Self-Supporting Hierarchical Porous PtAg Alloy Nanotubular Aerogels

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Self-supporting Hierarchical Porous PtAg Alloy Nanotubular Aero-gels as Highly Active and Durable Electrocatalysts Wei Liu, Danny Haubold, Bogdan Rutkowski, Martin Oschatz, Rene Huebner, Matthias Werheid, Christoph Ziegler, Luisa Sonntag, Shaohua Liu, Zhikun Zheng, AnneKristin Herrmann, Dorin Geiger, Bürgehan Terlan, Thomas Gemming, Lars Borchardt, Stefan Kaskel, Aleksandra Czyrska-Filemonowicz, and Alexander Eychmueller Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01394 • Publication Date (Web): 29 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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Chemistry of Materials

Self-supporting Hierarchical Porous PtAg Alloy Nanotubular Aerogels as Highly Active and Durable Electrocatalysts Wei Liu,*[a] Danny Haubold,[a] Bogdan Rutkowski,[b] Martin Oschatz,[c] René Hübner, [d] Matthias Werheid,[a] Christoph Ziegler,[a] Luisa Sonntag,[a] Shaohua Liu, [a] Zhikun Zheng, [a] AnneKristin Herrmann,[a] Dorin Geiger,[f] Bürgehan Terlan,[a] Thomas Gemming,[e] Lars Borchardt,[c] Stefan Kaskel,[c] Aleksandra Czyrska-Filemonowicz,[b] Alexander Eychmüller*[a] [a] Physical Chemistry and Center for Advancing Electronics Dresden (cfaed), TU Dresden, Bergstrasse 66b, 01062 Dresden, Germany [b] Faculty of Metals Engineering and Industrial Computer Science & International Centre of Electron Microscopy for Material Science, AGH University of Science and Technology, Al. A. Mickiewicza 30, 30-059 Kraków, Poland [c] Inorganic Chemistry, TU Dresden, Bergstrasse 66, 01062 Dresden, Germany [d] Helmholtz-Zentrum Dresden - Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstraße 400, 01328 Dresden, Germany [e] IFW Dresden, Helmholtzstraße 20, 01069 Dresden [f] Electron Microscopy Group of Materials Science, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm KEYWORDS: Aerogel, Nanotube, Porous Nanostructure, Fuel Cell, Formic Acid Oxidation

ABSTRACT: Developing electrocatalysts with low cost, high activity, and good durability is urgently demanded for the wide commercialization of fuel cells. By taking advantage of nanostructure engineering, we fabricated PtAg nanotubular aerogels (NTAGs) with high electrocatalytic activity and good durability via a simple galvanic replacement reaction between the in situ spontaneous gelated Ag hydrogel and the Pt precursor. The PtAg NTAGs have hierarchical porous network features with primary networks and pores from the interconnected nanotubes of the aerogel and secondary networks and pores from the inter-connected thin nanowires on the nanotube surface, and show very high porosities and large specific surface areas. Due to the unique structure, the PtAg NTAGs exhibit greatly enhanced electrocatalytic activity towards formic acid oxidation, reaching 19 times higher metal-based mass current density as compared to the commercial Pt black. Furthermore, the PtAg NTAGs show outstanding structural stability and electrochemical durability during the electrocatalysis. Noble metal based NTAGs are promising candidates for applications in electrocatalysis not only for fuel cells, but also for other energy-related systems.

Nanostructure engineering plays a vital role in nanotechnology. Porous metallic nanomaterials have attracted considerable interest in both research and industry, among which metallic aerogels are very promising materials due to their large surface areas and high porosities.1,2 Up to now, the construction of metallic aerogels with metal backbones has been realized by nanosmelting of hybrid polymer−metal oxide aerogels or by the direct sol−gel method.2-15 For example, Leventis obtained Fe, Co, Ni, Sn, and Cu aerogels by using the nanosmelting method, and some of them show promising applications as explosives and thermites.3-6 However, the precise control of the fine nanostructure and composition of the aerogels by this method still remains limited. Recently, our group developed direct sol−gel methods, either via a two-step gelation process or via a spontaneous one-step gelation process, and prepared mono- or multimetallic aerogels of various compositions containing Pt, Pd, Au, Ag and Ni.2,7-13 These metallic aerogels have three-dimensional (3D) nanowire or nanochain network-

like morphologies, and have very large specific surface areas and high porosities. Notably, they have shown excellent performances as electrocatalysts with both high catalytic activity and durability for alcohol oxidation8,13 and oxygen reduction reaction (ORR)9 which are important reactions in fuel cells or metal-air batteries. The outstanding performances of metal aerogels in the aforementioned applications stimulate great interest in enriching the diversity of the metallic aerogels where nanostructure engineering may play an eminent role and in extending their applications. The wide commercialization of fuel cells requires developing electrocatalysts taking into account their durability alongside their cost and activity.16,17 However, this still remains a great challenge. Beside our efforts discussed above, improving the activity and durability of electrocatalysts can be realized via the following routes: 1) by manipulating the structure at the atomic level via constructing Pt-skin, core-shell or alloy structures, or

