Development of novel PBI-blended membranes for SO2 electrolysis
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In light of the increased applicability of hydrogen as a clean alternative fuel source, the demand for hydrogen and the subsequent production thereof is expected to grow significantly in the near future. From the hydrogen production technologies investigated thus far, the Hybrid Sulfur (HyS) thermo-chemical cycle, using a proton exchange membrane- based (PEM) electrolyser, has shown potential at larger scale. During the HyS process, H2SO4 is decomposed in a high temperature step to produce SO2, O2 and H2O, of which the SO2 is then electrochemically oxidised to produce H2SO4 and H2 in the presence of water during SO2 electrolysis. For the electrolysis, a membrane is required that possesses outstanding thermal and chemical (H2SO4) stability while maintaining high proton conductivities in limited humidification. Since optimum electrolysis performance was predicted at temperatures above 120 °C, Nafion would not be suitable, whereas initial studies on PBI-based membranes seem promising. It was hence the purpose of this study to develop and evaluate novel PBI-based membranes in terms of membrane composition (polymer, cross-linking and ratio), H2SO4 stability (ex situ) and SO2 electrolyser performance (in situ) to identify suitable PEMs for future SO2 electrolyser applications above 100 °C. For this purpose, partially fluorinated and non-fluorinated bromo-methylated polymers (BrPAE-1 and BrPAE -2, respectively) were successfully synthesised and functionalised to be included as blend components with SFS and F6PBI. The H2SO4 stability (80 wt% H2SO4 at 100 °C for 5 days) of specifically the fluorinated polybenzimidazole (F6PBI) was excellent at effectively resisting sulfonation. The presence of partial sulfonation and dissolution (% wt changes) after H2SO4 treatment of the SFS, BrPAE-1 and -2 polymers, however, confirmed the need for further stabilisation by cross-linking. To study this, various combinations of polymers were combined. While this improved the stability somewhat, the introduction of ionic cross-links between, for example SFS and BrPAE-1/2 alone was not sufficient, requiring the addition of the highly stable F6PBI. Subsequently, ionic- and covalent cross-linking were combined to yield a 4-component PBI-based blend membrane. Twelve different 4-component combinations were prepared with varying acid-base ratios (A-, B-, and Ci-iv). The 4-component PBI-blend membranes containing only partially fluorinated acidic (SFS) and basic (F6PBI, BrPAE-1) polymers, again displayed the highest H2SO4 stability. The highest proton conductivity was measured for the blend membrane 1Ai (48 mS/cm at 120 °C). Three additional 4-component blend A type membranes (included sPPSU and PBIOO) were included, where blend 1Ai again displayed the highest H2SO4 stability (%wt change < 2 %; IECDirect change < 12 %, TGA degradation > 275 °C). The SO2 electrolysis performance (at 80 °C and 95 °C) of specifically the 1Ai blend surpassed that of the benchmark Nafion®115. At 120 °C, a significant decrease in cell voltage (up to 150 mV or nearly 50 %) was obtained for 1Ai when reaching the maximum current density of 1.0 A/cm2. When testing 1Ai MEAs with varying thicknesses and catalyst loadings At 120 °C, a trade-off was found to exist between the concentration of H2SO4 produced and the performance attainable (cell voltages at maximum current densities). Steady state measurements at 0.3 A/cm2 for 10 hours revealed an improvement in cell voltages (decrease of 60-175 mV). When comparing the voltage stepping at 80 °C and at 120 °C, a small current density increase (0.05 - 0.14 A/cm2) was noted at 120 °C. Post treatment characterisation (SEM and TGA-FTIR) confirmed that both the1Ai membrane and the hot-pressed catalyst remained stable during the electrolyser operations investiagted at 120 °C. It was concluded from the study that the combined fluorinated nature of acidic (SFS) and basic (F6PBI, BrPAE-1) polymers in the 4-component membrane 1Ai contributed to a more compatible blend with improved H2SO4 stability and sufficient conductivity at temperatures below and above 100 °C. This was found to be in agreement with the improved SO2 electrolyser performance noted at 80 °C and at 120 °C. This concludes that the aim set for developing a novel PBI-based blend membrane suitable for SO2 electrolyser application, both at 80 and 120 °C, was achieved.