Self-Assembly Synthesis of Cobalt- and Nitrogen-Coembedded

May 9, 2017 - Developing highly cost-effective catalysts for an oxygen reduction reaction (ORR) in fuel cells is highly significant but still full of ...
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Research Article pubs.acs.org/journal/ascecg

Self-Assembly Synthesis of Cobalt- and Nitrogen-Coembedded Trumpet Flower-Like Porous Carbons for Catalytic Oxygen Reduction in Alkaline and Acidic Media Hao Jiang,† Yisi Liu,†,‡ Jiayu Hao,† Yanqiu Wang,† Wenzhang Li,*,†,§,∥ and Jie Li*,†,§ †

School of Chemistry and Chemical Engineering, Central South University, 123 Qingshui Road, Changsha 410083, China Department of Mechanical and Materials Engineering, University of Western Ontario, 1151 Richmond Street, London, Ontario N6A 5B9, Canada § Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, 123 Qingshui Road, Changsha 410083, China ∥ Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Central South University, Ministry of Education, 932 Lushan South Road, Changsha, 410083, China ‡

S Supporting Information *

ABSTRACT: Developing highly cost-effective catalysts for an oxygen reduction reaction (ORR) in fuel cells is highly significant but still full of challenges. In this work, cobalt- and nitrogen-coembedded threedimensional (3D) trumpet flower-like porous carbons (CoNC) have been prepared by a simple two-step self-assembly technique, using carbon quantum dots (CQDs) as the carbon precursor and a supermolecular gel of self-assembled melamine and Co2+ ions as the nitrogen and cobalt sources. The resultant CoNC catalysts possess unique 3D trumpet flowerlike structures, efficient charge transfer ability, and abundant Co−Nx active sites. As a catalyst for ORR, the optimized CoNC-800 (pyrolyzed at 800 °C) exhibits efficient electrocatalytic activities, longer-term stability, and strong endurance to methanol both in acidic and alkaline media. It can be worked as a prospective substitute for a commercial Pt/C catalyst for ORR in the widespread implementation of fuel cells. KEYWORDS: Fuel cells, Oxygen reduction reaction (ORR), Cobalt and nitrogen coembedded, 3D Trumpet flower-like structure, Catalytic activity

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catalysts (M−N−C),8−10 transition metal oxides,11−14 sulfides,15 and their hybrids.16 Advanced carbon and heteroatom (N, P, and S)-functionalized carbon materials have been considered to be promising metal-free electrocatalysts for ORR due to their high activities and superior stability, as well as low cost. For instance, nitrogen-doped porous carbons with a flower-like hierarchical architecture exhibited outstanding ORR catalytic activity in alkaline media.17 However, most of the metal-free carbon catalysts exhibit an excellent ORR performance in alkaline media but dissatisfactory activity in acidic media because of the material structure affects.18 Thus, it is essential to explore advanced carbon-based catalysts with high cost performance and strong durability for ORR both in acidic media and alkaline media.

INTRODUCTION

Fuel cells and metal−air batteries have been considered as some of the most prospective renewable energy storage and conversion techniques to meet the rising energy demands and environmental pressure because of their high capacity and sustainable and environmental friendliness features. However, the energy-conversion efficiency of these renewable energy systems is seriously obstructed by the sluggish kinetic process of ORR occurring at the cathode. Therefore, a large amount of Pt-based materials have been adopted as cathode catalysts to surmount the slowness of the ORR process and improve working efficiency.1−4 Nevertheless, the rareness, high cost, and easy poisoning of Pt-based noble metal catalysts seriously obstruct the large-scale applications of these promising energy systems. Consequently, from a long-term point of view, developing inexpensive Pt substitutes with excellent ORR activity and strong durability is the inevitable trend. To date, many new materials have been developed to attempt replacing Pt-based precious metal catalysts, such as heteroatom-functionalized carbon materials,5−7 metal−nitrogen-doped carbon © 2017 American Chemical Society

