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Isothermal hydrogen production behavior and kinetics of bulk eutectic Mg–Ni-based alloys in NaCl solution
Journal Pre-proof Isothermal hydrogen production behavior and kinetics of bulk eutectic Mg–Ni-based alloys in NaCl solution Wei Tan, Yuan-e Yang, Yan-xiong Fang PII:
S0925-8388(19)33609-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.152363
Reference:
JALCOM 152363
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2019 Revised Date:
18 September 2019
Accepted Date: 19 September 2019
Please cite this article as: W. Tan, Y.-e Yang, Y.-x. Fang, Isothermal hydrogen production behavior and kinetics of bulk eutectic Mg–Ni-based alloys in NaCl solution, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152363. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Isothermal hydrogen production behavior and kinetics of bulk eutectic Mg-Ni-based alloys in NaCl solution Wei Tan a,b , Yuan-e Yang c, Yan-xiong Fang a
a
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, 510006 (China);
b
Department of Chemical and Biomolecular Engineering National University of Singapore, Singapore 117585 (Singapore) c
Guangdong Industry Polytechnic, Guangzhou, Guangdong,510300 (China)
Corresponding author: Wei Tan Tel.: +86 +86 20 39322231; fax: +86 20 39322231 E-mail: [email protected] ; [email protected] 1
Abstract To seek bulk Mg-based alloys with excellence H2 production properties, smelted eutectic Mg-Ni-based alloys are successfully prepared by metallurgical method. The phase compositions and microstructures are tested by X-ray diffraction (XRD) and scanning electron microscope (SEM). The isothermal H2 production curves of prepared alloys in 3.5 wt.% NaCl solution are collected at different temperatures by drainage method. H2 production kinetics and thermodynamics are investigated in combination with the microstructures results and fitting results of hydrolysis curves by Avrami-Erofeev equation and Arrhenius equation. The production capacity for Mg-23.5wt.%Ni (Mg23.5Ni) eutectic alloy is 706 mL·g-1 with 0.92 conversion yield at 303 K. When 10 wt.% La added, the production capacity and yield of Mg-23.5wt%Ni-10wt.%La (Mg23.5Ni10La) are 679 mL·g-1 and 0.98, respectively. The initial rate and final capacity for H2 production of experimental Mg-Ni-based eutectic alloys improve gradually with the rising reaction temperature. The hydrolysis apparent activation energies for Mg23.5Ni and Mg23.5Ni10La alloys are 40.56 kJ/mol and 31.72 kJ/mol, respectively. The refined lamellar microstructure and the introduced La2Mg17 active phase strongly promote the H2 production process of Mg23.5Ni10La alloy. The reported results provide promising way to synthesize bulk Mg-based H2 production alloys with high yield and rapid rate by controlling the microstructure and hydrolysis reaction. Key words: Hydrogen energy, H2 production, Mg-based alloys, Microstructures, Kinetics，Hydrolysis
2
1.
Introduction Fossil fuels are increasingly depleted and the environmental pollution is becoming
more serious, which force people to seek alternative green energy. Among numerous new energies, hydrogen energy attracts more and more attention mainly due to its various sources, recyclable, zero-emission and high energy density. Scientists and politicians from many countries, especially developed countries, have invested a lot of time and resources to promote the utilization of hydrogen [1-3]. Nowadays, the utilization of hydrogen energy is still seriously prevented by the production and storage problems [4, 5]. Much resources and efforts have been done to solve the challenges in H2 storage and the solid state H2 storage way emerging in recent years is considered to be ideal strategy [6-8]. The portable H2 storage alloys with high generation preparation capacity, low cost and enough safety are seriously demanded [9, 10]. Portable H2 generators designed based on the on-board H2 production strategy are proposed to equip on the numerous vehicles to solve the H2 storage problem. It is well known that photocatalytic H2 production and electrocatalytic H2 production means are ideal H2 preparation methods [11, 12]. However, the large energy-consumption, lacking catalysts and low efficiency hinder the utilization of electrolyze water [13]. Extensive H2 production through photolysis is severely prevented by the expensive catalysts and low efficiency [14, 15]. Besides, H2 production methods such as gas-reforming [16, 17] and biomass H2 production [18] are usually constrained by raw materials or high environmental requirements [19-21]. In comparison, the strategy of hydrolysis H2 production by active metals can be easily implement due to low requirements of raw materials and equipment [22-24]. Hydrolysis H2 production method can obtain a large amount of H2 easily and quickly, 3
which is regarded as a promising way of H2 production due to high efficiency and low pollution. Al-based and Mg-based alloys with enough hydrolysis reaction activity are selected to generate H2. Abundance in raw materials, high production capacity and simple system are the main advantages of the hydrolysis H2 production way. Active Al-based alloys with high theoretical H2 production capacities [25] are usually limited by the surface formed continuous and dense Al2O3 passivation layer [26]. In order to accelerate the hydrolysis H2 production process, acidic/alkaline [27] is directly introduced or the Ga, In and Sn elements with low melt points are added [28], increasing the risk of environmental pollution and causing waste of rare resources. There are many Mg-based engineering alloys used in automotive industry due to their lightweight, high-strength and extensive resources. In addition, the functional properties of Mg-based alloys are receiving more and more attention, such as in the fields of energy storage/preparation and conversion (H2 storage, H2 production, Mg-ion batteries, etc.) and the biomedical fields [29, 30]. Although Mg-based alloys still have some significant problems as H2 storage [6, 31] and H2 production materials, the theoretical H2 storage/ H2 production capacity [32], the natural reserves and the significant electrochemical advantages of Mg make it a significant advantage in the H2 production field. Mg is particularly attractive for H2 production in a neutral solution due to high theoretical H2 production capacity, abundant resources in earth's crust, high electrochemical activity, light weight mild reaction conditions [33, 34]. Besides, the non-toxic by-products during H2 production process are able to be utilized in other industries [35]. The H2 production reaction of Mg in neutral water can be expressed by equation (1) [36]. (1)
4
However, the hydrolysis of Mg can be rapidly interrupted by the formation of a passive magnesium hydroxide layer onto the surface of reactive materials [37]. The above-mentioned hydrolysis H2 production process is built on the chemical corrosion reaction of Mg. The hydrolysis process of pure Mg is mainly considered as the chemical corrosion process. Therefore, the H2 generation rate mainly depends on the matrix activity and the medium corrosivity. Cu, Si and rare earth elements as active elements are used to improve hydrolysis H2 production behaviour instead of adopting the acidic/alkaline media [36, 38]. Ball milling technique is also used to improve the H2 production characteristics of Mg-based alloys [39, 40]. However, it is worth noting that although the H2 production performance of the powder Mg-based alloys has been improved, it has been greatly confused for storage, transportation and safety. Mg-based alloys as powders have high specific areas, high possibility of flammable/explosive and easy contaminated characteristics, which will bring great troubles to storage, transportation, protection cost and safety. Hence, developing bulk Mg-based alloys with superior H2 production thermodynamics, high generation capacities and high generation yields is the sensible direction that should be worked hard in the future. Although, the H2 production of Mg/Mg2Ni lamellar composite has be reported by Li, SL et. al [41] and the fast hydrogen generation of Mg-Ni alloys has been carried out by Oh, S et al [42]. As rare earth element, La can form numerous second phases with Mg or Ni during the solidification process, which can composed micro-galvanic cells with the matrix Mg phase during the difference of potentials between second 5
phase and matrix phase. The La-doped on the hydrolysis H2 production thermodynamics of eutectic Mg-Ni alloy is regarded to promote the hydrolysis performance of Mg-Ni
binary alloys. In this work, the bulk Mg-23.5wt.%Ni
(Mg23.5Ni) and Mg-23.5wt.%Ni-10wt.%La (Mg23.5Ni10La) alloys are prepared and the isothermal hydrolysis H2 production behaviour as well as the thermodynamics in NaCl solution are investigated. The addition of 10 wt.% La in master Mg23.5wt.%Ni alloy to ameliorate the activities, introduce the activity phases, increase the grain/phase boundaries and enhance the mass-transfer to keep high hydrogen production capacity and rate. All conditions including initial sample size, hydrolysis H2 production medium, hydrolysis temperature, hydrolysis test platform, hydrolysis operation procedure were controlled consistently, and La content was set as the only variable. The influence of the rare earth La element on microstructures, hydrolysis H2 production thermodynamics and rate-limiting steps of the eutectic Mg-Ni alloy were focused. Combined the microstructures information and H2 production results, the microstructure-activity relationship of prepared Mg-based alloys in NaCl solution is established. 2.
