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Platinum nanoparticles decorated robust binary transition metal nitride–carbon nanotubes hybrid as an efficient electrocatalyst for the methanol oxidation reaction
Journal of Power Sources 326 (2016) 84e92
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Platinum nanoparticles decorated robust binary transition metal nitrideecarbon nanotubes hybrid as an efficient electrocatalyst for the methanol oxidation reaction Guohe Zhan a, Zhenggao Fu a, Dalei Sun a, **, Zhanchang Pan a, *, Chumin Xiao a, Shoukun Wu b, Chun Chen b, Guanghui Hu a, Zhigang Wei a a b
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, 510006, China Huizhou King Brother Electronic Technology Co., Ltd, Huizhou, 516083, China
h i g h l i g h t s CNTs@TiCoN hybrid support with tunable composition was successfully synthesized. Pt nanoparticles with small size were well dispersed on CNTs@TiCoN support. CNTs@TiCoN combines high conductivity and superb corrosion resistance. The catalyst shows remarkably enhanced methanol oxidation activity and durability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 23 June 2016 Accepted 27 June 2016
Titanium cobalt nitride (TiCoN)eCNTs hybrid support is prepared by a facile and efficient method, including a one-pot solvothermal process followed by a nitriding process, and this hybrid support is further decorated with Pt nanoparticles to catalyze the oxidation of methanol. The catalyst is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. Notably, Pt/CNTs@TiCoN catalyst exhibits a much higher mass activity and durability than that of the conventional Pt/C (JM) for methanol oxidation. The experimental data indicates that the CNTs@TiCoN hybrid support combines the merits of the CNTs’s high conductivity and the superb corrosion resistance of external TiCoN coating. © 2016 Elsevier B.V. All rights reserved.
Keywords: Titanium cobalt nitride Methanol oxidation reaction Enhanced stability Fuel cells
1. Introduction Direct methanol fuel cells (DMFCs) have arose much attention because of their high power density, low operating temperature, and reduced pollution as a new power source for portable electronic devices and automobiles [1e6]. Moreover, liquid methanol can be safely and inexpensively stored and transported, as compared with the gaseous hydrogen used for the proton exchange membrane fuel cells (PEMFCs). Thus, not surprisingly, DMFCs are suitable for a large scope of power applications [5,7,8]. However, to make PEMFCs economically viable, one of the main problems to be
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Sun), [email protected] (Z. Pan). http://dx.doi.org/10.1016/j.jpowsour.2016.06.112 0378-7753/© 2016 Elsevier B.V. All rights reserved.
solved is finding catalysts with sufficient activity and stability for the methanol oxidation reaction (MOR). Up to now, platinum (Pt) based catalysts are still the most effective for PEMFCs [9]. However, the Pt catalysts are severely poisoned by carbonaceous intermediates generated during the MOR process [10]. Furthermore, carbon black is still the most widely used catalyst support for noble materials in DMFCs. Unfortunately, it has been argued that the loss of performance for Pt/C catalyst is mainly ascribed to the corrosion of carbon supports, which further results in migration, aggregation, and Ostwald ripening of Pt NPs [11e19]. Therefore, the design and synthesis of highly active MOR catalysts with strong durability is extremely desirable. The introduction of novel support materials with co-catalytic functionality has proven to be a very effective approach for improving both the catalytic activity and the durability of Pt-based
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catalysts for MOR [20e23]. Transition metal nitrides (TMNs) are definitely suited to support noble metals in DMFCs because of their excellent electrical conductivity, thermal stability, exceptional hardness, corrosion resistance, and electrochemical durability in a wide range potential under fuel cell operations [23e31]. Pt or Pd supported on CrN [32] and TiN [29,31,33e35] as electrocatalyst in DMFCs has been reported, which showed higher performance and better stability than that of commercial Pt/C catalyst. In particular, bimetallic transition metal nitrides (BTMNs) demonstrate good synergistic effect with Pt on catalysis towards MOR activities. DiSalvo et al. reported Ti0.5Cr0.5N supported PdAg catalyst outperformed commercial Pt/C catalyst in terms of MOR activity while exhibiting high durability [36]. Recently, through the combination of solvothermal and post-nitriding process, we developed a Pt/ TiMoN hybrid material, which shows much higher MOR performance and durability than that of Pt/TiN and commercial Pt/C catalyst in acid solutions [21]. A significant advance of the BTMNs supported Pt catalysts was that the hybrid materials exhibit excellent stability due to the strong metal support interactions (SMSI) instead of the weak interactions between the carbon black and the Pt NPs. However, a drawback is that the discontinuity caused by the existence of defects and abundant grain boundaries of nanoparticles could lower its electronic conductivity and make the electron transport inefficient. Thus, catalytic performance of the catalysts was weakened. We assume that the combination of BTMN NPs and few-walled carbon nanotubes (CNTs) could overcome this issue as the hybrid materials with a BTMN shell and CNTs core are expected to offer many advantages, such as: i) combination of the robust BTMNs with good synergistic effect and the high conductivity of CNTs, ii) creating an interactive porous three-dimensional (3D) structure and constructing a high conductive electron pathway, and iii) the avoidance of direct contact between CNTs and the electrolyte to protect the core CNTs from corrosion. Herein, we report a facile method for the synthesis of titanium cobalt nitride/CNTs hybrid material (CNTs@TiCoN) as efficient support to anchor the Pt NPs. The resulting CNT@TiCoN supported Pt (Pt/CNTs@TiCoN) catalyst showed much higher MOR current density than our recently reported Pt/TiMoN and Pt/CNT@TiMoN prepared in the same method. This hybrid material affords a high potential for electrocatalyst support towards the MOR than did the carbon and BTMNs NPs. 2. Experimental procedures 2.1. Synthesis of CNT@TiCoN hybrid All chemicals were purchased commercially (analytical grade, Aladdin, China) and were used without further purification. CNTs (10e80 nm in diameter and 30e100 mm in length) were treated with concentrated HNO3 in an ultrasonic bath at 80 C for 6 h for surface functionalization, then followed by filtration and thoroughly washing with deionized water. 50 mg pre-treated CNTs were dispersed in a solution containing 30 mL ethanol, 5 mL deionized water and 5 mL benzyl alcohol under ultrasonication and stirring, then a uniformly mixed solution composed of 15 ml ethanol, 0.2 ml tetrabutyl titanate (C16H36O4Ti, TBOT) and appropriate amount of cobalt acetate tetrahydrate (Co(CH3COO)2$4H2O) was dipped into the CNTs suspension drop by drop and continuously stirred for 2 h, and then the mixture was transferred to a Teflon-lined autoclave and kept at 150 C for 3 h. The precipitate was cooled down to the room temperature, followed by filtering, washing thoroughly with deionized water, and then drying in air at 80 C overnight, which was labeled as CNTs@TiCoO2. Finally, appropriate amount of the powder precursors was placed in the
