Oxygen reduction reactions

The reaction between oxygen and hydrogen normally produces a great deal of heat  

However, if the reaction is carried out in a fuel cell, electricity is produced. A simple fuel cell is one in which hydrogen gas (fuel) is passed over one electrode (anode), oxygen is passed over the other (cathode) and the electrolyte is aqueous potassium hydroxide. The cell Pt/H2(g)/0H-(aq)/02(g)/Pt which produces 

The oxygen reduction reaction can proceed by two pathways in aqueous electrolytes. The first one, the so-called direct pathway, involves releasing four electrons per oxygen molecule to yield HgO. The indirect pathway involves releasing two electrons to yield hydrogen peroxide (HgOg) that in successive steps can produce water. Two further pathways, which are combinations of the above, can be envisaged. The series pathway implies sequential two or 

In the acid medium, Pt is the most active metal for the ORR and it drives the reaction via the four electron process. In fact, Pt was the preferred catalyst for fuel-cell applications from as early as the 1960s. Due to high Pt price and scarcity, many strategies were proposed for decreasing its usage within the fuel cell. 

The most promising ones are based upon (supported) Pt-based alloys. Numerous carbon supported bimetallic catalysts for the ORR have been studied, amongst them, PtCo, - PtAu, - PtV, PtFe, PtZn further PtM/C can be found in Refs. 12 and 178. 

A number of studies dealing with non-noble metal catalysts (NNMCs) have appeared in recent years. The main advantage of this type of catalyst is their low cost compared to Pt-based systems. In the next sections, relevant features to the ORR and the current status of catalyst development will be discussed. 

The oxygen reduction reaction (ORR) is the primary electrochemical reaction occurring at the cathode of a PEMFC, and is central to this promising technology for efficient and clean energy generation. The ORR is a multi-electron reaction that follows the direct four-electron mechanism on platinum-based electro-catalysts. It appears to occur in two pathways in acid electrolytes (Adzic and Lima, 2009)  

Considerable ongoing research aims to increase the electro-catalytic activity of platinum for the ORR. A particularly difficult problem to resolve is the large loss in potential (0.3. 4 V) mostly at the cathode, which is the source of major decline in the fuel cell s efficiency. Another drawback of existing electro-catalyst technology is the high Pt loading in cathode electro-catalysts, t)T)ically in the range of 0.1-0.5 mg cm.  

Many authors have been discussing the surface structure dependence of Pt toward the ORR under different conditions. Markovic et al (1995) found that the activity for ORR in 0.1 M HCIO4 decreases in the sequence of the Pt low-index planes (110) (111) (100), whereas the reactivity in H2 SO4 increased in the sequence (111) (100) (110). These differences in the sequence are ascribed to the strong bisulfate anion adsorption on the highly coordinated surfaces. Therefore, it is important to consider the strength of the anion adsorption and its surface-structure-dependent adsorption properties to understand the oxygen reduction reaction (Rabis et al, 2012). 

The effort to increase the active catalyst surface area per unit mass of Pt has centered in recent years on optimization of catalyst layer properties, aiming to maximize catalyst utilization in fuel cell electrodes based on Pt catalyst particle sizes of 2-5 nm (Gottesfeld, 2009). High catalyst utilization is conditioned on access to the largest possible percentage of the total catalyst surface area embedded in a catalyst layer by the three participants in the electrochemical process— gaseous reactants, protons and electrons—all at the rates called for by tiie demand current density. Fulfilling the latter condition requires a composite catalyst layer structure in which the electron 

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The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

If the potential of a metal surface is moved below line a, the hydrogen reaction line, cathodic hydrogen evolution is favored on the surface. Similarly a potential below line b, the oxygen reaction line, favors the cathodic oxygen reduction reaction. A potential above the oxygen reaction line favors oxygen evolution by the anodic oxidation of water. In between these two lines is the region where water is thermodynamically stable. 

Iron atoms pass into solution in the water as Fe leaving behind two electrons each (the anodic reaction). These are conducted through the metal to a place where the oxygen reduction reaction can take place to consume the electrons (the cathodic reaction). This reaction generates OH ions which then combine with the Fe ions to form a hydrated iron oxide Fe(OH)2 (really FeO, H2O) but instead of forming on the surface where it might give some protection, it often forms as a precipitate in the water itself. The reaction can be summarised by... 

