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Influence of chemisorbed carbon monoxide on the impedance of a smooth Pt wire at cathodic polarization in sulfuric acid solution
INFLUENCE OF CHEMISORBED CARBON MONOXIDE ON THE IMPEDANCE OF A SMOOTH Pt WIRE AT CATHODIC POLARIZATION IN SULFURIC ACID SOLUTION M. W. Institut fir Technische Elektrochemie,
BREITER
TlJ Wien, 9 Getreidemarkt,
A-1060 Wien, Austria
(Received 20 June 1984) Abstract-The ohmic and capacitive components of the impedance of a smooth Pt wire electrode were measured at a cathodic current density of -1 mAcm_’ at frequencies Y between 10,000 and 25H.z at different coverages of chemisorbed carbon monoxide in 0.5 M H’,SO,. The linear dependence upon w-“‘, characteristic for predominant H, diffusion on clean Pt electrodes, disappears at carbon monoxide coverages above 0.2. The hindrance of the Hz evolution reaction increases with the coverage of CO,, in a gradual fashion below 0.7 and rapidly at larger coverages. The hindrance is considerably larger on the Pt wire than on Pt foils, investigated previously. The change of the attenuation factor, caused by the blocking of the free surface, cannot account for the observed effect on the Pt wire electrode. Simple models, based on the Volmer-Tafel or Volmer-Heyrovsky mechanisms for H, evolution, cannot describe the experimental frequency dependence of the impedance at coverages above 0.2.
INTRODUCTION The effect of increasing coverage with chemisorbed carbon monoxide on the anodic oxidation of molecular hydrogen[l] and the cathodic hydrogen evolution[2] was previously studied for steady state conditions on smooth platinum electrodes in 0.5 M HzS04 at room temperature. Layers of CO,, are stable in the potential range of the said reactions. It was found in both cases that the reaction rate at constant potential, measured DSa hydrogen electrode in 0.5 M HzS04, decreased little with increasing CO, coverage below 0.7. The current remained largely determined by transport processes. At coverages above 0.7 the reaction rate decreased rapidly with 8,, The results[ i,2] are in agreement with the earlier suggestion[3] that the Hz evolution occurs mainly on sites with low heat of adsorption. The rate of hydrogen oxidation and evolution decreases only slightly at constant potential between 13,~ = 0 and 8,, = 0.7 because a sufficiently large number of sites with small heat of hydrogen adsorption is left. The change in the distribution of H adsorption sites can be assessed[4] as a function of 0,o by anodic charging curves. It is in agreement with the latter statement. The situation becomes more complicated at coverages above 0.7. The rapid decrease of the reaction rate results from two effects the separation of with 8, which is difficult: (a) decrease the number of reaction sites, (b) decrease of the rate constant of the heterogeneous step which is the slowest step in the overall reaction, disregarding mass transport processes. It is of interest to compare the inhibiting effect[ 1,2] of CO, on hydrogen oxidation. and evolution with theoretical predictions[5-71 on the blocking influence of adsorbed species on the diffusion current density under steady state conditions. An a priori calculation of the attenuation factor of the diffusion current
density was first performed in Refs[S] and[4] on the basis of Smythe’s cylinder model[8] and in[7] on the basis of a periodic-lattice model. In both cases, values of the attenuation factor between 1 and (1 -B) are predicted under the assumption that the process remains controlled by convective diffusion when the surface is partly blocked by other species. The attenuation factor approaches 1 if the dimension of the active sites is small and tends towards (1 - 13)if it is large in comparison with the thickness of the diffusion layer. Considering the results in[ l] and[2] for the electrode with the uniform current distribution it can be said that the attenuation factor is close to 1 for B,, G 0.7. processes rateSince mass transport are controlling[l, 23 the dimensions of the active sites have to be small relative to the thickness of the diffusion layer. The first derivation of the ac impedance of a partially blocked electrode, based on a concept of semispherical diffusion around a very small active site, was given in[9]. The next formulation[lO] is not considered[lI] satisfactory. A theory based on the periodic lattice model was presented in[lZ]. A different approach to the derivation of expressions for the electrode impedance was taken in[13]. Using the concept[ 1 I J that the non-linear diffusion problem for a partially blocked electrode is mathematically the same as that for linear diffusion coupled with a firstorder homogeneous reaction, the impedance was derived on the basis of the treatment[14] of that simpler case. Finally, a model for the transient response of electrodes with microscopic active and inactive sites was discussed in[iS]. This work is complementary to the previous[l l-l 31 considerations in which macroscopic surface inhomogeneities were treated. In the present communication the frequency dependence of the impedance of the interface of smooth platinum wire/OS M H,SO, was investigated as a function of the carbon monoxide coverage. The meas1725
M. W. BRELTER
1726
urements were always made at a cathodic dc current at room temperature (22 density of - 1 mAcm_’ t lDC). Earlier measurements[l6] of the impedance platinum wire/sulfuric acid solution demonstrated at cathodic polarization that the ohmic component 5s ,and the capacitive component l/c&Z, in a series circuit were determined by mass transport processes at frequencies between 30 and about 2OOOHz. The smooth Pt electrodes had been pretreated by anodic pulses. As expected from the theory[l6] the two components of the Warburg impedance were inversely proportional to the concentration of molecular hydrogen at the surface. When a cathodic current was maintained after the anodic pretreatment, the overvoltage increased[l7] slowly with time due to the adsorption of impurities from the electrolyte. The poisoning was redected[ 171 by a drastic change in the frequency dependence of the impedance. In contrast to the relatively uncontrolled poisoning[17], the chemisorption of carbon monoxide is carried out before the impedance measurements in the present work. The impedance is time-independent during the measurements at a given coverage with CO,,. The transition from a surface state of high reactivity to a state of low activity is accomplished in a well defined fashion on smooth platinum electrodes.
