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Ternary Pt9RhFex nanoscale alloys as highly efficient catalysts with enhanced activity and excellent CO-poisoning tolerance for ethanol oxidation Peng Wang, Shibin Yin, Ying Wen, Zhiqun Tian, Ningzhang Wang, Julian Key, Shuangbao Wang, and Pei Kang Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14947 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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ACS Applied Materials & Interfaces

Ternary Pt9RhFex Nanoscale Alloys as Highly Efficient Catalysts

with

Enhanced

Activity

and

Excellent

CO-poisoning Tolerance for Ethanol Oxidation Peng Wanga, b, Shibin Yina, Ying Wena, Zhiqun Tiana, Ningzhang Wangc, *, Julian Keya, Shuangbao Wanga, Pei Kang Shena, * a

Guangxi Key Laboratory of Electrochemical Energy Materials, Collaborative

Innovation Center of Renewable Energy Materials (CICREM), Guangxi University, Nanning 530004, China. b

State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning 530004, China. c

School of Computer, Electronics and Information, Guangxi University, Nanning 530004, China.

Corresponding author: E-mail: [email protected] (N.Z. Wang), Fax/Tel.: +86 771 3236385. E-mail: [email protected] (P.K. Shen), Fax/Tel.: +86-771-3237990.

ACS Paragon Plus Environment


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ABSTRACT: To address the problems of high cost and poor stability of anode catalysts in direct ethanol fuel cells (DEFCs), ternary nanoparticles Pt9RhFex (x = 1, 3, 5, 7 and 9) supported on carbon powder (XC-72R) have been synthesized via a facile method involving reduction by sodium borohydride followed by thermal annealing in N2 at ambient pressure. The catalysts are physically characterized by XRD, STEM, and XPS, and their catalytic performance for the ethanol oxidation reaction (EOR) is evaluated by cyclic and linear scan voltammetry, CO-stripping voltammograms and chronopotentiometry. All the Pt9RhFex/C catalysts of different atomic ratios produce high EOR catalytic activity. The catalyst of atomic ratio composition 9:1:3 (Pt/Rh/Fe) has the highest activity and excellent CO-poisoning tolerance. Moreover, the enhanced EOR catalytic activity on Pt9RhFe3/C when compared to Pt9Rh/C, Pt3Fe/C and Pt/C clearly demonstrates the presence of Fe improves catalytic performance. Notably, the onset potential for CO oxidation on Pt9RhFe3/C (0.271 V) is ~55, 75 and 191 mV more negative than on Pt9Rh/C (0.326 V), Pt3Fe/C (0.346 V) and Pt/C (0.462 V), respectively, which implies the presence of Fe atoms dramatically improves CO-poisoning tolerance. Meanwhile, compared to the commercial PtRu/C catalyst, the peak potential on Pt9RhFe3/C for CO oxidation was just slightly changed after several thousand cycles, which shows high stability against the potential cycling. The possible mechanism by which Fe and Rh atoms facilitate the observed enhanced performance is also considered herein, and we conclude Pt9RhFe3/C offers a promising anode catalyst for direct ethanol fuel cells (DEFCs). KEYWORDS: Pt9RhFex alloys, ternary nanoparticles, anode catalysts, CO-poisoning tolerance, ethanol oxidation


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1. INTRODUCTION: With impending global energy shortage and increasing pressure on the environment, fuel cells offer a sustainable and environmentally friendly alternative to the combustion of fossil fuels.1 In fuel cells, the use of ethanol compared to methanol offers higher energy density (8.0 vs. 6.1 kWh kg−1), lower toxicity, large-scale production from agricultural biomass, and is easily stored and transported.2-4 However, widespread use of direct ethanol fuel cells (DEFCs) remains obstructed by sluggish electrode kinetics and incomplete oxidation of ethanol at the anode. Accordingly, highly active catalysts for this process are crucial for the commercialization of DEFCs. Currently, Pt affords the most effective catalyst for the ethanol oxidation reaction (EOR). Nevertheless, complete oxidation of ethanol to CO2 requires C-C bond cleavage, which is difficult on pure Pt that is also easily poisoned by CO-like intermediate species produced during EOR.5 Binary Pt-based catalysts such as PtCu offer high tolerance for COads-like species,6 and others such as PtRu,7-8 PtFe,9 PtSn10 and PtRh11-12 have been extensively reported. In particular, Rh has lower EOR activity than Pt, but has high selectivity for C2 alcohols and facilitates their efficient oxidization to CO2.13-14 Many studies show that binary PtRh alloys are more efficient than pure Pt as EOR catalysts. Rao et al.15 synthesized graphene supported cubic PtRh alloys (PtxRhy/GN) with different atomic ratios of Pt and Rh using the modified polyol method with Br− as a shape-directing agent. The most active catalyst performance resulted from a 9:1 atomic ratio composition of Pt to Rh. In situ FTIR spectroscopy showed that Rh within the alloy promoted breakage of the ethanol C-C bond, and that a 1:1 atomic ratio of Pt to Rh most easily split the ethanol C-C bond and had the highest EOR selectivity for complete oxidation to CO2. Zhao and co-workers,16 using



