(330l) Hydrogen Sulfide And Carbon Dioxide-Selective Facilitated Transport Membranes For Fuel-Cell Fuel Processing

Hydrogen sulfide is a common contaminant in the hydrogen derived form fossil fuels, and it poisons the fuel cell anode catalyst. Since this poisoning mechanism is irreversible, even trace concentrations of H2S ( > 10 ppb) in the hydrogen can significantly degrade fuel cell performance. So it must be almost completely removed prior to feeding hydrogen to fuel cells. In this study, hydrogen sulfide-selective facilitated transport membranes containing amino groups were investigated to remove hydrogen sulfide in hydrogen purification for proton-exchange membrane (PEM) fuel cells. Facilitated transport membranes can achieve high selectivity for the gas of interest due to the reversible reaction of the target gas with the active carriers that are incorporated in the membranes. Compared to the conventional amine scrubbing, hydrogen sulfide and carbon dioxide removal by membrane separation has many advantages, such as low energy consumption, high weight and space efficiency, and operational simplicity as well as no regeneration step required and no moving parts involved in the pressure driven separation.

We have synthesized new membranes for the removal of hydrogen sulfide and carbon dioxide from hydrogen-containing synthesis gas by incorporating amino groups into polymer networks. The membranes are selective to hydrogen sulfide and carbon dioxide preferentially versus hydrogen since the acid gases permeate through the amine-containing membranes via the facilitated transport mechanism due to their reversible reactions with the amine whereas hydrogen has no such reaction. The membranes synthesized have shown high hydrogen sulfide and carbon dioxide permeabilities and selectivities vs. hydrogen. This type of membranes has the potential for hydrogen purification for environmentally friendly fuel cells.

Hydrogen sulfide has much higher reaction rate with the amine than carbon dioxide as the former reacts with the amine via proton transfer and the latter reacts with the amine via carbamate formation primarily. Thus, hydrogen sulfide can permeate through the membrane much faster than carbon dioxide. In the absorption of hydrogen sulfide and carbon dioxide into aqueous amine solutions, the hydrogen sulfide/carbon dioxide selectivity is about 3. Our initial membrane data have also shown hydrogen sulfide with about 3 times permeability of carbon dioxide. The modeling based on hydrogen sulfide permeability data has shown that less than 10 ppb hydrogen sulfide is achievable from typical synthesis gas with small membrane area requirement. In addition, our initial experiments with a limited membrane area have shown a nearly complete removal of hydrogen sulfide from 50 ppm in the synthesis gas feed to about 10 ppb in the hydrogen product, which is good for fuel cell applications.