Synchrotron based X-ray tomographic microscopy (XTM) is used for imaging and quantifying the redistribution of phosphoric acid (PA) in high temperature polymer electrolyte fuel cells (HT-PEFC) in-operando. The main focus of this work is the redistribution of phosphoric acid under dynamic load conditions. Therefore, two different load cycling protocols were applied and the transient redistribution within the fuel cell components was imaged. XTM, for the first time, revealed that the examined PBI based membrane system exhibits extensive electrolyte migration from cathode to anode under high current density operation. PA flooding of anode gas diffusion layer (GDL) and flow field channels occurred. Implications for technical applications and fuel cell degradation are discussed. Quantification of the migrated electrolyte is made by correlating in-operando grayscale values to ex-situ reference samples. High temperature polymer electrolyte fuel cells (HT-PEFC) are operating at temperatures up to 200• C using phosphoric acid (PA) doped polybenzimidazole (PBI) based membranes. This fuel cell technology has long been in the focus of research 1 due to advantages based on the higher operation temperature as compared to standard PEFCs, operating between 60-90• C. At the higher operating temperatures, the tolerance to fuel gas impurities increases significantly and operation with CO levels up to 3% and H 2 S up to 10 ppm can be achieved.2 This renders HT-PEFC especially suitable for stationary combined heat and power (CHP) applications, where a fuel processing unit can easily be thermally integrated and used for reforming hydrocarbon-based fuels without the need of additional gas clean-up. The advantageous characteristics of HT-PEFCs are also determined by the physico-chemical properties of the phosphoric acid electrolyte. First of all, PA has a low vapor pressure at these operating temperatures. This principally allows for long term operation without major electrolyte loss, an important aspect also specifically defined in the US Department of Energy's 2015 targets 3 (50'000 operating hours for stationary CHP systems). Phosphoric acid also exhibits excellent proton conductivity owing to a fast proton hopping mechanism. 4 Due to this inherent difference to PFSA-type membranes (water assisted shuttle mechanism), additional humidification is not necessary, which significantly reduces the complexity of HT-PEFC systems. However, only ca. 2 PA molecules per PBI repeating unit (PA/PBI) are directly interacting with the basic pyridinic nitrogen of PBI.5 However, PA doping levels are typically significantly higher, e.g., 5-10 for PA imbibed PBI films 6 and 20-40 for PBI membranes produced through the so-called poly-phosphoric acid (PPA) process.7 It can, therefore, be expected that the bulk of this phosphoric acid is more or less mobile within the molecular pores of the membrane. Hence, movement and redistribution of PA within the porous components (membrane, catalyst layer, micro-porous and gas diffusion layers) of the cell are expec...

Performance and durability of polymer electrolyte fuel cells are closely related to the water management. In gas diffusion layers (GDL), the presence of liquid water is associated with mass transport losses. For optimization of the materials, mechanisms and parameters influencing the water saturation have to be understood. Ex-situ water injection and withdrawal experiments, allowing for well-defined boundary conditions, have been performed with three different GDL materials, using X-ray tomographic microscopy to image the liquid water phase on the pore scale of the materials. The liquid saturation in the GDLs has been imaged as function of the capillary pressure. The results reveal that, due to the anisotropic structure of the GDLs, transport of water occurs mainly in the through-plane direction via parallel water paths. When the GDL is coated with a microporous layer (MPL), liquid saturation requires higher capillary pressure to overcome the MPL/GDL mixed region where pore and throat sizes are reduced and the water paths are restricted to the crack regions of the MPL.