UDC 543.272.1 : 541.136.8 : 546.21: 669 .14 A Survey on Industrial Applications of Oxygen Concentration Cells with Solid Electrolytes* By Kaz uhiro S . GOTO** 1. A H istory of Investigations on Oxygen Con­ Then, h e tried to determine rapidly the oxygen centration Cells at Elevated Temperatures concentration in molten steels from the electromotive The oxygen concentration cell can b e defined as force of the following cell; 7) a type of galvanic cells, whose electromotive force is Si02 Slag generated by the difference in the chemical potential Liquid I f + 'd Ag, H 2 or O 2 gas Fe- O A rom aCl open (reference elect'd.) of oxygen, or in other words, in the oxygen concen­ - , g hearth furnace tration, at the two electrodes under isothermal and ...... ..... ... .......... (3) isobaric conditions. As the electrolyte, used are the solid or liquid oxides Figure 1 shows the structure of this cell and Fig. 2 with predominant ionic conductivity a t elevated tem­ the relation between the electromotive force and the peratures. This category of galvanic cells can be ex­ oxygen content in steels determined by the conven­ pressed by the following generic cell formula ; tional vacuum fusion method. A similal' experiment was made by Sanbongi and ,:) , Solid or liquid I °2' 'd ' h P ~ .; , Ohtani supported by Fuji Iron a nd Steel Corp., using Electrode I OXI es WIt Eh:: ctrode 11 IOmc conduction solid magnesia as the electrolyteS) in 1958 . In J apan, these two investigations played a n im­ ........ .... .. .. .. ... ... ( I ) pOl·tant role in the development of the oxygen concen­ In 1908, H aber' ) m easured the electromotive force tration cel ls for the rapid oxygen a nalys is of molten of this type of cell, using Thuringer hard glass as the steels. The cells are now widely used in Japanese electrolyte. Eight years la ter, Treadwell2) tried to steel plants as discribed in the following section. All use fused borates, silicates, or so lid porcelain as the electrolyte and to control the oxygen pressure at the 1-1 2(- 1-1 20) gas electrodes by using the two phase mixtures of Fe and t FeO , or Cu20 and C uO. A similar study was in­ dependently reported by Baur, et al,3) After the extensive study by Treadwell,2) the oxygen concentration cells have been investigated in many 4 coun tries. Among them , Esin ) in Russia has con­ structed various types of the oxygen concentration cells. In 1957, Sanbongi, Ohtani a nd Omori5 ) reported Fe-O 550°C a review including all reports after Treadwell, and melt o hence the details will not be repeated here, except for o o the industrial applications. The first industrial applica tion of the oxygen con­ l centration cells was reported by Ona ka ) in 1944. To control the refining operation of steels, he tried to determine the oxidizing power of the slag of a n acid open hearth furnace from the electromotive force of the cell: Pt (1), Slag I Si02 Slag 1 Si0 2 Slag II Pt (II), o o Po, tube tube P ~ , (f) (f) ............. ........... (2) where the electrolyte was composed of solid silica containing iron oxide absorbed from slag 1. EMF Onaka showed that the electromotive force is ex­ Fig. I. Schematic diagram of galvanic cell to determine clusively generated by the difference in the oxygen oxygen pressure in liquid iron used by Onaka." potential of the two slags or in the ratio of Fe2 +- to The electrolyte is made of silica containing iron Fe3+ -content. oxide. * Originally published in Tetsu-to-Hagane, 62 ( 1976), 1265, inJapanese. English version received J anuar y 14, 1976. ** Department of M etallurgy, Tokyo J ns titute of Technology, O okayama, M egul'o-ku , Tokyo 152. Rev iew C469 1 ( 470 ) Transactions ISIJ, Vol. 16, 197 6 --I tions of the oxygen con centration cells and thus the present report has been m ade to supplement them. 160 /' The scope of the present survey is confined to the • following four types of applications; (l ) present state 140 of commercial oxygen concentration cells designed for 120 the a nalysis of waste gas from boilers and various in­ / dustrial furnaces, (2) d etermina tion of total oxygen 100 demands of the waste water from industrial plants .1· a nd NOx control of the exhaust gas of gasoline • 80 •• engines, (3) rapid oxygen analysis in copper a nd steels :--- at their refin eries, a nd (4) continuous oxygen analysis E 60 • in liq uid sodium for fast bleeder reactors. • 40 • • II. The Conduction M echanism in the Oxide 20 •• Electrolytes and the Electro-motive Force ./. 