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Gas phase hydrogen absorption andelectrochemical performance of La2(Ni,Co,Mg,M)10based alloys

H. Drulis a,*, A. Hackemer a, P. Głuchowski a, K. Giza b, L. Adamczyk b,H. Bala b

a Institute of Low Temperatures and Structure Research PAS, Wroclaw, PolandbDepartment of Chemistry, Czestochowa University of Technology, Czestochowa, Poland

a r t i c l e i n f o

Article history:

Received 15 July 2013

Received in revised form

14 November 2013

Accepted 22 November 2013

Available online 19 December 2013

Keywords:

Intermetallic hydrides

Pressureecomposition isotherms

Electrochemical charge/discharge

Hydrogen capacity

* Corresponding author. Tel.: þ48 71 343 502E-mail address: H.Drulis@int.pan.wroc.pl

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.11.0

a b s t r a c t

The effect of M ¼ In or Al on the hydrogenation behavior of the La2(Ni,Co,Mg,M)10 alloys at

room temperature is presented. The ceramic like samples have been prepared by powder

metallurgy route using pure Mg- and the La2Ni9�xMx alloy powder precursors. XRD analysis

revealed predominantly the CaCu5-type structure for all final alloys. Partial substitution of

Co by In in La2Ni8MgCo causes a slight decrease of hydrogen concentration whereas Al

addition increases this parameter. The highest hydrogen concentration of 1.87 wt.% has

been reached for La2(Ni8Co0.8Al0.2)Mg composition at hydrogen pressure of 10 bar. Indium

addition dramatically decreases the middle-plateau hydrogen equilibrium pressure from

peq ¼ 0.37 bar (In-free alloy) to peq ¼ 0.06 bar (1.7 at.% In). The electrochemical performance

of the studied materials has been characterized using chronoamperometric and chro-

nopotentiometric techniques. The galvanostatic hydrogenation experiments at 185 mA/g

discharge rate revealed the largest discharge current capacity of 355 mAh/g for La2(Ni8-Co0.8Al0.2)Mg alloy. The relative diffusivity factor of hydrogen ðDH=a2Þ varies for the tested

materials in the range of (2.0e5.4)$10�5 s�1.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Rare earthenickel (REeNi) based alloys are widely applied for

hydrogen storage, including the rechargeable metal hydride

(Ni/MH) batteries. Materials for the above applications should

reveal a high hydrogen capacity, moderate hydride stability

and reasonably high hydrogen absorption/desorption rates.

Most of these characteristics are usually derived from the

hydrogen pressureeconcentration (pec), isotherms [1,2] and

electrochemical charge/discharge measurements [3,4].

1; fax: þ48 71 344 1029.(H. Drulis).2013, Hydrogen Energy P92

The most spread and commercialized metal hydride elec-

trodes are mainly based on AB5-type alloys. Their hydrogen

capacities usually reach 300e330 mAh/g. Many methods such

as optimization of composition or doping were applied to

improve both the REeNi alloys discharge capacity and cycle

life [5]. In our recent papers we discussed the hydrogenation

properties of LaNi5�xInx [6] and LaNi4.3(Co, Al)0.7�xInx compo-

sitions [7]. Partial substitution of Ni by indium (x < 0.3)

significantly modifies the hydrogenation behavior. Particu-

larly, indium decreases the hydrogen equilibrium pressure

making In-doped alloys very interesting materials as negative

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 4 2 3e2 4 2 92424

MH electrodes in the Ni/MH batteries. The effect of indium is

especially distinct when part of nickel (3.3e6.7 at.%) is

substituted by cobalt [7].

Recent investigations of Kadir et al. [8,9] and De Negri et al.

[10] have shown that also themagnesium containing (RE,Mg)e

Ni alloys with the general formula of RE3�xMgxNi9 may serve

as promising materials for hydride electrodes owing to their

high hydrogen storage capacity and good electrochemical

properties. Electrochemical discharge capacities of the Mg

containing alloys with PuNi3-type structure are greater than

those with CaCu5-type structure. For example, the capacity of

410 mAh/g was observed by Kohno et al. [11] in the system

with the composition of La5Mg2Ni20Co3 whereas the capacity

of 380 mAh/g was observed by Tang et al. [12] in Mg modified

alloys with CaCu5-type structure. Generally, because of low

magnesium boiling point (1105 �C), the final composition and

hydrogen storage properties of the Mg containing alloys are

strongly affected by the metallurgical process used in alloys

manufacturing [13,14]. The main goal of this work focuses on

the relationship between the composition and both gas-

phase- and electrochemical charge/discharge hydrogenation

for LaeNieMg type alloys obtained by the so-called sintering

metallurgy. Our interest is to find Lae(Ni, Co)Mg-based alloys

with a discharge capacity greater than that of LaNi5 interme-

tallic compound and with hydrogen equilibrium pressure

lower than 1 bar.

