Ionic conductivity interconnects

Every ionic crystal can formally be regarded as a mutually interconnected composite of two distinct structures cationic sublattice and anionic sublattice, which may or may not have identical symmetry. Silver iodide exhibits two structures thermodynamically stable below 146°C sphalerite (below 137°C) and wurtzite (137-146°C), with a plane-centred I- sublattice. This changes into a body-centred one at 146°C, and it persists up to the melting point of Agl (555°C). On the other hand, the Ag+ sub-lattice is much less stable it collapses at the phase transition temperature (146°C) into a highly disordered, liquid-like system, in which the Ag+ ions are easily mobile over all the 42 theoretically available interstitial sites in the I-sub-lattice. This system shows an Ag+ conductivity of 1.31 S/cm at 146°C (the regular wurtzite modification of Agl has an ionic conductivity of about 10-3 S/cm at this temperature). 

The cathode of a battery or fuel cell must allow good ionic conductivity for the ions arriving from the electrolyte and allow for electron conduction to any interconnects between cells and to external leads. In addition these properties must persist under oxidizing conditions. An important strategy has been to employ layered structure solids in which rapid ionic motion occurs between the layers while electronic conductivity is mainly a function of the layers themselves. 

Cathode The primary functions of the cathode are catalyzing the O2 reduction and transporting the O2 ions to the electrolyte. Furthermore, the cathode should possess sufficient electronic conductivity to lead the electrons from the interconnect to the reaction sites. At practical operating conditions, the ionic conductivity of LSM is several orders of magnitude lower than its electronic conductivity. Therefore, in the single-phase and two-phase LSM type cathodes presented in the introduction the reaction sites are essentially at the TPBs of air, YSZ and LSM. Mixing the LSM with YSZ is one means, apart from refining the micro-... 

Figure 12.9 Arrhenius plots of the ionic conductivity of selected electrolytes. The temperature ranges of utilization of interconnects materials are also indicated. For electrolyte thicknesses > ISOum, the cell can be supported by the ionic membrane.

A major degradation mechanism in SOFC is poisoning of the cathode by chromium from volatilization of the interconnect material. The chromium deposition has been attributed to both chemical and electrochemical mechanisms. For an electrochemical reaction, deposition can occur only where both ions and electrons are available, which, for a purely ionic conducting electrolyte and a purely electronic conducting cathode, can occur only at the three-phase gas-electrolyte-electrode interface. However, the introduction of ionic conductivity into the cathode or electronic conductivity into the electrolyte can allow deposition to occur away from this... 

One of the most important characteristics that reflect the performance of a PEM in DMFC is proton conductivity. It has been reported that the proton conductivity depends on the DS, pretreatment of the membrane, hydration state, and ambient relative humidity and temperature. For ionomeric membranes, the proton conductivity depends on the amount of add groups attached to the polymer ring and their dissociation capability in water, which is accompanied by the generation of protons. The high ionic conductivity demonstrated by the membrane at high sulfonation level suggests that the water-swollen ionic domains in the membrane pores were interconnected to form a network structure. Water molecules also dissodate acid functionality and facilitate proton transport. Therefore, it can be deduced that water uptake is an important parameter in proton conductivity tests (Bauer et al. 2000). 

Equations 7.60 and 7.61 are the general representation of the ohmic resistance and ohmic voltage loss, respectively. The total ohmic voltage loss is the sum of all ohmic loss components owing to electronic resistances in interconnects and electrodes, and ionic conductivity in the electrolyte as shown in Figure 7.7. 

Generally, in a fuel cell, the electrolyte ohmic overpotential is the dominant component of the ohmic overpotential owing to the lower ionic conductivity value as compared to the electronic conductivity of electrodes and interconnect materials. Research effort to improve ohmic loss in a fuel cell is, therefore, focused on the improvement of the electrolyte in terms of higher ionic conductivity and lower thickness. Use of a thinner electrol5de is limited by a number of factors such as structural integrity, manufacturability and defects, increased parasitic loss owing to fuel crossover loss, and dielectric limit of the electrolyte. 

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