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C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes
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Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes Biyao Jin, Yiqi Li, Lianhua Zhao* Department of Chemistry, College of Science, Yanbian University, Yanji 133002, China
article info
abstract
Article history:
Bimetallic PdV/C nanoparticles have been synthesized by phase-transfer method for
Received 8 May 2018
catalyzing the oxygen reduction reaction (ORR) in alkaline electrolytes. PdV/C nano-
Received in revised form
particles with different V contents are spherical with a mean diameter of 3e4 nm, and the
28 August 2018
addition of V expands the lattice parameter of Pd as shown by XRD. XPS analysis indicated
Accepted 24 September 2018
the binding energy of Pd0 3d peak increased by ca. 1 eV. Density functional theory (DFT)
Available online xxx
calculations results shows d-band center of Pd down-shift after V-doping. Based on the electrocatalytic results, introducing V can improve the catalytic activity for ORR, methanol
Keywords:
crossover tolerance and stability. Especially, Pd4V/C shows higher initial potential
Palladium-vanadium/ carbon nano-
(Eonset ¼ 1.027 V), excellent methanol crossover tolerance (98.03% retained) and long-term
catalyst
stability (81.30% retained), which are comparable with Pt/CJM. This work provides a new
Phase transfer method
method of synthesizing PdV/C nanoparticles, which have the potential to be used as the
Oxygen reduction reaction
cathode electrocatalysts for fuel cells.
Alkaline electrolytes
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
d-band center
Introduction Oxygen reduction reaction (ORR) as the key step of energy conversion systems has received considerable attention [1e3]. Due to its slow kinetics, catalyst is critical for ORR [4e6]. Currently platinum (Pt) is extensively used for ORR. But Pt is expensive and has limited supply and long-time stability, which limits the large-scale production of fuel cells [7e10]. It is well known that Pd has similar electronic structure and chemical characteristics with Pt, and Pd shows catalytic activity for ORR in alkaline media [11e15]. In addition, the cost of Pd is only one third of Pt with abundant supply. Therefore, Pd is a probable element to replace Pt as an ORR catalyst.
However, Pd has a lower catalytic activity for ORR than that of Pt, because the d-band center of Pd is higher than Pt [16e18], which results in a stronger oxygen binding energy on Pd. The stronger oxygen binding inhibits the reduction of oxygencontaining intermediates to water or OH [19e21]. To improve the ORR catalytic activity of Pd, other metals or metal oxides are added to tune the electronic properties of Pd [14,22]. This modification would lead to the d-band center shift. Based on the relationship between the reaction activity and dband center reported by Nørskov et al. [23] and some experimental studies, adding heteroatoms (Cu, Co, Ag, Ni etc.) [24e27] could improve the catalytic activities by modification of their surface d-band [28]. For instance, Wang et al. [29] doped Pd/C with B of lower electronegativity, and certain electrons were
* Corresponding author. E-mail address: [email protected] (L. Zhao). https://doi.org/10.1016/j.ijhydene.2018.09.155 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
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transferred from B to Pd, which caused the downshift of d-band center and improved ORR catalytic activity. Shao et al. [30] found that PdeFe alloys have excellent catalytic activity for ORR, which could surpass that of carbon-supported Pt electrocatalysts. Moreover, researchers found that metallic oxide could modify the electronic structure of Pd and increase its catalytic activity. A layer of PdOx would form at the interface of Pd nanoparticles and metallic oxide, which modulates the electronic structure of Pd, decreases oxygen adsorption energy and improves the catalytic activity [31]. Through experiments and theoretical calculations, Ding et al. [32] found that PdOx layer is formed at the interface of Pd and ex-MMT in the Pd/ex-MMT nano-catalysts. In consequence, the Pd 4d shifts toward a lower energy state, leading to a larger gap between d orbital of Pd and p orbital of Oads and weakened bonding strength of Oads. Pd/ex-MMT showed remarkable ORR catalytic activity. The lattice constant of vanadium (3.02 A) is smaller than Pd (3.90 A). In addition, vanadium has smaller electronegativity than that of Pd. Therefore, an alloy of Pd and V could modulate the geometrical and electronic structure of Pd and enhance the ORR catalytic activity. Ang et al. [33] synthesized PdV/C alloy catalyst by wet chemical reduction and found that it has good ORR catalytic activity and stability in acidic electrolytes, while the catalytic activity of PdV/C toward ORR is not better than Pt/C. Meanwhile, the use of PdV/C as ORR catalyst in alkaline electrolytes has not been reported. The Pd-based catalysts are mostly prepared in aqueous solution [34e36]. It is easy to aggregate and leads to poor dispersion of catalystparticles. Although catalysts prepared in organic solvents have good dispersion property, Pd salts are difficult to be dissolved in organic solvents. We have prepared the well-dispersed Pd3Au/C alloy nanocatalyst by the phasetransfer method with PdCl2 and HAuCl4 as precursors. And it showed higher catalytic activity and excellent electrochemical stability toward the ethanol electro-oxidation [37]. This work prepared PdV/C nanoparticles in organic solvent by the phase-transfer method for the first time. The nanoparticles were dispersed uniformly, the addition of V caused the lattice expansion of Pd, and some V existed in the form of oxides. The PdV/C nanoparticles with different contents of V showed high ORR catalytic activity in alkaline electrolytes, high stability and methanol resistance. Especially, Pd4V/C nanoparticles showed the same ORR catalytic activity and stability as the commercially available Pt/CJM 20%.
Physical characterization Transmission electron microscopy (TEM, JEM-2100 F) was used to research the morphology of catalysts, and the size distribution was taken with 200 random selected nanoparticles. X-ray diffraction (XRD, Max-C, Rigaku) with a Cu Ka radiation source was used for crystal structure analysis. X-ray photoelectron spectroscopy (XPS, Lb250 UK) was used for investigating the binding energy of Pd 3d orbit.
