9 Thermodynamic Studies of Some Electrode Materials P. G. DICKENS
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Inorganic Chemistry Laboratory, Oxford University, Oxford QX1 3QR, England
The thermochemistry of a class of ternary oxide phases, the "oxide bronzes," A MO , is investigated. In this formulation MO is the highest oxide of a transition metal M = W, Mo, or V ; A is some other electropositive inclusion element such as Νa, K, or H; and x is a variable 0 < x < 1. The mate rials chosen for study are chemically inert towards nonoxi dizing acid media and are good electronic conductors; some have previously been examined as potential fuel cell elec trodes. Enthalpies of formation of a range of tungsten, molybdenum, and vanadium oxide bronzes are determined. The thermodynamic stability of these materials towards oxidation and disproportionation is examined, and their electrochemical characteristics are discussed. x
n
n
h e m a t e r i a l s r e f e r r e d t o i n t h e title a r e r e s t r i c t e d t o t h e " o x i d e A
b r o n z e s " (1, 2) Α^ΜΟ,», a class of t e r n a r y o x i d e phases d e r i v e d f o r
m a l l y b y t h e i n s e r t i o n of a n e l e c t r o p o s i t i v e e l e m e n t A i n t o t h e o x i d e matrix M O MO
n
of a t r a n s i t i o n m e t a l M . C o m m o n examples of p a r e n t oxides
w h i c h f o r m oxide b r o n z e s , a n d o n w h i c h s p e c i a l a t t e n t i o n is f o c u s e d
n
here, are W 0 , M o O , a n d V 0 . 3
s
2
8
T h e i n s e r t i o n e l e m e n t A is t y p i c a l l y a n
a l k a l i m e t a l , b u t analogous c o m p o u n d s Η * Μ Ο a n d these too a r e of c u r r e n t interest (3, 4).
Λ
are f o r m e d b y h y d r o g e n , T h e observed
electronic
p r o p e r t i e s of t h e a l k a l i m e t a l o x i d e bronzes suggest ( I ) t h a t t h e i n s e r t i o n e l e m e n t A is present as a c a t i o n , a n d t h e average o x i d a t i o n state of M is l o w e r e d a c c o r d i n g l y t o < 2 n . C o n s e q u e n t l y A J M O n behaves as a n elec t r o n i c c o n d u c t o r , [A *MO (e )] x
(χ >
n
x
y
e i t h e r m e t a l l i c as i n t h e case o f N a ^ W O e
0.3) o r s e m i c o n d u c t i n g , as f o r e x a m p l e w i t h β-ΝΆ Υ 0 . χ
2
6
Chemi
cally the alkali m e t a l oxide bronzes tend to b e rather inert, p a r t i c u l a r l y i n t h e case of A W0 , x
3
a n d resistant to attack b y n o n - o x i d i z i n g a c i d s . T h e 165
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
166
SOLID S T A T E
CHEMISTRY
c o m b i n a t i o n of m e t a l l i c c o n d u c t i v i t y a n d c h e m i c a l inertness has s t i m u l a t e d interest i n these m a t e r i a l s as p o t e n t i a l f u e l c e l l electrodes In Aa,MO
n
(5).
t h e p r o p o r t i o n χ of the i n s e r t e d element m a y v a r y c o n
tinuously over a w i d e range 0 < χ < c a l s t o i c h i o m e t r i c ratios o c c u r .
1, a n d l a r g e d e v i a t i o n s f r o m c l a s s i
F o r e x a m p l e , i n the w e l l k n o w n c u b i c
s o d i u m tungsten bronzes N a a . W 0
3
a single phase exists o v e r t h e r a n g e
0.37 ^ χ ^ 0.95, w h i c h m a y b e r e g a r d e d as a s o l i d s o l u t i o n of N a i n t h e W0
3
m a t r i x . P r o v i d e d t h a t t h e process of d i s s o l u t i o n of the i n s e r t i o n
e l e m e n t is sufficiently r a p i d a n d the r a n g e of h o m o g e n e i t y is sufficiently l a r g e , s u c h n o n - s t o i c h i o m e t r i c solids c o u l d f u n c t i o n as s o l i d - s o l u t i o n elec Downloaded by CORNELL UNIV on October 22, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0163.ch009
trodes i n a p p r o p r i a t e e l e c t r o c h e m i c a l cells a c t i n g , f o r e x a m p l e , as b a t t e r y cathodes.
