Shell Nanostructures as Advanced

Oct 9, 2014 - The fine balance between activity and durability is crucial for the development of high performance electrocatalysts. The importance of ...
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Letter pubs.acs.org/NanoLett

Multimetallic Core/Interlayer/Shell Nanostructures as Advanced Electrocatalysts Yijin Kang,† Joshua Snyder,† Miaofang Chi,‡ Dongguo Li,† Karren L. More,‡ Nenad M. Markovic,† and Vojislav R. Stamenkovic*,† †

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

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S Supporting Information *

ABSTRACT: The fine balance between activity and durability is crucial for the development of high performance electrocatalysts. The importance of atomic structure and compositional gradients is a guiding principle in exploiting the knowledge from well-defined materials in the design of novel class of core−shell electrocatalysts comprising Ni core, Au interlayer, and PtNi shell (Ni@Au@PtNi). This multimetallic system is found to have the optimal balance of activity and durability due to the synergy between the stabilizing effect of subsurface Au and modified electronic structure of surface Pt through interaction with subsurface Ni atoms. The electrocatalysts with Ni@Au@PtNi core-interlayer-shell structure exhibit high intrinsic and mass activities as well as superior durability for the oxygen reduction reaction with less than 10% activity loss after 10 000 potential cycles between 0.6 and 1.1 V vs the reversible hydrogen electrode. KEYWORDS: Core−shell, oxygen reduction reaction, electrocatalysis, nanoparticle, durability, activity

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The low pH and high oxygen content of the PEMFC cathode produce a highly corrosive environment with Pt−oxides readily forming at operational potentials, initiating at the high density of low-coordinated sites present on the surface of nanoparticulate-based catalysts. Surface oxides can readily move into the subsurface atomic layers through the place exchange mechanism43 where it can act to pull out the underlying transition metal and also promote dissolution of the surface Pt atoms. This leads to catalyst deactivation through both the loss of the favorable electronic effect induced by the dissolution of transition metal and the loss of electrochemically active surface area (ECSA) through the Ostwald ripening mechanism. It is expected then that durability of Pt alloy-based catalysts, for both extended surfaces and nanoscale materials, is best addressed by developing strategies to reduce or eliminate the place-exchange mechanism by changing the degree of both surface and subsurface oxidation potential. Zhang et al. demonstrated improved durability by adding Au clusters onto the surface of carbon-supported Pt nanoparticles where the ORR activity and ECSA remained nearly unchanged after 30 000 potential cycles.47 The observed stability was attributed to an increased nobility of the surface Pt atoms and increased

indering the global commercialization of polymer electrolyte membrane fuel cells (PEMFC) is the limitations of the current technology to provide long operational lifetimes at a minimized cost. These issues can be tied, to a large degree, to the sluggish kinetics of the cathodic oxygen reduction reaction (ORR) where significant quantities of precious metal-based catalyst are required to produce desired power.1−4 To date, substantial advancements have been made in the area of nanoscale catalyst design where strict control of shape and composition,5−29 guided by the study of well-defined single crystals and thin films,3,30−35 has yielded dramatic enhancements in ORR activity.3,13,15,30−46 Pt transition metal (Ni, Co, Fe, Cu, etc.) alloys demonstrate superior activity to pure Pt as a result of changes in the electronic structure of surface Pt atoms induced by neighboring transition metal atoms, optimizing the interaction strength with oxygenated intermediates during the ORR.31−34,41 The most active members of this subset are Pt(111)−skin extended surfaces that maintain a significant portion of the transition metal in the underlying atomic layers, which have been replicated at the nanoscale through synthesis with strict control of size, shape, composition, and compositional gradient.33,42,44 These binary Pt transition metal alloys mainly address the activity requirements; however, for the ideal cathode catalyst durability, issues are often omitted and more research focus is needed in order to provide valuable solutions. © XXXX American Chemical Society