Electrochemical measurements Electrochemical performance was experimented by the CHI660E electrochemical workstation (Chenhua Instruments Corp, Shanghai, China) with a glassy carbon rotating disk electrode (GC-RDE, Jiangsu Jiangfen Electroanalytical Instruments Co., Ltd., 3 mm diameter, 0.0707 cm2) decorated by catalysts in a single-compartment three-electrode cell. A platinum-wire electrode was used as the counter electrode and Ag/AgCl electrode (3 mol L1) served as the reference electrode. All potentials were translated into RHE, E(RHE) ¼ E(Ag/AgCl) þ 0.059 pH þ 0.2046 V. 4 mg catalyst was ultrasonic dispersing into 0.98 mL isopropanol and 20 mL 0.5 wt % Nafion aqueous solution (Sigma, USA) to obtain the catalyst ink. 10 mL ink was dropped onto the GC-RDE with catalyst loading was 0.15 mg cm¡2. In order to comparison, the commercial Pt/CJM (20 wt%) was also tested under the same conditions.
Theoretical basis The d-band center calculations were performed on the basis of density functional theory, which implemented in the QUANTUM-ESPRESSO package [38]. In order to describe the exhange-correlation interaction, Spin-polarized generalized gradient approximation of Perdew-Burke-Ernzerhof exchange-correlation functional was applied [39]. Vacuum regions was at least 20 A, which would avoid unphysical interactions between periodic images. All calculations were performed by a plane wave cutoff of 60 Ryto ensure convergence [40]. A quasi-Newton algorithm was used to Geometry optimization. A total energy convergence and residual forces A, respectively. The were 1.4 104 eV and below 0.02 eV/ method of calculating Gibbs free energy and the overpotential with DFT was shown in the supporting materials.
Experiment methods Chemicals and reagents
Results and discussion
Tetrabutylammonium hydroxide (TBAOH, 10% aqueous solution), PdCl2, NH4VO3 and NaBH4 was purchased from Aladdin Reagent (Shanghai). Vulcan XC-72 carbon was stirred for 5 h in the salpeter solution (HNO3: H2O ¼ 1: 1) for pretreatment. All chemicals were analytical reagent without further purification.
Materials characterization
Preparation of electrocatalysts PdnV/C (n ¼ 1, 2, 3, 4, 5) were prepared by the phase-transfer method with 20 wt% metal loading (shown in the supporting materials), just like our previous report [37].
The PdV/C nanoparticles with different contents of V were successfully prepared by the phase-transfer method. The morphology and size distribution of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were examined by TEM (Fig. 1). The nanoparticles were spherical with the average diameter of 3e4 nm and distributed on the carbon support, with little aggregation. The XRD patterns (Fig. 2) of the as-synthesized PdV/C nanoparticles with different contents of V, Pd showed the face-centered cubic (fcc) structure (JCPDF-46-1043)
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features. Compared with Pd/C, the corresponding XRD peaks of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were shifted negatively. The lattice parameters of Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C are listed in Table 1. The Pd lattice constants are 3.925, 3.928, 3.955, 3.950 and 3.962 A in Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C, Compared with Pd/C (3.914 A), the introduction of V modified the lattices of Pd in PdV/C nanoparticles with different contents of V dilate. It demonstrated that the addition of V modulated the geometric structure of Pd [33]. The reasons for lattice expansion maybe the V doped into the interstice of Pd lattice or a part of V existed in the form of VOx.
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The diffraction peaks belonging to Pd (111) were used to calculate the mean crystallite sizes with the Scherrer equation [12,41]. The smaller V in place of Pd should decrease the lattice average crystallite sizes of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/ C and PdV/C, which were 3.17, 2.66, 2.83, 2.43, 2.88 and 2.16 nm, respectively. They are smaller than those obtained from TEM data, because XRD data were obtained from crystals rather than amorphous structures. However, the mean particle size of TEM included crystals and amorphous particles. The lattice fringes at d ¼ 2.6 A might belong to PdOx (PdO (002) or PdO2 (101)) found in HR-TEM image (Fig. 3A), which identified the presence of layers. The fringes were at d ¼ 2.2 A and
Fig. 1 e TEM images of the different composite nanoparticles with particle size distribution: (a) Pd/C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C. Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
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Fig. 2 e XRD patterns of the different composite nanoparticles: (a) Pd/C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C.
1.9 A, which matched the Pd (111) and Pd (200), respectively [32]. The EDS element mapping (Fig. 3B) results showed the existence of V in PdV/C.
Electrocatalytic properties In order to investigate the electrocatalytic performance, the CVs of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were obtained in N2-saturated 0.1 mol L1 KOH solution (Fig. S1). The palladium oxide reduction peak potentials on Pd/C, Pd5V/ C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were 0.6282 V, 0.6862 V, 0.6691 V, 0.6669 V, 0.6799 and 0.6907 V, respectively. The reduction peak potentials of PdV/C nanoparticles with different contents of V were higher than that of the Pd/C. The reduction peak at more positive potential implied that the introduction of V accelerated the hydroxyl adsorption/ desorption on PdV/C catalyst [42], demonstrating an improved electrocatalytic behavior [2]. The ORR catalytic activity of PdV/C in alkaline electrolytes was subsequently investigated and compared with Pd/C and commercially available Pt/CJM catalysts. The ORR catalytic activity of the PdV/C nanoparticles with different contents of V in alkaline electrolytes (0.1 mol L1 KOH) was evaluated by
Table 1 e Crystal lattice parameter of PdV/C electrocatalysts with different component from XRD patterns. Catalysts 111 2q/ d/ A PdePd Distance/ a/ A Size/ deg. Pd/C Pd5V/C Pd4V/C Pd3V/C Pd2V/C PdV/C
39.86 39.74 39.72 39.43 39.48 39.35
nm 2.259 2.266 2.268 2.283 2.281 2.288
2.773 2.775 2.777 2.796 2.793 2.802
nm 3.914 3.925 3.928 3.955 3.950 3.962
3.17 2.66 2.83 2.43 2.88 2.16
Fig. 3 e A HRTEM and EDX images of Pd4V/C, B mapping of PdV/C.
