Specific features of the kinetics of gas-evolving reactions on highly active electrodes

SPECIFIC ,FEATURES OF THE KINETICS OF GAS-EVOLVING REACTIONS ON HIGHLY -ACTIVE ELECTRODES* V. V. LOSEV, N. YA BUNB and L. E. CHUVAEVA Karpov Institute of Physical Chemistry, Moscow 103064, U.S.S.R. (Received 1 September

1988; in revised form 24 November

1988)

Abstract-The equations of the steady-state polarization curve and the potential decay curve (pdc) upon current interruption were analysed for the electrochemical gas-evolving processes on the electrodes with high electrocatalytic activity under conditions when removal of reaction product is the rate-determining step of the overall process. It is shown that the criteria of the high electrocatalytic activity of these electrodes are the existence, on the polarization curve, of a Tafel section with “Nernstian” slope., 2.3 RT/nF. its position depending on the solution stirring intensity, at low and medium current densities; and a low polarizability section at high current densities under intensive gas evolution, as well as ihe existence, on pdc, of an almost horizontal section determined by gradual change of the equilibrium open circuit electrode potential in time, as a result of slow removal of reaction product from the near-electrode layer supersaturated with it. The above criteria were applied for kinetics analysis of the process of chlorine anodic evolution from acidified concentrated chloride solutions at high temperature (under chloralkali electrolysis conditions). It is shown that ruthenium based dimensionally stable anodes (DSA), as well as the smooth platinum anode activated by cyclic anodicsathodic treatment, have very high electrocatalytic activity with respect to the chlorine evolution process.

The aim of this paper is to through gas evolution. analyse the character of the steady-state polarization curve and the potential decay curve (pdc) upon current interruption for gas-evolving processes on highly electrocatalytically active electrodes under conditions when the reaction product removal is the rate-determining step of the overall process, as well as the experimental study of chlorine evolution kinetics on ruthenium baaed DSA and activated smooth Pt under industrial chloralkali electrolysis anode, conditions.

INTRODUCTION Due to the wide application of efficient metal oxide anodes, particularly, ruthenium based dimensionally stable anodes (DSA) covered with ruthenium and titanium oxides, having high electrocatalytic activity, in the industrial chloralkali electrolysis, in the recent decade there appeared a number of reviews dedicated to the kinetics of gas-evolving reactions on these electrodes[l-71. A peculiarity of the electrode process with gas evolution lies in the fact that the removal of the end product from the electrode surface may proceed not only through its diffusion into the bulk solution, but also by gas bubbles formation at the electrode surface. Thus, if.the rate of the electrochemical step of the overall process, say, anodic reaction: neR, + R,a0,

+ O,,

THEORETICAL The steady-state polarization curve equation accounting for both diffusion steps of the process (1) under intensive stirring and where CO], 4 [O],.,, ([0],, is the bulk concentration in saturated solution) has the following form[8]:

(1)

(where subscripts o and s refer to the bulk solution and electrode surface, respectively) is very high, and the preceding diffusion step does not affect its kinetics (eg under chlorine evolution from concentrated chloride solution, or oxygen evolution from water), then its rate-determining step is the removal of reaction product from the electrode surface. At relatively low current densities this removal will proceed only through diffusion of the reaction product molecules into the bulk solution; at high current densities, however, under supersaturation of the near-electrode solution layer by the reaction product and intensive evolution of gas bubbles, its removal would preferentially go

i = ia - i, = i,

[RI, ~expWFAE/R 0

COI.

- -exp(

co10

- aFAE/R

7’)

T)

1 ,

where: i, is the exchange current density; i. and i, are the partial anodic and cathodic current densities; a and j3 are the apparent transfer coefficients (a + fi = AE = E-E_; E,, = E’& + (R T/nF)ln([O]J ;i] ) T o analyse the effect of the subsequent diffusion step” *on the kinetics of process (1) when it is not complicated by the preceding diffusion step, we adopt the condition [R], = [RI,, which is easily realized (eg by [R], 9 CO],). With account for the known rela-

*Based partly on the paper presented at the Conference of the Chemical Society of G.D.R., Bad Stuer, 1987. 929

930

et al.

v. v. Losev

tion[8,

p. 1491

COl,/COl,= 1 + (i/if),

(3)

where iz is the cathodic diffusion limiting current of reaction product reduction corresponding to its bulk concentration, from Equation (2) we obtain, [9]: i = i, - i, = i,

exp(/3FAE/R

T)

i= i,

L

exp(pFAE/RT)

(4)

yields from Equation - exp( - ctFAE/R T)

1 + (iJiz)exp(

-

crFAE/R T)

(4):

COI, l”g[O,,

1

Igi

lgiO Qib

I_ (5)

AE=(=$)log(l+-$

nF

lgiL

1giz

Fig. 1. Schematic polarization curves: i’ = limiting current density (at supersaturation); i, = initial exchange current density; i: = exchange current density at supersaturation; (AE)’ = concentration polarization; (AE)” = electrochemical polarization; n = 2.

As follows from Equation (5), at relatively high AE a common linear section with Tafel slope, b, = 2.3 RT/pF, should appear on the anodic polarization curve[9]. However, in the case of a very fast electrode process (i, P iz), and at a small deviation from the equilibrium potential, Equation (5) is transformed into the common equation of concentration polarization:

2.3 RT

Ei.7 Ll

- [1 + (i/iz)]cxp(-nFAE/RT)]. A simple manipulation

E

(6)

ie at the electrode surface, the equilibrium of the electrochemical step of the overall process is practically maintained, and the potential shift from the equilibrium value, E,, (curve I, Fig. 1) is determined only by increase of the reaction product near-electrode concentration.* In this region of AE values, at i>iz we should observe on the anodic polarization curve a Tafel section with low (“Nernstian”) slope, b, = 2.3 R T/nF[ 10,111 (curve I, Fig. l).’ As follows from Equation (2), in region I the [0], increase leads to the positive shift of the partial cathodic polarization curve (Fig. 1), and the external current is equal to the difference of two high partial currents (i = i, - i,). The polarization curve’s character, however, should radically change when, upon increasing current density in this AE region, such supersaturation of the near-electrode solution layer by the reaction product molecules is reached that makes possible formation, on the electrode surface, of critical size gas bubble nuclei capable of growing (due to reaction product molecules transfer from the adjacent solution layer into them) and then breaking off from the surfaceC13, 143. Upon further increase of current density;removal of reaction product proceeds not only via diffusion but mainly via gas evolution. As a result, with growing i *For comparison, this figure shows also a polarization curve for the case. of the rate-determining electrochemical step with the same exchange current, but a very fast subsequent diffusion step (curve 1’). ‘The effect of the subsequent step of chlorine diffusion on its anodic evolution kinetics (uncomplicated by the slow preceding diffusion step) is analysed in[12].

