Copper-Organic Framework Fabricated with CuS Nanoparticles: Synthesis, Electrical Conductivity, and Electrocatalytic Activities for Oxygen Reduction Reaction.

To apply electrically nonconductive metal-organic frameworks (MOFs) in an electrocatalytic oxygen reduction reaction (ORR), we have developed a new method for fabricating various amounts of CuS nanoparticles (nano-CuS) in/on a 3D Cu-MOF, [Cu3 (BTC)2 ⋅(H2 O)3 ] (BTC=1,3,5-benzenetricarboxylate). As the amount of nano-CuS increases in the composite, the electrical conductivity increases exponentially by up to circa 109 -fold, while porosity decreases, compared with that of the pristine Cu-MOF. The composites, nano-CuS(x wt %)@Cu-BTC, exhibit significantly higher electrocatalytic ORR activities than Cu-BTC or nano-CuS in an alkaline solution. The onset potential, electron transfer number, and kinetic current density increase when the electrical conductivity of the material increases but decrease when the material has a poor porosity, which shows that the two factors should be finely tuned by the amount of nano-CuS for ORR application. Of these materials, CuS(28 wt %)@Cu-BTC exhibits the best activity, showing the onset potential of 0.91 V vs. RHE, quasi-four-electron transfer pathway, and a kinetic current density of 11.3 mA cm-2 at 0.55 V vs. RHE.