Embedding platinum-based nanoparticles within ordered mesoporous carbon using supercritical carbon dioxide technique as a highly efficient oxygen reduction electrocatalyst

Journal of Alloys and Compounds 741 (2018) 580e589

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Embedding platinum-based nanoparticles within ordered mesoporous carbon using supercritical carbon dioxide technique as a highly efficient oxygen reduction electrocatalyst Yazhou Zhou a, b, Chengzhou Zhu b, Guohai Yang b, c, Dan Du b, Xiaonong Cheng a, Juan Yang a, *, Yuehe Lin b, ** a b c

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, People's Republic of China School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920, United States School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou, 221116, People's Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2017 Received in revised form 26 December 2017 Accepted 3 January 2018 Available online 4 January 2018

Ordered mesoporous carbon (OMC) has shown great promising as superior support in the creating highly efficient and stable cathodic catalyst for oxygen reduction reaction (ORR) due to its many merits but there are still many challenges. We demonstrate in this work a facile and large-scale strategy to efficiently embed Pt-based crystals within OMC using supercritical carbon dioxide (scCO2) technique. Typically, PtFe/OMC catalysts with the highly dispersive, ultrafine sizes (1.3e2.3 nm), controllable compositions and loadings have been successfully fabricated. Through control of experimental process and loadings, most of crystals can be deposited into mesochannels of OMC. The integration of highly dispersive and ultrafine PtFe crystals as well as high surface area, mesoporous structure and good electrical conductivity of OMC supports make PtFe/OMC promising as active and stable electrocatalysts toward ORR. By careful comparison, PtFe/OMC catalysts show the overwhelmingly better or comparable electrochemical performance compared with previously reported mesoporous carbon supported Ptbased catalysts. These attractive materials hold great potential in cathodic electrocatalyst for fuel cells and the scCO2 technique is quite superior for constructing OMC with various embedded nanoparticles (such as PtPd, PtCu etc.) © 2018 Elsevier B.V. All rights reserved.

Keywords: Pt-based alloys Ordered mesoporous carbon Supercritical fluid Oxygen reduction reaction

1. Introduction Proton exchange membrane fuel cells (PEMFCs) are useful as clean power sources that represent a potential alternative to environmentally unfriendly fossil-fuel use owing to their fascinating features including high theoretical efficiency, high power density and green emission [1e3]. It always meets three major criteria: cost, performance and durability because most membrane electrode assembly (MEA) catalysts used currently are mainly based on platinum (Pt) [4]. Given the above challenges, most research efforts are focused on enhancement of the electrocatalytic activity of Pt-based catalysts towards oxygen reduction reaction

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Lin). https://doi.org/10.1016/j.jallcom.2018.01.053 0925-8388/© 2018 Elsevier B.V. All rights reserved.

(ORR) [5e7]. Specifically, the current performances of the Pt catalysts alloyed with the less noble late transition 3d metal M (M: Fe, Co, Ni, and Cu etc.) and with de-alloying, core-shell, hollow, or crystalline structure [8e10] are impressive, reflecting the higher mass and specific activities of PEMFCs compared to the current United States Department of Energy (US DOE) targets that have brought the technology close to pre-commercial viability [4]. Besides, the supports for Pt-based nanoparticles are also crucial for the design of highly efficient cathodic electrocatalysts for fuel cells. Up to now, porous carbon is one of the most widely adopted supports for design and fabrication of fuel cell catalysts [11,12]. Mesoporous carbon is one of the superior materials for supporting Pt catalysts that has shown the enhancement in electrochemical performances owing to the large specific surface area, mesoporous structure and excellent electrical conductivity [13]. Specifically, the large surface area of OMC is beneficial for highly dispersive of Pt-based nanoparticles, which can enhance the mass activity. The specific activity can be significantly improved by the

