Nickel nanowire arrays electrode as an efficient catalyst for urea peroxide electro-oxidation in alkaline media

Accepted Manuscript Title: Nickel nanowire arrays electrode as an efficient catalyst for urea peroxide electro-oxidation in alkaline media Author: Fen Guo Ke Ye Mengmeng Du Kui Cheng Yinyi Gao Guiling Wang Dianxue Cao PII: DOI: Reference:

S0013-4686(15)31044-6 http://dx.doi.org/doi:10.1016/j.electacta.2015.12.118 EA 26267

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-9-2015 16-12-2015 18-12-2015

Please cite this article as: Fen Guo, Ke Ye, Mengmeng Du, Kui Cheng, Yinyi Gao, Guiling Wang, Dianxue Cao, Nickel nanowire arrays electrode as an efficient catalyst for urea peroxide electro-oxidation in alkaline media, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.12.118 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nickel nanowire arrays electrode as an efficient catalyst for urea peroxide electro-oxidation in alkaline media

Fen Guo, Ke Ye, Mengmeng Du, Kui Cheng, Yinyi Gao, Guiling Wang, Dianxue Cao

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

 Corresponding authors. Tel: 00-86-451-82589036, Fax: 00-86-451-82589036, E-mail addresses: [email protected] (K. Ye); [email protected] (D. Cao). 1

Highlights:  Self-supporting

nickel

NWAs

electrode

was

templated

electroformed.  The reaction pathways of urea peroxide electro-oxidation were raised.  The KOH and urea peroxide concentrations were optimized.  Nickel NWAs electrode shows high stability and activity towards urea peroxide.

2

Abstract Urea peroxide as a solid-state fuel instead of liquid hydrogen peroxide is electro-catalytic oxidized by nickel nanowire arrays (NWAs) electrode. Nickel NWAs electrode is prepared by galvanostatically electrodepositing nickel into the pores and over-plating on the surface of polycarbonate template (PCT) from nickel bath solution. The NWAs are obtained after dissolving PCT in methylene chloride. Results show that urea peroxide decomposes into hydrogen peroxide and urea in aqueous solutions, then is electro-oxidized via two processes: i, hydrogen peroxide electro-oxidation at lower potential region; ii, both hydrogen peroxide and urea electro-oxidation at higher potential region. The onset oxidation potential and current density of urea peroxide electro-oxidation in 6.0 mol L-1 KOH and 1.0 mol L-1 urea peroxide are ca. -0.2 V and 700 mA cm-2 at 0.6 V, respectively. The nickel NWAs electrode demonstrates excellent stability for urea peroxide electro-oxidation under different polarization potentials.

Keywords: urea peroxide; nickel nanowire arrays; polycarbonate template; electro-oxidation; fuel cell.

3

1. Introduction Urea

peroxide

(CO(NH2)2•H2O2)

is

a

kind

of

adduct

synthesized by the reaction of urea with hydrogen peroxide. Urea holds together with hydrogen peroxide through hydrogen bond between nitrogen atom in urea and hydrogen atom in hydrogen peroxide [1, 2]. As an anhydrous source of hydrogen peroxide, urea peroxide has been used in a variety of oxidative transformations inorganic solvents [2-5]. Moreover, urea peroxide has a dual property in agriculture: hydrogen peroxide delivers oxygen and urea supplies fertilizer [1]. In recent decades, the electro-oxidation of hydrogen peroxide (H2O2) has been studied by many researchers [6-9]. Although H2O2 electro-oxidation on various catalysts reveals outstanding performance, its natures of liquid state, strong oxidability, high corrosivity and instability hinder itself as a universally applicable anode fuel. It is necessary to explore a kind of fuel that can meet both the safety and high electro-oxidation performance. Urea peroxide, in which urea stabilized with hydrogen peroxide, is inexpensive and mild. It is in solid-state and thus easy to preserve and transport. Moreover, urea peroxide is highly soluble and stable in the water with a pH value of ~7 [10]. Based on all these merits, urea peroxide is a superb candidate for anode fuel of fuel cells. Nickel has been utilized as an effective electro-catalyst for a wide range of small compounds electro-oxidation in alkaline medium, such as 4

