work were referenced to the reversible hydrogen electrode (RHE) potentials using the Nernst equation: ERHE = EAg/AgCl + 0.967 V. The ORR performance of N-PCN and N-C electrocatalysts was measured by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). By bubbling O2 or N2 for 30 min, the electrolyte was saturated with oxygen or nitrogen before the measurements. CV curves were recorded in a N2- and O2-saturated 0.1 M KOH solution between 0 and 1.15 V (vs. RHE) with a scan rate of 50 mV s-1, and LSV measurements were


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conducted in an O2-saturated 0.1 M KOH solution from 1.15 V to 0 V (vs. RHE) at a scan rate of 5 mV s-1 and a rotation rate of 1600 rpm. For the RRDE tests, the catalyst inks and electrodes were fabricated following the same procedure. In an O2-saturated 0.1 M KOH solution, the disk potential was scanned at 5 mV s-1 and the ring potential was set to 1.4 V vs. RHE at a rotation rate of 1600 rpm. The following equations (eqs 1 and 2) were used to calculate the electron transfer number (n) and the H2O2 yield:26,27 n=

4I D ID + (IR / N )

%H 2 O 2 = 100

(1)

2IR / N ID + (IR / N )

(2)

where ID and IR refer to the faradaic currents at the disk and ring electrodes, respectively, and N is the ring current collection efficiency with a value of 0.22 provided by the manufacturer. The stability of the catalysts was evaluated by means of the accelerated durability test protocol of the US Department of the Energy by cycling the catalysts between 0.6 and 1.0 V (vs. RHE) at 50 mV s-1 under O2 atmosphere in 0.1 M KOH.16 By CV with or without methanol, the methanol tolerance was assessed.

3. Results and Discussion 3.1. Structural properties



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The CTP was first pretreated by hexane to eliminate light aliphatic fractions, which allows the extracts with aromatic structure to be easily graphitized.25 Such aromatic precursors are believed to facilitate the incorporation of nitrogen-containing active sites into the graphitized carbon matrix in the presence of iron during the heat treatment.9 Then, mixed acid was chosen to oxidize the extracts to introduce oxygen-containing groups, which may act as ‘active sites’ for accelerating the disintegration of aromatic structures during activation.28 More importantly, the treatment of carbon precursors with mixed acid proved to be a workable way to introduce nitrogen groups into the carbon matrix. In spite of the common use as an activating agent, hydroxide, especially KOH, is extremely corrosive and toxic. The activation with FeCl3 as a mild activating agent, alternatively, can produce plentiful small micropores,29 facilitating the mass transport during the ORR process. From the typical SEM image of N-C (Figure 2a), block solid particles with macropores are observed. However, after activated by FeCl3, N-PCN becomes fine powder (Figure 2b). As it can be seen from the amplified SEM images (Figures 2c and 2d), the aggregates of N-PCN possess graphite-like lamellar structure and the flakes are stacked together. Furthermore, TEM images of the samples provide us similar results that N-C (Figures 3a and 3b) exhibits thick block structure but N-PCN (Figures 3c and 3d) appears to be thin, large and semitransparent nanosheets with various nanopores on the surface. The enormous structure difference between N-C and N-PCN materials should be attributed to the catalytic activation of FeCl3. It is reported that FeCl3 is easily codiffused with the carbon of PP to form a densely 10 ACS Paragon Plus Environment

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complanate Fe3C, which can be transformed into α-Fe completely at the higher temperature.30 The formation of α-Fe is observed in the XRD pattern of the product before acid leaching (Figure S1), and such self-generating Fe template can induce the growth of a dense 2D carbon atoms layer (nanosheets) during the carbonization. It is seen that there are two broad diffraction peaks at around 2θ = 26.1° and 43.2° in the XRD patterns of N-PCN and N-C (Figure 4a), ascribing to (002) and (100) diffractions of graphitic carbon.31 N-PCN with lower ratio of the D band to G band intensity (ID/IG) in the Raman spectra (Figure 4b) is more graphitic than N-C. Such special structure endows N-PCN with large surface area, active sites accessibility, high electron transfer rate and efficient mass transport.32-34



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Figure 2. (a) SEM image of N-C, SEM images of N-PCN at low magnification (b) and at high

(

(

magnification from both cross-sectional view (c) and plane view (d).

Figure 3. TEM images of N-C (a-b) and N-PCN (c-d).

(

( The

porosity

of

N-C

and

N-PCN

was

investigated

by

nitrogen

adsorption/desorption experiments at 77 K. The N2 sorption isotherms and the pore size distributions (PSD) curves calculated using nonlocal density functional theory (NLDFT) are presented in Figures 4c and 4d.


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Figure 4. XRD patterns (a), Raman spectra (b), N2 adsorption/desorption isotherms (c) and NLDFT pore size distributions (d) of N-PCN and N-C.

