Radially Phase Segregated PtCu@PtCuNi Dendrite@Frame

Dendrite@Frame Nanocatalyst for the Oxygen Reduction. Reaction .... In a typical synthesis of PtCu@PtCuNi dendrite@frame (PC@PCN), a slurry of Pt(acac...
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Radially Phase Segregated PtCu@PtCuNi Dendrite@Frame Nanocatalyst for the Oxygen Reduction Reaction Jongsik Park,†,§,‡ Mrinal Kanti Kabiraz,∥,‡ Hyukbu Kwon,§,‡ Suhyun Park,§ Hionsuck Baik,⊥ Sang-Il Choi,*,∥ and Kwangyeol Lee*,†,§ Downloaded via UNIV OF ALABAMA BIRMINGHAM on June 24, 2018 at 16:30:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul 02841, Korea Department of Chemistry and Research Institute for Natural Sciences, Korea University, Seoul 02841, Korea ∥ Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Korea ⊥ Korea Basic Science Institute (KBSI), Seoul 02841, Korea §

S Supporting Information *

ABSTRACT: Pt-based alloy nanoframes have shown great potential as electrocatalysts toward the oxygen reduction reaction (ORR) in fuel cells. However, the intrinsically infirm nanoframes could be severely deformed during extended electro-cyclings, which eventually leads to the loss of the initial catalytic activity. Therefore, the structurally robust nanoframe is a worthy synthetic target. Furthermore, ternary alloy phase electrocatalysts offer more opportunities in optimizing the stability and activity than binary alloy ones. Herein, we report a robust PtCuNi ternary nanoframe, structurally fortified with an inner-lying PtCu dendrite, which shows a highly active and stable catalytic performance toward ORR. Remarkably, the PtCu@PtCuNi catalyst exhibited 11 and 16 times higher mass and specific activities than those of commercial Pt/C. KEYWORDS: dendrite@frame, kinetic control, phase segregation, ternary alloy, electrocatalysis, oxygen reduction reaction

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might be more robust than a nanoframe without such a structural support. Enhanced catalytic activity and stability have been recently reported for PtCuNi ternary alloy nanoparticles toward ORR under acidic condition.25 The higher activity and durability of PtCuNi ternary alloy electrocatalysts over binary alloy phases make the PtCuNi nanoframe a worthy synthetic target, because the latter is expected to show both intrinsic stability of the PtCuNi phase and great activity derived from the high surface area of framework. Herein, we report the synthesis of a robust PtCuNi ternary frame, structurally fortified with an inner-lying PtCu dendrite, which shows highly active and stable catalytic performance toward ORR. From the previous studies, we and others have established that Pt atoms, trapped in a Ni matrix, can migrate to the outer surface of a nanoparticle, driven by the large Pt atom size.8,26−30 Furthermore, the differences in precursor decomposition kinetics can be advantageously exploited to form a core−shell

t-based alloy nanoparticles have been actively investigated as promising electrocatalysts in proton-exchange membrane fuel cells (PEMFC) because they greatly increase the reaction kinetics of the oxygen reduction reaction (ORR), which is a rate-determining step of fuel cell reactions.1−7 Various structural designs have emerged to maximize the catalytic potential of Pt-based alloy nanoparticles, and the structural concept of nanoframe has recently received great attention due to a very high electrocatalytically active surface area and accompanying high catalytic activity.8−13 Although ternary or quaternary alloy nanoparticles are being reported to exhibit excellent catalytic activity and stability,14−17 nanoframes of ternary or quaternary alloy phases are rare due to the dearth of rational synthetic approaches. On the other hand, flimsy nanoframe structures, however, suffer from an extensive structural disintegration during electrocatalysis; most of Pt-based nanoframes were synthesized based on leachable templates such as Ni and Cu nanoparticles, and further dissolution of leachable elements during electrocatalytic cycles can induce the structural deformation.18,19 Therefore, recent efforts have been focused on fortifying the nanoframe structures.4,20−24 We envisage that a nanoframe structure supported by a nanoframe-fortifying, inner-lying structure © 2017 American Chemical Society

Received: June 12, 2017 Accepted: October 12, 2017 Published: October 12, 2017 10844

DOI: 10.1021/acsnano.7b04097 ACS Nano 2017, 11, 10844−10851

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ACS Nano Scheme 1. Schematic Illustration of Formation of PC@PCN Dendrite@frame

nanostructure with different core and shell compositions. Combining these ideas, we formed a PtCuNi-based nanoframe structure with an inner structural support of a different composition, namely PtCu. Specifically, the decomposition of three different precursors of Pt(acac)2, Ni(acac)2, and Cu(acac)2 in one pot led to the initial formation of PtCu dendrite, which was subsequently covered by the Ni shell. The migration of Pt through the Ni matrix to the outer edge sites facilitated reduction of remnant Cu precursor on the Pt-rich edges, resulting in the formation of a PtCuNi nanoframe on the Ni matrix. Finally, removal of the leachable fraction of Cu and Ni in acid formed a PtCuNi nanoframe supported on a PtCu nanodendrite (see Scheme 1). The PtCu@PtCuNi dendrite@ frame exhibited an excellent catalytic activity and stability toward ORR, far surpassing those of commercial Pt/C.