Y. Zhou et al. / Journal of Alloys and Compounds 741 (2018) 580e589

abundant pores and high pore volume of OMC because of the facilitated mass diffusion. In addition, owing to the excellent electrical conductivity of OMC, the sufficient electron pathways can be provided by OMC, which also lead to high specific activity [14,15]. However, there are still many challenges. First, the wet chemical reduction is the most common method to disperse the Pt-based nanoparticles onto the OMC supports [16]. But, owing to the lack of surface functionalities, the ionic metal precursors are not easy to be adsorbed on the OMC, which causes the weak particle-carbon interaction and particle aggregation [17]. Consequently, the detachment of particles happens during potential cycling, which results in the substantial degradation of electrocatalytic performance. In order to get the highly dispersive of particles, surface modification for OMC is necessary [18,19]. However, the electrical conductivity of carbon supports, unfortunately, could be decreased due to the surfactants, and the interactions between particles and carbon supports are weakened. Second, for most of the OMCsupported Pt-based catalysts, particles are mainly dispersed on the surface of OMC [16e19]. Recent work showed that the embedment of Pt nanoparticles within the carbon pore walls is beneficial to improve the electrocatalytic performance owing to the advantages of mesoporous structure and the intimate interface between Pt and carbon [20]. For instance, Wu and co-workers reported the highly active and stable catalyst of embedment of Pt nanoparticles in OMC synthesized by the hard template, and followed by the carbon deposition, Pt precursor reduction and graphitization [21]. Importantly, such catalysts also have excellent methanol-tolerant property due to the novel carbon embedment property, which showed the great potential in cathodic catalyst for direct methanol fuel cells (DMFCs). However, the current methods are very complex and the electrocatalytic activity towards ORR is still needed to improve. Thirdly, catalysts embedded in OMC by current methods are still limited to Pt nanoparticles. Thus, it is highly desirable to develop new strategies to embed the Pt-based nanoparticles (such as PtFe, Pt Co, PtCu, etc.) within OMC that is probably further enhance the electrochemical performance. In our previous work, we have demonstrated making the supercritical carbon dioxide (scCO2) technique for the synthesis of three-dimensional graphene-supported Pt-based nanoparticles [22,23]. The organometallic precursors can dissolve into the scCO2 and be delivered into the small area owing to scCO2's low viscosity, zero surface tension and high diffusivity properties [15,24]. But, it is a challenge to controllably embed Pt-based nanoparticles within OMC by scCO2 technique has not been reported. Herein, we continue the scCO2 technique to address this challenge through controlling the synthesis process. The embedded PtM nanoparticles with ultrafine sizes, high dispersion, controllable loadings and composition can be achieved. The electrocatalytic performance of as-prepared PtFe/OMC catalysts were studied and compared with commercial Pt/C catalyst and previously reported OMC-supported Pt-based catalysts, which showed the overwhelmingly better performance of PtFe/OMC catalyst. 2. Experimental 2.1. Chemicals and materials OMC: cmk-3 (specific surface area: 1000 m2 g
1, pore diameter: 5.57 nm) was purchased from ACS Materials, LLC. Nafion perfluorinated resin solution (5%), perchloric acid (HClO4), borane tetrahydrofuran complex solution (borane-THF), isopropanol (99.5%), methanol, tetrahydrofuran (THF), Pt(hfa)2, Pd(hfa)2$xH2O, (hfa ¼ hexafluoroacetylacetonate, Fe(acac)2, Cu(acac)2 (acac ¼ aceylacetonate), were purchased from SigmaAldrich. High-purity CO2, oxygen (O2) and nitrogen (N2) gas

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were purchased from chemical store (Washington State University). Commercial Pt/C catalyst (20 wt%Pt, mainly 2.5e3.5 nm in diameter) was purchased from Alfa Aesar. 2.2. Embedment Pt-based nanoparticles within OMC using scCO2 technique The scCO2 reaction system can be seen in Fig. S1. The typical PtFe/OMC catalyst with Pt content of ~12.0 wt% and Pt:Fe atomic ratio of 1:1 was used to describe this synthesis process. The OMC (5 mg), the metal precursor Pt(hfa)2 (10 mg) and Fe(acac)2 (5.8 mg) with a small amount of THF as a modifier were put into a glass cell. This glass cell was then put into a high-pressure reaction cell located on a heater. The reaction cell was heated to 353 K. And then, CO2 gas was pressurized to 150 atm by pump, and introduced into the reaction cell. The metal precursor with CO2 and THF formed the supercritical fluid. The temperature was kept at 353 K for 2 h. After then, the temperature was cooled to 333 K, and vented out the CO2 quickly to remove the precursors without embedment within OMC. And then, the reaction cell was opened, and the borane-THF reducer was injected into cell (outside of glass cell). The system temperature increased to 353 K. After 0.5 h, the temperature was cooled, and then was depressurized by releasing CO2. The product was obtained by methanol washing under sonication for 5 times. In addition, Pt/OMC, Pt3Fe/OMC with Pt content of 13.0 wt%, PtFe/OMC with different Pt contents of 5.4 wt% and 25.6 wt%, PtPd/ OMC and PtCu/OMC composites were also synthesized by asdescribed scCO2 technique. 2.3. Characterization The scanning transmission electron microscope (STEM), the high-resolution TEM (HRTEM) analysis and electron diffraction pattern (EDP) were obtained using Titan 80-300 S/TEM operated at 300 kV (FEI Company). The transmission electron microscopy (TEM) was used to characterize the morphologies of products (Philips CM200 UT). The energy dispersive X-ray (EDX) was performed on a Hitachi-4700 field SEM operated at 20 kV. The elemental analysis of products was characterized by X-ray photoelectron spectroscopy (XPS) performed on a Kratos AXIS-165. The particle loading and composition were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES) performed on Thermo-Fisher iCAP 6300. The X-ray diffraction (XRD) obtained by powder X-ray diffractometer was used to analyse the crystal structure of catalysts (Siemens D5000). The mesoporous structures of products were characterized using nitrogen (N2) adsorption/desorption measurements (Quantachrome autosorb-6 automated gas sorption system). 2.4. Electrochemical measurements The three electrode system was used to study the electrocatalytic performance on a CHI 630E station (Shanghai CHI Instruments Co.). The counter electrode, reference electrode and working electrode are Pt wire, Ag/AgCl (3 M KCl) and a glassy carbon rotating disk electrode (GCRDE) modified by catalyst film (5 mm in diameter), respectively. For working electrode preparation, the catalyst ink (2 mg mL
1) was prepared firstly by dispersing catalyst in a mixture of isopropanol, 5 wt% Nafion and water with volume ratio of 4/1/160). And then, the surface of GCRDE was modified by dropping 10 mL catalyst ink, followed by drying at 60  C for 30 min. The target catalyst loadings on the GC RDEs are 1.08 mgPt for PtFe/HSG (~5.4 wt%Pt), 2.4 mgPt for PtFe/HSG (~12.0 wt%Pt), 5.12 mgPt for PtFe/HSG (~25.6 wt%Pt), 2.6 mgPt for Pt3Fe/HSG (~13.0 wt %Pt), and 4 mgPt for commercial Pt/C, respectively. Cyclic

