A Cubane-Like Cobalt Complex in Water Oxidation: Synthesis, True Catalyst, and Catalytic Mechanism

Abstract

In this study, we examine the OER performance of a known cuboidal Co complex, ([CoII 4(dpy{OH}O)4(OAc)3(H2O)](ClO4)(1.7H2O) dpk = di(2-pyridyl) ketone, (complex 1)). Our experiments show that at a pH of 11, complex 1 acts as a precursor for the true oxygen-evolution reaction (OER) catalyst. During the OER process, nanoparticles composed of Co, O, N, and C form on the electrode surface. These nanoparticles were analyzed using a range of techniques, including energy-dispersive spectroscopy (EDX), scanning electron microscopy (SEM), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), elastic recoil detection analysis, UV-visible spectroscopy (UV-vis), Raman spectroscopy, and electrochemical methods. XAS was utilized to confirm unequivocally the presence of CoO(OH) as the true catalyst under OER. Elastic recoil detection analysis (ERDA) detected a reduction in the overall N/Co ratio, indicating decomposition of the Co complex and ligand removal. Considering all the experimental data, it is proposed that the resulting nanoparticles are indeed CoO(OH) as the true OER catalysts, especially when combined with Fe impurities. This investigation offers a fresh perspective on the role of metal (hydr)oxide nanoparticles formed in the presence of multinuclear cobalt complexes during the OER process.