A direct electrochemical route from oxides to TiMn2 hydrogen storage alloy☆

Jing Zhu ,Lei Dai*,Yao Yu Jilin Cao ,Ling Wang *

1 School of Chemical Engineering and Technology,Hebei University of Technology,Tianjin 300130,China

2 College of Chemical Engineering,Hebei United University,Tangshan 063009,China

Keywords:Electro-deoxidation TiMn2 alloy CaCl2 melt Oxides Hydrogen storage

ABSTRACT This study is for investigating the direct electro-deoxidation of mixed TiO2-MnO2 powder to prepare TiMn2 alloy in molten calciumchloride.The influences of process parameters,such as sintering temperature,cell voltage,and electrolysis time,on the electrolysis process were examined to investigate the mechanism of alloy formation.The composition and morphology of the products were analyzed by XRD and SEM,respectively.The electrochemical property of TiMn2 alloy was investigated by cyclic voltammetry measurements.The results show that pure TiMn2 can be prepared by direct electrochemical reduction of mixed TiO2/MnO2 pellets at a voltage of 3.1 V in molten calcium chloride of 900°C for 7 h.The electro-deoxidation proceeds from the reduction of manganese oxides to Mn,which is reduced by TiO2 or CaTiO3 to form TiMn2 alloy.The cyclic voltammetry measurements using powder microelectrode show that the prepared TiMn2 alloy has good electrochemical hydrogen storage property.©2015 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

1.Introduction

AB2type intermetallic compounds can absorb and release hydrogen gasin large quantities.The reversible absorption and desorption in combination with large storage capacity makes these compounds very attractive as negative electrodes in rechargeable batteries[1,2].Among these materials,TiMn2based alloys have been studied extensively,because of their high hydrogen absorbing capacity of more than 1.0 H/M with a high reaction rate and their light weight[3-5].

In recent years,most researches about TiMn2based alloys have been focused on the improvement of their hydrogen absorption-desorption performance by element substitution[6-8],structural change[9,10]and multi-component[11].Mechanical alloying technique is still the dominant method for preparing TiMn2based alloys,consisting of melting,alloying,casting and powdering steps[12,13].Such processes are high energy consumption but low production efficiency,resulting in relatively high costs of the hydrogen storage materials.With continuous large scale use of hydrogen storage alloys,it is significant to develop new preparation methods with low energy consumption and simple operation.

Direct electro-deoxidation method provides a potential alternative[14].In this method,metals are extracted directly from their oxides as the cathodes in a molten salt electrolytic cell[15-17].This process not only offers economic production of numerous important metals and well-known alloys,but also provides a method for developing novel alloy compositions that cannot be produced by conventional metallurgical methods[18-22].

In this study,electrolytic reduction of TiO2and MnO2mixture into hydrogen storage alloy TiMn2is investigated.The products under different conditions are analyzed by using X-ray diffraction(XRD)and a scanning electron microscope(SEM).The mechanism of reduction reaction is discussed.

2.Experimental

Commercially available powder of titanium oxide,TiO2(≥99.99%),and manganese oxide,MnO2(≥99.99%),were used as raw materials for the cathode.The reagent grade oxide powder was mixed at given stoichiometric ratios(Ti/Mn=1:2)and ball-milled with anhydrous alcohol for 3 h.About2.0 g of the mixture was pressed to a cylindrical pellet(10 mm in diameter and 3 mm in thickness)under pressure load of 25 MPa.The pellet was then sintered in air at 900,1050 and 1250°C for 5 h separately.

Sintered samples were attached to Kanthal wires to form cathodes.The electrolyte used was dehydrated CaCl2, filled in an alumina crucible and introduced into a stainless steel tube reactor in a vertical furnace.Argon gas flowed through the reactor continuously.After the temperature was raised to the pre-set value,a pre-electrolysis was conducted under a constant voltage of 2.5 V for 0.5 h to remove the moisture and impurities.The electro-deoxidation experiments were conducted at a constant voltage with high purity graphite as anode electrode.The current passing the cell was collected by a PC computer aid system.After the electrolysis was finished,the cathode pellet was cooled to room temperature in the furnace,taken out of crucible,and then washed thoroughly in flowing water.The composition and morphology of the products were analyzed by XRD and SEM,respectively.

