H2 generation kinetics/thermodynamics and hydrolysis mechanism of high-performance La-doped Mg-Ni alloys in NaCl solution—A large-scale and quick strategy to get hydrogen

Xiojing Hou,Hongchng Shi,Lu Yng,Kiming Hou,Yi Wng,Lei Feng,?,Guoqun Suo,Xiohui Ye,Li Zhng,Ynling Yng

aSchool of Material Science and Engineering,Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials,Shaanxi

University of Science and Technology,Xi’an 710021,China

b State Key Laboratory of Solidification Processing,Northwestern Polytechnical University,Xi’an 710072,China

Abstract In this work,La-doped Mg-Ni multiphase alloys were prepared by resistance melting furnace(RMF)and then modified by high-energy ball milling(HEBM).The hydrolysis H2 generation kinetics/thermodynamics of prepared alloys in NaCl solutions have been investigated with the help of nonlinear and linear fitting by Avrami-Erofeev and Arrhenius equations.Combining the microstructure information before and after hydrolysis and thermodynamics fitting results,the hydrolysis H2 generation mechanism based on nucleation & growth has been elaborated.The final H2 generation capacities of 0La,5La,10La and 15 La alloys are 677,653,641 and 770 mL·g?1 H2 in 240min at 291K,respectively.While,the final H2 generation capacities of HEBM 0La,5La,10La and 15 La alloys are 632,824,611 and 653 mL·g?1 H2 in 20min at 291K,respectively.The as-cast 15La alloy and HEMB 5La alloy present the best H2 production rates and final H2 production capacities,especially the HEBM 5La can rapidly achieve high H2 generation capacity(670 and 824 mL·g?1 H2)at low temperature(291K)within short time(5 and 20min).The difference between the H2 generation capacities is mainly originated from the initial nucleation rate of Mg(OH)2 and the subsequent processes affected by the microstructures and phase compositions of the hydrolysis alloys.Relative low initial nucleation rate and fully growth of Mg(OH)2 nucleus are the premise of high H2 generation capacity due to the hydrolysis H2 generation process consisted by the nucleation,growth and contacting of Mg(OH)2 nucleus.To utilization H2 by designing solid state H2 generators using optimized Mg-based alloys is expected to be a feasible H2 generation strategy at the moment.? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:H2 generation;La-doped Mg-Ni alloys;Kinetics;Thermodynamics;Hydrolysis mechanism.

1.Introduction

Energy industries constitute the foundation of social civilization,which promotes the progress of technological innovation and human living standard[1].Every energy system transition gives birth to an unprecedented industrial revolution.The main energy structure of human has undergone the following stages of evolution:coal(C)→oil(-CH2)→natural gas(-CH4).Obviously,the utilization of the above-mentioned fossil energy sources is accompanied by a large amount emissions of carbon dioxide gas and serious impact to people’s living environment.Fortunately,during the replacement of energy,the C/H ratio in fuels continuously decreases,resulting in increasing the energy density while decreasing the emissions of CO2.In the last century,fossil fuels has taken on the most important role in energy utilization.Unfortunately,the fossil energy stored on the earth is too limited to further mine for a long time after excessive mining and utilization.Facing energy crisis and environment pollution,future energy with low-emission,high energy density and sustainability is strongly craved.With the situation of energy resource crisis and the issue of circumstance pollution being more and more emergent,exploitation and development of clean,sustainable energy resource becomes critical[1,2].

Now,people turned their attention to hydrogen energy with high thermal value,zero-emission and recyclability[3-6].Compared with traditional fossil energy,the thoroughly reduction of C/H ratio caused by removal of carbon increases the calorific value per unit mass and the CO2emission is effectively eliminated[7-9].Consisted by the most abundant and lightest H element in the world,H2has been regarded as a promising future energy carrier for green development after many generations unremitting efforts of scientists[10].

The main problems facing hydrogen energy application involve the H2preparation[11-14],H2storage[15-18]and H2utilization[6,19-21].Among them,the first problem needs to be solved is that the preparation of mass H2.Accompanied by the progress of science and technology,major achievements have been created and H2can be gradually produced massively in industry.At present,six approaches have been mainly explored to generate H2,which can be divided into two categories,namely H2production from water[22]and H2production from non-aqueous.The electrolysis of water,photo-catalysis of water and hydrolysis by active metals/hydrides and aqueous solution[23-28]belong to the former.And the reforming of fossil fuels,nuclear H2generation and biomass H2production processing[29]techniques return to the latter.The hot researched electrolysis and photolysis techniques are reported to be seriously restricted by their low energy efficiencies[30,31].Serious corrosion for equipment prevent the application of the nuclear H2production[32].Biomass technologies for H2production are still in laboratorial investigation stage restricted by the purification problem[33].At present,H2originated from the reforming of fossil fuels covers the vast majority of H2market share.However,the reforming way is not environmentally friendly due to lots of CO2is released.In additions,it is also unsustainable due to the finite of fossil fuels[34,35].Hydrogen generation and storage has become a rapidly developing research content[36,37],especially developing a feasible and easy implement H2generation way has become a real critical issue.

Recently,hydrolysis H2production via Mg and its hydrides become a more and more popular way due to low cost,abundance in reverse,high generation capacity,zeroemission,simple equipment[27,38,39]and easy regeneration by chemical reactions[40,41].The theoretical hydrolysis H2generation capacity of pure Mg is about 1000 mL·g?1at atmospheric environment,which is a promising candidate for on-board H2generators.

The hydrolysis reaction between Mg and H2O can be described by Eqs.(1)-(3)[42-45]:

The entire hydrolysis H2generation reaction is essential the nucleation & growth process of Mg(OH)2.Unfortunately,the aforementioned hydrolysis process is usually blocked or evenly interrupted by the surface formed Mg(OH)2passive layer,which normally causes sluggish kinetic and low H2capacity/conversion yield[42,46].The most problem to be solved in the system is that the Mg hydrolysis process will form a Mg(OH)2passivation layer,and the hydrolysis reaction is difficult to continue,resulting in poor hydrolysis kinetics[47-50].

