First principles computational study of highly stable ... - Nano Research

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DOI 10.1007/s12274-015-0839-2

1

First principles computational study of highly stable and active ternary PtCuNi nanocatalyst for oxygen reduction reaction

Seung Hyo Noh1, Byungchan Han2 ( ), and Takeo Ohsaka1 ( )

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0839-2 http://www.thenanoresearch.com on June 15, 2015 © Tsinghua University Press 2015

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TABLE OF CONTENTS (TOC)

First Principles

Computational Study of

Highly Stable and Active Ternary PtCuNi Nanocatalyst for Oxygen Reduction Reaction Seung Hyo Noh 1, Byungchan Han 2,*, and Takeo Ohsaka 1,* 1

Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259-G1-5 Nagatsuta, M idori-ku, Yokohama, 226-8502, Japan 2

Department of Chemical and Biomolecular Engineering, Yonsei University , Seoul, 120-749, Republic of Korea Durable and active PtCuNi ternary nanoparticle in segregation and dissolution phenomena

Nano Research DOI (automatically inserted by the publisher) Research Article

First Principles Computational Study of Highly Stable and Active Ternary PtCuNi Nanocatalyst for Oxygen Reduction Reaction Seung Hyo Noh1, Byungchan Han2,* ( ), and Takeo Ohsaka1,* ( )

Received: day month ye ar Revised: day month ye ar

ABSTRACT

Accepted: day month year

nanocatalyst with ternary transition metals for oxygen reduction reaction (ORR)

(automatically inse rte d by the publishe r) © Tsinghua University Press

Using density functional theory (DFT) calculations w e rationally design metallic in fuel cell application. We surround binary core-shell nanoparticles with a Pt skin layer. To overcome surface segregation problem of the core 3-d transition metal we identify a binary alloy Cu0.76Ni0.24, having strongly attractive atomic

and Springer-Ve rlag Be rlin

interactions by computationally screening 158 different alloy configurations

He ide lbe rg 2014

using energy convexhull theory. The Pt skinCu0.76Ni0.24 nanoparticle shows better electrochemical stability than pure Pt nanoparticle at ~3 nm size. We propose

KEYWORDS

that underlying mechanisms of the results are originated from favorable

density functional theory, ternary alloy, nanoparticle, durability, stability, alloy

compressive strain on Pt for ORR catalysis and atomic interactions among the nanoparticle shells for electrochemical stability. Our results will contribute to accurate identification and innovative design of promising nanomaterials for renewable energy systems.

1 Introduction A proton exchange membrane (PEM) fuel cell is a promising power system for electronic and locomotive devices harvesting renewable energy

sources [1, 2]. Performance of PEM fuel cell, however, significantly depends on its catalyst, the key component in the membrane electrode assembly (MEA) materials. Consequently, a rational design of Pt-based catalysts has been of paramount

Address correspondence to Takeo Ohsaka1, [email protected]; Byungchan Han2, [email protected]

Nano Res.

interest to overcome long-standing issues such as high material cost, degradations of catalytic activity toward oxygen reduction reaction (ORR) and electrochemical stability [3-5]. Nano-scale alloy catalysts are attractive due to several benefits compared to the bulk counterparts: reduced Pt loading and enhanced active surface sites for ORR catalysis [6, 7]. Indeed, binary alloy nanocatalysts such as PtCo [8], PtCu [9], and PtNi [10] consist of the archetypal examples. These catalysts form core-shell structures, at which Pt occupies the surface skin layers while cheaper 3-d transition metals reside inside the Pt skin layers. It was suggested by experimental measurements [10] and first principles calculations [11] that electronic structure or the compressive strain in a core-shell structure caused by size difference between the binary alloy components and Pt could be the origin of high ORR activities of Pt binary alloys, 3 ~ 10 times, compared with that of pure bulk Pt [12-15]. Accordingly, these core-shell alloy catalysts substantially relieved the problem of how to improve ORR. On the contrary, electrochemical stability of Pt-based metallic nanocatalysts are still far below the target for long-term system operation. It was reported that Pt nanoparticles electrochemically dissolve into acidic media [16], and the oxidative core metals inside Pt skin layers substantially segregate to surfaces followed by dissolution into acidic media via direct electrochemical reaction or surface oxide formation. Either of the scenarios for structural degradation eventually decreases ORR catalytic activity [17-20]. For instance, electrochemical dissolution potentials of Co, Ni, and Fe metals into their ionic states are respectively -0.28, -0.23, and -0.41 V (vs. NHE) at pH = 0 [21]. In our previous work [22], it was shown that Pt-Co core-shell nanocatalysts of about 1 nm sizes could be unstable under high oxygen coverage. Recent studies on ternary nanoparticles of core-shell structures proposed that Pt/Pd 3M (M: Ir, Co, Cr) catalysts could have substantially high activation barriers for the surface segregations of core elements depending on the metal M [23]. Simliary, Escaño et al. [24] reported that inserting

