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Time-Dependent Surface Structure Evolution of NiMo Films Electrodeposited Under Super Gravity Field as Electrocatalyst for Hydrogen Evolution Reaction Xiangtao Yu, Mingyong Wang, Zhi Wang, Xuzhong Gong, and Zhancheng Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03822 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Time-dependent Surface Structure Evolution of NiMo Films Electrodeposited Under Super Gravity Field as Electrocatalyst for Hydrogen Evolution Reaction Xiangtao Yua, Mingyong Wangb,∗∗, Zhi Wangb, Xuzhong Gongb, Zhancheng Guoa a

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing100083, China b 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, China Abstract The surface structures of the electrodeposited NiMo films from compact to porous structure are adjusted by the combination of gravity acceleration and electrodeposition time. The evolution mechanism of surface structure is discussed based on the protrusion growth theory. The dependence of catalytic activity of the NiMo films for hydrogen evolution reaction (HER) on surface structure evolution is studied. The results indicate that compact NiMo layer is firstly electrodeposited, and then protrusions are formed. Finally, the protrusions rapidly grow and form a porous structure. It is found that only the compact NiMo films are electrodeposited under normal gravity condition due to a long induced time for the protrusions formation. Under super gravity field, the induced time for the protrusions formation is only less than 5 mins owing to the enhanced mass transfer by the gravity-induced convection and hydrogen bubble agitation convection. So, a porous structure is easily formed under high gravity acceleration and long electrodeposition time. The HER activities of NiMo films are improved with the surface structure evolution from compact to porous

Corresponding author. Tel & Fax: 86-010-82544818, E-mail: [email protected] (M Y Wang) 1

ACS Paragon Plus Environment


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structure due to the increase of active area. All NiMo films exhibit a good long-term durability and the cell voltage of water electrolysis on porous NiMo films is lower. 1. Introduction Ni-based electrocatalysts consisting of earth-abundant elements are considered as a promising alternative to noble metal catalysts for hydrogen evolution reaction (HER).1-5 Generally, it is believed that the introduction of Mo into the NiMo alloy results in a pronounced increase of the density of states in Ni 3d orbitals at the Fermi level. Thus, the binding energy (∆GH) of H atom to the surface of catalyst is more close to the optimum value of zero.1,6-8 So, NiMo alloy possesses a high intrinsic catalytic activity for HER. In addition, the catalytic activity of electrocatalysts is determined by real active area. In order to further enhance the activity, it is necessary to enlarge the specific surface area of the NiMo alloy. Electrodeposition is a convenient and effective technique for the preparation of metal films. Various NiMo films were electrodeposited for HER.9-12 Electrodeposition conditions, such as bath composition, additives, electrodeposition time, and current density, were widely optimized to adjust the morphology of the NiMo films. However, the improvement of catalytic activity was still a challenge. Recently, the dynamic hydrogen bubble template (DHBT) method was considered as a facile and ideal method to prepare 3D porous metal films with a high specific surface area.13-18 The surface structure of porous films mainly depended on metal electrocrystallization13,19 and bubble behavior.13,20,21 In the past decade, super gravity field (also known as a centrifugal field or a 2


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gravitation field) attracted wide attention due to the enhanced mass transfer and rapid bubble separation during the electrochemical reaction. Metal electrocrystallization and bubble behavior can be adjusted by super gravity field in a large range. Therefore, the functional films with excellent properties can be electrochemically prepared. Eftekhari22 electrodeposited soft-magnetic CoNiFe alloys with high saturation magnetic flux density under a super gravity field. Chen electrodeposited Ni-CNTs23 and Ni-CeO224 composite cathodes with high catalytic activity for HER under a super gravity field. The recent progress toward the preparation of functional films by using super gravity field was reviewed in our previous review.25 Particularly, the porous NiMo films with a thickness of 180-240 µm were electrodeposited under super gravity field and exhibited a high catalytic activity for HER.26 However, the formation process and evolution mechanism of the porous structure are not analyzed. The dependence of HER activity on the surface structure is unclear. From the cross-section image of porous NiMo films,26 the porous structure varies with the change of distance perpendicular to electrode surface. It meant that the surface structure of NiMo films and the formation process of pores depended on the electrodeposition time, especially in the presence of bubbles. It may be related to the change of the local current density and ion concentration which determined the growth rate of metal film.27-30 Therefore, it was necessary to understand the surface structure evolution during the NiMo electrodeposition in order to maximize the catalytic activity for HER. In this work, the time-dependent surface structure evolutions for NiMo films 3



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electrodeposited under normal gravity condition and super gravity field were studied. The formation mechanism of the porous structure was discussed based on protrusion growth theory and the simulation analysis of the local distributions of current density and ion concentration by COMSOL software. The dependence of electrocatalytic activity for HER on surface structure was also studied. 2. Experimental The experimental equipment to obtain super gravity field has been described in detail in our previous papers.26,31 Gravity coefficient (G) was calculated by equation (1):

G=

ω2L g

=

N 2π 2 L 900 g

(1)

Where N was the rotating speed (rpm), g was the gravity acceleration under normal gravity condition (9.81 m s-2) and L was the distance between electrode center and axis (0.23 m in this experiment). The G value was adjusted by rotating an electrolytic cell with different speeds. The G value was 1 under normal gravity condition. The solution for NiMo electrodeposition was composed of 0.3 M NiSO4·6H2O (Sigma-Aldrich, 98.0%), 0.2 M Na2MoO4·2H2O (Sigma-Aldrich, 99.0%), and 0.3 M Na3C6H5O7 (Sigma-Aldrich, 99.0%). The pH was adjusted to 10.5 using ammonium hydroxide (Sigma-Aldrich, 25%-28%). The NiMo films were electrodeposited for 10-80 mins under normal gravity condition (G=1) and super gravity field (G=740). The electrodeposition was carried out in a standard three-electrode cell using a CHI 760C electrochemical workstation (CH Instrument, Inc.) at the cathode current 4


