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Controllable active sites and facile synthesis of cobalt nanoparticle embedded in nitrogen and sulfur co-doped carbon nanotubes as efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions
Journal of Energy Chemistry 38 (2019) 60–67
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Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Controllable active sites and facile synthesis of cobalt nanoparticle embedded in nitrogen and sulfur co-doped carbon nanotubes as efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions Taeseob Oh, Kwanwoo Kim, Jooheon Kim∗ School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 September 2018 Revised 19 December 2018 Accepted 27 December 2018 Available online 9 January 2019 Keywords: Oxygen reduction reaction Oxygen evolution reaction Nonprecious metal catalyst Nitrogen and sulfur co-doping Encapsulated structure
a b s t r a c t Development of efficient and promising bifunctional electrocatalysts for oxygen reduction and evolution reactions is desirable. Herein, cobalt nanoparticles embedded in nitrogen and sulfur co-doped carbon nanotubes (Co@NSCNT) were prepared by a facile pyrolytic treatment. The cobalt nanoparticles and codoping of nitrogen and sulfur can improve the electron donor-acceptor characteristics of the carbon nanotubes and provide more active sites for catalytic oxygen reduction and evolution reactions. The prepared Co@NSCNT, annealed at 900 °C, showed excellent electrocatalytic performance and better durability than commercial platinum catalysts. Additionally, Co@NSCNT-900 catalysts exhibited comparable onset potentials and Tafel slopes to ruthenium oxide. Overall, Co@NSCNT showed high activity and improved durability for both oxygen evolution and reduction reactions. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction Devices for energy harvesting, conversion, and storage, aimed at high efficiency, low cost, safety, and environmental friendliness, have attracted worldwide attention because they enable the increasingly inevitable shifting from a fossil fuel-based economy to an environmentally sustainable energy economy [1]. Energy storage and conversion devices, such as metal-air batteries and fuel cells, which have theoretically high energy densities, have been recognized as efficient alternatives [2,3]. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play important roles in metal-air batteries and fuel cells. However, both reactions have limitations due to their slow reaction kinetics on the cathode [4,5]. To activate the catalyzed ORR and OER, the activation energy should be lowered. Therefore, much attention has been focused on improving catalytic efficiency. In conventional oxygen reduction reactions, precious metals like platinum or palladium are most widely used as catalysts [6,7]. On the other hand, ruthenium oxide and iridium oxide-based materials are used as the most efficient catalysts in oxygen evolution reactions [8,9]. Ruthenium oxide
∗
Corresponding author. E-mail address: [email protected] (J. Kim).
and iridium oxide-based materials have low activity in oxygen reduction reactions and platinum-based catalysts exhibit poor performance in the oxygen evolution process. In addition, the precious metals are expensive, have poor stability, and sometimes are not environment-friendly. Therefore, bifunctional electrocatalysts, such as non-precious metals, and metal-free electrocatalysts must be developed for overcoming these challenges [10–13]. In recent years, efficient transition-metal-based catalysts for ORR and OER have been studied as alternative electrocatalysts owing to their variable valency states and structurally active sites for catalytic activity. However, their electrocatalytic performance is still insufficient due to the strong bonding between the intermediates of oxygen adsorption and poor electrical conductivity. To enhance electrical conductivity, carbon-based materials may also be used as support materials [14,15]. Since oxygen adsorption by the carbonaceous materials alone is not efficient, heteroatoms could be doped to enhance catalytic property. The charge density of the carbon atoms is redistributed upon doping heteroatoms into the carbonaceous material, facilitating oxygen adsorption, and in turn, improving catalyst properties [16]. Generally, nitrogen is doped as a heteroatom, but synergy is achieved by doping several heteroatoms like sulfur, boron, and phosphorus simultaneously [17–19]. Important factors like charge and spin density are affected by the chemical state between the carbon atoms and the heteroatoms. Accordingly, it has been shown that co-doping nitrogen and phosphorus
https://doi.org/10.1016/j.jechem.2018.12.021 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
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in carbon nanotube [20] and nitrogen boron in graphene [21] resulted in better catalytic performance for ORR and OER. Furthermore, for the development for bifunctional catalysts, heteroatom-doped carbon materials, such as cobalt-embedded nitrogen-doped carbon nanotubes [22], metallic cobalt encapsulated in nitrogen-enriched graphene shell [23], cobalt-iron sulfides in N-doped mesoporous carbon [24], and cobalt nanoparticles embedded in N-doped carbon [25], with different morphologies and structures from transition metals, have been investigated for ORR and OER [26]. However, these studies indicated that the catalysts required complex synthesis processes. Therefore, a facile method to synthesize heteroatom-doped carbonaceous material as a substrate for transition metal catalysts is desirable. In this study, cobalt nanoparticles embedded in nitrogen and sulfur co-doped carbon nanotubes (Co@NSCNT) was fabricated by a simple pyrolytic treatment. The prepared Co@NSCNT showed higher specific surface area for oxygen diffusion into the carbon structure and a well-defined nanostructure compared to commercial Pt/C catalyst. The nitrogen and sulfur atoms co-doped in carbon nanotubes can redistribute the charge density and facilitate ORR and OER catalysis. The cobalt nanoparticles in the carbon nanotubes can improve the chemical states of the active sites of Co@NSCNT to provide efficient ORR and OER catalytic activity. Among the prepared samples, Co@NSCNT annealed at 900 °C exhibited promising catalytic performance and better durability in alkaline solutions. 2. Experimental The Co@NSCNT was fabricated by a facile pyrolytic process in a tubular furnace. Dicyanamide (1.0 g) was thoroughly mixed with 0.2 g of cobalt chloride and 1.0 g of thiourea in a mortar. Then, the mixture was dispersed in 100 mL ethanol under stirring. This solution was stirred for 70 °C for 2 h in an oil bath with reflux and dried overnight at 80 °C. The dried mixture was transferred on an alumina boat in a tubular finance and heated at 500 °C with nitrogen gas flow for 2 h at a heating rate of 10 °C min−1 . The temperature of the tubular finance in this process was varied (700, 80 0, 90 0, and 10 0 0 °C) to obtain different Co@NSCNTs, denoted as Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0, respectively. Finally, the asobtained materials were dispersed in 0.5 M H2 SO4 for 1 day. After the acid treatment, the final product was washed with water and ethanol three times and dried overnight at 80 °C.