methanol [11, 12], glucose [13], hydrazine [14], urea [15, 16] and hydrogen peroxide [7, 8]. Yet, there are diverse expressions about how small compounds are electro-oxidized on nickel catalyst. Usually, Ni(OH)2/NiOOH shift reaction is used to describe their electro-catalytic mechanism [12]. Ni(OH)2 is electro-oxidized to NiOOH as potential sweeps accompanying with a chemical reaction that small compound reduces NiOOH back to Ni(OH)2, which is illustrated in the following equations: Ni(OH)2 + OH− - e− ⇄ NiOOH + H2O

Electrochemical (1) reaction

NiOOH + small compound →Ni(OH)2 + product Chemical

(2)

reaction A large number of studies have been focused on preparing nanowire arrays (NWAs) because of their high specific surface area and anisotropic magnetic properties [17-20]. Template electro-synthesis is a simple and versatile method that has been widely used to prepare nanowire arrays [21]. Anodized aluminum oxide (AAO) and track etched polycarbonate membranes are mostly two kinds of templates to prepare NWAs with defined morphology and size. Polymer membranes can face less impact when applied in acidic or alkaline solutions, which makes polycarbonate template (PCT) more commonly used than AAO. For electrodeposition, PCT has to be coated with a conductive metal layer on one side in ways 5

of thermal evaporation, ion sputtering or removable liquid mercury [22, 23]. Others abandoned conductive layer, instead, grinded out a convex metal for better contacting with PCT [24]. However, those methods are still costly and cumbersome. Inspired by the removable liquid mercury, the low melting pointing alloy is utilized as the conductive layer for preparing nickel NWAs catalyst in the present work. In this study, urea peroxide electro-oxidation was first investigated as

anode

fuel

on

the

nickel

NWAs

electrode.

The

unique

three-dimensional electrode contains two parts: nickel support and nickel NWAs. Nano arrays structure can supply plenty of active sites for urea peroxide electro-oxidation and allow reactants or products to diffuse rapidly. The electro-oxidation pathways of urea peroxide were also investigated based on the electrochemical behaviors of four kinds of fuels (urea, H2O2, urea&H2O2 and urea peroxide). Researches show that the nickel NWAs electrode exhibited high catalytic activity and excellent stability for urea peroxide electro-oxidation in alkaline media. 2. Experimental 2.1. Preparation of nickel NWAs electrode Polycarbonate membrane (50 nm pore diameter and 6 μm thickness) coated with PVP (polyvinylpyrrolidone) as wetting agent was purchased from Whatman Ltd. Bismuth-base alloy with a low melting pointing of 65 C was put in an oven. Above the melting pointing, the bismuth-base 6

alloy was in liquid state and then brushed onto one side of the PCT. After cooling down to the room temperature, the solid alloy attached with PCT was achieved and ready for electrodeposition. The nickel bath solution consisted of nickel sulfate, nickel chloride, boric acid, saccharin and sodium lauryl sulfate in proper concentrations [25, 26]. Boric acid helps to produce smoother and more ductile deposits; saccharine aids in film stress reduction; sodium lauryl sulfate is a brightening agent [27]. Bismuth-base alloy with PCT, electrolytic nickel sheet and saturated Ag/AgCl electrode served as working electrode, counter electrode and reference electrode, respectively. A current density of -40 mA cm-2 was constantly applied to the working electrode for about 4 hours. Appropriate magnetic stirring and a water bath temperature of 55 C were assisted in the electrodeposition apparatus. The one step electrodeposition process is lively shown in Fig. 1a. Bismuth-base alloy functioned as the current collector. Nickel ions were firstly electrodeposited into the ordered channels of PCT. After the pores were filled with nickel, the electrodeposition process continued to form a nickel layer on the surface of PCT, which is called over-plating. After electrodeposition, the bismuth-base alloy was peeled off from the PCT. Then, the electrode containing PCT was carefully placed in methylene chloride. It just took about 1 minute to dissolve PCT and free the NWAs. The obtained nickel NWAs electrode was further washed by ethanol for 7

