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Anodic characteristics of SrFe0.9M0.1O3(M: Ni, Co, Ti, Mn) electrodes
ANODIC SrFe,~,M,~,O,(M:
CHARACTERISTICS OF Ni, Co, Ti, Mn) ELECTRODES
Y. MATSUMOTO, I. KURIMOTO and E. SATO Department of Industrial Chemistry, Faculty of Engineering, Utsunomiya University. Ishiicho 2753, Utsunomiya, Japan (Received
3 August 1979)
Almtraet - Anodic characteristics of the substituted oxides, SrFe,,g M,,,Os(M : Ni, Co, Ti, Mn), were studied. It was found that the catalytic activities of the oxides substituted with Ni and Co ions for the oxygen evolution reaction are high in alkaline solution. The reaction mechanisms for the oxygen evolution reaction are proposed for all the substituted oxides under the assumption of Langmuirian adsorption condition. The current efficiencies of the anodic dissolution of the oxides were much higher in acidic solution than those in alkaline solution. The anodic dissolution is much suppressed by the substitution with Ni or Co ion in alkaline solution. Therefore, the oxides substitutedwith Ni and Co ions arc most suitable as anode materials in alkaline solution. The anodic dissolutionis based on the oxygen vacancy brmcd in the oxygen evolution process. The mechanism of the anodic dissolution of the oxides is proposed.
1. INTRODUCTION It was found in our laboratory
that SrFeO, with perovskite type structure has high catalytic actrvrty tor the oxygen evolution reaction[l]. The catalysis of the oxide for the oxygen evolution reaction will be primarily determined by whether the u* band is formed or not in the oxide bulk: the e* band formation is necessary for the high catalytic activitytl, 21.The high catalytic activity of the SrFeO, is explained by this concept, for this oxide has a vacant C* band[3]. However, a slight dissolution of this oxide which is undesirable as an anode material for practical use is also observed at high anodic overpotentiaf region. It was suggested in the previous pape.r[l] that the dissolution will be suppressed by the shortening of the life time of the electron trapped from the reducing species such as OH- and H,O into the er orbital existing as a surface state of the cr* band in the oxygen evolution reaction. This shortening may be brought about by the substitution of a part of the Fe cations in the SrFeOs with the other transition metal ions. Therefore, the anodic characteristics, ie, the anodic dissolution, the reaction mechanism for the oxygen evolution reaction and the electrocatalysis, are studied for the substituted oxides such as SrFe,,M,.,O,(M: Ni, Co, Ti, Mn) in this paper. 2. EXPERIMENTAL
SrFe,,,M,.,O, were synthesized by the similar method to that reported in the previous paper[l]. SrCOs, Fe20s, NiO, Co0 and TiOl were used as the starting materials. In the case of the preparation of SrFeo,9Mn,,,0,, aqueous solution of Mn(NO,), was used together with SrCO, and Fe,OJ. The above starting materials were stoichiometrically mixtured and then fired at 1000°C. The resulting mixtures were reground with agate mortar, folIowed by heating at 1300°C for 6 h. The prepared powders with the single phase of the perovskite type structure which were proved by X-ray analysis were pressed with 539
100kgcmP2 and then sintered at 1350°C. The resistivities of the sintering disc samples were measured by four probe method at room temperature. The porosities of the samples were about 20% and hence the real area will be greater than the apparent area. All samples were water-proofed by polystylene in order to obtain good reproducible results[2]. The current densities shown in this work will be indicated by the apparent one based on the geometric area. The solutions were pre-electrolyzed 1 M KOH, 0.5 M H,SO, and various concentrations of f&SOL which were of constant ionic strength but various pH values by adding a few drops of H,SO, or KOH solutions. The electrochemical tests were made at 25°C in the same manner as described in previous papers[l, 2, 4]. The amount of the Fe cation dissolved in the electrolyte was colorimetricaily determined by using ophenanthroline as the color-producing reagent. 3. RESULTS AND DISCUSSION Table 1 shows the resistivities and the types of the conductivities of the oxides in the vicinity of the room
temperature. The resistivities of the SrFeOs, SrFe,,,Ni,,,O, and SrFe,,,Co,,,O, were almost independent of the temperature. This type of the conductivity is represented as “metal-semi” in the third column in the Table. The resistivities of these oxides are lower than those of the StFeo,,Mn,,,O, and
Table 1. Resistivitiesand typg of conductivity of various oxides Oxide SrFeO, SrFeO.+iO,xO, SrFeO,sNtO.lOp SrFea,sCoo,iOs SrFeO.sTiO.lOs *semi fp) denotes ptype
Resistivity $2 cm)
Type of conductivity
1.3 6.3 7.0 6.5 1.2
metal-semi semi @)* metal-semi metal-semi semi (p)
x x x x x
10-2 10-Z 10-J lo- 3 lo- ’
semiconductor.
