Synthesis And Electrocatalytic Water Splitting Performance Of Transition Metal Nitride Based Hybrid Arrays

Electrocatalytic water splitting is an efficient and clean technology to produce high-purity hydrogen.The process of water splitting consists of the hydrogen evolution reaction（HER）on the cathode and oxygen evolution reaction（OER）on the anode.The suitable HER and OER electrocatalysts should be designed to reduce the required overpotential and improve the energy conversion efficiency.Although the noble metal-based catalysts exhibit outstanding electrocatalytic performance,their large-scale applications are limited by their high costs and scarce reserves.In addition,most reported electrocatalysts can only work at low current densities(<100 m A cm^-2),which cannot meet the requirement of actual industrial electrolyzers(>400 m A cm^-2).Therefore,the preparation of efficient non-noble metal-based electrocatalyst is important for promoting the rapid development of hydrogen production technology from electrocatalytic water splitting.Transition metal nitride（TMN）has the advantages of high conductivity,excellent chemical stability,and outstanding electrocatalytic activity.In view of the above advantages,we have synthesized a series of self-supporting TMN-based hybrid arrays electrocatalysts by optimizing composition and structure,and further studied their electrocatalytic performance.In this thesis,we have provided new design ideas for the development of non-noble metal-based electrocatalysts with low cost,high electrocatalytic activity,and high stability.The main contents are as follows:1.We have fabricated a core-shell heterostructure arrays on Ni foam（denoted as Ni Fe-MOF@Ni₃N/NF）by growing Ni Fe-MOF nanosheets onto the top of Ni₃N microsheets arrays,and which is used as a high-efficiency OER electrocatalyst for water splitting.In the Ni Fe-MOF@Ni₃N/NF,Ni Fe-MOF nanosheets and metallic Ni₃N microsheet arrays are served as the main OER catalytic components and self-supporting conductive scaffolds,respectively.The core-shell heterostructure architecture ensures high exposure and dispersion of active sites,and rapid charge transfer.In 1 M KOH electrolyte,Ni Fe-MOF@Ni₃N/NF electrode only needs an overpotential of 281 m V to deliver a current density of 200 m A cm^-2.This result indicates the feasibility of using metallic Ni₃N as scaffolds for MOFs’growth to enhance its electrocatalytic OER activity.2.We have prepared a 3D hierarchical nano/microsheet arrays on Ni foam（denoted as Ni Fe LDH@Ni₃N/NF）by electrodepositing Ni Fe LDH nanosheets onto the top of Ni₃N microsheets arrays,and which is used as a bifunctional electrocatalyst for overall water splitting.The typical 3D hierarchical heterostructure architecture of Ni Fe LDH@Ni₃N/NF affords abundant exposed active sites.The strong coupling effect between Ni Fe LDH nanosheets and Ni₃N microsheets provides a robust integrated structure and ensures fast electron transfer.The superhydrophilic/superaerophobic surface properties of Ni Fe LDH@Ni₃N/NF promote the mass transfer particularly at large current densities.Benefiting from the above advantages,Ni Fe LDH@Ni₃N/NF exhibits excellent HER and OER activities in alkaline media.By using Ni Fe LDH@Ni₃N/NF as a bifunctional electrocatalyst for overall water splitting,the alkaline aqueous electrolyzer exhibits a low cell voltage of1.80 V at a current density of 500 m A cm^-2 with a remarkable durability of 100 hours in 1M KOH electrolyte.3.A self-supported nickel-iron nitride microsheet arrays coated with carbon has been grown on Ni foam（Ni_xFe_yN@C/NF）,and which is used as an electrocatalyst for alkaline seawater splitting.The porous architecture and superhydrophilic/superaerophobic surface properties of Ni_xFe_yN@C/NF ensure sufficient exposure of active sites and fast mass transfer at large current densities.The synergetic coupling effect between the carbon coating and Ni_xFe_yN facilitates the electron transfer efficiency.When the molar ratio of Ni to Fe in Ni_xFe_yN@C/NF is 3,the catalyst（Ni₃Fe N@C/NF）has the best OER electrocatalytic performance in 1 M KOH.And the catalyst without Fe（Ni₃N@C/NF）has the best HER electrocatalytic performance.A two-electrode electrolyzer has been assembled by using Ni₃Fe N@C/NF and Ni₃N@C/NF as the anode and cathode,respectively.In the alkaline seawater electrolyte,the electrolyzer can deliver a large current density of 500 m A cm^-2 at a low cell voltage of 1.91 V.The abundant active sites,superior corrosion-resistance and electron conductivity of Ni_xFe_yN@C/NF are responsible for its excellent durability up to100 hours.