Fig. 1 e Evolution of XRD patterns of La2(Ni,Co)9Mg alloy

during consecutive steps of its synthesis: (a) arc-melted

La2(Ni,Co)9 precursor, (b) precursor and Mg powder mixture

after ball-milling and (c) final La2(Ni,Co)9Mg powder after

high temperature annealing.

2. Experimental

Five alloys of the composition of La2Ni9Mg, La2Ni8CoMg,

La2Ni7Co2Mg La2Ni8(Co0.8In0.2)Mg and La2Ni8(Co0.8Al0.2)Mg

were prepared by powder metallurgy using the mechanical

alloying (MA) route followed by annealing. The La2Ni9�xMx

(M ¼ Co and Al or In) alloy precursors and Mg (99.8 wt.%)

powder have been used. The precursor alloys were arc melted

from the individual metals: La (99.8%), Ni and M (99.9% purity)

in high purity argon gas atmosphere. As-cast precursor alloys

were mechanically crushed, milled into the powders and

mixedwith 8.3 at.% ofMg powder. A small excess (ca 5wt.%) of

Mg was introduced into starting powder mixtures to cover

partial magnesium evaporation. Then, the powder mixtures

were ball milled in a Fritsch mill under argon for 5 h with the

speed rate of 500 rpm and the revolution direction was being

changed every 30 min. After milling, the obtained amorphous

material was pressed into the pellets and sintered in 10�6 bar

vacuum. The sintering was carried out at 800 �C for 8 h, fol-

lowed by a second step at 600 �C for 8 h, analogously as in

paper [13]. The obtained sintered ceramics were characterized

by means of X-ray diffraction (XRD) using a CuKa radiation.

The gas-phase hydrogen absorptionedesorption properties of

the alloys were studied by the use of Sievert’s type equipment.

The sampleswere activated in vacuumat 250 �C for 1 h, cooled

to 23 �C and then charged with high purity hydrogen gas

(99.999% H2) at pH2 ¼ 20 bar. Several complete hydrogen

absorption-desorption cycles were performed prior to the

peceT measurements to ensure high rate of hydrogen ex-

change. The cyclic examination included the hydrogen ab-

sorption at 20 bar for 1 h and then the fast hydrogen

desorption with a rotary pump for next 1 h. Once these

processes had been performed three e four times, the pres-

sureeconcentration (pec) dependencies of hydrogen desorp-

tion were measured under hydrogen pressures from 20 to

0.02 bar at T ¼ 296 K.

The electrochemical charge/discharge tests were carried

out in a conventional three electrode cell, consisting of a

powder-composite metal hydride working electrode, a refer-

ence saturated calomel electrode (SCE) and a Pt wire counter

electrode, using a CHI 1140 A (Austin, Texas) workstation. The

powder composite electrodes were prepared by pressing a

homogenized mixture of 85 wt.% of corresponding alloy

powder, 10 wt.% of PVDF and 5 wt.% of C-45 carbon black into

pellets, 0.4e0.5 mm thick. The electrolyte was Ar-saturated,

6M KOH solution at a temperature of 23 �C. The chro-

nopotentiometric method was applied to determine the cur-

rent capacity and exchange current density variations as a

function of cycling. The electrodes were charged at a cathodic

current density of �185 mA/g for 3 h and discharged at

þ185 mA/g up to anodic potential of �0.6 V (vs SCE). The

relative hydrogen diffusivity factors ðDH=a2Þ, where a denotes

the mean particle radius and discharge capacities as function

of cycle number were determined by a multi-potential step,

chronoamperometric technique. In this method electrodes

were charged at Ech ¼ �1.2 V (vs SCE) for 104 s and discharged

at Edisch ¼ �0.6 V (SCE) for 104 s. More details concerning

experimental procedure can be found in our previously pub-

lished papers [6,7,19e21].