Y.Ma~su~uro. J. KURI~O
540
AND ESATO
100
10
I
I
1.3
Fig.
1.
1.4
I
I
1.5 16 E( V vs.RHE)
Polarization curve SrFe,.9Ni0.,0~
Fig. 3. pH us log i plot for the 0, evolution at 0.75 v “S nhe.
1
I
1.7
1.8
of the O2 evolution in 1 M KOH.
does not change with the substitution. On the other hand, Ti and Mn ions stably exist as M4* in oxides, therefore, the substitution with these ions brings about the increase of the amount of the Fe3+ ion, giving rise to the decrease of the hole density. Figures 1 and 2 show the typical polarization curves for the oxygen evolution reaction on the substituted oxides in 1 M KOH and 0.5 M H1S04, respectively. The other oxide electrodes also gave the similar polarization curves but the Tafel slopes were different from each other. The typical logi vs pH plot at a constant potential (us nhe) for the SrFeo,gNi,,,Os is shown in Fig. 3. The slope obtained from the linear relationship is about 2.2, as the Figure shows, so that the reaction order of OH- ion is concluded to be two in the oxygen evolution reaction. The other oxides also gave the good linear relationships in the same plot at a constant potential in which the Tafel relation is held. The Tafel slope, the exchange current and the slope of the (8 log i/d PH)~ obtained on the substituted oxides are summarized together with those for the SrFeOs in Tables 2 and 3 in alkaline and acidic solutions, respectively. The Tafel slope for the SrFe0.9Mn,.,0, electrode is near ZRT/F and for the other electrodes are near 2RT/3F in alkaline solution. In acidic solution, the Tafel slopes for all the oxides are near RT/F. In alkaline solutions, the following oxygen evolution reaction path is suggested for various anode oxide
on
which behave as p-type semiconSrFe,.,Ti,.,O,, ductors. It is apparent that the degeneracy occurs in the former oxides on account of the high density of the hole as the carrier. The hole is mainly formed in the x6= + KP+band by the Fe4+ ion in the oxide which contains Fe”+ and Fe4+ ions because of the presence of a large amount of the oxygen vacancy in the lattice[5]. Both Ni and Co ions stably exist as M2+ or M’” in oxides under the usual conditions. Therefore, the amount of the Fe4+ ion increases by the substitution with Ni or Co ion, if the composition of the
10
materialsrl, Table
QOl 1.7
1.9
20
21
E( V vs.RHE) Fig.
2.
Polarization curve of the 0, evolution SrFq,,Co,,,O, io 0.5 M H,SO*
on
2, 6-81. of kinetic parameters for the oxygen evolution reaction in 1 M KOH
2. Summary
Electrode
%
1.8
on SrFe,,,,Nii,,OS
SrFeQ* SrF+Wno.tO, SrFed%.tO~ SrFeo.&%OJ SrFeo.gTio.I03
*Referen@]
i0
b WI
(A .cm-*)
0.062 0.135 0.040 0.045 0.047
1.2 x 1.5 x 6.6 x 3.2x 5.6x
lo- 8 10-e lo-” lo-=’ lo-”
(a logi/dpH), 2.0 1.1 2.2 1.8 2.1
Anodic characteristics
of SrFe,.,
M,.,Oa(M:
Table 3. Summary of kinetic parameters for the oxygen evolution reaction in 0.5 M H,SOL
Electrode SrFeO,* SrFe,.&W.,O, SrFe,.,NLO, SrFe,.&o,.,O, SrFe,.,Ti,.,O,
b(V)
&I (A.cm -2)
0.078 0.087 0.080 0.072 0.065
7.9 X 10-g 3.5 X 10-a 2.0 X 10-a 1.4 x 10-a 8.0x 10-10
Electrode SrFcOo SrF%.,M,n,,,O, SrFeO.gN1O.IO, SrFe,.&&.,O, SrFe0.gTL.,03
*Reference[ 11.
