- Email: [email protected]
The electrochemistry of porous zinc
Surface Technology, 19 (1983) 335 - 346
335
T H E E L E C T R O C H E M I S T R Y OF POROUS ZINC
A. J. S. McNEIL and N. A. HAMPSON Chemistry Department, University of Technology, Loughborough, Leics. LE11 3TU (Gt. Britain)
(Received February 14, 1983)
Summary Commercially amalgamated porous zinc electrodes display a behaviour that resembles t hat of planar zinc electrodes rather than porous zinc electrodes. Th e dissolution behaviour of mercury-free solid and porous zinc electrodes was examined using specially constructed rotating disc electrodes in which the bulk solution processes were separated from those within the pores. Experiments were p e r f o r m e d under full automatic control of a bencht o p m i c r o c o m p u t e r . The results show that in the absence of m ercury the porous zinc behaves as an ideal semi-infinite porous electrode.
1. I n t r o d u c t i o n Experimental electrochemistry has always been amenable to instrumental methods, b u t this instrumentation, however sophisticated, has until recently always remained under manual control. The recent advances in computer t e c h n o l o g y have provided first minicomputers [1, 2] and now microc o mp u ter s th at are capable of total control of an electrochemical system. Although m i c r o c o m p u t e r s are being increasingly favoured as bench-top controllers and data loggers [ 3 - 8], some workers [ 9 - 1 3 ] prefer to base their system on a m i n i c o m p u t e r such as a PDP 11. A n u m b e r of significant advantages of digital systems'are widely recognized. (1) Experiments comprising repetitive sequences are well suited to machine control. (2) Digital c ont r ol and monitoring offer a precision and stability that m ay n o t be possible with manual methods. (3) All data are contained within the digital system and are thus fully accessible for manipulation and analysis. (4) The processed data can be very acceptably and accurately pl ot t ed as a hard copy. As digital techniques are being increasingly widely used, basic guidelines for their best application are being evolved [4, 6, 9, 11, 12, 14]. However, experimental electrochemistry is still in the early stages of this new 0376-4583/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
336
phase of development. Designs of c o m p u t e r hardware are constantly changing and incorporate incompatible operating systems. There is a general need for fully c o m m i t t e d scientific machines t hat are properly interfaced to m o n i t o r and control the analogue world. Little software has so far been developed. The c o m p u t e r algorithms (or instructions) for experimental applications are still being written and tested, a process that can be very laborious. However, we can envisage a time when general-purpose software routines will be established as standard laboratory expertise. There are two aspects t o be considered in the present development of digital systems. The first is to use digital m e t h o d s to examine well-characterized systems and to establish results c o n c o r d a n t with manual experiments. The second is to establish new areas of experi m ent at i on where manual m e th o d s o f monitoring and control are inadequate. With these objectives in view, a study of the anodic dissolution in alkaline solution of zinc, in bot h the solid and the porous conditions, is presented in this paper. Although the rotational speed de pe nde nc e of the dissolution has been well studied [15 - 18] for solutions up to 3 M strength, such experiments have not been p e r f o r m e d on solid zinc in stronger electrolytes and not at all on porous zinc. As we shall show in this paper, the experiments on porous zinc are not easily p e r f o r m e d manually and are m or e suited to the precise monitoring possible with machine control.
2. Experimental p r o c e d u r e
2.1. Materials and electrode preparation Anodic dissolution experiments on zinc were carried out in a threec o m p a r t m e n t cell. This comprised a central working electrode c o m p a r t m e n t c o n n e c t e d via a glass frit to a large area c o u n t e r e l e c t r o d e of platinum gauze and also c o n n e c t e d via a Luggin capillary and ground glass seal to an HgLHgO reference electrode. The electrolyte in all experiments was AnalaR grade KOH made up to 7 M in triply distilled water; a fresh batch was used for each ex p er imen t and was d e o x y g e n a t e d by bubbling oxygen-free nitrogen through it. Solid zinc electrodes were prepared from zinc rod (Koch-Light; purity, 99.999%) machined to a diameter of 3 mm (cross-sectional area, 0.0707 cm 2) and pressure fitted into a Teflon sheath. The solid polycrystalline electrodes were prepared by abrasion on 1200 grit carbide papers in running water and th en by briefly etching in 50 vol.% HC1, which removed the superficial d e f o r m e d metal layer [19] and e m b e d d e d abrasive and revealed the underlying grain structure. The etched electrode was t h o r o u g h l y rinsed in triply distilled water before being put wet into the alkaline electrolyte. Mercury-free porous zinc electrodes were prepared using AnalaR grade chemicals t h r o u g h o u t ; zinc dust (90% assay) and ZnO were used in a Zn:ZnO ratio of 25:75. The paste was made up with 0.01 M KOH in the p r o p o r t i o n I ml KOH to 2 g of dry p o w d e r mixture. This paste was pressed
335' into a recess 0.4 mm in depth and 3 mm in diameter, prepared from a solid zinc electrode abraded and t hen recessed with a shim into its Teflon shroud. T h e paste was reduced in 0.01 M KOH at 2 mA, equivalent to a current density of 30 m A c m 2. The variation in cell potential at this constant current was as shown in Fig. 1; th r ee distinct regions are displayed. In region A the resistance of the paste is falling rapidly as the ZnO particles are reduced, creating an increasing n u m b e r of conducting links. In region B the ohmic resistance of the paste has b e c om e insignificant, and the ZnO is being reduced steadily to zinc. In region C all the ZnO has been reduced and hydrogen evolution begins, associated with a higher and fluctuating potential due to the formation and periodic escape o f hydrogen within the electrode pores. Such a process has been considered by Korovin and coworkers [20, 21].
1 he~
~
~ B
-
-
~
~
"s 1,.~ los
C
(a)
(b)
Fig. 1. The variation in cell potential V with time for the reduction of Zn--ZnO paste to produce the porous zinc electrode: (a) the full reduction process; (b) the cell voltage fluctuations in hydrogen evolution. The charge passed for the full reduction of ZnO to zinc was 9 C corresponding to a wet paste mass o f 7.6 mg, in good agreement with measured masses of 7.3 mg, and to a total capacity for anodic dissolution of 12.7 C. It was f o u n d necessary t o contain the electrode paste within its recess by a cellophane separator to prevent it from disintegrating in the early stages o f reduction. The electrode faced upwards to allow the electrochemically generated h y d r o g en to escape. In the absence of the separator the hydrogen evolution reaction p r o d u c e d a stream of minute bubbles which could just be resolved with the naked eye and whose dimensions relate t o the pore size of the electrode. It was observed that there was a volume change on reduction. The paste th at was t r i m m e d flush with the Teflon surface became slightly convex and p r o ud of the Teflon sheath after reduction. 2. 2. I n s t r u m e n t a t i o n
The experimental arrangement comprised a K e m i t r o n PS40 potentiostat (Kemitron Electronics Ltd., Chester), Kemitron CM2 c o u l o m e t e r and Chemical Electronics RD1 m o t o r - g e n e r a t o r disc drive, all controlled by a K e m i t r o n K3 0 0 0E m i c r o c o m p u t e r , and a Lear Siegler ADM 3A+ visual
338 display unit (VDU). The K 3000E m i c r o c o m p u t e r is based on the 8-bit Z80 processor and has 64k bytes of dynamic random access m e m o r y , augmented by two 8 in f lo ppy disc drives, affor4ing a total of 512k bytes of disc storage space. The machine uses the Digital Research CP/M 2.2 operating system supporting Microsoft BASIC version 5.1. The digital c o m p u t e r was interfaced to the analogue instrumentation via 8-bit and 12-bit digital-to-analogue (D/A) and analogue-to-digital (A/D) converters. Th e 8-bit interface is the faster o f the two types with a signal conversion time of ab o ut 2.5 ms but with a digital resolution of only 1 in 2 s. An 8bit D/A interface was used for c o m p u t e r cont rol of the electrode rotational speed, b e t w e e n 0 and 48 Hz, with a resolution of less than 0.2 Hz. The 12bit interface is about ten times slower than the 8-bit interface, but the resolution is greatly increased to 1 in 212. The 12-bit A/D interface used in this work incorporated a zero-crossing det ect i on circuit and could read voltages between 0 and +-2047 mV with a resolution of 0.5 mV and, in BASIC, the rate was 50 readings s 1. The cell current measurements were made with a Fylde 255DA differential d.c. amplifier. The p o t e n t i o s t a t was controlled by a 12-bit D/A interface, with 0.5 mV resolution, and was stable to less than 0.1 mV. The converters used in this work provided only a voltage signal with no current.
