Mixed oxides of IrOz and ZrOz were prepared in various mole ratios and their cyclic voltammetric

behaviour, and Tafel behaviour for oxygen evolution reaction in acid and alkaline solutions were studied.

The service life of these electrodes were evaluated in 6 mol/dm3 NaOH solutions and it is found that

the electrodes containing 60 mole ,% of lrOz showed a maximum service life. The electrochemical

behaviour of these electrodes were also studied in ferricyanidelferrocyanide redox systems

Guruviah, S.

Subramanian, K.

IR@CECRI

Bulletin of Electrochemistry Vo1.2(61, Nov-Dec 1986 56-565 ELECTROCHEMICAL BEHAVlOUR OF TITANIUM SUPPORTED lr02/Zr02 ELECTRODES IN ACID AND ALKALINE SOLUTIONS AND IN FERRICYANIDE/FERROCYANlDE REDOX SYSTEMS K. SUBRAMANIAN and S GURUVIAH Central Electrochemical Research Institute, Karaikudi-623 006 ABSTRACT Mixed oxides of lrOz and ZrOz were prepared in various mole ratios and their cyclic voltammetric behaviour, and Tafel behaviour for oxygen evolution reaction in acid and alkaline solutions were studied. The service life of these electrodes were evaluated in 6 mol/dm3 NaOH solutions and it is found that the electrodes containing 60 mole ,% of lrOz showed a maximum service life. The electrochemical behaviour of these electrodes were also studied in ferricyanidelferrocyanide redox systems. Key W o r d s :  lrO2/2rO2 electrodes, ferri-ferro cyanides redox systems, cyclic voltammetry, oxygen evolution reaction T he growth of electrochemical industries forces electrochemists with serious tasks to develop high quality materials for a wide range of production processes. Since in most cases the anodes operate in extremely severe conditions (aggressive media, high anodic potentials, high temperature etc.) the development and choice of stable anodes with little dissolution poses the greatest difficulties. Though the pure precious metal oxides are the most efficient elec- trocatalysts [I-37, the life of these anodes were shortened [4,5] due to lack of stability of the oxide coating. The service life of these anodes could be increased by stabilising with valve metal oxides [6 ]such as Zr02, Ta20,, Ti02 etc. It was reported C7I that Ru02 was stabilised by the addition of Zr02 and gave a service life of about 200 hours in 6.0 mol/dm) NaOH at 80°C at 0.75 A/cm2 whereas for the pure Ru02 it was about 40 hours under the same conditions. The present paper mainly deals with the prepara- tion and studies on the behaviour of various compositions of Ir02/Zr02 in acid/alkaline solutions. The behaviour of these anodes was studied in ferricyanide/ferroqyanide redox systems. EXPERIMENTAL Preparation of electrodes The electrodes were prepared as per Indian Patent No.134375. Plain titanium sheets (ASTM Grade 11) of 1 x 18 x 0.1 cm were cleaned in trichloroethylene and etched in boiling 30% HCI for 15 minutes. The re- quired amounts of IrC13. 3H20 (51% metal content) and ZrOCI,. 8H20 were dissolved in minimum amount of concentrated HCI separately. The coating chelate was prepared by mixing the solutions depending upon the mole ratios of I r02 and Zr02 starting from Ir02 of 5 mole % to I00 mole Yo. The thickness of the film is fixed as 3 p for all anodes: isopropanol was used as an organic vehicle in all coating solutions. The coating solution was brushed on to the etched titanium support and dried at 110°C for 3 mins. and then fired at 460°C for 10 mins. in the presence of air. The process was continued until no more solution remain- ed. The anodes were finally baked at 500°C for50 mins.The coated film showed resistance to peeling and bend tests. Cyclic voltammetric oxygen evolution kinetics/redox behaviour To study the cyclic voltammetric behaviour of electrodes in alkaline solu- tions, and kinetics of oxygen evolution in alkaline/acid solutions, a two- compartment cell was used with a glass frit (G3) separating the working and counter electrode sections. In the study of ferricyanide - ferrocyanide redox systems, a single compartment cell was used as counter electrode potentials were measured with respect to saturated calomel (SCE) or Hg/HgO, NaOH electrodes depending upon the nature of the system. The cyclic voltammetric behaviour was studied in IM NaOH solutions. For ferri/ferro cyanide redox studies. 