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Effect of the state of the surface on the electrocatalysis of the anodic oxidation of hydrazine
Surface Technology, 5 (1977) 81 - 84 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
81
Short Communication
E f f e c t o f the state o f the surface on the electrocatalysis o f the a n o d i c oxidation of hydrazine
G. Bt~LANGER and A. K. VIJH Hydro-Quebec Institute of Research. Varennes, P.Q. (Canada) (Received May 11, 1976)
T h e e l e c t r o - o x i d a t i o n o f h y d r a z i n e has b e e n the subject o f n u m e r o u s studies and its e l e c t r o c h e m i c a l activity in alkaline solutions is well k n o w n ; several fuel cell p r o t o t y p e s based o n h y d r a z i n e have been p r o p o s e d [ 1 ]. In alkaline solutions, n o b l e metals [1 - 3 ] , some c a r b o n s u p p o r t e d catalysts, nickel [3 - 5] and nickel boride [1] have previously b e e n e x a m i n e d . As in our r e c e n t p a p e r on m e t h a n o l o x i d a t i o n [ 6 ] , the metals investigated in the present w o r k are Pt, Ir, Rh, Au, Ag, Ni, Co and Fe in 1M KOH. T h e electrodes were pre-polarized anodically t o o b t a i n o x i d e - c o v e r e d surfaces and the p o l a r i z a t i o n curves were r e c o r d e d , c o m m e n c i n g f r o m the high a n o d i c potentials. T h e high anodic p o t e n t i a l s a t t a i n e d could n o t be o f a c o n s t a n t value f o r every m e t a l since the h y d r a z i n e e l e c t r o - o x i d a t i o n r e a c t i o n was very fast on some metals at relatively low anodic potentials, as described below. Experimental method The e x p e r i m e n t s were c o n d u c t e d in a t h r e e - c o m p a r t m e n t P y r e x cell as described previously [7, 8 ] . H e l i u m was b u b b l e d in the working and the c o u n t e r c o m p a r t m e n t s . A reversible h y d r o g e n e l e c t r o d e in the same solution was used t h r o u g h o u t these e x p e r i m e n t s . T h e i n s t r u m e n t s and e x p e r i m e n t a l t e c h n i q u e have b e e n described b e f o r e [7, 9] and the p u r i t y and p r e p a r a t i o n o f the e l e c t r o d e s were the same [ 6 ] . T h e h y d r a z i n e (95% in water) was supplied b y E a s t m a n Organic Chemicals. T h e e l e c t r o d e s were pre-polarized at the highest p o t e n t i a l possible, as indicated in Table 1, f o r 30 min and t h e n the p o t e n t i a l was d e c r e a s e d by 3 m V steps and the log i r e c o r d e d a f t e r a pause o f 2 min at each new p o t e n t i a l value. Results and discussion The p o l a r i z a t i o n values a t t a i n e d for various metals at a fixed c u r r e n t density (50 m A cm -2 o f a p p a r e n t surface area) are given in Table 1, t o g e t h e r with the results o f G u t j a h r [ 1 ] . Also indicated in Table 1 are the a n o d i c potentials at which the various e l e c t r o d e s were pre-polarized and f r o m which the c u r r e n t - p o t e n t i a l curves c o m m e n c e d . T h e general a g r e e m e n t b e t w e e n our results and t h o s e o f G u t j a h r [1] m a y be n o t e d in Table 1; the small dis-
82 TABLE 1 Polarization data for different metals for hydrazine oxidation at 50 mA cm 2 in 1M KOH solutions Electrode
Pre-polarization potential (V(R.H.E.))
Potential at 5 0 m A c r o 2 (V(R.H.E.))
Previous results [ 1, 4 l (V(R.H.E.))
Rh Pd Ir Au Pt Ag Co Ni Fe
0.60 0.65 0.85 1.0 0.85 1.0 1.44 1.44 1.85
0.49 0.56 0.67 0.73 0.79 0.86 1.23 1.44 1.5
0.31 0.46 0.78 0.66 0.82
N o tes.
