Chronopotentiometric measurements on graphite anodes in cryolite—alumina melts

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Ati. 1969,Vol 14,pi 127to 134. ptrlcamonPrea hinted in Nmtbgn h’ehmd

CHRONOPOTENTIOMETRIC MEASUREMENTS ON GRAPHITE ANODES IN CRYOLITE-ALUMINA MELTS* J. THONSTAD The Engmeermg Research Fouudatton, Tecbmcal Umverstty of Norway, Trondberm, Norway Abstract-At cds above a crrtical cd, oxygen drscbarge on carbon m cryoltte-elumma melts ISl&ted bv dBuston-controlled deuletton of dtssolved alumma. followed bv the anode effect In melts wtth 2% pond6raux Al& 1+/*/C d6croit lm&irement a densite de courant crorssante pour des temps de transttion comprts entre envuon f8 et 1 ms, attetgnant une valeur constante pour des temps momdres. Ce comportement semble &re d0 B la drssocratton lente nuttale de l’alumme drssoute. La constante d’bquthbre de cette &action est trouv6e &galeilO, et les constantes de wtesses des composantes progresswe et regressive a 1400 et 2400 S-l respecttvement. Le wet&tent de diffusion de l’alumine dtssoute vaut 1,s x 10-6cm*/S a 1020°C La relation th&onque potentJ/temps des reactions trr&erstbles est satrsfatte a des densttes de courant mferteures a + 15 A/cm*, il des denstt6s de courant sup&reures, le potenttel croft plus raptdement que p&u. m_Dte Sauerstoffentladung an Kohle m Schmelxen von Kryol~th/Alummtumoxyd 1st oberhalb emer krtttschen Strom&&e durch eme drffustonskontrolherte Verarmung an geldstem Alumrmumoxyd, gefolgt vom Anodeneffekt, ltmtttert. In Scbmelzeu mtt wemger als 1 Gew % Al,O, 1st der Ausdruck i , +/*/I? fur Transtttonw&n 7 xwtchen 8 und 0,l ms konstant. Retr>der Gehah an Al,O, mehr als 2 Gew %, so nrmmt I &*/C nut stergender Stromdrchte fur Transttronszetten xwrschen ca 8 und 1 ms linear ab und erretcht her ktbxeren Zerten emen konstanten Wert. Die Ursaehe ftt dteses Verbalten schemt m emer vorgti tgen, laugsameu Drssoztatron des Alumtmumoxyds zu begen Fiir dte Gleichgewtchtskonstante piteser Reaktton fand man emen Wert von 0,60; dte Geschwmdigkeitskonstanteu der Vor- und Rtlckreaktron betrugeu 1400 bzw. 2400 s-r. Der Dtffustouskoetlizzent des gel&ten Alummmmoxyds betragt bet 1020°C 1,5 x 1O-6cmx/s Rer Strom&chten unterhalb 15 A/cm* 1st dte theorettsche Potenttal/Zettbextehung fur nrverstble Reakttonen effiilt, oberhalb dieses Wertes stelgt das Potential rascher an INTRODUCTION

THE ANODE reaction oxygen wrth reaction

on carbon in cryohte-ahmnna melts involves the discharge with carbon, yielding CO, under normal condttions,1

0”

+ &C+ *CO, + 2e

of

(1)

The oxygen ion may be bound in an Al-O-F complex.2 At a critical cd, the anode effect occurs, whereby the anode potential increases tenfold or more, fluorine is discharged and evolved as CF4, and arcing is sometimes observed at the anode surface. As shown previously3 the crttrcal cd 1s a lmear function of the alnnnna content of the melt, and the anode effect appears to be caused by a drfTuston-controlled depletion of oxygen-contammg ions at the anode surface. The * Mauuscrrpt received 25 January 1968. 127

128

J TXONSTAD

reactton preceding the anode effect should, therefore, be smted for chronopotentrometric study. The Sand equation4 r+C = &hFD+, (2) where z IS the cd, T the transttron trme, C the concentration and D the drffusron coefficient of the ion undergoing reaction, IS based on the assumption of linear dtffusion on to the electrode surface. In the present case gas evolution at the electrode may cause strrrmg m the drffusron layer. As shown in the following, this drfficulty 1s overcome at very short transitron times EXPERIMENTAL