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density of 0.6 or 2.4 A cm-2. The reference electrode was a solid state Ag/AgCl electrode (GD-IV, Beijing Research Institute of Chemical Engineering and Metallurgy). The potential difference between the reference electrode and saturated calomel electrode (SCE) was -56 ± 5 mV.31,32 The working electrode and counter electrode were a Cu foil with an exposed area of 5 mm×5 mm and a Pt foil, respectively. Potential-time curves for NiMo galvanostatic deposition were recorded. The surface morphologies and chemical composition of NiMo films were examined by the field emission scanning electron microscopy (FESEM) (JEOL JSM-7001F) and energy dispersive X-ray spectroscopy (EDS) (Oxford Instruments, UK INCA X-MAX), respectively. The EDS mapping images showed the distribution of the elements on the surface of films. The microstructure of NiMo films was investigated by high resolution transmission electron microscopy (HRTEM) (JEM-2100F). For the HRTEM analysis, the metal deposits were raked off from the substrate with tweezers, and then dispersed in ethanol by sonication for 30 mins. The crystal structure of NiMo films was analyzed by X-ray diffractometer (XRD) (RIGAKU D/max-RB). The average grain sizes were counted by Image-Pro plus software (IPP) according to SEM images in an area of about 0.035 mm2. The deposits were dissolved by the concentrated nitric acid, and diluted to 100 ml. Ni and Mo concentrations were determined by inductively coupled plasma atomic emission spectrometer (ICP) (PE Optima 6300DV). Then, Ni and Mo contents in deposits were calculated. The current efficiencies (θ) for metal electrodeposition were calculated by formula (2)：

5



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q = (

2mNi F 6mMo F ) ´ 100% + ItM Ni ItM Mo

(2)

Where mNi and mMo are the amount of Ni and Mo deposit in mass (g), respectively. F is the faraday constant (96485 C mol-1). I is the current intensity (A). t is the electrodeposition time (s). MNi and MMo are the molecular mass of Ni and Mo (g mol-1). The polarization curves were performed under various gravity conditions in 0.3 M Na2SO4 (Sigma-Aldrich, 99.0%) and 0.3 M Na3C6H5O7 solution with or without NiSO4 and Na2MoO4. For polarization curve measurements, a glassy carbon electrode (0.0314 cm2) with higher HER overpotential was used as working electrode. The reference electrode and counter electrode were the same to those used for the NiMo electrodeposition. The potential range was -0.7 V to -1.9 V and the scan rate was 30 mV s-1. The distributions of current density and the ion concentration near the cathode were simulated by the COMSOL software. The catalytic activities of various NiMo films for HER were investigated by linear sweep voltammetry (LSV), Tafel and electrochemical impedance spectroscopy (EIS) in 10 wt% NaOH (Sigma-Aldrich, 99.0%) solution under normal gravity condition. The LSV measurements were carried out at a scan rate of 10 mV s-1 in the potential range of -0.85 V to -1.60 V. For Tafel curves, the scan rate was 1 mV s-1. The EIS measurements were performed in the frequencies range of 100 kHz to 0.01 Hz at various overpotentials. The AC amplitude was 5 mV. The complex nonlinear least square (Cnls) fitting of the impedance data was carried out using the ZsimDemo 3.30d 6


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software. The long-term water electrolysis was performed in a H-type cell separated by a Fluorosulfonate Proton Exchange Membrane at 0.4 A cm-2 in 10 wt% NaOH solution. The cell voltages were recorded by paperless recorder to evaluate the durability of NiMo films. All experiments were repeated at least twice under the same conditions to ensure reproducibility and accuracy.

3. Results and discussion 3.1 The surface structure evolution of NiMo films during electrodeposition Figure 1 showed the SEM images of NiMo films electrodeposited under normal gravity condition (G=1) at -0.6 A cm-2 for different time. When the electrodeposition time was 40 mins, flat NiMo films with less spherical grains were obtained (Figure 1a and b). With the increase of the electrodeposition time, spherical grains grew (Figure 1c and d). However, after 80 mins, NiMo films were still compact and consisted of spherical grains (Figure 1e and f). The surface of the grains was cellular and rough (insets in Figure 1). With the increase of electrodeposition time, the grain number decreased, while the average diameter of the grains increased from 13 µm to 27 µm (Table S1 in the supporting information). Some microcracks were observed (arrows in Figure 1b and d), which may be ascribed to the release of the tensile stresses.33-35 The dependence of the surface morphology of the NiMo films electrodeposited under a super gravity field (G=740) on electrodeposition time was also studied and shown in Figure 2. At -0.6 A cm-2, the NiMo film was also compact in the first 5 mins (Figure 2a). However, lots of dispersed protrusions with a size of about 7 µm were generated on the film surface (Figure 2b and Table S1). After 10 mins, the porous 7



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NiMo film was obtained due to the rapid formation and growth of protrusions (Figure 2c). There were many channels around the protrusions (Figure 2d). The channels were produced due to the successive separation of hydrogen bubbles under a super gravity field. The porous structure could significantly increase the active sites for HER. When the electrodeposition time was increased to 40 mins, the grains grew to about 40 µm in size (Table S1). The rough surface of the grains became smooth (insets in Figure 2d and f). The pores with a diameter of about 4 µm were also formed on grains (arrows in Figure 2f) due to the separation of hydrogen bubbles. When the electrodeposition time was further increased, the grains grew and the surface became smoother. The porous structure of NiMo films was further developed (Figure 2g-j).

