연료 전지 전극 설계를 위한 거시 및 미시적 전산해석

Abstract

Increasing global energy demands and concern for the environment have been accelerating the research and development of electrochemical systems that can realize an efficient use of the renewable energy resources. Thus, fuel cells that directly convert chemical energy of fuel into electric energy are being considered as a potential power generator for stationary and transportation applications due to their high efficiency with low emission of pollutants. To promote their commercialization, it is necessary to develop system analysis and design technology for cost reduction and performance improvements. The main objective of this study is to obtain an insight into the influence of electrode design on the performance improvement in fuel cell-based electrochemical systems. In order to achieve this goal, a computational fluid dynamics (CFD) model was developed taking into account the transport phenomena in the fuel cells and the relationship between the electrochemical reaction and the morphology of electrodes. The model was then used to evaluate the electrode design parameters effects on the performance of solid oxide fuel cell (SOFC)-based systems. Furthermore, to characterize and analysis the microstructure of electrodes, three-dimensional (3-D) reconstruction technique was implemented using focused ion beam-scanning electron microscopy (FIB-SEM) tomography. Practically, this technique, which is applicable to the SOFC systems, was demonstrated on proton exchange membrane fuel cell (PEMFC) systems which can be validated by our previous experimental results. In chapter 2, a CFD electrochemical model considering theoretical morphology of SOFC electrodes is presented to account for the effect of grading, which affects both the active surface area and the effective diffusivity within the electrodes. The model was used to predict the cell polarization in porosity-graded and particle-size graded electrodes. As a result, the latter electrode exhibited better cell performance than the former electrode due to a larger active surface area. Detailed studies were also performed to investigate the effect of particle-size grading methods. Consequently, it can be confirmed that the overall cell polarization was mainly affected by lowered activation overpotentials when the particle-size grading was conducted. Chapter 3 describes a computational analysis of reversible electrochemical energy conversion (SOFC-based) system. An electrochemical modeling was applied for the simultaneous integration of different operating modes (namely, fuel cell and electrolysis cell modes) to enable more realistic predictions on the trade-off behavior of the influences of cell design parameters on the performance. Using the model, the effects of design variables (electrode thickness and interconnector rib size) on the current-potential characteristic and the round-trip efficiency were investigated. The predicted cell performance was significantly affected by the rib size, particularly when the electrode was thin, because of the uneven distribution of the reactant species. Overall, this chapter provides insights into key electrode design parameters that dominate the performance of reversible electrochemical cell. Chapter 4, 5 discuss a 3-D reconstruction method for microstructural analysis of electrodes. This approach allows a complete consideration of the electrode morphology, providing key pore-structural information (e.g. porosity, diffusivity and tortuosity). In chapter 4, the catalyst layer (CL) of PEMFC electrode was characterized by FIB–SEM tomography. The measured cross-section images were integrated to create the three-dimensionally reconstructed CL for calculating the microstructural properties. The structural properties obtained from the reconstruction were implemented into an electrochemical model to investigate their influence on the prediction of cell performance. In the predicted cell polarization, the reconstruction-based parameters resulted in maximum difference of 26% in the current density at 0.7 V. Consequently, it could be argued that the reconstruction method is crucial for the modeling and design of the electrodes to consider the effect of realistic microstructure. In chapter 5, to investigate the effect of CL structure on water transport in two-phase (gas and liquid) PEMFC system, the 3-D reconstruction technique was implemented on two CLs, namely conventional CL and nanosized dense-structured (NSDS) CL. The CLs were three-dimensionally reconstructed by FIB-SEM tomography, and the structural and transport properties were calculated at various water saturation levels of CLs. The results showed that the smaller pore size of the NSDS CL led to lower water permeability and lower saturation. In addition, the saturation-dependent structural and transport properties of CLs were introduced into a two-phase electrochemical model to confirm their influence on electrochemical model prediction. From the results, it was found that their effects were not negligible in the electrode modeling, in particular for the NSDS CL which has a complex nanostructure. In these studies, a computational analysis method for electrode design was developed by CFD electrochemical modeling and 3-D reconstruction of electrodes. It was demonstrated that this approach can be used as a guide for optimizing the performance of electrochemical cells with consideration of the macro and microscopic design variables of electrodes.