Platinum-Silver Alloy Nanoballoon Nanoassemblies with Super

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Platinum-Silver Alloy Nanoballoon Nanoassemblies with Super Catalytic Activity for the Formate Electrooxidation Shu-He Han, Hui-Min Liu, Juan Bai, Xin Long Tian, Bao Yu Xia, Jing-Hui Zeng, Jia-Xing Jiang, and Yu Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00004 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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ACS Applied Energy Materials

Platinum-Silver Alloy Nanoballoon Nanoassemblies with Super Catalytic Activity for the Formate Electrooxidation Shu-He Han,†,§ Hui-Min Liu,†,§ Juan Bai,† Xin Long Tian,‡ Bao Yu Xia,*, ‡ Jing-Hui Zeng,† Jia-Xing † ,† Jiang, and Yu Chen* †

Key Laboratory of Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, P.R. China

‡

Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, Wuhan National Laboratory for Optoelectronics, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China

ABSTRACT: Alkaline direct formate fuel cells (ADFFC) are emerging as a propitious candidate for the green energy conversion device. However, the poor electrocatalytic activity and stability of anodic electrocatalysts are immense challenges for its full-scale commercialization. In the current work, Pt-Ag alloy nanoballoon nanoassemblies (ANBNSs) were easily synthesized by the galvanic replacement reaction between three-dimensionally multibranched Ag nanoflowers and K2PtCl4. Electrochemical results exhibit that Pt-Ag ANBNSs have a nearly 19.3-fold activity enhancement for the formate oxidation reaction (FOR) over commercial Pt nanoparticles in alkaline media, as well as a higher resistance to CO poisoning and excellent durability for the FOR. This work demonstrates that Pt-Ag ANBNSs are indeed highly promising active and durable electrocatalysts for the FOR due to the integration of geometric/electronic effects and porous architecture.

KEYWORDS: galvanic reaction; Pt-Ag alloy; hollow nanoballoon; electrocatalysis; formate oxidation reaction Direct formic acid fuel cell (DFAFC), a highly competent green energy conversion system, has received widespread attention due to the high electromotive force, facile power-system integration, reasonable power density, and low 1-8 fuel crossover . Compared to the formic acid oxidation reaction in acidic media, the formate oxidation reaction (FOR) in alkaline media exhibits quick reaction kinetics, 9-13 smaller over-potential, and less poisoning effect . Additionally, various non-precious metal nanomaterials can be used as active cathodic electrocatalysts for the oxygen 14, 15 reduction reaction in alkaline media , which effectively reduce the overall cost of the fuel cells. Consequently, in the last few years, the direct alkaline formate fuel cell (DAFFC) 7, 9-13 got an all-embracing attention . The previous investigations have indicated that Pt and Pd 9, 16-18 are excellent electrocatalysts for the FOR . However, both of them have inherent shortcomings. Pd-based electrocatalysts have fine electrocatalytic activity for the FOR, yet the electrocatalytic stability is undesirable due to 9, 19 the vulnerable dissolution and poisoning of Pd . In contrast, Pt-based electrocatalysts exhibit higher stability, but their electrocatalytic activity for the FOR is 20-24 unsatisfactory . Recent studies have demonstrated that Pt-M alloys (M= Ag, Cu, Co, Ni etc.) can effectively improve the performance of Pt-based electrocatalysts for various electrocatalytic reactions such as formic acid oxidation reaction, oxygen reduction reaction, and methanol oxygen

reaction, since the corporation of other metals can momentously improve the utilization efficiency of Pt, and simultaneously optimize the electronic and geometrical 25-32 . However, investigations of the structures of Pt alloys FOR on the Pt-M alloy are rarely reported to the best of our knowledge. Besides the chemical composition control, the morphology tailoring of nanomaterials is also an effective 33, 34 way for improving the catalytic activity . Recently, the hollow and porous nanomaterials have been waged more and more attention due to their special advantages, such as abundant pores, large specific surface area, and enormous active sites, which facilitate the mass transfer of small 35-39 molecules and benefit to the enhancement of activity . In this work, Pt-Ag alloy nanoballoon nanoassemblies (ANBNSs) were synthesized by the galvanic reaction between K2PtCl4 and three-dimensionally (3D) multibranched Ag nanoflowers template. Due to the geometric/electronic effects and porous architecture, the Pt-Ag ANBNSs exhibit a 19.3-fold activity enhancement for the FOR compared to commercial Pt nanoparticles in KOH solution. In addition, the preferential dehydrogenation pathway of the FOR, good anti-CO poisoning capability, and 3D self-supported structures of the Pt-Ag ANBNSs result in excellent electrocatalytic stability for the FOR in alkaline media.

EXPERIMENTAL SECTION 1

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Reagents and Chemicals. Silver nitrate (AgNO3), potassium tetrachloroplatinate (II) (K2PtCl4), nitric acid (HNO3, 65%), ascorbic acid, polyacrylate sodium (SPA, average molecular weight 150,000), and potassium formate (HCOOK) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Commercial Pt nanoparticles and Pd nanoparticles were purchased from Johnson Matthey Corporation. All reagents used in this work −1 were of A.R. grade. Ultrapure water (resistance >18 MΩ cm ) was used in all experiments.

surface composition of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA). The specific surface areas of the samples were measured at 77 K with a Micromeritics ASAP 2020 HD88 system. UV-visible spectroscopy (UV-vis, Shimadzu UV-2600U) was used for + exploration the interaction of Ag ion with SPA and the growth process of Ag nanoflowers.

