Three-phase boundary region

Fig. II.1.24 Schematic representation of three types of electrochemical systems with two phases, a and in contact with the electrode surface, (a) Film deposit (b) island deposit with three-phase boundary regions and (c) emulsion system...

As in the case of H2 oxidation, the oxygen reduction reactions (ORR) on the cathode are also assumed to take place in a multi-step manner. The adsorption of O2 on the cathode surface is followed by the dissociation into two O atoms, and the surface diffusion to the three-phase boundary region. The O atoms take part in a number of electron transfer steps, reducing O to 0 . The rate limiting process, however, has not yet been identified conclusively. The overall oxygen reduction reaction and the incorporation of the ions into the electrolyte can be written in Kroger-Vink notation as... 

Surface diffusion to three-phase boundary regions... 

Figure 7.24 Illustration of the three-phase boundary regions of different SOFC anode materials. Similar extension of the boundary is obtained in mixed conducting cathode materials.

Sketch of the cross section through a sessile drop of liquid L on a flat solid S, both immersed in their common vapor atmosphere V. The D-faces Dlv, Dsv, and Dsl describe the corresponding phase boundaries. T marks the triple line as the new model element for the three-phase boundary region (. 0 between the phases S, L, and V. 0 = contact angle in thermodynamic equilibrium... 

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 8.6 The phase diagram for water (not to scale). The solid blue lines define the boundaries of the regions of pressure and temperature at which each phase is the most stable. Note that the freezing point decreases slightlv with increasing pressure. The triple point is the point at which three phase boundaries meet. The letters A and B are referred to in Example 8.3. 

The phase diagram features four phase regions, three phase boundaries, and two points of particular interest the triple point (TP) and the supercritical point (CP). Values for TP and CP from The International Association for the Properties of Water and Steam6 (IAPWS) are 273.16 K and 611.657 Pa (IAPWS, 2002) and 647.096 K and 22.064 MPa (IAPWS, 2002), respectively. Three of the phases (solid, liquid, and gas) are bounded by equilibrium... 

The amount of oxygen adsorbed in the three-phase region has been found to depend linearly on the exchange current density for different catalyst-electrodes under similar conditions.31,32 This indicates that the electrocatalytic reaction takes place at the three-phase boundary. Vayenas and co-workers pointed out that for less porous electrodes the charge-transfer reaction at the two-phase boundary might become important and that under some conditions oxygen on the electrolyte surface itself might play a role. 

Booth et al. (1996) compiled the data for all recovered hydrate samples and determined that 70% of the recovered samples were in the two-phase region, that is, at pressures higher (or temperatures lower) than the three-phase boundary, as shown in Figure 7.7. This result gives validity to the suggestion that... 

Theory would indicate, therefore, that electrodes should indeed have pores (and hence large numbers of menisci), but they should be very thin or else the catalyst should be concentrated only in a small region of high current density near the higher part of the three-phase boundary. The practical attainment of such a model was first reached in research carried out at the university level (Texas A M), and in a government research institute (Los Alamos National Laboratory). 

A third path, namely the ionization of the oxygen on the electrolyte surface followed by a direct incorporation into the electrolyte, can also not be excluded. In this case the electronic charge carriers, which are required in the oxygen reduction reaction, have to be supplied from the electrolyte. In solid electrolytes with very low electronic conductivity (e.g. zirconia), it can therefore be expected that the active zone is restricted to a region very close to the three-phase boundary. Hence, this path is, from a geometrical point of view, similar to the surface path discussed above. 

However, since gases generally have limited solubility in molten salts, corrosion will be restricted to a narrow region in the three-phase boundary. 

The performance of a fuel ceU is determined by the total surface area of the catalyst particles that participate in the reactions. Ideally, aU the surface of the Pt particles is used and the Pt achieves 100% dispersion (e.g., Pt exists as individual atoms). In reality, the ideal situation does not exist and only a small fraction of the Pt atoms can participate in the fuel cell reaction. The reasons are that the Pt can not achieve 100% dispersion and the fuel cell reactions require the so-called three-phase boundaries. It can be seen from Eqs. 1 and 2 that both the anode and the cathode reactions involve protons, electrons, and reactants. So, only the Pt surface that is accessible to protons, electrons, and the reactant is active, and such regions are often called catalyst-electrolyte-reactant three phase boundaries as illustrated in Fig. 2. All the other Pt surface area is basically wasted. For an electrode composed of Pt (or... 

Since both the anode and the cathode reactions involve the transport of electrons, protons, and reactants/products, only the catalyst surfaces at the catalyst-ionomer-reactant three-phase regions are electrochemically active. This area is called the electrochemical active surface area (ECSA). The remaining catalyst surfaces that do not meet this three-phase boundary condition will not be able to participate in the electrochemical reaction. An ideal electrode should be able to use all the catalyst surfaces to achieve 100% surface utilization. Measuring the ECSA and comparing it with the total surface area of the catalyst particles based on the particle size and flic catalyst loading reveals the actual utilization of the catalyst surface. 

A fuel cell electrode is porous and three-dimensional instead of flat. Its thickness varies at different locations. The reaction rates can be quite different from point to point due to the endless differences in the three-phase boundaries. The current distribution is not homogeneous at either micro- or macro-levels. For example, the inlet regions normally have higher current densities than the outlet regions. All these factors (three dimensions, uneven thickness, heterogeneous reaction rates, and uneven current distribution) could attribute to the double layers behaving differently from a pure capacitor. 

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