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Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction
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Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction Gang Zhao, T.S. Zhao*, Jianbo Xu, Zeng Lin, Xiaohui Yan Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
article info
abstract
Article history:
To investigate the impact of pore size of ordered mesoporous carbon (OMC) FDU-15-
Received 8 August 2016
supported Platinum (Pt) catalysts on oxygen reduction reaction (ORR), OMC FDU-15 with
Received in revised form
various pore sizes ranging from 4.0 nm to 8.1 nm are synthesized through a soft-template
25 October 2016
method, and then FDU-15-supported Pt catalysts are prepared by chemical impregnation
Accepted 11 November 2016
method. Characterizations show that increase in pore size enlarges the specific surface
Available online xxx
area and the pore volume of FDU-15, but decreases the electrical conductivity. Particle size of FDU-15-supported Pt catalyst is also influenced in a decreasing trend with the increase
Keywords:
of pore size. Electrochemical measurement demonstrates that FDU-15-supported Pt cata-
Ordered mesoporous carbons
lyst with a pore size of 6.5 nm yields the highest electrocatalytic activity for ORR, which is
FDU-15
further confirmed by single cell test on DMFC, as the catalyst for cathode.
Pore size
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Platinum Oxygen reduction reaction
Introduction Platinum (Pt) is generally used as catalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) for its superior activity and stability in acidic environment. However, due to the expensive price and the scarcity of Pt metal, costineffective of the system is unavoidable. To address this issue, supported Pt catalyst rather than pure Pt catalyst is used for the improved utilization, which makes it possible to decrease the total Pt amount needed. In this sense, an ideal support with high specific surface area, high electrical conductivity and high resistance to corrosion is needed.
Generally, Activated carbons are widely used as support materials for their low cost and extensive sources. Other than activated carbons, novel carbon materials such as carbon nanotube [1e6], carbon nanofiber [7e13], and ordered mesoporous carbon [14e28] have been reported to have potentials to be used as support materials for their unique morphologies and high specific surface areas. Among the above-mentioned carbon materials, ordered mesoporous carbon (OMC) is one of the most promising support materials for their unique ordered and tailored nanostructure and narrow pore size distributions in mesopore range [29e39]. Ryoo et al. [40] first prepared OMC with MCM-48 silica as hard template and sucrose as carbon source, through a nanocasting method, also known as hard-template method. After
* Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.ijhydene.2016.11.089 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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that, a family of OMCs, named CMK materials, was reported by his group, including CMK-1 to CMK-5 by using different templates. Joo et al. [41] first reported that Pt catalyst supported on CMK-5 showed much higher activity for the ORR than supported on carbon black. After that, extensive efforts have been made to improve the activity of OMC-supported Pt catalyst by controlling the morphology and the pore size of OMC. Joo et al. [42] synthesized OMC with various pore size by using ordered mesoporous silicas with various pore size as hard templates to investigated the effect of pore size of OMCsupported Pt catalyst's activity for ORR in DMFCs, and found larger pore size is good for ORR. Lu et al. [43] prepared OMC with controllable pore sizes in the range of 4e10 nm by means of a template procedure using 2D hexagonal MSU-H and 3D cubic KIT-6 as the hard templates and boric acid as the pore expanding agent; the analysis of two kinds of pore symmetries of OMC showed that the 3D cubic OMC demonstrated superior capacitive performance than the 2D hexagonal OMC. Song et al. [44] deposited Pt particles on CMK-3 with three different pore sizes obtained by using boric acid as the poreexpanding agent, and investigated their effect on the alcohol electrooxidation activity, and found smaller pore size is good for alcohol electrooxidation activity, due to the enhanced electric conductivity. Kim et al. [45] investigated the effect of different morphologies of OMC on ORR for supported Pt catalysts with three types of OMC, including CMK-3, CMK-3G, and CMK-5, and results indicated the Pt/CMK-3G exhibited the highest kinetic current density and highest oxygen oxidization reaction activity. D. Banham et al. [46e48] also investigated the influence of morphology and pore size of OMC on the supported Pt catalysts for their ORR activity by using mesoporous silicas as templates, and indicated that the inner wall thickness of OMC is critical to the design of support materials. Apparently, pore size and morphology of OMC play important roles on ORR performance, when OMC is used as support materials for Pt catalysts. It is noteworthy that above conclusions are based on the CMK-type ordered mesoporous carbons, prepared from hardtemplate route. The key drawback of this method lies in the fact that extra steps are required to remove the hard template, making it expensive and time-consuming. Different from the above mentioned hard-template method, Meng et al. [49] proposed an evaporation-induced self-assembly strategy to prepare OMC in 2006, known as soft-template method. The resultant OMC is called FDU materials including FDU-15 (2-D hexagonal), FDU-14 (3-D cubic), FDU-16 (3-D bicontinuous). It is noted that FDU-15 is completely the invert replicas of CMK-3, as shown in Fig. 1. As we know, research on CMK-3 used as support materials for Pt catalyst has been thoroughly investigated; however that on FDU-15 is seldom reported. Because of different morphologies of FDU-15 and CMK-3 (Fig. 1), the previous conclusion on CMK3 might not be right for FDU-15. In this work, OMC FDU-15, acting as supports for Pt catalysts, is prepared with resol and Pluronic F127 as carbon precursor and soft template. Tetraethyl orthosilicate (TEOS) and decane are used as pore-expanding agents, so that FDU-15 with various pore sizes can be formed. Subsequently, FDU15-supported Pt catalysts are then prepared by chemical impregnation method, with ethylene glycol (EG) as reducing
agent. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are conducted to evaluate the ORR performances of supported Pt catalysts, and the impact of pore size is discussed. Finally, a single DMFC is fabricated by employing the as-prepared FDU-15 supported Pt catalysts as cathode catalysts, and IeV performance and durability are tested.
