Understanding and Controlling Nanoporosity Formation for Improving the Stability of Bimetallic Fuel Cell Catalysts

Nanoporosity is a frequently reported phenomenon in bimetallic particle ensembles used as electrocatalysts for the oxygen reduction reaction (ORR) in fuel cells. It is generally considered a favorable characteristic, because it increases the catalytically active surface area. However, the effect of nanoporosity on the intrinsic activity and stability of a nanoparticle electrocatalyst has remained unclear. Here, we present a facile atmosphere-controlled acid leaching technique to control the formation of nanoporosity in Pt–Ni bimetallic nanoparticles. By statistical analysis of particle size, composition, nanoporosity, and atomic-scale core–shell fine structures before and after electrochemical stability test, we uncover that nanoporosity formation in particles larger than ca. 10 nm is intrinsically tied to a drastic dissolution of Ni and, as a result of this, a rapid drop in intrinsic catalytic activity during ORR testing, translating into severe catalyst performance degradation. In contrast, O₂-free acid leaching enabled the suppression of nanoporosity resulting in more solid core–shell particle architectures with thin Pt-enriched shells; surprisingly, such particles maintained high intrinsic activity and improved catalytic durability under otherwise identical ORR tests. On the basis of these findings, we suggest that catalytic stability could further improve by controlling the particle size below ca. 10 nm to avoid nanoporosity. Our findings provide an explanation for the degradation of bimetallic particle ensembles and show an easy to implement pathway toward more durable fuel cell cathode catalysts

Evolution of Structure and Activity of Alloy Electrocatalysts during Electrochemical Cycles: Combined Activity, Stability, and Modeling Analysis of PtIrCo(7:1:7) and Comparison with PtCo(1:1)

This study explores the changes in bulk composition/structure and oxygen reduction activity of two alloys, Pt₇IrCo₇ and PtCo, caused by Co leaching during electrochemical cycles and as a result of membrane electrode assembly (MEA) fabrication procedures. Exposure to liquid electrolyte and electrochemical cycles in a rotating disc electrode (RDE) environment resulted in substantial Co loss and no stabilization from the low levels of Ir used in the ternary material. The true composition of the ternary material was determined as Pt₈IrCo₃ following initial exposure to 0.1 M HClO₄ (before cycling) and Pt₁₁IrCo₄ after 5000 cycles. Density functional theory (DFT) modeling of the cycled catalyst compositions indicated that structures with Pt-rich upper layers would show the highest stability; however, addition of 0.25 ML oxygen adsorption favored Co segregation from second and third atomic layers. The high initial activities (>0.44A/mgPt) achieved in the RDE environment decreased with cycles and were not reproduced in MEAs. X-ray diffraction (XRD) analysis revealed a measurable increase in lattice parameter caused by the MEA preparation procedure, consistent with Co (and some Ir) leaching into the ionomer phase and relaxation of the lattice. MEA fabrication procedures and cycling in 1 M H₂SO₄ at 80^◦C showed greater changes to catalyst structure and increased Ir and Co loss compared to exposing the catalyst to RDE like conditions (0.1 M HClO₄, RT) explaining the observed discrepancy in activity between RDE and MEA