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the three-electrode system were glassy carbon (5.0 mm in diameter), platinum wire, and Ag/AgCl, respectively. Before measuring the electrochemical properties, the glassy carbon electrode was polished using 0.05 μL diamond and 1 μL of alumina polish kits and then rinsed several times with deionized water. To prepare the catalyst ink, 4.0 mg of the synthesized sample was added to 10 0 0 μL of a solution containing 800 μL of deionized water, 100 μL of absolute ethanol, and 100 μL of Nafion solution (5 wt%). This catalyst ink was ultrasonicated to obtain a homogenous solution. Then, 10 μL of the catalyst ink was dropped onto the center of the glassy carbon electrode and dried for 1 h. A rotating disk electrode (RDE) was used to investigate the catalytic activity in the experiments by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements. The CV measurements were carried out in O2 - or N2 -saturated 0.1 M potassium hydroxide with a potential range of 0.2 to − 1.0 V at a scan rate of 100 mV s−1 . To obtain more details about oxygen reduction by the electrocatalyst, the LSV curves from the rotating disk were recorded at a potential range between 0.2 and − 1.0 V and a scan rate of 10 mV s−1 , by varying the rotation speed between 400 and 2400 rpm in the O2 -saturated electrolyte. The number of transferred electrons can be obtained from the Koutecky–Levich equation [27,28]:
1 1 1 1 1 = + = + J JL JK JK Bw 1/2
(1)
B = 0.201nF CO2 DO2 2/3 ν −1/6
(2)
In Eq. (1), J is the measured current density, JL is the diffusionlimited current density, JK is the kinetics-limited current density, and w is the electrode rotation speed in rpm. B is related to the diffusion-limited current density and is given by Eq. (2). F is the Faraday constant (96,485 C mol−1 ), CO2 is the bulk concentration of O2 (1.2 × 10−6 mol cm−3 ), DO2 is the diffusion coefficient of O2 in 0.1 M KOH solution (1.9 × 10−5 cm2 s−1 ), and ν is the kinetic viscosity of the electrolyte (0.01 cm2 s−1 ). The LSV measurements of the OER were conducted at a scan rate of 10 mV s−1 , rotation speed of 1600 rpm, and a potential range of 0.2 to 1.0 V. To measure the electrode durability of the electrocatalysts, chronoamperometric measurements were performed at constant potentials of − 0.4 V and 0.8 V at 1600 rpm for 20,0 0 0 s. All electrochemical experimentals results were shown by reversible hydrogen electrode (RHE). The conversion Ag/AgCl electrode to RHE electrode is obtained by using the Nernst equation.
ERHE = EAg/ AgCl + E0 Ag/AgCl (reference) + 0.0591 pH 2.1. Material characterization The structure and morphology of the hybrid nanocomposites were analyzed using high-resolution transmission electron microscopy (HR-TEM, JEM-3010) and field-emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss). The pore volume, average pore size, and specific surface area of the nanocomposites were determined by the Brunauer–Emmett–Teller (BET) method using a surface area and pore size analyzer (BELSORP-max). X-ray diffraction (XRD) analysis was performed (New D8 ADVANCE, BrukerAXS) at a scan rate of 1° s−1 for a 2θ range of 5–80° using Fe Kα 1 radiation (λ = 0.1793 nm) to observe the crystal structure patterns. The binding energy and chemical composition of the synthesized catalyst were characterized using X-ray photoelectron spectroscopy (XPS, Thermo U.K. K-Alpha) with an Al Kα X-ray source (1486.6 eV). 2.2. Electrochemical measurements A potentiostat (CHI 600E) was used to characterize the electrochemical performance of the CFO/NS-MCS catalysts at room temperature. The working, counter, and reference electrodes used in
ERHE is the converted potential vs. RHE, EAg/AgCl is the experimental potential measured with respect to Ag/AgCl reference electrode, and E0 Ag/AgCl is the standard potential of Ag/AgCl at 25 °C (0.1976 V). 3. Results and discussion The Co@NSCNT samples were fabricated by simple pyrolysis of the precursors, which were in a homogeneous mixture state. During annealing of the homogeneous mixture at 500 °C, the precursors were transformed to sulfur-doped graphitic carbon nitride with Co2+ . In this state, dicyanamide was used as a source for both nitrogen and carbon, and thiourea was used to further increase the nitrogen and sulfur contents [29]. Additional high-temperature pyrolysis (70 0, 80 0, 90 0, and 10 0 0 °C) enhanced the graphitization process in nitrogen atmosphere. Finally, acid treatment can remove residual metal nanoparticles, which were not embedded in carbon nanotubes. The structure and morphology of the prepared Co@NSCNT900 catalyst were studied by FE-SEM and HR-TEM. The lowmagnification FE-SEM images of Co@NSCNT-700 showed that all
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Fig. 1. (a, b) Low and high-magnification FE-SEM image of Co@NSCNT-900, (c, d) low and high magnification HR-TEM image of Co@NSCNT-900, (e) nitrogen adsorptiondesorption isotherms of Co@NSCNT-900 and pore size distribution (the inset), (f) XRD pattern of Co@NSCNT-900.
the particles had well-organized nanotube structures (Fig. 1a). The high-magnification of FE-SEM image (Fig. 1b) showed that cobalt nanoparticles of size ∼90 nm were successfully embedded in the carbon nanotubes, and that there were no residual cobalt nanoparticles owing to acid treatment. In addition, the HR-TEM images in Fig 1(c and d) further confirm the Co@NSCNT-900 nanostructure. The carbon nanotubes showed an interlayer distance of 0.340 nm, corresponding to the (0 0 2) plane of carbon graphite. The lattice distance of metallic cobalt was showed to be 0.204 nm, corresponding to the (1 1 1) cubic crystal plane structure (inset of Fig. 1d). The Barrett–Joyner–Halenda (BJH) and Brunauer– Emmett–Teller (BET) measurements were performed to study the
specific surface area and pore size distribution of Co@NSCNT900. According to IUPAC categorization, this prepared sample exhibited the hysteresis curve of a typical IV type isotherm [30]. At the relative pressure of 0.4–1.0, mesopores can be identified in Fig. 1(e) [31]. The specific surface area of Co@NSCNT-900 was 251 m2 g−1 and the mesoporous pore size distribution was 20–30 nm (inset of Fig 1e). The mesopores of Co@NSCNT can provide efficient transport pathways for gaseous oxygen and increase the contact area through the electrolyte and electrode, thus improving the activity of the electrocatalyst. The X-ray diffraction (XRD) pattern of Co@NSCNT-900 was characterized to probe the cubic crystal structure in the 2θ range of 20° to 80°. A
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Fig. 2. (a) Raman spectra of Co@NSCNT-700, 800, 900 and 1000, (b) XPS wide scan spectra of Co@NSCNT-700, 800, 900, and 1000.
