Jacobsen, Torben

Mogensen, Mogens Bjerg

The size of electronic leak currents through the YSZ electrolyte of solid oxide cells have been calculated using basic solid state electrochemical relations and literature data. The distribution of the electromotive potential, of Galvani potential, of concentration of electrons, e, and electron holes, h, was also calculated as these parameters are the basis for the understanding of the electronic conductivity that causes the electronic leak currents. The results are illustrated with examples. The effects of electrolyte thickness, temperature and cell voltage on the electronic leak current density are reported

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Electromotive Potential Distribution and Electronic Leak Currents in Working YSZBased SOCsMogensen, Mogens Bjerg; Jacobsen, TorbenPublished in:E C S TransactionsLink to article, DOI:10.1149/1.3205660Publication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Mogensen, M. B., & Jacobsen, T. (2009). Electromotive Potential Distribution and Electronic Leak Currents inWorking YSZ Based SOCs. E C S Transactions, 25(2), 1315-1320. DOI: 10.1149/1.3205660Electromotive Potential Distribution and Electronic Leak Currents in Working YSZ Based SOCs  Mogens Mogensena, Torben Jacobsenb aFuel Cell and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, The Technical University of Denmark, DK - 4000 Roskilde, Denmark bDepartment of Chemistry, Building 2006, The Technical University of Denmark DK-2800 Lyngby, Denmark   The size of electronic leak currents through the YSZ electrolyte of solid oxide cells have been calculated using basic solid state electrochemical relations and literature data. The distribution of the electromotive potential, of Galvani potential, of concentration of electrons, e, and electron holes, h, was also calculated as these parameters are the basis for the understanding of the electronic conductivity that causes the electronic leak currents. The results are illustrated with examples. The effects of electrolyte thickness, temperature and cell voltage on the electronic leak current density are reported.  Introduction   A solid electrolyte will always possess a finite electronic conductivity. This has given rise to worries about the introduction of high oxygen partial pressures inside the electrolyte and about high leak currents through the yttria stabilized zirconia (YSZ) electrolyte of the solid oxide cell (SOC). Especially, problems may be foreseen in the solid oxide electrolyser cell (SOEC) in case of high cell voltages, which otherwise may be advantageous from an economic point of view, because the higher the cell voltage is, the higher is the hydrogen production rate.   Therefore, it is of considerable interest to get an estimate of which size of electronic current densities that will be present under various electrolysis conditions. The results reported here should be regarded as indicative only, because the precise properties of a zirconia based electrolyte will vary significantly with precise composition, and even low concentrations of some impurities may substantially alter the concentrations and mobility of electron carriers.    Concepts and Assumptions  Figure 1 outlines the basic concepts of electrical potentials in the context of a solid electrolyte. At a position outside a condensed phase in vacuum (see Fig. 1), the so called Volta potential, or outer potential, ψ, is defined as the work per unit charge needed to transport charge from infinity to the point in question. Differences in the Volta potential between two points in vacuum can be determined by the Kelvin probe technique.  The electric potential in the interior of the solid, the Galvani potential, φ, deviates from the Volta potential by the surface potential, χ. The electromotive potential, π, is ECS Transactions, 25 (2) 1315-1320 (2009)10.1149/1.3205660 ©  The Electrochemical Society1315Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jspdefined as e / F−π = −µ , i.e. the electrochemical potential of the electron divided by Faraday's number. This means that the electron concentration is an important contributor to the value of π.  For further details, please see ref. (1).   It is important to note that because the electron carrier (e and h) concentration inside the YSZ electrolyte is very low, and the oxide ion concentration is high and constant across the electrolyte, only a gradient in φ will cause migration of oxide ions.     Figure 1. Potentials in a solid conductor relative to the outer electric potential in vacuum.  The necessary equation for calculating the course of the electromotive potential across an SOC was established previously, and using the data for YSZ reported by Park and Blumenthal (2), the distribution of both the electromotive, the Galvani potentials and oxygen partial pressure across the cell was outlined in general (1).  This paper treats cells similar to the Risø DTU type of cells using data from cells measured in solid oxide fuel cell (SOFC) mode (3). The polarization resistances, Rp, are taken as independent of current density. This is a fair approximation for these cells. Further, 1 atm pressure, pure oxygen on the oxygen side, and a H2 + H2O mixture with an oxygen potential of -0.9 V (against pure O2 at 1 bar) on the hydrogen electrode were assumed. The potential of the LSM-YSZ (O2) electrode is taken as zero, i.e. as the reference.  Then, partly based on data from (3), the following numbers were used as basis for the calculations: At 850 °C Rp,Ni-YSZ = 0.05 Ω cm-2, the activation energy of the hydrogen electrode Ea,Ni = 0.8 eV, Rp,LSM-YSZ = 0.07 Ω cm-2, and the activation energy of the oxygen electrode Ea,LSM = 1.1 eV. All specific YSZ data were taken from Park and Blumenthal (2).   Results  The investigated parameters were cell voltage, temperature, and electrolyte thickness.  Apart from the curves of electromotive potential distribution, the cell current density (the oxide ion current density) and the electronic leak current densities were calculated. ECS Transactions, 25 (2) 1315-1320 (2009)1316Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp Figure 1 shows an example of a set of results including π, φ, and the concentrations ce and ch of the electrons and holes all as a function of the relative distance, x/L, from the interface between the Ni-YSZ (the H2O/H2) electrode and the electrolyte. x is the distance and L is the thickness of the electrolyte. It is seen that the electromotive potential, which to a large extent reflects the local concentration of e and h, and thus the "stability" of the YSZ, follows smooth curves, and does not assume any critical value. The concentration profiles of ce and cp show that the electrolyte part with the π-values left of the inflection points is predominantly n-type conductor (e is the charge carrier) and on the right side predominantly p-type conductor (h is the charge carrier).    Figure 2. Electromotive voltage, π, and electron and hole concentrations ce and ch, as functions of relative distance from the Ni-YSZ/YSZ interface at the open circuit voltage (OCV) of 0.9 V and the thermo-neutral voltage of 1.3 V. YSZ electrolyte thickness L = 20 µm, and temperature is 1000 °C. The calculated electronic leak current density, ieh = 16 mA cm-2, and the calculated ionic current density, iO = 11 A cm-2 at the cell voltage of 1.3 V. ieh = 4 mA cm-2 at OCV.  An electronic leak current density, ieh = 16 mA cm-2 and a cell current density = ionic current density, iO = 11 A cm-2 were calculated. The calculated cell current density becomes too high compared to experimental observation, because no mass transport limitations on the steam-hydrogen (Ni-YSZ) side was taken into account. The experimental observed iO at similar conditions is "only" about 3 A cm-2 (4). However, the actual value of iO is not very important for the value of ieh. The results of the calculations showed that the value of ieh is mainly determined by cell voltage, temperature and electrolyte thickness.  ECS Transactions, 25 (2) 1315-1320 (2009)1317Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jspFigure 3 illustrates some of the temperature effects. It is seen that the inflection point is close to the Ni-YSZ interface at 700°C, i.e. most of the electrolyte is p-type conductor. This inflection points moves noticeably towards the oxygen electrode at 1000 °C.     Figure 3. a) 700 °C, L = 20 µm, ieh = 0.26 µA cm-2, iO = 78 mA cm-2.  b) The same conditions as in Fig. 1: 1000 °C, L = 20 µm, ieh = 16 mA cm-2, iO = 11 A cm-2.  The electrolyte thickness of solid oxide cells reported in the literature varies from a few to a few hundreds of µm. Thus, ieh was calculated for the range of 2 to 200 ieh at 4 temperatures, 600, 700, 850 and 1000°C.  Steam electrolysis is an endothermic process. This means that electrolyser cell stack may be self cooling, if it is operated at a suitable voltage. If there is heat available from a non-electric source for evaporation water into steam, the thermo-neutral voltage cell is 1.3 V. If the cell stack also must produce the necessary heat for steam rising, then the voltage needed is close to 1.5 V. In case that there also is some heat loss in the system, then it may be necessary to increase the cell voltage to 1.6 V. Cell voltage higher than 1.6 V will usually not be advisable as this will result in a low energy conversion efficiency, see e.g. (4) and references therein. Thus, the relevant range of cell voltage is from OCV to 1.6  V.   Figure 4 shows the leak current density, ieh, as a function of electrolyte thickness at four temperatures at a cell voltage of 1.3 V. It is worth noting that the ordinate axis is logarithmic. The electron and hole mobility is thermal activated as well as the concentrations, and this causes an increase of ieh of about 500 times when going from 600 to 1000°C.  Further, it is observed that log ieh decreases approximately linearly with thickness above ca. 100 µm. Below this value, log ieh increases progressively with decreasing L.  Figure 5 presents ieh as a function of cell voltage for a 20 µm thick YSZ electrolyte at the same four temperatures as in Figure 4. It reveals that there a linear relationship between log ieh and cell voltage exists.  a) b)ECS Transactions, 25 (2) 1315-1320 (2009)1318Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp Figure 4. The leak current density, ieh, on log scale as a function of electrolyte thickness at four temperatures at a cell voltage of 1.3 V.  Figs. 4 and 5 together indicate that at a temperature of 1000°C, a thickness of 2 µm, and a potential of 1.6 V will give a substantial leak current density of e few 100 mA cm-2. Thus, this seems to be about or above the limit of what will be technological (economical) acceptable. Even at 850°C, a thickness of 2 µm may give a too high leak current density. However, for 10 µm at 850°C and cell voltage of 1.6 V, ieh will not come much above ca. 10 mA cm-2. As iO at such conditions will be 1 A cm-2 or more, the loss of efficiency due to the electron leakage will be below 1%, which probably will be acceptable.   Concluding Remarks  Calculations of electromotive potentials, electron carrier concentration, and from this, electronic leak current densities through the YSZ electrolyte of a solid oxide electrolyser cell revealed that at temperature of about 850°C or below the leakage will not pose a serious problem as long as a clean YSZ electrolyte is used.  However, if the electrolyte is contaminated with e.g. electrode materials during the cell fabrication, then the situation may be significantly worse. Thus, there is a need for more work – experimental as well as modeling - on this subject.  ECS Transactions, 25 (2) 1315-1320 (2009)1319Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp Figure 5.  Electronic leak current density, ieh, on a log scale as a function of cell voltage for a 20 µm thick YSZ electrolyte at the same four temperatures as in Figure 4.   Acknowledgments  This work was supported financially by The Programme Commission on Sustainable Energy and Environment, The Danish Council for Strategic Research, via the Strategic Electrochemistry Research Center (www.serc.dk), contract no. 2104-06-0011.   References  1. T. Jacobsen and M. Mogensen, ECS Transactions, 13, (26), 259 (2008). 2. J. H. Park and R. N. Blumenthal, J. Electrochem. Soc., 136, 2867 (1989). 3. R. Barfod, M. Mogensen, T. Klemenso, A. Hagen, Y-L. Liu, P. V. Hendriksen, J. Electrochem. Soc., 154, B371 (2007). 4. S. H. Jensen, P. H. Larsen, M. Mogensen, Int. J. Hydrogen Energy, 32, 3253 (2007).  ECS Transactions, 25 (2) 1315-1320 (2009)1320Downloaded 30 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

Electromotive Potential Distribution and Electronic Leak Currents in Working YSZ Based SOCs

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 17, 2017Electromotive Potential Distribution and Electronic Leak Currents in Working YSZBased SOCsMogensen, Mogens Bjerg; Jacobsen, TorbenPublished in:Meeting Abstracts - Electrochemical SocietyPublication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Mogensen, M. B., & Jacobsen, T. (2009). Electromotive Potential Distribution and Electronic Leak Currents inWorking YSZ Based SOCs. In Meeting Abstracts - Electrochemical Society (pp. Abstract 1283). ElectrochemicalSociety, Incorporated.Electromotive potential distribution and electronic leak currents in working YSZ based SOCs  M. Mogensen1, T. Jacobsen2 1Fuel Cell and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, The Technical University of Denmark, DK - 4000 Roskilde, Denmark, 2Deparment of Chemistry, Building 2006, The Technical University of Denmark, DK-2800 Lyngby, Denmark  A solid electrolyte will always possess a finite electronic conductivity. This given rise to worries about the introduction of high partial pressures inside and about too high leak currents through the yttria stabilized zirconia (YSZ) electrolyte of the solid oxide cell (SOC). Especially, problems have been predicted for the case of solid oxide electrolyser cell (SOEC).  The necessary equation for calculating the course of the electromotive potential across an SOC was established in a previous paper [1], and using the data for YSZ reported by Park and Blumenthal [2], the distribution of both the electromotive, the Galvani potentials and oxygen partial pressure across the cell was outlined in general.  This paper gives calculations for cells similar to the Risø DTU type of cells using data from cells measured in solid oxide fuel cell (SOFC) mode [3]. The polarization resistances, Rp, are taken as independent of current density. This is a fair approximation for these cells. Further, 1 atm pressure, pure oxygen on the oxygen side, and a mixture of 50 % steam, 50 % hydrogen on the hydrogen electrode were assumed. Then, partly based on data from [3] the following numbers were used as basis for the calculations: At 850 °C Rp,Ni-YSZ = 0.05 Ω cm-2, Ea = 0.8 eV, and Rp,LSM-YSZ = 0.07 Ω cm-2, Ea = 1.1 eV. All YSZ data were taken from [2]. The varied parameters were cell voltage, temperature, and electrolyte thickness.   Figure 1: Electromotive potential, π, as function of relative distance from the interface between the negative electrode and the electrolyte in a cell operating in SOEC mode at the thermo-neutral cell voltage of 1.3 V compared to open circuit conditions at 1000 °C. x/L = 1 is the interface to the oxygen electrode. The electrolyte thickness was set to 200 µm, which gave an oxide ion current density of -2.1 A cm-2. The electron leak current density is ∼ 10-4 A cm-2 under the SOEC operation condition.  Apart from the curves of electromotive potential, the cell current density (the oxide ion current density) and the electronic leak current densities were calculated.  Figs. 1 and 2 show two examples. The calculated electromotive potential as function of the distance from the interface between the Ni-YSZ (the H2O/H2) electrode and the electrolyte is given. The potential of the LSM-YSZ (O2) electrode is taken as zero, i.e. as the reference.   Figure 2: Electromotive potential, π, plotted like in Fig. 1 for 700 °C. The electrolyte thickness was set to 20 µm, which gave an oxide ion current density of -0.12 A cm-2. The electron leak current density is ∼4x10-6 A cm-2 under the SOEC operation at 1.3 V.  The electromotive potential, which reflects the local concentration of n and p (free electrons and holes), and thus the "stability" of the YSZ, follows smooth curves, and does not assume any critical value. The electrolyte part with the π-values left of the inflection points is predominantly n conductor and on the right side predominantly p conductor. The cell current densities look realistic, and most importantly, the electronic leak currents stay at insignificant low values compared to the current density of the cell. The calculated parameters as functions of cell voltage, temperatures and electrolyte thickness will be given in the paper.   Acknowledgements This work was supported financially by The Programme Commission on Sustainable Energy and Environment, The Danish Council for Strategic Research, via the SERC project (www.serc.dk), contract no. 2104-06-0011  References [1] T. Jacobsen and M. Mogensen, ECS Transactions, 13 (no.26), 259 (2008). [2] J.H. Park and R.N. Blumenthal, J. Electrochem. Soc., 136 2867 (1989). [3] R. Barford, M. Mogensen, T. Klemenso, A. Hagen, Y-L. Liu, P. V. Hendriksen, J. Electrochem. Soc., 154, B371 (2007).  216th ECS Meeting, Abstract #1283, © The Electrochemical Society

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Electromotive Potential Distribution and Electronic Leak Currents in Working YSZ Based SOCs

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