Anion Exchange Membrane with Thermally Convertible Unit for Alkaline Water Electrolyzer
Authors: Graha Hafis Pratama Rendra , Miyanishi Shoji , Ando Shinji , Yamaguchi Takeo 
Published: February 2020

Sustainability is a big aim of humanity in this era. United Nations establishes 17 Sustainable Development Goals (SDGs) as a guideline for achieving sustainability. Searching for affordable and clean energy is one of those goals1. Hydrogen acts as a clean energy carrier between renewable energy sources and the end user. It is easily transported and stored making it suitable for vehicles and portable devices. Water Electrolysis (WE) using polyelectrolyte membrane opens the path for sustainable hydrogen production. Presently, proton exchange membrane water electrolysis (PEMWE) is well-studied technologies and mature for small scale hydrogen production. However, the cost of this cell is still expensive due to the usage of Platinum Group Metal (PGM) as catalysts. Recently, anion exchange membrane water electrolysis (AEMWE) operating under alkaline condition attracts significant attention. This technology is more affordable and advantageous over conventional WE systems since cheap non-PGM electrode catalyst can be used. However, commercialization of AEMWE is still hindered by the lack of high-performance anion exchange membranes (AEMs). Ideally, AEMs should have high ionic conductivity as well as high durability in alkaline medium. Although ionic conductivity can be improved by increasing ion exchange capacity (IEC), high IEC AEMs are commonly suffered from excessive water uptake and then leads to poor conductivity and mechanical property. Meanwhile, recent studies show that the polymer backbone structure is very important for the durability of AEMs. Typical aromatic AEMs containing ether, ketone, or sulfone group in the backbone are unstable and suffered from chain scission in high pH environment. Aliphatic polymer has also poor durability against oxidative environment with a lot of radical species. For that sense, ether-free aromatic polymer attracts attention as a highly durable material in harsh alkaline condition. However, this kind of rigid polymer membrane is difficult to be prepared since solubility in common organic solvent is very low due to strong intermolecular interaction.

To overcome these issues, we developed a new AEM system with thermally leaving group (TLG) on the precursor polymer (Fig.1.). Anthracene derivative is selected as a core of TLG. TLG disrupts anthracene π–π stacking and makes the structure bulkier, resulting in the highly soluble compound. Therefore, the introduction of bulky TLG drastically enhance the solubility of the precursor polymer and allows the polymer to be cast as a film. TLG then removed by performing solid-state retro Diels-Alder reaction on the membrane film to afford ether-free aromatic membrane. Removal of TLG brings back the stacking of the polymer, which can suppress the water uptake of the AEMs despite very high IEC. As a first stage, we’ve developed the poly(2,6-anthracene-alt-1,4-para-phenylene) (PAPP) AEM as a benchwork polymer. Although PAPP membrane showed high ion conductivity and chemical durability, making membrane electrode assembly (MEA) for water electrolyzer cell was difficult due to lack of flexibility. Then, we refined the molecular design by adding a longer phenylene chain unit and reducing the ion exchange capacity (IEC) to increase the membrane flexibility (TPP-V2 and TP-V2). In fact, the obtained TPP-V2 and TP-V2 membrane can be bendable and easily handled for cell preparation. The ion conductivity of this AEM keeps initial value even after soaked in 8 M NaOH aq at 80 oC and fenton solution at 60 oC, indicating high alkaline and oxidative stability. These results show that the developed TP-V2 membrane is a promising candidate for water electrolyzer’s AEM.

Figure 1