Multilayered electrodes

In addition to the above criteria, an insulator may be used to mask the final electrode layer and to define the size of the geometric area exposed to the solution. 

---

Apart from molecular, or supramolecular design, modified electrodes can be improved by microstructuring. Two different strategies have been developed multilayered electrodes 272) of microelectrodes 6,273)... 

The combination of various SOFC component performance, microstructural, and property requirements has led to a variety of structures, such as the composite, graded, and multilayered electrodes and electrolytes described above. The need... 

Gauging catalysis by reference to an electrode where electrons are delivered (or eaten up) in an outer-sphere manner, redox catalysis is not expected to operate at a monolayer coated electrode (Figure 4.10), since, as discussed in Section 4.2.1, redox catalysis results from the three-dimensional dispersion of the catalyst. In contrast, there is no reason that chemical catalysis could not be operative at a monolayer coated electrode. For the same reasons, both redox catalysis and chemical catalysis are expected to function at multilayer electrode coatings (Figure 4.10). 

FIGURE 4.10. Catalysis at monolayer and multilayer electrode coatings. 

Catalysis at multilayered electrode coatings is then addressed. Besides the rate of the catalytic reaction within the film and the diffusion of the substrate and products between the bulk of the bathing solution and the film-solution interface, the current response depends on two additional factors permeation of the substrate through the film, and transport of electrons through the film. Analysis of the first of these factors also involves a discussion of the inhibition of the electrode electron transfer that the presence of a film on the electrode surface may cause, whether the electrode is covered by a monolayer or by a thicker film. This discussion also addresses the important case where inhibition is due to deposition onto the electrode surface of one of the reaction products. 

Combining an Electron-Shuttling Mediator with a Chemical Catalyst in a Multilayer Electrode Coating... 

Figure 81. Schematic of multilayer electrode geometry with nanometer scale insulator spacing (AI2O3) between metal electrodes (Ni, NiFe, Ta, and Au). [(pzTp)Fe (CN)3]4[Ni ((pz)3C(CH2) S)]4 " molecules are bridging across insulator to provide molecular current path. [Adapted from (280)]...

Electrode surfaces modified with a multilayered surface architecture prepared by a layer-by-layer repeated deposition of several enzyme mono-layers show a modulated increase of surface-bound protein with a subsequent increase in output current, which is directly correlated with the number of deposited protein layers. The versatility of this approach allows alternate layers of different proteins for the manufacture of electrode surfaces, which are the basis for multianalyte sensing devices with multiple substrate specificities. The surface chemistry used for the manufacture of multilayered electrode surfaces is similar to that previously described for the preparation of affinity sensors, and is based on the stabilization of self-assembled multilayer assemblies by specific affinity interactions, electrostatic attraction, or covalent binding between adjacent monolayers. 

The linear approach described here is expandable to multienzyme electrodes as well as multilayer electrodes. At least for the stationary case, multilayer models of bienzyme electrodes may be easily treated, too. The whole system is readily adaptable to potentiometric electrodes (Carr and Bowers, 1980). It must be noted, however, that the superiority over purely numerical solution procedures decreases with increasing number of enzyme species and in the multilayer model. The advantage in calculation speed using the sum formulas described (e.g., in Section 2.5.2) amounts to about two orders of magnitude. With multilayer electrodes and formulas containing double and triple sums it is reduced to one order of magnitude. 

Fig. 92. Schematic and response curve of successive determination of hypoxanthine (HX) and inosine (HXR) with a multilayer electrode. l xanthine oxidase, 2 cellulose triacetate membrane, 3 nucleoside phosphorylase, 4 oxygen electrode. (Redrawn from Watanabe et al., 1986).

Fig. 7 Mechanism of cathodic current generation by 13Zn/ITO multilayer electrodes. Adapted from [36]...

Over the subsequent LF insertion/extraction cycles, each column remains attached to the current collector and at the same time does not crack along its vertical axis, which explains the excellent cycling behavior of these electrodes. It also is possible to produce silicon-metal multilayer structures by means of sequential sputtering of silicon and other metals such as cobalt, iron, and zirconium. However, such thin multilayer electrodes might be useful only in thin-film microbatteries, because the thickness of these films is in the order of 120 nm, which means a capacity of about 0.05 mA h cm. Consequently, the working current density is around 30 p.A cm... 

Figure 7.11 Electron micrographs of a cross section of a multilayer electrode composed of bilayers of Pt on Sb-doped tin oxide and multiwalled carbon nanotubes in different magnifications [147].

The basic elements of a multilayered electrode are shown in Figure 12.3. Each part has its own design specification, depending on end use. First, electrode materials have to withstand both oxidizing and reducing conditions. Second, total electrode resistance should be below 10 Q to minimize errors associated with/S drop. Third, electrodes must have electroactive areas which are reproducible within 5% coefficient of variation. Fourth, during manufacture, all materials must withstand repetitive thermal cycling to T > 110 °C. Fifth, all the electrode materials must be mutually adhesive. Sixth, there has to be a complete absence of electroactive impurities. 

Later work involved use of solid-state materials and more sophisticated microscopic assemblies. The ever-expanding role of ultramicroelectrodes in studies of modified electrodes is also discussed. Modification of electrodes with monolayers and biological macromolecules is discussed briefly in the context of multilayered films, but their individual histories are not included. The focus of this ten-year, conceptual history is on the development of multilayered electrode coatings and related microstructures 135 references are included. 

ESs of both double-layer and battery-Uke materials such as porous carbon and RUO2, respectively, form multilayer electrode stractuies. The two electrodes either combine the two materials or use them separately. Device configuration consists of basic electrode, separator with electrolyte, and electrode design. 

The modeling domain should be adjusted to numerically model the effects of using a multilayer electrode. In this case, the electrochemical reaction must occur at two different layers the inner and outer catalyst... 

In 1997 and 1999 U.S. patents of Wilkinson et al. it was suggested that a multilayer electrode be used as the anode in DMFCs. The first layer consists of catalyst applied to carbon paper. Most of the methanol is oxidized in this layer. The second catalytic layer, applied directly to the membrane, already has a diluted methanol solution. Other ways of limiting methanol crossover are discussed in the next section when we discuss improvements in water management in fuel cells. 

Liu et al. (2004) used multilayer electrodes. Closer to the membrane, a thin layer (about 5 p,m) with Pt-IrOi catalyst for oxygen evolution was arranged. This layer is completely hydrophilic, so that good contact of water with the catalyst is secured. Adjacent to this inner catalytic layer is an outer catalytic layer containing platinum and Nafion that is supported by a hydrophobic gas-diffusion layer as in an ordinary fuel cell. In this way, oxygen evolution at the inner layer has little impact on the activity of the outer catalytic layer. The authors reported 25 cycles of successful alternative operations of this device. 

The deposition of a thin (2-3 nm) covering YSZ layer leads to a suppression of the oxygen adsorption and decomposition. Additionally, the deposition of the covering layer leads to an increase of the amount of nanosize Ni grains located in the near interface boimdary porous region between the grains of NiO and YSZ. As a result, electrochemical reactors with the functional multilayer electrode show much better selectivity for NO gas decomposition even with respect to the electrochemical cells with electro-catalytic electrode but without a covering layer. 

Awano, M., Bredikhin, S., Aronin, A., Abrosimova, G., Katayama, S. Hiramatsu, T., "NOx decomposition by electrochemical reactor with electrochemically assembled multilayer electrode". Solid State Ionics 175,605-608, (2004). 

---