Ni Foam: an Efficient Binder-Free

Aug 9, 2018 - Hongjing Wang , Hongjie Yu , Shuli Yin , You Xu , Xiaonian Li , Hairong Xue , and Liang Wang. ACS Sustainable Chem. Eng. , Just Accepted...
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Integrated Mesoporous PtPd Film/Ni Foam: An Efficient Binder-Free Cathode for Zn−Air Batteries Hongjing Wang, Hongjie Yu, Shuli Yin, You Xu, Xiaonian Li, Hairong Xue,* and Liang Wang* State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, 18 Chaowang Road, Hangzhou, Zhejiang 310014, P. R. China

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ABSTRACT: Constructing self-standing all-metal mesoporous electrodes is very important for energy storage and energy conversion. Herein, we report the in situ fabrication of a continuous mesoporous PtPd film on macroporous Ni foam (mPtPd-NF) by a very simple and efficient soaking method. The mPtPd-NF shows enhanced catalytic activity and high durability for oxygen reduction reaction, benefiting from its self-standing all-metal mesoporous structure and bimetallic compositions. The mPtPd-NF, as a binder-free cathode for Zn−air batteries (ZABs), exhibits superior battery performance. The proposed approach is truly facile for directly fabricating mesoporous metal film on the metal substrate, compared with previous complex methods (i.e., dealloying or template methods). This newly developed one-step method is very powerful for constructing mesoporous all-metal materials with controllable compositions and desired performances on a large scale for various electrocatalytic applications. KEYWORDS: In situ fabrication, Bimetallic PtPd, Mesoporous film, Catalyst, Zn−air batteries

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INTRODUCTION

To fabricate the mesoporous metals, intensive research efforts have been devoted for the past few years.19−21 The dealloying method is a traditional approach for the preparation of the mesoporous metals by selectively dissolving a lesser metal from a bi- or multialloy.22,23 However, it is difficult to control the size and distribution of the mesopores and construct the robust mesopore walls. Alternatively, another general route for the synthesis of mesoporous metals is the template method (i.e., hard and soft template methods).24,25 As a common hard template, mesoporous silica has been used to prepare mesoporous metals, which is based on direct replication from the template.26,27 Lyotropic liquid crystals (LLCs) are used as alternative soft templates, formed by the surfactant (and/or polymers) with very high concentrations.28 Both hard and soft template methods involve multiple steps are very complex, and the removal of the template may destroy the mesoporous structure. Furthermore, it is difficult to directly fabricate mesoporous metals on the substrate with these template methods. In the preparation process for the conventional electrode in ZABs, an additive polymer binder is used to bond the catalyst on the current collector, which increases the electrode’s weight and charge-transfer resistance.29−31 A binder-free electrode for ZABs is of a great advantage.32−34 Therefore, it is necessary to develop an

Oxygen reduction reaction (ORR) is a key cathodic process in Zn−air batteries (ZABs), which determines their electrochemical performance.1−4 As a matter of fact, the highly efficient catalysts are strongly required because of the intrinsically sluggish ORR kinetics.5 Pt-based materials are known to act as the ideal choices for the catalysts toward ORR.6,7 Up to now, depletable reserves, high cost, and poor stability for catalytic activity still severely limit the Pt-based ORR catalysts’ large-scale application.8,9 To break through these bottlenecks, developing Pt-based alloy catalysts is an effective strategy. The utilization efficiency of the expensive Pt can be greatly improved by the incorporation of other metal into Pt.10−12 More importantly, the alloying contribution can modulate the electronic configuration of Pt, improving the activity and stability for ORR.13,14 Also, creating extended mesoporous architectures is another effective approach. Owing to the large surface areas, applicable pore volume, and metallic framework, mesoporous metals have been considered as promising electrocatalysts with high catalytic performance for ORR.15−17 The Chen group reported a PtPd alloy porous nanorod, which shows high catalytic activity and durability for ORR.18 Its mesoporous structure can not only observably increase the surface area but can also reduce the content of precious metals. Therefore, integrating all-metal Pt-based alloy nanomaterials with mesoporous structures is expected to obtain high-performance ORR catalysts for ZABs. © XXXX American Chemical Society

