A new electrochemical synthesis route was developed to prepare spinel-type ZnCo2O4 and Co3O4 as high quality thin film-type electrodes for use as electrocatalysts for oxygen evolution reaction (OER). Whereas Co3O4 contains Co(2+) in the tetrahedral sites and Co(3+) in the octahedral sites in the spinel structure, ZnCo2O4 contains only Co(3+) in the octahedral sites; Co(2+) in the tetrahedral sites is replaced by Zn(2+). Therefore, by comparing the catalytic properties of ZnCo2O4 and Co3O4 electrodes prepared with comparable surface morphologies and thicknesses, it was possible to examine whether Co(2+) in Co3O4 is catalytically active for OER. The electrocatalytic properties of ZnCo2O4 and Co3O4 for OER in both 1 M KOH (pH 13.8) and 0.1 M phosphate buffer (pH 7) solutions were investigated and compared. The results suggest that the Co(2+) in Co3O4 is not catalytically critical for OER and ZnCo2O4 can be a more economical and environmentally benign replacement for Co3O4 as an OER catalyst.
New phases of Zn-Co-layered double hydroxides (Zn-Co-LDHs) were synthesized for the first time via a co-precipitation reaction using hydrogen peroxide as an oxidant. According to powder X-ray diffraction and field emission-scanning electron microscopy, both nitrate-and sulfate-forms of the Zn-Co-LDHs crystallized with the brucite-type layer structure having interlayer nitrate and sulfate anions, respectively, and commonly showed plate-like morphology with a crystal size of several hundred nanometers. dc magnetic susceptibility measurements revealed that the Zn-Co-LDHs displayed ferromagnetic/antiferromagnetic transitions below 15 K and the magnetic moment calculated from the paramagnetic region (30-300 K) indicated the co-existence of weak field Co 2+ and strong field Co 3+ ions. The mixed oxidation state of Co 2+ /Co 3+ was confirmed by the results of iodometry and X-ray absorption near-edge structure spectroscopy. The heat-treatment for the Zn-Co-LDHs at elevated temperatures produced mixed metal oxide nanocomposites composed of spinel ZnCo 2 O 4 and wurzite ZnO phases. The colloidal suspension of exfoliated Zn-Co-LDH nanosheets could be synthesized by dispersion of the pristine LDH materials in formamide, which was confirmed by the Tyndall phenomenon, high resolution-transmission electron microscopy/selected area electron diffraction, and UV-vis spectroscopy. The Zn-Co-LDH film fabricated with the restacked nanosheets exhibited pseudocapacitive behavior with a large specific capacitance and a good capacitance retention. The present findings underscore that the newly synthesized mixed valence Zn-Co-LDH phases showed promising functionality as a supercapacitor electrode material and also showed interesting magnetic coupling behavior.
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