Carbon- and Binder-free Core-Shell Nanowire Arrays for Efficient

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Carbon- and Binder-free Core-Shell Nanowire Arrays for Efficient Ethanol Electro-Oxidation in Alkaline Medium Fen Guo, Yiju Li, Baoan Fan, Yi Liu, Lilin Lu, and Yang Lei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16615 • Publication Date (Web): 15 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018

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ACS Applied Materials & Interfaces

Carbon- and Binder-free Core-Shell Nanowire Arrays for Efficient Ethanol Electro-Oxidation in Alkaline Medium Fen Guo1, Yiju Li2,3, Baoan Fan1, Yi Liu1, Lilin Lu1, Yang Lei1 1

Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials, School of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, P. R. China 2

Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, P. R. China 3

Department of Materials Science and Engineering, University of Maryland at College Park, College Park, Maryland 20742, USA KEYWORDS. Core-Shell Nanowires; Template-Assistant Electrodeposition; Ethanol Electro-Oxidation; Carbon- and Binder-Free; Fuel Cell ABSTRACT: To achieve highelectrochemical surface area (ECSA) and avoid carbon support and binder in the anode catalyst of direct ethanol fuel cell, herein, we design freestanding core-shell nickel@palladium-nickel nanowire arrays (Ni@Pd-Ni NAs) without carbon support and binder for high-efficiency ethanol electro-oxidation. Bare Ni nanowire arrays (Ni NAs) are first prepared using the facile template-assistantelectrodeposition method. Subsequently, the Ni@PdNi NAs are formed using one-step solution-based alloying reaction. The optimized Ni@Pd-Ni NAs electrode witha high ECSA of 64.4 m2 g–1Pdexhibits excellent electrochemical performance (peak current density: 622 A g–1Pd) and cycling stability for ethanol electro-oxidation. The facilely obtained yet high-efficiency core-shell Ni@Pd-Ni NAs electrode is a promising electrocatalyst, which can be utilized for oxygen reduction reaction, urea, hydrazine hydrate and hydrogen peroxide electro-oxidation, not limited to the ethanol electro-oxidation.

electron density had weakened the adsorption of CH3COads intermediate and thus enhanced the activity toward ethanol electro-oxidation. Note that, as one of the oxophilic metals, Ni can spontaneously generate several atomic layers of NiO and Ni(OH)2 on the surface in the atmospheric or aqueous conditions.19 By alloying oxophilic metals with Pd or Pt, more oxygen-containing adspecies can be formed, which drive the oxidation of alcohols without the production of poisoning intermediates.20-21 In this regard, Pd-Ni bimetallic alloy is considered to be a highly active and durable catalyst toward ethanol electro-oxidation.

1. INTRODUCTION Direct ethanol fuel cell (DEFC) is an energy-conversion device that allows the chemical energy of ethanol to be efficiently transferred to electric energy. Liquid ethanol as the fuel possesses several benefits of easy storage and transportation, nontoxicity and facile synthesis, when compared with other fuels such as hydrogen, hydrazine hydrate and methanol.1-3 The electrocatalyst is the most vital component of DEFC because it plays a decisive role in the output performance of DEFC.4 As the nonplatinum noble metal, Pd is an attractive catalyst for ethanol electro-oxidation owing to its high activity and more abundance on the earth’s crust.5-7 With the maturity of anion-exchange membrane technology,8 the alkaline medium has brought a huge interest in consideration of faster ethanol electro-oxidation kinetics,9 which thereafter allows the adoption of non-noble metals.

Conductive support with high specific surface area is also critical to ensure the favorable electrocatalytic performance, since the support not only supplies sites for the metals anchoring, but also acts as conductive framework for electron transport.22 However, in many researches, carbon materials are usually used as the supports for the ethanol electro-oxidation (Table 1). Although carbon materials, such as graphene, carbon nanotubes and active carbon, possessrelatively high surface and favorable electron conductivity, they have severe corrosion issue at high potential values and in harsh acidic medium.23-24 Zadick et al. also found that the ECSA degradation of carbon supported catalyst in alkaline medium is severe, because of the weak chemical bonding

Pd-M (M=Fe, Co, Ni, Cu, Sn, Sb, Mo, Bi, Te, etc.) was fabricated to lower the cost, increase the electrochemical surface area (ECSA) and electrocatalytic activity of the Pd catalyst.10-18For example, Li et al. prepared a carbon supported oxide-rich Pd-Cu bimetalliccatalyst, of which 74% of Cu atoms were in the oxide form (CuO or Cu2O).16 Such constituent results in the spill-over of oxygencontaining species from CuOx to Pd. The low Pd 3d 1

