Cathode electrocatalysis

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

ELECTROCATALYSIS OF OXYGEN REDUCTION IN THE FUEL CELL CATHODE NEW INSIGHTS AND NEW OUESTIONS... 

ELECTROCATALYSIS OE OXYGEN REDUCTION IN THE EUEL CELL CATHODE... 

With the four-electron ORR process in the fuel cell cathode well recognized as the principal challenge at both the theoretical and experiniental/technical levels, it is interesting to examine the effects of the most recent theoretical developments on the fundamental understanding of ORR electrocatalysis. Such examination would naturally focus on the nature and quahty of the links with experimental work, as reflected by ... 

The idea that the cathode potential with respect to ]lt(H20)/Pt-0Hads determines the value of the pre-exponential factor in the ORR rate expression was inspired by a comment by Andy Gewirth (Urbana) in his talk in Leiden, pointing to the value of Pourbaix diagrams for understanding ORR electrocatalysis. Indeed, the information on these ORR-mediating and facilitating M/M-OH surface redox systems is to be found in Pourbaix s Atlas. 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

Figure 6.7 illustrates the voltammetric response of the third-generation SOD-based 02 biosensors with Cu, Zn-SOD confined onto cystein-modified Au electrode as an example. The presence of 02" in solution essentially increases both the cathodic and anodic peak currents of the SOD compared with its absence [150], Such a redox response was not observed at the bare Au or cysteine-modified Au electrodes in the presence of 02". The observed increase in the anodic and cathodic current response of the Cu, Zn-SOD/cysteine-modified Au electrode in the presence of 02 can be considered to result from the oxidation and reduction of 02, respectively, which are effectively mediated by the SOD confined on the electrode as shown in Scheme 3. Such a bi-directional electromediation (electrocatalysis) by the SOD/cysteine-modified Au electrode is essentially based on the inherent specificity of SOD for the dismutation of 02", i.e. SOD catalyzes both the reduction of 02 to H202 and the oxidation to 02 via a redox cycle of its Cu (II/I) complex moiety as well as the direct electron transfer of SOD realized at the cysteine-modified Au electrode. Thus, this coupling between the electrode and enzyme reactions of SOD could facilitate the development of the third-generation biosensor for 02". ...

Moreover, despite the many advances in electrochemical measurement and modeling, our understanding of SOFC cathode mechanisms remains largely circumstantial today. Our understanding often relies on having limited explanations for an observed phenomenon (e.g., chemical capacitance as evidence for bulk transport) rather than direct independent measures of the mechanism (e.g., spectroscopic evidence of oxidation/reduction of the electrode material). At various points in this review we saw that high-vacuum techniques commonly employed in electrocatalysis can be used in some limited cases for SOFC materials and conditions (PEEM, for example). New in-situ analytical techniques are needed, particularly which can be applied at ambient pressures, that can probe what is happening in an electrode as a function of temperature, P02, polarization, local position, and time. 

Trasatti, S. (1992) Electrocatalysis of hydrogen evolution progress in cathode activation, in Advances in Electrochemical Science and Engineering (eds H. Gerischer and C.W. Tobias), VCH Verlag GmbH, Weinheim. 

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. 

The ZnO/eosinY hybrid films were prepared by cathodic electrodeposition from a hydrogen peroxide oxygen precursor in chloride medium [484,] and the role of reduced eosin bound to ZnO in the electrocatalysis was discussed. 

Electrocatalysis in fuel cells requires as well substances capable of catalyzing the anodic oxidation of fuels as catalysts for the cathodic reduction of oxygen. Several dyestuffs that catalyze oxygen reduction are known, but up to now only one has been described as active in anodic reactions. All these dyestuffs are N4-chelates. 

Table 5. Activities of some metal chelates in the catalytic decomposition of hydrogen peroxide and in electrocatalysis with an oxygen cathode 38>...

Cells can be made in which the cathode-anode distance is only 10-3 cm. Such cells have the advantage that the total impurity present is veiy small and may not be enough to cover more than 0.1% of the electrode surface if they were all adsorbed. Thus, suppose the impurity concentration were 10-6 mol liter-1 or 10-9 mol cc 1 or 10 12 mol in the cell Because an electrode surface can cany (at most) about 10-9 mol cm-2, the maximum fraction of the surface covered with impurity molecules is 0.1%. Does work with thin-layer cells eliminate the inpurity problem in electrode kinetics It improves it. However, active sites on catalysts may occupy less than 0.1% of an electrode and preferentially attract newly arriving impurities, so that even thin-layer cells may not entirely avoid the impurity difficulty,32 particularly if the electrode reaction concerned (as with most) involves adsorbed intermediates and electrocatalysis. 

Further Observations on the Technique of Steady-State Electrochemical Kinetic Measurements 1. In potentiostatic measurements, the appropriate interval of potential between each measurement depends on the total range of potential variation. It may be between 10 and 50 mV and can be automated and computer controlled (Buck and Kang, 1994). It is helpful to observe a series of steady-state currents at, say, 20 potentials taken from least cathodic to most cathodic, and the same series taken from most cathodic to the least cathodic. The two sets of current densities should be equal at each of the chosen constant potentials. In practice, with reactions involving electrocatalysis, a degree of disagreement up to 25% in the current density at constant potential is to be tolerated. 

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... 

Also for cathodic oxygen reduction in low-temperature fuel cells, platinum is indispensible as a catalyst whereas the cathodic electrocatalysts in MCFCs and SOFCs are lithiated nickel oxide and lanthanum-manganese per-ovskite, respectively. Appleby and Foulkes in the Fuel Cell Handbook (101) reviewed the fundamental work as well as the technologically important publications covering electrocatalysis in fuel cells till 1989. 

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