Indian Journal of Chemistry Vol.39A, July 2000, pp. 690-696

Electrochemical characterization of LaNis hydride electrode

R S Achyuthlal Babu', B Viswanathan*' & S Srinivasamurthyh • Department of Chemistry, h Department of Mechanical Engineering

Indian Institute of Technology, Chennai 600 036, India

Received 30 November1999; revised 10 May 2000

Galvanostatic, cyclic voltammetric and linear polarization characteristics of LaNij electrode with respect to cycling are presented. Variation in voltammetric charge-discharge current and potential values observed during cycling are discussed. Cy­clic voltammetric redox currents are correlated with respective steps in P-C isotherms of LaNij - Hz system. The stepwise reactions taking place at the LaNij electrode like in normal gaseous mode of hydrogenation is established. Values of the exchange current density of LaNij electrode increases during cycling. Evolved gas analysis establishes the electrochemical hydrogen storage. Evolution of hydrogen from b-hydride phase takes place at room temperature, while from a-phase, hydrogen evolves at higher temperatures. Microscopic analysis indicates that microcracks in LaNij electrodes appear due to electrochemical cycling.

Research and developments on metal hydrides were initially intended for hydrogen storage purpose alone l . The conventional method of hydrogenation of the promi­nent alloy, namely LaNis can be represented as

. . . ( 1 )

where PH2 i s the equilibrium pressure of hydrogen re­quired for hydr�de formation (plateau pressure) . Continous search on metal hydrides revealed that they possess desired characteristics required for other appli­cations like rechargeable batteries, heat pumps and ca­talysis2-4. Metal hydride based electrodes have received attention because of their characteristics similar to the cadmium electrode in Ni-Cd rechargeable batteries . When compared to the existing cadmium electrode, metal hydride electrodes possess some specific advantages like high storage capacity, low toxicity, good amenability to fast charge-discharge rates, no memory effect and long life. In general, it is convenient to charge and discharge any suitable electrode with small element transfer, like lithium ion transport in lithium batteries.Hydrogen is the smallest element and hence it is convenient to have suit­able reversible hydrogen storage electrode_The hydro­gen transfer responsible for charge storage in metal hy­dride electrode can be as high as I : I atomic ratio with

respect to number of host metal atoms in the lattice (Eq. l ) and hence can have high charge storage capacity.

Metal hydride electrodes are characterized using liner polarization and cyclic voltammetric methods to assess the performance

s- 13• Galvanostatic method is used to es­timate the charge and discharge capacity_ These studies are mainly concerned with the study of variations of charge-discharge rates with respect to alloy composition, surface coating, effect of binder and effect of activation of the electrode. The present study deals with a system­atic investigation of the performance of LaNis electrode.

Materials and Methods LaNis alloy (HYSTOR 205) was procured from

Ergenics Inc. NJ, USA. The alloy was ground mechani­cally in air and sieved through 1 00 mesh.Single phase of the alloy was confirmed by XRD.For electrode appl i­cation, the sieved LaNis powder was mixed with elec­trolytic copper powder (AR grade) in 1 :3 or 1 :4 ratio. For characterization of the electrode, LaNis and copper mixutre was pelletized to a diameter of 1 2 mm and a thickness of about 0.5 mm. The pellet was held on a glass holder using an epoxy adh�sive. One side of the electrode was connected to a leading wire, with silver paint at the point of contact to reduce ohmic resistance. The other side was exposed to the electrolytic solution for electrochemical reaction.

)

BABU et at.: CHARACfERISTICS OF LaNi HYDRIDE ELECTRODE 5 69 1

Galvanostatic measurements were carried out using ELICO potentiostat (Model CL 95). Cyclic voltammetric, l inear polarization and Tafel plot experiments were car­ried out us ing Wenking Instruments ( Wenking Potentioscan POS 73 ) and Wenking log output current sink (Model MLS 8 1 ) . All experiments were carried out with LaNis electrode as working- electrode, Pt foil as counter electrode and Hg/HgO ( in 6M KOH; 0.926 V at 298 K) as reference electrode using 6M (KOH electro­lyte in fully flooded condition in an open cell. Before characterization, the electrode was kept in freshly pre­pared 6M KOH for about 1 2 h for equilibration and con­ditioning.

Evolved gas analysis (EGA) was carried out for elec­trochemically charged pure LaNis (without copper) us­ing quadruple mass spectrometer attached with tempera­ture prog.rammer (Model Baltzers-GAM 44 1 ) . For evolved gas analysis, about 800 mg ofLaNi with one or s two drops of PTFE suspension ( as binder) was pellet-ized and used as working electrode. All other conditions were same as in other experiments. Small amount of

..... LaNis was taken out from electrochemically charged elec­trode in wet condition and loaded into the EGA cham­ber for analysis.

