Seediscussions,stats,andauthorprofilesforthispublicationat:https://www.researchgate.net/publication/272391001

Facileone-stepelectrodepositionpreparationofporousNiMofilmaselectrocatalystforhydrogenevolutionreaction

ARTICLEinINTERNATIONALJOURNALOFHYDROGENENERGY·FEBRUARY2015

ImpactFactor:3.31·DOI:10.1016/j.ijhydene.2014.12.022

CITATION

1

READS

26

4AUTHORS,INCLUDING:

MingyongWang

ChineseAcademyofSciences

31PUBLICATIONS374CITATIONS

SEEPROFILE

ZhiWang

SunYat-SenUniversity

185PUBLICATIONS1,221CITATIONS

SEEPROFILE

Zhan-chengGuo

UniversityofScienceandTechnologyBeijing

160PUBLICATIONS875CITATIONS

SEEPROFILE

Allin-textreferencesunderlinedinbluearelinkedtopublicationsonResearchGate,

lettingyouaccessandreadthemimmediately.

Availablefrom:MingyongWang

Retrievedon:14January2016

ww.sciencedirect.com

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 en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 1

Available online at w

ScienceDirect

journal homepage: www.elsevier .com/locate/he

Facile one-step electrodeposition preparation ofporous NiMo film as electrocatalyst for hydrogenevolution 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 Chinab State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083,

PR China

a r t i c l e i n f o

Article history:

Received 31 October 2014

Received in revised form

5 December 2014

Accepted 8 December 2014

Available online 31 December 2014

Keywords:

NiMo film

Electrodeposition

Catalytic activity

Hydrogen evolution reaction

Super gravity field

* Corresponding author. Tel./fax: þ86 010 82E-mail address: mywang@ipe.ac.cn (M. W

http://dx.doi.org/10.1016/j.ijhydene.2014.12.00360-3199/Copyright © 2014, Hydrogen Ener

a b s t r a c t

Porous NiMo film was prepared by one-step electrodeposition under super gravity field and

was used as electrocatalyst for hydrogen evolution reaction (HER). NiMo films possessed

three-dimensional porous structure. The thickness of porous layer was up to 180e240 mm.

Porous NiMo films with extremely large real active area exhibited high catalytic activity for

HER. The overpotential was very low and only 47 mV at 100 mA cm�2. Meanwhile, porous

NiMo films possessed good long-term stability by accelerated degradation studies.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

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

544818.ang).22gy Publications, LLC. Publ

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-basedmaterials for HERwas 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 nanoporousNi filmswith

the thickness of about 10 mm. However, the complexed

ished by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 12174

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 s�2)

and super gravity field (much higher than 9.8m s�2) 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, three-

dimensional porous NiMo films with high thickness were

directly electrodeposited under super gravity field at larger

current density than 0.6 A cm�2. Porous NiMo films possessed

extremely high active area and exhibited good catalytic

properties for HER.

Experimental

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:

G ¼ u2Lg

¼ N2p2L900g

(1)

Where N was rotating speed (rpm) of centrifuge, g was

gravity acceleration (9.8 m s�2) and L was the distance be-

tween 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 0.30M

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

similarwith the published data [1]. All chemical reagentswere

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 WYK-

3010 DC Power Supply. Unless marked, the total electric

quantity was 2880 C cm�2. NiMo films were characterized by

SEM (JEOL, JSM6700F), EDS (FEI MLA 250) and XRD (RIGAKU D/

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 andMowere 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 solu-

tion. 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 poten-

tial of �1.0 V was applied to activate NiMo film and steady-

state was reached. Tafel curves with a scan rate of 1 mV s�1

were measured to evaluate electrocatalytic activity of NiMo

films for HER. IR compensationwas 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 s�1.

Results and discussion

Characterization of NiMo films

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 elec-

trodeposited at larger current density than 0.6 A cm�2. The

morphology and crystal structure were shown in Fig. 1. Under

normal gravity condition (G¼ 1), when current density was up

to 2.4 A cm�2, 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 structurewith

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 sur-

face 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

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.

