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Oxygen gas evolution on hydrous oxides — An example of three-dimensional electrocatalysis?
J. Electroanal. Chem., 117 (1981) 155--160 Elsevier Sequoia S.A., Lausanne - - P r i n t e d in The Netherlands
155
Preliminary note O X Y G E N GAS E V O L U T I O N ON H Y D R O U S OXIDES - - AN EXAMPLE OF THREE-DIMENSIONAL ELECTROCATALYSIS?
LAURENCE D. BURKE and EUGENE J.M. O'SULLIVAN
Chemistry Department, University College, Cork (Ireland) (Received 21st October 1980)
The behaviour of hydrous oxide films produced by potential cycling techniques [ 1--5] is of considerable interest at the present time in such areas as electrocatalysis, electrochromic devices, and charge storage behaviour of oxides. Some of these films are unusually reactive with regard to oxygen gas evolution [6,7] -- particularly as compared with uncycled metal s u b s t r a t e s and we have suggested earlier [7] that increased surface area may be an important factor in accounting for this enhanced reactivity in the case of a cycled rhodium substrate. Our interpretation was disputed recently by Frazer and Woods [ 8], although towards the end of their article these authors also assumed that the oxide produced on cycling had a surface area significantly greater than that of the original solid-electrolyte interface. Conway [9], however, regards enhanced catalytic activity for chlorine gas evolution at cycled ruthenium and iridium anodes as being totally unrelated to change of real surface area o f these surfaces. Instead, an intrinsic catalytic effect is assumed to operate, involving a change in the distribution of cation valence states with potential. The object of the present article is to point o u t the complex nature of these interfaces after cycling and to show that there are elements of validity in each of the apparently conflicting viewpoints outlined above. It has frequently been demonstrated both with iridium [1,10,11] and rhodium [3,12] that the growth of the hydrous layer on cycling is n o t accompanied by any notable increase in charge associated with the double layer and hydrogen adsorption regions. This suggests that the electrode area remains constant -- despite the formation of a thick, quite visible, oxide film whose porous nature has apparently been demonstrated in the case of iridium by electronmicroscopy [ 5], and for which some evidence of crystallinity has been obtained by electron diffraction techniques [10]. When hydrated oxides are produced electrochemically on transition metal surfaces, either by constant highly anodic polarization (as in the case of gold [13]) or by potential cycling (as demonstrated by us recently [14] in the case of a Pt/Rh alloy), a duplex oxide film is obtained. The inner oxide layer is usually extremely thin, ca. 1--5 monolayers in thickness and largely anhydrous. The outer layer may be regarded as extensively hydrated with both b o u n d and trapped water present, as well as anions and cations. F o r instance in the case of iridium the shoulder at a b o u t 0.74 V on the main anodic charge storage peak in sulphuric acid solution is n o t observed in phosphoric acid solution [15], and is probably due to 0022-0728]81/0000-0000/$02.50, © 1981, Elsevier Sequoia S.A.
156
some sulphate ion incorporation in the film (either homogeneous incorporation or adsorption on the internal surface of the highly dispersed oxide) in the former electrolyte. The electrochemical behaviour of these films will be explained with the aid of the simplified model outlined in Fig. 1. The compact film in these systems is assumed to be highly conducting, with negligible potential drop across this inner layer. The major potential drop is assumed to occur, as illustrated in Fig. 1, at the outer surface of this compact film. The situation as represented here is similar to that proposed in the case of the duplex film on gold where the outer layer is assumed to be porous (fig. 9(c), ref. 13), except that in the present case the potential drop across the inner oxide is assumed to be negligible. One possible explanation of the constancy of the double layer and.hydrogen adsorption - desorption currents (especially the former} in oxide growth experiments is that this is the only interface between two bulk phases in these electrodes systems where a change in charge carrying species (i.e. a faradaic reaction) takes place. Changes in charge carrying species may also occur (as discussed later) at cations in the hydrated layer, but in our view the absence of a contribution to double layer charging in this case suggests that the material in the latter is so highly dispersed that it does not behave as a discrete separate phase. The nature of hydrous oxides has been discussed recently by Kennedy [16]. Hydrous deposits, produced for example by precipitation of such species as TiO(OH)~ on addition of ammonia to TIC14 solution, usually contain small quantities of foreign ions. These oxyhydroxide systems tend to be polymeric, the cations being linked by partially covalent oxygen (M--O--M) and hydroxy Compact
~xide
Hydrous oxide
Metol
"
_
e /" \ I
I
Fig. 1. Schematic representation of an in situ duplex oxide film formed on an iridium electrode under potential cycling conditions in acid. The suggested potential variation within the layers is outlined in the lower portion of this diagram. (o) iridium; (Q) oxygen; ( 0 ) sulphate; (@) hydrated proton.