Numerous strategies have been tried to ameliorate H2generation performance of Mg[38,51-54].Ball milling,alloying and changing solution compositions have been employed to enhance the hydrolysis process of Mg[2,42,55-57].Most of previous reports focus on hydrolysis reaction of Mg milled with metals,chlorides,oxides,hydrides and carbon materials[51,52,56,58,59].The acids,chloride salts or tap water can improve the hydrolysis kinetics and generation capacity of Mg and its alloys.Unfortunately,the environmental pollution,equipment corrosion as well as costs rising issues are brought during the implementation of the aforementioned modification ways[2,60-62].

Besides,microalloying combined high energy ball milling(HEBM)strategies have been selected to elevate the hydrolysis H2production kinetics and conversion yields of Mg-based alloys[18,63,64].The H2production characteristics of Mgbased alloys with reasonable alloying elements to optimize the microstructures,phase compositions as well as reaction reactivities[65-67].Unfortunately,the bulk Mg-based alloys with small specific surface areas cannot rapidly reach high conversion yields[68-71].The HEBM technique improves the hydrolysis rate and H2generation capacity of the Mgbased alloys by reducing particle sizes,elevating reaction contacting areas and shortening medium diffusion distance.Unfortunately,with the progress of the hydrolysis process,an integral surface colloidal Mg(OH)2layer forms,which seriously prevent the smoothly and quickly diffusion of aqueous medium into the alloy particles.The integrity destroy of the formed Mg(OH)2or the nucleation & growth control are effective strategies to quickly achieve high conversion yields.

The determining factors of high H2production capacity and the specific microscopic process of hydrolysis H2generation reaction are not clear enough.In this study,ascast(Mg10Ni)1-xLax(x=0,5,10,15wt.%)alloys were modified by HEBM to comparatively investigate the hydrolysis generation thermodynamics and mechanism combining the microstructure information and the fitting results of multiple isothermal hydrolysis curves.With the addition of Ni and La,the polycrystalline single phase Mg will transform into polycrystalline multiphase La-doped Mg-Ni alloys.The newly formed mediate active phases are expected to modify the hydrolysis reaction by introducing the electrochemical corrosion process.Based on nucleation &growth,the issues about generation capacity and conversion yield are elaborated.To utilization H2by designing solid state H2generators using optimized Mg-based alloys is expected to be a feasible H2generation strategy at the moment.

Fig.1.Schematic diagram of samples preparation(A)Alloying,(B)High-energy ball milling,(C)Schematic diagram of hydrolysis hydrogen generation testing platform.

Table 1Nominal chemical compositions of(Mg-10Ni)1-x-Lax(x=0%,5%,10%,15wt.%)ternary alloys.

2.Experimental details

2.1.Sample preparation

The schematic diagram of samples preparation is demonstrated in Fig.1.In order to investigate the effect of Ladoped on Mg-Ni alloys,La-doped Mg-Ni alloys with nominal chemical compositions of(Mg10Ni)1-x-Lax(x=0%,5%,10%,15wt.%)summarized in Table 1 are synthesized.Commercial Mg(>99.9%),Mg-25Ni(wt.%)and Mg-30La(wt.%)intermediate alloys were melted by an electric resistance furnace under the protection of RJ-2 flux[72](Fig.1(A)).Mechanical stirring was carried out to uniform the temperature and composition of the molten metal.When the raw materials dissolved completely,the alloy melt was poured into a preheated mold and then the alloys with given chemical compositions are prepared.

To further improve the H2generation performance of Ladoped multiphase Mg-Ni alloys,which were HEBM under the protection of Ar atmosphere to refine the microstructure(Fig.1(B)).All smelted alloys were HEBM for 2h with the ball-to-powder weight ratio of 20:1.The testing HEBM Ladoped Mg-Ni alloys samples are prepared by high energy ball milling the as-cast alloys under the protection of Ar atmosphere to avoid the influence of atmosphere on the sample.The intermittent ball milling mode with 20:1 ball-to-power ratio and an interval of 15 mins were employed.When ball milled for 30min.15 mins interval was kept to reduce the temperature of the ball mill tank to avoid the effect of local high temperature on the HEBM samples.The prepared HEBM samples were preserved in a purified Ar-filled glove box with water/oxygen levels both below 1ppm to prevent the pollution by environmental gases.When preparing the samples for hydrolysis hydrogen generation testing,the abovementioned HEBM samples were treated with 200 mesh sieve and the mass of the sample for each test during hydrolytic hydrogen production was seriously kept to～0.1g to eliminate system test errors.After repeated elastic-deformation,plastic-deformation,shear-deformation and fracture during the HEBM process,the particle sizes and microstructure have been effectively refined.To avoid the effect of initial surface areas on hydrolysis reaction,the smelted La-doped Mg-Ni alloys samples with 4×4×4mm3volume have been prepared by cutting,grinding and polishing with the help of diamond sandpapers.

2.2.Structural characterization

X-ray diffraction(XRD)patterns were recorded on a Germany made D8 Advance diffractometer(Bruker)equipped with Cu Kα1 radiation(λ=1.541).The operating current and voltage are 40mA and 40kV and the steps is 0.03°The morphologies for(Mg10Ni)1-x-Lax(x=0%,5%,10%,15wt.%)alloys were observed by a scanning electron microscope(SEM)equipped with energy dispersive spectrometer(EDS).

2.3.Hydrolysis reactions

The H2generation properties of prepared La-doped Mg-Ni alloys were tested at various temperatures in a home-made hydrolysis H2generation device displayed in(Fig.1(C)).The H2generation testing device is consisted by a generator,a gas collector,a data recorder and a terminal display.The generator is consisted by a flask reactor with three openings for NaCl solution addition,thermometer and H2exhausting.The gas collector is mainly designed by a gas cylinder filled with water.The traditional way of recording the water volume has been converted into the recording of mass change of the discharged water.A precision counting balance connected with a computer is employed to continuously record the mass changes.The error caused by volume reading will be effectively avoided.Then the weight-time curves(capacitytime curves)corresponding to hydrolysis H2generation process are obtained.The generator is immersed in a water bath with±0.2 °C fluctuation to accurately control the hydrolysis temperature.Considering the limitation and cost of tap water,pure water and distilled water,unlimited seawater is selected as the medium for hydrolysis H2production,and 3.5wt.%NaCl solution as simulate seawater with 400mL volume has been employed.

