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Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction
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Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction Jing Cai a, Jing Xu a, Jianming Wang a,*, Liying Zhang a, Huan Zhou a, Yuan Zhong a, Dan Chen a, Huiqing Fan a, Haibo Shao a, Jianqing Zhang a,b, Chu-nan Cao a,b a
Department of Chemistry, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China State Key Laboratory for Corrosion and Protection of Metal, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China b
article info
abstract
Article history:
Three-dimensional (3D) nanoporous nickel films were fabricated by a novel and facile
Received 22 July 2012
method. The fabrication process involved the heat treatment of the electrodeposited zinc
Received in revised form
layer on nickel substrate and the subsequent electrochemical dealloying. The mutual
3 October 2012
diffusion of Ni and Zn during the heat treatment resulted in the formation of the Ni2Zn11
Accepted 15 October 2012
alloy surface film. The 3D nanoporous nickel films with open pores and interconnected
Available online 19 November 2012
ligaments were obtained by the electrochemical dealloying of relatively active zinc from the alloy surface film. As the electrodeposited zinc amount increased, the thickness, pore
Keywords:
diameter and pore density of the nanoporous nickel films became larger. In our experi-
Nanoporous nickel film
mental range, the thickest nanoporous nickel film presented a thickness of 8 mm and an
Three-dimensional nanostructure
average pore diameter of 700 nm. The as-prepared 3D nanoporous nickel films exhibited
Electrochemical dealloying
much higher electrocatalytic activity for hydrogen evolution reaction (HER) than smooth
Hydrogen evolution reaction
nickel foil, and their electrocatalytic activities for HER enhanced with increase in the
ZneNi alloy film
porosity and thickness. It was concluded that the enhanced electrocatalytic activity and excellent electrochemical stability for HER of the as-prepared 3D nanoporous nickel films can be ascribed to their unique nanostructured characteristics. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nickel is a technologically important material with extensive applications in alkaline water electrolysis [1,2], batteries [3], fuel cells [4], electrochemical capacitors [5,6], and electrochemical hydrogenation of organic species [7]. The electrochemical reactions involved in the above applications are inherently surface/interface processes, thus their rates largely depend on the electrochemically active surface area of the
nickel electrode/substrate. Due to its high surface-to-volume ratio, nanoporous/microporous nickel has attracted considerable attention [8]. Among all the constructed methods, the dealloying technique has been identified as an effective preparation method for porous nickel [9]. The difference in chemical potential between the elements of an alloy in the applied electrolyte can lead to selective etching of the more active component and the formation of a porous metal. This natural corrosion process is known as
* Corresponding author. Tel.: þ86 571 87951513; fax: þ86 571 87951895. E-mail address: [email protected] (J. Wang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.084
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Scheme 1 e Schematic illustration for the fabrication of three-dimensional nanoporous nickel films.
dealloying, which has been exploited in the fabrication of porous nickel with a high surface area [10,11]. Generally, porous Raney nickel can be fabricated by caustic leaching of the Raney nickel precursor alloys (Ni/Al or Ni/Zn) obtained by cathodic codeposition [10,12,13], gas atomization [14,15], composite coating [16e18] and thermal spray [19e21]. The dissolution process results in the formation of the nanoporous structure of the remaining noble metal. However, the preparation of the Raney nickel precursor alloys is complicated, and the control of their compositions is difficult. At the present, there have been few systematic investigations towards controlling the morphology and dimensions of porous nickel films. It is still a great challenge to develop a convenient and tunable method to fabricate nanoporous nickel films with a high surface area [19,22]. Electrocatalysis of the hydrogen evolution reaction (HER) is critical to the operation of water-alkaline electrolyzers and chlor-alkali electrolyzers [2,8,13,23e29]. These two technologies are highly energy-intensive and are known to account for more than 25% of the total electrical energy consumption by industrial processes in the United States [27]. The searching on the nanoporous nickel film with low cost and high electrocatalysis performance is one of the potential approaches to reduce the related energy consumption. Here, we report a novel strategy for the fabrication of threedimensional nanoporous nickel films with tunable nanoporosity, as illustrated in Scheme 1. Zinc is first electrodeposited on nickel substrate, and then NieZn alloy surface film is formed due to the mutual diffusion of Ni and Zn during the heat treatment. In the subsequent electrochemical dealloying, the leaching of zinc results in the formation of nanoporous nickel films. We demonstrate that the pore density and dimensions of the porous nickel films can be conveniently tuned by tailoring the amount of electrodeposited zinc. The as-prepared nanoporous nickel films exhibit a much enhanced electrocatalytic activity for HER. Furthermore, the
as-constructed design strategy is verified to be effective for the formation of the nanoporous film on nickel foam.
2.
Experimental
2.1.
