Research Article www.acsami.org
Electrochemically Identifying Degradation Pathways of CarbonSupported Pt Catalysts Assists in Designing Highly Durable Catalysts Jing-Fang Huang* and Hsin-Ying Hsiao Department of Chemistry, National Chung Hsing University, 145 Xingda Road, Taichung 402, Taiwan, R.O.C. S Supporting Information *
ABSTRACT: Supported Pt catalysts are considered highly efficient in many applications because of their unique catalytic properties. Their poor durability hampers their use in practical applications, particularly in novel energy-conversion devices such as fuel cells. A facile electrochemical procedure that combines the evaluation of the electrochemical surface area with a breakthrough in direct electrochemical quantification of the Pt content was utilized. Catalytic performance-related factors and kinetics of Pt nanoparticle (Ptnano) growth on a carbon substrate were probed under hightemperature annealing and ambient-temperature potential polarization, respectively. Apart from the Pt dissolution/redeposition pathway, we demonstrated that the crystal migration/coalescence pathway in catalyst degradation could not be ignored at ambient temperature. We report the enhanced durability and long-term activity of carbon-supported Pt catalysts, where the Ptnano surface was partially encapsulated by nonspecific noble metal clusters; inhibition of the migration/coalescence pathway and effective exposure of Ptnano surface active sites led to such enhancements. KEYWORDS: fuel cell, platinum, catalyst, degradation, noble metal cluster, ORR
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INTRODUCTION Platinum has long been considered as the most effective catalyst to facilitate both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR), both of which are important for proton-exchange membrane fuel cells (PEMFCs). With rapid progress, PEMFCs have become a promising power source for terrestrial applications, the military, use in automobiles, and stationary and portable devices. Extensive research into fuel cells has focused on decreasing the cost of electrocatalyst materials while maintaining their activity during long-term operations.1−5 Low catalyst stability and degradation are major barriers in the development of fuel cells or other catalyst-requiring applications.6−16 Mechanistically, three main pathways for Pt-based catalyst degradation (electrochemically active surface area (ECSA) loss) have been proposed:17,18 (1) electrochemical Ostwald ripeningthe loss of ECSA due to Pt dissolution from smaller particles and redeposition of soluble Pt ions onto larger particles on the carbon support due to the different thermodynamic stabilities of Pt species of different size; (2) reduction of Pt dissolution at the cathode by crossover hydrogen from the anode forms large crystallites in the proton-conducting membrane; and (3) Pt nanoparticle (Ptnano) coalescence/sintering via nanocrystallite migration and agglomeration on the carbon support surface. In these proposed pathways, the decrease in ECSA was mainly caused by Pt loss from Pt dissolution and Ptnano growth from redeposition of dissolved Pt ions or Ptnano sintering. While Pt loss from the catalyst was possible in the first two pathways, the decrease in ECSA in the third pathway was primarily due to Ptnano growth. © XXXX American Chemical Society
Whether the coarsening of Ptnano on carbon occurs by Ostwald ripening via Pt dissolution/redeposition or by crystal migration/coalescence has been a long-standing debate.18 Most commonly used microimaging analytic or spectroscopic techniques employed to probe catalyst degradation, such as transmission electron microscopy (TEM), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and anomalous small-angle X-ray scattering (ASAXS), cannot monitor the Pt loss and micromorphological variations simultaneously.19−26 Additionally, the image analysis from TEM is also limited in representing the average properties of entire samples based on data obtained from these characterization techniques.27−29 In our previous studies,27 a facile electrochemical procedure that monitors the ECSA and Ptnano weight (Wpt) was proposed to simultaneously determine the Ptnano size and the number of Ptnano (NPt) (or particle density, NPt/Ag, where Ag is the geometric area of the electrode) (Supporting Information). This presented a breakthrough in tracking the Pt loss during electrocatalyst operations from a facile electrochemical Wpt quantification by the Cl−-complexing ability of Pt ions that facilitates anodic electrodissolution of the active supported Pt metal with the requisite “nano-size” (particle diameter < 10 nm).22,27,30,31 Rather than using a trial-and-error or combinatorial approach in the electrocatalyst design, we identified the pathway that mainly contributed to catalyst degradation, using our new Received: October 15, 2016 Accepted: November 18, 2016 Published: November 18, 2016 A