I n v i e w of t h i s i t is c l e a r l y d e s i r a b l e t h a t q u a n t i t a t i v e t h e r m o
d y n a m i c d a t a f o r s u c h m a t e r i a l s b e a v a i l a b l e . P r i o r to 1970 n o p u b l i s h e d t h e r m o d y n a m i c d a t a f o r a n y o x i d e b r o n z e system existed. T h e o b j e c t of this p a p e r is t o present a n d a n a l y z e t h e t h e r m o d y n a m i c d a t a f o r some o x i d e b r o n z e phases of v a n a d i u m , m o l y b d e n u m , a n d t u n g s t e n o b t a i n e d r e c e n t l y b y the a u t h o r a n d c o - w o r k e r s i n O x f o r d ( 2 ) .
B a c k g r o u n d for
this s t u d y is p r o v i d e d b y a b r i e f a c c o u n t of t h e m a i n s t r u c t u r a l features of the phases e x a m i n e d .
(a)
w
Β ο \>—j X (c)
Ce)
Figure 1. (a) The single ReO chain, (b) perovskite structure, and (c) projection along chain axis s
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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9.
DICKENS
Figure
2.
Structures
Thermodynamic
(a) Hexagonal
of
Oxide
A WO T
Bronzes
167
Studies
s
structure, structure
and (b) Tetragonal
(I)
A WO x
s
(2)
I n A t f M O n the t r a n s i t i o n m e t a l M is u s u a l l y i n a h i g h o x i d a t i o n state a n d has a s m a l l c r y s t a l r a d i u s . It exerts s t r o n g d i r e c t i o n a l b o n d i n g effects o n its nearest n e i g h b o r s i n the c r y s t a l .
Accordingly tunnel and layer
structures o c c u r , c o n s i s t i n g of l i n k e d p o l y h e d r a .
S u c h s t r u c t u r a l types
c a n a c c o m m o d a t e a large v a r i a t i o n of A c o n t e n t e i t h e r i n interstices o r b e t w e e n layers. T h e c o m m o n s t r u c t u r a l u n i t of the b r o n z e - f o r m i n g p a r e n t oxides is t h e M O
e
o c t a h e d r o n w h i c h is essentially r e g u l a r i n W 0
M 0 O 3 b u t severely d e f o r m e d i n V 0 . 2
and
3
T h e b r o n z e s o f t e n possess s t r u c
5
tures closely r e l a t e d to the p a r e n t o x i d e , a n d these c a n b e d e s c r i b e d i n terms of t h e a r r a n g e m e n t of a f e w s i m p l e groups of o c t a h e d r a as i l l u s t r a t e d i n F i g u r e s 1 a n d 2. Tungsten Bronzes. A l l t h e k n o w n t u n g s t e n b r o n z e structures c o n t a i n a t h r e e - d i m e n s i o n a l t u n n e l f r a m e w o r k of t h e host l a t t i c e , of s t o i c h i o m e t r y W 0 , c o n s i s t i n g of s i n g l e R e 0 3
3
c h a i n s ( F i g u r e 1) w h i c h share
a l l e q u a t o r i a l vertices w i t h f o u r other c h a i n s . I d e a l i z e d projections
of
t h e t u n g s t e n - b r o n z e structures are s h o w n i n F i g u r e s 1 a n d 2. A
atoms
o c c u p y t u n n e l sites l e v e l w i t h t h e a p i c a l o x y g e n atoms. C o m p l e t e
filling
of the tunnels i n a l l cases corresponds to the c o m p o s i t i o n A M 0 .
The
3
hexagonal tungsten bronze structure ( F i g u r e 2)
is f o u n d i n t u n g s t e n
b r o n z e s w i t h t h e largest i n s e r t i o n ions K , R b , C s , TÏÏ, I n , a n d N H 4 .
In
this s t r u c t u r e o n l y the s i x - s i d e d tunnels a r e o c c u p i e d , a n d the m a x i m u m
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
168
SOLID S T A T E
CHEMISTRY
c o m p o s i t i o n c o r r e s p o n d s to A0.33WO3. T h e t e t r a g o n a l t u n g s t e n b r o n z e s t r u c t u r e ( T i ) s h o w n i n F i g u r e 2 contains three types of t u n n e l : t r i a n g u lar ( T ) , square ( S ) , a n d pentagonal ( P ) .