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hybrid metal heterostructures;16,18-23 2) by synthesizing unconventionally shaped Pt nanocrystals with welldefined and high-index facets;24,25 3) by using supporting materials more corrosion-proof than carbon or forming self-supporting catalysts;17,26 4) by creating one dimensional (1D) materials like nanowires (NWs) and nanotubes (NTs), or fabricating comprehensive materials combining multiple length scales.20,27-31 For example, Pt nanotubes20 and Pt3Ni nanoframes29 have shown outstanding electrocatalytic activity and durability as ORR catalysts, owning to their unique characteristics such as 1D nanotubular morphology, 3D molecular accessibility, and self-supporting structure. Despite this, the extension of these favorable nanostructures and correspondingly transferring/magnifying their excellent activities into macroscale via forming a metal aerogel remains unexplored. Herein, we aim to take the advantage of nanostructure engineering and integrate 1D metal NWs with the NT geometry and further extend this to 3D aerogel networks to form hierarchical porous nanotubular metallic aerogels. PtAg nanotubular aerogels (NTAGs) have been generated via the facile galvanic replacement between the in situ spontaneous gelated Ag hydrogel and the Pt precursor H2PtCl6. The PtAg NTAGs display high porosities and large specific surface areas, and have shown excellent electrocatalytic activity toward formic acid oxidation and long lasting electrochemical and structural durability.

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veal that the PtAg NTAGs are highly crystalline and point to a face-centered cubic (fcc) polycrystalline structure. Xray powder diffraction (XRD) characterization confirms the fcc crystalline structure of the NTAGs (Figure S4). There is only one set of fcc diffraction patterns for both the PtAg NTAGs, their respective reflection positions shift towards smaller angles and their unit cell parameters are larger with respect to the Pt black, indicating the alloying of Pt with Ag i.e. a lattice expansion due to alloying.32 Additionally, a set of sharp reflections is also observed in the PtAgHCl NTAG, indicating the presence of a cubic of AgCl (JCPDS file: 31-1238) in it.33

Scheme 1. Schematic illustration on the synthesis of the PtAgHCl and PtAgNH4OH nanotubular aerogels. A schematic illustration on the preparation of the PtAg NTAGs is given in Scheme 1. Figure 1 and Figure S1-S2 show scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the Ag aerogel and the PtAgHCl and PtAgNH4OH NTAGs. In contrast to the solid nanowire interconnected network structure of the Ag aerogel, the PtAgHCl and PtAgNH4OH NTAGs show both hierarchical porous structures and hierarchical network structures. The nanotubes (shell thickness of ~ 7-10 nm) are interconnected with each other, forming the primary network of the PtAg NTAGs. On the nanotube surface some very thin nanowires (diameter of ~ 2.6 nm) are interconnected with each other, forming a secondary network (Figure 1 (j) (k) and Figure S2). The pores of the PtAg NTAGs are composed of hollow cavities of the nanotubes, very small pores surrounding the secondary network on the nanotube surface, and the widely distributed pores from the primary network of the aerogel. High resolution TEM (HRTEM) images and the corresponding Fast Fourier Transform (FFT) (Figure S3) re-

Figure 1. SEM (a, d, g) and TEM images (b, c, e, f, h, i, j, k) at different magnifications of the Ag aerogel (a-c), PtAgHCl (d-f, j) and PtAgNH4OH (g-i, k) NTAGs.


