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Energies 2020, 13, 5198 2 of 11 slow electrochemical reactions that occur on both sides of the cell and methanol crossover from the anode to the cathode side [1,2]. In order to overcome these challenges, different approaches such as improving the cell performance through changes on the different components structure and materials have been studied [2–16]. Although the diffusion layers (DLs) do not have a direct contribution on the electrochemical reactions, they provide the reactants access to the catalyst layers (CLs), remove the products and heat, and electrically connect the catalyst layers to the current collectors and the mechanical support to the membrane electrode assembly (MEA). Therefore, the DLs should be electrically and thermally conductive and porous to allow the reactants transport and products removal and rigid to support the MEA, but should also have some flexibility to provide a good electrical contact. Carbon-based materials, such as carbon paper and carbon cloth, have been widely used as DL in pDMFCs, since they met all the DL requirements. Additionally, the DLs may have a single-layer structure or a dual-layer structure, where one layer is similar to the one of the single-layer, backing layer (BL), and the other is a microporous layer (MPL). The BL allows the reactants diffusion and give mechanical support to the DL, whereas the MPL allows a uniform distribution of the reactants on the catalyst layer surface, decreasing the contact resistance between the CL and the BL. Therefore, the MPL must be porous to allow the reactants access and the products removal and should be electrically and thermally conductive. It is commonly accepted that the structural parameters of the DLs that have a clear effect on the pDMFC behavior are its thickness (linked to the mass transport resistance), its porosity (related to the species transport), and its wettability and roughness (responsible for the droplet/bubble attachment on the DL). Therefore, DLs with different structures, thicknesses, porosities, permeabilities, and surface wettability will have different transport characteristics and will lead to a different fuel cell behavior [2–16]. Based on that, the aim of this work was to analyze the effect of the anode diffusion layer (ADL) characteristics (thickness, material, and structure) on the performance of a pDMFC using a low-cost MEA, through the reduction of the catalyst loading on the anode and cathode sides. The loadings used were 3 mg/cm2 Pt/Ru at the anode and 1.3 mg/cm2 of Pt at the cathode, instead of the usual 4 mg/cm2 at both sides [3]. Most of the work, concerning the experimental evaluation of the behavior of a pDMFC have been limited to the evaluation of the polarization curves, which are very useful in pointing out the different losses that negatively affect the fuel cell behavior, but fail on evaluating the impact of each one, as the information given by the polarization curves is a sum of the various losses. Contrariwise, the electrochemical impedance spectroscopy (EIS) measurements allow the determination/quantification of each loss that affect the fuel cell behavior [17–20]. Such evaluation signalizes the major parameters that have a negative impact on the cell performance, such as the methanol crossover, and provides useful information for its optimization [19,20]. In this work, EIS measurements were carried out as a complementary diagnostic tool, to evaluate the effect of using different ADLs, with different materials, thicknesses, and structures on the performance of a pDMFC. 2. Materials and Methods 2.1. Fuel Cell Layout and Experimental Conditions The cell used in the experimental studies has an active area of 25 cm2 and is composed by an acrylic end plate, an isolating rubber plate, and a stainless-steel current collector, with an open ratio of 41%, on both anode and cathode sides (Figure 1). The anode end plate has a reservoir of 12.5 cm3 and the cathode end plate an open window frame of 25 cm2. A 3-layer MEA was used, acquired to QuinTech, with a Nafion 117 membrane, and 3 mg/cm2 of Pt/Ru and 1.3 mg/cm2 of Pt as, respectively, anode and cathode catalysts. At the ADL, different carbon-based materials with different properties, such as DL structure, porosity, and thickness, were tested. Four of them were made of carbon cloth and were designated as CC (carbon cloth without MPL; thickness: 0.4 mm; porosity: 0.83), CC_TPDF Image | Anode Diffusion Layer Properties on Direct Methanol Fuel Cell
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