Extended area electrodes

L. Lipp and D. Pletcher. Extended area electrodes based on stacked expanded titanium meshes. Electrochimica Acta 42 (1997) 1101-1111. 

Figure 6.2 Various configurations for extended-area polymer batteries. From left to right concertino, Swiss-roll and flat-plate versions. Electrode 1 (anode), electrolytic membrane, electrode 2 (cathode), current collector.

An effectiveness value greater than one indicates that the porous electrode is more effective than an electrode of the same geometric surface area, and that the reaction extends into the porous electrode stmcture. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Work in this area has been conducted in many laboratories since the early 1980s. The electrodes to be used in such a double-layer capacitor should be ideally polarizable (i.e., all charges supplied should be expended), exclusively for the change of charge density in the double layer [not for any electrochemical (faradaic) reactions]. Ideal polarizability can be found in certain metal electrodes in contact with elelctrolyte solutions free of substances that could become involved in electrochemical reactions, and extends over a certain interval of electrode potentials. Beyond these limits ideal polarizability is lost, owing to the onset of reactions involving the solvent or other solution components. 

The third microhotplate introduced in Sect. 4.3 was designed to extend the operation temperature limit imposed by the CMOS-metallization contacts in the heated area. A new heater design was devised, and a microfabrication sequence that enables the realization of Pt temperature sensors and Pt-electrodes was developed. This microhotplate was also monolithically integrated with circuitry as presented in Sect. 5.2, and operating temperatures of up to 500 °C have been achieved. 

In order to extend the effective electrode area in principle three-dimensional electrodes are possible, for example, by using a packed particle bed, a sintered or foamed metal, or a graphite fiber felt. But the depth of the working electrode volume usually is only small (it is dependent on the ratio of the electrode and electrolyte conductivity, for example, [45]). 

Most practical electrodes are a complex composite of powders composed of particles of the active material, a conductive diluent (usually carbon or metal powder), and a polymer binder to hold the mix together and bond the mix to a conductive current collector. Typically, a composite battery electrode has 30% porosity with a complex surface extending throughout the volume of the porous electrode. This yields a much greater surface area for reaction than the geometric area and lowers polarization. The pores of the electrode structures are filled with electrolyte. 

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