Porous metal conductor

We consider the porous metal catalyst film shown in Figure 11.12 which is interfaced with an O2" conductor. When a positive current, I, is applied between the catalyst and a counter electrode, oxide ions O2 are supplied from the solid electrolyte to the three phase boundaries (tpb) solid electrolyte-metal-gas at a rate I/2F. Some of these O2 will form 02 at the tpb and desorb ... 

In addition to the modified electrodes described in the previous sections, which usually involve a conductive substrate and a single film of modifying material, more complicated structures have been described. Typical examples (Figure 14.2.4) include multiple films of different polymers (e.g., bilayer structures), metal films formed on the polymer layer (sandwich structures), multiple conductive substrates under the polymer film (electrode arrays), intermixed films of ionic and electronic conductor (biconductive layers), and polymer layers with porous metal or minigrid supports (solid polymer electrolyte or ion-gate structures) (6,7). These often show different electrochemical properties than the simpler modified electrodes and may be useful in applications such as switches, amplifiers, and sensors. 

The principle behind all galvanic cells can be explained with reference to one of the simplest, the Daniell cell, which was invented in 1836. It consists of a zinc rod in a solution of zinc sulphate, and a copper rod in a solution of copper sulphate. To complete the circuit, a porous solid layer, which allows ions to pass between the sulphate electrolytes, and an external metallic conductor between the zinc and copper, are needed. In this case, electrons then pass around the external circuit and ions travel through the electrolyte solutions (Figure 9.2). 

A trnly innovative soft ionization source, based on a nanometer-thick membrane, was developed. The gas sample passes through a porous membrane that is coated on both sides with a metallic conductor film. A low voltage (10 V) produces a large electric field (>10 V cm- ) that causes soft and efficient ionization of molecnles passing through the membrane. Despite its apparent advantages, this ionization method has not found its way to commercial devices. 

Figure 4.1.50 shows the structure of a composite electrode. The composite is made of a mixture of electrolyte and electronically conducting phases and has a thickness of 5 to 50 um. Since this layer usually has insufficient in-plane electronic conductivity for current collection, it is covered with a current collecting layer of porous electronic conductor. This can be made of the same substance as the electronic component of the composite, or another substance of high electronic conductivity. For laboratory testing, precious metal pastes are convenient for this purpose. The thickness of the current collecting layer is typically in the region of 50)an. 

Table 18.4 also contains those substances, which are used in the fluid state at normal temperatures for cathodes. Their features were already described when we dealt with them as electrolytes. They are used with and without a co-solvent, they build up on the lithium metal s surface stable passivation layers which are cracked only under electrical load when during discharge lithium ions leave the surface. These cathodes are especially powerful if combined with highly porous cathodic conductors. 

The basic experimental setup is shown schematically in Figure 13.10a. The metal working catalyst electrode, usually in the form of a porous metal film 3 to 20 pm in thickness, is deposited on the surface of a ceramic solid electrolyte (e.g., YSZ, an conductor, or P"-AI2O3, a Na+ conductor). Catalyst, counter, and reference electrode preparation and characterization details have been presented in detail elsewhere, together with the analytical system for on-line monitoring of the rates of catalytic reactions by means of gas chromatography, mass spectrometry and IR spectroscopy. 

If the two electrodes are connected through an external metallic conductor, a steady current of electrons will flow from the Zn electrode to the Cu electrode. At the Zn anode, a steady oxidation of Zn(s) takes place, cf. eqn. (6.21), and at the Cu cathode, a simultaneous reduction of Cu++ takes place, cf. eqn. (6.20). In the electrolyte the electron flow is balanced by transfer of ions through the porous partition. The net reaction, therefore, corresponds to the redox reaction, eqn. (6.19) the Zn electrode is dissolved by an anode process and metallic copper is precipitated on the Cu electrode through a cathode process. 

The anode material in SOF(7s is a cermet (rnetal/cerarnic composite material) of 30 to 40 percent nickel in zirconia, and the cathode is lanthanum rnanganite doped with calcium oxide or strontium oxide. Both of these materials are porous and mixed ionic/electronic conductors. The bipolar separator typically is doped lanthanum chromite, but a metal can be used in cells operating below 1073 K (1472°F). The bipolar plate materials are dense and electronically conductive. 

For example, consider a system in which metallic zinc is immersed in a solution of copper(II) ions. Copper in the solution is replaced by zinc which is dissolved and metallic copper is deposited on the zinc. The entire change of enthalpy in this process is converted to heat. If, however, this reaction is carried out by immersing a zinc rod into a solution of zinc ions and a copper rod into a solution of copper ions and the solutions are brought into contact (e.g. across a porous diaphragm, to prevent mixing), then zinc will pass into the solution of zinc ions and copper will be deposited from the solution of copper ions only when both metals are connected externally by a conductor so that there is a closed circuit. The cell can then carry out work in the external part of the circuit. In the first arrangement, reversible reaction is impossible but it becomes possible in the second, provided that the other conditions for reversibility are fulfilled. 

The author of this book has been permanently active during his career in the held of materials science, studying diffusion, adsorption, ion exchange, cationic conduction, catalysis and permeation in metals, zeolites, silica, and perovskites. From his experience, the author considers that during the last years, a new held in materials science, that he calls the physical chemistry of materials, which emphasizes the study of materials for chemical, sustainable energy, and pollution abatement applications, has been developed. With regard to this development, the aim of this book is to teach the methods of syntheses and characterization of adsorbents, ion exchangers, cationic conductors, catalysts, and permeable porous and dense materials and their properties and applications. 

Mechanical Passivity.—In certain instances the dissolution of an anode is prevented by a visible film, e.g., lead dioxide on a lead anode in dilute sulfuric acid this phenomenon has been called mechanical passivity, but it is probably not fundamentally different from the forms of passivity already discussed. The film is usually not completely impervious, but merely has the effect of decreasing the exposed surface of the electrode to a considerable extent the effective c.d. is thus increased until another process in which the metal is involved can occur. At a lead anode in sulfuric acid, for example, the lead first dissolves to form plumbous ions which unite with the sulfate ions in the solution to form a porous layer of insoluble lead sulfate. The effective c.d. is increased so much that the potential rises until another process, viz., the formation of plumbic ions, occurs. If the acid is sufficiently concentrated these ions pass into solution, but in more dilute acid media lead dioxide is precipitated and tends partially to close up the pores the layer of dioxide is somewhat porous and so it increases in thickness until it becomes visible. Such an oxide is not completely protective and attack of the anode continues to some extent it is, however, a good conductor and so hydroxyl ions are discharged at its outer surface, and oxygen is evolved, in spite of its thickness. 

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