Cathodes electrocatalytic activity

Strictly speaking I0 is a measure of the electrocatalytic activity of the tpb for a given electrocatalytic reaction. It expresses the rates of the forward (anodic) and reverse (cathodic) electrocatalytic reaction under consideration, e.g. reaction (4.1), when there is no net current crossing the metal-solid electrolyte or, equivalently, the tpb. In this case the rates of the forward and the reverse reactions are obviously equal. It has been recently shown that, in most cases, as one would intuitively expect, I0 is proportional to the length, tpb, of the tpb.8... 

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

Battery applications Titanium containing y-Mn02 (TM) hollow spheres synthesis and catalytic activities in Li-air batteries [123] Orthorhombic LiMn02 nanorods for lithium ion battery application [124] Electrochemical characterization of MnOOH-carbon nanocomposite cathodes for metal—air batteries [125] Electrocatalytic activity of nanosized manganite [126]... 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

The defect chemistry of La, xSrxMn03l(5 under cathodic polarization conditions has been subjected to intensive studies due to its importance in the fundamental understanding of the 02 reduction mechanism and electrocatalytic activity. Yasumoto... 

Various strategies have been developed to improve the electrocatalytic activities of the LSM-based cathodes. Murray and Barnett [68, 69] showed that the addition of YSZ and gadolinia-doped ceria (GDC) phase to LSM significantly reduced the electrode polarization resistance. Figure 3.7 shows the electrode polarization resistance of the LSM/GDC tested in the air [69], The electrode polarization resistance... 

The electrocatalytic activity of MIEC cathodes also depends strongly on the properties of the electrolyte, as shown by Liu and Wu [109], The electrode polarization resistances, RE, or area specific resistance (ASR) measured by the electrochemical... 

To develop an alternative MIEC cathode not only the ex situ properties, e.g., cr, TEC, /), and k, but also the electrocatalytic activity, structural and chemical stability, and Cr-tolerance must be considered. Beyond testing in small SOFC button cells, the viability of new cathode materials must ultimately be proven in large-scale stack cells under practical current and temperature gradients. The issues involved in the development of cathode materials for large-scale stacks are significantly more complex than those in the small button cells briefly reviewed in this chapter. However, this does provide serious challenges as well as opportunities for materials scientists and engineers in the development of commercially viable ITSOFCs. 

These results confirmed the enhancement effect of tin on the electrocatalytic activities for the methanol oxidation. As stated before, tin has the main effect on the oxidation of eoCOad (Easily Oxidized COad) using water as the oxygen source while it has little effect on the oxidation of AoCOad (hard to-oxidize COad). which requires Ptr-OH as the oxygen source. This methanol oxidation enhancement effect is. therefore, also likely because of the enhancement of the utilization of water. Although it is not known whether this water utilization is through a path that has eoCOad as an intermediate or another path that has other active intermediates, there is no practical difference between the two because both paths proceed at more cathodic potentials than a path that has hoCOad as an intermediate. 

The nanostructured Au and AuPt catalysts were found to exhibit electrocatalytic activity for ORR reaction. The cyclic voltammetric (CV) curves at Au/C catalyst reveal an oxidation-reduction wave of gold oxide at +200 mV in the alkaline (0.5 M KOH) electrolyte but little redox current in the acidic (0.5 M H2SO4) electrolyte. Under saturated with O2, the appearance of the cathodic wave is observed at -190 mV in the alkaline electrolyte and at +50 mV in the acidic electrolyte. This finding indicates that the Au catalyst is active toward O2 reduction in both electrolytes. From the Levich plots of the limiting current vs. rotating speed data, one can derive the electron transfer number (w). We obtained n = 3.1 for ORR in 0.5 M KOH electrolyte, and 2.9 for ORR in 0.5 M H2SO4 electrolyte. The intermittent n-value between 2 and 4 indicates that the electrocatalytic ORR at the Au/Ccatalyst likely involved mixed 2e and 4e reduction processes. 

Anfolini et al. [161] also compared SAB and Vulcan XC-72 as possible candidates in MPLs, but in this case they used carbon cloth DLs with two MPLs. From their results, it was concluded that the Vulcan XC-72 gave slightly higher electrocatalytic activity for fhe ORR. On the other hand, MPLs near the FF that used SAB had better performance. Thus, it was suggested that for improved fuel cell performance af high pressures (around 3 atm), the ideal cathode MPL compositions would use the Vulcan XC-72 in the MPL next to the CL and SAB in the MPL next to the flow fields. 