extra atomic layers with Ru, Rh, Pd, Os, and Ir into the Pt-skinned Pt 3Co nanoparticles could prevent the surface segregation of Co. Indeed, it was shown that PtNiCu and PtCuCo manifested similar behaviors extending a long-term stability and ORR catalytic activity [25, 26]. Even though these ternary materials seem promising to solve the degradation of stability and activity of binary nanocatalysts, the features are still not commercial grades. It may be largely caused by insufficient understanding in the manipulation of catalytic properties on an atomic level. For example, quantitative characterization of mutual interactions between each shell with varying alloying elements may be the key descriptor in rational design of active and durable catalysts. In addition, to idenify thermodynamically stable configurations for the multi-component alloy nanoparticles could be complicated but important step toward advancing our fundamental knowledges on catalysis in general. In this paper, we proposed that PtCuNi ternary nanoparticle is a promising ORR catalyst with high electrochemical durability. Atomic level mechanism of the origin was elucidated using first principles density functional theory (DFT) calculations to guide a rational design of high performance catalysts for PEM fuel cells.

2

DFT Calculations

2.1 Model System Figure 1(a) illustrates our model system with its overall and cross sectional structures. The binary alloy components were simulated with Fe, Ni, Cr, and Cu metallic elements under Pt skin layer as reported by previous literatures [27-30]. Our model was based on an icosahedral nanoparticle composed of only (111) facet with three atomic shells (including one Pt shell) and approximately 1.5 nm size [31]. It was widely observed for Cr, Fe, Ni and Cu nanoparticles [32-36], and known that ORR activity are better than octahedrons [32]. We classified six atomic sites for atomic oxygen adsorption as depicted in Fig.1(b): Bridge1, Bridge2, FCC, HCP1, HCP2, and Top. Figure 1(c) is the schematic picture

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for a surface segregation of an elem ent inside Pt shell. We described stability of our model with varying size and adsorption sites of O in Figs. S1-3 of Electronic

Methfessel-Paxton method was employed as implemented in VASP to calculate density of states [44].

Supplementary Material (ESM) with the relevant previous works [37-40].

3

Results and discussion

3.1 Durability of Binary Alloys (PtCr, PtNi, PtFe, and PtCu)

Figure 1 (a) Illustration of Pt skin nanoparticles. The drawn 1.5 nm nanoparticles consist of Pt skin, 1st layer, 2nd layer, 3rd layer, and core atom. (b) Oxygen adsorption sites on the surface of Pt skin nanoparticles. (c) Illustration of segregation model in both binary and ternary alloys.

2.2 Computational Details Vienna ab-initio simulation package (VASP) [41] was utilized to calculate total ground state energies, and the generalized gradient approximation (GGA) was applied to deal with the exchange-correlation energy [42]. The interaction potentials of the core electrons were substituted by projector augmented wave (PAW) pseudopotentials [43]. Each atom was relaxed to get the optimized nanostructure with a cutoff energy of 520 eV for the plane wave basis. The spin-polarized calculations were employed. A gamma-point mesh of 1  1  1 k-point was used for a unit cell, which were composed of a ternary component nanoparticle with vacuum space of 1 nm around it to hinder interactions with imaginary particles appearing by periodic boundary condition. The global break condition of ionic relaxation loop was 10 -3 eV/unit cell in all calculations.

