Thermal electrocatalytic activity

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. 

Figure 7.11 Electrocatalytic activity of thermal oxides for O2 evolution as a function of heat of oxide formation [59].

Fig. 21. Influence of thermal activation on the electrocatalytic activity of Co-N4-complexes...

Relatively little information is available on the electrocatalytic activity of thermally prepared rhodium oxide for oxygen evolution, this oxide having been investigated for the most part in conjunction with other oxide catalysts [242, 243], A Tafel slope of ca. 50 mV decade-1 has been observed at low... 

Besides the effects of the typical carbon functional groups, the role of nitrogen and sulfur functionalities, introduced on carbons by chemical and thermal treatments, on the electrochemical performance of Pt catalysts for oxygen reduction in direct methanol fuel cells was investigated [47]. Once again, the metal-support interaction influences the size and chemical state of platinum particles and, as a consequence, the electrocatalytic activity. The introduction of nitrogen and sulphur functionalities was reported to improve the catalytic activity, but this result was mainly ascribed to the Pt particle size. 

Carbons exhibit a low electrocatalytic activity for the hydrogen electrode reaction (HER). Structural characteristics have significant electrocatalytic effects on the HER as changes from 2 X 10 to 2.5 x 10 A/cm on going from the basal plane to the side face of pyrolytic graphite. On glassy carbon, the HER overpotential decreases as the pretreatment temperature is increased. This thermal treatment leads to stmctural and chemical transformations from carbonization, precrystaUization, and to graphitization. 

The changes in the anode composition and morphology are also responsible for the erratic response of the current density vs. potential difference tendency and the decrease in the electrocatalytic activity. Thermo-corrosion and thermal stress failure, stress corrosion cracking during gas evolution, and overcritical local pressure conditions can cause extensive pitting and large ohmic voltage drops. 

PANI-NTs synthesized by a template method on commercial carbon cloth have been used as the catalyst support for Pt particles for the electro-oxidation of methanol [501]. The Pt-incorporated PANl-NT electrode exhibited excellent catalytic activity and stabUity compared to 20 wt% Pt supported on VulcanXC 72R carbon and Pt supported on a conventional PANI electrode. The electrode fabrication used in this investigation is particularly attractive to adopt in solid polymer electrolyte-based fuel cells, which arc usually operated under methanol or hydrogen. The higher thermal stabUity of y-Mn02 nanoparticles-coated PANI-NFs on carbon electrodes and their activity in formic acid oxidation pomits the realization of Pt-free anodes for formic acid fuel cells [260]. The exceUent electrocatalytic activity of Pd/ PANI-NFs film has recently been confirmed in the electro-oxidation reactions of formic acid in acidic media, and ethanol/methanol in alkaline medium, making it a potential candidate for direct fuel cells in both acidic and alkaline media [502]. 

This further demonstrates that the micropores formed during the thermal treatment are necessary for oxygen to access the Pt-nanoparticle active sites. Authors also evaluated the durability of the Pt C/MC hybrids through repeated CV cycles with the appropriate lower and upper potential limits in an 02-saturated electrolyte containing 0.5 mol of methanol. The results showed that the variation in the current density was only about 4 % after 40 cycles, which means that the Pt C/MC electrode has a considerable stable electrocatalytic activity for ORR in the presence of methanol [24]. 

A third school, led by Wiesener, proposed that the Co or Fe ions of the adsorbed N4 chelates promote the decomposition of the chelate upon thermal treatment followed by the formation, at high temperature, of a special form of carbon that would be the true catalyst. In this scenario the metal is only an intermediate and has no active role in the electroreduction of oxygen. In a later publication, they concluded that nitrogen is involved in the electrocatalytic active group on carbon. 

INEOS chlor offers both precious metal and nickel alloy coatings [192]. The precious metal coatings are deposited on a nickel substrate, and the electrocatalytically active layers are applied to the smface by thermal deposition or electroplating. Thermal spraying or vacuum deposition techniques form the nickel-alloy coating. These materials have been used in the FM 21 electrolyzers for over four membrane cycles. 

The investigation of properties of polymeric phthalocyanines concentrates on thermal stability [239,244,245,254], electrical conductivity and redox behaviour of thin films [30,240,250,251,255], catalytic activity [253], electrocatalytic activity for the O2 reduction [30] and photochemical properties [253]. 

Wang, Zhenbo et al. [26] studied the electrocatalytic activity of the Pt-Ru/C catalyst that was formed by thermal reduction with the acidic and alkaline Pt(NH3)2(N02)2 solutions and the same acidic Ru compound as precursors. It was found that the XRD patterns (Figure 10.3) of the two catalysts showed Pt reflections for an fee crystalline alloy structure. The catalyst prepared from the acidic Pt(NH3)2(N02)2 as a precursor has a more homogeneous distribution of Pt-Ru metal partieles on carbon. Its size is relatively small, about 3.7 nm. Its chemical composition is quite similar to the theoretical value of 1 1 (Pt Ru). The catalyst prepared from the alkaline Pt(NH3)2(N02)2 as a precursor has an uneven distribution of Pt-Ru particles on carbon, its size is relatively large, about 6.0 nm,... 

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