Modifying Interface Solvation to Improve the Oxygen Reduction Reaction Electrocatalysis
Authors:   , Gawas Ramchandra , Snyder Joshua , Tang Maureen 
Published: October 2023

Increasing the accessibility of green, affordable hydrogen and reducing the cost of polymer electrolyte membrane fuel cells (PEMFC) devices are critical for the widespread commercialization of hydrogen powered fuel cells. The high cost of PEMFCs is in part due to the high Pt leadings in the cathode catalyst layer (CCL) to compensate for kinetic and transport losses during the sluggish oxygen reduction reaction. Therefore, substantial research efforts have been focused on modifying the CCL and its constituent components to achieve cost parity with other advanced energy technologies, particularly in the transportation sector. Hydrophobic ionic liquids (ILs) have been used as interfacial additives for oxygen reduction reaction (ORR) electrocatalysts, demonstrating enhanced catalyst activity and durability. However, incorporating ILs or IL-modified catalysts into the electrodes of a PEMFC membrane electrode assembly (MEA) has proven to be challenging. To address this limitation, we develop a new ionomer chemistry with orthogonal properties of protonic conductivity and ionic liquid functionality: sulfonated poly(ionic liquid) block copolymers (S-PILBCPs). The new ionomer in the Pt/C CCLs yields a two-fold improvement in the kinetic activity, both in the half-cell and MEA. Furthermore, a Nafion/S-PILBCP composite ionomer substantially improves the high current density performance as well. Despite remarkable improvement in performance at half-cell and MEA-level, the mechanism of this improvement is not completely understood. We combine single crystal voltammetry with microkinetic modeling to analyze the mechanism of ORR enhancement in presence of ionic liquids. With iterative feedback between single crystal voltammetry and microkinetic model output, we establish the importance of accounting for lateral interactions between adsorbed oxygenated species by using real data informed adsorption isotherms. With our experimentally validated model, we show that the mechanism of impact of ILs on ORR activity is through exclusion of water and reduction in electrochemical interface solvation, yielding lower spectator hydroxyl coverages and a reduced barrier for hydroxyl removal, resulting in higher surface site availability during the ORR. The ORR activity of electrocatalysts is traditionally calculated using the current density measured at 0.9 V vs. RHE during the anodic sweep of the ORR polarization curve. This approach enables a quick and reliable screening technique for novel ORR electrocatalysts. However, linear sweep techniques often conceal transient features which could arise during the constant current or potential operation in real devices. We observe that ORR current significantly decays over time at constant potentials as a result of dynamic competition for active sites between the spectator hydroxyl and reactant oxygen species. We employ well-defined single crystal surfaces with the help of microkinetic modeling to get insights into this transient phenomenon observed during the ORR. This improved understanding will help design Pt-interfaces to moderate the interaction of the interfacial water network with Pt active sites, providing another strategy for improving ORR kinetics.