Received: March 2, 2017 Revised: April 11, 2017 Published: May 9, 2017 5341

DOI: 10.1021/acssuschemeng.7b00655 ACS Sustainable Chem. Eng. 2017, 5, 5341−5350

Research Article

ACS Sustainable Chemistry & Engineering

Synthesis of PCFs and NC. For comparison, nitrogen-doped porous carbon (denoted as NC) was synthesized using the same method only without adding Co(NO3)2·6H2O. Simplex porous carbon frameworks (denoted as PCFs) were prepared by direct pyrolysis of the CQDs at 800 °C under argon protection. Materials Characterization. X-ray diffraction (XRD) patterns were obtained from an X-ray diffractometer (SIMENS d500) to investigate the crystal structure of each sample. The surface morphologies of the products were analyzed with a scanning electron microscope (SEM, Nova NanoSEM 230). The detailed microstructures of the samples were analyzed by a transmission electron microscope (TEM, FEI TECNAI G2 F20). N2 adsorption/desorption tests were conducted at 77 K by a gas adsorption analyzer (JWBK132F, Beijing). Raman spectra of CoNC catalysts were obtained from a LabRAM Hr800 confocal Raman microscopic system under an excitation laser of 532 nm. The surface elemental composition and bonding configuration of NC and CoNC-800 were detected by an Xray photoelectron spectroscopy (XPS) spectrometer (K-Alpha 1063). The UV−vis absorption spectrum of the dispersed GQDs ethanol solution was measured by a UV−vis spectrophotometer (UV-1801). The loading of Co on CoNC-800 was detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES, PS-6, Baird). Electrode Preparations. To prepare the catalyst ink for electrochemical analysis, 4 mg of catalytic powders were dispersed in a 10 μL of Nafion (5 wt %) and 990 μL of ethanol (98 vol %) mixed solution. Subsequently, the mixed solution was sonicated for 10 min to form a homogeneous catalytic ink. Finally, 10 μL of catalytic ink was placed on a glassy carbon rotating disk electrode (RDE) (5 mm, Pine). After being dried at ambient temperature, the catalyst with a content of 0.2 mg cm−2 was loaded on the working electrode. As a comparison, a Pt/C (20 wt %) electrode was obtained with the similar method. As a comparison, a commercial Pt/C catalyst (20 wt % platinum on carbon black; platinum particle size < 3.5 nm) obtained from Alfa-Aesar was used in our work as a standard catalyst. The loading content of Pt/C on the electrode and the test conditions are consistent with CoNC samples. Electrochemical Measurements. To analyze the ORR activities of the as-prepared catalysts, a series of electrochemical tests (including CV, LSV, EIS, and chronoamperometry) were recorded on a Zahner (Zennium, Germany) electrochemical analyzer attached with an RDE. All the ORR electrochemical tests were executed in a normal threeelectrode system. Throughout the testing process, an Ag/AgCl/3 M KCl electrode was chose as reference electrode. Here, 0.1 M KOH and 0.5 M H2SO4 aqueous solutions were, respectively, used as the electrolyte in the process of electrochemical measurements and deaerated by high purity O2 and N2 (99.99% pure) according to concrete conditions. All the potentials in this work were converted to a reversible hydrogen electrode (RHE), E(RHE) = E(Ag/AgCl) + 0.976 V. Cyclic voltammetry (CV) experiments were recorded with a scan rate of 50 mV s−1. Linear sweep voltammetry (LSV) experiments were executed at 5 mV s−1 under rotating speeds varying from 400 to 1600 rpm. The ORR onset potential Eonset is defined as the potential at the intersection of the tangents just before and after the onset of rise in the disc current in the RDE LSV curve.26 The Koutechy−Levich (K-L) equations (eqs 1−3) are used to calculate the kinetic current density (Jk) and transferred electron numbers (n):

It is noteworthy that transition metal (Ni, Co, Fe)- and nitrogen-functionalized carbon materials (M−N−C) are only one of several successful examples to exhibit comparable or even better performance to noble metal-based catalysts both in acidic and alkaline media. Very recently, nitrogen-functionalized graphene spheres with a hollow structure and decorated with cobalt nanoparticles exhibited highly ORR activities and superior stability in a wide pH range.19 Fe3C wrapped into nitrogen-doped carbon frameworks with hierarchical porous structures have been fabricated and confirmed to be an effective and robust ORR catalyst both in alkaline and acidic environments.20 However, these hierarchically porous M−N− C catalysts were generally synthesized with the aid of external templates such as silica templates (SiO2 spheres and SBA15),19,20 polystyrene microspheres,21,22 solid NaCl,23 and oxides templates (MgO and porous alumina),24 which often have some shortcomings such as multisteps, rigorous conditions, and high cost. Therefore, facile synthesis of novel M−N−C catalysts with unique structures and excellent ORR performance over a wide pH range of aqueous media is still of great importance but full of challenges. Herein, we report a facile self-assembly approach to synthesis cobalt- and nitrogen-coembedded three-dimensional trumpet flower-like porous carbons (CoNC). Compared to the cumbersome template method, the proposed synthesis strategy in this work mainly contains two self-assembly processes, namely, melamine, HNO3, and Co2+ ions self-assembled to be a supermolecular gel and carbon quantum dots (CQDs) as “building blocks” to self-assemble into cross-linked 3D trumpet flower-like porous carbons. The resultant CoNC-800 is constructed with interconnected carbon sheets with a 3D trumpet flower-like porous structure and has uniformly Co and N coembedded and strong electron transfer ability, all of which are advantageous for enhancing its ORR activities. As would be expected, except for the superior durability and methanol tolerance, the ORR activity of CoNC-800 is equally outstanding to a Pt/C catalyst not only in alkaline but also in acidic media.