S
+OH-+SOH+e-
(1) (2)
SOH + OH-
+
SO-*SO
+e-
(3)
+ 2s
(4)
250 +
02
SO-
+ H,O
Under Langmuirian adsorption conditions, the kinetic parameters in Table 2 suggest that the step (1) is rate controlling on the SrFe0.gMn,,,03 and the step (3) is rate controlling on the other oxides except for the SrFeOs in alkaIine solution. The rate controlling step (l), on the SrFe,,,Mn,,,O, isconsistent with those on electrodes[Z]. The Mn ion on the the La 1 _ &,MnOs electrode surface of the SrFeO,pMn,.,O, is concluded to dominate to the reaction mechanism of the oxygen evolution reaction in alkaline solution. Probably, Mn3* or Mn4’ ion will be rich on the oxide surface. In acidic solution+ the first step, (5) of the following Wade and Hackerman path[9] is presumed to be rate controlling on all the oxides from their kinetic parameters under Langmuirian conditions. 2s + 2H,O +
SO + SOH,
+ 2H+ + 2e-
SO+SOH,-+2S+O,+2H+++e-
(5) (6)
As described in the previous paper[l], one of the S in (5) implies the oxygen vacancy on the oxide surface; the oxygen evolution on all the oxides proceed Ga the oxygen vacancy in acidic solution. Damjanovic et al[lO], also propose the responsibility of the participation of the oxygen vacancy to the oxygen evolution reaction on the platinum oxide film. Thus, the oxygen vacancy will be formed on the surfaces of most of the oxides in the oxygen evoIution process in acidic solution. However, the formed oxygen vacancy indirectly gives the anodic dissolution of the oxides in acidic solution as described in the latter section. In the previous paper[l], it was shown that the equations for the anodic dissolution of SrFeO, are represented as follows. SrFe03
+ 20H-
Table 4. Current efficiencies for the anodic dissolution under anodic polarization at 10 mA .crn-’ in 0.5 M HzSOI, and at lOOmA *cm-’ in 1 M KOH
(a logi/apH), 0 0.2 0.3 0.3 0.1
- 2e- -+ Sr2* + FeO:-
541
Ni, Co, Ti, Mn) electrodes
Efficiency (%) 0.5 h4 H,SO, 5.49 6.78 8.31 6.5 1 4.83
Efficiency (%) 1M KOH 0.40 > 0.50 8.8 x 10-b 1.3 X 10-S 2.2 X 10-J
dissolution equations for the substituted metal cations in the oxides are not clear. Therefore, in order to evaluate the degree of the anodic dissolution of the substituted oxides, we determined only the current efficiency of the anodic dissolution for the Fe ion in the substituted oxides after the anodic electrolyzing of the oxides. The current efficiency of the dissolution was determined by using the above equations, (7) and (8). The current efficiencies of the anodic dissolution obtained for the Fe ion are shown in Table 4 for the various electrodes at 25°C under constant anodic current densities of 10 mA/cm’ and 100mA/cm2 in 0.5 M H,SO, and 1 M KOH, respectively. The current efficiencies for the all B site cations in the substituted oxides are higher than those shown in Table 4, but will not exceed a few times those shown in Table 4 on account of the small ratio of the amount of the substiluted ion: x = 0.1. The current efficiency in the case of the SrFe,,gMn,.,Ob in 1 M KOH could not be reproducibly determined, but always exceeded 0.5 per cent. The current efficiencies of the dissolution are much higher in 0.5 M H&O. than those in 1 M KOH. It is noteworthy that the dissolution of the SrFeO, is extremely suppressed by the substitution with the Ni, Co and Ti ions in 1 M KOH. The anodic dissolution of the oxides in acidic solution is based on the oxygen vacancy formed on the surface during the oxygen evolution reaction as described in the previous section. The slight band bending with thin depletion layer occurs under the anodic vias, because the oxides show “semi” or “metalsemi” type of the conductivity in nature. The field imposed at the depletion layer is much lower than that at the double layer. In this stage, the negative charge on the oxygen ion at the oxide surface is lower than that in the bulk. Soon, the neutral oxygen atom will be resulted by the electron tunnelling from the surface
+ H,O (7)
in KOH solution and ace Bond
SrFeO,
+ HZ0 + SOS- 2e- -+SrSO,
+ FeOi-
+ 2H+ (8) in H,SO, solution. Fe and Sr ions in the substituted oxides will also dissolve anodically with the same processes as shown in the above equations, but the
Fig. 4. Model of the band bendingand the electron tunnelling
process.
Y. MATSUMOTO,J. KURIMOTOAND E. SATO
542
Sr
Table
Sr
‘0
0’
‘0 ‘/
‘Fe’
Sr
Sr+-Sr
/ \
/‘“, /CO)
‘0,
pi
Sr--+
Sr
(step1 1
(step21
at 10mA .cm-”
for various
Overvoltage
(V)
0.30 0.35 0.22 0.40
SrFe0,gJ%I03 SrFeO.&oO.,O, DSA(RuO,)* Ti/RuG,/MnO,*
,
*Reference[l].
Cstep4)
(step 3) Fig. 5. Mechanism
overvoltages
electrodesin 1M KOH
Electrode
-0,
Sr
,. .,Sr--.
5. Oxygen
0’
of the anodic dissolution. oxygen vacancy.
