- Email: [email protected]
Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction Mingyong Wang a,*, Zhi Wang a, Xiangtao Yu b, Zhancheng Guo a,b a
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China
article info
abstract
Article history:
Porous NiMo film was prepared by one-step electrodeposition under super gravity field and
Received 31 October 2014
was used as electrocatalyst for hydrogen evolution reaction (HER). NiMo films possessed
Received in revised form
three-dimensional porous structure. The thickness of porous layer was up to 180e240 mm.
5 December 2014
Porous NiMo films with extremely large real active area exhibited high catalytic activity for
Accepted 8 December 2014
HER. The overpotential was very low and only 47 mV at 100 mA cm2. Meanwhile, porous
Available online xxx
NiMo films possessed good long-term stability by accelerated degradation studies. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Keywords: NiMo film Electrodeposition Catalytic activity Hydrogen evolution reaction Super gravity field
Introduction Hydrogen as the clean energy carrier is considered as the best alternative to fossil fuel. Alkaline water electrolysis derived by renewable energy is a promising technology to produce hydrogen. However, the technology is not popularized in large scale due to high energy consumption. Energy can be saved by developing cathode materials with high catalytic activity to reduce overpotential of hydrogen evolution reaction (HER). Electrodeposited Ni-based materials [1e6] have obtained more and more attention due to low cost and good catalytic
properties for HER. Generally, the catalytic activities were improved by enlarging real active surface area and enhancing intrinsic activity of electrode materials [6]. It was found that intrinsic catalytic activity of Ni-based materials for HER was in the following order: NieMo > NieZn > NieCo > NieW > NieFe > NieCr > Ni [6]. In order to enlarge real surface area, porous metal films were prepared by dealloying and alumina template methods based on two-step process [7e9]. Cai [7] electrodeposited Zn film on Ni foil and heat treatment was carried out to obtain NieZn alloy film. Then, Zn was dissolved selectively from alloy film to prepare nanoporous Ni films with the thickness of about 10 mm. However, the complexed
* Corresponding author. Tel./fax: þ86 010 82544818. E-mail address: [email protected] (M. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.12.022 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
process and addition of impurities were key issues [8]. In addition, real active area depended on structure and thickness of porous layer. In previous studies, the thicknesses of porous Ni-based cathodes were only less than tens of micrometers [2,7e10]. Xia [2] prepared NieMoeCu coatings by constant current electrodeposition and film thickness was only about 8.92 mm. Lee [9] electrodeposited Ni nanowires into porous alumina template and the length was about 20 mm. Therefore, a promising route for the improvement of catalytic activity was to utilize three-dimensional space of electrode by increasing the thickness of porous layers. However, it was difficult to obtain porous Ni-based metal films with high thickness by one-step electrodeposition. Recently, it has been found that metal film structure, bubble size and disengagement rate during electrochemical reaction can be adjusted by changing gravity acceleration [11e15]. Therefore, microgravity (much lower than 9.8 m s2) and super gravity field (much higher than 9.8 m s2) were used to advance electrochemical reaction or understand reaction mechanism. The grains of metal films electrodeposited under super gravity field were refined based on enhanced micro mixing and mass transfer [11,12]. In this paper, threedimensional porous NiMo films with high thickness were directly electrodeposited under super gravity field at larger current density than 0.6 A cm2. Porous NiMo films possessed extremely high active area and exhibited good catalytic properties for HER.
max-RB). NiMo films were dissolved using the solution with the composition of 30 ml H3PO4 þ 15 mL HNO3 þ 55 mL H2O. The contents of Ni and Mo were examined by ICP-OES (Optima 5300DV). The chemical compositions of NiMo films were calculated. All electrochemical measurements were carried out on a CHI 604B electrochemical working station in 10% NaOH solution. Working electrodes were NiMo films. Platinum foil and solid state electrode (GD-IV, Beijing Research Institute of Chemical Engineering and Metallurgy) were used as counter electrode and reference electrode, respectively. Solid state electrode can be used under high pressure and temperature. It was prepared by sealing Ag/AgCl electrode using conductive polymer. The potential of solid state electrode was 0.19 V vs SHE. Before electrochemical measurements, constant potential of 1.0 V was applied to activate NiMo film and steadystate was reached. Tafel curves with a scan rate of 1 mV s1 were measured to evaluate electrocatalytic activity of NiMo films for HER. IR compensation was carried out to correct Tafel curves. The electrochemical impedance spectroscopy (EIS) measurements for HER were performed in the frequencies range of 100 kHz to 0.01 Hz at various overpotentials. The AC amplitude was 5 mV. All experiments were repeated at least twice under same conditions to ensure reproducibility and accuracy. Long-term stability tests were performed by cyclic voltammetry (CV) without IR compensation in the potential range of 1.8 V to 0.2 V. Scan rate was 50 mV s1.
Results and discussion Experimental Characterization of NiMo films Super gravity field was obtained by centrifuge with a 100 mL electrolytic cell [11,13]. The electrochemical signals were transferred by gold slip ring (EC 3848-10, MOOG Inc.) which was fixed on the top of axis. Gravity coefficient (G) was calculated as follows: G¼
u2 L N2 p2 L ¼ g 900g
(1)
Where N was rotating speed (rpm) of centrifuge, g was gravity acceleration (9.8 m s2) and L was the distance between electrode center and axis (0.25 m in this experiment). G value was 1 under normal gravity condition. The electrolytic cell was horizontal under super gravity field, while it was perpendicular under normal gravity condition. The solution for NiMo electrodeposition consisted of 0.30 M NiSO4$6H2O, 0.20 M Na2MoO4$2H2O and 0.30 M Na3C6H5O7$2H2O. pH value was adjusted to 10.5 using ammonia. The solution composition and pH value were similar with the published data [1]. All chemical reagents were analytical grade. Fresh double-distilled water was used throughout this work. Cathode and anode were pure copper foil and Pt foil, respectively. During electrodeposition, copper foil was positioned on the bottom of electrolytic cell and its surface for electrodeposition was perpendicular to gravity direction. The electrodeposition was controlled by a WYK3010 DC Power Supply. Unless marked, the total electric quantity was 2880 C cm2. NiMo films were characterized by SEM (JEOL, JSM6700F), EDS (FEI MLA 250) and XRD (RIGAKU D/
In order to obtain porous metal films, hydrogen bubbles were used as dynamic template by increasing current density of metal electrodeposition [16e19]. NiMo films were also electrodeposited at larger current density than 0.6 A cm2. The morphology and crystal structure were shown in Fig. 1. Under normal gravity condition (G ¼ 1), when current density was up to 2.4 A cm2, NiMo film was still relatively compact in macroscopic view (Fig. 1A). Further increase of current density would lead to very high cell voltage and solution temperature. For metals with lower melting point and higher exchange current density (such as Ag, Cu and Au), it was easier to form porous structure due to dendrite growth. However, higher melting point metals (such as Ni, Fe, Pd) with lower exchange current density tended to form compact metal films with granular grains. It was difficult to obtain porous structure with good adhesion under normal conditions. Grain sizes were statistically measured during SEM observation. From the magnified image (Fig. 1B), NiMo film consisted of granular grains with the diameter of about 5e9 mm, which was similar with those in other papers [1,20]. Although microscopic surface of NiMo film electrodeposited under normal gravity condition was rough, NiMo film was still two-dimensional. Beside electrocrystallization, the formation of porous metal films by hydrogen bubble template was mainly affected by size, quantity and disengagement rate of hydrogen bubbles [21]. It was well known that the disengagement of bubbles from electrolytic system was controlled by interphase
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
3
Fig. 1 e SEM images of electrodeposited NiMo films. (A, B) 2.4 A cm¡2, G ¼ 1; (C) 0.6 A cm¡2, G ¼ 740; (D, E) 1.2 A cm¡2, G ¼ 740; (F) 2.4 A cm¡2, G ¼ 740. Inserts in (A), (D) and (F) were XRD patterns of NiMo films.
buoyancy term, Drg [22]. In our previous study [13], it was found that the size of hydrogen bubbles was reduced obviously by super gravity field. Meanwhile, buoyancy force exerted on bubbles was much larger. Therefore, based on the adjustment of hydrogen bubbles by super gravity field, a feasible method to control the morphology of electrodeposited NiMo films was developed. NiMo films were electrodeposited under super gravity field (G ¼ 740). When current density was only 0.6 A cm2, NiMo film was porous and consisted of spherical particles with the diameter of about 50e80 mm (Fig. 1C). The surface of particles was cellular and smooth. The porosity was larger than that of NiMo film electrodeposited under normal gravity condition. At 1.2 A cm2, spherical particles were refined and cellular surface became more clear (Fig. 1D). Some paths (arrows in Fig. 1E) for bubble disengagement were observed. Microcracks were developed around the paths, which led to further rupture of NiMo particles. When current density was up to 2.4 A cm2 (Fig. 1F), the diameter of NiMo particles was only about 10e20 mm. Meanwhile, the surface of particles became rougher.
