Catalyst-Electrode Film Characterization

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

When one starts NEMCA experiments only r0 is important to measure and this is very easy. However the subsequent measurement of NG and Io is quite important for a better understanding the system and this we will discuss here. The measurement of tPb and Ntpb is discussed in Chapter 5. 

1 Catalytic Characterization Measurement of the Metal/Gas Interface Area Ac 

On the other hand metal films deposited on p -Al203, a Na+ conductor, are usually found after calcination to contain on their surface large amounts of sodium, which can nevertheless be easily pumped backed into the p -Al203 lattice via electrical current application.32,33 

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Relatively little work has been done on ORR catalysis by self-assembled mono-layers (SAMs) of metalloporphyrins. The advantages of this approach include a much better defined morphology, structure, and composition of the catalytic film, and the surface coverage, and the capacity to control the rate at which the electrons ate transferred from the electrode to the catalysts [CoUman et al., 2007b Hutchison et al., 1993]. These attributes are important for deriving the catal5d ic mechatfism. The use of optically transparent electrodes aUows characterization of the chemical... 

The basic experimental setup is shown schematically in Figure 13.10a. The metal working catalyst electrode, usually in the form of a porous metal film 3 to 20 pm in thickness, is deposited on the surface of a ceramic solid electrolyte (e.g., YSZ, an conductor, or P"-AI2O3, a Na+ conductor). Catalyst, counter, and reference electrode preparation and characterization details have been presented in detail elsewhere, together with the analytical system for on-line monitoring of the rates of catalytic reactions by means of gas chromatography, mass spectrometry and IR spectroscopy. 

Fig. 2a-c. Kinetic zone diagram for the catalysis at redox modified electrodes a. The kinetic zones are characterized by capital letters R control by rate of mediation reaction, S control by rate of subtrate diffusion, E control by electron diffusion rate, combinations are mixed and borderline cases b. The kinetic parameters on the axes are given in the form of characteristic currents i, current due to exchange reaction, ig current due to electron diffusion, iji current due to substrate diffusion c. The signpost on the left indicates how a position in the diagram will move on changing experimental parameters c% bulk concentration of substrate c, Cq catalyst concentration in the film Dj, Dg diffusion coefficients of substrate and electrons k, rate constant of exchange reaction k distribution coefficient of substrate between film and solution d> film thickness (from ref. 

Simple Fe porphyrins whose catalytic behavior in the ORR has been smdied fairly extensively are shown in Fig. 18.9. Literature reports disagree substantially in quantitative characterization of the catalytic behavior overpotential, stability of the catalysts, pH dependence, etc.). It seems plausible that in different studies the same Fe porphyrin possesses different axial hgation, which depends on the electrolyte and possibly specific residues on the electrode surface the thicknesses and morphologies of catalytic films may also differ among studies. AU of these factors may contribute to the variabUity of quantitative characteristics. The effect of the supporting surface on... 

Indirect bipolar (IB) polarization of the catalyst film in a ring-shaped electrochemical cell is realized by using the two gold electrodes as feeder electrodes. For advanced characterization of the cell, a third electric connection may be added. This latter, connected to the catalyst film itself, permits measurements also in the direct (monopolar) polarization mode, which is useful for the determination of the current bypass. 

In this paper, we had employed binary carbon supports to fabricate thin film electrodes in DMFCs. The roles of binary carbon supports and an optimal mixing ratio will be evaluated and characterized through cyclic voltammetry measurements. It will be shown that with the usage of two carbon supports, electrochemical activities and loading contents of catalysts can be enhanced. This improvement is further exemplified by the enhanced electrode kinetics of methanol oxidation for a binary carbon support-electrode in comparison to a single support-electrode. 

Anodic Electrochemical Oxidation. The anodic electropolymerization of thiophene presents several distinct advantages such as the absence of catalyst, direct grafting of the doped conducting polymer onto the electrode surface (which is of particular interest for electrochemical applications), easy control of the film thickness by deposition charge, and possibility to perform a first in situ characterization of the growing process or of the polymer by electrochemical and/or spectroscopic techniques. 

The most common electrode design currently employed is the thin-film design, characterized by the thin Nafion film that binds carbon-supported catalyst particles. The thin Nafion layer provides the necessary proton transport in the catalyst layer. However, this is a significant improvement over the PTFE-bound catalyst layer, which requires the less effective impregnation of Nafion . Sputter deposited catalyst layers have been shown to provide some of the lowest catalyst loadings, as well as the thinnest layers. The short conduction distance of the thin sputtered layer dissipates the requirement of a proton-conducting medium, which can simplify production. The performance of the state of the art sputtered layer is only slightly lower than that of the present thin-film convention [125]. 

Electrochemical polymerization offers particular advantages in that polymerized porphyrins can form electroactive, adherent and stable films on solid electrodes. Oxidative electropolymerization of several porphyrins and metalloporphyrins have been reported . Special focus has been placed on amino-substituted porphyrins due to the propensity of aniline to form electroactive polymers. Murray et al. reported on the electropolymerization of tetrakis(o-aminophenyl)porphyrin and several para-, ortho-, and meta-substituted tetrakis(aminophenyl)porphyrins with Co as a central metal s. They found that poly-Co(o-NH2)TPP films are effective catalysts for the electroreduction of oxygen in aqueous solution. Metalloporphyrin films on solid electrodes have been mainly characterized by voltammetry and resonsance Raman spectroscopy. The electrochemistry of ruthenium paradiethylamino substituted tetraphenylporphyrins recently have been investigated . This study reports the ac impedance and UV-visible reflectance spectroscopic studies of paradiethylamino substituted tetra-phenylporphyrin films formed via an oxidative electropolymerization process. 

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