Copper-Induced Formation of Structurally Ordered PtâFeâCu Ternary

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Copper Induced Formation of Structurally Ordered Pt-Fe-Cu Ternary Intermetallic Electrocatalysts with Tunable Phase Structure and Improved Stability Jing Zhu, Yao Yang, Lingxuan Chen, Weiping Xiao, Hongfang Liu, Héctor D. Abruña, and Deli Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02172 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Chemistry of Materials

Copper Induced Formation of Structurally Ordered Pt-Fe-Cu Ternary Intermetallic Electrocatalysts with Tunable Phase Structure and Improved Stability

Jing Zhu,†,# Yao Yang,‡,# Lingxuan Chen,† Weiping Xiao,† Hongfang Liu,† Héctor D. Abruña‡,* Deli Wang†,* †Key

laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P.R. China. ‡ Department

of Chemistry and Chemical Biology, Baker Laboratory, Cornell University,

Ithaca, New York, 14853, United States. Abstract In this work, the atomic arrangement in PtFexCu1-x ternary ordered intermetallic nanoparticles has been shown to represent an accurate and flexible structure-controlled strategy to optimize its electrocatalytic performance in the methanol oxidation. The presence of Cu in PtFexCu1-x facilitates the phase transformation from a chemically disordered face-centered cubic (fcc) structure to an ordered body-centered tetragonal (bct) PtFe phase at lower annealing temperatures. These ordered structures exhibit significantly enhanced activity and stability when compared to the disordered counterparts, because of not only the robust bct-PtFe ordered structure, but also the relatively inert Cu partially replacing Fe to mitigate the dissolution of the non-noble metals. Among these series of ordered PtFexCu1-x materials, PtFe0.7Cu0.3 exhibited the best durability, arising, in part, from the optimal Cu concentration (Cu:Fe=3:7). When additional Fe was replaced by Cu atoms, the ordered structure was destroyed, triggered by the loss of Cu, leading to severe leaching of the transition metals and the loss of durability. Incorporation of Cu also led to the lattice changes and the stronger adsorption of CO-like species，thereby decreasing the electrocatalytic activity for the methanol oxidation reaction (MOR). However, this represents an acceptable compromise given the remarkable improvement in durability. 1

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Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are considered to be one of the most promising energy conversion devices because they are environmentally benign and highly efficient.1,2 Taking into account safety and portability, the use of lowmolecular-weight alcohols, such as methanol and ethanol, to replace hydrogen as fuel, represents a generation of environmentally friendly fuel cell technologies.3,4 The development of appropriate fuel cell systems, such as direct methanol and direct ethanol fuel cells (DMFCs and DEFCs) have recently generated increasing interest in transportation since it represents a considerable fraction of world energy consumption and contributes significantly to air pollution.5,6 However, enormous challenges still remain in the development of such systems. One of the main obstacles is the identification of effective electrocatalysts to facilitate both the oxygen reduction reaction (ORR) and the methanol oxidation reaction (MOR), where reaction rates are far slower than that of the hydrogen oxidation reaction (HOR), especially in acid media.7,8 Pt is considered to exhibit the highest electrocatalytic activity for ORR among monometallic catalysts for alcohol oxidation at low temperatures in an acidic environment. However, the electrocatalytic performance of Pt is still not sufficiently high enough to enable the commercialization of DMFCs. Tremendous research efforts have been devoted to improve its performance and minimize the usage of Pt, including metal alloying (binary alloy PtM9-11 and ternary alloy PtMN12-14), atomic arrangement ordering (bimetallic15-17 and trimetallic18-20), structural optimization (porous structure21, hollow structure22, framework23), and preferential crystal face orientation24. To date, the most successful results have been achieved through the alloying strategy.25,26 It has four main effects on the electrocatalytic performance of noble metal materials including strain effect, electronic effect, ensemble effect and bifunctional effect.27 The strain effect plays a critical role in the ORR by incorporating 3d-transition metals (e.g. Ni, Co, Fe, Cu, Cr, etc.28-31) into the Pt lattice, especially in core-shell structure. The alloy core leads to a compressive strain of the Pt-rich shell, which 2


