Scalable Nanoporous (Pt1–xNix)3Al Intermetallic Compounds as

Nov 11, 2016 - Scalable Nanoporous (Pt1–xNix)3Al Intermetallic Compounds as Highly Active and Stable Catalysts for Oxygen Electroreduction. Gao-Feng...
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Scalable Nanoporous (Pt1−xNix)3Al Intermetallic Compounds as Highly Active and Stable Catalysts for Oxygen Electroreduction Gao-Feng Han,† Lin Gu,‡ Xing-You Lang,*,† Bei-Bei Xiao,† Zhen-Zhong Yang,‡ Zi Wen,† and Qing Jiang*,† †

Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, and School of Materials Science and Engineering, Jilin University, Changchun 130022, China ‡ Beijing National Laboratory for Condensed Matter Physics, The Institute of Physics, Chinese Academy of Science, Beijing 100190, China S Supporting Information *

ABSTRACT: Author: Bimetallic platinum−nickel (Pt−Ni) alloys as oxygen reduction reaction (ORR) electrocatalysts show genuine potential to boost widespread use of low-temperature fuel cells in vehicles by virtue of their high catalytic activity. However, their practical implementation encounters primary challenges in structural and catalytic durability caused by the low formation heat of Pt−Ni alloys. Here, we report nanoporous (NP) (Pt1−xNix)3Al intermetallic nanoparticles as oxygen electroreduction catalyst NP (Pt1−xNix)3Al, which circumvents this problem by making use of the extraordinarily negative formation heats of Pt−Al and Ni−Al bonds. The NP (Pt1−xNix)3Al nanocatalyst, which is mass-produced by alloying/dealloying and mechanical crushing technologies, exhibits specific activity of 3.6 mA cm−2Pt and mass activity of 2.4 A mg−1Pt at 0.90 V as a result of both ligand and compressive strain effects, while strong Ni−Al and Pt−Al bonds ensure their exceptional durability by alleviating evolution of Pt, Ni, and Al components and dissolutions of Ni and Al atoms. KEYWORDS: platinum alloys, intermetallic compounds, oxygen reduction reaction, electrocatalysis, nanoporous metals, dealloying, fuel cells

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Ni,17−31 Cu,32−36 Y,9,37 and Zr9 with improved ORR activity beyond that of state-of-the-art Pt/C for significantly reducing the usage of scarce Pt metal. These Pt-TM ORR nanocatalysts generally form a structure with a pure Pt overlayer on a Pt-TM core (Pt-TM/Pt) when the less-noble TMs are exposed to acid solution and spontaneously dissolve from the surface.3,5,6,38 The TM solute atoms at the subsurface more or less adjust the HO* adsorption energy of surface Pt atoms by altering the dband center (Ed) through the Pt-5d and TM-3d or −4d electron overlapping and/or the compressive/tensile strain effects,39,40 i.e., Ed ∝ −Vddm1/2 = −[ηddmℏ2(rdPtrdTM)3/2/(mDPt‑TM5)]1/2. Here, Vddm is the matrix element between Pt and TM atoms, rdPt and rdTM denote the spatial extents of their d orbitals, ηddm

INTRODUCTION Low-temperature proton exchange membrane (PEM) fuel cells are energy conversion devices that can deliver high-density energy with zero emission for many applications, such as portable electronic devices and transportation vehicles.1−3 They produce electricity via hydrogen oxidation and oxygen reduction on the anode and cathode, respectively, in which platinum catalysts, typically carbon-supported Pt nanoparticles (Pt/C NPs), are generally used to lower the overpotential for high-voltage output.3−6 However, the widespread commercialization of PEM fuel cells is essentially impeded by the highmass loadings of Pt required to catalyze the sluggish oxygen reduction reaction (ORR) because of strong adsorption of hydroxyl intermediate (HO*) on Pt/C nanocatalysts.7 In this regard, there has been considerable interest in exploring nanostructured bimetallic alloys of platinum and less-noble transition metals (Pt-TMs)3,5,6,8 such as Ti,9 Fe,10−13 Co,14−16 © XXXX American Chemical Society

Received: October 1, 2016 Accepted: November 11, 2016 Published: November 11, 2016 A

DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

respectively.44 A low critical potential of the Pt-TM alloys directly results in the dissolution of TM atoms. Although this dissolution process could be alleviated by the instant formation of pure Pt shell on Pt-TM alloys, a low formation energy cannot restrict the TM atoms from continuously diffusing from the interior to the surface and then dissolving in acid media. Suppression of the diffusibility of interior solution atoms, in particular less-noble TM atoms, is expected to be one of key strategies to improve the stability of Pt-TM nanocatalysts. Considering that the diffusion activation barrier of atoms at an applied potential (ϕ) is proportional to the sum of bondinteraction energies (Eb) to all neighboring atoms (n) according to a bond-breaking model, i.e., D ∝ exp[−(nEb + ϕ)/kBT] ∝ exp(−ΔU/kBT), the alloy formation energy (ΔU) is the pivotal parameter to judge the stability of alloys.41,45 Here, D is the diffusion coefficient; kB is the Boltzmann constant, and T is the absolute temperature. For Pt−Ni alloys, the ΔU = −0.1 eV/atom is too low to inhibit the interdiffusion of both Pt and Ni atoms, which eventually leads to the dissolution of Ni atoms and the structure evolution after exposure to a HClO4 solution (Figure 1a). While the

is a dimensionless constant, m is the mass of an electron, and DPt‑TM is the interatomic distance of Pt-TM.39,40 As a result of their HO* adsorption energy close to the optimal value, which is weaker than that of Pt(111) by ∼0.1 eV,6,37 nanostructured Pt−Ni catalysts have specific activity locating at the peak of the volcano-shape behavior for Pt-TM/Pt alloys,6,38,41 representing one class of the most important ORR electrocatalysts.17−31 However, they usually suffer from structure degradation due to the continuous dissolution of Ni atoms under detrimentally corrosive ORR conditions, although the ORR activity of Pt−Ni nanocrystals has been remarkably improved by precisely controlling their size, shape, and composition.17−31 Even under the protection of a pure Pt overlayer, it cannot be effectively alleviated due to the low formation energy of Pt−Ni alloys (−0.1 eV/atom),42 which is insufficient to restrict the interdiffusion between surface Pt atoms and subsurface Ni atoms, especially at the operation temperature of PEM fuel cells.17,19,43 This eventually results in the rapid performance losses of ORR catalysis of Pt−Ni nanostructures in the cycling. Moreover, their practical application in PEM fuel cells confronts one more challenge, which is the scalable synthesis in cost-effective and environmentally friendly pathways owing to the high affinity of the less-noble Ni constituent to oxygen.3,5,8 Whereas tiny Pt−Ni nanocrystals can be synthesized in bottom-up methods, most of them generally involve bulky capping agents (such as surfactants, polymers, and ionic or fatty ligands) or even resort to nonaqueous surroundings.17,18,22,23,28,43 Such strategies not only undergo numerous problems in high-volume production with low yield and quality but also introduce the capping agents to prevent O2 from freely accessing the surface of nanocrystals, giving rise to the undesirable decrease in ORR activity.17,21,23 Thus, developing Pt−Ni-based nanocatalysts with high catalytic activity and durability by a facile and scalable synthesis approach remains an important challenge. Here, we report a novel class of nanoporous ternary (Pt1−xNix)3Al (x = 0.167) intermetallic nanoparticles which are mass-produced by a facile top-down method for highperformance electrocatalysis toward ORR. Therein, Al is introduced to improve the structure stability of Pt−Ni alloys by utilizing the strongly bonded natures of Pt−Al (−0.74 eV/ atom) and Ni−Al (−0.42 eV/atom) while the dissolution of excessive Al facilitates the formation of bimodal nanoporous structure. As a consequence of the ligand and compressive strain effects exerted on the surface Pt atoms by the intermetallic subsurface, the NP (Pt1−xNix)3Al/C nanocatalyst is demonstrated to exhibit specific and mass activities of up to 3.6 mA cm−2Pt and 2.4 A mg−1Pt at 0.90 V versus a reversible hydrogen electrode (RHE), much higher than those of NP Pt60Ni40/C (1.9 mA cm−2Pt and 0.68 A mg−1Pt) and Pt/C (0.27 mA cm−2Pt and 0.14 A mg−1Pt). At the same time, the strong Pt−Al and Ni−Al bonds significantly suppress the evolution of Pt, Ni, and Al atoms, which protects the inside Al and Ni atoms against the further dissolution, enlisting the NP (Pt1−xNix)3Al/ C nanocatalysts to exhibit high catalytic durability. These extraordinary performances make it an attractive ORR nanocatalyst for the next-generation PEM fuel cells.

