A Study On Improving The Microstructure Of Proton Exchange Membranes

Proton exchange membrane fuel cell(PEMFC) is considered to be a promising power source for portable electronics, distributed stationary power stations and electric vehicles because of its high efficiency of energy conversion, high power density, low/zero emission and rapid startup. The proton exchange membrane(PEM) is one of the important components in the PEMFC. Its performance is greatly dependent on the proton conductivity of the PEM used. PEMs usually consist of a hydrophobic backbone and dispersed hydrophilic ionic clusters. While these ionic clusters form proton-conducting channels, the hydrophobic backbone is responsible for the mechanical integrity of the membrane. In this dissertation, we describe our attempts to improve the microstructure of PEMs by constructing wide and aligned proton-conducting channels so that the conductivity of the PEMs can be enhanced.For the first time, the template molecules 3,4-dimethylbenzaldehyde(DMBA) have been introduced to the casting solution of sulfonated poly(ether ether ketone)(SPEEK) membrane. In the solution, the hydrophobic benzyl rings of DMBA promote an ordered aggregation of SPEEK backbone, while the polar aldehyde groups facilitate the gathering of SPEEK ionic clusters. The resultant membranes show bigger ionic clusters, as well as greatly enhanced dimensional stability and mechanical performance. Therefore, DMBA provides an effective means to overcome the dilemma of excess swelling at high degree of sulfonation and poor conductivity at low sulfonation of SPEEK.For the first time, SPEEK membranes have been prepared via membrane casting solution consisting polar dimethyl formamide(DMF) and low polarity nonslovent diethyl carbonate(DEC). DEC induces the self-assembly of SPEEK molecules and turns the casting solution into an opaque mixture. The membranes cast from the dual solvent mixture show higher proton conductivity and water uptake in comparison to the conventional SPEEK membrane cast from DMF. SPEEK membranes with aligned structure have also been cast from the dual solvent mixture with the assistance of either alternating or stationary electric field. The aligned SPEEK membrane prepared under 3500 V·cm-1 and 1Hz presents the highest conductivity among the membranes prepared under different field strength and frequency. Alternating electric field is found to be superior to stationary field in regards to the improvement of membrane performance, as the negative effect of electrophoresis can be effectively eliminated under the condition of alternating field.The nonsolvent para-xylene(PX) was first introduced into Nafion/DMF casting solution, which leads to self-organization of Nafion ionomer and turns the casting solution into an opaque mixture. As a result, the membranes cast from dual slovent show bigger ionic clusters, higher proton conductivity and water uptake than Nafion membranes cast from DMF. Nafion membranes with aligned structure have also been cast from the dual solvent mixture with the assistance of alternating electric field. The aligned Nafion membrane prepared under 4000 V·cm-1 and1 Hz presents the highest conductivity among the membranes prepared under different field strength and frequency. Furthermore, the through-plane conductivity and mechanical performance of the aligned Nafion membrane increase with the electric field strength。...