Phosphoric acid containing poly(phenylene oxide) electrolytes for fuel cells and water electrolysis

Abstract

Phosphoric acid is a suitable material for high-temperature conditions which has high thermal stability and its amphoteric characteristic could help to transport the hydrogen at the anhydrous condition. Phosphoric acid is widely using with Poly(benzimidazole) which makes acid-base interaction. However, this interaction is not strong and leaching out of phosphoric acid in the membrane is a big problem to reduce fuel cell performance and stability. Here, we report the new type of polymer electrolyte membrane containing phosphoric acid which applies at polymer electrolyte membrane fuel cell (PEMFC) and polymer electrolyte membrane water electrolysis (PEMWE). To prevent phosphoric acid leaching, phosphoric acid is connected with a polymer backbone via chemical bonding. There is no leaching out of phosphoric acid of membrane immersed in DI water for 14 days. To control the side chain length, it could make the well-defined phase separation. The aggregation of phosphoric acid on the polymer chain end make the effective hydrogen transport way. PPO-HPA, which has the longest side chain, showed the highest proton conductivity under all temperature and humidity conditions. Especially, at anhydrous condition, PPO-HPA has the best proton conductivity (2.7 × 10 -3 S/cm at 120 oC and anhydrous condition) in spite of the low IEC value (3.00 mequiv/g). A well-defined separation of the hydrophilic and hydrophobic phase suggests the phosphonic acid groups to form proton conduction channels via interchain hydrogen bonding. To increase the proton conductivity, ion-pair interaction between quaternary ammonium and phosphoric acid is introduced. Moreover, the crosslinked structure is contributed to preventing the leaching out of phosphoric acid. In this experiment, there are 6 kinds of membranes that the ratio and length of crosslinker are controlled. The lower degree of crosslinking ratio with short crosslinker shows high proton conductivity. cE-20, which has ethyl crosslinker and 20% crosslinking ratio, shows the highest proton conductivity in all temperature ranges. Phosphonic acid contained polymers have also been applied to polymer electrolyte membrane water electrolysis. Usually, Nafion is widely used for the membrane which has good proton conductivity. In this experiment, we used phosphonic acid contained polymer for the PEMWE membrane. As a side chain length, hydrogen and oxygen permeability were controlled. PPO-HPA, which has the longest side chain, showed the lower oxygen permeability. This result showed a possibility to apply for the water electrolysis membrane which has phosphonic acid contained polymer.