polarization curves (Fig. 4A). The onset potential was confirmed by the potential at the intersection of the tangent line of the largest slope on the curve and the straight line at 0 mA cm2. The initial potentials (Eonset) varied in the following order: Pd4V/C (1.027 V)> Pd3V/C (0.971 V)> Pd5V/C (0.969 V)> Pd2V/C (0.932 V)> Pt/CJM (0.918 V)> PdV/C(0.863 V)> Pd/C (0.851 V). Eonset of Pd4V/C was even higher than Pt/CJM. Meanwhile, the half-wave potentials (E1/2) were in the following order: Pd4V/C (0.947 V) > Pd5V/C (0.902 V)> Pd3V/C (0.890 V)> Pd2V/C (0.844 V)> Pt/CJM (0.839 V)> PdV/C (0.808 V)> Pd/C (0.777 V). In Fig. 4A, a strong limiting diffusion current similar to Pt was observed from 0.8 V to 0.3 V, which represented a diffusion-controlled process of the efficient fourelectron pathway [43]. With the increase of vanadium content, the current density based on mass of Pd at the potential of 0.95 V, 0.90 V and 0.85 V (Fig. 4B) all exhibited a “volcano -
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Fig. 5 e Tafel curve of different composites catalysts: (a) Pd/ C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C and (g) Pt/CJM 20%.
Fig. 4 e A ORR polarization curves of different composites catalysts: in O2-saturated 0.1 mol L¡1 KOH solution. (a) Pd/ C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C, (g) V/C and (h) Pt/CJM 20%. B Current density of different composites catalysts in 0.1 mol L¡1 KOH at 0.85 V, 0.90 V and 0.95 V.
type” shape, and the highest mass specific activity found for the Pd4V/C nanoparticles at different potentials. The Tafel plot (Fig. 5) showed that the slopes of PdV/C catalysts were close to that of Pt/CJM suggesting the PdV/C nanoparticles with different contents of V exhibited the same ORR catalytic mechanism such as reaction path, the ratelimiting step, etc. Meanwhile, the slope of Pd4V/C (71 mV$ dec1) was smaller in comparison with Pt/CJM catalyst (85 mV$ dec1), indicating an enhanced ORR catalytic activity of Pd4V/C nanocomposites. The mechanism of Pd4V/C nanocomposite in catalyzing ORR was evaluated at varying rotation rates of the working electrode (Fig. 6). It shows increasing limiting current density due to a faster O2 diffusion rate with the increase of rotation rate. The slope of the linear curve at different potentials (the inset in Fig. 6) was obtained based on
the Koutecky´eLevich plot [8]. The n value close to 4.0 was determined, indicating that ORR on Pd4V/C nanocomposite underwent a four-electron transfer process with O2 reduced to OH [44]. K-L plots were mostly linear, indicating the firstorder kinetics of molecular oxygen [45]. The as-prepared Pd4V/C indeed exhibited remarkably enhanced catalytic activity for ORR. An ideal ORR catalyst requires remarkable stability and satisfactory resistance to methanol poisoning. The methanol tolerance of PdV/C nanoparticles with different contents of V and Pt/CJM catalyst were studied in 0.1 mol L1 KOH aqueous solution. Chronoamperometric measurement was carried out at 0.7 V and 3 mol L1 methanol was injected into the solution in 350 s. The relative current was equal to the current density
Fig. 6 e ORR polarization curves at different RPMs in O2saturated 0.1 mol L¡1 KOH solution: (a) 800 rpm, (b) 1200 rpm, (c) 1600 rpm, (d) 2000 rpm and (e) 2500 rpm. The inserted figure shows KouteckyeLevich Plots of Pd4V/C at different potentials.
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Fig. 7 e A Chronoamperometric response of different composites catalysts in O2-saturated 0.1 mol L¡1 KOH and 3 mL methanol (3 mol L¡1) is injected at 350 s, B I-t curve of different catalysts in O2-saturated 0.1 mol L¡1 KOH,: (a) Pd/ C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C and (g) Pt/CJM 20%.
at 1000 s divided by the current density value at 350 s, as shown in Fig. 7A. The relative currents (%) of these catalysts were measured and they were in the order of: Pd4V/C (98.03%) > PdV/C (93.15%) > Pd5V/C (91.08%) > Pd3V/C (91.00%) > Pd/C (89.98%) > Pd2V/C (89.75%). The loss of current density was due to methanol oxidation on the catalysts. PdV/C nanoparticles with different contents of V displayed negligible current changes comparing with Pt/CJM catalyst (42.62%). Meanwhile, the introduction of V increased the relative current rate, indicating that the introduction of V prevented methanol poisoning with the addition of methanol. Furthermore, the stability of the catalysts was examined by I-t curve at 0.7 V for 10800 s (Fig. 7B). The long-term stability of these catalysts were measured and they showed the order of: Pd4V/ C (81.30%)> Pd/C (75.75%)> PdV/C (71.42%)> Pd5V/C (70.85%)> Pd2V/C (64.92%)> Pd3V/C (52.96%) > Pt/CJM (35.24%). The PdV/C nanoparticles were much more stable than Pt/CJM catalyst. Obviously, the introduction of V improved the stability and resistance to methanol poisoning of Pd. Pd4V/C showed the best stability and poison tolerance.