Experimental details Eutectic
Mg-Ni-based
alloys
Mg-23.5wt.%Ni
(Mg23.5Ni)
and
Mg-23.5wt.%Ni-10wt.%La (Mg23.5Ni10La) were prepared using commercial Mg (> 99.5 wt.%), Ni (99.5 wt.%) and Mg-30wt.%La intermediate alloys by flux covering protection melting method. During the melting, mechanical stirring using a homemade mechanical stirrer was carried out to ensure the homogenization of 6
composition. The alloy melt were poured into a preheated mold and air cooling to room temperature after the dissolving of raw materials. XRD patterns were collected by a Bruker XRD diffractor (Germany). The Cu Kα1 radiation with λ = 1.541 Å wavelength is employed and the steps of 0.03° is used to identify phase compositions of experimental samples. A field emission scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) at 20 kV is employed to observe the morphologies of prepared alloys.
Fig. 1 The schematic diagram of home-made hydrolysis H2 generation testing device.
The isothermal hydrolysis hydrogen production curves of experimental alloys were tested by a home-made drainage device at different temperatures, which has been widely employed to collect the hydrogen production curves [43-45]. The schematic diagram of the home-made hydrolysis hydrogen generation testing device has been provided in Fig. 1, which is mainly consisted by a reactor, a gas collector and a data recorder. The hydrolysis reactor is placed in a constant temperature water bath for heating operation. Temperatures of water are important to hydrolysis hydrogen production reaction, which theoretically change as the reaction proceeding due to the 7
exothermic hydrolysis reaction. However, the reactor of hydrolysis hydrogen production reactor mentioned in manuscript is very small. In additions, in order to accurately keep hydrolysis temperatures, the hydrolysis reactor is heated by a water tank with ±0.3 oC fluctuation. To minimize errors originating from the volumes reading, an online balance is employed to count continuously. 200 mL NaCl solution with 3.5 wt.% concentration were used to generate H2 with samples with about 2×2×2 mm3. Larger samples are firstly cut from the Mg23.5Ni and Mg23.5Ni10La ingots. Then, the cut samples mechanically and manually ground by grinding wheels coupling with diamond sandpaper. The grinding process is combined with measuring by a vernier calipers to ensure the consistent shape and size of the prepared alloy samples. In order to avoid the surface roughness of the sample and the effect of the surface oxide on the hydrolysis hydrogen production performance, all the hydrolysis testing samples were finally surface-polished with 7000 # diamond sandpaper. The final surface polishing process was gentle to avoid the formation of surface oxide, and the well-polished samples were kept in an argon filled glove box with moisture and oxygen levels kept below 1 ppm. To accurately count continuously, a precision scale connecting with computer is utilized. Then the weight-time hydrolysis hydrogen production curves are continuously recorded. 3.
Results and discussion
3.1 Microstructure characterization
The microstructure and phase compositions of smelted eutectic Mg23.5Ni are shown in Fig. 2. The images displayed in Fig. 2(a) and Fig. 2(b) indicate that the 8
Mg23.5Ni with a typical eutectic microstructure. And the eutectic is consisted by α-Mg and Mg2Ni confirmed by the EDS results displayed in Fig. 2(c). Combined the Mg-Ni binary phase diagram and the solidification theory, it can be determined that the eutectic forms by the alternate nucleation & growth of α-Mg phase and Mg2Ni phase during solidification process of molten metal. The lamellar eutectic structure provides a large number of phase interfaces between Mg2Ni and α-Mg, which have a positive impact on the mass transfer process during the hydrogen production process. The XRD patterns of smelted eutectic Mg23.5Ni alloy presented in Fig. 2(d) shows that the metallic Mg phase is the dominant phase and some diffraction peaks corresponding to Mg2Ni phase are detected. The result of XRD is consistent with SEM and EDS results. Correlated corrosion theory and relative researches have shown that the corrosion process of pure metal with medium are basically considered to be dominated by chemical corrosion, and the difference between grain boundaries and crystals is neglected. When two phases with different electrochemical activities are present in the same solution, the corrosion process of the alloy is controlled by electrochemical corrosion translation. In general, the electrochemical corrosion process of the alloy is faster than the chemical corrosion process. Considering the electrochemical activity and mass transfer process of the alloy during hydrogen hydrolysis of the alloy, the microstructure and phase composition of the eutectic alloy have significant advantages, and it is expected to achieve rapid high-capacity H2 production of the bulk alloy.
9
Fig. 2 SEM and XRD of as-cast eutectic Mg23.5Ni alloy.
The results of microstructure and phase compositions of as-cast eutectic Mg23.5Ni10La alloy are demonstrated in Fig. 3. The SEM and EDS results displayed in Fig. 3(a-c) indicate that the smelted Mg23.5Ni10La alloy is composed by lamellar eutectic (α-Mg-Mg2Ni) matrix, the black polygonal La2Mg17 and the gray strip Mg2Ni. The corresponding phase composition is displayed in Fig. 3(d), the XRD diffraction pattern is dominated by matrix Mg phase. In addition, Mg2Ni phase is also observed in as-cast Mg23.5Ni10La alloy. It is worth noting that La2Mg17 phase is introduced when 10 wt.% La adding. Compared with the Mg-Ni eutectic binary alloy, the Mg-Ni-La alloy retains the advantages of the eutectic matrix multiphase interface, and introduces the active mesophase La2Mg17 phase. This change in microstructure will inevitably affect the hydrolysis characteristics of La-doped Mg-Ni alloy. 10
Fig. 3 Microstructure and phase compositions of as-cast eutectic Mg23.5Ni10La alloy.