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weakens the binding strength of the oxygenated species. From electronic effect perspective, the d-band center and orbital filling of Pt electrons also change through alloying, affecting the interaction between active Pt sites and adsorbed oxygenated species.32 In the ensemble effect, metals such as Zn, Bi and Ru33-35 can form a surface alloy with Pt atoms affecting the Pt atomic arrangement continuity. This continuity is widely believed to be necessary to generate COads during the formic acid oxidation, and promote the dehydration path instead of the dehydrogenation path. Concomitantly, these metals are usually more active than Pt in the water activation reaction that produces OHads, a required reactant when combined with Pt-COads, to yield CO2, which is often referred to bifunctional effect. It has been shown that the intrinsic properties of different alloying elements play various roles for different electrochemical reactions. However, only limited reports exist for ternary alloys, especially structurally ordered intermetallic catalysts, partially due to the difficulty of explicitly establishing the effects of every element. It is expected that ternary alloys, with tunable structural parameters, could achieve the optimization of the activity and durability. For example, the presence of Au in FePtAu facilitates the phase transformation of FePt and the dehydration path in the formic acid oxidation reaction, resulting in a significantly enhanced activity.36 Here, ternary alloy PtFexCu1-x nanoparticles have been systematically prepared and studied by using Cu to partially replace Fe in PtFe nanoparticles, which, in return, have been widely studied for both ORR and MOR37-41. Further, in order to obtain ordered phases, particle sintering is often employed since the high-temperature annealing is generally necessary though it often leads to Ostwald ripening and/or particle coalescence.42,43 In this study, we have found that Cu facilitates the phase transformation from a disordered fcc-PtFe to an ordered bct-PtFe phase at lower annealing temperatures, providing a rational design strategy for preparing structurally ordered Pt-based nanocatalysts. The roles of Cu in the ternary PtFe xCu1-x alloys have been systematically investigated, including both the disordered and ordered structures, revealing the effect of the Cu concentration on the particle size distribution, lattice 3



strain, and Fe dissolution behavior. All of these points can affect, in turn, the activity and durability. Significant improvements in the durability of ordered PtFe xCu1-x/C are ascribed to both, the replacement of Fe atoms with the relatively inert Cu, and having a robust bct-PtFe ordered intermetallic structure, which could mitigate the dissolution of Fe in acidic media. The systematic study of structurally ordered Pt-Fe-Cu ternary intermetallics provides

valuable insights

for optimally designing Pt-based

electrocatalysts in fuel cell applications. Experimental Sections Catalysts preparation Carbon-supported PtFexCu1-x/C nanoparticles were prepared by using an impregnation reduction method15. In a typical synthesis of PtFe0.5Cu0.5/C, 53.0 mg of H2PtCl6·6H2O, 8.3 mg of FeCl3 and 8.7 mg of CuCl2·2H2O were dissolved in de-ionized water, and 74 mg of Vulcan XC-72 carbon support were dispersed in the solution. After ultrasonic mixing for 30 min, the suspension was heated under magnetic stirring to allow the solvent to evaporate and form a smooth, thick slurry. The slurry was dried in a vacuum oven at 60 oC for 12 h. After grinding in an agate mortar, the resulting dark and freeflowing powder was heated in a tube furnace at 200 oC under flowing H2 for 2 h. Finally, the powder was cooled to room temperature under N 2. Subsequently, the samples were annealed at 300 oC for 20 h (denoted as PtFexCu1-x/C-300), 500 oC (PtFexCu1-x/C-500) or 700 oC (PtFexCu1-x/C-700) for 2 h under H2 atmosphere. Other nanoparticles were also prepared in the same way. Physical characterization The as-prepared nanoparticles were characterized by powder X-ray diffraction (XRD) using an X'Pert PRO diffractometer, and diffraction patterns were collected at a scanning rate of 4o/min with a step size of 0.02°. The composition of the catalysts was determined by X-ray fluorescence (XRF) using an EAGLE III spectrometer. Scanning transmission electron microscopy (STEM) images, and electron energy loss 4