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Table 1. Supports, catalytic performance and corresponding testing conditions of different catalysts Ethanol Concenc tration

Scan d Rate

– – 1.96

1.0 1.0 1.0

10 10 50

– 0.84

0.5

50

27

1.0

50

22

– 3000

– 8.3

– 1.0

– 50

– –

~75 ~350

– –

1.0 1.0

5 50

29

CeO2 MWCNT Graphene

35–50 14 77

– – 614.6

– – 0.80

– – 0.1

– – 50

23

No Support Carbon Fiber Cloth

23.23

570

1.0

50

33

26.95

495.8

2.45 1.84

1.0

50

24

a

b

Catalyst

Support

ECSA

Activity

PdSn/MWCNT PdSn/C Pd/Cu/Graphene

MWCNT Active Carbon Graphene

– – 20.22

39 24 392.6

Pd-Cu Alloy Pd/B and N-doped Graphene

No Support

–

392.16

Graphene

55.3

464.5

PdTex/C Pd/N-doped CNT

Active Carbon CNT

48.8 36

Pd/Ti Pd-Polyaniline-Pd/Ti

Ti Ti

Pd/CeO2 Pd-Bi/MWCNT Ni-Pd /Graphene Pd-Ag alloy Pd/Carbon Fiber Cloth

b*

Activity

Ni@Pd-Ni Nanowire 0.97 Self-Supported 64.4 622 0.5 50 Arrays 2 –1 –1 –2 –1 –1 a: m g Pd; b: Pd mass peak current /A g Pd; b*: peak current normalized to ECSA /mA cm ; c: mol L ; d: mV s

Ref. 25 25 26

13 28

30

31 32

This Work

Scheme 1. Schematic of the preparation process of the core-shell Ni@Pd-Ni NAs electrode. Nickel is first electrodeposited into the channels of polycarbonate template to form Ni nanowire arrays and then over-plated to form the Ni substrate. The freestanding Ni NAs electrode was immersed in HCl/PdCl2 solution to alloy with Pd. Ethanol electro-oxidation occurs on the Pd-Ni alloy shell of the self-supported core-shell Ni@Pd-Ni NAs electrode.

between carbon support and the catalyst particles.34 Moreover, for the catalyst powder, extra binders are needed, which will block the ion pathways and decrease the active sites.35 Therefore, carbon- and binder-free

electrocatalysts with high activity toward ethanol electrooxidation are urgently needed. In our work, we prepared a freestanding core-shell Ni@Pd-Ni NAs electrode for high-efficiency ethanol electro-oxidation. The self-supported Ni nanowire arrays 2


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were first prepared by template-assistant electrodeposition method. The Ni@Pd-Ni NAs were then successfully formed using one-step solution-based alloying reaction (Scheme 1). The freestanding Ni@Pd-Ni NAs electrode is carbon- and binder-free, which can avoid the degradation of ECSA and formation of “dead” sites. The optimized Ni@Pd-Ni NAs electrode with a high ECSA of 64.4 m2 g–1Pd displays a high catalytic current density of 622 A g–1Pd. The facile yet high-efficiency threedimensional (3D) Ni@Pd-Ni NAs electrode can be applied in many other electrocatalytic fields, such as oxygen reduction reaction and urea electro-oxidation, not limited to the ethanol electro-oxidation.

respectively. The front of the electrode is the Ni nanowire arrays and the back is the self-supported Ni substrate. The SEM image in Figure 1c shows the cross-section of the Ni NAs electrode, of which the Ni nanowires tightly attach to the Ni substrate which has a thickness of ~150 m. Figure 1d-f show the Ni nanowire arrays at different SEM magnifications. The Ni nanowires stand on the Ni substrate and gather together to form the random nanowire clusters, which are caused by the random nanochannels in the polycarbonate template.36 The TEM image reveals the diameter of single Ni nanowire is about 99.5 nm (Figure 1g). After alloyed with Pd, the Ni@Pd-Ni NAs remain the morphology of well-defined nanowire clusters (Figure 1h). The enlarged SEM image reveals that porous gauze-like shell surrounds each nanowire (Figure 1i).TEM image of the Ni@Pd-Ni nanowires further confirms the core-shell structure (Inset in Figure 1i). The TEM image in Figure 1j shows Ni@Pd-Ni nanowire con-

2. RESULTS AND DISSCUSSION Physico-chemical characterization of the Ni NAs and Ni@Pd-Ni NAs electrode. Figure 1a and b show the front and back photo images of the Ni NAs electrode,

