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Discharge behaviour of plane nickel hydroxide electrodes
DISCHARGE
BEHAVIOUR OF PLANE ELECTRODES G.
DAVOLIO
NICKEL
HYDROXIDE
and E. SORAGNI
lstituto di Chimica Fisica, UmversitA di Modena, Via Campi n. 183, 41100 Modena,
Ilaly
(Rectrioed 5 May 1982) Abstract-Plane nickel oxideelectrodes were prepared by electrochemicaldeposition of nickel hydroxide on flexible nickel strips. Compact and adherent deposits were obtained, with thickness up to 4 x lo-’ em. They were cycled in 6 M KOH solution and, owing to the flexibility of the supporting nickel strips, did not show detaching. The results obtained from galvanostatic discharge curves fit the Peukert equation i”t = constant, riving an n value of 1.23 for the 1 x lo-’ em thick film and a value of 1. IO for the 4 x 10m3 cm thick film. The &ffuGon coefficients of moving species are in the order of magnitude of IO- 5-1Om6 cm’s_ ‘; the rate of discharge does not seem to be limited by proton diffusion. The proton diffusion in solid phase is not the only factor to be taken into account in the discharge process: diffusion and migration of OH- and H,O species play an important role in plane thick nickel oxide film discharge.
carried out in a glass cell with a polymethylmetacrylate electrode holder; the auxiliary electrode was a nickel wire; a Luggin capillary placed near the electrode surface was connected to an Hg/HgO reference electrode in the same solution (6 M KOH). The constant current was imposed by an AMEL Galvanostat (Mod. 555), the voltage between working and reference electrode was recorded (through a high impedence AMEL Electrometer mod. 667/KM) by a high speed recorder obtaining the voltage-time curves. The electrodes were cycled in 6 M KOH, charged at - 1C rate, then discharged al constant current.
INTRODUCTION Improvements in high-rate discharge behaviour of porous nickel hydroxide plaques can be achieved by using conducting substrates as felts[l] or reticular foams[2] which have coarser porosity than the well known nickel carbonyl powder sintered plates. Coarse structures show lower concentration polarization and a better ratio between active mass and inactive support, thus leading to a higher energy density; owing to their lower surface area, these plates require thicker deposits in order to attain the same capacity density as sintered plates. For example, sintered nickel plates exhibit a specific area of - 0.4 m2 cm 3 and a specific capacity of - 0.3 Ah crnm3 with a nickel hydroxide deposit 1 x lo-* cm thick[3]; the same capacity density is obtained by reticular foam structures with a thickness of deposit ranging from 1 x IO-' to 4 x 10e3 cm. The purpose of this work is to investigate the role played by thickness (up to 4 x IO- ’ cm) in the discharge behaviour of plane nickel hydroxide electrodes, the final aim being to predict the performance of a coarse porous electrode.
RESULTS
AND
DISCUSSION
Galvanostatic E- I curves of nickel hydroxide electrode discharge show the typical potential plateau. The capacity (Q) delivered at low current density (C/5) reaches 100% of the available capacity (Q,J; the behaviour of various discharge rates of two electrodes with deposit thicknesses 1.0 x 10e3 cm and 4.0 in Figs 1 and 2 where the x 10-3 cm is reported electrode potential 0.7 Hg/HgO is plotted against the percentage of utilization of active material (C %). In the graph of Fig. 3 the percentage of utilization is plotted us the discharge rate C expressed as h-’ (A Ah- ‘): up to 1OC rate the utilization is unaffected by thickness; at higher rates the thinner deposits (l-2 fall in the utilization x 10-j cm) show a gradual attaining about 50 7; at a rate of about 50 C, whereas the utilization percentage of the thickest deposit (4 x lO_ 3 cm) drops to 40 “/;.at 30C rate. The results fit well the Peukert equation[4] i”t = C, relating the discharge time (t) at a given current (i) to the delivered capacity (C); n is an empirical constant (n > 1) which is obtained from the slopes of the straight lines of the plot of log i us log t (Fig. 4). In spite of the empirical nature of this relation the deviance of n from unity (n - 1) may be seen to represent the total hindrance to discharge due to all the types of polarization (ohmic, activation, dimusion, migration). The