Preparation of 3D Multibranched Ag Nanoflowers Template. In a typical synthesis, 0.2 mL of 0.1 M AgNO3 and

Synthesis and Characterization of 3D Multibranched Ag Nanoflowers. The preparation of high-quality templates

0.4 mL of 0.5 M SPA were added into 40 mL of ultrapure water with stirring. Then, 0.4 mL of 0.5 M ascorbic acid solution was added in the mixture solution (pH 8.0), and then stirred for 1.5 h at room temperature.

is pivotal because it determines the morphology of the 38, 40 targeted nanomaterials . Ag nanoflowers were synthesized using ascorbic acid as reducing agent and SPA as a complexant and surfactant, respectively. The morphology and structure of the products were characterized by SEM and TEM. As observed, the products exhibit a flower-like 3D-multibranched morphology with a uniform size of 0.8 μm (Figure 1A and 1B). The crystalline structure of the products was characterized by XRD. All diffraction peaks match well with the typical face centered cubic (fcc, PDF#04-0783) structure of Ag (Figure 1C). It is worth noting that SPA plays a key role in the formation of the multibranched structure. In the absence of SPA, the obtained Ag nanocrystals severe aggregate and exhibit irregular morphologies (Figure S1), indicating that SPA not only is related to the formation of branches but also effectively avoids the aggregation of nanocrystals. To explore the reason, UV-vis measurements were performed. After adding SPA into AgNO3 solution, UV-vis spectrum of AgNO3/SPA mixture solution was significantly different from that of initial AgNO3 solution, I indicating the generation of SPA-Ag complex (Figure 1D). Further linear sweep voltammetry measurements also shown the introduction of SPA effectively decrease the reduction I I potential of Ag precursor (Figure S2), indicating the SPA-Ag I complex formed can retard the reduction rate of Ag precursor and allow the consequent kinetically controlled 41-43 synthesis .

RESULTS AND DISCUSSION

Preparation of Pt-Ag Alloy Nanoballoon Nanoassemblies (ANBNSs). 0.2 mL of 0.024 M K2PtCl4 was dropped into the as-prepared Ag nanoflowers suspension. After adjusting solution pH to 3.0, the mixture solution was stirred for 120 min. Then, 1.0 mL H2O2 was Injected into a mixture solution and stirred for 8 h to remove the unreacted Ag nanoflowers. After the reaction, the obtained Pt-Ag ANBNSs were separated by centrifugation at 15000 rpm for 3 min, washed with ultrapure water and ethanol for 3 times, respectively. Electrochemical Instruments. The whole electrochemical experiments were performed at CHI-660 o electrochemical apparatus at 30 C. The standard three-electrode system was used for all electrochemical experiments. The Pt plate served as the auxiliary electrode, the saturated calomel reference electrode served as the reference electrode, and the electrocatalyst modified glassy carbon electrode with 3 mm diameter served as the working electrode. All potentials in this work were calibrated to the reversible hydrogen electrode (RHE). Uniformly dispersed suspension of electrocatalyst was prepared by ultrasonic the mixture of 20 mg of electrocatalyst, 2 mL of isopropyl alcohol and 8 mL of H2O for 0.5 h. And the working electrode was prepared by dropping 4 μL of the suspension on to the glassy carbon electrode surface. Finally, 10 μL of 0.5 wt % Nafion solution was covered on the surface of the electrocatalyst modified electrode surface. Electrochemical measurements were preceded in N2-saturated 0.1 M HClO4 solution or N2-saturated 1 M KOH + 1 M HCOOK solution. The electrochemically surface area (ECSA) of electrocatalysts were calculated by integrating the charge associated with H desorption on Pt surface according to following equation (ECSA=Q/C×m). Where, Q was Coulombic charge of the H desorption peak area, C was the hydrogen adsorption constant for polycrystalline Pt (210 μC –2 cm ), and m was the mass of Pt on the electrode surface. Characterization. The structure and morphology of the samples were characterized by the transmission electron microscopy (TEM, JEM-2100F), scanning electron microscopy (SEM, JSM-2010) with energy-dispersive X-ray (EDX), and X-ray diffraction (XRD, DX–2700). The chemical state and 2


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Figure 1. (A) SEM image, (B) TEM image, and (C) XRD pattern of Ag nanoflowers. Insert in 1B: TEM image of individual Ag nanoflower. (D) UV-vis spectra of SPA solution, AgNO3 solution and AgNO3/SPA mixture solution.

to be ca. 0.229~0.230 nm (Figure 2H), which is larger than that of Pt (111) (0.226 nm) and smaller than that of Ag (111) (0.236 nm), implying the generation of Pt-Ag alloy. The fast Fourier transform (FFT) images exhibit the spots with 6-fold rotational symmetry, confirming Pt-Ag ANBNSs have (111) facets.

Additionally, the pH value of the reaction solution is another important factor for the generation of Ag nanoflowers (Figure S3). After decreasing solution pH to 3, the size of Ag nanocrystals increases, and the branches disappear completely (Figure S3A). After increasing solution pH to 11, the size and branch degree of Ag nanoflowers decrease simultaneously (Figure S3B). Likely, the solution pH affects the nucleation and growth rate of Ag nanocrystals, resulting 44, 45 in the different morphologies . Synthesis and Characterization of Pt-Ag ANBNSs. The Pt-Ag ANBNSs were obtianed through the galvanic reaction between K2PtCl4 and Ag nanoflowers template, and the subequent oxidation etching process (Scheme 1). Because the θ 2– PtCl4 /Pt redox couple has a higher Nernst potential (φ = 46 I θ 47 1.19 V) than that of the Ag /Ag redox couple (φ = 0.8 V) , II which provides a driving of galcanic reaction. Pt ions contact with Ag surface and obtain the electrons from Ag. Subsequently, Pt atoms deposit on the surface of Ag 48 nanoflowers while Ag atoms dissolve into the solutions . During the reaction process, Pt atoms can diffuse across the surface and prefer to form Pt-Ag alloy in order to minimize 49, 50 the surface energy . Finally, the residual Ag core can be 45, 46 removed by hydrogen peroxide etching .

