Fuel-oxidant interface

Spent fuels vary in microstructure, and phase and elemental distribution depending on the in-core reactor operating conditions and reactor history. The chemical stability of spent U oxide fuel is described by local pH and Eh conditions, redox being the most important parameter. However, the redox system will also evolve with time as various radionuclides decay and the proportion of oxidants and reductants generated at the fuel/water interface changes with the altering a-, (J-, y-radiation field and with the generation of other corrosion products that can act as... 

Depending on the water composition other radical species are formed, such as carbonate and chloride radicals. This imposes net oxidizing conditions at the water—fuel interface because the generated oxidants, molecular oxygen and hydrogen peroxide, predominate under a radiation, and other radical species like OH- or CQf- are more active than the generated reductants, mainly molecular hydrogen. This is why we propose that the spent fuel-water interface is a dynamic redox system, independently of the conditions imposed on the near field (Merino et al. 2001). 

The different behaviours of gas diffusion electrodes with different catalyst loadings were studied by Paganin et al. [4], EIS measurements of 0.5 and 1 cm2 single cells were conducted with H2/02 (air) as fuel/oxidant. In their measurements, a Solartron 1250 frequency response analyzer and a 1286 electrochemical interface were employed. The amplitude of the AC signal was 10 mV and the frequency range was typically from 10 mHz to 10 kHz. Representative EIS results are shown in Figures 6.1 and 6.2. 

The anode (fuel electrode) must provide a common interface for the fuel and electrolyte, catalyze the fuel oxidation reaction, and conduct electrons from the reaction site to the external circuit (or to a current collector that, in turn, conducts the electrons to the external circuit). 

Corrosion develops as pitting at the fuel-water interface. Like any other type of corrosion, it follows an electrochemical mechanism. Oxidation of kerosene by bacteria releases organic acids that modify the pH of the medium. Microbial deposits form anodic sites by local acidification. The oxidation reaction consumes the oxygen dissolved in kerosene and in water. 

The anode must be an excellent catalyst for the oxidation of fuel (H2, CO), stable in the reducing environment of the fuel, electronically conducting, and must have sufficient porosity to allow the transport of the fuel to and the transport of the products of fuel oxidation away from the electrolyte/anode interface where the fuel oxidation reaction takes place [58]. The reaction taking place on the anode side is that of the fuels, such as hydrogen, which react with oxide ions that are delivered from the electrolyte. Electrons are the reaction product, accompanied by the formation of water ... 

D. M. Tricker and W. M. Stobbs, in High Temperature Electrochemical Behavior of Fast Ions and Mixed Conductors, eds. F. W. Poulsen, J. J. Bentzen, T. Jacobsen, E. Skou and M. J. L. Ostergard. Rise National Laboratory, Roskilde, 1993, pp. 453-460 D. M. Tricker. The Microstructure of Solid Oxide Fuel Cells and Related Metal/Oxide Interfaces, Thesis, University of Cambridge, 1993. 

Like the cathode, the anode must combine catalytic activity for fuel oxidation with electrical conductivity. Catalytic properties of the anode are necessary for the kinetics of the fuel oxidation with the oxide ions coming through the solid electrolyte. Ionic conductivity allows the anode to spread the oxide ions across a broader region of anode/electrolyte interface, and electronic conductivity is necessary to convey the electrons resulting from the electrode reaction out into the external circuit. 

A signihcant problem in tire combination of solid electrolytes with oxide electrodes arises from the difference in thermal expansion coefficients of the materials, leading to rupture of tire electrode/electrolyte interface when the fuel cell is, inevitably, subject to temperature cycles. Insufficient experimental data are available for most of tire elecuolytes and the perovskites as a function of temperature and oxygen partial pressure, which determines the stoichiometty of the perovskites, to make a quantitative assessment at the present time, and mostly decisions must be made from direct experiment. However, Steele (loc. cit.) observes that tire electrode Lao.eSro.rCoo.aFeo.sOs-j functions well in combination widr a ceria-gadolinia electrolyte since botlr have closely similar thermal expansion coefficients. 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

Jiang J, Kucernak A. 2004. Investigations of fuel cell reactions at the composite microelectrode solid polymer electrol3de interface. I. Hydrogen oxidation at the nanostructured Pt Nafion membrane interface. J Electroanal Chem 567 123-137. 

Basu RN, Tietz F, Wessel E, and Stover D. Interface reactions during co-firing of solid oxide fuel cell components. J. Mater. Process. Technol. 2004 147 85-89. 

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