Solid-electrolyte interface properties

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

Figure 5.17. Schematic representation of a metal crystallite deposited on YSZ and of the changes induced in its electronic properties upon polarizing the catalyst-solid electrolyte interface and changing the Fermi level (or electrochemical potential of electrons) from an initial value p to a new value p -eri30 31 Reprinted with permission from Elsevier Science.

It is thus clear from the previous discussion that the absolute electrode potential is not a property of the electrode material (as it does not depend on electrode material) but is a property of the solid electrolyte and of the gas composition. To the extent that equilibrium is established at the metal-solid electrolyte interface the Fermi levels in the two materials are equal (Fig. 7.10) and thus eU 2 (abs) also expresses the energy of transfering an electron from the Fermi level of the YSZ solid electrolyte, in equilibrium with po2=l atm, to a point outside the electrolyte surface. It thus also expresses the energy of solvation of an electron from vacuum to the Fermi level of the solid electrolyte. 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

The lithium-storage properties of these Si SiOx/C nanocomposite electrodes were investigated in different electrolyte systems and compared to pure Si nanoparticles. From all the analyzed systems, the Si SiOx-C nanocomposite in conjunction with the solvent vinylene carbonate (VC) to form the solid-electrolyte interface showed the best lithium storage performance in terms of a highly reversible lithium-storage capacity (1100 mAh g-1), excellent cycling performance, and high rate capability (Fig. 7.9). 

The electrochemical current and potential parameters are connected to the properties of the solid electrolyte interface and we will devote this section to the structure of the electrode-electrolyte solution interface at the molecular scale. 

In several previous papers, the possible existence of thermal anomalies was suggested on the basis of such properties as the density of water, specific heat, viscosity, dielectric constant, transverse proton spin relaxation time, index of refraction, infrared absorption, and others. Furthermore, based on other published data, we have suggested the existence of kinks in the properties of many aqueous solutions of both electrolytes and nonelectrolytes. Thus, solubility anomalies have been demonstrated repeatedly as have anomalies in such diverse properties as partial molal volumes of the alkali halides, in specific optical rotation for a number of reducing sugars, and in some kinetic data. Anomalies have also been demonstrated in a surface and interfacial properties of aqueous systems ranging from the surface tension of pure water to interfacial tensions (such as between n-hexane or n-decane and water) and in the surface tension and surface potentials of aqueous solutions. Further, anomalies have been observed in solid-water interface properties, such as the zeta potential and other interfacial parameters. 

As already mentioned, salt-containing liquid solvents are typically used as electrolytes. The most prominent example is LiPF6 as a conductive salt, dissolved in a 1 1 mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as 1 molar solution. It should be mentioned that this electrolyte is not thermodynamically stable in contact with lithium or, for example, LiC6. Its success comes from the fact that it forms an extremely stable passivation layer on top of the electrode, the so-called solid-electrolyte interface (SEI) [35], Key properties of such SEI layers are high Li+ and very low e conductivity - that is, they act as additional electrolyte films, where the electrode potential drops to a level the liquid electrolyte can withstand [36],... 

This expression is of interest as far as it relates to the diffusional properties through the solid film with proton concentration at the solid/electrolyte interface. 

Prelot, B. et al. Structural -chemical disorder of manganese dioxides. 1. Influence on surface properties al lire solid-electrolyte interface, J. Colloid Interf. Sci., 257, 77, 2003. 

Momma, T Nara, H. Yamagami, S. Tatsumi, C. Osaka, T. Effect of the atmosphere on chemical composition and electrochemical properties of solid electrolyte interface on electro-deposited Li metal, J. Power Sources 2011,1%, 6483-6487. 

An ideal electrode-electrolyte interface with an electron-transfer process can be described using Randle equivalent circuit shown in Fig. 2.7. The Faradaic electron-transfer reaction is represented by a charge transfer resistance and the mass transfer of the electroactive species is described by Warburg element (W). The electrolyte resistance R is in series with the parallel combination of the double-layer capacitance Cdi and an impedance of a Faradaic reaction. However, in practical application, the impedance results for a solid electrode/electrolyte interface often reveal a frequency dispersion that cannot be described by simple Randle circuit and simple electronic components. The interaction of each component in an electrochemical system contributes to the complexity of final impedance spectroscopy results. The FIS results often consist of resistive, capacitive, and inductive components, and all of them can be influenced by analytes and their local environment, corresponding to solvent, electrolyte, electrode condition, and other possible electrochemically active species. It is important to characterize the electrode/electrolyte interface properties by FIS for their real-world applications in sensors and energy storage applications. 

IR absorption, emission, and reflection spectra for molecular species either in solid, liquid, or gas phases arise mostly from various changes in energy due to transitions of molecules from one vibrational or rotational energy state to another. The frequency or wavelength of this energy transition is characteristic of the specific chemical bond vibration and/or rotation in the molecule which are determined by the molecular structure, the masses of the atoms, and the associated vibrational energy coupling. Attenuated total reflectance (ATR) and reflection-mode of IR in conjunction with electrochemical methods allow samples to be examined directly in the solid or liquid state without further preparation and are widely used in the characterization of electrode-electrolyte interface properties. Most of ILs are IR-active molecules. Since ILs are stable and chemically inert, the IR characterization can be easily performed on the IL-based system directly. 

Traditionally, the chemical stability of the electrode/electrolyte interface and its electronic properties have not been given as much consideration as structural aspects of solid electrolytes, in spite of the fact that the proper operation of a battery often depends more on the interface than on the solid electrolyte. Because of the high ionic conductivity in the electrolyte and the high electronic conductivity in the electrode, the voltage falls completely within a very narrow region at the electrolyte/electrode interface. 

---