Received: June 15, 2018 Revised: August 8, 2018 Published: August 9, 2018 A

DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION The self-standing all-metal mesoporous mPtPd-NF are prepared through a very simple method under ambient tempreture. The Ni foam are soaked in a reaction mixture aqueous solution, which serves as a reductant and a substrate (Figure 1). As observed, the original macroporous structure of

effective approach to construct an integrated binder-free cathode for ZABs, in which the mesoporous metallic catalyst is directly coated on the current collector. Recently, as an important advance, we have successfully synthesized various mesoporous metallic films on a flat substrate (e.g., gold film and ITO glass), such as Pt, PtPd, and Pt/Pd films, by using electrodeposition.35−37 This developed synthesis method is based on the concept of the electrochemical micelle assembly strategy (EMAS). Even though the mesoporous metallic film/support composite can be obtained, the support only serves as a substrate in the electrodeposition process. Moreover, the gold film and ITO glass are not the ideal choices for a current collector in a battery application. It would be very interesting if the substrate possesses multiple functions, which can act not only as a current collector for an electrode but also as a reductant for in situ preparation of mesoporous metal films. On the basis of the above concept, inexpensive and porous Ni foam is a promising candidate. As a current collector, the porous structure and high conductivity of Ni foam are very suitable features for application in electrodes for ZABs. Importantly, Ni foam has a low reduction potential, which can reduce the precious metal (i.e., Pt and Pd) ions by a galvanic replacement reaction. Therefore, it is possible to explore an in situ micelle assembly method to construct all-metal mesoporous films on a metal substrate. Herein, a new, very simple, and efficient route based on an in situ micelle assembly method is proposed for synthesizing mesoporous PtPd film on Ni foam (mPtPd-NF). The obtained mPtPd-NF shows enhanced activity and excellent durability for ORR. As a binder-free electrode, the mPtPd-NF exhibits high power and energy density, large specific capacity, and superior stability for ZABs.

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Figure 1. Schematic illustration of the fabrication for mPtPd-NF.

Ni foam remains after the fabrication, which is covered with a uniformly conformal coating formed by a large-scale mPtPd film (Figure S1). From the top view, the uniformly distributed mPtPd film without any cracks or defects can be clearly seen on the top surface (Figure 2a). It can be found from the crosssectional view that the entire film (∼580 nm in thickness) uniformly distributes the mesopores (Figure 2b). The mesopores with a uniform pore size distribution (15 nm in diameter) are separated by thin pore walls (15 nm in thickness), resulting in a high surface area and thus providing plenty of exposed active sites (Figure 2c and d). The nitrogen adsorption−desorption isotherms and pore size distribution curve of the mPtPd film further confirm its mesoporous structure (Figure S2). It is noted that the pore wall consists of numerous nanoparticles with high crystallization, evidenced by their clear lattice fringes (Figure 2e). The metallic polycrystalline characteristics of the mesopores film are confirmed by its SAED pattern (inset in Figure 2c). The constant interval of these lattice fringes is 0.23 nm, which are assigned to the (111) plane of a PtPd alloy with an fcc structure (inset of Figure 2e). The elemental mappings further clearly confirm the mPtPd film’s alloy form (Figure 2f), whose atomic ratio for Pt/Pd is 54.4/45.6 (Figure S3). There are five obvious diffraction peaks of the mPtPd film on the XRD pattern, which are attributed to the metallic fcc structure of PtPd crystal diffractions (Figure 3a). No additional diffraction peaks can be found, which suggests no undesired phase separation of this well-alloyed single-phase mPtPd film. These slightly shifting diffraction peaks are between the standard diffraction peaks of pure Pt and Pd, which implies the formation of a lattice contraction after introducing the Pd atoms into Pt. XPS measurements are carried out to further explore the valence states and surface compositions of the mPtPd film (Figure 3b). As shown in Figure 3c, two strong peaks of the Pt 4f spectrum at 75.2 and 71.9 eV are assigned to the Pt 4f5/2 and Pt 4f7/2 components, respectively, corresponding to the Pt (0) and Pt (II) species.38,39 Similarly, the binding energies of Pd 3d5/2 (341.5 eV) and Pd 3d3/2 (336.2 eV) are in good agreement with that of Pd (0) and Pd (II) (Figure 3d).40,41 The existence of oxidation states for Pt and Pd are in agreement with the previously reported papers.9,16,42 The chloride ion can be easily removed after the washing process. The above results indicate that most of the metal salt precursors (Pt4+ and Pd4+ species) are reduced and form metallic Pt and Pd. Notably, the binding energy values for Pt 4f peaks of mPtPd are slightly higher than those of the pure Pt. These shifting Pt 4f peaks suggest that the PtPd nanocrystals are in the form of the atomically mixed alloy.

