Catalyst layers gaseous reactants

A porous electrode offers a far higher true working surface area and thus a much lower true current density (current per unit surface area of the electrode). Such an electrode consists of a metal or carbon-based screen or plate serving as the body or frame, current collector, and support for active layers, containing a highly dispersed catalyst for the electrode reaction. The pores of this layer are filled in part with the liquid electrolyte, and in part with the reactant gas. The reaction itself occurs at the walls of these pores along the three-phase boundaries between the solid catalyst, the gaseous reactant, and the liquid electrolyte. 

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed beds. 

The generation of electric current in modern catalyst layers proceeds at nanoparticles of Pt that are randomly dispersed on a porous carbon substrate with pores of nanoscopic dimension (1-10 nm). A certain fraction of larger pores (10-100 nm) is needed for the supply of gaseous reactants and... 

Painted-on electrodes likewise need no binder. This is the type best suited for comparing various catalysts when working with gaseous reactants. The thin layer of the material and the direct contact between the catalyst particle and the gas phase at the hydrophobic covering layer ensures maximum utilization... 

In such a three-phase system, a different spatial concentration profile occurs, as shown in Figure 4.1.11, as compared to a two-phase system, cf. Figure 4.1.2. The gaseous reactant molecules first have to cross the boundary layer between the gas and liquid phase. In the liquid phase, they have to diffuse through the boundary layer between the liquid and the solid particle. The remaining way is the same as for a heterogeneous catalyst in the gas phase, as described in a previous Section 4.1.1.2. 

Two GDL, whose main task is to allow uniform access of gaseous reactants to the catalyst layer, are located on both anode and cathode side, and can be considered as an integrant part of the MEA. They are interposed between the catalyst layer and bipolar plates, and are constituted by a porous carbonaceous material, such as... 

P is in direct proportion to n/V, which is the molar concentration of the gas. Therefore, a lower concentration of a gaseous reactant within the catalyst layer will result in a lower partial pressure and then a lower electrode potential. 

The use of porous catalyst layer (CL) is based on the consideration that pores are needed for transporting gaseous reactants and water because a material diffuses much faster through empty space than through liquid or solid materials, and both hydrophobic and hydrophilic pores are needed for transporting gases and water, respectively. Numerous work has been done in order to create and maintain both kinds of pores within the CL, but an ideal situation is not yet achieved. A major problem encountered with such a CL with a thickness around 10 -20 pm is that flooding appears to be unavoidable. In... 

The main transport mechanism for the gaseous reactants through the GDL is diffusion in the concentration gradient between the gas channel and the surface of the catalyst layer and is described by Pick s first law ... 

The oxygen transport is based on several mechanisms. The gaseous reactants enter the fuel cell typically through the gas channels, and then diffuse through the GDL and into the catalyst layer. The gases diffuse into the catalyst layer either to the catalyst surface where they react or are dissolved into the ionomer in the catalyst layer. The gas phase mass transport in the GDL is due to diffusion governed by Pick s law, in which the rate of diffusion is a function of the concentration gradient, the thickness of the GDL, and the diffusion coefficient for the species [21]. 

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