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Oxygen evolution on NixFe3−xO4 electrodes
J. Electroanal. Chem,, 95 (1979) 233--235
© ElsevierSequoia S.A., Lausanne"-- Printed in The Netherlands Preliminary note OXYGEN EVOLUTION ON NixFe3-xO4 ELECTRODES .1
J. O R E H O T S K Y .2, H. H U A N G .3, C.R. D A V I D S O N and S. S R I N I V A S A N Department of Energy and Environment, Brookhaven National Laboratory, Upton, N. Y. 11973 (U.S.A.) (Received 10th October 1978)
Investigations of the influence of the magnetic properties of catalysts on adsorption and heterogeneous catalysis have been rather extensive and well documented [1--4]. By comparison, very little work has been done on electrochemical activity, as it relates to either the magnetic state or the magnetic properties of the electrode. Some recent work [5] has shown that the transfer coefficient for oxygen evolution on anodized nickel electrodes changes rapidly to higher values at about 210--260°C. A similar change has been recorded for the behavior of the exchange current density for oxygen reduction on lithiated nickel oxide electrodes at 150°C [6]. Since these temperatures roughly correspond to the Neel temperature (TN) of these two antiferromagnetic materials, the change in the electrocatalytic activity was attributed to the transformation of the electrode to the paramagnetic state from the antiferromagnetic state [5]. These results suggest that the kinetic parameters are dependent on the magnetic state of the electrode. The object of this particular investigation was to determine if the electrocatalytic activity for oxygen evolution on a ferrimagnetically ordered electrode in its demagnetized states is related to the magnetic properties of the electrode material. The magnetic property selected for the evaluation was the room temperature saturation magnetization. The experimental evaluation involved measuring the room temperature oxygen evolution kinetic parameters on a series of electrodes with different values for their room temperature magnetization. Ferrimagnetically ordered, fine particle NixFe3-xO4 spinels were selected as the electrode materials for this evaluation. The room temperature saturation magnetization of these spinels are significantly different and have a bell-shaped compositional dependency centered about the NilFe204 stoichiometry [7]. This allows for an easy separation of magnetic effects from systematic compositional effects on the electrode kinetic parameters. Five NixFe3-xO4 spinel powders of compositions Ni0.49Fe2.slO4, NiFe204, Nil.2sFe~.TsO4, and Nil.6sFe~.3204 were prepared by a freeze drying technique *1This work was performed under the auspices of the U.S. Department of Energy. .2 Visiting Scientist at BNL. Permanent address: Department of Engineering, Wilkes College, Wilkes-Barre, Pa. 18703, U.S.A. .3 Guest Scientist at BNL. Permanent address: Department of Chemistry and Physics, Middle Tennessee State University, Mursfreesboro, Tenn., U.S.A.
234
[8]. Crystals of Ni(NO3)2-6H20 and Fe(NO3)2-9H20 in the appropriate amounts were dissolved in doubly distilled water and the solution was sprayed into a liquid nitrogen bath. The resulting frozen salt crystals were then placed in flasks which were immersed in a dry ice--ethylene glycol mixture. The flasks were vacuum pumped to sublimate the associated ice. The resulting particles were then heated in vacuum to approximately 250°C for several hours to decompose the hydrated nickel iron nitrates and then heated in air at 350°C for one hour followed by heating at 850°(; for an additional hour to obtain the oxygen stoichiometry and spinel crystal structure. Debye-Scherrer X-ray photographs of the resulting particles confirmed the spinel structure. A nitrogen adsorption (BET) analysis showed that the particles had a specific surface area of about 60 m2/g. The electrodes were fabricated from each spinel powder by the Teflon bonding technique [6] onto a nickel screen connected to a Teflon encapsulated nickel lead wire. The Teflon cell used in this investigation is adequately described elsewhere [5]. The electrochemical study consisted of steady-state potentiostatic measurements, using a PAR 371 potentiostat, for oxygen evolution on these electrodes in a 30% KOH solution at 25°C. The electrolyte was prepared from reagent grade potassium hydroxide and doubly distilled water. The current potential measurements were made from high to low current and the potential of the spinel electrodes were measured relative to a dynamic hydrogen reference electrode (DHE) [9]. The counter electrode was a nickel wire. The IR corrected potential--current relationship for oxygen evolution on the five nickel ferrite spinel electrodes are shown in Fig. 1. A Tafel region is apparent for each curve from which the transfer coefficient (ha) was determined for oxygen evolution on each ferrite electrode. These transfer coefficients along with the room temperature saturation magnetization of each
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1.4
~.
.
i.4
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~
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.
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Fig. 1. Potential--current relationship for o x y g e n evolution o n various nickel ferrites at 25o(;.
