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The discharge process of nickel hydroxide electrodes used in batteries: A dynamic analysis study by EIS
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
33 (2008) 3493 – 3495
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The discharge process of nickel hydroxide electrodes used in batteries: A dynamic analysis study by EIS S.G. Real, M.G. Ortiz, E.B. Castro, A. Visintin Facultad de Ciencias Exactas, Instituto de Investigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, C.C. 16, Suc. 4, 1900 La Plata, Argentina
art i cle info
ab st rac t
Article history:
The discharge process of nickel hydroxide electrodes obtained by electrodeposition is
Received 2 August 2007
studied by electrochemical impedance spectroscopy (EIS). The results are discussed
Received in revised form
according to a physicochemical model, taking into account the porous nature of the
12 March 2008
electrode, the charge transfer process and the H transport in the active material. The
Accepted 12 March 2008
parameter identification procedure allows estimating of values related to the interfacial
Available online 6 May 2008
area per unit volume, the solid and solution conductivities as well as diffusion and kinetic
Keywords: Impedance
constants related to the discharge process. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Porous electrodes Nickel hydroxide
1.
Introduction
The nickel hydroxide electrode is widely used as an active material in nickel electrodes. The electrochemical energy storage in the nickel hydroxide electrodes is related to the reversible insertion of H into the nickel hydroxide/oxyhydroxide. The process reversibility is an important requirement for batteries electrode materials. The nickel electrode has been the subject of many publications, however there are still many factors not well understood, as the ‘‘sudden death’’ of the batteries that are dependent on: the concentration of the alkaline solution, the preparation method and the global kinetics of the process. In this study, the results obtained by electrochemical impedance spectroscopy (EIS) measurements for different states of discharge (SOD) of electrodeposited nickel hydroxide electrodes are presented and discussed according to physicochemical models developed in the laboratory.
2.
Experimental
Electrochemical experiments were performed in a conventional three compartment glass cell. The electrolyte was 7 M KOH at 25 1C. Potentials in the text are referred to Hg/HgOss. EIS measurements were performed using a PAR potentiostat and a Schlumberger 1250 frequency response analyzer (1 mHzofo65 kHz). EIS experiments were carried out potentiostatically for different SOD of the electrodes. First, the electrodes were charged completely and then discharged galvanostatically up to different open circuit potentials. The working electrodes consisted of nickel hydroxide electrodeposited on 82% porosity sintered nickel substrate employing:
(a) electrode I: alcoholic solution, 50% : 1:8 M ½NiðNO3 Þ2 þ 0:2 M Co ðNO3 Þ2 þ 0:15 M ½CdðNO3 Þ2 , (b) electrode II: solution: 1.8 M [Ni(NO3)2]+0.2 M [Co(NO3)2].
Corresponding author.
E-mail addresses: [email protected], [email protected] (S.G. Real). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.017
ARTICLE IN PRESS 3494
I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
3.
Zi implies the double layer capacitance impedance (Zdl) linked in parallel with the faradaic reaction impedance (ZF), i.e.
Results and discussion
The impedance response corresponding to Ni hydroxide electrodes was analysed in terms of a physicochemical model which accounts for the charge/discharge process, taking place in the porous structure of the electrode. The charge transfer reaction at the active material/electrolyte interface is coupled to H transport in the active material. The impedance function of the porous structure, Zp may be expressed as [1–3] L 2 þ ðs=k þ k=sÞ cosh n 1þ kþs n sinh n k þ s1=2 1=2 n¼L Zi ks
Zp ¼
(1)
being, L the electrode thickness, k and s the effective conductivities of the liquid and solid phases and Zi the impedance of the solid/liquid interface per electrode unit volume (O cm3).
1 1 Z1 i ¼ Zdl þ ZF
(2)
where Zdl ¼
1 ioCdl ae
(3)
ZF ¼
Zf aa
(4)
Cdl being the double layer capacitance per unit interfacial area (F cm2), and ae the interfacial area per unit volume (cm1), o ¼ 2pf (f, frequency of the perturbing signal). Zf is the faradaic impedance per unit interfacial area (O cm2), and aa the active area per unit volume, the active material electrodeposited in the pores of the sintered Ni substrate is modelled as a packing of cylinders of internal radius ra and external radius rb. The active material/electrolyte interface is located at ra and the active material/Ni substrate interface is located at rb. The active material is assumed to consist of a single phase solid solution and the faradaic current is a function of the
0
0
-10
-10
-20
-20
-30
-30
Φ
Φ
33 (2008) 3493 – 3495
-40
experimental
-50 -60
-3
-2
-1
-40
0.250V 0.300V 0.350V 0.400V 0.490V
1 0 log f/Hz
2
3
theoretical
0.250V 0.300V 0.350V 0.400V 0.490V
-50 -60 -4
4
-2
-3
-1
0 1 log f/Hz
2
3
4
Fig. 1 – Experimental and theoretical Bode diagrams as a function of discharge potentials for electrode I.