Complete
tunnels corresponds to ( T ) . 4 ( S ) . 2 ( P ) o . 4 W 0 . 3
0
0
filling
of a l l t h e
I n the tungsten bronzes
o n l y S a n d Ρ t u n n e l s a r e o c c u p i e d , a n d the s t r u c t u r e is f o u n d f o r A ions of
m e d i u m size.
E x a m p l e s are
Na^WOa
(0.21^x^0.35),
K^WOg
(0.40 ^ χ ^ 0 . 5 9 ) . D i s t o r t e d p e r o v s l d t e structures ( F i g u r e 1) are f o u n d i n m a n y t u n g s t e n b r o n z e s at l o w χ v a l u e s . A s the χ c o n t e n t increases, a p r o g r e s s i v e t r a n s i t i o n occurs f r o m a l a t t i c e of l o w s y m m e t r y to o n e of h i g h e r s y m m e t r y , a n d f o r c e r t a i n systems a c u b i c p h a s e is r e a c h e d (e.g.,
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N a ^ W O g is c u b i c f o r 0.37 ^ χ ^ 0 . 9 5 ) .
D i s t o r t i o n s of t h e W 0
3
frame
w o r k o c c u r i n t h e a c t u a l structures, a n d i n the case of the c u b i c h y d r o g e n tungsten bronze H a , W 0
3
t h e h y d r o g e n a t o m is d i s p l a c e d f r o m t h e i d e a l
p e r o v s l d t e site a n d is r a n d o m l y a t t a c h e d to o x y g e n atoms as h y d r o x y l g r o u p s ; i.e., this c o m p o u n d is m o r e c o r r e c t l y f o r m u l a t e d as W 0 . , ( O H ) 3
i )
(4). Molybdenum Bronzes. M o 0 consist of d o u b l e R e 0
3
3
itself has a l a y e r s t r u c t u r e . T h e l a y e r s
c h a i n s s h a r i n g edges i n t h e s t a g g e r e d m a n n e r
s h o w n i n F i g u r e 3. T h e b a s i c f r a m e w o r k p r o b a b l y s u r v i v e s i n t a c t i n t h e hydrogen molybdenum bronze that Η
Η*Μο0
3
i n w h i c h i t is suggested
atoms are i n s e r t e d as h y d r o x y l g r o u p s
between
(6)
layers.
The
c a t h o d i c b a t t e r y m a t e r i a l L i ^ M o O s m a y also b e c l o s e l y r e l a t e d s t r u c t u r a l l y to t h e p a r e n t o x i d e .
I n contrast, t h e w e l l c h a r a c t e r i z e d m o l y b
d e n u m b r o n z e s , r e d K0.33M0O3 a n d b l u e Κ * Μ ο 0
3
(0.28^x^0.30)
h a v e structures d e r i v e d f r o m o c t a h e d r a l clusters, s i m i l a r t o t h e
Figure 3.
(a) The double ReO chain, (b) projection and (c) MoO layer structure s
finite
along chain axis,
s
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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9.
DICKENS
Thermodynamic
169
Studies
(b)
Figure
4.
(a) Six-unit cluster in red K MoO , (b) tion, and (c) red K MoO structure 0
0
ss
ss
s
projec
s
p o l y n u c l e a r g r o u p s of the i s o p o l y m o l y b d a t e anions. T h e r e d K0.33M0O8 p h a s e contains a cluster of six o c t a h e d r a s h o w n i n F i g u r e 4. T h i s c l u s t e r of c o m p o s i t i o n M o 0 2 forms infinite r i b b o n s i n t h e v e r t i c a l d i r e c t i o n b y 6
2
vertex s h a r i n g w i t h s i m i l a r units. T h e r i b b o n s t h e n share f u r t h e r vertices h o r i z o n t a l l y to f o r m infinite t w o - d i m e n s i o n a l sheets
(Figure 4).
The
p o t a s s i u m ions c o m p l e t e l y o c c u p y positions of i r r e g u l a r e i g h t f o l d c o o r d i n a t i o n a n d b o n d t h e layers together.
