- Email: [email protected]
In situ preparation and high electrocatalytic activity of binary Pd-Ni nanocatalysts with low Pd-loadings
Electrochimica Acta 182 (2015) 96–103
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In situ preparation and high electrocatalytic activity of binary Pd-Ni nanocatalysts with low Pd-loadings Qingfeng Yi* , Qinghua Chen School of chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 July 2015 Received in revised form 5 September 2015 Accepted 9 September 2015 Available online 11 September 2015
Palladium nanoparticles supported on multi-walled carbon nanotube (MWCNT), with ultra-low Pd loadings (wt%) of 1.1%-1.9%, have been prepared through an in situ reduction of Pd2+ by Ni nanoparticles immobilized on MWCNT. That is, Ni nanoparticles are firstly loaded on MWCNT by chemical reduction method to fabricate the catalyst nanoNi/MWCNT. Then, aqueous PdCl2 solution with various concentrations is added to the nanoNi/MWCNT catalyst stepwise, leading to the formation of Pd nanoparticles and subsequent in situ deposition on the nanoNi/MWCNT. The as-synthesized nanocatalysts (Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT) have been characterized by SEM, XRD and XPS. Their electrocatalytic activity for ethanol oxidation in alkaline media has been investigated. Compared to the Pd/MWCNT catalyst prepared by the conventional NaBH4 reduction method, the asprepared Pd-Ni/MWCNT catalysts present low Pd-loading and significantly high electroactivity for ethanol oxidation. According to the cyclic voltammetric data, the forward-scan anodic peak current density j(Pd) on the Pd3.7Ni1/MWCNT catalyst is 130.9 mA cm2 mg1, which is over 11 times higher than on the Pd/MWCNT. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Pd Ni Ethanol oxidation Fuel cell
1. Introduction Direct liquid fuel cells (DLFCs) use liquid fuels to feed fuel cells and they possess more advantages than H2/O2 fuel cell [1]. Among the DLFCs, direct alcohol fuel cells (DAFCs) use liquid alcohols like methanol and ethanol, as fuels to keep the anodic reactions of the DAFCs. Further, compared to methanol, ethanol is considered to exhibit obvious superiority such as non-toxicity, natural availability and renewability. In addition, a direct ethanol fuel cell (DEFC) possesses the advantages of higher power density and zero greenhouse contribution to the atmosphere. In order to guarantee the normal operation of the DEFC, electro-oxidation of ethanol should be catalyzed on efficient catalysts. Pt and Pt-based materials have been considered to be efficient electro-catalysts for ethanol oxidation both in acidic and in alkaline media [2–4]. However, owing to the reasons known to all like high cost and scare resources of Pt, and the poisoning effect of Pt by some intermediates formed during electro-oxidation of ethanol, practical application of Pt catalysts to fuel cells is severely limited [5,6]. Development of other catalysts with high electroactivity for ethanol oxidation, therefore, is of very significance. Among them,
* Corresponding author. E-mail address: [email protected] (Q. Yi). http://dx.doi.org/10.1016/j.electacta.2015.09.053 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Pd and Pd-based catalysts are alternatives to Pt catalysts for ethanol oxidation in alkaline media [7–13]. Ethanol electrooxidation in an alkaline medium on a Pd catalyst takes place via the mechanism of the removal of the adsorbed ethoxi by the adsorbed hydroxyl on the Pd electrode [8], which is different from the mechanism on a Pt catalyst [6]. Various Pd catalysts have been prepared to investigate their electroactivity for ethanol oxidation in alkaline media. Dispersing Pd into nano-scale sizes particles and alloying of Pd with other metals like Ni [12–16], Ru [11], and Sn [3–5] et al., are common methods used to enhance the electroactivity of the Pd catalysts for ethanol oxidation. Although Pd is relatively low-cost compared with Pt, it still belongs to a noble metal. The large scale practical application of Pd catalysts will be limited due to the higher cost of Pd. Thus, it is significant to reduce the Pd loading on catalysts but also to maintain or improve their electroactivity for ethanol oxidation. Marchionni et al. reported a Pd-based composite catalyst Pd-(NiZn)/C with Pd-loading of 6.4% (wt%), which was prepared through the chemical deposition of Pd nanoparticles on (Ni-Zn)/C solid that was fabricated with the reaction of Zn powder with Ni2+ [17]. This catalyst exhibited excellent electroactivity for electro-oxidation of ethylene glycol (EG) and glycerol (G) in alkaline media. Multi-walled carbon nanotubes (MWCNTs) possess high specific surface areas and are extensively used as excellent supports of catalyst particles. In this work, we firstly synthesized
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
97
Table 1 Contents of Pd and Ni in the samples according to the analyses of EDS and ICP Pd4.1Ni/MWCNT
Pd3.7Ni/MWCNT
Pd1.3Ni/MWCNT
EDS analysis
Atomic ratio (Pd/Ni) Mass ratio (Pd / Ni)
4.14 7.2
3.71 6.8
1.28 2.3
ICP analysis
Mass ratio (Pd / Ni) Pd%(wt%) Ni%(wt%)
5.1 1.1 0.2
4.6 1.2 0.3
1.2 1.9 1.7
MWCNT- supported Ni nanoparticles (nanoNi/MWCNT) by a chemical reduction method. Then, an aqueous PdCl2 solution was mixed with the nanoNi/MWCNT to form Pd nanoparticles, followed by the in situ deposition of the Pd nanoparticles on the nanoNi/MWCNT. The as-obtained Pd-Ni/MWCNT catalysts possess low Pd loading and have been characterized by XRD, SEM and XPS. Their electrocatalytic activity for ethanol oxidation in alkaline media was investigated. 2. Experimental The electrolytes were prepared with NaOH (96%), ethanol (99.9%) and pure water purified using a Nanopure water system (18.2 MV cm). PdCl2 (Pd wt% 60%), Ni(NO3)26H2O (99.9%), N2H4H2O (60%) and MWCNT powder (diameter 1525 nm) were purchased from Sinopharm Group Chemical Reagent Co. Ltd. To synthesize the Pd-Ni/MWCNT catalysts, the nanoNi/MWCNT solid powder was firstly prepared by the following steps. MWCNTs were firstly pre-treated by the conventional oxidation in concentrated H2SO4/HNO3 mixture [12]. 166 mg of NiCl26H2O solid and 200 mg of the pretreated MWCNT particles were mixed with 14 mL ethanol and 17 mL water under stirring. To the mixture, 2 molL1 NaOH solution was added dropwise until the pH of the mixture was close to 14. Then, the mixture was heated to 50 C in the water bath and 20 mL of 10% (wt%) N2H4H2O solution was added under stirring. After that, the mixture was kept at 50 C under stirring for
Pd/MWCNT
11.1
0.5 h. After filtering, the solid was washed with ethanol and water until the filtrate pH was close to be neutral. The solid was transferred into a vacuum drying oven and remained at 40 C for 5 h to obtain the MWCNT-supported Ni nanoparticles (nanoNi/ MWCNT). PdxNiy/MWCNT catalysts were prepared by the reaction of the nanoNi/MWCNT particles with the aqueous PdCl2 solution. The detailed procedures were as follows: 20 mg of the nanoNi/ MWCNT particles were well dispersed in 5 mL H2O by ultrasonication. 2 molL1 NaOH solution was added dropwise to V mL of 5 mmol L1 PdCl2 until the pH was 3 4. This PdCl2 solution was slowly added to the above dispersion of the nanoNi/MWCNT in H2O under stirring at room temperature. The mixture was then stirred for 12 h, followed by filtering and washing with water until a neutral filtrate was obtained. The particles were dried in a vacuum oven at 40 C for 5 h to synthesize the catalyst sample PdxNiy/MWCNT. For different volume V (mL) of PdCl2 solution, the as-synthesized PdxNiy/MWCNT catalyst is Pd4.1Ni1/MWCNT at V = 19.7 mL, Pd3.7Ni1/MWCNT at V = 9.8 mL and Pd1.3Ni1/MWCNT at V = 3.9 mL. For comparison, MWCNT-supported Pd nanocatalyst (Pd/MWCNT) was also fabricated by using the conventional NaBH4 reduction method. That is, 10 mL of 5 mmol L1 PdCl2 was mixed with the pre-treated MWCNTs and the mixture was well dispersed under sonication. 10 mL of 2% NaBH4 solution was added dropwise under stirring. After stirring for 1 h, the mixture was filtered and the solid powder was washed with water until a neutral filtrate was obtained. The obtained particles were the Pd/MWCNT catalyst.
Fig. 1. SEM images of nanoNi/MWCNT (a), Pd4.1Ni1/MWCNT (b), Pd3.7Ni1/MWCNT (c) and Pd1.3Ni1/MWCNT (d) samples.