3.2 Hydrolysis hydrogen production kinetics
The isothermal hydrolysis H2 production behavior of eutectic Mg-Ni-based alloys are demonstrated in Fig. 4. Fig. 4(a) and Fig. 4(c) are capacity-time kinetics, which present the change of H2 production capacities vary with hydrolysis time. Fig. 4(b) and Fig. 4(d) are yield-time curves, which shows the yields, namely the conversion rate of hydrolysis alloys, vary with the hydrolysis time. The H2 production yield (%) is generally expressed as the ratio between the produced H2 capacity and the theoretical H2 production capacity. It can be seen from Fig. 4(a) that the as-cast Mg-Ni binary alloy can react with NaCl solution to generate H2 and the initial hydrolysis kinetics and the final generation capacities increase with the rising of reaction temperatures. The final H2 produced by as-cast eutectic Mg23.5Ni at 288, 11
293, 298 and 303 K are 410, 498, 606 and 706 mL/g, respectively. The corresponding yields for as-cast Mg23.5Ni at above-mentioned temperatures are 0.54, 0.65, 0.79 and 0.92, respectively.
Fig. 4 Hydrogen production capacities and yields of smelted eutectic Mg-Ni-based alloys (a) (b) Mg23.5Ni, (c) (d) Mg23.5Ni10La, (e) Kissinger plots. 12
After 10 wt.% La introduced, the hydrogen production curves of as-cast Mg23.5Ni10La at different temperatures are demonstrated in Fig. 4(c). The hydrolysis phenomenon of Mg23.5Ni10La alloy is similar to that of as-cast Mg23.5Ni alloy. Rising reaction temperature can promote the H2 production process and the initial rate and final capacity also increase. It can be seen that the rising temperature not only accelerates the rate of H2 production by eutectic Mg-Ni alloys hydrolysis, but also increases the H2 production yield of the whole process. The H2 capacities for as-cast Mg23.5Ni10La alloys at 288, 293, 298 and 303 K are 480, 537, 651 and 679 mL/g. The corresponding yields are 0.69, 0.78, 0.94 and 0.98 of as-cast Mg23.5Ni10La alloys with the temperature rising from 288 to 303 K. It is worth noting that the initial reaction rate of as-cast Mg23.5Ni10La is faster than that of as-cast Mg23.5Ni alloy at the same reaction temperature. In order to investigated the kinetics and thermodynamics, the corresponding hydrolysis kinetics curves displayed in Fig. 4(a) and Fig. 4(c) were fitted by the Avrami-Erofeev equation [37, 46] and the fitting results are shown in Fig. 4(b) and Fig. 4(d). The rate constant k summarized in Table 1 is also fitted by Arrhenius equation to calculate activation energy Ea of eutectic Mg-Ni-based and the fitted plots are presented in Fig. 4(e). The testing H2 production curves displayed in Fig. 4(b) and Fig. 4(d) of eutectic Mg-Ni-based alloys are well-fitted by nucleation and growth type Avrami-Erofeev equation (Equation(2)): (2) Where α(t), k, m and t are constant and reaction time. The fitted k, m and correlation 13
coefficient R are summarized in Table 1, which are consistent with testing curves, confirming that the hydrolysis H2 production processes of the smelted eutectic Mg-Ni-based alloys follow the nucleation & growth mechanism expressed by Avrami-Erofeev equation. The rate-controlling steps of the nucleation & growth reaction varies from material to material. Related research shows that m near 0.62 is a one-dimensional diffusion process, while close to 1.07 represents a three-dimensional interface reaction process [47]. Table 1 displays m of eutectic Mg-Ni-based alloys hydrolysis in 3.5 wt.% NaCl solution at different temperatures. The m value of eutectic Mg23.5Ni alloy close 1.07 indicates that the H2 production process of Mg23.5Ni alloy is controlled by the three-dimensional interface reaction. While m close to 0.67 of eutectic Mg23.5Ni10La alloy is obtained, indicating that the hydrolysis of Mg23.5Ni10La alloy in NaCl solution is a one-dimensional diffusion controlled reaction The hydrolysis apparent activation energy (Ea) of eutectic Mg-Ni-based alloys are able to calculated by the Arrhenius equation (Equation (3)) [23, 48]. (3) where T is hydrolysis temperature (K), k and k0 are reaction rate constants and R0 is the molar gas constant (8.314 J mol-1·K-1), and Ea is the hydrolysis apparent activation energy (kJ·mol−1). To a certain extent, Low Ea means higher hydrolysis activity of Mg resulting from the added Ni and La. The hydrolysis apparent activation energies of eutectic Mg-Ni-based alloys shown in Fig. 4(e) were calculated to be 40.56 and 31.72 kJ·mol-1, respectively. Which are all lower than that of MgH2 14
(58.06 kJ·mol-1) reaction in deionized water [49]. Table 1 The fitting values of rate constant k and m and correlation coefficient R.
k
288 K
293 K
298 K
303 K
0.45
0.564
0.753
1.039
Mg23.5Ni
R2
0.99 m
1.07
0.93
1.01
1.27
k
0.50
0.62
0.77
0.97
Mg23.5Ni10La
0.97 m
0.40
0.27
0.43
0.53
3.2 Products analysis and hydrolysis mechanism
Fig. 5 XRD patterns of hydrolysis products for smelted eutectic Mg-Ni-based alloys.
The hydrolysis products of H2 production by smelted eutectic Mg-Ni-based alloys at 303 K in NaCl solution are characterized by XRD and the phase compositions of hydrolysis products are presented in Fig. 5. The main phase in XRD patterns of hydrolysis products for Mg23.5Ni alloy is Mg(OH), which is the main products of hydrolysis H2 production. In addition, diffraction peak around 2θ = 33.0o belonging to Mg2Ni is observed. The hydrolysis products phase composition of 15
Mg23.5Ni10La demonstrated in Fig. 5 shows the same main phase Mg(OH)2 and second phase Mg2Ni phase in the XRD pattern. Besides, the diffraction peak around 2θ = 34.7o corresponding to La2Mg17 phase is presented. It can be seen that in the process of hydrolyzing H2 production of the eutectic Mg-Ni-based alloys, only Mg phase has undergone hydrolysis reaction, the second phases Mg2Ni and La2Mg17 as catalysis mainly promote the hydrolysis reaction and accelerate the entire hydrolysis H2 production process.
Fig. 6 Hydrolysis morphology of smelted eutectic Mg-Ni-based alloys (a) Mg23.5Ni, (b) Mg23.5Ni10La.
