Polybenzimidazoles (PBI) doped with phosphoric acid (PA) are promising electrolytes for medium temperature fuel cells. Their significant disadvantage is a partial or complete loss of mechanical properties and an increase in hydrogen permeability at elevated temperatures. Covalent silanol crosslinking is one possible way to stabilize PBI membranes in the presence of PA. Three organo-substituted silanes, namely (3-Bromopropyl)trimethoxysilane (SiBr), trimethoxy [2-(7-oxabicyclo [4.1.0]hept-3-yl)ethyl]silane (Si-biC) and (3-Glycidyloxypropyl)trimethoxysilane (KH 560), were used as covalent crosslinkers of PBI-O-PhT in order to determine the effect of the silane structure and crosslinking degree on membrane properties. The crosslinking degree was 1–50%. All crosslinked membranes were characterized by impedance and IR-spectroscopy. The mechanical properties, morphology, stability and hydrogen permeability of the membranes were determined. In the case of silanes with linear substituents (SiBr, KH 560), a denser structure is formed, which is characterized by greater oxidative stability and lower hydrogen permeability in comparison to the silane with a bulk group. All the crosslinked membranes have a higher mechanical strength compared with the initial PBI-O-PhT membrane both before and after doping with PA. Despite the hardening of the polymer matrix of the membranes, their proton conductivity changes insignificantly. It was shown that cross-linked membranes can be used in fuel cells.
The further development of high temperature polymer electrolyte membrane (HT-PEM) fuel cells largely depends on the improvement of all components of the membrane–electrode assembly (MEA), especially membranes and electrodes. To improve the membrane characteristics, the cardo-polybenzimidazole (PBI-O-PhT)-based polymer electrolyte complex doped with phosphoric acid is reinforced using an electrospun m-PBI mat. As a result, the PBI-O-PhT/es-m-PBInet · nH3PO4 reinforced membrane is obtained with hydrogen crossover values (~0.2 mA cm−2 atm−1), one order of magnitude lower than the one of the initial PBI-O-PhT membrane (~3 mA cm−2 atm−1) during HT-PEM fuel cell operation with Celtec®P1000 electrodes at 180 °C. Just as importantly, the reinforced membrane resistance was very close to the original one (65–75 mΩ cm2 compared to ~60 mΩ cm2). A stress test that consisted of 20 start–stops, which included cooling to the room temperature and heating back to 180 °C, was applied to the MEAs with the reinforced membrane. More stable operation for the HT-PEM fuel cell was shown when the Celtec®P1000 cathode (based on carbon black) was replaced with the carbon nanofiber cathode (based on the pyrolyzed polyacrylonitrile electrospun nanofiber mat). The obtained data confirm the enhanced characteristics of the PBI-O-PhT/es-m-PBInet · nH3PO4 reinforced membrane.
Получены и исследованы новые жидкокристаллические композиты на основе пленок поливинилового спирта, пластифицированного глицерином с диспергированными в нем каплями холестерического жидкого кристалла. Изучены механооптические свойства полученных композитов и показано, что под действием растяжения происходит сдвиг пика селективного отражения света в коротковолновую область спектра. Амплитуда сдвига зависит от степени растяжения и достигает 100 нм. Полученные полимерные композиты перспективны для создания оптомеханических сенсоров деформации.Ключевые слова: ЖК-композиты, поливиниловый спирт, холестерическая мезофаза, селективное отражение света.
The development of phosphorylated polybenzimidazoles (PBI) for high-temperature polymer–electrolyte membrane (HT-PEM) fuel cells is a challenge and can lead to a significant increase in the efficiency and long-term operability of fuel cells of this type. In this work, high molecular weight film-forming pre-polymers based on N1,N5-bis(3-methoxyphenyl)-1,2,4,5-benzenetetramine and [1,1′-biphenyl]-4,4′-dicarbonyl dichloride were obtained by polyamidation at room temperature for the first time. During thermal cyclization at 330–370 °C, such polyamides form N-methoxyphenyl substituted polybenzimidazoles for use as a proton-conducting membrane after doping by phosphoric acid for H2/air HT-PEM fuel cells. During operation in a membrane electrode assembly at 160–180 °C, PBI self-phosphorylation occurs due to the substitution of methoxy-groups. As a result, proton conductivity increases sharply, reaching 100 mS/cm. At the same time, the current-voltage characteristics of the fuel cell significantly exceed the power indicators of the commercial BASF Celtec® P1000 MEA. The achieved peak power is 680 mW/cm2 at 180 °C. The developed approach to the creation of effective self-phosphorylating PBI membranes can significantly reduce their cost and ensure the environmental friendliness of their production.
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