157 (M-O--H--O--M) bridges. Water molecules are also assumed to be present both in the b o u n d and free states. Bound water consists of H20 molecules bonded by dipole-ion or partially covalent bonds to metal cations where they complete the octahedral, or other energetically stable coordination state of the latter. If this type of material is annealed one obtains very stable, corrosionresistant structures similar to the rutile form of RuO2 (and RuO2/TiO2) used as dimensionally stable anodes [17]. However, the unannealed, largely amorphous deposits prepared by potential cycling are far less cross-linked, i.e. much more open in structure. Similar conclusions as to the hydrated nature of the charge storage film were drawn recently by McIntyre and co-workers [ 5] whose work confirms the charge storage model in a hydrated oxyhydroxide system originally proposed by Buckley and Burke [1,2]. Although s o m e ordering of the oxide structure was observed when a portion of the surface layer was subjected to electron diffraction analysis [10], this could have arisen as a result of partial dehydration of the sample under high vacuum conditions. Ellipsometric investigations [6] of these films in situ gave very low values for the apparent refractive index and this, together with the very low apparent density values [5] (2.0 g cm -3 for the film as compared with 11.7 g cm -3 for anhydrous IrO2) supports the idea of a very open structure for the outer layer. In view of the absence of a significant contribution to the double layer charging, we believe that the material in the outer layer may be regarded as a type of protonic zeolite [18] with an irregular three-dimensional open structure whose cages contain both water molecules and electrolyte. Crystallization of these films in the aqueous environment is inhibited by the presence of coordinated water at cation sites, and the fact that the film appears granular under the high vacuum, electron beam b o m b a r d m e n t conditions encountered in the electron microscope [11] does not necessarily imply that the in situ film has a regular rutile-type structure. While the zeolite-type material may of course be present in granular form in these films, we would stress the open structure of the material present in these microparticles. It would appear that the degree of dispersion is such that with hydrous films of limited thickness there is no discrete oxide phase with such bulk characteristics as a three-dimensional electronic band structure or a well-defined oxide--solution phase boundary. With regard to electrochemical behaviour, the layers grown on iridium and rhodium under potential cycling conditions display charge storage properties which, as outlined earlier [1,3], involve redox reactions of cations in the hydrated film, viz. - O - - I r - O ( h y d ) + H + + e - = --O--Ir--OH(hyd)
(1)
--O--Rh--O(hyd) + H20 + e - = - - O - - R h - - O H ( h y d ) + O H -
(2)
Conway and coworkers [19] have discussed h o w the profile of a voltammetric peak is influenced by interaction between sites undergoing redox transitions. The sharpness and high degree of reversibility at moderate sweep rates (30 mV s -1 ) of the peaks associated with the reactions represented by eqns. (1) and (2) suggest that interactions between the cations in the outer layer do n o t vary greatly with the extent of reduction or oxidation of these
158
films whose redox behaviour appears surprisingly fast and uncomplicated. The cations throughout the film (with possible exceptions of thicker films, where for example in the case of rhodium [14], under certain conditions, portion of the film remains electrochemically inert with regard to the main charge storage reaction} react at a rather well-defined, thermodynamically reversible potential, electrons passing rapidly along the oxygen-bridged cation network, and ion transport (mainly proton switching} in the hydrous layer being quite fast in the acidic or basic media used in our experiments. The voltammetric behaviour of these hydrous films contrasts sharply with the extremely broad, ill-defined peaks