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Electrode kinetics of nickel hydroxide in alkaline solution
Elcctmchimica
Acta. 1971, Vol. 16, pp. 833 to 843. Pcrgatnon Press. Printed in Northern Ireland
ELECTRODE KINETICS OF NICKEL HYDROXIDE IN ALKALINE SOLUTION* Z. TAKEHARA, M. KATO and S. YOSHIZAWA Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto, Japan Abstract-The eIectrode kinetics of a nickel hydroxide electrode and effects of Lif ions and rare-earth compounds were studied by the means of observation of decay and growth of polarization and measurement of electrode impedance. From the experimental results, the rate-determining step of the charge and discharge reaction is considered to be the diffusion process of protons and/or defects in the hydroxide layer. The values of %%/I# or 40. L in the hydroxide layer can be obtained from theoretical treatment of the observed data. These values may be used as the measure of activity of the nickel-hydroxide electrode. By addition of Li+ ions in the electrolyte, the values of 40. L were increased for charge, but slightly decreased for the discharge. Such phenomena may be explained as the effect of the Li+ ion, which has lower valency than the nickel ion in nickel hydroxide, an n-type semiconductor during charge and p-type semiconductor during discharge. And then, by addition of rare-earth compounds to the electrolyte solution, the values of d/D . L were slightly increased for charge and discharge, probably due to the increase of active centres on the electrode surface.
R&sum&-La cinktique de l’ilectrode d’hydroxide de nickel et aussi les effets des ions Li+ et des compos& d’616ments des terres rares ont 6tC &udib en mesurant les d&sint&grationet augmentation de la polarisation et Ia imp&lance d’&ctrode. Les rksultats confirment que 1’Ctage dkterminant la r&action de charge et d&charge est le processus de diffusion des protons et/au leurs dhfauts dans la couche d’hydroxide. On peut obtenir les valeurs de 1/D/+ ou do _ L dam la couche d’hydroxide par le traitement thkorique des rdsultats expdrimentaux. Ces valeurs sont utilids pour la mesure de la activit6 de la &&rode d’hydroxide de nickel. Par addition des ions Li+ dans la solution, les valeurs de 40. L ont augment& en cas de la charge, tar&s que dimin& faiblement en cas de la d&charge. Tels lea ph&om&nes peuvent &re expliquks par l’effet des ions Li+ qui ont mains de valence que les ions Ni*+ dans l’hydroxide de nickel qui est le semicondicteur de type-n en cas de la charge et celui de type-p en cas de la dkcharge. Par additions des composBs d’el6ments des terres rares dans la solution, les valeurs de 45 . L ont augment6 faiblement non seulement pour la charge, mais aussi pour la d&charge, probablement dii a I’augmentation des centres activiteurs sur la surface d’&ctrode.
Zusammenfassung-Die Elektrodenkinetik an Nickelhydroxyd-Electroden und die Einfliisse von Li+Ponen auf die Verbindungen Seltene Erden-Metalle wurden mit der Methoden der Verfolgung des zeitlichen Auf- und Abbaues der Polarisation und der Messung der Elektrodenimpedanz studiert. Gema den experimentellen Ergebnissen wird als der bestimmende Schritt fiir die Ladungs- und Entladungsreaktion die Diffusion von Prrtonen und/oder der Verlust von Protonen in der Hydroxydschicht angesehen. Die Werte von d/o/+ oder do . L in der Hydroxydschicht k&men von der theoretischen Behandlung der beobachteten Daten erhalten werden. Diese Werte kSnnen als ein Mass fiir die AktivitSit der Nickelhydroxyd-Elektrode benutzt werden. Durch die AuRasung von Li+-Ionen im Elektrolyt nahmen die Werte von 1/ 5. L fiir die Ladung zu und die fiir die Entladung etwas ab. Diese Erscheinungen kiinnen durch den Einflti von Li+-Ionen, die eine kleinere Valenz als NickelIonen in den Nickelhydroxyd-Elektroden des p-Halbleiters fiir die Ladung und des n-Halbleiters fiir die Entladung haben, erklLrt werden. Durch die Auf&ung von Verbindungen Seltene Erden-Metalle im Elektrolyt nahmen dann die Werte von d/D. L fi.ir die Ladung und Entladung etwas zu. Diese Erscheinungen kiinnen durch die Zunahme von aktiven Zentren auf der Elektrodenobefiche erkllrt werden. * Presented at the 17th meeting of CITCE, 1969; as amended, 19 January 1970.
Tokyo, September 1966; 833
manuscript received 18 July
834
2. TAKE-,
M. KATO and S. YOSHIZAWA
INTRODUCTION
known that the charge and discharge reaction of a nickel-hydroxyide in the alkaline battery isl+
IT IS well
Ni(II)
7
charge
Ni(II1)
discharge
+ H+ 7
//\ 0
OH
charge
Ni(IV)
discharge
electrode
+ 2 Hf.
