Local Characterisation of Solid Oxide Fuel Cells

Kurzfassung

Fuel cells offer high electrical efficiencies and the solid oxide fuel cell is particularly interesting because easily accessible fuels containing hydrocarbons like natural gas and reformates can be used. Nevertheless the desired high efficiency and high fuel utilisation lead to strong gradients in gas composition along the cell which can in turn lead to lower power densities and increased local degradation. To better understand the local distribution and its effects a combined experimental and modelling study was done. Segmented planar anode-supported cells were characterised in a setup with 4 x 4 segments and a detailed two-dimensional model representing this setup was developed and validated. The model was calibrated for hydrogen/nitrogen mixtures with various water contents as well as for reformate gases and internal reforming of methane. A very good agreement between the model and the experiments was achieved. Measurements with CO/CO2 were done to validate the CO oxidation in the model. Here the influence of very high fuel dilution could also be observed that led to a plateau-like behaviour in the current-voltages curves. This behaviour was attributed to the oxidation of nickel at the anode. Methane reforming leads to an additional variation in gas phase species along the flow path. A great part of the reforming process takes place at the nickel contact mesh and not at the anode of the cell which could be seen in comparison to experiments with a non-catalytic contact mesh. It was also shown that a dilution of the fuel with water or nitrogen can lead to a more homogenous distribution within the cell depending on control parameters. The model was used to asses the influence of cell parameters. A variation of the gas channels did not show a significant influence. A variation of the electrode thickness showed an increase of power density for thinner electrodes. The model was also used to asses the effect of the segmentation in comparison to a non-segmented cell. A non-segmented cell shows a more even distribution of voltage and a greater variance of current density while the gas composition along the flow path is similar for segmented and unsegmented cells. The validated model can be used further to estimate optimal and critical operating parameters. Through a variation of fuel flow rate and temperature the influence on gas composition along the cell and differences in fuel distribution at the side and in the middle of the flow field could be seen as well as its influence on cell performance. Higher load and higher temperatures lead to a stronger decrease of fuel along the cell. The segments located at the side of the cell showed a different performance than those in the middle, due to an unequal distribution of the fuel into the flow field. This influence increases with lower flow rates. During these experiments a correlation between poor local performance and electrolyte defects as well as local nickel oxidation at the anode was observed. It was also shown that extreme local conditions can lastingly damage the cell and local distributions have to be taken into account for stack development.