Nitrogen and Phosphorus Codoped Mesoporous Carbon Derived from

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Nitrogen and phosphorus codoped mesoporous carbon derived from polypyrrole as superior metal-free electrocatalyst towards the oxygen reduction reaction Zhengping Zhang, Junting Sun, Meiling Dou, Jing Ji, and Feng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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ACS Applied Materials & Interfaces

Nitrogen and Phosphorus Codoped Mesoporous Carbon Derived from Polypyrrole as Superior MetalFree Electrocatalyst towards the Oxygen Reduction Reaction

Zhengping Zhang, Junting Sun, Meiling Dou, Jing Ji, Feng Wang* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China.

KEYWORDS: nitrogen and phosphorus codoped, mesoporous carbon, carbon-based materials, metal-free, electrocatalysts, oxygen reduction reaction

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Abstract

To replace high-cost platinum group metal (PGM) electrocatalysts for oxygen reduction reaction (ORR) that is the crucial cathode reaction in fuel cell technology and metal-air battery, the development of low-cost and high-efficiency non-PGM catalysts for ORR has attracted much attention during the past decades. As one of the promising candidates, the N-doped carbon is highly desirable for its strong designability and outstanding catalytic activity and stability. In this work, a facile and rational strategy is demonstrated for preparation of N, P-codoped mesoporous carbon (N,P-MC) for ORR by the direct pyrolysis treatment of polypyrrole with phytic acid as Pdopant and polystyrene sphere as template. The resultant N,P-MC exhibits a mesoporous structure with the optimized ORR active sites originating from the N, P codoping. Owing to these features, N,P-MC exhibits excellent ORR activity, remarkable electrochemical stability and superior methanol tolerance, comparable or even better than that of commercial Pt/C catalyst. The origin of enhanced ORR performance can be attributed to both the increased active sites and the mesoporous structure, which is expected to guide the future preparation of more capable carbon-based electrocatalysts for oxygen reduction and other electrocatalytic application.

1. Introduction As one kind of the multi-step electrochemical reaction with sluggish kinetic, oxygen reduction reaction (ORR) has been gained more and more concerns with the development of numerous renewable energy devices (e.g., fuel cells and metal-air batteries).1-3 Although the platinum group metals (PGMs) have been utilized as the state-of-the-art electrocatalysts for oxygen reduction the expensive and scarcity of PGMs have greatly hindered the application and


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commercialization of these energy conversion devices.4 An additional drawback of PGMs is the poor selectivity, which can be poisoned by CO and methanol. To replace PGMs for ORR, numerous non-PGMs catalysts have been extensively investigated for the better ORR performance.5,

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As potential substitutes for PGMs catalysts, the N-doped carbon is highly

desirable for its outstanding catalytic activity and stability and strong designability, as well as the excellent selectivity to both methanol crossover effects and CO poisoning.3, 7-9 However, the ORR performance of pure N-doped carbon catalysts is still unsatisfactory, due to inadequate mass and electron transfer as well as less ORR active site amounts.7, 10 It is valuable and necessary to introduce the porous structure to N-doped carbon for improving the mass transfer and increasing the catalytically active sites of N-doped carbon.11-14 Meanwhile, the rich mesoporosity can promote the efficient reactant and electrolyte transport in the catalyst layer.10 As an effective strategy, utilizing the templates (e.g., silica and montmorillonite) can easily produce mesoporous carbon;10, 15 however, the removal of templates often needs some kinds of toxic or hazard reagents (e.g., HF or concentrated alkali for removing silicon template), which will pollute the environment and damage human health.13 Moreover, codoping carbon materials with nitrogen and heteroatom (e.g., P, S and B) may play an important role in improving electron transfer and in providing more active sites different from N-site, which can make N-doped carbon as meta-free catalyst more active and efficient.16-19 Among the (N, heteroatom)-codoped carbon, N, P-codoped carbon materials have attracted much attention in recent year.17,

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As one of the most important metalloid elements, P can promote the

carbonization for N-doped carbon during the high-temperature heat treatment, which can improve the electrical conductivity of N-doped porous carbon.21 Most importantly, doping P with N-doped porous carbon can effectively enhance the ORR catalytic activity. Although P has the


