Preparation And Electro/Photocatalytic Water Splitting Performance Of Transition Metal Phosphide

Water splitting is a promising strategy to produce clean and sustainable hydrogen energy.However,water splitting process belongs to a thermodynamic uphill reaction,which requires external forces such as electric energy and solar energy to drive oxygen evolution reaction（OER）and the hydrogen evolution reaction（HER）.Noble metal catalysts can accelerate HER and OER processes with negligible reaction overpotentials,but high cost and scarcity limit their large-scale applications in electro/photocatalysis.In recent years,transition metal phosphides have emerged as alternatives to noble metal catalysts due to their tunable crystal structure,good conductivity and low cost.However,the transition metal phosphides have some problems,such as little active sites,low intrinsic activity,poor stability at high current density,corrosion/passivation in OER test and low OER selectivity in seawater splitting.In this thesis,a variety of electro/photocatalysts related to transition metal phosphides with high activities and stabilities were successfully prepared by morphology control,element doping,vacancy introduction,interface engineering and electrode structure design.The research contents are as follows:（1）Large-size,porous,ultrathin bimetallic Ni Co Ox nanosheets are prepared using GO as template by a facile p H-controlled wet-chemical process,followed by phosphatization to obtain a series of Ni Co P nanosheets with the thickness from 3.5 to 12.8 nm.The prepared Ni Co P nanosheets with the thickness of 3.5 nm exhibit excellent HER and OER electrocatalytic activities with extremely low overpotentials of 34.3 and 245.0 m V to afford a current density of 10 m A/cm²in 1.0 M KOH,respectively,outperforming most of the state-of-art binary and ternary phosphides catalysts.Furthermore,the as-prepared ultrathin Ni Co P nanosheets combined with commonly used semiconductors（Cd S or C₃N₄）exhibit remarkably high photocatalytic HER performances under visible-light（λ≥420 nm）irradiation.Among them,the photocatalytic HER rate of Ni Co P/Cd S is 238.2 mmol h^-1g^-1,which is 37times higher than that of pure Cd S(6.3 mmol h^-1g^-1)and also better than that of Pt/Cd S(81.7 mmol h^-1g^-1).Experimental results and density functional theory calculations reveal that ultrathin Ni Co P nanosheets possess abundant active sites and high intrinsic conductivity,and are able to facilitate hydrogen adsorption/desorption,thus exhibiting good electrocatalytic/photocatalytic activities for water splitting.（2）Self-supporting Co-doped Ni P(Ni_xCo_1-xP)nanosheet array electrodes were fabricated by a controllable cation exchange strategy.The Ni P nanosheet array electrodes with abundant P vacancies and Co doping can be obtained without complex chemical reactions and post-processing steps by using Co²⁺to replace the Ni²⁺of the Ni P nanosheet array.Interestingly,P vacancies and Co atoms are enriched on the surface of the Ni P nanosheet array electrode,and the content of P vacancies can be fine-tuned simply by changing the cation exchange concentration.The as-obtained electrode has a rough nanosheet array structure and thus superaerophobic-hydrophilic capability.As a result,the optimal Ni_0.96Co_0.04P nanosheet array electrode has low overpotentials of249.7 m V for HER and 281.7 m V for OER at 1000 m A/cm²,small Tafel slopes of 49.6 m V/dec for HER and 36.5 m V/dec for OER,and good stability in alkaline medium.Further,the Ni_0.96Co_0.04P nanosheet array electrode at 500m A/cm²exhibits an ultralow potential of 1.71 V for overall water splitting,in keeping with an outstanding stability for 500 h.（3）A series of core-shell Ni P/Fe（OH）₃electrocatalysts were prepared by oxidation treatment of Ni P nanosheet array electrodes with K₂Fe O₄.The thickness of the oxide shell can be effectively controlled by adjusting the concentration of K₂Fe O₄,and then the interfacial coupling effect between Ni P and oxide shell can be controlled.Specially,the shielding effect of the oxide shell can prevent the Ni P core from being oxidized during the OER process,ensuring the intrinsic conductivity and morphology stability of the catalyst.In addition,the oxide shell was restructured to form more active Ni Fe O_x（OH）_yat high oxidation potentials.Due to the highly active Ni Fe O_x（OH）_yoxide shell,high conductive Ni P core,the unique nanosheet array morphology,and the interfacial interaction between Ni P and Ni Fe O_x（OH）_y,Ni P/Fe（OH）₃-2nanosheet array electrodes with oxide shell thicknesses of 7～12 nm exhibit excellent OER performance in alkaline media with overpotentials of 146.4,211.6 and 269.9 m V to afford current densities of 10,100 and 500 m A/cm²,respectively.In addition,the electrocatalytic OER stably lasts for a long time（500 h）at a large current density（500 m A/cm²）.Further,two-electrode cell using Ni P/Fe（OH）₃-2 for anode and Ni P for cathode at 500 m A/cm²exhibits an ultralow potential of 1.81 V for overall water splitting,in keeping with an outstanding stability for 500 h.（4）A highly efficient and stable NP-NCF-60 electrode was prepared by surface phosphating and electrodeposition.The electrode consists of surface Ni Co Fe-LDH layer,intermediate Ni P_xlayer and bottom Ni layer.The ultrathin nanosheet features,nanosheet array morphology,and tightly bound interface with Ni P_xof Ni Co Fe-LDH enable the NP-NCF-60 electrode to exhibit high electron transfer efficiency,abundant active sites,and excellent OER intrinsic activity.During the OER process,a large amount of PO_x^n-was released by intermediate Ni P_xlayer to form the anion-rich protective layer on the surface of the Ni Co Fe-LDH layer,realizing the repulsion of chloride ions by the electrode.In alkaline seawater electrolyte solution,the optimal NP-NCF-60electrode has low overpotentials of 252.8 and 286.5 m V at 500 and 1000m A/cm²,respectively.Further,the NP-NCF-60 electrode exhibits nearly 100%OER selectivity and durable stability（500 h）at 500 m A/cm²for overall seawater splitting.