further increase of the near-electrode reaction product concentration, CO],, and of the electrode potential is sharply decelerated, ie a sharp decrease of the polarization curve slope should occur[l5]. It should be noted, that the value b, = 2.3 RT/nF represents the lowest possible Tafel slope under conditions when the rate of the overall electrode process is determined by some charge transfer, or recombination, or subsequent diffusion step. Therefore, this sharp decrease of the polarization curve slope cannot be caused by a change of the electrode process mechanism. To obtain an approximated assessment of the polarization curve character at further increase of current density let us assume that near the electrode surface with growing i a certain practically constant limiting concentration of reaction product molecules, O:, is reached corresponding to a limiting supersaturation of the near-electrode solution layer with them. The existence of such limiting supersaturation practically independent of current density was found with different techniques under cathodic hydrogen evolution from acidic solutions on palIadium[16] and platinum[l7, IS], under anodic oxygen evolution on platinum[l8], as well as under anodic chlorine evolution on platinum[l9] and Co-Ni spine[20]. As follows from Equation (3), this limiting concentration and a more positive equilibrium potential, E&, corresponding to it, will be reached at a certain limiting value of current density, i’, (Fig. 1) to which corresponds a limiting value of concentration polarization: (AE)’

= EL, -E,,

and a limiting

(i:, Fig. l).*

position

= T,o,g,

0

of the partial cathodic

(7) curve

With increasing near-electrode reaction product concentration up to [O]:, the exchange current density increases respectively from i, to ii (Fig. 1). Consequently, the anodic polarization curve at E > EL,

*In the case of the initial solution saturated with reaction product (E,, = Ez), from the value (AE)’ = EL, - E:p,Lone can calculate by the Nernst equation the limiting supersaturation of near-electrode layer with this product.

931

Kinetics of gas-evolving electrodes (Curves II and III, Fig. 1) will be described Equation (4) but by: i = ia - i’c = ii

not by

expjIF(AE)“/RT)

-exp(-crF(AE)“i/RT)

1,

into the bulk solution is strongly affected by the convection fluxes existing near the electrode surface, determined not only by breaking-off and removal of the gas bubbles from the surface, but also by the process of their growth occurring both at the surface and after their break-off in the supersaturated bulk solution in the vicinity of the electrode[19, 21-231. Since a precise account of the influence of these factors on the mass transfer is very complicated, a quantitative description of complete pdc under these conditions is difficult. It is doubtless, however, that for an electrode with extremely high electrocatalytic activity (ii & ib) the whole pdc should correspond only to the reaction product removal step. In the case of a less active electrode, when the current densities under gas evolution are comparable with the exchange current density or exceed it noticeably, an overall pdc should also have an initial section of fast potential decay determined by the overvoltage of the charge transfer step. Since decay of the mentioned convection fluxes (and consequently, change of the reaction product near-electrode concentration) upon current interruption proceed much more slowly than relaxation of the kinetics of the charge transfer step of overall process, it permits, in principle, distinction on the overall pdc of an initial section, undistorted by mass transfer, corresponding to decay of the charge transfer step, overvoltage[ 167. Let us consider quantitative regularities of potential decay upon current interruption for the case of a very fast electrode gas-evolving reaction. The literature thoroughly discusses the case when pdc’s are measured in the current densities’ region where the polarization curve of the studied process obeys the Tafel equationC24; 8, p. 4341, ie at i B iw The solution of the differential equation

(8)

where (AE)ei = E - EL, is the electrochemical polarization. Thus, the overall polarization equals AE = (AE)’ + (AE)” (Fig. 1). It is easily proven that under condition i’ < i d ib the E value under current flow will only slightly differ from the new value of equilibrium potential (EL,) corresponding to i’. Thus, eg according to the following equationC8, p. 1341:

for n = 2 even at i = 0.1 ii, the (AEF’ value is 1.2 mV, and at i = 0.01 i: this value is ca 0.1 mV. So under such conditions even a small positive shift of potential from EL, should cause a very significant current density increase (region II, Fig. l), ie appearance of a low polarizability section (LPS) on the polarization curve. Only at i 2 it, with further increase of potential there begins a transition to the charge transfer section on the polarization curve with the slope b, = 2.3 R T/flF (region III, Fig. 1). As follows from the above, only this section of the polarization curve can be used to determine the kinetic parameters (ii, 8) of Equation (1). So, the specific feature of the gas-evolving process on a highly electrocatalytically active electrode is appearence of a LPS on the polarization curve, the length of LPS being inversely proportional to the i’/iL ratio. The existence of such section on the polarization curve indicates that the rate-determining step of the overall process is removal of reaction product. In this region of the polarization curve i 4 ib and, consequently, the equilibrium of the electrochemical step of overall process is practically maintained, and the electrode potential deviation from the initial equilibrium value should be determined only by increasing reaction product near-electrode concentration.* Let us consider the character that pdc upon current interruption should have under these conditions. On the electrode, after the instant ohmic potential drop (Eohm)[8, p. 4341, an equilibrium open-circuit potential, E,, = ES,, should set up corresponding to the increased (supersaturated) near-electrode concentration of reaction product molecules. Their removal from the electrode surface, upon current interruption, oia diffusion into the bulk solution and transfer into the gas bubbles on the electrode should be rather slow and, respectively, E,, would slowly decay to the initial value of the equilibrium potential, Eeq, corresponding to the reaction product concentration in the bulk solution. As is well known, under intensive gas evolution on the electrode the mass transfer of reaction product *As is seen from Fig. 1, in the region 11 the conditions realized are quite similar to those corresponding to polarization curve I’ at E - Ecq[S].

CdtAE), +i = dt’

0

’

where (AE), and i, are the values of overvoltage and current density at time t upon current interruption, C is the double layer capacity, has the form[8, p. 434; 241: (BE),,,-(AE),

= bln(%+

l),

(11)

where b is the Tafel slope of polarization curve. As follows from Equation (1 l), at t S Cb/i,=, a linear relationship should be observed between (AE), and In t.11 The shape of pdc should be different when the current density i, in Equation (9) is lower than or comparable with the exchange current density. In this case, corresponding in Fig. 1 to the non-Tafel transition section of polarization curve from region II to III, the dependence of i, on the electrochemical polarization (AE)S’ is expressed by Equation (9). Substitution of i, from this equation into Equation (10) yield

“A peculiar feature of this case is that the initial pdc section (t Q G/i,=,) is described by a linear relationship between (AE), and t, and transition from this section to the log relation between (AE), and time occurs earlier at higher current densities[ZS, 261.

