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Impedance studies of nickel hydroxide microencapsulated by cobalt
\ PERGAMON
International Journal of Hydrogen Energy 13 "0888# 862Ð879
Impedance studies of nickel hydroxide microencapsulated by cobalt X[!Y[ Wang \ J[ Yan\ H[!T[ Yuan\ Y[!S[ Zhang\ D[!Y[ Song Institute of New Energy Material Chemistry\ Nankai University\ Tianjin 299960\ People|s Republic of China
Abstract Nickel hydroxide is used as an active material in pasted!type nickel hydroxide electrodes for rechargeable alkaline batteries[ The electrochemical impedance spectroscopes of di}erent nickel hydroxide electrodes were measured[ The results were analyzed using a standard equivalent circuit model including a Warburg impedance term[ The Nyquist plots were used to interpret the characteristics of the di}erent electrodes[ The nickel hydroxide electrode deposited by cobalt has shown lower charge transfer resistance and higher electronic and protonic conductivity[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[ Keywords] Nickel hydroxide electrode^ Electroless cobalt plating^ Electrochemical impedance^ Conductivity
0[ Introduction The nickel hydroxide electrodes have been extensively used as the positive plate of rechargeable battery systems such as nickelÐcadmium\ nickelÐiron\ nickelÐzinc\ nickelÐ hydrogen\ and nickelÐmetal hydrides[ Recent studies have concentrated on the development of rechargeable alkaline batteries with high energy densities ð0Ł[ Oshitani developed a high!performance and low!cost pasted nickel electrodes made from a metal _ber substrate ð1Ł[ In order to improve the cell performance\ such as energy density\ Watanabe studied pasted!type nickel electrodes which were fabricated by _lling the pasted!type of nickel hydroxide as an active material in a nickel foam substrate ð2Ł[ Now\ the active material of most pasted nickel elec! trode is the spherical b!nickel hydroxide acquired by chemical precipitation[ However\ b!nickel hydroxide is a low conductivity p!type semiconductor ð3Ł[ Oshitani found that the conductivity of the nickel electrode decreased with the reduction in the oxidation state during discharge process and considered that the reason was due to poor current conduction between the nickel hydroxide electrode and the substrate ð1Ł[ During discharge\ nick!
Corresponding author
el"II# species accumulated at the hydroxide:electrolyte interface[ These species were insulating and consequently prevented discharge of the crystallite core ð4Ł[ Zimm! erman observed the formation of an isolating boundary layer between the current collector and the uncharged active material by means of impedance spectroscopy and considered that this layer caused the voltage to drop and resistive characteristics of the layer control electrode polarization ð6Ł[ Continual discharge resulted in layer growth and a resultant decrease in voltage[ In order to improve the performance of nickel hydrox! ide electrodes\ there have been numerous studies con! cerning the addition of cobalt to the lattice of nickel hydroxide in the form of cobalt hydroxide ð0\ 4\ 5Ł[ Cobalt has been considered to be the most e}ective in delaying formation of high impedance layers at the current col! lector interface[ Zimmerman and E}a have studied the e}ect of cobalt additives on the kinetics of nickel hydrox! ide electrode with electrochemical impedance spec! troscopy ð6Ł[ Cobalt can reduce the oxidizing and reduc! ing potentials of nickel hydroxide and increase the overpotential of oxygen evolution\ thus improving uti! lization of active material ð7Ł[ Cobalt\ being an element with a variable valence\ is considered to improve the protonic conductivity of nickel hydroxide[ In addition\ during the charge process cobalt can be oxidized into a highly conductive bÐCoOOH\ which henceforth remains
9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 2 9 ! X
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X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
as bÐCoOOH because of the irreversibility of Co"II#:Co"III# and thus improves the electronic con! ductivity and protonic conductivity of nickel hydroxide[ Besides the addition of cobalt into the lattice of nickel hydroxide\ many battery manufacturers still add adequate quantities of cobalt powder to their electrodes as a conductor[ Japanese researchers have developed a foam type nickel electrode in which the active paste "75) Ni"OH# 1\ 09) nickel powder and 3) cobalt powder# is impregnated directly into the highly porous metal foam structure[ However\ the distribution of cobalt powder on the surface of nickel hydroxide is nonuniform\ and thus\ can not e}ectively decrease the contact resistance between nickel hydroxide and the substrate[ Therefore\ in our previous studies ð8Ł we presented a new method of improving electrode performance by electroless cobalt plating at the surface of nickel hydroxide particles and found that the electrode would give excellent perform! ance[ In this paper\ we will study the characteristics of nickel hydroxide deposited by cobalt with elec! trochemical impedance spectroscopy and compare the behaviors of impedance of nickel hydroxide\ nickel hydroxide deposited by cobalt and nickel hydroxide elec! trodes added cobalt powder as a conductor[
1[ Experimental 1[0[ Preparation of electrodes The spherical nickel hydroxide was bÐNi"OH# 1 obtained by chemical precipitation\ which coprecipitated about 0[4 wt ) cobalt in the form of cobalt hydroxide in its lattice[ Electroless cobalt plating was carried out according to our previous study ð8Ł[ Nickel foam "0×0 cm# was used as the nickel hydrox! ide electrode substrate[ A nickel ribbon was spot welded with nickel foam as a current collector[ Three kinds of electrodes were prepared[ 1[0[0[ Electrode A The spherical nickel hydroxide powder was mixed directly with appropriate amount 0 wt) poly! tetra~uoroethylene "PTFE# aqueous suspension and kne! aded to obtain paste[ The paste was _lled in the nickel foam substrate with a spatula\ dried at 59>C for 0 h\ and then pressed at 19 MPa for 0 min to assure a good electronic contact between the substrate and active material[ 1[0[1[ Electrode B The spherical nickel hydroxide powder was uniformly mixed with 4 wt) cobalt metal powder to increase and stabilize the utilization of the active material\ and then an appropriate amount of PTFE was added to the mix!