To narrow down the number of configurations with ternary components we adopted firstly the binary core-shell nanocatalysts for ORR (PtCr, PtCu, PtNi, and PtFe [9, 15, 45]), and then rationally chose the third element to make ternary nanocatalysts. As represented in Table 1 our DFT calculations indicated that HCP1 site depicted at Fig.1 is thermodynamically the most stable site for atomic oxygen adsorptions in all core-shell nanoparticles considered in this paper agreeing with previous report [46]. Oxygen binding energies on PtNi (4.335 eV/atom) and PtCu (4.390 eV/atom) nanoparticles are slightly less than on bulk Pt (4.41 eV/atom) [47] implying better ORR activity than Pt being in a good agreement with previous calculations and experiments [11, 48, 49]. Table 1 Oxygen binding energies (eV/atom) on possible sites of PtM binary alloys Adsorption site PtCr

PtFe

PtNi

PtCu

Bridge1

4.090

3.657

4.268

4.273

Bridge2

3.588

3.343

4.128

4.178

FCC

3.828

3.529

4.115

4.279

HCP1

4.176

3.830

4.335

4.390

HCP2

3.624

3.461

4.047

4.213

TOP

4.119

3.542

4.153

4.158

To investigate whether adsorbed oxygen atoms induce surface segregation of core element or not we defined surface segregation energy, Eseg, as Eq. (1) Eseg  E OPtM seg   E OPtM ini 

where,

E(OPtMini)

and

E(OPtMseg )

(1)

mean

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DFT

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calculated energies before and after a surface segregation of a 3-d transition metal M (i.e., Cr, Fe, Ni, Cu in this paper), respectively. The more negative Eseg, the easier surface segregation. Table 2 Calculated surface segregation energies of a core 3-d transition metal in binary core-shell nanoparticles at O coverage of 0.166 M L and experimentally measured formation enthalpies of bulk metal oxides. Binary NPs (PtM)

Eseg Bulk metal (Cal. meV) oxides

Pt

-

Eform (Exp. kJ/mol)

PtO

-101

PtO 2

-80

pH = 0, and the ion concentration of Pt 2+ = 10 -6 M [16]. E(Pt n-nshell Mm ), E(Ptbulk ), and E(Pt nM m) are all calculated using DFT calculations, meaning total energies for a Pt skin dissolved nanoparticle, bulk Pt, and the binary alloy nanoparticle before the dissolution, respectively. The number of Pt shell atoms was indicated by nshell. Table 3 Dissolution potentials of 1.5 nm sized Pt-skinned binary alloys such as PtNi, PtFe, PtCu, and PtCr in (a) and in (b) dissolution potentials of bulk M (M = Ni, Co, Fe, Cu, and Cr). Bond distances between the nearest neighbor Pt atoms calculated in Pt skin-M catalysts were represented at (C). (a) Dissolution (b) Dissolution (c) Pt-Pt Metal potential of Ptskin-M potential of bulk M distance (M) (V vs. RHE) (Cal.) (V vs. NHE) (Exp.) (nm)

PtNi

-258

Ni2O 3

-490

PtFe

-294

Fe2O 3

-824

Pt

0.75

1.19

0.277

PtCu

126

CuO

-157

Ni

0.90

-0.23

0.266

Cu2O

-169

Fe

1.03

-0.41

0.269

Cr2O 3

-1140

Cu

0.86

0.34

0.268

Cr

1.13

-0.74

0.270

PtCr

-1051

Table 2 represents DFT calculated Eseg and experimentally measured oxide formation enthalpies of M [50, 51]. It is clear that the propensity of the surface segregation is well correlated with the oxide formation energy of M: in the order of PtCr > PtFe > PtNi > PtCu. In addition, Table 2 revealed that Cu is very promising for stable core element due to its high resistance against the surface segregation behavior, even with 0.166 monolayer (ML) of oxygen coverage. Surface segregation energies at higher coverages of O are discussed in section 3.3 . Since it was known that Pt nanoparticle of up to 2 nm lost its stability in acidic media via electrochemical dissolution into Pt 2+ ionic state [16] we calculated dissolution potentials of the core-shell type binary nanoparticles. Electrochemical dissolution potential was defined as the onset potential completely dissolving an outermost Pt skin layer into Pt 2+ ions in acidic aqueous solution at pH = 0 as described in Eq. (2). Ptbulk shell U diss  U diss 





