Morphological Instability in Topologically Complex, Three-Dimensional Electrocatalytic Nanostructures

Advances in electrocatalyst functionality have resulted from the evolution of complex nanostructured materials with increasing degrees of compositional and morphological complexity. Focused almost entirely on pushing the boundaries of intrinsic activity, electrocatalytic material development often overlooks stability. Operating in parallel to the typical mechanisms of electrochemical material degradation, three-dimensional nanomaterials are susceptible to an additional degradation process known as coarsening. Driven by the reduction of surface free energy, surface diffusion evolves the nano porous morphology toward a solid spherical particle. Here, using nanoporous NiPt alloy nanopartides (np-NiPt/C) as a representative three-dimensional electrocatalytic material, we demonstrate that coarsening is the dominant mechanism of degradation as observed during accelerated durability testing (ADT). The upper potential limit (UPL) of the ADT protocol is found to have a significant impact on coarsening, with the rate roughly scaling with the UPL. Here we demonstrate the viability of a methodology to limit the coarsening process by decoration of the surface with a foreign metal impurity, Ir, possessing a surface diffusivity lower than that of the catalytic species. Ir, present in a low coverage with negligible impact on the intrinsic activity, dramatically slows morphology evolution. This strategy is shown to result in significant improvements in the electrochemically active surface area and transition metal alloying component retention up to a UPL of 1.1 V versus the reversible hydrogen electrode. This proof-of-concept result demonstrates the utility of this strategy for improving the balance between activity and stability for three-dimensional electrocatalytic nanomaterials with potential application to a broad range of nanoscale geometries and compositions.