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Molybdenum-Doped PdPt@Pt Core−Shell Octahedra Supported by Ionic Block Copolymer-Functionalized Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst Kie Yong Cho,†,‡ Yong Sik Yeom,† Heun Young Seo,† Pradip Kumar,⊥ Albert S. Lee,‡ Kyung-Youl Baek,‡,§ and Ho Gyu Yoon*,†,∥ †
Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea ⊥ Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India § Nanomaterials Science and Engineering, University of Science and Technology, Daejeon 34113, Korea ∥ Graduate School of Management of Technology, Korea University, Seoul 02841, Korea ‡
S Supporting Information *
ABSTRACT: Development of highly active and durable electrocatalysts that can effectively electrocatalyze oxygen reduction reactions (ORR) still remains one important challenge for high-performance electrochemical conversion and storage applications such as fuel cells and metal-air batteries. Herein, we propose the combination of molybdenum-doped PdPt@Pt core−shell octahedra and the pyrenefunctionalized poly(dimethylaminoethyl methacrylate)-b-poly[(ethylene glycol) methyl ether methacrylate] ionic block copolymer-functionalized reduced graphene oxide (MoPdPt@Pt/IG) to effectively augment the interfacial cohesion of both components using a tunable ex situ mixing strategy. The rationally designed Mo-PdPt@Pt core−shell octahedra have unique compositional benefits, including segregation of Mo atoms on the vertexes and edges of the octahedron and 2−3 shell layers of Pt atoms on a PdPt alloy core, which can provide highly active sites to the catalyst for ORR along with enhanced electrochemical stability. In addition, the ionic block copolymer functionalized graphene can facilitate intermolecular charge transfer and good stability of metal NPs, which arises from the ionic block copolymer interfacial layer. When the beneficial features of the Mo-PdPt@Pt and IG are combined, the Mo-PdPt@Pt/IG exhibits substantially enhanced activity and durability for ORR relative to those of commercial Pt/C. Notably, the Mo-PdPt@Pt/ IG shows mass activity 31-fold higher than that of Pt/C and substantially maintains high activities after 10 000 cycles of intensive durability testing. The current study highlights the crucial strategies in designing the highly active and durable Pt-based octahedra and effective combination with functional graphene supports toward the synergetic effects on ORR. KEYWORDS: molybdenum doping, multimetallic nanocrystals, functionalized graphene, octahedra, core−shell structures, electrocatalysts, fuel cells
1. INTRODUCTION Demand for highly enhanced energy conversion and storage devices continually mount as humans continue to increase energy consumption, rendering next generation energy materials a crucial technology for a clean and sustainable future.1−5 Tireless efforts toward this end have led to significant development of high-performance fuel cells, supercapacitors, and batteries.6−9 Specifically, proton-exchange membrane fuel cells (PEMFCs) have become focal as a promising alternative for environmentally clean energy systems.10−13 However, commercialization of PEMFCs has been hampered by the high cost, sluggish kinetics of the ORR, and insufficient durability because combination of platinum nanoparticles (NPs) with the carbon support (Pt/C) has been the only recognized strategy for obtaining a feasible cathode catalyst.14,15 © XXXX American Chemical Society
Furthermore, the Pt/C catalyst suffers from severe limitations, including carbon corrosion, aggregation/dissolution of Pt NPs, and Ostwald ripening because of the high surface energy of Pt and generation of radical species during ORR.16−18 To overcome the inherent limitations of Pt/C, upgraded catalyst designs have been widely introduced.19−23 Alloying Pt with other transition metals such as Ni,24 Cu,25 Fe,26 Co,27 and Pd28−30 has proven advantageous for reducing the usage of Pt as well as for enhancing the ORR activity and durability compared to those of single Pt NP congeners because of the strain/electronic coupling effects.31−33 Specifically, Received: October 19, 2016 Accepted: December 19, 2016 Published: December 19, 2016 A
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
cohesion arising from ionic block copolymer as an interfacial linker, the well-developed Mo-PdPt@Pt octahedra were successfully incorporated with ionic block copolymer-functionalized rGO by an ex situ mixing process. In addition, the interfacial linker facilitates to enhance the stability of the MoPdPt@Pt octahedra during the ORR. With the beneficial features of the Mo-PdPt@Pt and IG, the Mo-PdPt@Pt/IG exhibited synergetic effects on ORR, notably 31-fold enhanced initial mass activity and 200-fold higher mass activity after 10 000 cycles of intensive durability tests in comparison to those of commercial Pt/C. To this end, the combination of the newly developed Mo-PdPt@Pt core−shell octahedra and the advanced support with well-defined interfacial interactions can be a highly promising candidate to solve the previous issues arising from both metal and carbon supports.
palladium exhibits desirable properties such as sharing the same facets and having an almost identical lattice constant (only 0.77% mismatch) to Pt.28,33,34 Previous works in designing effective ORR catalysts have centered on structural parameters such as certain facets, shapes, and sizes.28,35,36 Notably, smallsized (diameter: ≤ 5 nm) Pt-based octahedra with Pt(111) facets have exhibited maximal ORR activities because of their intrinsically more active surface properties, which can be achieved due to the minimized total interfacial free energy.28,37,38 In addition, the strategies for reducing scarce Pt content and enhancing activities have been studied by controlling the number of Pt layers on the Pd core, with studies revealing that 2−3 Pt atoms layers with octahedral and icosahedral structures exhibiting great catalytic activities due to optimal tensile strain.39,40 However, synthesis of Pd@Pt core−shell nanocrystals with well-defined unique structures has been hindered by the self-nucleation and dendritic growth of Pt arising from the bonding energy of Pd−Pt (191 kJ mol−1) relatively lower than that of Pt−Pt (307 kJ mol−1).28,40 To address this issue, Park et al. introduced a water-based system with use of a mild reducing agent to induce a low reduction speed of Pt precursors, giving well-defined Pd@Pt core−shell octahedra with a conformal Pt shell.40 Other strategies for achieving high ORR activity and durability have entailed the utilization of various dopants and supports into the catalyst.41−43 Pt-based nanocrystals doped with various transition metals, including Cr, Mn, Fe, Co, Mo, or Re, have provided a new pathway to achieve great ORR activity and durability.38 Notably, the introduction of Mo as a dopant onto the Pt3Ni octahedra resulted in the highest