Oxygen reduction reaction overpotential

The oxygen reduction reaction overpotential loss in alkaline membrane fuel cells is very similar to that in proton exchange membrane fuel cells, i.e., the cathode overpotential loss remains an important factor limiting the efficiency and performance of alkaline membrane fuel cells [6, 7]. However, switching to an alkaline medium as in alkaline membrane fuel cells allows for the use of... 

In the case of iron phthalocyanine catalyst dispersed on carbon, infrared spectroscopy measurements enabled to confirm the series mechanistic pathway for the ORR in the high overpotential domain (below 0.7 V vs RHE). Two intermediates species, 02 and H02 and two products, water (majority product) and hydrogen peroxide could be identified. However, since only few studies are led on the oxygen reduction reaction by infrared spectroscopy, a confident attribntion of the different vibrational transitions observed to a particnlar intermediate remains difficult. 

Figure 3.3 shows the oxygen overpotential dependence on pH. Assuming that the applied potential of platinum remains at 0.515 V vs. SCE, the oxygen reduction reaction occurs only when the pH is less than or equal to 8. 

Norskov JK, Rossmeisl J, Logadottir A, Lindquist L, Kitchin JR, Bligaard T, Jonsson H (2004) Origin of the overpotential for the oxygen reduction reaction at a fuel cell cathode. J Phys ChemB 108 17886-17892... 

In a low-temperature fuel cell, hydrogen gas is oxidized into protons, electrons, and other by-products when other fuels are used at the anode. At the cathode of the fuel cell, the oxygen is reduced, leading to formation of water. Both the anodic and cathodic reactions require electrocatalysts to reduce the overpotentials and increase reaction rates. In the state-of-the-art low-temperature fuel cells, Pt-based materials are used as the electrocatalysts for both the reactions however, the high cost and limited resources of this precious metal are hindering the commercialization of fuel cells. Recent efforts have focused on the discovery of electrocatalysts with little or no Pt for oxygen reduction reaction (ORR) [1-3]. 

The x-shapes [Eqs. (23.17-23.19)] allow us to construct a model for this process [14]. Our main assumption will be that the Tafel slope of the reaction in Eq. (23.22) is close to the Tafel slope of the oxygen reduction reaction (ORR) running in the electrode. This means that the shape of the overpotential which drives the VCH reduction follows the shape of the ORR. The equihbrium potential of the VCH reduction differs from that of the ORR however, this difference only shifts the overpotential as a whole along the potential axis. Hence the rate of Cr2 O3 deposition also peaks at the electrolyte interface and the thickness of the domain of fast Cr deposition is equal to the thickness of the ORR conversion domain f. ... 

Cheng M-J, Head-Gordon M, Bell AT. How to chemically tailor metal-porphyrin-like active sites on carbon nanotubes and graphene for minimal overpotential in the electrochemical oxygen evolution and oxygen reduction reactions. J Phys Chem C 2014 118 29482-91. 

For a given electrochemical system, the increase of the voltage efficiency is directly related to the decrease of the overpotentials of the oxygen reduction reaction, t]c, and alcohol oxidation reaction, T]a, which needs to enhance the activity of the catalysts at low potentials and low temperature, whereas the increase of the faradic efficiency is related to the ability of the catalyst to oxidize completely or not the fuel into carbon dioxide, i.e. it is related to the selectivity of the catalyst. Indeed, in the case of ethanol, taken as an example, acetic acid and acetaldehyde are formed at the anode [10], which corresponds to a number of electrons involved of 4 and 2, respectively, against 12 for the complete oxidation of ethanol to carbon dioxide. The enhancement of both these efficiencies is a challenge in electrocatalysis. 

As already mentioned above, the oxygen reduction reaction (ORR) at the PEFC cathode suffers from poor kinetics, resulting in overpotentials of about 0.3-0.4 V. In acidic environment, Pt-based catalysts are still recognized the materials of choice for the ORR which leads to water formation according to the following equation ... 

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