NO/NO2 Contamination of HT-PEM Fuel Cells – A Degradation Study with Evaluation of Regeneration Strategies

Abstract

In view of a sustainable energy supply, polymer electrolyte membrane fuel cell technology is used in several stationary and mobile application fields of power generation. High temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are operated at increased temperatures of around 160 °C to achieve a simplified water and heat management and to enhance the tolerable CO concentration in fuels. HT-PEMFCs are especially attractive in combination with methanol reforming in combined heat and power systems for auxiliary power units and further application fields. [1]Air pollution is a world-wide problem, whereby especially in urban areas ambient air contains nitrogen oxides (NOx) mainly originating from traffic. It has been shown in several studies that such NOx contamination affects the performance of low temperature PEMFC systems. [2] NO/NO2 enters the cathode through the supply with ambient air and reversibly adsorbs onto the platinum surface leading to catalytic deactivation. Immediate regeneration of low temperature PEM cells is possible by supplying clean air. [3] Although the HT-PEMFC system accounts for an enhanced tolerance towards CO contamination, NO/NO2 has a significant impact on its cell performance. On one hand, cell degradation is not completely understood yet, and on other hand, cell regeneration is not given by simple switching to clean air as in case of LT-PEMFCs. [4]For this purpose, systematic NOx contamination of commercially available HT-PEM single cells was carried out followed by analysing performance changes in dependence of several cell operational modes. First, the cell start-up was performed for two days at 160 °C and constant load of 0.3 A cm-2. Second, a controlled contamination using 10 ppm NO or NO2 inside the cathode air stream was applied. Last, after significant decrease of the voltage different cell operation modes were applied and tested as regeneration strategies, while the voltage change was continuously monitored. Cell operation under open circuit voltage (OCV), N2/N2 supply to both electrodes, cyclic voltammetry (CV up to 1.0, 1.2 and 1.5 VRHE) and an increased temperature of 180 °C were utilized. To evaluate the cause of voltage changes, the cells were electrochemically characterized using polarization curves, electrochemical impedance spectroscopy, cyclic voltammetry and linear sweep voltammetry at begin of test, after operation with contaminated air and after switching to the defined cell modes.Figure 1a) shows the voltage over time of cell break-in followed by operation under NO2 contamination resulting in a significant degradation of -364 µV h-1. Figure 1b) shows the voltage monitored during applying the different cell modes after cell contamination. NO2 is verified to have a dramatic impact on the HT-PEMFC without any regenerating effect by any of the cell modes. In conclusion, this study demonstrates the strong impact of NO/NO2 on HT-PEMFC systems and furthermore points out the difficulty to regenerate the cell after contamination. This is an important evidence for industry that less a suitable cell operation strategy and more a system engineering approach in view of appropriate filter systems is one possible way of solving the NOx contamination issue.[1] R. E. Rosli, A. B. Sulong, W. R. W. Daud, M. A. Zulkifley, T. Husaini, M. I. Rosli, E. H. Majlan, M. A. Haque, Int. J. Hydrogen Energy 2017, 42, 9293-9314.[2] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J. Shen, J. Power Sources 2007, 165, 739-756.[3] A. Talke, U. Misz, G. Konrad, A. Heinzel, J. Electrochem. Soc. 2018, 165, F3111-F3117.[4] Final Project Report „HT-Kathodenluft“ 2016. Figure 1: Voltage and current density of cell break-in followed by operation under NO2 contamination (a) and voltage monitoring during applying different cell modes (b). Figure 1