Proton exchange membrane fuel cells (PEMFCs) are potential energy conversion power devices with the advantages not only of using renewable and eco-friendly energy sources (e.g., hydrogen or methanol) as fuel but are also highly efficient. However, the majority of current PEMFCs use noble metals (e.g., Pt, Ru) supported on activated carbons as the electrocatalyst at both the anode and cathode, which limits mass production and cost and jeopardizes their commercialization [1, 2, 3, 4, 5, 6, 7]. Thus, the research and development R&D on the electrocatalyst for fuel cells directed towardsdecreasing or avoiding the utilization of a noble metal is a needed task.
The electrocatalysts for the methanol oxidation reaction (MOR) at the anode of DMFCs normally utilize Pt-based bifunctional noble catalysts supported on activated carbons or carbon porous materials (CPMs) [8, 9, 10, 11, 12, 13].The incorporation of a second noble metal (e.g., Ru) was found to enhance the structural, electronic, and chemical properties of the supported catalyst. Indeed, supported PtRu/C catalysts were found to show improved stability and tolerance to CO poisoning, hence, exhibiting superior electrocatalytic activity for the MOR [10, 11, 12, 13]. Nonetheless, severe Ru metal dissolution and the cross-over of the fuel (methanol) through the polyelectrolyte membrane have been observed with most anodic PtRu/C catalysts during the longterm operation of DMFCs [14, 15, 16]. Thus, further investigation on the stability of the supported PtRu/C electrocatalyst remains an urgent task. In addition, from the viewpoints of the cost effectiveness of the membrane electrode assembly (MEA) [17, 18, 19] and overall performance of the PEMFC, the further reduction in production cost while maintaining a satisfactory MOR activity at the anode are critical issues that can benefit both fundamental research and practical industrial applications.
Due to their unique high surface area, tunable meso-scale pore size, and tailorable surface functionalities, which give good molecular transport and enhanced chemical properties, ordered mesoporous carbons (OMCs) synthesized by casting from a hard or soft silica template [20, 21, 22, 23, 24] have drawn considerable research attention. They have been widely applied for various applications such as catalyst support, adsorbent, sensors, and electrode matrices [20, 21, 22, 23, 24, 25, 26]. For examples, OMCsand CPMs have been exploited as the electrode support for Pt-based electrocatalystsfor DMFCs/PEMFCs [9, 10, 11, 27, 28, 29, 30, 31, 32, 33, 34] and biomolecular sensors [25, 35, 36, 37]. Aside from optimizing the performance of the Pt-based electrocatalysts in fuel cells, intensive R&D attention has focused on reducing the loading of the precious metal or replacing it with a non-noble metal [38, 39, 40, 41, 42] or metal-free [43, 44, 45, 46, 47, 48, 49] catalyst.
Previously, we reported a direct procedure to prepare well dispersed, highly stable mono-(Pt) and bimetallic (PtRu) metal nanoparticles (NPs, 2-3 nm) on OMCs using SBA-15 as the template, furfuryl alcohol (FA) and trimethylbenzene (TMB) as the primary carbon source, and platinum and/or ruthenium acetylacetonate as the second metal and (secondary) carbon precursor [9, 10]. These Pt- and PtRu-OMC catalysts were found to possess excellent electrocatalytic properties and stability that surpassed those of common commercial catalysts for the MOR, and can have practical and cost effective applications in PEMFCs/DMFCs. Here, the above direct replication method was employed to prepare a series of bifunctional Pt-based PtM-OMCs, where the second metal was Ru or a transitionmetal such as Fe or Mo. The structure, composition, morphology, and textural properties of the OMC-supported PtM (M = Ru, Fe, Mo) alloy NP materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS), and N2 adsorption-desorption. Their electrocatalytic properties were assessed by cyclic voltammetry (CV) measurements. The PtM-OMC electrodes showed excellent methanol oxidation activity and stability, especially the PtFe-OMC catalyst, which has good perspective practical applications in DMFCs due to its cost effectiveness.
