Type-I electrode

The absence of a distinct catalytic activity in the case of the type I electrode at the HNO3 concentrations studied can be considered to be normal behavior if we take into consideration the results of some... 

Type I electrodes, the prevailing type, are three-phase composite media that consist of a solid phase of Pt and electronic support material, an electrolyte phase of ionomer and water, and the gas phase in the porous medium. Gas diffusion is the most effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Xp 30-60%, have to be made with thickness of Icl — 10 pm. In this thickness range, proton transport cannot be provided outside of the electrolyte environment. Porous gas diffusion electrodes are, therefore, impregnated with proton-conducting ionomer. The concept of a triple-phase boundary, often invoked for such electrodes, is however inadequate. The amount of the electrochemically active interface is usually controlled by two-phase boundary effects at the interface between Pt and water. 

The design space for CLs is divided by the thickness regime considered. Layers with IcL in the range of 1-10 pm must be fabricated as type I electrodes. Ultrathin layers... 

This chapter is devoted entirely to performance models of conventional catalyst layers (type I electrodes), which rely on reactant supply by gas diffusion. It introduces the general modeling framework and employs it to discuss the basic principles of catalyst layer operation. Structure-based models of CCL rationalize distinct regimes of performance, which are discernible in polarization curves. If provided with basic input data on structure and properties, catalyst layer models reproduce PEFC polarization curves. Consistency between model predictions and experimental data will be evaluated. Beyond polarization curves, performance models provide detailed maps or shapes of reaction rate distributions. In this way, the model-based analysis allows vital conclusions about an optimal design of catalyst layers with maximal catalyst utilization and minimal transport losses to be drawn. 

The type I electrode is an ideal case for the reversible electrochemical reaction where the transport of matter occurs through a double flux of charged species... 

Fig. 3.6 Shape of the potential-composition curve for type-I electrode...

Or both types of sites are simultaneously limiting y, = ye = 3 = potential for the type I electrode is given by ... 

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