Nitrogen-Doped Carbon NanoparticleâCarbon Nanofiber Composite

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Nitrogen-Doped Carbon Nanoparticle-Carbon Nanofiber Composite as an Efficient Metal-Free Cathode Catalyst for Oxygen Reduction Reaction Gasidit Panomsuwan, Nagahiro Saito, and Takahiro Ishizaki ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10493 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 24, 2016

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ACS Applied Materials & Interfaces

Nitrogen-Doped Carbon Nanoparticle− −Carbon Nanofiber Composite as an Efficient Metal-Free Cathode Catalyst for Oxygen Reduction Reaction GasiditPanomsuwan,*,†,¶ Nagahiro Saito,‡,♯,§ and Takahiro Ishizaki†,§ †

Department of Materials Science and Engineering, Faculty of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan ‡

Department of Materials, Physics and Energy Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

♯

Social Innovation Design Center (SIDC), Institute of Innovation for Future Society, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan §

Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

¶

Present address: NU-PPC Plasma Chemical Technology Laboratory, The Petroleum and

Petrochemical College, Chulalongkorn University, Soi Chulalongkorn 12, Phayathai Road, Pathumwan, Bangkok 10330, Thailand

ACS Paragon Plus Environment

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ABSTRACT Metal-free nitrogen-doped carbon materials are currently considered at the forefront of potential alternative cathode catalysts for the oxygen reduction reaction (ORR) in fuel cell technologies. Despite numerous efforts in this area over the past decade, rational design and development of a new catalyst system based on nitrogen-doped carbon materials via an innovative approach still present intriguing challenges in ORR catalysis research. Herein, a new kind of nitrogen-doped carbon nanoparticle−carbon nanofiber (NCNP−CNF) composite with highly efficient and stable ORR catalytic activity has been developed via a new approach assisted by a solution plasma process. The integration of NCNPs and CNFs by the solution plasma process can lead to a unique morphological feature as well as modifying physicochemical properties. The NCNP−CNF composite exhibits a significantly enhanced ORR activity through a dominant four-electron pathway in an alkaline solution. The enhancement in ORR activity of NCNP−CNF composite can be attributed to the synergistic effects of good electron transport from highly graphitized CNFs as well as abundance of exposed catalytic sites and meso/macroporosity from NCNPs. More importantly, NCNP−CNF composite reveals excellent long-term durability and high tolerance to methanol crossover compared with those of a commercial Pt/C catalyst. We expect that NCNP−CNF composite prepared by this synthetic approach can be a promising metal-free cathode catalyst candidate for ORR in fuel cells and other electrochemical energy technologies.

KEYWORDS: nitrogen-doped carbon materials, oxygen reduction reaction, solution plasma process, fuel cells


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INTRODUCTION The rational design and development of highly efficient cathode catalysts for the oxygen reduction reaction (ORR) are highly crucial for achieving high-performance fuel cells.1,2 So far, Pt nanoparticles supported on carbon materials (Pt/C) have been recognized as the state-of-theart cathode catalysts in fuel cells because of their excellent ORR catalytic activity through a desirable four-electron reduction pathway in both alkaline and acidic electrolytes. However, high price, poor long-term durability, methanol crossover effect, and carbon monoxide (CO) poisoning of Pt are serious hindrances for further advancement and wider commercialization of fuel cells.3,4 Therefore, a key technological challenge in the current fuel-cell research is focused on development and exploration of low-cost Pt-free cathode catalysts with excellent ORR performance in terms of both catalytic activity and durability.5−7 In recent years, metal-free heteroatom-doped carbon materials (e.g., nitrogen,8−10 boron,11 phosphorus,12 sulfur,13 fluorine14) have been explored and considered as one of the most prominent families of materials for advanced ORR catalysts ultimately to replace the commercial Pt/C catalysts because of their low cost, competitive ORR activity, and excellent durability. Among various heteroatoms, nitrogen doping has been found to play the most critical role in tailoring electronic structure and improving intrinsic ORR catalytic activity of carbon materials.15−17 Both theoretical calculations and experiments demonstrated that nitrogen doping into the sp2 carbon lattice could induce a charge redistribution around the nitrogen atoms owing to the difference in electronegativity between carbon (χC = 2.50) and nitrogen (χN =3.04). Such charge redistribution resulted in the creation of a net positive charge on the adjacent carbon atoms, which could change the chemisorption mode of O2 molecules from end-on adsorption (Pauling model) to side-on adsorption (Yeager model). The parallel diatomic adsorption of O2