1 E ( Ptn  nshell M m )  nshell E ( Ptbulk )  E ( Pt n M m ) 2nshell e

(2)

Pt shell where, U diss and U diss are dissolution potentials of a Pt skin and bulk Pt into Pt 2+ (aq), respectively. Pt was obtained from Pourbaix’s diagram at 25 ℃, U diss bulk

Our results shown in Table 3 indicated that PtCr nanoparticle has the highest dissolution potential (1.13 V vs RHE). Interestingly, the PtCu shows easier dissolution than PtFe, PtNi, and PtCr nanoparticles. It can be explained by the energy difference between E(Pt n-nshell Mm ) and E(Ptn Mm ) in Eq.(2). Cr and Fe have very strong affinity to oxygen to form oxides leading to large differences between E(Ptn-nshe llM m) and E(Pt nMm ) and thus, high dissolution potentials of the Pt skin shells. On the contrary, since Cu is stabler than Cr and Fe in acidic media, the energy difference between before and after electrochemical dissolution of a Pt skin layer in PtCu is smaller. Consequently, the dissolution potential of PtCu is also lower than those of PtFe, PtCr and PtNi. The high electrochemical dissolution potential of Pt-skin in the PtCr core-shell nanoparticle, however, does not imply good electrochemical stability due to serious surface segregations of Cr under oxidation environment as shown in Table 1. Thus, it is rationalized that PtCu should be a better candidate in spite of slightly lower dissolution potential than PtCr or PtFe core-shell nanoparticles. We added Ni to the PtCu core-shell nanoparticle to make ternary ORR catalyst on the ground that Ni is

bulk

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more resistant to the surface segregation than Fe and Cr (Table 2). Furthermore, our DFT calculations showed that atomic distances of two nearest neighbored Pt atoms in pure Pt, PtCu, and PtNi are 0.277, 0.268, and 0.266 nm, respectively (Table 3). It means that added Ni induces compressive strain on Pt, which is favorable for enhancing the ORR activity as proposed by the electronic structure theory of d-band center energy model [52, 53]. Before calculations of detailed electronic properties we identified thermodynamically stable alloy configuration of PtNiCu nanoparticles as a function of Cu (or Ni) composition.

We applied thermodynamic energy convexhull theory to 1 nm sized PtCu and PtNi nanoparticles with varying alloy compositions as shown in Fig. 2. Energy convexhull of a binary nanoparticle was defined as Eq. (3),

Figure 2 Energy convex hull of (a) PtCu and (b) PtNi nanoparticles. Filled (■) and empty (□) squares mean the stable and unstable configurations, respectively.

Figure 3 Energy convex hull of CuNi nanoparticles. Filled (■) and empty (□) squares mean the stable and unstable configurations, respectively.

3.2 Thermodynamic stability of PtCu and PtNi nanoparticles

3.3 Thermodynamic nanoparticles

Econvex 

1 E ( PtM )  xPt E ( Pt )  (1  xPt ) E (M ) ntot

(3)

where, Econvex is an extra thermodynamic stability by alloying against structural decomposition into pure nanoparticles, while x Pt is composition of Pt, and ntot denotes total number of atoms in the nanoparticle. Using cluster expansion theory [22], we generated 158 different configurations of PtCu and PtNi to identify ground state structures. Figure 2(a) illustrates that Cu thermodynamically prefers to stay inside of a Pt skin layer until its composition becomes 0.236, above which some Cu start to occupy the outmost Pt shell as well. PtNi nanoparticle (Fig. 2(b)) also forms a single Pt skin shell at below Ni composition of 0.236. Since both Ni and Cu form Pt skinned core-shell structures we assumed that the ternary component PtCuNi nanoparticle also prefers Pt skinCuNi configuration as well.

