Numerical Analysis on Transport Properties of Electrochemical Cell via 3-D Reconstruction Technique

Abstract

The worldwide demand for environmentally friendly and efficient energy use has accelerated research on electrochemical systems using renewable energy. Fuel cells, which are electrochemical energy conversion devices that can generate electricity directly from hydrogen-based fuels with high efficiency with low pollutant emissions and are expected to replace conventional power supplies in a variety of applications. For their commercialization and expansion, the development of fuel cell systems and optimization of electrodes are necessary for cost reduction and performance improvement. Lithium-ion batteries (LIBs) are secondary batteries with high operating voltage and energy density and are being applied in various fields from portable devices to electric transportation. It is also necessary to understand the structural characteristics of the electrode and separator as a porous medium. The main objective of this study is to examine the effects of electrodes and separators for performance improvement in fuel cell and lithium-ion battery-based electrochemical systems. To achieve this goal, a computational fluid dynamics (CFD) model in the proton exchange membrane fuel cell (PEMFC) system was developed considering the correlation between the morphology of the catalyst layer of the fuel cell and the electrochemical reaction and the transport characteristics in the fuel cell. This model was applied to analyze how the microstructural properties of the electrodes contribute to the performance improvement in the PEMFC system. In addition, an electrolyte imbibition model in a quasi-static state was developed to predict the behavior of the electrolyte in the separator and the electrode of a lithium-ion battery, and the model was used to analyze whether the transport properties vary depending on the electrode structure and the interfacial characteristics of the electrolyte. Additionally, a three-dimensional (3-D) reconstruction technique was applied to capture the microstructure characteristics of electrodes and separators based on Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) tomography. In chapter 2, to investigate the effect of catalyst layers of PEMFC, a CFD electrochemical model considering transport properties of electrodes with two phase modeling is presented based on 3-D reconstruction technique for microstructural analysis of electrodes. Developing a fuel cell model with fundamental structural properties such as distribution of pore size, geometrical network of individual phase, and volume-specific interfacial area are critical in evaluating the accurate cell performance. Therefore, herein, by FIB-SEM tomography, 3-D microstructure of catalyst layers is reconstructed from two real-time samples: (i) high tortuosity humidifying catalyst layer (HTH CL) and (ii) standard catalyst layer. From the reconstructed microstructure, water imbibition behavior at different levels of capillary pressure is simulated and the effective transport properties such as gas permeability, gas diffusivity, surface area and water permeability are derived as well. By coupling the effective structural and transport properties, a two-dimensional (2-D) model is developed to predict the performances of the two CLs, at relative humidity (RH) levels of 20% and 100%. Since the effective transport properties are derived from real-time samples, this 2-D model is expected to have a greater accuracy in predicting the fuel cell performance. Finally, the mechanism of self-humidifying membrane electrode assembly (MEA) at lower and higher RH conditions (20 % RH and 100 % RH) is demonstrated as a function of liquid water saturation in the cathode CL and water dry-out in the anode CL Chapter 3 describes a comprehending the relationship between transport properties and microstructure characteristics in construction of Li-ion battery. Since electrolyte filling in separator and electrode is a decisive step that primarily governs the storage capacity, battery life cycle and safety, herein the study was extended to identify the quantity of residual gas in the separator/electrode microstructure for facilitating better wetting and infilling of electrolyte into the electrode and separator. Moreover, the effect of structural and transport properties with different graphite content (50 %, 60 %, and 70 %) in the electrode were investigated followed by contact angle (CA) effect with electrolyte in the separator and graphite is presented. From the results of the study, it is claimed that the graphite contents of anode and contact angle of electrolytes explicitly impact the transport properties, in addition the conductivity of the electrolyte diminishes with respect to the microstructure of separator and anode. In these studies, numerical analysis of transport properties for electrode and separator was developed by CFD electrochemical modeling and electrolyte imbibition modeling via 3-D reconstruction by FIB-SEM tomography. The development of this approach is expected to contribute to the optimization of the performance of electrochemical cells considering macro and microscopic microstructural parameters of electrodes and separators.