Fluorine-doped nickel cobalt oxide spinel as efficiently bifunctional catalyst for overall water splitting

Exploring highly active, low cost, and robust electrocatalysts for overall water splitting remains a great challenge. Herein, a fluorine-doped nickel cobaltate spinel (NiCo2O4) is derived from the bimetallic nickel/cobalt metal-organic framework (CoNi-MOF) by a facile solvothermal approach and followed by an annealing process. It displays that the proposed NiCo2O4 nanostructure is combined with electrocatalytic active NiO and CoF2 nanoparticles. All of these nanoparticles are embedded homogeneously within the mesoporous carbon matrix to form abundant catalytic sites, leading to a strong synergetic effect with ionic/semi-ionic F and N atoms (denoted as NiCo2O4/NiO/CoF2@mC). It shows that the NiCo2O4/NiO/CoF2@mC hybrid obtained under 700 °C (denoted as NiCo2O4/NiO/CoF2@mC700) can be employed as efficiently bifunctional catalysts for hydrogen evolution (HER) and oxygen evolution reactions (OER) in alkaline solution. The NiCo2O4/NiO/CoF2@mC700 hybrid presents the faster and more efficient charge transfer, thus displaying an excellent electrocatalytic activity with low overpotentials of −45.3 mV and 1.47 V to reach a current density of 10 mA cm−2 and Tafel slopes of 29 and 78 mV dec−1 for the HER and OER, respectively. A two-electrode electrolyzer based on the NiCo2O4/NiO/CoF2@mC700 catalyst as both the anode and cathode exhibits outstanding stability for sustained water splitting with a cell voltage of only 1.56 V at 10 mA cm−2. This study not only opens a new avenue for designing efficient and flexible catalysts but also extends the application of metal organic frameworks in renewable energy resources.