Adsorption/Desorption Behaviors of Sulfur Species at Pt Single Crystal Surfaces

Abstract

Platinum is an excellent electrocatalyst for polymer electrolyte membrane (PEM) fuel cells because it drives hydrogen oxidation and oxygen reduction with very high efficiency. However, when sulfur species contained as a trace amount of impurity in hydrogen fuel and/or pollutant in air adsorb on platinum surfaces, power output significantly decrease because of reduced electrochemically active surface area. Such negative effect caused by the adsorption of sulfur species, so-called sulfur poisoning, is a serious problem because interaction between sulfur species and platinum surfaces is very strong. Thus, understanding and control of adsorption behavior of sulfur species at platinum surfaces are very important to maintain the cell performance of PEM fuel cells. In the present study, adsorption/desorption behaviors of sulfur species at well-defined platinum single crystal surfaces in acidic aqueous solutions are studied by electrochemical measurements, x-ray photoelectron spectroscopy (XPS), and surface x-ray diffraction (SXRD).Prior to each experiment, platinum single crystal surfaces were annealed by induction heating method [1, 2, 3] at 1600°C more than 1 h under the flowing of Ar and H2 mixed gas, followed by cooling under the Ar and H2 flow for 7 min. The surface was immersed in a 1 mM Na2S aqueous solution saturated with Ar and H2 mixed gas, for 1 h. After rinsing with ultrapure water, electrochemical measurements were carried out in an aqueous solution of 0.1 M HClO4 or H2SO4 deaerated with pure N2 gas.At the bare Pt(111) surface, characteristic current responses such as adsorption/desorption of hydrogen and hydroxyl group were observed. After immersing in the sulfur-containing solution, however, those characteristic current responses disappeared, confirming the adsorption of sulfur species and decrease of electrochemically active surface area. XPS in the S 2p region of the Pt(111) surface immersed in the sulfur-containing solution showed three major adsorbed species; adsorbed hydrosulfide ion (Pt-HS- ad) at 161.8 eV, adsorbed sulfur (Pt-Sad) at162.7 eV, and adsorbed polysulfide (Pt-Sn) at 164.3 eV. While Pt-HS- ad reductively desorbed from the surface by the potential sweeping up to -0.2 V vs. Ag/AgCl, Pt-Sad and Pt-Sn oxidatively desorbed by the potential sweeping up to the potential more positive than 0.7 V vs. Ag/AgCl. SXRD showed that the sulfur species adsorbed at the Pt(111) surface in the (√3×√3)R30° structure.At the Pt(100) surface, the oxidative desorption of sulfur species occurred at more positive potential than that at the Pt(111) surface. This indicates that the recovery from sulfur poisoning at the Pt(100) surface is more difficult than that at the Pt(111) surface. Unlike the Pt(111)-(√3×√3)R30° structure, SXRD showed that the sulfur species adsorbed at the Pt(100) surface in the (√2×√2)R45° structure. While the sulfur species occupies a three-fold hollow site of the Pt(111)-(√3×√3)R30° structure, it occupies a four-fold hollow site of the Pt(100)-(√2×√2)R45° structure. The different coordination number of adsorbed sulfur species is considered as the origin of different oxidative desorption behaviors.