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TiN–CNT) as efficient oxygen reduction reaction catalysts in acidic medium
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PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium Shengzhou Chen a,*, Qiuchan Huang a, Wei Yang a, Hanbo Zou b, Huangwang Mai a, JiaHai Wang b a b
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, China Guangzhou Key Laboratory for New Energy and Green Catalysis, Guangzhou University, Guangzhou, China
article info
abstract
Article history:
Herein, new catalysts for the oxygen reduction reaction (ORR) (PteCoN nanoparticles
Received 12 April 2018
supported on TiNecarbon nanotubes (CNTs)) were successfully prepared via the simple
Received in revised form
polymerized complex method and heat treatment in NH3. Through this facile two-step
30 May 2018
synthesis, metal chlorides were first transformed to the corresponding oxides, then
Accepted 1 June 2018
reduced to metal particles or nitrides. The synthesized PteCoN/TiNeCNT electrocatalysts
Available online xxx
showed excellent electrochemical activity for oxygen reduction and high tolerance against
Keywords:
RHE) is higher than that of 40 wt% commercial Pt/C (0.789 V vs. RHE) due to the synergistic
Oxygen reduction reaction
effect between nano-platinum particles and metal nitrides. The TiN-coated CNT support
Electrocatalyst
provided high electron transfer rate and catalytic tolerance in methanol solution. The
Direct methanol fuel cell
number of transferred electrons was examined by rotating ring-disk electrode measure-
Low-Pt loading
ments, which concurred with the result of the KouteckyeLevich equation for a four-
TiN
electron mechanism.
methanol in acidic medium. The ORR onset potential of Pt0.3CoN/TiNeCNT (0.812 V vs.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs), including direct methanol fuel cells (DMFCs), are widely researched because of their lower operating temperature, high energy conversion efficiency, and low emissions. Compared with other PEMFCs, DMFCs use a liquid-feed system and do not require heat management, and are therefore preferable for portable power applications [1]. However, a slow methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) with conventional Pt-based electrocatalysts demand plenty of
Pt, whose high cost limits the commercialization of the DMFCs. Additionally, methanol is permeable in perfluorosulfonate membranes from the DMFC anode to the cathode and produces a mixed potential between methanol oxidation and oxygen reduction at Pt/C, which lowers the activity of the Pt/C catalyst for ORR and reduces fuel efficiency. Developing a high-activity and methanol-tolerant cathode catalyst has become one of the main objectives for the development of DMFCs. Extensive studies have been performed in recent years for lowering the cost and enhancing the methanol tolerance of
* Corresponding author. E-mail address: [email protected] (S. Chen). https://doi.org/10.1016/j.ijhydene.2018.06.003 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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several types of catalysts, such as PteM (M ¼ transition metals, such as Ti, V, Fe, Co, and Ni) alloy nanocrystals [2e6], non-noble metal catalysts [7,8], and non-metal catalysts [9,10]. According to reported results [2,4,6], PteM/C has higher ORR activity than Pt/C in the presence of methanol due to the strong interaction between Pt and the transition metal. Consequently, the ORR activity of PteM/C is strongly influenced by the Pt:M molar ratio [6]. Among carbonsupported PteM alloy catalysts, PteCo/C has the highest activity for ORR [5], and mass activities in HClO4 solution increase in the order: Pt < PtCo < Pt3Co < PtCo3 [6]. Yang et al. synthesized PteCo nanowire networks (NWNs) using CTAB as a surfactant-assisted soft template and found that PteCo NWNs exhibit higher catalytic activity towards ORR than commercial Pt/C [3]. Currently, porous carbon black (Vulcan X72) is commercially used as an electrocatalyst support material for ORR. However, the presence of micropores limits this practical application because the metallic particles are trapped in the micropore and become electrochemically inaccessible [11]. Therefore, various materials have been researched to substitute carbon black, such as graphene [12,13], carbon nanotubes (CNTs) [14e18], and transition metal-based nitrides and oxynitrides [19,20]. Owing to their high surface area, good electronic conductivity, and high aspect ratio, CNTs are promising support materials for fuel cell catalysts. Alexeyeva et al. [16] examined Pt loaded on clean CNTs (both single- and multiwalled CNTs) as ORR catalysts and reported that CNTsupported nanoparticles (3.8 ± 1.1 nm) exhibited high ORR activity in both acid and alkaline solutions. Additionally, a series of different non-noble transition metal catalysts MeNe C supported on nitrogen doped CNT (M ¼ Fe, Co, Mn, and Ni) with a transferred electron number per O2 molecule of 4 in alkaline medium were prepared by Gao et al. [18]. In practice, the durability of the carbon support under the oxidizing conditions operating in PEMFCs in acid medium should be improved, since it causes the loss of Pt particles [21,22]. Recently, TiN nanoparticles, which have excellent electrochemical inertia and electrical conductivity, and high mechanical hardness and melting point, have attracted much attention as new support materials for ORR catalysis [23,24]. The high stability of TiN-supported Pt catalysts was ascribed to the oxide or oxynitride layers formed under PEMFC conditions [25] and strong catalystesupport interaction [23]. However, these studies were limited to pure-phase TiN supports such as thin films or nanoparticles [23,26]. The specific surface areas of these TiN supports synthesized at high nitridation temperature, e.g. TiN nanoparticles (47 m2/g) are difficult to improve [27]. Herein, complex support materials composed of CNT-coated nanoparticles effectively increased the dispersion of TiN, and presented high electronic conductivity. In this study, we synthesized low-platinum-content, highstability platinum-based electrocatalysts (TiNeCNT-supported PteCoN) via complex precipitation using citric acid and subsequent NH3 treatment. Platinum, cobalt, and titanium oxides were reduced or nitrided to Pt NCs, CoN, and TiN, respectively, through a two-stage NH3 treatment at 773 K and 1173 K. The structural characterization and electrochemical performance of the PteCoN/TiNeCNT catalyst in ORR were studied.
Materials and methods Chemicals All reagents purchased were of analytical grade and used without further purification. A carbon-supported Pt catalyst (40 wt%) was obtained from Johnson Matthey Fuel Cells Co., Ltd. Nafion (5 wt% in a mixture of alcohols and water) was purchased from DuPont American Corporation, and hydroxylfunctionalized multi-walled carbon nanotubes (MWCNTs) (diameter ¼ 20e30 nm, length ¼ 10e30 mm, purity>98 wt%,) were from Chengdu Organic Chemical Corporation. H2PtCl6$6H2O, CoCl2$6H2O, TiCl4, and citric acid were all purchased from Aladdin Chemical Corporation.
