Scalable Preparation of the Chemically Ordered Pt–Fe–Au

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Scalable Preparation of the Chemically Ordered Pt-Fe-Au Nano-Catalysts with High Catalytic Reactivity and Stability for Oxygen Reduction Reactions Hong Zhu, Yezheng Cai, Fanghui Wang, Peng Gao, and Jidong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05114 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Scalable Preparation of the Chemically Ordered Pt-Fe-Au Nano-Catalysts with High Catalytic Reactivity and Stability for Oxygen Reduction Reactions Hong Zhu*, Yezheng Cai, Fanghui Wang, Peng Gao, Jidong Cao State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Department of Organic Chemistry, Beijing Engineering Center for Hierarchical Catalysts, School of Science, Beijing University of Chemical Technology, Beijing 100029, China E-mail: [email protected]; Tel: +86-10-82161887; Fax: +86-10-64444919

Highlights The introduction of trace Au significantly improves the oxygen reduction reaction (ORR) catalysis (in acid media) of the Pt-Fe system catalysts. Compared with the oil bath heating reduction process, the microwave-assisted heating reduction process is more effective for synthesizing chemically ordered Pt-based catalyst with high ORR catalytic activity. The batch prepared Au-PtFe/C-H and Au-PtFe3/C-H catalysts have improved ORR catalysis, comparing with that of commercial JM Pt/C catalyst. Supporting Information

Abstract Carbon supported Au-PtxFey nanoparticles were synthesized via microwave heating polyol process, following by annealing for the formation of the ordered structure. The structure characterizations indicate that Au is alloyed with intermetallic Pt-Fe nanoparticles and therefore the surface electronic properties are tuned. The electrochemical tests show that the microwave heating polyol process is more effective than oil bath heating polyol process for synthesizing the highly active catalysts. The introduction of trace Au (0.2 wt.% Au) significantly improves the ORR catalytic activity of PtxFey catalysts. Au-PtFe/C-H (0.66 A/mgPt) and Au-PtFe3/C-H (0.63 A/mgPt) prepared in a batch of 10.0 g show significantly improved catalytic activities than their counterparts (PtFe/C-H and PtFe3/C-H) as well as commercial Johnson Matthey Pt/C (0.17 A/mgPt). In addition, the as prepared Au-PtFe/C-H and Au-PtFe3/C-H display highly enhanced stability towards the ORR compared to the commercial Pt/C. The superior catalytic performance is attributed to the synergistic effect of chemically ordered intermetallic structure and Au. This work provides a scalable synthesis of the multimetallic chemically ordered Au-PtxFey catalysts with high ORR catalytic performance in acidic condition.

Key words: Intermetallic; Au; ORR; Pt, Catalyst 1 Introduction ACS Paragon Plus Environment

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Pt-based nanoparticles (NPs) still play an irreplaceable role as the cathodic oxygen reduction reaction (ORR) catalysts used at the cathode of fuel cell (FC)1-6. Considering the severe resource security and economic issues of Pt resources, lots of scientific researches have been devoted to incraese the intrinsic catalytic capability of Pt in unit mass, aiming at decreasing the usage of Pt in the applied FC. To advance the practical application of Pt base catalysts, the catalytic activity should be greater than or equal to the target of 0.44 A/mgPt set by the U.S. Department of Energy7. So far, various types of highly active catalysts such as bimetallic and/or multimetallic alloy, core-shell, nanowire, and octahedra etc. structure Pt-based catalysts, have been developed3, 6, 8-24. The activity issue of Pt-based catalysts is seem to be resolved. However, a considerable number of Pt-based catalysts are limited in practical use due to the lack of sufficient catalytic stability in the harsh conditions of the FC operation5, 25-28 . Taking the cost, catalytic activity, and stability of the catalyst into account together, there is plenty of room for improvement of the ORR catalyst technology. Chemically ordered intermetallic Pt-based NPs, which have high alloying degree and good electrochemical stability, show an increasing potential for FC catalysts 29-37. However, the formation of intermetallic NPs usually depends on calcination at a suitable high temperature, causing vigorous growth and agglomeration of NPs and therefore significant decline in Pt utilization of these catalysts. It is conceivable that their catalytic ORR catalysis is not satisfactory and needs to be further enhanced. Note that only a few types of Pt alloys (such as Pt-Fe, Pt-Cu, PtCo et al.) can be converted to chemically ordered intermetallics. Considering that increasing the Pt utilization of the ordered catalysts can improve their catalysis, the synthesis of relatively small Pt base intermetallic NPs is very desirable38. In addition, increasing the activity of intermetallic catalysts can also be achieved by modulating their electronic structures. By introducing an additional 3d-transition metals (Cu, Ni, etc.) to the Pt-Fe intermetallic NPs, the catalysis of the intermetallic Pt-Fe nanocrystalline can be effectively improved 29, 31, 39. Nevertheless, the ORR catalysis of intermetallic catalysts is expected to be further enhanced 29, 39. Au has been reported and confirmed to be a stability element that improves the catalysis of Pt-based nanomaterials40-44. However, there are few reports on the synergistic effect of intermetallic and Au for simulately boosting the intrinsic catalytic capability and electrochemical stability of Pt-based catalysts toward the ORR. Furthermore，very limited study was reported on the scale-up synthesis of chemically ordered Pt-based catalysts. Combining the characteristics of the stabilization effect of Au in Pt-based NPs and the advantages of Pt-Fe intermetallic in catalyzing ORR, here we proposed a practical means for generally synthesizing ternary intermetallic Au-PtxFey catalysts with high catalytic performance toward the ORR in a way that can be scaled up. For the preparation of the intermetallic Au-PtxFey catalysts, carbon supported Au-PtFe NPs were first synthesized via two methods of microwave heating polyol process and oil bath heating polyol process; the obtained products were marked as Au-PtFe/C and Oil-Au-PtFe/C, respectively. Then, these two products were annealed at high temperature under inert gas for the crystal structure transformation from disordered alloy to ordered intermetallic; the final samples were marked as Au-PtFe/C-H and

