A voltammetric investigation of the charge storage reactions of hydrous iridium oxide layers

J Electroanal Chem, 162 (1984) 121-141

121

Elsevier Sequoia S A , Lausanne - Printed in The Netherlands

A V O L T A M M E T R I C I N V E S T I G A T I O N OF THE CHARGE S T O R A G E R E A C T I O N S OF H Y D R O U S IRIDIUM OXIDE LAYERS

L.D BURKE and D.P WHELAN

Chemtstry Department, Unwerst(v College, Cork (Ireland) (Received 21st April 1983, in revised form 31st August 1983)

ABSTRACT The unusual potentlal/pH behavlour of hydrous oxide layers grown on iridium under potentxal cychng conditions is interpreted on the basis of a model wluch describes the interaction between the acid-base and redox propertxes of tins type of system. It is pointed out that the suggested dxsperse nature of the material m the film is in agreement with earher conclusions as to the nature of transition metal hydrous orades m general based on magnetic susceptibility data Kinetic stu&es show that at low film thicknesses and slow sweep-rates m 1.0 tool d m - 3 H2 SO4 the main ano&e and cathodic charge storage reactions occur in a reversible manner However with thacker films and faster sweep-rates a marked diffusion llmatatlon is observed for the cathodic process; slow transfer of electrons between redox sites in the dispersed oxide material is assumed to be involved here Since, for the same film tinckness, diffusion llmatatlon during the posltxve sweep arises only at much higher sweep-rates (there appears to be a second mode of electron transfer m tins case due to the fact that in the higher, Ir(IV), oxadatlon state the oxide is slightly non-stolcinometnc), the anodic peak current densxties for thick film/fast sweep-rate condmons are generally much large The large &splacement of the anodic peak potential from its reversible value (hmltlng Tafel slope value --200 mV decade 1) demonstrates that some other form of inhabltlOn (possibly related to chemical rearrangement wltinn the surface layer) is involved here.

There is significant interest at the present ttme in the possibility of developing electrochromic display devices based on the phenomenon of electrochemically induced colour changes in species localized at electrode surfaces [1]. Following the report by Buckley and Burke [2] of such colour changes associated with a redox transition in hydrous indium oxide film produced by potential cycling, much effort has been devoted [3,4] to producing an actual display device based on this system. Redox activity of this type as of widespread interest in other areas, e.g. the battery and electrocatalysis fields, and sirmlar effects have now been described for a wide range of other hydrous metal oxide systems [5-9]. In some cases, e.g. Pt [10,11] and Au [12,13], electrochromic behaviour is not observed, the hydrous layer reducing directly to the metal with no distinct intermediate redox transition. In the case of at least one other metal examined in this laboratory, Ru [14], none of the intermediate transitions appeared to be accompanied by an electrochromic effect. Hydrous oxide films differ from their compact anhydrous analogues in a variety of ways, largely due to differences in the degree of dispersion of the material present in the surface layer. Thus according to Maclntyre and co-workers [15] the mean 0022-0728/84/$03 00

© 1984 Elsevier Sequoia S A.

122 density of hydrous iridium oxide films (ca. 2.0 g c m - 3 ) is significantly less than that of anhydrous IrO 2 (11.68 g cm 3). This dispersion, probably arises at two levels: (a) macrodlsperston due to the exastence of material in poorly crystalline form with pores filled with water and electrolyte penetrating the film to a significant degree; (b) mlcrodlsperston with oxycation chains or planes containing a high concentration of coordinated O H and H 2 0 species. A good illustration of mlcrodispersion is the aand ,/-forms of nickel oxide [16,17] where planes of brucite-type material are separated by layers of water molecules. This dispersion permits rapid injection or expulsion of counterions - a process necessary to neutralize space-charge effects associated with the addition of withdrawal of electrons during the course of redox reactions of these films. Differences in electrical conductivity between the reduced and oxidized forms of hydrous iridium oxide films have been reported by a number of authors [18,19]. The high resistance of the reduced material has been attributed [20] to the difficulty of site-to-site transfer of electrons when the oxycations in the dispersed material are in the same oxidation state. In general the hydrous oxides show an unusual p o t e n t i a l / p H response (ca. 88.6 mV per p H unit) for charge storage reactions recorded under voltammetric conditions [20]. Recent work with gold [12,13] indicated that this is due to an increase in the acidic properties of the oxide as the oxidation state of the transition metal ion in the hydrous oxide is increased. The present work arose originally from the observation (outlined in more detail later) that significant displacement of the main anodlc peak (resulting eventually in its overlap with oxygen gas evolution currents) occurred in the case of hydrous iridium oxide films at high film thicknesses and fast sweep-rates; the effect on the corresponding cathodic peak was far less dramatic. This behavlour was examined on the basis of a model outlined recently by Lavlron [21,22] to describe the behaviour of chemically modified electrodes. The transition from monolayer to multllayer-type behaviour (basically a transition from diffusionless to diffusion controlled conditions) with increasing sweep-rate a n d / o r film thickness is a feature also clearly observed with hydrous oxide layers. In the former case (diffuslonless conditions) the peak current 0 p ) is a linear function of sweep-rate (v). while in the latter, where reaction is limited by the transfer of charged species in the thick film, tp is proportional [22,23] to v to the power of 0.5 to 0.6. A further posslbihty to be borne in mind is slow electrochemical reaction m the surface layer; as outlined recently by Conway and Mozota [24] this can give rise to Tafel behavaour in the case of hydrous iridium oxide layers. Any model proposed to account for the electrochemical behaviour of hydrous oxide systems must also of course take into account their unusual p o t e n t i a l / p H behavlour. EXPERIMENTAL The working electrode consisted of a 3.63 cm length of iridium wire (0.5 m m diam.) sealed directly into glass. A conventional three compartment cell was used with a sintered glass frit separating the counter and working compartments. A