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molecules on the catalysts could effectively weaken the O–O bonding, thereby promoting the ORR process through a four-electron pathway.8,17−19 Although nitrogen doping on various types of carbon materials (e.g., graphene,9,20 carbon nanotubes,8,21 carbon nanocages22, carbon nanoparticles,23 carbon nanohorns24) have shown significant improvement in ORR activity with excellent durability, most of them are usually inferior to Pt/C catalysts in the aspect of either onset potential or current density. In a continuing effort to pursue highly efficient ORR catalysts, integration of two different carbon materials into a composite system with nitrogen doping (e.g., graphene−carbon nanotube,25−29 carbon nanotube−carbon nanoparticle,30 meso/nanoporous carbon−graphene31,32) has recently been proposed as an effective way to enhance ORR activity beyond the individual constituent carbon material. Such a positive improvement in ORR of the composites could be attributed to their unique architectural structure and the combined advantages of each constituent material responsible for ORR, such as large specific surface area, meso/macroporosity, and high electrical conductivity.25−32 Several synthetic approaches have been used to prepare the carbonbased composites for ORR, such as chemical vapor deposition (CVD),25,29 hydrothermal synthesis,26 solution self-assembly process,27 and post-treatment of mixed two-carbon materials with nitrogen-containing organic precursor at high temperature.28,30 Despite significant progress in this field, development of a new kind of nitrogen-doped carbon-based composite through an innovative synthetic approach is still an ongoing challenge. Solution plasma, nonequilibrium plasma generated in a liquid phase medium, has emerged as an efficient tool for synthesis of various nanomaterials.33−37 In our previous works, we have demonstrated the applicability of the solution plasma process for the synthesis of nitrogen-doped carbon nanoparticles (NCNPs) from nitrogen-containing organic precursors without the


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involvement of a metal catalyst precursor.38−40 The resultant NCNPs possessed a uniform incorporation of nitrogen atoms owing to in situ doping during synthesis. This synthetic approach has not only provided a new insight for the design and synthesis of metal-free nitrogendoped carbon materials, but was also further extended to a series of heteroatom doping simply by selecting appropriate organic precursors containing the desired heteroatoms (e.g., phosphorus,41 boron,42 fluorine43). However, the solution plasma process is still at a very young stage in the area of metal-free carbon-based ORR catalysts. Until now, to the best of our knowledge, the formation of a carbon-based composite system by utilizing an innovative solution plasma process has not been explored. Herein, we have demonstrated the synthesis of nitrogen-doped carbon nanoparticle−carbon nanofiber (NCNP−CNF) composite through an unprecedented approach assisted by the solution plasma process. The carbon nanofibers (CNFs) synthesized by a reactant floating method were first dispersed in a nitrogen-containing organic precursor, which is 2-cyanopyridine in this work. A subsequent synthetic process was conducted by generating plasma inside a homogeneous mixture of CNFs and 2-cyanopyridine. The NCNPs were directly produced from 2cyanopyridine with in situ nitrogen doping through dissociation and recombination processes, and then attached to the CNF surface, forming a composite structure. Interestingly, highly energetic CN radicals generated from plasma could also result in the nitrogen doping at the open edge-planes exposed on the CNF surface. Owing to the synergistic effects of NCNPs and CNFs as well as unique doping process under the solution plasma, NCNP−CNF composite shows an enhanced ORR catalytic activity with excellent durability and high tolerance to the methanol crossover in an alkaline solution. Moreover, morphological, structural, and physicochemical


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properties of NCNP−CNF composite and its components were investigated in detail to elucidate the factors contributing to enhanced ORR activity of NCNP−CNF composite.

EXPERIMENTAL SECTION Materials. Ethanol (C2H5OH, ≥99.5% purity), methanol (CH3OH, ≥99.5% purity), concentrated hydrochloric acid (HCl, 35.0−37.0%), and 0.1 M potassium hydroxide aqueous solution (0.1 M KOH, ≥99.5% purity) were purchased from Kanto Chemical Co., Inc. Ferrocene (Fe(C2H5)2, >98.0% purity) was purchased from Wako Pure Chemical Industries Ltd. 2Cyanopyridine (C6H4N2, ≥99.0% purity), Nafion® DE 521 solution (5 wt % in a mixture of lower aliphatic alcohols and water), and 20 wt % Pt on Vulcan XC-72 (Pt/C) were purchased from Sigma Aldrich. Ultrapure water (18.2 MΩ cm at 25 °C) was obtained by purification with an Advantec RFD 250 NB system. All chemicals were of analytical grade and used without further purification. Synthesis of CNFs. The CNFs were prepared by a floating reactant method following the procedure reported by Endo et al.44. Briefly, ferrocene and hydrogen sulfide were both used as the catalyst precursor, while natural gas was used as a carbon feedstock in the synthesis. Graphitization of the CNF was performed at 3000 °C for 10 min under a high-purity Ar atmosphere in a graphite-resistant furnace. The resulting CNFs were subsequently treated by HCl solution at 80 °C for 6 h to remove the residual Fe catalysts. Synthesis of NCNP− −CNF Composite. 100 mg of CNFs were dispersed in 100 mL of 2cyanopyridine, followed by stirring and sonication for 30 min each. The homogeneous suspension was poured into a reactor vessel, in which a pair of 1-mm diameter tungsten electrode


