Single-Atom to Ultrasmall\nAu Nanoparticles Anchored\non NiFe Layered Double Hydroxide as Catalyst for Oxygen and Hydrogen\nEvolution Reactions

Owing to the limited supply and high cost, the rational\ndesign\nof precious metal-based catalysts is of essential importance for boosting\nthe electrocatalytic activity. Herein, the variation of Au species\nfrom single-atoms to ultrasmall nanoparticles (3–6 nm) deposited\non NiFe-layered double hydroxide (NiFe-LDH) was investigated as a\nbifunctional electrocatalyst for oxygen and hydrogen evolution reactions.\nThe brucite-like layered structure of NiFe-LDH was verified by X-ray\ndiffraction. The existence of Au single-atom and ultrasmall nanoparticles\nas well as the local bonding environment of Au species were evaluated\nby high-resolution transmission electron microscopy and X-ray photoelectron\nspectroscopy. The integration of Au into NiFe-LDH as single atoms\nand metallic nanoparticles shows significant improvement in the electrocatalytic\nactivity in 1 M KOH. Single-atom Au/NiFe-LDH (2Au/NiFe-LDH, 0.21 wt\n% Au) delivers the lowest overpotential of 209 mV to catalyze 10 mA\ncm<sup>–2</sup> OER current density with the Tafel slope of\n46.5 mV dec<sup>–1</sup>. On the other hand, Au nanoparticles/NiFe-LDH\n(50Au/NiFe-LDH, 3.22 wt % Au) possesses an overpotential of 150 mV\nat 10 mA cm<sup>–2</sup> HER current density with the Tafel\nslope of 134.5 mV dec<sup>–1</sup>. For overall water splitting,\nunder the optimum conditions, the 2Au/NiFe-LDH//50Au/NiFe-LDH couple\nrequires the cell potentials of 1.62 and 1.78 V to deliver the current\ndensity of 10 and 100 mA cm<sup>–2</sup> in 1 M KOH. These\nfindings provide insights into the effect of decoration of Au species\non the NiFe-LDH catalyst surface for the efficient performance in\nthe production of O<sub>2</sub> and H<sub>2</sub> in the alkaline\nelectrolyte.