EXPERIMENTAL Nickel hydroxide was electrochemically precipitated on thin strips of nickel metal or of nickelated steel from hot nickel nitrate hydratated salt [Ni(NO,), .6H,O] by cathodic polarization. By painting one side and the edges of lhe strip with a suitable insulating varnish, it was possible to obtain a controlled deposit surface area (about 1 cm’) on the free side of the electrode. This procedure enables the electrode to be subjected to the charge-discharge cycles without detaching the active material owing to the flexibility of the thin metal support, which can follow the elongation produced by volume variations as the oxidalion state of nickel hydroxide changes. The nickel hydroxide thicknesses investigated were 1.0, 1.5, 2.0 and 4.0 x lo-’ cm. The experiments were 335
336
G. DAVOLIOAND
E.SORAGNI
values of n - 1 are reported in the last column of Table 1; they show an unexpected trend decreasing as the thickness of the deposit increases as if the overall hindrance to discharge would be lower for higher thicknesses. The diffusion coefficient D of moving species was calculated by using the equation proposed by Tysyachnyi and Ksenzhek[S]
where Q is the capacity delivered at the current density is the maximum available charge, 1 is the i, Q,, thickness and D the diffusion coefficient of moving species. Since Q = it we have li if = Q,;-,,,
t=Q,t-,b, Fig. 1. E
DS %
capacity plots. Deposit thickness x lo-” cm. Discharge rate: 206.OC c. .-...), 20.6 C(-‘-.-. lO.OC(-------), 4.1C(----).
1.0 -),
1
thus, plotting t us l/i, we obtain a straight line whose slope gives Q,, and whose intercept gives - (l/30). The experimental data are plotted in Fig. 5, the correlation coefficients are always greater than 0.98. The values of D and Q,,, are reported in Table 1. For the higher thicknesses a satisfactory agreement is observed between experimental (C/5 discharge rate) calculated and theoretical values of Q,,,; at low thicknesses (1.0 and 1.5 x lo- 3 cm) the observed capacities at C/5 are substantiaby higher and this may be attributed to the contributions of the nickel substrate or of a more active first layer of hydroxide: the fact deserves further investigation. The values of D are in the order of magnitude of 10-5-10-” cm’s_’ as for the ions in aqueous solutions since the slow step may be attributed to a process which takes place in solution rather than in the solid phase. The proton diffusion in the solid phase as required by the discharge reaction, NiOOH+H++e-tNi(OH),,
Fig. 2. E US%capacily plots. Deposit thickness4 x lo-’ cm. ), 6.3C Discharge rate: 32.OC(-.-.---), 12.7C((-).
may be the slow step in the case of the reduction of very thin compact layers of nickel hydroxide produced by cathodic polarization from diluted nickel nitrate solutions as found by MacArthur[6] and Briggs and Fleischmann[7] who have found diffusion coefficient values of moving species ranging from lo- lolo-l3 cm2 s-l.
Fig, 3. Plot of % utilization of active mass DS discharge rate. Deposit thickness: 4.0 x 10m’ cm 0, 2.0
v 10-3cmo,
1.5x 10-3cmV,
1.0x 10-3cmA.
Dmcharge behaviour of plane nickel hydroxide electrodes Table Deposit thickness
337
1
Q Ux theor.* (C Cm-2)
Q maxexp.*
(cm x 10’)
(C cm-2)
Q ,,c&d.~ (C cmm2)
(cm2 s-1 x IO”)
Diff. coeff.
1.0 1.5 2.0 4.0
2.5 3.75 5.0 10.0
4.1 4.3 5.5 10.6
3.05 2.90 5.33 10.11
2.5 3.4 5.9 8
n-l
0.235 0.200 0.127 a.099
* Maximum available capacity calculated by volume of deposit assuming density of Ni(OH), = 2.5 gcne3 and molecular weight of Ni(OH), -XH,O ‘= 105[8j. ’ Experimental capacity obtained at C/5 discharge rate. t Maximum available capacity calculated from the slope of f vs l/i plots of Fig. 5.