ture and it was kneaded to obtain a paste[ The rest of the following procedure was the same as for Electrode A[ 1[0[2[ Electrode C The spherical nickel hydroxide powder deposited 4 wt) cobalt at the surface was directly mixed with an appropriate amount of PTFE and kneaded to obtain a paste\ which was treated as for Electrode A[ 1[1[ Electrochemical measurements Prior to experimental use\ the electrodes were immersed in the electrolyte for 13 h at ambient tempera! ture[ The electrolyte consisted of 5 M KOH¦9[5 M LiOH[ Then\ the electrodes were given a constant current charging and discharging for three cycles to oxidize any cobalt present to the Co"III# state and to activate working electrode[ Electrochemical studies were performed in a three!elec! trode thermostatic electrolytic cell at ambient tem! perature "14>C#[ Two nickel hydroxide sheet coun! terelectrodes were placed in the side and the working electrode was positioned in the center\ A Hg:HgO ref! erence electrode was used via luggin capillary\ which was made up in the same alkaline solution as that used in the working cell[ All potentials in this paper were given relative to the Hg:HgO electrode[ AC impedance measurements were made using an EG and G PARC Model 162 Potentiostat:Galvanostat\ a Model 4109 Lock!in!Ampli_es and an IBM computer[ The measurements were made at the open circuit poten! tial with a superimposed 4 mV sinusoidal voltage in the frequency range 09 kHzÐ09 mHz[ Electrochemical Imp! edance Systems Software "Model 277# was used to collect data[ The data of the real and imaginary components obtained experimentally were analyzed using EQUI! VALENT CIRCUIT developed by Boukamp ð09Ł[
2[ Equivalent circuit analysis Various circuits may be proposed to represent the bat! tery impedance and the circuit parameters can be deter! mined by _tting the data to the circuit[ Di}erent portions of the data can be _tted either with a circle or a line in the impedance plane[ An arc in the impedance plane can be represented by a parallel "RQ# circuit\ where Q is a constant phase element "CPE#[ A line in the impedance plane can be represented by a series of RQ circuit[ Fitting was carried out starting at the high frequency end with the data in the impedance form[ The points at the high frequency end were _tted with an arc by partial non!linear least squares _tting[ After removing the high frequency scatter\ the data was _tted with a straight line[ The CPE corresponding to the double layer capacitance was subtracted\ and the data transferred to the impedance
X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
plane[ The series circuit RQ of the charge!transfer resist! ance and the CPE corresponding to the Warburg element was subtracted[ Uniform scatter around the origin was indicative of the good nature of the _t[ A total nonlinear least squares _t was performed on the data using the subtraction code from the subtraction _le[ At the end of the _t\ the data and simulated results were plotted in the impedance plane plot[
3[ Results Fig[ 0 shows the Nyquist plot for Electrode A[ The plot consists of only a depressed semicircle within entire frequency range measured\ indicating the electrode reac! tion is under charge!transfer control[ Fig[ 1 shows the Nyquist plot for Electrode B[ At high frequencies the semicircle is characteristic of the charge! transfer resistance acting in parallel with the double!layer
capacitance[ At intermediate frequencies a straight line having an angle of 34> with the real axis is seen\ being characteristics of the semi!in_nite di}usion[ At still lower frequencies\ the _nite length e}ects are observed and there is a transition from the 34> line toward a vertical line through the transition region\ the angle is about 44>[ Fig[ 2 shows the complex plane impedance plot for Electrode C[ The impedance spectrum consists of a depressed arc with a smaller diameter in the high fre! quency range\ a line inclined at approximately 34> to the real axis in the low frequency range[ The high frequency arc is probably due to the charge transfer reaction and the inclined line in the low frequency range is attributable to Warburg impedance associated with proton di}usion[ But\ the overall plot mainly shows a shape which sub! tends an angle of 34> with the real axis within frequency region studied[ Comparing the plots in Figs 0Ð2\ it is observed that the behaviors of impedance for the three electrodes are di}erent from each other[ The nickel hydroxide electrode "Electrode A# only exhibits a capacitive arc\ but the diam! eter of the capacitive semicircle is apparently larger than that of Electrode B and Electrode C[ This indicates that the reaction occurring at Electrode A is under a charge! transfer control and has very large charge!transfer resist! ance[ Besides\ Electrode B has a linear Warburg portion and a capacitive line at lower frequencies except for capacitive semicircle\ and Electrode C has only a linear Warburg portion and a relatively small charge!transfer resistance[ Generally\ at higher frequencies a semicircle is characteristic of the charge!transfer resistance acting in parallel with the double!layer capacitance ð00Ł[ This means that Electrode A has the largest charge!transfer resistance among three electrodes\ then Electrode B and lastly Electrode C[ At low frequencies the linear Warburg portion due to slow di}usion process can be seen for Electrode B and Electrode C\ whereas in Electrode A can
Fig[ 0[ Nyquist plot of Electrode A[
Fig[ 1[ Nyquist plot of Electrode B[
864
Fig[ 2[ Nyquist plot of Electrode C[
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X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