(V) denotes
band edge of the z valence band to the x$ conduction band, leading to the oxygen vacancy. Figure 4 shows the band bending and the above process. The dissolution mechanism in acidic solution is illustrated in Fig. 5. The formed oxygen vacancy is compensated by the oxygen atom of H,O, but the strength of the Fe-(O) bond formed newly will be weaker than the original Fe-O bond; the compensated oxygen ion is represented as (0) (step 2). In time, the compensated oxygen ion (O), will surround a Fe cation (step 3). In this stage, FeOi- ion easily removes from the oxide surface to the electrolyte (step * 4). If the dissolution of the oxides in alkaline solution is also based on the formed oxygen vacancy, it concludes that the amount of the formed oxygen vacancy is much smaller in alkaline solution than that in acidic solution. The main reason is that the band bending as shown in Fig. 4 is little formed on the oxide surface in alkaline solution. Therefore, the probability of the electron tunnelling is very small leading to little dissolution in alkaline solution. Even if the probability of the electron tunnelling is very small in alkaline solution, the probability is determined by the degree of the band bending formed by the anodic vias. The degree of the band bending decreases with the increasing of the carrier density which leads to the decreasing of the thickness of the depletion layer. As for the substitution with Ni and Co, the suppression of the dissolution is well explained by
this concept, because it is judged from the values of the resistivities that the carrier densities of the oxides substituted with Ni and Co ions are higher than that of the SrFeO,. However, this concept is not proper for the SrFe,,,Ti,,,O,. For the dissolution ofthe oxides in alkaline solution, the other explanation is proposed in the previous paper[ 11.When thee, orbital existing as a surface state of the b* band traps the electron from the OH- ion, the e bond of M-O on the surface will temporarily destruct. The 0 bond is kept, if the trapped electron instantly flows to the u* band in the bulk. However, if the flowing rate of the trapped electron is slow, the dissolution will proceed on account of the long time keeping of the u bond destruction. The mobility of the electron in the B+ band will he a measure for the electron flowing rate. The oxides substituted with Ni, Co and Ti ions will have higher mobility of the electron in the g+ band than those of SrFeO, and SrFe, .,Mnn ,O,. We believe that this is a reason-for the d&~olut& suppression by the substitution in alkaline solution. The variations of the potentials with time during the anodic polarization at 100mA/cm2 in 1 M KOH for the SrFe,,Ni,.,O, and SrFe,.,Coe.,O, electrodes are shown in Fig. 6. The preferable result at the anode material, that the potentials were roughly invariant for both electrodes+ was obtained. In the case of SrFe0.9Ti, ,O,, the potential varied to more positive with time ‘in the same experiment. Therefore, this electrode is not suitable as the anode material in alkaline solution. Table 5 shows the overvoltages for the various oxide electrodes at lOmA/cm’ in 1 M KOH. It is clear that the SrFe,,,Ni,.,O, and SrFeo.sCo0.103 have high catalytic activity. In conclusion, the SrFe,.,Ni0.103 and SrFe,,,Co,.,O, are most suitable for the anode materials in alkaline solution on account of the high catalytic activity and little dissolution.
29-
2 I_..-_______ g 16J d > z
1.6-
REFERENCES 1. Y. Matsumoto, J. Kurimoto Chem. 102, 77 (1979).
-----
2. Y. Matsumoto
-
SrFe Ni 0 SrFet,9C00,03 0.9 01 3 I
0
6
Fig. 6. Potential-time
curve
under
18
2L
hr)
anodk
100 mA .cmWZ in 1 M KOH.
polarization
and E. Sato, Electrochim.
Acta 24, 421
(1979).
I
12 Time(
and E. Sato, 1. electroanal.
at
J. appl. Phys. 37, 1415 (1966). 3. I. B. Goodenough, 4. Y. Matsumoto, H. Yoneyama and H. Tamura, J. electroamll. them. 79, 319 (1977). R. C. Sherwood and J. F. Peter, J. 5. J. B. MacChesney, them. Phys. 43, 1907 (1965). 6. M. H. Miles, Y. H. Huaug and S. Srinivasan, J. electrochem. Sot. 125, 1931 (1978).
Anodic characteristics of SrFe0.9M0.103(M 7. C. Iwakura, K. Fukuda and H. Tamura, Electrochim. Acta 21. 501 (1976). 8. M. H. Miles, G. Kissel, P. W. T. Lu and S. Srinivasan, J. electtocizem.SIX. 123, 332 (1976). 9. 7F(.H. Wade and N. Hackerman, Trans. Faraday Sac. 53,
: Ni, Co, Ti, Mn) electrodes
543
1636 (1957). 10. A. Damjanovic and B. Jovanovic, J. electrochem. Sot. 123, 374 (1976). 11. M. Morita, C. lwakura and H. Tamura, Elecrrochim. Acta 23, 33I (1978).
