Mesoporous AgPdPt Nanotubes as Electrocatalysts for the Oxygen

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Mesoporous AgPdPt Nanotubes as Electrocatalysts for the Oxygen Reduction Reaction Yaoyao Deng, Shuli Yin, Yayuan Liu, Yidong Lu, Xueqin Cao, Liang Wang, Hongjing Wang, Youliang Zhao, and Hongwei Gu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02206 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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ACS Applied Nano Materials

Mesoporous AgPdPt Nanotubes as Electrocatalysts for the Oxygen Reduction Reaction Yaoyao Deng‡,a, Shuli Yin‡,b, Yayuan Liua, Yidong Lua, Xueqin Caoa, Liang Wangb, Hongjing Wang*,b, Youliang Zhao*,a, Hongwei Gu*,a

a

Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,

Chemical Engineering and Materials Science & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China. b

State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology,

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R. China. ‡ Yaoyao Deng and Shuli Yin contributed equally to the work.

KEYWORDS: nanotube; mesoporous nanoshell; trimetallic alloy; electrocatalyst; oxygen reduction reaction

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ABSTRACT: One-dimensional multimetallic nanotubes with mesoporous walls are promising electrocatalysts for fuel cells on account of their distinct morphology and tunable composition. In the present work, we adopt micelle assisted galvanic replacement strategy to synthesize one-dimensional mesoporous AgPdPt nanotubes (AgPdPt MNTs) with uniform morphology and size in aqueous solution. On account of their distinct mesoporous nanotube structure and synergistic effect among different metals, the as-synthesized trimetallic AgPdPt MNTs exhibit enhanced catalytic performance toward the oxygen reduction reaction. This work offers a feasible method to design one-dimensional mesoporous nanotubes with tunable composition for various promising applications.

INTRODUCTION

Fuel cells with low emission and high energy conversion efficiency have attracted widespread attention as clean energy conversion devices.1 Nevertheless, the kinetics of the cathode oxygen reduction reaction (ORR) is very sluggish that vastly restrict the extensive applications of fuel cells.2-3 Therefore, developing efficient catalysts for


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breaking O=O bond is highly desired.4 Although platinum (Pt) remains the best electrocatalyst for the ORR so far, its poor durability and scarcity are still pivotal obstacles to widespread application in fuel cells.5 To resolve these issues, alloying Pt with other metals is a reliable approach to promote the ORR performance and reduce the Pt usage simultaneously.6-10

Introducing other metals into Pt can tune the d-band structure of Pt and weaken adsorption of oxygen species on Pt,11-16 thus improving the stability and activity.17-18 For example, rhombic dodecahedral Pt-Ni nanoframe showed enhanced catalytic activity owing to increased Ni content in the near-surface region and the extended twodimensional sheet structure within the nanoframe.19-20 Zigzag-like PtFe nanowires can greatly promote electrocatalytic performance for the ORR due to their ligand and strain effects.21 Therefore, it is a promising approach to control the composition of alloy nanostructures to optimize catalytic properties.

In addition, the morphology of metal nanostructures has a strong influence on the catalytic properties. Among various morphologies of nanostructures, one-dimensional


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(1D) nanostructure has received increasing attention because its anisotropic structure not only facilitates mass transfer but also avoids dissolution and aggregation of catalysts, which can heavily enhance their catalytic performance.22-30 Moreover, mesoporous nanotube structures can provide sufficient active sites for the ORR.31-38 Therefore, combination of 1D nanotube geometry with mesoporous structure is an efficient method to improve the ORR performance.

Herein, we represent a straightforward approach to prepare mesoporous AgPdPt nanotubes (AgPdPt MNTs) with uniform size and high yield in aqueous solution by coupling micelle template with galvanic replacement reaction. We employ Ag NWs as a self-sacrificial template and F127 as a soft template to synthesize AgPdPt MNTs. Owing to the mesoporous nanotube structure and trimetallic compositions, the obtained AgPdPt MNTs exhibit superior electrocatalytic activity and durability for the ORR to commercial Pt/C catalyst.

EXPERIMENTAL SECTION


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Synthesis of Ag NWs. The synthetic process was based on previously reported literature with minor modification.39 Firstly, 160 mg of PVP (Mw = 55000) and 160 mg of PVP (Mw = 360000) were dissolved in EG (44 mL) under nitrogen atmosphere. Then FeCl3 solution (2.5 mL, 1.2 mM) and AgNO3 solution (6 mL, 60 mg mL-1) were added. The Ag NWs were obtained when the mixture was stirred at 130 oC for 150 min. The obtained Ag NWs were washed and collected by centrifugation. Finally, Ag NWs were dispersed in water for further use (1 mg mL-1).