recorded activities. This formidable ORR performance was explained in terms of experimental observations and computational studies, illustrating local changes in the oxygen binding energies and strong Mo−Pt and Mo−Ni bonds.38 However, use of carbon black in this study precluded the fact that for the ORR applications, carbon black leads to carbon corrosion during ORR, which should be addressed for stable PEMFC operating potentials. As such, adoption of advanced supports for catalysts tailored for ORR to give beneficial effects on enhancing the durability as well as its electrocatalytic performance is of great importance. The properties toward ideal support materials can be generalized as follows: high electronic conductivity, good distribution of metal catalysts on the support, high corrosion resistance, high surface area, and strong cohesion to metal catalysts.11,12,42−44 Among the various approaches, Zeng et al. recently reported that ionic polymer-doped graphene as a support for Pt NPs led to catalytic activity and durability better than that of graphene without the ionic polymer.45 This result can be ascribed to the stable structure during the electrocatalytic reaction due to the high bonding strength between the metal and functional anchoring groups along with the intermolecular charge-transfer effect induced by the ionic polymers.42,45−47 From these reports, we can conclude that development of carefully designed catalysts with custom designed metal/support interfaces still remain as ongoing challenges. In this study, we reported the rationally designed Mo-doped PdPt@Pt core−shell octahedra with compositional benefits derived from segregation of Mo atoms on the vertexes and edges of octahedron and 2−3 shell layers of Pt atoms on a PdPt alloy core, which can provide the active surface and good stability in ORR. With great advantages of strong interfacial
2. EXPERIMENTAL SECTION 2.1. Materials. The reagents, including polyvinylpyrrolidone (PVP) (average MW: 10 000 g/mol), commercial Pt/C catalyst (Vulcan XC-72, E-TEK, 20 wt % Pt, 3−4 nm diameter), benzoic acid (>99.5%), sodium tetrachloropalladate (Na2PdCl4, 98%), chloroplatinic acid hydrate (H 2 PtCl 6 xH 2 O, 99.995%), potassium tetrachloroplatinate(II) (K2PtCl4, 99.99%), molybdenumhexacarbonyl (Mo(CO)6, 99.9%), N,N-dimethylformamide (99.8%), isopropyl alcohol (>99.7%), and Nafion (45% water solution), were purchased from E-TEK and Sigma-Aldrich and were used as received unless otherwise noted. Dimethylaminoethyl methacrylate (DMAEMA) monomer was refined by distillation after removal of the moisture with calcium hydride. Poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) monomer was refined using the inhibitor remover resin (AL-154, Sigma-Aldrich). 2.2. Characterization. X-ray diffraction patterns were acquired on a Rigaku diffractometer (Rigaku Smart Lab, Rigaku Co., Japan) operated at 45 kV and 40 mA with Cu Kα radiation (λ = 1.5406 A°) using a diffracted beam monochromator. Data were collected between 2θ = 5° and 100° at 0.01° intervals. The phase was identified by matching each characteristic peak with the JCPDS files. The synthesized graphene supports were characterized using a Raman spectrometer (LabRam ARAMIS IR2, Horiba, Japan). X-ray photoelectron spectroscopy (XPS) analysis was performed under reduced pressure using an X-ray photoelectron spectrometer (X-TOOL, ULVAC-PHI) with a monochromatic Al Kα source. The latticeresolved images were obtained by high-resolution transmission electron microscopic analysis (HR-TEM, FEI Titian 80-300) at an accelerating voltage of 300 kV. Scanning transmission electron microscopy (STEM) was applied for analysis of elemental mapping, energy dispersive X-ray (EDX), and line-scanning profile (FEI Talos F200X, maximum accelerating voltage of 200 kV) analyses. The compositional ratios of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Varian, 710 ES) analysis. Electrochemical measurements were performed using a glassy carbon rotating disk electrode (RDE, Ltd., RRDE-3A, ALS. Co.) connected to a potentiostat (VSP-300, Bio Logic Science Instruments). 2.3. Synthesis of PdPt Alloy Octahedra. In the typical method for synthesis of the octahedral PdPt alloy catalyst, polyvinylpyrrolidone (PVP, 420 mg) and benzoic acid (29.3 mg) were dissolved in 10 mL of N,N-dimethylformamide (DMF) by sonication for 10 min. After the well-dissolved solution was cooled in an ice bath, 2 mL of disodium tetrachloropalladate (Na2PdCl4, 20 mM aqueous solution) and 2 mL of chloroplatinic acid (H2PtCl6, 20 mM aqueous solution) were sequentially added to the prepared solution. Thereafter, the mixed solution was heated at 130 °C for 5 h in an oil bath after sonication for 2 min. The resulting product was diluted with an ethanol/acetone (2/ 1) mixture and purified by three cycles via centrifugation and washed with an ethanol/acetone (2/1) mixture. The final precipitated product was dried at RT for 24 h under reduced pressure. B
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces 2.4. Synthesis of PdPt@Pt Core−Shell Octahedra. The octahedral PdPt@Pt core−shell catalyst was synthesized by the further growth of Pt on the preformed octahedral PdPt alloy catalyst. An additional 1 mL of chloroplatinic acid (H2PtCl6 20 mM aqueous solution) was sequentially added into the suspension of the unpurified PdPt alloy catalyst prepared by using 1 mL of Na2PdCl4 (20 mM aqueous solution) and 0.5 mL of H2PtCl6 (20 mM aqueous solution) after the prepared solution was cooled to room temperature. Thereafter, strong agitation followed in an ice bath for 1 min. The reaction solution was heated at 130 °C for 24 h. The resulting product was diluted with an ethanol/acetone (2/1) mixture and purified by three cycles of centrifugation and washed with an ethanol/acetone (2/ 1) mixture. The final precipitated product was dried at room temperature (RT) for 24 h under reduced pressure. 2.5. Synthesis of Mo-Doped PdPt@Pt Core−Shell Octahedra. The octahedral PdPt@Pt core−shell catalyst was synthesized by the further growth of Pt and Mo on the preformed octahedral PdPt alloy catalyst. An additional 1 mL of chloroplatinic acid (H2PtCl6 20 mM aqueous solution) and 6 mg of molybdenumhexacarbonyl (Mo(CO)6) were sequentially added to the suspension of unpurified PdPt alloy catalyst prepared by using 2 mL of Na2PdCl4 (20 mM aqueous solution) and 0.5 mL of H2PtCl6 (20 mM aqueous solution) after the prepared solution was cooled to room temperature. Thereafter, strong agitation was followed in an ice bath for 1 min. The reaction solution was heated at 130 °C for 24 h. The resulting product was diluted with an ethanol/acetone (2/1) mixture, purified by centrifugation three times, and washed with ethanol/acetone (2/1) mixture. The final precipitated product was dried at RT for 24 h under reduced pressure. 