On the Effectiveness Factor of Flow-Through Porous Electrodes

An effectiveness factor, Φ, was introduced to investigate the effects of different parameters on the utilization extent of a flow-through porous electrode operating for simultaneous reactions. A mathematical model helped to study the effects of ohmic resistance, mass transfer, kinetics, and bubble formation on the effectiveness factor for the main reaction (metal deposition taken as an example). This was presented as a set of dimensional and dimensionless groups. These groups are the following: total theoretical limiting current, I L; conductivity group, σ; kinetic ratio, I o; and the bubble group, Ψ. The results showed that Φ increases at higher σ, lower I L and I o, and higher Ψ (high Ψ means lower extents of bubble formation). The ohmic resistance pushed the reaction to a small layer at the front of the electrode, resulting in a situation in which the reaction is limited at the front of the electrode (higher polarizations) but operates at lower currents at the back of the electrode (lower polarizations). Improvement of Φ values at higher I L and/or I o (i.e., at higher rates) could be obtained by using higher values of σ. Because σ depends on structure, transport, and kinetic parameters, it could help to guide the design of the porous electrode. A simultaneous side reaction (hydrogen evolution reaction, HER) retarded operating the cell at higher currents and confined estimation of the limiting currents of the main reaction. The bubble formation accentuates the ohmic effects. A satisfactory agreement between the model predictions and the experimental data was obtained.