Effect of the pore size variation in the substrate of the gas diffusion layer on water management and fuel cell performance
Introduction
As the problems of air pollution and energy starvation have been recently issued, fuel cells draw attention as promising future power sources due to zero emissions, high efficiency, and abundant resource of hydrogen. Especially, the proton exchange membrane (PEM) fuel cell has the characteristics of low operating temperature, quiet operation, rapid start-up, and shut down; therefore, it is expected to be applied to the powertrain of next generation vehicles or stationary power plants. However, water management in a PEM fuel cell is important for stable performance and durability of fuel cell. Thus, water management must be solved before PEM fuel cell can be commercialized [1], [2], [3], [4], [5].
The gas diffusion layer (GDL) is a crucial part of the fuel cell system. The GDL acts as a pathway of reactant gases and liquid water, electric conductor, thermal conductor, and mechanical damper. In particular, it acts as a path of two-phase mass transport; hence it is no exaggeration to state that the GDL is a key component in solving the water management problem [6], [7], [8], [9], [10].
Research studies of GDL have been attracting attention for a decade, due to the significant role of the GDL [8]. In particular, studies regarding the passage design of GDL to enhance mass transport ability are required. However, the previous research studies have limitations that many of research studies changed only the hydrophobicity in the GDL by controlling the amount of poly-tetrafluoroethylene (PTFE) to enhance the water removal ability of GDL [11], [12], [13], [14], [15], [16]. This approach is a passive means of water management. In addition, investigation of the effect of the substrate layer type, such as carbon cloth, felt, and paper, was reported in the past instead of the design the passage of the GDL in a direct way [17], [18], [19]. Currently, the focus of the research is on designing the passage of the GDL in an active way. Most of these attempts are related to MPL design because the change of the MPL characteristics is simpler than that of the substrate characteristics [10], [20], [21], [22], [23], [24]. In contrast, the attempts to design the structural characteristics of the substrate layer have been rarely reported in the literature. Zhang et al. [25] developed a metallic GDL that can control the porosity. The major difference between conventional GDLs is that it does not have a randomly formed pore structure but instead has a straight-pore structure. Furthermore, Cho et al. [26] introduced the activated carbon fibers and controlled the pore-size distribution of the substrate layer. They developed GDLs that have small macro-pores and large macro-pores and investigated the water balance inside of the GDL. Oh et al. [27] introduced the pore size gradient GDL to control the local capillary pressure gradient inside of the GDL and found that the pore size gradient structure is advantageous for the improvement of the fuel cell performance under various relative humidity (RH) conditions.
However, these previous studies could not directly connect the GDL characteristics to the mass transport behavior with fuel cell performance. Only qualitative hypotheses were adopted to explain the effect of the GDL characteristics on the fuel cell performance. Furthermore, many other experimental studies [28], [29], [30], [31], [32], [33], [34], [35], [36], such as neutron imaging, synchrotron X-ray, and optical photography, focus on liquid water visualization; however, these studies have limitations regarding the spatial and temporal resolution of liquid water behavior; in addition, these studies could not explain the correlation between the GDL structural design parameter and the fuel cell performance. Furthermore, numerical studies, e.g., based on the pore network model and the lattice Boltzmann model [37], [38], [39], [40], [41], [42], only focused on the GDL itself, making them useful for investigating the mass transport characteristics with microscopic view. However, these models also are unable to directly connect the mass transport behavior of the GDL with the cell performance due to difficulties of the extension of the calculation domain. Because the GDL characteristics, such as porosity, pore size, contact angle, and thickness, mainly affect the two-phase mass transport inside of the GDL, elucidation of the quantitative analysis of the mass transport characteristics as well as the fuel cell performance are required. In particular, the capillary pressure gradient, which is a driving force of liquid water inside of the GDL, and diffusion coefficient (according to liquid water saturation of each GDL part) should be analyzed to evaluate the active passage design of the GDL.
The objective of this study is to investigate the GDL structural characteristics involving different pore sizes in the substrate using experimental tests and simulations. The experimental results, such as the dynamic and steady-state performance of the fuel cell, are elucidated by the simulation results via analysis of the two-phase mass transport inside of the GDL. First, the basic characteristics of the GDLs, which have two different pore sizes in the substrate, were analyzed. Through SEM image analysis and porosimetry and water permeability tests, the porosity, pore size, internal contact angle, and thickness of the GDL were determined. In particular, the internal contact angle, which reflects the structural characteristics of the GDL, was measured instead of the surface contact angle. It is difficult to measure an accurate surface contact angle due to the rough surface of the substrate and the effect of the water droplet weight. After analysis of the basic GDL structural characteristics, the values were adopted in the model. The model used in this study was developed by MATLAB®–Simulink®. One of the features of this study is that the PEM fuel cell model is based on macro-scale two-phase flow equations with Darcy’s law and the concept of macroscopic capillary pressure and relative permeability. The model is simple and is able to capture dynamic characteristics extended to the cell performance. The model simulates the liquid water saturation in each part of the GDL and determines the cell performance based on the two-phase mass transport through the GDL. After validation of the simulation results with the experimental data, it is possible to elucidate the cell performance behavior according to structural characteristics of the GDL. The ingenuity of this work presents the quantitative effect of GDL structural feature on the cell performance with the change of water saturation profiles and simulated activation and ohmic overvoltages.
Section snippets
GDLs
To control the pore size in the substrate, two types of carbon fibers, whose lengths are 13 mm and 6 mm, were used. As shown in Fig. 1, relatively large macro-pores are formed when long carbon fibers are used. Conversely, short carbon fibers form relatively small macro-pores. Given this mechanism, two types of GDLs with a controlling portion of each fiber length were designed. The two GDLs have nearly the same areal weight. However, the weight portion of the two different carbon fiber lengths is
Model description
In this study, a dynamic, non-isothermal, and quasi-three-dimensional model of the PEM fuel cell was developed. The model used in this study was developed by modifying the previous model [46], [47] to consider the structural characteristics of the GDL and two-phase mass transport. For simplicity of the model, the unit cell was first discretized in the through-plane (cross-sectional) direction (z), and then the model was arranged along the in-plane (stream-wise) direction (x, y). Using this
Analysis of the FE-SEM images
To investigate and validate the structural characteristics of two samples, the images of FE-SEM were analyzed. The surface FE-SEM images of S8L2 and S2L8 are shown in Fig. 5. The magnifications of the images are 50× and 100×. The macro-pores of the substrate are formed between fibers. In the figure, the short carbon fibers of length of 6 mm overlapped more frequently than of the long carbon fibers of length of 13 mm, i.e., relatively small macro-pores are formed in S8L2 with short carbon fibers,
Conclusions
In this study, the structural characteristics of the GDL were investigated using both experimental work and simulation work. The structural parameters of the GDL from the experimental work were used to refine the developed model. As a result, the model is able to predict the effects of the various structures of the GDL on water management and fuel cell performance. Specifically, the structural characteristics of two GDLs, which have different macro-pore sizes in the substrate, were analyzed.
In
Acknowledgments
This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20143010031880), and SNU Institute of Advanced Machines and Design (IAMD).
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