(b)

(a)

100µm

10µm

(d)

(c)

10µm

100µm

10µm

(f)

(e)

10µm

10µm

100µm

8


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Figure 1. SEM images of NiMo films electrodeposited under normal gravity condition (G=1) at -0.6 A cm-2. (a,b) 40 mins, (c,d) 60 mins, (e,f) 80 mins. (b), (d), and (f) are the magnification of (a), (c) and (e). The insets in (b), (d) and (f) are the magnification of corresponding aggregate grains. When the current density was increased to -2.4 A cm-2, the surface structure evolution was similar to that at -0.6 A cm-2 for NiMo films electrodeposited under normal gravity condition (Figure 3a and b). The films were still compact, and a porous structure was not formed. For NiMo films electrodeposited under super gravity field, the structure evolution process was also similar with that at -0.6 A cm-2 (Figure 3c and d). The compact film with the protrusions was firstly electrodeposited, and then was transformed to porous metal film. However, the gain sizes were much smaller at -2.4 A cm-2 (Table S1). In addition, it was worth noting that the grain numbers per mm2 for the NiMo films electrodeposited under normal gravity condition were larger than that of NiMo films electrodeposited under super gravity field (Table S1). It suggested that the nucleation probability of NiMo films was decreased under super gravity field, which was beneficial to the rapid growth of the grains. So, the grains were larger under super gravity field and can be refined by increasing current density at the same electric quantity.

9



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(b)

(a)

20µm

100µm

(c)

(d)

20µm

100µm

(f)

(e)

10µm

20µm

100µm

(g)

(h)

10µm

20µm

100µm

(j)

(i)

10µm

20µm

100µm

Figure 2. SEM images of NiMo films electrodeposited under super gravity field (G=740) at -0.6 A cm-2. (a,b) 5 mins, (c,d) 10 mins, (e,f) 40 mins, (g,h) 60 mins and 10


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(i,j) 80 mins. (b), (d), (f), (h) and (j) are the magnification of (a), (c), (e), (g) and (i). The insets in (b), (d), (f), (h) and (j) are the magnification of corresponding aggregate grains. The surface structure of the electrodeposits was affected by the chemical compositions which were determined by the electrochemical reactions on the electrode. Podlaha and Landolt36-38 proposed a surface-adsorbed intermediate mechanism for NiMo coelectrodeposition. The molybdate was firstly reduced to Mo(IV) oxide (reaction (3)) as an intermediate, and then was reduced to molybdenum by the catalytic action of inducing nickel (reaction (4) and (5)). Therefore, NiMo alloys were electrodeposited. The chemical compositions and element distribution of NiMo films were shown in Figure 3b, Figure 3d, Table S2, and Figure S1 in the supporting information. It was well known that the high Ni2+ concentration in the solution contributed to the deposition of the metallic molybdenum instead of Mo(IV) or Mo(V) oxides.39 Only Ni and Mo were detected by EDS. The similar results were also observed in previous study.39 Ni was the main component in NiMo films (Figure 3b and d) and the content in the film electrodeposited under super gravity field (84.7 at%) was higher than that (76.5 at%) under normal gravity condition (Table S2). From EDS mapping images (Figure S1), Ni and Mo elements distributed uniformly in the films and phase separation did not occur. In order to study the effect of electrodeposition time on the composition distribution, the EDS linear scanning spectra for the cross-section of NiMo films was recorded and shown in Figure S2 in the supporting information. Both Ni and Mo distributed uniformly in the direction of 11



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growth. That is, the composition was hardly changed with the surface structure evolution. MoO42- + 2e- + 4H + = MoO2 + 2H2O

(3)

Ni2+ + 2e- = Ni

(4)

MoO2 + 4e- + 4H + = Mo + 2H2O

(5)

The cross-sectional SEM images of NiMo films were shown in Figure 3e and f. For NiMo films electrodeposited under normal gravity condition (Figure 3e), the compact layer with a thickness of about 41 µm was firstly electrodeposited at the bottom, and then the semi-spherical protrusions were formed on the top of the film. The total film thickness was about 77 µm. The porous structure was not formed and only the microcracks appeared on the compact NiMo layer due to the release of the tensile stresses (arrow in Figure 3e).33-35 For NiMo films electrodeposited under super gravity field (Figure 3f), a 3D porous structure consisting of NiMo dendrites was formed. The compact layer was very thin. The particles size increased with the increase of the distance to the substrate (i.e. the increase of electrodeposition time), which was consistent with the results in Table S1. The channels between the dendrites were formed due to a successive separation of hydrogen bubbles under super gravity field. The thickness of NiMo films was up to 217 µm due to the developed porous structures and was significantly larger than that under normal gravity condition. The current efficiency of NiMo electrodeposition under super gravity field at -2.4 A cm-2 was 6.63% which was much higher than that (1.05%) under normal gravity condition (Table S2). In order to construct porous structure by using bubbles as template, the 12


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electrodeposition of NiMo must be carried out at large current density. The serious hydrogen evolution reaction led to the low current efficiency.40,41 Although the amount of hydrogen evolution was larger under normal gravity condition than that under super gravity field, compact NiMo films were only produced. According to our previous study,32 the size of bubbles was much smaller under super gravity field than that under normal gravity condition. So, the number of bubbles may be much larger under super gravity field. Therefore, the successive and stable bubble channels were formed easily, which was beneficial to the formation of the oriented porous structure.

