Effects of Humidified Gas Environments on the Electrode Behavior of La0.6Sr0.4CoO3-

Effects of Humidified Gas Environments on the Electrode Behavior of La0.6Sr0.4CoO3-δ

T. J. McDonalda, S. B. Adlera

a Department of Chemical Engineering, University of Washington, Seattle, WA 98195,

USA

The degradation of La1-xSrxCoO3-δ electrodes upon humidification of the gas environment remains a poorly understood phenomenon. Nonlinear Electrochemical Impedance Spectroscopy is used to measure the full nonlinear response of a La0.6Sr0.4CoO3-δ electrode in dry and humidified gas environments to determine the likely sources of degradation.

Introduction

Previous work has indicated a high rate of degradation for SOFC cathodes when exposed to humidified gas environments (1,2). Understanding this degradation is an important aspect of SOFC commercialization, but for the mixed conducting perovskite La1-xSrxCoO3-δ (LSC) the underlying phenomenon remains unclear. To gain additional insights into the effects of the humidity driven degradation, the full nonlinear response of the electrode is measured and compared to previous modeling work on similarly fabricated electrodes (3). By studying the effects of humidified gas environments on LSC electrodes, with NonLinear Electrochemical Impedance Spectroscopy (NLEIS), it may be possible to improve the understanding of the electrode behavior.

Theory

For a linear EIS measurement the impedance (Z) of the system can be written as the ratio of the applied voltage perturbation amplitude ( ̃) and the resulting complex current amplitude ( )̃: ̃ ̃ ̃ [1]

where and denote the real and imaginary components of the complex current amplitude. Impedance spectra, plotted in Nyquist form, are characterized by their shape. For oxygen reduction on mixed conducting electrodes, there is often a single arc corresponding to electrode behavior, which is defined by a characteristic resistance (arc width Rchem or height Rc), and a characteristic frequency (peak frequency ωchem). Arc height, rather than arc width, is used here because it is less susceptible to errors from overlapping arcs or frequency dispersion.

For an NLEIS measurement, a current perturbation produces both a linear and a nonlinear voltage response. The nonlinear portion of the response can be characterized by harmonic spectra. As explained in more detail in (4), harmonic spectra are Nyquist plots

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of fourier coefficients for select harmonics, and are normalized to the magnitude of the linear impedance (Rc):

and [2]

and represent the unscaled and scaled fourier coefficients of the second order harmonic with a squared dependence on the perturbation amplitude, while and represent the unscaled and scaled fourier coefficients of the third order harmonic with a cubic dependence on the perturbation amplitude.

Scaling the nonlinear harmonics to the magnitude of the linear impedance removes the dependence of and on the magnitude of kinetic/transport rates. Harmonic spectra are therefore sensitive to the mechanism of electrode processes, but not sensitive to the value of the overall rate. As an example, oxygen reduction on the mixed conductor LSC, assuming a dissociative adsorption reaction mechanism, can be written as

⁄ , [3]

where is the overall rate of oxygen reduction, is the temperature-dependent rate constant, is the partial pressure of oxygen, is the concentration of oxygen vacancies, is the thermodynamic driving force for the reaction, R is the ideal gas constant, and T is the temperature (5). will depend on the linear portion of the rate law, and will be sensitive to equilibrium values of and , as well as . Conversely, harmonic spectra for and will only be dependent on the form of the rate law, and will be sensitive to the reaction orders and thermodynamic driving force but independent of the equilibrium values and .

Experimental

The cell studied consisted of a Ce0.9Gd0.1O1.95 (GDC) electrolyte pellet (with a cylindrical geometry), porous La0.6Sr0.4CoO3-δ (LSC-64) working and counter electrodes on the flat faces of the cylinder, and a La0.6Sr0.4CoO3-δ (LSC-64) reference electrode on the radial surface (illustrated in Figure 1). Electrolyte pellets, ~ 3 mm thick, were made by pressing GDC powder (Praxiar Specialty Ceramics) uniaxially at 3x104 kPa and then sintering at ~ 1400°C for 4 hours. Working (WE) and counter (CE) electrodes were screen printed using an ink mixture of LSC-64 powder (Praxair), alpha-terpineol, ethyl cellulose, ethanol and oleic acid (Alfa-Aesar), with a 1:1 ratio of electrode powder to organics. The reference electrode (RE) was hand painted on the sides of the electrolyte pellet with the electrode ink. Electrodes were sintered at 1090°C, in an active flow of dry air, for 2 hours. Both the WE and CE had a surface coverage of ~ 0.2 cm2 and were ~ 8-10 µm thick (verified by SEM).