Figure 2. (A) SEM and (B) TEM images of Pt-Ag ANBNSs. (C) HRTEM and (D) HAADF-STEM images of individual Pt-Ag ANBNSs. (E) The scheme of the balloon assemblies. (F) SAED pattern of Pt-Ag ANBNSs. (G) HRTEM of Pt-Ag ANBNSs at the edge region. (H) Magnified HRTEM images and the corresponding FFT images of Pt-Ag ANBNSs.

Scheme 1. Model diagram for the formation mechanism of Pt-Ag ANBNSs.

Further evidence for the alloy phase can be confirmed by XRD measurement (Figure 3A and Figure S4). The diffraction peaks of the ANBNSs locate at the sites between the pure Pt (PDF 04-0802) and Ag (PDF 04-0783), confirming again the formation of Pt-Ag alloy. According to Vegard’s law, Pt content in Pt-Ag alloy is calculated to be ca. 57.6 at.%, very close to the bulk Pt/Ag ratio detected by EDX (nPt: nAg = 56.5: 53 43.5, Figure S5), indicating a high alloy degree . EDX mapping images reveal that the very similar distribution of Pt and Ag element (Figure 3B). XPS investigations display that 0 0 the metallic Pt and Ag are dominant species in Pt-Ag ANBNSs (Figure 3C and 3D). The molar ratio of Pt/Ag was measured to be 59.2: 40.8, which is very close to the EDX results. XPS and EDX results further confirm the high alloy degree of Pt-Ag ANBNSs.

SEM and TEM images show the ANBNSs have uniform size and branched structure (Figure 2A and 2B), which is similar to the morphology of the Ag nanoflowers template (Figure 1A and 1B). TEM and HAADF-STEM images of the individual Pt-Ag ANBNSs show the strong luminance difference between the edge and the center (Figure 2C and 2D), indicating Pt-Ag ANBNSs have the hollow structure. The overall morphology of Pt-Ag ANBNSs is very similar to the shape of balloon assemblies (Figure 2E). The SAED pattern shows Pt-Ag ANBNSs have high crystallinity because of the clear and discrete diffraction dots (Figure 2F). The local magnified HRTEM image shows a shell thickness of ca. 4.6 nm (Figure 2G). Another magnified HRTEM image shows the continuous lattice fringes (Figure 2H), suggesting the structure of ANBNSs is probably formed by epitaxial growth 51, 52 rather than the accumulation of small nanoparticles . The lattice spacing in a series of HRTEM images were measured 3



ANBNSs. The red lines indicate the voids in (C) and the yellow lines shows the lattice dislocations in (D).

Electrocatalytic Performance Measurements. With high surface area, enormous holes, and abundant active sites, 57, 58 Pt-Ag ANBNSs are expected to be ideal electrocatalysts . The electrochemical properties of Pt-Ag ANBNSs and commercial Pt nanoparticles were invsetigated by cyclic voltammetry (CV) in N2-saturated 1 M KOH solution (Figure 5A). Pt-Ag ANBNSs show a bigger hydrogen adsorption/desorption peak than Pt nanoparticles, indicating Pt-Ag ANBNSs have a bigger ECSA value. To obtain the exact ECSA, CV tests were also performed in N2-saturated 0.1 M HClO4 solution (Figure S7). ECSA of Pt-Ag ANBNSs and 2 commercial Pt nanoparticles were measured to be 25.5 m 2 -1 -1 gPt and 16.5 m gPt , respectively. Pt mass-normalized activity (the current is normalized with Pt metal mass) of Pt-Ag ANBNSs and commercial Pt nanoparticles for the FOR were investigated by CV in N2-saturated 1 M HCOOK + 1 M KOH solution (Figure 5B). Compared to Pt nanoparticles catalysts, Pt-Ag ANBNSs showed a remarkably enhanced activity. For example, at potential of 0.66 V (vs RHE), the FOR current -1 (830 mA gPt ) at Pt-Ag ANBNSs is 19.3 times higher than that -1 (43 mA gPt ) of Pt nanoparticles. In addition, the initial potential of the FOR at Pt-Ag ANBNSs exhibits a negative shift of roughly 91 mV than that at Pt nanoparticles by the enlarged local drawing (insert in Figure 5B), showing a better 59 reactivity of Pt-Ag ANBNSs for the FOR . Futhermore, the ECSA-normalized activity (the current densities are normalized with ECSA) of Pt-Ag ANBNSs for the FOR is also –2 12.5 times higher than that of Pt nanoparticles (32.6 mA cm –2 vs. 2.6 mA cm ) at potential of 0.66 V (Figure 5C), demonstrating that the inherent electrocatalytic activity of Pt-Ag ANBNSs has been greatly enhanced. The obvious activity enhancement originates from the structural effect and electronic effect. Firstly, the unique 3D porous structure of Pt-Ag ANBNSs faciliates the fast mass transfer, improving the reaction rate; secondly, Pt-Ag ANBNSs own abundant low-coordination atoms, which is beneficial to the 54, 55 enhancement of electrocatalytic activity . Finally, the formation of Pt-Ag alloy results in the modulation of electronic structure of Pt atoms. XPS measurements show that Pt 4f peak of Pt-Ag ANBNSs has a negative shift (0.27 eV) compared to Pt nanoparticles (Figure S8). This means the electrons migration from Ag to Pt, leading to a down-shift in the d-band center, and thus a lower affinity 57 between Pt and oxygen containsing species . The weak – adsorption of HCOO on Pt surface is beneficial for the FOR 7, 60 in alkine media . Thus, the FOA activity of Pt-Ag ANBNSs can be greatly improved.