Results and Discussion A typical variation in discharge capacity of LaNi

electrode w ith respect to cycle number obtained b; galvanostatic cycling up to 50 cycles. The electrode was charged galvanostatically at a current of 10 rnA (-0.85 C ) for a total theoretical charge of 500 mAhlg (- 1 20% Cth) . The discharge was carried out up to 650 m V ( vs Hg/HgO) during each cycling. The LaNi electrode got . s activated u� to a saturation point of 308 mAhlg and then degraded on further cycling.

Based on extent of charging and discharging, the elec­trode stability and charge-discharge ranges (cathodic­anodic current ranges) cyclic voltammetric experiments were carried out between - 1 200 and -600 mV (vs Hgi HgO). This potential range is the best-suited range for metal hydride electrode in copper matrix . Because cop­per is inert in this potential range and can act as a well dispersing medium for active LaNi� material, the char­acteristics of LaNis can be easily stUdied.Narrowing of potential ranges may lead to partial charging and dis­charging. Wider ranges may lead to high cathodic or anodic currents. Assuming a six hydrogen atom transfer (or six-electron transfer) per formula unit of alloy, the

reversible reaction that is taking place at LaNis electrode can be represented as,

OH-� LaNi5H� + 60H- EO = -0.828 V

The above reaction, for clarity can be split as

6Hp + 6 e-OH-. LaNis; step I

� (6H)iI(] -60H-

Electrolysis of Hp

OH-, LaNis; step 2 � LaNisHo

+LaNis

HI absorption

_ . . (2)

. - . (3)

Electrochemical hydrogenation of LaNis alloy in­volves an electron transfer reaction and a bulk reaction (hydrogenation-dehydrogenation) in contrast to most other general electrochemical reactions wherein electron transfer reactions alone are involved.

Normal electron transfer reaction can be monitored with high voltammetric scan rates.But diffusion limita­tions of OR ion or Hp in the electrolyte solution or at the surface of the electrode, or hydrogen atom inside the electrode are inherently imposed on bulk reaction, moni­toring for example M-H2 reaction, and hence such reac­tions have to be monitored at slow scan rates.

Electrochemical characteristics of LaNi electrode with respect to number of voltammetric cy�les at 0.2 m V Isec are shown in Fig. l . In the cyclic voltammograms two peaks (cathodic and anodic) corresponding to the hydrogenation and dehydrogenation of LaNi electrode are observed at h igher number of cycles. Thi� indicates the reversible nature of LaNis - H2 reaction. In initial c��les, as the extent of hydrogen absorption i s not sig­mflcant, no perceptible cathodic hydrogenation peak is observed. Same characteristics are observed for L<iNi electrodes scanned at 0. 1 and 0.3 m Vis.

5

At all scan rates, as the number of cycles increases ?�t� anodi� and cathodic current values increase during ImtIal cychng due to pulverization and the creation of fresh active surface of LaNis.It is interesting to note that it is not only the hydrogen

-absorption-desorption cur­

rents, which increase, but the hydrogen evolution cur­rent also during cycling.Normalized values of absolute charge and discharge current values from cyc l ic voltammograms for a l l three-scan rates are plotted ver­sus number of cycles and typical data are given in Fig.2. In all three-scan rates studied, there is an initial increase followed by a decrease of both charge and discharge current values.

692 INDIAN J CHEM, SEC. A, JULY 2000

88

78

65

50 .. 1; 38 E :I c: � 27 u >-

Cathod ic

r6.6 6mA

� c: o Q. ... c: � :; u o � ..

!! «

2000 r---------------------�----, � , cq,y

� Q) co E 1500

M\A � 6-Ad6 A � A A 0 0

(t) (t) � o <0

o 0

0 0.1 mV/s A 0 ·2 m Y 15 0 0 ·3 mV/s

��----�2 5:-�--�5�O��--�7�5------,�OO u .� Cyc l E' num bE'r

14 ... u � II) � Fig.2- Influence of [monomer] on Rp

5

' 4

3

2

-600 -700 Pot �nt i"a l VS H g / HgO (mY)

-1100 -1200

Fig. l - Steady state rate of polymerization. Rp x I ()I mol dm·l S·I versus time in min. [Monomer] = 2.0M, [TBPC] = 0.02M, [PMS] = 0.02M, 11 = O.OSM, temp = SO'C SO for comparison of charge-discharge current val­

ues of different electrodes, it is preferred to compare the current values of different electrodes for same number of cycle or similarly activated electrodes. The initial in­crease in current values is due to the creation of fresh surfaces due to pulverization2 or increase in the extent of hydrogen absorption and desorption. Decrease in charge and discharge currents in latter part of cycling may be due to the creation of inactive surface oxides or hydroxides [Lap3 or La (OH) 3] and the reduction in effective amount of hydrogen storing materiap· 15-20. It is clear that both charging and discharging current values increase with increase in scan rates. This means that the time available under the instantaneous applied potential is not sufficient to either charge or discharge completely, even at such slow scan rates.