Preparation of PteCoN/TiNeCNT catalysts The Pt0.3CoN/TiNeCNT electrocatalyst was prepared according to the polymerized complex (PC) method and subsequently nitrided under an NH3 flow. Briefly, 1.16 mL TiCl4, 0.25 g CoCl2$6H2O, and 4.50 g citric acid (CA) were mixed with 40 mL ethanol at a molar ratio of Co:Ti ¼ 1:10 under continuous magnetic stirring for 10 min. Subsequently, 4.10 mL H2PtCl6$6H2O aqueous solution (40 mg mL1) was added to the above mixture at a molar ratio of Pt:Co ¼ 0.3:1 and then stirred for 1 h. The resulting solution was then concentrated at 393 K for about 1 h and polymerized at 423 K until a dry gel formed. Thereafter, 0.065 g CNTs and 50 mL ethanol were added into the gel under vigorous stirring in open air and the resulting mixture was stirred overnight. The CNT and TiN mass ratio was set to CNT:TiN ¼ 1:10. After stirring for 12 h, the mixture was dried at 573 K to obtain a precursor powder. The asprepared precursor was calcined at 573 K under a N2 gas flow at 100 mL min1 for 3 h, followed by NH3 treatment at 50 mL min1 and 773 K for 2 h (reduction of platinum). Finally, the sample was held at 1173 K for 5 h (production of metal nitrides). Previously published methods [28,29], which use a heat treatment under inert gas followed by an ammonia heat treatment, confirmed this as an efficient synthetic strategy for the production of stable metal nitrides. The catalysts with varying platinum and cobalt molar ratios of 0.1:1, 0.3:1, and 0.5:1 were labeled as Pt0.1CoN/TiNeCNT, Pt0.3CoN/TiNeCNT, and Pt0.5CoN/TiNeCNT, respectively. For comparison, the TiNeCNT, CoN/TiNeCNT, and Pt/TiNeCNT support catalysts were synthesized similarly.
Characterization The crystalline structures of the PteCoN/TiNeCNT catalysts were determined using a PW3040/60 X-ray diffractometer (XRD) equipped with Cu radiation, operating at 60 kV and 60 mA and a scanning rate of 2 /min. Scanning electron microscopy (SEM) analysis was performed with a JSM 7001F highresolution microscope. The morphological and microstructural characterizations were conducted using a Tecnai G2 F20 high resolution transmission electron microscope (HRTEM). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermoescalab 250Xi spectrometer with monochromatic Al Ka X-rays (1486.6 eV) at 150 W.
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Electrochemical measurements Electrochemical measurements were conducted with Princeton Applied Research PMC1000 from AMETEK Company. The catalyst ink for electrochemical testing was prepared by adding 5 mg of 40 wt% commercial Pt/C or some amounts of PteCoN/TiNeCNT catalyst powder into a 1 mL mixture of 53.75 vol% double-distilled water, 30 vol% isopropyl alcohol, 20 vol% acetone, and 6.25 vol% Nafion (5 wt%, DuPont). Then, the ink was sonicated for 30 min to disperse the catalyst. Subsequently, 10 mL of catalyst ink was coated onto a glassy carbon (GC) disk electrode (0.25 cm2 geometrical area, Pine Research Instrument) and dried under an infrared lamp. The combined mass of platinum and cobalt, the active ingredients of the PteCoN/TiNeCNT catalyst, on the GC disk was equal to that of platinum in commercial Pt/C (0.08 mg cm2). All electrochemical measurements were performed in a conventional three-electrode glass cell in 0.5 M H2SO4 at room temperature. An Ag/AgCl electrode (in a saturated KCl solution) and Pt foil were used as reference and counter electrodes, respectively. All potentials in this paper are reported relative to the potential of reversible hydrogen electrode (RHE). Cyclic voltammetry (CV) and rotating disc electrode (RDE) measurements from 0 to 1 V vs. RHE at 10 mV/s were performed to evaluate ORR activities. The hydrogen peroxide production was determined using the rotating ring disk electrode (RRDE) technique (E7R8 electrode, GC as the disk electrode with an outside diameter (OD) of 4.57 mm and Pt as the ring electrode with an OD of 5.38 mm, 22% collection efficiency, Pine Research Instrument).
Results and discussion Structural characterization The X-ray diffraction (XRD) pattern for CNT (Fig. 1a) displays four broad peaks at 26.4, 44.4, 54.5, and 77.2 , corresponding to carbon (PDF#41-1487) in the hexagonal crystal form. The TiN phase on the TiNeCNT support was further confirmed by XRD (Fig. 1a) by sharp and intense peaks at 36.9, 42.8, 62.2, 74.5, and 78.4 , referenced to TiN (PDF#87-0630) in the face centered cubic (fcc) crystalline form. The XRD pattern of Pt0.5CoN/
3
TiNeCNT is also shown in Fig. 1a. It suggests the existence of a fcc crystalline for CoN after the NH3 treatment. The diffraction peaks of CoN (PDF#16-0116) at 36.2, 42.2, 61.3, and 73.3 are consistent with the XRD pattern obtained, and the lattice planes correspond to a fcc unit cell with a space group of F43 m. Diffraction peaks assigned to pure cobalt or oxide species were not observed. In addition, fcc Pt nanoparticles with a space group of Fm-3m are confirmed by the peaks at 39.8, 46.2, 67.5, 81.3, and 85.7 (PDF#74-1214), belonging to (111), (200), (220), (311), and (222) reflection lattice planes, respectively. Using the PC method and subsequent heat-treatment at 773 K under NH3 flow, Pt4þ species were reduced into Pt0. Slow scanning at 0.2 /min over 60e71 (the 220 lattice planes of the fcc structure of Pt, CoN, and TiN nanoparticles) was used to investigate the structural interaction of platinum, cobalt, and titanium (Fig. 1b). The related structural parameters including the Pt (220) peak positions, lattice parameters, and average particle sizes are all listed in Table 1. The average particles sizes of Pt particles on the Pt0.1CoN/TiNeCNT, Pt0.3CoN/ TiNeCNT, and Pt0.5CoN/TiNeCNT catalysts estimated from the (220) reflection by the Scherrer equation were 12, 10.3, and 8.4 nm, respectively. The relatively large particle sizes indicate that high heating temperature (1173 K) under an NH3 flow induced the aggregation of metal and metal nitrides. In addition, the lattice parameters of Pt in the PteCoN/TiN-CNT catalysts decreased to around 3.85 A from the standard 3.92 A (PDF#74-1214), indicating contraction and Pt alloy formation. This resulted in smaller interatomic PtePt distances, which favorably modulate the adsorption strength of oxygen molecules and enhance ORR kinetics. The SEM micrograph shown in Fig. 2a demonstrates that the hydroxyl-functionalized CNTs adopt pure and dispersed stripe morphologies with a width of around 30 nm. These were utilized as substrates for PteCoNeTiN loading. Upon coating with TiN or PteCoNeTiN synthesized by the PC method and heat treatment, as shown in Fig. 2b and c, respectively, the surface of the CNTs roughened, indicating the uniform dispersion of Pt, CoN, and TiN nanoparticles on the CNT surfaces. This was further confirmed by the HRTEM images. The TEM and HRTEM images of the TiNeCNT and Pt0.3CoN/TiNeCNT samples are shown in Fig. 3. The TiN-CNT sample has a narrow size distribution of TiN particles on the
Fig. 1 e (a) XRD patterns of Pt0.5CoN/TiNeCNT, TiNeCNT, and CNTs; (b) XRD patterns of Pt0.1CoN/TiNeCNT, Pt0.3CoN/ TiNeCNT, and Pt0.5CoN/TiNeCNT with detailed 2q-scan about (220) fcc reflection. Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Table 1 e Structural parameters of Pt particles in different PteCoN/TiNeCNT catalysts. Sample Pt0.1CoN/TiNeCNT Pt0.3CoN/TiNeCNT Pt0.5CoN/TiNeCNT
(220) Peak position 2q( )
FWHM ( )
Particle size (nm)
Fcc Lattice parameter ( A)
68.853 68.906 68.530
0.804 0.933 1.152
12 10.3 8.4
3.853 3.851 3.869
Fig. 2 e SEM images of (a) CNTs, (b) TiN/CNT, and (c) Pt0.3CoN/TiNeCNT.