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Figure 2. STEM-HAADF images (a, f) and Ag, Pt and Cl STEM-EDX elemental maps (b-e, g-j) of the PtAgHCl (a-e) and PtAgNH4OH (f-j) NTAGs. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) as well as energy-dispersive X-ray spectroscopy (EDX) were further utilized to characterize the spatial elemental distribution in the PtAg NTAGs (Figure 2 and Table S1). The PtAgHCl NTAG is composed of Pt, Ag, and Cl distributed over the entire nanotubular networks, confirming the XRD results in that the Pt and Ag form an alloy. The average contents of Pt, Ag, and Cl of the PtAgHCl NTAG determined by STEM from the tubular image center positions (position 3-6 in Figure 2 (b) and Table S1), which comprise signals from both the upper and the bottom side of the 3D tubular structure due to the 2D projection in STEM-EDX analysis, are 58 at%, 35 at%, and 7 at%, respectively. The average contents of Pt, Ag, and Cl of the outer layer positions (position 7-12 in Figure 2 (b) and Table S1) of the tubular structure are 53 at%, 39 at%, and 8 at%, respectively. The Ag contents on these positions are a little higher than those for the tubular image center positions. Overall the distribution of Ag and Cl is not fully uniform as it shows a higher content at some localized positions where less Pt is found (e.g. position 1, Pt 38 at%, Ag 43 at%, Cl 19 at%). This indicates the existence of AgCl residues on the PtAgHCl nanotube surface with a certain degree of accumulation at some localized positions. The deviations from the otherwise uniform element distribution in the PtAgHCl NTAG could be due to the formation of the unique chemical structure of this aerogel under the chosen synthetic conditions. For the PtAgNH4OH NTAG, the contents of Pt, Ag and Cl are 70 at%, 21 at%, and 9 at% on the tubular image center positions (positions 1-9 in Figure 2 (g) and Table S1), respectively. The average contents of Pt, Ag, and Cl of the outer layer positions (positions 10-14 in Figure 2 (g) and Table S1) of the tubular structure are 77 at%, 17 at%, and 6 at%, respectively. The relatively uniform distribution of Pt and Ag together with the XRD results indicates the formation of PtAg alloy also in this aerogel. From the macroscopic point of view, as seen from the photographs during the PtAg aerogel preparation in Figure S5, despite the huge macroscale 3D porous network structure, the galvanic replacement reaction took place uniformly around the whole hydrogel, and no hetero-nanostructure formed independent of the reaction time, the amount of H2PtCl6 and the shape of the Ag hydrogel, which is different from the CdSe/Ag2Se gel of

hetereo-nanostructures produced by partial cation exchange.34 The specific surface area and porosity of the PtAg NTAGs, the Ag aerogel, and the commercial Pt black were determined from N2 physisorption isotherms. As shown in Figure 3 (a), the isotherm of the Ag aerogel is a combination of a type II and a type IV curve, similar to those for the metallic aerogels reported earlier.2,8,9 For the PtAg NTAGs, the isotherms are also a combination of a type IV and a type II curve, what is different is that they both show a large H2 type hysteresis loop typical for cage-like porous structures.35,36 The distinct adsorption of N2 of the PtAgNH4OH NTAG at low relative pressure indicates the presence of narrow pores whereas the uptake at high relative pressures results from aerogeltype pore systems. Such shape of the adsorption isotherm is typical for aerogels with hierarchical pore structure.37 The enlarged hysteresis loop results from pore blocking phenomena,38 leading to delayed desorption of the nitrogen adsorbed in the hollow structures of the NTAGs by the narrow pores into the nanotube shells. In such a case, the size distribution of the hollow cages is to be calculated from the adsorption branch and the size distribution of the smaller pores around them can be calculated from the desorption branch of the isotherms. As shown in Figure S6, the cummulative Barrett-Joyner-Halenda (BJH) pore size distributions calculated from the adsorption branches of the isotherms indeed show a step increase in the cummulative pore volume at 25-50 nm for the PtAg NTAGs. These sizes correspond to the inner hollow areas of the nanotubes, being in good agreement with TEM investigations. Smaller pores are also detected but without significant contribution to the cummulative pore volume. In contrast, the Ag aerogel and the Pt black do not show such a step increase in the pore size since their porosity arises exclusively from the primary network. As shown in Figure 3 (b, c) of the BJH pore size distributions of the desorption branches, the PtAgHCl and NTAGs exhibit considerably the PtAgNH4OH