235 ferrite is p r e s e n t e d in T a b l e I w h e r e it is a p p a r e n t t h a t t h e t r a n s f e r c o e f f i c i e n t s are a p p r o x i m a t e l y t h e s a m e a n d are n o t s y s t e m a t i c a l l y r e l a t e d t o e i t h e r t h e c o m p o s i t i o n or t h e r o o m t e m p e r a t u r e s a t u r a t i o n m a g n e t i z a t i o n o f t h e s e d e m a g n e t i z e d ferrite e l e c t r o d e s . TABLE1 Ferrite composition
Saturation magnetization (emu/g)
Transfer coefficient R T alni nOl = - -
F
Ni0.49 F%.sl O 4 Ni0.s5 Fe=.ls O4 Ni 1 Fe 2 04 Nil.2s Fel.~s 04 Nil.6s Fel.3~ 04
22.7 27.7 48.9 35.7 24.7
--
aE
1.4 1.5 1.5 1.6 1.2
REFERENCES 1 2 3 4 5 6 7 8 9
P.W. S e l w o o d , A d s o r p t i o n a n d CoLlective P a r a m a g n e t l s m . A c a d e m i c Press, N e w Y o r k , 1 9 6 2 . A. F a r k a s , O ~ h o h y d ~ o g e n , P a x a h y d r o g e n , a n d Heavy H y d r o g e n . C a m b r i d g e University Press, 1 9 3 5 . J.A. Hedvall, R. H e d l m a n d O. Persson, Z. Physlk. Chem., B27 ( 1 9 3 4 ) 1 9 6 . E.R.S. Winter, J. Catalysis, 6 ( 1 9 6 6 ) 35. M.H. Miles, G. Kissel, P.W.T. L u a n d S. Srinivasan, J. E l e c t r o c h e m . S o t . , 1 2 3 ( 1 9 7 6 ) 332. H.C. Bevan a n d A.C.C. Tscung, Electzochim. Aeta, 9 ( 1 9 7 4 ) 201. J. O r e h o t s k y a n d C.R. Davidson, a c c e p t e d f o r p u b l i c a t i o n in Magnetism Letters. F.J. S c h n e t t l e r , F.R. M o n f o r t e , a n d W.W. R h o d e s , Sei. Ceramics, 4 ( 1 9 6 8 ) 79. J. Giner, J. E l e c t r o c h e m . Soc., 111 ( 1 9 6 4 ) 3 7 6 .
© ElsevierSequoia S.A., Lausanne"-- Printed in The Netherlands Preliminary note OXYGEN EVOLUTION ON NixFe3-xO4 ELECTRODES .1
J. O R E H O T S K Y .2, H. H U A N G .3, C.R. D A V I D S O N and S. S R I N I V A S A N Department of Energy and Environment, Brookhaven National Laboratory, Upton, N. Y. 11973 (U.S.A.) (Received 10th October 1978)
Investigations of the influence of the magnetic properties of catalysts on adsorption and heterogeneous catalysis have been rather extensive and well documented [1--4]. By comparison, very little work has been done on electrochemical activity, as it relates to either the magnetic state or the magnetic properties of the electrode. Some recent work [5] has shown that the transfer coefficient for oxygen evolution on anodized nickel electrodes changes rapidly to higher values at about 210--260°C. A similar change has been recorded for the behavior of the exchange current density for oxygen reduction on lithiated nickel oxide electrodes at 150°C [6]. Since these temperatures roughly correspond to the Neel temperature (TN) of these two antiferromagnetic materials, the change in the electrocatalytic activity was attributed to the transformation of the electrode to the paramagnetic state from the antiferromagnetic state [5]. These results suggest that the kinetic parameters are dependent on the magnetic state of the electrode. The object of this particular investigation was to determine if the electrocatalytic activity for oxygen evolution on a ferrimagnetically ordered electrode in its demagnetized states is related to the magnetic properties of the electrode material. The magnetic property selected for the evaluation was the room temperature saturation magnetization. The experimental evaluation involved measuring the room temperature oxygen evolution kinetic parameters on a series of electrodes with different values for their room temperature magnetization. Ferrimagnetically ordered, fine particle NixFe3-xO4 spinels were selected as the electrode materials for this evaluation. The room temperature saturation magnetization of these spinels are significantly different and have a bell-shaped compositional dependency centered about the NilFe204 stoichiometry [7]. This allows for an easy separation of magnetic effects from systematic compositional effects on the electrode kinetic parameters. Five NixFe3-xO4 spinel powders of compositions Ni0.49Fe2.slO4, NiFe204, Nil.2sFe~.TsO4, and Nil.6sFe~.3204 were prepared by a freeze drying technique *1This work was performed under the auspices of the U.S. Department of Energy. .2 Visiting Scientist at BNL. Permanent address: Department of Engineering, Wilkes College, Wilkes-Barre, Pa. 18703, U.S.A. .3 Guest Scientist at BNL. Permanent address: Department of Chemistry and Physics, Middle Tennessee State University, Mursfreesboro, Tenn., U.S.A.
