High Temperature Proton Exchange Membranes With Enhanced Proton Conductivities At Low Humidity and High Temperature Based On Polymer Blends and Block Copolymers of Poly(1,3-Cyclohexadiene) and Poly(ethylene Glycol)

Hot (at 120 °C) and dry (20% relative humidity) operating conditions benefit fuel cell designs based on proton exchange membranes (PEMs) and hydrogen due to simplified system design and increasing tolerance to fuel impurities. Presented are preparation, partial characterization, and multi-scale modeling of such PEMs based on cross-linked, sulfonated poly(1,3-cyclohexadiene) (xsPCHD) blends and block copolymers with poly(ethylene glycol) (PEG). These low cost materials have proton conductivities 18 times that of current industry standard Nafion at hot, dry operating conditions. Among the membranes studied, the blend xsPCHD-PEG PEM displayed the highest proton conductivity, which exhibits a morphology with higher connectivity of the hydrophilic domain throughout the membrane. Simulation and modeling provide a molecular level understanding of distribution of PEG within this hydrophilic domain and its relation to proton conductivities. This study demonstrates enhancement of proton conductivity at high temperature and low relative humidity by incorporation of PEG and optimized sulfonation conditions

Dictating Pt-Based Electrocatalyst Performance in Polymer Electrolyte Fuel Cells, from Formulation to Application.

In situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with O2 transport resistance for low-loaded (0.05 mgPt cm-2) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites. Electrochemical techniques (CO displacement and ac impedance) show that catalyst inks with higher water content increase ionomer (sulfonate) interactions with Pt sites without significantly affecting ionomer coverage on the carbon support. Surprisingly, the higher anion adsorption is shown to have a minor impact on specific activity, while exhibiting a complex relationship with oxygen transport. Ex situ characterization of ionomer suspensions and catalyst/ionomer inks indicates that the lower ionomer coverage can be correlated with the formation of large ionomer aggregates and weaker ionomer/catalyst interactions in low-water content inks. These larger ionomer aggregates resulted in increased local oxygen transport resistance, namely, through the ionomer film, and reduced performance at high current density. In the water-rich inks, the ionomer aggregate size decreases, while stronger ionomer/Pt interactions are observed. The reduced ionomer aggregation improves transport resistance through the ionomer film, while the increased adsorption leads to the emergence of resistance at the ionomer/Pt interface. Overall, the high current density performance is shown to be a nonmonotonic function of ink water content, scaling with the local gas (H2, O2) transport resistance resulting from pore, thin film, and interfacial phenomena

Electrocatalysis of Oxygen Reduction with in-Situ formed Pt Nano-Rafts on Molybdenum Carbide Support

Proton exchange membrane fuel cell (PEMFC), is a technology that has the potential to economically replace combustion engines for transport with high efficiency, and clean (only water emission) energy. The US department of energy (DOE) identifies two remaining major hurdles to the deployment of this alternative: cost and durability of the cathode. Reducing the amount of platinum, still the only material with the needed catalytic activity for oxygen reduction reaction on the cathode, and the most expensive component, will help overcome the first problem and the creation of a new, ‘non-carbon’, more oxidation-resistant catalyst support material could overcome the second.US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technology and Fuel Cell Technology Program

Preparation and characterization of PdFe nanoleaves as electrocatalysts for oxygen reduction reaction

Novel PdFe-nanoleaves (NLs) have been prepared through a wet chemistry-based solution phase reduction synthesis route. High-resolution transmission electron microscopy (HR-TEM) and scanning transmission electron microscopy (S/TEM) coupled with high-spatial-resolution compositional analysis clearly show that this newly developed structure is assembled from Pd-rich nanowires (Pd-NWs) surrounded by Fe-rich sheets. The Pd-NWs have diameters in the range of 1.8-2.3 nm and large electrochemical surface areas of \u3e 50 m 2/g. Their length (30-100 nm) and morphology can be tuned by altering the nanostructure synthesis conditions and the Fe amount in the NLs. With increasing Fe content, thinner and longer sheet-enveloped nanowires can be prepared. The side surfaces of Pd-NWs observed by HR-TEM are predominantly Pd(111) facets, while the tips and ends are Pd(110) and Pd(100) facets. By etching away the enveloping Fe-rich sheets using an organic acid, the Pd-rich NWs are exposed on the surfaces of the nanoleaves, and they demonstrate high reactivity toward electrocatalytic reduction of oxygen in a 0.1 M NaOH electrolyte, i.e., a factor of 3.0 increase in the specific activity and a factor of 2.7 increase in the mass activity, compared to a commercial Pt/C catalyst (at 0 V vs. Hg/HgO). The electrocatalytic activity enhancement can be attributed to the unique nanoleave structure, which provides more Pd(111) facets, a large surface area, and more resistance to Pd oxide formation. © 2011 American Chemical Society