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Dendrite-embedded Pt-Ni Multiframes as Highly Active and Durable Electrocatalyst towards the Oxygen Reduction Reaction Hyukbu Kwon, Mrinal Kanti Kabiraz, Jongsik Park, Aram Oh, Hionsuck Baik, Sang-Il Choi, and Kwangyeol Lee Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00270 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Dendrite-embedded Pt-Ni Multiframes as Highly Active and Durable Electrocatalyst towards the Oxygen Reduction Reaction Hyukbu Kwon†,§ Mrinal Kanti Kabiraz,‡,§ Jongsik Park†,§ Aram Oh, †,⊥ Hionsuck Baik,⊥ Sang-Il Choi,*,‡ and Kwangyeol Lee*,†,∥

†

Department of Chemistry, Korea University, Seoul 02841, Korea

‡

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National

University, Daegu 41566, Korea ∥

Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Seoul

02841, Korea ⊥

§

Korea Basic Science Institute (KBSI), Seoul 02841, Korea

These authors contributed equally to this work.

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ABSTRACT: Pt-based nanoframe catalysts have been explored extensively due to their superior activity towards the oxygen reduction reaction (ORR). Herein, we report the synthesis of Pt-Ni multiframes, which exhibit the unique structure of tightly fused multiple nanoframes and reinforced by an embedded dendrite. Rapid reduction and deposition of Ni atoms on Pt-Ni nanodendrites induce the alloying/dealloying of Pt and Ni in the overall nanostructures. After chemical etching of Ni, the newly formed dendrite-embedded Pt-Ni multiframes show an electrochemically active surface area (ECSA) of 73.4 m2 g-1Pt and a mass ORR activity of 1.51 A mg-1Pt at 0.93 V, which is 30-fold higher than that of the state-of-the-art Pt/C catalyst. We suggest that high ECSA and ORR performances of dendrite-embedded Pt-Ni multiframes/C can be attributed to the porous nanostructure and numerous active sites exposed on surface grain boundaries and high-indexed facets.

KEYWORDS: Platinum · multiframes · porous nanostructure · electrocatalyst · oxygen reduction reaction


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Oxygen reduction reaction (ORR) is the rate-determining step for the actual use of hydrogen to produce electricity in the proton electrolyte membrane fuel cell (PEMFC).1-7 For the commercialization of the PEMFC, Pt-based alloy nanocrystals (Pt-M, M = Co, Ni, Cu, etc) have been investigated intensively to boost sluggish ORR kinetics.8-16 Control of compositions in the Pt-M alloys has been considered for the efficient use of Pt not only to reduce the Pt loading but also to optimize the Pt d-band structure with strained surfaces for activating ORR intermediates.17,

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In addition, the Pt-M nanocatalysts with hollow morphologies such as

nanoframes19-21 and nanocages22-24 are known to be effective structures due to their large surface area to volume ratio and increased number of low-coordinated active sites. However, the tenuous edges of hollow structures can collapse and agglomerate under high potentials and acidic ORR conditions. Leaching of dissolvable elements from the Pt-M nanocatalysts in harsh catalytic cycles would further aggravate the catalytic performances in terms of the activity and durability.25, 26 To overcome these problems, various strategies to fortify alloy nanoframes and cages have been developed, including formation of ternary nanocrystal phase and double nanoframes.27-34 Recently, we demonstrated the concept of dendrite@frame structure, in which the outer Pt-CuNi nanoframe is fortified by inner-residing Pt-Cu dendrite.27 The network of branches and fused joints can mechanically strengthen the multiframed structure and maintain the same structural motif even under severe structure deforming operating condition. While the structural stability during the catalysis has been drastically improved for the nanoframed structure, the meager catalytic activity of inner lying Pt-Cu was the major drawback. To increase the utilization of Pt, it would be desirable to have the more active Pt-Ni alloy phase for both the inner-lying structural support and the outer multiframes. In addition, a large number of nanosized pores in the