Figure 3. (A) XRD pattern, (B) EDX, (C) Pt 4f XPS spectrum and (D) Ag 3d XPS spectrum of Pt-Ag ANBNSs. The N2 adsorption/desorption curve of Pt-Ag ANBNSs displays the typical IV-isotherms with H3 hysteresis loop (Figure 4A), which demonstrates the hollow and porous nanostructures of Pt-Ag ANBNSs (Figure 4B). According to the Brunauer-Emmett-Teller method, the specific surface 2 -1 area of Pt-Ag ANBNSs is measured to be 29.41 m g , which is 2 -1 bigger than that of Pt nanoparticles (18.69 m g , Figure S6). Furthermore, a lot of apertures and channels can be clearly observed on the ANBNSs surface (Figure 4C). Such porous structure is beneficial for improving the Pt utilization, because the Pt atoms of inner surface can also participate in 38, 39, 54 the catalytic reaction . Meanwhile, many lattice dislocations are found on the surface of Pt-Ag ANBNSs (Figure 4D), which may originate from the introduction of Pt atoms. When Pt atoms with larger atomic radius are inserted into the lattice of Ag crystal, the original atomic arrangements are certainly affected, resulting in the 55, 56 formation of the defects and the lattice dislocations .

Figure 4. (A) N2 sorption isotherms, (B) the pore size distribution, (C) TEM and (D) HRTEM images of Pt-Ag

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initial activity of Pt nanoparticles for the FOR. After the chronoamperometry test, CV test, TEM, HRTEM images and EDX data that the the ECSA, morphology and composition of the PtAg ANBNSs still maintain well (Figure S10). For comparasion, PtAg nanoparticles have obvious aggrengation after 5000 s chronoamperometry test (Figure S11). This result strongly confrim that the hollow structure could significantly inhibit the Ostwald ripening effect, resulting high electrocatayic stability. At present, Pd is generally recognized as the best 5. electrocatalysts for the FOR in alkaline media To explore the practicability of Pt-Ag ANBNSs as the Pd-alternative electrocatalysts for the FOR, we further compared the electrocatalytic performance of Pt-Ag ANBNSs and commercial Pd nanoparticles for the FOR by CV in 1 M HCOOK + 1 M KOH solution (Figure 6). The FOR peak -1 current at Pt-Ag ANBNSs (830 mA gPt ) is slightly higher -1 than that at Pd nanoparticles (606 mA gPd ). However, the FOR peak potential (0.66 V) at Pt-Ag ANBNSs is much lower than that (0.75 V) at the Pd nanoparticles. The higher current and lower potential indicate that Pt-Ag ANBNSs have a better electrocatalytic activity for the FOR. Meanwhile, CV curve of Pd nanoparticles show a smaller If/Ib value compared to Pt-Ag ANBNSs, (0.48 vs. 2.13), suggesting that the FOR at Pd nanoparticles simultaneously undergo the dehydrogenation and dehydration pathways. Furthermore, chronoamperometry results display that the stability of Pd nanoparticles for the FOR is very poor. As illustrated, the FOR current at Pd nanoparticles disappears completely after only 2000 s (Figure 6B). In contrast, Pt-Ag ANBNSs remains 36% of their initial activity for the FOR current at 10,000 s (Figure 6B). Thus, CV and chronoamperometry tests clearly demonstrate that Pt-Ag ANBNSs have an improved 64-67 electrocatalytic activity and durability for the FOR .

Figure 5. The Pt mass- (A, B) and ECSA-normalized CV curves (C) for Pt-Ag ANBNSs and commercial Pt nanoparticles in N2-saturated (A) 1 M KOH solution and (B, C) 1 M HCOOK + 1 M KOH solution at the scan rate of 50 mV –1 s . (D) After 600 s chronoamperometry tests in 1 M HCOOK + 1 M KOH solution, CV curves for Pt-Ag ANBNSs and commercial Pt nanoparticles in N2-saturated 1 M KOH solution. Differing from bi-path oxidation behavior of formic acid at Pt surface in acidic media, FOR mainly undergoes the – – 2– dehydrogenation pathway (HCOO + 3OH →CO3 + 2H2O + – 10 2e ) in alkline media . Obviously, it is attractive that the ideal catalyst can effectively avoid the generation of CO intermediates during the FOR. However, it is difficult to completely carry out such ideal oxidation pathway. In the actual FOR process, it will produce CO or other small 61 carbon-molecules species . The peak current ratio If/Ib between forward scan (If) and backward scan (Ib) is an important parameter for deducing the reaction pathway of the FOR. According to CV curves, the If/Ib (ca. 2.13) of Pt-Ag ANBNSs is much higher than that (If/Ib = ca. 1.05) at the Pt nanoparticles, which indicates the FOR at Pt-Ag ANBNSs is 57 more complete than at Pt nanoparticles . To prove this claim, after 600 s chronoamperometry tests for Pt-Ag ANBNSs and commercial Pt nanoparticles, we further conduct the CV tests for Pt-Ag ANBNSs and commercial Pt nanoparticles. The accumulated COads on the Pt nanoparticles exhibit an obvious peak whereas the accumulated COads on Pt-Ag ANBNSs only shows a negligible small peak (Figure 5D). This result confirms that dehydromenation pathway of formate at the surface of Pt-Ag ANBNSs is more predominant than that at the Pt nanoparticles, which is more beneficial for electrocatalytic stability of Pt-Ag ANBNSs due to the less CO accumulation 57, 62, 63 . Chronoamperometry tests were performed to further investigate the activity and durability of the electrocatalysts. Obviously, Pt-Ag ANBNSs has superior electrocatalytic stability for the FOR (Figure S9). The FOR current of Pt nanoparticles completely disappears at 6000 s test. However, Pt-Ag ANBNSs preserves 47% of their initial value. After prolonging the test to 10,000 s, the remaining activity of -1 Pt-Ag ANBNSs (213 mA gPt ) is still 11.8 times higher than the