Fig. 3 e (a) TEM image and histogram of TiN nanoparticle size distribution of composite support TiNeCNT, (b) HRTEM image of TiNeCNT, (c) TEM, and (d) HRTEM images of Pt0.3CoN/TiNeCNT catalyst.
CNT surface, as shown in Fig. 3a, with an average particle size of ca. 6.44 nm. The lattice fringes are evident in the HRTEM image (Fig. 3b). This indicates that the characteristic spacing of 0.211 nm on the CNT surface matches that of the (200) lattice plane of the TiN nanoparticles. As shown in Fig. 3c, the CNT surfaces are uniformly covered with
nanoparticles. These were identified as Pt, CoN, and TiN with evidence shown in Fig. 3d. The observed spacings of 0.211, 0.227, and 0.248 nm in Fig. 3d correspond to TiN (200), Pt (111), and CoN (111) lattice planes, respectively. The distances of lattice fringes measured on the HRTEM images were consistent with the XRD results.
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 4 e TEM image and elemental images of Pt0.3CoN/TiNeCNT catalyst showing the presence of carbon, nitrogen, cobalt, platinum, and titanium.
The elemental distributions on the Pt0.3CoN/TiNeCNT sample surface were examined using a TEM instrument equipped with an energy dispersive spectroscopy (EDS) mapping system. Fig. 4 shows a typical TEM image of the Pt0.3CoN/ TiNeCNT catalyst and the corresponding elemental mapping images. The mapping images clearly show that the particles contain carbon, nitrogen, cobalt, platinum, and titanium. The images of the carbon-, nitrogen-, and titanium-rich particles indicate that Ti and N components are uniformly dispersed on the CNT surface and that the dispersion of Pt and Co elements on the CNTs is almost identical. The TEM mapping images further confirm that there are TiN nanoparticles and PteCo alloy particles on the catalyst.
Spectroscopic analysis of catalysts X-ray photoelectron spectroscopy (XPS) was performed to probe the chemical compositions and valence states of the PteCoeN/TiNeCNT composite. The binding energies (BE) of the Pt 4f, Co 2p, Ti 2p, C 1s, and N 1s of the Pt0.3CoN/ TiNeCNT sample were calibrated with respect to C 1s peak at 284.8 eV. The deconvolution of the Pt 4f spectra produced four peaks (Fig. 5a). The binding energies were observed at 71.8 and 75.2 eV, indicating that Pt is in the Pt0 state. Furthermore, a small amount of Pt in the þ2 oxidation state (PtO or Pt (OH)2) existed as evidenced by the peaks at 73.0 and 76.5 eV [30,31]. The spectrum of Co 2p revealed four peaks, as shown in Fig. 5b. The peaks at 781.3 and 797.1 eV were attributed to Co2þ 2p3/2 and Co2þ 2p1/2, respectively. In addition, the peaks at around 786.8 and 803.2 eV are satellite peaks [32,33]. These suggest the existence of Co2þ without indication of metallic Co in the Pt0.3CoN/TiNeCNT sample, which is consistent with the XRD results. The XPS spectrum of Ti 2p is shown in Fig. 5c. The two peaks of titanium located at 455.9 (Ti 2p3/2) and 461.5 eV (Ti 2p1/2) were associated with TiN particles [34]. Curve fitting the C 1s XPS region revealed two species, as shown in Fig. 5d. The deconvoluted spectrum displays peaks at 284.8 and 285.8 eV, which are attributed to CeC and CeN bonds, respectively [35]. Simultaneously, the surface content of N was
accurately measured by XPS (Fig. 5e) and can be divided into four electronic states: the BE at around 396.1 eV related to CoN [36]; the BE at around 396.9 eV assigned to TiN [34]; the BE at around 399.1 eV associated with NeH; and the BE at around 401.3 eV corresponding to graphitic-N [35,37]. Thus, the XPS spectra results indicate the formation of CeN bonds and the aggregation of Pt and metal nitrides on the carbon nanotubes during the high-temperature NH3 treatment.
Electrochemical performances The electrocatalytic properties of the synthesized PteCoN/ TiNeCNT catalysts were evaluated by cyclic voltammetry (CV) in either N2-or O2-saturated 0.5 M H2SO4 electrolyte solutions at 10 mV/s over 0e1 V (vs. RHE); the CV curves are shown in Fig. 6a. Compared with the curve of the Pt0.3CoN/TiNeCNT catalysts in the N2-saturated solution, a well-defined oxygen reduction peak was observed in that of the O2-saturated solution. The CV curve of Pt0.3CoN/TiNeCNT revealed a remarkable cathodic reduction peak at 0.716 V (vs. RHE), comparable with that of the Pt0.5CoN/TiNeCNT sample (0.708 V). Additionally, its onset potential (0.812 V) was superior to that of Pt0.1CoN/TiN-CNT (0.768 V), Pt/TiN-CNT (0.808 V), CoN/TiN-CNT (0.679 V), and commercial Pt/C (0.789 V) (Fig. 6b). The ORR onset potential herein is defined as the potential at which the ORR current is 5% of that measured at 0 V, considering that the diffusion-limited current is reached at this potential. Table 2 summarizes the values of onset potential, limiting current, and platinum loading of catalysts for ORR at 2000 rpm. Compared with commercial Pt/ C, as-synthesized PteCoN/TiNeCNT catalysts have lower Pt content but present better ORR activities in acidic medium. This may be attributed to the CoN component of the PteCoN/ TiNeCNT catalysts, as non-platinum CoN/TiNeCNT displayed little oxygen reduction activity. Kinetic-controlled current densities were determined, and the Tafel plots of PteCoN/TiNeCNT and Pt/C are provided in Fig. 6c. The results showed that in the low current density region, Tafel slopes of 73, 62, 59, and 61 mV dec1 were observed for Pt0.1CoN/TiNeCNT, Pt0.3CoN/TiNeCNT, Pt0.5CoN/
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 5 e (a) Deconvolution of Pt 4f, (b) Co 2p, (c) Ti 2p, (d) C 1s, and (e) N 1s of Pt0.3CoN/TiNeCNT sample.
TiNeCNT, and commercial Pt/C, respectively. The Tafel slope for Pt0.3CoN/TiNeCNT, Pt0.5CoN/TiNeCNT, and commercial Pt/C catalysts approximated the Tafel slope value of 60 mV dec1 obtained from literature data for bulk, single platinum crystals, and Pt nanoparticles in acid medium [6,38,39]. The pretreated Pt0.1CoN/TiNeCNT exhibited a slightly higher Tafel slope. The difference in the Tafel slopes
at the low current density can be explained by the change in adsorption behavior of hydroxide or oxide species due to the increased amount of Co on the surface of the catalyst [6,40]. To obtain clearer insight on the catalyst kinetics, the RDE polarization curves of oxygen reduction recorded at different rotating speeds (600e2000 rpm) were analyzed using the KouteckyLevich (KL) equation:
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 6 e (a) CV curves of PteCoN/TiNeCNT and commercial Pt/C in either N2e or O2-saturated 0.5 M H2SO4 solution at 10 mV/ s; (b) Linear sweep voltammetry (LSV) curves of PteCoN/TiNeCNT and commercial Pt/C in O2-saturated 0.5 M H2SO4 solution at 10 mV/s and 2000 rpm; (c) Tafel plots of PteCoN/TiNeCNT and commercial Pt/C in O2-saturated 0.5 M H2SO4 solution at 2000 rpm.
