- Email: [email protected]
Reference electrode placement and seals in electrochemical oxygen generators
Solid State Ionics 134 (2000) 35–42 www.elsevier.com / locate / ssi
Reference electrode placement and seals in electrochemical oxygen generators c, a a a b S.B. Adler *, B.T. Henderson , M.A. Wilson , D.M. Taylor , R.E. Richards a
Ceramatec, 2425 S. 900 W., Salt Lake City, UT 84119, USA Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195, USA c Chemical Engineering Department, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA b
Abstract We report measurements and numerical calculations of the potential distribution within a thin solid electrolyte near active (current-bearing) electrodes. These studies demonstrate two principles: (1) In a flat-plate geometry, the electrolyte is approximately equipotential beyond a distance of about three electrolyte thicknesses from the edge of the active electrodes. (2) If one of the active electrodes on one side of the electrolyte extends beyond the other, it strongly biases the potential of the electrolyte far from the active region. We show that these effects make it challenging to measure electrode overpotential accurately on thin cells. However, we also show that these effects can be useful for protecting glass-ceramic seals in an oxygen generator stack against electrochemical degradation / delamination. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In aqueous electrochemical systems, it is well established that the location of the reference electrode can significantly affect the accuracy of electrode polarization measurements. Inaccuracy is particularly pronounced when the ohmic drop in the electrolyte is non-uniform [1], the electrode kinetics are linear [2], and the reference electrode must be placed far away from the active region of the working electrode [3]. These adverse criteria combine when making a.c. polarization measurements on thin solid electrolytes, the most common geometry for solid-oxide fuel cells or electrically driven O 2 separation membranes. Nagata et al. [4] have shown *Corresponding author. Fax: 11-216-368-3016. E-mail address: [email protected] (S.B. Adler).
that popular electrode geometries (including splitelectrode configurations), can result in large inaccuracies in measured electrode overvoltage. More recently, Winkler et al. [5] have quantified these effects by solving Laplace’s equation in a thin electrolyte for various electrode geometries and reference electrode locations. In our own studies of mixed-conducting oxygen electrodes, we have examined the potential distributions in thin electrolytes experimentally and theoretically using finite element calculations. Our observations corroborate the findings of Winkler et al. [5] and illustrate two additional principles: 1. In a flat-plate geometry, the inactive portion of the electrolyte becomes approximately equipotential beyond about three electrolyte thicknesses from the edge of the active region.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00711-6
36
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
2. If one of the active electrodes on one side of the electrolyte extends beyond the other, it strongly biases the potential of the inactive electrolyte. While these effects make it challenging to measure electrode kinetics accurately, we show that these effects can be useful for controlling the electrolyte potential along the perimeter of a cell. This technique can be used to protect glass– ceramic seals in an oxygen generator stack against electrochemical degradation / delamination [6].
2. Potential distributions in a flat cell with a thin electrolyte If an electrolyte has constant composition and conductivity, and operates below its dielectric response frequency, the electrical state will obey Laplace’s equation: 0 5= 2 F,
(1)
where F is the quasielectrostatic potential based on the majority charge carrier (i.e. oxygen ion vacancies in the case of an oxide-ion conductor). With appropriate boundary conditions, this equation can be solved for a specific geometry using analytical or numerical methods, and yields the current density: i 5 2 k=F,
(2)
which flows everywhere perpendicular to gradients in potential. Typical boundary conditions are: i ' 5 k n ' ?=F 50 at any insulating surface i ' 5 k n ' ?=F 52k(F 2 Felectrode ) at any electrode surface
(3) where for simplicity of this discussion we have assumed linear electrode kinetics. In the special case where k5infinity (such that F 5 Felectrode along any electrode surface), the solution yields the primary current distribution. If electrode k5finite, the solution yields a secondary current distribution. In order to better understand the potential distributions in a flat-plate oxygen generator cell, we solved this system of equations for a thin twodimensional electrolyte using the TOPAZ-2D finite element package [7] on a HP720 computer
Fig. 1. Calculated potential distributions in a thin electrolyte.