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 en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 1 2175

buoyancy term, Drg [22]. In our previous study [13], it was

found that the size of hydrogen bubbles was reduced obvi-

ously 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 electro-

deposited NiMo films was developed.

NiMo films were electrodeposited under super gravity field

(G ¼ 740). When current density was only 0.6 A cm�2, 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 cm�2, spherical particles were refined and cellular sur-

face 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 cm�2 (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 perpendic-

ular to substrate surface (Fig. 2C). The oriented porous struc-

turewas different to conventional 3D porous structure [16e19]

and can defend against the damage of the shockwave gener-

ated 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 chan-

nels 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 electrodeposi-

tion proceeded around bubble channels in a long time, which

was beneficial to the formation of the oriented porous metal

Fig. 3 e EDS of NiMo films electrodeposited under G value

of 1 (A) and 740 (B) at 2.4 A cm¡2.

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.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 12176

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].

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 elec-

trodeposition (Fig. 4B). Tafel kinetic parameters and over-

potentials of HER at 100 mA cm�2 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. Espe-

cially, at 2.4 A cm�2, overpotential of HERwas only 47mV. The

value was much lower than those of Ni-based electrocatalysts

in previous reports (Table 2) [2,3,11,23e28]. In addition,

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 dec�1. Higher Tafel slopes were also observed in

many papers, especially on porous materials [29e31]. The

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.

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 en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 1 2177

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 semicirclewas observed for every

Table 1 e HER kinetic parameters from Tafel curves,overpotential at current density of 100 mA cm¡2 andsurface roughness (Rf) of NiMo films electrodepositedunder different condition.

i (mA cm�2) 2.4 0.6 1.2 2.4

G 1 740 740 740

h100 (mV) 132 72 52 47

a (V) 0.29 0.26 0.26 0.19

b (mV dec�1) 158.6 194.9 189.4 136.7

io (mA cm�2) 14.9 44.7 41.5 42.9

Cdl (mF cm�2) 82.80 97.94 104.94 392.80

Rf 4140 4897 5247 19,640

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 equiv-

alent circuit (EEC) with one time constant was used (Fig. 6).

During fitting, double layer capacitances were replaced by

constant phase element (CPE). T (F sn�1 cm�2) 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 three-

dimensional 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 semi-

circles. 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]:

Cdl ¼"

T�R�1s þ R�1

ct

�ð1�nÞ

#1=n

(2)

WhereRswas solution resistance (U cm2).Cdl for everyNiMo

film was obtained according to EIS results. Surface roughness

(Rf) was calculated by double layer capacitances (Cdl), which

was compared with 20 mF cm�2 for smooth surface [1,23]. Cdl

and Rf were given in Table 1. Rf value of NiMo film electro-

deposited 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 cm�2 and 1.2 A cm�2 were 4897 and 5247, respectively.

When current density was 2.4 A cm�2, 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 over-

potential 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 cm�2 and 11.9 mA cm�2 for NiMo films electrodeposited

under G value of 1 and740 at 2.4 A cm�2, respectively. It indi-

cated that intrinsic catalytic activity of NiMo films was almost

unchangedbysupergravityfield.The improvementof catalytic

activitywasascribed to the increaseof real surfacearea. Porous

NiMo films with high thickness possessed three-dimensional

spatial structure. Meanwhile, porous structure was formed

byhydrogenbubble templateduringNiMoelectrodeposition at

large current density. Therefore, active surface in porous film

was more available to catalyze HER than those obtained by

dealloyingandalumina template. So, efficient active siteswere

increased obviously, which led to high electrocatalytic activity

for HER. In previous studies, nano-scale Ni-based catalysts

werepreparedandexhibitedgoodcatalytic activities [27,34,35].

Although porous NiMo films (Fig. 2) consisted of micro-scale

grains, film thicknesses were much higher. Therefore, three-

dimensional porous NiMo films possessed better activity for

HER due to efficient active surface.

Table 2 e Overpotentials of representative Ni-based catalysts for HER in alkaline solution.