98
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
X-ray diffractograms (XRD) of the catalysts were obtained in a D/MAX2500X diffractometer (Japan), operating with Cu K a radiation generated at 40 kV and 250 mA (l = 0.15418 nm). SEM images and energy dispersive spectra (EDS) of the catalysts were recorded in a JSM-6380LV scanning electron microscopy. X-ray photoelectron spectroscopic (XPS) data of the prepared catalysts were obtained on an ESCALAB 250Xi spectrometer (VG Scientific Ltd., England) using Al Ka radiation (1486.6 eV). Contents of Pd and Ni in samples were obtained by inductively coupled plasma -atomic emission spectroscopy (ICP-AES-7510, Shimadzu). The Pd: Ni atomic ratios of the Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT samples (4.1:1, 3.7:1 and 1.3:1) were obtained according to the EDS analyses. Corresponding analytical data were listed in Table 1. Electrochemical measurements were carried out on AutoLab PGSTAT30/FRA electrochemical instrument (the Netherlands). A glassy carbon (GC) electrode (3 mm diameter, from LanLiKe, TianJing, China) was used as a substrate for the nano-catalyst particles. The GC electrode was polished with a 0.3 mm alumina suspension to give a mirror surface. Typically, 5 mg of a catalyst sample was dispersed in 0.95 mL of ethanol solution containing 50 mL of Nafion solution (5 wt% in water) by sonication for 40 min to obtain an ink. 12 mL of this ink was then dropped onto the GC electrode and dried at room temperature before the electrochemical measurement. Conventional three-electrode cell was used where the GC electrode modified with a catalyst sample, platinum foil and Hg/HgO (1 mol L1 KOH) electrode were used as working, counter and reference electrodes, respectively. All potentials reported in this paper were against the Hg/HgO. Before experiments, pure nitrogen gas (99.99%) was bubbled through the solution to remove the dissolved oxygen in the solution. And N2 gas was continuously flushed over the surface of the electrolyte during the experiments. 3. Results and discussion Redox couple Pd2+/Pd has the high equilibrium potential and the reduction of the Pd2+ ion in an aqueous solution to Pd nanoparticles by Ni nanoparticles will take place. According to the synthesis of the samples, the formation of Pd nanoparticles can be shown as the following reaction: Pd
2þ
þ NiðnanoNi=MWCNT Þ ! PdðnanoparticlesÞ þ Ni2þ
ð1Þ
The as-formed Pd nanoparticles are in situ deposited on the surface of the unreacted Ni particles. Therefore, the morphological structure of the samples would be related to the nanoNi/MWCNT. It is seen from Fig. 1a that the Ni particle of the nanoNi/MWCNT (the white particle with the diameter of ca. 200 nm) is formed by the aggregation of large amounts of smaller sizes of Ni
Fig. 2. EDS response of Pd3.7Ni1/MWCNT catalyst as a typical sample.
nanoparticles. After the reaction of the nanoNi/MWCNT with Pd2+, corresponding SEM images change with the different reaction conditions. Fig. 1b shows a similar morphological texture of the Pd4.1Ni1/MWCNT to the nanoNi/MWCNT (Fig. 1a) except that a small amount of Pd nanoparticles are deposited on the Ni particle. However, for Pd3.7Ni1/MWCNT (Fig. 1c) and Pd1.3Ni1/MWCNT (Fig. 1d) catalysts, they exhibit obvious different SEM images where the surface of the Ni particle is eroded and Pd nanoparticles with smaller sizes of ca.10-25 nm are deposited. This may be illustrated through their preparation. According to the reaction (1), the aqueous Pd2+ solution was added to the nanoNi/MWCNT dropwise to form Pd nanoparticles and Ni2+. However, if the Pd2+ solution continued to be added, the formed Ni2+ ions on the surface of the Ni particle were transformed to Ni2+ oxides or hydroxides due to the higher pH of the Pd2+ solution. For the preparation of Pd4.1Ni1/MWCNT catalyst, the total Pd2+ solution volume added is 19.7 mL, much greater than those for other catalysts. This leads to the formation of the larger Ni particle as indicated in Fig. 1b. Energy dispersive spectroscopy (EDS) is a powerful technique used to firm the presence of Pd and Ni metals in the catalysts. Fig. 2 displays the EDS response of the Pd3.7Ni1/MWCNT catalyst as a typical sample. It is observed from Fig. 2 that the energy peak at 2.8 keV is attributed to Pd, and the energy peaks at 0.8, 7.4 and 8.2 keV are ascribed to the Ni. EDS analyses also give the mass percentages of Pd and Ni on the surface of the catalysts, as shown in Table 1. Particle sizes of the samples were further determined by TEM technique. Fig. 3 reveals the TEM images of the nanoNi/MWCNT and Pd3.7Ni1/MWCNT as a typical binary Pd-Ni/MWCNT catalyst. The grey and long tubes with the average diameter of 29 nm are MWCNTs while the dark dots are catalyst particles. The particles of the nanoNi/MWCNT catalyst is about 17 nm in average diameter (Fig. 3a), showing that the large aggregates as indicated in Fig. 1a are composed of small particles. On the other hand, the Pd3.7Ni1/ MWCNT catalyst has an average diameter of about 5 nm (Fig. 3b). From the image of Fig. 3b, the Pd-Ni particles are confirmed to be immobilized on the surface of MWCNT.
Fig. 3. TEM images of nanoNi/MWCNT (a) and Pd3.7Ni1/MWCNT (b) samples.
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 4. XRD patterns of nanoNi/MWCNT (a), Pd4Ni1/MWCNT (b), Pd3.7Ni1/MWCNT (c) and Pd1.3Ni1/MWCNT (d) samples.
XRD patterns of the prepared samples including the nanoNi/ MWCNT are shown in Fig. 4. The diffraction peak at 2u value of 25.8 is ascribed to the (0 0 2) crystal plane of MWCNTs. Three diffraction peaks at 2u value of 39.9 , 46.5 0 and 68.1 are indexed to the (111), (2 0 0) and (2 2 0) planes of Pd face-centered cubic (fcc) crystal structure, respectively. The prepared samples Pd4.1Ni1/ MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT present similar Ni characteristic diffraction peaks to the nanoNi/MWCNT at 2u values of 44.4, 51.9 and 77.9 corresponding to the (111), (2 0 0) and (2 2 0) crystal faces, respectively, showing that Ni nanoparticles are facecenter-cubic crystalline [18]. No Pd-Ni alloy phase diffraction peak was observed. In addition, compared to the nanoNi/MWCNT, the three binary Pd-Ni/MWCNT catalysts present a characteristic diffraction peak of NiO at 2u of 81.9 corresponding to the (2 2 2) plane of NiO cubic structure [19]. Presence of Ni(II) oxides on the binary Pd-Ni/MWCNT catalysts is attributed to the formation of Ni (II) species by the reaction of Pd(II) with Ni particles as shown in reaction (1). XPS of Ni(2p) core level regions of the prepared nanoNi/ MWCNT and binary Pd-Ni/MWCNT catalysts are shown in Fig. 5. Ni (2p3/2,1/2) peak at 860.8 eV and two weak Ni(2p3/2,1/2) peaks at 852.3 and 869.9 eV for the nanoNi/MWCNT catalyst are assigned to Ni metal. Ni(2p3/2,1/2) peaks at 856.04 and 873.7 eV can be attributed to the presence of Ni oxides [20]. This is consistent with the XRD analysis. Compared to the nanoNi/MWCNT, intensity of the Ni oxide peaks on the binary Pd-Ni/MWCNT catalysts declines, showing the deposition of Pd nanoparticles on the nanoNi/MWCNT. XPS of Pd(3d) core level regions of the binary Pd-Ni/MWCNT catalysts are shown in Fig. 6. Pd(3d5/2,3/2) peaks at 337.4 and 342.7 eV can be ascribed to the Pd2+ in Pd oxides. Pd(3d5/2,3/2)
Fig. 5. XPS of Ni(2p) core level region in the prepared samples.
99
Fig. 6. XPS of Pd(3d) core level region in Pd4Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT samples.
peaks at 335.3 and 340.4 eV are attributed to the Pd metal. Intensity of Pd oxides peak is larger than that of Pd metal, which could be caused by the partial hydrolysis of PdCl2 in solution with higher pH value. Electrochemical properties of the prepared catalysts in 1 mol L1 NaOH solution were examined by cyclic voltammetry as indicated in Fig. 7. All catalysts reveal characteristic cathodic reduction peaks of Pd and Ni oxides despite different peak potentials on these samples. On Pd4.1Ni1/MWCNT, Pd3.7Ni1/ MWCNT and Pd1.3Ni1/MWCNT catalysts, the peak rPd potential for the reduction of Pd oxides formed during the positive-going scan is 0.286 V, 0.312 V and 0.375 V respectively, and the potential of the cathodic peak rNi, corresponding to the reduction of Ni(III) oxides to Ni(II) species [12], is 0.400 V, 0.342 V and 0.324 V respectively. An unique exception was observed on the Pd1.3Ni1/ MWCNT catalyst where a well-defined cathodic peak at 0.095 V arises, which is attributed to the reduction of Ni(II) species to Ni metal. As for the Pd1.3Ni1/MWCNT, it presents a larger Ni content and the surface of the Ni nanoparticles is not totally covered by the Pd nanoparticles. This leads to the obvious electrode reactions involved in the Ni species on the catalyst surface. For other catalysts Pd4.1Ni1/MWCNT and Pd3.7Ni1/MWCNT, most of the Ni nanoparticles surfaces are covered by Pd nanoparticles. This may hinder the further reduction of Ni(II) species because of the following reasons: after the rNi peak, which is related to the reduction of Ni(OH)3 (or Ni(III) oxide) to Ni(OH)2 (or Ni(II) oxide),
Fig. 7. Cyclic voltammograms of the prepared samples in 1 mol L1 NaOH solution at a scan rate of 50 mV s1.
