Impact of Platinum Primary Particle Loading on Fuel Cell Performance: Insights from Catalyst/Ionomer Ink Interactions

X-ray Photoelectron Spectroscopy Analysis of Nafion-Containing Samples: Pitfalls, Protocols, and Perceptions of Physicochemical Properties

X-ray photoelectron spectroscopy (XPS) is one of the most common techniques used to analyze the surface composition of catalysts and support materials used in polymer electrolyte membrane (PEM) fuel cells and electrolyzers, providing important insights for further improvement of their properties. Characterization of catalyst layers (CLs) is more challenging, which can be at least partially attributed to the instability of ionomer materials such as Nafion during measurements. This work explores the stability of Nafion during XPS measurements, illuminating and addressing Nafion degradation concerns. The extent of Nafion damage as a function of XPS instrumentation, measurement conditions, and sample properties was evaluated across multiple instruments. Results revealed that significant Nafion damage to the ion-conducting sulfonic acid species (>50% loss in sulfur signal) may occur in a relatively short time frame (tens of minutes) depending on the exact nature of the sample and XPS instrument. This motivated the development and validation of a multipoint XPS data acquisition protocol that minimizes Nafion damage, resulting in reliable data acquisition by avoiding significant artifacts from Nafion instability. The developed protocol was then used to analyze both thin film ionomer samples and Pt/C-based CLs. Comparison of PEM fuel cell CLs to Nafion thin films revealed several changes in Nafion spectral features attributed to charge transfer due to interaction with conductive catalyst and support species. This study provides a method to reliably characterize ionomer-containing samples, facilitating fundamental studies of the catalyst-ionomer interface and more applied investigations of structure-processing-performance correlations in PEM fuel cell and electrolyzer CLs.

Confinement of ionomer for electrocatalytic CO2 reduction reaction via efficient mass transfer pathways

Gas diffusion electrodes (GDEs) mediate the transport of reactants, products and electrons for the electrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assemblies. The random distribution of ionomer, added by the traditional physical mixing method, in the catalyst layer of GDEs affects the transport of ions and CO2. Such a phenomenon results in elevated cell voltage and decaying selectivity at high current densities. This paper describes a pre-confinement method to construct GDEs with homogeneously distributed ionomer, which enhances mass transfer locally at the active centers. The optimized GDE exhibited comparatively low cell voltages and high CO Faradaic efficiencies (FE > 90%) at a wide range of current densities. It can also operate stably for over 220 h with the cell voltage staying almost unchanged. This good performance can be preserved even with diluted CO2 feeds, which is essential for pursuing a high single-pass conversion rate. This study provides a new approach to building efficient mass transfer pathways for ions and reactants in GDEs to promote the electrocatalytic CO2RR for practical applications.

Solvent Effects on the Catalyst Ink and Layer Microstructure for Anion Exchange Membrane Fuel Cells

Understanding the complex solvent effects on the microstructures of ink and catalyst layer (CL) is crucial for the development of high-performance anion exchange membrane fuel cells (AEMFCs). Herein, we study the solvent effects within the binary solvent ink system composed of water, isopropyl alcohol (IPA), commercial anion exchange ionomer, and Pt/C catalyst. The results show that the Pt/C particles and ionomer tend to form large aggregates wrapped with a thick ionomer layer in IPA-rich ink and promote the formation of large mesopores within the CL. With the increase of the water content in the ink, Pt/C particles are more likely to bridge to each other through wrapped FAA to form a well-connected three-dimensional network. The CL fabricated using water-rich ink shows smaller pores, higher porosity, and a more homogeneous ionomer network without the formation of large aggregates. Based on these results, we propose that the properties of the solvent mixture, including dielectric constant (ε) and solubility parameter (δ), affect the coulomb interaction of charged particles and surface tension at interfaces, which in turn affects the microstructure of ink and CL. By leveraging the solvent effects, we optimize the CL microstructures and improve the performance of AEMFC. These results may guide the rational design and fabrication of AEMFCs.

Hydrogen Promotes the Growth of Platinum Pyramidal Nanocrystals by Size-Dependent Symmetry Breaking

The growth of pyramidal platinum nanocrystals is studied by a combination of synthesis/characterization experiments and density functional theory calculations. It is shown that the growth of pyramidal shapes is due to a peculiar type of symmetry breaking, which is caused by the adsorption of hydrogen on the growing nanocrystals. Specifically, the growth of pyramidal shapes is attributed to the size-dependent adsorption energies of hydrogen atoms on {100} facets, whose growth is hindered only if they are sufficiently large. The crucial role of hydrogen adsorption is further confirmed by the absence of pyramidal nanocrystals in experiments where the reduction process does not involve hydrogen.