Cross-sectional views of NiMo films were shown in Fig. 2. For compact NiMo film electrodeposited under G value of 1, film thickness was about 50e80 mm (Fig. 2A) and pores were not observed (Fig. 2B). However, porous NiMo film was composed of oriented NiMo clusters which were perpendicular to substrate surface (Fig. 2C). The oriented porous structure was different to conventional 3D porous structure [16e19] and can defend against the damage of the shockwave generated by hydrogen bubbles. The thickness of porous NiMo film was up to about 180e240 mm which was much higher than those in previous studies [2,7,9,10]. The channels between clusters were formed due to bubble disengagement. That is, metal electrodeposition only proceeded around bubble channels and NiMo clusters were grown along the direction of bubble disengagement (i.e. buoyancy direction). As shown in Fig. 2D, the disengagement direction of bubbles from electrode surface was parallel to the direction of buoyancy force or gravity. So, successive and stable bubble channels were formed easily under super gravity field. Metal electrodeposition proceeded around bubble channels in a long time, which was beneficial to the formation of the oriented porous metal
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Fig. 2 e The cross-section of NiMo films electrodeposited under G value of 1 (A, B) and 740 (C) at 2.4 A cm¡2, (B) was the magnification of (A); (D) The formation mechanism of porous metal films under super gravity field.
films with high thickness. Three-dimensional porous NiMo films with spatial structure must possess large real active area. Furthermore, only element Ni and Mo were found in NiMo films by EDS (Fig. 3). The content of Mo in NiMo film electrodeposited under super gravity field was slightly lower than that under normal gravity condition. Crystal structure of NiMo films by XRD were shown in inserts of Fig. 1A, D and F. There was only one broad diffraction peak for NiMo film. It means that NiMo films electrodeposited at higher current density show a large degree of amorphousness. XRD patterns of amorphous NiMo films were similar with those in previous papers [1,10].
exchange current densities (io) of porous NiMo films were much higher than that of compact NiMo film (Table 1). The results indicated that three-dimensional porous NiMo films possessed good catalytic activity for HER. Tafel slopes of all NiMo films were higher than the theoretical value of 116.3 mV dec1. Higher Tafel slopes were also observed in many papers, especially on porous materials [29e31]. The
Electrocatalytic activity for HER The electrocatalytic activities of amorphous NiMo films for hydrogen evolution reaction (HER) were investigated and Tafel polarization curves were shown in Fig. 4. At same overpotential, current densities of HER increased with the increase of G value (Fig. 4A) or current density for NiMo electrodeposition (Fig. 4B). Tafel kinetic parameters and overpotentials of HER at 100 mA cm2 were given in Table 1. For NiMo film electrodeposited under normal gravity condition, overpotential was 132 mV. The value was also lower than that in other study [2] due to rough surface. Overpotential was reduced obviously on three-dimensional porous NiMo films (Table 1). Meanwhile, with the increase of current density for NiMo electrodeposition, HER overpotentials decreased. Especially, at 2.4 A cm2, overpotential of HER was only 47 mV. The value was much lower than those of Ni-based electrocatalysts in previous reports (Table 2) [2,3,11,23e28]. In addition,
Fig. 3 e EDS of NiMo films electrodeposited under G value of 1 (A) and 740 (B) at 2.4 A cm¡2.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
5
NiMo film. It meant hydrogen evolution reaction was mainly controlled by electron transfer process. The deviation of depressed semicircles from ideal semicircles was related to the rough surface of NiMo films [33]. EIS experimental data were fitted using ZSimWin software and the electrical equivalent circuit (EEC) with one time constant was used (Fig. 6). During fitting, double layer capacitances were replaced by constant phase element (CPE). T (F sn1 cm2) was used as capacitive parameter. Electrochemical circuit parameters were presented in Table 3. It was found that charge transfer resistances (Rct) of HER were very low, especially at larger overpotentials. In addition, Rct values were lower on threedimensional porous NiMo films electrodeposited under G value of 740. It further confirmed that porous NiMo films exhibited better HER activity. During fitting, n was dispersion effect value of Nyquist plot and was used to represent the deviation degree of depressed semicircles from ideal semicircles. The n value was 1 for ideal semicircle, while the values were diminished on rough electrode. From Table 3, the n values were close to 0.5. The results also indicated that NiMo films were porous. In order to characterize real active area of NiMo films, Cdl was calculated according to following equation [1,23]: "
T
Cdl ¼ 1 ð1nÞ R1 s þ Rct
Fig. 4 e Tafel polarization curves of NiMo films electrodeposited at 2.4 A cm¡2 under various gravity condition (A) and at different current density (B) in 10% NaOH solution.
increase of Tafel slopes may be ascribed to the surface coverage by adsorbed hydrogen or the formation of oxides, which impeded the charge transfer [29,32]. The improvement of catalytic activity was attributed to porous structure of NiMo films. To examine real active area of metal film, electrochemical impedance spectroscopy (EIS) was usually used by estimating double layer capacitance (Cdl) [1,23]. Representative Nyquist plots of NiMo films were given in Fig. 5. Only one depressed semicircle was observed for every
Table 1 e HER kinetic parameters from Tafel curves, overpotential at current density of 100 mA cm¡2 and surface roughness (Rf) of NiMo films electrodeposited under different condition. i (mA cm2)
2.4
0.6
1.2
2.4
G h100 (mV) a (V) b (mV dec1) io (mA cm2) Cdl (mF cm2) Rf
1 132 0.29 158.6 14.9 82.80 4140
740 72 0.26 194.9 44.7 97.94 4897
740 52 0.26 189.4 41.5 104.94 5247
740 47 0.19 136.7 42.9 392.80 19,640
#1=n (2)
Where Rs was solution resistance (U cm2). Cdl for every NiMo film was obtained according to EIS results. Surface roughness (Rf) was calculated by double layer capacitances (Cdl), which was compared with 20 mF cm2 for smooth surface [1,23]. Cdl and Rf were given in Table 1. Rf value of NiMo film electrodeposited under normal gravity condition was 4140 and was similar with Navarro-Flores's result [1]. However, Rf values of porous NiMo films electrodeposited under G value of 740 at 0.6 A cm2 and 1.2 A cm2 were 4897 and 5247, respectively. When current density was 2.4 A cm2, Rf was up to 19,640. The value was much higher than those in previous studies [1,23]. Current densities with respect to real surface area at overpotential of 80 mV (i.e. i80/Rf) were calculated to compare intrinsic catalytic activity of porous NiMo film and compact NiMo film. It was found that the values of i80/Rf were 11.3 mA cm2 and 11.9 mA cm2 for NiMo films electrodeposited under G value of 1 and740 at 2.4 A cm2, respectively. It indicated that intrinsic catalytic activity of NiMo films was almost unchanged by super gravity field. The improvement of catalytic activity was ascribed to the increase of real surface area. Porous NiMo films with high thickness possessed three-dimensional spatial structure. Meanwhile, porous structure was formed by hydrogen bubble template during NiMo electrodeposition at large current density. Therefore, active surface in porous film was more available to catalyze HER than those obtained by dealloying and alumina template. So, efficient active sites were increased obviously, which led to high electrocatalytic activity for HER. In previous studies, nano-scale Ni-based catalysts were prepared and exhibited good catalytic activities [27,34,35]. Although porous NiMo films (Fig. 2) consisted of micro-scale grains, film thicknesses were much higher. Therefore, threedimensional porous NiMo films possessed better activity for HER due to efficient active surface.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Table 2 e Overpotentials of representative Ni-based catalysts for HER in alkaline solution. Catalyst Porous NiMoa Compact NiMob NiCox NiW NiMo NiMoCu Ni2P NiCo NiMo nanopowers1 NiMo nanopowers2 Porous Ni3AleMo a b
Current density (mA cm2)
Overpotential (mV)
Electrolyte
T ( C)
Reference
100 100 100 82 140 240 20 100 20
47 132 110e130 150 250 310 250 166 70
10% NaOH 10% NaOH 30% KOH 10% NaOH 6 M NaOH 6 M NaOH 1 M KOH 30% KOH 2 M KOH
25 25 30 25 25 25 25 30 25
This paper This paper 3 11 2 2 24 26 27
130
100
2 M KOH
25
27
200
500
6 M KOH
25
28
Electrodeposited under super gravity field (G ¼ 740) at 2.4 A cm2. Electrodeposited under normal gravity condition (G ¼ 1) at 2.4 A cm2.