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EXPERIMENTAL SECTION

Synthesis of CQDs. The synthesis of CQDs was based on a previous procedure with some modifications.25 Typically, 4 g of NaOH and 20 mL of acetone were mixed under vigorous magnetic stirring for 1 h, and then, reddish brown powders were obtained after the mixture was placed at room temperature for at least 3 days without any other additional treatments. The powders were washed with concentrated HCl (37 wt %) and deionized water several times. Finally, CQDs were obtained after being dried at 60 °C. Synthesis of CoNC. In a typical preparation, 500 mg of melamine and 50 mg of Co(NO3)2·6H2O were dissolved in 19.5 mL of deionized water under vigorous stirring and heated to form a transparent solution. Subsequently, 0.5 mL of concentrated HNO3 (68 wt %) was put into the above transparent solution and churned for 5 min. A Co2+−melamine−HNO3 hydrogel was formed when the mixture was naturally cooled to ambient temperature. To prepare 3D trumpet flower-like CoNC samples, the obtained xerogel and 2 g of CQDs (reddish brown powders without acid treatment) were ground about 10 min to form a fine mixture. Then, the mixture was pyrolyzed at different temperatures (700, 800, and 900 °C, named CoNC-700, CoNC-800, and CoNC-900, respectively) for 2 h under an Ar atmosphere. Finally, the CoNC samples were acid treated in 1 M HCl for 8 h to remove any unstable and ORR-nonreactive phases and then washed several times with deionized water. The final products were dried at 60 °C for further use.

1 1 1 = + J Jk Bω1/2

(1)

B = 0.2nFD02/3ν−1/6C0

(2)

Jk = nFkC0

(3)

where J and Jk are the measured current density and the kinetic current density, respectively. Here, ω is the electrode rotation speed, F is the Faraday constant (96485 C mol−1), D0 is the diffusion coefficient of O2 (1.9 × 10−5 cm2 s−1 for 0.1 M KOH and 1.15 × 10−5 cm2 s−1 for 5342

DOI: 10.1021/acssuschemeng.7b00655 ACS Sustainable Chem. Eng. 2017, 5, 5341−5350

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Figure 1. Schematic illustration for the synthesis of CoNC.

Figure 2. (a−c) SEM, (d) TEM, and (e, f) HRTEM images of CoNC-800. (g) STEM image and corresponding elemental mapping of CoNC-800. 0.5 M H2SO4), ν is the kinetic viscosity (0.01 cm2 s−1), and C0 is the concentration of O2 (1.2 × 10−6 mol cm−3). Electrochemical impedance spectroscopy (EIS) tests were implemented on a Zahner (Zennium, Germany) electrochemical analyzer in the frequency range from 100 kHz to 10 mHz. The applied potential and excitation amplitude were set as 0.68 V (vs RHE) and 5 mV, respectively. Chronoamperometry. The stabilities of CoNC-800 and Pt/C were tested by the current−time (i-t) chronoamperometric responses at 0.68 V (vs RHE) in 0.1 M KOH and 0.62 V (vs RHE) in 0.5 M H2SO4 for 12,000 s. A methanol tolerance test was implemented through an i-t response at 0.68 V (vs RHE) in 0.1 M KOH and 0.62 V

(vs RHE) in 0.5 M H2SO4 with a 0.5 mL of methanol (3 M) addition at 300 s.

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RESULTS AND DISCUSSION

The overall self-assembly synthesis procedure for CoNC catalysts is schematically illustrated in Figure 1. During the preparation process, melamine not only served as a nitrogen source for CoNC catalysts but also acted as skeletons to form a supermolecular gel. This is because each melamine molecule possesses nine H-bond sites and several unpaired electrons, which can be used as the H-bond acceptor.27,28 As for HNO3, it works as a pH regulator for self-assembling into a supra5343

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Figure 3. (a) N2 absorption/desorption isotherms and (b) pore size distributions of PCFs and CoNC-800. (c) XRD pattern and (d) Raman spectra of CoNC-700, CoNC-800, and CoNC-900.