(1)
PA 0
0
The reaction progressing on the electrode surface may be considered as follows, H+,+
OH-a01-s
charge
I%0 + q + e-2
(2)
OH;, is OH- ion in aqueous where, H+# is proton in solid phase (= Ni(OH)& electrolyte, 0 is defect of proton in solid phase (= NiOOH or NiO,) and e- is electron. Thus, equilibrium potential at constant concentration of electrolyte is expressed as follows
where L, and Ln are equivalent fractions of 0 and H+, at the electrode surface per unit area, respectively. In order to ascertain the above reaction mechanisms and to aquire the ratedetermining step, electrode kinetics of nickel hydroxide electrode and effects of Lit ions and rare-earth compounds were studied. EXPERIMENTAL
TECHNIQUE
The nickel-hydroxide electrodes were prepared as follows: the nickel plate (3 ems) or nickel wire (dia 0.5 mm, length 20 mm) were electroplated at 50°C 16-7 mA/cm2 for 30 min in a solution of pH 5.6-6.2 containing NiSO,*7 H,O (150 g/l), NiCI,+ H,O (15 g/l) and H&W, (15 g/l) was electroplated by cathodic and anodic cyclic treatments at 25”C, 2 mA/cma for 30 min (1-8: repeated period, 30 s, 9-14: repeated period, 1 min, 15-20: repeated period, 2 min, 21-22: repeated period, 4 min) in a solution containing NiSO,.7 Hz0 (75 g/l), CH&OONa (150 g/l) and NaOH (added till the formation of a white precipitate), and then, was repeated cyclic charge and discharge treatment of 4th time at 30°C O-33 mA/cm2 in 4 N KOH. Solutions were prepared with extra pure KOH and LiOH and COs2- was removed with Ba(OH),. In the study of the effects of rare-earth compounds, these compounds, l-5 g (purity > 99.9 %), such as Ce(OH),, Pr60,r and Nd(OH), were added so that the solution was saturated. The potentials US Hg/HgO/l N KOH of the plate electrode of nickel hydroxide during charge and discharge at constant cd in alkaline solution were observed in the electrolytic cell previously described,5 and the time dependences on potential after opening the circuit in charge and discharge were observed and analysed as mentioned later. The electrode impedances under polarization due to ac (I = i sin WC, i = 3 x 10W2mA/cm%) of the wire electrodes were observed by the use of the previous technique.6 From changes of potential more rapid than 1 ,us, the ohmic resistances in the eIectrode and electrolyte solution were calculated, and from these ohmic resistance
Electrode kinetics of nickel hydroxide in alkaline solution
835
and the Lissajous figures at the impedance measurement, the faradaic impedances as a simple series resistance and capacitance (R and C) were calculated. RESULTS
As shown in Fig. 1, the open-circuit potentials at steady state are changed gradually in the course of charge and discharge. The potential changes slowly, Fig. 1, suggesting that the diffusion of proton in the electrode is the rate-determining step 0.6
/i/--rL A’
A
$
0.5 -
g
<0
0.4
I”2 >
3
2
-
03 0
1 /I-
5
---
/---
4
P
3’
-
2’
t’ 10
15
20
0
5
IO
8’
Time,
min
Time, min
FIG. 1. Time/potential curves during constant current charge or discharge and potential decay after opening the circuit. A, charge; A’, after stopping current B, discharge B’, after stopping current - - - -, Steady state potential after opening the circuit 4 N KOH 30°C. Charge or discharge cd, 0.33 mA/cnP.
in the charge and discharge reaction ; agitation of solution scarcely affects the time dependences. As shown in Fig. 2, the decay of potential after opening the circuit changes with the temperature ; the potential decay becomes smaller and faster with the increase of temperature. Figure 3 shows that the overpotential increases gradually with the increase of the quantity of charge or discharge; then, in the case of charge, the potential increases, oxygen gas being evolved on the electrode surface. This tendency becomes more rapid at lower temperatures. The overpotential and the potential decay after opening the circuit are scarcely dependent on cd either on charge or discharge, as shown in Fig. 4. These results suggest that the electrode reaction becomes faster with higher cd at comparatively high temperature. Faradaic impedances of the nickel hydroxide electrode under ac polarization with nearly linear are shown in Fig. 5. R and l/wC increase with increase of l/6, relations with l/6; the slopes of these relations decrease with temperature. The slope of ~/WC is steeper than that of R.
2. Tm,
836
1 0
I 2
M. K&o
I 4
Time,
and S.
YOSHIZAWA
I
I
t
6
8
IO
min
FIG. 2. Potential decays after opening the circuit after the charge or dischargeof l-67 mA . min/cmainvarious electrolytetemperatures. A, charge. B, discharge: a, 5°C; b, 30°C; c, 50°C. 4 N KOH. Charge or dischargecd, O-33 mA/cm*.
Time,
min
3. Relation between charge or discharge time and over potenti& at various temperatures. A, charge. B, discharge; a, 5°C; b, 30°C; c, 50°C. 4 N KOH. Charge or dischargecd, O-33mA/un~. Fkc.
By addition of Li+ ions to the solution, the overpotential decreases during charge and slightly increases during discharge, as shown in Fig. 6. The discharge capacity becomes larger, due to larger formation of NiOOH or NiO, during charge. The same effects were found with Ce(OH),, Pr,O, and Nd(OH), added to the solution. These effects were the more marked at the lower cds of charge and discharge. DISCUSSION Analysis from potential decay after opening the circuit
From the above-mentioned results, it is suggested that the rate-determining step of the charge and discharge reaction is the diffusion process of proton and its defect in the electrode. Therefore, we assume that the electrode is kept at the partial
Electrode kinetics of nickel hydroxide in alkaline solution
837
0.20
Capacity,
mA.min/cm’
Time,
min
4. Relation between charge or discharge time and the overpotential, and potential decays after opening the circuit after the charge or discharge of 1.67 mA . min/cmsat various cd. A, charge; A’, after stopping current. B, discharge; B’, after stopping current. a, O-33mA/cnP; b, 0.67 mAlcma; c, 1.33 mA/cm8. 4 N KOH. 30°C. FIG.
equilibrium expressed by (3) as mentioned above. We shall deduce the theoretical equation for the potential decay after opening the circuit in charge. The defects of proton (0) are formed on the electrode surface by reaction (2) during the charge process and these defects diffuse into the interior of electrode, Fig. 7. First, assuming the one-dimensional diffusion of proton into the interior
3.0-
Fro. 5. Relations between l/V% and R or l/wC at various temperatures calculated for R and C in series. A, 0°C; B, 50°C. --o_, R; --t_, I/WC. 4 N KOH.
838 g
Z. TAKBHARA, M. KATO and S. YOSIUZAWA
0.7.