0.05 M K3 Fe(CN)6 in 1M KN03 or 0.1M H2S04 solution was used. The steady state experiments were car- ried out in 1M H2S04 and 6M ~ a 0 ~ ' f o r  acid and alkaline media respec- tively. In the evaluation of order of reaction, 2M H2S04 and 3M NaOH solutions were used. The data for Tafel curves were those obtained after waiting for 3 mins. at each point of measurement. All experiments were carried out at 30°C and by degassing the solutions with high purity nitrogen. All solutions were made up from Analar grade chemicals and triple distill- ed water. Accelerated service life Test Service Life tests of the electrodes under study were carried out in 6M NaOH solution at 30°C. at 1 A.cm-2 current density. Service life values were measured by recording the cell potential as a function of time during elec- trolysis under galvanostatic conditions. The service life for a given elec- trode is the time required for total failure of electrocatalytic surface layer. The time at which the latter occurred was immediately apparent as it resulted in a large increase in potential associated with complete failure of catalyst and the formation of an inactive anodic film on the titanium support. RESULTS AND DISCUSSION Cyclic voltammetric bebaviour In the cyclic voltammogram for Ir02/Zr02 electrodes in 1M NaOH at 30°C (Fig. I) a large background over the entire potential range (-1.1 V to -0.6 V vs Hg/HgO) was observed for the electrodes containing more than lomole To of Ir02. The amount of charging'current is directly pro- portional to the roughness of electrodes 18.97. Subramanian and Guruviah - Electrochemical behaviour of titanium supported lr02/Zr0, electrodes E, V vs HgIHgO, NaOH Fig.1: Cyclic voltammogram in 1 rn NaOH solutionsfor (a) 20 mole % IrO, and 80 mole % Zr02 on titanium substrate, (b) 25 mole % Ir02 and 75 mole ZrO, on titanium sustrate. Sweep rate = 50 mvlsec, temp: 30°C Two pairs of peaks are seen at about -0.3V vs Hg/HgO and + 0.3V rs Hg/HgO (Fig.1). The same observation has been made by earlier workers _ 101 on pure I r 0 2  The peaks 1 and 4 can be attributed to the reaction :111 : 2 110, + 2 H +  + 2 e + Ir203 + H20; E = 0.923 - 0.059 pH ... (1) whereas the peaks 2 and 3 may be due to the reaction : Ir02+ 2H20 IrOa-$4 H + 2e; E = 1.68 - 0.0985 pH ... (2) and Ir203 + 5H20 + 21r042- + 10 H +  + 6e-; E = 1.68 - 0.0985 pH ... (3) respectively. The qualitative and quantitative reproducibility of the cyclic voltam- mogram of Ir02/Zr02 is excellent. Repeated potential cycling between ox- ygen and hydrogen evolution does not affect the shape of the curve. But in 1M H2SO4 solution, only two peaks are observed at 0 . d ~  (NHE) (Fig.2) and it may be attributed to Ir02/lr203 redox couples (Eq.1). Fig.2: Cyclic voltammogram in 1 M H2S04 solutions for 10 mole % IrO, and 90 mole% Zr02 on titanium sustrate. Sweep rate:50 mV sac- l ,  Temp:30°C Overvoltane behaviour - Figure 3 shows the overvoltage variation with Ir02 content at constant cur- rent density of 10 m A . ~ m - ~  in 1M H2SO4 and 6M NaOH respectively. Fig. 3 : Overvoltage at constant current density of 10 mA cm-2 vs lr02 in 1M H2SO4 and 6M NaOH respectively It is noted in both cases that electrodes containing 5 mole % of Ir02 has maximum overvoltage, and it goes on decreasing up to 35 mole %; afterwards the overvoltage was constant over a wide range up to I00 mole % of Ir02 . This type of observation is noticed in acid as well as in alkaline solutions. Kinetics and reaction mechanism for oxygen evolution Typical Tafel curves for electrodes of 30 mole % of Ir02 obtained galvanostatically in 1M H2SO4 and 6M NaOH respectively were well reproduced and their anodic Tafel slopes were 60 mV and 40 mV repsec- tively (Fig. 4). i ,~.cm-' Fig. 4 : Typical Tafel curves for electrode of 30 mole % of lr02 and 70 mole % of Zr02 in 1M H2S04 and 6M NaOH respectively, Temp: 30°C !562 Bulletin of Electrochemistry Vo1.2(6) Nov-Dec 1986 Subrammian and Guruviah - Electrochemical behaviour of titanium supported lr02/Zr02 electrodes The Tafel value obtained for electrodes of 5 mde  T o  of IrOz is greater than 