DOI: 10.1021/acsami.6b13135 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. TEM images of Pt20/C (a) before and (b) after annealing at 250 °C for 60 h in an Ar atmosphere, (c) 3 000 cycle potential scan of ADT in an O2-saturated 0.1 M HClO4 aqueous solution at room temperature (∼28 °C), and potentiostating at (d) Edl = 0.4 V and (e) Ea1 = 1.0 V for 100 000 s in Ar-purged 0.1 M HClO4 aqueous solution at room temperature. (f) CVs of the Pt/C@GCE in an Ar-purged 0.2 M H2SO4 aqueous solution at a scan rate of 0.2 V·s−1 were recorded before and after two different potential polarizing treatments, including (ADT 30 000 cycles) the ADT, potentiostating on (CA (0.4 V) 300 000 s) Edl = 0.4 V and (CA (1.0 V) 300 000 s) Ea1 = 1.0 V under ambient temperature. paper were referenced to the reversible hydrogen electrode (RHE). The Pt/C electrocatalyst-modified glassy carbon electrode (Pt/C@ GCE) was used as the working electrode. The Pt/C catalyst suspension was prepared by mixing 5 mg of commercial Pt/C electrocatalyst powder in 5 mL of deionized water (specific resistivity = 18.2 MΩ cm−1), followed by the gradual addition of 1 mL of isopropyl alcohol, 30 μL of a 5% NF solution, and 20 min ultrasonication to obtain the Pt/C suspension. A GCE (BAS, 3.0 mm diameter, 0.07 cm2 for the preparation of postcatalyst-treated Pt/ C samples; Pine, 5.0 mm diameter, 0.196 cm2 for the examination of catalyst performance for ORR) served as the substrate electrode for the Pt/C electrocatalyst suspension. The Pt/C@GCE was fabricated using a drop-coating procedure. Briefly, a GCE was polished successively with 1.0, 0.3, and 0.05 μm alumina powder cloth (Buchler) followed by sonication in deionized water and drying prior to use. Pt/C suspension (2−7 μL) was pipetted onto the surface of the GCE as a circle with a geometric area of 0.07 or 0.2 cm2. A Pt/C@ GCE was obtained after drying under an Ar flow at room temperature for solvent evaporation. Pt/C Catalyst Durability Treatments. The durability of Pt/C catalyst was investigated in various treatments including hightemperature annealing and ambient-temperature potential polarization. In the ambient-temperature potential polarization, a Pt20/ C@GCE was used as a working electrode to prepare relative posttreated Pt/C samples. The microstructure of post-treated Pt/C was examined by the electrochemical characterization and image analysis. Image analysis of post-treated Pt/C was performed using a JEOL JEM1400 transmission electron microscope (TEM) and a JEOL JEM2100F field-emission TEM. High-Temperature Annealing. The Pt20/C catalyst was annealed in a tube oven in an Ar atmosphere at 225 and 250 °C. The rate of heating and cooling the samples was 2 °C s−1. The sample could not be annealed at temperatures above 275 °C, due to rupture of the carbon support at high annealing temperature. Accelerated Durability Test (Potential Scanning). The accelerated durability test (ADT) was conducted by the linear potential sweeping from 0.6 to 1.1 V vs RHE at a scan rate of 50 mV/s for various cycles
electrochemical procedure. This procedure probed the factors related to catalytic performance and kinetics of Ptnano growth during different treatments, such as high-temperature annealing and ambient-temperature potential polarization. Apart from the well-accepted Pt dissolution/redeposition pathway, results indicated that the significant contribution of the crystal migration/coalescence pathway in catalyst degradation could not be ignored at ambient temperature. Preparation of carbonsupported Pt catalysts (Pt/C) with sustained activity and extended durability is an urgent requirement. Although most research on the design of Pt-based catalysts was based on constraining the first degradation pathway (Ptnano dissolution) under potential cycling regimes,1,8,32,33 identifying the proper degradation pathway could extend the route for designing costviable and durable catalysts. We also demonstrated that the Ptnano crystal migration/coalescence pathway was effectively inhibited through enhancement of the Ptnano−carbon support interaction by surface modification with metal clusters. Furthermore, both maintained a high activity for Pt/C and significantly improved the durability.