1 1 1 ¼ þ j jk Bu1=2
(1)
where j represents the measured current density (mA/cm2), jk is the kinetically controlled current density (mA/cm2), u refers to the angular rotating frequency (rpm), and B is the Levich constant defined as: B ¼ 0:2nFCO2 DO2
2=3
y1=6
(2)
where n denotes the electron transfer number per O2, F is referred to as the Faraday constant (96485 C/mol), CO2is the
Table 2 e ORR activity and active component content of different catalysts. Sample
CoN/TiNeCNT Pt0.1CoN/TiNeCNT Pt0.3CoN/TiNeCNT Pt0.5CoN/TiNeCNT Pt/TiNeCNT Pt/C
Onset potential (V) 0.679 V 0.768 V 0.812 V 0.814 V 0.808 V 0.789 V
Limiting Pt þ CoN Pt current (wt%) (wt%) (mA cm2) 2.446 5.386 7.040 7.443 5.501 5.771
11.6 19.4 31.5 40.4 24.7 40
0 8.8 22.5 32.6 24.7 40
bulk O2 concentration (1.3106mol/L), DO2is the diffusion coefficient of O2 in the solution (1.7105cm2/s), and n is the kinetic viscosity of the electrolyte (1.0102cm2/s). The KeL analysis for Pt0.3CoN/TiNeCNT was performed on the RDE measurements at various rotating speeds (Fig. 7a). The linearity of the KeL plots suggests first-order reaction kinetics with respect to the concentration of dissolved oxygen. Table 3 shows that the electron transfer numbers for the ORR on the Pt0.3CoN/TiNeCNT sample are 3.97, 3.98, and 3.95 at the potential of 0.2, 0.4, and 0.6 V, respectively. These values suggest that the ORR is dominated by the four-electron process and O2 is directly reduced to H2O on the Pt0.3CoN/ TiNeCNT catalyst. RRDE measurements were performed to further understand the kinetics of the ORR on the Pt0.3CoN/TiNeCNT sample. The ring current, which is the maximum current for the H2O2 to H2O reduction [41], was investigated on pure Pt ring at 1.2 V (vs. RHE). The disk current density of Pt0.3CoN/TiNeCNT at 900 rpm significantly increased at 0.812 V (shown in Fig. 8a), the same as the onset potential obtained from the RDE measurements. Minimal ring disk current was detected, suggesting that H2O2 was barely produced during the operation. This confirmed that O2 was directly reduced to H2O on the Pt0.3CoN/ TiNeCNT catalyst. The electron transfer number, n, was also
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 7 e (a) LSV curves for Pt0.3CoN/TiNeCNT in O2-saturated 0.5 M H2SO4 solution at 10 mV/s with different rotating rates. (b) KouteckyeLevich plots for Pt0.3CoN/TiNeCNT at various electrode potentials.
Table 3 e Kinetic parameters deduced from the KouteckyeLevich plot and number of electrons transferred for ORR on Pt0.3CoN/TiNeCNT at different electrode potentials. Potential/V 0.2 0.4 0.6
Slope
Intercept
n
7.04 7.03 7.09
0.00192 0.0238 0.0455
3.97 3.98 3.95
calculated from n ¼ 4ID/(ID þ IR/N). Here, ID and IR are the disk and ring currents, respectively, where the current collection efficiency, N, is 0.22, according to the electrode used. Fig. 8b shows that the value of n varies from 3.94 to 3.98 over a potential ranging from 0 to 0.9 V indicating that a semi-complete four-electron pathway exists, consistent with the results of the RDE tests.
Fig. 8 e (a) RRDE voltammograms of Pt0.3CoN/TiNeCNT, tested in O2-saturated 0.5 M H2SO4 at 10 mV/s and 900 rpm with a constant Pt ring voltage of 1.2 V. (b) Curves of electron transfer number (n) and H2O2 yield of the Pt0.3CoN/TiNeCNT over 0.0e0.9 V.
Fig. 9 e (a) Chronoamperometric curves of PteCoN/TiNeCNT and commercial Pt/C in O2-saturated 0.5 M H2SO4 solution at 0.8 V under 1000 rpm for 18,000 s. (b) Chronoamperometric response for methanol tolerance of PteCoN/TiNeCNT and commercial Pt/C at 0.8 V under 1000 rpm. Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 9a presents the stability of Pt0.3CoN/TiNeCNT, Pt0.5CoN/TiNeCNT, and Pt/C catalysts, evaluated by chronoamperometry at 0.8 V. Correspondingly, the catalytic activity of Pt0.3CoN/TiNeCNT and Pt0.5CoN/TiNeCNT catalysts had little attenuation persisting above 91% of the initial values for 18,000 s, while that of commercial Pt/C catalyst retained only 70%. For DMFCs, the stability of the ORR catalysts in methanol solution is important since methanol diffuses from the anode to the cathode [42,43]. Herein, the methanol tolerance of the catalyst was evaluated. The performances of Pt0.3CoN/ TiNeCNT and Pt0.5CoN/TiNeCNT catalysts in methanol solution were examined based on chronoamperometric responses at 0.8 V (vs. RHE) and compared with that of Pt/C, as shown in Fig. 9b. Observably, when 3 M methanol was added to the solution at 1000 s, the current dramatically decreased for the commercial Pt/C electrode. In contrast, those of the Pt0.3CoN/TiNeCNT and Pt0.5CoN/TiNeCNT electrodes slightly increased upon the addition of methanol and remained almost unchanged after 1200 s. Hence, the methanol resistances of the Pt0.3CoN/TiNeCNT and Pt0.5CoN/TiN-CNT samples are better than that of commercial Pt/C, and the cocatalysts, CoN and TiN, contribute to this. These results demonstrate that the PteCoN/TiNeCNT electrocatalysts exhibit significantly higher stability than the Pt/C catalyst with and without methanol.
Conclusion In conclusion, we investigated and developed a simple route to synthesize novel electrocatalysts composed of PteCoN nanoparticles loaded on a TiNeCNT composite support. The XRD, TEM, EDS mapping, and XPS analyses confirmed that Pt, CoN, and TiN were synthesized successfully through the PC method and subsequent NH3 treatment. Furthermore, varying the platinum and cobalt molar ratios in PteCoN/TiNeCNT results in different electrocatalytic activities towards ORR. Moreover, the interaction of Pt with metal nitrides enhanced the catalytic stability in methanol. This complex multifunction approach can be applied to other low-platinum, high-stability catalysts, progressing the study of electrocatalysts in the acidic medium effectively.
Acknowledgments This work was financially supported by Yangcheng Scholars Program of Guangzhou, China (1201541563).
