Gas-electrolyte interface

The relationship between the five peroxide mechanism reaction steps can be seen in the reaction mechanism graph in Figure 4. As defined above, each step occurs at one of the five nodes, and the directed edges give the forward direction for the mechanism. Current-carriers for the overall mechanism are in boxes, while carbonate ions that continue from one cycle to the next are circled. Dashed vertical lines represent interfaces between phases. Nodes on the gas-electrolyte interface represent reaction steps occurring at that interface nodes attached to the electrolyte-solid interface represent reaction steps occurring at sites on the surface of the solid phase. The location of each reaction on this reaction mechanism graph follows the description of the... 

If gaseous, electrochemicaUy active components of the measuring environment are not dissolved in the electrode, then the electrode process will consist of the following stages (also shown in Figure 1.18). They are adsorption-desorption of electrochem-icaUy active gaseous components on gas-electrolyte (GE) and gas-metal (GM) interfaces, ionization reaction (with electron transfer) on the metal-electrolyte (ME) and gas-electrolyte interfaces, and mass-transfer processes on all boundaries of three phases (gas-metal, gas-electrolyte, and metal-electrolyte). Furthermore, mass transfer of electrons and holes on the surface electrolyte layer may also occur. It is evident that the quantity of the current in the stationary state is equal to the quantity of the nonmetal component adsorbing on the gas-metal and gas-electrolyte surfaces as a result of ionization of this component on the ME and GE surfaces. 

Discharge of the oxygen ions to subions O diffusion of subions O by the metal-electrolyte and gas-electrolyte interfaces and disproportion of subions 0 on and O on the gas-electrolyte interface with following desorption of oxygen molecules into the gaseous phase and transfer of ions into the zirconia electrolyte. This case can take place at the surface diffusion of subions (see Figure 1.18, c). 

The experiment done by Kellogg [70] in 1949 confirms these values. He describes how the gas film surface behaves like a vibrating structure (see Fig. 4.11). Kellogg also stressed that once the gas film is formed, no further bubble generation can be observed. He concluded that the electrochemical reactions must now take place at the gas-electrolyte interface. 

The nucleation process starts at defined active centers of the surface called the nucleation sites. Even in the case of an electrolyte supersaturated with gas the activation of the nucleation site is required. It depends not only on the nature of the gas and the electrolyte but also on the interfacial tension and number of substrate neighbors on the electrocatalyst. In summary, the existence of a gas-electrolyte interface besides the electrolyte-solid interface is required. Some authors explained this process as nucleate boiling [73] and others [74] as the super saturation of the electrolyte with the gas, developing different equations for the current density. Another important factor to consider is the number of surface active sites available for the bubble formation and the geometry of the bubble. Moreover, the surface roughness is certainly a factor to be considered for the stability of the nucleation site. The ageing of the nucleation site also has to be considered since it results in the loss of activity after a long time. 

A theoretical analysis of the current distribution and overpotential-current density relations for two models of porous gas diffusion electrodes—the simple pore and thin film models—has been carried out (108,109). The results of the analysis for the simple pore model will be summarized here. The reactant gas diffuses through the pore to the gas-electrolyte interface at 2 = 0, where it dissolves in the electrolyte and the dissolved gas diffuses through the electrolyte to the various electrocatalytic sites along the pore at which the reaction occurs (Fig. 25). It is assumed that the first and second steps of diffusion of reactant gas through the electrolyte-free part of the pore (z < 0) and of dissolution of gas at the gas-electrolyte interface are fast. 

The electrode processes in solid-electrolyte systems consist always of a number of serial and/or parallel steps. The characteristic steps of the gas electrode reactions include transport in the gas phase to (or from) the gas/electrode or gas/electrolyte interface, adsorption (or desorption) at these surfaces, diffusion to (or from) the reaction zone, and transfer reactions [14-24]. As a rule, the electrochemical reaction is believed to occur in the vicinity (within a few microns) of triple-phase boundary (TPB), the junction of the gas, electronic or mixed ionic-electronic conductor (electrode), and ionic conductor (electrolyte) the TPB length is mostly determined by the electrode microstructure formed during the cell fabrication. Actual location of the electrochemicaUy active sites depends generally on the bulk and surface transport properties of the electrode and solid-electrolyte materials. When the current I is passed or drawn through the cell, the working electrode potential vve deviates from the equilibrium value E. This deviation is characterized by the quantity of overpotential r] = we (see Chap. 1). The electrode polarization resistance defined as... 