Stability of NiMo films The electrochemical stability of electrode materials was another key criterion for practical application, especially for porous metal films with high thickness. Accelerated degradation studies by cyclic voltammetry (CV) [4,36e38] were performed to examine long-term stabilities of NiMo films in 10% NaOH solution. The first CV curve for different catalyst was given in Fig. 7. The electrocatalytic activities of NiMo films were much higher than those of pure Cu and Ni. Comparing to Cu and Ni materials, the intrinsic activity of NiMo materials was higher due to the synergistic catalytic effect between Ni and Mo. Current density on porous NiMo film electrodeposited under super gravity field (G ¼ 740) was obviously larger than that of NiMo film electrodeposited under normal gravity condition (G ¼ 1) at certain potential (Fig. 7). For example, current density of HER on porous NiMo films was 0.098 A cm2 at 1.3 V, while the value was only 0.056 A cm2 on compact NiMo film. The results further confirmed that porous NiMo films possessed good catalytic activity. Current densities at 1.8 V in CV curves after different cycle were collected and shown in Fig. 8. For NiMo films electrodeposited under normal gravity condition (G ¼ 1), three regions were observed (Fig. 8A). In the first 50 cycles, current densities of HER decayed rapidly due to the blockage of active surface by hydrogen bubbles [21,31]. Then, current densities were almost constant. After about 300 cycles, current densities began to decay again (Fig. 8A), which may mean the degradation of NiMo films. However, on porous NiMo films electrodeposited under super gravity field (G ¼ 740), current
Fig. 5 e Representative Nyquist curves of NiMo films. (A) Electrodeposited under G value of 1 at 2.4 A cm¡2. Overpotential (h): ¡95 mV. (B) Electrodeposited under G value of 740. Overpotential: ¡100 mV. Symbols were experimental data and solid lines were fitted curves.
Fig. 6 e The equivalent circuit with one time constant to describe HER.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
7
Table 3 e Electrochemical circuit parameters of NiMo films for HER at different overpotential. h (V)
Rs (U cm2) T (F sn1 cm2) n (0 < n < 1) Rct (U cm2)
G ¼ 1, 2.4 A cm2 0.045 1.33 0.070 1.36 0.095 1.50 0.120 1.32 0.170 1.43 G ¼ 740, 0.6 A cm2 0.040 0.82 0.065 1.08 0.090 1.12 0.115 1.15 0.140 0.88 0.165 0.85 G ¼ 740, 1.2 A cm2 0.050 1.17 0.075 1.32 0.100 1.24 0.125 1.58 0.150 1.26 G ¼ 740, 2.4 A cm2 0.050 1.22 0.075 1.40 0.100 1.26 0.125 1.62 0.150 1.39
0.61 0.48 0.42 0.33 0.24
0.4696 0.4729 0.4793 0.4658 0.627
1.26 1.12 0.99 0.95 0.75
0.37 0.31 0.29 0.30 0.23 0.21
0.4339 0.5380 0.6044 0.5515 0.5459 0.5355
1.08 0.86 0.67 0.66 0.64 0.62
0.38 0.37 0.34 0.35 0.28
0.5180 0.5657 0.5562 0.6719 0.6200
0.90 0.81 0.73 0.56 0.56
1.93 2.15 1.02 0.84 0.66
0.3858 0.4796 0.5297 0.8061 0.6666
0.85 0.53 0.39 0.24 0.29
densities of HER also decayed in the first 50 cycles due to the blockage of hydrogen bubbles (Fig. 8B). Then, current densities exhibited a periodic fluctuation in a certain range. The fluctuation was ascribed to the growth and disengagement of hydrogen bubbles. Further decay of current density did not happen, even after 500 cycles. The results indicated that porous NiMo films possessed good long-term stability and adhesion on substrate. After long-term stability test, surface morphologies of NiMo films were examined and shown in Fig. 9. The surface of NiMo film electrodeposited under normal
Fig. 8 e The longeterm stability of NiMo films electrodeposited under G values of 1 (A) and 740 (B) at 2.4 A cm¡2.
gravity condition (G ¼ 1) became coarser (Fig. 9A). Successive passive layer (arrows in Fig. 9B) covered entire surface during CV test, which led to the decrease of HER activity after 300 cycles. However, surface morphologies of porous NiMo films were not changed obviously (Fig. 9C and D). For compact NiMo films, real current density in positive scan during CV test was higher due to smaller active area, which led to the passivation or dissolution of NiMo. Although apparent current density was higher on porous NiMo film, real current density was lower due to larger real active area. Therefore, surface structure of porous NiMo films was hardly damaged during CV test.
Conclusions
Fig. 7 e CV curve of first cycle in 10% NaOH solution on pure Cu foil, pure Ni and electrodeposited NiMo films under G value of 1 and 740 at 2.4 A cm¡2.
An effective and feasible method to prepare thick and porous NiMo films was developed. Three-dimensional spatial NiMo film with the thickness of 180e240 mm was obtained by onestep electrodeposition under super gravity field. Surface
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Fig. 9 e SEM images of NiMo alloys after stability measurement by CV. (A, B): G ¼ 1; (C, D): G ¼ 740. Current density for deposition: 2.4 A cm¡2.
roughness of porous NiMo film was up to 19,640, which was about 4.7 times as large as that of NiMo film electrodeposited under normal gravity condition. The adjustment of bubble size and disengagement rate by super gravity field may contribute to the formation of porous structure. Threedimensional porous NiMo film exhibited good catalytic activity for HER and overpotential was only 47 mV at 100 mA cm2. The enhancement of catalytic activity was ascribed to extremely high real active area. Based on accelerated degradation studies, porous NiMo films also possessed good longterm stability. This method was promising to prepare functional metal films which were used as electrode materials in the field of water electrolysis, fuel cell and super-capacitors.
Acknowledgments This work is supported by Natural Science Foundation of China under the grant 51274180 and 50804043.
references
[1] Navarro-Flores E, Chong ZW, Omanovic S. Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium. J Mol Catal A Chem 2005;226:179e97.