932

k’. v.

the differential

where (AE):’

LOSEV

equation:

= E, - E&. Its integration (AE):’

= (AE):L,exp(

- ki:t),

yields: (13)

where k = nF/CR T. Thus, unlike the semilogarithmic pdc (11) describing the potential-time relationship in the Tafel region (i * i,), the pdc measured upon current interruption in the mentioned non-Tafel region of polarization curve, should have a short section of exponential E-t relation (13). Consequently, if in the case of a fast gas-evolving reaction a limiting supersaturation of the near-electrode layer with reaction product is reached (CO]: > [O]..,), then depending on relative values of current density and exchange current density, i:, three different pdc types can be realized. (a) At i Q ib, upon current interruption a reversible open-circuit potential, E,, = EL,, corresponding to elevated near-electrode reaction product concentration, should be reached. At that, pdc should have after the instant decay of ohmic potential drop only the mentioned section of the very slow reversible open-circuit potential change determined by a slow removal of reaction product from the near-electrode layer. (b) At i - it, pdc should comprise two sections: (i) a short one of the fast initial exponential potential decay (13) corresponding to the charge transfer overvoltage decay (due to double layer capacity discharge) till Et, is reached; (ii) one of the slow change of reversible open-ciicuit potential, analogous to the type (a) pdc.

et a[

reactions are the existence on the polarization curve of a section with the “Nernstian” slope, its position depending on stirring intensity, and LPS in the region of gas evolution, as well as the unusual pdc shape upon current interruption: the existence of almost horizontal section on this curve. The aim of the experimental part of this paper is to hefine possibilities of assessment of electrocatalytic activity of gas-evolving electrodes based on the analysis of the character of steady-state polarization curves and pdc’s upon current interruption illustrated by the reaction of chlorine evolution on ruthenium-based DSA as well as on an activated smooth platinum anode.

EXPERIMENTAL The experiments were performed on vertically positioned RuO,-TiO, DSA mainly with five-layer coatings containing 30 mol% RuO, (5 g m-’ Ru), as well as on a smooth platinum anode in concentrated NaCl solutions at 87°C. The measured potentials were corrected for the ohmic potential drop, and pdc’s were recorded with the C-8-13 storage oscilloscope. and a current interrupting unit. Figure 2 shows polarization curves measured in solutions with different chlorine content[27]. At a chlorine pressure of 1 atm, the anodic curve has a short quasilinear section with low slope (ca 20 mV).n To obtain the anodic branch of polarization curve in a wide potential range the chlorine concentration in the solution was lowered by continuous purging with argon+hlorine mixtures. One can see that at a chlorine partial pressure of 0.02 atm on the anodic polarization curve at low and medium current densities appears a clearcut linear section with 36 mV slope, ie

(c) At i % ib, first, a common section of fast semilogarithmic potential decay should be observed on pdc described by Equation (1 I), and then, close to EL,, a short transition section of exponential E-t relation. Finally, a second section of the slow change of reversible open-circuit potential, analogous to type (a) pdc. As noted above, change of the near-electrode reaction product concentration upon current interruption occurs much more slowly than relaxation of the kinetics of charge transfer step of the overall process, thus permitting the type (b) pdc processing to obtain quantitative information about the character of polarization curve in the region III (Fig. 1). For this purpose, it is necessary to set on such pdc a moment of time t = r, corresponding to transition from the first (activation) section to the second one. The difference of potentials on pdc between the values of the opencircuit potential at the moment of current interruption (after introduction of ohmic correction AE,,,) and at T is the charge transfer step overvoltage (AE)” (13). Measuring the type (b) pdc in the region III on the polarization curve (Fig. 1) at different current densities it is possible to use respective (AE)” values for plotting the polarization curve of a very fast electrode process in this region under intensive gas evolution and obtaining its true kinetic parameters (/3, ii, cf: Fig. 1). Thus, the criteria of the high electrocatalytic activity of electrodes towards electrochemical gas-evolving

jl,I, ,

::::[, -5

-4

-3

lgi

-*

-,

(a.cm-2)

Fig. 2. Polarization curves on a 30% RuO, electrode in 4.9 M NaCl with pH = 1.9 (87°C) at chlorine partial pressures: (1) 1; (2) 0.1; (3) 0.02 atm.

“Polarization curves with Tafel slopes lower than 2.3 RT/2F were obtained in concentrated chloride solutions at high temperatures on the ruthenium-based DSA[28-301, as well as on the Co,O,-based metaloxide anode[31].

933

Kinetics of gas-evolving electrodes

g

a”

::,_;____;_y

;

0 5

2

3

4 -1gI

I

0

ta.cm*)

Fig. 3. Polarization curves on a 30% RuO, electrode in 4.3 M NaCl with pH = 1.6 (87°C) saturated with chlorine (1) and argon (4); pdc at i = 100 mA cm- * (2) and A&log i relationship (3) in this solution.

2.3 RT/2F at 87°C. At higher i, the experimental points deviate from the linear section to lower potentials, and at i 3 0.1 A cm-’ on all three polarization curves a LPS appears. Let us first consider the initial linear section nature. It is well known that a similar section of anodic polarization curve with 2.3 RT/ZF slope has been observed for chlorine evolution by a number of researchers. Generally speaking, it may correspond not only to the subsequent diffusion step, but also to the mechanism with the rate-determining step of chlorine atoms recombination, or to a stepwise KrishtalikErenburg mechanism[l, 51. An unambiguous criterion of the diffusion nature of this Tafel slope is the dependence of its position on stirring intensity. It has been shown that under the mentioned conditions at chlorine pressure of 0.01 atm this linear section shifted to higher current densities with increasing rotation speed of the RuO,-TiO, disc DSA[32].** So, in the region of this Tafel section the rate of anodic chlorine evolution from DSA is determined under these conditions not by the charge transfer kinetics or Cl atoms recombination, but by subsequent chlorine diffusion into the bulk solution. As is clear from Fig. 2, at i > 20 mA cm-’ the polarization curve slope decreases-here we observe intensive gas evolution-further growth of the nearelectrode molecular chlorine concentration practically stops and removal of the produced chlorine proceeds mainly via gas evolution. ++ Consequently, in agreement with the results of theoretical analysis of the gasevolving reaction kinetics on highly active electrodes, on the polarization curve after the linear section with a

**Similar influence of stirring on the position of the polarization curve with Tafel slope 2.3 RT/2F was observed in hydrogen evolution on a palladium cathode from solution purged with nitrogen[16]. “In this region of the polarization curve its position ceases to depend on the disc rotation speed.

