Hydrogen oxidation reaction anode overpotential

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

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... 

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 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 adsorption, desorption, and electro-oxidation reactions of hydrogen and carbon monoxide are discussed. The hydrogen oxidation reaction requires the dissociation of the hydrogen molecule onto bare platinum sites. The CO molecules will also adsorb on the Ft sites, and require oxidation at higher electrode potentials in the range of 0.6 to 0.9 V for removal. Since this potential does not readily occur on the anode, the hydrogen oxidation occurs on a reduced number of Ff sites, resulting in increased anode overpotential. 

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... 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

The formation condition for PS can be best characterized by i-V curves. Figure 2 shows a typical i-V curve of silicon in a HF solution.56 At small anodic overpotentials the current increases exponentially with electrode potential. As the potential is increased, the current exhibits a peak and then remains at a relatively constant value. At potentials more positive than the current peak the surface is completely covered with an oxide film and the anodic reaction proceeds through the formation and dissolution of oxide, the rate of which depends strongly on HF concentration. Hydrogen evolution simultaneously occurs in the exponential region and its rate decreases with potential and almost ceases above the peak value. 

The technically desirable conditions of anode potentials smaller than 1(X) mV vs. RHE, imply very small rates of process (5d) at either platinum or platinum-alloy PEFC anode catalysts, as can be seen, for example, from the RDE results reported in [18d,e]. The PEFC anode catalyst is thus required to electro-oxidize hydrogen in the presence of significant coverage by CO. The rate of sequence (5b) -I- (5c) can be enhanced by anodic overpotential as long as process (5c) significantly limits the rate of this sequence. Since reaction (5c) is a fast and potential-driven process, at relatively low anodic overpotentials the rate of sequence (5b) -I- (5c) could become fully controlled by the rate of chemisorption of H atoms (Eq. (5b)) on a catalyst surface with few CO-free sites. 

There is another fuel cell working under the ambient condition, that is, direct methanol fuel cells (DMFCs). Difference in the PEFCs and DMFCs is their anode fuels (the cathode fuel is oxygen in both cases). In the DMFCs, methanol (CH3OH) is supplied to the anode instead of the hydrogen for the PEFCs and this difference is crucial for their ceU performances. Although the Pt is known to be an active catalyst for both HOR and methanol oxidation reaction (MOR), kinetics of the MOR is much slower than that of the HOR and ORR on the Pt catalyst, which increases anode overpotential and gives an inferior cell performance in the DMFCs as demonstrated in Fig. 1. Therefore, an important research topic is... 

The carbon anode showed an overpotential reaction, adsorbing hydrogen and preventing gas evolution until -0.65 V versus NHE. The proton absorption could block dihydrogen evolution until it became more thermodynamically feasible [83,86]. The reversible Mn02 oxidation reactions at the cathode show oxygen overpotential to 1.4 V versus NHE, which allows the device s potential window to be extended. 

The hydrogen gain experiment is carried out at the anode to gauge the increase in the anode overpotential due to the presence of CO. Since CO can strongly adsorb onto the surface of Pt to block the reaction sites for H2 oxidation, it seriously poisons the anode even at ppm levels. The poisoning leads to a higher overpotential for the oxidation of H2. 

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