To study the effect of electronic structure on ORR activity, XPS was tested and analyzed. Fig. 8A was XPS spectra of Pd/C and Pd4V/C. On the surface of catalyst, the detected binding energies of C 1s, Pd 3d, V 2p and O 1s located at ca. 285, 537, 516 and 533 eV. As shown in Fig. 8A, V only exists in Pd4V/C. Comparing the binding energy peaks of Pd/C and Pd4V/C obtained by XPS (Fig. 8B and C), the Pd0 3d5/2 and Pd0 3d3/2 peaks had shifted about 1 eV to higher binding energy after introduction of V. Meanwhile, the peak belonging to Pd4þ (338.30 eV) was found in the Pd4V/C XPS spectra, further confirming the presence of a PdOx layer. The formation of PdOx was via the d orbit of Pd binding with the p orbit of O (VOx). The higher core-level energy could be due to two aspects reasons: (1) the positive charges on the PdOx layer remained in the photoemission final state and (2) a small amount of electron transfer from V to Pd atoms [46]. The direction of electron transfer is determined by electronegativity (EN), which has EN (Pd) ¼ 2.20 and EN (V) ¼ 1.63. The binding energy shift direction is in line with the valence band shift; therefore, the binding energy of Pd 3d orbit increases [27,47]. It is known that the ORR activity of Pd is a function of both the O and OH binding energy. The high d-band center leads to high oxygen adsorption energy, which has negative effects on the ORR catalytic activity [29]. The Pd 3d binding energy peak shift to high binding energy implies the d-band center shift down, indicating that oxygen adsorption energy is lowered [48], which enhances the ORR catalytic activity of Pd4V/C. In addition, according to the XPS results of V 2P3/2 orbits (Fig. 8D), three peaks were found at 514.94 eV, 516.49 eV and 517.85 eV, which belong to vanadium oxides at the oxidation states of V3þ, V4þ and V5þ [49,50], respectively. The atomic contents of different valence states of V were V3þ: V4þ: V5þ ¼ 28.76: 44.25: 26.99. It further demonstrated that V existed in the form of oxide with polyvalence states, and this result is consistent with XRD analysis.
Calculated the d-band center of PdV(111) and VOx/Pd(111) DFT calculations have been conducted to gain insight into the activity improvement arising upon V introduction into Pd system. The d-band center is an important parameter of the distribution of transition metal d orbitals. It determines the strength of transition metal surfaces bonding to O/OH and thus is generally used to rationalize corresponding ORR activity [51,52]. Here, two models, i.e., PdV(111) and VOx/Pd(111), were considered on the assumption that V may either enter Pd lattice or form VOx oxide on surface due to its lower electronegativity. In PdV(111), it was found that V atoms strongly prefer staying in the bulk with the energy difference between a V atom incorporated in the surface and in a subsurface layer being 0.85 eV. Accordingly, PdV(111) with V in the subsurface layer was studied further. Fig. 9 shown the d-band center positions of Pd surface atoms in the two modeled systems along with pure Pt(111) and Pd(111) surfaces. In both PdV(111) and VOx/Pd(111), the d-band center positions of Pd atoms were significantly lower than that in pristine Pd(111) surface. Our calculated results suggested that the electronic structure of surface Pd atoms could be modified by introducing V atoms to resemble that of Pt, rationalizing the experimentally obtained high ORR activity in PdV samples. Meanwhile, according to the
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Fig. 8 e XPS spectra of Pd/C (a), Pd4V/C (b) (A); the deconvoluted Pd 3d region of Pd/C (B); the deconvoluted Pd 3d region for Pd4V/C (C) and the deconvoluted V 2P3/2 region for Pd4V/C (D).
Fig. 9 e Top views (a1)-(a4) and corresponding d-band center positions (b): (a1)Pt(111), (a2) Pd(111), (a3) PdV(111) and (a4)PdVOx(111). Dark grey, dark green, blue and red balls represent Pd, Pt, V and O atoms, respectively. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.)
theory of Norskov et al. [53], the over-potentials during actions would characterize the catalytic activities for ORR. To explore the over-potential and the effect of the electrode potential on the ORR activity, we have calculated the free energy changes of the ORR under various electrode potentials (Fig. S2). Table S1 shows binding energies of oxygenated intermediates (DEO*, DEOH*, and DEOOH* in eV) and ORR overpotentials (hORR in V). As shown in Table S1, the over-potential was in the order of V2O5Pd(111) < VPd(111) < Pt(111) < Pd (111). Due to the smaller
the sORR, the higher the ORR activity was, V2O5Pd(111) had the highest ORR activity.
Conclusions PdV/C nanoparticles with different V contents were prepared by the phase-transfer method for use as ORR catalysts in alkaline electrolytes. XRD analysis indicated that introduction
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of V modulated the geometry, enlarged the lattice constants of Pd and expanded the lattice. XPS results showed that introducing V into Pd would adjust the electronic structure, and DFT calculation results in line with it. PdV/C nanoparticles exhibited high ORR catalytic and the Eonset and E1/2 of PdnV/C (n ¼ 2, 3, 4) were higher than that of Pt/CJM 20%, indicating PdnV/C (n ¼ 2, 3, 4, 5) had higher ORR catalytic activity than Pt/ CJM 20%. Moreover, the methanol tolerance of Pd was improved after introduction of V, and PdnV/C (n ¼ 1, 2, 3, 4, 5) nanoparticles showed higher resistance to methanol than Pt/ CJM 20%. PdnV/C (n ¼ 1, 2, 3, 4, 5) nanoparticles also showed excellent long-term stability. These results indicate that PdV/ C nanoparticles prepared by phase transfer method have the potential to be used as the cathode catalyst for fuel cells.
Acknowledgements We gratefully acknowledge the financially supported by the National Natural Science Foundation of China (21461027).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.09.155.