CHARACTERISTICS OF Ni, Co, Ti, Mn) ELECTRODES
Y. MATSUMOTO, I. KURIMOTO and E. SATO Department of Industrial Chemistry, Faculty of Engineering, Utsunomiya University. Ishiicho 2753, Utsunomiya, Japan (Received
3 August 1979)
Almtraet - Anodic characteristics of the substituted oxides, SrFe,,g M,,,Os(M : Ni, Co, Ti, Mn), were studied. It was found that the catalytic activities of the oxides substituted with Ni and Co ions for the oxygen evolution reaction are high in alkaline solution. The reaction mechanisms for the oxygen evolution reaction are proposed for all the substituted oxides under the assumption of Langmuirian adsorption condition. The current efficiencies of the anodic dissolution of the oxides were much higher in acidic solution than those in alkaline solution. The anodic dissolution is much suppressed by the substitution with Ni or Co ion in alkaline solution. Therefore, the oxides substitutedwith Ni and Co ions arc most suitable as anode materials in alkaline solution. The anodic dissolutionis based on the oxygen vacancy brmcd in the oxygen evolution process. The mechanism of the anodic dissolution of the oxides is proposed.
1. INTRODUCTION It was found in our laboratory
that SrFeO, with perovskite type structure has high catalytic actrvrty tor the oxygen evolution reaction[l]. The catalysis of the oxide for the oxygen evolution reaction will be primarily determined by whether the u* band is formed or not in the oxide bulk: the e* band formation is necessary for the high catalytic activitytl, 21.The high catalytic activity of the SrFeO, is explained by this concept, for this oxide has a vacant C* band[3]. However, a slight dissolution of this oxide which is undesirable as an anode material for practical use is also observed at high anodic overpotentiaf region. It was suggested in the previous pape.r[l] that the dissolution will be suppressed by the shortening of the life time of the electron trapped from the reducing species such as OH- and H,O into the er orbital existing as a surface state of the cr* band in the oxygen evolution reaction. This shortening may be brought about by the substitution of a part of the Fe cations in the SrFeOs with the other transition metal ions. Therefore, the anodic characteristics, ie, the anodic dissolution, the reaction mechanism for the oxygen evolution reaction and the electrocatalysis, are studied for the substituted oxides such as SrFe,,M,.,O,(M: Ni, Co, Ti, Mn) in this paper. 2. EXPERIMENTAL
SrFe,,,M,.,O, were synthesized by the similar method to that reported in the previous paper[l]. SrCOs, Fe20s, NiO, Co0 and TiOl were used as the starting materials. In the case of the preparation of SrFeo,9Mn,,,0,, aqueous solution of Mn(NO,), was used together with SrCO, and Fe,OJ. The above starting materials were stoichiometrically mixtured and then fired at 1000°C. The resulting mixtures were reground with agate mortar, folIowed by heating at 1300°C for 6 h. The prepared powders with the single phase of the perovskite type structure which were proved by X-ray analysis were pressed with 539
100kgcmP2 and then sintered at 1350°C. The resistivities of the sintering disc samples were measured by four probe method at room temperature. The porosities of the samples were about 20% and hence the real area will be greater than the apparent area. All samples were water-proofed by polystylene in order to obtain good reproducible results[2]. The current densities shown in this work will be indicated by the apparent one based on the geometric area. The solutions were pre-electrolyzed 1 M KOH, 0.5 M H,SO, and various concentrations of f&SOL which were of constant ionic strength but various pH values by adding a few drops of H,SO, or KOH solutions. The electrochemical tests were made at 25°C in the same manner as described in previous papers[l, 2, 4]. The amount of the Fe cation dissolved in the electrolyte was colorimetricaily determined by using ophenanthroline as the color-producing reagent. 3. RESULTS AND DISCUSSION Table 1 shows the resistivities and the types of the conductivities of the oxides in the vicinity of the room
temperature. The resistivities of the SrFeOs, SrFe,,,Ni,,,O, and SrFe,,,Co,,,O, were almost independent of the temperature. This type of the conductivity is represented as “metal-semi” in the third column in the Table. The resistivities of these oxides are lower than those of the StFeo,,Mn,,,O, and
Table 1. Resistivitiesand typg of conductivity of various oxides Oxide SrFeO, SrFeO.+iO,xO, SrFeO,sNtO.lOp SrFea,sCoo,iOs SrFeO.sTiO.lOs *semi fp) denotes ptype
Resistivity $2 cm)
Type of conductivity
1.3 6.3 7.0 6.5 1.2
metal-semi semi @)* metal-semi metal-semi semi (p)
x x x x x
10-2 10-Z 10-J lo- 3 lo- ’
semiconductor.