RESULTS AND DISCUSSION In a typical synthesis of PtCu@PtCuNi dendrite@frame (PC@ PCN), a slurry of Pt(acac)2 (0.020 mmol), Ni(acac)2 (0.045 mmol), Cu(acac)2 (0.015 mmol), cetyltrimethylammonium chloride (CTAC) (0.06 mmol), and oleylamine (15 mmol) was prepared in a 100 mL Schlenk tube with magnetic stirring. After placing the solution under vacuum at 60 °C for 5 min, the Schlenk tube was directly placed in a hot oil bath, which was preheated to 270 °C. After heating at the same temperature for 30 min, the reaction mixture was cooled down to room temperature with magnetic stirring. After the addition of 15 mL toluene and 25 mL ethanol, the reaction mixture was centrifuged at 4000 rpm for 5 min. The PC@PCN was prepared via selective removal of Ni and a small amount of Cu components from the resulting nanocrystals by using hydrochloric acid (HCl) at 60 °C for 1 h (see Materials and Methods section for details). Representative transmission electron microscopy (TEM) image of PC@PCN is shown in Figure 1a. The enlarged TEM image in the inset indicates that the PC@PCN has a rhombic dodecahedral morphology. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping analysis in Figure 1b suggest that the composition of inner dendrite nanocrystal mainly consists of PtCu and that the outer frame consists of a PtCuNi ternary phase. We further studied energy dispersive X-ray spectroscopy (EDS) line profile and quantitative composition of a single nanocrystal as shown in Figure 1c−e. Line scan elemental profile of PC@PCN for Pt (yellow line), Ni (red line), and Cu (blue line) contents demonstrates that the core-dendrite site is mainly composed of PtCu binary alloy and the edge frame is of PtCuNi ternary alloy phases, respectively. The Figure 1e indicates that the compositions are Pt 34%, Ni 2%, Cu 64% at the core site and Pt 31%, Ni 44%, Cu 25% at the edge site. The lattice spacings of {111}, {200} of the edge part (Figure 1f (i))

Figure 1. Characterization of PtCu@PtCuNi dendrite@frame (PC@PCN). (a) TEM image of PC@PCN (inset: enlarged TEM image of PC@PCN). (b) STEM image and corresponding elemental mapping analysis of PC@PCN. (c) Combined elemental mapping analysis of PC@PCN. (d) Cross-sectional line profiles of PC@PCN along the yellow arrow marked in (c). (e) Quantitative analysis data of core and edge parts marked in (c). (f) HRTEM image of PC@PCN. The FFT pattern in white dotted square demonstrates the existence of two different phases in a single nanocrystal. (i−ii) Enlarged HRTEM images and corresponding FFT patterns of white square section in (f).

of PC@PCN are measured as 0.208 and 0.182 nm, while those of the core part (Figure 1f (ii)) are 0.217 and 0.187 nm, respectively. To demonstrate the validity of alloy lattice spacing analysis, we calculated the theoretical lattice spacings by using Vegard’s law.31 The calculated lattice spacings of {111} and {200} planes at the edge site are 0.210 and 0.182 nm, and those planes at the core site are 0.214 and 0.185 nm, respectively, which are similar to the values from HRTEM analysis. The X10845

