Electrolytes phase states

In solid-state systems it is often advantageous to have some of the electrolyte material mixed in with the reactant. There are two general advantages that result from doing this. One is that the contact area between the electrolyte phase and the electrode phase (the electrochemical interface) is greatly increased. The other is that the presence of the electrolyte material changes the thermal expansion characteristics of the electrode structure so as to be closer to that of the pure electrolyte. By doing so, the stresses that arise as the result of a difference in the expansion coefficients of the two adjacent phases that can use mechanical separation of the interface are reduced. 

Physical-chemical studies require traces of additives (reactants, catalysts, electrolytes) with respect to the concentration of the basic components of the microemulsion, and this causes only a minor change in the phase behavior of the system. However, when the amounts of additives are on the scale used in organic synthesis, the phase behavior, which is very sensitive to the concentration of the reactants, is sometimes difficult to control and the reaction is carried out in a one-, two- or three-phase state. 

The rate of an electrochemical reaction depends, not only on given system parameters (composition of the catalyst and electrolyte, temperature, state of the catalytic electrode surface) but also on electrode potential. The latter parameter has no analog in heterogeneous catalytic gas-phase reactions. Thus, in a given system, the potential can be varied by a few tenths of a volt, while as a result, the reaction rate will change by several orders of magnitude. 

A more direct approach to the photoinduced ET dynamics involves monitoring the lifetime of the excited state at the interface. By illuminating the interface in TIR from the electrolyte phase containing the quencher species, the generation of excited state is limited to the characteristic penetration depth given by the evanescent wave (/ ) [127],... 

The methodology of surface electrochemistry is at present sufficiently broad to perform molecular-level research as required by the standards of modern surface science (1). While ultra-high vacuum electron, atom, and ion spectroscopies connect electrochemistry and the state-of-the-art gas-phase surface science most directly (1-11), their application is appropriate for systems which can be transferred from solution to the vacuum environment without desorption or rearrangement. That this usually occurs has been verified by several groups (see ref. 11 for the recent discussion of this issue). However, for the characterization of weakly interacting interfacial species, the vacuum methods may not be able to provide information directly relevant to the surface composition of electrodes in contact with the electrolyte phase. In such a case, in situ methods are preferred. Such techniques are also unique for the nonelectro-chemical characterization of interfacial kinetics and for the measurements of surface concentrations of reagents involved in... 

Three-dimensional electrode nanoarchitectures exhibit unique structural features, in the guise of amplified surface area and the extensive intermingling of electrode and electrolyte phases over small length scales. The physical consequences of this type of electrode architecture have already been discussed, and the key components include (i) minimized solid-state transport distances (ii) effective mass transport of necessary electroreactants to the large surface-to-volume electrode and (iii) magnified surface—and surface defect—character of the electrochemical behavior. This new terrain demands a more deliberate evaluation of the electrochemical properties inherent therein. 

In redox electrodes an inert metal conductor acts as a source or sink for electrons. The components of the half-reaction are the two oxidation states of a constituent of the electrolytic phase. Examples of this type of system include the ferric/ferrous electrode where the active components are cations, the ferricyanide/ferrocyanide electrode where they are anionic complexes, the hydrogen electrode, the chlorine electrode, etc. In the gaseous electrodes equilibrium exists between electrons in the metal, ions in solution and dissolved gas molecules. For the half-reaction... 

Almost all modern practical aqueous primaries are referred to as dry cells . This designation should not be confused with the rather specialized solid state cells which make use of the recently discovered true solid electrolytes. Rather, the term implies that the aqueous electrolyte phase has been immobilized by the use of gelling agents or by incorporation into microporous separators. Such procedures permit the cells to operate in any orientation and reduce the effects of leakage should the container become punctured. 

Numerous other battery chemistries have evolved over time. The most prominent ones are assembled in Table 3.5.2. One possible categorization of battery technologies can be made according to the class of electrolyte they use. Here, we will distinguish between liquid aqueous, liquid nonaqueous, and solid electrolytes. To a certain degree, the phase state of the electrolyte determines the state of the electrodes. In general, it is advantageous to have a solid/liquid phase boundary between electrode and electrolyte because of much lower contact resistance in comparison to solid/solid contacts. Therefore, if the electrodes are solids, the electrolyte should be preferably liquid and vice versa. 

Additional experiments at various electrolyte contents showed that an increase in NaCl concentration did not influence the foam bilayer thickness, hence, these transitions are not of electrostatic origin and are probably due to the occurrence of phase transitions in foam bilayers. The data in Fig. 3.93 suggest the existence of different types of DMPC foam bilayers, which are distinguished by their phase state. 

Satyanarayana, L., Choi, G.P., Noh, W.S., Lee, W.Y and Park, J.S. (2007) Characteristics and performance of binary carbonate auxiliary phase CO2 sensor based on Li3PO4 solid electrolyte. Solid State Ionics, 177 (39-40). 3485-90. 

For the strong-interaction electron-transfer reactions, substantial quantum mechanical resonance splitting occurs in the activated state, and the electron becomes delocalized—i.e., smeared out between the electrode and the electrolyte phase reactants. The electrode surface has a strong catalytic effect, and such reactions are sensitive to the electrode surface conditions. The theoretical treatments of electron transfer for the strong interaction case are in a very early state (35). 

The semiconductor and the electrolyte phases are conveniently characterized through macroscopic relations. A microscopic model is required for the interface between the bulk phases. This model can be arbitrarily complex but is restricted by the requirement that thermodynamic relationships among the bulk phases hold. A convenient model for the interfacial region is represented in Fig. 2. The interface is represented by four planes, inner and outer Helmholtz planes on the electrolyte side of the interface and inner and outer surface states on the semiconductor side. The outer Helmholtz plane (OHP) is the plane of closest approach for (hydrated) ions associated with the bulk solution. The inner Helmholtz plane (IHP) passes through the center of ions specifically adsorbed on the semiconductor surface. The outer surface state (OSS) represents the plane of closest approach for electrons (and holes) associated with the bulk of the semiconductor. The inner surface state (ISS) is a plane of surface sites for adsorbed electrons. If surface sites are neglected, the ISS and the OSS are coincident. 

The porous electrode is one dimensional and both solid and electrolyte phase are continuous media with uniform effective conductivities, isothermal and steady state conditions apply. 

Lee, M.-H., Kim, H.J., Kim, E., Rhee, S.B., Moon, M.J., 1996. Effect of phase separation on ionic conductivity of poly(methyl methacrylate)-based solid polymer electrolyte. Solid State Ionics 85,91-98. 

Figure 3.1 Electronic equilibrium between a metallic phase and an electrolyte phase. The electronic energy states in the metal are described by the energy band (Section 2.9). The occupied states are and The density of states of electrons in the electrolyte are the energy distribution functions of the reduced and oxidized components of a redox system, e.g., Fe and Fe ions (Section 2.9.10). The equilibrium condition is equal values of the electrochemical potentials /x of the electrons in both phases. An alternative condition is equal values of the Fermi energy Ep in both phases.

In recent years, there has been growing interest in electrochemical studies in the sohd state, that is, in the absence of liquid electrolyte phase [126-128]. 

---