Effects of mass-transport and oxide films on the cathodic reduction of O2 on Ni

Corrosion Science, 1970. Vol. 10, pp. 775 to 784. Pergamon Press. Printed in Great Britain

EFFECTS OF MASS-TRANSPORT AND OXIDE FILMS ON THE CATHODIC REDUCTION OF 02 ON Ni* J. POSTLETHWAITEt and J. SEPHTON Department of Chemical Engineering, University of Manchester Institute of Science and Technology, Manchester 1, England Abstract--A study has been made of the effects of oxide films and mass-transport on the reduction of 02 at rotating cylindrical Ni electrodes. In alkaline solutions the film of Ni(OH)2 effectively stifles the steady-state mass-transport controlled O3 reduction that occurs on bare Ni. The film is very persistent and its effects are observed at potentials below its legitimate potential range unless the metal is subjected to considerable cathodic pretreatment. In acid solutions Ni(OH)s, said to form a prepassive porous film, has less effect on the reaction. It appears that there is a complete change in the kinetics with the formation of an oxide film and the reaction passes from mass transport control to severe activation control without an intervening mixed mass-transport--charge transfer control. R6sum6---Al'aide d'61ectrodes cylindriques rotatives en Ni, on a 6tudi6 les effets de pellicules d'oxyde et du transfert de masse. En solutions alcalines, la pellicule de Ni(OH), bloque le transfert de masse de r6gime. La pellicule est tenace et ses effets persistent b. des potentiels inf6rieurs ~t la normale, sauf si le mgtal est soumis /tun pr6traltement cathodique intense. En solutions acides Ni(OH)2, sens6 former une pellicule poreuse pr6passive, a moins d'effet sur la r6action. II apparalt qu'une modification complete de la cin6tique se produit du fait de la formation d'une pellicule d'oxyde; la r6action passe du contrfle par transfert de masse au contr61e par forte activation sans un interm~de 6 contrfle mixte transfert de masse/transfert de charge. Zusammenfassung--Eine Untersuchung des Einflusses von Oxidfilmen und des Stofftransports auf die R.eduktion yon 03 art rotierenden zylindrischen Niekelelektroden wurde ausgefiihrt. In alkalischen L6sungen unterdriickt die Bildung einer Schicht aus Ni(OH)2 die durch stationaren Stofftransport kontrollierte O2-R.eduktion,die an unbedeckten Nickelelektroaen auftritt. Der Film ist sehr best~indig und sein Einflul3 wird noch bei Potentialen unterhalb des theoretischen Bestfindigkeitspotentials beobachtet, auch wenn das Metall einer kathodischen Vorbehandlung ausgesetzt wurde. In sauren L6sungen hat Ni(OH)~, das sich als prapassive Schicht bilden muB, weniger EinfluB auf die R.eaktion. Es scheint, dab die Bildung des Oxidfilms die Kinetik der R.eaktion grunds~itzlich ~ndert, indem sic einen Obergang yon stoffiibergangskontrolliertem Reaktionstyp zu durchtrittskontrollierter R.eaktion bewirkt. Ein Bereich mit gemischter Reaktionskontrolle tritt nicht auf. INTR.ODUCTION IN A PREVIOUSstudy 1 involving the effect of flow o n the reduction o f O~. o n N i i n acid solutions steady-state mass-transport conditions were determined, which were in reasonable agreement with those predicted from established mass-transport correlations. However, the results relating to sub-limiting c.ds. could n o t be explained a n d the work has been extended to alkaline solutions where it was hoped that the absence of a c c o m p a n y i n g Ni dissolution would render the sub-limiting currents more a m e n a b l e to analysis. However, it was realized at a n early stage that the presence of a n oxide film which can persist below its legitimate potential range was having a very pron o u n c e d effect o n the O~-reduction reaction a n d results have been obtained with electrodes pretreated to be oxide-free a n d covered with a stable oxide film. Bianchi e t al. 2 have published a series of papers reporting studies o f O2-reduction in acid, neutral and alkaline solutions o n the following metals: Pt, Pd, It, Au, Ag, Cu, *Manuscript received 3 December 1969. "l'Present address: Faculty of Engineering, University of Saskatchewan, R.egina, Canada. 775

776

J. POSTLETHWAITE and J. SEPHTON C

ji I

nverted steel cud and ~mercury well --r/z in.steel shaft

Polythene tubing

- PTFE

- Nickel cylinder I in. o.d,x3 in.

