Surfactant assisted geometric barriers on PtNi@C electrocatalyst for phosphoric acid fuel cells

Abstract

The alloying strategy of Pt with Ni is commonly employed to improve the catalytic activity of electrocatalysts owing to the higher activity required to lower the binding energy of oxygen with Pt. When Pt is alloyed with Ni, electron transfer from Ni to Pt is conducive to metallic Pt, lowering the oxygen binding energy. Although the alloying strategy can increase the catalytic activity, confirming that the surface state has the intended electronic structure is difficult because of the oxidation or dissolution of metals. To determine the surface state of catalyst nanoparticles for a more efficient structure, the concept of physical blocking of the reactive surface was applied to determine the geometric effect. The oxygen reduction reaction in phosphoric acid electrolytes is more sensitive to the geometric effect due to anion adsorption. In this work, we synthesized carbon-supported PtNi alloying nanoparticles with different carbon shell thicknesses (PtNi@Cx; x = 0.5, 1, 2, 4, where x refers to the concentration of the surfactant) from various concentration of organic surfactants. Among the nanoparticles with various carbon shell thicknesses, PtNi@C2 showed the best oxygen reduction reaction performance in the presence of phosphoric acid with an appropriate geometric effect, ensuring a synergetic effect with alloying.