Hollow Echinus-like PdCuCo Alloy for Superior Efficient Catalysis of

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Hollow Echinus-like PdCuCo Alloy for Superior Efficient Catalysis of Ethanol Yalan Shu, Xiaoqin Shi, Yuanyuan Ji, Ying Wen, Xiaoyu Guo, Ye Ying, Yiping Wu, and Hai-Feng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17731 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces 1

Hollow Echinus-like PdCuCo Alloy for Superior Efficient Catalysis of Ethanol

Yalan Shu, Xiaoqin Shi, Yuanyuan Ji, Ying Wen*, Xiaoyu Guo, Ye Ying, Yiping Wu, and Haifeng Yang*

The Education Ministry Key Lab of Resource Chemistry, Department of Chemistry, Shanghai Normal University, Shanghai, 200234, P. R. China. E-mail: [email protected] (Haifeng. Yang); [email protected] (Ying Wen)

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ABSTRACT Large-scale preparation of hollow echinus-like PdCuCo alloy nanostructures with high surface area-to-volume ratio, rich active sites and relatively efficient catalytic activity have attracted considerable research interest. Herein, we present an economic and facile approach to synthesize hollow echinus-like PdCuCo alloy nanostructures (HENSs) by galvanic exchange reactions using Co nanospheres as sacrificial templates. Moreover, the catalytic activity could be adjusted via changing the composition of catalyst. The composition, morphology, and crystal structure of as-obtained nanomaterials are characterized by various techniques, such as inductively coupled plasma atomic emission spectrometry, transmission electron microscope and X-ray diffraction. Electrochemical catalytic measurement results prove that the catalyst of Pd75Cu8Co3 obtained by optimal preparation condition exhibits 10-fold higher activity for ethanol oxidation in comparison with the commercially available 20% Pd/C catalyst. The eminent performance of Pd75Cu8Co3 electrochemical

catalysis

could

be

ascribed

to

the

peculiar

echinus-like

nanostructures. KEYWORDS: sacrificial templates, hollow, echinus-like nanostructures, ethanol, higher activity


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1. INTRODUCTION Direct ethanol fuel cells (DEFCs) have been regarded as an ideal choice for the development of new-generation mobile power, owing to their environmental friendliness, high efficient, near ambient temperature operation, low emission, as well as abundant fuel sources.1-5 Pt has been deemed as the most effective catalyst for oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR), but it confronts the problem of prohibitive costs, limited reserves and electrochemical instability, which severely hampers the commercialization of DEFCs.6-8 Therefore, it calls for developing low cost non-Pt catalysts with promising catalytic activity and durability. Very recently, Pd and Pd-based nanomaterials have been explored because of their relatively good catalytic properties and lower cost as compared with Pt-based materials low cost, high electrocatalytic activity, and good resistance to CO poisoning.9-11 However, the activity and stability Pd-based nano-catalyst still need to be explored for further improvement for commercial applications. Therefore, design of Pd/non-noble-metal catalysts, such as PdFe,12-14PdCu,15,16 PdCo,17,18 and PdNi19,20 has an efficient strategy to diminish cost and simultaneously promote catalytic activity. In contrast to unitary or binary catalysts, in literature,21-24 Pd-based ternary catalyst exhibits enhancement of catalytic activity, due to adjusting the electronic structure and rearranging the surface atoms. For instance, Guo et al. fabricated a novel Core/Shell



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Ag (Au)/CuPd nanomaterials with exceptional activity for the oxygen reduction in 0.1 M KOH solution.25 Zhang and coworkers reported the synthesis of superior CuFePd electrocatalyst by one-pot reduction.6 On the other aspect, hollow metal nanostuctures have attracted great deal of attention due to the increased surface area and cost savings of material.

26-29

For

example, Wang and co-workers employed direct galvanic displacement to synthesize ternary PdRuCo hollow nanocrystals, which showed eminent activity and stability for polyhydric alcohol oxidation reactions (PAOR) in alkaline medium.30 Additionally, the morphology of catalysts not only greatly influences their catalytic activity but also their stability.31,32 Herein, the hollow echinus-like structure firstly possess a large surface area, low mass-density, and high surface permeability.33,34 Secondly, such hollow echinus-like structures not only offer more active sites owing to ultrahigh density of the branch-like tips, but also improve the mass transfer. Notably, the engineering to produce Pd-based ternary alloys and control their well-defined shapes is highly challenging. Based on the above considerations, in this work, we synthesized PdCuCo catalyst with hollow echinus-like alloy nanostructures (HENSs) by the galvanic replacement in mass production way and cobalt nanoparticles was used as the sacrificial templates. Compared with commercial 20% Pd/C, the PdCuCo HENSs catalyst exhibited the enhanced specific activity and durability. 2. RESULTS AND DISCUSSION 2.1 Morphology Characterization