EXPERIMENTAL The experiments were made in a Pyrex glass vessel of conventional design. Since the values of the ohmic and capacitive component of the interfacial impedance are proportional to the geometric surface area A, a Pt wire with 0.55 cm2 and a charge equivalent SQH = 0.10 mCcm- ’ for the removal of a monolayer of H atoms was chosen as the test electrode. The test electrodes of the previous studies of the influence of CO,, on hydrogen adsorption[4], hydrogen evolution[2] and hydrogen dissolution[l] could not be used because of the relatively large values of A. The Pt wire was located in the axis of a Pd wire spiral of the assembly shown in Fig. I of [4]. The purification of the electrolyte was described there[4]. The overvoltage q was measured us a hydrogen electrode in the same electrolyte (0.5 M HZS04) as the test electrode. The impedance measurements were always made at a cathodic current density of - 1 mAcm- z at room temperature (22 f 1YZ). The procedure for the establishment of different coverages with CO, was as follows: (a) After cycling the Pt wire a few times between 0.05 and 1.2 V at constant current (+ 36 ~Acm-“) the test electrode was kept at 0.1 V and the Pd wire spiral at 0.8 V during the stirring of the electrolyte with CO of CP grade at 1 cm’s_‘. (b) The CO stirring was followed by stirring with purified N, at 1 cm3s-’ for 1000 s. The test electrode remained at 0.1 V and the Pd spiral at 0.8 v. (c) The electrolyte was replaced once with fresh electrolyte, presaturated with N,. Both the test electrode and the Pd spiral stayed at open circuit. (d) An anodic charging curve was recorded at 36 PA cm-’ up to about 0.4 V. The oxidation of
CO,
was avoided in this fashion. The coverage:
eco = I- sQ’,lsQ,
(1)
was determined. Here sQ;I is the charge equivalent of a monolayer of H atoms in the presence of CO,, 1 The electrolyte was saturated with purified Ha at 1 cm’ s- ’ For about 600 s. Both the Pt electrode and the Pd electrode remained at open circuit. A cathodic current of - 1 mAcmm2 was applied. After 100 s the two components of the impedance were determined successively at 10,000,3ooO, 1000, 400, 200, 100, 70, 50, 40, 30, 25 and 10 Hz. The superimposed ac signal was always smaller than 2 mV,. The apparatus for the impedance measurements was similar to the circuit in earlier studies[ IS]. Finally the overvoltage was measured. Nitrogen stirring replaced the hydrogen stirring. The Pd spiral stayed at 0.8 V and the test electrode at open circuit. After the open-circuit potential of the Pt electrode reached 0.05 V another charging curve was initiated at the current density given above. This charging curve was extended to such potentials that a part of CO, was removed by oxidation to CO,. Then the test electrode was taken back to 0.05 V by a cathodic current pulse. Steps (d)-(g) were repeated until the surtace was tiee of CO,. The last impedance measurements were made at zero coverage with carbon monoxide. By comparing the 61, value from step (d) with the 6r,o value computed according to Equation (1) from the hydrogen branch of the charging curve of step (g), it was verified experimentally that the coverage with CO,, was not affected by the measurements during step (f). As in the previous studyC181, the electrolytic resistance R, between the test electrode and the tip of the Luggin capillary, leading to the reference electrode compartment, was determined as the ohmic component of the impedance at 10,000 Hz at 1.3 V in the oxygen region. Since the ac circuit is a galvanostatic one, the correction for R,, was applied in the bridge during the impedance measurements arrangement[lS 1 at -lmAcm-. EXPERIMENTAL
RESULTS
The two components of the impedance in a series circuit are plotted as a function of w-l’* at various coverages with CO, in Fig. l(a) and (b). The data in Fig. l(a) correspond to the coverage range in which the dc measurements[l, 21 demonstrated a relativeiy smali decrease of the reaction rate at constant potential with increasing coverage (@,o < 0.7). In contrast, the impedance behaviour in Fig. l(b) results in the coverage range where the cathodic current of H2 evolution at constant potential decreases at first rapidly in a narrow region and afterwards more slowly with B,,. Two different scales had to be employed for the abscissae in Fig. l(a) and (b) to accommodate all the experimental points. Solid symbols are used for the ohmic component and open symbols for the capacitive component. The data in Fig. l(a)and (b) were not COrreCted for the double layer capacity. As to be expected from previous work[16] the two components coincide practicatly at B,, = 0 and
Influence of chemisorbed CO
1727
the slope of the straight lines in the presence of CO, are larger by a factor of 2.2, 2.5 and 1.6 than the respective values of 0.13/(1 - f&J for 8,, = 0.3X,0.64 and 0.80. The preceding consideration was made to check if the experimental R,-uJ-‘/~ plots and 1/cs+L- If2 plots at Iower frequencies may be interpreted as the left portion of theoretical curves[13] on partially blocked electrodes. The disagreement between experimental and theoretical slopes implies that this is not the case. The inaccuracy of the impedance data cannot be held responsible for the shape of the experimental Rs_w-“’ curves and l/oCs~‘I* curves at f&o 1 0 in Fig. l(a) and (b). The data in Fig. I(b) reflect the rapid increase of R, and ~/WC, with 8,o in a narrow range. There is a marked difference between the curves at B,, = 0.8 and e ,-o = 0.9 and 0.97. The shape of the latter two curves looks similar to that of theoretical curves[14, 161 for a rate-controlling heterogeneous reaction. The ratios ~(~,o~lf1(~co = 0) and
for 100, 1000 and 10,000 Hz are given as a function of 8 co in Fig. 2. A double layer correction was not applied. The ratios represent relative measures of the degree to which the H2 evolution is hindered by the increasing coverage with t&o. DISCUSSION The dependence of the absolute value of the electrode impedance upon the carbon monoxide coverage
%I* 0.02
3000
II
000400
I1
0.04
200
00
w-l%&l
0.06 70
50
40
I
1
30
25
Hz
Fig. 1. Ohmic (solid symbols) and capacitive(open symbols) components of the impedance in a series circuit as a function of CL-“’ at different coverages of carbon monoxide. A correction for the double layer capacity is not audied. (a) e~0 = 0, 0 , n ; 0.38, A, & 0.64, o, . . (b): 9, zb:g0, &‘i; 0.90, Cl , I : 0.97, 0, . .
