Anion exchange membranes are an important component in alkaline electrochemical energy conversion and storage devices, and their alkaline stability plays a crucial role for the long-term use of these devices. Herein, a systematic study was conducted for the analysis of polymer backbone chemical stability in alkaline media. Nine representative polymer structures including poly­(arylene ether)­s, poly­(biphenyl alkylene)­s, and polystyrene block copolymers were investigated for their alkaline stability. Polymers with aryl ether bonds in their repeating unit showed poor chemical stability when treated with KOH and NaOCH3 solutions, whereas polymers without aryl ether bonds [e.g., poly­(biphenyl alkylene)­s and polystyrene block copolymers] remained stable. Additional NMR studies and density functional theory (DFT) calculations of small molecule model compounds that mimic the chemical structures of poly­(arylene ether)­s confirmed that electron-withdrawing groups near to the aryl ether bonds in the repeating unit accelerate chemical degradation. Results from this study suggest that the use of all-carbon-based polymer repeating units (i.e., polymers not bearing aryl ether bonds) can enhance long-term alkaline stability of anion exchange membranes in electrochemical energy devices.

A series of manganese complexes were synthesized with a variety of ligands and ligand substituents. These complexes were then studied using ultraviolet–visible spectroscopy, cyclic voltammetry, density functional theory calculations, and bulk electrolysis. The UV–vis, cyclic voltammetry, and calculation data show that the bipyridine π* level is modulated by the incorporation of different substituents on the bipyridine and through this interaction moderates the observed catalytic activity of the complex toward CO2 reduction. The calculations were correlated to the experimental UV–vis data and cyclic voltammetry data to demonstrate the relationship among these data, and a Hammett plot showed a good correlation between the substituent identity and the MLCT wavelength from UV–vis (R 2 = 0.96). When aliphatic substituents were placed on the 4,4′-positions of the bipyridine, the location of the bpy π* was not significantly altered. However, when more electron withdrawing substituents were placed on the 4,4′-positions the bpy π* level was altered more significantly. This alteration in the bpy π* level had a profound effect on the rate of CO production determined from bulk electrolysis. While complexes whose bpy π* level were similar or more blue shifted in comparison to the parent manganese complex did not display significantly altered efficiencies or rates for the conversion of CO2 to CO, those species whose bpy π* energies were significantly red shifted in comparison to the parent manganese complex displayed far poorer catalysis. This is postulated to be a combination of two factors. First, the singly reduced complex’s ability to lose the axial bromide ligand is diminished when electron-withdrawing groups are placed on the bpy ligand due to an increasing gap between the bpy π* and the Mn–Br σ*. Second, the decreased electron density of the HOMO of the doubly reduced complex with electron-withdrawing groups makes the binding of a molecule of CO2 less energetically favorable.

The increased interest in the use of anion exchange membranes (AEMs) for applications in electrochemical devices has prompted significant efforts in designing materials with robust stability in alkaline media. Most reported AEMs suffer from polymer backbone degradation as well as cation functional group degradation. In this report, we provide comprehensive experimental investigations for the analysis of cation functional group stability under alkaline media. A silver oxide-mediated ion exchange method and an accelerated stability test in aqueous KOH solutions at elevated temperatures using a Parr reactor were used to evaluate a broad scope of quaternary ammonium (QA) cationic model compound structures, particularly focusing on alkyl-tethered cations. Additionally, byproduct analysis was employed to gain better understanding of degradation pathways and trends of alkaline stability. Experimental results under different conditions gave consistent trends in the order of cation stability of various QA small molecule model compounds. Overall, cations that are benzyl-substituted or that are near to electronegative atoms (such as oxygen) degrade faster in alkaline media in comparison to alkyl-tethered QAs. These comprehensive model compound stability studies provide valuable information regarding the relative stability of various cation structures and can help guide researchers towards designing new and promising candidates for AEM materials.

A mechanistic analysis showing the critical importance of an intramolecular hydrogen bond for improved insight and understanding in CO2 electroreduction.

A cyanide-bridged di-manganese complex, {[Mn(bpy)(CO)3]2(μ-CN)}+, is introduced as an efficient electrocatalyst and photochemically active for proton-assisted of CO2 reduction to CO.