A Mo sheet(width 10 mm,thickness 0.5 mm,length 20 mm,purity 99.9%)with one circular hole(1.0 mm diameter)through the foil was used to make the metallic cavity electrodes(MCEs).The oxide powder was manually filled into the MCE cavity by repeatedly finger-pressing and used as the working electrode,on which cyclic voltammetry(CV)was carried out in CaCl2meltat900°C.High purity graphite and Kanthal wire were used as the counter electrode and pseudo-reference electrode,respectively.All electrochemical experiments were operated under the protection of an argon flow.The CVs were recorded by ZAHNER IM6e electrochemical station.

The electrochemical hydrogen storage property of TiMn2alloy was investigated by CV experiments in a classical three-electrode cell.The working electrode is a powder microelectrode made with a 50μmdiameter Pt wire and glass tube,and the depth of the cavity is about 40 μm.Tin is used to connect the Cu lead with Pt wire for good electronic conduction.The tail of the electrode is sealed with epoxy resin in order to fix the Cu lead.The TiMn2alloy powder was filled in the cavity of the powder microelectrode used as working electrode.A Pt plate and Hg/HgO electrode were used as the counter electrode and reference electrode,respectively.

3.Results and Discussion

3.1.The influence of sintering temperature

After the electro-deoxidation process,the oxide pellets are usually sintered to increase the sample strength and avoid powdering of pellets in molten salt.The mixed TiO2/MnO2pellets with Ti/Mn molar ratio of 1:2 sintered at 900,1050 and 1250°C are analyzed using XRD and the results are shown in Fig.1.After being sintered at900°C,TiO2in the pellet is stable,while MnO2disappears and completely converts to Mn2O3due to the negative Gibbs free energy(ΔG0)values of reaction(1).It should be noted that the formation of MnTiO3follows reaction(2)with the sintering temperature of 1050°C,corresponding to the decrease of peak intensities of Mn2O3and TiO2.With sintered temperature of 1250°C,TiO2disappears and part of Mn2O3is further reduced to Mn3O4following reaction(3).

with ΔG0=-40.646 kJ·mol-1at 900 °C.

Fig.1.XRD spectra of the mixed oxide pellets sintered at various temperatures.

with ΔG0=-30.37 kJ·mol-1at 1050 °C and ΔG0=-51.54 kJ·mol-1at 1250°C.

with ΔG0=-5.065 k J·mol-1at 1050 °C and ΔG0=-13.436 kJ·mol-1at 1250 °C.

Pellet porosity and particle size play important roles in the electrodeoxidation process.Fig.2 shows the SEM images of the mixed oxide pellets sintered at different temperatures.The mixed powder sintering at 900°C is mainly cobble-like particles with porous structure in submicrometers,which is beneficial for the diffusion of CaCl2meltto the interior of oxide pellet to promote the electro-deoxidation process.At the sintering temperature of 1050°C,particles are bigger,which means a longer distance for oxygen ion transportation from particle interior to surface and hence a lower electro-deoxidation rate[15].At sintering temperature of 1250°C,the oxide pellet is in quite compact structure,which will hinder the expansion of the three-phase interlines(metal/oxide/electrolyte)along the depth direction.

Fig.3 shows the XRDspectra for mixed oxide pellets after being electrolyzed at 3.1 V for 7 h.The mixed TiO2/MnO2pellet sintered at 900°C is completely reduced and pure TiMn2alloy is obtained.Sintered at 1050°C,besides TiMn2phase,there exist Mn2TiO4,MnTiO3,CaTiO3and Mn.Sintered at 1250°C,CaTiO3and Mn are dominant.This is consistent with the results in Fig.2.Higher sintering temperature will lead to lower reduction rate due to larger particle and more compact structure.

