Cationic polymers developed for alkaline fuel cell applications

Abstract

Alkaline fuel cells (AFCs) recently have gained renewed interest because of their facile electrode reaction kinetics, reduced fuel crossover and better water management compared to their protonic fuel cell counterpart. The emerging anionic exchange membrane fuel cell (AEMFC) adopts cationic group-functionalized polymers as the solid electrolyte instead of liquid potassium hydroxide or sodium hydroxide used in the traditional AFC, avoiding leakage problems, bicarbonate and carbonate salt induced electrode degradation issues that accompany liquid electrolytes. The polymer electrolyte applied in fuel cells should have good mechanical properties as well as high thermal and chemical stabilities, considering the basic, humid and elevated temperature operating conditions and the constant start-ups and shutdowns in fuel cell operation. The chemical stability of guanidinium ion under an alkaline environment was studied in this research with the aim of using guanidinium ion in alkaline exchange membranes. The high pKa value (~13) of protonated quanidinium suggests a high dissociation of hydroxide ions from a hexa-alkylated guanadinium resulting in a high hydroxide conductivity of a functionalized polymer electrolyte. Model guanidinium compounds were synthesized to study the alkaline stability under different temperatures. In order to form membranes with ion conducting channels, block copolymers can be designed that contain a hydrophilic cationic functional block separated from a hydrophobic block. This type of material can undergo microphase separation and form continuous ion conductive channels with the mechanical properties determined by the hydrophobic block material. In the present research, novel diblock copolymers were designed to combine an aromatic polymer block with a functionalized styrenic polymer block, aiming for combined mechanical and chemical merits from the good film forming, thermally and chemically stable poly(2,6-dimethyl-1,4-phenylene oxide) block (PPO) and the hydrophilic conductive poly(vinylbenzyltrimethylammonium) (PVBTMA) block. A poly(vinylbenzyl chloride) (PVBC) block was grown from the end of a PPO block through nitroxide mediated polymerization. The final quaternized PPO-b-PVBTMA film was obtained through hot pressing PPO-b-PVBC powder into films, followed by quaternizing in aqueous trimethylamine solution. Phase separation was indicated for the PPO-b-PVBC polymers based on the evidence of two Tgs from differential scanning calorimetry analysis. The PPO-b-PVBTMA membranes exhibited hydroxide conductivities as high as 166 ± 5 mS/cm at 60 °C under fully hydrated conditions. Comparing to the reported anionic exchange membranes, PPO-b-PVBTMA membranes showed promising conductivity with suppressed water uptake. In a similar design to the PPO-b-PVBTMA membranes, triblock copolymers were synthesized through NMP with polysulfone as the center block and PVBC as the outer blocks. The PVBC blocks were quaternized by Menshutkin reaction to produce PVBTMA blocks. PPO and polysulfone are both well known engineering polymers that function as the hydrophobic domains in these membranes, providing mechanical support, while the PVBTMA block(s) form hydrophilic domains to transport anions. Diblock copolymers and triblock copolymers with different weight percentage of PVBTMA block were synthesized and characterized by electrochemical impedance spectroscopy to study the effect of temperature, different counter ions, ion exchange capacity (IEC) and water uptake on ionic conductivity.