Fast kinetic redox process in layered cobaltous terephthalate MOF-type for aqueous hybrid devices. Magnetic properties as sensor of Co–Co interactions

Abstract

The understanding of the fast-redox reactions origin is crucial for developing highly stable hybrid devices for energy storage. In this work two layered structures were considered, labelled as Co–H2O and Co-Pyz, where water and pyrazine molecules are occupying the axial coordination sites (pillars), respectively. Electrochemical characterization clearly revealed that these materials exhibit two faradic processes. The first one is controlled by diffusion (slow process) while the second process is meanly of superficial nature (fast process). These results are in accordance with apparent diffusion coefficient of charge-compensating ions estimated by Electrochemical Impedance Spectroscopy. In-situ XANES and FT-IR characterization unravelled that Co3+/Co2+ redox process paired with hydroxyl ions insertion and desertion from the MOF are involved in the fast redox process. When pyrazine is present as pillar between cobaltous terephthalate lamellas, the solid exhibits larger currents and lower charge transfer resistance than in presence of water as pillar. The improved performance of the pyrazine containing solid, is mainly ascribed to increased exchange interactions and a weaker spin-orbit coupling for the cobalt ion, which contributes to delocalize its electrons (unpaired electrons redistribution during redox reaction). This favours an easier electron mobility when an external stimulus is applied, resulting in a lower resistance and higher current, in comparison to the hydrated analogue. Finally, a hybrid device was assembled with Co-Pyz and commercial activated carbon, exhibiting a paramount stability by retaining its initial capacitance after 5000 GCD cycles at 2.5 Ag−1.