Remarkable Improvement of the Catalytic Performance of PtFe

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Remarkable Improvement of the Catalytic Performance of PtFe Nanoparticles by Structural Ordering and Doping Yang He, Yan Lin Wu, Xin Xing Zhu, and Jian Nong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01810 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Applied Materials & Interfaces

Remarkable

Improvement

of

the

Catalytic

Performance of PtFe Nanoparticles by Structural Ordering and Doping Yang He, Yan Lin Wu, Xin Xing Zhu, and Jian Nong Wang* Nanocarbon and Manufacturing Innovation Center, School of Mechanical and Power Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China.

ABSTRACT In order to achieve fuel cell commercialization, the performance improvement and cost reduction of catalysts are still the main challenges. To enhance the catalytic activity and durability for oxygen reduction reaction (ORR), we prepare Au-PtFe particles entrapped in a porous carbon and then convert them to have a fine grained and highly ordered intermetallic structure. The optimal Au-PtFe particles toward catalyzing ORR exhibit initial specific and mass activities 9 times higher than the commercial catalyst of Pt/C. Such a large enhancement is much higher than most of the Pt-based ordered intermetallic catalysts reported in the literature. Accelerated durability testing induces little degradation of the catalytic activity to the ordered structure, particularly the Au-doped one, after potential cycling for many thousands of cycles under harsh electrochemical conditions involving an acidic medium and high potential range of 0.66~1.3 V. This is in big contrast with the large degradation shown by most previous catalysts. The excellent activity and durability are attributed to synergistic effects of the fine-grained and ordered structure of the particles, the confining support of the porous carbon, and the homogeneous incorporation of a trace amount of Au. The new intermetallic catalyst of Au-PtFe/C represents a new strategy for performance enhancement and cost reduction and thus promotes practical applications of proton exchange membrane fuel cells.

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KEYWORDS: Oxygen reduction reaction (ORR) ‧ PtFe nanoparticles ‧ ordered structure ‧ synergistic effects ‧ Au doping

INTRODUCTION

Proton exchange membrane fuel cell (PEMFC), as one of the most promising new power sources, have attracted world-wide studies owing to their high energy density, high energy conversion efficiency and environmental friendliness. However, the oxygen reduction reaction (ORR) at the cathode is sluggish and has to be triggered and driven by an active catalyst. The catalyst having been used so far is still Pt-based, although non-Pt ones have been under intensive research. However, the Pt catalyst is of high cost and suffers from insufficient durability. The durability issue originates from the harsh fuel cell operation conditions involving acidic media and high voltages.1-3 It is well known that PEMFCs are exposed under not only a dynamic driving load, but also multiple start-stops. It’s believed that start-stop cycles have a stronger effect on the process of Pt dissolution than the other aging cycles at the voltage of about 1.169 V. In addition, the dissolution rate of Pt was found to be greatly dependent upon the cycling potential up to 1.15 V and the temperature from 40 to 80°C.4-7 This is one of the main reasons why PEMFCs have not yet reached the stage of wide commercialization. Many efforts8,9 have been devoted to solving the above mentioned problems, including alloying Pt with 3d-transition metals, constructing core-shell structures, and promoting the disordered alloy structure to an ordered intermetallic one. The alloy catalysts,10-15 particularly, the bimetallic Pt-M (M=Fe, Co, Ni) ones, offer the opportunity to obtain enhanced selectivity, activity, and stability, due to their electronic and chemical properties that are distinct from those of their parent metals. Catalysts with a Pt shell16-19 have the potential to lower Pt usage and achieve ultrahigh ORR activity. However, these catalysts were mostly cycled at potentials lower than 1.0 V. The stability of the Pt shell and thus the durability of the catalytic activity at potentials higher than 1.0 V await detailed investigation. Pt-M intermetallic compounds20-25 with a fully ordered structure have demonstrated better performance as ORR electrocatalysts in comparison with disordered alloys. However, high temperature annealing (generally >700°C) is necessary to transform the disordered (face-centered cubic-fcc) structure into the ordered one (face-centered-tetragonal-fct), which inevitably causes the sintering and coarsening26,27 of the originally fine particles (Table S1).