Figure 1. (a, b) Photo images showing the front and back of the Ni NAs electrode. (c) SEM image of the cross-section of the Ni NAs electrode. The Ni nanowires attach to the Ni substrate which has a thickness of ~150 µm. (d-f) SEM images of the Ni NAs. The Ni nanowires gather together to form the random nanowire clusters. (g) TEM image of the single Ni nanowire with a diameter of 99.5 nm. (h, i) SEM images of the Ni@Pd-Ni NAs. Porous gauze-like shell surrounds each nanowire. Inset in Figure 1i shows the TEM image of the core-shell Ni@Pd-Ni nanowires. (j) TEM image of the single Ni@Pd-Ni nanowire. The Ni@Pd-Ni nanowire consists of the solid Ni core and gauze-like shell with a thickness of ~40 nm. 3



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and 2.063 Å ) and (200) (1.815 and 1.786 Å ) planes are larger than those of corresponding standard distances of Ni (111) and (200) planes. The increased interplanar distances are caused by the alloying of large-atom-radius Pd with Ni. Earlier studies also demonstrated Pd and Ni tend to form the substitutional alloy because they have approximate enthalpy of vaporization values (Ni: 370 kJ mol–1, Pd: 357 kJ mol–1), similar atomic sizes (Ni: 124.6 pm, Pd: 137.6 pm) and same crystal structures (fcc).32, 37-38 Owing to the low content of Pd in the Pd-Ni alloy, there is no obvious Pd diffraction peak shown in the whole XRD pattern. However, the XRD peaks of Ni@Pd-Ni NAs shift to smaller angle when compared with pure Ni NAs, which further confirms the formation of Pd-Ni alloy (Figure 2g).12 It is noteworthy that both electrodes are grown along the (200) planes preferentially. The mean crystalline sizes of Ni@Pd-Ni NAs and Ni NAs electrodes calculated by Scherrer Formula are 19.70 and 19.73 nm, respectively.39

sists of the solid Ni core and gauze-like shell with a thickness of ~40 nm. Figure S1 shows the elemental mappings and EDS spectrum of the top surface of the Ni@Pd-Ni NAs (Supporting Information), which evidences the existence of Pd element in the nanowire arrays. TEM and the corresponding EDS elemental mappings are exploited to further characterize the element distributions on single Ni@Pd-Ni nanowire. Figure 2a and b display the uniform distributions of Ni and Pd elements on the single nanowire. The overlapped distributions of Ni and Pd elements on the nanowire are shown in Figure 2c. The amplified overlay in Figure 2d further confirms the existence of Ni and Pd in the shell. The line scan mapping is used to investigate the lateral distribution and content of Ni and Pd. From the results of line scan spectra in Figure 2e, the atomic ratio of Pd to Ni is ~0.8 at%. Figure 2f displays the high-resolution TEM image of the Ni@Pd-Ni nanowire. The lattice fringe distances of the (111) (2.146

Figure 2. (a-d) TEM-EDS mappings of the Ni@Pd-Ninanowire. Ni and Pd elements are uniformly distributed on the nanowires. (e) Line scanning spectra of the Ni@Pd-Ninanowire. The atomic ratio of Pd to Ni is ~0.8 at%. (f) High-resolution TEM image of the Ni@Pd-Ni nanowire. The increased lattice fringe distances of the (111) and (200) planes of Ni indicate the alloying with Pd. (g) XRD patterns of the Ni NAs and Ni@Pd-Ni NAs electrodes. The XRD peak shifts of the Ni@Pd-Ni NAs electrode when compared with pure Ni NAs electrode confirm the formation of Pd-Ni alloy. 4


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nent would adsorb the dissociative water molecules to generate more OH adspecies, which also promotes the catalytic stability.41

Figure 3a represents the XPS survey of the Ni NAs and Ni@Pd-Ni NAs electrodes. The C 1s, O 1s and Ni 2p signals were recorded for both electrodes while Pd 3d signal only exists in Ni@Pd-Ni NAs. C element is detected because of the residual hydrolysis production of polycarbonate. Figure 3b is the high-resolution XPS spectra of Pd 3d. The Pd 3d3/2 and Pd 3d5/2 spectra are divided into two pairs of peaks, which are assigned to the metallic Pd and Pd2+. It is obvious that the peak of metallic Pd is predominant among the Pd species, indicating Pd metal is the main form. Figure 3c compares the high-resolution Ni 2p XPS peaks of Ni and Ni-Pd alloy. The deconvolutions reveal the existence of metallic Ni, NiO and Ni(OH) 2. Table 2 collects the positions and contents of different Ni species. Compared with those of Ni NAs, all the positions of Ni species in Ni@Pd-Ni NAs shift to larger binding energy, indicating the decreased electron density. Owing to the lower electronegativity of Ni (1.9) than that of Pd (2.2), the electron will transfer from Ni to Pd, leading to the increasing proportion of oxygen containing species in Ni (from 57.72% of Ni NAs to 60.52% of Ni@Pd-Ni NAs). It is referred the oxygen containing species, such as OH, can facilitate the oxidation of ethanol and its intermediates (as shown in Figure 3d).20-21, 40 Furthermore, due to the suitable bonding strength with OH, the Ni(OH)2 compo-

Optimization of immersing time. Ni@Pd-Ni NAs electrode was fabricated by directly immersing the Ni NAs electrode in HCl/PdCl2 solution for a duration of time to replace Ni with Pd. Too short repalcement time will lead to few Pd active sites. Overlong repalcement time will result in surplus Pd loading and increase the cost. Here, we chose a series of repalcement time according to the recording of open circuit potential (OCP) of Ni NAs electrode in HCl and HCl/PdCl2 solutions, respectively, since the OCP reflects the state change on the electrode surface.42 As Figure 4a shows, the OCP of Ni NAs in bare HCl was declined slowly, which relates to the neutralization reactions of Ni oxides or hydroxides with HCl. By contrast, the OCP in HCl/PdCl2 solution fell rapidly during the first 500 s and then slowly decreased, until was equal to that in HCl solution. The rapid OCP drop before 500 s attributes to the fast replacement reaction of PdCl42– with Ni. According to this changing trend, seven kinds of repalcement time were selected, i.e., 100 s, 200 s, 300 s,400 s, 500 s, 2,000 s and 7,200 s. Figure 4b and 4c demonstrate the ECSA, and electroactive Pd % (also known as Pd utilization, i.e., the proportion of the

Figure 3. (a) Wide scan XPS spectra of the Ni NAs (i) and Ni@Pd-Ni NAs (ii). The C 1s, O 1s and Ni 2p peaks exist for both electrodes while Pd 3d peak only exists in the Ni@Pd-Ni NAs. (b) High-resolution Pd 3d peaks of the Ni NAs (i) and Ni@Pd-Ni NAs (ii). The peak of the metallic Pd is predominant among the Pd species. (c) High-resolution Ni 2p peaks of the Ni NAs (i) and Ni@Pd-Ni NAs (ii). The proportion of oxygen containing species in Ni element is increased after alloying Pd. (d) Schematic illustration of ethanol electro-oxidation on the Pd-Ni alloy. The oxygen containing species, such as OH, can facilitate the oxidation of ethanol and its intermediates.

Table 2. XPS peak location and the content of different Ni species in the Ni NAs and Ni@Pd-Ni NAs

Ni NAs Ni@Pd-Ni NAs

Ni Position/ eV 855.46

NiO Position/ eV 856.99

Ni(OH)2 Position/ eV 861.42

Ni Content /% 42.28

NiO Content/ % 14.49

Ni(OH)2 Content/ % 43.23

855.55

857.08

861.46

39.48

16.14

44.38

am2

g–1

5



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tion of reactants.44 The mean pore diameters obtained from the differential pore size distributions are listed in Table S1. Notably, the mean pore diameter with longer replacement time increases.

palladium on the surface that provides active sites) of seven Ni@Pd-Ni NAs samples. The Ni@Pd-Ni NAs sample with an repalcement time of 2,000 s has the largest ECSA (64.4 m2 g–1Pd), which is also much higher than most of the catalysts in Table 1. Moreover, this sample reaches a best Pd utilization of 14.38% (see the calculation steps of electroactive Pd % in Ref. 38).

Figure 4d reveal the peak catalytic current density (jP) of "7,200 s" sample in one CV cycle is the largest. After normalized to ECSA, the peak current of “2,000 s” sample is ~0.97 mA cm–2, slightly lower than that of “7,200 s” sample and much higher than other five samples (Figure S3). However, the catalytic stability of “7,200 s” sample is unfortunately inferior to that of “2,000 s” sample (Figure 4e). This phenomenon can be well explained by the following: Ethanol oxidation on Pd-based catalyst is split into many steps, where the combination of Pd-CH3COads intermediate and M-OHads (Pd-CH3COads + M-OHadsPdCH3COOH + M, M: Pd or other metal) is thought to be the rate determining step.20, 38, 45 As reported, the OH species adsorbed on Ni sites benefits the removal of CH3COads intermediate to some extent. Additionaly, the “7,200 s” sample has the most Pd sites that exposed on the electrode surface, which may lead to the decreased Ni