several times and dried in air. 2.2. Characterization of nickel NWAs electrode and electrochemical tests for urea peroxide electro-oxidation The structure of nickel NWAs electrode was analyzed using an X-ray diffractometer (XRD, Rigaku TTR III) with Cu Kα radiation (λ=0.1514178 nm). The morphology was characterized by a scanning electron microscope (SEM, JEOL JSM-6480) and a transmission electron microscope (TEM, JEOL JEM-2010F). For TEM characterization, the obtained nickel NWAs were scratched down from nickel support and dispersed in ethanol by sonication. Electrochemical measurements of cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) for urea peroxide electro-oxidation were performed in a typical three-electrode electrochemical cell. The as-prepared nickel NWAs electrode, platinum foil and a saturated Ag/AgCl electrode (0.1981 V vs. SHE) were used as working electrode, counter electrode and reference electrode, respectively. All potentials in this work were referred to this reference electrode. The reported current densities were normalized by geometrical area of the electrode. For comparison, both the active surface areas of nickel sheet and nickel NWAs electrodes were estimated by measuring the electrochemical double-layer capacitance. Prior to use, nickel sheet was polished with alumina powder to 0.3 μm, and washed by 8

water thoroughly. The EIS tests were operated after an equilibrium time of 300 s at different fixed potentials. The frequency region was 100 kHz ~ 10 mHz with 5 mV potential amplitude. 3. Results and discussion 3.1. Characterization of the nickel NWAs electrode The micromorphology of nickel NWAs electrode was characterized by SEM. Figs. 1b and c show the top views of nickel NWAs electrode. As seen in Fig. 1b, the nanowires arrayed like the roof ridges with spaces between every “roof”. The spaces between every “roof” and single nanowire can offer reactants or products easy access to transfer into or away from the electrode. At higher magnification in Fig.1c, every single nanowire stood on and contacted tightly with the nickel support. The nanowires in sight were uniform in length and diameter, and they were intersected with each other in random directions. The inset of Fig. 1b is the side view of nickel NWAs electrode before dissolving PCT. The white wires are thought as nickel nanowires because of the electrical conductivity difference between nickel and polycarbonate. The nanowires were embedded in PCT and contacted with the rough nickel support. As known, the channels of PCT are created by means of bombarding the polycarbonate membrane with accelerating ion beam to produce defect tracks in random space. The angle between channels and surface can be as large as 34 [21]. As a result, nanowires themselves were not in one 9

direction and were not vertical with respect to the nickel support. For further observation, single nanowire was analyzed by TEM. The inset of Fig. 1c evidenced the diameter of nanowire was ~50 nm, which was in accordance with the pore diameter of PCT. To confirm both compositions of the electrode, nickel support and nickel NWAs were separately monitored by XRD. From the XRD patterns (Fig.2), it is seen that except for the broad peak of polycarbonate between 10 and 40, other diffraction peaks for nickel NWAs were indexed to the face-centered cubic phase of nickel (JCPDS No. 04-0850) and no other purity peaks were observed. Both nickel support and nickel NWAs had three peaks of 44.51°, 51.85° and 76.37°, corresponding well to the (111), (200) and (220) planes of nickel. 3.2. Surface electrochemical analysis of nickel NWAs electrode The continuous cyclic voltammograms for nickel NWAs electrode in 2.0 mol L-1 KOH, measured for the purpose of activating the catalyst, are shown in Fig. 3a. In the first scan, two anodic peaks and one cathodic peak appeared at -0.66 V (peak a1), -0.51 V (peak a2) and -1.09 V (peak c). Following the previous cases of literatures [28-32], the peaks were attributed to the following reactions: a1: Hads+ abs→ H+ + e-