3.2.The influence of cell voltage

The electro-deoxidation method is an environment-friendly process because the reduction of oxides and prevention of salt decomposition can be achieved synchronously.For relatively high electro-deoxidation rate,the choice of cell voltages is very important[23].The thermodynamic data of the possible reactions during the electro-deoxidation process at 900°C are calculated.

with ΔG=627.365 kJ·mol-1and ΔE=3.25 V.

with ΔG=67.169 kJ·mol-1and ΔE=0.12 V.

with ΔG=336.727 kJ·mol-1and ΔE=0.87 V.

with ΔG=-83.830 kJ·mol-1.

with ΔG=420.557 kJ·mol-1and ΔE=1.09 V.

In this work,cell voltages of 2.5,2.8 and 3.1 V were chosen for the electro-deoxidation of mixed oxide pellets.The XRD spectra of electrolysis products at different voltages are shown in Fig.4.After being electrolyzed at 2.5 V for 7 h,there are traces of raw materials(Mn2O3and TiO2)in the product.Most of Mn2O3is converted to Mn,as well as minor amounts of TiMn2alloys.CaTiO3forms at the expense of TiO2,which might result from the spontaneous reaction between TiO2and dissolved CaO following reaction(7)due to the negative ΔG value.At the cell voltage of 2.8 V,no Mn2O3is detected and TiMn2is dominant,while Mn,CaTiO3and traces of TiO2phase still exist,which means that higher cell voltage is needed for the formation of pure TiMn2phase.The XRD spectrum of the product after electrolysis at 3.1 V shows the formation of pure TiMn2phase.

Fig.2.SEM images of TiO2-MnO2 pellets sintered at 900 °C(A),1050 °C(B)and 1250 °C(C).

Fig.3.XRD spectra of the products from electrolysis of pellets sintered at different temperatures(3.1 V,900°C CaCl2 melt,7 h).

Fig.4.XRD spectra of products from electrolysis of the pellets sintered at 900°C at different cell voltages in 900°C CaCl2 melt for 7 h.

3.3.Analysis of electro-deoxidation process

The variations of phase composition during the electro-deoxidation determined by XRD are shown in Fig.5.It is suggested that the electrodeoxidation process has the following stages:Mn2O3and TiO2→Mn,CaTiO3→TiMn2.Based on the experimental results and previous studies on the mechanism of the electro-deoxidation of solid oxides[22,24,25],the reaction mechanism involving a compounding process and an electro-deoxidation process is proposed as follows.

Stage I:It is composed of electrochemical and chemical reactions.Mn2O3is first electro-deoxidized and forms Mn following reaction(5),since the decomposition voltage of Mn2O3is lower than that for TiO2(reaction 6).Meanwhile,TiO2reacts with Ca2+and O2-dissolved in molten CaCl2following reaction(7),forming CaTiO3.Stage II:It is characterized by the decrease of Mn and CaTiO3peaks and the increase of TiMn2peaks,which means that CaTiO3is electro-deoxidized.Interestingly,no Tiis detected in this stage,indicating the formation of TiMn2from direct reduction of CaTiO3on pre-formed Mn.It is noted that the background current is still about 0.6 A after forming the pure TiMn2phase,as shown in Fig.5B,which is due to the electronic conduction through molten CaCl2resulted from the drop of graphite particles and the impurity of the molten salt[16].

Fig.6 shows typical SEM micrographs of the products after electrolysis in 900°C CaCl2molten for different reaction time.In the first 1 h of the process,the outer part became looser because the raw materials were reduced continuously with the participation of CaCl2molten,accompanied by the fast escape of ionized oxygen from the layer surface,which could be assisted by the high temperature of the molten salt.Meanwhile,the interior of pellet was still in compact structure,which means that the electro-deoxidation did not occur yet in this area.This phenomenon indicates that the oxides of the surface are reduced first and then the reaction area(three-phase interlines)moves into the interior of pellet along the depth direction.With the electrodeoxidation moving into the interior of oxide layer,the product presents a porous structure(Fig.6C).In Fig.6D,the product obtained after electrolysis for 7 h shows a spongy and porous structure with nodular particles,in agreement with the previous results in most metallized products from the electro-reduction of solid metal oxides in CaCl2melt.The reduced pellet can be easily ground into powder with pestle and mortar.

