Liquid diffusion electrode

In the pores of the electrodes, practically no natural convection of the liquid takes place. Reactants dissolved in the liquid can be supplied in two ways from the external surface to the internal reaction zones (and reaction products transported away in the opposite direction) (1) by diffusion in the motionless liquid diffusion electrode),... 

The macrokinetics of processes in gas-diffusion electrodes is analogous to that in liquid-phase electrodes. In calculations, one must take into account, however, that the electric current and the solute species will be carried only through that part of pore space which is electrolyte filled, whereas gas supply is accomplished primarily not by diffusion through the liquid but by flow in the gas channels. 

The beginning of modeling of polymer-electrolyte fuel cells can actually be traced back to phosphoric-acid fuel cells. These systems are very similar in terms of their porous-electrode nature, with only the electrolyte being different, namely, a liquid. Giner and Hunter and Cutlip and co-workers proposed the first such models. These models account for diffusion and reaction in the gas-diffusion electrodes. These processes were also examined later with porous-electrode theory. While the phosphoric-acid fuel-cell models became more refined, polymer-electrolyte-membrane fuel cells began getting much more attention, especially experimentally. 

Studies by Sedlak (j6) have shown a similar response-flow relationship for liquid electrolyte cells which utilize a teflon-bonded diffusion electrode. The empirical equations and relationships derived generally apply to the SPE sensor cells. 

Figure 18.4—Liquid membrane electrode and gas diffusion electrode. The gas diffusion electrode is constructed in part from a pH electrode that is plunged into the internal solution.

When the electrode is completely immersed in the electrolyte solution, only a two-phase interface (i.e., liquid-solid) is present in the electrode structure. In form it may be either a consolidated powdered active carbon or a confined but unconsolidated bed of carbon particles. These are u.sed for flow-through porous electrodes in many electrochemical systems. The other mode of operation is the gas-diffusion electrode, in which the electrode pores contain both the electrolyte solution and a gaseous phase. Numerous publications [29-31] have reported on a theoretical analysis of flow-through porous electrodes and gas-diffusion electrodes, which takes into account the physicochemical characteristics of carbon electrode materials. There does not seem to be a uniform explanation for the effects of structural and chemical heterogeneity in carbons. 

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. 

The design of an optimum interface is strongly dependent on the pore structure of the active layer of the gas diffusion electrode. According to Fig. 6, the liquid electrolyte has to penetrate into the pores and to wet the pores, so that a thin layer of electrolyte covers the pore wall (low contact angle). This electrolyte film should be as thin as possible to allow a short diffusion path for the reactant gases to exist. A high solubility of the reactant gases in the electrolyte film is also favourable. 

Considerable changes are needed in the anodic part of the membrane-electrode assemblies in order to accommodate the first two of the above-mentioned points. Instead of the porous gas diffusion layer that in polymer electrolyte membrane fuel cells ensures a uniform distribution of hydrogen across the surface, a gas-liquid diffusion layer that contains a set of hydrophilic as well as a set of hydrophobic pores is needed here. Through the hydrophilic pores, this layer must secure the unobstructed access of the aqueous methanol solution to the reaction zone and its uniform distribution. Through the hydrophobic pores, this layer must secure the unobstructed elimination of carbon dioxide, as the gaseous reaction product, from the reaction zone. Analogous changes must be made in the catalytically active anode layer of the membrane-electrode assemblies, where the gas is actually formed, and must be removed toward the gas-liquid diffusion layer. 

S. Nakamatsu, N. Furuya, K. Saiki, H. Aikawa, and A. Sakata, Liquid-Permeable Gas Diffusion Electrode for Chlor-Alkali Membrane Cell. In H.S. Bumey, N. Furuya, F. Hine, and K.-I. Ota (eds.), Chlor-Alkali and Chlorate Technology R.B. MacMullin Symposium, Proc. vol. 99-21, The Electrochemical Society, Pennington, NJ (1999), p. 196. 

Beming et al. [58] performed a parametric study using their previously described singlephase, three-dimensional model [55]. The effect of various operational parameters such as the temperature and pressure on the fuel cell performance was investigated. In addition, geometrical and material parameters such as the gas diffusion electrode thickness and porosity as well as the ratio between the channel width and the land area were investigated. It was found that in order to obtain physically realistic results experimental measurements of various modeling parameters were needed. In addition, the contact resistance inside the cell was found to play an important role for the evaluation of impact of such parameters on the fuel cell performance. The impact of liquid water on transport in the gas-diffusion electrode was, however, not account for. 

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