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49:51. Recently, a similar PtCu dendrite was investigated for the morphology evolution from preformed multipods by further elongation along specific facets.35 In this study, we also observed similar multipodal seeds at 2 min of reaction, as shown in Figure S6. At 10 min, PtCu@Ni dendrite@shell nanostructures could be found in Figure 2b. The gray contrast in HAADF-STEM image of Figure 2e with EDS mapping of Ni implies the growth of Ni on PtCu dendrites. The EDS line profile analysis in Figure S3b,e also reveals the PtCu@Ni dendrite@shell nanostructure. It is worth noting that two different fcc{200} lattices were observed in HRTEM image and FFT pattern of Figure S5d−f, indicating the PtCu (d = 0.188 nm) and Ni (d = 0.180 nm) phases. XRD pattern in Figure 3b also shows the existence of a Ni shell. The atomic composition of intermediates at 10 min was found to be 27.5% Pt, 55.2% Ni, and 17.3% Cu by EDS (Figure S4b). It is well-known that halide ions under inert gas conditions strongly bind with Pt{110} facets, resulting in rhombic dodecahedral morphology.4,36,37 Therefore, the intermediate nanocrystals with an irregular shape at 10 min are plausibly due to the lack of pure Pt composition during the reaction. The gradual growth of Pt on the surface, however, induces the transformation of irregularly shaped PtCu@Ni dendrite@shell nanostructures into rhombic dodecahedral PC@PCN at 30 min (Figure 2c), of which atomic composition was found to be 33.7% Pt, 25.7% Ni, and 40.6% Cu by EDS (Figure S4c). The Moiré pattern in Figure S7 also suggests the existence of two different phases in a single nanocrystal. To investigate the role of Pt precursor, the intermediates at 5 min were washed and introduced to a new reaction mixture containing Ni precursor and CTAC, but excluding the Pt precursor. During the reaction, an irregularly shaped PtCu@Ni dendrite@shell could not be converted to shapely dodecahedron (Figure S8a), and only PtCu dendrite structures remained after the etching process (Figure S8b). On the other hand, when the irregularly shaped PtCu@Ni nanocrystals were subjected to a reaction mixture in the presence of the Pt precursor, the rhombic dodecahedral PC@PCN core@shell nanocrystals could be obtained (Figure S8c), and the further formation of PC@PCN core@frame was determined after etching (Figure S8d). These results indicate that the shape evolution into rhombic dodecahedral nanocrystal requires the incorporation of Pt into the growing Ni shell. Furthermore, the edge confinement of Pt component indicates the fast outward movement of large Pt atoms in the PtNi phase to the outer edge sites; similar Pt segregation behavior has been demonstrated previously by us and other groups.3,8,26 However, Cu segregation was not observed during the formation of the PtCu@Ni, indicating that the Cu content in the PtCuNi nanoframe might not have originated from the outward diffusion of Cu from the PtCu dendrite to the PtNi nanoframe. It could be more practical that the remaining Cu precursor, now of much lower concentration, in the reaction mixture might be preferentially reduced on the Pt sites, promoting the PtCuNi phase. To identify the role of each metal precursor for the shape evolution of resulting nanocrystals, we studied the concentration effects of metal precursors. In the absence of Pt precursor, no discernible nanoparticles were observed (Figure S9a), indicating that the reduction of Ni and Cu is facilitated by Pt seeds; 3d transition metals could be underpotentially deposited in the presence of noble metal seeds.38 On the

ray diffraction (XRD) pattern as shown in Figure S1 exhibits broad and asymmetric peaks, resulting from overlapping of peaks from PtCu and PtCuNi alloy phases. The two samples of PC@PCN, namely before and after chemical etching, were subjected to X-ray photoelectron spectroscopy (XPS) measurements (Figure S2). Black and red lines indicate the XPS data of before and after chemical etching samples, respectively. We found that there are no big changes in Pt 4f and Cu 2p binding energies between before and after chemical etching samples (Figure S2b). This demonstrates that the etching process does not affect greatly the electronic state of Pt and Cu. In Figure S2c, we could observe that the Ni 2p peaks appear at different binding energy values for before and after chemical etching samples. The relative intensity of the Ni metal 2p peak decreased, while that of the Ni2+ 2p peak slightly increased after chemical etching, demonstrating that the dissolution of Ni metal phase in the acid involves the conversion of Ni metal phase to Ni2+ oxidized state. The N 1s XPS peak (Figure S2d), indicating the existence of oleylamine, was barely observed after the chemical etching, because the chemical etching process also removes the surfactants.32−34 To investigate the detailed formation process of PC@PCN, temporal TEM images for reaction intermediates were obtained as shown in Figure 2. At the earlier stage of reaction, well-

Figure 2. (a−c) Temporal TEM images and (d−f) elemental mapping analysis obtained at (a,d) 5 min, (b,e) 10 min, and (c,f) 30 min. Scale bars in (d−f) are 30 nm.

defined PtCu cubic dendrites could be found (Figure 2a). The elemental mapping (Figure 2d) and EDS line profile analyses (Figure S3a,d) of reaction intermediates at 5 min suggest that the nanocrystals are composed of mainly Pt and Cu elements. The atomic composition of the PtCu dendrite was found to be 41.2% Pt, 0.4% Ni, and 58.4% Cu by EDS (Figure S4a). The low content of Ni in the PtCu dendrite is due to the slow decomposition kinetics of Ni precursor compared to those of Pt and Cu precursors. HRTEM image and FFT pattern with a [100] zone axis of the intermediate nanocrystal at 5 min are shown in Figure S5a−c. The lattice spacings of {200} and {220} planes are 0.188 and 0.134 nm, respectively, which lie between Pt lattice spacings (d{200} = 0.194 nm, d{220} = 0.138 nm) and Cu lattice spacings (d{200} = 0.181 nm, d{220} = 0.128 nm), indicating the formation of the PtCu alloy phase. The XRD pattern also showed peaks of 5 min intermediates between the Pt and Cu reflections (Figure 3a). The composition ratio of Pt and Cu calculated by Vegard’s law is 10846

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Figure 3. (a) XRD patterns of intermediates for PC@PCN at different reaction times. Extended views of XRD pattern in the range between 35° and 55° in order to examine the relative contributions of their distinctive phases at (b) 10 and (c) 30 min.