(1) Co, Ni and Fe are definitely covered by phase oxides at the above-indicated potentials, i.e. 1.44, 1.44 and 1.85 V, respectively. (2) See text for the possible reason for the discrepancies between our results and the previous work [1, 4]. crepancies as well as the inversion in the o r d e r o f the catalytic activity o f Pt and A u in the t w o studies are perhaps due t o the presence o f surface oxides on o u r electrodes. In the e x p e r i m e n t s o f Gutjahr, the e l e c t r o d e p o t e n t i a l s were s c a n n e d f r o m c a t h o d i c to a n o d i c p o t e n t i a l values, whereas in o u r w o r k the reverse situation obtained. F o r all the metals e x a m i n e d b y us, the corrosion c u r r e n t s are at least one to t w o orders of m a g n i t u d e smaller t h a n the h y d r a z i n e o x i d a t i o n currents. In Figs. 1 and 2, the actual p o l a r i z a t i o n curves are illustrated f o r Ni, Co, Fe and Ag in KOH, in t h e absence and presence o f h y d r a z i n e . In t h e absence of h y d r a z i n e , the observed currents originate f r o m c o r r o s i o n o f t h e metal (Fe, Co) as well as f r o m the o x y g e n e v o l u t i o n reaction. F r o m these Figures, the i m p o r t a n t e l e c t r o c h e m i c a l activity o f c o b a l t at low potentials (below 0.5 V) m a y be n o t e d . This high activity is c o n s i s t e n t with the result observed previously for the a n o d i c o x i d a t i o n o f m e t h a n o l on this electrode [ 6 ] . T h e high catalytic activity for the nickel electrode is n o t observed on t h e oxidecovered e l e c t r o d e as it was for the m e t h a n o l o x i d a t i o n u n d e r similar conditions [ 6 ] . However, for p o l a r i z a t i o n curves o b t a i n e d on oxide-free surfaces at low potentials ( n o t p r e s e n t e d here), nickel is m u c h m o r e e l e c t r o c a t a l y t ically active than o t h e r metals, e x c e p t Co, Ir and Rh. Its activity is an o r d e r of m a g n i t u d e higher t h a n t h a t o f Pt at 0.1 V (R.H.E.). The p o l a r i z a t i o n curves for the n o b l e metals are n o t r e p o r t e d here since t h e y are essentially similar to the ones r e p o r t e d previously [1, 4 ] . The e x a c t m e c h a n i s m for the anodic o x i d a t i o n o f h y d r a z i n e is still a m a t t e r of s o m e d e b a t e [1 - 5] b u t the data gathered so far b y d i f f e r e n t w o r k e r s t e n d to focus on the d e h y d r o g e n a tion o f the a b s o r b e d h y d r a z i n e as the r a t e - d e t e r m i n i n g step. This m e c h a n i s m thus leads us to e x p l o r e w h e t h e r a correlation b e t w e e n the rate o f h y d r a z i n e
83 t.8
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r
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j/
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Co/'
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, ~e
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.)
1.0 _==0.8 W
0.6
.
j
~-L.
-.
,,
/"
0.4 0.2
Iiili] 10
e
10 -5
I
I Illllrl
10-"
i
t0-3
i I~lllll
t0-2
I
I IIIIII
I
t0-~
log i (Acm -=)
Fig. 1. S t e a d y - s t a t e p o t e n t i o s t a t i c polarization curves for d i f f e r e n t e l e c t r o d e s in 1M KOH; t e m p e r a t u r e 25 °C, d e s c e n d i n g p o t e n t i a l steps o f 3 m V per 2 min. Polarization o f 30 rain at t h e highest p o t e n t i a l indicated o n the Figure. t.8
i
i iJ,,ll
I
~
iii1,,i
I
i
i1,11,11
i
i iii,u
I
i
i1,1111]
1.6
/J
ii*Tu
/
IM KOH 2M N=H4
/;e /
1.4 0. -'f/" /
-
~J
1.2 I.C
f
1"
. . . .
g
,,"
.-'"
Ag
0.6 0.4
\.