Figure 6. (A) The mass-normalized CV curves for Pt-Ag ANBNSs and Pd nanoparticles in N2 saturated 1 M HCOOK + –1 1 M KOH solution at the scan rate of 50 mV s . (B) Chronoamperometry curves of Pt-Ag ANBNSs and Pd nanoparticles in N2 saturated 1 M HCOOK + 1 M KOH solution at 0.56 V.

CONCLUSIONS In summary, we present a facile synthesis of Pt-Ag ANBNSs by galvanic reduction between 3D multibranched Ag nanoflowers template and K2PtCl4. The as-prepared Pt-Ag ANBNSs have unique structural features, such as porosity, self-supported structure, large specific surface area, and numerous defected atoms. In alkaline media, Pt-Ag ANBNSs 5



(5) Wu, D.; Zheng, Z.; Gao, S.; Cao, M.; Cao, R. Mixed-phase PdRu bimetallic structures with high activity and stability for formic acid electrooxidation. Phys. Chem. Chem. Phys. 2012, 14 (22), 8051-8057. (6) Shen, M.; Huang, Y.; Wu, D.; Lü, J.; Cao, M.; Liu, M.; Yang, Y.; Li, H.; Guo, B.; Cao, R. Facile ultrafine copper seed-mediated approach for fabricating quasi-two-dimensional palladium-copper bimetallic trigonal hierarchical nanoframes. Nano Res. 2017, 10 (8), 2810-2822. (7) Wu, D.; Cao, M.; Shen, M.; Cao, R. Sub-5 nm Pd-Ru Nanoparticle Alloys as Efficient Catalysts for Formic Acid Electrooxidation. Chemcatchem 2014, 6 (6), 1731-1736. (8) Ho, S. F.; Mendoza-Garcia, A.; Guo, S.; He, K.; Su, D.; Liu, S.; Metin, Ö.; Sun, S. A facile route to monodisperse MPd (M= Co or Cu) alloy nanoparticles and their catalysis for electrooxidation of formic acid. Nanoscale 2014, 6, 6970−6973. (9) Yu, X.; Manthiram, A. Catalyst-selective, scalable membraneless alkaline direct formate fuel cells. Appl. Catal. B: Environ. 2015, 165, 63−67. (10) Li, Y.; Feng, Y.; Sun, X.; He, Y. A Sodium-Ion-Conducting Direct Formate Fuel Cell: Generating Electricity and Producing Base. Angew. Chem. 2017, 129, 5828−5831. (11) Bartrom, A. M.; Ta, J.; Nguyen, T. Q.; Her, J.; Donovan, A.; Haan, J. L. Optimization of an anode fabrication method for the alkaline Direct Formate Fuel Cell. J. Power Sources 2013, 229, 234−238. (12) Joo, J.; Uchida, T.; Cuesta, A.; Koper, M. T.; Osawa, M. Importance of acid-base equilibrium in electrocatalytic oxidation of formic acid on platinum. J. Am. Chem. Soc. 2013, 135, 9991−9994. (13) John, J.; Wang, H.; Rus, E. D.; Abruña, H. D. Mechanistic Studies of Formate Oxidation on Platinum in Alkaline Medium. J. Phys. Chem. C 2012, 116, 5810−5820. (14) Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y. Highly efficient nonprecious metal catalysts towards oxygen reduction reaction based on three-dimensional porous carbon nanostructures. Chem. Soc. Rev. 2016, 45, 517−531. (15) Tong, Y.; Chen, P.; Zhou, T.; Xu, K.; Chu, W.; Wu, C.; Xie, Y. A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B, N-Decorated Graphene. Angew. Chem. Int. Ed. 2017, 56, 7121−7125. (16) Zhang, L. Y.; Zhao, Z. L.; Li, C. M. Formic acid-reduced ultrasmall Pd nanocrystals on graphene to provide superior electocatalytic activity and stability toward formic acid oxidation. Nano Energy 2015, 11, 71−77. (17) Kannan, P.; Maiyalagan, T.; Opallo, M. One-pot synthesis of chain-like palladium nanocubes and their enhanced electrocatalytic activity for fuel-cell applications. Nano Energy 2013, 2, 677−687. (18) Li, Y.; Wu, H.; He, Y.; Liu, Y.; Jin, L. Performance of direct formate-peroxide fuel cells. J. Power Sources 2015, 287, 75−80. (19) Iyyamperumal, R.; Zhang, L.; Henkelman, G.; Crooks, R. M. Efficient electrocatalytic oxidation of formic acid using Au@ Pt dendrimer-encapsulated nanoparticles. J. Am. Chem. Soc. 2013, 135, 5521−5524. (20) Ge, X.; Chen, L.; Kang, J.; Fujita, T.; Hirata, A.; Zhang, W.; Jiang, J.; Chen, M. A Core-Shell Nanoporous Pt-Cu Catalyst with Tunable Composition and High Catalytic Activity. Adv. Funct. Mater. 2013, 23, 4156−4162. (21) Yang, S.; Lee, H. Atomically dispersed platinum on gold nano-octahedra with high catalytic activity on formic acid oxidation. ACS Catal. 2013, 3, 437−443. (22) Jiang, Y.; Jia, Y.; Zhang, J.; Zhang, L.; Huang, H.; Xie, Z.; Zheng, L. Underpotential Deposition-Induced Synthesis of Composition-Tunable Pt Cu Nanocrystals and Their Catalytic Properties. Chem. Eur. J. 2013, 19, 3119−3124. (23) Kang, Y.; Qi, L.; Li, M.; Diaz, R. E.; Su, D.; Adzic, R. R.; Stach, E.; Li, J.; Murray, C. B. Highly active Pt3Pb and core–shell Pt3Pb–Pt electrocatalysts for formic acid oxidation. ACS Nano 2012, 6, 2818−2825.

