Oxygen Reduction Reaction Thesis

Oxygen Reduction Reaction Thesis-63
Ag is reported to show less electrocatalytic activity towards ORR compared to Pt, but is more stable than Pt cathodes during long-term operations [4].There are several metals other than noble metals that were also evaluated as cathode catalysts for ORR.

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They suggest that ORR proceeds along two parallel reaction pathways with rates that are comparable. The ORR is alkaline media versus reversible hydrogen electrode (RHE) at 25°, and its thermodynamic potentials at standard conditions are presented as follows [3, 8, 9]: It is desirable for the ORR to occur at potentials close to thermodynamic potentials as much as possible.

For the thermodynamic potentials to be obtained, the charge transfer kinetics of the ORR must be quick.

It has been reported that the kinetics of fuel cells at cathode are slow, hence show over-potential ƞ as in Eq. This function takes into account the electrolytic resistivity of an environment when the anodic and cathodic elements of a corrosion reaction are separated by this environment while still electrically coupled.

The graph in Figure 1 depicts a polarisation curve showing the electrochemical efficiency of a fuel cell. The first term in the Butler-Volmer equation represents the anodic reaction/metal dissolution, while the second term represents the cathodic reaction/metal deposition.

It has been showed that Pd/C is less sensitive to alcohol contamination compared to Pt/C, hence more tolerant to alcohol crossover [16, 24, 25].

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However, when comparing the ORR activity of Pd/C to Pt/C, the former has less activity and high potentials of amount 0.8 V versus NHE, hence less stability, which prevents its replacement as the preferred ORR catalyst over Pt/C [26].

It is also reported that Pt still shows over-potentials of over 400 m V from the equilibrium reversible potentials (1.19 V at 80°C) [21].

These high potentials result in the formation of adsorbed species on a platinum surface that restrain the ORR and hence result in performance loss [2].

Oxygen reduction reaction (ORR) has been the subject of extensive investigation over the last century [1].

This is largely because ORR is of major importance to energy conversion, in particular in the field of fuel cells and metal-air batteries [1, 2, 3].


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