A novel approach for the preparation of Ni–CeO₂ composite cathodes with enhanced electrocatalytic activity

Abstract

In this work, supergravity fields were utilized to prepare Ni–CeO₂ composite cathodes from a nickel sulphamate bath containing suspended nano-sized CeO₂ particles. The prepared Ni–CeO₂ composite coatings exhibit a significant enhancement in electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline solutions. The crystal structure, morphology and chemical compositions of the composite coatings were characterized by XRD, SEM and EDS measurements. It was shown that the prepared Ni–CeO₂ composite coatings displayed a fine grain size and high contents of CeO₂ incorporated into the Ni matrix. The electrochemical activity of the composite cathodes for HER was determined by polarization measurements and electrochemical impedance spectroscopy in 1.0 M NaOH solution. The results indicate that the catalytic activity of the Ni–CeO₂ composite coatings is enhanced significantly, and the highest value of the exchange current density reaches 338.4 μA cm⁻², which is obviously higher than the values previously reported in the literature. Meanwhile, the effects of both the CeO₂ concentration and the intensities of the supergravity fields on the properties of the Ni–CeO₂ coatings are investigated.

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1. Introduction

With the current increasingly serious energy crisis and environmental concerns, hydrogen is considered as the most promising alternative chemical fuel to fossil fuel due to its numerous advantages such as high specific energy, high specific power and long storage time.^1–4 Nevertheless, at present, most of the methods for hydrogen generation are still based on fossil fuels, such as natural gas reforming and gasification of coal and petroleum coke.⁵ Hence, hydrogen generation from water electrolysis is thought to be one of the most ideal methods which can produce hydrogen of high purity with zero pollutants emission.^6,7 The alkaline water electrolysis (AEL) and the proton exchange membrane electrolysis (PEMEL) are the most advanced electrolysis technologies, which produce H₂ by electrolysis in alkaline and acid conditions, respectively.⁸ Though the PEMEL technology benefits from higher energy efficiency, the high cost of membranes and equipment maintenance due to operation in strong acidic environment makes it difficult to substitute the AEL in large scale industrial production of H₂ up to now.⁹ Instead, AEL dominates the market of H₂ production in industrial scale due to cheaper cell components and abundant Ni-based and Fe-based materials having been used as commercial catalysts.^8,9 However, both of these technologies are restrained because of the high energy consumption caused by the large overpotential for hydrogen evolution reaction (HER).¹⁰ In this case, plenty of researches are focused on the exploration of active and economical electrode materials to reduce HER overpotential.

Up to now, Ni and Ni-based electrode materials have been widely studied and considered as the most suitable candidates due to their high activity and good stability for HER in alkaline solutions, such as Ni–Mo,^11,12 Ni–Co,^13,14 Ni–W.^15,16 Recently, Ni-based composite coatings containing active solid particles have also shown excellent electrocatalytic activity for HER and attracted more and more attention, such as Ni–MoO₃, Ni–MoO₂,^17,18 Ni–RuO₂,¹⁹ Ni–polyaniline²⁰ and Ni–rare earth (RE) compounds²¹ composite electrodes. The embedded active solid particles in Ni matrix can significantly facilitate the formation of nanocrystalline, the increase of the surface area of coating, and the enhancement of the intrinsic activity of materials. Among various active particles, the rare earth oxide is one kind of the most excellent composite phases due to the fact that many rare earth elements have abundant half-filled and empty d orbitals to form an adsorptive bond with H. As a result, an effective synergetic effect between Ni matrix and rare earth oxide particles for HER will come into being.^5,22,23 Herein, the nano-sized CeO₂ particles are chosen for the research due to the lightest molecular mass of Ce among rare earth elements which is beneficial to the dispersion and suspension in electrolyte.

It is well known that the composite electrodeposition is an effective technology for the fabrication of Ni-based composite coatings by dispersing the active solid particles into electrolyte. Recently, we have found that the mass transfer process during electrodeposition can be significantly enhanced under supergravity field, leading to the obvious changes in the microstructure and morphology of materials. Thus, an improvement on the electrochemical activity of the electrode materials was obtained based on these changes.^24–26 Inspired by this, supergravity field is utilized to the composite electrodeposition process to prepare Ni–CeO₂ composite coatings for HER in this paper. The influences of both the intensity of supergravity fields and the concentration of nano-sized CeO₂ particles on the crystal structure, morphologies and electrocatalytic performances of the composite coatings are systematically studied.

2. Experimental

2.1. Preparation of Ni–CeO₂ composite coatings

The electrodeposition of Ni–CeO₂ composite coatings were performed using a supergravity equipment which has been described detailedly in our previous paper.²⁴ A Ø 10 cm × 2 cm copper foil circular ring was used as substrate for composite electrodeposition and a pure nickel pipe was used as anode. All of the copper substrates were mechanically polished, degreased and activated successively before electrodeposition. The composite coatings were electrodeposited under various supergravity fields from electrolyte containing 350 g dm⁻³ Ni(NH₂SO₃)₂·4H₂O, 25 g dm⁻³ NiCl₂·6H₂O, 20 g dm⁻³ NH₄Cl and various concentrations of nano-sized CeO₂ (average particle size 30 nm) particles. The pH value of the plating bath was maintained at 3.5 ± 0.5 by NH₂SO₃H. Firstly, the composite electrodeposition was carried out at a constant supergravity field 2700 rpm to determine an optimum concentration of CeO₂ added in electrolyte. The used supergravity field was chosen by experience from large numbers of our previous experiments. Then, various intensities of supergravity fields (1300/1650/2000/2350/2700/3050 rpm) were performed through adjusting the value of rotating speed N to prepare composite coatings from the electrolyte with optimum CeO₂ concentration. The electrodeposition was carried out at a constant current density of 3 A dm⁻² for 1 h at 318 K. Prior to each electrodeposition, the electrolyte was subjected to continuous stirring and sonication for 3 h to get the nano-CeO₂ particles fully disperse and suspend.