0.20
Time,
min
Time,
min
6. Effects of Li+ ion on time/potential curves and relation between charge or dischargetime and over-potential, A, charge; B, dischar e. a, 4 N KOH; b, O-1 N
FIG.
LiOH + 3.9 N KOH; and infinite diffusion,
c, 1 N LiOH + 3 N KOH; d, 4 R LiOH. dischargecd, O-33 mA/em*.
the equivalent
30°C. Charge or
of the defects on the electrode surface
fraction
can be expressed ass L&L
= (2$/V%)
* (v5F
-
(4)
A&),
where L is the sum of L, and LH, 4 the equivalent fraction of the defect or proton per unit area and unit time (i/U’), D the diffusion coefficient, t the time measured after opening of the circuit and 7 the charge or discharge period. At the electrode surface, &fL,=L. (5) From (3), (4) and (5) E = E, -
(RT/F)
. In [{(~/295)/~
-
fi))
-
11.
(6)
As mentioned above, (6) is derived on the assumption that the thickness of the electrode is infinite, but, practically, the thickness is finite. With the above assumption, the equivalent of the defect on the electrode must be zero after infinite time, but this equivalent can approach actually to a certain value, L, (equivalent of proton defect in equilibrium). Therefore, next it must be assumed that the portion which takes part in diffusion corresponds to t,/L where the diffusion is carried out as the case of infinite diffusion. HZ0 Charge
/ H+-
-0 -
OH-
eElectrolyte sqlution
Electrode e-----c-
H+
H+-cOH-
Disc horge
LI RG.
\ H2O
X
0
7. Section of the electrode (Schematic).
839
Electrode kinetics of nickel hydroxide in alkaline solution
Thus (6) is converted E = E,, -
(RTjF)
to (7), . In [((V%/Z+)/(dt
+ T-
t,, -
l/t + d<)}
11,
-
(7)
where to is the time at which the potential after closing the circuit is equal to the steady state potential after opening the circuit. For the discharge process, the following equation for the potential decay after opening the circuit is obtained in the same way, E = E,, + (RT/F)
. In [((fi/2#)/(4r
+ T-
t,, -
v”? + l/sl;>} -
l].
(8)
TABLE~.~COVERYOFPOTENT~AL~TIMB~OPBNIMG THECIRCUIT 4 N KOH. Charge or discharge : l-67 mA . min/cm*. 30°C Current mA/cm*
Time after opening circuit slfP min L, equiv/cm* - slrp]
Electrode potential (mV vs potential at t = 0) Observed Calculated
G/+, [d/o.
Charge 27.9 19.55 x lo-*]
0.33
1.33
21.7 P-99 x 10-71
Discharge 0.33
43.3 [1.48x lo-‘]
18.6 12.56 x lo-‘]
1.33
As shown in Table 1, the observed
1 2 3 4 2:
-24.2 -30.0 -33-7 -350 -56.5 -36.7
-24.3 -29.7 -33-6 -36.1 -54.9 -38.0
1 2 3 4 205
-26.1 -30.0 -32.2 -33.7 -41-5 -34.7
-254 -29.6 -32.5 -345 -41.0 -36.1
:
15.6 18-O
14.2 17-o
;: 5 20
19.2 18.9 19-5 19.5
19.8 18.7 20.8 22.3
1
26-5 29-o 29*9 29-9 29.9 29.9
25-2 28.0 29-l 30.0 31.2 36.7
: 4 5 20
data from the potential change after opening
the circuit agree fairly well with (7) and (8). be obtained.
The values of 1/z/+
These values may be used as the measure
and $6
of activity
. L can
of the nickel
hydroxide electrodes. The values of l/o . L are increased with the increase of temperature, as shown in Fig. 8(a). From these temperature-dependences of D . L2 and assumption of temperature independency of L, the apparent activation energy for diffusion of defect and proton can be calculated to be 5.0 Kcal/equiv for the charge and 3-2 Kcaljequiv for the discharge, Fig. 9. The plots in Fig. 9 are not linear at comparatively high temperature. It shows that multi-diffusion layers at steady state polarization may exist. From detinition L=
effective surface area equivalent X apparent surface area unit volume
thickness of unit cell X
unit length
.
Z. TAKEIG#A, M. KATO and S. YOSIUZAWA
840
Temperature,
0
0.2
0.4
‘C
0.6
Cd.
mA/cm’
and d/o. L in various temperature(a), and at various cd (b). a, charge; b, discharge. 4 N KOH. 30°C. Charge or discharge, l-67 mA . min/cms.
FIG. 8. 1/z/+
IO-‘3
3.0
3.2
I/T
3.4
x 103,
3.6 I/OK
FIG. 9. Relations between 1JTand log (LX)*. a, values obtained from the potential decay (cd, O-33 mA/cm*); b, values obtained from the faradaic impedance (max cd, 0.03 mA/cm’). 4 N KOH.
Therefore, L is proportional effective surface area of the electrode per apparent unit L are increased with the increase of cd, Fig. 8 (b). 6 area. The values of 16. and/or I, are increased with the increase of cd. This phenomenon may be due to the increases of the lattice defect in the electrode and the effective surface area caused by the increase of cd.
841
Electrode kineticsof nickel hydroxidein alkaline solution
Analysis from the measurement of faradaic impedance of the electrode An electrode having proton defects of constant equivalent fraction L,/L is polarized by sine wave current (I = i sin cot, i = FL&) and the defects are formed and then removed from the electrode surface as in the model of Fig. 7. Assuming the one-dimensional diffusion of the defects in the electrode and infinite diffusion, the equivalent of defects on the electrode surface can be expressed L,
(s)
= L, +
sin (tir -
T) ,
(9)
and LH
=
L -
L,
(-$!S)
-
sin (WI -
a) +
On the electrode surface polarized by ac, the partial equilibrium expressed by (3) may not be kept. At the electrode reaction with mass-transfer i = i.
crF(E -
LH
L - L, exp
Eeq - i&j RT
1 (a -
Ln -
L e exp
l)F(E - I&, RT
iR,l)
I) ,
t11j
where i. is the exchange cd, CC the transfer coefficient, Ees the equilibrium potential at LH = L - L,, L, = L, and Rel the ohmic resistance in the electrode. When (E - E,, - iR,,I < RT/aF [or
i.
