Role of water environment in chemical degradation of a covalent organic framework tethered with quaternary ammonium for anion exchange membranes

The anion exchange membrane (AEM) is a main component for AEM fuel cells. Recently, a series of electrolytes based on covalent organic frameworks (COFs) functionalized with quaternary ammonium (QA) of showed extraordinary ionic conductivities thanks to the intrinsic porosity of the COF structures, which also provide a robust backbone for good mechanical strength. However, the chemical stability of the COF-based AEMs in alkaline conditions is yet to be understood. Here we systematically investigate the chemical degradation of the COF-based structures tethered with alkyl spacers by combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations. We find that the water environment protects the cationic groups from chemical degradation in terms of both physical and chemical effects, which play a synergistic role. Moreover, we introduce the effective density of water as an order parameter to quantitatively characterize the level of degradation of the COF-based systems with similar design of architecture. The results provide guidance for estimation of the chemical stability of COF-based AEMs.

Overcoming Acidic H2O2/Fe(II/III) Redox-Induced Low H2O2 Utilization Efficiency by Carbon Quantum Dots Fenton-like Catalysis

Fenton reaction has important implications in biology- and environment-related remediation. Hydroxyl radicals (^•OH) and hydroxide (OH^-) were formed by a reaction between Fe(II) and hydrogen peroxide (H₂O₂). The acidic H₂O₂/Fe(II/III) redox-induced low H₂O₂ utilization efficiency is the bottleneck of Fenton reaction. Electron paramagnetic resonance, surface-enhanced Raman scattering, and density functional theory calculation indicate that the unpaired electrons in the defects of carbon quantum dots (CQDs) and the carboxylic groups at the edge have a synergistic effect on CQDs Fenton-like catalysis. This leads to a 33-fold higher H₂O₂ utilization efficiency in comparison with Fe(II)/H₂O₂ Fenton reaction, and the pseudo-first-order reaction rate constant (k_obs) increases 38-fold that of Fe(III)/H₂O₂ under equivalent conditions. The replacement of acidic H₂O₂/Fe(II/III) redox with CQD-mediated Fe(II/III) redox improves the sluggish Fe(II) generation. Highly effective production of ^•OH in CQDs-Fe(III)/H₂O₂ dramatically decreases the selectivity of toxic intermediate benzoquinone. The inorganic ions and dissolved organic matter (DOM) in real groundwater show negligible effects on the CQDs Fenton-like catalysis process. This work presents a process with a higher efficiency of utilization of H₂O₂in situ chemical oxidation (ISCO) to remove persistent organic pollutants.

Investigation into the performance decay of proton-exchange membranes based on sulfonated heterocyclic poly(aryl ether ketone)s in Fenton's reagent

Sulfonated N-heterocyclic poly(aryl ether) proton-exchange membranes have potential applications in the fuel-cell field due to their favorable proton conduction capacity and stability. This paper investigates the changes in mass and performance decay, such as proton conduction and mechanical strength, of sulfonated poly(ether ether ketone)s (SPEEKs) and three sulfonated N-heterocyclic poly(aryl ether ketone) (SPPEK, SPBPEK-P-8, and SPPEKK-P) membranes in Fenton's oxidative experiment. The SPEEK membrane exhibited the worst oxidative stability. The oxidative stability of the SPPEK membrane is enhanced due to the introduction of phthalazinone units in the chains. The SPPEKK-P and SPBPEK-P-8 membranes exhibit better radical tolerance than the SPPEK membrane, with proton conductivity retention rates of 66% and 73% for 1 h oxidative treatment, respectively. In addition, the molecular chains of SPPEKK-P and SPBPEK-P-8 exhibit relatively little disruption. The pendant benzenesulfonic groups enhance the steric effects for reducing radical attacks on the ether bonds and reduce the hydration of molecular chains. The introduction of phthalazinone units decreases the rupture points in the main chain. Therefore, the radical tolerance of the membranes is improved. These results provide a reference for the design of highly stable sulfonated heterocyclic poly(aryl ether) membranes.

Are Radicals Formed During Anion-Exchange Membrane Fuel Cell Operation?

In this paper we present a study on stable radicals and short-lived species generated in anion-exchange membrane (AEM) fuel cells (AEMFCs) during operation. The in situ measurements are performed with a micro-AEMFC inserted into a resonator of an electron paramagnetic resonance (EPR) spectrometer, which enables separate monitoring of radicals formed on the anode and cathode sides. The creation of radicals is monitored by the EPR spin trapping technique. For the first time, we clearly show the formation and presence of stable radicals in AEMs during and after long-term AEMFC operation. The main detected adducts during the operation of the micro-AEMFC are DMPO-OOH and DMPO-OH on the cathode side, and DMPO-H on the anode side. These results indicate that oxidative degradation involving radical reactions has to be taken into account when stability of AEMFCs is investigated.