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EXPERIMENTAL SECTION
Chemicals. Commercial carbon-supported platinum catalysts (Pt20/C), 20 wt % Pt on XC-72 Valcan carbon (E-TEK), 70% HClO4 (JT-Baker), 37% HCl (Aldrich), 95−98% H2SO4 (Aldrich), 99.7% Cu(NO3)2 (JT-Baker), 99.9% K2PtCl4 (Alfa), 99.9% K2PdCl4 (Alfa), 99.99% HAuCl4 ·3H2O (Alfa), and 5% Nafion (NF) perfluorinated resin in a mixture of lower aliphatic alcohols and water (Aldrich) were used as received. Electrochemical Measurements. The electrochemical experiments were conducted using a CHI 760C potentiostat/galvanostat and a three-electrode electrochemical cell. Hg/HgSO4 (0.5 M H2SO4) (to prevent Cl− interference from the typical Ag/AgCl reference electrode) was used as a reference electrode, and graphite rod or platinum wire was used as a counter electrode. All potentials in this B
DOI: 10.1021/acsami.6b13135 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces in an O2-saturated 0.1 M HClO4 aqueous solution at room temperature. Potentiostating Treatment (Potential Holding). The underpotential adsorption and desorption of hydrogen (Hupd) were observed at lower potentials (0.8 V vs RHE). The featureless region between these potentials is the doublelayer region. For the potentiostating treatments, Pt20/C@GCE was under potential control at Ec1 = 0.2 V, Edl = 0.4 V, and Ea1 = 1.0 V vs RHE, respectively, in the Ar-purged 0.1 M HClO4 aqueous solution. Preparation of Surface Metal Cluster (Mc) Modified Pt/C Catalysts (Mc/Pt/C Includes Ptc/Pt/C, Pdc/Pt/C, and Auc/Pt/C). The noble metal (NM) cluster was prepared by galvanic displacement with NM atoms of an underpotentially deposited Cu monolayer (Cuupd) on a Ptnano surface on the Pt/C@GCE from a 0.2 M H2SO4containing 0.6 mM Cu(NO3)2 solution by a potential scan of 0.8−0.2 V vs RHE (scan rate, 0.02 V s−1). The potential was stopped at 0.2 V for 5 min to allow the Cuupd to completely cover the whole surface of the Ptnano surface. All of these operations were carried out in the threeelectrode electrochemical cell in an Ar atmosphere. The as-prepared Cuupd-modified Pt/C@GCE (Cuupd/Pt/C@GCE) was rinsed with deionized water and immersed in a 0.2 M H2SO4 solution containing 5.0 mM Mc ion for ∼5 min to displace the Cuupd with Mc, where Mc ion sources are K2PtCl4, K2PdCl4, and HAuCl4, respectively.
Pt redeposition. The ECSA of Ptnano is commonly measured from the charge under the voltammetric peaks for the underpotential adsorption or desorption of hydrogen (Hupd, 0.3−0.0 V vs RHE) on Pt20/C@GCE in an Ar-purged 0.2 M H2SO4 aqueous solution (Figure 1f), using the reported value of 0.21 mC·cm−2 for a clean Pt surface.34 These TEM morphological analyses are also consistent with expectations and results from monitoring the ECSA losses on Pt20/C catalysts after potential polarizing protocols (Figures 1f and S2 indicate a 30% loss in ECSA). The ECSA loss of the electrocatalyst was mainly attributed to Ptnano coarsening. Although similar Ptnano coarsening phenomena were found in various treatments, Pt20/C exhibited different micromorphological changes between high-temperature annealing and ambient-temperature potential polarization. Many examples of necked Ptnano (two or more particles connected to each other) on Pt20/C result from potential polarization (Figure. 1c−e) and are different from the solid and integrated spherical particle structure grown on Pt20/C after high-temperature annealing (Figure 1b). Interestingly, without the Pt redox reaction (Edl) or with only PtOx formation (Ea1), the fact that Ptnano coarsening occurred during potentiostatic treatments implies that Pt dissolution/redeposition cannot fully illustrate the Ptnano coarsening process (Figure 1d, e). The variation of WPt and the average radius of Ptnano (rav) on Pt20/C were simultaneously traced electrochemically with increasing treatment periods (Figure 2). In the high-temperature annealing, rav grew from 1.6