The hydrolysis H2 production products morphologies of smelted eutectic Mg-Ni-based alloys at 303 K in NaCl solution are shown in Fig. 6. Fig. 6(a) displays the product morphology of hydrolysis H2 production of Mg23.5Ni, it can be seen that the whole metal of as-cast eutectic Mg23.5Ni is corroded by NaCl solution and appears dark black, and the surface of Mg(OH)2 formed after corrosion is rugged and there are some micro cracks. Compared with the morphology of Mg23.5Ni after corrosion, the morphology of Mg23.5Ni10La hydrolysis H2 production in NaCl solution shown in Fig. 6(b), display the loose surface of Mg(OH)2 and there are a lot 16
of micropores and corrosion pits. Obviously, in the latter stage of hydrolysis H2 production, NaCl aqueous solution can still enter into the Mg23.5Ni10La alloy easily, which promotes the complete hydrolysis of the bulk alloy and reaches higher H2 production yield.
Fig. 7 Polarization curves of smelted eutectic Mg-Ni-based alloys. Table 2 Tafel fitting results of smelted eutectic Mg-Ni-based alloys.
Electrodes
Mg23.5Ni
Mg23.5Ni10La
Ecorr (V)
-1.153
-1.206
Icorr ((mA/cm2))
5.036
5.237
Fig. 7 is the polarization curves of eutectic Mg-Ni-based alloys. The corrosion current density Icorr and Tafel constants ba and bc are fitted parameters originated from the Butler-Volmer Equation (4) [50, 51].
(4) where I and η are experimental current and overpotential. The results of Ecorr and Icorr 17
are presented in Table 2. The potentials for as-cast eutectic Mg23.5Ni and Mg23.5Ni10La electrodes are determined to be -1.153 and -1.206 V, which indicate that La-doped can enhance the corrosion tendency of Mg23.5Ni alloy. And the corrosion currents of Mg23.5Ni and Mg23.5Ni10La alloys are 5.036 and 5.237 mA, respectively. The bulk Mg23.5Ni10La possesses more negative potential and larger corrosion current than those of Mg23.5Ni alloy. Which provides the reason why the initial hydrolysis reaction rate of as-cast Mg23.5Ni10La is faster than that of Mg23.5Ni alloy. The hydrolysis H2 production process of Mg-based alloys is mainly affected by key factors such as surface characteristics of the alloy, matrix activity, medium transport and hydrolysis temperature. The surface activity affects the initial H2 production rate of hydrolysis, that is, the response speed of the alloy after contact with the medium. The matrix activity depends on the phase composition of the alloy, which mainly determines the type of hydrolysis reaction of the alloy, that is, mainly chemical corrosion or electrochemical corrosion. The medium transport involves two processes: the initial medium penetrates the surface passivation layer into the interior of the alloy and the medium penetrates into the Mg(OH)2 colloid layer at the late stage of hydrolysis. The hydrolysis temperature mainly affects the diffusion process of the medium, providing the necessary energy for the hydrolysis reaction, which breaks through the limitation of the hydrolysis activation energy barrier. Corresponding to Mg23.5Ni alloy, the introduced active La2Mg17 phase strongly promote the electrochemical hydrolysis H2 production process of Mg23.5Ni10La. 18
The active La2Mg17 phase affects the formation of the subsequent colloidal Mg(OH)2, resulting a loose Mg(OH)2 layer, which ensures the continuous transport of the medium during the late hydrolysis process. Then higher hydrogen yields are presented in hydrolysis H2 production process of Mg23.5Ni10La. Hence, by adjusting the amount of added La, the second phase is able to be adjusted, thereby affecting the primary cells, and finally regulating the electrochemical H2 production process. 4.
Conclusions (1) The as-cast Mg23.5Ni alloy is consisted of lamellar Mg-Mg2Ni eutectic and
there are lots of phase boundaries. When added 10 wt.% La, the active La2Mg17 phase is introduced to Mg23.5Ni10La alloy. (2) Both Mg23.5Ni and Mg23.5Ni10La alloys can hydrolyze in NaCl solution to produce H2 and the initial H2 production rate and final capacity increase with the increasing of hydrolysis temperatures. (3) The initial reaction kinetic and final hydrogen yield of as-cast Mg23.5Ni10La is superior than that of as-cast Mg23.5Ni alloy at the same reaction temperature. The hydrolysis apparent activation energies of eutectic Mg-Ni-based alloys are 40.56 and 31.72 kJ/mol, respectively. The fitting results demonstrate that the hydrolysis reaction in NaCl solution of Mg23.5Ni alloy is a three-dimensional interface reaction process. While the hydrolysis of Mg23.5Ni10La alloy is a one-dimensional diffusion controlled process. (4) Faster initial rates and higher H2 yields are presented in hydrolysis H2 production process of Mg23.5Ni10La due to the positive effect of active La2Mg17 phase on the electrochemical hydrolysis H2 reaction and the formation of the subsequent colloidal Mg(OH)2. 19
Acknowledgement This work is financially supported by the Ministry of National science and technology, China, (Grant No. 2015BAK44B0).
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26
Captions of Table and Figures: Table 1 The fitting values of rate constant k and m and correlation coefficient R. Table 2 Tafel fitting results of smelted eutectic Mg-Ni-based alloys. Fig. 1 The schematic diagram of home-made hydrolysis H2 generation testing device. Figure 2 SEM and XRD of as-cast eutectic Mg23.5Ni alloy. Figure 3 Microstructure and phase compositions of as-cast eutectic Mg23.5Ni10La alloy. Figure 4 Hydrogen production capacities and yields of smelted eutectic Mg-Ni-based alloys (a)
(b) Mg23.5Ni, (c) (d) Mg23.5Ni10La, (e) Kissinger plots, (a) (b)
Mg23.5Ni, (c) (d) Mg23.5Ni10La, (e) Kissinger plots. Figure 5 XRD patterns of hydrolysis products for smelted eutectic Mg-Ni-based alloys. Figure 6 Hydrolysis morphology of smelted eutectic Mg-Ni-based alloys (a) Mg23.5Ni, (b) Mg23.5Ni10La. Fig. 7 Polarization curves of smelted eutectic Mg-Ni-based alloys.
27
Table 1 The fitting values of rate constant k and m and correlation coefficient R.
k
288 K
293 K
298 K
303 K
0.45
0.564
0.753
1.039
Mg23.5Ni
R2
0.99 m
1.07
0.93
1.01
1.27
k
0.50
0.62
0.77
0.97
Mg23.5Ni10La
0.97 m
0.40
0.27
0.43
0.53
Table 2 Tafel fitting results of smelted eutectic Mg-Ni-based alloys.
Electrodes
Mg23.5Ni
Mg23.5Ni10La
Ecorr (V)
-1.153
-1.206
Icorr ((mA/cm2))
5.036
5.237
28
Bulk Mg-Ni eutectic alloys with excellence H2 production performance are prepared. Capacities of Mg23.5Ni and Mg23.5Ni10La are 711 mL/g and 688 mL/g at 293K. Hydrolysis activation energies of Mg23.5Ni/ Mg23.5Ni10La are 40.56/ 31.72 kJmol-1. Faster initial rates and higher hydrogen yields are presented of Mg23.5Ni10La.