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multiframes could be beneficial to trap the gaseous reactants into the nano-confined spaces, deriving higher attempt of catalytic events.35, 36 Herein, we report the successful synthesis of bimetallic Pt-Ni multiframes with embedded Pt-Ni dendrite, which tightly support the Pt-Ni multiframes. Tumbleweed resembling Pt-Ni structure was generated from the agglomeration of initially formed nanodendrites. The burst reduction and deposition of abundant Ni, followed by Pt-Ni alloying/dealloying, created multiple Pt-Ni bridges among the Pt-Ni dendrites as well as over-lying inter-fused multiframes. After Ni etching process, Pt-Ni multiframes with embedded Pt-Ni dendrites with an average size of 72.18 nm were obtained (Scheme 1). Pt-Ni multiframes showed larger electrochemically active surface area (ECSA) of 73.4 m2 gPt-1 than 65.5 m2 gPt-1 of the state-of-the-art Pt/C catalyst due to the highly porous morphology. Electrocatalytic performances of the Pt-Ni multiframes exhibited greatly enhanced ORR mass activity of 5.03 A mgPt-1 at 0.9 V and the long-term stability, as judged by a slight decrease of 15.3 % in mass activity over 10000 cycles of the ORR test.

Results and Discussion In a typical synthesis of the Pt-Ni multiframes, a slurry of Pt(acac)2 (0.010 mmol), Ni(acac)2 (0.045 mmol), cetyltrimethylammonium chloride (CTAC) (0.045 mmol), and oleylamine (15 mmol) was prepared in an 100 mL Schlenk tube with a magnetic stirring at 50 oC for 7 min. The Schlenk tube was then placed in a hot oil bath and kept at 270 oC for 30 min. After cooling down the mixture, a product was precipitated by adding 15 mL of toluene and 25 mL of ethanol, and separated from the supernatant after centrifugation at 4000 rpm for 5 min. Chemical etching of


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Ni component from the precipitates was proceeded in a 3 M HCl solution at 60 oC for 1 h (see details in the Supporting Information (SI)). Transmission electron microscopy (TEM) images and the corresponding powder X-ray diffraction (PXRD) patterns demonstrate the formation mechanism of the Pt-Ni multiframes. At the initial stage of the reaction at 2.5 min (Figure 1a), dendritic Pt-Ni nanostructures were formed due to the fast reduction and aggregation of small Pt and Ni nuclei at high temperature.3739

High-resolution TEM (HRTEM) image in Figure S1a further implies the polycrystalline nature

of a dendritic Pt-Ni nanocrystal. Enlarged HRTEM image in Figure S1b shows the twin boundaries marked by white dashed lines. The {111} lattice distance of dendritic Pt-Ni nanocrystal is revealed to be 0.21 nm, implying 45.8% of Pt atomic composition according to Vegard’s law; {111} lattice distances of Pt and Ni are 0.23 nm and 0.20 nm, respectively. Atomic compositions of Pt and Ni in the dendritic Pt-Ni nanocrystals were 48.77 and 51.22%, obtained by energy-dispersive X-ray spectroscopy (EDS) measurement (Figure S2a). It is wellknown that the decomposition and reduction kinetics of Pt precursor are much faster than those of Ni. However, the preferential binding of chloride ions in CTAC with Pt ions slows down the formation speed of Pt seeds and leads to the formation of Pt-Ni bimetallic alloy nanodendrites in the early stage of the reaction.40, 41 Further reaction to 5 min, excess Ni ions were reduced to fill the space among the branches of dendrites. As shown in Figure 1b, a Pt-Ni complex nanostructure formed due to the deposition of Ni atoms on the several aggregated Pt-Ni dendrites. The Ni-deposited Pt-Ni dendrites (Pt-Ni complex) show {111} diffractions of Pt-Ni and Ni in PXRD patterns of Figure 1e. The atomic ratio of Pt/Ni in the Pt-Ni complex is 0.20, measured by EDS measurement (Figure S2b), which is similar to the ratio of precursors in the reaction mixture. EDS elemental mapping analysis of


