Interface Engineering with Ionic Liquid Composite Materials for Efficient and Durable Electrocatalysis
Authors: Li Yawei , Snyder Joshua David 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) are among the most promising candidates in carbon neutral renewable energy systems because of their superior efficiency, environmentally friendly principles, as well as a wide range of potential applications 1. The development of catalytic materials for electrochemical oxygen reduction has received significant attention as the sluggish kinetics of cathodic oxygen reduction reaction (ORR) is the major source of efficiency loss in current PEMFCs. To this end, significant achievements have been made in the past decade to improve both intrinsic and mass-based activity through the evolution of nanoscale morphology and the compositional profile 2-4. However, these adjustments on geometric and compositional profiles remain insufficient for the widespread integration of fuel cells into industry applications 5. It is necessary to look for new strategies, such as shifting focus away from the catalyst and move toward the optimization of electrocatalytic interfaces.
Electrocatalyst development for reactions involving multiple charge transfer steps and more than one adsorbed intermediate is severely limited by the scaling relations that govern the free energy of adsorption of those intermediates. Adsorbed intermediate scaling relations create a thermodynamic minimum overpotential of 0.37 V due to the fixed difference in free energy of adsorption (ΔΔGad) for *OH and *OOH of ~3.2 eV which is independent of the identity of the catalyst 1,6. It is found that by decreasing the degree of intermediate solvation, ΔΔGad can approach 2.46 eV which is the value required for ORR to take place at zero overpotential 7. Here we present our progress in addressing intermediate scaling for the ORR through the manipulation of metal/electrolyte interface with chemically tailored ionic liquids (ILs). Figure 1 demonstrates the impact of IL on the ORR on Pt(111). A significant increase in the ORR current is observed at both low overpotential and the potential region at which an adsorbed monolayer of hydrogen is formed on Pt. At low overpotential we attribute the enhanced activity to the disruption of *OH-H2O hydrogen bonded structure due to the control of the interaction of water with the catalytic surface in the presence of the IL. We argue that it is this change in water interaction with the surface, potentially in combination with other properties of the IL including reactant solubility, that results in the enhanced ORR activity.
We also studied the manner by which ILs impact the stability of 3-dimensional, nanoporous nanoparticle electrocatalysts, such as nanoporous PtNi nanoparticles (np-NiPt). In addition to the standard mechanisms of catalyst degradation including Pt dissolution/Ostwald ripening and coalescence/aggregation, nanoporous materials can lose electrochemically active surface area (ECSA) through surface smoothening driven coarsening 8. With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we have developed strategies to mitigate coarsening through incorporating ILs at the metal/electrolyte interface to directly mitigate electrochemically enhanced surface diffusion, which leads to ECSA loss. The hydrophobicity of these ILs is known to change the interaction of water with the surface, positively shifting the onset potential of Pt oxidation. The interfacial IL acts to limit the charge dependent formation of a surface metal/electrolyte anion complex which is responsible for the potential dependent enhancement in surface diffusion. The oxygen reduction reaction activity, ECSA of these complex composite nanoporous electrocatalysts, as shown in Figure 2, was tracked as a function of cycle number during accelerated durability testing (ADT) providing insight into the mechanism of catalyst degradation and the effect of ILs on catalyst stability.

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Figure 1