Surface-Selective Pt Enrichment for Stabilizing Ultrathin 2D PtCu Nanosheets Toward Durable Oxygen Reduction

Ultrathin Pt-based 2D alloy nanosheets are promising electrocatalysts for the oxygen reduction reaction (ORR) due to their high surface-to-volume ratios and favorable facet-dependent activity. However, their thermodynamic instability and susceptibility to transition metal dissolution present critical barriers to their practical application. While surface Pt enrichment could mitigate such degradation, constructing Pt-rich surfaces on 2D nanosheets remains challenging, as conventional approaches often induce structural collapse or undesired thickening. Herein, we report a surface-selective Pt enrichment strategy via galvanic replacement to stabilize ultrathin PtCu nanosheets (PtCu NS/C) while preserving their unique 2D integrity. PtCu@Pt NS/C features a Pt-enriched outer region and an alloyed subsurface, which effectively suppresses Cu exposure while maintaining beneficial electronic interactions. PtCu@Pt NS/C delivers high ORR activity, outperforming commercial Pt/C. More importantly, PtCu@Pt NS/C exhibits remarkable durability, with 17.5% mass activity loss after 30 000 accelerated stress test cycles. Notably, in single-cell PEMFC configurations, PtCu@Pt NS/C demonstrates outstanding voltage stability with a minimal loss of 4 mV at 0.2 A cm^-2, whereas PtCu NS/C and Pt/C suffer much larger losses of 41 and 38 mV, respectively. This work demonstrates that preferential near-surface Pt enrichment effectively stabilizes PtCu nanosheets, offering a surface-engineering approach for improving the durability of ultrathin 2D alloy electrocatalysts.