3. Results and discussion

3.1. Structure characterization

X-ray diffraction (XRD) with a CuKa radiation was used to

identify the phase structure and composition of the alloys. The

XRD data were collected using diffractometer X’Pert PRO

PANalytical. Fig. 1 shows the XRD patterns evolution obtained

Table 1 e Unit cell parameters of the main component ofthe tested La2(Ni,Co,Mg)10LxMx alloys from XRD analysis.

Sample Space group Unit cellparameters, nm

Abundance

a c

La2Ni9Mg P6/mmm 0.5022039 0.3981173 >95%

La2Ni8CoMg P6/mmm 0.5027354 0.4000330 >95%

La2Ni8(Co0.8In0.2)Mg

P6/mmm 0.5041739 0.3998451 >95%

La2Ni8(Co0.8Al0.2)Mg

P6/mmm 0.5035055 0.3992600 >95%

La2Ni7Co2Mg P6/mmm 0.5033665 0.3984836 >95%

LaNi5 ref. P6/mmm 0.50125(3) 0.39873(2) 100%

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 4 2 3e2 4 2 9 2425

for La2Ni7Co2Mg composition, as an example, at three

consecutive stages used during the alloy fabrication: (a) for the

arc-meltedprecursor, (b) aftermillingofMgwithprecursorand

(c) for the final alloy after the annealing procedure. The initial

arc-melted precursor, La2Ni7Co2 exhibits (Fig. 1a), rather com-

plex diffraction pattern. The precursor andMgmilled together

in themechanical alloying (MA) route show the pattern typical to

almost amorphousmaterial (Fig. 1b). Fig. 1c presents the X-ray

pattern of a full recrystallized material after its high tempera-

ture annealing. All final experimental XRD data were analyzed

with Rietveldmethodusing theXPert HighScore Plus software.

Fig. 2 gives (as anexample) theXRDprofiles of Rietveldanalysis

for one of the studied alloys with La2Ni9Mg formula.

X-ray spectra for precursors were not analyzed. The phase-

structural analysis proves that all studied LaeNieMg and

LaeNieCoeMg samples consist of mainly (>95%) with the

phases that crystallize in the CaCu5 type structure. Apart from

the main CaCu5-type structure pattern there are visible tiny

patterns belonging to the impurity phases of approximate

La2Ni7 composition whose abundance is on the level of ca

5 wt.%. The unit cell parameters of the main component

determined fromXRDdataanalysis are summarized inTable1.

The results of structural analysis of the tested materials

indicating their CaCu5-type structure are rather surprising. As

it wasmentioned in Section 2, following the synthesismethod

proposed in [13] we expected to obtain the material predom-

inantly with PuNi3 e type structure. Besides, it is believed that

LaNi5 compound does not dissolve magnesium.

The Rietveld refinement of the XRD pattern of the main

component in Fig. 2 showed that Mg forms solid solution in

our AB5-type material by occupying some quantity of 1a po-

sitions of P6/mmm space group: La1.98Mg0.02Ni10. We cannot

expect anything much different in other samples (see X-ray

results in Table 1). Therefore, we restrict such Rietveld anal-

ysis for one sample only. Similar results (La0.97Ni5Mg0,03) for

Fig. 2 e Rietveld refinement of the XRD pattern of the La2Ni9Mg a

and calculated lines. Vertical bars correspond to the Bragg peak

impurity phase are omitted.

AB5-Mg doped alloys have also been reported by Li et al. [15].

To confirm the presence of the rest of Mg in the alloys the EDX

analysis has been carried out. In Fig. 3 the EDX pattern regis-

tered for the sample with nominal composition of La2Ni7-Co2Mg is presented. Evidently, the peak of Mg is visible

at 125 keV. Average chemical composition calculated from

the normalized EDX peak intensities corresponds to La17.6Ni58.0Co13.8Mg10.6 formula (where the numbers are atomic

percentages) and it is close to the target composition of

La16.7Ni58.3Co16.6Mg8.3 formula unit (LaNi5-type). It is worth

mentioning that similar LaeNieMg composite materials

based on LaNi5þx structure have already been manufactured

and their electrochemical properties studied by Tang et al.