surface of CQDs.32 After annealing the mixtures of CQDs and supermolecular gel at 800 °C under an Ar atmosphere, the asobtained CoNC-800 exhibited unique 3D trumpet flower-like open-cell architectures, which were composed of interconnected carbon nanosheets only about 10−20 nm thick (Figure 2a and c). The SEM images of CoNC-700 and CoNC-900 are presented in Figure S4. The corresponding TEM and HRTEM images of CoNC-800 (Figure 2d and f) further revealed that numerous metallic Co nanoparticles (NPs) with the size of only a few nanometers were homogeneously and thoroughly incorporated into the flat and thin carbon nanosheets. The lattice spacings of 0.201 and 0.215 nm (Figure 2f) correspond to the (111) and (220) planes of metallic cobalt, respectively.33,34 The elemental mapping results (Figure 2g) reconfirmed the existence and even distribution of C, O, N, and Co elements in CoNC-800. Such small size and well-dispersed Co NPs of CoNC-800 can be mainly attributed to the following reasons: (1) During the formation of supramolecular hydrogel, the weak interactions between melamine and Co2+ ions prevented Co NPs to aggregate.35 (2) During the pyrolysis process, the decomposition of xerogel and the carbonization of CQDs further segregated and encapsulated the Co NPs into graphite frameworks, protecting them from agglomeration even under high temperature. Compared to the large-sized metallic Co anchored on the surface of carbon supports, the CoNC-800 with a small size and fully embedded Co NPs may have better activity and stability in acid media. Unlike the CoNC samples, only a large number of ultrathin carbon nanosheets were formed and stacked together when the CQDs were solely pyrolyzed at 800 °C under an Ar atmosphere (Figure S5). It is indicated that the large amounts of gas and instantaneous high internal pressure generated by the decomposition of the supermolecular gel boosts the formation of 3D trumpet flowerlike porous architecture. Such a unique 3D trumpet flower-like structure of CoNC-800 is not only conducive to reducing the diffusion distance and facilitating the oxygen molecules access the active centers in all directions but also to contributing to

molecular hydrogel with melamine. Meanwhile, Co2+ ions can be adsorbed by the melamine molecules with the aid of weak electrostatic interactions.29 In this way, under the actions of the H-bond and weak electrostatic interactions, a supermolecular gel was formed when melamine mixed with HNO3 and Co2+ ions (Figure S1).27,28 The interaction between melamine, HNO3, and Co2+ ions is beneficial to uniformly introduce Co and N atoms into graphite structures and prevent the agglomeration of metallic Co.27 Moreover, a large amount of Na-containing compounds existed in the obtained CQDs (without acid treatment) after an Aldol reaction. Previous studies have suggested that Na-containing compounds are good catalysts to stimulate a reaction potential and undermine the C−O bond to accelerate the carbonization reaction speed and increase the carbon conversion efficiency.25,30 Thus, under the high temperature and Na catalysis effect, the well-dispersed and small-sized CQDs powders with the linkage of numerous Orich functional groups were rapidly decomposed to generate abundant carbon atoms, which then act as building blocks to self-assemble into cross-linked carbon nanosheets. After pyrolyzing the above homogeneous mixture, large amounts of gas and instantaneous high internal pressure were generated by the decomposition of the supermolecular gel and the carbonization of CQDs, which boosts the formation of a 3D trumpet flower-like porous architecture and facilitates cobalt and nitrogen uniformly coembedded into the graphite matrix. SEM and TEM are exploited to investigate the detailed morphologies and microstructures of the CQDs, PCFs, and CoNC samples. As observed from Figure S2, the CQDs are evenly distributed, and the size of them mainly focused in the range from 3 to 5 nm. The lattice spacing of 0.208 nm was further revealed by the high-resolution TEM (HRTEM) image of CQDs (Figure S2b), which corresponds to the (100) plane of graphite.31 The UV−vis absorption spectrum of the dispersed brown CQDs ethanol solution exhibited a distinct absorption peak at ca. 310 nm (Figure S3), which may be assignable to the n−π* electronic transition occurring on the 5344

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Figure 4. (a) Wide XPS survey spectra of NC and CoNC-800. (b,c) High resolution N 1s spectra of NC and CoNC-800. (d) Co 2p spectra of CoNC-800.