Preparation of nanoporous nickel film electrodes
A zinc film was prepared by a cathodic electrodeposition in the electrolyte solution with 0.6 M ZnSO4$7H2O, 0.1 M (NH4)2SO4 and 10 mM sodium dodecyl sulfate (SDS) at room temperature. The deposition process was conducted in a twoelectrode cell with a platinum counter electrode. A nickel foil with an exposed area of 2 cm2 was assembled as the working electrode. Prior to deposition, the nickel foil was polished with 2000# grit waterproof abrasive paper, and ultrasonically cleaned in acetone and deionized water for 10 min, respectively. The zinc film was deposited at a constant current density of 10 mA cm2, and the deposition time was controlled to be 15, 30 and 45 min, respectively. After rinsed extensively with deionized water and dried in a nitrogen stream, the nickel substrates with the electrodeposited zinc coatings were annealed at 400 C for 4 h in an Ar-filled tube furnace to obtain the NieZn alloy films. The selective dissolution of Zn from the alloy film was carried out in 1 M KOH solution at a selected potential, using a three-electrode cell with a platinum counter electrode and a HgO/Hg reference electrode. Consequently, nanoporous nickel films were obtained by the preceding procedure. In the following discussions, the obtained nanoporous nickel films were designated as PNF-15, PNF-30 and PNF-45, corresponding to different electrodeposition time of zinc, respectively. A 3D nanoporous film was also constructed on nickel foam by the above experimental procedure. The zinc film was deposited on nickel foam at a constant current density of 60 mA cm2 for 90 min. Other experimental procedures were
Fig. 1 e SEM images of electrodeposited zinc film on nickel substrate before (a) and after (b) heat treatment.
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Fig. 3 e Typical cyclic voltammograms (CV) of the annealed NieZn alloy film in 1 M KOH solution at room temperature at a scan rate of 20 mV sL1. The initial scan potential is L1.100 V.
2.3.
Electrochemical tests
Electrochemical measurements were performed in a typical three-electrode glass cell with a platinum counter electrode and a HgO/Hg reference electrode by a potentiostat (CHI 1140A). Various film electrodes were used as the working electrode. The electrolyte was oxygen free 1 M KOH solution achieved by bubbling N2 for 20 min before the experiments. The cathodic currentepotential curves were potentiodynamically tested at a scan rate of 5 mV s1. The overpotentialetime dependence of the electrodes was recorded in 1 M KOH solution at a constant current density of 20 mA cm2 to characterize the electrochemical stability for HER. The solutions were prepared from analytical reagents and deionized water. The solution temperature was controlled at 30 C.
Fig. 2 e XRD patterns of electrodeposited zinc film (a), annealed surface film (b) and dealloying film (c). The electrodeposition time of zinc on nickel substrate is 45 min.
identical with those for the fabrication of nanoporous films on the nickel foil.
2.2.
Physical characterization
The X-ray diffraction (XRD) patterns of film electrodes were recorded using a Rigaku D/Max 2550 X-ray diffractometer with Cu Ka radiation at 40 kV and 300 mA. The morphologies and microstructures of various film electrodes were examined by scanning electron microscopy (SEM, SIRION-100, FEI Co. Ltd.) and transmission electron microscopy (TEM/HRTEM, JEM-2010, JEOL). The surface composition of the electrodes was analyzed by energy dispersive X-ray (EDX) spectroscopy (GENE IS 4000).
Fig. 4 e Currentetime curves for the selective dissolution of the annealed NieZn alloy films with various zinc electrodeposition times in 1.0 M KOH solution at L0.830 V at room temperature. The cutoff current is 20 mA.
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Fig. 5 e Top-view and cross-sectional SEM images of PNF-15 (a, d), PNF-30 (b, e) and PNF-45 (c, f).
3.
Results and discussion
The SEM images and XRD patterns of the electrodeposited zinc films on nickel substrate before and after the heat treatment are illustrated in Figs. 1 and 2, respectively. The cathodic
electrodeposition produces a dense deposition film on nickel substrate (Fig. 1a). Fig. 2a confirms that the as-prepared film consists of metallic zinc (JCPDS card no. 65-5973). It can be seen from Fig. 1 that the dense zinc film is turned into a coarse and loose layer by the heat treatment at 400 C. As shown in Fig. 2b, the annealed surface film is composed of Ni2Zn11
Fig. 6 e TEM image (a) and SAED pattern (b) of the materials scraped from PNF-45.