Synthesis of H-AgPdPt MNTs. In a typical synthesis, Na2PdCl4 (0.06 mmol), K2PtCl4 (0.072 mmol), H2PtCl6 (0.108 mmol) and HCl (1.8 mmol) solution were mixed with F127 (300 mg) under stirring. After complete dissolution, we added the as-prepared Ag NWs (10 mL) and AA solution (15.0 mL, 0.1 M) to the above mixture, and maintained at 50 oC for 3 h. The products were washed and collected by centrifugation, which were denoted as AgPdPt MNTs. Finally, the above products were treated with HNO3 solution for 12 h, which were denoted as H-AgPdPt MNTs.


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Electrochemical investigations. All samples were homodispersed in a mixed solution with a concentration of 2 mg mL-1, and the catalyst ink was dropped onto the glass carbon (GC) electrode with the same Pt loading of 2 μg, followed by coating 3 μL of Nafion (0.5 wt%). Electrochemical testing methods are similar to our previously reported work.40

Scheme 1. The illustration of synthesis of the H-AgPdPt MNT. RESULTS AND DISCUSSION

Scheme 1 exhibits the general preparation strategy of the AgPdPt MNT, in which Ag NWs as a self-sacrificial template decide the morphology of final products. Ag NWs are successfully synthesized with the uniform morphology and the diameter of about 40 nm (Figure S1). Then Ag NWs are mixed with reaction solution under stirring at 50 oC. Through galvanic replacement reaction and reduction reaction, AgPdPt MNTs are


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obtained.34, 41-42 To heighten the availability of Pt on the inner and exterior surfaces, the AgPdPt MNTs are etched under HNO3 for 12 h to obtain H-AgPdPt MNTs.43 Figure 1a shows that uniform H-AgPdPt MNTs have well-developed mesoporous structures on the entire surface of the nanotube. The diameter of the H-AgPdPt MNTs is about 90 nm (Figure 1b). The SEM image (inset in Figure 1b) displays the hollow structure of HAgPdPt MNTs. TEM image (Figure 1c) further demonstrates the hollow structure and the shell thickness is around 25 nm. Moreover, the nanoshells of H-AgPdPt MNTs consist of connected porous structures (Figure 1c). Selected-area electron diffraction (SAED) pattern confirms the polycrystalline nature of the H-AgPdPt MNTs (Figure 1d). The HRTEM image exhibits obvious lattice fringes on MNTs (Figure 1e), suggesting their high crystallinity. In the fast Fourier transform (FFT) image, the lattice distance is measured to 0.22 nm, which is attributed to the (111) plane of AgPdPt alloy (Figure 1f). These results successfully support the synthesis of AgPdPt mesoporous nanotube architecture.


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Figure 1. (a and b) SEM images of the H-AgPdPt MNTs. The inset in (b) displays the SEM image of the cross section of the H-AgPdPt MNTs. (c) TEM, (d) SAED, (e) HRTEM and (f) the lattice fringes and the corresponding FFT pattern image of the HAgPdPt MNTs. The structure and component of the H-AgPdPt MNTs were further investigated. The HAADF-STEM image reveals the hollow and mesoporous structure of the H-AgPdPt MNTs (Figure 2a). In the corresponding element mapping images (Figure 2b-f), the Ag, Pd, and Pt are evenly dispersed throughout the nanotube, suggesting the formation of trimetallic alloy structure. The mass ratio of Ag/Pd/Pt in the H-AgPdPt MNTs is calculated to 12/19/69 from ICP-OES analysis (Table S1). The XRD pattern of the HAgPdPt MNTs shows four obvious diffraction peaks (Figure 3a), which have a slight shift compared with pure Ag (PDF No. 04-0883), Pd (PDF No. 46-1043), and Pt peaks


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(PDF No. 04-0802), indicating the formation of AgPdPt alloy. Besides, the XPS spectra characterize the metallic states of Ag, Pd and Pt in H-AgPdPt MNTs (Figure 3b-d). The above results indicate that the trimetallic H-AgPdPt MNTs are successfully fabricated, which are promising electrocatalyst toward the ORR because of their mesoporous nanotube structure and trimetallic composition.

