Electron-conducting substrate

Most often, these disperse metal catalysts are supported by an electronically conducting substrate or carrier that should provide for uniform supply or withdrawal of electrons (current) to or from all catalyst crystallites. The substrate should also serve to stabilize the disperse state of the catalyst and retard any spontaneous coarsening of the catalyst crystallites. Two situations are to be distinguished (1) the disperse metal catalyst is applied to a substrate consisting of the same metal, and (2) it is applied to a chemically different substrate (a foreign substrate). Platinized platinum is a typical example of the former situation. 

Because no single homogeneous phase could fulfill these conflicting needs simultaneously, CLs require composite morphologies that consist of several interpenefrafing phases. A minimum of fwo distinct phases is needed, including a solid phase of nanoparticle catalyst (Pt) and electronically conducting substrate (carbon) and a liquid water phase in the void spaces of the substrate for diffusion and permeation of protons, water, and reactant molecules. 

Pt/Ru electrocatalysts are currently used in DMFC stacks of a few watts to a few kilowatts. The atomic ratio between Pt and Ru, the particle si2 e and the metal loading of carbon-supported anodes play a key role in their electrocatalytic behavior. Commercial electrocatalysts (e.g. from E-Tek) consist of 1 1 Pt/Ru catalysts dispersed on an electron-conducting substrate, for example carbon powder such as Vulcan XC72 (specific surface area of 200-250 m g ). However, fundamental studies carried out in our laboratory [13] showed that a 4 1 Pt/Ru ratio gives higher current and power densities (Figure 1.6). 

Electrociystallization denotes nucleation and crystal growth in electrochemical sterns under the influence of an electric field [1.1-1.21]. Electrocrystallization of metals takes place at an electronic conducting substrate / ionic conducting electrolyte interface including, in general, three stages ... 

The anode and cathode should be stable in the electrolysis medium, allow the desired oxida-tion/reduction reactions at the highest possible rates with miiumal by-product formation, and be of reasonable cost. In actuality, the electrodes may corrode or undergo physical wear during reactor operation, which may limit their lifetime. Often, if an expensive electrode material is needed for a given reaction, it can be plated or physically coated on a less costly, inert, and electronically conducting substrate. Common anode and cathode materials are listed in Table 26.8. 

The length scale i a 50 — 100 nm determines the effectiveness of catalyst utilization for spherical agglomerates. Analogous relations apply for ultrathin planar catalyst layers with similar thickness, L 100 — 200 nm. We consider layers that consist of Pt, water-filled pores and potentially an electronically conducting substrate. With these assumptions, we can put/(dfptc, XfXptc = 1 and g Sr) = 1. The volumetric exchange current density is, thus. 

Currently used electrode-catalysts (anode and cathode) consist of an assembly of metallic nanoparticles usually deposited on an electronic conducting substrate and embedded in a hydrated membrane [10, 11], which is the polymer electrolyte proton-conductive material (Figure 17.1). What differs between cathode and anode is the catalyst material, and also the significantly slow kinetics of the cathode oxygen reduction reaction compared to that of the anode hydrogen oxidation reaction. For this reason, several... 

Metal electrodeposition occurs at the interface between an electronically conducting substrate and an ionically conducting... 

Electrodes used in PEM fuel cells typically employ an electrocatalyst layer with a porous, carbonaceous, electronically-conductive substrate that has been rendered hydrophobic. The catalyst layer usually comprises platinum, or a platinum-containing alloy, on a carbonaceous support (typically carbon black), dispersed ionomeric material similar in constituency to the electrolyte-membrane, and dispersed hydrophobic polymer such as polytetrafluoroethylene. The substrate, which serves as a reactant-gas diffusion layer, may be a carbon paper or a woven or non-woven cloth. The catalyst layer may be deposited directly onto the electrolyte-membrane or onto the substrate and later placed in contact with the membrane. 

The internal solution in conventional ion-selective membrane (ISE) and reference electrodes has two roles. One is to support ion-to-electron transduction, or ion-to-electron current, between the internal solution and the electronically conducting substrate, e.g., metal, which is (or is a part of) the lead to the meter. For instance, in a silver-silver chloride reference electrode, it is the chloride ion in the potassium chloride solution that makes the transduction process possible, according to the reaction (see Sect. 5.2 and Chapter 6) ... 

Surface-modified electrodes — In order to alter the properties of an electron-conducting substrate, i.e., a metal or graphite or semiconductor used as a part of an electrode, different chemical compounds are produced/deposited/attached/immobilized on the surface. These electrodes are most frequently called surface-modified, chemically-modified, or polymer-modified electrodes, depending on the methods and materials used for the modification. The obvious purpose of these efforts is the production of electrodes with novel and useful properties for special applications, but also to help gain a better understanding of the fundamental charge transfer processes at the interfaces. Usually the enhancement of the rate of the electrode reaction... 

In order to carry out electrochemistry, nanostructures need to be back contacted to form an electrode. Electrodes are fabricated by various routes that coat an electronically conducting substrate with a thin layer of the semiconducting material. This thin layer formation is either based on deposition of the already synthesized nanostructured materials (often in a random arrangement such as sintered nanoparticle layers), or it is based on growth of nanostructured layers directly on a conducting substrate. The methods can generally be classified into non-electrochemical or electrochemical techniques, which are briefly discussed below. 

The addition of ferric ions to an aqueous solution of Fe(II) hexacyanide results in the formation of a highly colored colloidal precipitate known as Prussian blue (PB), a material regarded as the oldest coordination compound reported in the scientific literature.t It has been found recently that films of PB deposited on electronically conducting substrates are capable of undergoing a reversible blue-to-transparent color transition when the electrode potential is changed between two appropriate values. This is an illustration of what has been referred to as electrochromism, a phenomenon that may find wide application in connection with electronically controlled color display devices. 

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