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This work studies the long-term durability of high-temperature polymer electrolyte membrane fuel cells based on acid-doped polybenzimidazole membranes. The primary focus is on acid loss via the evaporation mechanism, which is a major cause of degradation in applications that involve long-term operation. Durability is assessed for 16 identically fabricated membrane electrode assemblies (MEAs), and evaluations are carried out using operating parameters as stressors with gas stoichiometries ranging from 2 to 25, current densities from 200 to 800 mA cm −2 , and temperatures of 160 or 180 • C. Cell diagnostics are composed of time resolved polarization curves, post mortem analysis, and in situ temperature measurements. A major part of the cell degradation during these steady-state tests can be ascribed to increasing area-specific series resistance. By means of post mortem acid-loss measurements, the degradation is correlated to the temperature and to the accumulated gas-flow volume. Such relations are indicative of acid loss via evaporation. Current density also plays a critical role for the acid loss and, thus, for the overall cell degradation. The effect of current is likely tied to mechanisms that involve water generation, migration of electrolyte ions, and locally elevated temperature inside the Since the polybenzimidazole (PBI) polymer acts as a Brønsted base in relation to phosphoric acid, membranes of PBI can be doped with phosphoric acid through acid-base complexation. Some of the H 3 PO 4 molecules, 2 per polymer repeat unit, are then bound to the basic functional sites of the polymer. However, excess acid is essential in order to attain a conductivity that is high enough for use as electrolyte in a polymer electrolyte membrane fuel cell (PEMFC). Fuel cells based on this type of electrolyte are known as high-temperature (HT-)PEMFCs, typically operating at temperatures of 140-180• C. Phosphoric acid-doped PBI membranes can be prepared either by direct (or so-called sol-gel) casting from a solution of PBI dissolved in polyphosphoric acid (PPA) or by solution casting of a PBI film via solvent evaporation, for which acid-doping follows subsequently. 1 For HT-PEMFCs, the cell performance usually increases to a stable state within the first few hundred hours of operation. This activation of the cell might be a consequence of acid redistribution in the membrane electrode assembly (MEA).2-4 Such redistribution of acid also implies susceptibility to its loss, however, because it is a testament to the mobility of the doping-acid. [5][6][7] As reviewed by Jakobsen et al., a number of mechanisms have been proposed to explain the loss of doping-acid from PBI membranes, including evaporation into the gas phase and a so-called steam distillation mechanism, the latter of which has been proposed governed by temperature as well as humidity. 9 Migration of electrolyte ions might also be an issue with respect to acid loss, particularly if the proton transference number deviates significantly from the maximum value for phosphori...

The passage of an electrical current through phosphoric acid doped polymer membranes involves parasitic migration of the acid, which imposes a critical issue for long-term operation of the high temperature polymer electrolyte membranes fuel cell (HT-PEMFC). To elucidate the phenomenon, a three-layered membrane is constructed with embedded micro reference electrodes to measure phosphoric acid redistribution in a polybenzimidazole based membrane. Under a constant load, a concentration gradient develops due to the acid migration, which drives the back diffusion of the acid and eventually reaches a steady state between migration and diffusion. The acid gradient is measured as a difference in local ohmic resistances of the anode-and cathode-layer membranes by electrochemical impedance spectroscopy. The phosphoric acid diffusion coefficient through the acid doped membrane is about 10 −11 m 2 s −1 , at least one order of magnitude lower than that of aqueous phosphoric acid solutions. The anion (H 2 PO 4 − ) transference number is found to range up to 4% depending on current density, temperature and atmospheric humidity of the cell, implying that careful control of the operating parameters is needed in order to suppress the vehicular proton conduction as a degradation mitigation strategy.

This is the first comprehensive review of the impact of water impurities on PEM, AEM, and alkaline water electrolysers.

Max Beckmann's ''Pierrette und Clown'' (Pierrette and Clown), 1925, is undoubtedly one of the major works in the collection of the Kunsthalle Mannheim, Germany. In this painting, damages can be seen, described as protrusions. These are blisters and crater-like holes, filled with metallic soap aggregates. Painting samples and cross-sections of the ground layer have been examined by environmental scanning electron microscopy (ESEM) and by confocal synchrotron radiation micro-X-ray fluorescence (CSRmXRF) in order to get compositional and spatial distribution information about the components and their behaviour. The analyzed samples of the unpainted ground layer consist of lithopone (ZnS$BaSO 4 ), as several bulges resembling small blisters, presumably protrusions in the nascent state. Direct CSRmXRF measurements on closed blisters showed that these blisters have a high concentration of zinc in the centre with surrounding layers of barium and strontium. Relatively small amounts of lead have been recorded. In order to determine the composition of the organic compounds (e.g. fatty acids) and of the inorganic pigments (lead white, zinc white, lithopone), Raman spectroscopy has been successfully applied to flakes of an opened protrusion. The obtained Raman spectra could be assigned to derivatives of fatty acids and lithopones. These results allow conclusions to be made about the chemical behaviour by measuring the identical sample spot of the protrusion with CSRmXRF as well as Raman spectroscopy.

Q. (2017). Probing phosphoric acid redistribution and anion migration in polybenzimidazole membranes. Electrochemistry Communications, 82, 21-24. AbstractMicro platinum electrodes embedded in a laminated phosphoric acid doped polybenzimidazole membrane are employed to monitor the acid migration during hydrogen pump mode operation. Upon application of a constant current, an immediate ohmic resistance decrease of the membrane near the anode is observed, accompanied by a corresponding increase near the cathode side. This is a direct evidence of migration of the acid anions via the vehicle mechanism of the conductivity, resulting in an accumulation of acid molecules at the anode side and depletion at the cathode side. Both resistances reach a steady state value after a prolonged period of measurement, apparently balanced by the back diffusion of the acid molecules. The phenomenon is magnified at higher current densities and with increased thickness of the overall membrane, which is of significance in quantitative understanding of the proton conductivity mechanism e.g. for determination of the anionic transference number.The finding provides a technique to monitor the acid redistribution within the membrane as a basis for an engineering solution to address the long-term durability of fuel cells built around phosphoric acid doped polymer membranes.

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