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TiN NTs derived from metal organic frameworks as high-performance electrocatalyst for methanol electrooxidation
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Short Communication
Pt-CeO2/TiN NTs derived from metal organic frameworks as high-performance electrocatalyst for methanol electrooxidation Qiuman Zhou a, Zhanchang Pan a,*, Deyou Wu a, Guanghui Hu a, Shoukun Wu b, Chun Chen b, Luhua Lin b, Yingsheng Lin b a
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, 510006, China b Huizhou King Brother Electronic Technology Co., Ltd, Huizhou, 516083, China
article info
abstract
Article history:
Metal organic frameworks (MOFs) have attracted tremendous attention in recent years
Received 9 November 2018
owing to their high-specific surface area (SSA) and variable porous structures. Owing to the
Received in revised form
strong interaction between Pt and CeO2, Pt combined steadily with CeO2. Furthermore, the
15 January 2019
surface of CeO2 can activate water to produce hydroxyl groups, which can accelerate the
Accepted 25 January 2019
removal of catalytic intermediate CO. But the bad conductivity of metal oxide is still a huge
Available online 16 March 2019
obstacle. More importantly, utilizing TiN with excellent conductivity as support can strengthen conductivity of catalyst and improve catalytic activity. Herein, a novel Pt-CeO2/
Keywords:
TiN Nanotubes (TiN NTs) catalysts derived from Ce-MOF was fabricated for the first time. In
Metal organic frameworks
the synthesis process of the targeted catalyst, the compounds of Ce-MOF and TiN NTs was
Porous materials
prepared via the hydrothermal method and post-nitriding treatment, and implemented as
Methanol oxidation
the Pt support. Scanning electron microscopy (SEM), transmission electron microscopy
Fuel cells
(TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption and electrochemical measurements were carried out to characterize the catalyst. Notably, the peak current density of Pt-CeO2/TiN NTs (0.67 A mg1Pt) was approximately 3 times higher than Pt/C (0.28 A mg1Pt) during methanol oxidation test, showing the exceptional properties toward methanol oxidation reaction (MOR). Remarkably, electrochemical testing data verified the superior tolerance to CO and enhanced catalytical activity of Pt-CeO2/TiN NTs and it could be attributed to the porous structures and the interaction between TiN NTs and CeO2. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Although direct methanol fuel cells (DMFCs) are under booming development, they are still faced with sluggish situations,
including rare resources of precious metal, low catalytic activity due to easily poison of Pt by carbon monoxide, etc [1e6]. To speed up the commercialization of DMFCs, researchers are devoted to synthesis effective and robust catalysts leading to a
* Corresponding author. E-mail address: [email protected] (Z. Pan). https://doi.org/10.1016/j.ijhydene.2019.01.231 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 4 4 ( 2 0 1 9 ) 1 0 6 4 6 e1 0 6 5 2
scope of advanced works. For instances,(I) developing a new kind of alternative to traditional Pt/C that Pt was alloyed with other metals to cut down the expense and break through the CO poisoning problem [7e15]; (II) setting down to manufacturing a favorable non-precious metal materials to take the place of conventional precious metal catalysts [16e20]. Titanium nitride (TiN) is an outstanding support due to its intrinsically prominent corrosion resistance to CO [21e23], brilliant electrochemical stability [24,25], and supernal conductivity [26]. In addition, the powerful mutual effect between Pt and TiN has been verified in recent years [11]. It is worth to talk about the catalysts that compounded with TiN as the supports exhibits high catalytical performance. TiN has become an ideal support compared to conventional carbon support. Owing to its intrinsic structures, metal organic frameworks (MOFs) have extensively emerged as prospective materials for energy transformation systems in recent years such as fuel cells, supercapacitors and so on [27e31]. Exploiting MOFs as precursor is one of desirable way to enhance the electrocatalytic activation [16,26,32,33]. However, how to make full use of MOF-based materials to design a novel catalyst to overcome the CO poisoning problem is still a huge challenge. Impressively, current researches have confirmed that, due to the synergistic effect between Pt and transition-metal oxide nanoparticles such as CeO2, ZrO2 and so on, Pt-based catalysts performances fascinating CO-tolerance [34]. On the other hand, the surface of CeO2 can activate water to produce hydroxyl groups, which can promote the removal of CO [35e41]. However, CeO2 nanoparticles are easily agglomerated, and it is difficult to obtain a high dispersion and a large specific surface area of CeO2. Furthermore, utilizing Ce-MOF as the precursor can anchor and restrict Ce atoms, and finally can obtain CeO2 with good dispersibility. Although CeO2 is an ideal material to support oxygen while Pt was under CO poisoning, the bad conductivity of metal oxide is still a huge obstacle. More importantly, using TiN with excellent conductivity as support can strengthen conductivity of catalyst and improve catalytic activity. To our best of knowledge, utilizing Ce-MOF as precursor to work in coordination with Pt, supported on TiN NTs, is still unreported. Herein, we fabricated a novel Pt-CeO2/TiN NTs derived from MOFs, aiming to strengthen the stability and electrochemical activity.