2.2. Characterizations

The crystal structure of the prepared composite coatings was characterized by X-ray diffraction (XRD) on a RigakuSmart Lab diffractometer with Cu Kα irradiation (λ = 1.5418 Å). The morphology of the deposits was studied using scanning electron microscopy (SEM) on the Zeiss Supra 55 type field emission scanning electron microscope at an accelerating voltage of 20 kV. The chemical compositions of the coatings were analyzed using the energy dispersive spectrometer (EDS) coupled with the SEM.

2.3. Electrochemical measurements

All the electrochemical measurements were performed using the CHI 660E electrochemical workstation in a standard three-electrode cell in 1 M NaOH solution at room temperature. The prepared circular ring cathode was cutted into several pieces (2 cm × 2 cm) and sealed by insulating epoxy resin, leaving an exposed surface area of 1 cm² as the working electrode. A platinum foil and an Hg/HgO electrode (0.0977 V vs. NHE) were used as the counter electrode and reference electrode, respectively. Tafel polarization curves were recorded by sweeping the potential from −0.875 to −1.225 V vs. Hg/HgO (−0.05 V ≤ η ≤ 0.3 V) at a scan rate of 5 mV s⁻¹. IR compensation was carried out to correct the polarization data by determining the solution resistance value from the electrochemical impedance spectroscopy (EIS) measurements. The EIS measurements were carried out at a potential of −1.075 V vs. Hg/HgO (i.e., an HER overpotential (η) of 150 mV) in the same configuration from 100 kHz to 0.01 Hz with an AC voltage amplitude of 5 mV. Cyclic voltammetric (CV) curves were recorded from 0.5 V to −1.5 V (vs. Hg/HgO) with a scan rate of 50 mV s⁻¹ at room temperature. Before the electrochemical measurements, the working electrode was held at a constant current density of 50 mA cm⁻² for 30 min to reduce the oxide film formed on the coating surface and establish stable conditions for HER, and then the solution was purged with N₂ for 30 min.

3. Results and discussion

3.1. Structural characterization

Fig. 1a shows the XRD patterns of Ni–CeO₂ composite coatings prepared from nickel sulfamate bath containing various concentrations of nano-sized CeO₂ particles under rotating speed 2700 rpm. For comparison, the pattern of pure Ni film electrodeposited under same condition is also present in Fig. 1a, which exhibits three strong diffraction peaks of (111), (200) and (220) crystal planes of Ni. The patterns of Ni–CeO₂ composite coatings all yield a series of reflections of CeO₂ phase, indicating that the nano-CeO₂ particles are successfully incorporated into Ni matrix. Compared with the pure Ni film, all diffraction peaks of Ni in composite coatings decrease significantly and become broader, which can be concluded that a finer grain size is obtained for Ni–CeO₂ composite coating. This change of grain refinement is common in composite electrodeposition and mainly attributed to the fact that the nano-sized particles dispersed in coating can increase the nucleation sites for reduction of metal ions and prevent the coarsening growth of crystal grains.^27–29 Furthermore, it can been seen that the intensities of diffraction peaks of CeO₂ are enhanced initially with the increase of CeO₂ content in solutions, followed by a slight decrease as the CeO₂ content exceeds 7 g dm⁻³. On the other hand the characteristic peaks of Ni exhibit a contrary tendency. This is due to that at first the content of CeO₂ particles incorporated in film depends to a considerable extent on the concentration in electrolyte. As the content in electrolyte increases, more nano-particles are embedded into Ni matrix and finer grain size is obtained. However, further increasing the concentration of CeO₂ will lead to an aggravation of agglomeration for nano-particles, which hinders the process of being embedded into coatings. Hence, the limiting effect of CeO₂ particles on the growth of Ni grain decreases. Fig. 1b exhibits the XRD patterns of Ni–CeO₂ composite coatings prepared from the electrolyte bath containing 7 g dm⁻³ nano-sized CeO₂ particles under different supergravity fields. It can be seen that the diffraction peaks of Ni in composite coatings prepared under supergravity fields decrease apparently and become much broader than that under the normal gravity field. In addition, the intensities of characteristic peaks of CeO₂ increase obviously under supergravity fields, indicating much more CeO₂ are incorporated in Ni matrix relative to normal condition. The enhancement of supergravity field on mass transfer process significantly promote the movement of Ni²⁺ ions and CeO₂ particles to cathode surface, resulting in the finer grains and higher CeO₂ content.

Fig. 1 (a) XRD patterns of Ni–CeO₂ composite coatings prepared at various concentrations of nano-sized CeO₂ particles in electrolyte under rotating speed 2700 rpm. (b) XRD patterns of Ni–CeO₂ composite coatings prepared under various gravity fields containing 7 g dm⁻³ CeO₂ in electrolyte.