ACS Paragon Plus Environment

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Oil-Au-PtFe/C-H catalysts. These catalysts synthesis processes do not require the use of protecting and capping agents. The Au content in Au-PtxFey NP was controlled at 0.1～0.4 wt.% Au in order to avoid introducing excessive additional cost. The physical and chemical characterizations indicate that the batch prepared Au-PtFe/C-H and Au-PtFe3/C-H catalysts with chemically ordered structure show significantly higher ORR catalysis (both catalytic activity and stability) than the commercially available Pt/C catalyst; and the microwave heating polyol process is an effective strategy for synthesizing high active catalyst. This work demonstrated the scalable synthesis of efficient ORR catalysts, which combined the synergistic effect of Au and chemically ordered structure on promoting the catalysis of Pt-Fe system nano-catalysts.

2 Experimental 2.1 Chemicals and Materials Ethylene glycol (EG), chloroplatinic acid (H2PtCl6·6H2O), iron nitrate nonahydrate (Fe(NO3)3·9H2O), chloroauric acid sodium (HAuCl4·4H2O), and potassium hydroxide (KOH) are analytical reagents and were provided by the Beijing Chemical Factory of China. Nafion solution (5 wt%) was perchased from Dupont. All the obtained reagents were directly used without any treatment. Carbon black (EC-300J) purchased from Sigma Aldrich was activated using hydrogen peroxide (Beijing Tongguang Fine Chemicals Company) and used as catalyst support. 2.2 Preparation of carbon supported common PtxFey and Au-PtxFey NPs Highly-dispersed carbon supprted Au-PtxFey NPs (20 wt%) were generally prepared using microwave-assisted reduction method similar to our previous work 36, 45. The Au-PtxFey NPs compositions were controlled by tuning the molar ratio of metal precursors. For the synthesis of 10.0 g Au-PtFe carbon supported Au-PtFe NPs with 0.2 wt.% Au, a mixture of 8.0 g activated carbon black and 400 ml EG was ultrasonicated to form uniformly dispersed suspension. Then, under magnetic stirring, 134.0 ml of Fe(NO3)3·9H2O (24 g/L), 206.0 ml of H2PtCl6·6H2O/EG (20 g/L), and 2.5 ml of HAuCl4·4H2O (20 g/L) were slowly added into the above suspension. The suspension was futher stirred for 3 h for well dispersion of metal precursors on the carbon balck. Then, right amount of KOH/EG (1 M) solution was added to the suspension to control the pH value at about 9.0. Next, the obtained suspension obove was fast heated to 140 °C using a microwave reactor (Apex, China) and maintained at this temperature for 40 min for completely reducing the metal precursors. The obtained sample was marked as Au-PtFe/C. To comprehend the effect of heating process on the ORR catalysis of the synthesed catalysts, another obtained suspension was also heated via a slow heating oil bath to 140 °C and maintained at this temperature for 40 min; the obtained sample was marked as Oil-Au-PtFe/C. After cooling to room temperature, the obtained products were centrifuged and washed with a mixtrue of ethanol and deionized water, and dried at a temperature of 80 °C for one day. Au-PtFe3/C was similarly synthesized via the synthesis process of Au-PtFe/C with



