Electric Field Assisted Fabrication Of Proton Exchange Membranes With Oriented Proton Channels

Polymer exchange membrane fuel cells (PEMFC) are considered to be a very promisingalternative power source for a wide spectrum of applications. As one of the key components inPEMFCs, the proton exchange membranes (PEMs) consist of a hydrophilic proton conductivephase and a hydrophobic non-conductive phase. These two phases are randomly distributed inhydrated PEMs, and the hydrated hydrophilic phase forms tortuous channels for protontransportation within the PEMs. One attractive new approach of improving conductivity ofPEMs is to construct straight proton conducting channels. In this work a new method of electricfield treatment is proposed and tested to prepare PEMs with oriented proton channels.As the phase separation has been found a critical factor for the external electric field to beeffective, a novel solution casting method of membrane preparation is explored to improve theconductivity of PEMs. A high AC electric field is applied to a heterogeneous Nafion solutionwhile evaporating the solvents, leaving aligned proton channels in the solidified membrane, andthe SAXS and WAXS have been given as direct evidences. Therefore, the trans-planeconductivity of the PEM is increased. A Non-solvent of low polarity carbon tetrachloride (CCl4)causes phase separation in the casting solution, which facilitates the Nafion ionomer to respondto the applied electric field. Despite the severe phase separation in the casting solution, theresultant electro-casting membrane shows a high mechanical strength of11.8MPa, as a resultof the orientation induced high crystallinity fo the molecular chains. The most interestingdiscovery is that the conductivity of PEMs can be increased by60%by just adding somenon-solvent into the casting solution, such a great improvement by the cheap and conveniencemethod.Two diblock copolymers of poly(1H,1H-pentafluoro-n-propyl acrylate-b-sulfonate styrene acid)(Polymer1) and poly (allyl acrylate-b-sulfonate styrene acid)(Polymer3) have beensynthesized by RAFT polymerization which can be used as PEMs. Polymer1is a typicalamphiphilic diblock copolymer which can be self-assembled into some ordered phase structures,and an external electric field makes the hydrophilic phase oriented in the trans-plane direction.The trans-plane conductivity of PEM increases by200%after the electric field treatment.Polymer3contains plenty of allyl groups as cross-linker in the diblock copolymer, which can beused to improve the solvent resistance and mechanical property of PEM. The Polymer3with anIEC of2.3can have a conductivity twice as that of Nafion at room temperature saturatedhumidity after cross-linking.With the aim to improve the conductivity of PEMs, an external AC electric field is applied to asulfonated poly (ether ether ketone)(SPEEK)-titanium (IV) dioxide (TiO2) hybrid castingsolution during solvent evaporation, and resulting in increased conductivity by75%in thetrans-plane direction versus normal methods. In addition, reciprocation of TiO2under the ACelectric field prevents the nanoparticles from aggregating and improves mechanical strength ofthe PEMs. Factors affect the proton channels’ alignment degree such as the applied fieldmagnitude and frequency are also investigated, and an AC electric field of low frequency andhigh magnitude which make the electric field more effective has been suggested.