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In situ solution plasma synthesis of silver nanoparticles supported on nitrogen-doped carbons with enhanced oxygen reduction activity
Materials Letters 251 (2019) 135–139
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
In situ solution plasma synthesis of silver nanoparticles supported on nitrogen-doped carbons with enhanced oxygen reduction activity Gasidit Panomsuwan a,⇑, Jidapa Chantaramethakul a, Chayanaphat Chokradjaroen b, Takahiro Ishizaki c a
Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand SIT Research Laboratory, Shibaura Institute of Technology, Tokyo 135-8548, Japan c Department of Materials Science and Engineering, Faculty of Engineering, Shibaura Institute of Technology, Tokyo 135-8548, Japan b
a r t i c l e
i n f o
Article history: Received 14 February 2019 Received in revised form 11 May 2019 Accepted 13 May 2019 Available online 13 May 2019 Keywords: Silver nanoparticles Nitrogen-doped carbon Electrocatalyst Oxygen reduction reaction Solution plasma
a b s t r a c t Silver nanoparticles supported on nitrogen-doped carbons (Ag/NC) were in situ synthesized by a solution plasma process. In the solution plasma, Ag nanoparticles were produced via the sputtering of Ag electrode, while the NC supports were simultaneously synthesized from 2-cyanopyridine (C6H4N2). The results of the characterization show that Ag nanoparticles had good crystallinity and the NC supports possessed an amorphous structure. The oxygen reduction reaction (ORR) catalyzed on Ag/NC proceeded via the co-existence of two and four-electron pathways in alkaline solution, with the four-electron pathway being found to be more dominant. An enhanced ORR activity of Ag/NC was attributed to the synergistic effect of Ag nanoparticles and NC supports. Moreover, Ag/NC exhibited long-term durability and high resistance to methanol oxidation in comparison with the commercial Pt/C catalyst. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Oxygen reduction reaction (ORR) is recognized as the key electrochemical reaction at the cathode which determines fuel cell performance and reliability [1]. Pt-based catalysts have been considered as the most efficient ORR catalysts that enable reduction of O2 molecules via a direct four-electron pathway, in both alkaline and acidic media [2]. Yet from economical and technical perspectives, the high price, limited resources, methanol poisoning effect, and poor stability of Pt-based catalysts are significant obstacles limiting the practical and widespread commercialization of future fuel cells [3]. To address this critical issue, various alternative non-Pt based catalysts with high ORR activity and low cost have been developed and reported with the aim of ultimately realizing commercially viable and practical fuel cell [4–7]. When considering the cost-to-activity balance of the other noble metals besides Pt (i.e., Ag, Au, and Pd), Ag is the best option for ORR catalysts due to its lower costs and greater natural abundance when compared with Pt. In addition, Ag also exhibits excellent ORR activity through a four-electron pathway in alkaline medium [8]. In the existing literature and practical experiments, Ag nanoparticles have been synthesized and loaded on carbon
⇑ Corresponding author. E-mail address: [email protected] (G. Panomsuwan). https://doi.org/10.1016/j.matlet.2019.05.052 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
supports using various methods, including (i) chemical reduction of silver salt via reducing agents [9], (ii) thermal treatment of silver salt in an inert atmosphere [10,11], (iii) magnetron sputtering [12], and (iv) electrodeposition [13]. The majority of synthetic methods typically involve multi-step processes and have long processing times which result in preparation difficulties and high costs. Developing a simpler and more convenient method to prepare Ag-based catalysts would therefore be of great significance. The solution plasma process (SPP) has recently emerged as a powerful method to synthesize various noble metal nanoparticles (e.g., Au [14], Ag [15], and Pt [16]) and carbon materials [17,18] owing to its simple, fast, and cost-effective method. Nonetheless, there are no previous studies in the literature which have used SPP to integrate the synthesis of Ag nanoparticles and carbon materials. The present study reports the in situ synthesis of Ag nanoparticles supported on nitrogen-doped carbons (Ag/NC) by SPP. The Ag nanoparticles were produced via the sputtering of Ag electrodes (top-down synthesis) while concurrently forming NC from 2-cyanopyridine (C6H4N2) through dissociation and recombination processes (bottom-up synthesis). The electrocatalytic activity toward ORR of Ag/NC was evaluated using a three-electrode system in alkaline solution. In addition, comprehensive characterizations of the physical and chemical properties of Ag/NC were made to further elucidate their effect on ORR activity.