Fig. 3. High-resolution N 1 s XPS spectra of (a) Co@NSCNT-700, (b) Co@NSCNT-800, (c) Co@NSCNT-900 and (d) Co@NSCNT-10 0 0.
sharp diffraction peak at 26.5° corresponded to the (0 0 2) plane of the graphitic carbon lattice, which was in agreement with the interlayer distance of 0.340 nm obtained by HR-TEM. The well-defined peaks at 44.2°, 51.6°, and 75.9° could be attributed to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively (JCPDS card no. 15-0806). The lattice distance of metallic cobalt in Fig 1(d) could be attributed the highest diffrac-
tion peak at 44.2°. Thus, the XRD pattern and HR-TEM images confirmed the formation of Co nanoparticles in the carbon nanotubes. Raman spectroscopy was used to show the degree of graphitization in the graphitic structures prepared (Fig. 2a). The two welldefined peaks at 1349 and 1581 cm−1 were named as D band and G band, respectively, which provided information on graphitic car-
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Fig. 4. High-resolution S 2p XPS spectra of (a) Co@NSCNT-700, (b) Co@NSCNT-800, (c) Co@NSCNT-900 and (d) Co@NSCNT-10 0 0.
bon disorder and sp3 and sp2 hybridization of the carbon defect sites. The ratio of ID and IG has been widely used to evaluate the defects in degree of graphitization [32]. The ID /IG value decreased from Co@NSCNT-700 to Co@NSCNT-10 0 0, attributed to the increase in pyrolysis temperature. This decrease indicated that the doping contents of nitrogen and sulfur were decreased, as identified by Xray photoelectron spectroscopy (XPS) (Fig. 2b). The XPS spectra of Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0 showed S 2p peaks at 164 eV, C 1s peaks at 285 eV, N 1s peaks at 398 eV, O 1s peaks at 533 eV, and Co 2p peaks at 780 eV, confirming the successful co-doping of nitrogen and sulfur by pyrolysis in the presence of thiourea. The ratio of carbon to nitrogen (N/C ratio) was 6.69% in Co@NSCNT-700 and 3.35% in Co@NSCNT-10 0 0. With increasing annealing temperature, the amount of nitrogen decreased. The amounts of sulfur and oxygen (S/C ratio and O/C) also decreased from 2.33% to 0.96% and from 8.10% to 3.20% when the pyrolysis temperature was increased. Additionally, it could be shown that the number of carbon peaks increased owing to graphitization, corresponding to the Raman G band. The reduction in ID /IG obtained by Raman spectroscopy is attributed to the decrease of nitrogen and sulfur amounts with increasing pyrolysis temperature. To further characterize the chemical configurations of nitrogen, sulfur, and cobalt in Co@NSCNT, we performed deconvolution of the XPS peaks to understand the chemical bonding states. As shown in Fig. 3(a), the deconvolution peak of N 1s exhibited annealing temperature between 700 °C and 1000 °C. There are four chemical bonding states of doped nitrogen: pyridinic N at bonding energy 398.6 eV, pyrrolic N at 400.1 eV, graphitic N at
401.1 eV, and oxidized N at 402.6 eV [13,33]. The contents of the various states of doped nitrogen were different for different annealing temperatures for Co@NSCNT. For Co@NSCNT-700, pyridinic N, pyrrolic N, graphitic N, and oxidized N constituted 46.1%, 14.8%, 27.9%, and 11.2% of the total nitrogen amount, respectively. When the annealing temperature was 800 °C, the contributions of pyridinic N, pyrrolic N, and oxidized N decreased to 44.1%, 13.0%, and 10.4%, while that of graphitic N increased to 32.5%. When annealing temperatures were 900 °C and 1000 °C, results similar to those obtained for an annealing temperature of 800 °C were obtained. The graphitic N content increased steadily, which indicated that graphitic N was stable at high temperatures. In contrast, the pyridinic N, pyrrolic N, and oxidized N contents were decreased owing to lower stability than that of graphitic N at high pyrolysis temperatures. The peaks of S 2p in the deconvoluted XPS spectra of Co@NSCNT at annealing temperature from 700 °C to 10 0 0 °C are shown in Fig. 4. There were two species of chemical bonding states in sulfur: thiophene S at a bonding energy 164.1 eV and oxidized S at 165.2 eV. The two sharp peaks are S 2p3/2 and 2p1/2 , assigned to the –C=S– conjugated bonds and –C–S–C– bonds, respectively, due to the bonds of carbon and sulfur. The other three broad peaks corresponded to the –C–SOx –C– bonds, owing to the bonds of sulfur and oxygen [34]. For Co@NSCNT-700, thiophene S and oxidized S were 68.8% and 31.2% of the total sulfur contents. When the annealing temperature was 800 °C, thiophene S content increased to 72.2%, while oxidized S content decreased to 27.8%. Similar results were obtained for Co@NSCNT-900 and Co@NSCNT-10 0 0. The oxi-
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Fig. 5. (a) CV curves of the Co@NSCNT-70 0, 80 0, 90 0 and 10 0 0 at scan rate of 100 mV s−1 in the O2 saturated 0.1 M KOH aqueous solution. (b) LSV curves of the Co@NSCNT70 0, 80 0, 90 0, 10 0 0 and Pt/C in the O2 saturated 0.1 M KOH aqueous solution at 1600 rpm and scan rate of 10 mV s−1 . (c) Onset and half-wave potentials of Co@NSCNT-700, 80 0, 90 0, 10 0 0 and Pt/C. (d) LSV curves of Co@NSCNT-900 at various rotation rates from 400 to 2400 rpm. (e) Koutecky–Levich plots for Co@NSCNT-900 from 0.15 to 0.55 V. (f) The electron transfer numbers at different electrode potentials from 0.15 to 0.55 V.
Fig. 6. (a) LSV curves of the Co@NSCNT-900, Pt/C and RuO2 in the N2 saturated 0.1 M KOH aqueous solution at 1600 rpm and scan rate of 10 mV s−1 . (b) Tapel plot of the Co@NSCNT-900, Pt/C and RuO2 .
dized S content decreased steadily, indicating that oxygen group in oxidized S was unstable at high pyrolysis temperatures. The above XPS deconvolution results indicated that chemical bonding states and doping levels of nitrogen and sulfur could be controlled by varying the pyrolysis temperature. 3.1. Electrochemical performance To characterize catalysts for OER measurement was was performed at
the electrochemical activity of the electroand ORR, the rotating disk electrode (RDE) performed. Firstly, cyclic voltammetry (CV) a scan rate of 0.1 V s−1 in oxygen-saturated
0.1 M KOH solution for Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0 (Fig. 5a). All the prepared Co@NSCNT catalysts indicated that cathodic reaction was obtained owing to ORR. On increasing the annealing temperature, the current density was improved except Co@NSCNT-10 0 0. The linear sweep voltammetry (LSV) curves of Pt/C and Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0 were obtained on RDE at a scan rate of 10 mV s−1 in oxygen-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm. As shown in Fig. 5(b), onset and half wave potentials were improved in the order of Co@NSCNT-10 0 0 < Co@NSCNT-700 < Co@NSCNT800 < Co@NSCNT-900 < Pt/C. The current density of Co@NSCNT900 and Pt/C at 0.1 V showed a comparable value.