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same number of valence electrons as N, the lower electronegativity of P atom might make the polarity of P-C bond different from that of N-C bond,22, 23 resulting in a high catalytic activity of C-P bond. Moreover, codoping of N and P also shows a synergetic effect,17 which is expected to increase the number of P-C bond as the catalytically active site and further improve the catalytic activity. To improve the porous structure and ameliorate surface functionalities for codoping carbon materials, a facile and rational strategy is demonstrated for preparation of N, P-codoped mesoporous carbon (N,P-MC) by the direct pyrolysis treatment of phytic acid (PA)-doped polypyrrole (PPy) with polystyrene (PS) microsphere as the organic templates. As shown in Scheme 1, we utilize phytic acid, which can complex with up to pyrrole monomers, as the nontoxic and protonic P-dopant. Meanwhile, the PS microsphere as the organic templates can be spontaneously removed during the pyrolysis treatment. After direct pyrolysis treatment, the resultant N,P-MC sample exhibits mesoporous structure with a large special surface area, as well as the superior activity, remarkable stability and excellent methanol tolerance, comparable or even better than commercial Pt/C catalyst.

2. Experimental section 2.1 Preparation of N, P-codoped mesoporous carbon (N,P-MC) The PS microspheres were prepared by emulsion polymerization according to a published procedure.24 Then 3 mL pyrrole monomers were mixed with 1 mL phytic acid (PA, 5 wt% aqueous solution) and 100 mg PS microsphere in 20 mL ethanol/water (1/4, wt/wt) solution


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under stirring for 1 h. Subsequently, 10 mL ferric chloride aqueous (0.03 g mL-1) was added into the mixed solution under stirring for 20 h to produce the PA-doped PPy/PS composite. The resultant composite was filtrated and washed with ultrapure water many times, and then dried at 80 °C for 12 h. Finally, the separated PA-doped PPy/PS composite was heated by a directly pyrolysis method in a tube furnace at 900 °C in Ar atmosphere for 2 h, and denoted as N, Pcodoped mesoporous carbon (N,P-MC). For comparison, the N-doped carbon (N-C), N, Pcodoped carbon (N,P-C) and N-doped mesoporous carbon (N-MC) materials were fabricated in the similar condition, but during the polymerization procedure without the presence of PA&PS, PS and PA, respectively. The ratios of N to P and pyrolysis temperature were optimized. It was found that the sample (derived from 3 mL pyrrole with 1 mL 5 wt% phytic acid, and 900 °C pyrolysis treatment) exhibited the best ORR activity (Supporting Information). Therefore, the samples in only this synthetic condition were used for subsequent analysis. 2.2 Electrochemical measurement Electrochemical measurements were obtained at room temperature with a normal three-electrode system: a Pt wire electrode (counter electrode), a potassium chloride saturated calomel electrode (SCE, reference electrode) and a modified glassy carbon (GC, d = 40 mm, working electrode). We dispersed and sonicated 10 mg of the as-prepared electrocatalyst in 2 mL ethanol with 5 µL 5 wt% Nafion mixtures. The catalyst ink was dripped onto the polished GC electrode, and modified GC electrode dried at room temperature for 20 min. The commercial 20 wt% Pt/C (Johnson Matthey) catalyst was also measured for comparison. The loading mass for all samples on the modified working electrode is 0.2 mg cm-2.


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3. Results and discussion We carried out ultraviolet (UV) spectra and scanning electron microscopy (SEM) images to reveal a formation of PA-doped PPy/PS composites. The UV spectra indicate that the adsorption peak of PA-doped PPy has a slight red-shift about ca. 20 nm compared with that of PPy (ca. 410 nm) attributable to the interaction between PPy and PA (Figure S1).25,