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spectroscopy (EELS) elemental maps were acquired using a fifth-order aberrationcorrected FEI Titan Themis STEM operated at 300 keV with a sub-ångstrom spatial resolution. Electrochemical measurements Electrochemical measurements were performed at room temperature using a CHI 760e Potentiostat/Galvanostat and a three-electrode electrochemical cell. The working electrode was a 5mm diameter glassy carbon disk. 5 mg of samples were dispersed in 1 mL isopropanol/Nafion solution (0.05 wt. %) via ultrasonic dispersion to form a homogeneous ink. 3 μL of ink were dropped onto the glassy carbon electrode, and allowed to dry. The Pt metal loading of the catalysts on the electrode was about 15 μg cm-2. A large-area Pt wire was used as the counter electrode, and a reversible hydrogen electrode (RHE) was used as the reference electrode. The electrocatalytic activities of the different catalysts for methanol oxidation, were studied by cyclic voltammetry (CV) and chronoamperometry (CA), in 1 M methanol containing 0.5 M H 2SO4. CV was performed between 0.05 and 1.2 V at a scan rate of 50 mV s-1, and CA was performed at a constant potential of 0.8 V for 3000 s. For CO stripping voltammetry, the electrode potential was held at 0.1 V in CO-saturated 0.5 M H 2SO4 solution for 15 min. N2 was bubbled into the solution for 15 min to remove excess CO dissolved in solution. Finally, the CO stripping voltammogram was measured over the potential range from 0.05 V to 1.2 V at a scan rate of 50 mV s -1.

Results and Discussion Fig. 1a displays the XRD patterns of Pt-Fe-Cu/C samples annealed at 500 oC and 700 oC.

The curve of PtFe/C-500 shows typical Pt face-centered cubic (fcc) features with

(111), (200), (220), and (311) planes, which are consistent with those of pure Pt (lattice planes in standard XRD pattern are presented as black lines in Figure S1). After annealing at 700 °C, the (001) and (100) superlattice peaks became visible, 5



demonstrating the formation of an ordered intermetallic bct-type PtFe phase. Although high-temperature (700 °C) annealing is generally necessary to obtain ordered phases, PtFe0.9Cu0.1/C-500, with a small amount of Cu replacing Fe, shows an ordered bct-PtFe phase just like PtFe/C-700, indicating that Cu is able to promote the phase transformation from a disordered fcc-PtFe to an ordered bct-PtFe at lower annealing temperatures. Cu was used to partially replace Fe to tune the crystal structure of PtFe/C. The XRD patterns of PtFexCu1-x/C (x = 1, 0.9, 0.7, 0.5, 0.3, 0.1, and 0) annealed at 300 oC for 2 h are shown in Fig. S1. Longer annealing time was expected to yield a more homogenous elemental distribution across the alloy particles. All PtFexCu1-x/C annealed at 300 oC for 20 h (Fig. S2) exhibited pure Pt fcc features. With increasing Cu concentration, the peaks become sharper and shift to higher 2θ values, indicating a growth in particle size and revealing the formation of bonding between Pt and Fe (Cu) with a lattice contraction, caused by the incorporation of the smaller Fe and Cu atoms into the Pt fcc structure. The detailed XRD results of 2θ values, lattice parameters, and mean domain sizes are summarized in Table S1. Noticeably, the mean domain size of PtFe (3.6 nm) is usually smaller when compared to PtCu (5.4 nm). Most of PtFexCu1-x/C annealed at 700 °C, such as x = 1, 0.9, 0.7, 0.5, and 0.3, exhibit a bct-PtFe intermetallic structure with a larger lattice parameter “a” and a smaller lattice parameter “c”, along with increasing Cu concentration (Fig. 1a and Table S2). This is evidenced by the negatively shifted (112) and positively shifted (201) peaks, respectively. XRD patterns of PtCu/C-700 and PtFe0.1Cu0.9/C-700 indicate an intermetallic phase of PtCu (Rhombohedral, a = b = 1.0703 nm, c = 1.3197 nm, and α = β = 90 °, γ = 120 °, and PDF card #00-042-1326) as shown in Fig. S3. PtFexCu1-x/C500 (x = 0.9, 0.7, 0.5, 0.3), annealed at 500 °C (Fig. S4), exhibit an ordered bct-PtFe phase, different from PtFe/C-500, which exhibits a disordered alloy structure. The relative intensity of the superlattice ordering (100) plane of bct-PtFe, compared to the major (101) plane, can be used to represent the level of ordering between the Pt and Fe 6


