Migration, solid electrolytes

Small fuel cells can be exploited most effectively if they can stand by and operate at ambient temperatures (and can therefore start rapidly), can operate on ambient air, can respond rapidly to load changes, have a non-migrating (solid) electrolyte, and have a reasonably high power density and specific-power. The fuel cell type that best suits these criteria is clearly the PEMFC, despite a drawback related to the fact that liquid water embodied in the solid polymer tends to freeze, and thereby impedes proton conduction, when its temperature drops below the freezing point. The PEMFC can stand by under freezing conditions. 

An instructive use has been made of the solid electrolyte, Agl, which conducts by the migration of silver ions. If this material is used as an electrolyte in the cell... 

It is important to realize that the migration in an electric field depends on the magnitude of the concentration of the charged species, whereas the diffusion process depends only on the concentration gradient, but not on the concentration itself. Accordingly, the mobility rather than the concentration of electrons and holes has to be small in practically useful solid electrolytes. This has been confirmed for several compounds which have been investigated in this regard so far [13]. 

Thus, as will be shown in this book, the effect of electrochemical promotion (EP), or NEMCA, or in situ controlled promotion (ICP), is due to an electrochemically induced and controlled migration (backspillover) of ions from the solid electrolyte onto the gas-exposed, that is, catalytically active, surface of metal electrodes. It is these ions which, accompanied by their compensating (screening) charge in the metal, form an effective electrochemical double layer on the gas-exposed catalyst surface (Fig. 1.5), change its work function and affect the catalytic phenomena taking place there in a very pronounced, reversible, and controlled manner. 

Figure 1.5. Schematic representation of a metal electrode deposited on a 02 -conducting (left) and on a Na -conducting (right) solid electrolyte, showing the location of the metal-electrolyte double layer and of the effective double layer created at the metal/gas interface due to potential-controlled ion migration (backspillover).

Then let us examine the rate relaxation time constant x, defined as the time required for the rate increase Ar to reach 63% of its steady state value. It is comparable, and this is a general observation, with the parameter 2FNq/I, (Fig. 4.13). This is the time required to form a monolayer of oxygen on a surface with Nq sites when oxygen is supplied in the form of 02 This observation provided the first evidence that NEMCA is due to an electrochemically controlled migration of ionic species from the solid electrolyte onto the catalyst surface,1,4,49 as proven in detail in Chapter 5 (section 5.2), where the same transient is viewed through the use of surface sensitive techniques. 

It also shows that electrochemical promotion is due to electrochemically controlled migration (backspillover) of ions (acting as promoters) from the solid electrolyte to the gas-exposed catalytically active catalyst-electrode surface. 

As already analysed in Chapter 5, once the backspillover species originating from the solid electrolyte have migrated at the metal/gas interface, then they act as normal (chemical) promoters for catalytic reactions. For example, Lambert and coworkers via elegant use of XPS18 have shown that the state of sodium introduced via evaporation on a Pt surface interfaced with P"-A1203 is indistinguishable from Na5+ introduced on the same Pt surface via negative (cathodic) potential application. 

Both questions have been recently addressed via a surface diffusion-reaction model developed and solved to describe the effect of electrochemical promotion on porous conductive catalyst films supported on solid electrolyte supports.23 The model accounts for the migration (backspillover) of promoting anionic, O5, species from the solid electrolyte onto the catalyst surface. The... 

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

The conductivity of solid salts and oxides was first investigated by M. Faraday in 1833. It was not yet known at that time that the nature of conduction in solid salts is different from that in metals. A number of fundamental studies were performed between 1914 and 1927 by Carl Tubandt in Germany and from 1923 onward by Abram Ioffe and co-workers in Russia. These studies demonstrated that a mechanism of ionic migration in the lattice over macroscopic distances is involved. It was shown that during current flow in such a solid electrolyte, electrochemical changes obeying Faraday s laws occur at the metal-electrolyte interface. 

The above two mechanisms may be regarded as isolated ion hops. Sometimes, especially in solid electrolytes, cooperative ion migration occurs. An example is shown in Fig. 2.1(c) for the so-called interstitialcy or knock-on mechanism. A Na" ion. A, in an interstitial site in the conduction plane of j -alumina (see later) cannot move unless it persuades one of the three surrounding Na ions, B, C or D, to move first. Ion A is shown moving in direction 1 and, at the same time, ion B hops out of its lattice site in either of the directions, 2 or 2. It is believed that interstitial Ag" ions in AgCl also migrate by an interstitialcy mechanism, rather than by a direct interstitial hop. 

The operating principle of a SOFC is schematically shown in Fig. 11.25. When an external load is applied to the cell, oxygen is reduced at the porous air electrode to produce oxide ions. These ions migrate through the solid electrolyte to the fuel electrode, and oxide ions react with the fuel, or CO, to produce HjO or COj. 

Adams, St. and Maier, J. (1998). Ag migration pathways in crystalline and glassey solid electrolytes Agl, AgM , 0. Solid State Ionics 105, 61-lA. 

When ions migrate through a solid electrolyte, they diffuse from this onto the gas-exposed surface of the metal electrode. These ions form a double layer (and hence a potential difference) at the metal/gas interface. I Iowcver, this potential difference (which varies with the electrode potential) in turn changes the work function at the gas/metal interface. The ease of availability of electrons in the bonding of radicals adsorbed from the gas phase onto the electrode increases as the electronic work function of the solid decreases. The chemical reaction rate of the catalyzed reaction depends on the bonding strength of these radicals to the electrode catalyst, which involves electrons from the metal and is therefore dependent on the work function of the metal this itself is a function of the electrode potential. In this way, a dependence of the rate of the chemical reaction upon the potential of the working electrode can be rationalized. 

The basic elements of a SOFC are (1) a cathode, typically a rare earth transition metal perovskite oxide, where oxygen from air is reduced to oxide ions, which then migrate through a solid electrolyte (2) into the anode, (3) where they combine electrochemically with to produce water if hydrogen is the fuel or water and carbon dioxide if methane is used. Carbon monoxide may also be used as a fuel. The solid electrolyte is typically a yttrium or calcium stabilized zirconia fast oxide ion conductor. However, in order to achieve acceptable anion mobility, the cell must be operated at about 1000 °C. This requirement is the main drawback to SOFCs. The standard anode is a Nickel-Zirconia cermet. 

Once the working electrode (W) is covered by the ionic conducting product or the entire solid electrolyte (E) is covered by the electronic conducting product, no electrically shorted surface exists. Thus, further growth in thickness has to involve diffusion of both the ionic species and electrons to the surface to react with the gas phase. Practically, diffusion of one species is much faster than the other. However, electroneutrality must be maintained under this open circuit condition. The growth rate is determined by either migration of electrons or mobile ionic reactants in the deposit (D). In both cases, the increase in thickness should follow the parabolic law. ... 

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