Metal electrodes deactivation

The theoretical treatment of electron transfer at metal electrodes has much in common with that for homogeneous electron transfer described in 12.2.3. The role of one of the reactants is taken by the electrode surface, which provides a rigid two-dimensional environment where reaction occurs. In some respects, electrode reactions represent a particularly simple class of electron-transfer reactions because only one redox center is required to be activated prior to electron transfer, and the proximity of the electrode surface often may yield only a weak, nonspecific influence on the activation energetics of the isolated reactant. As with homogeneous electron transfer, it is useful to consider that simple electrochemical reactions occur in two steps (1) formation from the bulk reactant of a precursor state with the reacting species located at a suitable site within the interphasial region where electron transfer can occur (2) thermal activation of the precursor species leading to electron transfer and subsequent deactivation to form the product successor state. 

Deactivation of metal electrodes in CO2 reduction has been reported by many workers, and regarded as an imavoidable defect irrespective of the potential importance of CO2 reduction. For example, the deactivation was described that the formation of CH4 and C2H4 at Cu electrode decays rapidly and hydrogen evolution prevails in 10 to 30 minutes after the start of the CO2 reduction. However, the features of the deactivation have been various, depending on the research groups. No agreement is found for the reason of the deactivation as well. Bard et al. and Vielstich et al. reported that the surface of the deactivated copper electrode is blackened after electrolysis. Bard et al. analyzed the surface by X-ray photoelectron spectroscopy (XPS), and reported that the... 

Hori et al. pointed out that the deactivation takes place due to the presence of heavy metal impurities originally contained in chemical reagents used as the electrolytes. Heavy metal ions in the electrolyte solution are cathodically reduced and deposited on the electrode surface during the CO2 reduction, deteriorating the electrocatalytic properties of metal electrodes. They apphed a classically established technique of preelectrolysis to purification of electrolyte solutions since their early works. Frese also referred to the impurity heavy metals, and mentioned the presence of Fe and Zn on the Cu electrode after electrolysis on the basis of the surface analysis by XPS. The importance of the purity of the electrolyte solution was mentioned in Section I1.2(zz) as well. The mechanism of the deactivation was recently established, and sununarized below. ... 

Deactivation of Cu and other metal electrodes is derived primarily from electrodeposition on the electrode surface of impurity metals originally contained in chemical reagents used for the electrolyte, such as Fe and Zn. Contamination of the electrode surface leads to severe deactivation of the electrodes. Neither intermediate species nor product from CO2 reduction causes the deactivation. Purification of the electrolyte solution is effective to prevent the deactivation process. 

Clearly, the larger peak separations (AEp) for the metal electrodes, especially in MeCN, indicate that the apparent irreversibility for the 02/02 - couple is due to the reaction of 02 -, or its disproportionation product (HOO"), with the metal surface. This effect is largest in MeCN because it is the poorest solvating agent for 02"-, which is equivalent to minimizing its deactivation. 

Voltammetric measurements in MeCN at stationary Pt, Au and Hg electrodes showed that the electrodes became deactivated by reaction of radicals formed by reduction of 1-naphthalenediazonium tetrafluoroborate with the metallic electrodes. Similar electrode... 

In the late 1970 s, Yamamoto et al. reported on the use of chemically modified metal electrodes for potentiometric investigations of antibody-antigen complexes. A titanium wire was chemically activated in an aqueous CNBr solution and then treated with the specific antiserum to the antigen under study, i.e., human chorionic gonadotropin (hCG). The electrode was then soaked in urea to deactivate any remaining surface active sites and prevent any... 

Among the oxygen reduction catalysts, Pt, Pt-based catalyst, or Ag dispersed in carbon is the most powerful catalyst. However, there are some problems such as the cost for the material, cohesion of the metals, and deactivation from dropping out of the catalyst from the base electrode. Nano-sized perovskite-type oxide electrocatalyst, which could be synthesized by a wet chemical route, is one of the most... 