(3)

a2 and c: Ni+2OH-⇄ α-Ni(OH)2+ 2e-

(4)

α-Ni(OH)2 is unstable in alkaline electrolyte and will dehydrate into 10

β-Ni(OH)2. It is proposed that β-Ni(OH)2 cannot be reduced to nickel, thus, except for the first scan, the reduction peak of c at -1.09 V disappeared in the subsequent scans. There was a pair of quasi reversible peaks between 0 V and 0.5 V, corresponding to the reaction of Ni(OH)2/NiOOH. A progressive increase of anodic and cathodic current density was observed (seen from the rising trend of two red arrows) when continuous scans were carried out. XPS analysis [30, 33, 34] pointed out that nickel metal in the air or aqueous solution conditions was instantaneously covered by NiO and Ni(OH)2 from inside to outside. The gradual increase of current density in the region of Ni(OH)2/NiOOH was due to that OH- ions would penetrate into nickel NWAs electrode at more positive potential, afterwards, Ni(OH)2 was enriched because Ni or NiO were transformed to Ni(OH)2 [9, 29, 35]. The active surface areas of nickel NWAs and nickel sheet electrode were estimated by cyclic voltammetry in a narrow potential range. The current density (j) in the middle of potential range was plotted as the function of scan rate (v). Only nonfaradaic path is assumed during the range and a formula to calculate the electric double-layer capacitance (Cd) can be used under this condition [36-39]: d j= Cdd v

(5)

According to the references [38, 39], specific double-layer capacitance(C*) of nickel in alkaline electrolyte is 60 μF cm-2. The result 11

active surface area (A) in a nominal area of 1.0 1.0 cm can be obtained: Aac = Cd / C*

(6)

Cyclic voltammetry was applied on the nickel NWAs electrode in 5.0 mol L-1 KOH at different scan rates between 0 V and 0.2 V in Fig. 3b. The negative current density at 0.1 V was plotted with the scan rate. Linear fitting in Fig. 3c was adopted to attain the slope, i.e., Cd. After dividing Cd by C*, the active surface area of nickel NWAs electrode was 26.33 cm2. It should be noted that the measured active surface area is not an absolute value due to that the surface area determined by electrochemical capacitance measurements was found to depend on pH and/or voltage range, or other challenges as reviewed by Trasatti and Petrii [36]. As a result, we additionally measured the active surface area of nickel sheet electrode in a geometrical surface area of 1.01.0 cm under the same conditions. Fig. 3c shows the linear fitting of current density versus scan rate of nickel sheet electrode. The active surface area turned out to be 2.75 cm2. The active surface area of nickel NWAs was 8.57 times as larger as that of nickel sheet. Fig. 3d displays cyclic voltammograms of nickel NWAs and sheet electrode in 2.0 mol L-1 KOH and 0.04 mol L-1 urea peroxide at scan rate of 10 mV s-1. The peak current densities of nickel NWAs was 55.6 mA cm-2, up to 4.38 times as large as that of nickel sheet electrode. After the peak current densities are 12

divided by active surface area, true peak current density of nickel NWAs was not increased by comparison with that of nickel sheet electrode. It shows that the true peak current density was not increased linearly with the increment of active surface area because not all active sites of nickel NWAs are utilized for urea peroxide electro-oxidation. However, in relation to same apparent surface area of 1.0  1.0 cm, the nickel NWAs has a larger active surface area, able to provide much more active sites than that of nickel sheet. 3.3. Urea peroxide electro-oxidation on nickel NWAs electrode In order to get deep insight into how electro-oxidation of urea peroxide happens on nickel NWAs electrode, four kinds of fuel: urea, H2O2, urea&H2O2, urea peroxide were studied by CV and EIS techniques. Since urea peroxide can decompose into H2O2 and urea in aqueous solutions at a mole ratio of 1:1, both urea and H2O2 concentrations were kept the same as that of urea peroxide. Fig. 4a displays cyclic voltammograms of nickel NWAs electrode in 2.0 mol L-1 KOH with and without 0.04 mol L-1 urea, H2O2, urea&H2O2 and urea peroxide, respectively. As seen, the nickel NWAs electrode in 2.0 mol L-1 KOH showed a pair of peaks, referring to the Ni(OH)2/NiOOH reaction. The addition of urea facilitated oxidation peak and suppressed the reduction peak at same time. Urea oxidation involved a push for NiOOH formation. This phenomenon was consistent with the electro-catalytic oxidation 13

