Single-Atom to Ultrasmall Au Nanoparticles Anchored on NiFe Layered Double Hydroxide as Catalyst for Oxygen and Hydrogen Evolution Reactions

Owing to the limited supply and high cost, the rational design of precious metal-based catalysts is of essential importance for boosting the electrocatalytic activity. Herein, the variation of Au species from single-atoms to ultrasmall nanoparticles (3–6 nm) deposited on NiFe-layered double hydroxide (NiFe-LDH) was investigated as a bifunctional electrocatalyst for oxygen and hydrogen evolution reactions. The brucite-like layered structure of NiFe-LDH was verified by X-ray diffraction. The existence of Au single-atom and ultrasmall nanoparticles as well as the local bonding environment of Au species were evaluated by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The integration of Au into NiFe-LDH as single atoms and metallic nanoparticles shows significant improvement in the electrocatalytic activity in 1 M KOH. Single-atom Au/NiFe-LDH (2Au/NiFe-LDH, 0.21 wt % Au) delivers the lowest overpotential of 209 mV to catalyze 10 mA cm^–2 OER current density with the Tafel slope of 46.5 mV dec^–1. On the other hand, Au nanoparticles/NiFe-LDH (50Au/NiFe-LDH, 3.22 wt % Au) possesses an overpotential of 150 mV at 10 mA cm^–2 HER current density with the Tafel slope of 134.5 mV dec^–1. For overall water splitting, under the optimum conditions, the 2Au/NiFe-LDH//50Au/NiFe-LDH couple requires the cell potentials of 1.62 and 1.78 V to deliver the current density of 10 and 100 mA cm^–2 in 1 M KOH. These findings provide insights into the effect of decoration of Au species on the NiFe-LDH catalyst surface for the efficient performance in the production of O₂ and H₂ in the alkaline electrolyte.