Catalyst Current density(mA cm�2)

Overpotential(mV)

Electrolyte T (�C) Reference

Porous NiMoa 100 47 10% NaOH 25 This paper

Compact NiMob 100 132 10% NaOH 25 This paper

NiCox 100 110e130 30% KOH 30 3

NiW 82 150 10% NaOH 25 11

NiMo 140 250 6 M NaOH 25 2

NiMoCu 240 310 6 M NaOH 25 2

Ni2P 20 250 1 M KOH 25 24

NiCo 100 166 30% KOH 30 26

NiMo nanopowers-

1

20 70 2 M KOH 25 27

NiMo nanopowers-

2

130 100 2 M KOH 25 27

Porous Ni3AleMo 200 500 6 M KOH 25 28

a Electrodeposited under super gravity field (G ¼ 740) at 2.4 A cm�2.b Electrodeposited under normal gravity condition (G ¼ 1) at 2.4 A cm�2.

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.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 12178

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 degra-

dation 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

weremuch 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

andMo. 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 filmswas 0.098 A cm�2

at �1.3 V, while the value was only 0.056 A cm�2 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 (G¼ 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 den-

sities 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. 6 e The equivalent circuit with one time constant to

describe HER.

Table 3 e Electrochemical circuit parameters of NiMofilms for HER at different overpotential.

h (V) Rs (U cm2) T (F sn�1 cm�2) n (0 < n < 1) Rct (U cm2)

G ¼ 1, 2.4 A cm�2

�0.045 1.33 0.61 0.4696 1.26

�0.070 1.36 0.48 0.4729 1.12

�0.095 1.50 0.42 0.4793 0.99

�0.120 1.32 0.33 0.4658 0.95

�0.170 1.43 0.24 0.627 0.75

G ¼ 740, 0.6 A cm�2

�0.040 0.82 0.37 0.4339 1.08

�0.065 1.08 0.31 0.5380 0.86

�0.090 1.12 0.29 0.6044 0.67

�0.115 1.15 0.30 0.5515 0.66

�0.140 0.88 0.23 0.5459 0.64

�0.165 0.85 0.21 0.5355 0.62

G ¼ 740, 1.2 A cm�2

�0.050 1.17 0.38 0.5180 0.90

�0.075 1.32 0.37 0.5657 0.81

�0.100 1.24 0.34 0.5562 0.73

�0.125 1.58 0.35 0.6719 0.56

�0.150 1.26 0.28 0.6200 0.56

G ¼ 740, 2.4 A cm�2

�0.050 1.22 1.93 0.3858 0.85

�0.075 1.40 2.15 0.4796 0.53

�0.100 1.26 1.02 0.5297 0.39

�0.125 1.62 0.84 0.8061 0.24

�0.150 1.39 0.66 0.6666 0.29

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.

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 en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 1 2179

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 fluc-

tuation 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. 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.

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 struc-

ture of porous NiMo filmswas hardly damaged during CV test.

Conclusions

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 one-

step electrodeposition under super gravity field. Surface

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.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 12180

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. Three-

dimensional porous NiMo film exhibited good catalytic activ-

ity for HER and overpotential was only 47 mV at 100 mA cm�2.

The enhancement of catalytic activity was ascribed to

extremely high real active area. Based on accelerated degra-

dation studies, porous NiMo films also possessed good long-

term stability. This method was promising to prepare func-

tional 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.

r e f e r e n c e s

[1] Navarro-Flores E, Chong ZW, Omanovic S. Characterizationof Ni, NiMo, NiW and NiFe electroactive coatings aselectrocatalysts 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 electrocatalytichydrogen evolution performance of Ni-Mo-Cu alloy coatingelectrode. Int J Hydrogen Energy 2014;39:4794e802.

[3] Herraiz-Cardona I, Gonz�alez-Buch C, Valero-Vidal C,Ortega E, P�erez-Herranz V. Co-Modification of Ni-based typeRaney electrodeposits for hydrogen evolution reaction inalkaline 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 anelectrocatalyst for the hydrogen evolution reaction. J AmChem Soc 2013;135:9267e70.

