Extrapolated Conductivity Data (taken Directly From Ref. [24] Without...

Figure 1 - uploaded by Yanhao DongContent may be subject to copyright.DownloadView publicationCopy referenceCopy captionEmbed figureExtrapolated conductivity data (taken directly from Ref. [24] without subtracting O  − ) Source publication +3 Oxygen Potential Transition in Mixed Conducting Oxide ElectrolyteArticleFull-text available

• Jul 2018

• Yanhao Dong
• I-Wei Chen

It is generally assumed that oxygen potential in a thin oxide electrolyte follows a linear distribution between electrodes. Jacobsen and Mogensen have shown, however, that this is not the case for thin zirconia membranes in solid oxide electrochemical cells. Here we demonstrate that there is a ubiquitous oxygen potential transition rooted in the p-...CiteDownload full-text

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Context 1... we need to specify not only the boundary potentials but also the flux, which can be jcharge, O J , or their linear combination. This is the essence of the problem. The non-linear conductivity in Fig. 1 makes finding the solution more cumbersome and the transition sharper but does not fundamentally alter the nature of the ...View in full-textContext 2... have associated O − diffusion to either an electron-mediated mechanism-a lattice O 2− exchanges position with an electron-tagged oxygen vacancy ( ) Fig. 1. Following the notation of σe* and σh* above and setting the total electronic conductivity the same as Park and Blumenthal's ...View in full-textContext 3... respectively; without O − , αe=αh=0. (As it will become clear later in this subsection, this is but one form of internal reactions between electronic species and ionic species.) Evidence for internal reactions between lattice defects and electrons/holes was already reviewed in Introduction. From Fig. 6, it is clear that a substantial αe or αh is needed for an appreciable effect of O − diffusion, hence to explain our experimental results summarized in the previous subsection. This in turn requires a large degree of association of oxygen vacancy with electron (e.g., ( ) .) Note that if there were no association/trapping at lattice/defect sites, this result would be very surprising because the mobilities of electron and hole should have been much larger than that of oxygen ion. But a strong association is consistent with the glassy energy landscape (analogous to a "compositional glass") of YSZ, which offers many lattice sites for possible electron/hole association. [39,40] It is also consistent with the observation that the activation energy of oxygen diffusion varies from 0.5 eV above 1000 o C, to 0.79 eV at lower temperature, because a stronger association is expected at lower temperature when the configurational entropy against association is less important. These observations support our opinion that the data in Fig. 1, which has been attributed to electrons and holes in the past, is likely to have a substantial contribution from O − or the like, especially at lower temperature; i.e., αe and αh increases at lower temperature. However, the fact that the two branches in dependent. This is reasonable since the concentrations of electrons and holes are usually rather low compared to that of oxygen ...View in full-textContext 4... That is, the sum of the red and blue curves in Fig. 1 is not eh  + but ...View in full-textContext 5... we already proposed in Ref. [16], the transition can be conceptually visualized as an p-n junction: A p-type region with a high hole conductivity, an n-type region with a high electron conductivity, joined by a junction with a huge junction resistance because of minimal electronic conductivity as shown in Fig. 1. Without any internal reaction, the electronic current is totally decoupled from the ionic current and cannot receive any assistance from ionic conductivity, so it must face the junction resistance alone despite the fact that the ionic conductivity well exceeds the electronic conductivity everywhere. As the huge junction resistance demands a huge driving force, which is provided by the steep slope of the potential, it gives rise to the potential ...View in full-textContext 6... Given the potential transition that is approximately antisymmetric and lying in the mid-section, the resulting strain profile should also be antisymmetric with a "neutral axis" lying at the mid-section (at the potential of the conductivity minimum in Fig. 1), which is the same profile as seen in a bent beam. In fact, fixing the transition near the mid-section will cause a different amount of bending from that caused by a transition near the electrodes, which will in turn lead to a different compensating elastic bending to make the entire section free of bending moment overall. Since it is the latter stress that remains and may possibly result in cracking [18,19], oxygen potential transition and how it depends on the defect charge states may affect device ...View in full-textContext 7... transitions in Fig. 4c-d and Fig. 6a-e again occur at the potential at the minimum of the combined red-blue curve in Fig. 1 Fig. 4, this falls at 2 eV, which explains why there is a wider flat region at such potential in the figure, hence cathode localization therein. Obviously, it is entirely possible to "switch side" to anode localization by adjusting boundary potentials so that the anode side offers a larger ** eh  + . This is illustrated in Fig. 6f. The switch-over can be quite abrupt, e.g., from curves ...View in full-textContext 8... this picture, we can also see that, if the experiment is performed under such condition that the terminal potentials do not traverse the two sides of the conductivity bottom in Fig. 1, then there is no p-n junction at all. Indeed, when we performed the experiment in hydrogen gas or argon [16], whose oxygen potential is expected to always lie to the left of the conductivity bottom, we observed a much more gradual variation in grain size without a sharp transition, which corresponds to having the entire electrolyte placed into the n-type region. (The calculated oxygen potential distributions, with O − conduction, for the above two cases were previously reported as Fig. 9 in Ref. [16].) These observations provided further support to our analysis of oxygen potential ...View in full-textContext 9... of mode and current density, solutions in Fig. 3a display a transition over a short distance (on the order of 10 μm), from a low oxygen potential on one side to a high oxygen potential on the other side. The inflection point of the transition is always around −2 eV, which corresponds to the potential where the minimum electronic conductivity lies at 1000 o C in Fig. 1. This is not coincidental. While