TECHNIQUE

The experimental cell has been described prevrously.3 The anode consisted of a graphite rod enclosed m boron nitride. The surface of the graphite exposed to the melt was varred from 0.05 to 1 cm2. An unshrelded graphite rod of 3 mm diameter was also used as anode. The graphrte crucrble which contained the cryohte-alumma melt and served as the cathode was rotated at 105 rev/min during the measurements. A transistorized constant current supply was used. The current remained constant wrthm f 1% during normal electrolysrs, but rt decreased by the abrupt voltage increase at the onset of the anode effect At this pomt the current was normally interrupted by a voltage cut-out which was incorporated m the current supply. Another current supply consrstmg of lead storage batteries of 115 V m serves wrth resrstors was also used The potentral/trme curves were traced on a storage oscilloscope triggered from the current supply. Measurements were made m cryohte-alumma melts containing from 0 25 to 12 wt-% AlaO3 (0.5 x lv to 24 x 10m4mole/cm3) at 1020°C with cds from 2 to 60 A/cm2. In order to obtam a reproducible anode surface, pre-electrolyas at O-5 A/cm2 was performed before each series of measurements The stnrmg of the melt, achieved by rotating the crucrble, bettered the reproducrblhty and permitted measurements to be carried out m rapid succession, but the results were the same as obtained m an unstrrred melt. When no stnrmg was apphed, It was necessary to wart for mmutes between each measurement. Otherwrse a porsomng of the electrode nnght occur, characterrzed by a drastic shortening of the transttlon trmes to almost ml. A poisoned electrode could not be restored to normal operation. The voltage cut-out that interrupted the current at the onset of the anode effect proved to be effective m preventing the polsomng. It 1s believed that the poisoning was caused by discharge of fluorine followed by the formation of a C-F surface compound.5 RESULTS

At cds above the critrcal, potentral/trme curves as shown m Figs. 1 and 2 were obtained. The breaks m the curves were well defined for transmon times longer than about 0 1 ms. At shorter times the breaks became mdlstmct and drfficult to locate. The transrtlon times were determmed by the graphrcal method proposed by Delahay.4 The experimental curves grve the total voltage drop across the cell. By mserting a separate graphrte cathode and usmg the crucrble as reference, rt was found that the entrre voltage increase durmg a measurement was associated with the anode. Generally, the left hand term m (2), id/C, increased rapidly wrth mcreasmg T for T > 10-8 ms. For T > cu 10 ms, the curves exhrbited a honzontal section with

Chronopotentlometrx

measurements on pph~te anodes m cryol&-ahumna I

I

I

melts

129

I

/

6-

v

4-

2-

0. I

0

I 4

3

2 7

w

FIG 1 Potentlal/tlme curve at 4 A/cm* m melt wth 0 5 wt-% A&O, I

I

I1

I’

12 -

v

a-

4-G

OU r.

ms

FIG. 2. Potentuxl/tune curve at 60 A/cm* m melt with 12 wt-% A1108.

ripples, as shown in Fig. 3. The length of this section varied m an unsystematic manner. Meaningful results were thus obtamed for transition times between 10 and O-1 ms. In this range the transition tunes were determmed wrth a precision better than f10 per cent withm a smgle run, while the results obtained m duphcate runs nught differ with as much as f20 per cent. The relatively poor reproducibihty was ascribed to the problem of obtaimng a reproducible anode surface and to possible current leakage through the BN shield 6 In melts with less than 0.5 wt-% A&O, and at low cds, two additional steps were observed on the potential/time curve, Fig. 1. At higher cds these steps disappeared or

0

40

20 T,

60

ms

FIG. 3. Potentml/tnne curve at transltlon tune longer than 10 ms. 10 A/cm*, 4 wt-% A&O,.