depend linearily upon w- “‘. Essentially the Warburg impedance of the diffusion of molecular hydrogen is measured. The Rs~u-“2 curves and l/oCs+-‘I* curves in Fig. 1 (a) at 8,o = 0.38 and 0.64 differ in their shape from the lower curve. Nevertheless the values of Rs and l/cuCs at a given frequency are relatively close to each other for each of the upper curves in Fig. 1(a). The last statement also applies to the low&r curve in Fig. l(b). Straight lines which are not shown in Fig. l(a) were drawn in a rough approximation through the I/WC, points of the two upper curves. The extension of these lines went through the origin of the coordinate system. The slopes of the straight lines amounted to 0.13, 0.46, 0.91, 1.58 for 0,, = 0, 0.38, 0.64 and 0.80. The slope is given in cmcm- I, using the same scale for data in Fig. l(a) and (b). The values of
the Fig. 2. Plot of ratios (sco)/~(&xJ = 0) and ( JR: + I/~zC$)t&( JR; + l/w’C&, -0 at ditlkrent frequencies as a function of 6~0. o, overvoltage ratio; A, impedance ratio at 10,000 Hz; q , impeaanCeratio at loo0 Hz; V, impedance ratio at 100 Hz
1728
M. W. BREITER
is discussed first. The discussion of the frequency dependence of the components at constant coverage follows. The latter discussion is restricted to Qco < 0.8 because the dc measurements indicate(21 that H3 evolution becomes feasible at about 8, = 0.9 on top of the adsorbed layer of carbon monoxide. Dependence of the absolute value ofthe impedance carbon monoxide coverage
upon
The results in Fig. 2 are in general agreement with those of the previous dc studies[l, 21. There is a gradual increase of the hindrance of the H2 evolution reaction up to about 6$-o = 0.7. This increase is followed by a rapid increase above oco = 0.7. However, there is a remarkable difference between the present results and those in[l] and [2] for tic0 < 0.7. The ratios rl (e,,)lrl
(O,, = 0)
and ( JR;+
WJ~G),,/(
JR:
+ W’C,z),
=o
at low frequencies increase considerably more in Fig. 2 of the present communication than the reaction rate decreases with &o in fig. 3 of[2]. Carbon monoxide has a considerably larger impact on the H2 evolution reaction at the smooth Pt wire electrode than at the two smooth electrodes[2], involving Pt foils. Possibly there is a correlation between the achievement of a high activity of the Pt wire electrodes by anodic pretreatment and the high sensitivity to poisoning. In contrast, Pt foils proved more difficult to activate over the years of experience of the author. They appear to possess less sensitivity to poisoning. The absolute value of the impedance at 100 Hz is given as an example for the behdviour at the lower frequency range in Fig. 2 as a function of &,. The behaviour is nearly identical with the dr behaviour. Although the values of the resistance ratio at 25 Hz are not shown i? Fig. 2, they are larger than g(tl,,)//1(6rco = 0) at a given value of 0,. This suggests that a decrease of l/rtiC, occurs with decreasing frequency below 25 Hz. The overvoltage ratio reflects the situation at 0 Hz. As it might be expected, the dependence of the absolute value of the impedance upon 8,-o becomes smaller with increasing frequency. The closeness of the curves for 1000 and 10,ooO Hz in Fig. 2 at oco $ 0.7 is mainly due to the fact that a double layer correction was not applied to the data at 10,000 Hz. This was not done because the low-frequency bchaviour where double layer effects are negligible is of main interest. The results in Fig. 2 demonstrate that the contribution of H2 diffusion to the electrode impedance becomes rapidly smaller with tl,, on the Pt wire electrode. Frequency
dependence
of the
impedance
frequency dependence of the impedance of a partially blocked electrode on which an electrochemical reaction occurs may be discussed in terms of: (a) a purely blocking effect[9-13, 151 and (h) a transition[l7] from mass transport to kinetic control. The assumptions of approach (a) are given in the The
Introduction. Since the frequency dependence of the two components of the impedance of a poisoned platinum electrode displayedt 171 a certain similarity with the theoretical dependence in the case of a ratecontrolling heterogeneous reaction[l9,20] the interpretation (b) was advanced[l7]. Essentially it is assumed in[19] and[20] that the rate constants of the heterogeneous reaction are constant[19, 201 across the surface or that the surface possesses[19] a uniform[21] heterogeneity. The results in Fig. l(a) of this paper are discussed on the basis of the approaches (a) and (b). It was derived in[13] for the approach (a) that the impedance at high frequencies behaves similarly to that at the homogeneous electrode with the surface area A/( 1 - 6). The value of 0 can be determined from the slope of the plot R, and/or ~/WC, DSw-~‘* in this frequency range. At intermediate frequencies the Rs+-“’ plots and l/oCs+m’r* plots display a relatively large curvature. At low frequencies the two plots become linear and have a slope smaller than that at high frequencies. The ohmic component is always larger than the capacitive one in a series circuit. A glance at the two upper curves in Fig. 1 (a) reveals that the frequency dependence of the experimental curves differs greatly from the dependence derived in[l3]. As pointed out in the section Experimental Results, it cannot be argued that, because of a certain inaccuracy of the experimental data, two straight lines with the slope 0.13/( 1 - 0,) are measured. These lines would correspond to the left Rs-cK’!’ plots and part of the theoretical l/oCs-W -“’ plots. It is suggested that the discrepancy between experimental results and theoretical predictions of approach (a) is found because the basic assumption[13,15] of a constant exchange current density of the electrode reaction on the free surface area is not fulfilled for the hydrogen evolution reaction on smooth platinum. In contrast, the gold model eiectrode[ll] which was partially covered with a photoresist layer displayed the predicted behaviour[13] during impedance studies[13] of a redox reaction. For approach (b) a detailed analysis of the frequency dependence of the impedance of the H2 evolution reaction is carried out for the Volmer-Tafel mechanism: H+ +e-
= H,,
(2)
H,, + H,, = Hz
(3)
and for the Volmer-Heyrovsky
H+ +eH+ +H,+e-
mechanism:
= H,
(4)
= H,.