[2] Xia M, Lei T, Lv NL, Li NF. Synthesis and electrocatalytic hydrogen evolution performance of Ni-Mo-Cu alloy coating electrode. Int J Hydrogen Energy 2014;39:4794e802. lez-Buch C, Valero-Vidal C, [3] Herraiz-Cardona I, Gonza rez-Herranz V. Co-Modification of Ni-based type Ortega E, Pe Raney electrodeposits for hydrogen evolution reaction in alkaline media. J Power Sources 2013;240:698e704. [4] Popczun EJ, McKone JR, Read CG, Biacchi AJ, Wiltrout AM, Lewis NS, et al. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J Am Chem Soc 2013;135:9267e70. [5] Warren EL, McKone JR, Atwater HA, Gray HB, Lewis NS. Hydrogen-evolution characteristics of Ni-Mo-coated, radial junction, nþp-silicon microwire array photocathodes. Energy Environ Sci 2012;5:9653e61. [6] Safizadeh F, Ghali E, Houlachi G. Electrocatalysis developments for hydrogen evolution reaction in alkaline solutions-A review. Int J Hydrogen Energy 2014. http://dx.doi. org/10.1016/j.ijhydene.2014.10.109. [7] Cai J, Xu J, Wang JM, Zhang LY, Zhou H, Zhong Y, et al. Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction. Int J Hydrogen Energy 2013;38:934e41. [8] Yi Y, Lee JK, Uhm S, Nam SC, Lee J. A single-step approach to create nano-pottery structures for efficient water electrocatalysis. Electrochem Comm 2009;11:2121e4. [9] Lee JK, Yi Y, Lee HJ, Uhm S, Lee J. Electrocatalytic activity of Ni nanowires prepared by galvanic electrodeposition for hydrogen evolution reaction. Catal Today 2009;146: 188e91. [10] Ohgai T, Washio R, Tanaka Y. Anisotropic magnetization behavior of electrodeposited nanocrystalline Ni-Mo Alloy
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22] [23]
[24]
[25]
[26]
thin films and nanowires array. J Elelctrochem Soc 2012;159:H800e4. Wang MY, Wang Z, Guo ZC, Li ZJ. The enhanced electrocatalytic activity and stability of NiW films electrodeposited under super gravity field for hydrogen evolution reaction. Int J Hydrogen Energy 2011;36:3305e12. Tong H, Kong LB, Wang CM. Electroless deposition of Ag onto p-Si(100) surface under the condition of the centrifugal fields. Thin Solid Films 2006;496:360e3. Wang MY, Wang Z, Guo ZC. Understanding of the intensified effect of super gravity on hydrogen evolution reaction. Int J Hydrogen Energy 2009;34:5311e7. Sakuma G, Fukunaka Y, Matsushima H. Nucleation and growth of electrolytic gas bubbles under microgravity. Int J Hydrogen Energy 2014;39:7638e8645. Nishikawa K, Fukunaka Y, Chassaing E, Rosso M. Electrodeposition of metals in microgravity conditions. Electrochim Acta 2013;100:342e9. Kim J, Kim R, Kwon H. Preparation of copper foam with 3dimensionally interconnected spherical pore network by electrodeposition. Electrochem Comm 2008;10:1148e51. Shin H, Dong J, Liu M. Nanoporous structure prepared by an electrochemical deposition process. Adv Mater 2003;15:1610e4. Cherevko S, Xing X, Chung C. Electrodeposition of threedimensional porous silver foams. Electrochem Comm 2010;12:467e70. Cherevko S, Chung C. Direct electrodeposition of nanoporous gold with controlled multimodal pore size distribution. Electrochem Comm 2011;13:16e9. Sun S, Podlaha EJ. Electrodeposition of Mo-Rich MoNi alloys from an aqueous electrolyte. J Electrochem Soc 2011;159:D97e102. Li Y, Jia WZ, Song YY, Xia XH. Superhydrophobicity of 3D porous copper films prepared using the hydrogen bubble dynamic template. Chem Mater 2007;19:5758e64. Ramshaw C. The opportunities for exploiting centrifugal fields. Heat Recovery Syst CHP 1993;13:493e513. NV, Jovic VD, Gajic -Krstajic Lj, Jovic BM, Antozzi AL, Krstajic Martelli GN. Electrodeposition of Ni-Mo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution. Int J Hydrogen Energy 2008;33:3676e87. Feng LG, Vrubel H, Bensimon M, Hu X. Easily-prepared dinickel phosphide (Ni2P) nanoparticles as an efficient and robust electrocatalyst for hydrogen evolution. Phys Chem Chem Phys 2014;16:5917e21. Mckone JR, Marinescu SC, Brunschwig BS, Winkler JR, Gray HB. Earth-abundant hydrogen evolution electrocatalysts. Chem Sci 2014;5:865e78. lez-Buch C, Herraiz-Cardona I, Ortega E, Garcı´aGonza n J, Pe rez-Herranz V. Synthesis and characterization of Anto
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
9
macroporous Ni, Co and Ni-Co electrocatalytic deposits for hydrogen evolution reaction in alkaline media. Int J Hydrogen Energy 2013;38:10157e69. Kone JR, Sadtler BF, Werlang CA, Lewis NS, Gray HB. Ni-Mo nanopowders for efficient electrochemical hydrogen evolution. ACS Catal 2013;3:166e9. Wu L, He YH, Lei T, Nan B, Xu NP, Zou J, et al. Characterization of the porous Ni3Al-Mo electrodes during hydrogen generation from alkaline water electrolysis. Energy 2013;63:216e24. Kubisztal J, Budniok A, Lasia A. Study of the hydrogen evolution reaction on nickel-based composite coating containing molybdenum powder. Int J Hydrogen Energy 2007;32:1211e8. Dong HX, Lei T, He YH, Xu NP, Huang BY, Liu CT. Electrochemical performance of porous Ni3Al electrodes for hydrogen evolution reaction. Int J Hydrogen Energy 2011;36:12112e20. Xie ZW, He P, Du LC, Dong FQ, Dai K, Zhang TH. Comparison of four nickel-based electrodes for hydrogen evolution reaction. Electrochim Acta 2013;88:390e4. Vǎduva CC, Vaszilcsin N, Kellenberger A, Medeleanu M. Catalytic enhancement of hydrogen evolution reaction on copper in the presence of benzylamine. Int J Hydrogen Energy 2011;36:6994e7001. Coleman EJ, Co AC. Galvanic displacement of Pt on nanoporous copper: an alternative synthetic route for obtaining robust and reliable oxygen reduction activity. J Catal 2014;316:191e200. zquez-Go mez L, Cattarin S, Guerriero P, Musiani M. Va Hydrogen evolution on porous Ni cathodes modified by spontaneous deposition of Ru or Ir. Electrochim Acta 2008;53:8310e8. zquezAntozzi AL, Bargioni C, Iacopetti L, Musiani M, Va mez L. EIS study of the service life of activated cathodes Go for the hydrogen evolution reaction in the chlor-alkali membrane cell process. Electrochim Acta 2008;53:7410e6. Ahn SH, Choi I, Park H, Hwang SJ, Yoo SJ, Cho E, et al. Effect of morphology of electrodeposited Ni catalysts on the behavior of bubbles generated during the oxygen evolution reaction in alkaline water electrolysis. Chem Comm 2013;49:9323e5. Deng J, Ren PJ, Deng DH, Yu L, Yang F, Bao XH. Highly active and durable non-precious-metal catalysts encapsulated in carbon nanotubes for hydrogen evolution reaction. Energy Environ Sci 2014;7:1919e23. Wang TY, Liu L, Zhu ZW, Papakonstantinou P, Hu JB, Liu HY, et al. Enhanced electrocatalytic activity for hydrogen evolution reaction from self-assembled monodispersed molybdenum sulfide nanoparticles on an Au electrode. Energy Environ Sci 2013;6:625e33.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction Mingyong Wang a,*, Zhi Wang a, Xiangtao Yu b, Zhancheng Guo a,b a
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China b State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, PR China
article info
abstract
Article history:
Porous NiMo film was prepared by one-step electrodeposition under super gravity field and
Received 31 October 2014
was used as electrocatalyst for hydrogen evolution reaction (HER). NiMo films possessed
Received in revised form
three-dimensional porous structure. The thickness of porous layer was up to 180e240 mm.