Experimental details Fabrication of Ce-MOF precursor Ce-MOF was fabricated from Ce(NO3)3$6H2O and PTA (PTA ¼ p-Phthalic acid [36]. First of all, Ce(NO3)3$6H2O (434 mg,1 mmol) and p-Phthalic acid（108 mg,0.5 mmol）was mixed in 50 mL mixed solution, containing of 30 mL DI （deionized water） ）and 25 mL absolute ethyl alcohol. Simultaneously, the suspension was under sonication treatment and 30 min stir to mix homogeneously. Subsequently, white mixture was under reflux condensation at 90 C for 3 h. Then the resulting products was washed with absolute ethyl alcohol and deionized water. Last, the white power was obtained after
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heat treatment in a vacuum drying oven at 65 C for overnight. Consequently, the Ce-MOF precursor was collected for following usage.
Synthesis of the CeO2/TiN NTs, TiN NTs and CeO2 (Ⅰ) Above all, Ce-MOF precursor from the first step（15 mg）and titanyl sulfate (2 g) were mixed in a uniform solution system including glycerol (22 mL), EG (ethylene glycol, 18 mL) and ethyl ether (15 mL). Then, it is crucial to carry out ultrasonic concussion treatment for 40 min and intensely stir for 1 h. After that, all reactants were transferred into a hydrothermal reactor and heated up to 150 C overnight. Subsequently, turbid liquid was percolated to acquire white precipitate, follo-wed by washing completely with deionized water and dried at 90 C. Next, the moderate quantities of white farinose samples were putted towards tube furnace to heat up to 700 C under NH3 gas flow (70 sccm) for 2 h. The rate of heat addition was 5 C per minute. Finally, the CeO2/TiN NTs was under refrigeration treatment to room temperature. TiN NTs was obtained in the same procedure without the adding of Ce-MOF precursor. As for the preparation of CeO2 (Ⅰ), Ce-MOF precursor from the first step (15 mg) were putted towards tube furnace to heat up to 700 C under NH3 gas flow (70 sccm) for 2 h. The rate of heat addition was 5 C per minute. Ultimately, CeO2 (Ⅰ) was gained.
Synthesis of the Pt-CeO2/TiN NTs, Pt-CeO2/C and Pt/TiN NTs catalysts Typically, sodium citrate (40 mg), 80 mg CeO2/TiN NTs, EG (ethylene glycol，18 mL) and H2PtCl6/EG (40 mg/mL,1.33 mL) were mixed and stirred vigorously. Next, the mixture was placed into an autoclave with a 30 mL Teflon lining, followed by centrifugation, washing with DI water (deionized water), ultimately, overnight heating in a vacuum drying oven at 90 C. Pt/TiN NTs was obtained in the same procedure but CeO2/TiN NTs was replaced with TiN NTs while Pt-CeO2/C was gained in the same procedure but the CeO2/TiN NTs was replaced with CeO2 (Ⅰ) from the second step and 51 mg carbon dust.
Material characterization The crystalline structures of the as-above compounded PtCeO2/TiN NTs was analyzed by X-ray diffraction (XRD, RigakuUltima III with Cu Ka radiation). The microstructures of asabove compounded samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL 2100F，Japan) and high resolution TEM (HR-TEM, JEOL 2100F, Japan). The BrunauerEmmett-Teller (BET, Tristar II 3020 with nitrogen adsorptiondesorption analyzer) was used to measure the specific surface areas and pore distribution of the samples. The inductively coupled plasma optical emission spectrometry (ICPOES，Leeman) was utilized to measure the loading of Pt. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) was performed to study the electronic effect, the interaction between Pt NPs and different supports and the interaction between Pt and CeO2.
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Electrochemical testing All electrochemical measurements were evaluated on Model PGSTAT302 N equipped with a three-electrode system, in which a glassy carbon (GCE, 0.196 cm2) performed as working electrode, a Ag/AgCl（saturated KCl solution）served as reference electrode and a Pt wire used as a counter electrode, at 25 ± 1 C. 3 mg catalysts were dissolved thoroughly in 1 mL ethanal solution, followed by sonication for 15 min. Above 5 mL well-proportioned catalyst ink was tiled equally on the GCE surface, then, air-drying for a few minutes later. Before measurement, a drop of 0.5 wt% Nafion-ethanol mixed solution was added, and dried again. The nominal catalyst loading was 25 mg cm2, and the real Pt contents were analyzed by ICP-OES. Cyclic voltammetry (CV) was carried out in N2saturated 0.5 M H2SO4 solution or 1 M CH3OHþ0.5 M H2SO4 solution at 50 mV/s, while Chronoamperometry was held at 0.4 V (vs Ag/AgCl) in 1 M CH3OHþ0.5 M H2SO4 solution for 5000s.