(5)
The derivation is based on the following assumptions: (a) The rates of the different reactions may be expressed as iv = i,(K
B,, B,,)
(6)
i, = i, (%, B,,)
(7)
i, = i, (E, 8,, eco).
(8)
Here 0, is the coverage of the ti atoms which are involved in hydrogen evolution. 0, need not be
Influence of chemisorbed CO
1729
identical with the total hydrogen coverage on that part of the surface which is free of CO,,. (b) The current distribution is uniform. (c) The influence of H2 diffusion is negligibly small. The assumption (a) is valid for a homogeneous surface for which a given rate constant has the same value on any site or for a surface with uniform heterogeneity[21], for instance according to Temkin’s model[22]. The geometry of the cell assembly guarantees the fultilment of the assumption (b) on a macroscopic scale. The boundary conditions are for the Volmer-Tafel mechanism: --FT$=
Aiv-Ai, Si,
‘v
= TAq+&Atl,-~A8, 'H
Ai, = Ai,
(91 H
= Ai,
and for the Volmer-Heyrovsky
(10) 0.4
mechanism:
1,
0.2 1
0.4 I
11
0.6 I 1,
11
3000 KMCI4CCI200 100 70
5040
0.8 1 G+-$ISEL&I
EI
30 25 HI
Fig. 3. Reciprocal values of the ohmic {solid symbols) and capacitive (open symbols) components of the impedance in a
parallel circuit after correction 0, l , e,, = 0.38; A, A, B,,
Ai, = Ai, + Ai,.
(12)
Here FT designates the maximum amount of H in Ccm-2 and i, is the faradaic current. The expressions for the reciprocal values Ai,/Aq of the complex impedance are:
ties. The component UK; increases from 0 at large frequencies, passes through a maximum at wM = u/F l” and tends towards 0 at small frequencies. In order to compare the theoretical and experimental results a correction for the double layer capacity has to beapplied. This was done in an approximate way by attributing I/WC, at 10,OOOHz to the double layer impedance ~/WC,. The correlation between experWC,) and the faradaic comimental values (l/R,, ponents is: 1/R’,
+($+$)/(g-%+jc0Fr>.
(14)
It was necessary to derive the reciprocal expressions because the Volmer and Heyrovsky reactions involve discharge steps occurring in parallel. The present derivation for the Volmer-Heyrovsky mechanism is more general than the one in[23]. The first terms in Equations (13) and (14) represent the discharge resistances which are approached at large frequencies. When separating the complex expressions of Equations (13)and (14)intoa real component (l/RL)andan part &K’,,) it is found that imaginary both components may be written in the same formal way for the Volmer-Tafel and Volmer-Heyrovsky mechanism:
(15) (16) The frequency dependence is similar for both mechanisms of Hz evolution under the assumptions stated before. The component l/R’p increases from l/r at large frequencies to (l/r + A/a) at very small frequen-
for the double layer capacity. = 0.64; 0, I, O,, = 0.90.
= l/R,
(171
UK’, = w(C,-Co).
(18)
Using theabove procedure the impedance data for 8,, = 0.38, 0.64 and 0.9 are given in the semilogarithmic plot of Fig. 3. A glance at the results in Fig. 3 demonstrates that I/R, displays the opposite dependence upon the frequency as is expected on the basis of Equation (IS). It is difficult to say if the capacitive component shows the correct frequency dependence because the location of the maximum is not known. Although the mechanisms for H, evolution were formulated in a way which assumed that only a part of the H atoms participates in the reaction, the experimental results in Fig. 3 do not agree with the theoretical prediction of the frequency dependence. The question arises whether hydrogen evolution occurs on the poisoned Pt wire electrode by another mechanism or whether the assumptions (a) and (b) are not fulfilled. The present investigation does not allow a decision.
REFERENCES 1. M. W. Brdter, J. electroanal. Chem 65, 623 (1975). 2. M. W. Breiter, ibid. 115, 45 (1980).
I730
M. W. BRE~TER
3. M. W. Breiter, in Tmns. Symp. on Elecfrode Processes (Edited bv E. Yeaaerl D. 307. Wilev. New York (19611. 4. k. W. Br&r, Pro: S$&. on Electr&atalysis, p. 11‘5.Thk Electrochem. Sot., Princeton (1974). 5. R. Landsberg and R. Thiele, Elecrrochim. Acta 11, 1243 (1966). 6. F. Scheller, S. Miiller, R. Landsberg and H. J. Spitzcr, .f. electroanal. Chem. 19, 187 (1968). 7. E. Levart. D. Schuhmann, 0. Contamin and M. Etman, ibid 70, 117 (1976). 8. W. R. Smvthe. J. aawl. Phvs. 24. 70 (1953). 9. K. J. Vet&r, i. phhys. C&I. 199, 3od (1952). 10. J. Lindemann and R. Landsberg, J. electroanal. Chem. 25, 20 (I 970). 11. T. Gueshi, K. Tokuda and H. Matsuda, ibid. 89, 247 (1978). 12. M. Etman, E. Levart and D. Schuhmann, ibid. 101, 14 (1979).
13. K. Tokuda, T. Gueshi and H. Matsuda, ibid. 102, 41 (1979). 14. H. Gerischer, Z. phys. Chem. 198, 286 (1951). 15. C. Am&ore, J. M. Saveant and D. Tessier, J. elecfroanal. Chem. 147, 39 (1983). 16. M. Breiter. H. Kammermaier and C. A. Knorr. Z. Elektrocheh. 60, 647 (1956). 17. C. A. Knorr, ibid. 59, 647 (1955). 18. M. W. Breiter, .f. electrounal. Chem. 7, 38 (1964). 19. H. Gerischer, 2. phys. Chem. 202, 55 (1952). 20. M. Breiter. H. Kammermaier and C. A. Knorr, Z. Elekwochem. 60, 454 (1956). 21. A. N. Frumkin, Advances in Elecwochemiswy (Edited by P. Delahay), Voi. 3, Chapter 5. Interscience, New York (1963). 22. M. Temkin, Z.fiz. Khirn. 15, 296 (1941). 23. H. Gerischer and W. Mehl. 2. Elekrrorhem. 59. 1049 (1955).
