Oxygen reduction reaction constants

Scheme 5. General scheme of oxygen reduction reaction with the kinetics constant associated with each pathway.

Even more important is the fact that most PEM materials (see below) are also quite permeable for water and methanol. Thus, thin membranes lead to substantial transport of these molecules from the anode side to the cathode (e. g., [25-29]). The permeation of methanol in the DMFC is undesirable for the obvious reason that it reduces the cell power ( mixed potential formation ), because no electrical work is generated in a cathodic oxidation reaction. Furthermore methanol on the cathode is unfavorable because it can block adsorption sites needed for the oxygen reduction reaction. The presence of methanol may even alter the rate constant of the oxygen reduction reaction. A typical solution to the problem of methanol transport is the use of dilute aqueous solutions of methanol, which assures almost complete oxidation when the anodic catalyst loading is high enough [30],... 

It becomes evident from this last expression that E°22 will shift toward more positive values as i , or equivalently, the magnitude of the rate constant of association k2 is increased. It is conceivable that the half-wave potential for the oxygen reduction reaction can be shifted by as much as 100 mV from E rf however, larger shifts are not likely, as a subsequent electron transfer step may become rate-limiting. 

Fig. 7 CeU G2 cathode catalyst-layer and GDL mass-transport overpotentials, oxygen reduction reaction (ORR) overpotential, and iR-corrected cell voltage as a function of current density at different durability testing times (1,054 total hours at constant 0.90 A cm Gore PRIMEA series 5620 MEA (50.0-cm active area, 0.40/0.60mg Pt cm, 35- am Gore SELECT membrane) SGL SIGRACET GDL 24BC (5wt%/23wt% PTFE substrate/MPL) at both sides. See the Appendix for durability testing conditions. RH Relative humidity, A Anode, C Cathode...

Figure 9,10 Nyquist plot of the Faradaic impedance Z .- of the oxygen reduction reaction simulated from the above reaction model and corresponding equivalent circuit. The model describes a competition between surface adsorption and incorporation, depending on the ratio of the rate constants for the respective processes.

The reaction rate of Co3+ with benzaldehyde was measured in independent experiments from the consumption of Co3+ in the absence of oxygen. The rate constant of this bimolecular reaction was found to coincide with k. Thus, in this process the limiting step of initiation is the reduction of Co3+ by aldehydes, and the complete cycle of initiation reactions includes the reactions [50,51] ... 

A point that seems to have been coming out of all of our work is that in interactions, especially oxidation-reduction reactions involving oxygenated species, we have to consider such condensations as this. I shouldn t be surprised if they were involved in a lot of the reactions involving simple metal ions which are hydrated. A recent article (2) states that bichromate also condenses with an aquo complex of cobalt with a much higher formation constant than that for CrSCV2 and with... 

Both e, q and H- rapidly reduce 02 to form 02 with second-order rate constants of 1.9xl010 and 2.1 xlO10 M 1 s 1, shown in Eqs. (11) and (12), respectively [3]. A dose of 10 kGy produces 60 pM H- and 270 pM of e-aq, in pure water. A dissolved oxygen concentration of 3.7 mg L-1 (120 pM) would scavenge approximately a third of the reducing species. In general, processes relying on reductive reactions for treatment are more... 

The oxidation-reduction potentials and reaction constants of oxidation of iron and manganese differ and these reactions can occur in different amounts of oxygen. That is why the level of appearance of particulate manganese is situated higher than that of particulate iron [63]. Bacteria have been shown to oxidize manganese [64], whereas iron oxidation is possible without bacteria but can be carried out with bacteria [50]. Reduced iron can be oxidized by particulate manganese, forming complex compounds [65]. 

Chromates are excellent inhibitors of oxygen reduction in near neutral and alkaline solutions. In these environments, they can stifle corrosion by suppressing this cathodic partial reaction. The inhibition mechanism appears to involve reduction of Cr(VI) to Cr(III) at a metal surface and formation of Cr(III)-0-substrate metal bonds (38). This surface complex is likely to be substitutionally inert and a good blocker of oxygen reduction sites, as suggested by the exceedingly small water exchange rate constant for the first coordination sphere of Cr3+ (39). 

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