observed with thermally prepared RuO2 oxides in acid [17] ; the redox reaction in the case of these anhydrous films is confined largely to surface cations. The difference is obviously due to the much higher degree of oxygen-bridged cross-linking in the rutile lattice of the latter. In the hydrous layers most of the cation coordination sites are occupied by water and the behaviour of the open polymeric n e t w o r k is probably n o t greatly different from that of its oxymetal components. At potentials just below the charge storage peaks the main processes are charging and discharging of the double layer at the c o m p a c t oxide-hydrous oxide (plus electrolytic solution} interface; the area of the latter does n o t alter on cycling. At more cathodic potentials t w o faradaic processes occur, partial reduction and restoration of the compact oxide layer and adsorption-desorption Of hydrogen. A t potentials above the charge storage peaks in the case of iridium there is further cation oxidation in the hydrous film -- this is shown by the growth of the upper reduction peak at a b o u t 1.37 V with increasing value of the upper sweep limit (fig. 7, ref. 1). The oxidation step involved here is probably IrO2(hyd) + H20 = IrO3(hyd} + 2H + + 2 e -
(3)
(total film conversion is n o t assumed) and oxygen gas evolution probably involves decomposition of the higher oxide [ 8,20], viz. IrO3(hyd) = IrO2(hyd) + ½0~
(4)
Frazer and Woods [8] have suggested that atomic oxygen [the p r o d u c t of their reaction (4)] diffuses from the bulk to the surface of the hydrated film. In view of the strong chemisorption or coordination of such radical species by cations in the film, we regard such a step as unlikely, and propose, as an alternative, that oxygen gas is produced within the open structure of the porous oxide film. This process m a y involve interaction at a single, or possibly two neighbouring cation sites 2 0 = 02
(5)
or
O + O H = [O2H] = 02 + H + + e -
(6)
The electrons involved in these reactions at sites along the polymer n e t w o r k are transferred to or from the electrode surface via the oxide "skeleton" of the hydrous layer.
159 The skeletal nature of the oxide is obviously a major advantage from an electrocatalytic viewpoint as it permits a major increase in the number of oxycations participating in the electrode reaction. The three dimensional character of the electrocatalytic performance of these anodes is also observed, to varying degrees, with such other electrode systems as platinized platinum, g a n e y nickel, thermally prepared RuO2 and porous graphite. There is, however, an important distinction in that the latter usually show an increase in charge in the double layer region -- this may be masked to some extent, as with RuO2, by surface redox transitions [17] -- but in most cases the finely divided material in the surface layer is present as a separate phase. In the hydrous layers the oxycations appear to be h y d r a t e d even at the individual level, especially below the oxygen evolution region, and the electrochemical evidence suggests that a distinct oxide phase is n o t present when these electrodes exist in an aqueous environment. There is some evidence for a modification of these hydrous oxide films (apart from dissolution [1,7]) u n d e r highly anodic conditions, and also at high film thicknesses. The decrease in oxygen evolution current with time under constant anodic polarization conditions in the case of iridium [ 8], the appearance of new peaks on the cyclic voltammogram (together with some loss in charge storage capacity) for rhodium [14], and the decrease in the rate of oxide growth on cycling with increasing film thickness for both metals [1,3] probably reflect increasing cross-linking of chains (or increasing crystallinity of the less hydrated layer) due to limited access of water to the inner regions of the outer film. Some discharge of bound water or h y d r o x y l groups probably occurs under these circumstances. As shown elsewhere [ 14] some of these changes can be reversed on slow cycling. It is largely a m a t