Electrocatalytic H2 Oxidation Reaction

With respect to fuel cell catalysis, most research has been focused on cathode ORR catalysts development, because the ORR kinetics are much slower than flic anodic HOR kinetics in other words, the fuel cell voltage drop polarized by load is due mainly to the cathode ORR overpotential [7, 8]. However, in some cases the overpotential of the anodic HOR can also contribute a non-negligible portion of the overall fuel cell voltage drop [8]. Therefore, the catalytic HOR on the fuel cell anode catalyst is also worth examining. 

On the other hand, aside from its importanee in fuel eell applieations, hydrogen eleetrooxidation eatalysis is also a model system for the fundamental understanding of eleetroehemieal kineties, eleetroeatalysis, and eleetroehemieal surfaee seienee, whieh have been studied for over a eentury [3, 5]. Indeed, the hydrogen evolution/oxidation reaetion (HER/HOR) is the simplest and most widely studied eleetroehemieal proeess. Almost all the basie laws of eleetrode kineties and the eoneepts of eleetroeatalysis were developed and verified by the examination of these two reaetions. 

This ehapter summarizes the kineties and meehanisms of the eleetroeatalyzed HOR on different eleetrode materials, ineluding platinum group metals, earbides, and transition metals. Advanees in CO-tolerant eleetroeatalysis for the HOR in fuel eells are also briefly introdueed. Despite its wide range of topies, the main purpose of this ehapter is to provide a fundamental understanding of the eleetroeatalysis of the HOR, the most important reaetion other than the ORR in the PEM H2 fuel eell. 

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In general the Nyquist plots of Fig. 27 can be simulated by two semicycles which rather correspond to the anodic (high frequencies arc) and cathodic (low frequencies arc) electrocatalytic processes (Table 5) 8,119,120 anodic H2 oxidation reaction being faster than the cathodic O2 reduction reaction appears at higher frequencies and with lower polarization resistance. 

When an electrocatalytic reaction involves a primary step of molecular dissociative chemisorption, for example, a c,e mechanism, then the electrocatalysis arises more directly, in the same way as for many regular catalytic processes that involve such a step of dissociative chemisorption. In this type of electrocatalytic reaction, the dissociated adsorbed fragments, for example, adsorbed H in H2 oxidation, become electrochemically ionized or oxidized in one or more charge-transfer steps following the initial dissociation. The rate... 

The Sabatier principle of catalysis also finds extensive application in the area of electrocatalysis reactants should be moderately adsorbed on the catalyst/electro-catalyst surface. Very weak or very strong adsorption leads to low electrocatalytic activity. This has been demonstrated repeatedly in the literature by the use of volcano plots (Figs 23-25). In these plots, the electrocatalytic activity is plotted as a function of the adsorption energy of the key reactant or some other parameter related to it in a linear or near-linear fashion, such as the work function of the metal [5], or the metal—H bond strength when discussing the H2 evolution reaction (Fig. 24) [54] or the enthalpy of the lower-to-higher oxide transition when examining the O2 evolution reaction (Fig. 25) [55]. 

Therefore, the analysis of the product distribution in the H2, HD, and D2 mixtures obtained during electrocatalytic hypophosphite oxidation on nickel electrode suggests that the hydride mechanism, assuming the release of hydride ion and instantaneous reaction with water, is unlikely due to HD content lower than the equilibrium values (hydride mechanism should lead to HD as a prevailing component [77]). Furthermore, this also puts to a question the electrochemical mechanism, according to which equilibrium H2, HD, and D2 mixtures must be formed due to the statistical recombination of H and D atoms for equally accessible electrode surface. To clarify this issue, computer simulations for the H2, HD, and D2 formed by the recombination of H and D atoms were performed. 

This experiment shows the role of H2O as proton carrier not only in the proton conductivity through the electrolyte membrane but also as a promoter for the electrocatalytic oxidation of H2 through reaction (6). Certainly reaction (6) is well established in aqueous systems, however in the present case it has been possible to discriminate between humidified and dry conditions. Based on the cyclic voltammogram of Fig. 29 the estimated H2 uptake corresponds to a surface area of 10.6 m /gr. This value was estimated... 

Pt SO far was proved to be the most active electrocatalytic material for both H2 oxidation and O2 reduction reactions being at the same time the most resistive to corrosion in both alkaline and acidic media. However even Pt has been shown to corrode at the cathode of PEMFC especially at operating voltages above 0.7 V with respect to a reference hydrogen electrode (RHE). Polymer electro-... 

The commercialization of proton exchange membrane fiiel cells (PEMFCs) is now challenged by the high cost of noble metal catalysts such as Pt. For H2 fueled PEMFCs, H2 oxidation is rapid at the Pt anode whereas the oxygen reduction is slow at the Pt cathode. In addition to the attempts to facilitate the electrocatalytic reaction, alloying of Pt as well as replacement of Pt have been pursued. Another attempt is to increase the efficiency of the Pt catalyst and thus to decrease the amount... 

This permits efficient SOFC operation with CH4, and also natural gas, as the fuel. Direct electrocatalytic CH4 oxidation at the tpb is too slow for practical SOFC units with any known electrocatalyst, but, due to the catalytic properties of the Ni surface, CH4 is converted to COj and H2 via Reactions (13.4) and (13.5) and the latter is then easily oxidized electrocatalytically at the tpb. 

For a Pt(lll) surface, with a surface density of 1.5 x 1015 atoms cm-2, the current density corresponding to a TOF of Is-1 is 0.18 mA cm-2 for a one-electron charge-transfer process. Such exchange current densities based on the real electrocatalyst surface area are quite typical for a decent electrocatalyst for H2 evolution or oxidation. Thus, one may conclude that the order of magnitude of the TOFs of catalytic and electrocatalytic reactions are quite similar. In the latter... 

Another notable electrocatalytic reaction where surface oxides take part is the electrooxidation of hydrazine (N2H4). The use of hydrazine as a fuel in the direct hydrazine fuel cell has attracted research interest because the hydrazine/02 fuel cell exhibits a high open circuit potential of 1.61 V (as opposed to 1.23 V in H2/O2 fuel cell) and its theoretical energy-conversion efficiency (AG/AH) is 100 % (as opposed to 83 % in H2/O2 fuel cell) [85]. The electrooxidation of hydrazine occurs via a 4-electron oxidatirm process yielding molecular N2 [86] ... 

Here the role of the particle is to couple the anodic oxidation of the reduced relay with H2 generation from water. The choice of the catalytic material may be based on the same considerations which apply for electrocatalytic reagents used on macroelectrodes the exchange current densities for the anodic and cathodic electron transfer steps must be high. Colloidal platin xm would then appear to be a suitable candidate to mediate reaction (6o). This fact was recognized already at the end of the last century when numerous examples for the intervention of finely divided Pt in the process of water reduction by agents such as Cr and V appeared in the german colloid literature. ... 

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