(739i) Microscopic Structures in Ion Exchange Fuel Cell Membranes

Ion exchange membrane (IEM) fuel cells transform chemical energy into electrical energy. Comparing to traditional petroleum based energy source, they generate water rather than more harmful carbon-based emissions on operation. In general, ion transport through the polymer membrane is dictated by the organization of the phase-segregated ion-transport domains at length scales ranging from nanometers to microns. And as a result, the rational design of any IEM requires a clear connection between the chemical arrangement of monomers within the copolymers and the resulting molecular-level organization of the ion-transport domains.

We therefore aim to provide detailed microscopic structural information for the design of fuel cell membranes, using polyimide-PEG (polyethylene glycol) random copolymer as the model system. Combining aromatic polyimide chains with PEG chains, the polyimide-PEG copolymers have great potential in making good IEMs. These copolymers are conveniently synthesized from the random co-polymerization of aromatic dianhydrides, aromatic diamines, and PEG oligomers (M.W. around 1 kDa) that are functionalized on both ends with amine groups. The flexibility in the choice of aromatic dianhydrides and aromatic diamines thus offer the resulting copolymer potentially tunable properties that are suitable in versatile operating conditions. As an IEM, the phase-segregated PEG domains provide ion transport channels, while the aromatic polyimide networks form mechanically and thermally stable matrix.

In this presented work, the polyimide-PEG random copolymers in the form of self-assembled membranes were synthesized and characterized using a variety of methods. The thermal stability and mechanical strength of the membranes were characterized using Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and tensile tests. The microscopic structures were investigated using a combination of Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), as well as Small Angle X-ray Scattering (SAXS). In order to further interpret the SAXS results, theoretical analysis was collaboratively employed. By predicting the self-assembled microscopic structure of worm like random copolymer chains, this theoretical study helps us to quantitatively understand the spatial distribution of PEG domains. Membranes conductivity was measured using Electrochemical Impedance Spectroscopy (EIS) in conjunction with Cyclic Voltammetry (CV). In addition, electrolytes such as ethylammonium nitrate (EAN) were incorporated into the membranes and the resulted doped membranes were compared with those bare membranes in terms of both microscopic structures and performances. The observed structure-property relations in various polyimide-PEG membranes are discussed.