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Newly Designed Graphene Cellular Monolith Functionalized with Hollow Pt-M (M = Ni, Co) Nanoparticles as the Electrocatalyst for Oxygen Reduction Reaction Yazhou Zhou, Juan Yang, Chengzhou Zhu, Dan Du, Xiaonong Cheng, Clive Hsu Yen, Chien M. Wai, and Yuehe Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04963 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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ACS Applied Materials & Interfaces

Newly Designed Graphene Cellular Monolith Functionalized with Hollow Pt-M (M = Ni, Co) Nanoparticles as the Electrocatalyst for Oxygen Reduction Reaction Yazhou Zhou,†,‡ Juan Yang,*,† Chengzhou Zhu,‡ Dan Du,‡ Xiaonong Cheng, † Clive Hsu Yen,§ Chien M Wai,§ and Yuehe Lin*,‡ †

School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, People’s

Republic of China ‡

School of Mechanical and Materials Engineering, Washington State University, Pullman,

Washington 99164-2920, United States §

Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States

*

E-mail: [email protected]; [email protected]

KEYWORDS: graphene cellular monolith; porous architecture; hollow structure; bimetallic nanoparticle; electrocatalyst; oxygen reduction reaction

ABSTRACT: Newly designed graphene cellular monoliths (GCMs) functionalized with hollow Pt-M nanoparticles (NPs) (Pt-M/GCM, M = Ni, Co) have been successfully achieved by a facile and powerful method on the basis of sonochemical-assisted reduction and gelatinization

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processes. Firstly, hollow Pt-M (M = Ni, Co) NPs were synthesized and distributed on graphene oxide sheets (Pt-M/GO) by sodium borohydride reduction of metal precursors in the ultrasonic environment. Secondly, the hollow structure was further formed by ascorbic acid (AA) reduction of Pt precursors in gelatinization process. Meanwhile, GO sheets with hollow Pt-M NPs were reduced to graphene, and were assembled into Pt-M/GCM hydrogels by gelatinization process. The Pt-M/GCM (M = Ni, Co) electrocatalysts have a factor of 9.4-18.9 enhancement in electrocatalytic activity and higher durability towards oxygen reduction reaction (ORR), compared with those of commercial Pt/C catalyst. In detail, the mass activities for Pt-Ni/GCM and Pt-Co/GCM are 1.26 A mgPt-1 and 1.79 A mgPt-1, respectively; meanwhile, the corresponding specific activities are 1.03 mA cm-2 and 2.08 mA cm-2. The successful synthesis of such attractive materials paves the way to explore a series of porous materials in widespread applications.

1. INTRODUCTION Highly porous graphene cellular monolith (GCM), constructing with two-dimensional graphene sheets, shows a great potential in widespread applications as an advanced supporting material of nanoparticles (NPs) due to its excellent mechanical, electronic and thermal properties, and large specific surface area.1-5 Integration of NPs into GCM is not only important for enhancement of monolith materials’ merits, but also crucial for building complicated macroscopic graphene monolith with some interesting properties.6-8 In recent years, GCM was used to design the porous electrode material for fuel cells through deposition of the Pt-based NPs.9,10 Theoretically, the porous architectures are able to provide higher specific surface areas and larger pore volumes. Due to these properties, the availability of electrocatalyst’s surface area can be maximized, which is good for electron transfer. The porous structure also can provide the better mass


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transport of reactants to electrocatalyst, resulting in enhanced electrocatalytic activity.11 However, the electrocatalytic performances of such porous electrode materials reported in previous work still need to be further improved. The alloying of Pt NPs with less noble late transition 3d metal M (M=Fe, Co, Ni, and Cu, etc.) is a key strategy for creating the superior and low-cost catalysts in fuel cells.12,13 The electronic structure of Pt can be modified on the effects of alloying process, as illustrated by ligand effects and strain effects, which can weaken the binding of the Pt surface to intermediates such as O*, HO* and HOO* during the oxygen reduction reaction (ORR).14 Therefore, the electrocatalytic activity and durability of alloyed Pt can be significantly improved. Thus, integration of above alloyed Pt-M NPs into the GCM may be a key strategy for creating the excellent electrocatalysts for fuel cells. A number of methods,15-18 such as self-assembly method, hydrothermal method, metal ion induced self-assembly method, and substrate-assisted reduction and assembly method, have recently been developed to functionalize GCM with noble metal nanocrystals (Pt, Au, Ag, Pd, Ir, Rh, etc.),16 metal oxide NPs (Fe2O3, TiO2, Co3O4 etc.),17,19,20 metal-metal oxide NPs (Cu-CuO, Ni-NiO, Pb-PbO, etc.),18 and alloys (Pd2/PtFe, etc.).21 However, as with most of the existing methods, very few successes have been achieved to construct above-mentioned Pt-M/GCM. Furthermore, the electrochemical performances of alloyed Pt catalysts are significantly affected by the particle morphology and structure. In particular, control Pt NPs with the caged, hollow or porous structure offers a promising method for achievement the desirable electrochemical performance.22,23 It has been proven that the hollow interior can decrease the number of buried non-functional precious metal atoms, and their unique geometry can provide a pathway for tailoring physical and chemical properties.24 Therefore, control the hollow Pt-M NPs in GCM at


