Aging in electrochemical double layer capacitors : an experimental and modeling approach

Electrochemical double layer capacitors (EDLC) are widely used in many different applications due to their high power capability, high cycle life, and mechanical robustness. Thus, they are an interesting option for applications with the need of high power at high current rates for a short time, e.g. regenerative breaking, peak shaving in combination with batteries or in uninterruptible power supply systems, e.g. as back up for pitch control in wind power plants. Theoretically, EDLCs store energy purely electrostatically. However, parasitic chemical reactions occur. These are predominantly temperature and voltage driven and lead to accelerated aging. In this thesis, the aging behavior of different commercially available EDLCs was investigated. Therefore, long-term accelerated ageing tests were performed, lasting up to more than three years. Calendric aging tests show a strong impact of cell voltage and temperature on EDLC’s lifetime. However, the temperature impact observed in this thesis is even higher than the impact presented in previous studies. Thus, high temperatures respectively hot spots in an EDLC module have even a more severe influence on aging than expected. Nevertheless, calendric aging results indicate very complex aging processes that cannot be easily explained by simple factors. Since EDLCs store energy electrostatically, it may be concluded that charging and discharging do not affect the EDLC’s lifetime. Within the cyclic aging tests performed in this thesis more than 1,000,000 cycles were carried out. However, by comparing the cyclic aging results with the expected calendric aging under the same conditions, it becomes clear that there is an additional impact on cyclic aging which has to be further investigated. Furthermore, cells in cyclic aging tests do not decrease in performance continuously but die a sudden death. This fact makes it very difficult to predict EDLC lifetime under cyclic operation. For a deeper understanding of the complex aging processes inside the investigated EDLC, post mortem analyses of selected aged cells were carried out. By means of laser microscopy and SEM images significant changes in the structure of both electrodes are observed. The negative electrode shows adhesion loss between activated carbon and current collector, whereas the positive electrode seems to be less porous at its surface. Impedance spectroscopy on coin cells, made from the aged electrode material, indicate that the negative electrode’s performance suffers from contact loss of active material, whereas the positive electrode dominates the poor dynamic behavior of an aged cell. Since high temperatures shorten EDLC lifetime significantly, optimized system design and cooling strategies must be considered in applications. An electro-thermal module model was developed in this thesis that allows simulating electrical performance and thermal behavior of various module configurations. Although, the ageing mechanisms are very complex and have to be further investigated, a first approach was done to enlarge this model to an ageing model. This model is parameterized by the results of the accelerated ageing tests and is able to represent the measured capacitance and resistance characteristics during ageing. Such a model is a useful tool for lifetime prediction under the considered operating conditions and helps to improve system design