Contacts, solid electrolytes

In solid-state batteries, it is extremely favorable to use the solid electrolyte for mechanical support. Despite the larger thickness, which lowers the relative amount for active material in the battery, the advantages are the absence of pinholes of the solid electrolyte, high electronic resistance, and simple multistack fabrication, since the individual cells may be contacted by their electronically conducting current collectors. 

Equation (40) relates the lifetime of potential-dependent PMC transients to stationary PMC signals and thus interfacial rate constants [compare (18)]. In order to verify such a correlation and see whether the interfacial recombination rates can be controlled in the accumulation region via the applied electrode potentials, experiments with silicon/polymer junctions were performed.38 The selected polymer, poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether, or technically (Hydrine-T), to which lithium perchlorate or potassium iodide were added as salt, should not chemically interact with silicon, but can provide a solid electrolyte contact able to polarize the silicon/electrode interface. 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

Several approaches have been proposed to measure the three phase boundary (tpb) length, Ntpb in solid state electrochemistry. The parameter Ntpb expresses the mol of metal electrode in contact both with the solid electrolyte and with the gas phase. More commonly one is interested in the tpb length normalized with respect to the surface area, A, of the electrolyte. This normalized tpb length, denoted by Ntpb,n equals Ntpt/A. 

A first approximate approach for estimating N,pb, or /tpb.n is to use scanning electron microscopy to estimate the average grain size, d, of the electrode. One then assumes spherical grains for the electrode film and semispherical grains in contact with the solid electrolyte to obtain ... 

As shown in Fig. 12.7 application of a potential between the two terminal Au electrodes induces NEMCA on the Pt film which is not connected to any metal wire but is simply in contact with the YSZ solid electrolyte. 

A considerable decrease in platinum consumption without performance loss was attained when a certain amount (30 to 40% by mass) of the proton-conducting polymer was introduced into the catalytically active layer of the electrode. To this end a mixture of platinized carbon black and a solution of (low-equivalent-weight ionomeric ) Nafion is homogenized by ultrasonic treatment, applied to the diffusion layer, and freed of its solvent by exposure to a temperature of about 100°C. The part of the catalyst s surface area that is in contact with the electrolyte (which in the case of solid electrolytes is always quite small) increases considerably, due to the ionomer present in the active layer. 

When a solid electrolyte is in contact with an electrolyte solution, with common anion Az (z <0), then the Galvani potential difference between the solid electrolyte and the solution is given by the equation... 

Two solid electrolytes in contact with a common ion show behaviour analogous to two metals in contact. 

The electrode is considered to be a part of the galvanic cell that consists of an electronic conductor and an electrolyte solution (or fused or solid electrolyte), or of an electronic conductor in contact with a solid electrolyte which is in turn in contact with an electrolyte solution. This definition differs from Faraday s original concept (who introduced the term electrode) where the electrode was simply the boundary between a metal and an electrolyte solution. 

A solid electrolyte is an ionic conductor and an electronic insulator. Ideally, it conducts only one ionic species. Aside from a few specialty applications in the electronics industry, solid electrolytes are used almost exclusively in electrochemical cells. They are particularly useful where the reactants of the electrochemical cell are either gaseous or liquid however, they may be used as separators where one or both of the reactants are solids. Used as a separator, a solid electrolyte permits selection of two liquid or elastomer electrolytes each of which is matched to only the solid reactant with which it makes contact. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Currently interest has now been directed toward a similar high temperature system, the ZEBRA Battery, which also uses P-alumina as a Na ion conductor. The sulfur electrode is replaced by nickel chloride or by a mixture of ferrous and nickel chlorides. Contact between the NiCl2 electrode and the solid electrolyte is poor as they are both solids, and current flow is improved by adding a second liquid electrolyte (molten NaAlCb) between this electrode and the P-alumina. The overall cell reaction is now ... 

This electrode, also called the glass electrode, is specific to H+ ions. Glass in this case does not refer to the material of the electrode body but to the membrane that ensures contact with the solution. The membrane is a thin wall of glass that has a very high sodium content (25%). In the presence of water, hydration occurs and the membrane s surface becomes comparable to a gel while its interior corresponds to a solid electrolyte. 

The lambda sensor, which is found in cars with catalytic converters, is an example of an oxygen probe based on the principle of selective electrodes. This sensor, which looks like a spark plug, has a zirconium sleeve (Zr02) that behaves as a solid electrolyte. The external wall is in contact with emitted gas while the internal wall (the reference) is in contact with air. Two electrodes measure the potential difference between the two walls, which is indicative of the difference in concentration of oxygen. 

Electrodes of this form have a minimum of two phases, namely a metal (either in pure form, or in an amalgam or solid solution) in contact with an electrolyte phase (solution, melt, solid electrolyte, glass, etc.) containing cations of the metal. 

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