only be seen a capacitive arc due to poor conductivity of nickel hydroxide[ The Warburg slope of Electrode C is approximately 34>C[ The Warburg slope is interpreted here as an emirical parameter related qualitatively rather than quantitatively to the di}usion resistance where a higher slope signi_es a slower rate of di}usion\ and a low slope a more rapid rate of di}usion ð01Ł[ In general\ for a planar electrode\ the Warburg slope is proportional to 0:CD0:1 and is about 34> where C is the concentration of di}using species and D is the di}usion coe.cient[ This model is satisfactory for simple reactions on planar electrodes but is not adequate for more com! plicated reactions and porous electrodes[ The behavior of an electrode with a porous electrode is more complicated[ Karumathilaka and Hampson ð02Ł considered that the Warburg slope of a porous electrode is between about 11[4> and 34>\ based on characteristics of an electrode with semi!in_nite pores[ It can be seen from Fig[ 2 that the behavior of Electrode C is similar to that of a planar electrode[ As seen from Figs 0Ð2\ the plots at high frequencies in the complex plane are depressed semicircles having a center below the real axis[ Therefore\ to _t the data to an equivalent circuit model containing a constant phase element "CPE# Q should be used[ Its impedance is described ð03\ 04Ł as ZCPE
0 Y" jv ¹ #n
"0#
Where v is the angular frequency in rad s−0\ Y and n are adjustable parameters of CPE[ The value of n 0 corresponds to capacitance\ n 9 corresponds to resist! ance and n 9[4 corresponds to Warburg di}usion[ The plots "a# and "b# in Fig[ 3 represent the equivalent
Fig[ 3[ Equivalent circuits of the nickel hydroxide electrodes for the impedance spectra[
circuit used to represent the processes occurring at the three electrodes to assist in analysing the impedance data[ R0\ R1\ Q0 and Q1 are the ohmic resistance\ charge transfer resistance\ constant phase element representing double layer capacitance and constant phase element rep! resenting Warburg impedance\ respectively[ Fig[ 3"a# is the simplest equivalent circuit taking into account the constant phase element "CPE#[ It shows that ohmic resistance R0 is in series with the parallel con! nection of a constant phase element "CPE# and a faradaic impedance R1[ Fig[ 3"b# represents the equivalent circuit consisting of a semicircle and a Warburg linear portion in the complex plane plot[ Analysis of the experimental data was performed by _tting equivalent circuits ð09Ł[ Fig[ 4 shows experimental and _tted plots of Fig[ 0[ It can be observed that the experimental Nyquist diagram for Electrode A is in good agreement with the _tted diagram according to Fig[ 3"a#[ The results in Fig[ 4 show that the electrode reaction occurring at Electrode A is mainly controlled by charge transfer process[ The experimental impedance data cor! responding to Electrode B and Electrode C in Figs 1 and 2 according to the model shown in Fig[ 3"b# are compared in Figs 5 and 6 with the theoretical impedance response that is expected for this equivalent circuit\ using a non! linear least squares _t procedure to obtain the theoretical transfer function that best approaches experimental results[ As it is evident from Figs 5 and 6\ the equivalent circuit response succeeds to simulate correctly the exper! imental data[ We feel that the models in Fig[ 3"a# and "b# are a good physical approximation to Electrode A\ Electrode B and Electrode C[ Of course\ it is worth noting that none of these equivalent circuit models can represent the true situation in the highly complex porous nickel hydroxide\ but in the absence of an adequate theoretical model\ these models can approximately represent the electrode process occurring at the electrodes[ Based on the preceding models\ Table 0 shows the results\ using non!linear least squares _t\ the data of Nyquist diagrams "EQUIVCRT[PAS program# accord! ing to the plots in Figs 4Ð6[ Comparing the results in Table 0\ it can be found that from Electrode A to Elec! trode C the values of R0 increase gradually and those of R1 decrease gradually\ and for Electrode C\ n0 0 and n1 9[4\ hence this signi_es that Q0 is a perfect capaci! tance and Q1 in Fig[ 3"b# is a perfect Warburg impedance[ At the same time\ these results also indicate that from Electrode A to Electrode C the magnitudes of semicircle at high frequencies decrease apparently\ and at low fre! quencies the linear Warburg region appears more and more apparently^ the behavior of Electrode B is in agree! ment with a porous electrode\ whereas that of Electrode C is similar to a planar electrode[ Therefore\ the elec! trochemical reaction of Electrode C is controlled by charge transfer and Warburg di}usion[ Analogy of Elec!
X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
866
Fig[ 4[ Nyquist plot of impedance data with equivalent circuit R0"R1Q# "in Fig[ 3"a## for Fig[ 0[
Fig[ 5[ Nyquist plot of impedance data with equivalent circuit R0"Q0"R1Q1## "in Fig[ 3"b## for Fig[ 1[
trode C to a planar electrode can ascribe to compress more compactly when the electrode is prepared owing to the existence of a cobalt coating on the surface of the nickel hydroxide[
4[ Discussion The impedance spectra show good agreement between the theoretical predications and the experimental data
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X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
Fig[ 6[ Nyquist plot of impedance data with equivalent circuit R0"Q0"R1Q1## "in Fig[ 3"b## for Fig[ 2[
Table 0 Values of equivalent circuit parameters for di}erent electrodes Electrode
R0:V
R1:V
Y0
n0
Y1
n1
Electrode A Electrode B Electrode C
0[2×09−1 8[6×09−2 0×09−2
9[61 0[4×09−1 4×09−2
9[22 1[7 1
9[7 9[64 0
* 046 29
* 9[64 9[4
