The development of an active and efficient electrocatalyst for the oxygen evolution reaction remains indispensable for the smooth running of an electrolyzer. Herein, we have synthesized two cobalt metal–organic frameworks (Co-MOFs) with the formulas [C6H6CoN2O4] (compound 1) and [C12H10CoN2O4] (compound 2) using pyrazine and 4,4′-bipyridine as linkers in dimethylformamide medium by a solvothermal method. Both Co-MOFs shows strong antiferromagnetic interactions with Θp = −70 and −61 K for compounds 1 and 2, respectively. The in situ transformation of both compounds catalyzes the OER efficiently in alkaline medium, affording a current density of 10 mA/cm2 at overpotentials of 276 ± 3 and 302 ± 3 mV by compounds 1 and 2, respectively. Moreover, compound 1 shows a very high turnover frequency (15.087 s–1), lower Tafel slope (56 mV/dec), and greater Faradaic efficiency of 95.42% in comparison to compound 2. The transformations of the Co-MOFs have been accessed by employing powder X-ray diffraction (PXRD), high-resolution transmission electron microscopic (HRTEM) analysis, and X-ray photoelectron spectroscopy, which reveal the formation of uniform hexagonal Co­(OH)2 plates. Therefore, the as-developed Co-MOF is found to be an efficient pre-electrocatalyst for the OER in alkaline medium. These results not only reveal the preparation of OER electrocatalysts from a Co-MOF but also establish a method to derive a potentially active electrocatalyst to substitute for the traditional noble-metal-based materials.

No abstract

Ternary oxides/sulfides have long been investigated as promising electrode materials for charge storage applications. However, it is important to rationally design nanostructured hybrid composites for superior charge storage performance as electrodes in devices. In this work, MoO 3 @NiCo 2 S 4 hybrid composites materials are synthesized by the hydrothermal method followed by annealing at different temperatures. The charge storage properties of these materials are tested by cyclic voltammetry, galvanostatic charge-discharge curves and electrochemical impedance spectroscopy. It is found that the structure of the hybrid composite material not only assists electron and charge transportation but also precisely control the volume expansion during redox reactions, contributing to superior electrochemical behavior. Among all the electrodes, the electrode fabricated with MoO 3 @NiCo 2 S 4 composite material annealed at 400 • C (MoO 3 @NiCo 2 S 4 -400) is the best for charge storage applications. At 400 • C, MoO 3 spreads as a thin layer of surface polymeric molybdates on NiCo 2 S 4 as seen in the XRD pattern. Significantly, it delivers the highest capacitance of 1622 F g −1 at 1 A g −1 in 2 M aqueous KOH electrolyte compared to other hybrid composite electrodes, NiCo 2 S 4 (962 F g −1 ), MoO 3 @NiCo 2 S 4 -500 (1412 F g −1 ) and MoO 3 @NiCo 2 S 4 -600 (970 F g −1 ), under the same measurement conditions. Furthermore, the MoO 3 @NiCo 2 S 4 -400 hybrid electrode shows better cyclic stability with 93% capacitance retention after 3000 charge-discharge cycles at 8 A g −1 . The synergistic effect of two components and annealing temperature plays important role in enhancing the charge storage performance. This work shows the importance of the synthesis temperature on the functional character of ternary sulfide/oxide composite materials for charge storage applications.

No abstract