LH -
-L3
=a + L.
F(E - E,, RT
iRel)
WI
By the replacement of (9) and (10)
(13) Therefore,
E -
E,, =
+L
wr -
WL (14)
Assuming a simple series resistance R and capacitance l/c&, (15)
(16) 12
Z. TAKEHARA, M. KATO
842
and S. YOSHIZAWA
Equations (15) and (16) show that R and 1/WC have linear relations having the same slope with l/l/z. Figure 5 shows that the slope of I/WC is steeper than that of R. This may occur because of the changes of R,i and/or i,, due to the change of frequency of the sine-wave current. At high frequency, an insulating zone may be produced in the electrode and Rel may be increased. If Rel and/or i,, is not changed, the frequencydependence of R may be shown by dotted line in Fig. 5. Surface area may be increased with the increase of frequency of sine-wave current. From the application of (15) and (16) to the dotted line in Fig. 5, the values of i0 and 2/o. L are obtained as shown in Table 2.
Temperature “C
3: 50
18
High frequency
125 232 262
mA/cm*
Low frequency
154 322 415
2/z . L (8ssuming L,IL = O-5) equiv/cm* - &/a
3-06 x IO-8 4.28 x lo-” 5.26 x lo-’
The exchange cd is comparatively large, and these results suggest that the diffusion of the defect and/or proton in the electrode is the rate-determining step. From comparison of Table 2 and Fig. 8, the values of l/o impedance are of the same order as those obtained temperature-dependence of D . La in Fig. 9, and dependency of L, the apparent activation energy proton can be calculated to be about 3.9 Kcal/equiv. as the values obtained from the potential decay.
. L obtained from the faradaic from the potential decay. From assumption of temperature-infor diffusion of the defect and This value is also the same order
The effects of various additives With addition of Li+ ions to the solution, the values of 2/o. L potential decay are increased for the charge, but slightly decreased for as shown in Fig. 10(a). For example, in the solution containing 1 N LiOH the apparent activation energy for diffusion of the defect and proton
obtained from the discharge, and 3 NKOH, obtained from
temperature-dependence of G . L is 3.9 Kcal/equiv for the charge and 3.8 Kcal/equiv for the discharge. By addition of Li+ ions, the charge activation energy decreases and the discharge activation energy increases in comparison with those in 4 N KOH. According to the measurement of Seebeck effect by Tuomi,’ NiOOH and NiOz formed by charge are n-type semiconductors. Products formed by discharge are p-type semiconductors containing Ni(IV), Ni(III) and Ni(II). According to X-ray analysis by Tuomi .’ Li+ ions are easily introduced in the crystal lattice of the nickel-hydroxide electrode. Therefore, by addition of Li+ ions to the solution, a nickel-hydroxide electrode having Li+ ions is formed, and since Li+ ions have lower valency than nickel ions, the diffusion rate of the defect and proton in the electrode may well be increased during charge and decreased during discharge. By addition of rare-earth slightly increased for charge
compounds to the solution, the values of 6 and discharge, Fig. 10(b). The increase of fi
. I, are . L is
Electrode kinetics of nickel hydroxide in alkaline solution
843
0.08
0.08
(b)
(a 1 >
O-06
E fg
O-44
s E ,” 0
0.02
0
0
I 4.0
i 3.0
I 2.0
KOH.
I I.0
I 0
_
N
1 0
I-O
2.0
LiOH. Concentration
“d -10 x
3.0
N of
5
0
0.1
O-2
0.3
Cd.
0.4
0.5
0.6
0.7
mA /cm2
electrolyte
FIG. 10. Effects of variousadditiveson over-potentials and 45. L.(a), effcctof Lifions; (b), effectof rare-earthcompoundsa, charge; b, discharge. 1, no additive; 2, saturated with Ce(OH)*; 3, saturatedwith PreOll; 4, saturatedwith Nd(OH), 4 N ICOH (in (b)) 30°C. Charge or dischargecd, O-33 mA/cm*. Charge or discharge, l-67 mA . min/cm*.
due to the increase of D and/or L. In both charge and discharge, the same effects are observed, and the relation between the quantity of charge or discharge and the steady-state potential after opening the circuit is not changed by the addition of rare earth compounds. These facts suggest that rare-earth compounds are not introduced into the electrode. The results may be considered due to the change of effective surface area, and the increase of G . L may be due to the increase of surface concentrations of proton and defect, ie active centres. These effects are considered to be so important for performance of alkaline batteries that more study of the mechanism is necessary. 1. 2.
3. 4. 5.
REFERENCES G. W. D. BRIGGS, E. JONESand W. F. K. WYNNE-JONES, Trans. Faraday Sue. 51,1433 (1955). G. W. D. BRIoGs, G. W. S~orr and W. F. K. WYNNE-JONFB, Electrochim.Actu 7,241 (1962). D. TUOMI,J. electrochem. Sue. 112, 1 (1965). E. J. CASEY,A. R. DUBOIS.P. E. LAKE and W. J. MOROZ, J. electrochem. Sot. 112, 371 (1965). S. YOSHUAWA and Z. TAKEHARA,Efectrochim. Actu 5,240 (1961). Z. TAKBHARA,Y. NAMBAand S. YOSIIIZAWA, Electrochim. Acta 13, 1395 (1968).
6. 7. D. TUOMI, J. electrocfiem. Sot. 112, 371 (1965).