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A schematic illustration of the proposed growth mechanism of the PdCuCo HENSs is presented in Scheme 1. In detail, the standard reduction potential of Co(II)/Co (-0.28 V vs standard hydrogen electrode (SHE) is lower than both of the PdCl42-/Pd (0.59 V vs SHE) and Cu(II)/Cu (0.34 V vs SHE). While dihydrogen tetrachloropalladate(II) solution was added, the galvanic replacement occurred immediately (Co + PdCl42- → Co2+ + Pd + 4Cl-, Co+Cu2+→ Cu + Co2+ ) and Co nanoparticles served as the template, which were gradually consumed by Pd and Cu ions, thereby forming hollow echinus-like alloy nanostructures of PdCuCo HENSs. As shown in Figure S1, the energy dispersive X-ray spectrometer (EDS) spectrum also confirms that the as-synthesized nanoparticles are made of Pd, Cu and Co. The precise compositional characterization was carried out with an inductively coupled plasma atomic emission spectrometry (ICP-AES), and the determined element ratio for the PdCuCo HENSs with the best catalytic performance should be Pd75Cu8Co3. As seen in Figure 1A, a scanning electron microscopy (SEM) image of Pd75Cu8Co3 HENSs made by optimal Co-nanopaticles-template-based galvanic replacement in massive way was taken, which also could be referred to the SEM images with different scales given in Figure S2. Transmission electron microscopy (TEM) was employed to characterize the detailed structure of Pd75Cu8Co3 HENSs. In Figure 1B, the resultant HENSs are mostly well dispersed and a bright center surrounded by a darker edge indicates the hollow-shaped nanoparticles. The magnified image given in Figure 1C shows a hollow Pd75Cu8Co3 nanospheres with the branch-like tips. The prepared Pd75Cu8Co3 HENSs with a rough surface show relatively uniform with average size ca. 90 nm.



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High resolution TEM (HRTEM) image (Figure 1D) exhibits the ordered and continuous lattice fringes with the lattice spacing, calculated to be ~0.228 nm, which corresponds to the (111) plane of Pd face centered cubic structure (FCC). The selected-area electron diffraction (SEAD) pattern (inset of Figure 1D) hints the high crystallinity of the Pd75Cu8Co3 HENSs. By close examination with high-angle annular dark field-scanning transmission electron microscope (HAADF-STEM), elemental mapping results (Figure 1E–I) prove uniform distributions of Pd, Cu, and Co elements throughout the entire nanostructures. Figure S3 displays the SEM images of two HENSs with different elemental compositions of Pd94Cu3Co2 and Pd52Cu5Co3 made by adding the amounts of precursors (H2PdCl4 against CuCl2) as control experiments. Evidently, similar echinus-like structures could be obtained by the molar ratio of 2:1, 1:2 and 1:0 (Figure S3), but some nanospheres form solid PdCuCo alloy particles without tips. Obviously, Pd/Cu ratio pays a key role in the formation of well-shaped PdCuCo HENSs. The crystalline structure of the as-synthesized Pd75Cu8Co3 HENSs was characterized by X-ray diffraction (XRD) as shown in Figure 2. The five obvious peaks can be assigned to crystallographic planes of (111), (200), (220), (311), and (222), respectively, meaning the face-centered-cubic structure of Pd. No peaks corresponding to the metal oxides in XRD results. Compared with the peak of pure Pd (JCPDS No.46-1043), the diffraction peaks of Pd75Cu8Co3 HENSs have slightly shifted to higher 2θ direction. For instance, in inset of Figure 2, the enlarged (111) band has a clear shift in peak position, indicating that the lattice parameter happens to