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the bright dendritic branches in a high-angle annular dark-field scanning TEM (HAADF-STEM) image shows the overall distribution of Pt and Ni, which are thoroughly mixed (Figure S3). According to the line profile analysis, the exposed surface of Pt-Ni complex nanostructure is mainly composed of Ni, which originated from the deposition of Ni precursor. After the chemical etching of Ni from the Pt-Ni complex nanostructure, a radially outgrown and highly porous Pt-Ni dendritic structure was observed as shown in Figure S4. This finding demonstrates that the Pt-Ni alloying process at the Pt-Ni/Ni interface is extremely fast under the reaction condition. As the reaction continued to 30 min, the size distribution of the Pt-Ni complex became narrower due to the Ostwald ripening42 as shown in Figure 1c. Compared with the PXRD pattern of the Pt-Ni complex at 5 min, the {111} diffraction for the product at 30 min indicates the reduced heterogeneity of the nanomaterial phase with weaker shoulder intensity at around 42o, demonstrating further Pt/Ni mixing. EDS elemental mapping, HAADF, and line profile analyses in Figure S5 reveal that Pt elements are predominantly located on the outer regions of the Pt-Ni complex nanostructure. This finding reveals the migration of Pt atoms from the dendritic core to Ni-rich area and further to edges and vertices during the prolonged reaction. The outward migration of Pt atoms in a Pt-Ni bimetallic nanocrystal has been previously discussed in recent literatures;36, 43-45 the difference of atomic size, high reaction temperature, crystal defects, and surface binding moieties have been found to be the deciding factors. Due to the larger atomic size of Pt than that of Ni, Pt atoms prefer less crowded and lower coordinated sites such as edges and vertices in the nanocrystal. The infiltration of Pt through Ni-rich phase to make a bimetallic alloy is an entropy positive reaction. Therefore, the reaction temperature over 270 oC may provide enough energy to overcome the energy barrier of Pt migration. In addition, Pt migration


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would occur preferentially at the grain boundaries and defect sites of the polycrystalline intermediate nanocrystal as revealed in Figure S1. Pt-Ni multiframes could be obtained after etching the Ni-rich phases as shown in Figure 1d. EDS data of the acid-etched sample in Figure S2d shows a significant dissolution of Ni, making the Pt/Ni ratio of the Pt-Ni multiframes increased to 1.8, which matches with Pt/Ni = 2.0 obtained by the inductively coupled plasma atomic emission spectroscopy (ICP-AES). PXRD pattern of the acid-etched sample in Figure 1e shows the peak-shift toward Pt richer phase. The PXRD peak broadening was also observed due to the reduced width of crystal grain of the Pt-Ni multiframes during the etching. Figure 1f shows the correlations between average diameter and corresponding elemental composition of Pt-Ni intermediates obtained by EDS. Average diameter of Pt-Ni intermediate is drastically increased from 52.7 to 78.5 nm within 5 min, due to the fast deposition of Ni in the early stage of the reaction. While no significant increase of the diameter was observed for the Pt-Ni intermediate at 30 min, the Pt migration to the surface edges and vertices has occurred during this time. After the chemical etching process, the dissolution of Ni decreased the average diameter of Pt-Ni multiframes to 72.2 nm. HRTEM analysis and the corresponding fast Fourier transformation (FFT) pattern demonstrate the polycrystallinity of the Pt-Ni multiframes (Figure 2a). These results are consistent with broad PXRD peak of the Pt-Ni multiframes in Figure 1e and clearly prove the existence of a number of grain boundaries and defect sites with high energy states. Moreover, atomic arrangement on the surface of the Pt-Ni multiframes represents the existence of high-indexed facets such as steps and kinks, which are advantageous to enhancing catalytic activity (Figure 2b).46-48 HAADF-STEM image shows the nanoarchitecture of multiply stacked frames with the thickness of approximately 2 to 3 nm and the EDS elemental mapping images represent the Pt-Ni alloyed


