Electrode material effects

The significant effects of media and electrode material on the reduction potentials and voltammetric peak currents for dissolved dioxygen require substantial knowledge of the media and appropriate calibration of the electrochem- 

The addition of f-BuOOH to the Cu (bpy) /02 system induces the formation of a reactive intermediate that oxygenates saturated hydrocarbons 35 

TABLE 9.12 Electrochemical Reduction Potentials for Oj, Metal Complexes FeL , Cul(bpy 2, Con(bpy)2(py)2+], and Their Combination in Aprotic Media 

The stimulated electroreduction of 02 by Feu(PA)2 [Table 9.12, reactions (18) and (19)] can be used to induce the ketonization of methylenic carbons 31,36 

The same chemistry can be accomplished with the electrons and acid replaced by reductases (PhNHNHPh or PhCH2SH) 37 

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Redox Thermodynamics for Oxygen Species Electrode material effects... 

Electrode materials and shapes may have a profound effect on cell designs. Anode materials encountered ia electrochemical processes are... 

The metallic electrode materials are characterized by their Fermi levels. The position of the Fermi level relative to the eneigetic levels of the organic layer determines the potential barrier for charge carrier injection. The workfunction of most metal electrodes relative to vacuum are tabulated [103]. However, this nominal value will usually strongly differ from the effective workfunction in the device due to interactions of the metallic- with the organic material, which can be of physical or chemical nature [104-106]. Therefore, to calculate the potential barrier height at the interface, the effective work function of the metal and the effective ionization potential and electron affinity of the organic material at the interface have to be measured [55, 107],... 

Working electrode materials have an appreciable effect on the oxidation potentials and the resulting current densities (see also Ref. [139]). 

TABLE 1. Macroelectrolytes of (4-aminophenyl) phenyl sulphone (I) and (2-aminophenyl) phenyl sulphone (II) showing the effect of the electrode material and the influence of the ortho substituent ... 

In the other examples, the electrode materials are not involved in the reactions chemically, but constitute the source [sink] of electrons. Such electrodes are called nonconsumable. The term inert electrodes sometimes used is unfortunate insofar as the electrode itself is by no means inert rather, it has a strong catalytic effect on the electrode reaction. For reactions occurring at such electrodes, the terms oxidation- reduction... 

So far, certain biomimetic catalysts (1 and 2b in Fig. 18.17) have been shown to reduce O2 to H2O under a slow electron flux at physiologically relevant conditions (pH 7,0.2-0.05 V potential vs. NHE) and retain their catalytic activity for >10" turnovers. Probably, only the increased stability of the turning-over catalyst is of relevance to the development of practical ORR catalysts for fuel cells. In addition, biomimetic catalysts of series 1,2,3, and 5, and catalyst 4b are the only metalloporphyrins studied in ORR catalysis with well-defined proximal and distal environments. For series 2, which is by far the most thoroughly studied series of biomimetic ORR catalysts, these well-defined environments result in an effective catalysis that seems to be the least sensitive among all metalloporphyrins to the electrode material (whether the catalyst is adsorbed or in the film) and to chemicals present in the electrolyte or in the O2 stream, including typical catalyst poisons (CO and CN ). 

The photovoltaic effect is initiated by light absorption in the electrode material. This is practically important only with semiconductor electrodes, where the photogenerated, excited electrons or holes may, under certain conditions, react with electrolyte redox systems. The photoredox reaction at the illuminated semiconductor thus drives the complementary (dark) reaction at the counterelectrode, which again may (but need not) regenerate the reactant consumed at the photoelectrode. The regenerative mode of operation is, according to the IUPAC recommendation, denoted as photovoltaic cell and the second one as photoelectrolytic cell . Alternative classification and terms will be discussed below. 

The boundary between effects thus defined is, however, not sharp. If, for instance, light is absorbed by a layer of a photoactive adsorbate attached to the semiconductor electrode, it is apparently difficult to identify the light-absorbing medium as a solution or electrode material . Photoexcited solution molecules may sometimes also react at the photoexcited semiconductor electrode this process is labelled photogalvanovoltaic effect. 

Iridium as an electrode material has received considerable attention in the last decade not only because of its excellent catalytic properties but also in relation to the electrochromic effect observed for anodic iridium oxide films (AIROF). Electrochromism of iridium was thought to be of technical relevance for display applications and triggered several studies of the electrochemical and optical properties of AlROFs [67, 85-88],... 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

Finally, the intrinsic features of the ultrasonic field, frequency and power, should also be taken into account in the design of the experimental arrangement. It is obvious that the mechanical and chemical effects derived from a low frequency field are quite different than those provided by high frequency fields, and these features should match with features of the electrode materials such dimension, structure and physical and chemical properties [30]. 

The orientation of the electroactive substance (the material undergoing electron-transfer at the electrode) with respect to the electrode surface can very substantially affect its electrochemical reactivity. This ought not be surprising electron transfer is a heterogeneous process, and ought therefore to be substantially dependent upon the exact nature of the contact between the electroactive species and the electrode. Orientational effects ought to be particularly important when the electroactive species is adsorbed upon, and hence in intimate contact with, the electrode surface. What kinds of effects are associated with orientation of substances at an electrode surface Generally what one observes is... 

Now let us consider a model for a SC device that comprises two electrodes (anode and cathode), each of them being electrically connected to a current collector fabricated of A1 foil. Let two of such collectors have a certain thickness of SAi- As an electrode material, an activated carbon powder is considered below. Anode and cathode are interposed with a separator of thickness Ss. The electrodes and separator are impregnated with electrolyte. In this paper we mostly focus on the optimization of SC performance by varying the electrode thickness, while some other effects will briefly be considered in the next section. 

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