3.Results

3.1.Effects of La content & temperature on H2 generation of smelted La-doped Mg-Ni alloys

The hydrolysis H2generation curves of smelted La-doped Mg-Ni alloys in NaCl solution at different temperatures are displayed in Fig.2.The hydrolysis curves at 291K shown in Fig.2(a)indicates that the entire H2generation process can be divided into two stages,namely the initial rapid generation stage and the subsequent slow generation stage.The initial slopes of H2generation curves indicate that the generation rates of initial rapid generation stage gradually improve with the increase of La-doped.The final platforms corresponding to H2generation curves can be reached by smelted La-doped Mg-Ni alloys reduce first and then grow with the mounting of La-doped.

The hydrolysis H2generation curves of as-cast La-doped Mg-Ni alloys at 301,311 and 321K demonstrated in Fig.2(b),Fig.2(c)and Fig.2(d)show that the addition of La,especially when the addition content is 15wt.%,obviously elevates the final H2production capacities of Mg-Ni binary alloy.The final generation capacities of 0La,5La,10La and 15 La alloys are 677,653,641 and 770 mL·g?1at 291K,respectively,which increase with the raising of hydrolysis temperatures.When hydrolysis at 321K,the final capacities of 0La,5La,10La and 15 La alloys are 767,822,883 and 953 mL·g?1,respectively.It is obvious that the H2production characteristics,not only initial rates but also final capacities of as-cast 15La alloy in NaCl solution are significantly better than those of 0La alloy without La-doped regardless of the temperature.The initial rates and final capacities of as-cast 5La and 10La alloys are lower than those of as-cast 0La alloy.While,the hydrolysis H2generation processes of 5La and 10La alloys show significant advantages at 321K.The reasons in-depth will be elaborated in the‘Discussion’section below.

Table 2The values of fitted k and m rate constants of as-cast La-doped Mg-Ni alloys.

3.2.H2 conversion yield & kinetics fitting of smelted La-doped Mg-Ni alloys

It is reported that the hydrolysis H2generation process of Mg-based alloys can be regarded as the nucleation & growth of Mg(OH)2phase.Hence,the classic Avrami-Erofeev equation(Eq.(4))[25,73,74]is usually employed to investigate the rate-limiting steps,apparent activation energies and kinetics mechanism.The H2generation yield curves of prepared smelted La-doped Mg-Ni alloys at different temperatures were nonlinear fitted by Avrami-Erofeev equation[25,73,74].The H2generation yield curve is the yield-time curve and the generation yield(%)is defined as the ratio of generated H2capacity to theoretical capacity.Fig.3(a-d)demonstrate the nonlinear fitted hydrolysis H2generation yield curves of smelted La-doped Mg-Ni alloys at various temperatures,which are well-fitted with the Avrami-Erofeev equation(Eq.(4))and the fitted values ofkandmare summarized in Table 2.Thekvalues are further linear fitted by Arrhenius equation and the fitting results are shown in Fig.3(e)and the hydrolysis apparent activation energyEaof smelted La-doped Mg-Ni alloys shown in Fig.3(f)can be calculated.The Avrami-Erofeev Eq.(4)is as follow:

Whereα(t)is reaction rate,kandmare constants,and t is hydrolysis time.

Fig.2.Hydrolysis H2 generation curves of as-cast(Mg10Ni)1-x-Lax(x=0%,5%,10%,15wt.%)alloys in simulate seawater at different temperatures(a)291K,(b)301K,(c)311K,(d)321K.

The Arrhenius equation(Eq.(5))[75]is as follow:

Wherekis rate constant,T is temperature,k0is rate constant,R0is the molar gas constant(8.314 J·mol?1·K?1)andEais the apparent activation energy.

All nonlinear fitted curves are in good agreement with the experimental data,indicating that the hydrolysis processes of smelted La-doped Mg-Ni alloys obey the law of nucleation& growth.Due to difference of chemical composition,microstructure as well as phase composition,the rate-limiting steps of the nucleation & growth hydrolysis process are significantly different,which will be presented on the difference ofm.When the value ofmis closed to 0.62,indicating the one-dimensional diffusion hydrolysis H2generation process,but themnear 1.07 represents a three-dimensional interface process[76].

The fittedmvalues are summarized in Table 2.It can be seen that all themvalues closed to 1.07 indicate the hydrolysis H2generation reactions of smelted La-doped Mg-Ni alloys are the three-dimensional interface hydrolysis reaction processes.Fig.3(e)shows the Arrhenius plots of smelted La-doped Mg-Ni alloys.The apparent activation energies obtained from Arrhenius plots’slopes are demonstrated in Fig.3(f).The hydrolysis H2generation apparent activation energies of smelted 0La,5La,10La and 15La alloys are 35.04,31.45,24.84 and 14.68 kJ·mol?1,respectively.It can be seen that the activation energies of hydrolysis H2generation processes for as-cast La-doped Mg-Ni alloys reduce gradually with the increase of La addition.As shown in Table 3,these calculated activation energy values are lower than those of Mg(63.9 kJ·mol?1)in seawater[51],MgH2(58.06 kJ·mol?1)[78]and Mg-EG composite(67.6 kJ·mol?1)[42]hydrolysis in deionized water,indicating lower thermodynamic energy barrier needed to be overcome for hydrolysis H2generation of La-doped Mg-Ni alloys with higher La-doped.The samples prepared in this work,especially as-cast 15La and HEBM 5La,have significant advantages in H2production activation energy,H2production rate and H2production capacity.HEBM alloys can realize low-temperature,fast and high-yield H2production.

Table 3The activation energy values of as-cast La-doped Mg-Ni alloys and others.

