Catalyst layer preparation

Table 4. Modes of catalyst layer preparation and application.

The apparent transfer coefficient of the cathodic reaction, ac, is a measure of the sensitivity of the transition state to the drop in electrostatic potential between electrolyte and metal [109,112]. According to Ref. 113, it is ac = 0.75 for the O2 reduction on Pt in aqueous acid electrolytes. In Ref. Ill the value ac = 1.0 was reported instead. Since the cathodic reaction is a complex multistep process, it might follow several reaction pathways, and the competition between them is affected by the operation conditions (rj, p, T). Therefore, different values of ac have been reported in different regimes of operation. Although in the simple reactions the transfer coefficient is a microscopic characteristic of the elementary act [112], for complex multistage reactions in fuel cell electrodes it is rather an empirical parameter of the model. The dependence of effective a for methanol oxidation on the catalyst layer preparation was recently studied [114]. 

On the same topic of DMFC performance with supported vs. unsupported catalysts Smotkin and co-workers concluded that at 363 Kthe supported PtRu (1 1) catalyst with a toad of 0.46 mg cm performed as welt as an unsupported PtRu (1 1) with over four times higher load, i.e., 2 mg cm [266]. It is likely that these differences between various studies are related not only to the intrinsic activity of the respective anode catalys layers but also to the manufacturing procedures such as catalyst layer preparation and application techniques, MEA hot pressing conditions (temperature, pressure and time), presence or absence of other binders (such as PTFE) and fuel cell compression. All these MEA manufacturing variables can affect, in a poorly understood manner at present, the structure, morphology and composition of the catalyst layer in the operating fuel cell. Therefore, in fuel cell experiments it is difficult to isolate the truly physico-chemical effect of the support on the catalytic activity. 

Unfortunately, while catalyst components and structures in low-temperature MEAs have attracted considerable attention, optimization of high-temperature catalyst layer structures and components seems little studied. Lobato et al. [83] investigated the effect of the catalytic ink preparation method on the performance of HT-PEMFCs. They employed two methods for catalyst layer preparation the solution method and the colloid method. In the solution method, catalyst ink was prepared by mixing the catalyst (20% Pt/C) and PBl solution (5% PBl in dimethylacetamide). In the colloid method, acetone was added to the mixture of catalyst and PBI solution, which made the PBI form a colloid suspended in the solvent. They found that electrodes prepared by the solution method showed better performances at 150 °C and 175 °C, and that the electrodes prepared by die colloid method gave a better performance at 125 °C. This is probably due to differences in catalyst layer structure (see Section 18.2.7). 

In the literature, few studies have focused on performance improvement and mitigation of high-temperature catalyst layers. For LT-PEMFCs, materials used in the catalyst layer preparation are commercially available. For HT-PEMFCs, materials are not only different from those in LT-PEMFCs but also differ from study to study. For example, in PBI membrane-based MEAs, Pt/C catalyst and PBI ionomer were used in the catalyst layer [80-83]. However, in a CSH2PO4 membrane-based catalyst layer, no ionomer was used [22]. It is expected that improvement and mitigation of a high-temperature catalyst layer should depend on the materials used, and the catalyst layer structures should be optimized according to the materials employed. 

Such a PTFE-bonded hydrophobic catalyst layer is a breakthrough in catalyst layer preparation technology for PEMFCs. First, this technique uses a carbon-... 

Membrane-based Hydrophilic Catalyst Layer. Wilson and Gottesfeld [8, 21-23] suggested an ionomer-bonded hydrophilic catalyst layer prepared with the decal transfer method. The so-called decal transfer process includes two key steps (1) coating catalyst ink onto a blank substrate (e.g., PTFE film) then (2) transferring the coat onto the membrane (as shown in Figure 19.6). A typical preparation procedure is as follows ... 

A colloid method such as this should be more suitable for applying catalyst ink onto a porous GDL because the larger catalyst agglomerates do not easily penetrate deeply into the GDL. Improved cell performance was observed with the catalyst layer prepared using this colloid method, which could be attributed to the increased reaction area, the reduced internal resistance, and the enhanced reactant mass transport. 

The location of the Nafion in the catalyst layer prepared by RSDT is believed to be different from that in a catalyst layer produced by a more standard ink process. It is hypothesized that the Nafion and ink form a matrix, upon which the platinum particles deposit in the vapor plume. In order to verify the location of the Nafion within the network, a representative catalyst layer was stained with lead. 

Su PH, Lin HL, Lin YP et al (2013) High temperature membrane electrode assembly catalyst layer preparation using various molecular weight polybenzimidazole binders. Int J Hydrogen Energy 38 13742-13753... 

Electrode preparation catalyst layer preparation/morphology... 

Figure 3.12. (a) TEM image of the Pt/C catalyst layer prepared with the isopropanol Nafion solution,... 

Voltammetric measurement of HOR/HER are usually performed using RDE setup with bulky metal disk electrode considering disk material as HOR/HER catalyst or thin catalyst layer, prepared in a same way as for ORR measurements (Figure 11). Due to faster electrode reaction kinetics much lower metal catalyst loading is required, typically under 10 pg cm. Also voltammetric measurements are performed under quasi-stationary conditions (potential sweep rate imder 10 mV s ) and Ohmic drop has to be compensated. 

Figure 6.4 Tape casting technique for catalyst layer preparation.

Catalyst layer preparation Catalyst utilization depends on the Pt dispersion, electrolyte availability and fabrication techniques. 

A typical catalyst layer preparation for a functional PEMFC cell is illustrated in Fig. 6. There are several other techniques for catalyst synthesis which includes electro- and electroless deposition, sol-gel and sputtering. 

The other new techniques for catalyst layer preparation are impregnation of polypyrole with Nafion (Park et al., 2004), grafting of polymer into the catalyst (Mizuhata, 2004), incorporation of organic solvents (Yang et al., 2004). 

Ethanol electro- oxidation/cyclic voltammetry, electrochemical impedance spectroscopy Pt and Pt/Ru NaOH solution Electrodeposition of noble metal on CuNi alloys Higher electrocatalytic activity is found for ethanol oxidation for the catalyst layer prepared from PTFE suspension of noble metal salts rather without PTFE suspension. The charge transfer resistance is greatly reduced in the Pt/Ru-modilied CuNi electrodes Gupta et al. (2004)... 

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