Fig.5.XRD spectra of products from electrolysis of the pellets(sintering at 900 °C in air for 5 h)at 3.1 V in 900 °C CaCl2 melt for different time(A)and current-time plot(B).

Fig.6.SEM image of the product from electrolysis of pellets sintered at 900 °C for different times(3.1 V,900 °C CaCl2 melt,7 h).A:1 h;B:3 h;C:5 h;D:7 h.

The voltammetric behavior of pure TiO2powder,pure Mn2O3powder,and mixed TiO2/Mn2O3powder on the MCE in molten CaCl2at 900°C is investigated,to further understand the electro-reduction of the initial powder in solid state,as shown in Fig.7.In the absence of oxide,calcium deposition is clearly seen on the CV by the large reduction peaks c1 and c2 between-0.4 and-0.9 V,and is confirmed by typical reoxidation or stripping peak,a1,upon reversing the potential scan[23].The voltammetric features of pure TiO2powder at 0.6-0.9 V are similar to those of the bare MCE.Then the current increases from the background level and a reduction peak c3 occurs at 0.4 V,suggesting that TiO2or CaTiO3is reduced.The CV of pure Mn2O3powder begins to exhibit larger current than the bare MCE from 0.8 V and a reduction peak c4 occurs at 0.1 V.Because c4 is absent on the CVs of the bare MCE and pure TiO2,it could be attributed to the reduction of Mn2O3to Mn metal.The reduction current on the mixed TiO2and Mn2O3powder CV starts at about 0.8 V,similar to Mn2O3powder.Then the current increases from 0.4 V to the reduction peak c4,which is likely caused by the formation of TiMn2compound resulted from the reduction of TiO2or CaTiO3on the newly formed Mn metal particles.

3.4.The electrochemical hydrogen storage property

Fig.7.Cyclic voltammograms of the MCE without and with TiO2 powder,Mn2O3 powder and mixed TiO2 and Mn2O3 powder in 900 °C CaCl2 melt(potential scanning rate:10 mV·s-1).

CV curves were measured in the potential range-1200 to 0 mV(versus Hg/HgO electrode)to investigate the electrochemical hydrogen storage property of TiMn2powder.Fig.8A illustrates the CV curves of the TiMn2alloy powder microelectrode.The anodic peak occurs at-0.6 V,due to the oxidation of desorbed hydrogen atoms at the surface and is used for evaluating discharge processes of TiMn2alloy.The discharge capacities for samples could be calculated from the cyclic voltammograms based on previously reported method[26].Fig.8B shows that discharge capacity depends on cycle number.The activation of TiMn2is needed and almost completed by 30 cycles.After 200 cycles the discharge capacity retention is approximately 97%.

4.Conclusions

(1)Pure TiMn2alloy can be obtained by electrolyzing a mixture of TiO2/MnO2in 900°C CaCl2melt with 3.1 V applied for 7 h.With the increase of sintering temperature,the particles are bigger and the pellets are more compact,resulting in lower electro-deoxidation rate.Higher cell voltage favors the electrodeoxidation process.

(2)According to XRD and CV analysis,the mechanism for the formation of TiMn2is a multi-step process,which could be divided into two steps:reduction of mixed oxide from manganese oxides to Mn,and reduction of TiO2or CaTiO3on the pre-formed Mn to form TiMn2alloy.

Fig.8.Cyclic voltammograms of TiMn2 alloy powder microelectrode in a 6 mol·L-1 KOH solution(A)and discharge capacity versus cycle number(B)(potentials canning rate:50 mV·s-1).

(3)The TiMn2alloy powder for hydrogen storage shows good activity and cycling stability and the capacity retention is approximately 97%after 200 cycles.