S0925-8388(19)33609-6
DOI:
https://doi.org/10.1016/j.jallcom.2019.152363
Reference:
JALCOM 152363
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2019 Revised Date:
18 September 2019
Accepted Date: 19 September 2019
Please cite this article as: W. Tan, Y.-e Yang, Y.-x. Fang, Isothermal hydrogen production behavior and kinetics of bulk eutectic Mg–Ni-based alloys in NaCl solution, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152363. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Isothermal hydrogen production behavior and kinetics of bulk eutectic Mg-Ni-based alloys in NaCl solution Wei Tan a,b , Yuan-e Yang c, Yan-xiong Fang a
a
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong, 510006 (China);
b
Department of Chemical and Biomolecular Engineering National University of Singapore, Singapore 117585 (Singapore) c
Guangdong Industry Polytechnic, Guangzhou, Guangdong,510300 (China)
Corresponding author: Wei Tan Tel.: +86 +86 20 39322231; fax: +86 20 39322231 E-mail: [email protected] ; [email protected] 1
Abstract To seek bulk Mg-based alloys with excellence H2 production properties, smelted eutectic Mg-Ni-based alloys are successfully prepared by metallurgical method. The phase compositions and microstructures are tested by X-ray diffraction (XRD) and scanning electron microscope (SEM). The isothermal H2 production curves of prepared alloys in 3.5 wt.% NaCl solution are collected at different temperatures by drainage method. H2 production kinetics and thermodynamics are investigated in combination with the microstructures results and fitting results of hydrolysis curves by Avrami-Erofeev equation and Arrhenius equation. The production capacity for Mg-23.5wt.%Ni (Mg23.5Ni) eutectic alloy is 706 mL·g-1 with 0.92 conversion yield at 303 K. When 10 wt.% La added, the production capacity and yield of Mg-23.5wt%Ni-10wt.%La (Mg23.5Ni10La) are 679 mL·g-1 and 0.98, respectively. The initial rate and final capacity for H2 production of experimental Mg-Ni-based eutectic alloys improve gradually with the rising reaction temperature. The hydrolysis apparent activation energies for Mg23.5Ni and Mg23.5Ni10La alloys are 40.56 kJ/mol and 31.72 kJ/mol, respectively. The refined lamellar microstructure and the introduced La2Mg17 active phase strongly promote the H2 production process of Mg23.5Ni10La alloy. The reported results provide promising way to synthesize bulk Mg-based H2 production alloys with high yield and rapid rate by controlling the microstructure and hydrolysis reaction. Key words: Hydrogen energy, H2 production, Mg-based alloys, Microstructures, Kinetics，Hydrolysis
2
1.
Introduction Fossil fuels are increasingly depleted and the environmental pollution is becoming
more serious, which force people to seek alternative green energy. Among numerous new energies, hydrogen energy attracts more and more attention mainly due to its various sources, recyclable, zero-emission and high energy density. Scientists and politicians from many countries, especially developed countries, have invested a lot of time and resources to promote the utilization of hydrogen [1-3]. Nowadays, the utilization of hydrogen energy is still seriously prevented by the production and storage problems [4, 5]. Much resources and efforts have been done to solve the challenges in H2 storage and the solid state H2 storage way emerging in recent years is considered to be ideal strategy [6-8]. The portable H2 storage alloys with high generation preparation capacity, low cost and enough safety are seriously demanded [9, 10]. Portable H2 generators designed based on the on-board H2 production strategy are proposed to equip on the numerous vehicles to solve the H2 storage problem. It is well known that photocatalytic H2 production and electrocatalytic H2 production means are ideal H2 preparation methods [11, 12]. However, the large energy-consumption, lacking catalysts and low efficiency hinder the utilization of electrolyze water [13]. Extensive H2 production through photolysis is severely prevented by the expensive catalysts and low efficiency [14, 15]. Besides, H2 production methods such as gas-reforming [16, 17] and biomass H2 production [18] are usually constrained by raw materials or high environmental requirements [19-21]. In comparison, the strategy of hydrolysis H2 production by active metals can be easily implement due to low requirements of raw materials and equipment [22-24]. Hydrolysis H2 production method can obtain a large amount of H2 easily and quickly, 3
which is regarded as a promising way of H2 production due to high efficiency and low pollution. Al-based and Mg-based alloys with enough hydrolysis reaction activity are selected to generate H2. Abundance in raw materials, high production capacity and simple system are the main advantages of the hydrolysis H2 production way. Active Al-based alloys with high theoretical H2 production capacities [25] are usually limited by the surface formed continuous and dense Al2O3 passivation layer [26]. In order to accelerate the hydrolysis H2 production process, acidic/alkaline [27] is directly introduced or the Ga, In and Sn elements with low melt points are added [28], increasing the risk of environmental pollution and causing waste of rare resources. There are many Mg-based engineering alloys used in automotive industry due to their lightweight, high-strength and extensive resources. In addition, the functional properties of Mg-based alloys are receiving more and more attention, such as in the fields of energy storage/preparation and conversion (H2 storage, H2 production, Mg-ion batteries, etc.) and the biomedical fields [29, 30]. Although Mg-based alloys still have some significant problems as H2 storage [6, 31] and H2 production materials, the theoretical H2 storage/ H2 production capacity [32], the natural reserves and the significant electrochemical advantages of Mg make it a significant advantage in the H2 production field. Mg is particularly attractive for H2 production in a neutral solution due to high theoretical H2 production capacity, abundant resources in earth's crust, high electrochemical activity, light weight mild reaction conditions [33, 34]. Besides, the non-toxic by-products during H2 production process are able to be utilized in other industries [35]. The H2 production reaction of Mg in neutral water can be expressed by equation (1) [36]. (1)
4
However, the hydrolysis of Mg can be rapidly interrupted by the formation of a passive magnesium hydroxide layer onto the surface of reactive materials [37]. The above-mentioned hydrolysis H2 production process is built on the chemical corrosion reaction of Mg. The hydrolysis process of pure Mg is mainly considered as the chemical corrosion process. Therefore, the H2 generation rate mainly depends on the matrix activity and the medium corrosivity. Cu, Si and rare earth elements as active elements are used to improve hydrolysis H2 production behaviour instead of adopting the acidic/alkaline media [36, 38]. Ball milling technique is also used to improve the H2 production characteristics of Mg-based alloys [39, 40]. However, it is worth noting that although the H2 production performance of the powder Mg-based alloys has been improved, it has been greatly confused for storage, transportation and safety. Mg-based alloys as powders have high specific areas, high possibility of flammable/explosive and easy contaminated characteristics, which will bring great troubles to storage, transportation, protection cost and safety. Hence, developing bulk Mg-based alloys with superior H2 production thermodynamics, high generation capacities and high generation yields is the sensible direction that should be worked hard in the future. Although, the H2 production of Mg/Mg2Ni lamellar composite has be reported by Li, SL et. al [41] and the fast hydrogen generation of Mg-Ni alloys has been carried out by Oh, S et al [42]. As rare earth element, La can form numerous second phases with Mg or Ni during the solidification process, which can composed micro-galvanic cells with the matrix Mg phase during the difference of potentials between second 5
phase and matrix phase. The La-doped on the hydrolysis H2 production thermodynamics of eutectic Mg-Ni alloy is regarded to promote the hydrolysis performance of Mg-Ni
binary alloys. In this work, the bulk Mg-23.5wt.%Ni
(Mg23.5Ni) and Mg-23.5wt.%Ni-10wt.%La (Mg23.5Ni10La) alloys are prepared and the isothermal hydrolysis H2 production behaviour as well as the thermodynamics in NaCl solution are investigated. The addition of 10 wt.% La in master Mg23.5wt.%Ni alloy to ameliorate the activities, introduce the activity phases, increase the grain/phase boundaries and enhance the mass-transfer to keep high hydrogen production capacity and rate. All conditions including initial sample size, hydrolysis H2 production medium, hydrolysis temperature, hydrolysis test platform, hydrolysis operation procedure were controlled consistently, and La content was set as the only variable. The influence of the rare earth La element on microstructures, hydrolysis H2 production thermodynamics and rate-limiting steps of the eutectic Mg-Ni alloy were focused. Combined the microstructures information and H2 production results, the microstructure-activity relationship of prepared Mg-based alloys in NaCl solution is established. 2.