2.3. Experimental algorithm The basic algorithm according to which the experi m ent was c o n d u c t e d comprised a p r e d e t e r m i n e d set of electrode potentials V and a set of rotational speeds R. The c o m p u t e r worked t hrough this two-dimensional array (V, R) and d e t e r m i n e d the steady state current I(V, R) for all values o f V and R. Thus at any potential the c o m p u t e r would set a rotational speed and then log the current response of the electrode. This was done by taking a n u m b e r N1 of current readings at the m a x i m u m speed of the A/D interface, which were averaged to give a value /mean' The c o m p u t e r then t o o k its last N 2 values o f Imean and did a least-squares calculation of di/dt, the slope of the c u r r e n t - t i m e curve. Values of N1 and N2 were selected before the experiment. The operating speed was such that the machine could, each second, give a value of Imean based on 15 readings of I as well as a value of the slope di/dt based on the last ten values of/mean" Thus the c o m p u t e r continuously m o n i t o r e d di/dt and when it calculated a value di/dt < 0.001 (in milliamperes per reading interval of /mean) it accepted this as representative of a steady state and recorded the arithmetic mean of the last ten current readings as I( V, R), the current for potential V and rotational speed R. When the experimental programme was com pl et ed the recorded data were immediately written to a f l oppy disc file rather than left exposed in an insecure m e m o r y , and the control of the electrode potential was then assigned to a subroutine which sought and maintained a rest potential by constantly minimizing the cell current. The m i c r o c o m p u t e r was also used in the analysis of data and greatly facilitated what has classically been a tedious process. In addition to the
339
numeric analysis a simple graphical representation of the data was put on the VDU using the LINE and TAB facilities of Microsoft BASIC. In this way, i -~ v e r s u s w -1/2 plots were made for each of the working potentials, from which a slope and an intercept were obtained by linear regression. The slope gave the speed dependence S of the anodic dissolution reaction and the intercept gave the notional dissolution current i~ at infinite rotational speed. These calculated values were then utilized in preparing slope dependence plots (log S against potential E) and Tafel plots (log i~ against E). With the help of the visual screen representation of these plots, points could be selected for the calculation of slopes and intercepts for both slope dependence and Tafel plots. The results of all these calculations were written to disc to complete the data file for each experiment. The data thus obtained were finally plotted as a hard copy using a Kemitron K2000 microcomputer (specifications as for the K3000E microcomputer but with both 5¼ in and 8 in floppy disc drives) driving a Watanabe WX 4636R digital x - y plotter. The experimental plots presented here were prepared using this system. A fuller description of the digital instrumentation and algorithms in experimental electrochemistry is to be presented elsewhere [22].
3. Results and discussion 3.1. S o l i d z i n c
A portion of a typical c u r r e n t - t i m e ( i - t ) log of the behaviour of a solid zinc electrode is shown in Fig. 2. At A the microcomputer has set a new electrode potential, doing this in five steps of 2 mV to avoid gross perturbation of the system. In B the electrode is held stationary (co = 0) as the current at the new potential reaches a steady value. Once this is achieved the machine proceeds through the regime (from C to G) of five standard speeds ( w -1/2 values of 0.22, 0.18, 0.14, 0.10 and 0.06 sl/2), only moving on to the next speed when d i / d t < 0.001 mA s-1. With the solid zinc electrode this is quickly achieved. The length of the current plateau at each rotational speed is only slightly longer than the time needed to make ten readings of Imean for the first calculation of d i / d t . For these experiments on solid zinc, 15 readings of I were made to calculate/mean, and ten values of/mean were used to derive d i / d t . The i -1 v e r s u s co- ~/2 plots for such an experiment are shown in Fig. 3. Linearity was good, with correlation coefficients of 0.999 being found at almost every potential. From the calculated slopes S and intercepts 1/Lo the slope dependence plot (Fig. 4) and Tafel plot (Fig. 5) for solid zinc were constructed. The proposed mechanism [15, 16, 23] for anodic zinc dissolution in alkaline solution is for two one-electron charge transfer steps with the second step, giving the divalent solution species, being the slower:
340 6
E
.
-
-
,
q
~
5
r --
I 0
--
~o~
r-
j
,
V_
//
.
4..
/
/J
r~
f_J
i
2, i I
i
i
.
b
B
~
C
[
0
i
E
[
F
0.
i
,OS
G
.I
.IS
.25
.2
w~l ~2 / s1/2
Fig. 2. A p o r t i o n of t h e e x p e r i m e n t a l c u r r e n t t i m e log illustrating t h e b e h a v i o u r of solid zinc at all e l e c t r o d e p o t e n t i a l s w i t h t h e d i s s o l u t i o n c u r r e n t ranging f r o m 0.1 to 10 m A . Fig. 3. Plots o f i i against co -1/2 for solid zinc at various p o t e n t i a l s (giving i n t e r c e p t i~ ~ a n d slope S): I - - 1 3 9 0 m V ; a , - - 1 3 8 5 m V ; i , - - 1 3 7 5 m V ; O, - - 1 3 6 5 m V ; 1 [ , - - 1 3 4 0 mV. 1.5.
1.2~
7 ~
.9,
.6.
\
/
-.5
--7/.
- 1
i
-14.00
I
-1380
--1360
I
I
i
--1320 E
/
-l,oo
/.
,
I
--134"0
/
/
- -i~8o
t
-13BO
/mY
-1320
--1340 E
/mY
Fig. 4. Plot of log S against E for solid zinc ( p o t e n t i a l d e p e n d e n c e , - - 3 0 m V decade 1). Fig. 5. Plot of log i~ against E for solid zinc (Tafel slope, 40 m V d e c a d e 1).