Results and discussion SEM confirmed the porous structure of CeO2/TiN NTs. Fig. 1(a) presented the length of the TiN NTs were between 5 mm and 10 mm. There were many pores on the TiN NTs surface which may expose to many active sites for Pt loading (Fig. 1(b)). The hollow structure was shown in Fig. 1(c) with a diameter of ca. 496 nm and wall thickness of ca. 92 nm. The lowmagnification TEM results were depicted in Fig. 1(d) and (e). The structural morphology of CeO2/TiN NTs was observed in Fig. 1(d) and the porous CeO2/TiN NTs wall is composed of nanocrystals. Furthermore, from Fig. 1(e), nanocrystals were tightly combined with each other. HRTEM demonstrated the lattice distance was 0.252 nm, which was indexed to (111) plane of the face-centered cubic (fcc) TiN [42]. The XRD patterns and BET results of the sample were exhibited in Fig. 1(g) and (h). The XRD patterns results were coincidence with the (fcc) TiN (JCPDS No. 38-1420) and the cubic CeO2(JCPDS No.
Fig. 1 e (a~c) SEM image CeO2/TiN NTs; (d~f) TEM image CeO2/TiN NTs; (f) HRTEM image CeO2/TiN NTs; (g) X-ray diffraction patterns of CeO2/TiN NTs, Ce-MOF, Ce-MOF processed in NH3 environment and commercial CeO2; (h) The N2 adsorptiondesorption isotherm of CeO2/TiN NTs, the insert of (h) is the relevant pore distributions of CeO2/TiN NTs; (i) CV curves of with different cycle numbers CeO2/TiN NTs in 0.5 M H2SO4 solution at a scan rate of 50 mVs¡1.
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43-1002), suggesting the purity of the samples and the formation of CeO2. Furthermore, Fig. 1(h) showed a notable TypeⅣ isotherm that the curve exhibited a convex upward in low P/ P0, while the isotherm rose rapidly in higher P/P0 due to capillary adsorption occurred in this area until the relative pressure was closed to 1, confirming the existence of mesopores. In addition, the major pore size of CeO2/TiN NTs was at ca. 9 nm by the pore size distribution curve (insert of Fig. 1(h)), which was coincident with SEM images. Furthermore, the consequence of BET demonstrated the specific surface area was measured to 93 m2g-1. Meanwhile, Fig. 1(i) exhibited the sample was evaluated in 0.5 M H2SO4 solution, which gives strongly circumstantial evidence for the electrochemical stability of CeO2/TiN NTs. As follow, Transmission electron microscope (TEM) image of Pt-CeO2/TiN NTs was revealed in Fig. 2 (a~c) that Pt nanoparticles (NPs) were well-distributed. Notably, current researches had testified that the shapes of Pt NPs were significant for the electrochemical performance and the optimized Pt NPs was about 3 nm [43,44]. In this research, the sizes of Pt nanoparticles was calculated to 3.0 ± 0.3 nm on account of counting the diameter of 200 Pt NPs in TEM image, indicating the high electrochemical performance was ascribed to the intriguing catalyst. In addition, the shapes of Pt was close to Pt/C (3.1 ± 0.4 nm). From the insert of Fig. 2(c), in view of standard lattice distance, 0.227 nm was accord with Pt (111) plane and 0.305 nm conforms to CeO2(111) plane. The electrochemical properties of Pt-CeO2/TiN NTs was investigated as follow. The electrochemically active area (ECSA) of as-prepared catalyst was evaluated on the basis of equation ECSA ¼ QH/(210 WPt), where QH and WPt are the adsorption charge quantity of H (mC) and the total loading amount of Pt (mgcm2) on the electrode surface, respectively, and 210 represents the charge quantity (mCcm2Pt) demands on oxidizing a monolayer of hydrogen on a Pt surface [45]. As calculated, the ECSA of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN
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NTs and Pt/C is 59, 58, 55 and 54 m2g-1 Pt1, respectively. The similar ECSA of the targeted catalyst, Pt-CeO2/C, Pt/TiN NTs and Pt/C were ascribed to the well-distributed and little size of Pt NPs on different carrier. The electroactivity of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C to methanol oxidation was measured in 1 M CH3OH and 0.5 M H2SO4 solution. Fig. 3(b) depicted that the onset potential of as-resulted catalyst (0.3 V), was negative shifted compared with Pt-CeO2/C (0.35 V), Pt/C (0.4 V) and Pt/TiN NTs (0.41 V), confirming the advantageous electrochemical catalytic conditions for MOR. Commonly, there are two anodic peaks that observed in curves (for Ptbased catalyst). First peak (if) current density is relevant to the methanol oxidation process and the next peak current density (ib) is regarded as oxidation of the intermediate product (CO) that generated