0
-20
-20
-40
-40
φ
φ
0
E=0.4 V E=0.35 V E=0.3 V E=0.25 V
-60
-60 E=0.40 V E=0.35 V E=0.30 V E=0.25 V
Experimental
-80 -2
-1
0
1 log f/Hz
2
3
-80 4
theoretical
-2
-1
0
1 log f/Hz
2
3
Fig. 2 – Experimental and theoretical Bode diagrams as a function of discharge potentials for electrode II.
4
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
3495
33 (2008) 3493 – 3495
Table 1 – Fitting parameters for electrode I E (V)
Cdlae (F cm3)
Rt/aa (O cm3)
DNi (cm2 s1)
ra (cm)
rb (cm)
53 53 53 45 2
o5 103 o5 103 o5 103 o5 103 5 102
o5 1011 o5 1011 o5 1011 o5 1011 o5 1011
1 103 1 103 1 103 1 103 7 104
44 103 44 103 44 103 44 103 44 103
0.49 0.4 0.35 0.3 0.25
Table 2 – Fitting parameters for electrode II Cdlae (F cm3)
E (V) 0.40 0.35 0.30 0.25
Rt/aa (O cm3) 3
473 470 450 1.3
6 10 6 103 2 102 0.52
being M ¼ DCi/DJi the mass transfer function [3]. and Ji the proton flux at film/electrolyte interface, r ¼ ra. The expression for M(o), is derived by solving Fick’s laws for radial diffusion in a cylinder of internal radius, ra and external radius rb [3], and with the boundary condition at rb, determined by a blocking interface condition FDJi ¼ Dif ¼ FD
r ¼ rb ;
DJ ¼ 0
accordingly "
ra (cm)
rb (cm)
o5 10 o5 1011 o5 1011 o5 1011
o2.2 10 o2.2 103 o2.2 103 o2.2 103
2.6 103 2.6 103 2.6 103 2.6 103
11
overpotential, Z, at the film/electrolyte interface, and of the hydrogen concentration, Ci, in the first monolayer of active material at the film/electrolyte interface. Under these conditions the faradaic impedance may be derived as 1 Di ðoÞ qif qif DJ ðoÞ ¼ (5) ¼ f þ MðoÞ i Zf DZðoÞ DZðoÞ qZ Ci qCi Z
r ¼ ra ;
DNi (cm2 s1)
dDCi , dr
3
probably due to the volume expansion taking place during discharge [4].
4.
Conclusions
The analysis of results according to this physicochemical model allows to conclude:
The changes observed during discharge in the impedance response of porous nickel electrodes are mainly due to changes in the interfacial and active areas of the electrode.
Acknowledgements (6) #
I0 ðc ÞK1 ðcb Þ þ K0 ðca ÞI1 ðcb Þ ; MðoÞ ¼ pffiffiffiffiffiffiffiffiffi a DjoðI1 ðca ÞK1 ðcb Þ þ I1 ðcb ÞK1 ðca ÞÞ rffiffiffiffiffi rffiffiffiffiffi io io ; cb ¼ rb ca ¼ ra D D
(7)
where I0, K0, I1 and K1 are Bessel functions and D is the diffusion coefficient of H in the Ni hydroxide film. The final expression for Zf may be derived from Eqs. (5)–(7): MðoÞ 1 qif qif ; ¼ ; A ¼ Rt Zf ¼ Rt A F Rt qZ Ci qCi Z Figs. 1 and 2 exhibit a fairly good agreement between experimental and theoretical EIS data. The results in Tables 1 and 2 indicate that for electrodes I and II the double layer capacity per volume unit (Cdl ae) diminishes with increasing SOD. The diminishment of ae is
This work was supported by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET) of Argentina and the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica. The authors also thank financial support through CONICET–Comisio´n Nacional de actividades espaciales (CONAE) agreement. R E F E R E N C E S
[1] De Levie R. In: Delahay P, editor. Advances in electrochemistry and electrochemistry engineering, vol. 6. NY: Interscience; 1967. p. 329. [2] Meyers JP, Doyle M, Darling RM, Newman J. J Electrochem Soc 2000;147:2930. [3] Novak I, Ortiz M, Castro B, Real S. SIBAE 2006;1:398. [4] Bernard P, Gabrielli C, Keddam M, Takenouti H, Leonardi J, Blanchard P. Electrochim Acta 1991;36:743.