123 Luggin capillary whose tip was set at a distance of about 1 m m beneath the end of the working electrode was used to minimize errors associated with t R drop in the solution• The cell was thermostatted at 25 _+ 0.1°C and the solution in the working electrode c o m p a r t m e n t was purged just prior to recording v o l t a m m o g r a m s with purified nitrogen gas. The electrolyte (1.0 mol d m -3 H2SO4) was made up using A n a l a r grade acid and triply distilled water. Potentials were measured, and are reported, with respect to a hydrogen reference electrode in the same solution. The potential of the working electrode was controlled using a H i - T e k (Model D T 2101) potentiostat p r o g r a m m e d with the aid of a H e w l e t t - P a c k a r d (Model 3310A) function generator. Current was recorded as a potential drop across a standard resistor. Voltammetric charge values, which were taken as an approximate value of hydrous oxide film thickness, were measured as the charge associated with that portion of the negative sweep between 1.50 to 0.40 V, the sweep being recorded at a slow scan-rate (usually 10 mV s -1) between 1.50 and 0 V.

Q2

Fa at1 0 d l c

o.1

% \

E

El 0.0

0.5

1 .O

i 1.5

E/V(n.ttF9 Fig.

1. Typical cychc voltammogram for a hydrous oxide-coated mdmm electrode m 1 0 mol d m -3

H2SO4, scan conditions, 0 tO 1 50 V at 7.5 mV s-I; oxide grown at 0 to 1.50 V, 400 cycles at 3.0 V s -1.

124 RESULTS A typical v o l t a m m o g r a m for a h y d r o u s o x i d e - c o a t e d i n d i u m electrode in a q u e o u s sulphuric acid is shown in Fig. 1. T h e m a i n feature is the redox t r a n s i t i o n at a b o u t 0.98 V which gave rise to the a n o d i c a n d c a t h o d i c p e a k s E a a n d E c. A further c a t h o d i c peak, E'c, is evident m Fig. 1 at a b o u t 1.40 V; its a n o d i c c o u n t e r p a r t , E'~, was occasionally o b s e r v e d as a s h o u l d e r on the c u r r e n t increase associated m a i n l y with oxygen gas e v o l u t i o n at potenUals a b o v e a b o u t 1.30 V. A s is usual with the h y d r o u s i r i d i u m o x i d e electrode system in a q u e o u s s u l p h u n c acid media, a shoulder, E~, was observed d u r i n g the positive sweep on the c a t h o d i c side of the m a i n a n o d l c peak. A novel feature In the p r e s e n t w o r k was the occamonal a p p e a r a n c e (most c o m m o n l y at low film thicknesses a n d slow sweep-rates) of the c a t h o d i c c o u n t e r p a r t (E~') of E~. G e n e r a l l y b o t h peaks, E~ a n d E'a', d i s a p p e a r e d as the h y d r o u s layer increased in thickness (see Fig. 2). W i t h thick films a n d fast scan-rates c a t h o d i c

"+t : \ . 2 ~ I J !

/." / •

, \

. /.." ~,] "

° • .

E o

0

\o •-

°

°.

\

..... ~--

/1"

\

,25

°,,

"..°°° \

]

, -

~ v.

\.

•

;

.50 0-5

10

E /

1 5

V(RHE.)

Fig 2 Effect of charge capacity on the cyclic voltammograms (0.0 to 1 5 V, 10 mV S-1) recorded for oxade films grown at 10 Hz, 0.0. to 1.5 V (30 V s -1) in 1 0 mol dm 3 H2SO4 at 25°C Charge capacity values ( ). 28.76 mC cm -2 (1000 cycles), (. . . . . . ), 214.6 mC cm -2 (8400 cycles, i x 0 1), ( - - ), 387 9 mC cm -2 (17000 cycles, t × IM).

125 currents were generally observed (as for e x a m p l e in Fig. 1) at p o t e n t i a l values over the region 0.1 to 0.6 V d u r i n g the positive sweep. T h e p e a k s on the positive sweep in the p o t e n t i a l region b e l o w 0.5 V usually altered little on cycling; the processes occurring here (as is clear f r o m recent w o r k with p l a t i n u m [10,11]) involve a c o m b i n a t i o n of c o m p a c t film f o r m a t i o n a n d surface h y d r i d e r e m o v a l reactions, the reverse process occurring at low p o t e n t i a l s on the negative sweep. T h e influence o f charge c a p a c i t y (or h y d r o u s o x i d e film thickness) on the v o l t a m m e t r i c response of the i n d i u m oxide electrode at c o n s t a n t sweep-rate in a q u e o u s sulphuric acid is illustrated in Fig. 2. T h e m o s t n o t a b l e feature is the positive shift of E a with increasing film thickness. T h e effect was far less m a r k e d in the case of the two m a i n c a t h o d i c peaks, E c a n d E'c. A s u m m a r y of p e a k m a x i m u m p o t e n t i a l d i s p l a c e m e n t s with oxide film charge capacity, t a k e n f r o m a m o r e extensive series of v o l t a m m e t r i c experiments, is given in Fig. 3. T h e p o t e n t i a l value for the u p p e r c a t h o d i c p e a k (E'~) was f o u n d to be virtually i n d e p e n d e n t of film thackness a n d for the r e m a i n d e r of the p r o j e c t a t t e n t i o n was, therefore, c o n c e n t r a t e d on the two lower peaks, E a a n d E¢. A significant difference in b e h a v l o u r for the processes involved in these two p e a k s is clear f r o m Fig. 3. The d i s p l a c e m e n t of Ea, the a n o d i c p e a k potential, f r o m the reversible p o t e n t i a l value, with increasing film thickness,

1,4

~*

*

~

2 1.2

I'C

X

X

I 200

0

Q~

I 400

mC crff 2

Fig. 3 Variation, with the anodlc charge storage capacity (Q), of the peak potentials (Ep) for the main charge storage reactions of a hydrous oxade-coated indmm electrode The films were produced by potential cycling (0.0 to 1 5 V, 30 V s -1, 10 Hz) in 1 0 mol dm -3 H2SO4 at 25°C Cyclic voltammograms were recorded using the same llnUts at scan-rates of 1 5 (O, e), 10 ([3, B), 30 (zx,v) and 140 (+, × ) mV s -1 The symbols in brackets refer to the main anodlc (Ea) and corresponding cathodic (Ec) peaks in Fig 1 Only the mean value (*) is given for the upper cathodic peak (E~) as the potential maxamum for the latter showed only a manor variation with sweep-rate.