[12]. Their EDX results indicated that the Mg content in the

regions of LaNi5eMg- contained solid solution reaches value

as high as 12 at.% close to the value of 10.6 at.% estimated in

our alloys. The presence of LaNi5eMg doped phase

(La0.78Nd0.18Mg0.03Ni3.99Mn0.19Co0.36Al0.33) have also been re-

ported by Ozaki et al. [16] for La0.8Mg0.2Ni3.4�xCo0.3(MnAl)x

composition with x ¼ 0.4.

lloy. Lower plot is a difference profile between experimental

positions for the constituent phases. Bars for La2Ni7

Fig. 3 e EDX pattern of La2Ni7Co2Mg alloy.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 4 2 3e2 4 2 92426

3.2. Hydrogen pressureehydrogen concentration (pec)isotherms

Fig. 4 shows the pec isotherms (296 K) of hydrogen gas

desorption for four representative intermetallic phases of

magnesium modified materials with final La2Ni9Mg, La2Ni8-CoMg, La2Ni8(Co0.8In0.2)Mg and La2Ni8(Co0.8Al0.2)Mg stoichi-

ometry. From the crystallographic point of view, all samples

had practically the same phase composition so one could

expect that they possess similar hydrogenation properties. On

the other hand, the precursors applied had different chemical

composition because of partial substitution of Ni by Co and Al/

In additions. To follow through the role of Mg element on

hydrogen absorption properties of the individual LaNi5etype

alloys the hydrogenation and pec characteristics of the pre-

cursor hydride phases are additionally shown.

Under hydrogen pressure of 10 bar the largest hydrogen

concentration equal to 1.87 wt.% has been obtained for

La2Ni8(Co0.8Al0.2)Mg composite. After the first cycle, the

hydrogen capacity used to drop up to ca 10% of its initial value,

depending on the alloy composition. Such a behavior in-

dicates that part of the composite material is hydrogenated

irreversibly. Thus, part of the material cannot be involved in

hydrogen desorption process as long as absorption/desorption

cycles are carried out at room temperature. This is likely

Mg2NiH4 hydride, which appears when composite material

decomposes upon hydrogenation. Generally, it is worth noting

the dramatic difference in hydrogenation ability between the

precursor alloys (without Mg) and magnesium modified ma-

terials. For example, the hydride capacity of La2Ni8(Co0.8In0.2)

Mg final alloy is over 3 times greater than that of its La2Ni8(-

Co0.8In0.2) precursor. The reason for these different properties

one can explain by a very complex multiphase state of the

precursor alloys. The corresponding situation is illustrated in

Fig. 5 by X-ray diffraction spectra for Mg-free precursor

(pattern “a”) and magnesium containing alloy (pattern “b”).

From the application point of view, the so-called reversible

hydrogen capacity (RHC) is of great importance. In this work

the RHC is assumed to be the amount of hydrogen gas

(expressed in mAh/g) that can be derived from fully hydro-

genated material during hydrogen isothermal desorption be-

tween two points shown in Fig. 4: one marked by CH,1 bar and

second with hydrogen pressure equal to pH2 ¼ 0. Such defined

RHC parameter has a very practical meaning because from

NiMH battery stability point of view the negative electrode

material should exhibit maximum hydrogen equilibrium

pressures no higher than 1 bar at room temperature. As it

results from Fig. 4(aed) all of the tested alloys containing 8.3

at.% Mg satisfactorily fulfill this criterion.

In Table 2 the mid-plateau H2 equilibrium pressures,

maximum hydrogen concentration in the tested materials

(arbitrary assumed to correspond to p ¼ 10 bar), hydrogen

content in the alloys at 1 bar and the calculated RHC values

determined from pec curves in Fig. 4 are collected. As it has

been already mentioned, the RHC values presented in Table 2

were estimated from the width of plateau part of pec iso-

therms between 1 bar and the vacuum. Therefore, the values

of RHC given in Table 2 are certainly somehow over-

estimated. In Table 2, the Qel,disch values determined electro-

chemically (see Section 3.3) are also collected for comparative

purpose. In practice, the values of RHC parameter reflect the

expected discharge current capacities of corresponding hy-

dride electrode for an “open” Ni/MH battery.