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2. Experiment
2.3. Electrochemical measurement
2.1. Synthesis of Ag/NC by SPP
A suspension of 5.0 mg catalyst in 490 lL ultrapure water, 490 lL ethanol, and 20 lL NafionÒ DE 521 solution was sonicated to obtain a homogeneous dispersion (5 mg mL 1). 10 lL of the catalyst suspension was drop cast onto a glassy carbon disk (4 mm disk diameter) of a rotating disk electrode (RDE). A suspension of NC, Ag/C, and 20 wt% Pt loading on Vulcan XC-72 (Pt/C) was also prepared using the same procedure for comparison. Electrochemical measurements were performed on a CHI-704E electrochemical analyzer with an RRDE-3A rotating ring disk electrode apparatus in 0.1 M KOH solution. A platinum coil and Ag/AgCl (saturated KCl) were used as the counter and reference electrodes, respectively.
A pair of Ag electrodes (1 mm in diameter) covered with an insulating ceramic tube was placed at the center of a glass reactor with a gap distance of 0.8 mm (Fig. S1). A bipolar pulsed voltage was applied to the electrodes using an MPP-HV04 Pekuris power generator (Kurita Seisakusho Co., Ltd.). The pulse duration and repetition frequency were set at 0.80 ls and 20 kHz, respectively. Plasma was generated within the gap between the Ag electrodes submerged in 100 mL of 2-cyanopyridine under vigorous stirring conditions for 30 min. The black solid product was then collected and washed using ethanol until the washed solvent turned colorless. The black solid product was subsequently thermally treated in a tubular furnace at 700 °C for 1 h under Ar flow (0.5 L min 1). Fig. 1a is a schematic depiction of the overall synthesis procedure of Ag/NC. For comparison, the NC without Ag and the Ag supported on undoped carbon (Ag/C) were also synthesized by SPP. Further details of these preparation procedures can be found in the Supplementary Information section.
2.2. Characterization A number of different tools were used to perform the characterizations. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) mapping images were taken on a JEOL JSM-IT300. Transmission electron microscopy (TEM) images were acquired using a JEOL JEM-3100F microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV diffractometer with Cu Ka radiation (k = 0.154 nm). N2 adsorption–desorption isotherm was measured with a Micromeritics 3Flex surface characterization analyzer at liquid N2 temperature ( 196 °C). X-ray photoelectron spectroscopy (XPS) measurement was carried out on a JEOL JPS-9010 MC spectrometer using an Mg Ka radiation source (1253.6 eV).
3. Results and discussion The SEM image of Ag/NC in Fig. 1b reveals the aggregated nanosized carbon particles with an interparticle pore network. The EDS mapping images (Fig. 1c) display uniform distributions of the Ag signal throughout the investigated area, thereby confirming the existence of Ag nanoparticles. The Ag/NC was composed of C (91.51 at%), O (6.72 at%), N (1.69 at%), and Ag (0.08 at%). Thermal gravimetric analysis (TGA) was also used to confirm the Ag content as 1.45 wt% (Fig. S2). The TEM image (Fig. 1d) shows that the Ag nanoparticles were dispersed on the NC supports, as indicated by multiple dark regions. The Ag nanoparticles were estimated to be about 10–15 nm in size. At a higher magnification, a lattice fringe is clearly seen on the Ag region, indicating good crystallinity of the Ag nanoparticles. The crystalline structure of the Ag nanoparticles and NC supports were further examined by XRD. As shown in Fig. 2a, the XRD patterns of both NC and Ag/NC show main broad peaks at about 23°, which is a characteristic of amorphous carbon. Additional small sharp peaks are also observed for Ag/NC at 38.1° and 44.3°, corresponding to the (1 1 1) and (2 0 0) planes of the face-centered cubic of Ag crystal, respectively. The pore structure of Ag/NC was examined using the N2 adsorption–desorption isotherm (Fig. 2b). This exhibited a combination of
Fig. 1. (a) Schematic diagram illustrating the synthesis of Ag/NC by SPP, (b) SEM, (c) EDS mapping, and (d) TEM images of Ag/NC.