The sorption isotherms (Figure 4c) of the samples are both characteristic of type-I according to the International Union of Pure and Applied Chemistry (IUPAC) classification. The steep increase of N2 uptake at very low relative pressures suggests the existence of abundant micropores in both samples. This result is further confirmed by the PSD curves (Figure 4d) which reveal that both samples consist of micropores and mesopores, and N-PCN owns much more micropores than N-C. Table 1 summarizes the textural properties, showing that N-PCN possesses much higher Brunauer-Emmett-Teller (BET) surface area of 1985.1 m2 g-1 and larger pore volume of 1.07 cm3 g-1 than N-C (882.6 m2 g-1 and 0.51 cm3 g-1, respectively). Obviously,



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FeCl3 plays an important role in enhancing the activation and promoting the formation of numerous pores for N-PCN,35 which facilitate the mass transport to and from the electrocatalyst.13,36

Table 1. Physical-chemical properties of the N-C and N-PCN. Sample

SBETa (m2 g-1)

Vpb (cm3 g-1)

Vmic (cm3 g-1)

Vmesod (cm3 g-1)

Smic (m2 g-1)

Ce (wt%)

He (wt%)

Ne (wt%)

CTP

-f

-f

-f

-f

-f

92.3

4.4

0.9

PP

16.5

0.09

-f

0.09

-f

49.5

1.9

9.1

N-C

882.6

0.51

0.26

0.25

663.7

73.8

2.4

2.6

N-PCN

1985.1

1.07

0.56

0.51

1361.7

84.4

1.2

3.5

a

Surface area calculated by the BET method at P/P0 = 0.005-0.05.

b

Total pore volume at P/P0 = 0.99.

c

Micropore volume and micropore surface area obtained by the t-plot method applied to the N2

adsorption branch. d

Mesopore volume determined by the difference between pore volume (Vp) and micropore

volume (Vmicro). e

Elemental analysis results of all samples.

f

No porosity.

The EA and XPS results respectively indicate the element contents and the chemical bonds between the elements. Table 1 gives the N contents of N-PCN and N-C determined by EA, which are both higher than that of CTP (0.9 wt%), showing the successful incorporation of nitrogen into both samples. The N content of N-PCN (3.5 wt%) is higher than that of N-C (2.6 wt%), by reason that transition metals 14 ACS Paragon Plus Environment

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catalyze the graphitization process of nitrogen/carbon precursors to form nanocarbon with more favorable nitrogen doping.9 The XPS results in Figure 5a exhibit the peaks at 284.8 eV, 400.1 eV and 531.8 eV corresponding to the peaks of C1s, N1s and O1s, respectively. The surface atomic percentage of N in N-PCN is about 4.1 at.%, which again proves that N has been introduced into the carbon framework. The only possible source of nitrogen is believed to be the nitro group of PP (9.1 wt%, Table 1) derived from HNO3/H2SO4 mixed acid treatment.37 The bonding configurations are further evaluated by high resolution XPS. The high resolution C1s spectrum (Figure 5b) of the N-PCN exhibits an asymmetric peak centered at 284.8 eV with a tail at high binding energy, indicating the presence of C atoms bonding to N or O heteroatoms. The C1s spectrum could be deconvoluted into five peaks located at 284.7 eV, 286.0 eV, 287.4 eV, 289.3 eV and 291.3 eV, which can be assigned to sp2-C aromatic carbon, sp3 C-C bonds, nitrogen incorporation or C-O bonds, C=O bonds and aromatic C structure (partly from the C=O or adsorbed oxygen components), respectively.38 In the high resolution N1s spectrum of N-PCN (Figure 5c), five N species are doped, namely pyridinic-N (398.3 eV), pyrrolic-N (399.8 eV), graphitic-N (401.0 eV), pyridine-N-oxide (403.2 eV) and the chemisorbed nitrogen oxide species (405.8 eV), respectively,38,39 while there are only pyridinic-N (398.3 eV) and graphitic-N (400.9 eV) in the high resolution N1s spectrum of N-C (Figure 5d).39,40 It is noteworthy that pyridinic-N and graphitic-N are regarded as the ORR catalytic activity contributors.8 In addition, the result of ICP-AES shows N-PCN contains an



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extremely low content of Fe (0.26 wt%, Table S1), which can also contribute to the ORR activity.8

Figure 5. (a) XPS survey of the N-PCN, the corresponding high resolution C1s (b) and N1s (c) of the N-PCN. (d) The high-resolution N1s XPS spectrum of the N-C.

