Hydrogen oxidation reaction densities

Electrocatalytic Oxidation of Hydrogen The rate constant of the hydrogen oxidation reaction (HOR), as measured by the exchange current density jo (i.e. the current... 

In this chapter we discnssed the reaction mechanisms of the most important electrochemical reactions involved in low temperature fuel cells. Apart from the hydrogen oxidation reaction, which is relatively easy, and does not lead to a large overpotential even at high current densities, the oxidation of low weight alcohols... 

This particular volcano plot is indeed nearly symmetric and demonstrates that Pt, Rh, Ir, and Re have the optimal M—H bond strength for maximum activity (exchange current density) for the hydrogen evolution/hydrogen oxidation reaction (HER/HOR). 

M HCIO4 for different disk electrodes as indicated in the labels, (bottom) Exchange-current densities in acid electrolyte (pH = 1) for the hydrogen oxidation reaction as a function of Pd coverage on the Pt( 111) substrate. 293 K. 

Yasuyuki Ishikawa, Juan J. Mateo, Donald A. Tryk, Carlos R. Cabrera Direct molecular dynamics and density-functional theoretical study of the electrochemical hydrogen oxidation reaction and underpotential deposition of H on Pt(l 11), "Journal of Electro-analytical Chemistry", 607, 37-46 (2007). 

In organic chemistry, oxidation and reduction processes are different from ordinary redox reactions because in many cases they do not involve direct electron transfer but may involve a decrease in electron density around a molecule or loss/gain of hydrogen. Oxidation reactions are useful to convert alcohols into carbonyl compounds, nitriles into acids, and amines into imines. SSA along with a suitable reagent such as oxone or sodium nitrite serves as a powerful oxidant. This part of the chapter encompasses the oxidation reactions catalyzed by SSA. 

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 ... 

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. 

Table 5.1 Approximate Exchange Current Density (i ) for the Hydrogen Oxidation Reaction on Different Metais at 25°C...

A cm , whereas the exchange current density for the hydrogen oxidation reaction (HOR) is in the range of 10 " -10 A cm . This significant difference suggests that the overall electrochemical kinetics of PEM fuel cells is limited by the relatively slow ORR. 

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