Obviously, it is not very easy to measure voltage variations inside a piece of iron, but we can artificially transport the oxygen-reduction reaction away from the metal by using a piece of metal that does not normally undergo wet oxidation (e.g. platinum) and which serves merely as a cathode for the oxygen-reduction reaction. 

Many thousands of miles of steel pipeline have been laid under, or in contact with, the ground for the long-distance transport of oil, natural gas, etc. Obviously corrosion is a problem if the ground is at all damp, as it usually will be, and if the depth of soil is not so great that oxygen is effectively excluded. Then the oxygen reduction reaction... 

A sheet of steel of thickness 0.50 mm is tinplated on both sides and subjected to a corrosive environment. During service, the tinplate becomes scratched, so that steel is exposed over 0.5% of the area of the sheet. Under these conditions it is estimated that the current consumed at the tinned surface by the oxygen-reduction reaction is 2 X 10 A m -. Will the sheet rust through within 5 years in the scratched condition The density of steel is 7.87Mg m . Assume that the steel corrodes to give Fe " ions. The atomic weight of iron is 55.9. 

If, however, it is assumed from Eq. (2-40) that the protection current density corresponds to the cathodic partial current density for the oxygen reduction reaction, where oxygen diffusion and polarization current have the same spatial distribution, it follows from Eq. (2-47) with = A0/7 ... 

The hydrogen evolution reaction (h.e.r.) and the oxygen reduction reaction (equations 1.11 and 1.12) are the two most important cathodic processes in the corrosion of metals, and this is due to the fact that hydrogen ions and water molecules are invariably present in aqueous solution, and since most aqueous solutions are in contact with the atmosphere, dissolved oxygen molecules will normally be present. 

The mechanism of the oxygen reduction reaction is by no means as fully understood as the h.e.r., and a major experimental difficulty is that in acid solutions (pH = 0) E02/H20 = 1 23, which means that oxygen will start to be reduced at potentials at which most metals anodically dissolve. For this reason accurate data on kinetics is available only for the platinum metals. In the case of an iridium electrode at which oxygen reduction is relatively rapid, a number of reaction sequences have been proposed, of which the most acceptable appear to be the following ... 

It has been emphasised that the oxygen reduction reaction is diffusion controlled, and it might be thought that the nature of the metal surface is unimportant compared with the effect of concentration, velocity and temperature that all affect /Y and hence. However, in near-neutral solutions the surface of most metals will be coated (partially or completely) with either... 

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffiision, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.5), but little effect at temperatures below 50 C. This study shows how unwise it is to separate these various... 

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

Cao D, Wieckowski A, Inukai J, Alonso-Vante N (2006) Oxygen reduction reaction on rathe-nium and rhodium nanoparticles modified with selenium and sulfur. J Electrochem Soc 153 A869-A874... 

Lewera A, Inukai J, Zhou WP, Cao D, Duong HT, Alonso-Vante N, Wieckowski A (2007) Chalcogenide oxygen reduction reaction catalysis X-ray photoelectron spectroscopy with Ru, Ru/Se and Ru/S samples emersed from aqueous media. Electrochim Acta 52 5759-5765... 

Zaikovskii VI, Nagabhushana KS, Kriventsov VV, Loponov KN, Cherepanova SV, Kvon RI, Bdnnemann H, Kochubey DI, Savinova ER (2006) Synthesis and structural characterization of Se-modified carbon-supported Ru nanoparticles for the oxygen reduction reaction. J Phys ChemB 110 6881-6890... 

Lee JW, Popov BN (2007) Ruthenium-based electrocatalysts for oxygen reduction reaction—a review. J Solid State Electrochem 11 1355-1364... 

Zhu L, Susac D, Teo M, Wong KC, Wong PC, Parsons RR, Bizzotto D, Mitchell KAR, Campbell SA (2008) Investigation of CoSa-based thin films as model catalysts for the oxygen reduction reaction. J Catal 258 235-242... 

Lee K, Zhang L, Zhang J (2007) Ternary non-noble mefal chalcogenide (W-Co-Se) as electrocatalyst for oxygen reduction reaction. Electrochem Commun 9 1704-1708... 

Gochi-Ponce Y, Alonso-Nunez G, Alonso-Vante N (2006) Synthesis and electrochemical characterization of a novel platinum chalcogenide electrocatalyst with an enhanced tolerance to methanol in the oxygen reduction reaction. Electrochem Commun 8 1487-1491... 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

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