Coupling thulium 4f orbitals with Ni3Fe LDH loaded with Pt to form an electronic buffer band for catalyzing alkaline overall water splitting

Abstract

Nickel–iron layered bimetallic hydroxides (Ni₃Fe LDHs) are regarded as excellent materials for electrodes for electrolyzing water but still require high overpotentials because of their weak adsorption/desorption of oxygen intermediates by metal–oxygen bonding (M–O). Herein, the rare earth element thulium (Tm) was selected for incorporation into Ni₃Fe LDH owing to its particular electronic structure of 4f¹³6s², and the composite LDH was regulated using an electron-compensated buffer to avoid the overoxidation of M–O and was proven to be an excellent OER electrocatalyst. By further loading platinum (Pt) on the Tm-doped LDH (Ni₃Fe LDH/NiFe₂O₄/Pt–Tm), the HER performance was also found to be greatly improved, and overall water splitting catalysts could be constructed utilizing the two types of Ni₃Fe LDH-based materials. The state-of-the-art optimized Ni₃Fe LDH/NiFe₂O₄/Pt–Tm composite in 1.0 M KOH solution achieved an OER overpotential as low as 224.0 mV at a current density of 10.0 mA cm⁻², which was one of the lowest among previously reported LDHs, with a Tafel slope of 54.81 mV dec⁻¹ and a turnover frequency (0.1969 s⁻¹). For HER catalysis, Ni₃Fe LDH/NiFe₂O₄/Pt–Tm exhibited an even lower overpotential of 125.0 mV (vs. 238.0 mV for bare Ni₃Fe LDH) at 10.0 mA cm⁻², and only 1.69 V was required to drive overall water splitting. The electrocatalytic stability was evaluated for 5000 cycles of cyclic voltammetry scanning and chronoamperometry tests. Raman and XPS spectra confirmed that structural regulation promoted the generation of NiOOH and the adsorption of intermediates. In particular, oxygen vacancies and unsaturated ligands were preserved, providing abundant active sites for electrocatalysis.