100
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 8. Cyclic voltammetric responses of the samples in 1 mol L1 NaOH solution in the presence of 0.5 mol L1 ethanol at a scan rate of 50 mV s1.
volume shrinkage of the Ni nanoparticles takes place and subsequently, the formed Ni(II) species are covered by the neighboring Pd nanoparticles. In order to investigate the electrochemical properties of the prepared Pd-Ni/MWCNT catalysts, we use ethanol oxidation reaction in alkaline media to evaluate their electrocatalytic activity. Fig. 8 shows cyclic voltammetric responses of the samples in 1 mol L1 NaOH solution in the presence of 0.5 mol L1 ethanol (C2H5OH). Fig. 8 reveals that electroactivity of the Pd-Ni/MWCNT catalysts changes with the atomic ratios of Pd to Ni. Pd3.7Ni1/ MWCNT shows the highest anodic peak current density during the forward scan among the samples while Pd4.1Ni1/MWCNT shows the lowest anodic peak current density. According to their ICP data, the Pd loading is not much different among the binary Pd-Ni samples. Results indicate that the electroactivity of these Pd-based catalysts may be related to the dispersion of Pd nanoparticles and the “bi-functional effect” of Pd-Ni [14–16]. Further, it is found from Fig. 8 that the anodic current density for forward scan on the Pd3.7Ni1/MWCNT catalyst is 89.4 mA cm2, which is higher than that on the Pd/MWCNT. It is also seen from Table 1 that the Pd loading of the Pd/MWCNT is much larger than the binary Pd-Ni/ MWCNT catalysts, showing that a considerable part of Pd on the Pd/MWCNT catalyst is not used to catalyze the ethanol oxidation. This can be further observed from Fig. 9, where current density is referenced against Pd loading on electrode. The current density for
Fig. 9. Cyclic voltammetric responses of the samples where the current density is referenced against Pd-loading. Others same as Fig. 8.
ethanol oxidation as indicated in Fig. 9 reveals the usage efficiency of Pd particles. Fig. 9 shows that the anodic peak current density (mA cm2 mg1) during the forward scan on the Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT catalysts is 38.8, 130.9 and 61.1, respectively, which are significantly larger than that on the Pd/ MWCNT. In view of the role that the dispersions of Pd nanoparticles play in the electrocatalytic activity for ethanol oxidation, we reckoned in the Pd active surface areas of the samples. According to the coulombs consumed for the reduction of Pd oxides (involved in the peak rPd) on a smooth polycrystalline Pd electrode (CV not shown) and the prepared catalysts (shown in Fig. 7) [10], the Pd active surface areas of the Pd/MWCNT, Pd4.1Ni1/MWCNT, Pd3.7Ni1/ MWCNT and Pd1.3Ni1/MWCNT catalysts loaded on the GC electrode (geometrical area 0.071 cm2) are figured out to be 2.4, 0.4, 2.6 and 1.6 cm2, respectively. The similarity of the Pd active surface area between Pd/MWCNT and Pd3.7Ni1/MWCNT catalysts suggests that the Pd nanoparticles of the Pd3.7Ni1/MWCNT are highly dispersed due to its low Pd loading as shown in Table 1. Cyclic voltammograms of the Pd3.7Ni1/MWCNT catalyst in 1 mol L1 NaOH solution containing various concentrations of ethanol ranging from 0.1 to 2.0 mol L1 are presented in Fig. 10a. The forward-going anodic peak (pa) current density exhibits a rapid increase with the ethanol concentration up to 1 mol L1 while it exhibits a slow increase when the ethanol concentration is greater than 0.7 mol L1, as indicated in Fig. 10b. According to the electrocatalytic oxidation mechanism of alcohols [10,11,21,22],
Fig. 10. Cyclic voltammograms of the ethanol oxidation on the Pd3.7Ni1/MWCNT catalyst at a scan rate of 50 mV s1 in 1 mol L1 NaOH solution containing ethanol solutions of various concentrations (a) and plot of the anodic peak (pa) current density vs ethanol concentration (b).
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 11. Chronoamperometric responses of 0.5 mol L1 C2H5OH in 1 mol L1 NaOH solution on the prepared samples at a potential step of 0.4 V.
ethanol oxidation on Pd-based catalysts in alkaline media is mainly involved in three processes: (1) formation of adsorbed ethoxi Pd(CH3CO)ad, (2) formation of adsorbed hydroxyl ion Pd-OHad, and (3) reaction of the Pd-(CH3CO)ad and Pd-OHad to produce acetate as the main product and release free active catalytic sites. In addition, the process (3) is considered to be the rate-determining step. Thus, numbers of the Pd-(CH3CO)ad play an important role in the electrooxidation rate of ethanol. The coverage of the adsorbed ethoxi species CH3COad on the catalyst surface is close to saturation at higher ethanol concentrations and therefore, the current density will no longer increase, as indicated in Fig. 10b where the anodic peak current density at 1.0 mol L1 ethanol is close to that at 2.0 mol L1 ethanol. It is further found from Fig. 10a that the reduction peak (pc) current density declines with the increase of the ethanol concentration. Considering that the peak pc is caused by the reduction of Ni oxides (Ni hydroxides) formed during the anodic sweep at higher potential, the formation of these Ni oxides is suppressed at higher ethanol concentration. This suggests that the ethanol oxidation is a dominant reaction at higher ethanol concentration during the anodic sweep event at more positive potentials. Electrocatalytic activity of the samples for ethanol oxidation was further investigated by chronoamperometric responses. Fig. 11 shows the j–t curves at a potential of 0.4 V vs Hg/HgO in 1 mol L1 NaOH+0.5 mol L1 C2H5OH solution. It is seen from Fig. 11 that all
Fig. 12. Electrochemical impedance spectra of the prepared catalysts in 1 mol L1 NaOH containing 0.5 mol L1 C2H5OH at 0.3 V. Inset is the zoomed main panel of the Nyquist plots.
101
samples, including Pd/MWC NT, show efficient activity for ethanol electro-oxidation, however, the activity of these Pd-Ni/MWCNT catalysts changes greatly with the changing Pd:Ni ratios. The final current density values after holding the potential at 0.4 V for 600 s on Pd/MWCNT, Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT catalysts are 2.7, 0.5, 8.6 and 3.1 mA cm2, respectively, showing the following electroactivity order: Pd3.7Ni1/MWCNT > Pd1.3Ni1/MWCNT > Pd/MWCNT > Pd4.1Ni1/ MWCNT. Result suggests that an appropriate ratio of Pd to Ni is very important to the electrocatalytic activity of the binary Pd-Ni catalysts. Similarly, if the Pd loading in these catalysts is considered, the final mass current density (mA cm2 mg1) is 0.40 on Pd/MWCNT, 0.78 on Pd4.1Ni1/MWCNT, 12.64 on Pd3.7Ni1/ MWCNT and 2.61 on Pd1.3Ni1/MWCNT, showing that the Pd nanoparticles of the Pd3.7Ni1/MWCNT catalyst are more fully used as the electrocatalyst for ethanol oxidation than other catalysts. Electrochemical impedance spectra (EIS) of the prepared catalysts in 1 mol L1 NaOH+0.5 mol L1 C2H5OH solution were also investigated to further study their electroactivity for ethanol oxidation. Fig. 12 presents the Nyquist diagrams of the catalysts at -0.3 V in 1.0 mol L1 NaOH + 0.5 mol L1 ethanol solution. Among the prepared catalysts, Pd4.1Ni1/MWCNT catalyst presents the largest electrochemical resistance. A semicircle with much lower electrochemical impedances develops on the Pd3.7Ni1/MWCNT, Pd1.3Ni1/MWCNT and Pd/MWCNT catalysts. It is clearly seen from Fig. 12 that the charge transfer resistance of ethanol oxidation follows the order: Pd4.1Ni1/MWCNT >> Pd/MWCNT > Pd1.3Ni1/ MWCNT > Pd3.7Ni1/MWCNT, showing that their electroactivity order is consistent with the analysis of CA data in Fig. 11. For the highly dispersed nanoparticles, their stability is the key problem that must be paid much attention to. Here, we measured the successively cyclic potential scanning of the Pd3.7Ni1/MWCNT catalyst in 1 mol L1 NaOH containing 0.5 mol L1 C2H5OH to evaluate its electrocatalytic stability. Fig. 13 shows 500 consecutive cyclic voltammograms (CVs) of the Pd3.7Ni1/MWCNT catalyst at a potential scan rate of 100 mV s1. The CV profile of each cyclic scanning is similar for each other. It is seen from the comparison of the 1st and 500th cyclic voltammograms that only a small amount of decline in the anodic current density for the forward scan is observed. The anodic peak current density on the 1st and 500th CVs is 104 and 77 mA cm2 respectively. This current density decay is partially caused by the consumption of ethanol due to the
Fig. 13. 500 consecutive sweeps of cyclic voltammograms on the Pd3.7Ni1/MWCNT catalyst in 1 mol L1 NaOH containing 0.5 mol L1 C2H5OH at a potential scan rate of 100 mV s1.
102
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 14. Schematic diagram for the formation of the Pd-Ni/MWCNT catalysts.