2.6. Synthesis of Ionic Block Copolymer-Functionalized rGO (IG). Pyrene-functionalized poly(dimethylaminoethyl methacrylate)-bpoly[(ethylene glycol) methyl ether methacrylate] (Py-PDMAEMA-bPPEGMEMA) ionic block copolymer (Mn: 19k, PDI: 1.09, 16 wt % of PDMAEMA, and 14 pyrene groups per chain) was synthesized using a combination of the atom transfer radical polymerization (ATRP) and the Menshutkin reaction (Scheme S1, Supporting Information).42,48 Graphene oxide (GO) was prepared by a modified Hummer’s method, and reduced graphene oxide (rGO) was then obtained after stepwise thermal treatment up to 1000 °C at a rate of 10 °C/min (Supporting Information).49 To accomplish effective integration of rGO with the ionic block copolymer, 30 mg of rGO was dispersed in 200 mL of NMP by sonication for 30 min. Thereafter, 80 mg of ionic block copolymer was added to the rGO suspension and then stirred in an ice bath overnight. Subsequently, the mixture was sonicated for 20 min. After the foregoing procedure, well-dispersed IG was obtained, and free ionic block copolymer was removed by simple filtration under reduced pressure and washing with acetone. The obtained powdery products were dried at RT for a day under reduced pressure. The ionic block copolymer introduced into the resulting product was 50 wt %. 2.7. Fabrication of Synthesized Octahedra on IG (Catalyst/ IG). To achieve rational integration of the IG support with the synthesized octahedra, 2 mg of IG support was dispersed in NMP by sonication for 5 min, and 4 mg of each synthesized octahedra (PdPt, PdPt@Pt, and Mo-PdPt@Pt, respectively) was dispersed in NMP by sonication for 30 min. Thereafter, the prepared IG and catalyst solutions were mixed, followed by sonication for 30 min and stirring at RT for 12 h sequentially. After the foregoing procedure, the mixture was filtered under reduced pressure to remove free metal catalysts, and the powdery resulting products were rinsed with acetone. The final precipitated product was dried at RT for 24 h under reduced pressure. 2.8. Electrochemical Measurements. The ORR measurements were performed using a three-electrode system including a glassy carbon rotating disk electrode (RDE) (diameter: 3 mm, area: 0.0706 cm2) as a working electrode, Ag/AgCl (3 M KCl) as a reference electrode, and Pt mesh (1 × 1 cm2) as a counter electrode. The working electrode was prepared by loading the ink on a glassy carbon electrode, where the loaded ink was prepared by dispersion of the synthesized catalysts (different optimal contents) in a mixture of 2propanol (412.16 μL), water (82.43 μL), and 5 wt % Nafion solution (5.41 μL). After optimization tests, the loaded Pt mass for Pt/C, PdPt, PdPt@Pt, Mo-PdPt@Pt, PdPt/rGO, PdPt/IG, PdPt@Pt/IG, and Mo-
PdPt@Pt/IG was determined as follows: 13.9, 29.9, 31.7, 4.8, 1.8, 2.1, and 1.4 μg/cm2, respectively (Table S1). The cyclic voltammograms were acquired by cycling between 0.03 and 1.2 VRHE at a potential scan rate of 50 mV s−1 in the electrolyte comprising 0.1 M HClO4 aqueous solution after purging with Ar. The reference electrode was converted to a reversible hydrogen electrode (RHE). The RHE calibration was conducted in a H2-saturated electrolyte with platinum wire as a working electrode. The potential was swept near the thermodynamic potential (− 0.268 V) for the H+/H2 reaction and calculated using E(RHE) = E(Ag/AgCl) + 0.268 V. The electrochemically active surface areas (ECSAs) were calculated by integrating the hydrogen desorption charge (QH) on the CV curves with a reference value of 210 μC cm−2 for desorption of a monolayer of hydrogen from a Pt surface and then divided by the mass of catalysts loaded on the working electrode (Supporting Information, eq S2). The electrochemical impedance measurements were performed in the frequency range of 10kHz to 100 mHz under signal amplitude of 0.6 V in HClO4 aqueous solution (0.1 M). The electron-transfer number (n) and ORR activities were calculated using eqs S3−S5, respectively (Supporting Information). ORR measurements were performed in O2-saturated HClO4 aqueous solution (0.1 M) at a potential scan rate of 10 mV s−1 and a rotation speed of 1600 rpm. The accelerated durability tests (ADT) were performed by acquiring the CV and ORR polarization curves while sweeping from 2000 to 10 000 cycles between 0.6 and 1.1 VRHE at a potential scan rate of 50 mV s−1 in O2-saturated HClO4 aqueous solution (0.1 M) at room temperature.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Mo-Doped PdPt@Pt Octahedra. The Mo-doped PdPt@Pt core−shell octahedra were synthesized by a two-step and facile one-pot process. As a first step, aqueous solutions of disodium tetrachloropalladate (Na2PdCl4) and chloroplatinic acid (H2PtCl6) as a precursor, N,N-dimethylformamide (DMF) as a solvent and a reducing agent, and polyvinylpyrrolidone (PVP) and benzoic acid (BA) as structure-directing agents were sequentially added to the reaction flask to synthesize a PdPt alloy core. For the second step, the PdPt@Pt core−shell structure and doping with Mo were completed by addition of molybdenumhexacarbonyl (Mo(CO)6) and extra aqueous solution of H2PtCl6 to the prepared PtPd alloy unpurified suspension. Introducing a low content of Pt into the core was strategically designed for avoiding self-nucleation and dendritic growth during synthesis of the Pt shell.28,40 The structure of Mo-PdPt@Pt was evaluated by STEM and HR-TEM analyses (Figures 1A and B, respectively). The STEM image of Mo-PdPt@Pt revealed the octahedral structure with its typical facets and good uniformity in size with an average edge length of 5.2 ± 0.5 nm. The lattice-resolved HRTEM image of the individual Mo-PdPt@Pt oriented along the [011] zone axis clearly exhibited the octahedral structure with well-defined fringes and a lattice d-spacing (approximately 0.22 nm). The defined edge lattice d-spacing was almost consistent with the lattice d-spacing of the (111) plane of the facecentered cubic (FCC) structure derived from the powder X-ray diffraction pattern of Mo-PdPt@Pt (Figure S2). The crystal lattice in the HR-TEM image and corresponding fast Fourier transform (FFT) pattern suggest good crystallinity (Figure 1B). Elemental analysis of Pt, Pd, and Mo was performed by STEMEDX (Figure S3). The resulting ratio was 45.7/53.2/1.1, which is similar to the results obtained from inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (Pt/ Pd/Mo = 42.6/55.9/1.5) (Table S2). The core−shell structure of the Mo-PdPt@Pt was confirmed by high resolution STEM (HR-STEM) (Figure 1C). From the C
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Structural and compositional analyses of the Mo-PdPt@Pt core−shell octahedra. (A) Representative STEM image; (B) latticeresolved HR-TEM image (inset: the FFT pattern); and (C) HRSTEM image (green dots: Mo-doped Pt atoms). (D) Elemental linescanning profiles along the direction of an arrow marked in the inset. (E) STEM-EDX elemental mapping images of Pd, Pt, and Mo and the merged image.