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Fig. 7. Durability evaluation of Co@NSCNT-900 and Pt/C electrodes for 20,000 s at 0.6 V (a) and 1.7 V (b) with rotation rate of 1600 rpm.
More details for the comparison of onset and half wave potentials are shown in Fig. 5(c). The onset and half wave potentials of Co@NSCNT-700 were 0.781 V and 0.852 V, and both potentials increased up to Co@NSCNT-900. However, the Co@NSCNT-10 0 0 catalyst showed the lowest onset and half wave potential of 0.760 V and 0.820 V. Surprisingly, the onset and half-wave potentials of Co@NSCNT-900 (0.833 V and 0.906 V) were competitive, compared to those of Pt/C (0.840 V and 0.931 V). These results showed that the catalytic activity of Co@NSCNT-900 for ORR was close to that of commercial Pt/C. The improved ORR activity was due to nitrogen and sulfur co-doping. Nitrogen doping into carbon nanotubes changed the carbon structure to sp2 -hybridized structure, which can affect the electroneutrality of carbon bonding, thus making active sites available for oxygen adsorption due to the large difference with the electronegativity of nitrogen. Additionally, the electron lone pairs in sulfur can facilitate active interaction with oxygen and sulfur can also cause charge redistribution in the carbon structure owing to the large atomic radius of sulfur. To further investigate the ORR activity of Co@NSCNT-900, different rotation rates from 400 to 2400 rpm were studied in LSV at a scan rate of 10 mV s−1 in oxygen-saturated 0.1 M KOH electrolyte. As shown in Fig. 5(d), the current density of Co@NSCNT900 increased with increasing rotation rate. This increment indicated that diffusion was controlled by varying the rotation speed. The Koutecky–Levich (K-L) plot in Co@NSCNT-900 was shown in Fig. 5(e). All K-L lines showed well-defined linearity at potential between 0.15 and 0.55 V. Fig. 5(f) showed the number of electrons transferred (n), which was derived from the Koutecky–Levich (KL) equation. The Pt/C catalyst showed the highest electron transfer number among the catalysts. Due to the effect of metallic Co and co-doping of heteroatoms, the Co@NSCNT-900 catalyst exhibited electron transfer number of 3.8 at potentials between 0.15 V and 0.55 V. The Co@NSCNT-900 catalyst showed comparable electron transfer number with Pt/C, and importantly, the ORR process involved 4-electron transfer, compared with the other catalysts. According to the XPS spectra and ORR results, the large amounts of nitrogen and sulfur were not needed to enhance the catalytic reaction. Additionally, pyridinic N and graphitic N were identified as active chemical bonds and thiophene S exhibited conjugated bond formation with the carbon structure, which facilitated the catalytic reaction [35]. Oxidized N and oxidized S groups were known as inactive chemical bonds due to the relation of oxygen in carbon structure [36]. The performances of the Co@NSCNT-900 nanocomposites in the OER are shown in Fig. 6(a). The RDE was characterized at a speed
of 1600 rpm and a scan rate of 10 mV s−1 for LSV measurements in nitrogen-saturated solution. The OER activities of Co@NSCNT-900, RuO2 , and Pt/C were measured at a current density of 10 mA cm−2 and onset potential. The Pt/C showed that the potential was 1.87 V when the current density was 10 mA cm−2 and the onset potential was 1.67 V. Surprisingly, the OER performance of the Co@NSCNT-900 nanocomposite showed lower potential (1.75 V) at 10 mA cm−2 and onset potential of 1.58 V than that of Pt/C, which were comparable to those of RuO2 (1.69 V and 1.55 V, respectively). This can be explained in terms of the higher electronic conductivity of the Co@NSCNT due to the metallic coupling of the Co nanoparticles in NSCNT [37,38]. The OER performances of the catalysts were also characterized using Tafel plots (Fig. 6b). The Tafel slopes of Pt/C were 192 mV decade−1 , implying low OER rate. Co@NSCNT-900 showed a Tafel slope of 154 mV decade−1 , comparable to that of RuO2 (115 mV decade−1 ). These results showed that the Co@NSCNT-900 had OER characteristics. Long-term durability is an important indicator of electrocatalyst performance. Fig. 7(a), obtained using chronoamperometric measurements at a rotation speed of 1600 rpm, shows the plot of relative current with time for Co@NSCNT-900 and commercial Pt/C. For ORR durability, current change was obtained in a continuously oxygen-saturated electrolyte at a constant potential of 0.6 V for 20,0 0 0 s. The commercial Pt/C electrocatalyst exhibited 29.1% decrease in relative current compared with the initial current, indicating unstable durability. In contrast, the Co@NSCNT-900 catalyst showed 89.1% relative current and only slight drop after 20,0 0 0 s. As shown in Fig. 7(b), Co@NSCNT-900 also exhibited promising durability for OER. At a constant potential of 1.7 V, the relative current of Co@NSCNT-900 was 82.0% after 20,0 0 0 s, whereas Pt/C produced 41.1% relative current. These results showed that Co@NSCNT-900 had promising catalytic durability compared to commercial Pt/C. 4. Conclusions In conclusion, Co@NSCNT catalysts were prepared by a simple pyrolytic treatment method. The dicyanamide, which was a source for carbon and nitrogen, was used to grow carbon nanotubes from cobalt nanoparticles, and thiourea was also used as a nitrogen and sulfur precursor. Among the prepared samples, Co@NSCNT-900 showed excellent ORR activity in terms of an onset potential of 0.833 V and half wave potential of 0.906 V. The electron transfer number was confirmed to four electrons. The resultant ORR performance, which was assigned to the active sites, was comparable
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with those of Pt/C. In OER, Co@NSCNT-900 exhibited comparable onset potential of 1.58 V and Tafel slope of 153 mV decade−1 , compared to ruthenium oxide. Moreover, this bifunctional electrocatalyst showed high durability for both ORR and OER. Considering the facile fabrication, low cost, high performance for ORR and OER, and long-term durability, Co@NSCNT-900 could be a promising bifunctional electrocatalyst for energy devices. Acknowledgments This work was supported by the Human Resources Development (No. 20184030202070) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Controllable active sites and facile synthesis of cobalt nanoparticle embedded in nitrogen and sulfur co-doped carbon nanotubes as efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions Taeseob Oh, Kwanwoo Kim, Jooheon Kim∗ School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 156-756, Republic of Korea
a r t i c l e
i n f o