6-
2-
I .
Fig. 4. Testing of Peukert equation[4]. Deposit thickness: 4.0x 10m3cm l (n =l.O!W) 2.0x 10-3cm r(n = 1.127), 1.5 x 10m3cm r(n x 1.200), 1.0 x 10-3cm A(n x 1.235).
The activation polarization contribution does not seem to be influenced by thickness up to 40% of the discharge: the plot of discharge potentials against the log of discharge current density (Fig. 6) gives a Tafel slope of about 0.060 V. The results show that the discharge process of nickel hydroxide electrode is controlled by the diffusion of OH from the reaction layer to the solution through the reduced active material; the latter isa rather porous
ti
05
0
I/~iml
Fig. 5. Discharge time I 4.0x10~3cmo,2.0x10~3cm
cm’)
vs l/i plot. Deposit
thickness: •,l.OxlO~JcmA.
mass permeable to the electrolytic solutions as may be seen in the SEM photography of the fracture plane of the deposit (Fig. 7). To interpret the results of piane thick nickel hydroxide electrodes, the discharge reaction is written as NiOOH+H,O+e+Ni(OH),+OH-
400-: , I
=%
1
2
log
ih++P
Fig. 6. Discharge potentlal vs log i plols. Deposit thickness: 4.0 x 10 3 cmo l , 2.0 x 10e3 cm q R x 10-j cm AA. Open symbols akr 5 “/,, of urilization. Full symbols after 407, of utilization.
,
1.0
G.DAVOLIOANDE.
Fig.7. SEM mxrograph
&-ig. 8. SEM micrograph
SORAGNI
of nickel hydroxide deposit fracture (5000 X, marker division 1 pm).
of nickel hydroxide deposit plane surface (5000 X, marker division 1 am).
(omitting the reticular water molecules). The reaction involves water diffusion from solution, through the reduced mass, to the oxidized active material and diffusion and migration of OHions produced in reduction reaction in the opposite direction. These processes occur in the solution phase entrapped in the active mass bypassing the tow proton diffusion in the solid phase. The increase of D values and the decrease
of n - 1 values with the deposit thickness may be explained by the appearance of microcracks in the nickel hydroxide active mass during cycling (Fig. 8). These cracks are more developed in the thicker deposits and represent an easier way for diffusion of moving species than the porous active mass. It is concluded that the deposit thicknesses up to 2x 10 ‘cm can be used on coarse substrate with
Discharge
behaviour of plane nickel hydroxide electrodes
acceptable losses in the solid phase even at high discharge rates (up to 50C). Thicker deposits can be satisfactorily used for plaques of high capacity density, but at lower discharge rate (1OC). Acknowledgement-The Centro Strumenti of Modena University is acknowledged for the SEM micrographs. REFERENCES 1. T. S. Turner, N. J. Williams and T. A. Ashcroft, Power Sources 3 (Edited by D. H. Collins) p. 169. Oriel Press, Newcastle-upon-Tyne (1971).
339
2. G. Crespy, R. Schmitt, M. A. Gutjahr and H. Sufferer. Power Sources 7 (Edited by J. Thompson)p. 219. Academic Press, London (1979). 3. P. C. Milner and U. B. T~OIIX+S, Advances iti Ekctrochemisrry and Electrochemical Engineenng (Edited by C. W. Tobias) Vol. 5, p. 1. Interscience, New York (1967). 4. W. Peukert, 2. Elecfrochem. 18, 287 (1897). 5. V. P. Tysyachnyi and 0. S. Ksenzhek. Eirktrukhimiya 12, 1161 (1976). 6. D. M. MacArthur, J. rlectrochem. Sm. 117, 729 (1970). 7. G. W. D. Briggs and M. Fleischmann, Truns. Faraday Sot. 67, 2397. 8. D. M. MacArthur, Power Sources 3 (Edited by D. H. Collins) p. 91. Oriel Press, Newcastle-upon-Tyne (1971).