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Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium Shengzhou Chen a,*, Qiuchan Huang a, Wei Yang a, Hanbo Zou b, Huangwang Mai a, JiaHai Wang b a b
School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, China Guangzhou Key Laboratory for New Energy and Green Catalysis, Guangzhou University, Guangzhou, China
article info
abstract
Article history:
Herein, new catalysts for the oxygen reduction reaction (ORR) (PteCoN nanoparticles
Received 12 April 2018
supported on TiNecarbon nanotubes (CNTs)) were successfully prepared via the simple
Received in revised form
polymerized complex method and heat treatment in NH3. Through this facile two-step
30 May 2018
synthesis, metal chlorides were first transformed to the corresponding oxides, then
Accepted 1 June 2018
reduced to metal particles or nitrides. The synthesized PteCoN/TiNeCNT electrocatalysts
Available online xxx
showed excellent electrochemical activity for oxygen reduction and high tolerance against
Keywords:
RHE) is higher than that of 40 wt% commercial Pt/C (0.789 V vs. RHE) due to the synergistic
Oxygen reduction reaction
effect between nano-platinum particles and metal nitrides. The TiN-coated CNT support
Electrocatalyst
provided high electron transfer rate and catalytic tolerance in methanol solution. The
Direct methanol fuel cell
number of transferred electrons was examined by rotating ring-disk electrode measure-
Low-Pt loading
ments, which concurred with the result of the KouteckyeLevich equation for a four-
TiN
electron mechanism.
methanol in acidic medium. The ORR onset potential of Pt0.3CoN/TiNeCNT (0.812 V vs.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs), including direct methanol fuel cells (DMFCs), are widely researched because of their lower operating temperature, high energy conversion efficiency, and low emissions. Compared with other PEMFCs, DMFCs use a liquid-feed system and do not require heat management, and are therefore preferable for portable power applications [1]. However, a slow methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) with conventional Pt-based electrocatalysts demand plenty of
Pt, whose high cost limits the commercialization of the DMFCs. Additionally, methanol is permeable in perfluorosulfonate membranes from the DMFC anode to the cathode and produces a mixed potential between methanol oxidation and oxygen reduction at Pt/C, which lowers the activity of the Pt/C catalyst for ORR and reduces fuel efficiency. Developing a high-activity and methanol-tolerant cathode catalyst has become one of the main objectives for the development of DMFCs. Extensive studies have been performed in recent years for lowering the cost and enhancing the methanol tolerance of
* Corresponding author. E-mail address: [email protected] (S. Chen). https://doi.org/10.1016/j.ijhydene.2018.06.003 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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several types of catalysts, such as PteM (M ¼ transition metals, such as Ti, V, Fe, Co, and Ni) alloy nanocrystals [2e6], non-noble metal catalysts [7,8], and non-metal catalysts [9,10]. According to reported results [2,4,6], PteM/C has higher ORR activity than Pt/C in the presence of methanol due to the strong interaction between Pt and the transition metal. Consequently, the ORR activity of PteM/C is strongly influenced by the Pt:M molar ratio [6]. Among carbonsupported PteM alloy catalysts, PteCo/C has the highest activity for ORR [5], and mass activities in HClO4 solution increase in the order: Pt < PtCo < Pt3Co < PtCo3 [6]. Yang et al. synthesized PteCo nanowire networks (NWNs) using CTAB as a surfactant-assisted soft template and found that PteCo NWNs exhibit higher catalytic activity towards ORR than commercial Pt/C [3]. Currently, porous carbon black (Vulcan X72) is commercially used as an electrocatalyst support material for ORR. However, the presence of micropores limits this practical application because the metallic particles are trapped in the micropore and become electrochemically inaccessible [11]. Therefore, various materials have been researched to substitute carbon black, such as graphene [12,13], carbon nanotubes (CNTs) [14e18], and transition metal-based nitrides and oxynitrides [19,20]. Owing to their high surface area, good electronic conductivity, and high aspect ratio, CNTs are promising support materials for fuel cell catalysts. Alexeyeva et al. [16] examined Pt loaded on clean CNTs (both single- and multiwalled CNTs) as ORR catalysts and reported that CNTsupported nanoparticles (3.8 ± 1.1 nm) exhibited high ORR activity in both acid and alkaline solutions. Additionally, a series of different non-noble transition metal catalysts MeNe C supported on nitrogen doped CNT (M ¼ Fe, Co, Mn, and Ni) with a transferred electron number per O2 molecule of 4 in alkaline medium were prepared by Gao et al. [18]. In practice, the durability of the carbon support under the oxidizing conditions operating in PEMFCs in acid medium should be improved, since it causes the loss of Pt particles [21,22]. Recently, TiN nanoparticles, which have excellent electrochemical inertia and electrical conductivity, and high mechanical hardness and melting point, have attracted much attention as new support materials for ORR catalysis [23,24]. The high stability of TiN-supported Pt catalysts was ascribed to the oxide or oxynitride layers formed under PEMFC conditions [25] and strong catalystesupport interaction [23]. However, these studies were limited to pure-phase TiN supports such as thin films or nanoparticles [23,26]. The specific surface areas of these TiN supports synthesized at high nitridation temperature, e.g. TiN nanoparticles (47 m2/g) are difficult to improve [27]. Herein, complex support materials composed of CNT-coated nanoparticles effectively increased the dispersion of TiN, and presented high electronic conductivity. In this study, we synthesized low-platinum-content, highstability platinum-based electrocatalysts (TiNeCNT-supported PteCoN) via complex precipitation using citric acid and subsequent NH3 treatment. Platinum, cobalt, and titanium oxides were reduced or nitrided to Pt NCs, CoN, and TiN, respectively, through a two-stage NH3 treatment at 773 K and 1173 K. The structural characterization and electrochemical performance of the PteCoN/TiNeCNT catalyst in ORR were studied.
Materials and methods Chemicals All reagents purchased were of analytical grade and used without further purification. A carbon-supported Pt catalyst (40 wt%) was obtained from Johnson Matthey Fuel Cells Co., Ltd. Nafion (5 wt% in a mixture of alcohols and water) was purchased from DuPont American Corporation, and hydroxylfunctionalized multi-walled carbon nanotubes (MWCNTs) (diameter ¼ 20e30 nm, length ¼ 10e30 mm, purity>98 wt%,) were from Chengdu Organic Chemical Corporation. H2PtCl6$6H2O, CoCl2$6H2O, TiCl4, and citric acid were all purchased from Aladdin Chemical Corporation.