Ni(acac)2, the corner-emphasized nanocrystals are formed, and hierarchical nanocrystals with hexapod core and framework shell appear after chemical etching (Figure 4b). When 2 equiv of Ni(acac)2 was added to the reaction mixture, the overall volume of nanocrystal was increased, and the edge thickness of framework was slightly decreased, plausibly due to the limited amount of Pt (Figure 4c). The role of Cu precursor for the shape evolution was also studied. Ultrathin PtNi nanoframe could be obtained without adding Cu precursor to the reaction mixture (Figure 4d). This framework structural feature has originated from the sequential decomposition of Ni precursor, followed by decomposition of Pt precursor. The reduction potential of Pt is higher than that of Ni, however, the Cl− ion from the CTAC might form the [PtCl4]2− complex and impede the reduction kinetics of Pt.40 This inverse reduction sequence makes it possible to synthesize an ultrathin PtNi nanoframework. Similarly, PC@PCN nanocrystals could be observed in the presence of 0.5 equiv of Cu precursor (Figure 4e) due to the sequential decomposition of Cu and Pt, followed by Ni with residual Cu and Pt. When 2 equiv of Cu(acac)2 was added to the reaction mixture (Figure 4f), the size of nanoparticles became irregular, while no discernible differences were observed for structural motifs when compared to the product from 0.5 equiv of Cu precursor. Therefore, we conclude that the amount of Cu precursor to the Pt is critical in the formation of dendritic core and frame; some amount of Pt precursor facilitated the PtCu alloy phase, and the residual Pt was used to form PtCuNi ternary frame structures. Finally, we investigated the effect of CTAC on the PC@PCN morphology control (Figure S10). Irregularly shaped nanoparticles are obtained from a standard protocol except for adding CTAC (Figure S10a,d). By adding 0.5 equiv of CTAC, much smaller PC@PCN nanocrystals were observed compared to that obtained in the condition of 1 equiv of CTAC, and the dendrite@frame structural features are rarely observed after etching (Figure S10b,e). At a higher concentration of CTAC (2 equiv), the nanocrystal size is slightly increased (Figure S10c,f). The presence of halide ion can induce slow reduction kinetics of metal precursor.42 Therefore, CTAC might slow down the

other hand, the nanocrystals synthesized at a higher concentration of Pt precursor exhibit edge-emphasized nanoframe morphology after the etching process (Figure S9b,c). In the absence of Ni precursor, PtCu dendrite nanocrystals were observed, and those formed initially showed no structural variation during the reaction and even after the chemical etching process (Figure 4a). The exterior of as-prepared PtCu dendrites is not a rectangular-like shape, implying that a Ni2+ ion could induce morphological evolution from Pt-based multipods to cubes.39 Therefore, the cube-like morphology in Figure 2a might have originated from the small degree of Ni mixing into the PtCu core. In the presence of 0.5 equiv of

Figure 4. Effect of the concentration of Ni and Cu precursor on the morphology of unetched nanocrystals. (a−c) TEM images of unetched nanocrystals prepared from Ni(acac)2 concentrations of (a) 0 equiv, (b) 0.5 equiv, and (c) 2 equiv at a fixed Pt(acac)2 and Cu(acac)2 standard concentration. (d−f) TEM images of unetched nanocrystals prepared from Cu(acac)2 concentrations of (d) 0 equiv, (e) 0.5 equiv, and (f) 2 equiv at a fixed Pt(acac)2 and Ni(acac)2 standard concentration. All of TEM images in the inset of (a−f) indicate the nanocrystal morphologies after chemical etching. 10847

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ACS Nano reduction kinetics of Pt by forming complex such as [PtCl4]2−, limiting the number of Pt seeds formed in the initial stage and thereby resulting in larger nanocrystals. The electrocatalytic performance of acid-etched PC@PCN/ C catalyst was evaluated and compared with those of PtCu dendrite/C (PC/C), PtCuNi frame/C (PCN/C), and commercial Pt/C as shown in Figure 5. TEM and EDS

remarkably enhanced ORR activity of PC@PCN/C among other catalysts might be due to the optimization of the Pt dband center position. We suppose that the lattice mismatch between dendrite and frame, inducing surface strain and synergistic effect of PCN ternary phase of PC@PCN/C, could be the critical factor in enhancing the ORR activity. In the case of PC/C, the dendritic structural features provide high surface area, but the PtCu composition is less active than the PCN ternary alloy. It is also difficult to control the optimized ternary composition in the absence of PC dendritic core, although the PCN/C also exhibits high surface area, originating from the frame structural features. In addition, the PC@PCN/C delivers 6 times of mass activity to the 2017 target, set by the U.S. Department of Energy (DOE) (0.44 A/mgPt at 0.90 V for MEA). Table S2 represents the comparison of mass activities of the state-of-the-art electrocatalysts at 0.9 V, indicating competitively high mass activity of the as-synthesized PC@ PCN/C. We conducted the accelerated durability test for the PC@ PCN/C, PC/C, PCN/C, and Pt/C in an Ar-saturated 0.1 M HClO4 solution with a potential range from 0.6 to 1.0 V vs RHE at a scan rate of 100 mV s−1. CVs before and after cycling tests for 5000 cycles of PC@PCN/C are shown in Figure 6.