obtained with the models shown in Fig[ 3"a# and "b#[ For the nickel hydroxide electrode\ the complex plane plot only shows a capacitive arc at entire frequencies "Fig[ 4#^ while for nickel hydroxide electrodes added cobalt powder as a conductor and deposited cobalt coating\ the complex plane plots exhibit a semicircle and a Warburg linear region "Figs 5 and 6#\ and the impedance behavior ultimately becomes dominated by the Warburg imped! ance[ According to the reported mechanisms ð6\ 05Ł\ the nickel hydroxide electrode reaction follows di}usion kin! etics[ MacArthur and Zimmerman ð6Ł proposed that dur! ing discharge a proton di}uses from the _lmÐelectrolyte interface into the active material and an electron enters across the conducting substrateÐ_lm interface[ During charge the proton di}uses to the _lmÐelectrolyte interface to react with a hydroxyl ion to form water[ Zimmerman described the overall process as follows ð6Ł H1 O \ OH"aq#¦H¦ "s#
"1#
H¦ "s# \ H¦ "s?#
"2#
H¦ "s?#¦NiOOH"s?#¦e"s?# \ Ni"OH# 1 "s?#
"3#
Reaction "1# represents the formation of a proton at catalytic site s at the electrode:electrolyte interface^ reac! tion "2# involves di}usion of the proton from site s into the electrode to the charge transfer site s?^ and reaction "3# is the charge transfer process involving the reduction of one of the higher valence species of active material in the lattice\ represented here as simply NiOOH[ It is well known that the high!charge phase is an n!type sem! iconductor and the low!charge one is an electronic insu! lator or a low conductive p!type semiconductor ð3Ł[ Therefore\ during discharge nickel "II# species accumu! late at the hydroxideÐelectrolyte interface[ These species are insulating or poor conductivity and consequently pre! vent discharge of the crystallite core[ Although the addition of cobalt powder into nickel hydroxide as a conductor can\ to a certain extent\ decrease the contact
X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
resistance between nickel hydroxide particles and foam nickel substrate\ it can not fully increase the utilization of active material due to nonuniform distribution of cobalt powder at the surface of nickel hydroxide particles[ How! ever\ cobalt coating deposited at the surface of nickel hydroxide particles is of uniform distribution and can be oxidized into high conductive cobalt oxyhydroxide during charge\ which henceforth remains as cobalt oxyhydroxide because of irreversibility of Co"II#:Co"II! I#couple ð4Ł[ Thus\ this will provide good electrical con! duction between the nickel hydroxide particles and the substrate[ As previously mentioned\ for Electrode C the Q0 and Q1 in the equivalent circuit in Fig[ 3"b# is similar to a capacitance and a Warburg impedance[ Accordingly\ Q0 and Q1 can be represented by capacitance Cdl and War! burg impedance W\ respectively[ The total impedance\ Z\ of any network is given by ð07Ł Z Rs −
j vCs
"4#
where Rs\ Cs are the e}ective series resistance and capaci! tance\ respectively[ For the equivalent circuit shown in Fig[ 3"b#\ the series resistance Rs may be expressed by R1 ¦ Rs R0 ¦ 1
s zv
1
1 dl
"0¦zvCdl s# ¦v C
0
R1 ¦
"5#
1
s zv
1
b
The capacitance is given by
0
"0¦zvCdl s#¦vCdl R1 ¦
s
1
1
zv zv 0 vCs s 1 1 b "0¦zvCdl s# 1 ¦v1 Cdl R1 ¦ zv
0
"6#
1
where W
0
s zv
−
js
1
zv
RT nFI9
Studies of electrochemical impedance spectra of the three electrodes were performed[ Two equivalent circuits shown in Fig[ 3 were used to _t experimental data[ The experimental data were in good agreement with theor! etical models[ It was clearly shown that the electrode reaction occurring at the nickel hydroxide "Electrode A# was mainly controlled by charge transfer\ while the elec! trode reaction occurring at the nickel hydroxide added cobalt powder as a conductor "Electrode B# and deposited cobalt coating "Electrode C# were controlled by charge transfer and Warburg di}usion[ The behavior of the nickel hydroxide electrode deposited cobalt coating is similar to a planar electrode[ Due to a good electrical contact between nickel hydroxide particles and foam nickel substrate\ provided by cobalt coating\ the electrode reaction has a larger exchange cur! rent density\ and thus\ during charge:discharge process\ the active material of electrode reaction will be fully used and exhibit higher utilization of active material and larger discharge capacity[
"7#
and s the Warburg coe.cient[ The expression for the Warburg coe.cient includes terms containing the concentrations of the reactants and products\ as well as their di}usion coe.cients[ The charge transfer resistance\ R1\ is related to the concentration term through the exchange current density\ since R1
rent density[ The impedance parameters will vary according to the concentration of the reacting and the product species[ The variation of the impedance par! ameters will be prominent in the low frequency region\ because the parameters s and R1 are observed here[ Theoretical analysis of Electrode A and Electrode B can also be carried out based on the above consideration[ According to the results in Table 0 the values of R1 from Electrode A to Electrode C apparently decrease[ It is known from Eq[ "8# that the exchange current density gradually increases[ Hence\ cobalt coating can act as a current collector and promote transfer reaction[ Owing to a good electrical contact provided by cobalt coating the exchange current density of electrode reaction is large enough compared with those of Electrode B so that the electrode reaction is prominently controlled by Warburg di}usion[
5[ Conclusion
0 1
s
868
"8#
Where R is the gas constant\ T is the absolute tempera! ture\ F is the Faraday constant\ n is the number of elec! trons involved in the relation and I9 is the exchange cur!