Figure 2. (A) Elemental line-scanning profiles along the directions (a and b) of arrows marked on the STEM image (A1) of the Mo-PdPt@ Pt. (B1−B3) Pt 4f, Pd 3d, and Mo 3d XPS spectra of the Mo-PdPt@ Pt, respectively.
octahedra was 50/50 for the appropriate comparison with the Mo-PdPt@Pt (Pd/Pt = 55.9/42.6; Table S2). The composition of the PdPt and PdPt@Pt octahedra were determined by ICPAES, resulting in the ratios of Pd/Pt = 51.6/48.4 and 48.7/51.3, which correspond to the PdPt and the PdPt@Pt, respectively (Table S2). In particular, the PdPt@Pt exhibited substantially higher Pt composition than that of the Mo-PdPt@Pt. This result can originate from the different mole ratios of the Pd and Pt precursors as mentioned in experimental section for the synthesis of the PdPt@Pt and the Mo-PdPt@Pt. The structure of the acquired PdPt and PdPt@Pt was investigated by STEM, exhibiting the octahedral structure with its typical facets and good uniformity in size (4.3 ± 0.3 and 5.0 ± 0.6 nm, respectively) (Figures 3A1 and B1, respectively). The latticeresolved HR-STEM images and their corresponding FFT patterns of both the individual PdPt and PdPt@Pt octahedra
difference in contrast between the Mo-doped Pt shell and the PdPt alloy core, it could be determined that the PdPt alloy core was encapsulated in a shell comprising 2−3 layers of Mo-doped Pt. The Pt shell thickness was determined to be approximately 0.8 nm, which corresponds well with 2−3 Pt atom layers.39 The STEM-EDX line-scanning profile of Mo-PdPt@Pt along the direction of the arrow marked in Figure 1D inset was applied to confirm its core−shell structure and indicated that the PdPt alloy core was covered by Pt and Mo atoms as a shell, and Mo atoms were primarily located at the edge of the octahedra (Figure 1D). To visualize and confirm these compositional results, STEM-EDX elemental mapping analysis was performed, demonstrating that the distribution of Pd atoms was mainly at the core region, whereas larger contents of Pt and Mo were observed particularly at the shell region of the octahedron (Figure 1E). As recent reports have shown that the atomic arrangement in Mo-doped Pt−Ni nanocrystals preferred to exist on the surface calculated via density functional theory (DFT), and Mo is more stable on the vertex site as absorbed oxygen can induce Mo to segregate to the surface in its oxidized state.38 To compare with this theory, we performed EDX line-scanning profiles of our Mo-PdPt@Pt catalyst following the directions a and b of the arrows on the vertex sites (Figure 2A). As shown, higher Mo contents were observed, particularly at the vertex sites relative to the facets (Figures 2Aa and b). In addition, XPS analysis of the Mo-PdPt@Pt catalyst also noticeably supported the aforementioned computational results, in which higher composition of Mo was detected relative to corresponding EDX. The Mo 3d XPS spectrum of Mo-PdPt@Pt revealed that the major state of Mo on the surface was the oxidized state (mainly MoO3), which largely differed from the mainly metallic state of Pt and Pd (Figures 2B). The PdPt alloy and PdPt@Pt core−shell octahedra, which are intermediate forms of the Mo-PdPt@Pt, were synthesized for comparative evaluation of Mo-PdPt@Pt in ORR. The target Pd and Pt composition ratio for the PdPt and the PdPt@Pt
Figure 3. (A1 and B1) Representative STEM image, (A2 and B2) lattice-resolved HR-STEM image (inset: the FFT pattern), and (A3 and B3) elemental line-scanning profiles along the direction of arrow marked in the inset STEM image of (A) the PdPt alloy octahedra and (B) the PdPt@Pt core−shell octahedra. D
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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aggregation were adhered on the rGO surface (Figure 4B). From these investigations, we can conclude that the role of the ionic block copolymer is significantly important in the control of the interface of the graphene-based nanocomposite catalysts. For the effective integration of the Mo-PdPt@Pt with a graphene support, we employed the tunable ex situ mixing system using IG by exploiting enhanced interfacial interactions between graphene and metal NPs. Compositional tuning of Mo-PdPt@Pt to IG was performed by changing the mixing weight ratios from 0.2 to 4.0, and the morphological changes were evaluated by STEM (Figures 5A and S4). The morphology of the Mo-PdPt@Pt/IG showed that the MoPdPt@Pt octahedra were adhered on certain regions of the IG support. The number of adhered Mo-PdPt@Pt increased with an increase in the Mo-PdPt@Pt fraction in the mixture. The rationally adaptable morphology for the catalyst application was observed at the Mo-PdPt@Pt to IG ratio of 2.0, where MoPdPt@Pt was well-dispersed and the entire IG surface was covered with an adequate metal content (16.7 wt % determined by STEM-EDX) (Figure 5A). However, a few regions found to be aggregated between metal NPs were observed, probably originating from hydrogen bonds between residual BAs and PVPs on the surface of metal NPs. Meanwhile, although the ratio (Mo-PdPt@Pt to IG) exceeded 2.0, the average metal content of the particles adhering to IG (Mo-PdPt@Pt/IG = 4.0) did not increase significantly (19.2 wt %), and there was an increase in the amount of aggregated regions (Figure S4D). To be clear, the reason with respect to enhanced interfacial interactions, the STEM-EDX elemental mapping of Mo-PdPt@ Pt/IG for C, O, and N atoms originated from primarily ionic block copolymers and structure directing agents was performed, demonstrating that relatively higher quantity of C, O, and N atoms was observed in the certain regions which were almost consistent with the location of the Mo-PdPt@Pt octahedra on the IG (Figure 5B). These results indicated that the introduced organic reagents acted as an interfacial adhesive control unit between the rGO support and the