Figure 5. Electrochemical properties of PC@PCN. Comparison of (a) CVs and (b) ORR polarization curves for the PC@PCN/C, PC/ C, PCN/C, and commercial Pt/C. (c) Mass activities given as kinetic current densities (jk) normalized against the Pt loading. (d) Mass and specific activities recorded at 0.9 VRHE for the catalysts.

mapping analyses of PC dendrite and PCN frame are shown in Figures S11 and S12. The cyclic voltammograms (CVs) and ORR polarization curves for the different catalysts normalized by the area of the rotating disk electrode (RDE) are shown in Figure 5a,b. The electrochemically active surface area (ECSA) was estimated by the charge associated with hydrogen underpotential deposition (Hupd)41 in the potential range of 0.05−0.4 V (vs RHE) and CO stripping method.42,43 (Figure S13). The Pt/C showed the highest ECSA value, followed by PC@PCN/C, PCN/C, and PC/C (Table S1). The Hupd-based ECSAs of PC@PCN/C, PC/C, PCN/C, and commercial Pt/C were 33.8, 25.3, 28.5, and 50.8 m2 gPt−1, respectively, comparable to those values obtained from CO stripping method, namely, 41.7, 36.4, 38.7, and 65.4 m2 gPt−1. Because ECSA is strongly associated with the particle size, a higher ECSA of commercial Pt/C (particle size of 2−3 nm) was found than those of PC@PCN/C, PC/C, and PCN/C catalysts. The ORR polarization curves showed that the half-wave potential for the PC@PCN/C was 0.933 V, which is higher than those of the PC/C (0.922 V), PCN/C (0.920 V), and commercial Pt/C (0.895 V), indicating the greatly reduced ORR overpotential for the PC@PCN/C (Figure 5b). In order to compare the kinetics of ORR, we normalized the current density based on the Pt loading and calculated the kinetic current density using Koutecky−Levich equation (see Materials and Methods section for details) to exhibit the Tafel plot in Figure 5c. Figure 5d shows the comparison of both the mass and specific activities at 0.9 VRHE for the catalyst. Remarkably, the PC@PCN/C exhibited 11 times and 16 times higher mass and specific activities than commercial Pt/C. The PC/C also showed 7 times and 14 times of mass and specific activities, respectively, than Pt/C. Mass and specific activities of the PCN/C showed 7 times and 12 times of commercial Pt/C, respectively. The

Figure 6. (a) CVs and (b) ORR polarization curves of PC@PCN/C and commercial Pt/C before and after the cycling test for 5000 cycles in an O2-saturated, 0.1 M aqueous HClO4 solution.

The ORR polarization curves indicate that the PC@PCN/C shows well-sustained catalytic performance as compared to that of the commercial Pt/C, with its half-wave potential. We found that the mass and specific activities of PC@PCN/C decreased to 70% of the initial level. The half-wave potential of PC@ PCN/C shifted from 0.933 to 0.924 V, which still recorded a higher value than that of the PC/C (from 0.922 to 0.916 V) and, PCN/C (from 0.920 to 0.916 V) (Figure S14). The ECSAs of PC@PCN/C before and after cycling tests were 33.8 and 28.5 m2 gPt−1, respectively. ECSA of all the catalysts before and after the stability tests are shown in Table S3. TEM images of PC@PCN/C, PC/C, and PCN/C after 5000 cycles of durability tests reveal no significant morphological changes in all three of the different structures (Figure S15). To investigate the elemental composition after the durability test, we conducted elemental mapping and line profile analyses as shown in Figure S16. It is clearly shown that the compositions at both edges and corners of the PC@PCN nanocrystal after the durability test are similar to those of fresh ones. Through this analysis, we also found that the dendritic core and frame structures are totally segregated even after the durability test. The elemental mapping of PC/C after the cycling test shows no great differences as compared to that of the before test as shown in Figure S17a,c. As shown in Figure S17b,d, some dissolution was found for Cu and Ni in PCN/C. 10848

