Electronic and Ionic Conduction

An electric current results from the motion of electrically charged particles in response to forces that act on them from an externally applied electric field. Positively charged particles are accelerated in the field direction, negatively charged particles in the direction opposite. Within most solid materials a current arises from the flow of electrons, which is termed electronic conduction. In addition, for ionic materials, a net motion of charged ions is possible that produces a current this is termed ionic conduction. The present discussion deals with electronic conduction ionic conduction is treated briefly in Section 18.16. 

In all conductors, semiconductors, and many insulating materials, only electronic conduction exists, and the magnitude of the electrical conductivity is strongly dependent on the number of electrons available to participate in the conduction process. However, not all electrons in every atom accelerate in the presence of an electric field. The number of electrons available for electrical conduction in a particular material is related to the arrangement of electron states or levels with respect to energy and the manner in which these states are occupied by electrons. A thorough exploration of these topics is complicated and involves principles of quantum mechanics that are beyond the scope of this book the ensuing development omits some concepts and simplifies others. 

Figore 18.2 Schematic plot of electron energy versus interatomic separation for an aggregate of 12 atoms N = 12). Upon close approach, each of the Is and 2s atomic states splits to form an electron energy band consisting of 12 states. 

The number of states within each band is equal to the total of all states contributed by the N atoms. For example, an s band consists of N states and a p band of 2 N states. With regard to occupancy, each energy state may accommodate two electrons that must have oppositely directed spins. Furthermore, bands contain the electrons that resided in the corresponding levels of the isolated atoms for example, a 4s energy band in the solid contains those isolated atoms 4s electrons. Of course, there are empty bands and, possibly, bands that are only partially filled. 

The electrical properties of a solid material are a consequence of its electron band structure—that is, the arrangement of the ontermost electron bands and the way in which they are filled with electrons. 

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One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

The relatively high mobilities of conducting electrons and electron holes contribute appreciably to electrical conductivity. In some cases, metallic levels of conductivity result ia others, the electronic contribution is extremely small. In all cases the electrical conductivity can be iaterpreted ia terms of carrier concentration and carrier mobiUties. Including all modes of conduction, the electronic and ionic conductivity is given by the general equation ... 

During electrochemical reduction (charge) of the carbon host, lithium cations from the electrolyte penetrate into the carbon and form a lithiated carbon Li rCn. The corresponding negative charges are accepted by the carbon host lattice. As for any other electrochemical insertion process, the prerequisite for the formation of lithiated carbons is a host material that exhibits mixed (electronic and ionic) conductance. 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

Thus, the enhanced activity of the electrode can be attributed to the presence of both electrons and oxygen vacancies (mixed electronic and ionic conduction) in the electrode material. 

The electrocrystallization and characterization of a novel molecular metal which displays both electronic and ionic conduction has been reported. The complex Li0.6(15-crown-5-ether)[Ni-(dmit)2] H20 is composed of stacks of [Ni(dmit)2] units which provide pathways for electronic conduction. The stacks are separated by parallel stacks of 15-crown-5-ether moieties in a channellike formation which facilitates ion conduction. The salt has a room temperature conductivity of 240 Scm-1. Temperature-dependent magnetic susceptibility and NMR measurements were used to prove the existence of Li+ movement within the crown ether channels.1030... 

The mentioned method for synthesis of oxide-hydroxide compounds (Ni, Cr, Co) is more controllable and enables with production of electrode films definite amounts of components. Therefore it guarantees the reproducibility of their compositions and properties. Using the above method we were able to produce the following oxide compounds zero valence metal and lowest oxidation state oxide-hydroxide compounds in cathode process and oxide-hydroxide compounds (in anode process the oxide compounds consist of highest oxidation state oxide-hydroxide compounds). Both type compounds possesses electronic and ionic conductivity. 

Defect populations and physical properties such as electronic conductivity can be altered and controlled by manipulation of the surrounding atmosphere. To specify the exact electronic conductivity of such a material, it is necessary to specify its chemical composition, the defect types and populations present, the temperature of the crystal, and the surrounding partial pressures of all the constituents. Brouwer diagrams display the defect concentrations present in a solid as a function of the partial pressure of one of the components. Because the defect populations control such properties as electronic and ionic conductivity, it is generally easy to determine how these vary as the partial pressure varies. 

Metals and semiconductors are common examples of electronic conductors, and under certain circumstances even insulators can be made electronically conducting, for example by photoexcitation. Electrolyte solutions, molten salts, and solid electrolytes are ionic conductors. Some materials have appreciable electronic and ionic conductivities,... 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

Carbon-sulfur hybrid materials, i.e., porous carbons [58,59] or CNTs having nanosized S in the pores or channels, are the most promising solution for the Li-S battery to increase the electronic and ionic conductivity of sulfur or sulfide, and prevent, to a great extent, the solubility of the polysulfide ions formed on reduction of S or upon oxidation of insoluble sulfides [60]. An intimate contact between carbon and sulfur is essential [61]. 

In a crystal, the electronic and ionic conductivities are generally tensor quantities relating the current density Iq to the applied electric field E in accordance with Ohm s law. The scalar expression for the mobile-ion current density in the different principal crystallographic directions has the form... 

Let us imagine, for instance, that we measured the defect population of a generic oxide MO composed essentially of Schottky defects (such a measure could be obtained, for instance, by thermogravimetry at various T and Po conditions or by electronic and ionic conductivity measurements at various values of T and Pq2)- then write... 

Implications of Electronic and Ionic Conductivities of Polyimide Films in Integrated Circuit Fabrication... 

Because electronic and ionic conduction are so structure-sensitive, the simple rule-of-mixtures approach to estimating the conductivity and resistivity of composites is not normally of use. As a result, the conductivity of specific composites for specific applications must be experimentally determined. In the next two sections, we examine two examples of how composites can be used in electrical applications, and we describe the influence of each component on the electrical properties. The first example involves the electrical insulating properties of polymers, and the second one involves enhancing the electrically conducting properties of polymers. 

The n-p Junction. Before beginning a discussion of electron transfer at interfaces between H-type semiconductor/solution interlaces, it is helpful to describe something of the theory of the famous n-p junction. This is not a part of electrode-process chemistry (which deals with electron-transfer reactions between electronically and ionically conducting phases), but it is the basis of so much modem technology (e.g., the transistor in computers) that an elementary version of events at the junction should be understood. Further, knowing about the n-p junction makes it easier to understand electrochemical interfaces involving semiconductors. 

Study of (ZrO2)o.85(CuO)o.i5—electronic and ionic conduction ZrO2 shows the following successive phase transitions on heating ... 

Both the anode and cathode are assumed to be made of porous materials providing electronic and ionic conductivity. As a consequence, there is a concomitant trans-... 

Figure 3.2 represents a schematic of the anode structure, highlighting the electronic and ionic conductive sites, while Figure 3.3 represents the schematic representation used for modeling purposes. 

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