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phase (JCPDS card no. 65-5310). The mutual diffusion of Ni and Zn during the heat treatment results in the formation of the NieZn alloy surface film, responsible for the change in the morphology and phase structure of the deposition film. It is noted by comparing Fig. 2a and b that the XRD peaks for Ni (JCPDS card no. 01-070-1849) are obviously enhanced by the heat treatment, which could be one evidence for the mutual diffusion of Ni and Zn. The cyclic voltammogram (CV) of the annealed NieZn alloy film electrode in 1 M KOH solution is shown in Fig. 3. For the initial scan, three oxidation peaks (a1, a2 and a3) and two reduction peaks (c2 and c3) occur in the investigated potential region. The oxidation peaks a1 (0.830 V) and a2 (0.656 V) result from the dissolution of zinc [30,31] and the transformation from metallic Ni to a-Ni(OH)2 [32e34], respectively. Note that the current gradually decreases from 0.550 to 0.420 V in the initially positive scan, which generally originates from the irreversible transformation from a-Ni(OH)2 to b-Ni(OH)2 by dehydration [32]. The oxidation peak (a3) of Ni(OH)2/NiOOH couple occurs at 0.506 V, and the corresponding reduction peak (c3) occurs at 0.408 V [35,36]. The rapid increase in current from 0.606 to 0.800 V is due to the evolution of oxygen [35]. The weak reduction wave c2 at 0.803 V can be ascribed to the transformation from Ni(OH)2 to Ni [32]. In the second scan, the peak a2 is absent, and the peak current of a1 obviously decreases. The formation of Ni(OH)2 layer in the initial CV scan leads to the passivation of Ni and the inhibition of zinc dissolution, responsible for the absence of the peak a2 and the decrease in the peak current of a1 in the second scan. Based on the preceding CV results, the dealloying potential of the annealed NieZn alloy film is determined to be 0.830 V. Fig. 4 displays the currentetime curves for the selective dissolution of zinc from the annealed alloy films with various amounts of electrodeposited zinc in 1.0 M KOH solution at 0.830 V. The three recorded curves present almost the same variation trend, that is, the dissolution current sharply increases in initial period, and then gradually decreases after passing a maximum value, and henceforth tends to level off at a very low value. The initial increase in dissolution current could result from the increase of active sites due to removal of the surface layer [37]. The decrease of zinc content in the alloy films is mainly responsible for the decay of dissolution current. It is noted from Fig. 4 that the dissolved amount of zinc from the annealed alloy films is basically proportional to the amount of electrodeposited zinc on nickel substrate. It can be reasonably concluded that the larger dissolved amount of zinc implies the higher porosity of the obtained nickel film. As shown by the XRD pattern of the dealloying film (PNF45) in Fig. 2c, the XRD peaks of Ni2Zn11 phase are absent, and the XRD peaks for Ni are greatly enhanced. Synchronously, some new XRD peaks representing for NiZn phase (JCPDS card no.47-1019) occur at 2q ¼ 43 , 47 , 58 and 68 . The EDX result reveals that the surface layer is composed of 67.51 wt% nickel, 27.53 wt% oxygen and 4.96 wt% zinc. This suggests that the dealloying film consists of a mass of Ni and a small quantity of NiZn phase, with a thin hydroxide/oxide surface layer. The stripping of most zinc during the electrochemical dealloying results in the formation of the porous nickel film and synchronously the electrochemically active Ni2Zn11 phase is
Fig. 7 e (a) Cathodic currentepotential curves of PNF-15, PNF-30, PNF-45 and smooth Ni foil in 1 M KOH solution at 30 C. (b) The dependence of the overpotential of PNF-45 on electrolysis time in 1 M KOH solution at 30 C under a constant current density of 20 mA cmL2.