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be shrunk because Pd atoms were replaced by smaller atoms of Cu or Co. The valence state and surface composition of Pd, Cu, and Co in the Pd75Cu8Co3 HENSs were determined by X-ray photoelectron spectroscopy (XPS). Figure 3A presents the XPS for Pd 3d. The principle peaks located at 335.6 and 340.9 eV, corresponding to Pd3d5/2 and Pd3d3/2 of metallic Pd (0), indicates that metallic Pd (0) are the predominate species. The smaller diffraction peaks observed at 336.9 and 342.4 eV are originated from PdO in the surface of the sample.23 The Cu 2p XPS spectrum for the as-prepared Pd75Cu8Co3 catalyst is given in Figure 3B. The Cu 2p peaks can be fitted to Cu (0) and Cu (II) binding energies. The primary peak and its sub-peak locate at 934.6 and 954.7 eV corresponding to Cu 2p3/2 and Cu 2p1/2 of Cu (0). The minor doublet at 942.7and 962.8 eV and the presence of satellite signals indicate the inevitable presence of a tiny amount of CuO species, which are possibly adsorbed on the alloy surfaces, due to the much higher sensitivity of XPS for analysis of surface species.35 As shown in Figure 3C, curve fitting of the Co 2p3/2 signals gives two cobalt species. The Co 2p3/2 XPS peaks at the binding energies of 781.3 and 785.2eV are ascribed to Co(OH)2 and CoOOH, respectively,36 meaning it is the oxidized cobalt species predominantly existing on the surface of PdCuCo sample. As seen in Figure S4, the Pd 3d XPS spectra of recorded from Pd75Cu8Co3, Pd78Co2, and commercial 20% Pd/C catalysts also validate that doping Cu and Co could alter the electron structure of Pd.

As above discussed, XPS results further confirms the

formation of an alloys phase, and to verify the modification of the surface electronic state of Pd by Cu or Co doping.



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2.2 Electrochemical characterization We estimated the electrochemical activities of the as-obtained PdCuCo HENSs made by different usages of Pd, Cu and Co contents, comparing with a commercial 20% Pd/C. Figure 4A displays the cyclic voltammetry (CV) curves for 4 kinds of PdCuCo HENSs and commercial 20% Pd/C in a N2-saturated NaOH solution (1.0 mol. L-1) with a scan rate of 50 mV.s-1. During the negative scan, reduction peaks of Pd oxide species for the Pd75Cu8Co3 HENSs had positive shift about 41 mV, remarking weak binding of the intermediates of oxide species on the surface of Pd75Cu8Co3 HENSs.37 The electrochemically-active surface area (ECSA) of the catalyst is calculated by using the following equation: ECSA = Q / (0.405mPd)

(1)

where mPd is the loading amount of Pd and the ECSA is obtained by integrating the charges (Q) related to the reduction peak of PdO, assuming that 0.405mC.cm-2 is the charge for electroreduction of a PdO monolayer.38 The calculated ECSA results are tabulated in Table 1. The specific ECSA of the Pd75Cu8Co3 HENSs (57.0 m2/gPd) is highest among the synthesized samples and commercial 20% Pd/C catalyst (24.0 m2/gPd). The catalytic activity of the PdCuCo HENSs to oxidation of ethanol was checked with CV method in 1.0 M NaOH electrolyte containing 1.0 M ethanol. Figure 4 B shows the CVs of the PdCuCo HENSs with the commercial 20% Pd/C as the comparison. It may be noted, as comparison with the commercially available 20% Pd/C, all PdCuCo HENSs demonstrate a higher oxidation peak current. In particular,


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while forward scanning, the Pd-mass-normalized peak current (3948 mA.mgPd-1) of Pd75Cu8Co3 HENSs was 10 times greater than that of the commercially available 20% Pd/C (375 mA.mgPd-1). In addition to evaluation of the activity by Pd mass, the catalytic activity was also normalized by total metal mass of Pd, Cu, and Co in the loading of catalyst, given in Figure S5. The electrocatalytic activity of the echinus-like PdCuCo nanostructures is greater than the commercial 20% Pd/C, which could be attributed to co-contribution of their larger ECSA and higher densities of active sites. The onset potential and peak current of each catalysts are summarized in Table 1. Pd75Cu8Co3 HENSs exhibit a more negative onset potential and higher peak current than others. Hence, it is reasonable to conclude that a suitably doping of Cu and Co contents is advantaged for improvement of the catalytic activity. Figure S6A shows the CV curves of Pd75Cu8Co3 HENSs for ethanol oxidation recorded at different scan rates. The catalytic current density of Pd75Cu8Co3 HENSs gradually elevates, and the peak potential has slightly positive movement. As illustrated in Figure S6B, a linear relationship ranges between the square root (v1/2) of the scan rate and the forward peak current value (jp). It indicates the diffusion-controlled process for ethanol oxidation occurred on Pd75Cu8Co3 HENSs. CV curves for ethanol oxidation by using the commercial 20% Pd/C modified electrode recorded via varying scan rates were given in Figure S6C and Figure S6D is the linear relationship plotted by jp and v1/2. The slope value of Pd75Cu8Co3 HENSs is larger than that of commercial 20% Pd/C catalysts, revealing the enhancement of oxidation kinetics of the Pd75Cu8Co3 HENSs. It should be contributed to open



