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Porous, electrodes three phase boundary

We consider the porous metal catalyst film shown in Figure 11.12 which is interfaced with an O2" conductor. When a positive current, I, is applied between the catalyst and a counter electrode, oxide ions O2 are supplied from the solid electrolyte to the three phase boundaries (tpb) solid electrolyte-metal-gas at a rate I/2F. Some of these O2 will form 02 at the tpb and desorb ... [Pg.503]

The amount of oxygen adsorbed in the three-phase region has been found to depend linearly on the exchange current density for different catalyst-electrodes under similar conditions.31,32 This indicates that the electrocatalytic reaction takes place at the three-phase boundary. Vayenas and co-workers pointed out that for less porous electrodes the charge-transfer reaction at the two-phase boundary might become important and that under some conditions oxygen on the electrolyte surface itself might play a role. [Pg.12]

The GDL is located on the back of the CL in order to improve gas distribution and water management in the cell. This layer has to be porous to the reacting gases, must have good electronic conductivity, and has to be hydrophobic so that the liquid produced water does not saturate the electrode structure and reduce the permeability of gases. The GDL needs to be resilient and the material of choice for the PEMFC is usually carbon fiber, paper or cloth, with a typical thickness of 0.2-0.5mm [74,75], This macroporous support layer is coated with a thin layer of carbon black mixed with a dispersed hydrophobic polymer, such as P I LL, in order to make it hydrophobic. This latter compound can, however, reduce the electronic conductivity of the GDL, and limit the three-phase boundary access. [Pg.404]

The efficacy of porous electrodes depends on two factors The pores allow an external geometric electrode area of 1 cm2 to be increased many times and the meniscus-like structure of the three-phase boundary gently increases the local limiting current density. Assume a model electrode with pores having a uniform... [Pg.381]

Since the reactants in a fuel cell are normally gaseous products, the electrodes for the PEFCs are normally porous to guarantee the supply of the reactant (gases) to the active zones. As explained in the case of the SOFCs, the electrodes in the case of the PEFC normally consist of a porous matrix, since the electrochemical reactions take place in the three-phase boundary (see Figure 8.26), that is, at the interface between the electrode, the electrolyte, and the reactant gas [168,170],... [Pg.413]

Usually, the working electrode (W) is a porous metallic electrode in PEVD. Thus, reactant (B) in the vapor phase can reach the surface of the solid electrolyte for initial electrochemical reaction at a three-phase boundary of solid electrolyte (E), working electrode (W) and sink vapor phase (S) as shown in Eigure 3 (location II). All reactants for the sink side electrochemical reaction (1) or (2) are only available there. Subsequent reaction and deposition of the product (D) requires both electrons and ions to travel through product (D) to the surface to react with vapor phase reactant(s) electrochemically at location III in Eigure 3. [Pg.108]

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. [Pg.32]

Electrodes The anodes of SOFC consist of Ni cermet, a composite of metallic Ni and YSZ, Ni provides the high electrical conductivity and catalytic activity, zirconia provides the mechanical, thermal, and chemical stability. In addition, it confers to the anode the same expansion coefficient of the electrolyte and renders compatible anode and electrolyte. The electrical conductivity of such anodes is predominantly electronic. Figure 14 shows the three-phase boundary at the interface porous anode YSZ and the reactions which take place. The cathode of the SOFC consists of mixed conductive oxides with perovskite crystalline structure. Sr doped lanthanum manganite is mostly used, it is a good /7-type conductor and can contain noble metals. [Pg.442]

Hence, the sodium activity - and thus the cell potential - is related to the SO2 partial pressure. The auxiliary electrode must be in contact with both the electrolyte (with the mobile sodium ions) and the metal electrode (to measure the electrical signal), as well as the gas consequently, porous electrodes are typically used to provide a large three-phase-boundary area. It is also possible to mix the auxiliary electrode with the electrode either only near the surface (where it is needed) or throughout the electrolyte (which is sometimes easier to fabricate) this is referred to as a composite electrolyte. [Pg.430]

The alkaline fuel cell (AFC) with its liqnid alkaline electrolyte KOH uses gas diffusion electrodes with a hydrophobic porous part, which is not flooded by the alkaline electrolyte, and a hydrophilic part containing electrolyte and thus leading to a three-dimensional three-phase boundary layer. As the electrode potentials in alkaline electrolyte are shifted towards more negative values, corrosion is less problematic. Raney Nickel and silver are the state-of-the-art catalysts. The practical use... [Pg.157]

Gas-diffusion electrodes with these catalysts are prepared by a few methods metalceramic porous hydroclosing layer used to work with enhanced pressure, as well as to work on air hydroclosing layer is hydrophobic Teflon or hydrophobic soot. Thus three-phase boundary (reaction s zone) created by pressure difference between electrolyte and gas hollow or by gradient of layer s wetting. Nickel net 400 micron is current conductor for all types of electrodes. [Pg.182]

Porous ceramics, with high electronic conduction and chemical stability in the fuel gas are required for anode and cathode. The electrode should be porous and have homogeneous microstructures because the electrode reaction occurs on three phase boundary (TPB), which consists of electrolyte, electrode and gas. The reaction site increases with TPB length. Using a new technique of spray dry process, a new... [Pg.238]

A porous electrode offers a far higher true working surface area and thus a much lower true current density (current per unit surface area of the electrode). Such an electrode consists of a metal or carbon-based screen or plate serving as the body or frame, current collector, and support for active layers, containing a highly dispersed catalyst for the electrode reaction. The pores of this layer are filled in part with the liquid electrolyte, and in part with the reactant gas. The reaction itself occurs at the walls of these pores along the three-phase boundaries between the solid catalyst, the gaseous reactant, and the liquid electrolyte. [Pg.132]

Another important feature of AFC porous electrodes is the control of the location and extent of the three-phase boundary. This can be achieved by adding a... [Pg.509]

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... [Pg.132]

A simplified scheme of the electrochemical heart of a PEFC is displayed in Fig. 3, where the central solid electrolyte is contacted by two porous gas diffusion electrodes (GDLs), which are in intimate contact to the membrane surface (see below, three phase boundary). At the interface to the membranes, the GDLs contain nanoparticles of platinum (black dots) as electrocatalyst. [Pg.1657]


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