Experimental details Eutectic
Mg-Ni-based
alloys
Mg-23.5wt.%Ni
(Mg23.5Ni)
and
Mg-23.5wt.%Ni-10wt.%La (Mg23.5Ni10La) were prepared using commercial Mg (> 99.5 wt.%), Ni (99.5 wt.%) and Mg-30wt.%La intermediate alloys by flux covering protection melting method. During the melting, mechanical stirring using a homemade mechanical stirrer was carried out to ensure the homogenization of 6
composition. The alloy melt were poured into a preheated mold and air cooling to room temperature after the dissolving of raw materials. XRD patterns were collected by a Bruker XRD diffractor (Germany). The Cu Kα1 radiation with λ = 1.541 Å wavelength is employed and the steps of 0.03° is used to identify phase compositions of experimental samples. A field emission scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) at 20 kV is employed to observe the morphologies of prepared alloys.
Fig. 1 The schematic diagram of home-made hydrolysis H2 generation testing device.
The isothermal hydrolysis hydrogen production curves of experimental alloys were tested by a home-made drainage device at different temperatures, which has been widely employed to collect the hydrogen production curves [43-45]. The schematic diagram of the home-made hydrolysis hydrogen generation testing device has been provided in Fig. 1, which is mainly consisted by a reactor, a gas collector and a data recorder. The hydrolysis reactor is placed in a constant temperature water bath for heating operation. Temperatures of water are important to hydrolysis hydrogen production reaction, which theoretically change as the reaction proceeding due to the 7
exothermic hydrolysis reaction. However, the reactor of hydrolysis hydrogen production reactor mentioned in manuscript is very small. In additions, in order to accurately keep hydrolysis temperatures, the hydrolysis reactor is heated by a water tank with ±0.3 oC fluctuation. To minimize errors originating from the volumes reading, an online balance is employed to count continuously. 200 mL NaCl solution with 3.5 wt.% concentration were used to generate H2 with samples with about 2×2×2 mm3. Larger samples are firstly cut from the Mg23.5Ni and Mg23.5Ni10La ingots. Then, the cut samples mechanically and manually ground by grinding wheels coupling with diamond sandpaper. The grinding process is combined with measuring by a vernier calipers to ensure the consistent shape and size of the prepared alloy samples. In order to avoid the surface roughness of the sample and the effect of the surface oxide on the hydrolysis hydrogen production performance, all the hydrolysis testing samples were finally surface-polished with 7000 # diamond sandpaper. The final surface polishing process was gentle to avoid the formation of surface oxide, and the well-polished samples were kept in an argon filled glove box with moisture and oxygen levels kept below 1 ppm. To accurately count continuously, a precision scale connecting with computer is utilized. Then the weight-time hydrolysis hydrogen production curves are continuously recorded. 3.
Results and discussion
3.1 Microstructure characterization
The microstructure and phase compositions of smelted eutectic Mg23.5Ni are shown in Fig. 2. The images displayed in Fig. 2(a) and Fig. 2(b) indicate that the 8
Mg23.5Ni with a typical eutectic microstructure. And the eutectic is consisted by α-Mg and Mg2Ni confirmed by the EDS results displayed in Fig. 2(c). Combined the Mg-Ni binary phase diagram and the solidification theory, it can be determined that the eutectic forms by the alternate nucleation & growth of α-Mg phase and Mg2Ni phase during solidification process of molten metal. The lamellar eutectic structure provides a large number of phase interfaces between Mg2Ni and α-Mg, which have a positive impact on the mass transfer process during the hydrogen production process. The XRD patterns of smelted eutectic Mg23.5Ni alloy presented in Fig. 2(d) shows that the metallic Mg phase is the dominant phase and some diffraction peaks corresponding to Mg2Ni phase are detected. The result of XRD is consistent with SEM and EDS results. Correlated corrosion theory and relative researches have shown that the corrosion process of pure metal with medium are basically considered to be dominated by chemical corrosion, and the difference between grain boundaries and crystals is neglected. When two phases with different electrochemical activities are present in the same solution, the corrosion process of the alloy is controlled by electrochemical corrosion translation. In general, the electrochemical corrosion process of the alloy is faster than the chemical corrosion process. Considering the electrochemical activity and mass transfer process of the alloy during hydrogen hydrolysis of the alloy, the microstructure and phase composition of the eutectic alloy have significant advantages, and it is expected to achieve rapid high-capacity H2 production of the bulk alloy.
9
Fig. 2 SEM and XRD of as-cast eutectic Mg23.5Ni alloy.
The results of microstructure and phase compositions of as-cast eutectic Mg23.5Ni10La alloy are demonstrated in Fig. 3. The SEM and EDS results displayed in Fig. 3(a-c) indicate that the smelted Mg23.5Ni10La alloy is composed by lamellar eutectic (α-Mg-Mg2Ni) matrix, the black polygonal La2Mg17 and the gray strip Mg2Ni. The corresponding phase composition is displayed in Fig. 3(d), the XRD diffraction pattern is dominated by matrix Mg phase. In addition, Mg2Ni phase is also observed in as-cast Mg23.5Ni10La alloy. It is worth noting that La2Mg17 phase is introduced when 10 wt.% La adding. Compared with the Mg-Ni eutectic binary alloy, the Mg-Ni-La alloy retains the advantages of the eutectic matrix multiphase interface, and introduces the active mesophase La2Mg17 phase. This change in microstructure will inevitably affect the hydrolysis characteristics of La-doped Mg-Ni alloy. 10
Fig. 3 Microstructure and phase compositions of as-cast eutectic Mg23.5Ni10La alloy.