stability

of

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Energy convexhull of 1.0 nm sized NiCu alloy nanoparticle was plotted in Fig. 3. It indicates that there is one stable configuration at the intermediate composition, Cu0.76Ni0.24, which consists of almost same stoichiometry as the L1 2 structure [54]. This compound-type structure with favorable atomic interactions of Ni and Cu will be beneficial for thermodynamic stability of the inner shells of the ternary Pt skinCuNi nanoparticle. Interestingly, the energy convex hull of Cu-Ni shows that the outermost shell is almost occupied with Cu except at very high Cu composition, which can be explained by lower surface energy of Cu (1.825 J/m 2) than Ni (2.450 J/m 2) [55].

the Pt skinCuNi nanoparticle obviously came from the favorable Ni-Cu atomic interaction. As shown at Fig. 3, the Cu0.76Ni0.24 nanoparticle has about 50 meV more thermodynamic stability with respect to pure Cu or Ni. In addition, this specific atomic interaction energy of Ni-Cu also turned out to reduce binding strength with O. For example, our DFT calculations indicated that the bond energies of Cu-O, Ni-O, and CuNi-O were 5.30, 6.02, and 5.11 eV, respectively as shown in Fig 4(c). By increasing the number of adsorbed O on Pt skinCuNi, PtCu and PtNi nanoparticles in a stepwise manner we tried to discover “the onset coverage” inducing surface segregations of Cu to the outmost Pt shells. Figure 5 den otes that O adsorption increases driving force for the surface segregation of core 3-d transition metals (Cu or Ni) almost linearly with its coverage. The onset coverages are, however, very sensitive to the kinds of nanoparticles: 0.1 ML (PtNi), 0.22 ML (PtCu) and 0.34 ML (Pt skinCuNi). Hansen et al. found that 0.5 ML of O could adsorb on bulk Pt at 1.23 V (vs. SHE) and pH = 0 [56]. According to the results 0.33 ML of O will be available at 0.99 ~ 1.10 V implying that Pt skinCuNi nanoparticle should be stable over the entire electrochemical potential window of PEM fuel cell operation [25, 26, 29].

Figure 4 (a) Illustration of Pt skinCuNi nanoparticle model (b) Segregation energy of PtCuNi nanoparticle at 0.166 oxygen concentration, and (c) O xygen binding energy between metal and oxygen (M -O).

We surrounded the surface of the Cu 0.76Ni0.24 nanoparticle with a Pt skin layer to make the ternary Pt skinCuNi nanoparticle model as shown in Fig. 4(a). Our DFT calculations showed that the surface segregation energy of Cu to the Pt skin layer of the Pt skinCuNi nanoparticle is +176 meV at 0.166 ML of O coverage as depicted in Fig. 4(b). Surprisingly, this is +49 meV higher than that in the PtCu core-shell nanoparticle. This extra-stability of Cu in

Figure 5 Segregation energy as a function of surface oxygen concentration. The segregation energies of PtCuNi, PtCu, and PtNi nanoparticles are calculated from 0 to 0.5 M L.

Using DFT calculations we obtained electrochemical

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dissolution potentials (vs. RHE) of a Pt skin layer in pure Pt, Pt skin Cu and Pt skinCuNi nanoparticles as a function of size. Figure 6(a) shows that Pt skin layers in both binary and ternary core-shell nanoparticles have substantially higher electrochemical stabilities than in pure Pt. It is noteworthy that at around 3.0 nm the nanoparticles of Pt skinCu and Pt skinCuNi approach to the electrochemical stability of bulk Pt (~1.01 V), while pure Pt nanoparticles still dissolve at 0.88 V. Furthermore, in the case of Pt skinCuNi, the slope of the dissolution potential versus particle size is steeper than that for a pure Pt meaning that Pt skinCuNi nanoparticle can meet the electrochemical stability targets with much smaller size than pure Pt nanoparticle as well. From the extrapolation of the dissolution potential versus particle size into larger size regimes as shown in Fig. 6(b) we believe that the electrochemical dissolution potentials of Pt skinCu and Pt skinCuNi nanoparticles will be 1.0 V at 2.8 and 3 nm, respectively, whereas pure Pt particle should be larger than the Pt skin alloys to reach the same potential. Consequently, our results propose that Pt skinCuNi nanoparticle with size above 3 nm will have the electrochemical stability to at least the same extent as a bulk Pt catalyst in acidic media of PEM fuel cells.