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(purity 99.9%, Nilaco Corporation) shielded with an insulating ceramic tube was placed at the center with a gap distance of 1.0 mm (Figure S1). A bipolar pulsed voltage was applied to the tungsten electrodes using an MPP-HV04 Pekuris power generator (Kurita Seisakusho Co., Ltd.). The pulse duration and repetition frequency were optimally fixed at 0.80 µs and 20 kHz, respectively, to obtain a stable and continuous plasma with a minimizing effect of electrode erosion during synthesis. Plasma was generated and stably maintained inside 100 mL of 2cyanopyridine dispersed with CNFs under vigorous stirring for 30 min. The simultaneous generation of C2 and CN radicals from dissociation of the 2-cyanopyridine molecules under plasma can act as the carbon and nitrogen sources, respectively, for growth and formation of NCNPs (Figure S2).38−40 It can be postulated that the CN radicals play the key role in both in situ doping throughout the entire structure of NCNPs and ex situ doping on the CNF surface. The synthesized NCNPs diffused into the liquid phase and subsequently attached to the surface of the dispersed CNFs, forming a composite with a NCNP:CNF mass ratio of approximately 2:1. The resulting NCNP−CNF composite was separated by filtration and repeatedly washed with ethanol until the wash solvent became colorless. The NCNP−CNF composite was dried at 60 °C for 12 h under ambient conditions and physically ground with an agate mortar and pestle. Finally, the NCNP−CNF composite was loaded into a ceramic boat and then transferred into a quartz tube placed inside a tubular furnace. The subsequent heat treatment was performed at 900 °C for 1 h with a heating rate of 5 °C min−1 under a high-purity Ar flow of 500mL min−1 and cooled naturally to room temperature under the same environment. The synthetic procedure of NCNP−CNF composite is depicted in Scheme 1. For comparison, NCNPs were also synthesized using the same procedures and conditions with use of 2-cyanopyridine without the dispersion of CNFs as a precursor.


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Characterizations. Field-emission scanning electron microscopy (FESEM) images were taken on a Hitachi S-4800 microscope operated at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and selected area electron diffraction (SAED) patterns were acquired using a JEOL JEM-2010 microscope operated at an accelerating voltage of 120 kV. N2 adsorption−desorption isotherms were measured on a Belsorp-mini II sorption analyzer at liquid N2 temperature (−196 °C) to investigate specific surface area, pore volume, and pore size distribution of the catalysts. Prior to the measurement, all catalysts were heated at 150 °C for 90 min under vacuum. The specific surface area was determined by the Brunauer−Emmett−Teller (BET) method at relative pressures (P/P0) between 0.05 and 0.30. The pore volume and pore size distribution were determined by the Barrett−Joyner−Halenda (BJH) method. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 0.15406 nm) at 40 kV and 40 mA. Raman spectra were recorded on a JASCO NRS-5100 spectrometer with a laser-excitation wavelength of 532.1 nm. Elemental analysis (EA) was carried out on a Perkin Elmer 2400 Series II CHNS/O analyzer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a JEOL JPS-9010MC spectrometer with monochromatic Mg Kα radiation (1253.6 eV) as an Xray source. The emission current and anode voltage were operated at 25 mA and 10 kV, respectively. Electrochemical Measurements. The catalyst ink was prepared by dispersing 5 mg of catalyst in a mixture of 480 µL ultrapure water, 480 µL ethanol, and 40 µL of Nafion® DE521 aqueous solution followed by sonication for 60 min to form a 5 mg mL−1 suspension. A 10-µL portion of the well-dispersed suspension was carefully dropped on a glassy carbon disk electrode (disk diameter: 4 mm, Adisk = 0.126 cm2) surrounded by a Pt ring (inner/outer-ring diameter:


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5.0/7.0 mm, Aring = 0.188 cm2) with a Teflon separator (rotating ring disk electrode, RRDE, ALS Co.), yielding a catalyst loading of 0.4 mgcat cm−2.The catalyst-modified electrodes were dried naturally under ambient conditions for 6 h prior to the electrochemical measurements. For comparison, a Pt/C suspension was also prepared and dropped on a glassy carbon disk of RRDE as a benchmark following the same procedure as described above, yielding a catalyst loading of 40 µgPt cm−2. The electrochemical measurements, including cyclic voltammetry (CV), linear sweep voltammetry (LSV), and current−time chronoamperometry were carried out on a CHI 832A electrochemical workstation (CH Instruments Inc.) equipped with an RRDE-3A rotating ring disk electrode apparatus (ALS Co.). A Pt coil (ALS, Co.) and Ag/AgCl electrode filled with a saturated KCl aqueous solution (ALS, Co.) were used as the counter and reference electrodes, respectively. All electrochemical measurements were performed in 0.1 M KOH solution at room temperature. High-purity N2 or O2 gas was purged into 0.1 M KOH solution with a constant flow rate of 50 mL min−1 for at least 30 min to obtain the N2- or O2-saturated solution prior to the measurement.

RESULTS AND DISCUSSION The morphology and detailed structure of CNFs, NCNPs, and NCNP−CNF composite were characterized by FESEM and TEM investigations. Figures 1a and 1b display typical FESEM and TEM images of NCNP−CNF composite, respectively. It is observed that most of NCNPs attach to the CNF surface throughout almost area investigated, giving rise to a unique morphology with an interconnected porous structure. Pristine CNFs exhibit a hollow core structure with severalmicrometer long straight fibers. The inner and outer diameters of CNFs are in the ranges of about