Nernstian slope appears a non-linear LPS. The LPS on anodic polarization curves in chloride solutions was observed also on ruthenium based DSA’s of other compositions[33, 341. Thus, the results of polarization measurements testify very high electrocatalytic activity of ruthenium based DSA in chlorine evolution reaction under the chloralkali electrolysis conditions: at current densities up to ca 0.5 Acm- 2 the equilibrium of the charge transfer step of the overall process seems to be maintained (i, > 1 Acm-‘). Consequently, the condition i -+ it, is realized not only in the region of initial Tafel section of the polarization curve, but also in the region of low polarizability (O.lHl.5 AcmW2). In this region of the polarization curve the experimentally observed deviation of potential from the equilibrium value in chlorine saturated solution (Ez) is determined only by increased near-electrode chlorine concentration, C,, and pdc upon current interruption should have an unusual shape, corresponding to the above case (a): it should not have the initial section of fast potential change caused by the charge transfer overvoltage decay due to the double layer capacity discharge. The pdc’s upon current interruption till to establishment of constant initial equilibrium potential at chlorine pressure of 1 atm were recorded at different current densities in the course of steady-state polarization measurements on ruthenium based DSA in chlorine-saturated solution[35, 367. Figure 3 shows a polarization curve (curve 1) that has an LPS and pdc measured at i = 100 mAcm_’ (curve 2): as can be seen, upon the instant ohmic potential decay (Eohm = E’ - EF=,) the shape of pdc (curve 2) corresponds to case (a)---the oscillogram E; - t is almost horizontal (in the initial 3.5 ms the EF changes by less than by 2 mV), ie the open-circuit potential E; shifts very slowly to negative values. The constant value of EP is reached in ca 15 s, does not depend on i and coincides with the initial equilibrium potential Ez = 1.293 V. The bottom of the figure shows the relationship between the overall open-circuit potential shift (AE

934

V. V. LOSEV

= ./?;=a - E$) and the current density (curve 3). As is seen, this curve is very similar to polarization curve 1. In fact, if one transfers the AE values from curve 3 in the E-log i coordinates (squares) on polarization curve 1, they will completely coincide with the respective E values on the polarization curve. This coincidence unambiguously proves that up to the highest current densities (ca 0.5 A cm)‘, the electrochemical equilibrium is retained on DSA, and the anode potential deviation from E’“’ polarization in under chlorine-saturated solztion is completely determined by increased near-electrode chlorine concentration. The shape of anodic polarization curve 1 indicates that due to the influence of cathodic process in the chlorine-saturated solution the DSA polarization is very low. To obtain an anodic curve in a wide potential range the chlorine content in the bulk solution was lowered by continuous argon purging of the solution. At that, at low and medium i a linear section (curve 4) with “Nernstian” slope (37 mV) appears on the polarization curve, and at high i this curve practically coincides with the polarization curve measured in the chlorine-saturated solution. Similar polarization curves were obtained at DSA with 100% RuO, coating (cf: Fig. 8, curve 1). An important polarization curve parameter measured under argon purging is the very current density at which the curve starts to deviate from linearity to a lower slope. This i value corresponds to the beginning of formation on the anode surface of critical size gas bubbles nuclei, and from the corresponding AE = E-E;’ value it is possible to assess approximately (by the Nernst equation) that critical near-electrode layer supersaturation with chlorine molecules at which starts their removal via gas evolution. Thus, deviation i= from of curve 4 linearity starts at and AE = 22 mV, ie the critical super10mAcm-2 saturation is 4, for a 100% RuO, electrode this deviation starts already at 5 mA cm-* (cJ curves 1 and 2, Fig. 8). From AE (Fig. 3) one can calculate by Equation (7) the limiting near-electrode layer supersaturation with chlorine (CJC,,,) under intensive gas evolution. Thus, at i = 0.5 A cm -’ for 30% RuO, electrode AE = 56 mV and CJC,,, = 34, and for lob% RuO, electrode AE = 35 mV and C./C... = 9.:: Apparently, in the case of DSA with pure Rub, coating having higher surface roughness than the ruthenium-based DSA of common composition, formation of critical size chlorine bubbles nuclei starts at a lower supersaturation level. The assessment of the near-electrode solution layer supersaturation with gaseous reaction product by pdc seems to be more accurate than that by the rotating double-ring electrode, since upon contact of this solution with the bubbles during its transfer from the internal ring to the external one the gaseous product molecules concentration in the solution can lower quite significantlyC37, 381. “Due to difficulties with reliable assessment of AE.,,, under intensive gas evolution (error is here + 4 mV) the accuracy of supersaturation measurement at i = 0.5 Acm-’ is CCI25%. Decreased supersaturation of near-electrode layer with chlorine is also observed under transition from the smooth platinum anode to the rough platinized titanium anode[19].

et al.

The pdc shape is very sensitive to changes in the catalytic activity of ruthenium-based DSA. Thus, judging by polarization measurements, the chlorine evolution rate on these anodes decreases sharply upon addition to the chloride solution of phosphate which is specifically adsorbed at the anode surface[39]. As is seen in Fig. 4a, on the polarization curve measured in chloride solution containing phosphate ions (curve 2) LPS is absent; this indicates a considerable retardation of chlorine evolution testifying the preferential phosphate adsorption on ruthenium-based electrode compared to Cl- ions predominant in the solution. Along-side with this, the shape of pdc for this electrode radically changes[35, 361. The pdc (Fig. 4b, curve 2’) measured upon phosphate addition to the solution has two sections apparently corresponding to the above mentioned case (b): after the initial section of fast potential decay confirming the activation control of the electrode process on ruthenium-based electrode in the presence of phosphate[39], a section of slow potential change appears on pdc determined by gradual slow decrease of the near-electrode chlorine concentration under conditions close to the charge transfer sten equilibrium. It has been- shown earlier, that under chlorine evolution on ruthenium-based DSA[Z, 3,32, 39a, 401, on nickel cobaltite and cobalt oxide anoder5,41, 421 the rate of this reaction noticeably decreases with growing solution acidity. As is seen from Fig. 5a, in the case of strongly acidic chlorine-saturated solution the polarization curve (curve 2) does not have an LPS, moreover, the polarization curve measured in this solution upon- argon purging (curve 4) has a slightly higher slope of the Tafel section, b = 40 mV (its position is independent of stirring intensity). On the corresponding pdc (Fig. Sbfiunlike the weakly acidic solution (curve l’tan initial section of fast potential change appears (curve 2’)[35, 363. Figure 6 shows the same pdc’s as a EP-log? plot in a wider time range. In the case of the weakly acidic solution this pdc (curve 1) has a section of a very slow change of open-circuit potential (2 mV in the first 250 ps); in the case of strongly acidic solution (curve 2)

w

6

I

I

4

3

-1gi

J

I

2

I

0

(o-cm-2)

Fig. 4. Polarization curves (a) and pdc at i = 100 mA crKz (b) on a 30% RuO, in 4.3 M NaCl (1, 1’) and 4.3 M NaCl + 0.1 M NaH,PO, (2, 2’1, pH = 1.6 (87°C) saturated with argon.

Kinetics of gas-evolving electrodes

935

1.24

I

I

I

1

I

I

3

4

3

2

I

a

Y

,

-lgila.cm-=t

Fig. 5. Polarization curves (a) and pdc at i = 200 mA cmm2 (b) on a 100% RuO, electrode at 87°C in 4.3 M NaCl with pH=1.6 (1, l’, 3) and 3.7MNaCl+0.6MHCl (2, 2, 4) saturated with chlorine (1, I’, 2, 2’) and argon (3, 4).

