Asymmetric ruthenium-iron dipole enabling fast alkaline water splitting on ruthenium-doped iron-nickel layered double hydroxides

Abstract

Electrically driven water splitting is an efficient method for green hydrogen production; however, its practical application is substantially constrained by the kinetically sluggish anodic oxygen evolution reaction (OER). Ruthenium (Ru) and its oxides are widely recognized as highly active OER catalysts. Although Ru is significantly cheaper than iridium (Ir), further reducing its content remains desirable. Herein, atomically dispersed Ru is doped into iron-nickel layered double hydroxides (Ru-FeNi-LDH) to decrease the Ru usage. We found that the Ru doping limit is roughly 9 wt%, and the Ru doping content significantly alters the OER kinetics—note that the high Ru concentration remarkably damages the Ru-FeNi-LDH structure and leads to agglomeration formation. By optimizing the Ru doping content to 3.3 wt%, the Ru-FeNi-LDH presents a low overpotential of 230 mV to reach a current density of 10 mA cm⁻² in 1 M KOH, which is far better than the reference FeNi-LDH (280 mV) and RuO₂ (350 mV). In the overall water splitting test, the current density of 10 mA cm⁻² can be reached at a low voltage of 1.52 V, with stable operation for 80 h. Interestingly, Ru and Fe form an asymmetric Ru-Fe dipole, which is likely doped together into the LDH because the content of Fe instead of Ni is dependent on Ru content in experimental results. The electron-deficient feature of the Ru-Fe dipole thus facilitates the OER process. This work demonstrates a dual-transition metal synergy, providing a design strategy for OER and related catalysts.