O2 reduction

Figure 21-6 shows the possibility of reducing the overvoltage of cathodic hydrogen evolution. One can also reduce restrictions in the O2 reduction hy using copper in lead alloys. Such alloying elements can be very effective because they... 

Which cathodic reaction is preferred for any combination of metal and environment depends on the relative amounts of O2 and H+ available, and kinetics of O2 reduction and H+ reduction on the metal surface. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Before considering these catalysts, some background on the O2 reduction reaction will be reviewed ( ). 

During the last two decades a variety of transition metal macrocycles have been shown to exhibit activity for O2 reduction In alkaline and acid media when adsorbed, chemically anchored or physically dispersed on electrode surface (Jf5). This class of compounds provides a unique opportunity to examine In detail some of the factors Involved In the activation and further reduction of 02 These would Include the effects associated with axial, peripheral... 

The first In situ MBS Investigation of molecules adsorbed on electrode surfaces was aimed primarily at assessing the feasibility of such measurements In systems of Interest to electrocatalysis (18). Iron phthalocyanlne, FePc, was chosen as a model system because of the availability of previous situ Mossbauer studies and Its Importance as a catalyst for O2 reduction. The results obtained have provided considerable Insight Into some of the factors which control the activity of FePc and perhaps other transition metal macrocycles for O2 reduction. These can be summarized as follows ... 

In case of fuel cell cathodes, theoretical considerations were directed towards optimizing catalysts for O2 reduction [103]. This has led to the synthesis of Pt3Co/C nanocatalyst systems and preliminary results again indicate perfect agreement between the calculations and the wet electrochemical results obtained with metal nanoparticles of the composition which theory had recommended [106]. 

Zhang J, Mo Y, Vukmirovic MB, Klie R, Sasaki K, Adzic RR. 2004. Platinum monolayer electrocatalysts for O2 reduction Pt monolayer on Pd(lll) and on carbon-supported Pd nanoparticles. J Phys Chem B 108 10955-10964. 

Figure 4.15 Overall barrier for O2 reduction to OOH over the Pt(ll 1) surface as a function of electrode potential. The inset shows the transition state for reduction at 1 V/NHE.

Damjanovic A, SepaDB. 1990. An analysis of the pH dependence of enthalpies and Gibbs energies of activation for O2 reduction at Pt electrodes in acid solutions. Electrochim Acta 35 1157-1162. 

Toda T, Igarashi H, Watanabe M. 1999. Enhancement of the electrocatalytic O2 reduction on Pt-Fe alloys. J Electroanal Chem 460 258-262. 

RECENT DEVELOPMENTS IN TTIE ELECTROCATALYSIS OF THE O2 REDUCTION REACTION... 

There are several factors through which anions can influence the pathway and O2 reduction kinetics. The main factors are competition with O2 for surface sites changes in the activity coefficients of the reactants, intermediates, and transition states and the acidity and dielectric properties of the electrolyte side of the interface [Adzic, 1998]. For example, perfluoro acids have higher O2 solubility and lower adsorbability than... 

Figure 9.14 Kinetic current density (squares) at 0.8 V for O2 reduction on the Pt monolayer deposited on various metal single-crystal surfaces in a 0.1 M HCIO4 solution, and calculated binding energies (circles) of atomic oxygen (BEq), as a function of calculated d-band center (relative to the Fermi level, ej — sp) of the respective surfaces. The data for Pt(lll) were obtained from [Markovic et al., 1999] and are included for comparison. Key 1, PIml/ Ru(OOOl) 2, PtML/Ir(lll) 3, PtML/Rh(lH) 4, PtML/Au(lll) 5, Pt(lll) 6, PIml/ Pd(lll). (Reproduced with permission from Zhang et al. [2005a].)...

TABLE 9.4 A Comparison of Surface Area and the Catalytic Activity Data for Pt/C and Au/Pt/C Before and After 30,000 Potential Cycles from 0.6 to 1.1 V under the Oxidizing Conditions of the O2 Reduction Reaction. Reprinted with Permission from Zhang et al. [2007b]... 

Adzic RR, Wang JX. 2000. Structure and inhibition effects of anion adlayers during the course of O2 reduction. Electrochim Acta 45 4203-4210. 

Anderson AB. 2002. O2 reduction and CO oxidation at the Pt-electrolyte interface. The role of H2O and OH adsorption bond strengths. Electrochim Acta 47 3759-3763. 

Sasaki K, Mo Y, Wang JX, Balasubramanian M, Uribe E, McBreen J, Adzic RR. 2003. Pt submonolayers on metal nanoparticles—Novel electrocatalysts for H2 oxidation and O2 reduction. Electrochim Acta 48 3841-3849. 

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