Polymer exchange membrane fuel cells modelingfrom kinetic models to water management

Resumen

Polymer electrolyte fuel cells, based on proton exchange membranes, also known as PEM fuel cells (acronym for polymer electrolyte membrane or proton-exchange membrane), constitute a real alternative for the generation of sustainable energy. Their versatility allows them to be used in mobile systems (from small portable electronic devices to vehicles) or in fixed stations (auxiliary power generators for buildings or small remote unattended stations). The biggest attraction of this type of fuel cells is that they can operate at low temperatures (0-100 C), which requires the use of catalysts based nobel metals such as platinum, which allows then even to start from freezing temperatures. The operation of a PEM fuel cell is based on the electrooxidation of the fuel at the anode and the electroreduction of the oxygen at the cathode. The anodic and cathodic reactions take place independently, with both electrodes separated by a polymer membrane that allows only the transfer of protons. This forces the electrons to travel through an external circuit generating an electric current. The proton conductivity of the membrane depends strongly on the water content of the membrane. For a correct operation of the cell, the electrochemical reactions and the water management appear therefore as fundamental aspects in its modeling and operation. Depending on the fuel used, there are two types of PEM fuel cells: those fueled with hydrogen and those fed with alcohols. Hydrogen fuel cells stand out because of their high power density, which makes them ideal for the automotive industry. When operating at full load, the performances of this type of batteries are dominated by the water management in the membrane. By contrast, direct alcohol fuel cells (DAFCs) generate currents that are significantly lower, making them a possible alternative to hydrogen fuel cells as power sources for small portable devices. However, DAFC batteries still have two major drawbacks: the slow kinetics of the electrooxidation of alcohol molecules and the crossover of water and alcohol from anode to cathode through the membrane. Both phenomena severely limit the performances of this type of fuel cells. The aim of this thesis is to study the influence of both electrochemistry and water management on the behavior of PEM type cells using simplified mathematical models. The theoretical framework that supports fuel cell modeling is presented first: this includes thermodynamic considerations, electrochemical reactions, mass transport, and membrane behavior are summarized. Next, a kinetic model for the description of the ethanol electrooxidation reaction in direct ethanol fuel cells is proposed. The kinetic model is then integrated into a 1D across-the-channel model for the anode of a direct ethanol fuel cell that is able to accurately predict the generated current and product selectivity data. The anode model is further extended to a full-length 1D Across-the-channel + 1D Along-the-channel model including a simple advective description for transport along the channels. In the second part of the thesis, an experimental and theoretical analysis of the water balance in a hydrogen fuel cell is performed. The testing campaign presented was conducted at the DLR-Stuttgart facility during a couple of quarterly stays during the years 2013 and 2014. The tests consisted in subjecting the operation of a segmented hydrogen cell to different relative humidity conditions of the feeding streams in order to identify stable working conditions at the lowest possible relative humidity at different temperatures. After the presentation of the experimental results, a global water balance model is proposed to correlate the operational stability of a fuel cell with the global water balance in the interior. After validating the global model with the experimental results, a parametric study is carried out to extract information about the dependencies of the stability frontier with the flow, relative humidity and stoichiometry of the feed currents for different operating temperatures.