EFFECTS OF ELECTROLYTE SOLUTION TO THE PROPERTIES OF RECHARGEABLE BATTERY lani4 5 ge0 5

Figure 1.2: Dependence of LnP H2 to 1/T Figure 1.3: Shematic representation of an interphase for a hydrogen absorbing metal aabsorption plan; t charge tranfer plane; l lattice[9].. Figur

VIETNAM NATIONAL UNIVERSITY, HANOI

VNU UNIVERSITY OF SCIENCE

FACULTY OF PHYSICS

Nguyen Thi Thu Huyen

EFFECTS OF ELECTROLYTE SOLUTION TO THE PROPERTIES OF RECHARGEABLE

BATTERY LaNi4.5 Ge0.5

International Standard Program

VIETNAM NATIONAL UNIVERSITY, HANOI

VNU UNIVERSITY OF SCIENCE

FACULTY OF PHYSICS

Nguyen Thi Thu Huyen

EFFECTS OF ELECTROLYTE SOLUTION TO THE PROPERTIES OF RECHARGEABLE

BATTERY LaNi4.5 Ge0.5

International Standard Program

Supervisor: Prof Dr Luu Tuan Tai

Hanoi - 2016

First and foremost, I would like to express my special thanks ofgratitude to my teacher Prof Dr Luu Tuan Tai who gave me the preciousopportunity to do and finish this wonderful project My sincere thanks foryour continuous support, your patience, enthusiasm and immense knowledge

I could not have imagined having better advisors for my study

My sincere thanks also go to teachers, officers and friends at Faculty ofPhysics University of Natural Sciences for always supply best conditions to

Numerical calculation and graphed

Figure 1.1: Crystal structure of LaNi 5 intermetallic compound[9] Figure 1.2: Dependence of LnP H2 to 1/T

Figure 1.3: Shematic representation of an interphase for a hydrogen absorbing metal (a)absorption plan; (t) charge tranfer plane; (l) lattice[9].

Figure 1.4: Schematic charge/discharge of Ni-MH battery[8].

Figure 1.5: Schematic representation of the concept of a sealed rechargeable Ni- MH battery[8]

Figure 1.6: Charge characteristics of Ni-MH battery

Figure 1.7 : Discharge characteristic of Ni-MH battery

Figure 1.8: Schematic representation of the hydride

formation/decomposition via a gas phase(a) and electrochemical charge transfer reaction(b)

Figure 1.9 : Reaction scheme proposed by Bode et al[8] for the Ni electrode reactions in alkaline solutions.

Figure 1.10: Schematized solid- state transition mechanism for the Ni(OH) 2 /NIOOH charge transfer reaction

Figure 2.1: Making sample system using arc melting(ITIMS)

Figure 2.2: X-diffraction device

Figure 2.3: Three electrodes electrochemical cell

Figure 2.4 : E 0 measurement of an working electrode.

Figure 2.5: Charge- discharge measurement schema

Figure 2.6 Cyclic Voltammetric measurement system

Table 3.1: Lattice parameters of sample before and after

charge/discharge cycles

Figure 3.1 The sites for H in lattice LaNi 5 intermetallic compound Figure 3.2 : X-ray diffraction spectra of LaNi 4.5 Ge 0.5 and its hydride

Figure 3.3: Magnetization curve of LaNi 4.5 Ge 0.5 before charge/ discharge

Figure 3.4: Magnetization curve of LaNi 4.5 Ge 0.5 after 5 th cycle in KOH (6M)

Figure 3.5: Magnetization curve of LaNi 4.5 Ge 0.5 after 5 th cycle in KOH(5M)and LiOH(1M)

Figure 3.6: Magnetization curve of LaNi 4.5 Ge 0.5 after 5 th cycle in KOH(5,1M)and LiOH(0.9M)

Figure 3.7: Cycle performance of LaNi 4.5 Ge 0.5 in KOH (6M ) and LiOH(1M)

Figure 3.8: Cycle performance of LaNi 4.5 Ge 0.5 in KOH(5M) and LiOH(1M)

Figure 3.9: Cycle performance of LaNi 4.5 Ge 0.5 in KOH(6M)

Figure 3.10: Cycle performance of LaNi 4.5 Ge 0.5 in KOH (5.1M ) and LiOH(0.9M)