Y.Ma~su~uro. J. KURI~O
540
AND ESATO
100
10
I
I
1.3
Fig.
1.
1.4
I
I
1.5 16 E( V vs.RHE)
Polarization curve SrFe,.9Ni0.,0~
Fig. 3. pH us log i plot for the 0, evolution at 0.75 v “S nhe.
1
I
1.7
1.8
of the O2 evolution in 1 M KOH.
does not change with the substitution. On the other hand, Ti and Mn ions stably exist as M4* in oxides, therefore, the substitution with these ions brings about the increase of the amount of the Fe3+ ion, giving rise to the decrease of the hole density. Figures 1 and 2 show the typical polarization curves for the oxygen evolution reaction on the substituted oxides in 1 M KOH and 0.5 M H1S04, respectively. The other oxide electrodes also gave the similar polarization curves but the Tafel slopes were different from each other. The typical logi vs pH plot at a constant potential (us nhe) for the SrFeo,gNi,,,Os is shown in Fig. 3. The slope obtained from the linear relationship is about 2.2, as the Figure shows, so that the reaction order of OH- ion is concluded to be two in the oxygen evolution reaction. The other oxides also gave the good linear relationships in the same plot at a constant potential in which the Tafel relation is held. The Tafel slope, the exchange current and the slope of the (8 log i/d PH)~ obtained on the substituted oxides are summarized together with those for the SrFeOs in Tables 2 and 3 in alkaline and acidic solutions, respectively. The Tafel slope for the SrFe0.9Mn,.,0, electrode is near ZRT/F and for the other electrodes are near 2RT/3F in alkaline solution. In acidic solution, the Tafel slopes for all the oxides are near RT/F. In alkaline solutions, the following oxygen evolution reaction path is suggested for various anode oxide
on
which behave as p-type semiconSrFe,.,Ti,.,O,, ductors. It is apparent that the degeneracy occurs in the former oxides on account of the high density of the hole as the carrier. The hole is mainly formed in the x6= + KP+band by the Fe4+ ion in the oxide which contains Fe”+ and Fe4+ ions because of the presence of a large amount of the oxygen vacancy in the lattice[5]. Both Ni and Co ions stably exist as M2+ or M’” in oxides under the usual conditions. Therefore, the amount of the Fe4+ ion increases by the substitution with Ni or Co ion, if the composition of the
10
materialsrl, Table
QOl 1.7
1.9
20
21
E( V vs.RHE) Fig.
2.
Polarization curve of the 0, evolution SrFq,,Co,,,O, io 0.5 M H,SO*
on
2, 6-81. of kinetic parameters for the oxygen evolution reaction in 1 M KOH
2. Summary
Electrode
%
1.8
on SrFe,,,,Nii,,OS
SrFeQ* SrF+Wno.tO, SrFed%.tO~ SrFeo.&%OJ SrFeo.gTio.I03
*Referen@]
i0
b WI
(A .cm-*)
0.062 0.135 0.040 0.045 0.047
1.2 x 1.5 x 6.6 x 3.2x 5.6x
lo- 8 10-e lo-” lo-=’ lo-”
(a logi/dpH), 2.0 1.1 2.2 1.8 2.1
Anodic characteristics
of SrFe,.,
M,.,Oa(M:
Table 3. Summary of kinetic parameters for the oxygen evolution reaction in 0.5 M H,SOL
Electrode SrFeO,* SrFe,.&W.,O, SrFe,.,NLO, SrFe,.&o,.,O, SrFe,.,Ti,.,O,
b(V)
&I (A.cm -2)
0.078 0.087 0.080 0.072 0.065
7.9 X 10-g 3.5 X 10-a 2.0 X 10-a 1.4 x 10-a 8.0x 10-10
Electrode SrFcOo SrF%.,M,n,,,O, SrFeO.gN1O.IO, SrFe,.&&.,O, SrFe0.gTL.,03
*Reference[ 11.