Received: July 23, 2014 Revised: September 23, 2014

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Figure 1. TEM images and cartoons (gray, Pt; red, Ni; and yellow, Au) illustrating the structures and compositional profiles of different nanoparticles: (a) 5.0 nm (σ < 5%) alloy PtNi, and (b,c) 5.1 nm (σ = 8%) and 5.4 nm (σ = 14%) PtNi−Au at the surface Au coverage of approximately (c) 10% and (d) 50%, respectively, and (d) 5.8 nm (σ = 9%) core−shell Au@PtNi; (e) EDX data showing the Pt peak (Pt-Lα line) and Au peak (Au-Lα line) of the nanoparticles corresponding to the structures shown in panels a−d; (f) the corresponding ORR polarization curves showing that surface Au deactivates the ORR activity of PtNi; inset of panel f, the CO-stripping curves (CV curves subtracted) for estimating ECSA and Au-coverage. ORR curves are taken in O2-saturated 0.1 M HClO4, on rotating disc electrode at a rotating speed of 1600 rpm and at a scan rate of 20 mV/s. Scale bars (a−d): 20 nm.

Figure 2. (a,b) TEM images of as-synthesized, (c,d) STEM images, and (e) EDX line-scan of electrolyte exposed to 5.0 nm (σ = 6%) Ni@Au@PtNi nanoparticles, which possess the optimized ORR electrocatalytic performance. (a) TEM image and inset fast Fourier transform (FFT) pattern showing a long-range ordered superlattice of Ni@Au@PtNi nanoparticles due to the high monodispersity. (c) Bright-field and (d) dark-field STEM images as well as (e) EDX data show the core−shell structure. In addition, the EDX line-profile shows a Pt-rich skeleton structure over the Au-coated Ni core. Scale bars: (a) 0.2 μm, (b) 20 nm, and (c,d) 2 nm.

core was coated by a multilayered Pt alloy shell with a goal of maintaining the high activity of Pt alloy catalysts with the improved durability.43 On the basis of surface energies, it is expected that Au tends to segregate over Pt; however, under oxidizing conditions relevant for the ORR, it was found that Au remained near the surface in the underlying atomic layers. The

oxidation potential, induced by the presence of neighboring Au atoms, which adjust the Pt electronic structure, lowering the dband center.47 However, Au on the surface may act to block active Pt sites, negatively impacting the activity, especially at high current densities. Our previous work addressed this issue by developing a core−shell structured catalyst where an Au B

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Figure 3. Correlation between durability (retained activity after 10 000 electrochemical cycles between 0.6 and 1.1 V) and the Au content in the subsurface of nanoparticles. The activity of corresponding Pt−Au−Ni nanostructures are presented as the kinetic current densities measured at both 0.95 V (black square) and 0.9 V (red circle). Blue star, cyan triangle, and magenta triangle represent the retained activity (measured at 0.95 V) of PtNi/C with a multilayered Pt−skin,42 Pt/C, and PtNi/C with Pt−skeleton surface morphology, respectively. The activity after cycles drops significantly if the Au content is below 5% because Ni atoms are leached out through the area that has no Au atoms.