t e r of semantics as to whether one can attribute surface area to an oxide polymer chain -- the important point is t h a t oxycation-solution contact is greatly increased as a result of potential cycling. There appears also to be an enhancement of intrinsic electrocatalytic activity associated with the formation of these hydrous layers. Evidence for this may be seen in the decrease in Tafel slope for oxygen gas evolution on iridium [6], from about 90 mV per decade in the absence of the h y d r o u s film to about 50 mV per decade in the presence of such a layer (similar behaviour is observed with rhodium [7]). The behaviour of the former substrate approaches that of platinum where the first electron transfer is generally assumed to be rate-limiting, viz. H20
=
OHads
+
H+ + e-
(7)
The behaviour in the case of the hydrous layer is similar to that observed with nickel in base [21] (or RuO2 in both acid and base [22]) where the ratedetermining step is assumed to involve an electrochemically generated oxide species. The change in the nature of the rate-limiting step is probably related to the fact that cations in the uncycled substrate are in a far lower state of hydration as compared with those in the hydrous film. The skeletal model of electrocatalysis presented here suggests an interesting avenue for research in this area. Various attempts are being made nowadays [23] to bond redox groups to inert substrates, i.e. to chemically m o d i f y the
160
latter in order to introduce specific electrocatalytic activity. If stable bonding linkages can be developed to give a reasonable thickness of a very open network of conducting polymer chains, with active sites at regular intervals along the chains, then a very effective utilization of active material and an increase in electrolytic cell efficiency should be possible. An electronically conducting open framework with a high density of groups or sites capable of undergoing redox transitions would also appear to be an ideal battery material for energy storage purposes. In conclusion it appears that the skeletal model of these hydrous oxide systems can reconcile the conflicting views of these electrode systems outlined in the first paragraph. The capacity of the electrochemical double layer scarcely alters during the growth of the hydrous film, primarily because there is no double layer associated with the latter which is permiated, down to the molecular level, by the aqueous phase. However, even though there is little change in area of the compact oxide/hydrous oxide interface on cycling the vastly increased contact in the outer layer between oxycations and the aqueous phase is obviously a major factor in the enhancement of the electrocatalytic activity of these electrode systems. Along with this, there is an enhancement of intrinsic catalytic activity as demonstrated by the decrease in Tafel slope for oxygen gas evolution on going from the uncycled to the cycled substrates. This feature will be discussed more thoroughly shortly in an article dealing with the anodic behaviour of cycled rhodium electrodes.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
D.N. Buckley and L.D. B u r k e , J. Chem. Soc. F a r a d a y 1, 71 ( 1 9 7 5 ) 1 4 4 7 . D.N. Buckley, L.D: Burke and J.K. Mulcahy, J. Chem. S o c . , Faraday 1, 72 ( 1 9 7 6 ) 1 8 9 6 . L.D. Burke and E.J.M. O'Sullivan, J. Electroanal. Chem., 93 ( 1 9 7 8 ) 11. D.A.J. R a n d and R. W o o d s , J. Electroanal. Chem., 55 ( 1 9 7 4 ) 375. J.D.E. M c I n t y r e , W.F. P e c k a n d S. N a k a h a r a , J. E l e c t r o c h e m . Soc., 1 2 7 ( 1 9 8 0 ) 1 2 6 4 . S. G o t t e s f e l d and S. Srinivasan, J. Electroanal. Chem., 86 ( 1 9 7 8 ) 89. L.D. Burke and E.J.M. O'sullivan, J. Electroanal. Chem., 9 7 ( 1 9 7 9 ) 123. E.J. Frazer and R. W o o d s , J. Electroanal. Chem., 1 0 2 ( 1 9 7 9 ) 127. B.E. C o n w a y in E. V e c c h i (Ed.), E x t . Abstr. 31st Meet. ISE, V e n i c e , 1 9 8 0 . Vol. 1, p p . 