Anode hydrogen oxidation reaction

Currently used electrode-catalysts (anode and cathode) consist of an assembly of metallic nanoparticles usually deposited on an electronic conducting substrate and embedded in a hydrated membrane [10, 11], which is the polymer electrolyte proton-conductive material (Figure 17.1). What differs between cathode and anode is the catalyst material, and also the significantly slow kinetics of the cathode oxygen reduction reaction compared to that of the anode hydrogen oxidation reaction. For this reason, several... 

The development of cheap and efficient electrocatalysts, especially the oxygen reduction electrocatalyst, is another task for the large-scale commer-ciahzation of PEMFCs. In any PEMFCs, there are two types of electrocatalysts one catalyzes the anodic hydrogen oxidation reaction and the other the cathodic oxygen reduction reaction. Both electrocatalysts rely heavily on the use of precious metals, especially the Pt-based electrocatalysts. The tasks in the development of electrocatalysts include the improvement of electrocatalytic activity, the reduction of the loading of precious metals, or even the replacement of precious metals with cheap metals, and the... 

A schematic of the fuel cell reactions and the main components of the MCFC is given in Figure 7.12. The global anode hydrogen oxidation reaction (HOR) is... 

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. 

When pure hydrogen is used as the fuel, the overpotential for the hydrogen oxidation reaction (HOR) at the Pt anode is negligibly small. 

These kinetic expressions represent the hydrogen oxidation reaction (HOR) in the anode catalyst layer and oxygen reduction reaction (ORR) in the cathode catalyst layer, respectively. These are simplified from the general Butler-Volmer kinetics, eq 5. The HOR... 

The mechanism of anodic hydrogen oxidation is much simpler than that of oxygen reduction. Reaction pathways would be the reverse of the Volmer-Heyrovsky or the Volmer-Tafel mechanism, that is, with anodic adsorption... 

Water vapour is produced at the anode diluting the fuel. The hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) occur at the triple phase boundary (TPB) zone where the electrode (electronic phase), electrolyte (ionic... 

As discussed above, if the exchange current density of anode hydrogen oxidation is i nMc =0,1 Acni 2 and that of the cathode oxygen reduction reaction (ORR) is... 

The hydrogen oxidation reaction (HOR) occurs at the anode electrode of an H/O2 PEM fuel cell (reaction 1) ... 

The hydrogen oxidation reaction (HOR) at the anode proceeds on Pt-based catalysts and is one of the simplest reaction systems. ° Nonetheless, fundamental information of the mechanism and kinetics of HOR is still lacking. The most common mechanisms are the so-called Heyrovsky-Volmer and Tafel-Volmer mechanisms involving the following steps ... 

The anode overpotential for hydrogen oxidation reaction is low, i.e., ija 0.05 V, so that Eq. (78) may be linearized, while the cathode overpotential is very high, i.e., 0.4 V in normal hydrogen fuel... 

While it is expected that electrocatalytic reactions on Ru surfaces should be strongly structure-sensitive, the first report on structural effects on hydrogen oxidation and evolution reactions appeared only recently The structural effects in the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER) may be factors affecting the performance of hydrogen fuel cell anodes. 

Effect of (Tion on the Hydrogen Oxidation Reaction Rate at Porous Pt Anode... 

Here hcathode and Panode Stand for overpotentials at the cathode and anode, respectively. In a fuel cell fed with pure dihydrogen, Panode is small, due to the fast kinetics of the hydrogen oxidation reaction (12.1a) on Pt catalysts, and is often neglected [4]. An overpotential may be separated into the reaction overpotential... 

The degree to which electrode potentials are nonequilibrium values depends on the relative rates of the underlying electrode reactions. Under comparable conditions, the rate of Reaction (16.3)—cathodic oxygen reduction—is 10 orders of magnitude lower than that of Reaction (16.2)—anodic hydrogen oxidation. 

Fuel cell CLs are the key components in the entire fuel cell device because the reactions such as hydrogen—oxidation reaction (HOR) at anode and the ORR at cathode occur inside the CLs. Particularly, in order to carry out the ORR, the catalyst particles inside the cathode CL must be in contact with each other for electrical conductivity and also in contact with protonic conducting (in acidic PEM fuel cells), or hydroxide conducting (in alkaline PEM fuel cells) ionomer for ionic conductivity. In addition, there must be some channels within the CL for transporting the reactants and the products. In other words, the catalyst particles must be in close contact with each other, with the electrolyte, and also with the adjacent diffusion medium (DM). Moreover, the reactants gas O2) and the produced water travel mainly through the voids, so the CL must be porous enough to allow gas to diffuse to the reaction sites and liquid water to wick out. 

However, for technical use of AFC, the long-term behavior of AFC components is important, especially that of the electrodes. Nickel can be used for the hydrogen oxidation reaction (catalyst in the anode) and on the cathode silver can be used as catalyst (see next section), no expensive noble metal (platinum) is necessary, because the oxygen reduction reaction kinetics are more rapid in alkaline electrolytes than in acids and the alkaline electrochanical environment in AFC is less corrosive compared to acid fuel cell conditions. Both catalysts and electrolyte represents a big cost advantage. The advantages of AFC are not restricted only to the cheaper components, as shown by Giilzow [1996]. 

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