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atoms in the lattice.44 The level of ordering of PtFexCu1-x/C-700 (I100/I101 varied from 16% to 22%) increases by around 3% relative to the samples annealed at 500 °C (13% to 15%), as shown in Table S2 and S3, indicating that higher annealing temperatures can improve the level of ordering. Typical overview TEM images of PtFe/C, PtFe0.5Cu0.5/C and PtCu/C samples, annealed at 300 oC for 20 h, are shown in Figs. S5a-c. The detailed TEM results of average particle sizes are summarized in Table S4. Overall, nanoparticles were well dispersed on the Vulcan carbon support. PtFe/C-300 and PtFe0.5Cu0.5/C-300 nanoparticles showed an average particle size of 2.3 nm and 3.4 nm, respectively, based on an analysis of more than 200 nanoparticles. However, PtCu/C-300 had a broader size distribution with an average particle size of 5.7 nm, indicating that Cu element can lead to larger particle sizes, when compared to Fe, for Pt based alloys annealed at 300 oC.

Metal nanoparticles generally exhibit more significant particle aggregation under

higher annealing temperatures, which are generally necessary to obtain ordered phases. Figs. S5d-h show TEM images of PtFexCu1-x/C-700 (x = 1, 0.7, 0.5, 0.3 and 0) annealed at 700 oC for 2 h. PtFe/C-700 exhibited both an increased average particle size up to 4.9 nm and a broader size distribution, compared to PtFe/C-300. PtCu/C-700 showed an average particle size of 6 nm, indicating a negligible difference in particle size, when compared to PtCu/C-300. PtFexCu1-x/C-700 (x =0.7, 0.5 and 0.3) had narrower size distributions and smaller average sizes, suggesting the ternary alloys trend to suppress particle aggregation at higher annealing temperatures. The crystal structure of PtFe0.5Cu0.5/C-700 was examined by atomic-scale high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging at 300 keV. Fig. 1b shows the ordered intermetallic structure of PtFe0.5Cu0.5 viewed along the [010] zone axis. Two perpendicular d-spacings were assigned to be the (100) and (001) planes (a and c directions of the unit cell, respectively) based on the standard XRD pattern of PtFe (PDF # 01-089-2051). Since the intensities of HAADF-STEM images are proportional to the atomic number (I ∝ Z1.7), Pt atom 7



columns are brighter than the non-noble metal atom columns, with Fe and Cu exhibiting little difference since they have very similar atomic numbers (Fig. 1b). The magnified atom columns (Fig. 1c) clearly show the periodic rectangular array of Pt atom columns surrounding the Fe and Cu atom columns, which matches the crystal model of PtFe projected on the [010] zone axis (b direction of the unit cell) (Figs. 1d-e). It can also be seen that the periodic intermetallic PtFe0.5Cu0.5 has several atomic layers on the surface with the same intensity as Pt (marked by the white arrows), suggesting the possible existence of a segregated pure Pt-rich shell, rather than a disordered PtFe0.5Cu0.5 alloy, as confirmed by EELS elemental mapping (vide infra). The chemical composition of PtFe xCu1-x/C nanoparticles was investigated using electron energy loss spectroscopy (EELS) elemental mapping. Fig. 2a shows another PtFe0.5Cu0.5 nanoparticle on the [110] zone axis with two perpendicular d-spacings, (001) and (1-10), which match the crystal model of the PtFe intermetallic shown in the inset. Figs. 2b-d present the EELS elemental maps of Pt (red), Fe(green) and Cu (blue) for the particle in Fig. 2a. Figs. 2e-g show the composite EELS maps of Pt vs. Fe, Pt vs. Cu and Fe vs. Cu. Both Pt vs. Fe and Pt vs. Cu EELS maps indicate a structure in which PtFe0.5Cu0.5 nanoparticles are surrounded by a thin Pt-rich shell. Fe vs. Cu maps show that Fe and Cu have a relatively homogenous elemental distribution and are intimately mixed with each other at the atomic-scale. The line profiles in Fig. 2h, corresponding to the white dashed boxes in Figs. 2b-d, show that the thickness of the Pt shell is about 0.5 nm, about a 2-3 atomic-layer Pt-rich shell. A similar trend for Fe and Cu in line profiles indicates that Fe and Cu have similar elemental distributions. PtFe0.7Cu0.3 also exhibits a similar Pt-rich shell with a thickness of 0.6 nm and a homogenous elemental distribution of Fe and Cu as shown in Fig. S6. This EELS chemical mapping study combined with previous analysis of atomic-scale STEM images in Fig. 1, unambiguously demonstrate that PtFexCu1-x has an ordered intermetallic structure core with a 2-3 atomic-layer Pt-rich shell, which may serve as a protective shell to mitigate the leaching of Fe and Cu and improve the durability of 8