2 1 - - 2 6 . D. Michell, D.A.J. R a n d and R. W o o d s , J. Electroanal. C h e m . , 84 ( 1 9 7 7 ) 117. S. G o t t e s f e l d and J.D.E. McIntyre, J. E l e c t r o c h e m . Soe., 1 2 6 ( 1 9 7 9 ) 742. S. G o t t e s f e l d , J. E l e c t r o e h e m . Soc., 127 ( 1 9 8 0 ) 2 7 2 . M.M. Lohrengel and J.W. Schultze, E l e c t r o c h i m . A c t a , 21 ( 1 9 7 6 ) 9 5 7 . L.D. Burke and E.J.M. O'Sullivan, J. Electroanal. Chem., 1 1 2 ( 1 9 8 0 ) 2 4 7 . J.O. Z e r b i n o and A.J. Arvis, J. E l e c t r o c h e m . Soc., 1 2 6 ( 1 9 7 9 ) 93. J.F. K e n n e d y , C h e m . Soc. Rev., 8 ( 1 9 7 9 ) 2 2 1 . L.D. Burke and O.J. Murphy, J. Eleetroanal. C h e m . , 9 6 ( 1 9 7 9 ) 19. J. T u r k e v i e h and Y. Ono, A d v . in Catal,, 2 0 ( 1 9 6 9 ) 135. H. A n g e r s t e i n - K o z l o w s k a , J. Klinger and B.E. C o n w a y , J. Electroanal. Chem., 75 ( 1 9 7 7 ) 45. D.N. B u c k l e y and L.D. Burke, J. Chem. Soe. F a r a d a y 1, 72 ( 1 9 7 6 ) 2 4 3 1 . A.L Krasfl'shchikov, Russ. J. Phys. C h e m . , 37 ( 1 9 6 3 ) 273. L.D. Burke, O.J. M u r p h y , J.F. O'Neill and S. V e n k a t e s a n , J. Chem. Soc. F a r a d a y 1, 73 ( 1 9 7 7 ) 1 6 5 9 . R.W. M u r r a y , Acc. Chem. Res., 13 ( 1 9 8 0 ) 135.
155
Preliminary note O X Y G E N GAS E V O L U T I O N ON H Y D R O U S OXIDES - - AN EXAMPLE OF THREE-DIMENSIONAL ELECTROCATALYSIS?
LAURENCE D. BURKE and EUGENE J.M. O'SULLIVAN
Chemistry Department, University College, Cork (Ireland) (Received 21st October 1980)
The behaviour of hydrous oxide films produced by potential cycling techniques [ 1--5] is of considerable interest at the present time in such areas as electrocatalysis, electrochromic devices, and charge storage behaviour of oxides. Some of these films are unusually reactive with regard to oxygen gas evolution [6,7] -- particularly as compared with uncycled metal s u b s t r a t e s and we have suggested earlier [7] that increased surface area may be an important factor in accounting for this enhanced reactivity in the case of a cycled rhodium substrate. Our interpretation was disputed recently by Frazer and Woods [ 8], although towards the end of their article these authors also assumed that the oxide produced on cycling had a surface area significantly greater than that of the original solid-electrolyte interface. Conway [9], however, regards enhanced catalytic activity for chlorine gas evolution at cycled ruthenium and iridium anodes as being totally unrelated to change of real surface area o f these surfaces. Instead, an intrinsic catalytic effect is assumed to operate, involving a change in the distribution of cation valence states with potential. The object of the present article is to point o u t the complex nature of these interfaces after cycling and to show that there are elements of validity in each of the apparently conflicting viewpoints outlined above. It has frequently been demonstrated both with iridium [1,10,11] and rhodium [3,12] that the growth of the hydrous layer on cycling is n o t accompanied by any notable increase in charge associated with the double layer and hydrogen adsorption regions. This suggests that the electrode area remains constant -- despite the formation of a thick, quite visible, oxide film whose porous nature has apparently been demonstrated in the case of iridium by electronmicroscopy [ 5], and for which some evidence of crystallinity has been obtained by electron diffraction techniques [10]. When hydrated oxides are produced electrochemically on transition metal surfaces, either by constant highly anodic polarization (as in the case of gold [13]) or by potential cycling (as demonstrated by us recently [14] in the case of a Pt/Rh alloy), a duplex oxide film is obtained. The inner oxide layer is usually extremely thin, ca. 1--5 monolayers in thickness and largely anhydrous. The outer layer may be regarded as extensively hydrated with both b o u n d and trapped water present, as well as anions and cations. F o r instance in the case of iridium the shoulder at a b o u t 0.74 V on the main anodic charge storage peak in sulphuric acid solution is n o t observed in phosphoric acid solution [15], and is probably due to 0022-0728]81/0000-0000/$02.50, © 1981, Elsevier Sequoia S.A.
