Ceramic electrode current density

The vapor pressure of potassium is much higher than that of lithium, and potassium can form an undesirable gas phase. Lithium dissolved in the electrolyte migrates toward the positive electrode where it is consumed in an unproductive chemical reaction. For an interelectrode distance of 1 mm, the associated self-discharge is equivalent to a leakage current density of 1-10 mA/cm. Moreover, dissolved lithium causes disintegration of the ceramic separators. The solubility of lithium greatly increases with increasing temperature. 

Figure I.6a also reveals the timeline of milestones in fuel cell design. The leftmost curve is the performance curve of the first practical H2/O2 fuel cell, built by Mond and Langer in 1889 (Mond and Langer, 1889). The electrodes consisted of thin porous leafs of Pt covered with Pt black particles with sizes of 0.1 lam. The electrol)de was a porous ceramic material, earthenware, that was soaked in sulfuric acid. The Pt loading was 2 mg cm and the current density achieved was about 0.02 A cm at a fuel cell voltage of 0.6 V. The next curve in Figure I.6a marks the birth of the PEFC, conceived by Grubb and Niedrach (Grubb and Niedrach, 1960). In this cell, a sulfonated cross-linked polystyrene membrane served as gas separator and proton conductor. However, the proton conductivity of the polystyrene PEM was too low and the membrane lifetime was too short for a wider use of this cell. It needed the invention of a new class of polymer electrolytes in the form of Nafion PFSA-type PEMs to overcome these limitations.

The efficiency of this cell depends on the electrode materials and their ability to produce hydrogen at low potentials, on the performance of the membrane, and on the electrolytic solution. Various relatively inexpensive electrode materials have been developed, and hydrogen production has been achieved at potentials as low as 0.5 V with ceramic carbon electrodes (Ranganathan and Easton, 2010). Additionally, several membranes have been investigated to identify those with lower copper diffusion rates but similar proton conductivities (Naterer et al., 2014 Ranganathan and Easton, 2010). Depending on these aspects and hence the current density, efficiencies ranging from 15% to 95% have been achieved (Hall et al., 2014). 

Table II. Comparison of modulation wavelengths calculated from Faraday s law (Ap) and from x-ray satellite spacings (A ) for ceramic superlattices deposited at various current densities (J) and dwell times (t). X represents the individual layer thickness calculated from Faraday s law. The electrode area ranged from 0.78 to 1.96 cm. ...

Fig. 8 Change of capacitance of CNTs, PPy and PPy-CNTs composite electrode on ceramic fabric as a function of the number of charge-discharge cycles at current density of 1 mA cm (Reproduced with permission from [48])...

Figure 6.11 Mathematical modelling of dependence of polarisation on electrode thickness (after [29]). volumefraction of the electronic conductor (nickel) in the cermet exchange current density i = 0.l Ajcm - conductivity of the metal component a= 2 x lO S/m, oxygen ionic conductivity of the ceramic component a,= 15 S/m, grainsizeofbothcomponents 1 pm.

Given the fuel utilization factor, the temperature drop causes a decrease in ionic conductivity on the ceramic phases (especially in the electrolyte layer, but also on the electrodes) and in electronic conductivity on the electrodes (albeit almost negligible). Also, the reaction kinetic is reduced (reduction in the macroscopic parameter exchange current density). Finally, the diffusion capability of the chemical species is reduced both on the bulk flow in the channels, and especially on the porous electrode (also in terms of the adsorption mechanism at the catalyst site), reducing the macroscopic parameter anode limiting current density. 

Fig. 16-10. As the gap sparks over, power-follow current is initiated and a magnetic field is established, setting up magnetic flux in the gap area. Flux-field reinforcers, with a higher permeability than air, lower the reluctance of the flux path, causing an increase in flux density in the arc path. Reinforcement of the field behind the arc causes the follow-current arc to move rapidly away from the spark gap toward the auxiliary electrodes. Courtesy Mykroy Ceramics Co.)...

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