J. SONSTAD

130

merged with the main curve. The lower step was very short and is beheved to be attributable to the water content of the melt, ze, to the discharge of hydroxide ions. The upper step exhihted well-defined curves at alumina contents below 0.25 %. The corresponding transrtlon times were not related to the alumina content m any meamngful sense, neither did the term ZT*attain a constant value. This step was believed to be associated with the onset of the anode effect. Smular multrple step curves have been observed by Piontelli et aZ7at low alumina contents. In Fig. 4, the term ~T~/C(A/cm? . s*/(mole/cma) is plotted us i m (A/cm?. Curve I is typrcal for melts with 1 wt-% Al,4 or less and curve II for melts with more than about 2 wt- % Al,O,. For melts with alummium contents from 0.25 to 1 wt- % (melts with less than 0.25 % A.&O3were not investigated systematrcally), the term z~+/Cattained a constant value of 2000 to 2200 for 7 < 8-5 ms, as shown by curve I.

I

IO

f

I

20

30 4

I

I

40

50

A/cm2

FIG. 4. Plot of i#/C vs f. Curve I, melts with 1 wt- % AllOt or less, curve II, melts wth more than 2 wt-% A&O,.

At alumina contents above 2 wt-%, the ir* vs i plot gave a straight but dechning curve for transitron times shorter than lo-8 ms and a horizontal curve for times shorter than 1.5-0.7 ms, as shown by curve II. The length of the mchned section vaned as indicated; it almost disappeared at high alumma contents (8-12 wt.-% Al*OJ and correspondmgly high cds. The horizontal section occurred at zrf/C = 8001000 mdependently of the alumma content. In melts with from 1 to 2 wt- % alumina, the horizontal sectron at hi/C m 2000 became very short and the curves continued to decline, but without eventually reaching a constant value as observed at higher alumma contents. In this concentration range the reproducibrhty was poor. DISCUSSION

When there is convection m the diffusron layer at the electrode, the term zr*/C wrll increase with increasing 7. The raprd increase m ir*/C for T > 10-8 ms is probably due to the formation of carbon &oxide gas at the electrode. The horizontal section with npples observed in the curves when T > 10 ms, Fig. 3, may represent

Chronopoteatiometricmeasurementson graphrteanodes m cryohte+alummamelts

131

a semi-stable state where the depletion of dissolved alumma at the anode is temporarily delayed because the rate of transport IS enhanced by turbulence created by the formation of gas. At transition times shorter than 10-8 ms this kmd of convection apparently did not interfere. Assuming a surface roughness of 10, the amount of current passed, eg i = 10 A/cm* and T = 05 ms, corresponds to the formation of approximately one monolayer of chemisorbed oxygen at the anode. Discrete gas bubbles thus cannot be formed at sufficiently short transition times. At transition times shorter than 10 ms, the thickness of the diffusion layer is less than lo-8 cm,* which corresponds to the lower hmit below which external stirrmg normally does not exert any influence upon diffusron-controlled electrode processes. This serves to explain the observed non-dependence of external stnrmg in this case. For very short transition tunes possible errors due to the finite rise-time of the current and to the charging of the double layer must be considered. The rise-tune, ie the time needed for the current to rise from 10 to 90% of the preset value, varred from 8 to 30 ,us for currents from 05 to 30 A. At high currents the error then was about 10% at T = 0~1msandl%at~=lms. According to Drossbach et UP the double-layer capacitance of a carbon anode in cryohte-alumma melts is about 20 pF/cma. Assummg an increase in anodic potential of 2 V, the time needed to charge the double layer at i = 10 A/cma is

The maximum error due to current rise-time and double-layer charging then amounts to15%atT=O*lmsand1*5%atT= 1 ms. In fact, the errors are considerably smaller, since the transition times are being calculated from the f&t break in the curves and not from the time of current ‘on’, as assumed m thrs estimate. The correction term that is to be apphed to the Sand equation when using electrodes of cyhndrrcal shapei was negligible in the present case because of the very short transition tunes. As shown by Delahay and 13erzms,” a decrease in iT&when plotted us i may be an indication of a slow preceding chemical reaction. For a simple first order reaction the general equation is id = +rhFCD* - (#di/K(k,

+ k,)f)(erf [(k, +

&)*T*]),

(3)

where k, and k, are the forward and backward rate constants and K = k,/k, is the equilibrmm constant of the chemical reaction, the other symbols having then usual meanings. The last term, the error function, erf (A), IS approximately unity when the argument il = (k, + k&+ > 2. Then iT* becomes a linear function of i with slope d lT+/di = --?r’/2K(k, + k&t

(4)

Extrapolation to zero current gives the value of 27’ that would be found if no slow chemical reaction mterfered. When iz < O-1, the error functron becomes approximately equal to 2++

= 2(k, + kb)*+*

,

and (3) simphfies tollJa ir* = +r*nFCD*/(l + l/K).