[email protected],53 On the basis of our strategy, the morphology-forming mechanism may be suggested as follows: the O and N atom enriched ionic block copolymers can play the role of an adhesive by interacting with benzoic acid on the surface of the metal NPs, leading to stable confinement of the metal NPs on the IG support (Figure 5C). The PdPt alloy and PdPt@Pt core−shell octahedral catalysts were introduced to IG using the same ex situ protocol used for the Mo-PdPt@Pt/IG and characterized by STEM and STEMEDX, demonstrating morphological similarity to the corresponding Mo-PdPt@Pt/IG with slightly higher metal contents (20.2 and 19.4 wt %, respectively) (Figures 6A and S5, respectively). To confirm the ionic block copolymer doping effects, the rGO without the ionic block copolymer was also employed as a support for comparison and fabricated with PdPt octahedra (PdPt/rGO). Although the fabricating protocols for PdPt/rGO were similar to those used for the PdPt/IG synthesis, STEM images of PdPt/rGO showed that the irregularly distributed and severely aggregated PdPt octahedra were observed, indicating the important role of the ionic block copolymers on the support for the proper interfacial cohesion between the metal and the support (Figure 6B1). The morphology forming mechanism for the PdPt/rGO can be similar to that of the synthesized catalysts except for one thing: the interfacial interaction between metal NPs was dominated because of the weak interfacial interaction between metal NP and rGO (Figure 6B2). This finding confirms that the
indicated good crystallinity (Figures 3A2 and B2, respectively). The HR-STEM image of the PdPt octahedron showed uniform contrast, while that of the PdPt@Pt octahedron showed clear borders derived from the contrast difference of the facets from the edges, which can be generally observed in the core−shell structure.39,40 The compositional distribution of Pd and Pt atoms on the PdPt and PdPt@Pt octahedra were investigated by STEM-EDX line-scanning profile, measured along the direction of the arrows marked on the STEM images of Figures 3A3 and B3, insets. The PdPt octahedron exhibited uniform compositional distribution in the both edge and facet. In contrast, the PdPt@Pt octahedron showed Pt composition at the edges relatively higher than that of Pd. These results indicated that the PdPt alloy and PdPt@Pt core−shell octahedra were well-synthesized as reference materials. 3.2. Effective Integration of Mo-Doped PdPt@Pt Octahedra with IG via an ex Situ Method. Controlled methods for effective integration of shaped Pt-based metal NPs with graphene supports have yet to be investigated in depth. Most approaches were performed using the in situ reaction, which is one universal method because of the simplicity and efficacy of this procedure,50 but revealed such limitations in the uniform integration and synthesis of well-defined shape and size of metal NPs on graphene. With regard to these issues, Bai et al. recently reported the electrocatalyst incorporating PtPd nanocubes with rGO. Although they successfully synthesized shape-controlled PtPd nanocubes, a very low number of PtPd nanocubes were irregularly introduced on the rGO support due to the weak interfacial interaction.51 In addition, Hu et al. also was dedicated to the development of three-dimensional graphene supported shaped Pt/PdCu ternary nanoboxes. However, control of the shape and size of nanoboxes was not well-defined. This can be anticipated due to the interference of graphene during the in situ synthesis of nanoboxes.52 We also tried to confirm the in situ method for the synthesis of the MoPdPt@Pt on the IG using the similar procedures used for synthesis of the Mo-PdPt@Pt, resulting in the STEM image showed the uncontrolled shape and size with mass aggregation of metal NPs on IG (Figure 4A). In addition, the use of rGO as a support for the synthesis of the Mo-PdPt@Pt/rGO using an in situ method showed that much fewer metal NPs with severe
Figure 4. Representative STEM images at the different magnifications of (A) the Mo-PdPt@Pt/IG and (B) Mo-PdPt@Pt/rGO synthesized by an in situ method (Supporting Information). E
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. (A) Representative STEM images of the Mo-PdPt@Pt/IG at the compositional ratio of 2.0 (Mo-PdPt@Pt to IG) at different magnifications. (B) STEM-EDX elemental mapping images of the Mo-PdPt@Pt/IG for C, O, N, and their merged image. (C) Schematic illustration for the plausible morphology forming mechanism of the Mo-PdPt@Pt/IG.
Figure 6. Representative STEM images of (A) PdPt/IG and (B1) PdPt/rGO at the compositional ratio of 2.0 (metal to support) at different magnifications. (B2) Schematic illustration for the plausible morphology forming mechanism of the PdPt/rGO.
of all synthesized catalysts exhibited a positive shift relative to corresponding Pt/C, indicating ORR activity higher than that of Pt/C (Figure 7B). Notably, the Mo-PdPt@Pt/IG exhibited the largest positive shift (47 mVRHE) from that of commercial Pt/C and followed sequentially by PdPt@Pt/IG (30 mVRHE), PdPt/IG (13 mVRHE), and PdPt/rGO (3 mVRHE). This can be anticipated owing to 2−3 Pt shell layers and the change in the electronic structure by Mo surface doping, which can lead to
enhanced interfacial interaction derived from the ionic block copolymer is a key parameter to form effective integration of the Mo-PdPt@Pt octahedra with a graphene support followed by the suggested morphology forming mechanism. 3.3. Evaluation of Electrocatalytic Activity and Durability. Electrochemical measurements were performed with the Mo-PdPt@Pt/IG, PdPt@Pt/IG, PdPt/IG, PdPt/rGO, and commercial Pt/C (Figures 7A and B). After the initial kinetic-diffusion control region, the half-wave potential (E1/2) F
