Kinetics of the Hydrogen Oxidation Reaction

For the hydrogen oxidation reaction in Equations 3.1 and 3.2 in acidic and alkaline media, respectively, the Volmer expression deals with the pure charge-transfer process (Had + e )- total (or net) current density for this two-directional charge-transfer process, i = ij- —, could be expressed as follows  

Aeeording to the reaetion expressed in Equation 3.6 in an alkaline solution, the eurrent-potential relationships ean be expressed as Equations 3.23 and 3.24, respeetively  

If the redox reaction equilibrium can be established, the partial forward current density will equal the partial backward current density. Equations 3.21 and 3.22 could also be expressed in terms of the exchange current density, i, as in Equations 3.25 and 3.26, respectively. These / are the values at the equilibrium potential () where the net current is zero  

When taking into consideration the overpotential ( T] = E-E g), which designates the hydrogen overpotential. Equations 3.21-3.24 may be rewritten as 3.27  

Note that in general, the value of 6 is current-density dependent. 

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The kinetics of the hydrogen oxidation reaction under excess 02 satisfactorily obeyed the following kinetic equation, typical of a two-step redox mechanism ... 

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... 

In this section, we construct several analytical polarization curves of PEMFCs and high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). In these types of cell, owing to the excellent kinetics of the hydrogen oxidation reaction, the polarization voltage of the anode is negHgible. The voltage loss in a PEMFC is determined by the oxygen transport, ORR kinetics, and the cell resistivity. 

The kinetics of the hydrogen oxidation reaction (HOR) (1.1) are very fast and the respective polarization voltage can be ignored. However,... 

Studies have shown that the decrease in membrane conductivity can explain only part of the total loss in fuel cell performance, typically 5 to 15%. Uribe et al. [84] and Soto et al. [86] attributed some performance loss to decreased ionomer conductivity in the catalyst layer. However, other studies [85,89] indicated that NH3 could also affect the kinetics of the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR). Halseid et al. [85] found that the HOR in a symmetric H2/H2 cell was affected by NH3 and they suggested that the adsorbed species could partially block the anode catalyst surface. They also found that the... 

The kinetics of the hydrogen oxidation reaction in acidic PEM fuel cells above room temperature are often so fast that they contribute a negligible voltage to the overall activation overpotential, which is therefore often assumed to be wholly attributable to the ORR [41]. This allows catalyst loadings on the anode to be as low as 0.05mgptcm without affecting overall fuel cell performance significantly [42] and means that catalyst development is mainly focused on the cathode. However, in alkaline media, the HOR on polycrystalline Pt has been... 

Eley-Rideal) mechanism, one of the reactants comes directly from the fluid phase to react with the other, which is already chemisorbed. This procedure was devised to explain the kinetics of the hydrogen-deuterium reaction on certain metals (see Section 9.2), but has also been suggested for other reactions. The Mars-van Krevelen mechanism applies to oxidations catalysed by oxides that are easily reducible, and are therefore able to release their lattice oxide ions for the purpose of oxidising the other reactant they are then replaced by the dissociation of molecular oxygen. With gold catalysts supported on such oxides, it is sometimes proposed that this mechanism plays a part in the total process. 

The kinetics of the hydrogen electrode reaction on dense porous graphite electrodes in molten KHSO4 from 245° C to 280°C [88-90] showed that the cathodic and anodic reactions are not strictly conjugated processes. The cathodic reaction was discussed in terms of conventional mechanisms, but the anodic reaction involves the simultaneous oxidation of hydrogen and graphite surface. The reaction exhibits a one-half power dependence on hydrogen pressure. 

In the present author s view, the Gaussian distribution function based on the solvent fluctuation model, which is developed for a simple redox couple, is used too often even when the basic assumption is not valid. For example, this type of distribution function is often drawn for the hydrogen evolution reaction where the oxidized state is H+ and reduced state is H2.105 Certainly the nature of the solvation is completely different between H+ and H2. Moreover, when one considers the kinetics of the hydrogen evolution reaction, one should consider not the energy level of H+/H2 but that of H+/H(a) as Gurney did. 

The kinetics of the preferential oxidation reaction of carbon monoxide (PrOx), for the hydrogen oxidation reaction (H2OX) and for the reverse water-gas shift reaction (RWGS) were provided, which had been determined in the relevant parameter space [398] ... 

Mello RMQ, TicianeUi EA. Kinetic study of the hydrogen oxidation reaction on platinum and Nafion covered platinum electrodes. Electrochim Acta 1997 42 1031-9. 

Lin R, Shih S. Kinetic analysis of the hydrogen oxidation reaction on Pt-black/Nafion electrode. J SohdState Electrochem 2006 10(4) 243-9. 

In PEM fuel cells, the exchange current density for the electrochemical oxygen reduction reaction (ORR, 10 -10 A cm ) is much smaller than that of the hydrogen oxidation reaction (HOR, 10 -10 A cm ). Due to the larger HOR exchange current density, the HOR at the anode Pt nanoparticle/PEM interface is much faster than the ORR at the cathode interface [14]. In other words, the overpotential for the HOR is negligibly small compared with that of the ORR when the anode is adequately hydrated. The overall electrochemical kinetics of PEMFCs is therefore dominated by the relatively slow oxygen reduction reaction. 

The kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

The advantage of AFCs over the other systems lies in the fact that the reduction of oxygen to OH- is much faster than the acidic equivalent of oxygen to H20 due to a better kinetics, which makes the AFC a more efficient system [15]. The hydrogen oxidation reaction in alkaline medium, however, is slower. 

The capability of SECM to detect and to image regions with different catalytic activities is well known [120-122]. So far, this technique has been applied to studies of mainly two electrocatalytic reactions, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), which have important implications for fuel cells. Unlike reversible redox mediators usually employed in SECM experiments, the kinetics of oxygen and hydrogen reactions are strongly dependent on the catalytic activity of the substrate surface. 

Extensive studies have been made of the oxidations of all the halides by hydrogen peroxide. Mellor in 1904 was already able to cite fourteen investigations of kinetics of the hydrogen peroxide-hydrogen iodide reaction including studies of the temperature dependence of the rates and the kinetic form of catalysis by salts of molybdenum and iron. Since the processes (1) and (2)... 

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