(Ceramatec). An example, Fig. 1a shows the numerical solution for the primary distribution in the active region of a thin symmetric cell. Since the geometry is one dimensional, the potential varies linearly with position across the electrolyte, as we would predict analytically. Near the edge of the electrodes, however, the potential profile becomes highly nonlinear. Fig. 1b shows a numerical calculation of the primary distribution in the same cell, near the edge of the active region, assuming electrodes are perfectly aligned. The potential along the gas-exposed surface of the electrolyte near the edges of the electrodes varies strongly with position, until a distance of about three electrolyte thickness. Beyond this, the potential is approximately uniform everywhere, with all equipotential surfaces referencing the same plane in the active region. This result holds even with finite electrode kinetics, provided the position is beyond the edge of both electrodes. This result indicates that any placement of a reference electrode beyond about three electrolyte thicknesses will measure the same equipotential surface in the electrolyte. Even if there is a PO 2 difference across the electrolyte (as in a fuel cell, for
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
example), the difference in voltage measured between reference electrodes located opposite each other on either side of the cell reflect a Nernst potential, not a potential gradient across the electrolyte. Assuming this PO 2 difference is known, then reference electrodes on the anode and cathode sides provide redundant information. In general one prefers to put reference electrodes in this outer region since it avoids potential gradients, and makes the measurement insensitive to exact location of the reference electrode. Fig. 1c shows the primary distribution assuming one electrode terminates, and the other overhangs to infinity. Again we see a region of uniform potential beyond about three electrolyte thicknesses from the edge of the terminating electrode. However, since the electrolyte in this region is in contact with the overhanging electrode, it shares the same potential. If we were to place a reference electrode in this region, we would measure (erroneously) that the overpotential of the overhanging electrode is zero. While the situation becomes more complex with finite electrode kinetics and finite overhang, the basic conclusion is the same: enormous errors in measurement can result when the current distribution is asymmetric. In order to determine experimentally the significance of this overhang effect, we made and tested cells with overhanging electrodes, as shown in Fig. 2a. These cells were similar to the symmetric cells described in reference [8], but were fabricated with special undersized electrodes of constant overhang distance d. The reference electrode was placed in the equipotential region at least three electrolyte thicknesses away from the edge of the overhanging electrode. Results (Fig. 2b) show that with little or no overhang (d , L / 2), the potential difference between the active and reference electrodes is approximately half the cell potential, which is expected for a symmetric cell under conditions of linear polarization. Beyond d 5 L, the potential difference decreases dramatically as overhang is increased, and yields values consistent with our simulations of the secondary current distribution. Finally beyond d 5 4L, where simulations predict a potential difference of zero, the measured value levels-off at a minimum of about 2 mV (a small Nernst potential of 2–3 mV
37
Fig. 2. Reference potential vs. electrode overhang.
remains due to gas-phase polarization of the active electrode).
3. Effects of potential distributions on a.c. impedance measurements Figs. 1c and 2b show that a significant overhang of one electrode beyond the other on a thin electrolyte will tend to bias the reference potential toward the overhanging electrode. Studies by Winkler et al. [5] of secondary potential distributions in thin cells show that this effect can create large errors in measurement of the electrode overpotential. They also show that when the electrolyte is very thin, and the kinetics at the two electrodes are different, large errors occur even when the electrode alignment is exact. Anecdotal observations in our own studies of mixed-conducting oxygen electrodes [8] appear to support these conclusions. As shown in Fig. 3a, one
38
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
Fig. 3. Effects of electrolyte thickness on a.c. impedence.
of our standard test configurations is a thin symmetric cell fabricated with identical electrodes, printed using the same process as a full-scale cell. We usually measure the combined impedance of both electrodes by measuring the total cell impedance at zero bias and subtracting the total ohmic (v 5 infinity) contribution of the electrolyte. In principle, we can also determine the impedance of each individual electrode by measuring impedance between electrode and reference electrode. As sketched qualitatively in Fig. 3b, we normally find the combined electrode impedance of the cell to be independent of electrolyte thickness. Also, when printed on a thick electrolyte, both electrodes normally have equal impedance as separated using the reference electrode. However, with a very thin electrolyte, we often observe differences of up to 200% in electrode resistance between the two electrodes, even when the electrodes are identical and have been made in the same manner as electrodes on a thick electrolyte. Our explanation for this behavior is shown in Fig. 4. In the process of printing the electrodes, it is possible for the anode and cathode to be slightly
Fig. 4. Why reference potential drifts with frequency. (a) Thick electrolyte — small reference drift (b) Thin electrolyte — large reference drift.