Acta. 1971, Vol. 16, pp. 833 to 843. Pcrgatnon Press. Printed in Northern Ireland
ELECTRODE KINETICS OF NICKEL HYDROXIDE IN ALKALINE SOLUTION* Z. TAKEHARA, M. KATO and S. YOSHIZAWA Department of Industrial Chemistry, Faculty of Engineering, Kyoto University, Kyoto, Japan Abstract-The eIectrode kinetics of a nickel hydroxide electrode and effects of Lif ions and rare-earth compounds were studied by the means of observation of decay and growth of polarization and measurement of electrode impedance. From the experimental results, the rate-determining step of the charge and discharge reaction is considered to be the diffusion process of protons and/or defects in the hydroxide layer. The values of %%/I# or 40. L in the hydroxide layer can be obtained from theoretical treatment of the observed data. These values may be used as the measure of activity of the nickel-hydroxide electrode. By addition of Li+ ions in the electrolyte, the values of 40. L were increased for charge, but slightly decreased for the discharge. Such phenomena may be explained as the effect of the Li+ ion, which has lower valency than the nickel ion in nickel hydroxide, an n-type semiconductor during charge and p-type semiconductor during discharge. And then, by addition of rare-earth compounds to the electrolyte solution, the values of d/D . L were slightly increased for charge and discharge, probably due to the increase of active centres on the electrode surface.
R&sum&-La cinktique de l’ilectrode d’hydroxide de nickel et aussi les effets des ions Li+ et des compos& d’616ments des terres rares ont 6tC &udib en mesurant les d&sint&grationet augmentation de la polarisation et Ia imp&lance d’&ctrode. Les rksultats confirment que 1’Ctage dkterminant la r&action de charge et d&charge est le processus de diffusion des protons et/au leurs dhfauts dans la couche d’hydroxide. On peut obtenir les valeurs de 1/D/+ ou do _ L dam la couche d’hydroxide par le traitement thkorique des rdsultats expdrimentaux. Ces valeurs sont utilids pour la mesure de la activit6 de la &&rode d’hydroxide de nickel. Par addition des ions Li+ dans la solution, les valeurs de 40. L ont augment& en cas de la charge, tar&s que dimin& faiblement en cas de la d&charge. Tels lea ph&om&nes peuvent &re expliquks par l’effet des ions Li+ qui ont mains de valence que les ions Ni*+ dans l’hydroxide de nickel qui est le semicondicteur de type-n en cas de la charge et celui de type-p en cas de la dkcharge. Par additions des composBs d’el6ments des terres rares dans la solution, les valeurs de 45 . L ont augment6 faiblement non seulement pour la charge, mais aussi pour la d&charge, probablement dii a I’augmentation des centres activiteurs sur la surface d’&ctrode.
Zusammenfassung-Die Elektrodenkinetik an Nickelhydroxyd-Electroden und die Einfliisse von Li+Ponen auf die Verbindungen Seltene Erden-Metalle wurden mit der Methoden der Verfolgung des zeitlichen Auf- und Abbaues der Polarisation und der Messung der Elektrodenimpedanz studiert. Gema den experimentellen Ergebnissen wird als der bestimmende Schritt fiir die Ladungs- und Entladungsreaktion die Diffusion von Prrtonen und/oder der Verlust von Protonen in der Hydroxydschicht angesehen. Die Werte von d/o/+ oder do . L in der Hydroxydschicht k&men von der theoretischen Behandlung der beobachteten Daten erhalten werden. Diese Werte kSnnen als ein Mass fiir die AktivitSit der Nickelhydroxyd-Elektrode benutzt werden. Durch die AuRasung von Li+-Ionen im Elektrolyt nahmen die Werte von 1/ 5. L fiir die Ladung zu und die fiir die Entladung etwas ab. Diese Erscheinungen kiinnen durch den Einflti von Li+-Ionen, die eine kleinere Valenz als NickelIonen in den Nickelhydroxyd-Elektroden des p-Halbleiters fiir die Ladung und des n-Halbleiters fiir die Entladung haben, erklLrt werden. Durch die Auf&ung von Verbindungen Seltene Erden-Metalle im Elektrolyt nahmen dann die Werte von d/D. L fi.ir die Ladung und Entladung etwas zu. Diese Erscheinungen kiinnen durch die Zunahme von aktiven Zentren auf der Elektrodenobefiche erkllrt werden. * Presented at the 17th meeting of CITCE, 1969; as amended, 19 January 1970.
Tokyo, September 1966; 833
manuscript received 18 July
834
2. TAKE-,
M. KATO and S. YOSHIZAWA
INTRODUCTION
known that the charge and discharge reaction of a nickel-hydroxyide in the alkaline battery isl+
IT IS well
Ni(II)
7
charge
Ni(II1)
discharge
+ H+ 7
//\ 0
OH
charge
Ni(IV)
discharge
electrode
+ 2 Hf.