I.36

1.34

0 f ’

0,’

1.32

-

I. 30

t L

s

I

I

I

I

4

3

2

1

a

-1g t Is)

Fig. 6. The pdc at i = 200mAcrn2 on a 100% RuO, electrode at 87°C in 4.3 MNaCl with pH = 1.6 (1), and 3.7 M NaCl+O.6 M HCl (2), saturated with chlorine.

this potential shifts by 6 mV in the same period, and then a linear E$log t section appears. This character of pdc is determined by the charge transfer overvoltage decay due to double layer discharge according to equation (11). In ca 20 ms curves 1 and 2 practically converge and, consequently, the section of pdc 2 at t > 20 ms is determined by the gradual decrease of the near-electrode chlorine concentration to the value, corresponding to the chlorine-saturated solution. As is seen in Fig. 6, in 30 ps after current interruption the open-circuit potential in strongly acidic solution is by 23 mV more positive than in the weakly acidic one. This value is in satisfactory agreement with the potential difference between the polarization curves 2 and 1 at i = 200 mA cm-2 (Fig. 5) and is the electrochemical polarization (AE)” in strongly acidic solution. Thus, pdc 2 refers to the mentioned case (c)--its initial section corresponds to the charge transfer overvoltage decay and the second one-to slow removal of chlorine molecules from the near-electrode layer. A significant decrease of the catalytic activity of DSA is observed with lowered temperature (Figs 7a and b). At that, the slopes of Tafel sections on polar-

3

4

3

-1gi

2

0

(a an-Z>

Fig. 7. Polarization curves (a) and pdc at i = 100 mA cmL2 (b) on a 30% RuO, electrode in 4.3 M NaCl with pH = 1.6 saturated with argon, at 87°C (1’) and 20°C (2’).

ization curves remain close to 2.3 RT/2F = 36 mV at 87°C (curve 1) and 31 mV at 20°C (curve 2). But at room temperature the polarization curve (curve 2) has no LPS, and the respective pdc (curve 2’) has an initial section of fast potential decay (almost 20 mV in at high temperature 3.5 ms),w while pdc measured (curve 1’) has only the practically horizontal sectionC35, 361. Similar changes in the character of polarization curve at high current densities, as well as of pdc, are observed in the case of DSA with low RuO, content, and DSA of common composition with thin coatings[35]. In the first case (Fig. S), unlike with the anodes with high RuO, content (polarization curves 1 and 2), on the polarization curve measured on a 10% RuO, electrode (curve 3), at i = 10 mA cm _ * a section with increasing slope is observed indicating a considerable irreversibility of chlorine evolution. There is also a drastic change in the pdc shape (curve 3’)---a long initial section of fast potential decay appears that is never observed on pdc’s for anodes with higher RuO, content (curves 1’ and 2’)[35, 361. In the second case (Fig. 9), a considerable change in the character of polarization curve at higher current densities, as well as of the pdc, indicating a sharp retardation of chlorine evolution, occurs under diminishing coating thickness of 30% RuO, electrodes[35]. Figure 9a clearly shows that all polarization curves, except the one for the anode with the thinnest coating (d = 0.03 pm), at low and medium current densities (up to i = 10 mAcm_*) have long linear sections with almost equal Tafel slopes (36-39 mV). At i>20mAcm‘, however, the shape of the curves measured on anodes with thin (0.0341 pm) and thick @PThepdc measured in a chloride solution on the 30% RuO, anode at 25°C and different current densities (up to 150 mA cm- *) in the range of 50 ms-1 $437. have only the section of relatively slow open-circuit potential decay determined apparently by gradual removal of chlorine from the supersaturated near-electrode layer under conditions close to the equilibrium of the charge transfer step of chlorine evolution.

936

v.

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b

1.26

1.22

I-

I

I

I

I

I

5

4

3

2

I

0

-Igi

(o.cm-2)

Fig. 8. Polarization curves (a) and pdc at i = 200 mA cm- * (b) on 100% RuO, (1, 1’). 30% (2, 2’) and 10% (3, 3’) electrodes in 4.3 M NaCl with pH = 1.6 (87°C) saturated with argon.

06 I 56

1.52 b I .46

I .44 f C

.40

? w 1.36

I 32

et al.

S-shape: first there appears a slight deviation to a lower slope, but already at i > 50 mA cm-’ its slope increases, and at i = 500 mA cm-’ this anode potential is 25 mV higher than that of the anode with d = 3 pm, ie there is a noticeable retardation of the charge transfer step of the overall process apparently due to a significant shift of the kinetic section III of polarization curve to lower current densities (cf: Fig. 1). Consequently, the most reliable information on the relationship between the DSA catalytic activity and coating thickness, and, apparently, on other factors affecting the chlorine evolution kinetics (pH, surfaceactive anions adsorption, temperature) can be obtained basing on the analysis of the polarization curve character at high current densities. The conclusion about chlorine evolution retardation on ruthenium based DSA at transition from thick coatings to thin ones agrees well with respective change in the pdc shape (Fig. 9b). In the case of thick coatings (curves I’-3’) the pdc has, after A&,, decay,” II only a horizontal section indicating complete reversibility of chlorine evolution. With thinner coatings, the pdc has first the section of fast open-circuit potential change (curves q-6’) and only then the second section of slow potential change corresponding to a gradual decrease of near-electrode chlorine concentration. As can be seen, the length of the first section increases sharply with diminishing coating thickness testifying a considerable loss of DSA catalytic activity at thicknesses d < 0.1 IAm. Figure 10 presents a comparison of Er-log t plot of pdc’s for the standard thickness anode (5 /cm) and the one with a thin coating in a wide time range. In the case of thick coating (curve 1) the EP value, setting in 10 ps after current interruption, changes only by 2 mV in the next 90 PCS proving the equilibrium nature of the opencircuit potential. For the thin-coated anode, EP changes by over 20 mV in the same time interval, and the relation E;-log t is linear, ie corresponds to the charge transfer overvoltage decay due to double layer discharge in agreement with Equation (11). The open-

1.26

1.24

1.20

J

I

I

I

I

I

5

4

3

2

I

0

-

lgi (a.cm-al

Fig. 9. Polarization curves (a) and pdc at i = 100 (b) on 30% RuO, electrodes with coating thickness (1, l’), 5 pm (2, 2’), 3 pm (3, 3’), I /Irn (4,4’), 0.1 ,um 0.03 pm (6, 6’) in 4.3 M NaCl with pH = 2.0 (87°C) with argon (a) and chlorine (b).

mA cm-’ of: 10 pm (5, 5’) and saturated

(3-10 pm) coatings is absolutely different: in the case of thin coatings (curves 5 and 6) the slope of polarization curve increases and they do not have LPS, what indicates the retardation of chlorine evolution, while in the case of thick coatings (curves 1-3) the slope of polarization curve decreases sharply indicating the high activity of these anodes. As can be seen, the intermediate curve 4 (d = 1 pm) has a more complex

Fig. 10. The pdc at i = 100 mA cm-* on 30% RuO, electrodes with coating thickness of: 5 pm (1) and 0.1 pm (2) in 4.3 M NaCl with pH L 1.6 (87°C) saturated with chlorine.

““The open-circuit potential upon current interruption, I?;=,, has different values for anodes with different coating thickness.

