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Tesi etd-01152020-113557

Thesis type
Tesi di laurea magistrale
Preparation and characterization of polymeric anion exchange membranes for green hydrogen production
Corso di studi
relatore Dott.ssa Martinelli, Elisa
relatore Prof. Galli, Giancarlo
Parole chiave
  • hydrogen
  • anion exchange membrane
  • electrolyzers
  • grafted copolymers
Data inizio appello
Data di rilascio
Riassunto analitico
Vinylbenzyl chloride (VBC) polymer chains were grafted from a commercially available poly(styrene-b-butadiene‐b‐styrene) (SBS) thermoplastic elastomer to produce two sets of VBC-grafted SBS copolymers by following two different grafting-from synthetic approaches, namely the conventional free radical polymerization and the controlled radical polymerization mediated by TEMPO. Special attention was devoted to varying the experimental parameters in the latter approach, including reagent concentration, reaction time, amount of initiator, TEMPO/initiator and VBC/TEMPO mole ratios. It was thus possible to modulate the mole content of grafted VBC (or degree of functionalization) in the final copolymers in the optimal range 6%–8% and to better control the length of the VBC grafts. The chemical-physical properties of the copolymers obtained by the two different grafting-from methods were compared and the differences highlighted. Moreover, by taking advantage of the controlled nature of TEMPO-mediated polymerization styrene-VBC random copolymer chains and styrene-VBC block copolymer chains were grafted from SBS.
Graft copolymers were used for the preparation of films by solution casting. Several parameters, including polarity and volatility of the solvent, controlled evaporation of the solvent, substrate of deposition, addition of a polymerization inhibitor, were systematically changed in order to optimize the solution casting procedure to high quality polymer membranes.
The –CH2Cl functional groups of the VBC grafts were then quantitatively converted to –CH2(CH3)3N+ quaternary ammonium groups by reaction with trimethylamine directly on the films, and the films therefrom were investigated as anion exchange membranes for potential application in a water electrolyzer. Mechanical tests confirmed the elastomeric character of the SBS-based membranes, with mechanical parameters, such as stress and strain at break and Young modulus, depending on the structure and chemical composition of the graft polymer chains. Water uptake for both hydroxyl and bicarbonate forms as well as electrochemical properties were also evaluated for the anion exchange membranes. In particular, the determined values of ionic conductivity ( 3.8 mS/cm at 20 °C) and ion exchange capacity ( 0.75 at 20 °C) were generally high and, in some cases, much higher than those of the commercial benchmark polymer membrane. However, hydrogen crossover was in any case  3.2 at 20 °C, much larger than that of the benchmark membrane. Such a hydrogen crossover appears to be a limitation for the actual use of these membranes in water electrolyzers and needs to be lowered by developing modified VBC-g-SBS copolymers in which the hydrogen permeability is significantly reduced.