\Wire connection

~ FIG. 1.

PTFE bearin 9 block

R o t a t i n g electrode assembly.

Ni, Co, Cr, stainless steel, graphite, magnetite, A1, Zr, Ti and Ta. They found that the presence of oxide films which are difficult to reduce on metals such as Ni and Al severely hinder the reaction. Golovkin e t al. 3 have studied O2-reduction on Ni in 5.3N KOH at 20-200°C and found that the reaction is controlled by the presence of Ni oxides and that the polarization was reduced by raising the temperature and promoting the electrodes with Li. EXPERIMENTAL

A rotating cylindrical electrode was used to obtain steady and non-steady-state electrolysis data under controlled conditions of mass-transport. The solutions studied were 0.1N NaOH and 0.0IN H~SO4 -I- 0.SM Na~SO+. The rotating electrode assembly is shown in Fig. 1. The Ni electrode was supported oft a 0"5 in. dia. steel shaft by two PTFE compression gaskets, the lower gasket fitting into a PTFE bearing block secured to the base of the cylindrical perspex electrolysis cell. Electrical contact was maintained between the electrode and the shaft by a soldered wire connection. Art inverted mild steel cup in a Hg well provided electrical contact between the potentiostat and the shaft. The complete electrolysis cell is shown in Fig. 2. In alkaline solutions a concentric Ni cylinder which lay flush with the cell

Effects of mass-transport and oxide films on cathodic reduction of O~ on Ni

777

;heel D

rrotJs

ibutor

FIG. 2.

Cylindrical perspex electrolysis cell.

wall was used as the counter electrode. This arrangement was not suitable for the acid solutions, where the Ni anode corroded and Ni plated out on the cathode, and a concentric loop of platinum wire was used. H g - H g O and SCE reference electrodes were used in the alkaline and acid solutions, respectively. The reference electrodes were connected to the cell by means of a solution bridge which terminated in a capillary near the working electrode. A sintered porous plastic distributor ensured good distribution of the 02 and N2 gases. The N~ used for deoxygenation was purified in a "Nilox" apparatus. The potentiostat had the following characteristics: voltage 4 - 2 V, 300 mA cathodic to 1 A anodic, with control ! 1 inV. The constant-current square wave generator used for the galvanostatic and transient studies had the following specification; continuously variable current range 0-500 mA, maximum continuous power output 25 W, square wave frequency 0.1-I000 Hz, current rise time < 10 ~s. The modes of operation were d.c., single pulse and continuous pulsing. The generator was used in conjunction with a storage oscilloscope and the circuit used for the transient analysis shown in Fig. 3 was also used to determine iR drops as previously described. 4, 6 The Ni specimens were cut from standard 1 in. o.d. Ni tubing and polished with a succession of carborundum papers down to 600 grade, degreased with acetone and washed with distilled water prior to installation in the cell. The electrode was blanked off with "Lacomit" to give a working area of 10 cm ~. The nominal composition was: Ni 99% minimum and maximum per cent Cu 0.25, Fe 0.4, C 0-15, Mg 0.2, Mn 0.35, Si 0.15 and S 0.1.

7~

J. POSTLE'rHWAITE and J.

SEPHTON

Reference electrode

Oscilloscope

(

Working electrode

I ConstantcurrentI square wave generatot j FIG.

Ammeter

3. Electrical circuit for transient galvanostatic analysis.

In 0.1N NaOH the values of iR drop were found to be reproducible and independent of flow and also independent of the state of the electrode surface following the different methods of pretreatment. These measurements were used to correct the potentiostatic data. In 0.01N HzSO~ the values of iR drop were large and very sensitive to minor alterations in the probe position. For this reason 0.01N H2SOa + 0"5M Na2SO4 was used, which gave very small iR drops. A more detailed account of the apparatus and procedure is given elsewhere, e RESULTS AND DISCUSSION Steady-state analysis in O.IN NaOH Following preliminary experiments it was realized that the oxide film which forms spontaneously on Ni when it is placed into an alkaline solution was persisting at potentials well below the potential range, predicted by purely thermodynamic considerations, and was severely interfering with the O~-reduction reaction. The initial experiments were conducted galvanostatically and it was found that on the application of <10 g.A/cm2 the potential fell from the rest potential, around 0.080 V, to values below the H equilibrium potential EH+/H2. The results shown in Fig. 4 were obtained after the Ni electrodes had been immersed in the test solution for 15 h to permit stabilization of the oxide. It was found that if the procedure was reversed after the electrodes had been subjected to a period of H evolution, and the current successively decreased, potentials above EH*/H. were attained corresponding to substantialcathodic activity. These cathodic currents at potentials above EH+/H=, which were obviously due to O2-reduction, increased with the total current passed at potentials below EH*/Hr It was found that complete elimination of the oxide film could be attained with 30 mA/cm 2 for 2-3 h, and that following such a treatment steady-state mass-transport O2-reduction could be attained. This is illustrated in Fig. 5, which gives the results of the potentiostatic analyses of electrodes after they had been subjected to a cathodic