during the MOR process [46]. Consequently, the figure of if/ib ratio can be used to measure the CO anti-poisoning ability of studied catalyst. Moreover, the if of Pt-CeO2/TiN NTs (0.61 A mg1Pt) was the highest among all studied catalysts, demonstrating the superior electrocatalytic efficiency for MOR. Furthermore, the value of ib of Pt-CeO2/TiN NTs (0.67 A mg1Pt) was approximately 2 times than Pt/TiN NTs (0.31 A mg1Pt), while the value of ib of Pt-CeO2/C (0.59 A mg1Pt) was nearly 2 times than Pt/C (0.28 A mg1Pt). Noteworthily, the if/ib ratio of Pt-CeO2/TiN NTs (0.91) were higher than Pt-CeO2/C (0.89), illustrating that TiN NTs had prominent advantage to MOR. To further explore the surface poisoning of the targeted catalyst, the chronoamperometry curves of Pt-CeO2/TiN NTs, Pt/TiN NTs, PtCeO2/C and Pt/C was carried out at the potential of 0.4 V. Compared to Pt/TiN NTs, Pt-CeO2/TiN NTs showed a slower drop of current, a better steady current, and higher initial currents. Compared to Pt/C, Pt-CeO2/C showed a slower drop of current, a better steady current, which indicates the better ability of CeO2 to anti-poisoning to the intermediates of CO again (Fig. 3(c)). As a conclusion, the well ability of the catalyst with accelerating removal of CO was confirmed.
Fig. 2 e (a~c) TEM image Pt-CeO2/TiN NTs, the insert of (c) is the HRTEM image Pt-CeO2/TiN NTs; (d) the corresponding histogram of Pt-CeO2/TiN NTs.
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Fig. 3 e (a) CV curves Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C in N2-saturated 0.5 M H2SO4 with a scan at 50 mVs¡1 and the insert is the relevant comparison of ECSA; (b) CV curves of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C in 1 M CH3OH and 0.5 M H2SO4; (c) Chronoamperometry curves of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C in N2-saturated 0.5 M H2SO4 and 1 M CH3OH at 0.4 V with a scan at 50 mVs¡1.
X-ray photoelectron spectroscopy (XPS) analyses were performed to study the electronic effect, the interaction between Pt NPs and different supports and the interaction between Pt and CeO2. Fig. 4 shows that each Pt 4f7/2 peak and Pt 4f5/2 peak could be deconvoluted to two pairs of doublets. The specific analysis result of Pt-CeO2/TiN NTs indicated that a large quantity of Pt atoms existed as Pt (0), which is beneficial for strengthening electroactivity towards MOR. Compare to
Pt/C, a positive shift of ~0.10 eV in binding energy of Pt/TiN NTs was observed, which confirmed the strong interaction between Pt and TiN support. In addition, an apparent negative shift of ~0.29 eV in binding energy of Pt-CeO2/TiN NTs was observed compared with Pt/TiN NTs, demonstrating CeO2 can provide electrons to Pt atoms and being good for a reduced dband vacancy of Pt, thereby weakened the interaction between Pt and intermediate CO [2,47]. Compared to Pt-CeO2/C, a
Fig. 4 e Pt 4f XPS surveys of the Pt-CeO2/TiN NTs, Pt/TiN NTs, Pt-CeO2/C, and Pt/C.
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Fig. 5 e (a) The CV of Pt/C and (b) CV of Pt-CeO2/TiN NTs before and after the durability test. (c) Comparison of ECSA loss between Pt/C and Pt-CeO2/TiN NTs.
negative shift of ~0.15 eV was observed in the binding energy of Pt-CeO2/TiN NTs, indicating TiN support is beneficial for electronic transferring owing to its supernal conductivity. Current researches have suggested that the catalyst perform high stability with comparison to Pt and carbon carrier owing to the stronger interaction between Pt and titanium nitride [11,48]. For the sake of exploring service life of the asresulted catalyst, accelerated durability test (ADT) was executed. Amazingly, the ECSA of Pt-CeO2/TiN NTs was basically maintained accordance after 2000 cycles while the ECSA of Pt/C is 61% lost after 2000 cycles comparing with its original ECSA (Fig. 5(c)). Compared to Pt/C, the superior stability of PtCeO2/TiN NTs was confirmed.
Conclusions In summary, we fabricated Pt-CeO2/TiN NTs catalyst derived from MOFs with porous structure. The long-term durability, interaction, and high-performance of Pt-CeO2/TiN NTs derived from MOFs were demonstrated in this paper. We convince that these results will arouse intensively investigative curiosity on MOF-based electrochemical catalysts and unfold perspectives for the progress of DMFCs.
Acknowledgments This work was supported by Natural Science Foundation of Guangdong Province, China (No. 2016A030313704), Science and Technology Planning Project of Guangdong Province, China (No. 2016B020240003).
