Spectroscopic in situ Measurements of the Relative Pt Skin Thicknesses and Porosities of Dealloyed PtM n (Ni, Co) Electrocatalysts.

X-ray adsorption near edge structure (XANES) data at the Co or Ni K-edge, analyzed using the Δμ difference procedure, are reported for dealloyed PtCo x and PtNi x catalysts (six different catalysts at different stages of life). All catalysts meet the 2017 DOE beginning of life target Pt mass activity target (>0.44 A mgPt-1), but exhibit varying activities and durabilities. The variance factors include different initial precursors, dealloying in HNO3 vs H2SO4, if a postdealloying thermal annealing step was performed, and different morphologies (some with a multi PtM x core and porous Pt skin, some single core with nonporous skin). Data are obtained at the initial beginning of life (BOL, ~200 voltage cycles) and after 10k and 30k (end of life, EOL) voltage cycles following DOE protocol (0.6-1.0 V vs reversible hydrogen electrode). The Δμ data are used to determine at what potential (Vpen) the Pt skin is penetrated by O. The durability, related to a drop in the electrochemical surface areas (ECSAs) after extensive voltage cycling, directly correlates with the Vpen at BOL. The data indicate that cycling produces a "characteristic" Pt skin robustness (porosity or thickness). When the Pt skin at BOL is "thin" (Vpen < 0.9 V) it grows to a "characteristic" thickness consistent with a Vpen of ≈1.1 V, and if it begins very thick, it thins to the same "characteristic" thickness. Particles dealloyed in H2SO4 appear to have a thicker Pt skin at BOL than those dealloyed in HNO3, and a postdealloying annealing procedure appears to produce a particularly nonporous skin with high Vpen, but not necessarily thicker. Furthermore, the PtM3 catalysts exhibited a fast skin "healing" process whereby the initial porous skin appears to become more nonporous after holding the potential at 0.9 V. This work is believed to be the first in situ XAS study to shed light on the nature of the Pt skin, its thickness, and/or porosity, and how it changes with respect to operating electrochemical conditions.