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Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transition-metal doping
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Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transition-metal doping Dengfeng Wu, Yang Yang, Changqing Dai, Daojian Cheng* State Key Laboratory of Organic-Inorganic Composites & Beijing Key Laboratory of Energy Environmental Catalysis & Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
highlights The octahedral or spherical PtCu nanoparticles are selectively synthesized. Different metal-doped PtCu nanooctahedrons are obtained by a one-pot strategy. PtCuSc nanooctahedrons exhibit remarkable mass activity and specific activity.
article info
abstract
Article history:
Developing active and durable electrocatalysts for oxygen reduction reaction (ORR) is of
Received 25 July 2019
great significance in proton exchange membrane fuel cells (PEMFCs). Herein, we develop a
Received in revised form
facile strategy to synthesize PtCu nanoparticles with enhanced ORR performance through
27 November 2019
morphology tuning and transition-metal doping. Two distinct PtCu nanoparticles, namely
Accepted 2 December 2019
nanooctahedrons (NOs) and nanospheres (NSs), are selectively synthesized in presence or
Available online xxx
absence of W(CO)6 via a facile one-pot method. Furthermore, by introducing a small amount of third transition metal, M-doped (M ¼ Sc, Y, La, Gd, Fe) PtCu NOs are obtained.
Keywords:
Electrocatalytic results suggest that the ORR performance of PtCu NOs is better than that of
Morphology tuning
PtCu NSs due to the morphology advantages. And the ORR performance of PtCuM NOs is
Transition-metal doping
further promoted since the doping effect of transition metals compared to that of PtCu
Nanooctahedrons
NOs. Particularly, PtCuSc NOs exhibit remarkable mass activity (1.652 mA mg1Pt) and
Oxygen reduction reaction
specific activity (2.093 mA cm2), which are 9.9 and 7.2 times higher than that of commercial Pt/C catalysts at 0.8 V (vs. RHE). Moreover, after accelerated stability tests, the loss of mass activity for PtCuSc NOs is only 9.2%, which is much lower than that of PtCu NOs (16.5%) and commercial Pt/C (44.3%). This work provides a feasible idea to boost the ORR performances of Pt-based nanoparticles. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) are promising candidates as clean energy conversion devices owning to their
preponderances such as high energy density, low operating temperature and environmental friendly [1,2]. However, the further commercialization of PEMFCs depends on overcoming high price and unsatisfying performance of Pt/C catalyst towards oxygen reduction reaction (ORR) on cathode [3e5]. In
* Corresponding author. E-mail address: [email protected] (D. Cheng). https://doi.org/10.1016/j.ijhydene.2019.12.003 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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the last couple of decades, great efforts have been done to develop various Pt-based nanoalloys, such as PtPd [6e8], PtCu [9,10], PtFe [11,12], PtCo [13e15] and PtNi [16,17] etc., to effectively enhance the electrocatalytic performance and sharply reduce the Pt usage. Among these bimetallic nanoalloys, PtCu represent a promising class of electrocatalysts for various electrocatalytic reactions, including ORR, thus have been attracting great attention [18e22]. Previous studies suggested that ORR is quite structuresensitive on the surfaces of Pt-based nanocrystals [23e25]. Therefore, engineering the structure (i.e., morphology) of nanoparticles is an efficient and promising approach to develop novel Pt-based catalysts with high Pt utilization and activity. Choi et al. demonstrated that the uniform 9 nm PtNi nanoparticles with octahedral morphology exhibited an excellent specific activity, which is about 51 times higher than that of commercial Pt/C catalysts for ORR [26]. Zhang et al. recently reported the facile synthesis of five-fold-twinned PtCu nanoframes with highly anisotropic by a one-pot method. These PtCu nanoframes exhibited enhanced electrocatalytic performances in comparison with other morphologies and commercial Pt/C [27]. Not limited to the above examples suggested that the morphology control of Pt-based alloy nanoparticles with exposed (111) crystal faces is highly efficient for enhancing ORR performance. More recently, doping a third transition metal into bimetallic NPs has been regarded as an efficient way to further modify the electronic structure of Pt sites, and thus can enhance the specific activity effectively [28e30]. Sun et al. reported that the ORR catalytic stability of octahedral PtCu alloy nanocrystals could be improved availably by trace Au doping (Au/Pt ¼ 0.0005) [31]. Beermann et al. presented a synthetic technique for Rh-doped PteNi catalysts with welldefined shapes, which exhibited an exceptional activity and stability behavior [32]. Huang et al. developed the strategy of doping a third metal into to prepare MoePt3Ni/C catalyst, which exhibited tens of times enhancements compared with the commercial Pt/C catalyst for ORR [33]. The transition metal doping can not only increase the activity of electrocatalyst but also improve their stability. However, it is of significant but remains a challenge to select suitable candidate from a series of third transition metals for PtCu nanoalloy with controlled morphology. In this work, an efficient synthetic strategy to enhance the ORR performance of PtCu nanoparticles by morphology tuning and transition-metal doping is reported. Firstly, PtCu nanoparticles with octahedral and spherical morphologies are selectively synthesized in presence or absence of W(CO)6. Electrocatalytic results suggest that the ORR performance of PtCu NOs is better than that of PtCu NSs since the morphology advantages. Then, by introducing a small amount of third transition metal, such as Sc, Y, La, Gd, Fe, PtCuM NOs are obtained with enhanced ORR activity compared to bimetallic PtCu and commercial Pt/C. Moreover, among these PtCuM NOs catalysts, PtCuSc NOs shows the best ORR activity and durability. Our results would shed new light on promoting the ORR performances of Pt-based electrocatalysts.
Experimental section Synthesis of nanomaterials For PtCu NSs, a typical synthesis is as follows. 20 mg Pt(acac)2, 10.2 mg Cu(acac)2, 9.0 mL OAm (oleylamine) and 1.0 mL OA (oleic acid) were mixed in a 50 mL three-necked flasks. The mixture was heated to 130 C in high purity N2 with magnetic stirring for 30 min. The reaction solution was then continuously heated to 230 C with magnetic stirring and kept at this condition for 40 min. The reaction was cooled down to room temperature naturally, and the black sediment was centrifuged and washed by the mixture of hexane and ethanol. The resulting product was obtained and dispersed into hexane as standby. The synthesis of PtCu NOs is similar to that of PtCu NSs except the additional W(CO)6 (0.142 mmol) was added in the reaction solution. Also, the synthesis of PtCuM (M ¼ Sc, Y, La, Gd, Fe) NOs is similar to that of PtCu NSs except the additional W(CO)6 (0.142 mmol) and M(acac)3 (M ¼ Sc, Y, La, Gd, Fe, 0.011 mmol) were added in the reaction solution. The electrocatalysts were prepared by supporting selfsynthesized nanomaterials on carbon black. Typically, 40 mg carbon black was added into dispersion. The mixture was then sonicated for 60 min. The resulting carbon supported samples was centrifuged and dried naturally in a vacuum oven overnight before electrochemical measurements.
Material characterization Transmission electron microscopy (TEM) images were collected on Tecnai G2 20 operating at 200 kV. High-resolution TEM (HRTEM) images were collected on a JEOL JEM-2100 transmission electron microscope. The microstructure and elemental distribution were characterized by high-angle annular bright/dark Field (HAADF or HAABF) in Scanning Transmission Electron Microscopy (STEM) mode. Elemental analyses were performed using inductively coupled plasma spectrometer (ICP) on Thermo Scientific iCAP6000 and energy dispersive X-ray spectroscopy (EDX) from a Hitachi S-4800 SEM operated at 20 kV. Powder X-ray diffraction (PXRD) patterns were recorded using Bruker D8 Advance X-ray diffractometer with CuKa (l ¼ 0.15418 nm) radiation source at 40 kV. Electronic environment of the catalysts were obtained by the X-ray photoelectron spectroscopy (XPS) on Thermofisher ESCALAB 250.
Electrochemical measurements Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were taken on a three electrode system connected to a CHI760e (Chen-Hua, China) electrochemical work station at room temperature. Rotating disk electrode (RDE) with glassy carbon (GC) was polished and cleaned before surface modifying by electrocatalysts. 10 mL catalyst ink with catalyst concentration of 2 mg mL1 was coated on the surface of GC and dried at room temperature for 2 h. The CV measurements
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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were performed in N2-saturated 0.1 M HClO4 solution at a potential range from 0.05 to 1.10 V with a scanning rate of 50 mV s1. ORR polarization curves were obtained in O2saturated 0.1 M HClO4 solution at a scan rate of 10 mV s1 on the GC-RDE rotating at rotating rate of 1600 rpm. The durability tests were carried out at full potential ranges for 1000 cycles at a scanning rate of 50 mV s1.
Results and discussion As shown in Scheme 1, the well-defined Pt-based nanocrystals were synthesized by co-reduction of Pt(acac)2 and Cu(acac)2 by using the mixture of OAm and AA as both solvent and reducing agent. Typically, PtCu NOs, PtCu NSs and M-doped PtCu NOs could be selectively synthesized in the presence of W(CO)6 and/or transition metal salts (see experiment section for more experimental details). Fig. 1A shows a typical TEM image of PtCu NSs. It is found that the monodisperse nanoparticles are of an average size around 9.9 nm and spherical appearance. The HRTEM image of single particle shows that the lattice fringe is 0.219 nm (Fig. 1B), corresponding to the interplanar distance of (111) plane of PtCu alloy with facet-centered cubic (fcc) structure. Fig. 1C shows the representative HAADF-STEM image of PtCu NS. It can be seen that the obtained PtCu NS are highly crystallized with clearly identifiable lattice. The Pt and Cu distribution on PtCu NSs were confirmed by EDX elemental mapping. As shown in Fig. 1D, both Cu and Pt are distributed on the PtCu NSs homogeneously. Moreover, the crosssectional compositional line-scanning profile of PtCu NS in Fig. 1E shows that both Cu and Pt contents are high in middle of the line-scanning of the PtCu NS and low at the edge of the PtCu NS. It is a typical EDX line scanning of alloy structure [34], which is consistent with the HAADF-STEM measurements. Above results confirm the formation of bimetallic PtCu NSs. Fig. 2A shows a TEM image of the as-synthesized PtCu NOs with an average diameter of 8.3 nm. Moreover, the HAADFSTEM image of PtCu NOs in Fig. S1 shows that more than 80% of the NPs are of octahedral morphology. Fig. 2B reveals a HAABF-STEM image of a single PtCu NO with the lattice fringe of 0.219 nm, which should be index to the (111) interplanar distance in fcc PtCu alloyed structure. The same lattice fringes of PtCu NSs and PtCu NOs indicate that the elemental components of them should be close. Indeed, both EDX analysis and ICP results suggest that the atomic ratio in PtCu NO and
Scheme 1 e Schematic illustration of the growth conditions to the controlled synthesis route for PtCu NOs, PtCu NSs and PtCuM (M ¼ Sc, Y, La, Gd and Fe) NOs.
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PtCu NS is about 1:1 (Fig. S2). Moreover, the mass percentage of Pt in carbon supported PtCu NOs is 9.64%. Fig. 2C displays the HAADF-STEM image of single PtCu NO. At the middle area of the PtCu NO, some knots and sections can be observed. The atoms in this area are coordinative unsaturated, which are more active to adsorb small molecules than other coordinative saturated atoms [35]. In addition, EDX elemental mapping and the cross-sectional compositional line-scanning profile for PtCu NO confirm the complete overlap of Pt and Cu elements through the whole particle (Fig. 2D and E). Above results confirm the formation of bimetallic PtCu with morphology of octahedron. It should be noted that W(CO)6 plays a crucial role in the growth of the PtCu NOs. Previous investigation suggested that W(CO)6 is an efficient shape-directing agent for Pt-based nanocrystals with specific morphology [36,37]. However, the residual CO would cause surface toxicity, thus reduce the catalytic performance of Pt-based nanoparticles [38]. Infrared spectroscopy was performed to check adsorption state on the surface of PtCu NOs (Fig. S3). It demonstrates that the PteCO valence bond is not found at the wave number of 2000 cm1, which illustrates that complete desorption of CO on the surface of the prepared PtCu NOs after treatment. The third transition metal was doped into PtCu NOs through the same synthesis protocol for PtCu NOs by adding a small amount of the third transition metal M (M ¼ Sc, Y, La, Gd, Fe) salts into the reaction solution. Typically, TEM image in Fig. 3A shows that the Sc-doped PtCu NOs exhibit monodisperse characteristic in solution and more than 60% of the particles keep octahedral morphology. The particle size of PtCuM NOs is similar to that of PtCu NOs, and the average size is still within 10 nm. Fig. 3B displays that the atomic ratios of Pt:Cu in PtCuM NOs/C remains 1:1. While, the content of Sc obtained from EDX and ICP is as low as 0.03%, indicating that the presence of third transition metal would not affect the coreduction of Pt and Cu salts subsequent with shape-control for its minute quantity. The distribution of element in PtCuSc NO was further examined by EDX line scanning. As shown in Fig. 3C, the presence of Sc signal peaks indicates that the third transition metal Sc was successfully doped into PtCu NOs/C. Other transition metals, such as Sc, Y, La, Gd, Fe, are also confirmed in PtCu NOs with thimbleful, as shown in Fig. S4. Based on the above analysis, a conclusion can be drawn that the synthesized PtCuM (M ¼ Sc, Y, La, Gd, Fe) NOs/C catalysts are trimetallic alloy catalysts with octahedral morphology, and the content of the third metal M (M ¼ Sc, Y, La, Gd, Fe) is less than 1%. To further verify the influence of third metal for crystal structure, powder XRD was performed and the corresponding curves for all samples are present in Fig. 3D. It can be seen that all peaks could be indexed as those of fcc PtCu structure, which is in accordance with the result of HRTEM. Moreover, peaks of these M-doped PtCu NOs/C are of a slightly negative shift relative to that of PtCu NOs/C. This should be origin from the third transition metal M doping into PtCu NOs. Moreover, the addition of a third metal may also bring about changes in the electronic structure of Pt and Cu. To this end, the element valence information in these nanomaterials was checked by XPS analysis. As shown in Fig. S5, the peaks of Pt4f7/2 and Pt4f5/ 2 in PtCu NSs are located at 70.93 and 74.32 eV, which are
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Fig. 1 e (A) TEM image of PtCu NSs. (B) HRTEM and (C) HAADF-STEM images of an individual PtCu NS. (D) The corresponding EDX mapping of the elements Cu (red), Pt (green) and the overlap. (E) EDX linescan profile of an individual PtCu NS. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2 e (A) TEM image of PtCu NOs. (B) HAABF-STEM and (C) HAADF-STEM images of an individual PtCu NO. (D) The corresponding EDX mapping of the elements Cu (red), Pt (green) and the overlap. (E) EDX linescan profile of an individual PtCu NO. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
respectively shifted to the lower binding energy compared to those in Pt(111) crystal face (71.00 and 74.35 eV) obtained from XPS database in National Institute of Standards and Technology (NIST). The alloy effect should be responsible to this shift. While, the peaks of Pt4f7/2 and Pt4f5/2 in PtCu NOs are further negatively shifted to 70.65 and 73.90 eV, indicating the electronic structure changes of Pt atom due to the morphology effect. Accordingly, the XPS peaks of Cu2p3/2 and Cu2p1/2 in PtCu NOs exhibit a positive shift by 0.22 and 0.29 eV compared to those in PtCu NSs. Moreover, the additional of third transition metal would further modify the electronic structure of Pt and Cu in PtCu NOs (Fig. S6). Typically, the XPS peaks of Pt4f7/2 in PtCuSc NOs, PtCuY NOs, PtCuLa NOs, PtCuGd NOs
and PtCuFe NOs are located at 70.72, 70.76, 70.81, 70.83 and 70.91 eV, respectively, which are shifted to the middle position between those in PtCu NOs and PtCu NSs. It should be noted that the incorporation of Sc has the most significant effect on modifying the electronic structure of Pt in PtCu NOs, indicating the promising ORR activity according to the Sabatier volcano of ORR catalysts [39]. To evaluate the catalytic activity towards ORR, CV curves were obtained firstly. Fig. S7 shows the CV curves of commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuM NOs/C catalysts. The electrocatalytically active surface areas (ECSAs) are evaluated from the hydrogen adsorption/desorption region (between 0.05 and 0.35 V) on CV curves. The ECSAs of
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Fig. 3 e TEM images of (A) PtCuSc NOs/C. (B) EDX spectra and corresponding elemental constituent of PtCu NSs/C. (C) EDX line scan profiles of an individual PtCuSc NO. (D) Typical XRD patterns for PtCu NSs/C, PtCu NOs/C, PtCuSc NOs/C, PtCuY NOs/C, PtCuLa NOs/C, PtCuGd NOs/C and PtCuFe NOs/C.
commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C, PtCuY NOs/C, PtCuLa NOs/C, PtCuGd NOs/C and PtCuFe NOs/C are calculated to be 57.9, 35.6, 59.1, 46.3, 52.4, 45.2, 54.6 and 47.1 m2 gPt1, respectively. It can be found that PtCu NOs/C has a larger ECSA than commercial Pt/C, PtCu NSs/C since the morphology advantage of octahedron, in which the PtCu (111) crystal faces get a total exposure. While, the additional transition metal doping would cause the ECSA loss for PtCu NOs since the corresponding replacement of Pt active sites on the surface. Fig. 4A shows the ORR polarization curves of commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C. The half wave potentials of these catalysts are 0.788 V, 0.804 V, 0.823 V and 0.863 V, respectively. This result suggests that the ORR performance of PtCu nanoparticles catalyst could be significantly affected by morphology tuning, in which the octahedral morphology is a better choice to enhance electrochemical catalytic performance. The better ORR performance of PtCu NOs than that of Pt NSs should mainly attribute to two respects. One is the total exposure of (111) crystal faces, on which the ORR reaction activity is improved, while the ORR reaction activity would be hindered on (100) or (110) crystal faces for Pt-based eletrocatalysts [40]. The other is the metal alloy effect, in which the incorporation of Cu could downshift of d-band center of Pt thus dramatically enhance the catalyst activity for ORR [41]. In addition to morphological and alloy effects, the performance of PtCu NOs/C could be further improved by doping a third transition metal. The mass activities and specific activities of these catalysts are calculated by normalizing the current by mass loading and
electrochemical active area of Pt on GC. As shown in Fig. 4B, the catalytic activity for ORR of these three catalysts orders: commercial Pt/C < PtCu NSs/C < PtCu NOs/C < PtCuSc NOs/C. In particular, the PtCuSc NOs/C catalyst shows the mass activity of 1.652 mA mg1Pt and the specific activity of 2.093 mA cm2 at 0.8 V (vs. RHE), which are 9.9 times and 7.2 times higher than commercial Pt/C catalysts, respectively. After doping the third transition metal M into the PtCu NOs/C, PtCuM NOs/C catalysts have been greatly improved the mass activity and specific activity for ORR (Fig. 4C). Obviously, PtCuSc NOs/C shows the highest ORR activity among these PtCuM NOs/C catalysts. Hence, PtCuSc NOs/C was chosen as a representative to explore the influence of the third transition metals on catalytic activity. According to the Sabatier volcano of ORR catalysts, ORR performance would be maximized when the oxygen binding energy is about 0.2 eV less than the binding energy on Pt(111) [39,42]. The oxygen binding energy on Pt(111) is too strong while oxygen binding energy on PtCu(111) is too weak [31,43]. Moreover, by alloying Sc and Pt, the overpotential towards ORR could be reduced because the binding energy of OeO bond is weakened [44]. According to the above XPS results, the incorporation of Sc on PtCu(111) can effectively lower the d-band center of PtCu(111), and thus may silently increase the oxygen binding energies of adjacent active sites on PtCu(111). As a result, some sites may become highly active for ORR. Above results suggest that the electrochemical performance of PtCu NOs/C catalyst could be improved tremendously by doping a small amount of the Sc atoms.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Fig. 4 e (A) ORR polarization curves for the commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C in O2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV s¡1 and a rotation rate of 1600 rpm. (B) The mass activity and specific activity of ORR for the commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C. (C) The mass activity and corresponding enhancement factors at 0.8 V of (a) commercial Pt/C, (b) PtCu NSs/C, (c) PtCu NOs/C, (d) PtCuSc NOs/C, (e) PtCuY NOs/C, (f) PtCuLa NOs/C, (g) PtCuGd NOs/C and (h) PtCuFe NOs/C catalysts. (D) Histograms of comparative mass activities at 0.80 V versus RHE for the commercial Pt/C, PtCu NOs/C and PtCuSc NOs/C before and after 1000 cycles of ADTs.
Durability determines the practicability for catalysts. Therefore, the catalytic stabilities of commercial Pt/C, PtCu NOs/C and PtCuSc NOs/C were tested, and the results are shown in Fig. S9. It can be seen that the deviation degree of ORR curves before and after the 1000 cycles in the stability test is given in the following trend: commercial Pt/C > PtCu NOs/ C > PtCuSc NOs/C. It is obvious that the stability of the PtCuSc NOs/C catalyst is the highest among these three catalysts. Under the condition of 0.8 V, the mass activity degradation of commercial Pt/C, PtCu NOs/C and PtCuSc NOs/C are as shown in Fig. 4D. It is found that after the stability test, the mass activity of commercial Pt/C decreased by 44.3%, and the mass activity of PtCu NOs/C declined by 16.5%. Correspondingly, PtCuSc NOs/C decreased the mass activity by 9.2% after the stability test. Obviously, the loss of mass activity for PtCuSc NOs/C was only 9.2%, which is much smaller than that of PtCu NOs/C (16.5%), and commercial Pt/C (44.3%) after stability test. Our results show that a small amount of the third transition metal Sc (less than 5%) contributes to effectively enhance the activity and stability of bimetallic PtCu NOs/C and commercial Pt/C in the electrocatalytic process, which will greatly promote the development of new trimetallic NPs catalysts.
Conclusions In conclusion, we demonstrated an efficient synthetic strategy to enhance the ORR performance of PtCu
nanoparticles by the combination of morphology tuning and transition-metal doping. Firstly, bimetallic PtCu NOs and PtCu NSs by the ligand of W(CO)6 to control the morphology of NPs were synthesized. The electrochemical test shows that the ORR performance of PtCu NPs catalyst could be significantly affected by tuning morphology and the octahedral morphology is a better choice because of the higher electrochemical catalytic performance. Secondly, by doping a small amount of the third transition metal M into the PtCu NOs/C catalyst, five PtCuM NOs/C were successfully synthesized and confirmed. PtCuSc NOs/C shows the highest ORR activity among the PtCuM NOs/C catalysts. Hence, PtCuSc NOs/C was chosen as a representative to explore the influence of the third transition metals on catalytic activity. The ORR activity and stability of commercial Pt/C, PtCu NOs/ C and PtCuSc NOs/C are tested. The PtCuSc NOs/C catalyst shows the mass activity of 1.652 mA mg1Pt and the specific activity of 2.093 mA cm2 at 0.8 V (vs RHE), which are 9.9 times and 7.2 times higher than commercial Pt/C catalysts, respectively. The mass activity loss of PtCuSc NOs/C was only 9.2%, much smaller than those of PtCu NOs/C (16.5%), and commercial Pt/C (44.3%) after 1000 cycles. It is found that doping a small amount of the third transition metal Sc can improve greatly the catalytic performance and stability of PtCu NOs/C in the electrocatalytic process. Our results provide a new method for the synthesis of trimetallic Pt-based catalysts under controlled morphology, which will promote the development of new trimetallic NPs catalysts.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Acknowledgments This work is supported by the National Natural Science Foundation of China (21822801, 21576008, 91634116) and the Fundamental Research Funds for the Central Universities (XK1802-1 and XK180301). D. Wu is grateful to the Project funded by China Postdoctoral Science Foundation (2018M641165).
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[15]
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.003.
[17]
references [18] [1] Wee J-H. Applications of proton exchange membrane fuel cell systems. Renew Sustain Energy Rev 2007;11:1720e38. [2] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349e84. [3] Mukerjee S, Srinivasan S. Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells. J Electroanal Chem 1993;357:201e24. [4] de Bruijn FA, Dam VAT, Janssen GJM. Review: durability and degradation issues of PEM fuel cell components. Fuel Cells 2008;8:3e22. [5] Martin S, Jensen JO, Li Q, Garcia-Ybarra PL, Castillo JL. Feasibility of ultra-low Pt loading electrodes for high temperature proton exchange membrane fuel cells based in phosphoric acid-doped membrane. Int J Hydrogen Energy 2019;44:28273e82. [6] Lu Y, Du S, Steinberger-Wilckens R. Three-dimensional catalyst electrodes based on PtPd nanodendrites for oxygen reduction reaction in PEFC applications. Appl Catal B Environ 2016;187:108e14. ndez JR, Gochi-Ponce Y, Salazar-Gaste lum MI, [7] Zapata-Ferna Reynoso-Soto EA, Paraguay-Delgado F, Lin SW, et al. Ultrasonic-assisted galvanic displacement synthesis of PtePd/MWCNT for enhanced oxygen reduction reaction: effect of Pt concentration. Int J Hydrogen Energy 2017;42:9806e15. [8] Liao M, Li W, Xi X, Luo C, Fu Y, Gui S, et al. Highly active Pt decorated Pd/C nanocatalysts for oxygen reduction reaction. Int J Hydrogen Energy 2017;42:24090e8. [9] Coleman EJ, Chowdhury MH, Co AC. Insights into the oxygen reduction reaction activity of Pt/C and PtCu/C catalysts. ACS Catal 2015;5:1245e53. [10] Fu S, Zhu C, Shi Q, Xia H, Du D, Lin Y. Highly branched PtCu bimetallic alloy nanodendrites with superior electrocatalytic activities for oxygen reduction reactions. Nanoscale 2016;8:5076e81. [11] Chung DY, Jun SW, Yoon G, Kwon SG, Shin DY, Seo P, et al. Highly durable and active PtFe nanocatalyst for electrochemical oxygen reduction reaction. J Am Chem Soc 2015;137:15478e85. [12] Du XX, He Y, Wang XX, Wang JN. Fine-grained and fully ordered intermetallic PtFe catalysts with largely enhanced catalytic activity and durability. Energy Environ Sci 2016;9:2623e32. [13] Eid K, Wang H, Malgras V, Alshehri SM, Ahamad T, Yamauchi Y, et al. One-step solution-phase synthesis of
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
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bimetallic PtCo nanodendrites with high electrocatalytic activity for oxygen reduction reaction. J Electroanal Chem 2016;779:250e5. Yang D, Yan Z, Li B, Higgins DC, Wang J, Lv H, et al. Highly active and durable PteCo nanowire networks catalyst for the oxygen reduction reaction in PEMFCs. Int J Hydrogen Energy 2016;41:18592e601. Li Z, Zeng R, Wang L, Jiang L, Wang S, Liu X. A simple strategy to form hollow Pt3Co alloy nanosphere with ultrathin Pt shell with significant enhanced oxygen reduction reaction activity. Int J Hydrogen Energy 2016;41:21394e403. Gong W, Jiang Z, Huang L, Shen PK. PtNi alloy hyperbranched nanostructures with enhanced catalytic performance towards oxygen reduction reaction. Int J Hydrogen Energy 2018;43:18436e43. ~ iz AM, Barbosa R, Miki Rosado G, Verde Y, Valenzuela-Mun Yoshida M, Escobar B. Catalytic activity of Pt-Ni nanoparticles supported on multi-walled carbon nanotubes for the oxygen reduction reaction. Int J Hydrogen Energy 2016;41:23260e71. Yang T, Pukazhselvan D, da Silva EL, Santos MC, Meng L, Ramasamy D, et al. Highly branched PtCu nanodandelion with high activity for oxygen reduction reaction. Int J Hydrogen Energy 2019;44:174e9. Li W, Hu Z-Y, Zhang Z, Wei P, Zhang J, Pu Z, et al. Nano-single crystal coalesced PtCu nanospheres as robust bifunctional catalyst for hydrogen evolution and oxygen reduction reactions. J Catal 2019;375:164e70. Li C, Liu T, He T, Ni B, Yuan Q, Wang X. Composition-driven shape evolution to Cu-rich PtCu octahedral alloy nanocrystals as superior bifunctional catalysts for methanol oxidation and oxygen reduction reaction. Nanoscale 2018;10:4670e4. Cheng D, Wu D, Xu H, Fisher A. Compositionecontrolled synthesis of PtCuNPs shells on copper nanowires as electrocatalysts. ChemistrySelect 2016;1:4392e6. Alekseenko AA, Moguchikh EA, Safronenko OI, Guterman VE. Durability of de-alloyed PtCu/C electrocatalysts. Int J Hydrogen Energy 2018;43:22885e95. Gan L, Cui C, Heggen M, Dionigi F, Rudi S, Strasser P. Element-specific anisotropic growth of shaped platinum alloy nanocrystals. Science 2014;346:1502e6. Wang C, Daimon H, Lee Y, Kim J, Sun S. Synthesis of monodisperse Pt nanocubes and their enhanced catalysis for oxygen reduction. J Am Chem Soc 2007;129:6974e5. Wu R, Tsiakaras P, Shen PK. Facile synthesis of bimetallic PtPd symmetry-broken concave nanocubes and their enhanced activity toward oxygen reduction reaction. Appl Catal B Environ 2019;251:49e56. Choi S-I, Xie S, Shao M, Odell JH, Lu N, Peng H-C, et al. Synthesis and characterization of 9 nm PteNi octahedra with a record high activity of 3.3 A/mgPt for the oxygen reduction reaction. Nano Lett 2013;13:3420e5. Zhang Z, Luo Z, Chen B, Wei C, Zhao J, Chen J, et al. One-pot synthesis of highly anisotropic five-fold-twinned PtCu nanoframes used as a bifunctional electrocatalyst for oxygen reduction and methanol oxidation. Adv Mater 2016;28:8712e7. Wu D, Cheng D. Core/shell AgNi/PtAgNi nanoparticles as methanol-tolerant oxygen reduction electrocatalysts. Electrochim Acta 2015;180:316e22. Wu Y, Zhao Y, Liu J, Wang F. Adding refractory 5d transition metal W into PtCo system: an advanced ternary alloy for efficient oxygen reduction reaction. J Mater Chem 2018;6:10700e9. Lokanathan M, Patil IM, Kakade B. Trimetallic PtNiCo nanoflowers as efficient electrocatalysts towards oxygen reduction reaction. Int J Hydrogen Energy 2018;43:8983e90.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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[31] Lu B-A, Sheng T, Tian N, Zhang Z-C, Xiao C, Cao Z-M, et al. Octahedral PtCu alloy nanocrystals with high performance for oxygen reduction reaction and their enhanced stability by trace Au. Nano Energy 2017;33:65e71. [32] Beermann V, Gocyla M, Willinger E, Rudi S, Heggen M, DuninBorkowski RE, et al. Rh-doped PteNi octahedral nanoparticles: understanding the correlation between elemental distribution, oxygen reduction reaction, and shape stability. Nano Lett 2016;16:1719e25. [33] Huang X, Zhao Z, Cao L, Chen Y, Zhu E, Lin Z, et al. Highperformance transition metaledoped Pt3Ni octahedra for oxygen reduction reaction. Science 2015;348:1230e4. [34] Wang C, Chi M, Wang G, van der Vliet D, Li D, More K, et al. Correlation between surface chemistry and electrocatalytic properties of monodisperse PtxNi1-x nanoparticles. Adv Funct Mater 2011;21:147e52. [35] Lim B, Lu X, Jiang M, Camargo PHC, Cho EC, Lee EP, et al. Facile synthesis of highly faceted multioctahedral Pt nanocrystals through controlled overgrowth. Nano Lett 2008;8:4043e7. [36] Yang H, Zhang J, Sun K, Zou S, Fang J. Enhancing by weakening: electrooxidation of methanol on Pt3Co and Pt nanocubes. Angew Chem Int Ed 2010;49:6848e51. [37] Porter NS, Wu H, Quan Z, Fang J. Shape-control and electrocatalytic activity-enhancement of Pt-based bimetallic nanocrystals. Accounts Chem Res 2013;46:1867e77.
[38] Chung DY, Kim H-i, Chung Y-H, Lee MJ, Yoo SJ, Bokare AD, et al. Inhibition of CO poisoning on Pt catalyst coupled with the reduction of toxic hexavalent chromium in a dualfunctional fuel cell. Sci Rep 2014;4:7450. [39] Rossmeisl J, Karlberg GS, Jaramillo T, Nørskov JK. Steady state oxygen reduction and cyclic voltammetry. Faraday Discuss 2009;140:337e46. [40] Li K, Li Y, Wang Y, He F, Jiao M, Tang H, et al. The oxygen reduction reaction on Pt(111) and Pt(100) surfaces substituted by subsurface Cu: a theoretical perspective. J Mater Chem 2015;3:11444e52. [41] Chen B, Cheng D, Zhu J. Synthesis of PtCu nanowires in nonaqueous solvent with enhanced activity and stability for oxygen reduction reaction. J Power Sources 2014;267:380e7. [42] Nørskov JK, Bligaard T, Rossmeisl J, Christensen CH. Towards the computational design of solid catalysts. Nat Chem 2009;1:37e46. [43] Ichiya T, Koiwa N, Ohma A, Tada S, Fushinobu K, Okazaki K. Surface electronic/atomic structure and activation energy on Pt(111), Pt3Cu(111), and PtCu(111) for PEFC cathode. Nanoscale Microscale Thermophys Eng 2010;14:110e22. [44] Garapati MS, Sundara R. Highly efficient and ORR active platinum-scandium alloy-partially exfoliated carbon nanotubes electrocatalyst for Proton Exchange Membrane Fuel Cell. Int J Hydrogen Energy 2019;44:10951e63.
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Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transition-metal doping Dengfeng Wu, Yang Yang, Changqing Dai, Daojian Cheng* State Key Laboratory of Organic-Inorganic Composites & Beijing Key Laboratory of Energy Environmental Catalysis & Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
highlights The octahedral or spherical PtCu nanoparticles are selectively synthesized. Different metal-doped PtCu nanooctahedrons are obtained by a one-pot strategy. PtCuSc nanooctahedrons exhibit remarkable mass activity and specific activity.
article info
abstract
Article history:
Developing active and durable electrocatalysts for oxygen reduction reaction (ORR) is of
Received 25 July 2019
great significance in proton exchange membrane fuel cells (PEMFCs). Herein, we develop a
Received in revised form
facile strategy to synthesize PtCu nanoparticles with enhanced ORR performance through
27 November 2019
morphology tuning and transition-metal doping. Two distinct PtCu nanoparticles, namely
Accepted 2 December 2019
nanooctahedrons (NOs) and nanospheres (NSs), are selectively synthesized in presence or
Available online xxx
absence of W(CO)6 via a facile one-pot method. Furthermore, by introducing a small amount of third transition metal, M-doped (M ¼ Sc, Y, La, Gd, Fe) PtCu NOs are obtained.
Keywords:
Electrocatalytic results suggest that the ORR performance of PtCu NOs is better than that of
Morphology tuning
PtCu NSs due to the morphology advantages. And the ORR performance of PtCuM NOs is
Transition-metal doping
further promoted since the doping effect of transition metals compared to that of PtCu
Nanooctahedrons
NOs. Particularly, PtCuSc NOs exhibit remarkable mass activity (1.652 mA mg1Pt) and
Oxygen reduction reaction
specific activity (2.093 mA cm2), which are 9.9 and 7.2 times higher than that of commercial Pt/C catalysts at 0.8 V (vs. RHE). Moreover, after accelerated stability tests, the loss of mass activity for PtCuSc NOs is only 9.2%, which is much lower than that of PtCu NOs (16.5%) and commercial Pt/C (44.3%). This work provides a feasible idea to boost the ORR performances of Pt-based nanoparticles. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) are promising candidates as clean energy conversion devices owning to their
preponderances such as high energy density, low operating temperature and environmental friendly [1,2]. However, the further commercialization of PEMFCs depends on overcoming high price and unsatisfying performance of Pt/C catalyst towards oxygen reduction reaction (ORR) on cathode [3e5]. In
* Corresponding author. E-mail address: [email protected] (D. Cheng). https://doi.org/10.1016/j.ijhydene.2019.12.003 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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the last couple of decades, great efforts have been done to develop various Pt-based nanoalloys, such as PtPd [6e8], PtCu [9,10], PtFe [11,12], PtCo [13e15] and PtNi [16,17] etc., to effectively enhance the electrocatalytic performance and sharply reduce the Pt usage. Among these bimetallic nanoalloys, PtCu represent a promising class of electrocatalysts for various electrocatalytic reactions, including ORR, thus have been attracting great attention [18e22]. Previous studies suggested that ORR is quite structuresensitive on the surfaces of Pt-based nanocrystals [23e25]. Therefore, engineering the structure (i.e., morphology) of nanoparticles is an efficient and promising approach to develop novel Pt-based catalysts with high Pt utilization and activity. Choi et al. demonstrated that the uniform 9 nm PtNi nanoparticles with octahedral morphology exhibited an excellent specific activity, which is about 51 times higher than that of commercial Pt/C catalysts for ORR [26]. Zhang et al. recently reported the facile synthesis of five-fold-twinned PtCu nanoframes with highly anisotropic by a one-pot method. These PtCu nanoframes exhibited enhanced electrocatalytic performances in comparison with other morphologies and commercial Pt/C [27]. Not limited to the above examples suggested that the morphology control of Pt-based alloy nanoparticles with exposed (111) crystal faces is highly efficient for enhancing ORR performance. More recently, doping a third transition metal into bimetallic NPs has been regarded as an efficient way to further modify the electronic structure of Pt sites, and thus can enhance the specific activity effectively [28e30]. Sun et al. reported that the ORR catalytic stability of octahedral PtCu alloy nanocrystals could be improved availably by trace Au doping (Au/Pt ¼ 0.0005) [31]. Beermann et al. presented a synthetic technique for Rh-doped PteNi catalysts with welldefined shapes, which exhibited an exceptional activity and stability behavior [32]. Huang et al. developed the strategy of doping a third metal into to prepare MoePt3Ni/C catalyst, which exhibited tens of times enhancements compared with the commercial Pt/C catalyst for ORR [33]. The transition metal doping can not only increase the activity of electrocatalyst but also improve their stability. However, it is of significant but remains a challenge to select suitable candidate from a series of third transition metals for PtCu nanoalloy with controlled morphology. In this work, an efficient synthetic strategy to enhance the ORR performance of PtCu nanoparticles by morphology tuning and transition-metal doping is reported. Firstly, PtCu nanoparticles with octahedral and spherical morphologies are selectively synthesized in presence or absence of W(CO)6. Electrocatalytic results suggest that the ORR performance of PtCu NOs is better than that of PtCu NSs since the morphology advantages. Then, by introducing a small amount of third transition metal, such as Sc, Y, La, Gd, Fe, PtCuM NOs are obtained with enhanced ORR activity compared to bimetallic PtCu and commercial Pt/C. Moreover, among these PtCuM NOs catalysts, PtCuSc NOs shows the best ORR activity and durability. Our results would shed new light on promoting the ORR performances of Pt-based electrocatalysts.
Experimental section Synthesis of nanomaterials For PtCu NSs, a typical synthesis is as follows. 20 mg Pt(acac)2, 10.2 mg Cu(acac)2, 9.0 mL OAm (oleylamine) and 1.0 mL OA (oleic acid) were mixed in a 50 mL three-necked flasks. The mixture was heated to 130 C in high purity N2 with magnetic stirring for 30 min. The reaction solution was then continuously heated to 230 C with magnetic stirring and kept at this condition for 40 min. The reaction was cooled down to room temperature naturally, and the black sediment was centrifuged and washed by the mixture of hexane and ethanol. The resulting product was obtained and dispersed into hexane as standby. The synthesis of PtCu NOs is similar to that of PtCu NSs except the additional W(CO)6 (0.142 mmol) was added in the reaction solution. Also, the synthesis of PtCuM (M ¼ Sc, Y, La, Gd, Fe) NOs is similar to that of PtCu NSs except the additional W(CO)6 (0.142 mmol) and M(acac)3 (M ¼ Sc, Y, La, Gd, Fe, 0.011 mmol) were added in the reaction solution. The electrocatalysts were prepared by supporting selfsynthesized nanomaterials on carbon black. Typically, 40 mg carbon black was added into dispersion. The mixture was then sonicated for 60 min. The resulting carbon supported samples was centrifuged and dried naturally in a vacuum oven overnight before electrochemical measurements.
Material characterization Transmission electron microscopy (TEM) images were collected on Tecnai G2 20 operating at 200 kV. High-resolution TEM (HRTEM) images were collected on a JEOL JEM-2100 transmission electron microscope. The microstructure and elemental distribution were characterized by high-angle annular bright/dark Field (HAADF or HAABF) in Scanning Transmission Electron Microscopy (STEM) mode. Elemental analyses were performed using inductively coupled plasma spectrometer (ICP) on Thermo Scientific iCAP6000 and energy dispersive X-ray spectroscopy (EDX) from a Hitachi S-4800 SEM operated at 20 kV. Powder X-ray diffraction (PXRD) patterns were recorded using Bruker D8 Advance X-ray diffractometer with CuKa (l ¼ 0.15418 nm) radiation source at 40 kV. Electronic environment of the catalysts were obtained by the X-ray photoelectron spectroscopy (XPS) on Thermofisher ESCALAB 250.
Electrochemical measurements Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests were taken on a three electrode system connected to a CHI760e (Chen-Hua, China) electrochemical work station at room temperature. Rotating disk electrode (RDE) with glassy carbon (GC) was polished and cleaned before surface modifying by electrocatalysts. 10 mL catalyst ink with catalyst concentration of 2 mg mL1 was coated on the surface of GC and dried at room temperature for 2 h. The CV measurements
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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were performed in N2-saturated 0.1 M HClO4 solution at a potential range from 0.05 to 1.10 V with a scanning rate of 50 mV s1. ORR polarization curves were obtained in O2saturated 0.1 M HClO4 solution at a scan rate of 10 mV s1 on the GC-RDE rotating at rotating rate of 1600 rpm. The durability tests were carried out at full potential ranges for 1000 cycles at a scanning rate of 50 mV s1.
Results and discussion As shown in Scheme 1, the well-defined Pt-based nanocrystals were synthesized by co-reduction of Pt(acac)2 and Cu(acac)2 by using the mixture of OAm and AA as both solvent and reducing agent. Typically, PtCu NOs, PtCu NSs and M-doped PtCu NOs could be selectively synthesized in the presence of W(CO)6 and/or transition metal salts (see experiment section for more experimental details). Fig. 1A shows a typical TEM image of PtCu NSs. It is found that the monodisperse nanoparticles are of an average size around 9.9 nm and spherical appearance. The HRTEM image of single particle shows that the lattice fringe is 0.219 nm (Fig. 1B), corresponding to the interplanar distance of (111) plane of PtCu alloy with facet-centered cubic (fcc) structure. Fig. 1C shows the representative HAADF-STEM image of PtCu NS. It can be seen that the obtained PtCu NS are highly crystallized with clearly identifiable lattice. The Pt and Cu distribution on PtCu NSs were confirmed by EDX elemental mapping. As shown in Fig. 1D, both Cu and Pt are distributed on the PtCu NSs homogeneously. Moreover, the crosssectional compositional line-scanning profile of PtCu NS in Fig. 1E shows that both Cu and Pt contents are high in middle of the line-scanning of the PtCu NS and low at the edge of the PtCu NS. It is a typical EDX line scanning of alloy structure [34], which is consistent with the HAADF-STEM measurements. Above results confirm the formation of bimetallic PtCu NSs. Fig. 2A shows a TEM image of the as-synthesized PtCu NOs with an average diameter of 8.3 nm. Moreover, the HAADFSTEM image of PtCu NOs in Fig. S1 shows that more than 80% of the NPs are of octahedral morphology. Fig. 2B reveals a HAABF-STEM image of a single PtCu NO with the lattice fringe of 0.219 nm, which should be index to the (111) interplanar distance in fcc PtCu alloyed structure. The same lattice fringes of PtCu NSs and PtCu NOs indicate that the elemental components of them should be close. Indeed, both EDX analysis and ICP results suggest that the atomic ratio in PtCu NO and
Scheme 1 e Schematic illustration of the growth conditions to the controlled synthesis route for PtCu NOs, PtCu NSs and PtCuM (M ¼ Sc, Y, La, Gd and Fe) NOs.
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PtCu NS is about 1:1 (Fig. S2). Moreover, the mass percentage of Pt in carbon supported PtCu NOs is 9.64%. Fig. 2C displays the HAADF-STEM image of single PtCu NO. At the middle area of the PtCu NO, some knots and sections can be observed. The atoms in this area are coordinative unsaturated, which are more active to adsorb small molecules than other coordinative saturated atoms [35]. In addition, EDX elemental mapping and the cross-sectional compositional line-scanning profile for PtCu NO confirm the complete overlap of Pt and Cu elements through the whole particle (Fig. 2D and E). Above results confirm the formation of bimetallic PtCu with morphology of octahedron. It should be noted that W(CO)6 plays a crucial role in the growth of the PtCu NOs. Previous investigation suggested that W(CO)6 is an efficient shape-directing agent for Pt-based nanocrystals with specific morphology [36,37]. However, the residual CO would cause surface toxicity, thus reduce the catalytic performance of Pt-based nanoparticles [38]. Infrared spectroscopy was performed to check adsorption state on the surface of PtCu NOs (Fig. S3). It demonstrates that the PteCO valence bond is not found at the wave number of 2000 cm1, which illustrates that complete desorption of CO on the surface of the prepared PtCu NOs after treatment. The third transition metal was doped into PtCu NOs through the same synthesis protocol for PtCu NOs by adding a small amount of the third transition metal M (M ¼ Sc, Y, La, Gd, Fe) salts into the reaction solution. Typically, TEM image in Fig. 3A shows that the Sc-doped PtCu NOs exhibit monodisperse characteristic in solution and more than 60% of the particles keep octahedral morphology. The particle size of PtCuM NOs is similar to that of PtCu NOs, and the average size is still within 10 nm. Fig. 3B displays that the atomic ratios of Pt:Cu in PtCuM NOs/C remains 1:1. While, the content of Sc obtained from EDX and ICP is as low as 0.03%, indicating that the presence of third transition metal would not affect the coreduction of Pt and Cu salts subsequent with shape-control for its minute quantity. The distribution of element in PtCuSc NO was further examined by EDX line scanning. As shown in Fig. 3C, the presence of Sc signal peaks indicates that the third transition metal Sc was successfully doped into PtCu NOs/C. Other transition metals, such as Sc, Y, La, Gd, Fe, are also confirmed in PtCu NOs with thimbleful, as shown in Fig. S4. Based on the above analysis, a conclusion can be drawn that the synthesized PtCuM (M ¼ Sc, Y, La, Gd, Fe) NOs/C catalysts are trimetallic alloy catalysts with octahedral morphology, and the content of the third metal M (M ¼ Sc, Y, La, Gd, Fe) is less than 1%. To further verify the influence of third metal for crystal structure, powder XRD was performed and the corresponding curves for all samples are present in Fig. 3D. It can be seen that all peaks could be indexed as those of fcc PtCu structure, which is in accordance with the result of HRTEM. Moreover, peaks of these M-doped PtCu NOs/C are of a slightly negative shift relative to that of PtCu NOs/C. This should be origin from the third transition metal M doping into PtCu NOs. Moreover, the addition of a third metal may also bring about changes in the electronic structure of Pt and Cu. To this end, the element valence information in these nanomaterials was checked by XPS analysis. As shown in Fig. S5, the peaks of Pt4f7/2 and Pt4f5/ 2 in PtCu NSs are located at 70.93 and 74.32 eV, which are
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Fig. 1 e (A) TEM image of PtCu NSs. (B) HRTEM and (C) HAADF-STEM images of an individual PtCu NS. (D) The corresponding EDX mapping of the elements Cu (red), Pt (green) and the overlap. (E) EDX linescan profile of an individual PtCu NS. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2 e (A) TEM image of PtCu NOs. (B) HAABF-STEM and (C) HAADF-STEM images of an individual PtCu NO. (D) The corresponding EDX mapping of the elements Cu (red), Pt (green) and the overlap. (E) EDX linescan profile of an individual PtCu NO. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
respectively shifted to the lower binding energy compared to those in Pt(111) crystal face (71.00 and 74.35 eV) obtained from XPS database in National Institute of Standards and Technology (NIST). The alloy effect should be responsible to this shift. While, the peaks of Pt4f7/2 and Pt4f5/2 in PtCu NOs are further negatively shifted to 70.65 and 73.90 eV, indicating the electronic structure changes of Pt atom due to the morphology effect. Accordingly, the XPS peaks of Cu2p3/2 and Cu2p1/2 in PtCu NOs exhibit a positive shift by 0.22 and 0.29 eV compared to those in PtCu NSs. Moreover, the additional of third transition metal would further modify the electronic structure of Pt and Cu in PtCu NOs (Fig. S6). Typically, the XPS peaks of Pt4f7/2 in PtCuSc NOs, PtCuY NOs, PtCuLa NOs, PtCuGd NOs
and PtCuFe NOs are located at 70.72, 70.76, 70.81, 70.83 and 70.91 eV, respectively, which are shifted to the middle position between those in PtCu NOs and PtCu NSs. It should be noted that the incorporation of Sc has the most significant effect on modifying the electronic structure of Pt in PtCu NOs, indicating the promising ORR activity according to the Sabatier volcano of ORR catalysts [39]. To evaluate the catalytic activity towards ORR, CV curves were obtained firstly. Fig. S7 shows the CV curves of commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuM NOs/C catalysts. The electrocatalytically active surface areas (ECSAs) are evaluated from the hydrogen adsorption/desorption region (between 0.05 and 0.35 V) on CV curves. The ECSAs of
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Fig. 3 e TEM images of (A) PtCuSc NOs/C. (B) EDX spectra and corresponding elemental constituent of PtCu NSs/C. (C) EDX line scan profiles of an individual PtCuSc NO. (D) Typical XRD patterns for PtCu NSs/C, PtCu NOs/C, PtCuSc NOs/C, PtCuY NOs/C, PtCuLa NOs/C, PtCuGd NOs/C and PtCuFe NOs/C.
commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C, PtCuY NOs/C, PtCuLa NOs/C, PtCuGd NOs/C and PtCuFe NOs/C are calculated to be 57.9, 35.6, 59.1, 46.3, 52.4, 45.2, 54.6 and 47.1 m2 gPt1, respectively. It can be found that PtCu NOs/C has a larger ECSA than commercial Pt/C, PtCu NSs/C since the morphology advantage of octahedron, in which the PtCu (111) crystal faces get a total exposure. While, the additional transition metal doping would cause the ECSA loss for PtCu NOs since the corresponding replacement of Pt active sites on the surface. Fig. 4A shows the ORR polarization curves of commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C. The half wave potentials of these catalysts are 0.788 V, 0.804 V, 0.823 V and 0.863 V, respectively. This result suggests that the ORR performance of PtCu nanoparticles catalyst could be significantly affected by morphology tuning, in which the octahedral morphology is a better choice to enhance electrochemical catalytic performance. The better ORR performance of PtCu NOs than that of Pt NSs should mainly attribute to two respects. One is the total exposure of (111) crystal faces, on which the ORR reaction activity is improved, while the ORR reaction activity would be hindered on (100) or (110) crystal faces for Pt-based eletrocatalysts [40]. The other is the metal alloy effect, in which the incorporation of Cu could downshift of d-band center of Pt thus dramatically enhance the catalyst activity for ORR [41]. In addition to morphological and alloy effects, the performance of PtCu NOs/C could be further improved by doping a third transition metal. The mass activities and specific activities of these catalysts are calculated by normalizing the current by mass loading and
electrochemical active area of Pt on GC. As shown in Fig. 4B, the catalytic activity for ORR of these three catalysts orders: commercial Pt/C < PtCu NSs/C < PtCu NOs/C < PtCuSc NOs/C. In particular, the PtCuSc NOs/C catalyst shows the mass activity of 1.652 mA mg1Pt and the specific activity of 2.093 mA cm2 at 0.8 V (vs. RHE), which are 9.9 times and 7.2 times higher than commercial Pt/C catalysts, respectively. After doping the third transition metal M into the PtCu NOs/C, PtCuM NOs/C catalysts have been greatly improved the mass activity and specific activity for ORR (Fig. 4C). Obviously, PtCuSc NOs/C shows the highest ORR activity among these PtCuM NOs/C catalysts. Hence, PtCuSc NOs/C was chosen as a representative to explore the influence of the third transition metals on catalytic activity. According to the Sabatier volcano of ORR catalysts, ORR performance would be maximized when the oxygen binding energy is about 0.2 eV less than the binding energy on Pt(111) [39,42]. The oxygen binding energy on Pt(111) is too strong while oxygen binding energy on PtCu(111) is too weak [31,43]. Moreover, by alloying Sc and Pt, the overpotential towards ORR could be reduced because the binding energy of OeO bond is weakened [44]. According to the above XPS results, the incorporation of Sc on PtCu(111) can effectively lower the d-band center of PtCu(111), and thus may silently increase the oxygen binding energies of adjacent active sites on PtCu(111). As a result, some sites may become highly active for ORR. Above results suggest that the electrochemical performance of PtCu NOs/C catalyst could be improved tremendously by doping a small amount of the Sc atoms.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Fig. 4 e (A) ORR polarization curves for the commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C in O2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV s¡1 and a rotation rate of 1600 rpm. (B) The mass activity and specific activity of ORR for the commercial Pt/C, PtCu NSs/C, PtCu NOs/C and PtCuSc NOs/C. (C) The mass activity and corresponding enhancement factors at 0.8 V of (a) commercial Pt/C, (b) PtCu NSs/C, (c) PtCu NOs/C, (d) PtCuSc NOs/C, (e) PtCuY NOs/C, (f) PtCuLa NOs/C, (g) PtCuGd NOs/C and (h) PtCuFe NOs/C catalysts. (D) Histograms of comparative mass activities at 0.80 V versus RHE for the commercial Pt/C, PtCu NOs/C and PtCuSc NOs/C before and after 1000 cycles of ADTs.
Durability determines the practicability for catalysts. Therefore, the catalytic stabilities of commercial Pt/C, PtCu NOs/C and PtCuSc NOs/C were tested, and the results are shown in Fig. S9. It can be seen that the deviation degree of ORR curves before and after the 1000 cycles in the stability test is given in the following trend: commercial Pt/C > PtCu NOs/ C > PtCuSc NOs/C. It is obvious that the stability of the PtCuSc NOs/C catalyst is the highest among these three catalysts. Under the condition of 0.8 V, the mass activity degradation of commercial Pt/C, PtCu NOs/C and PtCuSc NOs/C are as shown in Fig. 4D. It is found that after the stability test, the mass activity of commercial Pt/C decreased by 44.3%, and the mass activity of PtCu NOs/C declined by 16.5%. Correspondingly, PtCuSc NOs/C decreased the mass activity by 9.2% after the stability test. Obviously, the loss of mass activity for PtCuSc NOs/C was only 9.2%, which is much smaller than that of PtCu NOs/C (16.5%), and commercial Pt/C (44.3%) after stability test. Our results show that a small amount of the third transition metal Sc (less than 5%) contributes to effectively enhance the activity and stability of bimetallic PtCu NOs/C and commercial Pt/C in the electrocatalytic process, which will greatly promote the development of new trimetallic NPs catalysts.
Conclusions In conclusion, we demonstrated an efficient synthetic strategy to enhance the ORR performance of PtCu
nanoparticles by the combination of morphology tuning and transition-metal doping. Firstly, bimetallic PtCu NOs and PtCu NSs by the ligand of W(CO)6 to control the morphology of NPs were synthesized. The electrochemical test shows that the ORR performance of PtCu NPs catalyst could be significantly affected by tuning morphology and the octahedral morphology is a better choice because of the higher electrochemical catalytic performance. Secondly, by doping a small amount of the third transition metal M into the PtCu NOs/C catalyst, five PtCuM NOs/C were successfully synthesized and confirmed. PtCuSc NOs/C shows the highest ORR activity among the PtCuM NOs/C catalysts. Hence, PtCuSc NOs/C was chosen as a representative to explore the influence of the third transition metals on catalytic activity. The ORR activity and stability of commercial Pt/C, PtCu NOs/ C and PtCuSc NOs/C are tested. The PtCuSc NOs/C catalyst shows the mass activity of 1.652 mA mg1Pt and the specific activity of 2.093 mA cm2 at 0.8 V (vs RHE), which are 9.9 times and 7.2 times higher than commercial Pt/C catalysts, respectively. The mass activity loss of PtCuSc NOs/C was only 9.2%, much smaller than those of PtCu NOs/C (16.5%), and commercial Pt/C (44.3%) after 1000 cycles. It is found that doping a small amount of the third transition metal Sc can improve greatly the catalytic performance and stability of PtCu NOs/C in the electrocatalytic process. Our results provide a new method for the synthesis of trimetallic Pt-based catalysts under controlled morphology, which will promote the development of new trimetallic NPs catalysts.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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Acknowledgments This work is supported by the National Natural Science Foundation of China (21822801, 21576008, 91634116) and the Fundamental Research Funds for the Central Universities (XK1802-1 and XK180301). D. Wu is grateful to the Project funded by China Postdoctoral Science Foundation (2018M641165).
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.003.
[17]
references [18] [1] Wee J-H. Applications of proton exchange membrane fuel cell systems. Renew Sustain Energy Rev 2007;11:1720e38. [2] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349e84. [3] Mukerjee S, Srinivasan S. Enhanced electrocatalysis of oxygen reduction on platinum alloys in proton exchange membrane fuel cells. J Electroanal Chem 1993;357:201e24. [4] de Bruijn FA, Dam VAT, Janssen GJM. Review: durability and degradation issues of PEM fuel cell components. Fuel Cells 2008;8:3e22. [5] Martin S, Jensen JO, Li Q, Garcia-Ybarra PL, Castillo JL. Feasibility of ultra-low Pt loading electrodes for high temperature proton exchange membrane fuel cells based in phosphoric acid-doped membrane. Int J Hydrogen Energy 2019;44:28273e82. [6] Lu Y, Du S, Steinberger-Wilckens R. Three-dimensional catalyst electrodes based on PtPd nanodendrites for oxygen reduction reaction in PEFC applications. Appl Catal B Environ 2016;187:108e14. ndez JR, Gochi-Ponce Y, Salazar-Gaste lum MI, [7] Zapata-Ferna Reynoso-Soto EA, Paraguay-Delgado F, Lin SW, et al. Ultrasonic-assisted galvanic displacement synthesis of PtePd/MWCNT for enhanced oxygen reduction reaction: effect of Pt concentration. Int J Hydrogen Energy 2017;42:9806e15. [8] Liao M, Li W, Xi X, Luo C, Fu Y, Gui S, et al. Highly active Pt decorated Pd/C nanocatalysts for oxygen reduction reaction. Int J Hydrogen Energy 2017;42:24090e8. [9] Coleman EJ, Chowdhury MH, Co AC. Insights into the oxygen reduction reaction activity of Pt/C and PtCu/C catalysts. ACS Catal 2015;5:1245e53. [10] Fu S, Zhu C, Shi Q, Xia H, Du D, Lin Y. Highly branched PtCu bimetallic alloy nanodendrites with superior electrocatalytic activities for oxygen reduction reactions. Nanoscale 2016;8:5076e81. [11] Chung DY, Jun SW, Yoon G, Kwon SG, Shin DY, Seo P, et al. Highly durable and active PtFe nanocatalyst for electrochemical oxygen reduction reaction. J Am Chem Soc 2015;137:15478e85. [12] Du XX, He Y, Wang XX, Wang JN. Fine-grained and fully ordered intermetallic PtFe catalysts with largely enhanced catalytic activity and durability. Energy Environ Sci 2016;9:2623e32. [13] Eid K, Wang H, Malgras V, Alshehri SM, Ahamad T, Yamauchi Y, et al. One-step solution-phase synthesis of
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
7
bimetallic PtCo nanodendrites with high electrocatalytic activity for oxygen reduction reaction. J Electroanal Chem 2016;779:250e5. Yang D, Yan Z, Li B, Higgins DC, Wang J, Lv H, et al. Highly active and durable PteCo nanowire networks catalyst for the oxygen reduction reaction in PEMFCs. Int J Hydrogen Energy 2016;41:18592e601. Li Z, Zeng R, Wang L, Jiang L, Wang S, Liu X. A simple strategy to form hollow Pt3Co alloy nanosphere with ultrathin Pt shell with significant enhanced oxygen reduction reaction activity. Int J Hydrogen Energy 2016;41:21394e403. Gong W, Jiang Z, Huang L, Shen PK. PtNi alloy hyperbranched nanostructures with enhanced catalytic performance towards oxygen reduction reaction. Int J Hydrogen Energy 2018;43:18436e43. ~ iz AM, Barbosa R, Miki Rosado G, Verde Y, Valenzuela-Mun Yoshida M, Escobar B. Catalytic activity of Pt-Ni nanoparticles supported on multi-walled carbon nanotubes for the oxygen reduction reaction. Int J Hydrogen Energy 2016;41:23260e71. Yang T, Pukazhselvan D, da Silva EL, Santos MC, Meng L, Ramasamy D, et al. Highly branched PtCu nanodandelion with high activity for oxygen reduction reaction. Int J Hydrogen Energy 2019;44:174e9. Li W, Hu Z-Y, Zhang Z, Wei P, Zhang J, Pu Z, et al. Nano-single crystal coalesced PtCu nanospheres as robust bifunctional catalyst for hydrogen evolution and oxygen reduction reactions. J Catal 2019;375:164e70. Li C, Liu T, He T, Ni B, Yuan Q, Wang X. Composition-driven shape evolution to Cu-rich PtCu octahedral alloy nanocrystals as superior bifunctional catalysts for methanol oxidation and oxygen reduction reaction. Nanoscale 2018;10:4670e4. Cheng D, Wu D, Xu H, Fisher A. Compositionecontrolled synthesis of PtCuNPs shells on copper nanowires as electrocatalysts. ChemistrySelect 2016;1:4392e6. Alekseenko AA, Moguchikh EA, Safronenko OI, Guterman VE. Durability of de-alloyed PtCu/C electrocatalysts. Int J Hydrogen Energy 2018;43:22885e95. Gan L, Cui C, Heggen M, Dionigi F, Rudi S, Strasser P. Element-specific anisotropic growth of shaped platinum alloy nanocrystals. Science 2014;346:1502e6. Wang C, Daimon H, Lee Y, Kim J, Sun S. Synthesis of monodisperse Pt nanocubes and their enhanced catalysis for oxygen reduction. J Am Chem Soc 2007;129:6974e5. Wu R, Tsiakaras P, Shen PK. Facile synthesis of bimetallic PtPd symmetry-broken concave nanocubes and their enhanced activity toward oxygen reduction reaction. Appl Catal B Environ 2019;251:49e56. Choi S-I, Xie S, Shao M, Odell JH, Lu N, Peng H-C, et al. Synthesis and characterization of 9 nm PteNi octahedra with a record high activity of 3.3 A/mgPt for the oxygen reduction reaction. Nano Lett 2013;13:3420e5. Zhang Z, Luo Z, Chen B, Wei C, Zhao J, Chen J, et al. One-pot synthesis of highly anisotropic five-fold-twinned PtCu nanoframes used as a bifunctional electrocatalyst for oxygen reduction and methanol oxidation. Adv Mater 2016;28:8712e7. Wu D, Cheng D. Core/shell AgNi/PtAgNi nanoparticles as methanol-tolerant oxygen reduction electrocatalysts. Electrochim Acta 2015;180:316e22. Wu Y, Zhao Y, Liu J, Wang F. Adding refractory 5d transition metal W into PtCo system: an advanced ternary alloy for efficient oxygen reduction reaction. J Mater Chem 2018;6:10700e9. Lokanathan M, Patil IM, Kakade B. Trimetallic PtNiCo nanoflowers as efficient electrocatalysts towards oxygen reduction reaction. Int J Hydrogen Energy 2018;43:8983e90.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003
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[31] Lu B-A, Sheng T, Tian N, Zhang Z-C, Xiao C, Cao Z-M, et al. Octahedral PtCu alloy nanocrystals with high performance for oxygen reduction reaction and their enhanced stability by trace Au. Nano Energy 2017;33:65e71. [32] Beermann V, Gocyla M, Willinger E, Rudi S, Heggen M, DuninBorkowski RE, et al. Rh-doped PteNi octahedral nanoparticles: understanding the correlation between elemental distribution, oxygen reduction reaction, and shape stability. Nano Lett 2016;16:1719e25. [33] Huang X, Zhao Z, Cao L, Chen Y, Zhu E, Lin Z, et al. Highperformance transition metaledoped Pt3Ni octahedra for oxygen reduction reaction. Science 2015;348:1230e4. [34] Wang C, Chi M, Wang G, van der Vliet D, Li D, More K, et al. Correlation between surface chemistry and electrocatalytic properties of monodisperse PtxNi1-x nanoparticles. Adv Funct Mater 2011;21:147e52. [35] Lim B, Lu X, Jiang M, Camargo PHC, Cho EC, Lee EP, et al. Facile synthesis of highly faceted multioctahedral Pt nanocrystals through controlled overgrowth. Nano Lett 2008;8:4043e7. [36] Yang H, Zhang J, Sun K, Zou S, Fang J. Enhancing by weakening: electrooxidation of methanol on Pt3Co and Pt nanocubes. Angew Chem Int Ed 2010;49:6848e51. [37] Porter NS, Wu H, Quan Z, Fang J. Shape-control and electrocatalytic activity-enhancement of Pt-based bimetallic nanocrystals. Accounts Chem Res 2013;46:1867e77.
[38] Chung DY, Kim H-i, Chung Y-H, Lee MJ, Yoo SJ, Bokare AD, et al. Inhibition of CO poisoning on Pt catalyst coupled with the reduction of toxic hexavalent chromium in a dualfunctional fuel cell. Sci Rep 2014;4:7450. [39] Rossmeisl J, Karlberg GS, Jaramillo T, Nørskov JK. Steady state oxygen reduction and cyclic voltammetry. Faraday Discuss 2009;140:337e46. [40] Li K, Li Y, Wang Y, He F, Jiao M, Tang H, et al. The oxygen reduction reaction on Pt(111) and Pt(100) surfaces substituted by subsurface Cu: a theoretical perspective. J Mater Chem 2015;3:11444e52. [41] Chen B, Cheng D, Zhu J. Synthesis of PtCu nanowires in nonaqueous solvent with enhanced activity and stability for oxygen reduction reaction. J Power Sources 2014;267:380e7. [42] Nørskov JK, Bligaard T, Rossmeisl J, Christensen CH. Towards the computational design of solid catalysts. Nat Chem 2009;1:37e46. [43] Ichiya T, Koiwa N, Ohma A, Tada S, Fushinobu K, Okazaki K. Surface electronic/atomic structure and activation energy on Pt(111), Pt3Cu(111), and PtCu(111) for PEFC cathode. Nanoscale Microscale Thermophys Eng 2010;14:110e22. [44] Garapati MS, Sundara R. Highly efficient and ORR active platinum-scandium alloy-partially exfoliated carbon nanotubes electrocatalyst for Proton Exchange Membrane Fuel Cell. Int J Hydrogen Energy 2019;44:10951e63.
Please cite this article as: Wu D et al., Enhanced oxygen reduction activity of PtCu nanoparticles by morphology tuning and transitionmetal doping, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.003