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Short Communication
Pt-CeO2/TiN NTs derived from metal organic frameworks as high-performance electrocatalyst for methanol electrooxidation Qiuman Zhou a, Zhanchang Pan a,*, Deyou Wu a, Guanghui Hu a, Shoukun Wu b, Chun Chen b, Luhua Lin b, Yingsheng Lin b a
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, 510006, China b Huizhou King Brother Electronic Technology Co., Ltd, Huizhou, 516083, China
article info
abstract
Article history:
Metal organic frameworks (MOFs) have attracted tremendous attention in recent years
Received 9 November 2018
owing to their high-specific surface area (SSA) and variable porous structures. Owing to the
Received in revised form
strong interaction between Pt and CeO2, Pt combined steadily with CeO2. Furthermore, the
15 January 2019
surface of CeO2 can activate water to produce hydroxyl groups, which can accelerate the
Accepted 25 January 2019
removal of catalytic intermediate CO. But the bad conductivity of metal oxide is still a huge
Available online 16 March 2019
obstacle. More importantly, utilizing TiN with excellent conductivity as support can strengthen conductivity of catalyst and improve catalytic activity. Herein, a novel Pt-CeO2/
Keywords:
TiN Nanotubes (TiN NTs) catalysts derived from Ce-MOF was fabricated for the first time. In
Metal organic frameworks
the synthesis process of the targeted catalyst, the compounds of Ce-MOF and TiN NTs was
Porous materials
prepared via the hydrothermal method and post-nitriding treatment, and implemented as
Methanol oxidation
the Pt support. Scanning electron microscopy (SEM), transmission electron microscopy
Fuel cells
(TEM), X-ray diffraction (XRD), nitrogen adsorption/desorption and electrochemical measurements were carried out to characterize the catalyst. Notably, the peak current density of Pt-CeO2/TiN NTs (0.67 A mg1Pt) was approximately 3 times higher than Pt/C (0.28 A mg1Pt) during methanol oxidation test, showing the exceptional properties toward methanol oxidation reaction (MOR). Remarkably, electrochemical testing data verified the superior tolerance to CO and enhanced catalytical activity of Pt-CeO2/TiN NTs and it could be attributed to the porous structures and the interaction between TiN NTs and CeO2. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Although direct methanol fuel cells (DMFCs) are under booming development, they are still faced with sluggish situations,
including rare resources of precious metal, low catalytic activity due to easily poison of Pt by carbon monoxide, etc [1e6]. To speed up the commercialization of DMFCs, researchers are devoted to synthesis effective and robust catalysts leading to a
* Corresponding author. E-mail address: [email protected] (Z. Pan). https://doi.org/10.1016/j.ijhydene.2019.01.231 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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scope of advanced works. For instances,(I) developing a new kind of alternative to traditional Pt/C that Pt was alloyed with other metals to cut down the expense and break through the CO poisoning problem [7e15]; (II) setting down to manufacturing a favorable non-precious metal materials to take the place of conventional precious metal catalysts [16e20]. Titanium nitride (TiN) is an outstanding support due to its intrinsically prominent corrosion resistance to CO [21e23], brilliant electrochemical stability [24,25], and supernal conductivity [26]. In addition, the powerful mutual effect between Pt and TiN has been verified in recent years [11]. It is worth to talk about the catalysts that compounded with TiN as the supports exhibits high catalytical performance. TiN has become an ideal support compared to conventional carbon support. Owing to its intrinsic structures, metal organic frameworks (MOFs) have extensively emerged as prospective materials for energy transformation systems in recent years such as fuel cells, supercapacitors and so on [27e31]. Exploiting MOFs as precursor is one of desirable way to enhance the electrocatalytic activation [16,26,32,33]. However, how to make full use of MOF-based materials to design a novel catalyst to overcome the CO poisoning problem is still a huge challenge. Impressively, current researches have confirmed that, due to the synergistic effect between Pt and transition-metal oxide nanoparticles such as CeO2, ZrO2 and so on, Pt-based catalysts performances fascinating CO-tolerance [34]. On the other hand, the surface of CeO2 can activate water to produce hydroxyl groups, which can promote the removal of CO [35e41]. However, CeO2 nanoparticles are easily agglomerated, and it is difficult to obtain a high dispersion and a large specific surface area of CeO2. Furthermore, utilizing Ce-MOF as the precursor can anchor and restrict Ce atoms, and finally can obtain CeO2 with good dispersibility. Although CeO2 is an ideal material to support oxygen while Pt was under CO poisoning, the bad conductivity of metal oxide is still a huge obstacle. More importantly, using TiN with excellent conductivity as support can strengthen conductivity of catalyst and improve catalytic activity. To our best of knowledge, utilizing Ce-MOF as precursor to work in coordination with Pt, supported on TiN NTs, is still unreported. Herein, we fabricated a novel Pt-CeO2/TiN NTs derived from MOFs, aiming to strengthen the stability and electrochemical activity.
