Development of Highly Durable Anion Conductive Membrane with All-Aromatic Backbone for Alkaline Fuel Cell Application
Authors: Miyanishi Shoji , Yamaguchi Takeo 
Published: February 2020

Introduction:

Solid alkaline fuel cell using anion conductive membrane (ACM) attracts increasing attention since non-noble metal electrode catalyst and high density liquid fuel can be used in this system. However, development of chemically durable membrane is critical for practical device application. Currently, benzyl trimethylammonium modified polyarylene ethers (i.e. polyethersulfone, polyphenyleneoxide, polyetherketon) are used as typical aromatic ACMs for alkaline fuel cell application. However, aryl ether bond in the backbone is not stable and suffered from chain scission under alkaline environment [1], leading to a loss of mechanical strength and conductivity of the membrane
For this sense, ACM consisting of only aromatic backbone such as polyphenylene ACM is potential candidate for highly durable membranes [2-4]. However, these membranes are usually suffered from low solubility because of the rigid linear backbone and strong intermolecular π-π interaction. This drawback results in the difficultly for processing membranes
To overcome these problems, in this work, we have synthesized a novel ACM with all-aromatic backbone for alkaline fuel cell application. The designed polymer backbone is composed of alternative m-phenylene and p-phenylene unit, changing the polymer structure to 2D zigzag conformation to suppress strong intermolecular π-πstacking. (Fig 1). Also, branched alkyl side chain was introduced for further enhancing solubility.
We expected that AEM with both high durability and solubility can be obtained by using this membrane.

Method:

The precursor polymer for target membrane was synthesized by Pd-catalyzed Suzuki-Miyaura polymerization between m-phenylene and p-phenylene derivatives [5]. Structure and molecular weight of this precursor polymer were evaluated by 1H-NMR and gel permeation chromatography (GPC). Bromide groups at the polymer side chain were reacted with trimethylamine to afford the target membrane. Ion exchange capacity (IEC) of the membrane was evaluated by both 1H-NMR and titration. Ion conductivity of the membrane was calculated by alternative current impedance measurement. Alkaline stability of this membrane was evaluated by comparing 1H-NMR spectra before and after soaking into 8M NaOH aq, at 80 oC.

Results and Discussion:

The number average molecular weight (Mn) and polydistersity (PDI) of the precursor polymer were Mn=18000 and PDI=1.6, respectively.
Quaternization of the precursor polymer proceeded almost quantitatively from 1H-NMR spectra. IEC of target polymer was 2.6 meq from NMR analysis and similar value (2.4) was obtained in titration.
The synthesized membrane can be dissolved well into methanol, dimethylsulfoxide.and also 1-propanol/water (1:1 vol) mixture, which is typically used as suitable solvent for membrane electrode assembly (MEA) preparation. This indicates that the developed polymer is also promising candidate for an ionomer of fuel cells.
Bormide and bicarbonate conductivity of the membrane were 52 mS/cm and 41 mS/cm (70 oC, RH100%), respectively. The conductivity value is high enough for fuel cell application.
Chemical stability of the developed membrane was evaluated in harsh alkaline condition (8M NaOH, 80 oC).　Even after one week, no detectable change was observed in the membrane. In fact, 1H-NMR spectra of the membrane after the alkaline stability test shows that both the anion exchange group and the backbone are stable in this condition.
These results indicate that the developed all-aromatic ACM is promising candidate for a membrane and an ionomer of alkaline fuel cell.
Reference
[1] S. Miyanishi, T. Yamaguchi, Phys. Chem. Chem. Phys., 2016,18, 12009-12023
[2] E. J. Park and Y. S. Kim, J. Mater. Chem. A, 2018, 6, 15456-15477
[3] S. Miyanishi, T. Yamaguchi, J. Mater. Chem. A, 2019,7, 2219-2224
[4] H. P. R. Graha, S. Ando, S. Miyanishi and T. Yamaguchi, Chem. Commun., 2018, 54, 10820–10823.
[5] B. Deffner and A.D. Schluter, Poly. Chem. 2015, 6, 7833-7840

Figure 1