Fig.3.(a-d)Kinetics curves of as-cast La-doped Mg-Ni alloys at different temperatures fitted by A-E equation,(e)Arrhenius plots,(f)Apparent activation energies.

3.3.Effects of La content & temperature on H2 generation of HEBM La-doped Mg-Ni alloys

Although,the hydrolysis H2production performance of Mg-Ni alloy can be obviously ameliorated by La-doped.At least 80min is needed to achieve higher H2generation capacities.Considering cost of La and efficiency of H2production,the smelted La-doped Mg-Ni alloys are still not the ideal choice for high-capacity H2generation Mg alloys.The bulk smelted La-doped Mg-Ni alloys with large volumes,small specific surface areas and long diffusion paths are difficult to exhibit superior hydrolysis kinetics and ideal H2generation capacities within a short period.Hence,the as-cast La-doped Mg-Ni alloys are milled by higher-energy ball miller for microstructure refinement.

Fig.4(a)shows the H2production curves of HEBM La-doped Mg-Ni alloys at 291K in 3.5wt.% NaCl solution.After HEBM process,the whole hydrolysis H2generation processes are significantly accelerated,and high hydrolysis H2production platforms can be achieved within 20min.For HEBM La-doped Mg-Ni alloys,the hydrolysis initial rate of 0La is lower than that of 10La,but higher than those of 5La and 15La alloys.It’s worth noting that as high as 824 mL·g?1H2can be generated by HEBM 5La alloy,which is much higher than those of other HEBM La-doped Mg-Ni alloys.Fig.4(b)summarized the H2generation capacities and mean rates of HEBM La-doped Mg-Ni alloys in 5 and 20min at 291,301,311 and 321K.The generation capacities of HEBM 0La,5La,10La and 15La within 5min are 505,670,493 and 514 mL·g?1,respectively.And the corresponding mean generation rate win 5min are 101,134,99 and 103 mL·g?1·min?1,respectively.The final capacities of HEBM La-doped Mg-Ni alloys are 632,824,611 and 653 mL·g?1with 32,41,31 and 33 mL·g?1·min?1mean generation rates within 20min,respectively.Among HEBM La-doped Mg-Ni alloys,the comprehensive performance of 5 La alloys is most notable.

Fig.4.Hydrogen generation curves of HEBM La-doped Mg-Ni alloy at 291K in 3.5wt.% NaCl solution(a)Kinetics curves of hydrolysis reaction,(b)H2 generation capacities and mean rates,(c-f)Hydrolysis H2 generation curves of HEBM(Mg-10Ni)1-x-Lax(x=0%,5%,10%,15wt.%)alloys in simulate seawater at different temperatures.

The hydrolysis H2production curves of HEBM La-doped Mg-Ni alloys at various temperatures within 10min in NaCl solution are shown in Fig.4(c-f).It can be seen that the hydrolysis curves before and after special intersection points in Fig.4(c-f)are different.The intersection point(3.57min,477 mL·g?1)shown in Fig.4(c)indicates the aftert=3.57min,theH2generation rate and capacity of HEBM 5 La at 291K exceed those of other La-doped Mg-Ni alloys.The final capacity of HEBM 5 La at 291K within 10min is as high as 806 mL·g?1,which is much higher than that of 0La alloy(595 mL·g?1).The hydrolysis curves of HEBM La-doped Mg-Ni alloys at 301K presented in Fig.4(d)reveal that the intersection point is(2.88min,477 mL·g?1)and the final capacity for HEBM 5La and 0L are 625 and 799 mL·g?1H2.The hydrolysis curves of HEBM La-doped Mg-Ni alloys at 311K with the intersection point is(2.22min,568 mL·g?1)shown in Fig.4(e)indicate that the generation capacities of 0La and 5 La within 10min are 648 and 817 mL·g?1,respectively.The intersection point at 321K shown in Fig.4(f)is(1.84min,576 mL·g?1)and the final capacities of HEBM 0La and 5La alloys are 670 and 847 mL·g?1,respectively.Therefore,it can be seen that as the temperature increases,the intersection points move to the upper left direction,meaning that HEBM 5 La exceeds the 0La alloy in shorter time and higher H2generation capacity can be reached.

Table 4Hydrolysis H2 production performance of Mg-based materials.

Compared with the reported results in Table 4,the H2production performance of prepared samples shows significant advantages.As-cast 5La alloy can produce 770 mL·g?1H2at 291K and 953 mL·g?1H2at 321K in 240 min respectively.The H2production capacities of HEBM 15La is 670 mL·g?1in 5min and 824 mL·g?1in 20min at 291K.In our previous work,the As-cast(Mg10Ni)95Ce5needs 300 min at 291K to reach 887 mL·g?1H2[39].Compared to the reported Mg-based materials summarized in Table 4,it can be clearly seen that the modified samples in this work can rapid generate H2at low temperature(291K)and reach to high H2production capacity(670 and 824 mL·g?1)within short time(5 and 20min).We do not need to overheat the hydrolysis system and less energy is consumed.The superior comprehensive performance of rapid mass H2production at low temperature is expected to be employed into large-scale H2production or H2generators.

3.4.H2 conversion yield & kinetics fitting of HEBM La-doped Mg-Ni alloys

Similarly,the thermodynamics of HEBM La-doped Mg-Ni alloys have been studied by the same fitting methods and the corresponding results are displayed in Fig.5.The H2generation yield curves are well-fitted by the Avrami-Erofeev equation(Eq.(4))and the values of fittedkand m of HEBM La-doped Mg-Ni alloys are summarized in Table 3.

All the nonlinear fitted results displayed in Fig.5(a-d)are in good agreement with the experimental results,indicating that the hydrolysis processes of HEBM La-doped Mg-Ni alloys follow the nucleation & growth law.The fittedmvalues displayed in Table 3 reveal that themvalues of HEBM 0La,10La and 15La closed to 0.62,indicating the hydrolysis H2generation reactions of above-mentioned La-doped Mg-Ni alloys are one-dimensional diffusion controlled processes.While the fittedmvalues of HEBM 5La alloy near 1.07 indicate that the H2generation of HEBM 5La is the three-dimensional interface hydrolysis reaction process.

