A rotating disc electrode study of oxygen reduction at platinised nickel and cobalt coatings

Platinized nickel and cobalt coatings, Pt(Ni) and Pt(Co), have been prepared on glassy carbon, GC, rotating disc electrode substrates by a two-step room temperature procedure that involved the electrodeposition of nickel and cobalt layers and their spontaneous partial replacement by platinum ("transmetalation") when immersed into a chloroplatinic acid solution. By tuning the quantity of initially deposited nickel and cobalt, Pt(Ni) and Pt(Co) bimetallic coatings having a 26% atom Ni and 30% atom Co composition have been prepared. For both materials typical Pt surface electrochemistry was recorded during fast voltammetry in deaerated acid, pointing to the existence of a continuous Pt skin over a Pt-Ni and Pt-Co core. Oxygen reduction at the Pt(Ni)/GC and Pt(Co)/GC electrodes was studied by means of steady-state voltammetry at a rotating disc electrode and the construction of Tafel plots from corresponding voltammetric data. It was found that, when the initial potential of the voltammetric sweep allowed the formation of a complete Pt oxide monolayer, then oxygen reduction was hindered for low overpotentials at Pt(Ni) and Pt(Co), compared to pure bulk Pt. On the other hand, when the initial potential was less positive (thus leading to the formation of a fraction of surface oxide monolayer) the presence of Ni and Co enhanced the kinetics of oxygen reduction. The former behaviour is attributed to a decrease in oxide reduction ability of Pt in the presence of Ni and Co, while the latter to an increase in dissociative oxygen chemisorption due to Ni and Co.