The Pt-based OMC-supported bifunctional catalysts (PtM-OMC; M = Ru, Fe, Mo) were prepared by a direct replication procedure reported elsewhere [9, 10]. In brief, SBA-15 was first synthesized based on the receipt reported by Zhao et al. [50]. Then, the as-synthesized SBA-15 sample was subjected to a dehydration treatment at 423 K under vacuum for 12 h before it was used as the hard template. The direct replication synthesis was carried out by mixing known amounts of the primary metal precursor, i.e., platinum (II) acetylacetonate (Pt(CH(COCH3)2)2, denoted as Pt(acac)2) and a second metal (M = Ru, Fe, Mo) precursor, viz., ruthenium (III) acetylacetonate (denoted as Ru(acac)3), iron (III)acetylacetonate (denoted as Fe(acac)3), or molybdenum (IV) dioxy acetylacetonate (denoted as MoO2(acac)2), in the primary carbon sources, namely, FA and TMB, under ultrasonication. To initiate the polymerization of the carbon precursors, a desired amount of oxalic acid was used as the catalyst. In this context, the two metal (Pt and M) acetylacetonates also served as the secondary carbon sources. Subsequently, the mixed solution was used to impregnate 0.5 g of dehydrated SBA-15 at room temperature by incipient wetness impregnation. This was followed by polymerization under ambient condition at 333 K, then at 353 K, both for 16 h. Typically, the carbonization procedure was performed under a N2 environment, first at 423 K for 2 h, followed by increasing the temperature to 573 K with a ramped rate of 1 K/min, then, increased to 1073 K with a rate of 5 K/min and maintained at the final temperature for an additional 4 h. The resulting silicon-carbon (Si-C) composite was etched by HF (1wt%) aqueous solution for 24 h to remove the silica template, then thoroughly washed with a mixture of distilled water and alcohol, and dried at 373 K to obtain the final products, which were denoted as Pt-OMC (without the second metal) and PtM-OMC (M = Ru, Fe, Mo).
The structural integrity of the various samples was examined by small and large angle XRD measurements performed on a PANalytical X’Pert PRO instrument. In addition, their textural properties were monitored by N2 adsorption-desorption isotherms measured at 77 K using a Quantachrome Autosorb-1 physisorption apparatus. The specific surface area (SBET), total pore volume (Vtot), and pore size (dBJH) were derived using the conventional BET and BJH methods. TEM images of the catalyst samples were recorded by a JEOL JEM-2100 instrument operating at 200 kV. The metal composition in each sample wasdeterminedby inductively coupled plasma-mass spectroscopy (ICP-MS; PE-SCIEX ELAN 6100 DRC).
X-ray absorption near edge structure(XANES) and EXAFS spectra of the metal-supported OMCs were recorded at Beamline 01C1 at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The electron storage ring was operated at 1.5 GeV and 300 mA. A double Si(111) crystal monochromator was employed for energy selection with a resolution (ΔE/E) better than 1 × 10-4 at the LIII-edge for Pt and K-edge for the second metal species (M = Ru, Fe, Mo). The calibration of the XANES and EXAFS spectra were made by recording the spectra of standard compounds such as Pt, Ru, Fe, or Mo metal foil. The EXAFS function in energy space, c(E), was obtained by subtracting the post-edge background from the raw absorption data and followed by normalization with respect to the edge jump step. Then, the normalized c(E) was transformed from energy space to k space (k denotes the photoelectron wave vector). All data in the c(k) function were multiplied by a factor k2 to compensate for the damping of EXAFS oscillations in the high k region. Subsequently, the k2-weighted c(k) data, which ranged from 3.6 to 12.5 Å-1 for the Pt LIII-edge and 3.6 to 11.6 Å-1 for the Ru and Mo K-edges, were Fourier transformed to r space to get the EXAFS contributions from the various coordination shells. A nonlinear least squares algorithm was applied to fit the EXAFS spectra in r space in the absence of phase correction in the range of 1.7-3.2 Å for Pt and 1.5-3.2 Å for Ru and Mo, respectively. All computer programs were implemented by a UWXAFS 3.0 package [51] with the backscattering amplitude and phase shift for the specific atom pairs calculated by the FEFF7 code [52].