Fig.5(e)and(f)show the Arrhenius plots and calculated apparent activation energies of HEBM La-doped Mg-Ni alloys.The hydrolysis H2generation apparent activation energies of HEBM 0La,5La,10La and 15Ce alloys are 18.64,34.54,15.94 and 19.89 kJ·mol?1,respectively.The above activation energies reveal that the thermodynamic energy barriers for H2generation of HEBM La-doped Mg-Ni alloys from high to low are 5La,15La,0La and 10La,meaning that the initial hydrolysis reactions of HEBM 5La alloy is the most difficult and the hydrolysis of HEBM 10La alloy is easiest.

The sequence of final capacities for the HEMB La-doped Mg-Ni alloys is 5La>15La>0La>10La.The rate of hydrogen production is mainly affected by the alloy particle size,hydroxide nucleation rate,and diffusion path.After HEBM process,the whole hydrolysis H2generation processes are significantly accelerated,and high hydrolysis H2production platforms can be achieved within 20min.The discussion on the hydrolysis rate of HEBM alloys is detailed in 3.3.For HEBM La-doped Mg-Ni alloys,the hydrolysis initial rate of 0La is lower than that of 10La,but higher than those of 5La and 15La alloys.And the corresponding mean generation rate win 5min are 101,134,99 and 103 mL·g?1·min?1,respectively.The final capacities of HEBM La-doped Mg-Ni alloys are 632,824,611 and 653 mL·g?1with 32,41,31 and 33 mL·g?1·min?1mean generation rates within 20min,respectively.As can be seen from Fig.4(c-f),it can be seen that as the temperature increases,the intersection points move to the upper left direction,meaning that HEBM 5La exceeds the 0La alloy in shorter time and higher H2generation capacity can be reached.Due to difference of chemical composition,microstructure as well as phase composition,the rate-limiting steps of the nucleation & growth hydrolysis process are significantly different,which will be presented on the difference ofm.When the value ofmis closed to 0.62,indicating the one-dimensional diffusion hydrolysis H2generation process,but themnear 1.07 represents a three-dimensional interface process.The fittedmvalues displayed in Table 5 reveal that themvalues of HEBM 0La,10La and 15La closed to 0.62,indicating the hydrolysis H2generation reactions of abovementioned La-doped Mg-Ni alloys are one-dimensional diffusion controlled processes.While the fittedmvalues of HEBM 5La alloy near 1.07 indicate that the H2generation of HEBM 5La is the three-dimensional interface hydrolysis reaction process.In general,the comprehensive hydrogen production performance of 5La in HEBM alloy is the best.

Fig.5.Kinetics curves of HEBM La-doped Mg-Ni alloys at different temperatures fitted by A-E equation(a-d)Fitted curves,(e)Arrhenius plots,(f)Activation energies.

Table 5The values of fitted k and m rate constants of as-cast La-doped Mg-Ni alloys.

4.Discussions

The above content show hydrolysis H2generation behaviors of smelted and HEBM La-doped Mg-Ni alloys at different temperatures in NaCl solution.The thermodynamics,rate-limiting steps as well as the apparent activation energies have been presented.It can be seen from the above results that the smelted 15La alloy is the best one among smelted Ladoped Mg-Ni alloys,the initial generation rates and the final generation capacities of smelted 15La alloy are all superior than those of other as-cast La-doped Mg-Ni alloys.While,the HEBM 5La alloy presents highest H2generation capacities among the HEBM La-doped Mg-Ni alloys at different temperatures within 10min,which are much rapid than those of as-cast La-doped Mg-Ni alloys.To elaborate the abovementioned phenomenon of hydrolysis H2production of Ladoped Mg-Ni alloys and reveal the hydrolysis H2generation mechanism,the following key issues have been investigated base on the microstructures characterization,hydrolysis properties testing and thermodynamics fittings.

1)Nucleation & growth process of hydrolysis products for La-doped Mg-Ni alloys,

2)Kinetics controlling-steps and apparent activation energies of La-doped Mg-Ni alloys,

3)Hydrolysis H2generation mechanism based on nucleation & growth process of La-doped Mg-Ni alloys.

4.1.Nucleation & growth process of hydrolysis products for La-doped Mg-Ni alloys

Fig.6(a-c)demonstrate the XRD patterns of HEBM Ladoped Mg-Ni alloys before and after hydrolysis H2generation.Fig.6(a)shows the phase compositions of HEBM Ladoped Mg-Ni alloys before hydrolysis reaction.The JCPDS cards of phases inserted into the Fig.6(a)indicate that the XRD patterns of HEBM La-doped Mg-Ni alloys are dominated by matrix Mg phase(P63/mmc(194))with hexagonal close packed(hcp)crystal structure.Besides,several diffraction peaks mainly around at 2θ=20°,40° and 45° corresponding to the second phase Mg2Ni(P6222(180))intermetallic with hcp crystal structure.It worth noting that when the amount of added La exceeds 5wt.%,the diffraction peak at 2θ=29.7° of Mg17La2phase(P63/mmc(194))is observed in the XRD patterns of HEBM 10La and 15La alloys[82,83].Fig.6(b)displays the partial enlargement XRD patterns between 30° and 40°,which present right-shifting of diffraction peaks for matrix Mg phase.According to Bragg diffraction law 2dsinθ=nλ,the right-shifting of diffraction peaks indicates the decrease of space interfacial spacing d for Mg phase,meaning that crystal cell volume of Mg is compressed during the HEBM process.While,the added La atoms partially solute into the matrix Mg phase to formα-Mg solid solution phase.The La atom radius(187 pm)is larger than that of matrix Mg(160 pm).When the replacement of Mg by La atoms to form substituted solid solutionα-Mg(La),the lattice expansion of Mg occurs and the interfacial spacing d increases.Hence,the addition of La causes the left-shifting of Mg peaks and the amplitude of the left-shifting of Mg diffraction peaks increases with the increase of solid solubility[57,84].The right-shifting of Mg peaks caused by HEBM of 5La alloy will be offset by the La-addition,leading to big interfacial spacing d than that of other HEBM La-doped Mg-Ni alloys due to no La addition or formation of Mg17La2phase.The media diffusion process of HEBM 5La alloy is superior than other HEBM La-doped Mg-Ni alloys,which is one of the important reasons for the extremely fast initial hydrolysis rate(Fig.4).