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voltammograms (CVs) measurements were obtained by scanning at the rate of 50 mV s
1 in a N2-saturated 0.1 M HClO4 solution when the CV profiles had stable shape. The ORR activities were analyzed by ORR polarization curves that are obtained by scanning in an O2-saturated 0.1 M HClO4 solution at a scan rate of 10 mV s
1 with different rotation rates. The durability was measured using accelerated durability test (ADT) according to the previous work. In addition, all potentials were relative to hydrogen electrode (RHE). 3. Results and discussion The general idea in this paper is shown in Fig. 1. For the synthesis of PtFe/OMC catalyst, the Pt(hfa)2, Fe(acac)2 and OMC with THF were loaded into the reactor cell (Fig. 1A). When the temperature and pressure are increased to 353 K and 150 atm, respectively, the scCO2 environment can be formed; meanwhile the organometallic precursors and THF started to dissolve into scCO2 and be delivered into mesochannels of OMC with scCO2 in this high pressure system (Fig. 1B). The unembedded metal precursors were removed by releasing CO2 (Fig. 1C). This process is crucial for embedment of nanoparticles within OMC, but not on the surface of OMC. In our previous work of deposition of Pt-based nanoparticles in 3D graphene, we have to cool down the system temperature lower than supercritical temperature before releasing CO2 to keep the metal precursors adsorbed onto the graphene sheets [23e25]. In this work, however, the system temperature was still maintained in 353 K when the CO2 is releasing, which has two advantages for the preparation of Pt-based nanoparticles embedded within OMC. One is that the unembedded metal precursors were still in supercritical status, which can be removed from system with releasing CO2 easily (the metal precursors were difficult to be adsorbed onto the surface of OMC owing to the lack of surface functionalities); the other is that the embedded metal precursors were not easy to diffuse outside of mesochannels owing to the lower precursor concentration. The embedded metal precursors then were reduced to crystals by borane-THF (Fig. 1D). The Pt/OMC was synthesized at first using the modified scCO2 technique, which was used to demonstrate the designed feasibility. The crystal structure of Pt/OMC catalyst was characterized by XRD, which is shown as Fig. 2d. The four obvious peaks appeared at 2q of 39.8 , 463 , 67.8 , and 81.6 can be found, corresponding to distinct