(a)

(b) Ni

100µm

100µm Mo

Ni

Ni (at.%) Mo (at.%) 76.5

23.5

Ni

10µm

0

5

10

15

10µm

20

Energy /keV

(d)

(c)

Ni

100µm

100µm Ni

Mo

Ni

20µm 0

5

Ni (at.%) Mo (at.%) 84.7 15.3

20µm 10

15

20

Energy /keV

(e) NiMo

(f) 77 µm NiMo 217 µm

Compact layer Cu

50µm

Cu 13


50µm


Figure 3. Top-view and cross-sectional SEM images of NiMo films electrodeposited at -2.4 A cm-2 under G value of 1 (a,b,e) and G value of 740 (c,d,f) for various electrodeposition time: (a,c) 10 mins and (b,d,e,f) 20 mins. The inset SEM images in (a), (b), (c) and (d) are the minification of the corresponding films, and the inset spectra in (b) and (d) are the corresponding EDS spectra and chemical compositions. The microstructure and crystal structure of NiMo films were investigated by HRTEM. The grain size of NiMo films was about 4 µm (Figure 4a), which was consistent with the results in Table S1. The HRTEM image and corresponding selected area electron diffraction (SAED) pattern were shown in Figure 4b and c, respectively. There was no obvious lattice space in the HRTEM image and only a diffuse halo was detected in the SAED pattern of NiMo films. The results indicated that the NiMo films may be mainly amorphous.

(a)

(b)

(c)

10 nm

G=1

(e)

Cu(111) Ni(111) Cu(220) Cu(200)

40 min

Intensity (a.u.)

(d) Intensity (a.u.)

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G=210 G=416

30

Cu(220)

60 min

80 min

G=740 20

Cu(111) Ni(111) Cu(200)

40

50

60

70

80

20

90

30

40

2θ / deg

50 60 2θ / deg

70

80

90

Figure 4. The TEM images (a), HRTEM image (b) and SAED pattern (c) of the NiMo 14


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film electrodeposited under G value of 740, at -2.4 A cm-2. The XRD patterns of NiMo films electrodeposited under different G value (d) and different time under G value of 740 (e) at -2.4 A cm-2. The crystal structures of NiMo films were further examined by XRD. Besides the peaks of Cu substrate, a broad and less sharp peak was seen in the XRD patterns. With the increase of the G value (Figure 4d) or the electrodeposition time (Figure 4e), the broad peak of NiMo film became wider. The results were similar with those for amorphous NiMo films in previous works.42,43 In addition, a shoulder peak at 44.8º was also found, and indicated that crystalline Ni particles may be embedded in amorphous NiMo. The similar coexistence phenomenon of amorphous and crystalline structures was also found in electrodeposited Co-P films.44 3.2 The formation mechanism of porous NiMo films The surface structure of NiMo films was related to metal electrocrystallization and hydrogen evolution reaction during the electrodeposition process. The structure evolution and the bubble behavior on the electrode surface can be clearly reflected by potential-time (E-t) curves (Figure 5a).45-47 For the E-t curve of NiMo electrodeposition under normal gravity condition, a large potential fluctuation with the amplitude of about 0.8 V was observed due to the growth and separation of hydrogen bubbles with large break-off diameter.47 In the first 1400 s, the average value of the potential was almost constant. Then, the potential shifted positively after 1400 s. According to the Nernst Equation, the reduction potential of metal ions was related to the concentration. However, bulk concentration decreased slightly due to 15



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the tiny consumption of metal ions in NiMo solution. Therefore, the reduction potential of metal ions was hardly changed. It meant that the potential shift should be ascribed to the surface structure evolution. That is, the real surface area increased due to the generation of the protrusions, which resulted in the decrease of real current density.48 Because the electrodeposition was carried out in galvanostatic mode, the overpotential for metal electroreduction decreased with the decrease of real current density according to the Tafel equation.45 In addition, the increase of surface roughness due to the growth of protrusions was beneficial to improve the wettability of film surface.49 So, it was easier for hydrogen bubbles to be separated from film surface. Therefore, the break-off diameters of bubbles were decreased, leading to low bubble coverage on the film surface.47 Consequently, the ohmic resistance due to the adsorption of bubbles on the NiMo film surface was decreased. It also led to the positive shift of the potential.19,50 For the E-t curve under super gravity field (G=740), the potentials were much more positive than those under normal gravity condition due to the enhanced mass transfer of metal ions and the rapid disengagement of bubbles.25,31 No obvious potential fluctuation was found due to the small break-off diameter of bubbles. Three regions in E-t curve appeared under super gravity field: the growth region of compact layer in the first 200 s, protrusion growth region in the time from 200 s to 600 s and porous structure evolution region after 600 s. In the growth region of compact layer, the potential was relatively negative. Then, the protrusions grew rapidly. Hence, the overpotential and ohmic resistance decreased due to the increase of the real surface area and better wettability. So, the potential rapidly 16


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shifted positively. After 600 s, a porous structure was formed and surface structure became relatively stable. Therefore, the potential slowly shifted positively. The results were consistent to those of SEM images.