Figure 1. Schematic illustration of the atomic structure evolution before and after longer electrochemical cycles. (a) PtNi solid solution and (b) (Pt1−xNix)3Al intermetallics.

amphoteric Al is introduced into Pt−Ni alloys, there forms an intermetallic (Pt1−xNix)3Al compound with exceptionally strong Pt−Al (−0.74 eV/atom) and Ni−Al (−0.42 eV/atom) bonds. This is anticipated to protect interior Ni and Al atoms against dissolving during the long-term electrochemical cycling by depressing their diffusibilities to the surface (Figure 1b).41 Synthesis and Structure Characterization. The massproduction of NP (Pt1−xNix)3Al nanoparticles is carried out in a top-down procedure which involves alloying/dealloying and mechanical crushing technologies, as schematically illustrated in Figure S1. This protocol can be generalized for designing and mass-producing other multimetallic systems composed of lessnoble TMs or rare-earth constituents. Typically, an ingot of Pt10Ni2Al88 (atom %) alloy is first made by co-melting Pt, Ni, and Al metals, wherein Pt and Ni atoms and partial Al atoms engender the (Pt1−xNix)8Al21 (x = 0.167) intermetallic compound, and excessive Al metal exists in α-phase (Figure S2). Precursor Pt10Ni2Al88 ribbons with cross sections of ∼1 mm × ∼20 μm are produced by a melt-spinning method and then chemically etched in 1 M NaOH.45−47 While the α-Al is rapidly dissolved from the composite,45,48 the less-noble Al component in the as-etched (Pt1−xNix)8Al21 alloy is further slowly and selectively leached for the formation of more stable

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RESULTS AND DISCUSSION Theoretical Analysis on Alloy Stability. The degradation of Pt-TM alloys is caused by both the dissolution of TM atoms and the evolution of Pt or TM atoms, which are determined by the critical potentials and the alloy formation energy, B

DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Microstructure and chemical characterizations. (a) Low magnification HAADF-STEM image of mechanically crushed NP (Pt1−xNix)3Al nanoparticles. (b) Atomic-resolution HAADF-STEM image of (Pt1−xNix)3Al nanoparticles. (c) Line profiles for the columns of atoms in the boxed areas marked with layers 1−5 in panel b, which correspond to atomic layers 1−5 from the outer to inner. The Al atoms could be recognized by the signal which is weaker than their two neighboring atoms. (d) HAADF-STEM image of cross-sectional (Pt1−xNix)3Al ligaments. Inset: the fast Fourier transform pattern of the selected square region by the red dashed line. (e) STEM-EDS mapping shows uniform distribution of Al atoms in Pt−Ni matrix with a slightly low concentration at the rims of the skeletons. (f) Typical XPS survey spectrum for the as-dealloyed NP (Pt1−xNix)3Al. (g) XRD patterns of NP Pt3Al and NP (Pt1−xNix)3Al intermetallic compounds. The line pattern shows card 65-3255 for Pt3Al according to JCPDS. (h) Schematic structure of NP (Pt1−xNix)3Al units, wherein Ni atoms randomly and partly replace Pt atoms in ordered FCC L12 Pt3Al. Light gray, blue, and red spheres present Pt, Al, and Ni atoms, respectively.

(Pt1−xNix)3Al (x = 0.167) intermetallic compound. A typical field-emission scanning electron microscope (FESEM) image of the dealloyed Pt−Ni−Al ribbons displays a bimodal nanoporosity consisting of nanopore channels with pore sizes of ∼40 nm and ∼3 nm (Figure S3), which are generated in the consecutive dissolution of Al from α-phase region and the (Pt1−xNix)8Al21 intermetallic compound, respectively. When mechanically crushed, the (Pt1−xNix)3Al ligaments associated with large pore channels rupture at their intersection and become NP nanoparticles with characteristic length of ∼3 nm, as shown in the high-angle annular-dark-field scanning transmission electron microscope (HAADF-STEM) image (Figure 2a). As a result of chemical dealloying, the Al atoms distribute only in the subsurface layers, while the Pt and Ni atoms locate on the surface, wherein partial atoms in the Pt columns show faint contrast probably because they are substituted by Ni atoms (Figures 2b and c). In the unique planes of {010} and {110}, the bright Pt columns and dark Al