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RESULTS AND DISCUSSION TEM images of commercial carbon-supported Pt electrocatalysts (Pt20/C) (20 wt % Pt on XC-72 Valcan carbon, ETEK) were used to track the micromorphological changes before and after high-temperature annealing at 250 °C for 60 h under an Ar atmosphere (Figure 1a, b, and Figure S1). Under ambient temperature (Figure 1c−e), two different potential polarizing protocols, including linear potential sweeping and potentiostating, were used to mimic possible electrode operations. A Pt20/C modified glassy carbon (GC) working electrode (Pt20/C@GCE) was used to prepare relative TEM samples. In Figure 1a, a typical bright-field TEM of a pristine Pt20/C reveals a relatively uniform dispersion of Ptnano with sizes on the order of 2−4 nm (diameter). After hightemperature annealing, Ptnano coarsening expectedly occurred on Pt20/C, leading to growth of the Ptnano size (diameter increased to ∼5 nm) and decrease in the Ptnano density (Figure 1b). The Ptnano coarsening phenomenon also occurred under ambient temperature (Figure 1c−e). Figure 1c displays the TEM image of Pt20/C after treatment with the well-known accelerating durability test (ADT), by applying a linear potential sweep from 0.6 to 1.1 V vs RHE at a scan rate of 50 mV/s for 3 000 cycles in an O2-saturated 0.1 M HClO4 aqueous solution at room temperature.1,8,32,33 In many studies, in addition to ORR, ADT cycling between two redox potential limits covering both formation of Pt oxides (1.1 V) and their reduction (0.6 V) revealed that Ptnano growth could be dominated by preferential dissolution (or loss) of small particles and redeposition of dissolved Pt ions onto existing particles. Surprisingly, Ptnano coarsening was observed by potentiostating at a double-layer potential, Edl = 0.4 V, and an anodic potential, Ea1 = 1.0 V, for 100 000 s in an Ar-purged 0.1 M HClO4 aqueous solution at room temperature (∼28 °C) (Figure 1d, e). Edl was especially considered as a double-layer region without any faradaic reaction in an Ar-purged electrolyte solution (without both Pt redox reaction and ORR). The formation of PtOx or trace Pt dissolution (0.8−1.2 V vs RHE) was expected at the more positive anodic potential, Ea1, without
Figure 2. Electrochemical procedure evaluating changes in (a, b) the average Ptnano radius (rav) and (c, d) Ptnano weight (Wpt) of Pt20/C after (a, c) high-temperature annealing in an Ar atmosphere and (b, d) potentiostating at Ec1 = 0.2 V, Edl = 0.4 V, and Ea1 = 1.0 V (vs RHE) in an Ar-purged 0.1 M HClO4 aqueous solution at room temperature.
to 1.9−2.0 nm at 225 °C and to 2.2−2.4 nm at 250 °C. NPt/Ag decreased with the Ptnano growth during high-temperature annealing (Figure S3), and the growth rate of rav depended on the annealing temperature. Under ambient temperature, the growth of rav was recorded while potentiostating at three different polarization potentials: a cathodic potential (Ec1) = 0.2 V, Edl = 0.4 V, and Ea1 = 1.0 V vs RHE. Although Hupd and PtOx formation occurred at Ec1 and Ea1, respectively, with the exception of Edl, rav growth was observed with a similar rate under these polarization potentials. The dispersion of Pt20/C in C
DOI: 10.1021/acsami.6b13135 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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the method from the Adzic’s group, are displayed in Figure 3c. The Auc/Pt/C catalyst displayed a negligible 3% ECSA loss and
isopropanol solution without any extra treatment was chosen as a control experiment and showed no obvious change in the rav; the rav increased from 1.6 to 2.2−2.4 during potential polarizations. These rav growth trends all concur with TEM observations. A similar phenomenon was observed in Pt20/C after ADTs in our previous studies. Besides characterizing the factors related to catalytic activity, such as ECSA, rav, and NPt/ Ag, the unique advantage of WPt quantification was used for monitoring the Pt content during catalyst treatments. A reduction in the ECSA during the growth in Ptnano mean size was observed without any Pt loss during ADTs at room temperature. These results imply that Ptnano coarsening could be mainly attributed to crystal migration/coalescence; however, the Pt dissolution/redeposition pathway cannot be ignored due to repetition of the Pt redox reaction during ADTs. Parts c and d of Figure 2 illustrate WPt monitoring vs time by electrochemical quantification during various treatments. No obvious WPt loss was expectedly observed during high-temperature annealing (Figure 2c). Under potentiostatic conditions at ambient temperature, there was also no WPt loss at polarization potentials Ec1 and Edl, as similar results were observed in the control experiment of Pt20/C dispersion in isopropanol solution; negligible Pt oxidation or dissolution occurred (Figure 2d). Interestingly, the Pt content was >92% under potentiostatic conditions at Ea1 for 600 000 s (∼167 h) (Figure 2d). It also implied that Pt dissolution was an ultrasluggish process at a more negative anodic potential than 1.0 V vs RHE, and a mechanism other than the well-known Pt dissolution/ redeposition was needed to illustrate Ptnano coarsening processes. Kinetic analysis of rav is often described asymptotically by the power law:35
ravn (t ) = ravn (0) + Ct