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multiframes with relatively denser Pt distribution on the surface of frames as shown in Figure 2c. The highly porous multiframed structure would be advantageous to attaining internal accessibility of chemical reactants and thus promoting the catalytic active sites. Above observations show the role of Ni as a space filler promoting the aggregation of Pt-Ni dendrites to interconnected structure and the effect of fast Pt atom migration to the surface resulting in the multiframed morphology. The control of porosity and degree of surface segregation of Pt-Ni multiframes might be critically related to the performance of electrocatalysts, because the catalyst composition and catalyst surface area are the determining factors for the catalyst activity. The amounts of Ni precursor might control the porosity of nanoframes, because the fast decomposing Ni would determine the overall nanoparticle growth pattern. The reaction temperature could also influence the degree of surface segregation by modulating the mobility of Pt atoms which are initially located at the inner dendrite region. To demonstrate the influence of these factors, we conducted several experiments as discussed below. Figure 3a-c show the different Pt-Ni multiframes obtained by adding 0.25, 0.5, and 2 equiv. of Ni precursors compared to that used in the standard protocol. When 0.25 equiv. of Ni was used, the lack of Ni atoms neither aggregate the dendritic particles nor the structural changes during the Ni decomposition and etching process. In contrast, dendritic core and shell segregated Pt-Ni multiframes with larger voids were observed by adding 2 equiv. of Ni in the reaction mixture. As a result, it is concluded that the increased amount of Ni precursor enhances the volume of the voids and the porosity of the framed structure. PXRD patterns in Figure S7a showed the peak-shift of products toward Ni-richer composition as the amount of Ni precursor increases. However, after etching of the abundant Ni element, compositions of Pt and Ni in all the different Pt-Ni multiframes showed similar Pt/Ni ratios,


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indicating 55~65 atomic percentage of Pt as thermodynamically stable phase under acid etching condition (Figure S7b and Figure S8). The effect of temperature was also investigated to show that the attachments among dendrites and thus the formation of multiframes in Pt-Ni complex can be promoted only at higher temperatures, which provide sufficient energy for the Pt migration (Figure 3d-f). The reduction rate of Pt precursor is found to affect the final multiframe structures. Pt reduction rate is easily controlled by adding different amount of halides; due to the strong binding of Pt-halide complex, Pt precursor decomposition rate can be significantly slowed. By adding different amounts of CTAC, different morphologies of Pt-Ni bimetallic nanocrystal were produced (Figure S9). Interestingly, dendritic core-shell segregated Pt-Ni multiframes was formed when 2 equiv. of CTAC was used as shown in Figure S9d. This structure resembles the structure shown in Figure 3c, suggesting that Pt deposition takes place only after Ni deposition is finished. Similar morphology can also be found when 0.010 mmol of K2PtCl4 or 0.045 mmol of NiCl2 was used (Figure S10a and b). In the presence of bromide, Pt reduction rate was reduced as much as in chloride and therefore the segregated Pt-Ni multiframes were also observed (Figure S10c). Above results imply that the amount of halide ion is important factor in determining the internal structure, namely, the connectivity of the Pt-Ni multiframes. Electrocatalytic features of the Pt-Ni multiframes were assessed and benchmarked against the state-of-the-art Pt/C catalyst. The Pt-Ni multiframes were loaded on carbon supports (Pt-Ni multiframes/C, Figure S11) and then treated in acetic acid to remove residual surfactants. Catalysts were loaded on rotating disk electrodes (RDEs) containing glassy carbon (GC) with the Pt loading of 10.2 µg cm-2 for the Pt-Ni multiframes/C and 20.4 µg cm-2 for the Pt/C catalysts, respectively. Electrochemical cleaning process (see details in the SI) was utilized not only to


