Effect of Model Fuel Impurities for Reformed Jet Fuels on the Hydrogen Oxidation at Platinum Based Catalyst under HT-PEMFC Conditions

Jet fuels like the Jet-A and Jet-A1 fuel for commercial aviation or the military JP-8 fuel are important fuels for many types of application. Recently the replacement of these fuels by synthetic drop-in fuels is being considered. As fuel cell based auxiliary power units are of interest in many of the applications using jet fuels for the main propulsion, the question arises if avoiding some of the impurities which need to be expected in reformed fossil jet fuels can enhance the operation of the fuel cells. To further investigate this question tests have been performed on catalyst and single cell level. Catalyst tests were performed with commercial 20 wt.% Pt/C catalyst from Johnson & Matthey using circular gas diffusion electrodes with about 1 cm diameter which were mounted in a special test cell allowing for inline mass spectrometry during the electrochemical characterisation (1, 2). Parts of the measurements were verified by single cell measurements in a commercial fuel cell test bed (Evaluator C050, Fuelcon AG, Germany). The tested MEAs were commercially procured from either Danish Power Systems (Denmark) or Advent (Greece). For the tests an accelerated stress tests based on start-stop cycling described elsewhere (3) was employed. Three types of impurities were considered each represented by one or two model substances. The first class of impurities are alkenes which can result from the reforming of hydrocarbon fuels of any composition. The effect of alkenes was studied using ethene as model substance. The second type of impurities is aromatic compounds which are contained in fossil jet fuels but not in all replacement fuels. The effect of aromatic compounds was tested with toluene. The last type of impurities is sulphurous compounds which are not prersent in most replacement fuels. Here two types of model substances were investigated. Beside hydrogen sulphide thiophene was tested in combination with toluene as it is both an aromatic compound and a sulphur organic compound. Gaseous impurities were tested using pre-mixed gases purchased from Linde-Gase Germany. Toluene and thiophene were added by bubbling the gas feed through a flask with the toluene solution containing thiophene at ambient temperature so that the saturation partial pressure off 2.9 kPa should have been established. The addition of small amounts of ethene two the hydrogen feedstock caused slightly increased currents in the catalyst testing. The MS data also showed a slight increase of the CO2 production at high potentials (cf. fig 1). Tests with Advent MEAs and hydrogen with 10 ppm of ethene confirmed that no effect became apparent during the test phase of 130 start-stop cycles (cf. fig 2). At low potentials toluene has no effect of the hydrogen oxidation at Pt/C catalyst. At high potentials the activity is however slightly reduced (cf. fig 3). This reduction could be attributed to the oxidation of the thiophene which was present in the toluene. This can be concluded from the observed reduction of the thiophene peak at m/z 83 in the mass spectrum at the relevant potentials shown in fig 4. The effect of H2S has already been studied by Schmidt and Baurmeister (4) as well as by our group (2, 5). On single cell level H2S concentration of 10 – 20 ppm can be tolerated for Pt/C anodes. In total sulphur remains to be the most critical impurity if it occurs at to high concentrations, smaller amounts even of sulphur-aromatic compounds can however be tolerated. The use of fuel cells should thus profit from the introduction of sulphur reduced drop in fuels. In the contribution further results of ongoing single cell tests will be reported. Acknowledgements Financial support by the German Federal Ministry of Defence, Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB) under contract E/E210/AF020/CF062 is gratefully acknowledged. The authors would like to thank Ms. Florian Jung for her support in electrode preparation. References: 1. C. Niether, M. S. Rau, C. Cremers, D. J. Jones, K. Pinkwart and J. Tübke, Journal of Electroanalytical Chemistry, 747, 97 (2015). 2. M. Rau, C. Cremers and J. Tübke, International Journal of Hydrogen Energy, 40, 5439 (2015). 3. M. S. Rau, A. Niedergesäß, C. Cremers, S. Alfaro, T. Steenberg and H. A. Hjuler, Fuel Cells (submitted). 4. T. J. Schmidt and J. Baurmeister, ECS Transactions, 3, 861 (2006). 5. M. Rau, C. Cremers, K. Pinkwart and J. Tübke, ECS Transactions, 64, 983 (2014). Figure 1