Reference electrodes, aqueous solutions solid contact

Conventional reference electrodes consist of a solid reversible electrode and an aqueous electrolyte solution. To measure the individual contributions from the anode and the cathode of a PEM fuel cell, the electrolyte solution of the reference electrode must either be in direct contact with one side of the solid proton exchange membrane or be located in a separate compartment with electrical contact between the reference electrode and the solid membrane by means of a salt bridge [66], As a result, two different types of reference electrode configurations are employed for the study of fuel cells internal and external. 

The experimental setup is shown in Figure 9.23. The Pt-black catalyst film also served as the working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H2-02 mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. The Pt catalyst-working electrode potential, Urhe (=Uwr)> was measured with respect to a reversible reference H2 electrode (RHE) via a Luggin capillary in contact with the Pt-free side of the Nafion membrane. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

In polymer-based ISEs, electrical contact between the membrane and inner reference electrode is made via an inner filling electrolyte. This type of ISE is the most common and they are usually referred to as liquid contact ISEs or very often simply ISEs. On the other hand, the contact can be obtained by the substitution of the aqueous inner solution with another polymeric material, to produce so-called solid-contact ISEs Table 2.1 provides current achievements in trace level... 

A new kind of all solid-state sensors was first reported by Cattrall and Preiser in 1971 in which the internal reference element was in direct contact with the sensing membrane and thus contained no aqueous solution. The first group of such simplified sensors was those of the so-called coated wire construction type (CWE). In this approach, a metal wire was dipped with a solution of PVC in THF containing also a suitable electroactive material. During evaporation of the solvent, a PVC film on the metal wire surface was formed. Although different materials such as platinum, silver or sliver chloride, and aluminum could serve as central conductors, the nature of the wire support had no substantial influence on the electrode performance if it did not react with the membrane components. ... 

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