(496d) Multi-Scale Modelling of Low-Temperature Electrolysis of Steam in Molten Carbonate Cells

High-temperature electrochemical cells, such as solid oxide and molten carbonate cells, have been recently receiving attention for both electricity generation (fuel cells) and hydrogen or CO production (electrolysis cells). Different scales of models may be found in the literature. 0D models, which allow to describe the correlation between current density and voltage, can be useful to gain some insight on the main phenomena responsible for overpotential in a given cell; however, the results cannot be extrapolated to cells operating under different operating conditions or with different geometries. 1D models allow to describe activation and ohmic overpotentials, but not the effect of concentration gradients of gaseous species participating in the electrohcemical reactions. On the other hand 2D models allow to take into account the different electrochemical and transport phenomena determining the performance of a cell and, once developed, they allow to analyze the effect of changes in operating conditions. Obviously, the difficulty in developing these models goes hand-in-hand with their degree of completeness and versatility. It is interesting to note that 2D models for electrochemical cells are rare in the liteature, although models of similar degrees of complexity exist for chemical reactors in which no electrochemical processes occur. The difficulty in finding thorough models is more pronounced for electrolysis cells than for fuel cells.

Molten carbonate electrolysis cells (MCECs) are currently being considered for the high temperature electrolysis of steam; however, studies on these cells are also very scarce.The MCEC configuration studied consists of two porous electrodes, separated by a molten carbonate electrolytic mixture. Gas is fed to the the cathode and anode sides of the cells and flows through channels running parallel to the electrodes themselves. The model was employed to study the performance of the cell operating both at the high temperatures traditionally proposed for MCECs of approximately 600°C and at temperatures around 500°C. In terms of cell assembly, the main difference when operating at lower temperatures lies in the need to use a different mixture of carbonates, having a melting temperature lower than 400°C and that is therefore liquid under the operating conditions envisaged. Changing the composition of the electrolyte mixture has an effect on the ionic conductivity both in the electrolyte layer and within the porous electrode.

In the present work different models have been developed to analyze the behavior of a planar molten carbonate electrolysis cell. The results were compared with experimental data available in the literature. The 0D model allowed to determine that, under common operating conditions, the performance of these cells is limited by activation and ohmic overpotentials. To gain further insight, a 1D model was developed in which it was possible to distinguish between (i) the anodic and cathodic activation overpotentials and (ii) ohmic overpotentials due to ion transport in the electrolyte present in the cathode, anode, and electrolyte matrix, as well as electron transport in the electrodes. The results revealed that the overpotentials could be mainly attributed to activation at the anode and ohmic losses due to the resistance of ion transport within the electrolyte matrix.

The 2D model developed takes into account

1. Mass transport in the gas channels and in the porous electrodes;
2. Charge transport in the electrodes and electrolytes;
3. Heat transport throughout the cell.

With regards to mass transport, the phenomena considered where convection, diffusion and reaction. In the cathodic gas channel the possible occurrence of reverse-water-gas shift (rWGS) was considered. The electrochemical semi-reactions of hydrogen and oxygen production were accounted for in the cathode and anode, respectively. Charge transport by electrons was described in the solid fraction of the electrodes; whereas charge transport by ions was described in the liquid electrolyte, both within the electrode porosity and in the electrolyte layer itself. As for heat transfer, the non-uniformity of temperature is due to the heat effect of both the rWGS reaction and the electrochemical water-splitting reaction as well as that of Joule heating.

The 2D model explicitly accounts for concentration gradients of the gaseous species in the directions both parallel and orthogonal to the direction of the main gas flow. In so doing, it was found that the activation losses at the anode had been slightly overpredicted by the 1D model.

Overall, it was found that ohmic losses due to ion transport in the electrolyte matrix are high, and should be reduced to improve the performance of the cell. This can be done by acting on the geometric configuration of the cell, so to reduce the distance between the electrodes.

The present work presents, to the best of the authors' knowledge, the first report of multi-scale modelling of MCECs. The results have allowed to determine the factors limiting the behavior of these electrochemical cells and thus provide a guide to the design of more efficicient cells.