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clean the surface of the all catalysts but also to leach out the surface Ni atoms from the Pt-Ni multiframes/C, facilitating the Pt skin-like structure.49 STEM image, corresponding elemental mapping, and line profile analysis of Pt-Ni multiframes before and after the cyclic voltammetry test demonstrate the formation of Pt-skin like structure during the cleaning process (Figure S12). Cyclic voltammetry (CV) was performed in an Ar-saturated 0.1 M HClO4 solution with a potential range between 0.08 and 1.10 V (vs. reversible hydrogen electrode, VRHE) at a scan rate of 50 mV s-1 (inset of Figure 4a). ECSA was determined from the underpotentially deposited hydrogen (Hupd) adsorption/desorption peak areas (0.08 to 0.4 VRHE) in a CV curve.50, 51 The PtNi multiframes/C and the Pt/C showed ECSAHupd values of 73.4 and 65.5 m2 gPt-1, respectively. The competitive ECSA of the Pt-Ni multiframes/C to that of the Pt/C grants access of more reactants in contact with internal and external surfaces of multiframes. ECSA of the catalysts was also measured by CO stripping method as shown in Figure S13.52, 53 The ECSACO/ECSAHupd ratio for the Pt-Ni multiframes/C (1.58) was 1.35 times higher than that of the Pt/C (1.17) (Figure S13d).54 It should be noted that the surface Pt structure can be probed based on the ECSACO/ECSAHupd ratio. ECSACO based on the strong adsorption of CO on Pt surfaces reflects reliable ECSA value of Pt-based catalysts. However, due to the weaker Hupd energy on Pt-skin surfaces than on bimetallic Pt-Ni and pure Pt surfaces, the value of ECSAHupd of Pt-skin will be smaller than those of the counterparts. According to the ECSACO/ECSAHupd ratio in literatures and our measurements, we could demonstrate that the CV cycled Pt-Ni multiframes/C shows Ptskin like surfaces.55 Onset potential of a CO stripping peak for the Pt-Ni multiframes/C was measured to be 0.13 V lower than that of the Pt/C (Figure S13c). This phenomenon can be correlated directly with the downshifting of Pt d-band center, thereby the weaker binding with oxygen-containing species, promoting ORR.56, 57


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ORR performance of the Pt-Ni multiframes/C and the state-of-the-art Pt/C were measured in O2-saturated 0.1 M HClO4 solutions and the resulting polarization curves were normalized by geometric surface area of RDE containing GC (0.196 cm2). As shown in Figure 4a, the ORR polarization curve for the Pt-Ni multiframes/C showed a positive shift in half wave potential of 46 mV than that for the Pt/C, indicating a greatly reduced overpotential. The Koutecky-Levich equation (see details in the SI) was used to calculate the kinetic current density, which was then normalized by total mass of Pt loading and ECSA of the catalysts to obtain the mass and specific ORR activities, respectively. Mass ORR activities for the Pt-Ni multiframes/C and the Pt/C were 5.03 and 0.26 A mg-1Pt at 0.9 VRHE (Figure 4b and Table S1) respectively. Because the polarization curve for the Pt-Ni multiframes/C at 0.9 VRHE reached the diffusion limiting current, we also compared the mass activities at 0.93 VRHE which showed an enhancement of 30 times for the Pt-Ni multiframes/C (1.51 A mg-1Pt) than that for the Pt/C catalyst (0.05 A mg-1Pt). The mass ORR activity for the Pt-Ni multiframes/C was reproducible even at lower Pt loading on GC (5.1 µg cm-2). Specific ORR activity normalized by ECSAHupd showed an enhancement of 17 and 21 times for the Pt-Ni multiframes/C (7.06 and 2.09 mA cm-2Pt) than the Pt/C (0.41 and 0.1 mA cm2

Pt)

at 0.9 and 0.93 VRHE (Table S2), respectively. Tafel plots of mass and specific activities

showed that the Pt-Ni multiframes/C catalyst exhibits nearly 12 and 10 times than U.S. Department of Energy (DOE) set (2017 target, purple dashed line) mass (0.44 A mg-1Pt) and specific (0.7 mA cm-2Pt) activities, respectively, at 0.9 VRHE for MEA (Figure 4d and Figure S14c). It is evident that after 10000 cycles, the Pt-Ni multiframes/C catalyst can still deliver the DOE required mass and specific activities even at higher potential of 0.94 VRHE, thereby reducing the overpotential for ORR.