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Moreover, we

conducted SEM to investigate the morphologies of PS microsphere before and after PPy coating (SEM, Figure 1a and Figure S2). The result indicates that the uncoated PS microspheres present an average diameter of ca. 70 nm with smooth surface, while the PPy coating PS microspheres exhibit the rough surface with an average particular size of ca. 100 nm. After the direct pyrolysis treatment, most of the N,P-MC kept the well-defined spherical feature even after the high-temperature heat-treatment, while some of the spherical feature collapsed and formed into hemispheres, due to the decomposition and volatilization of PS microspheres (Figure 1b). We further investigated the porous structure with transmission electron microscopy (TEM, Figure 1c). It reveals that the N,P-MC sample exhibits spherical mesopores with a carbon-shell thickness of ca. 10 nm and an average diameter of ca. 70 nm. Besides, the high-magnification TEM image (Figure 1d) of N,P-MC exhibits the relatively disordered carbon layers with numerous edge-like graphitic structures, which is expected to affect the catalytic activity.15 N2 adsorption-desorption tests were carried out to further investigate the porous structure of all the as-prepared samples (Figure 2a). Contrary to N,P-C and N-C (68 and 31 m2 g-1, respectively), the N,P-MC and N-MC samples exhibit type-IV isotherm characteristics for mesoporous materials with the similar Brunauer-Emmett-Teller (BET) surface area of 305 and 300 m2 g-1, respectively. Additionally, a hysteresis loop at high relative pressures (p/p0 > 0.8) reveals the presence of


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macropores.17 Furthermore, density functional theory (DFT) pore size distribution plots show a increased pore volume from 0.05/0.09 cm3 g−1 for N,P-C/N-C to 0.66/0.47 cm3 g−1 for N,PMC/N-MC, owing to the numerous large mesopores with diameters >10 nm (Figure 2b). Besides, the pore size distribution plots reconfirm that N,P-MC and N-MC possess a hierarchical porous structure (micropores, centered at 0.9 nm; mesopores, centered at >10 nm), which can offer numerous ORR active sites and facilitate the reactant and electrolyte transfer.10, 27, 28. The numerous mesopores are attributed to the spontaneous removal of PS templates (an average diameter of ca. 70 nm) during the pyrolysis treatment, demonstrating a controlled synthesis of the mesoporous carbon. The microstructures of carbon layer were investigated using X-ray diffraction (XRD) patterns and Raman spectra. The corresponding (002) reflection of a graphitic-type lattice (JCPDS card: no. 41-1487) have an obvious high-angle-shift from 22.4° to 25.2°, due to the presence of P element (Figure 2c). This phenomenon can be attributed to that P element, one kind of metalloid elements, can catalytically activate the carbonization process during the pyrolysis treatment.21 In addition, the Raman spectra were conducted and deconvoluted into four graphitic carbon-related peaks: I-line (ca. 1180 cm-1, attributed to impurities or heteroatoms), D-line (ca. 1350 cm-1, attributed to defects on the graphene plane), D″-line (ca. 1470 cm-1, attributed to defects in graphene layer stacking) and G-line (ca. 1560 cm-1, attributed to the graphitic carbon).29,

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Subsequently, we calculated the proportion of IG/II, IG/ID and IG/ID″ for all the as-prepared samples to evaluate the heteroatoms-doping content, the graphene-plane defects and the stack defects, respectively. As shown in Figure 2e, after doping with P, the proportions of IG/ID for N,P-MC and N,P-C increase as the (002) reflection shifts to high angle in their XRD patterns, which indicates that the N/P-codoped carbon possess high graphitization carbon structures. In