(5)

132

J THONSTAD

At sufficiently small values of 7, the term ZT&may thus eventually attain a constant value. A graphical presentation by Delahay and Berzu# of theoretical ZT~us z curves demonstrates that a horizontal hne IS already obtamed when il < 0.5 and an mchned straight hne when iz > 1. At alumma contents above 2 wt-%, the slope of the id vs z plots, exemphfied by curve II m Fig. 4, was found to vary between -0.020 and -0.026. Extrapolation to zero current yielded values of zr*/C from 1750 to 2000, which IS slightly lower than those obtained at lower alumma contents, 2000-2200. These results Indicate that (2), (4) and (5) can be applied m this case to calculate the constants K, k, and k, for a slow chermcal reaction prccedmg the oxygen discharge reaction at the anode. It was then found that m the present case the change from the mchned to the horizontal curve (Fig. 4, curve II) occurred when il = l-2, ie at somewhat higher values than expected from theory. Because ;Z> O-1, (5) is not strictly vahd. A series expansion of the error functiorWs shows that the value of K obtamed from (5) m this case is too hrgh by a factor of approxrmately 1.3. By dividing (2) and (5), and mtroducmg the corresponding average experimental values that pertam to the respective equations and usmg the correction factor, the eqmhbnum constant K can be calculated 1 + l/1*3 K = 2000/900, K fi 0.60. Due to scatter in the experimental values of ZT), the hnuts of error are f15 per cent. The rate constants can now be calculated from (3) usmg the average value of -0.024 for the slope -n*/2K(k, + kJt = -0.024, giving kf = 1400 s-r and k, = 2400 s-l In this case the hmits of error are f40 per cent. The nature of this slow precedmg chermcal reaction is not known. (3), (4) and (5) are valid for a first order chermcal reaction of the type A rs B. If the reaction deviates from this simple scheme, the calculated constants may not be correct. The slow reaction is probably connected with the dissociation of alumina. Previous potential-sweep measurement$ also indicate that there occurs a slow preceding chemical reaction at alumma contents above 1-wt%. In this concentration range the melt probably contains ions with different numbers of oxygen atoms,” and the dissociation prior to discharge may then include several steps. Smce the composition of the ions is not known, it IS not possible to attribute the slow step to any specific reaction DzJiiszon coeficzent

The diffusion coefficient can be calculated from (1) Smce the nature of the dissolved species IS not known, it should be considered merely as the diffusion coefficient of oxygen-containing ions formed by the dissolution of alumma m molten cryohte at 1020°C. Introducmg the average value of zr*/C! = 2000, and n = 6, one obtams D = 1.5 x lo4 cmz/s. The hrmts of error are f15 per cent.

Chronopotentlometrlc measurements on graph& anodes m cryol&+hunma

melts

133

Withm the experimental error the diffusion coefficient is constant over the entire concentration range that has been Investigated (O-25-12 wt-% A&O&. The shghtly higher values of id/C obtamed at low concentrations may be due to the presence of oxrde impurities m the cryohte.3 The observed constancy of the diffusion coefficrent IS m disagreement with the data of Shungm et all6 obtamed m a study of the kinetics of the dissolution of alumina m cryohte melts. At 1080°C and 15 wt- % alumma the diffusion coefficient was found to be 0.72 x 106 cm%/s, and it was said to decrease with decreasing alumma content. Potentud/time relationshzp The theoretical potential/time evaluated by DelahayP E = (RT/anJ)

