Anion Exchange Polymer for Hydrogen Production and Application

Abstract

This dissertation is related with development of anion exchange polymer (AEPs) with the relation between membrane structure, mechanical and electrochemical properties for high cell performances. Prior to the development of anion exchange polymer, a fundamental understanding of anion exchange polymer structure, anion and water molecules transport mechanism, alkali stability of polymer, a basic principle of hydrogen production, and application were summarized in chapter 1. In Chapter 2, blended membranes composed of a series of N3-butyl-substituted imidazolium-based poly(1-vinyl-3-imidazole-co-styrene) (PIS), prepared via radical polymerization, and poly(vinyl alcohol) (PVA), were used as anion exchange membranes (AEM) for alkaline water electrolysis (AWE). PISPVAx blend membranes (where x denotes the ratio of imidazolium group and styrene in PIS being 4:6, 3:7, and 2:8) were doped with 6 M KOH solution to prepare OH- conductive AEMs and their thermal and mechanical properties, chemical stabilities, and hydration properties were analyzed. The highest ionic conductivity of 89.7 mS cm-1 was observed for the PISPVA46 membrane when using with 0.5 M KOH solution at 60 °C. The outstanding electrochemical performance of the PISPVA46 membrane, compared with those of PISPVA37 and PISPVA28, was reflected in a current density of 547.7 mA cm-2 at a cell voltage of 2.0 V. The higher imidazole ratio in the blend membrane produced an increase in the elongation at break and hydration property, such as water uptake and IEC, that obviously enhances the ion conducting capability of the resulting AEMs. Additionally, the KOH molecules were introduced into the membrane through PVA, resulting in high KOH uptake. Therefore, the PISPVAx blend membranes are useful candidates for anion exchange membrane water electrolysis (AEMWE) systems. In Chapter 3, we investigated the effect of two different cationic copolymers based on poly(phenylene oxide) (PPO) with N-cyclic quaternary ammonium (QA) groups, including six-membered dimethyl piperidinium (DMP) and bis-six-membered azonia-spiro undecane (ASU) as a binder (or catalyst ionomer) for AEMFCs. An earlier report on the same polymers for membranes in AEMFCs indicated the better electrochemical performance of PPO-ASU compared with PPO-DMP. Therefore, we would like to investigate these two polymers for catalyst ionomers. The outcome in this study using these two copolymers as catalyst ionomers indicates the opposite result; the electrochemical performance of the PPO-DMP ionomer is much better than the PPO-ASU ionomer. The commercial Fumion ionomer was used for the qualitative comparison. The density functional theory (DFT) calculation of the adsorption energy according to different orientations of the cationic groups on the catalyst surface shows that there is no difference between the adsorption energy of DMP and ASU cations, in compliance with the orientations of the cations. Although the PPO-ASU ionomer membrane has the highest hydroxide conductivity at 60 oC in liquid water, the hydrogen oxidation/reduction (HOR) activity of PPO-DMP and PPO-ASU showed similar values with the Fumion ionomer. While the PPO-DMP ionomer membrane shows relatively large fuel gas (hydrogen) permeability in dry and wet conditions, due to the chain flexibility and the presence of two methyl groups compared to the single methyl groups and lower flexibility of the PPO-ASU and Fumion ionomers. The electrochemical performance of a membrane electrode assembly (MEA) using the PPO-DMP ionomer exhibited an exceptional peak power density of 335 mW/cm2 compared to lower peak power densities the of PPO-ASU and Fumion ionomers under 60 oC and a fully humidified condition (H2/O2). The SEM images of MEAs after testing supports the conclusion that the PPO-DMP ionomer forms a uniform catalyst interface that is very well bound between the electrode and membrane, unlike the PPO-ASU and Fumion ionomers. The PPO-DMP ionomer membrane also showed better tensile strength and elongation at break than the PPO-ASU ionomer membrane. Therefore, we conclude that the well-prepared three-phase boundary structure played a critical role for the catalyst ionomer in each electrode, overcoming one of the critical performance-limiting factors. In Chapter 4, the conclusion and direction for further studies regarding advanced anion exchange polymer binder and AEM with chemically and electrochemically stable, and durability under alkali conditions.