Electrodes porous matrix

Gas diffusion electrodes have assumed a fundamentally important role in the technology of PEMFC. The treatment of the gas diffusion electrode is very complex, involving the physics of the porous matrix, the analysis of its equilibrium with the electrolyte and the gas as well as the thermodynamics and kinetics of the approximate transport processes. The surface of a gas electrode depends on the successfiil maintenance of a three phase equilibrium involving the electrode, porous matrix, the reactant gas and the electrolyte. 

The theory on the level of the electrode and on the electrochemical cell is sufficiently advanced [4-7]. In this connection, it is necessary to mention the works of J.Newman and R.White s group [8-12], In the majority of publications, the macroscopical approach is used. The authors take into account the transport process and material balance within the system in a proper way. The analysis of the flows in the porous matrix or in the cell takes generally into consideration the diffusion, migration and convection processes. While computing transport processes in the concentrated electrolytes the Stefan-Maxwell equations are used. To calculate electron transfer in a solid phase the Ohm s law in its differential form is used. The electrochemical transformations within the electrodes are described by the Batler-Volmer equation. The internal surface of the electrode, where electrochemical process runs, is frequently presented as a certain function of the porosity or as a certain state of the reagents transformation. To describe this function, various modeling or empirical equations are offered, and they... 

The porous matrix of gold-black electrode has enabled ferrocene-modified glucose oxidase to perform the smooth electron transfer by means of easy access between self-assembled molecules and electrode surface. 

In the SECM measurement (Figure 18), a small microelectrode (typically a metal or carbon electrode) is rastered across the surface of interest, and the current resulting from a Faradaic reaction is mea-sured. 220 q-j g experiment is arranged such that the tip current is proportional to the local concentration of a redox species, which in turn may reflect molecular transport rates within a porous matrix (Figure ISa) or the electron-transfer activity at an electrode (Figure 18b). 

Fig. 3.29 Schematic view of the inteiphase at a porous matrix air electrode...

Pocket plate design is not suitable for the positive electrode because of the infiltration of soluble zincate and the consequent decrease in positive electrode capacity. Porous matrix positives do not suffer so badly from... 

Whereas current-producing reactions occur at the electrode surface, they also occur at considerable depth below the surface in porous electrodes. Porous electrodes offer enhanced performance through increased surface area for the electrode reacdon and through increased mass-transfer rates from shorter diffusion path lengths. The key parameters in determining the reaction distribution include the ratio of the volume conductivity of the electrolyte to the volume conductivity of the electrode matrix, the exchange current, the diffusion characteristics of reactants and products, and the total current flow. The porosity, pore size, and tortuosity of the electrode all play a role. 

The above equations for the diffusion coefficients do not take into account the volume fraction of porosity and the tortuous nature of the path through porous bodies. When the transport occurs through a porous body, as in fuel cell electrodes, effective diffusion coefficients accounting for the interaction of gaseous species with the porous matrix must be employed. Different theoretical approaches for the determination of the effective diffusion have been proposed in the literature. The Bruggemann correction allows the evaluation of these coefficients, through the following expression [47] ... 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

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],... 

The most common electrode material used in LC-EC is carbon, either as solid glassy carbon disks in thin-layer cells, or as a high-surface-area porous matrix through which the mobile phase can flow. Gold electrodes are useful to support a mercury film and these are primarily used to determine thiols and disulfides, and also for carbohydrates using pulsed electrochemical detection... 

Hydrogen gas fuel and air (O2) are fed to anode and cathode Pt catalyst powder layers, respectively. The Pt catalysts is Teflon-bonded to porous carbon sheets to form gas-diffusion electrodes, with a catalyst loading of about 1.0 mg/cm. The Pt anode and cathode are separated by a thin inert porous matrix that is filled with concentrated phosphoric acid. The cell operates at 200°C (to improve the electrode kinetics), with a cell voltage of about 0.67 V at a current density of 0.150 A/cm. Most voltage losses occur at the air cathode. The hydrogen gas must be pure because sulfur and carbon monoxide poison the Pt anode catalyst. This type of fuel cell is commercially available today, with more than 200 systems installed all over the world in hospitals, hotels, office buildings, and utility power plants. 

During extended cell operation, electrolyte may be lost from the L1A102 matrix and this will lead to gradual decay in cell performance and, ultimately, to fuel leakage through to the positive electrode. It is therefore essential that the porous matrix should remain full of electrolyte at all times. Typically, 60 wt.% of carbonate is constrained within 40wt.% of matrix. Carbon monoxide present in the cell may undergo disproportionation to form carbon via the Boudouard reaction, i.e.. 

Novel conductive composite films have been developed by Kaplin and Qutubuddin [101] using a two-step process microemulsion polymerization to form a porous conductive coating on an electrode followed by electropolymerization of an electroactive monomer such as pyrrole. The porous matrix was prepared by polymerizing an SDS microemulsion containing two monomers, acrylamide and styrene [102], The electropolymerization of pyrrole was performed in an aqueous perchlorate or toluenesulfonate solution. The effects of polymerization potential on the electropolymerization, morphology, and electrochemical properties were reported [101]. The copolymer matrix improves the mechanical behavior of the polypyrrole composite film. 

Luminescence from Porous Electrodes As in bulk systems, photogeneration of charge carriers in a porous electrode or injection of minority carriers from solution can lead to light emission [25]. In a macroporous system in which the depletion layer can follow the contours of the porous matrix, one does not expect significant differences between bulk and porous electrodes with regard to the potential dependence of the emission. If, however, the porosity is high and the dimensions of the structures become very small (e.g. <5 nm) then special effects may be expected. These are indeed found as, for example, with nanoporous silicon. 

Alkaline mercury-zinc batteries were manufactured as sealed cells of low capacity (0.05-15 Ah). They contain mercury oxide HgO and a limited amount of electrolyte (about 1 ml/Ah) absorbed in a porous matrix, so they operate only according to the secondary process of the zinc electrode. Modern mercury-zinc batteries were developed by S. Ruben in the beginning of the 1940s. His button construction was so effective that large-scale production started in the United States as early as World War II and after the war in other countries. A schematics of the button construction is shown in Figure 4.1... 

At the working temperature of the battery (400-600°C) pure lithium is liquid. Two main construction types of lithium electrodes have been reported liquid lithium in a porous matrix and a solid alloy of lithium with another metal. The matrix of the... 

Kinetics of Reactant Transport Near and within Porous Matrix Electrode Layer ... 

Figure 2.12 (A) Schematic of the electrode/electrolyte interface of the porous matrix layer, and (B) equivalent electrode/electrolyte interfaces of the porous matrix layer and the oxidant distribution within the interfaces. (For color version of this figure, the reader is referred to the online version of this book.)...

It is worth pointing out that Eqns (4.13)—(4.20) are for the ORR on a smooth planar electrode or catalyst surface rather than in a porous matrix catalyst layer. It is expected that the situation in the catalyst layer may be more complicated than on the planar surface. However, it is believed that with modification using the apparent parameters as well as the real electrochemical active surface, the equations are still valid for quantitative treatment of experimental data. 

---