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catalyst. It could also serve to enhance electrocatalytic activity. Based on the aforementioned ordered PtFexCu1-x core with a Pt-rich shell structure, the effect of Cu concentration on the strain effect towards the Pt shell was further analyzed. The smaller Fe or Cu atoms incorporating into the Pt fcc structure resulted in a lattice contraction. As shown in Fig. 3a, when the disordered fcc structure is transformed to an ordered fct structure, the value of the lattice parameter “c” of fct structure is smaller than “a”, and the bct structure is equivalent to a fct structure with a simpler unit cell. Based on the XRD results, as the Cu concentration increases, “c” gradually decreases since Cu atoms are smaller than Fe, and “a” increases, possibly as a response to the expansion of the unit cell in “c” direction. Fig. 3b presents a schematic atomic distribution of an individual particle observed in Fig. 1b, which has the surface covered by the (101) and (001) planes of a bct-Pt shell (Figs. 3c-d). The nearest Pt-Pt distances of these planes of the bct-Pt shell are marked as d1Pt-Pt and d2Pt-Pt, respectively, in Fig. 3e. Based on the XRD results, d1Pt-Pt increased from 2.70 Å to 2.75 Å while d2PtPt

decreased from 2.66 Å to 2.64 Å (Note that dPt-Pt of pure Pt is 2.77 Å). The effect of

Cu concentration on the lattice strain is probably dominated by the expansion effect, which could have an impact on the binding strength of reaction intermediates, thus influencing its electrocatalytic activity. CV properties in 0.5 M H2SO4 solution of PtFe/C annealed at 500 oC and 700 oC and PtFe0.9Cu0.1/C annealed at 500 oC are shown in Fig. S7. The hydrogen region of PtFe0.9Cu0.1/C-500 is similar to that of PtFe/C-700, suggesting that the outer Pt shell is remained intact and the surface areas of the particles are comparable. The electrocatalytic activity for methanol oxidation was evaluated in 1 M methanol solutions (Fig. 4a). All the currents were normalized to the electrochemical surface area (ECSA) of Pt, determined from integrating the charge under the hydrogen adsorption/desorption region from +0.05 to +0.4 V. The positive-going (anodic) sweep peak-current density of PtFe/C-500 is about six times higher than that of Pt/C. we believe the enhancement in electrocatalytic activity is, at least in part, due to the phase 9



transformation, of both PtFe/C-700 and PtFe0.9Cu0.1/C-500, with the bct-PtFe intermetallic phase exhibiting higher activities than the PtFe/C-500. It is also possibly ascribed to an optimal structure with an ordered bct-structure core, combined with a 23 atomic-layer Pt-rich shell, and a strain effect from the smaller non-noble metals incorporating into the Pt core. To assess the stability of these electrocatalysts, chronoamperometry at 0.8 V was conducted (Fig. 4b). Although the current density of all the catalysts decreased with time, PtFe/C-700 and PtFe0.9Cu0.1/C-500 exhibited higher current densities at all times relative to PtFe/C-500. Furthermore, PtFe0.9Cu0.1/C500 exhibited much better stability than PtFe/-700, suggesting that Cu plays an important role in maintaining the stability during electrochemical testing. To further understand the role of Cu in PtFe xCu1-x/C catalysts towards the electrochemical oxidation of methanol, a systematic electrochemical performance measurement of the Pt-Fe-Cu ternary catalysts was conducted. Fig. S8 presents CV profiles for Pt/C and PtFexCu1-x/C annealed at 300 oC for 20 h. The hydrogen regions were depressed by the incorporation of Fe and Cu into the Pt lattice and the area gradually decreased with increasing Cu concentrations, which could be ascribed, at least in part, to the larger particle size at higher Cu concentration. The electrocatalytic activities for methanol oxidation of PtFexCu1-x/C-300 and Pt/C are presented in Fig. S9a. All the currents were normalized to the mass of Pt on the electrodes. The positivegoing sweep peak-current densities of PtFe/C-300 and PtFe0.9Cu0.1/C-300 reached the highest values among the studied samples, about three times as high as that of Pt/C. Their activities also gradually decreased with increasing Cu concentration. To account for the effects of particle size, the specific activities (currents normalized to the ECSA) at 0.8 V are presented in Fig. S10 where it is evident that the Cu atoms replacing Fe actually improved the specific activity for the PtFe alloy. The stability of PtFexCu1-x/C-300 was assessed by chronoamperometric methods carried out at 0.8 V (Fig. S9b) with PtFe0.5Cu0.5/C-300 and PtFe0.7Cu0.3/C-300 exhibiting the best stability. These catalysts, loaded onto carbon paper, were also 10


