Evaluation of process parameters and membranes for SO2 electrolysis
Krüger, Andries Johannes
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The environmentally unsafe by-products (CO2, H2S, NOx and SO2 for example) of using carbon-based fuels for energy generation have paved the way for research on cleaner, renewable and possibly cheaper alternative energy production methods. Hydrogen gas, which is considered as an energy carrier, can be applied in a fuel cell setup for the production of electrical energy. Although various methods of hydrogen production are available, sulphur-based thermochemical processes (such as the Hybrid Sulfur Process (HyS)) are favoured as alternative options for large scale application. The SO2 electrolyser is applied in producing H2 gas and H2SO4 by electro-chemically converting SO2 gas and water. This study focused firstly on the evaluation of the performance of the SO2 electrolyser for the production of hydrogen and sulphuric acid, using commercially available PFSA (perfluorosulfonic acid) (Nafion®) as benchmark by evaluating i) various operating parameters (such as cell temperature and membrane thickness), ii) the influence of MEA (membrane electrode assembly) manufacturing parameters (hot pressing time and pressure) and iii) the effect of H2S as a contaminant. Subsequently, the suitability of novel PBI polyaromatic blend membranes was evaluated for application in an SO2 electrolyser. The parametric study revealed that, depending on the desired operating voltage and acid concentration, the optimisation of the operating conditions was critical. An increased cell temperature promoted both cell voltage and acid concentration while the use of thin membranes resulted in a reduced voltage and acid concentration. While an increased catalyst loading resulted in increased cell efficiency, such increase would result in an increase in manufacturing costs. Using electrochemical impedance spectroscopy at the optimised operating conditions, the MEA manufacturing process was optimised with respect to hot press pressure and time, while the effect of selected operating conditions was used to evaluate the charge transfer resistance, ohmic resistance and mass transport limitations. Results showed that the optimal hot pressing conditions were 125 kg.cm-2 and 50 kg.cm-2 for 5 minutes when using 25 and 10 cm2 active areas, respectively. The charge transfer resistance and mass transport were mostly influenced by the hot pressing procedure, while the ohmic resistance varied most with temperature. Applying the SO2 electrolyser in an alternative environment to the HyS thermochemical cycle, the effect of H2S on the SO2 electrolyser anode was investigated for the possible use of SO2 electrolysis to remove SO2 from mining off-gas which could contain H2S. Polarisation curves, EIS and CO stripping were used to evaluate the transient voltage response of various H2S levels (ppm) on cell efficiency. EIS confirmed that the charge transfer resistance increased as the H2S competed with the SO2 for active catalyst sites. Mass transport limitations were observed at high H2S levels (80 ppm) while the ECSA (electrochemical surface area obtained by CO stripping) showed a significant reduction of active catalyst sites due to the presence of H2S. Pure SO2 reduced the effective active area by 89% (which is desired in this case) while the presence of 80 ppm H2S reduced the active catalyst area to 85%. The suitability of PBI-based blend membranes in the SO2 electrolyser was evaluated by using chemical stability tests and electrochemical MEA characterisation. F6PBI was used as the PBI-containing base excess polymer which was blended with either partially fluorinated aromatic polyether (sFS001), poly(2,6-dimethylbromide-1,4-phenylene oxide (PPOBr) or poly(tetrafluorostyrene-4-phosphonic acid) (PWN) in various ratios. Some of the blend membranes also contained a cross-linking agent which was specifically added in an attempt to reduce swelling and promote cross-linking within the polymer matrix. The chemical stability of the blended membranes was confirmed by using weight and swelling changes, TGA-FTIR and TGA-MS. All membranes tested showed low to no chemical degradation when exposed to 80 wt% H2SO4 at 80°C for 120 h. Once the MEA doping procedure had been optimised, electrochemical characterisation of the PBI MEAs, including polarisation curves, voltage stepping and long term operation (> 24 h) was used to evaluate the MEAs. Although performance degradation was observed for the PBI membranes during voltage stepping, it was shown that this characterisation technique could be applied with relative ease, producing valuable insights into MEA stability. Since it is expected that the SO2 electrolyser will be operated under static conditions (cell temperature, pressure and current density) in an industrial setting (HyS cycle or for SO2 removal), a long term study was included. Operating the SO2 electrolyser under constant current density of 0.1 A cm-2 confirmed that PBI-based polyaromatic membranes were suitable, if not preferred, for the SO2 environment, showing stable performance for 170 hours. This work evaluated the performance of commercial materials while further adding insights into both characterisation techniques for chemical stability of polymer materials and electrochemical methods for MEA evaluation to current published literature. In addition to the characterisation techniques this study also provides ample support for the use of PBI-based materials in the SO2 electrolyser.