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Novel method for determining the triple phase boundary width
SOSI-13828; No of Pages 3 Solid State Ionics xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Novel method for determining the triple phase boundary width Tzvia Beitner a, Sioma Baltianski b, Ilan Riess c, Yoed Tsur b,⁎ a b c
Grand Technion Energy Program, Technion-IIT, Haifa 3200003, Israel Chemical Engineering Department, Technion-IIT, Haifa 3200003, Israel Physics Department, Technion-IIT, Haifa 3200003, Israel
a r t i c l e
i n f o
Article history: Received 16 July 2015 Received in revised form 13 November 2015 Accepted 23 November 2015 Available online xxxx Keywords: Triple phase boundary Fuel cells Electrolysis cells Impedance spectroscopy
a b s t r a c t In order to improve the performance of solid oxide fuel cells (SOFCs), there is a need to deepen our understanding of the cathode reactions, which is rate-limiting. The purpose of this work is to develop and demonstrate a direct electrochemical technique for estimating the cathode triple-phase boundary (TPB) between electrode, electrolyte and gas phase. Interdigitated gold micro-electrodes are fabricated on YSZ for this purpose as gold is oxygen-blocking, forcing the oxygen to enter into the solid electrolyte at the TPB. Three-electrode measurements are performed, allowing ac impedance measurement between two interdigitated gold electrodes while at the same time driving dc current between one of the gold electrodes and a silver counter electrode. Impedance spectroscopy (IS) measurements are performed at 300 °C. IS results demonstrate two main features — a large low-frequency (LF) arc and a smaller high-frequency (HF) arc, which actually consists of two overlapping arcs. The results suggest that the HF arc reflects mainly the surface impedance between the opposing TPBs and that the TPB width is much less than 8 μm and estimated to be at most 1 μm. The TPB width increases under high anodic polarization. The LF arc reflects the electrode impedance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The performance of SOFCs is limited by the cathodic reaction, which requires adsorption and dissociation of oxygen molecules on the cathode surface and diffusion of the adsorbed atomic or ionic oxygen to the active electrode area, where it is caught by an oxygen vacancy in the electrolyte. When the cathode is a poor ionic conductor, the active electrode area is limited to the triple phase boundary (TPB) between the gas, electrode, and solid oxide electrolyte phases. Various mathematical models have been proposed to describe the TPB [1,2]. O'Hayre et al. developed a mathematical model taking into account coupled reactions, diffusion phenomena and TPB “effective width” [3]. Experimentally, it has been demonstrated that the TPB width may be estimated using oxygen isotope tracing [4,5]. Using secondary ion mass spectrometry analysis (SIMS) on gold mesh cathodes on YSZ, Horita et al. estimated the cathode TPB width to be less than 1 μm for a cathodic overpotential of η = − 0.5 V at 700 °C [6]. Lower temperature analysis has been performed by Fleig et al. on the Pt/YSZ/ O2 TPB — it was estimated to have a width on the order of 1.0–1.5 μm at 325 °C under a high cathodic overpotential of η = −2.18 V [7]. Despite the TPB width being an important factor in numerical modeling of fuel cells, it has only been estimated in the past. The aim of this work is to develop a direct method of characterizing the active area of the cathode using electrical measurements. This is ⁎ Corresponding author. E-mail address: [email protected] (Y. Tsur).
demonstrated using gold micro-electrodes on an Yttria-stabilized Zirconia (YSZ) solid electrolyte (SE). Gold is known to block oxygen transport through it. Oxygen has to be introduced to the YSZ at the TPB. The temperature of the measurements was limited to 300 °C due to instability of both the Au and Ag electrodes at higher temperatures for extended time scales. 2. Experimental 2.1. Sample preparation Pairs of interdigitated gold micro-electrodes were fabricated using lift-off patterning on an YSZ (001) single crystal. Each electrode consisted of a connecting bar and 20 stripes, with each stripe having a width ΔE of 10 μm or 16 μm with an inter-stripe spacing ΔG of 2, 4, 8 or 16 μm. A schematic of an interdigitated electrode pair is given in Fig. 1 and a magnification of a typical arrangement having an electrode stripe width of ΔE = 16 μm and spacing between adjacent stripes ΔG = 4 μm is shown in Fig. 2. A silver counter electrode (CE) was painted on the back of the YSZ. 2.2. Three-electrode measurement system A three-electrode arrangement (demonstrated previously in [8,9] and illustrated in Fig. 3) allowed biasing one of the gold electrodes (Au1) with respect to the Ag CE on the opposite side of the YSZ crystal. Impedance spectroscopy (IS) was then performed between the two
http://dx.doi.org/10.1016/j.ssi.2015.11.026 0167-2738/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: T. Beitner, et al., Novel method for determining the triple phase boundary width, Solid State Ionics (2015), http:// dx.doi.org/10.1016/j.ssi.2015.11.026
2
T. Beitner et al. / Solid State Ionics xxx (2015) xxx–xxx
ΔE ΔG
Fig. 1. Schematic of interdigitated gold electrode stripes. The width of the electrode stripe is given by ΔE and the spacing between adjacent stripes by ΔG.