Kinetics of gas-evolving electrodes circuit potential for this electrode in 10 ps after current interruption is 80 mV higher than for the one with thick coating (Fig. IO). This value practically coincides with the difference of potentials between the polarization curves of these electrodes at the same current density i = 100 mA cm-r (Fig. 9a, curves 2 and 5), equal to the electrochemical polarization for the thin electrode. Consequently, the initial section of pdc 2 in Fig. 10 corresponds to the charge transfer overvoltage decay of the overall process (AE”, Fig. l).nn The cause of lowered catalytic activity of the thinnest coatings is, apparently, the lower RuO, content on their external surface due to titanium diffusion from the Ti-support into the coating and oxygen counter-diffusion in the process of oxide film formation at high temperature[35,44]. The lowered content of RuO, in the thin coating is also indicated by the character of the relationship between A&,, values measured from pdc (Fig. 3) and the current density (Fig. 11). These curves are linear, as is seen, and for thick coatings (l-10 pm, clirve 1) the experimental points fall on one and the same line, and AE,,, is obviously the potential drop in the solution between the tip of the Luggin capillary and anode surface. At coating thickness of 0.1 pm (curve 2) and 0.03 pm curve increases by (curve 3) the slope of the AE,,,-i 30% and twice, respectively, indicating emergence of an additional resistance inside the coating, apparently, mainly at the coating/Ti-support interface[44]. The results cited testify a very high exchange current of chlorine evolution on ruthenium based DSA in chloralkali cells. This, in principle, permits a certain lowering of the exchange current, ie lowering of the electrocatalytic activity of commercial DSA’s, eg by additional solution acidification or doping the coatings with respective additions so as to lower the oxygen content in chlorine and DSA corrosion rate[45].

I

1

300

400

I 0

100

200

I 500

i (mA-cm-‘) Fig. 11. Ohmic potential drop UScurrent density for 30% RuO, electrodes with coating thickness of (1) 5; (2) 0.1; (3) 0.03 pm in 4.3 M NaCl with pH = 1.6 (87°C).

ssmtersection of curves 2 and 1 is apparently caused by the slower shift of the open-circuit potential of the thick electrode due to slow chlorine removal from the coating pores.

937

It seems interesting to consider application of the discussed criteria of the electrodes high catalytic activity in gas-evolving electrochemical reactions for assessment of the Pt-anode activity to chlorine evolution. This process proceeds on the Pt-anode with relatively low overvoltage; under prolonged polarization, however, a gradual decrease of its electrocatalytic activity is observed, ie passivation of the process of chlorine evolution, indicating a changing state of platinum surfaceC19, 46, 471. For instance, anodic polarization of a platinized Ti-anode at i = 250 mAcm-* in 0.5 M HCI at 80°C for 1 h leads to potential increase almost by 400 mV.*** At the same time, cathodic polarization of such passive anode at i = 3 mA cm-* leads to a considerable decrease of the overvoltage at the subsequent anodic polarization of platinum[46]. In a concentrated chloride solution, at high temperature, retardation of chlorine evolution on a smooth Pt-anode caused by an adsorbed oxygen film is accompanied by anode corrosion[47]. Since the electrocatalytic activity of ruthenium based DSA to chlorine evolution is significantly increased under conditions of chloralkali electrolysis (weakly acidic concentrated chloride solution, high temperature), it looks interesting to study the Ptanode behaviour under such conditions. The experiments were performed in 4.3 M NaCl solution with pH = 1.6, at 87°C on a vertically positioned smooth Pt-anode pretreated in a H,O, + The steady-state anodic H,S04 (1: 1) solution[49]. polarization curve (Fig. 12a, curve 1) shows that the overvoltage, AE, at i = 200 mA cm-’ is ca 55 mV. The pdc measured at this i (Fig. 12b, curve 1’) has the initial section of fast open-circuit potential change, EF, and the following section of a very slow change of EP. The following five-time cathodic-anodic treatment of this anode in the same solution at i = 20 mAcm_r (cycle duration 2 min) leads to its considerable activation: an LPS appears on the polarization curve (curve 2). The shape of pdc (curve 2’) also changes radically-it only has an almost horizontal section of slow EF shift. The constant value EP = Ez is reached in 20 s. The bottom of Fig. 12a carries the relation between the overah open-circuit E shift (AE = EP=,, - Ezy), and the current density (curve 3), very much similar to polarization curve 2. The AE values from curve 3 marked on the polarization curve 2 in AE-log i coordinates (squares) coincide with respective E values of this polarization curve. Consequently, as in the case of ruthenium based DSA (compare Fig. 3), the electrochemical equilibrium is retained up to the highest current densities and the Pt-anode potential deviation from Ez under polarization is determined only by increased near-electrode chlorine concentrationttt Under continuous argon purging a clear-cut linear section with the “Nernstian” slope, curve (curh, = 37 mV, appears on the polarization ve 4) which shifts to lower i with decreasing stirring intensity. ***Increased chlorine evolution overvoltage is also observed upon anodic prepolarization of platinum in 0.5 M H,SO,[48]. tttThe limiting near-electrode layer supersaturation with chlorine at i = 500 mA crne2 was assessed from the AE value by the Nemst equation: AE = 5lmV C$C_ = 24.

938

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et al.

Fig. 12. Polarization curves (a; 1, 2.4) and pdc at i = 200 mA cm mz(b, I’, 2’) and relationship AE DSlogi (3) on a Pt-electrode in 4.3 M NaCl with pH = 1.6 (87°C) saturated with chlorine (1.2, 3) and argon (4); 1, l’-without electrochemical pretreatment; 2, 2’, 3, L-after electrochemical pretreatment.

Thus, it is possible to achieve very high electrocatalytic activity with a Pt-anode under chloralkali electrolysis conditions by its cyclic electrochemical pretreatment. At the same time, under these conditions, as well as in diluted chloride solutions and at low temperature, a gradual passivation of an activated Pt-anode to chlorine evolution is also observed. As shown in Fig. 13, upon exposure of this anode at high current density its potential gradually shifts to higher values reaching an almost constant value after approximately one hour. If we assume, that the near-electrode layer supersaturation with chlorine does not change at that, then this potential shift ( AE = 45 mV) should be completely determined by increased charge transfer overvoltage of the overall anodic process, ie AE = (AE)” (cf Fig. 1). Figure 14a shows, alongside the polarization curve fdr an activated Pt-anode (curve l), the polarization curve measured on the anode passivated as described above (curve 2). This polarization curve does not have an LPS. As Fig. 14b shows, passivation of the platinum anode leads to a significant change in the shape of pdc measured at i = 500 mA cm 2 (curve 2’). This pdc has the initial section of fast potential decay indicating a considerable charge transfer overvoltage, and a subsequent section of slow open-circuit potential change. Figure 15 shows pdc’s of these anodes measured at i = 500 mA cm - 2 in a wide time interval. In the case of the activated anode (curve 1) the opencircuit potential shifts by 7 mV in the first 100 ps, and only by 2 mV in the next ea 300 ps, ie on the &-log t curve transition from kinetic section of pdc to the section of slow EP change occurs at T - 100 ps, and hence AEe’ = 7 mV. In the Lase of the passivated anode (curve 2) the open-circuit potential decays in the first 100~s by 60 mV, and the ET-log t plot of this initial section of pdc is linear. According to Equation (11) describing the charge transfer overvoltage decay due to double layer discharge, the slope of this section (b = 50 mV) is in

agreement with the Tafel section slope of polarization at curve (Fig. 14a, curve 2). As Fig. 15 i2,lOOmACIK~ shows. a distinct break on curve 2 coincides with the

3

E J

20

a

0 0

20

40

60

60

t (min)

Fig. 13. Pt-electrode potential at i = 500 mA cn-’ UStime in 4.3 M NaCl with pH = 1.6 (87°C) saturated with chlorine.