columns arrange alternately in a super period, demonstrating the L12 ordered intermetallic structure of (Pt1−xNix)Al (Figure 2d). STEM energy-dispersive spectroscopy (STEM-EDS) mapping further illustrates the slightly lower concentration of Al atoms at the rims of the skeleton compared with the uniformly distributed Al atoms in Pt−Ni matrix (Figure 2e). These indicate that, in each NP (Pt1−xNix)3Al nanoparticle, the interconnected ligaments mainly comprise the Pt−Ni alloy surface and ∼3 nm intermetallic (Pt1−xNix)3Al subsurface. The presence of Pt, Ni, and Al elements was confirmed by an X-ray photoelectron spectroscopy (XPS) survey (Figure 2f), and their overall atomic ratio of 62:13:25 was determined by the EDS spectrum (Figure S4) and inductively coupled plasma mass spectrometry (ICP-MS). Figure 2g displays an X-ray diffraction pattern of NP (Pt1−xNix)3Al nanoparticles with the characteristic diffraction peaks that correspond to the feature planes of an FCC L12 structure (JCPDS 65-3255). These are in accordance with the superordered structure of the Pt3Al C

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Figure 3. Electrochemical measurements. (a) CV curves of NP (Pt1−xNix)3Al/C and Pt60Ni40/C as well as commercially available Pt/C catalysts recorded in N2-purged 0.10 M HClO4 at a scan rate of 50 mV s−1. (b) ORR polarization curves on NP (Pt1−xNix)3Al/C, NP Pt60Ni40/C, and Pt/C in O2-saturated 0.10 M HClO4 at room temperature with a rotation rate of 1600 rpm and a scan rate of 10 mV s−1. (c) Kinetic current densities of NP (Pt1−xNix)3Al/C, NP Pt60Ni40/C, and Pt/C as a function of applied potential (Tafel plots). (d) Specific activities of NP Pt−Al and Pt−Ni series alloys as a function of their Pt content (atom %). Error bars represent standard deviations of at least four independent measurements.

intermetallic compound but with a slight shift of peak positions to a higher angle.45 This indicates that Al atoms strongly interact with both Pt and Ni atoms to form a thermodynamically stable (Pt1−xNix)3Al intermetallic compound with an ordered Pt3Al structure because of the negative formation heats of Pt−Al (−0.74 eV/atom) and Ni−Al (−0.42 eV/ atom),42,49,50 as schematically illustrated in Figure 2h. Owing to the electron transfer in the following order of Pt−Pt < Pt− Ni < Pt−Al, the Pt 4f XPS spectrum of NP (Pt1−xNix)3Al nanoparticles exhibits a shift less negative than that of the NP Pt3Al ones relative to Pt/C (Figure S5), further confirming the formation of (Pt1−xNix)3Al intermetallic compound. Electrochemical Characterizations. The catalyst ink is prepared by mixing NP (Pt1−xNix)3Al nanoparticles with carbon black (NP (Pt1−xNix)3Al/C), which was then loaded on a glassy carbon electrode for electrochemical measurements. After pretreatment by potential cycling between 0.05 and 1.0 V (vs RHE), the electrocatalytic properties of NP (Pt1−xNix)3Al/ C were investigated for comparison with NP Pt60Ni40/C (pore size ∼3 nm, Figure S6) and commercial state-of-the-art Pt/C (20 wt %, Johnson Matthey) nanocatalysts. Figure 3a shows cyclic voltammetry (CV) curves of these different catalysts at a scan rate of 50 mV s−1 in a nitrogen-purged 0.10 M HClO4 solution. Resembling the observations of Pt/C and NP Pt60Ni40/C nanocatalysts,17−31 there are two distinctive potential regions for NP (Pt1−xNix)3Al/C, which are associated with the Hupd adsorption/desorption processes (H+ + e− = Hupd) in 0.05 < E < 0.40 V and the formation of an HO* layer (2H2O = HO* + H3O+ + e−) in E > 0.70 V. This strongly suggests a Pt shell structure in which there are not any residual Ni and Al atoms on the surface layer.25,51 The structure is further verified by the ECSACO/ECSAHupd ratio of 1.2 for NP (Pt1−xNix)3Al/C (Figure S7a),51 where ECSACO and ECSAHupd