(1)
PA 0
0
The reaction progressing on the electrode surface may be considered as follows, H+,+
OH-a01-s
charge
I%0 + q + e-2
(2)
OH;, is OH- ion in aqueous where, H+# is proton in solid phase (= Ni(OH)& electrolyte, 0 is defect of proton in solid phase (= NiOOH or NiO,) and e- is electron. Thus, equilibrium potential at constant concentration of electrolyte is expressed as follows
where L, and Ln are equivalent fractions of 0 and H+, at the electrode surface per unit area, respectively. In order to ascertain the above reaction mechanisms and to aquire the ratedetermining step, electrode kinetics of nickel hydroxide electrode and effects of Lit ions and rare-earth compounds were studied. EXPERIMENTAL
TECHNIQUE
The nickel-hydroxide electrodes were prepared as follows: the nickel plate (3 ems) or nickel wire (dia 0.5 mm, length 20 mm) were electroplated at 50°C 16-7 mA/cm2 for 30 min in a solution of pH 5.6-6.2 containing NiSO,*7 H,O (150 g/l), NiCI,+ H,O (15 g/l) and H&W, (15 g/l) was electroplated by cathodic and anodic cyclic treatments at 25”C, 2 mA/cma for 30 min (1-8: repeated period, 30 s, 9-14: repeated period, 1 min, 15-20: repeated period, 2 min, 21-22: repeated period, 4 min) in a solution containing NiSO,.7 Hz0 (75 g/l), CH&OONa (150 g/l) and NaOH (added till the formation of a white precipitate), and then, was repeated cyclic charge and discharge treatment of 4th time at 30°C O-33 mA/cm2 in 4 N KOH. Solutions were prepared with extra pure KOH and LiOH and COs2- was removed with Ba(OH),. In the study of the effects of rare-earth compounds, these compounds, l-5 g (purity > 99.9 %), such as Ce(OH),, Pr60,r and Nd(OH), were added so that the solution was saturated. The potentials US Hg/HgO/l N KOH of the plate electrode of nickel hydroxide during charge and discharge at constant cd in alkaline solution were observed in the electrolytic cell previously described,5 and the time dependences on potential after opening the circuit in charge and discharge were observed and analysed as mentioned later. The electrode impedances under polarization due to ac (I = i sin WC, i = 3 x 10W2mA/cm%) of the wire electrodes were observed by the use of the previous technique.6 From changes of potential more rapid than 1 ,us, the ohmic resistances in the eIectrode and electrolyte solution were calculated, and from these ohmic resistance
Electrode kinetics of nickel hydroxide in alkaline solution
835
and the Lissajous figures at the impedance measurement, the faradaic impedances as a simple series resistance and capacitance (R and C) were calculated. RESULTS
As shown in Fig. 1, the open-circuit potentials at steady state are changed gradually in the course of charge and discharge. The potential changes slowly, Fig. 1, suggesting that the diffusion of proton in the electrode is the rate-determining step 0.6
/i/--rL A’
A
$
0.5 -
g
<0
0.4
I”2 >
3
2
-
03 0
1 /I-
5
---
/---
4
P
3’
-
2’
t’ 10
15
20
0
5
IO
8’
Time,
min
Time, min
FIG. 1. Time/potential curves during constant current charge or discharge and potential decay after opening the circuit. A, charge; A’, after stopping current B, discharge B’, after stopping current - - - -, Steady state potential after opening the circuit 4 N KOH 30°C. Charge or discharge cd, 0.33 mA/cnP.
in the charge and discharge reaction ; agitation of solution scarcely affects the time dependences. As shown in Fig. 2, the decay of potential after opening the circuit changes with the temperature ; the potential decay becomes smaller and faster with the increase of temperature. Figure 3 shows that the overpotential increases gradually with the increase of the quantity of charge or discharge; then, in the case of charge, the potential increases, oxygen gas being evolved on the electrode surface. This tendency becomes more rapid at lower temperatures. The overpotential and the potential decay after opening the circuit are scarcely dependent on cd either on charge or discharge, as shown in Fig. 4. These results suggest that the electrode reaction becomes faster with higher cd at comparatively high temperature. Faradaic impedances of the nickel hydroxide electrode under ac polarization with nearly linear are shown in Fig. 5. R and l/wC increase with increase of l/6, relations with l/6; the slopes of these relations decrease with temperature. The slope of ~/WC is steeper than that of R.
2. Tm,
836
1 0
I 2
M. K&o
I 4
Time,
and S.
YOSHIZAWA
I
I
t
6
8
IO
min
FIG. 2. Potential decays after opening the circuit after the charge or dischargeof l-67 mA . min/cmainvarious electrolytetemperatures. A, charge. B, discharge: a, 5°C; b, 30°C; c, 50°C. 4 N KOH. Charge or dischargecd, O-33 mA/cm*.
Time,
min
3. Relation between charge or discharge time and over potenti& at various temperatures. A, charge. B, discharge; a, 5°C; b, 30°C; c, 50°C. 4 N KOH. Charge or dischargecd, O-33mA/un~. Fkc.
By addition of Li+ ions to the solution, the overpotential decreases during charge and slightly increases during discharge, as shown in Fig. 6. The discharge capacity becomes larger, due to larger formation of NiOOH or NiO, during charge. The same effects were found with Ce(OH),, Pr,O, and Nd(OH), added to the solution. These effects were the more marked at the lower cds of charge and discharge. DISCUSSION Analysis from potential decay after opening the circuit
From the above-mentioned results, it is suggested that the rate-determining step of the charge and discharge reaction is the diffusion process of proton and its defect in the electrode. Therefore, we assume that the electrode is kept at the partial
Electrode kinetics of nickel hydroxide in alkaline solution
837
0.20
Capacity,
mA.min/cm’
Time,
min
4. Relation between charge or discharge time and the overpotential, and potential decays after opening the circuit after the charge or discharge of 1.67 mA . min/cmsat various cd. A, charge; A’, after stopping current. B, discharge; B’, after stopping current. a, O-33mA/cnP; b, 0.67 mAlcma; c, 1.33 mA/cm8. 4 N KOH. 30°C. FIG.
equilibrium expressed by (3) as mentioned above. We shall deduce the theoretical equation for the potential decay after opening the circuit in charge. The defects of proton (0) are formed on the electrode surface by reaction (2) during the charge process and these defects diffuse into the interior of electrode, Fig. 7. First, assuming the one-dimensional diffusion of proton into the interior
3.0-
Fro. 5. Relations between l/V% and R or l/wC at various temperatures calculated for R and C in series. A, 0°C; B, 50°C. --o_, R; --t_, I/WC. 4 N KOH.
838 g
Z. TAKBHARA, M. KATO and S. YOSIUZAWA
0.7.