Experimental Preparation of OMC FDU-15 with different pore sizes Preparation of OMC FDU-15 is according to the literature [49]. Firstly, Resol is prepared to as carbon precursor for OMC FDU-15. Typically, 1.0 g of phenol, 0.21 g of NaOH (20 wt. %) and 1.7 g of formaldehyde solution (37 wt. %) were mixed and heated at 75 C for 1 h. After cooled to room temperature, pH value of the mixture was controlled at 7.0 with 1 M of HCl solution. And then water in the mixture was removed through vacuum distillation. Ethanol was added to form the resol solution (20 wt. %). When preparing FDU-15, 1.0 g of Pluronic F127 and 5.0 g of resol were dissolved and mixed in 20.0 g of ethanol. After that, the mixture was transferred to a dish and evaporated for 8 h at room temperature, before heated at 100 C for 24 h for thermopolymerization. The resultant product was calcinated at 800 C for 3 h in argon atmosphere at a rate of 1 C min1. The as-prepared sample was crashed to powders and denoted as FDU-15-1. To prepare FDU-15 with larger pore size, 1.0 g of TEOS [50] was used as a pore-expanding agent and added into the mixture before it was transferred to the dish. The following steps were the same as those of FDU-15-1. The resultant product was treated with 3 M KOH solution at 80 C for 8 h to remove silica. The final sample is denoted as FDU-15-2. Decane was used as a second pore-expanding agent to further increase the pore size of FDU-15-2. 1.0 g and 2.0 g of decane were added into the mixture after the adding of TEOS, and the following process was the same as in FDU-15-2. The as-prepared FDU-15 samples are denoted as FDU-15-3 and FDU-15-4.
Preparation of FDU-15-supported Pt catalysts FDU-15-supported Pt catalysts were prepared by impregnation method with EG as reducing agent [51e53]. Specifically, 3.38 mL of chloroplatinic acid EG solution (7.4 mgPt mL1) and 100 mg of FDU-15 carbon were dissolved and mixed in a certain amount of EG, in which ratio of Pt is designed at 20 wt.%. 1 M of NaOH solution was subsequently added to adjust the pH value of mixture to 12. After that, the mixture was heated to 140 C for 3 h for the completely reduction of Pt catalyst. The final catalyst was obtained after filtering, washing, and drying at 80 C for 6e8 h in a vacuum oven. The as-prepared Pt catalysts are denoted as Pt/FDU-15-1, Pt/FDU-15-2, Pt/FDU-15-3, and Pt/FDU-15-4, according to the supporting materials.
Physical characterizations Nitrogen sorption isotherms were conducted on a Beckman Coulter SA3100 surface area analyzer. BJH model was used to
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 1 e Schematic illustrations of FDU-15 (a) and CMK-3 (b).
deduce the parameters of pore structure, in which the value for the pore volume (Vt) was calculated by the adsorption branches at a relative pressure of 0.9814 and pore size distribution was from the adsorption branches of the isotherms. Xray diffraction (XRD) measurement was recorded on an X'pert Pro (PANalytical) with Ni-filtered Cu Ka radiation. Thermogravimetric (TG) test was measured on a TGAQ5000 from room temperature to 800 C under air flow with a ramp rate of 5 C min1. Transmission electron microscopy (TEM) images were obtained from a TEM 2010F (JEOL), and the samples for TEM test were prepared as follows: the sample was suspended in ethanol through ultrasonic oscillation and dropped onto a holey carbon film supported on a Cu grid. Measurements on electrical conductivity of the asprepared FDU-15 samples were according to the literature [54,55]. Briefly, a certain amount of sample was pressed to a pellet under a fixed pressure, and then the pellet was placed between two gold plates, connected with an Autolab instrument. Electrical conductivity of the sample was achieved by impedance spectroscopy.
Electrochemical measurements An Autolab instrument was employed for the electrochemical measurements by using a Pt mesh as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Working electrode was made by dispersing 2.0 mg of the asprepared catalyst and 15 mL of Nafion solution (5 wt.%) into 2.0 mL of ethanol under ultrasonication; and then 20 mL of the resultant ink was dropped onto a polished glassy carbon electrode for several times and dried at ambient conditions. CV measurements were obtained under nitrogen saturated 0.5 M H2SO4 solution. LSV measurements were conducted in oxygen-saturated 0.5 M H2SO4 solution with a rotating speed of 1600 rpm and a scan rate of 10 mV s1 to examine the oxygen reduction reaction (ORR).
Evaluation of single cell on DMFC Preparation of membrane electrode assembly (MEA) for a single DMFC Process of preparation of MEA for a single DMFC is similar to the previous work [56].
Typically, a certain amount of Vulcan XC-72 carbon and 20% PTFE were mixed in ethanol and ultrasonicated to get a homogeneous ink; the resultant ink was brushed onto a PTFEtreated Toray carbon paper until the loading of Vulcan XC-72 carbon reached 2 mg cm2, which served as diffusion layer for both anode and cathode. Catalyst layer was fabricated in a similar way by mixing a certain amount of catalyst and 15% Nafion in ethanol, the resultant catalyst ink was brushed onto the above as-prepared diffusion layer. For anode, commercial 60% PtRu/C (ratio of Pt:Ru is 1:1) from Johnson Matthey was used as catalyst with a loading of 4 mg cm2; for cathode, the as-prepared FDU-15 supported Pt catalysts were used as catalysts with an amount of 2 mg cm2. MEA was obtained by hot-pressing the anode and the cathode on each side of a pretreated Nafion 115 membrane, which included boiling for 1 h in 5 vol. % H2O2 at 80 C and another 1 h in 0.5 M H2SO4 at 80 C before washing it in DI water.
Test on single DMFC Single DMFC was fabricated by fixing the MEA between two stainless steel plates with a single serpentine flow field [56]. 1 M of methanol solution and air from a cylinder were supplied to the anode and the cathode of the DMFC with a flow rate of 1 mL min1 and 150 mL min1, respectively. Temperature of the DMFC was kept at 75 C with help of auxiliary heat system, and an Arbin instrument was used as the electric load.