Raman spectra, as shown in Figure 3d. The ratio of the D band (1350 cm−1) and G band (1588 cm−1) is generally adopted to detect the graphitization and disorder of carbon materials.37 Obviously, the ID/IG ratios of CoNC catalysts decreased with increasing heat treatment temperature, indicating that a higher heat treatment temperature tends to obtain a CoNC sample with a higher graphitization degree, which is consistent with the analysis of XRD patterns. The elemental composition and bonding configuration of NC and CoNC-800 were executed by XPS. Figure 4a distinctly reveals that CoNC-800 was composed of C, N, O, and Co elements, but only C, N, and O existed in the NC sample. All results of the atomic percentages are listed in Table S2. Observed from the XPS N 1s spectra of NC and CoNC-800 (Figure 4b and c), N types can be mainly classified into pyridinic N (398.4 eV), pyrrolic N (400 eV), graphitic N (401.4 eV), and pyridinic N oxide (403−406 eV).11 Besides, a satellite peak at 399.2 eV (Figure 4c) can be assigned to Co− Nx species, which is caused by the slight difference between the binding energy of pyridinic N and Co−Nx.38,39 Among them, pyridinic N, Co−Nx, and graphitic N are perceived as the active centers for ORR. Although the nitrogen content (N 1s) of CoNC-800 is lower than that of NC (Table S2), the total amount of pyridinic N (including Co−Nx) and graphitic N for CoNC-800 is much higher than that of NC (Table S3), indicating that the existence of cobalt is in favor of generating pyridinic N (including Co−Nx) and graphitic N. The core level spectra of Co 2p is composed of five peaks (Figure 4d). The low energy peak at 778.4 eV is attributed to metallic Co (0). The characteristic peaks at 780.5 eV is attributed to Co−Nx components, which are very beneficial to enhancing the ORR activities of catalysts.40 The other peaks at 780.5 and 796.2 eV were assigned to Co (II) (namely, Co−O species).19 Moreover, shakeup peaks of Co 2p1/2 have been observed at 802.4 eV. Generally, Co−O sites are not stable in acidic environments and have been recognized as inactive toward ORR.41

expose more active centers to the electrolyte to obtain preferable electrocatalytic performance.36 The surface area and porous texture of the resultant catalysts were characterized on the basis of N2 adsorption/desorption analysis. As illustrated in Figure 3a, both of the PCFs and CoNC-800 displayed the type IV adsorption/desorption isotherm with an obvious type-H4 hysteresis loop at P/P0 between 0.45 and 1.0, which is characteristic of mesoporous and slit-shaped pores.10 Unlike the PCFs, a distinct N2 uptake at low relative pressure P/P0 < 0.1 was observed for CoNC-800, suggesting the formation of some micropores.29 Moreover, both CoNC-800 and PCFs have a wide pore size distribution (1−300 nm), as presented in Figure 3b. According to the BET results shown in Table S1, the specific surface area and pore volume of CoNC-800 are much larger than those of PCFs. The formation of a hierarchical porous structure and large surface area of CoNC-800 is mainly attributed to the fast-moving gas generated by the decomposition of the supermolecular gel and the carbonization of CQDs. The resultant CoNC-800 with a unique 3D trumpet flower-like architecture and high specific surface area could provide desired support for the Co−Nx active sites and facilitate the rapid transportation of ORRrelevant species. The crystalline structures of CoNC samples were explored by the XRD characterization technique. As displayed in Figure 3c, a pronounced C (002) characteristic peak can be observed at around 26.5°, revealing the formation of graphitic carbon derived from CQDs and the high graphitization degree for CoNC samples.6 In addition, three strong diffraction peaks appear at 44.09°, 51.59°, and 75.80°, which were well indexed to the (111), (200), and (220) planes of Co NPs (JCPDS 150806).9 Obviously, the peak intensity of Co NPs was elevated with increasing pyrolysis temperature. It was suggested that the higher pyrolysis temperature contributed to reducing Co2+ ions into Co NPs and improving its crystallinity. Further structural information about the CoNC catalysts was observed from 5345

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Figure 5. (a) CV curves for PCFs, NC, CoNC-700, CoNC-800, and CoNC-900 in O2-saturated 0.1 mol L−1 KOH electrolyte with a scan rate of 50 mV s−1. (b) LSV curves for PCFs, NC, CoNC-700, CoNC-800, CoNC-900, and Pt/C in O2-saturated 0.1 mol L−1 KOH electrolyte with a rotate speed of 1600 rpm. (c) Electrochemical activity given as the kinetic current density (Jk) at 0.58 V for all CoNC samples and Pt/C catalyst. The numeral on the bar represents the corresponding electron transfer number. (d) Tafel plots of CoNC-700, CoNC-800, CoNC-900, and Pt/C catalysts.