Experimental details Fabrication of Ce-MOF precursor Ce-MOF was fabricated from Ce(NO3)3$6H2O and PTA (PTA ¼ p-Phthalic acid [36]. First of all, Ce(NO3)3$6H2O (434 mg,1 mmol) and p-Phthalic acid（108 mg,0.5 mmol）was mixed in 50 mL mixed solution, containing of 30 mL DI （deionized water） ）and 25 mL absolute ethyl alcohol. Simultaneously, the suspension was under sonication treatment and 30 min stir to mix homogeneously. Subsequently, white mixture was under reflux condensation at 90 C for 3 h. Then the resulting products was washed with absolute ethyl alcohol and deionized water. Last, the white power was obtained after
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heat treatment in a vacuum drying oven at 65 C for overnight. Consequently, the Ce-MOF precursor was collected for following usage.
Synthesis of the CeO2/TiN NTs, TiN NTs and CeO2 (Ⅰ) Above all, Ce-MOF precursor from the first step（15 mg）and titanyl sulfate (2 g) were mixed in a uniform solution system including glycerol (22 mL), EG (ethylene glycol, 18 mL) and ethyl ether (15 mL). Then, it is crucial to carry out ultrasonic concussion treatment for 40 min and intensely stir for 1 h. After that, all reactants were transferred into a hydrothermal reactor and heated up to 150 C overnight. Subsequently, turbid liquid was percolated to acquire white precipitate, follo-wed by washing completely with deionized water and dried at 90 C. Next, the moderate quantities of white farinose samples were putted towards tube furnace to heat up to 700 C under NH3 gas flow (70 sccm) for 2 h. The rate of heat addition was 5 C per minute. Finally, the CeO2/TiN NTs was under refrigeration treatment to room temperature. TiN NTs was obtained in the same procedure without the adding of Ce-MOF precursor. As for the preparation of CeO2 (Ⅰ), Ce-MOF precursor from the first step (15 mg) were putted towards tube furnace to heat up to 700 C under NH3 gas flow (70 sccm) for 2 h. The rate of heat addition was 5 C per minute. Ultimately, CeO2 (Ⅰ) was gained.
Synthesis of the Pt-CeO2/TiN NTs, Pt-CeO2/C and Pt/TiN NTs catalysts Typically, sodium citrate (40 mg), 80 mg CeO2/TiN NTs, EG (ethylene glycol，18 mL) and H2PtCl6/EG (40 mg/mL,1.33 mL) were mixed and stirred vigorously. Next, the mixture was placed into an autoclave with a 30 mL Teflon lining, followed by centrifugation, washing with DI water (deionized water), ultimately, overnight heating in a vacuum drying oven at 90 C. Pt/TiN NTs was obtained in the same procedure but CeO2/TiN NTs was replaced with TiN NTs while Pt-CeO2/C was gained in the same procedure but the CeO2/TiN NTs was replaced with CeO2 (Ⅰ) from the second step and 51 mg carbon dust.
Material characterization The crystalline structures of the as-above compounded PtCeO2/TiN NTs was analyzed by X-ray diffraction (XRD, RigakuUltima III with Cu Ka radiation). The microstructures of asabove compounded samples were characterized by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL 2100F，Japan) and high resolution TEM (HR-TEM, JEOL 2100F, Japan). The BrunauerEmmett-Teller (BET, Tristar II 3020 with nitrogen adsorptiondesorption analyzer) was used to measure the specific surface areas and pore distribution of the samples. The inductively coupled plasma optical emission spectrometry (ICPOES，Leeman) was utilized to measure the loading of Pt. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) was performed to study the electronic effect, the interaction between Pt NPs and different supports and the interaction between Pt and CeO2.
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Electrochemical testing All electrochemical measurements were evaluated on Model PGSTAT302 N equipped with a three-electrode system, in which a glassy carbon (GCE, 0.196 cm2) performed as working electrode, a Ag/AgCl（saturated KCl solution）served as reference electrode and a Pt wire used as a counter electrode, at 25 ± 1 C. 3 mg catalysts were dissolved thoroughly in 1 mL ethanal solution, followed by sonication for 15 min. Above 5 mL well-proportioned catalyst ink was tiled equally on the GCE surface, then, air-drying for a few minutes later. Before measurement, a drop of 0.5 wt% Nafion-ethanol mixed solution was added, and dried again. The nominal catalyst loading was 25 mg cm2, and the real Pt contents were analyzed by ICP-OES. Cyclic voltammetry (CV) was carried out in N2saturated 0.5 M H2SO4 solution or 1 M CH3OHþ0.5 M H2SO4 solution at 50 mV/s, while Chronoamperometry was held at 0.4 V (vs Ag/AgCl) in 1 M CH3OHþ0.5 M H2SO4 solution for 5000s.