Article history: Received 28 September 2018 Revised 19 December 2018 Accepted 27 December 2018 Available online 9 January 2019 Keywords: Oxygen reduction reaction Oxygen evolution reaction Nonprecious metal catalyst Nitrogen and sulfur co-doping Encapsulated structure
a b s t r a c t Development of efficient and promising bifunctional electrocatalysts for oxygen reduction and evolution reactions is desirable. Herein, cobalt nanoparticles embedded in nitrogen and sulfur co-doped carbon nanotubes (Co@NSCNT) were prepared by a facile pyrolytic treatment. The cobalt nanoparticles and codoping of nitrogen and sulfur can improve the electron donor-acceptor characteristics of the carbon nanotubes and provide more active sites for catalytic oxygen reduction and evolution reactions. The prepared Co@NSCNT, annealed at 900 °C, showed excellent electrocatalytic performance and better durability than commercial platinum catalysts. Additionally, Co@NSCNT-900 catalysts exhibited comparable onset potentials and Tafel slopes to ruthenium oxide. Overall, Co@NSCNT showed high activity and improved durability for both oxygen evolution and reduction reactions. © 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
1. Introduction Devices for energy harvesting, conversion, and storage, aimed at high efficiency, low cost, safety, and environmental friendliness, have attracted worldwide attention because they enable the increasingly inevitable shifting from a fossil fuel-based economy to an environmentally sustainable energy economy [1]. Energy storage and conversion devices, such as metal-air batteries and fuel cells, which have theoretically high energy densities, have been recognized as efficient alternatives [2,3]. The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play important roles in metal-air batteries and fuel cells. However, both reactions have limitations due to their slow reaction kinetics on the cathode [4,5]. To activate the catalyzed ORR and OER, the activation energy should be lowered. Therefore, much attention has been focused on improving catalytic efficiency. In conventional oxygen reduction reactions, precious metals like platinum or palladium are most widely used as catalysts [6,7]. On the other hand, ruthenium oxide and iridium oxide-based materials are used as the most efficient catalysts in oxygen evolution reactions [8,9]. Ruthenium oxide
∗
Corresponding author. E-mail address: [email protected] (J. Kim).
and iridium oxide-based materials have low activity in oxygen reduction reactions and platinum-based catalysts exhibit poor performance in the oxygen evolution process. In addition, the precious metals are expensive, have poor stability, and sometimes are not environment-friendly. Therefore, bifunctional electrocatalysts, such as non-precious metals, and metal-free electrocatalysts must be developed for overcoming these challenges [10–13]. In recent years, efficient transition-metal-based catalysts for ORR and OER have been studied as alternative electrocatalysts owing to their variable valency states and structurally active sites for catalytic activity. However, their electrocatalytic performance is still insufficient due to the strong bonding between the intermediates of oxygen adsorption and poor electrical conductivity. To enhance electrical conductivity, carbon-based materials may also be used as support materials [14,15]. Since oxygen adsorption by the carbonaceous materials alone is not efficient, heteroatoms could be doped to enhance catalytic property. The charge density of the carbon atoms is redistributed upon doping heteroatoms into the carbonaceous material, facilitating oxygen adsorption, and in turn, improving catalyst properties [16]. Generally, nitrogen is doped as a heteroatom, but synergy is achieved by doping several heteroatoms like sulfur, boron, and phosphorus simultaneously [17–19]. Important factors like charge and spin density are affected by the chemical state between the carbon atoms and the heteroatoms. Accordingly, it has been shown that co-doping nitrogen and phosphorus
https://doi.org/10.1016/j.jechem.2018.12.021 2095-4956/© 2019 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.
T. Oh, K. Kim and J. Kim / Journal of Energy Chemistry 38 (2019) 60–67
in carbon nanotube [20] and nitrogen boron in graphene [21] resulted in better catalytic performance for ORR and OER. Furthermore, for the development for bifunctional catalysts, heteroatom-doped carbon materials, such as cobalt-embedded nitrogen-doped carbon nanotubes [22], metallic cobalt encapsulated in nitrogen-enriched graphene shell [23], cobalt-iron sulfides in N-doped mesoporous carbon [24], and cobalt nanoparticles embedded in N-doped carbon [25], with different morphologies and structures from transition metals, have been investigated for ORR and OER [26]. However, these studies indicated that the catalysts required complex synthesis processes. Therefore, a facile method to synthesize heteroatom-doped carbonaceous material as a substrate for transition metal catalysts is desirable. In this study, cobalt nanoparticles embedded in nitrogen and sulfur co-doped carbon nanotubes (Co@NSCNT) was fabricated by a simple pyrolytic treatment. The prepared Co@NSCNT showed higher specific surface area for oxygen diffusion into the carbon structure and a well-defined nanostructure compared to commercial Pt/C catalyst. The nitrogen and sulfur atoms co-doped in carbon nanotubes can redistribute the charge density and facilitate ORR and OER catalysis. The cobalt nanoparticles in the carbon nanotubes can improve the chemical states of the active sites of Co@NSCNT to provide efficient ORR and OER catalytic activity. Among the prepared samples, Co@NSCNT annealed at 900 °C exhibited promising catalytic performance and better durability in alkaline solutions. 2. Experimental The Co@NSCNT was fabricated by a facile pyrolytic process in a tubular furnace. Dicyanamide (1.0 g) was thoroughly mixed with 0.2 g of cobalt chloride and 1.0 g of thiourea in a mortar. Then, the mixture was dispersed in 100 mL ethanol under stirring. This solution was stirred for 70 °C for 2 h in an oil bath with reflux and dried overnight at 80 °C. The dried mixture was transferred on an alumina boat in a tubular finance and heated at 500 °C with nitrogen gas flow for 2 h at a heating rate of 10 °C min−1 . The temperature of the tubular finance in this process was varied (700, 80 0, 90 0, and 10 0 0 °C) to obtain different Co@NSCNTs, denoted as Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0, respectively. Finally, the asobtained materials were dispersed in 0.5 M H2 SO4 for 1 day. After the acid treatment, the final product was washed with water and ethanol three times and dried overnight at 80 °C.