Zn Zn I
(i)
' ~" Zrl I + e rate-determining step
~ Zn H + e
(2)
The equilibrium of the zinc(II) solution species with the electrode under all experimental conditions is demonstrated by the --30 mV decade -1 depen-
341
dence of the plot of log S v e r s u s potential (Fig. 4), in full agreement with the results of previous workers [15 - 18]. However, the predicted Tafel slope of 40 mV decade -1, consistent with the above mechanism, is found only at moderate anodic overpotentials (Fig. 5 and Table 1). The observed Tafel plots are curved such that the Tafel slope (measured in millivolts per decade) increases with increasing anodic overpotential. These observations are wholly consistent with previous work [ 1 5 - 1 7 , 23]. The large slopes observed at high overpotentials can be explained in terms of the formation of a film of ZnO on the dissolving electrode surface with an increasing ohmic contribution to polarization [24, 25]. Armstrong and Bell [16] concluded from impedance measurements that at high overpotentials the presence of a surface-absorbed zinc(I) species made a significant contribution to the overall dissolution mechanism. The marked fall in Tafel slope at low anodic overpotentials can be accounted for in terms of the hydrogen evolution reaction which is significant for polycrystalline zinc in alkaline solutions even in the active region, where there is an overall anodic dissolution current. Under these conditions any increase in overpotential would produce both an increase in the anodic zinc dissolution current and a decrease in the hydrogen evolution current. The result is a greater dependence of the measured current on the potential than for pure zinc dissolution. Armstrong and Bulman [15] and Marshall [17] obtained small Tafel slopes at low overpotential. Marshall also found straighter Tafel plots for alkaline solutions containing zincate, where the experimental potential region was removed from that for significant hydrogen evolution. However, if the measured dissolution current at low overpotentials is the resultant of two competing processes, then it is interesting that the observed i 1 v e r s u s ~ 1/2 plots (Fig. 3) for these potentials are not curved. This requires that the hydrogen evolution reaction is independent of the rotational speed, which would occur if hydrogen were evolved only from water in the electrolyte.
TABLE 1 Tafel slopes for t h e solid zinc a n d p o r o u s zinc e l e c t r o d e s
Potential
(my)
--1390 --1380 --1370 --1360 --1350 --1340
Solid zinc
Porous zinc
ip/i s
Current is (mA)
Tafel slope (mV decade -l)
Current ip (mA)
Tafel slope ( m V d e c a d e -1)
0.63 1.41 2.69 4.36 6.31 9.12
23.5 32.5 41.1 51.6 62.5 78.4
2.95 4.36 5.89 7.41 9.12 12.6
53.3 64.4 80.1 99.3 122.0 148.5
4.68 3.09 2.19 1.69 1.44 1.38
342 3.2. Porous
zinc
A p o r t i o n o f t h e c u r r e n t log for a p o r o u s zinc e l e c t r o d e is s h o w n in Fig. 6. Its f o r m is a n a l o g o u s to t h a t f o r t h e solid e l e c t r o d e e x c e p t f o r t h e o b v i o u s d i f f e r e n c e t h a t t h e a c h i e v e m e n t o f s t e a d y d i s s o l u t i o n c u r r e n t s is a slow process. Figure 6 s h o w s t h e b e h a v i o u r at a fairly low a n o d i c o v e r p o t e n tial, w h e r e t h e r e are significant c u r r e n t f l u c t u a t i o n s t o be averaged o u t b u t w h e r e s t e a d y state c u r r e n t s w e r e r e a c h e d c o m p a r a t i v e l y q u i c k l y . At high o v e r p o t e n t i a l s this p r o c e s s c o u l d t a k e m u c h longer, even up to 1 0 0 s. Whereas t h e i I v e r s u s co-1/2 plots for solid zinc were linear o v e r t h e w h o l e r o t a t i o n a l s p e e d range, this was n o t t h e case f o r p o r o u s zinc (Fig. 7). It can be a s s u m e d t h a t t h e r o u g h f r o n t surface o f t h e p o r o u s e l e c t r o d e causes t u r b u l e n c e at t h e higher r o t a t i o n a l speeds w i t h b r e a k d o w n o f t h e d i f f u s i o n l a y e r a n d a c o n s e q u e n t a n o m a l o u s increase in c u r r e n t .
J
"\\
s~ B
1
C
// I
a
1
E
1
F
k
G
I
.05
.I
.15
m ~12,
sH2
.2
.25
Fig. 6. A portion of the experimental current time log illustrating the behaviour of porous zinc. Dissolution currents were higher than for solid zinc and could exceed 20 mA. Fig. 7. Plots of i ~ against co 1/2 for porous zinc at various low anodic overpotentials: i , --1406 mV; ~, --1400 mV; A , - 1 3 9 4 mV; e), --1385 mV;][,--1376 mV. S l o p e d e p e n d e n c e a n d Tafel p l o t s w e r e c o n s t r u c t e d f r o m i -1 v e r s u s c~-1/2 p l o t s using c u r r e n t m e a s u r e m e n t s at t h e first f o u r r o t a t i o n a l speeds. A l t h o u g h n o p r o b l e m s w e r e e n c o u n t e r e d w i t h t h e p a s s i v a t i o n o f p o r o u s zinc e l e c t r o d e s , even u p to p o t e n t i a l s as a n o d i c as - - 1 2 8 0 m V , it was n o t possible to w o r k o v e r this entire r a n g e w i t h a single e l e c t r o d e . Each e x p e r i m e n t was l i m i t e d b y t w o factors. T h e first was t h e finite c a p a c i t y o f t h e e l e c t r o d e , o f a b o u t 13 C, o f w h i c h a b o u t 4 C c o u l d r e a s o n a b l y be used b e f o r e t h e elect r o d e b e c a m e u n r e p r e s e n t a t i v e b e c a u s e o f excessive loss o f m a t e r i a l . T h e s e c o n d was t h e n e c e s s i t y o f w o r k i n g t h r o u g h t h e e x p e r i m e n t a l a l g o r i t h m no faster t h a n t h e p o r o u s e l e c t r o d e c o u l d e q u i l i b r a t e at each n e w d i s s o l u t i o n c o n d i t i o n . Figures 8 a n d 9 s h o w slope d e p e n d e n c e and Tafel p l o t s f o r
343 1,4
1.2
i
#
-1410
-1400
B o
o o
.4
41420
i
)
-1410
i
i
-1400
=
i
-1390
i
i
J
-1300
O
41 420
-1570
-1390
-1380
E /mY
-1370 E
/mY
Fig. 8. Plot of log S against E for porous zinc (potential dependence, - - 3 0 mV decade-l). (From Fig. 7.) Fig. 9. Plot of log i~ against E for porous zinc (Tafel slope, 40 mV decade-I). (From Fig. 7.) 2
1.5.
r;
I .5. .5
-.5.
-14,0
-1~10
-1~00
-1350
-1320
E /
-1290
mY
-_Ii 440
/ ) -1410
I -1380
i -1350
-1 20
-1290
E/mY
Fig. 10. Plot of log S against E for porous zinc over a wide potential range built up from
a number of individual experiments such as that represented in Fig. 8 (potential depend e n c e , - - 3 0 mV decade-]). Fig. 11. Plot of log io~ against E for porous zinc over a wide potential range built up as for Fig. 10 (Tafel slope, 80 mV decade-I).
p o r o u s zinc at a range of l o w anodic overpotentials. Data from a n u m b e r of individual e x p e r i m e n t s such as these were used to build up c o m p o s i t e plots (Figs. 10 and 1 1 ) , with few inconsistencies. While the slope d e p e n d e n c e is fairly stable at - - 3 0 m V decade -1 over a wide p o t e n t i a l range, the Tafel slope ( 4 0 m V decade -1 in Fig. 9) rapidly increases with anodic overpotential (Fig. 11).