The N2-adsorption-desorption isotherm properties for all of the prepared samples are studied. Unlike the electrochemical adsorption (coulometric method used in this study), which can only obtain the surface area that originated from Pd atoms; the physical adsorption method can obtain the surface area of the whole sample. Clearly, all of the eight samples exhibit IV type adsorption–desorption isotherms with a hysteresis loop at P/P0> 0.45, indicating the existence of mesoporous structure (Figure S2).43 The surface areas calculated by using the Brunauer–Emmett– Teller (BET) model are summarized in Table S1. The surface area of “2,000 s” sample is as high as 29.74 m2 per 1.01.0 cm2 geometric surface area, which can provide a sufficient electrode–electrolyte interface for the adsorp-

–1

–1

–1

Figure 4. (a) OCP recording of the Ni@Pd-Ni NAs electrode in 6.0 mmol L HCl and 6.0 mmol L HCl/ 0.5 mmol L PdCl2 solutions, respectively. (b, c) ECSA and Electroactive Pd % of A, B, C, D, E, F and G samples. (d) CV curves and forward peak -1 -1 –1 current plots (jp, the inset) of A, B, C, D, E, F and G samples in 1.0 mol L KOH/0.5 mol L ethanol, scan rate: 50 mV s . (e) For-1 -1 –1 ward jp retention of A, B, C, D, E, F and G samples for 50 cycles in 1.0 mol L KOH/0.5 mol L ethanol, scan rate: 50 mV s . The replacement time of A, B, C, D, E, F and G sample is 100, 200, 300, 400, 500, 2,000 and 7,200 s, respectively. 6


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based catalyst is CH3COO– (the reaction mechanism within losing of 4 electrons is summarized in Figure S9), which is manifested by the following reaction equation:32,

sites. Therefore, the “7,200 s”sample that with excessive Pd sites and little Ni sites, will hinder the removal process, and thus cause its poorer catalytic stability than that of the “2,000 s” sample. The “2,000 s”sample exhibited the most excellent catalytic stability (jP retention after 50 CV cycles: 96.7%), even greater than that of the reported PdP/MWCNT catalyst.46 For long-term consideration, 2,000 s is the optimal replacement time.

38,45

CH3CH2OH + 5OH––4e–CH3COO– + 4H2O

(1)

–

As is shown, ethanol reacts with OH in a fixed molar ratio. Either too high or too low ratio has a negative effect on the reaction velocity. Besides, an excess of ethanol would lead to the formation of more strongly-adsorbed CH3COads species, and thus mitigate the catalytic current. Back to our experiment results, it is concluded that on the basis of 1.0 mol L–1 KOH, 0.5 mol L–1 ethanol is the most suitable. CV curves at different scan rates are shown in Figure 5e. With the increasing scan rate, the forward jP is distinctly increased as well. The forward peak potentials (EP) vs. ln of scan rates (v) are plotted in Figure 5f. Their linear relationship demonstrates methanol electrooxidation is irreversible.32 The plots of jP vs. v1/2 also exhibit a linear relation, which signifies methanol electrooxidation on Ni@Pd-Ni NAs is controlled by mass transfer (Figure 5g).32 Since ethanol electro-oxidation is irreversible, its current density can be formulated as follows:

Mechanism and Kinetics of ethanol electro-oxidation. The mechanism of ethanol electro-oxidation focused on the optimized Ni@Pd-Ni NAs is investigated. Figure 5a compares the CV curves of Ni NAs and Ni@Pd-Ni NAs in 1.0 mol L–1 KOH solution. The quasi-reversible peaks of 3 and 3’, 4 and 4’ are associated with the electrochemical conversion of Ni(OH)2 and NiOOH. Ni@Pd-Ni NAs electrode also shows other characteristic electrochemical signals as labeled by 1, 1’, 1’’ and 2. Signal 1’ and 1’’ include the reactions of hydrogen adsorption, absorption and hydrogen evolution. Peak 1 represents the hydrogen desorption reaction. Importantly, peak 2 originates from the reduction of Pd-O (Pd-O + H2O + 2e–Pd + 2OH–). By combining with the Pd mass loading, the coulombic charge of peak 2 can be used to calculate the ECSA of Ni@Pd-Ni NAs electrode (Figure S4, S5and S6, Supporting Information).45

j  nFk  exp

Figure 5b displays the CV curves of Ni NAs and optimized Ni@Pd-Ni NAs electrode in the solution containing 1.0 mol L–1 KOH and 0.5 mol L–1 ethanol. Ni NAs electrode virtually shows no catalytic activity in the potential range. In contrast, Ni@Pd-Ni NAs electrode gives two oxidation peaks (forward and backward peak). These phenomenon indicates that Pd is the effective catalyst in the potential range. Only when increasing the upper potential over 0.35 V, Ni expresses its catalytic activity toward ethanol through the E-C coupline mechanism (Figure S7). In the forward scan, the oxidation current is gradually increased to the highest value (jP), and then goes downhill. From Figure 5c, it is observed that Pd-O (Pd + 2OH–Pd-O + H2O+ 2e–) formed at the potentials where the catalytic current drops.38 Such phenomenon indicates the downslope catalytic current after the forward peak is owing to the increasing generation of Pd-O, instead of Pd-CH3CH2OHads.32, 47 In the backward scan, a sharp current peak at ~-0.3 V is witnessed, the location of which is coincidently the same as that of Pd-O reduction peak. This directs us to reasonably assume that Pd sites were recovered, and then the ethanol molecule and intermediates were adsorbed again to release electrons.