the electronic current can be readily supported by the large electronic conductivity available at the high/low oxygen potential on the two sides, it is hampered by the minimum electronic conductivity in the middle, at about −2 eV, of [25], it must be traversed and it holds the steepest oxygen potential gradient when forcing through a steady-state electronic current. Such is the origin of the oxygen potential ...View in full-textContext 10... we need to specify not only the boundary potentials but also the flux, which can be jcharge, O J , or their linear combination. This is the essence of the problem. The non-linear conductivity in Fig. 1 makes finding the solution more cumbersome and the transition sharper but does not fundamentally alter the nature of the ...View in full-textContext 11... have associated O − diffusion to either an electron-mediated mechanism-a lattice O 2− exchanges position with an electron-tagged oxygen vacancy ( ) Fig. 1. Following the notation of σe* and σh* above and setting the total electronic conductivity the same as Park and Blumenthal's ...View in full-textContext 12... respectively; without O − , αe=αh=0. (As it will become clear later in this subsection, this is but one form of internal reactions between electronic species and ionic species.) Evidence for internal reactions between lattice defects and electrons/holes was already reviewed in Introduction. From Fig. 6, it is clear that a substantial αe or αh is needed for an appreciable effect of O − diffusion, hence to explain our experimental results summarized in the previous subsection. This in turn requires a large degree of association of oxygen vacancy with electron (e.g., ( ) .) Note that if there were no association/trapping at lattice/defect sites, this result would be very surprising because the mobilities of electron and hole should have been much larger than that of oxygen ion. But a strong association is consistent with the glassy energy landscape (analogous to a "compositional glass") of YSZ, which offers many lattice sites for possible electron/hole association. [39,40] It is also consistent with the observation that the activation energy of oxygen diffusion varies from 0.5 eV above 1000 o C, to 0.79 eV at lower temperature, because a stronger association is expected at lower temperature when the configurational entropy against association is less important. These observations support our opinion that the data in Fig. 1, which has been attributed to electrons and holes in the past, is likely to have a substantial contribution from O − or the like, especially at lower temperature; i.e., αe and αh increases at lower temperature. However, the fact that the two branches in dependent. This is reasonable since the concentrations of electrons and holes are usually rather low compared to that of oxygen ...View in full-textContext 13... we already proposed in Ref. [16], the transition can be conceptually visualized as an p-n junction: A p-type region with a high hole conductivity, an n-type region with a high electron conductivity, joined by a junction with a huge junction resistance because of minimal electronic conductivity as shown in Fig. 1. Without any internal reaction, the electronic current is totally decoupled from the ionic current and cannot receive any assistance from ionic conductivity, so it must face the junction resistance alone despite the fact that the ionic conductivity well exceeds the electronic conductivity everywhere. As the huge junction resistance demands a huge driving force, which is provided by the steep slope of the potential, it gives rise to the potential ...View in full-textContext 14... That is, the sum of the red and blue curves in Fig. 1 is not eh  + but ...View in full-textContext 15... Given the potential transition that is approximately antisymmetric and lying in the mid-section, the resulting strain profile should also be antisymmetric with a "neutral axis" lying at the mid-section (at the potential of the conductivity minimum in Fig. 1), which is the same profile as seen in a bent beam. In fact, fixing the transition near the mid-section will cause a different amount of bending from that caused by a transition near the electrodes, which will in turn lead to a different compensating elastic bending to make the entire section free of bending moment overall. Since it is the latter stress that remains and may possibly result in cracking [18,19], oxygen potential transition and how it depends on the defect charge states may affect device ...View in full-textContext 16... this picture, we can also see that, if the experiment is performed under such condition that the terminal potentials do not traverse the two sides of the conductivity bottom in Fig. 1, then there is no p-n junction at all. Indeed, when we performed the experiment in hydrogen gas or argon [16], whose oxygen potential is expected to always lie to the left of the conductivity bottom, we observed a much more gradual variation in grain size without a sharp transition, which corresponds to having the entire electrolyte placed into the n-type region. (The calculated oxygen potential distributions, with O − conduction, for the above two cases were previously reported as Fig. 9 in Ref. [16].) These observations provided further support to our analysis of oxygen potential ...View in full-textContext 17... transitions in Fig. 4c-d and Fig. 6a-e again occur at the potential at the minimum of the combined red-blue curve in Fig. 1 Fig. 4, this falls at 2 eV, which explains why there is a wider flat region at such potential in the figure, hence cathode localization therein. Obviously, it is entirely possible to "switch side" to anode localization by adjusting boundary potentials so that the anode side offers a larger ** eh  + . This is illustrated in Fig. 6f. The switch-over can be quite abrupt, e.g., from curves ...View in full-textContext 18... of mode and current density, solutions in Fig. 3a display a transition over a short distance (on the order of 10 μm), from a low oxygen potential on one side to a high oxygen potential on the other side. The inflection point of the transition is always around −2 eV, which corresponds to the potential where the minimum electronic conductivity lies at 1000 o C in Fig. 1. This is not coincidental. While the electronic current can be readily supported by the large electronic conductivity available at the high/low oxygen potential on the two sides, it is hampered by the minimum electronic conductivity in the middle, at about −2 eV, of [25], it must be traversed and it holds the steepest oxygen potential gradient when forcing through a steady-state electronic current. Such is the origin of the oxygen potential ...View in full-text