To probe the electrocatalytic properties of the Pt-OMC and PtM-OMC catalysts, CV measurements were conducted on a galvanostat/potentiostat (Autolab, PGSTAT30) equipped with a rotating disk electrode (RDE). This was operated with three electrodes, namely, the Ag/AgCl reference electrode, the working electrode, and the counter electrode using a Pt wire. Prior to each run, the glassy carbon thin film electrode was polished to a mirror finish with a 0.05 mm alumina suspension. The catalyst ink was prepared by adding 5 mg of Pt- or PtM-OMC sample into 2.5 mL of deionized water, followed by ultrasonic treatment for 0.5 h. Subsequently, 20 μL of the resulting suspension mixture was withdrawn and pipetted onto the glassy carbon electrode, followed by drying in air at 333 K for 0.5 h. Finally, 7.5 μLof 1% Nafion (DuPont) solution was added as a binder under a N2 environment. Electrooxidation of methanol (MeOH) was carried out at room temperature (298 K) in an electrolyte solution consisting of 0.5 mol/L H2SO4 and 1 mol/L MeOH. The data were recorded in the potential range between -0.2 and 1.0 V with a typical scan rate of 10 mV/s. Prior to each CV measurement, the electrolyte solution was purged with N2 (99.9% purity) for at least 0.5 h to remove dissolved O2. Then, the experiment was conducted under purging N2 condition with an electrode rotating speed of 1600 r/min. For comparison, the CV measurement of a commercial PtRu/C catalyst (Johnson-Matthey, 20 wt%Pt and 10 wt%Ru supported on Vulcan XC72 carbon black, hereafter denoted as PtRu/XC-72 ), were also recorded.
Figure 1 displays the small-and large-angle XRD patterns of the pristine OMC (without metal loading), monometallic Pt-OMC, and various bimetallic PtM-OMC catalysts. As shown in Fig. 1(a), the small-angle XRD pattern of pristine OMC exhibited wellresolved diffraction peaks at 2q = 1.0°, 1.6°, and 1.9°, which were indexed to the (100), (110), and (200) reflection, respectively, corresponding to the two dimensional (2D) hexagonal space group (p6mm) resembling the mesoporous CMK-3 carbon material [53, 54]. Except for the marginal shift of the (100) peak towards lower 2q angles, similar lowangle XRD patterns were observed for the Pt-OMC and other PtM-OMC catalysts. This indicated that the mesoporous structure remained unchanged even in the presence of metal NPs. Unlike the large angle XRD pattern of the pristine OMC, which revealed two main diffraction peaks at 2q = 25° and 44° from amorphous carbon (Fig. 1(b)), the Pt-OMC sample showed additional peaks at 2q = 39°, 47°, 68°, and 82°, which were the (111), (200), (220) and (311) diffraction of crystalline Pt(0) metal with a face centered cubic (fcc) lattice. On the other hand, slight shifts in the diffraction peaks for most of the PtM-OMCs were observed, indicating changes in their lattice constants from the alloyed PtM NPs in the OMC support. This assignment is supported by the fact that no characteristic diffraction peak for Ru, Fe, Mo, and their metal oxides [10, 56, 57, 58, 59, 60] was observed. Due to the substitution of Pt by a smaller second metal (M = Ru, Fe, Mo), the PtRu, PtFe, and PtMo alloys as supported catalysts showed ananticipated reduction in particle size (Dp) [10, 55], as deduced from the Scherrer formula and the data (Table 1).
In terms of their textural properties, all samples showed a well defined hysteresis loop in their N2 adsorption-desorption isotherm (see Fig. S1(a) in the Supporting Information, hereafter denoted as SI†), which is typical for ordered mesoporous materials. All isotherms were Type IV according to IUPAC classification. Accordingly, their SBET, Vtot and dBJH deduced from the adsorption branch of the isotherms are depicted in Table 1 along with the metal loading ratio (M/Pt) deduced from ICP-MS. Clearly, the pristine OMC, Pt-OMC, and PtM-OMC catalysts all possessed high surface areas (900 m2/g) and high pore volumes (0.70 cm3/g) with a uniform pore size distribution (PSD; see Fig. S1(b); SI†). However, notable decreases in the values of SBET, Vtot, and dBJH for Pt-OMC and PtM-OMCs compared to the pristine OMC were observed, which were due to the metal/alloy NPs in the mesopore channels of the carbon support. Nonetheless, the small variation in the pore sizes reflected that no serious pore blocking occurred in the support. We hypothesized that the well-dispersed alloyed metal NPs were likely partially embedded or studded in the pore walls of the OMC [9, 10], which may be responsible for the extraordinary stability observed (vide infra).