Pt (111), Pt (200), Pt (220), and Pt (311) diffraction peaks. The result is highly consistent with those of standard Pt (JCPDSICDD card No. 04-0802), indicating that the face-centered cubic (fcc) metallic Pt nanoparticles were deposited into OMC using the scCO2 technique. The morphology and porous structure of Pt/OMC are characterized by TEM, which are shown in Fig. 2a and b. We can clearly see the highly ordered structure of OMC with the pore size of 5e6 nm, wherein the Pt nanoparticles were uniformly deposited, and no particle aggregation can be found. The higher magnification TEM image shown in Fig. 2c provides firm evidence that almost Pt nanoparticles were loaded into mesochannels of OMC. In addition, the average particle size of Pt nanoparticles is determined as 4.4 nm that is more or less smaller than the pore size of OMC (5-6 nm), implies that the Pt nanoparticles may be embedded within OMC. For comparison, Pt deposited onto the surface of OMC (Pt@OMC) sample was synthesized in the liquid phase. As shown in Fig. S2, the Pt nanoparticles can be found on OMC support. However, The Pt nanoparticles in Pt@OMC sample have much larger size and particle aggregations compared to Pt/OMC catalyst synthesized by scCO2 technique. Due to very few functional groups existing on the surface of OMC, the metal precursors are difficult to scatter and be adsorbed onto the OMC, which results in the particle aggregations and larger particle size. Even through these issues can be addressed by using the surfactants or acid treatment on OMC, the Pt nanoparticles are still very difficult to be dispersed into mesochannels of OMC in liquid phase because that the metal precursors cannot scatter uniformly into the mesochannels of OMC due to the high interfacial tension and viscosity of precursor solution. The results indicate the superior method of scCO2 technique for depositing Pt nanoparticles into OMC. We then deposited the bimetallic PtFe nanoparticles into OMC using scCO2 technique. There are two main reasons for choosing Fe as a second less noble metal to form the alloyed Pt. One is ORR activity of Pt catalyst can be significantly enhanced by being alloyed with Fe according to the previous work [26,27]; second is that it is difficult to fabricate the PtFe alloy nanoparticles via scCO2 technique and the harsh system conditions such as high temperature and pressure are needed due to the active metal of Fe. Therefore, the successful PtFe/OMC catalyst synthesized using our modified scCO2 technique under the relative experimental conditions would pave the way to design a variety of Pt-based/OMC materials and

Fig. 1. Synthesis mechanism of embedment PtFe within OMC by scCO2 technique. (A) Loading of metal precursors in reactor cell, (B) delivery of metal precursors into mesochannels of OMC, (C) removing the unembedded metal precursors by venting CO2, (D) reducing process, (E) washing process.

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Fig. 2. (aec) TEM images of Pt/OMC catalyst, inset of (c) is particle size distribution of Pt in the catalyst, (d) typical XRD pattern of the catalyst.

develop them in widespread applications. The PtFe/OMC catalysts with controllable composition and loading were obtained by scCO2 technique. In a typical synthesis of catalyst, PtFe/OMC catalyst with the Pt content of ~12.0 wt% was firstly prepared. Fig. 3a shows the EDX data of PtFe/OMC and Pt/ OMC catalysts. The Pt and Fe elements can be found in PtFe/OMC catalyst. According to the EDX data, the molar ratio between Pt and Fe is 11:12, and the Pt content in the catalyst is 12.4 wt%, which are close to ICP-AES data (Pt:Fe ¼ 49:50, Pt loading ¼ 12.0 wt%). The crystalline structure of PtFe/OMC catalyst is firstly characterized by XRD. As shown in Fig. 3b, the PtFe/OMC catalyst has the similar diffraction patterns compared with that of Pt/OMC catalyst, implying that the PtFe nanoparticles synthesized by scCO2 technique have fcc structure. By the detailed observation of the diffraction patterns, XRD peaks for PtFe/OMC catalyst obviously split to a higher angle comparing to Pt/OMC catalyst, which corresponds to the compression of the lattice by induced Fe. Furthermore, there is no any major peak of Fe can be found for PtFe/OMC catalyst, implying that Fe was alloyed into the particles. On the basis of XRD pattern, the crystalline size of PtFe nanoparticles in Pt2Fe3/ OMC catalyst was calculated using Scherer equation (1) [28]:

L¼

0:9lKa1 B2q cos qmax

(1)

where L is the mean size of crystalline PtFe nanoparticle, lKa1 is the X-ray wavelength (0.154 nm), B2q is the half-peak width, and qmax is the angle of the peak. After calculating according to the equation, the crystalline size of PtFe in PtFe/OMC catalyst is 1.6 nm. Besides,

XPS was also employed to further characterize the composition of PtFe nanoparticles in the catalyst. As shown in Fig. 3c, the Pt 4f XPS spectrum of Pt/OMC catalyst exhibits two pairs of doublets at 71.5 eV and 74.8 eV, which are attributed to Pt0 4f7/2 and Pt0 4f5/2, respectively, suggesting that the Pt nanoparticles in Pt/OMC catalyst are in metallic form [23]. However, the Pt 4f XPS spectrum peaks of PtFe/OMC catalyst have a positive shift of 0.4 eV, implying that the PtFe nanoparticles in catalyst have alloyed structure. The Fe 2p XPS spectrum of PtFe/OMC catalyst can be clearly seen in Fig. 3d. The Fe 2p XPS spectrum could be deconvoluted into double peaks: one is Fe 2p3/2 (ca. 708.7 eV); another is Fe 2p1/2 (ca. 720.7 eV). The Fe 2p3/2 XPS peak is further deconvoluted into two pairs of doublets at 708.4 eV and 711.4 eV, which are assigned to metallic Fe (0) and oxidized Fe (FeOx). However, no FeOx XRD peaks can be observed in XRD pattern of Pt2Fe3/OMC catalyst. We consider that the FeOx is probably very little or is in noncrystalline oxidative states formed on the surface and subsurface of ultrafine PtFe nanoparticles. The XRD and XPS results prove that alloyed PtFe nanoparticles with typical fcc structure are successfully deposited into OMC using the scCO2 technique. The morphologies of PtFe/OMC products are characterized by TEM. The TEM images of PtFe/OMC catalyst with Pt content of ~12.0 wt% can be seen in Fig. 4c and d. The highly dispersive PtFe nanoparticles can be observed in OMC support. The PtFe nanoparticles have the ultrafine particle size of 1.5 nm that is close to XRD result, and narrow particle size distribution (Fig. 4d). Remarkably, the PtFe nanoparticles are mostly embedded in the mesochannels of OMC instead of dispersed out of the pore carbon walls. We then synthesized the PtFe/OMC with different Pt contents