－

G RT

(2)

a b cethanol cKOH

Where n is the number of electron transferred. F is the Faraday constant. k is the standard rate constant. R, T and G are the ideal gas constant, Kelvin temperature and activation energy, respectively. a and b are the reaction orders of ethanol and KOH, respectively. Figure 5h shows the plots as well as the linear fittings of ln j vs. ln cethanol, where the j values are the data fetched at -0.3, -0.4 and 0.5 V in Figure S8 (Supporting Information). As seen, all the slopes approach to 1, affirmatively implying that ethanol reacts in the 1st order. By holding the concentrations of ethanol and KOH, the apparent activation energy (G) can be obtained according to the following transformation of Equation 2: ln j  －

G 1  A R T

(3)

Where A is a constant, R equals to 8.314 J mol–1 K–1. Figure 5i is a section of forward CV scan at different temperatures and a relatively low scan rate of 20 mV s–1. The j values are the data fetched at -0.34 V where the reaction is under kinetic control. The plots of ln j vs. 1/T are fitted linearly in the inset. After multiplying the slope by R, the apparent activation energy is calculated to be 11.1 kJ mol–1. The apparent activation energy of Ni@Pd-Ni NAs is significantly lower than those reported on Pd/C (26.6 kJ mol– 1 48 ), Pd-Pb/C (27.87 kJ mol–1),49 Pd-Bi/C (19.68 kJ mol–1),14 and Pd-Ni-Cu-P metallic glass nanowires (17.12 kJ mol–1),50 which means the higher reactivity toward ethanol electrooxidation. The intrinsic activation energy of ethanol electro-oxidation on the Ni@Pd-Ni NAs is obtained according to the Tafel curves (Figure S10 and Table S2). The linear fitting results of lgj0 vs. 1/T show the intrinsic activation energy is 134.1 kJ mol–1.

The kinetics of ethanol electro-oxidation focused on the optimal Ni@Pd-Ni NAs electrode is further discussed. Figure S8 presents the CV responses when the ethanol concentration is varied (Supporting Information). As seen in Figure 5d, the forward peak current (jP) was raised as the ethanol concentration increased from 0.1 to 0.5 mol L – 1 . Higher concentration did not improve the catalytic activity. On the contrary, the jP is rapidly declined from 0.5 to 0.7 mol L–1. It has been generally accepted that the predominant product of ethanol electro-oxidation on Pd7



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–1

Figure 5. (a, b) CV curves of Ni NAs and optimized Ni@Pd-Ni NAs electrodes in 1.0 mol L KOH solutions without and with 0.5 –1 –1 –1 mol L ethanol, respectively. (c) Amplified CV curves of Ni NAs in 1.0 mol L KOH, Ni@Pd-Ni NAs electrode in 1.0 mol L KOH –1 –1 with and without 0.5 mol L ethanol. (d) Plots of forward jP vs. cethanol. (e) CV curves of Ni@Pd-Ni NAs electrode in 1.0 mol L –1 –1 KOH/0.5 mol L ethanol at the scan rate of x V s (x=0.05, 0.10, 0.15, 0.20 and 0.25). (f) Plots of forward EP vs. ln v. (g) Plots of 1/2 forward jP vs. v . (h) Plots of ln j vs. ln cethanol at -0.3,-0.4 and -0.5 V obtained from Figure S8. (i) A section of forward scan at 303, –1 –1 –1 313, 323 and 333 K in 1.0 mol L KOH/0.5 mol L ethanol, scan rate: 20 mV s . The inset is the plots of ln j vs. 1/T.

Electrochemical impedance spectroscopy (EIS) is a sensitive method to investigate the kinetics of small molecule electro-oxidation reactions, such as methanol, urea, 2isopropanol and glucose electro-oxidation.51-54 Figure 6a shows the Nyquist plots of Ni@Pd-Ni NAs catalyst in 1.0 mol L–1 KOH/0.5 mol L–1 ethanol solution. Several potentials during the forward CV scan were selected as the polarization potentials, and under each potential, the polarization time was set to 600 s to reach a quasi-stable state. As observed, the Nyquist arc gradually shrunk from -0.7 to -0.2 V and then expanded over -0.2 V. After fitting by the typical Randles circuit,55 the charge transfer resistance (Rct) was obtained and the results are shown in Figure 6b.

