Electrocatalysis on bimetallic surfaces: modifying catalytic reactivity for oxygen reduction by voltammetric surface dealloying.

The surface electrocatalytic reactivity of noble metals, for instance Pt, has frequently been modified by alloying Pt with less noble metal atoms within the top surface and/or subsurface layer.1,2 Monolayers of pure Pt deposited on top of non-Pt substrates also showed significantly altered surface catalytic reactivity.3 Here, we report a distinctly different synthetic strategy to modify the surface reactivity of Pt. The synthesis involves electrochemical surface dealloying, that is, selective electrodissolution, of non-noble metal atoms from bimetallic precursors. In particular, we report on significant activity enhancements for the oxygen reduction reaction (ORR) after Cu dealloying from carbon-supported Pt-Cu alloy nanoparticle electrocatalysts. After removal of Cu atoms from the surface region, the resulting particle catalysts showed previously unachieved 4-6-fold activity improvements over pure Pt.4 Pt has been the ORR electrocatalyst of choice for decades, yet the search for more active electrocatalysts continues to be a key scientific and technological challenge in the area of electrochemical energy conversion and has become a “conditio sine qua non” in polymer electrolyte membrane fuel cell research. The electrocatalytic activity of dealloyed Pt-Cu catalysts for the oxygen reduction reaction (ORR) is demonstrated by sweep voltammetric measurements in O2 saturated electrolyte using a rotating disk electrode (Figures 1, 2, and Supporting Information Figure S1). Cyclic voltammetric (CV) characterization of the dealloyed Pt-Cu catalyst surfaces in O2 free electrolyte (inset Figure 1) resembled those of pure Pt, indicating no residual Cu atoms near the surface after dealloying. The CV peak associated with the formation of oxygenated adsorbates (0.8-0.9 V) was shifted to more anodic potentials suggesting the delayed formation of Pt-oxides after dealloying.2,5 The steep portion of the voltammetric ORR curves (Figure 1) were considerably shifted to higher potentials indicating significant ORR activity (negative reduction current density) at lower overpotentials (higher electrode potentials) compared to pure Pt. Figure 2 shows a quantitative comparison of the Pt-mass based (A/mgPt) (Figure 2a) and the Pt surface-area based (μA/cmPt) (Figure 2b) activities of the dealloyed Pt-Cu catalysts in the kinetically controlled regime. At 0.9 V the Pt-Cu nanoparticle catalysts outperformed pure Pt by a unprecendented4 factor of 4-6 times. The catalysts synthesized from a Pt25Cu75 precursor that was annealed at 800 °C even exceeded catalyst performance targets of ORR fuel cell catalysts (0.44 A/mgPt and 720 μA/cmPt at 0.9 V) set by the Department of Energy4 (Figure S1). The electrochemical surface areas (Figure S1c and Table S1) of the dealloyed Pt25Cu75 catalyst showed no significant increase compared to pure Pt, ruling out pure surface area enhancement effects. The synthesis of the electrocatalyst is a two-step process involving the preparation of carbon-supported Cu-rich precursor alloys (Pt/Cu stoichiometry of 1:3, Pt25Cu75) followed by electrochemical dissolution of Cu (dealloying). The Cu-rich catalyst precursors were prepared by a conventional impregnation-reductive annealing method. We compare results obtained from three different catalyst precursors annealed at 600, 800, and 950 °C. Structural characterization of the three precursors was carried out using X-ray diffraction (XRD) (Figure 3). All three precursor compounds showed reflection profiles consistent with a face-centered cubic (fcc) crystal symmetry. The shift of the strong Pt-Cu(111) reflections (at 2θ ≈ 42°) to higher angles compared to the pure Pt(111) peak (2θ ) 39.7°) indicates that Cu and Pt atoms formed disordered Pt-Cu alloy phases with reduced Pt-Pt interatomic distance. With higher temperatures, the (111) peaks became narrower suggesting larger particles. The 600 and 800 °C precursors showed peaks of an unalloyed pure Cu phase. More detailed XRD peak analysis is presented in Figure S2 and Table S1. Profile analysis revealed that all three precursors consisted of two distinct alloy phases (“fcc1”and “fcc2” in Table S1). The compositional differences between phases fcc1 and fcc2 decreased with temperature. The peak intensity ratio between the alloy peaks and the pure Cu peaks increased with temperature. The peak intensity ratio between the Cu-rich alloy phase fcc 2 and the PtFigure 1. Sweep voltammetry of dealloyed Pt25Cu75 catalysts, annealed at 600 °C (black), 800 °C (red), 950 °C (blue), compared to Pt (dotted). The inset shows the cyclic voltammograms of the catalysts in O2 free electrolyte. The horizontal black dotted line indicates the positive and negative capacitance currents. Arrows indicate scan directions.