Thermal and Electrochemical Degradation of Anion Exchange Membranes

Open Access
Author:
Mendoza, Alfonso Julio
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 29, 2012
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Qing Wang, Committee Member
  • James Patrick Runt, Committee Member
  • Chao Yang Wang, Special Member
Keywords:
  • AEMs
  • anion exchange membranes
  • GDLs
  • Raman spectroscopy
  • IR spectroscopy
  • Electrochemistry
Abstract:
Anion exchange membranes (AEM) must be designed to withstand alkaline environments and electrochemical stress over long periods of time during device operation. Today, most AEMs employ quaternary ammonium cations functionalized onto polymers with benzylic moieties since these types of nitrogen-based cations are easy to attach to polymers and have shown promising conductivity and reasonable stability. Vibrational spectroscopy was primarily used in this dissertation to probe the stability of a series of quaternary ammonium-based AEMs. Research on the spectral assignments of the AEMs studied is not comprehensive and new assignments were needed for accurate spectral interpretation of the degradation processes of the materials. Moieties such as C-N, CH2 and CH3 are present in many polymeric backbones and cationic groups. Similar groups in different parts of the polymer structure leads to overlapping vibrational frequencies in the Raman and IR spectra. The Raman and IR spectra of quaternary ammonium based aliphatic and aromatic AEMs were systemically compared to determine their peak assignments and uncover key peaks that gave information on the degradation of these species. Based on the complete vibrational assignments of three key AEMs, two separate degradation experiments were employed to utilize the different strengths and advantages of IR and Raman spectroscopy. The rates of degradation were measured by analyzing the vibrational spectra over time to quantify the relative stability of quaternary ammonium under the range of conditions tested. In the IR spectra of each AEM, the asymmetric stretching of quaternary ammonium was present as three degenerate peaks in the 900 – 980 cm-1 region and was used to compare the stability of the following AEMs: quaternary ammonium functionalized poly(vinylbenzyl chloride), Q-PVBC, quaternary ammonium functionalized poly(styrene)-b-poly(ethylene-r-butylene)-b-poly(styrene), Q-SEBS, and quaternary ammonium functionalized RADEL®-based poly(sulfone), Q-RADEL. Both Q-PVBC and Q-SEBS employ styrene-based moieties in the ionic phase and were expected to exhibit similar stabilities because of their similar chemical structures. The amount of quaternary ammonium remaining after 10 h of degradation at 135 °C was 80 % for Q-PVBC, 42 % for Q-RADEL, and 5 % for Q-SEBS. When examining the bicarbonate anion peaks over 10 h, its intensity was most stable in Q-PVBC, followed by Q-RADEL and Q-SEBS. Additionally, the carbonyl moieties were present in the IR spectra of Q-RADEL and Q-SEBS. It is suggested that Q-PVBC, the most hydrophilic of the three, remains hydrated throughout degradation contributing to the stability of quaternary ammonium. The effect of temperature, humidity, and hydrogen oxidation reaction on Q-SEBS was studied using a modified channel flow double electrode (CFDE) cell, capable of in situ collection of Raman spectra. Key benzene vibrations characteristic of styrenes were used to analyze the in situ spectra. The effect of temperature and hydration on the stability of quaternary ammonium in Q-SEBS showed that the material degraded faster at higher temperature and lower hydration. The amount of quaternary ammonium functionalized styrene remaining was 20 % (Tcell = 135 °C, pH2O = 18.9 kPa), 43% (Tcell = 100 °C, pH2O = 13.9 kPa), 49 % (Tcell = 135 °C, pH2O = 79.8 kPa), and 85 % (Tcell = 135 °C, pH2O = 79.8 kPa, -0.8 V vs. RHE). Electrochemical degradation was not found to accelerate degradation due to water production in the cell which stabilized Q-SEBS. When the samples were degraded at high temperatures, the formation of carbonyl functionalized styrene residues in Q-SEBS and Q-RADEL were observed in both Raman and IR spectra. A degradation mechanism was presented where a benzyl alcohol group on styrene forms, by S2N substitution of hydroxide at the benzyl position of the quaternary ammonium, and subsequently attacks another quaternary ammonium moiety to form an ether linkage to yield carbonyl and methyl-functionalized styrene residues. Lastly, Raman microspectroscopy was used to gather spatially resolved chemical information on the carbon and PTFE distribution of fuel cell gas diffusion layers. GDLs with varying PTFE content were imaged over areas of 1000 x 1000 μm and it was found that PTFE morphology was readily observed on the surface of the GDLs having greater than 20 wt. % PTFE where the PTFE was more likely to be present at the intersection of multiple carbon fibers. Spectroscopic measurements were performed on green and sintered gas diffusion layers having 1.8 to 44 wt. % PTFE with point-by-point resolution of 10 to 500 μm. It was found that the average PTFE signal detected on the surface of the GDL increased monotonically for both green and sintered GDLs with increasing bulk PTFE loading. Additionally, green GDLs had more PTFE on their surface compared to sintered GDLs.