ECS Transactions, 58 (3) 153-162 (2013)



Figure 1. Illustration of the button cell used for electrochemical testing. Cross section is depicted, with LSC-64 electrodes, GDC electrolyte, and Au current collectors (mesh for WE/CE, single wire for RE).

Electrode behavior was studied using both Linear EIS and NLEIS half-cell measurements. EIS measurements used a voltage perturbation with an amplitude of 10mV, over a range of frequencies from 0.01 to 100,000 Hz. NLEIS measurements were performed with a current perturbation over a range of amplitudes (10-20 per frequency) and frequencies (0.02 to 100,000 Hz). The amplitude range was chosen based on the measured size of the nonlinearities, to yield at-least a 3rd order harmonic, but no more than a 5th order harmonic, at the peak frequency (determined by EIS). A more detailed description of the NLEIS technique is given in (4).

Linear EIS was measured repeatedly until reproducible results were obtained to ensure the cell had reached equilibrium. Once reproducible impedance spectra had been obtained, an NLEIS measurement was performed; followed by Linear EIS to assess any system drift. Experimental conditions tested were oxygen partial pressures (PO2) of 0.01, 0.1, and 1.0 atm, and temperatures of 550, 650 and 750°C. Only testing at 550°C included humidified environments of ~ 2-2.5 mol%. A specific order of conditions was chosen to first characterize the dry electrochemical behavior of the cell, then to determine the humidified behavior, and finally to determine the dry behavior after humidified testing (referred to as dry post-humidified).

Results

Linear Results

Figure 2 gives impedance spectra for half-cell measurements on an LSC-64 porous electrode in dry, humidified, and post-humidified environments, at 550°C and at oxygen partial pressures of 0.01, 0.1, and 1 atm. There are 3 distinct features for all impedance spectra obtained. A high frequency offset (>100 kHz), a moderate frequency arc (~ 1-100 kHz), and a low frequency arc (~ 0.01-100 Hz). Values of the high frequency offset fall within the range of the calculated ohmic resistance do to ionic transport in GDC (20-30 Ohms based on electrode geometry and the conditions tested) (6). The moderate frequency arc is assumed to be interfacial phenomena because of its weak PO2 dependence, while the low frequency arc is assumed to correspond to the dominant electrode processes.




Figure 2. Impedance spectra for porous LSC-64 electrode at 550°C. Gas conditions vary across oxygen partial pressures of 0.01 atm (x), 0.1 atm (cross), 1 atm (diamond), dry (a), humidified (b), and dry post-humidifed (c). Peak frequencies of the electrode processes arc are indicated on all spectra, while the moderate frequency arc frequencies are only indicated in (a) and remain unchanged in (b) and (c).

As mentioned previously, the impedance arc of the electrode processes can be characterized by the arc shape and important dimensions (arc height and peak frequency). A Gerischer shaped impedance is seen for all conditions, except for humidified and post-humidified environment at the lowest PO2 tested (0.01 atm), where the impedance became distinctly semi-circular. The Gerischer shape is generally attributed to co-limitation of transport and kinetics on electrode behavior. Transition to a semi-circular shape at low PO2 in a humidified environment, coupled with an increasing characteristic resistance, indicates that the kinetics are disproportionately inhibited by the presence of water, and that the process is non-reversible at the conditions tested. Figure 3 illustrates the time dependence of Rc for changing humidity levels at 1.0 atm of oxygen.

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025 35 45 55 65

Im Z

(Ω

)

Re Z (Ω)

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~150 Hz

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025 50 75 100 125

Im Z

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Re Z (Ω)

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0.016 Hz

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(a) (b)

(c)




Figure 3. Changes in the characteristic resistance (Rc) of the impedance response for a porous LSC-64 electrode over time; in changing experimental conditions. All measurement data points are taken in 1.0 atm PO2. Dotted lines indicate a change in testing environment, where section I corresponds to initial dry testing, section II corresponds to high temperature testing, section III corresponds to humidified testing, and section IV corresponds to dry, post-humdified testing.

Introduction of water into the environment results in a significant increase in Rc over a period of several hundred hours. This effect was only partially reversible upon removal of the water, which shows a small, immediate decrease in Rc. Characteristic frequency (ωchem) is also seen to increase upon introduction of water in Figure 2, with partial reversibility upon water removal.

While the absolute value of the characteristic parameters changed drastically upon introduction of water, the PO2 dependence remained largely unchanged (Figure 4). Both parameters exhibit a power law dependence on oxygen partial pressure , with n~0.65-0.85 for ωchem and n~(-0.1)–(-0.3) for Rc. Using the analytical solutions for characteristic parameters from (7), the expected PO2 dependence for co-limited diffusion and dissociative adsorption kinetics (calculated without a surface diffusion pathway) is indicated as solid lines in Figure 4.

0

5

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15

0 200 400 600 800

Rc

(Ω)

Hours

I II III IV




Figure 4. Characterstic resistance (Rc) and frequency (ωchem), for a porous LSC-64 electrode at 550°C, as a function of oxygen partial pressure. Gas conditions are varied from dry (diamonds), to humidified (triangles), to dry, post-humidified (circles). Lines indicate calculated PO2 dependence for co-limited kinetics and transport with a dissociative adsorption rate law and no surface diffusion.

In addition to absolute changes in the parameters, and relatively consistent trends in PO2, the relationship between the measured parameters changes upon humidification at the lowest PO2 measured (0.01 atm), as shown in Figure 5. For the majority of the data collected the power law dependence between the parameters is very close to n=2, indicated by the solid line. However, at the lowest PO2 for humidified and post humidified conditions, the power law dependence is closer to n=1 (dashed line).

Figure 5. Characterstic resistance (Rc) versus characteristic time (tchem=1/ ωchem), for a porous LSC-64 electrode at 550°C, as a function of oxygen partial pressure, with 1.0 atm (diamonds), 0.1 atm (triangles), and 0.01 atm (circles). Gas conditions are also varied from dry, to humidified, to dry post-humidified within a single set of symbols. Lines indicate power law dependencies ( ) of n=2 (solid) and n=1 (dashed).

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Rc

(Ω)

pO2 (atm)

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ωch

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Non-Linear Results

Higher harmonic spectra can generally be characterized by the low frequency intercept of the real axis, the overall magnitude of the harmonic, and the location of the phasor line. 2nd and 3rd harmonic spectra collected at 550°C and varying gas conditions are given in Figure 6. The main feature of the harmonic spectra is a spiral that originates at the origin (high frequency), spirals out, and terminates on the real axis (low frequency). The frequency range of the spiral is on the same order as the low frequency impedance arc. The only exception is for dry conditions at 1.0 atm PO2 where a second very low frequency arc can be seen in the 2nd harmonic, occurring at frequencies that are on the same order as the last few frequencies in the semicircular portion of the low frequency impedance arc (where the arc meats the real axis).

Figure 6. 2nd and 3rd harmonic spectra for a porous LSC-64 electrode at 550°C, and in dry gas environments of 0.01 atm (blue), 0.1 atm (green), and 1 atm (red) PO2. Phasor lines indicate frequencies of ωchem/2 and ωchem/3 for the 2nd and 3rd harmonics respectively.




Figure 7. Comparison of 2nd and 3rd harmonic spectra for a porous LSC-64 electrode at 550°C, in humified (a) and dry, post-humidified (b) gas environments, and 0.01 atm (blue), 0.1 atm (green), and 1 atm (red) PO2. Phasor lines indicate frequencies of ωchem/2 and ωchem/3 for the 2nd and 3rd harmonics respectively.

Similar to the impedance response, the introduction of humidity causes irreversible changes in the harmonic spectra (Figure 7), however the magnitude of the changes is significantly less than seen in the impedance. The main effects of humidification are: slight reductions in the magnitude of the 2nd harmonic and in the high 3rd harmonic at 1 atm PO2, and shifts towards the positive real for the 3rd harmonic at 1.0 and 0.1 atm PO2. Interestingly, some of the 3rd harmonic shifts continued even after the humidity was removed.

Discussion

The observed change in linear impedance from Gerischer like to semi-circular, can be explained by a transition from a co-limited (transport and kinetics) to a primarily kinetically limited regime. This also accounts for the change in power law dependence between Rc and ωchem seen in Figure 5. Previous work (7) has shown co-limitation of kinetics and transport to predict a similar dependence to that seen for the co-limited data, but for a kinetically limited regime the entire electrode is activated, giving a constant

(a)

(b)




chemical capacitance, and a characteristic time constant that scales directly with the characteristic resistance.

LSC electrodes have previously been shown to primarily follow a dissociative adsorption reaction rate law, with evidence of significant surface transport (3). The expected dependence of Rc and ωchem is in close agreement to the actual dependence, measured by linear EIS, for all conditions studied. This matches previous findings of a dissociative adsorption mechanism, but indicates that the surface diffusion pathway is less prevalent in the cell tested for this study. It should also be noted that Rc shows a small deviation from strictly power law dependence on oxygen content (at the lowest PO2) after the addition of water. This is likely due to the change from co-limitation to primarily kinetically limited.

Changes in harmonic spectra have previously been explained by several different phenomena (3). Alteration of the surface thermodynamics through segregation of Sr within the perovskite lattice can cause small to large changes in the low frequency intercept depending on the size of the segragation. A change in reaction mechanism or transport pathway would also lead to changes in harmonic spectra, but on a much larger magnitude than the changes seen within this study. Finally, changes in the relative rates of surface/bulk transport as well as the relative rates of transport/kinetics will cause small changes in the trends and magnitudes of harmonic spectra.

Conclusions

Both the linear and nonlinear response, for a porous LSC-64 electrode, show distinct, irreversible changes upon exposure to a humidified gas environment at 550°C. The linear response parameters, such as the characteristic resistance (Rc), were found to change dramatically and the impedance was seen to transition from a co-limited (Gerischer) response to primarily kinetically limited (semi-circular). This was attributed to selective inhibition of the surface exchange reaction. Changes in the nonlinear harmonics were also observed, but were much smaller in comparison to those seen in the linear impedance. Given the relatively small changes in harmonics compared to the large changes seen in the linear impedance, it is likely that the predominant effect of humidification is a scaling of the absolute rate of electrode processes and not a change in mechanisms.

The hypothesis found to be the most consistent with the observed trends was that the oxygen reduction reaction rate was primarily affected in a manner which suppressed the overall rate but did not result in a change in mechanism. A phenomenon like active site blocking would affect the linear exchange rate, causing an inhibition of overall kinetic rate, while having no direct effect on the nonlinear response. Inhibition of the reaction rate would then lead to changes in the relative rates of kinetics and transport which would have small nonlinear effects. This hypothesis can be further tested by comparing observed changes in nonlinear harmonics to those predicted by large changes in relative rates.




Acknowledgments

This work was supported, in part, by the National Science Foundation, Divisions of Engineering (CBET) and Materials Research (Ceramics) under Grant numbers 0907662, 0542874, 0829171, and 0412076, and by the U.S. Dept. of Energy, Fossil Energy SECA program, under cooperative agreement number DE-FC26-02NT41566. TJM also acknowledges the Ford Motor Company for fellowship support.

References

1. A. Arregui et al., Electrochimica Acta, 58, 312–321 (2011).

2. J. Nielsen and M. Mogensen, Solid State Ionics (2011).

3. C. R. Kreller, thesis, University of Washington (2011).

4. J. R. Wilson, D. T. Schwartz, and S. B. Adler, Electrochimica Acta, 51, 1389–1402 (2006).

5. S. B. Adler, X. Y. Chen, and J. R. Wilson, Journal of Catalysis, 245, 91–109 (2007).

6. D. J. L. Brett, A. Atkinson, N. P. Brandon, and S. J. Skinner, Chemical Society reviews, 37, 1568–78 (2008).

7. Y. Lu, C. R. Kreller, and S. B. Adler, Journal of The Electrochemical Society, 156, B513 (2009).




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Effects of Humidified Gas Environments on the Electrode Behavior of La0.6Sr0.4CoO3-