-~___ /
0.2
/'
/
-
i
i i iliad
/ j
0 -e
i iiiiflJ
i
i0-5
i i tlllll
t0-4
L J i Jtlll]
i
t0-~ log i (Acm -z)
i t t,atll
10-2
i
i i ii~f~l
t0 -~
Fig. 2. S t e a d y - s t a t e p o t e n t i o s t a t i c polarization curves for d i f f e r e n t e l e c t r o d e s in t h e presence o f 2M N2H 4 in 1M K O H ; t e m p e r a t u r e 25 °C, d e s c e n d i n g p o t e n t i a l steps o f 3 m V per 2 min. Polarization o f 30 min at the highest p o t e n t i a l indicated o n t h e Figure.
oxidation and the rate of the hydrogen evolution reaction on various metals can be established. The log i at 0.4 V for the hydrazine oxidation was plotted against the log i0 (the logarithm of the current at the reversible potential) for the hydrogen evolution reaction (HER) [10] for the different metals, as in Fig. 3. The approximate linear relationship observed in Fig. 3 seems to confirm the proposed mechanism in which the dehydrogenation of the hydrazine plays a predominant role in the kinetics of the reaction. The potential chosen to establish the correlation {i.e. 0.4 V) represents an oxide-
84 I0-'~
Rh o
t0-* Ir
Pd
tO-S
o
Pt
o
z
>0 10_4 L~I
t 0 -5
Au 0
t 0 -6
7
6
5 4 3 -(log io), pH=O, HER
2
I
Fig. 3. Correlation between the anodic oxidation current density of hydrazine with the exchange current density for the hydrogen evolution reaction; the latter values refer to pH = 0 [ 1 0 ] .
free surface, which would be the most appropriate for discussions involving HER or dehydrogenation. Cobalt, nickel and iron, on which an oxide is still present at this potential, do not follow the proposed correlation. No definite trend or correlation could be established between the current density for the hydrazine oxidation and other quantities such as metalmetal bond energies, heats of chemisorption of hydrogen on metals and metal-hydrogen bond energies in gaseous hydrides, etc. Also, the electroactivity of hydrazine on oxide-covered electrodes was found to be unrelated to the quantities such as heats of formation or atomization (taken as per equivalent) of the corresponding oxides. 1 M. A. G u t j a h r , in G. S a n d s t e d e (ed.), F r o m Electrocatalysis to Fuel Cells, University of Washington Press, Seattle, 1972. 2 J. A. Harrison and Z. A. Khan, J. Electroanal. Chem., 26 ( 1 9 7 0 ) 1. 3 M. Petek and S. Bruckenstein, J. Electroanal. Chem., 47 ( 1 9 7 3 ) 329. 4 M. A. G u t j a h r a n d W. Vielstich, Chem. Ing. T e c h n o l . , 40 ( ] 9 7 2 ) 180. 5 M. F l e i s c h m a n n , K. K o r i n e k and D. Pletcher, J. Electroanal. Chem., 34 ( 1 9 7 2 ) 499. 6 A. K. Vijh, J. Catalysis, 37 ( 1 9 7 4 ) 410. 7 A. B~langer, G. B~langer a n d A. K. Vijh, J. E l e c t r o c h e m . Soc., 118 ( 1 9 7 1 ) 1543. 8 A. K. Vijh, J. E l e c t r o c h e m . Soc., 119 ( 1 9 7 2 ) 679. 9 G. B~langer, J. E l e c t r o c h e m . Soc., 118 ( 1 9 7 1 ) 583. 10 A. B~langer a n d A. K. Vijh, E l e k t r o k h i m . , 10 ( 1 9 7 4 ) 1 8 5 4 ; Soviet E l e c t r o c h e m . , 10 ( 1 9 7 4 ) 1754.