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Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes Biyao Jin, Yiqi Li, Lianhua Zhao* Department of Chemistry, College of Science, Yanbian University, Yanji 133002, China
article info
abstract
Article history:
Bimetallic PdV/C nanoparticles have been synthesized by phase-transfer method for
Received 8 May 2018
catalyzing the oxygen reduction reaction (ORR) in alkaline electrolytes. PdV/C nano-
Received in revised form
particles with different V contents are spherical with a mean diameter of 3e4 nm, and the
28 August 2018
addition of V expands the lattice parameter of Pd as shown by XRD. XPS analysis indicated
Accepted 24 September 2018
the binding energy of Pd0 3d peak increased by ca. 1 eV. Density functional theory (DFT)
Available online xxx
calculations results shows d-band center of Pd down-shift after V-doping. Based on the electrocatalytic results, introducing V can improve the catalytic activity for ORR, methanol
Keywords:
crossover tolerance and stability. Especially, Pd4V/C shows higher initial potential
Palladium-vanadium/ carbon nano-
(Eonset ¼ 1.027 V), excellent methanol crossover tolerance (98.03% retained) and long-term
catalyst
stability (81.30% retained), which are comparable with Pt/CJM. This work provides a new
Phase transfer method
method of synthesizing PdV/C nanoparticles, which have the potential to be used as the
Oxygen reduction reaction
cathode electrocatalysts for fuel cells.
Alkaline electrolytes
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
d-band center
Introduction Oxygen reduction reaction (ORR) as the key step of energy conversion systems has received considerable attention [1e3]. Due to its slow kinetics, catalyst is critical for ORR [4e6]. Currently platinum (Pt) is extensively used for ORR. But Pt is expensive and has limited supply and long-time stability, which limits the large-scale production of fuel cells [7e10]. It is well known that Pd has similar electronic structure and chemical characteristics with Pt, and Pd shows catalytic activity for ORR in alkaline media [11e15]. In addition, the cost of Pd is only one third of Pt with abundant supply. Therefore, Pd is a probable element to replace Pt as an ORR catalyst.
However, Pd has a lower catalytic activity for ORR than that of Pt, because the d-band center of Pd is higher than Pt [16e18], which results in a stronger oxygen binding energy on Pd. The stronger oxygen binding inhibits the reduction of oxygencontaining intermediates to water or OH [19e21]. To improve the ORR catalytic activity of Pd, other metals or metal oxides are added to tune the electronic properties of Pd [14,22]. This modification would lead to the d-band center shift. Based on the relationship between the reaction activity and dband center reported by Nørskov et al. [23] and some experimental studies, adding heteroatoms (Cu, Co, Ag, Ni etc.) [24e27] could improve the catalytic activities by modification of their surface d-band [28]. For instance, Wang et al. [29] doped Pd/C with B of lower electronegativity, and certain electrons were
* Corresponding author. E-mail address: [email protected] (L. Zhao). https://doi.org/10.1016/j.ijhydene.2018.09.155 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
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transferred from B to Pd, which caused the downshift of d-band center and improved ORR catalytic activity. Shao et al. [30] found that PdeFe alloys have excellent catalytic activity for ORR, which could surpass that of carbon-supported Pt electrocatalysts. Moreover, researchers found that metallic oxide could modify the electronic structure of Pd and increase its catalytic activity. A layer of PdOx would form at the interface of Pd nanoparticles and metallic oxide, which modulates the electronic structure of Pd, decreases oxygen adsorption energy and improves the catalytic activity [31]. Through experiments and theoretical calculations, Ding et al. [32] found that PdOx layer is formed at the interface of Pd and ex-MMT in the Pd/ex-MMT nano-catalysts. In consequence, the Pd 4d shifts toward a lower energy state, leading to a larger gap between d orbital of Pd and p orbital of Oads and weakened bonding strength of Oads. Pd/ex-MMT showed remarkable ORR catalytic activity. The lattice constant of vanadium (3.02 A) is smaller than Pd (3.90 A). In addition, vanadium has smaller electronegativity than that of Pd. Therefore, an alloy of Pd and V could modulate the geometrical and electronic structure of Pd and enhance the ORR catalytic activity. Ang et al. [33] synthesized PdV/C alloy catalyst by wet chemical reduction and found that it has good ORR catalytic activity and stability in acidic electrolytes, while the catalytic activity of PdV/C toward ORR is not better than Pt/C. Meanwhile, the use of PdV/C as ORR catalyst in alkaline electrolytes has not been reported. The Pd-based catalysts are mostly prepared in aqueous solution [34e36]. It is easy to aggregate and leads to poor dispersion of catalystparticles. Although catalysts prepared in organic solvents have good dispersion property, Pd salts are difficult to be dissolved in organic solvents. We have prepared the well-dispersed Pd3Au/C alloy nanocatalyst by the phasetransfer method with PdCl2 and HAuCl4 as precursors. And it showed higher catalytic activity and excellent electrochemical stability toward the ethanol electro-oxidation [37]. This work prepared PdV/C nanoparticles in organic solvent by the phase-transfer method for the first time. The nanoparticles were dispersed uniformly, the addition of V caused the lattice expansion of Pd, and some V existed in the form of oxides. The PdV/C nanoparticles with different contents of V showed high ORR catalytic activity in alkaline electrolytes, high stability and methanol resistance. Especially, Pd4V/C nanoparticles showed the same ORR catalytic activity and stability as the commercially available Pt/CJM 20%.
Physical characterization Transmission electron microscopy (TEM, JEM-2100 F) was used to research the morphology of catalysts, and the size distribution was taken with 200 random selected nanoparticles. X-ray diffraction (XRD, Max-C, Rigaku) with a Cu Ka radiation source was used for crystal structure analysis. X-ray photoelectron spectroscopy (XPS, Lb250 UK) was used for investigating the binding energy of Pd 3d orbit.