are the electrochemical surface areas determined by integrating charges from CO stripping (electro-oxidation of preadsorbed CO) and underpotentially deposited hydrogen, respectively. To evaluate accurately the ORR catalytic activity, the specific ECSA of NP (Pt1−xNix)3Al/C is determined to be 65 m2 g−1Pt according to the CO stripping, which is higher than those of NP Pt60Ni40/C (36 m2 g−1Pt) and Pt/C (53 m2 g−1Pt) catalysts (Figures S7b and c). The ORR measurements of NP (Pt1−xNix)3Al/C are performed in an O2-saturated 0.1 M HClO4 solution using a rotating disk electrode (RDE) at room temperature. The current−potential curves at different rotation rates are shown in Figure S8a. They display two distinct potential regions: the first is the diffusion-limiting current region below 0.80 V, and the second is the mixed kinetic diffusion control region between 0.80 and 1.05 V. The parallel linear relationship between j−1 and ω−1/2 at 0.775−0.850 V suggests first-order reaction kinetics of the molecular oxygen (Figure S8b). According to the slopes of the Koutecky−Levich (K−L) plots,52 the transfer electron number of ∼4.0 demonstrates the nearly complete reduction of O2 to H2O via a four-electron process on the surface of (Pt1−xNix)3Al.4,14 Figure 3b compares the ORR polarization curve of NP (Pt1−xNix)3Al/C with those of NP Pt60Ni40/C and Pt/C at a rotation rate of 1600 rpm, wherein the electrochemical areas of these nanocatalysts are almost the same (∼6 cm2Pt cm−2geo). In the mixed region, the half-wave potential (ΔE1/2) for NP (Pt1−xNix)3Al/C positively shifts ∼81 mV relative to that of Pt/C. In contrast, NP Pt60Ni40/C represents a small positive shift of ΔE1/2 (∼62 mV). Figure 3c shows the kinetic current densities of NP (Pt1−xNix)3Al/C, Pt60Ni40/C, and Pt/C as a function of potential in the Tafel plot with the slopes at low overpotentials in the sequence NP (Pt1−xNix)3Al/C (52 mV dec−1) < NP Pt60Ni40/C (60 mV D

DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Electrochemical stability of the NP (Pt1−xNix)3Al/C catalyst. (a) ORR polarization curves of NP (Pt1−xNix)3Al/C catalyst in a longer cycling test of 50 000 potential cycles collected at a rotation rate of 1600 rpm and a scan rate of 10 mV s−1 in O2-saturated 0.10 M HClO4. (b) CV curves for the 50 000-cycle stability test of NP (Pt1−xNix)3Al/C catalyst in N2-purged 0.10 M HClO4 at 50 mV s−1 within a potential window of 0.60 to 1.0 V. (c) Evolutions of ECSA and (d) Pt mass activity retentions for NP (Pt1−xNix)3Al/C, NP Pt60Ni40/C, and Pt/C nanocatalysts as a function of cycle number. (e) Geometrical illustrations depicting the segregation of Al and Ni atoms from the core to the Pt monolayer for DFT calculations. The positive value indicates a less stable state.

dec−1) < Pt/C (71 mV dec−1), implying dramatically accelerated ORR kinetics in NP (Pt1−xNix)3Al/C. This is verified by the exchange current density of NP (Pt1−xNix)3Al/C (7.2 × 10−6 mA cm−2), which is higher than the values of both NP Pt60Ni40/C (6.7 × 10−6 mA cm−2) and Pt/C (5.8 × 10−6 mA cm−2) (Figure S9). To demonstrate the evolution of the specific activity as a function of Pt content (atom %) in intermetallic Pt−Al-based catalysts, NP Pt5Al/C and Pt3Al/C with the same nanoporous structures but without any Ni atoms were fabricated by chemically dealloying the Pt−Al alloy precursor, and the atomic ratios of Pt/Al were tuned by controlling the dealloying temperature and time.45 Compared with that of Pt/C (0.2772 nm),14 NP Pt5Al and Pt3Al have average nearest-neighbor Pt− Pt distances of 0.2755 and 0.2741 nm, respectively.43 When 13 atom % Pt in Pt3Al is replaced by Ni atoms, the nearestneighbor Pt−Pt distance of (Pt1−xNix)3Al is further compressed to 0.2721 nm. This gives rise to an enhanced compressive strain effect (1.8% relative to Pt/C), which couples with the additional ligand effect of the Ni constituent to adjust d-band structure of Pt surface atoms for realizing a balance between HOO* and HO* adsorption energies. Therefore, the NP (Pt1−xNix)3Al/C exhibits a specific activity up to 3.6 mA cm−2Pt at 0.90 V versus RHE, which is much higher than those of NP