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30−40 and 70−80 nm, respectively. On the other hand, primary particles of NCNPs have a quasispherical shape with a diameter of about 20−40 nm and aggregate to each other. The corresponding SAED pattern of CNFs presents a well-defined concentric ring, indicating good crystallinity (Figure S3). In contrast, a diffuse ring pattern is observed for NCNPs (Figure S4), suggesting a predominantly amorphous structure. More detailed structural information of NCNPs and CNFs was obtained by HRTEM. The HRTEM image of CNFs in Figure 1c clearly shows long-range order of the (002) basal planes of graphite stacked at an angle of 20−30° with respect to the fiber axis. This feature is the characteristic of a cup-stacked structure, which is in agreement with the previous reports.44−46 The open-edge planes exposed on the external surface of CNF can possibly serve as appropriate sites for nitrogen doping.46 The HRTEM image of NCNPs in Figure 1d allows visualization of short-range order structure, again suggesting the dominant amorphous nature of NCNPs. The catalysts were further characterized by N2 adsorption−desorption measurements, which can give information on their specific surface area, pore volume, and pore-size distribution (Table S1). Figures 2a and 2b present the N2 adsorption−desorption isotherms and corresponding pore size distributions of all catalysts, respectively. As can be seen, both NCNPs and NCNP−CNF composite show almost identical N2 adsorption−desorption isotherms with a type IV characteristic and a type H3 hysteresis loop.47 A steep increase in adsorption−desorption curves with a narrow hysteresis loop at high relative pressures (P/P0 = 0.80−0.99) suggests the main contributions of large-sized mesopores and interparticle macropores.48 The specific surface areas of CNFs, NCNPs, and NCNP−CNF composite determined by the BET method are 47.4, 171.9, and 159.8 m2 g−1, respectively. This result means that CNFs possess low specific surface area and lack of porous structure. The corresponding pore size distribution of NCNPs and


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NCNP−CNF composite derived by the BJH method clearly reveal high intensity in the pore diameters larger than 10 nm, indicating the existence of meso- and macropores. The mean pore diameters of NCNPs and NCNP−CNF composite are estimated to be 39.0 and 53.0 nm, respectively. The pore volumes of NCNPs and NCNP−CNF composite are 0.94 and 0.84 cm3 g−1, respectively. A slight decrease in pore volume of NCNP−CNF composite is due to more presence of macropores in the composite. This result indicates that the surface morphology and porous feature of NCNP−CNF composite are mainly dominated by NCNPs rather than CNFs. It is known that high specific surface area can lead to greater exposure of ORR catalytic sites, while large pore volume and pore diameter play crucial roles in facilitating the diffusion and mass transfer of electrolyte during ORR electrocatalysis.48−50 The XRD patterns of CNFs, NCNPs, and NCNP−CNF composite are presented in Figure 2c. Pristine CNFs clearly show the narrow diffraction peak at 2θ angle of 26.0°, corresponding to the (002) plane of the graphitic carbon. In contrast, a broader and weaker 002 diffraction peak at a lower 2θ angle of 23.7° is evident for NCNPs, which is characteristic of amorphous carbon. The interlayer spacing (d002) of CNFs and NCNPs calculated from the 002 diffraction peaks by Bragg’s law are about 0.342 and 0.375 nm, respectively. The d002 value, close to that of graphite (0.335 nm),51 with a concomitant narrow 002 diffraction peak of CNFs reflects its high degree of graphitization in comparison with NCNPs. The XRD pattern of NCNP−CNF composite comprises both narrow and broad peaks arising from CNFs and NCNPs, respectively. This result implies that NCNP−CNF composite contains a combination of highly crystalline phase from CNFs and amorphous phase from NCNPs. The presence of highly graphitized CNFs in the composite can serve as an electrically conductive pathway for charge transportation, which is beneficial for the ORR. Structural information was additionally acquired by Raman


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spectroscopic measurement. The Raman spectra in Figure 2d show two pronounced peaks of the characteristic D band at 1350 cm−1 and G band at 1590 cm−1. The G band is assigned to the graphitic E2g mode corresponding to the in-plane bond-stretching motion of a pair of sp2carbon atoms, while the D band is attributed to breaking of the symmetry caused by structural disorder and defects.52 The intensity ratio of the D band to the G band (ID/IG) is usually employed as an indicator for evaluating the degree of disorder in carbon materials.52 From the Raman spectra, the ID/IG values are estimated to be 0.83, 1.05, and 0.97 for CNFs, NCNPs, and NCNP−CNF composite, respectively. This result is in accordance with the above discussions on SAED, HRTEM, and XRD data that CNFs have a high degree of graphitization, while NCNPs exhibit a low degree of graphitization (more disorder structure). The incorporation of nitrogen atoms in the catalysts were further confirmed by both XPS and EA measurements. The XPS survey spectra of NCNPs and NCNP−CNF composite in Figure 3a show the presence of dominant C 1s peak along with N 1s and O 1s peaks without discernable signals from other elements, suggesting the metal-free nature of the catalysts. The emergence of the N 1s peak for NCNPs and NCNP−CNF composite is evidence that nitrogen atoms have been successfully incorporated into their carbon structure. The atomic contents of carbon, oxygen, and nitrogen for CNFs, NCNPs, and NCNP−CNF composite derived from the XPS quantitative analysis are summarized in Table S2. The surface nitrogen contents of NCNP and NCNP−CNF are found to be 1.33 and 1.35 at %, respectively, which are almost at the same level. In contrast, bulk nitrogen content of NCNPs and NCNP−CNF composite acquired from EA are 1.49 and 0.51 wt %, respectively. The surface/bulk ratio of nitrogen for NCNPs is close to unity (1.01), suggesting a uniform distribution of nitrogen atoms at the surface and in the bulk of the carbon particles. On the other hand, the surface/bulk ratio of nitrogen for NCNP−CNF composite is