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tubular furnace, annealed at 800 C under an NH3 gas flow (100 sccm) for 2 h with a progressive heating rate (room temperature to 800 C, 5 C min1), and the sample was labeled as CNTs@TiCoN. For comparison, CNTs@TiMoN was synthesized with the same process without the addition of Co(CH3COO)2$4H2O, instead of MoCl5, and the atomic ratio of Ti/M (Co or Mo) was maintained at 4:1 for both CNTs@TiCoN and CNTs@TiMoN. 2.2. Preparation of Pt/CNTs@TiCoN and Pt/CNTs@TiMoN CNTs@TiCoN and CNTs@TiMoN supported Pt catalysts were synthesized by the ethylene glycol (EG) reduction method, labeled as Pt/CNTs@TiCoN and Pt/CNTs@TiMoN, respectively, and the total Pt content was controlled ca. 20 wt%. Briefly, 80 mg of the support, 60 mg of sodium citrate and 1.35 mL of H2PtCl6$6H2O solution (40 mg mL1) were mixed with 30 mL of EG, followed by vigorous stirring and ultrasonic for 30 min. Then the mixture was transferred to a flask and heated at 160 C for 3 h. Subsequently, the suspension was filtered and washed thoroughly with deionized water, and then dried at 60 C in a vacuum drying oven overnight. The commercial Johnson Matthey Pt/C (JM Pt/C, 3.3 nm on Vulcan XC-72R carbon, 20 wt%) was used for comparison in this work. 2.3. Materials characterization X-ray diffraction (XRD) was operated on a Rigaku-Ultima III Xray diffractometer with Cu Ka radiation (l ¼ 1.5405 Å) in the Bragg angle ranging from 20 to 86 . Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were performed with a field-emission scanning electron microscope (FE-SEM, Hitachi S4800). Transmission electron microscopy (TEM) and highresolution Transmission electron microscopy (HR-TEM) images were acquired with a JEOL 2100 microscope. X-ray photoelectron spectroscopy (XPS) was performed on an Axis Ultra DLD X-ray photoelectron spectrometer employing a monochromated Al-Ka Xray source (hv ¼ 1486.6 eV). The precise Pt loading was carried out by inductively coupled plasma optical emission spectrometry (ICPOES, Leeman PROFILE SPEC) measurements. 2.4. Electrode preparation All electrochemical experiments were carried out on an Autolab electrochemical workstation (Model PGSTAT302N, Metrohm) at room temperature (25 ± 1 C), using a three-electrode electrochemical cell. The cell consisted of a glassy carbon working electrode (GC electrode, 5 mm inner diameter, 0.196 cm2), a platinum foil counter electrode and an Ag/AgCl (3 M NaCl) reference electrode. In this study, all potentials are referenced with respect to Ag/ AgCl electrode. A thin film of the electrocatalyst was prepared as follows: 5 mg catalyst was dispersed ultrasonically in 1 mL ethanol for 30 min in ice water bath to form a uniform catalyst ink (1 mg Pt mL1). A total of 5 mL of well-dispersed catalyst ink was spread onto the pre-polished GC electrode. After drying at room temperature, the electrode was then covered by a drop of Nafion solution (0.1 wt % in ethanol solution) and dried in air before measurements. The loading amount of Pt for Pt/C, Pt/CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiMoN were controlled at 25 mg cm2 normalized to the geometric electrode area, and the actual loadings are confirmed by ICP-OES. Cyclic Voltammetry (CV) performance of the catalysts was carried out in N2-saturated 0.5 M H2SO4 solution in the potential range from 0 to 1 V at a scan rate of 50 mV s1. Before the ORR activity test, all of the electrodes were pretreated by cycling the potential between 0 and 1.2 V at a sweep rate of 50 mV s1 for 20 cycles so as to remove any surface contamination. The
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electrocatalytic properties of the catalysts toward methanol oxidation were investigated in 1 M CH3OH þ 0.5 M H2SO4 solution, and the chronoamperometry curves of all the catalysts were held at 0.6 V for 5000 s in 1 M CH3OH þ 0.5 M H2SO4 solution. The accelerated durability test (ADT) for the catalysts was performed in 0.5 M H2SO4 exposed to air in the potential range from 0.6 to 1.05 V at room temperature.
3. Results and discussion The morphology and structure of the as-prepared CNTs@TiCoN were first examined by SEM and TEM. The TiCoN coating was found to cover the CNTs surface uniformly with insignificant aggregation, and only few aggregated TiCoN nanoparticles were observable (Fig. 1a and b). TEM images of pre-treated pure CNTs were shown in
Fig. 1. (a) SEM and (b) enlarged SEM image of as-prepared CNTs@TiCoN samples, (c) TEM and (d) enlarged TEM image of as-prepared CNTs@TiCoN, and the insert is the corresponding HR-TEM images. (e) XRD patterns of CNTs, as-prepared CNTs@TiCoO2, CNTs@TiCoN and Pt/CNTs@TiCoN, (f) EDS profile of Pt/CNTs@TiCoN.
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Fig. S1, and CNTs@TiCoN were shown in Fig. 1 c and d, respectively. Fig. 1d clearly showed the formation of nanoparticles with average size of ~7.5 nm on CNTs, with almost no nanoparticles detached from nanotubes. The detailed structural features of the samples were characterized by HR-TEM, as shown in the inset of Fig. S1b and Fig. 1d. The lattice spacing of ca. 0.34 nm belongs to the graphite (002) plane, and the value of ca. 0.25 nm of CNTs@TiCoN are corresponding well to a growth direction along the (111) plane of face centered cubic (fcc) TiN(JCPDS No. 38-1420) [17,24]. Fig. 1e shows the XRD patterns of CNTs, as-prepared CNTs@TiCoO2, CNTs@TiCoN and Pt/CNTs@TiCoN samples, respectively. The diffraction peaks of CNTs@TiCoO2 confirmed that TiCoO2 NPs belonged to the anatase phase (JCPDS No. 21-1272) [37], and the relative weak graphite (002) peak compared with the CNTs sample due to the TiCoO2 coating. For the CNTs@TiCoN sample, the diffraction peaks centered at 36.7, 42.6, 61.8 , 74.1, and 77.9 are corresponding to the (fcc) structure of TiN [17,24]. Only the TiN phase could be observed after the nitriding process, implying that all the anatase TiO2 nanocrystals in the precursors were wholly converted to TiN
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nanostructures. Furthermore, the absence of any single metallic phase of Co or phase separation between Ti and Co nitrides, demonstrating that Co element was incorporated into the TiN structure to form a single-phase solid solution. For the Pt/ CNTs@TiCoN sample, the locations of diffraction peaks for Pt are identical to PDF 04-0802, and the grain size were calculated to be around 3.3 nm. The EDX profile of CNTs@TiCoN sample was depicted in Fig. S1c, demonstrating the presence of Co element and a Ti:Co atomic ratio of 18.75:4.57, which agrees closely with the feeding atomic ratio of 4:1. The EDX date implied that the composition of the (Ti:Co) binary metal nitride can be tuned in a facile manner by changing the feeding ratios of cobalt acetate tetrahydrate and TBOT in the precursors. The Pt/CNTs@TiCoN were further analyzed by EDX. In the EDX profile of Pt/CNTs@TiCoN sample (Fig. 1e), the clear peaks of Pt, Co and Ti confirm the successful doping of Co and deposition of Pt. Quantitative analysis from EDX shows 19.81 wt% Pt in the sample, which agrees well with the feeding Pt ratio (20 wt%). Fig. 2 shows the TEM and HR-TME images of Pt/CNTs, Pt/
Fig. 2. The TEM images of (a,b) Pt/CNTs, (d,e) Pt/CNTs@TiCoN and (g,h) JM Pt/C, and (c, f, i) are the corresponding histograms of Pt NPs diameter.