344 T h e slope d e p e n d e n c e p l o t s f o r p o r o u s zinc (Fig. 10) still s h o w t h e - - 3 0 m V d e c a d e -1 p o t e n t i a l d e p e n d e n c e , indicating t h a t the e l e c t r o c h e m i s t r y is t h e s a m e as for solid zinc. Similarly, t h e T a f e l p l o t for p o r o u s zinc (Fig. 11) shows a c u r v a t u r e similar t o t h a t f o r solid zinc for w h i c h t h e s a m e e x p l a n a tions can be a d d u c e d . H o w e v e r , t h e i n s t a n t a n e o u s Tafel slope d E / d ( l o g i~) for p o r o u s zinc is c o n s i s t e n t l y t w i c e t h a t f o r solid zinc o v e r t h e full experim e n t a l p o t e n t i a l range. In this r e s p e c t t h e b e h a v i o u r o f t h e p o r o u s e l e c t r o d e c o r r e s p o n d s to t h e ideal semi-infinite cylindrical p o r e c o n s i d e r e d b y de Levie [ 2 6 ] . This agrees w i t h t h e q u a l i t a t i v e e v i d e n c e o f the c u r r e n t - t i m e experim e n t a l log (Fig. 6) w h i c h s h o w s t h e slow p r o c e s s of e q u i l i b r a t i o n w i t h i n t h e e l e c t r o d e interior a f t e r t h e setting o f each new p o t e n t i a l or r o t a t i o n a l speed. H o w e v e r , it is i m p l i c i t in de Levie's t r e a t m e n t and in its p r e d i c t i o n of a d o u b l i n g of Tafel slope in t h e t r a n s i t i o n f r o m a solid to a p o r o u s e l e c t r o d e t h a t t h e t w o Tafel p l o t s will i n t e r s e c t , i.e. for a n o d i c d i s s o l u t i o n t h e r e will be s o m e p o t e n t i a l w h e r e t h e d i s s o l u t i o n c u r r e n t will be t h e s a m e f o r b o t h a p o r o u s and a solid e l e c t r o d e . This can b e seen in t h e e x p e r i m e n t a l Tafel p l o t s (Figs. 5 a n d 11) b y t a k i n g d u e a c c o u n t o f t h e i r c u r v a t u r e . T h e i n s t a n t a n e o u s Tafel slopes f o r a range o f p o t e n t i a l s , as well as t h e c o r r e s p o n d i n g value o f i~, are given in T a b l e 1. T h e p l o t s o f t h e r e c i p r o c a l Tafel slopes d(log i ~ ) / d E v e r s u s E (Fig. 12) clearly s h o w t h a t b o t h p l o t s a p p r o a c h o n e a n o t h e r a n d a p p r o a c h zero w i t h increasing a n o d i c p o t e n t i a l , i.e. b o t h e l e c t r o d e s display a similar limiting c u r r e n t . In this limiting case t h e i n t e r i o r o f t h e p o r o u s elect r o d e d o e s n o t c o n t r i b u t e significantly to t h e s t e a d y state c u r r e n t w h i c h is d e t e r m i n e d b y t h e p r o j e c t e d e l e c t r o d e area. T h e p l o t o f t h e ratio ip/i s o f s t e a d y state c u r r e n t s at d i f f e r e n t p o t e n t i a l s (Fig. 13), t a k e n f r o m t h e respec-
L~S .04
~l~ .03 e-
.02
.0|
~I~00
-1350
-1300
E /
mV
-1400
-1350
-1300 E I mV
Fig. 12. Plots of the reciprocal instantaneous Tafel slopes d(log ioo)/dE against E: e, solid zinc electrode; m porous zinc electrode. Fig. 13. Plot of the ratio ip/i s of limiting currents against E for porous and solid electrodes. (From Figs. 5 and 11 and Table 1.)
345
rive Tafel plots, shows h o w the benefit of the internal surface area of the porous electrode is gradually lost. The current-time log for these high anodic overpotentials is similar to Fig. 6, clearly indicating a porous nature, but n o w the internal area delays the achievement of the steady state rather than contributes any significant faradaic current. The region of current generation is confined to the front face of the electrode in agreement with the findings of Breiter [27] and Coates e t al. [ 2 8 ] . For solid zinc the successful operation of the computer algorithm is demonstrated, and the mechanism of zinc dissolution in 7 M KOH is shown to match that determined for less concentrated alkaline solutions. For porous zinc the computer easily performed an experiment which would be very difficult to perform by manual means. For both solid and porous zinc electrodes the rates of anodic dissolution are dependent on rotational speed. In a further c o m m u n i c a t i o n we shall show h o w this dependence can be eliminated.
References 1 M. W. Breiter, J. Electrochem. Soc., 112 (1965) 845. 2 R. A. Osteryoung, in J. S. Mattson, H. B. Mark, Jr., and H. C. MacDonald, Jr. (eds.), Computers in Chemistry and Instrumentation, Vol. 2, Dekker, New York, 1972. 3 Personal computers in chemistry. In P. Lykos (ed.), Proc. American Chemical Society Meet., Washington, DC, August 1979, Wiley, New York, 1981. 4 O. R. Brown, Electrochim. Acta, 27 (1982} 33. 5 N. A. Hampson and M. J. Willars, Surf. Technol., 7 (1978) 247. 6 P. J. Mitchell, S. A. G. R. Karunathilaka and N. A. Hampson, Surf. Technol., 14 (1981) 45. 7 M. I. Cohen and P. A. Heimann, J. Res. Natl. Bur. Stand., 83 (1978) 429. 8 T. E. Edmonds, Anal. Chim. Acta, 108 (1979) 155. 9 D. Britz, Anal. Chim. Acta, 143 (1982) 95. 10 J. Mozota, B. Barnett, D. Tessier, H. Angierstein-Kozlowska and B. E. Conway, Anal. Chim. Acta, 133 (1981} 191. 11 J. A. Harrison and C. E. Small, Electrochim. Acta, 25 (1980) 447. 12 J. A. Harrison, J. Power Sources, 8 (1982) 99. 13 J. A. Harrison, Electrochim. Acta, 27 (1982) 1113. 14 E. B. James, Chem. Br., 18 (1982) 620. 15 R. D. Armstrong and G. M. Bulman, J. Electroanal. Chem. Interface Electrochem., 25 (1970) 121. 16 R. D. Armstrong and M. F. Bell, J. Electroanal. Chem. Interface Electrochem., 55 (1974) 201. 17 A. Marshall, Ph.D. Thesis, University of Technology, Loughborough, 1975. 18 A. Marshall, N. A. Hampson, J. S. Drury and J. P. G. Farr, Surf. Technol., 5 (1977) 149. 19 L. E. Samuels, Metallographic Polishing by Mechanical Means, Pitman, London, 1971. 20 N. V. Korovin and A. F. Feoktistov, Elektrokhimiya, 6 (1970) 338. 21 A. F. Feoktistov, G. N. Maksimov and N. V. Korovin, Elektrokhimiya, 10 (1974) 1676. 22 A. J. S. McNeil and P. J. Mitchell, t o be published. 23 J. O'M. Bockris, Z. Nagy and A. Damjanovic, J. Electrochem. Soc., 119 (1972) 285. 24 N. A. Hampson and M. J. Tarbox, J. Electrochem. Soc., 110 (1963) 95.
346 25 R. N. Elsdale~ N. A. Hampson, P. C. Jones and A. N. Strachan, J. Appl. Electrochem., i (1971) 213. 26 R. de Levie, Adv, Electrochem. Electrochem. Eng., 6 (1967) 329. 27 M.W. Breiter, Electrochim. Acta, 15 (1970) 1297. 28 G. Coates, N. A. Hampson, A. Marshall and D. F. Porter, J. Appl. Electrochem., 4 (1974) 75.