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a microwave-polyol method, prepared PtRh/C catalysts for ethanol oxidation in alkaline media. Pt2Rh/C had better catalytic ethanol oxidation performance than Pt/C regarding both onset potential and peak current density. Again, the superior performance could be ascribed to the presence of Rh offering improved C-C bond cleavage. Meanwhile, PtFe catalysts have been widely investigated. Huang et al.17 synthesized Pt-Fe alloy supported on an ordered mesoscopic carbon (OMC) via a one-pot replication method. The Pt-Fe/OMC catalysts formed narrow, uniformly dispersed nanoparticles, and produced higher methanol oxidation catalytic activity than Pt/C. Also, Lv et al.18 prepared a PtFe bulk-surface differential nano-catalyst by a chemically dealloying procedure. As an anode catalyst for direct methanol fuel cells (DMFCs), the PtFe had 2.8 times higher catalytic activity than that of Pt/C due to the electronic effect of Fe. Furthermore, possibly owing to the bulk-surface differential structure of the PtFe catalyst, its onset potential for CO oxidation was more negative than that of Pt/C. In addition to binary Pt-based catalysts, an abundance of ternary Pt-based alloys have been designed including PtRuSn,19 PtRuFe,1 PtSnRh20 and PtRuNi,21 which demonstrate notably high catalytic performance. Their outstanding activity is thought to arise from bifunctional effects and/or the modification of Pt electronic structure originating from interactions between Pt and other atoms. Shen et al.22 assembled an EOR catalyst of PtRhNi particles on pristine graphene nanosheets (GNSs), which produced excellent activity and stability attributed to bifunctional effects and modification of Pt electronic structures. Adzic’s group23 reported PtRhSnO2 electrocatalysts of outstanding EOR efficiency owing to synergistic effects between the three constituents: i.e. where Pt facilitates ethanol dehydrogenation; Rh facilitates C-C bond breakage; and SnO2 together with H2O, provides OH species to oxidize


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dissociated CO at Rh sites as well as modifying Rh electronic structure to adsorb ethanol, intermediates, and products. In this work, we synthesized ternary Pt9RhFex (x = 1, 3, 5, 7 and 9) nanoparticles with varying Fe contents supported on carbon powders (XC-72R). The synthesis route involves impregnation-reduction followed by thermal annealing in N2 at ambient pressure. All the Pt9RhFex/C catalysts produced high EOR catalytic activity and high stability, with dramatically improved CO anti-poisoning performance. The optimal atomic ratio for Pt, Rh and Fe was found to be 9:1:3.

2. EXPERIMENTAL SECTION

2.1 Catalyst synthesis Pt9RhFex/C catalysts were prepared by the impregnation-reduction method followed by a high-temperature treatment. Total metal loading on carbon was kept consistent at 20 % wt. wherein the metal Fe content was varied and the atomic ratio of Pt to Rh at 9:1 was maintained. Firstly, 117.3 mg H2PtCl6·6H2O (containing 43.4 mg of Pt), 7.1 mg Rh(NO3)3·2H2O (containing 2.5 mg of Rh) and 121.4 mg FeCl3·6H2O (containing 4.1 mg of Fe) as starting precursors were added to 150 ml deionized water in an ultrasonic bath. Secondly, 200 mg carbon powders (XC-72R) were added to the mixture and the pH value was adjusted to pH 9 by addition of NaOH aqueous solution. A well-dispersed slurry was obtained after stirring and ultrasonication for 15 min, which was then reduced by 10 mL NaBH4 aqueous solutions (containing 121.4 mg of NaBH4). After impregnation for several hours, the resulting black solid was filtered, washed, and dried at 80 oC for 12 h in a vacuum oven. The obtained powder was then reduced in a tube furnace at 300 oC under N2 atmosphere at ambient pressure for 2 h. The as-prepared catalyst was denoted as Pt9RhFe3/C. The other Pt9RhFex/C (x = 1, 5,