Pt60Ni40/C (1.9 mA cm−2Pt) with the similar Pt content and commercially available Pt/C (0.27 mA cm−2Pt) (Figure 3d) as well as other nanostructured bimetallic Pt-TM/C catalysts (TM = Ti,9 Fe,10−13 Co,14−16 Ni,17−26 Cu,32−36 Y,9,37 and Zr9) (Figure S10a) but lower than Pt3Ni nanoframes25 and Modoped Pt3Ni nanoparticles. 26 Furthermore, the specific activities of the Pt−Al-based intermetallic compounds successively increase with the decrease in Pt content, in distinct contrast with the observation for NP Pt−Ni solid solutions,20,28 which exhibit a maximum activity at NP Pt75Ni25/C and begin to reduce when the Pt content deviates from 75 atom % (Figure 3d). The specific activity of NP (Pt1−xNix)3Al/C is ∼3 times higher than that of NP Pt86Ni14/C with the almost same Pt:Ni ratio.20 This distinguishable difference reveals the significant role of Al in improving the ORR activity of NP (Pt1−xNix)3Al/C: the incorporation of Al not only significantly reduces the usage of scarce Pt but also produces compressive strain and ligand effects by forming strong Pt−Al and Ni−Al bonds, which can weaken the HO* adsorption energy of Pt surface atoms relative to that of Pt−Ni. Therefore, the mass activity of NP (Pt1−xNix)3Al/C (2.4 A mg−1Pt) is significantly enhanced relative to that of the commercial Pt/C (0.14 A mg−1Pt) at 0.90 V versus RHE (Figure S10b).3 E

DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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0.90 V. Furthermore, formation of strong Pt−Al and Ni−Al bonds in the NP (Pt1−xNix)3Al intermetallic compound restricts the diffusion of Pt, Ni, and Al atoms, which prevents the further dissolution of Ni and Al atoms in a longer potential cycling. The extraordinary performance makes NP (Pt1−xNix)3Al a promising candidate as a cathode nanocatalyst in a PEM fuel cell.