120 mV. The Tafel slopes obtained for other electrodes of IrOz/ZrOz are found to  be the same within an error of a few millivolts per decade both in acid and alkaline solutions. From the current - potential curves obtained galvanostatically in acid solution of different pH, the anodic potentials at  given current densities are plotted against activity of H +  ions (Fig. 5) by using the activity data for H2S04 aqueous solution and a slope [ e ) i  of about 4.06V was obtained. The dependence of current density at  the given potential on activity of H+ ions is shown in Fig. 5, and the order of reaction ( 6 logi/ 6 log a,, + bi with respect to  H +  ions is found to be -1. Figure 5 : Activity (acr + dependence of anodic potential a t  const. cur- rent density and of current densitil at  constant potential for 30 mole % Id2 and 70 mole % Zr02 electrode in H2SO4 solutions, Temp. 300C The same type of behaviour was observed in all electrodes containing more than 10 mole % of IrO,. Table I: Electrochemical Parameters of lr02 Electrodes Solution Parameters Ideal value Observed value Acidic Anodic Tafel slope 0.06 V 0.059 V ( a E / s l ~ g a ~ + ) ~  + 0 . 0 6 ~  + 0.063 v (6 log i /b log aH)E - 1.0 - 1.20 Alkaline Anodic Tafel slope 0.040 V 0.041 V ( b ~ / s l o g % , - ) ~  - 0.040V - 0.040 V ( '&El6  log kH-), 1.0 O.% Figure 6 : Act~ i ty  (aoH- dependence of anodic potential at  constant cur- rent density and, of current density at constant anodic potential for 30 mole % Id2 and 70 mob % Zr02 electrode, Temp. 30°C The typical electrochemical parameters both in acid and alkaline soh- tions for the 50 mole rlo 1- : 50 mole rlo ZrOz electrodes are summark- ed in Table I. On the basis of experimental parameters obtained, the theoretical rate ex pression for oxygen evolution in acid solution may be presented in a morc convenient way as Bulletin of Electrochemistry,Vol.2(6), Nov-Dec 1986 563 Subramanian and Guruviah - Electrochemical behaviour of titanium supported lrO2/2rO2 electrodes E = Constant + (RT/F) In i + (RT/F) In a .+ ......... (4) The reaction mechanism may be deduced from tke observed parameters under Langmuirian conditions of adsorption [12,13] The suggested mechanism ~14Jinvolving surface site 'S' is: *; S + HzO -,-I: SOH + H +  + e-  (5) SOH k" S O  + H + (6) SO *SO + e-  (7) 2S0 * 2s + 0, (8) The second step given in Eq. 5 is rate determining, and the overall cur- rent density is given as : i = n~ . + ) ,, , . . . ' 9 )  k -  I This is in good agleement with data obtained. The same observation was made on pure IrOz 115 1. The mechanism for oxygen evolution in alkaline solution may be deter- mined as above. From i-E relationships in alkaline solution, the plots of log i vs 1013 %H- and E vs log %H- (Fig.6) can be obtained and respective slope values, ( d lo& S log %H.)E (equal to about 1) and ( s E/ S log ao, )i (equal to about -0.04V) are obtained. Based on thc experimental parameters, the theoretical rate expression may be written as : E = Constant + 2 x 2. 3 R T ~ O ~  i - log a ...( 10) 3F Under langmuirian conditions, the following mechanism has been pro- posed El63 S Z + O H  -% S O H Z + ~ -  ...( I I) SOH' + O H  (SOH)Z+ + e-  .. .(12) SOHz+' + OH- + SZ-OH-OH- SOHI+ 1 + OH- f Sz-OH-OH- -3 2SZ + 0, + 2H20 ...( 13) The second step (Eq.12) is assumed to be the rate-determining step and the overall current density is given as i = nF k~ k ~ i  (I + 0) EF .a, - exp -k - ~  , RT This agrees well with Eq.10 by assuming '0' is equal to !h. Accelerated service life From the plot of service life versus mole % of IrD2 content (Fig.7) it is found that the service life increases up to 35 mole Qo of Ir02, beyond which the service life becomes constant up to 80 mole % of Ir02 content and decreases thereafter. The 5 mole % Ir02 electrode showed a very high overvoltage from the beginning and it failed within 10 hours of iesting. But the maximum service life of about 500 hours was obtained for elec- trodes of 35 mole % to 80 mole % of Ir02 content and only 300 hours was obtained for pure Ir02 electrode. The above data and test give an idea that the Ir02 can be well stabilis- ed by addition of Zr02 if the Ir02 content is