126 1,5 °.

1.0

//\

,..

• u

-

/

O5

E O--

.. ..........

•

•

05

!

_

I O"5

10

I 1•5

E/V(RHE) Fig 4. Effect of scan-rate on the cyclic v o l t a m m o g r a m s (0 0 to 1 5 V) recorded for an l n d m m o x i d e fdm g r o w n at 10 Hz, 0.0 to 1 5 V (30 V s 1 ), f o r 8400 cycles in 1 0 m o l drn - 3 H 2 S O 4 at 25°C, charge c a p a o t y w a s 214 6 m C c m - 2 Scan-rates of the c y c h c v o l t a m m o g r a m s . ( ), 3 0 m V s - 1 , ( . . . . . . ), 20 m V s-l(t×01),( - - ),100mVs l(/x002)

was far greater, especially at high sweep rates, than that for the corresponding cathodic peak. The influence of sweep-rate on the voltammetric behaviour of an iridium oxide electrode of constant charge capacity (or hydrous oxide film thickness) in aqueous sulphuric acid is illustrated in Fig. 4. Peak maximum potential variation with sweep-rate, for films of different oxide thickness, are illustrated in Fig. 5. As in Fig. 3, the peak potential displacement values for the cathodic process lie close to one another; for the sake of clarity, data for the cathodic peak are given for only two tbackness values in Fig. 5. The major point emerging from Figs. 3 and 5 is that there is a considerable difference in the peak displacement, and thus kinetic behaviour, between the main anodic and cathodic charge storage reactions of these hydrous films• Peak current density variations for the main anodic (Ea) and cathodic (Ec) processes are shown, for a series of four different sweep-rates, as a function of charge capacity (or hydrous oxide film thickness) in Figs. 6 and 7, respectwely. For the upper cathodic peak (E'c), the data for winch are not shown here, excellent hnear

127

~

4

~

t~

1-2

o o

o

o

•

1.0

'

'

X

I.

I

0

50

__

I

100 v /

mV

150

s~

Fig 5. Effect, for different oxide thicknesses, of scan-rate on the peak potentials (Ep) of cychc voltammograms recorded for a hydrous oxade-coated indium electrode 1.0 mol dm -3 H2SO4 at 25°C The films were produced in s]tu by potential cychng (00 to 1 5 V, 30 V s ], 10 Hz) The cychc voltammograms were recorded between the same potential llrmts (D) (28.76 mC cm-2), (A) (52 22 mC cm 2), (O) (89 96 mC cm-2), (a) (214 6 mC cm-2), (I) (290 2 mC cm -2) and (I) (387 9 mC cm -2) refer to the roam anodlc peak, Ea, (× (2876 mC cm 2) and (+) 2146 mC cm -2) refer to the mare cathodic peak, E,; (*) refers to the mean values for the upper cathodic peak E'c

b e h a v i o u r was o b s e r v e d at all sweep-rates a n d charge c a p a c i t y values used in the p r e s e n t investigation. E v i d e n t l y all species u n d e r g o i n g the transition giving rise to this p e a k d i d so in a r a p i d m a n n e r . F o r the a n o d i c p e a k (Ea) r e a s o n a b l y linear b e h a v l o u r was also o b s e r v e d (Fig. 6), the only significant d e v i a t i o n arising with the thickest films at the fastest sweep-rate used in this investigation. A s is clear f r o m Fig. 7 the d e p a r t u r e s f r o m linearity were m u c h m o r e m a r k e d in the case of the m a i n c a t h o d i c p e a k (E c). F u r t h e r results of scan-rate e x p e r i m e n t s are s u m m a r i z e d in Figs. 8 a n d 9. In these log l / l o g v plots slopes of a p p r o x i m a t e l y u n i t y were o b s e r v e d in the case of b o t h the a n o d i c a n d c a t h o d i c p e a k s at low film thicknesses, as expected [22] for thin layer c o n d i t i o n s (absence of diffusion limitation). A similar slope was also o b s e r v e d at low scan-rates for a m u c h thicker film in the case of the a n o d i c p e a k (Fig. 8). However, at sweep-rate values a b o v e a b o u t 85 mV s - I a c h a n g e o v e r to a significantly lower slope of ca. 0.72 (indicative [22] of diffusion control) was observed. A s illustrated in Fig. 9 the onset of diffusion c o n t r o l occurred at significantly lower film thicknesses a n d sweep-rate values in the case of the m a i n c a t h o d i c peak. Since the d a t a shown in Figs. 8 a n d 9 for E a a n d Ec at a n y p a r t i c u l a r thickness a n d sweep-rate value were

128

taken from the same voltammogram, it is obvious that diffusion is more rapid during the anodic reaction. Although diffusion control was not significant in the case of the main anodic peak at low sweep-rates, departures of the peak potential from the reversible value were quite marked (Fig. 5) under these conditions. Plots of peak potential vs. the logarithm of either scan-rate (Fig. 10) or peak current density (tp was found to be a linear function of scan-rate for the main anodic reaction, some deviation occurring, however, with quite thick films and fast scan-rates) were linear, with a mean Tafel slope (taken from the linear sections of the anodic peak data for the three thicker films in Fig. 10) of ca. 200 mV decade-1. Much smaller deviations from reversible behaviour were observed (Fig. 10) for the process associated with the main cathodic peak at all film thicknesses examined in the present work. For films of relatively low

