Electrocatalysis of Hydrogen Oxidation

Studies on HOR electrocatalysis have built up a foundation for all modem electrocatalysis. Since the pioneering work of the 1960s, when flie dependence of hydrogen adsorption upon the crystallographic orientation of the platinum surface was discovered, the study of the relationship between electrochemical activity and surface structure has been the main theme of electrochemical research [50]. In this section, we first introduce the electrocatalysis of the HOR on Pt and Pt-alloy electrodes, with detailed discussion of the stmcture-sensitivity of hydrogen adsorption on well-defined Pt surfaces. Then we discuss two types of non-noble catalysts for the HOR carbides and Raney nickels. 

As discussed above, the mechanism for the HOR on a platinum electrode in acid electrolyte proceeds through two pathways, Tafel-Volmer and Heyrovsky-Volmer, both of which involve the adsorption of molecular hydrogen (Had), followed by a fast charge-transfer step  

Despite significant achievements in alloying Pt with other metal(s) to form active and CO-tolerant catalysts, there remain some unsolved problems for the applicability of these alloy catalysts to real fuel cell systems. One of the major 

Since catalyst performance for the HOR is strongly dependent on the total active surface area, supported catalysts have been developed to maximize the catalyst surface area. Compared to bulk Pt catalysts, supported catalysts show higher activity and stability due to fine dispersion, high utilization, and stable nanoscale metallic particles. 

Nanostruetured earbon sueh as carbon nanotubes (CNTs) [76, 77-79] is another type of earbon support. CNTs are a promising kind of material for eatalyst support in fuel eell catalysis applications, due to their unique eleetrieal and struetural properties. The reported studies have shown that CNTs are superior to earbon blaeks as eatalyst supports for PEM fuel eells [80]. Matsumoto et al. [81] reported that a CNT-supported Pt eatalyst with 12 wt% Pt loading eould give a 10% higher 

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Santos E, Schmickler W. 2007b. Electrocatalysis of hydrogen oxidation—Theoretical foundations. Angew Chem Int Ed 46 8262-8265. 

Zhou, J., Y. Zu, and A. J. Bard, Scanning electrochemical microscopy. Part 39. The proton/hydrogen mediator system and its application to the study of the electrocatalysis of hydrogen oxidation, J. Electroanal. Chem., Vol. 491, 2000 pp. 22-29. 

Santos, E. and Schmickler, W. 2007. Electrocatalysis of hydrogen oxidation-theoretical foundations. Angew. Chem. Int. Ed. 46 8262-8265. 

Quaino, R, Santos, E., Wohschmidt, H., Montero, M., and Stimming, U. 2011. Theory meets experiment Electrocatalysis of hydrogen oxidation/evolntion at Pd-An nanostructnres. Catal. Today 177 55-63. 

Chen SL, Kucemak A. 2004b. Electrocatalysis under conditions of high mass transport investigation of hydrogen oxidation on single submicron Pt particles supported on carbon. J Phys ChemB 108 13984-13994. 

Wendt, H. and Plzak, V. (1990) Electrode kinetics and electrocatalysis of hydrogen and oxygen electrode reactions. 2. Electrocatalysis and electrocatalysts for cathodic evolution and anodic oxidation of hydrogen, in Electrochemical Hydrogen Technologies (ed. H. Wendt), Elsevier, Amsterdam, Chapter 1. 2. 

Trasatti, Electrocatalysis of Hydrogen Evolution Progress in Cathode Activation A. Hammou, Solid Oxide Fuel Cells... 

Our discussion of PEFC anode electrocatalysis can be summarized as follows. The rate of hydrogen oxidation at an impurity-free, well-humidified Pt/ionomer interface is sufficiently high to generate negligible anode voltage losses in PEFCs. At most, losses of the order of 30-50 mV at 1 A/cm are expected under such conditions. 

In this final section, electrocatalysis of hydrogen, oxygen, and organic oxidation processes is briefly summarized, and practical implications discussed. 

The electrocatalysis of HCOOH oxidation on Pd constitutes a special case and represents an exception when compared to CH3OH and C2H5OH. Some of the early work on this topic has been presented in Section 4.1. The cyclic voltammogram of Pd(lll) in 0.1 M H2SO4 is shown by Figure 4.32 [167], Key features are i) pronunced underpotentially deposited hydrogen peaks (adsorption/desorption) around -0.05 V vs. SCE, ii) ordered SOd adlayer formation at +0.1 V vs. SCE, and iii) Pd surface oxidation at 0.8 V vs. SCE and reduction of the surface oxides at 0.4 V on the cathodic scan, generating irreversible surface defects [167]. 