Consequently, the ORR activity of CoNC-800 in acidic media may be mainly attributed to the Co−Nx sites. Remarkably, the ICP-AES test revealed that the total loading of Co on CoNC800 was approximately 14.2 wt % (Table S4), which was higher than that detected by XPS (ca. 3.15 wt % converted from 0.66 at. % of Co, Table S2). The main reason for this phenomenon is that the deeply embedded Co species (Co NPs or Co2+) in the carbon matrix were hard to detect by a surface composition analytical technique (XPS).42 To appraise the ORR performance of the as-prepared samples, CV measurements were first performed in a 0.1 mol L−1 KOH electrolyte (Figure 5a). Compared to a characterless voltammetric current without any palpable reductive peak shown in an N2-saturated electrolyte, an obvious reduction peak was detected at around 0.82 V in an O2-saturated electrolyte, indicating the outstanding ORR activity of CoNC800 (Figure S6). To further investigate the electrocatalytic activities of each sample, RDE tests were executed to obtain the corresponding LSV curves (Figure 5b). The CV and LSV curves of the Pt/C electrode in alkaline media are shown in Figure S7. Some ORR performance parameters of each catalyst are presented in Table 1. Remarkably, the CoNC-800 exhibited prominent ORR activity with the most positive onset potential Eonset at 0.93 V to those of CoNC-900 (0.89 V), CoNC-700 (0.86 V), NC (0.83 V), and PCFs (0.76 V). Not only that but the Eonset and E1/2 (half-wave potential) of CoNC-800 both were only 10 mV less than those of Pt/C and exceed some M− N−C catalysts reported in the literature (Table S5). The prominent ORR activities for CoNC-800 in alkaline media may benefit from the synergetic effect of the following factors: (1) The unique 3D trumpet flower-like porous architecture is conducive to exposing more of the active sites and facilitating the accessibility of oxygen. (2) The imbedding of heteroatoms

Table 1. ORR Performance Parameters of Different Samples Tested in Alkaline Media catalyst PCFs NC CoNC-700 CoNC-800 CoNC-900 20% Pt/C

Ep Eonset (V vs RHE) (V vs RHE) 0.64 0.68 0.75 0.82 0.79 0.85

0.76 0.83 0.86 0.93 0.89 0.94

E1/2 (V vs RHE)

diffusion-limiting current (mA cm−2)

0.66 0.71 0.79 0.82 0.80 0.83

2.84 3.63 4.61 5.34 4.98 5.46

into the graphitic lattice can induce charge redistribution and facilitate the adsorption of oxygen and the following reduction reaction on carbon. (3) The presence of metallic Co may form a Co−Nx active center and selectively promote the formation of N−C active sites. Moreover, the RDE tests with various rotating velocities from 400 to 1600 rpm were implemented to thoroughly explore the ORR kinetics of the CoNC catalysts. A set of LSV curves and Koutecky−Levich (K-L) plots of CoNC-700, CoNC-800, and CoNC-900 were displayed in Figure S8. Due to the reduced diffusion distance at high velocity, the current density augments with increasing rotation speeds.29,43 The transferred electron number (n) and the kinetic current density (Jk) of each catalyst can be calculated from the K-L equation. As shown in Figure 5c, the value of n is 3.99 at 0.58 V (vs RHE), suggesting that CoNC-800 is more inclined to choose a 4e− pathway to reduce oxygen. Meanwhile, the corresponding Jk of CoNC-800 is 29.7 mA cm−2, which is very close to that of Pt/C (36.1 mA cm−2) and outperforms CoNC-700 and CoNC-900 (Figure 5c). The ORR kinetics of the above catalysts was also discussed by drawing their Tafel plots, as shown in Figure 5d. A same slope 5346

DOI: 10.1021/acssuschemeng.7b00655 ACS Sustainable Chem. Eng. 2017, 5, 5341−5350

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Figure 6. (a) CV curves for PCFs, NC, CoNC-700, CoNC-800, and CoNC-900 in O2-saturated 0.5 mol L−1 H2SO4 electrolyte with a scan rate of 50 mV s−1. (b) LSV curves for PCFs, NC, CoNC-700, CoNC-800, CoNC-900, and Pt/C in O2-saturated 0.5 mol L−1 H2SO4 electrolyte with a rotate speed of 1600 rpm, (c) Electrochemical activity given as the kinetic current density (Jk) at 0.36 V (vs RHE) for all CoNC samples and Pt/C catalyst. The numeral on the bar represents the corresponding electron transfer number. (d) Tafel plots of CoNC-700, CoNC-800, CoNC-900, and Pt/C catalysts.