exhibit extraordinary activity for the FOR compared to Pt nanoparticles, due to the structural effect and electronic effect. More significantly, Pt-Ag ANBNSs significantly enhance the dehydrogenation pathway of the FAOR, resulting in less CO accumulation. Consequently, the chemical inertness of Pt metal and less CO accumulation result in outstanding stability of Pt-Ag ANBNSs for the FOR. Excellent electrocatalytic activity and stability make Pt-Ag ANBNSs an advanced FOR electrocatalyst for the FOR in alkaline media. Our work indicates that Pt-based alloy nanostructres may have potential application in the DAFFC.

ASSOCIATED CONTENT Supporting Information Additional information for TEM images of Ag with different size. EDX data of Pt-Ag ANBNSs. N2 adsorption/desorption isotherm of Pt black. Pt 4f XPS spectra of Pt-Ag ANBNSs and Pt black. And many electrochemistry texts for Pt-Ag ANBNSs, Pt black and Pd black.

AUTHOR INFORMATION Corresponding Authors * Email: [email protected] (Y. Chen) * Email: [email protected] (B.Y. Xia)

Author Contributions §

S. H. Han and H. M. Liu contributed equally to this work. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was sponsored by National Natural Science Foundation of China (21473111), Fundamental Research Funds for the Central Universities (GK201602002, GK201701007 and 2017KFXKJC002), and the 111 Project (B14041). The authors also thank the financial support by National 1000 Young Talents Program of China, the Innovation Foundation of Shenzhen Government (JCYJ20160408173202143), and the Innovation Research Funds of HUST (3004013109, 0118013089, and 2017KFYXJJ164).

REFERENCES (1) Zhang, S.; Shao, Y.; Yin, G.; Lin, Y. Electrostatic Self-Assembly of a Pt-around-Au Nanocomposite with High Activity towards Formic Acid Oxidation. Angew. Chem. Int. Ed. 2010, 49, 2211−2214. (2) Wang, R.; Liu, J.; Liu, P.; Bi, X.; Yan, X.; Wang, W.; Ge, X.; Chen, M.; Ding, Y. Dispersing Pt atoms onto nanoporous gold for high performance direct formic acid fuel cells. Chem. Sci. 2014, 5, 403−409. (3) Joo, J.; Choun, M.; Jeong, J.; Lee, J. Influence of Solution pH on Pt Anode Catalyst in Direct Formic Acid Fuel Cells. ACS Catal. 2015, 5, 6848−6851. (4) Wu, D.; Cao, M.; Cao, R. Ru-assisted synthesis of {111}-faceted Pd truncated bipyramids: a highly reactive, stable and restorable catalyst for formic acid oxidation. Chem. Commun. 2014, 50, 12970-12972.