Preparation of PteCoN/TiNeCNT catalysts The Pt0.3CoN/TiNeCNT electrocatalyst was prepared according to the polymerized complex (PC) method and subsequently nitrided under an NH3 flow. Briefly, 1.16 mL TiCl4, 0.25 g CoCl2$6H2O, and 4.50 g citric acid (CA) were mixed with 40 mL ethanol at a molar ratio of Co:Ti ¼ 1:10 under continuous magnetic stirring for 10 min. Subsequently, 4.10 mL H2PtCl6$6H2O aqueous solution (40 mg mL1) was added to the above mixture at a molar ratio of Pt:Co ¼ 0.3:1 and then stirred for 1 h. The resulting solution was then concentrated at 393 K for about 1 h and polymerized at 423 K until a dry gel formed. Thereafter, 0.065 g CNTs and 50 mL ethanol were added into the gel under vigorous stirring in open air and the resulting mixture was stirred overnight. The CNT and TiN mass ratio was set to CNT:TiN ¼ 1:10. After stirring for 12 h, the mixture was dried at 573 K to obtain a precursor powder. The asprepared precursor was calcined at 573 K under a N2 gas flow at 100 mL min1 for 3 h, followed by NH3 treatment at 50 mL min1 and 773 K for 2 h (reduction of platinum). Finally, the sample was held at 1173 K for 5 h (production of metal nitrides). Previously published methods [28,29], which use a heat treatment under inert gas followed by an ammonia heat treatment, confirmed this as an efficient synthetic strategy for the production of stable metal nitrides. The catalysts with varying platinum and cobalt molar ratios of 0.1:1, 0.3:1, and 0.5:1 were labeled as Pt0.1CoN/TiNeCNT, Pt0.3CoN/TiNeCNT, and Pt0.5CoN/TiNeCNT, respectively. For comparison, the TiNeCNT, CoN/TiNeCNT, and Pt/TiNeCNT support catalysts were synthesized similarly.
Characterization The crystalline structures of the PteCoN/TiNeCNT catalysts were determined using a PW3040/60 X-ray diffractometer (XRD) equipped with Cu radiation, operating at 60 kV and 60 mA and a scanning rate of 2 /min. Scanning electron microscopy (SEM) analysis was performed with a JSM 7001F highresolution microscope. The morphological and microstructural characterizations were conducted using a Tecnai G2 F20 high resolution transmission electron microscope (HRTEM). X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermoescalab 250Xi spectrometer with monochromatic Al Ka X-rays (1486.6 eV) at 150 W.
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Electrochemical measurements Electrochemical measurements were conducted with Princeton Applied Research PMC1000 from AMETEK Company. The catalyst ink for electrochemical testing was prepared by adding 5 mg of 40 wt% commercial Pt/C or some amounts of PteCoN/TiNeCNT catalyst powder into a 1 mL mixture of 53.75 vol% double-distilled water, 30 vol% isopropyl alcohol, 20 vol% acetone, and 6.25 vol% Nafion (5 wt%, DuPont). Then, the ink was sonicated for 30 min to disperse the catalyst. Subsequently, 10 mL of catalyst ink was coated onto a glassy carbon (GC) disk electrode (0.25 cm2 geometrical area, Pine Research Instrument) and dried under an infrared lamp. The combined mass of platinum and cobalt, the active ingredients of the PteCoN/TiNeCNT catalyst, on the GC disk was equal to that of platinum in commercial Pt/C (0.08 mg cm2). All electrochemical measurements were performed in a conventional three-electrode glass cell in 0.5 M H2SO4 at room temperature. An Ag/AgCl electrode (in a saturated KCl solution) and Pt foil were used as reference and counter electrodes, respectively. All potentials in this paper are reported relative to the potential of reversible hydrogen electrode (RHE). Cyclic voltammetry (CV) and rotating disc electrode (RDE) measurements from 0 to 1 V vs. RHE at 10 mV/s were performed to evaluate ORR activities. The hydrogen peroxide production was determined using the rotating ring disk electrode (RRDE) technique (E7R8 electrode, GC as the disk electrode with an outside diameter (OD) of 4.57 mm and Pt as the ring electrode with an OD of 5.38 mm, 22% collection efficiency, Pine Research Instrument).
Results and discussion Structural characterization The X-ray diffraction (XRD) pattern for CNT (Fig. 1a) displays four broad peaks at 26.4, 44.4, 54.5, and 77.2 , corresponding to carbon (PDF#41-1487) in the hexagonal crystal form. The TiN phase on the TiNeCNT support was further confirmed by XRD (Fig. 1a) by sharp and intense peaks at 36.9, 42.8, 62.2, 74.5, and 78.4 , referenced to TiN (PDF#87-0630) in the face centered cubic (fcc) crystalline form. The XRD pattern of Pt0.5CoN/
3
TiNeCNT is also shown in Fig. 1a. It suggests the existence of a fcc crystalline for CoN after the NH3 treatment. The diffraction peaks of CoN (PDF#16-0116) at 36.2, 42.2, 61.3, and 73.3 are consistent with the XRD pattern obtained, and the lattice planes correspond to a fcc unit cell with a space group of F43 m. Diffraction peaks assigned to pure cobalt or oxide species were not observed. In addition, fcc Pt nanoparticles with a space group of Fm-3m are confirmed by the peaks at 39.8, 46.2, 67.5, 81.3, and 85.7 (PDF#74-1214), belonging to (111), (200), (220), (311), and (222) reflection lattice planes, respectively. Using the PC method and subsequent heat-treatment at 773 K under NH3 flow, Pt4þ species were reduced into Pt0. Slow scanning at 0.2 /min over 60e71 (the 220 lattice planes of the fcc structure of Pt, CoN, and TiN nanoparticles) was used to investigate the structural interaction of platinum, cobalt, and titanium (Fig. 1b). The related structural parameters including the Pt (220) peak positions, lattice parameters, and average particle sizes are all listed in Table 1. The average particles sizes of Pt particles on the Pt0.1CoN/TiNeCNT, Pt0.3CoN/ TiNeCNT, and Pt0.5CoN/TiNeCNT catalysts estimated from the (220) reflection by the Scherrer equation were 12, 10.3, and 8.4 nm, respectively. The relatively large particle sizes indicate that high heating temperature (1173 K) under an NH3 flow induced the aggregation of metal and metal nitrides. In addition, the lattice parameters of Pt in the PteCoN/TiN-CNT catalysts decreased to around 3.85 A from the standard 3.92 A (PDF#74-1214), indicating contraction and Pt alloy formation. This resulted in smaller interatomic PtePt distances, which favorably modulate the adsorption strength of oxygen molecules and enhance ORR kinetics. The SEM micrograph shown in Fig. 2a demonstrates that the hydroxyl-functionalized CNTs adopt pure and dispersed stripe morphologies with a width of around 30 nm. These were utilized as substrates for PteCoNeTiN loading. Upon coating with TiN or PteCoNeTiN synthesized by the PC method and heat treatment, as shown in Fig. 2b and c, respectively, the surface of the CNTs roughened, indicating the uniform dispersion of Pt, CoN, and TiN nanoparticles on the CNT surfaces. This was further confirmed by the HRTEM images. The TEM and HRTEM images of the TiNeCNT and Pt0.3CoN/TiNeCNT samples are shown in Fig. 3. The TiN-CNT sample has a narrow size distribution of TiN particles on the
Fig. 1 e (a) XRD patterns of Pt0.5CoN/TiNeCNT, TiNeCNT, and CNTs; (b) XRD patterns of Pt0.1CoN/TiNeCNT, Pt0.3CoN/ TiNeCNT, and Pt0.5CoN/TiNeCNT with detailed 2q-scan about (220) fcc reflection. Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Table 1 e Structural parameters of Pt particles in different PteCoN/TiNeCNT catalysts. Sample Pt0.1CoN/TiNeCNT Pt0.3CoN/TiNeCNT Pt0.5CoN/TiNeCNT
(220) Peak position 2q( )
FWHM ( )
Particle size (nm)
Fcc Lattice parameter ( A)
68.853 68.906 68.530
0.804 0.933 1.152
12 10.3 8.4
3.853 3.851 3.869
Fig. 2 e SEM images of (a) CNTs, (b) TiN/CNT, and (c) Pt0.3CoN/TiNeCNT.
