Superoxide-peroxide mechanism

As reaction mechanisms become more complicated, the need for effective graphical depictions increases. To illustrate this, let us consider the superoxide-peroxide mechanism of Adanuvor, White and Appleby (1990).25 The overall reaction for this mechanism is the same as in the previous cases 02 + 2C02 + 4e 2C03. This mechanism consists of seven steps, three of which appear to be the same as in the peroxide mechanism  

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As a first example of the use of reaction mechanism graphs, consider the electrochemistry of molten carbonate fuel cell (MCFC) cathodes. These cathodes are typically nickel-oxide porous electrodes with pores partially filled with a molten carbonate electrolyte. Oxygen and carbon dioxide are fed into the cathode through the vacant portions of the pores. The overall cathodic reaction is 02 + 2C02 + 4e / 2C03=. This overall reaction can be achieved through a number of reaction mechanisms two such mechanisms are the peroxide mechanism and the superoxide-peroxide mechanism, and these are considered next. 

Figure 7. Reaction route graphs for the peroxide and superoxide-peroxide mechanisms reaction steps occur on directed edges nodes n, represent the component potentials, the difference between these potentials for adjacent nodes is the affinity of the associated reaction step and terminal nodes are open, intermediate nodes, closed.

Figure 8. Equivalent component-potential reaction route graph for the peroxide and superoxide-peroxide mechanisms.

The reaction route graphs, however, do have certain limitations. It is not in general possible, for example, to depict the physical location of the various reactions and species. It is not easy to distinguish on reaction route graphs that the peroxide ions, which must move across the electrolyte in the peroxide mechanism, exist only on the phase interfaces (gas-electrolyte and electrolyte-solid) in the superoxide-peroxide mechanism. This depiction is one of the strong points for reaction mechanism graphs. 

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