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 7. Electrocatalytic properties of the Mo-PdPt@Pt/IG, PdPt@Pt/IG, PdPt/IG, PdPt/rGO, and commercial Pt/C. (A) CV profiles in Arsaturated 0.1 M HClO4 at a scan rate of 50 mV s−1. (B) ORR polarization curves in O2-saturated 0.1 M HClO4 at a scan rate of 10 mV s−1. (C) Nyquist plots for the electrochemical impedance measurements in the frequency range of 10kHz to 100 mHz under signal amplitude of 0.6 V (inset: the equivalent circuit). (D) Specific ECSAs. (E) Specific activity and (F) mass activity calculated at 0.9 VRHE. In panels A and B, current densities were normalized in reference to the geometric area of a RDE (0.0706 cm2). 2 −1 g−1 Pt ), and PdPt/rGO (30.8 m gPt ) (Figure 7D). These results confirmed that the Mo-PdPt@Pt/IG exhibited an electrochemically active surface area much higher than that of the other synthesized catalysts. The kinetic current (ik) was calculated by using the K−L equation (1/i = 1/ik + 1/id, where i is the measured current and id is the diffusion limiting current) and normalized by ECSA for specific activity and by the loaded Pt mass for mass activity, derived from eqs S4 and S5, respectively.42 All synthesized catalysts exhibited greatly enhanced ORR performances in terms of the specific and mass activities at 0.9 VRHE in comparison to those of the Pt/C (Figures 7D and E). The MoPdPt@Pt/IG showed highest values in both specific and mass −1 activities (jk,specific: 1.67 mA cm−2 Pt and jk,mass: 2.46 mA μgPt ), exhibiting both 7-fold and 31-fold enhancement in specific and mass activities relative to those of the Pt/C. Notably, the MoPdPt@Pt/IG showed high enhancements in mass activity of 5.8, 2.0, and 1.5-fold higher than that of the PdPt/rGO, PdPt/ IG, and PdPt@Pt/IG, respectively, followed sequentially by PdPt@Pt/IG > PdPt/IG > PdPd/rGO (Figure 7E). Furthermore, the specific and mass activities of Mo-PdPt@ Pt/IG were found to be significantly higher than the target value of the U.S. Department of Energy (DOE) (jk,specific: 0.720 −1 mA cm−2 Pt and jk,mass: 0.44 mA μgPt at 0.9 VRHE), indicating its 1,42,59,60 superior performance in ORR. From these electrocatalytic studies, the large enhancement of the Mo-PdPt@Pt/ IG in electrocatalytic activity can be anticipated because of the ligand effect reflecting the electronic coupling, the optimal passivation of the PdPt core with 2−3 Pt atom layers, and the Mo-doping effect.38−40,45,47,54,55 The rGO and IG also can play as an active material because of their intrinsic electrocatalytic properties, as shown in Figure S8. The IG showed ORR activity slightly better than that of the rGO, specifically exhibiting that the onset potential and E1/2 of the IG showed 12 and 27 mVRHE positive shifts, respectively, relative to those of the rGO. It can be anticipated by the intermolecular charge transfer effect derived from the doped ionic block copolymer.42 To confirm
the abundant active sites on the surface of the Mo-PdPt@ Pt.54,55 The charge transfer capabilities of the electrocatalysts were investigated by electrochemical impedance spectroscopy (Figure 7C). The Nyquist plots exhibited semicircular and linear regions which corresponded to the charge transfer limited region and the diffusion control region, respectively.56 On the basis of the equivalent circuit model, the electrode charge transfer resistance (Rct) was acquired (Figure 7C, inset). From the semicircle in the Nyquist plots, the Rct values were determined to be 147.1, 138.6, 119.6, 112.4, and 94.6 Ω, which correspond to Pt/C, PdPt/rGO, PdPt/IG, PdPt@Pt/IG, and Mo-PdPt@Pt/IG, respectively. These results confirm that the Mo-PdPt@Pt/IG showed the substantially lower charge transfer resistance in comparison to the other electrocatalysts, and the differences in the Rct values corresponded well to the results of half-wave potential in the investigated electrocatalysts. The polarization curves of the Mo-PdPt@Pt/IG, PdPt@Pt/ IG, PdPt/IG, and Pt/C according to the rotation rate were measured to produce the Koutecky−Levich (K−L) plots for the calculation of electrons transfer number (n) (Figure S6).42 The K−L plots of all catalysts at different potentials showed the linear fitting lines corresponding to first-order reaction kinetics with regard to the concentration of dissolved and diffused O2 on the catalysts (Figure S7).47 The number of electrons transferred for Mo-PdPt@Pt/IG was calculated to be ∼4.0 at 0.7−0.9 VRHE by using eq S3, demonstrating that the 4e− ORR mechanism is primarily preferred. This value was almost similar to the other synthesized catalysts PdPt/IG and PdPt@Pt/IG (∼3.9 at 0.7−0.9 VRHE) and commercial Pt/C (∼3.7 at 0.7−0.9 VRHE).57,58 CV profiles were utilized to calculate the ECSA described in eqs S1 and S2 (Figure 7A).42,47 All synthesized catalysts exhibited ECSA values lower than those of the corresponding Pt/C (71.5 m2 g−1 Pt ), and the Mo-PdPt@Pt/IG notably showed the closest value (67.6 m2 g−1 Pt ) to that of Pt/C, followed 2 sequentially by PdPt@Pt/IG (51.5 m2 g−1 Pt ), PdPt/IG (34.6 m G
DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Table 1. Electrocatalytic Properties of Synthesized Catalysts and Several Representative Electrocatalytic Results from Reported State-of-the-Art Pt or/and Pd-Based Catalysts with Various Carbon Supportsa specific activity @ 0.9 VRHE (mA cm−2 Pt )
ECSA (m2 g−1 Pt ) catalyst
a
support
initial
Mo-PdPt@Pt octahedra
IG
67.6
PdPt@Pt octahedra
IG
51.5
PdPt octahedra
IG
34.6
commercial Pt NP
carbon black
71.5
[email protected] icosahedra
carbon black
47.1
Pd@Pt2−3L octahedra
carbon black
53.6
Pd@Pt2−3L nanocube
carbon black
47.6
Pt-on-Pd dendrite
carbon black
48.5
PdPt nanorod
graphene
23.6
Pt NPs
ionomer-doped GO
Pt nanocluster
DNA-doped GO
100.5 66.6
after ADT (cycle #) 63.4 (10k) 41.5 (10k) 23.3 (10k) 30.0 (10k) 26.5 (10k) 46.6 (10k) 30.9 (10k) 24.3 (10k)
initial 1.67 1.17 1.10 0.23
NA 40.6 (10k) 62.6 (10k)
after ADT (cycle #) 1.53 (10k) 1.01 (10k) 0.83 (10k) 0.07 (10k)
mass activity @ 0.9 VRHE (mA μg−1 Pt ) initial 2.46 1.70 1.22 0.08
after ADT (cycle #) 1.99 (10k) 1.35 (10k) 0.62 (10k) 0.01 (10k) 0.36 (10k) 0.43 (10k) 0.19 (10k)
ref this work this work this work E-TEK
1.36
NA
0.64
39
0.91
NA
0.49
0.49
NA
0.24
0.42
NA
0.20
NA
28
0.60
0.52 (1k)
0.14
0.12 (1k)
61
0.50
NA
0.36
NA
45
0.22
NA
0.32
NA
47
40 54
NA: not available.
Figure 8. Electrochemical durability of Mo-PdPt@Pt/IG, PdPt@Pt/IG, and PdPt/IG catalysts versus Pt/C measured in the range of 0.6−1.1 VRHE at a scan rate of 50 mV s−1 in O2-saturated 0.1 M HClO4 solution. ORR polarization curves and corresponding cyclic voltammograms of the (A) MoPdPt@Pt/IG, (B) PdPt@Pt/IG, (C) PdPt/IG, and (D) Pt/C catalysts before and after 10k potential cycles. (E) Normalized ECSA, specific activity, and mass activity of catalysts as a function of potential cycles.