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NTAG exhibits only one dominant pore size within the shells of the cavities at about 7.3 nm (Figure 3 (c)). The cumulative pore volume and the ABET of the PtAgNH4OH NTAG are much higher than those of the PtAgHCl NTAG. These differences combined with the STEM-EDX results indicate that the AgCl residue in the PtAgHCl NTAG most probably covers preferentially the small pores on the nanotube surface of this aerogel. The content of Ag before and after the HNO3 treatment of the PtAgHCl NTHG did not change, but was reduced by ~ 32% for the PtAgNH4OH NTHG (Table S2), further confirming the above assumption and implying that the AgCl residue may form a protective layer in the PtAgHCl NTAG. The ABET are 24.7 m2 g-1, 83.4 m2 g-1, 10.0 m2 g-1, and 30.8 m2 g-1 for the PtAgHCl NTAG, the PtAgNH4OH NTAG, the Ag aerogels, and the Pt black, respectively. The much lower ABET of the as compared to PtAgNH4OH NTAG is likely resulting from the blocking of the smaller pores by the AgCl residue. In view of such blocking phenomena, accssibility limitations of the nitrogen molecules to the hollow interior of the PtAgHCl NTAG can not be ruled out. Despite the presence of AgCl residue, the ABET of the PtAgHCl NTAG is, however, significantly higher as compared to the Ag aerogel.

Figure 3. N2 physisorption isotherms (a), pore size distribution (b, c) evaluations from the desorption branches of the isotherms of the Ag aerogel, PtAgHCl and PtAgNH4OH NTAGs, and the Pt black. more mesopores in the range of 3-20 nm than the Ag aerogel. The size of these pores corresponds to the shells surrounding the hollow cavities. According to its rather dense structure, the Pt black does not contain pores with this specific size. The pore volumes of the PtAgHCl (0.165 cm3 g-1) and the PtAgNH4OH NTAGs (0.300 cm3 g-1) are much higher than that for the Ag aerogel (0.086 cm3 g-1). These results are consistant with the hollow nanotubular structure of the NTAGs and confirm their hierarchical pore architecture. Despite the similar morphologies of the PtAgHCl and the PtAgNH4OH NTAGs, there are obvious differences between them considering the pore size distributions, the cumulative pore volumes, and the Brunauer–Emmett–Teller specific surface areas (ABET). The PtAgNH4OH NTAG has a higher volume of mesopores than the PtAgHCl NTAG (Figure 3 (c) and Figure S6 (b)), and it exhibits two dominant pore sizes at about 4.3 nm and 5.6 nm, while the PtAgHCl

The electrocatalytic activity of the PtAg NTAGs was tested with the model reaction of formic acid oxidation. Commercial Pt black was also investigated for comparison. Before that, all the modified electrodes were firstly cycled in 0.1 M HClO4 in the range of -0.23 V to 1.20 V (vs Ag/AgCl) under N2 until steady-state electrode surfaces were obtained. As shown in Figure S7, the anodic peak at above 0.7 V for the PtAg NTAGs rapidly decayed with an increasing number of cyclic voltammetry (CV) scans, which is associated with the oxidative dissolution/leaching (voltammetric dealloying) of Ag from the alloyed PtAg NTAGs. At the same time, the cathodic peak associated with the reduction of oxidized Pt and Ag decayed and shifts positively by 80 mV and 110 mV for the PtAgNH4OH and PtAgHCl NTAGs, respectively, which suggests that the dealloying process is accompanied by a rearrangement of Pt atoms at the metal surface with the increase of the number of CV scans.39 After some cycles, steady-state electrode surfaces were obtained and the voltammetric features of the PtAg NTAGs are very similar to that of the Pt black.40 For the PtAgHCl NTAG, the first and the following few scans show a more complex behavior than those for the PtAgNH4OH NTAG. The peak at 0.61 V, which appeared only in the first scan can be assigned to the physisorbed AgCl residue as discussed above. In the case of PtAgNH4OH, only a very small amount of AgCl exists in the sample, and correspondingly, no such peak was observed in the respective CVs. TEM, HAADF-STEM imaging, STEM-EDX as well as SEM-EDX were further utilized to monitor the morphologies and compositions of the PtAg NTAGs after the electrochemical treatment. The hierarchical porous nanotubular 3D network structure, the size of the secondary nanowires and the alloy state of Pt and Ag in both of the PtAg NTAGs remained but the chemical compositions changed dramatically (Figure S8 and S9, Table S3 and S4). For the PtAgHCl NTAG, the elemental contents changed to 88


