Electrochemical MEA Characterization: Area Specific Resistance Corrected to Fuel Utilization as Universal Characteristic for Cell Performance

Membrane-electrode-assembly (MEA) is a heart component of SOFC and defines the limits for power density, efficiency and durability available for exploitation in stacks and systems. MEA performance is often given as measure for possibilities of fuel cell technology. Most widespread way to communicate the MEA performance is to present the I-U-characteristic at a constant temperature and to estimate the area specific resistance from the slope of this curve. Other methods for characterization of performance like current density, impedance and local resistance measurement under defined operating conditions also have been used for this purpose. The fuel composition worldwide used for MEA tests (from dry hydrogen to H₂:H₂O=50:50) is connected with open circuit voltage which has great impact on measured performance. Moreover the realized fuel utilization strongly depends on operating conditions and varies in broad range. For these reasons it is difficult to compare results achieved by different researchers. It is beneficial to define unified value which would characterise electrochemical MEA performance. To resolve this challenge the voltage drop during the cell performance have been mathematically separated in two parts: (i) voltage drop due to change of Nernst voltage as a result of fuel utilization (and humidification) by current flow and (ii) voltage drop due to ohmic and polarization losses in the cell. This approach allows to separate the influence of non-linearity of Nernst voltage dependence from current on estimated cell resistance. It was found that at temperatures above 750°C the area specific resistance of the cell corrected to the Nernst voltage drop (fuel utilization) is a constant value, which only weakly depend on fuel composition. Favourable operating conditions as well as influence of operating conditions for estimation of cell resistance will be presented.