Sensor technologyy for advanced aircraft fuel cell systems

Abstract

For aircraft fuel cell systems an early detection of errors and unflavorunfavorable ed (geschmacklos? Undisturbed, unhindered, unimpeded) operation conditions are needed to achieve sufficient reliability and life time. An important aspect of fuel cells in air planes is the operation with reformats reformate gases as fuel. The reformate composition can significantly varyies depending on the used fuel and the history and operation state of the reformer. Unfortunately, PEFC stacks reacts very sensitivelye on impurities in the fuel gas, whereasand the impurities reduce not only can reduce the electrochemical performance in a reversiblerecoverable way but also can may poisoning the catalysts irreversiblye. Therefore, a sensor technologyy for the gas composition and its impact of on the PEFC° system is necessary. For the analysis of the fuel composition various methods e.g. gas chromatography or mass spectrometry can be used, whereas the mass spectrometry allows faster measurements. The second part for the reformat analysis – the determination of the influence on the fuel cell – can be realized with a monitoring sensor cell which is exposed to the same gas composition through a branch pipe. The main problem challenge is the need that this is this cell should be reacts much faster with respect to on impurities in the reformate gas than the fuel cell system. Various compositions of the reaction layers and different gas diffusion layers have various properties and reacts in different ways with regard toon impurities in the fuel gas. This can be used to enhance the sensitivity of a sensor cell compared with the sensitivity of the stack. Consequently, thus such sensor cells can predict a decrease of the stack performance. In order to optimize the prediction of the impact of the reformate composition on a PEFC stack, various sensor cells with different MEAs must be used, whereas the different cells haveing different sensitivities for the various impurities. At the DLR a dry preparation technique for MEAs was developed which is very flexible for regarding the variation of the MEA composition, noble metal loading and structure., cConsequently MEAs with specific various electrochemical properties can be prepared which are adapted to the detection requirements. In addition, this technique allows the preparation of graded MEA structures with which allows preparing MEAs with locally different electrochemical performance. For on-line analytic and investigation of fuel cell processes tools for current density measurements were developed at the DLR. The combination of the current density measurement with graded MEAs allow to analyze the local activity or performance on the cell, which means, the performance and the influence of impurities in a cell can be determined depending on the local position on a segmented MEA related to various MEA compositions with different compositions in the segments. So a multi sensor fuel cellcontrol system can be realized in a single cell – reducing the space and weight requirements for thus analytic system which are important factors for aircraft applications. In a first version of a multi sensor cell 25 sensors in a single cell with 5 by 5 cm2 should be building up and will be tested with various gas mixtures.