Unit cell porous electrode

This primary current distribution becomes important in practical electrochemical devices, e.g., fuel cells. Here, one uses porous electrodes to try to increase the active electrode area for a unit apparent external area. This seems a good idea at first, but in reality the resistance of the solution down the pores prevents ions produced in the fuel cell reaction from "getting out and often only a small length of the pore in a porous electrodes is active. 

However, there is another kind of influence on current distribution that may even the score. This is called secondary current distribution and describes the resistances set up at the interface of the working electrodes in a cell in which the interface tends to be polarizable. For example, it was shown [Eq. (7.36)] that when f) < RT/F, the interfacial resistance per unit area is RT/igF. If i0 is very small (e.g., 10-10 A cm-2, hence, an interfacial resistance cm-2 of 2.6 x 10 ohms), it is this interfacial resistance and not the ohmic resistance in the bulk solution that detennines the current distribution. Thus, in an extreme case of high solution concentration (low solution resistance) and low i q, a substantial fraction of the length of the pores in a porous electrode remains active.34 Considerations such as these, together with resistance effects at edges, all count in cell design. 

Equivalent circuits for the catalyst layer are similar to those for porous electrodes, where charge-transfer resistance, capacitance, and Warburg resistance should be considered. The catalyst layer can be conceived of as a whole uniform unit or as a non-uniform circuit. In the case of a uniform unit, the equivalent circuits are similar to the modified ones discussed in Section 4.2.2 2, and the equations in that section apply. In many cases, such as in the presence of adsorbents, the surface is covered by the adsorbed species. For example, in direct methanol fuel cells and in H2/air fuel cells, CO adsorption should be considered. One example is illustrated in Ciureanu s work [7], as shown in Figure 4.31. 

Therefore the utilization of porous materials with high surface areas serving as electrodes enables capacitors to be made with high capacitance. For example, the use of an activated carbon (10 pF cm ) with the specific surface area of 1000 m g gives a capacitance as high as 100 F g . It should be noted that the capacitance measured in a unit cell corresponds to a quarter of the capacitance per unit weight or volume of the single electrode (F g or F cm ), because the... 

The problem of ohmic drops by diaphragms has been studied for a long time. A laboratory scale diaphragm-less water electrolyzer was developed for hydrogen production at large pressures of up to 140 kPa by electrolysis in an alkaline solution. Porous electrodes with a nickel catalyst and a copper cover layer serve as cathodes, whereas nickel sheets are used as anodes. Modular construction of the electrolyzer permits simple combination of its cells into larger units. Thus, up to 20 cells with diskshaped electrodes of 7 cm in diameter were connected in series and provided with electrolyte manifolds, automatic pressure, and electrolyte level control devices. The dimensions of the electrolyte manifolds were optimized based on the calculations of parasitic currents [50],... 

The SOFC unit has a three-layer sandwich structure two porous electrode, anode and cathode, serving as the chemical reaction, and the electrolyte, serving as the diffusion layer of oxygen ions but electrically nonconductive. A typical SOFC structure is shown in Figure 5-1. The anode and cathode should be porous to allow the diffusion of oxygen ions. A single SOFC unit cannot provide enough power therefore, the interconnection between stacks of cells is required. 

Porous electrodes are used in fuel cells in order to maximize the interfacial area of the catalyst per unit superficial area. As described above, this is critical not only for lowering the total amount of catalyst needed for the fuel cell but also to increase the power that can be delivered per unit superficial area. The... 

In the above sections, we have presented the electrode kinetics of electron-transfer reaction and reactant transport on planar electrode. However, for practical application, the electrode is normally the porous electrode matrix layer rather thtin a planner electrode siuface because of the inherent advantage of large interfacial area per unit volume. For example, the fuel cell catalyst layers are composed of conductive carbon particles on which the catalyst particles with several nanometers of diameter are attached. On the catalyst particles, some proton or hydroxide ion-conductive ionomer are attached to form a solid electrolyte, which is uniformly distributed within the whole matrix layer. Due to the electrode layer being immersed into the electrolyte solution, this kind of electrode layer is called the flooded electrode layer . 

An adaptation of a particulate bed cell which enables the production of metal onto a particulate substrate without fusing the bed together is the pulsed percolated porous electrode (pppe). This cell uses a regular pulse of upflowing electrolyte to unsettle a particulate bed during electrodeposition. An industrial prototype has been built in France by the Martineau Co. who specialise in the recovery of photographic fixing salts. The unit has also been tested on a cadmium effluent [18]. 

In the industrial applications of electrochemistiy, the use of smooth surfaces is impractical and the electrodes must possess a large real surface area in order to increase the total current per unit of geometric surface area. For that reason porous electrodes are usually used, for example, in industrial electrolysis, fuel cells, batteries, and supercapacitors [400]. Porous siufaces are different from rough surfaces in the depth, /, and diameter, r, of pores for porous electrodes the ratio Hr is very important. Characterization of porous electrodes can supply information about their real surface area and electrochemical utilization. These factors are important in their design, and it makes no sense to design pores that are too long and that are impenetrable by a current. Impedance studies provide simple tools to characterize such materials. Initially, an electrode model was developed by several authors for dc response of porous electrodes [401-406]. Such solutions must be known first to be able to develop the ac response. In what follows, porous electrode response for ideally polarizable electrodes will be presented, followed by a response in the presence of redox processes. Finally, more elaborate models involving pore size distribution and continuous porous models will be presented. 

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... 

The specific capacitance of a capacitor cell depends on the number of ions accumulated on the surface of unit mass of porous electrode. On... 

Campbell SA, Stumper J, Wilkinson DP, Davis MT, inventors Ballard Power Systems Inc, assignee. Porous electrode substrate for an electrochemical fuel cell. United States patent US 5863673.1999 Jan 26. 

In the case of iilterpress cells, it was mentioned in Chapter 2 that the most common, practical method of enhancing mass transport is the use of plastic-mesh turbul cei nonioters. It is rarely convenient to use such devices in metal-recovery cells due to the problems of the mesh being incorporated into the deposit or else promoting non-uniform deposits. One possibility is to utilize a porous electrode, such as a packed bed or a metal mesh or foam in these cases, the electrode itself acts as a turbulence promoter (in addition to providing a high electrode area per unit volume). 

The general layout of a cell includes a proton-conducting polymer electrolyte membrane (PEM), sandwiched between the anode and the cathode. Each electrode compartment is composed of (i) an active catalyst layer (CL), which accommodates finely dispersed nanoparticles of Pt that are attached to the surface of a highly porous and electronically conductive support, (ii) a gas diffusion layer (GDL), and (iii) a flow field (FF) plate that serves at the same time as a current collector (CC) and a bipolar plate (BP). This plate conducts current between neighboring cells in a fuel cell stack. At the cathode side, usually a strongly hydrophobic microporous layer (MPL) is inserted between CL and GDL, which facilitates the removal of product water from the cathode CL. The central unit including PEM and porous electrode layers, excluding the bipolar plates, is called the membrane electrode assembly (MEA). 

Electrochemical reactions are heterogeneous reactions which occur on the electrolyte-electrolyte interface. In fuel cell systems, the reactants are supplied from the electrolyte phase to the catalytic electrode surface. In battery systems, the electrodes are usually composites made of active reactants, binder and conductive filler. In order to minimize the energy loss due to both activation and concentration polarizations at the electrode surface and to increase the electrode efficiency or utilization, it is preferred to have a large electrode surface area. This is accomplished with the use of a porous electrode design. A porous electrode can provide an interfacial area per unit volume several decades higher than that of a planar electrode (such as 10" cm ). 

A US company has been developing a fuel cell based on a ceramic oxide electrolyte to operate at about 1000° C. It is able to consume either hydrogen or hydrogen-carbon monoxide mixtures (reformed natural gas, etc.) as the anode fuel. Unlike all the other systems which are based on stacks of parallel porous electrodes, the design here is centred on a ceramic oxide tube and cylindrical electrodes. A number of 3 kW units with 144 cells are under test to check performance and to ascertain lifetime under practical conditions. A high electrical efficiency of > 50% has been achieved in early tests and very useful waste heat is available from this high-temperature cell. 

Fig. 4.20 (a) The structure of unit cell (b) polymer electrolyte memrbane with porous electrodes that are composed of platinum paticles uniformly supported on carbon particles [78]... 

Practical units of fuel cells could not operate without porous electrode structures. Porous electrodes with their large electrochemically active surface allow reasonable currents to be supplied at acceptable losses due to polarization (see section 2 of chapter II). Although a few properties, like maximum available surface of electrocatalyst and hydrogenation and dehydrogenation of carbonaceous species for Teflon-bonded platinum black electrodes, and formation of oxygen layers for Raney nickel electrodes, have been discussed in preceding chapters, a discussion of the parameters that determine the operation of porous electrodes had to be offered in a separate chapter. While the empirical aspects concerning the operation of porous electrodes are covered in this chapter, theoretical aspects are dealt with in chapter XVI. 

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