Results and discussion Physical characterization of the as-prepared OMC FDU-15 samples The information of specific surface areas and pore size distributions of the FDU-15 samples were obtained from nitrogen sorption isotherms. As shown in Fig. 2a, there is a condensation at the middle range PS/P0 for all the samples, implying a pattern of type IV, confirming a typical mesoporous structure is formed with a narrow pore size distribution. The pore size distributions in Fig. 2b reveal an increasing trend of pore size from 4.0 nm to 5.5 nm, 6.5 nm, and 8.1 nm with the adding of
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 2 e Nitrogen sorption isotherms (a) and pore size distribution (b) curves of the as-prepared OMC FDU-15 samples. TEOS and decane. Table 1 provides the physical properties of the as-prepared FDU-15 samples. The specific surface areas (SBET) of OMC FDU-15 samples increase from 581 m2 g1 to 921 m2 g1, following the same trend of pore volume (Vt) from 0.443 mL g1 to 1.329 mL g1, suggesting that the increases in the specific surface areas and pore volumes are responses to enlargement of the pore sizes from 4.0 nm to 8.1 nm. It is noted that the function of TEOS is to mitigate the shrinkage of carbon framework during the process of carbonization, so that larger pore sizes can be retained [41]. However, further addition of TEOS cannot further enlarge the pore size, but break down the pore structure of FDU-15. So we have to resort to another strategy. Decane was chosen as another swelling agent because it can enlarge micelles in the solution which ultimately settles as pores, so that pore size is enlarged without any destruction on the pore structure. Small-angle XRD patterns of the FDU-15 samples are shown in Fig. 3. All samples demonstrate the main diffraction peaks [100] at 2q z 0.8 , indicating the presence of an ordered mesoporous structure with a two-dimensional (2-D) hexagonal symmetry. Furthermore, TEM images (Fig. 4aed) provide an evidence of this unique textural structure of the asprepared samples in large domain with good regularities, confirming the morphology of FDU-15. It is noteworthy that pore sizes in Fig. 4aed illustrate an increasing trend, which is consistent with the results from the pore size distributions in nitrogen sorption isotherms. Electrical conductivities for the as-prepared FDU-15 samples were measured to investigate the effect of pore size on electrical conductivity, as shown in Table 1. It is found that the electrical conductivities of the FDU-15 samples decrease from 4.46 to 2.21 S cm1 with the increase of pore sizes from 4.0 to 8.1 nm. The reason could be ascribed to that the increased pore size undoubtedly leads to the growth of the pore volume
Table 1 e Physical properties of as-prepared OMC FDU-15 samples.
FDU-15-1 FDU-15-2 FDU-15-3 FDU-15-4
SBET (m2 g1)
Vt (mL g1)
d (nm)
s (S cm1)
581 755 865 921
0.443 0.953 1.008 1.329
4.0 5.5 6.5 8.1
4.46 3.89 3.05 2.21
Fig. 3 e Small-angle XRD patterns of as-prepared OMC FDU-15 samples.
(Vt) and the porosity of FDU-15 samples, which inevitably decreases the electrical conductivity.
Characterization of FDU-15-supported Pt catalysts XRD patterns of FDU-15-supported Pt catalysts are illustrated in Fig. 5. All catalysts show individual [111], [200], [220], and [311] diffraction peaks at 2q z 39.9 , 46.1 , 67.8 , and 81.4 respectively, in line with the reflections of a typical Pt nanocrystal exhibiting a face-centered cubic phase. Generally, Sherrer equation is used to calculate the average particle size of Pt particles from the peak of [220] at 2q z 67.8 or [111] at 2 q z 39.9 . However, [220] peaks of Pt crystals for the asprepared supported Pt catalysts are so small that it is difficult for Sherrer equation to calculate the average particle size of Pt catalysts, implying very small average Pt particle sizes for all the samples. On the other hand, the [111] peaks of Pt catalysts are suffered from the disturbance of the [200] peaks of Pt catalysts at 2q z 46.1 and the [100] reflection of pure FDU-15 at 2q z 43.80 , according to our previous work [57], making it impractical to calculate the particle size from XRD patterns. TG analyses were conducted to determine the amount of Pt in the as-prepared Pt catalysts. According to Fig. 6, all the Pt catalysts showed a same behavior of losing weight dramatically in the temperature range from 400 C to 500 C, due to the
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 4 e TEM images of as-prepared FDU-15-1 (a), FDU-15-2 (b), FDU-15-3 (c), and FDU-15-4 (d).
Fig. 5 e XRD patterns of FDU-15-supported Pt catalysts.
Fig. 6 e TG analyses of Pt catalysts.
carbon oxidization. It can be concluded that the Pt amounts in the Pt catalysts are quite similar from 18.85% to 20.65%, which is consistent with the experimental design (20%), indicating the loss of Pt in the process of experimental can be neglected. TEM images of FDU-15-supported Pt catalysts are shown in Fig. 7aed. The sharp contrasts in TEM images clearly show the morphologies of FDU-15 and Pt particles. Domains of 2-D ordered hexagonal arrays can be observed in all the as-prepared catalysts, suggesting that the unique pore structure of FDU-15 is preserved during Pt catalyst preparation. For all the
supported Pt catalysts, Pt particles are well dispersed on the surface of the FDU-15 without severe agglomeration. Based on the measurements of 200 particles in random regions, the average Pt particle sizes in Pt/FDU-15-1, Pt/FDU-15-2, Pt/FDU15-3, and Pt/FDU-15-4 are estimated to be 2.9 nm, 2.3 nm, 2.0 nm, and 2.0 nm respectively (Fig. 7eef). The corresponding histograms reveal that the Pt particle size distributions are rather narrow and illustrate the typical features of a Gaussian distribution. Interestingly, the declining trend of the average Pt particle size is the opposite to that of the pore size and the specific surface areas. In addition, the dominant Pt particle
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 7 e TEM images of Pt particles in Pt/FDU-15-1 (a), Pt/FDU-15-2 (b), Pt/FDU-15-3 (c), and Pt/FDU-15-4 (d), and particle size distributions of the four samples (eef).
sizes for Pt/FDU-15-3 and Pt/FDU-15-4 are similar. A likely explanation is that the specific area of FDU-15 enlarges in response to an increase in the pore size, ensuring a better dispersion of Pt particles on the larger surface area of the FDU15 samples. However, when the pore size of FDU-15 reaches a
value greater than 6.5 nm, the particle size of the FDU-15supported Pt catalysts become independent to the specific surface area of the FDU-15 samples, while related to the preparation method and the amount of Pt's loading. Thus, the dominant Pt particle sizes remain similar (2.0 nm) for the last
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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two samples, despite that the pore size increases from 6.5 nm (FDU-15-3) to 8.1 nm (FDU-15-4).