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the atomic level, and optimizing the structural and compositional effect between GCM and NPs would be an available strategy for meeting these performance goals. Sonochemical-assisted reduction method, since being discovered, has an advantage in synthesis of nanomaterials, especially metal or bimetallic NPs.25,26 In our precious work, deposition of Pt, Rh, PtPd, and PtRu NPs on multiwalled carbon nanotubes has been achieved with sonochemical-assisited reduction method.27-29 In the case of ultrasound-initiated reduction of metal ions in aqueous solution, the acoustic cavitation can cause the extreme but transient local conditions, which not only can faster reduction rate leading to smaller, more uniform particles, and high dispersion on supports, but also can reduce metal ions to the bimetallic NPs.26 Above analysis suggests that sonochemical-assisited reduction method gives it a great potential in synthesis of alloyed Pt with late transition 3d metals. However, integration of such hollow PtM (M = Ni, Co) NPs into GCM with this method has not been reported. Herein, we demonstrate for the first time, a facile and powerful method to integrate hollow Pt-M (M = Ni, Co) NPs into GCM using sonochemical-assisted reduction process, together with gelatinization reaction, as illustrated in Scheme 1. We firstly reduced the Pt2+-M2+/GO by sodium borohydride (NaBH4) in sonochemical environment. The hollow Pt-M NPs are obtained and coated on GO sheets. We then assembled the Pt-M/GO sheets into Pt-M/GCM with gelatinization reaction using ascorbic acid (AA). The as-obtained materials have the well-defined macroscopic porous structure. The hollow Pt-Ni or Pt-Co NPs with 10-17 nm distribute uniformly into GCM. Clearly, the unique structure and morphology of Pt-M/GCM materials are expected to be porous electrodes with enhanced electrochemical performances for ORR.


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Scheme 1. Schematic Illustration of the Synthesis Procedure of Hollow Pt-M/GCM (M = Ni, Co) Porous Electrocatalysts. 2. MATERIALS AND METHODS 2.1. Chemicals and Materials. Potassium tetrachloroplatinate (II) K2PtCl4, Nickel (II) acetate tetrahydrate Ni(OAC)2·4H2O (99.9%), Cobalt(II) acetate Co(OAC)2, NaBH4, AA, potassium chloride (KCl 99%), perchloric acid (HClO4 70%), absolute ethyl alcohol, and Nafion (5% in a mixture of lower aliphatic alcohols and water) were all purchased from Sigma Aldrich. Commercial Pt/C catalyst (Pt loading: 20 wt%, Pt on carbon black) was purchased from Alfa Aesar. The particle size of Pt on black carbon is 2-3 nm. GO was prepared by oxidation of natural graphite powder according to the modified Hummers’ method, and was dissolved in water to form the 2 mg mL-1 of homogeneous GO suspension by sonication. 2.2. Preparation of Pt-M/GCM (M = Ni, Co) Electrocatalysts: The procedures of preparation of Pt-M/GCM materials involve two steps: sonochemical-assisted reduction process and gelatinization process. The typical synthesis of hollow Pt-Ni/GCM materials is as following: Sonochemical-Assisted Reduction Process: 5 mL of as-prepared homogeneous GO suspension was stored in a 10 mL cylindrical sampler vial. 100 µL of an aqueous solution of mixing 0.1 M K2PtCl4(aq) and 0.1 M Ni(OAC)2(aq) was injected into the GO suspension, and the mixed solution was sonicated for 0.5 h with an ultrasonic reactor (Cleaning bath, FS60H,