converted into the relatively inactive NiZn phase. The thin hydroxide/oxide surface layer could result from the oxidation by oxygen during the fabrication of the porous nickel film. The effects of electrodeposited zinc amount on the morphologies and surface structures of the porous nickel films obtained by the electrochemical dealloying are demonstrated in Fig. 5. All the obtained nickel films exhibit a threedimensional nanoporous structure with open pores and interconnected ligaments on nanoscale, although their surface morphologies are significantly different. PNF-15, obtained by dealloying the annealed film with low zinc content, shows a film thickness of 3 mm, an average pore diameter of 500 nm and a relatively low pore density. As the zinc content increases, the thickness, pore diameter and pore density of the nanoporous nickel films become larger, suggesting their more electrochemically active sites. PNF-30 presents a film thickness of 6 mm and an average pore diameter of 600 nm, and the corresponding values for PNF-45 are 8 mm and 700 nm, respectively. The above SEM results imply that the porosity and thickness of the nanoporous nickel films can conveniently tuned only by tailoring the amount of electrodeposited zinc. It is also noted from Fig. 5 that some cracks with
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200e300 nm width, which could be induced by residual stress in the annealed alloy films, appear in the nanoporous nickel films. These cracks as well as the mentioned interconnected nanoporous structure can enhance the accessibility of the electrolyte and promote the transport of reactive species within the electrode, thereby resulting in increase in the electrochemically active sites [10,38]. The microstructure of PNF-45 was further probed by TEM observations, and the results are illustrated in Fig. 6. The TEM image in Fig. 6a exhibits that the interconnected ligaments of PNF-45 consist of many nanoparticles with diameters of 2e15 nm. The selected-area electron diffraction (SAED) pattern in Fig. 6b shows three sets of obvious diffraction rings that can be indexed as the (111), (200) and (220) crystal planes of metallic Ni from the inner to the outer. This indicates that the nanoparticles from PNF-45 are mainly composed of metallic nickel with a polycrystalline structure, which coincides with the XRD pattern in Fig. 2c. The electrocatalytic activities for HER of the as-prepared 3D nanoporous nickel films as well as smooth nickel foil are
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showed in Fig. 7a. The as-prepared nickel films exhibit much higher electrocatalytic activity (lower overpotential and larger current density) for HER than smooth nickel foil, resulting from their unique 3D nanoporous structure. The electrocatalytic activities for HER of the 3D nanoporous Ni films are in the following order: PNF-15 < PNF-30 < PNF-45. This is consistent with the increase sequence for the porosity and thickness of the nanoporous nickel films [38] (Fig. 5). The electrochemical stability of electrode materials during electrolysis is critical for their practical application [10,18]. Fig. 7b shows the overpotentialetime dependence of PNF-45 in 1 M KOH solution at 30 C under a constant current density of 20 mA cm2. The overpotential (h) value represents the difference between the reversible HER potential (vs. HgO/Hg) at the given conditions [39] and the resulting potential at 20 mA cm2. The h value gradually decreases in the initial 1 h period, and it almost keeps constant in the subsequent electrolysis for 99 h. This implies that the as-prepared 3D nanoporous nickel film (PNF-45) has the excellent electrochemical stability for HER. The initial decrease in h is probably due to
Fig. 8 e SEM images of Ni foam (a, b) and porous Ni foam (c, d, e, f) obtained by the as-constructed dealloying procedure.
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ligaments on nanoscale. The interconnected ligaments consist of many nanoparticles with diameters of 2e15 nm and have a polycrystalline nature. It is demonstrated that the thickness, pore diameter and pore density of the 3D nanoporous nickel films can be conveniently tuned by tailoring the amount of electrodeposited zinc. The as-prepared 3D nanoporous nickel films exhibit much enhanced electrocatalytic activity and excellent electrochemical stability for HER, resulting from their unique 3D nanoporous structure. The nanoporous film is also successfully constructed on nickel foam by the same design strategy, and the as-obtained nanoporous nickel foam exhibits the markedly enhanced electrocatalytic activity for HER. Further work will aim at the optimization of the present work and other applications of the 3D nanoporous nickel films. Fig. 9 e Cathodic currentepotential curves in 1 M KOH solution at 30 C of Ni foam and porous Ni foam obtained by the as-constructed dealloying procedure.
removal of hydroxide/oxide from the electrode surface [18]. It is noted that the potential fluctuation occurs on the overpotentialetime curve, which originates from the formation and escape of H2 bubbles. Nickel foam as an engineering material has been extensively applied in some hydrogen generation fields such as alkaline water electrolysis [40] and chemical hydrolysis [41]. For these applications, the nickel foam with high surface area is expected. To verify the applicability of our design approach for hydrogen generation materials, the fabrication strategy shown in Scheme 1 was applied to nickel foam. The SEM images in Fig. 8 shows that the applied design approach results in a significant morphology change of the ligaments of nickel foam, from a relatively smooth surface to a coarse and nanoporous structure. Note that the nanoporous structure on nickel foam is similar with that on the nickel foil (Fig. 5). Associated with the favorable morphology change, the electrocatalytic activity for HER of the treated nickel foam is markedly enhanced, as shown in Fig. 9. The treated nickel foam with a nanoporous structure exhibits much higher current densities for HER at potentials lower than 0.95 V, compared to the untreated nickel foam. For example, the treated nickel foam presents a current density of 0.283 A cm2 at the potential of 1.310 V, over four times higher than that of the untreated nickel foam. The superior electrocatalytic activity for HER of the as-constructed nanoporous nickel foam suggests its potential application in the hydrogen generation fields.
4.
Conclusions
The three-dimensional (3D) nanoporous nickel films have been successfully fabricated on nickel foils by a novel and facile method based on the electrochemical dealloying. The Ni2Zn11 surface layer is formed by the mutual diffusion of Ni and Zn during the heat treatment of the electrodeposited zinc film on nickel substrate. The stripping of zinc during the electrochemical dealloying results in the formation of the 3D nanoporous nickel films with open pores and interconnected
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 50972128, 51174176).