Fig. 3 e (a) TEM image and histogram of TiN nanoparticle size distribution of composite support TiNeCNT, (b) HRTEM image of TiNeCNT, (c) TEM, and (d) HRTEM images of Pt0.3CoN/TiNeCNT catalyst.
CNT surface, as shown in Fig. 3a, with an average particle size of ca. 6.44 nm. The lattice fringes are evident in the HRTEM image (Fig. 3b). This indicates that the characteristic spacing of 0.211 nm on the CNT surface matches that of the (200) lattice plane of the TiN nanoparticles. As shown in Fig. 3c, the CNT surfaces are uniformly covered with
nanoparticles. These were identified as Pt, CoN, and TiN with evidence shown in Fig. 3d. The observed spacings of 0.211, 0.227, and 0.248 nm in Fig. 3d correspond to TiN (200), Pt (111), and CoN (111) lattice planes, respectively. The distances of lattice fringes measured on the HRTEM images were consistent with the XRD results.
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 4 e TEM image and elemental images of Pt0.3CoN/TiNeCNT catalyst showing the presence of carbon, nitrogen, cobalt, platinum, and titanium.
The elemental distributions on the Pt0.3CoN/TiNeCNT sample surface were examined using a TEM instrument equipped with an energy dispersive spectroscopy (EDS) mapping system. Fig. 4 shows a typical TEM image of the Pt0.3CoN/ TiNeCNT catalyst and the corresponding elemental mapping images. The mapping images clearly show that the particles contain carbon, nitrogen, cobalt, platinum, and titanium. The images of the carbon-, nitrogen-, and titanium-rich particles indicate that Ti and N components are uniformly dispersed on the CNT surface and that the dispersion of Pt and Co elements on the CNTs is almost identical. The TEM mapping images further confirm that there are TiN nanoparticles and PteCo alloy particles on the catalyst.
Spectroscopic analysis of catalysts X-ray photoelectron spectroscopy (XPS) was performed to probe the chemical compositions and valence states of the PteCoeN/TiNeCNT composite. The binding energies (BE) of the Pt 4f, Co 2p, Ti 2p, C 1s, and N 1s of the Pt0.3CoN/ TiNeCNT sample were calibrated with respect to C 1s peak at 284.8 eV. The deconvolution of the Pt 4f spectra produced four peaks (Fig. 5a). The binding energies were observed at 71.8 and 75.2 eV, indicating that Pt is in the Pt0 state. Furthermore, a small amount of Pt in the þ2 oxidation state (PtO or Pt (OH)2) existed as evidenced by the peaks at 73.0 and 76.5 eV [30,31]. The spectrum of Co 2p revealed four peaks, as shown in Fig. 5b. The peaks at 781.3 and 797.1 eV were attributed to Co2þ 2p3/2 and Co2þ 2p1/2, respectively. In addition, the peaks at around 786.8 and 803.2 eV are satellite peaks [32,33]. These suggest the existence of Co2þ without indication of metallic Co in the Pt0.3CoN/TiNeCNT sample, which is consistent with the XRD results. The XPS spectrum of Ti 2p is shown in Fig. 5c. The two peaks of titanium located at 455.9 (Ti 2p3/2) and 461.5 eV (Ti 2p1/2) were associated with TiN particles [34]. Curve fitting the C 1s XPS region revealed two species, as shown in Fig. 5d. The deconvoluted spectrum displays peaks at 284.8 and 285.8 eV, which are attributed to CeC and CeN bonds, respectively [35]. Simultaneously, the surface content of N was
accurately measured by XPS (Fig. 5e) and can be divided into four electronic states: the BE at around 396.1 eV related to CoN [36]; the BE at around 396.9 eV assigned to TiN [34]; the BE at around 399.1 eV associated with NeH; and the BE at around 401.3 eV corresponding to graphitic-N [35,37]. Thus, the XPS spectra results indicate the formation of CeN bonds and the aggregation of Pt and metal nitrides on the carbon nanotubes during the high-temperature NH3 treatment.
Electrochemical performances The electrocatalytic properties of the synthesized PteCoN/ TiNeCNT catalysts were evaluated by cyclic voltammetry (CV) in either N2-or O2-saturated 0.5 M H2SO4 electrolyte solutions at 10 mV/s over 0e1 V (vs. RHE); the CV curves are shown in Fig. 6a. Compared with the curve of the Pt0.3CoN/TiNeCNT catalysts in the N2-saturated solution, a well-defined oxygen reduction peak was observed in that of the O2-saturated solution. The CV curve of Pt0.3CoN/TiNeCNT revealed a remarkable cathodic reduction peak at 0.716 V (vs. RHE), comparable with that of the Pt0.5CoN/TiNeCNT sample (0.708 V). Additionally, its onset potential (0.812 V) was superior to that of Pt0.1CoN/TiN-CNT (0.768 V), Pt/TiN-CNT (0.808 V), CoN/TiN-CNT (0.679 V), and commercial Pt/C (0.789 V) (Fig. 6b). The ORR onset potential herein is defined as the potential at which the ORR current is 5% of that measured at 0 V, considering that the diffusion-limited current is reached at this potential. Table 2 summarizes the values of onset potential, limiting current, and platinum loading of catalysts for ORR at 2000 rpm. Compared with commercial Pt/ C, as-synthesized PteCoN/TiNeCNT catalysts have lower Pt content but present better ORR activities in acidic medium. This may be attributed to the CoN component of the PteCoN/ TiNeCNT catalysts, as non-platinum CoN/TiNeCNT displayed little oxygen reduction activity. Kinetic-controlled current densities were determined, and the Tafel plots of PteCoN/TiNeCNT and Pt/C are provided in Fig. 6c. The results showed that in the low current density region, Tafel slopes of 73, 62, 59, and 61 mV dec1 were observed for Pt0.1CoN/TiNeCNT, Pt0.3CoN/TiNeCNT, Pt0.5CoN/
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 5 e (a) Deconvolution of Pt 4f, (b) Co 2p, (c) Ti 2p, (d) C 1s, and (e) N 1s of Pt0.3CoN/TiNeCNT sample.
TiNeCNT, and commercial Pt/C, respectively. The Tafel slope for Pt0.3CoN/TiNeCNT, Pt0.5CoN/TiNeCNT, and commercial Pt/C catalysts approximated the Tafel slope value of 60 mV dec1 obtained from literature data for bulk, single platinum crystals, and Pt nanoparticles in acid medium [6,38,39]. The pretreated Pt0.1CoN/TiNeCNT exhibited a slightly higher Tafel slope. The difference in the Tafel slopes
at the low current density can be explained by the change in adsorption behavior of hydroxide or oxide species due to the increased amount of Co on the surface of the catalyst [6,40]. To obtain clearer insight on the catalyst kinetics, the RDE polarization curves of oxygen reduction recorded at different rotating speeds (600e2000 rpm) were analyzed using the KouteckyLevich (KL) equation:
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
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Fig. 6 e (a) CV curves of PteCoN/TiNeCNT and commercial Pt/C in either N2e or O2-saturated 0.5 M H2SO4 solution at 10 mV/ s; (b) Linear sweep voltammetry (LSV) curves of PteCoN/TiNeCNT and commercial Pt/C in O2-saturated 0.5 M H2SO4 solution at 10 mV/s and 2000 rpm; (c) Tafel plots of PteCoN/TiNeCNT and commercial Pt/C in O2-saturated 0.5 M H2SO4 solution at 2000 rpm.