different amount of metal precursors. PtFe/C and PtFe3/C were synthesized via the similar synthesis process of Au-PtFe/C/Au-PtFe3/C using microwave reactor without the addition of HAuCl4·4H2O. 2.3 Preparation of chemically ordered intermetallic NPs To increase the alloying degree of Pt-based NPs and to convert the structure of common NPs from disordered alloy to ordered intermetallic, the above synthesized Au-PtFe/C, Au-PtFe3/C, PtFe/C, and PtFe3/C mentioned above were then annealed at 500 °C. After annealing for 60 min, the annealed smaples were then controllably cooled down to 25 °C. The final samples were marked as Au-PtFe/C-H, Au-PtFe3/C-H, PtFe/C-H, and PtFe3/C-H catalysts. 2.4 Physical characterizations and electrochemical measurements Powder X-ray diffraction (XRD) patterns of the as prepared catalysts in this work were recorded using a Japan Shimadzu XD-3A diffractometer with a Cu Kα radiation (λ= 1.54 Å) for understanding the crystal structure. Transmission electron microscopy (TEM) images were photographed using a JEOL TEM 2010 microscope to reveal the morphologies. The actual compositions were determined using energy dispersive X-ray spectrometer (EDS). The surface electronic properties of Pt and Au were measured by an USA Thermo Fisher X-ray photoelectron spectrometer (LAB250 ESCA System). The distribution of different elements in catalyst NP was collected by using JEOL JEM ARM200F high-angle annular dark field scanning transmission electron microscopy (HAADF/STEM) equipped with an EDS36. 2.5 Electrochemical measurements The electrochemical measurements of the catalysts were carried out on a ZAHNER ENNIUM potentiostat with a three electrode cell at room temperature, which consists of a reference electrode (Ag/AgCl/KCl electrode), a counter electrode (platinum wire), and a working electrode. The working electrode was prepared by coating 20 ul of catalyst slurry on the glassy carbon rotating disk electrode, which has been polished and cleaned before use. The reported potentials in this work have been normalized to reversible hydrogen electrode (RHE). For the preparation of catalyst slurry, 1mg of the as prepared catalyst was ultrasonically dispersed in 1 ml of a mixed solution composed of isopropyl alcohol (250 µl), Nafion solution (5 ul), and deionized water (745 µl). The cyclic voltammetry (CV) curves were collected in N2-saturated HClO4 solution (the molar concentration is 0.1 M) at a scan rate of 50 mV/s, and the linear sweep voltammetry (LSV) curves were collected O2-saturated HClO4 solution (0.1 M) at a scan rate of 10 mV/s and a a rotation rate of 1600 rpm.

3. Results and discussion 3.1 Structural characterizations of the as prepared catalysts To reveal the crystal structure, XRDcharacterization tests were performed on the as prepared catalysts. The XRD patterns of the catalysts before annealing are shown in


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Fig. 1 (a) XRD patterns comparison between PtFe/C-H and Au-PtFe/C-H; (b) XRD patterns comparison between PtFe3/C-H and Au-PtFe3/C-H; (c) Synthesis scheme of Au-PtFe/C-H and Au-PtFe3/C-H NPs.

Fig. S1. These catalysts exhibit diffraction peaks with relatively low intensity and there is nearly no shift in the diffraction peak positions compared to pure Pt standard, suggesting low degree of alloying46. Fig. 1 shows the XRD patterns comparison among the catalysts after annealing as well as the synthesis scheme of Au-PtFe/C-H and Au-PtFe3/C-H NPs. It can be seen that the XRD diffraction peaks become sharp after annealing. As shown in Fig. 1(a), PtFe/C-H and Au-PtFe/C-H show the characteristic peaks of pure Pt. The characteristic peaks located at about 2θ=40°, 46°, 67°, and 82° are assigned to the Pt characteristic crystal palnes of (111), (200), (220), and (311) 33. The positions of these four characteristic peaks shift to higher 2θ angles when compared to those of pure Pt standard. This shift is attributed to the incorporation of small Fe atoms into Pt lattice, forming bimetallic alloy phase with lattice contraction47. Note that the Au-PtFe/C-H shows a negative shift of peak positions when compared to PtFe/C-H (seen in Fig. 1(a) and Fig.S2(a), which is the result of addition of relatively large Au atoms14, 48; this negative shift indicates the existence of alloying between Au and PtxFey. However, Au-PtFe3/C-H show the positive shift of peak positions than PtFe3/C-H, indicating higher alloying degree (seen in Fig. 1(b) and Fig.S2(b)). This phenomenon is ascribed to that the Au can promote the alloying degree of Au-PtFe3/C-H, which has much more Fe atomic ratio than Au-PtFe/C-H48. However, as shown in Fig. S3, the positions of Pt characteristic peaks for Oil-Au-PtFe/C-H are nearly consistent with those of pure Pt standard, indicating low degree of alloying. In addition, the diffraction peaks at about 23° and 33°, which are the characteristic peaks of chemically ordered structure, are small; this indicates low degree of ordering of Oil-Au-PtFe/C-H. Combining the XRD peak positions of Au-PtFe/C-H, we can infer that the microwave heating obtains NPs with



