Electrodes, bilayer-coated

B. Fabre, S. Burlet, R. Cespuglio, and G. Bidan, Voltammetric detection of NO in the rat brain with an electronic conducting polymer and Nafion bilayer-coated carbon fibre electrode. J. Electroanal. Chem. 426, 75-83 (1997). 

An anisotropic photoinduced electron flow is important to produce an efficient photocurrent. The anisotropic arrangement of the reaction components is necessary for that purpose. Polymer materials are very useful to achieve such arrangement. The arrangement of Ru(bpy)j+ and MV2+ was attained by coating electrode at first with a polymer pendant Ru complex film and then with a polymer pendant MV2+ (28) film. The photocurrent induced by this bilayer coating system is shown in... 

Figure 5. A Nafion/cellulose acetate bilayer coated electrode. (Reproduced from ref. 14. Copyright 1986 American Chemical Society.)...

Thin elastomeric coatings or coverings, which also serve as a moisture barrier, act as skin, preventing evaporation and leakage of the electrolyte solution(s). This allows these actuators to be fiiUy operational anywhere. Using a bilayer coating can provide for a compact actuator, where the innermost layer (the layer closer to the EAP) is a conductive layer and serves as the external electrode (Fig. 4.27). 

Gulla et al. have demonstrated superior performance and stabUily of carbonless thin layer electrodes made by a dual ion beam assisted deposition (IBAD) technique that combines physical vapor deposition (PVD) with ion beam bombardment [47]. They found that bilayered coatings on GDL with either a Co or a Cr iimer layer ( 50 nm thin) and a Pt outer layer ( 50 nm thin, and 0.08 mg Pt cm ) showed a more than 50% higher Pt mass activity at 900 mV than a Pt single layer. 

Ding et al. have reported [95] a novel potassium ion sensor based on valinomycin-incorporated bilayers supported on a gold electrode modified by a hexadecylmercaptan monolayer. The ion selectivity for K+ is 400 times greater than for Na. The system is stable for two months at -10 °C which is much more practical than the reported stability of many cationic bilayer membranes with longevities of less than 120 hrs [113]. The lipid membrane is prepared by the painted-frozen method [114] in which the SAM modified electrode is coated with the lipid solution and allowed to freeze (removing the solvent) the supported bilayers are then immersed into a solution of valinomycin to create the K+ sensor. 

A bilayer-coated electrode has two electroactive films, each having different reduction potentials. The inner layer is in direct contact with the carrier electrode surface and acts as a mediator to the outer layer which is mainly in contact with the solution. Provided that the redox levels in the two layers are appropriate, the interface between the two polymer films acts analogously to a semiconductor junction as a charge rectifying junction. The method of preparing first the inner layer on the electrode and then, in a second step, the outer layer may in principle be deduced from the conventional methods already described. 

Fig. 16. Schematic electron energy diagram for a bilayer coated electrode...

Fig. 44. Estimation of electron flux in photoelectrochemical processes at bilayer coated electrodes. Units of the values mol cm" s" ...

In the water photolysis system composed of an aqueous solution of colloidal Prussian Blue (PB, see Sect. 5.1) and Ru(bpy) 413,414) photoinduced electron transfer was considered to occur with electrons being transferred from the excited Ru complex adsorbed on the PB colloidal particle to the PB. Such a photochemical reaction must also be applicable to the coated electrode system used for photogalvanic cells. This kind of coated electrode system for photoelectrochemical conversion must in addition give information on heterogeneous photochemical reactions. Thus a basal plane pyrrolytic graphite (BPG) electrode was first coated with a polymer-pendant Ru(bpy)3 (7) membrane and then with a PB membrane by reductive electrodeposition from the aqueous mixture of Fe(CN) and Fe . The bilayer-coated system... 

Polymer pendant MtPcs (M Fe, Co, Np, Cu +) were used as films to produce a photoresponse. MV present in the solution was not effective as an acceptor, but Fe EDTA worked as an acceptor to enhance the photocurrent about 10 times A polymer bilayer coated electrode consisting of an electron-mediator film in the inner layer 49) and a sensitizer film (50) in the outer layer produced a rectified photocurrent In the presence of a sacrificial reductant (triethanolamine) in the solution, the bilayer coated electrode gave exclusively anodic photocurrent independent of the applied potential between —0.2 and —0.3 V vs SCE. 

Fig. 5. Schematic representation of a Pt electrode coated with succesive layers of redox polymers A and B a bilayer transistor electrode. Arrows indicate directions in which communication of the electrode and the outer layer is possible (from ref. ).

Additional improvements can be achieved through the use of multilayers (based on different overlaid films). Such combination of the properties of different films has been documented with bilayers of Nation/CA (14) and Nafion/collagen (29). The former allows selective measurements of the neurotransmitter dopamine in the presence of the slightly larger epinephrine and the anionic ascorbic acid (Figure 5). In addition to bilayers, mixed (composite) films, such as PVP/CA (75) or polypyrrole/Eastman Kodak AQ (30) layers can offer additional permselectivity advantages, such composites exhibit properties superior to those of their individual components. Also promising are sensor arrays, based on electrodes coated with... 

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. 

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

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