The Synthesis And Properties Towards Hydrogen Evolution Reaction In Hybrid-water Electrolysis Of Ni-base Composites Coated On Ni Foam Substrate

The energy demand and environmental pollution brought about by the development of science and technology and the increase in population have made traditional fossil fuels unable to meet needs.Hydrogen has a large energy density,a wide range of sources,and no pollution to the environment.It is a research hotspot in recent years,and it is also an internationally recognized clean energy with huge application potential.Hydrogen production by electrolyzed water is one of the most promising hydrogen production methods currently developed.It consists of two half reactions:cathode hydrogen evolution reaction（HER）and anode oxygen evolution reaction（OER）.Among them,OER has a complex proton-coupled electron transfer process and a high theoretical oxidation potential,which will significantly reduce energy efficiency and limit large-scale production of hydrogen.Therefore,the anode reaction with a lower theoretical potential can be used instead of OER to achieve more efficient hydrogen evolution.For example,the ammonia oxidation reaction（AOR）,urea oxidation reaction（UOR）,and hydrazine oxidation reaction（Hz OR）（theoretical oxidation potentials are-0.77,0.37,and-0.33 V）.At the same time,urea is the main component of urine in urban wastewater and will be converted into toxic ammonia and nitrogen-containing compounds in natural processes.Ammonia is the main component of feed and fertilizer wastewater,causing eutrophication of water and causing the growth of certain microorganisms such as algae.Hydrazine is a major component of pharmaceutical and petrochemical wastewater and has permanent damage to human and animal skin.Through electrolysis,urea,ammonia or hydrazine can be effectively decomposed,reducing environmental pollution.UOR,AOR and Hz OR mixed water electrolysis can not only produce hydrogen efficiently,but also purify wastewater to meet the needs of energy conversion and environmental protection.The most effective way to reduce energy consumption is to reduce the overpotential of the electrode.At present,Ir（or Ru）and Pt-based materials are the best anode and cathode catalysts,respectively,but these catalysts have low storage capacity and high price,which greatly limits their practical applications.Therefore,the development of new and efficient dual-function platinum-free electrocatalysts that simultaneously catalyze HER and UOR（or AOR or Hz OR）has become the key to electrocatalytic hydrogen production technology.In recent years,due to its unique structure and electronic characteristics,the potential of foamed nickel-based catalysts in the field of efficient hydrogen evolution has attracted attention.The main content of this thesis includes the following aspects:（1）Preparation of nickel nitride and its performance in water-urea electrolysisThe synthesis method of Ni₃N/NF nanospheres is firstly a nanowire array on Ni（OH）₂/NF nickel foam is prepared as a template by hydrothermal method at 120℃,and then autocatalytically reduced to Ni/NF foamed nickel on a 90°C oil bath The nanosphere array continued to be used as a template,and finally calcined into a nanosphere on Ni₃N/NF foamed nickel by calcination under an ammonia atmosphere at 350℃.The composition,structure and morphology of the prepared nickel nitride were characterized by X-ray diffraction,X-ray photoelectron spectroscopy,Raman spectroscopy,and scanning electron microscopy.Their electrocatalytic performances were compared by linear sweep voltammetry,cyclic voltammetry,electrochemical impedance spectroscopy and chronoamperometry.the electrocatalytic hydrogen evolution performance of synthetic materials are tested,and found that the Ni₃N/NF produced in the alkaline solution,UOR showed super high activity and stability,and also had excellent catalytic properties and durability in HER.In a two-electrode electrolytic cell,the battery voltage can reach 100 m A·cm^-2 with only 1.423 V（vs·RHE）.Therefore,it can be used as a bifunctional catalyst for efficient urea oxidation and electrocatalytic hydrogen evolution.（2）Preparation of nickel nitride and MOF composites and their performance in water-urea electrolysisNovel MOF-derived nickel nitride on nickel foam catalysts were successfully synthesized at different temperature（300℃,350℃,400℃）（denoted as C-T,T=300,350,400）.The C-350was demonstrated as a highly active and durable 3D catalyst electrode for urea oxidation reaction（UOR）,with a required potential of 1.340 V to achieve a geometric catalytic current density of 10 m A/cm².It is worth noting that C-350 has high hydrogen evolution reaction（HER）activity.It provides a low overpotential of 47 m V at 10 m A/cm²,which enables it to become a bifunctional catalyst of UOR and HER for overrall water-urea splitting,and its two-electrode alkaline electrolyzer only needs a voltage of 1.503 V to obtain 100 m A/cm²,which is 0.160 V lower than that of pure water splitting and the current density can be maintained for at least 24hours,which demonstrates that its excellent activity and stability have commercial feasibility.（3）Preparation of nickel molybdate and nickel hydroxide composites and their Performance in water-urea electrolysisNi MoO₄-Ni（OH）₂/NF is synthesized by hydrothermal reactions and explored as both hydrogen evolution reaction（HER）and UOR catalyst electrodes.This composite catalyst shows high catalytic bifunctional activities towards both HER and UOR.To validate both catalytic UOR and HER activities and stability,a two-electrode water-urea electrolyzer composed of Ni MoO₄-Ni（OH）₂/NF as both anode and cathode materials is constructed（Ni MoO₄-Ni（OH）₂/NF||Ni MoO₄-Ni（OH）₂）.Experiments show that at a current density of 10m A·cm^-2,a voltage of 1.341 V with a high stability（over 3000 cycles）can be achieved,which are much better than those obtained using a Pt/CllIr O₂.（4）Preparation of nitrogen-doped nickel-zinc-copper ternary hydrotalcite and reduced graphene oxide composites and their performance in hybrid water electrolysisA bifunctional catalyst N-NiZnCu LDH/r GO on nickel foam is synthesized.To validate its electrochemical performance and stability,a two-electrode electrolyzer composed of N-NiZnCu LDH/r GO as both anode and cathode materials is constructed（N-NiZnCu LDH/r GO||N-NiZnCu LDH/r GO）.Experiments show that at a current density of10 m A·cm^-2,the voltages of AOR,UOR,and Hz OR are 0.489,1.305,and 0.010 V with a high stability（over 3000 CV cycles）,which are much better than those of Pt/C||Ir O₂.This study demonstrates N-NiZnCu LDH/r GO can replace precious metals for commercial hydrogen energy production in the hybrid-water electrolysis and be employed for treatment of industrial wastewater.