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CNTs@TiCoN and Pt/C, respectively. From the images, it can be seen that the aggregation of Pt nanoparticles is minimal and they are highly dispersed on both the CNTs and CNTs@TiCoN supports as well as the carbon black. The averaged size of the particles for Pt/ CNTs, Pt/CNTs@TiCoN and Pt/C catalysts were 3.32 ± 0.6, 3.32 ± 0.4 and 3.34 ± 0.6 nm, respectively, calculated by counting the diameter of 100 random picked Pt nanoparticles, as shown in the corresponding histograms (Fig. 2 (c,f, i)). In order to unambiguously identify the Pt and TiCuN nanoparticles, the microstructure of the Pt/CNTs@TiCoN was examined by HR-TEM, and the images were shown in Fig. S2. It can be observed that the small, uniformly distributed, darkly contrasting spherical Pt nanoparticles (3～4 nm diameter) are highly dispersed and well adhered to the CNTs@TiCoN support, though no posttreatment (e.g., annealing treatment, acid treatment, or polymer wrapping) steps were used. Furthermore, the TiCoN nanoparticles were also can be distinguished easily (yellow cycles marked) in the HR-TEM images. The CVs of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN catalysts are shown in Fig. 3a, and the electrochemically active area (ECSA) was calculated by integrating the charge passing the electrode during the hydrogen adsorption/desorption, after correction for double-layer formation and assuming a value of 210 mC cm2 for the adsorption of a hydrogen monolayer [14,38]. The ECSA was measured to be 57.3 and 55.9 m2 g1 for Pt/CNTs and Pt/CNTs@TiCoN, compared with 54.6 m2 g1 for JM Pt/C catalyst. The similar ECSA was owing to both the same size and good dispersion of the Pt NPs on the three supports. Fig. 3b shows the electrocatalytic properties of the catalysts toward methanol oxidation in 1 M CH3OH þ 0.5 M H2SO4 solution, and Pt/TiCoN and Pt/CNTs@TiMoN were also investigated
for comparisons. The enlarged vertical axis (positive scans) in the potential range from 0.25 to 0.6 V in Fig. 3b was shown in Fig. 3c, as can be seen, Pt/CNTs@TiCoN has a negative onset potential than that of Pt/TiCoN and Pt/CNTs@TiMoN, which were much more negative than that of the JM Pt/C and Pt/CNTs catalysts, indicating that it is more favorable for methanol oxidation on Pt/CNTs@TiCoN. The peak current density of Pt/CNTs@TiCoN catalyst is 0.92 A mg1Pt, which is 4.5-fold of Pt/C catalyst. Furthermore, Pt/ CNTs@TiCoN exhibited better catalytic activity towards the MOR than our recently reported Pt/TiMoN [21] and Pt/CNTs@TiMoN prepared in the same method, indicating the positive effect of the CNTs supports and the Co doping. To demonstrate the electronic effect and the interaction between Pt NPs and the various supports, the samples were analyzed by XPS (Fig. 4). The peaks of Co 2p and Mo 3d in CNTs@TiCoN and CNTs@TiMoN samples confirmed the successful doping of Co and Mo element, respectively, and the high-resolution of Co 2p and Mo 3d scans also demonstrated the existence of CoeN and MoeN bond [39e42]. From the XPS data of Ti 2p (Fig. 4c), it can be clearly seen that the bonding of TieC (463.1 eV), TieOeN (457.3 eV), TieN (461.6 eV) and TieO (463.1 eV) were coexisted [43e47]. However, the XRD data in Fig. 1e showed evidence of only TiN, confirming that the surface TiO2 layer was very thin. The high-resolution of Pt 4f of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN were provided in Fig. 4d, and the all the scans showed two states of Pt. The Pt 4f binding energy (BE) of Pt/CNTs is almost the same with Pt/C. However, a clear negative shift of ca. 0.47 eV in the BE of Pt4f7/2 for Pt/CNTs@TiCoN with respect to the Pt/C, indicating the strong interaction between anchored Pt atoms and the CNTs@TiCoN support. The onset
Fig. 3. CV curves of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN catalysts, (b) the mass activities of Pt/C, Pt/CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiCoN catalysts toward the methanol oxidation in 1 M CH3OH þ 0.5 M H2SO4 solution, (c) the enlarged image in Fig. 3b (rang from 0.25 to 0.6 V).
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Fig. 4. XPS spectra of (a) Mo 3d and (b) Co 2p for CNTs@TiCoN and CNTs@TiMoN samples, respectively, (c) Ti 2p of CNTs@TiN and CNTs@TiMoN, (d) Pt 4f of Pt/C, Pt/CNTs and Pt/ CNTs@TiCoN catalysts, (e) C 1s of CNTs, CNTs@TiN and CNTs@TiCoN.
potential for oxide formation and reduction peak potential of the Pt/CNTs@TiCoN catalyst obviously shifted toward positive potential could also be observed from the comparison of CV curves (Fig. 3a), compared with the other catalysts, suggesting that the electronic state (d state) of the surface Pt was changed through the interaction with the CNTs@TiCoN support. The negative shifted Pt 4f binding energy suggested that the Pt atoms can get electrons donated from the CNTs@TiCoN supports [48], inducing a down shift of the Pt d states relative to the Fermi level [20,49e51], which will result in a reduced intermediate adsorptive coupling for Pt in the ratedetermining step during the MOR process. Thus, a faster catalytic electrode reaction was obtained for Pt/CNTs@TiCoN catalyst than
that of the Pt/C and Pt/CNTs catalyst. The performance enhancement observed for Pt/CNTs@TiCoN than that of Pt/TiMoN and Pt/ TiCoN without the CNTs can most likely be attributed to not only the Ptesupport interactions, but also the specific properties of the TiCoNeCNTs composites. On one hand, the formation of a 3-D like morphology assembled from the intersecting 1-D CNTs@TiCoN, which enhance the mass transfer of reactant and resultant molecules. On the other hand, the direct growth of TiCoN nanostructures on CNTs may afford strong electrical and chemical coupling between TiCoN nanoparticles and the core CNTs, which constructs a fast electron transport network during the MOR process [52,53]. Fig. 4e shows the high revolution spectrums of C 1 s in CNTs,
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CNTs@TiMoN and CNTs@TiCoN samples. Compared with CNTs, the apparent TieC and NeC bond are confirmed by the C 1 s (BE 281.5 and 287.8 eV) [54e56], respectively, suggesting the formation of TieNeC structure in CNTs@TiMoN and CNTs@TiCoN. The results illustrated the strong electrical and chemical coupling between TiMoN/TiCoN nanoparticles and the carbon network, that constructs a rapid and continuous electron transfer pathway, which is definitely do good to the MOR process. The CO stripping voltammograms was carried out in 0.5 M H2SO4 solution at room temperature, as depicted in Fig. 5a. Before the test, High-purity CO was bubbled into the electrolyte solution for 30 min when the electrode potential was kept at 0.2 V to achieve maximum coverage of CO on the Pt nanoparticles. The onset potential (0.32 V) of CO oxidation on Pt/CNTs@TiCoN is obviously more negative than that on the Pt/C (0.47 V) and Pt/CNTs (0.46 V) in the first forward scan, confirming the Pt/CNTs@TiCoN catalyst can facilitate removal of CO from the surface, which could be due to the co-catalyst effect reported previously (OH groups can be adsorbed on the TiN surface and help oxidize the poisonous intermediates adhered on Pt surface), thus CO removal was accelerated. To further evaluate the rate of surface poisoning, chronoamperometry curves of Pt/C, Pt/CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiCoN catalysts were measured in 1 M CH3OH þ 0.5 M H2SO4 solution and the potential was held at 0.60 V during the measurements (Fig. 5b). What’s more, normalized decay corresponding to the current at 5000s and the initial (I_5000s/I _initial) was also conducted, as shown in Fig. 5c. It was clear that Pt/ CNTs@TiCoN, Pt/CNTs@TiMoN and Pt/TiCoN catalysts exhibited a slower current decay over time in comparison with Pt/C and Pt/ CNTs, confirming a higher tolerance to the carbonaceous species generated during methanol oxidation of the catalysts with binary
