Oxygen Reduction Reaction Thesis

Oxygen Reduction Reaction Thesis-67
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].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].

Transition metals such as Fe, Ni, Co and Cr have been extensively studied due to their improved ORR electrocatalytic activity as alloys for Pt in the presence of a support [17, 28]. [17] prepared Pt Fe/C using the impregnation method.

Current densities of Pt Fe/C for ORR in methanol were higher (78.6 m A cm), but was lower in methanol-free solutions, indicating that Pt Fe/C is a better methanol-tolerant catalyst compared to Pt/C.

An electrocatalyst is used to induce a four-electron reduction of O to water by utilising the protons that permeate from the anode.

Pt is the electrocatalyst that is currently used for ORR reactions, as it is the only commercially available catalyst with sensible activity and stability for PEMFCs, although it offers limited commercialisation of fuel cells due to its limited availability and high cost [4, 20].

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.Oxygen reduction in aqueous solutions occurs mainly through two different pathways: either a four-electron reduction pathway from O.The most accepted mechanism of ORR was first proposed by Damjanovic et al. [7], making it easier to understand the complicated reaction pathway of oxygen on the metal surface.Considerable research has been conducted to try to (1) reduce the costs of fuel cells, which is one of the stumbling blocks in fuel cell commercialisation using low-cost non-Pt catalysts such as supported platinum group metals Pd, Ir and Ru; (2) improve the electrocatalytic activity of the cathode catalyst, which includes using bimetallic alloy catalysts, transition metal macrocyanides, transition metal chalcogenides and metal oxides in order to improve the ORR kinetics on the new catalyst; and (3) fabricate Pt with novel nanostructures such as nanotubes, graphene and carbon nanofibres (CNFs), as it is known that supports may significantly affect the performance of the catalyst.However, these efforts are still in the research stage, as their activity and stability are still lower than that of the Pt catalyst.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.Figure 3 shows a comparison of the activities of various catalysts as a function of binding energy.These catalysts showed less catalytic activity towards ORR compared to Pt, with less electrochemical stability [28].For ORR, two Tafel slopes, 60 m V dec The kinetics of the ORR at the cathode are very important, as they are the factors for the performance of PEMFCs [14, 15].There are several issues that need to be addressed, including slow reaction kinetics at the cathode, which are due to highly irreversible ORR, and fuel crossover in the cathode, which causes a mixed potential, leading to potential loss and 25% reduction in efficiency, hence reducing the ORR performance [16, 17, 18, 19].


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