15G

e~

'E

ulO 0 < E

50

0

" 0

~

~

~ 200

-I 400

Q / m C cm-2 Fig 6 Peak current density (lp) dependence on the charge capacity (Q) for the main anodlc charge storage peak (E a in Fig. 1) in cyclic voltammograms recorded for hydrous ordde-coated r a d i u m electrodes The films were produced by potential cycling (0 0 to 1.5 V, 30 V s -1, 10 Hz) in 1 0 tool d m -3 H2SO 4 at 25°C. The cyclic voltammograms were recorded using the same hmlts: (D) 1 5 mV s -1, ( O ) 10 mV s - I , (zx), 30 mV s - i ; (I) 140 mV s -1

129

,

20

•

•

~E u E

"-

IO

0

200

Q /mC

400

c m -2

Fig 7 Peak current density (lp) dependence on the charge capaoty (Q) for the lower cathodic charge storage peak (E c in Fig 1) in cyclic voltammograms recorded for hydrous oxide-coated iridium electrodes Oxide growth condmons, symbols, etc., are identical to those given with Fig. 6

3

~0

'E

1.0

0

< E cL

~-

O.G

O~ 0

- lh

I 0.0

I

I

l.O log

(v/mV

I

2.0

S-1)

Fig. 8 Effect of scan-rate (o) on the peak current density (t) for the mare ano&c reaction (peak E a in Fig. 1) for hydrous lrldmm oxide electrodes In 1.0 mol dm 3 H2SO 4 at 25°C. The films were produced by cycling m SltU (00 to I 50 V, 10 Hz, 30 V s 1) for (n) 1000 cycles (28 76 mC cm-2); (11) 17,000 cycles (387 9 mC cm-2). The figures on the diagram give the computer-estimated slope values

130

20

,~

= . . ~ . r "'4' ~o~OO j~.=....--,,'~" ~o....e~o-'°'° C--m.'/22 r-~ 1o a

Iz u

10-

'~

~

[:3"0"0"~ ''''~r n =1

O /

- 1,0 - -

I

0.0

I

I

10 log (v/mY

I

2.0

s- I )

F i g 9. Effect o f scan-rate (v) on the peak current density 0 ) for the mare cathodic reaction (peak E c m

Fig 1) Con&tlons were identical to those outhned for Fig 8 except that data for a film of intermediate thickness (grown for 4,000 cycles, 108.8 m C c m - 2 ) is included ( O ) to dlustrate the change an slope value for a given film with increasing scan-rate 1.4

./,,.-

~1.0

./A

i T

0,0

~"'~

I

1,0 l o g ( v / m V s" I )

~ -t-~

. . . ~ i - ,_,~j:Ts..~"

/

~ ~x ~-X.xJ

+~+'tI I 2.0

Fig. 10 Vanatlon of peak potential (Ep) with the l o g a n t h m of the sweep-rate (o) for the main anodic and cathodic peaks (E a and E c m Fig 1) m cychc voltammograms (0 to 1 50 V) recorded for hydrous oxade-coated indium electrodes m 1 0 mol d m -3 H2SO 4 at 25°C For oxJde growth conditions and symbols, etc., see Fig. 5

131

40

O

~

E 020(]

• • O 0

" "

I 50

"

O"

~5-----ET----~-----O I 100

0 150

V / mV gl Fig 11. Variation of the measured cathodic charge storage capacity (Q) with increasing scan-rate m cychc voltammograms (0 to 1 50 V) recorded for hydrous oxide-coated iridium electrodes m 1.0 mol d m 3 H2SO 4 at 25°C Oxide film formation conditions were slmdar to those quoted for Fig 3 No of oxide growth cycles (rn) 1000; ( O ) 4000; (zx) 17000

thickness a changeover, with increasing sweep-rate, from reversible to Tafel behaviour is clearly evident, Fig. 10, in the case of the anodic reaction. The effect of scan-rate on the cathodic charge capacity of hydrous iridium oxide films is shown in Fig. 11. In contrast to the two thinner films, where little variation was observed on increasing the scan-rate, the charge capacity values for the thickest film decreased rapidly on increasing the sweep-rate, especially at lower values for the latter. It is interesting to note that the ratio of the charge values measured at the lowest sweep-rate for these three films (1 : 3 : 14.5) is in reasonable agreement with the ratio of the number of oxide growth cycles ( 1 : 4 : 1 7 ) used to produce these layers. DISCUSSION

The difference in p o t e n t i a l / p H response between hydrous and anhydrous oxides is one of the most significant features of oxide electrochemistry [20]. Our earlier claim that the pH response of hydrous iridium oxide films, under cyclic voltammetry conditions, is in excess of 59 m V / p H unit is borne out by the recent work of Conway and Mozota [24]. Such behavlour may be explained by analogy with the recent idea proposed in the case of gold [13], namely that there is an interaction in these systems between the acidic and redox properties of the hydrous material. In the case of compact anhydrous oxide materials with strong, three-dimensional bonding (i.e. extended oxygen bridging) between cations, both hydration (or hydroxylation) and redox reactions are confined to surface oxycations. Even in the latter

132 case potential/pH responses may be higher than 59 m V / p H unit in cyclic voltammetry [25], though not in open-circuit [26], experiments with thermally prepared oxides such as RuO 2 where partial hydration of surface oxycations is apparently involved. With hydrous oxides produced by potential cycling many of the coordinated oxygen species are terminal (OH or OH2), rather than bridging. This of course is an essential requirement for (or the direct result of) the disperse nature of these materials. In some respects a hydrous oxide may be regarded as an initially anhydrous oxide with an extremely large area, hydroxylated, surface. In fact many oxides in aqueous media are probably intermediate in character between these two extremes. The hydrous oxides, because of their more open structure, d~splay much greater specific charge storage values (i.e. charge capacity per g or per mole of oxide). Since little is known of the structure of hydrous oxides in general, our speculation in this area is based on the following guidelines: (a) It is quite difficult to distinguish in the case of aquo ions in aqueous media [27] between water molecules in the inner coordination sphere and m the solvation shell, H20 exchange between these two regions is usually extremely rapid. (b) All di- and tri-positive aquo ions of the first transition series (and probably those of the second and third series) are octahedral [M(H20)6] 2+ or3+ in aqueous media. They all tend to be acidic, i.e. prone to hydrolysis [27]:

[M(H20)6 ] n+= [M(H20),(OH)](.-a)+ + H+

(1)

(c) Studies of the magnetic properties of hydrous oxides [28], in particular the higher susceptibility values of gels as compared with the corresponding crystalline materials, support the idea that these are highly dispersed amorphous materials; they undergo a vigorous exothermal reaction on strong heating and this reaction is accompanied by a major decrease in specific surface area as the material alters to the crystalline state. The magnetic susceptlblhty data for hydrous chromla and ferric oxide indicate such a degree of attenuation that thread-type structures have been proposed. In view of the assumption of a dimer species made earlier in the case of gold [13], and in the present case (see below) for the reduced form of the hydrous iridium species, it is interesting that the magnetic data led to a similar claim [28] in the case of Fe(III) gels. It was demonstrated [13] earlier that a reasonable explanation of the potential/pH behaviour of gold is possible on the basis of eqn. (2). Au(OH)(mm-3)- + 3 e - = Au + m O H -

(2)

Nernstian analysis of this reaction showed that the observed slope of 3 / 2 (2.303

RT/F) V / p H unit is possible if m = 4.5, and on the basis of this result a species of composition [Au203(OH)3.3 H20] 3- was suggested as the basis of the hydrous film. The above analysis for the composition of the hydrous layer is relatively simple in the case of gold where one form of the couple (i.e. the metal) is of known

133

composition. For iridium (and most other metals) the composition of neither the reduced or oxidized form of the surface layer is known, and the analysis outlined below (which is similar to that previously given for gold) gives only the difference in composition between the reduced and oxidized forms of the surface layer. The reaction involved in the case of the main charge storage peak of the hydrous iridium oxide layer is regarded as a classical redox process: Ir4++ e - = Ir 3+

(3)

in which the metal ion activities are modified by interaction with O H - ions:

(4) (5)

Ir 3+ + q O H - = Ir(OH)(qq- 3)Ira++ r O H - = Ir(OH)(f -4)The corresponding stability constants are: glll

= ~, ~I (r q( O - 3H) -) /q,/~~ I r 3+z~ q "~OH

(6)

g l v ~-~ _ia.(irr-(-O4 H ) -)-r/ /~U ir 4+ 6t ~ rO H -

Using the latter expressions to substitute for the metal ion activities in the Nernst equation for the electrode reaction represented by eqn. (3), viz.: E = E ° - (RTfF)

(7)

ln(alr3+/alr,,+)

it is easily shown that: E=E °+

-~1

zT(r--4)---Ir(OH)r

n ~

~Ir(OH)q

Kill

KIVP(r-q)

2.3RT

-ff

~r-q)pH

(8)

where P is the ionic product of water. Hence, if the activities of the surface species are pH-independent (and they appear to be, judging by the constant slope of the p o t e n t i a l / p H graph [20]): 8E/rpH

= -(2.3RT/F)(r-

q) = - 0 . 0 5 9 ( r - q ) V / p H unit at 25°C

(9)

Experimentally 6 E / S p H ~ - 0 . 0 8 8 V / p H unit [20]; hence r - q--- 1.5. This suggests that the charge storage reaction for the film should be written as: Ir(OH)(r r-4)- + e - = Ir(OH)(qq- 3)- + 1.5 O H -

(10)

Since values for r and q cannot be determined by the present tech0ique, the assumption made here is that the hydroxide complex of Ir(III) is similar to that of Au(III). The composition of the reduced state is then wntten as Ir2(OH) 3- or [Ir203(OH)2 • 3 H20] 3- and the net charge storage reaction may be represented as follows: 2 [IrO2(OH)2- 2 H 2 0 ] 2 - + 2 e - = [Ir203(OH)3 • 3 H 2 0 ] 3 - + 3 O H -

(11)

The reaction may be written in structural terms as outlined in eqn. (12). The counter ions in acid are assumed to be protons, and in base monovalent alkali metal ions such as Na +. It should be borne in mind that the above reactions may

134

H20

. \ j.o< Ir

-o

o Ir

Qo.---o H20

H20

H20

H20

[

+2 e'-" - 3 OH--~2e - + 3 OH-

H20

/ /t

I >oIr

Ir

o OH

I oH H20 (12)

also be written as proton addition reactions; subsequent transfer of water molecules between the coordination sphere and hydration shell would yield the same chemical species in both cases. It is important to bear in mind here that with regard to oxide electrochemistry in general departures from 59 m V / p H unit behaviour are not a manifestation of non-Nernstian behaviour. The unusual p H / d e p e n d e n c e in the case of the hydrous iridium oxide system is related in the first instance to the more acidic character of the Ir(IV) species, i.e. its greater tendency to hydrolyse due to the increased charge density around the central metal ion. As the pH of the solution is increased, the equilibria represented by eqns. (4) and (5) are stufted more to the right, i.e. the hydroxy complexes become more stable or the metal ion activity is reduced. This effect is more marked in the case of the Ir(IV) species as r is greater than q. Thus, as postulated earlier [13] in the case of the hydrous gold oxide system, the unusual p o t e n t l a l / p H response is due to this additional lowering of the activity of the oxidized form of the redox couple; however, for thin films at slow sweep-rate the redox behaviour is still clearly reversible (E, = E c, Fig. 1) and, as outlined above, a Nernstian interpretation is possible. While the structures proposed here for the hydrous film are largely speculative, their main function or advantage is to account (in terms of eqn. (12)) for the unusual p o t e n t i a l / p H dependence which appears to be a central feature of hydrous oxide electrochemistry. The model proposed here for the charge storage reaction of the hydrous film is assumed to relate to the main charge storage peaks, E, and E c in Fig. 1. The peaks labelled E~ and E'c are assumed to be due, as in the case of ruthenium [14], to a I V / V I transition. The remaining pair of peaks, labelled E~ and E~', observed (Fig. 2) only with relatively thin layers, is assumed to be due to a I I I / I V transition in species containing sulphate anions within the coordination sphere. It is assumed that the formation of such species is increasingly inhibited at higher film thicknesses due to exclusion of bulky anionic species from the metal surface by the surrounding gelatinous oxide layer. Conway and Mozota [24] have pointed out recently that the shape, location and resolution of peak E~ are greatly influenced by the nature of the ions in the solution, a further indication that the species involved here are complexes formed by incorporation of the anion produced by dissociation of the acid. It is clear from the nature of the peak potential displacement data illustrated in Figs. 2-5 that there is a considerable difference between the anodic and cathodic