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at% Pt, 9 at% Ag, 3 at% Cl on the outer layer positions (positions 1-9 in Figure S9 (b)) and 87 at% Pt, 10 at% Ag, 3 at% Cl on the tubular image center positions (positions 10-13 in Figure S9 (b)). As compared with the original PtAgHCl NTAG (Figure 2 (b) and Table S1), the Pt content increased by about 29 (35) at%, while the Ag content decreased by around 25 (30) at%, and Cl decreased around 4 (5) at% on the center (outer) positions. In general the elements became more uniformly distributed. In the case of the PtAgNH4OH NTAG, the element contents are also 88 at% Pt, 9 at% Ag, 3 at% Cl on the outer layer positions (positions 1-9 in Figure S9 (g)); and 89 at% Pt, 8 at% Ag, 3 at% Cl on the tubular image center positions (positions 10-13 in Figure S9 (g)). The Pt content increased by about 19 (11) at%, the Ag content decreased by around 13 (8) at%, and Cl decreased by around 6 (3) at% on its center (outer) positions, as compared with the original PtAgNH4OH NTAG (Figure 2 (g) and Table S1). These results show that the electrochemical treatment leads to electrochemical cleaning of the PtAg NTAG surfaces (including the removal of the AgCl residues), to dealloying of Ag from the aerogel, and to the rearrangement of Pt and Ag atoms at the metal surface. The electrochemical active surface area (ECSA) before electrocatalysis estimated from the 25th cycle in 0.1 M HClO4 for the PtAgHCl (19.4 m2 g-1metal) and PtAgNH4OH (14.9 m2 g-1metal) are 6.9 times and 5.3 times that of the Pt black (2.8 m2 g-1metal). After the electrochemical treatment, the catalyst modified electrodes were utilized for the electrocatalytic oxidation of formic acid. As is known, the formic acid oxidation can take place via dual pathways, namely the direct oxidation to CO2 by a dehydrogenation reaction (equation S1, reflected as f1 peak) and the indirect oxidation by a dehydration reaction via the poisoning CO intermediate (equation S2, reflected as f2 peak).21,41-43 As shown in Figure 4 and Table S5, the metal-based mass current density jf1 of the PtAgHCl and PtAgNH4OH NTAGs are 19.0 and 9.0 times and the jf2 are 4.5 and 7.9 times higher than those of the Pt black, respectively, suggesting that both NTAGs have much higher electrocatalytic activities toward the formic acid oxidation than the Pt black. The jf1/jf2 for the PtAg NTAGs are also higher than that for the Pt black, indicating that the formation of the poisoning intermediate COads is more suppressed on the nanotubular aerogels, probably due to the so-called third-body effect.21,41,43 The small fraction of surface Ag (third body) reduces the number of ensembles of adjacent Pt atoms (the adsorption sites for CO) due to geometrical hindrance and correspondingly the surface is less poisoned by the COads than the pure Pt surface. Compared with the reported data, as seen from Table S6, the PtAg NTAGs show higher activity than cataysts such as Pt nanotubes, Pt hollow nanospheres or Pt nanocrystals44-46 and are among the most active Pt containing catalysts for formic acid oxidation, benefiting from their unique hierarchical porous nanotubular network structure.

Figure 4. (a) The forward 30th CV scans of the electrodes modified with the PtAgHCl NTAG, PtAgNH4OH NTAG, and Pt black in 0.1 M HClO4 – 0.5 M HCOOH in the potential range of -0.20 V to 0.95 V (vs Ag/AgCl) under N2. Scan rate: 50 mVs-1. ‘|—|’ shows the standard deviation. (b) Chronoamperometry curves for formic acid oxidation at 0.18 V (vs Ag/AgCl) on the modified electrodes after 40 CV scans in 0.1 M HClO4–0.5 M HCOOH under N2. The long-term stabilities of the modified electrodes were investigated by continuous CV scans and by chronoamperometry. As shown in Figure S10, the catalytic currents of the PtAgHCl and PtAgNH4OH NTAGs for the 1st CV scan are 30.5 and 21.2 times higher than that of the Pt black, respectively. After that the catalytic current decreases quickly in the first 15 scans. Despite the initial faster decrease rate than that of the Pt black, the decrease slows down considerably with increasing number of scans. Over the entire range of CV scans the catalytic currents of the PtAgHCl and the PtAgNH4OH NTAGs remain at least 13.2 times and 8.5 times that of the Pt black, respectively. As for the chronoamperometry test (Figure 4 (b)), both the PtAg NTAGs exhibit much higher initial polarization current densities and steady-state current densities than the Pt black electrode does. Notably, the current density ratios between the PtAg NTAGs and the Pt black also increase with the increase of test time (Table S7). These results further confirm the higher catalytic activity and better tolerance to poisonous intermediate species of the PtAg NTAGs toward formic acid electrooxidation than the Pt black. TEM was further utilized to monitor the structural stabilities of the catalysts during the electrocatalysis



