(23e) New Membranes for Energy and Bio Applications
This presentation covers three areas of new membranes for energy and bio applications: (1) carbon dioxide-selective membranes for hydrogen purification, (2) proton-exchange membranes (PEMs) for fuel cells, and (3) nanoporous polycaprolactone membranes for controlled release. On hydrogen purification for fuel cells, the membranes have been synthesized by incorporating amino groups into crosslinked polyvinylalcohol polymer matrixes. The membranes have shown high carbon dioxide permeability and selectivity vs. hydrogen up to 170oC. Using the membrane, we have obtained <10 ppm carbon monoxide in the hydrogen product in water-gas-shift membrane reactor experiments via carbon dioxide removal. The data have been in good agreement with modeling prediction. We also removed carbon dioxide from a syngas containing 17% carbon dioxide to <30 ppm. The carbon dioxide captured on the permeate side in the acid gas stream had a dry concentration of greater than 98%, by using steam as the sweep gas, which is good for sequestration. Similar carbon dioxide concentration was obtained from nitrogen-containing gas. PEMs to operate at high temperatures (above 100oC) and low humidities (below 50% relative humidity) are needed to increase anode's tolerance to carbon monoxide poisoning and to eliminate the water flooding problem at the cathode. For the PEMs, we have synthesized sulfonated polyimide copolymers containing hydrophilic soft segments to increase the water retention of the membranes at high temperatures and low humidities. In fuel cell performance testing, the new membrane showed similar performance as Nafion® 112 at 70oC and 80% RH, but much better performance than Nafion® 112 at 120oC and 50% RH. Recently, we have also synthesized new sulfonated polybenzimidazole (SPBI) copolymer-based membranes. The membrane has exhibited a very high conductivity (> 0.1 S/cm) at high temperatures (> 120oC) and low humidities (even anhydrous). In addition, this membrane has possessed excellent thermal, oxidative, chemical and hydrolytic stabilities even at high temperatures. Thus, it has the great potential for the PEM fuel cell application at high temperatures and low humidities. All of these new membranes should be much more cost-effective since the starting materials are more than two orders of magnitude less expensive than those for Nafion® membranes.
On nanoporous polycaprolactone (PCL) membranes for controlled release, a constant drug release rate is of paramount importance to implantable drug delivery systems in finding the remedy against chronic diseases. We have synthesized nanoporous PCL membranes to achieve the zero-order release rate. Nanoporous PCL membranes were prepared via the combination of thermally- and nonsolvent-induced phase separations. In the membrane preparation, 1,4-dioxane and 2-methoxyethanol were used as solvent and nonsolvent, respectively, resulting in uniform nanoporous membranes and consistent lysozyme diffusion using a Teflon plate for membrane casting. Pore connectivity was improved significantly when coagulation bath temperature was lowered from 35 to 5°C. By using a 5°C water coagulation bath, the average pore size reduced from about 90 nm to 55 nm while increasing the casting solution concentration from 15 wt% to 25 wt% PCL. Thus, by varying the polymer concentration of the casting solution, the drug release rate can be well controlled with the constant zero-order rate using the higher polymer concentration.