In addition to the high specific and mass activities, the NP (Pt1−xNix)3Al/C nanocatalysts exhibit remarkable durability in a longer potential cycling between 0.60 to 1.0 V at a scan rate of 50 mV s−1 in an O2-saturated 0.10 M HClO4 solution. Figure 4a shows the evolution of polarization curves for the ORR on the NP (Pt1−xNix)3Al/C electrode during the 50 000 cycles, where the half-wave potential negatively shifts by only ∼4, 8, 10, 12, and 14 mV per 10 000 cycles relative to the initial value. However, both the NP Pt60Ni40/C and the state-of-the-art Pt/C nanocatalysts suffer from substantial degradations in their ORR performance. After 50 000 cycles, notable shifts of 29 and 39 mV are observed in the half-wave potentials for NP Pt60Ni40/C (Figure S11a) and Pt/C (Figure S12a) due to the continuous dissolution of less-noble Ni atoms from the Pt60Ni40 subsurface20 and the aggregation of Pt nanoparticles,45,53 respectively. These processes are validated by their ECSA changes (Figures S11b and S12b): in the initial 10 000 cycles, the ECSA of NP Pt60Ni40/C increases to ∼110% of the pristine surface area, while for the Pt/C, the ECSA continuously decreases to 31% in the potential cycling, which is much lower than that of NP (Pt 1−x Ni x ) 3 Al/C (∼75%) (Figures 4b and c). As a consequence, the outstanding structure stability enables NP (Pt1−xNix)3Al/C to maintain ∼60% of the initial mass activity at 0.90 V after 50 000 cycles, far beyond NP Pt60Ni40/C (∼33%) and Pt/C (∼30%) (Figure 4d). Moreover, evident morphology change cannot be observed except for only 11% Al dissolution after 10 000 cycles (Figures S13 and S14). The superior durability of NP (Pt1−xNix)3Al/C results from the Pt shell structure of intermetallic compounds. To theoretically elucidate the enhancement of the stability, we perform density functional theory (DFT) calculations on (Pt1−xNix)3Al periodic supercells with pure Pt(111) and Pt−Al or Pt−Ni alloy surfaces (Figure 4e) to demonstrate the surface segregation energies of Al (Eseg,Al) and Ni (Eseg,Ni) atoms.54 Here, Eseg,Al = EPt shell − EPt−Al‑shell and Eseg,Ni = EPt shell − EPt−Ni‑shell, where EPt shell denotes the total energy of intermetallic (Pt1−xNix)3Al/Pt slab and EPt−Al‑shell or EPt−Ni‑shell is the total energy of the system with one Al or Ni atom moving onto the Pt shell, respectively. Both positive surface segregation energies of 0.58 and 0.72 eV for Ni and Al atoms indicate a more thermodynamically stable (Pt1−xNix)3Al/Pt, which is expected to spontaneously form in acidic electrolyte.17−31 Although Al and Ni thermodynamically tend to dissolve (the standard dissolution potentials of −1.7 and −0.26 V versus RHE for Al to Al3+ and Ni to Ni2+) under the conditions of a cathode in a PEM fuel cell,55 the favorable bulk formation heats of Pt−Al (−0.74 eV/atom) and Ni−Al (−0.42 eV/atom) are expected to suppress the transport of Al and Ni atoms from the interior of the alloy to the surface. Furthermore, they significantly restrain the evolution of the Pt shell, which in turn protects against the further dissolution of Al and Ni atoms in the core.

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METHODS

Fabrication of NP Nanoparticles of (Pt1−xNix)3Al Intermetallic Compounds and Pt60Ni40 Alloys. The Pt10Ni2Al88 (atom %) alloy ingots were made by co-melting commercially available Pt (99.99%), Ni (99.99%), and Al (99.95%) in an arc-melting furnace in an argon atmosphere (1.1 × 105 Pa). The precursor Pt10Ni2Al88 ribbons (∼20 μm × ∼1 mm) were prepared by a melting-spinning technique in an Ar atmosphere (3 × 104 Pa). The NP (Pt1−xNix)3Al intermetallic ribbons were fabricated by chemically dealloying Pt10Ni2Al88 ones in N2-bubbled 1 M NaOH aqueous solution at ambient temperature. After removal of the residual chemical substances in nanopore channels by rinsing in ultrapure water (18.2 MΩ cm) and drying in a vacuum oven, the NP nanoparticle catalysts were prepared by a mechanical crushing method. They were dispersed in a mixture of Nafion (1% v/v, Sigma-Aldrich), isopropanol (20% v/ v), and ultrapure water (79% v/v), as well as commercially available carbon black (Vulcan XC-72, Cabot) for electrochemical measurements. The fabrication of NP Pt60Ni40 nanoparticles was conducted by chemical dealloying Pt24Ni76 alloy nanoparticles in a N2-saturated 1 M HNO3 solution at room temperature for 15 h. Therein, the Pt24Ni76 alloy precursor was first prepared by thermal reduction of a mixture slurry containing 12 mL of 10 mM H2PtCl6, 18 mL of 20 mM Ni(NO3)2, and 94 mg of Vulcan XC-72 carbon in a H2/Ar atmosphere at 300 °C for 5 h and then at 950 °C for 15 h. Structural Characterization. High-resolution STEM characterization was performed on a field-emission transmission electron microscope (JEM-ARM200CF, JEOL, 200 keV), which was equipped with double spherical-aberration correctors for both condenser and objective lenses. A field-emission scanning electron microscope (JSM6700F, JEOL, 15 keV) was employed to investigate the microstructure of the catalysts. Atomic ratio analysis of elements was measured utilizing both EDS (Oxford) and ICP-MS (Thermo Electron). XPS characterizations were carried out on Thermo ECSALAB 250 with an Al anode. XRD patterns were collected using a D/max2500pc diffractometer with Cu Kα radiation. Electrochemical Measurements. All electrochemical experiments were carried out in a three-electrode setup in which a Pt foil was used as the counter electrode, and a Hg/Hg2Cl2 electrode was employed as the reference electrode. The working electrodes with a geometrical area of 0.196 cm2 were prepared by drop casting 10 μL of NP (Pt1−xNix)3Al/C, Pt3Al/C, Pt5Al/C, Pt60Ni40/C, or commercially available Pt/C (20 wt %, Johnson Matthey) onto a glassy carbon RDE (Pine Research Instrumentation). All potentials were calibrated with respect to RHE. Cyclic voltammetry was performed in the range of 0.05 to 1.0 V at a scan rate of 50 mV s−1 in N2-saturated 0.1 M HClO4 solution. The ECSA was estimated by subtracting double-layer contribution and using a conversion factor of ∼210 μC cm−2 in a potential range of 0.05−0.40 V for the hydrogen adsorption/ desorption.17,25 Considering that the weakened interaction between Had and surface Pt sites gives rise to an underestimation of ECSA values, CO stripping was performed in parallel experiments for calibrating the Pt-ECSA of NP (Pt1−xNix)3Al/C and Pt60Ni40/C. Before CO-stripping, the electrolyte was saturated with CO by bubbling it through the solution for 15 min at a potential of 0.05 V. The ORR polarization curves were collected by sweeping the potential from 0.05 to 1.05 V (RHE) at 10 mV s−1 in an oxygen-saturated 0.1 M HClO4 solution with various rotation rates. The kinetic current density (jk) was calculated according to the Koutecky−Levich equation