misaligned. For a thick electrolyte (Fig. 4a), any misalignment is relatively small compared to the electrolyte thickness, so the difference between the primary potential distribution (at v 5infinity) and the secondary potential distribution (at v 50) near the two electrodes is similar. The location of the equipotential surface measured by the reference electrode therefore stays approximately fixed as a function of frequency, and the fraction of the electrolyte resistance apportioned to each electrode overvoltage remains roughly constant. However, for a thin electrolyte (Fig. 4b), the same misalignment can be a large fraction (or multiple) of the electrolyte thickness. Thus in analogy to Fig. 1c, we expect a primary potential distribution in which the reference potential lies very close to the working electrode at v 5infinity. As the frequency sweeps to zero, and we move from a primary to a secondary distribution, the potential distribution across the overhanging portion of the electrode become significant, and thus the reference electrode no longer
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
measures an equipotential surface in the active region of the electrolyte. A significant part of the electrode overvoltage is misapportioned to the other electrode, resulting in an apparent difference in electrode overpotential between anode and cathode. We have found these effects to be unavoidable on thin cells, and thus we only use thick cells when attempting to isolate the impedance of a single working electrode.
4. Use of offset electrodes to protect gas-tight seals in an oxygen generator stack Although misalignment of electrodes on a thin electrolyte can wreak havoc on electrochemical measurements, it can also be useful for controlling potential in the inactive portion of an electrolyte. As described in a recent patent [6], we have used this principle as a method to protect gas-tight seals in an oxygen generator stack against electrochemical degradation / delamination. Fig. 5 shows a schematic of an oxygen generator stack used by Ceramatec and Air Products and
39
Chemicals to test the durability of glass–ceramic sealants. The stack consists of thin circular cells of 10% strontia-doped ceria (SCO), with La 12x Sr x CoO 32d (LSCO) electrodes. These cells are connected electrically in series with La 12x Srx MnO 3 (LSM) interconnects, which also serve to collect and channel O 2 through an internal manifold that runs the length of the stack. A critical part of this design is a thin layer of lithium aluminosilicate glass– ceramic (LAS) which serves as a gas-tight sealant between the lip of the O 2 manifold cavity of the interconnect and the edge of the SCO cell. Even a tiny leak through this seal at any one of the cells compromises the pressure integrity and O 2 purity of the entire stack. Prior to making and testing stacks, LAS had proven durable for years in sealing tubes made from LSM and ceria. When we first began making stacks, each cell was made with a coextensive anode and cathode. Although the stacks would appear well-bonded after fabrication, the LAS seal bonds would delaminate and the stacks would fall apart within 3 weeks of operation! We noticed that failure always occurred at the same interface: between the LAS sealant and the
Fig. 5. Anatomy of an oxygen generator.
40
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
ceria on the anode side of the cells, never on the cathode side. Further tests proved that electrochemical operation was required to cause failure. In order to better understand this effect, we began a series of experiments called ‘pellet tests’, intended to measure directly the durability of the seal against applied potential / current. As illustrated in Fig. 6a, a pellet is made from the same materials as a stack, and consists of an LSM disk bonded with LAS glass to a slightly larger disk of ceria (electroded on one side only). This pellet is put under the same operating conditions as a stack, with a positive voltage difference applied between the LSM and the electrode. In all cases the measured current is very small (mA), so the pellet must be checked manually for delamination after a fixed period of time. Fig. 6b shows the results. At small applied voltages (,25 mV), delamination does not occur even after relatively long period of time (3300 h). However, at applied voltages .50 mV, delamination occurs in less than 10 days. At an applied voltage of
700 mV, a single-lap shear version of this test (where we time the delamination using a weight hung from the pellet), delamination occurs in less than 60 s! At negative applied voltages, delamination never occurs. From these experiments, we began to suspect the failure mechanism illustrated in Fig. 7. In a standard stack cell, the anode and cathode are coextensive at the edge of the electrolyte plate near the anode-side seal. If the current through the seal is small, we expect (based on Fig. 2) that the edge of the electrolyte plate will be equipotential at about half the cell potential (2350 mV) relative to the LSM interconnect on the other side of the glass bondline. Since the glass itself is a semiconductor, at small currents we also expect the glass itself to be equipotential at about zero (0 mV) relative to the LSM. Thus nearly the entire voltage difference across the seal is a driving force for O 2 formation at the glass–ceria interface via 2O 22 4→e2 1O 2 , with an equilibrium driving force of: RT ] ln PO 2 5 FLSM 2 FCSO 4F
(4)
At 7508C, a voltage difference of 350 mV translates
Fig. 6. Pellet delamination test.
Fig. 7. Theory for anode seal delamination.