Content

Introduction 1

Chapter 1: Intermetallic hydride material and rechargeable nickel-metal hydride battery 2

1.1 The intermetallic hydride material 2

1.1.1Crystal structure of intermetallic compounds base on LaNi5 .2

1.1.2 Kinetics of sorption and desorption of hydrogen 3

1.1.3 Hydrogen adsorption capability of intermetallic compounds 4

1.1.4 Hydro sorption in electrochemnical systems 5

1.1.5 Magnetic ptoperties 6

1.2 Rechargeable Nickel-Metal hydride (Ni-MH) battery 6

1.2.1 The reactions 6

1.2.2 Structure of nickel Hydride Batteries 7

1.2.3 Charge charateristics 8

1.2.4 Discharge characteristics 9

1.2.5 Discharge characteristics 9

1.2.6 The nickeloxide electrode 12

Chapter 2: Experimental techniques 15

2.1 Sample preparation 15

2.2 X-ray diffraction measurement 16

2.3 Magnetic measurements 17

2.4 Electrochemical studies 17

2.4.1 Three electrodes electrochemical system 17

2.4.2 Open- circuit potential measurement 18

2.4.3 Galvanostatic charge-discharge cycles 19

2.4.4 Cyclic Voltammetric technique 20

Chapter 3: Results and discussion 23

3.1 Crystal structure analysis 23

3.2 Magnetic results 24

3.3 The electrochemical results 26

Conclusion 30

References 31

Hydrogen absorption capacity of the inter-metallic diatomiccompounds materials were first discovered in the late 60s of the 20th century.Since then, the compounds RT5 have been known and studied a lot because ofthe ability to absorb and disabsorb the very large amounts of hydrogen atatmospheric pressure and room temperature [9] which does not damage thelattice structure Hydrogen accumulation in the crystal lattice of the materialcreates a permanent-form hydrogen container and energy reserves Thisfeature has been applied in many fields of science and technology, one of theapplications that is built rechargeable battery cathode Ni-MH[[3,4] Theadvantages of Ni-MH battery are high-capacity battery and its waste does notpollute the environment[7] On the other hand, compared with Ni-Cd or thelithium battery are familiar products in the electronics and communicationshanded, Ni-MH battery have longer lifetime and lower cost.[7]

Currently, NiMH batteries are widely used, thus improving the qualityand innovation are necessary There are many ways to improve the batteryperformance has been studied as: doping 3d elements capable of absorbinghydrogen, reducing particle size which increase the surface area of theelectrode in contact with the electrolyte solution to increase the level ofhydrogen absorption, changes capable of releasing hydrogen absorption and

by acting on the electrolyte solution.The third way takes very few interested,earlier with NiCd batteries, the electrolyte solution has been carefully studiedand selected by the 6M KOH electrolyte solution thus selected now for thesame type of positive electrode is NaOH

In this work, we focus on the influence of the electrolyte solution

to the electrochemical properties of the LaNi4,5Ge0,5 compound

The thesis contains three chapters and some conclusions:

Chapter 1 : Intermetallic hydride material and rechargeable

nickel-metal hydride battery

Chapter 2: Experimental technique

Chapter 3: Results and discussion

Chapter 1: Intermetallic hydride material and

rechargeable nickel- metal hydride battery

1.1 The intermetallic hydride material

1.1.1Crystal structure of intermetallic compounds base on LaNi 5

The intermetallic compound system LaNi5 crystallizes with thehexagonal CaCu5- type structure The structure consists of two alternatingtypes of plane: the basal plane with Lanthanum and Nickel atom whichoccupy the 1a and 2c sites respectively the z=1/2 plane with only Nickelatoms on the 3g site[[1]

Lanthanum 1 a

Nickel I 2 c

Nickel II 3 g

Figure 1.1: Crystal structure of LaNi 5 intermetallic compound[9].