higher degree of alloying than oil bath heating. In addition, both Au-PtFe/C-H and Au-PtFe3/C-H have additional diffraction peaks at about 2θ=38°, which can be attributed to Au metallic (Au PDF # 04-0784)49. The appearance of Au metallic is attributed to that the Au can not be well alloyed with Fe and/or Pt during the reduction of the precursors. Because Au has a much higher standard electrode potential than that of of both Pt and Fe, the Au precursor is easily reduced into metallic Au when compared to Pt and Fe precursors during the first step of microwave-assisted reduction process; so most Au exists in the form of NP core. However, some Au could exists in the form of NP shell due to the rapid and uniform heating of microwave-assisted reduction process. Furthermore, after annealing, partial internal Au would like to segregate outward and alloy with Pt-Fe intermetallic, as evidenced from the differences of diffraction peak positions for PtFe/C-H and Au-PtFe/C-H as shown in Fig. 1(a), PtFe3/C-H and Au-PtFe3/C-H as shown in Fig. 1(b). In short, after annealing, most Au was well alloyed with Pt-Fe NP. The synthesis scheme of Au-PtFe/C-H and Au-PtFe3/C-H is shown in Fig. 1(c). The alloying among Au, Pt and Fe will be further confirmed using EDS in association with HAADF/STEM. Furthermore, the PtFe/C-H and Au-PtFe/C-H exhibit two small peaks at about 2θ=23° and 33°, which are attributed to the (001) and (110) superlattice planes of intermetallic phase29, 31, 39, 47. However, no clear superlattice plane can be found for both PtFe3/C-H and Au-PtFe3/C-H, which can be ascribed to that the formation of intermetallic requires special composition and that the content of intermetallic phase is less. Compared with the XRD patterns of PtFe/C-H and Au-PtFe/C-H in Fig. 1(a), the XRD patterns of PtFe3/C-H and Au-PtFe3/C-H in Fig. 1(b) are broader, suggesting a smaller crystal size based on the Scherrer’s equation. This phenomenon was attributed to the increase of Fe limits the particle growth or the incomplete alloying between Pt and Fe50. The difference in the diffraction peak shape in Fig. 1(a) and (b) is attributed PtFe3/C-H and Au-PtFe3/C-H have much higher Fe content than of PtFe/C-H and Au-PtFe/C-H. In addition, the Fe in PtFe3/C-H and Au-PtFe3/C-H was not completely alloyed with Pt. The crystallization of the unalloyed Fe was not well as that of alloyed Au-PtFe; the partial unalloyed Fe exists in the form of oxidized state. The TEM characterization results of PtFe/C-H and PtFe3/C-H catalysts are shown in Fig. S4. The NPs of these catalysts are well distributed on carbon black without agglomeration and have relatively small particle size for both PtFe/C-H (the average particle size d=4.4 nm) and PtFe3/C-H (d=3.3 nm), suggesting robustness of the synthetic strategy. The Au-PtFe/C-H (d=3.8 nm) and Au-PtFe3/C-H (d=3.2 nm) exhibit similar structural features as shown in Fig. S5. TEM characterization demonstrates that the introdution of Au will not basically change the macroscopic morphology of NPs. These features are beneficial to improve the Pt utilization of the catalysts and therefore enhance the ORR catalysis. In order to further observe the microstructure of the catalyst metal particles, we carried out HRTEM characterization. Fig. 2 shows the expanded HRTEM views of PtFe/C-H (Fig. 2(a)), PtFe3/C-H (Fig. 2(b)), Au-PtFe/C-H (Fig. 2(c)), and Au-PtFe3/C-H (Fig. 2(d)) catalysts; the focused planes are attributed to the Pt (111) plane. The spacings of Pt (111) plane for PtFe/C-H,


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PtFe3/C-H, Au-PtFe/C-H, and Au-PtFe3/C-H catalysts are measured to be 0.225, 0.220, 0.242, and 0.239 nm, respectively3, 10, 51-52. The Au-PtFe/C-H and Au-PtFe3/C-H show slightly larger Pt(111) spacings than PtFe/C-H and PtFe3/C-H; this is a result of lattice expansion caused by the introduction of Au, which has a larger atomic radius than Fe. HRTEM characterization indicates that the introduced Au in the Pt-Fe system regulates the microstructure structure of Pt to a certain degree. In other words, it also indicates the existence of alloying between Pt and Au. The compositions of the as prepared catalysts were analyzed by EDS spectrometer. The EDS spectra of these catalysts are shown in Fig. S6 and the results are listed in Table 1. Table 1 EDS analysis results of the as prepared catalysts Catalysts PtFe/C-H Au-PtFe/C-H PtFe3/C-H Au-PtFe3/C-H

Composition/atomic% Pt

Fe

Au

49 50 27 24

51 49 73 75

1 1

Fig. 2 Expanded HRTEM views of (a) PtFe/C-H, (b) PtFe3/C-H, (c) Au-PtFe/C-H, and (d) Au-PtFe3/C-H catalysts.



Fig. 3 Enlarged (a) and atomic-resolution (b) HAADF/STEM images of Au-PtFe/C-H NPs at different regions.