transition metal as the support. Additionally, much higher current density retainment was observed for the Pt/CNTs@TiCoN in the long run versus the other ones, indicating it was much more electroactive for MOR. ADT is conducted to test the electrochemical durability of the catalysts, since long-term stability of the electrocatalyst is still one of the top issues to be resolved before the commercialization of DMFCs. Fig. 6 a and b show the CV curve evolutions for Pt/C, Pt/ CNTs, Pt/TiCoN and Pt/CNTs@TiCoN catalysts, respectively. As observed, JM Pt/C and Pt/CNTs both lost more than 50% of their total value after 10, 000 cycles. However, Pt/TiCoN and Pt/CNTs@TiCoN catalysts showed great improvement in ECSA preservation, with 80% and 76% of the initial ECSA after the ADT (Fig. 6c), suggesting the higher durability of Pt/CNTs@TiCoN over the commercial JM Pt/ C under the fuel cell operate conditions. Moreover, to gain more understanding of the enhanced activity and durability of Pt/CNTs@TiCoN, the catalysts were collected after the ADT, and the structure was observed by TEM. As shown in Fig. 7, the average Pt NPs size of the commercial Pt catalyst increases from averaged 3.3 nm to 6e10 nm (Fig. 7a and b), indicating sustaining growth of Pt nanoparticles. In addition, a significant decrease of Pt nanoparticles density on carbon black/CNTs support are observed, demonstrating that the major cause for performance loss of commercial JM Pt and Pt/CNTs catalysts is Pt detaching, degradation and aggregation as the consequence of the corrosion of the carbon support. In contrast, these problems were avoided in considerable degree in Pt/CNTs@TiCoN catalyst, since no significant increase in the Pt NPs size and almost no variation of Pt nanoparticles density was observed (Fig. 7e and f), which was most likely owing to the good corrosion resistance of the CNTs@TiCoN support, and the strong interaction between the anchored Pt NPs and the support as
Fig. 5. (a) CO stripping of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN catalysts in 0.5 M H2SO4 at a scan rate of 50 mVs1 at room temperature. (b) Chronoamperometry curves of Pt/C, Pt/ CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiCoN catalysts held at 0.60 V (vs. Ag/AgCl) in 1 M CH3OH þ 0.5 M H2SO4 solution for 5000 s. (c) The normalized current density corresponding to the current at 5000s and the initial.
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Fig. 6. The evolution of CV curves after 10 000 cycles for (a) Pt/C and Pt/CNTs, (b) Pt/TiCoN, Pt/CNTs@TiMoN, and (c) Comparison of ECSA loss for the catalysts.
Fig. 7. (a, b) TEM images of collected Pt/C, (c, d) Pt/CNTs and (e,f) Pt/CNTs@TiCoN catalysts after the ADT.
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well. These synergetic effects slow down the aggregation and detachment of Pt nanoparticles from the CNTs@TiCoN support, introducing a super activity and stability for the Pt/CNTs@TiCoN catalyst. The results demonstrate that CNTs@TiCoN hybrid composition is indeed a promising support to improve Pt catalytic activity and durability for practical catalytic applications, for which exhibits the merits of high conductivity (CNTs), corrosion resistance and strong metal support interactions (TiCoN). 4. Conclusions In summary, we have developed a facile procedure to synthesize Pt/CNTs@TiCoN hybrid catalyst used the hybrid material, CNTs@TiCoN, as the support for Pt, and most importantly, the experimental data show that Pt/CNTs@TiCoN have a much higher catalytic activity and durability than commercial JM Pt/C for methanol oxidation, and the performance also outperformed Pt/ TiCoN and Pt/CNTs@TiMoN catalysts in the same conditions. The higher MOR performance and durability of the novel catalyst was probably due to the high electrical conductivity of the special properties of the TiCoNeCNTs composites, high stability of transition metal nitrides and the strong metal support interactions. These advances open up broad possibilities for the design and synthesis of various binary transition metal nitrides coated CNTs as the support for Pt based catalysts with applications in DMFCs. Acknowledgement This research was financially supported by Guangdong Natural Science Foundation (2016A030313704), Scientific and technological projects in Guangdong Province (2014A010105041, 2016A010103035), Huizhou Daya Bay technological projects (2015A01006) and Huizhou technological projects (2014C050012004). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.06.112. References [1] K. Sasaki, J.X. Wang, H. Naohara, N. Marinkovic, K. More, H. Inada, R.R. Adzic, Electrochim Acta 55 (2010) 2645e2652. [2] N. Kristian, Y. Yu, J.-M. Lee, X. Liu, X. Wang, Electrochim Acta 56 (2010) 1000e1007. [3] J. Guo, G. Sun, Q. Wang, G. Wang, Z. Zhou, S. Tang, L. Jiang, B. Zhou, Q. Xin, Carbon 44 (2006) 152e157. [4] T. Maiyalagan, C. Mahendiran, K. Chaitanya, R. Tyagi, F. Nawaz Khan, Research Chem. Intermed. 38 (2011) 383e391. [5] T. Maiyalagan, Int. J. Hydrogen Energy 34 (2009) 2874e2879. [6] H. Yan, C. Tian, L. Sun, B. Wang, L. Wang, J. Yin, A. Wu, H. Fu, Energy Environ. Sci. 7 (2014) 1939. [7] T. Maiyalagan, T.O. Alaje, K. Scott, J. Physical Chem. C 116 (2012) 2630e2638. [8] V. Bambagioni, C. Bianchini, A. Marchionni, J. Filippi, F. Vizza, J. Teddy, P. Serp, M. Zhiani, J. Power Sources 190 (2009) 241e251. [9] L.X. Ding, A.L. Wang, G.R. Li, Z.Q. Liu, W.X. Zhao, C.Y. Su, Y.X. Tong, J. Am. Chem. Soc. 134 (2012) 5730e5733. [10] M. Tian, G. Wu, A. Chen, ACS Catal. 2 (2012) 425e432. [11] A.-L. Wang, H. Xu, J.-X. Feng, L.-X. Ding, Y.-X. Tong, G.-R. Li, J. Am. Chem. Soc. 135 (2013) 10703e10709. [12] S.-Y. Huang, P. Ganesan, B.N. Popov, ACS Catal. 2 (2012) 825e831.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Platinum nanoparticles decorated robust binary transition metal nitrideecarbon nanotubes hybrid as an efficient electrocatalyst for the methanol oxidation reaction Guohe Zhan a, Zhenggao Fu a, Dalei Sun a, **, Zhanchang Pan a, *, Chumin Xiao a, Shoukun Wu b, Chun Chen b, Guanghui Hu a, Zhigang Wei a a b
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, 510006, China Huizhou King Brother Electronic Technology Co., Ltd, Huizhou, 516083, China
h i g h l i g h t s CNTs@TiCoN hybrid support with tunable composition was successfully synthesized. Pt nanoparticles with small size were well dispersed on CNTs@TiCoN support. CNTs@TiCoN combines high conductivity and superb corrosion resistance. The catalyst shows remarkably enhanced methanol oxidation activity and durability.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2016 Received in revised form 23 June 2016 Accepted 27 June 2016
Titanium cobalt nitride (TiCoN)eCNTs hybrid support is prepared by a facile and efficient method, including a one-pot solvothermal process followed by a nitriding process, and this hybrid support is further decorated with Pt nanoparticles to catalyze the oxidation of methanol. The catalyst is characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. Notably, Pt/CNTs@TiCoN catalyst exhibits a much higher mass activity and durability than that of the conventional Pt/C (JM) for methanol oxidation. The experimental data indicates that the CNTs@TiCoN hybrid support combines the merits of the CNTs’s high conductivity and the superb corrosion resistance of external TiCoN coating. © 2016 Elsevier B.V. All rights reserved.