81
Short Communication
E f f e c t o f the state o f the surface on the electrocatalysis o f the a n o d i c oxidation of hydrazine
G. Bt~LANGER and A. K. VIJH Hydro-Quebec Institute of Research. Varennes, P.Q. (Canada) (Received May 11, 1976)
T h e e l e c t r o - o x i d a t i o n o f h y d r a z i n e has b e e n the subject o f n u m e r o u s studies and its e l e c t r o c h e m i c a l activity in alkaline solutions is well k n o w n ; several fuel cell p r o t o t y p e s based o n h y d r a z i n e have been p r o p o s e d [ 1 ]. In alkaline solutions, n o b l e metals [1 - 3 ] , some c a r b o n s u p p o r t e d catalysts, nickel [3 - 5] and nickel boride [1] have previously b e e n e x a m i n e d . As in our r e c e n t p a p e r on m e t h a n o l o x i d a t i o n [ 6 ] , the metals investigated in the present w o r k are Pt, Ir, Rh, Au, Ag, Ni, Co and Fe in 1M KOH. T h e electrodes were pre-polarized anodically t o o b t a i n o x i d e - c o v e r e d surfaces and the p o l a r i z a t i o n curves were r e c o r d e d , c o m m e n c i n g f r o m the high a n o d i c potentials. T h e high anodic p o t e n t i a l s a t t a i n e d could n o t be o f a c o n s t a n t value f o r every m e t a l since the h y d r a z i n e e l e c t r o - o x i d a t i o n r e a c t i o n was very fast on some metals at relatively low anodic potentials, as described below. Experimental method The e x p e r i m e n t s were c o n d u c t e d in a t h r e e - c o m p a r t m e n t P y r e x cell as described previously [7, 8 ] . H e l i u m was b u b b l e d in the working and the c o u n t e r c o m p a r t m e n t s . A reversible h y d r o g e n e l e c t r o d e in the same solution was used t h r o u g h o u t these e x p e r i m e n t s . T h e i n s t r u m e n t s and e x p e r i m e n t a l t e c h n i q u e have b e e n described b e f o r e [7, 9] and the p u r i t y and p r e p a r a t i o n o f the e l e c t r o d e s were the same [ 6 ] . T h e h y d r a z i n e (95% in water) was supplied b y E a s t m a n Organic Chemicals. T h e e l e c t r o d e s were pre-polarized at the highest p o t e n t i a l possible, as indicated in Table 1, f o r 30 min and t h e n the p o t e n t i a l was d e c r e a s e d by 3 m V steps and the log i r e c o r d e d a f t e r a pause o f 2 min at each new p o t e n t i a l value. Results and discussion The p o l a r i z a t i o n values a t t a i n e d for various metals at a fixed c u r r e n t density (50 m A cm -2 o f a p p a r e n t surface area) are given in Table 1, t o g e t h e r with the results o f G u t j a h r [ 1 ] . Also indicated in Table 1 are the a n o d i c potentials at which the various e l e c t r o d e s were pre-polarized and f r o m which the c u r r e n t - p o t e n t i a l curves c o m m e n c e d . T h e general a g r e e m e n t b e t w e e n our results and t h o s e o f G u t j a h r [1] m a y be n o t e d in Table 1; the small dis-
82 TABLE 1 Polarization data for different metals for hydrazine oxidation at 50 mA cm 2 in 1M KOH solutions Electrode
Pre-polarization potential (V(R.H.E.))
Potential at 5 0 m A c r o 2 (V(R.H.E.))
Previous results [ 1, 4 l (V(R.H.E.))
Rh Pd Ir Au Pt Ag Co Ni Fe
0.60 0.65 0.85 1.0 0.85 1.0 1.44 1.44 1.85
0.49 0.56 0.67 0.73 0.79 0.86 1.23 1.44 1.5
0.31 0.46 0.78 0.66 0.82
N o tes.