The full width at half maximum (FWHM) of (111), (200) and (220) diffraction peaks of Ni obtained from XRD patterns is presented in Table 1 and used to calculate the Ni crystalline size of the corresponding crystal plane by Scherrer's equation. The average crystalline size D of each coating derived from averaging the sum of the crystalline sizes of above three crystal planes is also listed in Table 1. It can be concluded from the values of FWHM and average crystalline sizes of the coatings prepared at the same supergravity fields in Table 1 that with the increase of CeO₂ content in electrolyte the Ni grain size decreases and reaches a minimum for 7 g dm⁻³ followed by a slight increase, which is in accordance with the above analysis for Fig. 1a. Besides, it is important to note that the contribution of supergravity fields towards crystal refinement outweighs the impact of CeO₂ particles embedded in coating as the results shown in Table 1. For instance, although the composite coating prepared at rotating speed 2350 rpm exhibits a lowest content of CeO₂ particles in coating as shown in Fig. 1b, the grain size of the composite coating still decreases markedly and reaches a minimum. Meanwhile, as also shown in Table 1, for the samples prepared from the same CeO₂ content in electrolyte, the average grain sizes gradually reduce as the N value increases to 2350 rpm, followed by a slight increase. This phenomenon may be ascribed to the fact that the mass transfer process is enhanced with the increase of N value, which promotes the decrease of concentration polarization and increase of electrochemical polarization to get a high nucleation rate for finer grains coming into being. Besides, during electrodeposition lower hydrogen bubble coverage on cathode surface will be acquired by further enlarging the N value. Hence, the effective deposition surface area at high N value is much larger than that at low N value. In other words, the effective current density for electrodeposition decreases under high N value, which results in the coarsening of Ni grain to some extent again.²⁴

Table 1 XRD parameters of Ni and Ni–CeO₂ composite coatings

CeO2 concentration/g dm^−3	N value/rpm	BNi(111)/°	BNi(200)/°	BNi(220)/°	D/nm
0	2700	0.290	0.365	0.426	25
3	2700	0.298	0.330	0.533	24
5	2700	0.292	0.394	0.539	23
7	Normal	0.305	0.408	0.422	24
7	1300	0.358	0.468	0.596	20
7	1650	0.373	0.484	0.588	19
7	2000	0.384	0.498	0.615	18
7	2350	0.455	0.649	0.639	16
7	2700	0.366	0.519	0.585	19
7	3050	0.386	0.509	0.614	18
9	2700	0.314	0.429	0.592	21
11	2700	0.349	0.496	0.605	20

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3.2. Morphology of Ni–CeO₂ composite coating

The effects of CeO₂ concentration and supergravity field on the morphology of Ni–CeO₂ composite coating are presented in Fig. 2. The magnified image of the local area as indicated by the red arrow for each sample is shown in the inset. The pure Ni film prepared at rotating speed 2700 rpm is shown in Fig. 2a, which displays a typical hemispheric morphology with uniform grains and rare flaws. Fig. 2b–d show the morphologies of Ni–CeO₂ composite coatings electrodeposited at rotating speed 2700 rpm containing 3 g dm⁻³, 7 g dm⁻³ and 11 g dm⁻³ CeO₂ in electrolyte respectively. As Fig. 2b shows, it still can be seen hemispheric morphology characteristic of Ni but with obvious nano-CeO₂ particles dispersed in the coating from the magnified image inset. Further increasing the CeO₂ concentration in solutions, the hemispheric morphology gradually disappears and large amounts of nano-CeO₂ are dispersed in the coatings as presented in Fig. 2c and d and the corresponding magnified images. This phenomenon can be attributed to the effect of grain refinement of nano-CeO₂ particles incorporated in Ni matrix. However, as discussed in XRD analysis that too many CeO₂ particles in solution will lead to a deterioration of agglomeration. As a result, many agglomerated CeO₂ particles embedded in some regions of the coating can be seen clearly from Fig. 2d. The morphologies of composite coatings prepared under various gravity fields containing same concentration of 7 g dm⁻³ nano-CeO₂ in solutions are exhibited in Fig. 2c and e–g. As shown in Fig. 2e, the coating prepared under normal gravity field exhibits a hemispheric surface morphology with small amounts of CeO₂ particles aggregating in local area. Fig. 2f, c and g display entirely changed morphologies with large amounts of nano-CeO₂ dispersed in coatings for Ni–CeO₂ films prepared under rotating speed 2350, 2700 and 3050 rpm respectively. As the rotating speed increases, agglomerated CeO₂ particles gradually appear in local areas, which are especially apparent in rotating speed 3050 rpm. The schematic illustration of the proposed behaviors of Ni²⁺ and nano-CeO₂ under normal gravity and supergravity conditions during electrodeposition is given in Fig. 3. As shown in Fig. 3a, the H₂ generated from the side reaction during normal electrodeposition can tightly adsorb on the cathode surface and gradually accumulate into large volume bubbles due to continuous evolution of H₂. And then, the adsorbed large bubbles with additional gas/liquid interface will block the path for Ni²⁺ penetrating through and reaching the electrode surface to reduce to Ni. In contrast, the nano-CeO₂ particles can get through the H₂ bubbles and gradually aggregate at the locations of the bubbles due to their relatively large mass, which results in the finally heterogeneously distributed morphology feature as shown in Fig. 3a. Unlike the normal condition, the large electrolyte scouring force and buoyancy force under supergravity field can effectively facilitate the bubbles to disengage from the cathode surface despite of the continuous and random generation of H₂, which timely eliminates the barrier for the Ni²⁺ reduction on the electrode surface and contributes to the formation of uniform coatings as Fig. 3b depicts. Besides, the mass transfer process is enhanced prominently under supergravity field, far more CeO₂ particles tend to be embedded into composite coatings and fill along the interfacial gaps and grain boundaries to prevent the coarsening growth of Ni grains, leading to the change of morphologies.²⁹

Fig. 2 (a–d) SEM images of Ni–CeO₂ composite coatings prepared at various concentrations of nano-sized CeO₂ particles in electrolyte: (a) pure Ni, (b) 3 g dm⁻³, (c) 7 g dm⁻³, (d) 11 g dm⁻³; (e–g) SEM images of Ni–CeO₂ composite coatings prepared under various gravity fields: (e) normal, (f) 2350 rpm, (g) 3050 rpm; (h) SEM image and EDS elemental mapping of Ce, O, and Ni for the composite coating prepared at rotating speed 2000 rpm.