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Therefore, the fct-structured catalysts have not yielded the maximal activity for ORR. In addition, accelerated durability tests have shown that the intermetallic catalysts, even having an ordered structure, still undergo more or less activity loss after several thousands of potential cycles even at potentials below 1.0 V (Table S2). In this study, we report successful synthesis of carbon supported PtFe intermetallic compounds with an average size of only 3.65 nm by using an impregnation and solid state-diffusion approach. Differing to our previous work22, we use a new carbon support and thus greatly simplify the synthesis process. Furthermore, we prepare the Au-PtFe compounds with different amounts of Au doped. The optimized Au-PtFe catalysts are shown to have an excellent activity and extra high durability toward the catalysis of ORR under harsh electrochemical conditions in comparison with typical Pt-based alloy or intermetallic catalysts reported so far. RESULTS AND DISCUSSION Structure Characterization of Catalysts. The formation process of Au-doped intermetallic PtFe embedded in a carbon shell is illustrated in Figure 1. The Fe/C powder originated from the simultaneous pyrolysis of iron carbonyl and acetylene and contained amorphous Fe clusters homogeneously distributed in amorphous carbon (Figure S1a). The metallic composition of the Fe/C powder was measured as 33% (wt.%) by TGA in air (Figure S2). The Fe clusters served as the nucleation site and reducing agent of the dissolved Pt and Au precursors. The as-prepared PtFe and Au-PtFe nanoparticles underwent structural transformation at a temperature of about 900 °C. The atomic ratios of Pt:Fe:Au were measured as 1:1:0.02 for Au-PtFe/C, and that of Pt:Fe as 1:1 for PtFe/C and 1:2 for PtFe2/C by ICP-AES. The total metal content in each sample was about 40 wt.%. As shown in Figure S1b and S1c, carbon-coated Au-PtFe particles after the thermal annealing had a size of 3.65 nm and distributed uniformly on carbon. This size is close to the initial size (2.5 nm) of the unheated Au-PtFe particles. Such a small variation of the particle size before and after thermal annealing reveals that the carbon shell could prevent Au-PtFe particles from sintering and coarsening even when they are heated at a temperature as high as 900 °C. In addition, HRTEM images (Figure S3) clearly showed the structural changes of AuPtFe/C nanoparticles before and after the heat treatment. The presence of the carbon shell after heat treatment indicates that the final carbon material acted as both the protective coating and support for the metallic catalyst particles (Figure S3e, S3f). This is the unique feature of the present Fe/C powder favourable for achieving fine intermetallic catalysts with high activity, as will be



shown below. However, the heat treatment at temperatures higher than 900 °C had led to larger particle sizes and thicker carbon shells and thus loss of catalytic activity.

Figure 1. Schematic for the synthesis of Au-PtFe/C intermetallic compound nanoparticle.

