Iridium-doped Nickel Vanadium Layered Double Hydroxide For Overall Water Splitting

Increasingly energy issues urgently require us to develop new green energy.Bifunctional electrocatalysts for both hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）are very crucial for converting water into clean fuel through overall water splitting.A bifunctional catalyst should contain catalytic sites that satisfies both HER and OER,has optimal adsorption energy for both HER and OER intermediates.Therefore,we aimed at designing separate catalytic sites for HER and OER on the surface of Ni-based LDH,so that the intermediates could be adsorbed by such sites and adjusted independently.First,Ir doped Ni V-LDH samples were prepared on Ni foam and the nanosheet array structure was obtained.Ir doped did not change theα-Ni（OH）₂ phase.The detailed characterizations showed that Ir atoms entered the lattice of Ni V-LDH by substituting Ni atoms.Ni VIr-LDH exhibits excellent performance when the Ir content is 14%.The resultant Ni VIr-LDH exhibited excellent performance with the OER and HER overpotential values of 203 m V@10 m A cm^-2 and 42 m V@10m A cm^-2,respectively.Thereby,the newly prepared Ni VIr-LDH catalyst achieved an overall water splitting current of 10 m A cm^-2 under an applied voltage of 1.49 V（as both anode and cathode）,which is much lower than that of the Pt/C–Ir/C couple(1.60V@10 m A cm^-2).Further,A series of Ir-doped LDH with different Ir contents in the Ni VIr-LDH material was prepared.The samples possessed the phase structure ofα-Ni（OH）₂and flake-like morphology.Once the Ir content exceeded 19%,the crystal structure was found to collapse and become morphous.Interestingly,both HER and OER performance first increased and then decreased with the increase in Ir contents.The result shows that water dissociation,HER and OER occur on Ir,bridge O and V atoms,respectively.Therfore,the H*and O*intermediates could be adsorbed by such sites and adjusted independently.The dopant Ir ions were found to play a triple role:（i）Ir ions serve as the catalytic sites for water dissociation;（ii）they reduce charge density on adjacent bridge oxygen,thus facilitating HER there;and（iii）finally they increase charge density on V ions,which in turn boosts OER.As a result,the novel catalyst achieves an ultralow applied voltage for overall water splitting.