We further used HAADF/STEM to analyze the microstructure of the as-prepared catalysts. Fig. 3 shows an enlarged HAADF/STEM images of NPs in different regions of Au-PtFe/C-H catalyst. As shown in Fig. 3(a) and (b), the Au-PtFe/C-H metal particles have an ball-like shape and relatively small a particle size. According to the measured lattice spacing of the NP in (a), the lattice fringes are attributed to the (001) superlattice of the PtFe intermetallic, further confirming the formation of chemically ordered structure; the HAADF/STEM analysis result is consistent with the aforementioned XRD analysis result. Atomic-resolution HAADF/STEM characterization of another catalyst NP as dispalyed in Fig. 3(b) showed that the intensity of bright spots on the focused crystal surface is mostly consistent, indicating that the different elements of Au, Pt, and Fe are uniformly distributed and are highly alloyed in the metal NP. According to HAADF/STEM analysis, we can initially infer that the NP of Au-PtFe/C-H possessed an alloy structure. In order to reveal its microstructure as much as possible and to further confirm the existing alloying form of different elements in Au-PtFe/C-H, we characterized the element distribution of the individual NPs using HAADF/STEM. Fig. 4 shows the enlarged HAADF/STEM image of the catalyst and the EDS elemental distributions. As seen from Fig. 4(b), (c), and (d), different elements of Au, Pt, and Fe appear at the same positions of the metal particles in Fig. 4(a), further confirming the uniform element distribution and alloying form. This mainly attributed to the rapid reduction process of metal precursors using microwave heating. The high dispersion of Au in Pt-Fe NPs favors its effect on the electronic properties of Pt and thus changes their catalytic performance. In addition, as shown in Fig. 4(b), the signal belonging to Au concentrates in the middle of the NP and gradually decreases to the edge, suggesting that the NP has an ultra-fine Au core and a well alloyed Au-Pt-Fe phase (shell). Nevertheless, it is still difficult to precisely determine the NP microstructure of Au-PtFe/C-H and Au-PtFe3/C-H due to the very low Au content and the ultra-fine core Au.


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Fig. 4 (a) Enlarged HAADF/STEM image of Au-PtFe/C catalyst; (b, c, d) the EDS elemental distributions of Au, Pt, and Fe.

The surface electronic properties of PtFe/C-H, Au-PtFe/C-H, PtFe3/C-H, and Au-PtFe3/C-H were characterized by XPS. The XPS results of these catalysts are shown in Fig. 5 (a). Clearly the binding energies of Au-PtFe/C-H and Au-PtFe3/C-H are slghtly lower than those of PtFe/C-H, suggesting a decrease of the surface oxygen affinity13. This decrease is beneficial to the dissociation of the oxygen-containing species from Pt in the process of catalyzing ORR, which will help to release the catalytic active sites and therefore promote the ORR. The XPS of Au-PtFe/C and Au-PtFe3/C-H catalysts in Au 4f region are shown in Fig. 5 (b); the black vertical lines represent the binding energies of metallic Au0. The Au 4f peaks shift to higher binding energies, suggesting electronic interaction between Au and PtFe through alloying, which is consistent with XRD results49, 53-54. Previous report has pointed out that the electronic properties of Pt on the NP surface may not be tuned by the inner Au core44. Therefore, the change in the surface electronic properties of Au-PtFe/C-H and Au-PtFe3/C-H can be attributed to the alloying between Pt-Fe and Au in the NP



shell44.

Fig. 5 (a) XPS of PtFe/C-H, Au-PtFe/C-H, PtFe3/C-H, and Au-PtFe3/C-H in the Pt 4f region; (b) Au 4f XPS of Au-PtFe/C and Au-PtFe3/C-H.

3.2 ORR performance evaluation of the as prepared catalysts The ORR catalytic performance of the as prepared catalysts were evaluated by CV and LSV at 25 °C. In order to select the preparation route of chemically ordered Au-PtxFey catalysts, we recorded the CV and LSV curves of two representative catalysts (Oil-Au-PtFe/C-H and Au-PtFe/C-H) prepared using oil bath heating reduction process and microwave-assisted heating reduction process; the results are shown in Fig. S7. The ORR catalytic performance of JM Pt/C was so evaluated and provided for comparision. It can be seen from Fig. S7(a) that the as prepared catalysts display Pt redox peaks and relatively high hydrogen adsorption peaks (HAP), indicating the Pt rich surface. To understand the Pt utilization, the electrochemically active surface area (ECSA) values of different catalysts were measured from the HAP charge (0.05 to 0.4 V) with 210 µC/cm2Pt as a conversion factor34. Au-PtFe/C-H (109.44 m2/gPt) shows a slightly increase in HAP (also means a slightly increase of Pt utilization) than Oil-Au-PtFe/C-H (103.51 m2/gPt). However, Au-PtFe/C-H shows an obviously higher half-wave potential (E1/2) than Oil-Au-PtFe/C-H as seen in Fig. S7(b), showing much increased catalytic activity toward the ORR. For quantitatively revealing the catalytic ability of these catalysts, their kinetic currents was normalized to Pt loading and ECSA of Pt on the electrode to obtained mass and specific activity, respectively; the kinetic current can be obtained via the Koutecky-Levich equation. As seen in Fig. S7(c), whether mass activity or specific activity, the ORR catalytic ability of Au-PtFe/C-H is clearly higher than that of Oil-Au-PtFe/C-H. One explanation for this phenomenon is that the former has a higher degree of alloying than the latter as indicated in the XRD analysis described above. The initial evaluation of catalytic activity for Oil-Au-PtFe/C-H and Au-PtFe/C-H catalysts indicates that microwave heating polyol process is more effective than oil bath heating polyol process for synthesizing Pt-based catalyst with high ORR catalytic activity.


