Cathode/electrolyte interface

Oxide ions are formed at the interface cathode/electrolyte from oxygen molecules according to the net reaction ... 

Sol-gel technique has been used to deposit solid electrolyte layers within the LSM cathode. The layer deposited near the cathode/electrolyte interface can provide ionic path for oxide ions, spreading reaction sites into the electrode. Deposition of YSZ or samaria-doped ceria (SDC, Smo.2Ceo.8O2) films in the pore surface of the cathode increased the area of TPB, resulting in a decrease of cathode polarization and increase of cell performance [15],... 

The consequences of the electrochemical reduction of high valence chromium species would be the precipitation of Cr203 solid phase at the cathode-electrolyte interface boundary. These led to the hypothesis that the degradation mechanism of LSM cathode is dominated by an electrochemical reduction of high valence vapor species of chromium (Cr03 and C OH O to solid phase Cr203 in competition with the 02 reduction reaction, followed by the chemical reaction with LSM to form (Cr,Mn)304 phases at the TPB, blocking the active sites [174-180], The process is written as follows [174] ... 

Hence, the presence of trace impurities, which either pre-exist in pristine electrode and bulk electrolyte or are introduced during the handling of the sample, could profoundly affect the spectroscopic images obtained after or during certain electrochemical experiments. This complication due to the impurities is especially serious when ex situ analytic means were employed, with moisture as the main perpetrator. For cathode/electrolyte interfaces, an additional complication comes from the structural degradation of the active mass, especially when over-delithiation occurs, wherein the decomposition of electrolyte components is so closely entangled with the phase transition of the active mass that differentiation is impossible. In such cases, caution should always be exercised when interpreting the conclusions presented. 

This sharp decline in cell output at subzero temperatures is the combined consequence of the decreased capacity utilization and depressed cell potential at a given drain rate, and the possible causes have been attributed so far, under various conditions, to the retarded ion transport in bulk electrolyte solutions, ° ° - ° ° the increased resistance of the surface films at either the cathode/electrolyte inter-face506,507 Qj. anode/electrolyte interface, the resistance associated with charge-transfer processes at both cathode and anode interfaces, and the retarded diffusion coefficients of lithium ion in lithiated graphite anodes. - The efforts by different research teams have targeted those individual electrolyte-related properties to widen the temperature range of service for lithium ion cells. 

The latter authors used anode and cathode symmetrical cells in EIS analysis in order to simplify the complication that often arises from asymmetrical half-cells so that the contributions from anode/ electrolyte and cathode/electrolyte interfaces could be isolated, and consequently, the temperature-dependences of these components could be established. This is an extension of their earlier work, in which the overall impedances of full lithium ion cells were studied and Ret was identified as the controlling factor. As Figure 68 shows, for each of the two interfaces, Ra dominates the overall impedance in the symmetrical cells as in a full lithium ion cell, indicating that, even at room temperature, the electrodic reaction kinetics at both the cathode and anode surfaces dictate the overall lithium ion chemistry. At lower temperature, this determining role of Ra becomes more pronounced, as Figure 69c shows, in which relative resistance , defined as the ratio of a certain resistance at a specific temperature to that at 20 °C, is used to compare the temperature-dependences of bulk resistance (i b), surface layer resistance Rsi), and i ct- For the convenience of comparison, the temperature-dependence of the ion conductivity measured for the bulk electrolyte is also included in Figure 69 as a benchmark. Apparently, both and Rsi vary with temperature at a similar pace to what ion conductivity adopts, as expected, but a significant deviation was observed in the temperature dependence of R below —10 °C. Thus, one... 

As has been shown in Eigure 68, since the time constants for these two electrochemical components, Rsei and Ra, are comparable at anode/electrolyte and cathode/electrolyte interfaces, respectively, the impedance spectra of a full lithium ion could have similar features in which the higher frequency semicircle corresponds to the surface films on both the anode and the cathode, and the other at lower frequency corresponds to the charge-transfer processes occurring at both the anode and the cathode. ... 

GaP/electrolyte interface. The electrolyte is 0.15M HN03 and the current density flowing through the interface is 20 mA/cm2. The low-energy limit of the spectrum is determined by the photomultiplier sensitivity, (b) Strongly cathodically biased p-GaP/electrolyte interface. Hot electrons are created by tunneling from valence to conduction bands. These may decay radioactively to fill empty states created by cation injection or drive other redox reactions. 

Fig. 3.9 Schematic showing both ohmic and activation losses, and the modeled discretized potential jump at the cathode-electrolyte interface.

Since electrons are produced at the anode-electrolyte interface, they proceed from this interface toward the current collector above the anode as shown in Figure 3.8. (The readeris reminded that it is a common convention to considerthe electric current direction as opposite to that of electron flow.) Due to ohmic losses, a potential decrease takes place as the current flows within the anode. At the cathode-electrolyte interface, a mass flux occurs, due to reaction (3.16). 

The reduction of the current flowing along the cell is furtllcr shown in Fig. ure 4.27, where the current distribution along the cathode-electrolyte interface is depicted, when the cell voltage is 0.7. 

Fig. 4.27 Current density (A/m2) distribution along the cathode/electrolyte interface for case 2 and V = 0.7.

It must be noted that the Comsol Multiphysics simulation does not take into account the electrochemical reactions occurring at the cathode-electrolyte and electrolyte-anode interfaces, with corresponding activation losses (which will be treated separately in Section 6.2.2). 

Cathode In contrast to the anode the cathode operates in an oxidizing environment but, like the anode, it must have high electronic conductivity and a pore structure enabling the gaseous oxidant to reach the cathode/electrolyte interface. 

LaMnOs perovskite. The Sr dopant provides for oxygen transfer to the cathode-electrolyte interface. 

Horita, T., Yamaji, K., Sakai, N., Xiong, X.P., Kato, T., Yokokawa, H., Kawada, T. Imaging of oxygen transport at SOFC cathode/electrolyte interfaces by a novel technique. J. Power Sources 2002,106, 224-30. 

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