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Accelerated ORR durability test was conducted for both the Pt-Ni multiframes/C and the stateof-the-art Pt/C catalysts under a scan rate of 100 mV s-1 between 0.6 and 1.0 VRHE in an O2saturated 0.1 M HClO4 solution for 5000 and 10000 cycles. ORR polarization curves of the Pt-Ni multiframes/C before and after cycles are presented in Figure 4c, indicating a very slight shift in half wave potential after durability test. As shown in the inset of Figure 4c, the mass activity losses at 0.9 VRHE for the Pt-Ni multiframes/C were only 6.36 and 15.3% after 5000 and 10000 cycles, respectively, compared to the initial value. In the case of the Pt/C, the mass activity losses at 0.9 VRHE were 30.7 and 50% after 5000 and 10000 cycles, respectively, compared to the initial value (Figures 4d, S14, and S15). ECSAsHupd of the Pt/C were reduced to 21.69 and 38.5% after 5000 and 10000 cycles (Table S2), respectively. Remarkably, ECSAsHupd of the Pt-Ni multiframes/C were reduced to only 5.51 and 14.8% after 5000 and 10000 cycles, respectively. Thicker Pt-skin and a lower coverage of oxygenated intermediates during the long-term CV cycling may result in the highly durable ORR performance for the Pt-Ni multiframes/C.54, 58 After 10000 cycles, loss of specific activity for the Pt-Ni multiframes/C was only 6.23% than initial activity (Table S2). TEM images taken after stability test of the Pt/C reveals agglomeration of the nanoparticles, while that of the Pt-Ni multiframes/C show no observable changes (Figure S16). We also studied the electrochemical performances of the Pt-Ni intermediates obtained at 2.5, 5, and 30 min of reaction time, and the electrochemical results are presented in Figure S17. As Pt migration is more pronounced at longer reaction times, surface of the Pt-Ni intermediates of longer reaction times became more available for the electrocatalysis, as proved by the timedependent ECSA and ORR mass activities. Therefore, it is concluded that thorough Pt migration plays a significant role in accomplishing high ECSA and ORR performance of the Pt-Ni


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intermediate/C. After 5000 cycles of the durability test, ORR mass activities of Pt-Ni intermediates/C at 2.5 and 5 min were reduced to 35 and 50%, respectively (Table S1 and S2). However, the Pt-Ni intermediates at 30 min with higher degree of Pt migration showed only 14% loss of the initial ORR mass activity after 5000 cycles, revealing that the higher level of Pt segregation helped the long-term stability of Pt-Ni based nanocatalysts. Finally, further Pt enrichment by chemical etching of the Pt-Ni intermediate/C (30 min) achieved the high active and durable Pt-Ni multiframes/C. Nanoframe structures provide the opportunity to utilize protic ionic liquid (IL) in an electrochemical measurement, which is a useful method to investigate whether the nanocatalyst has a hollow/open structure or not.19, 27, 59 The protic [MTBD][NTf2] ionic liquid (IL) was used to encapsulate the Pt-Ni multiframes/C (Pt-Ni multiframes/C/IL; see the details in the SI), where the [MTBD][NTf2] IL exhibits approximately two-fold O2 solubility than HClO4 electrolyte.19, 59 Due to capillary force, [MTBD][NTf2] IL was pulled inside of the Pt-Ni multiframes, resulting in more availability of O2 reduction for the Pt-Ni multiframes/C. As a result, ORR polarization curve of the Pt-Ni multiframes/C/IL was slightly shifted to higher potential than that of the Pt-Ni multiframes/C (Figure S18). The mass ORR activity of the Pt-Ni multiframes/C/IL at 0.93 VRHE exhibited mass activity of 2.94 A mg-1Pt, which is nearly 2 times to that of the Pt-Ni multiframes/C (Figure S18c), and closely reached to the record mass activity of 2.87 A mg-1Pt at 0.935 VRHE for the Pt-Ni nanowires in previous literature.49 After the [MTBD][NTf2] IL encapsulation, the Pt/C/IL showed no significant enhancement of the mass activity as Pt/C has no hollow structure. The Pt-Ni multiframes/C/IL showed stability over 5000 cycles with 25% loss of mass activity (Figure S18d), which still records 1.4 times higher than that of the fresh PtNi multiframes/C.