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Fig. 2. Characterization of NC and Ag/NC: (a) XRD patterns, (b) N2 adsorption–desorption isotherm, (c) full XPS spectra and enlarged view of XPS Ag 3d region, and (d) highresolution N 1s XPS spectra with peak deconvolution.
type I and II isotherms with a H3 loop according to the IUPAC classification, indicating the presence of micropores, mesopores, and macropores. The Brunauer–Emmett–Teller (BET) method was used to determine the specific surface area of Ag/NC. Using BET, the specific surface area was found to be 309 m2 g 1, which was divided into micropore (30%) and mesopore/macropore (70%). Meanwhile, an average pore size determined using the Barrett–Joy ner–Halenda (BJH) method was about 22 nm. XPS measurement was conducted to further investigate the surface chemical composition and bonding configuration. The full XPS spectra of Ag/NC was composed of C 1 s, O 1 s, N 1 s, and Ag 3d peaks (Fig. 2c). Doublet peaks at 368.2 and 374.2 eV were associated with Ag 3d5/2 and Ag 3d3/2 of Ag in a zero-valent state, respectively [9]. The C, O, N, and Ag contents of Ag/NC were 82.55, 16.37, 1.03, and 0.05 at%, respectively. The XPS N 1 s peak of both NC and Ag/NC was deconvoluted into four components, including pyridinic N (N1), pyrrolic N (N2), graphitic N (N3), and pyridinic N–oxide (N4), with Table S1 summarizing their relative percentages. Pyridinic N and graphitic N were found to be major components, while pyrrolic N and pyridinic N–oxide were minor components. There was no shift in the binding energy of nitrogen bonding was observed for Ag/NC when compared with NC. This result could indicate a very low or absent formation of Ag–N bonds on Ag/NC, potentially as a result of low Ag content. The XPS C 1 s and O 1 s
spectra consisting of several subpeaks that were obtained from deconvolution are shown in Figs. S3 and S4, respectively. The surface of Ag/NC was largely composed of oxygen moieties in the forms of epoxide, phenolic, and alcoholic groups, which were each found to have less effect on ORR activity [20]. The cyclic voltammetry (CV) curves of Ag/NC measured in both N2 and O2-saturated 0.1 M KOH solutions at room temperature are shown in Fig. 3a. A pronounced ORR peak emerged at about 0.28 V when measuring in the O2-saturated solution, but this peak was absent in the N2-saturated solution. Owing to low Ag loading, there was no detection of an anodic peak from Ag oxidation. From the linear sweep voltammetry (LSV) curve which was recorded at 1600 rpm (Fig. 3b), the onset potential of Ag/NC occurred at 0.174 V, which was more positive than for NC ( 0.186 V) and Ag/C ( 0.203 V). The current density in a diffusion-controlled region was found in the following order: Ag/ NC > NC > Ag/C. Although Ag/NC had better ORR activity in terms of onset potential and current density in comparison to NC and Ag/C, it was still inferior to Pt/C. To further evaluate the ORR pathway on the catalysts, RDE measurement was performed at various rotation speeds ranging from 225 to 2500 rpm (Figs. 3c and S5). The electron transfer number (n) can be calculated by the Koutecky–Levich (K–L) plot using the RDE data (more details are available in the Supplementary Information). Fig. 3d shows the
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Fig. 3. Electrochemical measurements of NC, Ag/NC, Ag/C, and Pt/C in 0.1 M KOH solution: (a) CV curves of Ag/NC (50 mV s 1) in both N2 and O2-saturated solutions (N2: dashed line, O2: solid line), (b) LSV curves at 1600 rpm (10 mV s 1), (c) the K–L plots at 0.45 V, and (e) the long-term durability and (f) methanol resistance tests at applied potential of 0.4 V and rotation speed of 1600 rpm.