0.20
Time,
min
Time,
min
6. Effects of Li+ ion on time/potential curves and relation between charge or dischargetime and over-potential, A, charge; B, dischar e. a, 4 N KOH; b, O-1 N
FIG.
LiOH + 3.9 N KOH; and infinite diffusion,
c, 1 N LiOH + 3 N KOH; d, 4 R LiOH. dischargecd, O-33 mA/em*.
the equivalent
30°C. Charge or
of the defects on the electrode surface
fraction
can be expressed ass L&L
= (2$/V%)
* (v5F
-
(4)
A&),
where L is the sum of L, and LH, 4 the equivalent fraction of the defect or proton per unit area and unit time (i/U’), D the diffusion coefficient, t the time measured after opening of the circuit and 7 the charge or discharge period. At the electrode surface, &fL,=L. (5) From (3), (4) and (5) E = E, -
(RT/F)
. In [{(~/295)/~
-
fi))
-
11.
(6)
As mentioned above, (6) is derived on the assumption that the thickness of the electrode is infinite, but, practically, the thickness is finite. With the above assumption, the equivalent of the defect on the electrode must be zero after infinite time, but this equivalent can approach actually to a certain value, L, (equivalent of proton defect in equilibrium). Therefore, next it must be assumed that the portion which takes part in diffusion corresponds to t,/L where the diffusion is carried out as the case of infinite diffusion. HZ0 Charge
/ H+-
-0 -
OH-
eElectrolyte sqlution
Electrode e-----c-
H+
H+-cOH-
Disc horge
LI RG.
\ H2O
X
0
7. Section of the electrode (Schematic).
839
Electrode kinetics of nickel hydroxide in alkaline solution
Thus (6) is converted E = E,, -
(RTjF)
to (7), . In [((V%/Z+)/(dt
+ T-
t,, -
l/t + d<)}
11,
-
(7)
where to is the time at which the potential after closing the circuit is equal to the steady state potential after opening the circuit. For the discharge process, the following equation for the potential decay after opening the circuit is obtained in the same way, E = E,, + (RT/F)
. In [((fi/2#)/(4r
+ T-
t,, -
v”? + l/sl;>} -
l].
(8)
TABLE~.~COVERYOFPOTENT~AL~TIMB~OPBNIMG THECIRCUIT 4 N KOH. Charge or discharge : l-67 mA . min/cm*. 30°C Current mA/cm*
Time after opening circuit slfP min L, equiv/cm* - slrp]
Electrode potential (mV vs potential at t = 0) Observed Calculated
G/+, [d/o.
Charge 27.9 19.55 x lo-*]
0.33
1.33
21.7 P-99 x 10-71
Discharge 0.33
43.3 [1.48x lo-‘]
18.6 12.56 x lo-‘]
1.33
As shown in Table 1, the observed
1 2 3 4 2:
-24.2 -30.0 -33-7 -350 -56.5 -36.7
-24.3 -29.7 -33-6 -36.1 -54.9 -38.0
1 2 3 4 205
-26.1 -30.0 -32.2 -33.7 -41-5 -34.7
-254 -29.6 -32.5 -345 -41.0 -36.1
:
15.6 18-O
14.2 17-o
;: 5 20
19.2 18.9 19-5 19.5
19.8 18.7 20.8 22.3
1
26-5 29-o 29*9 29-9 29.9 29.9
25-2 28.0 29-l 30.0 31.2 36.7
: 4 5 20
data from the potential change after opening
the circuit agree fairly well with (7) and (8). be obtained.
The values of 1/z/+
These values may be used as the measure
and $6
of activity
. L can
of the nickel
hydroxide electrodes. The values of l/o . L are increased with the increase of temperature, as shown in Fig. 8(a). From these temperature-dependences of D . L2 and assumption of temperature independency of L, the apparent activation energy for diffusion of defect and proton can be calculated to be 5.0 Kcal/equiv for the charge and 3-2 Kcaljequiv for the discharge, Fig. 9. The plots in Fig. 9 are not linear at comparatively high temperature. It shows that multi-diffusion layers at steady state polarization may exist. From detinition L=
effective surface area equivalent X apparent surface area unit volume
thickness of unit cell X
unit length
.
Z. TAKEIG#A, M. KATO and S. YOSIUZAWA
840
Temperature,
0
0.2
0.4
‘C
0.6
Cd.
mA/cm’
and d/o. L in various temperature(a), and at various cd (b). a, charge; b, discharge. 4 N KOH. 30°C. Charge or discharge, l-67 mA . min/cms.
FIG. 8. 1/z/+
IO-‘3
3.0
3.2
I/T
3.4
x 103,
3.6 I/OK
FIG. 9. Relations between 1JTand log (LX)*. a, values obtained from the potential decay (cd, O-33 mA/cm*); b, values obtained from the faradaic impedance (max cd, 0.03 mA/cm’). 4 N KOH.
Therefore, L is proportional effective surface area of the electrode per apparent unit L are increased with the increase of cd, Fig. 8 (b). 6 area. The values of 16. and/or I, are increased with the increase of cd. This phenomenon may be due to the increases of the lattice defect in the electrode and the effective surface area caused by the increase of cd.
841
Electrode kineticsof nickel hydroxidein alkaline solution
Analysis from the measurement of faradaic impedance of the electrode An electrode having proton defects of constant equivalent fraction L,/L is polarized by sine wave current (I = i sin cot, i = FL&) and the defects are formed and then removed from the electrode surface as in the model of Fig. 7. Assuming the one-dimensional diffusion of the defects in the electrode and infinite diffusion, the equivalent of defects on the electrode surface can be expressed L,
(s)
= L, +
sin (tir -
T) ,
(9)
and LH
=
L -
L,
(-$!S)
-
sin (WI -
a) +
On the electrode surface polarized by ac, the partial equilibrium expressed by (3) may not be kept. At the electrode reaction with mass-transfer i = i.
crF(E -
LH
L - L, exp
Eeq - i&j RT
1 (a -
Ln -
L e exp
l)F(E - I&, RT
iR,l)
I) ,
t11j
where i. is the exchange cd, CC the transfer coefficient, Ees the equilibrium potential at LH = L - L,, L, = L, and Rel the ohmic resistance in the electrode. When (E - E,, - iR,,I < RT/aF [or
i.
