Design of New Anion Exchange Membranes for Electrochemical Applications

Open Access
Author:
Zhu, Liang
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 02, 2016
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Michael Anthony Hickner, Committee Chair
  • Qing Wang, Committee Member
  • Evangelos Manias, Committee Member
  • Enrique Daniel Gomez, Committee Member
Keywords:
  • Anion exchange membranes
  • poly(2
  • 6-dimethyl-1
  • 4-phenylene oxide)s
  • fuel cells
  • fluorene side chains
  • Suzuki-Miyaura coupling
Abstract:
Anion exchange membranes (AEMs) are polymer-based electrolyte solids that conduct anions (OH, HCO3, Cl, et al.), with positively charged groups bound covalently to the polymer backbones. There has been a strong and growing worldwide interest in the use of anion exchange membranes for electrochemical energy conversion and storage systems. Anion exchange membrane fuel cells (AEMFCs) have been regarded as promising energy conversion devices for stationary and mobile applications due to their potential low cost. To realize high-performance AEMFCs, new polymeric membranes are needed that are highly conductive and chemically stable. Herein, cross-linked, multication side chain, and fluorene side chain AEMs based on poly(2,6-dimethyl-1,4-phenylene oxide)s (PPO) were synthesized. PPO was chosen as an AEM substrate because of its ease of functionalization at large scale and relatively good stability and membrane properties. To produce anion conductive and durable polymer electrolytes for alkaline fuel cell applications, a series of cross-linked quaternary ammonium functionalized poly(2,6-dimethyl-1,4-phenylene oxide)s with mass-based ion exchange capacities (IEC) ranging from 1.80 to 2.55 mmol/g were synthesized via thiol-ene click chemistry. From small angle X-ray scattering (SAXS), it was found that the cross-linked membranes developed micro-phase separation between the polar PPO backbone and the hydrophobic alkyl side chains. The ion conductivity, dimensional stability, and alkaline durability of the cross-linked membranes were evaluated. The hydroxide ion conductivity of the cross-linked samples reached 60 mS/cm in liquid water at room temperature. The chemical stabilities of the membranes were evaluated under severe, accelerated aging conditions and degradation was quantified by measuring ion conductivity changes during aging. The cross-linked membranes retained their relatively high ion conductivity and good mechanical properties both in 1 M and 4 M NaOH at 80 °C after 500 h. Attenuated total reflection (ATR) spectra were used to study the degradation pathways of the membranes, and it was discovered that β-hydrogen (Hofmann) elimination was likely to be the major pathway for degradation in these membranes. Side-chain containing AEMs with one, two or three cations per side chain were designed and synthesized, enabling a study of how the degree of polymer backbone functionalization and arrangement of cations on the side chain impact AEM properties. A systematic study of anion exchange membranes (AEMs) with multiple cations per side chain site was conducted to demonstrate how this motif can boost both the conductivity and stability of poly(2,6-dimethyl-1,4-phenylene oxide)-based AEMs. The highest conductivity, up to 99 mS/cm at room temperature, was observed for a triple-cation side chain AEM with 5 or 6 methylene groups between cations. This conductivity was considerably higher than AEM samples based on benzyltrimethyl ammonium or benzyldimethylhexyl ammonium groups with only one cation per side chain site. In addition to high conductivity, the multication side chain AEMs showed good alkaline and dimensional stabilities. High retention of ion exchange capacity (IEC) (93% retention) and ionic conductivity (90% retention) were observed for the triple-cation side chain AEMs during degradation testing in 1 M NaOH at 80 °C for 500 h. Based on the high-performance triple-cation side chain AEM, a Pt-catalyzed fuel cell with a peak power density of 364 mW/cm2 was achieved at 60 °C under 100% related humidity. Anion-conductive copolymers, poly(2,6-dimethyl-1,4-phenylene oxide)s containing fluorene side chains with pendant alkyltrimethylammonium groups, were synthesized via Suzuki-Miyaura coupling of aryl bromides with fluorene-boronic acids. The quaternized copolymers produced ductile, transparent membranes which were soluble in dimethyl formamide, dimethyl sulfoxide and methanol at room temperature. The fluorene side chain-containing membranes showed considerably higher hydroxide ion conductivities, up to 176 mS/cm at 80 °C, compared to that of typical anion exchange membranes based on the benzyltrimethyl ammonium moiety. The results of titration and hydroxide ion conductivity measurements demonstrated excellent chemical stability of the fluorene side chain-containing anion exchange membranes (AEMs), even after 1000 h immersion in 1 M NaOH at 80 C. The results of this study suggest a scalable route for the preparation of AEMs for practical alkaline fuel cell applications. A unique approach was employed to toughen AEMs by crosslinking the AEMs using commercial Jeffamine additives. Compared to the BTMA40 membrane, the 10% Jeffamine cross-linked membrane demonstrated significantly higher elongation at break. To be specific, the hydrated BTMA40 membrane showed a 51.7% elongation at break, while the 10% Jeffamine cross-linked membrane had a 166.8 % elongation at break. Clearly, the introducing of hydrophilic cross-linked network greatly enhanced the toughness of the AEMs. Overall, this thesis details a number of strategies for the large-scale production of PPO-based anion exchange membranes. These strategies will be useful in going forward in the design and deployment of hydroxide, bromide, bicarbonate, and chloride-conducting membranes for water purification and electrochemical technology.