The structure, morphology, and pore structure of the catalysts can be inferred from the TEM results, as shown in Fig. 2 and S1(c) (SI†). Large domains of ordered mesoporous channel sembedded with PtM alloy (or Pt) NPs with typical sizes of 2-3 nm were evident. Again, this was due to the direct replication synthesis method employed for the preparation of these supported catalysts. The nature and sizes of the metal/alloy NPs buried in the Si-C composites remained practically unchanged after the removal of the silica hard templateby acid leaching. As a result, these metal/alloy NPs, which were confined in the pore channels of the carbon skeleton (average dBJH~ 3.0 nm; see Table 1), were highly dispersed and most likely partially embedded or studded on the inner pore surfaces of the OMC. These results obtained from the TEM analysis were in excellent agreement with those from the XRD and physisorption studies.
To study the electronic states and atomic environments of the alloyed PtRu, PtFe, and PtMo NPs in the OMCsupport, additional experiments by X-ray absorption spectroscopy (XAS) were performed using a synchrotron radiation light source. The absorption spectra exhibited fine structures both above and below the corelevel absorption edges. In particular, the near edge (XANES) spectra provide information on the excitation of a core electron to quasi-bound states, and the valence of the central atom can be determined from the edge position [9, 61]. The normalized XANES spectra at the Pt LIII-edge of Pt-OMC and the PtM-OMC catalysts are displayed in Fig. 3 along with that of the model compound (Pt foil). The absorption peaks near 11567 eV, namely, the white lines, were mainly the result of electron transition from the 2p3/2 band to the 5d band of the Pt metal. The corresponding absorption peak intensity reflected the d-orbital occupancy. That the intensities of the absorption peak maxima in the white lines region for the PtM-OMC catalysts were comparable with that of the Pt foil (Inset; Fig. 3) indicated their similar local electronic structure and oxidation state of Pt. This indicated the presence of the nearly fully reduced state of Pt in the supported alloy catalysts. Likewise, similar absorption peak intensities were observed near the white lines region of the normalized XANES spectra at the Ru K-edge of PtRu-OMC compared to that of the reference Ru foil (Fig. S2(a); SI†),confirming the existence of reduced Ru in the alloyed NPs. On the other hand, notable differences in the absorption curves at the Mo K-edge of PtRu-OMC and Mo metal foil were observed (Fig. S2(b); SI†), revealing the occurrence of the partial oxidation state of Mo. There was no detectable XANES spectrum at the Fe K-edge, probably due to the insufficient energy of the beam source.
Based on the k2-weighted EXAFS spectra at the Pt LIII-edge of Pt foil, Pt-OMC, and PtM-OMC catalysts (Fig. 4) together with the Ru and Mo K-edge spectra (not shown; also, the spectrum at the Fe K-edge was not detected) and Fourier transforms, the parameters of the coordination number (N), bond distance (R), Debye-Waller factor (σ2), and inner potential shift (ΔEo) were deduced and summarized in Table 2. The weaker intensities in the EXAFS oscillations observed at the Pt LIII-edge for the PtM-OMCs compared to Pt-OMC and Pt foil indicated a smaller cluster domain size of the second metal (M) compared to Pt. This may be inferred from the smaller coordination number (N) of the homometallic coordination shell (Pt-Pt) observed with the PtM-OMCs compared to Pt-OMC (Table 2). In addition, a preferred bonding for homometallic (Pt-Pt) over heterometallic (Pt-M) coordination was also observed, as revealed by the larger differences in N, indicating the occurrence of aggregation of Pt atoms prior to the formation of alloy PtM NPs upon carbonization treatment. All the PtM-OMC catalysts exhibited a lower sum of nearest neighbors around the absorbing Pt (ΣNPt-i= NPt-Pt + NPt-M; Table 3) compared to the commercial PtRu/XC-72 catalyst (ΣNPt-i = 8.9), in good agreement with the observed decreases in Dp for PtM-OMCs compared to Pt-OMC deduced from the XRD results (Table 1).