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subjected to chronoamperometry at 0.8 V, followed by XRD and XRF, as shown in Figs. S11 and S16a, respectively. The corresponding XRD results of the 2θ values, lattice parameters, and domain sizes are summarized in Table S5. The alloy structures were perturbed since the lattice constants of PtFe xCu1-x/C-300 were larger than their initial values, indicating that the degree of alloying decreased and that the non-noble metals partially dissolved. The average particle sizes of PtFe/C-300 and PtCu/C-300 also increased from 3.6 nm and 5.4 nm to 5.1 nm and 6.4 nm, respectively, after the stability tests. Based on XRF results in Fig. S16a, the dissolution of the non-noble metals in PtFexCu1-x/C-300 ternary alloys, such as PtFe0.5Cu0.5/C-300 and PtFe0.3Cu0.7/C-300 was mitigated when compared to the binary alloys. Nevertheless, the PtFexCu1-x/C-300 ternary alloys exhibited a smaller change in the average particle size, and especially small for PtFe0.5Cu0.5/C-300, suggesting a higher resistance to Ostwald ripening, which also probably contributed to their enhanced stability. For PtFexCu1-x/C annealed at 700 °C, the electrocatalytic activity for methanol oxidation also gradually decreased with increasing Cu concentration when the currents were normalized to the mass of Pt on the electrodes (Fig. 4c). All the blank CV profiles for PtFexCu1-x/C-700 and Pt/C are shown in Fig. S12 and the ECSA calculated from the hydrogen adsorption and desorption regions are presented in Table S6. Fig. S13a clearly shows that ECSA values gradually increased along with increasing Cu concentration for PtFexCu1-x/C-700 (x = 1, 0.9, 0.7, 0.5, and 0.3), indicating that the activity degradation is not due to the particle growth. The above analysis indicates that the influence of Cu concentration on the strain effect and the lattice expansion were triggered by the smaller Cu atoms replacing Fe in bct-structure. One can argue that the combined effects could lead to a stronger adsorption of reaction intermediates resulting in decreasing activities. In the CV profiles (Fig. S12), the hydrogen-desorption peak at the higher potential suggests a stronger hydrogen binding and possibly more severe CO-like poisoning.45-46 In Fig. S13b, the hydrogen-desorption peak at lower potential gradually disappeared and another at higher potential became more prominent along 11



with increasing Cu concentration. CO stripping voltammograms can provide evidence for the binding strength of the CO on surfaces. As shown in Figs. 4d and S14, the CO stripping potential for PtFexCu1-x/C-700 moved to more positive potential values with increasing Cu concentration, suggesting stronger CO binding. Data are summarized in Table S6. The stability of PtFexCu1-x/C-700 was examined by chronoamperometric measurements at 0.8 V (Fig. 4e), and the mass activities, after the stability test (3000 s) are demonstrated in Fig. 4f. The stability increased in the sequence: PtFe0.3Cu0.7/C-700 < PtFe/C-700 < PtFe0.5Cu0.5/C-700 < PtFe0.9Cu0.1/C-700 < PtFe0.7Cu0.3/C-700. The superior durability of PtFe0.7Cu0.3/C-700 was ascribed to a robust bct-PtFe ordered structure combined with an optimal Cu concentration. XRF and XRD, following chronoamperometric measurements, were conducted to reveal the relationship between the durability and the metal dissolution process. As shown in Fig. 5a, PtFe/C-300 (with a Fe/Pt ratio of 23/77) exhibited a higher level of transition metal dissolution relative to PtCu/C-300 (Cu/Pt ratio of 34/66), since Cu is more stable than Fe in acid media. In contrast, ordered PtFe/C-700 (preserved Fe/Pt ratio of 44/56) showed a less amount of transition metal dissolution than PtCu/C-700 (preserved Cu/Pt ratio of 41/59), indicating that the bct-PtFe ordered structure has a very significant effect on suppressing the non-noble metal dissolution relative to the PtCu intermetallic structure. When compared to the ordered PtFe/C-700, PtFe0.9Cu0.1/C-700 and PtFe0.7Cu0.3/C-700 (all had a M (Fe, Cu)/Pt ratio of 46/54) exhibited an enhanced resistance to M dissolution, suggesting that the presence of Cu could suppress Fe dissolution. Taking into account the various Fe and Cu initial concentrations, their remaining contents, after the stability tests, were calculated assuming no Pt loss. This yielded a “volcano” shaped profile for the Fe composition in Fig. 5b. PtFe0.7Cu0.3/C-700 exhibited the highest Fe retention, again demonstrating that the Cu atoms replacing Fe in bct-structure can further suppress the Fe dissolution, thus accounting, at least in part, for its optimal durability. In Figs. 5c-d, the XRD patterns of PtFe0.7Cu0.3/C-700 and PtFe0.5Cu0.5/C12