relationship

for n-reversible reactions has been

In (nFC~&z) + (RT/anJ in (1 - (t/T)*),

(6)

where a and n, are the transfer coefficient and the number of electrons m the ratedetermining step respectively, and k,,, is the forward rate constant for the electrochenucal reaction, the other terms havmg their usual meaning. (6) is valid also when the reaction product 1s insoluble m the electrolyte, If rt is formed at unit activrty.l‘r In Fig. 5 E ISplotted USlog [l - (t/T)*]. Smce a reference electrode was not used, the potentials are arbitrarily referred to the first break m the potential/time curves. At cds from 2 to about 15 A/cm* straight lines with slopes of 0.42 to 048 were obtained, as shown by curve I. According to (6) this corresponds to aq, = 060 to O-52. For discharge of single oxygen ions, n, = 2, and a, the transfer coefficient, is then approximately O-3. Tafel plots of the anodic overvoltage m cryolite-alumina melts17 gave an, % 1 at cds below about 1 A/cm 2. At higher cds the overvoltage mcreases more rapidly than corresponding to a logarrthmic relationship.

FIG. 5 Plot of E us log (1 - (r/+/4) I, cd below 15 A/cm*, II, cd above 20 A/cm*

134

J. THONSTAD

With mcreasmg cd above 8 A/cm*, the appearance of the potentml/tune curves gradually changed from curved to almost straight lines, as shown m Fig. 2, and the potential increase between the breaks m the curves was about 2 V as against about 0.8 V at lower cds. The correspondmg plot m Fig. 5, curve II, 1s curved. This discrepancy may be caused by, eg, slow desorption of carbon Qoxlde or increased ohmic resistance through a gas film on the anode. author wishes to thank A S An-lal og Sunndal Verk for financial support and for the perrmssron to pubhsh this work The constant current supply was constructed at the Dinsron of Automatic Control. The work was done m the laboratories of Professor J Brun, head of the Department of Industrial Electrochermstry. The assistance of Mr. J. Gulbrandsen m carrymg out the experrments is acknowledged.

Acknowledgements-The

REFERENCES 1. T G PEARSON and J. WADDINOTON, Drrcuss. Faraday Sot. 1,307 (1947). 2. J. BRYNESTAD, K. GRJOTHEIM, F. GRBNVOLD, J L. HOLMand S. URNES,DLWUSSFaruuby Sot 32, 90 (1961) 3. J THONSTAD, EIectrochim Actu 12,1219 (1967). 4. P. DELAHAY,New Instrumental Metho& in Electrochemutry, pp. 184, 187, 209 Interscience, New York (1954). 5. P. MI3RoAuLTand A. AHMADI,C r hebd. Seanc Acad. Sci. Pans 250,849 0960) 6. J. THOSE, Elrctrochem Techn 6, 346 (l%S). 7. R. PIOEITBLLI, B Mm and P Psnaranm, EZectrochzm. Actu 10, II17 (1965) 8. K. Varraa, Eiektrockemische Kmet& pp. 170,183. Sprmger, Berlm (1961). 9. P. DROSSBACH, T. -0, P. KRAHLand W. PPBIPPBR, Chemre Zngr Tech 33,84 (1961) 10. D. G. Parsas and J. LINOANE,J. electrmI. Chem. 2, I (l%l). ;; E DA-y and T BERZINS,J. Am. them. Sot. 75,2486 (1953). LNTINENand R A. Osma~ou~o, m Fused Salts, Ed. by B. R Sundheim, p 258. . McGraw-Hdl, New York (1964). 13 Handbook of Mathematical Functwns, Ed. by M. AllRAMowrTZand I A. STEOUN,p. 310. Dover, New York (1964) 14. M. ROL~Nand M. Ray, Bull sot. Chum. Fr. 2749 (1966). 15. P. M. Sr-ruaro~~,L M. BARMIN and V. N. B~RONENKOV, Zzo uyssh ucheb. Zaued , Tswt Met. 5, (4), 106 (1962) 16. M. PAUNO~IC, J. electroanal Chem. 14,447 (1967). 17. J. THONSTAD and E. HOVE,Con J. Chem 42,1542 (1964).