S
+OH-+SOH+e-
(1) (2)
SOH + OH-
+
SO-*SO
+e-
(3)
+ 2s
(4)
250 +
02
SO-
+ H,O
Under Langmuirian adsorption conditions, the kinetic parameters in Table 2 suggest that the step (1) is rate controlling on the SrFe0.gMn,,,03 and the step (3) is rate controlling on the other oxides except for the SrFeOs in alkaIine solution. The rate controlling step (l), on the SrFe,,,Mn,,,O, isconsistent with those on electrodes[Z]. The Mn ion on the the La 1 _ &,MnOs electrode surface of the SrFeO,pMn,.,O, is concluded to dominate to the reaction mechanism of the oxygen evolution reaction in alkaline solution. Probably, Mn3* or Mn4’ ion will be rich on the oxide surface. In acidic solution+ the first step, (5) of the following Wade and Hackerman path[9] is presumed to be rate controlling on all the oxides from their kinetic parameters under Langmuirian conditions. 2s + 2H,O +
SO + SOH,
+ 2H+ + 2e-
SO+SOH,-+2S+O,+2H+++e-
(5) (6)
As described in the previous paper[l], one of the S in (5) implies the oxygen vacancy on the oxide surface; the oxygen evolution on all the oxides proceed Ga the oxygen vacancy in acidic solution. Damjanovic et al[lO], also propose the responsibility of the participation of the oxygen vacancy to the oxygen evolution reaction on the platinum oxide film. Thus, the oxygen vacancy will be formed on the surfaces of most of the oxides in the oxygen evoIution process in acidic solution. However, the formed oxygen vacancy indirectly gives the anodic dissolution of the oxides in acidic solution as described in the latter section. In the previous paper[l], it was shown that the equations for the anodic dissolution of SrFeO, are represented as follows. SrFe03
+ 20H-
Table 4. Current efficiencies for the anodic dissolution under anodic polarization at 10 mA .crn-’ in 0.5 M HzSOI, and at lOOmA *cm-’ in 1 M KOH
(a logi/apH), 0 0.2 0.3 0.3 0.1
- 2e- -+ Sr2* + FeO:-
541
Ni, Co, Ti, Mn) electrodes
Efficiency (%) 0.5 h4 H,SO, 5.49 6.78 8.31 6.5 1 4.83
Efficiency (%) 1M KOH 0.40 > 0.50 8.8 x 10-b 1.3 X 10-S 2.2 X 10-J
dissolution equations for the substituted metal cations in the oxides are not clear. Therefore, in order to evaluate the degree of the anodic dissolution of the substituted oxides, we determined only the current efficiency of the anodic dissolution for the Fe ion in the substituted oxides after the anodic electrolyzing of the oxides. The current efficiency of the dissolution was determined by using the above equations, (7) and (8). The current efficiencies of the anodic dissolution obtained for the Fe ion are shown in Table 4 for the various electrodes at 25°C under constant anodic current densities of 10 mA/cm’ and 100mA/cm2 in 0.5 M H,SO, and 1 M KOH, respectively. The current efficiencies for the all B site cations in the substituted oxides are higher than those shown in Table 4, but will not exceed a few times those shown in Table 4 on account of the small ratio of the amount of the substiluted ion: x = 0.1. The current efficiency in the case of the SrFe,,gMn,.,Ob in 1 M KOH could not be reproducibly determined, but always exceeded 0.5 per cent. The current efficiencies of the dissolution are much higher in 0.5 M H&O. than those in 1 M KOH. It is noteworthy that the dissolution of the SrFeO, is extremely suppressed by the substitution with the Ni, Co and Ti ions in 1 M KOH. The anodic dissolution of the oxides in acidic solution is based on the oxygen vacancy formed on the surface during the oxygen evolution reaction as described in the previous section. The slight band bending with thin depletion layer occurs under the anodic vias, because the oxides show “semi” or “metalsemi” type of the conductivity in nature. The field imposed at the depletion layer is much lower than that at the double layer. In this stage, the negative charge on the oxygen ion at the oxide surface is lower than that in the bulk. Soon, the neutral oxygen atom will be resulted by the electron tunnelling from the surface
+ H,O (7)
in KOH solution and ace Bond
SrFeO,
+ HZ0 + SOS- 2e- -+SrSO,
+ FeOi-
+ 2H+ (8) in H,SO, solution. Fe and Sr ions in the substituted oxides will also dissolve anodically with the same processes as shown in the above equations, but the
Fig. 4. Model of the band bendingand the electron tunnelling
process.
Y. MATSUMOTO,J. KURIMOTOAND E. SATO
542
Sr
Table
Sr
‘0
0’
‘0 ‘/
‘Fe’
Sr
Sr+-Sr
/ \
/‘“, /CO)
‘0,
pi
Sr--+
Sr
(step1 1
(step21
at 10mA .cm-”
for various
Overvoltage
(V)
0.30 0.35 0.22 0.40
SrFe0,gJ%I03 SrFeO.&oO.,O, DSA(RuO,)* Ti/RuG,/MnO,*
,
*Reference[l].
Cstep4)
(step 3) Fig. 5. Mechanism
overvoltages
electrodesin 1M KOH
Electrode
-0,
Sr
,. .,Sr--.
5. Oxygen
0’
of the anodic dissolution. oxygen vacancy.