Figure 3. ORR polarization curves for (a) Auc/Pt/C, (b) Ptc/Pt/C, and (c) Pdc/Pt/C catalysts on a GC rotating disk electrode (0.196 cm2, ∼2.2 μg Pt loading) before (black dashed line) and after potential polarizing treatments, including (blue solid line) 30 000 potential cycles of ADT and (red dashed-dotted line) potentiostating at Edl = 0.4 V for 300 000 s. A sweep rate of 10 mV/s and rotation rate of 1 600 rpm were used. Insets shown in (a, b, c) are CVs of the electrodes in 0.5 M H2SO4 at a scan rate of 0.2 V·s−1 and were recorded before and after potential polarizing treatments under ambient temperature. (d) Possible mechanism illustrates how NM clusters enhance the Ptnano−support interaction to improve the stability of Pt-containing catalysts.
a stable catalytic performance for ORR after 30 000 cycles of ADT. In this work, other metal clusters (Pt and Pd) replaced the Au cluster to create surface metal cluster-modified Pt/C (Mc/Pt/C), Ptc/Pt/C, and Pdc/Pt/C, by simply changing the gold source (HAuCl4) to K2PtCl4 and K2PdCl4, respectively, in the galvanic replacement process (Figure 3a, b). Unexpectedly, the Mc/Pt/C catalysts all exhibited ultrahigh stable ORR catalytic performance, including stable ECSA and unaffected half-wave potential (E1/2) for ORR after ADT for 30 000 cycles, even though their catalytic activity was similar to pure Pt/C catalyst (Figure 3). It was noteworthy that the Mc/Pt/C catalyst maintained an ultrahigh stable ORR catalytic performance not only during ADT but also while potentiostating at Edl for 300 000 s. The metallic clusters, Ptc or Pdc, should not inhibit Pt dissolution during treatments because the ultratrace Ptc modification does not affect the intrinsic solubility of Pt and Pdc, which is even more soluble than Pt. The significant improvement of catalyst durability during potentiostating at Edl should imply that Ptnano coarsening through crystal migration/ coalescence was effectively hindered by metal cluster modification. A possible mechanism was proposed in Figure 3d, where Ptnanos on the Pt/C were electrochemically covered by a Cu UPD monolayer in the initial stage, resulting in the encapsulation of Ptnanos by a thin layer of Cu. However, the encapsulation of Ptnanos could reduce their activity due to coverage of their active sites. Subsequently, the Cu UPD was replaced by a more noble metal (NM), e.g., Au, Pt, and Pd, in the galvanic replacement process. As the galvanic replacement proceeds, NM cluster nuclei formed by NM ions become NM adatoms and diffuse over the surface to incorporate into NM clusters. The replacement could induce the formation of both
(1)
where rav(t) is the time evolution of the mean particle radius, rav(0) is the average radius at t = 0, n is the exponent, and C is the coefficient, which depends on the growth kinetics (or the Ptnano coarsening mechanism). For surface Ostwald ripening with 2D detachment, diffusion, and attachment, the classical Lifshitz−Slyozov−Wagner (LSW) theory36,37 predicted n = 3 and 4 for kinetic- and diffusion-limited growth, respectively. For particle migration/coalescence, n is expected to be between 5 and 7.38 The variation of rav vs time from various treatments illustrated in Figure 2a, b were subjected to kinetic analysis. In these cases, use of eq 1 to fit the experimentally determined kinetics yielded n ≈ 2.1 and n ≈ 2.4 for high-temperature annealing at 225 and 250 °C, respectively. The results showed that Ptnano coarsening under high-temperature annealing was similar to Ostwald ripening with kinetic-limited growth. This conclusion is consistent with previous reports on TEM image analysis35 and also suggests that the facile electrochemical procedure was successfully used for Ptnano coarsening kinetic analysis. Interestingly, the equation fitting results were n = 6.3− 7.7 under potentiostatic conditions and are larger than those for kinetic- and diffusion-limited growth (n = 3 or 4). This also suggests that particle coarsening should favor migration/ coalescence pathways under ambient temperature. The pioneering works by Adzic’s group1 demonstrated that the stability of Pt ORR catalysts were significantly improved by modifying the Ptnano surface with Au clusters (Auc/Pt/C), which were deposited onto Pt/C through a galvanic replacement reaction. The reproducible results for the preparation and ORR performance of Auc/Pt/C, according to D
DOI: 10.1021/acsami.6b13135 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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catalytic performance, as well as another outlook on the design of novel, highly stable catalysts.