6


Page 6 of 9

Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Peng, Z.; You, H.; Yang, H. An electrochemical approach to PtAg alloy nanostructures rich in Pt at the surface. Adv. Funct. Mater. 2010, 20, 3734−3741. (25) Yin, A. X.; Min, X. Q.; Zhang, Y. W.; Yan, C. H. Shape-selective synthesis and facet-dependent enhanced electrocatalytic activity and durability of monodisperse sub-10 nm Pt− Pd tetrahedrons and cubes. J. Am. Chem. Soc. 2011, 133, 3816−3819. (26) Zhang, J.; Yang, H.; Fang, J.; Zou, S. Synthesis and oxygen reduction activity of shape-controlled Pt3Ni nanopolyhedra. Nano Lett. 2010, 10, 638−644. (27) Zhang, H.; Jin, M.; Wang, J.; Li, W.; Camargo, P. H.; Kim, M. J.; Yang, D.; Xie, Z.; Xia, Y. Synthesis of Pd-Pt bimetallic nanocrystals with a concave structure through a bromide-induced galvanic replacement reaction. J. Am. Chem. Soc. 2011, 133, 6078−6089. (28) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Marković, N. M. Improved oxygen reduction activity on Pt3Ni (111) via increased surface site availability. Science 2007, 315, 493−497. (29) Tian, X. L.; Wang, L.; Deng, P.; Chen, Y.; Xia, B. Y. Research advances in unsupported Pt-based catalysts for electrochemical methanol oxidation. Journal of Energy Chemistry 2017, 26, 1067-1076. (30) Tian, X. L.; Xu, Y. Y.; Zhang, W.; Wu, T.; Xia, B. Y.; Wang, X. Unsupported Platinum-Based Electrocatalysts for Oxygen Reduction Reaction. ACS Energy Lett. 2017, 2 (9), 2035-2043. (31) Hammer, B.; Nørskov, J. K. Theoretical surface science and catalysis—calculations and concepts. Adv. Catal. 2000, 45, 71−129. (32) Nørskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the computational design of solid catalysts. Nat. Chem. 2009, 1, 37−46. (33) Ye, Z. Q.; Tan, M. Q.; Wang, G. L.; Yuan, J. L. Preparation, characterization, and time-resolved fluorometric application of silica-coated terbium (III) fluorescent nanoparticles. Anal. Chem. 2004, 76, 513−518. (34) Hirano, M.; Ota, K.; Preparation of photoactive anatase-type TiO2/silica gel by direct loading anatase-type TiO2 nanoparticles in acidic aqueous solutions by thermal hydrolysis. J. Mater. Sci. 2004, 39, 1841−1844. (35) Guo, Y.; Xu, Y. T.; Zhao, B.; Wang, T.; Zhang, K.; Yuen, M. M.; Fu, X. Z.; Sun, R.; Wong, C. P. Urchin-like Pd@ CuO-Pd yolk-shell nanostructures: synthesis, characterization and electrocatalysis. J. Mater. Chem. A 2015, 3, 13653−13661. (36) Liu, H. l.; Nosheen, F.; Wang, X. Noble metal alloy complex nanostructures: controllable synthesis and their electrochemical property. Chem. Soc. Rev. 2015, 44, 3056−3078. (37) González, E.; Merkoçi, F.; Arenal, R.; Arbiol, J.; Esteve, J.; Bastús, N. G.; Puntes, V. Enhanced reactivity of high-index surface platinum hollow nanocrystals. J. Mater. Chem. A 2016, 4, 200−208. (38) Wang, X.; Feng, J.; Bai, Y.; Zhang, Q.; Yin, Y. Synthesis, properties, and applications of hollow micro-/nanostructures. Chem. Rev. 2016, 116, 10983−11060. (39) Zhu, C.; Du, D.; Eychmüller, A.; Lin, Y. Engineering Ordered and Nonordered Porous Noble Metal Nanostructures: Synthesis, Assembly, and Their Applications in Electrochemistry. Chem. Rev. 2015, 115, 8896−8943. (40) Li, Y.; Shi, J. Hollow-Structured Mesoporous Materials: Chemical Synthesis, Functionalization and Applications. Adv. Mater. 2014, 26, 3176−3205. (41) Hsu, S. C.; Chuang, Y. C.; Sneed, B. T.; Cullen, D. A.; Chiu, T. W.; Kuo, C. H. Turning the halide switch in the synthesis of Au–Pd alloy and core–shell nanoicosahedra with terraced shells: Performance in electrochemical and plasmon-enhanced catalysis. Nano Lett. 2016, 16, 5514−5520. (42) Su, N.; Chen, X.; Yue, B.; He, H. Formation of palladium concave nanocrystals via auto-catalytic tip overgrowth by interplay of reduction kinetics, concentration gradient and surface diffusion. Nanoscale 2016, 8, 8673−8680.

(43) Meng, M.; Fang, Z.; Zhang, C.; Su, H.; He, R.; Zhang, R.; Li, H.; Li, Z. Y.; Wu, X.; Ma, C. Integration of kinetic control and lattice mismatch to synthesize Pd@ AuCu core-shell planar tetrapods with size-dependent optical properties. Nano Lett. 2016, 16, 3036−3041. (44) Carcouet, C. C.; van de Put, M. W.; Mezari, B.; Magusin, P. C.; Laven, J.; Bomans, P. H.; Friedrich, H.; Esteves, A. C.; Sommerdijk, N. A.; van Benthem, R. A.; de With, G. Nucleation and growth of monodisperse silica nanoparticles. Nano Lett. 2014, 14, 1433−14338. (45) Völkle, C. M.; Gebauer, D.; Cölfen, H. High-resolution insights into the early stages of silver nucleation and growth. Faraday Discuss. 2015, 179, 59−77. (46) García, A. M.; Hunt, A. J.; Budarin, V. L.; Parker, H. L.; Shuttleworth, P. S.; Ellis, G. J.; Clark, J. H. Starch-derived carbonaceous mesoporous materials for the selective adsorption and recovery of critical metals. Green Chem. 2015, 17, 2146−2149. (47) Steinberg, S.; Hodge, V.; Schumacher, B.; Sovocool, W. Sampling for silver nanoparticles in aqueous media using a rotating disk electrode: evidence for selective sampling of silver nanoparticles in the presence of ionic silver. Environ. Monit. Assess. 2017, 189, 99. (48) Yang, X.; Yang, M.; Pang, B.; Vara, M.; Xia, Y. Gold nanomaterials at work in biomedicine. Chem. Rev. 2015, 115, 10410−10488. (49) Chen, S.; Thota, S.; Wang, X.; Zhao, J. From solid to core@ shell to hollow Pt–Ag nanocrystals: thermally controlled surface segregation to enhance catalytic activity and durability. J. Mater. Chem. A 2016, 4, 9038−9043. (50) Chen, J.; Wiley, B.; McLellan, J.; Xiong, Y.; Li, Z. Y.; Xia, Y. Optical properties of Pd-Ag and Pt-Ag nanoboxes synthesized via galvanic replacement reactions. Nano Lett. 2005, 5, 2058−2062. (51) Zhang, Z.; Yang, Y.; Nosheen, F.; Wang, P.; Zhang, J.; Zhuang, J.; Wang, X. Fine Tuning of the Structure of Pt–Cu Alloy Nanocrystals by Glycine-Mediated Sequential Reduction Kinetics. Small 2013, 9, 3063−3069. (52) Jang, A. R.; Hong, S.; Hyun, C.; Yoon, S. I.; Kim, G.; Jeong, H. Y.; Shin, T. J.; Park, S. O.; Wong, K.; Kwak, S. K. Wafer-scale and wrinkle-free epitaxial growth of single-orientated multilayer hexagonal boron nitride on sapphire. Nano Lett. 2016, 16, 3360−3366. (53) Salgado, J. R.; Antolini, E.; Gonzalez, E. R. Structure and activity of carbon-supported Pt− Co electrocatalysts for oxygen reduction. J. Phys. Chem. B 2004, 108, 17767−17774. (54) Yang, X.; Roling, L. T.; Vara, M.; Elnabawy, A. O.; Zhao, M.; Hood, Z. D.; Bao, S.; Mavrikakis, M.; Xia, Y. Synthesis and Characterization of Pt-Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction. Nano Lett. 2016, 16, 6644−6649. (55) Wen, Y. H.; Huang, R.; Li, C.; Zhu, Z. Z.; Sun, S. G. Enhanced thermal stability of Au@Pt nanoparticles by tuning shell thickness: Insights from atomistic simulations. J. Mater. Chem. 2012, 22, 7380. (56) Yong Ding; Fengru Fan; Zhongqun Tian; Wang, Z. L. Atomic Structure of Au - Pd Bimetallic Alloyed Nanoparticles. J. Am. Chem. Soc. 2010, 132, 12480−12486. (57) Li, H. H.; Fu, Q. Q.; Liang, X.; Ma, S. Y.; Zheng, Y.; Yu, S. H. Highly crystalline PtCu nanotubes with three dimensional molecular accessible and restructured surface for efficient catalysis. Energ. Environ. Sci. 2017, 10, 1751−1756. (58) Ma, S. Y.; Li, H. H.; Hu, B. C.; Cheng, X.; Fu, Q. Q.; Yu, S. H. Synthesis of Low Pt-Based Quaternary PtPdRuTe Nanotubes with Optimized Incorporation of Pd for Enhanced Electrocatalytic Activity. J. Am. Chem. Soc. 2017, 139, 5890−5895. (59) Megan E. Scofield; Christopher Koenigsmann; Lei Wang; Haiqing Liua; Wong, S. S. Tailoring the composition of ultrathin, ternary alloy PtRuFe nanowires for the methanol oxidation reaction and formic acid oxidation reaction. Energ. Environ. Sci. 2015, 8, 350−363.