335
T H E E L E C T R O C H E M I S T R Y OF POROUS ZINC
A. J. S. McNEIL and N. A. HAMPSON Chemistry Department, University of Technology, Loughborough, Leics. LE11 3TU (Gt. Britain)
(Received February 14, 1983)
Summary Commercially amalgamated porous zinc electrodes display a behaviour that resembles t hat of planar zinc electrodes rather than porous zinc electrodes. Th e dissolution behaviour of mercury-free solid and porous zinc electrodes was examined using specially constructed rotating disc electrodes in which the bulk solution processes were separated from those within the pores. Experiments were p e r f o r m e d under full automatic control of a bencht o p m i c r o c o m p u t e r . The results show that in the absence of m ercury the porous zinc behaves as an ideal semi-infinite porous electrode.
1. I n t r o d u c t i o n Experimental electrochemistry has always been amenable to instrumental methods, b u t this instrumentation, however sophisticated, has until recently always remained under manual control. The recent advances in computer t e c h n o l o g y have provided first minicomputers [1, 2] and now microc o mp u ter s th at are capable of total control of an electrochemical system. Although m i c r o c o m p u t e r s are being increasingly favoured as bench-top controllers and data loggers [ 3 - 8], some workers [ 9 - 1 3 ] prefer to base their system on a m i n i c o m p u t e r such as a PDP 11. A n u m b e r of significant advantages of digital systems'are widely recognized. (1) Experiments comprising repetitive sequences are well suited to machine control. (2) Digital c ont r ol and monitoring offer a precision and stability that m ay n o t be possible with manual methods. (3) All data are contained within the digital system and are thus fully accessible for manipulation and analysis. (4) The processed data can be very acceptably and accurately pl ot t ed as a hard copy. As digital techniques are being increasingly widely used, basic guidelines for their best application are being evolved [4, 6, 9, 11, 12, 14]. However, experimental electrochemistry is still in the early stages of this new 0376-4583/83/$3.00
© Elsevier Sequoia/Printed in The Netherlands
336
phase of development. Designs of c o m p u t e r hardware are constantly changing and incorporate incompatible operating systems. There is a general need for fully c o m m i t t e d scientific machines t hat are properly interfaced to m o n i t o r and control the analogue world. Little software has so far been developed. The c o m p u t e r algorithms (or instructions) for experimental applications are still being written and tested, a process that can be very laborious. However, we can envisage a time when general-purpose software routines will be established as standard laboratory expertise. There are two aspects t o be considered in the present development of digital systems. The first is to use digital m e t h o d s to examine well-characterized systems and to establish results c o n c o r d a n t with manual experiments. The second is to establish new areas of experi m ent at i on where manual m e th o d s o f monitoring and control are inadequate. With these objectives in view, a study of the anodic dissolution in alkaline solution of zinc, in bot h the solid and the porous conditions, is presented in this paper. Although the rotational speed de pe nde nc e of the dissolution has been well studied [15 - 18] for solutions up to 3 M strength, such experiments have not been p e r f o r m e d on solid zinc in stronger electrolytes and not at all on porous zinc. As we shall show in this paper, the experiments on porous zinc are not easily p e r f o r m e d manually and are m or e suited to the precise monitoring possible with machine control.
2. Experimental p r o c e d u r e
2.1. Materials and electrode preparation Anodic dissolution experiments on zinc were carried out in a threec o m p a r t m e n t cell. This comprised a central working electrode c o m p a r t m e n t c o n n e c t e d via a glass frit to a large area c o u n t e r e l e c t r o d e of platinum gauze and also c o n n e c t e d via a Luggin capillary and ground glass seal to an HgLHgO reference electrode. The electrolyte in all experiments was AnalaR grade KOH made up to 7 M in triply distilled water; a fresh batch was used for each ex p er imen t and was d e o x y g e n a t e d by bubbling oxygen-free nitrogen through it. Solid zinc electrodes were prepared from zinc rod (Koch-Light; purity, 99.999%) machined to a diameter of 3 mm (cross-sectional area, 0.0707 cm 2) and pressure fitted into a Teflon sheath. The solid polycrystalline electrodes were prepared by abrasion on 1200 grit carbide papers in running water and th en by briefly etching in 50 vol.% HC1, which removed the superficial d e f o r m e d metal layer [19] and e m b e d d e d abrasive and revealed the underlying grain structure. The etched electrode was t h o r o u g h l y rinsed in triply distilled water before being put wet into the alkaline electrolyte. Mercury-free porous zinc electrodes were prepared using AnalaR grade chemicals t h r o u g h o u t ; zinc dust (90% assay) and ZnO were used in a Zn:ZnO ratio of 25:75. The paste was made up with 0.01 M KOH in the p r o p o r t i o n I ml KOH to 2 g of dry p o w d e r mixture. This paste was pressed
335' into a recess 0.4 mm in depth and 3 mm in diameter, prepared from a solid zinc electrode abraded and t hen recessed with a shim into its Teflon shroud. T h e paste was reduced in 0.01 M KOH at 2 mA, equivalent to a current density of 30 m A c m 2. The variation in cell potential at this constant current was as shown in Fig. 1; th r ee distinct regions are displayed. In region A the resistance of the paste is falling rapidly as the ZnO particles are reduced, creating an increasing n u m b e r of conducting links. In region B the ohmic resistance of the paste has b e c om e insignificant, and the ZnO is being reduced steadily to zinc. In region C all the ZnO has been reduced and hydrogen evolution begins, associated with a higher and fluctuating potential due to the formation and periodic escape o f hydrogen within the electrode pores. Such a process has been considered by Korovin and coworkers [20, 21].
1 he~
~
~ B
-
-
~
~
"s 1,.~ los
C
(a)
(b)
Fig. 1. The variation in cell potential V with time for the reduction of Zn--ZnO paste to produce the porous zinc electrode: (a) the full reduction process; (b) the cell voltage fluctuations in hydrogen evolution. The charge passed for the full reduction of ZnO to zinc was 9 C corresponding to a wet paste mass o f 7.6 mg, in good agreement with measured masses of 7.3 mg, and to a total capacity for anodic dissolution of 12.7 C. It was f o u n d necessary t o contain the electrode paste within its recess by a cellophane separator to prevent it from disintegrating in the early stages o f reduction. The electrode faced upwards to allow the electrochemically generated h y d r o g en to escape. In the absence of the separator the hydrogen evolution reaction p r o d u c e d a stream of minute bubbles which could just be resolved with the naked eye and whose dimensions relate t o the pore size of the electrode. It was observed that there was a volume change on reduction. The paste th at was t r i m m e d flush with the Teflon surface became slightly convex and p r o ud of the Teflon sheath after reduction. 2. 2. I n s t r u m e n t a t i o n
The experimental arrangement comprised a K e m i t r o n PS40 potentiostat (Kemitron Electronics Ltd., Chester), Kemitron CM2 c o u l o m e t e r and Chemical Electronics RD1 m o t o r - g e n e r a t o r disc drive, all controlled by a K e m i t r o n K3 0 0 0E m i c r o c o m p u t e r , and a Lear Siegler ADM 3A+ visual
338 display unit (VDU). The K 3000E m i c r o c o m p u t e r is based on the 8-bit Z80 processor and has 64k bytes of dynamic random access m e m o r y , augmented by two 8 in f lo ppy disc drives, affor4ing a total of 512k bytes of disc storage space. The machine uses the Digital Research CP/M 2.2 operating system supporting Microsoft BASIC version 5.1. The digital c o m p u t e r was interfaced to the analogue instrumentation via 8-bit and 12-bit digital-to-analogue (D/A) and analogue-to-digital (A/D) converters. Th e 8-bit interface is the faster o f the two types with a signal conversion time of ab o ut 2.5 ms but with a digital resolution of only 1 in 2 s. An 8bit D/A interface was used for c o m p u t e r cont rol of the electrode rotational speed, b e t w e e n 0 and 48 Hz, with a resolution of less than 0.2 Hz. The 12bit interface is about ten times slower than the 8-bit interface, but the resolution is greatly increased to 1 in 212. The 12-bit A/D interface used in this work incorporated a zero-crossing det ect i on circuit and could read voltages between 0 and +-2047 mV with a resolution of 0.5 mV and, in BASIC, the rate was 50 readings s 1. The cell current measurements were made with a Fylde 255DA differential d.c. amplifier. The p o t e n t i o s t a t was controlled by a 12-bit D/A interface, with 0.5 mV resolution, and was stable to less than 0.1 mV. The converters used in this work provided only a voltage signal with no current.