Fig. 3 Schematic illustration of the formation process of Ni–CeO₂ composite coatings: (a) under normal gravity field and (b) under supergravity field.

EDS elemental mapping analysis of composite coating prepared under rotating speed 2000 rpm containing 7 g dm⁻³ CeO₂ in the bath is presented in Fig. 2h. The mapping images of Ce, O and Ni shown in figure indicate that many nano-CeO₂ particles are distributed throughout the Ni matrix. The CeO₂ contents in various composite coatings measured by EDS are compared in Fig. 4. As shown in Fig. 4a, with an increase of particles in the electrolyte the content of CeO₂ particles in coatings prepared at rotating speed 2700 rpm increases and reaches the maximum of 12.22 at% for 7 g dm⁻³, followed by a slight decrease due to the aggravation of agglomeration at high concentration. Moreover, it can be seen from Fig. 4b that the CeO₂ content in coatings prepared under various supergravity fields decrease first and then increase with the enhancement of rotating speed. This phenomenon can be attributed to the combined effects of electrolyte scouring force and centrifugal force. Although the former one removes the particles from the cathode surface, the latter is always much larger and plays a dominant role in electrodeposition process to promote the particles transport to the cathode surface. Both of their influences increase along with the enhancement of rotating speed, however the rising tendency of centrifugal force is inferior to electrolyte scouring force at first and then increases by further enlarging the rotating speed, which leads to the gradual decrease of CeO₂ content in coating at first and then increases again. In addition, the CeO₂ content in coatings prepared under various supergravity fields all exhibits a high value varies from 11.69 at% to 14.05 at% as displayed in Fig. 4b, which is significantly higher than the value only 2.37 at% for normal gravity condition. Meanwhile, we are delighted to find that these values are much higher than the results ever reported in other literature. For example, Zheng et al.²³ used normal electrodeposition with 25 g dm⁻³ nano-CeO₂ adding in the solution to obtain Ni–CeO₂ composite coating with 6.47 at% CeO₂ in coating and Zhou et al.²⁹ used a ultrasound-assisted pulsed electrodeposition technique to acquire Ni–CeO₂ coating with a CeO₂ content of 8.12 at% in coating from the electrolyte containing 15 g dm⁻³ nano-CeO₂. Thereby, it means that to obtain a similar nano-particle content in coating as the literature above, we can dramatically decrease the concentration of particles added in electrolyte with the help of supergravity field, which can immensely reduce the cost of preparation of Ni–CeO₂ composite coating or even other composite coatings.

Fig. 4 (a) CeO₂ content in composite coatings prepared at various concentrations of nano-sized CeO₂ particles in electrolyte under rotating speed 2700 rpm. (b) CeO₂ content in composite coatings prepared under various gravity fields containing 7 g dm⁻³ CeO₂ in electrolyte.

3.3. Electrochemical characterization

3.3.1. Polarization measurements. Fig. 5 shows the Tafel polarization curves of Ni–CeO₂ composite coatings for HER in 1.0 M NaOH solution at room temperature. The effects of nano-CeO₂ concentration and supergravity fields on the electrocatalytic activities of composite films are shown in Fig. 5a and b respectively. A typical Tafel region confirms that HER on Ni–CeO₂ composite coatings is kinetically controlled by charge transfer, and the kinetic parameters for HER can be derived from the Tafel equation:where η_c is the cathode overpotential, j is the current density, j₀ is the exchange current density, R is the universal gas constant, T is the absolute temperature, b is the Tafel slope and a is the intercept correlated with j₀. α and F are the charge-transfer coefficient and the Faraday constant respectively.

Fig. 5 Tafel curves for HER in 1.0 M NaOH solution at 298 K on the composite coatings (a) prepared at various concentrations of nano-sized CeO₂ particles in electrolyte under rotating speed 2700 rpm and (b) prepared under various gravity fields containing 7 g dm⁻³ CeO₂ in electrolyte.

The calculated kinetic parameters of composite coatings for HER are listed in Table 2. Evidently, a significant improvement on HER catalytic activity for Ni–CeO₂ composite coatings is observed compared with the pure Ni coating according to the increased j₀ values and the reduced Tafel slope b. The improved HER catalytic activity should be attributed to the change of morphology feature caused by the incorporation of CeO₂ particles and the synergistic effect between Ni matrix and CeO₂ particles. The embedded nano-CeO₂ particles have been proposed to act as the active sites for more H_ad to form Ce–H_ad on Ni–CeO₂ composite coating instead of only Ni–H_ad on pure Ni coating, which can effectively accelerate the HER process.²³ It can be found that under a same supergravity field the j₀ value increases obviously with the increased CeO₂ concentration in electrolyte and reaches 208.2 μA cm⁻² as the concentration increases to 7 g dm⁻³. Further increasing the concentration leads to a slight decrease in j₀, but the value is still much higher than the pure Ni coating. This result is mainly due to the deterioration of agglomeration in electrolyte at high concentration, which subsequently aggravates the agglomeration of CeO₂ particles in coatings to affect the electrocatalytic activities.