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Fabrication of three-dimensional nanoporous nickel films with tunable nanoporosity and their excellent electrocatalytic activities for hydrogen evolution reaction Jing Cai a, Jing Xu a, Jianming Wang a,*, Liying Zhang a, Huan Zhou a, Yuan Zhong a, Dan Chen a, Huiqing Fan a, Haibo Shao a, Jianqing Zhang a,b, Chu-nan Cao a,b a
Department of Chemistry, Zhejiang University, Zheda Road 38, Hangzhou 310027, PR China State Key Laboratory for Corrosion and Protection of Metal, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China b
article info
abstract
Article history:
Three-dimensional (3D) nanoporous nickel films were fabricated by a novel and facile
Received 22 July 2012
method. The fabrication process involved the heat treatment of the electrodeposited zinc
Received in revised form
layer on nickel substrate and the subsequent electrochemical dealloying. The mutual
3 October 2012
diffusion of Ni and Zn during the heat treatment resulted in the formation of the Ni2Zn11
Accepted 15 October 2012
alloy surface film. The 3D nanoporous nickel films with open pores and interconnected
Available online 19 November 2012
ligaments were obtained by the electrochemical dealloying of relatively active zinc from the alloy surface film. As the electrodeposited zinc amount increased, the thickness, pore
Keywords:
diameter and pore density of the nanoporous nickel films became larger. In our experi-
Nanoporous nickel film
mental range, the thickest nanoporous nickel film presented a thickness of 8 mm and an
Three-dimensional nanostructure
average pore diameter of 700 nm. The as-prepared 3D nanoporous nickel films exhibited
Electrochemical dealloying
much higher electrocatalytic activity for hydrogen evolution reaction (HER) than smooth
Hydrogen evolution reaction
nickel foil, and their electrocatalytic activities for HER enhanced with increase in the
ZneNi alloy film
porosity and thickness. It was concluded that the enhanced electrocatalytic activity and excellent electrochemical stability for HER of the as-prepared 3D nanoporous nickel films can be ascribed to their unique nanostructured characteristics. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Nickel is a technologically important material with extensive applications in alkaline water electrolysis [1,2], batteries [3], fuel cells [4], electrochemical capacitors [5,6], and electrochemical hydrogenation of organic species [7]. The electrochemical reactions involved in the above applications are inherently surface/interface processes, thus their rates largely depend on the electrochemically active surface area of the
nickel electrode/substrate. Due to its high surface-to-volume ratio, nanoporous/microporous nickel has attracted considerable attention [8]. Among all the constructed methods, the dealloying technique has been identified as an effective preparation method for porous nickel [9]. The difference in chemical potential between the elements of an alloy in the applied electrolyte can lead to selective etching of the more active component and the formation of a porous metal. This natural corrosion process is known as
* Corresponding author. Tel.: þ86 571 87951513; fax: þ86 571 87951895. E-mail address: [email protected] (J. Wang). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.10.084
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Scheme 1 e Schematic illustration for the fabrication of three-dimensional nanoporous nickel films.
dealloying, which has been exploited in the fabrication of porous nickel with a high surface area [10,11]. Generally, porous Raney nickel can be fabricated by caustic leaching of the Raney nickel precursor alloys (Ni/Al or Ni/Zn) obtained by cathodic codeposition [10,12,13], gas atomization [14,15], composite coating [16e18] and thermal spray [19e21]. The dissolution process results in the formation of the nanoporous structure of the remaining noble metal. However, the preparation of the Raney nickel precursor alloys is complicated, and the control of their compositions is difficult. At the present, there have been few systematic investigations towards controlling the morphology and dimensions of porous nickel films. It is still a great challenge to develop a convenient and tunable method to fabricate nanoporous nickel films with a high surface area [19,22]. Electrocatalysis of the hydrogen evolution reaction (HER) is critical to the operation of water-alkaline electrolyzers and chlor-alkali electrolyzers [2,8,13,23e29]. These two technologies are highly energy-intensive and are known to account for more than 25% of the total electrical energy consumption by industrial processes in the United States [27]. The searching on the nanoporous nickel film with low cost and high electrocatalysis performance is one of the potential approaches to reduce the related energy consumption. Here, we report a novel strategy for the fabrication of threedimensional nanoporous nickel films with tunable nanoporosity, as illustrated in Scheme 1. Zinc is first electrodeposited on nickel substrate, and then NieZn alloy surface film is formed due to the mutual diffusion of Ni and Zn during the heat treatment. In the subsequent electrochemical dealloying, the leaching of zinc results in the formation of nanoporous nickel films. We demonstrate that the pore density and dimensions of the porous nickel films can be conveniently tuned by tailoring the amount of electrodeposited zinc. The as-prepared nanoporous nickel films exhibit a much enhanced electrocatalytic activity for HER. Furthermore, the
as-constructed design strategy is verified to be effective for the formation of the nanoporous film on nickel foam.