more than 35 mole % and less than 80 mole To. And above 80 mole To, the electrode could not get the required mechanical stability and a low service life of 300 hours was achieved; but below 35 mole %, the required amount of catalyst might not be available, and hence the life is insignificant. It was found that iridium got deposited on the cathode during prolong- ed electrolysis. The presence of Ir was qualitatively identified with the help of electron probe microanalyser. In NaOH solution, the I r02 becomes IrOj-and got deposited at the cathode; hence it may be assumed that during prolonged operation in 6M NaOH solution deterioration of I r02 takes place r l q  to form 110- as given below IrO, + 2 0 H -  IrOj- + 2 H +  + 2e ...( 14) (solution) IrOj- + 4H20 + 6e- + Ir + 8 OH- ... (15) (cathode)  Fig.7 : Variation of service life in hours with mole % of Ir02 content Fig.8 : Cyclic voltammogram for 20 mole % lr02 and 80 mole % Zr02 elec- trode in 0.05M Kg FeKN)6 in 1M KNOB solutions, sweep rate = 50 mV/sec, Temp. 30°C 564 Bulletin of Electrochemistry Vo1.2(6) Nov-Dec 1986 ~ubramanian and Guruviah - Electrochemical behaviour of 'titanium supported lrOz/Zr02 electrodes Ferri/ferrocyaalde redox studies Figure 8 shows a typical cyclic voltammogram on I r02 electrode in 0.05 M K3 Fe (CN)6 in 1M KNO, solutions. A value of Ep = 60 mV was ob- tained for one electron chargotransfer reaction of ferri-ferrocyanide system. The formal reduction potential was obtained as 0.2 V vs SCE. The same type of cyclic voltammograms and Ep = 60 mV were obtain- ed in almost all electrodes containing more than 30 mole % of Ir02. The same behaviour was observed in 0.05 K3 Fe(CN)6 in 1M NaOH solution having En = 60 mV and at the sanle (0.2 V vs SCE) formal reduc- tion potential-(~6. 9). r Fig.9 : Cyclic voltammogram for 20 mole % I r02  and $U mole % ZrOz elec- trode in 0.05 M K3Fe(CNI6 in 1M NaOH solutions, sweep rate = 50 mvlsec, Temp. 30°C E ,VwSCE Fig.10 : Cyclic voltammogram for 20 mole % lrOz and 80 mole % Zr02 electrode in 0.05 M K3Fe(CNj6 in IM HZSo4 solutions, Sweep rate = 50 mvlsec, Temp. 30°C. In O.05M K3Fe(CN)6 in 1M H2SO4 solutions (Fig.10) Ep value was about 60 mV, but the formal reduction potential was shifted to  more than some 150 mV as the ferricyanide/ ferrocyanide complex is not stable in acid medium and it represents purely a Fe3+/Fe2+ system only p 1 SUMMARY (i) The cyclic voltammetric behaviour of Ir02/Zr02 electrodes are similar to  that of pure I r02 electrodes and four peaks are observed, which cor- respond to 11-0, to  Ir203; I r02 to  I r O a - ; l a -  to Ir203 and Ir203 to Ir02 reactions.. (ii) The reaction mechanlsrns for oxygen evolution are similar to that of pure IrOz electrodes in acid solutions, but the kinetic parameters and reac- tion mechanism observed in this investigation greatly deviates from earlier workers [13] in alkaline solutions in which a reaction order of 2 was obtained with respect to  OH- ion activity ( lM to  2M KOH). In the present studies (3M to  6M NaOH) the order of reaction is 1 and the reaction is followed by a different mechanism [14 1. (iii) The service life of IrO, (35 moIe qo) electrodes can be .veil stabilised by the addition of Zr02 and life can be enhanced. During prolonged operation in alkaline media, deterioration of IrO, to soluble IO4-  OC- curs with possible deposition at  the cathode as Ir or Ir02. (iv) The ferri/ferrocyanide redox reaction was reversile even at  the elec- trode of 10 mole alo I r02 stabiliaed with Zr02 in neutral as well as in alkaline solutions. REFERENCES 1. R S Y e ,  J Orehotsky, W Visscher and S Srinivasan, J Electrochem Soc, 128 (1981) 1900 2. S Trasatti and G Buzzaca, J Electroanal Chem, 29 (1971) Appendix-1 3. D V Kokoulina, Y I Krasovitskya and T V Ivanova, Electrokhimiya, 14 (1978) 267 4. M M Jacksic, A K Despic, I M Csonka and B Z Nikolic, Jl?lectrochem Soc, 116 (1%9) 1316 5. F Hine, M Yasuda, T.Nada. 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Electrochemical behaviour of titanium supported Ir02/Zr02 electrodes in acid and alkaline solutions and in ferricyanide/ferrocyanide redox systems

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