(V) denotes
band edge of the z valence band to the x$ conduction band, leading to the oxygen vacancy. Figure 4 shows the band bending and the above process. The dissolution mechanism in acidic solution is illustrated in Fig. 5. The formed oxygen vacancy is compensated by the oxygen atom of H,O, but the strength of the Fe-(O) bond formed newly will be weaker than the original Fe-O bond; the compensated oxygen ion is represented as (0) (step 2). In time, the compensated oxygen ion (O), will surround a Fe cation (step 3). In this stage, FeOi- ion easily removes from the oxide surface to the electrolyte (step * 4). If the dissolution of the oxides in alkaline solution is also based on the formed oxygen vacancy, it concludes that the amount of the formed oxygen vacancy is much smaller in alkaline solution than that in acidic solution. The main reason is that the band bending as shown in Fig. 4 is little formed on the oxide surface in alkaline solution. Therefore, the probability of the electron tunnelling is very small leading to little dissolution in alkaline solution. Even if the probability of the electron tunnelling is very small in alkaline solution, the probability is determined by the degree of the band bending formed by the anodic vias. The degree of the band bending decreases with the increasing of the carrier density which leads to the decreasing of the thickness of the depletion layer. As for the substitution with Ni and Co, the suppression of the dissolution is well explained by
this concept, because it is judged from the values of the resistivities that the carrier densities of the oxides substituted with Ni and Co ions are higher than that of the SrFeO,. However, this concept is not proper for the SrFe,,,Ti,,,O,. For the dissolution ofthe oxides in alkaline solution, the other explanation is proposed in the previous paper[ 11.When thee, orbital existing as a surface state of the b* band traps the electron from the OH- ion, the e bond of M-O on the surface will temporarily destruct. The 0 bond is kept, if the trapped electron instantly flows to the u* band in the bulk. However, if the flowing rate of the trapped electron is slow, the dissolution will proceed on account of the long time keeping of the u bond destruction. The mobility of the electron in the B+ band will he a measure for the electron flowing rate. The oxides substituted with Ni, Co and Ti ions will have higher mobility of the electron in the g+ band than those of SrFeO, and SrFe, .,Mnn ,O,. We believe that this is a reason-for the d&~olut& suppression by the substitution in alkaline solution. The variations of the potentials with time during the anodic polarization at 100mA/cm2 in 1 M KOH for the SrFe,,Ni,.,O, and SrFe,.,Coe.,O, electrodes are shown in Fig. 6. The preferable result at the anode material, that the potentials were roughly invariant for both electrodes+ was obtained. In the case of SrFe0.9Ti, ,O,, the potential varied to more positive with time ‘in the same experiment. Therefore, this electrode is not suitable as the anode material in alkaline solution. Table 5 shows the overvoltages for the various oxide electrodes at lOmA/cm’ in 1 M KOH. It is clear that the SrFe,,,Ni,.,O, and SrFeo.sCo0.103 have high catalytic activity. In conclusion, the SrFe,.,Ni0.103 and SrFe,,,Co,.,O, are most suitable for the anode materials in alkaline solution on account of the high catalytic activity and little dissolution.
29-
2 I_..-_______ g 16J d > z
1.6-
REFERENCES 1. Y. Matsumoto, J. Kurimoto Chem. 102, 77 (1979).
-----
2. Y. Matsumoto
-
SrFe Ni 0 SrFet,9C00,03 0.9 01 3 I
0
6
Fig. 6. Potential-time
curve
under
18
2L
hr)
anodk
100 mA .cmWZ in 1 M KOH.
polarization
and E. Sato, Electrochim.
Acta 24, 421
(1979).
I
12 Time(
and E. Sato, 1. electroanal.
at
J. appl. Phys. 37, 1415 (1966). 3. I. B. Goodenough, 4. Y. Matsumoto, H. Yoneyama and H. Tamura, J. electroamll. them. 79, 319 (1977). R. C. Sherwood and J. F. Peter, J. 5. J. B. MacChesney, them. Phys. 43, 1907 (1965). 6. M. H. Miles, Y. H. Huaug and S. Srinivasan, J. electrochem. Sot. 125, 1931 (1978).
Anodic characteristics of SrFe0.9M0.103(M 7. C. Iwakura, K. Fukuda and H. Tamura, Electrochim. Acta 21. 501 (1976). 8. M. H. Miles, G. Kissel, P. W. T. Lu and S. Srinivasan, J. electtocizem.SIX. 123, 332 (1976). 9. 7F(.H. Wade and N. Hackerman, Trans. Faraday Sac. 53,
: Ni, Co, Ti, Mn) electrodes
543
1636 (1957). 10. A. Damjanovic and B. Jovanovic, J. electrochem. Sot. 123, 374 (1976). 11. M. Morita, C. lwakura and H. Tamura, Elecrrochim. Acta 23, 33I (1978).