Keywords: Titanium cobalt nitride Methanol oxidation reaction Enhanced stability Fuel cells
1. Introduction Direct methanol fuel cells (DMFCs) have arose much attention because of their high power density, low operating temperature, and reduced pollution as a new power source for portable electronic devices and automobiles [1e6]. Moreover, liquid methanol can be safely and inexpensively stored and transported, as compared with the gaseous hydrogen used for the proton exchange membrane fuel cells (PEMFCs). Thus, not surprisingly, DMFCs are suitable for a large scope of power applications [5,7,8]. However, to make PEMFCs economically viable, one of the main problems to be
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Sun), [email protected] (Z. Pan). http://dx.doi.org/10.1016/j.jpowsour.2016.06.112 0378-7753/© 2016 Elsevier B.V. All rights reserved.
solved is finding catalysts with sufficient activity and stability for the methanol oxidation reaction (MOR). Up to now, platinum (Pt) based catalysts are still the most effective for PEMFCs [9]. However, the Pt catalysts are severely poisoned by carbonaceous intermediates generated during the MOR process [10]. Furthermore, carbon black is still the most widely used catalyst support for noble materials in DMFCs. Unfortunately, it has been argued that the loss of performance for Pt/C catalyst is mainly ascribed to the corrosion of carbon supports, which further results in migration, aggregation, and Ostwald ripening of Pt NPs [11e19]. Therefore, the design and synthesis of highly active MOR catalysts with strong durability is extremely desirable. The introduction of novel support materials with co-catalytic functionality has proven to be a very effective approach for improving both the catalytic activity and the durability of Pt-based
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catalysts for MOR [20e23]. Transition metal nitrides (TMNs) are definitely suited to support noble metals in DMFCs because of their excellent electrical conductivity, thermal stability, exceptional hardness, corrosion resistance, and electrochemical durability in a wide range potential under fuel cell operations [23e31]. Pt or Pd supported on CrN [32] and TiN [29,31,33e35] as electrocatalyst in DMFCs has been reported, which showed higher performance and better stability than that of commercial Pt/C catalyst. In particular, bimetallic transition metal nitrides (BTMNs) demonstrate good synergistic effect with Pt on catalysis towards MOR activities. DiSalvo et al. reported Ti0.5Cr0.5N supported PdAg catalyst outperformed commercial Pt/C catalyst in terms of MOR activity while exhibiting high durability [36]. Recently, through the combination of solvothermal and post-nitriding process, we developed a Pt/ TiMoN hybrid material, which shows much higher MOR performance and durability than that of Pt/TiN and commercial Pt/C catalyst in acid solutions [21]. A significant advance of the BTMNs supported Pt catalysts was that the hybrid materials exhibit excellent stability due to the strong metal support interactions (SMSI) instead of the weak interactions between the carbon black and the Pt NPs. However, a drawback is that the discontinuity caused by the existence of defects and abundant grain boundaries of nanoparticles could lower its electronic conductivity and make the electron transport inefficient. Thus, catalytic performance of the catalysts was weakened. We assume that the combination of BTMN NPs and few-walled carbon nanotubes (CNTs) could overcome this issue as the hybrid materials with a BTMN shell and CNTs core are expected to offer many advantages, such as: i) combination of the robust BTMNs with good synergistic effect and the high conductivity of CNTs, ii) creating an interactive porous three-dimensional (3D) structure and constructing a high conductive electron pathway, and iii) the avoidance of direct contact between CNTs and the electrolyte to protect the core CNTs from corrosion. Herein, we report a facile method for the synthesis of titanium cobalt nitride/CNTs hybrid material (CNTs@TiCoN) as efficient support to anchor the Pt NPs. The resulting CNT@TiCoN supported Pt (Pt/CNTs@TiCoN) catalyst showed much higher MOR current density than our recently reported Pt/TiMoN and Pt/CNT@TiMoN prepared in the same method. This hybrid material affords a high potential for electrocatalyst support towards the MOR than did the carbon and BTMNs NPs. 2. Experimental procedures 2.1. Synthesis of CNT@TiCoN hybrid All chemicals were purchased commercially (analytical grade, Aladdin, China) and were used without further purification. CNTs (10e80 nm in diameter and 30e100 mm in length) were treated with concentrated HNO3 in an ultrasonic bath at 80 C for 6 h for surface functionalization, then followed by filtration and thoroughly washing with deionized water. 50 mg pre-treated CNTs were dispersed in a solution containing 30 mL ethanol, 5 mL deionized water and 5 mL benzyl alcohol under ultrasonication and stirring, then a uniformly mixed solution composed of 15 ml ethanol, 0.2 ml tetrabutyl titanate (C16H36O4Ti, TBOT) and appropriate amount of cobalt acetate tetrahydrate (Co(CH3COO)2$4H2O) was dipped into the CNTs suspension drop by drop and continuously stirred for 2 h, and then the mixture was transferred to a Teflon-lined autoclave and kept at 150 C for 3 h. The precipitate was cooled down to the room temperature, followed by filtering, washing thoroughly with deionized water, and then drying in air at 80 C overnight, which was labeled as CNTs@TiCoO2. Finally, appropriate amount of the powder precursors was placed in the
