Structural, electronic, and electrochemical analyses of sputter-coated Pt and Pt–Co/GCE electrodes with ultra-low metal loadings for PEM fuel cell applications

Abstract: In this study, unsupported platinum and platinum–cobalt ultra-thin catalyst layers with varying metal loadings were prepared by a sequential magnetron sputtering technique. The deposited catalyst layers were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), X-ray diffraction, cyclic voltammetry, and rotating disk electrode methods. Ultra-low Pt loadings, in the range of 6–22 µg cm⁻², were used to examine the kinetic properties of the deposited catalyst layers. The results revealed that the highest electrochemically active surface area, 89.7 m² g⁻¹, was obtained for Pt:Co (1:3) nanocatalyst with an optimum Pt loading of 10 µg cm⁻². The XPS and UPS results indicated that the catalyst layers prepared at elevated temperatures contained a high degree of Pt–Co interaction. It was seen that the incorporation of Co atoms into the Pt lattice improved the oxygen reduction reaction (ORR) activity of the catalysts at certain Pt:Co concentrations and sputtering temperatures. Among all catalyst layers, the highest mass and specific current density values (793.08 A gPt-1 and 1163.18 µA cm⁻², respectively) were obtained from the electrode with 10 µg cm⁻² Pt loading prepared at 300 °C. Although the introduction of Co atoms in the catalyst layer did not result in a systematic enhancement in the ORR performance, the Pt:Co (1:1) composition deposited at 300 °C had the highest ORR performance with a mass activity of 571.17 A gPt-1 and a specific activity of 835.59 µA cm⁻² among the all Co-containing catalysts. Our results indicated that heat treatment, surface structure, and composition have profound effects on the performance of the catalyst layers. Graphical Abstract: [Figure not available: see fulltext.]