Carbon supports catalysts/catalyst layers

Start-stop Cathode catalyst sirrface area loss Catalyst pruticle agglomeration due to carbon support corrosion Catalyst layer water accirmulation Catalyst layer morphology change due to carbon support corrosion Membrane pinhole formation Mechanical stress by hydration/dehy- dration ... 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

Time courses of rate of hydrogen generated from decalin with carbon-supported platinum catalyst at various feed rates in bench-scale continuous operation. Catalyst platinum nanoparticles supported on ACC (5 wt-metal%), 0.29 g (one layer, ), 0.58 g (two layers, A), and 0.87 g (three layers, O). Feed rate of decalin 1.5, 2.0, 2.5, 3.0, and 5.0 mL/min. Reaction conditions boiling and refluxing by heating at 280°C and cooling at 25°C. 

In practice, the catalyst used in the thin-layer CLs for both anode and cathode is carbon-supported Pt catalyst (Pt/C) or Pt alloy, such as PtRu/C, although nonsupported catalysts can be used. In terms of the overall electrode structure, an electrode with a thin CL generally contains three layers ... 

Using a carbon-supported Pt catalyst to replace Pt black can reduce the platinum loading by a factor of 10—from 4 to 0.4 mg/cm [74]. However, the platinum utilization in this PTFE-bound catalyst layer still remains low in the vicinity of 20% [75,76]. 

The electromembrane reactor used in the study was a flow-through undivided electrocatalytic cell. The principal feature of the cell is the ceramic-based sheet, which was coated with the carbon-supported catalyst on one side. The coated side was used as the anode and the cathodic side was not coated with any electroconductive substance or catalyst. Current was supphed to the anode and cathode by means of backing layers, which are connected, to the external power source by means of a conducting wire. The backing layers that were used in this study are carbon cloths 6100-200 purchased from Lydall, United States. 

Fig. 28 Mass distributions of carbon-supported Pt catalyst particles (a) in the as-cast catalyst layer, (b) after 1300 hours of operation, and (c) after 2200 hours of operation as PEFC cathode catalyst [51].

We have found that cathodes with significantly reduced Pt loading seem to benefit from the use of carbon-supported catalysts. A comparison of the activity of unsupported Pt cathode catalyst with the activity of carbon-supported Pt catalyst (40% Pt by weight) indicates that carbon-supported catalysts outperform unsupported catalysts as long as Pt loading remains below ca. 1 mg cm . Also, carbon-supported Pt and Pt-X cathodes both require careful optimization of the ionomer content in the catalyst layer, with the best results obtained at a weight fraction of recast Nafion between 30 and 40%. 

However, the activity of these metal-loaded pillared clays is more than 10 times lower that for carbon-supported catalysts (15) for both unsaturated nitriles. This loss of activity could be explained in two ways the first supposes a lower accessibility of the metallic surface in the case of the pillared clays catalyst as compared with carbon based catalysts. We may suppose some strong diffusional effect. This would indicate that, assuming a good repartition of the metal into the layers of the pillared clay, the most active accessible metallic sites would just be those near the outer edge of the clay. 

We find that a layer model analysis can adequately describe the Pt NMR spectrum of nanoscale electrode materials. The shifts of the surface and sub-surface peaks of Pt NMR spectra correlate well with the electronegativity of various adsorbates, while the Knight shift of the adsorbate varies linearly with the f-LDOS of the clean metal surface. The Pt NMR response of Pt atoms from the innermost layers of the nanoparticles does not show any influence of the adsorbate present on the surface. This provides experimental evidence, which extends the applicability of the Friedel-Heine invariance theorem to the case of metal nanoparticles. Further, a spatially-resolved oscillation in the s-like E( -LDOS was observed via Pt NMR of a carbon-supported Pt catalyst sample. The data indicate that much of the observed broadening of the bulk-like peak in Pt NMR spectra of such systems can be attributed to spatial variations of the A( f). The oscillatory variation in A(A) beyond 0.4 nm indicates that the influence of the metal surface goes at least three layers inside the particles, in contrast to the predictions based on the Tellium model. 

The comparative data of the cell performances obtained in methanol/air (Fig. 7) shows that the best performance was achieved for cell 3 with carbon supported catalyst. As it is shown in Fig. 7, the performance of the cell 3 improved with time, which was not observed for the cells 1 and 2 tested under similar conditions. This effect can be explained by the presence of a higher concentration of Nafion in the Pt/C cathode catalyst layer, which in this case probably needs time to approach equilibrium and humidification. 

The DMFC based on carbon supported catalyst with low catalyst loading (1.3 mg/cm ) has been successfully tested in a methanol/air environment. The cell shows better performance in comparison to the cell based on unsupported catalyst with twice the Pt-black loading. These results are explained by the higher surface area of Pt carbon supported catalyst and are in good correlation with CV and BET data. The results show that carbon supported catalyst can be successfully used as the electrode material for the fabrication of relatively cheap cathode catalyst layers in DMFC. Further work is needed to estimate the lower concentration limit of the catalyst, which is sufficient to maintain stable performance and long-term endurance. 

ETEK electrodes are prepared for carbon-supported Pt-catalysts with electrocatalytic layers of about 25-30 pm thick. Such thick layers prevent... 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

Accompanying the oxidation of carbon support, the double layer capacitance of the catalyst layer increases gradually, and a new redox process may appear at ca. 0.6 V from quinone/hydroquinone couple. 

Basically, there are two methods to form the MEA of a PEM fuel cell. One alternative is using appropriate techniques to add the carbon-supported catalyst to a porous and conductive material, such as carbon cloth or carbon paper, called a gas diffusion layer (GDL). Normally, polytetrafluoroethylene (PTFE) and Nafion... 

Additionally, Ni and CuNi supports were also explored for ethanol oxidation in alkaline media using RRu and PtMo eatalysts [201, 298]. EDX analysis showed that Ni was mostly present in metallie state, with some contribution from an oxide layer. The ethanol oxidation current density increased linearly on a logarithmic scale with the NaOH concentration (10 M to 2 M) for both PtRu and PtMo supported on CuNi (70 30 wt%) [201]. Unfortunately, no direct comparison was performed with carbon-supported catalysts. Thus, the contribution of the support to the observed electrocatalytic effect could not be assessed. PtMo had a higher initial activity however, after about 200 minutes its activity dropped below fliat of PtRu. Anodes with PtRu atomic ratios between 1.1 1 and 2.1 1 supported on Ni gave the lowest Tafel slopes for ethanol oxidation [298]. 

In the case of PtMo on carbon support catalysts. Molybdenum is also dispersed in the gas diffusion layer. The use of such a technique promotes the water-gas shift reaction (WGSR) in the gas diffusion layer. Thus, the concentration of CO in the gas channel is lowered [65,66]. To study the use of PtMo/C as the catalyst numerically, the WGSR (equation (7.27)) should be incorporated in the model. 

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