Table 2 Kinetic parameters for HER derived from Tafel curves recorded in 1 M NaOH solution at 298 K

CeO2 concentration/g dm^−3	N value/rpm	b mV^−1 dec^−1	j0/μA cm^−2	α
0	2700	188.0	1.842	0.31
3	2700	195.3	19.43	0.30
5	2700	166.9	123.9	0.35
7	Normal	220.8	4.670	0.27
7	1300	159.3	147.2	0.37
7	1650	148.3	196.2	0.40
7	2000	141.8	232.3	0.42
7	2350	146.6	338.4	0.40
7	2700	165.1	208.2	0.36
7	3050	161.9	191.8	0.37
9	2700	171.4	174.4	0.34
11	2700	173.8	132.6	0.34

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Hence, the catalytic activities of composite coatings prepared under various gravity fields from the relatively optimal CeO₂ concentration of 7 g dm⁻³ in the solution are compared. It is apparent to find that the j₀ values of coatings electrodeposited under supergravity fields are much larger compared with the normal one. And the j₀ increases to a maximum of 338.4 μA cm⁻² as the rotating speed increases to 2350 rpm, which reveals the highest electrocatalytic activity among the studied samples. The significantly enhanced catalytic activity of the samples prepared under supergravity fields relative to the normal one can be mainly attributed to the following reasons. First, the prominently enhanced CeO₂ content in coating will provide more active sites for H_ad during HER process. Second, the uniform distribution of CeO₂ particles can give rise to a more efficient synergistic effect between Ni matrix and CeO₂ for HER. Finally, the thorough change of morphology may increase the catalytic surface area of the coating, leading to a sufficient contact between the electrode and the electrolyte. Whereas, further increasing the rotating speed results in a drop of j₀ value. This can be attributed to the gradual coarsening of grain size and too many CeO₂ particles incorporated in coatings. Though the HER intermediate product H atom can easily adsorb on Ce atom to form Ce–H_ads due to the presence of half-filled and empty d orbitals.²³

However, on the contrary too many CeO₂ particles embedded in coating will affect the H_ads desorption process and lead to the decrease of catalytic activity. Benefiting from the supergravity fields, the HER catalytic activities of the Ni–CeO₂ composite coatings are enhanced prominently and most of the j₀ values are much higher than those ever reported in literature.²³ Furthermore, the highest j₀ value of Ni–CeO₂ composite coating in this paper is also superior to the values of some other Ni and Ni-based cathodes reported for HER in alkaline solutions as shown in Table 3.^30–38

Table 3 HER exchange current density for some Ni and Ni-based electrocatalysts

Catalyst	Method	Electrolyte	T (°C)	j0 (mA cm^−2)	Ref.#
a Electrodeposited under supergravity field (N = 2350 rpm).
Ni–CeO2^a	Electrodeposition	1 M NaOH	25	0.338	This paper
Ni54Mo27B19	Melt spinning	1 M KOH	25	0.016	29
Fe–Ni–graphene	Electrodeposition	6 M NaOH	25	0.133	30
Ni0.90Dy0.10	Arc melting	8 M KOH	25	3.67 × 10^−3	31
Ni0.90Ce0.10	Arc melting	8 M KOH	25	6.70 × 10^−3	32
Ni/MWCNT	Pulsed laser ablation	1 M KOH	25	0.110	33
Ni60Fe30Cu10	Ball milling	1 M NaOH	25	0.095	34
Nanocrystalline Ni	Magnetron sputtering	1 M NaOH	25	0.307	35
Ni–P + Fe2O3–TiO2	Electroless plating	32% NaOH	30	6.02 × 10^−6	36
Nano-Zr67Ni33	Melt-spinning	6 M KOH	25	0.250	37

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Tafel slope b is another important parameter which can provide insights into the HER mechanism on the electrodes and illustrate the rate of j change with η_c. As listed in Table 1, the Tafel slopes of composite coatings prepared under rotating speed 1650, 2000 and 2350 rpm display more favorable values of 148.3, 141.8 and 146.6 mV dec⁻¹ respectively, revealing better electrochemical activities for HER among the studied coatings. The Tafel slopes of all Ni–CeO₂ coatings prepared under supergravity fields are higher than the theoretical value of 118 mV dec⁻¹. The higher Tafel slope values are consistent with other researches for HER on the Ni-based composite coatings,^{5,20,22,23,39} which may be attributed to the oxide film and adsorbed intermediates covered on the surface or the presence of low electron conductivity of CeO₂ particles that slightly hinder the charge transfer.^12,20 It is known that the HER in alkaline solutions is predominated by three principal reactions, commonly named the Volmer, Heyrovsky and Tafel reactions. The Tafel slope b yield a series values of −118, −40 and −30 mV dec⁻¹ respectively for each reaction being the rate determining step (RDS). Accordingly, it can be speculated that the RDS of HER on the Ni–CeO₂ composite coatings may be the Volmer step or the Volmer step coupled with one of the other two steps.²³

3.3.2. Electrochemical impedance spectroscopy. EIS measurements are utilized to further investigate the electrocatalytic kinetics of HER on the Ni–CeO₂ composite coatings. The complex plane Nyquist plots are obtained at cathode overpotential of 150 mV as shown in Fig. 6. In order to analyze the impedance data, the impedance spectra are fitted by the Armstrong's equivalent electric circuit model based on the single-adsorbate mechanism as depicted in the inset of Fig. 6,⁴⁰ in which the R_s is the solution resistance, R_ct is the charge transfer resistance and the R_p is basically related to the resistance of the adsorbed intermediate H_ads. Herein, the constant phase element (CPE) is used to replace the double layer capacitance due to the deviation from the ideal capacitance behavior caused by the nonuniform electrode surface state.^41,42 Thus, CPE1 is corresponding to the double layer capacitance C_dl, and CPE2 is the pseudo-capacitance associated with the adsorption of intermediate H_ads.^23,36,43 The double layer capacitance C_dl of the electrode is determined using the equation proposed by Brug et al.:^12,13,44where Q₁ is the capacitance coefficient and n is the CPE exponent. The surface roughness R_f which characterizes the real electrochemically active surface area of the electrode can be calculated by comparing the C_dl value with 20 μF cm⁻² for smooth electrode surface.^11–13 All the values of the electrochemical parameters simulated from the impedance measurements are given in Table 4.