BREITER
TlJ Wien, 9 Getreidemarkt,
A-1060 Wien, Austria
(Received 20 June 1984) Abstract-The ohmic and capacitive components of the impedance of a smooth Pt wire electrode were measured at a cathodic current density of -1 mAcm_’ at frequencies Y between 10,000 and 25H.z at different coverages of chemisorbed carbon monoxide in 0.5 M H’,SO,. The linear dependence upon w-“‘, characteristic for predominant H, diffusion on clean Pt electrodes, disappears at carbon monoxide coverages above 0.2. The hindrance of the Hz evolution reaction increases with the coverage of CO,, in a gradual fashion below 0.7 and rapidly at larger coverages. The hindrance is considerably larger on the Pt wire than on Pt foils, investigated previously. The change of the attenuation factor, caused by the blocking of the free surface, cannot account for the observed effect on the Pt wire electrode. Simple models, based on the Volmer-Tafel or Volmer-Heyrovsky mechanisms for H, evolution, cannot describe the experimental frequency dependence of the impedance at coverages above 0.2.
INTRODUCTION The effect of increasing coverage with chemisorbed carbon monoxide on the anodic oxidation of molecular hydrogen[l] and the cathodic hydrogen evolution[2] was previously studied for steady state conditions on smooth platinum electrodes in 0.5 M HzS04 at room temperature. Layers of CO,, are stable in the potential range of the said reactions. It was found in both cases that the reaction rate at constant potential, measured DSa hydrogen electrode in 0.5 M HzS04, decreased little with increasing CO, coverage below 0.7. The current remained largely determined by transport processes. At coverages above 0.7 the reaction rate decreased rapidly with 8,, The results[ i,2] are in agreement with the earlier suggestion[3] that the Hz evolution occurs mainly on sites with low heat of adsorption. The rate of hydrogen oxidation and evolution decreases only slightly at constant potential between 13,~ = 0 and 8,, = 0.7 because a sufficiently large number of sites with small heat of hydrogen adsorption is left. The change in the distribution of H adsorption sites can be assessed[4] as a function of 0,o by anodic charging curves. It is in agreement with the latter statement. The situation becomes more complicated at coverages above 0.7. The rapid decrease of the reaction rate results from two effects the separation of with 8, which is difficult: (a) decrease the number of reaction sites, (b) decrease of the rate constant of the heterogeneous step which is the slowest step in the overall reaction, disregarding mass transport processes. It is of interest to compare the inhibiting effect[ 1,2] of CO, on hydrogen oxidation. and evolution with theoretical predictions[5-71 on the blocking influence of adsorbed species on the diffusion current density under steady state conditions. An a priori calculation of the attenuation factor of the diffusion current
density was first performed in Refs[S] and[4] on the basis of Smythe’s cylinder model[8] and in[7] on the basis of a periodic-lattice model. In both cases, values of the attenuation factor between 1 and (1 -B) are predicted under the assumption that the process remains controlled by convective diffusion when the surface is partly blocked by other species. The attenuation factor approaches 1 if the dimension of the active sites is small and tends towards (1 - 13)if it is large in comparison with the thickness of the diffusion layer. Considering the results in[ l] and[2] for the electrode with the uniform current distribution it can be said that the attenuation factor is close to 1 for B,, G 0.7. processes rateSince mass transport are controlling[l, 23 the dimensions of the active sites have to be small relative to the thickness of the diffusion layer. The first derivation of the ac impedance of a partially blocked electrode, based on a concept of semispherical diffusion around a very small active site, was given in[9]. The next formulation[lO] is not considered[lI] satisfactory. A theory based on the periodic lattice model was presented in[lZ]. A different approach to the derivation of expressions for the electrode impedance was taken in[13]. Using the concept[ 1 I J that the non-linear diffusion problem for a partially blocked electrode is mathematically the same as that for linear diffusion coupled with a firstorder homogeneous reaction, the impedance was derived on the basis of the treatment[14] of that simpler case. Finally, a model for the transient response of electrodes with microscopic active and inactive sites was discussed in[iS]. This work is complementary to the previous[l l-l 31 considerations in which macroscopic surface inhomogeneities were treated. In the present communication the frequency dependence of the impedance of the interface of smooth platinum wire/OS M H,SO, was investigated as a function of the carbon monoxide coverage. The meas1725
M. W. BRELTER
1726
urements were always made at a cathodic dc current at room temperature (22 density of - 1 mAcm_’ t lDC). Earlier measurements[l6] of the impedance platinum wire/sulfuric acid solution demonstrated at cathodic polarization that the ohmic component 5s ,and the capacitive component l/c&Z, in a series circuit were determined by mass transport processes at frequencies between 30 and about 2OOOHz. The smooth Pt electrodes had been pretreated by anodic pulses. As expected from the theory[l6] the two components of the Warburg impedance were inversely proportional to the concentration of molecular hydrogen at the surface. When a cathodic current was maintained after the anodic pretreatment, the overvoltage increased[l7] slowly with time due to the adsorption of impurities from the electrolyte. The poisoning was redected[ 171 by a drastic change in the frequency dependence of the impedance. In contrast to the relatively uncontrolled poisoning[17], the chemisorption of carbon monoxide is carried out before the impedance measurements in the present work. The impedance is time-independent during the measurements at a given coverage with CO,,. The transition from a surface state of high reactivity to a state of low activity is accomplished in a well defined fashion on smooth platinum electrodes.