LH -
-L3
=a + L.
F(E - E,, RT
iRel)
WI
By the replacement of (9) and (10)
(13) Therefore,
E -
E,, =
+L
wr -
WL (14)
Assuming a simple series resistance R and capacitance l/c&, (15)
(16) 12
Z. TAKEHARA, M. KATO
842
and S. YOSHIZAWA
Equations (15) and (16) show that R and 1/WC have linear relations having the same slope with l/l/z. Figure 5 shows that the slope of I/WC is steeper than that of R. This may occur because of the changes of R,i and/or i,, due to the change of frequency of the sine-wave current. At high frequency, an insulating zone may be produced in the electrode and Rel may be increased. If Rel and/or i,, is not changed, the frequencydependence of R may be shown by dotted line in Fig. 5. Surface area may be increased with the increase of frequency of sine-wave current. From the application of (15) and (16) to the dotted line in Fig. 5, the values of i0 and 2/o. L are obtained as shown in Table 2.
Temperature “C
3: 50
18
High frequency
125 232 262
mA/cm*
Low frequency
154 322 415
2/z . L (8ssuming L,IL = O-5) equiv/cm* - &/a
3-06 x IO-8 4.28 x lo-” 5.26 x lo-’
The exchange cd is comparatively large, and these results suggest that the diffusion of the defect and/or proton in the electrode is the rate-determining step. From comparison of Table 2 and Fig. 8, the values of l/o impedance are of the same order as those obtained temperature-dependence of D . La in Fig. 9, and dependency of L, the apparent activation energy proton can be calculated to be about 3.9 Kcal/equiv. as the values obtained from the potential decay.
. L obtained from the faradaic from the potential decay. From assumption of temperature-infor diffusion of the defect and This value is also the same order
The effects of various additives With addition of Li+ ions to the solution, the values of 2/o. L potential decay are increased for the charge, but slightly decreased for as shown in Fig. 10(a). For example, in the solution containing 1 N LiOH the apparent activation energy for diffusion of the defect and proton
obtained from the discharge, and 3 NKOH, obtained from
temperature-dependence of G . L is 3.9 Kcal/equiv for the charge and 3.8 Kcal/equiv for the discharge. By addition of Li+ ions, the charge activation energy decreases and the discharge activation energy increases in comparison with those in 4 N KOH. According to the measurement of Seebeck effect by Tuomi,’ NiOOH and NiOz formed by charge are n-type semiconductors. Products formed by discharge are p-type semiconductors containing Ni(IV), Ni(III) and Ni(II). According to X-ray analysis by Tuomi .’ Li+ ions are easily introduced in the crystal lattice of the nickel-hydroxide electrode. Therefore, by addition of Li+ ions to the solution, a nickel-hydroxide electrode having Li+ ions is formed, and since Li+ ions have lower valency than nickel ions, the diffusion rate of the defect and proton in the electrode may well be increased during charge and decreased during discharge. By addition of rare-earth slightly increased for charge
compounds to the solution, the values of 6 and discharge, Fig. 10(b). The increase of fi
. I, are . L is
Electrode kinetics of nickel hydroxide in alkaline solution
843
0.08
0.08
(b)
(a 1 >
O-06
E fg
O-44
s E ,” 0
0.02
0
0
I 4.0
i 3.0
I 2.0
KOH.
I I.0
I 0
_
N
1 0
I-O
2.0
LiOH. Concentration
“d -10 x
3.0
N of
5
0
0.1
O-2
0.3
Cd.
0.4
0.5
0.6
0.7
mA /cm2
electrolyte
FIG. 10. Effects of variousadditiveson over-potentials and 45. L.(a), effcctof Lifions; (b), effectof rare-earthcompoundsa, charge; b, discharge. 1, no additive; 2, saturated with Ce(OH)*; 3, saturatedwith PreOll; 4, saturatedwith Nd(OH), 4 N ICOH (in (b)) 30°C. Charge or dischargecd, O-33 mA/cm*. Charge or discharge, l-67 mA . min/cm*.
due to the increase of D and/or L. In both charge and discharge, the same effects are observed, and the relation between the quantity of charge or discharge and the steady-state potential after opening the circuit is not changed by the addition of rare earth compounds. These facts suggest that rare-earth compounds are not introduced into the electrode. The results may be considered due to the change of effective surface area, and the increase of G . L may be due to the increase of surface concentrations of proton and defect, ie active centres. These effects are considered to be so important for performance of alkaline batteries that more study of the mechanism is necessary. 1. 2.
3. 4. 5.
REFERENCES G. W. D. BRIGGS, E. JONESand W. F. K. WYNNE-JONES, Trans. Faraday Sue. 51,1433 (1955). G. W. D. BRIoGs, G. W. S~orr and W. F. K. WYNNE-JONFB, Electrochim.Actu 7,241 (1962). D. TUOMI,J. electrochem. Sue. 112, 1 (1965). E. J. CASEY,A. R. DUBOIS.P. E. LAKE and W. J. MOROZ, J. electrochem. Sot. 112, 371 (1965). S. YOSHUAWA and Z. TAKEHARA,Efectrochim. Actu 5,240 (1961). Z. TAKBHARA,Y. NAMBAand S. YOSIIIZAWA, Electrochim. Acta 13, 1395 (1968).
6. 7. D. TUOMI, J. electrocfiem. Sot. 112, 371 (1965).