The Nemst equation above for the dependence of the equilibrium potential of redox electrodes on the activity of solution species is also valid for uncharged species in the gas phase that take part in electron exchange reactions at the electrode-electrolyte interface. For the specific equilibrium process involved in the reduction of chlorine ... 

The electrical double-layer structure at Ga/DMF, In(Ga)/DMF, and Tl(Ga)/DMF interfaces upon the addition of various amounts of NaC104 as a surface-inactive electrolyte has been investigated by differential capacitance, as well as by the streaming electrode method.358 The capacitance of all the systems was found to be independent of the ac frequency, v. The potential of the diffuse layer minimum was independent of... 

In fact, the key to understand electrochemical promotion is to understand the mechanism by which the effect of polarization at the catalyst/electrolyte interface propagates to the catalyst/gas interface ... 

Solid electrolyte cells can be used to alter significantly the work function catalytically active, catalyst electrode surface by polarizing the catalyst-solid electrolyte interface. 

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. 

The non-zero value of e Fw-e FR in Eq. (5.35) implies that there are net surface charges on the gas exposed electrode surfaces. These charges (q+,q.) have to be opposite and equal as the cell is overall electrically neutral and all other charges are located at the metal-solid electrolyte interfaces to maintain their electroneutrality. The charges q+ = -q. are quite small in relation to the charges, Q, stored at the metal-electrolyte interface but nevertheless the... 

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. 

Porous electrodes are commonly used in fuel cells to achieve hi surface area which significantly increases the number of reaction sites. A critical part of most fuel cells is often referred to as the triple phase boundary (TPB). Thrae mostly microscopic regions, in which the actual electrochemical reactions take place, are found where reactant gas, electrolyte and electrode meet each other. For a site or area to be active, it must be exposed to the rractant, be in electrical contact with the electrode, be in ionic contact with the electrolyte, and contain sufficient electro-catalyst for the reaction to proceed at a desired rate. The density of these regions and the microstmcture of these interfaces play a critical role in the electrochemical performance of the fuel cells [1]. 

An important result of this study is the conclusion of a particle-size-dependent COads surface mobility. The value obtained for large Ft particles is significantly smaller than Deo at a solid/gas interface. However, Kobayashi and co-workers, using solid state NMR, performed measurements of the tracer diffusion coefficient Deo at the solid/electrolyte interface and for Ft-black particles (about 5nm grain... 

Electrode potentials are relative values because they are defined as the EMF of cells containing a reference electrode. A number of authors have attempted to define and measure absolute electrode potentials with respect to a universal reference system that does not contain a further metal-electrolyte interface. It has been demonstrated by J. E. B. Randles, A. N. Frumkin and B. B. Damaskin, and by S. Trasatti that a suitable reference system is an electron in a vacuum or in an inert gas at a suitable distance from the surface of the electrolyte (i.e. under similar conditions as those for measuring the contact potential of the metal-electrolyte system). In this way a reference system is obtained that is identical with that employed in solid-state physics for measuring the electronic energy of the bulk of a phase. 

Although the desorption energy at the metal-electrolyte interface should be different from that of the metal-gas phase, there is no reason to expect a different behavior for the change of d with 0. 

Henri CL, Parkinson L, Ralston JR, Craig VSJ (2008) A mobile gas-water interface in electrolyte solutions. J Phys Chem C 112 15094-15097... 

One possibility of circumventing the problem of the solvent absorption is to use Raman spectroscopy, where the probing light is in the visible, and this approach is detailed in section 2.1.7. However, the difficulties experienced with the application of Raman to the electrode/electrolyte interface (vide infra), refocused attention on the seductive simplicity of IR spectroscopy, particularly as the technique had proved invaluable in the study of species at the gas/solid and vacuum/solid interfaces. 

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