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Fig. 6 Summary of the electrochemical properties of different catalysts. (a, c, e) CV curves, LSV curves, and catalytic activity histograms of JM Pt/C, PtFe/C, Au-PtFe/C, PtFe3/C, and Au-PtFe3/C catalysts. (b, d, f) CV curves, LSV curves, and catalytic activity histograms of JM Pt/C, PtFe/C-H, Au-PtFe/C-H, PtFe3/C-H, and Au-PtFe3/C-H catalysts.

The summary of the ORR catalytic performance of different catalysts using microwave-assisted heating reduction process annealing is displayed in Fig. 6. The CV curves of the as prepared catalysts with and without annealing are shown in Fig. 6(a) and (b); the CV curve of JM Pt/C catalyst was also displayed for comparison. As can be seen from Fig. 6(a), the Au-PtFe/C and Au-PtFe3/C exhibited an obvious increase in HAP area than their counterparts (PtFe/C and PtFe3/C) without Au, suggesting an increase in Pt utilization with the introduction of Au. After annealing,



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the obtained PtFe/C-H, Au-PtFe/C-H, PtFe3/C-H, and Au-PtFe3/C-H catalysts even show considerable HAP areas. The LSV curves of as prepared catalysts without and with annealing are dispalyed in Fig. 6(c) and (d). The LSV curve of JM Pt/C is also displayed in Fig. 6(c) and (d) for comparison. The as prepared catalysts except PtFe3/C show different positive shifts of E1/2, suggesting superior ORR catalytic ability. The ECSA and E1/2 values of different catalysts before durability test are listed in Table 2. The annealed catalysts show only small decrease in ECSA, indicating good Pt utilization. In addition, the E1/2 values of the catalysts after annealing show larger positive shifts compared to those before annealing, displaying further improved catalytic ability for the ORR. The improvement of ORR catalysis is attributed to the increased alloying degree of PtFe/C-H, Au-PtFe/C-H, PtFe3/C-H, and Au-PtFe3/C-H after annealing. The Au-PtFe/C-H shows the highest catalytic capacity (highest E1/2 value) among the tested catalysts, being consistent with the previously reported results that the PtFe catalyst shows the best catalytic activity among Pt-Fe catalysts with different compositions55. The high catalytic activity of Au-PtFe/C-H is contributed by the chemically ordered structure along with relatively high content of Fe. Table2 ECSA and E1/2 values of different catalysts before durability test Catalysts

ECSA (m2/gPt)

E1/2 (V)

JM Pt/C PtFe/C Au-PtFe/C PtFe3/C Au-PtFe3/C

64.99 134.48 148.29 129.81 135.01

0.878 0.892 0.905 0.862 0.886

Catalysts

ECSA (m2/gPt)

E1/2 (V)

PtFe/C-H Au-PtFe/C-H PtFe3/C-H Au-PtFe3/C-H

113.05 109.44 130.68 160.00

0.905 0.926 0.882 0.913

Fig. 6(e) and (f) show the catalytic activity histograms of the as prepared catalysts with and without annealing, respectively; the ORR catalytic activity of JM Pt/C was also provided for comparison. The catalytic activities of the catalysts were significantly increased after annealing. The mass activity for Au-PtFe/C-H is 0.66 A/mgPt and 0.63 A/mgPt for Au-PtFe3/C-H, about 50% higher than that of the 2017 DOE′s target (0.44 A/mgPt). Surprisingly, Au-PtFe/C-H and Au-PtFe3/C-H show much higher catalytic activity (both mass activity and specific activity) than their counterparts (PtFe/C-H and PtFe3/C-H) without Au, suggesting that the addition of a small amount of Au has tuned the electronic property of Pt as well as the essentially catalytic capacity. The significantly improved catalytic activity for these two catalysts is attributed to the decrease of oxygen affinity (as described in XPS analysis) along with higher degree of alloying (as described in XRD analysis). Stamenkovic et al. have showed that the Au core cannot alter the electric property of the topmost Pt atoms as well as their catalytic activity. This difference in the Au effect on the catalysis of Pt is caused by the different nanostructure of metallic NP. For Au-PtFe/C-H and Au-PtFe3/C-H as-prepared in this work, trace amounts of Au are highly dispersed in the Pt-Fe phase and alloyed with Pt-Fe, forming chemically ordered ternary intermetallic and therefore tuning the electronic property of Pt.