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7 and 9) samples with different Fe atomic numbers were prepared in this way, maintaining the atomic ratio of Pt to Rh at 9:1 in all catalysts. Experimental control catalysts of varying compositions were also produced by the same overall process and included Pt9Rh/C, Pt3Fe/C, Pt/C, and Rh/C. 2.3 Catalysts characterization X-ray powder diffraction (XRD) measurements were carried out on a SmartLab3 X-ray diffractometer (Rigaku, Japan), with Cu Kα radiation (λ = 1.5405 Å) at 40 kV and 30 mA. The 2θ angular regions between 10o and 90o were finely scanned at 5o min−1 to obtain the crystalline sizes and lattice parameters according to the Scherrer equation and Vegard’s law.24 Scanning transmission electron microscopy (STEM) was carried out using a TITAN G2 (FEI, American) microscope at 200 kV. High-resolution STEM and elemental mapping were performed using a 60-300 electron microscope (200 kV) equipped with image corrector and a high-angle annular dark field (HAADF) detector. X-ray photoelectron spectroscopy (XPS) measurements were employed on an ESCALAB 250 Xi (Thermo Fisher Scientific, USA) with an Al X-ray source operated at 150 W. Survey spectra were collected at a pass energy (PE) of 100.0 eV over the binding energy range of 0-1350 eV. All electrochemical measurements were carried out on a PINE (PINE, USA) electrochemical workstation in a thermostatically-controlled standard three-electrode cell at 30 oC, using a carbon rod and a saturated calomel electrode (SCE) as counter electrode and reference electrode respectively. The working electrode was a catalyst-modified glassy carbon electrode of 5.0 mm diameter. The catalyst slurry was prepared as a mixture containing 10.0 mg catalyst, 1980 µL ethanol and 20 µL Nafion solution (5.0 % wt.) dispersed for 15 min in an ultrasonic bath to obtain a highly-dispersed ink. The thin ink film was then coated onto the surface of a glassy


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carbon electrode using a micropipette, and dried at room temperature. Cyclic voltammetry (CV) experiments were carried out in 0.1 mol L−1 HClO4 aqueous solutions with and without 1.0 mol L−1 C2H5OH de-aerated with N2. For CO-stripping experiments, CO adsorption was conducted at −0.12 V in a CO saturated 0.1 mol L−1 HClO4 aqueous solution for 15 min, and subsequently purged with N2 for 30 min to remove the dissolved CO from solution, followed by scans between −0.23 and 0.87 V at a rate of 50 mV s−1 for 2 cycles. The stability of the as-prepared catalysts was evaluated by chronoamperometry measurements and continuous CV cycling. In this work, all the potentials are referenced to SCE, and all current densities are normalized by the mass of Pt loading except for CO-stripping tests where the current densities are normalized by the total mass of noble metal loadings.

3. RESULTS AND DISCUSSION

Figure 1a shows the XRD patterns of the as-prepared Pt9RhFe3/C and various control catalysts. All the catalysts produced peaks typical of crystalline face centered cubic (fcc) structure. The 2θ angles (39.99o (111), 46.45o (200), 67.73o (220) and 81.55o (311)) correspond to polycrystalline Pt (PDF#65-3259); 2θ angles (41.25o (111), 48.01o (200), 70.12o (220) and 84.32o (311)) correspond to polycrystalline Rh (PDF#05-0685); 2θ angles (40.58o (111), 47.33o (200), 68.87o (220) and 82.88o (311)) correspond to polycrystalline PtRh (PDF#65-7938); and 2θ angles (40.60o (111), 46.47o (200), 67.82o (220) and 82.14o (311)) correspond to polycrystalline PtFe (PDF#89-2051). Figure S1a shows the XRD patterns of all the Pt9RhFex/C (x = 1, 3, 5, 7 and 9) catalysts, which notably produced no distinct peaks relating to Fe oxides. Figure 1b shows the corresponding curve fitting for (220) diffraction peaks in part of Figure 1a, in which the position of the peak maximum (θmax) and the full-width at



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half-maximum (B2θ) could be obtained precisely. The lattice parameters of the Pt fcc crystallite were calculated according to Vegard’s rule. α=

2 λkαl sin θ max

(1)