Fig. 14. Polarization curves (a) and pdc at i = 500 mA cn-* (b) on the activated (1,l’) and passivated (2,2’) Pt-electrode in 4.3 M NaCl with pH = 1.6 (87°C) saturated with chlorine.

939

Kinetics of gas-evolving electrodes

x

b

1.46

.‘3o ~ 5

4

3 -1g

-1gt

(5)

Fig. 15. The pdc at i = 500 mAcm- * on the activated (1) and passivated (2,2’) Pt-electlode in 4.3 M NaCl with pH = 1.6 (87°C) saturated with chlorine.

nearly horizontal section of curve 1; the constant value of the open-circuit potential is reached for both electrodes in ca 10s and coincides with the initial equilibrium potential in chlorine-saturated solution. Consequently, near-electrode supersaturation with chlorine is practically equal for both the activated and passivated anodes. Thus, pdc’s of these anodes correspond to the cases (b) and(c), respectiviely, discussed in the theoretical part. An approximated assessment of the exchange current density of chlorine evolution process on an activated Ptanode in chlorine-supersaturated near-electrode layer from the respective pdc in Fig. 15 (curve 1) gives by Equation (9) (AEf’ = 7 mV at i = 500 mA cm - *) the value of ii= 1.1 Acme*; the i: assessment for the passivated Pt-anode under the same condition from Fig. 14a by extrapolation of the above mentioned Tafel section of curve 2 to El, yields the value cu 50 mA cm- *. Consequently, the exchange current density in the chlorinesupersaturated solution layer decreases under passivation ca 20-fold, apparently caused by surface oxides formation on platinum[4~. As in the case of ruthenium based DSA, the electrocatalytic activity of Pt anode to chlorine evolution decreases in concentrated chloride solution with falling temperature. Figure 16a shows, that at room temperature the cyclic cathodic-anodic pretreatment of platinum anode leads to its slight activation (cf: curves 1 and 2). Under solution purging with argon, at low and medium current densities a well defined Tafel section with 29 mV slope appears on the polarization curve for the activated anode (curve 3).*** Curves 2 and 3, however, do not have LPS-at i> 100 mAcm_* their slopes increase, and at i = rrtAnodic polarization curves with low T&J slope, dependent of the stirring intensity, were observed at room temperature on a rotating porous Pt disc anode in 1 M HCl, and impedance measurements showed that the process’ rate is determined by the subsequent diffusion step, while with smooth platinum the charge transfer is the rate-determining step[50].

2

0

i la-cm-*)

Fig. 16. Polarization curves (a) and pdc at i = 200 mA cm-* (b) on a Pt-electrode in 4.3 M NaCl with pH = 1.6 (23”C), saturated with chlorine (1, l’, 2, 2’) and argon (3); 1, l’without electrochemical pretreatment; 2, 2’, 3-after electrochemical pretreatment.

-1gt

(5)

Fig. 17. The pdc at i = 2OOmAcm-’ on an activated Ptelectrode in 4.3 M NaCl with pH = 1.6 (23°C) saturated with chlorine.

800 mA cm-’ the potential of anode shifts to positive values in time, ie further retardation of chlorine evolution is observed. A proof of increased Pt-anode activity upon cathodic-anodic pretreatment even at room temperature provides the shape of pdc (Fig. 16b, curves 1’ and 2’). The pdc 1’ measured on Pt at i = 200 mA cm-* upon chemical treatment has a section of fast potential decay of ca 70 mV per 0.5 ms, and upon cycling it is only 18 mV (curve 2’). This is in agreement with the polarization measurements data (curves 1 and 2): the potential shift at i = 200 mA cm- * is also ca 50 mV. As can be seen even on the activated anode (curve 2’) the pdc section of fast E; decay is preserved (18 mV) indicating retained significant retardation of chlorine evolution. This effect is most distinct manifested on the EF-log t plot of pdc (Fig. 17) measured in a wide time range: in a period of 5 to 100 ps a linear section related apparently to the charge transfer retardation according to Equation (11) is observed on pdc, and at t > 100~s slow diffusion removal of chlorine from the near-electrode layer occurs. From the shift of the open-circuit potential from z = 100 ps to the equilib-

v. v.

LOSEV

rium potential in the chlorine-saturated solution after 60 s (AE = EF - Eg) the chlorine supersaturation in the near-electrode solution layer was calculated: AE = 25 mV, C&Z,,, = 7. As shown above, in the case of ruthenium based DSA a considerable retardation of chlorine evolution with increasing solution acidity is observed. Therefore, it seemed interesting to investigate the effect of acidity on the electrocatalytic activity of Pt-anode to this reaction. After the above mentioned common chemical pretreatment the platinum anode was placed in 3.7 M NaCl + 0.6 M HCl solution at 87°C (platinum was not cathodically-anodically activated). Figure 18 shows that polarization curves measured on the Ptanode in weakly acidic solution upon electrochemical activation (curve l), and in acidic solution (curve 2) have similar shapes: on both curves at high current densities well defined extended LPs’s are observed!00 Polarization curve measured in strongly acidic solution purged with argon (curve 3) has a linear section with “Nernstian” slope (b = 36 mV) at Iow and medium current densities, and at i > 10 mAcm_’ the anode potential deviates from Tafel relationship to negative potentials, and, same as in chlorine-saturated solution, a LPS appears on the polarization curve. As mentioned above, to obtain a highly active Pt-anode, in the case of strongly acidic solution, there is no need in its anodic
et al.