(a)

-1.5 E / V (vs Ag/AgCl)

-2.0 G = 740

-2.5 -3.0

porous structure evolution

Protuberance growth

-3.5 Compact layer -4.0

0.8 V

-4.5 -5.0

G=1

-5.5 0

500 1000 1500 2000 2500 3000 3500

t/ s

0

Mass transport control

(b)

Activation control

A

i / mA cm

-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-20 B C D

-40 G

H

E F

Hydrogen evolution

I

A, E: G = 1

-60

B, F: G = 210 C, H: G = 416

-80

D, I: G = 740

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

E / V(vs Ag/AgCl) Figure 5. (a) Potential-time curves for NiMo galvanostatic deposition under various gravity conditions at -2.4 A cm-2. (b) Polarization curves under various gravity conditions: (A-D) in Ni/Mo-free solution with 0.3 M Na2SO4, 0.3 M Na3C6H5O7, 17



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pH=10.5, (E-I) in solution with 0.3 M NiSO4·6H2O, 0.2 M Na2MoO4·2H2O, 0.3 M Na2SO4, 0.3 M Na3C6H5O7, pH=10.5. According to the above results, during NiMo electrodeposition at large current density, compact films were firstly electrodeposited under all gravity conditions. Then, the protrusions were produced on the film surface. An induced time must be achieved for the protrusion growth. From the results in Figure 1 and Figure 2, the induced time for protrusions under super gravity field was much shorter than that under normal gravity condition. That is, super gravity field accelerated the formation of protrusions and porous NiMo films were easily prepared. Based on the protrusion growth theory of metal electrocrystallization, the protrusions

growth

was

influenced

by

the

kinetics

factors

of

metal

electroreduction.50,51 The relationships of the protrusion height (Y), the induced time for protrusions growth (τ) and the kinetics factors (such as the limiting diffusion current density id, diffusion layer thickness δ and overpotential η) were expressed as follows:50,51

t Y = Y 0 exp( )

(6)

τ

   id nF   τ= +δ  V   α nF   η i exp   0   RT 

2

  nF   id 1 − exp  − η   RT   

(7)

At high η, formula (7) can be simplified to formula (8):50,51

τ ∝ δ 2 C*

(8)

where Y0 is the initial height of protrusion, t is the deposition time, n is the number of 18


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electron transfer, F is Faraday constant (96485 C mol-1), V is the molar volume of metal (m3 mol-1), i0 is the exchanged current density (A m-2), R is the constant, T is the temperature (K) and C* is the interfacial concentration of the metal ions (M). For polarization curves obtained in Ni/Mo-free solution ((A)-(D) in Figure 5b, only hydrogen evolution reaction happened. At a certain potential, the current densities of HER under super gravity field were much higher than that under normal gravity condition, and increased slowly with the increase of G value. Apparently, the hydrogen evolution reaction was enhanced by super gravity field. In our previous papers,25,31,32 it was confirmed that hydrogen bubbles would be disengaged rapidly from electrode surface under super gravity field due to smaller break-off diameters and larger interphase buoyancy force (∆ρg).52 The agitation induced by strong bubble separation can enhance the transfer of metal ions from bulk solution to the interface region of cathode.53 Moreover, according to Nikolić,20 the bubble agitation effect would be enhanced by the decrease of the break-off diameter, which can reduce the δ value. As a result, the C* value was increased under super gravity field. It was beneficial to shorten the induced time of protrusion formation according to formula (8). For the polarization curves of NiMo electrodeposition ((E)-(I) in Figure 5b), it was found that the electrodeposition of NiMo started at about -1.2 V. Then, the current density rapidly increased with the negative shift of the potential. It indicated that NiMo electrodeposition was an activation control process in this stage.20,53 With further negative shift of potential, an obvious platform of current density appeared in 19



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the potential range from about -1.25 V to -1.4 V. The phenomenon indicated that NiMo electrodeposition was controlled by mass transfer.20,53 The slow increases of current density with the negative shift of the potential in the platform stage should be attributed to slight HER. When the potential was more negative than about -1.4 V, the current density rapidly increased due to the occurrence of serious HER on the freshly generated NiMo films surface. Compared to that under normal gravity condition, the onset potentials of the limiting diffusion current density plateau shifted positively with the increase of G value. The limiting diffusion current density increased from about 11.3 mA cm-2 (G=1) to about 14.7 mA cm-2 (G =210). When G value was up to 416 and 740, the limiting diffusion current densities were up to about 15.9 mA cm-2 and 17.3 mA cm-2, respectively. The results also confirmed that the mass transfer of metal ions was enhanced by super gravity field due to the gravity-induced convection and hydrogen bubble agitation convection.54-56 That is, the thickness of diffusion layer (δ) reduced and the interfacial concentration of the metal ions (C*) increased under super gravity field. According to formula (8), the induced time (τ) for protrusion formation was reduced. It was seen from Figure 2 that the surface structure of NiMo films changed from a compact layer to a porous structure after the formation of protrusions. To better understanding the evolution of surface structure, the distributions of current density and metal ion concentration on electrode/solution interface were simulated. The 2D model geometry was shown in Figure S3 and SI.1 in the supporting information. Under normal gravity condition, the effective protrusion was hardly produced because 20


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of the long induced time for protrusion formation. So, the cathode surface was flat (Figure S3a). However, the induced time for protrusion formation was reduced under super gravity field, and the protrusions were easily formed. Semi-ellipse protrusions were generated on the cathode surface (Figure S3b and SI.1), and the profile of the protrusions in the model was built according to the SEM images. A concentration-dependent kinetic model was assumed. The relationship between the local current density iLOC and the overpotential η, metal concentration CM was shown as follows:57,58