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detector. XRD patterns were collected to understand the crystal structures of PtCu@PtCuNi (PC@PCN) nanocrystals with a Rigaku Ultima III diffractometer system using a graphite-monochromatized Cu−Kα radiation at 40 kV and 40 mA. Metal contents in PC@PCN/ C catalyst were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Preparation of PtCu@PtCuNi Dendrite@frame Nanocrystal (PC@PCN). A slurry of Pt(acac)2 (0.020 mmol), Ni(acac)2 (0.045 mmol), Cu(acac)2 (0.015 mmol), CTAC (0.06 mmol), and oleylamine (15 mmol) was prepared in a 100 mL Schlenk tube with magnetic stirring. After placing the solution under vacuum at 25 °C for 5 min, the solution was charged with 1 atm Ar. Then the Schlenk tube was directly placed in a hot oil bath, which was preheated to 270 °C. After heating at the same temperature for 30 min, the reaction mixture was cooled down to room temperature with magnetic stirring. After cooling to room temperature and adding 15 mL of toluene and 25 mL of ethanol, the reaction mixture was centrifuged at 4000 rpm for 5 min. The resulting precipitates were further purified 2 times by washing with ethanol/toluene (v/v = 10 mL/5 mL). Then the resulting precipitates were dispersed in a mixture of 2 mL of toluene, 2 mL of ethanol, and 2 mL of 3 M HCl solution. The mixture was placed in preheated oil bath at 60 °C for 1 h. Finally, the precipitated PC@PCN were centrifuged and washed with ethanol (10 mL) for 2 times and then dried under vacuum. Preparation of PtCuNi Frame Nanocrystal (PCN). In a preparation of PCN nanoframe, we modified a previously reported protocol for the synthesis of the PtCu nanoframe.18 Pt(acac)2 (20.0 mg), Ni(acac)2 (50.0 mg), CuCl2·2H2O (50.0 mg), glucose (60.0 mg), and 2 mL of oleic acid, 8 mL of oleylamine were added into 50 mL vial. After the vial was capped, the mixture was ultrasonicated for 2 h to give a reddish homogeneous solution. Then the vial was directly placed in a hot oil bath, which was preheated to 230 °C. After heating at the same temperature for 3 h, the reaction mixture was cooled down to room temperature with a magnetic stirring. After cooling to room temperature, the reaction mixture was washed with 15 mL of toluene and 20 mL of ethanol several times and collected by centrifugation at 4000 rpm for 5 min. The resulting precipitates were dispersed in a mixture of 5 mL of toluene and 5 mL of acetic acid solution. The mixture was placed in a preheated oil bath at 70 °C for 1 h. Finally, the resulting PCN nanoframe product was washed with ethanol (20 mL) for 2 times, collected by centrifugation, and then dried under vacuum. Preparation of the PC@PCN/C, PC/C, and PCN/C Catalysts. Suspension of 20 mg of PC@PCN (PC or PCN) nanocrystals and 80 mg of Ketjen black carbon was dispersed in 30 mL of chloroform, and the mixture was then magnetically stirred and ultrasonicated for 5 min. After centrifugation, the resulting PC@PCN/C (PC/C or PCN/C) catalyst was redispersed in 30 mL of acetic acid and then heated at 60 °C or 30 min to clean the residual surfactants. PC@PCN/C (PC/C or PCN/C) catalyst was washed with ethanol 3 times and dried under vacuum. Preparation of [MTBD][NTf2] Ionic Liquid (IL). 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene [MTBD] purchased from Sigma-Aldrich was neutralized by adding 10.6 M HNO3 solution at 0 °C. Lithium salt of bis(trifluoromethane)sulfonimide [NTf2] purchased from SigmaAldrich was dissolved in DI water. Then, the neutralized [MTBD] and [NTf2] aqueous solution was mixed with a molar ratio of 1:1 to produce [MTBD][NTf2] IL. A viscous [MTBD][NTf2] IL was washed several times with DI water after the residual water from the mixture was removed. The [MTBD][NTf2] IL was then dried in a vacuum oven at 70 °C for 12 h. Electrochemical Characterization. Electrochemical characterization of samples was carried out at room temperature under atmospheric pressure using a three-compartment electrochemical cell connected to a CHI600E potentiostat (CH Instruments, USA). An Ag/AgCl electrode was used as the reference electrode, and potentials of as-received data were converted to reversible hydrogen electrode (RHE). Pt mesh (1 × 1 cm2) was used as a counter electrode. For the oxygen reduction reaction (ORR) measurement, a rotating disk electrode (RDE, Pine Research Instrumentation) with a glassy carbon disk (GC, 5 mm in diameter) was used as the working