To understand the ordered intermetallic fct structure of Au-PtFe after the annealing and the effect of Au-doping, XRD spectra were obtained and are presented in Figure 2. Several observations are worthy to note. 1) Both fct Au-PtFe and fct PtFe demonstrate the characteristic (100) and (110) peaks for the superlattice21,24,28 (as marked with asterisks) of the ordered fct structure in comparison with the as-deposited PtAuFe/C and fcc-PtFe2/C. The fcc structure for PtFe2 reveals that a proper atomic ratio is necessary to generate the ordered fct structural transformation. 2) By taking close examination of the XRD patterns (Figure 2, inset), we can see that the (111) peak for fct Au-PtFe has downshifted slightly relative to that of the fct PtFe, and the interplanar spacings of (111) planes are calculated to be 0.219 and 0.222 nm for PtFe and Au-PtFe, respectively, according to Bragg’s law. In comparison with the ordered fct-PtFe lattice structure, the (111) diffraction peak of the disordered fcc-PtFe had a large positive shift, indicating that Au doping, even by a trace amount (Au:Pt=1:50), induced slight expansion to the ordered fct lattice with the ordered fct structure maintaied. 3) The peaks at 2θ = 22.9o and 73.7o regarded as (001) and (003) superlattice peaks in the ordered AuFe intermetallic structure can’t be found in the XRD pattern of fct-Au-PtFe. Thus, Au atoms tend to be in a disordered state in the intermetallic lattice. 4) The degree of fct structure ordering, or fct ratio, may be estimated based on the ratio of the diffraction intensities of the superlattice peak and the strongest peak (for details, see Table S3). For the PtFe and Au-PtFe samples, the fct ratios are 86.5% and 81.4%, respectively, suggesting that these samples were not fully ordered. However, as shown below, such deviations (1) larger than the value (=0.95) for fct-PtFe (Table S5). Therefore, CO-striping results further verify the existence of a Pt-skin structure. Origin of the Enhanced Durability. A catalyst degrades in activity during electrochemical cycling mainly by two mechanisms, particle agglomeration (or coarsening) and dissolution


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into the electrolyte. The enhanced durability of the present PtFe/C, particularly Au-PtFe/C, may be closely related to the protective effects of the carbon shell, intermetallic structure and Au doping imposed on these degradation processes. Our unique preparation method is beneficial to the formation of Au-PtFe particles with thin carbon shells during the process of high-temperature annealing for structural ordering. The observation of the retention of the initial uniform distribution of fine particles after long-term cycling is indicative of little coarsening (only from 3.65 to 5.40 nm, Figure S11c). In addition, the carbon shell after ADTs (as shown in the HRTEM images, Figure S12c, S12d) demonstrated the graphitic structure, which is nearly the same as the structure before ADTs (Figure S3d, S3e). Such limiting effect from the carbon shell avoids the particle coarsening by surface diffusion. In contrast, Pt nanoparticles in Pt/C catalyst not only dissolve and agglomerate into larger particles (from 2.3 to 9.18 nm, Figure S17) via Ostwald ripening process, but also are detached from the carbon support due to the weak Pt-carbon interaction. As a result, the Au-PtFe catalyst degraded very slowly, but the Pt/C one very quickly as can be seen from the variation of particle size with cycling number. Our initial AuPtFe/C catalyst was heat-treated at 900°C in a tube furnace under vacuum to achieve the ordered intermetallic fct structure. Both the limited particle coarsening and very low Pt contents in the acidic electrolyte after prolonged cycling even within the high potential range of 0.66~1.3 V (Table S4) are the evidence that the wide dissolution of Pt didn’t occur with the intermetallic particles. That is, the catalyst is very corrosion-resistant under the harsh electrochemical conditions. The enhanced corrosion resistance and catalytic stability can be attributed to the heteroatomic bonding in the intermetallic structure, which tends to have a more negative enthalpy of formation relative to the random alloys. As a result, it is much more difficult for Fe atoms to leave from the ordered particles than from the disordered ones during the electrochemical cycling. In addition, the dissolution of Fe is usually much severer than Pt. But the protection by the Pt-skin shell can mitigate the dissolution of Fe. The slower dissolution of Fe is beneficial for maintaining the fct structure (Figure S12) and thus results in higher durability. The previous method used for Au-doping44 included the synthesis of initial Pt-M (M=transition metals) alloys and subsequent Au-doping by an M replacement reaction. Our strategy involves simultaneous Pt and Au reduction by Fe and thus is simpler and more direct, leading to uniform distribution of Au in the intermetallic lattice. Although only a trace amount of gold (Au:Pt=1:50) is doped, an apparent improvement of ORR durability was



