Fabrication of Acid-Grafted Polybenzimidozole Membranes for High Temperature Polymer Fuel Cell and Their Properties
Authors: Han Hoon , Fujigaya Tsuyohiko , Nakashima Naotoshi 
Published: February 2020

Introduction

Polymer electrolyte membrane fuel Cells (PEMFC) have received much attention, because PEMFC are promising power generator for many applications such as automobiles and portable devices. One of the key componenet for PEMFC is polymer electrolyte membrane (PEM) that can transfer protons from cathode to anode, and the performance of PEM highly depends on their proton conductivity. Most-frequently studied PEM is Nafion due to their high proton conductivity even at room temperature. However, Nafion has serious drawbacks such as high cost and high humidity dependency, and also the operation temperature is limited up to 100 oC. Since higher temperature can give better reaction kinetics and reduce poisoning of Pt catalyst, materials that can show high proton conductivity at lower humidity and higher temperature have been required.
Polybenzimidazole (PBI) doped by Phosphoric acid (PA) has been developed to provide high proton conductivity at high temperature under low humidity. However, leakage of PA was observed after long-term operation1, and this hinders further applications of PA doped PBI.
To solve this problem, Kawahara et al2 synthesized propanesulfonic acid-grafted PBI (PBI-PS) by substitution of benzimidazoles of PBI. PBI-PS showed improved solubility and 10-3 S/cm as its proton conductivity over 100 oC. However, no fuel cell test was conducted, and higher proton conductivity is required to be commercialized.
In this research, poly(2,5-benzimidazole) (ABPBI) was chosen instead of PBI due to the shorten distance between benzimidazole groups, which enables to shorten the distance between grafted propanesulfonic acids. In addition, ABPBI has advantages in the cost and the ease of synthesis compared to PBI since ABPBI can be easily obtained from the self-condensation of one monomer. ABPBI grafted by propanesulfonic acid (ABPBI-PS) was synthesized, and their properties were compared with PBI-PS.

Experiment

Synthesis of PBI-PS
In 30 mL of dimethylacetamide solution of PBI (32.4 mM), NaH (0.051 g, 2.13 mmol) was added, and the temperature was kept at 90 oC for 3 h. Then, 3-bromopropanesulfonic acid sodium salt (0.877 g, 3.44 mmol) was added to the solution, and the temperature was kept at 30 oC for 2 h and 80 oC for 22 h. The solution was reprecipitated in acetone-water mixture (9:1) and then, the precipitate was filtrated and washed with acetone. After drying under vacuum at 60 oC for 24 h, yellow powder (0.262 g, 45.2 %) was obtained.
Synthesis of ABPBI-PS
In 30 mL of dimethyl sulfoxide solution of ABPBI (86.1 mM), NaH (0.186 g, 7.75 mmol) was added, and the temperature was kept at 120 oC for 24 h. Then, 3-bromopropanesulfonic acid sodium salt (1.74 g, 7.73 mmol) was added to the solution, and the temperature was kept at 30 oC for 24 h. The solution was reprecipitated in acetone-water mixture (9:1) and then, the precipitate was filtrated and washed with acetone. After drying under vacuum at 60 oC for 24 h, brown powder (0.668 g, 99.4) was obtained.
Membrane fabrication
DMSO solutions of ABPBI-PS (10 mL, 20 mg/mL) were poured into petri dishes and heated at 60 oC for 24 h. The obtained film (45.4 μm) was treated with HCl (1.0 M). PBI-PS membrane was prepared in similar manner.

Result and Discussion

PBI-PS and ABPBI-PS were synthesized based on previous reports2,3. From FT-IR measurements, both compounds showed new peaks at 1190 and 1050 cm-1 assignable to asymmetric and symmetric stretching of SO3. Based on the 1H NMR proton integration, the grafting ratio of PBI-PS and ABPBI-PS are 94.5 % and 65.0 %, respectively.
The obtained membranes were yellow and self-standable. We found that the thickness of the membrane could be controlled by changing the concentration of the solutions. Proton conductivity measurements were conducted from 30 oC to 120 oC at 100 % relative humidity. Similar to the result from Kawahara, PBI-PS showed 1.5 mS/cm-1 as its maximum conductivity at 110 oC. Interestingly, the maximum conductivity of ABPBI-PS was 16 mS/cm-1 at 120 oC, which was much higher than PBI-PS. This gap was probably brought by the shorter distance between grafted propanesulfonic acids of ABPBI-PS.

References

(1) Oono, Y.; Sounai, A.; Hori, M. J. Power Sources
2013, 241, 87–93.
(2) Kawahara, M.; Rikukawa, M.; Sanui, K.; Ogata, N. Solid State Ionics
2000, 136, 1193–1196.
(3) Namazi, H.; Ahmadi, H. J. Memb. Sci.
2011, 383, 280–288.

Figure 1