(282c) Improved Activity and Durability of Ionic Liquid Composite Nanoporous Nanoparticle Electrocatalysts for Oxygen Reduction Reaction

The development of efficient and stable catalysts for the cathodic oxygen reduction reaction (ORR) is one of the greatest challenges for the exploitation of polymer electrolyte membrane fuel cells (PEMFCs). Dealloying has shown increasing utility in the field of electrocatalysis as a tool for the synthesis and development of nanoporous materials possessing high surface-to-volume ratios with controlled morphology and compositional gradient (core-shell structure) [1]. After electrochemical dealloying, the open, bicontinuous, three-dimensional nanoporous nanoparticle electrocatalysts exhibit dramatically enhanced electrocatalytic properties [2,3]. We have taken advantage of the free volume within these nanoporous nanoparticles and created a composite catalyst architecture through the incorporation of a hydrophobic, protic ionic liquid (IL), specifically [MTBD][beti]. Modification of the interface through the formation of IL thin film on the surface of an electrocatalyst yields enhanced catalytic rates by chemically biasing reactants to the surface and changing the degree of solvation/stabilization of adsorbed intermediates.

In the development of efficient electrocatalysts for ORR, durability is too often ignored in the pursuit of higher activities. Here we studied the manner by which ILs impact the stability of 3-dimensional, nanoporous nanoparticle electrocatalysts. 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. We use a combination of in-situ and ex-situ experimental techniques, under accelerated degradation conditions, to develop insight into the structural and compositional evolution of nanoporous PtNi nanoparticles (np-NiPt) formed through the dealloying of Pt₂₀Ni₈₀precursor nanoparticles, and demonstrate that facilitated diffusion of stable surface atoms under relevant electrochemical conditions will drive evolution of the particle morphology from a porous to a nonporous structure. 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 [3]. 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 and residual transition metal content of these complex composite nanoporous electrocatalysts was tracked as a function of cycle number during accelerated degradation testing providing insight into the mechanism of catalyst degradation and the effect of ILs on catalyst stability.

References

[1] Snyder, J.; McCue, I.; Livi, K. & Erlebacher, J., J. Am. Chem. Soc. 134,8633-8645 (2012).

[2] Snyder, J.; Fujita, T.; Chen, M. W. & Erlebacher, J., Nature Mater. 9,904-907 (2010).

[3] Snyder, J.; Livi, K. & Erlebacher, J., Adv. Funct. Mater. 23, 5494-5501 (2013).