Polybenzimidazole and Phosphonic Acid Groups-Functionalized Polyhedral Oligomeric Silsesquioxane Composite Electrolyte for High Temperature Proton Exchange Membrane

Sung-Kon Kim

doi:10.1155/2016/2954147

Abstract

Here, we report composite membrane consisting of poly[2,2′-(m-phenylene)-5,5′-(bibenzimidazole)] (PBI) and polyhedral oligomeric silsesquioxane functionalized with phosphonic acid groups (PO(OH)2-POSS) for high temperature proton exchange membrane. ~7 phosphonic acid groups are incorporated into the phenyl rings of POSS via bromination in a high yield (~93%), followed by substitution of the bromine elements by phosphonate ester groupsviaa Pd(0) catalyzed P–C coupling reaction. Phosphonic acid groups are formed by the hydrolysis of the phosphonate ester groups in hydrobromic acid solution. At a 50 wt% of PA content in the membranes, PBI/PO(OH)2-POSS composite membrane shows larger proton conductivity of 3.2 × 10−3 S cm−1than 2.8 × 10−3 S cm−1of PBI membrane at 150°C and anhydrous conditions, owing to the multiple phosphonic acid groups of PO(OH)2-POSS that can function as proton transport medium at high temperature and low humidity conditions.

Highlights

Phosphoric acid (PA)-doped polybenzimidazoles (PBIs) have gained great interests for proton exchange membrane (PEMs) due to their intrinsic attributes such as high proton conductivity, low gas permeability, excellent oxidative and thermal stabilities, and almost zero water drag coefficient [1,2,3,4,5]
The bromination of Polyhedral oligomeric silsesquioxane (POSS) results in the broadening of proton peaks associated with the phenyl rings of POSS (δ = 8.1∼6.3 ppm), while the peaks of isooctyl moiety (δ < 2.0 ppm) remain unchanged, demonstrating that bromine elements are incorporated into the phenyl rings of POSS
The extent of bromine elements incorporated was calculated by the comparison of integrals of proton peaks associated with the phenyl ring and the isooctyl moieties

Summary

Introduction

Phosphoric acid (PA)-doped polybenzimidazoles (PBIs) have gained great interests for proton exchange membrane (PEMs) due to their intrinsic attributes such as high proton conductivity (up to 200∘C), low gas permeability, excellent oxidative and thermal stabilities, and almost zero water drag coefficient [1,2,3,4,5]. Phosphonic acid groups are attracting much attention because of their high thermal, hydrolytic, and oxidative stabilities [19,20,21,22] They undergo autodissociation due to intrinsically amphoteric character, leading to the formation of hydrogen bond network [23, 24], and can participate in the transport of protons from site to site without carrier molecules via Grotthuss-type mechanism [22, 23, 25]. These distinctive properties are of specific interest to proton conducting electrolytes that operate at high temperature and low humidity conditions. The functionalized POSS was incorporated with PBI for use in high temperature PEMs

Experimental

Results and Discussion

Conclusions