prolonged sweep. Result shows the high electroactivity stability of the Pd3.7Ni1/MWCNT catalyst. According to the preparation processes of the catalysts, the formed Pd nanoparticles may be directly deposited on the surface of Ni particles that have been immobilized on the MWCNTs. The high surface energy of Pd and Ni particles would decline due to the interaction between Pd and Ni particles. This favors the stability of these particles. Very low Pd loading and high ethanol oxidation electroactivity of the Pd-Ni/MWCNT catalysts may be related to their special morphological structure. According to reaction (1), the formation of Pd nanoparticles was accompanied by Ni2+ species. The produced Ni2+ ions then leave the surface of the nanoNi/MWCNT and subsequently, the formed Pd particles fill the vacancies left by Ni particles. Possible formation mechanism of the Pd-Ni/MWCNT catalysts can be illustrated with Fig. 14. Ni2+ ions are reduced to Ni particles by N2H4 and the as-formed Ni particles are immobilized on the surface of MWCNTs to obtain the nanoNi/MWCNT. Reaction between Pd2+ and the nanoNi/MWCNT results in deposition of Pd nanoparticles on Ni particles and leads to the formation of a binary Pd-Ni composite that has been extensively reported to exhibit better electrocatalytic activity for alcohol oxidation than Pd [12– 16]. Highly dispersed Pd nanoparticles also enhance their electrochemical activity. Therefore, the excellent electroactivity of the Pd-Ni particles may be ascribed to the following three reasons: (1) small sizes of Pd nanoparticles with high active surfaces, (2) “bi-functional effect” of Pd-Ni [15], and (3) electroactivity of Ni nanoparticles themselves for alcohol oxidation [23]. According to this possible mechanism as indicated in Fig. 14, the assynthesized Pd-Ni/MWCNT catalysts are different from the conventional binary Pd-Ni nanoparticles on their morphological structures because the Pd nanoparticles are distributed on the surface of Ni nanoparticles. The “bi-functional effect” comes from where the Pd nanoparticles and Ni particles are in contact. Considering that the Pd/MWCNT and Pd3.7Ni1/MWCNT have an approximate Pd active surface area (2.4 and 2.6 cm2), the significant difference of their anodic peak current densities as shown in Fig. 9 is caused by the so-called “bi-functional effect”. In other words, combination of Pd and Ni particles, performed by the in situ deposition of Pd nanoparticles on Ni particles, greatly enhances the electrocatalytic activity of the Pd nanocatalyst for ethanol oxidation. Further, the formation mechanism of the samples as indicated in Fig. 14 can be reasonably confirmed by the analyses of EDS and ICP. EDS technique can be used to obtain the semi-quantitative data of the metal contents on the surface of the samples. That is, mass percentages of Pd and Ni, obtained by EDS analyses, only reflect the
surface compositions of the samples. However, ICP data exhibit the overall compositions of the samples. According to Fig. 14 where Pd nanoparticles are deposited on the surface of the Ni particles, Pd mass percentage based on the catalyst surface is considerably larger than that based on the whole catalyst. Therefore, the Pd masses (or Ni masses) obtained by EDS are different from those by ICP. Table 1 shows the results of EDS and ICP analyses for Pd and Ni. It is seen from Table 1 that for the four samples, the mass ratios of Pd to Ni obtained from EDS analyses are much higher than those from ICP analyses. This proves the rationality of the proposed mechanism, that is, Pd nanoparticles are immobilized on the surface of Ni nanoparticles. On the other hand, the high electroactivity of the binary Pd-Ni catalysts for ethanol oxidation may be also related to the presence of NiO [24]. For ethanol oxidation on NiO-supported Pd-based catalysts in alkaline media, the competitive adsorption on the catalysts is favorable for ethanol instead of intermediates produced during ethanol oxidation, and no CO intermediate is formed. Results show that the intermediate poisoning of the present catalysts reduces due to the presence of NiO, leading to their high electrocatalytic activity and stability. 4. Conclusions Novel binary Pd-Ni catalysts with low Pd-loadings and high electroactivity for ethanol oxidation were prepared by an in situ deposition of Pd nanoparticles on Ni particles. Main conclusions are as follows. (1) Ni particles, immobilized on the surface of MWCNTs, are used as the reduction agent of Pd2+, and the as-formed Pd nanoparticles are in situ deposited on the Ni particles to synthesize the MWCNT-supported binary Pd-Ni catalysts (Pd-Ni/MWCNT) with various atomic ratios of Pd:Ni. (2) The obtained Pd-Ni/MWCNT catalysts have low Pd-loadings (1.11.9 wt%) and the Pd3.7Ni1/MWCNT presents the best electrocatalytic activity for ethanol oxidation in alkaline media among the prepared samples. (3) The excellent electroactivity of the Pd3.7Ni1/MWCNT catalyst may be ascribed to both the highly dispersed Pd nanoparticles on Ni particles and the so-called “bi-functional effect” of the binary Pd-Ni catalysts with an appropriate atomic ratio of Pd:Ni. (4) The method proposed in this work provides a new approach to the fabrication of binary or ternary Pd-based (or other noble metals-based) nano-composites with low noble metal-loadings and enhanced electroactivity. Acknowledgement This work was supported by The National Natural Science Foundation of China (21376070 and 20876038), Hunan Provincial Natural Science Foundation of China (14JJ2096), and A Project supported by Scientific Research Fund of Hunan Provincial Education Department (11K023). References [1] S. Song, P. Tsiakaras, Recent progress in direct ethanol proton exchange membrane fuel cells (DE-PEMFCs), Appl. Catal. B: Environ. 63 (2006) 187–193. [2] Q. He, B. Shyam, K. Macounova, P. Krtil, D. Ramaker, S. Mukerjee, Dramatically enhanced cleavage of the C-C bond using an electrocatalytically coupled reaction, J. Am. Chem. Soc. 134 (2012) 8655–8661. [3] J. Ribeiro, D.M. dos Anjos, K.B. Kokoh, C. Coutanceau, J.-M. L’eger, P. Olivi, A.R. de Andrade, G. Tremiliosi-Filho, Carbon-supported ternary PtSnIr catalysts for direct ethanol fuel cell, Electrochim. Acta 52 (2007) 6997–7006. [4] X. Xue, J. Ge, T. Tian, C. Liu, W. Xing, T. Lu, Enhancement of the electrooxidation of ethanol on Pt–Sn–P/C catalysts prepared by chemical deposition process, J. Power Sources 172 (2007) 560–569.
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103 [5] L. Jiang, L. Colmenares, Z. Jusys, G.Q. Sun, R.J. Behm, Ethanol electrooxidation on novel carbon supported Pt/SnOx/C catalysts with varied Pt:Sn ratio, Electrochim. Acta 53 (2007) 377–389. [6] A. Santasalo-Aarnio, Y. Kwon, E. Ahlberg, K. Kontturi, T. Kallio, M.T.M. Koper, Comparison of methanol, ethanol and iso-propanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC, Electrochem. Commun. 13 (2011) 466–469. [7] J. Liu, J. Ye, C. Xu, S.P. Jiang, Y. Tong, Kinetics of ethanol electrooxidation at Pd electrodeposited on Ti, Electrochem. Commun. 9 (2007) 2334–2339. [8] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Mechanism study of the ethanol oxidation reaction on palladium in alkaline media, Electrochim. Acta 54 (2009) 2203–2208. [9] Q.F. Yi, L.Z. Sun, In situ synthesis of palladium nanoparticles on multi-walled carbon nanotubes and their electroactivity for ethanol oxidation, Rare Metals 32 (2013) 586–591. [10] Q.F. Yi, F.J. Niu, L.Z. Sun, Fabrication of novel porous Pd particles and their electroactivity towards ethanol oxidation in alkaline media, Fuel 90 (2011) 2617–2623. [11] Q.F. Yi, F.J. Niu, L.H. Song, X.P. Liu, H.D. Nie, Electrochemical activity of novel titanium-supported porous binary Pd–Ru particles for ethanol oxidation in alkaline media, Electroanal. 23 (2011) 2232–2240. [12] Q.F. Yi, L.Z. Sun, X.P. Liu, H.D. Nie, Palladium–nickel nanoparticles loaded on multi-walled carbon nanotubes modified with b-cyclodextrin for electrooxidation of alcohols, Fuel 111 (2013) 88–95. [13] L.Z. Sun, Q.F. Yi, X.P. Liu, Electrocatalytic oxidation of propanol and butanol isomers on palladium-nickel catalyst, J. Fuel Chem. Tech. 41 (2013) 1274–1280. [14] S.Y. Shen, T.S. Zhao, J.B. Xu, Y.S. Li, Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells, J. Power Sources 195 (2010) 1001–1006.