The studies of the absorption and the disabsorption have been carriedout on their compounds showed that in the process of hydrogenation material,elements entered the hole tetrahedron, octahedron and network failures filledinterstitial mechanism alters the lattice constant without changing the materialstructure

1.1.2 Kinetics of sorption and desorption of hydrogen

Hydrogen absorption process can be studied by isotherm of pressurebalance as a function of concentration (x) of the oxidizing compound.However, according to Bureau, Planagan and Oast, its kinetic process can bestudied by a simpler way When hydrogenation occurs with 2-phases todistinguish the values ΔH and ΔF can be obtained from the temperatureH and ΔH and ΔF can be obtained from the temperatureF can be obtained from the temperaturedependence of the pressure balance Hydrogenation reaction occurs betweenLaNi5 and hydrogen compounds are represented as follows:

LaNi5 + mH2 = LaNi5H2m

In thermodynamics, kinetics Vanhoff equation is represented:

LnPH2 = -ΔH and ΔF can be obtained from the temperatureF/R + ΔH and ΔF can be obtained from the temperatureH/RT

where R is the gas constant, the value of ΔH and ΔF can be obtained from the temperatureH and ΔH and ΔF can be obtained from the temperatureF is the thermodynamicquantities corresponding to 1 mol hydrogen Considering the temperaturerange can be considered small enough isothermal, then ΔH and ΔF can be obtained from the temperatureH and ΔH and ΔF can be obtained from the temperatureF will notdepend on the temperature By plotting the dependence of RnH2 with theinverse of temperature (1 / T) will be obtained 1 superlative line Based on thegraph it is easy to find the value of ΔH and ΔF can be obtained from the temperatureH (corresponding to the slope of the line)and the value ΔH and ΔF can be obtained from the temperatureS ΔH and ΔF can be obtained from the temperatureH can get different values, it can have a positive ornegative value Hydrogenation occurs in two phases: the first phase to theprocess of decomposition of hydrogen molecules into atoms, this processconsumes energy (ΔH and ΔF can be obtained from the temperatureH> 0) The second stage occurs as hydrogenation, theprocess is radiating energy (ΔH and ΔF can be obtained from the temperatureH <0) Thus, depending on the dominantprocess that ΔH and ΔF can be obtained from the temperatureH yet received a positive or negative value For Entropy (ΔH and ΔF can be obtained from the temperatureS),

on the other, its value does not depend on inter-metallic compounds Thestudy showed that the entropy of hydrogenation mainly contributed by theentropy of hydrogen (ΔH and ΔF can be obtained from the temperatureSgas = 130 J / mol H2 at room temperature)

Considering all of the hydrogenation reaction has advantages in terms

of energy (exothermic reaction, ΔH and ΔF can be obtained from the temperatureH <0) for the reaction to occur, so whenplotting the dependence of LnPH2 to 1 / T, toys market will look like thefigure below:

2.2 2.4 2.6 2.8 3.0 3.2 0

10 20 30 40 50

1.1.3 Hydrogen adsorption capability of intermetallic compounds

In kinetic and catalysts, it is known that transition metals, such asNickel , Conalt, Iron…are capable to a adsorb amounts of hydrogen on thesurface Due to the unfill shell they can establish weak bonds with hydrogenatoms Adsorbed amount depends on the chemical potential of each d element

on the surface area of the metal, the hydrogen pressure and the temperature

It is also necessary to mention here the surface effect phenomena.There are several processes causing the composition of the surface ofintermetallics to be different from the bulk concentration It is the fact that thesurface energy of 3d element is bigger than that of the rare earth, which canlead the surface equilibrium concentration of the rare earth atoms to exceedsthat of the bulk Surface segregation is quite a general phenomenon, and isexpected to occur whenever the constituent elements have sufficientlydifferent properties Often small amounts of oxygen or water present asimpurities in the hydrogen gas These give rise to the formation of rare earthoidation enenly results in a surface relatively rich in 3d transition metals.Under this point of view we can assume that the hydrogen adsorption ofintermetallic compounds is dominated by the 3d elements on the surface

1.1.4 Hydro sorption in electrochemical systems

Due to the non-autonomous character of the electrode/electrolyteinterphase, a substantial greater number of factors affect the sorption processthan in the gas phase An interphase region is formed whenever an electrode

is in contact with an electrolyte In the simplest case, the interphase regiontakes the form of the electrical double layer In the more complex cases and inparticular, during the charge tranfer reaction, it consists of layers, eachassociated with a participating elementary process