References ð0Ł Watanabe K\ Koseki M\ Kumagai N[ J Power Sources 0885^47]12[ ð1Ł Oshitani M\ Tufu H\ Takashima K\ Tsuji S\ Matsumaru Y[ J Electrochem Soc 0878^025]0489[ ð2Ł Watanabe K\ Kikuoka T[ J Appl Electrochem 0884^14]108[ ð3Ł Barnard R\ Randell C[F\ Tye FL[ J Appl Electrochem 0873^09]098[ ð4Ł Armstrong RD\ Briggs GWD\ Charles EA[ J Appl Elec! trochem 0877^07]104[ ð5Ł Zimmerman AH[ J Power Sources 0873^01]122[ ð6Ł Zimmerman AH\ E}a\ PK[ J Electrochem Soc 0873^020]698[
879
X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
ð7Ł Barton RT\ Mitchell PJ\ Hampson NA[ Surf Coat Technol 0875^17]0[ ð8Ł Wang XY\ Yan J\ Zhou Z\ et al[ J Power Sources 0887^41]114[ ð09Ł Boukamp BA[ Equivalent Circuit Users Manual[ 1nd ed[ The Netherlands] University of Twente\ 0878[ ð00Ł Musilova M\ Jindra J\ Mrha J\ et al[ J Power Sources 0876^10]56[ ð01Ł Reid MA\ Loyselle PL[ J Power Sources 0877^16]174[ ð02Ł Karunathilaka SAGR\ Hampson NA[ J Appl Electrochem 0870^00]254[
ð03Ł Brug GJ\ Van Den Eeden ALG\ Sluyters!Rehobach M\ et al[ J Electroanal Chem 0873^065]164[ ð04Ł Viswanathan VV\ Salkind AJ\ Kelley JJ\ et al[ J Appl Elec! trochem 0884^14]605[ ð05Ł Motupally S\ Streinz CC\ Weidner JW[ J Electrochem Soc 0884^031]0390[ ð06Ł MacArthur DM[ J Electrochem Soc 0869^006]311[ ð07Ł Murugesamoorthi KA\ Srinivasan S\ Appleby AJ[ J Appl Electrochem 0880^10]84[
International Journal of Hydrogen Energy 13 "0888# 862Ð879
Impedance studies of nickel hydroxide microencapsulated by cobalt X[!Y[ Wang \ J[ Yan\ H[!T[ Yuan\ Y[!S[ Zhang\ D[!Y[ Song Institute of New Energy Material Chemistry\ Nankai University\ Tianjin 299960\ People|s Republic of China
Abstract Nickel hydroxide is used as an active material in pasted!type nickel hydroxide electrodes for rechargeable alkaline batteries[ The electrochemical impedance spectroscopes of di}erent nickel hydroxide electrodes were measured[ The results were analyzed using a standard equivalent circuit model including a Warburg impedance term[ The Nyquist plots were used to interpret the characteristics of the di}erent electrodes[ The nickel hydroxide electrode deposited by cobalt has shown lower charge transfer resistance and higher electronic and protonic conductivity[ Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved[ Keywords] Nickel hydroxide electrode^ Electroless cobalt plating^ Electrochemical impedance^ Conductivity
0[ Introduction The nickel hydroxide electrodes have been extensively used as the positive plate of rechargeable battery systems such as nickelÐcadmium\ nickelÐiron\ nickelÐzinc\ nickelÐ hydrogen\ and nickelÐmetal hydrides[ Recent studies have concentrated on the development of rechargeable alkaline batteries with high energy densities ð0Ł[ Oshitani developed a high!performance and low!cost pasted nickel electrodes made from a metal _ber substrate ð1Ł[ In order to improve the cell performance\ such as energy density\ Watanabe studied pasted!type nickel electrodes which were fabricated by _lling the pasted!type of nickel hydroxide as an active material in a nickel foam substrate ð2Ł[ Now\ the active material of most pasted nickel elec! trode is the spherical b!nickel hydroxide acquired by chemical precipitation[ However\ b!nickel hydroxide is a low conductivity p!type semiconductor ð3Ł[ Oshitani found that the conductivity of the nickel electrode decreased with the reduction in the oxidation state during discharge process and considered that the reason was due to poor current conduction between the nickel hydroxide electrode and the substrate ð1Ł[ During discharge\ nick!
Corresponding author
el"II# species accumulated at the hydroxide:electrolyte interface[ These species were insulating and consequently prevented discharge of the crystallite core ð4Ł[ Zimm! erman observed the formation of an isolating boundary layer between the current collector and the uncharged active material by means of impedance spectroscopy and considered that this layer caused the voltage to drop and resistive characteristics of the layer control electrode polarization ð6Ł[ Continual discharge resulted in layer growth and a resultant decrease in voltage[ In order to improve the performance of nickel hydrox! ide electrodes\ there have been numerous studies con! cerning the addition of cobalt to the lattice of nickel hydroxide in the form of cobalt hydroxide ð0\ 4\ 5Ł[ Cobalt has been considered to be the most e}ective in delaying formation of high impedance layers at the current col! lector interface[ Zimmerman and E}a have studied the e}ect of cobalt additives on the kinetics of nickel hydrox! ide electrode with electrochemical impedance spec! troscopy ð6Ł[ Cobalt can reduce the oxidizing and reduc! ing potentials of nickel hydroxide and increase the overpotential of oxygen evolution\ thus improving uti! lization of active material ð7Ł[ Cobalt\ being an element with a variable valence\ is considered to improve the protonic conductivity of nickel hydroxide[ In addition\ during the charge process cobalt can be oxidized into a highly conductive bÐCoOOH\ which henceforth remains
9259!2088:88:,19[99 Þ 0888 International Association for Hydrogen Energy[ Published by Elsevier Science Ltd[ All rights reserved PII] S 9 2 5 9 ! 2 0 8 8 " 8 7 # 9 9 0 2 9 ! X
863
X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
as bÐCoOOH because of the irreversibility of Co"II#:Co"III# and thus improves the electronic con! ductivity and protonic conductivity of nickel hydroxide[ Besides the addition of cobalt into the lattice of nickel hydroxide\ many battery manufacturers still add adequate quantities of cobalt powder to their electrodes as a conductor[ Japanese researchers have developed a foam type nickel electrode in which the active paste "75) Ni"OH# 1\ 09) nickel powder and 3) cobalt powder# is impregnated directly into the highly porous metal foam structure[ However\ the distribution of cobalt powder on the surface of nickel hydroxide is nonuniform\ and thus\ can not e}ectively decrease the contact resistance between nickel hydroxide and the substrate[ Therefore\ in our previous studies ð8Ł we presented a new method of improving electrode performance by electroless cobalt plating at the surface of nickel hydroxide particles and found that the electrode would give excellent perform! ance[ In this paper\ we will study the characteristics of nickel hydroxide deposited by cobalt with elec! trochemical impedance spectroscopy and compare the behaviors of impedance of nickel hydroxide\ nickel hydroxide deposited by cobalt and nickel hydroxide elec! trodes added cobalt powder as a conductor[
1[ Experimental 1[0[ Preparation of electrodes The spherical nickel hydroxide was bÐNi"OH# 1 obtained by chemical precipitation\ which coprecipitated about 0[4 wt ) cobalt in the form of cobalt hydroxide in its lattice[ Electroless cobalt plating was carried out according to our previous study ð8Ł[ Nickel foam "0×0 cm# was used as the nickel hydrox! ide electrode substrate[ A nickel ribbon was spot welded with nickel foam as a current collector[ Three kinds of electrodes were prepared[ 1[0[0[ Electrode A The spherical nickel hydroxide powder was mixed directly with appropriate amount 0 wt) poly! tetra~uoroethylene "PTFE# aqueous suspension and kne! aded to obtain paste[ The paste was _lled in the nickel foam substrate with a spatula\ dried at 59>C for 0 h\ and then pressed at 19 MPa for 0 min to assure a good electronic contact between the substrate and active material[ 1[0[1[ Electrode B The spherical nickel hydroxide powder was uniformly mixed with 4 wt) cobalt metal powder to increase and stabilize the utilization of the active material\ and then an appropriate amount of PTFE was added to the mix!