103
[15] Z. Qi, H. Geng, X. Wang, C. Zhao, H. Ji, C. Zhang, J. Xu, Z. Zhang, Novel nanocrystalline PdNi alloy catalyst for methanol and ethanol electro-oxidation in alkaline media, J. Power Sources 196 (2011) 5823–5828. [16] C. Jin, X. Sun, Z. Chen, R. Dong, Electrocatalytic activity of PdNi/C catalysts for allyl alcohol oxidation in alkaline solution, Mater. Chem. Phys. 135 (2012) 433–437. [17] A. Marchionni, M. Bevilacqua, C. Bianchini, Y.-X. Chen, J. Filippi, P. Fornasiero, A. Lavacchi, H. Miller, L. Wang, F.o Vizza, Electrooxidation of Ethylene Glycol and Glycerol on Pd-(Ni-Zn)/C Anodes in Direct Alcohol Fuel Cells, Chem. Sus. Chem. 6 (2013) 518–528. [18] C.-T. Hsieh, Y.-W. Chou, J.-Y. Lin, Fabrication and electrochemical activity of Niattached carbon nanotube electrodes for hydrogen storage in alkali electrolyte, Int. J. Hydrogen Energy 32 (2007) 3457–3464. [19] Z. Liang, H. Sun, Y. Cui, Catalytic oxidation of cyclohexanol on the Ti/Ni/NiO electrode, Chin. J. Catal. 30 (2009) 254–258. [20] T. Shobha, C.L. Aravinda, P. i Bera, L. Gomathi Devi, S.M. Mayanna, Characterization of Ni–Pd alloy as anode for methanol oxidative fuel cell, Mater. Chem. Phys. 80 (2003) 656–661. [21] S.T. Nguyen, H.M. Law, H.T. Nguyen, N. Kristian, S. Wang, S.H. Chan, X. Wang, Enhancement effect of Ag for Pd/C towards the ethanol electro-oxidation in alkaline media, Appl. Catal. B: Environ. 91 (2009) 507–515. [22] Q.F. Yi, H. Chu, Q.H. Chen, Z. Yang, X.P. Liu, High Performance Pd, PdNi, PdSn and PdSnNi Nanocatalysts Supported on Carbon Nanotubes for Electrooxidation of C2-C4 Alcohols, Electroanal 27 (2015) 388–397. [23] D.K. Liguras, K. Goundani, X.E. Verykios, Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ni catalysts, J. Power Sources 130 (2004) 30–37. [24] F. Hu, C. Chen, Z. Wang, G. Wei, P.K. Shen, Mechanistic study of ethanol oxidation on Pd–NiO/C electrocatalyst, Electrochim. Acta 52 (2006) 1087–1091.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
In situ preparation and high electrocatalytic activity of binary Pd-Ni nanocatalysts with low Pd-loadings Qingfeng Yi* , Qinghua Chen School of chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, Hunan, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 July 2015 Received in revised form 5 September 2015 Accepted 9 September 2015 Available online 11 September 2015
Palladium nanoparticles supported on multi-walled carbon nanotube (MWCNT), with ultra-low Pd loadings (wt%) of 1.1%-1.9%, have been prepared through an in situ reduction of Pd2+ by Ni nanoparticles immobilized on MWCNT. That is, Ni nanoparticles are firstly loaded on MWCNT by chemical reduction method to fabricate the catalyst nanoNi/MWCNT. Then, aqueous PdCl2 solution with various concentrations is added to the nanoNi/MWCNT catalyst stepwise, leading to the formation of Pd nanoparticles and subsequent in situ deposition on the nanoNi/MWCNT. The as-synthesized nanocatalysts (Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT) have been characterized by SEM, XRD and XPS. Their electrocatalytic activity for ethanol oxidation in alkaline media has been investigated. Compared to the Pd/MWCNT catalyst prepared by the conventional NaBH4 reduction method, the asprepared Pd-Ni/MWCNT catalysts present low Pd-loading and significantly high electroactivity for ethanol oxidation. According to the cyclic voltammetric data, the forward-scan anodic peak current density j(Pd) on the Pd3.7Ni1/MWCNT catalyst is 130.9 mA cm2 mg1, which is over 11 times higher than on the Pd/MWCNT. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Pd Ni Ethanol oxidation Fuel cell
1. Introduction Direct liquid fuel cells (DLFCs) use liquid fuels to feed fuel cells and they possess more advantages than H2/O2 fuel cell [1]. Among the DLFCs, direct alcohol fuel cells (DAFCs) use liquid alcohols like methanol and ethanol, as fuels to keep the anodic reactions of the DAFCs. Further, compared to methanol, ethanol is considered to exhibit obvious superiority such as non-toxicity, natural availability and renewability. In addition, a direct ethanol fuel cell (DEFC) possesses the advantages of higher power density and zero greenhouse contribution to the atmosphere. In order to guarantee the normal operation of the DEFC, electro-oxidation of ethanol should be catalyzed on efficient catalysts. Pt and Pt-based materials have been considered to be efficient electro-catalysts for ethanol oxidation both in acidic and in alkaline media [2–4]. However, owing to the reasons known to all like high cost and scare resources of Pt, and the poisoning effect of Pt by some intermediates formed during electro-oxidation of ethanol, practical application of Pt catalysts to fuel cells is severely limited [5,6]. Development of other catalysts with high electroactivity for ethanol oxidation, therefore, is of very significance. Among them,
* Corresponding author. E-mail address: [email protected] (Q. Yi). http://dx.doi.org/10.1016/j.electacta.2015.09.053 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Pd and Pd-based catalysts are alternatives to Pt catalysts for ethanol oxidation in alkaline media [7–13]. Ethanol electrooxidation in an alkaline medium on a Pd catalyst takes place via the mechanism of the removal of the adsorbed ethoxi by the adsorbed hydroxyl on the Pd electrode [8], which is different from the mechanism on a Pt catalyst [6]. Various Pd catalysts have been prepared to investigate their electroactivity for ethanol oxidation in alkaline media. Dispersing Pd into nano-scale sizes particles and alloying of Pd with other metals like Ni [12–16], Ru [11], and Sn [3–5] et al., are common methods used to enhance the electroactivity of the Pd catalysts for ethanol oxidation. Although Pd is relatively low-cost compared with Pt, it still belongs to a noble metal. The large scale practical application of Pd catalysts will be limited due to the higher cost of Pd. Thus, it is significant to reduce the Pd loading on catalysts but also to maintain or improve their electroactivity for ethanol oxidation. Marchionni et al. reported a Pd-based composite catalyst Pd-(NiZn)/C with Pd-loading of 6.4% (wt%), which was prepared through the chemical deposition of Pd nanoparticles on (Ni-Zn)/C solid that was fabricated with the reaction of Zn powder with Ni2+ [17]. This catalyst exhibited excellent electroactivity for electro-oxidation of ethylene glycol (EG) and glycerol (G) in alkaline media. Multi-walled carbon nanotubes (MWCNTs) possess high specific surface areas and are extensively used as excellent supports of catalyst particles. In this work, we firstly synthesized
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
97
Table 1 Contents of Pd and Ni in the samples according to the analyses of EDS and ICP Pd4.1Ni/MWCNT
Pd3.7Ni/MWCNT
Pd1.3Ni/MWCNT
EDS analysis
Atomic ratio (Pd/Ni) Mass ratio (Pd / Ni)
4.14 7.2
3.71 6.8
1.28 2.3
ICP analysis
Mass ratio (Pd / Ni) Pd%(wt%) Ni%(wt%)
5.1 1.1 0.2
4.6 1.2 0.3
1.2 1.9 1.7
MWCNT- supported Ni nanoparticles (nanoNi/MWCNT) by a chemical reduction method. Then, an aqueous PdCl2 solution was mixed with the nanoNi/MWCNT to form Pd nanoparticles, followed by the in situ deposition of the Pd nanoparticles on the nanoNi/MWCNT. The as-obtained Pd-Ni/MWCNT catalysts possess low Pd loading and have been characterized by XRD, SEM and XPS. Their electrocatalytic activity for ethanol oxidation in alkaline media was investigated. 2. Experimental The electrolytes were prepared with NaOH (96%), ethanol (99.9%) and pure water purified using a Nanopure water system (18.2 MV cm). PdCl2 (Pd wt% 60%), Ni(NO3)26H2O (99.9%), N2H4H2O (60%) and MWCNT powder (diameter 1525 nm) were purchased from Sinopharm Group Chemical Reagent Co. Ltd. To synthesize the Pd-Ni/MWCNT catalysts, the nanoNi/MWCNT solid powder was firstly prepared by the following steps. MWCNTs were firstly pre-treated by the conventional oxidation in concentrated H2SO4/HNO3 mixture [12]. 166 mg of NiCl26H2O solid and 200 mg of the pretreated MWCNT particles were mixed with 14 mL ethanol and 17 mL water under stirring. To the mixture, 2 molL1 NaOH solution was added dropwise until the pH of the mixture was close to 14. Then, the mixture was heated to 50 C in the water bath and 20 mL of 10% (wt%) N2H4H2O solution was added under stirring. After that, the mixture was kept at 50 C under stirring for
Pd/MWCNT
11.1
0.5 h. After filtering, the solid was washed with ethanol and water until the filtrate pH was close to be neutral. The solid was transferred into a vacuum drying oven and remained at 40 C for 5 h to obtain the MWCNT-supported Ni nanoparticles (nanoNi/ MWCNT). PdxNiy/MWCNT catalysts were prepared by the reaction of the nanoNi/MWCNT particles with the aqueous PdCl2 solution. The detailed procedures were as follows: 20 mg of the nanoNi/ MWCNT particles were well dispersed in 5 mL H2O by ultrasonication. 2 molL1 NaOH solution was added dropwise to V mL of 5 mmol L1 PdCl2 until the pH was 3 4. This PdCl2 solution was slowly added to the above dispersion of the nanoNi/MWCNT in H2O under stirring at room temperature. The mixture was then stirred for 12 h, followed by filtering and washing with water until a neutral filtrate was obtained. The particles were dried in a vacuum oven at 40 C for 5 h to synthesize the catalyst sample PdxNiy/MWCNT. For different volume V (mL) of PdCl2 solution, the as-synthesized PdxNiy/MWCNT catalyst is Pd4.1Ni1/MWCNT at V = 19.7 mL, Pd3.7Ni1/MWCNT at V = 9.8 mL and Pd1.3Ni1/MWCNT at V = 3.9 mL. For comparison, MWCNT-supported Pd nanocatalyst (Pd/MWCNT) was also fabricated by using the conventional NaBH4 reduction method. That is, 10 mL of 5 mmol L1 PdCl2 was mixed with the pre-treated MWCNTs and the mixture was well dispersed under sonication. 10 mL of 2% NaBH4 solution was added dropwise under stirring. After stirring for 1 h, the mixture was filtered and the solid powder was washed with water until a neutral filtrate was obtained. The obtained particles were the Pd/MWCNT catalyst.
Fig. 1. SEM images of nanoNi/MWCNT (a), Pd4.1Ni1/MWCNT (b), Pd3.7Ni1/MWCNT (c) and Pd1.3Ni1/MWCNT (d) samples.