Very early studies of dye sensitization have been on metal electrodes and aromatic hydrocarbon crystals [6,7]. On metal electrodes, the excited states of dye molecules are rapidly deactivated (by energy transfer to the broad continuum states of the metal) and there are no evidences of electron transfer quenching. Organic crystals are insulators with a wide bandgap (> 3eV) and narrow conduction and valence bands. Hole injection in the fully occupied valence bands of organic crystals such as anthracene, perylene or phenanthrene through excited dye molecules (e.g, Rhodamine B) was discovered in 1963 and the process has been examined extensively. The efficiency of sensitized hole generation in these cases is directly related to the... 

Thus, besides its toxicity, mercury cannot be used at potentials more positive than -0.3 to +0.4 V versus the saturated calomel electrode (SCE) because of the ease with which Hg is oxidized. At relatively high anodic potentials, noble metal electrodes are also subject to a loss of activity as a result of surface oxide formation [T3] and adsorption of partially oxidized reactant molecules [4,5]. Glassy carbon (GC) is normally deactivated after a long-time exposure to electrolyte solutions, although many methods for its reactivation have been suggested (see ref. 6 and references therein). The use of carbon (or graphite) electrodes in the anodic voltammetry is also rendered difficult by interference from oxidation background currents that are not precisely reproducible. 

However, the CO tolerance at Pt-Co degraded at 70 °C. As seen in Fig. 10.9, the HOR activity of Pt-Co at a given dco is close to that of pure Pt, although the deceleration effect on the CO adsorption rate was still observed to some extent at 70 °C. Such a deactivated electrode cannot recover the original CO tolerance. This can certainly be ascribed to a severe dealloying of the nonprecious metal component (Co) in hot acid solution. We will discuss this in Section 10.3.2. 

A remaining crucial technological milestone to pass for an implanted device remains the stability of the biocatalytic fuel cell, which should be expressed in months or years rather than days or weeks. Recent reports on the use of BOD biocatalytic electrodes in serum have, for example, highlighted instabilities associated with the presence of 02, urate or metal ions [99, 100], and enzyme deactivation in its oxidized state [101]. Strategies to be considered include the use of new biocatalysts with improved thermal properties, or stability towards interferences and inhibitors, the use of nanostructured electrode surfaces and chemical coupling of films to such surfaces, to improve film stability, and the design of redox mediator libraries tailored towards both mediation and immobilization. 

Raney Ni with additives is also used [77, 276]. In particular, valve metals are added to stabilize the catalyst structure [102,410, 411], thus decreasing the recrystallization and sintering which always takes place as the solution temperature is raised [412] (which points to the high energy state of such an electrode structure). In this respect, potential cycling has also been observed to be detrimental since it can induce recrystallization [407]. This is probably the reason why surface oxidation may be deleterious with Raney structures [390] while it normally results in improved electrocatalytic properties with bulk Ni electrodes [386]. However, after prolonged cathodic load resulting in deactivation, Raney Ni electrodes can be reactivated (temporarily) by means of anodic sweeps [405]. 

Another source of deactivation is the usual presence of metallic impurities, in particular Fe, in technical solutions [25, 151, 446]. Fe is deposited on NiSx and can produce deactivation, but the effect is reduced on account of the larger surface area and the semimetallic nature of the surface. The addition of MoS2 is reported to result in an improvement in this direction as well. Ni impurities have been found not to deactivate NiSx electrodes. 

Well-established anode materials are Ni cermets such as Ni/YSZ composites. The presence of the second phase increases the contact area and prevents the catalytically active Ni particles from aggregating. The use of the composite becomes problematic if hydrocarbons are to be directly converted Ni catalyzes cracking, and the resulting carbon deposition deactivates the fuel cells. Therefore either pure H2 has to be used or the fuel has to be externally reformed. A third way is internal conversion of CHV with H20 to synthesis gas. The necessary steam addition, however, reduces the overall efficiency. Another problem of Ni cermets, if they are to be used at lower temperatures, is a potential oxidation of the Ni. Alternatives are Cu/Ce02 cermets in which Cu essentially provides the electronic conductivity and Ce02 the catalytic activity. Note that an efficient current collecting property of the electrode presupposes a metal concentration above the percolation threshold. 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

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