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Fig. 3. (a) EDX and (b) XRD patterns, (c) Pt 4f XPS spectra of Pt/OMC and PtFe/OMC catalysts, and (d) Fe 2p XPS spectrum of PtFe/OMC catalyst.

Fig. 4. The typical TEM images of PtFe/OMC catalysts with different Pt contents (a,b) 5.4 wt%, (c,d) 12.0 wt% and (e,f) 25.6 wt%, the insets in (b), (d) and (f) are corresponding particle size distributions, (g) HAADF-STEM and HR-STEM images, (h) EDP of PtFe/OMC catalyst with Pt content of 25.6 wt%.

using scCO2 technique. Fig. 4a and b shows the TEM images of PtFe/ OMC with lower Pt content of 5.4 wt%. The most of the PtFe nanoparticles are also clearly embedded in mesochannels of OMC. The particle size decreases to 1.3 nm. Fig. 4e and f shows the TEM images of PtFe/OMC with higher Pt content of 25.6 wt%. Due to the high nanoparticle loading, the PtFe nanoparticles not only disperse within mesochannels, but also aggregate out of the pore walls. The particle size increases to 2.3 nm. The results show that particle

loading in OMC can be easily controlled by adjustment of metal precursors with scCO2 technique. In our experiments, in order to load the most particles into carbon walls, the optimal Pt content in PtFe nanoparticles is probably around 12.0 wt%. The morphology and crystalline structure of PtFe/OMC catalyst are further characterized by the atomic resolution STEM, performing using high-angle annular dark field (HAADF). The STEM image of PtFe/OMC catalyst with Pt content of ~25.6 wt% is shown

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in Fig. 4g. The PtFe nanoparticles evenly distribute in OMC support, which is similar to TEM image of Fig. 4f. The HR-STEM images of PtFe nanoparticles are also shown in the inset of Fig. 4g. The lattice fringes of PtFe nanoparticles can be clearly seen, which exhibits the good crystallinity of PtFe nanoparticle. In addition, the spacing of the adjacent fringes presented in HRSTEM image is measured with a value of around 2.2 A, which is attributed to the {111} interplane distance of fcc PtFe. The EDP was also employed to characterize the alloyed structure of PtFe nanoparticles. Both EDP peaks of OMC and PtFe nanoparticles can be found in Fig. 4h. For PtFe nanoparticles, the ring diffraction spots can be attributed to (111), (200), (220) and (311) planes, which indicate the fcc PtFe. Although the scCO2 has been developed to synthesize the metal and alloy nanoparticles, it is still difficult to obtain the alloyed Pt with late transition 3d metal and high temperature is necessary [29e31]. In this paper, the borane-THF that has the high solubility in scCO2 and strong reducing activity can reduce the metal precursors into the nanoparticles in the modest temperature. Besides of the controllable particle loading, the particle composition can be also adjusted. By this superior scCO2 technique, the Pt3Fe/OMC (~13.0 wt%Pt), PtPd/OMC (12.3 wt%Pt) (Fig. S3) and PtCu/OMC (12.5 wt%Pt) (Fig. S4) were also successfully synthesized, which further represents the scCO2 technique to be the powerful and large-scale manufacturing method to explore the new porous materials. The ultrafine particle size, controllable content and composition suggest that the scCO2 technique may provide the way to create the active electrocatalysts. The porous structure of pure OMC, PtFe/OMC (5.4 wt%) and PtFe/ OMC (12.0 wt%) was analyzed by N2 adsorption/desorption measurement, as shown in Fig. 5. The typical N2 adsorption/desorption isotherms of OMC without particles, PtFe/OMC catalysts with Pt contents of 5.4 wt% and 12.0 wt% can be seen in Fig. 5a. The clear type IV N2 sorption isotherms and H2 hysteresis loops can be obtained for all the samples. The pore size distributions were obtained using the Barret-Joyner-Halenda (BJH) theory, which are shown in Fig. 5b. We can see the uniform mesopores centered at ~5.3 nm for pure OMC. In contrast, the PtFe/OMC products exhibit the smaller mesoporous sizes: ~4.4 nm for PtFe/OMC (5.4 wt%) and to ~4.0 nm for PtFe/OMC (12.0 wt%). The decrease of mesopore size of OMC might be attributed to the embedment of PtFe nanoparticles into the carbon walls, which is consistent with our initial conception. The embedment of nanoparticles also causes the lower total pore volume and specific surface area of PtFe/OMC: 0.78 cm3 g
1, 815.8 m2 g
1 for PtFe/OMC (5.4 wt%) and 0.68 cm3 g
1, 728.7 m2 g
1 for PtFe/OMC (12.0 wt%), compared to the pure OMC (1.2 cm3 g
1, 920.2 m2 g
1). The understandable microporosity in the PtFe/OMC product can be formed due to the embedment of PtFe nanoparticles into carbon walls. The as-prepared PtFe/OMC products that have the integration of highly-dispersive and spatially embedded PtFe nanoparticles in mesochannels of OMC, regular mesopores as well as high specific surface area are respected as an efficient electrocatalyst for ORR, exhibiting the high electrocatalytic activity and good durability. The cyclic voltammetry is firstly employed to characterize the electrocatalytic properties. The typical CVs of the PtFe/OMC catalyst with Pt content of 12.0 wt% and Pt3Fe/OMC catalyst are shown in Fig. 6a. The Pt/OMC and commercial Pt/C catalysts are also provided for comparisons. We can see the typical current response occurred in the potential range of 0.05e0.45 V is attributed to hydrogen adsorption/desorption processes. The Pt/OMC, PtFe/OMC and Pt3Fe/ OMC catalysts with OMC as supports have much larger doublelayer capacitances than that of commercial Pt/C catalyst because of the larger specific surface area and more oxygenated surface groups of OMC support than the activated carbon [32]. The electrochemically active surface areas (ECSAs) of as-prepared catalysts

