ABSTRACT
Understanding the degradation mechanisms of organic cations under basic conditions is extremely important for the development of durable alkaline energy conversion devices. Cations are key functional groups in alkaline anion exchange membranes (AAEMs), and AAEMs are critical components to conduct hydroxide anions in alkaline fuel cells. Previously, we have established a standard protocol to evaluate cation alkaline stability within KOH/CD3OH solution at 80 °C. Herein, we are using the protocol to compare 26 model compounds, including benzylammonium, tetraalkylammonium, spirocyclicammonium, imidazolium, benzimidazolium, triazolium, pyridinium, guanidinium, and phosphonium cations. The goal is not only to evaluate their degradation rate, but also to identify their degradation pathways and lead to the advancement of cations with improved alkaline stabilities.
ABSTRACT
Highly base-stable cationic moieties are a critical component of anion exchange membranes (AEMs) in alkaline fuel cells (AFCs); however, the commonly employed organic cations have limited alkaline stability. To address this problem, we synthesized and characterized the stability of a series of imidazolium cations in 1, 2, or 5 M KOH/CD3OH at 80 °C, systematically evaluating the impact of substitution on chemical stability. The substituent identity at each position of the imidazolium ring has a dramatic effect on the overall cation stability. We report imidazolium cations that have the highest alkaline stabilities reported to date, >99% cation remaining after 30 days in 5 M KOH/CD3OH at 80 °C.
ABSTRACT
A strategy has been devised to study the incorporation and exchange of anions in a candidate alkaline anion exchange membrane (AAEM) material for alkaline fuel cells using the electrochemical quartz crystal microbalance (EQCM) technique. It involves the electro-oxidation of methanol (CH3OH) under alkaline conditions to generate carbonate (CO3(2-)) and formate (HCOO(-)) ions at the electrode of a quartz crystal resonator coated with an AAEM film, while simultaneously monitoring changes in the frequency (Δf) and the motional resistance (ΔR(m)) of the resonator. A decrease in Δf, indicating an apparent mass increase in the film, and a decrease in ΔR(m), signifying a deswelling of the film, were observed during methanol oxidation. A series of additional QCM experiments, in which the effects of CH3OH, CO3(2-), and HCOO(-) were individually examined by changing the solution concentration of these species, confirmed the changes to be due to the incorporation of electrogenerated CO3(2-)/HCOO(-) into the film. Furthermore, the AAEM films were found to have finite anion uptake, validating the expected tolerance of the material to salt precipitation in the AAEM. The EQCM results obtained indicated that HCOO(-) and CO3(2-), in particular, interact strongly with the AAEM film and readily displace OH(-) from the film. Notwithstanding, the anion exchange between CO3(2-)/HCOO(-) and OH(-) was found to be reversible. It is also inferred that the film exhibits increased swelling in the OH(-) form versus the CO3(2-)/HCOO(-) form. Acoustic impedance analysis of the AAEM-film coated quartz resonators immersed in water showed that the hydrated AAEM material exhibits significant viscoelastic effects due to solvent plasticization.
ABSTRACT
A tetrakis(dialkylamino)phosphonium cation was evaluated as a functional group for alkaline anion exchange membranes (AAEMs). The base stability of [P(N(Me)Cy)(4)](+) was directly compared to that of [BnNMe(3)](+) in 1 M NaOD/CD(3)OD. The high base stability of [P(N(Me)Cy)(4)](+) relative to [BnNMe(3)](+) inspired the preparation of AAEM materials composed of phosphonium units attached to polyethylene. The AAEMs (hydroxide conductivity of 22 ± 1 mS cm(-1) at 22 °C) were prepared using ring-opening metathesis polymerization, and their stabilities were evaluated in 15 M KOH at 22 °C and in 1 M KOH at 80 °C.