Electrochemical measurements of the supported Pt catalysts CV measurements were performed for the as-prepared Pt catalysts in nitrogen-saturated 0.5 M H2SO4 solution. It is clear that hydrogen underpotential deposition area could be seen for all the samples, indicating a typical behavior of Pt catalyst in acid environment, as illustrated in Fig. 8a. We can calculate the value of electrochemical surface area (SECSA) by integrating the charge (Q) in hydrogen underpotential deposition with the following Eq. (1): SECSA ¼
Q mC
(1)
where SECSA is the electrochemical surface area for Pt catalyst, Q is the integrated charges in hydrogen underpotential deposition area, in which the double capacity charge is deducted, m is the Pt mass, and C is the hydrogen underpotential deposition constant on the smooth Pt surface (0.21 mC cm2). Accordingly, the values of SECSA for the FDU-15-supported Pt catalysts are 47.1 m2 g1Pt (Pt/FDU-15-1), 54.1 m2 g1Pt (Pt/ FDU-15-2), 70.2 m2 g1Pt (Pt/FDU-15-3) and 63.4 m2 g1Pt (Pt/ FDU-15-4) respectively. With the increase in pore size, the value of SECSA also enlarges, caused by the decreasing trend of Pt particle size. The peak value of the SECSA is as high as 70.2 m2 g1Pt, with the Pt catalyst supported on FDU-15 with an average pore size of 6.5 nm. However, the value of SECSA declines to 63.4 m2 g1Pt with a pore size of 8.1 nm. As discussed in 3.2, when the pore size is under 6.5 nm, the increase of pore size results in the smaller Pt particle size of the as-prepared FDU-15-supported Pt catalysts yielding a larger value of SECSA. However, the particle size of Pt catalyst remains constant when the pore size becomes larger than 6.5 nm. In addition, an increase in pore size of FDU-15 tends to decrease the electrical conductivity, in response to increased porosity, as shown in Table 1. Therefore, combined the above mentioned effects, the peak value of the SECSA was reached with the pore size of 6.5 nm. LSV measurements are conducted to compare the effect of pore size on the ORR performances (Fig. 8b). The onset potential of FDU-15-supported Pt catalysts moves slightly positively with an increase in the pore size of FDU-15 samples,
Table 2 e Electrochemical properties of as-prepared FDU15-supported Pt catalysts. Sample Pt/FDU-15-1 Pt/FDU-15-2 Pt/FDU-15-3 Pt/FDU-15-4
Pt particle size (nm)
SECSA Eonset ikin (m2 g1) (V) (mA mg1)
2.9 2.3 2.0 2.0
47.1 54.1 70.2 63.4
0.98 1.0 1.02 1.02
138 189 276 235
which is consistent with the trend of the average Pt particle sizes' decreasing (Table 2). The onset potentials for Pt/FDU-153 and Pt/FDU-15-4 are almost the same (1.02 V), explained by the fact that they both have similar average Pt particle sizes. The ORR mass activities at 0.9 V of the as-prepared samples can be calculated from following equation: MAðmass activityÞ ¼ ikin ¼
ilim *iobv ilim iobv
(2)
where ikin is the kinetic current density, ilim is the limiting diffusion current density, and iobv is the current density at the potential of 0.9 V. The highest mass activity (276 mA mg1Pt) can be attained when the pore size is 6.5 nm, consistent with its largest value of SECSA (70.2 m2 g1). Although the dominant Pt particles' size in Pt/FDU-15-4 is the similar as those in Pt/ FDU-15-3, the mass activity declines as a result of the decreased value of SECSA (Table 2). Therefore, the pore size of FDU-15 has a close impact on the ORR mass activity for the as-prepared FDU-15-supported Pt catalysts. First of all, the increase of pore size leads to the enlargement of the specific surface area and the pore volume, which provides more sites for Pt deposition, and brings about smaller Pt particle size. However, the electrical conductivity reduces with the increase of pore size, as a negative respect. Combined with the advantage and disadvantage, the highest mass activity of FDU-15-supported Pt catalyst was achieved with the pore size of 6.5 nm.
Single cell test ORR is generally the cathode reaction for PEMFCs and DMFCs. Therefore, single cell test on DMFC was used to further evaluate the impact of pore size on the performance of DMFC, in which the as-prepared FDU-15-supported Pt catalysts were used as the
Fig. 8 e CV (a) and LSV (b) curves of FDU-15-supported Pt catalysts in 0.5 M H2SO4 solution. Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 9 e IeV curves (a) and power density (b) of a single DMFC cell with as-prepared FDU-15-supported Pt catalysts as cathode catalysts, and stability test of Pt/FDU-15-3 in single cell at a constant current density of 0.1 A cm¡2 (ced).
catalysts for cathode of DMFC. According to Fig. 9aeb, the best performance was obtained from the sample of Pt/FDU-15-3, with a highest peak power density of 0.141 W cm2. The trend in single cell test is highly consistent with that in the electrochemical measurement, implying the reason should be the combination of particle size of Pt catalysts and electrical conductivity, which caused by pore size of FDU-15. In addition, the difference in the area of mass transportation of IeV curves should be caused by the variety of pore size, in which larger pore size brings about larger limiting current density. As the best catalyst in this study, stability test was also conducted for the Pt/FDU-15-3 by operating the single cell for continuously 73 h at a current density of 0.1 A cm2, compared with that of commercial 20% Pt/C from E-TEK. According to Fig. 9ced, the voltage of the single cell changes from 0.534 V to 0.471 V in the continuously 73 h's operation, decreased by 12.9%, which is very similar to that of the commercial Pt catalyst, implying a similar degradation rate. The cell voltage of Pt/FDU-15-3 is a little higher than that of the commercial Pt catalyst, possibly due to the optimized pore size. Therefore, this result shows the great potential of the as-prepared Pt/FDU15-3 as the catalyst for cathode in DMFC, compared with the commercial Pt catalyst. It is noted that the measured degradation rate is a little different from that of the literature [58,59], which may be caused by the difference test condition used.
Conclusion In this paper, impact of pore size of FDU-15-supported Pt catalyst is investigated by adopting a soft-template method.
The FDU-15 samples with four different pore sizes, ranging from 4.0 nm to 8.1 nm, are prepared by the addition of poreexpanding agents. By using the above FDU-15 samples as supports, Pt catalysts are synthesized by the impregnation method, with EG as the reducing agent. The relationship between the pore size of FDU-15 and the ORR activity of FDU-15supported Pt catalysts is studied. It is noted that the pore size of FDU-15 has an influence on the electrical conductivity and the particle size of Pt catalysts. CV and LSV measurements illustrate that the FDU-15-supported Pt catalyst with a pore size of 6.5 nm delivers the largest value of SECSA (70.2 m2 g1) and the highest ORR mass activity at 0.9 V (276 mA mg1Pt). Single cell test on DMFC further confirms the best performance is achieved from the single cell which Pt/FDU-15-3 (6.5 nm) is used as the cathode catalyst.