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Conclusions In conclusion, we introduced a facile one-pot synthesis of the dendrite-embedded Pt-Ni multiframes. Precise kinetic controls regarding the nucleation of nanodendrites and their agglomeration via abundant Ni are the key for the formation of the highly porous hierarchical multiframed structure. The dendrite-embedded Pt-Ni multiframes/C with abundant mass diffusion pathways and catalytically active surfaces of high-indexed facets promotes the ORR mass activity with 30-fold higher than the state-of-the-art Pt/C at 0.93 VRHE. Furthermore, the inner-residing dendrite core, highly fused multiframes on the surface, as well as interconnectivity between them contribute synergistically to the maintenance of the framed structure under the catalysis, leading to a high catalytic durability even after 10000 catalytic cycles. The structural motif and synthetic strategy might be further extended to develop more fine-tuned electrocatalysts.


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Scheme 1. Schematic illustration on the formation of Pt-Ni multiframes. At initial stage, small dendritic Pt-Ni nanoparticles (Figure 1a) are synthesized via the co-reduction of Pt and Ni precursors. Then, fast Ni deposition facilitates the aggregation of Pt-Ni dendrites and results in Pt-Ni complex structures (Figure 1b, c). After the migration of Pt atoms to grain boundaries and defect sites, the Pt-Ni multiframed structure (Figure 1d) is built in the complex structures and can be obtained through the chemical etching process.


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Figure 1. Temporal TEM images obtained at (a) 2.5, (b) 5, (c) 30 min, and (d) after chemical etching. (e) Corresponding PXRD patterns of each reaction stages. References: Pt for PDF#03065-2868 and Ni for PDF#01-071-9414. (f) Correlations between average diameter and corresponding elemental composition of Pt-Ni intermediates and Pt-Ni multiframes obtained by EDS.


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Figure 2. (a) HRTEM image of the Pt-Ni multiframes and the inset showing the FFT pattern. (b) Enlarged HRTEM image showing the surface atomic steps and kink sites of Pt-Ni multiframes. (c) HAADF-STEM image and the corresponding elemental mapping analysis of the Pt-Ni multiframes.


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Figure 3. Effects of concentration of Ni precursor and reaction temperature on the morphology of the Pt-Ni multiframes. TEM images of chemically etched Pt-Ni multiframes obtained by adding (a) 0.25, (b) 0.5, and (c) 2 equiv. of Ni(acac)2 compared to that used in standard protocol. TEM images of chemically etched Pt-Ni nanocrystals synthesized in different reaction temperatures of (d) 210, (e) 230, and (f) 250 oC.


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Figure 4. Electrochemical properties of the Pt-Ni multiframes/C catalyst. Comparison of (a) ORR polarization curves for the Pt-Ni multiframes/C and the state-of-the-art Pt/C. The inset showing the corresponding cyclic voltammograms. (b) Comparison of mass and specific activities of the catalysts recorded at 0.9 and 0.93 VRHE. (c) ORR polarization curves of the Pt-Ni multiframes/C before and after 5000 and 10000 cycles in an O2-saturated 0.1 M HClO4 solution. The inset showing changes in mass activities of the Pt-Ni multiframes/C before and after potential cycles. (d) Tafel plots of mass activities given as kinetic current densities (jk) normalized against the Pt loading.


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ASSOCIATED CONTENT SUPPORTING INFORMATION This supporting information is available free of charge on the ACS Publications websites via the Internet at http://pubs.acs.org. Figures S1- S18 and tables S1 – S3 give more details on characterization of the synthesized materials and their electrocatalytic performance data. For example, additional TEM, HRTEM, XRD, elemental mapping analysis, EDS, ICP-AES, and electrocatalytic performance data are given.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (S.–I.C. for electrochemical measurements) * E-mail: [email protected] (K.L. for synthesis and characterization) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §

H. Kwon, M. K. Kabiraz, and J. Park contributed equally to this work.

ACKNOWLEDGMENT This

work

was

supported

by

IBS-R023-D1,

NRF-2017R1A2B3005682,

NRF-

2016H1D5A1910726, KBSI project E37300, and Korea University Future Research Grant. The


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authors thank Korea Basic Science Institute (KBSI) for the usage of their HRTEM and ICP-AES instruments.

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Table of contents Hierarchical structure of Pt-Ni multiframe nanocrystals exhibit excellent electrocatalytic activity and durability towards the oxygen reduction reaction.

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Dendrite-embedded Pt-Ni Multiframes as Highly ... - ACS Publications

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