New Materials and Flow Field Design for Middle-Temperature Direct Methanol Fuel Cell with Low Cathode Pressure V. Gogel 1 , M. Sakthivel 2 , J. Bender 3 , B. Salzmann 3 , J. Kerres 3 , J. Scholta 1 , J.-F. Drillet 2 * 1 Zentrum fu ¨r Sonnenenergie- und Wasserstoff-Forschung (ZSW), Helmholtzstr. 8, 89081 Ulm, Germany 2 DECHEMA-Forschungsinstitut (DFI), Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany 3 University of Stuttgart, Institute of Chemical Engineering (ICVT), Boeblingerstr. 78, 70199 Stuttgart, Germany Received November 29, 2018; accepted May 02, 2019; published online ¢¢¢ Abstract A partially fluorinated polyether ionomer (SFS) has been prepared and blended with partially fluorinated polybenzi- midazole (F 6 -PBI), yielding acid-base blend membranes with different weight ratios of the blend components. Best mem- branes in terms of conductivity and mechanical stability were characterized towards their solubility in methanol at high pressure and temperature as well as their ex situ and in situ conductivity. Optimized acid-base SFS-PBI blend membrane and carbon-supported Pt 3 Pd cathode catalyst were then tested in several DMFC cell designs. The results from preli- minary membrane electrode assembly (MEA) tests turned out to be completely scalable from 25 cm 2 up to 100 cm 2 active area in a single-cell configuration set-up. During ulti- mate test in a 100 cm 2 4-cell DMFC stack at 130 °C, a maximal power of about 60 W was yielded with methanol feed of 2.8 bar and air feed close to ambient pressure. This was possi- ble due to repeated reduction of channel cross-section geome- try combined with forced convection effects that led to enhanced oxygen transport at rib/GDE interface and higher catalyst utilization in starve zones. Keywords: Cation-exchange Polymer-PBI Blend Membrane, Flow Field, Low Pressure Cathode, Mesoporous Carbon Sup- port, Middle-temperature DMFC, Partially Fluorinated Mem- brane, Pt 3 Pd Alloy, Sulfonated Poly(arylene)s Blend Mem- brane 1 Introduction Hydrogen fuel cells are attractive technologies for energy conversion and storage [1]. However, safe storage, transporta- tion and distribution of hydrogen are still problematic. To satisfy the demand for e.g. portable power generators, small on-board APUs in campervans, boats and lighting, huge research efforts have been done to develop more practicable direct methanol fuel cells (DMFC) technology in the last past three decades. Methanol as liquid fuel is easier to handle than gaseous hydrogen and has a twofold energy density com- pared to that of compressed hydrogen at 700 bar (19 MJ L –1 vs. 9 MJ L –1 ). However, despite an approximate 10 times higher Pt catalyst loading, cell power density of DMFC is still by at least a factor 10 below that of a hydrogen polymer electrolyte fuel cell (H 2 -PEMFC). This is mostly due to sluggish activity of PtRu for the methanol oxidation and methanol permeation through the polymer membrane. Since the discovery of per- fluorinated Nafion â in the late 1960 [2], considerable works focused on improving methanol Nafion-based composite membranes [3–5] and developing cheaper products. Nafion is a commercially available perfluorinated mem- brane which is mechanically and chemically very stable, but has wide proton-conducting channels which allow methanol crossover from anode to cathode [6]. As a consequence, addi- tional methanol oxidation reaction (MOR) occurs at the cath- ode and led to so-called mixed potential formation and furthermore to reduced cell performance and fuel efficiency [3, 7]. Nonetheless, fuel yields up to 80–90% can be reached in DMFC by correctly balancing methanol concentration with power density in order to significantly sink methanol concen- tration at the anode/membrane interface [8]. Therefore mem- branes with low methanol permeation rate, high ionic conduc- tivity and long-term chemical/mechanical stability are still required. Multitude of systems were synthesized and tested in – [ * ] Corresponding author, drillet@dechema.de FUEL CELLS 00, 0000, No. 0, 1–12 ª 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 ORIGINAL RESEARCH PAPER DOI: 10.1002/fuce.201800172