Figure 2. (a and b) HAADF-STEM and (c-f) EDX elemental mapping images of the HAgPdPt MNTs.


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Figure 3. (a) XRD and (b-d) XPS patterns of the H-AgPdPt MNTs. To explore the possible formation process of the AgPdPt MNTs, the morphology of Ag-Pd-Pt trimetallic nanotubes changing with time are investigated (Figure 4). At the initial stage, we find that the nanowires are gradually converted into hollow structures (Figure 4a and b). With the reaction time prolonging to 60 min, ultrafine nanoparticles are aggregated on the nanotube surface to form rough surface, and these interconnected nanoparticles form the mesoporous walls of nanotubes (Figure 4c). As the reaction time proceeds, mesoporous walls become thicker (Figure 4d-f). Based on the above results, we can speculate the formation process of AgPdPt MNTs. Firstly, Ag NWs are used as a self-sacrificial template to synthesize metallic nanotubes by galvanic


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replacement reactions.44-47 Then, F127 serves as a soft template to generate micelles to bond with metal ions. After reduction by AA, mesoporous structures are formed on the surface of nanotubes.34 Finally, mesoporous AgPdPt nanotubes are successfully fabricated by the HNO3 etching. In the absence of F127, the obtained products have poor nanotube morphology without porous structure (Figure S2). Therefore, the above results indicate that the F127 can serve as the pore-directing agent and protecting agent for generating mesoporous structures and preventing the particle aggregations.34, 48

The redox potentials of the [PtCl4]2−/Pt (0.76 V vs. SHE) and [PtCl6]2−/[PtCl4]2− (0.68 V vs. SHE) are much higher than that of the [AgCl]/Ag (0.22 V vs. SHE).49-51 The galvanic replacement reaction between the Pt precursors and the Ag nanowires spontaneously occurs, resulting in the nanotube structures. Meanwhile, the F127 micelles direct the formation of the mesoporous walls and the addition of AA facilitates the full reduction of the metal ions, which favors to form the AgPdPt MNTs. After partly etching the Ag from the AgPdPt MNTs, the H-AgPdPt MNTs are obtained.


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Figure 4. The morphology of AgPdPt MNTs at different stages: (a) 15, (b) 30, (c) 60, (d) 90, (e) 120, and (f) 180 min. It has been reported that the molar ratios of K2PtCl4/H2PtCl6 have great effect on the morphology of the nanomaterials.52 Figure S3 shows the SEM and TEM images of the samples synthesized by altering the molar ratios of K2PtCl4/H2PtCl6. When K2PtCl4 is used as the only Pt precursor, the average diameter of the nanotube is about 58 nm and there are no obvious porous structures on the nanotube surface (Figure S3a and f). When certain amounts of H2PtCl6 are fed together with K2PtCl4 (K2PtCl4/H2PtCl6 = 8:2 or K2PtCl4/H2PtCl6 = 6:4), the average diameter of the nanotube gradually increases (Figure S3b, c, g and h), and porous structures are began to develop on the nanotube surface. When the molar ratio of K2PtCl4/H2PtCl6 is reduced to 4:6, the obtained sample


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has uniform morphology and well-developed mesoporous structure (Figure 1a-c). Further increasing the amount of H2PtCl6 (K2PtCl4/H2PtCl6 = 2:8 or K2PtCl4/H2PtCl6 = 0:10), the obtained products have poor nanotube morphology with obvious mesoporous structure (Figure S3d, e, i and j). Therefore, the optimized ratio of K2PtCl4/H2PtCl6 facilitates the formation of well-developed mesoporous nanotube structures.

In addition, we also investigated the morphology and composition of bimetallic HAgPd MNTs and H-AgPt MNTs for comparison. The H-AgPd MNTs were prepared without the Pt precursors. As shown in Figure S4a and b, the average diameter of HAgPd MNTs is about 66 nm and there are no obvious mesoporous structures on the nanotube surface. The mass ratio of Ag/Pd in H-AgPd MNTs is calculated to 54:46 (Table S1). Similarly, H-AgPt MNTs were prepared without the addition of Pd precursors. The obtained H-AgPt MNTs possess uniform mesoporous nanotubular structures (Figure S4c and d). The mass ratio of Ag/Pt in H-AgPt MNTs is also measured to 23:77 (Table S1).