EXPERIMENTAL The experiments were made in a Pyrex glass vessel of conventional design. Since the values of the ohmic and capacitive component of the interfacial impedance are proportional to the geometric surface area A, a Pt wire with 0.55 cm2 and a charge equivalent SQH = 0.10 mCcm- ’ for the removal of a monolayer of H atoms was chosen as the test electrode. The test electrodes of the previous studies of the influence of CO,, on hydrogen adsorption[4], hydrogen evolution[2] and hydrogen dissolution[l] could not be used because of the relatively large values of A. The Pt wire was located in the axis of a Pd wire spiral of the assembly shown in Fig. I of [4]. The purification of the electrolyte was described there[4]. The overvoltage q was measured us a hydrogen electrode in the same electrolyte (0.5 M HZS04) as the test electrode. The impedance measurements were always made at a cathodic current density of - 1 mAcm- z at room temperature (22 f 1YZ). The procedure for the establishment of different coverages with CO, was as follows: (a) After cycling the Pt wire a few times between 0.05 and 1.2 V at constant current (+ 36 ~Acm-“) the test electrode was kept at 0.1 V and the Pd wire spiral at 0.8 V during the stirring of the electrolyte with CO of CP grade at 1 cm’s_‘. (b) The CO stirring was followed by stirring with purified N, at 1 cm3s-’ for 1000 s. The test electrode remained at 0.1 V and the Pd spiral at 0.8 v. (c) The electrolyte was replaced once with fresh electrolyte, presaturated with N,. Both the test electrode and the Pd spiral stayed at open circuit. (d) An anodic charging curve was recorded at 36 PA cm-’ up to about 0.4 V. The oxidation of
CO,
was avoided in this fashion. The coverage:
eco = I- sQ’,lsQ,
(1)
was determined. Here sQ;I is the charge equivalent of a monolayer of H atoms in the presence of CO,, 1 The electrolyte was saturated with purified Ha at 1 cm’ s- ’ For about 600 s. Both the Pt electrode and the Pd electrode remained at open circuit. A cathodic current of - 1 mAcmm2 was applied. After 100 s the two components of the impedance were determined successively at 10,000,3ooO, 1000, 400, 200, 100, 70, 50, 40, 30, 25 and 10 Hz. The superimposed ac signal was always smaller than 2 mV,. The apparatus for the impedance measurements was similar to the circuit in earlier studies[ IS]. Finally the overvoltage was measured. Nitrogen stirring replaced the hydrogen stirring. The Pd spiral stayed at 0.8 V and the test electrode at open circuit. After the open-circuit potential of the Pt electrode reached 0.05 V another charging curve was initiated at the current density given above. This charging curve was extended to such potentials that a part of CO, was removed by oxidation to CO,. Then the test electrode was taken back to 0.05 V by a cathodic current pulse. Steps (d)-(g) were repeated until the surtace was tiee of CO,. The last impedance measurements were made at zero coverage with carbon monoxide. By comparing the 61, value from step (d) with the 6r,o value computed according to Equation (1) from the hydrogen branch of the charging curve of step (g), it was verified experimentally that the coverage with CO,, was not affected by the measurements during step (f). As in the previous studyC181, the electrolytic resistance R, between the test electrode and the tip of the Luggin capillary, leading to the reference electrode compartment, was determined as the ohmic component of the impedance at 10,000 Hz at 1.3 V in the oxygen region. Since the ac circuit is a galvanostatic one, the correction for R,, was applied in the bridge during the impedance measurements arrangement[lS 1 at -lmAcm-. EXPERIMENTAL
RESULTS
The two components of the impedance in a series circuit are plotted as a function of w-l’* at various coverages with CO, in Fig. l(a) and (b). The data in Fig. l(a) correspond to the coverage range in which the dc measurements[l, 21 demonstrated a relativeiy smali decrease of the reaction rate at constant potential with increasing coverage (@,o < 0.7). In contrast, the impedance behaviour in Fig. l(b) results in the coverage range where the cathodic current of H2 evolution at constant potential decreases at first rapidly in a narrow region and afterwards more slowly with B,,. Two different scales had to be employed for the abscissae in Fig. l(a) and (b) to accommodate all the experimental points. Solid symbols are used for the ohmic component and open symbols for the capacitive component. The data in Fig. l(a)and (b) were not COrreCted for the double layer capacity. As to be expected from previous work[16] the two components coincide practicatly at B,, = 0 and
Influence of chemisorbed CO
1727
the slope of the straight lines in the presence of CO, are larger by a factor of 2.2, 2.5 and 1.6 than the respective values of 0.13/(1 - f&J for 8,, = 0.3X,0.64 and 0.80. The preceding consideration was made to check if the experimental R,-uJ-‘/~ plots and 1/cs+L- If2 plots at Iower frequencies may be interpreted as the left portion of theoretical curves[13] on partially blocked electrodes. The disagreement between experimental and theoretical slopes implies that this is not the case. The inaccuracy of the impedance data cannot be held responsible for the shape of the experimental Rs_w-“’ curves and l/oCs~‘I* curves at f&o 1 0 in Fig. l(a) and (b). The data in Fig. I(b) reflect the rapid increase of R, and ~/WC, with 8,o in a narrow range. There is a marked difference between the curves at B,, = 0.8 and e ,-o = 0.9 and 0.97. The shape of the latter two curves looks similar to that of theoretical curves[14, 161 for a rate-controlling heterogeneous reaction. The ratios ~(~,o~lf1(~co = 0) and
for 100, 1000 and 10,000 Hz are given as a function of 8 co in Fig. 2. A double layer correction was not applied. The ratios represent relative measures of the degree to which the H2 evolution is hindered by the increasing coverage with t&o. DISCUSSION The dependence of the absolute value of the electrode impedance upon the carbon monoxide coverage
%I* 0.02
3000
II
000400
I1
0.04
200
00
w-l%&l
0.06 70
50
40
I
1
30
25
Hz
Fig. 1. Ohmic (solid symbols) and capacitive(open symbols) components of the impedance in a series circuit as a function of CL-“’ at different coverages of carbon monoxide. A correction for the double layer capacity is not audied. (a) e~0 = 0, 0 , n ; 0.38, A, & 0.64, o, . . (b): 9, zb:g0, &‘i; 0.90, Cl , I : 0.97, 0, . .
