Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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3D Robust Carbon Aerogels Immobilized with Pd3Pb Nanoparticles for Oxygen Reduction Catalysis Gengtao Fu,† Yu Liu,† Zexing Wu,‡ and Jong-Min Lee*,† †
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore State Key Laboratory Base of Eco-Chemicmaal Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science & Technology, Qingdao 266042, P. R. China
‡
S Supporting Information *
ABSTRACT: Development of desired electrocatalysts with high activity, excellent stability, and high selectivity for oxygen reduction reaction (ORR) is of significant importance for fuel cells. Herein, a high-efficiency electrocatalyst for the ORR is reported which utilizes a cross-linked carbon aerogel of reduced oxide graphene (rGO), and carbon nanotubes (CNTs) functioned as a three-dimensionally (3D) scaffold for anchoring ordered Pd3Pb nanoparticles (Pd3Pb/rGO-CNTs). The Pd3Pb/rGO-CNTs were synthesized by a facile and scalable route, mainly depending on the formation of a poly(vinyl alcohol) cross-linked GO-CNTs double-network hydrogel that allows for the efficient capture of highly active Pd3Pb particles after pyrolysis. The resulting composite exhibits remarkable ORR performances in terms of high half-wave potential, excellent stability, and methanol tolerance ability, resulting from the synergistic effect of ordered structure of Pd3Pb phase and 3D interconnected double-network of rGO-CNTs aerogels. The ordered Pd3Pb intermetallic compounds mainly work as the active centers for the ORR, while rGO-CNTs aerogels not only prevent Pd3Pb nanoparticles from aggregation but also provide open and accessible pores for fast transportation of reactants to the active centers. KEYWORDS: double-network hydrogels, cross-linked carbon aerogels, Pd3Pb intermetallic compounds, oxygen reduction, synergetic effects sites distribution due to their ordered atom arrangements.30−35 Such ordered-PdM induced enhancement of the electrocatalytic reactions has been observed in PdFe,33,36,37 PdPb,30,32,34 PdCo,38 and PdSn39 systems. To build the intermetallic ordered phases, thermal treatments at high temperatures are necessary, but which generally results in serious agglomeration of nanoparticles. Additionally, carbon black is the most commonly used catalyst−support among these ordered catalysts. Nevertheless, traditional methods that deposit particles on carbon black failed to give well-anchored particles with controllable particle size. Therefore, it is extremely critical to develop a suitable strategy for the synthesis of ordered PdM phase with controllable particle size and firm integration on carbon supports but remains challenging. In this contribution, we developed an effective synthetic route for large-scale synthesis of 3D interconnected rGO-CNTs aerogels supported ordered Pd3Pb nanoparticles (Pd3Pb/rGOCNTs) building from Pd2+/Pb2+-containing PVA/GO-CNTs hydrogel as a cathode catalyst for the ORR. The formation of PVA cross-linked GO-CNTs hydrogel allows for the efficient capture of highly active Pd3Pb nanoparticles after pyrolysis, which refrains from peeling off and agglomeration of particles
1. INTRODUCTION Fuel cells have long been regarded as one of most potential power generators for a variety of energy conversion devices because of their high energy density and low pollutant emission.1−6 Oxygen reduction reaction (ORR) is a critical half-reaction for fuel cells, however, which generally suffers from sluggish kinetics and requires effective catalysts to drive.7−11 Although noble metal platinum (Pt)-based catalysts possess excellent electrocatalytic properties for the ORR, the commercially large-scale applications have been limited by their scarcity and high cost.12−14 Intensive efforts so far have been committed to replace precious Pt-based catalysts through various strategies without compromising the catalytic activity.15−20 The Sabatier principle indicates that palladium is the second best active metal toward ORR,21 and its activity can be further enhanced by alloying with transition metals M (M = Co, Ni, Fe, and so forth), as verified by both experimental and theoretical evidence,22−26 owing to the ligand effect and strain effect.27,28 However, the poor structure stability of these widely studied PdM alloys is a severe limitation for their practical applications.29,30 This is because that PdM alloys present the disordered atomic structures in which atom positions are occupied by Pd and M randomly,29,31 resulting in the atom migration easily under the electrochemical reaction conditions. Relative to the disordered alloys, the intermetallic compounds possess chemically robust structure and uniform active © XXXX American Chemical Society
Received: February 22, 2018 Accepted: April 10, 2018
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DOI: 10.1021/acsanm.8b00275 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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ACS Applied Nano Materials
Figure 1. Schematic illustration of the preparation of Pd3Pb/rGO-CNTs aerogels.
Figure 2. (a) XRD pattern of Pd3Pb/rGO-CNTs; inset shows crystal structure of ordered Pd3Pb phase. (b) TGA curve of Pd3Pb/rGO-CNTs. (c) EDX spectrum. High-resolution (d) C 1s, (e) Pd 3d, and (f) Pb 4f XPS spectra. were dissolved in 625 μL of poly(vinyl alcohol) (PVA, 1750 ± 50) solution (16 mg mL−1). It was then mixed with a 2.5 mL aqueous solution containing graphene oxide (GO, 4 mg mL−1) and carbon nanotubes (CNTs, 4 mg mL−1), and the mixture was shaken violently to form a hydrogel. After continuous ultrasonic for 5 min, the homogeneous Mn+-PVA/GO-CNTs hydrogel hydrogel was formed. Subsequently, the Mn+-PVA/GO-CNTs aerogel was obtained through a freeze-drying process (24 h) by using the hydrogel as a precursor. The products were calcined at 600 °C under a flow of 10% H2/Ar for 12 h and resulted as Pd3Pb/rGO-CNTs aerogels. 2.2. Characterizations. X-ray powder diffraction (XRD) was used to identify phase on a Rigaku diffractometer with Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) was performed using a PerkinElmer thermal analysis system, through heating from 40 to 900
from carbon support during the electrochemical reaction. The 3D interconnected architectures of GO-CNTs hydrogels not only provide accessible and open pores for rapid transportation of reactants to active sites but also are conducive to maintaining the high conductivity of catalysts. As a result, the Pd3Pb/rGOCNTs aerogels exhibit outstanding electrocatalytic performances for the ORR, including high activity, reliable stability, and robust poison tolerance that are superior to those of carbonsupported Pt and Pd catalysts.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Pd3Pb/RGO-CNTs Aerogels. Typically, 8.0 mg of palladium chloride (PdCl2) and 5.0 mg of lead nitrate (Pb(NO3)2) B
DOI: 10.1021/acsanm.8b00275 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 3. (a, b) SEM images of Pd3Pb/rGO-CNTs. (c−e) TEM images of Pd3Pb/rGO-CNTs at different magnification. (f) Particle-size distribution histogram recorded from panel e. (g) HRTEM image and corresponding FFT pattern. (h) STEM image, line scanning profiles, and EDX element mappings. °C (rate: 10 °C min−1) under air atmosphere. X-ray photoelectron spectroscopy (XPS) was obtained with a Thermo VG Scientific ESCALAB 250 spectrometer, and the binding energy was calibrated via the peak energy of C 1s (284.6 eV). N2 adsorption−desorption curves were tested on a Micromeritics ASAP 2050 system at 77 K. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrum (EDX) were carried out with a Hitachi S4800 SEM. Transmission electron microscopy (TEM) and scanning TEM (STEM) were performed with a JEOL 2010F TEM/STEM (200 kV). 2.3. Electrocatalytic Methods. Electrocatalysis was performed in CHI 760E electrochemical analyzer, in which a standard threeelectrode system was used. The rotating disk electrode (RDE) or rotating ring−disk electrode (RRDE), Pt wire, and calomel reference electrode (SCE) worked as the working electrode, auxiliary electrode, and reference electrode, respecttively. The ink of catalysts was prepared through dispersing the mixture of catalyst (5 mg), ethanol (1 mL), and 5 wt % Nafion solution (20 μL). Then, 10 μL of ink was spread onto the electrode surface, and the loading was 250 μg cm−2. The ORR activities of catalysts were measured in O2-saturated 0.1 M KOH at predefined rotation speed with a scan rate of 5 mV s−1 by the RDE voltammograms.
noteworthy that owing to amphiphilic nature of GO and π−π attraction between GO and CNTs, the CNTs could be well dispersed and adsorbed onto the GO surface.40,41 Hydrogen bonding between the rich-hydroxyl groups of PVA chains and oxygen-containing groups of GO and carboxylated CNTs has been considered to be a main driving force for the formation of hydrogels. Moreover, the abundant oxygenated functionalities of GO and CNTs with a lone electron pair can also bind Mn+ precursors efficiently to form GO-Mn+-CNTs complex through coordination and electrostatic interaction,42 resulting in the desired Mn+-PVA/GO-CNTs hybrid hydrogels. The obtained hybrid hydrogels were then dehydrated through the freezedrying to obtain Mn+-PVA/GO-CNTs aerogels with a highly 3D porous structure (Figure S1). The insertion of tortuous CNTs in Mn+-PVA/GO-CNTs aerogels can bridge the adjacent GO nanosheets and prevent them from restacking as well as increase specific surface area. After pyrolysis in H2 at 600 °C for 6 h, ordered intermetallic Pd3Pb particles could be formed in 3D porous rGO-CNTs aerogels by reduction of Pd and Pb precursors. The experimental details are presented in the Experimental Section. The phase identification of Pd3Pb/rGO-CNTs aerogels was performed with XRD measurement. As shown in Figure 2a, five typical diffraction peaks assigning to (111), (200), (220), (311), and (222) planes with additional six superlattice peaks corresponding to (100), (110), (210), (211), (300), and (310) planes were consistent with ordered intermetallic Pd3Pb phase (Pm-3m (221); a = b = c = 4.035 Å; JCPDS No. 50-1631). Furthermore, five typical diffraction peaks for ordered Pd3Pb/
3. RESULTS AND DISCUSSION 3.1. Structure and Composition of Electrocatalysts. The Pd3Pb/rGO-CNTs aerogels were synthesized via a facile sol−gel polymerization of PVA, GO, carboxylated CNTs, and metal precursors (Mn+), followed by the freeze-drying and pyrolysis treatments, which is schematically illustrated in Figure 1. In the first step, a stable black Mn+-PVA/GO-CNTs hydrogel was formed via mixing two kinds of aqueous solution of GOCNTs and PVA-Mn+ under the continuous ultrasonication. It is C
DOI: 10.1021/acsanm.8b00275 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 4. ORR performances of Pd3Pb/rGO-CNTs, Pt/C, and Pd/C: (a) ORR polarization curves in O2-saturated 0.1 M KOH; (b) Tafel plots derived from (a); (c) rotation-rate-dependent current−potential curves for Pd3Pb/rGO-CNTs, and (d) corresponding K−L plots at different potentials. (e) Electron transfer number n and (f) percentage of H2O2 at different potentials.
ordered Pd3Pb/rGO-CNTs aerogels was determined with TGA (Figure 2b), during which Pd3Pb was oxidized to PdPbO2 and PdO completely (Figure S4). Based on the content of metal oxide, the original Pd3Pb content was calculated to be approximately 28.1 wt %. The molar ratio of Pd and Pb is about 75.9:24.1 as confirmed by the EDX spectrum (Figure 2c), which is similar to that of the content of the precursors. The corresponding weight percentages of Pd and Pb were calculated to be 61.7% and 38.3% (Figure 2c, inset), respectively. The surface composition of ordered Pd3Pb was obtained by XPS measurement. According to the full XPS spectrum, the Pd/Pb surface atomic ratio was determined to be 75.2:24.8 (Figure S5), close to that from EDX analysis, indicating that Pd3Pb phase prepared by the present method has the homogeneous element distribution in the bulk and surface. Meanwhile, the high-resolution C 1s XPS spectrum (Figure 2c) can be deconvoluted into three signals at binding
rGO-CNTs noticeably shift to lower angles relative to those of pure Pd/rGO-CNTs sample (Figure S2), indicating the increase of Pd lattice constant after introducing Pb atoms.30,34 Different from cubic Pd phase, Pd atoms in Pd3Pb occupy half of the octahedral holes and Pb atoms occupy the corner positions (inset in Figure 2a).30,32 A broad diffraction peak at 25.9° was also observed, corresponding to the (002) reflection of the graphitic carbon. The results of XRD pattern confirm that ordered intermetallic Pd3Pb phase was successfully introduced into rGO-CNTs aerogels. The intermetallic structure of Pd3Pb/rGO-CNTs greatly depends on the pyrolysis temperature, as shown in Figure S3. Clearly, the ordered Pd3Pb phase cannot be formed at a relatively low temperature (300 °C) rather than the disordered alloy phase. With increasing the pyrolysis temperature, the ordered superlattice peaks become visible, demonstrating that the intermetallic phases are generated. The Pd3Pb content in the D
DOI: 10.1021/acsanm.8b00275 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Commercial Pt/C and Pd/C catalysts were tested under the same condition for references. Figure 4a shows the ORR polarization curves of three catalysts. As observed, the half-wave potential was changed in the following order: Pd3Pb/rGOCNTs > Pt/C > Pd/C. The Pd3Pb/rGO-CNTs exhibits the highest half-wave potential of 0.862 V, which is about 21 and 32 mV higher than those of commercial Pt/C (0.841 V) and Pd/C (0.830 V), respectively, indicating that O2 was more easily reduced on Pd3Pb/rGO-CNTs catalyst. This half-wave potential value is also comparable to that of many other noble-metal-based ORR electrocatalysts (Table S1), proving high ORR performance of Pd3Pb/rGO-CNTs. On the basis of the Koutecky−Levich (K−L) equation, the kinetic current density (ik, specific activity) was calculated from the polarization curves (Figure 4b). It is easily observed that the ik of Pd3Pb/rGO-CNTs is much bigger than those of Pt/C and Pd/ C catalysts between 0.80 and 0.95 V, manifesting that Pd3Pb/ rGO-CNTs has the best kinetic behavior. Figure 4c shows the rotation-rate-dependent polarization curves of Pd3Pb/rGOCNTs. On the basis of the K−L equation from the diffusioncontrolled region, a series of plots were obtained by the linear relation between current density (j) and ω1/2 at different potentials (Figure 4d). The electron transfer number (n) was calculated to be approaching 4 from the slopes of K−L plots. This result indicates that Pd3Pb/rGO-CNTs undergoes a fourelectron transfer path dominated for the ORR, as further confirmed by the RRDE test (Figure S9). As deduced from Figure 4e, the average electron transfer number (n) of Pd3Pb/ rGO-CNTs is about 3.84 over a range of potentials (0.2−0.7 V), accompanied by the low HO2− yield of approximately 8.0%, which is very close to that of Pt/C catalyst (n: 3.86; HO2−: 6.6%) but apparently larger than that of Pd/C catalyst (n: 3.75; HO2−: 12.3%). On the basis of the structural and compositional advantages of Pd3Pb/rGO-CNTs, it can be speculated that three beneficial factors are responsible for their remarkable ORR activity (Figure 5): First, an ordered intermetallic phase
energies of 284.6, 286.0, and 289.1 eV, which are assigned to C−C, C−O, and OC−O, respectively.43 Compared with GO (Figure S6), the peak intensity related to the oxidized carbon species (C−O) in Pd3Pb/rGO-CNTs was dramatically reduced while C−C species correspondingly increased, clearly indicating that most of the oxygen groups on graphene had been removed after pyrolysis. Additionally, the XPS results also show that Pd is mainly consisted of metallic state (Figure 2e) while Pb is composed with Pb0 and Pb2+ (Figure 2f), which is probably derived from partial oxidation of Pb on Pd3Pb surface. Therefore, it is reasonable to conclude that ordered intermetallic Pd3Pb and graphitic carbon coexist in the resulting Pd3Pb/rGO-CNTs aerogels. The morphology and structure of Pd3Pb/rGO-CNTs aerogels were first characterized with SEM (Figure 3a,b). It is observed clearly that Pd3Pb/rGO-CNTs aerogels exhibit a welldefined and interconnected 3D network structure, in which the wrinkled rGO sheets are uniformly covered by randomly oriented and entangled CNTs. This result was also confirmed via TEM (Figure 3c,d), which further proves the robust interconnection between rGO and CNTs. According to N2 adsorption−desorption isotherms (Figure S7), the Brunauer− Emmett−Teller (BET) surface area of Pd3Pb/rGO-CNTs is up to of 134 m2 g−1 with a narrow pore-size distribution (22−50 nm). Such 3D porous rGO-CNTs aerogels can not only increase the specific contact area but also offer more exposed active sites for the anchoring of Pd3Pb particles. Close inspection displays that Pd3Pb particles are uniformly, closely anchored onto rGO-CNTs surface, and few particles scatter out of rGO-CNTs aerogels (Figure 3e). The histogram of particle diameters (Figure 3f) shows a relatively narrow particle-size distribution with an average size of 7.2 nm. The small particle sizes may be ascribed to the confinement of rGO-CNTs dual network, which suppresses the agglomeration of particles.44 To better verify the ordered intermetallic structure of Pd3Pb particle, we further investigated the atomic-level arrangement of Pd and Pb with high-resolution TEM (HRTEM). Figure 3g reveals the lattice spacing of 0.225 nm, well matching up with the interplanar spacing of (111) facet of cubic Pd3Pb. Furthermore, the superlattice spots can be recognized in corresponding FFT pattern (inset in Figure 3g), which were assigned to the superlattice reflection at (100) and (110) facets. The diffraction pattern aligns with the simulated intermetallic phase (Figure S8). The results further confirm the ordered intermetallic structure of Pd3Pb particles, in agreement with XRD analysis. Figure 3h shows scan-TEM (STEM) image of a single Pd3Pb particle and the corresponding line scanning profiles and EDX element mappings. It is obvious that both Pd and Pb atoms are uniformly dispersed in the prepared Pd3Pb particles. 3.2. Electrocatalytic ORR Activity, Stability, and Selectivity. Unlike the disordered Pd3Pb alloys in which atom positions are occupied randomly by Pd and Pb, the ordered intermetallic Pd3Pb has definite structure and composition, resulting in uniform active sites and significantly enhanced electrocatalytic performance.30−32,45−47 Although the electrocatalytic properties of some ordered Pd3Pb particles have been investigated, the 3D-structural rGO-CNTs aerogels supported Pd3Pb phase has never been reported as electrocatalyst. Here, the ORR was chosen as proof-of-concept reaction to evaluate the electrocatalytic properties of the resulting Pd3Pb/rGO-CNTs catalyst, which was first investigated with the RDE in an O2-saturated 0.1 M KOH solution.
Figure 5. Schematic illustration of the compositional and structural advantages of Pd3Pb/rGO-CNTs as an efficient ORR catalyst.
significantly boosts ORR activity owing to the ordered active sites, the modification of electron configuration, and the change in the Pd−Pd bond distance.30,32 In comparison to the disordered alloy catalyst, the ordered Pd3Pb/rGO-CNTs catalyst shows a significant 36 mV shift to more positive potential (Figure S10), making the benefits of the ordered Pd3Pb phase apparently. However, a better understanding of ORR mechanism at the Pd3Pb surface requires more detailed modeling of many possible reaction pathways. Second, the strong couple of Pd3Pb particles with 3D porous graphitized rGO-CNTs aerogels offers the excellent electronic contact to the external circuit and enables the active species to diffuse easily to/from the ordered Pd3Pb particles. The synergistic E
DOI: 10.1021/acsanm.8b00275 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Figure 6. (a) Chronoamperometric responses of four catalysts at 0.7 V in O2-saturated 0.1 M KOH (percentage of current retained versus operation time). (b) Bar plots of the degradation percentage for the catalysts after the stability tests. ORR polarization curves of (c) Pd3Pb/rGO-CNTs and (d) commercial Pd/C in O2-saturated 0.1 M KOH with and without the presence of 0.5 M CH3OH (rotation rate: 1600 rpm; sweep rate: 5 mV s−1).
Pb elements within Pd3Pb/rGO-CNTs are still present in the form of metallic state. These results demonstrate that Pd3Pb phase is electrochemically stable under the present ORR condition. In addition, the 3D interconnected network structure of Pd3Pb/rGO-CNTs was well preserved (Figure S13), further demonstrating the robust structure. It is well documented that the methanol tolerance of ORR catalysts is a great challenge in fuel cells because methanol permeation to the cathode dramatically decreases the power efficiency of fuel cells.16 Therefore, the effect of methanol crossover of Pd3Pb/ rGO-CNTs was then evaluated by LSV measurement in O2saturated 0.1 M KOH containing 0.5 M methanol solution. As shown in Figure 6c, the Pd3Pb/rGO-CNTs kept no dramatic changes in the kinetically controlled region with the addition of methanol. In contrast, the commercial Pd/C catalyst exhibits an obvious methanol oxidation peaks in the ORR polarization curve while the corresponding limiting current density was also greatly reduced (Figure 6d), which can be attributed to the competition between methanol oxidation and ORR on Pd/C surface.48 The result demonstrates the high selectivity of Pd3Pb/rGO-CNTs for the ORR with an excellent methanol tolerance at the cathode.
combination of CNTs and rGO within Pd3Pb/rGO-CNTs cannot be neglected, likely contributing to the enhanced ORR activity. Compared with Pd3Pb/CNTs and Pd3Pb/rGO prepared by similar methods (Figure S11a,b), Pd3Pb/rGOCNTs reveals better ORR properties (Figures S11c) in low current densities and half-wave potentials (Figures S11d), demonstrating the synergistically improved ORR activity. Apart from activity, the long-term stability is another important criterion for practical fuel cells. Figure 6a displays the chronoamperometric curves of three catalysts at 0.7 V in O2-saturated 0.1 M KOH solution with 1600 rpm for 10 000 s. It can be observed that the current density on Pd3Pb/rGOCNTs is much larger than those on commercial Pt/C and Pd/ C catalysts over the entire time course, hinting its improved ORR activity and stability. According to the resulted relative current, the Pd3Pb/rGO-CNTs shows a more stable curve with lost only 17.6% of its initial activity after 10 000 s running (Figure 6b). In contrast, continuous operations for 10 000 s on commercial Pt/C and Pd/C electrodes cause about 29.7% and 34.2% decrease in current density, respectively (Figure 6b). The enhanced stability of Pd3Pb/rGO-CNTs may arise from, on the one hand, the 3D porous rGO-CNTs network that offers robust chemical and mechanical stability, thus significantly suppressing the possible aggregation/dissolution of active particles during the stability testing. On the other hand, the ordered intermetallic structure makes the Pd3Pb have a good chemical stability in alkaline medium, as evidenced by previous research.30,45 To confirm the structural stability of the ordered Pd3Pb phase, XPS spectra of Pd3Pb/rGO-CNTs have been carried out after the continuous stability test. Through the composition analysis, the surface atomic ratio of Pd/Pb is about 76.9:23.1 (Figure S12a,b), close to the origin proportion of Pd/ Pb (75.2:24.8). The percentage of Pd0/(Pd0 + Pd2+) and Pb0/ (Pb0 + Pb2+) in Pd3Pb/rGO-CNTs was calculated to be 80.3% and 68.5% (Figure S12c,d), respectively, indicating that Pd and
4. CONCLUSION In summary, we successful design an effective approach for the synthesis of a novel Pd3Pb/rGO-CNTs electrocatalyst derived from Pd2+/Pb2+-containing PVA/GO-CNTs hydrogel. The 3D interconnected rGO-CNTs aerogels supported Pd3Pb phase with ordered atomic structure offer great advantages for constructing an advanced ORR catalyst. Compared with commercial catalysts, the Pd3Pb/rGO-CNTs presents excellent electrocatalytic properties for the ORR including high activity, reliable stability, and unusual methanol tolerance ability. It is believed that the introduction of ordered Pd3Pb plays a very important role in significantly enhanced ORR activity via the F
DOI: 10.1021/acsanm.8b00275 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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encased in graphitic layers as oxygen reduction catalysts. Angew. Chem., Int. Ed. 2014, 53, 3675−3679. (9) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. EarthAbundant Nanomaterials for Oxygen Reduction. Angew. Chem., Int. Ed. 2016, 55, 2650−2676. (10) Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J. Y.; Su, D. Biaxially strained PtPb/Pt core/shell nanoplate boosts oxygen reduction catalysis. Science 2016, 354, 1410−1414. (11) Liu, M.; Lu, Y.; Chen, W. PdAg Nanorings Supported on Graphene Nanosheets: Highly Methanol-Tolerant Cathode Electrocatalyst for Alkaline Fuel Cells. Adv. Funct. Mater. 2013, 23, 1289− 1296. (12) He, D. S.; He, D.; Wang, J.; Lin, Y.; Yin, P.; Hong, X.; Wu, Y.; Li, Y. Ultrathin Icosahedral Pt-Enriched Nanocage with Excellent Oxygen Reduction Reaction Activity. J. Am. Chem. Soc. 2016, 138, 1494−1497. (13) Shang, L.; Yu, H.; Huang, X.; Bian, T.; Shi, R.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. WellDispersed ZIF-Derived Co,N-Co-doped Carbon Nanoframes through Mesoporous-Silica-Protected Calcination as Efficient Oxygen Reduction Electrocatalysts. Adv. Mater. 2016, 28, 1668−1674. (14) Deng, Y.; Dong, Y.; Wang, G.; Sun, K.; Shi, X.; Zheng, L.; Li, X.; Liao, S. Well-Defined ZIF-Derived Fe−N Codoped Carbon Nanoframes as Efficient Oxygen Reduction Catalysts. ACS Appl. Mater. Interfaces 2017, 9, 9699−9709. (15) Han, Y.; Wang, Y.-G.; Chen, W.; Xu, R.; Zheng, L.; Zhang, J.; Luo, J.; Shen, R.-A.; Zhu, Y.; Cheong, W.-C.; et al. Hollow N-Doped Carbon Spheres with Isolated Cobalt Single Atomic Sites: Superior Electrocatalysts for Oxygen Reduction. J. Am. Chem. Soc. 2017, 139, 17269−17272. (16) Mahmood, A.; Xie, N.; Ud Din, M. A.; Saleem, F.; Lin, H.; Wang, X. Shape controlled synthesis of porous tetrametallic PtAgBiCo nanoplates as highly active and methanol-tolerant electrocatalyst for oxygen reduction reaction. Chem. Sci. 2017, 8, 4292−4298. (17) Ye, W.; Chen, S.; Ye, M.; Ren, C.; Ma, J.; Long, R.; Wang, C.; Yang, J.; Song, L.; Xiong, Y. Pt4PdCu0.4 Alloy Nanoframes as Highly Efficient and Robust Bifunctional Electrocatalysts for Oxygen Reduction Reaction and Formic Acid Oxidation. Nano Energy 2017, 39, 532−538. (18) Jiang, K.; Wang, P.; Guo, S.; Zhang, X.; Shen, X.; Lu, G.; Su, D.; Huang, X. Ordered PdCu-Based Nanoparticles as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts. Angew. Chem., Int. Ed. 2016, 55, 9030−9035. (19) Erikson, H.; Sarapuu, A.; Solla-Gullón, J.; Tammeveski, K. Recent progress in oxygen reduction electrocatalysis on Pd-based catalysts. J. Electroanal. Chem. 2016, 780, 327−336. (20) Chen, Y.; Jiang, X.; Li, Y.; Li, P.; Liu, Q.; Fu, G.; Xu, L.; Sun, D.; Tang, Y. General Strategy for Synthesis of Pd3M (M = Co and Ni) Nanoassemblies as High-Performance Catalysts for Electrochemical Oxygen Reduction. Adv. Mater. Interfaces 2018, 5, 1701015. (21) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 2004, 108, 17886−17892. (22) Ramanathan, M.; Li, B.; Greeley, J.; Prakash, J. MicrostructureORR activity relationships in Pd3M (M = Cu, Ni, Fe) electrocatalysts synthesized at various temperatures. ECS Trans. 2010, 33, 181−190. (23) Wang, Y.; Balbuena, P. B. Design of Oxygen Reduction Bimetallic Catalysts: Ab-Initio-Derived Thermodynamic Guidelines. J. Phys. Chem. B 2005, 109, 18902−18906. (24) Chen, A.; Ostrom, C. Palladium-Based Nanomaterials: Synthesis and Electrochemical Applications. Chem. Rev. 2015, 115, 11999− 12044. (25) Chen, X.; Si, C.; Wang, Y.; Ding, Y.; Zhang, Z. Multicomponent platinum-free nanoporous Pd-based alloy as an active and methanoltolerant electrocatalyst for the oxygen reduction reaction. Nano Res. 2016, 9, 1831−1843.
strengthened geometric and electronic effect. On the other hand, the 3D interconnected network of rGO-CNTs aerogels allow fast accessibility of reactants to the active Pd3Pb sites while preventing them from aggregation and leaching out. We believe that the present synthetic approach can be further extended to the more-general synthesis of 3D porous carbonbased materials with metals or metal oxides for energy storage application.
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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/acsanm.8b00275. Additional material characterizations and eletrocatalytic measurements, including SEM images, TEM images, XRD patterns, XPS spectra, nitrogen adsorption− desorption isotherms, and ORR curves as well as a table comparing ORR activity of Pd3Pb/rGO-CNTs with precious-metal-based electrocatalysts previously reported (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (J.-M.L.). ORCID
Gengtao Fu: 0000-0003-0411-645X Jong-Min Lee: 0000-0001-6300-0866 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by Singapore Maritime Institute Maritime Sustainability (MSA) R&D Programme (grant M4061829). The authors thank Mr Sng Yeow Poo at Trinity Offshore Pte Ltd. and Mr Ravindra Pallaniappan for their support.
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