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Materials 2021, 14, 1617 5 of 20 calculated under pseudo steady-state conditions by the time dependence of using the formula shown below: CB(t)= A DKCA(t−to) (1) VB L where A and L are the membrane area and thickness respectively; D, K, and to are the ethanol diffusivity, solubility, and the measurement time lag, respectively. The product DK is the membrane permeability and was calculated from the slope of the linear plot of CB against t according to the following equations: slope = d(CB) = A DKCA (2) dt VBL VB L Permeability = DK = slope x A C 2.4.3. Ion-Exchange Capacity (IEC), Alcohol and Water Uptake, and Dimensional Stability Determination (3) The ion-exchange capacities (IECs) of the cast membranes were determined by acid- base titrations using 1-phenolphtalein as the end-point indicator. Typically, a membrane after vacuum drying at 70 ◦C for 24 h was equilibrated with 1 M NaCl to fully exchange the protons (H+) with sodium ions. The IEC was calculated according to the formula: IEC= MNaOHxVNaOH (4) mdry (i) Water uptake was calculated by the difference in weights between dry and wet mem- brane samples. The dry weight, mdry, was measured after a sample was placed in a vacuum oven at 80 ◦C for 24 h. The wet weight, mwet, was measured after equilibration of the dry sample in deionized water for another 24 h. (ii) Alcohol uptake was calculated by the difference in weights between dry and wet membrane samples. The dry weight, mdry, was measured after a sample was placed in a vacuum oven at 80 ◦C for 24 h. The wet weight, mwet, was measured after equilibration of the dry sample in alcohols (methanol and ethanol) for another 24 h. The dimensional stability of the membranes was estimated by the differences in the three linear dimensions (length, width, and thickness) between dry and wet samples in the liquid uptake tests. 2.4.4. Examination of Membrane Morphology The cross-section of the dry membrane was examined by FE-SEM on a JEOL JSM- 6700F (JEOL, Tokyo, Japan) operating at 5 kV. All SEM samples had been vacuum-dried at 70 ◦C for 24 h before the examination. The cross-section was obtained by fracturing a sample membrane sample in liquid nitrogen. The SEM sample pretreatment involved Pt shadowing on a JEOL JFC-1300 (JEOL, Tokyo, Japan) auto fine coater at 10 mA for 60 s. 2.4.5. MEA Preparation and DMFC Tests The preparation of membrane electrode assemblies (MEAs) using the cross-linked diblock membranes and Nafion 117 membranes followed these procedures: for the diblock membrane, a sample was sandwiched between two 5 cm2 commercial electrodes and hot-pressed at 90 ◦C and 16 kgf/cm2 for 60 s. For the preparation of Nafion 117-based MEA, hot-pressing was carried out at 140 ◦C and 16 kgf/cm2 for 90 s. The resulting MEAs were kept in tightly sealed containers before use. DMFC testing was carried out in a single microcell with an effective membrane area of 25 cm2 supplied by Fuel Cell Technologies Inc, NM, USA. The feed to the cell was regulated by a micropump. For this study, the feed was methanol solutions in ultrapure water with the following concentrations: 1.0, 2.0, 4.0, 8.0, and 16.0 M. The methanol solution was delivered to the anode compartment at a fixed flow rate of 5 cc/min, whereas dry oxygen was fed to the cathode compartment at a fixed APDF Image | Hydrophilic Cross-Linked Aliphatic Hydrocarbon Diblock Copolymer
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