2.
Experimental
2.1.
Preparation of nanoporous nickel film electrodes
A zinc film was prepared by a cathodic electrodeposition in the electrolyte solution with 0.6 M ZnSO4$7H2O, 0.1 M (NH4)2SO4 and 10 mM sodium dodecyl sulfate (SDS) at room temperature. The deposition process was conducted in a twoelectrode cell with a platinum counter electrode. A nickel foil with an exposed area of 2 cm2 was assembled as the working electrode. Prior to deposition, the nickel foil was polished with 2000# grit waterproof abrasive paper, and ultrasonically cleaned in acetone and deionized water for 10 min, respectively. The zinc film was deposited at a constant current density of 10 mA cm2, and the deposition time was controlled to be 15, 30 and 45 min, respectively. After rinsed extensively with deionized water and dried in a nitrogen stream, the nickel substrates with the electrodeposited zinc coatings were annealed at 400 C for 4 h in an Ar-filled tube furnace to obtain the NieZn alloy films. The selective dissolution of Zn from the alloy film was carried out in 1 M KOH solution at a selected potential, using a three-electrode cell with a platinum counter electrode and a HgO/Hg reference electrode. Consequently, nanoporous nickel films were obtained by the preceding procedure. In the following discussions, the obtained nanoporous nickel films were designated as PNF-15, PNF-30 and PNF-45, corresponding to different electrodeposition time of zinc, respectively. A 3D nanoporous film was also constructed on nickel foam by the above experimental procedure. The zinc film was deposited on nickel foam at a constant current density of 60 mA cm2 for 90 min. Other experimental procedures were
Fig. 1 e SEM images of electrodeposited zinc film on nickel substrate before (a) and after (b) heat treatment.
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Fig. 3 e Typical cyclic voltammograms (CV) of the annealed NieZn alloy film in 1 M KOH solution at room temperature at a scan rate of 20 mV sL1. The initial scan potential is L1.100 V.
2.3.
Electrochemical tests
Electrochemical measurements were performed in a typical three-electrode glass cell with a platinum counter electrode and a HgO/Hg reference electrode by a potentiostat (CHI 1140A). Various film electrodes were used as the working electrode. The electrolyte was oxygen free 1 M KOH solution achieved by bubbling N2 for 20 min before the experiments. The cathodic currentepotential curves were potentiodynamically tested at a scan rate of 5 mV s1. The overpotentialetime dependence of the electrodes was recorded in 1 M KOH solution at a constant current density of 20 mA cm2 to characterize the electrochemical stability for HER. The solutions were prepared from analytical reagents and deionized water. The solution temperature was controlled at 30 C.
Fig. 2 e XRD patterns of electrodeposited zinc film (a), annealed surface film (b) and dealloying film (c). The electrodeposition time of zinc on nickel substrate is 45 min.
identical with those for the fabrication of nanoporous films on the nickel foil.
2.2.
Physical characterization
The X-ray diffraction (XRD) patterns of film electrodes were recorded using a Rigaku D/Max 2550 X-ray diffractometer with Cu Ka radiation at 40 kV and 300 mA. The morphologies and microstructures of various film electrodes were examined by scanning electron microscopy (SEM, SIRION-100, FEI Co. Ltd.) and transmission electron microscopy (TEM/HRTEM, JEM-2010, JEOL). The surface composition of the electrodes was analyzed by energy dispersive X-ray (EDX) spectroscopy (GENE IS 4000).
Fig. 4 e Currentetime curves for the selective dissolution of the annealed NieZn alloy films with various zinc electrodeposition times in 1.0 M KOH solution at L0.830 V at room temperature. The cutoff current is 20 mA.
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Fig. 5 e Top-view and cross-sectional SEM images of PNF-15 (a, d), PNF-30 (b, e) and PNF-45 (c, f).
3.
Results and discussion
The SEM images and XRD patterns of the electrodeposited zinc films on nickel substrate before and after the heat treatment are illustrated in Figs. 1 and 2, respectively. The cathodic
electrodeposition produces a dense deposition film on nickel substrate (Fig. 1a). Fig. 2a confirms that the as-prepared film consists of metallic zinc (JCPDS card no. 65-5973). It can be seen from Fig. 1 that the dense zinc film is turned into a coarse and loose layer by the heat treatment at 400 C. As shown in Fig. 2b, the annealed surface film is composed of Ni2Zn11
Fig. 6 e TEM image (a) and SAED pattern (b) of the materials scraped from PNF-45.