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addition, the proportions of IG/ID″ of N,P-MC and N-MC are also much smaller than those of N,P-C and N-C, implying an increased graphene stack defects that are promoted by utilizing the PS spheres as templates. However, the heteroatoms-doping content keeps in a stable value without getting affected by the P element and PS templates. These results indicate that after codoping and template removing, the carbonaceous structure of N,P-MC transforms from turbostratic stacking layers into grain refined structure with numerous edge defects, being in favor of providing active sites for ORR and improving the electrical conductivity.8, 10, 15 The X-ray photoelectron spectroscopy (XPS) measurements were carried out for the surface chemical structures of all the samples. The XPS survey spectrum (Figure S4a, Table S1) of the N,P-MC sample exhibits the presence of C (82.49 at%), N (3.56 at%), P (0.60 at%) and O (13.05 at%). The high-resolution XPS C 1s scan shows that N,P-MC consists primarily of sp2hybridized carbon, along with some C-P, C-N, C-O and -COOH moieties (Figure 3a). The highresolution XPS N 1s and P 2p spectra of N,P-MCs (Figure 3b, 3c) present that most doping elements bind with carbon to form various bonding species (pyridinic N, 398.7 eV; pyrrolic N, 399.8 eV; graphitic N, 401.3 eV; P-C, 131.8 eV).17, 20 In addition, the pronounced O 1s peak (Figure 3d) was also deconvoluted into two peaks O-C (533.0 eV) and O-H (531.2 eV), attributable to the chemically bonded oxygen and physically adsorbed oxygen, respectively.27, 31 The observed N, P-codoping structure in pyrolytic carbon provides the active sites for the ORR process. In addition, the Fe 2p peak is not found in the high-resolution XPS Fe 2p spectrum of N,P-MC, demonstrating the element Fe has been removed from the final product (Figure S4b). We firstly conducted cyclic voltammetry (CV) measurements to investigate the ORR activity of the as-prepared electrodes in N2- and O2-saturated 0.1 M KOH solution (Figure 4a). The obvious cathodic peaks corresponding to ORR between 0.7 and 0.8 V (versus the reversible hydrogen


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electrode, vs. RHE), are observed in O2-satured solution, but not in N2-satured solution. The potentials of cathodic peak move to more positive area for the carbons codoped with N and P (N,P-C and N,P-MC), and the cathodic current densities increase for the carbons with mesoporous structure (N-MC and N,P-MC). Additionally, the electrochemically active surface areas (ECSAs) for the above four samples were calculated from the electrochemical double-layer capacitance (Cdl, Figure 4b and Figure S6, S7). Compared with N,P-C and N-C, the mesoporous carbon samples (i.e., N,P-MC and N-MC) exhibit much larger surface areas, which allow facile diffusion of reactants and electrolytes. The rotating ring-disk electrode (RRDE) measurements were further carried out to investigate the catalytic activity of the above samples in O2-saturated 0.1 M KOH solution (Figure 5a and Figure S9), and compared with commercial Pt/C. After the mesoporous carbon codoped with N and P, the N,P-MC electrode exhibits the excellent ORR activity with a positive half-wave potential (E1/2) of 0.84 V and a large diffusion limited current density (Jd) of 5.0 mA cm-2, similar with the commercial Pt/C electrode (E1/2 = 0.84 V and Jd = 5.0 mA cm-2). Meanwhile, N,P-C and N-MC also exhibit the better ORR activities than the pure N-C electrocatalyst, most probably due to the N and P codoping and mesoporous structure. In addition, the percentage of peroxide (% HO2-) and the transferred electron number (n) calculated from the RRDE measurements exhibit that the N,P-MC electrocatalyst can effectively reduce the intermediate yield of 2-electron pathway (ca. 1.5 %), indicating a 4-electron transfer pathway (Figure 5b). The n derived from the Koutecky-Levich (K-L) equation for the N,P-MC electrode also reconfirms a direct 4-electron transfer pathway in the ORR process (Figure 5c). Furthermore, Tafel plots (Figure 5d) in the low over-potential region indicates that the N,P-MC electrode shows the fast kinetics process and high transfer coefficient for ORR (Tafel slope of 58 mV dec-1), even smaller