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the three-electrode system were glassy carbon (5.0 mm in diameter), platinum wire, and Ag/AgCl, respectively. Before measuring the electrochemical properties, the glassy carbon electrode was polished using 0.05 μL diamond and 1 μL of alumina polish kits and then rinsed several times with deionized water. To prepare the catalyst ink, 4.0 mg of the synthesized sample was added to 10 0 0 μL of a solution containing 800 μL of deionized water, 100 μL of absolute ethanol, and 100 μL of Nafion solution (5 wt%). This catalyst ink was ultrasonicated to obtain a homogenous solution. Then, 10 μL of the catalyst ink was dropped onto the center of the glassy carbon electrode and dried for 1 h. A rotating disk electrode (RDE) was used to investigate the catalytic activity in the experiments by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements. The CV measurements were carried out in O2 - or N2 -saturated 0.1 M potassium hydroxide with a potential range of 0.2 to − 1.0 V at a scan rate of 100 mV s−1 . To obtain more details about oxygen reduction by the electrocatalyst, the LSV curves from the rotating disk were recorded at a potential range between 0.2 and − 1.0 V and a scan rate of 10 mV s−1 , by varying the rotation speed between 400 and 2400 rpm in the O2 -saturated electrolyte. The number of transferred electrons can be obtained from the Koutecky–Levich equation [27,28]:
1 1 1 1 1 = + = + J JL JK JK Bw 1/2
(1)
B = 0.201nF CO2 DO2 2/3 ν −1/6
(2)
In Eq. (1), J is the measured current density, JL is the diffusionlimited current density, JK is the kinetics-limited current density, and w is the electrode rotation speed in rpm. B is related to the diffusion-limited current density and is given by Eq. (2). F is the Faraday constant (96,485 C mol−1 ), CO2 is the bulk concentration of O2 (1.2 × 10−6 mol cm−3 ), DO2 is the diffusion coefficient of O2 in 0.1 M KOH solution (1.9 × 10−5 cm2 s−1 ), and ν is the kinetic viscosity of the electrolyte (0.01 cm2 s−1 ). The LSV measurements of the OER were conducted at a scan rate of 10 mV s−1 , rotation speed of 1600 rpm, and a potential range of 0.2 to 1.0 V. To measure the electrode durability of the electrocatalysts, chronoamperometric measurements were performed at constant potentials of − 0.4 V and 0.8 V at 1600 rpm for 20,0 0 0 s. All electrochemical experimentals results were shown by reversible hydrogen electrode (RHE). The conversion Ag/AgCl electrode to RHE electrode is obtained by using the Nernst equation.
ERHE = EAg/ AgCl + E0 Ag/AgCl (reference) + 0.0591 pH 2.1. Material characterization The structure and morphology of the hybrid nanocomposites were analyzed using high-resolution transmission electron microscopy (HR-TEM, JEM-3010) and field-emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss). The pore volume, average pore size, and specific surface area of the nanocomposites were determined by the Brunauer–Emmett–Teller (BET) method using a surface area and pore size analyzer (BELSORP-max). X-ray diffraction (XRD) analysis was performed (New D8 ADVANCE, BrukerAXS) at a scan rate of 1° s−1 for a 2θ range of 5–80° using Fe Kα 1 radiation (λ = 0.1793 nm) to observe the crystal structure patterns. The binding energy and chemical composition of the synthesized catalyst were characterized using X-ray photoelectron spectroscopy (XPS, Thermo U.K. K-Alpha) with an Al Kα X-ray source (1486.6 eV). 2.2. Electrochemical measurements A potentiostat (CHI 600E) was used to characterize the electrochemical performance of the CFO/NS-MCS catalysts at room temperature. The working, counter, and reference electrodes used in
ERHE is the converted potential vs. RHE, EAg/AgCl is the experimental potential measured with respect to Ag/AgCl reference electrode, and E0 Ag/AgCl is the standard potential of Ag/AgCl at 25 °C (0.1976 V). 3. Results and discussion The Co@NSCNT samples were fabricated by simple pyrolysis of the precursors, which were in a homogeneous mixture state. During annealing of the homogeneous mixture at 500 °C, the precursors were transformed to sulfur-doped graphitic carbon nitride with Co2+ . In this state, dicyanamide was used as a source for both nitrogen and carbon, and thiourea was used to further increase the nitrogen and sulfur contents [29]. Additional high-temperature pyrolysis (70 0, 80 0, 90 0, and 10 0 0 °C) enhanced the graphitization process in nitrogen atmosphere. Finally, acid treatment can remove residual metal nanoparticles, which were not embedded in carbon nanotubes. The structure and morphology of the prepared Co@NSCNT900 catalyst were studied by FE-SEM and HR-TEM. The lowmagnification FE-SEM images of Co@NSCNT-700 showed that all
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Fig. 1. (a, b) Low and high-magnification FE-SEM image of Co@NSCNT-900, (c, d) low and high magnification HR-TEM image of Co@NSCNT-900, (e) nitrogen adsorptiondesorption isotherms of Co@NSCNT-900 and pore size distribution (the inset), (f) XRD pattern of Co@NSCNT-900.
the particles had well-organized nanotube structures (Fig. 1a). The high-magnification of FE-SEM image (Fig. 1b) showed that cobalt nanoparticles of size ∼90 nm were successfully embedded in the carbon nanotubes, and that there were no residual cobalt nanoparticles owing to acid treatment. In addition, the HR-TEM images in Fig 1(c and d) further confirm the Co@NSCNT-900 nanostructure. The carbon nanotubes showed an interlayer distance of 0.340 nm, corresponding to the (0 0 2) plane of carbon graphite. The lattice distance of metallic cobalt was showed to be 0.204 nm, corresponding to the (1 1 1) cubic crystal plane structure (inset of Fig. 1d). The Barrett–Joyner–Halenda (BJH) and Brunauer– Emmett–Teller (BET) measurements were performed to study the
specific surface area and pore size distribution of Co@NSCNT900. According to IUPAC categorization, this prepared sample exhibited the hysteresis curve of a typical IV type isotherm [30]. At the relative pressure of 0.4–1.0, mesopores can be identified in Fig. 1(e) [31]. The specific surface area of Co@NSCNT-900 was 251 m2 g−1 and the mesoporous pore size distribution was 20–30 nm (inset of Fig 1e). The mesopores of Co@NSCNT can provide efficient transport pathways for gaseous oxygen and increase the contact area through the electrolyte and electrode, thus improving the activity of the electrocatalyst. The X-ray diffraction (XRD) pattern of Co@NSCNT-900 was characterized to probe the cubic crystal structure in the 2θ range of 20° to 80°. A
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Fig. 2. (a) Raman spectra of Co@NSCNT-700, 800, 900 and 1000, (b) XPS wide scan spectra of Co@NSCNT-700, 800, 900, and 1000.