mechanism mentioned before (Eq. (1) and (2)). Botte et al. [15, 40, 41] stated that this indirect electro-oxidation process of urea on nickel was E-C (electrochemical-chemical) mechanism. H2O2 electro-oxidation on nickel NWAs electrode started at more negative potential (~ -0.2 V) than urea. The oxidation current density gradually increased until a pair of peaks emerged in the potential region of Ni(OH)2/NiOOH reaction. This implied that H2O2 electro-oxidation and Ni(OH)2/NiOOH reactions both existed during the whole potential scan. 0.04 mol L-1 urea and 0.04 mol L-1 H2O2 was dissolved together in the 2.0 mol L-1 KOH for CV test. According to the curve, only sole H2O2 electro-oxidation reaction proceeded before the onset oxidation potential of Ni(OH)2/NiOOH. Later, the sudden rising oxidation peak was assigned to the occurrence of urea electro-oxidation, declaring both H2O2 and urea electro-oxidation reactions happened at higher potential region. Urea peroxide was catalytically electro-oxidized by nickel NWAs electrode as well. The CV curve of urea peroxide resembled that with urea&H2O2, proving that after dissolving urea peroxide in KOH, the hydrogen bond was broken, and then H2O2 and urea were separately electro-oxidized in their own ways. The concentration of KOH was varied for the cause of confirming the urea electro-oxidation in urea peroxide happened only after NiOOH formed. In Fig. 4b, the dashed curves showed that as the concentration of KOH increased from 1.0 mol L-1 to 5.0 mol L-1, the oxidation and 14

reduction peaks moved to the negative potential. The potential shift was in first order with respect to the OH- concentration [42]. Higher the KOH concentration, more negative the reduction peak. After adding urea peroxide, there was a one-to-one correspondence between reduction peak of solid curves and that of dashed curves at different KOH concentrations. More than the potential shift of Ni(OH)2/NiOOH, the onset oxidation potential of urea peroxide electro-oxidation also moved negatively as the increase of KOH concentration. Fig. 5 shows the Nyquist plots of nickel NWAs electrode in 2.0 mol L-1 KOH with and without 0.04 mol L-1 urea, H2O2, urea&H2O2 and urea peroxide, respectively. The potential applied on the three-electrode system was not open circuit potential, therefore the parameter of equilibration time was set as 300s in order to reach a quasi-stationary condition before capturing the first EIS data. At 0.1 V in Fig. 5a, the Nyquist graphs of H2O2, urea&H2O2, urea peroxide demonstrated one suppressed semi-circle for each. The frequency at the end of the semi-circle was marked in order to identify different electrochemical reactions. The frequencies of H2O2&urea and urea peroxide were very close to that of H2O2, which illustrated that H2O2 was electro-oxidized at 0.1 V on nickel NWAs electrode for three kinds of fuel. The diameter of semi-circle was charge transfer resistance (Rct), which could be used to evaluate the electrochemical reaction rate. The diameters of H2O2, 15