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than commercial Pt/C (62 mV dec-1). Moreover, we have also investigated the ORR activity of N-C, N,P-C, N-MC and N,P-MC in a O2-saturated acidic electrolyte, along with Pt/C as the reference electrode (Figure S10). Compared with the N-C, N,P-C, N-MC samples, the catalytic activity of N,P-MC is enhanced, despite a little inferior performance than commercial Pt/C. To better elucidate the enhanced ORR performance and of N,P-MC, we compared the N and P contents of the as-prepared samples (Table S1) and deconvoluted the corresponding highresolution XPS N 1s and P 2p spectra with their relative contents of N-sites (i.e., pyridinic N, pyrrolic N and graphitic N) and P-sites (i.e., P-C and P-O bonds) (Figure S5, Table S2, S3). According to the literatures, as to N-doped carbon, the planar N-containing moieties, especially for the carbon atoms near the pyridinic N, are treated as the active sites for ORR.32, 33 Meanwhile, as to the presence of N, P-codoped structure, some carbon atoms near the P-C bond and graphitic N also exhibits high catalytic activity (Figure 6a).17 Herein, we compared the contents of pyridinic N and P-C bond as ORR active sites (Figure 6b). Despite the less content of pyridinic N, the N, P-codoped pyrolytic carbon still possess more amounts of active sites than merely Ndoped carbon, attributing to the existence of P-C bonds. It indicates that the amount of active sites in these carbon materials can be increased by codoping with nitrogen and phosphorus. In addition, the mesoporous structure with a large surface area plays an important role in faciliating electrolyte and reactant diffusion, which is also an essential factor to affect the ORR catalysis.13 Therefore, we calculated the enhancements of both the active sites content and BET surface area as the enhancement factor, compared with the N-C sample (Figure 6b, and see the associated discussions in Supporting Information for details). Compared with the E1/2 and kinetic current density (Jk) at 0.85 V following the trend of N-C (0.62 V, 0.30 mA cm-2) > N,P-C (0.77 V, 0.95 mA cm-2) > N-MC (0.81 V, 1.62 mA cm-2) > N,P-MC (0.84 V, 3.47 mA cm-2), the calculated


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enhancement factors (1.23-fold (N,P-C), 1.97-fold (N-MC) and 2.14-fold (N,P-MC) compared with the N-C sample) are consistent with the result of electrochemical testing for ORR, demonstrating that the origin of the enhanced catalytic activity for N,P-MC mainly attribute to the increased active sites and large surface area. Moreover, the high graphization structure also improves the electrical performance of this carbon material (Table S4). Considering the stability and selectivity of electrocatalyst are important indicators of practical applications, we evaluated the electrochemical stability and selectivity of N,P-MC with chronoamperometric (i-t) measurements in O2-saturated alkaline electrolyte. The commercial Pt/C electrode was also tested for comparison. The current responses at the ORR potential of 0.85 V were recorded for 10,000 s (Figure 7a), indicating that the N,P-MC electrode presents the remarkable stability (maintains the 95.8 % activity), much better than Pt/C (63.2 %). Additionally, unlike commercial Pt/C, the N,P-MC electrode shows the excellent methanol tolerance, exhibiting a much better fuel selectivity (Figure 7b).

4. Conclusion In summary, we demonstrate the fabrication of the N, P-codoped mesoporous carbon by the direct pyrolysis treatment of PPy with PA as non-toxic P-dopant and PS sphere as self-eliminated template. After direct pyrolysis treatment, the N,P-MC exhibits hierarchical porous structure with the pyridinic N and P-C bond co-doping structure as effective ORR active sites. Owing to these features, the N,P-MC exhibits the superior ORR performance and much better than the commercial Pt/C catalyst. Therefore, together with its low synthetic cost and rational preparation procedure, the N,P-MC can be expected to be a promising candidate of non-PGM catalysts for


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the further large-scale application and commercialization. In addition, the origin of enhanced ORR performance attributes to both the increased active sites and mesoporous structure, guiding for future design of more efficient carbon-based materials for oxygen reduction and other electrocatalytic application.

ASSOCIATED CONTENT Supporting Information Detailed materials, preparation of polystyrene (PS) microsphere, characterizations and calculations; SEM, TEM, UV spectra, FT-IR spectra, XPS survey spectra, high resolution XPS scans of N 1s and P 2p, electrochemical tests for the as-prepared samples are shown in Supporting Information, including the optimization of codoped carbon for ORR. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENT This work was supported by the National Natural Science Funds of China (51432003, 51125007).

