Bilayered electrodes

In addition to bilayered electrodes with a functional layer and a support layer, electrodes have also been produced with multilayered or graded structures in which the composition, microstructure, or both are varied either continuously or in a series of steps across the electrode thickness to improve the cell performance compared to that of a single- or bilayered electrode. For example, triple-layer electrodes commonly utilize a functional layer with high surface area and small particle size, a second functional layer (e.g., reference [26]) or diffusion layer with high porosity and coarse structure, and a current collector layer with coarse porosity and only the electronically conductive phase (e.g., reference [27]) to improve the contact with the interconnect. 

Figure 6.14 illustrates an OLED microcavity structure that comprises a stack of organic layers for providing EL, an upper electrode, and a bottom bilayer electrode of metal transparent conductive layer. The thickness of the transparent conductive layer (e.g., ITO) in the OLED structures can be varied across the substrate surface so as to achieve color tuning. One typical structure of the devices is glass/Ag/ITO (with a graded film... 

Another concept, first introduced by Johnson Matthey [66], is the bilayer concept. Here, the CO oxidation function is separated from the hydrogen oxidation function. Figure 14.18, measured by using home-made electrodes at ECN, shows the benefit of this bilayer concept. Using bilayer electrodes composed of a PtRu layer closest to the membrane, and a PtMo layer on top of that, a CO concentration of 200 ppm in a gas consisting of 75% H2 and 25% CO2 leads to a voltage decline of 50 mV at 350 mA cm 2 versus more than 250 mV for PtRu electrodes [67]. 

The solid-state electrochemical formation of bilayer electrode structures of different hexacyanoferrates was studied, and a theoretical model was proposed for cyclic voltammetric behavior of the transformation of PB into cadmium hexacyanoferrates [411]. 

Fig. 14.32. Cyclic voltammogram of coenzyme Q within the bilayer electrode. Phosphate buffer (pH 7.4, ionic strength 0.15), scan rate =100 mV/s. (Reprinted from Y. Xiaoling, J. Cullson, S. Sun and F. M. Hawkridge, Interfacial Electron Transfer Reactions of heme Proteins, Charge and Field Effects in Biosystems, M. J. Allen, S. F. Cleary, and F. M. Hawkridge, eds., vol. 2, p. 87, Plenum, 1989.)...

Figure 4.39. Nyquist plot of a Pt and Pt/PPY PPY/PSS bilayer electrode in 0.2 M NaC104 solution. Potential Pt OCV conducting polymer o, 0.1 , -0.2 A, -0.3 0, -0.4,, -0.5 V [13]. (Reprinted with permission from J Phys Chem 1993 97 3941-3. 1993 American Chemical Society.)...

Gulla et al. reported several thin-layer electrodes with superior performance and stability. Using a dual-ion beam-assisted deposition technique, they coated a Pt outer layer ( 50 nm thick, 0.08 mg Pt/cm ) directly onto GDLs with either a Co or Cr inner layer ( 50 nm thick). These bilayered electrodes showed a mass-specific Pt activity more than 50% higher at 900 mV than that for a single Pt layer on GDLs. No ionomers were present in the electrodes. 

Amperometric glucose biosensors based on co-immobiUzation of GOx with [Os(bpy)2(4-VP)ioCl]Cl in electrochemically generated polyphenol film [72]. A bilayer electrode in which Os-pol3nmer is first adsorbed on a Pt electrode and then electrochemically deposited on polyphenol-GOx fihn showed sensitivity of 1.63-1.79 /jA/cm /mM to 20 mM glucose at 0.4 V and 6-7 mM. Low background current and less interference by common electroactive compounds were noted. 

The term "bilayer" means two spatially segregated macromole-cular layers. Examples of applications of single component and bilayer electrodes to interesting chemical processes can be found in the individual references found in ref. 1. 

Figure 14.2.4 More complex modified electrode structures based on electroactive polymers, (a) Sandwich electrode (b) array electrode (c) microelectrode d, e) bilayer electrodes if) ion-gate electrode. [Reprinted with permission from C. E. D. Chidsey and R. W. Murray, Science, 231, 25 (1986), copyright 1986, American Association for the Advancement of Science.]...

Fig. 4 Energy level diagram for electron-transfer events at the bilayer electrode consisting of a Nafion film containing ferrocenylmethyltrimethylamnnonium ions and a Prussian-blue film containing 14 wt% Ti02 particles [55]. (With permission by the Electrochemical Society, Inc.).

Current (left panels) and mass flux (right panels) responses for a polybithiophene/polyxylylviologen (PBT/PXV) bilayer. Electrode Au (area = 0.23 cm ) on 10-MHz AT-cut quartz crystal. Solution 1 moldm tetraethylammonium perchlorate/acetonitrile. Voltammetric experiment, scan rate 50 mV s (upper panels),... 

K. Teshima, K. Yamada, N. Kobayashi, R. Hirohashi, Photopolymerization of aniline with a tris(2,2 -bipyridyl)ruthenium complex-methylviologen polymer bilayer electrode system, Chemical Communications 1996, 7, 829. 

SPECIAL MODIFICATIONS WITH ELECTROACTIVE POLYMER FILMS Bilayer Electrodes... 

Oyama and F. C. Anson, Facile attachment of transition metal complexes to graphite electrodes coated with polymeric ligands Observation and control of metal-ligand coordination among reactants confined to electrode surfaces, J.Am.Chem., 101 739 (1979). Denisevich, K. W. Willman, and R. W. Murray, Undirectional current flow and charge state trapping at redox polymer interfaces on bilayer electrodes Principles, experimental demonstration, and theory, J.Am.Chem.Soc., 103 4727 (1981). 

FIGURE 2.29. Typical RDE profiles recorded for the reduction of 1 mM Fe (aq) in 0.1 M HCl at (a) a glassy carbon electrode coated with a polyhydroxyphenazine film (F = 6 X 10" mol cm ) and at (b) a bilayer electrode consisting of an inner polyhydroxyphenazine layer [same surface coverage as in (a)] and an outer electroactive ruthenium-loaded polyvinylpyridine polymer (F = 1 X 10 mol cm ). Rotation speed in each case is 1000 rpm (Ref. 80). 

Other catalysts, such as PtMo, mainly favor the CO oxidation. This is effective only if sufficient CO is oxidized at low potential, which is not always the case. Bilayer electrodes have been proposed to separate the CO oxidation from the H2 oxidation. In this concept, CO oxidation takes place in a layer adjacent to the GDL, catalyzed, for example, by PtMo, and hydrogen oxidation in Pt-Ru layer adjacent to the membrane. This improved the CO tolerance significantly compared to the tolerance obtained with monolayer system of either catalyst [134, 135]. 

A p-n-heterojunction type device can be fabricated on the basis of the junction formed between poly(pyrrole) and PT derivatives (e.g. PT or PMT) [784]. A PODT/Cfio junction device with a photo-induced charge transfer between PODT and Cgo is shown in Fig. 22 [449]. Rectifying bilayer electrodes with sequential bilayer structures are prepared from any pair of PBT/poly(pyrrole) and poly(3-bromothiophene)/poIy(pyrrole) by anodic electropolymerization on platinum electrodes [785]. 

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