Figure 6 Dissolution potentials as a function of the size of nanoparticle of Pt, PtCu, and PtCuNi in (a) and in (b) their extrapolations into larger particles. For bulk model calculations, bottom two layers are fixed, and (3 x 3) unit cell was calculated using a gamma-point mesh of 5 x 5 x 1 k-point.

3.4 ORR Activity of PtskinCuNi nanoparticle Experimentally it has been shown that PtCuNi is highly active toward ORR [27, 57, 58]. We adopted d-band center energy model to roughly estimate relative ORR activity of Pt skinCuNi with respect to those of Pt skinCu, Pt skinNi and pure Pt nanoparticles. To utilize d-band center theory [49] we calculated electronic structures of the projected density of state (PDOS) for d-states of the nanoparticles and 2p state of adsorbed O as shown in Fig. 7(a). It illustrates that 2p state of O in the pure Pt nanoparticle system is split into filled bonding and empty antibonding states by interacting with 5d orbital of Pt , while 2p anti-bonding states are partially filled in the Pt skinCu, Pt skinNi, and Pt skinCuNi nanoparticles. Since electron filling in the anti-bonding state decreases bond strength of Pt with O, the Pt skinCu, Pt skinNi, and Pt skinCuNi nanoparticles should have better ORR activities than a pure Pt nanoparticle. Figure 7(b) illustrates equilibrium lattice constants of the nanoparticles of Pt (0.392 nm), Pt skin Cu (0.379 nm), Pt skinNi (0.376 nm) and Pt skinCuNi (0.376 nm) calculated by DFT method. It implies that the Pt skinCuNi nanoparticle has the strong compressive strain on Pt leading to the high ORR activity among the nanoparticles, according to the d-band center energy model.

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These results can help to guide high performance electrocatalysts for PEM fuel cells and other kinds of materials exposed to the similar electrochemical environment.

Acknowledgements This research was supported by Global Frontier Program

through

the Global

Interface

Materials

(GFHIM)

Frontier of

the

Hybrid National

Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT & Future Planning (grant number 2013M3A6B1078882). The New and Renewable Energy R&D Program (20113020030020) under the Ministry of Knowledge Econo my, Republic of Ko rea partially supported this work.

This research was

performed using Tsubame 2.5 at the Global Scientific Information and Computing Center of the Tokyo Institute of Technology as a R esearch Project of HPCI systems (Project ID: hp140038). Author, Seunghyo Noh, also appreciates the Government of Japan for MEXT scholarship.

Figure 7 (a) Projected density of state of catalysts at 1.5 nm size. Vertex atoms at which oxygen is adsorbed were investigated. O xygen 2p state and Pt 5d state are colored with yellow and brown, respectively. (b) Lattice constants of binary and ternary catalysts are shown with the ball and stick models of catalysts.

4

Electronic Supplementary Material: Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher).

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Extensively utilizing DFT calculations we have showed that Pt skinCuNi nanoparticle could be a very promising ternary catalyst for ORR with high electrochemical stability in acidic media. Rationally bottom-upping binary core-shell nanoparticles with a Pt skin layer enabled us to design the novel ternary Pt skinCuNi nanoparticle. Our results indicate that the serious surface segregation issue of 3d-transition metals could be substantially relieved by alloying with another element forming a compound, such as Cu-Ni. We show that Pt skinCuNi nanoparticle with particle size around 3 nm can reach (or even better) stability of bulk Pt. The favorable compressive strain and atomic interactions among nanoparticle shells are proposed as the underlying mechanism.

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