Multiscale Design & Dynamics of PEM Fuel Cell Systems
Polymer electrolyte membrane fuel cells (PEMFC) convert directly the energy of chemical substances, like hydrogen, into electrical energy which can be used in mobile or stationary applications. Currently, hydrogen is mainly produced from hydrocarbons via steam reforming and water gas shift reaction. During the reforming reaction, carbon monoxide (CO) is continuously produced as undesired by-product and must be removed because it acts as poison for the PEMFC anode catalyst. In the PSE group as an alternative to the classical catalytic preferential oxidation (PrOx) reactor, a membrane reactor for the electrochemical preferential CO oxidation (ECPrOx) has been investigated [1,3]. Therein, the energy produced by selective CO oxidation and undesired H2oxidation can be harvested partly as electrical energy. The fascinating feature of this ECPrOx reactor is the autonomous oscillatory behavior of the cell voltage at galvanostatic operation [1,3]. The oscillations arise from an anodic self-cleaning mechanism occurring during the selective CO oxidation in a certain window of operating parameters . PSE group has shown that oscillatory regime favors higher CO conversion and carbon dioxide selectivity . Furthermore, in cooperation with Prof. Hamilton Varela (U Sao Paulo, Brazil) autonomous cell voltage oscillations were for the first time reported for direct methanol fuel cell . This finding is in agreement with our investigations of the dynamic behavior of PEMFC fed with H2/CO gas mixtures [1,3] and confirms our theoretical explanations .
PSE group also developed a novel dynamic analysis method, concentration-alternating Frequency Response Analysis (cFRA). Conventional electrochemical impedance spectroscopy (EIS) sometimes fails to separately identify certain transport phenomena, occurring in PEMFC, due to similar time constants. Unlike EIS, cFRA uses periodic perturbations of the partial pressure of specific reactants (water and/or oxygen) at the inlet of the cathode of the PEMFC. As output variable either the current or the cell voltage are observed, corresponding to voltastatic or galvanostatic operation, respectively. Our theoretical study  has revealed the capabilities of cFRA for selective identification of different transport phenomena. In order to validate the theoretical findings, an experimental setup (Fig.1) able to generate simultaneous periodic inputs of oxygen and water partial pressure directly at the cathode inlet was developed. Furthermore, a procedure which allows decoupling of water and oxygen variations in the electrical output signal was implemented such that cFRA transfer functions for oxygen and water perturbations can be determined separately.
 Peña Arias, I. K., Hanke-Rauschenbach, R., & Sundmacher, K. (2017). Influence of the autonomous oscillations and the CO concentration on the performance of an ECPrOx reactor. Electrochimica Acta, 251, 602-612.
 Nogueira, J. A., Pena Arias, I., Hanke-Rauschenbach, R., Vidaković-Koch, T., Varela, H., & Sundmacher, K. (2016). Autonomous voltage oscillations in a Direct Methanol Fuel Cell. Electrochimica Acta, 212, 545-552.
 Kirsch, S., Hanke-Rauschenbach, R., Stein, B., Kraume, R. & Sundmacher, K. (2013) .The electro-oxidation of H2, CO in a model PEM fuel cell: Oscillations, chaos, pulses. J. Electrochem. Soc., 160(4), F436.
 Sorrentino, A., Vidaković-Koch, T., Hanke-Rauschenbach, R., & Sundmacher, K. (2017). Concentration-alternating frequency response: A new method for studying polymer electrolyte membrane fuel cell dynamics. Electrochimica Acta, 243, 53-64.