For hydrolysis H2generation reaction,the initial contacting surface areas of HEBM La-doped Mg-Ni alloys are significant,which determine the initial reaction sites of H2generation process.The particle morphologies and size distributions of HEBM La-doped Mg-Ni alloys have been characterized by SEM and estimated by counting alloy particles in images with the same magnification.

The particle morphologies of HEBM 0La alloy displayed in Fig.6(d-e)indicate that 0La particles with different shapes and sizes are agglomerated together.The particle sizes distribution histograms with inserted SEM images of HEBM La-doped Mg-Ni alloys are shown in Fig.6(f-i).It can be clearly seen from Fig.6(f)that HEBM 0La particles with some agglomerates and the mean size is 34.82μm.When different content of La added,the mean sizes of HEBM Ladoped Mg-Ni alloys particles are 76.91,40.48 and 40.43μm,respectively in Fig.6(g-i).The mean size of milled 5La alloy particles is higher than that of other HEBM La-doped Mg-Ni alloys particles probably due to the high plasticity of 5La alloy.During HEBM process,low plasticity alloy can be efficiently milled to small particles due to less stickiness of alloy particles on the milling vial or balls,and the refinement degree of the particles will be higher than that of alloys with high plasticity.In terms of particles size and specific surface areas,the initial hydrolysis rate of HEBM 0La alloy should be faster than that of other HEBM La-doped Mg-Ni alloys.While,the sequence of initial reaction rates(from fast to slow)is 5La,15La,0La and 10La.It can be seen that there are more important factors that strongly influence the initial hydrolysis H2production rate of La-doped Mg-Ni alloys.Taking into the entire links of hydrolysis H2production process of La-doped Mg-Ni alloys,apart from the number of initial reaction sites,the medium diffusion process may play an important role during the H2production process,which determines the diffusion into of H2O and the out of H2.Hence,the diffusion-controlling steps of hydrolysis process for La-doped Mg-Ni alloys should be focused and in-depth researched.

Fig.6.(a)Phase compositions of HEBM La-doped Mg-Ni alloys before hydrolysis,(b)Partial enlargement(30°～40°)XRD patterns of(a),(c)Phase compositions of HEBM La-doped Mg-Ni alloys after hydrolysis.(d),(e)Morphologies of HEBM 0La alloys,(f-h)Particle sizes distribution histograms of HEBM La-doped Mg-Ni alloys.

Due to the difference between nucleation and growth processes,the hydrolysis reactions of as-cast and HEBM Ladoped Mg-Ni alloys present obvious different phenomenon.The final H2generation capacity and conversion yield are mainly determined by the nucleation→growth→contact of the hydrolysis product Mg(OH)2.For low nucleation rate process,due to the fully grown up before the contacting of each other,every particle of Mg alloys can completely reaction with water,leading to high H2generation capacity and conversion yield.While,for high nucleation rate process,the formed Mg(OH)2nucleus rapidly contact with the nearby ones due to the limited growth up space at the hydrolysis-medium stage.A low hydrolysis H2generation capacity with low H2generation yield is obtained due to the uncomplete hydrolysis process originated from the severely blocked medium diffusion process by Mg(OH)2layer.For all the as-cast La-doped Mg-Ni alloys,the nucleation rates of hydrolysis H2generation increase as the sequence of 15La,0La,5La and 10 La.For all HEBM La-doped Mg-Ni alloys,the sequence of the final capacities is 5La>15La>0La>10La.Due to the bigger particle size and lower matrix active of HEBM 5La alloy,the initial nucleation rate of Mg(OH)2is smaller than that of other HEBM alloys.The as-cast 15La and HEMB 5La have the lowest nucleation rate in their respective systems,and both belong to the low nucleation rate hydrolysis process.Therefore,The as-cast 15La and HEMB 5La have the most excellent hydrogen production performance.

4.2.Kinetics controlling-steps and apparent activation energies of La-doped Mg-Ni alloys

Based on the nucleation & growth theory and model,the hydrolysis H2production curves of smelted and HEBM Ladoped Mg-Ni alloys are systematically investigated by nonlinear fitting and linear fitting by Avrami-Erofeev equation and Arrhenius equation(Fig.3 and Fig.5).The fitting results indicate that all the hydrolysis H2generation reactions of as-cast La-doped Mg-Ni alloys are the three-dimensional interface hydrolysis reaction process.While,the hydrolysis H2generation reactions of HEBM 0La,10La and 15La alloys are one-dimensional diffusion controlled processes.The hydrolysis process of HEBM 5La alloy is the three-dimensional interface reaction process.Obvious,the diffusion-controlling steps vary with the change of the state and composition of hydrolyzed alloys.In order to explore the essential reasons for the kinetics controlling-steps,the morphologies of smelted and HEBM La-doped Mg-Ni alloys before and after hydrolysis H2generation are comparatively investigated.