The extent of alloying in the PtM NPs and their corresponding intra-particle dispersion of Pt and M in the PtM-OMC catalysts were further examined using a quantitative approach developed by Hwang and co-workers [62, 63]. The relative structural coordination and quantitative parameters, namely Pobs, Mobs, ΣNPt-i, ΣNM-i, JPt, and JM derived are summarized in Table 3. The value observed for JM being higher than JPt in each PtM-OMC (M = Ru, Mo; corresponding values for Fe were unavailable) catalyst was consistent with a core-shell structural model of a bimetallic nanocluster with Pt in the inner core and M (second metal) in the outer shell [62]. This picture was supported by the results obtained from the above coordination number analysis.
The electrocatalytic performance of the PtM-OMCs as anodic catalysts for the MOR was evaluated by electrochemical analysis using CV with a RDE rotating speed of 1600 r/min 0.5 mol/L H2SO4 and 1.0 mol/L MeOH electrolyte solution at room temperature. The CV profiles observed for Pt-OMC and the PtM-OMC modified electrodes exhibited the typical voltammetric characteristics for the electrooxidation of methanol, as shown in Fig. 5(a). Typically, a methanol oxidation peak at 0.6 V was observed in the curve recorded during the forward scan, while an anodic peak at 0.25-0.35 V was evident during the reverse scan. The latter can be attributed to the removal of the incompletely oxidized carbonaceous species formed during the forward scan [9, 10, 64, 65, 66, 67]. As depicted in Table 4, a higher current density was observed for the supported bimetallic PtM-OMC catalysts compared to the monometallic supported Pt-OMC. This indicated that the incorporation of a second metal indeed led to significant enhancement in MOR activity. For example, the anodic peak current (If) observed for the PtRu-OMC catalyst was higher than that of Pt-OMC by nearly 2.5 times. Likewise, the If value observed for the PtFe-OMC and PtMo-OMC catalysts were about 1.5 times higher than that of Pt-OMC.
On the other hand, a lower onset potential (Vo) and anodic peak potential (Vap) were detected for all the PtM-OMC catalysts compared to the Pt-OMc (Table 4). Note that despite the higher metal loading (nominal Pt:Ru = 20 wt%:10 wt%) in the commercial PtRu/XC-72 catalyst over that of the PtM-OMCs (nominal Pt:Ru = 10 wt%:10 wt%), a higher Vap value was observed for the former (0.71 V) than the latter (typically 0.60 V; see Table 4). This indicated that the PtM-OMCs reported here possessed well-dispersed alloy NPs with high metal surface areas [10] (vide infra).
The catalytic performance of the electrocatalyst can also be assessed in terms of the ratio of the measured forward versus reverse anodic peak current (i.e., If/Ir). A high If/Ir ratio indicates a superior electrocatalytic activity during MOR and less catalyst deactivation due to the deposition of carbonaceous species on the active sites, hence, better CO tolerance [68]. The If/Ir values deduced from the Pt-OMC and different PtM-OMC catalysts are summarized in Table 4 together with literature data of other supported bimetallic catalysts using various carbon substrates. First, it is clear that the If/Ir ratios observed for the carbon supported bimetallic catalysts were larger than that of the monometallic catalyst. For examples, the If/Ir ratios obtained for the PtM-OMCs (typically > 2.2) as well as from the commercial PtRu/XC-72 (2.8; but with a higher metal loading) were higher than that of the Pt-OMC catalyst (2.0). This clearly indicates the benefit of incorporating a second metal (M) for CO tolerance and overall MOR activity of the catalyst. From the viewpoint of material synthesis strategy, the PtRu-OMC catalyst here prepared by the direct replication method gave rise to a much higher value of If/Ir (3.5) compared to that of PtRu/XC-72 (2.8) prepared by the conventional impregnation method even though the latter possessed a higher Pt loading. Clearly, the catalysts prepared by the direct method not only showed improved MOR activity but also less vulnerability to coke deactivation and better tolerance against CO poisoning. This gave an effective reduction in the polarization overpotential (Vo) [69] and a desirable increase in the current density (Fig. 5(a) and Table 4). This may be ascribed to the better alloying and dispersion of the metal NPs in the OMC support with a high BET surface area. In this regard, the PtFe-OMC catalyst, which was synthesized by using a more cost effective Fe metal precursor to replace the noble (Ru) metal precursor, gave an overall performance in terms of Vo, Vap, and If/Ir ratio (cf., Table 4) that was superior or at least comparable to that of the commercial PtRu/XC-72 and other carbon supported bimetallic catalyst systems even though these other catalysts have much higher metal loadings (Table 4).