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CONCLUSIONS In summary, we developed a facile and scalable strategy to mass-produce nanoporous ternary (Pt1−xNix)3Al intermetallic compounds for high-performance catalysis toward ORR. Relative to the NP Pt−Ni solid solutions and the commercially available Pt/C catalysts, the incorporation of Al in NP (Pt1−xNix)3Al not only substantially reduces the usage of Pt metal but also aptly modulates the adsorption energies of oxygen intermediates by introducing both compressive strain and ligand effects. This enlists the NP (Pt1−xNix)3Al catalysts to have high ORR activities of 3.6 mA cm−2Pt and 2.4 A mg−1Pt at

1 1 1 = + jm ECSAPtjk Ageojd F

DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces where jm is the measured current density, Ageo is the geometrical area of the electrode (0.196 cm2), and jd is the diffusion limited current density.27 Computational Method. DFT calculations on (Pt1−xNix)3Al supercells with Pt and alloy shells were performed on a four-layer p(2 × 2) periodic slab using a spin-unrestricted method in CASTEP code with ultrasoft pseudopotentials. The (Pt1−xNix)3Al supercells were constructed by randomly substituting 13% of Pt atoms by Ni atoms in structure-ordered Pt3Al structure. The generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional was employed to describe exchange and correlation effects. The accurate electronic convergence was obtained with a smearing of 0.1 eV to the orbital occupation. The convergence tolerance of energy was 2.0 × 10−5 eV/atom; the maximum force was 0.05 eV/Å, and maximum displacement is 0.002 Å in CASTEP. The structural stability of alloy surfaces with different composition is evaluated according to the relative energy, ΔE = EPt shell − Ealloy shell, where EPt shell and Ealloy shell denote the total energies of alloy slabs with Pt shell and alloy shell.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12553. XRD patterns of precursor Pt10Ni2Al88, FESEM image, EDS of NP (Pt1−xNix)3Al, XPS spectra, TEM and EDS of Pt60Ni40/C, CO stripping curves, ORR polarization curves, and durability measurements of NP (Pt1−xNix)3Al, NP Pt60Ni40/C and Pt/C (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Gao-Feng Han: 0000-0001-5943-0492 Xing-You Lang: 0000-0002-8227-9695 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grants 51422103, 51201069, and 51631004), the Top-notch Young Talent Program of China, Key Project of Chinese Ministry of Education (Grant 313026), the Research Fund for the Doctoral Program of Higher Education of China (Grant 20120061120042), and the Program for Innovative Research Team (in Science and Technology) at the University of Jilin Province.

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REFERENCES

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DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b12553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX