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Porous Cu-rich@Cu3Pt alloy catalyst with a low Pt loading for enhanced electrocatalytic reactions
Journal of Alloys and Compounds 691 (2017) 26e33
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Porous Cu-rich@Cu3Pt alloy catalyst with a low Pt loading for enhanced electrocatalytic reactions Jin-Yeon Lee, Sang-Beom Han, Da-Hee Kwak, Min-Cheol Kim, Seul Lee, Jin-Young Park, In-Ae Choi, Hyun-Suk Park, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul, 156-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2016 Received in revised form 29 July 2016 Accepted 22 August 2016 Available online 24 August 2016
The core-shell nanostructures are used to improve the specific activity of catalysts in electrooxidation reactions. We report a porous Cu-rich@Cu3Pt core-shell using a seeding method. In this method, a hetero-nucleation process is carried out in the presence of cubic Cu2O seeds. Cu-rich@Cu3Pt core-shell is used as an electrocatalyst in electrooxidation reactions. According to the data obtained using transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD), Cu-rich@Cu3Pt has a porous nanostructure and dominant {111} facets. It consists of a well-defined coreshell structure in which a Cu-rich phase is the core and Cu3Pt alloy is the shell. The porous Cu-rich@Cu3Pt exhibits an excellent electrocatalytic performance in methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). © 2016 Elsevier B.V. All rights reserved.
Keywords: Core shell Pt Cu Methanol oxidation reaction Oxygen reduction reaction
1. Introduction Direct methanol fuel cells (DMFCs) have received intensive interests as a promising candidate of clean energy source for powering portable electronic devices and electric vehicles [1e3]. At the anode of DMFCs, methanol oxidation reaction (MOR) is a complex process involving the exchange of six electrons and formation of intermediates such as CHO or CO [4e6]. In this reaction, active catalytic sites are required for adsorption and oxidation of methanol. Furthermore, active catalytic sites are also required for the oxidation and desorption of adsorbed intermediates of MOR [7]. Currently, Pt-based alloy or core-shell nanoparticles have been comprehensively studied and utilized as anodic electrocatalysts in MOR, which takes place in DMFCs [8e12]. Moreover, proton exchange membrane fuel cells (PEMFCs) have also been considered as a promising alternative power source because they have a fast startup procedure, high energy efficiency, low operating temperature, and environmental friendliness [13,14]. In PEMFCs, Pt is the most effective catalyst that facilitates oxygen reduction reaction (ORR) [15e19]. However, owing to the high cost and scarcity of Pt, the application of ORR has been hindered. As a result, we have not
* Corresponding author. E-mail address: [email protected] (K.-W. Park). http://dx.doi.org/10.1016/j.jallcom.2016.08.221 0925-8388/© 2016 Elsevier B.V. All rights reserved.
been able to achieve widespread commercialization of fuel cell technologies [20,21]. In these systems, Pt and Pt-based nanostructure materials are efficiently used in acidic media as anode and cathode [22,23]. Pt-based bimetallic nanoparticles (NPs), such as core-shell and its alloys, have well-controlled morphology and structure that exhibits optimized electrochemical properties in catalytic reactions [24e26]. Moreover, the core-shell structured NPs are also capable of generating enhanced electrocatalytic properties, which are driven by electronic interactions between the Pt shell and the second metal core as well as the Pt shell nanostructures with different geometries, such as islands, monolayer, and intermixing phase [27,28]. Furthermore, in order to improve specific mass activity in electrocatalytic reactions, the core-shell nanostructures are placed on the exposed surface structures of platinum atoms, while the thin layer of shell structures is in the core-shell NPs [29e31]. Especially, the core-shell cathode catalysts having transition metals such as Fe, Co, Ni, and Cu as a core have intensively been studied. Mukerjee et al. reported fundamental aspects of Cu dissolution and contamination in CuSO4-doped 0.1 N solutions of HClO4 and H2SO4 under both inert and oxygenated conditions [32]. Strasser and coworkers proposed Pt-Cu dealloyed core-shell catalysts for an improved ORR activity, which result in a shift of the electronic band structure of Pt [33]. In addition, Jia et al. showed that the atomic distribution of Pt-based bimetallic NPs under operating conditions
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was strongly dependent on the initial atomic ratio [34]. In particular, dendritic and flower-like Pt-based nanostructures have shown an enhanced electrocatalytic activity and stability in electrochemical power sources, because they have porous structures with particular facets that favor electrocatalytic reactions [35e37]. In this study, we synthesized porous core-shell NPs (denoted as porous Cu-rich@Cu3Pt), which were used as electrocatalysts through a seeding method in which hetero-nucleation process was carried out in the presence of Cu2O nanocube seeds. The crystal structure, elemental composition, morphology, and chemical states of porous Cu-rich@Cu3Pt were characterized by XRD analysis, field-emission transmission electron microscopy (FETEM), EDX spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of the as-prepared electrocatalysts in MOR and ORR were measured and compared using a potentiostat. 2. Experimental 2.1. Synthesis of porous Cu-rich@Cu3Pt NPs 0.01 M copper(II) chloride dihydrate (CuCl2$2H2O, 99.0%, Aldrich) and 2 M sodium hydroxide (NaOH, 98.0%, Samchun) were mixed in de-ionized (DI) water at 50 C. Then, 0.6 M L-ascorbic acid (C6H8O6, 99.0%, Aldrich) was added in the solution and stirred for 3 h. After completing the reaction, the final precipitate was obtained as a seed for porous Cu-rich@Cu3Pt. The precipitate (0.04 g) as a seed and 0.01 g hydrogen hexahydroxyplatinate(IV) (H2Pt(OH)6, 99.9%, Aldrich) were mixed in 450 mL of DI-water at 50 C with continuous stirring. Then, 0.6 M L-ascorbic acid was added in the mixed solution containing the seed in the absence of surfactant. The solution of Cu-rich@Cu3Pt was kept at 50 C for 3 h until the Pt salts were completely reduced on the core. The resulting colloidal solution, which was black in color, was cooled down to 25 C. To prepare Cu-rich@Cu3Pt nanoparticles (40 wt%) supported on carbon black (Vulcan XC-72R), carbon power was mixed and stirred in the colloid solution. The pH of the mixture solution was adjusted to 2, and then, washed it with ethanol and de-ionized water. Thereafter, the products were dried in an oven at 50 C. 2.2. Structural and chemical analysis The morphology and size of the catalysts were characterized by FE-TEM using a Tecnai G2 F30 system, which was operated at 300 kV. The TEM samples were prepared by placing drops of catalyst suspension, which was dispersed in ethanol on a carboncoated nickel grid. The EDX analysis of the catalysts was performed on a FE-TEM. To determine the structure of the catalysts, XRD analysis was carried out using a Bruker X-ray diffractometer (D2 PHASER, Bruker AXS) with a Cu Ka (l ¼ 0.15418 nm) source and a Ni filter. The source was operated at 30 kV and 10 mA. The 2q angular scan was performed from 20 to 80 at a scan rate of 0.02 min1. XPS (Thermo Scientific, K-Alpha) analysis was carried out using an Al Ka X-ray source of 1486.6 eV in a chamber pressure below 1 108 Torr and 200 W beam power. The high resolution spectra were obtained using a pass energy of 46.95 eV. The step size and time per step were chosen to be 0.025 eV and 100 ms, respectively. Both the ends of the baseline were set sufficiently far apart, to avoid distortion in the shape of the spectra, including the tails. A small variation in the range of the baseline did not affect the relative number of fitted species (less than 1%). The C 1s electron binding energy was referenced at 284.6 eV, and a GaussianLorentzian production function was applied using a nonlinear least-squares curve-fitting program.
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2.3. Electrochemical analysis The electrochemical properties of the as-prepared electrocatalysts for MOR and ORR were measured in a three-electrode cell at 25 C using a potentiostat (Eco Chemie, AUTOLAB). The electrocatalyst inks were prepared by mixing the electrocatalyst powder, DI water, 2-propanol solution (99.5%, Aldrich), and 5 wt% Nafion® solution (Aldrich). The glassy carbon working electrode was coated with electrocatalyst inks and dried at 50 C in an oven. The Pt loading amounts of porous Cu-rich@Cu3Pt catalysts were 5.2 and 10.0 mg cm2 in MOR and ORR, respectively. And loading amount of comm-Pt catalysts were 40.0 mg cm2 in MOR and ORR, respectively. In addition, Pt wire and Ag/AgCl (in saturated 3 M KCl) were used as counter and reference electrodes, respectively. To compare the electrochemical properties and MOR activity of the samples, cyclic voltammograms (CVs) of the electrocatalysts were obtained in Ar-saturated 0.1 M HClO4 and 0.1 M HClO4 þ 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. To evaluate the electrocatalytic stability of MOR, the electrocatalysts were kept at 0.45 V for 7200 s in 0.1 M HClO4 þ 2.0 M CH3OH and CVs were then obtained in 0.1 M HClO4 þ 2.0 M CH3OH after the stability test. The current-potential curves of ORR were obtained in O2-saturated 0.1 M HClO4 solution by sweeping the potential from 0.8 to 0.0 V with a rotation speed of 1600 rpm at a scan rate of 5 mV s1. The stability test of ORR was carried out by applying a linear potential sweep for 2000 cycles between 0.4 and 0.9 V with a rate of 50 mV s1 in O2-saturated 0.1 M HClO4 solution at 25 C. The oxygen reduction currentpotential curves after the stability test of the electrocatalysts were obtained by sweeping the potential from 0.8 to 0.0 V at a scan rate of 5 mV s1 and a rotating disk speed of 1600 rpm. 3. Results and discussion Fig. 1(a)-(c) show TEM images of Cu2O, which was synthesized through a chemical reaction. The as-prepared Cu2O exhibited a cubic shape (denoted as cubic-Cu2O), which had an average size of ~111.5 nm (Fig. 2(a)). The cubic-Cu2O exhibited a d-spacing of 0.216 nm, which corresponded to (200) plane of the reference Cu2O (JCPDS No. 77e0199) (Fig. 1(c)). Fig. 1(d) and (e) show TEM images of the as-synthesized NPs having a porous spherical shape with the agglomeration of many small NPs and an average particle size of ~41.9 nm (Fig. 2(b)). Interestingly, the shell region of porous Curich@Cu3Pt NP exhibited {111} facets, having a d-spacing of 0.213 nm. The Cu3Pt alloy phase exhibited a face-centered-cubic (fcc) crystal structure (Fig. 1(f)). On the other hand, in the core region of the Cu-rich alloy phase, Pt represents the {111} facets with a d-spacing of 0.211 nm. To identify the elemental distribution of NPs, a linear profile and an elemental mapping image of the core-shell NPs were obtained as shown in Fig. 1(g) and (h), respectively. The as-prepared NPs contain dominant Cu (red) in the core region, while Pt (green) mixed with Cu in the shell region. As shown in Fig. 1(i), the core-shell NPs were synthesized by a seeding method in which Pt was grown through heterogeneous nucleation. The cubic-Cu2O NPs as seeds in this process were reduced to Cu-rich phase as a core material (Eq. (1) of disproportionation process) and the Cu3Pt alloy shell was subsequently formed on the Cu-rich core (Eqs. (2-1) and (2-2)) [38,39]. 3Cu2O þ 6Hþ / 3Cu2þ þ 3Cu0 þ 3H2O
(1)
H2Pt(OH)6 þ 2C6H8O6 / Pt0 þ 6H2O þ 2C6H6O6
(2-1)
2Cu0 þ H2Pt(OH)6 / 2Cu2þ þ Pt0 þ 4OH þ 2H2O
(2-2)
Furthermore, as confirmed by EDX, the NPs are elemental
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Fig. 1. (a)e(c) TEM and HR-TEM images of the as-prepared Cu2O. (d)e(f) TEM and HR-TEM images of core-shell NPs synthesized by a seeding method using the as-prepared cubicCu2O NPs. (g), (h) Line profile and elemental mapping image of the core-shell NPs. (i) Schematic illustration of synthesis of the porous Cu-rich@Cu3Pt NPs using a seeding method in the presence of cubic-Cu2O.
Fig. 2. Particle size distribution curves of the as-prepared (a) Cu2O and (b) Cu-rich@Cu3Pt.
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Fig. 3. Comparison of elemental compositions of the samples.
components, consisting of 13.0 at% of Pt and 87.0 at% of Cu, which is in good agreement with an experimentally intentional ratio (Fig. 3). As shown in Fig. 4(a), the Cu2O powder, which was used as a seed, exhibited a cubic crystal structure (PDF No. 77e0199) without other crystal structures such as Cu and CuO. The unit cell parameters of Cu2O samples were determined to be a ¼ b ¼ c ¼ 4.258 Å, which is in agreement with those of a typical Cu2O. In the case of the NPs, the XRD peaks seem to consist of (111), (200), and (220), indicating an fcc crystal structure (Fig. 4(a)). It is interesting that the XRD peaks of the NPs contained no Cu2O phase and could be evidently fitted by two peaks (denoted as A and B, respectively) as indicated in Fig. 4(b). Huang et al. also reported that the asprepared Cu@PtCu exhibited a similar XRD peaks to our result, which could be evidently distinguishable [40]. Assuming a substitutional solid solution between metallic phases, the angle shift of the XRD peaks indicates alloy formation between Pt and Cu. Based on the Vegard's law using the equation of dPtCu ¼ X$dPt þ (1-X)$dCu, where dPtCu, dPt, and dCu are d-spacings of PtCu, pure Pt, and pure Cu, respectively, and X is atomic ratio of Pt, the atomic ratios of Pt and Cu in A and B are determined to be 25.9:74.1 and 6.5:93.5, respectively [41,42]. The chemical states and elemental compositions of NPs were
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determined by XPS, as shown in Fig. 5. The Pt 4f7/2 and 4f5/2 peaks typically appear at ~71 and ~75 eV, respectively, with a theoretical area ratio of 4:3. In the as-prepared NPs, the Pt 4f peaks consist of metallic and oxide states; i.e. the peaks for Pt0 and Pt2þ at ~71.8 and ~74.1 eV, respectively (Fig. 5(b)). On the other hand, the Cu 2p3/2 and 2p1/2 peaks typically appear at ~933 and ~953 eV, respectively, with a theoretical area ratio of 2:1. The Cu 2p peaks consist of dominant metallic and oxidized states; the peaks for Cu0 and Cu2þ appear at 932.7 and 934.1 eV, respectively (Fig. 5(c)). XPS results indicated that the as-prepared NPs contained 28.7 at% of Pt and 71.3 at% of Cu, which was in good agreement with the composition of the diffraction peak (A) in the XRD data (Fig. 4(b)). Recently, Li and co-workers synthesized self-supported Pt nanoclusters, which comprised of 2e3 nm Pt NPs, by conducting the galvanic replacement process between Cu2O nanocubes and PtCl2 4 in the presence of a small amount of acid. Thereafter, Cu2O was completely transformed into Pt, which did not contain Cu residual phases, such as Cu2O or Cu [43]. However, in this study, by comparing the EDX, XRD, and XPS data, it can be inferred that the Cu-rich@Cu3Pt NPs contain a core of Cu-rich phase and a shell of Cu3Pt alloy. Using Cu2O as a seed, we prepared porous Cu-rich@Cu3Pt NPs having a homogeneous distribution of Pt and Cu atoms in both the bulk and the surface. To compare the electrochemical properties of the porous Curich@Cu3Pt and commercial Pt/C (40 wt%, E-TEK Co., denoted as comm-Pt), the CVs were obtained in 0.1 M HClO4 with a scan rate of 50 mV s1 at 25 C as shown in Fig. 6(a). The hydrogen desorption curves of catalysts exhibit remarkable oxidation peaks corresponding to the {111} facets of Pt crystal [44]. This indicates that the porous Cu-rich@Cu3Pt has a well-defined formation on the electrode surface, which is enclosed by {111} facets. Furthermore, to compare the electrocatalytic performance of catalysts, CVs were obtained in 0.1 M HClO4 þ 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. The electrochemical characteristic curves of MOR were normalized by electrochemical active surface area (EASA) and loading amount of Pt catalyst, as shown in Fig. 6(b) and (c), respectively. The EASAs estimated from the hydrogen adsorption area of porous Cu-rich@Cu3Pt and comm-Pt were 0.29 and 1.24 cm2, respectively. The porous Cu-rich@Cu3Pt exhibited a lower onset potential and higher oxidation current density per EASA and loading amount than those of comm-Pt, representing an improved electrocatalytic activity in MOR (Fig. 6(b) and (c)). Fig. 6(d) shows oxidation current densities at near the kinetically controlled potential of þ0.4 V obtained from Fig. 6(b) and (c). The porous Cu-
Fig. 4. (a) Wide-range XRD patterns of cubic-Cu2O and core-shell NPs. (b) XRD peak of (220) plane in the core-shell NPs.
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Fig. 5. (a) Wide-range, (b) Pt 4f, and (c) Cu 2p XPS spectra of the core-shell NPs.
Fig. 6. (a) CVs of porous Cu-rich@Cu3Pt and comm-Pt catalysts in an Ar-saturated 0.1 M HClO4 with a scan rate of 50 mV s1 at 25 C. Plots of current densities normalized by (b) EASA and (c) Pt loading versus potential for the samples in an Ar-saturated 0.1 M HClO4 þ 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. (d) Comparison of current densities of the samples measured at 0.4 V for MOR.
Fig. 7. Comparison of the MOR of (a) porous Cu-rich@Cu3Pt and (b) comm-Pt catalysts before and after the stability test in an Ar-saturated 0.1 M HClO4 containing 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. (c) Comparison of current densities of the samples in MOR at 0.6 V before and after the stability test.
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Fig. 8. (a) Current-potential curves of porous Cu-rich@Cu3Pt and comm-Pt catalysts in an O2-saturated 0.1 M HClO4 between 0.8 and 0.0 V with a rotation speed of 1600 rpm at a scan rate of 5 mV s1. Plots of current densities normalized by (b) EASA and (c) Pt loading versus potential of catalysts.
Fig. 9. Comparison of the ORR of (a) porous Cu-rich@Cu3Pt and (b) comm-Pt catalysts before and after the stability test in an O2-saturated 0.1 M HClO4 between 0.8 and 0.0 V with a rotating speed of 1600 rpm and a scan rate of 5 mV s1. (c) Comparison of half-wave potentials of the samples in ORR before and after the stability test.
Fig. 10. (a,b) TEM images and (c) EDX spectrum of Cu-rich@Cu3Pt after the stability test.
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rich@Cu3Pt exhibited 1.9 time higher specific activity and 10.0 times higher mass activity than those of comm-Pt. The improved electrocatalytic activity of the porous Cu-rich@Cu3Pt core-shell nanostructure can be attributed to low amount of Pt loading in the shell, Cu3Pt alloy phase favorable for MOR, porous structure with a large specific surface area, and dominant {111} surface structure [45e50]. Zhang et al. reported that the PtxCu1-x samples (Pt35Cu65, Pt53Cu47, and Pt68Cu32) with controllable atomic composition prepared by using a one-pot chemical route and the Pt35Cu65 catalyst having a Cu-rich alloy phase exhibited improved electrocatalytic activities for MOR due to synergetic effect of the alloy formation and porous structure [51]. To evaluate the electrocatalytic stability of MOR, the catalysts were maintained at 0.45 V for 7200 s in 0.1 M HClO4 þ 2.0 M CH3OH at 25 C (Fig. 7). As shown in Fig. 7(a) and (b), porous Cu-rich@Cu3Pt maintained a high oxidation current density after the stability test, exhibiting the enhanced electrocatalytic stability in MOR. In the case of comm-Pt, the oxidation current density after the stability test significantly decreased, representing the considerable reduction ratio from the initial values. Therefore, we conclude that both improved activity and stability of porous Cu-rich@Cu3Pt in MOR are due to the porous core-shell nanostructure, which favors MOR. Fig. 8(a) shows the ORR activity of the as-prepared samples using a linear sweep voltammetry (LSV) with a rotating disk speed of 1600 rpm in O2-saturated 0.1 M HClO4 solution at 25 C. The porous Cu-rich@Cu3Pt exhibited high half-wave potential (0.52 V) and current density (1.51 mA cm[email protected] V), compared to comm-Pt (0.49 V and 0.99 mA cm[email protected] V). Especially, as indicated in Fig. 8(c), the mass activity of porous Cu-rich@Cu3Pt in the range of the kinetically controlled potential was much higher than that of comm-Pt, resulting from the Cu3Pt shell as an active site in porous Cu-rich@Cu3Pt as a core-shell nanostructure. According to the literature, Pt(111) surface showed an improved E1/2 by 20 mV compared to the current state-of ethe-art Pt/C catalyst [52]. The Cu-rich@Cu3Pt revealed an enhanced E1/2 by 30 mV compared to the current state-of -the-art Pt/C catalyst. Thus, the improved ORR activity of porous Cu-rich@Cu3Pt relative to comm-Pt might be mainly due to the preferential exposure of {111} facets and a porous nanostructure facilitating the mass transport of oxygen and byproducts of the reduction process. In addition, the porous structure provides highly effective active sites and a confinement effect within the porous structure that result in increased residence time at the active surface [53]. In order to characterize the electrochemical stability of electrocatalysts in ORR, a stability test was performed using linear potential sweeps between 0.4 and 0.9 V for 2000 cycles in O2-saturated 0.1 M HClO4. The LSV curves of the electrocatalysts after the ORR stability test were compared (Fig. 9(a) and (b)). After 2000 cycles, the porous Cu-rich@Cu3Pt exhibited a slight loss (~1.06 mV) in the half-wave potential, suggesting an improved ORR stability of the nanostructured catalyst. The TEM images and EDX spectrum of the Cu-rich@Cu3Pt after the cycling process were obtained as shown in Fig. 10. The NPs maintained a porous spherical shape and ORR activity despite the dissolution of Cu (Pt:Cu ¼ 68:32) during the stability test [33]. In contrast, commPt showed a serious drop (~47.06 mV) in the half-wave potential, most likely due to the agglomeration of comm-Pt in the cycling test (Fig. 9(c)). Therefore, highly improved activity and stability in MOR and ORR of porous Cu-rich@Cu3Pt, i.e. high current densities and high retentions both before and after the stability test are mainly ascribed to both the well-defined core-shell porous structure and the homogeneous distribution of Pt-Cu alloy phases in the preferential exposed {111} facets. However, since the core-shell NPs have a relatively large size, it is challenging to use them as an electrocatalyst in fuel cells. Therefore, the downsizing of NPs should be carried out as in future studies.
4. Conclusions In summary, we have synthesized porous Cu-rich@Cu3Pt as a core-shell catalyst for MOR and ORR using a seeding method and cubic-Cu2O seeds. The porous Cu-rich@Cu3Pt exhibited a welldefined core-shell nanostructure consisting of a Cu-rich phase in the core and Cu3Pt alloy in the shell. The improved electrocatalytic performance of porous Cu-rich@Cu3Pt NPs in MOR and ORR may be attributed to the porous core-shell structure, which has preferentially exposed {111} facets of Cu3Pt alloy shell. Acknowledgments This work was supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20148520120160). References [1] H. Wang, S. Ji, W. Wang, R. Wang, S. Afr. J. Chem. 66 (2013) 17e20. [2] Q. Lv, Y. Xiao, M. Yin, J. Ge, W. Xing, C. Liu, Electrochim. Acta 139 (2014) 61e68. [3] J. Xu, X. Liu, Y. Chen, Y. Zhou, T. Lu, Y. Tang, J. Mater. Chem. 22 (2012) 23659e23667. [4] Y. Paik, S.-S. Kim, O.H. Han, Angew. Chem. Int. Ed. 47 (2008) 94e96. [5] A. Hamnett, Catal. Today 38 (1997) 445e457. [6] T. Vidakovi c, M. Christov, K. Sundmacher, J. Electroanal. Chem. 580 (2005) 105e121. [7] L. Li, Y. Xing, Energies 2 (2009) 789e804. [8] B.T. Sneed, A.P. Young, D. Jalalpoor, M.C. Golden, S. Mao, Y. Jiang, Y. Wang, C.K. Tsung, ACS Nano 8 (2014) 7239e7250. [9] S.W. Kang, Y.W. Lee, Y. Park, B.-S. Choi, J.W. Hong, K.-H. Park, S.W. Han, ACS Nano 7 (2013) 7945e7955. [10] Y.-W. Lee, A.-R. Ko, S.-B. Han, H.-S. Kim, K.-W. Park, Phys. Chem. Chem. Phys. 13 (2011) 5569e5572. [11] H.-L. Liu, F. Nosheen, X. Wang, Chem. Soc. Rev. 44 (2015) 3056e3078. [12] H.-J. Qiu, H.T. Xu, X. Li, J.Q. Wang, Y. Wang, J. Mater. Chem. A 3 (2015) 7939e7944. [13] C.L. Do, T.S. Pham, N.P. Nguyen, V.Q. Tran, Adv. Nat. Sci. Nanosci. Nanotechnol. 4 (2013) 035011. [14] Y. Shao, G. Yin, Y. Gao, J. Power Sources 171 (2007) 558e566. [15] C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H.L. Xin, J.D. Snyder, D. Li, J.A. Herron, M. Mavrikakis, M. Chi, K.L. More, Y. Li, N.M. Markovic, G.A. Somorjai, P. Yang, V.R. Stamenkovic, Science 343 (2014) 1339e1343. [16] C. Zhang, W. Sandorf, Z. Peng, ACS Catal. 5 (2015) 2296e2300. [17] M.K. Debe, Nature 486 (2012) 43e51. [18] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345e352. ~ a, J. Am. [19] D. Wang, H.L. Xin, Y. Yu, H. Wang, E. Rus, D.A. Muller, H.D. Abrun Chem. Soc. 132 (2010) 17664e17666. [20] B. Lim, M. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X. Lu, Y. Zhu, Y. Xia, Science 324 (2009) 1302e1305. [21] V.R. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C.A. Lucas, G. Wang, P.N. Ross, N.M. Markovic, Nat. Mater. 6 (2007) 241e247. [22] J.-N. Zheng, S.-S. Li, X. Ma, F.-Y. Chen, A.-J. Wang, J.-R. Chen, J.-J. Feng, J. Power Sources 262 (2014) 270e278. [23] J.-N. Zheng, L.-L. He, C. Chen, A.-J. Wang, K.-F. Ma, J.-J. Feng, J. Power Sources 268 (2014) 744e751. [24] Y.W. Lee, M. Kim, Z.H. Kim, S.W. Han, J. Am. Chem. Soc. 131 (2009) 17036e17037. [25] X. Liu, W. Wang, H. Li, L. Li, G. Zhou, R. Yu, D. Wang, Y. Li, Sci. Rep. 3 (2013) 1404. [26] C. Zhang, S.Y. Hwang, A. Trout, Z. Peng, J. Am. Chem. Soc. 136 (2014) 7805e7808. [27] S. Duan, P.-P. Fang, F.-R. Fan, I. Broadwell, F.-Z. Yang, D.-Y. Wu, B. Ren, C. Amatore, Y. Luo, X. Xu, Z.-Q. Tian, Phys. Chem. Chem. Phys. 13 (2011) 5441e5449. [28] D.-J. Chen, A.M. Hofstead-Duffy, I.-S. Park, D.O. Atienza, C. Susut, S.-G. Sun, Y.J. Tong, J. Phys. Chem. C 115 (2011) 8735e8743. [29] Y.-W. Lee, J.-Y. Lee, D.-H. Kwak, E.-T. Hwang, J.I. Sohn, K.-W. Park, Appl. Catal. B 179 (2015) 178e184. [30] J.W. Hong, D. Kim, Y.W. Lee, M. Kim, S.W. Kang, S.W. Han, Angew. Chem. Int. Ed. 50 (2011) 8876e8880. [31] A. Oh, H. Baik, D.S. Choi, J.Y. Cheon, B. Kim, H. Kim, S.J. Kwon, S.H. Joo, Y. Jung, K. Lee, ACS Nano 9 (2015) 2856e2867. [32] Q. Jia, D.E. Ramaker, J.M. Ziegelbauer, N. Ramaswamy, A. Halder, S. Mukerjee, J. Phys. Chem. C 117 (2013) 4585e4596.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Porous Cu-rich@Cu3Pt alloy catalyst with a low Pt loading for enhanced electrocatalytic reactions Jin-Yeon Lee, Sang-Beom Han, Da-Hee Kwak, Min-Cheol Kim, Seul Lee, Jin-Young Park, In-Ae Choi, Hyun-Suk Park, Kyung-Won Park* Department of Chemical Engineering, Soongsil University, Seoul, 156-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 April 2016 Received in revised form 29 July 2016 Accepted 22 August 2016 Available online 24 August 2016
The core-shell nanostructures are used to improve the specific activity of catalysts in electrooxidation reactions. We report a porous Cu-rich@Cu3Pt core-shell using a seeding method. In this method, a hetero-nucleation process is carried out in the presence of cubic Cu2O seeds. Cu-rich@Cu3Pt core-shell is used as an electrocatalyst in electrooxidation reactions. According to the data obtained using transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD), Cu-rich@Cu3Pt has a porous nanostructure and dominant {111} facets. It consists of a well-defined coreshell structure in which a Cu-rich phase is the core and Cu3Pt alloy is the shell. The porous Cu-rich@Cu3Pt exhibits an excellent electrocatalytic performance in methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). © 2016 Elsevier B.V. All rights reserved.
Keywords: Core shell Pt Cu Methanol oxidation reaction Oxygen reduction reaction
1. Introduction Direct methanol fuel cells (DMFCs) have received intensive interests as a promising candidate of clean energy source for powering portable electronic devices and electric vehicles [1e3]. At the anode of DMFCs, methanol oxidation reaction (MOR) is a complex process involving the exchange of six electrons and formation of intermediates such as CHO or CO [4e6]. In this reaction, active catalytic sites are required for adsorption and oxidation of methanol. Furthermore, active catalytic sites are also required for the oxidation and desorption of adsorbed intermediates of MOR [7]. Currently, Pt-based alloy or core-shell nanoparticles have been comprehensively studied and utilized as anodic electrocatalysts in MOR, which takes place in DMFCs [8e12]. Moreover, proton exchange membrane fuel cells (PEMFCs) have also been considered as a promising alternative power source because they have a fast startup procedure, high energy efficiency, low operating temperature, and environmental friendliness [13,14]. In PEMFCs, Pt is the most effective catalyst that facilitates oxygen reduction reaction (ORR) [15e19]. However, owing to the high cost and scarcity of Pt, the application of ORR has been hindered. As a result, we have not
* Corresponding author. E-mail address: [email protected] (K.-W. Park). http://dx.doi.org/10.1016/j.jallcom.2016.08.221 0925-8388/© 2016 Elsevier B.V. All rights reserved.
been able to achieve widespread commercialization of fuel cell technologies [20,21]. In these systems, Pt and Pt-based nanostructure materials are efficiently used in acidic media as anode and cathode [22,23]. Pt-based bimetallic nanoparticles (NPs), such as core-shell and its alloys, have well-controlled morphology and structure that exhibits optimized electrochemical properties in catalytic reactions [24e26]. Moreover, the core-shell structured NPs are also capable of generating enhanced electrocatalytic properties, which are driven by electronic interactions between the Pt shell and the second metal core as well as the Pt shell nanostructures with different geometries, such as islands, monolayer, and intermixing phase [27,28]. Furthermore, in order to improve specific mass activity in electrocatalytic reactions, the core-shell nanostructures are placed on the exposed surface structures of platinum atoms, while the thin layer of shell structures is in the core-shell NPs [29e31]. Especially, the core-shell cathode catalysts having transition metals such as Fe, Co, Ni, and Cu as a core have intensively been studied. Mukerjee et al. reported fundamental aspects of Cu dissolution and contamination in CuSO4-doped 0.1 N solutions of HClO4 and H2SO4 under both inert and oxygenated conditions [32]. Strasser and coworkers proposed Pt-Cu dealloyed core-shell catalysts for an improved ORR activity, which result in a shift of the electronic band structure of Pt [33]. In addition, Jia et al. showed that the atomic distribution of Pt-based bimetallic NPs under operating conditions
J.-Y. Lee et al. / Journal of Alloys and Compounds 691 (2017) 26e33
was strongly dependent on the initial atomic ratio [34]. In particular, dendritic and flower-like Pt-based nanostructures have shown an enhanced electrocatalytic activity and stability in electrochemical power sources, because they have porous structures with particular facets that favor electrocatalytic reactions [35e37]. In this study, we synthesized porous core-shell NPs (denoted as porous Cu-rich@Cu3Pt), which were used as electrocatalysts through a seeding method in which hetero-nucleation process was carried out in the presence of Cu2O nanocube seeds. The crystal structure, elemental composition, morphology, and chemical states of porous Cu-rich@Cu3Pt were characterized by XRD analysis, field-emission transmission electron microscopy (FETEM), EDX spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of the as-prepared electrocatalysts in MOR and ORR were measured and compared using a potentiostat. 2. Experimental 2.1. Synthesis of porous Cu-rich@Cu3Pt NPs 0.01 M copper(II) chloride dihydrate (CuCl2$2H2O, 99.0%, Aldrich) and 2 M sodium hydroxide (NaOH, 98.0%, Samchun) were mixed in de-ionized (DI) water at 50 C. Then, 0.6 M L-ascorbic acid (C6H8O6, 99.0%, Aldrich) was added in the solution and stirred for 3 h. After completing the reaction, the final precipitate was obtained as a seed for porous Cu-rich@Cu3Pt. The precipitate (0.04 g) as a seed and 0.01 g hydrogen hexahydroxyplatinate(IV) (H2Pt(OH)6, 99.9%, Aldrich) were mixed in 450 mL of DI-water at 50 C with continuous stirring. Then, 0.6 M L-ascorbic acid was added in the mixed solution containing the seed in the absence of surfactant. The solution of Cu-rich@Cu3Pt was kept at 50 C for 3 h until the Pt salts were completely reduced on the core. The resulting colloidal solution, which was black in color, was cooled down to 25 C. To prepare Cu-rich@Cu3Pt nanoparticles (40 wt%) supported on carbon black (Vulcan XC-72R), carbon power was mixed and stirred in the colloid solution. The pH of the mixture solution was adjusted to 2, and then, washed it with ethanol and de-ionized water. Thereafter, the products were dried in an oven at 50 C. 2.2. Structural and chemical analysis The morphology and size of the catalysts were characterized by FE-TEM using a Tecnai G2 F30 system, which was operated at 300 kV. The TEM samples were prepared by placing drops of catalyst suspension, which was dispersed in ethanol on a carboncoated nickel grid. The EDX analysis of the catalysts was performed on a FE-TEM. To determine the structure of the catalysts, XRD analysis was carried out using a Bruker X-ray diffractometer (D2 PHASER, Bruker AXS) with a Cu Ka (l ¼ 0.15418 nm) source and a Ni filter. The source was operated at 30 kV and 10 mA. The 2q angular scan was performed from 20 to 80 at a scan rate of 0.02 min1. XPS (Thermo Scientific, K-Alpha) analysis was carried out using an Al Ka X-ray source of 1486.6 eV in a chamber pressure below 1 108 Torr and 200 W beam power. The high resolution spectra were obtained using a pass energy of 46.95 eV. The step size and time per step were chosen to be 0.025 eV and 100 ms, respectively. Both the ends of the baseline were set sufficiently far apart, to avoid distortion in the shape of the spectra, including the tails. A small variation in the range of the baseline did not affect the relative number of fitted species (less than 1%). The C 1s electron binding energy was referenced at 284.6 eV, and a GaussianLorentzian production function was applied using a nonlinear least-squares curve-fitting program.
27
2.3. Electrochemical analysis The electrochemical properties of the as-prepared electrocatalysts for MOR and ORR were measured in a three-electrode cell at 25 C using a potentiostat (Eco Chemie, AUTOLAB). The electrocatalyst inks were prepared by mixing the electrocatalyst powder, DI water, 2-propanol solution (99.5%, Aldrich), and 5 wt% Nafion® solution (Aldrich). The glassy carbon working electrode was coated with electrocatalyst inks and dried at 50 C in an oven. The Pt loading amounts of porous Cu-rich@Cu3Pt catalysts were 5.2 and 10.0 mg cm2 in MOR and ORR, respectively. And loading amount of comm-Pt catalysts were 40.0 mg cm2 in MOR and ORR, respectively. In addition, Pt wire and Ag/AgCl (in saturated 3 M KCl) were used as counter and reference electrodes, respectively. To compare the electrochemical properties and MOR activity of the samples, cyclic voltammograms (CVs) of the electrocatalysts were obtained in Ar-saturated 0.1 M HClO4 and 0.1 M HClO4 þ 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. To evaluate the electrocatalytic stability of MOR, the electrocatalysts were kept at 0.45 V for 7200 s in 0.1 M HClO4 þ 2.0 M CH3OH and CVs were then obtained in 0.1 M HClO4 þ 2.0 M CH3OH after the stability test. The current-potential curves of ORR were obtained in O2-saturated 0.1 M HClO4 solution by sweeping the potential from 0.8 to 0.0 V with a rotation speed of 1600 rpm at a scan rate of 5 mV s1. The stability test of ORR was carried out by applying a linear potential sweep for 2000 cycles between 0.4 and 0.9 V with a rate of 50 mV s1 in O2-saturated 0.1 M HClO4 solution at 25 C. The oxygen reduction currentpotential curves after the stability test of the electrocatalysts were obtained by sweeping the potential from 0.8 to 0.0 V at a scan rate of 5 mV s1 and a rotating disk speed of 1600 rpm. 3. Results and discussion Fig. 1(a)-(c) show TEM images of Cu2O, which was synthesized through a chemical reaction. The as-prepared Cu2O exhibited a cubic shape (denoted as cubic-Cu2O), which had an average size of ~111.5 nm (Fig. 2(a)). The cubic-Cu2O exhibited a d-spacing of 0.216 nm, which corresponded to (200) plane of the reference Cu2O (JCPDS No. 77e0199) (Fig. 1(c)). Fig. 1(d) and (e) show TEM images of the as-synthesized NPs having a porous spherical shape with the agglomeration of many small NPs and an average particle size of ~41.9 nm (Fig. 2(b)). Interestingly, the shell region of porous Curich@Cu3Pt NP exhibited {111} facets, having a d-spacing of 0.213 nm. The Cu3Pt alloy phase exhibited a face-centered-cubic (fcc) crystal structure (Fig. 1(f)). On the other hand, in the core region of the Cu-rich alloy phase, Pt represents the {111} facets with a d-spacing of 0.211 nm. To identify the elemental distribution of NPs, a linear profile and an elemental mapping image of the core-shell NPs were obtained as shown in Fig. 1(g) and (h), respectively. The as-prepared NPs contain dominant Cu (red) in the core region, while Pt (green) mixed with Cu in the shell region. As shown in Fig. 1(i), the core-shell NPs were synthesized by a seeding method in which Pt was grown through heterogeneous nucleation. The cubic-Cu2O NPs as seeds in this process were reduced to Cu-rich phase as a core material (Eq. (1) of disproportionation process) and the Cu3Pt alloy shell was subsequently formed on the Cu-rich core (Eqs. (2-1) and (2-2)) [38,39]. 3Cu2O þ 6Hþ / 3Cu2þ þ 3Cu0 þ 3H2O
(1)
H2Pt(OH)6 þ 2C6H8O6 / Pt0 þ 6H2O þ 2C6H6O6
(2-1)
2Cu0 þ H2Pt(OH)6 / 2Cu2þ þ Pt0 þ 4OH þ 2H2O
(2-2)
Furthermore, as confirmed by EDX, the NPs are elemental
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Fig. 1. (a)e(c) TEM and HR-TEM images of the as-prepared Cu2O. (d)e(f) TEM and HR-TEM images of core-shell NPs synthesized by a seeding method using the as-prepared cubicCu2O NPs. (g), (h) Line profile and elemental mapping image of the core-shell NPs. (i) Schematic illustration of synthesis of the porous Cu-rich@Cu3Pt NPs using a seeding method in the presence of cubic-Cu2O.
Fig. 2. Particle size distribution curves of the as-prepared (a) Cu2O and (b) Cu-rich@Cu3Pt.
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Fig. 3. Comparison of elemental compositions of the samples.
components, consisting of 13.0 at% of Pt and 87.0 at% of Cu, which is in good agreement with an experimentally intentional ratio (Fig. 3). As shown in Fig. 4(a), the Cu2O powder, which was used as a seed, exhibited a cubic crystal structure (PDF No. 77e0199) without other crystal structures such as Cu and CuO. The unit cell parameters of Cu2O samples were determined to be a ¼ b ¼ c ¼ 4.258 Å, which is in agreement with those of a typical Cu2O. In the case of the NPs, the XRD peaks seem to consist of (111), (200), and (220), indicating an fcc crystal structure (Fig. 4(a)). It is interesting that the XRD peaks of the NPs contained no Cu2O phase and could be evidently fitted by two peaks (denoted as A and B, respectively) as indicated in Fig. 4(b). Huang et al. also reported that the asprepared Cu@PtCu exhibited a similar XRD peaks to our result, which could be evidently distinguishable [40]. Assuming a substitutional solid solution between metallic phases, the angle shift of the XRD peaks indicates alloy formation between Pt and Cu. Based on the Vegard's law using the equation of dPtCu ¼ X$dPt þ (1-X)$dCu, where dPtCu, dPt, and dCu are d-spacings of PtCu, pure Pt, and pure Cu, respectively, and X is atomic ratio of Pt, the atomic ratios of Pt and Cu in A and B are determined to be 25.9:74.1 and 6.5:93.5, respectively [41,42]. The chemical states and elemental compositions of NPs were
29
determined by XPS, as shown in Fig. 5. The Pt 4f7/2 and 4f5/2 peaks typically appear at ~71 and ~75 eV, respectively, with a theoretical area ratio of 4:3. In the as-prepared NPs, the Pt 4f peaks consist of metallic and oxide states; i.e. the peaks for Pt0 and Pt2þ at ~71.8 and ~74.1 eV, respectively (Fig. 5(b)). On the other hand, the Cu 2p3/2 and 2p1/2 peaks typically appear at ~933 and ~953 eV, respectively, with a theoretical area ratio of 2:1. The Cu 2p peaks consist of dominant metallic and oxidized states; the peaks for Cu0 and Cu2þ appear at 932.7 and 934.1 eV, respectively (Fig. 5(c)). XPS results indicated that the as-prepared NPs contained 28.7 at% of Pt and 71.3 at% of Cu, which was in good agreement with the composition of the diffraction peak (A) in the XRD data (Fig. 4(b)). Recently, Li and co-workers synthesized self-supported Pt nanoclusters, which comprised of 2e3 nm Pt NPs, by conducting the galvanic replacement process between Cu2O nanocubes and PtCl2 4 in the presence of a small amount of acid. Thereafter, Cu2O was completely transformed into Pt, which did not contain Cu residual phases, such as Cu2O or Cu [43]. However, in this study, by comparing the EDX, XRD, and XPS data, it can be inferred that the Cu-rich@Cu3Pt NPs contain a core of Cu-rich phase and a shell of Cu3Pt alloy. Using Cu2O as a seed, we prepared porous Cu-rich@Cu3Pt NPs having a homogeneous distribution of Pt and Cu atoms in both the bulk and the surface. To compare the electrochemical properties of the porous Curich@Cu3Pt and commercial Pt/C (40 wt%, E-TEK Co., denoted as comm-Pt), the CVs were obtained in 0.1 M HClO4 with a scan rate of 50 mV s1 at 25 C as shown in Fig. 6(a). The hydrogen desorption curves of catalysts exhibit remarkable oxidation peaks corresponding to the {111} facets of Pt crystal [44]. This indicates that the porous Cu-rich@Cu3Pt has a well-defined formation on the electrode surface, which is enclosed by {111} facets. Furthermore, to compare the electrocatalytic performance of catalysts, CVs were obtained in 0.1 M HClO4 þ 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. The electrochemical characteristic curves of MOR were normalized by electrochemical active surface area (EASA) and loading amount of Pt catalyst, as shown in Fig. 6(b) and (c), respectively. The EASAs estimated from the hydrogen adsorption area of porous Cu-rich@Cu3Pt and comm-Pt were 0.29 and 1.24 cm2, respectively. The porous Cu-rich@Cu3Pt exhibited a lower onset potential and higher oxidation current density per EASA and loading amount than those of comm-Pt, representing an improved electrocatalytic activity in MOR (Fig. 6(b) and (c)). Fig. 6(d) shows oxidation current densities at near the kinetically controlled potential of þ0.4 V obtained from Fig. 6(b) and (c). The porous Cu-
Fig. 4. (a) Wide-range XRD patterns of cubic-Cu2O and core-shell NPs. (b) XRD peak of (220) plane in the core-shell NPs.
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J.-Y. Lee et al. / Journal of Alloys and Compounds 691 (2017) 26e33
Fig. 5. (a) Wide-range, (b) Pt 4f, and (c) Cu 2p XPS spectra of the core-shell NPs.
Fig. 6. (a) CVs of porous Cu-rich@Cu3Pt and comm-Pt catalysts in an Ar-saturated 0.1 M HClO4 with a scan rate of 50 mV s1 at 25 C. Plots of current densities normalized by (b) EASA and (c) Pt loading versus potential for the samples in an Ar-saturated 0.1 M HClO4 þ 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. (d) Comparison of current densities of the samples measured at 0.4 V for MOR.
Fig. 7. Comparison of the MOR of (a) porous Cu-rich@Cu3Pt and (b) comm-Pt catalysts before and after the stability test in an Ar-saturated 0.1 M HClO4 containing 2.0 M CH3OH with a scan rate of 50 mV s1 at 25 C. (c) Comparison of current densities of the samples in MOR at 0.6 V before and after the stability test.
J.-Y. Lee et al. / Journal of Alloys and Compounds 691 (2017) 26e33
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Fig. 8. (a) Current-potential curves of porous Cu-rich@Cu3Pt and comm-Pt catalysts in an O2-saturated 0.1 M HClO4 between 0.8 and 0.0 V with a rotation speed of 1600 rpm at a scan rate of 5 mV s1. Plots of current densities normalized by (b) EASA and (c) Pt loading versus potential of catalysts.
Fig. 9. Comparison of the ORR of (a) porous Cu-rich@Cu3Pt and (b) comm-Pt catalysts before and after the stability test in an O2-saturated 0.1 M HClO4 between 0.8 and 0.0 V with a rotating speed of 1600 rpm and a scan rate of 5 mV s1. (c) Comparison of half-wave potentials of the samples in ORR before and after the stability test.
Fig. 10. (a,b) TEM images and (c) EDX spectrum of Cu-rich@Cu3Pt after the stability test.
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rich@Cu3Pt exhibited 1.9 time higher specific activity and 10.0 times higher mass activity than those of comm-Pt. The improved electrocatalytic activity of the porous Cu-rich@Cu3Pt core-shell nanostructure can be attributed to low amount of Pt loading in the shell, Cu3Pt alloy phase favorable for MOR, porous structure with a large specific surface area, and dominant {111} surface structure [45e50]. Zhang et al. reported that the PtxCu1-x samples (Pt35Cu65, Pt53Cu47, and Pt68Cu32) with controllable atomic composition prepared by using a one-pot chemical route and the Pt35Cu65 catalyst having a Cu-rich alloy phase exhibited improved electrocatalytic activities for MOR due to synergetic effect of the alloy formation and porous structure [51]. To evaluate the electrocatalytic stability of MOR, the catalysts were maintained at 0.45 V for 7200 s in 0.1 M HClO4 þ 2.0 M CH3OH at 25 C (Fig. 7). As shown in Fig. 7(a) and (b), porous Cu-rich@Cu3Pt maintained a high oxidation current density after the stability test, exhibiting the enhanced electrocatalytic stability in MOR. In the case of comm-Pt, the oxidation current density after the stability test significantly decreased, representing the considerable reduction ratio from the initial values. Therefore, we conclude that both improved activity and stability of porous Cu-rich@Cu3Pt in MOR are due to the porous core-shell nanostructure, which favors MOR. Fig. 8(a) shows the ORR activity of the as-prepared samples using a linear sweep voltammetry (LSV) with a rotating disk speed of 1600 rpm in O2-saturated 0.1 M HClO4 solution at 25 C. The porous Cu-rich@Cu3Pt exhibited high half-wave potential (0.52 V) and current density (1.51 mA cm[email protected] V), compared to comm-Pt (0.49 V and 0.99 mA cm[email protected] V). Especially, as indicated in Fig. 8(c), the mass activity of porous Cu-rich@Cu3Pt in the range of the kinetically controlled potential was much higher than that of comm-Pt, resulting from the Cu3Pt shell as an active site in porous Cu-rich@Cu3Pt as a core-shell nanostructure. According to the literature, Pt(111) surface showed an improved E1/2 by 20 mV compared to the current state-of ethe-art Pt/C catalyst [52]. The Cu-rich@Cu3Pt revealed an enhanced E1/2 by 30 mV compared to the current state-of -the-art Pt/C catalyst. Thus, the improved ORR activity of porous Cu-rich@Cu3Pt relative to comm-Pt might be mainly due to the preferential exposure of {111} facets and a porous nanostructure facilitating the mass transport of oxygen and byproducts of the reduction process. In addition, the porous structure provides highly effective active sites and a confinement effect within the porous structure that result in increased residence time at the active surface [53]. In order to characterize the electrochemical stability of electrocatalysts in ORR, a stability test was performed using linear potential sweeps between 0.4 and 0.9 V for 2000 cycles in O2-saturated 0.1 M HClO4. The LSV curves of the electrocatalysts after the ORR stability test were compared (Fig. 9(a) and (b)). After 2000 cycles, the porous Cu-rich@Cu3Pt exhibited a slight loss (~1.06 mV) in the half-wave potential, suggesting an improved ORR stability of the nanostructured catalyst. The TEM images and EDX spectrum of the Cu-rich@Cu3Pt after the cycling process were obtained as shown in Fig. 10. The NPs maintained a porous spherical shape and ORR activity despite the dissolution of Cu (Pt:Cu ¼ 68:32) during the stability test [33]. In contrast, commPt showed a serious drop (~47.06 mV) in the half-wave potential, most likely due to the agglomeration of comm-Pt in the cycling test (Fig. 9(c)). Therefore, highly improved activity and stability in MOR and ORR of porous Cu-rich@Cu3Pt, i.e. high current densities and high retentions both before and after the stability test are mainly ascribed to both the well-defined core-shell porous structure and the homogeneous distribution of Pt-Cu alloy phases in the preferential exposed {111} facets. However, since the core-shell NPs have a relatively large size, it is challenging to use them as an electrocatalyst in fuel cells. Therefore, the downsizing of NPs should be carried out as in future studies.
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