Electrochemical impedance spectroscopy investigation of spinel type cobalt oxide thin film electrodes in alkaline medium

Synthesis, structure, oxygen evolution reaction (OER) and visible-light assisted organic reaction studies on A2M2TeB2O10 (A = Ba and Pb; M = Mg, Zn, Co, Ni, Cu, and Fe)

Compounds with the general formula A2M2TeB2O10 (A = Ba and Pb; M = Mg, Zn, Co, Ni, Cu, and Fe) have been synthesised via solid-state techniques and characterised. The structure exhibits M2B2O10 layers connected by TeO6 octahedra giving rise to a three-dimensional structure with voids, where Ba2+ ions reside. Substitution of Mg by transition elements (M = Co, Ni, and Cu) in Ba2Mg2TeB2O10 and (Ba0.5Pb1.5)Mg2TeB2O10 gives rise to interesting colored compounds. NIR reflectivity studies indicated that white-colored compounds exhibited good NIR reflectivity, which was is comparable to that of TiO2. Dielectric studies indicated reasonable values with low dielectric loss at low frequencies. The cobalt-substituted compounds Ba2(MgCo)TeB2O10 and (Ba0.5Pb1.5)(MgCo)TeB2O10 were explored towards the oxygen evolution reaction (OER) in an alkaline medium. The compound (Ba0.5Pb1.5)(MgCo)TeB2O10 was found to be a good electrocatalyst for the OER with a faradaic efficiency of ∼96%. The Cu-substituted compound Ba2(Mg1.5Cu0.5)TeB2O10 was found to be a good photocatalyst for the formation of α-chloroketones under visible light in the presence of molecular oxygen.

Operando Identification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution

Co oxides and oxyhydroxides have been studied extensively in the past as promising electrocatalysts for the oxygen evolution reaction (OER) in neutral to alkaline media. Earlier studies showed the formation of an ultrathin CoO_x(OH)_y skin layer on Co₃O₄ at potentials above 1.15 V vs reversible hydrogen electrode (RHE), but the precise influence of this skin layer on the OER reactivity is still under debate. We present here a systematic study of epitaxial spinel-type Co₃O₄ films with defined (111) orientation, prepared on different substrates by electrodeposition or physical vapor deposition. The OER overpotential of these samples may vary up to 120 mV, corresponding to two orders of magnitude differences in current density, which cannot be accounted for by differences in the electrochemically active surface area. We demonstrate by a careful analysis of operando surface X-ray diffraction measurements that these differences are clearly correlated with the average thickness of the skin layer. The OER reactivity increases with the amount of formed skin layer, indicating that the entire three-dimensional skin layer is an OER-active interphase. Furthermore, a scaling relationship between the reaction centers in the skin layer and the OER activity is established. It suggests that two lattice sites are involved in the OER mechanism.