The electrochemical measurement of PdPt/rGO was carried out to confirm the ionic block copolymer doping effect, with PdPt/IG showing ECSA value (1.1-fold increase), specific activity (1.15-fold increase), and mass activity (2.8-fold increase) higher than those of the corresponding PdPt/rGO (Figures 7C−E). Lower ORR properties of PdPt/rGO can be expected mainly owing to the severely aggregated PdPt octahedra on the rGO support and the absence of the intermolecular charge-transfer effect originating from the ionic block copolymer doping.42,45,47 These results are in close agreement with the results reported by Zeng et al, where ionic
the compositional effect, the electrocatalytic performance of the PdPt, PdPt@Pt, and Mo-PdPt@Pt without carbon supports was also evaluated, as shown in Figure S8. The results exhibited a trend similar to those of the catalysts with the IG supports. However, their ORR activities were substantially lower than those of the catalysts with the IG support (Figures 7 and S8). From these investigations, we can conclude that the compositional changes in the metal nanostructure and the presence of the rationally designed carbon support highly impact the enhanced ORR activity. H
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ACS Applied Materials & Interfaces polymer-doped GO with Pt NPs achieved ∼1.3-fold higher specific activity and ∼1.4-fold higher mass activity relative to those of corresponding Pt NPs/GO (Table 1).45 This finding indicated that the use of the IG as a support can lead to better morphology as well as higher ORR performance than those of corresponding rGO without the ionic block copolymer. Comparative evaluation of the synthesized catalysts in ORR properties was performed in contrast with the previously reported state of the art Pt or/and Pd-based electrocatalysts with various carbon supports is summarized in Table 1. The ORR activity of the PdPt@Pt/IG showed remarkable ORR activities in comparison to state of the art Pt or/and Pd-based electrocatalysts and was comparable to that of Pd@Pt core− shell octahedra and icosahedra supported by carbon black, exhibiting that these catalysts showed similar ECSA and specific activity with lower mass activity relative to those of the PdPt@ Pt/IG (Table 1).39,40 This was explained by the 4-fold smaller particle size of PdPt@Pt octahedra and ionic polymer doping effects. However, the Mo-PdPt@Pt/IG showed ORR properties much higher than those of the PdPt@Pt/IG, indicating that the strategy for effective integration of the Mo-doped PdPt@Pt core−shell octahedra with the ionic block copolymer-functionalized graphene support is a promising candidate for highperformance ORR electrocatalysts (Table 1).28,39,40,45,47,54,61 The accelerated durability test (ADT) of the Mo-PdPt@Pt/ IG, PdPt@Pt/IG, PdPt/IG, and Pt/C was performed in the range of 0.6−1.1 VRHE at a scan rate of 50 mV s−1 in O2saturated 0.1 M HClO4 solution. Using the protocol for ADT described above, CV and ORR polarization curves of the MoPdPt@Pt/IG, PdPt@Pt/IG, and PdPt/IG were acquired as a function of the potential cycles to confirm retention of the ORR performance (Figures S9 and S10). After 10k potential cycles, the Mo-PdPt@Pt/IG achieved exceptionally high electrochemical stability, exhibiting only a mere 6 mV negative shift in E1/2 (Figure 8A). In contrast, although the PdPt@Pt/IG and PdPt/IG showed negative shifts (16 and 36 mV, respectively) larger than those of corresponding Mo-PdPt@ Pt/IG, these synthesized electrocatalysts exhibited stability substantially higher in comparison to that of Pt/C (ΔE1/2 = 118 mVRHE), indicating the largely enhanced electrochemical stability of our electrocatalysts (Figures 8B−D). To investigate the support effect on durability, the PdPt/IG was compared to the reported PdPt nanorod with neat graphene supports, exhibiting that the PdPt/IG showed retention in ECSA (91.5%) better than that of the corresponding PdPt nanorod/graphene (∼87% after 1k potential cycles) albeit larger potential cycles (2k) (Table 1).61 In addition, this comparison was in good agreement with the result obtained by Tiwari et al, where they demonstrated that the DNA doping effect resulted in higher retention of the ECSA value (94%) for Pt NPs supported by DNA-doped GO compared that of Pt NPs/GO (67%) after 10k potential cycles.47 This was indicative of the increased interfacial cohesion between the metal and the support and its effects in boosting durability in ORR compared to those without the interfacial linker.42,47 We plotted the normalized ECSA, specific activity, and mass activity of the Mo-PdPt@Pt/IG, PdPt@Pt/IG, PdPt/IG, and Pt/C as a function of potential cycles (Figure 8E). A decline in the ECSA values of all synthesized catalysts with increasing potential cycles was observed, following a linear and steady trend, whereas the ECSA profile of Pt/C declined abruptly with a reduction as high as 30% after 2k potential cycles, exhibiting
only 41% retention after 10k potential cycles. To confirm this, structure changes of Pt/C before and after ADT were performed through the TEM analysis. After 10k potential cycles, the size of Pt NPs in Pt/C increased up to several times relative to initial size (3−4 nm), and the severe aggregation and the reduced number of Pt NPs were also observed, indicating its weak stability in ORR (Figure S11). Whereas, after 10k potential cycles, the ECSA value of the Mo-PdPt@Pt/IG was significantly retained (93.8%), followed by the PdPt@Pt/IG (80%) and PdPt/IG (67%) (Figure 8E). This high retention capability of the Mo-PdPt@Pt/IG in ECSA can be attributed to the following reasons: (1) Pt passivation, (2) Mo-doping effects, and (3) anchoring ability. The morphological and compositional characterizations of the synthesized catalysts after 10k potential cycles were carried out by STEM and STEM-EDX to confirm our proposed reasons. The PdPt/IG showed large structural changes, exhibiting mostly connected and largely aggregated PdPt NPs with the greatly reduced Pd content from 54.2 to 33.1% (Figure S12A1). Furthermore, the reduced Pd content at the shell region led to the structure change to the core−shell structure defined by the STEM-EDX elemental line-scanning profile analysis (Figure S12A2). Meanwhile, despite the full shape change from octahedra to sphere-like, an increase of the aggregated regions, and the loss of Pd composition (∼12%), PdPt@Pt/IG showed improved stability in ORR over PdPt/IG because of Pt passivation on the PdPt core (Figure S12B). In the case of the Mo-PdPt@Pt/IG, we observed a larger retention in shape, metal distribution, and Pd content (∼7% loss) in comparison to PdPt/IG and PdPt@Pt/IG, which could be explained by the Pt passivation effect along with the Modoping effect (Figure S12C).38 The morphological structure change and loss in ECSA of the various types of catalysts after ADT influenced retention of ORR activities. After 10k potential cycles, Mo-PdPt@Pt/IG showed the highest retention of the mass activity (81.5%) and specific activity (91.2%), followed in sequence by PdPt@Pt/IG, PdPt/IG, and Pt/C, which corresponded well to the above morphological/compositional analyses and degradation of ECSA (Figure 8E). Despite the fact that Mo-PdPt@Pt/IG achieved highly enhanced electrochemical durability after 10k potential cycles, exhibiting retention percentage in mass activity 6.3-fold higher than that of corresponding commercial Pt/C (12.8%), the changes in the structure and elemental composition derived from the Pd dissolution should be addressed for the long-term application. Xie et al. provided important insight into the Pd dissolution phenomenon, demonstrating that the ORR durability of the Pd@Pt core− shell nanocubes was largely dictated by the number of Pt shell layers. Although Pd@Pt core−shell nanocubes with 2−3 Pt atom shell layers showed ORR activity much higher than that with 6 atom shell layers, the shell comprising 6 Pt atom layers resulted in electrochemical durability (6% loss in mass activity) almost 4-fold higher than that of the shell comprising 2−3 Pt atom shell layers (21% loss in mass activity) after 10k cycles of durability testing owing to better Pt passivation of the Pd core.54 The results indicated that the electrochemical durability of the present core−shell structured catalysts (2−3 Pt atom layers) can prospectively be further enhanced by controlling the thickness of the Mo-doped Pt shell layers onto the PdPt core. I
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(2) Winter, M.; Brodd, R. J. What Are Batteries, Fuel Cells, and Supercapacitors? Chem. Rev. 2004, 104, 4245−4270. (3) Higgins, D.; Zamani, P.; Yu, A.; Chen, Z. The Application of Graphene and its Composites in Oxygen Reduction Electrocatalysis: a Perspective and Review of Recent Progress. Energy Environ. Sci. 2016, 9, 357−390. (4) Steele, B. C.; Heinzel, A. Materials for Fuel-cell Technologies. Nature 2001, 414, 345−352. (5) Cho, K. Y.; Cho, A.; Kim, H. − J.; Park, S. − H.; Koo, C. M.; Kwark, Y. J.; Yoon, H. G.; Hwang, S. S.; Baek, K.-Y. Control of hard Block Segments of Methacrylate-based Triblock Copolymers for Enhanced Electromechanical Performance. Polym. Chem. 2016, 7, 7391−7399. (6) Liu, Z.; Pulletikurthi, G.; Endres, F. A Prussian Blue/Zinc Secondary Battery with a Bio-Ionic Liquid-Water Mixture as Electrolyte. ACS Appl. Mater. Interfaces 2016, 8, 12158−12164. (7) Li, X.; Wei, B. Q. Supercapacitors based on Nanostructured Carbon. Nano Energy 2013, 2 (2), 159−173. (8) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-ion Batteries: a Review. Energy Environ. Sci. 2011, 4, 3243−3262. (9) Larcher, D.; Tarascon, J. M. Towards Greener and more Sustainable Batteries for Electrical Energy Storage. Nat. Chem. 2014, 7, 19−29. (10) Chandan, A.; Hattenberger, M.; El-kharouf, A.; Du, S.; Dhir, A.; Self, V.; Pollet, B. G.; Ingram, A.; Bujalski, W. High Temperature (HT) Polymer Electrolyte Membrane Fuel Cells (PEMFC) − A Review. J. Power Sources 2013, 231, 264−278. (11) Neburchilov, V.; Martin, J.; Wang, H.; Zhang, J. A Review of Polymer Electrolyte Membranes for Direct Methanol Fuel Cells. J. Power Sources 2007, 169, 221−238. (12) Sharma, S.; Pollet, B. G. Support Materials for PEMFC and DMFC Electrocatalysts - a Review. J. Power Sources 2012, 208, 96− 119. (13) Wu, J. F.; Yuan, X. Z.; Martin, J. J.; Wang, H. J.; Zhang, J. J.; Shen, J.; Wu, S. H.; Merida, W. A Review of PEM Fuel Cell Durability: Degradation Mechanisms and Mitigation Strategies. J. Power Sources 2008, 184, 104−119. (14) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. HighPerformance Electrocatalysts for Oxygen Reduction derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443−447. (15) Kou, Z.; Cheng, K.; Wu, H.; Sun, R.; Guo, B.; Mu, S. Observable Electrochemical Oxidation of Carbon Promoted by Platinum Nanoparticles. ACS Appl. Mater. Interfaces 2016, 8, 3940−3947. (16) Chen, S.; Wei, Z.; Qi, X.; Dong, L.; Guo, Y. G.; Wan, L.; Shao, Z.; Li, L. Nanostructured Polyaniline-Decorated Pt/C@PANI CoreShell Catalyst with Enhanced Durability and Activity. J. Am. Chem. Soc. 2012, 134, 13252−13255. (17) Sun, S.; Zhang, G.; Geng, D.; Chen, Y.; Li, R.; Cai, M.; Sun, X. A Highly Durable Platinum Nanocatalyst for Proton Exchange Membrane Fuel Cells: Multiarmed Starlike Nanowire Single Crystal. Angew. Chem., Int. Ed. 2011, 50, 422−426. (18) Xia, B. Y.; Ng, W. T.; Wu, H. B.; Wang, X.; Lou, X. W. SelfSupported Interconnected Pt Nanoassemblies as Highly Stable Electrocatalysts for Low-Temperature Fuel Cells. Angew. Chem., Int. Ed. 2012, 51, 7213−7216. (19) Chen, Z. W.; Higgins, D.; Yu, A. P.; Zhang, L.; Zhang, J. J. A Review on Non-precious Metal Electrocatalysts for PEM Fuel Cells. Energy Environ. Sci. 2011, 4, 3167−3192. (20) Wu, J.; Yang, H. Platinum-based Oxygen Reduction Electrocatalysts. Acc. Chem. Res. 2013, 46, 1848−1857. (21) Wang, X.; Figueroa-Cosme, L.; Yang, X.; Luo, M.; Liu, J.; Xie, Z.; Xia, Y. Pt-based Icosahedral Nanocages: Using a Combination of {111} Facets, Twin Defects, and Ultrathin Walls to Greatly Enhance Their Activity toward Oxygen Reduction. Nano Lett. 2016, 16, 1467− 1471. (22) Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y. Highly Efficient Nonprecious Metal Catalysts towards Oxygen Reduction Reaction
4. CONCLUSIONS The Mo-doped PdPt@Pt core−shell octahedra were effectively synthesized by using a facile one-pot process. The obtained Mo-PdPt@Pt octahedra comprised a PdPt alloy core, 2−3 Pt atom shell layers, and Mo-doping mainly on the surface particularly segregated on the vertexes and edges, leading to the highly active surface. To the best of our knowledge, this is the first example of the synthesis of the Mo-PdPt@Pt core−shell octahedra and demonstration of their unique features. The developed Mo-PdPt@Pt octahedra were successfully integrated with ionic block copolymer-doped graphene using the ex situ mixing strategy by exploiting the strong interfacial cohesion. The all-encompassing features of Mo-PdPt@Pt combined with the advanced support augmenting the interfacial cohesion contributed to give compelling ORR performance, exhibiting large enhancement in the specific activity (7-fold increase) and mass activity (31-fold increase) relative to those of commercial Pt/C and far surpassing the target values suggested by U.S. DOE and even superior to previous studies centering on either development of the metal NP or support. Furthermore, the ADT results of the Mo-PdPt@Pt indicated its robust and promising durability after intensive 10k potential cycles. We believe that this fundamental research can provide a good platform for designing various Pt-based metal NPs and their effective integration with advanced functional carbon-based supports, which may afford enhanced performance in nextgeneration energy storage devices such as fuel cells, metal-air batteries, and beyond.
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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/acsami.6b13299. Detailed experimental procedures, characterization of IG support, calculation of specific ECSA, electron-transfer numbers, and ORR activities (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ho Gyu Yoon: 0000-0003-3083-4826 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (Project 10041850, Development of Prototype 154 kV Compact Power Cables with Insulation Thickness Decreased by 15% or More Based on Ultrasuper Smooth Semiconductive Materials) funded by the Ministry of Trade, Industry & Energy (MI, Korea) and partially supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (Project 2016M3A7B4027805).
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DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.6b13299 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