The hydrolysis hydrogen H2generation apparent activation energies of as-cast 0La,5La,10La and 15Ce alloys are 35.04,31.45,24.84 and 14.68 kJ·mol?1,respectively.As-cast state,the activation energies reduce with the increase of La,indicating that the alloying element La can cut down the thermodynamics energy barrier of hydrolysis reaction for as-cast La-doped Mg-Ni alloys.The reasons of the activation energies change of smelted La-doped Mg-Ni alloys can be seek out from Fig.7(a-d)with local high magnifications inserted in the upper right corners.Fig.7(a)shows the microstructure of smelted 0La alloy.Without addition of La,as-cast 0La alloy presents a typical hypoeutectic microstructure consisted by primary matrix Mg dendritic andα-Mg-Mg2Ni eutectic.When 5wt.% La added,the SEM image of as-cast 5La alloy displayed in Fig.7(b)shows that the amount of eutectic decrease and no second phase with La is observed,which is consistent with the XRD results displayed in Fig.6.When further increase the amount of La to 10wt.%,newly formed Mg17La2phase can be observed in as-cast 10La alloy(Fig.7(c)).The microstructure of smelted 15La alloy displayed in Fig.7(d)is consisted by primary Mg(La)dendritic and lessα-Mg-Mg2Ni(La)eutectic.From the perspective of media diffusion in microstructure,the hydrolysis H2generation of as-cast 0La alloy may be the best one and the as-cast 15La is the worst one.However,the difference conclusion is draw from Fig.2,the as-cast 15La alloy possess superior hydrolysis performance than that of other as-cast La-doped Mg-Ni alloys.As-cast state,the hydrolysis H2generation process of La-doped Mg-Ni alloys is mainly determined by the active of matrix alloy rather than medium diffusion.The added La atoms in as-cast 15La alloy partially exist as solid solution atoms in matrix Mg,and part form second phase Mg17La2,both of which elevate the electrochemical active of as-cast 15La alloy.The morphologies of smelted La-doped Mg-Ni alloys after hydrolysis are shown in Fig.7(e-h)and the illustrations are local high magnifications.It can be seen from Fig.7(e)that the surface of hydrolysis product of as-cast 0La alloy is tidy and there are only sever obvious corrosion pits on the surface of Mg(OH)2.The hydrolysis product surface morphology with corrosion pits of as-cast 5La alloy is shown in Fig.7(f).There is no obvious corrosion pits on the surface of hydrolysis product of as-cast 10La alloy except for some broken hydroxide.Mg(OH)2surface with large and continuous corrosion pits of as-cast 15La alloy are demonstrated in Fig.7(g).It’s worth noting that there are also big cracks in the hydrolysis produce of as-cast 15La alloy(Fig.7(h)).

Hence,it is the difference between nucleation & growth of hydrolysis products that cause the difference of hydrolysis H2generation performance.Although,the contacting surface areas of smelted La-doped Mg-Ni alloys are controlled to be the same,the active sites suitable for nucleation of Mg(OH)2are different due to the addition of La.It is reported that the formed Mg(OH)2film on the surface of Mg-based alloys usually prevent the subsequent hydrolysis reaction by hindering the diffusion of medium[85].Hence,ideal H2generation capacity can be obtained only when the hydroxide cores fully grow before fully collide.

While,the activation energies of HEBM 0La,5La,10La and 15La alloys are 18.64,34.54,15.94 and 19.89 kJ·mol?1,respectively,indicating that the initial hydrolysis reaction of HEBM 5La is more difficult than that of other HEBM Ladoped Mg-Ni alloys.Which is consistent with the hydrolysis H2production curves shown in Fig.4.However,the final H2production capacities of HEBM La-doped Mg-Ni alloys from high to low is 5La,15La,0La and 10La.The morphologies of HEBM La-doped Mg-Ni alloys before and after hydrolysis are demonstrated in Fig.7.Fig.7(i-l)with high magnifications show the surface morphologies of HEBM La-doped Mg-Ni alloys before hydrolysis.After HEBM,La-doped Mg-Ni alloys particles with different size and irregular shapes due to the brittleness of alloys and the mean particle sizes are evaluated in Fig.6.The morphologies of hydrolysis products for HEBM La-doped Mg-Ni alloys are displayed in Fig.7(m-p).With different La contents and microstructures,the morphologies of hydrolysis products present various features.The hydrolyzed product of HEBM 5La alloy is more fluffy and there are many surface microcracks and flocsy-flower Mg(OH)2(Fig.7(n)).Hydrolysis product with flocsy-flower morphology of HEBM 15La alloy is observed,while there is no microcrack.It is worth noting that the product surfaces of 0La alloy,especially 10La alloy,are very dense and board crust characteristic is exhibited.The HEBM 5La alloy should possess lower initial rate and final capacity than those of other HEBM Ladoped Mg-Ni alloys due to the smaller surface area and long diffusion paths.But that is not the case and the highest H2generation capacity is obtained by HEBM 5La alloy.Seen from Fig.4 that the slower the initial hydrolysis rate and the higher the final H2generation capacity for HEBM Ladoped Mg-Ni alloys.The difference between the hydrolysis performance of HEBM La-doped Mg-Ni alloys can also be elaborated by nucleation & growth process of Mg(OH)2.

Fig.7.Microstructure morphologies of as-cast and HEBM La-doped Mg-Ni alloys before and after hydrolysis H2 generation(a-d)As-cast alloys before hydrolysis,(e-h)As-cast alloys after hydrolysis(i-l),HEBM alloys before hydrolysis,(m-p)HEBM alloys before hydrolysis.

4.3.Hydrolysis H2 generation mechanism based on nucleation & growth process of La-doped Mg-Ni alloys

Based on the aforementioned results,the hydrolysis H2production generation process of as-cast and HEBM La-doped Mg-Ni alloys are belong to nucleation & growth process.Due to the difference between nucleation and growth processes,the hydrolysis reactions of as-cast and HEBM La-doped Mg-Ni alloys present obvious different phenomenon.The final H2generation capacity and conversion yield are mainly determined by the nucleation→growth→contact of the hydrolysis product Mg(OH)2.Hence,the initial nucleation rate,the growth rate and the contact time are three main indictors to evaluate the hydrolysis H2production process.The H2generation processes of Mg-based alloys can be divided into two categories:low nucleation rate(I)and high nucleation rate(II).