Fig. 3. High-resolution N 1 s XPS spectra of (a) Co@NSCNT-700, (b) Co@NSCNT-800, (c) Co@NSCNT-900 and (d) Co@NSCNT-10 0 0.
sharp diffraction peak at 26.5° corresponded to the (0 0 2) plane of the graphitic carbon lattice, which was in agreement with the interlayer distance of 0.340 nm obtained by HR-TEM. The well-defined peaks at 44.2°, 51.6°, and 75.9° could be attributed to the (1 1 1), (2 0 0), and (2 2 0) planes, respectively (JCPDS card no. 15-0806). The lattice distance of metallic cobalt in Fig 1(d) could be attributed the highest diffrac-
tion peak at 44.2°. Thus, the XRD pattern and HR-TEM images confirmed the formation of Co nanoparticles in the carbon nanotubes. Raman spectroscopy was used to show the degree of graphitization in the graphitic structures prepared (Fig. 2a). The two welldefined peaks at 1349 and 1581 cm−1 were named as D band and G band, respectively, which provided information on graphitic car-
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Fig. 4. High-resolution S 2p XPS spectra of (a) Co@NSCNT-700, (b) Co@NSCNT-800, (c) Co@NSCNT-900 and (d) Co@NSCNT-10 0 0.
bon disorder and sp3 and sp2 hybridization of the carbon defect sites. The ratio of ID and IG has been widely used to evaluate the defects in degree of graphitization [32]. The ID /IG value decreased from Co@NSCNT-700 to Co@NSCNT-10 0 0, attributed to the increase in pyrolysis temperature. This decrease indicated that the doping contents of nitrogen and sulfur were decreased, as identified by Xray photoelectron spectroscopy (XPS) (Fig. 2b). The XPS spectra of Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0 showed S 2p peaks at 164 eV, C 1s peaks at 285 eV, N 1s peaks at 398 eV, O 1s peaks at 533 eV, and Co 2p peaks at 780 eV, confirming the successful co-doping of nitrogen and sulfur by pyrolysis in the presence of thiourea. The ratio of carbon to nitrogen (N/C ratio) was 6.69% in Co@NSCNT-700 and 3.35% in Co@NSCNT-10 0 0. With increasing annealing temperature, the amount of nitrogen decreased. The amounts of sulfur and oxygen (S/C ratio and O/C) also decreased from 2.33% to 0.96% and from 8.10% to 3.20% when the pyrolysis temperature was increased. Additionally, it could be shown that the number of carbon peaks increased owing to graphitization, corresponding to the Raman G band. The reduction in ID /IG obtained by Raman spectroscopy is attributed to the decrease of nitrogen and sulfur amounts with increasing pyrolysis temperature. To further characterize the chemical configurations of nitrogen, sulfur, and cobalt in Co@NSCNT, we performed deconvolution of the XPS peaks to understand the chemical bonding states. As shown in Fig. 3(a), the deconvolution peak of N 1s exhibited annealing temperature between 700 °C and 1000 °C. There are four chemical bonding states of doped nitrogen: pyridinic N at bonding energy 398.6 eV, pyrrolic N at 400.1 eV, graphitic N at
401.1 eV, and oxidized N at 402.6 eV [13,33]. The contents of the various states of doped nitrogen were different for different annealing temperatures for Co@NSCNT. For Co@NSCNT-700, pyridinic N, pyrrolic N, graphitic N, and oxidized N constituted 46.1%, 14.8%, 27.9%, and 11.2% of the total nitrogen amount, respectively. When the annealing temperature was 800 °C, the contributions of pyridinic N, pyrrolic N, and oxidized N decreased to 44.1%, 13.0%, and 10.4%, while that of graphitic N increased to 32.5%. When annealing temperatures were 900 °C and 1000 °C, results similar to those obtained for an annealing temperature of 800 °C were obtained. The graphitic N content increased steadily, which indicated that graphitic N was stable at high temperatures. In contrast, the pyridinic N, pyrrolic N, and oxidized N contents were decreased owing to lower stability than that of graphitic N at high pyrolysis temperatures. The peaks of S 2p in the deconvoluted XPS spectra of Co@NSCNT at annealing temperature from 700 °C to 10 0 0 °C are shown in Fig. 4. There were two species of chemical bonding states in sulfur: thiophene S at a bonding energy 164.1 eV and oxidized S at 165.2 eV. The two sharp peaks are S 2p3/2 and 2p1/2 , assigned to the –C=S– conjugated bonds and –C–S–C– bonds, respectively, due to the bonds of carbon and sulfur. The other three broad peaks corresponded to the –C–SOx –C– bonds, owing to the bonds of sulfur and oxygen [34]. For Co@NSCNT-700, thiophene S and oxidized S were 68.8% and 31.2% of the total sulfur contents. When the annealing temperature was 800 °C, thiophene S content increased to 72.2%, while oxidized S content decreased to 27.8%. Similar results were obtained for Co@NSCNT-900 and Co@NSCNT-10 0 0. The oxi-
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Fig. 5. (a) CV curves of the Co@NSCNT-70 0, 80 0, 90 0 and 10 0 0 at scan rate of 100 mV s−1 in the O2 saturated 0.1 M KOH aqueous solution. (b) LSV curves of the Co@NSCNT70 0, 80 0, 90 0, 10 0 0 and Pt/C in the O2 saturated 0.1 M KOH aqueous solution at 1600 rpm and scan rate of 10 mV s−1 . (c) Onset and half-wave potentials of Co@NSCNT-700, 80 0, 90 0, 10 0 0 and Pt/C. (d) LSV curves of Co@NSCNT-900 at various rotation rates from 400 to 2400 rpm. (e) Koutecky–Levich plots for Co@NSCNT-900 from 0.15 to 0.55 V. (f) The electron transfer numbers at different electrode potentials from 0.15 to 0.55 V.
Fig. 6. (a) LSV curves of the Co@NSCNT-900, Pt/C and RuO2 in the N2 saturated 0.1 M KOH aqueous solution at 1600 rpm and scan rate of 10 mV s−1 . (b) Tapel plot of the Co@NSCNT-900, Pt/C and RuO2 .
dized S content decreased steadily, indicating that oxygen group in oxidized S was unstable at high pyrolysis temperatures. The above XPS deconvolution results indicated that chemical bonding states and doping levels of nitrogen and sulfur could be controlled by varying the pyrolysis temperature. 3.1. Electrochemical performance To characterize catalysts for OER measurement was was performed at
the electrochemical activity of the electroand ORR, the rotating disk electrode (RDE) performed. Firstly, cyclic voltammetry (CV) a scan rate of 0.1 V s−1 in oxygen-saturated
0.1 M KOH solution for Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0 (Fig. 5a). All the prepared Co@NSCNT catalysts indicated that cathodic reaction was obtained owing to ORR. On increasing the annealing temperature, the current density was improved except Co@NSCNT-10 0 0. The linear sweep voltammetry (LSV) curves of Pt/C and Co@NSCNT-70 0, 80 0, 90 0, and 10 0 0 were obtained on RDE at a scan rate of 10 mV s−1 in oxygen-saturated 0.1 M KOH solution at a rotation speed of 1600 rpm. As shown in Fig. 5(b), onset and half wave potentials were improved in the order of Co@NSCNT-10 0 0 < Co@NSCNT-700 < Co@NSCNT800 < Co@NSCNT-900 < Pt/C. The current density of Co@NSCNT900 and Pt/C at 0.1 V showed a comparable value.
