Dynamic Operation of HT-PEFC: In-Operando Imaging of Phosphoric Acid Profiles and (Re)distribution

Abstract

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...

Operando X-ray Tomographic Microscopy Imaging of HT-PEFC: A Comparative Study of Phosphoric Acid Electrolyte Migration

“…10 For better understanding of this phenomena, the influence of current density (see Table I) and effect of different membranes (see Table II When phosphoric acid is quantified, it refers to PA droplets in the macro pores of the GDL. The spatial resolution of the imaging (2.3 μm voxel size) prevents the segmentation of PA droplets in the pores of the MPL and the catalyst layer.…”

“…10,11,[16][17][18][19][20][21] In this work, operando X-ray tomographic microscopy (XTM) is used to image the electrolyte redistribution as a function of membrane material (conventional film casted m-PBI 22 and PPA processed p-PBI membranes 23 ), electrolyte loading and current density.…”

Correlating Electrolyte Inventory and Lifetime of HT-PEFC by Accelerated Stress Testing

“…The thickness of the membrane after cell assembly is approximately 100 μm. 4 During MEA preparation PA gets partially transferred to the electrodes and the membrane acid content defines the MEA beginning of life (BoL) acid content. The electrodes consist of Pt/Vulcan XC-72 supported platinum catalyst with a loading of 1 mg Pt cm −2 on anode and cathode, respectively, coated onto SGL 38 carbon paper gas diffusion layers (GDL) including a microporous layer.…”

“…The concentration of the water/phosphoric acid mixture was subsequently analyzed by ion chromatography (IC) (Metrohm 882 Compact IC plus System; Metrosepp A Supp 5 150 anion separation column). The IC system measures the HPO 4 2− concentrations in the range of 1-20 ppm (mg L −1 ) with an accuracy of ±1.5%, determined by periodically measuring standards. The loss rate of H 3 PO 4 from the MEA can be calculated as follows: ; the term corrects for the difference in molar mass between the two species), A the active area of the fuel cell and t the time the bottle was connected to the fuel cell.…”

“…We have recently demonstrated that PBI based membrane systems exhibit extensive electrolyte migration from cathode to anode under high current operation. 4 This was attributed to the high mobility of free hydrogen phosphate anions which carry part of the ionic current. While this work focuses on phosphoric acid loss by evaporation and its implication on lifetime and fuel cell performance, it cannot be excluded that the high PA mobility has an effect on electrolyte evaporation as it can influence the PA resupply and saturation of the electrodes.…”