Fig. 6 Nyquist plots of Ni–CeO₂ composite coatings (a) prepared at various concentrations of nano-sized CeO₂ particles in electrolyte under rotating speed 2700 rpm and (b) prepared under various gravity fields containing 7 g dm⁻³ CeO₂ in electrolyte. Symbols were experimental data and solid lines were fitted curves.

Table 4 Electrochemical circuit parameters obtained from the EIS measurements

CeO2 concentration/g dm^−3	N value/rpm	Rs/Ω cm^−2	Rct/Ω cm^−2	Rp/Ω cm^−2	Q1/mΩ^−1 s^n cm^−2	Cdl/mF cm^−2	Rf
0	2700	3.07	833.20	1046.00	0.102	0.033	2
3	2700	3.15	14.33	63.29	9.342	0.935	47
5	2700	3.88	8.27	29.24	19.350	1.651	82
7	Normal	3.63	429.90	178.40	1.742	0.073	4
7	1300	3.65	3.19	26.06	13.670	1.711	85
7	1650	3.55	3.51	21.88	15.810	3.428	171
7	2000	3.80	2.81	20.90	37.780	6.335	317
7	2350	3.22	2.39	15.86	54.970	8.528	426
7	2700	3.15	3.21	22.32	41.760	5.074	254
7	3050	3.51	4.40	23.52	28.240	2.971	148
9	2700	3.33	3.37	26.64	27.610	2.672	133
11	2700	3.74	3.35	28.01	27.280	1.843	92

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As the results listed in Table 4, the electrochemical reaction resistance (R_ct + R_p) of the composite coatings decrease significantly compared with the pure Ni coating. The value of the composite coatings prepared at the same rotating speed 2700 rpm decreases to 25.53 Ω cm⁻² as the CeO₂ concentration increases to 7 g dm⁻³, almost 74 times lower than the value of pure Ni coating, followed by a slight increase. It is known that a low electrochemical reaction resistance value represents a high catalytic activity for HER. Thus, the Ni–CeO₂ coating prepared at the solution containing 7 g dm⁻³ CeO₂ exhibits the highest catalytic activity. The phenomenon is in accordance with the result presented in the Tafel analysis. Besides, comparison of the (R_ct + R_p) values of the composite coatings prepared under various supergravity fields shows a similar trend with the increase of rotating speed, in which the electrochemical reaction resistance decreases first and reaches a minimum of 18.25 Ω cm⁻² for 2350 rpm, then increasing again to some extent. The charge transfer resistances R_ct of the composite coatings prepared under various supergravity fields are between 2.39 and 4.40 Ω cm⁻², which are found to be markedly lower than the value 429.90 Ω cm⁻² for the composite coating prepared under normal condition. Lower value of R_ct means more rapid of the Volmer reaction for HER. It further verifies the high electrocatalytic activity of Ni–CeO₂ coatings prepared under supergravity fields, especially the sample prepared at rotating speed 2350 rpm.

It has been considered that the catalytic activity depends strongly on the active surface area available for HER.^20,45 In order to characterize the active sites on the electrode surface, the surface roughness R_f based on the double layer capacitance C_dl is also estimated and given in Table 4. Compared with the Ni coating, the R_f values of Ni–CeO₂ composite coatings prepared under different CeO₂ concentrations increase prominently and reach the highest of 254 for 7 g dm⁻³. Therefore, the impact of supergravity fields on the surface roughness of the composite coatings prepared under the optimum CeO₂ concentration is also studied and shown in the Table 4. It can be seen that all of the R_f values of coatings prepared under supergravity fields are far more higher than that prepared at normal condition, which confirms the significant improvement on the electrocatalytic activity by supergravity fields. This result is mainly attributed to the finer grain size, more homogeneous distribution of CeO₂ particles and higher content of CeO₂ in coating obtained with the help of supergravity fields. Meanwhile, with the enhancement of supergravity field, the R_f values increase first and reach a maximum of 426 for preparation under 2350 rpm, followed by a decrease. As discussed above, excess nano-CeO₂ particles in coating will hinder the desorption of H_ads, thus large amount of H_ads adsorbed on electrode surface may increase the thickness of the double layer and lead to the decrease of C_dl value,²³ which consequently results in the decrease of R_f.