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Fisher Scientific, USA, 150 W, 40 kHz). After that, 300 µL of NaBH4 (0.5 M) was added into aforementioned mixed solution during the sonication. This step was finished after 10 min sonication. Gelatinization Process: 10 mg of AA was added to the mixed solution obtained by sonochemical-assisted reduction process, and with 2 min sonication. And then this vial was placed in an oil bath for 4 h at 90 oC without stirring. The Pt-Ni/GCM hydrogel was obtained, and washed with distilled water for 5 times. Finally, the hydrogel was treated by liquid nitrogen for 5 min, and then freeze-dried for 24 h (Freeze drying machine FD-1-50, Beijing Boyikang Laboratory Instruments Co., Ltd China) to form Pt-Ni/GCM nanomaterial. By the same processes, the Pt/GCM and Pt-Co/GCM nanomaterials were also synthesized. 2.3. Characterization. FEI Sirion field emission scanning electron microscope was employed to characterize the porous structures and morphologies. Transmission electron microscopy (FEI/Philips CM-200) was used to further characterize the morphologies of products. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS-165 with a monochromatized AlKα X-ray anode (1486.6 eV). The X-ray diffraction (XRD) analyses were performed on a Siemens D5000 powder X-ray diffractometer with Cu KR radiation at 40 kV and 30 mA. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to measure the actual metal loading and composition (Thermo-Fisher iCAP 6300, Thermo Fisher Scientific Inc.). The porous structures of catalysts were further characterized using nitrogen adsorption/desorption measurements performed on a Quantachrome autosorb-6 automated gas sorption system. 2.4. Electrocatalysis for ORR. The electrochemical performances were measured by rotating disk electrode (RDE) on a CHI 630E station (CH Instruments, Inc., USA) with a three-


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electrode system. The Pt wire and Ag/AgCl (3 M KCl) were used as the counter electrode and reference electrode, respectively, and a glassy carbon (GC) disk electrode (5 mm in diameter) was served as the working electrode. Catalyst preparation for electrochemical measurements can be seen in supporting information. In this paper, all potentials were relative to hydrogen electrode (RHE). Cyclic voltammograms (CVs) of catalysts were recorded in a N2-saturated 0.1 M HClO4 solution at 50 mV s-1 when the CV profiles had stable shape. And then, the ORR polarization curves were recorded at a scan rate of 10 mV s-1 and rotation rate of 1600 rpm after purging oxygen into 0.1 M HClO4 solution for 30 min. 3. RESULTS AND DISCUSSION The photos of the reaction solution after being subjected to this method at different steps are shown in Figure 1. Firstly, homogeneous mixed suspension was obtained by adding metal precursors into GO aqueous suspension with the sonication (Figure 1A). During this process, metal ions of Pt2+ and M2+ can be absorbed on the GO sheets uniformly. After that, sonochemical-assisted reduction was processed to reduce the metal precursors to form Pt-M NPs. By adding NaBH4, the drastic reaction along with many bubbles was observed immediately, and mixed solution turned to dark colour. In surprise, the suspension was still maintained in homogeneous status without any precipitates after 10 min sonication (Figure 1B). Secondly, PtM/GCM hydrogels can be accomplished by gelatinization of as-prepared homogeneous suspension with AA for 4 h at temperature of 90 oC (Figure 1C). Finally, Pt-M/GCM (M = Ni, Co) porous electrocatalysts can be obtained by freeze drying of hydrogels (Figure 1D). The sonochemical process is a key for this newly designed method of Pt-M/GCM nanomaterials with several advantages: (1) the sonochemical-assisted process is useful to uniformly deposit the NPs with small size, high dispersion on graphene sheets; (2) by the NaBH4 reduction of metal ions


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under sonochemical process, the alloyed Pt-M NPs can be obtained; (3) Pt-Ni and Pt-Co NPs have hollow structure under sonochemical process; (4) the sonochemical process is useful to maintain the high dispersibility of mixed solution, which is crucial for further self-assembly of GO sheets into cellular monolith in the gelatinization step. One thing should be noticed that the dosage of NaBH4 we used is 7.5 times higher molarity than metal ions. Such dosage of NaBH4 is not enough to reduce GO, therefore the dispersibility of the mixed solution is not affected. Otherwise, the higher NaBH4 would remove the oxygen-containing functional group of GO, and the monolith would not be accomplished (Figure S1).

Figure 1. Photos of Metal Precursors and GO Mixed Solution After Being Subjected to Porous Electrocatalyst at Different Steps. (A) Pt2+-M2+/GO Dispersion, (B) Pt-M/GO Prepared by NaBH4 Reduction of Dispersion under Sonication Environment, (C) Pt-M/GCM Hydrogel Prepared by AA Reduction of Pt-M/GO at 90 oC without Stirring, (D) Pt-M/GCM Obtained after Freeze Drying of Pt-M/GCM Hydrogel. We firstly synthesized the Pt/GCM nanomaterial by this method. The cross-sectional structure of Pt/GCM can be observed from the typical scanning electron microscopy (SEM)


