Porous Gas Electrodes

To reduce and overcome this concentration polarization, several techniques are available. For reactants soluble in the electrolyte, high bulk concentrations are used and also the electrolyte is circulated by pumping which uses a fraction of the electricity produced by the fuel cell and hence reduces the available power. For gaseous reactants, porous gas electrodes are used to achieve larger contact of the three phases, namely the gas, the liquid and the solid phases. There are different types of such electrode and two examples are shown in Figures 3a et 3b. 

The main reason a porous gas electrode is so active,7 therefore, is that it allows particularly large maximum diffusion currents by diffusion through (fairly) thin meniscus layers. But this thesis brings a corresponding antithesis because it implies that farther up the pore where there is no meniscus but bulk solution, the gaseous... 

For porous gas electrodes, shown schematically in Fig. 2(b), there can be many different steps involved. For example, the overall cathodic reaction on a Pt electrode on YSZ is 02(gas) + 4e (Pt) 20 (YSZ). One can suggest that the overall reaction consists of the following series of elementary steps diffusion of O2 in the gas phase toward the TPB, O2 adsorption on the Pt, O2 dissociation, electron transfer to to form diffusion of... 

The form of the electrode is also modified to meet the needs of particular processes. Thus, for example, electrodes are commonly constructed from meshes, expanded metal and related materials in order to maximize surface area, reduce cost and weight and also enhance the release of gaseous products. It will also be seen in the later discussion of cell design that various types of particulate bed electrodes have been proposed and the possibilities for porous gas electrodes, which reduce mass transport restrictions for gaseous reactants, must also be considered. 

The question of the frequency dependence of the current distribution and its effect on the measured impedance of a solid state electrochanical system has been hardly considered, although it is important in discussing the impedance of, for example, porous gas electrodes on anion conductors, of rough electrodes (discussed below), and also perhaps of polycrystalline materials. In aqueous electrochemical situations the effects has been considered with respect to the rotating disk electrode, where there may be severe current distribution problems. 

When the reactant is a low-solubility gas it is advantageous to minimize mass-transport effects by employing a porous-gas electrode. The electrode is manufactured by compressing catalyst, conducting powder (e.g. graphite) and perhaps a... 

Pt/Ru Catalyst Polymer Pt Catalyst Porous Gas Layer Electrolyte Layer Diffusion Membrane Electrode... 

A porous platinum electrode is used so that the oxygen can pass through the electrode and react with the gas sensor material. Oxygen ions move... 

Stable, conductive electrodes would also be a problem. Preliminary experiments, were carried out in a cell, using simulated flue gas nearly identical to that shown in Fig. 24. In these tests, the membranes were hot-pressed from mixed powders of electrolyte (ternary eutectic of [Na, Li, K]2 S04) with LiA102 as matrix. The electrodes were constructed of cold-pressed Li20-9Cr203, partially sintered to give a highly-porous gas-diffusion structure. The tests were encouraging up to 50% of the S02 was removed from the simulated flue gas with the application of current. Simultaneously, a stream of concentrated S03 and Oz was evolved at the anode. 

Uribe et al.117 examined the reduction of CO in liquid NH3-0.1 M KI at -50°C, using various working electrodes such as Pt, Ni, C, and Hg. The reaction of CO with electrogenerated solvated electrons produced dimeric species, which precipitated as K2C202. Electrochemical reduction of CO in an aqueous solution at porous gas-diffusion and wet-proof electrodes of Co, Ni, and Fe was carried out,178 and Cj to C3 hydrocarbons and ethylene were reported to be the products. 

The next important stage in the development of porous gas-diffusion electrode is an investigation of influence of thickness of PANI layer (or more easy controlled parameters like PANI mass or electrochemical capacity) on the local currents of O2 electroreduction (table 3). 

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

Figure 12. Modeling and measurement of oxygen surface diffusion on Pt. (a) Model I adsorbed oxygen remains in equilibrium with the gas along the gas-exposed Pt surface but must diffuse along the Pt/YSZ interface to reach an active site for reduction. Model II adsorbed oxygen is reduced at the TPB but must diffuse there from the gas-exposed Pt surface, which becomes depleted of oxygen near the TPB due to a finite rate of adsorption, (b) Cotrell plot of current at a porous Pt electrode at 600 °C and = 10 atm vs time. The linear dependence of current with at short times implies semi-infinite diffusion, which is shown by the authors to be consistent only with Model II. (Reprinted with permission from ref 63. Copyright 1990 Electrochemical Society, Inc.)...

The structure of a SPE cell is shown in Fig. 2.3. The basic unit of a SPE electrolyzer is an electrode membrane electrode (EME) structure that consists of the polymer membrane coated on either side with layers (typically several microns thick) of suitable catalyst materials acting as electrodes [43,49,50], with an electrolyzer module consisting of several such cells connected in series. The polymer membrane is highly acidic and hence acid resistant materials must be used in the structure fabrication noble metals like Pt, Ir, Rh, Ru or their oxides or alloys are generally used as electrode materials. Generally Pt and other noble metal alloys are used as cathodes, and Ir, Ir02, Rh, Pt, Rh-Pt, Pt-Ru etc. are used as anodes [43,46]. The EME is pressed from either side by porous, gas permeable plates that provide support to the EME and ensure... 

A potentiometric electrochemical cell consisting of a reference electrode, solid-state electrolyte(s), and an indicator electrode can provide information about the partial pressure of gas in the same way as the cells utilizing ion-selective electrodes and liquid electrolytes can. The general mechanism is as follows. A sample gas, which is part of a redox couple, permeates into the solid-state structure usually through the porous metal electrode and sets up a reversible potential difference at that interface according to the reaction... 

Usually, rather than using a hydrogen gas electrode, a glass membrane electrode is used for the measurement. As discussed in Sec. 8, the potential across such a membrane can be proportional to the difference in pH s of the solutions on each side of the membrane. One design for a membrane-type pH electrode, which incorporates a Ag/AgCl reference electrode in a tube concentric to the membrane electrode, is shown in Fig. 6. The electrode is immersed in the solution whose pH is to be measured, with the solution level above the porous plug. 

Electrocatalytic possibilities with C02 reduction in aqueous solution are surprising. On copper at 0 °C, CH4 is the main product from electrolysis, and on a molybdenum cathode at room temperature, it is methanol (Hori, 1980). Using lead in a porous gas diffusive electrode, it has been possible to obtain HCOOH at 100 mA cm-2 (Hallmann, 1991). Macrocyclic compounds catalyze the reduction of C02 to CO to... 

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