PtM/C (M = Ni, Cu, or Ag) electrocatalysts: effects of alloying components on morphology and electrochemically active surface areas

The microstructures of Pt/C and PtM/C (M = Ni, Cu, or Ag) electrocatalysts were studied using X-ray diffraction and transmission electron microscopy (TEM). The electrochemically active surface areas of the prepared materials were estimated by cyclic voltammetry in 1 M H2SO4. The materials, with metal contents ranging from 30 to 35 wt.%, were synthesized by chemically reducing the metal precursors in water-ethylene glycol solutions. The actual composition of the bimetallic nanoparticles corresponds to a theoretical (1:1) composition for the PtAg/C catalysts, whereas in the PtNi/C and PtCu/C materials, a portion of the alloying component exists in an oxide form. Decreasing the average metallic crystallite sizes from 3.5 to 1.6 nm does not increase the electrochemically active surface area. This apparent contradiction is because a majority of the PtNi and PtCu nanoparticles consist of 2-4 disordered crystallites. In addition, a portion of the PtNi or PtCu nanoparticle surface is covered by nickel or copper oxides, respectively. PtAg nanoparticles, which have a smaller size relative to other bimetallic particles according to the TEM data, are characterized by an intense platinum surface segregation. The agglomeration processes are lowest for the PtAg nanoparticles.