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| Home > Publications database > Characterization of Phosphoric Acid Doped Polybenzimidazole Membranes |
| Home > Publications database > Characterization of Phosphoric Acid Doped Polybenzimidazole Membranes |
| Book/Dissertation / PhD Thesis | FZJ-2018-06977 |
2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-364-8
Please use a persistent id in citations: http://hdl.handle.net/2128/20404 urn:nbn:de:0001-2018121804
Abstract: A polymer electrolyte fuel cell (PEFC) is an energy converter that converts the chemical energy of a fuel directly into electrical energy through electrochemical reactions. Compared to classic PEFCs, high temperature PEFCs (with operating temperatures between 140-180 °C) have several advantages, such as high carbon monoxide tolerance and easier water management. A key component of HT-PEFCs is the phosphoric acid-doped polybenzimidazole (PBI) membrane. The interactions between the components of the PBI-H$_{3}$PO$_{4}$-H$_{2}$O ternary system have an influence on cell behavior. In this work, this system was investigated by means of ex situ, in situ and operando methods. In ex situ doping experiments, the water and acid uptake of the membranes were determined at different temperatures and acid concentrations. The water uptake of the membranes increased slightly in the range of 30 to 70 °C, but decreased significantly in the range of 70 to 110 °C, while the acid doping level of the membrane increased across the entire temperature range. The acid doping and water absorption increased with increasing acid concentration of the solution in the doping bath within the investigated range of acid concentration of 1.66-14.67 M. At low acid concentrations, the water absorption is independent of the acid absorption. In the case of high acid concentrations in the doping bath, the water absorption is coupled with acid absorption. As shown by in-situ current density measurements, the partial pressure of water vapor along the channel can have a significant impact on the local current density distribution. In the gas flow direction, the partial pressure of water vapor on the cathode side increases with dry input gases and a stoichiometry of two from 0 up to a maximum of 210 mbar. Accordingly, a local phosphoric acid concentration in the cell of between 76 wt .-% P$_{2}$O$_{5}$ and 68 wt .-% P$_{2}$O$_{5}$ at 160 °C arises. The resulting local differences in proton conductivities can affect the performance of a cell. It was found that due to the high water vapor partial pressure at the cell exit and the associated high proton conductivity, the effect of oxygen depletion can be overcompensated. After an initial decrease in the current density, this increased again in the direction of the cathode-side gas outlet. Long-term measurements with single cells showed that based on impedance measurements, which the ohmic resistance of the cells decreased during the measurement period, whereas charge transfer and mass transport increased. The solubility of the PBI membranes in the hot phosphoric acid environment can be responsible for this. The dissolution phenomena of the membrane results, on the one hand, in a reduction in the membrane thickness, with the result of decreasing ohmic resistance. On the other hand, membrane material penetrates into the catalyst layer and reduces the usable active area of the catalyst. However, these two opposite effects ultimately lead to the degradation of the cell.By operando-neutron radiography experiments with H$_{2}$ and D$_{2}$ as fuel gas, different H-to-D and D-to-H exchange times were detected in the phosphoric acid-doped membrane. On the basis of statistical considerations, the assumption of quantum-mechanical H-D transition probabilities and that only phosphoric acid is involved in the exchange, the different exchange times can be explained while maintaining the charge retention. The estimated times for the complete replacement of H$_{3}$PO$_{4}$ with D$_{3}$PO$_{4}$ in the phosphoric acid are in good agreement with the neutron radiographically-measured values.
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