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Fig. 7 CV (a, c, e) and LSV (b, d, f) curves of JM Pt/C, Au-PtFe/C-H, and Au-PtFe3/C-H catalysts before and after durability test.

To evaluate the catalytic stability of the as prepared catalysts, a durability test was carried out by applying 5,000 sweeps potential cycling in O2-saturated HClO4 solution. The scanning potential range was 0.6-1.0 V, and the scan rate was 50 mV/s. Fig. 7 shows the summary of catalytic properties of JM Pt/C, Au-PtFe/C-H, and Au-PtFe3/C-H before and after 5,000 sweeps. After durability test, it can be seen from Fig.7(a), (c), and (e) that these three catalysts show a decrease in HAP area, indicating the active site loss. The comparison of LSV curves before and after durability test are displayed in Fig. 7(b), (d), and (f). After durability test, the decreases of half-wave potential for JM Pt/C, Au-FePt/C-H, and Au-FePt3/C-H were measured to be 33, 5, and 6 mV, respectively. The ECSA values, mass activities, and specific activities comparisons of the tested catalysts before and after durability test are shown in Fig. 8. The as prepared catalysts (Au-PtFe/C-H and Au-PtFe3/C-H) still show much increased



catalytic activity than JM Pt/C after durability test. The decrease of mass activity for Au-PtFe/C-H (3.0%) and Au-PtFe3/C-H (8.0%) is much less than that (47.1%) of JM Pt/C. In addition, compared to JM Pt/C, these two catalysts show minor changes in specific activity, suggesting good stability of both the structure and little change in the surface electronic property of Pt. These observed phenomena strongly confirm high ORR catalytic stability of the Au-PtFe/C-H and Au-PtFe3/C-H catalysts. This improved ORR catalysis is the synergetic results of the increased alloying degree and the addition Au 23, 44, 56-57. To examine the effect of Au on catalytic stability, the catalytic properties of the catalysts without Au before and after durability test were also recorded and the results are displayed in Fig. S8. Clearly PtFe/C-H and PtFe3/C-H catalysts also exhibit high catalytic stability for the ORR, as evidenced from little change in both E1/2 and catalytic properties (Fig. S9) of these two catalysts after durability test. At this point, the as-synthesized catalysts have excellent ORR catalysis in acidic media, suggesting the feasibility of the synthetic strategy for efficient catalysts. However, note that Au-PtFe/C-H and PtFe/C-H show an decrease in specific activity after durability test, and Au-PtFe3/C-H and PtFe3/C-H show the opposite behavior compared to the formers. These different specific activity changes is mainly due to the difference of composition. The decrease of specific activity for the front catalysts is attributed to the decrease of ligand effect due to the leaching of surface active metal Fe and its oxides. However, thanks to the high Fe content, Au-PtFe3/C-H and PtFe3/C-H even exhibit an increase in specific activity after stability test, indicating an improvement of catalytic ability of a single Pt atom. For Au-PtFe3/C-H and PtFe3/C-H, the Fe was rich on the NP surface and may impedes the contact of oxygen with the active site Pt. After stability test, the surface Fe and its oxides gradually dissolve into the solution, promoting the contact of oxygen with the active site Pt.

Fig. 8 ECSA (a), mass (b) and specific (c) activity comparisons of the catalysts before and after durability test.

The catalytic mechanism of the as prepared catalysts was also studied by recording the LSVcurves at rotation rates of 100, 400, 900, 1600, and 2500 rpm in O2-saturated HClO4 solution; the measured results along with the Koutecky–Levich plots are dispalyed in Fig. S10. For Au-PtFe/C-H and Au-PtFe3/C-H catalysts, the electron-transferred numbers were calculated to be 3.8 and 4.1 at 0.60-0.80 V, respectively, meaning that the O2 was directly and efficiently reduced to produce H2O