ture and it was kneaded to obtain a paste[ The rest of the following procedure was the same as for Electrode A[ 1[0[2[ Electrode C The spherical nickel hydroxide powder deposited 4 wt) cobalt at the surface was directly mixed with an appropriate amount of PTFE and kneaded to obtain a paste\ which was treated as for Electrode A[ 1[1[ Electrochemical measurements Prior to experimental use\ the electrodes were immersed in the electrolyte for 13 h at ambient tempera! ture[ The electrolyte consisted of 5 M KOH¦9[5 M LiOH[ Then\ the electrodes were given a constant current charging and discharging for three cycles to oxidize any cobalt present to the Co"III# state and to activate working electrode[ Electrochemical studies were performed in a three!elec! trode thermostatic electrolytic cell at ambient tem! perature "14>C#[ Two nickel hydroxide sheet coun! terelectrodes were placed in the side and the working electrode was positioned in the center\ A Hg:HgO ref! erence electrode was used via luggin capillary\ which was made up in the same alkaline solution as that used in the working cell[ All potentials in this paper were given relative to the Hg:HgO electrode[ AC impedance measurements were made using an EG and G PARC Model 162 Potentiostat:Galvanostat\ a Model 4109 Lock!in!Ampli_es and an IBM computer[ The measurements were made at the open circuit poten! tial with a superimposed 4 mV sinusoidal voltage in the frequency range 09 kHzÐ09 mHz[ Electrochemical Imp! edance Systems Software "Model 277# was used to collect data[ The data of the real and imaginary components obtained experimentally were analyzed using EQUI! VALENT CIRCUIT developed by Boukamp ð09Ł[
2[ Equivalent circuit analysis Various circuits may be proposed to represent the bat! tery impedance and the circuit parameters can be deter! mined by _tting the data to the circuit[ Di}erent portions of the data can be _tted either with a circle or a line in the impedance plane[ An arc in the impedance plane can be represented by a parallel "RQ# circuit\ where Q is a constant phase element "CPE#[ A line in the impedance plane can be represented by a series of RQ circuit[ Fitting was carried out starting at the high frequency end with the data in the impedance form[ The points at the high frequency end were _tted with an arc by partial non!linear least squares _tting[ After removing the high frequency scatter\ the data was _tted with a straight line[ The CPE corresponding to the double layer capacitance was subtracted\ and the data transferred to the impedance
X[!Y[ Wang et al[ : International Journal of Hydrogen Energy 13 "0888# 862Ð879
plane[ The series circuit RQ of the charge!transfer resist! ance and the CPE corresponding to the Warburg element was subtracted[ Uniform scatter around the origin was indicative of the good nature of the _t[ A total nonlinear least squares _t was performed on the data using the subtraction code from the subtraction _le[ At the end of the _t\ the data and simulated results were plotted in the impedance plane plot[
3[ Results Fig[ 0 shows the Nyquist plot for Electrode A[ The plot consists of only a depressed semicircle within entire frequency range measured\ indicating the electrode reac! tion is under charge!transfer control[ Fig[ 1 shows the Nyquist plot for Electrode B[ At high frequencies the semicircle is characteristic of the charge! transfer resistance acting in parallel with the double!layer
capacitance[ At intermediate frequencies a straight line having an angle of 34> with the real axis is seen\ being characteristics of the semi!in_nite di}usion[ At still lower frequencies\ the _nite length e}ects are observed and there is a transition from the 34> line toward a vertical line through the transition region\ the angle is about 44>[ Fig[ 2 shows the complex plane impedance plot for Electrode C[ The impedance spectrum consists of a depressed arc with a smaller diameter in the high fre! quency range\ a line inclined at approximately 34> to the real axis in the low frequency range[ The high frequency arc is probably due to the charge transfer reaction and the inclined line in the low frequency range is attributable to Warburg impedance associated with proton di}usion[ But\ the overall plot mainly shows a shape which sub! tends an angle of 34> with the real axis within frequency region studied[ Comparing the plots in Figs 0Ð2\ it is observed that the behaviors of impedance for the three electrodes are di}erent from each other[ The nickel hydroxide electrode "Electrode A# only exhibits a capacitive arc\ but the diam! eter of the capacitive semicircle is apparently larger than that of Electrode B and Electrode C[ This indicates that the reaction occurring at Electrode A is under a charge! transfer control and has very large charge!transfer resist! ance[ Besides\ Electrode B has a linear Warburg portion and a capacitive line at lower frequencies except for capacitive semicircle\ and Electrode C has only a linear Warburg portion and a relatively small charge!transfer resistance[ Generally\ at higher frequencies a semicircle is characteristic of the charge!transfer resistance acting in parallel with the double!layer capacitance ð00Ł[ This means that Electrode A has the largest charge!transfer resistance among three electrodes\ then Electrode B and lastly Electrode C[ At low frequencies the linear Warburg portion due to slow di}usion process can be seen for Electrode B and Electrode C\ whereas in Electrode A can
