Inner electrode material effect

It is important to notice that the rate of a given outer sphere electrode redox reaction should be independent of the nature of the metal electrode if allowance is made for electrostatic work terms or double layer effects which will, of course, be dependent on the nature of the electrode material. Inner sphere reactions, on the other hand, are expected to be catalytic with kinetics strongly dependent on the electrode surface due to specific adsorption interactions. 

Additional alterations in the work terms with the electrode material for outer-sphere reactions may arise from discreteness-of-charge effects or from differences in the nature of the reactant-solvent interactions in the bulk solution and at the reaction plane. Thus metals that strongly chemisorb inner-layer solvent (e.g., HjO at Pt) also may alter the solvent structure in the vicinity of the outer plane, thereby influencing k bs variations in the stability of the outer-sphere precursor (and successor) states. Such an effect has been invoked to explain the substantial decreases (up to ca. 10 -fold) in the rate constants for some transition-metal aquo couples seen when changing the electrode materiaf from Hg to more hydrophilic metals such as Pt. Much milder substrate effects are observed for the electroreduction of more weakly solvated ammine complexes . 

The influence of varying the electrode material upon proton electroreduction has been the subject of extensive study . This reaction can be regarded as inner-sphere in that it often involves a rate-limiting, proton-transfer step to form an adsorbed hydrogen atom . Although superficially different, the conventional treatment of substrate effects upon such reactions is similar to that presented here. Thus the marked increase in the rate constants seen for Hj evolution as the metal-hydrogen atom bond energy... 

It is possible, however, that an ion can interact chemically with the electrode material. If this happens the ion may break through the solvent layers or, as in the case of the solid, become displaced from a normal lattice site. This possibility is known as specific adsorption. In aqueous electrochemistry the locus of the centers of the specifically adsorbed ions is known as the inner Helmholtz plane. Neutral molecules may also adsorb and hence affect the faradic current, for example by blockage of the reaction sites. Neutral molecule effects have not been studied in the case of solid systems and will therefore not be considered further. 

The inner surface of the medical back belt consists of cotton twill because it is directly applied onto the skin. In the centre of the inner surface the orthopaedic pad, called pelotte, is attached, which consists of foam or silicone material. These orthopaedic pads are often used because they can put pressure on dehned parts of the body to give these parts extra support. They are frequently attached in orthosis or artihcial limbs to prevent the product from shifting during use. This is a very important aspect for the medical back belt because on top of this pad the textile electrodes are embroidered, which have to stay in place, otherwise the treatment will not be effective enough. In the back belt a foam pelotte is used with a protective cover of 100% unbleached cotton. 

Typical construction and its output characteristics of ZrO -based sensors are shown in Fig. 2.7. The cell in oxygen sensors is usually shaped like a test tube where the inner and outer surfaces are each coated with ultrathin layers of porous platinum which act as the cathode and anode electrodes. The output of this potentiometric sensor is due to the combined effect of chemical and electrical processes. At high temperatures >650 °C, zirconium dioxide exhibits two mechanisms (Park et al. 2009) (1) ZrO partly dissociates to produce oxygen ions, which can be transported through the material when a voltage is applied and (2) ZrO behaves like a solid eleetrolyte for oxygen. 

The proton conductivity of the SPIES polymer membranes was measured using AC impedance spectroscopy and utilized a standard 4-electrode measuranent setup to eliminate electrode and interfacial effects. The Teflon sample fixture was placed inside a temperature and humidity controlled oven. The fabrication of the fixture allows complete exposure of the sample to the humidified air within the chamber. The two outer electrodes were made of platinum foil and these acted to source the current in the sample. Two inner platinum wire electrodes (spaced 1 cm apart) were then used to measure the voltage drop aaoss a known distance. By measuring the impedance of the material as a function of frequency at a set temperature and humidity, the conductivity of the membrane is obtained using the magnitude of the impedance in a region where the phase angle is effectively zero. The proton conductivities of the membranes were measured in the cell shown in Fig. 6.4. 

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