3.3.3. Cyclic voltammetric study. CV curves (without IR correction) of composite coatings prepared under different gravity fields were studied in same alkaline medium at a scan rate of 50 mV s⁻¹ between 0.5 V and −1.5 V. As depicted in Fig. 7, all of the CV curves show oxidation peaks at the potential range of −1.0 V to −0.5 V and reduction peaks at around −0.75 V (vs. Hg/HgO) from the zoom in the insert, which are associated with H atoms desorption/adsorption processes.^46,47 It's worth noting that when the H atoms adsorption process weakens on the electrode prepared under normal gravity condition, the corresponding oxidation peak decreases and shifts to the right, exhibiting poor intrinsic activity for HER. Besides, both the oxidation and reduction peaks of the samples prepared under supergravity fields were increased compared to the normal one, especially for the coatings prepared at rotating speed 2350 and 3050 rpm, which indicate that the H atoms desorption/adsorption processes are more activated on the electrodes surface. This phenomenon may be ascribed to the increased electrochemically active surface area concluded from the EIS analysis. The second oxidation peak in each curve at the potential range of −0.25 V to 0.25 V can be attributed to the oxidation of electrode surface in the anodic sweep. It can be also observed in Fig. 8 that the peak cathodic current densities (i_pc) of the studied Ni–CeO₂ coatings prepared under supergravity fields are all larger than the sample prepared at normal condition. Comparing with the other two rotating speeds, the sample prepared at the moderate rotating speed 2350 rpm shows the largest value of i_pc at −1.5 V, representing the highest intrinsic activity for HER. These results are in accordance with the Tafel and EIS measurements and further confirmed that significantly enhanced electrocatalytic activity for HER was achieved by introducing supergravity fields into the electrodeposition of Ni–CeO₂ composite coating.

Fig. 7 CV curves for HER in 1 M NaOH at 298 K on Ni–CeO₂ composite coatings prepared under different gravity fields.

Fig. 8 Chronopotentiometry curves of Ni and Ni–CeO₂ coatings recorded at 250 mA cm⁻² for 10 h in 1 M NaOH solution.

3.3.4. Chronopotentiometry study. In addition to the catalytic activity, stability is another important evaluation criterion to estimate the performance of Ni-based composite cathodes for practical applications. Thus, stability of the prepared Ni and Ni–CeO₂ cathodes was tested by chronopotentiometry measurement. Fig. 8 shows the E–t curves of all studied samples recorded under a constant current density of 250 mA cm⁻² for 10 h in 1 M NaOH solution. It can be found that the pure Ni electrode exhibits the highest cathodic potential for HER, indicating that the electrocatalytic activity is inferior to the Ni–CeO₂ composite cathodes. The Ni–CeO₂ composite coating prepared at normal condition presents improved catalytic activity and good stability, but still performs a high cathodic potential. Compared with above two samples, the curve of Ni–CeO₂ coating prepared at rotating speed 2350 rpm exhibits the lowest potential for HER, which reveals the best catalytic activity for HER and is in coincidence with the results found in above electrocatalytic analysis. In addition, it can be seen that the potential is a little high in the initial period and then gradually decreases and keeps stable. This phenomenon might be considered as the activated process of the electrode due to plenty of embedded CeO₂ particles and the entirely changed morphology.

4. Conclusions

This work displays a novel approach to prepare Ni–CeO₂ composite coating with significantly enhanced activity for HER in alkaline solution. The impacts of both the CeO₂ concentration and supergravity field on the crystal structure, morphology, chemical composition and electrocatalytic activity of the Ni–CeO₂ composite coatings were systematically investigated. With the help of supergravity field, the mass transfer process was markedly improved, which contributed to the formation of finer grains and the increase of nano-sized CeO₂ embedded in coatings and proved to be an effective method to reduce the cost of material preparation. The electrochemical measurements show that the catalytic activities of composite coatings are improved prominently compared with that prepared under normal gravity condition. The coating prepared at rotating speed 2350 rpm from the optimal CeO₂ concentration 7 g dm⁻³ exhibits the highest exchange current density and surface roughness R_f towards HER, the value of exchange current density is about 4 times higher than the best value reported in literature. And the R_f also yields a significant enhancement, which increases up to 426 about 107 times larger than the one prepared under normal condition. The enhancement of electrocatalytic activity should be radically attributed to the increase of homogeneously distributed CeO₂ particles and the modification of morphology feature under supergravity field. It further stimulates us to carry out more research works on the preparation of high HER performances Ni-based alloy composite cathodes under supergravity fields.

Acknowledgements

We are grateful for the financial support from the Natural Science Foundation of State key Laboratory of China and the Postgraduate Innovation Project of Hebei Province (00302-6370013).