In this presentation, the inter phase region is an open system in which anumber of consecutive process take place of which the slowest onedetermines the rate These processes include transport of the reactants fromthe bulk to the electrode surface by diffusion, absorption on the electrodesurface, charge transfer, desorption of the reaction products away from theelectrode surface In a discharging battery, these same processes occur,however, in a battery the electrons ultimately flow into an external circuitwhere the electrical work is delivered [3]

The role of the inter phase may vary from that of an inhibitor to anaccelerator of both charge tranfer and molecular transport This concept,although not explicitly stated, was implied in the discussion of hydrogenabsorption from the gas phase where it was shown that small metal alloyclusters absorb more hydrogen via a mechanism that becomes inoperative asthe cluster size increases The inter phase may be modified in the course ofbattery operation, especially as a result of cycling

Figure 1.3: Shematic representation of an interphase for a hydrogen absorbing metal (a)absorption plan; (t) charge tranfer plane; (l) lattice[9] 1.1.5 Magnetic properties

The LaNi5 is an enhance Pauli paramagnet with an almost independent susceptibility at low temperatures that amount to 3,7.10-3 µB T-1

field-at 4.2K [5] and is slowly decreasing with increasing temperfield-ature up to300K.The hydrogenation process of this compound changes their magnetismconsiderably Cyclic absorption and adsorption of hydrogen by LaNi5 causesuper paramagnetic Ni cluster to precipitate on the surface of the meltparticles Continued cycling eventually causes ferromagnetic behavior ofLaNi5 compound

1.2 Rechargeable Nickel-Metal hydride (Ni-MH) battery

1.2.1 The reactions

Nickel metal hydride batteries employ nickel hydroxide for the positiveelectrode similar to Ni-Cd batteries The hydrogen is stored in a hydrogenabsorbing alloy for the negative electrode, and an aqueous solution consistingmainly of potassium hydroxide for the electrolyte Their charge and dischargereaction shown below

Positive electrode Ni(OH)2 + OH-  NiOOH + H2O + e- (1)Negative electrode M + H2O + e-  MHab + OH- (2)

SOLUTIONPHASE

INTERPHASE

ELECTRODEINTERIOR/SOLIDPHASE

Overall electrode Ni(OH)2 + M  NiOOH + MHab (3)

(M : hydrogen absorbing alloy, Hab : absorbed hydrogen)

As can be seen by the overall reaction given above, hydrogen movesfrom the positive to negative electrode duringcharge and reverse duringdischarge with the electrolyte taking no part in the reaction, which mean thatthere is no accompanying increase in the electrolyte The H absorbing alloynegative electrode during overcharge by sufficiently increaseing the capacity

of negative electrode which is the same method employed by Ni-Cd batteries

By keeping battery’s internal pressure constant in this manner, it is possible toseal the battery

Figure 1.4: Schematic charge/discharge of Ni-MH battery[8] 1.2.2 Structure of nickel Hydride Batteries

A schematic representation of a Ni-MH battery containg an AB5-typehydride-forming electrode and a Ni electrode [8] The electrodes areelectrically insulated from each other by a separator Both separtor andelectrodes are impregnated with an alkaline solution that provides for theionic conductivity between the two electrodes

Figure 1.5: Schematic representation of the concept of a sealed

rechargeable Ni- MH battery[8].

1.2.3 Charge charateristics

Charge charactertics of nickel metal hidride batteries are affected bycurrent, time and temperature The battery voltage rise when the chargecurrent is increased or when the temperature is low The charge efficiencydiffer depending on the current, time, temperature and other factors

Figure 1.6: Charge characteristics of Ni-MH battery

1.2.4 Discharge characteristics

Discharge characteristics of nickel metal hidride depend on current,temperture…and the discharge voltage characteristics are flat at 1.2V which isalmost the same as for Ni-Cd batteries The discharge efficience and thedischarge voltage decrease in proportion as the current rises or temperaturedrops Compare with Ni-Cd batteries, nickel-metal hydride batteries haveinferior high-rate discharge characteristics, making them less suitable for use

in applications requiring high-current discharge As with Ni-Cd batteries,repeated charge and diccharge of these batteries under high discharge cut-offvoltage conditions causes a drop in the discharge voltage ( which issometimes accomplanied by a simultaneous drop in capacity) The dischargecharacteristics can be restored by charge and discharge to a discharge endvoltage of down to 1V per cell

Figure 1.7 : Discharge characteristic of Ni-MH battery.

