Ni–Co–S–Se Alloy Nanocrystals: Influence of the Composition on Their in Situ Transformation and Electrocatalytic Activity for the Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) is an important half reaction in various technologies, such as metal air batteries and electrochemical water splitting. Transition metal chalcogenides, especially those based on Co and Ni, are emerging as promising OER catalysts, thanks to their high activity and low cost. However, it is still being debated whether they act as actual catalysts or as catalyst precursors, undergoing structural and morphological changes under OER conditions. To gain a better comprehension of this topic, we have developed a simple colloidal synthesis method for alloy nanocrystals (NCs) based on Ni, Co, S, and Se with a tunable composition, and we studied their structural and morphological evolution during OER. We found that binary CoSe, ternary Ni-Co-Se and quaternary Ni-Co-S-Se NCs, with the exception of NiSe, each undergo structural and morphological changes under OER conditions, forming the corresponding metal oxides/hydroxides, which act as the actual active catalysts. Interestingly, we discovered that the composition of the starting metal chalcogenide NCs plays a major role in dictating the crystallinity, conductivity and activity of such oxide/hydroxide materials. This compositional tuning, that is going from CoSe to Ni0.25Co0.65SO4Se0.6 NCs, resulted in a similar to 7 fold improvement in the OER activity in terms of turnover frequency. Our results suggest that the compositional engineering of metal chalcogenide materials could potentially be used to control their inevitable transformation into the corresponding oxide/hydroxide counterparts, eventually improving their OER activity.