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phase (JCPDS card no. 65-5310). The mutual diffusion of Ni and Zn during the heat treatment results in the formation of the NieZn alloy surface film, responsible for the change in the morphology and phase structure of the deposition film. It is noted by comparing Fig. 2a and b that the XRD peaks for Ni (JCPDS card no. 01-070-1849) are obviously enhanced by the heat treatment, which could be one evidence for the mutual diffusion of Ni and Zn. The cyclic voltammogram (CV) of the annealed NieZn alloy film electrode in 1 M KOH solution is shown in Fig. 3. For the initial scan, three oxidation peaks (a1, a2 and a3) and two reduction peaks (c2 and c3) occur in the investigated potential region. The oxidation peaks a1 (0.830 V) and a2 (0.656 V) result from the dissolution of zinc [30,31] and the transformation from metallic Ni to a-Ni(OH)2 [32e34], respectively. Note that the current gradually decreases from 0.550 to 0.420 V in the initially positive scan, which generally originates from the irreversible transformation from a-Ni(OH)2 to b-Ni(OH)2 by dehydration [32]. The oxidation peak (a3) of Ni(OH)2/NiOOH couple occurs at 0.506 V, and the corresponding reduction peak (c3) occurs at 0.408 V [35,36]. The rapid increase in current from 0.606 to 0.800 V is due to the evolution of oxygen [35]. The weak reduction wave c2 at 0.803 V can be ascribed to the transformation from Ni(OH)2 to Ni [32]. In the second scan, the peak a2 is absent, and the peak current of a1 obviously decreases. The formation of Ni(OH)2 layer in the initial CV scan leads to the passivation of Ni and the inhibition of zinc dissolution, responsible for the absence of the peak a2 and the decrease in the peak current of a1 in the second scan. Based on the preceding CV results, the dealloying potential of the annealed NieZn alloy film is determined to be 0.830 V. Fig. 4 displays the currentetime curves for the selective dissolution of zinc from the annealed alloy films with various amounts of electrodeposited zinc in 1.0 M KOH solution at 0.830 V. The three recorded curves present almost the same variation trend, that is, the dissolution current sharply increases in initial period, and then gradually decreases after passing a maximum value, and henceforth tends to level off at a very low value. The initial increase in dissolution current could result from the increase of active sites due to removal of the surface layer [37]. The decrease of zinc content in the alloy films is mainly responsible for the decay of dissolution current. It is noted from Fig. 4 that the dissolved amount of zinc from the annealed alloy films is basically proportional to the amount of electrodeposited zinc on nickel substrate. It can be reasonably concluded that the larger dissolved amount of zinc implies the higher porosity of the obtained nickel film. As shown by the XRD pattern of the dealloying film (PNF45) in Fig. 2c, the XRD peaks of Ni2Zn11 phase are absent, and the XRD peaks for Ni are greatly enhanced. Synchronously, some new XRD peaks representing for NiZn phase (JCPDS card no.47-1019) occur at 2q ¼ 43 , 47 , 58 and 68 . The EDX result reveals that the surface layer is composed of 67.51 wt% nickel, 27.53 wt% oxygen and 4.96 wt% zinc. This suggests that the dealloying film consists of a mass of Ni and a small quantity of NiZn phase, with a thin hydroxide/oxide surface layer. The stripping of most zinc during the electrochemical dealloying results in the formation of the porous nickel film and synchronously the electrochemically active Ni2Zn11 phase is
Fig. 7 e (a) Cathodic currentepotential curves of PNF-15, PNF-30, PNF-45 and smooth Ni foil in 1 M KOH solution at 30 C. (b) The dependence of the overpotential of PNF-45 on electrolysis time in 1 M KOH solution at 30 C under a constant current density of 20 mA cmL2.
