Novel Approaches to Immobilized Heteropoly Acid Systems for High Temperature, Low Relative Humidity Polymer-Type Membranes (Final Report) (2012)

Original research was carried out at the CSM and the 3M Company from March 2007 through September 2011. The research was aimed at developing new to the world proton electrolyte materials for use in hydrogen fuel cells, in particular with high proton conductivity under hot and dry conditions (>100mS/cm at 120°C and 50%RH). Broadly stated, the research at 3M and between 3M and CSM that led to new materials took place in two phases: In the first phase, hydrocarbon membranes that could be formed by photopolymerization of monomer mixtures were developed for the purpose of determining the technical feasibility of achieving the program's Go/No-Go decision conductivity target of >100mS/cm at 120°C and 50%RH. In the second phase, attempts were made to extend the achieved conductivity level to fluorinated material systems with the expectation that durability and stability would be improved (over the hydrocarbon material). Highlights included: Multiple lots of an HPA-immobilized photocurable terpolymer derived from di-vinyl-silicotungstic acid (85%), n-butyl acrylate, and hexanediol diacrylate were prepared at 3M and characterized at 3M to exhibit an initial conductivity of 107mS/cm at 120°C and 47%RH (PolyPOM85v) using a Bekktech LLC sample fixture and TestEquity oven. Later independent testing by Bekktech LLC, using a different preheating protocol, on the same material, yielded a conductivity value of approximately 20mS/cm at 120°C and 50%RH. The difference in measured values is likely to have been the result of an instability of properties for the material or a difference in the measurement method. A dispersed catalyst fuel cell was fabricated and tested using a 150¼m thick HPA-based photocurable membrane (above, PolyPOM75v), exhibiting a current density of greater than 300mA/cm2 at 0.5V (H2/Air 800/1800sccm 70°C/75%RH ambient outlet pressure). Multiple lots of a co-polymer based on poly-trifluorovinylether (TFVE) derived HPA were synthesized and fabricated into films, Generation II films. These materials showed proton conductivities as high as 1 S/cm under high RH conditions. However, the materials suffered from compromised properties due to impure monomers and low molecular weights. Multiple lots of an HPA-immobilized fluoropolymer derived from preformed PVDF-HFP (Generation III films) were synthesized and formed into membranes at 3M and characterized at 3M to exhibit conductivity reaching approximately 75mS/cm at 120°C/40%RH using a Bekktech sample fixture and TestEquity oven (optimized membrane, at close of program). Initial fuel cell fabrication and testing for this new class of membrane yielded negative results (no measureable proton conductivity); however, the specific early membrane that was used for the two 5cm2 MEAs was later determined to have <1 mS/cm at 80°C/80%RH using the Bekktech fixture, vs. ca. 200 mS/cm at 80°C/80%RH for samples of the later-optimized type described above. Future work in this area (beyond the presently reported contract) should include additional attempts to fabricate and test fuel cells based on the later-optimized Generation II and III polymer. A manufacturing study was performed which predicted no difficulties in any future scale up of the materials.