8 High-Temperature Oxygen Sensors Based on Electrochemical Oxygen Pumping E. M. Logothetis and R. E. Hetrick Downloaded by PENNSYLVANIA STATE UNIV on May 21, 2013 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch008
Scientific Research Laboratories, Ford Motor Company, Dearborn, MI 48121-2053
This paper reviews the area of high temperature oxygen sensors based on oxygen pumping. After an initial discussion of the principle of oxygen pumping, the various sensor designs developed over the years are described and their properties are analyzed and compared in detail. The main characteristic of all these sensors is that their signal output is linearly proportional to the oxygen partial pressure. Their other characteristics, however, depend on the specifics of the sensor design. It is shown that these sensors have several advantages over other oxygen sensors such as the Nernst concentration cell and the metal oxide resistive sensors including higher sensitivity to oxygen and weaker dependence on temperature. H i g h temperature oxygen s e n s o r s a r e f i n d i n g a n i n c r e a s i n g use i n a v a r i e t y o f a p p l i c a t i o n s i n c l u d i n g m o n i t o r i n g and c o n t r o l o f i n d u s t r i a l p r o c e s s e s ( 1 ) and o f i n t e r n a l combustion engine o p e r a t i o n ( 2 ) . There a r e two t y p e s o f h i g h temperature s o l i d s t a t e d e v i c e s w h i c h have been d e v e l o p e d and a l r e a d y e x t e n s i v e l y u s e d f o r O2 s e n s i n g . One type i s an e l e c t r o c h e m i c a l N e r n s t oxygen c o n c e n t r a t i o n cell b a s e d on Zr02 s o l i d e l e c t r o l y t e ( 3 - 4 ) w h i c h g e n e r a t e s a v o l t a g e s i g n a l g i v e n b y EMF-(RT/4F)ln(P /Pn R ) 1 where F and R a r e r e s p e c t i v e l y t h e Faraday and i d e a l gas c o n s t a n t s , T i s the a b s o l u t e temperature and P and P a r e the oxygen p a r t i a l p r e s s u r e s i n the unknown and i n a r e f e r e n c e atmospheres. Sensors o f the second type(5) c o n s i s t o f a m e t a l o x i d e element such as T1O2 h a v i n g a r e s i s t a n c e t h a t depends on P a c c o r d i n g t o the r e l a t i o n s h i p R - R Q P Q - , where RQ depends on m a t e r i a l p r o p e r t i e s and i n c l u d e s a n e x p o n e n t i a l v a r i a t i o n w i t h t e m p e r a t u r e , w h i l e v a l u e s o f n a r e u s u a l l y i n the range 1/4 t o 1/6. A l t h o u g h t h e s e s e n s o r s have been s u c c e s s f u l l y used f o r a u t o m o t i v e engine c o n t r o l as w e l l as f o r o t h e r a p p l i c a t i o n s , t h e i r r e l a t i v e l y l o w s e n s i t i v i t y t o oxygen ( a s gauged b y t h e i r l o g a r i t h m i c o r f r a c t i o n a l power dependence on P ) r e s t r i c t s t h e i r use t o c a s e s where P shows l a r g e changes (e.g. d e t e c t i o n o f s t o i c h i o m e t r i c c o m b u s t i b l e gas m i x t u r e s ) o r t o cases where parameters a f f e c t i n g 0
0
0R
0
0
0
0097-6156/ 86/ 0309-0136506.00/ 0 © 1986 American Chemical Society
In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Downloaded by PENNSYLVANIA STATE UNIV on May 21, 2013 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch008
8.
