Carbon supports catalyst layers

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... 

The catalyst layer is composed of multiple components, primarily Nafion ion-omer and carbon-supported catalyst particles. The composition governs the macro- and mesostructures of the CL, which in turn have a significant influence on the effective properties of the CL and consequently the overall fuel cell performance. There is a trade-off between ionomer and catalyst loadings for optimum performance. For example, increased Nafion ionomer confenf can improve proton conduction, but the porous channels for reactanf gas fransfer and water removal are reduced. On the other hand, increased Pt loading can enhance the electrochemical reaction rate, and also increase the catalyst layer thickness. 

The catalyst layer usually consists of carbon-supported catalyst or carbon black mixed with PIPE and/or proton-conducting ionomer (e.g.. Nation iono-mer). Because the sizes of the pores in a t) ical DL are in the range of 1-100 pm and the average pore size of the CL is just a few hundred nanometers, the risk of having low electrical contact between both layers is high [129]. Thus, the MPL is also used to block the catalyst particles and does not let them clog the pores within the diffusion layer [57,90,132,133]. 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Nafion content in the catalyst layer plays an important role in electrode performance. Incorporation of Nafion ionomer into carbon-supported catalyst particles to form the catalyst layer for the gas diffusion electrode can establish a three-dimensional reaction zone, which has been proven by cyclic voltammetric measurements. An optimal Nafion content in the catalyst layer of the electrode may minimize the performance loss that arises from ohmic resistance and mass transport limitations of the electrode [6],... 

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. 

Under certain approximations, using the concepts of percolation theory, the basic parameters can be related to the volume portions of the components of the layer. This offers a relationship between the structure of the porous composite catalyst layer and its performance. An optimum composition (in terms of volume fractions of electrolyte material, carbon and carbon-supported catalyst, and pore space) is a Holy Grail here. Albeit this goal can still be far away in view of the simplified character of the models used, these models give at least some rational scheme for... 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

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%. 

The right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

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. 

Fig. 1 represents the in-situ electrochemical characterization of the cathode catalyst layers based on supported and unsupported catalysts. At two times lower loading of carbon supported catalyst (MEA 3), its limiting current density and thus, electrochemical surface area (ESA) is 3 times higher than the corresponding surface area of unsupported catalyst (MEA 1). This is due to the smaller size of the Pt partieles in carbon supported eatalyst and thus, higher surfaee area in contaet with Nafion (Table 1). 

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. 

Two major degradation processes are affecting the cathode catalyst layer when carbon supported catalysts are used ... 

Figure 1.8 shows a scheme of the microstructxu e of the catalyst layer, where the presence of TPRs allows the simultaneous flows of and electrons toward the PEM and GDL, respectively. Note that the thickness of the proton conducting membrane covering the carbon-supported catalyst nanoparticles is of the order of nanometers, depending of the concentration of proton conducting binder (usually Nafion) in the ink used to prepare the MEA. 

Several methods of applying the catalyst layer to the gas diffusion electrode have been reported. These methods are spreading, spraying, and catalyst power deposition. For the spreading method, a mixture of carbon support catalyst and electrolyte is spread on the GDL smface by rolling a metal cylinder on its surface [22]. In the spraying method, the catalyst and electrolyte mixture is repeatedly sprayed onto the GDL surface until a desired thickness is achieved. 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

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]. 

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