Modifying Electronic Structure Of Nickel-Based Transition Metal Layered Double Hydroxide For Photo/Electrocatalytic Water Splitting

The energy crisis and environmental pollution become more and more serious,which calls for the development of efficient,clean and cheap energy carriers.Hydrogen owns high energy density,light weight and non-polluting combustion products,thus it is considered to be an ideal alternative energy carrier for traditional fossil fuels.More-over,there are abundant water resources on the earth,making photo/electrocatalytic hydrogen evolution possible to alleviate the energy crisis.Generally,noble metal pro-moters or sacrificial reagents are usually required in the photocatalysis process.The popular catalyst of hydrogen evolution is platinum,and the best catalysts of oxygen evolution are iridium-based and ruthenium-based（Ir O₂,Ru O₂）catalysts in the electro-catalysis.However,noble metals are expensive and limited.Meanwhile,the use of sac-rificial agents also causes energy loss.Therefore,in this thesis,we adopt single-phase transition metal hydroxides as model catalysts to study the photo/electrocatalytic pro-cess for overall water splitting.In chapter 3,we employed an intensive laser to ablate a Ni Co alloy target im-mersed in alkaline solution,and produced so-called Ni Co(OH_1-x)₂ nanosheets with non-stoichiometric composition and O^2-/Co³⁺ions exposed on the surface.The nonstoichi-ometric composition broadens the band gap,while O^2-and Co³⁺ions boost hydrogen and oxygen evolution reactions,respectively.As such,the photocatalyst achieves a H₂evolution rate of 1.7μmol h^-1 under the AM 1.5G sunlight irradiation and apparent quantum yield（AQE）of 1.38%at 380 nm.Herein,we demonstrate that nonstoichio-metric nickel-cobalt double hydroxide could implement overall water splitting by itself upon sunlight irradiation,avoiding the consumption of noble-metal co-catalyst for the first time.In chapter 4,a hydrothermal method was adopted to synthesize different valence states of Ir doped Ni Fe-LDH.Ir³⁺-doped Ni Fe-LDH delivers an ultralow overpotential(19 m V@10 m A cm^-2)hydrogen evolution reaction（HER）,which is superior to Ir⁴⁺doped Ni Fe-LDH(44 m V@10 m A cm^-2)and even commercial Pt/C catalyst(40 m V@10 m A cm^-2),and reaches the highest level ever reported for Ni Fe-LDH-based catalysts.Theoretical and experimental analyses reveal that Ir³⁺ions donate more electrons to their neighboring O atoms than Ir⁴⁺ions,which facilitates the water dissociation and hydrogen desorption,eventually boosts hydrogen evolution reaction.The same va-lence-state effect is found for Ru and Pt dopants in Ni Fe-LDH,implying that chemical valence state should be considered as a common factor on modulating catalytic perfor-mance.In chapter 5,a hydrothermal method was employed to synthesize Ni Fe-LDH doped with amphoteric metals（Al,Zn and Cd）.The hydroxide containing metal vacan-cies are obtained after etching in hot concentrated alkaline solution.Among them,Cd-doped Ni Fe-LDH achieved the lowest overpotential,85 m V,for alkaline hydrogen evo-lution of at a current density of 10 m A cm^-2,while the performance of Zn and Al doped samples decreases sequentially.This is mainly because Cd²⁺can be used as an active site for H₂O adsorption,while metal vacancies affect the charge density of oxygen in the M-O-M bond,thereby optimize the adsorption of H protons.eventually improve the catalytic performance.In contrast,the Al and Zn dopants only generate metal vacancies,and do not cause active sites.In chapter 6,Ni Fe-LDH was doped with rare-earth elements（Sm,Ce and La）.Ni Fe Sm-LDH shows the best OER catalytic activity.At a current density of 10 m A cm^-2,the overpotential of Ni Fe Sm-LDH is only 210 m V,the Tafel is 11 m V dec^-1.Sm with the strongest electronegativity acquires more electrons from Ni than the other two ele-ments according to XPS analysis,which increases the valence of Ni,optimizes the ad-sorption of O intermediates,and effectively improves the catalytic activity.Based on the above studies,the photo/electrocatalytic activity can be effectively improved by adjusting the electronic structure of transition metal hydroxides.our work provides new ideas for the design of highly active catalysts in the future.