Atmosphere-directed surface composition control on plasma-synthesized PtNi/C nanoparticles: decoupling bulk alloying...

Maximizing the mass activity (MA) of Pt-alloy oxygen reduction reaction (ORR) catalysts requires simultaneous optimization of two often-conflicting parameters: specific activity (SA) and electrochemically active surface area (ECSA). Here, we demonstrate that the post-synthesis annealing atmosphere enables control of the surface composition of PtNi/C nanoparticles independently of their bulk alloy structure, thereby decoupling the SA-ECSA trade-off. A systematic study of 12 catalysts-prepared by solution plasma synthesis and annealed under N₂, dilute H₂ (10% H₂/Ar), and sequential N₂ + H₂ atmospheres at 300-600 °C-reveals that the annealing temperature governs bulk alloying (Pt(111)2θ shift), while the annealing atmosphere governs the near-surface Ni chemical state (XPS Ni 2p_3/2). Under N₂, near-surface NiO_x formation creates a trade-off: SA increases with alloying (r = +0.98 with 2θ), but ECSA decreases due to carbon graphitization (r = +1.00 with I_D/I_G), yielding r(SA, ECSA) = -0.97. Under H₂, the predominance of oxidized Ni species is suppressed, leading to uniformly high SA (1.61-1.74 mA cm_Pt^-2) independent of bulk alloying, while the carbon structural/electrochemical properties are better preserved, yielding r(SA, ECSA) = +0.05-no trade-off. Sequential N₂ + H₂ treatment also eliminates the trade-off (r = +0.49) but yields lower SA, because the prior N₂ annealing at 600 °C irreversibly over-alloys the particles (2θ > 40.5°) and damages the carbon support. The double-layer capacitance normalized by ECSA (Q_dl/ECSA) is ∼2-fold higher for H₂-treated catalysts at ≥400 °C, reflecting changes in both metal surface species and carbon-support surface chemistry. The MA of all 12 catalysts is quantitatively described by MA = SA × ECSA/100 (r = 1.00), confirming that SA and ECSA are independently tunable. The H₂-400 °C catalyst achieves the highest MA of 1424 A g_Pt^-1 at 0.9 V vs. RHE. These results establish atmosphere-directed surface composition control as a rational strategy for decoupling bulk and surface optimization in Pt-alloy ORR catalysts.