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images (Figure 2a,b). The well-defined three-dimensional (3D) network consists of interconnected micron-scale voids are observed clearly in the SEM images. Figure 2c,d show the transmission electron microscopy (TEM) images of Pt/GCM nanomaterial. It is clearly seen that the typical wrinkled and fibrous structures of GCM formed by folding of few layers of graphene sheets. The Pt NPs with an average size of 3 nm are uniformly deposited on graphene sheets. No particle aggregations are found, which indicates the high dispersion of Pt NPs in GCM. We knew that the GO concentration for constructing of GCM is usually not lower than 2 mg mL-1. Under the high concentration of GO solution, it is difficult for metal ions to uniformly distribute on single GO sheet, causing the particle aggregations and large particle size in GCM using some methods described in previous work.30,31 In our experiment, the sonochemical process can prevent particles from aggregation, decrease particle size, and improve the particle dispersion on graphene sheets. For comparison, we directly synthesized the Pt/GCM nanomaterial through AA reduction of precursors and without sonochemical process, and the particle aggregations can be found obviously in SEM image (Figure S2). Figure 2e shows the XRD pattern of Pt/GCM. Four main XRD peaks at ca. 39.9o, 46.6o, 67.8o, and 81.9o can be found, corresponding to the planes of Pt (111), Pt (200), Pt (220), and Pt (311), respectively. The XRD result is in agreement with the standard Pt (JCPDSICDD card No. 01-1194), which indicates the deposition of Pt NPs with the face-centered cubic (fcc) structure in GCM. Figure 2f shows the C 1s XPS spectra of GO and GCM. The C1s XPS profile of GCM shows the significant improvement in the intensities of C-C bonds (284.4 eV) and C=C bonds (285.3 eV), and decreased intensities of oxygen-containing carbon (hydroxyl and epoxy C-O, 286.5 eV, carbonyl C=O, 287.9 eV and carboxyl O=C-O, 289.2 eV) compared with those of GO


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(inset in Figure. 2f).32,33 The atomic ratio between C and O for GCM is 8.9, while it is 3.1 for GO, which indicates an efficient reduction reaction of GO using this method.

Figure 2. Morphologies and Structure Characterization of Pt/GCM. (a,b) SEM and (c,d) TEM Images, (e) XRD Pattern, and (f) C 1s XPS Spectra of GO and GCM.


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We then synthesized Pt-Ni/GCM nanomaterial using as-described method. Figure 3a shows the XRD patterns of Pt-Ni/GCM and Pt/GCM nanomaterials. The four main XRD peaks for PtNi can be observed to shift toward higher angle comparing to Pt/GCM, which indicates the lattice contraction induced by Ni. The XRD result proves that the fcc structured Pt-Ni alloys were deposited in GCM using this method. Furthermore, no Ni oxide peaks (such as NiO, JCPDSICDD card No. 780429 or Ni(OH)2, JCPDSICDD card No. 14-0117) is observed in PtNi/GCM nanomaterial, indicating that there are no such oxides presents in the bulk of catalyst. Under the sonochemical reduction environment, the strong NaBH4 can reduce the Pt and Ni precursors to alloyed Pt-Ni NPs. For comparison, we tried to synthesize Pt-Ni/GCM without sonochemical process. However, there is no lattice contraction occurred in the XRD pattern for this product (Figure S3), indicating that the synthesis of alloyed Pt-Ni without sonochemical process is failed. Furthermore, the particle size (47 nm) is much larger than that of Pi-Ni obtained with sonochemical process (Figure S4). The structure and composition of Pt-Ni/GCM nanomaterial were further characterized by XPS. Figure 3b shows the Pt 4f spectrum of Pt-Ni NPs. The Pt 4f7/2 and Pt 4f5/2 peaks can be seen at 71.3 eV and 74.7 eV, respectively.29 For the further deconvolution, the Pt 4f7/2 and Pt 4f5/2 peaks present two sets of peaks at 71.2, 71.9 eV and 74.7 and 76.0 eV, respectively. One set shows Pt 4f7/2 at 71.2 eV and Pt 4f5/2 at 74.7 eV, which is corresponding to Pt (0). The other set exhibits at Pt 4f7/2 at 71.9 eV and Pt 4f5/2 at 76.0 eV, which is attributed to the Pt (II). The deconvolution XPS result indicates that the Pt (0) are the predominant species in the Pt-Ni NPs. Figure 3c shows the Ni 2p spectrum of Pt-Ni NPs. Due to multielectron excitation, the Ni 2p spectrum has a complex structure with intense satellite signals of high binding energy (861.4 eV) adjacent to the main peaks.34,35 After these shake-up peaks are considered, the Ni 2p3/2 peak was deconvoluted into two peaks at 852.8 eV and 855.9


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eV, which are attributed to Ni(0), and Ni(OH)2, respectively.35 However, there is no Ni(OH)2 peak detected in the XRD pattern of the Pt-Ni/GCM. We consider that such oxides are noncrystalline, and should be surface and subsurface oxidative states. It has been proven that due to some desirable properties of Ni(OH)2 layer such as proton and electronic conductivity, Ni(OH)2 layer on surface of catalyst can improve the durability of catalyst through protecting catalyst against corrosion during electrocatalytic process.36 Therefore, Ni(OH)2 layer on Pt-Ni NPs may be also good for electrocatalytic activity with respect to ORR.