2.3. Experimental algorithm The basic algorithm according to which the experi m ent was c o n d u c t e d comprised a p r e d e t e r m i n e d set of electrode potentials V and a set of rotational speeds R. The c o m p u t e r worked t hrough this two-dimensional array (V, R) and d e t e r m i n e d the steady state current I(V, R) for all values o f V and R. Thus at any potential the c o m p u t e r would set a rotational speed and then log the current response of the electrode. This was done by taking a n u m b e r N1 of current readings at the m a x i m u m speed of the A/D interface, which were averaged to give a value /mean' The c o m p u t e r then t o o k its last N 2 values o f Imean and did a least-squares calculation of di/dt, the slope of the c u r r e n t - t i m e curve. Values of N1 and N2 were selected before the experiment. The operating speed was such that the machine could, each second, give a value of Imean based on 15 readings of I as well as a value of the slope di/dt based on the last ten values of/mean" Thus the c o m p u t e r continuously m o n i t o r e d di/dt and when it calculated a value di/dt < 0.001 (in milliamperes per reading interval of /mean) it accepted this as representative of a steady state and recorded the arithmetic mean of the last ten current readings as I( V, R), the current for potential V and rotational speed R. When the experimental programme was com pl et ed the recorded data were immediately written to a f l oppy disc file rather than left exposed in an insecure m e m o r y , and the control of the electrode potential was then assigned to a subroutine which sought and maintained a rest potential by constantly minimizing the cell current. The m i c r o c o m p u t e r was also used in the analysis of data and greatly facilitated what has classically been a tedious process. In addition to the
339
numeric analysis a simple graphical representation of the data was put on the VDU using the LINE and TAB facilities of Microsoft BASIC. In this way, i -~ v e r s u s w -1/2 plots were made for each of the working potentials, from which a slope and an intercept were obtained by linear regression. The slope gave the speed dependence S of the anodic dissolution reaction and the intercept gave the notional dissolution current i~ at infinite rotational speed. These calculated values were then utilized in preparing slope dependence plots (log S against potential E) and Tafel plots (log i~ against E). With the help of the visual screen representation of these plots, points could be selected for the calculation of slopes and intercepts for both slope dependence and Tafel plots. The results of all these calculations were written to disc to complete the data file for each experiment. The data thus obtained were finally plotted as a hard copy using a Kemitron K2000 microcomputer (specifications as for the K3000E microcomputer but with both 5¼ in and 8 in floppy disc drives) driving a Watanabe WX 4636R digital x - y plotter. The experimental plots presented here were prepared using this system. A fuller description of the digital instrumentation and algorithms in experimental electrochemistry is to be presented elsewhere [22].
3. Results and discussion 3.1. S o l i d z i n c
A portion of a typical c u r r e n t - t i m e ( i - t ) log of the behaviour of a solid zinc electrode is shown in Fig. 2. At A the microcomputer has set a new electrode potential, doing this in five steps of 2 mV to avoid gross perturbation of the system. In B the electrode is held stationary (co = 0) as the current at the new potential reaches a steady value. Once this is achieved the machine proceeds through the regime (from C to G) of five standard speeds ( w -1/2 values of 0.22, 0.18, 0.14, 0.10 and 0.06 sl/2), only moving on to the next speed when d i / d t < 0.001 mA s-1. With the solid zinc electrode this is quickly achieved. The length of the current plateau at each rotational speed is only slightly longer than the time needed to make ten readings of Imean for the first calculation of d i / d t . For these experiments on solid zinc, 15 readings of I were made to calculate/mean, and ten values of/mean were used to derive d i / d t . The i -1 v e r s u s co- ~/2 plots for such an experiment are shown in Fig. 3. Linearity was good, with correlation coefficients of 0.999 being found at almost every potential. From the calculated slopes S and intercepts 1/Lo the slope dependence plot (Fig. 4) and Tafel plot (Fig. 5) for solid zinc were constructed. The proposed mechanism [15, 16, 23] for anodic zinc dissolution in alkaline solution is for two one-electron charge transfer steps with the second step, giving the divalent solution species, being the slower:
340 6
E
.
-
-
,
q
~
5
r --
I 0
--
~o~
r-
j
,
V_
//
.
4..
/
/J
r~
f_J
i
2, i I
i
i
.
b
B
~
C
[
0
i
E
[
F
0.
i
,OS
G
.I
.IS
.25
.2
w~l ~2 / s1/2
Fig. 2. A p o r t i o n of t h e e x p e r i m e n t a l c u r r e n t t i m e log illustrating t h e b e h a v i o u r of solid zinc at all e l e c t r o d e p o t e n t i a l s w i t h t h e d i s s o l u t i o n c u r r e n t ranging f r o m 0.1 to 10 m A . Fig. 3. Plots o f i i against co -1/2 for solid zinc at various p o t e n t i a l s (giving i n t e r c e p t i~ ~ a n d slope S): I - - 1 3 9 0 m V ; a , - - 1 3 8 5 m V ; i , - - 1 3 7 5 m V ; O, - - 1 3 6 5 m V ; 1 [ , - - 1 3 4 0 mV. 1.5.
1.2~
7 ~
.9,
.6.
\
/
-.5
--7/.
- 1
i
-14.00
I
-1380
--1360
I
I
i
--1320 E
/
-l,oo
/.
,
I
--134"0
/
/
- -i~8o
t
-13BO
/mY
-1320
--1340 E
/mY
Fig. 4. Plot of log S against E for solid zinc ( p o t e n t i a l d e p e n d e n c e , - - 3 0 m V decade 1). Fig. 5. Plot of log i~ against E for solid zinc (Tafel slope, 40 m V d e c a d e 1).
