Simulation of the operation of a direct ethanol fuel cell anode

Abstract

Direct ethanol fuel cells (DEFCs) have motivated more and more interest in recent years due to the intrinsic advantages of ethanol such as its low toxicity, renewability, and its easy production in great quantity by the fermentation from sugar-containing raw materials [1-3]. In the present work, a mathematical model is developed by using a Fortran model in order to predict the behaviour of a DEFC anode [4]. This model considers the mass transport in the whole anode compartment and proton exchange membrane, together with the sluggish reaction rates of ethanol electrooxidation and the ohmic resistance effects in the catalyst layer. The related issues of DEFCs, such as ethanol crossover and the anodic overpotential were the main subjects of the present simulation. The results showed that at low current densities and high ethanol concentrations, the ethanol crossover rate seriously affects the DEFC performance. This could be attributed to the following two reasons: (i) a large portion of ethanol fed to the anode is wasted due to ethanol crossover without any contribution to the electrical energy, (ii) a decreased cathode effectiveness due to the reduced catalyst's active sites resulting from the adsorption of permeated ethanol, the poison by the intermediate products of ethanol and flooding. Furthermore, it was found that the anodic overpotential and the reaction rate of ethanol electrooxidation in the catalyst layer are more sensitive to the effective protonic conductivity than to the effective diffusion coefficient of ethanol. This suggests that the void volume in the catalyst layer should be filled with ionomer to a large extent. In addition, the thickness of the catalyst layer affects the anodic overpotential. As the thickness increases, the anodic overpotential of a DEFC is reduced to a certain extent due to the fact that in this case it can provide more active sites. It is worthy to be noted that the simulation results and the experimental ones [3] are in good agreement.