of CoNC-800 after long-term electrochemical tests in 0.5 mol L−1 H2SO4 was also obtained. As shown in Figure S11, the CoNC-800 catalyst shows a decrease in ORR current and negatively shifts in Eonset by 90 mV after long-term electrochemical tests in acid solution due to losing Co species (Co NPs, Co−Nx, and Co−O moieties), while previous studies showed that the Co NPs and Co−O moieties are quite unstable in an acid environment and contribute very little to ORR.41,44 This means that the Co−Nx moieties in CoNC samples play a key role to catalyze ORR in acidic media. On the basis of the above analysis and compared to the activity of metal-free carbon catalysts (PCFs and NC), we surmise that the enhanced catalytic activity on CoNC catalysts in acid media is largely attributed to the synergistic effect of unique 3D trumpet flowerlike structures and abundant Co−Nx active sites. The reaction kinetics of the CoNC catalysts were also studied by RDE measurements in a 0.5 mol L−1 H2SO4 electrolyte. A series of LSV curves and K-L plots of CoNC catalysts are presented in Figure S12. The kinetically controlled current was increased with increasing rotation rates. The K-L plots of CoNC catalysts displayed good linearity at the corresponding potential range, indicating an efficient electron transfer process. On the basis of the K-L plots of the above catalysts at 0.36 V (vs RHE), the calculated values of Jk and n per oxygen molecule during the ORR are shown in Figure 6c. Significantly, the kinetic current density (16.5 mA cm−2) and the transferred electron number (3.46) of CoNC-800 were the largest among all the CoNC catalysts investigated in this work and comparable to those of Pt/C catalyst. The small values of Jk and n of CoNC-700 may result from its low degree of graphitization, relatively poor electrical conductivity, and fragmentized structure, which are consistent with the conclusion obtained from Raman spectra and SEM images. Nevertheless, the increase in pyrolysis temperature could also result in the loss of doped heteroatoms and the agglomeration of metallic Co, thus reducing the electrocatalytic activity of

was observed for CoNC-800 and Pt/C (72 mV/dec), revealing that theypossess a very similar reaction mechanism and good kinetic process toward ORR.11 Except for the outstanding performance in alkaline media, the ORR activity of CoNC-800 is also highly competitive to precious metal catalysts in acidic media. As shown from the CV curves in Figure S9, a pronounced reduction peak approximately at 0.65 V can be observed in an O2-saturated 0.5 mol L−1 H2SO4 electrolyte. However, the reduction peak potentials of PCFs and NC were much more negative than that of CoNC800 (Figure 6a). The poor ORR activities of metal-free carbon catalysts (PCFs and NC) in acid media are mainly due to the insufficient active centers within them.18 At the same time, the LSV curves of these catalysts were given for further comparing their ORR activities (Figure 6b). The CV and LSV curves of the Pt/C catalyst in acidic media are presented in Figure S10. The ORR performance parameters of each catalyst in acidic media are summarized in Table 2. As expected, the CoNC-800 displayed an Eonset of 0.77 V and E1/2 of 0.63 V, which are better than those of other samples prepared in this work and also comparable to those of Pt/C and some M−N−C catalysts reported in the literature (Table S6). To investigate the dominative active sites for ORR in acidic media, the LSV curve Table 2. ORR Performance Parameters of Different Samples Tested in Acidic Media

catalyst

Ep (V vs RHE)

PCFs NC CoNC-700 CoNC-800 CoNC-900 20% Pt/C

0.05 0.20 0.49 0.65 0.59 0.75

Eonset E1/2 (V vs RHE) (V vs RHE) 0.29 0.42 0.63 0.77 0.74 0.80

0.17 0.24 0.46 0.63 0.55 0.69

diffusion-limiting current (mA cm−2) 1.65 3.44 2.98 4.83 3.79 4.67 5347

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Figure 7. Methanol crossover tolerance test of CoNC-800 and Pt/C conducted by chronoamperometric response in O2-saturated (a) 0.1 mol L−1 KOH and (b) 0.5 mol L−1 H2SO4. The i-t chronoamperometric curves of CoNC-800 and Pt/C catalysts in O2-saturated (c) 0.1 mol L−1 KOH and (d) 0.5 mol L−1 H2SO4.

After adding 0.5 mL methanol (3 M) at 300 s, the ORR relative current of Pt/C catalyst significantly decreased owing to methanol poisoning, whereas no significant current changed to CoNC-800. The experimental results clearly demonstrated the excellent resistance of the CoNC-800 catalyst toward methanol. The same conclusion can be obtained after a methanol tolerance test in 0.5 mol L−1 H2SO4 (Figure 7b). Except for methanol tolerance, the durability is also a crucial parameter to evaluate the practicability of a catalyst for fuel cells. The durability of CoNC-800 and Pt/C was tested by chronoamperometric measurements at 0.68 V (vs RHE) in a 0.1 mol L−1 KOH electrolyte (Figure 7c). Remarkably, the current of CoNC-800 still persisted 91.6% after 12,000 s, but the Pt/C catalyst preserved only 60.7% of its maximum current in the same amount of time. Thus, it can be concluded that the longer-term stability of Pt/C is much inferior to that of CoNC800 in an alkaline medium. Moreover, the durability tests of CoNC-800 and Pt/C were also carried out in a 0.5 mol L−1 H2SO4 electrolyte at the potential of 0.4 V. As shown in Figure 7d, the activity loss for CoNC-800 was also less than that of Pt/ C. Consequently, all of the results demonstrated that CoNC800 possesses much better durability compared to Pt/C both in alkaline and acidic media. The structure and composition of CoNC-800 after long-term electrochemical tests were further studied by TEM and ICP-AES. As shown in Figure S14, obvious aggregation of metallic Co NPs can be observed after long-term electrochemical tests. In addition, the total loading of Co on CoNC-800 shows a decrease after some operation time (Table S4), which is caused by the dissolution of Co species (Co or Co−O moieties) in acidic and alkaline electrolytes. The aggregation of Co NPs and the loss of Co species may be the main reason for the degradation activity of CoNC-800.

CoNC-900. The superior ORR performance of CoNC-800 was also confirmed from Tafel plots (Figure 6d). Obviously, the Tafel slope of CoNC-800 (107 mV/decade) was smaller than those of CoNC-700 (133 mV/decade) and CoNC-900 (121 mV/decade). At the low polarization region, the transfer of the first electron may be the rate-determining step on oxygen reduction.45 From the mechanistic point of view, a smaller Tafel slope for CoNC-800 means a quicker reaction kinetic and higher intrinsic ORR activity.9,11,45 This is because the overpotential is increased significantly with increasing current when the catalyst has a high Tafel slope. Electrochemical impedance spectroscopy (EIS) measurements were recorded at 0.68 V (vs RHE) in a 0.1 mol L−1 KOH electrolyte to explore the interfacial reactions and the charge transfer kinetics of the as-prepared catalysts during the ORR process. The Nyquist plots and the corresponding equivalent circuit diagram are presented in Figure S13. The impedance plots for all of the catalysts showed almost similar arc-like profiles. Then, an appropriate equivalent circuit was employed to fit the impedance spectra, as described in the inset of Figure S13a. According to the fitting results, the Rct (charge transfer resistance) of CoNC-800 (68.5 Ω) was much smaller than those of PCFs (567.2 Ω), NC (335.8 Ω), CoNC-700 (260.5 Ω), and CoNC-900 (196.8 Ω). The smallest Rct of CoNC-800 distinctly suggests the highest conductivity and best charge transport capability than other catalysts. Such a low interfacial charge transfer resistance for CoNC-800 is mainly attributed to its unique trumpet flower-like porous structures with multidimensional electron transport pathways, high degree of graphitization, and small amounts of cobalt embedded into the graphite structure.36,46 A super immunity to methanol crossover is another distinct advantage of CoNC catalysts. Figure 7a presents the current− time (i-t) chronoamperometric responses of the CoNC-800 and Pt/C catalyst in 0.1 mol L−1 KOH with methanol injection. 5348

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CONCLUSIONS In summary, cobalt and nitrogen coembedded three-dimensional trumpet flower-like porous carbons (CoNC) have been synthesized by a simple two-step self-assembly strategy. It is notable that the resulting CoNC-800 exhibits superior electrocatalytic activity, highly durability and methanol tolerance for ORR, not only in acidic media, but also in alkaline media. The ORR performance of CoNC-800 are also better than some transition metal−nitrogen-carbon catalysts reported in literatures in several evaluation indexes. The outstanding ORR performance of the CoNC-800 catalyst is largely attributed to its unique 3D trumpet flower-like structures, highly conductivity and multidimensional electron transport pathways, as well as the abundant and well dispersed Co−Nx active sites. This study not only opens up a facile and scalable strategy to direct synthesis of functionalized 3D trumpet flower-like porous carbons from zero-dimensional CQDs, but also demonstrates a promising low cost, high active and high stability nonprecious metal electrocatalyst for fuel cells and metal air batteries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00655. Information as mentioned in the text. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86-731-88879616. E-mail: [email protected] (W. Li). *Tel./Fax: +86-731-88879616. E-mail: [email protected] (J. Li). ORCID

Wenzhang Li: 0000-0002-6801-4105 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Nature Science Foundation of China (No. 51474255), Hunan Provincial Science and Technology Plan Project, China (No. 2016TP1007), Fundamental Research Funds for the Central Universities of Central South University (2017zzts114), OpenEnd Fund for the Graduate Student Research Innovation Project of Hunan Province (No. 150140008), and open research Fund Program of Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education.

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