A third catalytic route consists of using cathode materials with catalytic properties. The electrocatalytic activity of electrode materials towards the reduction of organic halides has been the object of many studies during the past few years [82], with silver having been shown to possess powerful electrocatalytic activities... 

In a technological perspective, electrocatalytic activity is not the only requirement for a cathode. The electrode must be stable under the working conditions, i.e, AV should... 

Different methods of preparation usually result in cathodes with different activity. In some cases, the authors have directly compared different preparation procedures [141]. However, the same procedure may produce different results in different laboratories. This is not surprising, since this has been the case also with activated anodes. Only by identifying the factors responsible for the electrocatalytic activity, and by finding suitable conditions to control them during the preparation, it will be possible to standardize procedures and perhaps to establish some standards for reference. 

In what follows, some of the approaches proposed to enhance the electrocatalytic activity of cathodes for H2 evolution will be scrutinized. Although the investigations reported are of fundamental interest, they should help to gain insight into the factors responsible for the activity of industrial electrodes. 

Studies on single crystal faces have indicated that, on the whole, the crystallographic orientation has a minor influence on the electrocatalytic activity. Of the possible explanations which can be offered [292], a reasonable one is that the high coverage with adsorbed hydrogen far from equilibrium smoothes down the differences in A//ad from face to face. Actually, quantum chemical calculations reproduce more closely the situation at 0H-O, which does not usually correspond to the real situation under cathodic load. Another possibility is that the hydridic... 

The study of inactive adatoms on noble (precious) metals has little impact on the practical problems of cathode activation for two reasons (i) deactivation is the more common occurrence (ii) adatoms are not stable in the absence of ions in solution where a finite level of precursors must be maintained, which in fact corresponds to the approach of in situ activation. The presence of ionic impurities in solution may pose serious technical problems. Studies of adatoms activation of Raney Ni, a material of current use in technology, can have a greater practical impact. It is interesting that the adsorption of Cd or Pb normally results in a sizable enhancement of the catalytic activity of Raney Ni [307-312]. The Tafel slope of the Raney Ni used by these authors is reported to decrease to ca. 30 mV as the catalyst is first soaked in a solution of the nitrates of the above metals [307, 308] (Fig. 15). The electrocatalytic activity is observed to increase slowly with time of adsorption as well as of polarization. 

Oxides are always present on the surface of transition metals in alkaline solution. At open circuit they are intermediates in the mechanism of corrosion. The resistance of Ni towards corrosion in base is better than Fe or mild steel, especially at high caustic concentration and high temperature [23, 24]. The role of surface oxides in the cathodic range of potentials depends on the conditions of their formation. Thus, a reducible layer of hydroxide Ni(OH)2 or even oxohydroxide NiOOH has been found [385] to be beneficial for the electrocatalytic activity. It has even been claimed [386] that some good performances are specifically due to the formation of oxide layers during the preparation (Fig. 19). An activation of the Ni surface by the application of anodic current pulses has been reported [387] to be beneficial owing to the formation of Ni(OH)2 layers. This has been confirmed by impedance studies of the mechanism [388]. 

A single-chamber solid oxide fuel cell (SC-SOFC), which operates using a mixture of fuel and oxidant gases, provides several advantages over the conventional double-chamber SOFC, such as simplified cell structure with no sealing required and direct use of hydrocarbon fuel [1, 2], The oxygen activity at the electrodes of the SC-SOFC is not fixed and one electrode (anode) has a higher electrocatalytic activity for the oxidation of the fuel than the other (cathode). Oxidation reactions of a hydrocarbon fuel can... 

The cathode has a higher electrocatalytic activity for the reduction of oxygen according to the reaction ... 

Anode materials are required to have a high electrocatalytic activity for the partial oxidation of the fuel in order to facilitate Reaction (1). Consequently, several anode materials have been tested including various compositions of Ni-cermets such as 70 wt.% Ni/30 wt.% YSZ (Ni + YSZ), 70 wt.% Ni/30 wt.% Sm0.2Ceo.8Ox (Ni + CSO), and 60 wt.% Ni/40 wt.% Gdo iCeo gOx (Ni + CGO). Because cathode materials exhibit a high electro-catalytic activity for the reduction of the oxygen in order to facilitate reaction (4), several cathode materials have also been tested, including various compositions of (La, Sr) (Co, Fe)C>3 (LSCF), and CSO-LSCF. 

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