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tubular furnace, annealed at 800 C under an NH3 gas flow (100 sccm) for 2 h with a progressive heating rate (room temperature to 800 C, 5 C min1), and the sample was labeled as CNTs@TiCoN. For comparison, CNTs@TiMoN was synthesized with the same process without the addition of Co(CH3COO)2$4H2O, instead of MoCl5, and the atomic ratio of Ti/M (Co or Mo) was maintained at 4:1 for both CNTs@TiCoN and CNTs@TiMoN. 2.2. Preparation of Pt/CNTs@TiCoN and Pt/CNTs@TiMoN CNTs@TiCoN and CNTs@TiMoN supported Pt catalysts were synthesized by the ethylene glycol (EG) reduction method, labeled as Pt/CNTs@TiCoN and Pt/CNTs@TiMoN, respectively, and the total Pt content was controlled ca. 20 wt%. Briefly, 80 mg of the support, 60 mg of sodium citrate and 1.35 mL of H2PtCl6$6H2O solution (40 mg mL1) were mixed with 30 mL of EG, followed by vigorous stirring and ultrasonic for 30 min. Then the mixture was transferred to a flask and heated at 160 C for 3 h. Subsequently, the suspension was filtered and washed thoroughly with deionized water, and then dried at 60 C in a vacuum drying oven overnight. The commercial Johnson Matthey Pt/C (JM Pt/C, 3.3 nm on Vulcan XC-72R carbon, 20 wt%) was used for comparison in this work. 2.3. Materials characterization X-ray diffraction (XRD) was operated on a Rigaku-Ultima III Xray diffractometer with Cu Ka radiation (l ¼ 1.5405 Å) in the Bragg angle ranging from 20 to 86 . Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were performed with a field-emission scanning electron microscope (FE-SEM, Hitachi S4800). Transmission electron microscopy (TEM) and highresolution Transmission electron microscopy (HR-TEM) images were acquired with a JEOL 2100 microscope. X-ray photoelectron spectroscopy (XPS) was performed on an Axis Ultra DLD X-ray photoelectron spectrometer employing a monochromated Al-Ka Xray source (hv ¼ 1486.6 eV). The precise Pt loading was carried out by inductively coupled plasma optical emission spectrometry (ICPOES, Leeman PROFILE SPEC) measurements. 2.4. Electrode preparation All electrochemical experiments were carried out on an Autolab electrochemical workstation (Model PGSTAT302N, Metrohm) at room temperature (25 ± 1 C), using a three-electrode electrochemical cell. The cell consisted of a glassy carbon working electrode (GC electrode, 5 mm inner diameter, 0.196 cm2), a platinum foil counter electrode and an Ag/AgCl (3 M NaCl) reference electrode. In this study, all potentials are referenced with respect to Ag/ AgCl electrode. A thin film of the electrocatalyst was prepared as follows: 5 mg catalyst was dispersed ultrasonically in 1 mL ethanol for 30 min in ice water bath to form a uniform catalyst ink (1 mg Pt mL1). A total of 5 mL of well-dispersed catalyst ink was spread onto the pre-polished GC electrode. After drying at room temperature, the electrode was then covered by a drop of Nafion solution (0.1 wt % in ethanol solution) and dried in air before measurements. The loading amount of Pt for Pt/C, Pt/CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiMoN were controlled at 25 mg cm2 normalized to the geometric electrode area, and the actual loadings are confirmed by ICP-OES. Cyclic Voltammetry (CV) performance of the catalysts was carried out in N2-saturated 0.5 M H2SO4 solution in the potential range from 0 to 1 V at a scan rate of 50 mV s1. Before the ORR activity test, all of the electrodes were pretreated by cycling the potential between 0 and 1.2 V at a sweep rate of 50 mV s1 for 20 cycles so as to remove any surface contamination. The
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electrocatalytic properties of the catalysts toward methanol oxidation were investigated in 1 M CH3OH þ 0.5 M H2SO4 solution, and the chronoamperometry curves of all the catalysts were held at 0.6 V for 5000 s in 1 M CH3OH þ 0.5 M H2SO4 solution. The accelerated durability test (ADT) for the catalysts was performed in 0.5 M H2SO4 exposed to air in the potential range from 0.6 to 1.05 V at room temperature.
3. Results and discussion The morphology and structure of the as-prepared CNTs@TiCoN were first examined by SEM and TEM. The TiCoN coating was found to cover the CNTs surface uniformly with insignificant aggregation, and only few aggregated TiCoN nanoparticles were observable (Fig. 1a and b). TEM images of pre-treated pure CNTs were shown in
Fig. 1. (a) SEM and (b) enlarged SEM image of as-prepared CNTs@TiCoN samples, (c) TEM and (d) enlarged TEM image of as-prepared CNTs@TiCoN, and the insert is the corresponding HR-TEM images. (e) XRD patterns of CNTs, as-prepared CNTs@TiCoO2, CNTs@TiCoN and Pt/CNTs@TiCoN, (f) EDS profile of Pt/CNTs@TiCoN.
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Fig. S1, and CNTs@TiCoN were shown in Fig. 1 c and d, respectively. Fig. 1d clearly showed the formation of nanoparticles with average size of ~7.5 nm on CNTs, with almost no nanoparticles detached from nanotubes. The detailed structural features of the samples were characterized by HR-TEM, as shown in the inset of Fig. S1b and Fig. 1d. The lattice spacing of ca. 0.34 nm belongs to the graphite (002) plane, and the value of ca. 0.25 nm of CNTs@TiCoN are corresponding well to a growth direction along the (111) plane of face centered cubic (fcc) TiN(JCPDS No. 38-1420) [17,24]. Fig. 1e shows the XRD patterns of CNTs, as-prepared CNTs@TiCoO2, CNTs@TiCoN and Pt/CNTs@TiCoN samples, respectively. The diffraction peaks of CNTs@TiCoO2 confirmed that TiCoO2 NPs belonged to the anatase phase (JCPDS No. 21-1272) [37], and the relative weak graphite (002) peak compared with the CNTs sample due to the TiCoO2 coating. For the CNTs@TiCoN sample, the diffraction peaks centered at 36.7, 42.6, 61.8 , 74.1, and 77.9 are corresponding to the (fcc) structure of TiN [17,24]. Only the TiN phase could be observed after the nitriding process, implying that all the anatase TiO2 nanocrystals in the precursors were wholly converted to TiN
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nanostructures. Furthermore, the absence of any single metallic phase of Co or phase separation between Ti and Co nitrides, demonstrating that Co element was incorporated into the TiN structure to form a single-phase solid solution. For the Pt/ CNTs@TiCoN sample, the locations of diffraction peaks for Pt are identical to PDF 04-0802, and the grain size were calculated to be around 3.3 nm. The EDX profile of CNTs@TiCoN sample was depicted in Fig. S1c, demonstrating the presence of Co element and a Ti:Co atomic ratio of 18.75:4.57, which agrees closely with the feeding atomic ratio of 4:1. The EDX date implied that the composition of the (Ti:Co) binary metal nitride can be tuned in a facile manner by changing the feeding ratios of cobalt acetate tetrahydrate and TBOT in the precursors. The Pt/CNTs@TiCoN were further analyzed by EDX. In the EDX profile of Pt/CNTs@TiCoN sample (Fig. 1e), the clear peaks of Pt, Co and Ti confirm the successful doping of Co and deposition of Pt. Quantitative analysis from EDX shows 19.81 wt% Pt in the sample, which agrees well with the feeding Pt ratio (20 wt%). Fig. 2 shows the TEM and HR-TME images of Pt/CNTs, Pt/
Fig. 2. The TEM images of (a,b) Pt/CNTs, (d,e) Pt/CNTs@TiCoN and (g,h) JM Pt/C, and (c, f, i) are the corresponding histograms of Pt NPs diameter.