observed (Au-PtFe/C vs. PtFe/C) (Figure 4c; Figure 6d, 6e; Table S2). We attribute the enhanced durability of this kind to the change of the behaviour of Pt dissolution by Au doping. First, when Au atoms are located on the Pt surface, the durability enhancement may be caused by the increase of the dissolution potential of Pt by Au doping (the dissolution potentials are 1.83 V for Au and 1.18 V for Pt45). Second, when Au atoms are within the fct-PtFe lattice, they can make additional contributions to suppress Pt dissolution. That is, Au atoms can’t be oxidized within the given potential ranges (0.6−1.0 V and 0.66−1.3 V), and therefore they are less energetically favourable for the formation of interior oxides in the crystal lattice. Our ICP results (Table S4) of the Pt content dissolved in the acidic solution after ADTs under different potential ranges are supportive of this suggestion. CONCLUSION In summary, this work reports a new and simple approach to fine-grained and well dispersed PtFe particles. The Fe clusters embedded in amorphous carbon may have functioned as a reducing agent and a nucleation site for Pt/Au deposition as well. Due to a confining effect provided by the porous structure of the carbon support, the entrapped Fe and Pt particles can be converted to an intermetallic phase with a fully ordered fct structure at high temperatures (800 ºC) without the unwanted grain growth usually observed before. A trace amount of Au at the Au/Pt level of 1:50 can be doped homogeneously in the PtFe lattice via synchronous reduction and post thermal annealing for the structural ordering. Compared with Pt-only crystals and other ordered or disordered Pt-M structures, the present highly ordered fct PtFe particles with or without Au doping show the highest activity and durability in catalyzing ORR in acidic media. Particularly, the Au doped ones illustrate little loss in catalytic activity during the prolonged cycling even within the high potential range of 0.66~1.3 V. These higher than ever activity and durability may be ascribed to the geometrical and electronic structures of the ordered PtFe crystals and the additional effects of Au doping beneficial for catalytic activity enhancement and chemical stabilization against Fe etching and Pt dissolution under harsh electrochemical conditions. The present study reveals that the synergistic effects from structural ordering and minor doping of multimetallic nanostructures in tuning the catalytic activity and durability, and this synthetic strategy may be extended to other Pt alloy systems with much desired catalytic properties and reduced Pt usage for practical applications in PEMFCs. EXPERIMENTAL SECTION Synthesis of Fe/C Support. The support used in this study was a home-made carbon material


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with fine Fe clusters entrapped, as shown in our previous work. Briefly, this carbon support was prepared by pyrolysis of the mixture of iron pentacarbonyl (Fe(CO)5) and acetylene (C2H2) at 700°C. The synthesis pathway was under the protection of N2 which was also used as the carrier gas. The flow rates of C2H2 and N2 were 100 ml min-1 and 60 L h-1, respectively. However, the gas path of C2H2 and N2 in this work was optimized so that the final Fe/C powder possessed a higher carbon content than that in our previous work. In addition, compared with the traditional carbon supports, such as carbon black and carbon nanotubes, our support had both a graphitic structure and relatively high surface area. Preparation of PtFe/C and Au-PtFe/C Catalysts. PtFe/C catalysts were prepared by an impregnation reduction method. Typically, the Fe/C support was sonicated in deionized water for 40 min. Then the mixture was put into the flask and refluxed under continual vigorous stirring. After having been heated up to 130°C, a certain amount of chloroplatinic acid (H2PtCl6·6H2O) solution was added into the carbon slurry and stirred for 3 h. After having been cooled down to room temperature, the slurry was filtered and washed with deionized water several times until no Cl- could be detected by an AgNO3 solution. The nominal Pt:Fe atomic ratio was designed to be 1:1 or 1:2. The as-prepared PtFe/C catalyst was further heat-treated at 900°C in a tube furnace under vacuum to achieve structural ordering. The Au-doped PtFe/C catalysts were prepared by a synchronous reduction reaction. H2PtCl6·6H2O and HAuCl4·4H2O solutions were added simultaneously into a stirred carbon slurry and heated to 130°C under stirring for 3 h. The sample was then washed using deionized water until the pH value was close to 7, and finally heat-treated at 900°C in vacuum for 30 min to form an intermetallic compound of Au-PtFe. In order to verify the role of Au on the PtFe particle, we prepared the Au-PtFe/C catalysts with different atomic ratios (Au:Pt = 1:10, 1:25 and 1:50). Characterization. The chemical contents were analyzed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, iCAP 6000 Radial, THERMO). Transmission electron microscopy (TEM, JEOL-2100) was applied to study the morphology and particle dispersion of the catalyst. The scanning transmission electron microscopy (STEM, JEM-ARM200F) with an energy dispersive X-ray spectroscopy (EDX) analyzer was used to analyze the chemical composition and atomic distribution of the synthesized particles. X-ray diffraction (XRD) analysis was carried out to record the crystallization of as-prepared catalysts. The X-ray diffractometer was operated at 35 KV and 200 mA with nickel-filtered Cu Kα radiation as an incident beam (D/max 2550 VL/PC).