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Fig. 5. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of pure OMC, PtFe/OMC with Pt contents of 5.4 wt% and 12.0 wt%.

were calculated using the hydrogen adsorption/desorption in CVs. The ECSA is 65.0 A mg
1 Pt for Pt/OMC catalyst, which is a little smaller than that of commercial Pt/C catalyst (70.0 A mg
1 Pt ). The smaller ECSA of Pt/OMC catalyst might be attributed to the larger Pt nanoparticle size (4.4 nm) compared with commercial Pt/C (2e3 nm). By introducing Fe, the ECSAs are up to 86.0 A mg
1 Pt and 80.0 A mg
1 Pt for PtFe/OMC and Pt3Fe/OMC catalyst, respectively. The increased ECSAs indicate the more active sites existing in catalysts, and PtFe/OMC catalyst is most active. ORR measurements were performed using a GC RDE in O2saturated 0.1 M HClO4 solution. As shown in Fig. 6b, the ORR polarization curves for PtFe/OMC and Pt3Fe/OMC catalysts are recorded at a scan rate of 10 mV s
1 and rotation speeds of 1600 rpm. The Pt/OMC catalyst and commercial Pt/C catalyst were used for comparisons. There are two distinguishable potential regions can be clearly seen from ORR polarization curves. One is attributed to diffusion-limiting current region (below 0.6 V); the other is attributed to the mixed kinetic diffusion control region (0.6e1.1 V). Specifically, the Pt/OMC catalyst and commercial Pt/C catalyst show the similar onset and half-wave potentials, indicates that the electrocatalytic activity Pt/OMC catalyst toward ORR is comparable with commercial Pt/C catalyst. However, Pt size in Pt/OMC catalyst is much larger than that of in Pt/C catalysts, which means the OMC is superior support for catalyst compared to the activated carbon due to the mesoporous structure. By introducing the Fe, the Pt3Fe/ OMC and PtFe/OMC catalysts show the significant enhancement in electrocatalytic activity compared with Pt/OMC catalyst and

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Fig. 6. Electrochemical performances of Pt/OMC, PtFe/OMC, Pt3Fe/OMC, and commercial Pt/C catalysts. (a) CV, (b) ORR polarization curves, (c) ORR polarization curves of PtFe/OMC with Pt content of 12.0 wt% catalyst recorded in O2-saturated 0.1 M HClO4 electrolyte at 225-1600 rpm, and (d) The detailed ORR polarization curves at 225-625 rpm from (c).

