Dealloyed binary PtM3 (M = Cu, Co, Ni) and ternary PtNi3M (M = Cu, Co, Fe, Cr) electrocatalysts for the oxygen reduction reaction: Performance in polymer electrolyte membrane fuel cells

Dealloyed Pt bimetallic nanoparticles are highly active electrocatalysts for the electroreduction of molecular oxygen (ORR), the key barrier to more efficient polymer electrolyte membrane fuel cells (PEMFCs). Most previous studies of dealloyed Pt alloys focused on the structure and mechanism of dealloyed Pt-Cu bimetallic materials. Also, stability concerns related to Cu prompted the search for alternative non-noble metal components for dealloying. Here, we report on a comparative study of dealloyed binary PtM3 (M = Co, Cu, Ni) electrocatalyst for use in PEMFC cathodes. We also study synergistic effects of a third metal in ternary PtNi3M (M Co, Cu, Fe, Cr) cathode electrocatalysts. All catalyst precursor materials were prepared by an impregnation, freeze-drying, annealing route. After deployment of the catalyst precursor in single PEM cells, the active dealloyed form of the catalysts was obtained through a voltammetric dealloying protocol. Dealloyed binary PtM3 catalysts showed more than a threefold activity improvement for ORR for M = Co, Cu, and close to a threefold improvement for M = Ni in terms of the Pt-mass activity (A mg(pt)(-1)) of the single fuel cell, compared to a 45 wt% Pt/C reference cathode catalyst. Improvements in specific surface area normalized activities (A cm(pt)(-2)) followed those in Pt-mass activity. All ternary catalysts, except the Fe containing one, showed clearly improved catalytic ORR performance compared to PtNi3, in particular PtNi3Co and PtNi3Cu. A previously unachieved four- to fivefold activity improvement in real single MEAs was observed. Near-surface (XPS) and bulk (EDS/ICP) compositional characterizations suggested that the degree of dealloying of Pt-Co and Pt-Ni binary precursors is lower than that of Pt-Cu compounds. Pt-Co and Pt-Ni still showed 15-20 at.% non-noble metal near the surface and in the bulk of the dealloyed particles, whereas, under the chosen dealloying conditions, Pt-Cu formed core-shell structures with a Pt-rich surface and a Pt-Cu core. Of the selectively characterized Pt-Ni-Co and Pt-Ni-Cu ternaries, the near-surface composition of dealloyed Pt-Ni compounds showed an atomic ratio of about 1:1, compared to about 5:1 in the bulk, pointing to a Ni enrichment at the surface with only small residual amounts of Co or Cu. Our study highlights a number of novel active cathode catalyst compositions and underscores the sensitive dependence of the ORR activity of dealloyed Pt binary and ternary nanoparticle electrocatalysts on the nature and initial composition of the non-noble alloy component. (C) 2010 Elsevier B.V. All rights reserved.