Fig. 4. IS results for Vdc b 0 at 300 °C, ΔG = 8 μm. The voltages indicated on the graph refer to the dc bias.
frequency range was 50 mHz–1 MHz. The results for negative dc bias and positive dc bias are shown in Figs. 4 and 5, respectively. A magnification of the high-frequency (HF) arc for the negative and positive bias measurements is shown in Figs. 6 and 7. Figs. 6 and 7 as well as ISGP analysis [10] reveal that the HF arc consists actually of two arcs with the smaller one at the highest frequency limit.
Fig. 2. Optical image of interdigitated gold electrode stripes with ΔE = 16 μm and ΔG = 4 μm.
interdigitated gold electrodes (Au1–Au2) and the effect of the Au1–Ag dc bias on the Au1–Au2 measurement was examined. 3. ac impedance results 3.1. dc voltage dependence IS was performed at 300 °C for electrodes with ΔE of 10 μm and ΔG of 8 μm. The amplitude of the ac voltage used was Va = 30 mV and the Fig. 5. IS results for Vdc N 0 at 300 °C, ΔG = 8 μm. The voltages indicated on the graph refer to the dc bias.
dc
IS
H
R
Au1
ΔE
ΔG YSZ
Au2
ΔE
Ag CE
Substrate (Alumina) Fig. 3. Illustration of the 3-electrode measurement setup.
Fig. 6. Magnified HF arc at 300 °C for Vdc b 0, ΔG = 8 μm.
Please cite this article as: T. Beitner, et al., Novel method for determining the triple phase boundary width, Solid State Ionics (2015), http:// dx.doi.org/10.1016/j.ssi.2015.11.026
T. Beitner et al. / Solid State Ionics xxx (2015) xxx–xxx
3
Fig. 7. Magnified HF arc at 300 °C for Vdc N 0, ΔG = 8 μm.
Fig. 8. Magnified HF arc at 300 °C for ΔG = 2, 4, 8 and 16 μm.
The low frequency (LF) arc depends strongly on the applied bias, and the decrease in the LF arc is larger for positive bias than for equivalent negative bias. The HF arc shows a weak dependence on the dc bias. A slight decrease in the HF impedance is observed for high negative bias; a more significant increase is observed for high positive bias.
stripes. However, simulations using a corresponding equivalent circuit show that this capacitance could contribute only to the small arc at the highest frequencies. Further details of this analysis will be given in a later publication. The essence of this analysis is that the LF arc is determined by electrochemical reaction at the TPBs while the main contribution to the HF arc comes from the YSZ resistance and the Au\YSZ capacitance. Since the latter does not change when the gap between the stripes is enhanced (ΔG), the measurements presented in Fig. 8 can help in refining the estimation of the TPB width. The TPB width is a parameter in a model that assumes high conductivity at the TPB and low conductivity at the YSZ bulk. The analysis yields an estimate of the TPB width of less than 1 μm.
3.2. Electrode spacing dependence IS was performed at 300 °C for interdigitated gold electrodes with ΔE = 16 μm and ΔG = 2, 4, 8 and 16 μm. The LF results are not shown, as no discernible dependence of the LF impedance on the spacing was observed. The HF arcs for the various spacings are shown in Fig. 8. The HF impedance increases with increasing spacing. This, along with its weak dependence on dc bias and the correct order of magnitude [11] suggest that the HF major arc reflects the impedance of oxygen ion motion in the YSZ between Au1 and Au2. Hence, Fig. 7 suggests that the TPB width is much smaller than the gap ΔG of 8 μm, as the impedance changes only slightly under high applied bias. This appears to be consistent with results obtained by Horita et al. using oxygen isotope exchange at higher temperatures (700 °C) under cathodic bias only (η = − 0.5 V) for Au/YSZ/O2, where the TPB width was estimated to be smaller than 1 μm [6]. We notice that the more significant change happens under anodic polarization, rather than cathodic polarization of the gold electrode on YSZ. 4. Discussion The LF results suggest that the decrease in the electrode impedance (decrease in the LF arc) is due to the exponential dependence of the current on voltage in the electrode reaction at the TPBs, forcing the ac impedance to decrease under dc bias applied together with the ac bias, a possible effect which has been shown previously both theoretically and experimentally [12]. The difference in the decrease rate in the LF arc height under opposite dc bias is due to a difference in stoichiometry under opposite polarity. Another possible contribution to the HF part of the impedance is the geometric capacitance of the Au electrode
Acknowledgments We acknowledge the support from the Nancy and Stephen Grand Technion Energy Program (GTEP) and Israel Science Foundation (ISF) under grants 699/11 and 2797/11. We thank R. Merkle for the helpful discussions and the Technology and Nanostructuring Service Groups of the MPI for Solid State Research Stuttgart, Germany, for the microelectrode fabrication.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
A.M. Gokhale, S. Zhang, M. Liu, J. Power Sources 194 (2009) 303. X. Deng, A. Petric, J. Power Sources 140 (2005) 297. R. O'Hayre, D.M. Barnett, F.B. Prinz, J. Electrochem. Soc. 152 (2005) A439. T. Horita, K. Yamaji, N. Sakai, H. Yokokawa, T. Kawada, T. Kato, Solid State Ionics 127 (2000) 55–65. H. Kishimoto, N. Sakai, K. Yamaji, T. Horita, M.E. Brito, H. Yokokawa, K. Amezawa, Y. Uchimoto, Solid State Ionics 179 (2008) 347–354. T. Horita, K. Yamaji, N. Sakai, Y. Xiong, T. Kata, H. Yokokawa, T. Kawada, J. Power Sources 106 (2002) 224–230. J. Fleig, A. Schintlmeister, A.K. Opitz, H. Hutter, Scr. Mater. 65 (2011) 78. J. Rutman, S. Raz, I. Riess, Solid State Ionics 177 (2006) 1771–1777. A. Hashibon, S. Raz, I. Riess, Solid State Ionics 149 (2002) 167–176. S. Hershkovitz, S. Baltianski, Y. Tsur, Solid State Ionics 188 (2011) 104–109. J. Kilner, Nat. Mater. 7 (2008) 838–839. Y. Cohen, A. Davidovich, I. Riess, Appl. Phys. Lett. 66 (1995) 706–708.
Please cite this article as: T. Beitner, et al., Novel method for determining the triple phase boundary width, Solid State Ionics (2015), http:// dx.doi.org/10.1016/j.ssi.2015.11.026
Contents lists available at ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
Novel method for determining the triple phase boundary width Tzvia Beitner a, Sioma Baltianski b, Ilan Riess c, Yoed Tsur b,⁎ a b c
Grand Technion Energy Program, Technion-IIT, Haifa 3200003, Israel Chemical Engineering Department, Technion-IIT, Haifa 3200003, Israel Physics Department, Technion-IIT, Haifa 3200003, Israel
a r t i c l e
i n f o
Article history: Received 16 July 2015 Received in revised form 13 November 2015 Accepted 23 November 2015 Available online xxxx Keywords: Triple phase boundary Fuel cells Electrolysis cells Impedance spectroscopy
a b s t r a c t In order to improve the performance of solid oxide fuel cells (SOFCs), there is a need to deepen our understanding of the cathode reactions, which is rate-limiting. The purpose of this work is to develop and demonstrate a direct electrochemical technique for estimating the cathode triple-phase boundary (TPB) between electrode, electrolyte and gas phase. Interdigitated gold micro-electrodes are fabricated on YSZ for this purpose as gold is oxygen-blocking, forcing the oxygen to enter into the solid electrolyte at the TPB. Three-electrode measurements are performed, allowing ac impedance measurement between two interdigitated gold electrodes while at the same time driving dc current between one of the gold electrodes and a silver counter electrode. Impedance spectroscopy (IS) measurements are performed at 300 °C. IS results demonstrate two main features — a large low-frequency (LF) arc and a smaller high-frequency (HF) arc, which actually consists of two overlapping arcs. The results suggest that the HF arc reflects mainly the surface impedance between the opposing TPBs and that the TPB width is much less than 8 μm and estimated to be at most 1 μm. The TPB width increases under high anodic polarization. The LF arc reflects the electrode impedance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The performance of SOFCs is limited by the cathodic reaction, which requires adsorption and dissociation of oxygen molecules on the cathode surface and diffusion of the adsorbed atomic or ionic oxygen to the active electrode area, where it is caught by an oxygen vacancy in the electrolyte. When the cathode is a poor ionic conductor, the active electrode area is limited to the triple phase boundary (TPB) between the gas, electrode, and solid oxide electrolyte phases. Various mathematical models have been proposed to describe the TPB [1,2]. O'Hayre et al. developed a mathematical model taking into account coupled reactions, diffusion phenomena and TPB “effective width” [3]. Experimentally, it has been demonstrated that the TPB width may be estimated using oxygen isotope tracing [4,5]. Using secondary ion mass spectrometry analysis (SIMS) on gold mesh cathodes on YSZ, Horita et al. estimated the cathode TPB width to be less than 1 μm for a cathodic overpotential of η = − 0.5 V at 700 °C [6]. Lower temperature analysis has been performed by Fleig et al. on the Pt/YSZ/ O2 TPB — it was estimated to have a width on the order of 1.0–1.5 μm at 325 °C under a high cathodic overpotential of η = −2.18 V [7]. Despite the TPB width being an important factor in numerical modeling of fuel cells, it has only been estimated in the past. The aim of this work is to develop a direct method of characterizing the active area of the cathode using electrical measurements. This is ⁎ Corresponding author. E-mail address: [email protected] (Y. Tsur).
demonstrated using gold micro-electrodes on an Yttria-stabilized Zirconia (YSZ) solid electrolyte (SE). Gold is known to block oxygen transport through it. Oxygen has to be introduced to the YSZ at the TPB. The temperature of the measurements was limited to 300 °C due to instability of both the Au and Ag electrodes at higher temperatures for extended time scales. 2. Experimental 2.1. Sample preparation Pairs of interdigitated gold micro-electrodes were fabricated using lift-off patterning on an YSZ (001) single crystal. Each electrode consisted of a connecting bar and 20 stripes, with each stripe having a width ΔE of 10 μm or 16 μm with an inter-stripe spacing ΔG of 2, 4, 8 or 16 μm. A schematic of an interdigitated electrode pair is given in Fig. 1 and a magnification of a typical arrangement having an electrode stripe width of ΔE = 16 μm and spacing between adjacent stripes ΔG = 4 μm is shown in Fig. 2. A silver counter electrode (CE) was painted on the back of the YSZ. 2.2. Three-electrode measurement system A three-electrode arrangement (demonstrated previously in [8,9] and illustrated in Fig. 3) allowed biasing one of the gold electrodes (Au1) with respect to the Ag CE on the opposite side of the YSZ crystal. Impedance spectroscopy (IS) was then performed between the two
http://dx.doi.org/10.1016/j.ssi.2015.11.026 0167-2738/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: T. Beitner, et al., Novel method for determining the triple phase boundary width, Solid State Ionics (2015), http:// dx.doi.org/10.1016/j.ssi.2015.11.026
2
T. Beitner et al. / Solid State Ionics xxx (2015) xxx–xxx
ΔE ΔG
Fig. 1. Schematic of interdigitated gold electrode stripes. The width of the electrode stripe is given by ΔE and the spacing between adjacent stripes by ΔG.
Fig. 4. IS results for Vdc b 0 at 300 °C, ΔG = 8 μm. The voltages indicated on the graph refer to the dc bias.
frequency range was 50 mHz–1 MHz. The results for negative dc bias and positive dc bias are shown in Figs. 4 and 5, respectively. A magnification of the high-frequency (HF) arc for the negative and positive bias measurements is shown in Figs. 6 and 7. Figs. 6 and 7 as well as ISGP analysis [10] reveal that the HF arc consists actually of two arcs with the smaller one at the highest frequency limit.
Fig. 2. Optical image of interdigitated gold electrode stripes with ΔE = 16 μm and ΔG = 4 μm.
interdigitated gold electrodes (Au1–Au2) and the effect of the Au1–Ag dc bias on the Au1–Au2 measurement was examined. 3. ac impedance results 3.1. dc voltage dependence IS was performed at 300 °C for electrodes with ΔE of 10 μm and ΔG of 8 μm. The amplitude of the ac voltage used was Va = 30 mV and the Fig. 5. IS results for Vdc N 0 at 300 °C, ΔG = 8 μm. The voltages indicated on the graph refer to the dc bias.
dc
IS
H
R
Au1
ΔE
ΔG YSZ
Au2
ΔE
Ag CE
Substrate (Alumina) Fig. 3. Illustration of the 3-electrode measurement setup.
Fig. 6. Magnified HF arc at 300 °C for Vdc b 0, ΔG = 8 μm.
Please cite this article as: T. Beitner, et al., Novel method for determining the triple phase boundary width, Solid State Ionics (2015), http:// dx.doi.org/10.1016/j.ssi.2015.11.026
T. Beitner et al. / Solid State Ionics xxx (2015) xxx–xxx
3
Fig. 7. Magnified HF arc at 300 °C for Vdc N 0, ΔG = 8 μm.
Fig. 8. Magnified HF arc at 300 °C for ΔG = 2, 4, 8 and 16 μm.
The low frequency (LF) arc depends strongly on the applied bias, and the decrease in the LF arc is larger for positive bias than for equivalent negative bias. The HF arc shows a weak dependence on the dc bias. A slight decrease in the HF impedance is observed for high negative bias; a more significant increase is observed for high positive bias.
stripes. However, simulations using a corresponding equivalent circuit show that this capacitance could contribute only to the small arc at the highest frequencies. Further details of this analysis will be given in a later publication. The essence of this analysis is that the LF arc is determined by electrochemical reaction at the TPBs while the main contribution to the HF arc comes from the YSZ resistance and the Au\YSZ capacitance. Since the latter does not change when the gap between the stripes is enhanced (ΔG), the measurements presented in Fig. 8 can help in refining the estimation of the TPB width. The TPB width is a parameter in a model that assumes high conductivity at the TPB and low conductivity at the YSZ bulk. The analysis yields an estimate of the TPB width of less than 1 μm.
3.2. Electrode spacing dependence IS was performed at 300 °C for interdigitated gold electrodes with ΔE = 16 μm and ΔG = 2, 4, 8 and 16 μm. The LF results are not shown, as no discernible dependence of the LF impedance on the spacing was observed. The HF arcs for the various spacings are shown in Fig. 8. The HF impedance increases with increasing spacing. This, along with its weak dependence on dc bias and the correct order of magnitude [11] suggest that the HF major arc reflects the impedance of oxygen ion motion in the YSZ between Au1 and Au2. Hence, Fig. 7 suggests that the TPB width is much smaller than the gap ΔG of 8 μm, as the impedance changes only slightly under high applied bias. This appears to be consistent with results obtained by Horita et al. using oxygen isotope exchange at higher temperatures (700 °C) under cathodic bias only (η = − 0.5 V) for Au/YSZ/O2, where the TPB width was estimated to be smaller than 1 μm [6]. We notice that the more significant change happens under anodic polarization, rather than cathodic polarization of the gold electrode on YSZ. 4. Discussion The LF results suggest that the decrease in the electrode impedance (decrease in the LF arc) is due to the exponential dependence of the current on voltage in the electrode reaction at the TPBs, forcing the ac impedance to decrease under dc bias applied together with the ac bias, a possible effect which has been shown previously both theoretically and experimentally [12]. The difference in the decrease rate in the LF arc height under opposite dc bias is due to a difference in stoichiometry under opposite polarity. Another possible contribution to the HF part of the impedance is the geometric capacitance of the Au electrode
Acknowledgments We acknowledge the support from the Nancy and Stephen Grand Technion Energy Program (GTEP) and Israel Science Foundation (ISF) under grants 699/11 and 2797/11. We thank R. Merkle for the helpful discussions and the Technology and Nanostructuring Service Groups of the MPI for Solid State Research Stuttgart, Germany, for the microelectrode fabrication.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
A.M. Gokhale, S. Zhang, M. Liu, J. Power Sources 194 (2009) 303. X. Deng, A. Petric, J. Power Sources 140 (2005) 297. R. O'Hayre, D.M. Barnett, F.B. Prinz, J. Electrochem. Soc. 152 (2005) A439. T. Horita, K. Yamaji, N. Sakai, H. Yokokawa, T. Kawada, T. Kato, Solid State Ionics 127 (2000) 55–65. H. Kishimoto, N. Sakai, K. Yamaji, T. Horita, M.E. Brito, H. Yokokawa, K. Amezawa, Y. Uchimoto, Solid State Ionics 179 (2008) 347–354. T. Horita, K. Yamaji, N. Sakai, Y. Xiong, T. Kata, H. Yokokawa, T. Kawada, J. Power Sources 106 (2002) 224–230. J. Fleig, A. Schintlmeister, A.K. Opitz, H. Hutter, Scr. Mater. 65 (2011) 78. J. Rutman, S. Raz, I. Riess, Solid State Ionics 177 (2006) 1771–1777. A. Hashibon, S. Raz, I. Riess, Solid State Ionics 149 (2002) 167–176. S. Hershkovitz, S. Baltianski, Y. Tsur, Solid State Ionics 188 (2011) 104–109. J. Kilner, Nat. Mater. 7 (2008) 838–839. Y. Cohen, A. Davidovich, I. Riess, Appl. Phys. Lett. 66 (1995) 706–708.
Please cite this article as: T. Beitner, et al., Novel method for determining the triple phase boundary width, Solid State Ionics (2015), http:// dx.doi.org/10.1016/j.ssi.2015.11.026