commercial Pt/C catalyst. Specifically, the Pt3Fe/OMC and PtFe/OMC catalysts have the corresponding 19 mV and 26 mV higher onset potentials compared with that of commercial Pt/C catalyst. The half-wave potentials of E1/2 were also determined to further evaluate the ORR activity of catalysts. The E1/2 values of Pt3Fe/OMC and PtFe/OMC catalysts are 0.880 V and 0.895 V, which are 13 mV and 28 mV higher compared with that of commercial Pt/C catalyst (0.867 V), respectively. According to the electrocatalytic activities of Pt/OMC, Pt3Fe/OMC and PtFe/OMC, we can see that the ORR activity is affected significantly by the atomic ratio between Pt and Fe in PtFe alloys. The PtFe/OMC shows the most active among the catalysts. Fig. 6c presents the ORR polarization curves of PtFe/OMC catalyst recorded at various rotation rates. There is an obvious current peak in the ORR polarization curve of such PtFe/OMC catalysts at 225 rpm, which is similar to the work reported by Wu [21]. It is probably because the O2 molecules did not diffuse deep into the mesoporous carbon walls due to the low rotation rate. However, the mesoporous size of PtFe.OMC (~12.0 wt%) catalyst is ~4.0 nm according to the N2 adsorption/desorption measurement, which does not affect the transfer of O2 into the carbon walls. And this phenomenon graduates away with accelerating the rotation rate, as shown in Fig. 6d. The results demonstrate indirectly that the most of PtFe nanoparticles have been embedded into carbon walls. In order to further evaluate the ORR electrocatalytic activity, the intrinsic ORR activities of all the catalysts were performed and compared. The kinetic current can be calculated using the wellknown mass-transport correction from the Levich-Koutecky equation (equation S1) according to the ORR polarization curves [23]. Fig. 7a shows the corresponding Tafel plots of catalysts. We can see clearly that the Pt/OMC catalyst has comparable electrocatalytic activity with commercial Pt/C catalyst. The Pt3Fe/OMC and PtFe/OMC catalysts have the significant improvement in ORR activities compared to commercial Pt/C catalyst. The specific activities of catalysts calculated at three different potentials are shown in Fig. 7b. Specifically, the specific activities of Pt3Fe/OMC catalyst (0.4 mA cm
2 at 0.90 V, 1.25 mA cm
2 at 0.85 V and 2.63 mA cm
2 at 0.80 V) are much higher than those of Pt/OMC catalyst

(0.22 mA cm
2 at 0.90 V, 0.85 mA cm
2 at 0.85 V and 2.28 mA cm
2 at 0.80 V) and commercial Pt/C catalyst (0.16 mA cm
2 at 0.90 V, 0.67 mA cm
2 at 0.85 V and 1.64 mA cm
2 at 0.80 V). The PtFe/OMC catalyst shows the highest activity of 0.57 mA cm
2 at 0.90 V, 2.33 mA cm
2 at 0.85 V and 3.9 mA cm
2 at 0.80 V, which are 2.4e3.7 times higher than those of commercial Pt/C catalyst. Moreover, the mass activities of catalysts show the similar trends with the specific activities, which can be seen in Fig. S5. In particular, the most active PtFe/OMC shows the much higher specific
1 activities of 0.49 A mg
1 Pt at 0.90 V, 2 A mgPt at 0.85 V and 3.35 A
1 mgPt at 0.80 V compared with those of commercial Pt/C catalyst
1
1 (0.11 A mg
1 Pt at 0.90 V, 0.47 A mgPt at 0.85 V and 1.15 A mgPt at 0.80 V). The reducing Pt loading on cathode without loss of electrocatalytic performance is an important strategy to reduce the catalyst cost and promote the commercialization of fuel cells [4]. The mass activities of PtFe/OMC catalysts with different Pt contents of 5.4 wt%, 12.0 wt% and 25.6 wt% were demonstrated, which can be seen in Fig. S6. Specifically, the limiting apparent mass activity of PtFe/OMC is high to ~800 mA mg
1 Pt when its Pt content in OMC is 5.4 wt%. With increasing Pt content, the mass activity is decreasing. However, even though the Pt content in the OMC is higher to ~25.6 wt%, a high limiting apparent mass activity value of ~240 mA mg
1 Pt can still be achieved. The superior mass activity compared with those of many reported catalysts indicates the PtFe/OMC catalyst prepared using scCO2 technique has great potential in cathodic catalyst for fuel cell [21,33,34]. The durability of PtFe/OMC catalyst in the electrochemical environment was also studied using an ADT and compared with commercial Pt/C. The PtFe/OMC catalyst was first pre-treated through 200 cycling scans between 0.05 and 1.2 V, which is sufficient time to allow the O2 molecules diffuse into the carbon walls. And then, CVs and ORR polarization curves were recorded after 10000 potential cycling scans. Fig. 8a and b show the CVs of commercial Pt/C and PtFe/OMC catalysts before and after ADT. We can see clearly that 10000 cycles cause both commercial Pt/C and PtFe/ OMC catalysts loss of electrocatalytic activity. But, the PtFe/OMC

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Fig. 7. Intrinsic electrocatalytic activities of Pt/OMC, Pt3Fe/OMC, PtFe/OMC and commercial Pt/C catalysts. (a) The corresponding Tafel plots and (b) specific activities at three different potentials.