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CNTs@TiCoN and Pt/C, respectively. From the images, it can be seen that the aggregation of Pt nanoparticles is minimal and they are highly dispersed on both the CNTs and CNTs@TiCoN supports as well as the carbon black. The averaged size of the particles for Pt/ CNTs, Pt/CNTs@TiCoN and Pt/C catalysts were 3.32 ± 0.6, 3.32 ± 0.4 and 3.34 ± 0.6 nm, respectively, calculated by counting the diameter of 100 random picked Pt nanoparticles, as shown in the corresponding histograms (Fig. 2 (c,f, i)). In order to unambiguously identify the Pt and TiCuN nanoparticles, the microstructure of the Pt/CNTs@TiCoN was examined by HR-TEM, and the images were shown in Fig. S2. It can be observed that the small, uniformly distributed, darkly contrasting spherical Pt nanoparticles (3～4 nm diameter) are highly dispersed and well adhered to the CNTs@TiCoN support, though no posttreatment (e.g., annealing treatment, acid treatment, or polymer wrapping) steps were used. Furthermore, the TiCoN nanoparticles were also can be distinguished easily (yellow cycles marked) in the HR-TEM images. The CVs of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN catalysts are shown in Fig. 3a, and the electrochemically active area (ECSA) was calculated by integrating the charge passing the electrode during the hydrogen adsorption/desorption, after correction for double-layer formation and assuming a value of 210 mC cm2 for the adsorption of a hydrogen monolayer [14,38]. The ECSA was measured to be 57.3 and 55.9 m2 g1 for Pt/CNTs and Pt/CNTs@TiCoN, compared with 54.6 m2 g1 for JM Pt/C catalyst. The similar ECSA was owing to both the same size and good dispersion of the Pt NPs on the three supports. Fig. 3b shows the electrocatalytic properties of the catalysts toward methanol oxidation in 1 M CH3OH þ 0.5 M H2SO4 solution, and Pt/TiCoN and Pt/CNTs@TiMoN were also investigated
for comparisons. The enlarged vertical axis (positive scans) in the potential range from 0.25 to 0.6 V in Fig. 3b was shown in Fig. 3c, as can be seen, Pt/CNTs@TiCoN has a negative onset potential than that of Pt/TiCoN and Pt/CNTs@TiMoN, which were much more negative than that of the JM Pt/C and Pt/CNTs catalysts, indicating that it is more favorable for methanol oxidation on Pt/CNTs@TiCoN. The peak current density of Pt/CNTs@TiCoN catalyst is 0.92 A mg1Pt, which is 4.5-fold of Pt/C catalyst. Furthermore, Pt/ CNTs@TiCoN exhibited better catalytic activity towards the MOR than our recently reported Pt/TiMoN [21] and Pt/CNTs@TiMoN prepared in the same method, indicating the positive effect of the CNTs supports and the Co doping. To demonstrate the electronic effect and the interaction between Pt NPs and the various supports, the samples were analyzed by XPS (Fig. 4). The peaks of Co 2p and Mo 3d in CNTs@TiCoN and CNTs@TiMoN samples confirmed the successful doping of Co and Mo element, respectively, and the high-resolution of Co 2p and Mo 3d scans also demonstrated the existence of CoeN and MoeN bond [39e42]. From the XPS data of Ti 2p (Fig. 4c), it can be clearly seen that the bonding of TieC (463.1 eV), TieOeN (457.3 eV), TieN (461.6 eV) and TieO (463.1 eV) were coexisted [43e47]. However, the XRD data in Fig. 1e showed evidence of only TiN, confirming that the surface TiO2 layer was very thin. The high-resolution of Pt 4f of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN were provided in Fig. 4d, and the all the scans showed two states of Pt. The Pt 4f binding energy (BE) of Pt/CNTs is almost the same with Pt/C. However, a clear negative shift of ca. 0.47 eV in the BE of Pt4f7/2 for Pt/CNTs@TiCoN with respect to the Pt/C, indicating the strong interaction between anchored Pt atoms and the CNTs@TiCoN support. The onset
Fig. 3. CV curves of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN catalysts, (b) the mass activities of Pt/C, Pt/CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiCoN catalysts toward the methanol oxidation in 1 M CH3OH þ 0.5 M H2SO4 solution, (c) the enlarged image in Fig. 3b (rang from 0.25 to 0.6 V).
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Fig. 4. XPS spectra of (a) Mo 3d and (b) Co 2p for CNTs@TiCoN and CNTs@TiMoN samples, respectively, (c) Ti 2p of CNTs@TiN and CNTs@TiMoN, (d) Pt 4f of Pt/C, Pt/CNTs and Pt/ CNTs@TiCoN catalysts, (e) C 1s of CNTs, CNTs@TiN and CNTs@TiCoN.
potential for oxide formation and reduction peak potential of the Pt/CNTs@TiCoN catalyst obviously shifted toward positive potential could also be observed from the comparison of CV curves (Fig. 3a), compared with the other catalysts, suggesting that the electronic state (d state) of the surface Pt was changed through the interaction with the CNTs@TiCoN support. The negative shifted Pt 4f binding energy suggested that the Pt atoms can get electrons donated from the CNTs@TiCoN supports [48], inducing a down shift of the Pt d states relative to the Fermi level [20,49e51], which will result in a reduced intermediate adsorptive coupling for Pt in the ratedetermining step during the MOR process. Thus, a faster catalytic electrode reaction was obtained for Pt/CNTs@TiCoN catalyst than
that of the Pt/C and Pt/CNTs catalyst. The performance enhancement observed for Pt/CNTs@TiCoN than that of Pt/TiMoN and Pt/ TiCoN without the CNTs can most likely be attributed to not only the Ptesupport interactions, but also the specific properties of the TiCoNeCNTs composites. On one hand, the formation of a 3-D like morphology assembled from the intersecting 1-D CNTs@TiCoN, which enhance the mass transfer of reactant and resultant molecules. On the other hand, the direct growth of TiCoN nanostructures on CNTs may afford strong electrical and chemical coupling between TiCoN nanoparticles and the core CNTs, which constructs a fast electron transport network during the MOR process [52,53]. Fig. 4e shows the high revolution spectrums of C 1 s in CNTs,