3.2. Electrocatalytic activity measurements The ORR catalytic activity was first examined by cyclic voltammetry experiments performed under N2 or O2 atmospheres in 0.1 M KOH electrolyte with a scan rate of 50 mV s-1. As shown in Figure 6a, featureless voltammetric curves are observed in the N2-saturated solution for both materials. However, when the 16 ACS Paragon Plus Environment

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electrolyte is saturated with O2, a well-defined cathodic peak at approximately 0.78 V vs. RHE is observed for N-PCN, which is indicative of its enhanced electrocatalytic activity toward the ORR in contrast to N-C without obvious peak current. To gain an in-depth understanding of the ORR process on N-PCN catalyst, RDE measurements were carried out in the O2-saturated 0.1 M KOH electrolyte at 1600 rpm in comparison with N-C and commercial Pt/C catalysts (Figure 6b). The linear sweep voltammograms (LSVs) show that N-PCN demonstrates higher activity than N-C in terms of current density and half-wave potential (E1/2). This may be mainly due to the larger surface area and relatively higher N doping contents of N-PCN than N-C as well as the trace of Fe-containing sites, providing more exposed active sites toward the ORR. Remarkably, N-PCN exhibits a higher diffusion-limited current density than that of commercial Pt/C (i.e. 5.8 vs. 5.6 mA cm-2). In addition, the ORR half-wave potential of N-PCN is located at 0.85 V (vs. RHE), which is more positive than that of Pt/C catalyst (~ 0.83 V vs. RHE). This positive half-wave potential shift (20 mV) of N-PCN in comparison with Pt/C catalyst is also superior or comparable to that of the other reported N-doped carbon ORR catalysts (Table S2).



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Figure 6. (a) CV curves of N-C and N-PCN in N2- (dash line) and O2- (solid line) saturated 0.1 M KOH aqueous solution at a scan rate of 50 mV s-1. (b) RDE voltammograms in an O2-saturated 0.1 M KOH solution at room temperature (rotation speed: 1600 rpm, scan rate: 5 mV s−1) for N-C, N-PCN, and commercial Pt/C. (c) RRDE voltammograms in an O2-saturated 0.1 M KOH solution at room temperature (rotation speed: 1600 rpm, scan rate: 5 mV s−1) for N-PCN. (d) Electron transfer number and hydrogen peroxide yield obtained from the RRDE curves for N-PCN.

RRDE system, which is able to determine the amount of H2O2 produced during the ORR, is used to further get insight into the reaction pathway of the ORR occurring on N-PCN. The RRDE results shown in Figure 6c reveal that ring currents are an order of magnitude lower than disk currents, indicating the ideal four-electron


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selectivity of N-PCN catalyst. Figure 6d illustrates the peroxide yields and electron transfer number derived from the RRDE results. It is observed that N-PCN exhibits the peroxide yields below 10% and the electron transfer number above 3.8 throughout the potential window from 0.27 V to 0.85 V (vs. RHE), verifying that the ORR mainly proceeds via the four-electron pathway on N-PCN. Such striking ORR performance of N-PCN should be attributed to the synergistic effects of large surface area, high nitrogen doping contents and Fe-containing sites as well as good conductivity. Durability and methanol tolerance play an important role in the practical use of an ORR catalyst. The stability of N-PCN can be evaluated by recording the linear sweep voltammograms (LSVs) between 3000 CV cycles shown in Figure 7a and compared with the result of the commercial Pt/C catalyst (Figure 7b). After 3000 continuous cycles, the half-wave potential of the N-PCN shifted negatively by only 10 mV, quite comparable to that of the Pt/C catalyst (9 mV), confirming the excellent stability of N-PCN. For direct methanol fuel cells (DMFCs), methanol crossover, which means methanol may permeate from the anode to the cathode, will happen. Therefore, it is crucial to check its electrocatalytic selectivity towards ORR against methanol electrooxidation in DMFCs, i.e. methanol tolerance. As revealed by the CV experiments in the O2-saturated 0.1 M KOH solution with or without 0.5 M methanol, N-PCN maintained its outstanding ORR electrocatalytic performance when methanol was introduced (Figure 7c), indicating good methanol tolerance of N-PCN. However,



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obvious anodic peaks corresponding to methanol oxidation were observed for the Pt/C catalyst (Figure 7d), exposing its poor methanol tolerance.

Figure 7. RDE voltammograms of N-PCN (a) and commercial Pt/C (b) before and after 3,000 cycles between 0.6 V–1.0 V (vs. RHE) at 50 mV s−1 in an O2-saturated 0.1 M KOH solution; Cyclic voltammograms of N-PCN (c) and commercial Pt/C (d) in an O2-saturated 0.1 M KOH solution at room temperature with and without 0.5 M methanol.


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4. Conclusions In summary, N doped porous carbon nanosheets (N-PCN) have been successfully prepared by a scalable and low-cost approach using coal tar pitch as a carbonaceous source and FeCl3 as activating agent. N-PCN displays a flaky structure with rich micropores/mesopores and has a high specific surface area of 1985.1 m2 g-1, which facilitate the efficient mass transport during the ORR process. In addition, the synergistic effects of high nitrogen contents, especially pyridinic-N and graphitic-N and Fe-containing sites as catalytically active sites boost its ORR performance with a positive half-wave potential of 0.85 V (vs. RHE) via a four-electron pathway. Moreover, the N-PCN catalyst exhibits superior methanol tolerance and comparable ORR catalytic stability to the commercial Pt/C catalyst, which makes it a promising candidate for the cathode catalysts in the applications of fuel cells and metal-air batteries.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Project No. 21376074 and 21676080).

Supporting Information XRD patterns, a table of chemical compositions and a table of ORR performance comparison between N-PCN and other carbon catalysts.



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