Figure 3. (a) The XRD Patterns of Pt-Ni/GCM and Pt/GCM, (b) Pt 4f and (c) Ni 2p XPS Spectra of Pt-Ni/GCM. Figure 4a,b show the interior microstructures of Pt-Ni/GCM characterized by SEM. The foam-like 3D structure of Pt-Ni/GCM is clearly observed, which is similar with Pt/GCM. The such network structure is further characterized by TEM, as shown in Figure 4c,d. With a lower magnification (Figure 4c), the well-defined porous structure assembled by wrinkled graphene sheets can be clearly seen. We can also see that the Pt-Ni NPs uniformly distribute on the graphene sheets. With a higher magnification (Figure 4d and inset in d), the hollow structured PtNi NPs with an average size of 10 nm can be clearly seen. The hollow Pt-Ni NPs synthesized by this simple method surprised us because such structures of Pt-M alloys are generally synthesized using the galvanic replacement,37-40 template-assisted method,24,41 and Kirbendall effect.42-44


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Recently, there is a popular method to synthesize hollow PtNi/C45,46 or PtCo/C47 nanocrystallites involving the galvanic replacement and the nanoscale Kirkendall effect. The hollow structure can be firstly formed during the galvanic replacement between Ptz+ ions and Ni-rich/C or Co-rich/C templates. And then, the hollow structure will be further formed by nanoscale Kirkendall effect through dissolution of unreacted Ni or Co elements. Their formation mechanism can be introduced to explain the possible mechanism of hollow structure for our work.

Figure 4. Morphology and Structure Characterization of Pt-Ni/GCM. (a,b) SEM and (c,d) TEM Images.


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In this work, we found that sonochemical-assisted reduction process plays a role in formation of hollow Pt-Ni alloys. Figure S5 shows the TEM image of Pt-Ni/GO obtained in the end of sonochemical-assisted reduction process, and both partially hollow structure and core-shell of NPs can be clearly observed. Moreover, the hollow structure cannot be obtained without this process in our system. That means hollow structure should be firstly formed during this process. Therefore, we consider that the possible formation mechanism of hollow Pt-Ni alloys in GCM should be attributed to two processes: (a) the hollow structure starts to form in the first step of sonochemical-assisted reduction process; (b) the growth of well-defined hollow Pt-Ni alloys is finished in the second step of gelatinization process. It has been found that the core-shell structured alloys, such as Fe-Co, Co-Ni, Au-Pd, Au-Ag, Au-Pt, and Pt-Ru, etc. can be obtained through the sonochemical co-reduction of metal ions in aqueous solution.26,48 Since the redox potential of [PtCl42−/Pt] (0.76 V vs SHE) is significantly higher than that of [Ni2+/Ni] (−0.25 V vs SHE), the core-shell Pt-Ni NPs with Pt as core and Ni as shell may be formed in the beginning of sonochemical-assisted reduction process (described in eqn 1 and 2).49 Due to the insufficient reducing agent of NaBH4 in this process, the galvanic replacement between PtCl42− and Ni should be happened (described in eqn 3), and cause the formation of pinholes and partially hollow structure. 2PtCl + BH + 4OH = 2Pt ↓ +BO + 2H O + 8Cl (1)

4Ni + BH + 8OH = 4Ni ↓ +BO + 6H O (2) PtCl + 4Cl (3) + Ni = Pt ↓ +Ni

As shown in Figure S5, pinholes in surface of NPs and partially hollow structure can be clearly seen, which indicate that the galvanic replacement is indeed happened in this step. Moreover, since the diffusivity of Pt is greater than that of Ni, and then hollow structure can be


