Benjamin D. Gould

Products of SO2 Adsorption on Fuel Cell Electrocatalysts by Combination of Sulfur K-Edge XANES and Electrochemistry

Electrochemical adsorption of SO(2) on platinum is complicated by the change in sulfur oxidation state with potential. Here, we attempt to identify SO(2) adsorption products on catalyst coated membranes (CCMs) at different electrode potentials using a combination of in situ sulfur K-edge XANES (X-ray absorption near-edge structure) spectroscopy and electrochemical techniques. CCMs employed platinum nanoparticles supported on Vulcan carbon (Pt/VC). SO(2) was adsorbed from a SO(2)/N(2) gas mixture while holding the Pt/VC-electrode potential at 0.1, 0.5, 0.7, and 0.9 V vs a reversible hydrogen electrode (RHE). Sulfur adatoms (S(0)) are identified as the SO(2) adsorption products at 0.1 V, while mixtures of S(0), SO(2), and sulfate/bisulfate ((bi)sulfate) ions are suggested as SO(2) adsorption products at 0.5 and 0.7 V. At 0.9 V, SO(2) is completely oxidized to (bi)sulfate ions. The identity of adsorbed SO(2) species on Pt/VC catalysts at different electrode potentials is confirmed by modeling of XANES spectra using FEFF8 and a linear combination of experimental spectra from sulfur standards. Results on SO(2) speciation gained from XANES are used to compare platinum-sulfur electronic interactions for Pt(3)Co/VC versus Pt/VC catalysts in order to understand the difference between the two catalysts in terms of SO(2) contamination.

Operational Performance Recovery of SO2-Contaminated Proton Exchange Membrane Fuel Cells

Airborne sulfur contaminants (SO 2 , H 2 S, and COS) cause the performance of proton exchange membrane fuel cells (PEMFCs) to degrade because they adsorb to the Pt catalysts and modify reaction sites for oxygen reduction. Electrochemical methods can be used for PEMFC performance recovery by oxidizing adsorbed sulfur species (SO x ) on the Pt catalysts to sulfate (SO 2- 4 ) at high potentials and then removing them as water-soluble anions at low potentials. We examine the effectiveness of five distinct methods for PEMFC performance recovery after 3 h of exposure to 1 ppm SO 2 in air at 60°C and 48.3 kPa g (7 psi) and relative humidity of 100 | 50% (anode | cathode). The methods are tested when the Pt surface is partially covered but not completely saturated with sulfur species. The methods include variations in the cathode potential and gas environment (N 2 or air). In the optimum method, the cells are switched from normal H 2 | air operation to H 2 | N 2 by electrochemically consuming O 2 in the air. The potential is then cycled between 0.09 and 1.1 V. vs the potential at the anode to restore 97% of the platinum catalyst electrochemical surface area. This in situ N 2 cycling method returns the polarization curves of contaminated PEMFCs to their original performance in less than 3 min.

Comparison of the Sulfur Poisoning of PBI and Nafion PEMFC Cathodes

The poisoning effect of H 2 S and SO 2 in air is compared for phosphoric-acid-doped polybenzimidazole (PBI) membrane and perfluorosulfonic acid (Nafion) proton exchange membrane fuel cells (PEMFCs). The cathodes of PBI PEMFCs are about 70 times more resistant to 1 ppm of H 2 S or S0 2 than the Nafion PEMFCs. The PBI PEMFCs only lose <2% in cell performance when exposed to 1 ppm of H 2 S or S0 2 and have 5.2 and 7.1% losses with 5 and 10 ppm H 2 S over 24 h, respectively. Purging the poisoned PBI cells with neat air leads to complete performance recovery.

Fabrication Method for Laboratory-Scale High-Performance Membrane Electrode Assemblies for Fuel Cells

Custom catalyst-coated membranes (CCMs) and membrane electrode assemblies (MEAs) are necessary for the evaluation of advanced electrocatalysts, gas diffusion media (GDM), ionomers, polymer electrolyte membranes (PEMs), and electrode structures designed for use in next-generation fuel cells, electrolyzers, or flow batteries. This Feature provides a reliable and reproducible fabrication protocol for laboratory scale (10 cm2) fuel cells based on ultrasonic spray deposition of a standard Pt/carbon electrocatalyst directly onto a perfluorosulfonic acid PEM.

Lightweight Titanium Metal Bipolar Plates for PEM Fuel Cells

Bipolar plates (BPPs) serve multiple roles in polymer electrolyte membrane fuel cells (PEMFCs). When assembled in a stack, they provide the structural backbone of the stack, plus serial electronic connections. They also provide gas (air and fuel) and coolant distribution pathways. Traditionally, bipolar plates have been made of carbon, but these are being replaced in favor of metal bipolar plates made of stamped foils. The Naval Research Laboratory has explored making titanium metal BPPs using 3D printing methods (direct metal laser sintering – DMLS) and superplastic forming, and then using a gold/TiO2 surface layer for corrosion resistance. The 3D printed plates are made as one piece with the coolant flow internal to the resulting 2-mm thick structure. Their surface roughness requires smoothing prior to coating to increase their cell-to-cell conductivity. We found that 3D printed cells with 22 and 66 cm2 active areas are slightly warped, preventing the robust sealing of the stacks. The formed plates are made in separate pieces and then joined. Despite the high temperatures required for superplastic forming, the resulting plates are thin and lightweight, making them highly attractive for lightweight compact PEMFC stacks.