T h i s c o m p o u n d has a t h e o r e t i c a l
c o m p o s i t i o n l i m i t of K0.33M0O3. B l u e K * M o 0
3
(0.28^x^0.30)
con
tains a different cluster c o n s i s t i n g of 10 e d g e - s h a r e d o c t a h e d r a ( Μ θ ι Ο ) 0
3 0
w h i c h c o n n e c t w i t h s i m i l a r groups t o f o r m the l a y e r s t r u c t u r e s h o w n i n F i g u r e 5.
T h e p o t a s s i u m ions a g a i n o c c u p y
sites b e t w e e n
t h e layers
w h i c h are f u l l y o c c u p i e d at χ = 0.30. T h e s t r u c t u r e of a l i t h i u m m o l y b d e n u m bronze L i J M o O s ( 0 . 3 1 ^ x ^ 0 . 3 9 ) ,
p r e p a r e d at h i g h t e m p e r a
tures ( 8 ) , has n o t b e e n d e t e r m i n e d , b u t t h e l a t t i c e p a r a m e t e r s suggest t h a t i t too consists of o c t a h e d r a l clusters. T h e o x y g e n - d e f i c i e n t d e n u m bronze N a M o O i 6
Vanadium Bronzes.
7
has a d i s t o r t e d p e r o v s l d t e s t r u c t u r e . The idealized V 0 2
5
structure ( F i g u r e 6)
b e g e n e r a t e d b y l i n k i n g together single o c t a h e d r a l shearing two R e 0
3
molyb
chains together a l o n g c o m m o n
ribbons
formed
can by
o c t a h e d r a l edges.
E a c h ribbon connects v i a its free vertices to f o u r s i m i l a r r i b b o n s . I n the a c t u a l s t r u c t u r e c o n s i d e r a b l e d i s t o r t i o n f r o m this s i m p l e r e p r e s e n t a t i o n occurs; t h e c o o r d i n a t i o n is closer to fivefold ( t r i g o n a l p y r a m i d a l ) , a n d the
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
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170
SOLID STATE
CHEMISTRY
(b)
Figure
5.
(a) Ten unit cluster in blue K MoO , tion, and (c) blue K MoO structure 0
0
S0
s0
(b)
s
projec
s
Figure 6. (a) The single octahedral ribbon, (b) projection, (c) V . 0 structure (idealized) represented as octahedra, and (d) represented as bipyramids 5
c o m p o u n d has a l a y e r s t r u c t u r e r a t h e r t h a n a t u n n e l structure ( F i g u r e 6 ) . O n e v a n a d i u m b r o n z e s t r u c t u r e , the α-phase, is b a s e d o n this s t r u c t u r e . I t is f o u n d f o r s m a l l concentrations of A atoms, e.g. N a a , V 0 2
0.02) w h e r e t h e i n t e r - l a y e r sites of a m o r e s y m m e t r i c a l V 0 2
occupied.
T h e a ' - N a V 0 p h a s e (0.70 ^ χ ^ x
L o w temperature i n s e r t i o n (7)
2
5
cathodic
materials based
are p r o b a b l y of this type.
5
5
(0 < χ
^
l a t t i c e are
1) has a s i m i l a r structure. on V 0 2
and alkali metal
5
T h e / ? - N a a . V 0 structure 2
5
(0.22
Goodenough and Whittingham; Solid State Chemistry of Energy Conversion and Storage Advances in Chemistry; American Chemical Society: Washington, DC, 1977.
9.
DICKENS
Thermodynamic
171
Studies
χ ^ 0 . 4 0 ) contains irregularly shaped tunnels enclosed b y b o t h double
^
(essentially octahedral) a n d single (trigonal b i p y r a m i d a l )
ribbons
(Fig
u r e 7 ) . T h e s o d i u m atoms i n t h e β - p h a s e o c c u p y t h e t u n n e l sites M i , b u t t h e p r o x i m i t y of n e i g h b o r i n g sites p r o h i b i t s t h e i r s i m u l t a n e o u s t i o n . T h e u p p e r c o m p o s i t i o n l e v e l is χ =
occupa
0 . 3 3 c o r r e s p o n d i n g to a z i g - z a g
a r r a n g e m e n t of the A ions d o w n t h e t u n n e l . T h e e x t r a p o s i t i o n M M
3
(0.44 ^
χ ^
Experimental
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2
and
m a y b e o c c u p i e d b y s m a l l e r i n s e r t i o n elements, e.g. i n j S ' - L i ^ V ^ O e 0.49)
and
J9-CU*V O 2
B
(0.26 ^
χ
