PTFE-Bonded Gas Diffusion Electrodes

Gulzow, E., Holzwarth, B., Schnurnberger, W., Schulze, M., Steinhilber, G., and Wagner, N. (1992) PTFE bonded gas diffusion electrodes for alkaline fuel cells, presented at the 9th World Hydrogen Energy Conference, 1992, Paris. 

Lifetime test for oxygen electroreduction electrode/XPS, electrochemical impedance spectroscopy) Ag KOH solution Catalyst preparation not given. Porous PTFE bonded gas diffusion layer is prepared by cold rolling method 12-15% performance loss of cell voltage in 5000 h of operation time due to decrease of the surface roughness of electrode and PTFE degradation effect. Wagner et al. (2003)... 

Fig. 16. Schematic presentation of the morphological features of gas diffusion electrodes for fuel cells of (A) PTFE-bonded and Pt-activatcd Hi anodes and O2 cathodes used for Oi reduction in acidic and alkaline fuel cells (a) support, (b) hydrophobic gas diffusion layer, (c) hydrophilic electrode layer, (d) electrolyte, (e) magnified schematic of PTFE-bonded soot electrode, (f) adjacent hydrophobic layer, (g) microporous soot particles, (h) gas channels (mesopores), (k) PTFE particles, (I) flooded micro- and mesopores, (B) Schematic presentation of the morphology of PTFE-bonded Raney-nickel anodes used in alkaline fuel cells ol the Siemens technology.

According to Raistrick (162), prefabricated gas diffusion electrodes (PTFE-bonded, Pt-activated soot, Prototech) are soaked with an alcohol solution of the monomer, and the solvent is subsequently evaporated. As the solution is wetting the electrode fairly well the active electrode layer is evenly impregnated by the ionomer. The impregnated electrode is subsequently glued to the membrane by hot pressing. 

Oxygen reduction studies and cyclic voltammetry were carried out at ambient temperature (22 2°C) in 1 cm gas diffusion electrodes as previously described (8). The catalyst, mixed with ca. 50 mass % PTFE as a binder (and Nafion solution in one case), was spread on carbon fibre paper (CFP Toray TGPH090), which was then hot-bond to a Nafion 117 membrane. In the cell, a controlled flow of gas (O2, or N2 for cyclic voltammetry) was passed over the CFP and the membrane was in contact with a 1 M H2S04(aq) solution containing counter and reference (SCE) electrodes. 

PTFE-bonded hydrophobic electrodes are the modified versions of gas diffusion electrodes developed for PAFCs. In preparation, the catalyst particles are mixed with PTFE emulsion to form a catalyst ink, which is then cast onto the GDL. In order to provide ionic transport to the catalyst sites, the PTFE-bonded catalyst layers are generally impregnated with an ionomer, commonly Nation, by brushing or spraying. A typical preparation process is detailed as follows ... 

The importance of microstructural optimization of hydrophobic gas diffusion electrodes was emphasized in an article by Tantram and Tseung (1969). Tantram and Tseung considered porous electrodes that consisted of mixtures of finely dispersed Pt black particles, bonded by polytetrafluoroethylene (PTFE). Hydrophobic and hydrophilic parts formed interconnected networks of porous PTFE and porous catalyst aggregates. The two authors recognized the importance of agglomeration and dual hydrophobic/hydrophilic porosity. 

Like the PEM fuel cell, the PAFC uses gas diffusion electrodes. In the mid-1960s, the porous electrodes used in the PAFC were PTFE-bonded Pt black, and the loadings were about 9 mg Pt cm on each electrode. Since then, Pt supported on carbon has replaced Pt black as the electrocatalyst, as for the PEMFC, as shown in Figure 4.6. The carbon is bonded with PTFE (about 30-50 wt%) to form an electrode-support structure. The carbon has important functions ... 

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