(1) Co, Ni and Fe are definitely covered by phase oxides at the above-indicated potentials, i.e. 1.44, 1.44 and 1.85 V, respectively. (2) See text for the possible reason for the discrepancies between our results and the previous work [1, 4]. crepancies as well as the inversion in the o r d e r o f the catalytic activity o f Pt and A u in the t w o studies are perhaps due t o the presence o f surface oxides on o u r electrodes. In the e x p e r i m e n t s o f Gutjahr, the e l e c t r o d e p o t e n t i a l s were s c a n n e d f r o m c a t h o d i c to a n o d i c p o t e n t i a l values, whereas in o u r w o r k the reverse situation obtained. F o r all the metals e x a m i n e d b y us, the corrosion c u r r e n t s are at least one to t w o orders of m a g n i t u d e smaller t h a n the h y d r a z i n e o x i d a t i o n currents. In Figs. 1 and 2, the actual p o l a r i z a t i o n curves are illustrated f o r Ni, Co, Fe and Ag in KOH, in t h e absence and presence o f h y d r a z i n e . In t h e absence of h y d r a z i n e , the observed currents originate f r o m c o r r o s i o n o f t h e metal (Fe, Co) as well as f r o m the o x y g e n e v o l u t i o n reaction. F r o m these Figures, the i m p o r t a n t e l e c t r o c h e m i c a l activity o f c o b a l t at low potentials (below 0.5 V) m a y be n o t e d . This high activity is c o n s i s t e n t with the result observed previously for the a n o d i c o x i d a t i o n o f m e t h a n o l on this electrode [ 6 ] . T h e high catalytic activity for the nickel electrode is n o t observed on t h e oxidecovered e l e c t r o d e as it was for the m e t h a n o l o x i d a t i o n u n d e r similar conditions [ 6 ] . However, for p o l a r i z a t i o n curves o b t a i n e d on oxide-free surfaces at low potentials ( n o t p r e s e n t e d here), nickel is m u c h m o r e e l e c t r o c a t a l y t ically active than o t h e r metals, e x c e p t Co, Ir and Rh. Its activity is an o r d e r of m a g n i t u d e higher t h a n t h a t o f Pt at 0.1 V (R.H.E.). The p o l a r i z a t i o n curves for the n o b l e metals are n o t r e p o r t e d here since t h e y are essentially similar to the ones r e p o r t e d previously [1, 4 ] . The e x a c t m e c h a n i s m for the anodic o x i d a t i o n o f h y d r a z i n e is still a m a t t e r of s o m e d e b a t e [1 - 5] b u t the data gathered so far b y d i f f e r e n t w o r k e r s t e n d to focus on the d e h y d r o g e n a tion o f the a b s o r b e d h y d r a z i n e as the r a t e - d e t e r m i n i n g step. This m e c h a n i s m thus leads us to e x p l o r e w h e t h e r a correlation b e t w e e n the rate o f h y d r a z i n e
83 t.8
i
i IlIHl[
r
t.6 _
] IlllJl
I
I
] l l ] ~ l
~A~/7~-
1.4
•
I ==llU[
I
IllIHq
-
I
I IIIII]'
~. Ko.
j/
Ni.," $" ..//
t.2
Co/'
......"
, ~e
\
.)
1.0 _==0.8 W
0.6
.
j
~-L.
-.
,,
/"
0.4 0.2
Iiili] 10
e
10 -5
I
I Illllrl
10-"
i
t0-3
i I~lllll
t0-2
I
I IIIIII
I
t0-~
log i (Acm -=)
Fig. 1. S t e a d y - s t a t e p o t e n t i o s t a t i c polarization curves for d i f f e r e n t e l e c t r o d e s in 1M KOH; t e m p e r a t u r e 25 °C, d e s c e n d i n g p o t e n t i a l steps o f 3 m V per 2 min. Polarization o f 30 rain at t h e highest p o t e n t i a l indicated o n the Figure. t.8
i
i iJ,,ll
I
~
iii1,,i
I
i
i1,11,11
i
i iii,u
I
i
i1,1111]
1.6
/J
ii*Tu
/
IM KOH 2M N=H4
/;e /
1.4 0. -'f/" /
-
~J
1.2 I.C
f
1"
. . . .
g
,,"
.-'"
Ag
0.6 0.4
\.