Results and discussion SEM confirmed the porous structure of CeO2/TiN NTs. Fig. 1(a) presented the length of the TiN NTs were between 5 mm and 10 mm. There were many pores on the TiN NTs surface which may expose to many active sites for Pt loading (Fig. 1(b)). The hollow structure was shown in Fig. 1(c) with a diameter of ca. 496 nm and wall thickness of ca. 92 nm. The lowmagnification TEM results were depicted in Fig. 1(d) and (e). The structural morphology of CeO2/TiN NTs was observed in Fig. 1(d) and the porous CeO2/TiN NTs wall is composed of nanocrystals. Furthermore, from Fig. 1(e), nanocrystals were tightly combined with each other. HRTEM demonstrated the lattice distance was 0.252 nm, which was indexed to (111) plane of the face-centered cubic (fcc) TiN [42]. The XRD patterns and BET results of the sample were exhibited in Fig. 1(g) and (h). The XRD patterns results were coincidence with the (fcc) TiN (JCPDS No. 38-1420) and the cubic CeO2(JCPDS No.
Fig. 1 e (a~c) SEM image CeO2/TiN NTs; (d~f) TEM image CeO2/TiN NTs; (f) HRTEM image CeO2/TiN NTs; (g) X-ray diffraction patterns of CeO2/TiN NTs, Ce-MOF, Ce-MOF processed in NH3 environment and commercial CeO2; (h) The N2 adsorptiondesorption isotherm of CeO2/TiN NTs, the insert of (h) is the relevant pore distributions of CeO2/TiN NTs; (i) CV curves of with different cycle numbers CeO2/TiN NTs in 0.5 M H2SO4 solution at a scan rate of 50 mVs¡1.
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43-1002), suggesting the purity of the samples and the formation of CeO2. Furthermore, Fig. 1(h) showed a notable TypeⅣ isotherm that the curve exhibited a convex upward in low P/ P0, while the isotherm rose rapidly in higher P/P0 due to capillary adsorption occurred in this area until the relative pressure was closed to 1, confirming the existence of mesopores. In addition, the major pore size of CeO2/TiN NTs was at ca. 9 nm by the pore size distribution curve (insert of Fig. 1(h)), which was coincident with SEM images. Furthermore, the consequence of BET demonstrated the specific surface area was measured to 93 m2g-1. Meanwhile, Fig. 1(i) exhibited the sample was evaluated in 0.5 M H2SO4 solution, which gives strongly circumstantial evidence for the electrochemical stability of CeO2/TiN NTs. As follow, Transmission electron microscope (TEM) image of Pt-CeO2/TiN NTs was revealed in Fig. 2 (a~c) that Pt nanoparticles (NPs) were well-distributed. Notably, current researches had testified that the shapes of Pt NPs were significant for the electrochemical performance and the optimized Pt NPs was about 3 nm [43,44]. In this research, the sizes of Pt nanoparticles was calculated to 3.0 ± 0.3 nm on account of counting the diameter of 200 Pt NPs in TEM image, indicating the high electrochemical performance was ascribed to the intriguing catalyst. In addition, the shapes of Pt was close to Pt/C (3.1 ± 0.4 nm). From the insert of Fig. 2(c), in view of standard lattice distance, 0.227 nm was accord with Pt (111) plane and 0.305 nm conforms to CeO2(111) plane. The electrochemical properties of Pt-CeO2/TiN NTs was investigated as follow. The electrochemically active area (ECSA) of as-prepared catalyst was evaluated on the basis of equation ECSA ¼ QH/(210 WPt), where QH and WPt are the adsorption charge quantity of H (mC) and the total loading amount of Pt (mgcm2) on the electrode surface, respectively, and 210 represents the charge quantity (mCcm2Pt) demands on oxidizing a monolayer of hydrogen on a Pt surface [45]. As calculated, the ECSA of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN
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NTs and Pt/C is 59, 58, 55 and 54 m2g-1 Pt1, respectively. The similar ECSA of the targeted catalyst, Pt-CeO2/C, Pt/TiN NTs and Pt/C were ascribed to the well-distributed and little size of Pt NPs on different carrier. The electroactivity of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C to methanol oxidation was measured in 1 M CH3OH and 0.5 M H2SO4 solution. Fig. 3(b) depicted that the onset potential of as-resulted catalyst (0.3 V), was negative shifted compared with Pt-CeO2/C (0.35 V), Pt/C (0.4 V) and Pt/TiN NTs (0.41 V), confirming the advantageous electrochemical catalytic conditions for MOR. Commonly, there are two anodic peaks that observed in curves (for Ptbased catalyst). First peak (if) current density is relevant to the methanol oxidation process and the next peak current density (ib) is regarded as oxidation of the intermediate product (CO) that generated during the MOR process [46]. Consequently, the figure of if/ib ratio can be used to measure the CO anti-poisoning ability of studied catalyst. Moreover, the if of Pt-CeO2/TiN NTs (0.61 A mg1Pt) was the highest among all studied catalysts, demonstrating the superior electrocatalytic efficiency for MOR. Furthermore, the value of ib of Pt-CeO2/TiN NTs (0.67 A mg1Pt) was approximately 2 times than Pt/TiN NTs (0.31 A mg1Pt), while the value of ib of Pt-CeO2/C (0.59 A mg1Pt) was nearly 2 times than Pt/C (0.28 A mg1Pt). Noteworthily, the if/ib ratio of Pt-CeO2/TiN NTs (0.91) were higher than Pt-CeO2/C (0.89), illustrating that TiN NTs had prominent advantage to MOR. To further explore the surface poisoning of the targeted catalyst, the chronoamperometry curves of Pt-CeO2/TiN NTs, Pt/TiN NTs, PtCeO2/C and Pt/C was carried out at the potential of 0.4 V. Compared to Pt/TiN NTs, Pt-CeO2/TiN NTs showed a slower drop of current, a better steady current, and higher initial currents. Compared to Pt/C, Pt-CeO2/C showed a slower drop of current, a better steady current, which indicates the better ability of CeO2 to anti-poisoning to the intermediates of CO again (Fig. 3(c)). As a conclusion, the well ability of the catalyst with accelerating removal of CO was confirmed.
