Direct Membrane Deposition with TiO2 Reinforced Nafion® for Medium Temperature – Polymer Electrolyte Membrane Fuel Cells

Abstract

Medium Temperature – Polymer Electrolyte Membrane Fuel Cells (MT-PEMFCs) operate at temperatures between 80 °C and 120 °C. They show various advantages compared to traditional low temperature PEMFCs operating up to 80 °C: The management of liquid water is simplified, the electrode kinetics are enhanced and the tolerance to impurities contained in the reactant stream is increased [1–6]. However, huge deficits in terms of cell performance and durability to date hinder the application of MT-PEMFCs in the field of hydrogen mobility [7]. We present a novel manufacturing method called ‘direct membrane deposition’ [8] in combination with TiO2 reinforcement which enables well performing MT-PEMFCs. The direct deposition of the reinforced membrane is realized by drop casting a dispersion of Nafion® and TiO2 nanoparticles onto anode and cathode gas diffusion electrodes. The fuel cell is assembled with the two membrane layers facing each other, completely substituting the commonly used membrane foil. A scheme of the membrane electrode assembly (MEA) fabricated for this work is depicted in Figure 1 a). MT-PEMFCs constructed this way allow stable fuel cell operation at 120 °C with a maximum power density of 2.0 W/cm2 (H2/O2; 0.5/0.5 L/min; 300/300 kPaabs., 90% RH). With increasing temperature from 80 °C to 120 °C, the membrane resistance of the TiO2 reinforced directly deposited membrane increases by only 10 % whereas the membrane resistance of a non-reinforced directly deposited membrane increases by 54 %. At 100 °C (120°C) the maximum power density of the fuel cell with directly deposited TiO2 reinforced membrane is 27 % (9 %) higher than the maximum power density of our reference system. The corresponding polarization curves are depicted in Figure 1 b). The reference fuel cell was manufactured with a Nafion® HP (DuPont) membrane, which, to our knowledge, is the thinnest commercially available reinforced membrane. As the main reason for the higher power densities compared to a state-of-the-art Nafion® HP membrane a lower membrane- and charge transfer resistance is found. In this work we show that TiO2 reinforcement has proven to effectively stabilize the membrane resistance of directly deposited Nafion® membranes at elevated fuel cell operation temperatures. Surprisingly, MT-PEMFCs constructed this way are able to outperform even state-of-the-art Nafion® HP membranes. Figure 1 : a) Illustration of the membrane electrode assembly (MEA) fabricated in this work. A thin TiO2 reinforced Nafion® layer is deposited directly on both anode and cathode gas diffusion electrodes. A thin subgasket prevents hydrogen and current crossover through the end faces of the active area. b) Shows a comparison of the polarization curves for a directly deposited membrane (DDM) to a commercial Nafion® HP (DuPont) membrane. For each membrane the current density characteristics power density is evaluated at 100°C (blue dashed curves) and 120°C (red curves). The operation conditions were: H2/O2; 0.5/0.5 L/min; 300/300 kPa, 90% RH. Acknowledgements This work was funded by the German Federal Ministry of Education BMBF within the project “Gecko” (03SF0454C).