H2SO4 stability of PBI–blend membranes for SO2 electrolysis
Schoeman, Johannes Gerhardus
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Alternative energy sources are needed if the current use of energy is to be sustained while reducing global warming. A possible alternative energy source that has significant potential is hydrogen. For hydrogen to become a serious contender for replacing fossil fuels, the production thereof has to be further investigated. One such process, the membrane–based Hybrid Sulphur (HyS) process, where hydrogen is produced from the electrolysis of SO2, has received considerable interest recently. Since H2SO4 is formed during SO2 electrolysis, H2SO4 stability is a prerequisite for any membrane to be used in this process. In this study, pure as well as high and low temperature blended polybenzimidazole (PBI), partially fluorinated poly(arylene ether) (sFS) and nonfluorinated poly(arylene ethersulphone) (sPSU) membranes were investigated in terms of their acid stability as a function of acid concentration by treating them in H2SO4 (30, 60 and 90wt%) for 120h at 1bar pressure. The high temperature blend membranes contain the basic polymer in excess (70 wt% basic PBI and 30wt% acid sPSU/sFS polymer) and require acid doping in order to conduct protons. In the doped state they are able to conduct protons up to 200°C. The low temperature blend membranes are also composed of the same PBI polymer used in the high temperature membranes, as well as the same acidic polymers with one of the membranes containing a fluorinated polymer and the other a nonfluorinated polymer (sFS or sPSU) in excess. These membranes do not require any acid doping to conduct protons but they are only stable at temperatures below 80°C. High temperature blend membranes were characterised using through–plane conductivity, GPC and IEC, whilst low temperature membranes were characterised using in–plane and through–plane proton conductivity, weight change, TGA, GPC, SEM, EDX and IEC techniques. The conductivity determination techniques (especially the in–plane technique) proved to be cumbersome, whilst all the other analysis techniques were deemed appropriate. H2SO4 exposure had a destabilising effect on the PBI membrane which presented as weight gain at the 30 and 60wt% H2SO4 concentrations due to salt formation and dissolution at the 90wt% acid treatment due to sulphonation. In the sFS membrane dissolution was observed at 30 and 60wt% as a result of oligomer loss that occurred during the post treatment washing process and partial dissolution, as a result of sulphonation, at the 90wt% treated membrane. The sPSU membrane showed great stability at 30 and 60wt%, though dissolution was observed at 90wt% because of membrane sulphonation due to a lack of fluorination. The sFS–PBI membrane blend proved to be stable with only slight degradation taking place at 90wt% treatment due to sulphonation. Similarly the sPSU–PBI blend membrane showed great stability at the 30 and 60wt% H2SO4 treatment concentrations however total dissolution occurred at 90wt% treatment again due to a lack of fluorination. Although both the low temperature blended membranes showed superb stability to H2SO4 concentrations expected in the SO2 electrolyser (30–40wt%), the low temperature blended sFS–PBI membrane seemed slightly more stable over the H2SO4 treatment concentration range (30–90wt%), due to the protective role of the fluorinated polymer. The superior acid stability of this membrane could prove vital for proper SO2 electrolysis, especially for prolonged periods of operation.
- ETD@PUK