33 (2008) 3493 – 3495
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The discharge process of nickel hydroxide electrodes used in batteries: A dynamic analysis study by EIS S.G. Real, M.G. Ortiz, E.B. Castro, A. Visintin Facultad de Ciencias Exactas, Instituto de Investigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, C.C. 16, Suc. 4, 1900 La Plata, Argentina
art i cle info
ab st rac t
Article history:
The discharge process of nickel hydroxide electrodes obtained by electrodeposition is
Received 2 August 2007
studied by electrochemical impedance spectroscopy (EIS). The results are discussed
Received in revised form
according to a physicochemical model, taking into account the porous nature of the
12 March 2008
electrode, the charge transfer process and the H transport in the active material. The
Accepted 12 March 2008
parameter identification procedure allows estimating of values related to the interfacial
Available online 6 May 2008
area per unit volume, the solid and solution conductivities as well as diffusion and kinetic
Keywords: Impedance
constants related to the discharge process. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Porous electrodes Nickel hydroxide
1.
Introduction
The nickel hydroxide electrode is widely used as an active material in nickel electrodes. The electrochemical energy storage in the nickel hydroxide electrodes is related to the reversible insertion of H into the nickel hydroxide/oxyhydroxide. The process reversibility is an important requirement for batteries electrode materials. The nickel electrode has been the subject of many publications, however there are still many factors not well understood, as the ‘‘sudden death’’ of the batteries that are dependent on: the concentration of the alkaline solution, the preparation method and the global kinetics of the process. In this study, the results obtained by electrochemical impedance spectroscopy (EIS) measurements for different states of discharge (SOD) of electrodeposited nickel hydroxide electrodes are presented and discussed according to physicochemical models developed in the laboratory.
2.
Experimental
Electrochemical experiments were performed in a conventional three compartment glass cell. The electrolyte was 7 M KOH at 25 1C. Potentials in the text are referred to Hg/HgOss. EIS measurements were performed using a PAR potentiostat and a Schlumberger 1250 frequency response analyzer (1 mHzofo65 kHz). EIS experiments were carried out potentiostatically for different SOD of the electrodes. First, the electrodes were charged completely and then discharged galvanostatically up to different open circuit potentials. The working electrodes consisted of nickel hydroxide electrodeposited on 82% porosity sintered nickel substrate employing:
(a) electrode I: alcoholic solution, 50% : 1:8 M ½NiðNO3 Þ2 þ 0:2 M Co ðNO3 Þ2 þ 0:15 M ½CdðNO3 Þ2 , (b) electrode II: solution: 1.8 M [Ni(NO3)2]+0.2 M [Co(NO3)2].
Corresponding author.
E-mail addresses: [email protected], [email protected] (S.G. Real). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.017
ARTICLE IN PRESS 3494
I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
3.
Zi implies the double layer capacitance impedance (Zdl) linked in parallel with the faradaic reaction impedance (ZF), i.e.
Results and discussion
The impedance response corresponding to Ni hydroxide electrodes was analysed in terms of a physicochemical model which accounts for the charge/discharge process, taking place in the porous structure of the electrode. The charge transfer reaction at the active material/electrolyte interface is coupled to H transport in the active material. The impedance function of the porous structure, Zp may be expressed as [1–3] L 2 þ ðs=k þ k=sÞ cosh n 1þ kþs n sinh n k þ s1=2 1=2 n¼L Zi ks
Zp ¼
(1)
being, L the electrode thickness, k and s the effective conductivities of the liquid and solid phases and Zi the impedance of the solid/liquid interface per electrode unit volume (O cm3).
1 1 Z1 i ¼ Zdl þ ZF
(2)
where Zdl ¼
1 ioCdl ae
(3)
ZF ¼
Zf aa
(4)
Cdl being the double layer capacitance per unit interfacial area (F cm2), and ae the interfacial area per unit volume (cm1), o ¼ 2pf (f, frequency of the perturbing signal). Zf is the faradaic impedance per unit interfacial area (O cm2), and aa the active area per unit volume, the active material electrodeposited in the pores of the sintered Ni substrate is modelled as a packing of cylinders of internal radius ra and external radius rb. The active material/electrolyte interface is located at ra and the active material/Ni substrate interface is located at rb. The active material is assumed to consist of a single phase solid solution and the faradaic current is a function of the
0
0
-10
-10
-20
-20
-30
-30
Φ
Φ
33 (2008) 3493 – 3495
-40
experimental
-50 -60
-3
-2
-1
-40
0.250V 0.300V 0.350V 0.400V 0.490V
1 0 log f/Hz
2
3
theoretical
0.250V 0.300V 0.350V 0.400V 0.490V
-50 -60 -4
4
-2
-3
-1
0 1 log f/Hz
2
3
4
Fig. 1 – Experimental and theoretical Bode diagrams as a function of discharge potentials for electrode I.