K–L plots of all catalysts at 0.45 V. The corresponding n values of NC, Ag/C, Ag/NC, and Pt/C were 3.00, 3.15, 3.41, and 4.03, respectively. This result confirms that the ORR of NC, Ag/C, and Ag/NC proceeds via the co-existence of two and four-electron pathways while the four-electron pathway was more dominant for Ag/NC. Enhanced ORR activity of Ag/NC over both NC and Ag/C can be explained in a number of ways. First, the existence of graphitic N on NC supports promotes the ORR activity [17,19]. Second, doping nitrogen atoms in carbon lattice structure results in good O2 adsorption on the NC support, which is able to supply more O2 to Ag for the ORR. Third, the presence of Ag can facilitate the ORR via a four-electron pathway. The ORR durability of Ag/NC was assessed and compared with Pt/C by measuring a chronoamperometric response at 0.4 V for 20000 s (1600 rpm) (Fig. 3e). Compared to Pt/C, the J/J0 of Ag/NC exhibited a slower rate of attenuation with a higher current density after 20000 s as compared to that of Pt/C. Additionally, resistance to methanol oxidation was also tested by introducing 1 M methanol during the chronoamperometric measurement (Fig. 3f). Upon introducing 1 M methanol, the J/J0 of Ag/NC dropped by about 5%, whereas that of Pt/C substantially degraded to 65% owing to methanol oxidation. These results provide evidence that when compared with Pt/C, Ag/NC has better long-term durability and higher resistance to methanol oxidation.
over, Ag/NC also possessed excellent long-term durability and high resistance to methanol oxidation. Although the ORR activity of Ag/ NC is either worse than or comparable to other catalysts that are reported in the literature, the results of the present study can be used as a guideline or reference for the advanced synthesis of Ag/ NC by SPP. Moreover, this in situ synthesis strategy can be further applied in the preparation of other metal nanoparticles supported on various heteroatom-doped carbons by simply changing the electrode and precursor. Declaration of Competing Interest None. Acknowledgement This work was financially supported by the Faculty of Engineering, Kasetsart University (Grant No. 60/11/MATE). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.05.052. References
4. Conclusions The Ag/NC catalyst was successfully synthesized by in situ SPP. The NC supports exhibited amorphous structures, while the Ag nanoparticles had good crystallinity. The enhanced ORR activity of Ag/NC is attributed to the synergistic effects of the Ag nanoparticles and NC supports, even when there is low Ag content. More-
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
In situ solution plasma synthesis of silver nanoparticles supported on nitrogen-doped carbons with enhanced oxygen reduction activity Gasidit Panomsuwan a,⇑, Jidapa Chantaramethakul a, Chayanaphat Chokradjaroen b, Takahiro Ishizaki c a
Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand SIT Research Laboratory, Shibaura Institute of Technology, Tokyo 135-8548, Japan c Department of Materials Science and Engineering, Faculty of Engineering, Shibaura Institute of Technology, Tokyo 135-8548, Japan b
a r t i c l e
i n f o
Article history: Received 14 February 2019 Received in revised form 11 May 2019 Accepted 13 May 2019 Available online 13 May 2019 Keywords: Silver nanoparticles Nitrogen-doped carbon Electrocatalyst Oxygen reduction reaction Solution plasma
a b s t r a c t Silver nanoparticles supported on nitrogen-doped carbons (Ag/NC) were in situ synthesized by a solution plasma process. In the solution plasma, Ag nanoparticles were produced via the sputtering of Ag electrode, while the NC supports were simultaneously synthesized from 2-cyanopyridine (C6H4N2). The results of the characterization show that Ag nanoparticles had good crystallinity and the NC supports possessed an amorphous structure. The oxygen reduction reaction (ORR) catalyzed on Ag/NC proceeded via the co-existence of two and four-electron pathways in alkaline solution, with the four-electron pathway being found to be more dominant. An enhanced ORR activity of Ag/NC was attributed to the synergistic effect of Ag nanoparticles and NC supports. Moreover, Ag/NC exhibited long-term durability and high resistance to methanol oxidation in comparison with the commercial Pt/C catalyst. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction Oxygen reduction reaction (ORR) is recognized as the key electrochemical reaction at the cathode which determines fuel cell performance and reliability [1]. Pt-based catalysts have been considered as the most efficient ORR catalysts that enable reduction of O2 molecules via a direct four-electron pathway, in both alkaline and acidic media [2]. Yet from economical and technical perspectives, the high price, limited resources, methanol poisoning effect, and poor stability of Pt-based catalysts are significant obstacles limiting the practical and widespread commercialization of future fuel cells [3]. To address this critical issue, various alternative non-Pt based catalysts with high ORR activity and low cost have been developed and reported with the aim of ultimately realizing commercially viable and practical fuel cell [4–7]. When considering the cost-to-activity balance of the other noble metals besides Pt (i.e., Ag, Au, and Pd), Ag is the best option for ORR catalysts due to its lower costs and greater natural abundance when compared with Pt. In addition, Ag also exhibits excellent ORR activity through a four-electron pathway in alkaline medium [8]. In the existing literature and practical experiments, Ag nanoparticles have been synthesized and loaded on carbon
⇑ Corresponding author. E-mail address: [email protected] (G. Panomsuwan). https://doi.org/10.1016/j.matlet.2019.05.052 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
supports using various methods, including (i) chemical reduction of silver salt via reducing agents [9], (ii) thermal treatment of silver salt in an inert atmosphere [10,11], (iii) magnetron sputtering [12], and (iv) electrodeposition [13]. The majority of synthetic methods typically involve multi-step processes and have long processing times which result in preparation difficulties and high costs. Developing a simpler and more convenient method to prepare Ag-based catalysts would therefore be of great significance. The solution plasma process (SPP) has recently emerged as a powerful method to synthesize various noble metal nanoparticles (e.g., Au [14], Ag [15], and Pt [16]) and carbon materials [17,18] owing to its simple, fast, and cost-effective method. Nonetheless, there are no previous studies in the literature which have used SPP to integrate the synthesis of Ag nanoparticles and carbon materials. The present study reports the in situ synthesis of Ag nanoparticles supported on nitrogen-doped carbons (Ag/NC) by SPP. The Ag nanoparticles were produced via the sputtering of Ag electrodes (top-down synthesis) while concurrently forming NC from 2-cyanopyridine (C6H4N2) through dissociation and recombination processes (bottom-up synthesis). The electrocatalytic activity toward ORR of Ag/NC was evaluated using a three-electrode system in alkaline solution. In addition, comprehensive characterizations of the physical and chemical properties of Ag/NC were made to further elucidate their effect on ORR activity.
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2. Experiment
2.3. Electrochemical measurement
2.1. Synthesis of Ag/NC by SPP
A suspension of 5.0 mg catalyst in 490 lL ultrapure water, 490 lL ethanol, and 20 lL NafionÒ DE 521 solution was sonicated to obtain a homogeneous dispersion (5 mg mL 1). 10 lL of the catalyst suspension was drop cast onto a glassy carbon disk (4 mm disk diameter) of a rotating disk electrode (RDE). A suspension of NC, Ag/C, and 20 wt% Pt loading on Vulcan XC-72 (Pt/C) was also prepared using the same procedure for comparison. Electrochemical measurements were performed on a CHI-704E electrochemical analyzer with an RRDE-3A rotating ring disk electrode apparatus in 0.1 M KOH solution. A platinum coil and Ag/AgCl (saturated KCl) were used as the counter and reference electrodes, respectively.
A pair of Ag electrodes (1 mm in diameter) covered with an insulating ceramic tube was placed at the center of a glass reactor with a gap distance of 0.8 mm (Fig. S1). A bipolar pulsed voltage was applied to the electrodes using an MPP-HV04 Pekuris power generator (Kurita Seisakusho Co., Ltd.). The pulse duration and repetition frequency were set at 0.80 ls and 20 kHz, respectively. Plasma was generated within the gap between the Ag electrodes submerged in 100 mL of 2-cyanopyridine under vigorous stirring conditions for 30 min. The black solid product was then collected and washed using ethanol until the washed solvent turned colorless. The black solid product was subsequently thermally treated in a tubular furnace at 700 °C for 1 h under Ar flow (0.5 L min 1). Fig. 1a is a schematic depiction of the overall synthesis procedure of Ag/NC. For comparison, the NC without Ag and the Ag supported on undoped carbon (Ag/C) were also synthesized by SPP. Further details of these preparation procedures can be found in the Supplementary Information section.
