Cyclic ammonium grafted poly (arylene ether ketone) hydroxide ion exchange membranes for alkaline water electrolysis with high chemical stability and cell efficiency
Introduction
Hydrogen is not only a future energy source but also an ideal energy carrier replaceable for fossil fuels which possess the inherent problems associated with unstable supply of resources, emission of carbon dioxide, and air pollution and global warming [[1], [2], [3]]. As the most of hydrogen resources, however, are supplied through the reforming of methane releasing various pollutants including carbon dioxide, they do not contribute to the “carbon-balanced” energy system [4]. Water electrolysis system produces hydrogen gases by the decomposition of water, and thus it is highly efficient process maintaining high gas purity compared with the conventional reforming system [5].
Among three kinds of water electrolysis systems - alkaline water electrolysis (AWE) [6,7], proton exchange membrane water electrolysis (PEMWE) [8,9] and solid oxide steam electrolysis (SOSE) [10], AWE and PEMWE are normally operated at mild temperatures from 60 to 80 °C. AWE uses basic electrolyte solution to transfer anions produced from the decomposition reaction of water. Thus, the anion transporting ability of electrolyte is a key factor determining the overall efficiency of water electrolyzer.
The traditional AWE requires highly concentrated KOH solution with separator to transfer hydroxide ions and to avoid mixing of produced gases. However, as KOH electrolyte is highly corrosive and produces insoluble carbonate salts via the reaction with environmental carbon dioxide, it dramatically reduces ion conductivity in long term operation [11]. Also, as the separator used for liquid electrolytes generally shows very high ion conducting resistance and imperfect gas barrier property, it often results in very poor cell performance [12]. PEMWE is mostly operated with a polymer electrolyte membrane which can transport protons. The polymer electrolyte membrane forms ion clusters to exchange and transfer ions in the direction of the opposite electrodes with high efficiency. However, PEMWE has a critical drawback in the cost because it often uses expensive commercial membranes (ex. Nafion) and precious metal catalysts such as platinum [5,13].
Recently, alkaline anion exchange membrane water electrolysis (AAEMWE) is suggested by combination of the advantages of AWE and PEMWE [14]. The AAEMWE has the advantage that the carbonate salt is not formed due to the transport of anion through the polymer electrolyte. Also, The polymer electrolyte membranes with thin thickness can achieve high ion transport performance while maintaining excellent durability with prevention of gas mixing [15]. Unlike PEMWE, non-precious metal catalyst can be used, so that a cheaper electrolyzer system can be established. The electrolyte membranes used in this low-cost system thus require not only high anion conductivity but also durability with excellent thermal, mechanical, and chemical stability for long-term operation in an alkaline environment.
The cationic functional groups present in the anion exchange membrane (AEM) can be sulfonium [16], phosphonium [17], and ammonium groups, as these groups transport hydroxide ions by forming channels with ionic clusters in the hydrated state. Fumasep FAA-3 is a well-known commercial AEM for the ion exchange of various electrochemical cells under alkaline conditions [[18], [19], [20], [21]]. However, it has some weaknesses attributed to the poor thermal stability of main chain and poor durability due to degradation of the quaternary ammonium functional group under alkaline conditions [22,23]. In order to solve problems, arylene based polymer electrolyte membranes with better thermal, mechanical, and chemical stability have been proposed [[24], [25], [26]]. As poly (arylene ether) ketone (PAEK) backbone is reported to be very stable in thermal, mechanical, and chemical aspects [27], it has been applied in preparation of many types of electrolyte membranes in fuel cell. In consideration of anion transferrable functional groups, ammonium groups with stable structure under alkaline conditions have been investigated [[28], [29], [30]]. The cyclic ammonium functional group may show higher chemical stability than the conventional quaternary ammonium because it has a steric hindrance that has difficulty to be placed in the anti-periplanar conformation for Hofmann elimination [28]. In this work, we synthesized a PEAK electrolyte membrane with a cyclic ammonium at the pendant site. The synthesized electrolyte membranes were analyzed in a variety of aspects such as anion conductivity, water uptake, thermal, mechanical, and chemical stability in addition to the fundamental chemical structure identification and morphological behavior. The AWE performance was also tested by electrochemical analysis to compare it with a liquid electrolyte system along with Zirfon porous separator as well as commercial Fumasep and QA functionalized membranes.
Section snippets
Materials
4,4-Bis(4-hydroxyphenyl)-valeric acid, N-hydroxysuccinimide (NHS), iodomethane, N,N′-dicyclohexylcarbodiimide (DCC), and 3-(dimethylamino)-1-propylamine were purchased from Aldrich Chemical Company (Milwaukee, WI, USA). 1-(3-Aminopropyl)-4-methylpiperazine (APMP) and dimethylsulfoxide (DMSO) were purchased from Alfa Aesar Chemical Company (Reston, VA, USA). 4,4′-Difluorobenzophenone was purchased from Tokyo Chemical Industry (Tokyo, Japan). Potassium carbonate (K2CO3), toluene and iso-propanol
Chemical structure identification
From the GPC analysis, the number and weight average molecular weights were 35000 and 60000 g mol−1 respectively. The synthesized PAEK-COOH and PAEK-NHS were analyzed using 1H NMR (500 MHz, DMSO‑d6) and the presence of carboxylic and NHS functional groups were confirmed as shown in Fig. 2. The protons in benzene ring in the polymer main chain revealed NMR signals at 7.07, 7.22 and 7.73 ppm. In Fig. 2(a), the NMR peak at 12.02 ppm originates from the proton in COOH-group of PAEK-COOH. It
Conclusions
The synthesis of a series of PAEK-APMP electrolyte membranes controls the IEC and ionic conductivity according to the feed amount of the APMP. The synthesized PAEK-APMP electrolyte membranes showed better thermal stability of main chain than the commercial FAA-3 membrane and thus it confirmed its suitable utilization in water electrolysis under wide operation temperature range. In addition, the pendant APMP groups lead to higher hydration affinity and thus higher anionic conductivity than QA.
Acknowledgements
This work was sponsored by the National Research Foundation of Korea Grant, funded by the Korean Government (MEST) (NRF-2015M1A2A2074669 and NRF-2017-R1A2B2008019).
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