1 1 1 ¼ þ j jk Bu1=2
(1)
where j represents the measured current density (mA/cm2), jk is the kinetically controlled current density (mA/cm2), u refers to the angular rotating frequency (rpm), and B is the Levich constant defined as: B ¼ 0:2nFCO2 DO2
2=3
y1=6
(2)
where n denotes the electron transfer number per O2, F is referred to as the Faraday constant (96485 C/mol), CO2is the
Table 2 e ORR activity and active component content of different catalysts. Sample
CoN/TiNeCNT Pt0.1CoN/TiNeCNT Pt0.3CoN/TiNeCNT Pt0.5CoN/TiNeCNT Pt/TiNeCNT Pt/C
Onset potential (V) 0.679 V 0.768 V 0.812 V 0.814 V 0.808 V 0.789 V
Limiting Pt þ CoN Pt current (wt%) (wt%) (mA cm2) 2.446 5.386 7.040 7.443 5.501 5.771
11.6 19.4 31.5 40.4 24.7 40
0 8.8 22.5 32.6 24.7 40
bulk O2 concentration (1.3106mol/L), DO2is the diffusion coefficient of O2 in the solution (1.7105cm2/s), and n is the kinetic viscosity of the electrolyte (1.0102cm2/s). The KeL analysis for Pt0.3CoN/TiNeCNT was performed on the RDE measurements at various rotating speeds (Fig. 7a). The linearity of the KeL plots suggests first-order reaction kinetics with respect to the concentration of dissolved oxygen. Table 3 shows that the electron transfer numbers for the ORR on the Pt0.3CoN/TiNeCNT sample are 3.97, 3.98, and 3.95 at the potential of 0.2, 0.4, and 0.6 V, respectively. These values suggest that the ORR is dominated by the four-electron process and O2 is directly reduced to H2O on the Pt0.3CoN/ TiNeCNT catalyst. RRDE measurements were performed to further understand the kinetics of the ORR on the Pt0.3CoN/TiNeCNT sample. The ring current, which is the maximum current for the H2O2 to H2O reduction [41], was investigated on pure Pt ring at 1.2 V (vs. RHE). The disk current density of Pt0.3CoN/TiNeCNT at 900 rpm significantly increased at 0.812 V (shown in Fig. 8a), the same as the onset potential obtained from the RDE measurements. Minimal ring disk current was detected, suggesting that H2O2 was barely produced during the operation. This confirmed that O2 was directly reduced to H2O on the Pt0.3CoN/ TiNeCNT catalyst. The electron transfer number, n, was also
Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 7 e (a) LSV curves for Pt0.3CoN/TiNeCNT in O2-saturated 0.5 M H2SO4 solution at 10 mV/s with different rotating rates. (b) KouteckyeLevich plots for Pt0.3CoN/TiNeCNT at various electrode potentials.
Table 3 e Kinetic parameters deduced from the KouteckyeLevich plot and number of electrons transferred for ORR on Pt0.3CoN/TiNeCNT at different electrode potentials. Potential/V 0.2 0.4 0.6
Slope
Intercept
n
7.04 7.03 7.09
0.00192 0.0238 0.0455
3.97 3.98 3.95
calculated from n ¼ 4ID/(ID þ IR/N). Here, ID and IR are the disk and ring currents, respectively, where the current collection efficiency, N, is 0.22, according to the electrode used. Fig. 8b shows that the value of n varies from 3.94 to 3.98 over a potential ranging from 0 to 0.9 V indicating that a semi-complete four-electron pathway exists, consistent with the results of the RDE tests.
Fig. 8 e (a) RRDE voltammograms of Pt0.3CoN/TiNeCNT, tested in O2-saturated 0.5 M H2SO4 at 10 mV/s and 900 rpm with a constant Pt ring voltage of 1.2 V. (b) Curves of electron transfer number (n) and H2O2 yield of the Pt0.3CoN/TiNeCNT over 0.0e0.9 V.
Fig. 9 e (a) Chronoamperometric curves of PteCoN/TiNeCNT and commercial Pt/C in O2-saturated 0.5 M H2SO4 solution at 0.8 V under 1000 rpm for 18,000 s. (b) Chronoamperometric response for methanol tolerance of PteCoN/TiNeCNT and commercial Pt/C at 0.8 V under 1000 rpm. Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003
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Fig. 9a presents the stability of Pt0.3CoN/TiNeCNT, Pt0.5CoN/TiNeCNT, and Pt/C catalysts, evaluated by chronoamperometry at 0.8 V. Correspondingly, the catalytic activity of Pt0.3CoN/TiNeCNT and Pt0.5CoN/TiNeCNT catalysts had little attenuation persisting above 91% of the initial values for 18,000 s, while that of commercial Pt/C catalyst retained only 70%. For DMFCs, the stability of the ORR catalysts in methanol solution is important since methanol diffuses from the anode to the cathode [42,43]. Herein, the methanol tolerance of the catalyst was evaluated. The performances of Pt0.3CoN/ TiNeCNT and Pt0.5CoN/TiNeCNT catalysts in methanol solution were examined based on chronoamperometric responses at 0.8 V (vs. RHE) and compared with that of Pt/C, as shown in Fig. 9b. Observably, when 3 M methanol was added to the solution at 1000 s, the current dramatically decreased for the commercial Pt/C electrode. In contrast, those of the Pt0.3CoN/TiNeCNT and Pt0.5CoN/TiNeCNT electrodes slightly increased upon the addition of methanol and remained almost unchanged after 1200 s. Hence, the methanol resistances of the Pt0.3CoN/TiNeCNT and Pt0.5CoN/TiN-CNT samples are better than that of commercial Pt/C, and the cocatalysts, CoN and TiN, contribute to this. These results demonstrate that the PteCoN/TiNeCNT electrocatalysts exhibit significantly higher stability than the Pt/C catalyst with and without methanol.
Conclusion In conclusion, we investigated and developed a simple route to synthesize novel electrocatalysts composed of PteCoN nanoparticles loaded on a TiNeCNT composite support. The XRD, TEM, EDS mapping, and XPS analyses confirmed that Pt, CoN, and TiN were synthesized successfully through the PC method and subsequent NH3 treatment. Furthermore, varying the platinum and cobalt molar ratios in PteCoN/TiNeCNT results in different electrocatalytic activities towards ORR. Moreover, the interaction of Pt with metal nitrides enhanced the catalytic stability in methanol. This complex multifunction approach can be applied to other low-platinum, high-stability catalysts, progressing the study of electrocatalysts in the acidic medium effectively.
Acknowledgments This work was financially supported by Yangcheng Scholars Program of Guangzhou, China (1201541563).
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Please cite this article in press as: Chen S, et al., PteCoN supported on TiN-modified carbon nanotubes (PteCoN/TiNeCNT) as efficient oxygen reduction reaction catalysts in acidic medium, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.06.003