135 behaviour of the main charge storage reaction of the hydrous iridium oxide layer. The more marked departure in linearity at faster sweep-rates with the thicker films in the case of peak current versus charge capacity plots for the main cathodic (Fig. 7), as compared with the main anodic (Fig. 6), process clearly indicates a more marked degree of inhibition during the reduction sweep. The effect of this inhibition is clear from Fig. 11; at fast sweep-rates much of the material present in the thicker film remains unreduced during the negative sweep. In view of the fact that the reduced form of the oxide is a poor electronic conductor [18,19] it seems reasonable to assume here, as did Conway and Mozota (who also observed a difference in sweep-rate response between the main anodic and cathodic charge storage reactions, see Fig. 7 in ref. 24), that this inhibition of the oxide reduction reaction is assooated with slow electron transfer through an increasing thickness of Ir(III) material, produced in the early stages of the negative sweep, separating the metal surface from the bulk of the outer Ir(IV) oxide. However, as discussed in more detail later, the resistance of the partly reduced layer in cyclic voltammetry experiments may be significantly less than that measured under steady state conditions as in the former case Ir(IV) centres may diffuse, by an electron hopping mechanism, to pick up electrons at the metal surface (such centres do not exist anywhere within a fully reduced layer). Incomplete reduction during the negative sweep is also the origin of the cathodic currents observed (Fig. 1) at the lower region of the positive sweep when voltammograms are recorded in a repetitive manner. Further reduction of Ir(IV) species is possible in these positive sweeps at potentials well below the main anodic peak due to slow transfer of electrons across the poorly conducting iimer layer of Ir(III) material. These cathodic currents at the lower end of the positive sweep are apparently not observed at fast (100 mV s -1) sweep-rates [24]. Indications of the effect can, however, be found in earlier work reported from this laboratory [2] where large discrepancies between the anodic and cathodic currents were observed in the region of 0.4 V in voltammograms for hydrous iridium oxide layers in acid recorded at 30 mV s -1. The effect is even more dramatic in the present case where the sweep-rate is only 7.5 mV s -1. Evidently there are two components in the overall current in this region; an anodic component due to charging of the metal/solution interface, which predominates at fast sweep-rates, and a cathodic component due to reduction of residual Ir(IV) material which predominates at slow sweep-rates. The presence of unreduced Ir(IV) species at the lower end of the sweep may be due to a combination of factors, e.g. a low bulk or intergranular electronic conductivity, slow counterion penetration into poorly dispersed regions of the film, or slow structural or chemical changes associated with the redox reaction in the latter type of material. The mechanism of charge transfer through polymer films is of considerable interest at the present time, not alone in the field of hydrous oxides but also in connection with the behaviour of chemically modified electrodes [29]. Although a band theory model has been proposed [19,24] to describe the transition from the conducting, Ir(IV), to the insulating, Ir(III), state in the case of hydrous oxides, there is an alternative view [20] in which charge transfer is xaewed in terms of electron

136

hopping from reduced to oxidized sites. This approach has also been discussed in some depth by Murray and coworkers [23,30] in connection with the redox behaviour of polymerized vlnylferrocene films on platinum. This area of research, the redox behaviour of inorganic species in organic polymers, is currently of great interest. Some of the systems, especially those based on the use of ion-exchange coatings [31-33], have the additional complication that the redox species can move within the film. It is assumed that such mass transfer of active material does not occur to a significant extent with oxycatlon groups during the course of measurements with hydrous oxide layers. It is worth noting also that loss of band structure can occur, even with metals, when the crystallite diameter drops below 10 nm [34]. The high degree of dispersion of hydrous films in general is clear from their marked electrocatalytic activity for various anodic reactions [35,36]. While this behaviour m a y be due to macrodisperslon, i.e. the presence of minute particles in the surface film giving rise to a very high surface area oxide/solution interface, the dispersion m a y well extend down to the molecular level; Glarum and Marshall, for instance, have suggested [18] that the interconnections in the hydrous polymer matrix (even in the stable or aged state) are continuously making and breaking. If such is the case then an extended band model would not be valid and, as outlined earlier [20], the difference in conductivity between the oxidized and reduced state is probably due to the fact that in the latter the metal ions are all in the III state whereas in the former (which is also the highly coloured state) there is a small degree of non-stoichlometry, i.e. traces of Ir(V) or Ir(VI) in equihbrlum with Ir(IV). The presence of nonstoichaometry in higher oxidation states of metal oxides, e.g. Ni(IV) in Ni(III) [37], is a well established feature in electrochemistry. The relationship between voltammetric peak current 0 p ) and sweep-rate (v) in the case of diffusion controlled reactions is given by eqn. (13) [38]. tp = 2.7 × 105

n3/2AD1/2CoOl/2

(13)