3.3. Electrochemical hydrogenation

In Fig. 6 the anodic current densities (in logarithmic scale)

versus discharge time recorded for cathodically charged

La2Ni8(Co0.8In0.2)Mg electrode are presented for 7 successive

cycles, as an example. Similar dependencieswere obtained for

La2Ni8CoMg, La2Ni8(Co0.8Al0.2)Mg and La2Ni7Co2Mg electrodes.

Integration of anodic current density of hydrogen oxidation

over the entire range of discharge time allows to evaluate the

changes of discharge capacity (Qdisch) at subsequent cycles.

The calculated discharge capacities vs cycle number for all of

the tested electrodes are shown in Fig. 7. As it can be seen, the

tested alloys usually need 2e4 cycles to reach their maximum

capacity. The lowest current capacity exhibits the La2Ni8CoMg

alloy (8.3 at.% Co) e its maximum value of 280 mAh/g corre-

sponds to fourth cycle. Partial substitution of Co by In or Al

(1.7 at.%) in this material is prone to distinct increase of

hydrogen absorption ability. For La2Ni8(Co0.8In0.2)Mg electrode

the discharge capacity was 340 mAh/g (3e4 cycle) whereas for

La2Ni8(Co0.8Al0.2)Mg the capacity was as large as 367 mAh/g

(2e3 cycle). Similarly great capacity (344 mAh/g for 4e5 cycle)

was observed for Co-rich alloy (16.7 at.% Co) of La2Ni7Co2Mg

composition. From Fig. 7 it appears that capacities of cobalt-,

cobalt/indium- or cobalt/aluminum substituted La2Ni9Mg-

based alloys are generally comparable to each other (ca 336 e

355mAh/g). The electrochemical discharge capacities of LaNi5e based alloys without Mg component used in commercial Ni/

MH batteries are usually on the level of 300 mAh/g. Thus,

some of LaNi5 -Mg -based alloys reported in this paper can be

considered as potential candidates for negative electrode

materials in the rechargeable NieMH batteries.

From the slope of the linear segments in Fig. 6 it is possible

to estimate the effective coefficient of the hydrogen diffusion

DH in the electrode using the following equation, which is

valid for sufficiently long discharge times [17]:

logi ¼ log

�� 6FD

da2ðC0 � CSÞ

�� p2DH

2:303a2t (1)

Fig. 5 e XRD patterns for La2Ni8(Co0.8In0.2) precursor and

the final La2Ni8(Co0.8In0.2)Mg alloy.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 4 2 3e2 4 2 9 2427

where i denotes the measured anodic current density, DH e

the effective hydrogen diffusion coefficient, d e density of the

alloy, a e average radius of the alloy particles, Co e the initial

hydrogen concentration in the alloy, Cs e the surface

hydrogen concentration and t the actual discharge time. The

sign � in Eq. (1) corresponds to the charge (�) or discharge (þ)

processes. Because, it is hard to determine the real average

particle size with satisfactory accuracy (and, thus its mean

diameter a) we use the DH=a2 fraction to evaluate hydrogen

diffusivity within the electrode material. The calculated

values of DH=a2 (we name them “relative diffusivity factors”)

are presented in Fig. 8 for subsequent cycles. As it is shown in

Fig. 8 the DH=a2 values determined from the chronoampero-

metric measurements are on the order of 10�5 s�1. The chro-

nopotentiometric method is also very useful for

determination of exchange current density of H2O/H2 system

for hydrogen storage material as a function of cycle number

[18]. According to [20,21] the exchange current density ioH2O=H2

can be obtained from the following relationship:

logioH2O=H2¼ 1

2logðiajicjÞ � DE

2b(2)

where ic, and ia, are the charge- and discharge current density,

DE is the potential jump that occurs during external current

switching from negative into positive values, and b e Tafel

slope of cathodic/anodic straight line for hydrogen electrode

(equal to 0.12 V at room temperature).

As seen from Fig. 9, the H2O/H2 exchange current density

increases with cycle number with certain tendency to settle

down after 7e8 cycle. Only for La2Ni8Co0.8Al0.2Mg electrodewe

can see a progressive increase of the exchange current density

with cycling. An increase in the exchange current densitywith

cycling reflects the charge transfer rate increase at the inter-

face between MH alloy powder and the electrolyte. The most

Fig. 4 e Hydrogen desorption isotherms of (a) La2Ni9Mg (b)

La2Ni8CoMg, (c) La2Ni8Co0.8In0.2Mg and (d)

La2Ni8Co0.8Al0.2Mg hydrides and their precursor hydride

phases at T [ 296 K.