urea&H2O2 were almost identical with each other and that of urea peroxide was a little bigger. It is indicated that the reaction rates of H2O2, urea&H2O2 mixture were faster than that of urea peroxide, which was in good agreement with CV results in Fig.4a. Urea was not electro-oxidized at 0.1 V and there was only a straight line through all frequency regions, almost coincided with the Nyquist plots without any fuel. In Fig. 5b, the EIS at 0.45 V was more complicated and it was worth a thorough analysis to explain four kinds of fuel electro-oxidation pathways respectively. As shown in Fig. 5b, the semi-circle at lower frequency region in 2.0 mol L-1 KOH represented the oxygen generation reaction. In the solution of 2.0 mol L-1 KOH and 0.04 mol L-1 H2O2, a semi-circle emerged which declared that H2O2 electro-oxidation reaction happened at 0.45 V. The inset of Fig. 5b was the partial enlarged detail of high frequency region. The Nyquist plots tested in 2.0 mol L-1 KOH without any fuel showed a semi-circle, referring to the reaction of Ni(OH)2/NiOOH, which was correspondent to the CV results. After adding four kinds of fuel, semi-circles still existed and the end frequencies were the same as that without any fuel, that was, 5.23 kHz, showing Ni(OH)2/NiOOH transfer reaction presented in parallel with fuel electro-oxidation reaction. Interestingly, the Nyquist plots of urea, urea&H2O2, urea peroxide at lower frequencies fell into second quadrant. The negative real impedance plots implied that when urea was participated in the electro-oxidation 16

reaction,

the

catalyst

was

probably

poisoned

and

thus,

the

rate-determining step (r.d.s) differed at 0.1 V and 0.45 V [43, 44]. Botte et al. [45] studied urea dissociation rates of urea on NiOOH by density function theory (DFT). Three reaction mechanism paths were put forward based on the loss of protons from different amine groups, different types of nitrogen atoms bonding, etc. The final structure NiOOH·CO·N2 was identical for all the pathways, rendering a common mechanism onward. The main common three steps were as follows (M denotes NiOOH ): [M·CO ·N2]ad + OH-→[M·CO· OH]ad + N2 + e-

(7)

[M·CO· OH]ad + OH-→ [M·CO2]ad + H2O + e-

(8)

[M·CO2]ad→ M + CO2

(9)

The rate constants for each step were 7.3×108 L mol-1 s-1, 1.6 L mol-1 s-1 and 4.3×10-65 s-1 successively. The r.d.s was found to be the removal of carbon dioxide with a rate constant of 4.3  10-65 s-1. The catalyst could be deactivated by the surface blockage of CO2 adsorption. Ojani et al [46] studied the methanol electro-oxidation reaction by EIS at different potentials. When the adsorbed intermediate CO passivated the catalyst surface, the location of Nyquist plots transformed from the first quadrant to the second quadrant. On the analogy of this, the appearance of negative real impedance suggested that the desorption of CO2 on the active sites of catalyst surface (Eq. (13)) might be the rate-determining step at 0.45V. Based on the discussion above, the reaction pathways of 17

urea peroxide electro-oxidation are illustrated as below: CO(NH2)2•H2O2→CO(NH2)2 + H2O2

Hydrogen bonds broken:

(10)

at 0.1 V:

H2O2 + 2OH-⇄ 2H2O + O2 + 2e-

(11a)

at 0.45 V:

H2O2 + 2OH-⇄ 2H2O + O2 + 2e-

(11b)

6NiOOH + CO(NH2)2 + 2OH- → 6Ni(OH)2 + CO32- +

(12)

N2 CO2ad + 2OH-→CO32- + H2O

(13)

A direct urea peroxide fuel cell (DUPFC) is composed of urea peroxide in anode and oxygen in cathode. The theoretical open circuit voltage of DUPFC is calculated as 1.059 V, including the hydrogen bonds breakage energy of 29 kJ mol-1 (O—H••• N) in aqueous solution. The solution reactions of DUPFC are as follows: Anode: CO(NH2)2•H2O2(aq) + 8OH-⇄N2 + O2 + CO2 + H2O +8e-, E = -0.658 V vs. SHE

(14)

Cathode: O2 + 2H2O + 4e-⇄ 4OH-, E= 0.401 V vs. SHE

(15)

Overall reaction: CO(NH2)2•H2O2(aq) + O2→ CO2 + N2 + 3H2O, E= 1.059 V According

to

Eq.