98
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
X-ray diffractograms (XRD) of the catalysts were obtained in a D/MAX2500X diffractometer (Japan), operating with Cu K a radiation generated at 40 kV and 250 mA (l = 0.15418 nm). SEM images and energy dispersive spectra (EDS) of the catalysts were recorded in a JSM-6380LV scanning electron microscopy. X-ray photoelectron spectroscopic (XPS) data of the prepared catalysts were obtained on an ESCALAB 250Xi spectrometer (VG Scientific Ltd., England) using Al Ka radiation (1486.6 eV). Contents of Pd and Ni in samples were obtained by inductively coupled plasma -atomic emission spectroscopy (ICP-AES-7510, Shimadzu). The Pd: Ni atomic ratios of the Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT samples (4.1:1, 3.7:1 and 1.3:1) were obtained according to the EDS analyses. Corresponding analytical data were listed in Table 1. Electrochemical measurements were carried out on AutoLab PGSTAT30/FRA electrochemical instrument (the Netherlands). A glassy carbon (GC) electrode (3 mm diameter, from LanLiKe, TianJing, China) was used as a substrate for the nano-catalyst particles. The GC electrode was polished with a 0.3 mm alumina suspension to give a mirror surface. Typically, 5 mg of a catalyst sample was dispersed in 0.95 mL of ethanol solution containing 50 mL of Nafion solution (5 wt% in water) by sonication for 40 min to obtain an ink. 12 mL of this ink was then dropped onto the GC electrode and dried at room temperature before the electrochemical measurement. Conventional three-electrode cell was used where the GC electrode modified with a catalyst sample, platinum foil and Hg/HgO (1 mol L1 KOH) electrode were used as working, counter and reference electrodes, respectively. All potentials reported in this paper were against the Hg/HgO. Before experiments, pure nitrogen gas (99.99%) was bubbled through the solution to remove the dissolved oxygen in the solution. And N2 gas was continuously flushed over the surface of the electrolyte during the experiments. 3. Results and discussion Redox couple Pd2+/Pd has the high equilibrium potential and the reduction of the Pd2+ ion in an aqueous solution to Pd nanoparticles by Ni nanoparticles will take place. According to the synthesis of the samples, the formation of Pd nanoparticles can be shown as the following reaction: Pd
2þ
þ NiðnanoNi=MWCNT Þ ! PdðnanoparticlesÞ þ Ni2þ
ð1Þ
The as-formed Pd nanoparticles are in situ deposited on the surface of the unreacted Ni particles. Therefore, the morphological structure of the samples would be related to the nanoNi/MWCNT. It is seen from Fig. 1a that the Ni particle of the nanoNi/MWCNT (the white particle with the diameter of ca. 200 nm) is formed by the aggregation of large amounts of smaller sizes of Ni
Fig. 2. EDS response of Pd3.7Ni1/MWCNT catalyst as a typical sample.
nanoparticles. After the reaction of the nanoNi/MWCNT with Pd2+, corresponding SEM images change with the different reaction conditions. Fig. 1b shows a similar morphological texture of the Pd4.1Ni1/MWCNT to the nanoNi/MWCNT (Fig. 1a) except that a small amount of Pd nanoparticles are deposited on the Ni particle. However, for Pd3.7Ni1/MWCNT (Fig. 1c) and Pd1.3Ni1/MWCNT (Fig. 1d) catalysts, they exhibit obvious different SEM images where the surface of the Ni particle is eroded and Pd nanoparticles with smaller sizes of ca.10-25 nm are deposited. This may be illustrated through their preparation. According to the reaction (1), the aqueous Pd2+ solution was added to the nanoNi/MWCNT dropwise to form Pd nanoparticles and Ni2+. However, if the Pd2+ solution continued to be added, the formed Ni2+ ions on the surface of the Ni particle were transformed to Ni2+ oxides or hydroxides due to the higher pH of the Pd2+ solution. For the preparation of Pd4.1Ni1/MWCNT catalyst, the total Pd2+ solution volume added is 19.7 mL, much greater than those for other catalysts. This leads to the formation of the larger Ni particle as indicated in Fig. 1b. Energy dispersive spectroscopy (EDS) is a powerful technique used to firm the presence of Pd and Ni metals in the catalysts. Fig. 2 displays the EDS response of the Pd3.7Ni1/MWCNT catalyst as a typical sample. It is observed from Fig. 2 that the energy peak at 2.8 keV is attributed to Pd, and the energy peaks at 0.8, 7.4 and 8.2 keV are ascribed to the Ni. EDS analyses also give the mass percentages of Pd and Ni on the surface of the catalysts, as shown in Table 1. Particle sizes of the samples were further determined by TEM technique. Fig. 3 reveals the TEM images of the nanoNi/MWCNT and Pd3.7Ni1/MWCNT as a typical binary Pd-Ni/MWCNT catalyst. The grey and long tubes with the average diameter of 29 nm are MWCNTs while the dark dots are catalyst particles. The particles of the nanoNi/MWCNT catalyst is about 17 nm in average diameter (Fig. 3a), showing that the large aggregates as indicated in Fig. 1a are composed of small particles. On the other hand, the Pd3.7Ni1/ MWCNT catalyst has an average diameter of about 5 nm (Fig. 3b). From the image of Fig. 3b, the Pd-Ni particles are confirmed to be immobilized on the surface of MWCNT.
Fig. 3. TEM images of nanoNi/MWCNT (a) and Pd3.7Ni1/MWCNT (b) samples.
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 4. XRD patterns of nanoNi/MWCNT (a), Pd4Ni1/MWCNT (b), Pd3.7Ni1/MWCNT (c) and Pd1.3Ni1/MWCNT (d) samples.
XRD patterns of the prepared samples including the nanoNi/ MWCNT are shown in Fig. 4. The diffraction peak at 2u value of 25.8 is ascribed to the (0 0 2) crystal plane of MWCNTs. Three diffraction peaks at 2u value of 39.9 , 46.5 0 and 68.1 are indexed to the (111), (2 0 0) and (2 2 0) planes of Pd face-centered cubic (fcc) crystal structure, respectively. The prepared samples Pd4.1Ni1/ MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT present similar Ni characteristic diffraction peaks to the nanoNi/MWCNT at 2u values of 44.4, 51.9 and 77.9 corresponding to the (111), (2 0 0) and (2 2 0) crystal faces, respectively, showing that Ni nanoparticles are facecenter-cubic crystalline [18]. No Pd-Ni alloy phase diffraction peak was observed. In addition, compared to the nanoNi/MWCNT, the three binary Pd-Ni/MWCNT catalysts present a characteristic diffraction peak of NiO at 2u of 81.9 corresponding to the (2 2 2) plane of NiO cubic structure [19]. Presence of Ni(II) oxides on the binary Pd-Ni/MWCNT catalysts is attributed to the formation of Ni (II) species by the reaction of Pd(II) with Ni particles as shown in reaction (1). XPS of Ni(2p) core level regions of the prepared nanoNi/ MWCNT and binary Pd-Ni/MWCNT catalysts are shown in Fig. 5. Ni (2p3/2,1/2) peak at 860.8 eV and two weak Ni(2p3/2,1/2) peaks at 852.3 and 869.9 eV for the nanoNi/MWCNT catalyst are assigned to Ni metal. Ni(2p3/2,1/2) peaks at 856.04 and 873.7 eV can be attributed to the presence of Ni oxides [20]. This is consistent with the XRD analysis. Compared to the nanoNi/MWCNT, intensity of the Ni oxide peaks on the binary Pd-Ni/MWCNT catalysts declines, showing the deposition of Pd nanoparticles on the nanoNi/MWCNT. XPS of Pd(3d) core level regions of the binary Pd-Ni/MWCNT catalysts are shown in Fig. 6. Pd(3d5/2,3/2) peaks at 337.4 and 342.7 eV can be ascribed to the Pd2+ in Pd oxides. Pd(3d5/2,3/2)
Fig. 5. XPS of Ni(2p) core level region in the prepared samples.
99
Fig. 6. XPS of Pd(3d) core level region in Pd4Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT samples.
peaks at 335.3 and 340.4 eV are attributed to the Pd metal. Intensity of Pd oxides peak is larger than that of Pd metal, which could be caused by the partial hydrolysis of PdCl2 in solution with higher pH value. Electrochemical properties of the prepared catalysts in 1 mol L1 NaOH solution were examined by cyclic voltammetry as indicated in Fig. 7. All catalysts reveal characteristic cathodic reduction peaks of Pd and Ni oxides despite different peak potentials on these samples. On Pd4.1Ni1/MWCNT, Pd3.7Ni1/ MWCNT and Pd1.3Ni1/MWCNT catalysts, the peak rPd potential for the reduction of Pd oxides formed during the positive-going scan is 0.286 V, 0.312 V and 0.375 V respectively, and the potential of the cathodic peak rNi, corresponding to the reduction of Ni(III) oxides to Ni(II) species [12], is 0.400 V, 0.342 V and 0.324 V respectively. An unique exception was observed on the Pd1.3Ni1/ MWCNT catalyst where a well-defined cathodic peak at 0.095 V arises, which is attributed to the reduction of Ni(II) species to Ni metal. As for the Pd1.3Ni1/MWCNT, it presents a larger Ni content and the surface of the Ni nanoparticles is not totally covered by the Pd nanoparticles. This leads to the obvious electrode reactions involved in the Ni species on the catalyst surface. For other catalysts Pd4.1Ni1/MWCNT and Pd3.7Ni1/MWCNT, most of the Ni nanoparticles surfaces are covered by Pd nanoparticles. This may hinder the further reduction of Ni(II) species because of the following reasons: after the rNi peak, which is related to the reduction of Ni(OH)3 (or Ni(III) oxide) to Ni(OH)2 (or Ni(II) oxide),
Fig. 7. Cyclic voltammograms of the prepared samples in 1 mol L1 NaOH solution at a scan rate of 50 mV s1.
