Insights into the active nickel centers embedded in graphitic carbon nitride for the oxygen evolution reaction

Experimental and theoretical studies have demonstrated that the use of single atom catalysts (SACs) for energy conversion processes is very promising. However, their stability under catalytic conditions is the main issue that hinders their commercial use. In this work, we report an oxygen evolution catalyst based on single nickel atoms stabilized in triazine-based carbon nitride (CN) and a detailed study of the evolution of the Ni centers under catalytic conditions. The nanostructured materials have been characterized by combining experimental techniques, such as X-ray diffraction, transmission electron microscopy, X-ray absorption and X-ray photoemission spectroscopy, with DFT theoretical calculations to determine the CN structure, the metal adsorption sites, the coordination of the Ni atoms, and the changes undergone under catalytic conditions. Electrochemical characterization showed a linear increase of the catalytic activity with Ni loading. The stability of the materials was studied by HR-TEM and XAS post-catalysis measurements and DFT simulations. Results indicated a partial chemical restructuring of the single Ni atoms under catalytic conditions with the formation of Ni-O-Ni moieties, stabilized in the CN cavities, which are the real catalytic species. Combination of experimental work and DFT calculations to establish the structural changes undergone by single Ni atoms stabilized in carbon nitride under OER conditions.