A Study On The Modified Sulfonated Poly(Ether Ether Ketone) Proton Exchange Membranes For DMFC

Direct methanol fuel cell (DMFC) is a novel power technology and is expected to find wide applications in areas from mobile phones to electric rehicles. Two obstacles, however, need to be surmounted before its commercialization. One is low methanol oxidation activity at the anode, and the other is methanol crossover from the anode to the cathode through the membrane. Therefore, developing new membranes to deal with the obstacles has been an important research subject in the fuel cell field. In this study, new methanol barrier and high temperature membranes for DMFC are explored.Sulfonated poly(ether ether ketone) (SPEEK) membranes are very promising alternative to Nafion membrane in DMFC for their high conductivity and better methanol resistance. However, in SPEEK membranes, there exits conflicts among ionic conductivity, methanol resistance and dimensional stability. The demand of high proton conductivity calls for the membranes to have a high degree of sulfonation (DS) and work at elevated temperature. Such membranes at elevated temperature, however, tend to swell excessively or even dissolve, thereby lowers applicable temperature.In order to overcome the conflicts, insolvable hydroxyapatite (HA) and functionalized silica (silica sol (SiO2) and silica with sulfonic acid groups (SiOx–S)) were incorporated into the SPEEK matrix with relative high DS to prepare novle composite membranes, respectively. Results show that the hydrogen bond happened between the adulterants and the polymers, which helps to reduce the swelling and methanol permeability of the composite membranes, and improve their dimensional stability. For the composite membranes, high conductivity can be achieved as the applicable temperature is improved. For the hygroscopic property of the adulterants, the conductivity of composie membranes under low relative humidity at high temperature(>100℃) was improved efficiently.Phenolphthalein poly(ether sulfone) (PES–C) is an engineering thermoplastic with high temperature resistance, and can be sulfonated to prepare SPES–C proton exchage membranes. The SPES–C membrane swelled somewhat (<13%) in boiling water, even with DS as high as 70%. The demand of high proton conductivity calls for SPES–C membranes to have a high degree of sulfonation (DS), which decreases mechanical strength of the membranres. In order to improve the properties of SPEEK membrane with high DS, SPES–C with a relative low DS (DS=53.7%) and PES–C were chosen and blended into SPEEK matrix to fabricate SPEEK/SPES–C and SPEEK/PES–C blend membranes, respectively. Results show that by controlling the DS of SPEEK and blend proportion, the blend membranes with dimensinoal stability, high conductivity and excellent methanol resistance can be fabricated. Furthermore, compared with pure SPEEK membrane with the same ion–exchange capacity, the blend membrane has better methanol resistance and dimensional stability.An electric field perpendicular to the membrane plane was applied during the course of preparing SPEEK/PES–C membranes by solution casting, and the influence of the electric parameters (e.g., electric field strength (E), frequency (f), waveform and DC/AC electric field) on the blend membranes'performance was explored preliminary. Results show that the electric field can change the membranes'microstructures, and can induce the membranes to orient along the electric field direction, thus improving obviously the trans–plane conductivity of the blend membranes. Furthermore, the electric field can also change the water content and the methanol permeability of the blend membranes.