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Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction Gang Zhao, T.S. Zhao*, Jianbo Xu, Zeng Lin, Xiaohui Yan Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
article info
abstract
Article history:
To investigate the impact of pore size of ordered mesoporous carbon (OMC) FDU-15-
Received 8 August 2016
supported Platinum (Pt) catalysts on oxygen reduction reaction (ORR), OMC FDU-15 with
Received in revised form
various pore sizes ranging from 4.0 nm to 8.1 nm are synthesized through a soft-template
25 October 2016
method, and then FDU-15-supported Pt catalysts are prepared by chemical impregnation
Accepted 11 November 2016
method. Characterizations show that increase in pore size enlarges the specific surface
Available online xxx
area and the pore volume of FDU-15, but decreases the electrical conductivity. Particle size of FDU-15-supported Pt catalyst is also influenced in a decreasing trend with the increase
Keywords:
of pore size. Electrochemical measurement demonstrates that FDU-15-supported Pt cata-
Ordered mesoporous carbons
lyst with a pore size of 6.5 nm yields the highest electrocatalytic activity for ORR, which is
FDU-15
further confirmed by single cell test on DMFC, as the catalyst for cathode.
Pore size
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Platinum Oxygen reduction reaction
Introduction Platinum (Pt) is generally used as catalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) for its superior activity and stability in acidic environment. However, due to the expensive price and the scarcity of Pt metal, costineffective of the system is unavoidable. To address this issue, supported Pt catalyst rather than pure Pt catalyst is used for the improved utilization, which makes it possible to decrease the total Pt amount needed. In this sense, an ideal support with high specific surface area, high electrical conductivity and high resistance to corrosion is needed.
Generally, Activated carbons are widely used as support materials for their low cost and extensive sources. Other than activated carbons, novel carbon materials such as carbon nanotube [1e6], carbon nanofiber [7e13], and ordered mesoporous carbon [14e28] have been reported to have potentials to be used as support materials for their unique morphologies and high specific surface areas. Among the above-mentioned carbon materials, ordered mesoporous carbon (OMC) is one of the most promising support materials for their unique ordered and tailored nanostructure and narrow pore size distributions in mesopore range [29e39]. Ryoo et al. [40] first prepared OMC with MCM-48 silica as hard template and sucrose as carbon source, through a nanocasting method, also known as hard-template method. After
* Corresponding author. E-mail address: [email protected] (T.S. Zhao). http://dx.doi.org/10.1016/j.ijhydene.2016.11.089 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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that, a family of OMCs, named CMK materials, was reported by his group, including CMK-1 to CMK-5 by using different templates. Joo et al. [41] first reported that Pt catalyst supported on CMK-5 showed much higher activity for the ORR than supported on carbon black. After that, extensive efforts have been made to improve the activity of OMC-supported Pt catalyst by controlling the morphology and the pore size of OMC. Joo et al. [42] synthesized OMC with various pore size by using ordered mesoporous silicas with various pore size as hard templates to investigated the effect of pore size of OMCsupported Pt catalyst's activity for ORR in DMFCs, and found larger pore size is good for ORR. Lu et al. [43] prepared OMC with controllable pore sizes in the range of 4e10 nm by means of a template procedure using 2D hexagonal MSU-H and 3D cubic KIT-6 as the hard templates and boric acid as the pore expanding agent; the analysis of two kinds of pore symmetries of OMC showed that the 3D cubic OMC demonstrated superior capacitive performance than the 2D hexagonal OMC. Song et al. [44] deposited Pt particles on CMK-3 with three different pore sizes obtained by using boric acid as the poreexpanding agent, and investigated their effect on the alcohol electrooxidation activity, and found smaller pore size is good for alcohol electrooxidation activity, due to the enhanced electric conductivity. Kim et al. [45] investigated the effect of different morphologies of OMC on ORR for supported Pt catalysts with three types of OMC, including CMK-3, CMK-3G, and CMK-5, and results indicated the Pt/CMK-3G exhibited the highest kinetic current density and highest oxygen oxidization reaction activity. D. Banham et al. [46e48] also investigated the influence of morphology and pore size of OMC on the supported Pt catalysts for their ORR activity by using mesoporous silicas as templates, and indicated that the inner wall thickness of OMC is critical to the design of support materials. Apparently, pore size and morphology of OMC play important roles on ORR performance, when OMC is used as support materials for Pt catalysts. It is noteworthy that above conclusions are based on the CMK-type ordered mesoporous carbons, prepared from hardtemplate route. The key drawback of this method lies in the fact that extra steps are required to remove the hard template, making it expensive and time-consuming. Different from the above mentioned hard-template method, Meng et al. [49] proposed an evaporation-induced self-assembly strategy to prepare OMC in 2006, known as soft-template method. The resultant OMC is called FDU materials including FDU-15 (2-D hexagonal), FDU-14 (3-D cubic), FDU-16 (3-D bicontinuous). It is noted that FDU-15 is completely the invert replicas of CMK-3, as shown in Fig. 1. As we know, research on CMK-3 used as support materials for Pt catalyst has been thoroughly investigated; however that on FDU-15 is seldom reported. Because of different morphologies of FDU-15 and CMK-3 (Fig. 1), the previous conclusion on CMK3 might not be right for FDU-15. In this work, OMC FDU-15, acting as supports for Pt catalysts, is prepared with resol and Pluronic F127 as carbon precursor and soft template. Tetraethyl orthosilicate (TEOS) and decane are used as pore-expanding agents, so that FDU-15 with various pore sizes can be formed. Subsequently, FDU15-supported Pt catalysts are then prepared by chemical impregnation method, with ethylene glycol (EG) as reducing
agent. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are conducted to evaluate the ORR performances of supported Pt catalysts, and the impact of pore size is discussed. Finally, a single DMFC is fabricated by employing the as-prepared FDU-15 supported Pt catalysts as cathode catalysts, and IeV performance and durability are tested.
