Stability Of Anion Exchange Membranes Research Articles

Anion-conducting ionomers: Study of type of functionalizing amine and macromolecular cross-linking

Anion exchange membranes (AEM) were prepared by chloromethylation and amination of polysulfone (PSU) and polyphenylsulfone (PPSU). The reaction pathways were studied by 1H and 13C NMR spectroscopy and ab-initio calculations. Various amines were used, including trimethylamine (TMA), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,4-diazabicyclo[2.2.2]octane (DABCO) to study the influence of the molecular volume and the stabilizing effect of a delocalization of the positive ammonium ion charge on a second nitrogen. Furthermore, the effect of a solvothermal cross-linking on the stability of AEM was investigated. The alkaline stability, measured after treatment in 2 M KOH at 60 °C during 168 h, is improved by DBN and polymer reticulation. The ionic conductivity in water reaches values above 10 mS/cm with DABCO. The hydroxide ion mobility depends linearly on the hydration number.

An attempt to generate anion exchange membranes by amination of the perfluorinated 3M precursor leads to the hydrolysis of the precursor

We report attempts to synthesize and to study the stability of anion exchange membranes (AEMs) from the perfluorinated 3M membrane precursor having terminal sulfonyl fluoride, –SO2F, groups in the side chains of the backbone. The precursor was obtained in powder form from which flexible films were prepared using hot pressing; the films were reacted with trimethylamine (TMA), 1,4-dimethylpiperazine (DMP), and quaternized 1,4-diazabicyclo[2.2.2]octane (DABCO) amines, following published protocols. The chemical stability of the generated membranes was determined by FTIR, and by 1H and 19F NMR, first after the reaction between the precursor and the amines, and then after addition of KOH. The FTIR spectra indicated the presence of the C–H bands of the amines after reaction, and their disappearance after addition of KOH, a result consistent with 1H NMR spectra of the membranes swollen by deuterated dimethylsulfoxide (DMSO-d6). The 19F NMR spectra indicated the presence of SO3− anions, in the membranes after the initial amination reaction and also after addition of KOH. This deduction was consistent with the detection of the band at 1052cm−1 in the FTIR spectra of membranes generated after amination, as well as after the addition of KOH. The main conclusion is that no AEMs were generated by the reaction of the 3M precursor with TMA, DMP, and DABCO: The amines acted as catalysts for the hydrolysis of the sulfonyl fluorides and led to the generation of the SO3− anions. The conclusions of this study are expected to be valid for synthetic efforts to obtain AEMs starting with other sulfonyl fluoride polymers, such as the Nafion™ precursor.

Hydroxide-ion induced degradation pathway for dimethylimidazolium groups in anion exchange membranes

The alkaline stability of imidazolium-based anion exchange membranes was studied by FT-IR and 13C solid state NMR. Imidazolium groups are susceptible to attack by hydroxide ions and undergo degradation in alkaline conditions through a ring-opening mechanism. Density functional theory (DFT) studies were performed to better understand the degradation pathway and the relative stability of imidazolium cations on exposure to hydroxide. The ring-opening degradation pathway follows the following three steps: (a) a nucleophilic reaction, (b) a ring-opening reaction, and (c) a rearrangement reaction. The structural changes induced in the imidazole ring were studied using a model of the dimethylimidazolium cation ([DMIM]+). The energy changes along the degradation pathway were obtained from reaction path calculations. The effect of hydroxide ion concentration and dielectric constant of surrounding medium on reaction energy barriers were studied. The hydroxide-ion induced degradation of the dimethylimidazolium cation was found to be more facile in high-pH environments. Good solvation of the dimethylimidazolium cation and the hydroxide ion was helpful in minimizing the degradation and enhancing the chemical stability of anion exchange membranes based on these cations.

Best Practices for Examining Anion Exchange Membrane Alkaline Stability for Solid-State Alkaline Fuel Cells

Alkaline fuel cells (AFCs) with an anion exchange membrane (AEM) electrolyte are promising electrochemical energy conversion devices since they can operate without platinum group metals. In the past decade, AEMAFC performance has improved substantially due to improvements in electrode binders and AEM ionic conductivity. However, the alkaline (in)stability of AEMs is a significant hurdle that precludes commercialization of this technology. To date, there have not been any satisfactory strategies to adequately assess an AEM’s suitability for an AFC. Here, we report a best practices approach to evaluate a leading AEM candidate (poly(2,6-dimethyl 1,4-phenylene) oxide (PPO) with quaternary ammonium groups) for AEMAFCs. In this work, PPO AEMs were immersed in alkaline solutions at elevated temperatures and were characterized via functional tests (ion-exchange capacity and mechanical properties) and via two-dimensional NMR techniques. The data generated from these tests provided mechanistic insights into the degradation of PPO AEMs in alkaline conditions.

Stability of composite anion exchange membranes with various functional groups and their performance for energy conversion

In this study, we report the relative stabilities of anion exchange membranes (AEMs) comprised of different functional groups. The base membrane was synthesized from vinylbenzyl chloride (VBC) that was cross-linked by divinylbenzene (DVB) using a porous polyethylene (PE) substrate, and subsequently quaternized by functional groups including ammonium, diammonium, and phosphonium types. Every synthetic process was confirmed by FTIR spectra, and hydroxide ion conductivity, ion-exchange capacity, and water uptake. For the various functional groups, the membrane stabilities were examined under alkaline and accelerated oxidative conditions. The membranes were found to be reasonably stable under alkaline conditions, whereas the oxidative stability was significantly dependent on the structures of the functional groups. Moreover, an alkaline fuel cell test was performed at 60°C for selected membranes. Membranes quaternized with trimethylamine (PE-TMA) and triethylamine (PE-TEA) showed the highest power density, while PE-TEA exhibited a higher oxidative stability. Also, the PE-TEA composite membrane was successfully tested for a non-aqueous vanadium redox flow battery.

Assessing the influence of different cation chemistries on ionic conductivity and alkaline stability of anion exchange membranes

Polysulfone (PSF) backbones were functionalized with reactive chloromethyl groups for preparing thin film anion exchange membranes (AEMs) with fixed benzyl quaternary cations. Three different cation chemistries of varying basicity were evaluated: 1,4-dimethylpiperazinium (DMP+), trimethylammonium (TMA+), and trimethylphosphonium (TMP+). The water uptake, ionic conductivity, and stability in alkaline media of these AEMs were assessed with both chloride and hydroxide counteranions. The results obtained revealed that the basicity value of the free base conjugate of the functionalized quaternary cations correlated well with gains in ionic conductivity. Cation basicity also correlated well with the alkaline stability of cations with the same inorganic atom, but was not an appropriate heuristic for comparing alkaline stability across cations with different inorganic atoms. The alkaline stability studies indicated that the primal degradation pathway of the TMA+ cation differed from that of the TMP+ cation (direct nucleophilic attack versus ylide formation). PSF with TMA+ and DMP+ cations were demonstrated to show alkaline fuel cell performance that reflected their respective ionic conductivity values.