EXPERIMENTAL SECTION

Synthesis of Mesoporous PtPd Film on Ni Foam (mPtPdNF). Briefly, for the pretreatment of Ni foam, Ni foam (1 cm × 1 cm) was cleaned with HCl (3 M), ethanol and pure water. Typically, mPtPd-NF was synthesized by a precursor solution containing 1.5 mL of K2PtCl4 (20 mM), 1.5 mL of NaPdCl4 (20 mM), 30 mg F127, and 25 μL of HCl (6M). The pretreatment of Ni foam involved soaking it in the mixture solution for 12 h under ambient conditions. At the end of the chemical reaction, synthetic mPtPd-NF was washed with ethanol and water to remove the metal ion and surfactant. For synthesis of Pt-NF, Pt5Pd-NF, PtPd5-NF, and Pd-NF, we just changed the molar ratio of Pt and Pd precursors. Characterizations. X-ray photoelectron spectroscopy (XPS) data were obtained on an ESCALABMK II X-ray photoelectron spectrometer. The crystal structure of the samples was investigated by X-ray diffraction (XRD) patterns conducted on a D8 ADVANCE (Bruker AXS, Germany). The morphologies of the samples were determined by scanning electron microscopy (SEM) images carried out at on a ZEISS SUPRA 55 and equipped with an energy dispersive X-ray analysis (EDX). Transmission electron microscopy (TEM) was performed with JEM-2010 microscopy operated at 200 kV. Zinc−Air Battery Test. The zinc−air battery test was conducted by a custom-made two-electrode Zn−air cell composed of a zinc anode, mPtPd-NF air cathode, and KOH electrolyte (6.0 M). The contrast sample, Pt/C-NF, was prepared by dropping Pt/C catalyst ink on Ni foam to form a uniform catalyst layer with a loading amount of about 1 mg cm−2. The LSV performances and power densities of the Zn−air battery were conduct on a CHI 660E workstation. The discharge performance of ZAB was carried out on a Land testing system. B

DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. (a)Top-side and (b) cross-sectional SEM images and (c) low magnification and (d) high magnification TEM images of mPtPd-NF. (e) HRTEM image, lattice fringes, and fast Fourier transform pattern of mPtPd-NF. (f) HAADF-STEM pattern and STEM-EDX element mapping patterns of mPtPd-NF. Inset of panel (c) reveals the SAED pattern.

Figure 3. (a) XRD and (b−d) XPS patterns of mPtPd.

