Investigation of the conduction properties of phosphoric and sulfuric acid doped polybenzimidazole

Abstract

When complexed with a strong inorganic acid such as sulfuric or phosphoric acid, ®lms of polybenzimidazole (PBI) show two types of behaviour, depending on the time of immersion in the corresponding acid bath. The ®rst type, prepared at shorter doping times, has conductivity in the range 10 25 ±10 24 S cm 21 , whilst the second, of conductivity w10 23 S cm 21 is formed after more prolonged immersion. The `switch-over' from one state to the other is at 10±11 h in H 3 PO 4 , and 2±3 h in H 2 SO 4 . PBI has a remarkable capacity to concentrate H 3 PO 4 and, even in an acid bath of concentration 3 mol dm 23 , the acid concentration within a PBI membrane is ca. 14.5 mol dm 23 . IR spectroscopy performed on hydrated PBI membranes as a function of temperature, and on acid-complexed membranes as a function of the amount of sorbed acid con®rms proton transfer from H 3 PO 4 to the imino groups of PBI and, at high doping levels, the presence of undissociated H 3 PO 4 .

High temperature proton exchange membranes based on polybenzimidazoles for fuel cells

“…Since then phosphoric aciddoped PBI membranes have been successfully developed and systematically characterized. The first patent was filed by Savinell and Litt [22] [29][30][31][32][33], good mechanical properties [34] and excellent thermal stability [35] have been reported at temperatures up to 200 • C under ambient pressure. Fuel cells and related technologies have been developed with operating features such as little humidification [36], high CO tolerance [37], better heat utilization [38,39] and possible integration with fuel processing units [40,41].…”

“…The infrared (IR) [30,31,[181][182][183][184][185][186][187][188], Raman spectroscopy [187][188][189] and nuclear magnetic resonance (NMR) spectra [181,182,186] have been made to investigate the polymer and its protonation by acids. For pristine PBI, the IR spectrum [183][184][185] from 2000 to 4000 cm −1 is of particular interest since most of the informative N-H stretching modes occur in this range, with typically three distinguishable bands at 3415, 3145 and 3063 cm −1 .…”

Blend membranes based on sulfonated poly(ether ether ketone) and polysulfone bearing benzimidazole side groups for proton exchange membrane fuel cells

“…In this system, an acid-base complex occurs between the nonprotonated, basic nitrogen of the PBI repeat unit and the absorbed phosphoric acid [3]. The protons hop between the nitrogen of benzimidazole and phosphoric acid by forming benzimidazolium cation and dihydrogenphosphate anion [4,5], facilitating proton conduction by a Grotthuss-type mechanism, unlike the vehicle-type mechanism in Nafion. Similar complexed systems have also been pursued by replacing water by heterocycles as proton solvents [6][7][8][9][10][11].…”

Bio-Inspired Membranes for Advanced Polymer Electrolyte Fuel Cells. Anhydrous Proton-Conducting Membrane via Molecular Self-Assembly

“…[33][34][35] Proton mobility in the electrolyte most likely depends on the distance between the hopping sites in the polymer material, [33][34][35] and usually, it is related to the activation energy of proton transfer under anhydrous condition. Several new polymers have been reported as possible electrolytes: polybenzimidazole(PBI)-acidic molecules, [49][50][51] imidazole group immobilized polymer-acidic molecules, [52][53][54][55][56] ionic liquid, [57][58][59][60] and inorganic-organic copolymers. 61 These electrolyte materials have high proton conductivity even under anhydrous and intermediate temperature conditions; however, some require the presence of liquid moieties, such as phosphoric or sulfuric acid solutions or an ionic liquid, to make proton conductivity high enough for the practical applications.…”