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further formed by Kirkendall mechanism. And, the extreme but transient local conditions and cavity bubbles caused by acoustic cavitation during the sonochemical process may enhance the Kirkendall effect, and accelerate the formation of hollow structure.50 After the sonochemical process, the unreacted PtCl42- can be reduced by AA, and grow on surface of hollow Pt-Ni NPs in gelatinization process, which can further promote the formation of hollow structure. As shown in inset of Figure 4d, the multi-shell structure in some hollow Pt-Ni NPs can be clearly seen, which is consist with our analysis. Figure S6 shows the high-resolution transmission electron microscope (HRTEM) image of hollow Pt-Ni NP. The lattice fringes with d-spacing of 0.22 nm and 0.25 nm marked in Figure S6 are indexed to the (111) planes of fcc Pt crystal and alloyed PtNi crystal, respectively, which indicates the formation of Pt-rich surface layers. It is difficult to reduce Ni precursors to metallic Ni by AA in this system, while Pt precursors can be easily reduced to Pt, which cause the Pt-rich in hollow Pt-Ni alloys. The element analysis is also obtained by ICP-AES. The molar ratio between Pt and Ni elements in Pt-Ni alloy increases from initial 50 : 50 to 75 : 25, which is consist with TEM result. Due to the similar character with Ni, Co is used to synthesize the hollow NPs using the template-assisted method or Kirbendall effect. Thus, the hollow Pt-Co NPs are expected to be synthesized using the same method which can also support the aforementioned formation mechanism. Figure 5a,b show the 3D network of Pt-Co/GCM characterized by SEM. The similar porous structure for each sample indicates that the NP species does not have much effect on the macroscopic porous structure. Figure 5c,d show the TEM images of Pt-Co/GCM nanomaterial. Besides porous structure, it can be clearly seen that the hollow Pt-Co NPs with an average size of 17 nm evenly scatter upon the graphene sheets. Figure 5e shows the XRD pattern of Pt-Co/GCM, indicates the fcc structure of Pt-Co NPs in GCM (Figure 5e). The structure and composition are


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further characterized by XPS, as shown in Figure 5 (f-h). The successful synthesis of PtCo/GCM with well-defined hollow structure has proven that this is a facile and powerful method to synthesize the hollow Pt-M/GCM (M = Ni, Co) porous catalysts. Such materials are expected to be porous electrode materials with respected to ORR.

Figure 5. Morphology and Structure Characterization of Pt-Co/GCM. (a,b) SEM and (c,d) TEM Images, (e) XRD Patterns, and (f) XPS Spectrum, (g) Pt 4f and (h) Co 2p XPS Spectra.


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The porous structures of Pt-Ni/GCM and Pt-Co/GCM porous catalysts are further characterized by N2 adsorption-desorption measurements. Figure 6a shows the typical nitrogen adsorption/desorption isotherms of porous catalysts and pure GCM. The pore distribution (Figure 6b) obtained using the Barret-Joyner-Halenda (BJH) theory shows the hierarchical porous structures of Pt-Ni/GCM and Pt-Co/GCM catalysts, including mesopores and micropores. The specific surface areas and total pore volumes are 512.3 m2 g-1 and 0.62 cm3 g-1 for Pt-Ni/GCM, 480.5 m2 g-1 and 0.58 cm3 g-1 for Pt-Co/GCM, respectively, which are much higher than those of pure GCM (169.5 m2 g-1, 0.32 cm3 g-1). Figure S7 show the SEM images of GCM. Without deposition of NPs, the serious aggregations of graphene sheets can be clearly seen. We considered that a lot of NPs scattering upon the graphene sheets not only can act as active sites to gelatinize GO sheet into porous monolith, but also can prevent graphene sheets from aggregation, which causing the much higher specific surface area and larger total pore volume.

Figure 6. (a) Nitrogen Adsorption/Desorption Isotherms and (b) Pore Size Distributions of Pure GCM, Pt-Ni/GCM, and Pt-Co/GCM. Inspired by the attractive properties (hollow structure, well-defined, highly dispersive, and porous network) of Pt-M/GCM (M = Ni, Co) porous catalysts, we initially assessed such


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electrocatalysts towards ORR. The electrocatalytic properties of these catalysts were firstly performed by cyclic voltammetry. Figure 7a shows the CVs of the Pt-M/GCM (M = Ni, Co) and the commercial Pt/C catalysts recorded in N2-saturated 0.1 M HClO4 solution at a sweep rate of 50 mV s-1. The double-layer capacitances of such porous catalysts are much larger than that of the commercial Pt/C catalyst because of the much larger specific surface area and more oxygenated surface groups of GCM than the activated carbon, which is crucial for mass activity enhancement of catalyst.51 The electrochemically active surface areas (ECSAs) of catalysts were evaluated by CVs, which can provide important information regarding the number of available active sites. The details are shown in supporting information. The Pt-Ni/GCM and Pt-Co/GCM catalysts show the comparable ECSAs (86.7 m2 gPt-1 and 85.6 m2 gPt-1, respectively) calculated from the hydrogen adsorption/desorption in CVs compared with that of the commercial Pt/C catalyst (84.2 m2 gPt-1). The higher ECSAs indicate that more electrochemical active sites exist in Pt-M/GCM (M = Ni, Co) catalysts. The electrocatalytic performance of Pt-M/GCM (M = Ni, Co) catalysts for ORR was then evaluated. Figure 7b shows the ORR polarization curves of Pt-M/GCM (M = Ni, Co) and the commercial Pt/C catalysts obtained using the RDEs. We can clearly see that the ORR polarization curves have two distinguishable potential regions: one below 0.6 V is attributed to the diffusion-limiting current region; the other one between 0.6 and 1.1 V is mixed kineticdiffusion control region. Both Pt-Ni/GCM and Pt-Co/GCM porous catalysts exhibit substantially improved ORR activities compared to the commercial Pt/C catalyst, including the higher on-set and half-wave potentials, and the mass limiting current densities.


