Electrochemical devices concepts

For purposes of verifying of the concept of a self-discharge due to the LEM oxidation by air, we have designed a coin cell with a zinc electrode and a thin PANI/TEG cathode. The typical curves of voltage change for such electrochemical device are given by Figure 6. 

Electrochemical biosensors are analytical devices in which an electrochemical device serves as a transduction element. They are of particular interest because of practical advantages, such as operation simplicity, low expense of fabrication, and suitability for real-time detection. Since the first proposal of the concept of an enzyme-based biosensor by Clark, Jr [1], significant progress in this field has been achieved with the inherited sensitivity and selectivity of enzymes for analytical purposes. 

The concept of electric transport in polymers due to the availability of polymeric materials with characteristics similar to those of metals is certainly fascinating and, indeed, many studies have been directed towards the preparation and the characterisation of these new electroactive conductors. The final goal is their use as new components for the realisation of electronic and electrochemical devices with exotic designs and diverse applications. 

So far, we have considered open electron systems. We would, however, like to apply the concepts introduced above to electrical and electrochemical devices that consist of metals and/or doped insulators. Solid phases, such as metals and insulators do not contain electrons only. For instance, a metal consists of a regular array of core ions, and more or less freely moving electrons (the electron gas). However, as long as we discuss the transfer or directed motion of solely electrons, the concept of the chemical potential of the electrons in a (solid) phase remains valuable. 

The usual procedures for the conception of electrochemical reactors arise from the mass conservation laws and the hydrodynamic structure of the device. In fact, four types of balances can be considered energy, charge, mass, and linear movement quantity. Since the reactor must include the anodic and the cathodic reactions, it is possible to make a complete balance for the mass. The temperature also governs the stability of a chemical reactor, but in the case of an electrochemical device, the charge involved in the entire process has to be considered first [3-5]. 

The first chapter focuses on the basic notions that need to be mastered before being able to go on and tackle the following chapters. The reader is reminded of the basic concepts, all defined in precise detail, as well as being introduced to certain experimental aspects. This chapter is therefore meant more or less for beginners in electrochemistry. The common electrochemical systems are described in the second chapter, which introduces the elementary laws so that they can be applied immediately by the reader. This chapter does not therefore provide any in-depth demonstrations. However, it is the last two chapters and the appendices that go into greater depth to tackle the key notions in a thorough and often original way. The third chapter focuses on aspects related to thermodynamic equilibrium, and the fourth chapter deals with electrochemical devices with a current flow, and which are therefore not in equilibrium. 

The last two decades have seen an explosion of interest in ionic liquids [1]. Their use as solvents has been the subject of widespread academic study [2] and they have been applied in a number of commercial processes [3]. Much of the interest in ionic liquids has centered on their possible use as green solvents [4]. However, this has been the subject of much controversy [5], and the concept of a green solvent itself is now somewhat dated. There have been many reviews of ionic liquids. Some of these have focused on particular applications, for example, analysis [6], biocatalysis [7], catalysis [8], electrochemical devices [9], or engineering fluids [10]. Others have concentrated on particular subgroups of ionic liquids, for example, task-specific ionic liquids [11]. This chapter summarizes what is known about the physicochemical properties that are of particular interest for supported ionic hquid phases (SILPs). 

Both Evans and Stem diagrams are included in order to compare and analyze simple and uncomplicated electrochemical systems. The concept of anodic control and cathodic control polarization is also introduced. In addition, the predetermined corrosion circuit is included in order to analyze corrosion using an electrochemical device containing an external circuit. 

Tattoo-based electrochemical devices have been recently described for evaluating different physiologically relevant compounds in sweat or skin. Wmdmiller et al. (2012) developed stamp transfer electrodes for electrochemical sensing on nonplanar and oversized surfaces. The concept was demonstrated towards the voltammetric detection of dopamine, ascorbic acid and acetaminophen, as well as to quantify increasing levels of uric acid in human skin. 

Second, in designing new molecule-based electronic devices, one of the major goals is the precise control of the current flowing between the terminals. Electrochemical molecular junctions allow for control of the potentials of the electrodes with respect to the redox potential of incorporated redox-active molecules with well-defined, accessible, tunable energy states. These junctions represent unique systems able to predict precisely at which applied potential the current flow will take off. Even though the presence of a liquid electrolyte represents a detriment towards possible applications, they provide the concepts for designing molecular devices that mimic electronic functions and control electrical responses. 

The latter concept implies providing local life support systems for unfriendly environments. By now, Ukrainian scientists and engineers have developed a variety of processes for potable water treatment by adsorption, electrochemical oxidation, electrocoagulation, electro-coprecipitation, electrodialysis, electrofloatation, floatation, membrane techniques etc. Each family must get small units for water purification, air cleaning and removal of hazardous substances from the food as soon as possible, for it may take decades to introduce cleaner production on a national scale. Here, we should follow the example of Western business people who bring with them to Ukraine devices enabling a safe existence in this unfriendly environment. 

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

The fuel cell in Figure 13.9 can be conceptually viewed as a combination of a Nafion film-coated cathode and a Nafion film-coated anode. Hence, the fuel cell is, in essence, a combination of two chemically modified electrodes. This idea is, in fact, more than just a concept, because electrochemical investigations of Nafion film-coated electrodes have been used to obtain fundamental chemical and electrochemical information that is relevant to the operation of such devices [93]. For example, the kinetics of 02 reduction in fuel cells can be investigated at such modified electrodes the solubility and diffusion coefficient for 02 in Nafion and the proton conductivity of this membrane material can also be determined. Chemically modified electrodes have made analogous contributions to battery development. 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

The use of ELISA is broad and it finds applications in many biological laboratories over the last 30 years many tests have been developed and vahdated in different domains such as clinical diagnostics, pharmaceutical research, industrial control or food and feed analytics for instance. Our work has been to redesign the standard ELISA test to fit in a microfluidic system with disposable electrochemical chips. Many applications are foreseen since the biochemical reagents are directly amenable from a conventional microtitre plate to our microfluidic system. For instance, in the last 5 years, we have reported previous works with this concept of microchannel ELISA for the detection of thromboembolic event marker (D-Dimer) [4], hormones (TSH) [18], or vitamin (folic acid) [24], It is expected that similar technical developments in the future may broaden the use of electroanalytical chemistry in the field of clinical tests as has been the case for glucose monitoring. This work also contributes to the novel analytical trend to reduce the volume and time consumption in analytical labs using lab-on-a-chip devices. Not only can an electrophoretic-driven system benefit from the miniaturisation but also affinity assays and in particularly immunoassays with electrochemical detection. 

---