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Fig. 7. Durability evaluation of Co@NSCNT-900 and Pt/C electrodes for 20,000 s at 0.6 V (a) and 1.7 V (b) with rotation rate of 1600 rpm.
More details for the comparison of onset and half wave potentials are shown in Fig. 5(c). The onset and half wave potentials of Co@NSCNT-700 were 0.781 V and 0.852 V, and both potentials increased up to Co@NSCNT-900. However, the Co@NSCNT-10 0 0 catalyst showed the lowest onset and half wave potential of 0.760 V and 0.820 V. Surprisingly, the onset and half-wave potentials of Co@NSCNT-900 (0.833 V and 0.906 V) were competitive, compared to those of Pt/C (0.840 V and 0.931 V). These results showed that the catalytic activity of Co@NSCNT-900 for ORR was close to that of commercial Pt/C. The improved ORR activity was due to nitrogen and sulfur co-doping. Nitrogen doping into carbon nanotubes changed the carbon structure to sp2 -hybridized structure, which can affect the electroneutrality of carbon bonding, thus making active sites available for oxygen adsorption due to the large difference with the electronegativity of nitrogen. Additionally, the electron lone pairs in sulfur can facilitate active interaction with oxygen and sulfur can also cause charge redistribution in the carbon structure owing to the large atomic radius of sulfur. To further investigate the ORR activity of Co@NSCNT-900, different rotation rates from 400 to 2400 rpm were studied in LSV at a scan rate of 10 mV s−1 in oxygen-saturated 0.1 M KOH electrolyte. As shown in Fig. 5(d), the current density of Co@NSCNT900 increased with increasing rotation rate. This increment indicated that diffusion was controlled by varying the rotation speed. The Koutecky–Levich (K-L) plot in Co@NSCNT-900 was shown in Fig. 5(e). All K-L lines showed well-defined linearity at potential between 0.15 and 0.55 V. Fig. 5(f) showed the number of electrons transferred (n), which was derived from the Koutecky–Levich (KL) equation. The Pt/C catalyst showed the highest electron transfer number among the catalysts. Due to the effect of metallic Co and co-doping of heteroatoms, the Co@NSCNT-900 catalyst exhibited electron transfer number of 3.8 at potentials between 0.15 V and 0.55 V. The Co@NSCNT-900 catalyst showed comparable electron transfer number with Pt/C, and importantly, the ORR process involved 4-electron transfer, compared with the other catalysts. According to the XPS spectra and ORR results, the large amounts of nitrogen and sulfur were not needed to enhance the catalytic reaction. Additionally, pyridinic N and graphitic N were identified as active chemical bonds and thiophene S exhibited conjugated bond formation with the carbon structure, which facilitated the catalytic reaction [35]. Oxidized N and oxidized S groups were known as inactive chemical bonds due to the relation of oxygen in carbon structure [36]. The performances of the Co@NSCNT-900 nanocomposites in the OER are shown in Fig. 6(a). The RDE was characterized at a speed
of 1600 rpm and a scan rate of 10 mV s−1 for LSV measurements in nitrogen-saturated solution. The OER activities of Co@NSCNT-900, RuO2 , and Pt/C were measured at a current density of 10 mA cm−2 and onset potential. The Pt/C showed that the potential was 1.87 V when the current density was 10 mA cm−2 and the onset potential was 1.67 V. Surprisingly, the OER performance of the Co@NSCNT-900 nanocomposite showed lower potential (1.75 V) at 10 mA cm−2 and onset potential of 1.58 V than that of Pt/C, which were comparable to those of RuO2 (1.69 V and 1.55 V, respectively). This can be explained in terms of the higher electronic conductivity of the Co@NSCNT due to the metallic coupling of the Co nanoparticles in NSCNT [37,38]. The OER performances of the catalysts were also characterized using Tafel plots (Fig. 6b). The Tafel slopes of Pt/C were 192 mV decade−1 , implying low OER rate. Co@NSCNT-900 showed a Tafel slope of 154 mV decade−1 , comparable to that of RuO2 (115 mV decade−1 ). These results showed that the Co@NSCNT-900 had OER characteristics. Long-term durability is an important indicator of electrocatalyst performance. Fig. 7(a), obtained using chronoamperometric measurements at a rotation speed of 1600 rpm, shows the plot of relative current with time for Co@NSCNT-900 and commercial Pt/C. For ORR durability, current change was obtained in a continuously oxygen-saturated electrolyte at a constant potential of 0.6 V for 20,0 0 0 s. The commercial Pt/C electrocatalyst exhibited 29.1% decrease in relative current compared with the initial current, indicating unstable durability. In contrast, the Co@NSCNT-900 catalyst showed 89.1% relative current and only slight drop after 20,0 0 0 s. As shown in Fig. 7(b), Co@NSCNT-900 also exhibited promising durability for OER. At a constant potential of 1.7 V, the relative current of Co@NSCNT-900 was 82.0% after 20,0 0 0 s, whereas Pt/C produced 41.1% relative current. These results showed that Co@NSCNT-900 had promising catalytic durability compared to commercial Pt/C. 4. Conclusions In conclusion, Co@NSCNT catalysts were prepared by a simple pyrolytic treatment method. The dicyanamide, which was a source for carbon and nitrogen, was used to grow carbon nanotubes from cobalt nanoparticles, and thiourea was also used as a nitrogen and sulfur precursor. Among the prepared samples, Co@NSCNT-900 showed excellent ORR activity in terms of an onset potential of 0.833 V and half wave potential of 0.906 V. The electron transfer number was confirmed to four electrons. The resultant ORR performance, which was assigned to the active sites, was comparable
T. Oh, K. Kim and J. Kim / Journal of Energy Chemistry 38 (2019) 60–67
with those of Pt/C. In OER, Co@NSCNT-900 exhibited comparable onset potential of 1.58 V and Tafel slope of 153 mV decade−1 , compared to ruthenium oxide. Moreover, this bifunctional electrocatalyst showed high durability for both ORR and OER. Considering the facile fabrication, low cost, high performance for ORR and OER, and long-term durability, Co@NSCNT-900 could be a promising bifunctional electrocatalyst for energy devices. Acknowledgments This work was supported by the Human Resources Development (No. 20184030202070) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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