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estimated to be 2.96, which reflects surface enrichment of nitrogen atoms. Such high nitrogen content on the surface of NCNP−CNF composite suggests good distribution and coverage of NCNPs on the CNF surface. Moreover, it may imply that the nitrogen atoms were partly incorporated onto the CNF surface rather than inside of the CNFs when exposed to plasma during synthesis. More detailed information on the surface chemical states of the catalysts was further examined by high-resolution XPS measurements on C 1s, O 1s, and N 1s peaks. High-resolution XPS C 1s spectra present an asymmetric peak shape with the main component at 284.5 eV corresponding to the sp2 hybridized graphitic carbon. A tail at higher binding energies is caused by the presence of carbon atoms bonded to nitrogen and different oxygen-containing moieties (Figure S5).20,28,53,54 The presence of XPS O 1s peak is possibly a result of partial oxidation of the catalyst surface (Figure S6). As shown in Figure 3b, high-resolution XPS N 1s spectra of NCNPs and NCNP−CNF composite reveal a double-peak shape, which can be deconvoluted into four peaks corresponding to various nitrogen states, including pyridinic N (398.4 ± 0.1 eV), pyrrolic N (400.3 ± 0.1 eV), graphitic N (401.0 ± 0.1 eV), and pyridinic N-oxide (403.5 ± 0.2 eV).15,20,55,56 There is no significant difference in distribution of nitrogen states detected on NCNPs and NCNP−CNF composite. Notably, the nitrogen atoms are predominantly bonded to the surrounding carbon atoms in the forms of graphitic N (63−67%) and pyridinic N (20−24%) (Figure 3b and Table S3). An almost absence of pyrrolic N (~2%) is possibly due to its thermal instability at high treatment temperatures.22,39,57,58 The absolute contents of various nitrogen states for NCNPs and NCNP−CNF composite are quantitatively shown in Figure 3c and Table S3. This result suggests that the surface chemistry and active sites of NCNP−CNF composite are mainly governed by NCNPs, which are largely exposed on the CNF surface in the composite.


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Another important finding that should be noted here is a slightly higher pyridinic N on NCNP−CNF composite (23.6%) compared with NCNPs (20.7%). This may be evidence that the nitrogen atoms were partly incorporated at the open-edge planes exposed on the CNF surface during exposure to plasma. Based on the conclusions in the recent literature, graphitic N and pyridinic N were proposed to be the highly catalytic sites to boost and facilitate the ORR process, whereas pyrrolic N and pyridinic N-oxide seemed to make negligible contributions to the ORR.17,18,56,59−61 The electrocatalytic activity toward the ORR of the catalysts was first evaluated by CV measurements in 0.1 M KOH solution saturated with N2 and O2 at room temperature (Figure 4a). For comparison, a commercial Pt/C was also tested under the same conditions. The CV curves of all catalysts reveal a featureless voltammetric current in the N2-saturated solution (dashed line), while a prominent peak of the ORR appears in the O2-saturated solution (solid line). The CV curve of CNFs exhibits two cathodic reduction peaks at −0.30 and −0.80 V. A wide second peak at a high negative potential is due to the reduction of peroxide, indicating poor catalytic activity of CNFs. A single cathodic reduction peak at −0.28 V can be observed for NCNPs and NCNP−CNF composite. Another important aspect is a higher current peak of NCNP−CNF composite compared with that of CNFs and NCNPs. These results suggest that NCNP−CNF composite possesses higher ORR activity than CNFs and NCNPs. To gain additional insight into the ORR process of the catalysts, LSV measurements were carried out on an RRDE in an O2-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm and a scan rate of 10 mV s−1 (Figure 4b). The corresponding ring current was simultaneously measured with a Pt ring electrode by applying a constant potential of 0.5 V for detecting peroxide species generated at the disk electrode. All LSV curves reveal two distinct sections


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within the potential range investigated: (i) a mixed kinetic−diffusion-controlled section at low overpotentials and (ii) a diffusion-controlled section at high overpotentials. The LSV curve of CNFs exhibits a two-step reduction process with low current density. The first and second onset potentials are found to be −0.21 and −0.64 V, respectively. This result indicates poor ORR catalytic activity of CNFs via two consecutive steps of a two-electron pathway with the generation of peroxide as intermediates. The onset potential of NCNP−CNF composite commences at −0.14 V, which is 30 mV more positive than that of NCNPs (−0.17 V). However, it is still 60 mV more negative than that of Pt/C (−0.08 V). The limiting current density of NCNP−CNF composite is much higher than that of CNFs and NCNPs, and even comparable to that of Pt/C throughout the potential range investigated. Notably, a single-step wide plateau of limiting current density is observed for NCNPs and NCNP−CNF composite, implying that the selectivity does not significantly change as a function of electrode potential. These results allow us to conclude that NCNP−CNF composite has better ORR activity than CNFs and NCNPs, as indicated by more positive onset potential and larger limiting current density. To investigate further the ORR catalytic pathway on the catalysts, the electron transfer number (n) per O2 molecule and HO2−% yield involved in the ORR can be determined on the basis of the disk and ring currents using the following equations:62

n = 4×

I disk

I disk , + I ring / N

HO2− % = 100 ×

2 I ring / N I disk + I ring / N

(1)

,

(2)

where Idisk and Iring are the faradic disk and ring currents, respectively. N is the collection efficiency of the ring electrode (0.43). Figures 4c and 4d show the n values and HO2− yields