Fig[ 0[ Nyquist plot of Electrode A[
Fig[ 1[ Nyquist plot of Electrode B[
864
Fig[ 2[ Nyquist plot of Electrode C[
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only be seen a capacitive arc due to poor conductivity of nickel hydroxide[ The Warburg slope of Electrode C is approximately 34>C[ The Warburg slope is interpreted here as an emirical parameter related qualitatively rather than quantitatively to the di}usion resistance where a higher slope signi_es a slower rate of di}usion\ and a low slope a more rapid rate of di}usion ð01Ł[ In general\ for a planar electrode\ the Warburg slope is proportional to 0:CD0:1 and is about 34> where C is the concentration of di}using species and D is the di}usion coe.cient[ This model is satisfactory for simple reactions on planar electrodes but is not adequate for more com! plicated reactions and porous electrodes[ The behavior of an electrode with a porous electrode is more complicated[ Karumathilaka and Hampson ð02Ł considered that the Warburg slope of a porous electrode is between about 11[4> and 34>\ based on characteristics of an electrode with semi!in_nite pores[ It can be seen from Fig[ 2 that the behavior of Electrode C is similar to that of a planar electrode[ As seen from Figs 0Ð2\ the plots at high frequencies in the complex plane are depressed semicircles having a center below the real axis[ Therefore\ to _t the data to an equivalent circuit model containing a constant phase element "CPE# Q should be used[ Its impedance is described ð03\ 04Ł as ZCPE
0 Y" jv ¹ #n
"0#
Where v is the angular frequency in rad s−0\ Y and n are adjustable parameters of CPE[ The value of n 0 corresponds to capacitance\ n 9 corresponds to resist! ance and n 9[4 corresponds to Warburg di}usion[ The plots "a# and "b# in Fig[ 3 represent the equivalent
Fig[ 3[ Equivalent circuits of the nickel hydroxide electrodes for the impedance spectra[
circuit used to represent the processes occurring at the three electrodes to assist in analysing the impedance data[ R0\ R1\ Q0 and Q1 are the ohmic resistance\ charge transfer resistance\ constant phase element representing double layer capacitance and constant phase element rep! resenting Warburg impedance\ respectively[ Fig[ 3"a# is the simplest equivalent circuit taking into account the constant phase element "CPE#[ It shows that ohmic resistance R0 is in series with the parallel con! nection of a constant phase element "CPE# and a faradaic impedance R1[ Fig[ 3"b# represents the equivalent circuit consisting of a semicircle and a Warburg linear portion in the complex plane plot[ Analysis of the experimental data was performed by _tting equivalent circuits ð09Ł[ Fig[ 4 shows experimental and _tted plots of Fig[ 0[ It can be observed that the experimental Nyquist diagram for Electrode A is in good agreement with the _tted diagram according to Fig[ 3"a#[ The results in Fig[ 4 show that the electrode reaction occurring at Electrode A is mainly controlled by charge transfer process[ The experimental impedance data cor! responding to Electrode B and Electrode C in Figs 1 and 2 according to the model shown in Fig[ 3"b# are compared in Figs 5 and 6 with the theoretical impedance response that is expected for this equivalent circuit\ using a non! linear least squares _t procedure to obtain the theoretical transfer function that best approaches experimental results[ As it is evident from Figs 5 and 6\ the equivalent circuit response succeeds to simulate correctly the exper! imental data[ We feel that the models in Fig[ 3"a# and "b# are a good physical approximation to Electrode A\ Electrode B and Electrode C[ Of course\ it is worth noting that none of these equivalent circuit models can represent the true situation in the highly complex porous nickel hydroxide\ but in the absence of an adequate theoretical model\ these models can approximately represent the electrode process occurring at the electrodes[ Based on the preceding models\ Table 0 shows the results\ using non!linear least squares _t\ the data of Nyquist diagrams "EQUIVCRT[PAS program# accord! ing to the plots in Figs 4Ð6[ Comparing the results in Table 0\ it can be found that from Electrode A to Elec! trode C the values of R0 increase gradually and those of R1 decrease gradually\ and for Electrode C\ n0 0 and n1 9[4\ hence this signi_es that Q0 is a perfect capaci! tance and Q1 in Fig[ 3"b# is a perfect Warburg impedance[ At the same time\ these results also indicate that from Electrode A to Electrode C the magnitudes of semicircle at high frequencies decrease apparently\ and at low fre! quencies the linear Warburg region appears more and more apparently^ the behavior of Electrode B is in agree! ment with a porous electrode\ whereas that of Electrode C is similar to a planar electrode[ Therefore\ the elec! trochemical reaction of Electrode C is controlled by charge transfer and Warburg di}usion[ Analogy of Elec!