where n is the number of electrons transferred per reacting species (n = 1 m the present case), A denotes the electrode area, D is the diffusion coefficient and c o is the initial concentration of reacting species. Values for ~lp/~U1/2 were taken only for the thicker films and faster sweep-rates as it is clear from Figs. 8 and 9 that these are the only conditions where definite evidence for diffusion control was obtained. For the cathodic process 8i/Sv 1/2 was approximately 0.04 A V-1/2 sl/2; the corresponding value for the anodic process was 0.33 A V - l / 2 s1/2. Co, the concentration of the iridium species in the film is effectively the density of the hydrous material divided by the molecular weight. McIntyre and coworkers [15] have suggested a value of 2.0 g cm -3 for the density of the hydrous iridium oxide layer; taking the molecular weight of the oxidized form (assumed here to be [IrO2(OH)2 • 2 H20] 2-) to be 294, c 0 was calculated to be approximately 6.8 × 10 -3 mol cm -3 (the same value was taken for the positive sweep; even though the molecular weight is probably different in this case, so is the density, and accurate values for the latter are not known). Using the value of 0.57 cm 2 (the geometric area of the electrode) for A, a value of 1.5 × 10 - 9 c m 2 S- I was estimated for the diffusion coefficient in the case of the

137 negative sweep and 1.0 × 1 0 - 7 c m 2 S-1 m the case of the positive sweep. The fact that the estimated value for the diffusion coefficient for the anodlc process is substantially larger than that for the cathodic process suggests that slow electron, rather than proton, transfer may limit the rate of response of these films, as it is well known [18,19] that the reduced form of the oxide has a substantially lower electronic conductivity. This correlation, however, must be treated with caution; the low conductivity of the reduced form was observed m low amplitude perturbation ac impedance [18] or steady-state electrolysis [19] investigations, i.e. conditions where the hydrous material was totally in the reduced state. In terms of the electron hopping mechanism, cathodic current flow across such a film from a metallic substrate would entail generation of thermodynamically unfavourable lower oxidation states, e.g. Ir(II). The situation is significantly different dunng the cychc voltammetry studies, where during the course of the main catho&c peak the overall process in the film is the outward movement of the Ir(III)/Ir(IV) boundary from the metal surface. Rather than electrons moving out through the very Ir(III)-rich inner region (generating high energy Ir(II) centres in the process), it is far more likely that electron transfer will first occur at the Ir(III)/Ir(IV) boundary in the film; the resulting Ir(IV) centres then diffusing by an electron hopping mechanism (as in a p-type semiconductor) until they eventually acquire electrons from the metal surface. The slow step in the overall reaction, determining the value of the diffusion coefficient, may well be the electron transfer between the oxycations (especially at discontinuities in the dispersed, highly hydrated, regions of the polymer), a process complicated, as outlined in eqn. (12), by changes in the ligand coordination sphere. The corresponding process during the posmve sweep is an outward movement of the Ir(IV)/Ir(III) boundary, i.e. electrons diffuse via the same type of hopping mechanism from the outer Ir(III) region, through an increasing thickness of the inner Ir(IV) layer, to the metal surface. The observed faster rate of diffusion in this case may be explained by postulatxng a second route for electron transfer, one that does not operate during the course of the reduction sweep. Such a route is possible for oxidation of the film due to the fact that the Ir(IV) state is apparently non-stoichiometric. Thus on the oxidation sweep not alone are electrons withdrawn from any Ir(III) species in the region of the m e t a l / o x i d e interface but in addition some are also withdrawn from Ir(IV) species in this region. Thus there is a continuous generation of small levels of Ir(V) or Ir(VI) which migrate rapidly (again by an electron hopping process) through the Ir(IV) region to the outer layer of Ir(III) material. Dunng the negative sweep the material about the electrode is predominantly in the stoichiometric Ir(III) state whereas during the positive sweep the material in this region is mainly in the non-stoicbaometnc Ir(IV) state; with thicker films and fast sweep-rates the degree of non-stoichiometry is probably enhanced due to the increase in potential for the positive transition. This additional route for electron transfer may be the reason for the higher diffusion coefficient values in the positive sweep. Evidence supporting the idea that charge transfer in acid grown films is limited by electron, rather than ion, flow was obtained recently in work carried out [39] with

138 similar films grown on iridium in dilute base (0.1 mol dm 3 NaOH). When these were cycled in the latter solution the protons produced during the main anodic transition (eqn. (14)) neutralized the dilute base locally, i.e. within the pores of the film, with the result that there was a marked transfer of charge, with increasing scan-rate, along the voltage axis from the typical base (ca. 0.7 V), to the acid (ca. 1.0 V), value. This unusual effect was not observed when acid-grown films were cycled in the same manner in dilute base. It appears that the acid grown films are of a more open texture; ion transfer is so rapid that significant pH variations do not arise with these films. Due to the higher conductivity of the oxidized material, the oxidation reaction is able to propagate (apart from fast sweep-rate and very thick films) rapidly throughout the film. On comparing the data at fast sweep-rates for the thickest film in Figs. 6 and 7 (and these were taken from the same voltammograms) it may be seen that the peak current density values were nearly an order of magnitude larger for the anodic, as compared with the cathodic, sweep. With increasing current density (Fig. 10) there is a gradual changeover in this case of the anodic reaction from reversible to Tafel conditions. Since the peak current densities for the cathodic process were limited (apparently by slow electron transfer) to relatively low values, peak potential displacement from the reversible value remained quite small (Fig. 10, also Figs. 3 and 5). A complete changeover to a Tafel slope of ca. 200 mV decade -1 was observed (Fig. 10) with the much higher current densities involved in the case of the anodic sweep. A full interpretation of such a high Tafel slope is apparently not possible [24] in terms of conventional principles of electrochemical kinetics; possibly the value may be influenced by structural changes associated with the alteration of the inner coordination sphere as represented in eqn. (12). To lmnimize the development of space charge In the surface layer, the redox reaction in the film must be accompanied by ion n'ngration processes. While it may appear from eqn. (11) that the migrating ion is O H - (and this has been a topic of some controversy [40,41]), proton migration by the Grotthus mechanism is far more likely at low pH. Proton involvement in the overall reaction may be taken into account by representing the redox reaction in the following form: 2 [ I r O 2 ( O n ) 2 . 2 H 2 0 ] 2- + 2 e - + 3 n + = [Ir203(OH)3 • 3 H 2 0 ] 3- q- 3 H 2 0

