Carbon-supported membrane electrode applications

Liu, D.-J. and Yang, J., Method of Fabricating Electrode Catalyst Layers with Directionally Oriented Carbon Support for Proton Exchange Membrane Fuel Cell, U.S. Patent Application 20060269827, November 30, 2006. 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

Taylor et al.8 were the first to report an electrochemical method for preparation of MEAs for PEMFCs. In their technique, Pt was electrochemically reduced and deposited at the electrode membrane interface, where it was actually utilized as an electrocatalyst. Nation, which is an ion exchange polymer membrane, is first coated on a noncatalyzed carbon support. The Nafion-coated carbon support is then immersed into a commercial acidic Pt plating solution for electrodeposition. Application of a cathodic potential results in diffusion of platinum cations through the active Nation layer. The migrated platinum species are reduced and form Pt particle at the electrode/membrane interface only on the sites which are both electronically and ionically conductive. The deposition of Pt particles merely at the electrode/membrane interface maximizes the Pt utilization. The Pt particles of 2-3.5 nm and a Pt loading of less than 0.05 mg cm-2 were obtained employing this technique.8 The limitation of this method is the difficulty of the diffusion of platinum... 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

Reflectance measurements provided an excellent means for building an ammonium ion sensor involving immobilization of a colorimetric acid-base indicator in the flow-cell depicted schematically in Fig. 3.38.C. The cell was furnished with a microporous PTFE membrane supported on the inner surface of the light window. The detection limit achieved was found to depend on the constant of the immobilized acid-base indicator used it was lO M for /7-Xylenol Blue (pAT, = 2.0). The response time was related to the ammonium ion concentration and ranged from 1 to 60 min. The sensor remained stable for over 6 months and was used to determine the analyte in real samples consisting of purified waste water, which was taken from a tank where the water was collected for release into the mimicipal waste water treatment plant. Since no significant interference fi-om acid compounds such as carbon dioxide or acetic acid was encountered, the sensor proved to be applicable to real samples after pH adjustment. The ammonium concentrations provided by the sensor were consistent with those obtained by ion chromatography, a spectrophotometric assay and an ammonia-selective electrode [269]. 

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