The structure of a commercial sulfonated poly(ether ether ketone) (sPEEK) membrane was analyzed by Small-Angle X-Ray Scattering (SAXS) for different water uptakes obtained after immersion in liquid water at various temperatures. For low membrane swelling, the SAXS profile displays only a wide-angle peak in the 0.2–0.3 Å⁻¹ region. As the membrane swells, two supplementary correlation peaks arise and shift towards small angles, which are the signature of a structural evolution of the membrane, whereas the wide angle peak remains stable. The SAXS spectra of sPEEK membranes can thus display three correlation peaks simultaneously. Therefore we propose a new interpretation of these SAXS spectra which conclude that the two small angle peaks are attributed to the so-called matrix and ionomer peaks and the wide-angle peak is ascribed to the mean separation distance between sulfonic acid groups grafted onto the polymer backbone. This peak attribution implies that the sPEEK nano-phase separation is triggered by an immersion in hot water (ionomer peak apparition). Our new peak attribution was confirmed by studying the impact of temperature, electron density contrast and ionic exchange capacity.

Via SAXS, herein, we studied how a sPEEK microstructure evolves when it is immersed in water at a wide range of temperatures (20–100 °C) and time scales (from a few hours to dozens of days). In particular, we scrutinized the behavior of sPEEK at the temperature and time associated with the appearance of a well-defined nanosegregated morphology. At 80 °C, we observed nanoscale swelling along with smoothing of the water/polymer interface over a long period of time (several days). Herein, two of the main membrane properties, i.e., water uptake and proton conductivity, were studied for different immersion times and temperatures. It was found that the abovementioned properties were remarkably correlated with the evolution of the membrane microstructure, which was partly conserved after drying. The present findings helped us to understand that the thermally activated evolutions observed at both the nanoscale and macroscale were associated with the sPEEK β-relaxation crossover. Therefore, the very different swelling behaviors of sPEEK and Nafion are correlated to the much higher β-relaxation of sPEEK vs. Nafion (75 °C vs. −20 °C – dry state). From a practical viewpoint, this study emphasizes, for membranes alternative to Nafion, the importance and impact of the membrane pretreatment on their functional properties.

Deep eutectic solvents (DESs) are of particular interest for electrometallurgy processes, since intrinsically conductive and electrochemically stable in a wide range of potentials. Cheaper and greener than conventional ionic liquids, DESs are often bio-sourced and exhibit a higher biodegradability. Ethaline, a DES composed of choline chloride (ChCl) as hydrogen bond acceptor and ethylene glycol (EG) as hydrogen bond donor in 1:2 molar proportions (Et 1:2), is commonly used in electrometallurgy thanks to its good transport properties. However, if ChCl can be considered as a “green” reactant, this is not the case for EG. A DES with a lower toxicity can be obtained by replacing EG by propylene glycol (PG), widely used in cosmetics and pharmacology, yielding a DES called Propeline. The present paper explores the potential of this lesser-known DES in the electrometallurgy of precious metals. Because changing the hydrogen bond donor leads to a modification in the DES bulk properties, the first part of this work deals with the determination of PG-based DESs’ density, viscosity, and conductivity, which are properties of interest for electrochemical processes. The influence of water and PG content is presented and values are compared to those of Ethaline. It appears that ChCl:PG in a molar ratio 1:3 (Pr 1:3) presents the best transport properties. The potentiality of this solvent for the electrometallurgy of precious metals is then discussed: electrochemical stability and electrochemical systems of Ag, Pd, and Au are compared in Pr 1:3 and Et 1:2. Finally, diffusion coefficients of the metallic species and the DES components are given, determined by electrochemical and NMR techniques, respectively.