0
-20
-20
-40
-40
φ
φ
0
E=0.4 V E=0.35 V E=0.3 V E=0.25 V
-60
-60 E=0.40 V E=0.35 V E=0.30 V E=0.25 V
Experimental
-80 -2
-1
0
1 log f/Hz
2
3
-80 4
theoretical
-2
-1
0
1 log f/Hz
2
3
Fig. 2 – Experimental and theoretical Bode diagrams as a function of discharge potentials for electrode II.
4
ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y
3495
33 (2008) 3493 – 3495
Table 1 – Fitting parameters for electrode I E (V)
Cdlae (F cm3)
Rt/aa (O cm3)
DNi (cm2 s1)
ra (cm)
rb (cm)
53 53 53 45 2
o5 103 o5 103 o5 103 o5 103 5 102
o5 1011 o5 1011 o5 1011 o5 1011 o5 1011
1 103 1 103 1 103 1 103 7 104
44 103 44 103 44 103 44 103 44 103
0.49 0.4 0.35 0.3 0.25
Table 2 – Fitting parameters for electrode II Cdlae (F cm3)
E (V) 0.40 0.35 0.30 0.25
Rt/aa (O cm3) 3
473 470 450 1.3
6 10 6 103 2 102 0.52
being M ¼ DCi/DJi the mass transfer function [3]. and Ji the proton flux at film/electrolyte interface, r ¼ ra. The expression for M(o), is derived by solving Fick’s laws for radial diffusion in a cylinder of internal radius, ra and external radius rb [3], and with the boundary condition at rb, determined by a blocking interface condition FDJi ¼ Dif ¼ FD
r ¼ rb ;
DJ ¼ 0
accordingly "
ra (cm)
rb (cm)
o5 10 o5 1011 o5 1011 o5 1011
o2.2 10 o2.2 103 o2.2 103 o2.2 103
2.6 103 2.6 103 2.6 103 2.6 103
11
overpotential, Z, at the film/electrolyte interface, and of the hydrogen concentration, Ci, in the first monolayer of active material at the film/electrolyte interface. Under these conditions the faradaic impedance may be derived as 1 Di ðoÞ qif qif DJ ðoÞ ¼ (5) ¼ f þ MðoÞ i Zf DZðoÞ DZðoÞ qZ Ci qCi Z
r ¼ ra ;
DNi (cm2 s1)
dDCi , dr
3
probably due to the volume expansion taking place during discharge [4].
4.
Conclusions
The analysis of results according to this physicochemical model allows to conclude:
The changes observed during discharge in the impedance response of porous nickel electrodes are mainly due to changes in the interfacial and active areas of the electrode.
Acknowledgements (6) #
I0 ðc ÞK1 ðcb Þ þ K0 ðca ÞI1 ðcb Þ ; MðoÞ ¼ pffiffiffiffiffiffiffiffiffi a DjoðI1 ðca ÞK1 ðcb Þ þ I1 ðcb ÞK1 ðca ÞÞ rffiffiffiffiffi rffiffiffiffiffi io io ; cb ¼ rb ca ¼ ra D D
(7)
where I0, K0, I1 and K1 are Bessel functions and D is the diffusion coefficient of H in the Ni hydroxide film. The final expression for Zf may be derived from Eqs. (5)–(7): MðoÞ 1 qif qif ; ¼ ; A ¼ Rt Zf ¼ Rt A F Rt qZ Ci qCi Z Figs. 1 and 2 exhibit a fairly good agreement between experimental and theoretical EIS data. The results in Tables 1 and 2 indicate that for electrodes I and II the double layer capacity per volume unit (Cdl ae) diminishes with increasing SOD. The diminishment of ae is
This work was supported by Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET) of Argentina and the Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica. The authors also thank financial support through CONICET–Comisio´n Nacional de actividades espaciales (CONAE) agreement. R E F E R E N C E S
[1] De Levie R. In: Delahay P, editor. Advances in electrochemistry and electrochemistry engineering, vol. 6. NY: Interscience; 1967. p. 329. [2] Meyers JP, Doyle M, Darling RM, Newman J. J Electrochem Soc 2000;147:2930. [3] Novak I, Ortiz M, Castro B, Real S. SIBAE 2006;1:398. [4] Bernard P, Gabrielli C, Keddam M, Takenouti H, Leonardi J, Blanchard P. Electrochim Acta 1991;36:743.