Gold Nanoparticles Incorporated in a Zinc-Based Metal-Organic Framework as Multifunctional Catalyst for the Oxygen Reduction and Hydrogen Evolution Reactions

Successful synthesis of gold nanoparticles incorporated in a zinc-based metal-organic framework (Au@Zn-MOF) is reported in this paper. The synthesis of Au@Zn-MOF is confirmed by UV-Vis, FT-IR and X-ray photoelectron spectroscopy (XPS) studies. The Au@Zn-MOF catalyst demonstrates electrocatalytic activity towards the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). The relative catalytic performance of Au@Zn-MOF towards ORR and HER has been studied under acidic condition. ORR proceeds via a two electron and two proton mechanism with hydrogen peroxide as the end product, while HER follows the Volmer mechanism i.e., adsorption of H+ on the catalyst's active sites. Au@Zn-MOF exhibits an ORR onset potential of 0.45V (vs. RHE) with two different Tafel slopes, -93 and -103.6 mV in acidic solution. Further, an excellent catalytic activity is observed for HER with an onset potential of 0.02 V (vs. RHE) and a Tafel slope of 87 mV in N-2 saturated 0.1 M HClO4 solution. However, in O-2 saturated 0.1 M HClO4 solution, a HER onset potential of 0.04 V (vs. RHE) is observed with two different Tafel slopes, 610 and 220 mV. The value of Tafel slopes in the presence of O-2 advocates the diminution of the ORR activity because of the HER. Thus, Tafel slope values of HER and ORR suggest that protons and O-2 compete to reach the electrode surface for getting reduced.