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on these catalysts. The structural features of the catalysts after durability test were also characterized by TEM for explaining their superior ORR catalysis. Fig. S11 shows the TEM characterization results of the catalysts. Clearly, there is nearly no change in the NP distribution and particle size for these two catalysts after durability test compared to the initial ones as described above. For Au-PtFe/C-H (3.3 nm) and Au-PtFe3/C-H (3.2 nm), their particle sizes were kept in a small range, showing good structural/geometric stability. The little change in particle size also indicates strong binding between carbon carrier and NPs and the geometry integrity of NPs for these catalysts. Because an introduction of a small amount of Au can effectly increase the catalytic ability of PtFe/C-H and PtFe3/C-H, we further examined the effect of different Au contents on the ORR catalysis. Au-PtFe/C-H with 0.1wt.% and 0.4wt.% of Au were also prepared with the similar synthesis process of Au-PtFe/C-H with different amount of Au precursors；the obtained smaples were marked as 0.1wt.% Au-PtFe/C-H and 0.4wt.% Au-PtFe/C-H. The EDS spectra of these two catalysts are displayed in Fig. S12. The ORR catalysis of 0.1wt.% Au-PtFe/C-H and 0.4wt.% Au-PtFe/C-H were also evaluated in O2-saturated HClO4 solution. For ease of identification and comaprison, Au-PtFe/C-H is noted as 0.2wt.% Au-PtFe/C-H. The summary of the ORR catalysis results of JM Pt/C, PtFe/C-H, and 0.2wt.% Au-PtFe/C-H with different Au contents catalysts are shown in Fig. S13. From Fig. S13(c), we can see that 0.2wt.% Au-PtFe/C-H exhibits the best ORR catalytic activity. In this regard, we think that too small content of Au cannot effectively regulate the Pt electronic structure, and excessive Au will inhibit the active site Pt of the catalyst. Therefore, the amount of Au introduced into the catalyst NP must be precisely controlled for maximizing the catalysis. Besides, to reveal the effect of annealing conditions on the ORR catalysis of the catalysts, we also study the microstructure change of catalysts prepared at harsher annealing condition and the ORR catalysis. Au-PtFe/C and Au-PtFe3/C were annealed up to 700 °C; the obtained samples were marked as Au-PtFe/C-700 and Au-PtFe3/C-700 catalysts. The TEM characterization results of these two catalysts are shown in Fig. S14. The particle sizes for both Au-PtFe/C-700 (d=6.5 nm) and Au-PtFe3/C-700 (d=5.6 nm) were sharply increased compared to those of their counterparts annealed at 500 °C. In addition, agglomeration phenomenon can be observed for these catalysts obtained by annealing at 700 °C. Considering excessive temperature will lead to a significant increase in particle size, which is not conducive to increase the Pt utilization of the catalyst, annealing above 700 °C is not recommended. The ORR catalysis of Au-PtFe/C-700 and Au-PtFe3/C-700 were also evaluated and the results are displayed in Fig. S15(c). Compared with Au-PtFe/C-H prepared at a annealing temperature of 500 °C, Au-PtFe/C-700 (0.39 A/mgPt) showed an significantly decrease in mass activity for ORR. Similarly, the ORR catalytic activity of Au-PtFe3/C-700 (0.37 A/mgPt) was greatly reduced compared to Au-PtFe3/C-H, which is a result of the increase in particle size due to excessive annealing temperature. Furthermore, the intrinsic catalytic abilities (specific activity) of Au-PtFe/C-700 and Au-PtFe3/C-700 were also weakened compared to their



counterparts prepared at a annealing temperature of 500 °C. The degradation of catalytic ability suggested that the effect of Au in regulating surface Pt is weakened due to the sharply increased particle size. Therefore, the annealing temperature should be controlled below 700 °C for catalysis optimization55.

4 conclusion This work demonstrated the synergy of Au and chemically ordered inermetallic structure on boosting the ORR catalysis of Pt-Fe system catalysts. The chemically ordered Au-Pt-Fe catalysts, prepareded by a general two-step method contains a polyol reduction process and calcination, show superior ORR catalysis than JM Pt/C catalyst. Compared with the oil bath heating reduction proccess, the microwave-assisted heating reduction proccess is more conducive to increase the degree of alloying as well as the ORR catalytic activity of Au-PtFe catalyst. The catalytic ability of PtFe/C-H and PtFe3/C-H was significantly increased through the introduction of Au. The as prepared chemically ordered Au-PtFe/C-H and Au-PtFe3/C-H catalysts showed about 50% higher mass catalytic actvity than the 2017 U.S. DOE′s target, together with highly enhanced electrochemical stability compared to JM Pt/C, displaying viable application prospect for FC. The improved catalytic performance is the synergetic result of the tuned surface electronic characteristics of the Au-PtFe/C-H and Au-PtFe3/C-H causing by the introduction of trace Au, relatively high Fe content, as well as the unique intermetallic structure. The strategy reported in this work can be a practical reference for the design of scalable synthesis of superior catalysts for fuel cell and other catalitic applications.

Acknowledgments We gratefully appreciate the financial support from the National Key Research and Development Program of the Beijing Municipal Science and Technology program (No.Z171100000917019), the National Natural Science Foundation of China (No. 21376022 and U170520055), National Key Research and Development Program of China (No. 2016YFB0101203), the International S&T Cooperation Program of China (No. 2013DFA51860), and the Fundamental Research Funds for the Central Universities (No. JC1504).

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Scalable Preparation of the Chemically Ordered Pt–Fe–Au

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