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Generally, the structure and composition of the catalysts have strong impact on electrocatalytic activity. The H-AgPdPt MNTs have nanotube structure with mesoporous nanoshells and trimetallic composition, which are expected to be a promising electrocatalyst for the ORR. The ORR performance of H-AgPdPt MNTs, AgPdPt MNTs, H-AgPd MNTs, H-AgPt MNTs, and commercial Pt/C is investigated in acidic electrolyte. The electrochemically active surface areas (ECSA) of different catalysts can be calculated by the obtained cyclic voltammograms (CVs) (Figure S5a). The ECSAs normalized by the mass of Pt for H-AgPdPt MNTs, AgPdPt MNTs, and commercial Pt/C are 54.7, 47.8, and 45.3 m2 g-1, respectively. The higher ECSA of H-AgPdPt MNTs is mainly due to the advantages of structure and composition. Importantly, the ECSA of the H-AgPdPt MNTs is higher than many other Pt-based catalysts, such as Pt1.3Ni NWs (42.5 m2 g-1),53 Pt cubic nanocages (46.8 m2 g-1),54 ultrathin Pt nanoplates (30.6 m2 g1).55

From the CVs in Figure S5b, a distinct cathodic peak can be detected. Comparing

with the other two samples, the peak of H-AgPdPt MNTs has a positive shift, indicating that the H-AgPdPt MNTs is more susceptible to the ORR. From the LSV curves, we can observe that the H-AgPdPt MNTs possess a highest onset potential (Eonset) and half


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wave potential (E1/2) (Figure 5a). The Eonset values of H-AgPdPt MNTs, AgPdPt MNTs, and commercial Pt/C are 0.99, 0.98, and 0.95 V, respectively. The E1/2 values of HAgPdPt MNTs, AgPdPt MNTs, and commercial Pt/C are 0.90, 0.88, and 0.86 V, respectively (Figure 5b). In order to demonstrate the superior ORR performance of the trimetallic H-AgPdPt MNTs, bimetallic H-AgPt MNTs and H-AgPd MNTs are measured for the ORR. Compared with H-AgPdPt MNTs, the H-AgPt MNTs have more negative values of Eonset and E1/2, demonstrating that the addition of Pd can facilitate the improvement of ORR performance (Figure S6). As can be seen from the LSV curves, the ORR performance of H-AgPd MNTs is very poor, indicating the excellent catalytic activity of Pt for the ORR (Figure S6).


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Figure 5. (a) LSV curves, (b) the Eonset and E1/2, (c) Tafel slopes, and (d) specific activities and mass activities of the three catalysts. (e) LSV curves of H-AgPdPt MNT at different RDE rotation rates and electron transfer numbers at corresponding potentials. (f) The electron transfer numbers of H-AgPdPt MNTs and commercial Pt/C. The ORR kinetics of different catalysts can be reflected by Tafel slopes, which were determined from LSV curves (Figure 5c). The Tafel slop of the H-AgPdPt MNTs is 59 mV dec-1, which is lower than that of the AgPdPt MNTs (61 mV dec-1) and the commercial Pt/C (64 mV dec-1). This result indicates that the H-AgPdPt MNTs is easier to adsorb and activate the O2 on the surface. The ECSA-normalized specific activity of


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the H-AgPdPt MNTs at 0.85 V is 1.11 mA cm-2, and it is 1.3 and 1.9 times that of the AgPdPt MNTs (0.84 mA cm-2) and commercial Pt/C (0.57 mA cm-2), respectively (Figure 5d). The mass activity normalized by the Pt loading for the H-AgPdPt MNTs (0.61 mA μg-1) at 0.85 V is 1.5 and 2.3 times as high as that of the AgPdPt MNTs (0.40 mA μg-1) and the commercial Pt/C (0.26 mA μg-1), respectively (Figure 5d). These information reveal the superior ORR activity of the H-AgPdPt MNTs.

LSV curves are commonly used to explore the ORR kinetics and measured at different rotating speeds. From the LSV curves of H-AgPdPt MNTs, we can find that the limiting current density enhances with the rotating speed raising (Figure 5e). KouteckyLevich (K-L) points at 0.4, 0.5, 0.6, and 0.7 V display good linear relation, reflecting that the ORR kinetics of H-AgPdPt MNTs is first order reaction (inset in Figure 5e).56 According to the K-L equation, the electron transfer number (n) of H-AgPdPt MNTs in ORR is determined to 3.97, 3.95, 3.97, and 3.96 at corresponding potentials (Figure 5f). The n values of commercial Pt/C are calculated to 3.94, 3.92, 3.94, and 3.98 at corresponding potentials (Figure S7). All the n values are close to 4, showing that the