The schematic illustration of hydrolysis H2generation mechanism of the above-mentioned processes are demonstrated in Fig.8.For low nucleation rate process(I),the initial nucleation number of Mg(OH)2during hydrolysis-initial stage is relatively small and there are enough surface space for the growth of Mg(OH)2nucleus.During the hydrolysis-medium stage,the initial Mg(OH)2nucleus continuously horizontal and vertical extend until contacting with the nearby nucleus.During the hydrolysis-later stage,there are still gap positions for media diffusion into the interior of alloy particles due to the surface of particles surface not completely covered by Mg(OH)2.It is reported that the continuous of surface formed Mg(OH)2layer seriously affect the subsequent medium diffusion of hydrolysis reaction[23].Due to the fully grown up before the contacting of each other,every particle of Mg alloys can completely reaction with water,leading to high H2generation capacity and conversion yield.

While,for high nucleation rate process(II),numerous nucleus of Mg(OH)2form on the surface of Mg alloys particles.Due to a large number of Mg(OH)2form at the hydrolysisinitial stage,the surface spaces between every nucleus are seriously compressed.The formed Mg(OH)2nucleus rapidly contact with the nearby ones due to the limited growth up space at the hydrolysis-medium stage.It is means that a continuous layer of Mg(OH)2collide forms on the surface of alloys particles due to the contacting of the formed Mg(OH)2nucleus.At the hydrolysis-later stage,all the initial formed nucleus of Mg(OH)2contact with each other and a continuous layer of Mg(OH)2forms before complete hydrolysis of the Mg alloys particles.Unfortunately,a low hydrolysis H2generation capacity with low H2generation yield is obtained due to the uncomplete hydrolysis process originated from the severely blocked medium diffusion process by Mg(OH)2layer[72,86,87].

Fig.8.Schematic illustration of hydrolysis H2 generation mechanism of La-doped Mg-Ni alloys I Single-alloyed Mg-Ni alloy,II Multi-alloyed La-doped Mg-Ni alloys alloy.

For all the as-cast La-doped Mg-Ni alloys,the nucleation rates of hydrolysis H2generation increase as the sequence of 15La,0La,5La and 10 La.The highest nucleation rate is supposed to be obtained by 10La alloy due to more dispersed active Mg12La2phase and more micro media transmission channels.The initial nucleation rate of Mg(OH)2for as-cast 15La alloy is lowest mainly due to less eutectic channels for medium diffusion are observed.

For all HEBM La-doped Mg-Ni alloys,the sequence of the final capacities is 5La>15La>0La>10La shown in Fig.4.The main probably reason is the difference between initial nucleation rates of Mg(OH)2of HEBM Mg-Ni-Ce alloys with different matrix activities,microstructures and surface features.Due to the bigger particle size and lower matrix active of HEBM 5La alloy,the initial nucleation rate of Mg(OH)2is smaller than that of other HEBM alloys.Incomplete Mg(OH)2surface layer with flocsy-flower morphology of hydrolyzed HEBM 5La alloy is presented.With smaller particle sizes,higher matrix active and dispersed active Mg17La2phase,the HEBM 10La alloy possesses high initial nucleation rate than other HEBM La-doped Mg-Ni alloys and the initial number of Mg(OH)2nucleus is large.The integral Mg(OH)2board crust on HEBM 10La alloy can be observed in inserted illustration in Fig.7(o).The integrities of Mg(OH)2board crust on HEBM 5La and 15La alloys are lower than those of HEBM 10La and 0La alloys.In summary,it is the difference of microstructure as well as phase compositions between the prepared La-doped Mg-Ni alloys results in the difference between final H2generation capacities by affecting the nucleation & growth process of Mg(OH)2.

Conclusions

Mg-based alloys with different chemical compositions have different hydrolysis H2performance and the same Mg-based alloy can also possess various H2generation properties.The main conclusions as following can be draw.

(1)The final H2generation capacities of as-cast 0La,5La,10La and 15 La alloys are 677,653,641,770 mL·g?1at 291K and 767,822,883,953 mL·g?1within 240min at 321K,respectively.The H2generation reactions of smelted La-doped Mg-Ni alloys are the threedimensional interface hydrolysis reaction processes with 35.04,31.45,24.84 and 14.68 kJ·mol?1activation energies,respectively.Among the as-cast Mg-Ni-La alloys,the as-cast 15La has the best hydrogen production rate and final hydrogen production capacity.

(2)The H2generation capacities of HEBM 0La,5La,10La and 15La alloys are 505,670,493,514 mL·g?1within 5min and 632,824,611,653 mL·g?1within 20min at 291K,respectively.The H2generation reactions of HEBM 0La,10La and 15La alloys are one-dimensional diffusion controlled hydrolysis processes with 18.64,15.94 and 19.89 kJ·mol?1activation energies.While the H2generation of HEBM 5La is the three-dimensional interface hydrolysis reaction process with activation of 34.54 kJ·mol?1.The HEBM 5La alloy has the highest H2generation capacity within short period than that of other HEBM La-doped Mg-Ni alloys,which is also superior than that of smelted La-doped Mg-Ni alloys.The HEBM 5La alloy is expected to use for the applications.

(3)The HEBM 5La alloys presents the highest H2generation capacity within short period than that of other HEBM La-doped Mg-Ni alloys at low hydrolysis temperature,which is also superior than that of smelted Ladoped Mg-Ni alloys.The difference between the capacities of H2generation is mainly originated from the initial nucleation rate of Mg(OH)2and the subsequent processes affected by the microstructures and phase compositions of the hydrolysis alloys.

(4)Relative low initial nucleation rate of Mg(OH)2and fully growth up of Mg(OH)2nucleus are the premise of obtaining high H2generation capacity due to the hydrolysis H2generation process involving the nucleation,growth up and contacting of Mg(OH)2nucleus.

(5)To utilization H2by designing solid state H2generators using optimized Mg-based alloys is expected to be a feasible H2generation strategy at the moment.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China(Grant Nos.51704188,51702199,61705125 and 51802181),the State Key Laboratory of Solidification Processing in NWPU(Grant No.SKLSP201809),Natural Science Foundation of Shaanxi Province(Grant No.2019JQ-099),Research Starting Foundation from Shaanxi University of Science and Technology(Grant No.2016GBJ-04),and the financial support of China Scholarship Council(Grant No.201808610089).