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... Since conductivity is proportional to the concentration of charge carriers, and electron and hole concentrations exhibit an exponential dependence on the chemical potential of the species of interest, electron and hole conductivities are highly nonlinear and can vary by several orders of magnitude. For example, in YSZ, electronic conductivity can fluctuate by 4-5 orders of magnitude even at high temperatures exceeding 1273 K [20]. Due to their temperature dependence, this variation becomes even more pronounced at lower temperatures, making electronic conductivities even more nonlinear at ambient temperatures relevant to solid-state batteries. ...... To derive a closed-form solution for the Li 0 chemical potential profile inside SE separators, we assume a hypothetical Li conductor with mixed ionic and electronic conductions and focus on a one-dimensional (1D) problem along the -direction. Assumptions similar to those made in Ref. [20] for the numerical modeling of oxygen conductors in solid oxide fuel/electrolysis cells are adopted. Specifically, we consider only the motion of Li + ions and electrons, given the limited mobilities of other species (e.g., La 3+ , Zr 4+ , and O 2− in Li 7 La 3 Zr 2 O 12 (LLZO)) at room temperature. ...Electrochemical potential in multilayer solid electrolytes and mechanical implicationsArticle

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• Qizhang Yan
• Jian Luo

Electric fields and currents, which are used in innovative materials processing and electro-chemical energy conversion, can often alter microstructures in unexpected ways. However, little is known about the underlying mechanisms. Using ZnO-Bi 2 O 3 as a model system, this study uncovers how an applied electric current can change the microstructural evolution through an electrochemically induced grain boundary transition. By combining aberration-corrected electron microscopy, photoluminescence spectroscopy, first-principles calculations , a generalizable thermodynamic model, and ab initio molecular dynamics, this study reveals that electrochemical reduction can cause a grain boundary disorder-to-order transition to markedly increase grain boundary diffusivities and mobilities. Consequently, abruptly enhanced or abnormal grain growth takes place. These findings advance our fundamental knowledge of grain boundary complexion (phase-like) transitions and electric field effects on microstructural stability and evolution, with broad scientific and technological impacts. A new method to tailor the grain boundary structures and properties, as well as the microstructures, electrochemically can also be envisioned.ViewShow moreGet access to 30 million figuresJoin ResearchGate to access over 30 million figures and 160+ million publications – all in one place.Join for freeAdvertisementJoin ResearchGate to find the people and research you need to help your work

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