1.5Fη C 0.5Fη iLOC = i0 (exp( ) − M exp(− )) RT CM , ref RT

(9)

Where CM is the concentration of Ni2+ in the primary reaction (or the concentration of MoO42- in the accompanying reaction) (M), CM,ref is the corresponding equilibrium concentration (M). Moving meshes were used to simulate the displacement of the cathode boundary. As shown in Figure 6a, the current density and the ion concentration distributed uniformly on the cathode surface under normal gravity condition. The ion transfer was controlled by linear diffusion. The uniform and flat film was preferentially formed. Moreover, the linear diffusion would lead to large overpotential due to serious concentration polarization. The relationships of nucleation formation energy A (J mol-1), nucleation probability W, critical nucleation radius hi (m) and overpotential η were expressed as:59

21



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16πσ 2V 2 3 z 2 F 2η 2

(10)

W = B exp(−b / η 2 )

(11)

zFη = 2σ V / hi

(12)

A=

Where z is the number of electron transfer, B and b are constants, σ is interfacial free energy (J m-2). With the increase of the overpotential, the nucleation formation energy and critical nucleation radius decreased, and the nucleation probability increased. So, the crystal size formed under normal gravity condition was relatively small (Table S1). Surface: Concentration (mol/m3) Contour: Concentration (mol/m3) Streamline: Electrolyte current density vector

(a)

Surface: Concentration (mol/m3) Contour: Concentration (mol/m3) Streamline: Electrolyte current density vector

(b)

22


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Figure 6. The ion concentration distribution (surface), the corresponding isoconcentration line (horizontal contour) and the current density distribution (vertical streamline): (a) before the protrusion formation (G=1) and (b) after the protrusion formation (G=740). Under super gravity field, the effective protrusions were easily formed on the cathode surface due to the short induced time. The ion diffusion process on the protrusions was changed from a linear diffusion to a micro-semispherical diffusion.27-29 On the other hand, current density was larger on the tip of the protrusions (Figure 6b), which was also beneficial to the ion transfer.60 So, the local ion concentration on the protrusions was significantly higher than that on the film surface without the protrusions. From Figure 6b, the local ion concentration on the tip of the protrusions was about 0.49 M, while it reduced to about 0.413 M on the film surface without the protrusions because of the limitation of ion transfer. Consequently, the tip of the protrusions grew rapidly, which would further enlarge the concentration difference. So, the film grew slowly in the region without protrusions. As a result, the oriented NiMo clusters which were perpendicular to substrate surface were electrodeposited (Figure 3b). NiMo films with porous structure were obtained. 3.3 The electrocatalytic activity of NiMo films with different surface structures for HER From the above results, the surface structure of NiMo films depended on the electrodeposition time and gravity condition. It is well known that the physical-chemical properties (such as catalytic activity) of metal films are determined 23



by the surface structure. Therefore, the dependences of HER activity of NiMo films on electrodeposition time and gravity coefficient were studied.

0.00

(a)

i / A cm-2

-0.02

B C

-0.04 -0.06

A D

-0.08 -0.10

E -0.12

Electrocatalysts A B C D E Onset Potential -1.13 -1.12 -1.1 -1.03 -1.02 (V)

-1.6

-1.4

-1.2

-1.0

E / V (vs. SCE)

-0.5

(b) C B

-1.0

log (i / A cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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D

45 mV

A

E 23 mV

-1.5 -2.0 -2.5 -3.0 -3.5 -0.25

-0.20

-0.15

-0.10

-0.05

0.00

η / V (vs. SCE) Figure 7. (a) Linear sweep curves, (b) Tafel polarization curves for HER of NiMo films electrodeposited under various gravity conditions and electrodeposition times at -2.4 A cm-2. (A) G=1, 10 mins, (B) G=1, 20mins, (C) G=740, 5 mins, (D) G=740, 10 mins, (E) G=740, 20 mins. The inset tables in (a) are the onset potentials of NiMo 24


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films for HER. The results of LSV measurements were shown in Figure 7a. The onset potentials (-1.03 V and -1.02 V) for HER on NiMo films electrodeposited under super gravity field were more positive than those (-1.13 V and -1.12 V) on NiMo films electrodeposited under normal gravity field for the same electrodeposition time. The onset potentials slightly shifted positively with the increase of electrodeposition time for NiMo films electrodeposited under all gravity conditions. At a fixed cathode potential, the current densities of HER were increased with the increase of electrodeposition time. It meant that the HER catalytic activity was improved. In addition, NiMo films electrodeposited under super gravity field possessed better HER catalytic activity than those electrodeposited under normal gravity condition. The Tafel polarization curves and the corresponding kinetic parameters of NiMo films for HER were shown in Figure 7b and Table 1, respectively. At the same current density for HER, the overpotentials of NiMo films electrodeposited under super gravity field were much lower than those of NiMo films electrodeposited under normal gravity field for the same electrodeposition time. The HER overpotentials decreased with the increase of electrodeposition time. Particularly, for the NiMo films electrodeposited under super gravity field, the effect of the electrodeposition time on the HER overpotentials was more obvious. For example, the overpotential (η100) of HER on NiMo film electrodeposited under super gravity field for 20 mins was only 47 mV at 100 mA cm-2. The value was much lower than those of NiMo electrocatalysts in previous reports (Table S3 in the supporting information). However, 25