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
to an equilibrium O 2 partial pressure of 10 7 atmospheres! This situation is not unlike the internal pressures that can develop at the interface between zirconia and ceria due to a discontinuity in electrical conductivity [9]. Since pressure is exponential with potential, we expect a steep increase in failure rate with potential drop across the seal interface. Based on these observations and ideas, we developed a resolution to the seal delamination problem illustrated in Fig. 8a. The principle of the method is to make cells with a cathode that is intentionally undersized by an overhang distance d relative to the anode. Thus during operation, the potential of the electrolyte at the edge of the cell is biased toward the potential of the anode-side interconnect. If the overhang is sufficient, the ceria and LSM will be nearly equipotential, such that there is no driving force for O 2 formation at the seal
41
interface. Based on Fig. 2, we redesigned the anode to have an overhang distance d of about ten times the electrolyte thickness. We built and tested stacks as shown in Fig. 5 both with and without overhang, with special reference electrodes located on the electrolyte across from the seal so that we could measure directly the actual potential the seal is subjected to (as in the pellet tests). In order to quantify stack leakage, we used a mass flow controller to measure O 2 flow-rate with a backpressure of O 2 on the stack. Any leak is detectable as a loss of flow efficiency, defined as O 2 flow relative to the rate of O 2 formation (determined by the current). The results of these experiments are shown in Fig. 8b. At a backpressure of 1.5 psig, a stack with coextensive anode and cathode begins to leak within a few hours, and this leak gets worse with time. After 3 weeks flow efficiency drops below 80%, and obvious seal delamination can be seen on postmortem analysis of the stack. On the other hand, a stack with an overhanging anode maintains a flowefficiency of 1.0 for over 5000 h, with no signs of delamination. These results have proven reproducible in dozens of stacks under a wide range of conditions and design configurations.
5. Conclusions The observations and calculations reported in this paper demonstrate that potential distributions in a thin electrolyte are extremely sensitive to the details of electrode geometry. We show how this sensitivity can be used to control the electrolyte potential in the inactive region of an oxygen generator stack, thereby protecting glass–ceramic sealants against electrochemical degradation / delamination. We also show that these effects can make it a challenge to measure electrode overpotential accurately in situ. In order to minimize inaccuracies introduced by current-distribution effects, we advocate the following:
Fig. 8. Problem solution (a) overhanging anode configuration, (b) seal performance at 1.5 psig backpressure.
1. To avoid potential gradients along the electrolyte surface, it is best to place the reference electrode at least three electrolyte thicknesses away from any active electrode. 2. When working with identical electrodes at zero bias on thin electrolytes, it is more accurate to
42
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
measure combined anode / cathode electrode resistance (and divide by two), than to try to measure anode and cathode individually. 3. For studies of cells with a different anode and cathode (or electrodes at nonzero bias), it is not possible to obtain meaningful results with a thin electrolyte. We have had some success printing electrodes of interest on a special ‘extra-thick’ electrolyte, where electrode geometry can be better controlled and symmetry effects are less pronounced. This approach has the advantage of closely reproducing the same materials and fabrication procedures as a thin cell, but does not necessarily eliminate the problems inherent to slab geometry. Alternatively, Winkler et al. [5] report specific electrolyte geometries that largely eliminate ohmic error. This approach allows true separation of anode and cathode effects, but requires effort to insure that materials and fabrications procedures are relevant to a thin electrolyte.
Acknowledgements The authors wish to thank Ceramatec, Inc. and Air Products and Chemicals Inc. for permission to
publish this work. Special thanks go to Mike Zoetmulder and Dave Padgen for their assistance in building and testing prototype stacks and cells studied in this work. We also with to thank Joe Hartvigsen for helpful discussions.
References [1] J. Newman, J. Electrochem. Soc. 113 (1966) 1235. [2] W.H. Tiedemann, J.S. Newman, D.N. Bennion, J. Electrochem. Soc. 120 (1973) 256. [3] A.C. West, J.S. Newman, J. Electrochem. Soc. 136 (1973) 3755. [4] M. Nagata, Y. Itoh, H. Iwahara, Solid State Ionics 67 (1994) 215. [5] J. Winkler, P.V. Hendriksen, N. Bonanos, M. Mogensen, J. Electrochem. Soc. 145 (1998) 1184. [6] S. Adler, B.T. Henderson, R.E. Richards, D.M. Taylor, M.A. Wilson, U.S. Patent No. 5868918, 1999. [7] For current information contact Livermore Software Technology Corporation at www.lstc.com. [8] S. Adler, Solid State Ionics 111 (1998) 125. [9] A.V. Virkar, J. Electrochem. Soc. 138 (5) (1991) 1481.