3D printed sample holder for in-operando EPR spectroscopy on high temperature polymer electrolyte fuel cells

Electrochemical cells contain electrically conductive components, which causes various problems if such a cell is analyzed during operation in an EPR resonator. The optimum cell design strongly depends on the application and it is necessary to make certain compromises that need to be individually arranged. Rapid prototyping presents a straightforward option to implement a variable cell design that can be easily adapted to changing requirements. In this communication, it is demonstrated that sample containers produced by 3D printing are suitable for EPR applications, with a particular emphasis on electrochemical applications. The housing of a high temperature polymer electrolyte fuel cell (HT-PEFC) with a phosphoric acid doped polybenzimidazole membrane was prepared from polycarbonate by 3D printing. Using a custom glass Dewar, this fuel cell could be operated at temperatures up to 140°C in a standard EPR cavity. The carbon-based gas diffusion layer showed an EPR signal with a characteristic Dysonian line shape, whose evolution could be monitored in-operando in a non-invasive manner.

Preparation and fuel cell performance of catalyst layers using sulfonated polyimide ionomers

Sulfonated polyimide (SPI-8) ionomers were used as binders in the catalyst layers, and their fuel cell performance was evaluated. SPI-8 ionomers functioned well in the anode with only minor overpotential even at low humidity (50% relative humidity (RH)). In contrast, the cathode performance was significantly dependent on the content and molecular weight of the ionomers and humidity of the supplied gases. Higher molecular weight of the ionomer caused larger potential drop at high current density at 80 and 100% RH since oxygen supply and/or water discharge became insufficient due to higher water uptake (swelling) of the ionomer. Similar results were obtained at higher ionomer content, because of the increase of thickness in the catalyst layer. The mass transport was improved with decreasing humidity, however, proton conductivity became lower. While the maximum values of j(@0.70 V) for all membrane electrode assemblies (MEAs) were ca. 0.35 A/cm(2), each electrode could have the different appropriate operating conditions. The results suggest that the parameters such as oxygen supply, proton conductivity, and water uptake and discharge need to be carefully optimized in the catalyst layers for achieving reasonable cathode performance with hydrocarbon ionomers.

Sulfonated poly(arylene ether sulfone ketone) multiblock copolymers with highly sulfonated blocks. Long-term fuel cell operation and post-test analyses

The stability of poly(arylene ether sulfone ketone) (SPESK) multiblock copolymer membranes having highly sulfonated hydrophilic blocks was tested in an operating fuel cell. The electrochemical properties and drain water were monitored during the test, followed by post-test analyses of the membrane. During a 2000-h fuel cell operation test at 80 °C and 53% RH (relative humidity) and with a constant current density (0.2 A cm(-2)), the cell voltage showed minor losses, with slight increases in the resistance. In the drain water, anions such as formate, acetate, and sulfate were observed. Post-test analyses of the chemical structure by NMR and IR spectra revealed that the sulfonated fluorenyl group with ether linkage was the most likely to have degraded during the long-term operation, producing these small molecules. The minor oxidative degradation only slightly affected the proton conductivity, water uptake, and phase-separated morphology.

Electron spin resonance. Part two: a diagnostic method in the environmental sciences

A review is presented of some of the ways in which electron spin resonance (ESR) spectroscopy may be useful to investigate systems of relevance to the environmental sciences. Specifically considered are: quantititave ESR, photocatalysis for pollution control; sorption and mobility of molecules in zeolites; free radicals produced by mechanical action and by shock waves from explosives; measurement of peroxyl radicals and nitrate radicals in air; determination of particulate matter polyaromatic hydrocarbons (PAH), soot and black carbon in air; estimation of nitrate and nitrite in vegetables and fruit; lipid-peroxidation by solid particles (silica, asbestos, coal dust); ESR of soils and other biogenic substances: formation of soil organic matter carbon capture and sequestration (CCS) and no-till farming; detection of reactive oxygen species in the photosynthetic apparatus of higher plants under light stress; molecular mobility and intracellular glasses in seeds and pollen; molecular mobility in dry cotton; characterisation of the surface of carbon black used for chromatography; ESR dating for archaeology and determining seawater levels; measurement of the quality of tea-leaves by ESR; green-catalysts and catalytic media; studies of petroleum (crude oil); fuels; methane hydrate; fuel cells; photovoltaics; source rocks; kerogen; carbonaceous chondrites to find an ESR-based marker for extraterrestrial origin; samples from the Moon taken on the Apollo 11 and Apollo 12 missions to understand space-weathering; ESR studies of organic matter in regard to oil and gas formation in the North Sea; solvation by ionic liquids as green solvents, ESR in food and nutraceutical research.