The XPS measurements were conducted to trace the changes of oxidation state of the elements during the cycles as shown in Figure S18. The oxidation state of Pt in PC@PCN/C moved to higher binding energy, plausibly due to the oxidation of some Ni contents during catalytic operation. However, the oxidation states of Pt and Cu in PC/C before and after testing show no big differences. On the other hand, the oxidation state of Pt in PCN/C moved to a higher binding energy, however, only slightly. It is interesting to note that although both PC@PCN/ C and PCN/C show the same Pt oxidation change trend, the oxidation and dissolution of Ni are more detrimental to the catalytic performance of PCN/C. We have also investigated the advantage of dendrite@frame structure of PC@PCN/C by electrochemical method. The PC@PCN/C was encapsulated with protic [MTBD][NTf2] ionic liquid (IL) (see Materials and Methods section for details), where it shows approximately a two-fold high O2 solubility than that of the HClO4 electrolyte.44 The PC@PCN/ C dendrite@frame structure induces capillary force which pulls the [MTBD][NTf2] IL inside the frame, resulting in more availability of O2 reduction for PC@PCN/C catalyst. The [MTBD][NTf2] IL encapsulated PC@PCN/C (PC@PCN/ C/IL) exhibited a mass activity of 3.87 A mg−1 which is 1.6 times that of the PC@PCN/C (Figure S19). The commercial Pt/C showed negligible enhancement of ORR mass activity after the [MTBD][NTf2] IL encapsulation. These results strongly support that the inside of the PC@PCN/C dendrite@ frame participated in the ORR, leading to the enhancement of electrocatalytic properties. In addition, PC@PCN/C/IL showed better stability over 5000 cycles with only 9% loss of mass activity (Figure S19d), indicating the beneficial effect of [MTBD][NTf2] IL which can support the frame structure of PC@PCN/C during the long period of ORR measurements.43

CONCLUSIONS We have developed a rational synthetic strategy for a PtCuNi ternary alloy nanoframe structure, which is structurally supported by an inner-lying PtCu dendrite, via understanding the reaction kinetics of precursors and utilizing the directional alloy component diffusion. The overall nanocatalyst structural motif was unaltered even after extended catalytic cycles, suggesting that the intrinsically infirm nanoframe can be structurally fortified by an inner-positioned structural support. Further studies on the structural fortification of nanoframes and their optimal catalyst compositions for a durable electrocatalytic performance are currently under way. MATERIALS AND METHODS Reagents. Pt(acac)2 (97%), Ni(acac)2 (95%), Cu(acac)2 (99.99+ %), and oleylamine (98%) were purchased from Sigma-Aldrich. Hexadecyltrimethylammonium chloride (CTAC) (98%) was purchased from Alfa Aesar. All reagents were used as received without further purification. Material Characterization. TEM and HRTEM studies were carried out with a TECNAI G2 F30ST microscope and Tecnai G2 20 S-twin microscope. Aberration-corrected imaging and high spatial resolution EDS were performed at FEI Nanoport in Eindhoven using a Titan Probe Cs TEM 300 kV with Chemi-STEM technology. EDS elemental mapping data were collected using a higher efficiency detection system (Super-X detector with XFEG); it integrates 4 FEIdesigned silicon drift detectors (SDDs) very close to the sample area. Compared to conventional EDX detector with Schottky FEG systems, ChemiSTEM produces up to 5 times the X-ray generation with the XFEG and up to 10 times the X-ray collection with the Super-X 10849

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ACS Nano electrode (E3TPK, model no. AFE3T050GCPK). The RDE was polished with 0.05 μm alumina suspensions to generate a mirror finish. The catalyst ink was prepared by mixing a catalyst powder, 5 wt % Nafion (5 μL, Aldrich), deionized water (1.0 mL), and isopropyl alcohol (0.25 mL, >99.5%, Duksan) using ultrasonication for at least 30 min. A 20.0 μL of the ink was dropped on GC electrode and dried to form a thin film. The total Pt loadings on GC electrode were 20.4 μg cm−2 for commercial Pt/C and 10.2 μg cm−2 for PC@PCN/C, PCN/C, and PC/C catalysts, respectively. Electrochemical cleaning prior to the electrochemical measurements was done in a potential cycle ranging from 0.08 to 1.10 V in an Ar-saturated solution of 0.1 M HClO4 for 50 cycles at a scan rate of 100 mV s−1. Changing the scan rate to 50 mV s−1 for 2 cycles, the final CV was obtained under the same condition. Hydrogen underpotential (Hupd)-based ECSA was calculated by measuring the charges (QH) generated from averaging desorption and adsorption regions of hydrogen between 0.08 and 0.45 VRHE with a reference value of 210 μC cm−2 and then divided by the mass of Pt loaded on GC electrode.45,46 For CO stripping experiments, the precleaned electrode held at potential of 0.05 V for 10 min in a CO saturated 0.1 M HClO4 solution. After purging Ar for 30 min, the CO stripping curve was taken with a scan rate of 50 mVs−1. The second CV cycle was used as a baseline for the CO stripping peak obtained in the first cycle. CO oxidation peak was absent in a second CV, which indicated full oxidation of absorbed CO to CO2. From CO stripping peak, measured charge (QCO) was normalized with a reference value of 420 μC cm−2 and then divided by the mass of Pt loaded on GC electrode.42 Linear sweep voltammetry curves for the ORR were carried out in an O2-saturated electrolyte with a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm. All the ORR measurement results were iR-drop corrected. The kinetic current density (jk) was derived from the Koutecky−Levich equation as follows:

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kwangyeol Lee: 0000-0003-0575-7216 Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by IBS-R023-D1, NRF2017R1A2B3005682, NRF-2015R1D1A3A01019467, and Korea University Future Research Grant. The authors thank Korea Basic Science Institute (KBSI) for the usage of their HRTEM and ICP-AES instrument. REFERENCES (1) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168−2201. (2) You, H.; Yang, S.; Ding, B.; Yang, H. Synthesis of Colloidal Metal and Metal Alloy Nanoparticles for Electrochemical Energy Applications. Chem. Soc. Rev. 2013, 42, 2880−2904. (3) Cui, C.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Compositional Segregation in Shaped Pt alloy Nanoparticles and Their Structural Behaviour during Electrocatalysis. Nat. Mater. 2013, 12, 765−771. (4) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339−1343. (5) Escudero-Escribano, M.; Malacrida, P.; Hansen, M. H.; VejHansen, U. G.; Velázquez-Palenzuela, A.; Tripkovic, V.; Schiøtz, J.; Rossmeisl, J.; Stephens, I. E. L.; Chorkendorff, I. Tuning the Activity of Pt alloy Electrocatalysts by Means of the Lanthanide Concentration. Science 2016, 352, 73−76. (6) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. Highperformance Transition Metal-doped Pt3Ni Octahedral for Oxygen Reduction Reaction. Science 2015, 348, 1230−1234. (7) Stephens, I. E. L.; Rossmeisl, J.; Chorkendorff, I. Toward Sustainable Fuel Cells. Science 2016, 354, 1378−1379. (8) Oh, A.; Baik, H.; Choi, D. S.; Cheon, J. Y.; Kim, B.; Kim, H.; Kwon, S. J.; Joo, S. H.; Jung, Y.; Lee, K. Skeletal Octahedral Nanoframe with Cartesian Coordinates via Geometrically Precise Nanoscale Phase Segregation in a Pt@Ni Core-Shell Nanocrystal. ACS Nano 2015, 9, 2856−2867. (9) He, D. S.; He, D.; Wang, J.; Lin, Y.; Yin, P.; Hong, X.; Wu, Y.; Li, Y. Ultrathin Icosahedral Pt-Enriched Nanocage with Excellent Oxygen Reduction Reaction Activity. J. Am. Chem. Soc. 2016, 138, 1494−1497. (10) Wang, X.; Figueroa-Cosme, L.; Yang, X.; Luo, M.; Liu, J.; Xie, Z.; Xia, Y. Pt-based Icosahedral Nanocages: Using a Combination of {111} Facets, Twin Defects, and Ultrathin Walls to Greatly Enhance Their Activity toward Oxygen Reduction. Nano Lett. 2016, 16, 1467− 1471. (11) Gan, L.; Heggen, M.; Cui, C.; Strasser, P. Thermal Facet Healing of Concave Octahedral Pt-Ni Nanoparticles Imaged in situ at the Atomic Scale: Implications for the Rational Synthesis of Durable High-Performance ORR Electrocatalysts. ACS Catal. 2016, 6, 692− 695. (12) Huang, X.; Zhu, E.; Chen, Y.; Li, Y.; Chiu, C. − Y.; Xu, Y.; Lin, Z.; Duan, X.; Huang, Y. A Facile Strategy to Pt3Ni Nanocrystals with Highly Porous Features as an Enhanced Oxygen Reduction Reaction Catalyst. Adv. Mater. 2013, 25, 2974−2979.

1 1 1 = + j jk jd

jk =

(j × jd ) jd − j

where j is the measured current density and jd is the diffusion limiting current. For the accelerated electrochemical stability test, the CVs and ORR polarization curves were measured after sweeping 5000 cycles between 0.6 and 1.0 VRHE at a rate of 100 mV s−1 in an O2-saturated 0.1 M HClO4 solution at room temperature. After the cycling, the CV and ORR activity was obtained in a fresh electrolyte under the same conditions. For the ORR measurements in the presence of [MTBD][NTf2] IL, a drop of IL was added onto GC electrode covered with catalysts such as commercial Pt/C and PC@PCN/C. Excess [MTBD][NTf2] IL was removed from GC electrode after 3 h, and further cleaning was done through electrochemical cycling in Ar-saturated 0.1 M HClO4. CVs, ORR polarization curves, and stability test were performed in similar conditions as outlined above in 0.1 M HClO4 solution.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04097. Figures S1−S19 and tables S1−S3 give more details on characterization of our synthesized materials and their electrocatalytic performance data. For example, additional TEM, line profile analysis, HRTEM, XRD, elemental mapping analysis, EDS, ICP-AES, and electrocatalytic performance data are given (PDF) 10850

DOI: 10.1021/acsnano.7b04097 ACS Nano 2017, 11, 10844−10851

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DOI: 10.1021/acsnano.7b04097 ACS Nano 2017, 11, 10844−10851