(Figure 1d). Elucidation of the true effect of the presence of Au requires strict control of deposition parameters during the synthesis and size/composition distribution in nanoparticles. Precise size-selective precipitation is used to prevent broadening of the size distribution after multistep seeded growth of the monodisperse nanoparticles.12 The Au@PtNi core−shell nanoparticles are synthesized by overgrowing a ∼1 nm thick PtNi alloy shell on a near-spherical Au core with a diameter of 5.1 nm (σ = 9%; Figure S1, Supporting Information), while the PtNi−Au nanoparticles are prepared by depositing Au atoms over 5.0 nm (σ < 5%) PtNi alloy nanoparticles (Figure S2, Supporting Information). By tuning the concentration of Au precursor and deposition reaction time, we effectively control the surface coverage of Au atoms to be 10% and 50% (evaluated by CO-stripping as shown in Figure S3, Supporting Information, and corresponding to ∼5% and 25% overall Au content, Figure 1e). The ORR activity trend associated with Au placement is shown in Figure 1f, where the Au@PtNi core− shell nanoparticles match the specific activity of PtNi alloy nanoparticles; however, with enhanced stability and lower Pt mass as Pt is only present in the ∼1 nm thick shells. In contrast to the observations made by Zhang et al. for Pt/C electrocatalysts,47 in the Pt−Au−Ni system we find that Au present on the surface results in lower specific activity where 10% surface coverage of Au leads to a decrease in half-wave by 40 mV, while 50% surface coverage leads to a loss of over 100 mV in the half-wave potential and a significant decrease in the

Pt-based mass activity of such core−shell catalysts was more than four times that of commercial Pt/C catalysts with negligible loss in ECSA, specific activity and change in morphology after 60 000 potential cycles to 1.1 V versus the reversible hydrogen electrode (RHE).43 Similarly, Pt monolayer on PdAu nanoparticles also exhibited great durability toward the ORR; however, durability enhancement was attributed to the surface Au atoms that occupy defect sites in the shell.48 These results suggest that, regardless of their location on the top or beneath the surface, Au atoms can effectively enhance the durability of Pt-based electrocatalysts by preventing oxidation and dissolution of Pt atoms. Here we report a systematic study related to composition and its gradients for maximized intrinsic activity and precious metals utilization in multimetallic electrocatalysts for the ORR. In Pt alloy nanoparticle electrocatalysts, Au can have both catalytic and durability roles owing to its high redox potential. Its effectiveness for both is greatly affected by the nanoparticle structure as well as compositional profile, and therefore, proper electrocatalyst design requires a fundamental understanding of the structure−function relationship, specifically how Au placement can affect the catalytic properties of Pt alloys. In this study we consider two different cases, one in which Au is deposited on PtNi nanoparticles (Figure 1a) to form nanostructures shown in Figure 1b,c, which we will denote as PtNi−Au, and the other one in which Au is located beneath the surface, within the “core”, which we will denote as Au@PtNi C

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less active Pt surface.42,43 All of the above points toward a unique multilayered compositional profile where the core is entirely composed of an inexpensive transition metal, while the interface between the core and Pt alloy shell contains an optimal amount of Au to enhance durability and limit the total mass of precious metals within the catalyst. We term these nanoparticles with such distinct compositional profile as core/ interlayer/shell nanoparticles. Through a controlled layer-by-layer growth during chemical synthesis of multimetallic particles we add Au to the interface between particle core and shell. In brief, 3.1 nm (σ < 5%) Ni nanoparticles are synthesized and used as core material (Figure S4, Supporting Information) to deposit Au in a desired quantity, forming an intermediate Ni@Au core/shell nanostructure. The final step in synthesis is deposition of a PtNi shell with the thickness of approximately 1 nm (∼6 MLs), which forms the desirable nanosegregated Ni@Au@PtNi nanoparticles with a diameter of 5.0 nm (σ = 6%), as shown in Figure 2. Size-selective precipitation is used after each step to ensure the high monodispersity, which is evidenced by the large area superlattices (Figures 2a and S5, Supporting Information). Before proceeding with further structural characterizations and electrochemical evaluation, Ni@Au@PtNi nanoparticles were deposited on high surface area carbon support and surfactants were removed by previously established protocol. The scanning transmission electron microscopy (STEM) images (Figure 2c,d) show that after exposure to acidic electrolyte the core− shell structure and the Ni core are preserved, while the linescan of energy-dispersive X-ray spectroscopy (EDX) confirms the formation of the Pt-rich topmost layers with Pt−skeletontype of surface due to dissolution of Ni (Figure 2e).42 Typically, annealing of the acid-leached nanoparticles is used to transform the surface from a rough Pt−skeleton to the ideal Pt−skin;42 however, this would inevitably induce diffusion of Au atoms to the surface because of their lower surface energy with respect to Pt (1.506 J·m−2 Au vs 2.489 J·m−2 Pt).49 In order to demonstrate not only that this fundamental principle is taking place at nanoscale, but also our ability to delineate once more Au-surface vs Au-subsurface case, we performed elemental analysis after annealing. On that basis, it was confirmed that Au has selectively diffused to the surface (Figures S6 and S7, Supporting Information) resulting in a significant decrease in activity (Figure S6, Supporting Information), similar to that seen when Au is intentionally placed on the surface of PtNi nanoparticles, indicating that the optimal Pt−skin structure for Pt−Au−Ni electrocatalysts is not achievable by our wellestablished annealing protocol. In our previous study, the durability enhancement in Au/ FePt ternary core/shell electrocatalyst was ascribed to the hindered place-exchange mechanism.43 Density functional theory (DFT) calculations43 indicated that Au concentration equivalent to 0.25 ML in the underlying atomic layer of a Pt surface reduces the adsorption strength of subsurface oxygen at all potentials relevant for the ORR. The presence of subsurface oxygen through the place-exchange mechanism is responsible for leaching out of transition metals from the subsurface as well as being the dominant mechanism by which Pt is dissolved. Therefore, placement of Au atoms with higher redox potential in the near-surface region lowers the probability for both transition metal (Fe, Co, and/or Ni) and Pt dissolution. Here we demonstrate that the amount of interlayered Au that surrounds Ni core can be successfully controlled from a complete monolayer down to about 0.25 ML (Figure 3).

