Electrodes porous catalytic

Porous catalytic pellets, 25 271-272 Porous electrodes, 3 428-429 Porous glass, 22 394 Porous graphite, 12 747 Porous hydrogels, for tissue engineering, 13 750-751... 

The first methanol-fed PEM EC working with an AEM was conceived by Hunger in 1960 [15,45]. This system contained an AEM with porous catalytic electrodes pressed on both sides and led to relatively poor electrical performance (1 mA cm at 0.25 Vat room temperature with methanol and air as the reactants). Since this first attempt, many studies have been carried out to develop alkaline membranes. 

A common type of oxygen sensor takes the form of an yttria stabilized zirconia (YSZ, see earlier) tube electroded on the inner and outer surfaces with a porous catalytic platinum electrode. The electrode allows rapid equilibrium to be established between the ambient, the electrode and the tube. Such a system is shown schematically in Fig. 4.36. 

In the first case, YSZ electrolyte of thickness 250 - 400 microns is covered with Ni-cermet anode and LSM cathode of thickness 25-50 microns. IPPE has 4 patents for technology of porous catalytic interfaces electrode-solid electrolyte . More than 200 single cells of this type were tested. At 950 C, power density of 700 mW/cm was achieved (Fig. 6). 

Decreasing operation temperature of solid oxide fuel cells (SOFCs) and electrocatalytic reactors down to 800-1100 K requires developments of novel materials for electrodes and catalytic layers, applied onto the surface of solid electrolyte or mixed conducting membranes, with a high performance at reduced temperatures. Highly-dispersed active oxide powders can be prepared and deposited using various techniques, such as spray pyrolysis, sol-gel method, co-precipitation, electron beam deposition etc. However, most of these methods are relatively expensive or based on the use of complex equipment. This makes it necessary to search for alternative synthesis and porous-layer processing routes, enabling to decrease the costs of electrochemical cells. Recently, one synthesis technique based on the use... 

The primary optimization parameter of porous electrodes is the ideal electrochem-ically active surface area per unit volume sa- rough approximation, the value ECSA is proportional to the amount of the electrocatalytically active material Pt, in the case of PEFC electrodes. It is inversely proportional to the feature size d, which could represent diameters of catalyst particles, of pores in a porous catalytic medium, or of rod-like structures (nanotubes or nanorods), onto which a thin film of catalyst is deposited. On the other hand, is also roughly proportional to the energy density... 

In our opinion, the new oxide supports demonstrated their suitability from a chemical point of view, but their intrinsic structure hinders their successful application in porous catalytic layers. In order to take into account the decisive role of the support morphology for a successful layer preparation, oxides with novel morphologies need to be tailored and the electrode design adapted to the new materials. This strategy is similar for pol)aner and oxide supports, respectively, and will be illustrated below. 

The electrochemical reduction of carbon dioxide to produce fuel has often been termed by some as artificial photosynthesis. Compared to the traditional researches using alkaline solution electrolytes, porous separators, and solid metallic electrode structures, there are numerous benefits to using a cell design based on a solid polymeric ion-conduction MEA with porous catalytic electrodes. In this part, we will introduce a typical example of hydroxide-ion-conduction membrane used for electrochemical conversion of carbon dioxide in alkaline PEM cells, conducted by Valdez and coworkers [132],... 

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

The catalytic-electrocatalytic reactor consists of a membrane electrode assembly, such as Pt-black/Nafion/Pd/C sandwiched between sheets of porous carbon cloth, housed in a fuel cell assembly. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

Weaver [40] studied alternate cathode materials at 650 °C, finding several that performed well. Steady-state polarization on Ni, Co and Fe porous electrodes operating as cathodes in a MCFC, with a standard (Li/K)2 C03 tile is shown in Figs. 30-32. Note that the oxidant gas fed to these cathodes is, in normal MCFC operation, the fuel, composed of 32.5% H2, 17.5% COz, 17.5% H20, the balance N2. Polarizations were first taken with this clean gas where the only reaction can be Eq. (35). After steady-state was attained, 0.65% H2S was added and sufficient time allowed for the electrode to convert to the sulfides. After 24 hours, the outlet H2S reached the inlet level and polarizations were measured. Note in Figs. 30-32, that the performance with H2S is significantly improved over the clean gas. (The Ni sample was a commercial (Gould) MCFC electrode the Co and Fe were pressed from powders. Each gas was 8 sq cm in superficial area). The improvement is probably due to a catalytic mechanism involving sulfur interactions with the electrode, as, for Co ... 

Analysis of data presented clearly suggests that a sharp increase of catalytic activity takes place in relatively thin layer of PANI, which corresponds to the mass of about 0.6...0.9 mg (or Q=108...164 mC) per geometric surface of electrode (0.5 cm2). Further increase of PANI thickness (or mass) cannot increase enough the catalytic activity of porous electrode. That is the reason why the developed by us PANI/graphite composites have relatively high content of graphite (Table 4). 

The air gas-diffusion electrode developed in this laboratory [5] is a double-layer tablet (thickness ca.1.5 mm), which separates the electrolyte in the cell from the surrounding air. The electrode comprises two layers a porous, from highly hydrophobic, electrically conductive gas layer (from the side of the air) and a catalytic layer (from the side of the electrolyte). The gas layer consists of a carbon-based hydrophobic material produced from acetylene black and PTFE by a special technology [6], The high porosity of the gas layer ensures effective oxygen supply into the reaction zone of the electrode simultaneously the leakage of the electrolyte through the electrode... 

The catalytic layer of the air electrode is made from a mixture of the same hydrophobic material and porous catalyst [2]. It comprises hydrophobic zones through which the oxygen is transported in gas phase and zones containing catalyst where the electrochemical reduction of oxygen is taking place. It must be noted that the overall structure of the electrode is reproducible when various kinds of carbon-based catalysts are used. 

As it follows from Table 5, many catalysts contain metallic platinum. We have developed bi-layer porous hydrophobic air electrodes, which do not contain platinum metals, are active and can be cycled [24, 25] (Figures 4-6). These bifunctional catalysts are pyrolized Co - macrocyclic compounds. Said catalyst has high catalytic activity for the oxygen reduction and also features acceptable stability, however its activity for the oxygen evolution is not high enough. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

The porous hydrophobic film of previous electrode designs has now been substituted with a new layer based on a mixture of particles of hydrophobic carbon and PTFE binder. This mixture is very similar in composition to the catalytic layer. This particular modification provides several advantages ... 

The functions of porous electrodes in fuel cells are 1) to provide a surface site where gas/liquid ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions, the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in high-temperature fuel cells because ionization reaction rates increase with temperature. It is also a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a one-sided manner (see latter part of next section). 

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