[5] Warren EL, McKone JR, Atwater HA, Gray HB, Lewis NS.Hydrogen-evolution characteristics of Ni-Mo-coated, radialjunction, nþp-silicon microwire array photocathodes. EnergyEnviron Sci 2012;5:9653e61.

[6] Safizadeh F, Ghali E, Houlachi G. Electrocatalysisdevelopments for hydrogen evolution reaction inalkaline solutions-A review. Int J Hydrogen Energy2015;40:256e74.

[7] Cai J, Xu J, Wang JM, Zhang LY, Zhou H, Zhong Y, et al.Fabrication of three-dimensional nanoporous nickel filmswith tunable nanoporosity and their excellentelectrocatalytic activities for hydrogen evolution reaction. IntJ Hydrogen Energy 2013;38:934e41.

[8] Yi Y, Lee JK, Uhm S, Nam SC, Lee J. A single-step approach tocreate nano-pottery structures for efficient waterelectrocatalysis. Electrochem Comm 2009;11:2121e4.

[9] Lee JK, Yi Y, Lee HJ, Uhm S, Lee J. Electrocatalytic activityof Ni nanowires prepared by galvanic electrodepositionfor hydrogen evolution reaction. Catal Today 2009;146:188e91.

[10] Ohgai T, Washio R, Tanaka Y. Anisotropic magnetizationbehavior of electrodeposited nanocrystalline Ni-Mo Alloy

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 en e r g y 4 0 ( 2 0 1 5 ) 2 1 7 3e2 1 8 1 2181

thin films and nanowires array. J Elelctrochem Soc2012;159:H800e4.

[11] Wang MY, Wang Z, Guo ZC, Li ZJ. The enhancedelectrocatalytic activity and stability of NiW filmselectrodeposited under super gravity field for hydrogenevolution reaction. Int J Hydrogen Energy 2011;36:3305e12.

[12] Tong H, Kong LB, Wang CM. Electroless deposition of Ag ontop-Si(100) surface under the condition of the centrifugal fields.Thin Solid Films 2006;496:360e3.

[13] Wang MY, Wang Z, Guo ZC. Understanding of the intensifiedeffect of super gravity on hydrogen evolution reaction. Int JHydrogen Energy 2009;34:5311e7.

[14] Sakuma G, Fukunaka Y, Matsushima H. Nucleation andgrowth of electrolytic gas bubbles under microgravity. Int JHydrogen Energy 2014;39:7638e8645.

[15] Nishikawa K, Fukunaka Y, Chassaing E, Rosso M.Electrodeposition of metals in microgravity conditions.Electrochim Acta 2013;100:342e9.

[16] Kim J, Kim R, Kwon H. Preparation of copper foam with 3-dimensionally interconnected spherical pore network byelectrodeposition. Electrochem Comm 2008;10:1148e51.

[17] Shin H, Dong J, Liu M. Nanoporous structure prepared by anelectrochemical deposition process. Adv Mater2003;15:1610e4.

[18] Cherevko S, Xing X, Chung C. Electrodeposition of three-dimensional porous silver foams. Electrochem Comm2010;12:467e70.

[19] Cherevko S, Chung C. Direct electrodeposition of nanoporousgold with controlled multimodal pore size distribution.Electrochem Comm 2011;13:16e9.

[20] Sun S, Podlaha EJ. Electrodeposition of Mo-Rich MoNi alloysfrom an aqueous electrolyte. J Electrochem Soc2011;159:D97e102.

[21] Li Y, Jia WZ, Song YY, Xia XH. Superhydrophobicity of 3Dporous copper films prepared using the hydrogen bubbledynamic template. Chem Mater 2007;19:5758e64.

[22] Ramshaw C. The opportunities for exploiting centrifugalfields. Heat Recovery Syst CHP 1993;13:493e513.