Figure 4. (a) CV curves and (b) ORR polarization curves of the optimized Ni@Au@PtNi/C electrocatalysts before and after 10 000 cycles between 0.6 and 1.1 V, demonstrating the high durability. CV curves are taken in Ar-saturated 0.1 M HClO4, at a scan rate of 50 mV/s. The ORR curves are taken in O2-saturated 0.1 M HClO4, on rotating disc electrode at a rotating speed of 1600 rpm and at a scan rate of 20 mV/s.

diffusion limited current, suggesting a partial shift from 4electron toward 2-electron oxygen reduction, which is the mechanism by which oxygen is reduced on Au, forming H2O2 rather than H2O. Even though surface Au atoms are effectively blocking active Pt sites resulting in decreased activity, the PtNi−Au nanoparticles are also found to have enhanced durability in comparison to the Au-free PtNi alloy catalysts. The same deactivation is observed on extended surfaces, (Figure S3, Supporting Information), where a Au surface coverage as low as 0.17 monolayer (ML) on a Pt3Ni thin film is found to substantially deactivate ORR. From these results it is clear that the beneficial effects of Au are more effectively utilized in Pt− Au−Ni ternary system when Au is located beneath the surface, and not directly exposed to the reactive environment. In principle, core−shell transition metal@Pt catalysts can minimize the mass of buried, electrocatalytically inactive Pt; however, if the core itself is high in precious metal content, such as Au, it fails to effectively address the cost per kilowatt in PEMFCs. While Au core−Pt-based shell particles have demonstrated adequate activity and superior durability,43,48 their activity per precious metal mass is still low because of the high Au content within the core. Nevertheless, when the composition of the core is deficient in precious metals and composed mostly of transition metals that have low redox potentials, thin layers of Pt (e.g., Pt monolayers) prove to be insufficient to protect the core from dissolution. Increasing the thickness of the pure Pt shell can improve the durability of the core−shell particles, but Pt shell thicknesses beyond a few atomic layers lose the beneficial electronic effect from subsurface transition metals such as Ni and Co and leads to D