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(24) Han, J.; Xu, G.; Ding, B.; Pan, J.; Dou, H.; MacFarlane, D. R. Porous Nitrogen-Doped Hollow Carbon Spheres Derived from Polyaniline for High Performance Supercapacitors. J. Mater. Chem. A 2014, 2, 5352-5357. (25) Zha, Z.; Yue, X.; Ren, Q.; Dai, Z. Uniform Polypyrrole Nanoparticles with High Photothermal Conversion Efficiency for Photothermal Ablation of Cancer Cells. Adv. Mater. 2013, 25, 777-782. (26) Yang, X.; Zhu, Z.; Dai, T.; Lu, Y. Facile Fabrication of Functional Polypyrrole Nanotubes via a Reactive Self-Degraded Template. Macromol. Rapid Commun. 2005, 26, 1736-1740. (27) Collins, P. G. Extreme Oxygen Sensitivity of Electronic Properties of Carbon Nanotubes. Science 2000, 287, 1801-1804. (28) Ferrero, G. A.; Preuss, K.; Fuertes, A. B.; Sevilla, M.; Titirici, M. M. The Influence of Pore Size Distribution on the Oxygen Reduction Reaction Performance in Nitrogen Doped Carbon Microspheres. J. Mater. Chem. A 2016, 4, 2581-2589. (29) Maldonado, S.; Morin, S.; Stevenson, K. J. Structure, Composition, and Chemical Reactivity of Carbon Nanotubes by Selective Nitrogen Doping. Carbon 2006, 44, 1429-1437. (30) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Pöschl, U. Raman Microspectroscopy of Soot and Related Carbonaceous Materials: Spectral Analysis and Structural Information. Carbon 2005, 43, 1731-1742. (31) Wang, S.; Zhang, L.; Xia, Z.; Roy, A.; Chang, D. W.; Baek, J. B.; Dai, L. BCN Graphene as Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2012, 51 (17), 4209-4212.


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Figures

Scheme 1. Illustration for the synthetic process of N,P-MC electrocatalyst.


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Figure 1. SEM images of the a) PA-doped PPy/PS composite and b) N,P-MC sample. c) Lowand d) high-magnification TEM images of N,P-MC sample.


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Figure 2. BET characterization, a) nitrogen adsorption-desorption isotherms and b) the corresponding DFT pore size distributions for N-C, N,P-C, N-MC and N,P-MC. c) XRD patterns and d) Raman spectra of the above four samples. e) 2-Theta, IG/ID, IG/ID″ and IG/II, as a function of nitrogen- and phosphorus-codoped and mesoporous carbon structure.


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Figure 3. High-resolution X-ray photoelectron spectroscopy scans of a) C 1s, b) N 1s, c) P 2p and d) O 1s for N,P-MC.


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Figure 4. a) CV curves of the as-prepared metal-free electrodes with a sweep rate of 50 mV s-1 in O2-saturated (solid line) or N2-saturated (dashed line). b) Linear fitting of current density (∆J, at a potential of 1.1 V vs. RHE) versus scan rate used to estimate the double-layer capacitance (Cdl). The corresponding data and high-magnification scale of Figure 4b for the N-C and N,P-C samples was shown in Figure S6 and S7.


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Figure 5. a) RRDE voltammograms of the as-prepared electrocatalysts and commercial Pt/C electrode at 1600 rpm in O2-saturated 0.1 M KOH. The ring current (Ir, the ring potential is constant at 1.5 V) and disk current (Id, the sweep rate is 5 mV s-1) were shown on the upper and lower half of the graph, respectively. b) The % HO2- and the n for the above five electrodes were shown on the upper and lower half of the graph, respectively. c) Linear polarization curves of N,P-MC with different rotation rates at a sweep rate of 5 mV s−1in O2-saturated 0.1 M KOH. The inset in c) demonstrates the K-L plots at different potentials. d) Tafel plots of all the above electrodes.


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Figure 6. a) Illustration of the ORR active sites of N, P-codoped carbon structure. b) The active sites (pyridinic N and P-C bond) content and BET surface area with the enhancement factor to N-C of the N-C, N,P-C, N-MC and N,P-MC electrocatalysts.

Figure 7. a) Electrochemical stability and b) methanol-crossover by chronoamperometric curves (i-t) of N,P-MC and commerical Pt/C in O2-saturated alkaline electrolyte. The arrow shows the addition of 3 mL methanol solution into the 120 mL electrolye after ca. 300 s.


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Abstract Graphic


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Nitrogen and Phosphorus Codoped Mesoporous Carbon Derived from

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