(14)

However, the latter equation must be modified due to the fact that some of the protons involved are already present, as counterlons ( H I ) , within the film (these are equivalent to electrostatically bounded counterions in a cation exchange resin). The net reaction for the reduction process should, therefore, be represented as: 2 ([IFO2(OH)2.aH2O12-'2H~}+ae

+2H+=[Ir203(OH)3-3H20] + 3 H20

3 .3H~ (15)

Thus for each electron injected into the film there is a simultaneous transfer of one proton (H~+) from the bulk solution into the hydrous material (at the same rime there is transfer locally of 1.5 protons into the hgand sphere of the central metal ion

139 per electron added to the latter; this is the important feature from the thermodynamic viewpoint). Such charge transfer is apparently also accompanied by changes in the ligand coordination spheres of the transition metal ions, e.g. expulsion of water molecules and alterations in the polymer linkages. Direct evidence for these latter alterations in the film is quite difficult to obtain at the present time but such behaviour is not improbable. It has been demonstrated [27] with 180 experiments, for instance, that water molecule exchange between the coordination sphere and bulk solution is frequently quite rapid with transition metal ions in homogeneous solution. In addition the rate of such exchange decreases with increasing charge on the central metal ion in solution (there IS a direct analogy here with the expulsion of water molecules, see for example eqn. (14), on going from the Ir(IV) to Ir(III) state of the hydrous layer). In view of the controversy surrounding the nature of the ionic species entering or leaving these films to preserve electroneutrality during the course of oxidation [40], It is worth pointing out that even in acid, where electron loss to the metal is most likely accompanied by proton loss from a bound water molecule in the hydrous layer, the hydroxide 1on formed in the latter process coordinates to the central metal ion. In other words hydrous ion insertion processes can occur locally 0.e. at a transition metal site) during such reactions, even though this species is unlikely to participate to any great extent in the ion diffusion step at low pH. An interesting point in the present work is the question of tR effects. As outlined earlier electron transfer through these films during the cathodic sweep is probably not as difficult as passing electrons through a totally reduced Ir(III) film. Therefore the resistance of the film may not be large until most of the surface layer is reduced. In addition, since the outer hydrous oxide layer ts evidently porous [35], the metal surface, with its compact oxide surface layer in contact with the aqueous phase is unlikely to be markedly affected by tR effects (the pores in the film, filled with acid electrolyte, acting as low resistance pathway for the current). The possible influence of marked tR effects on the diffusion coefficient values estimated from cyclic voltammetry data was checked more recently (after the original submission of this paper) using a potential step technique. For semi-infinite diffusion conditions the decay of current, l, with time, t, is given by the Cottrell equation [30]: , = n F A D 1 / 2 C o / ( ~ r t ) 1/2

(16)

n, F, A, D and c o denote the same quantities here as in eqn. (13). In these experiments relatively thick films, where diffusion control is more likely, were used and the potential was stepped to a value well beyond the reversible peak potential (1.200 to 0.700 V in the case of the cathodic step) to reduce the effect of ohmic resistance in the amperometnc response [30]. Linear t / t -a/2 behaviour was observed, yielding diffusion coefficient values (estimated using similar assumptions to those outlined earlier for the potential sweep experiments) of ca. 2.3 × 10-a0 cm 2 s-1 for the cathodic process and 3.0 X 10 10 to 5.1 × 10 -9 cm 2 s -~ for the anodic process. The value for the anodic step increased with increasing value of the

140

oxidation potential (Le. the value for the second portion of the step). This is probably due to two factors: (a) the electrochemical reaction in the case of the anodic process is significantly non-reversible, increasing the oxidation potential will decrease the contribution of slow electrochermcal discharge relative to that of diffusion; (b) increasing the oxidation potential probably enhances the degree of non-stolchiometry in the inner, Ir(IV), region of the hydrous film, thus increasing the contribution of the second mode of electron transfer dlscus~ed earlier in the case of electron diffusion during the anodic process under sweep conditions. The fact that at least the values for the cathodic process (2.3 × 10-10 cm 2 s-1 from the step and ].5 × 10 -9 cm 2 s -1 from the sweep technique) are reasonably close, particularly in view of the fact that the iridium electrodes used for the two sets of data were different and that these hydrous films are not highly regular in structure, supports the view that zR errors are not of major importance in the cyclic voltammetry experiments. In conclusion, the present work provides, for the first time, a detailed interpretation of the unusual p o t e n t i a l / p H dependence of the main redox reaction of hydrous iridium oxide layers as recorded by various authors [20,24,42-44] using cyclic voltammetry techniques. Although rigorous structural analysis was not carried out, the suggested disperse nature of the material present in these layers is in agreement with both their low apparent refractive index as measured by in situ ellipsometry [45] and conclusions drawn as to the nature of transition metal hydrous oxades m general on the basis of magnetic susceptibility measurements [28]. An interesting facet of these hydrous oxides is their similarity, from the electrochemical viewpoint, to chermcally modified electrodes where the same type of transition from fast (reversible) to diffusion-controlled voltammetrlc behaviour has been observed [23]. From a practical point of view the behaviour outlined here should be of interest in areas such as the development of battery systems and electrochromic devices. In addition, Fig. 11 clearly demonstrates that the use of charge capacity as a measure of oxide film thickness may be valid only for relatively thin film, unless the coulometric data is obtained at quite slow sweep-rates. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13

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