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DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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This reaction process can be described by the equations shown in Figure S10. Simultaneously, the F127 micelles can serve as the soft template, which direct the formation of the mesoporous structure. Finally, the mPtPd film can be obtained after removing F127 micelles. In the preparation process, the metal (Pt and Pd) ions and Cl− can form coordination compound ions (PtCl42− and PdCl42−, respectively) after adding HCl. The dissociation equilibrium of PtCl42− and PdCl42− can shift to the reverse reaction by the Cl−-rich species, thus slowing their dissociation and then controlling the reaction rate of the galvanic replacement reaction. In the process of the film formation, the optimized Pt and Pd precursor amounts combined with the addition of F127 and HCl are beneficial for fabricating a mPtPd film on Ni foam. Active research on the preparation of the porous metals has been underway for some years. A few routes for fabricating porous metals have been developed, such as the dealloying method, template method, and so on.47−49 However, multistep operations or highly corrosive conditions are involved. In the present synthesis, Ni foam is simultaneously used as the reductant and substrate. This in situ synthetic strategy can be readily used for fabrication of mesoporous PtPd film on Ni foam in a large scale. As a binder-free electrode, the mPtPd-NF is highly favorable for electrochemical energy conversions. Inspired by the advantages of structure and composition, the as-prepared mPtPd-NF is investigated as a promising cathode for full ZABs. We tested its catalytic performance for ORR prior to the application as an electrode material and benchmarked against the Pt-NF, Pt5Pd-NF, PtPd5-NF, PdNF, and commercial Pt/C catalyst. The LSV curve of mPtPdNF shows the higher positive onset potential (Eonset) and half potential potential (E1/2) (1.04, 0.932 V), as compared with those of Pt-NF (1.05, 0.918 V), Pt5Pd-NF (1.00, 0.906 V), PtPd5-NF (0.99, 0.89 V), Pd-NF (0.94, 0.85 V), and Pt/C (1.00, 0.896 V) (Figure S11). In addition, the limiting current density of mPtPd-NF (5.7 mA cm−2) is the highest among PtNF (5.4 mA cm−2), Pt5Pd-NF (4.8 mA cm−2), PtPd5-NF (5.2 mA cm−2), Pd-NF (4.3 mA cm−2), and Pt/C (5.0 mA cm−2) (Figure S12a). The highest limiting current density of mPtPdNF is mainly ascribed to its uniform mesoporous structure and optimized Pt/Pd ratio. Normally, Pt shows higher catalytic activity for ORR than that of Pd. When the ratio of Pt/Pd is high (Pt rich), although the Pt contents of the Pt and Pt5Pd increase, the accessible surface area decreases because of their nonporous structure, leading to the moderate limiting current density. Moreover, PtPd5 with a low Pt content also has a moderate limiting current density, owing to its distinctly mesoporous structure. For pure Pd, its sheet structure provides a limited effective surface area, resulting in its lowest limiting current density. These results of RDE experiments show the excellent ORR catalytic activity of PtPd-NF. Meanwhile, a Tafel plot is adopted to investigate the ORR kinetic process on the samples. The obtained Tafel slopes at low overpotentials are 99.5, 67.8, 66.4, 64.4, 65.5, and 63.8 mV dec−1 for Pt-NF, Pt5Pd-NF, mPtPd-NF, PtPd5-NF, Pd-NF, and Pt/C, respectively (Figure S12b). Similar values for mPtPd-NF and Pt/C imply fast kinetic processes for ORR because of their similar reaction pathway and the rate-determining step. With increasing current density, the ORR overpotential for mPtPd-NF increases slowly due to its low Tafel slope, indicating that O2 could be easily adsorbed and activated on its surface. Moreover, it is noted that mPtPd-NF shows a much higher kinetic current density (jk) obtained at 0.9 V relative to

The alloying between Pt and Pd induces the lattice expansion/ compression, resulting in the change of electronic structure. These investigations fully confirm that mPtPd film is successfully fabricated on Ni foam. In order to explain the synthetic factors for constructing mPtPd-NF, various control experiments are performed. By replacing F127 with Brij 58 or PVP, the nonuniform porous structure is obtained in the presence of Brij 58, while irregular sheet-like structures can be formed by adding PVP (Figure S4). Without adding any surfactant, the obtained PtPd film has a nonporous structure (Figure S4c and S4f). These results show the important role of F127 on the formation of the mPtPd film. The effect of surfactant on the morphology is in agreement with the previous research.35,37,43 In the reaction mixture, it is highly necessary to choose the moderate HCl. In the absence of HCl, the PtPd film without a porous structure is prepared. It has a much thicker thickness (3.6 μm) in comparison to the typical PtPd-NF (580 nm) because of its fast reaction rate (Figure S5a and S5d). When 5 μL of HCl is added, no distinctly porous structure of the PtPd film is observed, while its thickness is reduced to 3.0 μm (Figure S5b and S5e). With an excessive HCl amount (125 μL), the thickness of the PtPd film observably decreases (330 nm), and its pore density is very low (Figure S5c and S5f). In the reaction mixture, the Pt and Pd precursors’ reduction rates can be retarded in the presence of HCl, resulting in a slow addition of Pt and Pd atoms, which is beneficial for the formation of the mesoporus structure. Meanwhile, it can produce an unfavorable impact due to too slow of a reduction rate. For the fabrication of the mPtPd film, the selected HCl amount (25 μL) favors a uniform mesoporous structure. Furthermore, when HCl is replaced with HNO3 or H2SO4, nonporous PtPd films are formed (Figure S6). In order to investigate the mPtPd film’s formation process, we also explore the effect of reaction time. Increasing the reaction time, the color of the reaction solution changes from brown to black (Figure S7), and the more obvious pores and thicker thickness can be observed by SEM, displaying the shape evolution of the PtPd film (Figure S8). Too long of a reaction time results in the nonuniform mesoporous structure of the film with an increased thickness. The contents of metallic precursors (Pt and Pd) play an important role in the structure of the mPtPd film. By changing the Pt and Pd precursor amounts, it also can be found that both PtPd5 and mPtPd films show the mesoporous structure (Figure S9). However, compared with the typical mPtPd-NF, their pore size, uniformity, and film thickness are low in quality. The optimized contents of Pt and Pd precursors are critical to the preparation of uniform mesoporous mPtPd-NF film. For the formation process of mPtPd-NF, the micelle assembly and simultaneous galvanic replacement reaction are the critical factors. Its possible formation mechanism is speculated as follows. The F127 concentration of 1.0 wt % is higher than its critical micelle concentration of 0.3 wt % in the reaction mixture, which leads to the formation of spherical micelles.44 In the aqueous solution, the dissolved Pt and Pd ions are coordinated with the water molecules to form metal− aqua complexes.45 The F127 micelles can interact with these metal−aqua complexes due to the presence of a hydrophilic ethylene oxide (EO) group.46 As a result, the exterior EO layer of the micelles can adsorb the dissolved Pt and Pd ions. During the galvanic replacement reaction process, as a reducing agent, Ni foam can readily reduce Pt and Pd ions near its surface. D

DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) Schematic diagram of the structure of the primary ZABs, image of a small red screen (∼3.7 V) powered by three ZABs in series, and an open-circuit potential of about 4.57 V. (b) LSV curves and power densities of the Zn−air battery by using Pt/C-NF and mPtPd-NF as the air electrode, respectively. (c) Long-term discharge curves of the samples. (d) Galvanostatic discharge curves of a mPtPd-NF-based ZAB at various current densities (5, 10, and 20 mA cm−2). (e) Long-term galvanostatic discharge test of 10 mA cm−2. The battery is recharged by refilling Zn foil and the electrolyte.

negligible decrease for limiting current density. These results indicate the superior ORR stability of mPtPd-NF. Furthermore, chronoamperometric measurements are also used to further investigate the catalytic stability of mPtPd-NF. As observed, the initial catalytic current density of Pt/C shows a loss of 35.8% after testing for 5 h, while that for mPtPd-NF is 14.9% (Figure S16b), which confirms the high stability and good long-term durability of mPtPd-NF. Owing to the high catalytic performance for ORR, the prepared mPtPd-NF can be used as an efficient cathode for ZABs. To imitate the realistic application, we assemble a ZAB prototype, in which the mPtPd-NF is directly served as a binder-free cathode (Figure 4a). Three series-wound ZABs with mPtPd-NF electrodes can power a small red LED panel, profiting from the highly average open-circuit voltage (OCV) of 1.52 V. We further investigate the cell performance of the mPtPd-NF in ZABs, using a commercial Pt/C-based ZAB as a benchmark. As shown in Figure 4b, the higher OCV (1.52 V) and larger current density (108 mA cm−2; 1.0 V) of the mPtPd-NF-based ZAB can be found on its discharging polarization curve, compared with those of the Pt/C-based ZAB (0.618 V and 108 mA cm−2, respectively). Moreover, the mPtPd-NF-based ZAB has a higher power density of 111 mW cm−2 at a current density of 121.6 mA cm−2 in comparison to that of the Pt/C-based ZAB (67 mW cm−2). In addition, a PtPd-NF-based battery shows higher specific capacity (613 mAh gZn−1) and energy density (728 Wh kgZn−1) at a discharge

the compared samples, indicating its high instinct catalytic activity for ORR (Figure S13). To further research the ORR kinetics of mPtPd-NF, the LSV curves are tested at various rotation rates (Figure S14a). On the basis of the Koutecky−Levich equation, mPtPd-NF shows the high calculated electron-transfer numbers close to 4, which suggests a one-step four-electron pathway in the ORR process (Figure S14b). The rotating ring disk electrode (RRDE) measurements can obtain analogical results. It can be observed that mPtPd-NF shows a negligible current for H2O2 oxidation (ring current (ir)) in comparison to its current for ORR (disc current (id)) (Figure S15a). The small ir of mPtPd-NF is even lower than that of Pt/C, implying the markedly restrained H2O2 evolution during the process of ORR. Figure S15b shows the calculated H2O2 yield and electron-transfer numbers of mPtPd-NF and Pt/C, using their ir and id. The low HO2− yield (3.9) at the potential range of 0.3−0.8 V, which is slightly above that of Pt/C (Figure S15b). The high four-electron selectivity of mPtPd-NF is further confirmed by the low HO2− yield and dominated four-electron pathway in the ORR process, which is well consistent with the results of the K−L plots. We also conducted the accelerated durability test for mPtPd-NF to evaluate its stability for ORR. mPtPd-NF has the almost changeless Eonset observed on the LSV curve after testing for 1000 CV cycles (Figure S16a). It also can be found that mPtPd-NF shows only a 10 mV degradation for E1/2 and a E

DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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density of 10 mA cm−2 than those of a Pt/C-based battery (484 mAh gZn−1and 541 Wh kgZn−1, respectively), in which both higher specific capacity and energy density are calculated on the mass of consumed Zn (Figure 4c). Moreover, a high voltage of 1.27 V for a PtPd-NF-based battery can be found on its galvanostatic discharge curves (Figure 4d). After testing for 20 h (5 mA cm−2), this voltage is still very stable. Stable voltages of 1.25 (10 mA cm−2) and 1.15 V (20 mA cm−2), respectively, can also be observed. Interestingly, the Zn−air battery can be “recharged” by refilling the electrolyte and Zn foil after expending Zn. Thus, we test a long-term galvanostatic discharge for a PtPd-NF-based battery at 10 mA cm−2 (Figure 4e). During the test process, no obvious voltage drop can be observed on the galvanostatic discharge curves. All of the above results indicate that the as-prepared binder-free PtPdNF electrode has superior catalytic performance and stability in practical applications for ZABs. Benefiting from the favorable mesoporous structure and bimetallic PtPd compositions, this obtained mPtPd-NF as a binder-free cathode exhibits excellent electrochemical performance for ZABs (Figure S17). The mPtPd film, the same as commercial Pt/C, shows the typical four-electron pathway in the ORR process. The mPtPd film with the uniform mesopores has high specific surface area, which offers sufficient active sites. The durability and stability of the mPtPd film can be effectively enhanced by its self-supported metallic framework. Furthermore, the bimetallic composition of the PtPd alloy also favors high catalytic activity and can significantly improve its catalytic activity and durability. Alloying Pt with Pd is beneficial to accelerate the dissociation of adsorbed O2 on a mesoporous PtPd film, resulting in reduced electroreduction polarization for the dissociated O atoms. The macroporous Ni foam and mesoporous PtPd film form hierarchical porous structures, which not only provide efficiently hierarchical diffusion paths for O2 and ions but also facilitate the infiltration of the electrolyte. Moreover, the robust adhesion derived from the in situ fabrication ensures effective and fast electron transport due to the close sticking of the PtPd film on Ni foam. This mesoporous PtPd film strongly grown on Ni foam avoids the addition of a polymer binder, which ensures high electrical conductivity. Owing to the high catalytic activity for ORR, effective diffusion of O2 and ions, and fast charge transfer, the electrochemical performance of the mPtPd-NF ZABs can be significantly improved.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02834. Additional characterization data and electrochemical data. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mails: [email protected] (H. Xue). *E-mails: [email protected] (L. Wang). ORCID

Hongjing Wang: 0000-0003-0641-3909 Hairong Xue: 0000-0003-3856-4138 Liang Wang: 0000-0001-7375-8478 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21601154, 21776255, 21701141) and Natural Science Foundation of Zhejiang Province (Grant LQ18B010005).

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REFERENCES

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CONCLUSION In summary, mPtPd-NF is successfully fabricated in an aqueous solution under ambient temperature by using a very simple soaking method. Owing to the self-standing all-metal mesoporous structure and bimetallic PtPd compositions, mPtPd-NF shows enhanced catalytic activity together with high durability for ORR. mPtPd-NF can serve as an efficient binder-free cathode for ZABs with high power and energy density, large specific capacity, and superior stability. With the proposed synthetic approach, it is very easy to achieve in situ preparation of a mesoporous metal film on a metal substrate, which is very different from the traditional dealloying and template methods. The present in situ preparation strategy will work well for directly fabricating mesoporous all-metal materials in applications for various electrochemical fields. F

DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.8b02834 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX