Porous Electrode Reactor

The porous electrode is one dimensional and both solid and electrolyte phase are continuous media with uniform effective conductivities. 

According to the coupled equation, Eqs. (103) and (104), with the polarization Eq. (41), a charge balance coupled with a material balance for a redox pair in a volume element will be 

Assuming the potential in conductive solid phase and the potential of open-circuit, (f m and U in Eq. (42), are constant. According to Ohm law, the vector of local current density in the solution phase can be written as 

For a simplified one-dimensional model, namely, the transfer process in the porous electrode is dominated by diffusion with the effective coefficient De, Eqs. (113) and (114) become 

The is a dimensionless overpotential variable and is a dimensionless conduction parameter, in which the group ai Lp is a characteristic property for electrode reaction, while Lhc is a characteristic property for conduction resistance. 

Let Sj= 1 for O + e -o R and considering an anodic cnrrent only, the charge balance Eq. (106) and material balance Eq. (108) become 

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Two types of continuous flow solid oxide cell reactors are typically used in electrochemical promotion experiments. The single chamber reactor depicted in Fig. B.l is made of a quartz tube closed at one end. The open end of the tube is mounted on a stainless steel cap, which has provisions for the introduction of reactants and removal of products as well as for the insertion of a thermocouple and connecting wires to the electrodes of the cell. A solid electrolyte disk, with three porous electrodes deposited on it, is appropriately clamped inside the reactor. Au wires are normally used to connect the catalyst-working electrode as well as the two Au auxiliary electrodes with the external circuit. These wires are mechanically pressed onto the corresponding electrodes, using an appropriate ceramic holder. A thermocouple, inserted in a closed-end quartz tube is used to measure the temperature of the solid electrolyte pellet. 

An appreciable increase in working area of the electrodes can be attained with porous electrodes (Section 18.4). Such electrodes are widely used in batteries, and in recent years they are also found in electrolyzers. Attempts are made to use particulate electrodes which consist of a rather thick bed of particulate electrode material into which the auxiliary electrode is immersed together with a separator. Other efforts concern fiuidized-bed reactors, where a finely divided electrode material is distributed over the full electrolyte volume by an ascending liquid or gas flow and collides continuously with special current collector electrodes (Section 18.5). 

Cheng, C.Y. and Kelsall, G.H. (2007) Models of hypochlorite production in electrochemical reactors with plate and porous electrode. J. Appl. Electrochem. 37, 1203-1217. 

Despite their intrinsic simplicity, the plate-and-frame reactors require accurate selection of the materials, especially when the electrolytic processes are performed in non-aqueous solvents and/or deal with substrates which are currently used as solvents, as in the case of volatile organic chlorides. Special attention should be paid to the selection of the gaskets, for long-term operational stability. Moreover, in the case of the SPE scheme, porous electrodes should be adopted, to allow for optimal contact between electrode, electrolyte and reactants/products. In this case, mass-transfer limitations into the porous structure should be carefully considered. 

A tri-dimensional electrode reactor geometry was studied by He et al. (2004a, b) to overcome the problem of low conductivity media (>1 S m-1). The reduction of model carbon tetrachloride was performed on a porous copper foam with good conversion rates and almost total dehalogenation of the substrate. 

The formation of gas bubbles in a porous medium affects, e.g., the performance of anaerobic granular sludge particles containing entrapped gas (waste-water treatment), or the performance of electrochemical reactors where a gas (hydrogen, oxygen, chlorine) evolves inside a porous electrode. A related problem is that of bubble nucleation on structure surfaces, which can be as varied as specially designed surfaces for enhanced nucleate boiling heat transfer, or the... 

It should also be observed that the catalytic cell reactor (described in Section II.D), which is intended to be an alternative reactor to trickle beds for liquid-phase hydrogenations, is a further-developed electrochemical filter-press cell based on the firm Electro Cell AB s concept with respect to the preparation of thin, porous electrodes. 

Porous electrodes have the same advantages as particle bed reactors, but they have the difficulty of becoming plugged as the metal deposit builds up. Sometimes this is not important if the value of the deposited metal is high and the electrode may be sacrificed. An example of such a system is the Zadra cell used in gold electrowinning, where the cathode is a steel mat that can be melted away when gold is recovered. 

Leaching and electrolysis processes can be used for metal recovery from waste electrical and electronic equipment. Metals such as Ag, Au, Cu, Pb, Pd, Sn, are dissolved from shredded electronic scrap in an acidic aqueous chloride electrolyte by oxidizing them with aqueous dissolved chlorine species. In the electrochemical reactor, chlorine is generated at the anode for use as the oxidant in the leach reactor and the dissolved metals are deposited from the leach solution at the cathode. The very low concentrations of the precious metal ions require the use of porous electrodes with high specific surface areas and high mass transport rates to achieve economically adequate reactor productivities and space-time yields [72]. 

Porous metallic structures have been used for electrocatalysis (Chen and Lasia, 1991 Kallenberg et al., 2007). Porous electrodes are made with conductive materials that can degrade under high temperatures at high anodic potential conditions. This last problem is of less importance for fuel cell anodes, which operate at relatively low potentials, but it can be of importance for electrochemical reactors. Porous column electrodes prepared by packing a conductive material (carbon fiber, metal shot) forming a bar are frequently used. Continuous-flow column electrolytic procedures can provide high efficiencies for electrosynthesis or removal of pollutants in industrial situations. Theoretical analysis for the electrodeposition of metals on porous solids has been provided by Masliy et al. (2008). 

The single cells consist of a dense solid electrolyte membrane and two porous electrodes. In most cases, at least one of the electrodes is exposed to an oxygen-containing gas (often, ambient air), while the other electrode is exposed to an inert gas, a liquid metal, a partial vacuum, or a reacting mixture (hydrogen, water vapor, hydrocarbons, CO, CO2, etc.). The single-chamber reactor (SCR) has been also proposed either as a membrane reactor or as a fuel cell. In this case, the solid-electrolyte disk, with two different electrodes that are coated either on opposite sides or on the same side of the pellet, is suspended in a flow of the reacting mixture (see Section 12.6.3). 

Ion sources can be used as electrochemical reactors for oxidation or reduction reactions [157], They may be fitted with electrochemical cells in order to monitor such processes. Conventional electrochemical measurements do not provide direct information on inter-mediate/product species. Moreover, the reaction intermediates present between electrodes may be unstable. To enable thorough characterization of electrochemical processes, such devices can be coupled with MS (e.g., [158,159]). Since the 1970s many of the popular ion sources have been used for that purpose [160]. For example, in one early work, a porous electrode was introduced to sample and detect volatile products (NO and N2O) during electrochemical reduction of NOj [158]. In the differential electrochemical MS, electrochemical cells are interfaced with MS using a porous Teflon membrane [161]. Such systems can facilitate characterization of reaction intermediates as well as electrode processes with respect to the applied potentials [161]. 

Many of the problems of fluidized-bed cells appear to have been overcome by the development of three-dimensional contiguous-bed reactors (36,37) which mainly evolved from the porous electrode designs usi3 "Tn battery or fuel cell applications. These electrode designs are characterised by very high specific surface areas and space time yields. At the same time the ability to control... 

Electrical field effects are an example of a transport phenomenon that does not arise in most chemical reactors, and these field effects often dictate the current distribution. Usually, electrical field effects are more important in the (ionicaUy conducting) electrolyte than in the (electronically conducting) electrodes. However, as is the case of porous electrodes for fuel cells and batteries, significant potential variations in the electrodes may result if the electrodes are very thin, very large, or have high specific resistivity. Current distributions where the potential drop in the electrode is important were first studied in 1953 [4] the phenomenon is called the terminal effect or resistive substrate effect. ... 

Electrolyte flow may trickle through the porous electrode (as in the trickle-tower reactor (Fig. 2.26(a)) or be pumped upwards through a flooded-bed electrode eg. Fig, 2.35(b)). 

As shown in Fig. 5 7(b) the solid polymer electrolyte cell comprises a membrane, fuel cell type, porous electrodes and three further components z carbon collector, a platinized titanium anode support and a cathode support made from carbon-fibre paper The collector is moulded in graphite with a fluorocarbon polymer binder A 25 pm thick platinized titanium foil is moulded to the anode side to prevent oxidation. The purpose of the collector is to bnsure even fluid distribution over the active electrode area, to act as the main structural component of the cell, to provide sealing of fluid ports and the reactor and to carry current from one cell to the next E>emineralized water is carried across the cell via a number of channels moulded into the collector These channels terminate in recessed manifold areas each of which is fed from six drilled ports. The anode support is a porous conducting sheet of platinized titanium having a thickness of approximately 250 pm. The purpose of the support is to distribute current and fluid uniformly over the active electrode area. It also prevents masking of those parts of the electrode area which would be covered by the... 

It has already been mentioned that three-dimensional, porous, electrodes offer particularly high values of the electroactive area per unit reactor volume whilst also giving a moderate increase in the mass transport coefficient The result is a significantly increased performance from a given volume of reactor, compared to two-dimensional electrode materials, due to the high value of fct s-This performance may be utilized in various ways including ... 

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