5 December 2014
Porous NiMo films with extremely large real active area exhibited high catalytic activity for
Accepted 8 December 2014
HER. The overpotential was very low and only 47 mV at 100 mA cm2. Meanwhile, porous
Available online xxx
NiMo films possessed good long-term stability by accelerated degradation studies. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Keywords: NiMo film Electrodeposition Catalytic activity Hydrogen evolution reaction Super gravity field
Introduction Hydrogen as the clean energy carrier is considered as the best alternative to fossil fuel. Alkaline water electrolysis derived by renewable energy is a promising technology to produce hydrogen. However, the technology is not popularized in large scale due to high energy consumption. Energy can be saved by developing cathode materials with high catalytic activity to reduce overpotential of hydrogen evolution reaction (HER). Electrodeposited Ni-based materials [1e6] have obtained more and more attention due to low cost and good catalytic
properties for HER. Generally, the catalytic activities were improved by enlarging real active surface area and enhancing intrinsic activity of electrode materials [6]. It was found that intrinsic catalytic activity of Ni-based materials for HER was in the following order: NieMo > NieZn > NieCo > NieW > NieFe > NieCr > Ni [6]. In order to enlarge real surface area, porous metal films were prepared by dealloying and alumina template methods based on two-step process [7e9]. Cai [7] electrodeposited Zn film on Ni foil and heat treatment was carried out to obtain NieZn alloy film. Then, Zn was dissolved selectively from alloy film to prepare nanoporous Ni films with the thickness of about 10 mm. However, the complexed
* Corresponding author. Tel./fax: þ86 010 82544818. E-mail address: [email protected] (M. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.12.022 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
process and addition of impurities were key issues [8]. In addition, real active area depended on structure and thickness of porous layer. In previous studies, the thicknesses of porous Ni-based cathodes were only less than tens of micrometers [2,7e10]. Xia [2] prepared NieMoeCu coatings by constant current electrodeposition and film thickness was only about 8.92 mm. Lee [9] electrodeposited Ni nanowires into porous alumina template and the length was about 20 mm. Therefore, a promising route for the improvement of catalytic activity was to utilize three-dimensional space of electrode by increasing the thickness of porous layers. However, it was difficult to obtain porous Ni-based metal films with high thickness by one-step electrodeposition. Recently, it has been found that metal film structure, bubble size and disengagement rate during electrochemical reaction can be adjusted by changing gravity acceleration [11e15]. Therefore, microgravity (much lower than 9.8 m s2) and super gravity field (much higher than 9.8 m s2) were used to advance electrochemical reaction or understand reaction mechanism. The grains of metal films electrodeposited under super gravity field were refined based on enhanced micro mixing and mass transfer [11,12]. In this paper, threedimensional porous NiMo films with high thickness were directly electrodeposited under super gravity field at larger current density than 0.6 A cm2. Porous NiMo films possessed extremely high active area and exhibited good catalytic properties for HER.
max-RB). NiMo films were dissolved using the solution with the composition of 30 ml H3PO4 þ 15 mL HNO3 þ 55 mL H2O. The contents of Ni and Mo were examined by ICP-OES (Optima 5300DV). The chemical compositions of NiMo films were calculated. All electrochemical measurements were carried out on a CHI 604B electrochemical working station in 10% NaOH solution. Working electrodes were NiMo films. Platinum foil and solid state electrode (GD-IV, Beijing Research Institute of Chemical Engineering and Metallurgy) were used as counter electrode and reference electrode, respectively. Solid state electrode can be used under high pressure and temperature. It was prepared by sealing Ag/AgCl electrode using conductive polymer. The potential of solid state electrode was 0.19 V vs SHE. Before electrochemical measurements, constant potential of 1.0 V was applied to activate NiMo film and steadystate was reached. Tafel curves with a scan rate of 1 mV s1 were measured to evaluate electrocatalytic activity of NiMo films for HER. IR compensation was carried out to correct Tafel curves. The electrochemical impedance spectroscopy (EIS) measurements for HER were performed in the frequencies range of 100 kHz to 0.01 Hz at various overpotentials. The AC amplitude was 5 mV. All experiments were repeated at least twice under same conditions to ensure reproducibility and accuracy. Long-term stability tests were performed by cyclic voltammetry (CV) without IR compensation in the potential range of 1.8 V to 0.2 V. Scan rate was 50 mV s1.
Results and discussion Experimental Characterization of NiMo films Super gravity field was obtained by centrifuge with a 100 mL electrolytic cell [11,13]. The electrochemical signals were transferred by gold slip ring (EC 3848-10, MOOG Inc.) which was fixed on the top of axis. Gravity coefficient (G) was calculated as follows: G¼
u2 L N2 p2 L ¼ g 900g
(1)
Where N was rotating speed (rpm) of centrifuge, g was gravity acceleration (9.8 m s2) and L was the distance between electrode center and axis (0.25 m in this experiment). G value was 1 under normal gravity condition. The electrolytic cell was horizontal under super gravity field, while it was perpendicular under normal gravity condition. The solution for NiMo electrodeposition consisted of 0.30 M NiSO4$6H2O, 0.20 M Na2MoO4$2H2O and 0.30 M Na3C6H5O7$2H2O. pH value was adjusted to 10.5 using ammonia. The solution composition and pH value were similar with the published data [1]. All chemical reagents were analytical grade. Fresh double-distilled water was used throughout this work. Cathode and anode were pure copper foil and Pt foil, respectively. During electrodeposition, copper foil was positioned on the bottom of electrolytic cell and its surface for electrodeposition was perpendicular to gravity direction. The electrodeposition was controlled by a WYK3010 DC Power Supply. Unless marked, the total electric quantity was 2880 C cm2. NiMo films were characterized by SEM (JEOL, JSM6700F), EDS (FEI MLA 250) and XRD (RIGAKU D/
In order to obtain porous metal films, hydrogen bubbles were used as dynamic template by increasing current density of metal electrodeposition [16e19]. NiMo films were also electrodeposited at larger current density than 0.6 A cm2. The morphology and crystal structure were shown in Fig. 1. Under normal gravity condition (G ¼ 1), when current density was up to 2.4 A cm2, NiMo film was still relatively compact in macroscopic view (Fig. 1A). Further increase of current density would lead to very high cell voltage and solution temperature. For metals with lower melting point and higher exchange current density (such as Ag, Cu and Au), it was easier to form porous structure due to dendrite growth. However, higher melting point metals (such as Ni, Fe, Pd) with lower exchange current density tended to form compact metal films with granular grains. It was difficult to obtain porous structure with good adhesion under normal conditions. Grain sizes were statistically measured during SEM observation. From the magnified image (Fig. 1B), NiMo film consisted of granular grains with the diameter of about 5e9 mm, which was similar with those in other papers [1,20]. Although microscopic surface of NiMo film electrodeposited under normal gravity condition was rough, NiMo film was still two-dimensional. Beside electrocrystallization, the formation of porous metal films by hydrogen bubble template was mainly affected by size, quantity and disengagement rate of hydrogen bubbles [21]. It was well known that the disengagement of bubbles from electrolytic system was controlled by interphase
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
3
Fig. 1 e SEM images of electrodeposited NiMo films. (A, B) 2.4 A cm¡2, G ¼ 1; (C) 0.6 A cm¡2, G ¼ 740; (D, E) 1.2 A cm¡2, G ¼ 740; (F) 2.4 A cm¡2, G ¼ 740. Inserts in (A), (D) and (F) were XRD patterns of NiMo films.
buoyancy term, Drg [22]. In our previous study [13], it was found that the size of hydrogen bubbles was reduced obviously by super gravity field. Meanwhile, buoyancy force exerted on bubbles was much larger. Therefore, based on the adjustment of hydrogen bubbles by super gravity field, a feasible method to control the morphology of electrodeposited NiMo films was developed. NiMo films were electrodeposited under super gravity field (G ¼ 740). When current density was only 0.6 A cm2, NiMo film was porous and consisted of spherical particles with the diameter of about 50e80 mm (Fig. 1C). The surface of particles was cellular and smooth. The porosity was larger than that of NiMo film electrodeposited under normal gravity condition. At 1.2 A cm2, spherical particles were refined and cellular surface became more clear (Fig. 1D). Some paths (arrows in Fig. 1E) for bubble disengagement were observed. Microcracks were developed around the paths, which led to further rupture of NiMo particles. When current density was up to 2.4 A cm2 (Fig. 1F), the diameter of NiMo particles was only about 10e20 mm. Meanwhile, the surface of particles became rougher.
