C2 and N3 substituted imidazolium functionalized poly(arylene ether ketone) anion exchange membrane for water electrolysis with improved chemical stability

Highlights

• A series of C2 and N3 substituted imidazolium based PAEK membranes were prepared.

• Alkaline stability was significantly improved by the protection of C2 position.

• PAEK-APMBI showed superior chemical stability in highly basic condition.

• Flexible N3 butyl enhanced anion conductivity while maintaining lower water uptake.

• PAEK-APMBI exhibited excellent electrolytic performance up to 2 A cm⁻² at 2.53 V.

Abstract

The effect of substituents at C2 and N3 positions of imidazolium functional group on various properties of the poly(arylene ether) ketone based anion electrolyte membranes was studied. The synthesized membranes were applied for alkaline water electrolysis using zero gap designed cell in order to evaluate their cell performances. All of the membranes exhibited better ionic conductivity and electrochemical performance than the commercial Fumasep FAA-3 membrane. The C2 methyl and N3 butyl substituted imidazolium-based membrane (PAEK-APMBI) specifically showed quite high ionic conductivity of 0.0154 S cm⁻¹ and the voltage of 2.53 V at the current density of 2 A cm⁻² (in 10 wt% KOH solution) at 60 °C, while those of Fumasep FAA-3 membrane were 0.013 S cm⁻¹ and 2.63 V, respectively. Especially, the PAEK-APMBI membrane exhibited the excellent alkaline stability with the 96% IEC retention after long-term treatment in alkaline environment, while the Fumasep FAA-3 membrane illustrated a significant reduction in IEC to 38%. Based on those results, the PAEK-APMBI membrane would be a promising candidate for application in anion exchange membrane electrolysis system.

Introduction

As the global warming becomes more and more crucial due to the growth of consumption of conventional fuels [1,2], the development of renewable energy system has been considered as an urgent priority [3,4]. The hydrogen with the advantages of high power conversion efficiency and zero-carbon emission is recognized as a promising renewable and environmentally-friendly energy source alternative to fossil fuels [5,6]. However, the current hydrogen production methods such as the steam reforming of natural gas [7,8], the gasification and reforming of heavy oil, and the gasification of coal and petroleum coke result in serious emission of greenhouse gasses [[9], [10], [11]]. On the other hand, water electrolysis is a simple, reliable, and clean technology to produce extremely pure hydrogen from the electrolytic decomposition of water [12].

The water electrolysis system is commonly operated under acidic or alkaline conditions corresponding to types of ion exchange membranes [13]. The proton exchange membrane (PEM) water electrolysis is performed under acidic condition, which offers several advantages including high energy efficiency and a high hydrogen production rate [14]. The PEM water electrolysis, however, has a crucial drawback associated with the high cost of the catalysts [15]. On the other hand, the alkaline water electrolysis has cost competition compared to the PEM water electrolysis due to the adoption of a non-precious catalyst. Compared to the conventional alkaline water electrolysis which employs porous diaphragm separators, the alkaline anion exchange membrane (AEM) water electrolysis has the advantages of the safety and gas mixing prevention [16,17]. Several types of polymers including both aliphatic structure such as polyethylene (PE) [18], polypropylene (PP) [19], and aromatic structure such as poly(arylene ether ketone) (PAEK) [20], poly(phenylene oxide) (PPO) [21] and polysulfone (PSU) [22] have been used for anion exchange membranes. Among those, PAEK inherently has some advantageous properties associated with good mechanical and thermal stability and versatile chemical modification [23].

Various cation species including quaternary ammonium (QA) [24], imidazolium [25], phosphonium [26], guanidinium [27], pyridinium [28] and sulfonium [29] have been explored as functional groups for anion exchange membranes. Although QA has been most widely used due to its cost competiveness, simple preparation process, and high anion conductivity [30], it has some serious problems related with the toxicity by volatile trimethylamine and low alkali thermochemical stability [31]. Recently, imidazolium has gained a lot of attention due to its facile modification, synthetic convenience, and comparable ionic conductivities [32]. Moreover, as imidazolium was proved to be stabilized through charge delocalization, it may now replace ammonium cations. However, the improvement of its stability in strong alkaline environment remains essential for long-term applications. Several studies have revealed that the ring-opening mechanism by the hydroxide attack at C2 position was the primary degradation pathway of imidazolium cations [33]. Consequently, the C2 substitution strategy was performed by a few groups in order to improve its alkaline stability [25,33,34]. In addition, the installation of N3 alkyl substituents was reported as an another approach to enhance its chemical stability [32,35,36]. It was announced that the N3 alkyl substituent should be four or five carbons because the alkaline stability of imidazolium-based AEM was just comparable or even worse than N3-methyl substituted counterpart if the alkyl chain length of N3 substituent reaches to over six carbons [35,37].

In the present study, a series of PAEK membranes with imidazolium functional group were prepared via amidation of 1-(3-aminopropyl)-imidazole and 1-(3-aminopropyl)-2-methyl-1H-imidazole with carboxylic acid group on the side chain of PAEK. The ion-exchange capacity of the polymer was controlled by the mole ratio of the imidazolium functional group to the reactive COOH group of polymer backbone. The influences of C2 methyl and N3 butyl substituents on the alkaline stability of the synthesized PAEK based AEMs were investigated in detail. Other properties such as thermal stability, water uptake, swelling ratio, and mechanical properties were also studied in association with the effects of substituent types. Additionally, the water electrolysis cell performance was carried out to evaluate the application possibility of this type of AEM into the hydrogen production process.