The trend of Rct is accord with the reverse trend of the CV current in forward scan. Rct reached its lowest at -0.2 V, and correspondingly the catalytic current at -0.2 V was the highest. It is worth mentioning that ethanol electro-oxidation on Pt/C catalyst demonstrated an interesting Nyquist arc in the second quadrant, which is caused by the poisoning of CO (the strongly adsorbed intermediate).56 However, in our case, the Nyquist arcs always locate in the first quadrant, which means a very different reaction pathway when compared with Pt-based catalyst. Jiang et al. pointed out that Pd-Ni-P electrode catalyzed ethanol oxi8


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–1

–1

Figure 6. (a) Nyquist plots of the Ni@Pd-Ni NAs in 1.0 mol L KOH/0.5 mol L ethanol. The results imply the CO poisoning does not occur on the Ni@Pd-Ni NAs electrode. (b) A section of forward scan obtained from Figure 4d and plots of Rct vs. –1 Potentials. (c) CA curves of Ni@Pd-Ni NAs catalyst at different potentials and temperatures. Electrolyte: 1.0 mol L KOH + 0.5 –1 mol L ethanol.

dation via a 4-electron process to CH3COO– (Equation 1), implying there was no breakage of C-C bond.38 In other words, the CO poisoning does not occur on the Pd-based catalyst. Hence, in a certain respect, Pd-based material is more suitable as a high-durability catalyst for ethanol electro-oxidation.

self-supported Ni@Pd-Ni nanowire is composed of the solid Ni core (~99.5 nm in diameter) and gauze-like Pd-Ni alloy shell (~40 nm thick). The optimized Ni@Pd-Ni NAs electrode has a large ECSA of 64.4 m2 g–1Pd. In addition, the Ni in the Pd-Ni alloy offers oxygen-containing species, which facilitates the removal of ethanol molecules or intermediates. As a result, the catalyst demonstrated a high peak current density of 622 A g–1Pd and 96.7% retention after 50 cycles. The presented carbon- and binder-free Ni@Pd-Ni NAs electrode with open and low-tortuosity micro-structure can avoid the issues of ECSA degradation and active sites blocking, which is a promising anode catalyst for DEFC.

We have further studied the catalytic activity and durability of Ni@Pd-Ni NAs by chronoamperometric (CA) experiments under different conditions, and the results are shown in the following Figure 6c. Only one Ni@Pd-Ni NAs electrode was used throughout the 14400 s’ CA test. Due to the faster reaction velocity, the catalytic current is gradually increased with the increasing operating temperature. Four polarization potentials before (-0.5 V) and after (-0.1 V) the CV peaks, right at the backward peak (0.3 V) and forward peak (-0.2 V) are selected, respectively. The CA currents at -0.3 and -0.2 V are much higher than those at -0.5 and -0.1 V, well corresponded to the CV currents in Figure 5b. In general, the Ni@Pd-Ni NAs catalyst expresses remarkable stability at low temperature and not at CV peak location. By contrast, the CA currents at higher temperature and right at CV peak locations show slight degradation, which might be caused by the rapid evaporation of ethanol at high temperature and/or the excessive depletion of reactants near the electrode-electrolyte interface.

4. EXPERIMENTAL SECTION Reagents and materials. Nickel sulfate hexahydrate (NiSO46H2O), nickel chloride hexahydrate (NiCl26H2O), boric acid (H3BO3), lauryl sodium sulfate, saccharin, Potassium hydroxide (KOH) and hydrochloric acid (HCl) were purchased from Shanghai Aladdin biological technology Co,. Ltd (China). Palladium chloride (PdCl2), corn ethanol (synthesized by corn fermentation) and electrolytic nickel sheet were obtained from Sinopharm Chemical Reagent Co., Ltd (China). The track-etched polycarbonate template (channel diameter: 50 nm, thickness: 6 m) was purchased from General Electric Company (USA). The low-melting point alloy (TinBismuth based alloy) was obtained from Knight Welding Material Co., Ltd (China). All reagents were in analytical grade and directly used without further treatment. Thesolutions throughout were made up by the ultra-pure water (18.2 M cm).

3. CONCLUSION In our work, we designed a carbon- and binder-free coreshell Ni@Pd-Ni NAs electrode for high-efficiency ethanol electro-oxidation using the facile template-assistant electrodeposition and solution-based alloying methods. The 9



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HCl/PdCl2 solution lasting for 2,000 s except N2 adsorption/desorption and ICP measurements.