I.34

z-5

EG’

>_ 1.26 w

L-

I-

4

I

I

3

I

2

I

I

0

-1g i (o.cm+) Fig. 18. Polarization curves on a Pt-electrode at 87°C in 4.3 M NaCl with pH = 1.6 (1) and 3.7 M NaCl + 0.6 M HCI (2, 3) saturated with chlorine (1, 2) and argon (3).

quently, the mechanism of chlorine evolution on the highly active metallic electrode (platinum) differs considerably from the one on the metaloxide anodes[2, 3, 5,51,52]. Nevertheless, the above experimental results show that both on the metaloxide anodes and platinum, under chloralkali electrolysis conditions, chlorine evolution proceeds at a very high rate (the exchange current density is above 1 Acm-‘). At high current densities for ruthenium based DSA and smooth platinum the similar shape of anodic polarization curves manifested through the existence of LPS under intensive gas evolution (Figs 4 and 12a, respectiyely) convincingly proves that high exchange current densities observed in the case of these DSA are not related with their high porosity[5]: since the gaseous reaction product removal from the pores of active coating is impeded under these conditions, the ruthenium based DSA, as well as the smooth platinum anode, operates under conditions of mass transfer limitation in the bulk solution[52]. It is interesting to note, that in such technologically important electrochemical gas-evolving reactions as hydrogen and oxygen evolution, the so high catalytic activity of metallic and metaloxide electrodes is apparently never reached. This special feature of chlorine evolution is apparently determined by the fact that only one chlorine-containing particle-as showed the analysis of this reaction mechanismClG3, 5, 53]-is involved in the charge transfer steps of chlorine evolution.

CONCLUSIONS “‘The shift of equilibrium potential, as well as of the whole polarization curve, towards negative potentials in strongly acidic solution depends apparently on the changed potential difference at the liquid interface between the solution in the cell and in the reference electrode (in all experiments we used a silver chloride reference electrode in a saturated KC1 solution). 111111 From the overvoltage value, AE, at LPS in Fig. 18, by Nernst equation the chlorine supersaturation in the nearelectrode layer was computed that turned out equal in both solutions and was ca 24. ‘“restabilization of the platinum’s active state is apparently ensured by the instability of platinum surface oxide in a hot strongly acidic chloride solutionC471.

As follows from the above, in the case of very fast reactions on the electrodes with gas evolution, all the three considered criteria of high electrocatalytic activity of such electrodes should be simultaneously kept. (1) If the steady-state polarization curve has a linear section with the “Nernstian” slope (b = 2.3 RT/nF) till the start of gas bubbles formation corresponding to the rate-determining step of reaction product diffusion into the bulk solution (I in Fig. l), further experiments are required to prove the effect of stirring intensity on the position of this section. In the case of smooth (es metallic) electrodes with not very

941

Kinetics of gas-evolving electrodes

on a Pt-electrode at 87°C in 3.7 M NaCl + 0.6 M HCl (1) and Fig. 19. The pdc at i=2OOmAcm-* 4.3 M NaCl with pH = 1.6 (2) saturated with chlorine.

high exchange current density these sections, in principle, can be used to define position of the true kinetic polarization curve by the rotating disc electrode. (2) The existence, on the polarization curve, of an almost horizontal low polarizability section (II in Fig. 1) observed under intensive gas evolution proves that at this section the potential shift from the equilibrium in the bulk solution saturated with reaction product is determined only by supersaturation of the near-electrode layer with this product. At that, the equilibrium of the charge transfer step of the overall process is retained, and the process rate is determined by the subsequent step of reaction product removal. The assessment of the exchange current density in this case is possible if the polarization curve has a subsequent transition sectioti (between the regions II and III in Fig. 1). At high current densities, the experimental detection of these sections on the steady state polarization curve is complicated due to errors in determination of the ohmic potential drop under intensive gas evolution. (3) The characteristic feature of the method for assessment of the electrocatalytic activity of the electrode based on the shape of the potential decay curve upon current interruption consists in that it enables one to separate the kinetic component, (A-E)“, from the overall electrode polarization. Since the decay of ohmic potential drop, A&,,,,, proceeds much faster than the decay of charge transfer overvoltage due to discharge of the double layer capacity, a possible error under intensive gas evolin the LIE,,, determination ution should not affect the character of the relationship between the open-circuit potential and time after current interruption. Consequently, the analysis of this relationship enables one to reliably separate (AE)e’ from the subsequent gradual change of the open-circui’t potential determined by very slow removal of reaction product from the near-electrode layer supersaturated with it. The (AE)e’ values measured at different current densities can, in principle, be used for plotting of the true polarization curve (III in Fig. 1) of a very fast electrode process under intensive gas evolution and for determining its kinetic parameters.

REFERENCES 1. L. I. Krishtalik, Electrochim. Acta 26, 329 (1981). 2. L. I. Krishtalik, D. V. Kokoulina and R. G. Erenburg, Itooi Nauki tekh. Elektrokhimiva 20. 44 (1982). 3. L. i. Krishtalik, Charge Transfer Rkactibns in Electrochemical and Chemical Processes, Plenum Press, New York (1986). 4. S. Trasatti and G. Lodi, in Electrodes of Conductive Oxides, Part B, p. 521, Elsevier, .Amsterdam (1981). 5. S. Trasatti, Electrochim. Acta 32, 369 (1987). 6. L. J. J. Janssen, in Modern Chlor-Alkali Technology, Vol. 2, p. 271 (1983). 7. J. A. Harrison, in Modern Chlor-Alkali Technology, Vol. 2, p. 246 (1983). 8. K. J. Vetter, Elektrochemische Kinetik, S. 264, Springer, Berlin (19611. 9. V. V. Lose;, A. I. Molodov and V. V. Gorodetskii, Electrochim. Acta 12, 475.(1967). IO. 0. Esin, M. Loshkarev and K. Sophiisky, Acrn Physicochim. U.R.S.S. 7, 433 (1937). 11. J. N. Agar and F. P. Bowden, Proc. R. Sot. 169, 206 (1939). 12. L. J. J. Janssen, G. J. Visser and E. Barendrecht, Electrochim. Acta 28, 155 (1983). 13. H. Vogt, Chem.-lng.-Techn. 52, 418 (1980). 14. H. Vogt, Fortschr. Verfuhrenstechn. 16, 297 (1978); 20, 369 (1982). 15. V. V. Losev, Elektrokhimiya 17, 733 (1981). 16. R. Clamroth and C. A. Knorr, 2. Elektrochem. 57, 399 (1953). 17. Ya. M. Kolotyrkin and A. N. Chemodanov, Dokl. A.N. S.S.S.R. 134, 128 (1960). 18. S. Shibata, Bull. them. Sot. Jpn, 36, 53 (1963); Electrochim. Actn 23, 619 (1978). 19. L. Miiller, M. Krenr and R. Landsberg, J. electroanaZ. Chem. 180,453 (1984). 20. L. Miiller, M. Krenz and M. Zettler, 2. phys. Chem., Leipzig 265, 729 (1984). 21. L. J. J. Janssen and J. G. Hoogland, Electrochim. Acta 15, 1013 (1970). 22. H. V&t, in Comprehensive Treatise of Electrochemistry, Vol. 6, 445 (1983). 23. H. Vogt, Electrochim. Acta 29, 167; 175 (1984). 24. A. Frumkin, Acta Physicochim. U.R.S.S., 18, 23 (1943); Disc. Faraday Sot. 1, 57 (1947). 25. Ya. M. Kolotyrkin, Zh. Fiz. Khim. 20, 667 (1946). 26. V. E. Past and S. A. Iofa. Zh. Fiz. Khim. 33, 913 (1959).

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