Table 2 e Hydrogenation parameters of the tested La2Ni9Mg material partly substituted with Co and In/Al for nickel,determined from the pec measurements (23 �C).

Alloy composition apeq [bar] bCH,10 bar [%wt] cCH,1 bar [%wt] dRHC [mAh/g] eQel,disch [mAh/g]

Galvan. Chronoam.

La2Ni9Mg I/86s 0.91 1.63 1.29 347 314

La2Ni8CoMg I/98s 0.43 1.85 1.52 409 230 280

La2Ni7Co2Mg I/109s 0.60 1.75 1.34 360 325 344

La2Ni8Co0.8In0.2Mg I/101s 0.45 1.71 1.37 369 339 340

La2Ni8Co0.8Al0.2Mg I/106s 0.63 1.87 1.67 449 355 367

a Equilibrium pressure of H2 measured in the middle of plateau of pH2 ¼ f(cH) isotherm.b Hydrogen concentration absorbed by the fresh sample (first cycle at pH2 ¼ 10 bar).c Average hydrogen concentration in a sample when hydrogen gas pressure is equal 1 bar.d Reversible capacity read from pec isotherm along plateau between pH2 ¼ 1 bar and hypothetical vacuum.e Discharge capacity from galvanostatic- (at �0.5C/þ0.5C rate) and chronoamperometric measurements.

0 2000 4000 6000 8000 10000

1

10

100

1000

5, 6, 7

432

1

cycle number 1 2 3 4 5 6 7

La2Ni8Co0.8In0.2Mg

i, m

A/g

t, s

Fig. 6 e Chronoamperometric curves of La2Ni8Co0.8In0.2Mg

electrode at L0.6 V (SCE) for 7 subsequent cycles.

Fig. 7 e The discharge capacity of the studied electrodes vs

cycle number determined by chronoamperometric

method.

4 6 8 10

2

4

6

8

(D/a

2 ) x 1

05 , s-1

La Ni CoMg La Ni Co In Mg La Ni Co Al Mg La Ni Co Mg

cycle number

Fig. 8 e Relative hydrogen diffusivity factors of the tested

electrode materials versus cycle number.

Fig. 9 e Exchange current density of the H2O/H2 system for

the tested electrode materials versus cycle number.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 4 2 3e2 4 2 92428

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 4 2 3e2 4 2 9 2429

feasible reason of this increase seems to be a continuous

development of active surface with cycling. The tendency for

ioH2O=H2to stabilize may result from appearance of corrosion

products (oxide phases) at individual particles that inhibit

both hydrogen transport and charge transfer at the interfacial

areas.

4. Conclusion

This paper confirms the earlier observations that the final

composition and hydrogen storage properties of the Mg con-

taining REeNi based alloys are strongly affected not only by

the metallurgical process used in the alloys manufacturing

but also by the subtle details of their synthesis. Nevertheless,

from the application point of view the so-called reversible

hydrogen capacity (RHC) is of great importance. The RHCs

estimated in this paper, are quite close to the Qel,disch values

measured directly from electrochemical experiments. The

analysis of the mentioned RHCs shows that apart from

La2Ni8CoMg the best hydrogen (both gas-phase and electro-

chemical) desorption performance exhibit alloys with stoi-

chiometry of La2Ni8(Co0.8Al0.2)Mg and La2Ni8(Co0.8In0.2)Mg i.e.

those with part of cobalt (1.7 at.%) substituted by Al or In.

Exchange current densities of H2O/H2 system increase with

cycling. The greatest exchange currents (>70mA/g) have been

found for Al-doped alloy. Established structure- and hydro-

genation properties of the described La2(Ni,Co,Mg,M)10-type

composites will allow a better selection and composition

optimization in further development of the NieMH battery

negative electrode materials with improved electrochemical

performance. This optimization includes further sub-

stitutions and examination of their synergistic effects andwill

be a subject of our prospective investigations.

Acknowledgments

The work was supported by Wroclaw Research Centre EITþwithin the project “The Application of Nanotechnology in

Advanced Materials” e NanoMat (POIG.01.01.02-02-002/08) co-

financed by the European Regional Development Fund

(Operational Programme Innovative Economy, 1.1.2).

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