Electroactive catalyst layers

The alternative is to fabricate CCLs as ultrathin two-phase composites 100 mn -200 mn), in which electroactive Pt could form the electronically conducting phase, or Pt nanoparticles could be supported on a conductive substrate. The remaining volume should be filled with liquid water, as the sole medium for proton and reactant transport. The ultra-thin two-phase catalyst layer was explored by using the Poisson-Nemst-Planck (PNP) equations as employed for water-filled spherical agglomerates [69, 118]. The equations in Section 8.5.2 can be rewritten for the ID planar situation... 

In situ cyclic voltammograms are useful in assessing the EPSA and electroactive surface areas of a catalyst layer. In order to obtain comparable results, the process has to be performed identically in terms of the measurement method, operating conditions, and catalyst layer state. Otherwise, large variability in the results is to be expected. 

The objective of catalyst layer design is twofold from a materials scientist s perspective, the objective is to maximize the electrochemically active surface area (ECSA) per unit volume of the catalytic medium Secsa, by (i) catalyst dispersion in nanoparticle form or as an atomistically thin film and (ii) optimization of access to the catalyst surface for electroactive species consumed in surface reactions. From a fuel cell developers point of view, the objective is to optimize pivotal performance metrics like voltage efficiency, energy density, and power density (or specific power) under given cost constraints and lifetime requirements. These performance objectives are achievable by integration of a highly active and sufficiently stable catalyst into a structurally well-designed layer. 

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

The effect of the structure of the catalysts on their electroactivity was first evaluated by CO stripping. The oxidation of a saturated layer of adsorbed CO occurs at lower potentials on the codeposited Pt0 8+Ru0 2/C catalyst than on the Pt0 8Ru0 2/C coreduced one, and the mixture of a Pt/C catalyst with a Ru/C catalyst gives the worst activity (Figure 9.16a). The activity for methanol electro-oxidation is also higher at the nonalloyed codeposited catalyst than at the alloyed... 

The plot of i as a function of Xe exhibits a maximum in the electroactive fraction i /(i Xm) of catalyst particles (normalized to the metal content). For the specified percolation parameters of the layer, the catalyst utilization corresponding to this maximum is 42%, as obtained at = 0.5 (and Xrn = 0.3). This example illustrates the limitations imposed on the catalyst utilization by the percolation properties of the interpenetrating networks. 

Another alternative that would help to raise catalyst utilization would be to make CLs of extremely thin two-phase composites. Electroactive Pt (eventually deposited on a substrate) should form the electronically conductive phase. The remaining volume should be filled with liquid water. Since the layer is only a two-phase composite, not impregnated with ionomer, the problem of the protonic contact resistance at the PEM CL interface could be mitigated, making the CCL insensitive to the type of PEM. Using Pois-son-Nemst-Planck theory, it could be shown that close to 100% of the catalyst would be utilized, since transport of oxygen and protons would be unproblematic for such thicknesses ( 100 nm) [129],... 

In the first case, the voltammetric response can mainly be associated to reduc-tive/oxidative dissolution processes and topotactic or epitactic solid-to-solid transformations, eventually confined to thin surface layers of the parent material (Scholz and Meyer, 1998 Grygar et al., 2000 Scholz et al., 2005). In the second case, among other possibilities, the solid can act as a preconcentrating system for enhancing the signal of the electroactive probe in solution, but also as a catalyst with regard to this process. 

Schematic diagram of an enzymatically coupled potentiometric sensor is shown in Fig.6. Its basic operating principle is simple an enzyme (a catalyst) is immobilized inside a layer into which the substrate(s) diffuse. As it does it reacts according to the Michaelis-Menten mechanism and the product(s) diffuse out of the layer, into the solution. Any other species which participate in the reaction must also diffuse in and out of the layer. Because of the combined mass transport and chemical reaction this problem is often referred to as diffusion-reaction mechanism. It is quite common in electrochemical reactions where the electroactive...

An effective way to include a catalyst inside brucite-like layers is an LbL deposition technique [51, 52, 89, 90] exfoliated anionic clays constitute one of the components of the multilayers, while the actual electrocatalytic element possesses an opposite charge and acts as the second component of the film. This technique allows the deposition of very thin films with a homogenous and controlled disposition of the catalytic sites inside the coating. Moreover, it allows a higher number of electroactive sites to be effective in the catalytic reaction. 

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