Effects of mass-transport and oxide films on cathodic reduction of O~ on Hi

779

-06 -0.7 i - -- EHVH;t

o

o

i

:,% o a.

-0 8

-0-9 -I-0 -I.I

~ ond deoxygenated

a

t

e

d

"'~ - . , . R e ~ e

o xyg eno t t=~"

j---. -1'20

I

3

Currentdensity,

4

mA/cm z

FIG. 4. Steady-state cathodic behaviour of Hi electrodes initially oxide-covered. Galvanostatic analysis in 0.IN NaOH.

current of 30 mA/cm 2 for 3 h. Each analysis was commenced immediately following the cathodic pretreatment and without interruption of potentiostatic control. The potential was raised by 50 mV steps every 2 min. The results in Figs. 4 and 5 clearly show that the persistent oxide film effectively eliminated the steady-state mass-transport controlled O2-reduction exhibited by the oxide-free electrodes. The values of the iR drops were checked on the oxide-covered electrodes and found to be correct and the abnormal behaviour of these electrodes could not be accounted for by large iR drops across the oxide film. This is consistent with the findings of Golovkin, s who found that the slow rate of O2-reduction on nickel electrodes in KOH colutions could not be thus explained and the suggestions of Bianchi 2 are apparently not borne out. The current values in the mass-transport controlled O2-reduction region were attained quickly and were subsequently quite steady in contrast to the readings at potentials above the passivation potential where the currents were very time dependent decreasing steadily with time. This behaviour is presumably related to the growth of the passive film, and the "sub-limiting" current values shown in Fig. 5 are not the result of the reaction coming under mixed mass-transport--charge transfer control as would be expected from a rise in potential. They are most likely the result of a discontinuity in the kinetics for O2-reduction brought about by the formation of the passive film. The equilibrium potential 7 ENi/Ni(OH), in 0.IN NaOH is - - 0 . 6 4 V, and it appears that the film is formed much more easily than it is reduced, since the decrease in the O~.-reduction currents occurred within 100 mV and 4 min of this potential value being exceeded. The experimental values of the limiting current density iam for Oz-reduction on the oxide-free electrodes are in close agreement with those calculated from the relationship

780

J. POSTLETHWAITEand J. SEPHTON

-0.4

*7

o

g o

g o O.

-I.c

....

.'-.. ~

I

1....."-----._

2

5

Current density, FIG. 5.

~

rnA/crn2

Steady-state cathodic behaviour of N i electrodes initially oxide-free. Potentiostatic analysis in 0" 1N NaOH.

ilim ~ zfkCb using values o f the m a s s - t r a n s p o r t coefficient k given by the c o r r e l a t i o n due to Eisenberg et al. s k(~t~

°'64~ :

u koD]

0.0791 (duo) -°'3

\-~/

T h e values o f viscosity ~. density p a n d diffusivity D were t a k e n as 0.01 g cm-Xs -z, 1 g c m -3 a n d 2.34 × 10 -5 cm2s -1, respectively. T h e bulk c o n c e n t r a t i o n o f dissolved 02, cb a n d z were t a k e n to be 1.2 × 1 0 - 6 g mole cm -3 and 4, respectively. T h e experimental a n d calculated values are given in Table 1. TABLE 1

Limiting current for O=- reduction (mA/cm0 Reynolds number

Calculated

1680

0'48 0.78 1.08 1'66 2.04 2.78 3.33

3380 5060 10,200 13,600 20,300 27,400

Experimental 0-50 0'75 1-10 1'60 2'13 2.80 3"20

Effects of mass-transport and oxide films on cathodic reduction of 02 on Ni

781

Thus it appears that at potentials below ENi/Ni(OH), and in the absence of any persistent oxide, Ni can support steady-state mass-transport controlled O~-reduction. However, the presence of the oxide Ni(OH)~ above or b e l o w ENi/Ni(OH)= severely restricts the reaction.

