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Electrochemical pressure impedance spectroscopy (EPIS) has received the attention of researchers as a method to study mass transport processes in polymer electrolyte mem-brane fuel cells (PEMFC). It is based on analyzing the cell voltage response to a harmonic excitation of the gas phase pressure in the frequency domain. Several experiments with a single-cell fuel cell have shown that the spectra contain information in the frequency range typical for mass transport processes and are sensitive to specific operating condi-tions and structural fuel cell parameters. To further benefit from the observed features, it is essential to identify why they occur, which to date has not yet been accomplished. The aim of the present work, therefore, is to identify causal links between internal processes and the corresponding EPIS features.
To this end, the study follows a model-based approach, which allows the analysis of inter-nal states that are not experimentally accessible. The PEMFC model is a pseudo-2D model, which connects the mass transport along the gas channel with the mass transport through the membrane electrode assembly. A modeling novelty is the consideration of the gas vol-ume inside the humidifier upstream the fuel cell inlet, which proves to be crucial for the reproduction of EPIS. The PEMFC model is parametrized to a 100 cm² single cell of the French project partner, who provided the experimental EPIS results reproduced and in-terpreted in the present study.
The simulated EPIS results show a good agreement with the experiments at current den-sities ≤ 0.4 A cm–2, where they allow a further analysis of the observed features. At the lowest excitation frequency of 1 mHz, the dynamic cell voltage response approaches the static pressure-voltage response. In the simulated frequency range between 1 mHz – 100 Hz, the cell voltage oscillation is found to strongly correlate with the partial pressure oscillation of oxygen, whereas the influence of the water pressure is limited to the low frequency region.
The two prominent EPIS features, namely the strong increase of the cell voltage oscillation and the increase of phase shift with frequency, can be traced back via the oxygen pressure to the oscillation of the inlet flow rate. The phenomenon of the oscillating inlet flow rate is a consequence of the pressure change of the gas phase inside the humidifier and in-creases with frequency. This important finding enables the interpretation of experimen-tally observed EPIS trends for a variation of operational and structural fuel cell parame-ters by tracing them back to the influence of the oscillating inlet flow rate.
The separate simulation of the time-dependent processes of the PEMFC model through model reduction shows their individual influence on EPIS. The sluggish process of the wa-ter uptake by the membrane is visible below 0.1 Hz, while the charge and discharge of the double layer becomes visible above 1 Hz. The gas transport through the gas diffusion layer is only visible above 100 Hz. The simulation of the gas transport through the gas channel
without consideration of the humidifier becomes visible above 1 Hz. With consideration of the humidifier the gas transport through the gas channel is visible throughout the fre-quency range. The strong similarity of the spectra considering the humidifier with the spectra of the full model setup shows the dominant influence of the humidifier on EPIS.
A promising observation is the change in the amplitude relationship between the cell volt-age and the oxygen partial pressure oscillation as a function of the oxygen concentration in the catalyst layer. At a frequency where the influence of oxygen pressure on the cell voltage is dominant, for example at 1 Hz, the amplitude of the cell voltage oscillation could be used to indirectly measure the oxygen concentration in the catalyst layer.
Combined heat and power production (CHP) based on solid oxide fuel cells (SOFC) is a very promising technology to achieve high electrical efficiency to cover power demand by decentralized production. This paper presents a dynamic quasi 2D model of an SOFC system which consists of stack and balance of plant and includes thermal coupling between the single components. The model is implemented in Modelica® and validated with experimental data for the stack UI-characteristic and the thermal behavior. The good agreement between experimental and simulation results demonstrates the validity of the model. Different operating conditions and system configurations are tested, increasing the net electrical efficiency to 57% by implementing an anode offgas recycle rate of 65%. A sensitivity analysis of characteristic values of the system like fuel utilization, oxygen-to-carbon ratio and electrical efficiency for different natural gas compositions is carried out. The result shows that a control strategy adapted to variable natural gas composition and its energy content should be developed in order to optimize the operation of the system.