surface-active vacancies and surface NM clusters. Surface NM clusters could be located in more stable sites on the surface, and more kink or step sites could be present in the interface between the Ptnano and the carbon support; NM clusters could also favor these locations. According to this hypothesis, the subsequent NM replacement could both reactivate the Cu UPD-covered Ptnano surface and form NM clusters, to serve as surface wedges that stabilize Pt nanos by increasing the Pt nano−support interaction. The presence of these partially encapsulated NM clusters was clearly observed in the high-resolution TEM (HRTEM) images (Figure 4). The NM clusters were primarily
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13135. In situ electrochemical characterization of Ptnano, electrodissolution of Ptnanos, electrochemical characterization of ORR, TEM images of Pt20/C before and after hightemperature annealing, electrochemical procedure evaluating changes in the normalized ECSA and the Ptnano density (NPt/Ag) of Pt20/C, and TEM and HRTEM of the carbon-supported catalysts before and after ADT measurement (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jing-Fang Huang: 0000-0001-9700-8778 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of the Republic of China, Taiwan.
Figure 4. TEM images of (a) Ptc/Pt/C, (b) Pdc/Pt/C, and (c) Auc/ Pt/C catalysts made by displacement of a Cu UPD by Au, Pt, and Pd, respectively. High-resolution TEM images of (d) Ptc/Pt/C, (e) Pdc/ Pt/C, and (f) Auc/Pt/C collected from the dashed rectangular area in (a, b, c) show fringe orientations that are consistent with a Pt(111) single-crystal structure. The different structures in the areas indicated by the arrows are ascribed to Pt, Pd, and Au clusters.
REFERENCES
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located at the interfaces between Ptnano and the carbon substrate. After performing ADTs for 30 000 cycles, the particle size and particle density of NM cluster-modified Ptnano on the Mc/Pt/C resulted in no obvious change. Although the NM cluster layer gradually became thinner, it was still stably located in the interface between the Ptnano and carbon substrate (Figure S4). These observations strongly suggest that NM clusters enhance the Ptnano−support interactions and improve the stability of Pt-containing catalysts.
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CONCLUSION In summary, a facile electrochemical procedure was used to successfully assess the catalytic performance-related properties of typical Pt/C electrocatalysts, such as the mean particle size, particle density, and Pt content, as well as the Ptnano coarsening kinetics during various catalyst treatments. In addition to the well-accepted Pt dissolution/redeposition mechanism, Ptnano coarsening by crystal migration/coalescence was strongly evident by combining kinetics and image analyses in this work, especially for occurrences observed under ambient temperature. On the basis of the mechanism identified, the preparation of ultrastable Mc/Pt/C catalysts demonstrated that nonspecific NM cluster modification significantly improved the durability of Pt/C catalysts. The procedure NM cluster modification (galvanic replacement of Cu UPD from Pt/C surface by NM ions) could both enhance the Ptnano−support interactions and reactivate the Pt/C surface. More importantly, this work may provide a new strategy to quickly evaluate the E
DOI: 10.1021/acsami.6b13135 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
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DOI: 10.1021/acsami.6b13135 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