[23] Krstaji�c NV, Jovi�c VD, Gaji�c-Krstaji�c Lj, Jovi�c BM, Antozzi AL,Martelli GN. Electrodeposition of Ni-Mo alloy coatings andtheir characterization as cathodes for hydrogen evolution insodium hydroxide solution. Int J Hydrogen Energy2008;33:3676e87.

[24] Feng LG, Vrubel H, Bensimon M, Hu X. Easily-prepareddinickel phosphide (Ni2P) nanoparticles as an efficient androbust electrocatalyst for hydrogen evolution. Phys ChemChem Phys 2014;16:5917e21.

[25] Mckone JR, Marinescu SC, Brunschwig BS, Winkler JR,Gray HB. Earth-abundant hydrogen evolutionelectrocatalysts. Chem Sci 2014;5:865e78.

[26] Gonz�alez-Buch C, Herraiz-Cardona I, Ortega E, Garcı́a-Ant�on J, P�erez-Herranz V. Synthesis and characterization of

macroporous Ni, Co and Ni-Co electrocatalytic deposits forhydrogen evolution reaction in alkaline media. Int JHydrogen Energy 2013;38:10157e69.

[27] Kone JR, Sadtler BF, Werlang CA, Lewis NS, Gray HB. Ni-Monanopowders for efficient electrochemical hydrogenevolution. ACS Catal 2013;3:166e9.

[28] Wu L, He YH, Lei T, Nan B, Xu NP, Zou J, et al.Characterization of the porous Ni3Al-Mo electrodes duringhydrogen generation from alkaline water electrolysis. Energy2013;63:216e24.

[29] Kubisztal J, Budniok A, Lasia A. Study of the hydrogenevolution reaction on nickel-based composite coatingcontaining molybdenum powder. Int J Hydrogen Energy2007;32:1211e8.

[30] Dong HX, Lei T, He YH, Xu NP, Huang BY, Liu CT.Electrochemical performance of porous Ni3Al electrodes forhydrogen evolution reaction. Int J Hydrogen Energy2011;36:12112e20.

[31] Xie ZW, He P, Du LC, Dong FQ, Dai K, Zhang TH. Comparisonof four nickel-based electrodes for hydrogen evolutionreaction. Electrochim Acta 2013;88:390e4.

[32] Vǎduva CC, Vaszilcsin N, Kellenberger A, Medeleanu M.Catalytic enhancement of hydrogen evolution reaction oncopper in the presence of benzylamine. Int J HydrogenEnergy 2011;36:6994e7001.

[33] Coleman EJ, Co AC. Galvanic displacement of Pt onnanoporous copper: an alternative synthetic route forobtaining robust and reliable oxygen reduction activity. JCatal 2014;316:191e200.

[34] V�azquez-G�omez L, Cattarin S, Guerriero P, Musiani M.Hydrogen evolution on porous Ni cathodes modified byspontaneous deposition of Ru or Ir. Electrochim Acta2008;53:8310e8.

[35] Antozzi AL, Bargioni C, Iacopetti L, Musiani M, V�azquez-G�omez L. EIS study of the service life of activated cathodesfor the hydrogen evolution reaction in the chlor-alkalimembrane cell process. Electrochim Acta 2008;53:7410e6.

[36] Ahn SH, Choi I, Park H, Hwang SJ, Yoo SJ, Cho E, et al. Effectof morphology of electrodeposited Ni catalysts on thebehavior of bubbles generated during the oxygen evolutionreaction in alkaline water electrolysis. Chem Comm2013;49:9323e5.

[37] Deng J, Ren PJ, Deng DH, Yu L, Yang F, Bao XH. Highly activeand durable non-precious-metal catalysts encapsulated incarbon nanotubes for hydrogen evolution reaction. EnergyEnviron Sci 2014;7:1919e23.

[38] Wang TY, Liu L, Zhu ZW, Papakonstantinou P, Hu JB, Liu HY,et al. Enhanced electrocatalytic activity for hydrogenevolution reaction from self-assembled monodispersedmolybdenum sulfide nanoparticles on an Au electrode.Energy Environ Sci 2013;6:625e33.