1.2.5 Discharge characteristics

In gas phase reaction hydrogen gas is brought in to contact with ahydride-forming compound and the gaseous molecules dissociate at the solidinterface Atomic hydrogen is strongly adsorbed at the metal surface as themetal hydrogen interaction energetically very favorable The adsorbedhydrogen atoms can, subsequently, be converted into the absorbed state byjumping into the interstitial sites beneath the first atomic layer Furthertransportation into the bulk of the solid occurs via diffusion The occurence of

Figure 1.8: Schematic representation of the hydride formation/decomposition via a gas phase(a) and electrochemical charge

transfer reaction(b).

The electrochemical analogy is shown in fig 1.8.b The overallelectrochemical reaction has already been presented by hydride-formingreaction Here we will go into some detail and consider the various partialsteps involed in the electrochemical hydride formation reaction Thefollowing steps can successively distinguished:

i The supply of reactants by means of diffusion from the bulk (b) of the

electrolyte to the solid interface

H2O(s) H2O (4)

ii The charge transfer reaction occuring at interface

H2O+e-Had+OH- (5)

H(ad) is atomic hydrogen adsorption As the results of reductionreaction, atomic hydrogen Had is again adsorbed on the surface

iii The removal of electrochemical formed reaction products from the

interface by mean of diffusion This include absorption of adsorbedhydrogen Habs by the RT5 compound through which the hydride isformed

HadHabs And transport of OH- ions into the bulk of the electrolyte

OH

-sOH

-b

iv Depending on the materials composition and on the hydrogen

concentration in the solid, either an α phase is formed

Habs(α) Habs(β))

v Recombination of two Had atoms has to be taken into account Thislead to the formation of H2, Which is released, from the electodesurface as a gas

2HadH2

The hydrogen absorption and desorption properties of intermetalliccompound are generally characterized by pressure-composition isotherms Ithas long been recognized that there is a direct relationship between theequilibrium pressure for absorption and desorption of hydrogen gas and theequilibrium potential (E0) of metal hydride electrode This relationshiphasbeen expressed by:

E0 = -[RTln(P(H2))]/nF (6)where:

R: gas constant

n: number of electrons

E : measure against a reference electrode

The plateau pressure of the hydride-forming material employed in arechargeable Ni-MH battery may not be too high for different reasons (i) thegas pressure inside a battery should be relatively low for obvious safetyreason (ii) to reduce the battery self-discharge rate (iii)competition betweenthe hydrogen evolution reaction and the hydride formation reaction becomesmore severe when high plateau material are employed On the other hand, animportant electrochemical requirement to met for Ni-MH battery is a high cellvoltage.

1.2.6 The nickel oxide electrode

Numerous studies have been devoted to the Ni-electrode over the lastfew decades Excellent reviews on this topic have recently been writen [[1].These reviews include the various preparation methods and manufacturingprocesses of active electrode materials In contrast to what is suggested by Eq1.1, the charge and discharge reactions of a Ni- electrode are much morecomplex This is further accentuated by the fact that different electro-activemodifications of both divalent and trivalent Ni have to be considered Theexistance of these modification has first been extensively discussed by Bode

et al, and has, since then, been the subject of many investigations

Oxidation

Modification Ni α- Ni(OH)2 γ-NiOOH

Figure 1.9 : Reaction scheme proposed by Bode et al[8] for the Ni

electrode reactions in alkaline solutions.

Β-Ni(OH)2 β)-NiOOH

Figure summarizes the various Ni species, most relevant for energystorage applications Starting with metallic Ni the highly hydrated α-modification of divalent Ni(OH)2 can be electrochemically prepared atrelatively negative potentials, in accordance with thermodynamicconsiderations Upon cycling and ageing at more positive potentials thismodification is dehydrated in to the β) form, which can be further oxidized totrivalent β)-NiOOH During overcharging, β)-NiOOH can be irreversiblyconverted into the hydrated γ-modification, γ-NiOOH can beelectrochemically reduced into α-Ni(OH)2

The charge transfer reaction between the β)-modifications of anickeloxide battery electrode is generally considered to occur via a solid-statetransition mechanism is schematically in Fig 1.9

Figure 1.10: Schematized solid- state transition mechanism for the

Ni(OH) 2 /NIOOH charge transfer reaction.