converted into the relatively inactive NiZn phase. The thin hydroxide/oxide surface layer could result from the oxidation by oxygen during the fabrication of the porous nickel film. The effects of electrodeposited zinc amount on the morphologies and surface structures of the porous nickel films obtained by the electrochemical dealloying are demonstrated in Fig. 5. All the obtained nickel films exhibit a threedimensional nanoporous structure with open pores and interconnected ligaments on nanoscale, although their surface morphologies are significantly different. PNF-15, obtained by dealloying the annealed film with low zinc content, shows a film thickness of 3 mm, an average pore diameter of 500 nm and a relatively low pore density. As the zinc content increases, the thickness, pore diameter and pore density of the nanoporous nickel films become larger, suggesting their more electrochemically active sites. PNF-30 presents a film thickness of 6 mm and an average pore diameter of 600 nm, and the corresponding values for PNF-45 are 8 mm and 700 nm, respectively. The above SEM results imply that the porosity and thickness of the nanoporous nickel films can conveniently tuned only by tailoring the amount of electrodeposited zinc. It is also noted from Fig. 5 that some cracks with
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200e300 nm width, which could be induced by residual stress in the annealed alloy films, appear in the nanoporous nickel films. These cracks as well as the mentioned interconnected nanoporous structure can enhance the accessibility of the electrolyte and promote the transport of reactive species within the electrode, thereby resulting in increase in the electrochemically active sites [10,38]. The microstructure of PNF-45 was further probed by TEM observations, and the results are illustrated in Fig. 6. The TEM image in Fig. 6a exhibits that the interconnected ligaments of PNF-45 consist of many nanoparticles with diameters of 2e15 nm. The selected-area electron diffraction (SAED) pattern in Fig. 6b shows three sets of obvious diffraction rings that can be indexed as the (111), (200) and (220) crystal planes of metallic Ni from the inner to the outer. This indicates that the nanoparticles from PNF-45 are mainly composed of metallic nickel with a polycrystalline structure, which coincides with the XRD pattern in Fig. 2c. The electrocatalytic activities for HER of the as-prepared 3D nanoporous nickel films as well as smooth nickel foil are
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showed in Fig. 7a. The as-prepared nickel films exhibit much higher electrocatalytic activity (lower overpotential and larger current density) for HER than smooth nickel foil, resulting from their unique 3D nanoporous structure. The electrocatalytic activities for HER of the 3D nanoporous Ni films are in the following order: PNF-15 < PNF-30 < PNF-45. This is consistent with the increase sequence for the porosity and thickness of the nanoporous nickel films [38] (Fig. 5). The electrochemical stability of electrode materials during electrolysis is critical for their practical application [10,18]. Fig. 7b shows the overpotentialetime dependence of PNF-45 in 1 M KOH solution at 30 C under a constant current density of 20 mA cm2. The overpotential (h) value represents the difference between the reversible HER potential (vs. HgO/Hg) at the given conditions [39] and the resulting potential at 20 mA cm2. The h value gradually decreases in the initial 1 h period, and it almost keeps constant in the subsequent electrolysis for 99 h. This implies that the as-prepared 3D nanoporous nickel film (PNF-45) has the excellent electrochemical stability for HER. The initial decrease in h is probably due to
Fig. 8 e SEM images of Ni foam (a, b) and porous Ni foam (c, d, e, f) obtained by the as-constructed dealloying procedure.
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ligaments on nanoscale. The interconnected ligaments consist of many nanoparticles with diameters of 2e15 nm and have a polycrystalline nature. It is demonstrated that the thickness, pore diameter and pore density of the 3D nanoporous nickel films can be conveniently tuned by tailoring the amount of electrodeposited zinc. The as-prepared 3D nanoporous nickel films exhibit much enhanced electrocatalytic activity and excellent electrochemical stability for HER, resulting from their unique 3D nanoporous structure. The nanoporous film is also successfully constructed on nickel foam by the same design strategy, and the as-obtained nanoporous nickel foam exhibits the markedly enhanced electrocatalytic activity for HER. Further work will aim at the optimization of the present work and other applications of the 3D nanoporous nickel films. Fig. 9 e Cathodic currentepotential curves in 1 M KOH solution at 30 C of Ni foam and porous Ni foam obtained by the as-constructed dealloying procedure.
removal of hydroxide/oxide from the electrode surface [18]. It is noted that the potential fluctuation occurs on the overpotentialetime curve, which originates from the formation and escape of H2 bubbles. Nickel foam as an engineering material has been extensively applied in some hydrogen generation fields such as alkaline water electrolysis [40] and chemical hydrolysis [41]. For these applications, the nickel foam with high surface area is expected. To verify the applicability of our design approach for hydrogen generation materials, the fabrication strategy shown in Scheme 1 was applied to nickel foam. The SEM images in Fig. 8 shows that the applied design approach results in a significant morphology change of the ligaments of nickel foam, from a relatively smooth surface to a coarse and nanoporous structure. Note that the nanoporous structure on nickel foam is similar with that on the nickel foil (Fig. 5). Associated with the favorable morphology change, the electrocatalytic activity for HER of the treated nickel foam is markedly enhanced, as shown in Fig. 9. The treated nickel foam with a nanoporous structure exhibits much higher current densities for HER at potentials lower than 0.95 V, compared to the untreated nickel foam. For example, the treated nickel foam presents a current density of 0.283 A cm2 at the potential of 1.310 V, over four times higher than that of the untreated nickel foam. The superior electrocatalytic activity for HER of the as-constructed nanoporous nickel foam suggests its potential application in the hydrogen generation fields.
4.
Conclusions
The three-dimensional (3D) nanoporous nickel films have been successfully fabricated on nickel foils by a novel and facile method based on the electrochemical dealloying. The Ni2Zn11 surface layer is formed by the mutual diffusion of Ni and Zn during the heat treatment of the electrodeposited zinc film on nickel substrate. The stripping of zinc during the electrochemical dealloying results in the formation of the 3D nanoporous nickel films with open pores and interconnected
Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 50972128, 51174176).
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