where λkαl = 0.154056 nm. The lattice parameters of Pt9RhFe3/C, Pt9Rh/C, Pt3Fe/C, Pt/C and Rh/C were 3.8728 Å, 3.9120 Å, 3.8883 Å, 3.9156 Å and 3.7987 Å respectively. Clearly, due to differences of the lattice parameters as compared to Pt and Rh, alloys in the catalysts were formed during post-treatment at high temperature. Visibly, the lattice constants of Pt decreased in Pt alloys catalysts due to the incorporation of smaller atoms into Pt nanoparticles, which induced compressive strains on the crystal lattice. In Pt alloys catalysts, the introduction of Rh and Fe both reduced the distance of Pt-Pt, which caused lattice contraction, thus favoring the adsorption and/or desorption for reactants, and promoting their catalytic performance as reported.25 Furthermore, the peak positions of (220) facet shifted positively (Figure S1b), and the lattice parameters were reduced (Table S1), at the Fe atomic ratio increased. The results indicate that compressive strains were larger with higher Fe contents. Therefore, the relationship between the 2θ angles of peak positions of (220) facet and lattice parameters with the Fe atomic numbers is depicted in Figure S2. Moreover, the average crystalline sizes of Pt9RhFe3/C, Pt9Rh/C, Pt3Fe/C, Pt/C and Rh/C were 4.0 nm, 6.2 nm, 3.8 nm, 8.6 nm and 5.5 nm respectively, calculated using the Scherrer equation.24 Notably, smaller particle sizes coincided with the presence of increased Fe, which contributes to the dispersion of the catalysts. Details from the XRD analysis are summarized in Table 1. Figure 2a shows the bright field TEM image of the Pt9RhFe3/C catalyst, and it can be seen that the majority of the ternary Pt9RhFe3 particles were uniformly distributed


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on the carbon supports. Based on 300 particles measured in random regions, the average particle sizes calculated according to Gaussian fitting was 3.41 nm (in accordance with the XRD result), and the particle size distribution was narrow (Figure 2b). HRTEM was used to further examine the nanostructure of a single Pt9RhFe3 nanoparticle (Figure 2c). A clear lattice fringe of 0.22 nm was found, which matched the lattice spacing of the (111) plane of fcc PtRh (0.2230 nm). The Fast Fourier Transformation (FFT) image (inset, Figure 2c) shows a regular hexagon structure, which also confirms the observation. The HADDF-STEM and elemental mapping were then used to elucidate the composition and morphology of the ternary nanoparticles (Figure 2d-h). Pt, Rh and Fe elements proved to coexist in a uniform distribution within a fine particle size. Therefore, the ternary alloys were successfully formed as small and well dispersed particles. XPS was used to determine the elemental composition and electronic structures of the nanoparticles. Figure S3a-c show the wide survey spectrum of Pt9RhFe3/C, Rh3d and Fe2p in Pt9RhFe3/C spectrum. The result indicates the coexistence of Pt, Rh and Fe, which is in agreement with the elemental mapping results. Meanwhile, the weak peaks of Rh3d and Fe2p reveal that the alloyed Pt9RhFe3 nanoparticles have a Pt-rich outmost surface, which would thus facilitate structural stability in acidic solution. The spectrum of Pt4f contains doublets of 4f7/2 and 4f5/2, with a theoretical peak area ratio of 4:3 (Figure 3). Two paired peaks were obtained by the deconvolution of Pt4f, which could be assigned to Pt0 and Pt2+ species respectively (summarized in Table 2). Clearly, the Pt4f in Pt/C had binding energies (BEs) of 74.81 and 71.40 eV for Pt0 in 4f5/2 and 4f7/2 doublets respectively. Moreover, in the Pt alloys, the BEs of Pt0 shifted to higher energies, that is, the BEs of Pt0 in Pt9RhFe3/C, Pt9Rh/C and Pt3Fe/C were 74.84, 74.82 and 74.83 eV for 4f5/2 and 71.48, 71.42 and 71.46 eV for 4f7/2



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peaks, respectively. The BE shifts of Pt0 relate to two factors: lattice strain26 and charge transfer.27 The core level BE of Pt strongly correlated with the d-band center position based on d-band center theory. Consequently, the positive shifts in BE of Pt0 indicate lowering of the d-band centers through interaction with Fe. Notably, the XRD results confirmed the Pt alloys compressed the lattice parameters of Pt due to the smaller size of Fe atoms, which could lower their d-band centers.1 Furthermore, due to Pt (2.28) and Fe (1.90) having different electronegativity, charge transfer from Fe to Pt atoms could occur causing the positive shifts in the BEs of Pt0. Figure S4a and b show the CV curves of Pt9RhFex/C (x = 1, 3, 5, 7 and 9) catalysts in N2-purged 0.1 mol L−1 HClO4 aqueous solutions without and with 1.0 mol L−1 C2H5OH at 30 oC. Figure S4a shows that all the Pt9RhFex/C catalysts have similar electrochemical surface area (ESA). However, the catalyst with an atomic ratio of 9:1:3 (Pt/Rh/Fe) produced the highest peak current density value and had the most negative onset potential (Figure S4b). The relationship between peak current density and the Fe atomic ratio number is shown in Figure S6. Where the Fe atomic ratio number is relatively small (x ≤ 3), Fe dramatically improved the catalytic activity of catalysts thus indicating bifunctional effects. However, with Fe content > x = 3, the corresponding decreased Pt content provides insufficient active sites on for EOR, which leads lower overall catalytic activity. Nevertheless, Pt9RhFe3/C had the best catalytic activity among these catalysts. The CV curves of Pt9RhFe3/C and the control catalysts are shown in Figure 4a and b, and it can be seen that Rh/C (Figure 4b) produced almost no EOR activity due to its negligible dehydrogenation ability compared to Pt.15 The peak current densities for Pt9RhFe3/C, Pt9Rh/C, Pt3Fe/C and Pt/C were 968.26, 427.16, 417.96, and 238.50 mA mgPt−1 respectively. Pt9RhFe3/C clearly had the highest EOR activity, and its peak current density compared to