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Figure 7. Electrochemical Properties of Pt-M/GCM (M = Ni, Co) and the Commercial Pt/C Catalysts. (a) CVs, (b) ORR Polarization Curves, (c,d) the Corresponding Tafel Plots, and (e) Mass Activities and (f) Specific Activities Measured at 0.9 V.


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The intrinsic electrocatalytic activities of catalysts can be represented by the kinetic current calculated from the polarization curves using the well-known mass-transport correction according to the Levich-Koutecky equation (eqn (S2)).52 Figure 7c and 7d show the corresponding mass and specific activities of catalysts, which indicate the much higher intrinsic electrocatalytic activities of porous catalysts than those of the commercial Pt/C catalyst in the potential range of 0.88-0.95 V. In detail, the mass activities of the Pt-Ni/GCM and Pt-Co/GCM catalysts measured at 0.9 V are 1.26 A mgPt-1 and 1.79 A mgPt-1, respectively, which are 10.5 times and 14.9 times higher than that of the commercial Pt/C catalyst (0.12 A mgPt-1) (Figure 7e). In particular, the mass activities of Pt-M/GCM (M = Ni, Co) catalysts are much greater than the U.S. Department of Energy’s 2017 target (0.44 A mgPt-1).14 The specific activities of these different catalysts shown in Figure 7f also demonstrate similar trends to the mass activities. The specific activities of the Pt-Ni/GCM and Pt-Co/GCM porous catalysts are 1.03 mA cm-2 and 2.08 mA cm-2, respectively, which are 9.4 times and 18.9 times higher than that of the commercial Pt/C catalyst (0.11 mA cm-2) calculated at 0.9 V (Figure 7f). We also studied ORR durability of the Pt-Ni/GCM and Pt-Co/GCM catalysts by an accelerated durability test (ADT), and compared to the commercial Pt/C catalyst (for the details, Supporting Information). The catalysts were treated using 20,000 cycles CV scans by potential sweeping from 0.6 to 1.0 V (vs. RHE) at a scan rate of 100 mV s-1. The CVs and ORR polarization curves before and after ADT were recorded at scan rate of 50 mV s-1 and 10 mV s-1, respectively. The commercial Pt/C catalyst exhibits the substantial loss of ECSA and kinetic activities after 20,000 potential cycles due to the dissolution of Pt itself, agglomeration of Pt particles, and corrosion of the carbon support at high potentials (Figure S8). The mass activity of the commercial Pt/C decreased to 0.03 A mgPt-1 after ADT (Figure S9). In contrary, only a slight decrease of ECSAs


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was found for Pt-Ni/GCM (Figure 8a) and Pt-Co/GCM (Figure 8b) after the ADT, suggesting the high stability in electrochemical condition. Figure 8c and Figure 8d show the ORR kinetic activity durability for Pt-Ni/GCM and Pt-Co/GCM, respectively. And the mass and specific activities of Pt-Ni/GCM and Pt-Co/GCM catalysts calculated at 0.9 V before and after ADT were provided in Figure S8. Only a slight kinetic activity change was found for Pt-Ni/GCM. Compared to the Pt-Ni/GCM catalyst, the Pt-Co/GCM catalyst lost ~ 10% electrocatalytic activity. However, the Pt-Co/GCM catalyst still has the higher electrocatalytic activity compared to the Pt-Ni/GCM. Overall, the catalytic activity and durability of Pt-M/GCM (M = Ni, Co) porous catalysts are enhanced when compared with those of the commercial Pt/C catalyst

Figure 8. Electrochemical Durability of Catalysts. CVs of (a) Pt-Ni/GCM, (b) Pt-Co/GCM Catalysts, and ORR Polarization Curves of (c) Pt-Ni/GCM, (d) Pt-Co/GCM Catalysts Before and After ADT.