BEHAVIOUR OF PLANE ELECTRODES G.
DAVOLIO
NICKEL
HYDROXIDE
and E. SORAGNI
lstituto di Chimica Fisica, UmversitA di Modena, Via Campi n. 183, 41100 Modena,
Ilaly
(Rectrioed 5 May 1982) Abstract-Plane nickel oxideelectrodes were prepared by electrochemicaldeposition of nickel hydroxide on flexible nickel strips. Compact and adherent deposits were obtained, with thickness up to 4 x lo-’ em. They were cycled in 6 M KOH solution and, owing to the flexibility of the supporting nickel strips, did not show detaching. The results obtained from galvanostatic discharge curves fit the Peukert equation i”t = constant, riving an n value of 1.23 for the 1 x lo-’ em thick film and a value of 1. IO for the 4 x 10m3 cm thick film. The &ffuGon coefficients of moving species are in the order of magnitude of IO- 5-1Om6 cm’s_ ‘; the rate of discharge does not seem to be limited by proton diffusion. The proton diffusion in solid phase is not the only factor to be taken into account in the discharge process: diffusion and migration of OH- and H,O species play an important role in plane thick nickel oxide film discharge.
carried out in a glass cell with a polymethylmetacrylate electrode holder; the auxiliary electrode was a nickel wire; a Luggin capillary placed near the electrode surface was connected to an Hg/HgO reference electrode in the same solution (6 M KOH). The constant current was imposed by an AMEL Galvanostat (Mod. 555), the voltage between working and reference electrode was recorded (through a high impedence AMEL Electrometer mod. 667/KM) by a high speed recorder obtaining the voltage-time curves. The electrodes were cycled in 6 M KOH, charged at - 1C rate, then discharged al constant current.
INTRODUCTION Improvements in high-rate discharge behaviour of porous nickel hydroxide plaques can be achieved by using conducting substrates as felts[l] or reticular foams[2] which have coarser porosity than the well known nickel carbonyl powder sintered plates. Coarse structures show lower concentration polarization and a better ratio between active mass and inactive support, thus leading to a higher energy density; owing to their lower surface area, these plates require thicker deposits in order to attain the same capacity density as sintered plates. For example, sintered nickel plates exhibit a specific area of - 0.4 m2 cm 3 and a specific capacity of - 0.3 Ah crnm3 with a nickel hydroxide deposit 1 x lo-* cm thick[3]; the same capacity density is obtained by reticular foam structures with a thickness of deposit ranging from 1 x IO-' to 4 x 10e3 cm. The purpose of this work is to investigate the role played by thickness (up to 4 x IO- ’ cm) in the discharge behaviour of plane nickel hydroxide electrodes, the final aim being to predict the performance of a coarse porous electrode.
RESULTS
AND
DISCUSSION
Galvanostatic E- I curves of nickel hydroxide electrode discharge show the typical potential plateau. The capacity (Q) delivered at low current density (C/5) reaches 100% of the available capacity (Q,J; the behaviour of various discharge rates of two electrodes with deposit thicknesses 1.0 x 10e3 cm and 4.0 in Figs 1 and 2 where the x 10-3 cm is reported electrode potential 0.7 Hg/HgO is plotted against the percentage of utilization of active material (C %). In the graph of Fig. 3 the percentage of utilization is plotted us the discharge rate C expressed as h-’ (A Ah- ‘): up to 1OC rate the utilization is unaffected by thickness; at higher rates the thinner deposits (l-2 fall in the utilization x 10-j cm) show a gradual attaining about 50 7; at a rate of about 50 C, whereas the utilization percentage of the thickest deposit (4 x lO_ 3 cm) drops to 40 “/;.at 30C rate. The results fit well the Peukert equation[4] i”t = C, relating the discharge time (t) at a given current (i) to the delivered capacity (C); n is an empirical constant (n > 1) which is obtained from the slopes of the straight lines of the plot of log i us log t (Fig. 4). In spite of the empirical nature of this relation the deviance of n from unity (n - 1) may be seen to represent the total hindrance to discharge due to all the types of polarization (ohmic, activation, dimusion, migration). The