Zn Zn I
(i)
' ~" Zrl I + e rate-determining step
~ Zn H + e
(2)
The equilibrium of the zinc(II) solution species with the electrode under all experimental conditions is demonstrated by the --30 mV decade -1 depen-
341
dence of the plot of log S v e r s u s potential (Fig. 4), in full agreement with the results of previous workers [15 - 18]. However, the predicted Tafel slope of 40 mV decade -1, consistent with the above mechanism, is found only at moderate anodic overpotentials (Fig. 5 and Table 1). The observed Tafel plots are curved such that the Tafel slope (measured in millivolts per decade) increases with increasing anodic overpotential. These observations are wholly consistent with previous work [ 1 5 - 1 7 , 23]. The large slopes observed at high overpotentials can be explained in terms of the formation of a film of ZnO on the dissolving electrode surface with an increasing ohmic contribution to polarization [24, 25]. Armstrong and Bell [16] concluded from impedance measurements that at high overpotentials the presence of a surface-absorbed zinc(I) species made a significant contribution to the overall dissolution mechanism. The marked fall in Tafel slope at low anodic overpotentials can be accounted for in terms of the hydrogen evolution reaction which is significant for polycrystalline zinc in alkaline solutions even in the active region, where there is an overall anodic dissolution current. Under these conditions any increase in overpotential would produce both an increase in the anodic zinc dissolution current and a decrease in the hydrogen evolution current. The result is a greater dependence of the measured current on the potential than for pure zinc dissolution. Armstrong and Bulman [15] and Marshall [17] obtained small Tafel slopes at low overpotential. Marshall also found straighter Tafel plots for alkaline solutions containing zincate, where the experimental potential region was removed from that for significant hydrogen evolution. However, if the measured dissolution current at low overpotentials is the resultant of two competing processes, then it is interesting that the observed i 1 v e r s u s ~ 1/2 plots (Fig. 3) for these potentials are not curved. This requires that the hydrogen evolution reaction is independent of the rotational speed, which would occur if hydrogen were evolved only from water in the electrolyte.
TABLE 1 Tafel slopes for t h e solid zinc a n d p o r o u s zinc e l e c t r o d e s
Potential
(my)
--1390 --1380 --1370 --1360 --1350 --1340
Solid zinc
Porous zinc
ip/i s
Current is (mA)
Tafel slope (mV decade -l)
Current ip (mA)
Tafel slope ( m V d e c a d e -1)
0.63 1.41 2.69 4.36 6.31 9.12
23.5 32.5 41.1 51.6 62.5 78.4
2.95 4.36 5.89 7.41 9.12 12.6
53.3 64.4 80.1 99.3 122.0 148.5
4.68 3.09 2.19 1.69 1.44 1.38
342 3.2. Porous
zinc
A p o r t i o n o f t h e c u r r e n t log for a p o r o u s zinc e l e c t r o d e is s h o w n in Fig. 6. Its f o r m is a n a l o g o u s to t h a t f o r t h e solid e l e c t r o d e e x c e p t f o r t h e o b v i o u s d i f f e r e n c e t h a t t h e a c h i e v e m e n t o f s t e a d y d i s s o l u t i o n c u r r e n t s is a slow process. Figure 6 s h o w s t h e b e h a v i o u r at a fairly low a n o d i c o v e r p o t e n tial, w h e r e t h e r e are significant c u r r e n t f l u c t u a t i o n s t o be averaged o u t b u t w h e r e s t e a d y state c u r r e n t s w e r e r e a c h e d c o m p a r a t i v e l y q u i c k l y . At high o v e r p o t e n t i a l s this p r o c e s s c o u l d t a k e m u c h longer, even up to 1 0 0 s. Whereas t h e i I v e r s u s co-1/2 plots for solid zinc were linear o v e r t h e w h o l e r o t a t i o n a l s p e e d range, this was n o t t h e case f o r p o r o u s zinc (Fig. 7). It can be a s s u m e d t h a t t h e r o u g h f r o n t surface o f t h e p o r o u s e l e c t r o d e causes t u r b u l e n c e at t h e higher r o t a t i o n a l speeds w i t h b r e a k d o w n o f t h e d i f f u s i o n l a y e r a n d a c o n s e q u e n t a n o m a l o u s increase in c u r r e n t .
J
"\\
s~ B
1
C
// I
a
1
E
1
F
k
G
I
.05
.I
.15
m ~12,
sH2
.2
.25
Fig. 6. A portion of the experimental current time log illustrating the behaviour of porous zinc. Dissolution currents were higher than for solid zinc and could exceed 20 mA. Fig. 7. Plots of i ~ against co 1/2 for porous zinc at various low anodic overpotentials: i , --1406 mV; ~, --1400 mV; A , - 1 3 9 4 mV; e), --1385 mV;][,--1376 mV. S l o p e d e p e n d e n c e a n d Tafel p l o t s w e r e c o n s t r u c t e d f r o m i -1 v e r s u s c~-1/2 p l o t s using c u r r e n t m e a s u r e m e n t s at t h e first f o u r r o t a t i o n a l speeds. A l t h o u g h n o p r o b l e m s w e r e e n c o u n t e r e d w i t h t h e p a s s i v a t i o n o f p o r o u s zinc e l e c t r o d e s , even u p to p o t e n t i a l s as a n o d i c as - - 1 2 8 0 m V , it was n o t possible to w o r k o v e r this entire r a n g e w i t h a single e l e c t r o d e . Each e x p e r i m e n t was l i m i t e d b y t w o factors. T h e first was t h e finite c a p a c i t y o f t h e e l e c t r o d e , o f a b o u t 13 C, o f w h i c h a b o u t 4 C c o u l d r e a s o n a b l y be used b e f o r e t h e elect r o d e b e c a m e u n r e p r e s e n t a t i v e b e c a u s e o f excessive loss o f m a t e r i a l . T h e s e c o n d was t h e n e c e s s i t y o f w o r k i n g t h r o u g h t h e e x p e r i m e n t a l a l g o r i t h m no faster t h a n t h e p o r o u s e l e c t r o d e c o u l d e q u i l i b r a t e at each n e w d i s s o l u t i o n c o n d i t i o n . Figures 8 a n d 9 s h o w slope d e p e n d e n c e and Tafel p l o t s f o r
343 1,4
1.2
i
#
-1410
-1400
B o
o o
.4
41420
i
)
-1410
i
i
-1400
=
i
-1390
i
i
J
-1300
O
41 420
-1570
-1390
-1380
E /mY
-1370 E
/mY
Fig. 8. Plot of log S against E for porous zinc (potential dependence, - - 3 0 mV decade-l). (From Fig. 7.) Fig. 9. Plot of log i~ against E for porous zinc (Tafel slope, 40 mV decade-I). (From Fig. 7.) 2
1.5.
r;
I .5. .5
-.5.
-14,0
-1~10
-1~00
-1350
-1320
E /
-1290
mY
-_Ii 440
/ ) -1410
I -1380
i -1350
-1 20
-1290
E/mY
Fig. 10. Plot of log S against E for porous zinc over a wide potential range built up from
a number of individual experiments such as that represented in Fig. 8 (potential depend e n c e , - - 3 0 mV decade-]). Fig. 11. Plot of log io~ against E for porous zinc over a wide potential range built up as for Fig. 10 (Tafel slope, 80 mV decade-I).
p o r o u s zinc at a range of l o w anodic overpotentials. Data from a n u m b e r of individual e x p e r i m e n t s such as these were used to build up c o m p o s i t e plots (Figs. 10 and 1 1 ) , with few inconsistencies. While the slope d e p e n d e n c e is fairly stable at - - 3 0 m V decade -1 over a wide p o t e n t i a l range, the Tafel slope ( 4 0 m V decade -1 in Fig. 9) rapidly increases with anodic overpotential (Fig. 11).
