Porous carbon electrode

Figure 15.3 Simulated effectiveness factor for porous carbon electrode as a function of the exchange current density jo and DCo for Ip] = 0.4 V for a 10wt% Pt/C catalyst layer with 7= 10, A = 140m g p = 2gcm, Nafion volume fraction 0.6, thickness p,m, and ionic conductivity 0.05 Scm See the text for details. (Reproduced from Gloaguen et al. [1994], with kind permission from Springer Science and Business Media.)...

In order to see how the electrode thickness might be optimized in order to provide the lowest electrode resistivity, we have developed a theoretical model to describe the charge/discharge processes in porous carbon electrodes. As a first approximation, let us consider an electrode having two sets of cylindrical pores, namely, nanopores (NP) of less than 3 nm in diameter and transport channels (TC) of more than 20 nm in diameter, with each nanopore having an exit to only one TC. ... 

Figure 2. An equivalent circuit of a porous carbon electrode in SC.

Figure 7 illustrates the dynamics of fluid migration through porous carbon electrodes to obey the Hagen-Poiseuille equation that is normally used to describe the transport through membranes having the pores of cylinder-like shape. Therefore, this method can probably be used for express analysis of the electrolyte dynamics in different porous carbon materials. 

Porous carbon electrodes were used in the field of electrolyte-soluble (p-nitraniline) [34] and electrolyte -insoluble (nitrobenzene) [35] organic cathodic depolarizers. 

The use of porous carbon electrodes allows accurate control of the ratio of oxidized and reduced forms of p-nitriline by adjusting the electrolyte flow through the electrode. As a result, the reduction of p-nitraniline to p-phenylenediamine was carried out continuously at current densities three-fold greater than the current densities obtained from batch processes [34] and -45 % of the total yield was obtained, depending on the p-nitriline used [34]. 

The anodic stability of the Tf anion, as measured on a GC surface, was not found to be particularly high 130 inferior to Bp4 and PFe but better than C104 . Ab initio calculations yielded similar conclusions, and results measured on porous carbon electrodes were consistent with those measured on GC.81... 

Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte - the acid is contained in a Teflon-bonded silicon carbide matrix - and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types, the first to be used commercially, and features the most proven track record in terms of commercial applications with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. 

N.J. Forrow and S.W. Bayliff, A commercial whole blood glucose biosensor with a low sensitivity to hematocrit based on an impregnated porous carbon electrode, Biosens. Bioelectron., 21 (2005) 581-587. 

Philips has taken a different direction in the field of electrochemical fluorination. The Philips ECF process uses porous carbon electrodes and KF 2 HF as an electrolyte. Here, fluorination is effected in the pores of the anodes by electrochemically produced elemental fluorine. The process is therefore suitable for low-boiling products which are substantially insoluble in the electrolyte. The process, which has been successfully tested on the pilot scale, is reviewed by W. V. Childs in 75). 

The electro-active surface of the porous carbon electrode for EDLCs is accessible only through the cumulative resistance of the electrolyte inside the pore. Therefore, the porous structure of the porous carbon becomes one of the most important factors influencing the energy/power densities. Fractal analysis has proven to be useful to describe the geometric and structural properties of rough surfaces and pore surfaces.56 66... 

As a matter of fact, for porous carbon electrode it is still a troublesome issue to relate the determined surface fractal dimension dFss with the CPE exponent a. The effect of the surface inhomogeneity on the ion penetration into the pores during doublelayer charging/discharging will be discussed in detail in the following Section V.3. 

FIGURE 1.26 Three-electrode laboratory cell. (A) Porous carbon electrode film (carbon + binder) (B) treated A1 current collectors (C) porous separator (D) Teflon plates (for stack pressure) and (E) stainless steel clamps (not shown, see arrows). (From Gamby, J., et al., J. Power Sources, 101,109, 2001.)... 

Avarbz, R.G., Vartanova, A.V., Gordeev, S.K., Zjukov, S.G., Zelenov, B.A., Kravtjik, A.E., Kuznetsof, V.P., Kukusjkina, J.A., Mazaeva, T.V., Pankina, O.S., and Sokolov, V.V. Double layer capacitor with porous carbon electrodes and method for manufacturing these electrodes. European Patent WO 97/20333, 1996. 

Each electrode unit consists of a central porous carbon electrode, on either side of which is situated a reference electrode and a auxiliary electrode. As the pressure drop across the porous electrode is relatively small, these electrode units can be connected in series forming an array. Normally up to 16 units can be placed in series and these are commercially available. However, a sensor system that contains as many as 80 electrodes in the space of a few millimeters has also been constructed [18]. 

The advent of the porous carbon electrode made coulometric detection possible and thus opened the way for an effective electrode array detector. An example of the application of the detector to monitoring... 

Porous carbon electrodes were used as the anode and cathode and the operating temperatures varied from 700 to 920°C. A 98.8% removal efficiency of 0.65% H2S was reported at current densities around 35 mA/cm. ... 

The properties of active carbon render it a difficult material to use as an electrode. Electrochemical processes occur more often in the inner cavities (pore structure) of active carbon particles than on their outer, planar surface. For this reason, three-dimensional electrochemical activity is observed rather than the planar responses characteristic of solid carbon electrodes. It is generally a.s.sumed that the total area of the internal structure of the porous carbon electrode is completely wetted by electrolyte, although this may not be the case with high-surface-area carbons containing micropores inaccessible to electrolyte. The main difficulty is estimating the electrochemically active part of the total surface area of the active carbon electrode material. 

According to eq. 2 a constant current appears in the cyclic voltammogram (CV) when Q is plotted versus U. In real systems such as porous carbon electrodes, both load resistances due to the spatial distributed capacitance in the pores (circuit model in fig.l) and surface functional groups cause a deviation from the rectangular CV-shape. While the first induces a finite time constant in the charging process, the latter are identified by current peaks in the CV [14,6]. The voltage range used for cyclic voltammetry was -0.2 to 0.8 Volt vs.. g/, gCl at a scanrate of 5 mV/s, respectively. 

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. 

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