[PMS] = 0.02M, [TBPC] = 0.02M, 11 = O.OSM, temp = SO"C Line A: S+ log Rp versus [og[BMA]; line B : 6+ log Rp versus [og[BMAJ

t 1"0

:r . 0.

0l _ 6

f. H y steres'I S C1.. -- 8

t1T 1 \ I I [ I I H MH « 1 \ (X.

H M H (J

�des.

!�bS .

11

M Hoc H I M -

Fig.3- Influence of [PMS] ori 'R p

S o l i d sol. of �

111

jl I I I I I II I : \\ I 1 1\

des�iabS. M H �

[monomer] = 2.0 M, [TBPC] = 0.02M, 11 = O.OSM, temp = SO" C Line A: S+ log Rp versus 3+ [og[PMS]; line B : 6+[og Rp versus 3+ [og[PMS]

It is seen from Fig . 1 that the anodic peak potential moves towards more negative potential but the cathodic peak potential moves towards less negative potential with increasing number of cycles as the electrode surface changes with number of cycles. At all scan rates and for all cycle numbers, there is always a difference between charging and discharging potential values. This is simi-

......

BABU et at.: CHARACIERISTICS OF LaNis HYDRIDE ELECTRODE 693

1\ �1 - 6 00

-

Cyc le n u m be r 1 1 4

F rom B

1·66 mA Zero current

point .

-700 - 900 -1000 -1 1 00 -1200 Pot ent i a l v s H g / H g O ( m V )

Fig.4- Cyclic voltammogram of LaNis electrode at 0.02 m V /sec. The electrode was previously cycled for 1 1 3 cycles at 0.2 m V /sec: Weight of LaNis+Cu = 1 65.3 mg; weight of LaNis = 33 mg .

lar to the difference between the hydrogenation and de­hydrogenation plateau pressures observed in M-H2 re­actions I .2 l .

Variations in peak charge and discharge potentials with cycle number shows that as the electrode is cycled, the electrode is capable of getting charged at lower and lower negative potentials. Because of the overlapping nature of hydrogen absorption potential and hydrogen evolu­tion potential, no clear distinction could be made for charging potential with respect to scan rates . However, as the electrode is cycled, the electrode is capable of discharging at higher negative potentials. Increase in scan rate decreases the anodic peak (negative) potential val­ues. The difference between the charge and discharge potentials of respective cycles decreases with cycling. Similarly, decrease in hysteresis with cycling is seen in gaseous mode of hydrogenation of alloys also, atleast

. during initial cycling22.23 . After first five to ten cycles, a small additional ca­

thodic current is seen in cathodic scan (scanning from -600 mV to - 1200 mV (vs Hg/HgO in 6M KOH) (Fig.2), for all the three scan rates indicated by arrow marks in one of the cyclic voltammograms. In each cycle, a high cathodic current corresponding to bulk hydrogen absorp­tion and hydrogen evolution follows this small cathodic current .

25.-------------� ®

'" E

20

C 15 �. :J v v

-g 10 � o u

- - 67"'cycle

-- - 4 7t hCycle

-26thCycle

Fig.5-Linear ploarisation plots for charging (A) and discharging (B) of LaNis electrode for fully charged states. Weight of LaNis +Cu = 1 5 1 mg; weight of LaNis =30.2 mg.

To explain various steps involved in electrochemical hydrogenation of LaNi5 alloy electrode, one has to con­sider the pressure composition -isothermal (phase dia­gram) characteristics of M (solid)-H2 (gas) system. A typical P-C isotherm of M-H2 system is represented in Fig.3 . There are three distinct regions, namely a, a+� and � regions. The hydrogen content (HIM values) of LaNi5Hx is always considerably less than the hydrogen content of �.;LaNi5Hy (bulk hydride). Formation of a­hydride is spread over a wide pressure range. But the a �� transformation takes place at a constant pressure called plateau pressure. Metal O-hydrogen systems show a difference (hysteresis) between hydride formation and dissociation pressures. Formation of � -hydride phase is preceded by the formation of a-hydride and vice versa during decomposition of hydride phases. Solid solution of �-hydride is normally above 1 atm pressure and the variation in hydrogen content is very low.

