Kwan-Soo Lee

Morphological transformation during cross-linking of a highly sulfonated poly(phenylene sulfide nitrile) random copolymer

We present a new approach of morphological transformation for effective proton transport within ionomers, even at partially hydrated states. Highly sulfonated poly(phenylene sulfide nitrile) (XESPSN) random network copolymers were synthesized as alternatives to state-of-the-art perfluorinated polymers such as Nafion®. A combination of thermal annealing and cross-linking, which was conducted at 250 °C by simple trimerisation of ethynyl groups at the chain termini, results in a morphological transformation. The resulting nanophase separation between the hydrophilic and hydrophobic domains forms well-connected hydrophilic nanochannels for dramatically enhanced proton conduction, even at partially hydrated conditions. For instance, the proton conductivity of XESPSN60 was 160% higher than that of Nafion® 212 at 80 °C and 50% relative humidity. The water uptake and dimensional swelling were also reduced and mechanical properties and oxidative stability were improved after three-dimensional network formation. The fuel cell performance of XESPSN membranes exhibited a significantly higher maximum power density than that of Nafion® 212 under partially hydrated environments.

Fuel Cell Membrane Characterizations

Polymer electrolyte membranes (PEMs) play a crucial role for use in major polymer-based fuel cell applications. Key PEM properties such as ion conductivity, reactant permeability, and chemical/physical stability are strongly influenced by the chemical structure and processing conditions of PEMs. This paper presents the property measurement techniques of PEMs using stand-alone membranes and membrane electrode assembly (MEA) configurations. PEM properties such as ion exchange capacity, water uptake, ion conductivity, gas/liquid permeability, and chemical/physical stability are discussed with emphasis on measurement techniques. In addition, the measurement techniques for polymer electrolyte in the catalyst layer are briefly discussed. This review may give some insight to polymer scientists when novel PEM materials are designed, prepared, screened, or fine-tuned for advanced fuel cell systems.

Solvent-assisted thermal annealing of disulfonated poly(arylene ether sulfone) random copolymers for low humidity polymer electrolyte membrane fuel cells

Fundamental studies of a new thermal annealing strategy for maximizing proton conductivity and single cell performance of glassy sulfonated hydrocarbon fuel cell membrane materials under both fully and partially hydrated conditions are reported. Directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers (BisA-XX, XX is the mole percent of hydrophilic moieties, XX = 20 to 40) with 2,2′-isopropylidene diphenol swivel ((CH3)2–C) units in their polymer backbone were cast from N,N-dimethylacetamide (DMAc) and dried under two different protocols; one set was dried at 60 °C (BisA-XX_60 °C), and the other at 150 °C under vacuum immediately after the initial drying (BisA-XX_150 °C). Small amounts of DMAc solvent remained during the second drying step which depressed the glass transition temperatures (Tg) of the BisA copolymers lower than 150 °C. BisA-XX_150 °C samples were essentially thermally annealed when dried at 150 °C. This increased the density of BisA copolymer chains; their Tg values were increased. Moreover, T1 values of the protons in BisA-XX aromatic phenylene rings measured via solid-state NMR technique were lower as a result of the improved 1H–1H dipolar interaction. Interestingly, proton conductivity was improved after thermal annealing, possibly because the sulfonic acid density in the fully hydrated state (i.e., IECv(wet)) was enhanced. The synergistic effect of reduced water uptake and more developed hydrophilic–hydrophobic nanochannel formation may be important. The thermal annealing significantly influenced proton conductivity particularly at a low humidity (30 to 80% relative humidity (RH)). As a result, BisA-XX_150 °C samples derived from identical materials exhibited electrochemical polymer electrolyte membrane fuel cell (PEMFC) performances similar and superior to Nafion® 112 and BisA-XX_60 °C samples measured at 65% RH at 80 °C, respectively.

Intermediate temperature fuel cells via an ion-pair coordinated polymer electrolyte

Fuel cells are attractive devices that convert chemical energy into electricity through the direct electrochemical reaction of hydrogen and oxygen. Intermediate temperature fuel cells operated at 200–300 °C can simplify water and thermal managements, enable the use of non-precious or low-loading precious metal catalysts and provide insensitivity toward fuel and air impurities such as carbon monoxide. However, the performance of current intermediate temperature fuel cells is poor due to a lack of highly-conductive membrane electrolytes and optimal electrodes designed for these fuel cells. Here, we demonstrate high-performing intermediate temperature fuel cells that use SnP2O7–polymer composite membranes and a quaternary ammonium-biphosphate ion-pair coordinated polymer electrolyte in the electrodes. The peak power density of the fuel cell under H2 and O2 reached 870 mW cm−2 at 240 °C with minimal performance loss under exposure to 25% carbon monoxide.