Improving Proton Exchange Membrane Fuel Cells by Thiol-Capped Silver Nanoparticles

Hydrogen fuel cells have emerged as a sustainable and clean energy source, generating electricity through the electrochemical reaction between hydrogen and oxygen, emitting water vapor as a sole byproduct. This technology has been proposed as a resolution to the impact of carbon emissions on climate change. Proton exchange membrane fuel cells (PEMFCs) are a type of fuel cell that transports H+ from the anode to the cathode across a proton exchange membrane (PEM). PEMFCs have been noted for their low operating temperatures and high power densities. However, since existing PEMFCs use platinum (Pt) catalysts to enhance efficiency, the cost of Pt materials limits their commercialization. Furthermore, carbon monoxide (CO) binding with Pt catalysts, due to inevitable CO impurities in hydrogen gas, also leads to fewer active sites on the catalysts, lowering cell efficiency. This study aims to investigate octanethiol-stabilized silver nanoparticles (AgNPs) as a CO-resistant catalyst in PEMFCs and examine the impact of AgNPs at different thiol stabilization levels on PEMFC efficiency.

AgNPs were synthesized using the Brust-Schiffrin method. During the synthesis, 200, 400, or 600 μL of octanethiol (200th, 400th, 600th respectively) were added. The thiol molecules surrounded the AgNPs, forming stable Ag–S–C links, which prevented agglomeration and maintained their functional properties. Transmission electron microscopy (TEM) images of the 200th AgNP samples confirmed their quasi-spherical shapes and normal size distributions, with an average diameter of 2.7 ± 0.8 nm. However, TEM images of the 400th and 600th AgNPs demonstrated agglomeration, likely caused by incomplete reduction of Ag salt in the synthesis process or oxidation of AgNPs after synthesis. X-ray diffraction (XRD) analysis of all samples showed reflections at 38.1, 44.3, 64.4, and 77.3 degrees, which aligns with AgNP peaks in literature, representing the diffraction from (111), (200), and (220) planes of silver nanoparticles. The 200th nanoparticles’ XRD spectra exhibited the highest peak intensity of AgNP, which supported the TEM results.

Isothermal data of AgNPs at each thiol stabilization level, collected with a Langmuir-Blodgett trough (LBT), were analyzed to determine the surface pressures at which AgNP monolayers start to form. Deposition of 200th, 400th, and 600th AgNP monolayers on Nafion-117 membranes at 5 and 10 mN/m surface pressures was then completed using the LBT. The resulting six nanoparticle-deposited membranes were characterized for their PEMFC performances at a fuel cell test station. Results demonstrated 22.2% and 44.4% maximum power density enhancements (vs. the control) for the 200th AgNPs deposited at 5 mN/m and 10 mN/m pressures, respectively, and no significant enhancement by the 400th or 600th AgNPs at either pressure, as predicted by the TEM and XRD analyses. Further research on the 200th AgNPs deposited at 15 mN/m and 20 mN/m pressures demonstrated a 55.55% enhancement in the peak performance and better durability. To study the effect of the monolayer against CO poisoning, the 200th AgNPs deposited at 15 mN/m and 20 mN/m were tested, yielding a 66.66% enhancement in the fuel cell performance when compared to the control. Density Functional Theory (DFT) calculation is under study in order to model these reactions.

In conclusion, our findings demonstrated the effectiveness of 200th AgNPs in increasing the resistance against CO poisoning and the efficiency of PEMFCs, and provide valuable information for future studies of hydrogen fuel cells.

We would like to acknowledge the Louis Morin Charitable Trust for their help in making this research project possible.