Delamination catalyst layers

Catalyst Layer Cracking and Delamination Catalyst layers are typically sprayed, deposited, or spread onto the electrolyte from a viscous mixture. This mixture is then baked at an elevated temperature, which drives off volatile compounds in the catalyst mixture used to control mixture viscosity and dispersion. As a result, small fissures, or mudcracks are common in catalyst layers, as shown in Figure 6.32, with widths much greater than the average pore size in a continuous portion of the catalyst layer. Over time, and as a result of the electrolyte expansion and contraction with water content variation, these cracks can grow and lead to delamination or catalyst layer degradation. 

It must be appreciated, that surfaces of Nb, Ta, V and Zr will be immediately reoxidized or otherwise contaminated upon re-exposure to air, albeit with thitmer layers (or in the limit a monolayer) relative to thick native films often encountered before abrasion. As a minimum, monolayer adsorption of contaminants is almost certainly assured in all but the best ultra-high vacuum (< 1 x torr or <1 X 10 Pa) or in systems which simultaneously sputter substrate surfaces with argon or other inert ion during catalyst deposition. Some interfacial impurities between the substrate and the catalyst layer are tolerated in practice. However, the state of the substrate surface immediately prior to catalyst deposition is critical for wetting and adherence of the catalyst layers and for prevention of delamination. Theoretical flux maxima will not be achieved if thick impurity layers at the cata-lyst/substrate interface hinder hydrogen diffusion. 

Certain performance losses of fuel cells during steady-state operation can be fully or partially recovered by stopping and then restarting the life test. These recoverable losses are associated to reversible phenomena, such as cathode catalyst surface oxidation, cell dehydration or incomplete water removal from the catalyst or diffusion layers [85]. Other changes are irreversible and lead to unrecoverable performance losses, such as the decrease in the ECSA of catalysts, cathode contamination with ruthenium, membrane degradation, and delamination of the catalyst layers. 

Delamination of the MPL from the GDL substrate has not been widely reported but may occur during freeze-thaw cycles, as occurs with catalyst-layer delamination from the membrane [131, 132]. A different situation occurs in the GDL/MPL, where the pore diameters are on the order of a micron or larger and the water is not hydrating the sulfraiic acid of the ionomer. The volume expansion caused by ice formation can produce large isotropic stresses that can damage the structure of the catalyst layer, the MPL, or the GDL. 

The role of the bipolar plate in a stack is twofold it distributes reactants along the surface of adjacent cells and transports current from one cell to another. A serious problem in stacks is the formation of highly resistive (current-free) spots. These spots arise due to local corrosion of BP material, due to numerous processes which increase the local resistivity of the membrane-electrode assembly (e.g. delamination of the catalyst layer from the membrane (Liu et al., 2004)), or simply due to uneven distribution of clamping pressure. 

Gas diffusion layer (GDL) delamination and cracking, GDL fibre breakage. Catalyst layer cracking. 

Yan et al. (2006) investigated the effect of cold-start temperatures (-5 °C, -10 °C and -15 °C) on fuel cell performance. It was reported that the fuel cell was able to start at -5 °C if it was pre-purged with dry nitrogen and air, although at -10 °C the PEFC could start by using the highest air stoichiometry (4) and 100 mA cm current load. At -15 °C the fuel cell could not start. SEM studies of MEAs subjected to temperatures below 0 °C revealed severe damage on the membrane and gas diffusion layer as well as delamination of the catalyst layer from the membrane (Yan et al., 2006). 

AC impedance studies have indicated that DMFC performance loss due to interfacial failure can be linked to (1) increased ohmic resistance of the cell, (2) enhanced electrode overpotential, and (3) electrode flooding. Ohmic loss due to the interfacial resistance buildup can contribute several tens of milliohm square centimeters to the total ceU resistance. An increase in the overpotential is caused by the loss of contact between the membrane and the catalyst, which renders part of the catalyst layer unusable (a possible major performance loss). Electrode flooding can be attributed to the nonuniform current distribution, which is more significant when membrane-cathode delamination occurs. 

An important DMFC performance loss, which involves the membrane, is electrode delamination. The phenomenon, which is rooted in markedly different properties of the two layers, purely polymeric membrane and composite electrode, results in an increase in the cell resistance (enhanced ohmic loss), loss in the MEA area, and interfacial flooding. Expectably, membrane-electrode delamination is more likely in systems with different polymers in the membrane and catalyst layer. 

Electrolyte Fracture Electrolyte fracture can result from rapid or severe temperature and or humidity cycling, including frozen conditions. Electrolyte fracture is not always catastrophic but results in increased hydrogen crossover, leading to failure over time. Freeze-thaw cycling can potentially also result in catalyst layer delamination, as shown in Figure 6.60. 

Figure 6.60 Scanning electron micrograph of membrane electrode assembly that had catalyst layer delamination resulting from unrestrained freezing in liquid water. The gaps caused by this delamination can serve as pooling locations for liquid during subsequent operation.

A delaminated zeolite with an Si/Al ratio of 29, derived from the layered zeolite Nu-6(1), was employed as catalyst for dehydration of xylose at 170 °C, using a water-toluene biphasic reactor-system.140 This material, designated del-Nu-6(l), proved to be efficient for this transformation, giving 47% selectivity to furfural at 90% xylose conversion. 

A recent investigation has demonstrated the usefulness of ultrasonic irradiation in the preparation of delaminated zeolites, which are a particular type of modified oxides - microporous crystalline aluminosilicates with three-dimensional structures - having a greater catalytic activity than the layered structures (clays) and mesoporous catalysts. In an attempt to increase the pore size of zeolites, a layered zeolite precursor was... 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

Most of the zeolite syntheses carried out under hydrothermal conditions directly results in the formation of three-dimensional crystalline frameworks. However, several zeolites, like MCM-22 or ferrierite, can be synthesized in the form of layered precursors, which can be transformed by further thermal treatment into the three-dimensional crystal structure. These layered solids arouse an interest due to their ability to intercalate guest molecules between two neighboring zeolite layers. Using a proper treatment, layered zeolite materials can be delaminated while the structure of layers is preserved, which makes accessible all active sites located on the external surface of such catalyst. By adding proper inorganic guest molecules functioning as pillars, the control of the interlayer distance can be achieved. Such materials... 

As we discussed above, there are two major types of CL fabrication techniques. One is to apply the catalyst ink onto the gas diffusion layer to form a catalyzed diffusion medium (CDM), and the other is to apply the catalyst ink onto the PEM to form a CCM. Normally, applying the ink to the gas diffusion medium has the advantage of preserving the membrane from chemical attacks by the solvents in the catalyst ink. However, it seems that the CL does not come into close contact with the membrane and therefore the electrode is prone to delamination. Regarding CCM, there are two ways of applying the catalyst ink to the membrane, namely the decal transferring process and the direct coating process. In the former, the CL is cast onto a PTFE blank... 

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