2.2. Characterization A number of different tools were used to perform the characterizations. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) mapping images were taken on a JEOL JSM-IT300. Transmission electron microscopy (TEM) images were acquired using a JEOL JEM-3100F microscope. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima IV diffractometer with Cu Ka radiation (k = 0.154 nm). N2 adsorption–desorption isotherm was measured with a Micromeritics 3Flex surface characterization analyzer at liquid N2 temperature ( 196 °C). X-ray photoelectron spectroscopy (XPS) measurement was carried out on a JEOL JPS-9010 MC spectrometer using an Mg Ka radiation source (1253.6 eV).
3. Results and discussion The SEM image of Ag/NC in Fig. 1b reveals the aggregated nanosized carbon particles with an interparticle pore network. The EDS mapping images (Fig. 1c) display uniform distributions of the Ag signal throughout the investigated area, thereby confirming the existence of Ag nanoparticles. The Ag/NC was composed of C (91.51 at%), O (6.72 at%), N (1.69 at%), and Ag (0.08 at%). Thermal gravimetric analysis (TGA) was also used to confirm the Ag content as 1.45 wt% (Fig. S2). The TEM image (Fig. 1d) shows that the Ag nanoparticles were dispersed on the NC supports, as indicated by multiple dark regions. The Ag nanoparticles were estimated to be about 10–15 nm in size. At a higher magnification, a lattice fringe is clearly seen on the Ag region, indicating good crystallinity of the Ag nanoparticles. The crystalline structure of the Ag nanoparticles and NC supports were further examined by XRD. As shown in Fig. 2a, the XRD patterns of both NC and Ag/NC show main broad peaks at about 23°, which is a characteristic of amorphous carbon. Additional small sharp peaks are also observed for Ag/NC at 38.1° and 44.3°, corresponding to the (1 1 1) and (2 0 0) planes of the face-centered cubic of Ag crystal, respectively. The pore structure of Ag/NC was examined using the N2 adsorption–desorption isotherm (Fig. 2b). This exhibited a combination of
Fig. 1. (a) Schematic diagram illustrating the synthesis of Ag/NC by SPP, (b) SEM, (c) EDS mapping, and (d) TEM images of Ag/NC.
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Fig. 2. Characterization of NC and Ag/NC: (a) XRD patterns, (b) N2 adsorption–desorption isotherm, (c) full XPS spectra and enlarged view of XPS Ag 3d region, and (d) highresolution N 1s XPS spectra with peak deconvolution.
type I and II isotherms with a H3 loop according to the IUPAC classification, indicating the presence of micropores, mesopores, and macropores. The Brunauer–Emmett–Teller (BET) method was used to determine the specific surface area of Ag/NC. Using BET, the specific surface area was found to be 309 m2 g 1, which was divided into micropore (30%) and mesopore/macropore (70%). Meanwhile, an average pore size determined using the Barrett–Joy ner–Halenda (BJH) method was about 22 nm. XPS measurement was conducted to further investigate the surface chemical composition and bonding configuration. The full XPS spectra of Ag/NC was composed of C 1 s, O 1 s, N 1 s, and Ag 3d peaks (Fig. 2c). Doublet peaks at 368.2 and 374.2 eV were associated with Ag 3d5/2 and Ag 3d3/2 of Ag in a zero-valent state, respectively [9]. The C, O, N, and Ag contents of Ag/NC were 82.55, 16.37, 1.03, and 0.05 at%, respectively. The XPS N 1 s peak of both NC and Ag/NC was deconvoluted into four components, including pyridinic N (N1), pyrrolic N (N2), graphitic N (N3), and pyridinic N–oxide (N4), with Table S1 summarizing their relative percentages. Pyridinic N and graphitic N were found to be major components, while pyrrolic N and pyridinic N–oxide were minor components. There was no shift in the binding energy of nitrogen bonding was observed for Ag/NC when compared with NC. This result could indicate a very low or absent formation of Ag–N bonds on Ag/NC, potentially as a result of low Ag content. The XPS C 1 s and O 1 s