Cross-sectional views of NiMo films were shown in Fig. 2. For compact NiMo film electrodeposited under G value of 1, film thickness was about 50e80 mm (Fig. 2A) and pores were not observed (Fig. 2B). However, porous NiMo film was composed of oriented NiMo clusters which were perpendicular to substrate surface (Fig. 2C). The oriented porous structure was different to conventional 3D porous structure [16e19] and can defend against the damage of the shockwave generated by hydrogen bubbles. The thickness of porous NiMo film was up to about 180e240 mm which was much higher than those in previous studies [2,7,9,10]. The channels between clusters were formed due to bubble disengagement. That is, metal electrodeposition only proceeded around bubble channels and NiMo clusters were grown along the direction of bubble disengagement (i.e. buoyancy direction). As shown in Fig. 2D, the disengagement direction of bubbles from electrode surface was parallel to the direction of buoyancy force or gravity. So, successive and stable bubble channels were formed easily under super gravity field. Metal electrodeposition proceeded around bubble channels in a long time, which was beneficial to the formation of the oriented porous metal
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Fig. 2 e The cross-section of NiMo films electrodeposited under G value of 1 (A, B) and 740 (C) at 2.4 A cm¡2, (B) was the magnification of (A); (D) The formation mechanism of porous metal films under super gravity field.
films with high thickness. Three-dimensional porous NiMo films with spatial structure must possess large real active area. Furthermore, only element Ni and Mo were found in NiMo films by EDS (Fig. 3). The content of Mo in NiMo film electrodeposited under super gravity field was slightly lower than that under normal gravity condition. Crystal structure of NiMo films by XRD were shown in inserts of Fig. 1A, D and F. There was only one broad diffraction peak for NiMo film. It means that NiMo films electrodeposited at higher current density show a large degree of amorphousness. XRD patterns of amorphous NiMo films were similar with those in previous papers [1,10].
exchange current densities (io) of porous NiMo films were much higher than that of compact NiMo film (Table 1). The results indicated that three-dimensional porous NiMo films possessed good catalytic activity for HER. Tafel slopes of all NiMo films were higher than the theoretical value of 116.3 mV dec1. Higher Tafel slopes were also observed in many papers, especially on porous materials [29e31]. The
Electrocatalytic activity for HER The electrocatalytic activities of amorphous NiMo films for hydrogen evolution reaction (HER) were investigated and Tafel polarization curves were shown in Fig. 4. At same overpotential, current densities of HER increased with the increase of G value (Fig. 4A) or current density for NiMo electrodeposition (Fig. 4B). Tafel kinetic parameters and overpotentials of HER at 100 mA cm2 were given in Table 1. For NiMo film electrodeposited under normal gravity condition, overpotential was 132 mV. The value was also lower than that in other study [2] due to rough surface. Overpotential was reduced obviously on three-dimensional porous NiMo films (Table 1). Meanwhile, with the increase of current density for NiMo electrodeposition, HER overpotentials decreased. Especially, at 2.4 A cm2, overpotential of HER was only 47 mV. The value was much lower than those of Ni-based electrocatalysts in previous reports (Table 2) [2,3,11,23e28]. In addition,
Fig. 3 e EDS of NiMo films electrodeposited under G value of 1 (A) and 740 (B) at 2.4 A cm¡2.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
5
NiMo film. It meant hydrogen evolution reaction was mainly controlled by electron transfer process. The deviation of depressed semicircles from ideal semicircles was related to the rough surface of NiMo films [33]. EIS experimental data were fitted using ZSimWin software and the electrical equivalent circuit (EEC) with one time constant was used (Fig. 6). During fitting, double layer capacitances were replaced by constant phase element (CPE). T (F sn1 cm2) was used as capacitive parameter. Electrochemical circuit parameters were presented in Table 3. It was found that charge transfer resistances (Rct) of HER were very low, especially at larger overpotentials. In addition, Rct values were lower on threedimensional porous NiMo films electrodeposited under G value of 740. It further confirmed that porous NiMo films exhibited better HER activity. During fitting, n was dispersion effect value of Nyquist plot and was used to represent the deviation degree of depressed semicircles from ideal semicircles. The n value was 1 for ideal semicircle, while the values were diminished on rough electrode. From Table 3, the n values were close to 0.5. The results also indicated that NiMo films were porous. In order to characterize real active area of NiMo films, Cdl was calculated according to following equation [1,23]: "
T
Cdl ¼ 1 ð1nÞ R1 s þ Rct
Fig. 4 e Tafel polarization curves of NiMo films electrodeposited at 2.4 A cm¡2 under various gravity condition (A) and at different current density (B) in 10% NaOH solution.
increase of Tafel slopes may be ascribed to the surface coverage by adsorbed hydrogen or the formation of oxides, which impeded the charge transfer [29,32]. The improvement of catalytic activity was attributed to porous structure of NiMo films. To examine real active area of metal film, electrochemical impedance spectroscopy (EIS) was usually used by estimating double layer capacitance (Cdl) [1,23]. Representative Nyquist plots of NiMo films were given in Fig. 5. Only one depressed semicircle was observed for every
Table 1 e HER kinetic parameters from Tafel curves, overpotential at current density of 100 mA cm¡2 and surface roughness (Rf) of NiMo films electrodeposited under different condition. i (mA cm2)
2.4
0.6
1.2
2.4
G h100 (mV) a (V) b (mV dec1) io (mA cm2) Cdl (mF cm2) Rf
1 132 0.29 158.6 14.9 82.80 4140
740 72 0.26 194.9 44.7 97.94 4897
740 52 0.26 189.4 41.5 104.94 5247
740 47 0.19 136.7 42.9 392.80 19,640
#1=n (2)
Where Rs was solution resistance (U cm2). Cdl for every NiMo film was obtained according to EIS results. Surface roughness (Rf) was calculated by double layer capacitances (Cdl), which was compared with 20 mF cm2 for smooth surface [1,23]. Cdl and Rf were given in Table 1. Rf value of NiMo film electrodeposited under normal gravity condition was 4140 and was similar with Navarro-Flores's result [1]. However, Rf values of porous NiMo films electrodeposited under G value of 740 at 0.6 A cm2 and 1.2 A cm2 were 4897 and 5247, respectively. When current density was 2.4 A cm2, Rf was up to 19,640. The value was much higher than those in previous studies [1,23]. Current densities with respect to real surface area at overpotential of 80 mV (i.e. i80/Rf) were calculated to compare intrinsic catalytic activity of porous NiMo film and compact NiMo film. It was found that the values of i80/Rf were 11.3 mA cm2 and 11.9 mA cm2 for NiMo films electrodeposited under G value of 1 and740 at 2.4 A cm2, respectively. It indicated that intrinsic catalytic activity of NiMo films was almost unchanged by super gravity field. The improvement of catalytic activity was ascribed to the increase of real surface area. Porous NiMo films with high thickness possessed three-dimensional spatial structure. Meanwhile, porous structure was formed by hydrogen bubble template during NiMo electrodeposition at large current density. Therefore, active surface in porous film was more available to catalyze HER than those obtained by dealloying and alumina template. So, efficient active sites were increased obviously, which led to high electrocatalytic activity for HER. In previous studies, nano-scale Ni-based catalysts were prepared and exhibited good catalytic activities [27,34,35]. Although porous NiMo films (Fig. 2) consisted of micro-scale grains, film thicknesses were much higher. Therefore, threedimensional porous NiMo films possessed better activity for HER due to efficient active surface.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Table 2 e Overpotentials of representative Ni-based catalysts for HER in alkaline solution. Catalyst Porous NiMoa Compact NiMob NiCox NiW NiMo NiMoCu Ni2P NiCo NiMo nanopowers1 NiMo nanopowers2 Porous Ni3AleMo a b
Current density (mA cm2)
Overpotential (mV)
Electrolyte
T ( C)
Reference
100 100 100 82 140 240 20 100 20
47 132 110e130 150 250 310 250 166 70
10% NaOH 10% NaOH 30% KOH 10% NaOH 6 M NaOH 6 M NaOH 1 M KOH 30% KOH 2 M KOH
25 25 30 25 25 25 25 30 25
This paper This paper 3 11 2 2 24 26 27
130
100
2 M KOH
25
27
200
500
6 M KOH
25
28
Electrodeposited under super gravity field (G ¼ 740) at 2.4 A cm2. Electrodeposited under normal gravity condition (G ¼ 1) at 2.4 A cm2.