100
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 8. Cyclic voltammetric responses of the samples in 1 mol L1 NaOH solution in the presence of 0.5 mol L1 ethanol at a scan rate of 50 mV s1.
volume shrinkage of the Ni nanoparticles takes place and subsequently, the formed Ni(II) species are covered by the neighboring Pd nanoparticles. In order to investigate the electrochemical properties of the prepared Pd-Ni/MWCNT catalysts, we use ethanol oxidation reaction in alkaline media to evaluate their electrocatalytic activity. Fig. 8 shows cyclic voltammetric responses of the samples in 1 mol L1 NaOH solution in the presence of 0.5 mol L1 ethanol (C2H5OH). Fig. 8 reveals that electroactivity of the Pd-Ni/MWCNT catalysts changes with the atomic ratios of Pd to Ni. Pd3.7Ni1/ MWCNT shows the highest anodic peak current density during the forward scan among the samples while Pd4.1Ni1/MWCNT shows the lowest anodic peak current density. According to their ICP data, the Pd loading is not much different among the binary Pd-Ni samples. Results indicate that the electroactivity of these Pd-based catalysts may be related to the dispersion of Pd nanoparticles and the “bi-functional effect” of Pd-Ni [14–16]. Further, it is found from Fig. 8 that the anodic current density for forward scan on the Pd3.7Ni1/MWCNT catalyst is 89.4 mA cm2, which is higher than that on the Pd/MWCNT. It is also seen from Table 1 that the Pd loading of the Pd/MWCNT is much larger than the binary Pd-Ni/ MWCNT catalysts, showing that a considerable part of Pd on the Pd/MWCNT catalyst is not used to catalyze the ethanol oxidation. This can be further observed from Fig. 9, where current density is referenced against Pd loading on electrode. The current density for
Fig. 9. Cyclic voltammetric responses of the samples where the current density is referenced against Pd-loading. Others same as Fig. 8.
ethanol oxidation as indicated in Fig. 9 reveals the usage efficiency of Pd particles. Fig. 9 shows that the anodic peak current density (mA cm2 mg1) during the forward scan on the Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT catalysts is 38.8, 130.9 and 61.1, respectively, which are significantly larger than that on the Pd/ MWCNT. In view of the role that the dispersions of Pd nanoparticles play in the electrocatalytic activity for ethanol oxidation, we reckoned in the Pd active surface areas of the samples. According to the coulombs consumed for the reduction of Pd oxides (involved in the peak rPd) on a smooth polycrystalline Pd electrode (CV not shown) and the prepared catalysts (shown in Fig. 7) [10], the Pd active surface areas of the Pd/MWCNT, Pd4.1Ni1/MWCNT, Pd3.7Ni1/ MWCNT and Pd1.3Ni1/MWCNT catalysts loaded on the GC electrode (geometrical area 0.071 cm2) are figured out to be 2.4, 0.4, 2.6 and 1.6 cm2, respectively. The similarity of the Pd active surface area between Pd/MWCNT and Pd3.7Ni1/MWCNT catalysts suggests that the Pd nanoparticles of the Pd3.7Ni1/MWCNT are highly dispersed due to its low Pd loading as shown in Table 1. Cyclic voltammograms of the Pd3.7Ni1/MWCNT catalyst in 1 mol L1 NaOH solution containing various concentrations of ethanol ranging from 0.1 to 2.0 mol L1 are presented in Fig. 10a. The forward-going anodic peak (pa) current density exhibits a rapid increase with the ethanol concentration up to 1 mol L1 while it exhibits a slow increase when the ethanol concentration is greater than 0.7 mol L1, as indicated in Fig. 10b. According to the electrocatalytic oxidation mechanism of alcohols [10,11,21,22],
Fig. 10. Cyclic voltammograms of the ethanol oxidation on the Pd3.7Ni1/MWCNT catalyst at a scan rate of 50 mV s1 in 1 mol L1 NaOH solution containing ethanol solutions of various concentrations (a) and plot of the anodic peak (pa) current density vs ethanol concentration (b).
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 11. Chronoamperometric responses of 0.5 mol L1 C2H5OH in 1 mol L1 NaOH solution on the prepared samples at a potential step of 0.4 V.
ethanol oxidation on Pd-based catalysts in alkaline media is mainly involved in three processes: (1) formation of adsorbed ethoxi Pd(CH3CO)ad, (2) formation of adsorbed hydroxyl ion Pd-OHad, and (3) reaction of the Pd-(CH3CO)ad and Pd-OHad to produce acetate as the main product and release free active catalytic sites. In addition, the process (3) is considered to be the rate-determining step. Thus, numbers of the Pd-(CH3CO)ad play an important role in the electrooxidation rate of ethanol. The coverage of the adsorbed ethoxi species CH3COad on the catalyst surface is close to saturation at higher ethanol concentrations and therefore, the current density will no longer increase, as indicated in Fig. 10b where the anodic peak current density at 1.0 mol L1 ethanol is close to that at 2.0 mol L1 ethanol. It is further found from Fig. 10a that the reduction peak (pc) current density declines with the increase of the ethanol concentration. Considering that the peak pc is caused by the reduction of Ni oxides (Ni hydroxides) formed during the anodic sweep at higher potential, the formation of these Ni oxides is suppressed at higher ethanol concentration. This suggests that the ethanol oxidation is a dominant reaction at higher ethanol concentration during the anodic sweep event at more positive potentials. Electrocatalytic activity of the samples for ethanol oxidation was further investigated by chronoamperometric responses. Fig. 11 shows the j–t curves at a potential of 0.4 V vs Hg/HgO in 1 mol L1 NaOH+0.5 mol L1 C2H5OH solution. It is seen from Fig. 11 that all
Fig. 12. Electrochemical impedance spectra of the prepared catalysts in 1 mol L1 NaOH containing 0.5 mol L1 C2H5OH at 0.3 V. Inset is the zoomed main panel of the Nyquist plots.
101
samples, including Pd/MWC NT, show efficient activity for ethanol electro-oxidation, however, the activity of these Pd-Ni/MWCNT catalysts changes greatly with the changing Pd:Ni ratios. The final current density values after holding the potential at 0.4 V for 600 s on Pd/MWCNT, Pd4.1Ni1/MWCNT, Pd3.7Ni1/MWCNT and Pd1.3Ni1/MWCNT catalysts are 2.7, 0.5, 8.6 and 3.1 mA cm2, respectively, showing the following electroactivity order: Pd3.7Ni1/MWCNT > Pd1.3Ni1/MWCNT > Pd/MWCNT > Pd4.1Ni1/ MWCNT. Result suggests that an appropriate ratio of Pd to Ni is very important to the electrocatalytic activity of the binary Pd-Ni catalysts. Similarly, if the Pd loading in these catalysts is considered, the final mass current density (mA cm2 mg1) is 0.40 on Pd/MWCNT, 0.78 on Pd4.1Ni1/MWCNT, 12.64 on Pd3.7Ni1/ MWCNT and 2.61 on Pd1.3Ni1/MWCNT, showing that the Pd nanoparticles of the Pd3.7Ni1/MWCNT catalyst are more fully used as the electrocatalyst for ethanol oxidation than other catalysts. Electrochemical impedance spectra (EIS) of the prepared catalysts in 1 mol L1 NaOH+0.5 mol L1 C2H5OH solution were also investigated to further study their electroactivity for ethanol oxidation. Fig. 12 presents the Nyquist diagrams of the catalysts at -0.3 V in 1.0 mol L1 NaOH + 0.5 mol L1 ethanol solution. Among the prepared catalysts, Pd4.1Ni1/MWCNT catalyst presents the largest electrochemical resistance. A semicircle with much lower electrochemical impedances develops on the Pd3.7Ni1/MWCNT, Pd1.3Ni1/MWCNT and Pd/MWCNT catalysts. It is clearly seen from Fig. 12 that the charge transfer resistance of ethanol oxidation follows the order: Pd4.1Ni1/MWCNT >> Pd/MWCNT > Pd1.3Ni1/ MWCNT > Pd3.7Ni1/MWCNT, showing that their electroactivity order is consistent with the analysis of CA data in Fig. 11. For the highly dispersed nanoparticles, their stability is the key problem that must be paid much attention to. Here, we measured the successively cyclic potential scanning of the Pd3.7Ni1/MWCNT catalyst in 1 mol L1 NaOH containing 0.5 mol L1 C2H5OH to evaluate its electrocatalytic stability. Fig. 13 shows 500 consecutive cyclic voltammograms (CVs) of the Pd3.7Ni1/MWCNT catalyst at a potential scan rate of 100 mV s1. The CV profile of each cyclic scanning is similar for each other. It is seen from the comparison of the 1st and 500th cyclic voltammograms that only a small amount of decline in the anodic current density for the forward scan is observed. The anodic peak current density on the 1st and 500th CVs is 104 and 77 mA cm2 respectively. This current density decay is partially caused by the consumption of ethanol due to the
Fig. 13. 500 consecutive sweeps of cyclic voltammograms on the Pd3.7Ni1/MWCNT catalyst in 1 mol L1 NaOH containing 0.5 mol L1 C2H5OH at a potential scan rate of 100 mV s1.
102
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103
Fig. 14. Schematic diagram for the formation of the Pd-Ni/MWCNT catalysts.