3.2 Hydrolysis hydrogen production kinetics
The isothermal hydrolysis H2 production behavior of eutectic Mg-Ni-based alloys are demonstrated in Fig. 4. Fig. 4(a) and Fig. 4(c) are capacity-time kinetics, which present the change of H2 production capacities vary with hydrolysis time. Fig. 4(b) and Fig. 4(d) are yield-time curves, which shows the yields, namely the conversion rate of hydrolysis alloys, vary with the hydrolysis time. The H2 production yield (%) is generally expressed as the ratio between the produced H2 capacity and the theoretical H2 production capacity. It can be seen from Fig. 4(a) that the as-cast Mg-Ni binary alloy can react with NaCl solution to generate H2 and the initial hydrolysis kinetics and the final generation capacities increase with the rising of reaction temperatures. The final H2 produced by as-cast eutectic Mg23.5Ni at 288, 11
293, 298 and 303 K are 410, 498, 606 and 706 mL/g, respectively. The corresponding yields for as-cast Mg23.5Ni at above-mentioned temperatures are 0.54, 0.65, 0.79 and 0.92, respectively.
Fig. 4 Hydrogen production capacities and yields of smelted eutectic Mg-Ni-based alloys (a) (b) Mg23.5Ni, (c) (d) Mg23.5Ni10La, (e) Kissinger plots. 12
After 10 wt.% La introduced, the hydrogen production curves of as-cast Mg23.5Ni10La at different temperatures are demonstrated in Fig. 4(c). The hydrolysis phenomenon of Mg23.5Ni10La alloy is similar to that of as-cast Mg23.5Ni alloy. Rising reaction temperature can promote the H2 production process and the initial rate and final capacity also increase. It can be seen that the rising temperature not only accelerates the rate of H2 production by eutectic Mg-Ni alloys hydrolysis, but also increases the H2 production yield of the whole process. The H2 capacities for as-cast Mg23.5Ni10La alloys at 288, 293, 298 and 303 K are 480, 537, 651 and 679 mL/g. The corresponding yields are 0.69, 0.78, 0.94 and 0.98 of as-cast Mg23.5Ni10La alloys with the temperature rising from 288 to 303 K. It is worth noting that the initial reaction rate of as-cast Mg23.5Ni10La is faster than that of as-cast Mg23.5Ni alloy at the same reaction temperature. In order to investigated the kinetics and thermodynamics, the corresponding hydrolysis kinetics curves displayed in Fig. 4(a) and Fig. 4(c) were fitted by the Avrami-Erofeev equation [37, 46] and the fitting results are shown in Fig. 4(b) and Fig. 4(d). The rate constant k summarized in Table 1 is also fitted by Arrhenius equation to calculate activation energy Ea of eutectic Mg-Ni-based and the fitted plots are presented in Fig. 4(e). The testing H2 production curves displayed in Fig. 4(b) and Fig. 4(d) of eutectic Mg-Ni-based alloys are well-fitted by nucleation and growth type Avrami-Erofeev equation (Equation(2)): (2) Where α(t), k, m and t are constant and reaction time. The fitted k, m and correlation 13
coefficient R are summarized in Table 1, which are consistent with testing curves, confirming that the hydrolysis H2 production processes of the smelted eutectic Mg-Ni-based alloys follow the nucleation & growth mechanism expressed by Avrami-Erofeev equation. The rate-controlling steps of the nucleation & growth reaction varies from material to material. Related research shows that m near 0.62 is a one-dimensional diffusion process, while close to 1.07 represents a three-dimensional interface reaction process [47]. Table 1 displays m of eutectic Mg-Ni-based alloys hydrolysis in 3.5 wt.% NaCl solution at different temperatures. The m value of eutectic Mg23.5Ni alloy close 1.07 indicates that the H2 production process of Mg23.5Ni alloy is controlled by the three-dimensional interface reaction. While m close to 0.67 of eutectic Mg23.5Ni10La alloy is obtained, indicating that the hydrolysis of Mg23.5Ni10La alloy in NaCl solution is a one-dimensional diffusion controlled reaction The hydrolysis apparent activation energy (Ea) of eutectic Mg-Ni-based alloys are able to calculated by the Arrhenius equation (Equation (3)) [23, 48]. (3) where T is hydrolysis temperature (K), k and k0 are reaction rate constants and R0 is the molar gas constant (8.314 J mol-1·K-1), and Ea is the hydrolysis apparent activation energy (kJ·mol−1). To a certain extent, Low Ea means higher hydrolysis activity of Mg resulting from the added Ni and La. The hydrolysis apparent activation energies of eutectic Mg-Ni-based alloys shown in Fig. 4(e) were calculated to be 40.56 and 31.72 kJ·mol-1, respectively. Which are all lower than that of MgH2 14
(58.06 kJ·mol-1) reaction in deionized water [49]. Table 1 The fitting values of rate constant k and m and correlation coefficient R.
k
288 K
293 K
298 K
303 K
0.45
0.564
0.753
1.039
Mg23.5Ni
R2
0.99 m
1.07
0.93
1.01
1.27
k
0.50
0.62
0.77
0.97
Mg23.5Ni10La
0.97 m
0.40
0.27
0.43
0.53
3.2 Products analysis and hydrolysis mechanism
Fig. 5 XRD patterns of hydrolysis products for smelted eutectic Mg-Ni-based alloys.
The hydrolysis products of H2 production by smelted eutectic Mg-Ni-based alloys at 303 K in NaCl solution are characterized by XRD and the phase compositions of hydrolysis products are presented in Fig. 5. The main phase in XRD patterns of hydrolysis products for Mg23.5Ni alloy is Mg(OH), which is the main products of hydrolysis H2 production. In addition, diffraction peak around 2θ = 33.0o belonging to Mg2Ni is observed. The hydrolysis products phase composition of 15
Mg23.5Ni10La demonstrated in Fig. 5 shows the same main phase Mg(OH)2 and second phase Mg2Ni phase in the XRD pattern. Besides, the diffraction peak around 2θ = 34.7o corresponding to La2Mg17 phase is presented. It can be seen that in the process of hydrolyzing H2 production of the eutectic Mg-Ni-based alloys, only Mg phase has undergone hydrolysis reaction, the second phases Mg2Ni and La2Mg17 as catalysis mainly promote the hydrolysis reaction and accelerate the entire hydrolysis H2 production process.
Fig. 6 Hydrolysis morphology of smelted eutectic Mg-Ni-based alloys (a) Mg23.5Ni, (b) Mg23.5Ni10La.