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for NiMo films electrodeposited under normal gravity condition, the η100 only decreased from 155 mV to 132 mV with the increase of the electrodeposition time from 10 mins to 20 mins. The difference of overpotentials (23 mV) for NiMo films electrodeposited under normal gravity condition was one half of that (45 mV) electrodeposited under super gravity field (the mark in Figure 7b). It should be ascribed to the larger change of surface structure with electrodeposition time for NiMo films electrodeposited under super gravity field (Figure 3d). Moreover, the exchange current densities of HER on NiMo films electrodeposited under super gravity field increased with the electrodeposition time (Table 1). The NiMo film electrodeposited for 20 mins under super gravity field possessed the highest i0 (42.90 mA cm-2). The results showed that the HER catalytic activity of NiMo films was improved with the increase of the electrodeposition time, especially for NiMo films electrodeposited under super gravity field. The composition of the NiMo films was hardly changed with the electrodeposition time (Figure S2). So, the improvement of catalytic activity should be related to the time-dependent surface structure evolution of NiMo films. Tafel slopes (b) of the porous NiMo films were higher than 116.3 mV dec-1. It was difficult to judge the rate determining step.61,62 However, considering the high surface coverage by adsorbed hydrogen in a large overpotential, the rate determining step of porous NiMo films for HER may be the Heyrovsky reaction.61 Table 1 HER kinetic parameters of metal films from Tafel polarization curves. b (V dec-1) is the Tafel slope, i0 is the exchange current density, α is the charge-transfer 26


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coefficient and i150 is the current density at the overpotential of 150 mV. T

G

(mins)

b

io

a

η100

Cdl,av

(mV dec-1)

(mA cm-2)

(V)

(mV)

(mF cm-2)

Rf

10

1

193.6

15.7

0.35

155

58.5

2925

20

1

158.6

14.9

0.29

132

82.8

4140

5

740

155.3

14.2

0.29

129

53.4

2671

10

740

193.9

32.9

0.29

92

80.2

4012

20

740

136.7

42.9

0.19

47

392.8

19640

To further understand the enhanced mechanism for HER on NiMo films, EIS measurements were performed and shown in Figure 8a. The zoomed part of the Nyquist plot at high frequencies was also shown in Figure S4 in the supporting information. It could be clearly found that only one depressed semicircle was observed on the complex plane plots. It indicated that only one time constant existed.61,63,64 So, the classical Randles electrical equivalent circuit (EEC) was used to fit the EIS data (the inset in Figure 8a).61,63,64 Rs (Ω cm2) was the solution resistance between the film and the reference electrode. Rct (Ω cm2) was the resistance of charge transfer. The constant phase element (CPE, T (F sn-1 cm-2)) was used to replace the double layer capacitance of charge transfer process. n (0 ≤ n ≤ 1) was the dispersion effect value of Nyquist plot and was used to represent the deviation degree of depressed semicircles from ideal semicircles. When n was 0, CPE became a resistance. When n was 1, CPE was a pure capacitor.65 From Table S4 in the 27



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supporting information, all n values were much smaller than 1. It meant that semicircles were depressed and exhibited a large deviation from ideal semicircles due to the porous structure. The EEC parameters obtained by fitting EIS results were shown in Table S4. From the Nyquist plots and the EEC parameters, for NiMo films electrodeposited under all gravity conditions, the diameters of semicircles and the Rct values at the same overpotential decreased with the increase of the electrodeposition time. For NiMo films electrodeposited under super gravity field, the Rct value decreased from 1.08 Ω cm2 to 0.24 Ω cm2 with the increase of the electrodeposition time from 10 mins to 20 mins. The decrease percentage was up to 77.9%. However, for NiMo films electrodeposited under normal gravity condition, the percentage was only 32.0% from 1.40 Ω cm2 to 0.95 Ω cm2. The results showed that HER activity of NiMo films was improved with the increase of the electrodeposition time, especially for NiMo films electrodeposited under super gravity due to obvious surface structure evolution (Figure 3c,d). The results were consistent with those of Tafel measurements. It was well known that a high surface roughness (Rf) meant more active sites per geometric area for HER.66 Only one semi-circle appeared in Nyquist diagrams.67 The double layer capacitance (Cdl) was calculated based on the EEC parameters by formula (13) to evaluate the effective active area of NiMo films.10,65,67

(13)

where n is the dispersion effect value of Nyquist plot (1>n>0). The average double 28


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layer capacitance (Cdl(av)) values were calculated based on Cdl values at various overpotentials (Table 1). The Cdl value of smooth NiMo alloys surface was 20 µF cm-2.61 The Rf values can be obtained by the formula (14).19

(a)

-Z'' / Ω cm2

1.25 1.00 0.75 0.50

A

D C

0.25 0.00 0.0

0.5

1.0

E 2.0

1.5

B 2.5

Z' / Ω cm2 3.5

(b)

A

3.0

U/V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A 10 µm

D 2.5

D

2.0

E

E

20 µm

20 µm

1.5 0

5

10

15

20

t/h Figure 8. (a) Nyquist plots at the overpotential (η) of -125 mV, and (b) the cell voltage (U)-time (T) curves at 0.4 A cm-2 for HER on NiMo films electrodeposited under various conditions at -2.4 A cm-2. (A) G=1, 10 mins, (B) G=1, 20mins, (C) G=740, 5 mins, (D) G=740, 10 mins, (E) G=740, 20 mins. The inset in (a) is the 29



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Page 30 of 43

equivalent circuits used to describe HER, and the inset in (b) shows the morphology of NiMo films after long-term electrolysis. Rf =

C dl ( av )

(14)