7



(60) Joo, J.; Uchida, T.; Cuesta, A.; Koper, M. T. M.; Osawa, M. Importance of Acid–Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum. J. Am. Chem. Soc. 2013, 135, 9991−9994. (61) Cuesta, A.; Cabello, G.; Gutiérrez, C.; Osawa, M. Adsorbed formate: the key intermediate in the oxidation of formic acid on platinum electrodes. Phys. Chem. Chem. Phys. 2011, 13, 20091−20095. (62) Fu, G. T.; Xia, B. Y.; Ma, R. G.; Chen, Y.; Tang, Y. W.; Lee, J. M. Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction. Nano Energy 2015, 12, 824−832. (63) Cao, X.; Wang, N.; Han, Y.; Gao, C.; Xu, Y.; Li, M.; Shao, Y. PtAg bimetallic nanowires: Facile synthesis and their use as excellent electrocatalysts toward low-cost fuel cells. Nano Energy 2015, 12, 105−114. (64) Hong, J. W.; Kim, M.; Kim, Y.; Han, S. W. Trisoctahedral Au–Pd Alloy Nanocrystals with High-Index Facets and Their Excellent Catalytic Performance. Chem. Eur. J. 2012, 18, 16626−16630.

(63) Cao, X.; Wang, N.; Han, Y.; Gao, C.; Xu, Y.; Li, M.; Shao, Y. PtAg bimetallic nanowires: Facile synthesis and their use as excellent electrocatalysts toward low-cost fuel cells. Nano Energy 2015, 12, 105−114. (64) Hong, J. W.; Kim, M.; Kim, Y.; Han, S. W. Trisoctahedral Au–Pd Alloy Nanocrystals with High-Index Facets and Their Excellent Catalytic Performance. Chem. Eur. J. 2012, 18, 16626−16630. (65) Zhang, J.; Yang, H.; Martens, B.; Luo, Z.; Xu, D.; Wang, Y.; Zou, S.; Fang, Pt–Cu nanoctahedra: synthesis and comparative study with nanocubes on their electrochemical catalytic performance. J. Chem. Sci. 2012, 3, 3302−3306. (66) Xu, D.; Bliznakov, S.; Liu, Z.; Fang, J.; Dimitrov, N. Composition-Dependent Electrocatalytic Activity of Pt-Cu Nanocube Catalysts for Formic Acid Oxidation. Angew. Chem. 2010, 122, 1304−1307. (67) Kang, Y.; Murray, C. B. Synthesis and electrocatalytic properties of cubic Mn-Pt nanocrystals (nano cubes). J. Am. Chem. Soc. 2010, 132, 7568−7569.

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SYNOPSIS TOC Pt-Ag alloy nanoballoon nanoassemblies (ANBNSs) are synthesized by the galvanic reaction between K2PtCl4 and three-dimensionally (3D) multibranched Ag nanoflowers template. As an efficient electrocatalyst for fomate oxidation in KOH solution, Pt-Ag ANBNSs exhibit a 19.3-fold activity enhancement compared to commercial Pt nanoparticles due to the geometric/electronic effects and porous architectures. In particularly, the preferential dehydrogenation pathway of the FOR and good anti-CO poisoning capability together with 3D self-supported structures, Pt-Ag ANBNSs demonstrate the promising potential in alkaline direct formate fuel cell. Table of Contents Artwork


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Platinum-Silver Alloy Nanoballoon Nanoassemblies with Super

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