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Figure 5. Bar charts showing (a) electrochemically active surface areas, (b) specific activities, and (c) mass activities of Pt/C, PtNi/C (with Pt− skeleton), and Ni@Au@PtNi/C (with quasi Pt−skin) electrocatalysts before and after 10 000 cycles of electrochemical cycling up to 1.1 V. The activity is normalized kinetic current density measured at 0.95 V. The specific activity is the activity normalized to the ECSA measured by COstripping. (d) TEM images of Pt, PtNi, and Ni@Au@PtNi nanoparticles before and after 10 000 cycles up to 1.1 V. All the images are at a size of 100 nm × 100 nm.

durability, indicating that the threshold amount of Au required to sufficiently distribute among the subsurface regions and suppress the presence of subsurface oxygen is 5 at % for nanoparticles of approximately 5 nm diameter. The ORR activity of Ni@Au@PtNi/C electrocatalysts is tested and compared with commercial Pt/C and PtNi/C electrocatalysts. As shown in Figure 4, the Ni@Au@PtNi/C electrocatalysts possess superior durability toward the ORR without noticeable loss in either ECSA or specific activity. Surprisingly, the ratio of ECSA determined by CO-stripping to that under potential deposited hydrogen (Hupd) on Ni@Au@ PtNi is 1.40 (Table S2, Supporting Information), close to the value that is regarded as a characteristic signature of the Pt− skin structure, which is a reflection of the change in hydrogen adsorption strength induced by the underlying transition metal.42,50 The apparent change in hydrogen binding strength should be ascribed to the Au presence that induces formation of the “quasi-Pt−skin”. Subsurface Au has a tendency to segregate over Pt but never gets to the topmost surface under relevant ORR conditions due to strong interaction between Pt surface atoms and oxygenated species. This facilitates an electrochemically induced annealing effect and formation of the Pt−skin-like

Detailed electrochemical evaluation revealed that after 10 000 potential cycles in 0.1 M HClO4 between 0.6 and 1.1 V vs RHE, there is a negligible loss in activity for all of the nanoparticles containing some quantity of subsurface Au. As mentioned above, we varied the amount of Au from a solid Au core where Au makes up 47 at % (determined by EDX) of the nanoparticle, to an equivalent of ∼1 ML over the Ni core where Au is 28 at % down to a Au content of 5 at % , which is about to 0.25 ML on the Ni core (coverage is estimated based on an spherical model for the core−shell particles). Even for the lowest Au content, 5 at %, 95% and 92% of the activity at 0.95 and 0.90 V vs RHE, respectively, are retained (Figure 3), and there is minimal change to the morphology of the carbon supported nanoparticles. Analysis of the electrolyte after the durability testing with inductively coupled plasma mass spectrometry (ICP-MS) shows a minimal dissolution of both Pt and Ni for the Ni@Au@PtNi nanoparticles; see Table S1, Supporting Information. In contrast, both Pt/C and PtNi/C with Pt−skeleton surfaces retain less than 70% of their initial activity (0.95 V).42 However, it is important to emphasize that Au content below 5 at % at the interface between the Ni core and PtNi shell fails to provide a substantial enhancement in E

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Efficiency and Renewable Energy, Fuel Cell Technologies. Part of the microscopy was performed at the Center for Nanophase Materials Sciences, which is sponsored at ORNL by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

surface structure. Not only is the ratio from ECSA affected but also high specific activity that is exclusively reserved for electrocatalysts with a Pt-Skin surface structure is observed (Figure 5,a−c). The specific and mass activity enhancement of Ni@Au@PtNi/C is over 8-fold versus the Pt/C and outperforms multilayered Pt−skin PtNi/C42 due to the replacement of expensive Pt and/or Au with Ni in the core. Figure 5d exhibits transmission electron microscopy (TEM) images of Pt/C, PtNi−skeleton/C, and Ni@Au@PtNi/C nanoparticles before and after 10 000 cycles. After potential cycling, the size of Pt and PtNi nanoparticles are significantly changed: big particles (>10 nm) and small particles (