(14),

the

(16) products

of

urea

peroxide

electro-oxidation are benign nitrogen, oxygen and carbon dioxide, just as the air composition. The electron transfer number of urea peroxide electro-oxidation reaction is 8, higher than that of urea, H2O2 and 18

methanol electro-oxidation reactions. The mole ratio of urea peroxide and hydroxides should be kept at 1:8 for the optimal electro-catalytic performance. However, besides slight hydrolysis lessens urea peroxide, KOH not only functions as reactants but also the supporting electrolyte. Therefore, it is necessary to explore the best KOH and urea peroxide concentrations. Fig. 6 shows the effect of KOH concentration on the urea peroxide electro-oxidation

performance

at

nickel

NWAs

electrode.

The

concentration of urea peroxide was kept at 0.5 mol L-1. The onset potential was around -0.2 V and slightly reduced with the increase of KOH concentration up to 7.0 mol L-1. The oxidation current density rose from 1.0 mol L-1 to 6.0 mol L-1 and then remained unchanged with further increase of KOH concentration to 7.0 mol L-1. Excess KOH made no contribution to the enhancement of urea peroxide electro-oxidation and 6.0 mol L-1 was considered as the optimal KOH concentration. The appearance of CV curves exhibited a straight line because a large number of gaseous products produced (Eq. (14)), which stirred the solution and made the concentration gradient disappeared. Fig. 7 demonstrates the dependence of urea peroxide concentration on the urea peroxide electro-oxidation performance. The KOH concentration was fixed at the 6.0 mol L-1. At low urea peroxide concentration, the CV curves were obviously made of two parts, referring 19

to H2O2 electro-oxidation reaction at lower potential region as well as both urea and H2O2 electro-oxidation reactions at higher potential region. At 0.08 mol L-1 urea peroxide, there was a visible sharp peak in the reverse scan (dashed circle), which was correlated to the chemical desorption of adsorbed CO2. The adsorbed CO2 reacted with OH-, thus, the blockage active sites regenerated and the current density suddenly increased. The electro-oxidation performance of nickel NWAs electrode was enhanced as the urea peroxide concentration increased from 0.08 mol L-1 to 1.5 mol L-1. However, Δj (current density increment) at one specific potential decreased and even when the concentration of urea peroxide increased from 1.0 mol L-1 to 1.5 mol L-1, the increment of current density at 0.6 V was only 16 mA cm-2, which was much lower than the current density (700 mA cm-2) in 1.0 mol L-1 urea peroxide at 0.6 V. Since Δj from 1.0 mol L-1 to 1.5 mol L-1 was too low to compensate the urea peroxide costs, the optimal urea peroxide concentration was 1.0 mol L-1. Finally, in order to evaluate the stability of the nickel NWAs electrode for urea peroxide electro-oxidation, nickel NWAs electrode was examined in 6.0 mol L-1 KOH and 1.0 mol L-1 urea peroxide at different polarization potentials and the results are depicted in Fig. 8a. At -0.05 V and 0.1 V, the current density remained nearly constant within 1200s. The current densities of -0.05 V and 0.1 V were separately 82.55 mA cm-2 and 264.01 mA cm-2. Notably, the chronoamperometric curve at 0.4 V 20

displayed an upward trend rather than a degradation trend caused by the fuel consumption. The current density rose from 450.65 mA cm-2 at 6 s to 487.24 mA cm-2 at 1200 s. It was possibly due to that NiOOH with higher electrical conductivity than Ni(OH)2 [30] was continuously generated at 0.4 V, which accelerated the electron transfer of urea peroxide electro-oxidation reaction. After the CA tests, the nickel nanowires were also scratched down from nickel support for TEM characterization. Inset of Fig. 8b is the TEM image of nanowires after long-time polarizations. As can be seen, the nanowire morphology was almost unchanged compared with that before CAs. In addition, we observed the SEM morphology of nickel NWAs electrode after CAs. It is seen in Fig. 8b that nanowires

