A Nature-Inspired Antioxidant Strategy based on Porphyrin for Aromatic Hydrocarbon Containing Fuel Cell Membranes

The use of hydrocarbon-based proton conducting membranes in fuel cells is currently hampered by the insufficient durability of the material in the device. Membrane aging is triggered by the presence of reactive intermediates, such as HO⋅, which attack the polymer and eventually lead to chain breakdown and membrane failure. An adequate antioxidant strategy tailored towards hydrocarbon-based ionomers is therefore imperative to improve membrane lifetime. In this work, we perform studies on reaction kinetics using pulse radiolysis and γ-radiolysis as well as fuel cell experiments to demonstrate the feasibility of increasing the stability of hydrocarbon-based membranes against oxidative attack by implementing a Nature-inspired antioxidant strategy. We found that metalated-porphyrins are suitable for damage transfer and can be used in the fuel cell membrane to reduce membrane aging with a low impact on fuel cell performance.

Dynamics of Ion Pairing in Dilute Aqueous HCl Solutions by Spectroscopic Measurements of Hydroxyl Radical Conversion into Dichloride Radical Anions

The rate of formation of dichloride anions (Cl₂^•–) in dilute aqueous solutions of HCl (2–100 mmol·kg^–1) was measured by the technique of pulse radiolysis over the temperature range of 288–373 K. The obtained Arrhenius dependence shows a concentration averaged activation energy of 7.3 ± 1.8 kJ·mol^–1, being half of that expected from the mechanism assuming the ^•OHCl^– intermediate and supporting the ionic equilibrium-based mechanism, i.e., the formation of Cl₂^•– in the reaction of ^•OH with a hydronium–chloride (Cl^–·H₃O⁺) contact ion pair. Assuming diffusion-controlled encounter of the hydronium and chloride ions and including the effect of the ionic atmosphere, we showed that the reciprocal of τ, the lifetime of (Cl^–·H₃O⁺), follows an Arrhenius dependence with an activation energy of 23 ± 4 kJ·mol^–1, independent of the acid concentration. This result indicates that the contact pair is stabilized by hydrogen bonding interaction of the solvent molecules. We also found that at a fixed temperature, τ is noticeably increased in less-concentrated solutions (m_HCl < 0.01 m). Since this concentration effect is particularly pronounced at near ambient temperatures, the increasing pair lifetime may result from the solvent cage effect enhanced by the presence of large supramolecular structures (patches) formed by continuously connected four-bonded water molecules.

Ionic-Equilibrium-Based Mechanism of •OH Conversion to Dichloride Radical Anion in Aqueous Acidic Solutions by Kinetic and Theoretical Studies

A new mechanism for the dichloride radical anion (Cl₂^•-) formation in diluted acidic chloride solutions is proposed on the grounds of pulse radiolysis measurements of the optical absorption growth at 340 nm and the density functional theory and Hartree-Fock computations. We show that the rate of ^•OH conversion into Cl₂^•- is determined by the equilibrium concentration of the ionic pair H₃O⁺·Cl^-. According to the proposed mechanism, the diffusional encounter of ^•OH and H₃O⁺·Cl^- is followed by fast concerted charge/proton transfer ( k(25 °C) = 6.2 × 10¹² s^-1) to yield Cl^•, which then reacts with Cl^- to produce Cl₂^•-. The mechanism has been confirmed by the observed first-order growth of the Cl₂^•- absorption and a direct proportionality of the rate constant to the activities of H₃O⁺ and Cl^- ions. The salt effect on the rate of Cl₂^•- formation is due to the ionic strength effect on the equilibrium H₃O⁺ + Cl^- ⇄ H₃O⁺·Cl^-.

Rate constant for the H˙ + H2O → ˙OH + H2 reaction at elevated temperatures measured by pulse radiolysis

Maintaining the structural integrity of materials in nuclear power plants is an essential issue associated with safe operation. Hydrogen (H₂) addition or injection to coolants is a powerful technique that has been widely applied such that the reducing conditions in the coolant water avoid corrosion and stress corrosion cracking (SCC). Because the radiation-induced reaction of ˙OH + H₂ → H˙ + H₂O plays a crucial role in these systems, the rate constant has been measured at operation temperatures of the reactors (285-300 °C) by pulse radiolysis, generating sufficient data for analysis. The reverse reaction H˙ + H₂O → ˙OH + H₂ is negligibly slow at ambient temperature; however, it accelerates considerably quickly at elevated temperatures. Although the reverse reaction reduces the effectiveness of H₂ addition, reliable rate constants have not yet been measured. In this study, the rate constants have been determined in a temperature range of 250-350 °C by pulse radiolysis in an aqueous I^- solution.

Analysis of redox states of protic and aprotic solutions irradiated by low linear energy transfer carbon-ion beams using a 2,2-diphenyl-1-picrylhydrazyl radical

The redox states of protic and aprotic solutions were evaluated after carbon-ion irradiation.

Radiation induced environmental remediation of Cr(VI) heavy metal in aerated neutral solution under simulated industrial effluent

Cr(VI) compounds are major water contaminants in most industrial effluents, due to their carcinogenicity, while Cr(III) is an important element for human metabolism. In a previous work, we showed that Cr(VI) was radiolytically reduced to Cr(III) by the CO2ˉ• radical at pH 3 N2O-saturated solution in the presence of formate. Here in the present work, this removal was investigated by steady state irradiation and pulse radiolysis in aerated solution at neutral pH, which is close to natural conditions in most wastewaters, where the reducing agent is the superoxide radical anion O2ˉ•. The degradation of Cr(VI) increased linearly with the absorbed dose and was significantly enhanced by the added formate but not by the radiolitically produced hydrogen peroxide at this pH. The rate constant for this reduction was found to be 1.28×108 M−1 s−1 and the absorption spectrum of Cr(V) transient species was obtained. A partial recovery of Cr(VI) is observed over a period of ca. 5 ms following a second order kinetics with a rate constant 8.0×106 M−1 s−1. These outcomes suggest that gamma-irradiation of Cr(VI)-contaminated wastewaters and industrial effluents in presence of formate can be simple, effective and economical means for the remediation of this major contaminant.

Pilot-scale study of the radiation-induced silica removal from underground brackish water in Saudi Arabia

Silica scaling deposition in industrial water systems is one of the biggest challenges facing the water treatment industry due the low solubility of the scalants in the feed waters. In this preliminary work, we investigated the effectiveness of the ionizing radiation induced removal of silica in water sample from the Salbukh, Saudi Arabia, water treatment plant by using metallic iron as the source of ferric hydroxide to co-precipitate the silica. The influence of several reaction parameters, i.e. iron powder dosage, radiation dose, initial pH and equilibrium pH effect were investigated. In the optimum conditions, up to 75% of silica was removed. This preliminary study showed that this environmentally friendly process is effective in silica removal from underground water.

Revisited water radiolysis at elevated pH by accounting O3˙− kinetics at low and high LET

Recordings of O₃˙⁻ formation and decays under low and high LET radiations and at pH 13.2 allowed revisiting the rate constants of its reactions with O₂˙⁻ and HO₂⁻.