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structures from the branch-like tips, which is another factor to improve the catalytic performance. The durability and stability of catalysts are considered as the most important issues for the commercialization of fuel cells. Thus, the electrocatalytic stabilities of as-proposed catalysts were investigated by chronoamperometric (CA) measurements, which were conducted in a N2-saturated 1.0 M NaOH and 1.0 M C2H5OH solution at room temperature for 10000 s. Figure 5 displays CA curves for ethanol oxidation measured at a fixed potential of -0.25V for all of the observed catalysts. The Pd-mass-normalized current density of the EOR shows rapid decay at the initial stage due to the poisoning of the intermediate species. It is obvious that all of PdCuCo HENSs exhibit slower decay of current density over time in comparison commercial 20% Pd/C, suggesting their higher tolerances to the carbonaceous species, generated during ethanol oxidation. Additionally, the stability of the Pd75Cu8Co3 HENSs was carefully examined by the CVs acquired by 500 cycles in 1.0 M NaOH containing 1.0 M C2H5OH solution as a scan rate was set at 100 mV/s. As shown in Figure 6, with increasing scanning times, at start stage, the peak current density increased. After about 50 cycles, the peak current density by using Pd75Cu8Co3 gradually decayed with the increase of scanning times, which should be due to poisoning of CO-like species as well as some dissolution loss of Pd. In the end, the Pd-mass-normalized peak current density of the Pd75Cu8Co3 could still retain higher level than that of commercial 20% Pd/C after 500 cycles of ethanol oxidation. Therefore, the Pd75Cu8Co3 catalyst has eminent cycling stability for ethanol oxidation. Based on the


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current density normalized by total metal mass of Pd, Cu and Co, the stability of the catalysts were also observed as shown in figure S7. The possible mechanism for the improved stability could be tentatively stated as follows: the affinity of Cu with carbon monoxide (CO) is quite lower as comparison with Ru, Rh, Pd, and Pt, which should be beneficial to prolonging long-term stability of the electrocatalyst. Additionally, Co capturing the hydroxyl (OHad) acts as the crucial role in the excellent durability of PdCuCo HENSs because of the efficient oxidation and removal of adsorbed carbon monoxide (COad).38 The CO-stripping experiments were also performed to estimate the anti-poisoning ability of the catalysts. In Figure 7, the onset potential and peak potential of CO oxidation at Pd75Cu8Co3 HENSs negatively shift ~150 and 70 mV, respectively, with respect to those on commercial 20% Pd/C, manifesting that Pd75Cu8Co3 HENSs possess better COads intermediate tolerance capability toward the EOR. Furthermore, the catalytic activity and durability of Pd75Cu8Co3 HENSs in the ethylene glycol oxidation reaction (EGOR) under alkaline conditions were also evaluated. Figure S8A shows the mass activity of the Pd75Cu8Co3 HENSs toward oxidation of ethylene glycol. The mass peak current density of Pd75Cu8Co3 HENSs(5558 mA.mgPd-1) is almost 13-fold higher than that of commercial 20% Pd/C(434mA.mgPd-1), which is also better than 1800 mA.mgPt-1 of the reported PtCuCo hollow nanospheres.39 The chronoampherometric study of as-synthesized Pd75Cu8Co3 HENSs and commercial 20% Pd/C are represented in Figure S8B. CA studies were conducted in 1.0M NaOH solution containing 1.0 M ethylene glycol at-



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0.15V for a period of 10000 s. Surprisingly, even at the end of 10000 s test, the current density of Pd75Cu8Co3 HENSs is still 24-fold larger than that of the commercial 20% Pd/C. The above results unambiguously depict that the Pd75Cu8Co3 HENSs possess superior catalytic performance for E–EG/OR and thus can be expected to be a new type of efficient electro-catalyst material for direct fuel cells. 3. CONCLUSIONS In summary, we demonstrated an economic and facile method to fabricate hollow PdCuCo nanospheres with an echinus-like structure by using Co nanospheres as the sacrificial templates. The alloy of Pd75Cu8Co3 HENSs as an optimal catalyst exhibited 10-fold higher catalytic activity than commercial 20% Pd/C for EOR, which was attributed to the unique echinus-like morphology and the synergetic effect of multimetallic doping. Besides, the stability of the Pd75Cu8Co3 nanocatalyst was superior to commercial 20% Pd/C for ethanol oxidation. This synthesis strategy provided new insight for the large-scale production of high-performance and Pd-based catalysts for fuel-cell applications. ASSOCIATED CONTENT Supporting Information Reagents and methods, TEM-EDS image of Pd75Cu8Co3 HENSs, SEM images at different magnifications of Pd75Cu8Co3 HENSs, SEM images of the prepared PdCuCo HENSs with different compositions, XPS spectrum of Pd 3d regions in PdCuCo HENSs, the current density is normalized by total metal mass (Pd, Cu, and Co), the CV curves of Pd75Cu8Co3 HENSs and commercial 20% Pd/C for ethanol oxidation


