(16b) Deconvoluting Ionomer-Electrocatalyst Interactions for the Oxygen Reduction Reaction on Polycrystalline Pt Surfaces

Proton exchange membrane fuel cells (PEMFCs) are a key technology for transitioning to renewable energy by utilizing O₂ and H₂ as reactants. The primary limitation in PEMFC performance arises from the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. While many studies have focused on tuning electrocatalyst composition to optimize ORR activity, recent efforts highlight the critical role of the electrocatalyst-electrolyte interface in influencing reaction kinetics and mass activity across different catalyst structures.

This work investigates the impact of tetrabutylammonium (TBA⁺) cations within Nafion on polycrystalline (pc) Pt as a model system to understand their influence on ORR activity using rotating disk electrode experiments. We demonstrate the promotive kinetic effects of TBA⁺- Nafion formulations through variations in the Tafel slope, with Nafion (68 mV/dec) being slightly higher compared to TBA⁺-Nafion (62 mV/dec), and an increased O₂ mass transport current density for TBA⁺-Nafion compared to Nafion alone. Potentiostatic electrochemical impedance spectroscopy reveals that TBA⁺- Nafion films exhibit lower O₂ mass transport resistance (111.6 ohm) compared to Nafion alone (188.8 ohm) at 0.7 V RHE, further corroborating ORR polarization data

Ex situ X-ray photoelectron spectroscopy (XPS) indicates that Nafion undergoes greater degradation on Pt pc disks after ORR compared to TBA⁺- Nafion films, where the ionomer remains intact. Morphological analysis via atomic force microscopy infrared spectroscopy reveals distinct swelling dynamics between TBA⁺- Nafion and Nafion films post-reaction. In addition, ionomer film stability varies within the TBA⁺- Nafion system, where spectroscopically, Nafion chemical signatures remain consistent across the Pt interface. In contrast, Nafion films without TBA⁺ exhibit varied spectroscopic features, indicating ionomer inhomogeneity and dynamic changes after ORR. Pt stability was evaluated using inductively coupled plasma mass spectrometry, which revealed no Pt dissolution in TBA⁺-Nafion films (baseline dissolution), whereas Nafion films exhibited a Pt dissolution rate of 0.15 ng cm⁻² s⁻¹. Interestingly, while Pt dissolution is often correlated with increased oxide formation, both ex situ XPS and in situ X-ray absorption spectroscopy (XAS) indicate greater oxide features in the presence of TBA⁺. These oxide species are attributed to oxygenated adsorption states associated with ORR, suggesting a favorable microenvironment for enhanced reaction kinetics.

Altogether, these findings provide insight into how electrode-ionomer interactions influence activity, mass transport, and material stability under operating conditions. By systematically investigating well-defined electrocatalyst-ionomer interfaces, this study establishes a framework for optimizing PEMFC catalyst inks and demonstrates how TBA⁺ can modulate Pt surface structure to promote ORR electrocatalysis.