Electrochemical measurements Electrochemical performance was experimented by the CHI660E electrochemical workstation (Chenhua Instruments Corp, Shanghai, China) with a glassy carbon rotating disk electrode (GC-RDE, Jiangsu Jiangfen Electroanalytical Instruments Co., Ltd., 3 mm diameter, 0.0707 cm2) decorated by catalysts in a single-compartment three-electrode cell. A platinum-wire electrode was used as the counter electrode and Ag/AgCl electrode (3 mol L1) served as the reference electrode. All potentials were translated into RHE, E(RHE) ¼ E(Ag/AgCl) þ 0.059 pH þ 0.2046 V. 4 mg catalyst was ultrasonic dispersing into 0.98 mL isopropanol and 20 mL 0.5 wt % Nafion aqueous solution (Sigma, USA) to obtain the catalyst ink. 10 mL ink was dropped onto the GC-RDE with catalyst loading was 0.15 mg cm¡2. In order to comparison, the commercial Pt/CJM (20 wt%) was also tested under the same conditions.
Theoretical basis The d-band center calculations were performed on the basis of density functional theory, which implemented in the QUANTUM-ESPRESSO package [38]. In order to describe the exhange-correlation interaction, Spin-polarized generalized gradient approximation of Perdew-Burke-Ernzerhof exchange-correlation functional was applied [39]. Vacuum regions was at least 20 A, which would avoid unphysical interactions between periodic images. All calculations were performed by a plane wave cutoff of 60 Ryto ensure convergence [40]. A quasi-Newton algorithm was used to Geometry optimization. A total energy convergence and residual forces A, respectively. The were 1.4 104 eV and below 0.02 eV/ method of calculating Gibbs free energy and the overpotential with DFT was shown in the supporting materials.
Experiment methods Chemicals and reagents
Results and discussion
Tetrabutylammonium hydroxide (TBAOH, 10% aqueous solution), PdCl2, NH4VO3 and NaBH4 was purchased from Aladdin Reagent (Shanghai). Vulcan XC-72 carbon was stirred for 5 h in the salpeter solution (HNO3: H2O ¼ 1: 1) for pretreatment. All chemicals were analytical reagent without further purification.
Materials characterization
Preparation of electrocatalysts PdnV/C (n ¼ 1, 2, 3, 4, 5) were prepared by the phase-transfer method with 20 wt% metal loading (shown in the supporting materials), just like our previous report [37].
The PdV/C nanoparticles with different contents of V were successfully prepared by the phase-transfer method. The morphology and size distribution of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were examined by TEM (Fig. 1). The nanoparticles were spherical with the average diameter of 3e4 nm and distributed on the carbon support, with little aggregation. The XRD patterns (Fig. 2) of the as-synthesized PdV/C nanoparticles with different contents of V, Pd showed the face-centered cubic (fcc) structure (JCPDF-46-1043)
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features. Compared with Pd/C, the corresponding XRD peaks of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were shifted negatively. The lattice parameters of Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C are listed in Table 1. The Pd lattice constants are 3.925, 3.928, 3.955, 3.950 and 3.962 A in Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C, Compared with Pd/C (3.914 A), the introduction of V modified the lattices of Pd in PdV/C nanoparticles with different contents of V dilate. It demonstrated that the addition of V modulated the geometric structure of Pd [33]. The reasons for lattice expansion maybe the V doped into the interstice of Pd lattice or a part of V existed in the form of VOx.
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The diffraction peaks belonging to Pd (111) were used to calculate the mean crystallite sizes with the Scherrer equation [12,41]. The smaller V in place of Pd should decrease the lattice average crystallite sizes of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/ C and PdV/C, which were 3.17, 2.66, 2.83, 2.43, 2.88 and 2.16 nm, respectively. They are smaller than those obtained from TEM data, because XRD data were obtained from crystals rather than amorphous structures. However, the mean particle size of TEM included crystals and amorphous particles. The lattice fringes at d ¼ 2.6 A might belong to PdOx (PdO (002) or PdO2 (101)) found in HR-TEM image (Fig. 3A), which identified the presence of layers. The fringes were at d ¼ 2.2 A and
Fig. 1 e TEM images of the different composite nanoparticles with particle size distribution: (a) Pd/C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C. Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
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Fig. 2 e XRD patterns of the different composite nanoparticles: (a) Pd/C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C.
1.9 A, which matched the Pd (111) and Pd (200), respectively [32]. The EDS element mapping (Fig. 3B) results showed the existence of V in PdV/C.
Electrocatalytic properties In order to investigate the electrocatalytic performance, the CVs of Pd/C, Pd5V/C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were obtained in N2-saturated 0.1 mol L1 KOH solution (Fig. S1). The palladium oxide reduction peak potentials on Pd/C, Pd5V/ C, Pd4V/C, Pd3V/C, Pd2V/C and PdV/C were 0.6282 V, 0.6862 V, 0.6691 V, 0.6669 V, 0.6799 and 0.6907 V, respectively. The reduction peak potentials of PdV/C nanoparticles with different contents of V were higher than that of the Pd/C. The reduction peak at more positive potential implied that the introduction of V accelerated the hydroxyl adsorption/ desorption on PdV/C catalyst [42], demonstrating an improved electrocatalytic behavior [2]. The ORR catalytic activity of PdV/C in alkaline electrolytes was subsequently investigated and compared with Pd/C and commercially available Pt/CJM catalysts. The ORR catalytic activity of the PdV/C nanoparticles with different contents of V in alkaline electrolytes (0.1 mol L1 KOH) was evaluated by
Table 1 e Crystal lattice parameter of PdV/C electrocatalysts with different component from XRD patterns. Catalysts 111 2q/ d/ A PdePd Distance/ a/ A Size/ deg. Pd/C Pd5V/C Pd4V/C Pd3V/C Pd2V/C PdV/C
39.86 39.74 39.72 39.43 39.48 39.35
nm 2.259 2.266 2.268 2.283 2.281 2.288
2.773 2.775 2.777 2.796 2.793 2.802
nm 3.914 3.925 3.928 3.955 3.950 3.962
3.17 2.66 2.83 2.43 2.88 2.16
Fig. 3 e A HRTEM and EDX images of Pd4V/C, B mapping of PdV/C.