References

1. M. Dresselhaus and I. Thomas, Nature, 2001, 414, 332–337.
2. H. B. Gray, Nat. Chem., 2009, 1, 7.
3. O. I. Velikokhatnyi, K. Kadakia, M. K. Datta and P. N. Kumta, J. Phys. Chem. C, 2013, 117, 20542–20547.
4. M. Balat, Int. J. Hydrogen Energy, 2008, 33, 4013–4029.
5. Z. Zheng, N. Li, C.-Q. Wang, D.-Y. Li, F.-Y. Meng and Y.-M. Zhu, J. Power Sources, 2013, 222, 88–91.
6. M. Wang, Z. Wang, X. Gong and Z. Guo, Renewable Sustainable Energy Rev., 2014, 29, 573–588.
7. K. Zeng and D. Zhang, Prog. Energy Combust. Sci., 2010, 36, 307–326.
8. C. I. Müller, K. Sellschopp, M. Tegel, T. Rauscher, B. Kieback and L. Röntzsch, J. Power Sources, 2016, 304, 196–206.
9. D. Pletcher and X. Li, Int. J. Hydrogen Energy, 2011, 36, 15089–15104.
10. F. Safizadeh, E. Ghali and G. Houlachi, Int. J. Hydrogen Energy, 2015, 40, 256–274.
11. O. Aaboubi, Int. J. Hydrogen Energy, 2011, 36, 4702–4709.
12. M. Wang, Z. Wang, X. Yu and Z. Guo, Int. J. Hydrogen Energy, 2015, 40, 2173–2181.
13. C. González-Buch, I. Herraiz-Cardona, E. Ortega, J. García-Antón and V. Pérez-Herranz, Int. J. Hydrogen Energy, 2013, 38, 10157–10169.
14. C. Lupi, A. Dell'Era and M. Pasquali, Int. J. Hydrogen Energy, 2009, 34, 2101–2106.
15. M. A. Oliver-Tolentino, E. M. Arce-Estrada, C. A. Cortés-Escobedo, A. M. Bolarín-Miro, F. Sánchez-De Jesús, R. d. G. González-Huerta and A. Manzo-Robledo, J. Alloys Compd., 2012, 536, S245–S249.
16. M. Wang, Z. Wang, Z. Guo and Z. Li, Int. J. Hydrogen Energy, 2011, 36, 3305–3312.
17. N. Krstajić, L. Gajić-Krstajić, U. Lačnjevac, B. Jović, S. Mora and V. Jović, Int. J. Hydrogen Energy, 2011, 36, 6441–6449.
18. N. Krstajić, U. Lačnjevac, B. Jović, S. Mora and V. Jović, Int. J. Hydrogen Energy, 2011, 36, 6450–6461.
19. L. Vázquez-Gómez, S. Cattarin, P. Guerriero and M. Musiani, Electrochim. Acta, 2007, 52, 8055–8063.
20. D. A. Dalla Corte, C. Torres, P. dos Santos Correa, E. S. Rieder and C. de Fraga Malfatti, Int. J. Hydrogen Energy, 2012, 37, 3025–3032.
21. M. Domínguez-Crespo, A. Torres-Huerta, B. Brachetti-Sibaja and A. Flores-Vela, Int. J. Hydrogen Energy, 2011, 36, 135–151.
22. Z. Zheng, N. Li, C.-Q. Wang, D.-Y. Li, F.-Y. Meng, Y.-M. Zhu, Q. Li and G. Wu, J. Power Sources, 2013, 230, 10–14.
23. Z. Zheng, N. Li, C.-Q. Wang, D.-Y. Li, Y.-M. Zhu and G. Wu, Int. J. Hydrogen Energy, 2012, 37, 13921–13932.
24. J. Du, G. Shao, X. Qin, G. Wang, Y. Zhang and Z. Ma, Mater. Lett., 2012, 84, 13–15.
25. T. Liu, G. Shao and M. Ji, Mater. Lett., 2014, 122, 273–276.
26. Z.-H. Chen, Z.-P. Ma, J.-J. Song, L.-X. Wang and G.-J. Shao, J. Power Sources, 2016, 324, 86–96.
27. C. Cai, X. Zhu, G. Zheng, Y. Yuan, X. Huang, F. Cao, J. Yang and Z. Zhang, Surf. Coat. Technol., 2011, 205, 3448–3454.
28. C. P. Kumar, T. Venkatesha and R. Shabadi, Mater. Res. Bull., 2013, 48, 1477–1483.
29. X. Zhou and Y. Shen, Surf. Coat. Technol., 2013, 235, 433–446.
30. T. Rauscher, C. I. Müller, A. Schmidt, B. Kieback and L. Röntzsch, Int. J. Hydrogen Energy, 2016, 41, 2165–2176.
31. S. Badrayyana, D. K. Bhat, S. Shenoy, Y. Ullal and A. C. Hegde, Int. J. Hydrogen Energy, 2015, 40, 10453–10462.
32. D. Cardoso, L. Amaral, D. Santos, B. Šljukić, C. Sequeira, D. Macciò and A. Saccone, Int. J. Hydrogen Energy, 2015, 40, 4295–4302.
33. D. Santos, L. Amaral, B. Šljukić, D. Macciò, A. Saccone and C. Sequeira, J. Electrochem. Soc., 2014, 161, F386–F390.
34. M. McArthur, L. Jorge, S. Coulombe and S. Omanovic, J. Power Sources, 2014, 266, 365–373.
35. F. Rosalbino, G. Scavino and M. A. Grande, J. Electroanal. Chem., 2013, 694, 114–121.
36. Z. Grubač, M. Metikoš-Huković and R. Babić, Int. J. Hydrogen Energy, 2013, 38, 4437–4444.
37. S. Shibli and J. Sebeelamol, Int. J. Hydrogen Energy, 2013, 38, 2271–2282.
38. L. Mihailov, T. Spassov and M. Bojinov, Int. J. Hydrogen Energy, 2012, 37, 10499–10506.
39. J. Kubisztal, A. Budniok and A. Lasia, Int. J. Hydrogen Energy, 2007, 32, 1211–1218.
40. R. Armstrong and M. Henderson, J. Electroanal. Chem., 1972, 39, 81–90.
41. L. Chen and A. Lasia, J. Electrochem. Soc., 1992, 139, 3214–3219.
42. M. Okido, J. Depo and G. Capuano, J. Electrochem. Soc., 1993, 140, 127–133.
43. U. Lačnjevac, B. Jović, V. Jović, V. Radmilović and N. Krstajić, Int. J. Hydrogen Energy, 2013, 38, 10178–10190.
44. G. Brug, A. Van Den Eeden and M. Sluyters-Rehbach, J. Electroanal. Chem., 1984, 176, 275–295.
45. I. Herraiz-Cardona, E. Ortega, J. G. Antón and V. Pérez-Herranz, Int. J. Hydrogen Energy, 2011, 36, 9428–9438.
46. J. Tang, X. Zhao, Y. Zuo, P. Ju and Y. Tang, Electrochim. Acta, 2015, 174, 1041–1049.
47. E. A. Franceschini, G. I. Lacconi and H. R. Corti, Electrochim. Acta, 2015, 159, 210–218.

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