TY - THES ID - 138268241 TI - Evaluation of layered double hydroxides in anion exchange composite membranes for water vapor electrolysis AU - Stalmans, Leander AU - Rongé, Jan AU - Martens, Johan. AU - KU Leuven. Faculteit Bio-ingenieurswetenschappen. Opleiding Master in de bio-ingenieurswetenschappen. Milieutechnologie (Leuven) PY - 2018 PB - Leuven KU Leuven. Faculteit Bio-ingenieurswetenschappen DB - UniCat UR - https://www.unicat.be/uniCat?func=search&query=sysid:138268241 AB - In the search for a highly flexible energy conversion system for production of hydrogen gas from renewable energy sources, a water vapor-fed electrolyser is developed. The use of water vapor instead of liquid water establishes significant flexibility gains, but also invokes challenges for all components that make up the electrolysissystem. Assolidelectrolyte,ananionexchangemembraneisneededthat maintains good ionic conductivity at a varying relative humidity. For this purpose, composite membranes with poly(vinyl alcohol) (PVA), impregnated with 4 M KOH, and layered double hydroxides (LDHs) were investigated in this master thesis. In a first step, various LDHs were screened for ionic conductivity and water uptake, while more information on the mobility of the charge-balancing anion and the hydroxide ions in the LDH interlayer was gathered. Electrochemical impedance spectroscopy showed clear differences in ionic conductivity, with the valence to dehydrated ion radius ratio as a promising predictor. According to literature, only the hydroxide ions contribute to this conductivity, while the charge-balancing anion is assumed immobile. However, in-depth electrochemical characterization of the LDHs,inwhichthewatersplittingreactionwasusedtolooksolelyatthehydroxide ions, showed that gradients in hydroxide concentration can form within the LDHs. This concentration polarization effect, which is reported here for the first time, can only happen when the charge-balancing anion is mobile as well. A PVA membrane and two composite membranes with 10 wt% of either nitrateorcarbonate-intercalatedLDH,i.e. “PVAMgAlNO3”and“PVAMgAlCO3”respectively, were investigated thoroughly. The water uptake of the PVA membrane was shown to be enhanced by LDH addition. The total water uptake of the composite membrane even surpassed the expected uptake, based on the sum of the contributions of both constituents. This points towards interaction effects between the LDHs and the PVA matrix. In the electrochemical tests, the PVA MgAl CO3 showed slightly higher in-plane ionic conductivity at 60, 80 and 95 % relative humidity than the bare PVA membrane. The PVA MgAl NO3 membrane showed poor homogeneityduetoissuesinthesynthesismethod,andthereforeshowedpoorelectrochemical properties and performance. In cyclic voltammetric and chronopotentiometric experiments in an electrolysis cell at different relative humidities, the PVA MgAl CO3 membrane was outperformed by the PVA membrane. The differences were mainly ascribed to stronger diffusion limitation of water. Apart from the issues in the synthesis procedure, strong indications were found that slight differences in the procedure of impregnating the membrane with KOH can also cause large changes in performance, obscuring the actual effect of the LDH addition. Therefore, it is suggested to perform future testing with homogeneous anion exchange membranes, that have fixed cationic head-groups for ionic conductivity. ER -