were

contact

tightly

with

nickel

support

and

the

self-supporting structure of electrode owned high stability. Overall, the nickel NWAs electrode exhibited an excellent stability for urea peroxide electro-oxidation. 4. Conclusions Nickel NWAs electrode has been successfully fabricated via the polycarbonate template. The electrode with a unique three-dimensional structure consisted of two parts: nickel support and nickel NWAs, which benefits the diffusion of reactants or products. Solid urea peroxide can decompose into hydrogen peroxide and urea in aqueous solution, and they were separately electro-oxidized on nickel NWAs electrode. The 21

electrode demonstrated high catalytic activity for urea peroxide electro-oxidation and excellent stability in harsh alkaline media. The novel structure and high catalytic performance made the nickel NWAs electrode and urea peroxide as a promising anodic catalyst and fuel for DUPFCs, respectively. Acknowledgements We gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (21403044), the Natural Science Foundation of Heilongjiang Province of China (LC2015004), the China Postdoctoral Science Special Foundation (2015T80329), the Heilongjiang Postdoctoral Fund (LBH-Z14211), the China Postdoctoral Science Foundation (2014M561332), the Major Project

of

Science

and

Technology

of

Heilongjiang

Province

(GA14A101), the Project of Research and Development of Applied Technology of Harbin (2014DB4AG016) and the Fundamental Research Funds for the Central Universities (HEUCF20151004).

22

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Figure captions: Fig. 1. (a) Schematic of the fabrication process of nickel NWAs electrode; (b, c) SEM images of nickel NWAs electrode. Inset of b is the side view of PCT filled with nickel NWAs. Inset of c is the TEM image of nickel nanowires.

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Fig. 2. XRD patterns of PCT, nickel support and PCT filled with nickel NWAs.

32

Fig. 3. (a) Cyclic voltammograms of nickel NWAs electrode with continuous scans in 2.0 mol L-1 KOH at a scan rate of 50 mV s-1; (b) different scan rates of nickel NWAs electrode in narrow potential range of 0 V-0.2 V in 5.0 mol L-1 KOH; (c) plots of the current density at 0.1 V against the scan rates; (d) cyclic voltammograms of nickel NWAs and nickel sheet electrodes in 2.0 mol L-1 KOH and 0.04 mol L-1 urea peroxide at a scan rate of 10 mV s-1.

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Fig. 4. Cyclic voltammograms of nickel NWAs electrode: (a) in 2.0 mol L-1 KOH with and without 0.04 mol L-1 urea, H2O2, urea&H2O2, urea peroxide; (b) in different KOH concentrations with and without 0.04 mol L-1 urea peroxide at a scan rate of 10 mV s-1.

34

Fig. 5. Nyquist plots of nickel NWAs electrode at 0.1 V (a) and 0.45 V(b) in 2.0 mol L-1 KOH in the absence and presence of 0.04 mol L-1 urea, H2O2, urea&H2O2, urea peroxide.

35

Fig. 6. Cyclic voltammograms of nickel NWAs electrode in x mol L-1 KOH(x=1.0, 2.0, 3.0, 4.0, 5.0, 6.0 and 7.0) and 0.5 mol L-1 urea peroxide at a scan rate of 10 mV s-1.

36

Fig. 7. Cyclic voltammograms of nickel NWAs electrode in x mol L-1 urea peroxide(x=0.08, 0.15, 0.25, 0.5, 1.0 and 1.5) and 6.0 mol L-1 KOH at a scan rate of 10 mV s-1.

37

Fig. 8. (a) Chronoamperometric curves of nickel NWAs electrodes for urea peroxide electro-oxidation at different potentials in 6.0 mol L-1 KOH and 1.0 mol L-1 urea peroxide; (b) SEM of nickel NWAs and TEM image (inset) of nanowires after CA tests.

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