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within the potential range from −1.0 to −0.3 V calculated from Eq. (1) and (2), respectively. The n values of CNFs are in the range of 2.48−3.32, and the HO2– yields are about 80% at low overpotentials and substantially decrease to 34% at −1.0 V. A significant decrease in HO2– yields and concomitant increase in n values at high overpotentials suggest that the ORR catalyzed by CNFs mainly proceeds through two consecutive steps that each involve a two-electron pathway: (i) O2 + H2O + 2e− → OH− + HO2− and (ii) HO2− + H2O + 2e− → 3OH−. For NCNPs, the n values are calculated to be 2.99−3.02, suggesting that the ORR takes place through a coexisting pathway involving both the two-electron and four-electron transfers. The n values of NCNP−CNF composite are 3.21−3.51 and the corresponding HO2– yields are about 40%, which is much lower than those of CNFs and NCNPs. These results are evidence that the ORR catalyzed by NCNP−CNF composite proceeds dominantly through a four-electron pathway (O2 + 2H2O + 4e− → 4OH−) with less two-electron pathway. Moreover, the n values of NCNP−CNF composite are nearly unchanged over a wide potential range, indicating a smoother and more electrochemically stable ORR process. For Pt/C, the n values are found to be 3.74−3.95 with relatively low HO2– yields of 4−15%, confirming that the ORR proceeds almost entirely through a four-electron pathway. Rotating disk electrode (RDE) measurement was also carried out to verify the ORR pathway evaluated by the RRDE measurement. A series of LSV curves was recorded at various rotation speeds from 225 to 2500 rpm (Figure S7). Obviously, the limiting current density increases with an increase in rotation speeds, which can be explained by the shortened diffusion distance at high rotation speeds. The Koutecky−Levich (K−L) plots (J−1 versus ω−1/2) of NCNPs and NCNP−CNF composite acquired from the LSV curves at various rotation speeds exhibit good linearity and near parallelism in the potential range from −0.3 to −0.6 V (Figure S8), suggesting


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first-order reaction kinetics and similar n values for the ORR. The n values per O2 molecule involved in the ORR process at various potentials can be estimated from the slopes of the K−L plots (see Supporting Information for more details).62 At −0.4 V, the n values are calculated to be 2.16, 3.07, and 3.54 for CNFs, NCNPs, and NCNP−CNF composite, respectively (Figure 4e). The n values obtained from the K−L plots are consistent with those obtained from the above RRDE measurements (Figure S9). The mass transfer-corrected Tafel plots were also examined to evaluate the ORR kinetics of the catalysts. The Tafel plots for all catalysts (Figure 4f) can be obtained by plotting the logarithm of kinetic current density (Jk) of the disk current against potential using the LSV curves at 1600 rpm (Figure 4b). The Jk values can be calculated by the following equation:63

Jk =

JJ d , Jd − J

(3)

where J is the measured current density and Jd is the diffusion-limitting current density. At low overpotentials, the Tafel slopes of CNFs, NCNPs, NCNP−CNF composite, and Pt/C are −41, −60, −62, and −64 mV dec−1, respectively. It is found that the Tafel slopes of NCNPs and NCNP−CNF composite are very close to that of Pt/C. This result indicates that the ORR kinetics on NCNPs and NCNP−CNF composite in alkaline solution are nearly similar to that of Pt/C. A large deviation in Tafel slope of CNFs in comparison with NCNP−CNF composite and Pt/C indicates the difference and poor ORR kinetics of CNFs. Moreover, the Jk value for the ORR of NCNP−CNF composite is higher than that of CNFs and NCNPs in the mixed kinetic−diffusioncontrolled region. According to the electrochemical measurements, it is reasonable to conclude that the integration of NCNPs and CNFs into a composite through the solution plasma process


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can lead to significant enhancement of ORR activity in alkaline solution in spite of low nitrogen doping content (1.35 at %). Next, let us consider each individual constituent in the composite based on the above experimental results. The electrical conductivity of CNFs is about 9.1 S cm−1, which is about three times higher than that of NCNP (3.5 S cm−1) (Table S4), owing to its high degree of graphitization. However, the ORR activity of CNFs is very poor due to the absence of ORR active sites, low specific surface area, and lack of porosity. In the case of NCNPs, they possesses a uniform distribution of ORR active sites (i.e., pyridinic N and graphitic N) and high specific surface area with the presence of hierarchical meso/macroporosity; however, low electrical conductivity attributed to its amorphous nature limits the electron transfer in the ORR process as well as lowers the oxygen reduction rate. This means that the catalysts with either only high electrical conductivity or abundance of ORR active sites with high porosity cannot lead to highperformance ORR catalysts. The enhanced ORR activity of NCNP−CNF composite should thus be attributed to the combined advantages of both CNFs and NCNPs, which is discussed below. To elucidate further the factors responsible for enhanced ORR activity of NCNP−CNF composite, we also prepared a physical mixture of NCNPs and CNFs with the corresponding NCNP:CNF mass ratio of 2:1 to compare with NCNP−CNF composite prepared by the solution plasma process. The detailed preparation of NCNP/CNF mixture is given in the Supporting Information. Interestingly, the ORR activity of NCNP/CNF mixture in terms of both onset potential and current density is much inferior to that of NCNP−CNF composite (Figure S10). The LSV curve of NCNP/CNF mixture is similar to that of NCNPs in the mixed kinetic−diffusion-controlled region, but largely deviated in the diffusion-controlled region. This result proves that physical mixing of NCNPs and CNFs into a composite cannot improve the