The hydrolysis H2 production products morphologies of smelted eutectic Mg-Ni-based alloys at 303 K in NaCl solution are shown in Fig. 6. Fig. 6(a) displays the product morphology of hydrolysis H2 production of Mg23.5Ni, it can be seen that the whole metal of as-cast eutectic Mg23.5Ni is corroded by NaCl solution and appears dark black, and the surface of Mg(OH)2 formed after corrosion is rugged and there are some micro cracks. Compared with the morphology of Mg23.5Ni after corrosion, the morphology of Mg23.5Ni10La hydrolysis H2 production in NaCl solution shown in Fig. 6(b), display the loose surface of Mg(OH)2 and there are a lot 16
of micropores and corrosion pits. Obviously, in the latter stage of hydrolysis H2 production, NaCl aqueous solution can still enter into the Mg23.5Ni10La alloy easily, which promotes the complete hydrolysis of the bulk alloy and reaches higher H2 production yield.
Fig. 7 Polarization curves of smelted eutectic Mg-Ni-based alloys. Table 2 Tafel fitting results of smelted eutectic Mg-Ni-based alloys.
Electrodes
Mg23.5Ni
Mg23.5Ni10La
Ecorr (V)
-1.153
-1.206
Icorr ((mA/cm2))
5.036
5.237
Fig. 7 is the polarization curves of eutectic Mg-Ni-based alloys. The corrosion current density Icorr and Tafel constants ba and bc are fitted parameters originated from the Butler-Volmer Equation (4) [50, 51].
(4) where I and η are experimental current and overpotential. The results of Ecorr and Icorr 17
are presented in Table 2. The potentials for as-cast eutectic Mg23.5Ni and Mg23.5Ni10La electrodes are determined to be -1.153 and -1.206 V, which indicate that La-doped can enhance the corrosion tendency of Mg23.5Ni alloy. And the corrosion currents of Mg23.5Ni and Mg23.5Ni10La alloys are 5.036 and 5.237 mA, respectively. The bulk Mg23.5Ni10La possesses more negative potential and larger corrosion current than those of Mg23.5Ni alloy. Which provides the reason why the initial hydrolysis reaction rate of as-cast Mg23.5Ni10La is faster than that of Mg23.5Ni alloy. The hydrolysis H2 production process of Mg-based alloys is mainly affected by key factors such as surface characteristics of the alloy, matrix activity, medium transport and hydrolysis temperature. The surface activity affects the initial H2 production rate of hydrolysis, that is, the response speed of the alloy after contact with the medium. The matrix activity depends on the phase composition of the alloy, which mainly determines the type of hydrolysis reaction of the alloy, that is, mainly chemical corrosion or electrochemical corrosion. The medium transport involves two processes: the initial medium penetrates the surface passivation layer into the interior of the alloy and the medium penetrates into the Mg(OH)2 colloid layer at the late stage of hydrolysis. The hydrolysis temperature mainly affects the diffusion process of the medium, providing the necessary energy for the hydrolysis reaction, which breaks through the limitation of the hydrolysis activation energy barrier. Corresponding to Mg23.5Ni alloy, the introduced active La2Mg17 phase strongly promote the electrochemical hydrolysis H2 production process of Mg23.5Ni10La. 18
The active La2Mg17 phase affects the formation of the subsequent colloidal Mg(OH)2, resulting a loose Mg(OH)2 layer, which ensures the continuous transport of the medium during the late hydrolysis process. Then higher hydrogen yields are presented in hydrolysis H2 production process of Mg23.5Ni10La. Hence, by adjusting the amount of added La, the second phase is able to be adjusted, thereby affecting the primary cells, and finally regulating the electrochemical H2 production process. 4.
Conclusions (1) The as-cast Mg23.5Ni alloy is consisted of lamellar Mg-Mg2Ni eutectic and
there are lots of phase boundaries. When added 10 wt.% La, the active La2Mg17 phase is introduced to Mg23.5Ni10La alloy. (2) Both Mg23.5Ni and Mg23.5Ni10La alloys can hydrolyze in NaCl solution to produce H2 and the initial H2 production rate and final capacity increase with the increasing of hydrolysis temperatures. (3) The initial reaction kinetic and final hydrogen yield of as-cast Mg23.5Ni10La is superior than that of as-cast Mg23.5Ni alloy at the same reaction temperature. The hydrolysis apparent activation energies of eutectic Mg-Ni-based alloys are 40.56 and 31.72 kJ/mol, respectively. The fitting results demonstrate that the hydrolysis reaction in NaCl solution of Mg23.5Ni alloy is a three-dimensional interface reaction process. While the hydrolysis of Mg23.5Ni10La alloy is a one-dimensional diffusion controlled process. (4) Faster initial rates and higher H2 yields are presented in hydrolysis H2 production process of Mg23.5Ni10La due to the positive effect of active La2Mg17 phase on the electrochemical hydrolysis H2 reaction and the formation of the subsequent colloidal Mg(OH)2. 19
Acknowledgement This work is financially supported by the Ministry of National science and technology, China, (Grant No. 2015BAK44B0).
20
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26
Captions of Table and Figures: Table 1 The fitting values of rate constant k and m and correlation coefficient R. Table 2 Tafel fitting results of smelted eutectic Mg-Ni-based alloys. Fig. 1 The schematic diagram of home-made hydrolysis H2 generation testing device. Figure 2 SEM and XRD of as-cast eutectic Mg23.5Ni alloy. Figure 3 Microstructure and phase compositions of as-cast eutectic Mg23.5Ni10La alloy. Figure 4 Hydrogen production capacities and yields of smelted eutectic Mg-Ni-based alloys (a)
(b) Mg23.5Ni, (c) (d) Mg23.5Ni10La, (e) Kissinger plots, (a) (b)
Mg23.5Ni, (c) (d) Mg23.5Ni10La, (e) Kissinger plots. Figure 5 XRD patterns of hydrolysis products for smelted eutectic Mg-Ni-based alloys. Figure 6 Hydrolysis morphology of smelted eutectic Mg-Ni-based alloys (a) Mg23.5Ni, (b) Mg23.5Ni10La. Fig. 7 Polarization curves of smelted eutectic Mg-Ni-based alloys.
27
Table 1 The fitting values of rate constant k and m and correlation coefficient R.
k
288 K
293 K
298 K
303 K
0.45
0.564
0.753
1.039
Mg23.5Ni
R2
0.99 m
1.07
0.93
1.01
1.27
k
0.50
0.62
0.77
0.97
Mg23.5Ni10La
0.97 m
0.40
0.27
0.43
0.53
Table 2 Tafel fitting results of smelted eutectic Mg-Ni-based alloys.
Electrodes
Mg23.5Ni
Mg23.5Ni10La
Ecorr (V)
-1.153
-1.206
Icorr ((mA/cm2))
5.036
5.237
28
Bulk Mg-Ni eutectic alloys with excellence H2 production performance are prepared. Capacities of Mg23.5Ni and Mg23.5Ni10La are 711 mL/g and 688 mL/g at 293K. Hydrolysis activation energies of Mg23.5Ni/ Mg23.5Ni10La are 40.56/ 31.72 kJmol-1. Faster initial rates and higher hydrogen yields are presented of Mg23.5Ni10La.