Preparation And Characterization Of SPEEK Proton Exchange Membranes Doped With Acidified Inorganic Particles

Proton exchange membrane (PEM) is one of the key components of proton exchange membrane fuel cells (PEMFCs), serving to separate the fuel gas and the oxygen and to transfer proton simultaneously. The performance of the fuel cell is greatly dependent on the PEM. Due to the complicated synthesis, high cost and dramatic decline in conductivity at a high temperature, the traditional perfluorinated sulfonic acid membranes (such as Nafion) are facing great challenges. Sulfonated poly(ether ether ketone)(SPEEK) is an outstanding polymer and superior to Nafion in the synthesis cost and the methanol resistance, and thus is a promising alternative material of PFSA resin. However, when the degree of sulfonation (DS) is high, the SPEEK swells in water seriously and loses strength completely, while the DS is low, the SPEEK is poor in conductivity.Modification with hygroscopic acidic inorganic materials is an effective manner to enhance the conductivity and the performance under low humidity of SPEEK membrane. In this work, we firstly investigated the sulfonating conditions and the swelling resistance of the SPEEK of varied DS, by which to determine the scope of DS within which the SPEEK was suitable for inorganic modification. Then, silica sulfuric acid (SSA) nanoparticles were synthesized and the SPEEK/SSA composite membranes were prepared and characterized. Superacid tin oxide (SSnO2) particles were also synthesized and the SPEEK/SSnO2composite membranes were fabricated and their properties were investigated.Through the investigation of the sulfonation of PEEK, we found that when keeping the reaction temperature at50and60℃, we could obtain SPEEK of different DS by changing the reaction time conveniently. High-DS SPEEK has poor swelling resistance in water. When DS<50%, the SPEEK has a potential to be further sulfonated. When DS>60%, SPEEK swells in water seriously, or even dissolves in water. SPEEK of that high DS needs crosslinking modification. When60%>DS>50%, SPEEK has a good mechanical stability in water and is suitable for inorganic modification.In order to enhance the water uptake and conductivity, SSA nanoparticles were synthesized and doped into SPEEK (DS=51%) to prepare SPEEK/SSA composite membranes. The composite membrane with5wt%doping content showed the best performance:at80℃, the water uptake was56.4%, conductivity was0.13S cm-1,18.6%higher than that of SPEEK membrane. The composite membranes also exhibited higher water uptake and conductivity than SPEEK membrane. At80℃, when the relative humidity (RH) was40%and80%, the conductivity of SPEEK-5SSA was166.4%and48.5%higher than the pristine SPEEK membrane, respectively.In order to enhance the conductivity of the SPEEK membrane and improve its mechanic properties and stability in water, SSnO2particles were synthesized and added into SPEEK to prepare SPEEK/SSnO2composite membranes. As the increase of the SSnO2content, the elastic modulus increased, the swelling ratio decreased, and the conductivity increased firstly and then decreased. Compared with the pristine SPEEK membrane, the composite membrane with doping content of20wt%was58.4%higher in elastic modulus,14.1%higher in conductivity and38.4%lower in swelling ratio under a temperature of80℃in water.