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CNTs@TiMoN and CNTs@TiCoN samples. Compared with CNTs, the apparent TieC and NeC bond are confirmed by the C 1 s (BE 281.5 and 287.8 eV) [54e56], respectively, suggesting the formation of TieNeC structure in CNTs@TiMoN and CNTs@TiCoN. The results illustrated the strong electrical and chemical coupling between TiMoN/TiCoN nanoparticles and the carbon network, that constructs a rapid and continuous electron transfer pathway, which is definitely do good to the MOR process. The CO stripping voltammograms was carried out in 0.5 M H2SO4 solution at room temperature, as depicted in Fig. 5a. Before the test, High-purity CO was bubbled into the electrolyte solution for 30 min when the electrode potential was kept at 0.2 V to achieve maximum coverage of CO on the Pt nanoparticles. The onset potential (0.32 V) of CO oxidation on Pt/CNTs@TiCoN is obviously more negative than that on the Pt/C (0.47 V) and Pt/CNTs (0.46 V) in the first forward scan, confirming the Pt/CNTs@TiCoN catalyst can facilitate removal of CO from the surface, which could be due to the co-catalyst effect reported previously (OH groups can be adsorbed on the TiN surface and help oxidize the poisonous intermediates adhered on Pt surface), thus CO removal was accelerated. To further evaluate the rate of surface poisoning, chronoamperometry curves of Pt/C, Pt/CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiCoN catalysts were measured in 1 M CH3OH þ 0.5 M H2SO4 solution and the potential was held at 0.60 V during the measurements (Fig. 5b). What’s more, normalized decay corresponding to the current at 5000s and the initial (I_5000s/I _initial) was also conducted, as shown in Fig. 5c. It was clear that Pt/ CNTs@TiCoN, Pt/CNTs@TiMoN and Pt/TiCoN catalysts exhibited a slower current decay over time in comparison with Pt/C and Pt/ CNTs, confirming a higher tolerance to the carbonaceous species generated during methanol oxidation of the catalysts with binary
transition metal as the support. Additionally, much higher current density retainment was observed for the Pt/CNTs@TiCoN in the long run versus the other ones, indicating it was much more electroactive for MOR. ADT is conducted to test the electrochemical durability of the catalysts, since long-term stability of the electrocatalyst is still one of the top issues to be resolved before the commercialization of DMFCs. Fig. 6 a and b show the CV curve evolutions for Pt/C, Pt/ CNTs, Pt/TiCoN and Pt/CNTs@TiCoN catalysts, respectively. As observed, JM Pt/C and Pt/CNTs both lost more than 50% of their total value after 10, 000 cycles. However, Pt/TiCoN and Pt/CNTs@TiCoN catalysts showed great improvement in ECSA preservation, with 80% and 76% of the initial ECSA after the ADT (Fig. 6c), suggesting the higher durability of Pt/CNTs@TiCoN over the commercial JM Pt/ C under the fuel cell operate conditions. Moreover, to gain more understanding of the enhanced activity and durability of Pt/CNTs@TiCoN, the catalysts were collected after the ADT, and the structure was observed by TEM. As shown in Fig. 7, the average Pt NPs size of the commercial Pt catalyst increases from averaged 3.3 nm to 6e10 nm (Fig. 7a and b), indicating sustaining growth of Pt nanoparticles. In addition, a significant decrease of Pt nanoparticles density on carbon black/CNTs support are observed, demonstrating that the major cause for performance loss of commercial JM Pt and Pt/CNTs catalysts is Pt detaching, degradation and aggregation as the consequence of the corrosion of the carbon support. In contrast, these problems were avoided in considerable degree in Pt/CNTs@TiCoN catalyst, since no significant increase in the Pt NPs size and almost no variation of Pt nanoparticles density was observed (Fig. 7e and f), which was most likely owing to the good corrosion resistance of the CNTs@TiCoN support, and the strong interaction between the anchored Pt NPs and the support as
Fig. 5. (a) CO stripping of Pt/C, Pt/CNTs and Pt/CNTs@TiCoN catalysts in 0.5 M H2SO4 at a scan rate of 50 mVs1 at room temperature. (b) Chronoamperometry curves of Pt/C, Pt/ CNTs, Pt/TiCoN, Pt/CNTs@TiMoN and Pt/CNTs@TiCoN catalysts held at 0.60 V (vs. Ag/AgCl) in 1 M CH3OH þ 0.5 M H2SO4 solution for 5000 s. (c) The normalized current density corresponding to the current at 5000s and the initial.
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Fig. 6. The evolution of CV curves after 10 000 cycles for (a) Pt/C and Pt/CNTs, (b) Pt/TiCoN, Pt/CNTs@TiMoN, and (c) Comparison of ECSA loss for the catalysts.
Fig. 7. (a, b) TEM images of collected Pt/C, (c, d) Pt/CNTs and (e,f) Pt/CNTs@TiCoN catalysts after the ADT.
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well. These synergetic effects slow down the aggregation and detachment of Pt nanoparticles from the CNTs@TiCoN support, introducing a super activity and stability for the Pt/CNTs@TiCoN catalyst. The results demonstrate that CNTs@TiCoN hybrid composition is indeed a promising support to improve Pt catalytic activity and durability for practical catalytic applications, for which exhibits the merits of high conductivity (CNTs), corrosion resistance and strong metal support interactions (TiCoN). 4. Conclusions In summary, we have developed a facile procedure to synthesize Pt/CNTs@TiCoN hybrid catalyst used the hybrid material, CNTs@TiCoN, as the support for Pt, and most importantly, the experimental data show that Pt/CNTs@TiCoN have a much higher catalytic activity and durability than commercial JM Pt/C for methanol oxidation, and the performance also outperformed Pt/ TiCoN and Pt/CNTs@TiMoN catalysts in the same conditions. The higher MOR performance and durability of the novel catalyst was probably due to the high electrical conductivity of the special properties of the TiCoNeCNTs composites, high stability of transition metal nitrides and the strong metal support interactions. These advances open up broad possibilities for the design and synthesis of various binary transition metal nitrides coated CNTs as the support for Pt based catalysts with applications in DMFCs. Acknowledgement This research was financially supported by Guangdong Natural Science Foundation (2016A030313704), Scientific and technological projects in Guangdong Province (2014A010105041, 2016A010103035), Huizhou Daya Bay technological projects (2015A01006) and Huizhou technological projects (2014C050012004). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.06.112. References [1] K. Sasaki, J.X. Wang, H. Naohara, N. Marinkovic, K. More, H. Inada, R.R. Adzic, Electrochim Acta 55 (2010) 2645e2652. [2] N. Kristian, Y. Yu, J.-M. Lee, X. Liu, X. Wang, Electrochim Acta 56 (2010) 1000e1007. [3] J. Guo, G. Sun, Q. Wang, G. Wang, Z. Zhou, S. Tang, L. Jiang, B. Zhou, Q. Xin, Carbon 44 (2006) 152e157. [4] T. Maiyalagan, C. Mahendiran, K. Chaitanya, R. Tyagi, F. Nawaz Khan, Research Chem. Intermed. 38 (2011) 383e391. [5] T. Maiyalagan, Int. J. Hydrogen Energy 34 (2009) 2874e2879. [6] H. Yan, C. Tian, L. Sun, B. Wang, L. Wang, J. Yin, A. Wu, H. Fu, Energy Environ. Sci. 7 (2014) 1939. [7] T. Maiyalagan, T.O. Alaje, K. Scott, J. Physical Chem. C 116 (2012) 2630e2638. [8] V. Bambagioni, C. Bianchini, A. Marchionni, J. Filippi, F. Vizza, J. Teddy, P. Serp, M. Zhiani, J. Power Sources 190 (2009) 241e251. [9] L.X. Ding, A.L. Wang, G.R. Li, Z.Q. Liu, W.X. Zhao, C.Y. Su, Y.X. Tong, J. Am. Chem. Soc. 134 (2012) 5730e5733. [10] M. Tian, G. Wu, A. Chen, ACS Catal. 2 (2012) 425e432. [11] A.-L. Wang, H. Xu, J.-X. Feng, L.-X. Ding, Y.-X. Tong, G.-R. Li, J. Am. Chem. Soc. 135 (2013) 10703e10709. [12] S.-Y. Huang, P. Ganesan, B.N. Popov, ACS Catal. 2 (2012) 825e831.
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