In cyclic voltammograms, the initial small cathodic currents are attributed to the formation of a-solid solu­tion of LaNi5-H2 (a-LaNi5HJ The subsequent high ca­thodic currents can be attributed to the formation of �­hydride of LaNiy

694 INDIAN J CHEM, SEC. A, JULY 2000

For clarity, the cathodic scan of one cycle was recorded with higher sensitivity up to - 1 0 rnA of cathodic current from -600 mY. After that the sensitivity is lowered and recorded continuously. The amplified current curve clearly indicates the a-hydride formation with constant current values (Fig. I ) . Same characteristic is observed for electrodes scanned at 0. 1 and 0.3 mY/s. Clear dis­tinction between the anodic currents due to the decom­position of a-hydride and �-hydride is not seen because they overlap at these scan rates .

In P-C isotherms, the HIM values of a-hydride range varies constantly (almost small and constant d(HIM)/dp over a pressure range). Similarly, in voltammograms, a­hydride is formed with constant current values (analogues to constant d(HIM)ldp) over a potential range. In other words, the variation in HIM values with pressure or po­tential is constant.

It is seen that a-hydride formation is seen at lower negative potentials compared to the bulk hydride forma­tion. This means that LaNis acts as an electrocatalyst for electrolysis of water by forming the hydride. It is not only the available electrical potential gradient (applied) that is responsible for electrolysis of water but also the chemical potential gradient (hydrogen content gradient in the LaNis) present in the electrode. Probably, because of the high affinity towards hydrogen, LaNi5 weakens the H-O bond in the water molecule and electrolyses water at lower potentials.

Because d(HIM)ldp for �-hydride is high and because a�� conversion takes place at almost constant pres­sure , high current values are observed in a narrow po­tential range during �-hydride formation.

As the pressure required for the formation of solid solution of �-hydride is higher than I atm and as the rate of formation is slow under the electrochemical charging conditions, no corresponding distinct current profile is observed in cathodic scan. Hydrogen evolves at around I atm at the electrode-electrolyte interface.

The cyclic voltammograms of LaN is electrode re­corded at slow scan rates (0.02 mY/s and 0.03 mYls) are shown in FigA. These cyclic voitammograms, recorded with very slow scan rates, clearly display various steps taking place at the electrode. The hydrogen absorption and desorption corresponding to �-hydride appear at constant potentials (similar to constant pressure in P-C isotherms) with high current values (high HIM values). The potential differences between absorption and desorption stages (the hysteresis) is very low. The small constant anodic current seen over a wide potential range

9

-<­"j' 7 o ,... >< _ 5 c CII ... ... B 3 c o

o 2

8 � -,"-\ I \ I , 7'r- I ' 4. I \

� 6 ,"J \ '0 I \ .- 5 � \ >< \

" 6 8 T i me (mi n )

\ \ \

\ \

'\ "

' ......

- - - 1 . 0

- - - - 2 .0 -- - 1 8 . 0

( A )

---1 .0 --- - 2 .0

(8)

1 , ___ ...... ...... _ _ _ _ _ _ _ _ o l' -1- - -4- - -1 _ _ or =.::::1:=.::

... ...

o 2 " T ' 6 8 10 12 I m e ( min) 6 -

'" '"

.., '"

-_ - - 1 . 0

-- - - 2 · 0

;' /

;' .. "",'"

.,, '"

- "'" 0 +- - ---.:1 _�_ J. _

50 1 0 0 1 5 0 2 0 0 Te mperature ( t C )

/

I I

I

/ I

I ,

(C)

� 2 5 0

Fig.6- (A) Evolved gas profile of galvanostatically charged LaNil under isothermal condition at 303 K.

(B) Evolved gas profile of galvanostatically charged LaNi\ under isothermal condition at 303 K.H2 and H ion curreni profiles only are shown in expanded form from A.

(C) Evolved gas profile of galvanostatically charged LaNi, under programmed heating ( 5° / min) Recorded after iso­thermal analysis (A and B) at 303 K.