The electrochemical stability of the PtM-OMC (M = Ru, Fe, Mo) catalysts during MOR was also evaluated by chronoamperometric (CA) experiments at room temperature, as shown in Fig. 5(b). In general, when MeOH is oxidized on the surface of the catalyst, tenacious reaction intermediates such as COad also accumulate progressively if its rate of removal cannot keep pace up with the oxidation of MeOH. Thus, the catalyst’s tolerance to CO poisoning can be assessed by monitoring the variation of current density with time (Fig. 5(b)). Clearly, while the decay rates of the oxidation current density observed for the PtRu-OMC and PtMo-OMC samples were nearly the same, that obtained for the PtFe-OMC catalyst was notably slower, indicating a better tolerance to the CO poisoning.
To further assess the resistance to CO poisoning of the PtM-OMCs, we performed additional CO stripping studies from which information on the interaction between the adsorbed CO and surface of the Pt-based alloy NPs can be compared. The experiments were done by purging N2 containing 500 ppm of CO through the pre-activated electrode in the presence of an electrolyte (0.5 mol/L H2SO4 and 1.0 mol/L MeOH) for 15 min while keeping a constant potential (0.05 V) at the rotating working electrode (1600 ppm). This allowed for complete adsorption of CO onto the catalyst surface prior to the CV measurement. The results obtained from the CO stripping experiments over the various catalysts are depicted in Fig. 5(c). It is obvious that a notable decrease in the electrooxidation current density of methanol was observed for the Pt-OMC, PtMo-OMC, and PtRu/XC-72 catalysts after they were exposed to CO, indicating an inferior MOR activity due to CO poisoning. On the other hand, only a marginal decrease in the current density was found for the PtRu- and PtFe-OMC catalysts during MOR after adsorption of CO. Clearly, these two catalysts exhibited superior methanol electrooxidation stability, which may be ascribed due to their tolerance for CO poisoning [12, 70, 71, 72, 73]. Moreover, in the presence of CO, a much lower MOR activity was observed for the monometallic Pt-OMC compared to its bimetallic counterparts, indicating the role of the second metal on the electrocatalytic activity during MOR.
Based on the results above, the PtFe-OMC catalyst showed extraordinary electrocatalytic activity and CO tolerance during MOR which were comparable to that of PtRu-OMC having a similar metal loading. In this context, this OMC-supported Pt-based catalyst which used the incorporation of a more cost-effective second metal such as a transitionmetal, is a prospective material for practical applications in DMFC/PEMFCs as the anodeelectrode.
A series of Pt-based bifunctional metal (PtM, M = Ru, Fe, Mo) alloy catalysts supported on an ordered mesoscopic carbon (OMC) were synthesized by a one-pot replication method. The PtM-OMC catalysts possessed uniform and highly dispersed alloy nanoparticles (size 2-3 nm) in the pore channels of the OMC as evidenced by electron microscopy and X-ray absorption spectroscopy (XAS). These supported bimetallic catalysts exhibited superior electrocatalytic performance and excellent tolerance to CO poisoning during methanol oxidation reaction (MOR) compared to a typical commercial PtRu/XC-72 catalyst. In particular, the PtFe-OMC catalyst showed a comparable MOR activity to PtRu-OMC, which would allow the use of a cost effective second metal and has prospective applications as the electrode material for fuel cells, sensors, and energy storage devices.
The authors thank Prof. Dr. Bing-Joe Hwang and Mr. Jun-Ren Pan (National Taiwan University of Science and Technology) for their technical supports and helpful discussions on the X-ray absorption experiments.
2016, Vol. 37 Issue (1): 43-53
PDF (1790 KB)
2016, Vol. 37 Issue (1): 43-53
PDF (1790 KB)