Experimental Preparation of OMC FDU-15 with different pore sizes Preparation of OMC FDU-15 is according to the literature [49]. Firstly, Resol is prepared to as carbon precursor for OMC FDU-15. Typically, 1.0 g of phenol, 0.21 g of NaOH (20 wt. %) and 1.7 g of formaldehyde solution (37 wt. %) were mixed and heated at 75 C for 1 h. After cooled to room temperature, pH value of the mixture was controlled at 7.0 with 1 M of HCl solution. And then water in the mixture was removed through vacuum distillation. Ethanol was added to form the resol solution (20 wt. %). When preparing FDU-15, 1.0 g of Pluronic F127 and 5.0 g of resol were dissolved and mixed in 20.0 g of ethanol. After that, the mixture was transferred to a dish and evaporated for 8 h at room temperature, before heated at 100 C for 24 h for thermopolymerization. The resultant product was calcinated at 800 C for 3 h in argon atmosphere at a rate of 1 C min1. The as-prepared sample was crashed to powders and denoted as FDU-15-1. To prepare FDU-15 with larger pore size, 1.0 g of TEOS [50] was used as a pore-expanding agent and added into the mixture before it was transferred to the dish. The following steps were the same as those of FDU-15-1. The resultant product was treated with 3 M KOH solution at 80 C for 8 h to remove silica. The final sample is denoted as FDU-15-2. Decane was used as a second pore-expanding agent to further increase the pore size of FDU-15-2. 1.0 g and 2.0 g of decane were added into the mixture after the adding of TEOS, and the following process was the same as in FDU-15-2. The as-prepared FDU-15 samples are denoted as FDU-15-3 and FDU-15-4.
Preparation of FDU-15-supported Pt catalysts FDU-15-supported Pt catalysts were prepared by impregnation method with EG as reducing agent [51e53]. Specifically, 3.38 mL of chloroplatinic acid EG solution (7.4 mgPt mL1) and 100 mg of FDU-15 carbon were dissolved and mixed in a certain amount of EG, in which ratio of Pt is designed at 20 wt.%. 1 M of NaOH solution was subsequently added to adjust the pH value of mixture to 12. After that, the mixture was heated to 140 C for 3 h for the completely reduction of Pt catalyst. The final catalyst was obtained after filtering, washing, and drying at 80 C for 6e8 h in a vacuum oven. The as-prepared Pt catalysts are denoted as Pt/FDU-15-1, Pt/FDU-15-2, Pt/FDU-15-3, and Pt/FDU-15-4, according to the supporting materials.
Physical characterizations Nitrogen sorption isotherms were conducted on a Beckman Coulter SA3100 surface area analyzer. BJH model was used to
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 1 e Schematic illustrations of FDU-15 (a) and CMK-3 (b).
deduce the parameters of pore structure, in which the value for the pore volume (Vt) was calculated by the adsorption branches at a relative pressure of 0.9814 and pore size distribution was from the adsorption branches of the isotherms. Xray diffraction (XRD) measurement was recorded on an X'pert Pro (PANalytical) with Ni-filtered Cu Ka radiation. Thermogravimetric (TG) test was measured on a TGAQ5000 from room temperature to 800 C under air flow with a ramp rate of 5 C min1. Transmission electron microscopy (TEM) images were obtained from a TEM 2010F (JEOL), and the samples for TEM test were prepared as follows: the sample was suspended in ethanol through ultrasonic oscillation and dropped onto a holey carbon film supported on a Cu grid. Measurements on electrical conductivity of the asprepared FDU-15 samples were according to the literature [54,55]. Briefly, a certain amount of sample was pressed to a pellet under a fixed pressure, and then the pellet was placed between two gold plates, connected with an Autolab instrument. Electrical conductivity of the sample was achieved by impedance spectroscopy.
Electrochemical measurements An Autolab instrument was employed for the electrochemical measurements by using a Pt mesh as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Working electrode was made by dispersing 2.0 mg of the asprepared catalyst and 15 mL of Nafion solution (5 wt.%) into 2.0 mL of ethanol under ultrasonication; and then 20 mL of the resultant ink was dropped onto a polished glassy carbon electrode for several times and dried at ambient conditions. CV measurements were obtained under nitrogen saturated 0.5 M H2SO4 solution. LSV measurements were conducted in oxygen-saturated 0.5 M H2SO4 solution with a rotating speed of 1600 rpm and a scan rate of 10 mV s1 to examine the oxygen reduction reaction (ORR).
Evaluation of single cell on DMFC Preparation of membrane electrode assembly (MEA) for a single DMFC Process of preparation of MEA for a single DMFC is similar to the previous work [56].
Typically, a certain amount of Vulcan XC-72 carbon and 20% PTFE were mixed in ethanol and ultrasonicated to get a homogeneous ink; the resultant ink was brushed onto a PTFEtreated Toray carbon paper until the loading of Vulcan XC-72 carbon reached 2 mg cm2, which served as diffusion layer for both anode and cathode. Catalyst layer was fabricated in a similar way by mixing a certain amount of catalyst and 15% Nafion in ethanol, the resultant catalyst ink was brushed onto the above as-prepared diffusion layer. For anode, commercial 60% PtRu/C (ratio of Pt:Ru is 1:1) from Johnson Matthey was used as catalyst with a loading of 4 mg cm2; for cathode, the as-prepared FDU-15 supported Pt catalysts were used as catalysts with an amount of 2 mg cm2. MEA was obtained by hot-pressing the anode and the cathode on each side of a pretreated Nafion 115 membrane, which included boiling for 1 h in 5 vol. % H2O2 at 80 C and another 1 h in 0.5 M H2SO4 at 80 C before washing it in DI water.
Test on single DMFC Single DMFC was fabricated by fixing the MEA between two stainless steel plates with a single serpentine flow field [56]. 1 M of methanol solution and air from a cylinder were supplied to the anode and the cathode of the DMFC with a flow rate of 1 mL min1 and 150 mL min1, respectively. Temperature of the DMFC was kept at 75 C with help of auxiliary heat system, and an Arbin instrument was used as the electric load.