Fabrication of Ni@Pd-Ni nanowire arrays catalyst. The preparation steps of Ni@Pd-Ni nanowire arrays (Ni@Pd-Ni NAs) are depicted in Scheme 1. The lowmelting point alloy was evenly pasted on one side of the porous polycarbonate template in an oven at 90oC. The cooled alloy acted as the conductor in the following electrodeposition process. The physical photos of the alloy in its melting state, polycarbonate template and the cooled Alloy-Template electrode were displayed in Figure S11 (Supporting Information). The electrodeposition was run by a constant current procedure (196 mA, ~4 hours) on a Potentiostat and in a two-electrode electrolytic cell. The Alloy-Template electrode and the electrolytic nickel sheet worked as the anode and cathode, respectively. The electrodeposition bath was composed of NiSO46H2O (270 g L–1), NiCl26H2O (50 g L–1), H3BO3 (30 g L–1), saccharin (1.0 g L–1) and sodium lauryl sulfate (0.05 g L–1), which was almost the same as the Watt’s nickel formula. During electrodeposition, the bath was kept stirring and maintained at 55oC. Once the electrodeposition started, the white template exposed to the solution suddenly turned to black in the initial dozens of seconds, signifying the imbedding of nickel into the nanochannels. Deposition proceeded on the top of the nanowires until a layer of Ni was formed. Afterwards, the conductor was peeled off by hand and the template was dissolved in the warmed 6.0 mol L–1 KOH to free Ni nanowires.

Electrochemical Evaluations. The electrochemical measurements were carried on a CHI-604E electrochemical workstation and in a conventional three-electrode electrolytic cell. Ni NAs or Ni@Pd-Ni NAs electrode, platinum (1.02.0 cm2) and Ag/AgCl electrode (0.198 V vs. SHE) were selected as the working, counter and reference electrodes, respectively. Electrochemical impedance spectra were obtained by setting the frequency range of sinusoidal wave as 105~10–2 Hz and with an amplitude of 5 mV. Prior to testing, the solutions containing KOH or KOH/ethanol were deaerated by nitrogen gas flow.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” Additional SEM-EDS and N2 adsorption/desorption characterizations, true surface area, Pd mass loading, detailed calculation steps for ECSA and intrinsic activation energy, CV curves with extended potential range, CV curves normalized by ECSA, CV curves under different ethanol concentration, Scheme for ethanol electro-oxidation process, physical photos of low-melting point alloy, polycarbonate template and cooled Alloy-Template electrode.

AUTHOR INFORMATION

Ni@Pd-Ni NAs catalyst was then fabricated via a chemical replacement method. To be specific, Ni NAs electrode was immersed in 6.0 mmol L–1 HCl and 0.5 mmol L–1 PdCl2, where a replacement reaction took place as following:

Corresponding Author

PdCl4 + Ni Pd + Ni + 4Cl

All authors have given approval to the final version of the manuscript.

2–

2+

–

*Fen Guo. Email: [email protected]

Author Contributions

(4)

A portion of Ni atoms at the surface of Ni nanowires were replaced by Pd, leading to the alloying of Pd with Ni (as evidenced by the detailed physicochemical characterization). The as-prepared Ni@Pd-Ni nanowire is composed of solid Ni core and gauze-like Pd-Ni alloy shell.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the financial support by the National Natural Science Foundation of China (21206129),the Open Fund from Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials(WKDM201710) and the funding from Youth Science and Technology Backbone Training Program of Wuhan University of Science and Technology (250089). Yiju Li also acknowledges the financial support of China Scholar Council.

Physico-chemical Characterizations. The morphologies of Ni NAs and Ni@Pd-Ni NAs electrodes were investigated by field emission scanning electron microscope (FESEM, Hitachi-8010, Japan). Transmission electron microscopy (TEM) images, high resolution-TEM images and the element distributions of single Ni@Pd-Ni nanowire were obtained with JEOL-2100F (Japan) microscope. The crystalline structures of the samples were examined by Xray diffractometer (XRD, Philips-X'Pert PRO MPD, Netherlands) with Cu K radiation. X-ray photoelectron spectroscopy (XPS, Thermo Fisher-Escalab250Xi, UK) was performed to get information of the valence and content of different elements. Pd mass loadings were determined by inductively coupled plasma mass spectrometer (ICP, ThemoElemental-IRIS Advantage Radial, USA). N2adsorption/desorption measurements were characterized byN2 adsorption at 77 K on an ASAP 2460 (Micromeritics, USA). All samples were degassed at 150 C under vacuum for 10 h before the measurements. All the characterized Ni@Pd-Ni NAs electrodes were those immersed in

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Carbon- and Binder-free Core-Shell Nanowire Arrays for Efficient

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