Reference electrode placement and seals in electrochemical oxygen generators c, a a a b S.B. Adler *, B.T. Henderson , M.A. Wilson , D.M. Taylor , R.E. Richards a
Ceramatec, 2425 S. 900 W., Salt Lake City, UT 84119, USA Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195, USA c Chemical Engineering Department, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA b
Abstract We report measurements and numerical calculations of the potential distribution within a thin solid electrolyte near active (current-bearing) electrodes. These studies demonstrate two principles: (1) In a flat-plate geometry, the electrolyte is approximately equipotential beyond a distance of about three electrolyte thicknesses from the edge of the active electrodes. (2) If one of the active electrodes on one side of the electrolyte extends beyond the other, it strongly biases the potential of the electrolyte far from the active region. We show that these effects make it challenging to measure electrode overpotential accurately on thin cells. However, we also show that these effects can be useful for protecting glass-ceramic seals in an oxygen generator stack against electrochemical degradation / delamination. 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In aqueous electrochemical systems, it is well established that the location of the reference electrode can significantly affect the accuracy of electrode polarization measurements. Inaccuracy is particularly pronounced when the ohmic drop in the electrolyte is non-uniform [1], the electrode kinetics are linear [2], and the reference electrode must be placed far away from the active region of the working electrode [3]. These adverse criteria combine when making a.c. polarization measurements on thin solid electrolytes, the most common geometry for solid-oxide fuel cells or electrically driven O 2 separation membranes. Nagata et al. [4] have shown *Corresponding author. Fax: 11-216-368-3016. E-mail address: [email protected] (S.B. Adler).
that popular electrode geometries (including splitelectrode configurations), can result in large inaccuracies in measured electrode overvoltage. More recently, Winkler et al. [5] have quantified these effects by solving Laplace’s equation in a thin electrolyte for various electrode geometries and reference electrode locations. In our own studies of mixed-conducting oxygen electrodes, we have examined the potential distributions in thin electrolytes experimentally and theoretically using finite element calculations. Our observations corroborate the findings of Winkler et al. [5] and illustrate two additional principles: 1. In a flat-plate geometry, the inactive portion of the electrolyte becomes approximately equipotential beyond about three electrolyte thicknesses from the edge of the active region.
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00711-6
36
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
2. If one of the active electrodes on one side of the electrolyte extends beyond the other, it strongly biases the potential of the inactive electrolyte. While these effects make it challenging to measure electrode kinetics accurately, we show that these effects can be useful for controlling the electrolyte potential along the perimeter of a cell. This technique can be used to protect glass– ceramic seals in an oxygen generator stack against electrochemical degradation / delamination [6].
2. Potential distributions in a flat cell with a thin electrolyte If an electrolyte has constant composition and conductivity, and operates below its dielectric response frequency, the electrical state will obey Laplace’s equation: 0 5= 2 F,
(1)
where F is the quasielectrostatic potential based on the majority charge carrier (i.e. oxygen ion vacancies in the case of an oxide-ion conductor). With appropriate boundary conditions, this equation can be solved for a specific geometry using analytical or numerical methods, and yields the current density: i 5 2 k=F,
(2)
which flows everywhere perpendicular to gradients in potential. Typical boundary conditions are: i ' 5 k n ' ?=F 50 at any insulating surface i ' 5 k n ' ?=F 52k(F 2 Felectrode ) at any electrode surface
(3) where for simplicity of this discussion we have assumed linear electrode kinetics. In the special case where k5infinity (such that F 5 Felectrode along any electrode surface), the solution yields the primary current distribution. If electrode k5finite, the solution yields a secondary current distribution. In order to better understand the potential distributions in a flat-plate oxygen generator cell, we solved this system of equations for a thin twodimensional electrolyte using the TOPAZ-2D finite element package [7] on a HP720 computer
Fig. 1. Calculated potential distributions in a thin electrolyte.
(Ceramatec). An example, Fig. 1a shows the numerical solution for the primary distribution in the active region of a thin symmetric cell. Since the geometry is one dimensional, the potential varies linearly with position across the electrolyte, as we would predict analytically. Near the edge of the electrodes, however, the potential profile becomes highly nonlinear. Fig. 1b shows a numerical calculation of the primary distribution in the same cell, near the edge of the active region, assuming electrodes are perfectly aligned. The potential along the gas-exposed surface of the electrolyte near the edges of the electrodes varies strongly with position, until a distance of about three electrolyte thickness. Beyond this, the potential is approximately uniform everywhere, with all equipotential surfaces referencing the same plane in the active region. This result holds even with finite electrode kinetics, provided the position is beyond the edge of both electrodes. This result indicates that any placement of a reference electrode beyond about three electrolyte thicknesses will measure the same equipotential surface in the electrolyte. Even if there is a PO 2 difference across the electrolyte (as in a fuel cell, for
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
example), the difference in voltage measured between reference electrodes located opposite each other on either side of the cell reflect a Nernst potential, not a potential gradient across the electrolyte. Assuming this PO 2 difference is known, then reference electrodes on the anode and cathode sides provide redundant information. In general one prefers to put reference electrodes in this outer region since it avoids potential gradients, and makes the measurement insensitive to exact location of the reference electrode. Fig. 1c shows the primary distribution assuming one electrode terminates, and the other overhangs to infinity. Again we see a region of uniform potential beyond about three electrolyte thicknesses from the edge of the terminating electrode. However, since the electrolyte in this region is in contact with the overhanging electrode, it shares the same potential. If we were to place a reference electrode in this region, we would measure (erroneously) that the overpotential of the overhanging electrode is zero. While the situation becomes more complex with finite electrode kinetics and finite overhang, the basic conclusion is the same: enormous errors in measurement can result when the current distribution is asymmetric. In order to determine experimentally the significance of this overhang effect, we made and tested cells with overhanging electrodes, as shown in Fig. 2a. These cells were similar to the symmetric cells described in reference [8], but were fabricated with special undersized electrodes of constant overhang distance d. The reference electrode was placed in the equipotential region at least three electrolyte thicknesses away from the edge of the overhanging electrode. Results (Fig. 2b) show that with little or no overhang (d , L / 2), the potential difference between the active and reference electrodes is approximately half the cell potential, which is expected for a symmetric cell under conditions of linear polarization. Beyond d 5 L, the potential difference decreases dramatically as overhang is increased, and yields values consistent with our simulations of the secondary current distribution. Finally beyond d 5 4L, where simulations predict a potential difference of zero, the measured value levels-off at a minimum of about 2 mV (a small Nernst potential of 2–3 mV
37
Fig. 2. Reference potential vs. electrode overhang.
remains due to gas-phase polarization of the active electrode).
3. Effects of potential distributions on a.c. impedance measurements Figs. 1c and 2b show that a significant overhang of one electrode beyond the other on a thin electrolyte will tend to bias the reference potential toward the overhanging electrode. Studies by Winkler et al. [5] of secondary potential distributions in thin cells show that this effect can create large errors in measurement of the electrode overpotential. They also show that when the electrolyte is very thin, and the kinetics at the two electrodes are different, large errors occur even when the electrode alignment is exact. Anecdotal observations in our own studies of mixed-conducting oxygen electrodes [8] appear to support these conclusions. As shown in Fig. 3a, one
38
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
Fig. 3. Effects of electrolyte thickness on a.c. impedence.
of our standard test configurations is a thin symmetric cell fabricated with identical electrodes, printed using the same process as a full-scale cell. We usually measure the combined impedance of both electrodes by measuring the total cell impedance at zero bias and subtracting the total ohmic (v 5 infinity) contribution of the electrolyte. In principle, we can also determine the impedance of each individual electrode by measuring impedance between electrode and reference electrode. As sketched qualitatively in Fig. 3b, we normally find the combined electrode impedance of the cell to be independent of electrolyte thickness. Also, when printed on a thick electrolyte, both electrodes normally have equal impedance as separated using the reference electrode. However, with a very thin electrolyte, we often observe differences of up to 200% in electrode resistance between the two electrodes, even when the electrodes are identical and have been made in the same manner as electrodes on a thick electrolyte. Our explanation for this behavior is shown in Fig. 4. In the process of printing the electrodes, it is possible for the anode and cathode to be slightly
Fig. 4. Why reference potential drifts with frequency. (a) Thick electrolyte — small reference drift (b) Thin electrolyte — large reference drift.
misaligned. For a thick electrolyte (Fig. 4a), any misalignment is relatively small compared to the electrolyte thickness, so the difference between the primary potential distribution (at v 5infinity) and the secondary potential distribution (at v 50) near the two electrodes is similar. The location of the equipotential surface measured by the reference electrode therefore stays approximately fixed as a function of frequency, and the fraction of the electrolyte resistance apportioned to each electrode overvoltage remains roughly constant. However, for a thin electrolyte (Fig. 4b), the same misalignment can be a large fraction (or multiple) of the electrolyte thickness. Thus in analogy to Fig. 1c, we expect a primary potential distribution in which the reference potential lies very close to the working electrode at v 5infinity. As the frequency sweeps to zero, and we move from a primary to a secondary distribution, the potential distribution across the overhanging portion of the electrode become significant, and thus the reference electrode no longer
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
measures an equipotential surface in the active region of the electrolyte. A significant part of the electrode overvoltage is misapportioned to the other electrode, resulting in an apparent difference in electrode overpotential between anode and cathode. We have found these effects to be unavoidable on thin cells, and thus we only use thick cells when attempting to isolate the impedance of a single working electrode.
4. Use of offset electrodes to protect gas-tight seals in an oxygen generator stack Although misalignment of electrodes on a thin electrolyte can wreak havoc on electrochemical measurements, it can also be useful for controlling potential in the inactive portion of an electrolyte. As described in a recent patent [6], we have used this principle as a method to protect gas-tight seals in an oxygen generator stack against electrochemical degradation / delamination. Fig. 5 shows a schematic of an oxygen generator stack used by Ceramatec and Air Products and
39
Chemicals to test the durability of glass–ceramic sealants. The stack consists of thin circular cells of 10% strontia-doped ceria (SCO), with La 12x Sr x CoO 32d (LSCO) electrodes. These cells are connected electrically in series with La 12x Srx MnO 3 (LSM) interconnects, which also serve to collect and channel O 2 through an internal manifold that runs the length of the stack. A critical part of this design is a thin layer of lithium aluminosilicate glass– ceramic (LAS) which serves as a gas-tight sealant between the lip of the O 2 manifold cavity of the interconnect and the edge of the SCO cell. Even a tiny leak through this seal at any one of the cells compromises the pressure integrity and O 2 purity of the entire stack. Prior to making and testing stacks, LAS had proven durable for years in sealing tubes made from LSM and ceria. When we first began making stacks, each cell was made with a coextensive anode and cathode. Although the stacks would appear well-bonded after fabrication, the LAS seal bonds would delaminate and the stacks would fall apart within 3 weeks of operation! We noticed that failure always occurred at the same interface: between the LAS sealant and the
Fig. 5. Anatomy of an oxygen generator.
40
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
ceria on the anode side of the cells, never on the cathode side. Further tests proved that electrochemical operation was required to cause failure. In order to better understand this effect, we began a series of experiments called ‘pellet tests’, intended to measure directly the durability of the seal against applied potential / current. As illustrated in Fig. 6a, a pellet is made from the same materials as a stack, and consists of an LSM disk bonded with LAS glass to a slightly larger disk of ceria (electroded on one side only). This pellet is put under the same operating conditions as a stack, with a positive voltage difference applied between the LSM and the electrode. In all cases the measured current is very small (mA), so the pellet must be checked manually for delamination after a fixed period of time. Fig. 6b shows the results. At small applied voltages (,25 mV), delamination does not occur even after relatively long period of time (3300 h). However, at applied voltages .50 mV, delamination occurs in less than 10 days. At an applied voltage of
700 mV, a single-lap shear version of this test (where we time the delamination using a weight hung from the pellet), delamination occurs in less than 60 s! At negative applied voltages, delamination never occurs. From these experiments, we began to suspect the failure mechanism illustrated in Fig. 7. In a standard stack cell, the anode and cathode are coextensive at the edge of the electrolyte plate near the anode-side seal. If the current through the seal is small, we expect (based on Fig. 2) that the edge of the electrolyte plate will be equipotential at about half the cell potential (2350 mV) relative to the LSM interconnect on the other side of the glass bondline. Since the glass itself is a semiconductor, at small currents we also expect the glass itself to be equipotential at about zero (0 mV) relative to the LSM. Thus nearly the entire voltage difference across the seal is a driving force for O 2 formation at the glass–ceria interface via 2O 22 4→e2 1O 2 , with an equilibrium driving force of: RT ] ln PO 2 5 FLSM 2 FCSO 4F
(4)
At 7508C, a voltage difference of 350 mV translates
Fig. 6. Pellet delamination test.
Fig. 7. Theory for anode seal delamination.
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
to an equilibrium O 2 partial pressure of 10 7 atmospheres! This situation is not unlike the internal pressures that can develop at the interface between zirconia and ceria due to a discontinuity in electrical conductivity [9]. Since pressure is exponential with potential, we expect a steep increase in failure rate with potential drop across the seal interface. Based on these observations and ideas, we developed a resolution to the seal delamination problem illustrated in Fig. 8a. The principle of the method is to make cells with a cathode that is intentionally undersized by an overhang distance d relative to the anode. Thus during operation, the potential of the electrolyte at the edge of the cell is biased toward the potential of the anode-side interconnect. If the overhang is sufficient, the ceria and LSM will be nearly equipotential, such that there is no driving force for O 2 formation at the seal
41
interface. Based on Fig. 2, we redesigned the anode to have an overhang distance d of about ten times the electrolyte thickness. We built and tested stacks as shown in Fig. 5 both with and without overhang, with special reference electrodes located on the electrolyte across from the seal so that we could measure directly the actual potential the seal is subjected to (as in the pellet tests). In order to quantify stack leakage, we used a mass flow controller to measure O 2 flow-rate with a backpressure of O 2 on the stack. Any leak is detectable as a loss of flow efficiency, defined as O 2 flow relative to the rate of O 2 formation (determined by the current). The results of these experiments are shown in Fig. 8b. At a backpressure of 1.5 psig, a stack with coextensive anode and cathode begins to leak within a few hours, and this leak gets worse with time. After 3 weeks flow efficiency drops below 80%, and obvious seal delamination can be seen on postmortem analysis of the stack. On the other hand, a stack with an overhanging anode maintains a flowefficiency of 1.0 for over 5000 h, with no signs of delamination. These results have proven reproducible in dozens of stacks under a wide range of conditions and design configurations.
5. Conclusions The observations and calculations reported in this paper demonstrate that potential distributions in a thin electrolyte are extremely sensitive to the details of electrode geometry. We show how this sensitivity can be used to control the electrolyte potential in the inactive region of an oxygen generator stack, thereby protecting glass–ceramic sealants against electrochemical degradation / delamination. We also show that these effects can make it a challenge to measure electrode overpotential accurately in situ. In order to minimize inaccuracies introduced by current-distribution effects, we advocate the following:
Fig. 8. Problem solution (a) overhanging anode configuration, (b) seal performance at 1.5 psig backpressure.
1. To avoid potential gradients along the electrolyte surface, it is best to place the reference electrode at least three electrolyte thicknesses away from any active electrode. 2. When working with identical electrodes at zero bias on thin electrolytes, it is more accurate to
42
S.B. Adler et al. / Solid State Ionics 134 (2000) 35 – 42
measure combined anode / cathode electrode resistance (and divide by two), than to try to measure anode and cathode individually. 3. For studies of cells with a different anode and cathode (or electrodes at nonzero bias), it is not possible to obtain meaningful results with a thin electrolyte. We have had some success printing electrodes of interest on a special ‘extra-thick’ electrolyte, where electrode geometry can be better controlled and symmetry effects are less pronounced. This approach has the advantage of closely reproducing the same materials and fabrication procedures as a thin cell, but does not necessarily eliminate the problems inherent to slab geometry. Alternatively, Winkler et al. [5] report specific electrolyte geometries that largely eliminate ohmic error. This approach allows true separation of anode and cathode effects, but requires effort to insure that materials and fabrications procedures are relevant to a thin electrolyte.
Acknowledgements The authors wish to thank Ceramatec, Inc. and Air Products and Chemicals Inc. for permission to
publish this work. Special thanks go to Mike Zoetmulder and Dave Padgen for their assistance in building and testing prototype stacks and cells studied in this work. We also with to thank Joe Hartvigsen for helpful discussions.
References [1] J. Newman, J. Electrochem. Soc. 113 (1966) 1235. [2] W.H. Tiedemann, J.S. Newman, D.N. Bennion, J. Electrochem. Soc. 120 (1973) 256. [3] A.C. West, J.S. Newman, J. Electrochem. Soc. 136 (1973) 3755. [4] M. Nagata, Y. Itoh, H. Iwahara, Solid State Ionics 67 (1994) 215. [5] J. Winkler, P.V. Hendriksen, N. Bonanos, M. Mogensen, J. Electrochem. Soc. 145 (1998) 1184. [6] S. Adler, B.T. Henderson, R.E. Richards, D.M. Taylor, M.A. Wilson, U.S. Patent No. 5868918, 1999. [7] For current information contact Livermore Software Technology Corporation at www.lstc.com. [8] S. Adler, Solid State Ionics 111 (1998) 125. [9] A.V. Virkar, J. Electrochem. Soc. 138 (5) (1991) 1481.