polarization curves (Fig. 4A). The onset potential was confirmed by the potential at the intersection of the tangent line of the largest slope on the curve and the straight line at 0 mA cm2. The initial potentials (Eonset) varied in the following order: Pd4V/C (1.027 V)> Pd3V/C (0.971 V)> Pd5V/C (0.969 V)> Pd2V/C (0.932 V)> Pt/CJM (0.918 V)> PdV/C(0.863 V)> Pd/C (0.851 V). Eonset of Pd4V/C was even higher than Pt/CJM. Meanwhile, the half-wave potentials (E1/2) were in the following order: Pd4V/C (0.947 V) > Pd5V/C (0.902 V)> Pd3V/C (0.890 V)> Pd2V/C (0.844 V)> Pt/CJM (0.839 V)> PdV/C (0.808 V)> Pd/C (0.777 V). In Fig. 4A, a strong limiting diffusion current similar to Pt was observed from 0.8 V to 0.3 V, which represented a diffusion-controlled process of the efficient fourelectron pathway [43]. With the increase of vanadium content, the current density based on mass of Pd at the potential of 0.95 V, 0.90 V and 0.85 V (Fig. 4B) all exhibited a “volcano -
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Fig. 5 e Tafel curve of different composites catalysts: (a) Pd/ C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C and (g) Pt/CJM 20%.
Fig. 4 e A ORR polarization curves of different composites catalysts: in O2-saturated 0.1 mol L¡1 KOH solution. (a) Pd/ C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C, (g) V/C and (h) Pt/CJM 20%. B Current density of different composites catalysts in 0.1 mol L¡1 KOH at 0.85 V, 0.90 V and 0.95 V.
type” shape, and the highest mass specific activity found for the Pd4V/C nanoparticles at different potentials. The Tafel plot (Fig. 5) showed that the slopes of PdV/C catalysts were close to that of Pt/CJM suggesting the PdV/C nanoparticles with different contents of V exhibited the same ORR catalytic mechanism such as reaction path, the ratelimiting step, etc. Meanwhile, the slope of Pd4V/C (71 mV$ dec1) was smaller in comparison with Pt/CJM catalyst (85 mV$ dec1), indicating an enhanced ORR catalytic activity of Pd4V/C nanocomposites. The mechanism of Pd4V/C nanocomposite in catalyzing ORR was evaluated at varying rotation rates of the working electrode (Fig. 6). It shows increasing limiting current density due to a faster O2 diffusion rate with the increase of rotation rate. The slope of the linear curve at different potentials (the inset in Fig. 6) was obtained based on
the Koutecky´eLevich plot [8]. The n value close to 4.0 was determined, indicating that ORR on Pd4V/C nanocomposite underwent a four-electron transfer process with O2 reduced to OH [44]. K-L plots were mostly linear, indicating the firstorder kinetics of molecular oxygen [45]. The as-prepared Pd4V/C indeed exhibited remarkably enhanced catalytic activity for ORR. An ideal ORR catalyst requires remarkable stability and satisfactory resistance to methanol poisoning. The methanol tolerance of PdV/C nanoparticles with different contents of V and Pt/CJM catalyst were studied in 0.1 mol L1 KOH aqueous solution. Chronoamperometric measurement was carried out at 0.7 V and 3 mol L1 methanol was injected into the solution in 350 s. The relative current was equal to the current density
Fig. 6 e ORR polarization curves at different RPMs in O2saturated 0.1 mol L¡1 KOH solution: (a) 800 rpm, (b) 1200 rpm, (c) 1600 rpm, (d) 2000 rpm and (e) 2500 rpm. The inserted figure shows KouteckyeLevich Plots of Pd4V/C at different potentials.
Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
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Fig. 7 e A Chronoamperometric response of different composites catalysts in O2-saturated 0.1 mol L¡1 KOH and 3 mL methanol (3 mol L¡1) is injected at 350 s, B I-t curve of different catalysts in O2-saturated 0.1 mol L¡1 KOH,: (a) Pd/ C, (b) Pd5V/C, (c) Pd4V/C, (d) Pd3V/C, (e) Pd2V/C, (f) PdV/C and (g) Pt/CJM 20%.
at 1000 s divided by the current density value at 350 s, as shown in Fig. 7A. The relative currents (%) of these catalysts were measured and they were in the order of: Pd4V/C (98.03%) > PdV/C (93.15%) > Pd5V/C (91.08%) > Pd3V/C (91.00%) > Pd/C (89.98%) > Pd2V/C (89.75%). The loss of current density was due to methanol oxidation on the catalysts. PdV/C nanoparticles with different contents of V displayed negligible current changes comparing with Pt/CJM catalyst (42.62%). Meanwhile, the introduction of V increased the relative current rate, indicating that the introduction of V prevented methanol poisoning with the addition of methanol. Furthermore, the stability of the catalysts was examined by I-t curve at 0.7 V for 10800 s (Fig. 7B). The long-term stability of these catalysts were measured and they showed the order of: Pd4V/ C (81.30%)> Pd/C (75.75%)> PdV/C (71.42%)> Pd5V/C (70.85%)> Pd2V/C (64.92%)> Pd3V/C (52.96%) > Pt/CJM (35.24%). The PdV/C nanoparticles were much more stable than Pt/CJM catalyst. Obviously, the introduction of V improved the stability and resistance to methanol poisoning of Pd. Pd4V/C showed the best stability and poison tolerance.