Steady-state analysis h~ 0.01N H~SO4 + 0.5M Na2SO4 Oxide-free electrodes were prepared by cathodic pretreatment at 30 mA/cm 2 for 1 h and immediately followed by potentiostatic analysis during which the potential was increased by 50 mV steps every 2 min. The results obtained in O2-saturated and deoxygenated solutions are shown in Fig. 6. The electrolysis curves are of similar form to those obtained previously1 for Ni in 0.0IN H2SO4 in an annular flow cell. The limiting currents for steady-state mass-transfer controlled O~-reduction obtained by correcting the O2-saturated solution electrolysis curves for H2 evolution, are compared with the values determined from the mass-transport correlation given below in Table 2. The values of V-, P and D were taken as 0.011 g era-is -x, 1.0 g c m -z and 2.34 x 10-5 cm2s -1 respectively. As might be expected following such corrections the agreement is not always as close as that for alkaline solutions where distinct limiting currents were obtained above EH+/H= in the absence of Ni dissolution. The equilibrium potential for theNi/Ni(OH)= system in 0.01N H2SO4 is -- 0.008 V and above this potential the current values became time dependent decreasing with time. Sato arid Okamoto s consider that NiO (the heat of hydration for NiO is only 310 cal/mol) is formed as a porous pre-passive film in acid solutions in contrast to the truly passivating film it forms in alkaline solutions. The formation of such a porous film might be expected to have less effect

0.I 0 ~.~'-~

.

.

.

.

!

E N,/N, (OH)z

"6 -04 ~~!---

EH./Hz

-02 -6 -0.3 =,,

"6 -0.4 (a. -0.5 Solubon

deoxygenated 2 Current density,

FIG. 6.

3

4

mA/cm z

Steady-state cathodic behaviour of Hi electrodes initially oxide-free. Potentiostatic analysis in 0'0IN H=SO4 + 0-5M Na2SO4.

782

J. POSTLETHWAITEand J. SEPHTON TABLE

2

Limiting current for O3-reduction (mA/cm2) Reynolds number

Calculated

Experimental

1510 3040 6080 9180 12,240 18,270 24,660

0.49 0.81 1.32 1.71 2.10 2.80 3.40

0.55 1.00 1-60 1.85 2.20 2.80 3.40

on the O2-reduction reaction a n d this is reflected in the results, in which c o m p a r i s o n o f the curves in Figs. 5 and 6 a b o v e ENi/Ni(OH), shows a greater r e d u c t i o n o f current in the alkaline solutions. This is despite the fact that the results in acid solutions have not been corrected for Ni dissolution.

Unsteady-state analysis in O.1N N a O H The a p p l i c a t i o n o f currents in the form o f c o n t i n u o u s square waves, with the current on for half each cycle a n d off for the remainder, has been previously applied to the a n o d i c dissolution o f Fe a n d Ag. 4,5 In this type o f u n s t e a d y - s t a t e analysis the

t

o

-0.3

"E

i

>

-0.4

-0.5

cn

! . E~_~'/",JZo"k_

o

. . . .

g

8

-0-7 _ o

EH'/%

ol

c

o

~

t

j0xide-free~

-0-8 o

~

-0,9

vered

.~ c

¢, o.I

n°

I

Io Switching frequency,

Ioo

Hz

o

IOOO

Current density,

mA/cm a

FIG. 7. Potential decay from steady-state potential corresponding to 0.75 mA/cm -~at Reynolds number 1680. Potential decay at a given switching frequency is the decay from the steady-state potential during the current off times, where current off time = 0'5 × l/Hz s. Curves shown for oxide-free and oxide-covered Ni electrodes in 0.1N NaOH.

