Elsevier

Applied Energy

Volume 171, 1 June 2016, Pages 200-212
Applied Energy

Effect of the pore size variation in the substrate of the gas diffusion layer on water management and fuel cell performance

https://doi.org/10.1016/j.apenergy.2016.02.132Get rights and content

Highlights

  • Water management capability is varied according to the GDL structure.

  • Pore size variation in the substrate affects the density of MPL and MPL penetration.

  • GDL with small macro-pores in the substrate has water retention capability.

  • GDL with large macro-pores in the substrate has water removal capability.

  • A tool that can be used to design the GDL structure is developed in this study.

Abstract

The gas diffusion layer (GDL) is a key component of a proton exchange membrane (PEM) fuel cell due to its role as a pathway for fuel, air, and water. GDL determines the mass balance and water management in the PEM fuel cell. Thus, an investigation of the optimal structural characteristics of the GDL with improved water management is important to ensure high performance of the PEM fuel cell. In this study, a PEM fuel cell model that considers the structural characteristics of GDL is developed, and the various the GDL structural characteristics are analyzed and validated. Specifically, the effects of pore size variation in the substrate of the GDL on water management and cell performance are investigated. Two GDL samples with different pore sizes are evaluated according to the porosity, pore size, thickness, and internal contact angle to understand the basic structural characteristics of the GDL. The GDL structural parameters with the basic characteristics are incorporated into the developed model. The cell performance is predicted to be relative to the two-phase mass transport inside the GDL. The characteristics of the micro-porous layer (MPL) and MPL penetration part are found to be affected by the variation in the macro-pore size of the substrate. The water management capability of the GDL varies with these differences with respect to water retention and removal characteristics. Under the condition of relative humidity (RH) 100%, the averaged saturation value of the MPL and MPL penetration part of the GDL with small macro-pore in the substrate is 18.8% higher than that of the GDL with large macro-pore in the substrate. The cell performance is also affected by the operating conditions of relative humidity and current load. The voltage of the GDL with small macro-pore in the substrate at the current density of 1.6 A/cm2 is 10.3% lower than that of the GDL with large macro-pore in the substrate under RH 100%.

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).

References (55)

  • C. Lim et al.

    Effects of hydrophobic polymer content in GDL on power performance of a PEM fuel cell

    Electrochim Acta

    (2004)
  • T. Ioroi et al.

    Influence of PTFE coating on gas diffusion backing for unitized regenerative polymer electrolyte fuel cells

    J Power Sources

    (2003)
  • Y. Wang et al.

    Elucidating differences between carbon paper and carbon cloth in polymer electrolyte fuel cells

    Electrochim Acta

    (2007)
  • S. Park et al.

    Effect of a GDL based on carbon paper or carbon cloth on PEM fuel cell performance

    Fuel

    (2011)
  • C.-H. Liu et al.

    Effect of carbon fiber paper made from carbon felt with different yard weights on the performance of low temperature proton exchange membrane fuel cells

    J Power Sources

    (2008)
  • F.-B. Weng et al.

    Experimental study of micro-porous layers for PEMFC with gradient hydrophobicity under various humidity conditions

    Int J Hydrogen Energy

    (2011)
  • H. Tang et al.

    Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

    J Power Sources

    (2007)
  • M. Manahan et al.

    Laser perforated fuel cell diffusion media. Part I: related changes in performance and water content

    J Power Sources

    (2011)
  • J.H. Chun et al.

    Development of a novel hydrophobic/hydrophilic double micro porous layer for use in a cathode gas diffusion layer in PEMFC

    Int J Hydrogen Energy

    (2011)
  • J. Cho et al.

    Effect of the micro porous layer design on the dynamic performance of a proton exchange membrane fuel cell

    Int J Hydrogen Energy

    (2014)
  • F.-Y. Zhang et al.

    Performance of a metallic gas diffusion layer for PEM fuel cells

    J Power Sources

    (2008)
  • J. Cho et al.

    Study on the performance of a proton exchange membrane fuel cell related to the structure design of a gas diffusion layer substrate

    Int J Hydrogen Energy

    (2014)
  • H. Oh et al.

    Effects of pore size gradient in the substrate of a gas diffusion layer on the performance of a proton exchange membrane fuel cell

    Appl Energy

    (2015)
  • J. Zhang et al.

    In situ diagnostic of two-phase flow phenomena in polymer electrolyte fuel cells by neutron imaging: Part B. Material variations

    Electrochim Acta

    (2006)
  • K. Tüber et al.

    Visualization of water buildup in the cathode of a transparent PEM fuel cell

    J Power Sources

    (2003)
  • D. Spernjak et al.

    Experimental investigation of liquid water formation and transport in a transparent single-serpentine PEM fuel cell

    J Power Sources

    (2007)
  • D. Kramer et al.

    In situ diagnostic of two-phase flow phenomena in polymer electrolyte fuel cells by neutron imaging: Part A. Experimental, data treatment, and quantification

    Electrochim Acta

    (2005)
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