Stability of NiMo films The electrochemical stability of electrode materials was another key criterion for practical application, especially for porous metal films with high thickness. Accelerated degradation studies by cyclic voltammetry (CV) [4,36e38] were performed to examine long-term stabilities of NiMo films in 10% NaOH solution. The first CV curve for different catalyst was given in Fig. 7. The electrocatalytic activities of NiMo films were much higher than those of pure Cu and Ni. Comparing to Cu and Ni materials, the intrinsic activity of NiMo materials was higher due to the synergistic catalytic effect between Ni and Mo. Current density on porous NiMo film electrodeposited under super gravity field (G ¼ 740) was obviously larger than that of NiMo film electrodeposited under normal gravity condition (G ¼ 1) at certain potential (Fig. 7). For example, current density of HER on porous NiMo films was 0.098 A cm2 at 1.3 V, while the value was only 0.056 A cm2 on compact NiMo film. The results further confirmed that porous NiMo films possessed good catalytic activity. Current densities at 1.8 V in CV curves after different cycle were collected and shown in Fig. 8. For NiMo films electrodeposited under normal gravity condition (G ¼ 1), three regions were observed (Fig. 8A). In the first 50 cycles, current densities of HER decayed rapidly due to the blockage of active surface by hydrogen bubbles [21,31]. Then, current densities were almost constant. After about 300 cycles, current densities began to decay again (Fig. 8A), which may mean the degradation of NiMo films. However, on porous NiMo films electrodeposited under super gravity field (G ¼ 740), current
Fig. 5 e Representative Nyquist curves of NiMo films. (A) Electrodeposited under G value of 1 at 2.4 A cm¡2. Overpotential (h): ¡95 mV. (B) Electrodeposited under G value of 740. Overpotential: ¡100 mV. Symbols were experimental data and solid lines were fitted curves.
Fig. 6 e The equivalent circuit with one time constant to describe HER.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
7
Table 3 e Electrochemical circuit parameters of NiMo films for HER at different overpotential. h (V)
Rs (U cm2) T (F sn1 cm2) n (0 < n < 1) Rct (U cm2)
G ¼ 1, 2.4 A cm2 0.045 1.33 0.070 1.36 0.095 1.50 0.120 1.32 0.170 1.43 G ¼ 740, 0.6 A cm2 0.040 0.82 0.065 1.08 0.090 1.12 0.115 1.15 0.140 0.88 0.165 0.85 G ¼ 740, 1.2 A cm2 0.050 1.17 0.075 1.32 0.100 1.24 0.125 1.58 0.150 1.26 G ¼ 740, 2.4 A cm2 0.050 1.22 0.075 1.40 0.100 1.26 0.125 1.62 0.150 1.39
0.61 0.48 0.42 0.33 0.24
0.4696 0.4729 0.4793 0.4658 0.627
1.26 1.12 0.99 0.95 0.75
0.37 0.31 0.29 0.30 0.23 0.21
0.4339 0.5380 0.6044 0.5515 0.5459 0.5355
1.08 0.86 0.67 0.66 0.64 0.62
0.38 0.37 0.34 0.35 0.28
0.5180 0.5657 0.5562 0.6719 0.6200
0.90 0.81 0.73 0.56 0.56
1.93 2.15 1.02 0.84 0.66
0.3858 0.4796 0.5297 0.8061 0.6666
0.85 0.53 0.39 0.24 0.29
densities of HER also decayed in the first 50 cycles due to the blockage of hydrogen bubbles (Fig. 8B). Then, current densities exhibited a periodic fluctuation in a certain range. The fluctuation was ascribed to the growth and disengagement of hydrogen bubbles. Further decay of current density did not happen, even after 500 cycles. The results indicated that porous NiMo films possessed good long-term stability and adhesion on substrate. After long-term stability test, surface morphologies of NiMo films were examined and shown in Fig. 9. The surface of NiMo film electrodeposited under normal
Fig. 8 e The longeterm stability of NiMo films electrodeposited under G values of 1 (A) and 740 (B) at 2.4 A cm¡2.
gravity condition (G ¼ 1) became coarser (Fig. 9A). Successive passive layer (arrows in Fig. 9B) covered entire surface during CV test, which led to the decrease of HER activity after 300 cycles. However, surface morphologies of porous NiMo films were not changed obviously (Fig. 9C and D). For compact NiMo films, real current density in positive scan during CV test was higher due to smaller active area, which led to the passivation or dissolution of NiMo. Although apparent current density was higher on porous NiMo film, real current density was lower due to larger real active area. Therefore, surface structure of porous NiMo films was hardly damaged during CV test.
Conclusions
Fig. 7 e CV curve of first cycle in 10% NaOH solution on pure Cu foil, pure Ni and electrodeposited NiMo films under G value of 1 and 740 at 2.4 A cm¡2.
An effective and feasible method to prepare thick and porous NiMo films was developed. Three-dimensional spatial NiMo film with the thickness of 180e240 mm was obtained by onestep electrodeposition under super gravity field. Surface
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
8
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Fig. 9 e SEM images of NiMo alloys after stability measurement by CV. (A, B): G ¼ 1; (C, D): G ¼ 740. Current density for deposition: 2.4 A cm¡2.
roughness of porous NiMo film was up to 19,640, which was about 4.7 times as large as that of NiMo film electrodeposited under normal gravity condition. The adjustment of bubble size and disengagement rate by super gravity field may contribute to the formation of porous structure. Threedimensional porous NiMo film exhibited good catalytic activity for HER and overpotential was only 47 mV at 100 mA cm2. The enhancement of catalytic activity was ascribed to extremely high real active area. Based on accelerated degradation studies, porous NiMo films also possessed good longterm stability. This method was promising to prepare functional metal films which were used as electrode materials in the field of water electrolysis, fuel cell and super-capacitors.
Acknowledgments This work is supported by Natural Science Foundation of China under the grant 51274180 and 50804043.
references
[1] Navarro-Flores E, Chong ZW, Omanovic S. Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium. J Mol Catal A Chem 2005;226:179e97.