Fig. 8. The durability of catalysts before and after ADT. CVs for (a) commercial Pt/C catalyst and (b) PtFe/OMC catalyst, ORR polarization curves for (c) commercial Pt/C catalyst and (d) PtFe/OMC catalyst.

catalyst is more stable. Specifically, the ECSA degradation of commercial Pt/C and PtFe/OMC catalysts are about 52.2% and 21.3%, respectively. As shown in Fig. 8 c and Fig. 8d, the commercial Pt/C catalyst exhibits 34 mV negative potential shifts after ADT, while PtFe/OMC catalyst shifted only 17 mV. The results demonstrated that the PtFe/OMC catalyst has excellent durability in the electrochemical environment. The ordered mesoporous carbon, as above mentioned has a great potential in the design and fabrication of fuel cell catalyst because of its high specific surface area and porous structure. Currently, as with the most of the existing methods, Pt-based nanoparticles were deposited onto the surface of mesoporous carbon (Table 1, for instance). Few methods can be developed for embedment of Pt-based nanoparticles into the mesoporous carbon. In addition, many OMC supported Pt-based catalysts were used as anodic catalysts such as methanol oxidation reaction (MOR) or formic acid oxidation reaction (FAOR), but the electrocatalytic activity for ORR still needed to be improved. After carefully comparison, our PtFe/OMC catalyst prepared using scCO2 technique is a

combination of high ORR electrocatalytic activity and durability through control of the particle loading, alloying composition, holding a great promise in cathodic catalyst for PEMFCs (Table 1). Furthermore, scCO2 method described in this work is a superior method for embedment of Pt-based in mesoporous carbon due to relative simple procedures, lower temperature and environmentally friendly compared with current methods. However, there is still a big gap on the electrocatalytic performances between out PtFe/OMC catalyst and the state-of-the-art ORR catalysts such as Pt3Ni/C nanoframes [40] and octahedral Pt3Ni/C catalysts [41]. We consider that the ORR activity and durability of PtFe/OMC catalysts can be further enhanced by incorporating the third element, such as Ni, Co etc. And we will continue to do the work on this way. 4. Conclusions We have demonstrated a highly efficient and large-scale manufacturing method to embed the alloyed PtM (M ¼ Fe, Pd and Cu) into OMC using the scCO2 technique. The organometallic

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Table 1 Comparisons between catalysts of Pt-based supported by porous carbons.a Catalyst

Processing

Particle size (nm)

ECSA (m2 g
1)

Performance

Ref

Pt/GMC Pt50Ru50/CMM PtRu20/TFC PtPb-OMCS Pt@GC Pt@CNX/CNT PtFe/OMC

Phase-transfer Hard template Wet chemical reduction Atom transfer radical polymerization Hard template Polymerization, pyrolysis, reduction scCO2

3.37 ± 0.65 2e3 4.3 ± 0.8 13.1 4.0e7.5 3.12 ± 0.87 1.5 ± 0.3

69.00 / 70.89 / / 74.29 86.00

ORR MOR MOR FAOR ORR ORR ORR

[35] [36] [37] [38] [21] [39] This work

a GMC: ordered graphitic mesoporous carbon; CMM: carbon mesoporous material; TFC: ordered mesoporous carbon thin film; OMCS: ordered mesoporous carbon silica; GC: ordered mesoporous carbon; CNx/CNT: N-doped porous carbon/carbon nanotube.

precursors can dissolve into the scCO2 and be delivered into the mesochannels of OMC due to scCO2's low viscosity, zero surface tension and high diffusivity properties. After reduction reaction by borane-THF, the embedded PtM nanoparticles with ultrafine sizes, high dispersion and controllable loadings and compositions can be achieved. Specifically, the PtFe/OMC catalysts have the significant enhancement in electrocatalytic activities and durability toward ORR compared with commercial Pt/C catalysts. After careful comparison, the PtFe/OMC catalyst with Pt content of 12.0 wt% has the better electrochemical performance compared with most of reported OMC supported Pt-based catalysts. This work demonstrate several advantages: (1) OMC as supports are good to form the small and uniform size nanoparticles that is beneficial to the high electrocatalytic activity; (2) carbon walls can protect the nanoparticles from detachment/aggregation and dissolution in electrochemical environment, maintaining the stable catalytic activity; (3) the successful synthesis of such attractive materials paves the way to explore the new porous materials with some interesting properties by embedment different nanoparticles.

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Acknowledgements J.Y., X.C. and Y.Z. gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant 51572114, 51672112 and 51702129). Y.L. and D.D. thank the Washington State University (WSU) start-up grant for financial support. We also thank Franceschi Microscopy & Image Center at WSU for TEM measurements.

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Appendix A. Supplementary data

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Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.01.053.

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