Electrodes ionomer impregnation

Improving Catalyst Utilization by Ionomer Impregnation of Gas Diffusion Electrodes... 

Fig. 13. Effects of CO level (ppm) in the hydrogen feed stream on the performance of a PEFC at 80 °C. Both electrodes were based on an ionomer-impregnated Pt/C catalyst and thin sputtered platinum film, of total loading 0.45 mg Pt/cm [17].

Fig. 25. Air cathode catalyst utilization for different types of catalyst layers in contact with ionomeric membranes. , Platinum black/PTFE (4 mg/cm ) ionomer-impregnated gas-diffusion electrodes (0.45 mg Pt/cm ) A, thin film of Pt/C//ionomer composite (0.13 mg Pt/cm ). The advantage of thin-film catalyst layers increases particularly at high current density (lower cell voltage) because transport limitations within the catalyst layer are minimized.

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].

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. 

An anode configuration closely related to the AB approach is the so-called reconfigured anode, in which a thin layer of metal (such as Pt/C) or metal oxide (such as FeOx) is added to the outside of the anode GDL facing the flow field. - Unlike a normal anode electrode layer that is impregnated with ionomers for facile proton transport, this ionomer-free CO oxidation layer is hydrophobic for improved gas diffusion to help maximize the interaction between CO and O2. 

The fuel cell concept has been known for more than 150 years. It was Christian Friedrich Schonbein who recognized and described the appearance of inverse electrolysis [4] shortly before Sir William Grove, the inventor of the platinum/ zinc battery, constructed his first gas voltaic battery [5]. Grove used platinum electrodes and dilute sulfuric acid as a proton conducting electrolyte. Sulfuric acid is still used today for the impregnation of porous separators serving as the electrolyte in direct methanol laboratory fuel cells [6], but the most commonly used fuel cell electrolytes today are hydrated acidic ionomers. As opposed to aqueous sulfuric acid, where the dissociated protons and the diverse sulfate anions (conjugated... 

Gas diffusion is a much more effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Yp - 30% -60%, have to be made much thicker, 10 pm - 20 pm. At such thicknesses, proton diffusion in liquid water is not sufficient for providing uniform reaction conditions. Porous gas diffusion electrodes are therefore impregnated with proton-conducting ionomer, usually Nafion [1-2, 4]. Resulting CLs are random composite media of carbon/Pt, ionomer, and a complex pore space. 

Phosphoric acid-doped polybenzimidazole (PA-PBl) membrane is the most common type for HT-PEMFCs. It was also introduced in the catalyst layer as an ionomer to improve proton conductivity [8, 80-83]. Similar to die apphcation of Nafion ionomer in the catalyst layer, polybenzimidazole (PBl) solution was incorporated into the catalyst layer by an impregnation method (die electrode was soaked in a PBI solution), by a gluing method (the PBl membrane was coated by PBI solution), or by a brushing method (the PBl solution was brushed onto the electrode) [80]. 

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 ... 

Electrochemical-deposition Electrode. Vilambi-Reddy et al. [62] presented the early research on the electrochemical deposition electrode. They developed an electrochemical catalyzation (ECC) technique to deposit platinum catalyst particles selectively in the regions accessible to both ions and electrons. In the ECC technique, a hydrophobic porous carbon paper was first coated with dispersed carbon particles and PTFE to form a substrate. Then the Nafion ionomer was impregnated onto this carbon substrate. This substrate was then placed into a platinum acid-plating bath, along with a platinum counter electrode. One side of this substrate, without Nafion, was masked with a non-conducting film, which... 

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