Catalyst layer bonding

The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25-100 p,m thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper/cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. 

The catalyst layer is the most expensive part of this fuel cell. It is made from a mixture of platinum, carbon powder, and PEM powder, bonded to a conductive carbon fiber cloth. We obtained ours from E-Tek Inc. The cost for an order of their ELAT catalyst cloth sheet includes a setup charge. So get together with others for a larger order if you want to keep costs down. We paid 360 for a piece of ELAT 15.2 centimeters by 15.2 centimeters [6 inches by 6 inches] including the 150 setup charge. This piece provides enough for about twelve disks. Each fuel cell requires two disks of ELAT and one larger disk of PEM to make the sandwich, so you can make six cells from this size... 

Zhang, X., and Shi, P. Dual bonded catalyst layer structure cathode for PEMFC. Electrochemistry Communications 2006 8 1229-1234. 

In the proton-emitting membrane or proton electrolyte membrane (PEM) design, the membrane electrode assembly consists of the anode and cathode, which are provided with a very thin layer of catalyst, bonded to either side of the proton exchange membrane. With the help of the catalyst, the H2 at the anode splits into a proton and an electron, while Oz enters at the cathode. On the inside of the porous anode is a thin platinum catalyst layer. When H2 reaches this layer, it separates into protons (H2 ions) and electrons. One of the reasons why the cost of fuel cells is still high is because the cost of the platinum catalyst is rising. One ounce of platinum cost 361 in 1999 and increased to 1,521 in 2007. 

Fig. 1. Schematic presentation of a PEFC cross-section. The cell (left) consists of a membrane catalyzed on both sides (referred to as a membrane/electrode (M E) assembly ), gas-diffusion backing layers and current collectors with flow fields for gas distribution. The latter become bipolar plates in a fuel cell stack. The M E assembly described schematically here (right) shows catalyst layers made of Pt/C catalyst intermixed with ionomer and bonded to the membrane (large circles in the scheme correspond to 10 nm dia. carbon particles and small circles to 2 nm dia. platinum particles).

Fig. 23 Air cathode catalyst mass utilization (A mg-1 Pt) for different types of catalyst layers as developed chronologically for hydrogen/air PEFC. Squares PTFE-bonded Pt black at 4 mg Pt/cm2 circles ionomer-impregnated, PA- type electrodes (0.45 mg Pt/cm2) triangles thin-film Pt/C//ionomer composite (0.13 mg Pt/cm2). The relative advantage of thin-film catalyst layers is seen to increase with cell current density, as expected from the lower transport limitations involved (see Sect. 8.3.7.2.3) [10,11].

The anode GDM (with or without MPL), the anode CL bound to either the GDM or the PEM, the PEM, the cathode CE bound to either the GDM or the PEM, and the cathode GDM (with or without MPE) are often hot-bonded together to form a multilayered structure. Such a structure is called a membrane electrode assembly (MEA). When two catalyst layers are separately applied to either side of a PEM, the resulting PEM is a CCM. Hot-bonding is carried out under a suitable pressure and a suitable temperature for a few minutes. The temperature is chosen such that the membrane softens. For Nation, it is often around 130°C. The pressure is often around 100 bars. 

Acetate is often the main product of the ethanol oxidation reaction. However, for concentrated alkaline solutions, side products are formed, including polymerized acetaldehyde and carbonate. Polyacetaldehyde is regarded as an unwanted by-product because it coats and blocks the catalyst layer. The complete oxidation of ethanol to carbonate is highly desired because it doubles the number of electrons per reagent molecule. The catalytic cleavage of the C-C bond in aqueous media is a challenging task in catalyst research. At DLR, this is a research topic in collaboration with the University of Diisseldorf [31, 32]. 

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