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Pt9Rh/C, Pt3Fe/C, and Pt/C was 2.26, 2.31 and 4.06 times higher respectively (Table 3). Moreover, the Pt9RhFe3/C catalyst also exhibited higher mass activity and specific activity for EOR compared with the reported Pt-based catalysts, which were summarized in (Table S3). Furthermore, the onset potential of ethanol oxidation on Pt9RhFe3/C was more negative, indicating the Pt9RhFe3/C had the lowest over-potential for EOR. The enhanced activity of the ternary Pt9RhFe3/C catalyst is considered to relate to the following: (1) Bifunctional effects: The dissociation of water molecules on the metal surface follows in the sequence of Fe > Rh > Pt28-29 through theoretical calculation. The presence of Fe contributed to the removal of the intermediates formed during EOR, which was responsible for the enhanced activity of Pt9RhFe3/C catalyst. (2) Strain effects: XRD revealed that compressive strains occurred in the Pt alloy crystals, giving rise to downshifts of d-band centers with respect to Fermi levels.30 According to d-band theory, decrease in the density of d states of the metal near the Fermi level could be helpful to remove intermediates on the alloyed Pt surface,26 which thus facilitates the adsorption and/or desorption of ethanol. (3) Ligand effects: Ligand effects resulting from electron transfer from Fe and Rh atoms to Pt atoms also influences the catalytic activity of the catalysts.31 Furthermore, Fe as well as Rh, compared with Pt, is easier to stabilize the metal-CH2CH2O complex during ethanol oxidation owing to its electronic properties. Figure S5a shows the linear scan curves generated from all the Pt9RhFex/C catalysts in N2-purged 0.1 mol L−1 HClO4 with 1.0 mol L−1 C2H5OH aqueous solution at a scan rate of 2 mV s−1. Clearly, Pt9RhFe3/C had higher EOR activity than the other catalysts with different Fe atomic numbers. Tafel plots were used to evaluate the kinetic behavior of all the Pt9RhFex/C catalysts (Figure S5b). The Tafel slopes show



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over-potential decreases with reducing slope. Obviously, the Pt9RhFe3/C catalyst had the minimum value of Tafel slope, reflecting the lowest over-potential (Table S2). Figure S6 shows the trend curve between the Tafel slopes and Fe atomic numbers of all the Pt9RhFex/C catalysts, and Figure 5 shows the linear scan curves and Tafel plots of Pt9RhFe3/C and the other compared catalysts. Evidently, the onset potential on Pt9RhFe3/C was the most negative, indicating the lowest over-potential and improved EOR kinetics. Moreover, by comparing the slopes of Pt9Rh/C and Pt/C, the presence of Rh clearly decreases the over-potential and thus likely promotes the cleavage of the C-C bond in ethanol. Importantly, the onset potential on Pt3Fe/C was more negative than that on Pt/C and the Tafel slope of Pt9RhFe3/C also decreases dramatically compared to that of Pt9Rh/C (see Table 3). This may be because Fe exerts an electronic effect on Pt atoms due to its low electronegativity, which results in increased surface hydrophilicity, improved adsorption energy, and concentration of -OH species on the catalyst surface.32 These factors may thus modify the activation performance on Pt and lower EOR potentials. To evaluate resistance to CO-poisoning on the catalysts during EOR, CO-stripping tests were conducted in 0.1 mol L−1 HClO4 aqueous solutions at a scan rate of 50 mV s−1. In Figure 6a, the electrochemical surface areas (ESAs) of Pt9RhFe3/C and other compared catalysts were calculated by assuming a full coverage of CO monolayer on PtRh alloy (Rh adopted in Rh/C) surface associated with 420 µC cm−2 charge,16 which is also summarized in Table 3. The higher ESA of Pt/C accounts for its high Pt content that supplies the most active sites on its surface. Most importantly, the onset potential of CO oxidation on Pt9RhFe3/C was at 0.411 V, about 55 mV, 75 mV, 191 mV and 140 mV more negative than Pt9Rh/C, Pt3Fe/C, Pt/C and Rh/C respectively (Table 3). Comparing the onset potentials of Pt9RhFe3/C and Pt9Rh/C, it can be assumed


