Mathematical modeling of electrocatalytic layers in high temperature polymer electrolyte membrane fuel cells

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Date

2023-05

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Γιωτάκος, Παναγιώτης

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Abstract

In today’s modern era, the need for decarbonisation of energy production is imperative for the sustainable development and prosperity of human society. The development of phosphoric acid doped polybenzimidazole (PBI) electrolyte membranes in 1995 allowed the operation of Polymer Electrolyte Membrane (PEM) fuel cells (FC) at temperatures higher than 100◦C. This led to the development of High Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFC), one of the most potent electrochemical devices for efficient clean power production. Since then, there has been extensive work done on developing and optimizing HT-PEMFCs for improved performance, extending the lifetime of operation, reducing manufacturing costs and most importantly, increasing the understanding of the electrochemical processes occurring during operation. The state-of-the-art HT-PEMFCs can reach operating power densities up to 800 [mW/cm^2] with a maximum efficiency as high as 80%, when configured for Combined Heat and Power (CHP) generation.The aim of this dissertation is to provide a deeper understanding of the electrochemical processes inside the catalytic layers of the two electrodes, focusing especially on the cathodic electrode, where Oxygen Reduction Reaction (ORR) takes place. The ORR mechanism has been under the microscope for more than five decades, since it is solely responsible for introducing the largest potential losses, which in order to mitigate require high amounts of expensive Pt catalyst loadings. To this end, mathematical modeling (both 3D & 1D numerical and analytical) is successfully coupled with two electrochemical characterization methods; the steady state polarization (IV) curve characterization method and the Electrochemical Impedance Spectroscopy (EIS) characterization method. Various parameters that affect the performance of HT-PEMFCs are studied by applying IV-EIS, however the main contribution of this thesis comes from unraveling the sluggish kinetics and energetics of the so far elusive ORR. We performed meticulous/detailed experimental steady state (IV) and EIS measurements in the activation region of operation (where the ORR is complete) by carefully controlling the experimental operating conditions, in order to avoid mass transport limitations and excessive water production, which would otherwise severely affect the structure of the Cathodic catalytic layer (CCL). By experimentally observing that the limiting current density of the FC was controlled by kinetics (instead of mass transport), we employed the simplest ORR mechanism (the 4e− Dissociative Pathway) using microkinetic modeling, based on the Transition State Theory (TST). For the representation of the CCL a macro-homogeneous Transmission Line Model (TLM) was used to derive analytically both the IV and EIS, using only two somewhat arbitrary (assumed) parameters i.e., the symmetry factor β = 0.5 and the Langmuir isotherm kinetics for the adsorbed species on thecatalyst surface. We were able not only to achieve excellent fitting results (using our in house developed Monte Carlo Least Squares Fitting Algorithm) with the experimental data (IV & EIS), but to successfully extract the ORR kinetics (such as: the kinetic constants, the double layer capacitance, the ionic conductivity of the CCL etc.) and calculate the energetics of the elementary steps, i.e. both the activation energies (kinetics) and the reaction steps’ free energies (thermodynamics), bridging the gap between Density Functional Theory equilibrium analysis and electrochemical kinetics dynamic analysis. The excellent fitting results revealed that: i) all charge transfer reaction steps appear under the same high frequency arc, directly related to the Cdl of the electrochemical interface, while the low frequency arc originates from the ORR kinetic inertia, ii) both EIS and polarization resistance are dominated by the intrinsic ORR kinetic inertia, due to the competitive nature of the elementary reaction steps on the coverages of the adsorbed species (Oad and OHad), iii) with the help of the Degree of Rate Control (DRC) analysis, we identified O2(g) dissociative adsorption as the main limiting step responsible for the introduction of high ORR kinetic and thermodynamic overpotentials, iv) the independent calculation of H2(g) crossover current density from the anode side, was in excellent agreement with our model predictions and proved that hydrogen crossover is solely responsible for the deviation of the experimental open circuit potential from its thermodynamic Nernst value. Last but not least, the contributions of this thesis not only enrich the scarce literature of successfully extracting kinetic and energetic mechanistic information of an actual HT-PEMFC but also provide a guide, a methodology for screening and optimizing electrocatalysts for multistep electrochemical reactions.

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Mathematical modeling, High temperature polymer electrolyte fuel cells (HT-PEMFCs), Oxygen reduction reaction (ORR), Polarization curve (IV), Electrochemical impedance spectroscopy (EIS), Degree of rate control (DRC), Transmission line model (TLM), Transition state theory (TST), Kinetic inertia, Reaction mechanistic analysis (RMA), Microkinetic modeling, Numerical modeling, Analytical modeling, Hydrogen pumping, CO stripping voltammetry, X-ray diffraction (XRD), Activation energy, Thermodynamic overpotential, Kinetic overpotential, Kinetic limiting current density, Relaxation impedance, Charge transfer

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