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The enhancement in electrochemical performances for the ORR described in Pt-M/GCM (M = Ni, Co) porous catalysts should be attributed to the porous GCM support and hollow Pt-M alloys according to our experiments and the previous investigations. In order to demonstrate the effect of GCM support for ORR, the electrochemical performances of Pt/GCM catalyst were also studied, which can be seen in Figure S10. The Pt/GCM shows the higher ECSA of 85.4 m2 gPt-1 than that of commercial Pt/C catalyst. The ORR polarization curves (Figure S10b) also demonstrate the improved ORR activity of the Pt/GCM catalyst compared to the commercial Pt/C catalyst. The results proved that the GCM is indeed a promising support to improve catalytic activity. On the one hand, graphene has been proven to enhance the ORR activity and durability of Pt-based catalysts due to its high surface area, excellent electronic properties, high stability, and strong interaction with NPs;51,53-55 on the other hand, GCM as support has the porous architectures that not only maximizing the availability of catalyst surface area for electron transfer, but also providing better mass transport of reactants.56 Besides GCM support, the Pt-Ni (Co) NPs contribute mostly to the ORR activity. By introducing of Ni or Co, the d-band center of Pt is finetuned on basis of electronic effect of the second transition metal Ni or Co, which results in facilitating O2 adsorption, activation, and weakening the reaction intermediates on the surface of Pt.12 Therefore, the ORR activity of Pt-M catalyst has been significantly improved. Moreover, recent research showed that the electrochemical performance of Pt-M alloys can also be further improved by controlling the catalyst shape (octahedral shape), composition (multimetallic alloys) and unique structure (core-shell, hollow, and dendritic structures).57 In particular, formation hollow structured Pt-M NPs is a key strategy for creating excellent electrocatalysts for ORR owing to the unique properties of hollow structure such as increased surface area, low density, easy recovery, self-supporting capacity, and surface permeability.58-61 For example, Erlebacher


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and co-workers reported that nanoporous NiPt catalysts prepared by electrochemical dealloying of NiPt alloys showed the enhanced electrocatalytic activity for ORR;62 Recently, Yu and coworkers reported a facile approach to synthesize hollow PtPdCu catalyst with chemical dealloying process using mild acid of acetic acid;63 the hollow PtCo/C or PtNi/C nanocrystallites synthesized by galvanic replacement also showed significantly enhanced electrocatalytic activity and durability.47 Those work demonstrated that the enhanced ORR activity on the hollow Pt-M catalysts should be attributed to the opened porous architecture, “ensemble effect”, and “strain effect”.46,62 In this paper, the Pt-Ni (Co) alloys in GCM support have the well-defined hollow structure, which should also be beneficial for enhancement in electrocatalytic activity of PtM/GCM. Moreover, there are many previous reports on synthesis of hollow Pt-based catalysts, and most of them were loaded on the surface of carbon black through multistep. Compared with these catalysts, the hollow Pt-Ni (Co)/GCM catalysts in this work not only have hollow structured NPs, but also have porous architecture of support, which is good for improvement of electrocatalytic activity of catalyst and decrease of catalyst cost. 4. CONCLUSION In summary, we have demonstrated that the Pt catalysts alloyed with the less noble late transition 3d metal Ni, Co can be integrated into the GCM by the newly designed, powerful and large-scale manufacturing method, including the sonochemical-assisted reduction process and gelatinization process. The sonochemical-assited reduction and gelatinization processes guaranteed the formation of Pt-M alloys and porous monolith, respectively. The NPs can promote the formation of hierarchical porous structures and cause the high specific surface areas and large total pore volumes for Pt-Ni/GCM (512.3 m2 g-1, 0.62 cm3 g-1) and Pt-Co/GCM (480.5 m2 g-1, 0.58 cm3 g-1). Specially, the well-defined hollow Pt-Ni and Pt-Co alloys were obtained in Pt-M/GCM. The Pt-


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M/GCM (M = Ni, Co) porous catalysts have a factor of 9.4-18.9 enhancement in electrocatalytic activity and higher durability towards ORR, compared with those of commercial Pt/C catalyst. The successful synthesis of such attractive materials paves the way to explore a series of porous materials in widespread applications. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami. Experimental details of electrochemical performance. XRD pattern, SEM and TEM images of Pt-Ni/GCM prepared without sonochemical process, HRTEM image of hollow Pt-Ni NP, the electrocatalyst performance of Pt/GCM, and the durability of the commercial Pt/C catalyst are shown in Figure S1-S10. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant 51572114). Y.L. and D.D. thank the Washington State University start-up grant for financial support. We thank Franceschi Microscopy & Image Center at Washington State University for TEM measurements.


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BRIEFS Newly designed graphene cellular monoliths functionalized with hollow Pt-M alloys (M = Ni, Co) has been successfully achieved by a facile and powerful method on the basis of sonochemical-assisted reduction and gelatinization processes, which paves the way to create the new porous materials in widespread applications. SYNOPSIS


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M = Ni, Co - ACS Publications - American Chemical Society

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