Three-phase boundaries cathodes

In the case of SOFCs, a large volume of work shows that for many SOFC electrodes, overall performance scales with the ID geometric length of this three-phase boundary. As such, the TBP concept and electrode performance models based on it have proven to be some of the most useful phenomenological concepts for guiding design and fabrication of SOFC cathodes, particularly the microstructure. 

Although the SOFC community has generally maintained an empirical approach to the three-phase boundary longer than the aqueous and polymer literature, the last 20 years have seen a similar transformation of our understanding of SOFC cathode kinetics. Few examples remain today of solid-state electrochemical reactions that are not known to be at least partially limited by solid-state or surface diffusion processes or chemical catalytic processes remote from the electrochemical—kinetic interface. 

Bulk path at moderate to high overpotential. Studies of impedance time scales, tracer diffusion profiles, and electrode microstructure suggest that at moderate to high cathodic over potential, LSM becomes sufficiently reduced to open up a parallel bulk transport path near the three-phase boundary (like the perovskite mixed conductors). This effect may explain the complex dependence of electrode performance on electrode geometry and length scale. To date, no quantitative measurements or models have provided a means to determine the degree to which surface and bulk paths contribute under an arbitrary set of conditions. 

The hydrogen oxidation within a fuel cell occurs partly at the anode and the cathode. Different models were supposed for the detailed reaction mechanisms of the hydrogen at Ni-YSZ (yttria stabilised zirconia) cermet anodes. The major differences of the models were found with regard to the location where the chemical and electrochemical reactions occur at the TPB (three-phase boundary of the gaseous phase, the electrode and the electrolyte). However, it is assumed that the hydrogen is adsorbed at the anode, ionised and the electrons are used within an external electrical circuit to convert the electrical potential between the anode and the cathode into work. Oxygen is adsorbed at the cathode and ionised by the electrons of the load. The electrolyte leads the oxide ion from the cathode to the anode. The hydrogen ions (protons) and the oxide ion form a molecule of water. The anodic reaction is... 

Figure 17 shows different mechanistic pathways for the oxygen reduction at the LSM cathode on YSZ electrolyte. The adsorbed, partially fully ionized oxygen may move along the surface to the three phase boundary where it is transformed into the electrolyte. (In principle it may also reach this place directly via the gas phase.) The oxygen may also reach the electrolyte by diffusion through the LSM bulk via a counter motion of O2 and 2e . Note that LSM sandwiched between Pt (serving as a reversible electrode) and YSZ... 

A cathodically polarized air electrode (planar) has a limiting current of about 1(T4 A cm-2. In a fuel cell and the critical quantity that controls the magnitude of the current density is the thickness of the electrolyte in the meniscus of the three-phase boundaries. This varies with the shape of the meniscus and the contact angle, (d) Assume a section of the meniscus having a solution thickness of 10-5 cm and calculate the limiting current of this section, (e) In light of these zeroth approximation calculations, where do you think the maximum activity of a pore lies (Bockris)... 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

Bias-dependent measurements were performed in order to check to what extent the mechanism depends on the electrical operation conditions. Fig. 43 shows double-logarithmic plots of the electrode polarization resistance (determined from the arc in the impedance spectrum) versus the microelectrode diameter observed at a cathodic bias of —300 mV and at an anodic bias of +300 mV respectively. In the cathodic case the electrode polarization resistance again scales with the inverse of the electrode area, whereas in the anodic case it scales with the inverse of the microelectrode diameter. These findings are supported by I-V measurements on LSM microelectrodes with diameters ranging from 30-80 pm the differential resistance is proportional to the inverse microelectrode area in the cathodic regime and comes close to an inverse linear relationship with the three-phase boundary (3PB) length in the anodic regime [161]. 

The reduction of oxygen to takes place at the interface of the three phase boundary. The mixed cathode which allows both electron and to travel along the surface and bulk extends the interface of the three phase boundary to an active zone for the reduction of O2 to 0 . The extension of the active zone depends on the oxygen diffusion, surface exchange coefficient, the microstructure and porosity of the cathode layer, shown in Fig. 2 (a). ... 

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. 

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

Porous ceramics, with high electronic conduction and chemical stability in the fuel gas are required for anode and cathode. The electrode should be porous and have homogeneous microstructures because the electrode reaction occurs on three phase boundary (TPB), which consists of electrolyte, electrode and gas. The reaction site increases with TPB length. Using a new technique of spray dry process, a new... 

The direct methanol fuel cell (DMFC) is one of most interesting low-temperature fuel cell system for practical application in small or portable electronic devices. This is attributed to its high energy density, feasible operation conditions at ambient temperature, and easy availability of the liquid fuel [1]. In this type of fuel cell, the diffusion of methanol molecules to the three-phase boundary of the oxygen cathode can hardly be avoided. Under working conditions, this leads to a negative shift of... 

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... 

While it is important to maximize the electrode area accessible to O2, the cathode must also allow for contact with the electrolyte. A three-phase boundary comprised of O2, electrolyte and carbon with catalyst is central to the proper functioning of an air electrode. Maintaining a balance between access to atmospheric O2 and electrolyte solution is achieved by adequately dispersing just the right amoimt of a hydrophobic material, polytetrafluoroethlyene (PTFE or Teflon ), in the carbon powder mix. Too little PTFE or it is not dispersed well enough and the electrolyte solution could saturate the cathode. This reduces the area at which O2 can react with a resulting loss of performance. Too much PTFE and the electrolyte will not sufficiently wet the cathode. Again, this limits battery performance. 

There are, however, a number of important systems where this situation does not hold, for example ceramic-metal composites, and ceramic composites of electronic and ionic conductors, used as electrodes in sohd oxide fuel cells. Composite electrodes are important in a solid oxide fuel cell (SOFC), as they provide the contact area necessary for the electrode processes to occur. This is usually visualized as the three-phase boundary (TPB), the boundary line where electronic conductor, ionic conductor, and pores meet. A composite cathode is shown schematically in Figure 4.1.14, after Costamagna et al. [1998]. The processes occurring in a composite electrode are briefly as follows ... 

Point contact electrodes are made by contacting a point of electrode materia] on a dense electrolyte. They are simpler than composite cathodes due to (a) their well defined geometry, allowing the estimation of the three-phase boundary length and (b) the absence of complex diffusional processes. 

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... 

---