Porous gas diffusion electrodes

Clearly, the current density must increase first and then decrease as a function of the distance from the edge of the pore, reaching a maximum somewhere in between. This is the most active area of the pore. A great saving in expensive catalyst material can be realized if this area is identified and controlled and the catalyst material is applied only to it. 

It is important to understand diat increasing the roughness factor of a planar electrode increases the rate of chaise transfer but has little effect on the rate of mass transport. On the other hand, the use of correctly designed porous electrodes can increase the rates of both processes. Thus the use of porous electrodes will be essential whenever gaseous reactants (e.g., H2 or O2) are employed, even after a suitable electrocatalyst is found. 

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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). 

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... 

Inhibiting Mass Transport Limitations and Ohmic Limitations in Porous Gas Diffusion Electrodes... 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

The combination of anode/electrolyte/cathode in proton exchange membrane fuel cell is usually referred to as the membrane electrode assembly (MEA).51 Usually the MEA was produced by attaching a catalyst layer (frequently Pt, Pt alloys, or other noble metals) on one side of porous gas diffusion electrodes. The catalysts... 

Concentration overpotential, due to slow diffusion processes in the electrolyte. This arises, in particular, in porous gas diffusion electrodes. 

The methods for solving Equation 16.48 are found in the literature [5,7], We are not going to discuss further since we are especially interested in the case of wetting porous gas-diffusion electrodes with respect to the thin-film model. 

A theoretical analysis of the current distribution and overpotential-current density relations for two models of porous gas diffusion electrodes—the simple pore and thin film models—has been carried out (108,109). The results of the analysis for the simple pore model will be summarized here. The reactant gas diffuses through the pore to the gas-electrolyte interface at 2 = 0, where it dissolves in the electrolyte and the dissolved gas diffuses through the electrolyte to the various electrocatalytic sites along the pore at which the reaction occurs (Fig. 25). It is assumed that the first and second steps of diffusion of reactant gas through the electrolyte-free part of the pore (z < 0) and of dissolution of gas at the gas-electrolyte interface are fast. 

Fio. 25. Mode of operation of porous gas diffusion electrode is shown using the simple pore model (108). 

Significant improvements in MEA fabrication techniques have been made with the incorporation of Nafion -H into porous gas diffusion electrodes... 

There are several reports on the use of MPc complexes as electrocatalysts for the reduction of CO2, but reports on the use of these complexes for the oxidation of CO are scarce. Diffusion electrodes have been employed extensively for the use of MPc complexes as electrocatalysts for CO2 reduction. Using porous gas diffusion electrodes, Fumya and coworkers studied the activity of a series of MPc complexes towards the reduction of CO2 and found the activity to be depended on the nature of the central metal. Table 7.4 . On FePc and PdPc modified electrodes both hydrogen and CO were obtained, on ZnPc and AlPc, the main products were hydrogen, carbon dioxide and formic acid and on H2PC, MgPc, MnPc,... 

Hydrogen technical electrodes for low temperature fuel cells such as AFCs, PAFCs and PEFCs are porous gas diffusion electrodes (GDEs) [4]. These electrodes have a large area reaction zone with minimum mass transport hindrances, thus allowing the easy access of... 

Polymer Electrolyte Fuel Cells (PEFCs), Introduction, Fig. 3 Simplified scheme with an acidic solid polymer electrolyte, e.g., the polymer electrolyte fuel cell PEFC). Fuel, H2 Oxidant, O2. Only porous gas diffusion electrodes and electrolyte are shown cell housing is not shown [12]... 

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. 

Springer TE, Raistrick ID. Electrical impedance of a pore wall for the flooded-agglomerate model of porous gas-diffusion electrodes. J Electrochem Soc 1989 136 1594-603. 

Gas diffusion is a much more effective mechanism of reactant supply and water removal. Yet, CLs with sufficient gas porosity, usually in the range Yp - 30% -60%, have to be made much thicker, 10 pm - 20 pm. At such thicknesses, proton diffusion in liquid water is not sufficient for providing uniform reaction conditions. Porous gas diffusion electrodes are therefore impregnated with proton-conducting ionomer, usually Nafion [1-2, 4]. Resulting CLs are random composite media of carbon/Pt, ionomer, and a complex pore space. 

Srinivasan S, Hurwitz HD, Bockris JMO. Fundamental equations of electrochemical kinetics at porous gas-diffusion electrodes. J ChemPhys 1967 46(8) 3108-22. 

Srinivasan D, Hurwitz HD. Theory of a thin film model of porous gas-diffusion electrodes. Electrochim Acta 1967 12(5) 495-512. 

Fuel cell electrodes are porous gas diffusion electrodes, which are usually described in modeling using homogenization [144]. This means that the pore electrolyte and the electrode material share the same geometrical domain and the electric potential in the electronic and ionic conductors are present in the same geometrical domain. Also the concentration variables for the species in the gas phase, the species dissolved in the electrolyte, and the constituents of the electrolyte may be present in the same geometrical domain defined by the gas diffusion electrodes. The electrochemical reactions that occur at the interface between the pore electrolyte and the electrode are introduced as sources or sinks in the material and current balances. To calculate the chemical composition in the electrolyte and in the gas phase in every point in space in a geometrical domain, material balances for each of the species in the solution as well as a conservation of mass for the whole have to be defined. The conservation of mass for the whole solution may eliminate one of the species material balances, which for a dilute solution usually is the solvent s material balance. The constitutive relations in the electrolyte may be the... 

The flow in the gas channels and in the porous gas diffusion electrodes is described by the equations for the conservation of momentum and conservation of mass in the gas phase. The solution of these equations results in the velocity and pressure fields in the cell. The Navier-Stokes equations are mostly used for the gas channels while Darcy s law may be used for the gas flow in the GDL, the microporous layer (MPL), and the catalyst layer [147]. Darcy s law describes the flow where the pressure gradient is the major driving force and where it is mostly influenced by the frictional resistance within the pores [145]. Alternatively, the Brinkman equations can be used to compute the fluid velocity and pressure field in porous media. It extends the Darcy law to describe the momentum transport by viscous shear, similar to the Navier-Stokes equations. The velocity and pressure fields are continuous across the interface of the channels and the porous domains. In the presence of a liquid phase in the pore electrolyte, two-phase flow models may be used to account for the interaction between the gas phase and the liquid phase in the pores. When calculating the fluid flow through the inlet and outlet feeders of a large fuel cell stack, the Reynolds-averaged Navier-Stokes (RANS), k-o), or k-e turbulence model equations should be used due to the presence of turbulence. 

Markicevic, B., Bazylak, A., and Djilali, N., 2007, Determination of transport parameters for multiphase flow in porous gas diffusion electrodes using a capillary network model , J. Power Sources, 171 pp. 706. 

For this three-dimensional porous gas diffusion, electrodes are used in fuel cells to provide a three-dimensional reaction zone and the diffusion of the reactant species to the electro-active sites by radial diffusion. The pore sizes and particles used are on the order of nanometers, resulting in the effective diffusion layer thickness being several orders of magnitude smaller. Limiting current densities on the order of 1-10 A/cm can be reached with such designs. 

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