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ORR process is a four-electron pathway. The above result also can be further investigated by the RRDE tests (Figure 6a). Comparing with disk current (ID), ring current (IR) that represent the oxidation of H2O2 is negligible, demonstrating that the generation of H2O2 is highly limited by the H-AgPdPt MNTs. The H2O2 percentages of the H-AgPdPt MNTs and commercial Pt/C can be calculated by the IR and ID. The percentage of H2O2 (H2O2%) for H-AgPdPt MNTs (~0.9%) is much lower than that of commercial Pt/C (~3.8%). The n value also can be determined by IR and ID. The n value of H-AgPdPt MNTs is about 3.98, which is more close to 4 than commercial Pt/C (3.94) (Figure 6b). These results reveal that the H-AgPdPt MNTs have better four-electron selectivity than commercial Pt/C for the ORR.


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Figure 6. (a) RRDE tests of ORR and (b) H2O2% and electron transfer number of the HAgPdPt MNTs and commercial Pt/C. (c) LSV curves before and after stability tests for H-AgPdPt MNTs. (d) Chronoamperometric curves of the H-AgPdPt MNTs and commercial Pt/C at 0.5 V. For the sake of estimating the durability of the H-AgPdPt MNTs, long-term stability tests were carried out. After 5000 cyclic tests, the LSV curve of H-AgPdPt MNTs has a minor change compared with the initial curve (Figure 6c). Moreover, the Eonset value almost has no change, the E1/2 value only decreases by 11 mV, and the limiting current density only decreases by 0.11 mA cm-2. For comparison, the commercial Pt/C has great drops in the Eonset, E1/2, and limiting current density after 5000 cyclic tests (Figure S8). Furthermore, chronoamperometric tests are also used to investigate the durability of H-AgPdPt MNTs and commercial Pt/C. After 5 h stability test at 0.5 V, the H-AgPdPt MNTs remain 88.5% of its initial current density (Figure 6d), which is superior to the commercial Pt/C (59.8%). The above consequences demonstrate that the H-AgPdPt MNTs possess stronger long-term stability toward ORR.

From the above, the H-AgPdPt MNTs exhibit excellent catalytic activity and durability


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toward ORR that are greatly relative to their unique structure and trimetallic characteristics. The mesoporous nanotube structure not merely provides accelerated mass transfer and low diffusion resistance, but also enhances electrocatalytic surface area and active sites. Trimetallic structure can tune the electronic state of each metal, which is beneficial to improve the interaction between active sites and the oxygen species, therefore enhancing the ORR performance. The H-AgPdPt MNTs have superior ORR performance to the AgPdPt MNTs, which is mainly due to the fact that more active sites can be generated on the inner walls by partly etching Ag and Pd.

CONCLUSIONS

In summary, we have successfully synthesized one-dimensional mesoporous AgPdPt nanotubes (H-AgPdPt MNTs) in aqueous solution via micelle assisted galvanic replacement reaction, in which F127 micelles favor the formation of porous surface and the Ag nanowires is used as a self-sacrificial template to direct the nanotube shape. On account of the mesoporous nanotube structure and trimetallic characteristics, the obtained H-AgPdPt MNTs exhibit improved electrocatalytic properties toward the ORR.


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Therefore, the proposed micelle assisted galvanic replacement strategy is meaningful to achieve the metallic nanotube with porous surface for all kinds of hopeful applications.

ASSOCIATED CONTENT

Electronic Supplementary Information (ESI) available: Additional characterization data and electrocatalytic performance data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected], [email protected]

ORCID

Liang Wang: 0000-0001-7375-8478

Hongjing Wang: 0000-0003-0641-3909

Youliang Zhao: 0000-0002-4362-6244


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Hongwei Gu: 0000-0001-9962-4662

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (No. 21373006, 21601154, 21776255), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201708), the Science and Technology Program of Suzhou (SYG201732), the Priority Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES (1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. (2) Jiang, G.; Zhu, H.; Zhang, X.; Shen, B.; Wu, L.; Zhang, S.; Lu, G.; Wu, Z.; Sun, S. Core/Shell Face-Centered Tetragonal FePd/Pd Nanoparticles as an Efficient Non-Pt Catalyst for the Oxygen Reduction Reaction. ACS Nano 2015, 9, 11014-11022. (3) Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594-3657.


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Mesoporous AgPdPt Nanotubes as Electrocatalysts for the Oxygen

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