EXPERIMENTAL Nickel hydroxide was electrochemically precipitated on thin strips of nickel metal or of nickelated steel from hot nickel nitrate hydratated salt [Ni(NO,), .6H,O] by cathodic polarization. By painting one side and the edges of lhe strip with a suitable insulating varnish, it was possible to obtain a controlled deposit surface area (about 1 cm’) on the free side of the electrode. This procedure enables the electrode to be subjected to the charge-discharge cycles without detaching the active material owing to the flexibility of the thin metal support, which can follow the elongation produced by volume variations as the oxidalion state of nickel hydroxide changes. The nickel hydroxide thicknesses investigated were 1.0, 1.5, 2.0 and 4.0 x lo-’ cm. The experiments were 335
336
G. DAVOLIOAND
E.SORAGNI
values of n - 1 are reported in the last column of Table 1; they show an unexpected trend decreasing as the thickness of the deposit increases as if the overall hindrance to discharge would be lower for higher thicknesses. The diffusion coefficient D of moving species was calculated by using the equation proposed by Tysyachnyi and Ksenzhek[S]
where Q is the capacity delivered at the current density is the maximum available charge, 1 is the i, Q,, thickness and D the diffusion coefficient of moving species. Since Q = it we have li if = Q,;-,,,
t=Q,t-,b, Fig. 1. E
DS %
capacity plots. Deposit thickness x lo-” cm. Discharge rate: 206.OC c. .-...), 20.6 C(-‘-.-. lO.OC(-------), 4.1C(----).
1.0 -),
1
thus, plotting t us l/i, we obtain a straight line whose slope gives Q,, and whose intercept gives - (l/30). The experimental data are plotted in Fig. 5, the correlation coefficients are always greater than 0.98. The values of D and Q,,, are reported in Table 1. For the higher thicknesses a satisfactory agreement is observed between experimental (C/5 discharge rate) calculated and theoretical values of Q,,,; at low thicknesses (1.0 and 1.5 x lo- 3 cm) the observed capacities at C/5 are substantiaby higher and this may be attributed to the contributions of the nickel substrate or of a more active first layer of hydroxide: the fact deserves further investigation. The values of D are in the order of magnitude of 10-5-10-” cm’s_’ as for the ions in aqueous solutions since the slow step may be attributed to a process which takes place in solution rather than in the solid phase. The proton diffusion in the solid phase as required by the discharge reaction, NiOOH+H++e-tNi(OH),,
Fig. 2. E US%capacily plots. Deposit thickness4 x lo-’ cm. ), 6.3C Discharge rate: 32.OC(-.-.---), 12.7C((-).
may be the slow step in the case of the reduction of very thin compact layers of nickel hydroxide produced by cathodic polarization from diluted nickel nitrate solutions as found by MacArthur[6] and Briggs and Fleischmann[7] who have found diffusion coefficient values of moving species ranging from lo- lolo-l3 cm2 s-l.
Fig, 3. Plot of % utilization of active mass DS discharge rate. Deposit thickness: 4.0 x 10m’ cm 0, 2.0
v 10-3cmo,
1.5x 10-3cmV,
1.0x 10-3cmA.
Dmcharge behaviour of plane nickel hydroxide electrodes Table Deposit thickness
337
1
Q Ux theor.* (C Cm-2)
Q maxexp.*
(cm x 10’)
(C cm-2)
Q ,,c&d.~ (C cmm2)
(cm2 s-1 x IO”)
Diff. coeff.
1.0 1.5 2.0 4.0
2.5 3.75 5.0 10.0
4.1 4.3 5.5 10.6
3.05 2.90 5.33 10.11
2.5 3.4 5.9 8
n-l
0.235 0.200 0.127 a.099
* Maximum available capacity calculated by volume of deposit assuming density of Ni(OH), = 2.5 gcne3 and molecular weight of Ni(OH), -XH,O ‘= 105[8j. ’ Experimental capacity obtained at C/5 discharge rate. t Maximum available capacity calculated from the slope of f vs l/i plots of Fig. 5.