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700 after the stability tests indicated that PtFe0.7Cu0.3/C-700 retained its ordered structure well. However, PtFe0.5Cu0.5/C-700 had a noticeable degradation of the ordered structure, and a disordered phase, marked by red stars in Fig. 5d, emerged from the ordered intermetallic bct-PtFe structure, suggesting that excessive Cu replacement of Fe atoms could lower the long-term durability. PtFe0.5Cu0.5/C-700 (retained M (Fe, Cu)/Pt of 41/59) actually showed more severe leaching of the transition metals than PtFe0.7Cu0.3/C-700 (retained M (Fe, Cu)/Pt of 46/54). (Fig. 5a). Conclusions In summary, ordered PtFexCu1-x ternary intermetallic nanoparticles have been well prepared and characterized. They exhibited improved electrochemical activity and stability towards the methanol oxidation, when compared to both the ordered and disordered PtFe/C and PtCu/C. This is ascribed to an optimal structure with an ordered bct-structure core combined with a 2-3 atomic-layer Pt-rich shell and a strain effect of the smaller non-noble metals incorporating into the Pt lattice of the core. The ordered bct-PtFe structure significantly mitigates the Fe dissolution when compared to the disordered PtFe alloy. Introducing Cu to partially replace Fe in PtFe xCu1-x facilitates the phase transformation from a disordered fcc-structure to an ordered bct-PtFe phase at lower annealing temperatures. Ordered PtFe0.7Cu0.3/C-700, with an optimal Cu concentration, further suppressed Fe dissolution, which is likely due to the higher stability of Cu relative to Fe in acid media. When the Cu/Fe ratio was further increased to 50/50 in PtFe0.5Cu0.5/C-700, more severe degradation of the ordered structure was observed after stability test. Furthermore, the smaller Cu atoms, partially replacing Fe in bct-structure, induced the stronger adsorption of CO-like species, resulting in a slight loss of activity towards methanol oxidation. However, this can be considered to be an acceptable compromise compared to the remarkable enhancement in stability. Overall, Cu plays a significant role in the enhancement of the catalyst durability. This study represent a strategic approach to the development of ternary alloy catalysts for methanol oxidation, in particular and fuel cells, in general. 13



Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx. Additional XRD results of PtFexCu1-x/C annealed at 300, 500 and 700 oC, XRD results of PtFexCu1-x/C annealed at 300 and 700 oC before and after chronoamperometric measurements, overview TEM images of PtFexCu1-x/C, additional STEM images and EELS maps, CV profiles and the MOR activity evaluation of PtFexCu1-x/C annealed at 300 and 700 oC, CO stripping measurements of PtFexCu1-x/C annealed 700 oC, and XRF measurements of PtFexCu1-x/C annealed at 300 and 700 oC. Author Information Corresponding author: *corresponding author: D.W.: [email protected], H.D.A.: [email protected] Author Contributions #J.Z.

and Y.Y. contributed equally to this work.

Notes The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation (21573083), 1000 Young Talent (to Deli Wang), and the Innovation Research Funds of Huazhong University of Science and Technology (2017KFYXJJ164). This work was supported as part of the Energy Materials Center at Cornell, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0001086. Atomic-scale STEM images and EELS maps were acquired using of TEM facilities of the Cornell Center for Materials Research (CCMR) which are supported through the National Science Foundation Materials Research Science and Engineering Center (NSF MRSEC) program (DMR-1719875). We thank Analytical and Testing Center of Huazhong 14


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University of Science& Technology for allowing us to use its facilities. J.Z. performed the synthesis, XRD and electrochemical measurements with the help of L.X and W.X. under the guidance of D.W. Y.Y. conducted the STEM imaging and EELS mapping under the guidance of H.D.A.