20

The Rf values increased with the increase of the electrodeposition time for NiMo films electrodeposited under both normal and super gravity conditions (Table 1). For NiMo films electrodeposited under normal gravity condition, the Rf value (4140) for 20 mins was only about 1.5 times of that (2925) for 10 mins due to the slight change of surface structure. However, Rf values were 19640 for 20 mins and 4012 for 10 mins for NiMo films electrodeposited under super gravity field, respectively. The ratio was up to about 4.9 times due to very obvious surface structure evolution from flat to porous film. In addition, the intrinsic activity of catalysts was usually expressed by the ratio of i0 to Rf. It was found that intrinsic activity of HER was gradually decreased with the increase of roughness. The results indicated that the HER activity of NiMo films electrodeposited under super gravity field was enhanced due to the developed porous structure (i.e. the increase of the active area). The durability, as a critical parameter to evaluate the performance of NiMo films for practical applications, was studied by the long-term electrolysis in 10 wt% NaOH solution at a large current density of 0.4 A cm-2 for 20 h. Figure 8 showed the cell voltage-time curves (U-t). In the initial period of electrolysis, the cell voltages increased sharply for the NiMo film electrodeposited under normal gravity condition. However, the cell voltages dropped slowly for NiMo films electrodeposited under 30


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super gravity field. After about 6 hours, cell voltages for all NiMo films were hardly changed. It meant that the NiMo films exhibited a good long-term durability. It can be also seen that the cell voltages for porous NiMo films electrodeposited under super gravity field decreased with the electrodeposition time, and the values (about 2.42 V 2.2 V) were much lower than that (about 3.27 V) for the NiMo film electrodeposited under normal gravity condition. It also indicated that the porous NiMo films exhibited a better catalytic activity and the overpotentials of HER were much lower. In addition, it was worth noting that the fluctuation amplitudes of cell voltages for the porous NiMo film electrodeposited under super gravity field for 20 mins were much smaller. It meant that the break-off diameter of hydrogen bubbles on porous NiMo films was small,19,23 which was ascribed to the rapid separation of hydrogen bubbles due to the good wettability.49 Therefore, the ohmic voltage drop due to bubble coverage was decreased on porous NiMo films, which also led to low cell voltages. Furthermore, the surface morphologies of the NiMo films after long-time electrolysis were observed and shown in the inset of Figure 8b. The surface structures of all NiMo films were hardly changed. It further proved that all NiMo films exhibited a good long-term durability. Conclusions The surface structure evolution of the electrodeposited NiMo films and the catalytic properties for HER were studied based on the adjustment of the gravity acceleration and electrodeposition time. It was confirmed that the formation of 3D porous NiMo films included three stages: the induction period for the protrusion 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

formation (i.e. the growth time of compact layer), the protrusion growth period, and the porous structure evolution period. Under super gravity field, the induced time for the protrusion formation was reduced to less than 5 mins due to the gravity-induced and hydrogen bubble agitation convection. The ion diffusion mode on the tip of the protrusions was changed from a linear diffusion to a semispherical diffusion, which led to a rapid growth of the protrusions due to large local current density and high metal ion concentration. Therefore, 3D porous NiMo films were easily formed. Under normal gravity condition, only compact NiMo film was electrodeposited due to the long induced time for protrusions. The surface structure evolution from compact layer to porous film by increasing gravity acceleration and electrodeposition time obviously enlarged the effective active area. So, the catalytic activity for HER was improved. Meanwhile, a good long-term durability was obtained on all NiMo films. The cell voltage of water electrolysis on porous NiMo films was much lower due to the low HER overpotential and rapid separation of hydrogen bubbles. Supporting information Supplementary data associated with this article can be found in the online version. EDS mapping images and EDS spectrums of linear scanning analysis for NiMo films and the table for EEC parameters Acknowledgements This work is supported by the Natural Science Foundation of China (51274180, 51422405) and Youth Innovation Promotion Association, CAS (2015036).

32


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Films Prepared Using the Hydrogen Bubble Dynamic Template. Chem. Mater. 2007, 19, 5758-5764. (22) Eftekhari, A. Improving Cu Metallization of Si by Electrodeposition under Centrifugal Fields. Microelectron. Eng. 2003, 69, 17-25. (23) Chen, Z.; Ma, Z.; Song, J.; Wang, L.; Shao, G. Novel One-step Synthesis of Wool-ball-like

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Cherevko,

S.;

Xing,

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Chung,

C.

Hydrogen

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Assisted

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Determination

of

Effective

Capacitance

and

Film

Thickness

From

Constant-phase-element Parameters. Electrochim. Acta 2010, 55, 6218-6227. (66) Herraiz-Cardona, I.; Ortega, E.; Anton, J. G.; Perez-Herranz, V. Assessment of 41



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the Roughness Factor Effect and the Intrinsic Catalytic Activity for Hydrogen Evolution Reaction on Ni-based Electrodeposits. Int. J. Hydrogen Energy 2011, 36, 9428-9438. (67) Navarro-Flores, E.; Chong, Z.; Omanovie, 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, 179-197.

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TOC Graphic

-0.5

G=740, t=10 min

G=740, t=20 min

-1.0 G=1, t=20 min

-1.5

1.25

-Z'' / Ω cm2

log (i / A cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


G=740, t=5 min

-2.0 -2.5

1.00 0.75 0.50 0.25

-3.0

G=740, t=5 min G=1, t=20 min G=740, t=10 min G=740, t=20 min

-0.20

-0.15

-0.10

-0.05

0.00

0.00 0.0

0.5

η/V

1.0

1.5

Z' / Ω cm2

43


2.0

2.5

Time-Dependent Surface Structure Evolution of ... - ACS Publications

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