Two-dimensional (2D) Co-based MOF-on-MOF heterojunction nanostructures with improved electrocatalytic activity were successfully constructed via a mild two-step solution route, employing Co2+ ions as the center atoms, and 1,4-benzenedicarboxylate (BDC) and 4,4′-biphenyldicarboxylate (BPDC) as ligands. The as-obtained heterojunction nanostructures were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) technologies. Electrochemical measurements showed that as-prepared Co-BPDC/Co-BDC heterojunction nanostructures presented markedly enhanced OER electrocatalytic activity, compared with single Co-BPDC, Co-BDC, and/or their physical mixture. Also, the Co-BPDC/Co-BDC-3 heterojunction prepared after treatment for 3 h exhibited the strongest catalytic activity. To reach the current density j geo = 10 mA cm–2, the Co-BPDC/Co-BDC-3 heterojunction-modified glassy carbon electrode required an overpotential of 335 mV in 1 M KOH, which was reduced by 57 and 93 mV, compared to the electrodes modified by Co-BDC and Co-BPDC, respectively. Simultaneously, the heterojunction catalyst also displayed better long-term stability. The improvement of the above performances should be attributed to the increased structure stability, BET surface area, ECSA, and electron transfer ability of the heterojunction.

Metal−organic frameworks (MOFs) have attracted much interest in electrocatalysis as promising catalysts in recent years. However, it still remains a challenge to directly use MOF-based materials as bifunctional electrocatalysts for overall water splitting. Herein, a hierarchical Co,Fe-MOF-74/Co/carbon cloth (CC) threelayer hybrid electrode is synthesized by a simple electrochemical deposition technology with subsequent solvothermal treatment. Electrochemical performance investigations show that the asconstructed Co,Fe-MOF-74/Co/CC electrode exhibits impressive electrocatalytic activity for full water splitting, which only requires a low overpotential of 226 mV@20 mA cm −2 for the oxygen evolution reaction (OER) and 94 mV@−10 mA cm −2 for the hydrogen evolution reaction (HER), respectively. Employing the as-constructed electrode as the anode and cathode in a two-electrode setup simultaneously, only 1.65 V of the voltage is needed to deliver the current density of 20 mA cm −2 . The above excellent electrochemical performances should be attributed to the specific architecture, the synergistic effect of the Co and Fe species, and the superhydrophilic surface of the catalyst. More importantly, further investigations uncover that metal hydroxides and oxyhydroxides produced in situ during electrochemical tests are the veritable active species, which promote the high catalytic activity of the present catalyst.

A ZIF-67-self-sacrificing template strategy was designed for the synthesis of undoped/Cl-doped α-Co(OH)2 hollow nano-dodecahedrons with enhanced OER performance.

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