Characterization of gas diffusion layer transport properties by limiting current approach
Introduction
Low-temperature proton exchange membrane fuel cells (LT-PEMFC) are a family of electrochemical devices which may be considered as a replacement of the internal combustion technology in a long-term strategy towards a carbon-neutral economy. Using hydrogen as a fuel they produce zero-emission electricity, providing versatile lightweight systems with high energy conversion efficiency. However, in space-constrained applications, such as automotive or airborne, fuel cell should operate at current densities >1 A cm−2 to keep the total size and weight of the powertrain low. This stipulates fast consumption of reacting gases and intensive production of water in porous electrodes of the fuel cell. Thus, for operation at high current densities, efficient mass transport of reactants/products is a prerequisite.
In many regards, a gas diffusion layer (GDL) governs the mass transport of the entire fuel cell. Therefore, many numerical [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] and experimental [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] studies focus on water management and oxygen transport in the GDL. When pores of fuel cell electrodes are filled with water during the load cycle, oxygen diffusion to the active catalyst sites slows down and the overall fuel cell performance dramatically drops. To tackle this problem a hydrophobic fine/coarse structure of the GDL has been proposed [22]. Different pore-size distributions of carbon paper and microporous layers (MPLs) develop a capillary pressure gradient, forcing water removal from the catalyst layer and keeping low saturation of GDL [1]. Nevertheless, the efficiency of such GDL structure in water removal, oxygen transport can be largely affected by increasing amount of narrow pores and larger tortuosity.
Studying the gas transport in the GDL provides important insights into the optimization of the electrode structure and modeling of fuel cell. Mostly, GDL gas transport is studied in an ex-situ Loschmidt cell [11,21,23], where pressure difference in two chambers separated by the sample allows to calculate the diffusion coefficient. Despite providing a reliable and fast way of diffusivity measurements, this method does not fully account for the processes that take place in the real fuel cell, when water generation, flow-field compression, and uneven thermal distribution should be considered. In this regard, in-situ measurements [17,19,20] of GDL can be more beneficial.
A limiting current approach [14,19,[24], [25], [26]] allows fast in-situ measurements of diffusion properties. This method provides insights into the oxygen transport process in the catalyst layer, which allows to estimate different contributions of the transport resistance [27,28]. Moreover, a study of the local oxygen transport resistance paves the way for the optimization of low-platinum catalyst systems [29]. Though the limiting current method has a great potential for the gas transport measurements, this approach is limited to a low concentration of oxygen due to the influence of produced water. This problem can be overcome by using the limiting current technique in a “hydrogen pump” regime [14,30]. In this technique, hydrogen is oxidizing and reducing without a water surplus, which provides better measurement accuracy.
In this work, we study commercially available GDLs with different structure of the carbon paper substrates and microporous layers. Diffusion properties of GDLs were estimated by the limiting current techniques. To account for water management in GDL, we relate the relative diffusivity measured at different oxygen concentrations to the values measured for hydrogen at moderate humidity. Then diffusion properties were related to the structure of GDLs, and fuel cell performance at different relative humidities was measured.
Section snippets
Sample preparation
Transport properties and fuel cell performance of GDLs were studied in-situ using home-prepared catalyst coated membranes (CCM). Cathode catalyst layer of CCMs was prepared by the ultrasonic spraying (ExactaCoat, Sono-Tek) of the catalyst ink on a FS-715-RFS membrane (15 μm; FuMA-Tech) and NR-212 membrane (51 μm; FuelCellStore) for the fuel cell and limiting current density measurements, respectively. Thinner membrane for fuel cell testing (FS-715-RFS) was used to show the real-world
Structure study
Fig. 1(a–e) show a cross-sectional view of different GDLs. All studied GDLs have distinctive fine/coarse structures made of carbon nanoparticles and carbon fibres, respectively. However, the thickness ratio between fine and coarse regions differs amongst studied GDLs. Freudenberg GDLs H15C14, H24C5 and H23C3, for example, have the smallest and relatively close to each other MPL/substrate ratios of 25/161, 30/228 and 40/245 µm/µm, respectively. In contrast, MB-30 and 29BC GDLs have larger ratios
Conclusions
In this work, we focused on the transport properties of different commercial GDLs. A study by electron microscopy and mercury intrusion porosimetry revealed important insights into the structure and morphology of studied systems. Detailed analysis of oxygen and hydrogen transport allowed to investigate water management – a critical parameter for high fuel cell performance. It was defined that in many regards GDL thickness is a decisive parameter, which affects transport properties. Thicker GDLs
CRediT authorship contribution statement
Yurii V. Yakovlev: Conceptualization, Methodology, Investigation, Writing – original draft, Writing – review & editing. Miquel Gamón Rodríguez: Formal analysis, Investigation, Validation. Yevheniia V. Lobko: Investigation, Writing – original draft, Resources. Maryna Vorokhta: Investigation, Methodology. Peter Kúš: Investigation, Methodology. Iva Matolínová: Resources, Funding acquisition, Writing – original draft. Vladimír Matolín: Resources, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge financial support from the grant No. CZ.02.1.01/0.0/0.0/16_025/0007414 OP VVV project PaC NG.
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