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(Figure S11). The hierarchical porous nanotubular 3D network structure and the nanocrystallites on the secondary network of the tube surface of the PtAgHCl NTAG remained unaltered after 200 CV scans for the HCOOH electrooxidation, showing its excellent structural stability. As for the PtAgNH4OH NTAG, the hierarchical porous nanotubular 3D network structure was retained, too, but the size of the nanocrystals in the secondary network on the tube surface increased slightly from ~ 2.6 nm to ~ 3.9 nm and the size of the small pores increased obviously, indicating the existance of atom diffusion/Ostwald ripening during the electrocatalysis of this aerogel. The reason why the PtAgNH4OH NTAG shows lower structural stability than that of the PtAgHCl NTAG remains however elusive. In the case of Pt black, agglomeration is well observed both before and after electrocatalysis. When compared with PtAgNH4OH NTAG, as discussed above, the PtAgHCl NTAG contains less Pt and a higher content of AgCl, shows higher chemical corrosionresistance in HNO3 during the synthesis (Table S2), a lower ABET but a higher ECSA after the electrochemical treatment, a higher electrocatalytic activity, and improved structural stability (Figure S10). The reason of this behavior might be more complicated, but could be related to the presence of Cl (AgCl) caused by different synthesis strategies (using NH4OH or HCl to remove the AgCl byproduct). The outstanding electrocatalytic activity of the PtAg aerogels toward formic acid oxidation and their excellent durability are supposed to be attributed to the following aspects. Firstly, the hierarchical porous nanotubular aerogel structure, which combines the secondary nanowire networks with a nanotubular geometry and extends to the macroscale aerogel network, provides the aerogel with favorable active sites due to the small size of the secondary network and the existence of many defects and strained sites. Secondly, the interconnected hierarchical porous nanotubular 3D network structure endows these aerogels with a large porosity, which allows for efficient mass transport through the pores and easy access to active sites.47 Furthermore, the combination of dimensions in multiple length scales and the extended surface area characteristics inhibit the dissolution and migration of the metal atoms in the aerogel as well as Ostwald ripening of the nanostructures and lead to the high durability of the aerogels in the electrocatalysis. Finally, due to the incorporation of Ag, the number of ensembles with adjacent Pt atoms is decreased and, consequently, unfavorable dehydration pathways of the formic acid oxidation are suppressed.41 In this work, we demonstrate an effective strategy for designing (electro)catalysts with both good activity and durability, by combing the 1D metal nanowire structure with a nanotube geometry and further extension into 3D aerogel networks. PtAg nanotubular aerogels with hierarchical porous network structures were fabricated by the simple galvanic replacement reaction between the Ag hydrogel and the Pt precursor. The PtAg nanotubular aerogels show excellent electrocatalytic activities toward the formic acid oxidation, with the metal-based mass

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current densities reaching 19 times that of the commercial Pt black, and exhibit outstanding electrochemical and structural durability during the electrocatalysis. This catalyst design strategy represents a promising direction to develop highly active and stable catalysts, and is of great importance in unfolding their practical applications in fuel cells, and other energy conversion and storage devices.

ASSOCIATED CONTENT Supporting Information. Description of the Experimental methods, results of the HRTEM, and XRD investigations as well as TEM number averaged size distribution and the morphology change of the tested catalysts before and after electrocatalysis, additional electrochemical data, Tables for the contents of different components in the aerogels obtained by STEM-EDX and SEM-EDX and Tables for the summary of electrochemical results are collected in the SI. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail for A.E.: [email protected]. *E-mail for W.L.: [email protected].

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. /

Funding Sources

ACKNOWLEDGMENT We acknowledge the support from the European Research Council (ERC-2013-AdG 340419 AEROCAT), DFG-SNF (EY 16/18-1), DFG (EY 16/10-2, EY 16/18-1 and GRK 1401), and DFG within the Cluster of Excellence“Center for Advancing Electronics Dresden”(cfaed). The research leading to the STEM-HAADF and STEM-EDX results has received funding from the European Union Seventh Framework Programme under Grant Agreement 312483 ESTEEM2 (Integrated Infrastructure Initiative-I3). Support by the Structural Characterization Facilities Rossendorf at IBC is gratefully acknowledged.

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Self-Supporting Hierarchical Porous PtAg Alloy Nanotubular Aerogels

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