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866
Fig[ 4[ Nyquist plot of impedance data with equivalent circuit R0"R1Q# "in Fig[ 3"a## for Fig[ 0[
Fig[ 5[ Nyquist plot of impedance data with equivalent circuit R0"Q0"R1Q1## "in Fig[ 3"b## for Fig[ 1[
trode C to a planar electrode can ascribe to compress more compactly when the electrode is prepared owing to the existence of a cobalt coating on the surface of the nickel hydroxide[
4[ Discussion The impedance spectra show good agreement between the theoretical predications and the experimental data
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Fig[ 6[ Nyquist plot of impedance data with equivalent circuit R0"Q0"R1Q1## "in Fig[ 3"b## for Fig[ 2[
Table 0 Values of equivalent circuit parameters for di}erent electrodes Electrode
R0:V
R1:V
Y0
n0
Y1
n1
Electrode A Electrode B Electrode C
0[2×09−1 8[6×09−2 0×09−2
9[61 0[4×09−1 4×09−2
9[22 1[7 1
9[7 9[64 0
* 046 29
* 9[64 9[4
obtained with the models shown in Fig[ 3"a# and "b#[ For the nickel hydroxide electrode\ the complex plane plot only shows a capacitive arc at entire frequencies "Fig[ 4#^ while for nickel hydroxide electrodes added cobalt powder as a conductor and deposited cobalt coating\ the complex plane plots exhibit a semicircle and a Warburg linear region "Figs 5 and 6#\ and the impedance behavior ultimately becomes dominated by the Warburg imped! ance[ According to the reported mechanisms ð6\ 05Ł\ the nickel hydroxide electrode reaction follows di}usion kin! etics[ MacArthur and Zimmerman ð6Ł proposed that dur! ing discharge a proton di}uses from the _lmÐelectrolyte interface into the active material and an electron enters across the conducting substrateÐ_lm interface[ During charge the proton di}uses to the _lmÐelectrolyte interface to react with a hydroxyl ion to form water[ Zimmerman described the overall process as follows ð6Ł H1 O \ OH"aq#¦H¦ "s#
"1#
H¦ "s# \ H¦ "s?#
"2#
H¦ "s?#¦NiOOH"s?#¦e"s?# \ Ni"OH# 1 "s?#
"3#
Reaction "1# represents the formation of a proton at catalytic site s at the electrode:electrolyte interface^ reac! tion "2# involves di}usion of the proton from site s into the electrode to the charge transfer site s?^ and reaction "3# is the charge transfer process involving the reduction of one of the higher valence species of active material in the lattice\ represented here as simply NiOOH[ It is well known that the high!charge phase is an n!type sem! iconductor and the low!charge one is an electronic insu! lator or a low conductive p!type semiconductor ð3Ł[ Therefore\ during discharge nickel "II# species accumu! late at the hydroxideÐelectrolyte interface[ These species are insulating or poor conductivity and consequently pre! vent discharge of the crystallite core[ Although the addition of cobalt powder into nickel hydroxide as a conductor can\ to a certain extent\ decrease the contact
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resistance between nickel hydroxide particles and foam nickel substrate\ it can not fully increase the utilization of active material due to nonuniform distribution of cobalt powder at the surface of nickel hydroxide particles[ How! ever\ cobalt coating deposited at the surface of nickel hydroxide particles is of uniform distribution and can be oxidized into high conductive cobalt oxyhydroxide during charge\ which henceforth remains as cobalt oxyhydroxide because of irreversibility of Co"II#:Co"II! I#couple ð4Ł[ Thus\ this will provide good electrical con! duction between the nickel hydroxide particles and the substrate[ As previously mentioned\ for Electrode C the Q0 and Q1 in the equivalent circuit in Fig[ 3"b# is similar to a capacitance and a Warburg impedance[ Accordingly\ Q0 and Q1 can be represented by capacitance Cdl and War! burg impedance W\ respectively[ The total impedance\ Z\ of any network is given by ð07Ł Z Rs −
j vCs
"4#
where Rs\ Cs are the e}ective series resistance and capaci! tance\ respectively[ For the equivalent circuit shown in Fig[ 3"b#\ the series resistance Rs may be expressed by R1 ¦ Rs R0 ¦ 1
s zv
1
1 dl
"0¦zvCdl s# ¦v C
0
R1 ¦
"5#
1
s zv
1
b
The capacitance is given by
0
"0¦zvCdl s#¦vCdl R1 ¦
s
1
1
zv zv 0 vCs s 1 1 b "0¦zvCdl s# 1 ¦v1 Cdl R1 ¦ zv
0
"6#
1
where W
0
s zv
−
js
1
zv
RT nFI9
Studies of electrochemical impedance spectra of the three electrodes were performed[ Two equivalent circuits shown in Fig[ 3 were used to _t experimental data[ The experimental data were in good agreement with theor! etical models[ It was clearly shown that the electrode reaction occurring at the nickel hydroxide "Electrode A# was mainly controlled by charge transfer\ while the elec! trode reaction occurring at the nickel hydroxide added cobalt powder as a conductor "Electrode B# and deposited cobalt coating "Electrode C# were controlled by charge transfer and Warburg di}usion[ The behavior of the nickel hydroxide electrode deposited cobalt coating is similar to a planar electrode[ Due to a good electrical contact between nickel hydroxide particles and foam nickel substrate\ provided by cobalt coating\ the electrode reaction has a larger exchange cur! rent density\ and thus\ during charge:discharge process\ the active material of electrode reaction will be fully used and exhibit higher utilization of active material and larger discharge capacity[
"7#
and s the Warburg coe.cient[ The expression for the Warburg coe.cient includes terms containing the concentrations of the reactants and products\ as well as their di}usion coe.cients[ The charge transfer resistance\ R1\ is related to the concentration term through the exchange current density\ since R1
rent density[ The impedance parameters will vary according to the concentration of the reacting and the product species[ The variation of the impedance par! ameters will be prominent in the low frequency region\ because the parameters s and R1 are observed here[ Theoretical analysis of Electrode A and Electrode B can also be carried out based on the above consideration[ According to the results in Table 0 the values of R1 from Electrode A to Electrode C apparently decrease[ It is known from Eq[ "8# that the exchange current density gradually increases[ Hence\ cobalt coating can act as a current collector and promote transfer reaction[ Owing to a good electrical contact provided by cobalt coating the exchange current density of electrode reaction is large enough compared with those of Electrode B so that the electrode reaction is prominently controlled by Warburg di}usion[
5[ Conclusion
0 1
s
868
"8#
Where R is the gas constant\ T is the absolute tempera! ture\ F is the Faraday constant\ n is the number of elec! trons involved in the relation and I9 is the exchange cur!
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