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ORR activity, but merely compromises the onset potential of NCNPs and low current density of CNFs. Therefore, the better ORR activity of NCNP−CNF composite over NCNP/CNF mixture may not be only attributed to the combined advantages of both CNFs and NCNPs, but a specific interaction between NCNPs and CNFs in a composite structure as well as an additional effect from the plasma during synthesis also playing crucial roles. Based on the above finding, the enhanced ORR activity of NCNP−CNF composite can be explained as follows. First, a large specific surface area and the existence of meso/macroporosity with in the composite provide a number of active sites for catalyzing the ORR and also enhance electrolyte/reactant transport. Second, highly graphitized CNFs can serve as an electrically conductive path inside an interconnected network structure of the composite (Table S4), which thus promotes fast electron transport ability. Third, ex situ nitrogen doping on the open edgeplane exposed on the CNF surface during synthesis may partially contribute to an enhancement in the ORR activity of NCNP−CNF composite. However, it seems to play a minor role in the ORR activity of NCNP−CNF composite owing to a very small doping content. Fourth, a specific interaction at the interface between NCNPs and CNF surface may create unexpected properties that influence the ORR activity of NCNP−CNF composite. Long-term durability and methanol tolerance are the indispensable characteristics for ORR catalysts in practical fuel cell applications. The durability of NCNP−CNF composite was evaluated by measuring the loss in current density by mean of chronoamperometric response at a constant potential of −0.35 V in an O2-saturated 0.1 M KOH solution for 40000 s (Figure 5a). After continuous operation for 40000 s, the relative current density of NCNP−CNF composite still remained as high as 85%, whereas that of Pt/C suffered a significant decrease to 69%. A methanol tolerance test was also carried out by adding 3.0 M methanol to O2-saturated 0.1 M


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KOH solution during measurement of the chronoamperometric response (Figure 5b). Upon the addition of 3.0 M methanol, the current density of NCNP−CNF composite slightly changed by less than 7%, whereas an instantaneous drop in current density was clearly observed for Pt/C. This result suggests that NCNP−CNF composite has a higher tolerance to methanol crossover than Pt/C. Figures 5c and 5d show the CV curves before and after the addition of 3.0 M methanol for NCNP−CNF composite and Pt/C, respectively. There is no noticeable change in the ORR peak for NCNP−CNF composite. In contrast, the ORR peak of Pt/C disappeared, while a pair of anodic peaks at −0.15 and −0.08 V has emerged due to methanol oxidation. The electrochemical durability tests confirm that NCNP−CNF composite exhibits much better long-term durability and higher tolerance to methanol crossover than Pt/C.

CONCLUSION In summary, we have developed the NCNP−CNF composite as an efficient metal-free ORR catalyst through a new approach assisted by a solution plasma process. The NCNP−CNF composite exhibits enhanced catalytic activity toward ORR via a dominant four-electron reduction pathway in alkaline solution, which is much superior to both CNFs and NCNPs. The enhanced ORR activity of NCNP−CNF composite can be mainly attributed to the synergistic contributions of NCNPs and CNFs. In the NCNP−CNF composite, NCNPs provide high surface density of ORR active sites (i.e., pyridinic N and graphitic N) with meso/macroporosity, while CNFs serve as highly conductive paths for charge transport. Moreover, the NCNP−CNF composite shows excellent long-term operation durability and high tolerance to methanol crossover in comparison with the commercial Pt/C catalyst. Based on the electrochemical evaluation and stability tests, the NCNP−CNF composite can potentially be a promising cathode


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catalyst candidate to replace the commercial Pt/C in fuel cell applications. We believe that this successful synthetic approach offers a powerful impetus and also paves a new way for the design and synthesis of heteroatom-doped carbon-based composite for electrochemical energy applications.

ASSOCIATED CONTENT Schematic illustration of the solution plasma process system used for the synthesis of NCNP−CNF composite, FESEM and TEM images of CNFs and NCNPs, optical emission spectrum measured from the plasma generated in 2-cyanopyridine, XPS survey spectra, highresolution XPS C 1s and O1s spectra, surface and bulk elemental compositions obtained from XPS and EA measurements, percentages and absolute nitrogen contents of various nitrogen states of NCNPs and NCNP−CNF composite, electrical conductivity measurement, LSV curves at various rotation speeds from 225 to 2500 rpm of various catalysts, the K−L plots and calculated n values of various catalysts at the potentials from −0.4 to −1.0 V, comparison of ORR catalytic activity of NCNP−CNF composite and NCNP/CNF mixture.

AUNTHOR INFORMATION Corresponding author *E-mail: [email protected], [email protected] (G. Panomsuwan)

Notes


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The authors declare no competing financial interest

ACKNOWLEDGEMENTS This work was partially supported by NU-PPC Plasma Chemical Technology Laboratory, The Petroleum and Petrochemical College, Chulalongkorn University.

