Designing Nanoplatelet Alloy/Nafion Catalytic Interface for Optimization of PEMFCs: Performance, Durability, and CO Resistance

We have adapted the two-phase Brust method to synthesize large quantities of AuPd alloy nanoparticles with diameter of 1.86 ± 0.40 nm. When the particles were spread at the air/water interface of a Langmuir-Blodgett (LB) trough, they exhibited a distinct pressure area isotherm curve. The X-ray reflectivity (XRR) shows the formation of an incompressible monolayer with uniform thickness of 1.16 ± 0.02 nm at low pressures which collapses to form a second layer with 2.13 nm thickness at higher pressures. High resolution transmission electron microscopy (HRTEM) imaging of the monolayer indicates that the particles are highly crystalline, with well-defined atomic planes and self-assembly into a hexagonal structure. Extended X-ray absorption fine structure (EXAFS) analysis of the LB lift-off films only shows Au-Au and Au-Pd configurations, consistent with the formation of random alloy rather than core-shell structure. When the particle monolayer was lifted onto the Nafion membrane of a proton exchange membrane fuel cell (PEMFC), a maximum power output of 0.54 W/cm ² was obtained with a Pt loading of only 0.15 and 0.05 mg/cm ² at the cathode and anode, respectively. This represents a 15% enhancement, which persists even after 30K cycles, relative to a membrane electrode assembly (MEA) using a noncoated membrane. Density function theory (DFT) modeling of the hexagonally packed platelet nanoparticle alloy structure deposited on Nafion predicted that the SO ₃ functionality serves a similar function to metal oxide supports in reducing the activation barrier for the CO oxidation reaction. This was confirmed by measuring the power output of a PEMFC when 0.1% CO was mixed either into the input H ₂ stream at the anode or with O ₂ at the cathode. In the uncoated MEAs, a significant decrease of 72% and 61% was measured when CO was introduced at the anode and cathode, respectively, while MEAs with coated particle membranes only experienced a decrease of 21% and 17%, respectively. These results indicate that a synergy can be established between the nanoparticle platelets and the surface of the Nafion membrane, which can produce CO resistant PEMFC, that could operate at ambient temperatures on impure, but abundant hydrogen gas such as those produced by reformate or electrolysis processes. The availability of inexpensive H ₂ opens opportunities for gas distribution and greatly facilitates the commercialization of PEMFCs, which will conform to the ambitious goals set by the Department of Energy (DOE) for 2020.