6-
2-
I .
Fig. 4. Testing of Peukert equation[4]. Deposit thickness: 4.0x 10m3cm l (n =l.O!W) 2.0x 10-3cm r(n = 1.127), 1.5 x 10m3cm r(n x 1.200), 1.0 x 10-3cm A(n x 1.235).
The activation polarization contribution does not seem to be influenced by thickness up to 40% of the discharge: the plot of discharge potentials against the log of discharge current density (Fig. 6) gives a Tafel slope of about 0.060 V. The results show that the discharge process of nickel hydroxide electrode is controlled by the diffusion of OH from the reaction layer to the solution through the reduced active material; the latter isa rather porous
ti
05
0
I/~iml
Fig. 5. Discharge time I 4.0x10~3cmo,2.0x10~3cm
cm’)
vs l/i plot. Deposit
thickness: •,l.OxlO~JcmA.
mass permeable to the electrolytic solutions as may be seen in the SEM photography of the fracture plane of the deposit (Fig. 7). To interpret the results of piane thick nickel hydroxide electrodes, the discharge reaction is written as NiOOH+H,O+e+Ni(OH),+OH-
400-: , I
=%
1
2
log
ih++P
Fig. 6. Discharge potentlal vs log i plols. Deposit thickness: 4.0 x 10 3 cmo l , 2.0 x 10e3 cm q R x 10-j cm AA. Open symbols akr 5 “/,, of urilization. Full symbols after 407, of utilization.
,
1.0
G.DAVOLIOANDE.
Fig.7. SEM mxrograph
&-ig. 8. SEM micrograph
SORAGNI
of nickel hydroxide deposit fracture (5000 X, marker division 1 pm).
of nickel hydroxide deposit plane surface (5000 X, marker division 1 am).
(omitting the reticular water molecules). The reaction involves water diffusion from solution, through the reduced mass, to the oxidized active material and diffusion and migration of OHions produced in reduction reaction in the opposite direction. These processes occur in the solution phase entrapped in the active mass bypassing the tow proton diffusion in the solid phase. The increase of D values and the decrease
of n - 1 values with the deposit thickness may be explained by the appearance of microcracks in the nickel hydroxide active mass during cycling (Fig. 8). These cracks are more developed in the thicker deposits and represent an easier way for diffusion of moving species than the porous active mass. It is concluded that the deposit thicknesses up to 2x 10 ‘cm can be used on coarse substrate with
Discharge
behaviour of plane nickel hydroxide electrodes
acceptable losses in the solid phase even at high discharge rates (up to 50C). Thicker deposits can be satisfactorily used for plaques of high capacity density, but at lower discharge rate (1OC). Acknowledgement-The Centro Strumenti of Modena University is acknowledged for the SEM micrographs. REFERENCES 1. T. S. Turner, N. J. Williams and T. A. Ashcroft, Power Sources 3 (Edited by D. H. Collins) p. 169. Oriel Press, Newcastle-upon-Tyne (1971).
339
2. G. Crespy, R. Schmitt, M. A. Gutjahr and H. Sufferer. Power Sources 7 (Edited by J. Thompson)p. 219. Academic Press, London (1979). 3. P. C. Milner and U. B. T~OIIX+S, Advances iti Ekctrochemisrry and Electrochemical Engineenng (Edited by C. W. Tobias) Vol. 5, p. 1. Interscience, New York (1967). 4. W. Peukert, 2. Elecfrochem. 18, 287 (1897). 5. V. P. Tysyachnyi and 0. S. Ksenzhek. Eirktrukhimiya 12, 1161 (1976). 6. D. M. MacArthur, J. rlectrochem. Sm. 117, 729 (1970). 7. G. W. D. Briggs and M. Fleischmann, Truns. Faraday Sot. 67, 2397. 8. D. M. MacArthur, Power Sources 3 (Edited by D. H. Collins) p. 91. Oriel Press, Newcastle-upon-Tyne (1971).