Spectroscopic<i>in situ</i>Measurements of the Relative Pt Skin Thicknesses and Porosities of Dealloyed PtM_<i>n</i>(Ni, Co) Electrocatalysts

Abstract

X-ray adsorption near edge structure (XANES) data at the Co or Ni K-edge, analyzed using the Δ<i>μ</i> difference procedure, are reported for dealloyed PtCo _<i>x</i> and PtNi _<i>x</i> 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 mg_Pt^-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 _<i>x</i> 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 Δ<i>μ</i> data are used to determine at what potential (<i>V</i>_pen) 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 <i>V</i>_pen at BOL. The data indicate that cycling produces a "characteristic" Pt skin robustness (porosity or thickness). When the Pt skin at BOL is "thin" (<i>V</i>_pen < 0.9 V) it grows to a "characteristic" thickness consistent with a <i>V</i>_pen 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 <i>V</i>_pen, 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 <i>in situ</i> 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.