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CO-poisoning tolerance was significantly improved by the presence of Fe atoms. Here, electron transfer from Fe to Pt atoms may reduce the d state of Pt near the Fermi level, thus weakening the adsorption of Pt-CO,33 thus improving CO-poisoning resistance. Moreover, the CO-stripping curves of all the Pt9RhFex/C catalysts (Figure S7) reveal that Pt9RhFex/C catalysts shared similar onset potentials for CO oxidation. The ESAs the Pt9RhFex/C catalysts and their onset potentials are summarized in Table S-2. Figure 6b and c show the CO-stripping results on the commercial PtRu/C and Pt9RhFe3/C catalysts before and after 5000 cycles. It is clear that the peak potential for CO oxidation on the commercial PtRu/C catalyst is shifted positively with increasing the cycling number. It is possibly due to that Ru in PtRu/C catalyst is gradually dissolved during the potential cycling, and CO oxidation more occurs on the Pt atoms. 34 There are two peaks on the commercial PtRu/C catalyst after 500 cycles, and the second peak is close to the potential on Pt/C, which providing an evidence to the Ru dissolution. However, the peak potential for CO oxidation on Pt9RhFe3/C is just slightly shifted toward positive direction after 5000 cycles, which shows good stability against the potential cycling. This result indicates that the presence of both Rh and Fe could enhance the CO-poisoning tolerance due to their synergistic effects. Catalyst stability is a crucial factor for fuel cell performance. Catalyst stability was first measured using chronoamperometry over a period of 1 h with the potential held at 0.4 V in an aqueous solution of 0.1 mol L−1 HClO4 with 1.0 mol L−1 C2H5OH (Figure 7 and S8). Figure S8 shows that Pt9RhFe3/C had higher initial and final current densities than the other Pt9RhFex/C catalysts, indicating that it had the greatest stability. Furthermore, the curves in Figure 7 reflect gradual current density decrease occurred on all the catalysts with increasing time due to the effect of CO-like byproducts, formed during EOR, occupying the active sites of the catalysts. After the



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3600 s durability test, the final current densities on the catalysts were as follows: Pt9RhFe3/C > Pt9Rh/C > Pt3Fe/C > Pt/C. The presence of either Fe or Rh noticeably improved catalyst stability due to bifunctional effects. As previously reported, Rh facilitates cleavage of the ethanol C-C bond,11, 23 and in the case of Fe it may modify Pt electronic structure to lower the bond strength of Pt-adsorbate and adsorption of the intermediates resulting in the high stability of the catalyst. Furthermore, to examine the structural stability of Pt9RhFe3/C in the acidic HClO4 electrolyte, continuous CV cycling was next carried out, with STEM-EDX line-scan measurements (Figure S9) before and after cycling. Figure S9a and c show that the peak current densities of Pt9RhFe3/C after 5000 cycles decreased to 770.77 and 81.6 mA mgPt−1. Maintenance of the peak current density on Pt9RhFe3/C was 79.9% (Figure S9b) and much higher than that of Pt/C at 34.5% (Figure S9d). Figure S9e shows the STEM-EDX line scan of the Pt9RhFe3 nanoparticle prior to cycling, where Fe and Rh had relatively lower contents than Pt in the ternary particle, with Fe showing a higher concentration at the center of the particle than other regions. Besides, it has relatively higher Pt contents at the surface, which correlates with the XPS results. After 1000 cycles, the plot suggests, from the slight reduction in Fe peak intensity, that some Fe atoms at and near the catalyst surface may have dissolved. Overall the line-scan profile for all three elements does not significantly change after cycling (Figure S9f). Although, there is no typical core-shell structure formed, these results imply a Pt-rich property on the surface of the particle is maintained throughout cycling.35

4. CONCLUSIONS

Pt9RhFex (x = 1, 3, 5, 7 and 9) ternary nanoparticles supported on carbon powders


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(XC-72R) were successfully synthesized using a simple impregnation-reduction method followed by high temperature treatment. The physical properties of the nanoparticles were determined by XRD, STEM and XPS. Measurement of catalytic EOR activity in acidic media revealed that all the Pt9RhFex/C catalysts had excellent activity and high stability, and that the optimal atomic ratio of Pt to Rh to Fe was 9:1:3. The Pt9RhFe3/C catalyst had the highest catalytic activity and best durability when compared to the other catalysts (Pt9Rh/C, Pt3Fe/C, Pt/C and Rh/C), and exhibited excellent stability for CO oxidation compared to the commercial PtRu/C catalyst. The results indicate that Fe modifies the electronic structure of Pt and promotes low potential oxidation of CO-like intermediates. It is believed that the Pt9RhFe3/C thus provides a promising EOR catalyst for direct ethanol fuel cells (DEFCs).