depend linearily upon w- “‘. Essentially the Warburg impedance of the diffusion of molecular hydrogen is measured. The Rs~u-“2 curves and l/oCs+-‘I* curves in Fig. 1 (a) at 8,o = 0.38 and 0.64 differ in their shape from the lower curve. Nevertheless the values of Rs and l/cuCs at a given frequency are relatively close to each other for each of the upper curves in Fig. 1(a). The last statement also applies to the low&r curve in Fig. l(b). Straight lines which are not shown in Fig. l(a) were drawn in a rough approximation through the I/WC, points of the two upper curves. The extension of these lines went through the origin of the coordinate system. The slopes of the straight lines amounted to 0.13, 0.46, 0.91, 1.58 for 0,, = 0, 0.38, 0.64 and 0.80. The slope is given in cmcm- I, using the same scale for data in Fig. l(a) and (b). The values of
the Fig. 2. Plot of ratios (sco)/~(&xJ = 0) and ( JR: + I/~zC$)t&( JR; + l/w’C&, -0 at ditlkrent frequencies as a function of 6~0. o, overvoltage ratio; A, impedance ratio at 10,000 Hz; q , impeaanCeratio at loo0 Hz; V, impedance ratio at 100 Hz
1728
M. W. BREITER
is discussed first. The discussion of the frequency dependence of the components at constant coverage follows. The latter discussion is restricted to Qco < 0.8 because the dc measurements indicate(21 that H3 evolution becomes feasible at about 8, = 0.9 on top of the adsorbed layer of carbon monoxide. Dependence of the absolute value ofthe impedance carbon monoxide coverage
upon
The results in Fig. 2 are in general agreement with those of the previous dc studies[l, 21. There is a gradual increase of the hindrance of the H2 evolution reaction up to about 6$-o = 0.7. This increase is followed by a rapid increase above oco = 0.7. However, there is a remarkable difference between the present results and those in[l] and [2] for tic0 < 0.7. The ratios rl (e,,)lrl
(O,, = 0)
and ( JR;+
WJ~G),,/(
JR:
+ W’C,z),
=o
at low frequencies increase considerably more in Fig. 2 of the present communication than the reaction rate decreases with &o in fig. 3 of[2]. Carbon monoxide has a considerably larger impact on the H2 evolution reaction at the smooth Pt wire electrode than at the two smooth electrodes[2], involving Pt foils. Possibly there is a correlation between the achievement of a high activity of the Pt wire electrodes by anodic pretreatment and the high sensitivity to poisoning. In contrast, Pt foils proved more difficult to activate over the years of experience of the author. They appear to possess less sensitivity to poisoning. The absolute value of the impedance at 100 Hz is given as an example for the behdviour at the lower frequency range in Fig. 2 as a function of &,. The behaviour is nearly identical with the dr behaviour. Although the values of the resistance ratio at 25 Hz are not shown i? Fig. 2, they are larger than g(tl,,)//1(6rco = 0) at a given value of 0,. This suggests that a decrease of l/rtiC, occurs with decreasing frequency below 25 Hz. The overvoltage ratio reflects the situation at 0 Hz. As it might be expected, the dependence of the absolute value of the impedance upon 8,-o becomes smaller with increasing frequency. The closeness of the curves for 1000 and 10,ooO Hz in Fig. 2 at oco $ 0.7 is mainly due to the fact that a double layer correction was not applied to the data at 10,000 Hz. This was not done because the low-frequency bchaviour where double layer effects are negligible is of main interest. The results in Fig. 2 demonstrate that the contribution of H2 diffusion to the electrode impedance becomes rapidly smaller with tl,, on the Pt wire electrode. Frequency
dependence
of the
impedance
frequency dependence of the impedance of a partially blocked electrode on which an electrochemical reaction occurs may be discussed in terms of: (a) a purely blocking effect[9-13, 151 and (h) a transition[l7] from mass transport to kinetic control. The assumptions of approach (a) are given in the The
Introduction. Since the frequency dependence of the two components of the impedance of a poisoned platinum electrode displayedt 171 a certain similarity with the theoretical dependence in the case of a ratecontrolling heterogeneous reaction[l9,20] the interpretation (b) was advanced[l7]. Essentially it is assumed in[19] and[20] that the rate constants of the heterogeneous reaction are constant[19, 201 across the surface or that the surface possesses[19] a uniform[21] heterogeneity. The results in Fig. l(a) of this paper are discussed on the basis of the approaches (a) and (b). It was derived in[13] for the approach (a) that the impedance at high frequencies behaves similarly to that at the homogeneous electrode with the surface area A/( 1 - 6). The value of 0 can be determined from the slope of the plot R, and/or ~/WC, DSw-~‘* in this frequency range. At intermediate frequencies the Rs+-“’ plots and l/oCs+m’r* plots display a relatively large curvature. At low frequencies the two plots become linear and have a slope smaller than that at high frequencies. The ohmic component is always larger than the capacitive one in a series circuit. A glance at the two upper curves in Fig. 1 (a) reveals that the frequency dependence of the experimental curves differs greatly from the dependence derived in[l3]. As pointed out in the section Experimental Results, it cannot be argued that, because of a certain inaccuracy of the experimental data, two straight lines with the slope 0.13/( 1 - 0,) are measured. These lines would correspond to the left Rs-cK’!’ plots and part of the theoretical l/oCs-W -“’ plots. It is suggested that the discrepancy between experimental results and theoretical predictions of approach (a) is found because the basic assumption[13,15] of a constant exchange current density of the electrode reaction on the free surface area is not fulfilled for the hydrogen evolution reaction on smooth platinum. In contrast, the gold model eiectrode[ll] which was partially covered with a photoresist layer displayed the predicted behaviour[13] during impedance studies[13] of a redox reaction. For approach (b) a detailed analysis of the frequency dependence of the impedance of the H2 evolution reaction is carried out for the Volmer-Tafel mechanism: H+ +e-
= H,,
(2)
H,, + H,, = Hz
(3)
and for the Volmer-Heyrovsky
H+ +eH+ +H,+e-
mechanism:
= H,
(4)
= H,.
(5)
The derivation is based on the following assumptions: (a) The rates of the different reactions may be expressed as iv = i,(K
B,, B,,)
(6)
i, = i, (%, B,,)
(7)
i, = i, (E, 8,, eco).