1. For the Case oj the Oxide Electrolytes oj Uniform Com­ O~~ __~ __~ __L-~L--L __~ o 100 200 300 400 500 600 700 position %QX 10' ----> The cell can be described as follows. Fig. 2. R elation between the EMF of the ce ll of Fig. I An oxide, and the oxygen content determined by the conven­ P ~ o' f . C P:;" E lectrode I MO, 0 UTIllorm Electrode II ... (4) tional vacuum fus ion method') composition The charge carriers in the oxide, MO, would be the investigations cited above are quite unique, but 2 M2 +, 0 - , electrons or positive holes. Using the the fo llowing reasons limited their wider applications. assumption of the local equilibrium in the electrolyte, ( I ) The electric cond uction mechanism in the Wagner9l obtained the Gibbs fr ee energy change of electrolytes was not cl early known. Virtual cell re­ the thin slice of the electrolyte, when the infinitesimal actions were not definite accordingly. current was supplied to the cell under isobaric con­ (2) Because solid oxides with large ionic conduc­ ditions. Then, this infinitesimal Gibbs free energy tivity were not yet found, the internal resistance of cha nge was integrated from the leftmost to the reight­ the cells was too la rge to have a reproducible electro­ most side of the electrolyte to obtain the total free motive force. energy cha nge of the virtual cell reaction. Using the (3) When liquid oxides are u sed instead of solid relation, J G = - nFE, one gets the fo llowing expression ones, the interna l resistance of the cell can be reduced, for the electromotive force: but in many cases, th e liquid electrolytes react with the electrode m aterials. Therefore, the stability of RT P" E = 4F f p~ ' (t~t , .+ t u '- ) d l nPo , the electromotive force was not acceptable. 0 , These difficulties could be covered to some extent RT f a~; by the a rt of the construction of the galvanic cells and = - , (tM " + to,-)d In aM 2F aM the careful measurements of the electromotive forces. RT aM However, the difficultes limited the applications of = - f (l - te)dlnaM ··· .... ....... .... (5) the oxygen concentration cells only to specific experi­ 2F aM m ents. where, E: the electromotive force 9 To solve these theoretical problems, C. Wagner ,lOl F: the Faraday constant reported two theoretical works in 1933 and 1966, Pb, and P(f, : the oxygen pressures at the two which strictly related the conduction mechanism of electrodes the oxide electrolytes to the electromotive force at a:u and a~: the chemical activities of the elevated temperatures. metal at the two electrodes Furthermore, Kiukkola and Wagnerlll discovered t i : means the transference number of the that zirconia or thoria base solid solutions can be used charge carriers as the electrolyte to have very stable and reproducible For zi rconia base solid electrolytes , the transference electromotive forces, simply b ecause of their very number of oxygen is known to be unity at a higher large oxygen ion conductivity a bove 500°C. oxygen pressure, a nd thus all other transference num­ Tha nks to this celebrated publication, high tem­ bers become zero . Therefore, Eq. (5) gives simply perature electrochemistry has been extensively d e­ veloped a ll over the world in the 1960's. E = RT I P :;, Two review papers have been published on this ..... ....... ...... ... (6) 4F n P'0 , development in 1970 by R app and Shores 12l and in 1972 by Goto and Pluschkell 13l documenting 242 a nd At a n oxygen pressure lower th an tha t for chromium­ 295 publications, respectively. However, these re­ chromium oxide equilibrium, the transference num­ vi ew papers did not deal with the industrial a pplica- ber of electrons can not be neglected and is given as a R eview Transac tions ISIJ, Vol. 16, 1976 ( 471 ) function of oxygen press ure. In this case, Eq. (5) even at lower tempera tures. H owever, its use is very can be integrated to yield Eg . (12 ) in the following limited, because the second term can not be neglected section. d ue to ea y vaporization of a lkali metal oxides .16 ) 2. For the Case oj the B inary Oxide E lectrolytes of Non­ III. Industrial Production oj Zirconia Base Solid uniform Composition Electrolytes and Thier Impurity Contents Using AO- B02 as the binary oxides, the cell can In la boratory zirconia - lime solid solution can be be described by made from the aqueous solution of zirconium nitra te, (ZrO)(N0 3)2 a nd calcium carbonate, CaC0 3. Slow­ AO- B02 of El d II P~ " non-uniform ectro e , .. (7) ly drying the solution, the compounds deposit in very E lectrode I composition P;;, fin e powders. The decomposition of the nitra te a nd carbonate a t 700 0 to 800 0 e gives the mixture of zir­ where Po, denotes the chemical potential of oxygen , conia a nd lime.