ASSOCIATED CONTENT ◘Supporting Information The supporting information is available free of charge on the ACS Publication website at DOI: Additional data, figures and descriptions and tables. ■AUTHOR INFORMATION Corresponding Authors *(N.Z.W) E-mail: [email protected]. *(P.K.S.) E-mail: [email protected]

■ ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(NSFC) (21303260), Fundamental Research Funds for the Central Universities (2014ZDPY20), the Major International (Regional) Joint Research Project (51210002), the National Basic Research Program of China (2015CB932304), the Natural Science Foundation of Guangdong Province (2015A030312007) and Guangxi Science and Technology Project (AB16380030).


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Figures and Captions Figure 1. (a) XRD curves of the compared samples: (1) Pt9RhFe3/C, (2) Pt9Rh/C, (3) Pt3Fe/C, (4) Pt/C and (5) Rh/C, at a scan rate of 5o min−1. (b) The corresponding curve fitting for (220) diffraction peaks in part (a). Figure 2. (a) Bright field TEM image of Pt9RhFe3/C catalyst, (b) the corresponding particle size distribution histogram of (a), (c) HRTEM image of a single Pt9RhFe3 nanoparticle and the inset in (c) is the FFT pattern, (d) HAADF-STEM overview image based on three nanoparticles in random regions, (e, f, g) elemental mapping images of Pt, Rh, Fe, and (h) the combination of (e), (f) and (g). Figure 3. XPS spectra of Pt4f in (a) Pt9RhFe3/C, (b) Pt9Rh/C, (c) Pt3Fe/C and (d) Pt/C catalysts. Figure 4. CV curves for the as-prepared catalysts in N2-purged 0.1 mol L−1 HClO4 without (a) and with (b) 1.0 mol L−1 C2H5OH aqueous solution at 30 oC, scan rate 50 mV s−1. Figure 5. (a) Linear scan curves for the as-prepared catalysts in N2-purged 0.1 mol L−1 HClO4 + 1.0 mol L−1 C2H5OH aqueous solution at 30 oC, scan rate 2 mV s−1. (b) Tafel plots for ethanol oxidation. Figure 6. (a) CO-stripping voltammograms for the Pt9RhFe3/C, Pt9Rh/C, Pt3Fe/C, Pt/C, Rh/C and commercial PtRu/C catalysts in 0.1 mol L−1 HClO4 aqueous solutions at a scan rate of 50 mV s−1. Pre-adsorption of CO over Pt at −0.12V (vs. SCE) for 15 min. Peak potentials for CO-stripping on (b) the commercial PtRu/C and (c) Pt9RhFe3/C catalysts after different cycling numbers. The insets are the plots of the peak potentials for CO oxidation varying with cycling numbers. Figure 7. Chronoamperometry curves for the catalysts in 0.1 mol L−1 HClO4 + 1.0 mol L−1 C2H5OH aqueous solutions at 30 oC with the potential held at 0.40 V. Table 1. Structural parameters of different compared catalysts extracted from powder X-ray diffraction. Table 2. Summary of XPS results of different compared catalysts. Table 3. Summary of electrochemical parameters of the compared catalysts.



Figure 1

(a)

(111) (200)

Intensity / a.u.

(1)

(220)

(311)

(2) (3) (4) (5)

0

20

40

60

80

100

2 Theta / (θ )

(b) (1)

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(2)

(3)

(4) (5)

65

66

67

68

69

70

71

72

2 Theta / (θ )


73

74

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2



Figure 3

Pt P4f 7/2 (70.9 eV)

2+

Pt P4f 5/2 (76.3 eV)

85

80

(b)

0

0

Pt P4f 5/2 (74.4 eV)

2+

Pt P4f 7/2 (72.6 eV)

75

70

Intensity (a.u.)

Intensity (a.u.)

(a)

65

(c)

2+

Pt P4f 5/2 (76.1 eV)

0

85

80

2+

Pt P4f 7/2 (72.6 eV)

75

70

Binding energy (eV)

75

70

65

65

0

(d)

Pt P4f 7/2 (71.0 eV)

Pt P4f 7/2 (75.8 eV)

80

Binding energy (eV)

Intensity (a.u.)