(8)
Here 0, is the coverage of the ti atoms which are involved in hydrogen evolution. 0, need not be
Influence of chemisorbed CO
1729
identical with the total hydrogen coverage on that part of the surface which is free of CO,,. (b) The current distribution is uniform. (c) The influence of H2 diffusion is negligibly small. The assumption (a) is valid for a homogeneous surface for which a given rate constant has the same value on any site or for a surface with uniform heterogeneity[21], for instance according to Temkin’s model[22]. The geometry of the cell assembly guarantees the fultilment of the assumption (b) on a macroscopic scale. The boundary conditions are for the Volmer-Tafel mechanism: --FT$=
Aiv-Ai, Si,
‘v
= TAq+&Atl,-~A8, 'H
Ai, = Ai,
(91 H
= Ai,
and for the Volmer-Heyrovsky
(10) 0.4
mechanism:
1,
0.2 1
0.4 I
11
0.6 I 1,
11
3000 KMCI4CCI200 100 70
5040
0.8 1 G+-$ISEL&I
EI
30 25 HI
Fig. 3. Reciprocal values of the ohmic {solid symbols) and capacitive (open symbols) components of the impedance in a
parallel circuit after correction 0, l , e,, = 0.38; A, A, B,,
Ai, = Ai, + Ai,.
(12)
Here FT designates the maximum amount of H in Ccm-2 and i, is the faradaic current. The expressions for the reciprocal values Ai,/Aq of the complex impedance are:
ties. The component UK; increases from 0 at large frequencies, passes through a maximum at wM = u/F l” and tends towards 0 at small frequencies. In order to compare the theoretical and experimental results a correction for the double layer capacity has to beapplied. This was done in an approximate way by attributing I/WC, at 10,OOOHz to the double layer impedance ~/WC,. The correlation between experWC,) and the faradaic comimental values (l/R,, ponents is: 1/R’,
+($+$)/(g-%+jc0Fr>.
(14)
It was necessary to derive the reciprocal expressions because the Volmer and Heyrovsky reactions involve discharge steps occurring in parallel. The present derivation for the Volmer-Heyrovsky mechanism is more general than the one in[23]. The first terms in Equations (13) and (14) represent the discharge resistances which are approached at large frequencies. When separating the complex expressions of Equations (13)and (14)intoa real component (l/RL)andan part &K’,,) it is found that imaginary both components may be written in the same formal way for the Volmer-Tafel and Volmer-Heyrovsky mechanism:
(15) (16) The frequency dependence is similar for both mechanisms of Hz evolution under the assumptions stated before. The component l/R’p increases from l/r at large frequencies to (l/r + A/a) at very small frequen-
for the double layer capacity. = 0.64; 0, I, O,, = 0.90.
= l/R,
(171
UK’, = w(C,-Co).
(18)
Using theabove procedure the impedance data for 8,, = 0.38, 0.64 and 0.9 are given in the semilogarithmic plot of Fig. 3. A glance at the results in Fig. 3 demonstrates that I/R, displays the opposite dependence upon the frequency as is expected on the basis of Equation (IS). It is difficult to say if the capacitive component shows the correct frequency dependence because the location of the maximum is not known. Although the mechanisms for H, evolution were formulated in a way which assumed that only a part of the H atoms participates in the reaction, the experimental results in Fig. 3 do not agree with the theoretical prediction of the frequency dependence. The question arises whether hydrogen evolution occurs on the poisoned Pt wire electrode by another mechanism or whether the assumptions (a) and (b) are not fulfilled. The present investigation does not allow a decision.
REFERENCES 1. M. W. Brdter, J. electroanal. Chem 65, 623 (1975). 2. M. W. Breiter, ibid. 115, 45 (1980).
I730
M. W. BRE~TER
3. M. W. Breiter, in Tmns. Symp. on Elecfrode Processes (Edited bv E. Yeaaerl D. 307. Wilev. New York (19611. 4. k. W. Br&r, Pro: S$&. on Electr&atalysis, p. 11‘5.Thk Electrochem. Sot., Princeton (1974). 5. R. Landsberg and R. Thiele, Elecrrochim. Acta 11, 1243 (1966). 6. F. Scheller, S. Miiller, R. Landsberg and H. J. Spitzcr, .f. electroanal. Chem. 19, 187 (1968). 7. E. Levart. D. Schuhmann, 0. Contamin and M. Etman, ibid 70, 117 (1976). 8. W. R. Smvthe. J. aawl. Phvs. 24. 70 (1953). 9. K. J. Vet&r, i. phhys. C&I. 199, 3od (1952). 10. J. Lindemann and R. Landsberg, J. electroanal. Chem. 25, 20 (I 970). 11. T. Gueshi, K. Tokuda and H. Matsuda, ibid. 89, 247 (1978). 12. M. Etman, E. Levart and D. Schuhmann, ibid. 101, 14 (1979).
13. K. Tokuda, T. Gueshi and H. Matsuda, ibid. 102, 41 (1979). 14. H. Gerischer, Z. phys. Chem. 198, 286 (1951). 15. C. Am&ore, J. M. Saveant and D. Tessier, J. elecfroanal. Chem. 147, 39 (1983). 16. M. Breiter. H. Kammermaier and C. A. Knorr. Z. Elektrocheh. 60, 647 (1956). 17. C. A. Knorr, ibid. 59, 647 (1955). 18. M. W. Breiter, .f. electrounal. Chem. 7, 38 (1964). 19. H. Gerischer, 2. phys. Chem. 202, 55 (1952). 20. M. Breiter. H. Kammermaier and C. A. Knorr, Z. Elekwochem. 60, 454 (1956). 21. A. N. Frumkin, Advances in Elecwochemiswy (Edited by P. Delahay), Voi. 3, Chapter 5. Interscience, New York (1963). 22. M. Temkin, Z.fiz. Khirn. 15, 296 (1941). 23. H. Gerischer and W. Mehl. 2. Elekrrorhem. 59. 1049 (1955).