-

BABU et at.: CHARACfERISTICS OF LaNi5 HYDRIDE ELECTRODE 695

in Fig.4 are attributed to discharge of hydrogen from a­hydride phase. Formation or charging current of a-solid solution is not as clear as the decomposition current. Overlap of anodic currents corresponding to the desorption of hydrogen from a-phase and p-phase leads to a shoulder in Fig.4. At slightly higher scan rate, the shoulder disappears. So like in normal gaseous mode of hydrogenation (gas-solid reaction) electrochemical hy­drogenation also takes place through a stepwise reac­tion as given below

OH-, E. OR, E. LaNi5 + x'Hp+x'e- � a-LaNisHx• � p-LaNisHx .....

where x '« x"

Kinetic studies

-x· OH- +x"H20+x"e--x'OH-

Linear polarisation plots of charging and discharging at completely charged states are given as a function of cycle number in Fig.5. Before recording the kinetic plots, the electrodes were charged completely by scanning voltammetrically from -600 m V to - 1 200 m V. As the discharge current involves only discharge and charging current can involve both charging and hydrogen evolu­tion currents, the kinetic plots corresponding to charg­ing and discharging are represented separately. There is a linear correlation between the current density and over­potential of both charging and discharging as required by the equation,

I = I ( TlFIR1) II

The values of exchange current density were calcu­lated from the values of slopes of these plots and are given along with the kinetic plots. The values of exchange current densities corresponding to both charging and dis­charging increase with cycle number as the electrode surface is changing with respect to number of cycles. This means that LaNis electrode becomes more amena­ble to fast charge and discharge as the cycle number in­creases, provided the discharge capacity does not de­crease drastically. So for comparison, it is preferred to compare the kinetics of electrodes cycled for same number of cycle or similarly activated electrodes or fully activated electrodes.

Evolved gas analysis (EGA) Figure 6 shows the evolved gas profi les of

galvanostatically charged electrode recorded. As the sam-

pIe from electrode is taken out in wet condition for EGA current corresponding to mle = 1 8 is also seen. The presence of ion current at m/e=2 confirms the electrochemical storage of hydrogen in LaNis electrode. The evolution profile of hydrogen (mle set at 2) and water (mle set at 1 8) with time and temperature are shown in Fig.6. At room temperature, only one distinct hydrogen evolution peak is seen. Evolution of hydrogen at room temperature is attributed to the decomposition of hydride phase. But when the sample is heated, two more hydro­gen evolution profiles appear - one starting with the evaporation of water at around 373 K and the other above this temperature. These two peaks are attributed to the evolution of hydrogen from a-phase. At present it is not clear how the presence of water or PTFE in LaN is affect the hydrogen evolution profiles. However, it is to � noted that the glass transition point and melting point of PTFE are -398 K and 600 K respectively. A separate electrode was cycled by ten galvanostatic cycles and then dis­charged completely. This galvanostatically-discharged electrode was then charged potentiostatically at 865 m V vs Hg/HgO for 2 h wherein only the a-phase is formed (Figs ! and 4). Evolution of hydrogen from this electrode at room temperature is feeble. But the evolved gas analy­sis of this electrode shows the evolution of hydrogen at two different high temperatures, which corresponds to the decomposition of a-hydride. Continuos cyclic voltammetric measurements lead to microcracks in the electrode as seen from scanning electron micrographs.

Conclusion

The appearance of cathodic and anodic peaks after few cycles corresponding to charging (or hydrogenation) and discharging (or dehydrogenation) of LaNis indicates the reversible nature of the LaNis-H2 reaction. Both ca­thodic and anodic current densities increase with cycling till they reach a maximum value and then decreases on further cycling, corresponding to the activation and degradation.So, for comparison of anodic and cathodic current values of different electrodes, it is preferred to compare them at the same state of activation (i.e. in terms of cycle number) or in fully activated state. The cathodic and anodic peak potentials vary with cycling. Cathodic charging peak potential decreases but the anodic dis­charging peak potential increases with cycle number, which indicates that the electrode tends towards equi­librium state due to cycling.Electrochemical hydrogena­tion takes place in a stepwise manner. Similar to hyster­esis seen in gaseous mode of hydrogenation-dehydro-

696 INDIAN J CHEM, SEC. A, JULY 2000

genation, electrochemical hydrogenation and dehydro­genation corresponding to the hydride transformation takes place at two potential ranges.The values of ex­change current density increase with cycling. Exchange current density values can be compared of the same state of activation of the electrode in terms of cycle number or similarly activated electrodes or fully activated elec­trode. Tafel equation is not applicable due to the com­plexities of various steps involved in high overpotential values. Evolved gas analysis shows three different hy­drogen desorption profiles. Hydrogen evolution at room temperature is due to hydrogen from �- phase.Evolution of hydrogen at high temperature is attributed to hydro­gen evolution from a-phase.

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