156
some sulphate ion incorporation in the film (either homogeneous incorporation or adsorption on the internal surface of the highly dispersed oxide) in the former electrolyte. The electrochemical behaviour of these films will be explained with the aid of the simplified model outlined in Fig. 1. The compact film in these systems is assumed to be highly conducting, with negligible potential drop across this inner layer. The major potential drop is assumed to occur, as illustrated in Fig. 1, at the outer surface of this compact film. The situation as represented here is similar to that proposed in the case of the duplex film on gold where the outer layer is assumed to be porous (fig. 9(c), ref. 13), except that in the present case the potential drop across the inner oxide is assumed to be negligible. One possible explanation of the constancy of the double layer and.hydrogen adsorption - desorption currents (especially the former} in oxide growth experiments is that this is the only interface between two bulk phases in these electrodes systems where a change in charge carrying species (i.e. a faradaic reaction) takes place. Changes in charge carrying species may also occur (as discussed later) at cations in the hydrated layer, but in our view the absence of a contribution to double layer charging in this case suggests that the material in the latter is so highly dispersed that it does not behave as a discrete separate phase. The nature of hydrous oxides has been discussed recently by Kennedy [16]. Hydrous deposits, produced for example by precipitation of such species as TiO(OH)~ on addition of ammonia to TIC14 solution, usually contain small quantities of foreign ions. These oxyhydroxide systems tend to be polymeric, the cations being linked by partially covalent oxygen (M--O--M) and hydroxy Compact
~xide
Hydrous oxide
Metol
"
_
e /" \ I
I
Fig. 1. Schematic representation of an in situ duplex oxide film formed on an iridium electrode under potential cycling conditions in acid. The suggested potential variation within the layers is outlined in the lower portion of this diagram. (o) iridium; (Q) oxygen; ( 0 ) sulphate; (@) hydrated proton.
157 (M-O--H--O--M) bridges. Water molecules are also assumed to be present both in the b o u n d and free states. Bound water consists of H20 molecules bonded by dipole-ion or partially covalent bonds to metal cations where they complete the octahedral, or other energetically stable coordination state of the latter. If this type of material is annealed one obtains very stable, corrosionresistant structures similar to the rutile form of RuO2 (and RuO2/TiO2) used as dimensionally stable anodes [17]. However, the unannealed, largely amorphous deposits prepared by potential cycling are far less cross-linked, i.e. much more open in structure. Similar conclusions as to the hydrated nature of the charge storage film were drawn recently by McIntyre and co-workers [ 5] whose work confirms the charge storage model in a hydrated oxyhydroxide system originally proposed by Buckley and Burke [1,2]. Although s o m e ordering of the oxide structure was observed when a portion of the surface layer was subjected to electron diffraction analysis [10], this could have arisen as a result of partial dehydration of the sample under high vacuum conditions. Ellipsometric investigations [6] of these films in situ gave very low values for the apparent refractive index and this, together with the very low apparent density values [5] (2.0 g cm -3 for the film as compared with 11.7 g cm -3 for anhydrous IrO2) supports the idea of a very open structure for the outer layer. In view of the absence of a significant contribution to the double layer charging, we believe that the material in the outer layer may be regarded as a type of protonic zeolite [18] with an irregular three-dimensional open structure whose cages contain both water molecules and electrolyte. Crystallization of these films in the aqueous environment is inhibited by the presence of coordinated water at cation sites, and the fact that the film appears granular under the high vacuum, electron beam b o m b a r d m e n t conditions encountered in the electron microscope [11] does