[2] Xia M, Lei T, Lv NL, Li NF. Synthesis and electrocatalytic hydrogen evolution performance of Ni-Mo-Cu alloy coating electrode. Int J Hydrogen Energy 2014;39:4794e802. lez-Buch C, Valero-Vidal C, [3] Herraiz-Cardona I, Gonza rez-Herranz V. Co-Modification of Ni-based type Ortega E, Pe Raney electrodeposits for hydrogen evolution reaction in alkaline media. J Power Sources 2013;240:698e704. [4] Popczun EJ, McKone JR, Read CG, Biacchi AJ, Wiltrout AM, Lewis NS, et al. Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J Am Chem Soc 2013;135:9267e70. [5] Warren EL, McKone JR, Atwater HA, Gray HB, Lewis NS. Hydrogen-evolution characteristics of Ni-Mo-coated, radial junction, nþp-silicon microwire array photocathodes. Energy Environ Sci 2012;5:9653e61. [6] Safizadeh F, Ghali E, Houlachi G. Electrocatalysis developments for hydrogen evolution reaction in alkaline solutions-A review. Int J Hydrogen Energy 2014. http://dx.doi. org/10.1016/j.ijhydene.2014.10.109. [7] Cai J, Xu J, Wang JM, Zhang LY, Zhou H, Zhong Y, et al. Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction. Int J Hydrogen Energy 2013;38:934e41. [8] Yi Y, Lee JK, Uhm S, Nam SC, Lee J. A single-step approach to create nano-pottery structures for efficient water electrocatalysis. Electrochem Comm 2009;11:2121e4. [9] Lee JK, Yi Y, Lee HJ, Uhm S, Lee J. Electrocatalytic activity of Ni nanowires prepared by galvanic electrodeposition for hydrogen evolution reaction. Catal Today 2009;146: 188e91. [10] Ohgai T, Washio R, Tanaka Y. Anisotropic magnetization behavior of electrodeposited nanocrystalline Ni-Mo Alloy
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22] [23]
[24]
[25]
[26]
thin films and nanowires array. J Elelctrochem Soc 2012;159:H800e4. Wang MY, Wang Z, Guo ZC, Li ZJ. The enhanced electrocatalytic activity and stability of NiW films electrodeposited under super gravity field for hydrogen evolution reaction. Int J Hydrogen Energy 2011;36:3305e12. Tong H, Kong LB, Wang CM. Electroless deposition of Ag onto p-Si(100) surface under the condition of the centrifugal fields. Thin Solid Films 2006;496:360e3. Wang MY, Wang Z, Guo ZC. Understanding of the intensified effect of super gravity on hydrogen evolution reaction. Int J Hydrogen Energy 2009;34:5311e7. Sakuma G, Fukunaka Y, Matsushima H. Nucleation and growth of electrolytic gas bubbles under microgravity. Int J Hydrogen Energy 2014;39:7638e8645. Nishikawa K, Fukunaka Y, Chassaing E, Rosso M. Electrodeposition of metals in microgravity conditions. Electrochim Acta 2013;100:342e9. Kim J, Kim R, Kwon H. Preparation of copper foam with 3dimensionally interconnected spherical pore network by electrodeposition. Electrochem Comm 2008;10:1148e51. Shin H, Dong J, Liu M. Nanoporous structure prepared by an electrochemical deposition process. Adv Mater 2003;15:1610e4. Cherevko S, Xing X, Chung C. Electrodeposition of threedimensional porous silver foams. Electrochem Comm 2010;12:467e70. Cherevko S, Chung C. Direct electrodeposition of nanoporous gold with controlled multimodal pore size distribution. Electrochem Comm 2011;13:16e9. Sun S, Podlaha EJ. Electrodeposition of Mo-Rich MoNi alloys from an aqueous electrolyte. J Electrochem Soc 2011;159:D97e102. Li Y, Jia WZ, Song YY, Xia XH. Superhydrophobicity of 3D porous copper films prepared using the hydrogen bubble dynamic template. Chem Mater 2007;19:5758e64. Ramshaw C. The opportunities for exploiting centrifugal fields. Heat Recovery Syst CHP 1993;13:493e513. NV, Jovic VD, Gajic -Krstajic Lj, Jovic BM, Antozzi AL, Krstajic Martelli GN. Electrodeposition of Ni-Mo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution. Int J Hydrogen Energy 2008;33:3676e87. Feng LG, Vrubel H, Bensimon M, Hu X. Easily-prepared dinickel phosphide (Ni2P) nanoparticles as an efficient and robust electrocatalyst for hydrogen evolution. Phys Chem Chem Phys 2014;16:5917e21. Mckone JR, Marinescu SC, Brunschwig BS, Winkler JR, Gray HB. Earth-abundant hydrogen evolution electrocatalysts. Chem Sci 2014;5:865e78. lez-Buch C, Herraiz-Cardona I, Ortega E, Garcı´aGonza n J, Pe rez-Herranz V. Synthesis and characterization of Anto
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
9
macroporous Ni, Co and Ni-Co electrocatalytic deposits for hydrogen evolution reaction in alkaline media. Int J Hydrogen Energy 2013;38:10157e69. Kone JR, Sadtler BF, Werlang CA, Lewis NS, Gray HB. Ni-Mo nanopowders for efficient electrochemical hydrogen evolution. ACS Catal 2013;3:166e9. Wu L, He YH, Lei T, Nan B, Xu NP, Zou J, et al. Characterization of the porous Ni3Al-Mo electrodes during hydrogen generation from alkaline water electrolysis. Energy 2013;63:216e24. Kubisztal J, Budniok A, Lasia A. Study of the hydrogen evolution reaction on nickel-based composite coating containing molybdenum powder. Int J Hydrogen Energy 2007;32:1211e8. Dong HX, Lei T, He YH, Xu NP, Huang BY, Liu CT. Electrochemical performance of porous Ni3Al electrodes for hydrogen evolution reaction. Int J Hydrogen Energy 2011;36:12112e20. Xie ZW, He P, Du LC, Dong FQ, Dai K, Zhang TH. Comparison of four nickel-based electrodes for hydrogen evolution reaction. Electrochim Acta 2013;88:390e4. Vǎduva CC, Vaszilcsin N, Kellenberger A, Medeleanu M. Catalytic enhancement of hydrogen evolution reaction on copper in the presence of benzylamine. Int J Hydrogen Energy 2011;36:6994e7001. Coleman EJ, Co AC. Galvanic displacement of Pt on nanoporous copper: an alternative synthetic route for obtaining robust and reliable oxygen reduction activity. J Catal 2014;316:191e200. zquez-Go mez L, Cattarin S, Guerriero P, Musiani M. Va Hydrogen evolution on porous Ni cathodes modified by spontaneous deposition of Ru or Ir. Electrochim Acta 2008;53:8310e8. zquezAntozzi AL, Bargioni C, Iacopetti L, Musiani M, Va mez L. EIS study of the service life of activated cathodes Go for the hydrogen evolution reaction in the chlor-alkali membrane cell process. Electrochim Acta 2008;53:7410e6. Ahn SH, Choi I, Park H, Hwang SJ, Yoo SJ, Cho E, et al. Effect of morphology of electrodeposited Ni catalysts on the behavior of bubbles generated during the oxygen evolution reaction in alkaline water electrolysis. Chem Comm 2013;49:9323e5. Deng J, Ren PJ, Deng DH, Yu L, Yang F, Bao XH. Highly active and durable non-precious-metal catalysts encapsulated in carbon nanotubes for hydrogen evolution reaction. Energy Environ Sci 2014;7:1919e23. Wang TY, Liu L, Zhu ZW, Papakonstantinou P, Hu JB, Liu HY, et al. Enhanced electrocatalytic activity for hydrogen evolution reaction from self-assembled monodispersed molybdenum sulfide nanoparticles on an Au electrode. Energy Environ Sci 2013;6:625e33.
Please cite this article in press as: Wang M, et al., Facile one-step electrodeposition preparation of porous NiMo film as electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.022