-~___ /
0.2
/'
/
-
i
i i iliad
/ j
0 -e
i iiiiflJ
i
i0-5
i i tlllll
t0-4
L J i Jtlll]
i
t0-~ log i (Acm -z)
i t t,atll
10-2
i
i i ii~f~l
t0 -~
Fig. 2. S t e a d y - s t a t e p o t e n t i o s t a t i c polarization curves for d i f f e r e n t e l e c t r o d e s in t h e presence o f 2M N2H 4 in 1M K O H ; t e m p e r a t u r e 25 °C, d e s c e n d i n g p o t e n t i a l steps o f 3 m V per 2 min. Polarization o f 30 min at the highest p o t e n t i a l indicated o n t h e Figure.
oxidation and the rate of the hydrogen evolution reaction on various metals can be established. The log i at 0.4 V for the hydrazine oxidation was plotted against the log i0 (the logarithm of the current at the reversible potential) for the hydrogen evolution reaction (HER) [10] for the different metals, as in Fig. 3. The approximate linear relationship observed in Fig. 3 seems to confirm the proposed mechanism in which the dehydrogenation of the hydrazine plays a predominant role in the kinetics of the reaction. The potential chosen to establish the correlation {i.e. 0.4 V) represents an oxide-
84 I0-'~
Rh o
t0-* Ir
Pd
tO-S
o
Pt
o
z
>0 10_4 L~I
t 0 -5
Au 0
t 0 -6
7
6
5 4 3 -(log io), pH=O, HER
2
I
Fig. 3. Correlation between the anodic oxidation current density of hydrazine with the exchange current density for the hydrogen evolution reaction; the latter values refer to pH = 0 [ 1 0 ] .
free surface, which would be the most appropriate for discussions involving HER or dehydrogenation. Cobalt, nickel and iron, on which an oxide is still present at this potential, do not follow the proposed correlation. No definite trend or correlation could be established between the current density for the hydrazine oxidation and other quantities such as metalmetal bond energies, heats of chemisorption of hydrogen on metals and metal-hydrogen bond energies in gaseous hydrides, etc. Also, the electroactivity of hydrazine on oxide-covered electrodes was found to be unrelated to the quantities such as heats of formation or atomization (taken as per equivalent) of the corresponding oxides. 1 M. A. G u t j a h r , in G. S a n d s t e d e (ed.), F r o m Electrocatalysis to Fuel Cells, University of Washington Press, Seattle, 1972. 2 J. A. Harrison and Z. A. Khan, J. Electroanal. Chem., 26 ( 1 9 7 0 ) 1. 3 M. Petek and S. Bruckenstein, J. Electroanal. Chem., 47 ( 1 9 7 3 ) 329. 4 M. A. G u t j a h r a n d W. Vielstich, Chem. Ing. T e c h n o l . , 40 ( ] 9 7 2 ) 180. 5 M. F l e i s c h m a n n , K. K o r i n e k and D. Pletcher, J. Electroanal. Chem., 34 ( 1 9 7 2 ) 499. 6 A. K. Vijh, J. Catalysis, 37 ( 1 9 7 4 ) 410. 7 A. B~langer, G. B~langer a n d A. K. Vijh, J. E l e c t r o c h e m . Soc., 118 ( 1 9 7 1 ) 1543. 8 A. K. Vijh, J. E l e c t r o c h e m . Soc., 119 ( 1 9 7 2 ) 679. 9 G. B~langer, J. E l e c t r o c h e m . Soc., 118 ( 1 9 7 1 ) 583. 10 A. B~langer a n d A. K. Vijh, E l e k t r o k h i m . , 10 ( 1 9 7 4 ) 1 8 5 4 ; Soviet E l e c t r o c h e m . , 10 ( 1 9 7 4 ) 1754.