LOGOTHETIS AND HETRICK
High- Temperature Oxygen Sensors
137
device operation such as temperature and pressure are w e l l - c o n t r o l l e d (e.g. i n sampling systems). Another type of high temperature s o l i d state O2 sensor that has been developed i s based on the p r i n c i p l e of electrochemical pumping of oxygen with ZrC>2 e l e c t r o l y t e s . These sensors have higher sensit i v i t y (generally, a f i r s t power dependence on P Q ) than the Nernst c e l l and the r e s i s t i v e device and possess a number of other charact e r i s t i c s that make them very promising f o r many new applications. In this paper, we review the various types of sensors based on O2 pumping and discuss and compare t h e i r c h a r a c t e r i s t i c s . Since the commercialization of these devices i s j u s t beginning, some of the sensor performance c h a r a c t e r i s t i c s (e.g. accuracy, r e p r o d u c i b i l i t y and d u r a b i l i t y ) are not yet known and, consequently, w i l l not be included i n this discussion. I t i s pointed out that although our discussion i s l i m i t e d to O2-sensing with oxygen-ion conducting s o l i d e l e c t r o l y t e s , most of the concepts and sensor designs discussed here are equally applicable to sensing other ions using the corresponding ion-conducting s o l i d e l e c t r o l y t e s . Oxygen Pumping Consider a slab of an oxygen-ion-conducting material such as yttriumdoped Zr02 with platinum electrodes on both sides separating two regions with d i f f e r e n t oxygen concentration ( F i g . l ) . This difference i n the oxygen chemical p o t e n t i a l w i l l drive oxygen from the high oxygen concentration region to the low concentration region through the Zr02 e l e c t r o l y t e . At the lower oxygen p a r t i a l pressure (P^) side, two oxygen ions combine to give an oxygen molecule to the gas phase leaving four electrons on the Pt electrode: 2 0
=
(Zr0 )
> 0
2
(gas) +
2
4 e" (Pt)
(1)
The oxygen l o s t from the Zr02 material by reaction (1) i s recovered by the reverse reaction occurring at the gas/Pt/ Zr02 interface i n the higher oxygen p a r t i a l pressure (P2) side. The net r e s u l t of these processes i s the transfer of one oxygen molecule from the high to the low P side and of four electrons from electrode 2 to electrode 1. As a r e s u l t of this electron transfer, an e l e c t r i c f i e l d develops within the Zr02 and exerts an opposing force on the oxygen ions i n the e l e c t r o l y t e . At equilibrium, the net current through the Zr02 material i s zero and the open-circuit EMF developed between the two Pt electrodes i s given by the well-known Nernst equation(4). 0
EMF - (RT/4F)ln(P /P ) 1
2
(2)
where R, F and T have been previously defined. I f a load r e s i s t o r R L i s connected across the Zr02 electrochemical c e l l of F i g . l , a current w i l l continuously pass through the closed c i r c u i t given by I = EMF/(R + R ) L
t
(3)
where R^ i s the internal impedance of the c e l l ; R^ i s the sum of the resistance of the Zr02 slab and the resistance of the Pt electrodes
In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
138
FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS
( i n c l u d i n g any e f f e c t i v e r e s i s t a n c e r e l a t i n g t o t h e t r a n s p o r t o f O2 through the e l e c t r o d e s ) . I n t h i s case, the e l e c t r o c h e m i c a l c e l l o p e r a t e s as a f u e l c e l l . Suppose, n e x t , t h a t an e x t e r n a l v o l t a g e V i s a p p l i e d a c r o s s t h e Zr02 c e l l ( F i g . 2 ) . The c u r r e n t , i n t h i s c a s e , i s g i v e n b y I - (V + E M F ) / ( R + R )
Downloaded by PENNSYLVANIA STATE UNIV on May 21, 2013 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch008
(4)
±
L
Depending on t h e r e l a t i v e magnitude and s i g n o f V and EMF, t h e c u r r e n t w i l l t r a n s f e r (pump) oxygen from t h e h i g h t o t h e l o w o r from the l o w t o t h e h i g h oxygen p a r t i a l p r e s s u r e s i d e s . I n t h i s c o n f i g u r a t i o n , t h e e l e c t r o c h e m i c a l c e l l o p e r a t e s as a n oxygen pump. E q u a t i o n (4) i s v a l i d under t h e a s s u m p t i o n t h a t t h e oxygen pumping a c t i o n does n o t change a p p r e c i a b l y t h e v a l u e s o f and P2 a t the two gas/Pt/Zr02 i n t e r f a c e s . T h i s , however, i s n o t u s u a l l y valid. I n f a c t , as i t w i l l be seen s h o r t l y , i n most oxygen s e n s i n g d e v i c e s , one p u r p o s e l y c o n f i g u r e s t h e s t r u c t u r e so t h a t a t l e a s t one o f t h e ?i and P2 changes a t t h e gas/Pt/Zr02 i n t e r f a c e . C o n s i d e r , f o r example, t h e s t r u c t u r e s i n F i g . 3 . I n Fig.3a, a porous l a y e r i s p l a c e d on t o p o f e l e c t r o d e 2 and a c t s as a b a r r i e r t o t h e d i f f u s i o n o f O2 from t h e b u l k o f t h e gas t o t h e P t e l e c t r o d e 2. I n F i g . 3 b , a s i m i l a r s i t u a t i o n i s obtained by i n t r o d u c i n g adjacent t o e l e c t r o d e 2 an e n c l o s e d volume v w h i c h communicates w i t h t h e b u l k o f t h e gas t h r o u g h a r e s t r i c t i o n , an a p e r t u r e C. I n the cases i l l u s t r a t e d i n F i g . 3 , t h e pumping a c t i o n c a n change t h e oxygen p a r t i a l p r e s s u r e a t e l e c t r o d e 2 from t h e b u l k v a l u e P^ t o a l o w e r v a l u e P^ . The d i f f e r e n c e between P^ and P^ w i l l i n d u c e a d i f f u s i o n a l f l u x G o f O2 from t h e b u l k o f t h e gas t o t h e gas/Pt/Zr02 i n t e r f a c e : G =
