Three-Phase Boundary Issue

The three-phase boundary issue (problem) is a requirement to provide an intimate contact between all necessary phases of an electrochemical half-reaction. In the case of the anodic reaction of a PEMFC, the phases are (1) H2 gas, (2) H+ conductive membrane, and (3) e conductive Pt/C. If one of the phases is not available, the reaction does not take place. Note that in the case of the cathodic reaction of a PEMFC, there is a four-phase boundary issue. Therefore, the issue should generally be called the multiphase boundary. 

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Electrochemistry of PEMFC, DMFC, and SOFC is considered in detail, and the three-phase boundary issue is presented as a paramount problem in the development of the electrochemical energy conversion systems. 

The traditional electrochemical diagrams cannot properly address the three-phase boundary problem. A new type of electrochemical diagrams is proposed to properly describe the three-phase boundary issue. 

Following our approach in sec. 5.6, all contributions of nature (ii) are subsumed in the line tension, which has already been discussed. Hence, for the present section only item (i) remains. For the LG and LjL2 phase boundaries we have already treated the corresponding tensions in some detail in chapter 2. There is no need to repeat that. Interpretation of the two tensions involving solids is avoided because these are unmeasurable. The only issue left is how the various models and interpretations work out for a set of three phase boundaries. To that end, let us itemize the various interpretations of chapter 2. Where relevant, it is assumed that is the equilibrium value the SG interface may carry an adsorbate. 

Many processes occur in this three-phase boundary or in the overall fuel cell which may limit the overall performance. From a simple electrochemical point of view, the cell voltage E is associated with the fuel cell eflBciency t] as described by the following very simple equations for a more detailed treatment of this topic and a discussion of other efficiency issues, the reader is referred to the literature [5] ... 

We divide this program into three parts. The first issue is how to use the data accumulated at our chosen state point to infer the location of some point cx on the coexistence curve, for some finite N. The second is to map out the phase boundary emanating from that point. And the third is to deal with the corrections associated with the limited ( finite ) size of the simulation system. 

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