In a recent review paper by Shao (2011) the author mentions that because of limited resources and high cost, platinum electrocatalysts, used in many low-temperature fuel cells, hinder the commercialization of fuel cell power plants. Recent efforts have focused on the discovery of palladium-based electrocatalysts with no or little platinum. The paper overviews progress in electrocatalysis by palladium-based materials for the reaction of hydrogen oxidation as well for the reaction of oxygen reduction (see the next section). 

Chen, S. and Kucemak, A. (2004) Electrocatalysis under conditions of high mass transport investigation of hydrogen oxidation on single submicron Pt particles supported on carbon. The Journal of Physical Chemistry B, 108,13984-13994. Li, X. (2006) Principles of fuel cells. Platinum Metals Review, 50,200. Srinivasan, S., EnayetuUah, MA., Somasimdaram, S., Swan, D.H., Manko,... 

Chen S and Kucemak A (2004) Electrocatalysis under conditions of High Mass Transport Investigation of Hydrogen Oxidation on Single Submicron Pt Particles Supported on Carbon, J. Phys. Chem. B, 108, pp. 13984-13994. 

In the electron transfer theories discussed so far, the metal has been treated as a structureless donor or acceptor of electrons—its electronic structure has not been considered. Mathematically, this view is expressed in the wide band approximation, in which A is considered as independent of the electronic energy e. For the. sp-metals, which near the Fermi level have just a wide, stmctureless band composed of. s- and p-states, this approximation is justified. However, these metals are generally bad catalysts for example, the hydrogen oxidation reaction proceeds very slowly on all. sp-metals, but rapidly on transition metals such as platinum and palladium [Trasatti, 1977]. Therefore, a theory of electrocatalysis must abandon the wide band approximation, and take account of the details of the electronic structure of the metal near the Fermi level [Santos and Schmickler, 2007a, b, c Santos and Schmickler, 2006]. 

Pt is, of course, not a good electrocatalyst for the O2 evolution reaction, although it is the best for the O2 reduction reaction. However, also with especially active oxides of extended surface area, the theoretical value of E° has never been observed. For this reason, the search for new or optimized materials is a scientific challenge but also an industrial need. A theoretical approach to O2 electrocatalysis can only be more empirical than in the case of hydrogen in view of the complexity of the mechanisms. However, a chemical concept that can be derived from scrutiny of the mechanisms mentioned above is that oxygen evolution on an oxide can be schematized as follows [59] ... 

Electrocatalytic Reduction of Dioxygen and Hydrogen Peroxide These two processes must be emphasized because reduction of dioxygen, and eventually hydrogen peroxide, features the usually claimed pathway for reoxidation of reduced POMs after the participation of the latter in oxidation processes. As a consequence, electrocatalysis of dioxygen and hydrogen peroxide reduction is a valuable catalytic test with most new POMs [154, 156,161]. 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

Saturating the electrolyte with iron(lll) hydroxide (e.g., by addition of aqueous solutions of ferric nitrate) and simultaneously adding cobaltous salts leads to in situ formation of a mixed Fe(llI)/Co(ll)/Co(IIl) deposit, which exhibits catalytic activity comparable to that of Fe304 shown by the current voltage curve in Fig. 11. Such mixed oxidic catalyst coatings are composed of very small oxide crystals, which evidently are dissolved upon current interruption due to dissociative oxide dissolution. The transfer of dissolved metal ions to the cathode followed by cathodic deposition of the metal, however, can be completely prohibited, if the potential of the cathode due to optimal electrocatalysis of cathodic hydrogen evolution proceeds with an over-... 

V. Electrocatalysis of Cathodic Oxygen Reduction and Anodic Hydrogen Oxidation in Fuel Cells... 

The first step was the evolution away from the Schottky barrier model of photoelectrochemistry caused by the evidence from the late 1970s onward that the rate of photoelectrochemical reactions was heavily dependent on surface effects (Uosaki, 1981 Szklarczyk, 1983). This was followed by the use of both a photocathode and a photoanode in the same cell (Ohashi, 1977). Then the use of nonactive thin protective passive layers of oxides and sulfides allowed photoanodes to operate in potential regions in which they would otherwise have dissolved (Bockris and Uosaki, 1977). The final step was the introduction of electrocatalysis of both hydrogen and oxygen evolution by means of metal islets of appropriate catalytic power (Bockris and Szklarczyk, 1983). 

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