Electrochemical Testing. Electrochemical testing was conducted using an electrochemical workstation (CHI760 potentiostat, CH Instrument) with a conventional three-electrode cell at a constant temperature. The reference and counter electrodes were a saturated calomel electrode (SCE) and a large-area Pt plate, respectively. It should be noted that we used a graphite electrode as the counter electrode rather than Pt electrode during ADT cycles. The working electrode used was a glassy carbon electrode. The water bath was set within 25 ± 2°C to eliminate the effect of ambient temperature variation over different seasons. The working electrode was prepared as follows: 4 mg catalyst was dispersed in a 2 ml mixture of methanol and Nafion (5 wt.%) with the mass ratio of 50:1. The mixture was agitated by ultrasonication for 30 min to form an ink. A volume (6-12 μl) of the catalyst ink was then dropped on a glassy carbon electrode with a diameter of 5 mm and dried to yield a thin-film electrode. The total Pt (or Pt+Au) loading on the working electrode was controlled to be 20 μg cm-2. The catalyst film was dried under the ambient condition for several minutes before testing. Cyclic voltammetry (CV) measurements were carried out in N2-saturated 0.1 M HClO4 solution at 25°C after every 1000 (1k) cycles of potential sweep from 0.06 to 1.3 V (vs. RHE) at a rate of 100 mV s-1. The electrochemical active surface area (ECSA) was estimated by measuring the charge associated with H adsorption between 0.05 and 0.37 V. After the CV measurements, CO was supplied into the HClO4 solution for 30 min, and then N2 was bubbled for 1 h to remove the dissolved CO. Finally, CO-striping measurements were carried out at a scan rate of 50 mV s-1 in 0.1 M HClO4. The catalytic activity for ORR was tested using a rotating disk electrode (RDE) (Pine, AFMSRCE 3005) by linear sweep voltammetry (LSV). Polarization curves were obtained in an O2-saturated 0.1 M HClO4 solution at 25°C with a scan rate of 5 mV s-1 and rotation rate of 1600 rpm. The kinetic current (Ik) was calculated to characterize the ORR activity of a catalyst from the following Koutecky-Levich equation: ,

(1)

where I is the experimentally measured current and Id the diffusion limiting current. For the accelerated durability test (ADT), the RDE was subjected to potential cycling. Specifically, the electrode was scanned from 0.6 to 1.0 V or from 0.66 to 1.3 V (vs. RHE) at a rate of 100 mV s-1. CV and ORR curves were recorded at each set of 1000 (1k) cycles. ASSOCIATED CONTENT Supporting Information


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Particle size distributions of Au-PtFe/C, TEM images of Au-PtFe/C after cycling, ORR polarization curves for different catalysts, CV curves and ECSA results between 0.6−1.0 V and 0.66−1.30 V, stability comparison of various types of Pt-based catalysts. Corresponding Author Email: [email protected] ACKNOWLEDGMENTS This research is supported by National Key R&D Program of China (2018YFA0208404), Innovation Program of Shanghai Municipal Education Commission, and National Natural Science Foundation of China (51271077, U1362104).

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Tuning

of

Cu3Pt/C

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Intermetallic

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by

Selective

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Remarkable Improvement of the Catalytic Performance of PtFe

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