Fig. 2 e (a~c) TEM image Pt-CeO2/TiN NTs, the insert of (c) is the HRTEM image Pt-CeO2/TiN NTs; (d) the corresponding histogram of Pt-CeO2/TiN NTs.
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Fig. 3 e (a) CV curves Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C in N2-saturated 0.5 M H2SO4 with a scan at 50 mVs¡1 and the insert is the relevant comparison of ECSA; (b) CV curves of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C in 1 M CH3OH and 0.5 M H2SO4; (c) Chronoamperometry curves of Pt-CeO2/TiN NTs, Pt-CeO2/C, Pt/TiN NTs and Pt/C in N2-saturated 0.5 M H2SO4 and 1 M CH3OH at 0.4 V with a scan at 50 mVs¡1.
X-ray photoelectron spectroscopy (XPS) analyses were performed to study the electronic effect, the interaction between Pt NPs and different supports and the interaction between Pt and CeO2. Fig. 4 shows that each Pt 4f7/2 peak and Pt 4f5/2 peak could be deconvoluted to two pairs of doublets. The specific analysis result of Pt-CeO2/TiN NTs indicated that a large quantity of Pt atoms existed as Pt (0), which is beneficial for strengthening electroactivity towards MOR. Compare to
Pt/C, a positive shift of ~0.10 eV in binding energy of Pt/TiN NTs was observed, which confirmed the strong interaction between Pt and TiN support. In addition, an apparent negative shift of ~0.29 eV in binding energy of Pt-CeO2/TiN NTs was observed compared with Pt/TiN NTs, demonstrating CeO2 can provide electrons to Pt atoms and being good for a reduced dband vacancy of Pt, thereby weakened the interaction between Pt and intermediate CO [2,47]. Compared to Pt-CeO2/C, a
Fig. 4 e Pt 4f XPS surveys of the Pt-CeO2/TiN NTs, Pt/TiN NTs, Pt-CeO2/C, and Pt/C.
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Fig. 5 e (a) The CV of Pt/C and (b) CV of Pt-CeO2/TiN NTs before and after the durability test. (c) Comparison of ECSA loss between Pt/C and Pt-CeO2/TiN NTs.
negative shift of ~0.15 eV was observed in the binding energy of Pt-CeO2/TiN NTs, indicating TiN support is beneficial for electronic transferring owing to its supernal conductivity. Current researches have suggested that the catalyst perform high stability with comparison to Pt and carbon carrier owing to the stronger interaction between Pt and titanium nitride [11,48]. For the sake of exploring service life of the asresulted catalyst, accelerated durability test (ADT) was executed. Amazingly, the ECSA of Pt-CeO2/TiN NTs was basically maintained accordance after 2000 cycles while the ECSA of Pt/C is 61% lost after 2000 cycles comparing with its original ECSA (Fig. 5(c)). Compared to Pt/C, the superior stability of PtCeO2/TiN NTs was confirmed.
Conclusions In summary, we fabricated Pt-CeO2/TiN NTs catalyst derived from MOFs with porous structure. The long-term durability, interaction, and high-performance of Pt-CeO2/TiN NTs derived from MOFs were demonstrated in this paper. We convince that these results will arouse intensively investigative curiosity on MOF-based electrochemical catalysts and unfold perspectives for the progress of DMFCs.
Acknowledgments This work was supported by Natural Science Foundation of Guangdong Province, China (No. 2016A030313704), Science and Technology Planning Project of Guangdong Province, China (No. 2016B020240003).
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