234
[8]. Crystals of Ni(NO3)2-6H20 and Fe(NO3)2-9H20 in the appropriate amounts were dissolved in doubly distilled water and the solution was sprayed into a liquid nitrogen bath. The resulting frozen salt crystals were then placed in flasks which were immersed in a dry ice--ethylene glycol mixture. The flasks were vacuum pumped to sublimate the associated ice. The resulting particles were then heated in vacuum to approximately 250°C for several hours to decompose the hydrated nickel iron nitrates and then heated in air at 350°C for one hour followed by heating at 850°(; for an additional hour to obtain the oxygen stoichiometry and spinel crystal structure. Debye-Scherrer X-ray photographs of the resulting particles confirmed the spinel structure. A nitrogen adsorption (BET) analysis showed that the particles had a specific surface area of about 60 m2/g. The electrodes were fabricated from each spinel powder by the Teflon bonding technique [6] onto a nickel screen connected to a Teflon encapsulated nickel lead wire. The Teflon cell used in this investigation is adequately described elsewhere [5]. The electrochemical study consisted of steady-state potentiostatic measurements, using a PAR 371 potentiostat, for oxygen evolution on these electrodes in a 30% KOH solution at 25°C. The electrolyte was prepared from reagent grade potassium hydroxide and doubly distilled water. The current potential measurements were made from high to low current and the potential of the spinel electrodes were measured relative to a dynamic hydrogen reference electrode (DHE) [9]. The counter electrode was a nickel wire. The IR corrected potential--current relationship for oxygen evolution on the five nickel ferrite spinel electrodes are shown in Fig. 1. A Tafel region is apparent for each curve from which the transfer coefficient (ha) was determined for oxygen evolution on each ferrite electrode. These transfer coefficients along with the room temperature saturation magnetization of each
/ 1.5 I cl
~.s?.O,.
1.4
~.
.
i.4
~e20~
~
1.4
"
I
1.4162
I
I llllll
I
t6'
l
.
I
IIIIII
1
1o°
1
I
tllllJ
t
I
I I I~[
Io'
i /rnA
Fig. 1. Potential--current relationship for o x y g e n evolution o n various nickel ferrites at 25o(;.
235 ferrite is p r e s e n t e d in T a b l e I w h e r e it is a p p a r e n t t h a t t h e t r a n s f e r c o e f f i c i e n t s are a p p r o x i m a t e l y t h e s a m e a n d are n o t s y s t e m a t i c a l l y r e l a t e d t o e i t h e r t h e c o m p o s i t i o n or t h e r o o m t e m p e r a t u r e s a t u r a t i o n m a g n e t i z a t i o n o f t h e s e d e m a g n e t i z e d ferrite e l e c t r o d e s . TABLE1 Ferrite composition
Saturation magnetization (emu/g)
Transfer coefficient R T alni nOl = - -
F
Ni0.49 F%.sl O 4 Ni0.s5 Fe=.ls O4 Ni 1 Fe 2 04 Nil.2s Fel.~s 04 Nil.6s Fel.3~ 04
22.7 27.7 48.9 35.7 24.7
--
aE
1.4 1.5 1.5 1.6 1.2
REFERENCES 1 2 3 4 5 6 7 8 9
P.W. S e l w o o d , A d s o r p t i o n a n d CoLlective P a r a m a g n e t l s m . A c a d e m i c Press, N e w Y o r k , 1 9 6 2 . A. F a r k a s , O ~ h o h y d ~ o g e n , P a x a h y d r o g e n , a n d Heavy H y d r o g e n . C a m b r i d g e University Press, 1 9 3 5 . J.A. Hedvall, R. H e d l m a n d O. Persson, Z. Physlk. Chem., B27 ( 1 9 3 4 ) 1 9 6 . E.R.S. Winter, J. Catalysis, 6 ( 1 9 6 6 ) 35. M.H. Miles, G. Kissel, P.W.T. L u a n d S. Srinivasan, J. E l e c t r o c h e m . S o t . , 1 2 3 ( 1 9 7 6 ) 332. H.C. Bevan a n d A.C.C. Tscung, Electzochim. Aeta, 9 ( 1 9 7 4 ) 201. J. O r e h o t s k y a n d C.R. Davidson, a c c e p t e d f o r p u b l i c a t i o n in Magnetism Letters. F.J. S c h n e t t l e r , F.R. M o n f o r t e , a n d W.W. R h o d e s , Sei. Ceramics, 4 ( 1 9 6 8 ) 79. J. Giner, J. E l e c t r o c h e m . Soc., 111 ( 1 9 6 4 ) 3 7 6 .