Pt P4f 5/2 (74.4 eV)

0

Pt P4f 7/2 (70.9 eV) 2+

85

0

0

0

Pt P4f 5/2 (74.4 eV)

Pt P4f 7/2 (71.8 eV)

Binding energy (eV)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

0

Pt P4f 5/2 (74.2 eV)

Pt P4f 7/2 (71.0 eV)

2+

Pt P4f 7/2 (71.6 eV)

0

Pt P4f 5/2 (76.3 eV)

85

80

75

70

Binding energy (eV)


65

Page 25 of 32

Figure 4

j / mA mgPt

-1

(a) 100

Pt9RhFe3/C

80

Pt9Rh/C

60

Pt3Fe/C

40

Pt/C Rh/C

20 0 -20 -40 -60 -80 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.6

0.8

1.0

E / V vs. SCE

(b) 1000

Pt9RhFe3/C Pt9Rh/C

800

Pt3Fe/C

-1

j / mA mgPt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt/C Rh/C

600 400 200 0 -200 -0.4

-0.2

0.0

0.2

0.4

E / V vs. SCE



Figure 5

(a) 500 Pt9RhFe3/C Pt9Rh/C

400

j / mA mgPt

-1

Pt3Fe/C Pt/C Rh/C

300 200 100 0 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

E / V vs. SCE

(b)

0.42 0.40

E / V vs. SCE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

0.38

Pt9RhFe3/C Pt9Rh/C Pt3Fe/C Pt/C

0.36 0.34 0.32 0.30 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

−1

logj (mA.mgPt )


2.4

Page 27 of 32

(b)160

(a) 240

Pt9RhFe3/C

120

Pt/C Rh/C PtRu/C

80

j / mA mgPt

Pt3Fe/C

-1

Pt9Rh/C

160

40 0

120 100 80 60

0.6 0.5 0.4 0.3 0.2

0

1000 2000 3000 4000 5000

cycling numbers

40 20

-40

0 0.0

0.2

0.4

0.6

E / V vs. SCE (c) 140 120 100 80 60

0.8

1.0

-0.2

0.0

0.2

0.4

st

1 th 500 th 1000 th 2000 th 3000 th 4000 th 5000

0.6 0.5 0.4 0.3 0.2

0

1000 2000 3000 4000 5000

cycling numbers

40 20 0 -0.2

0.6

E / V vs. SCE peak potential / V vs. SCE

-0.2

-1

-80 -0.4

st

1 th 500 th 1000 th 2000 th 3000 th 4000 th 5000

140

j / mA mg

-1

200

peak potential / V vs. SCE

Figure 6

j / mA mg

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

0.2

0.4

0.6

0.8

E / V vs. SCE


1.0

0.8

1.0


Figure 7

250

Pt9RhFe3/C

-1

Pt9Rh/C

200

Pt3Fe/C Pt/C

j / mA mgPt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

150 100 50 0

600

1200

1800

2400

3000

Time / s


3600

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Table 1 2θ (degree)

Lattice parameter (Å)

Crystal size (nm)

(220) facet

α

Pt9RhFe3/C

68.51

3.8728

4.0

Pt9Rh/C

67.72

3.9120

6.2

Pt3Fe/C

68.19

3.8883

3.8

Pt/C

67.65

3.9156

8.6

Rh/C

70.03

3.7987

5.5

Name


d


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 2 Pt2+4f5/2

Pt04f5/2

Pt2+4f7/2

Pt04f7/2

Pt9RhFe3/C

75.69

74.84

72.11

71.48

Pt9Rh/C

75.74

74.82

72.31

71.42

Pt3Fe/C

75.88

74.83

72.07

71.46

Pt/C

75.91

74.81

72.05

71.40

Name


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Table 3 Name

Slope

Eo(V)

jm (mA mgPt−1)

SESA (m2 g−1)

js (mA cm−2)

Pt9RhFe3/C

0.1847

0.271

968.26

35.25

27.47

Pt9Rh/C

0.2185

0.326

427.16

20.38

20.96

Pt3Fe/C

0.1869

0.346

417.96

16.91

24.72

Pt/C

0.2581

0.462

238.50

60.49

3.94

Rh/C

━

0.411

━

36.81

━

PtRu/C

━

0.185

━

46.38

━

Onset potential (Eo) of CO stripping, mass activity (jm), electrochemical surface area (SESA), specific activity (js).



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Graphical Abstract

● Ternary Pt9RhFex (x = 1, 3, 5, 7 and 9) nanoparticles were synthesized firstly as EOR catalysts. ● All the Pt9RhFex/C showed excellent performance for EOR, and the optimal atomic ratio for Pt, Rh and Fe was 9:1:3. ● The onset potential for CO oxidation on Pt9RhFe3/C was 190 mV more negative than that on Pt/C, which also displayed outstanding stability than commercial PtRu/C.


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Ternary Pt9RhFex Nanoscale Alloys as Highly ... - ACS Publications

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