Results and discussion Physical characterization of the as-prepared OMC FDU-15 samples The information of specific surface areas and pore size distributions of the FDU-15 samples were obtained from nitrogen sorption isotherms. As shown in Fig. 2a, there is a condensation at the middle range PS/P0 for all the samples, implying a pattern of type IV, confirming a typical mesoporous structure is formed with a narrow pore size distribution. The pore size distributions in Fig. 2b reveal an increasing trend of pore size from 4.0 nm to 5.5 nm, 6.5 nm, and 8.1 nm with the adding of
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 2 e Nitrogen sorption isotherms (a) and pore size distribution (b) curves of the as-prepared OMC FDU-15 samples. TEOS and decane. Table 1 provides the physical properties of the as-prepared FDU-15 samples. The specific surface areas (SBET) of OMC FDU-15 samples increase from 581 m2 g1 to 921 m2 g1, following the same trend of pore volume (Vt) from 0.443 mL g1 to 1.329 mL g1, suggesting that the increases in the specific surface areas and pore volumes are responses to enlargement of the pore sizes from 4.0 nm to 8.1 nm. It is noted that the function of TEOS is to mitigate the shrinkage of carbon framework during the process of carbonization, so that larger pore sizes can be retained [41]. However, further addition of TEOS cannot further enlarge the pore size, but break down the pore structure of FDU-15. So we have to resort to another strategy. Decane was chosen as another swelling agent because it can enlarge micelles in the solution which ultimately settles as pores, so that pore size is enlarged without any destruction on the pore structure. Small-angle XRD patterns of the FDU-15 samples are shown in Fig. 3. All samples demonstrate the main diffraction peaks [100] at 2q z 0.8 , indicating the presence of an ordered mesoporous structure with a two-dimensional (2-D) hexagonal symmetry. Furthermore, TEM images (Fig. 4aed) provide an evidence of this unique textural structure of the asprepared samples in large domain with good regularities, confirming the morphology of FDU-15. It is noteworthy that pore sizes in Fig. 4aed illustrate an increasing trend, which is consistent with the results from the pore size distributions in nitrogen sorption isotherms. Electrical conductivities for the as-prepared FDU-15 samples were measured to investigate the effect of pore size on electrical conductivity, as shown in Table 1. It is found that the electrical conductivities of the FDU-15 samples decrease from 4.46 to 2.21 S cm1 with the increase of pore sizes from 4.0 to 8.1 nm. The reason could be ascribed to that the increased pore size undoubtedly leads to the growth of the pore volume
Table 1 e Physical properties of as-prepared OMC FDU-15 samples.
FDU-15-1 FDU-15-2 FDU-15-3 FDU-15-4
SBET (m2 g1)
Vt (mL g1)
d (nm)
s (S cm1)
581 755 865 921
0.443 0.953 1.008 1.329
4.0 5.5 6.5 8.1
4.46 3.89 3.05 2.21
Fig. 3 e Small-angle XRD patterns of as-prepared OMC FDU-15 samples.
(Vt) and the porosity of FDU-15 samples, which inevitably decreases the electrical conductivity.
Characterization of FDU-15-supported Pt catalysts XRD patterns of FDU-15-supported Pt catalysts are illustrated in Fig. 5. All catalysts show individual [111], [200], [220], and [311] diffraction peaks at 2q z 39.9 , 46.1 , 67.8 , and 81.4 respectively, in line with the reflections of a typical Pt nanocrystal exhibiting a face-centered cubic phase. Generally, Sherrer equation is used to calculate the average particle size of Pt particles from the peak of [220] at 2q z 67.8 or [111] at 2 q z 39.9 . However, [220] peaks of Pt crystals for the asprepared supported Pt catalysts are so small that it is difficult for Sherrer equation to calculate the average particle size of Pt catalysts, implying very small average Pt particle sizes for all the samples. On the other hand, the [111] peaks of Pt catalysts are suffered from the disturbance of the [200] peaks of Pt catalysts at 2q z 46.1 and the [100] reflection of pure FDU-15 at 2q z 43.80 , according to our previous work [57], making it impractical to calculate the particle size from XRD patterns. TG analyses were conducted to determine the amount of Pt in the as-prepared Pt catalysts. According to Fig. 6, all the Pt catalysts showed a same behavior of losing weight dramatically in the temperature range from 400 C to 500 C, due to the
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 4 e TEM images of as-prepared FDU-15-1 (a), FDU-15-2 (b), FDU-15-3 (c), and FDU-15-4 (d).
Fig. 5 e XRD patterns of FDU-15-supported Pt catalysts.
Fig. 6 e TG analyses of Pt catalysts.
carbon oxidization. It can be concluded that the Pt amounts in the Pt catalysts are quite similar from 18.85% to 20.65%, which is consistent with the experimental design (20%), indicating the loss of Pt in the process of experimental can be neglected. TEM images of FDU-15-supported Pt catalysts are shown in Fig. 7aed. The sharp contrasts in TEM images clearly show the morphologies of FDU-15 and Pt particles. Domains of 2-D ordered hexagonal arrays can be observed in all the as-prepared catalysts, suggesting that the unique pore structure of FDU-15 is preserved during Pt catalyst preparation. For all the
supported Pt catalysts, Pt particles are well dispersed on the surface of the FDU-15 without severe agglomeration. Based on the measurements of 200 particles in random regions, the average Pt particle sizes in Pt/FDU-15-1, Pt/FDU-15-2, Pt/FDU15-3, and Pt/FDU-15-4 are estimated to be 2.9 nm, 2.3 nm, 2.0 nm, and 2.0 nm respectively (Fig. 7eef). The corresponding histograms reveal that the Pt particle size distributions are rather narrow and illustrate the typical features of a Gaussian distribution. Interestingly, the declining trend of the average Pt particle size is the opposite to that of the pore size and the specific surface areas. In addition, the dominant Pt particle
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 7 e TEM images of Pt particles in Pt/FDU-15-1 (a), Pt/FDU-15-2 (b), Pt/FDU-15-3 (c), and Pt/FDU-15-4 (d), and particle size distributions of the four samples (eef).
sizes for Pt/FDU-15-3 and Pt/FDU-15-4 are similar. A likely explanation is that the specific area of FDU-15 enlarges in response to an increase in the pore size, ensuring a better dispersion of Pt particles on the larger surface area of the FDU15 samples. However, when the pore size of FDU-15 reaches a
value greater than 6.5 nm, the particle size of the FDU-15supported Pt catalysts become independent to the specific surface area of the FDU-15 samples, while related to the preparation method and the amount of Pt's loading. Thus, the dominant Pt particle sizes remain similar (2.0 nm) for the last
Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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two samples, despite that the pore size increases from 6.5 nm (FDU-15-3) to 8.1 nm (FDU-15-4).