prolonged sweep. Result shows the high electroactivity stability of the Pd3.7Ni1/MWCNT catalyst. According to the preparation processes of the catalysts, the formed Pd nanoparticles may be directly deposited on the surface of Ni particles that have been immobilized on the MWCNTs. The high surface energy of Pd and Ni particles would decline due to the interaction between Pd and Ni particles. This favors the stability of these particles. Very low Pd loading and high ethanol oxidation electroactivity of the Pd-Ni/MWCNT catalysts may be related to their special morphological structure. According to reaction (1), the formation of Pd nanoparticles was accompanied by Ni2+ species. The produced Ni2+ ions then leave the surface of the nanoNi/MWCNT and subsequently, the formed Pd particles fill the vacancies left by Ni particles. Possible formation mechanism of the Pd-Ni/MWCNT catalysts can be illustrated with Fig. 14. Ni2+ ions are reduced to Ni particles by N2H4 and the as-formed Ni particles are immobilized on the surface of MWCNTs to obtain the nanoNi/MWCNT. Reaction between Pd2+ and the nanoNi/MWCNT results in deposition of Pd nanoparticles on Ni particles and leads to the formation of a binary Pd-Ni composite that has been extensively reported to exhibit better electrocatalytic activity for alcohol oxidation than Pd [12– 16]. Highly dispersed Pd nanoparticles also enhance their electrochemical activity. Therefore, the excellent electroactivity of the Pd-Ni particles may be ascribed to the following three reasons: (1) small sizes of Pd nanoparticles with high active surfaces, (2) “bi-functional effect” of Pd-Ni [15], and (3) electroactivity of Ni nanoparticles themselves for alcohol oxidation [23]. According to this possible mechanism as indicated in Fig. 14, the assynthesized Pd-Ni/MWCNT catalysts are different from the conventional binary Pd-Ni nanoparticles on their morphological structures because the Pd nanoparticles are distributed on the surface of Ni nanoparticles. The “bi-functional effect” comes from where the Pd nanoparticles and Ni particles are in contact. Considering that the Pd/MWCNT and Pd3.7Ni1/MWCNT have an approximate Pd active surface area (2.4 and 2.6 cm2), the significant difference of their anodic peak current densities as shown in Fig. 9 is caused by the so-called “bi-functional effect”. In other words, combination of Pd and Ni particles, performed by the in situ deposition of Pd nanoparticles on Ni particles, greatly enhances the electrocatalytic activity of the Pd nanocatalyst for ethanol oxidation. Further, the formation mechanism of the samples as indicated in Fig. 14 can be reasonably confirmed by the analyses of EDS and ICP. EDS technique can be used to obtain the semi-quantitative data of the metal contents on the surface of the samples. That is, mass percentages of Pd and Ni, obtained by EDS analyses, only reflect the
surface compositions of the samples. However, ICP data exhibit the overall compositions of the samples. According to Fig. 14 where Pd nanoparticles are deposited on the surface of the Ni particles, Pd mass percentage based on the catalyst surface is considerably larger than that based on the whole catalyst. Therefore, the Pd masses (or Ni masses) obtained by EDS are different from those by ICP. Table 1 shows the results of EDS and ICP analyses for Pd and Ni. It is seen from Table 1 that for the four samples, the mass ratios of Pd to Ni obtained from EDS analyses are much higher than those from ICP analyses. This proves the rationality of the proposed mechanism, that is, Pd nanoparticles are immobilized on the surface of Ni nanoparticles. On the other hand, the high electroactivity of the binary Pd-Ni catalysts for ethanol oxidation may be also related to the presence of NiO [24]. For ethanol oxidation on NiO-supported Pd-based catalysts in alkaline media, the competitive adsorption on the catalysts is favorable for ethanol instead of intermediates produced during ethanol oxidation, and no CO intermediate is formed. Results show that the intermediate poisoning of the present catalysts reduces due to the presence of NiO, leading to their high electrocatalytic activity and stability. 4. Conclusions Novel binary Pd-Ni catalysts with low Pd-loadings and high electroactivity for ethanol oxidation were prepared by an in situ deposition of Pd nanoparticles on Ni particles. Main conclusions are as follows. (1) Ni particles, immobilized on the surface of MWCNTs, are used as the reduction agent of Pd2+, and the as-formed Pd nanoparticles are in situ deposited on the Ni particles to synthesize the MWCNT-supported binary Pd-Ni catalysts (Pd-Ni/MWCNT) with various atomic ratios of Pd:Ni. (2) The obtained Pd-Ni/MWCNT catalysts have low Pd-loadings (1.11.9 wt%) and the Pd3.7Ni1/MWCNT presents the best electrocatalytic activity for ethanol oxidation in alkaline media among the prepared samples. (3) The excellent electroactivity of the Pd3.7Ni1/MWCNT catalyst may be ascribed to both the highly dispersed Pd nanoparticles on Ni particles and the so-called “bi-functional effect” of the binary Pd-Ni catalysts with an appropriate atomic ratio of Pd:Ni. (4) The method proposed in this work provides a new approach to the fabrication of binary or ternary Pd-based (or other noble metals-based) nano-composites with low noble metal-loadings and enhanced electroactivity. Acknowledgement This work was supported by The National Natural Science Foundation of China (21376070 and 20876038), Hunan Provincial Natural Science Foundation of China (14JJ2096), and A Project supported by Scientific Research Fund of Hunan Provincial Education Department (11K023). References [1] S. Song, P. Tsiakaras, Recent progress in direct ethanol proton exchange membrane fuel cells (DE-PEMFCs), Appl. Catal. B: Environ. 63 (2006) 187–193. [2] Q. He, B. Shyam, K. Macounova, P. Krtil, D. Ramaker, S. Mukerjee, Dramatically enhanced cleavage of the C-C bond using an electrocatalytically coupled reaction, J. Am. Chem. Soc. 134 (2012) 8655–8661. [3] J. Ribeiro, D.M. dos Anjos, K.B. Kokoh, C. Coutanceau, J.-M. L’eger, P. Olivi, A.R. de Andrade, G. Tremiliosi-Filho, Carbon-supported ternary PtSnIr catalysts for direct ethanol fuel cell, Electrochim. Acta 52 (2007) 6997–7006. [4] X. Xue, J. Ge, T. Tian, C. Liu, W. Xing, T. Lu, Enhancement of the electrooxidation of ethanol on Pt–Sn–P/C catalysts prepared by chemical deposition process, J. Power Sources 172 (2007) 560–569.
Q. Yi, Q. Chen / Electrochimica Acta 182 (2015) 96–103 [5] L. Jiang, L. Colmenares, Z. Jusys, G.Q. Sun, R.J. Behm, Ethanol electrooxidation on novel carbon supported Pt/SnOx/C catalysts with varied Pt:Sn ratio, Electrochim. Acta 53 (2007) 377–389. [6] A. Santasalo-Aarnio, Y. Kwon, E. Ahlberg, K. Kontturi, T. Kallio, M.T.M. Koper, Comparison of methanol, ethanol and iso-propanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC, Electrochem. Commun. 13 (2011) 466–469. [7] J. Liu, J. Ye, C. Xu, S.P. Jiang, Y. Tong, Kinetics of ethanol electrooxidation at Pd electrodeposited on Ti, Electrochem. Commun. 9 (2007) 2334–2339. [8] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Mechanism study of the ethanol oxidation reaction on palladium in alkaline media, Electrochim. Acta 54 (2009) 2203–2208. [9] Q.F. Yi, L.Z. Sun, In situ synthesis of palladium nanoparticles on multi-walled carbon nanotubes and their electroactivity for ethanol oxidation, Rare Metals 32 (2013) 586–591. [10] Q.F. Yi, F.J. Niu, L.Z. Sun, Fabrication of novel porous Pd particles and their electroactivity towards ethanol oxidation in alkaline media, Fuel 90 (2011) 2617–2623. [11] Q.F. Yi, F.J. Niu, L.H. Song, X.P. Liu, H.D. Nie, Electrochemical activity of novel titanium-supported porous binary Pd–Ru particles for ethanol oxidation in alkaline media, Electroanal. 23 (2011) 2232–2240. [12] Q.F. Yi, L.Z. Sun, X.P. Liu, H.D. Nie, Palladium–nickel nanoparticles loaded on multi-walled carbon nanotubes modified with b-cyclodextrin for electrooxidation of alcohols, Fuel 111 (2013) 88–95. [13] L.Z. Sun, Q.F. Yi, X.P. Liu, Electrocatalytic oxidation of propanol and butanol isomers on palladium-nickel catalyst, J. Fuel Chem. Tech. 41 (2013) 1274–1280. [14] S.Y. Shen, T.S. Zhao, J.B. Xu, Y.S. Li, Synthesis of PdNi catalysts for the oxidation of ethanol in alkaline direct ethanol fuel cells, J. Power Sources 195 (2010) 1001–1006.
103
[15] Z. Qi, H. Geng, X. Wang, C. Zhao, H. Ji, C. Zhang, J. Xu, Z. Zhang, Novel nanocrystalline PdNi alloy catalyst for methanol and ethanol electro-oxidation in alkaline media, J. Power Sources 196 (2011) 5823–5828. [16] C. Jin, X. Sun, Z. Chen, R. Dong, Electrocatalytic activity of PdNi/C catalysts for allyl alcohol oxidation in alkaline solution, Mater. Chem. Phys. 135 (2012) 433–437. [17] A. Marchionni, M. Bevilacqua, C. Bianchini, Y.-X. Chen, J. Filippi, P. Fornasiero, A. Lavacchi, H. Miller, L. Wang, F.o Vizza, Electrooxidation of Ethylene Glycol and Glycerol on Pd-(Ni-Zn)/C Anodes in Direct Alcohol Fuel Cells, Chem. Sus. Chem. 6 (2013) 518–528. [18] C.-T. Hsieh, Y.-W. Chou, J.-Y. Lin, Fabrication and electrochemical activity of Niattached carbon nanotube electrodes for hydrogen storage in alkali electrolyte, Int. J. Hydrogen Energy 32 (2007) 3457–3464. [19] Z. Liang, H. Sun, Y. Cui, Catalytic oxidation of cyclohexanol on the Ti/Ni/NiO electrode, Chin. J. Catal. 30 (2009) 254–258. [20] T. Shobha, C.L. Aravinda, P. i Bera, L. Gomathi Devi, S.M. Mayanna, Characterization of Ni–Pd alloy as anode for methanol oxidative fuel cell, Mater. Chem. Phys. 80 (2003) 656–661. [21] S.T. Nguyen, H.M. Law, H.T. Nguyen, N. Kristian, S. Wang, S.H. Chan, X. Wang, Enhancement effect of Ag for Pd/C towards the ethanol electro-oxidation in alkaline media, Appl. Catal. B: Environ. 91 (2009) 507–515. [22] Q.F. Yi, H. Chu, Q.H. Chen, Z. Yang, X.P. Liu, High Performance Pd, PdNi, PdSn and PdSnNi Nanocatalysts Supported on Carbon Nanotubes for Electrooxidation of C2-C4 Alcohols, Electroanal 27 (2015) 388–397. [23] D.K. Liguras, K. Goundani, X.E. Verykios, Production of hydrogen for fuel cells by catalytic partial oxidation of ethanol over structured Ni catalysts, J. Power Sources 130 (2004) 30–37. [24] F. Hu, C. Chen, Z. Wang, G. Wei, P.K. Shen, Mechanistic study of ethanol oxidation on Pd–NiO/C electrocatalyst, Electrochim. Acta 52 (2006) 1087–1091.