An extensive treatment of the agglomerate model for porous electrodes in molten carbonate fuel cells-I. Qualitative analysis of the steady-state model

The agglomerate model for porous electrodes in molten carbonate fuel cells (MCFC) is subjected to an extensive analytical study. It is shown that for cathodes in MCFCs the agglomerates may be viewed as equivalent diffusion volumes instead of real geometric representations. In this view the shape of the agglomerates (eg cylindrical or planar) is less relevant. As a consequence, this paper considers the easiest-to-handle shape of semi-infinite slabs. Using analytical mathematical tools, the model is shown to be mathematically and physically well-posed. Indications for optimal electrode thickness and agglomerate size are given, based on general problem properties and analytic solutions for special cases. Of all process parameters a variation of the ionic conductivity was shown to have the largest impact on the overall polarization. The impact of the exchange current density is comparable with or even larger than found for the diffusion coefficients, even though the electrode process is probably diffusion-controlled. The diffusion coefficients have to be increased simultaneously for a significant improvement of performance. While the exact nature of the agglomerates is not very well defined, the predicted polarization depends strongly on the width that is chosen. This is an important disadvantage of the agglomerate model.