To study the effect of electronic structure on ORR activity, XPS was tested and analyzed. Fig. 8A was XPS spectra of Pd/C and Pd4V/C. On the surface of catalyst, the detected binding energies of C 1s, Pd 3d, V 2p and O 1s located at ca. 285, 537, 516 and 533 eV. As shown in Fig. 8A, V only exists in Pd4V/C. Comparing the binding energy peaks of Pd/C and Pd4V/C obtained by XPS (Fig. 8B and C), the Pd0 3d5/2 and Pd0 3d3/2 peaks had shifted about 1 eV to higher binding energy after introduction of V. Meanwhile, the peak belonging to Pd4þ (338.30 eV) was found in the Pd4V/C XPS spectra, further confirming the presence of a PdOx layer. The formation of PdOx was via the d orbit of Pd binding with the p orbit of O (VOx). The higher core-level energy could be due to two aspects reasons: (1) the positive charges on the PdOx layer remained in the photoemission final state and (2) a small amount of electron transfer from V to Pd atoms [46]. The direction of electron transfer is determined by electronegativity (EN), which has EN (Pd) ¼ 2.20 and EN (V) ¼ 1.63. The binding energy shift direction is in line with the valence band shift; therefore, the binding energy of Pd 3d orbit increases [27,47]. It is known that the ORR activity of Pd is a function of both the O and OH binding energy. The high d-band center leads to high oxygen adsorption energy, which has negative effects on the ORR catalytic activity [29]. The Pd 3d binding energy peak shift to high binding energy implies the d-band center shift down, indicating that oxygen adsorption energy is lowered [48], which enhances the ORR catalytic activity of Pd4V/C. In addition, according to the XPS results of V 2P3/2 orbits (Fig. 8D), three peaks were found at 514.94 eV, 516.49 eV and 517.85 eV, which belong to vanadium oxides at the oxidation states of V3þ, V4þ and V5þ [49,50], respectively. The atomic contents of different valence states of V were V3þ: V4þ: V5þ ¼ 28.76: 44.25: 26.99. It further demonstrated that V existed in the form of oxide with polyvalence states, and this result is consistent with XRD analysis.
Calculated the d-band center of PdV(111) and VOx/Pd(111) DFT calculations have been conducted to gain insight into the activity improvement arising upon V introduction into Pd system. The d-band center is an important parameter of the distribution of transition metal d orbitals. It determines the strength of transition metal surfaces bonding to O/OH and thus is generally used to rationalize corresponding ORR activity [51,52]. Here, two models, i.e., PdV(111) and VOx/Pd(111), were considered on the assumption that V may either enter Pd lattice or form VOx oxide on surface due to its lower electronegativity. In PdV(111), it was found that V atoms strongly prefer staying in the bulk with the energy difference between a V atom incorporated in the surface and in a subsurface layer being 0.85 eV. Accordingly, PdV(111) with V in the subsurface layer was studied further. Fig. 9 shown the d-band center positions of Pd surface atoms in the two modeled systems along with pure Pt(111) and Pd(111) surfaces. In both PdV(111) and VOx/Pd(111), the d-band center positions of Pd atoms were significantly lower than that in pristine Pd(111) surface. Our calculated results suggested that the electronic structure of surface Pd atoms could be modified by introducing V atoms to resemble that of Pt, rationalizing the experimentally obtained high ORR activity in PdV samples. Meanwhile, according to the
Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e9
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Fig. 8 e XPS spectra of Pd/C (a), Pd4V/C (b) (A); the deconvoluted Pd 3d region of Pd/C (B); the deconvoluted Pd 3d region for Pd4V/C (C) and the deconvoluted V 2P3/2 region for Pd4V/C (D).
Fig. 9 e Top views (a1)-(a4) and corresponding d-band center positions (b): (a1)Pt(111), (a2) Pd(111), (a3) PdV(111) and (a4)PdVOx(111). Dark grey, dark green, blue and red balls represent Pd, Pt, V and O atoms, respectively. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.)
theory of Norskov et al. [53], the over-potentials during actions would characterize the catalytic activities for ORR. To explore the over-potential and the effect of the electrode potential on the ORR activity, we have calculated the free energy changes of the ORR under various electrode potentials (Fig. S2). Table S1 shows binding energies of oxygenated intermediates (DEO*, DEOH*, and DEOOH* in eV) and ORR overpotentials (hORR in V). As shown in Table S1, the over-potential was in the order of V2O5Pd(111) < VPd(111) < Pt(111) < Pd (111). Due to the smaller
the sORR, the higher the ORR activity was, V2O5Pd(111) had the highest ORR activity.
Conclusions PdV/C nanoparticles with different V contents were prepared by the phase-transfer method for use as ORR catalysts in alkaline electrolytes. XRD analysis indicated that introduction
Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155
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of V modulated the geometry, enlarged the lattice constants of Pd and expanded the lattice. XPS results showed that introducing V into Pd would adjust the electronic structure, and DFT calculation results in line with it. PdV/C nanoparticles exhibited high ORR catalytic and the Eonset and E1/2 of PdnV/C (n ¼ 2, 3, 4) were higher than that of Pt/CJM 20%, indicating PdnV/C (n ¼ 2, 3, 4, 5) had higher ORR catalytic activity than Pt/ CJM 20%. Moreover, the methanol tolerance of Pd was improved after introduction of V, and PdnV/C (n ¼ 1, 2, 3, 4, 5) nanoparticles showed higher resistance to methanol than Pt/ CJM 20%. PdnV/C (n ¼ 1, 2, 3, 4, 5) nanoparticles also showed excellent long-term stability. These results indicate that PdV/ C nanoparticles prepared by phase transfer method have the potential to be used as the cathode catalyst for fuel cells.
Acknowledgements We gratefully acknowledge the financially supported by the National Natural Science Foundation of China (21461027).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.09.155.
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Please cite this article in press as: Jin B, et al., Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.155