not necessarily imply that the in situ film has a regular rutile-type structure. While the zeolite-type material may of course be present in granular form in these films, we would stress the open structure of the material present in these microparticles. It would appear that the degree of dispersion is such that with hydrous films of limited thickness there is no discrete oxide phase with such bulk characteristics as a three-dimensional electronic band structure or a well-defined oxide--solution phase boundary. With regard to electrochemical behaviour, the layers grown on iridium and rhodium under potential cycling conditions display charge storage properties which, as outlined earlier [1,3], involve redox reactions of cations in the hydrated film, viz. - O - - I r - O ( h y d ) + H + + e - = --O--Ir--OH(hyd)
(1)
--O--Rh--O(hyd) + H20 + e - = - - O - - R h - - O H ( h y d ) + O H -
(2)
Conway and coworkers [19] have discussed h o w the profile of a voltammetric peak is influenced by interaction between sites undergoing redox transitions. The sharpness and high degree of reversibility at moderate sweep rates (30 mV s -1 ) of the peaks associated with the reactions represented by eqns. (1) and (2) suggest that interactions between the cations in the outer layer do n o t vary greatly with the extent of reduction or oxidation of these
158
films whose redox behaviour appears surprisingly fast and uncomplicated. The cations throughout the film (with possible exceptions of thicker films, where for example in the case of rhodium [14], under certain conditions, portion of the film remains electrochemically inert with regard to the main charge storage reaction} react at a rather well-defined, thermodynamically reversible potential, electrons passing rapidly along the oxygen-bridged cation network, and ion transport (mainly proton switching} in the hydrous layer being quite fast in the acidic or basic media used in our experiments. The voltammetric behaviour of these hydrous films contrasts sharply with the extremely broad, ill-defined peaks observed with thermally prepared RuO2 oxides in acid [17] ; the redox reaction in the case of these anhydrous films is confined largely to surface cations. The difference is obviously due to the much higher degree of oxygen-bridged cross-linking in the rutile lattice of the latter. In the hydrous layers most of the cation coordination sites are occupied by water and the behaviour of the open polymeric n e t w o r k is probably n o t greatly different from that of its oxymetal components. At potentials just below the charge storage peaks the main processes are charging and discharging of the double layer at the c o m p a c t oxide-hydrous oxide (plus electrolytic solution} interface; the area of the latter does n o t alter on cycling. At more cathodic potentials t w o faradaic processes occur, partial reduction and restoration of the compact oxide layer and adsorption-desorption Of hydrogen. A t potentials above the charge storage peaks in the case of iridium there is further cation oxidation in the hydrous film -- this is shown by the growth of the upper reduction peak at a b o u t 1.37 V with increasing value of the upper sweep limit (fig. 7, ref. 1). The oxidation step involved here is probably IrO2(hyd) + H20 = IrO3(hyd} + 2H + + 2 e -
(3)
(total film conversion is n o t assumed) and oxygen gas evolution probably involves decomposition of the higher oxide [ 8,20], viz. IrO3(hyd) = IrO2(hyd) + ½0~
(4)
Frazer and Woods [8] have suggested that atomic oxygen [the p r o d u c t of their reaction (4)] diffuses from the bulk to the surface of the hydrated film. In view of the strong chemisorption or coordination of such radical species by cations in the film, we regard such a step as unlikely, and propose, as an alternative, that oxygen gas is produced within the open structure of the porous oxide film. This process m a y involve interaction at a single, or possibly two neighbouring cation sites 2 0 = 02
(5)
or
O + O H = [O2H] = 02 + H + + e -
(6)
The electrons involved in these reactions at sites along the polymer n e t w o r k are transferred to or from the electrode surface via the oxide "skeleton" of the hydrous layer.