344 T h e slope d e p e n d e n c e p l o t s f o r p o r o u s zinc (Fig. 10) still s h o w t h e - - 3 0 m V d e c a d e -1 p o t e n t i a l d e p e n d e n c e , indicating t h a t the e l e c t r o c h e m i s t r y is t h e s a m e as for solid zinc. Similarly, t h e T a f e l p l o t for p o r o u s zinc (Fig. 11) shows a c u r v a t u r e similar t o t h a t f o r solid zinc for w h i c h t h e s a m e e x p l a n a tions can be a d d u c e d . H o w e v e r , t h e i n s t a n t a n e o u s Tafel slope d E / d ( l o g i~) for p o r o u s zinc is c o n s i s t e n t l y t w i c e t h a t f o r solid zinc o v e r t h e full experim e n t a l p o t e n t i a l range. In this r e s p e c t t h e b e h a v i o u r o f t h e p o r o u s e l e c t r o d e c o r r e s p o n d s to t h e ideal semi-infinite cylindrical p o r e c o n s i d e r e d b y de Levie [ 2 6 ] . This agrees w i t h t h e q u a l i t a t i v e e v i d e n c e o f the c u r r e n t - t i m e experim e n t a l log (Fig. 6) w h i c h s h o w s t h e slow p r o c e s s of e q u i l i b r a t i o n w i t h i n t h e e l e c t r o d e interior a f t e r t h e setting o f each new p o t e n t i a l or r o t a t i o n a l speed. H o w e v e r , it is i m p l i c i t in de Levie's t r e a t m e n t and in its p r e d i c t i o n of a d o u b l i n g of Tafel slope in t h e t r a n s i t i o n f r o m a solid to a p o r o u s e l e c t r o d e t h a t t h e t w o Tafel p l o t s will i n t e r s e c t , i.e. for a n o d i c d i s s o l u t i o n t h e r e will be s o m e p o t e n t i a l w h e r e t h e d i s s o l u t i o n c u r r e n t will be t h e s a m e f o r b o t h a p o r o u s and a solid e l e c t r o d e . This can b e seen in t h e e x p e r i m e n t a l Tafel p l o t s (Figs. 5 a n d 11) b y t a k i n g d u e a c c o u n t o f t h e i r c u r v a t u r e . T h e i n s t a n t a n e o u s Tafel slopes f o r a range o f p o t e n t i a l s , as well as t h e c o r r e s p o n d i n g value o f i~, are given in T a b l e 1. T h e p l o t s o f t h e r e c i p r o c a l Tafel slopes d(log i ~ ) / d E v e r s u s E (Fig. 12) clearly s h o w t h a t b o t h p l o t s a p p r o a c h o n e a n o t h e r a n d a p p r o a c h zero w i t h increasing a n o d i c p o t e n t i a l , i.e. b o t h e l e c t r o d e s display a similar limiting c u r r e n t . In this limiting case t h e i n t e r i o r o f t h e p o r o u s elect r o d e d o e s n o t c o n t r i b u t e significantly to t h e s t e a d y state c u r r e n t w h i c h is d e t e r m i n e d b y t h e p r o j e c t e d e l e c t r o d e area. T h e p l o t o f t h e ratio ip/i s o f s t e a d y state c u r r e n t s at d i f f e r e n t p o t e n t i a l s (Fig. 13), t a k e n f r o m t h e respec-
L~S .04
~l~ .03 e-
.02
.0|
~I~00
-1350
-1300
E /
mV
-1400
-1350
-1300 E I mV
Fig. 12. Plots of the reciprocal instantaneous Tafel slopes d(log ioo)/dE against E: e, solid zinc electrode; m porous zinc electrode. Fig. 13. Plot of the ratio ip/i s of limiting currents against E for porous and solid electrodes. (From Figs. 5 and 11 and Table 1.)
345
rive Tafel plots, shows h o w the benefit of the internal surface area of the porous electrode is gradually lost. The current-time log for these high anodic overpotentials is similar to Fig. 6, clearly indicating a porous nature, but n o w the internal area delays the achievement of the steady state rather than contributes any significant faradaic current. The region of current generation is confined to the front face of the electrode in agreement with the findings of Breiter [27] and Coates e t al. [ 2 8 ] . For solid zinc the successful operation of the computer algorithm is demonstrated, and the mechanism of zinc dissolution in 7 M KOH is shown to match that determined for less concentrated alkaline solutions. For porous zinc the computer easily performed an experiment which would be very difficult to perform by manual means. For both solid and porous zinc electrodes the rates of anodic dissolution are dependent on rotational speed. In a further c o m m u n i c a t i o n we shall show h o w this dependence can be eliminated.
References 1 M. W. Breiter, J. Electrochem. Soc., 112 (1965) 845. 2 R. A. Osteryoung, in J. S. Mattson, H. B. Mark, Jr., and H. C. MacDonald, Jr. (eds.), Computers in Chemistry and Instrumentation, Vol. 2, Dekker, New York, 1972. 3 Personal computers in chemistry. In P. Lykos (ed.), Proc. American Chemical Society Meet., Washington, DC, August 1979, Wiley, New York, 1981. 4 O. R. Brown, Electrochim. Acta, 27 (1982} 33. 5 N. A. Hampson and M. J. Willars, Surf. Technol., 7 (1978) 247. 6 P. J. Mitchell, S. A. G. R. Karunathilaka and N. A. Hampson, Surf. Technol., 14 (1981) 45. 7 M. I. Cohen and P. A. Heimann, J. Res. Natl. Bur. Stand., 83 (1978) 429. 8 T. E. Edmonds, Anal. Chim. Acta, 108 (1979) 155. 9 D. Britz, Anal. Chim. Acta, 143 (1982) 95. 10 J. Mozota, B. Barnett, D. Tessier, H. Angierstein-Kozlowska and B. E. Conway, Anal. Chim. Acta, 133 (1981} 191. 11 J. A. Harrison and C. E. Small, Electrochim. Acta, 25 (1980) 447. 12 J. A. Harrison, J. Power Sources, 8 (1982) 99. 13 J. A. Harrison, Electrochim. Acta, 27 (1982) 1113. 14 E. B. James, Chem. Br., 18 (1982) 620. 15 R. D. Armstrong and G. M. Bulman, J. Electroanal. Chem. Interface Electrochem., 25 (1970) 121. 16 R. D. Armstrong and M. F. Bell, J. Electroanal. Chem. Interface Electrochem., 55 (1974) 201. 17 A. Marshall, Ph.D. Thesis, University of Technology, Loughborough, 1975. 18 A. Marshall, N. A. Hampson, J. S. Drury and J. P. G. Farr, Surf. Technol., 5 (1977) 149. 19 L. E. Samuels, Metallographic Polishing by Mechanical Means, Pitman, London, 1971. 20 N. V. Korovin and A. F. Feoktistov, Elektrokhimiya, 6 (1970) 338. 21 A. F. Feoktistov, G. N. Maksimov and N. V. Korovin, Elektrokhimiya, 10 (1974) 1676. 22 A. J. S. McNeil and P. J. Mitchell, t o be published. 23 J. O'M. Bockris, Z. Nagy and A. Damjanovic, J. Electrochem. Soc., 119 (1972) 285. 24 N. A. Hampson and M. J. Tarbox, J. Electrochem. Soc., 110 (1963) 95.
346 25 R. N. Elsdale~ N. A. Hampson, P. C. Jones and A. N. Strachan, J. Appl. Electrochem., i (1971) 213. 26 R. de Levie, Adv, Electrochem. Electrochem. Eng., 6 (1967) 329. 27 M.W. Breiter, Electrochim. Acta, 15 (1970) 1297. 28 G. Coates, N. A. Hampson, A. Marshall and D. F. Porter, J. Appl. Electrochem., 4 (1974) 75.