Triple phase boundaries

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]. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

D Mixed ionic electronic conductor (MIEC) o Triple-phase boundaries (TPB s)... 

FIGURE 6.1 Triple-phase boundaries (TPBs) in SOFC electrodes at which electrochemical reactions take place. Cathode mixed conductor materials have larger potentially electrochem-ically reactive surface areas (entire particle surfaces rather than only the TPBs). 

Fig. 14.20 Schematic representation of the relevant SOFC reactions. The steam reform reaction needs nickel as the catalyst. Oxidation of H2 and CO takes place at the triple phase boundaries (TPBs represented as smaller dots) in the anode, and is catalyzed by the Ni. At the cathode, 02 reduction also occurs at TPBs and is catalyzed by LSM.

Hayashi and co-workers [145] built an ideal triple phase boundary inside the mesopores of carbon support in order to examine the electrochemical reactions occurring in nanoscale. Depending on the solvent used (2-propanol) to dilute Nation , the reactivity toward oxygen reduction was different. Nation dissolved in 2-propanol was able to penetrate deeper into the mesopores and contact with more Pt particles... 

Main source terms prevailing in most transport equations for a fuel cell model are due to electrochemical reactions occurring in the electrode comprised of three phases electronic (s), electrolyte (e), and gas ( ). Electrochemical reactions occur at the triple-phase boundary according to the following general formula... 

The volumetric production rate of species k due to electrochemical reaction occurring at the triple-phase boundary is given by Faraday s law... 

The reactivity of iodine has been investigated at liquid-liquid-electrode triple-phase boundary systems. Scholz et al. [162] demonstrated that the reduction of iodine dissolved in a nitrobenzene microdroplet triggered the transfer of cations from the aqueous into the organic solution phase (Scheme 6). In addition, the formation of IzCl" interhalogen anions in the organic phase was discovered and shown to be the key to the overall process. 

Water vapour is produced at the anode diluting the fuel. The hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) occur at the triple phase boundary (TPB) zone where the electrode (electronic phase), electrolyte (ionic... 

For example, Bieberle and Gauckler [7] developed an electrochemical model for the Ni, H2-H2O-YSZ system (i.e. the anodic triple phase boundary). As a result they identify possible reaction mechanisms and calculate some kinetic parameters, thus providing valuable inputs and information for simulating the entire fuel cell. Moreover a better understanding of atomistic phenomena acting at the anode-electrolyte interface is provided. 

Simplification of the Model Triple-Phase-Boundary Reactions as Boundary Conditions... 

As previously explained, the triple-phase-boundary is the site where ions, electrons and gas coexist, thus enabling reactions (3.15) and (3.16) to take place. As can be observed in the schematic representation given in Figure 3.3, the triple-phase-boundary represents a small portion of the entire electrode domain. Moreover, since the electronic conductivity of the electrodes is much higher than the ionic conductivity, ion migration within the electrode domains is very limited in space, thus reactions (3.15) and (3.16) are likely to take place very close to the electrode-electrolyte interface. 

Boundary Conditions when the Triple-Phase-Boundary is Reduced to the Electrode/Electrolyte Interface... 

Equations (5.46) and (5.47) are referred to as steam methane reforming (SMR) reaction and Water Gas Shift (WGS) reaction respectively. These reactions are homogenous reactions that occur everywhere inside the anode, whereas Equations (5.48) and (5.49) only occur at the active triple phase boundary. Treatment of source terms due to electrochemical oxidation of H2 (Equation 5.49) is already covered in the previous sections and the treatment is similar for electrochemical oxidation of CO (Equation 5.48). Specie source terms due to homogenous reactions in the mechanism are given by ... 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

O Hayre R, Prinz FB (2004) The air/platinum/Nafion triple-phase boundary characteristics, scaling, and implications for fuel cells. J Electrochem Soc 15LA756-... 

Figure 15.1. Illusuation of the difference in location of the electrode reaction on two different SOFC electrode types. Upper In an electrode where the electrode material is exclusively an electronic conductor, the reaction zone is restrained to the vicinity of the triple phase boundary (TPB). Lower In a mixed ionic-electronic conductor (MIEC) the electrode reaction can take place on the entire electrode surface...

Although powder routes have been successfully developed for synthesis of catalysts used in low temperature fuel cells, electrodeposition offers the highest noble metal utilization and is preferred over the alternative chemical methods. Electrodeposition enables the formation of catalyst particles on specific sites where they can be essentially utilized, i.e., the triple phase boundary where the membrane (ionic conductor), electrode (electronic conductor), and reactants meet. Powder methods do not guarantee that all catalyst particles are in contact with both electrode and membrane materials, and therefore, a portion of catalyst particles may remain inactive. 

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