spectra consisting of several subpeaks that were obtained from deconvolution are shown in Figs. S3 and S4, respectively. The surface of Ag/NC was largely composed of oxygen moieties in the forms of epoxide, phenolic, and alcoholic groups, which were each found to have less effect on ORR activity [20]. The cyclic voltammetry (CV) curves of Ag/NC measured in both N2 and O2-saturated 0.1 M KOH solutions at room temperature are shown in Fig. 3a. A pronounced ORR peak emerged at about 0.28 V when measuring in the O2-saturated solution, but this peak was absent in the N2-saturated solution. Owing to low Ag loading, there was no detection of an anodic peak from Ag oxidation. From the linear sweep voltammetry (LSV) curve which was recorded at 1600 rpm (Fig. 3b), the onset potential of Ag/NC occurred at 0.174 V, which was more positive than for NC ( 0.186 V) and Ag/C ( 0.203 V). The current density in a diffusion-controlled region was found in the following order: Ag/ NC > NC > Ag/C. Although Ag/NC had better ORR activity in terms of onset potential and current density in comparison to NC and Ag/C, it was still inferior to Pt/C. To further evaluate the ORR pathway on the catalysts, RDE measurement was performed at various rotation speeds ranging from 225 to 2500 rpm (Figs. 3c and S5). The electron transfer number (n) can be calculated by the Koutecky–Levich (K–L) plot using the RDE data (more details are available in the Supplementary Information). Fig. 3d shows the
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Fig. 3. Electrochemical measurements of NC, Ag/NC, Ag/C, and Pt/C in 0.1 M KOH solution: (a) CV curves of Ag/NC (50 mV s 1) in both N2 and O2-saturated solutions (N2: dashed line, O2: solid line), (b) LSV curves at 1600 rpm (10 mV s 1), (c) the K–L plots at 0.45 V, and (e) the long-term durability and (f) methanol resistance tests at applied potential of 0.4 V and rotation speed of 1600 rpm.
K–L plots of all catalysts at 0.45 V. The corresponding n values of NC, Ag/C, Ag/NC, and Pt/C were 3.00, 3.15, 3.41, and 4.03, respectively. This result confirms that the ORR of NC, Ag/C, and Ag/NC proceeds via the co-existence of two and four-electron pathways while the four-electron pathway was more dominant for Ag/NC. Enhanced ORR activity of Ag/NC over both NC and Ag/C can be explained in a number of ways. First, the existence of graphitic N on NC supports promotes the ORR activity [17,19]. Second, doping nitrogen atoms in carbon lattice structure results in good O2 adsorption on the NC support, which is able to supply more O2 to Ag for the ORR. Third, the presence of Ag can facilitate the ORR via a four-electron pathway. The ORR durability of Ag/NC was assessed and compared with Pt/C by measuring a chronoamperometric response at 0.4 V for 20000 s (1600 rpm) (Fig. 3e). Compared to Pt/C, the J/J0 of Ag/NC exhibited a slower rate of attenuation with a higher current density after 20000 s as compared to that of Pt/C. Additionally, resistance to methanol oxidation was also tested by introducing 1 M methanol during the chronoamperometric measurement (Fig. 3f). Upon introducing 1 M methanol, the J/J0 of Ag/NC dropped by about 5%, whereas that of Pt/C substantially degraded to 65% owing to methanol oxidation. These results provide evidence that when compared with Pt/C, Ag/NC has better long-term durability and higher resistance to methanol oxidation.
over, Ag/NC also possessed excellent long-term durability and high resistance to methanol oxidation. Although the ORR activity of Ag/ NC is either worse than or comparable to other catalysts that are reported in the literature, the results of the present study can be used as a guideline or reference for the advanced synthesis of Ag/ NC by SPP. Moreover, this in situ synthesis strategy can be further applied in the preparation of other metal nanoparticles supported on various heteroatom-doped carbons by simply changing the electrode and precursor. Declaration of Competing Interest None. Acknowledgement This work was financially supported by the Faculty of Engineering, Kasetsart University (Grant No. 60/11/MATE). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.05.052. References
4. Conclusions The Ag/NC catalyst was successfully synthesized by in situ SPP. The NC supports exhibited amorphous structures, while the Ag nanoparticles had good crystallinity. The enhanced ORR activity of Ag/NC is attributed to the synergistic effects of the Ag nanoparticles and NC supports, even when there is low Ag content. More-
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