Investigation of CoS2-based thin films as model catalysts for the oxygen reduction reaction

Three CoS2-, NiS2- and (Co,Ni)S2-based thin films were prepared by magnetron sputtering and studied as catalysts for the oxygen reduction reaction (ORR). Electrochemical assessments indicate that all three films have significant ORR catalytic activities, with that of (Co,Ni)S2-type showing the best performance with regard to both open circuit potential (OCP) and current density. The ternary film has an OCP value of 0.89 V vs. the reversible hydrogen electrode, and shows a closer approach to values for Pt than have been obtained to date for other transition metal chalcogenides. The thin films assessed by electrochemistry have been characterized by a range of techniques including high-resolution X-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and scanning Auger microscopy. The CoS2-based film has a wrinkled surface, which appears relatively unchanged after an electrochemical durability test, while the NiS2-like film preferentially loses Ni and the (Co,Ni)S2-like film (based on nanoparticles with approximate composition Co0.6Ni0.4S2) undergoes a phase separation (spinodal decomposition). All these films show an excess of S in the as-prepared form and after electrochemistry; evidence is presented for the possible presence of some polysulfides.