Effects of mass-transport and oxide films on cathodic reduction of O~ on Ni

783

potential decay, during the "current off" periods is determined over a wide frequency range, 0.01-10000 Hz, and the presence of activation and concentration contributions to the overpotential can be discerned because of their different deca~ times. In fact, the activation and concentration decays overlap and absolute values of these quantities cannot be obtained directly from the curves. The results shown in Fig. 7 were obtained for oxide-free and oxide-covered electrodes in O2-saturated 0.1N NaOH. The c.d., 0.75 mA/cm 2, was greater than the limiting current at the chosen Reynold's number 1680, so that the switching curves contain contributions from both the O=-reduction reaction and from the h.e.r. The results for the oxide-covered electrode were obtained by starting at the low switching frequencies and switching the current off for 10 min intervals between frequency settings to permit the oxide to recover from any reduction that may have occurred whilst the current was on. Whereas with the oxide-free electrodes where the problem was to avoid oxide formation the readings were commenced at the highest frequency and current was set to 30 mA/cm 2 for 10 min intervals between readings. By these means it was hoped that the switching curves would wholly relate to oxide-free and oxide-covered electrodes, and indeed the results given in Fig. 7 show the two types of electrode to have very different decay properties. With the oxide-free electrode the potential decay from the steady-state potential to a value above EH+/H~ occurred at a frequency of around 10 Hz, i.e. during current off times of 5 × 10-2 s, whereas with the oxide-covered electrode the decay of the Eoz/H 2 0 8

_E

Y ~Mixed

potential

t = oo

f, a~

..-Mixed . . . . . .

potential t = o

(~.

~-u_

o~

fred

lax

FIG. 8. Mixed potential of O~ reduction and Ni-oxidation reactions at t = 0 and t = oo, where at t = 0 oxide starts to form on bare nickel surface on which oxygencan be reduced under mass-transport controlled conditions and at t = oo oxygenreduction on fully developed passive film.

784

J. POSTLETHWAITEand J. SEPHTON

overpotential which related almost wholly to H2 evolution was more sluggish. It might be thought on first consideration that the inflexion on the oxide-free curve was related to a change from activation overpotential decay to concentration overpotential decay. However, the situation is much more complex than that relating to the decay characteristics o f a single electrode reaction. Above ENi/Ni(OH), the decay potential is a mixed potential relating to the unique single potential at which the 02reduction and Ni-oxidation reactions balance at any given time. The rate of O~.reduction at ENi/Ni(OH), is the steady-state mass-transport controlled value, but above ENi/N~tOH)1 the rate is very much reduced as the oxide forms and the reaction comes under charge transfer control. Initially the mixed potential will correspond to a balance between the mass-transport controlled Q - r e d u c t i o n on bare Ni and the rate of oxidation o f the bare Ni surface to Ni(OH)2. As the oxide forms both rates decrease and the final value is given by the balance between activation controlled O2-reduction on the Ni oxide film and oxidation to maintain the passive film. Since this value is o f the order o f 0-1 ~.A/cm 2 in alkaline solutions 9 it is not surprising that the final rest potential is well above the inflexion point in the switching curve as shown in Fig. 8. CONCLUSIONS I. The kinetics o f O2-reduction on Ni exhibit a discontinuity corresponding to the formation of Ni(OH)2. 2. In alkaline solutions the passive film can persist below ENi/N~fOH), and stifle the mass-transport controlled O2-reduction which exists in its absence. 3. The effect o f the formation of Ni(OH)2 as a porous pre-passive film in acid solutions is less than the truly passive film it forms in alkaline solutions. 4. Because of the discontinuity in the kinetics of 02- reduction there is no steadystate effect o f mass-transport which could be analysed at sub-limiting current densities. R.EFER.ENCES 1. J. POSTLETHWAITEand D. R.. HURP, Corros. Sci. 7, 435 (1967). 2. G. BIANCHI, F. MAZZA and. T. MuSSINI, Proc. 2nd Int. Congr. Metallic Corrosion, NACE, Houston p. 893 (1966). 3. Yu. I. GOLOVKIN,N. P. VASILISTOVand N. A. FEDOTOV,Elektrokhimiya 3, 805 (1967). 4. J. POSTLETHWAITEand D. M. SHARP, Electrochim. Acta 13, 571 (1968). 5. J. POSTLETH~,VAITEand D. M. SHARP, Trans. Inst. Chem. Engrs 47, T 198 (1969). 6. J. SEPHTON,M.Sc. Thesis, University of Manchester (1969). 7. M. POURBAIX,E. DELTOMBEand N. DE ZOUBOV,Proc. 7th Meeting CITCE, Lindau (1955), p. 193. Butterworths, London (1957). 8. M. EXSENBERG,C. W. TOBIASand C. R.. W1LKF,Chert1. EngngProg. Syrup. Ser. 16, 51 (1955). 9. N. SATOand G. OKAMOTO,J. electrochent. Soc. 110. 605 (1963).