Enhancement of sulfonated poly ether ether ketone based electrolytes for bipolar membrane fuel cell

Abstract

The limitation of the existing bipolar membrane fuel cell (BPMFC) is the produced power output which is not efficient due to water flooding at the junction layer of the proton exchange membrane (PEM) and anion exchange membrane (AEM). In worse situations, this membrane fuel cell leads to delamination between the PEM and AEM layer of a bipolar membrane (BPM) or disintegration between the membrane layer and electrode part. The electrolyte material and the design of the hot-press parameters for BPM electrode assembly are influential factors in hydrated membrane disintegration that reduce the BPMFC performance. Thus, this research developed BPM-based sulfonated poly(ether ether ketone) (sPEEK) by considering the membrane synthesizing method and designing adhesion parameters to improve the compatibility of the PEM/AEM junction layer. The sPEEK was composited with two different materials; titanium dioxide (TiO2) with 0.5 – 2.0 wt.% and polyethersulfone (PES) with 5 – 20 wt.%, as PEM anode. Meanwhile, AEM cathode was prepared by the crosslinking of a sulfonic acid group from sPEEK with a quaternary ammonium group from trimethyl-amine that is known as crosslinked quaternary ammonium PEEK (cQAPEEK) membrane at three different chloromethylation times of 48, 72, and 96 hours. The phase inversion via heating technique was applied for preparing PEM and AEM electrolytes. The developed PEM and AEM electrolytes were characterized according to their structures, morphologies, thermal and mechanical stabilities, and physiochemical and electrochemical properties. Then, the PEM and AEM were hot-pressed to develop BPM. The design of the hot-pressed parameters was based on pressure and temperature while the time was made constant. To obtain the optimum parameters, the design was determined based on response surface methodology (RSM) analysis. The results for PEM showed that sPEEK/PES possessed the highest proton conductivity of 7.18 mS cm-1 for 5 wt.% PES, whereas sPEEK/TiO2 PEM obtained the highest proton conductivity of 9.08 mS cm-1 for 0.5 wt.% TiO2 with excellent mechanical and thermal stabilities. Both of these optimum composites membranes were used in BPM with the best cQAPEEK AEM which had the highest anion conductivity of 5.38 mS cm-1 when chloromethylized for 72 hours. Based on RSM analysis, the optimal pressure and temperature for hot-pressed PEM and AEM were 3 tons/square inch and 120 °C. It produced the best adhesion of membranes where no gap existed between AEM and PEM as proven through scanning electron microscopy analysis. Furthermore, there was no excessive attachment at the PEM/AEM junction and this provided low ionic resistance and created a better ion pathway at the junction. The ionic conductivity of BPM sPEEK/PES5-cQAPEEK72h was 8.16 mS cm-1, while BPM sPEEK/TiO2(0.5)-cQAPEEK72h showed 8.39 mS cm-1 of ionic conductivity. In terms of power output, the sPEEK/TiO2(0.5)-cQAPEEK72h showed higher peak power density, which is 53.12 mW cm-2 with increment of about 3.13 % due to better ionic conductivity than sPEEK/PES5-cQAPEEK72h (51.51 mW cm-2). However, the sPEEK/PES5-cQAPEEK72h showed lower hydrogen/oxygen fuel permeation during operation than sPEEK/TiO2(0.5)-cQAPEEK72h and Nafion 117-cQAPEEK72h according to voltage versus time graph which indicates that it has excellent membrane durability. By considering power output as the main investigated parameter, this study chose sPEEK/TiO2-cQAPEEK72h as the best BPM electrolyte due to its high performance and sufficient durability. Remarkably, all the developed membranes were in good condition without any disintegration of the layers after testing at various temperatures and environments. This research showed that the BPM electrolyte based modified PEEK provided the best BPM material and adhesion design of the 3 tons/square inch pressure and 120 °C temperature demonstrated a better degree of adhesion between the PEM and the AEM.