Porous electrode surface area

Co limited kinetics. As with platinum, porous mixed-conducting electrodes are co-limited by molecular dissociation and transport. For mixed conductors with high rates of bulk ionic transport, values of k vary from 0.4 to 20 fjim depending on Po2> temperature, and electrode surface area with typical values in the 3—5 fjim range. This result indicates that a significant portion of the electrode surface is active for oxygen reduction, not just material in the immediate vicinity of the TPB. 

In electrocatalysis, the activity of different electrocatalysts is usually expressed via the exchange current I0, and the specific activity, via the exchange current density, iQ (A cm-2), still often computed on the basis of the superficial electrode surface area. Only when the current is normalized using the true surface area of the electrode-electrolyte interface, the comparison between different electrocatalysts is truly meaningful. The determination of the true surface area of porous electrodes is discussed in Sect. 2.3.5. 

A typical example includes the yttria-stabilized-zirconia-based high-temperature potentiometric oxygen sensor which is widely used in automotive applications. Platinum thick films are applied, forming both the cathode and anode of the sensor. The thick electrode has a porous structure which provides a larger electrode surface area compared to non-porous structures. For current measurement, a porous electrode is desirable since it leads to a larger current output. If the metallic film serves as the electrocatalyst, a porous structure is also desirable, for it provides more catalytic active sites. On the other hand, electrodes formed by the thick-film technique do not have an exact, identical... 

Increased electrode surface area plays an important role in performance. Enhanced area allows more electrolyte ions to organize at the electrode surface. Earger pores and channels in the electrode layer increase the accessibility and speed at which ions can organize onto the electrode pores from bulk electrolyte. Area plays a significant role for deciding on the materials for the electrode layers and the electrolyte. The pore structures of materials vary (macroporous >50 nm, mesoporous <50 nm, microporous <2 nm) and the ability to control the type of porous area available will lead to increased optimization between accessible power and maximum charge storage. 

The effective exchange current density of the porous composite CL, given per unit of apparent (external) electrode surface area,y = J SecsaIcl = j Stotlci stat, is the key physical property of a CL. This parameterization reveals two main avenues for enhancing the value of f via improving the intrinsic electrocatalytic activity or via optimizing the structural CL design, embodied in Secsa and Icl-... 

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 ). 

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

The effectiveness of a porous electrode over a plane surface electrode is given by the product of the active surface area S in cm /mL and the penetration depth Tp of the reaction process into the porous electrode. 

An effectiveness value greater than one indicates that the porous electrode is more effective than an electrode of the same geometric surface area, and that the reaction extends into the porous electrode stmcture. 

Methods for the Determination of the Real Surface Area of Rough and Porous Electrodes... 

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

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