Electrochemical measurements of the supported Pt catalysts CV measurements were performed for the as-prepared Pt catalysts in nitrogen-saturated 0.5 M H2SO4 solution. It is clear that hydrogen underpotential deposition area could be seen for all the samples, indicating a typical behavior of Pt catalyst in acid environment, as illustrated in Fig. 8a. We can calculate the value of electrochemical surface area (SECSA) by integrating the charge (Q) in hydrogen underpotential deposition with the following Eq. (1): SECSA ¼
Q mC
(1)
where SECSA is the electrochemical surface area for Pt catalyst, Q is the integrated charges in hydrogen underpotential deposition area, in which the double capacity charge is deducted, m is the Pt mass, and C is the hydrogen underpotential deposition constant on the smooth Pt surface (0.21 mC cm2). Accordingly, the values of SECSA for the FDU-15-supported Pt catalysts are 47.1 m2 g1Pt (Pt/FDU-15-1), 54.1 m2 g1Pt (Pt/ FDU-15-2), 70.2 m2 g1Pt (Pt/FDU-15-3) and 63.4 m2 g1Pt (Pt/ FDU-15-4) respectively. With the increase in pore size, the value of SECSA also enlarges, caused by the decreasing trend of Pt particle size. The peak value of the SECSA is as high as 70.2 m2 g1Pt, with the Pt catalyst supported on FDU-15 with an average pore size of 6.5 nm. However, the value of SECSA declines to 63.4 m2 g1Pt with a pore size of 8.1 nm. As discussed in 3.2, when the pore size is under 6.5 nm, the increase of pore size results in the smaller Pt particle size of the as-prepared FDU-15-supported Pt catalysts yielding a larger value of SECSA. However, the particle size of Pt catalyst remains constant when the pore size becomes larger than 6.5 nm. In addition, an increase in pore size of FDU-15 tends to decrease the electrical conductivity, in response to increased porosity, as shown in Table 1. Therefore, combined the above mentioned effects, the peak value of the SECSA was reached with the pore size of 6.5 nm. LSV measurements are conducted to compare the effect of pore size on the ORR performances (Fig. 8b). The onset potential of FDU-15-supported Pt catalysts moves slightly positively with an increase in the pore size of FDU-15 samples,
Table 2 e Electrochemical properties of as-prepared FDU15-supported Pt catalysts. Sample Pt/FDU-15-1 Pt/FDU-15-2 Pt/FDU-15-3 Pt/FDU-15-4
Pt particle size (nm)
SECSA Eonset ikin (m2 g1) (V) (mA mg1)
2.9 2.3 2.0 2.0
47.1 54.1 70.2 63.4
0.98 1.0 1.02 1.02
138 189 276 235
which is consistent with the trend of the average Pt particle sizes' decreasing (Table 2). The onset potentials for Pt/FDU-153 and Pt/FDU-15-4 are almost the same (1.02 V), explained by the fact that they both have similar average Pt particle sizes. The ORR mass activities at 0.9 V of the as-prepared samples can be calculated from following equation: MAðmass activityÞ ¼ ikin ¼
ilim *iobv ilim iobv
(2)
where ikin is the kinetic current density, ilim is the limiting diffusion current density, and iobv is the current density at the potential of 0.9 V. The highest mass activity (276 mA mg1Pt) can be attained when the pore size is 6.5 nm, consistent with its largest value of SECSA (70.2 m2 g1). Although the dominant Pt particles' size in Pt/FDU-15-4 is the similar as those in Pt/ FDU-15-3, the mass activity declines as a result of the decreased value of SECSA (Table 2). Therefore, the pore size of FDU-15 has a close impact on the ORR mass activity for the as-prepared FDU-15-supported Pt catalysts. First of all, the increase of pore size leads to the enlargement of the specific surface area and the pore volume, which provides more sites for Pt deposition, and brings about smaller Pt particle size. However, the electrical conductivity reduces with the increase of pore size, as a negative respect. Combined with the advantage and disadvantage, the highest mass activity of FDU-15-supported Pt catalyst was achieved with the pore size of 6.5 nm.
Single cell test ORR is generally the cathode reaction for PEMFCs and DMFCs. Therefore, single cell test on DMFC was used to further evaluate the impact of pore size on the performance of DMFC, in which the as-prepared FDU-15-supported Pt catalysts were used as the
Fig. 8 e CV (a) and LSV (b) curves of FDU-15-supported Pt catalysts in 0.5 M H2SO4 solution. Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089
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Fig. 9 e IeV curves (a) and power density (b) of a single DMFC cell with as-prepared FDU-15-supported Pt catalysts as cathode catalysts, and stability test of Pt/FDU-15-3 in single cell at a constant current density of 0.1 A cm¡2 (ced).
catalysts for cathode of DMFC. According to Fig. 9aeb, the best performance was obtained from the sample of Pt/FDU-15-3, with a highest peak power density of 0.141 W cm2. The trend in single cell test is highly consistent with that in the electrochemical measurement, implying the reason should be the combination of particle size of Pt catalysts and electrical conductivity, which caused by pore size of FDU-15. In addition, the difference in the area of mass transportation of IeV curves should be caused by the variety of pore size, in which larger pore size brings about larger limiting current density. As the best catalyst in this study, stability test was also conducted for the Pt/FDU-15-3 by operating the single cell for continuously 73 h at a current density of 0.1 A cm2, compared with that of commercial 20% Pt/C from E-TEK. According to Fig. 9ced, the voltage of the single cell changes from 0.534 V to 0.471 V in the continuously 73 h's operation, decreased by 12.9%, which is very similar to that of the commercial Pt catalyst, implying a similar degradation rate. The cell voltage of Pt/FDU-15-3 is a little higher than that of the commercial Pt catalyst, possibly due to the optimized pore size. Therefore, this result shows the great potential of the as-prepared Pt/FDU15-3 as the catalyst for cathode in DMFC, compared with the commercial Pt catalyst. It is noted that the measured degradation rate is a little different from that of the literature [58,59], which may be caused by the difference test condition used.
Conclusion In this paper, impact of pore size of FDU-15-supported Pt catalyst is investigated by adopting a soft-template method.
The FDU-15 samples with four different pore sizes, ranging from 4.0 nm to 8.1 nm, are prepared by the addition of poreexpanding agents. By using the above FDU-15 samples as supports, Pt catalysts are synthesized by the impregnation method, with EG as the reducing agent. The relationship between the pore size of FDU-15 and the ORR activity of FDU-15supported Pt catalysts is studied. It is noted that the pore size of FDU-15 has an influence on the electrical conductivity and the particle size of Pt catalysts. CV and LSV measurements illustrate that the FDU-15-supported Pt catalyst with a pore size of 6.5 nm delivers the largest value of SECSA (70.2 m2 g1) and the highest ORR mass activity at 0.9 V (276 mA mg1Pt). Single cell test on DMFC further confirms the best performance is achieved from the single cell which Pt/FDU-15-3 (6.5 nm) is used as the cathode catalyst.
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Please cite this article in press as: Zhao G, et al., Impact of pore size of ordered mesoporous carbon FDU-15-supported platinum catalysts on oxygen reduction reaction, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.089