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recorded at different scan rates, total-metal-mass normalized chronoamperometry curves of EOR, the electrocatalytic activity and durability of Pd75Cu8Co3 HENSs in the EGOR under alkaline conditions. ACKONWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (No.21475088), International Joint Laboratory on Resource Chemistry (IJLRC), Shanghai Key Laboratory of Rare Earth Functional Materials, and Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors. REFERENCES (1) Hong, W.; Wang, J.; Wang, E. Facile Synthesis of Highly Active PdAu Nanowire Networks as Self-supported Electrocatalyst for Ethanol Electrooxidation. ACS Appl. Mater. Inter. 2014, 6, 9481-9487. (2) Pereira, J. P.; Falcão, D. S.; Oliveira, V. B.; Pinto, A. M. F. R. Performance of a Passive Direct Ethanol Fuel Cell. J. Power Sources 2014, 256, 14-19. (3) Yang, Z. Z.; Liu, L.; Wang, A. J.; Yuan, J.; Feng, J. J.; Xu, Q. Q. Simple Wet-Chemical Strategy for Large-scaled Synthesis of Snowflake-like PdAu Alloy Nanostructures as Effective Electrocatalysts of Ethanol and Ethylene Glycol Oxidation. Int. J. Hydrogen Energ. 2017, 42, 2034-2044. (4) Liu, C.; Cai, X.; Wang, J.; Liu, J.; Riese, A.; Chen, Z.; Sun, X.; Wang, S. D. One-Step Synthesis of AuPd Alloy Nanoparticles on Graphene as a Stable Catalyst for Ethanol Electro-oxidation. Int. J. Hydrogen Energ. 2016, 41, 13476-13484.



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Scheme 1 Synthesis route for the hollow echinus-like PdCuCo alloy nanostructures.



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Figure 1 SEM image (A) TEM image (B, C) and HRTEM image (D) of the as-obtained Pd75Cu8Co3 HENSs. Inset in C shows the corresponding SEAD Pattern. HAADF-STEM and elemental mapping (E-I).


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Figure 2 XRD patterns of Pd75Cu8Co3 HENSs; the inset is the magnified one from Pd (111).



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Figure 3 XPS spectra of (A) Pd 3d, (B) Cu 2p, and (C) Co 2p for Pd75Cu8Co3 HENSs.


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Figure 4 (A) CV curves of various PdCuCo HENSs and commercial 20% Pd/C catalysts in 1.0 M NaOH at the scan rate of 50 mV/s. (B) mass activity; (C) special activity in 1.0 M NaOH and 1.0 M C2H5OH solution at the scan rate of 50 mV/s. (D) Mass activity (black) and special activity (red) of PdCuCo HENSs, PdCo HENSs and commercial 20% Pd/C catalysts.



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Table 1 Electrochemical Parameters of PdCuCo HENSs and commercial 20% Pd/C. ECSA

Onset

Mass activity

Special

(m2 /g)

potential

(mA/mgPd)

Activity

Mass activity 2

(V)

(mA/cm )

(mA/mgmetal)

20% Pd/C

24.0

-0.43

375

1.5

375

Pd78Co2

39.5

-0.50

2075

5.2

2031

Pd75Cu8Co3

57.0

-0.61

3948

7.0

3640

Pd94Cu3Co2

48.5

-0.53

2871

5.9

2803

Pd52Cu5Co3

47.6

-0.54

2933

6.1

2776


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Figure 5 Chronoamperometry curves of different catalysts, recorded at -0.25 V towards EOR; the inset is the magnified one from 8000 to 10000 s.



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Figure 6 The 100th, 200th, 300th, 400th and 500th CVs of Pd75Cu8Co3 catalyst in 1.0 M NaOH and 1.0 M C2H5OH solution at a scan rate of 100 mV/s (A); The long-term durability of Pd75Cu8Co3 HENSs and commercial 20% Pd/C catalysts referring to peak current densities for ethanol oxidation for 500 circles (B).


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Figure 7 CO stripping curves on Pd75Cu8Co3 HENSs and commercial 20% Pd/C catalysts recorded in 1.0 M NaOH solution at the scan rate of 50 mV/s.



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Graphical Abstract


Hollow Echinus-like PdCuCo Alloy for Superior Efficient Catalysis of

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