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Isotherms and Heats of Adsorption. J. Electrochem. Soc. 1965, 112, 451-455.

Figure 1. (a) XRD patterns for PtFe/C and PtFe0.9Cu0.1/C annealed at 500 oC for 2 h, and PtFexCu1-x/C (x = 1, 0.9, 0.7, 0.5, and 0.3) annealed at 700 oC for 2 h, respectively. Short red lines are the peak positions of standard intermetallic bct-PtFe (PDF # 01-0892051). (b) Atomic-resolution ADF-STEM image of PtFe0.5Cu0.5/C annealed at 700 oC. White arrows indicate pure Pt shells. (c) The magnified super lattice features from the white dotted box area in (b). (d) Crystal model of PtFe projected on the [010] zone axis based on PDF #01-089-2051. (e) Unit cell of bct-type PtFe.

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Figure 2. (a) Atomic-scale image of PtFe0.5Cu0.5 on the [110] zone axis with two perpendicular d-spacings, (001) and (1-10) (inset: crystal model of PtFe projected on the same zone axis based on PDF #01-089-2051). (b-d) EELS chemical maps of Pt, Fe and Cu. (e-g) Composite EELS chemical maps of Pt vs. Fe, Pt vs. Cu and Fe vs. Cu. (h) Line profiles of Pt, Fe and Cu extracted from white dotted boxes in (b-d).

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Figure 3. (a) Crystal models of the equivalent fct- and bct-type Pt1M1 intermetallic compounds. Yellow and red areas represent crystal facets of (c) and (d), respectively. (b) Schematic atomic distribution of an individual particle in Figure 1b, the dark grey atoms are Pt, the blue ones are non-noble metals (Fe and Cu). (c-d) Atomic distribution diagrams of crystal facets of (c) and (d) extracted from green and red dashed boxes in (b), the first and second rows are the structures of ordered Pt1M1 and Pt shell, respectively. (e) Magnified atomic distribution diagrams of the crystal facets of Pt shell from (c-d) and the yellow lines mark the nearest Pt-Pt distances, respectively. (f) The nearest Pt-Pt distance of PtFexCu1-x/C (x = 1, 0.9, 0.7, 0.5, 0.3) annealed at 700 oC for 2 h, based on the XRD results.

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Figure 4. Electrochemical performance of Pt/C, PtFe/C annealed at 500 oC and 700 oC and PtFexCu1-x/C annealed at 700 oC. (a) Cyclic voltammograms in 0.5 M H2SO4 + 1 M CH3OH at a sweep rate of 50 mV s−1. (b) Current vs. time plots of PtFexCu1-x/C annealed at 700 oC and Pt/C catalysts measured by chronoamperometry at 0.8 V vs. RHE. (c) Cyclic voltammograms of PtFexCu1-x/C in 0.5 M H2SO4 + 1 M CH3OH at a sweep rate of 50 mV s−1. (d) CO stripping voltammograms of Pt/C and PtFexCu1-x/C annealed at 700 oC in 0.5 M H2SO4 solution at a scan rate of 50 mV s-1. (e) Current vs. time plots of PtFexCu1-x/C measured by chronoamperometry at 0.8 V vs. RHE. (f) Mass activities at 0.8 V vs. RHE after stability tests for 3000 s. 22


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Figure 5. (a) XRF profiles of PtFexCu1-x/C annealed at 300 oC for 20 h and 700 oC for 2 h after chronoamperometric measurements. (b) The remained contents of Fe and Cu of PtFexCu1-x/C annealed at 700 oC for 2 h after chronoamperometric measurements. (c) XRD patterns for PtFe0.7Cu0.3/C-700 loaded onto the carbon paper before and after chronoamperometry measurements. (d) XRD patterns for PtFe0.5Cu0.5/C-700 loaded onto the carbon paper before and after chronoamperometry measurements, as well as PtFe0.5Cu0.5/C-300 as a reference. The star marks represent the four peaks attributed to disordered solid alloys.

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Copper-Induced Formation of Structurally Ordered PtâFeâCu Ternary

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