Electrodes microstructure

In order to achieve good reversibility, the composite electrode microstructure... 

In the present case, the electron hopping chemistry in the polymeric porphyrins is an especially rich topic because we can manipulate the axial coordination of the porphyrin, to learn how electron self exchange rates respond to axial coordination, and because we can compare the self exchange rates of the different redox couples of a given metallotetraphenylporphyrin polymer. To measure these chemical effects, and avoid potentially competing kinetic phenomena associated with mobilities of the electroneutrality-required counterions in the polymers, we chose a steady state measurement technique based on the sandwich electrode microstructure (19). 

Due to the important relationship between particle size of starting powders and resulting electrode microstructure and corresponding performance, much work has been performed to modify the particle size and morphology of the starting powders used in SOFC processing. Additional methods have been investigated to better control the microstructure and properties of fuel cell components, which are discussed in more detail in Section 6.2. 

Bulk path at moderate to high overpotential. Studies of impedance time scales, tracer diffusion profiles, and electrode microstructure suggest that at moderate to high cathodic over potential, LSM becomes sufficiently reduced to open up a parallel bulk transport path near the three-phase boundary (like the perovskite mixed conductors). This effect may explain the complex dependence of electrode performance on electrode geometry and length scale. To date, no quantitative measurements or models have provided a means to determine the degree to which surface and bulk paths contribute under an arbitrary set of conditions. 

Virkar, A., Chen, J., Tanner, C. and Kim, J. (2000) The role of electrode microstructure on activation and concentration polarizations in solid oxide fuel cells, Solid State Ionics 131, 189-198. 

White et al. synthesized nanometric La2Cu04 through three techniques auto-ignition, Pechini method, and coprecipitation (White et al., 2008). The NPs were used to fabricate sensing electrodes for NO, and the effect of electrode microstructure on the sensitivity and response time was studied. The response times of the sensors were exponentially dependent on electrode grain size. Sensors with fine-grained electrodes were able to produce a steady-state and consistent voltage at lower temperatures, which improved their response sensitivity. 

During the past ten years, a great deal of research has been devoted to the improvement of electrode microstructure 7, 8], notably of the thickness, porosity and dual phase nature, as well as the use of functionally graded materials and interlayers. 

Yoon JW, GriUi ML, Bartolomeo ED, PoUni R, Traversa E (2001) The NO response of solid electrolyte sensors made using nano-sized LaFeOj electrodes. Sens Actuators B 76 483-488 Yoon SP, Nam SW, Kim SG, Hong SA, Hyun SH (2003) Characteristics of cathodic polarization at Pt/YSZ interface without the effect of electrode microstructure. J Power Sources 115 27-34 Yoon SP, Nam SW, Han J, Lim TH, Hong SA, Hyun SH (2004) Effect of electrode microstructure on gas-phase diffiision in solid oxide fuel cells. Solid State Ionics 166 1-11 Zhuiykov S (2007) Electrochemistry of zirconia gas sensors. CRC, Boca Raton, FL... 

The electrode processes in solid-electrolyte systems consist always of a number of serial and/or parallel steps. The characteristic steps of the gas electrode reactions include transport in the gas phase to (or from) the gas/electrode or gas/electrolyte interface, adsorption (or desorption) at these surfaces, diffusion to (or from) the reaction zone, and transfer reactions [14-24]. As a rule, the electrochemical reaction is believed to occur in the vicinity (within a few microns) of triple-phase boundary (TPB), the junction of the gas, electronic or mixed ionic-electronic conductor (electrode), and ionic conductor (electrolyte) the TPB length is mostly determined by the electrode microstructure formed during the cell fabrication. Actual location of the electrochemicaUy active sites depends generally on the bulk and surface transport properties of the electrode and solid-electrolyte materials. When the current I is passed or drawn through the cell, the working electrode potential vve deviates from the equilibrium value E. This deviation is characterized by the quantity of overpotential r] = we (see Chap. 1). The electrode polarization resistance defined as... 

A large group of chemical compositions has been investigated as potential candidates for IT SOFC cathode materials. A recent article by Skinner has provided an overview of the progress of perovskite type oxides for the SOFC cathode, with an emphasis on the role of chemical compositions [41]. On the contrary, microstructure plays a major role in the cathode function as well. This is particularly true when the composite cathode, which shows a better performance compared to a single composition cathode, is used. Several authors have shown that electrode microstructure and transport properties have a profound effect on polarization. Tanner et al. [42] have shown that polarization resistance (Rp) depends upon the grain size, d, of the ionic conductor in the composite electrode and the volume fraction porosity, which was further derived as in (1) by considering the monolayer gas adsorption. A similar relation has been proposed as [43] ... 

In order to achieve good reversibility, the composite electrode microstructure must have the ability to accommodate any volume changes that might result from the reaction that takes place internally. This can be taken care of by clever microstructural design and alloy fabrication techniques. 

Transport of gaseous species usually occurs by binary diffusion, where the effective binary diffusivity is a function of the fundamental binary difiiisivity -HaO. and microstructural parameters of the anode [3, 4]. In electrode microstructures with very small pore sizes, the possible effects of Knudsen diffusion, adsorption/desorption and surface diffusion may also be present. The physical resistance to the transport of gaseous species through the anode at a given current density is reflected as an electrical voltage loss . This polarisation loss is known as concentration polarisation, and is a function of several parameters, given as... 

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