159 The skeletal nature of the oxide is obviously a major advantage from an electrocatalytic viewpoint as it permits a major increase in the number of oxycations participating in the electrode reaction. The three dimensional character of the electrocatalytic performance of these anodes is also observed, to varying degrees, with such other electrode systems as platinized platinum, g a n e y nickel, thermally prepared RuO2 and porous graphite. There is, however, an important distinction in that the latter usually show an increase in charge in the double layer region -- this may be masked to some extent, as with RuO2, by surface redox transitions [17] -- but in most cases the finely divided material in the surface layer is present as a separate phase. In the hydrous layers the oxycations appear to be h y d r a t e d even at the individual level, especially below the oxygen evolution region, and the electrochemical evidence suggests that a distinct oxide phase is n o t present when these electrodes exist in an aqueous environment. There is some evidence for a modification of these hydrous oxide films (apart from dissolution [1,7]) u n d e r highly anodic conditions, and also at high film thicknesses. The decrease in oxygen evolution current with time under constant anodic polarization conditions in the case of iridium [ 8], the appearance of new peaks on the cyclic voltammogram (together with some loss in charge storage capacity) for rhodium [14], and the decrease in the rate of oxide growth on cycling with increasing film thickness for both metals [1,3] probably reflect increasing cross-linking of chains (or increasing crystallinity of the less hydrated layer) due to limited access of water to the inner regions of the outer film. Some discharge of bound water or h y d r o x y l groups probably occurs under these circumstances. As shown elsewhere [ 14] some of these changes can be reversed on slow cycling. It is largely a m a t t e r of semantics as to whether one can attribute surface area to an oxide polymer chain -- the important point is t h a t oxycation-solution contact is greatly increased as a result of potential cycling. There appears also to be an enhancement of intrinsic electrocatalytic activity associated with the formation of these hydrous layers. Evidence for this may be seen in the decrease in Tafel slope for oxygen gas evolution on iridium [6], from about 90 mV per decade in the absence of the h y d r o u s film to about 50 mV per decade in the presence of such a layer (similar behaviour is observed with rhodium [7]). The behaviour of the former substrate approaches that of platinum where the first electron transfer is generally assumed to be rate-limiting, viz. H20
=
OHads
+
H+ + e-
(7)
The behaviour in the case of the hydrous layer is similar to that observed with nickel in base [21] (or RuO2 in both acid and base [22]) where the ratedetermining step is assumed to involve an electrochemically generated oxide species. The change in the nature of the rate-limiting step is probably related to the fact that cations in the uncycled substrate are in a far lower state of hydration as compared with those in the hydrous film. The skeletal model of electrocatalysis presented here suggests an interesting avenue for research in this area. Various attempts are being made nowadays [23] to bond redox groups to inert substrates, i.e. to chemically m o d i f y the
160
latter in order to introduce specific electrocatalytic activity. If stable bonding linkages can be developed to give a reasonable thickness of a very open network of conducting polymer chains, with active sites at regular intervals along the chains, then a very effective utilization of active material and an increase in electrolytic cell efficiency should be possible. An electronically conducting open framework with a high density of groups or sites capable of undergoing redox transitions would also appear to be an ideal battery material for energy storage purposes. In conclusion it appears that the skeletal model of these hydrous oxide systems can reconcile the conflicting views of these electrode systems outlined in the first paragraph. The capacity of the electrochemical double layer scarcely alters during the growth of the hydrous film, primarily because there is no double layer associated with the latter which is permiated, down to the molecular level, by the aqueous phase. However, even though there is little change in area of the compact oxide/hydrous oxide interface on cycling the vastly increased contact in the outer layer between oxycations and the aqueous phase is obviously a major factor in the enhancement of the electrocatalytic activity of these electrode systems. Along with this, there is an enhancement of intrinsic catalytic activity as demonstrated by the decrease in Tafel slope for oxygen gas evolution on going from the uncycled to the cycled substrates. This feature will be discussed more thoroughly shortly in an article dealing with the anodic behaviour of cycled rhodium electrodes.
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