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(58) Parvez, K.; Yang, S.; Hernandez, Y.; Winter, A.; Turchanin, A.; Feng, X.; Müllen, K. Nitrogen-Doped Graphene and Its Iron-Based Composite as Efficient Electrocatalysts fr Oxygen Reduction Reaction. ACS Nano 2012, 6, 9541−9550. (59) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Exploration of the Active Center Structure of Nitrogen-Doped GrapheneBased Catalysts for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 7936−7942. (60) Zhang, C.; Hao, R.; Laio, H.; Hou, Y. Synthesis of Amino-Functionalized Graphene as Metal-Free Catalyst and Exploration of the Roles of Various Nitrogen States in Oxygen Reduction Reaction. Nano Energy 2013, 2, 88−97. (61) Liu, M.; Song, Y.; He, S.; Tjiu, W. W.; Pan, J.; Xia, Y.-Y.; Liu, T. Nitrogen-doped Graphene Nanoribbons as Efficient Metal-Free Electrocatalysts for Oxygen Reduction . ACS Appl. Mater. Interface 2014, 6, 4214−4222. (62) Xing, W.; Yin, G.; Zhang, J. Rotating Electrode Methods and Oxygen Reduction Electrocatalysts, Elsevier, Amsterdam, 2014, Chapter 6. (63) Wang, S.; Iyyamperumal, E.; Roy, A.; Xue, Y.; Yu, D.; Dai, L. Vertically Aligned BCN Nanotubes as Efficient Metal-Free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-Doping with Boron and Nitrogen. Angew. Chem. Int. Ed.2011, 50, 11756−11760.


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LIST OF FIGURES

Scheme 1. Schematic representation of the synthesis of NCNP−CNF composite via a solution plasma process.

Figure 1. (a) FESEM and (b) TEM images of NCNP−CNF composite. The corresponding SAED pattern of NCNP−CNF composite is shown in the inset of (b). HRTEM images at (c) CNFs and (d) NCNPs.


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Figure 2. (a) N2 adsorption−desorption isotherms and (b) pore size distribution of CNFs, NCNPs, and NCNP−CNF composite. (c) XRD patterns and (d) Raman spectra with the corresponding ID/IG values of CNFs, NCNPs, and NCNP−CNF composite.

Figure 3. (a) XPS survey spectra of CNFs, NCNPs, and NCNP−CNF composite. The arrows indicate the presence of N 1s peak. (b) High-resolution XPS N 1s spectra of NCNPs and NCNP−CNF composite. (c) Nitrogen doping contents and distribution of four types of nitrogen


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states in NCNPs and NCNP−CNF composite. (d) Schematic representation of bonding configurations of nitrogen atoms in the graphitic plane (i.e., pyridinic N, pyrrolic N, graphitic N, and pyridinic N-oxide).

Figure 4. The electrochemical evaluations of CNFs, NCNPs, NCNP−CNF composite, and Pt/C: (a) CV curves in N2 and O2-saturated 0.1 M KOH solutions at a scan rate of 50 mV s−1 (N2: dashed line, O2: solid line). (b) LSV curves on an RRDE in an O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1 after background correction by subtracting the measured current density in N2-saturated solution. The rotation speed was 1600 rpm, and the ring potential was fixed at 0.5 V. (c) Electron transfer number (n) and (d) HO2− yield derived from the disk and ring currents at the potential range from −1.0 to −0.3 V. (e) The K−L plots (J−1versusω−1/2) of


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various catalysts derived from the LSV curves at various rotation speeds from 225 to 2500 rpm at −0.4 V. (f) Tafel plots of various catalysts derived from the LSV curves at 1600 rpm (Figure 4b).

Figure 5. (a) Chronoamperometric responses of NCNP−CNF composite and Pt/C in O2saturated 0.1 M KOH solution at −0.35 V for 40000 s (1600 rpm). (b) Methanol tolerance test of NCNP−CNF composite and Pt/C in O2-saturated 0.1 M KOH solution at −0.35 V (1600 rpm). The arrow at 3000 s indicates the addition of 3.0 M methanol. CV curves of (c) NCNP−CNF composite and (d) Pt/C in O2-saturated 0.1 M KOH solution with and without 3.0 M methanol at a scan rate of 50 mV s−1.

TABLE OF CONTENT


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Nitrogen-doped carbon nanoparticle−carbon nanofiber (NCNP−CNF) composite has been developed via a new approach assisted by a solution plasma process. The resulting NCNP−CNF composite exhibits a significantly enhanced oxygen reduction reaction (ORR) activity through a dominant four-electron pathway with excellent electrochemical durability in alkaline solution. The NCNP−CNF composite can be a promising cathode catalyst candidate in fuel cell and electrochemical energy applications.


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Nitrogen-doped carbon nanoparticle−carbon nanofiber (NCNP−CNF) composite has been developed via a new approach assisted by a solution plasma process. The resulting NCNP−CNF composite exhibits a significantly enhanced oxygen reduction reaction (ORR) activity through a dominant four-electron pathway with excellent electrochemical durability in alkaline solution. The NCNP−CNF composite can be a promising cathode catalyst candidate in fuel cell and electrochemical energy applications. 40x27mm (600 x 600 DPI)


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Scheme 1 54x35mm (600 x 600 DPI)



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Figure 1 85x85mm (600 x 600 DPI)


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Figure 4 114x155mm (600 x 600 DPI)



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Nitrogen-Doped Carbon NanoparticleâCarbon Nanofiber Composite

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