Investigations on Functional Materials for Hybrid Sulfur Cycle for Thermochemical Hydrogen Production

Thermochemical water splitting cycles has shown great potential towards efficient production of hydrogen on an industrial scale. Hybrid Sulfur cycle (Hy-S) is one of the most preferably studied thermochemical cycle due to several advantages. Hy-S cycle involves two steps, i.e. sulfuric acid decomposition reaction and aqueous SO₂ electrolysis. Development of functional materials such as catalysts, electrocatalysts, membrane electrode assemblies etc., was undertaken with an objective to improve the reaction kinetics and efficiency of the Hy-S cycle.
     Iron oxide based catalyst was chosen and efforts were made to overcome the sintering issues encountered during its prolonged uses. For the purpose, dispersed iron oxide (15 wt.%) on various supports (SiO₂, TiO₂, CeO₂ and ZrO₂) were synthesized and evaluated their activity for sulfuric acid decomposition reaction. The catalytic activity for the acid decomposition at 750°C followed the order: Fe₂O₃/SiO₂> Fe₂TiO₅/TiO₂> Fe₂O₃/ZrO₂> Fe₂O₃/CeO₂. Various preparation methods like polyol, wet-impregnation, hydrothermal and equilibrium adsorption were also employed to maximize the catalyst performance of Fe₂O₃/SiO₂ catalyst for acid decomposition reaction. The Fe₂O₃ (15wt.%)/SiO₂ samples prepared by polyol method exhibited highest activity for the sulfuric acid decomposition reaction and the activity trend at 800 °C was found to be as follows: polyol > equilibrium-adsorption> wet-impregnation ~ hydrothermal. Further stability of Fe₂O₃ (15wt.%)/SiO2 catalyst during prolonged uses (100 h) for sulfuric acid decomposition reaction at 800 °C was evaluated.
     The catalytic properties of the dispersed iron oxide samples were correlated to nature of support, structure, morphology and the redox properties of iron oxide phase. Detailed investigations on both fresh and used catalyst were studied to elucidate the mechanistic aspects of the acid decomposition process. The most probable mechanism of sulfuric acid decomposition over dispersed iron oxide catalyst which involved formation and decomposition of surface sulfate species was proposed.
     Solar thermal sulfuric acid decomposition employing concentrated solar heat from a 1.8 m diameter dish and in-house developed quartz receiver-reactor was also successfully demonstrated. A maximum SO₂ yield of 38 % was achieved with Fe₂O₃/(15wt. %)/SiO₂ catalyst at weight hourly space velocity (WHSV) of ~ 28 g acid/gcat/h, at 750-850 °C.
     A series of Pt/C electrocatalysts with varying platinum content (10-40 wt.%) were successfully prepared and electrochemical properties were tested for hydrogen evolution reaction(HER) and aqueous SO₂ oxidation reaction. Amongst these, catalyst containing 20 wt. % Pt was found to be the most effective. Further, a single cell PEM based aqueous SO₂ electrolyzer (4cm2 active area) was designed, fabricated and tested with the membrane electrode assembly comprising of the most active Pt/C electrocatalyst. A current density of ~75 mAcm² was achieved at a cell voltage of 1 V. As a non-noble metal based electrocatalyst, molybdenum carbide electrocatalysts dispersed on carbon with varying Mo content (10-40 wt.%) were synthesized and evaluated for HER. Through these studies 20 wt% Mo content was found to be the optimum loading to attain maximum electroactivity for HER.